ACERC
Abstracts - 1998 |
ACERC
Abstracts - 1998 |
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Research Area 1: Combustion Chemistry |
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.
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.
Bartholomew, C.H.; Butala,
T.Q.; Medina, J.C.; Lee, M.L.; Taylor, S.J. and Andrus, D.B.
Proceedings of the International Conference on Coal Seam Gas and Oil,
Brisbane, Australia, March 23-25, 1998.
Coal seam reservoirs are important worldwide commercial sources of natural gas. It is commonly assumed that hydrocarbon gases are formed in coal seams by thermolysis (cracking) of coal organic matter. Recently, however, the reliability of this geologic process model has been questioned. In fact, results of artificial maturation experiments indicate that raw (mineral-containing) coal generates hydrocarbon gas at substantially higher rates than demineralized coal. This difference suggests that mineral catalysis could be a critical variable affecting hydrocarbon gas formation during coal maturation.
The objective of our combined literature and experimental study is to evaluate potential roles of minerals in catalyzing coal-bed methane formation. In the first phase of this study, rate and product selectivity data for hydrocarbon thermolysis and mineral-catalyzed cracking or synthesis reactions were compiled in a comprehensive review of technical literature sources. Kinetic models were used to predict conversion rates and product yields at typical low-temperature conditions of coal maturation. It was found that under these conditions hydrocarbon thermolysis reactions would be too slow to generate, even over geologic times, large, self-sourced coal seam natural gas deposits. By contrast, acid-mineral- and transition-metal-catalyzed reactions would occur at sufficiently high rates in geologic time and at geologic conditions to generate large quantities of natural gas, although the product distribution over acid-mineral catalysis is very different than for natural gas. Two potentially viable catalytic routes involving naturally occurring transition metal species and capable of forming large natural gas deposits within hours to several years are: (1) hydrogenolysis of alkanes and/or alkenes over iron and nickel and (2) CO2 methanation on iron and nickel. Selectivities of these catalysts in both reactions for methane are high, and the product distributions are similar to those of natural gases. We were also able to identify several geologically viable catalytic and noncatalytic routes for production of H2, a reactant typically found in coal gas and important in the catalytic production of methane from hydrocarbons or CO2.
In the second phase of this work, potential methane-forming reactions were conducted for 100-hour periods at 180°C in 1 atm of H2 in the presence or absence of reduced or unreduced iron-silica catalysts. Carbon dioxide and 1-dodecene, both found in coal beds, were utilized as model substrates. A computer-automated batch reactor system with Pyrex reactor, glass stirrer, and on-line GC analysis was used to measure reactant and product concentrations as a function of time.
Significant rates of methane formation are observed in both reactions in the presence of the prereduced catalyst after just a few hours. However, induction time and methane yield vary with substrate. In carbon dioxide methanation, the induction time is 1 h compared to 17 h for olefin hydrogenolysis, and the rate of methane production is an order of magnitude higher in CO2 methanation relative to olefin hydrogenolysis (262 and 32 mmol gFe^-1 d^-1 respectively). The latter rate compares favorably with data reported for C8 olefin hydrogenolysis. Production rates of light alkanes other than methane (i.e., ethane, propane, and butane) are also significant, although an order-of magnitude lower than for methane; thus the product distributions are characteristics of natural gas. On the other hand, no products are observed over 100 h for either reaction if no catalyst or the unreduced catalysts (Fe2O3/silica) is present.
These data suggest that natural gas may be formed in coal seams by either CO2 methanation or liquid hydrocarbon hydrogenolysis on reduced iron minerals present in the coal. An important implication of our analysis is that iron-mineral catalysis rather than homogeneous thermolysis leads to natural gas formation during coal maturation. This, in turn, suggests using coal minerals rather than currently used coal thermal maturity parameters for gas resource assessment and exploration.
Guo, F. and Hecker, W.C.
Twenty-Seventh Symposium (International) on Combustion, The Combustion
Institute, Pittsburgh, accepted.
The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and post-combustion 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, 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 ~ Pitts ~ UBC ~ Pocah >> 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 (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. While all chars exhibited the shift in EA, the values of the shift temperature and the EA within each temperature regime tended to increase with increasing rank. Also, the reactivity of chars seemingly depends not only on organic char surface area, but also on mineral content, specifically CaO surface area.
The data also indicate that the rate of the heterogeneous reduction of NO by char is significant and comparable with the rate of homogeneous reburning of NO by CHI, and thus, the heterogeneous reaction should be included in models of NO formation during coal combustion.
Hickenlooper, J.L.
Determination of Rh-CO Integrated Absorption Coefficients as a Function of
Temperature and Coverage for ZSM-5 Supported Rhodium, M.S./BYU, April 1998.
Advisor: Hecker
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