| Coal is the main part in China's energy mix and it will remain the foundation of China's national economic development for a long period in the future.Modern coal chemical industry is the main direction of clean and efficient utilization of coal,an important way to diversify petrochemical raw materials and a strategic choice to guarantee national energy security.China's modern coal chemical projects are mainly located in the mid-west region with the most distinctive feature of "coal-rich and waterscarce".With the development of industrial upgrading and demonstration,the shortage of water resources and "zero discharge" of wastewater have become an important bottleneck restricting the development of coal chemical industry.Modern coal chemical water system is mainly divided into process water,circulating water,power steam and wastewater,etc.In this paper,the theories and methods of chemical engineering,engineering thermophysics,environmental engineering and system engineering are applied to make a comprehensive analysis of coal chemical water system.The main process units and the operation characteristics of water system are studied,including: the chemical engineering principles of process water and reaction-generated water in the main units of coal chemical gasification,conversion and synthesis;the thermodynamic characteristics of heating steam and waste heat circulating water between units and units;analysis of the action process,production method and treatment route of coal chemical power steam system,and wastewater treatment and reuse system.On this basis,through a large number of literature analysis,the key research areas and contents of modern coal chemical water system optimization are proposed.Firstly,the circulating water system is the largest part of water consumption in coal chemical industry.This paper systematically compares and analyses the process flow of different open and closed circulating water cooling systems,and optimizes the topological structure of circulating water and air cooling systems.The technicaleconomic comparison model of circulating water cooling system based on the same boundary parameters is constructed and the key water consumption and economic indicators of different systems are analyzed.The selection ideas and method system of circulating water cooling system are proposed.The technical-economic comparison model of circulating water cooling system also provides theoretical basis and quantitative tools for the selection and design of technology and equipment.An optimization model based on air cooling system and circulating water cooling system is constructed,and a new circulating water system topology consisting of air cooler and water cooling tower is established.The ways to reduce the water consumption and annual average cost of the system are studied.The key points are to carry out the verification and analysis of the cooling water system considering the pre-air cooler and the cooling water system considering the branch pre-air cooler,and propose an optimization technical scheme.It is pointed out that the water-saving and anti-fog cooling system has obvious water-saving effect and has a broad promotion prospect.A water saving effect of 19% can be achieved.By optimizing the design of adding pre-air cooler to some high temperature circulating water backwater branches,water consumption can be reduced by 25%.Secondly,wastewater treatment and reuse is an important part of coal chemical water saving,and it is the key to whether the coal chemical project can truly "zero emissions".This paper systematically analyzes the characteristics of different wastewaters,focuses on the comparative analysis of the treatment process of high-salt wastewater,and builds a water-saving and techno-economic evaluation model based on “zero emission” of wastewater.Two integrated technical routes are emphatically analyzed,i.e.the integrated technical routes aiming at improving the utilization rate of wastewater and the integrated technical routes aiming at realizing the non-exhaustion of wastewater.An optimization scheme for deep treatment of concentrated brine is proposed,which can reduce the investment and operating cost of process technology and improve water reuse rate.It is found that among the seven process systems,mechanical enhanced evaporation crystallization technology has the lowest total annual cost and can achieve 75% recycling,which has great potential for promotion.Thirdly,using the method of whole life cycle analysis,the typical coal chemical technology industry chain and water treatment system are analyzed and compared,and the whole life cycle water consumption model of typical coal chemical technology industry chain is constructed,focusing on the energy consumption and water consumption analysis of the water system of typical coal chemical processes such as coal direct liquefaction,coal indirect liquefaction,coal to olefins,coal to methane,etc.The calculation results of circulating water cooling system are systematically analyzed and compared.Finally,in order to further understand the situation of water use and water consumption in modern coal chemical industry,understand the water system optimization and water saving potential of typical modern coal chemical projects,and verify the effects of innovative research on circulating water systems,wastewater treatment and reuse systems in this paper,an empirical study on the water system of a coal-to-olefins project(the first large-scale industrial demonstration project in China)is carried out.The water use units of the project are tested,the water system balance and main water indexes are analyzed,and the water system of the project is optimized according to the innovative content of the paper.The water consumption indicators of the project have been significantly improved: the fresh water replenishment of the circulating water system has been reduced from 1131 t/h before optimization to 479 t/h,with a drop of 57%;through advanced treatment and evaporation crystallization of saline wastewater,the recovery rate of reused water increased from 41% to 81.7%,so that the wastewater could not be discharged;the overall fresh water consumption of the project has been reduced from 2698 t/h to 1708 t/h after optimization.In addition,the water consumption per unit product has been reduced from 35.9 tons to 22.8 tons,and the comprehensive water-saving efficiency reached 36.7%.The characteristic analysis of water system in coal chemical industry,the study of circulating water system,wastewater treatment,reuse system and whole life cycle are discussed in this paper.At the source of water consumption,a characteristics for technical evaluation of circulating water cooling system is proposed.In the process of water consumption,a new topology optimization structure of circulating water and air cooler is proposed.At the terminal link,an optimization scheme for zero discharge of wastewater is proposed.These application innovative research results will provide important theoretical guidance for modern coal chemical water system optimization and specific engineering design. |
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