Methane reforming and partial oxidation was studied to evaluate the catalytic effects of coal chars and coal ashes on methane (CH4) conversion, sum selectivity (the sum of H2 and CO), and ratio selectivity (the ratio of H2/CO) in an atmospheric fluidized bed. The kinetics study presented the possibility of CH4 reforming and partial oxidation with a favorable H2/CO ratio, greater than 5. The higher H2/CO ratio in CH4 reforming and the partial-oxidation process can reduce the consumption of CH4 needed to adjust the H2/CO ratio during combined coal gasification and methane reforming. Coal ashes failed to be good candidates of catalysts on CH4 reforming and partial oxidation because of their very low specific surface area available for catalytic reactions. However, coal chars presented very promising catalytic performance on CH4 reforming and partial oxidation because of their larger specific surface area. In this study, no other constituents in coal fly ash or special surface properties of coal chars were correlated with the enhanced methane-conversion efficiency. It seems that the specific surface area is only variable in controlling methane-conversion efficiency.
Four types of biomass (chicken waste, wood pellets, coffee residue, and tobacco stalks) were cofired at 30 wt % with a U.S. sub-bituminous coal (Powder River Basin Coal) in a laboratory-scale fluidized bed combustor. A cyclone, followed by a quartz filter, was used for fly ash removal during tests. The temperatures of the cyclone and filter were controlled at 250 and 150 degrees C, respectively. Mercury speciation and emissions during cofiring were investigated using a semicontinuous mercury monitor, which was certified using ASTM standard Ontario Hydra Method. Test results indicated mercury emissions were strongly correlative to the gaseous chlorine concentrations, but not necessarily correlative to the chlorine contents in cofiring fuels. Mercury emissions could be reduced by 35% during firing of sub-bituminous coal using only a quartz filter. Cofiring high-chlorine fuel, such as chicken waste (Cl = 22340 wppm), could largely reduce mercury emissions by over 80%. When low-chlorine biomass, such as wood pellets (Cl = 132 wppm) and coffee residue (Cl = 134 wppm), is cofired, mercury emissions could only be reduced by about 50%. Cofiring tobacco stalks with higher chlorine content (Cl = 4237 wppm) did not significantly reduce mercury emissions. This was also true when limestone was added while cofiring coal and chicken waste because the gaseous chlorine was reduced in the freeboard of the fluidized bed combustor, where the temperature was generally below 650 degrees C without addition of the secondary air. Gaseous speciated mercury in flue gas after a quartz filter indicated the occurrence of about 50% of total gaseous mercury to be the elemental mercury for cofiring chicken waste, but occurrence of above 90% of the elemental mercury for all other cases. Both the higher content of alkali metal oxides or alkali earth metal oxides in tested biomass and the occurrence of temperatures lower than 650 degrees C in the upper part of the fluidized bed combustor seemed to be responsible for the reduction of gaseous chlorine and, consequently, limited mercury emissions reduction during cofiring. This study identified the important impacts of temperature profile and oxides of alkali metal (alkali earth metal) on mercury emissions during cofiring in the fluidized bed combustor.
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