Modeling of Oxy Gasification of High Ash Coal in Fluidized Bed Gasifier

Syngas derived from coal gasification has been a principal source for power and several chemicals [1]. Among various prevalent technologies for producing syngas from coal, fluidized bed gasification has been pioneered for applications involving high ash (20-40%). Based on the requirement of the syngas, the gasifying agent can be either oxygen or air. In applications such as power generation, the primary gasifying agent is a mixture of steam and air. Whereas, when the demand is for production of chemicals of high purity, such as methanol (and its derivatives like DME (dimethyl ether)), pure oxygen along with steam is employed for gasification. Even though the subsequent technology for conversion of syngas to methanol has been commercially developed worldwide, the real bottleneck lies in reliable gasification of high ash coal such that it ensures cheaper gas cleanup and its conversion to methanol without any detrimental effect on catalyst and reactor downstream. It has been observed that fluidized beds operated with high ash coal predominately suffer from low carbon conversion with intermittent bed agglomeration. In order to address such issues, it is thus required to develop an efficient yet simple modeling framework which should estimate the quality of syngas produced (specifically CO/H₂) along with non-isothermal bed behavior based on varying operating conditions.

With this context, the present study thus envisages on developing a mathematical model to predict the exit syngas composition, temperature distribution, and carbon conversion. The model considers eight significant reactions: four each of homogeneous and heterogeneous. To incorporate the effect of bed hydrodynamics, a two-phase description of fluidized beds is considered, wherein it is assumed that the coal (heterogeneous) reactions take place in the emulsion phase, whereas, the gaseous (homogenous) phase reactions occur in both the emulsion as well as the bubble phase [2,3]. It was further assumed that the devolatilization of the incoming fresh coal feed was instantaneous as compared to the actual char gasification reactions. Pure oxygen was used a gasifying medium with molar ratio with steam being maintained at 2. When the bed was run at atmospheric pressure and 900 ^oC, preliminary results indicate that with an increase in the height the molar concentrations of CO and H₂ increase in both bubbles as well as the emulsion phase. Once such an analysis is performed in a non-isothermal bed, it is further expected to yield temperature distributions of coal particles which can be indicative of their tendency to agglomerate. A detailed analysis of the model with the influence of pressure and inlet oxygen concentration will be discussed in the final presentation.

References

[1] L. Chen, S.Z. Yong, A.F. Ghoniem, Oxy-fuel combustion of pulverized coal: Characterization, fundamentals, stabilization and CFD modeling, Progress in Energy and Combustion Science. 38 (2012) 156–214. doi:10.1016/J.PECS.2011.09.003.

[2] M.L. de Souza-Santos, Comprehensive modelling and simulation of fluidized bed boilers and gasifiers, Fuel. 68 (1989) 1507–1521. doi:10.1016/0016-2361(89)90288-3.

[3] F. Chejne, E. Lopera, C.A. Londoño, Modelling and simulation of a coal gasification process in pressurized fluidized bed, Fuel. (2011). doi:10.1016/j.fuel.2010.06.042.