## 3 Application
### Thermochemical Conversion
#### 3.8.1 Gasification  
The gasification process converts solid fuel into high-value gaseous fuel and chemical products through a series of homogeneous and heterogeneous reaction routes. Unlike pyrolysis, gasification requires the participation of gasification agents, including oxygen, air, carbon dioxide and steam. The main objects of gasification are biomass and coal. Computational fluid dynamics and its related methods are effective tools for reactor design,  
Figure 7: Distribution of bubbles passing position and transport position [92]. performance prediction and structure optimization in the gasification process [100; 101; 102]. The prediction of the operating performance and mechanism in a gasification reactor is of great significance to solve energy and environmental problems. A reasonable modeling method determines the prediction accuracy of the gasification process since CFD is sometimes not directly applicable. [PERSON] et al. conducted computational fluid dynamics modeling for the four regions (drying, pyrolysis, oxidation and reduction) of the downdraft gasifier, used a step-by-step method to evaluate the gas composition of volatile decomposition in the gasification process and tested the thermochemical kinetics and robustness [103]. The study presents a model that accounts for time-limited wall reactions, which is essential for understanding the performance of entrained flow gasifiers. [PERSON] et al. used CFD-DDPM to establish the numerical model of coal gasification in a circulating fluidized bed. DDPM was used to simulate the flow of the particle phase in a coal gasification furnace. The coal particles with size distribution were tracked in the fluid velocity field, and the multidirectional coupling between particle fluid and particle was considered [104]. Considering the effects of the diffusion rate and kinetic rate to simulate the char reactions, [PERSON] et al. established an intrinsic reaction rate submodel for biomass entrained flow gasification. The finite rate/eddy dissipation model was applied to calculate the homogeneous reaction rates. The heterogeneous reaction rate was calculated by writing UDF [105]. The intrinsic reaction rate submodel might oversimplify the chemical kinetics involved in biomass gasification, potentially leading to inaccuracies. Slagging and fouling are important phenomena associated with ash handling and discharge in coal combustion and gasification. Improper treatment and discharge of slagging and fouling will cause safety problems. [PERSON] et al. developed a three-dimensional slag model using UDF. This model coupled VOF and DPM and considered the dispersion of particles to describe multiphase flow, which was applied to the slag flow simulation in three-dimensional pilot-scale facilities [106]. CFD simulation of the slag flow shows that the performance of slagging entrained flow gasification may be critically affected by the behavior of char/ash particles as they interact with the slag-covered wall [107; 108; 109]. [PERSON] et al. developed a compartmental model of entrained flow slagging gasifiers of solid fuels based on CFD-DPM. The model considered the near-wall phenomenon related to the transfer of coke and ash particles from the main body of the reactor to the reactor wall. Although the model cannot be used for prediction at this stage, it is of great positive significance for evaluating the correlation between particle separation and gasifier performance and considering particle-wall interaction [101]. However, due to the complexity of the gasification process, it is difficult to ensure high prediction accuracy. There is still a certain distance between the prediction results of CFD-DPM and the flow conditions in the real situation. The CFD-DPM method, as applied to gasification processes, offers a sophisticated approach to simulate the complex interactions between the gas and solid phases, enabling detailed analysis of particle behavior, heat transfer and chemical reactions.