Multifunctional Additives for NO_X Abatement in Fluidized Bed Biomass Combustion

摘要

Two major challenges in fluid bed combustion of biomass are increased NOX emissions and bed agglomeration. Different additives were employed to simultaneously reduce NOX emissions and bed agglomeration from the fluidized bed combustion of biomass. The base fuel was straw, and the additives included CaO, kaolin, MgCO3, coal fly ash, clay, (NH4)(2)Fe(SO4)(2), NH4Fe(SO4)(2), (NH4)(2)SO4, NH4MgPO4, AlNH4(SO4)(2), (NH4)(2)HPO4, (NH4)(3)[Fe(C2O4)(3)], and urea. The influence of (NH4)(2)SO4 particle size (35 and 106 mu m) and introduction method (batch addition or premixing with fuel) was additionally investigated. The most effective additives against NOX emissions and bed agglomeration were further studied in air staged straw combustion and unstaged sunflower husk combustion. During sunflower husk combustion, the influence of ash accumulation and incipient defluidization on NOX emissions were examined. The results show that kaolin, CaO, MgCO3, (NH4)(2)Fe(SO4)(2), NH4Fe(SO4)(2), AlNH4(SO4)(2), and NH4MgPO4 prevented defluidization during straw combustion under the investigated conditions. Of these, AlNH4(SO4)(2) and NH4MgPO4 reduced the fuel-N to NO conversion by 40%. The mechanism of reduction was related to the facilitation of thermal DeNO(X) reactions by the introduction of NH3-releasing additives. However, the NH-based additives resulted in higher emissions of N2O. The size of (NH4)(2)SO4 particles had a slight influence on the defluidization tendency and nitrogen chemistry, while no significant difference was observed between the two additive introduction methods. Air staging reduced the fuel-N to NO conversion by 40% during straw combustion. The use of NH4MgPO4 and AlNH4(SO4)(2) under air staged conditions increased the NO emission slightly. This was predominantly caused by the combustion of NH3 in the secondary air jet. In the case of unstaged sunflower husk combustion, NH4MgPO4 and AlNH4(SO4)(2) prevented defluidization while reducing the conversion of fuel-N to NO by 30%. During sunflower husk combustion, the accumulation of ash increased NO and decreased NH3 concentrations above the bed. This was related to the poor mixing as the bed approached defluidization and to the catalytic effect of ash forming elements on the oxidation of NH3 to NO.