Fate of Alkali Metal During Biomass Thermochemical Conversion

Combustion, gasification, and pyrolysis are all potential processes for producing electricity from biomass. The fate of the alkali metals initially present in biomass during each of these processes can affect the success and efficiency of electricity production. Alkali metals, in particular potassium, have been implicated as key ingredients for enhancing fouling and slagging of heat transfersurfaces in power generating facilities that convert biomass to electricity via direct biomass combustion. When biomass is used as a fuel in boilers, the deposits formed reduce efficiency, and in the worst case, lead to unscheduled plant downtime. Biomass pyrolysis oils have been targeted as future fuels in diesel engines, and ultimately, aeroderivative gas turbines. Alkali metals releasedduring the combustion of pyrolysis oils can lead to fouling of fuel injectors in diesel engines and fouling and corrosion of turbine blades. Alkali metals released during biomass gasification also pose challenges for using the product gas in combustors and turbines. The fate of alkali metal release during biomass combustion, gasification, and pyrolysis has been studied by directly sampling thehot gases liberated from combustion and gasification of small biomass samples in a variable-temperature quartz-tube reactor employing a molecular beam mass spectrometer system. We have successfully used this experimental technique to identify alkali species released during the combustion of 23 solid biomass feedstocks, four biomass chars, and seven biomass-derived pyrolysis oils. By studying thecombustion of biomass pyrolysis oils and chars it is possible to determine how the alkali metal is partitioned during biomass pyrolysis. Multiple combustion conditions have been investigated to target those conditions that minimize alkali metal release. The results of these laboratory studies indicate that during biomass combustion, initial feedstock composition has the most pronounced effect onalkali metal released during combustion. Four mechanisms of alkali metal released involving potassium sulfate, potassium chloride, potassium hydroxide, and potassium cyanate have been identified depending on the feedstock composition. Alkali metals are sequestered in the char during biomass pyrolysis. The fate of alkali metal during biomass gasification will be discussed. Contact (e-mail):[email protected]