Effect of high-flux solar irradiation on the gasification of coal in a hybrid entrained-flow reactor

Abstract

A mathematical model is developed and employed to investigate the mechanisms by which high-flux solar irradiation influences the gasification of coal particles in an entrained-flow reactor environment. Existing models from the literature for autothermal entrained-flow gasifiers were extended to include high-flux solar irradiation. Model predictions indicate that, during the very early stages of gasification, particle and gaseous temperatures are slightly different for the autothermal, solaronly, and mixed autothermal/solar cases, with the rate of temperature increase being greater the more oxidant is used. However, once devolatilization begins, the temperatures and gaseous species are calculated to be very different. During later stages of gasification, the calculated temperatures, gas compositions, and carbon conversions for the cases vary significantly. The presence of oxygen leads to earlier and higher peak temperatures as well as higher initial carbon conversions. For solar-only conditions, the later solid temperatures are higher, leading to faster conversion of carbon during the latter stages of gasification. Because of these differences in temperatures, together with the differences in the quantity of inlet oxygen and steam for each case, there are also substantial differences in the distribution of gas species with axial distance over the reactor. These differences include increasing the H₂ content from 29% for the autothermal case to 51% for the solar case while, at the same time, reducing the CO₂ content from 11 to 2% while keeping the CO content relatively constant (∼37−39% for all cases). Consequently, hybridizing with solar increases the H₂/CO ratio from 0.77 to 1.4 while, at the same time, decreasing the CO₂/CO ratio from 0.29 to 0.05 as the solar flux is increased from 0 to 100% of the maximum required amount. Additionally, increasing the solar flux from 0 to 100% of the maximum increases the instantaneous solar share from 0 to 37% and increases the upgrade factor from 78 to 140%. The overall efficiency also increases from autothermal to solar, but for most incident solar flux values (less than 3 MW/m²), the efficiency of the reactor is less than autothermal as a result of additional loss of heat from the reactor via re-radiation. As a result of the variation in both the composition and production rate of the product gas on the fractional solar input, the use of syngas storage and/or further development of Fischer−Tropsch liquid technology will be required to enable flexible operation that is likely to be required in a coal to liquid plant with a hybrid solar/oxygen entrained-flow gasifier.