| OCR Text |
Show 12 to reach high extents of burnout. We observe a significant loss of reactivity during the intermediate stages of char combustion for several bituminous coals, but not for subbituminous coals, lignites, or biomass. These experiments, and the analyses of residual carbon from boiler fly ash, suggest that the most important mechanism for this loss of global reactivity is a thermal, and perhaps oxidative, annealing process that occurs under boiler conditions. The role of thermal annealing, or char deactivation in the lower furnace, has not been fully appreciated in the design and optimization of today's combustion equipment. There is great potential for using these new insights to rationally optimize new furnace designs and retrofits for both control of NOx emissions and improvement of carbon burnout. None of the other possible explanations for char reactivity loss that were evaluated in this work playa major role. Although a small degree of inert maceral enrichment is noticeable in laboratorygenerated TIlinois No.6 char, retention of less reactive portions of the chars does not appear to be a comparable effect in this process. Available surface area for oxidation of combustible material, as indicated by macroporosity and lack of an ash diffusion barrier, does not appear to be a dominant factor in explaining char reactivity loss at low to intermediate conversion - although it can greatly affect loss-on-ignition values by diluting the residual carbon with inorganic material. Initial particle-to-particle variation in reactivity, although less important for most fuels than the loss of char reactivity during combustion, should also be considered when predicting conversion to the unburned carbon levels necessary for the economic operation of industrial boilers. REFERENCES Bailey, J.B., Tate, A.G., Diessel, C.F.K. and Wall, T.F. (1990). Fuel 69:225. Baxter, L.L., Fletcher, T.H. and Ottesen, D.K. (1988). Energy and Fuels 2:423. Charpenay, S., Serio, M.A. and Solomon, P.R. (1992). In the 24th Symposium (International) on Combustion, The Combustion Institute, p. 1189. Crelling, J.C., Hippo, E.J., Woerner, B.A. and West, D.P. (1992). Fuel 71: 151. Davis, K.A. and Hurt, R.H. (1994). in the 25th Symposium (International) on Combustion, The Combustion Institute. Essenhigh, R.H. (1981). in Chemistry of Coal Utilization, Second Supplementary Volume (ed. by M. A. Elliot), John Wiley & Sons, New York, p. 1153. Hottel, H.C. and Stewart, I.M. (1940). Ind. Eng. Chem.32:719. |
