Gasification is a thermos-chemical process through which a carbon based fuel such as biomass is broken down to its elemental gases. The combustible gases are combusted with air to produce heat for commercial applications. Gasification reaction, gas composition, and operating conditions are dictated by composition as well as the physical attributes of the biomass. For example, it has been found that the gasifier’s operating temperature decreases as the feedstock moisture content increases. When this happens, the equivalence ratio must be increased in order to increase the operating temperature in maintaining a stable bed-zone operating conditions.
The analysis of biomass physical attributes and chemical compositions is necessary for feeding, system’s performance, and downstream operations. Moisture content, calorific value, species, particle sizes, and absence of impurities such as plastics and sand, are the most necessary fuel parameters for wood chips for end-users.
Because biomass is a low energy density resource, therefore, considerable volumes of biomass must be collected over extensive areas to satisfy gasification requirements. This poses complex logistics as well as costly transportation. The consistency of biofuel attributes affects the cost of supply logistics. For example, using a covered storage can really help in controlling the fuel moisture content.
Research conducted by doctoral candiadte Ehsan Oveisi under the supervision of his research supervisors Professor Shahab Sokhansanj and C J Lim from The University of British Columbia in collaboration with two Canadian companies, Cliff Mui from Nexterra Systems Corp. and Robby Gill from Cloverdale Fuel Ltd.. in addition to Fernando Preto from CanmetENERGY (which is Canada’s leading research and technology organization in the field of clean energy) presented a comprehensive analysis of the feedstock attributes and improvement of the performance of the University of British Columbia gasifier in thermal output as well as operational reliability over a period of three years. Their research work is published in peer-reviewed journal, Biomass and Bioenergy.
The research team recorded feedstock quality data at the plant on a regular basis. Their objective was to quantify seasonal variations in the feedstock quality attributes, causes for variation in the feedstock quality, and the effect of feedstock quality on the system steam output and reliability. The selected gasifier with its fuel supply system provided a real case study and a basis for improving the design and reliability of future gasification systems.
An analysis of the feedstock attributes as well as operating conditions of the selected gasifier indicated that, in the absence of a dryer, blending the feedstock with varying moisture contents increased the variability in the feedstock delivered to the gasifier plant. A large variation in steam production over the three years was recorded and this was attributed to variations in the feedstock moisture, ash contents, and particle size.
Ehsan Oveisi and the research team also observed that protecting the biomass from rain reduced the level as well as variation in fuel moisture content. Providing a paved surface was critical in minimizing contamination during handling at the depot. Comminuting and fractionating the fuel applying three screens were effective in producing a more consistent feedstock size.
Ehsan Oveisi, Shahab Sokhansanj, Anthony Lau, C. Jim Lim, Xiaotao Bi, Mahmood Ebadian, Fernando Preto, Cliff Mui, Robby Gill. In-depot upgrading the quality of fuel chips for a commercial gasification plant. Biomass and Bioenergy, issue 108 (2018), pages 138–145.
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