Energy-saving Retrofit Of The Rectisol Unit And Total Site Integration Of The Waste Heat Utilization System In The Coal-to-SNG Process

Modern coal chemical industry is an important way for clean and efficient utilization of coal resources,which can alleviate the current state of high external dependence on oil and natural gas.Based on the energy structure and supply-demand relationship in China,modern coal chemical industry still plays the ballast role of strategic security.However,its high energy consumption and high carbon emission are the main problems that the coal chemical industry has been criticized for a long time.This paper first analyzes the reasons for the low resource and energy utilization efficiency of the coal chemical industry,and reviews its development status and energy-saving and emission reduction technologies.It was found that the temperature mismatch between heat source and heat sink greatly limits the energy-saving and emission reduction potential of coal chemical industry,resulting in huge energy loss and indirect carbon emission.In this background,this paper focuses on specific problems and technical challenges in both the process system and the waste heat utilization,aiming to develop corresponding energy-saving optimization methods and strategies.The Datang Keqi coal-to-SNG plant was used as the research object,and the total site energy utilization data was obtained after long-term on-site research.The energy analysis of the Rectisol unit reveals that its cold energy utilization process has a large potential for energy-saving and emission reduction.The-40°C refrigerant consumption is mainly around the CO₂absorption tower.The absorber temperature,concentration and absorption degree of the decarbonisation process all affect the heat duty of the propylene coolers.The retrofit system achieved initial CO₂ separation by condensation and liquefaction of the syngas,which can reduce the subsequent processing load and refrigeration demand.While keeping the circulating methanol flowrate and decarbonisation targets constant,the dissolved CO₂ content in the semi-depleted methanol will also be reduced,helping to decrease the low-pressure steam demand in the regeneration process.The condensation system consists of 2 stages condensation,whose main source of refrigeration capacity is CO₂-rich condensate throttling refrigeration.Since the CH₄ content was liquefied at the same time,a methane recovery column was added to enhance the CH₄ recovery to the original level.The utility costs of single equipment were decreased from 93.45 million to 89.51 million,accounting for 8.1%.An annual revenue of 15.08 million can be achieved in a coal-to-SNG plant with an annual production capacity of 1.33 billion Nm³.The total investment cost can be recovered within 4 years.In response to the situation of surplus low-grade waste heat,technological innovation and integration were carried out for the efficient recovery and waste heat utilization.A wider temperature interval of waste heat recovery and higher efficiency of waste heat utilization contribute to the improvement of the overall yield of waste heat utilization.Low-temperature waste heat divided by 105℃were used for waste heat recovery and utilization in the refrigeration and heating systems,respectively.In this paper,a rigorous return temperature of cooling water（33°C）was selected to enhance the adaptability of cooling condition.The innovative double-stage ammonia absorption refrigeration system recovers low-grade waste heat above 105°C through the optimization of bubble temperature at different pressures and concentrations,meeting the demand for-40°C refrigeration capacity in the Rectisol unit.For the waste heat recovery and utilization below 105℃,it is possible to realize the cascade recovery of waste heat by absorption heat pump and organic Rankine cycle.The condensing heat from waste heat generation can also be used in the low-temperature heat sink of the absorption heat pump.By optimizing the technical parameters of heat pumps through technologies combination,the efficiency and economic performance for heating and power generation can be improved.The COP of refrigeration and heating can reach 0.31 and 1.77.The techno-economic analysis shows that the proposed absorption refrigeration system decreased the utility costs and carbon emissions of original vapor compression refrigeration systems by84%and 87%,respectively.The proposed absorption heat pump system decreased the utility costs and carbon emissions of original heat exchange station by 55%and 50%,respectively.It shows that the waste heat utilization system has a good performance in energy-saving and emission reduction.The multi-period waste heat cascade recovery and utilization were carried out for the supply and demand contradiction between total site heat sources and heat sinks under different seasons and periods.The utilization strategy of total site waste heat was proposed,and the cascade waste heat recovery can be achieved through temperature interval classfication and technology combination.The main waste heat refrigeration solutions are direct refrigeration with low-temperature waste heat above 105°C and indirect refrigeration with integrated sub-low-temperature power generation below 105°C.The heating solution consists of 2 types of absorption heat pumps with waste heat recovery.The combination of low-pressure steam and condensing heat from waste heat power generation accounts for over 90%of the heating water supply.Surplus low-pressure steam in the non-heating season was recovered and utilized by a variable waste heat power generation system in addition to enhanced DARS refrigeration.In summary,waste heat refrigeration replaced 89.1%-97.5%of the-40°C propylene refrigeration capacity,absorption heat pumps completely replaced steam heating,and variable power generation achieved the reuse of 10-30 t/h surplus steam.The results show that the utility cost of refrigeration,heating and variable power generation can be reduced by 67%,59%and 20%,respectively.The total site waste heat utilization decreased the total cost from 186 million CNY/year to 67 million/year,accounting for 64%.The uncertainties in the coal chemical production process will be transferred to the generated low-grade waste heat.If only deterministic optimization of the system is performed,it will not only result in larger redundancy in equipment size,but also affect the stable operation of the system during larger fluctuations.In order to improve the stability of the waste heat cogeneration system,the system operation flexibility was enhanced by a two-stage flexible design.Taking the waste heat utilization of gas liquor separation and phenol ammonia recovery as an example,the design solution under uncertainty of each scenario was explored.The representative operating scenarios were obtained by data mining.Design parameters under robust optimization for each scenario were obtained through the first-stage flexible design,which ensures that the system can be operated under conservative situations.The overall system improves the flexibility of the waste heat utilization system to 86%at a smaller economic loss within 8%.The time series prediction of uncertain parameters was achieved by the second-stage flexible design.Then rolling optimization was carried out to further enhance the smooth variation between waste heat and refrigeration demand.The total cost for the selected period was decreased by 10.7%compared to the scenario without rolling optimization.Finally,the ORC real-time optimization scheme was explored to make full use of the waste heat,with an increase in power generation of 27%.