Reduction of HCl Emissions from Combustion of Biomass by Alkali Carbonate Sorbents or by Thermal Pretreatment

Abstract

Elevated emission of hydrogen chloride (HCl) from the combustion of biomass in utility boilers is a major issue, as it can cause corrosion, and in combination with the high alkali content often encountered in these fuels, it can deposit molten alkali salts on the boiler’s water tubes. Such deposition can impede the heat transfer and cause further corrosion. Previous work in this laboratory showed that the HCl emissions of biomass combustion can be drastically abated by burning torrefied biomass instead of raw biomass. Alternatively, the present work concentrated on using alkali-based sorbents to capture the HCl emissions from the combustion of raw biomass and reported on their effectiveness. It burned corn straw and corn-based distillers dried grains with solubles (DDGS), both pulverized to the size range 75–150 µm. Their combustion products were treated with calcium carbonate (CaCO3), sodium carbonate (Na2CO3), and potassium carbonate (K2CO3). The high-temperature evaluation of the sorbent’s performance was achieved by the coinjection of the sorbents with biomass in a drop tube furnace (DTF) at steady-state steady-flow conditions. Moderate- and low-temperature evaluation of the sorbent’s performance was achieved by allowing the effluents of the batch combustion of the biomass to pass through heated fixed beds of the alkali carbonates. In all experiments, the HCl reductions by the sorbents ranged from 10 to 57% at a molar alkali/chlorine ratio of ∼5 and a gas/sorbent contact time on the order of 1 s. The HCl reduction by the alkali carbonate sorbents at the high temperature (1,350 K) was comparable to that achieved at the moderate temperatures (773 and 573 K), whereas the reduction at room temperature (298 K) was lower. Potassium carbonate was more effective than sodium carbonate, which in turn was more effective than calcium carbonate in the HCl capture. However, under all the current experimental conditions, the replacement of the raw biomass fuel with thermally pretreated biomass (torrefied at 523 K) resulted in HCl emissions that were lower than those from treating the effluents of raw biomass with the previously described alkali carbonate sorbents.