Release of inorganic constituents from leached biomass during thermal conversion

Leaching of inorganic materials has recently been shown to substantially im prove the combustion properties of biomass fuels, especially straw but incl uding other herbaceous and woody fuels. Leaching with water removes large f ractions of alkali metals (typically >80% of potassium and sodium) and chlo rine (>90%). Smaller fractions of sulfur and phosphorus are also removed. A lkali metals are heavily involved in ash fouling and slagging in combustion and thermal gasification systems. Chlorine is a facilitator of alkali vola tilization, and contributes to corrosion and air pollution. The presence of these elements has reduced or eliminated the use of certain biomass fuels in many combustion applications, even where such use might provide signific ant environmental benefits. Leaching could mitigate the undesirable effects of biomass ash in thermal systems. Reported here for the first time are co mparative studies of volatile inorganic species evolving from leached and u nleached biomass fuels during thermal conversion. Leached and unleached sam ples of rice straw, wheat straw, switchgrass, commercial power plant wood f uel, and banagrass (Pennisetum purpureum) were tested in bench-scale combus tion studies using an alumina-tube flow reactor housed in a variable temper ature furnace and coupled to a molecular beam mass spectrometer (MBMS) syst em. Sugarcane bagasse, as the leached byproduct of sugar production, was al so tested. The MBMS system was used to monitor the combustion products, inc luding inorganic vapors, directly and in real time during each batch combus tion event. Total relative amounts of HCl(g), SO2(g), NaCl(g), KCl(g), and other species were compared for leached and unleached samples. The MBMS res ults were consistent with the levels of alkali metals and chlorine in the s amples as determined from the proximate, ultimate, and ash analyses of the samples. The more alkali and chlorine in a given sample, the more gas-phase HCl, KCl, and NaCl detected with the MBMS during combustion of that partic ular sample. The MBMS results clearly support earlier results, which indica ted that leaching biomass effectively reduces or eliminates the release of alkali metal vapors during combustion.