The Study On Low Temperature Waste Heat Utilization And Carbon Reduction Of Coal-based Methanol Process

China is the world’s largest methanol producer and consumer. Rich in coal,insufficient oil supply and limited natural gas resources is the current energy sources structure in China, thus, the main methanol production is from coal-to-methaonl process in China. Coal-to-methanol process is characterized of high energy consumption and high carbon emissions intensity. In the coal-to-methanol plant, a large amount of low grade heatis not used efficiently.In addition, on the CO₂ recovery and capture condition, CO₂ compressor consumes large amounts of electric energy and transfer into low temperature waste heateventually. Recovery and utilize the waste heat in the process can improve the energy efficiency and reduce the process carbon emissions. The low temperature waste heat utilization and carbon capture technology are studied in this study. Based on the process simulation, applying the heat integration, waste heat power generation technologies,the developed coal-to-methanol with low temperature waste heat utilization and integrated carbon capture process are anaylized.The coal-to-methaol process based on GE coal slurry gasification include the units followed: coal gasification, quench, water gas shift, chilled methanl wash, methanol synthesis and methanol distillation. 600 kt/a production capacity coal-to-methanol process is simulatedin AspenPlus process simulator. The process configuration of streams, process operating parameters is studied, which provide the data basisto waste heat utilization and carbon reduction process.There is huge amount of reaction heat and high temperature heat released in the coal to methanol process. Pinch analysis results showed thatthe threshold problems with onlycold utility required are existed in the heat exchange network of the gasification and ash-mixed water treatment, water gas shift, methanol synthesis and distillation units.Besides, there are a lot of low temperature waste heat in these units. The total mount of waste heat in methanol synthesis and the distillation is large, however the temperature is lower tha 70℃, which is difficult to be recovery and be used. There is a reasonable amount of waste heat with the temperation of 80-150℃in the gasification ash-mixed water treatment and water gas shift unit, which is the main waste heat in coa-based methanol process, and further be used for waste heat power generation.Organic Rankine cycle power generation is low temperature heat utilization technology, with application oflow boiling point organic fluid as the working medium transferringthe low grade heat energy into electricity. R600 a is selected as the working fuild in waste heat power generation for coal to methanol process, and the Pinch analysis and heat exchange network design are conducted. Results shown that waste heat power generation unit in coal-to-methanolis also a threshold problemwithjust cold utilities are needed, there is 64.2 MW heat flow form the waste source, the overall heat exchange between hot streams and cold streams is 32.2 MW.unit within the total heat generated by the heat source logistics 64.2 MW. In the 600 kt/a methanol plant, the gross electricity outputis 3.95 MW from waste heat power generation, net electricity output is around 3.55 MW, payback period of ORC investment is 3 years. The methanol product cost is reduced by the waste heat power generation applied in the coal-based methanol processFor reducing carbon emission in coal-based methanol process and improve efficiency of CO₂ capture, an integrated acid gas removal process with high CO₂ capture rate is proposed, which is featured by Rectisol desorption processs heat integrated with CO₂ multistage compression. By improving the methanol rich fluid temperature in CO₂ desorption, dropthe desorption pressure, the CO₂ capture rate can be improved significantly.The chilled methanol is heat exchanged with of CO₂ compressor, providing the cooling source to CO₂ liqufication, the CO₂ capturerate increase with saving 9.6% power consumption and reducing 36.1% cooling water consumption. For consider increase capture rate at a reasonable energy consumption, CO₂ capturerate of 80-90% is optimal range for the integrated processAs carbon capture technology applied to coal-to-methanol process, CO₂ multistage compression produceslots of low temperature waste heat. Recovering the waste heat from the conventional carbon capture in coal-to-methanol process in organic Ranke cycle power generation, the net electricity output is around 4.32 MW, the methanol productper ton cost decreases appropriate 39 CNY. In the condition of chilled methanol washing and CO₂ compression integration technology, CO₂ capture rate increases to 85%, four stages CO₂ compression is used in the process, and the waste heat amount is slight lower than the conventional carbon capture process, the net electricity output is around 4 MW in waste heat power generation unit. Thus, with the application of waste heat power generation.power consumption decreases and the carbon emissions isindirectly reducedin coal-based methanol with carbon capture plant.