| OCR Text |
Show 15 towards equilibrium. In the case of the waste paper, the natural gas preheat may have influenced the accuracy of the fmal NOx concentration value. The reason for the declining S02 emission with decreasing equivalence ratio is not entirely clear. Fly ash analyses were not performed at each equivalence ratio tested, and the trend in condensed phase sulfur retention or capture is unknown. Nor were other gas phase sulfur species (e.g. S03, H2S) measured. The increase in NOx production as equivalence ratio declines could be associated with the reduction in S02 emission, since both species compete with each other and with carbon for oxygen-containing radicals in the oxidation reactions (Bowman, 1991). Also possible is an enhancement at lower equivalence ratios of reactions with fly ash producing condensed phase sulfates and other species. This may be the reason for the reduced fuel S to S02 conversion with rice straw in comparison with the other fuels (Table 3). Rice straw contains up to 2% alkali in the dry fuel, and is the most alkaline fuel tested. Sulfate-forming reactions when burning rice straw may account for the reduced S02 emission rates. Additional experiments are required to confmn this. The uncontrolled emissions from the MFC are compared in Table 4 against emissions from open field burning of rice straw and Douglas frr wood residues from agriCUltural and forest operations, and with controlled emissions from commercially operating biomassfueled circulating fluidized bed (CFB) combustion power plants in California. The latter employ both ammonia injection for NOx control, and limestone injection in the bed for S02 control and to retard bed agglomeration. The field burn data were obtained through wind tunnel simulations of open fires spreading against the wind (straw) or in piles (wood). All field burn simulations were conducted at overall air-fuel ratios between 100 and 1000. The values listed for the MFC are at l/J = 0.85, yielding higher S02, CO and THC values, and lower NOx values compared to l/J = 0.3. CO and hydrocarbon emissions from commercial power plants are controlled primarily through stoichiometry, with sufficient secondary or tertiary air added to keep concentrations low. The MFC results at the higher equivalence ratios are an order of magnitude lower than the field burn results, suggesting in the case of open field fires limited oxygen transport to the flame and locally rich conditions near the fuel, and mixing related quenching in the turbulent flame. NOx and S02 emission levels are roughly comparable between the open frres and MFC experiments. NOx emissions from the CFB units are reduced about 4 times compared to the uncontrolled emissions, which is consistent with computed NOx reduction efficiencies in commercial units (Grass and Jenkins, 1994). Power plant S02 emissions |
