Tar formation and transformation in steam gasification of biomass in a dual fluidised bed gasifier.
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Gasification is a promising thermos-chemical process that converts biomass into syngas (producer gas). However, the generation of tar compounds during the gasification process limit the subsequent syngas application, since it causes operational problems by blocking downstream equipment. Furthermore, tar removal is one of the main challenges for the commercialisation of the biomass gasification technology. Since 2007, a 100kW dual fluidised bed (DFB) gasifier is under development by the Bioenergy Research Group at the University of Canterbury to assess the performance of syngas. But there is an apparent knowledge gap in understanding tar formation and conversion in the DFB gasifier, especially for the transformation of gas and tar production from initial devolatilization to subsequent gasification. This thesis aims to fundamentally understand and establish the tar formation and conversion during biomass gasification, with the objective of determining the operating parameters and selection of feedstocks and bed materials for reduction of the tar content in the producer gas.
To achieve the objective, the evolution of gas and tar compounds produced from initial devolatilization stage to subsequent gasification stage have been experimentally investigated in the 100kW DFB gasifier. N2 and steam were respectively introduced into the DFB gasifier as fluidisation agent for devolatilization and gasification process. The producer gas composition was analysed by a micro-gas chromatography (GC), while the tar compounds were identified and quantified by the GC analysis.
The first part of the thesis deals with the effect of operating parameters on tar conversion and reduction. The operating parameters included the gasification temperature (in the range of 700-800°C), mean gas residence time (from 0.19 to 0.25 s) and steam to biomass ratio (from 0.63 to 1.51). Pellets of radiata pine wood were used as the feedstock, and silica sand was used as the bed material.
The results demonstrated the correlations between tar and light gas from initial devolatilization to final gasification. During devolatilization, linear relations on the mass production between of light poly-aromatic hydrocarbon (PAH) tar compounds and CH4, and between heavy PAH tar compounds and C2 gas (C2H2+C2H4) were obtained. In the subsequent gasification, a linear relation between heavy PAH tar compounds and C2H2+C2H4 were also presented. Consequently, CH4, C2H2 and C2H4 could be used as indicators of tar production and speciation. These results provided an understanding of the tar conversion and transformation is taking place in the DFB gasifier, the correlation between tar and light gas could be used to estimate the PAH tar compounds during biomass gasification.
Besides, it was found that the total tar concentration in the producer gas was reduced by 34%, 36% and 46%, respectively, by changing the operating parameters (1) the elevating gasification temperature, (2) longer the residence time, and (3) increasing steam to biomass (S/B) ratio. Interestingly, within the testing range, the proportion of heavy PAH tar in the total tar production was increased from 30% to 48%, and from 33% to 42%, respectively, by changing the temperature and S/B ratio. However, the residence time had very few effects on the proportion of heavy PAH tar.
In the second part, the tar formation and conversion during the steam gasification of various biomass species at 700 and 800°C were studied. Radiata pine wood, corn stover and rice husk were selected as the biomass feedstock. Cellulose, hemicellulose and lignin are three main components in the biomass, whose chemical structure, composition and thermal decomposition properties are different. It was found that corn stove was rich in cellulose, rice husk had a high content of hemicellulose and pine had a high content of lignin.
In devolatilization process, the experimental results demonstrated that radiata pine generated a high proportion of toluene, while corn stover exhibited a high proportion of phenols, while in the subsequent gasification process, radiata pine wood produced a high proportion of naphthalene, while corn stover gave a large proportion of biphenyl. The results allow understanding of the main conversion mechanisms taking place in the gasifier. Consequently, two simplified chemical pathways of secondary tar conversion (phenols and toluene were used as the precursor, respectively) were proposed. In addition, the proportion of total PAH tar compounds in the producer gas from gasification of pine wood was the highest followed by gasification of rice husk and then corn stover, because lignin represents a potential precursor for PAH tar formation.
In the last part, the effects of bed material on tar reduction in the steam gasification were experimentally investigated. The selected catalytic bed materials were calcined olivine sand, Woodhill sand and limestone-silica blends (50-50 wt.%) while silica sand was used as control. The experimental results have demonstrated that catalytic bed materials effectively reduce the tar concentration. It was found that, in comparison with silica sand, the tar concentration was reduced by 24%, 28% and 43%, respectively, with calcined olivine, Woodhill sand and limestone-silica blends. In the meantime, H2 production was promoted by using catalytic bed material, since the effect on the equilibrium of water-gas shift reaction.
This research demonstrated the influence of operating parameter and selection of feedstock and bed material on the tar reduction in the DFB gasifier, providing the information for the further development on the downstream tar removal system and syngas application.