Numerical simulation of cleanliness of glass window in solar CO<sub>2</sub> coal gasification reactor

Abstract

Solar CO2 coal gasification technology uses highly concentrated solar radiation to generate high temperature above 1000°C, so that the pulverized coal particles can react with CO2 to generate CO in the solar reactor. Compared with common industrial coal gasification technology, solar CO2 coal gasification has the advantages of reducing CO2 emissions and producing high quality and high purity gaseous fuels in a clean and energy efficient manner. Solar coal gasification stores intermittent and unstable solar energy in clean fuels, which has promising application and has attracted the interest of many researchers around the world. As the core component of solar thermochemical, solar reactors are mainly divided into direct and indirect types. The direct reactor is easy to realize and has many advantages,which is convenient for transport and liquid slagging. However, a major problem of the direct solar reactor is that coal particles may contaminate the reactor glass window which will affect the efficiency and safety of the reactor. So keeping the reactor quartz glass clean is the key to increase reactor efficiency and safety. The reactor windows are typically purged with argon to avoid contamination by coal ash and tar, but this also results in reducing thermal efficiency and difficulties of separating from the gas. In this paper, CO2 gas is used as the reactant and also used to clean the quartz glass window in a solar gasification reactor. This paper establishes a three dimensional mathematical model of a directly-irradiated cyclone-flow solar thermochemical reactor. UDF is used to realize the flux ellipsoid distribution and concentrating in fluent software.A gas-solid two-phase flow model is established and the participating media radiation and chemical reaction kinetic models of coal particles and gases are coupled by Euler-Lagrange method. The numerical results are validated by comparing with the experimental results of ETH cyclone reactor. Then the effect of CO2 intake mass flow ratio and coal particle mass flow on the deposition of coal particles on the quartz glass window surface was studied. The calculation results show that maintaining the glass window cleaning mainly depends on the intake mass flow ratio of the sweeping glass and the cavity. When the sweeping glass window intake CO2 mass flow is too small, coal particles are easy to contaminate the glass. When the sweeping glass window intake CO2 mass flow is too large, the molar ratio of CO2 to coal particles will increase which reduce CO2 conversion. The coal particle inlet CO2 mass flow accounts for a small amount of total intake gas mass flow, mainly as a gas for transporting coal particles. When the molar ratio of CO2 to coal particles is in the range of 1.3−1.8 and the mass flow rate of the cavity, sweeping glass and conveying coal particles takes the percentage of the total CO2 intake mass flow rate in the range of 71.9%−78.2%, 15.7%−20.7% and 5%− 10%, respectively, the glass window cleanliness is optimal. The increase of mass flow rate of coal particles (1.5×10-4 −4×10-4 kg/s) can significantly reduce the molar ratio of CO2 to coal particles, but the turbulence of coal particle trajectory will increase, which causes a small amount of coal particles to move toward the glass window through the opening aperture.