Use of Alkali Carbonate Sorbents for Capturing Chlorine-Bearing Gases from Corn Straw Torrefaction

Combustion of torrefied biomass for power generation has many advantages over combustion of raw biomass, one of which is lower emissions of chlorine-bearing gases. This is because partial evolution of these gases takes place during the torrefaction process; hence, the resulting torrefied biomass has a lower chlorine mass fraction than its raw biomass precursor. Research showed that, during torrefaction of corn straw, the predominant chlorinated species in the evolving gas (torgas) are CH3Cl and HCl. The former is more prevalent when torrefaction takes place at temperatures under 350 degrees C, whereas the latter is more abundant at higher temperatures. In this work, corn straw was torrefied at a furnace temperature of 300 degrees C for 20 min under atmospheric pressure in an inert nitrogen flow. Under this condition, corn straw lost nearly 40% of its original mass, along with 73% of its chlorine mass to the gas phase. To control the emissions of the chlorinated species, the torrefaction gas was heterogeneously reacted with beds of alkali carbonate sorbents kept at temperatures in the range of 25-500 degrees C. It was determined that the sorbents captured HCl more effectively than CH3Cl, with amounts varying with both the type of sorbent and the sorbent/gas reaction temperature. HCl removal effectiveness varied in the range of 26-80%, whereas CH3Cl removal effectiveness varied in the range of 15-22% at the conditions implemented herein. Potassium carbonate (K2CO3) was found to be slightly more effective than sodium carbonate (Na2CO3) and significantly more effective than calcium carbonate (CaCO3), particularly at low reaction temperatures.