Investigations into a novel process for high ash Indian coal gasification- a two-stage gasifier

Abstract

India, in addressing climate change, has embarked on clean coal initiatives to reduce carbon emissions. Various measures are taken towards meeting the decarbonization process, and coal conversion through gasification is one of the cleanest and most efficient options for coal to electricity or chemicals. However, the high ash content of Indian coal poses many challenges when used in conventional gasification reactors. The most crucial issues in the conventional gasification reactor are ash agglomeration/clinker formation, low carbon conversion, and low-quality gas due to higher tar content. The present research focuses on addressing these issues using a two-stage gasification process. The novel method of staging the process involves a unique combination of reactors. In the first stage, a cyclone reactor is used for the devolatilization of coal and homogeneous combustion of the volatiles to generate a consistent pyrolyzed char with very low volatile content. The generated char is transferred to the second stage, i.e., a fixed bed reactor, where oxy-steam char gasification produces clean hydrogen-rich syngas. Unlike the conventional reactors, in the proposed configuration, the product of pyrolysis (followed by combustion) and gasification from different reactors do not mix. This unique mass flow configuration provides a high calorific value of gas, higher CO+ H2, and extremely low tar content in the syngas. For the process optimization and designing the reactors of the gasification system, the kinetic parameters at the single particle level, as well as the packed bed, are critical. Given that the proposed two stage gasifier comprises two distinct reactors—the cyclone and fixed bed—a thorough comprehension of the residence time of coal within each reactor is imperative. For the cyclone reactor, the residence time was derived from the flaming time observed in single particle studies and flow visualization analyses. Conversely, for the residence time in the packed bed reactor, insights were gleaned from fixed bed reactor studies conducted in both steam and oxy-steam environments. In all the studies, coal with an ash content in the range of 29 ± 2 % has been used. Single particle experiments have been conducted to determine a relation between coal particle size and pyrolysis time in an oxidative (air) environment in a heated furnace. The relation between flaming time and particle size was 0.99 d1.8, where time has been measured in seconds and diameter of the particle was measured in mm. This relation is used to predict the pyrolysis time of coal particles and, hence, an approximate residence in the cyclone reactor with a given particle size. Coupled with the results from the CFD analysis for a cyclone reactor, it provides insight into the design of the pyrolysis phase with necessary modifications in the standard cyclone dust collector. To understand the gas behaviour inside the reactor, a cold flow simulation of the two-phase flow in the cyclone reactor is carried out. The results of flow visualization validate the simulation. It has been found from the simulation studies that increasing the length and reducing the diameter of the vortex finder enhances the contact between the particle and gas. Hence, a longer and narrower vortex finder is suggested as compared to the standard design of the dust collector. The preliminary design of the reactor for bench scale testing was conceived using a conventional cyclone dust collector design, with standard dimensions for a cyclone reactor having a coal consumption of 4 kg/hr. Corroborative evidence through the experiments using a transparent acrylic cyclone model is used to determine the residence time of the particle in the model cyclone. Considering the 1 mm particle size of the particle, a flow visualization study has been done in a reduced-scale transparent reactor model with the help of high-speed imaging. The residence time of the particle in the reactor is found to be 0.32 ± 0.04 s in the cylindrical region and 1.82 ± 0.31 s in the conical region. These numbers have been used for scaling the size of the cyclone reactor. Besides, it has been found that with increasing inlet velocity of the flow, the particle residence time increases. Controlled parametric studies were carried out in a packed bed towards arriving at the conversion time required for the char. The reactivity of the high ash coal char is determined in an externally heated furnace for three particle size ranges: S1.2 (1-1.4 mm), S1.7 (1.4-2 mm), and S3.2 (2.4-4 mm). The gas composition has been found to be invariant with the particle size, suggesting volumetric participation of the steam with char. However, the reactivity of the particle has been increased with reducing particle size. Reducing the size from S3.2 to S1.7 range, a marginal increase of 5 % in the reactivity has been observed, and S1.7 to S1.2 shows the interesting transition from diffusion to kinetically controlled condition. Based on these experiments, the particle size has been fixed to 1 mm for pilot-scale experiments. Based on the experiments on simulated packed conditions, an autothermal fixed bed reactor with an optimized mixture of O2 and steam instead of pure steam was used to maintain the requisite temperature. These results suggested the entire operation can be sustained autothermally using oxy-steam for stage 2 for the syngas generation by sacrificing less than 8 % of the energy in the gas. Using the data from the fundamental studies, a 150 KWth two-stage gasification is designed, developed, and operated at 0.6 tonnes per day (TPD) capacity. The coal was fed into the cyclone reactor, after devolatilization the coal char was transferred to fixed bed reactor. Since the air was used as an oxidant in the cyclone reactor to facilitate the autothermal oxidative pyrolysis of the coal, the volumetric proportion of N2 was higher in the gas. Gas leaving the cyclone reactor typically contained the following compositions: CO 4.8%, CO2 15.8%, CH4 2.1%, H2 5.5%, and remaining N2 i.e., 71.9%. The effect of temperature, oxygen, and steam flow rates on the performance of the reactor has been observed. The typical volume percentage of CO, CO2, CH4, and H2 are 30-40 %, 15-20 %, 0.5-2 %, and 40-50 %, respectively, yielding a syngas of calorific value of up to 11.2 MJ/kg at ~940 ˚C. Carbon conversion greater than 80 % has been achieved with a negligible tar content in the syngas. The concentration of CO+H2 in the raw syngas from the second reactor can be as high as 86 %. Higher CO+H2 and low tar content in the gas make this configuration suitable for a wide range of chemical synthesis. This work, for the first time, exhibits a gas for a range of downstream fuels and chemicals that can be generated using high-ash Indian Coal.