Fletcher, TH

2000

A Global Free-Radical Mechanism for Light Gas Nitrogen Release from Coal during Devolatilization

Perry, S.T.; Fletcher, T.H.; Pugmire, R.J. and Solum, M.S.
Energy & Fuels, 14, 1094-1102 (2000).
Contact: Fletcher

The Use of Two Mixture Fractions to Treat Coal Combustion in Turbulent Pulverized-Coal Flames

Flores, D.V. and Fletcher, T.H.
Combustion Science and Technology, 150, 1-26 (2000).
Contact: Fletcher

Measurement of Soot and Char in Pulverized Coal Fly Ash

Veranth, J.M.; Fletcher, T.H.; Pershing, D.W. and Sarofim, A.F.
Fuel, 79(9), 1067-1075 (2000).
Contact: Veranth

Solid-State C-13 NMR Characterization of Matched Tars and Chars From Rapid Coal Devolatilization

Perry, S.T.; Hambly, E.M.; Fletcher, T.H.; Solum, M.S. and Pugmire, R.J.
Proceedings of the Combustion Institute, 28 (in press, 2000).
Contact: Fletcher

Modeling High Pressure Char Oxidation Using Langmuir Kinetics with an Effectiveness Factor

Hong, J.; Hecker, W.C. and Fletcher, T.H.
Proceedings of the Combustion Institute, 28 (in press, 2000).
Contact: Fletcher

Transformations of Coal-Derived Soot at Elevated Temperature

Rigby, J.R.; Ma, J.; Webb, B.W. and Fletcher, T.H.
Accepted for publication in Energy & Fuels (2000).
Contact: Webb

Improving the Accuracy of Predicting Effectiveness Factors for m-th Order and Langmuir Rate Equations in Spherical Coordinates

Hong, J.; Hecker, W.C. and Fletcher, T.H.
Energy & Fuels, 14, 663-670 (2000).
Contact: Fletcher

1999

Modeling Nitrogen Release During Devolatilization on the Basis of Chemical Structure of Coal

Genetti, D. and Fletcher, T.H.
Energy & Fuels, 13, 1082-1091 (1999).

C-13 NMR spectroscopy has been shown to be an important tool in the characterization of coal structure. Important quantitative information about the carbon skeletal structure is obtained through C-13 NMR analysis techniques have progressed beyond the mere determination of aromaticity, and can now describe features such as the number of aromatic carbons per cluster and the number of attachments per aromatic cluster. These C-13 NMR data have been used to better understand the complicated structure of coal, to compare structural differences in coal, tar, and char, and to model coal devolatilization. Unfortunately, due to the expense of the process, extensive C-13 NMR data are not available for most coals. A non-linear correlation has been developed that predicts the chemical structure parameters of both US and non-US coals generally measured by 13C NMR and often required for advanced devolatilization models. The chemical structure parameters correlated include: (i) the average molecular weight per side chain (Mdelta); (ii) the average molecular weight per aromatic cluster (Mcl); (iii) the ratio of bridges to total attachments (p0); and (iv) the total attachments per cluster (+1). The correlation is based on ultimate and proximate analysis, which is generally known for most coals. C-13 NMR data from 30 coals were used to develop this correlation. The correlation has been used to estimate the chemical structure parameters generally obtained from C-13 NMR measurements, and then applied to coal devolatilization predictions using the CPD model and compared with measured total volatiles and tar yields. The predicted yields compare well with measured yields for most coals.

The Use of Two Mixture Fractions to Treat Coal Combustion in Turbulent Pulverized-Coal Flames

Flores, D.V. and Fletcher, T.H.
Combustion Science and Technology (1999).

Previous coal combustion models using assumed-shape PDF's to treat turbulence-chemistry interactions have used only one progress variable to treat products from coal reactions. This assumes that the products of all coal reactions have the same composition. However, the composition of the combustion products of coal particles is known to vary with burnout, especially between devolatilization and char oxidation. In this work, two progress variables were implemented which distinguish between the products of devolatilization and those of char oxidation. This new approach requires as input the specified volatile content and elemental release during devolatilization. The values for these parameters were estimated based on elemental release data obtained in flat-flame burners. Predictions of the new and the old approaches for the major variables of the field were not appreciably different. However, NO pollutant predictions of the new method were, in general, better than those of the old method, particularly at downstream locations.

The new two-progress variable method is currently limited by the scientific understanding of nitrogen release during coal devolatilization and char oxidation; predictions should improve as better fundamental models of nitrogen release are developed.

Predicting C-13 NMR Measurements of the Chemical Structure of Coal Based on Elemental Composition and Volatile Matter Content

Genetti, D., Fletcher, T.H. and Pugmire, R.J.
Energy & Fuels, 13:60-68 (1999).

A model that predicts the amount and distribution between tar and light gas of nitrogen released during devolatilization has been developed and incorporated into the Chemical Percolation Devolatilization (CPD) model. This work represents the first volatile nitrogen release model developed based on C-13 NMR measurements of coal structure. This work also represents the first volatile nitrogen release model evaluated by comparing model predictions with chemical structural features of the char (determined by C-13 NMR spectral analyses). The model is limited to nitrogen release during primary pyrolysis, and assumes that all light gas nitrogen is HCN. Model predictions of nitrogen release compare well with measured values for most coals and devolatilization conditions tested.

Kinetics of High-Pressure Char Oxidation

Sawaya, R.J.; Allen, J.W.; Hecker, W.C.; Fletcher, T.H. and Smoot, L.D.
ACS Preprint, Div. Fuel Chem., 44, pp xx (Aug 1999).

The kinetics of char oxidation at atmospheric pressure have been much studied and are fairly well agreed upon. However, the kinetics of char oxidation at elevated pressures have not been studied to any significant extent, and standard kinetic models which work at low pressure do not work at high pressure. This paper reports the results of a study to determine the high-pressure kinetics of char oxidation for Pittsburgh #8 char under Zone I conditions. Rate data were obtained for total pressures from one to 64 atmospheres and oxygen mole fractions between 0.03 and 0.40. Temperature dependencies as well as oxygen partial pressure dependencies were determined and the suitability of using various Langmuir-Hinschellwood expressions to fit the data were explored.

Measurement of Soot and Char in Pulverized Coal Fly Ash

Veranth, J.M.; Fletcher, T.H.; Pershing, D.W. and Sarofim, A.F.
Fuel, In Review, 1999.

The unburned carbon in the fly ash produced by low-NOx pulverized coal combustion has been shown by electron microscopy to be a mixture of porous coal char particles and aggregates of submicron particles, which are thought to be soot. The carbon is bimodally distributed with large soot aggregates mixed with the char in the particles larger than 10 microns and dispersed soot found with the submicron particles. A method for determining the mass of soot and char by liquid-suspension gravity separation was used with both laboratory-scale and power plant fly ash samples. For low-NOx, staged, pilot-scale combustion of bituminous coal the soot in the soot in the furnace exit ash was estimated to be 0.2 to 0.6% of the fuel carbon, which was about 35% of the total unburned carbon.

1998

Development and Application of a Correlation of C-13 NMR Chemical Structural Analyses of Coal Based on Elemental Composition and Volatile Matter Content

Genetti, D. and Fletcher, T.H.
Accepted for publication, Energy and Fuels, 1998.

C-13 NMR spectroscopy has been shown to be an important tool in the characterization of coal structure. Important quantitative information about the carbon skeletal structure is obtained through C-13 NMR spectral analysis of coal. Solid-state C-13 NMR analysis techniques have progressed beyond the mere determination of aromaticity and can now describe features such as the number of aromatic carbons per cluster and the number of attachments per aromatic cluster. These C-13 NMR data have been used to better understand the complicated structure of coal, to compare structural differences in coal, tar, and char, and to model coal devolatilization. Unfortunately, due to the expense of the process, extensive C-13 NMR data are not available for most coals. A nonlinear correlation has been developed that predicts the chemical structure parameters of both U.S. and non-U.S. coals generally measured by C-13 NMR and often required for advanced devolatilization models. The chemical structure parameters correlated include (I) the average molecular weight per side chain (Mdelta); (ii) the average molecular weight per aromatic cluster (Mcl); (iii) the ratio of bridges to total attachments (p0); and (iv) the total attachments per cluster (sigma + 1). The correlation is based on ultimate and proximate analyses, which are generally known for most coals. C-13 NMR data from 30 coals were used to develop this correlation. The correlation has been used to estimate the chemical structure parameters generally obtained from C-13 NMR measurements, and then applied to coal devolatilization predictions using the CPD model and compared with measured total volatiles and tar yields. The predicted yields compare well with measured yields for most coals.

Nitrogen Transformation in Coal During Pyrolysis

Kelemen, S.R.; Gorbaty, M.L.; Kwiatek, P.J.; Fletcher, T.H.; Watt, M.; Solum, M.S. and Pugmire, R.J.
Energy & Fuels, 12:159-73 (1998).

X-ray photoelectron spectroscopy (XPS) was used to identify and quantify the changes in organically bound nitrogen forms present in the tars and chars of coals after pyrolysis. For fresh coal, pyrrolic nitrogen is the most abundant form of organically bound nitrogen, followed by pyridinic, quaternary, and amino types. Some of the quaternary nitrogen species initially present in coal are lost upon mild pyrolysis, prior to hydrocarbon devolatilization. These quaternary species are attributed to pyridinic or basic nitrogen species associated with hydroxyl groups from carboxylic acids or phenols. A portion of the quaternary nitrogen species is lost at the very earliest stage of pyrolysis. Upon devolatilization, the resultant tar and char contain mostly pyrrolic and pyridinic forms; however, a portion of the quaternary nitrogen initially present in the coal appears in the coal char and tar. The relatively strong bonding interactions associated with these quaternary species suggests that there may be other quaternary nitrogen, in addition to protonated pyridines, in low-rank coal. For low-rank coal, amino groups are preferentially released and concentrate in the tar. XPS analysis of chars and tars produced during rapid heat-up (10^14 deg/s) pyrolysis show similar trends. However, severe pyrolysis of the devolatilized char results in the appearance of an asymmetric carbon (1s) line shape indicative of very large polynuclear "graphitic-like" units. This transformation is accompanied by a rise in the relative number of quaternary nitrogen forms and occurs over a relatively narrow temperature range. Quaternary and pyridinic nitrogen forms become the dominant forms in severely pyrolyzed chars. The relatively low level of quaternary nitrogen in the rapid heat-up chars indicates that very large polynuclear aromatic structures are not fully developed under these pyrolysis conditions.

Modeling Soot Derived from Pulverized Coal

Brown, A.L. and Fletcher, T.H.
Energy & Fuels, 12:745-57 (1998).

A semiempirical model has been developed for predicting coal-derived soot. The main feature of the model is a transport equation for soot mass fraction. Tar prediction options include either an empirical or a transport equation approach, which directly impacts the source term for soot formation. Also, the number of soot particles per unit mass of gas may be calculated using either a transport equation or an assumed average. Kinetics are based on Arrhenius rates taken from published measurements. Radiative properties are calculated as a function of averaged optical constants, predicted gas temperatures, predicted gas densities, and the soot mass fractions. This model has been incorporated into a comprehensive coal modeling code and evaluated based on comparisons with soot, temperature, and NOx measurements for three experimental cases. Accurate predictions of soot yields have been achieved for both laminar and turbulent coal flames. Larger scale turbulent predictions illustrated that inclusion of a soot model changed the local gas temperatures by as much as 300 K and the local NOx concentration by as much as 250 ppm. These predictions demonstrate the necessity for an accurate soot model in coal combustion systems.

 

Evaluation of CH4/NOx Reduced Mechanisms Used for Modeling Lean Premixed Turbulent Combustion of Natural Gas

Mallampalli, H.P.; Fletcher, T.H. and Chen, J.Y.
Journal of Engineering for Gas Turbines and Power, 120:703-12 (1998).

This study has identified useful reduced kinetic schemes that can be used in comprehensive multidimensional gas-turbine combustor models. Reduced mechanisms lessen computational cost and possess the capability to accurately predict the overall flame structure, including gas temperatures and key intermediate species such as CH4, CO and NOx. In this study, four new global mechanisms with five, six, seven, and nine steps based on the full GRI 2.11 mechanism, were developed and evaluated for their potential to model natural gas chemistry (including NOx chemistry) in gas turbine combustors. These new reduced mechanisms with five, six, seven, and nine steps based on the full GRI 2.11 mechanism, were developed and evaluated for their potential to model natural gas chemistry (including NOx chemistry) in gas turbine combustors. These new reduced mechanisms were optimized to model the high pressure and fuel-lean conditions found in gas turbines operating in the lean premixed code calculations, the five-step reduced mechanism was identified as a promising model that can be used in a multidimentional gas-turbine code for modeling lean -premixed, high-pressure turbulent combustion of natural gas. Predictions of temperature, CO, CH4, and NO from the five- to nine-step reduced mechanisms agree within 5 percent of the predictions from the full kinetic model for 1 < pressure (atm) < 30, and 0.6 < phi < 1.0. If computational costs due to additional global steps are not severe, the newly developed nine step global mechanism, which is a little more accurate and provided the least convergence problems, can be used. Future experimental research in gas turbine combustion will provide more accurate data, which will allow the formulation of better full and reduced mechanisms. Also, improvement in computational approaches and capabilities will allow the use of reduced mechanisms with larger global steps, perhaps full mechanisms.

1997

Predicting C-13 NMR Measurements of Chemical Structure of Coal Based on Elemental Composition and Volatile Matter Content

Genetti, D. and Fletcher, T.H.
ACS Division of Fuel Chemistry Pre Prints, 42(1) 194-98, April 1997. Funded by US Department of Energy/University Coal Research and ACERC.

Devolatilization models based on quantitative measurements of chemical structure, such as available through C-13 NMR analysis, have been successful in predicting tar volatiles yields as a function of heating rate, temperature, pressure, and coal type. An example of such a devolatilization model is the CPD model, developed in ACERC. However, due to limited resources, C-13 NMR structural parameters have only been obtained for about 35 coals at the present time. Industrial interest in coal devolatilization has led to several attempts to correlate structural parameters affecting devolatilization as a function of the ultimate analysis of coals. A triangular (i.e., linear) interpolation technique is used to estimate the input parameters for one current devolatilization model, while another popular model uses a procedure that estimates the coal structural parameters based on simple linear correlations of ultimate analysis.

An extensive statistical analysis to determine the validity of linear correlations of C-13 NMR structural parameters based on ultimate analysis was performed. A database including elemental composition, the ASTM volatile matter content, and C-13 NMR structural parameters for 30 coals of widely varying rank and composition was used in the analysis. The database was closely examined using the SPSS® statistical computer package. Using SPSS®, a correlation matrix was calculated between all of the chemical structural parameters obtained from the NMR analysis. From the correlation matrix, the strength of relationships between the individual elements and the derived parameters were easily determined. The parameters were also examined for relationships among themselves. Multi-variate linear regression was then performed to derive equations that predict each of the parameters as a function of the elemental composition and volatile matter content. The r² value was the determined for each correlation. The r² value is the coefficient of determination which determines the relative strength of correlation (r²=1 is a perfect correlation). In this analysis the r² values ranged from 0.17 for sigma+1 to 0.59 for Md (r²=0.49 Po and r²=0.38 for MWcl). The low r² values indicate only a weak linear correlation between the C-13 NMR structural parameters and the ultimate analysis. However, even when r² is zero, a strong non-linear correlation is possible. As a result of this study, it was determined that correlations base on linear regressions of ultimate analysis are unsuitable for predicting C-13 NMR structural parameters with reasonable accuracy.

A non-linear correlation has now been developed that predicts the chemical structure parameters generally measured by C-13 NMR and required for the CPD devolatilization model: (1) the average molecular weight per side chain (Mdelta); (2) the average molecular weight per aromatic cluster (MWcl); (3) the ratio of bridges to total attachments (Po); and (4) the total attachments per cluster (sigma+1). The correlation is based on ultimate and proximate analyses, which are generally known for most coals.

The correlation has been used to estimate the chemical structure parameters generally obtained from C-13 NMR measurements, and then coal devolatilization predictions were performed using the CPD model and compared with measured total volatiles yields. The combination of the empirical model and the CPD model accurately predicts tar and total volatiles yields for coals with carbon content (daf) ranging from 65 percent to 94 percent.

Nitrogen Transformations in Coal During Pyrolysis

Kelemen, S.R.; Gorbaty, M.L.; Kwiatek, P.J.; Fletcher, T.H.; Watt, M.; Solum, M.S. and Pugmire, R.J.
Energy & Fuels, 12:159-72(1997). Funded by ACERC, Exxon and Federal Energy Technology Center.

X-ray photoelectron spectroscopy (XPS) was used to identify and quantify the changes in organically bound nitrogen forms present in the tars and chars of coal after pyrolysis. For fresh coal, pyrrolic nitrogen is the most abundant form of organically bound nitrogen, followed by pyridinic, quaternary, and amino types. Some of the quaternary nitrogen species initially present in coal are lost upon mild pyrolysis, prior to hydrocarbon devolatilization. These quaternary species are attributed to pyridinic or basic nitrogen species associated with hydroxyl groups from carboxylic acids or phenols. A portion of the quaternary nitrogen species is lost at the very earliest stage of pyrolysis. Upon devolatilization, the resultant tar and char contain mostly pyrrolic and pyridinic forms; however, a portion of the quaternary nitrogen initially present in the coal appears in the coal char and tar. The relatively strong bonding interactions associated with theses quaternary species suggests that there may be other quaternary nitrogen, in addition to protonated pyridines, in low-rank coal. For low-rank coal, amino groups are preferentially released and concentrate in the tar. XPS analysis of chars and tars produced during rapid heat-up (104 deg/s) pyrolysis show similar trends. However, sever pyrolysis of the devolatilized char results in the appearance of asymmetric carbon (1s) line shape indicative of very large polynuclear "graphitic-like" units. This transformation is accompanied by a rise in the relative number of quaternary nitrogen forms and occurs over a relatively narrow temperature range. Quaternary and pyridinic nitrogen forms become the dominant forms in severely pyrolyzed chars. The relatively low level of quaternary nitrogen in the rapid heat up chars indicates that very large polynuclear aromatic structures are not fully developed under these pyrolysis conditions.

N-15 NMR Spectroscopy of Coals and Pyrolysis Products

Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Fletcher, T.H. and Wind, R.A.
Proceedings of the 9th International Conference on Coal Science, Essen, Germany, September 7-12, 1997. Funded by ACERC, US Department of Energy/University Coal Research and New Energy and Industrial Technology Development Organization.

N-15 NMR spectra are reported for a number of coals from Pacific Rim countries. Arguments are presented to explain discrepancies between observations and conclusions obtained from NMR experiments and those obtained by XPS and XANES techniques. Detection of different types of nitrogen species is discussed in terms of cross polarization dynamics together with the effects of the large chemical shift anisotropy that are found in different types of nitrogen functional groups. Significant differences are observed in the types of nitrogen present in these coals and these variations are associated with coal rank as has been in a previous study of the Argonne Premium coals. We have also begun to examine pyrolysis char and tar samples. The NMR data indicate that significant differences exist between the types of nitrogen structures observed in coal, char and tar samples. These differences suggest that different mechanisms may exist for nitrogen release from tar and char samples.

Soot in Coal Combustion Systems

Fletcher, T.H.; Ma, J.; Rigby, J.R.; Brown, A.L. and Webb, B.W.
Prog. Energy Combust. Sci., 23:283-301(1997). Funded by ACERC.

Soot is generated from coal when volatile matter, tar in particular, undergoes secondary reactions at high temperatures. A description of soot in coal flames allows better calculations of radiative transfer and temperatures in near-burner regions, which in turn allows more accurate predictions of NOx formation in coal-fired furnaces. Experiments are reviewed that examine the formation, agglomeration and properties of coal-derived soot, including pyrolysis experiments and combustion experiments. This review includes the types of experiments performed, the soot yields obtained, the size of the soot particles and agglomerates, the optical properties of soot, the relationship between coal-derived soot and soot form simple hydrocarbons, and attempts to model soot in coal flames.

1996

Effects of Pyrolysis Heating Rate on Intrinsic Reactivities of Coal Chars

Gale, T.K.; Bartholomew, C.H. and Fletcher, T.H.
Energy & Fuels, 10(3):766-755, 1996. Funded by ACERC.

The main objective of this work was to determine the effects of pyrolysis heating rate on intrinsic O2 reactivity of coal chars. Relationships of intrinsic reactivity to other pyrolysis conditions and char physical and chemical structure were also investigated, and empirical correlations were obtained. Two different entrained flow reactors (a flat flame methane-air burner and a drop tube reactor) were used to prepare chars under a variety of different pyrolysis conditions at maximum particle temperatures and heating rates of 840-1627 K and 104 to 2 s 105 K/s, respectively. Intrinsic reactivities of a lignite and two bituminous coal chars decrease with increasing preparation heating rate. Maximum particle temperature and heating rate are difficult preparation parameters to separate and were closely coupled in this work, as in most entrained flow coal research. Indeed, much of the work described in the literature defining the effects of pyrolysis heating rate on coal char reactivity; has utilized vast residence time differences, comparing data from fixed bed (residence time of ~ 1 h) and entrained flow reactors (residence time of ~100 ms). It is concluded from this work that observations made on the basis of such experimentation are a function more of residence time and reactor variations (packed or fixed bed, as opposed to entrained flow) than particle heating rate. This work also provides evidence that intrinsic reactions of O2 with coal char (for the three coals observed in this study) are not significantly influenced by large differences in char meso- or micropore surface area obtained by varying pyrolysis conditions.

Nitrogen Release During Coal Combustion

Baxter, L.L.; Reginald, E.M.; Fletcher, T.H. and Hurt, R.H.
Energy & Fuels, 10:188-196. Funded by US Department of Energy/Sandia National Laboratories.

Experiments in entrained flow reactors at combustion temperatures are performed to resolve the rank dependence of nitrogen release on an elemental basis for a suite of 15 U.S. coals ranging from lignite to low-volatile bituminous. Data were obtained as a function of particle conversion, with overall mass loss up to 99% on a dry, ash-free basis. Nitrogen release rates are presented relative to both carbon loss and overall mass loss. During devolatilization, fractional nitrogen release from low-rank coals is much slower than fractional mass release and noticeably slower than fractional carbon release. As coal rank increases, fractional nitrogen release rate relative to that of carbon and mass increases, with fractional nitrogen release rates exceeding fractional mass and fractional carbon release rates during devolatilization for high-rank (low-volatile bituminous) coals. At the onset of combustion, nitrogen loss rates increase significantly. For all coals investigated, fractional nitrogen loss rates relative to those of mass and carbon pass through a maximum during the earliest stages of oxidation. The mechanism for generating this maximum is postulated to involve nascent thermal rupture of nitrogen-containing compounds and possible preferential oxidation of nitrogen sites. During later stages of oxidation, the fractional loss rate of nitrogen approaches that of carbon for all coals. Changes in the relative release rates of nitrogen compared to those of both overall mass and carbon during all stages of combustion are attributed to a combination of the chemical structure of coals, temperature histories during combustion, and char chemistry.

Measurements of the Optical Properties of Coal-Derived and Propane-Derived Soot in a Flat Flame Reactor

Rigby, J.R.; Webb, B.W. and Fletcher, T.H.
Proceedings of the Spring Meeting of the Western States Section of the Combustion Institute, Tempe, Arizona, March 11-12, 1996. Funded by ACERC.

All hydrocarbon-based fuels have the potential to form soot during combustion of devolatilization reactions. Soot resulting from incomplete combustion is the main contributor to luminosity in flames. Because of its high surface area and spectrally continuous radiation, sot is a very efficient thermal radiator. The optical properties of coal-derived soot have not received as much attention as soot derived from gaseous hydrocarbon fuels. The reason for this neglect may be the difficulty in separating the radiation effects of the coal-derived soot, char and fly ash. The radiative properties of coal-derived soot have not been characterized, nor have the influences of coal type, volume fraction and morphology been examined. The radiative properties of coal-derived soot char can be sued in combustion modeling and burner design. In the near-burner region, the stoichiometry is very fuel rich providing for the high soot volume fraction and for large radiative heat fluxes being transmitted to the burners and walls. In this region, neglecting soot can result in inaccurate radiant flux predictions, as well as inaccurate predictions of gas temperatures, species concentrations, pressure fields, and velocity profiles. Radiative properties of soot can also be used to determine soot volume fraction and soot temperature in-situ.

Based on preliminary results from this study, trends for C-lambda from coal-derived and propane-derived differ as a function of residence time in the post flame environment, even though the ranges of magnitudes of C-lambda overlap. All measurements indicate an increase in C-lambda at increasing wavelengths. Further work is needed to examine these trends for different coal-derived soots. Explanations of why the optical properties of soot change and parameters to characterize these changes are also needed. This work provides the basis for futures in-situ measurements that will measure soot volume fraction in a coal pyrolysis experiment.

Conversion of Coal Tar to Soot During Coal Pyrolysis in a Post-Flame Environment

Ma, J.; Fletcher, T.H. and Webb, B.W.
Twenty-Sixth Symposium (International) on Combustion, (in press) 1996. (Also presented at the Twenty-Sixth Symposium (International) on Combustion, Naples, Italy, July, 1996.) Funded by ACERC.

Coal pyrolysis experiments were performed in the post-flame region of CH4/H2/air flat flame burner running in fuel-rich conditions, where the temperature and gas compositions were similar to those in the near burner region of a large-scale coal-fired furnace. Volatiles released form the coal particles formed a cloud of soot particles at high temperatures in the absence of oxygen. The soot particles in the cloud were collected at different residence times using a water-cooled, nitrogen-quenched suction probe. The test variables included the reaction temperature and coal type. Soot yields in terms of weight percentage of dry ash-free coal were measured based on the bulk soot collection experiments. The measured soot yields were related to coal rank, reaction temperature, and residence time. Size changes of soot particles due to soot agglomeration were also observed. The information obtained bout coal-derived soot is useful in predictions of radiative heat transfer and pollutant formations in the near-burner region of pulverized coal-furnaces.

Chemical Structure of Coal Tar During Devolatilization

Fletcher, T.H.; Watt, M.; Bai, S.; Solum, M.S. and Pugmire, R.J.
Twenty-Sixth Symposium (International) on Combustion, (in press) 1996. (Also presented at the Twenty-Sixth Symposium (International) on Combustion, Naples, Italy, July, 1996.) Funded by ACERC and US Department of Energy/University Coal Research.

Three coals of different rank were pyrolyzed in a drop tube reactor at a maximum temperature of 900 K and residence time of 160 ms. The coal and char were analyzed with solid state C-13 NMR. The tar was dissolved in deuterated methylene chloride. It was found that the tar was only partially soluble in CD2CL2-. The non-soluble tar portion was analyzed using a recently developed high-resolution C-13 NMR technique developed for liquid phases. The tar structure was found to be significantly different from the structure of the char and coal. The number of bridges and loops per cluster in the tar was up to 65% lower than in the char. In addition, the number of aromatic carbons per cluster in the tar was significantly lower than that found in either the coal or the char. Since the molecular weight per cluster in the tar is lower than reported average tar molecular weights, these data imply that tar is made up of a number of multiple clusters (dimers, trimers, etc.) as well as single clusters (i.e., monomers). The mass of nitrogen per cluster in the tar was found to be significantly lower in the tar than in either the coal or the char. These experimental findings suggest that changes may be necessary in current network devolatilization models to accurately describe the changes in chemical structure.

Conversion of Coal Tar to Soot During Coal Pyrolysis in a Post-Flame Environment

Ma, J.; Fletcher, T.H. and Webb, B.W.
Twenty-Sixth Symposium (International) on Combustion, Naples, Italy, July 1996. Funded by ACERC.

Coal pyrolysis experiments were performed in the post-flame region of CH4/H2/air flat flame burner running in fuel-rich conditions, where the temperature and gas compositions were similar to those in the near burner region of a large-scale coal-scale coal-fired furnace. Volatiles released from the coal particles formed a cloud of soot particles at high temperatures in the absence of oxygen. The soot particles in the cloud were collected at different residence times using a water-cooled, nitrogen-quenched suction probe. The test variables included the reaction temperature and coal type. Soot yields in terms of weight percentage of dry ash-free coal were measured based on the bulk soot collection experiments. The measured soot yields were related to coal rank, reaction temperature, and residence time. Size changes of soot particles due to soot agglomeration were also observed. The information obtained about coal-derived soot is useful in predictions of rediative heat transfer and pollutant formations in the near-burner region of pulverized coal-furnaces.

Global Rate Expression for Nitric Oxide Reburning. Part 2

Chen, W.; Smoot, L.D.; Hill, S.C. and Fletcher, T.H.
Energy & Fuels, 10(5):1046-1052, 1996. Funded by ACERC.

An investigation of a global reburning-NO reaction, SigmaijCiHj + NO --> HCN + . . ., which is a reduction pathway of nitric oxide (NO) by reaction with gaseous hydrocarbons, was conducted. The global reburning-NO rate expression was deduced from a combination of elemental reactions. The global rate expression and its rate constants were then determined by correlating predicted species profiles from simple hydrocarbon flames. This global reburning-NO rate constant can be expressed as 2.7 x 106 exp(-18,800/RT) (gmol/cm³s). This expression and constants are applicable to atmospheric pressure with an equivalence ratio rang of 1.2-2.08 for light hydrocarbon reburning ghases (CH4 and C2H4).

A Computational Method for Determining Global Fuel-NO Rate Expressions.

Part 1

Chen, W.; Smoot, L.D.; Fletcher, T.H. and Boardman, R.D.
Energy & Fuels, 10(5):1036-1045, 1996. Funded by ACERC.

Global chemical reaction rates used in the modeling of NOx formation in comprehensive combustion codes have traditionally been obtained trough correlation of experimental data. In this paper, a computational approach for obtaining global rates is presented. Several premixed flames were simulated, and sensitivity analysis of species concentration profiles was used to suggest global pathways in fuel-nitrogen conversion to NO. Based on these analyses, the global reaction rates were formulated. The predicted species concentration profiles and their derivatives were then used in the determination of the global rate constants. The correlation of rate constants for the two fuel-NO global rates (HCN + NO N2 + . . . and HCN + O2 NO + . . .) are discussed. Comparisons of the computed global rate constants with those rate constants with those deduced from experimental data show good agreement. The global rates provide practical kinetics for simulating nitrogen pollutant chemistry in complex flames.

1994

The Structure and Reaction Process of Coal

Smith, L.K.; Smoot, L.D.; Fletcher, T.H. and Pugmire, R.J.
Plenum Press Co., New York, 1994. Funded by ACERC.

This book characterizes the properties and reaction rates of the eleven U.S. coals selected emphasis by ACERC. Eight of the eleven comprise the Argonne National Laboratory's Premium Coal Sample Bank. The book features the comprehensive measurement of organic and inorganic components of the coal structure by advanced methods (SEM, NMR, GC, porosimetry, Pycnometry, X-ray, and MS). The book features the measurement of coal devolatilization and char oxidation rates by advanced, optical methods and correlative relationships between the structure and reaction processes.

Decreases in the Swelling and Porosity of Bituminous Coals During Devolatilization

Gale, T.K.; Bartholomew, C.H. and Fletcher, T.H.
Combustion and Flame, 1994 (in press). (Also presented at the 25th International Symposium on Combustion, Irvine, CA, August 1994). Funded by ACERC.

Concern about comparability and validity of different methods for producing coal chars for reactivity experiments has led to research on the effect of devolatilization conditions on the char physical and chemical structure. Particle diameter and porosity changes during devolatilization significantly affect char oxidation rates. In particular, physical properties of chars prepared in drop tube reactors differ greatly from chars prepared in flat flame burner experiments. Recent data indicate that the presence of oxygen in the gas atmosphere has no effect on swelling until char oxidation has begun. The present research concentrates on the effects of heating rate, particle temperature and residence time on the swelling and porosity of a plastic coal, and compares these results with a non-plastic coal. The heating rate at which the transition from increasing swelling to decreasing swelling occurs in approximately 5 x 10³ K/s for swelling coals. Swelling coals also reach a maximum porosity near this heating rate. At low particle heating rates swelling gradually increases versus heating rate in contrast to a decline in the swelling at high heating rates in a narrow heating rate region of 2 x 10^4 to 7 x 10^4 K/s. Non swelling bituminous and lignite coals continue to increase in porosity beyond the heating rate of 2 x 10^4 K/s.

Effects of Pyrolysis Conditions on Internal Surface Areas and Densities of Coal Chars Prepared at High Heating Rates in Reactive and Non-Reactive Atmospheres

Gale, T.K.; Fletcher, T.H. and Bartholomew, C.H.
Energy & Fuels, 1994 (in press). Funded by ACERC.

Concern about comparability and validity of different methods for producing coal chars for research has motivated this investigation of the effects of devolatilization conditions on the physical properties of coal chars. It is evident from the findings of this study that care must be taken to prepare chars under conditions similar to those of full-scale coal combustion boilers prior to performing char oxidation studies. Two different entrained flow reactors were used to prepare chars under a variety of different pyrolysis conditions at maximum particle temperatures and heating rates between 840 to 1627 K and 10^4 to 2 x 10^5 K/s respectively. Under these conditions micro-pore (CO2) surface area generally increases with residence time and mass release for lignite and bituminous coals, as does true density. Micro-pore surface area also increases somewhat with increasing maximum particle temperature and heating rate. Meso-pore (N2) surface area is most affected by reactive gas atmospheres (carbon activation). The presence of steam in the post flame gases of methane/air flat flame burners is a significant factor in increasing meso-pore surface are of chars prepared in such burners, even though the mass conversion by steam gasification is small. Partial char oxidation with O2 significantly affects char N2 and CO2 surface area at these heating rates and residence times (50 to 100 ms), sometimes increasing and sometimes decreasing internal surface area. Low rank lignite and sub-bituminous coals have higher potentials for forming chars with increased N2 surface are than bituminous coals. The moisture content of low rank coals may be more important than rank. Lignite with high moisture content yields char with a significantly higher N2 surface area than char prepared from lower moisture content lignite. However, initial coal moisture has less effect on CO2 surface area.

Chemical Structure Changes of Coal, Char, and Tar During Devolatilization

Fletcher, T.H. and Pugmire, R.J.
Proceedings of the ACS Spring Meeting, San Diego, CA, March 1994, ACS Preprints, Division of Fuel Chemistry, 39:108-112. Funded by ACERC, Brigham Young University and University of Utah.

Enormous progress has been made in coal pyrolysis research during the last decade. Models of coal devolatilization have progressed from simple rate expressions based on total mass release to empirical relationships based on the elemental composition of the parent coal to models that attempt to describe the macromolecular network of the coal. Measurements of particle temperature during devolatilization have eliminated much of the controversy regarding overall rates of devolatilization. In the last several years, advancements in chemical analysis techniques have allowed quantitative investigations of the chemical structure of both coal and its pyrolysis products, including the nature of the resulting char. A prominent research goal is to accurately predict the rates, yields, and products of devolatilization from measurements of the parent coal structure. This goal necessitates modeling the reaction processes on the molecular scale, with activation energies that relate to chemical bond breaking rather than release of products from the coal. C-13 and H-1 NMR spectroscopy have proven particularly useful in obtaining average values of chemical structure features of coal, char and tar. This paper reviews experimental data regarding chemical structure features of coal, char, and tar during rapid devolatilization, and how these data have impacted the development and input parameters for devolatilization models. In particular, the relationship between pyridine extract yields and extract yields predicted purely from NMR chemical structure data discussed.

1993

Coal Characteristics, Structure, and Reaction Rates

Smith, L.K.; Smoot, L.D.; Fletcher, T.H. and Pugmire, R.J.
Chapter 3, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.

The purposes of this chapter are to document, correlate, synthesize, integrate and relate the structural characterization and reaction rates of the suite of ACERC coals. The focus has been on research projects sponsored by ACERC. However, related research work outside of ACERC has also been considered. This chapter (1) reviews the selection of the suite of research coals, (2) reviews the origin of coal which gives rise to the various structural moieties in coal, (3) reviews coal characterization programs and documents the structure and characteristics of the research coals, (4) reviews coal reaction mechanisms, (5) explores the relationships of coal structure to devolatilization and char oxidation reaction rates, and (6) considers the models being developed which predict reaction characteristics based on structurally dependent parameters. Research programs in the field are still very active, the models are still in the formative states, reaction rates for the selected research coals are being measured, and the reaction processes for these coals have yet to be fully explored using the structurally dependent models. Further results will undoubtedly be forthcoming. This chapter is a condensed version of a larger work to characterize the structure and conversion processes of the research coals.

The Structure and Reaction Processes of Coal

Smith, L.K.; Smoot, L.D.; Fletcher, T.H. and Pugmire, R.J.
Plenum Publishing Corp., The Netherlands, 1993 (in press). Funded by ACERC.

This new ACERC book documents and integrates the current understanding of the organic and inorganic structure of coal and its reaction processes. Work in ACERC forms the foundation while the book attempts to include pertinent worldwide results. The book cites more than eight hundred references, almost all within the past decade, while the large majority are from various researchers around the world.

Eleven U.S. coals of various rank are emphasized in the book. These commonly used and highly characterized eleven coals form the research coals for ACERC and include all eight coals of the Argonne National Laboratory's Premium Coal Sample Bank. Altogether, the book contains six chapters. After an introduction, Chapter 2 documents the selection and characteristics of the suite of eleven coals, and relates them to various national databases. Chapter 3 deals with the geochemical history of coal and its macromolecular structure. Chapter 4 describes advanced analytical methods for measuring organic and inorganic structure of coal and documents results for the eleven coals. Chapters 5 and 6 treat the reaction processes of coals and chars. Recent model developments that relate fuel structure to yields and reaction rates are presented and compared to rate and yield data. Important measurements from the coal suite and other coals are reported and related to coal structure.

Laying a foundation for the future, this book has been written at a time when progress in this area is dramatic. The authors acknowledge that new results will be published at a rapid rate. What we have sought to accomplish through the writing of this manuscript is to promote increasing cooperative focus regarding the understanding of coal structure and its reaction and conversion processes. From this perspective, the book is thought to be the first of its kind.

The Structure and Reaction Processes of Coal

Smith, L.K.; Smoot, L.D.; Fletcher, T.H. and Pugmire, R.J.
Plenum Press Co., New York, 1993 (in press). Funded by ACERC.

This book characterizes the properties and reaction rates of the eleven US coals selected for emphasis by ACERC. Eight of the eleven comprise the Argonne National Laboratory's Premium Coal Sample Bank. The book features the comprehensive measurement of organic and inorganic components of the coal structure by advanced methods (SEM, NMR, GC, porosimetry, pycnometry, X-ray, and MS). The book features the measurement of coal devolatilization and char oxidation rates by advanced, optical methods and correlative relationships between the structure and reaction processes.

An Overview of ACERC Reserach in Fuel Characterization and Reaction Mechanisms

Pugmire, R.J. and Fletcher, T.H.
Energy & Fuels, 7 (6):700-703, 1993. Funded by ACERC.

A major objective of the Advanced Combustion Engineering Research Center is the development and verification of data on fuel characterization and reaction mechanisms and rates that can be incorporated into submodels for use in the comprehensive combustion codes. As Technology has advanced, the levels of analytical sophistication has also advanced, making it possible to augment the existing body of information with new data. From this new data it is possible to draw new insights regarding the complex nature of coal and the various processes associated with combustion. The ACERC program has made it possible to bring different disciplines together to work on an integrated research program that is targeted at a few key strategic issues. The overall program has been divided into six areas of research, designated as Thrust Areas. The principal areas of focus in Thrust Area 1 have been in delineation of coal structure and those key factors that are important in developing fundamental knowledge of devolatilization and char oxidation processes. This article discusses the objectives, accomplishments, and plans of ACERC-sponsored research in these areas.

Chemical Structural Features of Pyridine Extracts and Residues of the Argonne Premium Coals Using Solid State C-13 NMR Spectroscopy

Fletcher, T.H.; Bia, S.; Pugmire, R.J.; Solum, M.S.; Woods, S. and Grant, D.M.
Energy & Fuels, 7 (6):734-742, 1993. (Presented at the Spring Meeting of the Western States Section of the Combustion Institute, Salt Lake City, UT, March 1993.) Funded by ACERC.

Soxhlet extractions were performed on the eight Argonne Premium coals using pyridine purged with argon and followed by a novel washing procedure to remove the pyridine. Mass closure (extracts plus residues) on duplicate experiments accounted for 94-96% of the original coal, repeatable to within 2%. Chemical structural features determined from C-13 NMR analyses of the extracts and residues showed more attachments per aromatic cluster for the residues, indicating a greater degree of covalent bonding in the residue than in the extract. H-1 NMR analysis of the extracts showed a gradual increase in the hydrogen aromaticity with rank, along with a maximum in the percentage of a-hydrogen in the high-volatile bituminous coals. Composite chemical features constructed from weighted averages of the features of the residues and extracts agree with many of the features of the parent coal. Chemical structural features of the extracts determined from H-1 NMR analyses agree with similar data reported previously for early coal tars during devolatilization at rapid heating rates.

Progress in Coal Pyrolysis

Solomon, P.R.; Fletcher, T.H. and Pugmire, R.J.
Fuel, 72:587-597, 1993. Funded, in part, by ACERC.

The heterogeneous nature of coal and the complexity of the pyrolysis process has made it very difficult to perform unambiguous experiments to determine the rates and mechanisms in coal pyrolysis. However, recent years have seen a number of new experimental and theoretical approaches that shed new light on the subject. This paper considers the recent progress on kinetics, the formation of volatile products, network models, crosslinking, rank effects, and the 'two-component model of coal structure.' Recent experiments that measured coal particle temperatures at high heating rates provide reasonable agreement on kinetic rate constants. These rates also agree with those derived from experiments at low heating rates. In tar formation and transport, a consensus is being reached on the central role of the volatility of tar molecules in explaining the variation with operating conditions (pressure, heating rate, particle size, etc.) of the amounts and molecular weight distribution of tars. Progress in the quantitative prediction of tar and char yields is being made through recently developed models for the fragmentation of the macromolecular coal network. These models, which provide quantitative descriptions of the relations between the chemical structure of the coal and the physical and chemical properties of the pyrolysis products (gas, tar, soot, and char), are an exciting advance in the understanding of the pyrolysis process. Such models are linking the occurrence of the plastic phrase with the 'liquid' fragments formed during pyrolysis. On the subject of retrogressive cross-linking reactions, both solvent swelling and NMR measurements confirm important rank-dependent differences in reaction rates: these appear to be related to the oxygen functionalities. Reasonable agreement is also seen for variations with coal rank of kinetics rates derived from measurements at low heating rates. Experiments suggest that the recently revived 'two-component' hypothesis of coal structure has application to low-rank coals which are mixtures of two distinct components: polymethylenes and a more aromatic network. Bituminous coals, however, appear far more homogeneous. Although experiments can distinguish loosely and tightly bound fractions these fractions appear to consist of similar materials and are differentiated primarily in their molecular weight and degree of connection to the network. These coals appear to behave in a manner that is described by the network decomposition models.

Heat Transfer From a Molten Phase to an Immersed Coal Particle During Devolatilization

Hurt, R.H.; Fletcher, T.H. and Sampaio, R.S.
ASME Journal of Heat Transfer, 115:717-723, 1993. Funded by Sandia Technology Maturation Program.

In several development and commercial processes, coal particles come into direct contact with a high-temperature molten phase. These processes include molten carbonate coal gasification and bath smelting for the production of iron. Recently, real-time X-ray fluoroscopic images have been published that show volatile matter evolving rapidly from coal particles immersed in molten phases, displacing the surrounding melt and producing a periodic cycle of formation, rise, and detachment of gas cavities. The present work makes use of these observations to develop a model of heat transfer from the melt to particles undergoing gas evolution. The model is developed for the general case and applied to predict melt-particle heat transfer coefficients under conditions relevant to bath smelting processes. The model shows that the presence of the gas film can actually increase the overall heat transfer rate under certain conditions.

Swelling Properties of Coal Chars During Rapid Pyrolysis and Combustion

Fletcher, T.H.
Fuel, 72:1485-1495, 1993. Funded by Sandia National Laboratories and ACERC.

Coal devolatilization experiments are commonly conducted at moderate temperatures (800 to 1300 K) and heating rates (10³ to 104 K/s) in inert environments in order to measure evolved species before secondary reaction in gas phase. However, chars from these experiments exhibit different physical characteristics than chars obtained under typical combustion conditions (1500 to 2000 K, 105 K/s, and 3 to 10 MOL% oxygen). Experiments were conducted in two laminar, entrained-flow reactors to determine characteristics of coal chars in inert and oxygen-rich environments. One flow reactor was heated electrically, with gas temperatures of 1250 K, and the mol% oxygen was varied from 0 to 10%. The other flow reactor used a flat flame burner as the heat source, with gas temperatures of 1600 K, and the post-flame oxygen content was varied from 0 to 12 mol%. In both reactors, sampling was limited to regions during and immediately following devolatilization. Five coals of different rank were examined; for a given coal, similar total volatile yields were obtained in both flow reactors, and similarities in chemical compositions of the resulting chars are discussed. For softening coals, the apparent densities of chars obtained in the electrically heated reactor are much lower than that of chars from the flat-flame reactor, regardless of the gas phase oxygen content. This implies that changes in particle swelling behavior between typical devolatilization experiments and char combustion experiments are not due to the pressure of oxygen, but to heating rate or post-flame gas species other than oxygen.

Chemical Structural Features of Coal Chars, Tars, and Char Extracts During Rapid Pyrolysis Using C-13 and H-1 NMR Spectroscopy

Fletcher, T.H.; Solum, M.S.; Pugmire, R.J.; Grant, D.M.; Bai, S.; Ma, J. and Woods, S.
7th International Conference on Coal Science, Banff, Alberta, Canada, September 1993 (in press). Funded by ACERC.

Structural characteristics have been determined for parent coals and for chars collected at different stages of pyrolysis. Recent work has focused on trying to understand the relationship between chemical structural features of the unreacted coal and the devolatilization and char oxidation phenomena. Models of coal devolatilization have recently related devolatilization behavior to the structure of the parent coal and the initial amount of pyridine extracts. Fong et al. used pyridine extraction methods to quantify the amount of metaplast formed during pyrolysis of a Pittsburgh #8 coal. These experiments demonstrated that under moderate heating conditions (~500 K/s to 873 K), as much as 80% of the initial coal was transformed into a combination of extractable material and volatiles. The work presented here is an examination of the pyridine extraction procedure of the Argonne Premium coal samples and the detailed study of the carbon skeletal structure of the extracts and the extraction residues from these coals. This is the first stage of an experimental program to examine the yield and chemical features of extracts of coal chars collected as a function of time during pyrolysis.

The Use of Solid State C-13 NMR Spectroscopy to Study Pyridine Extracted and Extraction Residues in the Argonne Premium Coals

Pugmire, R.J.; Solum, M.S.; Bai, S.; Fletcher, T.H.; Woods, S. and Grant, D.M.
Proceedings of the 205th ACS National Meeting, 38, no. 2, Denver, CO, March 1993. Funded by ACERC.

The relationship between coal structure and combustion behavior is a matter of on-going research in our laboratories. A great deal of effort has gone into obtaining data that is used for modeling studies of devolatilization behavior. We have also carefully studied the process of char formation. Our past work has focused on trying to understand the relationship between coal/char/tar formation as they relate to the devolatilization and char oxidation phenomena. The formation of metaplast during pyrolysis was studied by Fong and Howard in terms of extractable material obtained at different stages of the devolatilization process. We have recently turned our attention to metaplast formation in devolatilization and plan to conduct a series of experiments that will help define the formation and chemical structure of metaplast in coals of different rank.

Comparison of Reactivity and Physical Structure of Chars Prepared Under Different Pyrolysis Conditions, i.e. Temperature, Gas Atmosphere, and Heating Rate

Gale, T.K.; Bartholomew, C.H. and Fletcher, T.H.
Proceedings of the International Conference on Coal Science, Banff, Canada, September 1993. Funded by ACERC.

Coal combustion consists of basically two main steps: 1) pyrolysis and oxidation of the liquid and volatile matter, and 2) subsequent oxidation of the residual porous char matrix. Char oxidation is the slower of these two steps and is difficult to bring to completion. Pyrolysis significantly affects the resulting char structure, porosity, internal surface area and chemical composition (e.g. H/C ratios) and hence the char oxidation rate. A highly porous char particle is more accessible to reactant molecules and will, therefore, have a higher reactivity in the reaction zone influenced by pore diffusion. Under surface reaction controlled conditions, reactivity increases with increasing internal surface area and H/C ratio.

A number of different experimental methods and reactor types are currently used to produce chars for laboratory study. These different reactors typically operate under conditions that are quite reproducible from one run to another. However, variations in pyrolysis conditions from one method to another and from one reactor type to another may be large. Comparisons of data obtained in different laboratories are often rationalized by matching experimental conditions thought to be most critical such as temperature and residence time, or temperature and total volatiles yield. However, comparing chars at the same residence time or the same mass loss may not be valid, because at different heating rates and/or gas-phase oxygen concentrations, the chemical and physical nature of the pyrolysis will vary.

The objective of this research was to determine effects of variations in pyrolysis conditions on char structure and reactivity for a group of chars prepared from coals of low to high rank. Heating rate, temperature, residence time, and gas atmosphere during pyrolysis were the main variables in the study.

Coal and Char Structural Parameters Derived from Solid State C-13 NMR Studies

Pugmire, R.J.; Solum, M.S.; Fletcher, T.H. and Grant, D.M.
The 5th Australian Coal Science Conference, Melbourne, Australia, March 1993. (Also presented at the 5th Australian Coal Science Conference, Melbourne, Australia, November, 1992.) Funded by ACERC.

In contrast to previous efforts where coal-general devolatilization model input parameters describing chemical structure are adjusted to force agreement between predicted and measured tar and total volatiles yields, coal-dependent chemical structure coefficients for the Chemical Percolation Devolatilization (CPD) model developed by the authors and others are taken directly from C-13 NMR analyses of parent coals. This procedure, outlined in the paper eliminates most adjustable parameters from the model, and predictions of tar and total volatiles yields become true tests of the model and the NMR data, rather than mere results of curve fitting. Resulting model predictions of tar and total volatiles yields as a function of coal type, temperature, heating rate, and pressure comparable with available experimental data, showing the value of both the model and the NMR chemical structure data.

Properties of Soot from Coal Tar

Ma, J.; Dean, M.; Rossman, J.; Sastrawinata, T.; Webb, B.W. and Fletcher, T.H.
Meeting of the Western States Section of the Combustion Institute, October 1993, Menlo Park, CA. Funded by ACERC.

Soot properties and formation mechanisms have been extensively studied in gas flames such as acetylene and propane. However, relatively little information is known concerning soot properties in coal combustion. Coal tar is the precursor to soot in coal combustion, so that the aromatic ring structures are already present. Experiments are presented to show the size of soot particles generated from coal tar at high temperature. A flat flame burner is used to provide the high temperature environment. Coal particles are entrained along the centerline of the reactor, and release pyrolysis products into the hot surrounding gas. The tar/soot cloud diffuses radially away from the centerline as it is convected axially in the flow reactor. The soot sampling system inserts a carbon-coated microscope grid radially into the soot cloud at different residence times, and the soot particle deposit thermophoretically. Soot particles are then analyzed using transmission electron microscopy (TEM) at magnifications as high as 150,000. Distinct soot particles with approximate diameters of 25 nm were observed along with particle agglomerates consisting of multiple primary particles. The observed agglomerate size increases with residence time in the reactor. Liquid-like unstable deposits (believed to be condensed tar) were also observed. These qualitative observations are important for descriptions of soot radiation from coal flames.

Overview of ACERC Comprehensive Model Development

Fletcher, T.H. and Hill, S.C.
Energy & Fuels, 7, (6):870-873, 1993. Funded by ACERC.

A major objective of the Advanced Combustion Engineering Research Center (ACERC) is the development of comprehensive combustion models to help in the solution of critical national combustion problems. Computer models incorporate research and technology results from center projects and from external research programs. The synergistic integration of scientific knowledge that is expected from the NSF engineering research centers is demonstrated to a great extent at ACERC by the development of these software tools. The transfer of technology from ACERC to industry is also accomplished in part by the implementation of the models at industrial firms. The effort to develop such products requires significant integration and development, together with fundamental research. The development of comprehensive models also produces personnel and technology able to help address the challenge of synergistic cross-linkage among thrust areas within ACERC and provides an important means of transferring this technology to industry. This article is an overview of the purpose, accomplishments and goals of research at ACERC in comprehensive modeling. (Thrust Area 5.)

1992

The Structure and Reaction Process of Coal

Smith, L.K.; Smoot, L.D.; Fletcher, T.H. and Pugmire, R.J.
Plenum Press Co., New York, 1994. Funded by ACERC.

This book characterizes the properties and reaction rates of the eleven U.S. coals selected emphasis by ACERC. Eight of the eleven comprise the Argonne National Laboratory's Premium Coal Sample Bank. The book features the comprehensive measurement of organic and inorganic components of the coal structure by advanced methods (SEM, NMR, GC, porosimetry, Pycnometry, X-ray, and MS). The book features the measurement of coal devolatilization and char oxidation rates by advanced, optical methods and correlative relationships between the structure and reaction processes.

 

Decreases in the Swelling and Porosity of Bituminous Coals During Devolatilization

Gale, T.K.; Bartholomew, C.H. and Fletcher, T.H.
Combustion and Flame, 1994 (in press). (Also presented at the 25th International Symposium on Combustion, Irvine, CA, August 1994). Funded by ACERC.

Concern about comparability and validity of different methods for producing coal chars for reactivity experiments has led to research on the effect of devolatilization conditions on the char physical and chemical structure. Particle diameter and porosity changes during devolatilization significantly affect char oxidation rates. In particular, physical properties of chars prepared in drop tube reactors differ greatly from chars prepared in flat flame burner experiments. Recent data indicate that the presence of oxygen in the gas atmosphere has no effect on swelling until char oxidation has begun. The present research concentrates on the effects of heating rate, particle temperature and residence time on the swelling and porosity of a plastic coal, and compares these results with a non-plastic coal. The heating rate at which the transition from increasing swelling to decreasing swelling occurs in approximately 5 x 10³ K/s for swelling coals. Swelling coals also reach a maximum porosity near this heating rate. At low particle heating rates swelling gradually increases versus heating rate in contrast to a decline in the swelling at high heating rates in a narrow heating rate region of 2 x 10^4 to 7 x 10^4 K/s. Non swelling bituminous and lignite coals continue to increase in porosity beyond the heating rate of 2 x 10^4 K/s.

 

Effects of Pyrolysis Conditions on Internal Surface Areas and Densities of Coal Chars Prepared at High Heating Rates in Reactive and Non-Reactive Atmospheres

Gale, T.K.; Fletcher, T.H. and Bartholomew, C.H.
Energy & Fuels, 1994 (in press). Funded by ACERC.

Concern about comparability and validity of different methods for producing coal chars for research has motivated this investigation of the effects of devolatilization conditions on the physical properties of coal chars. It is evident from the findings of this study that care must be taken to prepare chars under conditions similar to those of full-scale coal combustion boilers prior to performing char oxidation studies. Two different entrained flow reactors were used to prepare chars under a variety of different pyrolysis conditions at maximum particle temperatures and heating rates between 840 to 1627 K and 10^4 to 2 x 10^5 K/s respectively. Under these conditions micro-pore (CO2) surface area generally increases with residence time and mass release for lignite and bituminous coals, as does true density. Micro-pore surface area also increases somewhat with increasing maximum particle temperature and heating rate. Meso-pore (N2) surface area is most affected by reactive gas atmospheres (carbon activation). The presence of steam in the post flame gases of methane/air flat flame burners is a significant factor in increasing meso-pore surface are of chars prepared in such burners, even though the mass conversion by steam gasification is small. Partial char oxidation with O2 significantly affects char N2 and CO2 surface area at these heating rates and residence times (50 to 100 ms), sometimes increasing and sometimes decreasing internal surface area. Low rank lignite and sub-bituminous coals have higher potentials for forming chars with increased N2 surface are than bituminous coals. The moisture content of low rank coals may be more important than rank. Lignite with high moisture content yields char with a significantly higher N2 surface area than char prepared from lower moisture content lignite. However, initial coal moisture has less effect on CO2 surface area.

Chemical Structure Changes of Coal, Char, and Tar During Devolatilization

Fletcher, T.H. and Pugmire, R.J.
Proceedings of the ACS Spring Meeting, San Diego, CA, March 1994, ACS Preprints, Division of Fuel Chemistry, 39:108-112. Funded by ACERC, Brigham Young University and University of Utah.

Enormous progress has been made in coal pyrolysis research during the last decade. Models of coal devolatilization have progressed from simple rate expressions based on total mass release to empirical relationships based on the elemental composition of the parent coal to models that attempt to describe the macromolecular network of the coal. Measurements of particle temperature during devolatilization have eliminated much of the controversy regarding overall rates of devolatilization. In the last several years, advancements in chemical analysis techniques have allowed quantitative investigations of the chemical structure of both coal and its pyrolysis products, including the nature of the resulting char. A prominent research goal is to accurately predict the rates, yields, and products of devolatilization from measurements of the parent coal structure. This goal necessitates modeling the reaction processes on the molecular scale, with activation energies that relate to chemical bond breaking rather than release of products from the coal. C-13 and H-1 NMR spectroscopy have proven particularly useful in obtaining average values of chemical structure features of coal, char and tar. This paper reviews experimental data regarding chemical structure features of coal, char, and tar during rapid devolatilization, and how these data have impacted the development and input parameters for devolatilization models. In particular, the relationship between pyridine extract yields and extract yields predicted purely from NMR chemical structure data d

The Impace of Pyrolysis in Combustion

Solomon, P.R. and Fletcher, T.H.
25th Symposium International on Combustion, 1994 (in press). (Also presented at the 25th Symposium (International) on Combustion, Irvine, CA, Jul-Aug, 1994.) Funded by ACERC.

The pyrolysis process has impacts throughout coal combustion. The roles of pyrolysis in various aspects of the coal combustion process are described including the devolatilization yield, nitrogen release, softening and swelling, soot formation, and char reactivity. These processes can be understood and quantitatively predicted using recently developed network pyrolysis models that describe the transformation of the coal's chemical structure. The models are described and examples of their predictive ability for important coal combustion phenomena are presented.

1991

Progress in Coal Pyrolysis

Solomon, P.R.; Fletcher, T.H. and Pugmire, R.J.
Fuel, 1991 (in press). Funded by US Department of Energy and ACERC.

The heterogeneous nature of coal and the complexity of the pyrolysis process has made it very difficult to perform unambiguous experiments to determine the rates and mechanism in coal pyrolysis. The last several years have, however, provided a number of new experimental and theoretical approaches that shed new light on the subject. This paper will consider the recent progress on the topics of: kinetics, the formation of volatile products, network models, crosslinking, rank effects, and the "two-component model of coal structure." In kinetics, recent experiments that measure coal particle temperatures at high heating rates provide reasonable agreement on kinetic rate constants. The rates also agree with those derived from low heating rate experiments. In tar formation and transport, a consensus is being reached on the central role of the tar molecule's volatility in explaining the variation with operating parameters (pressure, heating rate, particle size, etc.) in the tar's amount and molecular weight distribution. Progress in the quantitative prediction of tar and char is being made by recently developed models for the fragmentation of the macromolecular network. These models, which provide quantitative description of the relationship between the chemical structure of the coal and the physical and chemical properties of the resultant pyrolysis products (gas, tar, soot, and char), are an exciting advancement in the understanding of the pyrolysis process. Such models are linking the occurrence of the coal's plastic phase with the "liquid" fragments formed during pyrolysis. On the subject of retrogressive crosslinking reactions, both solvent swelling and NMR measurements confirm important rank dependent differences in reaction rates. These appear related to the oxygen functionalities. Reasonable agreement is also seen for rank variations of kinetics rates derived from low heating rate experiments. Experiments suggest that the recently revived "two-component hypothesis" of coal structure has application to low rank coals that are a mix of polymethylenes and a more aromatic network. Bituminous coals, however, appear far more homogeneous. These coals appear to behave in a manner that is described by the network decomposition models. The presentation will provide a brief report on these topics.

Structural Evolution of Matched Tar/Char Pairs in Rapid Pyrolysis Experiments

Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Fletcher, T.H.
Fuel, 70:414-423, 1991. Funded by Pittsburgh Energy Technology Center and ACERC.

Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the relationship between chemical structures of coal and the char particles and condensed tar vapors produced from coals of various ranks at rapid heating conditions. The C-13 NMR analysis of the coal chars indicate that significant amounts of aliphatic material is released from the coal during devolatilization with little initial change to the aromatic cluster size or number of cross links per cluster. The evolution of the char structure following tar release is a function of the time/temperature history of the char. The structures of the primary tars are compared to the parent coal and the gas phase evolution of the tar structure is followed with time.

Predicting Vapor Pressures of Tar and Metaplast During Coal Pyrolysis

Fletcher, T.H.; Grant, D.M. and Pugmire, R.J.
ACS Division of Fuel Chem. Preprints, 36(1):250-257, 1991 (201st ACS National Meeting, Atlanta, GA, April 1991). Funded by Pittsburgh Energy Technology Center and ACERC.

Models of coal pyrolysis have progressed from simple one or two step empirical Arrhenius expressions that correlate total mass release during devolatilization, as reviewed by Anthony and Howard, to detailed descriptions of hydrocarbon chemistry and mass transport. These models describe the yields and compositions of pyrolysis products from coal under a wide range of heating conditions and ambient pressures. During pyrolysis of softening coals, a liquid phase appears that is referred to as metaplast. Release of pyrolysis gases and tar vapors inside the particle cause bubble formation in the softened coal particle, followed by swelling (increase in the particle diameter) with large internal voids (cenosphere formation). The softened state is followed by crosslinking or repolymerization which solidify the char matrix. As the coal particle is heated to sufficiently high temperatures, the light species in the metaplast are released as hydrocarbon vapors, along with light gases. Coal tar is generally defined to consist of those species which are released from the coal during pyrolysis which condense at room temperature and pressure. Low rank coals and lignite generally give low tar yields, and do not exhibit much softening or swelling behavior; this non-softening behavior may be caused by early crosslinking reactions. High rank coals (i.e., anthracites and low volatile bituminous coals) contain low amounts of volatile matter, and hence coal particles remain relatively intact during pyrolysis unless fragmentation occurs.

Mass transport affects coal pyrolysis in two ways: (1) as the ambient pressure increases, the tar yield decreases, and (2) as particle size increases, the tar yield decreases. However, there seem to be regions where the two mass transport effects are not controlling. For instance, in vacuum, the small pressure generated inside the pyrolyzing coal particle from the release of light gases and tar vapors may control the process. Also, total volatiles yields from a lignite were observed to remain constant with increasing ambient pressure, although this is probably due to the low tar yield of the lignite. Changes in coal pyrolysis yields as a function of particle size for diameters less than 200 µm are small.

Progress in Coal Pyrolysis

Solomon, P.R.; Fletcher, T.H. and Pugmire, R.J.
8th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, October 1991. Funded by US Department of Energy and ACERC.

The heterogeneous nature of coal and the complexity of the pyrolysis process has made it very difficult to perform unambiguous experiments to determine the rates and mechanism in coal pyrolysis. The last several years have, however, provided a number of new experimental and theoretical approaches that shed new light on the subject. This paper will consider the recent progress on the topics of: kinetics, the formation of volatile products, network models, crosslinking, rank effects, and the "two-component model of coal structure." In kinetics, recent experiments that measure coal particle temperatures at high heating rates provide reasonable agreement on kinetic rate constants. The rates also agree with those derived from low heating rate experiments. In tar formation and transport, a consensus is being reached on the central role of the tar molecule's volatility in explaining the variation with operating parameters (pressure, heating rate, particle size, etc.) in the tar's amount and molecular weight distribution. Progress in the quantitative prediction of tar and char is being made by recently developed models for the fragmentation of the macromolecular network. These models, which provide quantitative description of the relationship between the chemical structure of the coal and the physical and chemical properties of the resultant pyrolysis products (gas, tar, soot, and char), are an exciting advancement in the understanding of the pyrolysis process. Such models are linking the occurrence of the coal's plastic phase with the "liquid" fragments formed during pyrolysis. On the subject of retrogressive crosslinking reactions, both solvent swelling and NMR measurements confirm important rank dependent differences in reaction rates. These appear related to the oxygen functionalities. Reasonable agreement is also seen for rank variations of kinetics rates derived from low heating rate experiments. Experiments suggest that the recently revived "two-component hypothesis" of coal structure has application to low rank coals that are a mix of polymethylenes and a more aromatic network. Bituminous coals, however, appear far more homogeneous. These coals appear to behave in a manner that is described by the network decomposition models. The presentation will provide a brief report on these topics.

1990

Chemical Percolation Model for Devolatilization: 2. Temperature and Heating Rate Effects on Product Yields

Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J. and Grant, D.M.
Energy & Fuels, 4 (54), 1990. Funded by Pittsburgh Energy Technology Center, US Department of Energy, National Science Foundation and ACERC.

The chemical percolation devolatilization (CPD) model previously developed to describe the devolatilization behavior of rapidly heated coal was based on the chemical structure of the parent coal. Percolation lattice statistics are employed to describe generation of finite tar clusters as labile bonds are cleaved in the infinite coal lattice. The model is used here to describe effects of heating rate and temperature on tar and gas release from coal. Coefficients for the net rate of competition between char formation and side-chain formation are generated from heated screen data performed at five different heating rates. The model also compares well with heated screen data obtained at 1000 K/s and different hold times at the final temperature as well as with data from entrained-flow reactors obtained at higher heating rates (104 K/s) where particle temperatures have been measured. Results indicate that the CPD model predictions yield good agreement with published data for a wide range of coals and particle heating rates.

Structural Evolution of Matched Tar/Char Pairs in Rapid Pyrolysis Experiments

Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Fletcher, T.H.
Fuel, 1990 (In press). Funded by ACERC.

Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the relationship between chemical structures of coal and the char particles and condensed tar vapors produced from coals of various ranks at rapid heating conditions. The C-13 NMR analysis of the coal chars indicate that significant amounts of aliphatic material is released from the coal during devolatilization with little initial change to the aromatic cluster size or number of cross links per cluster. The evolution of the char structure following tar release is a function of the time/temperature history of the char. The structures of the primary tars are compared to the parent coal and the gas phase evolution of the tar structure is followed with time.

A Chemical Percolation Model of Coal Devolatilization: 3. Direct Use of C-13 NMR Data to Predict Effects of Coal Type

Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J.; Solum, M.S. and Grant, D.M.
Polycyclic Aromatic Compounds, 1:251-264, 1990. Funded by Gas Research Institute and ACERC.

The chemical percolation devolatilization (CPD) model describes the devolatilization behavior of rapidly heated coal based on the chemical structure of the parent coal. Percolation lattice statistics are employed to describe the generation of tar precursors of finite size based on the number of cleaved labile bonds in the infinite coal lattice. The chemical percolation devolatilization model described here includes treatment of vapor-liquid equilibrium and a cross-linking mechanism. The cross-linking mechanism permits reattachment of metaplast to the infinite char matrix. A generalized vapor pressure correlation for high molecular weight hydrocarbons, such as coal tar, is proposed based on data from coal liquids. Coal-independent kinetic parameters are employed. Coal-dependent chemical structure coefficients for the CPD model are taken directly from C-13 NMR measurements, with the exception of one empirical parameter representing the population of char bridges in the parent coal. This is in contrast to the previous and common practice of adjusting input coefficients to precisely match measured tar and total volatiles yields. The CPD model successfully predicts the effects of pressure on tar and total volatiles yields observed in heated grid experiments for both bituminous coal and for lignite. Predicted tar molecular weights are consistent with size-exclusion chromatography (SEC) data and field ionization mass spectrometry (FIMS) data. Predictions of average molecular weights of aromatic clusters as a function of coal type agree with corresponding data from NMR analyses of parent coals. The direct use of chemical structure data as a function of coal type helps justify the model on a mechanistic rather than an empirical basis.

 

Mass Spectrometric Studies of the Chemical Composition of Coal Tars Produced in a Laminar Flow Reactor

Lo, R.; Pugmire, R.J.; Fletcher, T.H. and Meuzelaar, H.L.C.
Preprints for Papers Presented at the 200th ACS National Meeting, 35 (3), 697-704, Washington, D.C., 1990. Funded by ACERC and Consortium for Fossil Fuel Liquifaction Science.

Curie-point desorption in combination with Gas Chromatography/Mass Spectrometry (GC/MS) and, alternatively, with direct Low Voltage Mass Spectrometry (LV-MS) was used to investigate the chemical composition and structure of condensed tar vapors produced during rapid devolatilization (heating rate ~10,000 K/sec) of carefully sized coal particles representing the Beulah Zap, Big Blue, Illinois #6, Pittsburgh #8, and Pocahontas #3 seams, respectively, using the laminar flow reactor described by Fletcher et al at two gas temperatures (1050 K and 1250).

Tar samples were collected by means of a special probe at different points downstream of and corresponding to residence times between 70 and 250 ms. GC/MS analyses of the corresponding tars indicate that the degree of aromaticity increased rapidly as a function of residence time at the 1250 K gas temperature condition. Moreover, at 1250 K devolatilization is complete within 70 ms and beginning secondary gas phase reactions of tar vapors (viz. marked increases in PNAH content and corresponding decreases in phenolic components) are observed within less than 100 ms. However, at 1050 K the coal devolatilization process appears to be barely complete after 250 ms and little or no evidence of secondary gas phase reactions is found.

Solid-State C-13 and H-1 NMR Studies of the Evolution of the Chemical Structure of Coal Char and Tar During Devolatilization

Fletcher, T.H.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Pugmire, R.J.
Twenty-third Symposium (International) on Combustion, The Combustion Institute, France, 1990 (In press). Funded by US Department of Energy and ACERC.

Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the chemical structure of char particles and condensed tar vapors produced as pyrolysis products from an Illinois #6 coal at rapid heating conditions (~104 K/s) at two gas conditions (maximum gas temperatures of 1250 K and 1050 K). The temperature history of particles in the flow reactor is determined using a unique infrared sizing-pyrometry system. The C-13 NMR analyses of the coal chars indicate that significant amounts of aliphatic material are released from the coal during devolatilization, with little change to the aromatic cluster size or number of attachments per cluster. At long residence times, and at higher temperatures, small increases in the cluster size in the char are observed. The H-1 NMR analyses indicate that thermal decomposition of tar vapor occurs at the 1250 K gas condition, as evidenced by increases in the aromaticity and decreases in the peripheral aliphatic groups, such as methyl groups and aliphatic bridge material.

1989

Solid-State C-13 NMR Studies of Coal Char Structure Evolution

Solum, M.S.; Pugmire, R.J.; Grant, D.M.; Fletcher, T.H. and Solomon, P.R.
Fuel Fiv. Preprints, 34 (4), 1337-1346, 198th ACS National Meeting, Miami, 1989. (Also presented at the Western States Section, The Combustion Institute Spring Meeting, Pullman, Washington, 1989.) Funded by ACERC (National Science Foundation and Associates and Affiliates).

Solid state C-13 NMR techniques have been used to study the evolution of char structure during pyrolysis processes. The effects of residence time, heating rate, and final char temperature are observed. The NMR data demonstrates that extensive loss of aromatic ring bridge material precedes significant change in aromatic cluster size.

A Chemical Percolation Model for Devolatilization: Temperature and Heating Rate Effects

Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J. and Grant, D.M.
Fuel Div. Preprint, 34 (4), 1272-1279, 198th ACS National Meeting, Miami, 1989. Funded by US Department of Energy and ACERC (National Science Foundation and Associates and Affiliates).

It is well known that the yield of volatile matter obtained from a pulverized coal is dependent upon the temperature history of the particle. However, the effect of heating rate on volatiles yield is difficult to study independently of final temperature. For example, the volatile yields obtained in an entrained flow reactor study by Kobayashi, et al. increase with both temperature and heating rate, but the independent contribution of heating rate could not be assessed. Heated screen experiments were developed to study devolatilization behavior at different heating rates independently from the final particle temperature. The data of Anthony and Howard show little increase in volatiles yield when particles are heated to the same final temperature on a heated screen at different heating rates. In a more recent study, Gibbins-Matham and Kandiyoti show evidence for small increases in the volatiles yield from a Pittsburgh #8 coal as the heating rate is increased from 1 K/s to 1000 K/s on a heated screen. Coal samples were heated at 5 different heating rates to a final temperature of 700ºC and held for 30 s. Experiments were repeated several times in order to ensure accuracy of the data. The total volatiles yield increases from 41.5% at 1 K/s to 46.8% at 1000 K/s, a relative increase in yield of 13%. This increase in yield with increase in heating rate is small, but is larger than associated experimental errors.

The chemical percolation devolatilization (CPD) model was developed as a means to describe coal devolatilization behavior based upon the chemical structure of the patent coal. Some of the input parameters for this model are obtained from NMR characterizations of the parent coal. Percolation statistics are used to describe the probability of generating finite tar fragments from the infinite coal matrix. Pyrolysis yields of tar, gas, and char for three different types of coal are described using a single set of kinetic parameters: only chemical structure parameters are changed for the different coals. The initial description of the CPD model allowed for a temperature dependence of the competition between side chain formation and char formation. However, this option was not exercised in the initial study in order to demonstrate general utility of the model for one set of devolatilization data on three coals collected over a narrow range of temperatures and heating rates. In the present work, the Gibbins-Matham and Kandiyoti data are used to determine additional coefficients for the CPD model that accurately predict the changes in char and tar yield as a function of heating rate.

Chemical Percolation Model for Devolatilization: II. Temperature and Heating Rate Effects on Product Yields

Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J. and Grant, D.M.
Accepted for publication by Energy & Fuels, 1989. Funded by US Department of Energy and ACERC (National Science Foundation and Associates and Affiliates).

The CPD model previously developed to describe the devolatilization behavior of rapidly heated coal was based on the chemical structure of the parent coal. Percolation lattice statistics are employed to describe generation of finite tar clusters as labile bonds are cleaved in the infinite coal lattice. The model is used here to describe effects of heating rate and temperature on tar and gas release from coal. Coefficients for the net rate of competition between char formation and side chain formation are generated from heated screen data performed at five different heating rates. The model also compares well with heated screen data obtained at 1000 K/s and different hold times at the final temperature as well as with data from entrained flow reactors obtained at higher heating rates (104 K/s) where particle temperatures have been measured. Results indicate that the CPD model predictions yield good agreement with published data for a wide range of coals and particle heating rates.

1988-1986

A Study of Two Chemical Reaction Models in Turbulent Combustion

Smith, P.J. and Fletcher, T.H.
Accepted for publication in Combustion Science and Technology, 1988. 24 pgs. Not externally funded.

Research efforts with comprehensive computer models that have tried to predict the performance of coal combustors have either neglected the effect of the turbulence on the mean chemical properties or have used one of two approximate methods. This paper focuses on the impact of the turbulence on the chemical reactions of the volatile products of coal combustion processes. It is shown that by ignoring the effect of the turbulence on mean combustion properties significant differences occur as compared to experimental data and predicted by both of the more rigorous models. The first method, the volatile reactances model, is an extension of an approach for premixed gaseous combustion presented by Magnussen and Hjertager. The second method, the statistical, coal-gas mixture fraction model, is an extension of gaseous diffusion flame approaches. These two methods are examined, analyzed and evaluated by comparing predictions from each method with experimental data from three laboratory furnaces. It is shown that while the first method takes only half as much computational time, the second method is required if species and temperatures in zones containing other than mixtures of pure fuel, pure oxidant and pure stoichiometric product are needed. The distribution of eddy mixtures as formulated in the second method is shown to be more consistent with existing limited experimental data.

A Chemical Model of Coal Devolatilization Using Percolation Lattice Statistics

Grant, D.M.; Pugmire, R.J.; Fletcher, T.H. and Kerstein, A.R.
ACS Div. of Fuel Chemistry, 33, (2), 322-332, 1988. 10 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).

We have developed a model for coal devolatilization that incorporates the diversity of coal structure in such a way that the analytical data obtained from solid state NMR provides the initial input data. Using experimentally determined kinetic rate parameters, it is possible to fit the gas, tar and char production of a lignite and high volatile bituminous coal. We have employed percolation theory to provide analytical expressions for the lattice statistics required in devolatilization modeling. The percolation theory allows one to avoid the more time-consuming Monte Carlo technique with no loss of generality or important statistical features. Percolation theory analytically describes the size distribution of finite clusters of sites joined by intact bridges but isolated from all remaining sites by broken bridges. The theory specifies a critical bridge population, depending only on the site coordination number, above which infinite arrays will coexist with clusters of finite size. It is a simple matter to adapt the structural features of percolation theory to both the tar and gas obtained in coal pyrolysis. The infinite arrays of percolation theory are interpreted as the macroscopic lattice of unreacted coal and/or char while the relatively small tar molecules may be identified with the fine clusters of percolation theory. The details of the model will be discussed together with the results obtained in modeling devolatilization behavior of coals of various ranks.

Prediction of High Intensity Pulverized Coal Combustion

Suzuki, T.; Smoot, L.D.; Fletcher, T.H. and Smith, P.J.
Combustion Science and Technology, 45, (3&4), 167-183, 1986. 17 pgs. Funded by Brigham Young University and Kobe Steel Company, Japan.

The overall characteristics of high-intensity pulverized coal combustion have been predicted by a one-dimensional model. The mixing of the primary stream of pulverized coal and transport air with secondary combustion air was estimated by a growth angle of the primary jet. The coal particle burnout was strongly affected by the extent of devolatilization, which varies amount coals. The extent of devolatilization as characterized by variation in a devolatilization coefficient was correlated with either proximate volatiles percentage or H/C mass ratio of the virgin coal. The resulting comparisons of predictions with measurements for eight coal types and tree different combustors show that observed trends are generally predicted. The data used for these comparisons were obtained from a wide range of high-intensity combustion experiments. The proximate volatile matter in the virgin test coals ranged form 16 to 40 percent while the coal feed rate was varied from 12 to 290 kg/hr. Combustion air temperature varied from 297 to 1483 K while residence time ranged from 3 to 140 ms. Comparative results suggest that the predictive method can be useful in interpreting high intensity combustion test results.