A mechanistic description of the evolution of aromatic tar during catalytic upgrading of the raw gas produced from biomass steam gasification

Doktorsavhandling, 2017

The removal of tar from the raw gas produced during biomass gasification is a prerequisite for the viability of the process downstream of the gasifier. The tar, the composition of which is dominated by aromatics, readily condenses, leading to process disruption. For upgrading the raw gas to remove the tar, the catalytic method enables the conversion of the tar into useful permanent gases at operating temperatures lower than that used in the alternative thermal method without catalysts. Understanding how the tar evolves during the raw gas upgrading process is essential for implementation of the catalytic method. This work aims at improving the current understanding of the evolution of the tar during the raw gas upgrading. The focus is on capturing the principles of the product selectivity of the tar conversion with the presence of steam, H2, and CO2 in the raw gas as cracking and reforming agents, and on describing the main trends in the tar evolution. The catalytic method is the main focus, with the thermal method being investigated mainly for comparison. In terms of the main reaction pathways through which tar is converted, raw gas upgrading is a combination of different cracking and reforming processes used in petroleum refineries. Thus, the well-established knowledge of the relevant petrochemical processes is adopted as the basis for this work. Furthermore, to represent the real-life condition, raw gas that was produced in the Chalmers 2–4-MWth dual fluidized bed biomass gasifier (an industrial-scale gasifier) was used in the experiments. As the first step, a mechanism that explains the gradual conversion of tar and light hydrocarbons, as well as the main trends of product formation during the upgrading process was formulated. This mechanism was used to develop a kinetic model that provides a simplified description of the catalytic raw gas upgrading. The model takes into account eight groups of tar and light hydrocarbons that are present in the applied raw gas and that are indicative of the progress of tar evolution. The extents to which the parent tar/light hydrocarbons are converted into CO/CO2 and into smaller tar/light hydrocarbons are taken as an input. The applicability of the model was demonstrated for the upgrading of the Chalmers raw gas in the presence of a process-activated ilmenite catalyst that was obtained from the Chalmers 12-MWth boiler. The evolutionary profiles of the tar and light hydrocarbon groups were derived. The results confirm that this model is able to capture the features of the upgrading process. The tendency to produce polycyclic aromatic hydrocarbon (PAH) tar due to the mutual combination of carbon-containing intermediates, which is suggested in the mechanism, was investigated in relation to process severity in a steam and H2-containing reaction environment. The results show that the growth of PAH tar can be suppressed, given that the process severity is sufficient to convert steam and H2 into hydrogen intermediates, which prevents combination of the carbon-containing intermediates. The obtained results explain the fate of PAH tar during the late stage of tar maturation in steam gasification of biomass. Overall, this work provides a generalized understanding of the evolution of tar during the raw gas upgrading. The similarity between raw gas upgrading and petrochemical processes is confirmed, which encourages further applications of the mature knowledge of the petroleum refinery to biomass gasification. Furthermore, the results provide essential inputs for the future development of more-comprehensive models, in that the complicated features of the upgrading process can be gradually resolved.