Evaluation of the energy efficiency of the shell coal gasification process by coal type

The energy efficiency of the coal gasification process for five different coals (Pittsburgh #8, Illinois #6, Drayton coal, a coal from Montana Rosebud, and Wyoming coal) was evaluated using a rigorous dynamic model. The model considered a Shell entrained-flow gasifier with a membrane wall and a quenching system for a 300 MW-class integrated gasification combined cycle (IGCC) power plant. Parametric studies on gasifying agents (oxygen and steam) were conducted to identify the optimal ratios of oxygen and steam to each coal for maximum cold gas efficiency (CGE). The gasifier performance was evaluated in terms of the product gas flow rate, CGE, gas temperature, slag generation, and steam consumption. The optimal ratio of oxygen to coal flow for the maximum CGE varied from 0.704 to 0.871 depending on the coal type. Then, the maximum CGE of the coals was achieved in the range of 79.8-80.4% without the addition of steam. The CGEs of bituminous coals were improved by the addition of steam, resulting in 80.8-81.3% of CGEs. By contrast, sub-bituminous coals did not have any benefit to the CGE from the addition of steam, showing 79.8-80.3% of CGEs. Therefore, the optimal amount of both oxygen and steam for each coal was determined to maximize energy production in the gasification process. Based on the same lower heating value of syngas from the gasifier (739.5 MJ/s), the total recovered energy in the gasifier was 175.2-188.6 MJ/s for bituminous coals and 146.7-155.2 MJ/s for sub-bituminous coals at optimal gasifying agents. The energy demand of the gasification system and related units (air separating unit, coal treatment, and steam consumption) was in the range of 39.4-40.2 MW, which showed a small difference among coal types. Consequently, the energy efficiency of the gasifier strongly depended on the HHV of coal. However, considering the significantly lower energy density of sub-bituminous coals compared to bituminous coals, the performance of their gasification was considerably high in the Shell gasifier.