Investigation of the Performance of a Syngas Quench Cooling Design in a Downdraft Entrained-Flow Gasifier

Syngas coming out of a gasifier is usually very hot. Cooling is necessary to allow this syngas to be transported without damaging downstream piping or equipment. Furthermore, the existing cold syngas cleaning technology also requires that the syngas temperature to be reduced below 600 o F. Direct water quenching of syngas is one of the cooling schemes that can not only provide the necessary cooling, but also help to drive the water-gas shift (WGS) reaction to convert steam and carbon monoxide to carbon dioxide and hydrogen, which is an important process for systems implementing carbon capture and sequestration (CCS). To assist designing an adequate syngas quench cooling section, this study has employed computational fluid dynamics (CFD) to investigate the performance of a preliminary design of a water quench system in a downdraft entrained flow coal gasifier. The preliminary quench system design consists of a primary water curtain section located in the main entrance of the quench section and a secondary water spray section distributed in the outer annular passage with a counter-flow arrangement. The bottom water bath level can be adjusted to change the syngas’s local speed over the water surface to fine-tune the WGS reaction without using more sprayed water. The main goal of this study is to use the experimental results to calibrate the CFD model, especially the WGS reaction model, and then to use the calibrated model to help design different water spray strategies to achieve the targeted temperature and syngas composition. The result shows that three different reaction rates (Jones’ rate under a catalytic condition, Wade’s rate, and Sato’s rate under a non-catalytic condition) are all too fast when comparing the CFD results against the experimental data. The exponential constant value (A) of each reaction rate is therefore adjusted to match the experiment data within 2 percentage points (or 6%) in both CO conversion rate and H2 generation. The effect of the injection locations (primary vs. secondary) on WGS is marginal. Both locations result in 16% CO conversion rate. Spraying water in the primary location only provides a marginal advantage: an increase of 4% in H2 production, 3% in HHV value, and 30K in temperature. Using water bath level to fine-tune the WGS reaction is workable. When the water level gap decreases from 1050mm to 700 mm, the CO conversion rate decreases 7 percentage points (or 35%) from 20% to 13%, and the syngas outlet temperature decreases by 20K (from 891 K to 871 K.) Beyond the gap of 1050mm, the effect of the water bath level is not noticeable.