ACERC
Abstracts - 1999 |
ACERC
Abstracts - 1999 |
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Research Area 2: NOx/Pollutants |
Molina, A.; Sarofim, A.F.;
Eddings, E.G. and Pershing, D.W.
Progress in Energy and Combustion Science, to be submitted, November
1999.
The contribution of the nitrogen present in the char on the production of nitrogen oxides during char combustion was analyzed. A literature review summarized the current understanding of the mechanisms that account for the formation of NO and N2O from the nitrogen present in the char. The review focused on 1) The functionalities in which nitrogen is present in the coal and how they evolve during coal devolatilization; 2) The mechanism of nitrogen release from the char to the homogeneous phase and its further oxidation to NO; and 3) The reduction of NO on the surface of the char. The critical analysis of these three issues allowed to identify uncertainties and well-founded conclusions observed in the literature for this system.
The existent models for the production of nitrogen oxides from char-N were also reviewed. A critic analysis of the assumptions made in these models and how they affect the final predictions is presented. Finally, a simplified version of these models was used to perform a parametric analysis of the incidence of the rate of NO reduction on the char surface, the rate of carbon oxidation, and the instant during the char oxidation when the nitrogen is released; on the total conversion of char-N to NO. The results underscore the importance of the reaction of NO reduction on the char surface on the final conversion of char-N to NO.
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.
Cannon, S.M.; Brewster,
B.S. and Smoot, L.D.
Combustion & Flame, 119:233-252 (1999).
The velocity-composition probability density function (pdf) model coupled with a k-?-based mean flow computational fluid dynamics (CFD) model was used to describe the turbulent fluid flow, heat transfer, chemistry, and their interactions in a bluff-body, lean, premixed, methane-air combustor. Measured data [1,2] including velocity, temperature, and chemical species concentrations were used to evaluate the model. The chemistry calculations were performed with an in situ look-up tabulation method [3]. A reduced, 5-step chemical mechanism [4] for describing fuel oxidation, CO, and NO chemistry was used in the model. NO formation from thermal, N2O-intermediate, and prompt pathways was included in the 5-step mechanism. An axisymmetric, unstructured grid was used for solving the Eulerian, mean flow equations and the vertices were used to store mean statistics for solving the Lagrangian, fluid particle equations. Predicted velocity and composition mean statistics were compared to measurements in the bluff-body combustor for a lean equivalence ratio of 0.59. The predictions of major species matched measured and calculated equilibrium values in the recirculation zone. Comparisons of mean CO throughout the combustor were always within an order of magnitude and showed marked improvements over past predictions. Maximum discrepancies between measured and predicted NO concentrations were between 5 and 7 ppm (~50%). The accessed composition space in this turbulent combustion simulation represented the values of species mole fraction and enthalpy for each fluid particle at each time step and was found to lie in a relatively small, uniquely shaped region that was dictated by the mixing, reaction, and heat transfer in the combustor. This accessed composition region was obtained in situ and required about 35 megabytes of storage once a steady state was reached. This memory requirement was more than three orders of magnitude less than would be needed in a standard, a priori table. The in situ tabulation approach allowed for technically correct and efficient chemical kinetic calculations by using the 5-step mechanism in this pdf-method-based, multidimensional combustor model.
Xu, H.; Smoot, L.D. and
Hill, S.C.
Energy & Fuels, 13, 411-420 (1999).
Advanced reburning is a NOx reduction process wherein injection of a hydrocarbon fuel such as natural gas downstream of the combustion zone is followed by injection of a nitrogen-containing species such as ammonia. The authors recently reported a seven-step, 11-species reduced mechanism for NO reduction by advanced reburning processes. However, inclusion of even a seven-step reduced mechanism into a CFD code for turbulent combustion leads to substantial computational demands. IN this work, the authors have further simplified the kinetic mechanism. A simpler four-step, eight-species reduced mechanism for NO reduction by advanced reburning has been developed from a 312-step, 50-species full mechanism through the use of a systematic reduction method. The four-step reduced mechanism is in good agreement with the full mechanism for most laminar flow cases. It also agrees qualitatively with three sets of experimental data, which show the influences of temperature, CO concentration, O2 concentration, and the ratio (NH3/NO)in. It can be applied for coal-, gas-, and oil-fired combustion. The four-step reaction sequence has been integrated into a comprehensive CFD combustion code for turbulent combustion, PCGC-3. The method of integration is described. Several computations are reported with the combined code to demonstrate the predictive behavior of the advance reburning mechanism in turbulent, pulverized coal combustion. The model calculations show the effects of temperature and concentrations of CO, O2, and NH3 on NO reduction.
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