Process Synthesis Of Fixed-bed Coal Gasification Zero Liquid Discharge Based On Life Cycle And Multi-objective Optimization

There are many types and contents of pollutants in fixed-bed coal gasification wastewater,and the water quality is the worst in coal to natural gas,coal to olefin and coal to ethylene glycol process wastewater.The treatment of fixed-bed coal gasification wastewater is the key to technological breakthroughs in the zero liquid discharge treatment of wastewater from modern coal chemical processes.Therefore,it is of great significance to use the life cycle assessment method to analyze the zero liquid discharge process of fixed-bed coal gasification wastewater,to establish the simulation of the whole process,and to conduct comprehensive analysis of energy,exergy,economic and environmental impact comprehensively and multi-objective optimization.The life cycle water consumption,life cycle cost and total environmental impact of the six zero liquid discharge and the four standard liquid discharge process of the fixed-bed coal gasification wastewater with a capacity of 1800 t/h is analyzed.Based on life cycle water depletion analysis methods,after considering the indirect water consumption,the zero liquid discharge process reuses 1 m³ of water will consume an additional 1.64m³of fresh water.After taking into account external emissions,the zero liquid discharge process increases the life cycle cost by 76.6%.Based on greenhouse gases equivalent conversion method,the zero liquid discharge process also increases the CO_2-eq emissions by 22.5%.After internal normalization,the total environmental impact and total human health impact of the zero liquid discharge process are 1.15 and 1.03,respectively.It shows that the zero liquid discharge process of fixed-bed coal gasification wastewater can not meet the ideal requirements of water saving and environmental protection.The synergistic effects of different biochemical treatment technologies,such as upflow anaerobic sludge bed,membrane bioreactor,sequence batch activated sludge method and anaerobic aerobic process in the treatment of fixed-bed coal gasification wastewater are analyzed from the perspective of engineering,economic and environment（3E）.19 key performance indicators such as chemical oxygen demand removal rate and global warming potential are selected to build a 3E triangle model.Among them,the upflow anaerobic sludge bed process has the best 3E performance（0.5,0,0.5）,which is slightly better than sequential batch activated sludge process（0.42,0.58,0）,membrane bioreactor（0.28,0.46,0.28）and anaerobic aerobic process（0.3,0.04,0.67）.The weight dimension analysis shows that the change in engineering performance has the greatest impact on the 3E performance,and the chemical oxygen demand removal rate has the greatest impact on the results（0.16～0.33）.The whole process of zero liquid discharge treatment of fixed-bed coal gasification wastewater is simulated and a comprehensive analysis of energy,exergy,economy and environment is carried out.For the traditional process,the total energy consumption is403248 kW.The exergy output of the zero liquid discharge process is only 5.45%,the exergy loss of the phenol ammonia recovery unit is the largest,which accounts for89.60%.Economic analysis shows that the phenol ammonia recovery unit accounts for the largest part of the exergy loss and the highest cost.The results of environmental analysis show that the phenol and ammonia recovery,biochemical treatment and evaporation&crystallization unit have the serious impact on the environment.After external normalization,the eutrophication potential is one of the most important indicators and the total environmental impact is 4.78×10^-8.A low stripper pressure flow is proposed for the key unit of phenol and ammonia recovery.The results show that when the stripper pressure increased from 0.2 MPa to 0.6 MPa,the TEC increased from396969 kW to 403731 kW.In addition,the total energy consumption of multi-objective optimization scheme is 402764 kW and the CO₂ content is 0.1%.