Direct Observation of Alkali Vapor Release during Biomass Combustion and Gasification. 1. Application of Molecular Beam/Mass Spectrometry to Switchgrass Combustion

Abstract

Electricity from biomass and biomass-derived fuels has become an attractive and viable alternative energy source. Alkali metal release during biomass combustion can cause significant problems in terms of severe fouling and slagging of heat transfer surfaces in boilers thus reducing efficiency, and in the worst case, leading to unscheduled plant shutdown. Future biomass to electricity facilities will benefit from increased efficiencies by incorporating integrated gasification 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 the fouling and slagging problem is to develop methods of hot gas cleanup to reduce the amount of alkali vapor to acceptable levels. A detailed understanding of the mechanisms of alkali metal release during biomass combustion as well as identifying alkali metal containing vapors and how the vapors lead to fouling and slagging could greatly benefit the development of hot gas cleanup technology. This paper demonstrates the application of molecular beam/mass spectrometry to the study of alkali metal speciation and release during switchgrass combustion. We have successfully used this experimental technique to identify alkali metal containing species released during the combustion of switchgrass at four different conditions: 1100 °C in He/O2-(20%), 800 °C in He/O2(20%), 1100 °C in He/O2(5%), and 1100 °C in He/O2(10%)/steam(20%). These conditions were chosen to study the effect of temperature, oxygen concentration, and excess steam on alkali metal release and speciation. Initial feedstock composition is the most significant factor which affects the amount and species of alkali metal released during biomass combustion. The switchgrass sample screened in the present study is high in both alkali metal (potassium) and chlorine. As a result, the predominant alkali metal containing species released during switchgrass combustion is potassium chloride. Varying the combustion condition affects the amount of alkali metal released by a factor of 2 or less. Adding excess steam to the combustion environment tends to shift the form of alkali metal release from the alkali chloride to the hydroxide. © 1995, American Chemical Society. All rights reserved.