Char reactivity assessment with steam in packed bed and pilot scale under oxy-steam environment

Abstract

This paper focuses on char gasification in steam towards generating syngas as a part of two-stage gasification technology developed in-house to handle high-ash coal. Controlled experiments in a small-scale packed bed reactor are used to arrive at the dependence of char reactivity using parametric studies like the evolution of temperature, gas composition, flow rate, and carbon conversion rate with time and are compared for different particle sizes. The volume percentages of CO, CO2, CH4, and H2 were 7, 26, 1, and 66, respectively, in steam char experiments, over a particle size range from 1 mm to 4 mm. If the CO2 is separated, the H2 volume percentage reaches 90%. In small-scale experiments, the gas composition for all the particle sizes was invariable; however, the reactivity of small particles is higher than the bigger particles, i.e., 0.5 g/g-hr as against 0.38 g/g-hr for bigger particles due to the transition from diffusion to the kinetic regime, which establishes the suitability of small particles for pilot-scale operations.Finally, the experiments were carried out on pilot-scale char gasification in an oxy-steam environment, and the results were compared with simulated experiments for small-size particles. The consequences of adding oxygen with steam to keep the system auto-thermal are discussed. In the pilot scale under oxy-steam conditions, the volumetric concentrations of CO, CO2, CH4, and H2 were 10.8, 39.9, 4.7, and 44.6 %, respectively. In contrast, the reactivity increased from 0.46 to 0.90 g/g-hr with a decrease in the calorific value of gas from 9.7 to 8.6 MJ/Nm3.