## 3 Application
### Thermochemical Conversion
#### 3.8.3 Combustion  
Combustion is a high-temperature and exothermic oxidation process, which is often carried out in a combustion reactor. The numerical simulation of the combustion process is of positive significance to the design and performance evaluation of the combustion reactor [118, 119, 120]. For a better reaction, pyrolysis, gasification and mixing are often used as the initial steps of combustion. These processes contain a large number of particle and fluid phases, which have complex changes and interactions in the reaction process.  
The focus of combustion process simulation is to carry out comprehensive hydrodynamic modeling. This complex process includes flow modeling, discrete phase modeling, combustion and radiation modeling. In the flow and discrete phase modeling, DPM is used for the heat transfer and transport of particles in the combustion process under sparse conditions [119,121]. [PERSON] et al. combined the RNG k-\(\varepsilon\) turbulence model and DPM to model the thermal spray combustion. The flame flow characteristics, mass fraction of gas components and particle flight characteristics were calculated [122]. In dense phase combustion, [PERSON] et al. first applied the mixed Euler Lagrange dense discrete phase method for the modeling of the combustion process in a three-dimensional circulating fluidized bed [123]. The particle size distribution of real combustion particles and the interaction between particles in the dense region were considered. UDF is adopted to expand the modeling of DDPM in the combustion process by [PERSON] et al. UDF reinjected particles into the burner, and the pressure drop, circulation rate and mass load control of the combustor were calculated [124]. Regarding the modeling method of combustion,El-Sayed et al. chose the eddy dissipation model (EDM) to simulate the turbulent chemical interaction during the combustion of sesame and broad bean stalks in the freeboard zone in a bubbling fluidized bed combustor. The effects of chemical kinetic parameters, solid-phase parameters, gas-phase parameters and heat flux through walls on the numerical results were obtained and verified [96]. The EDM method can describe the interaction between turbulence and chemical reactions. Although the computational cost of the EDM method is moderate, the main disadvantage is that it reduces the chemical reaction mechanism [125]. The EDM method is improved by [PERSON]'s eddy dissipation concept (EDC) model [126]. The model describes the chemical reaction mechanism in more detail. The chemical reaction occurs on a finer time scale, and the calculation is more accurate, but it is also very time-consuming. As a novel combustion model, the steady laminar flamelet model (SFM) assumes that turbulent and non-premixed flames can be simulated by the collection of several one-dimensional laminar small flames [127; 128]. It combines the advantages of EDM and EDC, allows for the use of detailed chemical reaction mechanisms and significantly reduces the calculation time. As an important part of comprehensive computational fluid dynamics modeling, radiation modeling also has several different models to calculate the absorption characteristics of gas. The exponential wide band model (EWBM) is sometimes applied [129], but it requires large amounts of calculation. The more commonly used model is the weighted sum of gray gases model (WSGGM). Combined with the discrete ordinates (DO) radiation model, results with reasonable accuracy can be calculated [130].  
In conclusion, the method of integrating CFD-DPM, combustion modeling and radiation modeling is successful in simulating the combustion process, which effectively describe complex multiphase flow, heat and mass transfer and chemical reaction. However, due to the high requirements of combustion process simulation for spatial and temporal resolution, further turbulence simulation methods and discrete phase methods are needed to accurately deal with turbulence modulation, mixing and chemical problems.