Thermogravimetric characterisations of biomass and waste for gasification processes: Dissertation

Gasification, among the several thermochemical conversion processes, possesses great potential in the advanced utilisation of biomass and wastes as a source for energy and material production. This work deals with the characterisation of reactivity and ash sintering in the gasification of biomass and wastes using thermogravimetry (TG) as a tool. The gasification reactivity of biomass fuels depends on many factors; in particular the catalytic factors play a large role, and the behaviour is not easy to predict. On the basis of results obtained experimentally and information available in the literature, a method was developed for the characterisation of reactivity and ash sintering of fuels to be used in fluidised bed gasification processes, both atmospheric and pressurised. The purpose was to take into account the complexity of the phenomena rather than to find systematic correlations between the various fuel properties and reactivity and ash sintering, or to determine kinetic parameters. The observed complexity was the spur to develop a method, one that would be as simple as possible, to characterise biomass and solid recovered fuels or wastes intended as feedstocks for use in gasification while taking into account the conditions of the process. Thus, the temperature and pressure ranges, heating rates and gaseous environment for the thermobalance were selected so as to be relevant for the conditions existing in fluidised bed gasifiers, including pressurised conditions. The temperature maximum was thus 1000 C, and the heating rate of the fuel sample was adjusted to that when feedstock enters the reactor. The gaseous environment was selected so that it would sufficiently describe that existing in a gasifier. Solid recovered fuels were selected to represent waste-based fuels rather than waste as a whole. The gasification reactivity of solid recovered fuels was found to be similar to that of paper and higher than that of wood. Test results showed broad scattering before the samples were extracted by sample division. The method also provides information about ash sintering. The ash is the residue remaining in the thermobalance after the gasification reactions, which thus represents the conditions of gasification, even pressurised gasification. A separate ash does not need to be prepared in the laboratory. The ash residue can be classified into three categories: non-sintered ash, partly sintered ash and totally sintered or fused ash. For some of the biomasses, ash sintering was much stronger under pressurised conditions than at atmospheric pressure. The laboratory findings for ash sintering appeared in various ways in real scale since several factors affect the ash depositing in the reactor. Each time sintering (even weak) was detected in the ash residue of the TG tests, agglomerates or deposits were found in the reactor. The results of thermobalance tests on different fuels were in good correspondence with the behaviour of the fuels in fluidised bed gasifiers.