A Study On Carbon Emission Reduction And Energy Saving Technology Of Low Temperature Unit In Coal Gasification Integrated With Coking To Methanol Process

China’s primary energy has mainly been based on coal for a long time as its oil and natural gas consumptions are highly dependent on foreign countries.Abundant coal resources enable the coal chemical industry to develop rapidly,and many commercialized coal chemical projects have been constructed and operated.However,the mismatch of hydrogen and carbon atoms between coal resources and chemical products has resulted in low coal resources utilization and carbon redundancy,which makes the coal chemical industry face the problems of high carbon emission and energy consumption.In this paper,the typical coal-to-methanol process is studied,and the novel coal gasification integrated with coal coking to methanol process is further analyzed.It indicates that the integration of coal gasification and coking can realize the grading and staged utilization of coal.The process is further integrated with the dry methane reforming unit to improve the coal resources utilization and reduce carbon emissions.Furthermore,we found that the Rectisol wash unit and the syngas gas cryogenic separation unit are both operated under low-temperature and high-pressure conditions.The large amount of solvent circulation in the Rectisol process and large refrigerant consumption in the cryogenic separation unit leads to ineffective energy utilization and high energy consumption in the methanol production process.In order to achieve the goal of energy saving,this paper explored and improved the Rectisol and syngas cryogenic separation process to reduce the amount of absorbent and refrigerant.The techo-economic analysis was used to analyze the proposed new process,and find out the advantages and disadvantages of the new process,which can provide a reference for the energy-saving design and improvement of the cryogenic unit.This paper firstly analyzes the coal gasification and coking to methanol process,and demonstrates the rationality of the process and its advantages in carbon emissions reduction.In view of the low utilization of coke products in the coking process,a part of the coke is output as a product,and another part is gasified to produce syngas,which is then mixed with the syngas produced by coal gasification.The crude coke oven gas produced in the coking process is purified and separated by pressure swing adsorption to separate methane and hydrogen to obtain hydrogen with a purity of 99.9%.After that,methane and the pure CO₂ produced in the acid gas removal unit are converted into syngas through the methane dry reforming reaction.Then,the pure hydrogen,the clean syngas and the syngas from the methane dry reforming unit were mixed to obtain the methanol syngas with a hydrogen-to-carbon ratio of 2.0.It finally entered the methanol synthesis unit for methanol production.The integrated process combines the two processes of coal gasification and coal coking,where coke’s utilization is promoted and CO₂emission reduction can be achieved via effectively utilization of COG,realizing the utilization of coal with high efficiency and value.The integrated process can significantly enhance the carbon utilization efficiency and reduce the carbon emissions through the techno-economic analysis.The result shows that the carbon utilization efficiency is increased from 37.3%to 51.6%compared with CTM process,and the CO₂ emission is also reduced from 2.04t to 1.78t by 14.6%when producing 1t methanol.In terms of economic benefits,the internal rate of return is increased from 15.3%to 22.5%,showing higher economic benefits.Meanwhile,the intergated process is more flexible and can adjust the coke production ratio according to market price changes.In the era of high carbon taxes,the intergated process will further demonstrate its clean and low-carbon advantages.During the analysis of the coal-to-methanol process,it was found that the Rectisol wash unit had the problems of high energy consumption and inefficiency of energy utilization.This paper further studies the energy-saving scheme of Rectisol wash process from two aspects:heat exchange network modification and process integration.In the first place,the pinch analysis method is conducted to modify the heat exchange network without changing the existing heat exchange streams.The modified heat exchange network can reduce the cooling and heating duty by 237 k W,contributing to a reduction of the total utility costs by about 1.5%and the total product cost by 0.3 million CNY.Secondly,in view of the large consumption of methanol circulation and cooling capacity,this chapter further designs a Rectisol wash process integrated with CO₂ liquefaction.The techno-economic analysis is performed and the results are compared with the conventional Rectisol process to demonstrates its energy saving potential and feasibility.The results showed that the solvent circulation in the new process reduced by 46.5%and the energy consumption decreased from 389.1 MJ to 204.3 MJ for capturing 1kg of CO₂.In terms of the economic benefit,the newly developed process reduced the capital investment and the total product cost by 12.4%and 8.7%,respectively,indicating a higher economic performance.The new integrated Rectisol process proposed in this paper can be readily used for the real applications in the acid gas removal process of the chemical industry to increase the energy efficiency as well as economic performance.In order to solve the problems of high energy consumption and ineffective energy utilization in the syngs cryogenic separation unit,this paper also proposed a cryogenic separation process integrated with ammonia absorption refrigeration driven by compression waste heat.During the compression process,low tempreture compression waste heat（140℃～190℃）will be generated and is then removed by the cooling water in the traditional cryogenic separation process,resulting in a waste of energy.In this chapter,the compression waste heat is used as the heat source of the ammonia absorption refrigeration cycle.The ammonia absorption refrigeration can produce the cold energy of-29.5℃,which is used for the pre-cooling of the syngas and mixed refrigerant.The heat integration results of the new process showed that the ammonia absorption pre-cooling cycle can provide 16.9%of the refrigeration capacity for the mixed refrigerant refrigeration cycle,thereby reducing the energy consumption of the mixed refrigerant refrigeration cycle.With the integration of ammonia absorption pre-cooling cycle,the consumption of mixed refrigerant is reduced from 4700kmol/h to 5550 kmol/h by 15.3%.The reduction of mixed refrigerant consumption fuether reduced the total energy consumption for compressors from 27 MW to 20 MW by 25.8%.The exergy analysis showed that the new process reduced the total exergy loss from 25.8 MW to19.14 MW,and the exergy efficiency was increased from 50.9%to 54.5%,indicating that the the energy use in the new process was more effective and the irreversible losses was reduced.In terms of the economic performance,the total capital investment and the total production cost of the new process were reduced by 10.8%and 11.7%,indicating that greater economic advantages.