Alkali Release and Effects on Biomass Thermal Conversion Processes

Abstract

High alkali content is an important feature of biomass, and it has a series of implications for thermal conversion processes. This thesis focuses on the release of alkali during various biomass thermal conversion processes, including biomass pyrolysis, char gasification, co-conversion of different types of biomass, and thermal conversion of biomass mixed with fresh and used bed materials. Alkali release is also studied in combination with fuel conversion, to elucidate the underlying relationships. In addition, particle release during steam gasification of char is investigated. The studies were carried out on different reactor scales, ranging from the micro scale to pilot scale. A reliable methodology for simultaneous monitoring of alkali release and sample mass was developed based on the application of a thermogravimetric analyzer in combination with a surface ionization detector (TGA-SID). Using TGA-SID, a significant level of alkali release was observed when wood char conversion approaches completion during CO2 gasification, while the level of alkali release from straw char decreased continuously throughout the process. Alkali migration from straw to wood was observed at temperatures above 600°C during co-pyrolysis, based on online alkali measurements. Positive and negative synergistic effects were observed during the co-gasification at low and high conversions of char, respectively. This is attributed to alkali and silicon migration from the straw to the wood. Fresh bed materials affect wood and straw char gasification reactivities and rates of alkali release, and different bed materials may play different roles. For example, an alkali-containing bed material can enhance char gasification in the initial stage, a Si-containing bed material inhibits char gasification and alkali release at high conversions of char, an Al-containing bed material can inhibit char conversion when the char has a high silicon content, and Mg- and Ca-containing bed materials ensure that alkali persists in releasable form, thus favoring substantial alkali release from the char. Used silicon bed material has a coating layer that is abundant in Ca, Si, K, and Mg. These elements can migrate to the char surface during thermal conversion processes and affect char gasification. In addition, a comprehensive system based on diluters, particle sizers, and SID has been successfully used for particle and alkali measurements in laboratory- and pilot-scale reactors. The laboratory-scale steam gasification system shows that the released levels of alkali and particles significantly increase when char conversion approaches completion. Using steam as the gasifying agent instead of CO2 results in a higher level of alkali release during most of the gasification stage. Aerosol particles are also released during steam gasification, at rates that vary by more than one order of magnitude depending on the char composition. The present study improves our understanding of alkali release, migration, and reaction during biomass thermal conversion processes. The acquired fundamental knowledge can be used for reactor design, co-gasification optimization, and selection of bed materials.