Effect Of CO₂ Recycling To The Gasifier On Pressurized Fixed Bed Dry Bottom Gasification

Pressurized fixed bed dry bottom gasification is thought to be the most competitive gasification technology for the production of SNG As a mature technology, it has many advantages, such as investment saving, high CH4 content in the crude gas and so on. At the same time, there are some disadvantages from it, mainly focus on the CO2 and phenolic wastewater emissions. To solve the problem, using CO2, as the co-gasifying agent, to substitute part of steam is proposed in this thesis. It aimed at maximizing the utilization of CO2, making the gasification technology be water saving, CO2 and phenolic wastewater emissions reduction.In this thesis, Aspen Plus is used to simulate the pressurized fixed bed dry bottom gasification process. Kinetic characteristics of the coal char is considered in the model. Gasification reaction, corresponding reaction rate and kinetic parameters are given by a kinetic subroutine, written by Fortran language. Simulation results are compared with the actual plant operating data. It showed that the simulation results are in good agreements with the actual plant operating data, so the model is suitable to simulate and analysis the pressurized fixed bed dry bottom gasification process.Based on the validated model, effect of O2/Coal and Steam/O2 on pure steam gasification is studied respectively. The optimized O2/Coal and Steam/O2 is 0.175Nm3/kg and 6.5kg/Nm3, respectively. Based on the optimized O2/Coal and Steam/O2, effect of gasification pressure on pure steam gasification is also discussed. Results showed that CH4 content in the crude gas increased by 16.3% with the gasification pressure increased from 4MPaG to 6MPaGBased on the optimized O2/Coal and Steam/O2, effect of replacement ratio of CO2 to steam and the replacement ways on the gasification process are also studied when CO2, as the co-gasifying agent, is used to substitute part of steam. Results showed that the net CO2 emissions reduced, the amount of undecomposed steam in the crude gas decreased to reduce the phenolic wastewater when CO2, as the co-gasifying agent, is used to substitute part of steam for pressurized fixed bed dry bottom gasification, making the gasification technology be water saving, CO2 and phenolic wastewater emissions reduction technology. The maximum replacement ratio of CO2 to steam is determined on the premise that CH4 production decline is less than 15% and carbon conversion rate maintains at (90±5)%. Effect of O2/Coal, gasification pressure on the co-gasification process is also respectively studied at the same replacement ratio of CO2 to steam. Results showed that the gasification indexes are improved to varying degrees with the increasing O2/Coal; CH4 content in the crude gas increased by 9.1% under gasification pressure from 4MPaG to 6MPaG...