AbstractThe scarcity of fossil fuels and the environmental consequences associated to their usage highlight the necessity for the development of renewable energies. Bioenergy is one type of renewable energy particularly attractive due to its independence from the weather when compared to other renewable energy technologies such as wind and solar energy. Additionally, it can be used for the production of liquid, solid and gaseous fuels. However, production of energy from biomass remains constrained by technical and economical limitations.
Biomass gasification is a thermochemical conversion technology that, broadly speaking, consists on heating biomass under the presence of limited amounts of an oxidizer to yield syngas, a versatile mixture of hydrogen and carbon monoxide, as the primary product. A recurrent challenge for gasification is the production of pollutants during the process; some of the produced pollutants require complicated treatment. Tars particularly pose a challenge because they condense at high temperatures and deactivate the catalysts used to decompose either the tars or some other pollutants.
Extensive research has demonstrated that the most promising solution to tars is catalytic reforming. Alas, the catalysts are easily deactivated during the process. On the other hand, chars, which are a byproduct from gasification, have demonstrated their catalytic capabilities for tar reforming and are inexpensive. Although a variety of works have been conducted using char as a catalyst for tar reforming to test the effect of different parameters such as the char precursor biomass, particle size, inorganic contents in the char, etc., the effect of the char porous structure during tar steam reforming is still without study. Moreover, the key features of char that affect the process of deactivation by coke remain unclear. Additionally, works that analyse the efficiency of tar reforming downstream of gasification using char catalysts in an integrated system have not been reported.
The aim of this work is evaluating the performance of char as catalyst for tar reforming by studying the effects of the porous structure of char, analysing the biomass gasification with a char catalytic tar reforming system, based on the first and second laws of thermodynamics, and proposing novel concept for a system integrating energy and char recovery in the coupled gasification-tar reforming system.
Firstly, the performance of char as catalyst was evaluated by carrying out experiments during which regular and activated biomass gasification residual chars were used as chars with different porous structure. Experiments were conducted by injecting a laboratory produced syngas mixture in a reactor packed with a char bed at different temperatures. Results indicated that while at lower temperatures (650 and 750 °C) microporous chars provide the largest tar conversion, at temperatures of 850 °C almost complete tar conversion was achieved regardless of the char being microporous or mesoporous in nature. Moreover, although the conversion provided by microporous char was initially higher, mesoporous char proved to be more resistant to catalyst deactivation.
Secondly, from the thermodynamic analysis on the gasification and tar reforming coupled system, it was found that the coupled system is favoured by low gasification temperatures and high reforming temperatures. Moreover, the reforming temperatures might be adjusted to achieve different extents of tar removal, as it was found that at reforming temperatures of 850 °C, the tar found is mostly benzene, whose presence is acceptable for some applications such as gas turbines.
The findings from the experiments and thermodynamic analysis indicate that an integrated system, where the char produced during gasification constantly supplies the char bed for reforming, can be developed, and the efficiency of the process can be increased by integrating a heat recovery system within the process. Moreover, further increases in efficiency can be achieved by using the steam generated during biomass drying as a reforming agent. Therefore, a concept where gasification and tar reforming using auto-generated char and steam occur in the same reactor is proposed. However, since the design of a prototype, construction and testing of the proposed reactor is a complicated matter that could easily result in a stand-alone thesis, it is left beyond the scope of this work.
Overall, char is a promising catalyst that can effectively tackle the tar problem under adequate conditions. Further work needs to be conducted to determine the appropriateness of using a single reactor for the process, both technically and economically.
|Date of Award
|Marco Geron (Supervisor), Xiaolei Zhang (Supervisor) & Vivek Ranade (Supervisor)