On the efficiency of alkali based sorbents for removal of coal-combustion generated SO₂ and biomass-combustion generated HC1

• Online Access: http://hdl.handle.net/2047/D20236953

Acid gas emissions (e.g., SO₂, HCl and NO_x) from combustion of solid fuels, such as coal and biomass, have been associated with health-related issues and environmental degradation. As wet flue gas desulfurization is both space- and capital- intensive, in- furnace dry sorbent injection has been considered over the years to be a viable alternative. However, there is still uncertainty on the best selection of the sorbents for particular fuels and furnace operating conditions. The sulfur content of coals can vary widely from a fraction of 0.5 wt% to 7.5 wt% [1], while their chlorine contents are typically low (≤ 0.3 wt%) [2]. To the contrary the chlorine content of biomass can be high (as high as 0.66%) [3], whereas their sulfur content is typically low (≤ 0.2%) [1]. Since during combustion of solid fuels sulfur typically oxidizes to SO₂ and chlorine typically forms HCl, this study tested the effectiveness of various sorbents to capture the SO₂ emissions of coal and the HCl emissions of biomass. Different sorbents were co-fired with a high-sulfur bituminous coal (Illinois #6, containing 5.5% S) and a high- chlorine raw biomass (corn straw 0.63% Cl), respectively. The two fuels were pulverized to particles with sizes ranging from 75 to 150 μm, which were then blended with pulverized calcium-, sodium- and potassium-containing compounds, with particles ranging from 75 to150 μm, at different proportions. Combustion of the fuels occurred at high heating rates (10⁴4 -10⁵ K/s) in a laboratory-scale electrically-heated drop-tube furnace, operated at 1400 K under fuel-lean conditions. The efficiency of different sorbents in reducing SO₂ and HCl emissions in the combustion effluents was evaluated and compared. Results showed that in the cased of both coal and biomass combustion the effectiveness of the alkali-based sorbents for either SO₂ or HCl acid gas removal was that potassium and sodium are better than calcium. Thermodynamic analysis on the reaction of the acid gases with the solid sorbents supported the experimental results.