Comprehensive parametric study of fixed-bed co-gasification process through Multiple Thermally Thick Particle (MTTP) model

Co-gasification technology provides a promising solution for the energetic valorization of various biomass feedstocks, especially those not directly applicable for gasification owing to their low-calorific values or high ash content, but the complexity of co-gasification technology has put forward challenges to model formulation when using numerical methods to evaluate the performance of the gasifier. In the present work, the Multiple Thermally Thick Particle (MTTP) model is developed and validated for fixed-bed co-gasification process. The sub-phase treatment that extends the conventional Eulerian-Eulerian framework allows calculating the inhomogeneity in the solid phase, and the intra- and inter-particle heat and mass transfer sub-models enable quantitative evaluation of the conversion of different solid fuels with nonnegligible intraparticle gradients. Parametric analyses of moisture content and particle size were performed to evaluate how the degree of difference between each fuel would influence the reaction processes, gasification performances and steady-state operating conditions. Upon changing the moisture content of the fuel mixtures (biomass and MSW) from both 9.0 wt% to 9.0 wt% for MSW and 27.0 wt% for biomass, the maximum temperature difference between the solid particle surfaces of different fuels was nearly 150 K, and the regions of different conversion processes became highly overlapped. When increasing the moisture content of biomass from 9.0 wt% to 27.0 wt% under 1:1 mixing ratio, the cold gas efficiency, volume fraction of H2 and higher heating value of syngas increased from 65.26%, 12.03%, and 6.29 MJ/Nm3 to 68.39%, 15.62%, and 6.44 MJ/Nm3, respectively. Therefore, the quality and efficiency can be improved when increasing the moisture content in a certain range although the overall capacity of the gasifier will decrease. The intraparticle temperature profiles calculated by the MTTP model also revealed that decreasing the particle size from centimeter- to millimeter-level will significantly reduce the temperature difference between the surface and center of the fuel particles, thus accelerating the in-bed reaction rates and increasing the overall capacity of the gasifier. The MTTP model is flexible for various working conditions and solid fuel mixtures, which is a versatile and convenient tool to optimize the complicated co-gasification process.