ACERC
Abstracts - 1999 |
ACERC
Abstracts - 1999 |
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Research Area 1: Combustion Chemistry |
Perry, S.T.
A Global Free-Radical Mechanism for Nitrogen Release during Coal Devolatilization
Based on Chemical Structure, Ph.D./BYU, Chemical Engineering Department,
December 1999. Advisor: Fletcher
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.
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.
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.
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.
Guo, F. and Hecker, W.C.
Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute,
1999/pp.3085-3092
The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and postcombustion clean-up processes to control NOx emissions from combustion. The reaction is also important in understanding the formation and reduction of NO during coal combustion. In this study, the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah-Zap [NDL], Dietz, Utah Blind Canyon [UBC], Pittsburgh #8, and Pocahontas #3) were investigated in a packed-bed reactor at temperatures between 723 and 1173 K. Graphite and coconut char were also studied.
The low-rank chars were found to be significantly more reactive than the high-rank chars (NDL> Dietz>> coconut ~ Pittsburgh #8 ~ UBC ~ Pocahontas #3 >> graphite) with the T50 (temperature required for 50% NO conversion) varying from 870 K for NDL to 1100 K for graphite for a given set of conditions. For all chars studied, the reaction was found to be first order with respect to NO partial pressure and to exhibit an activation energy 0(EA) shift from 100-160 kJ/mol at low temperatures to 190-250 kJ/mol at high temperatures. The shift to distinctly different and higher EA's at higher temperature is opposite to what would be expected if a reaction is shifting from chemical rate control to mass transfer control and suggests different mechanisms or rate-determining steps at high and low temperatures. Although all chars exhibited the shift in EA, the shift temperature and the EA within each temperature regime tended to increase with increasing rank. Also, the relative reactivity of the chars depends not only on organic char surface area but also on inorganic content, specifically, CaO surface area.
Genetti, D.
An Advanced Model of Coal Devolatilization Based on Chemical Structure,
M.S./BYU, Mechanical Engineering Department, April 1999. Advisor: Fletcher
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