Release of inorganic constituents from leached biomass during thermal conversion
Leaching of inorganic materials has recently been shown to substantially improve the combustion properties of biomass fuels, especially straw but including other herbaceous and woody fuels. Leaching with water removes large fractions of alkali metals (typically >80% of potassium and sodium) and chlorine (>90%). Smaller fractions of sulfur and phosphorus are also removed. Alkali metals are heavily involved in ash fouling and slagging in combustion and thermal gasification systems. Chlorine is a facilitator of alkali volatilization, 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 significant environmental benefits. Leaching could mitigate the undesirable effects of biomass ash in thermal systems. Reported here for the first time are comparative studies of volatile inorganic species evolving from leached and unleached biomass fuels during thermal conversion. Leached and unleached samples of rice straw, wheat straw, switchgrass, commercial power plant wood fuel, and banagrass (Pennisetum purpureum) were tested in bench-scale combustion studies using an alumina-tube flow reactor housed in a variable temperature furnace and coupled to a molecular beam mass spectrometer (MBMS) system. Sugarcane bagasse, as the leached byproduct of sugar production, was also tested. The MBMS system was used to monitor the combustion products, including inorganic vapors, directly and in real time during each batch combustion 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 results were consistent with the levels of alkali metals and chlorine in the samples 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 particular sample. The MBMS results clearly support earlier results, which indicated that leaching biomass effectively reduces or eliminates the release of alkali metal vapors during combustion.