Effects of volatile-char interactions on char structure and reactivity during the gasification of low rank fuels
thesisposted on 02.02.2017 by Zhang, Shu
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Gasification is an efficient thermochemical conversion process to produce syngas that can then be used to generate electricity or to produce liquid fuels and chemicals. Gasification can be applied to many carbon-containing resources: biomass, peat, coal, anthracite, oil residues and municipal solid wastes. Among them, coal (especially brown coal due to its high reactivity) and biomass are the most reliable and abundant feedstocks for gasification. Advanced gasification technologies are ideal alternatives to the traditional way of fossil fuel utilisation because gasification-based technologies can greatly improve fuel utilization efficiency and meet the increasingly stringent environmental regulations, including CO2 emission. In the meanwhile, a tailored gasification process for biomass is also highly demanded considering the neutral release of greenhouse gas from biomass utilisation. More importantly, net reduction of CO2 could be achieved if biomass is utilised in conjunction with a carbon sequestration technology. Brown coal and biomass have many important unique features compared with the internationally traded black coals. The examples of these features include high contents of moisture, oxygen and aliphatic structure, low contents of aromatic structures, and the presence of well dispersed alkali and alkaline earth metallic species. These structural features and properties make the gasification behaviour of brown coal and biomass distinctly different from that of high-rank coals. Because the volatiles from brown coal and biomass are highly reactive and their chars are unstable, the interactions between volatiles and char can be one of the most important characteristics of brown coal and biomass gasification. The purpose of this study was to investigate the volatile-char interactions during the pyrolysis and gasification of Victorian brown coal and mallee biomass. Both inter-particle and intra-particle volatile-char interactions were examined in detail. Pyrolysis and gasification experiments were carried out in novel reactors where the extent of volatile-char interactions can be controlled relatively accurately. The structural features of char after various extents of volatile-char interactions were characterised with FT-Raman spectroscopy. The retention of alkali and alkaline earth metallic species in char was also quantified. In addition to the gasification rate of char with steam, the intrinsic reactivity of selected chars in air (O2) at low temperature was also measured. The results from this study indicate that the volatile-char interactions could inhibit char conversion during gasification through three possible ways: H radical occupation on reactive sites, changes in char structure and volatilisation of inherent catalysts. Intra- and inter-particle volatile-char interactions were all important for the gasification of char when using large particles. These results indicate that the volatile-char interactions must be considered in the design and operation of gasifiers for Victorian brown coal and biomass.