Inorganic Compounds in Biomass Feedstocks. 1. Effect on the Quality of Fast Pyrolysis Oils

Inorganic compoundsespecially potassium, calcium, sodium, silicon, phosphorus, and chlorineare the main constituents of the ash in biomass feedstocks. The concentrations of ash range from less than 1% in softwoods to 15% in herbaceous biomass and agricultural residues. During biomass pyrolysis, these inorganics, especially potassium and calcium, catalyze biomass decomposition and char-forming reactions. Chars formed during these reactions invariably end up in the biomass pyrolysis oils (biocrude oils) as suspended submicron particles. The presence of high concentrations of submicron char particles in biocrude oils will make them problematic for combustion in steam boilers, diesel engines, and turbine operations because of the potential release of the ash and alkali metals during combustion. We experimented with sequential cold filtering of biocrude oils using filters of varying pore size and this revealed that most of the ash and alkali metals detected in biocrude oils are trapped in the chars. Leaching studies conducted on the chars suspended in the oils showed no leaching of alkali metals from the chars into the oils. Cold filtration of the oils dissolved in acetone was ineffective in reducing the alkali metals content to acceptable levels, but hot gas filtration of the pyrolysis vapors reduced the alkali metals content below 10 ppm. Our data suggest that hot gas filtration can potentially reduce the ash and alkali metals content of the biocrude oils to acceptable levels so they can be used as turbine, diesel engine, or boiler fuels.