Modeling post-flame sulfation of KCl and KOH in bio-dust combustion with full and simplified mechanisms

Abstract

The gas-phase interaction between alkali volatiles and sulphur oxides has important implications for deposition and corrosion in combustion of biomass. In the present study, gas-phase transformation of KOH and KCl in the post-flame zone in bio-dust combustion has been studied by detailed chemical kinetic modeling for a woody and a herbaceous biomass, respectively. For both biomasses K > Cl on a molar basis, and KOH is the major alkali species at high temperature. The modeling indicates that KOH is readily converted to K2SO4 in the presence of SO2 at temperatures below 1500 K. Below 1100 K, gaseous K2SO4 nucleates homogeneously, promoting further sulfation. For the woody biomass, where K < Cl + 2S, the sulfation process is kinetically limited due to the competition with chlorination. For the herbaceous biomass, with K > Cl + 2S, the excess KOH facilitates internal equilibration among the alkali species. For both biomasses, the KCl concentration remains constant during sulfation, because any KCl consumed is rapidly replenished by the reaction KOH + HCl ⇄ KCl + H2O. The consequence is that the sulfation process under the investigated conditions does not help to remove the main corrosive agent, KCl. For use in CFD, a skeletal model for the gas-phase sulfation of KCl and KOH has been developed, based on systematic reduction of the detailed chemical kinetic model. However, for fuels with excess K compared to Cl + 2S, a simple equilibrium calculation may yield satisfactory results.