Experimental and numerical investigations into air and oxy-fuel combustion of single coal particles

摘要

The paper presents experimental and numerical results of single coal particle combustion in air and O_2/CO_2 gas mixtures. The combustion process was experimentally investigated in an electrically heated up reactor through which the flow of hot gases up to temperature of 1250 K is possible. Experimental investigations were performed for cubic particles having the size of around 2 mm burning in air and a gas mixture of O_2/CO_2 at temperature of 1223 K. The particle temperature was measured by a thermocouple on which the particle was placed. Particle heat up, devolatilization, ignition and combustion of volatiles in a laminar diffusion flame around the particle and coal char combustion are phenomena which occurred and were recorded by a high speed camera during the whole combustion process. The main differences which distinguish air and oxy-fuel combustion consist in the ignition delay of volatile matter and lower combustion temperatures at char combustion.The single coal particle combustion was also numerically modeled using the Implicit Continuous-fluid Eulerian (ICE) method combined with the numerical solution of the particle mass and energy conservation equations. To simplify the multidimensional problem, the process of combustion was assumed to be 1-dimensional and the coal particle was treated as a Lagrange discrete particle. The mass fluxes of individual gas species describing devolatilization and coal char combustion and particle temperature being the solution of the particle mass and energy conservation equations were applied as the boundary conditions in the ICE method which describes a multispecies chemically reactive unsteady compressible flow around the particle. To define the gas velocity and density, the unsteady mass and momentum equations have been combined like usually made in projection methods to get the Helmholtz equation which together with appropriate boundary conditions define gas pressure. Considering the spherical symmetry of the system assumed for the numerical simulation, the time-dependent 1-dimensional energy and species mass conservation equations and the pressure Helmholtz equation can be efficiently solved by the use of the tridiagonal matrix algorithm (TDMA).Numerical results were compared with the experimental ones. The numerical results, i.e. the particle temperature and combustion times, correctly match up with the whole combustion process experimentally observed.