Multifunctional Additives for NOX Abatement in Fluidized Bed Biomass Combustion
journal contribution
posted on 2021-07-15, 19:07 authored by Burak Ulusoy, Bozidar Anicic, Liyan Zhao, Weigang Lin, Bona Lu, Wei Wang, Kim Dam-Johansen, Hao WuTwo
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)2Fe(SO4)2, NH4Fe(SO4)2, (NH4)2SO4, NH4MgPO4, AlNH4(SO4)2, (NH4)2HPO4, (NH4)3[Fe(C2O4)3], and urea. The influence of (NH4)2SO4 particle size (<35 and <106
μm) 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)2Fe(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 DeNOX reactions by the introduction of NH3-releasing additives.
However, the NH-based additives resulted in higher emissions of N2O. The size of (NH4)2SO4 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.
