Some of the novel designs of gasifiers are shown in Fig. 16.11.

The chemical looping gasifier is a novel concept to produce a product gas rich in hydrogen and free from nitrogen and at the same time facilitate carbon dioxide capture using sorbent and its regeneration to produce a pure stream of carbon dioxide. It consists of two reactors; one is a bubbling fluidized bed gasifier where the biomass is gasified with steam in the presence of sorbent calcium oxide. The carbon dioxide produced during gasification is captured by calcium oxide. The calcium carbonate thus formed moves into another reactor: a regenerator working as a fast bed. The regenerator is heated to a temperature of 950°C. At this temperature, the calcium carbonate calcines to calcium oxide and carbon dioxide. The solid calcium oxide is separated in a cyclone and sent to the gasifier, while

16.11 Novel designs of gasifiers (a), Acharya et al. (2009); (b), Jinhu et al. (2004)).

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carbon dioxide is separated for sequestration. A laboratory scale 5 kW unit is being investigated by the authors.

A novel approach of integrating a fluidized bed with an entrained bed has been proposed by Wu et al. (2004). The lower portion of the reactor is the fluidized bed where, under a moderate temperature of 1000°C, coal with higher reactivity will be converted. The unconverted char in fly ash is trapped in the cyclone separator and is then fed into two chambers of entrained flow gasifiers, further converting it into useful gases. The entrained flow chamber is maintained at 1200-1400°C. The fly ash may contain 30-70% of the char. This system can be flexible in adjusting the composition of CO and H2 in the product gas by controlling gasification in the fluidized bed and the entrained bed. An experiment done in a bench scale shows an overall carbon conversion of 95% with a CO and H concentration in the product gas within the range of 78-82%. By adjusting the gasification taking place in fluidized bed and the entrained bed, the H2/CO ratio can be varied from 0.70 to 1.25.

The rotating fluidized bed process is based on a new concept of injecting fluidization gas tangentially in the fluidization chamber. The basic principal is that the drag force is overcome by the centrifugal force. With this method, a more uniform fluidization is obtained at high centrifugal forces. Finally, inter-particle van derWaals forces can be overcome, allowing fluidization of very fine particles, such as cohesive (Geldart group C) micro — and nano-particles. The fluidization gas enters tangentially and leaves from the central chimney.