Identification of Gas Phase Alkali Species Released During Biomass Combustion

Electricity from biomass and biomass-derived fuels has become an attractive and viable alternative energy source. Biomass combustion facilities, in conjunction with future dedicated feedstock supply systems, can yield self-sufficient power generating facilities, thus reducing the U.S. dependence on foreign oil and slowing the accumulation of greenhouse gases because CO2 released during thecombustion is taken in during crop growth. Biomass power facilities produce electricity by using the hot combustion gases to generate steam, which is expanded through a turbine. Alkali released during biomass combustion causes significant problems in terms of severe fouling and slagging of heat transfer surfaces in boilers, thus reducing efficiency, and in the worst case leads to unscheduledplant downtime. Future biomass-to-electricity facilities will benefit from increased efficiencies by incorporating integrated combined cycle systems that use biomass combustion gases to directly drive an aeroderivative turbine. These systems will have even lower tolerances for alkali vapor release because accelerated erosion and corrosion of turbine blades results in shorter turbine lifetimes.One solution to fouling and slagging problems is to develop methods of hot gas cleanup that reduce the amount of alkali vapor to acceptable levels. This requires (a) a detailed understanding of the mechanisms of alkali release during biomass combustion; and (b) identifying this alkali vapor species and how these vapors lead to fouling and slagging. Our approach is to directly sample the hotgases liberated from the combustion of small biomass samples in a variable temperature quartz tube reactor employing a molecular beam mass spectrometer system, constructed and operated at NREL, to monitor the combustion event. This system is ideal for studying high-temperature, ambient- pressure environments such as those encountered during these alkali screening studies. Chemical reactions arequenched and condensation is inhibited during the free-jet expansion of the high- temperature combustion gases. As a result, reactive and condensable alkali species remain in the gas phase at temperatures far below their condensation point. We have successfully used this experimen- tal technique to identify alkali species released during the combustion of selected biomass feedstocks used inlarger-scale combustion facilities and power generating facilities. The mass spectral results, in conjunction with multivariate data analysis, have been used to correlate alkali release with feed- stock composition in addition to changes in alkali speciation under various combustion conditions. Primary alkali release in the combustion of relatively low alkali woody feedstocks is through thealkali sulfates. Alkali chlorides are the primary alkali species released during combustion of herba- ceous feedstocks. The chlorine content of biomass has been identified as an important parameter that facilitates alkali release. These results will ultimately be used to develop a predictive model that relates feedstock composition to fouling and slagging behavior in power generating facilities.Contact (e-mail): [email protected]