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The design and operational behaviour of a laboratory scale fixed-bed gasifier
Conradie, Frederik Hendrik
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Fixed bed coal gasification and combustion operations form an important part in the global energy supply, and these processes are characterized by complex interactions which are difficult to study on an industrial scale. A laboratory scale fixed-bed coal combustion reactor (LSR) was therefore constructed and used to mimic pilot and industrial scale fixed bed combustion and gasification. The initial aims were to establish operating conditions comparable to that of previous pilot and commercial scale work. Secondary aims included keeping operations less expensive, more flexible, while using smaller sample and particle sizes while maintaining representativeness. Measurement capabilities inside the reactor and post experiment bed dissection have shown that bed temperature profiles and reaction zones were representative of both a pilot scale reactor (PSR), and an industrial scale reactor (ISR). The chemical transformational behaviour that coal particles undergo during conversion in deep packed beds was investigated. Experiments were performed in a transient overfeed mode, fed with 4, 6 and 8 mm coal particles. A post experiment dissection of the bed contents along with full chemical characterisation was undertaken. The dissection method was able to provide an accurate representation of the characteristic reaction zones. The residual volatile matter and the overlap in the reduction and pyrolysis zones were insensitive to particle size variation and mainly determined by the maximum temperature in each zone. The reaction front velocity and heating rates that particles experience in the different reaction zones were obtained and showed significant variation during the transient start-up stage, but are remarkably comparable once the stable reaction front is formed. The physical transformational behaviour that coal particles and the bed structure undergo in a fixed bed reactor was also investigated. The dissection method limited mechanical fragmentation by minimizing particle handling and bed structural disturbance. Particle size distribution and the particle size distribution width showed a significant variation in the oxidation zone and the ash bed which was not previously quantified on industrial scale experiments. The particles and the fixed bed macroscopic structural change was measured by the particle porosity and the fixed bed voidage. The role of particle agglomeration in the lower sections of the fixed bed was demonstrated in the bed voidage data and is tracked as the reaction front moves up through the fixed bed of particles. This current mode of operation represents fixed bed combustion and gasification operations particularly during the transient start-up stages and the subsequent particle and bed transformational behaviour
- Engineering