A Study On Nearly Zero Liquid Discharge Of High Concentrated Organic Wastewater Of Fixed-bed Coal Gasification

Currently,water resource is the major bottleneck restricting development of the coal chemical industry.To achieve zero liquid discharge of wastewater has become an imperative facing the coal chemical industry.Various individual wastewater treatment technologies used in the coal chemical industry,though having been tested by industrial practices,have not yet achieved satisfying effects in coal gasification wastewater treatment.At present,the insufficient of coal chemical wastewater treatment is not the study of the unit process but the whole system.Therefore,this paper conducts a quantitative and systematic research of how to achieve zero liquid discharge of coal chemical wastewater.To start with,benchmark case for fixed bed coal gasification wastewater treatment are established,which consist of six basic units,namely gas liquor separation,phenol&ammonia recovery process,biochemical treatment,advanced treatment,water reuse unit and multi-effect evaporation&crystallization.Next,a group of reference data of fixed bed coal gasification wastewater quality is proposed.The specific pollutants in the wastewater are divided into eight types,including insoluble oils,sour gases,ammonia nitrogen,phenols,heterocyclic compounds,monocyclic and polycyclic aromatic hydrocarbons,long-chain alkanes and fatty acids.After that,a mass balance calculation model for the benchmark case is built.Then,bottlenecks of the system are analyzed.Results show that incomplete elimination of polyphenols,PAHs,long-chain alkanes,and ammonia nitrogen is the main reason for substandard reuse water quality.In order to realize zero liquid discharge in the real sense,this research explores technical indexes for various key nodes.Finally,a new set of fixed bed coal gasification wastewater treatment processes,which shows good agreement with technical indexes of key nodes,is put forward.The export phenols concentration of the current process is as high as 500-700 mg/L.Most of them are polyphenols which easily lead to the broken down of the following biochemical treatment process due to its high toxicity for biochemical bacterial.This paper analyzes the drawback of the current process.The solvent used in the current process has a weak selectivity on polyphenols.Meanwhile,the pH value of the wastewater is in the range7.5-8.5 before extraction.Polyphenols are partly dissociated into ionic state which limits the extraction efficiency.Therefore,intensification and improvement of the phenols extraction process is carried out in this paper from the aspect of new solvent development and process innovation.The framework for this work design includes solvent development,extraction experiment,process synthesis,modeling and simulation.A compound solvent with 95%MIBK and 5%toluene for polyphenols removal is developed.Process synthesis of CO₂acidification and extraction is carried to develop a new polyphenols extraction process.Modeling and simulation of the process are conducted to determine the operation parameters.400 kg/h CO₂ is used to reduce the pH value of the wastewater to 5-6.Theoretical extraction stage number is set to 4 for the new solvent and the solvent/water ratio is 1:5.5?volume?.After the treatment of the new process,the polyphenols in raffinate could be reduced to 88mg/L.The removal efficiency of polyphenols reaches 96.5%.The most efficient treatment method for fixed bed gasification wastewater is distillation and extraction.Phenols and ammonia are recovered as products.However,energy consumption,in the form of low-pressure steam and mid-pressure steam,is relatively high in this process.Technological and economic performance of the wastewater treatment process developed by South China University of Technology?SCUT?was analyzed in this paper.Two integration processes based on SCUT wastewater treatment process are proposed in which solvent recovery system is thermally integrated.The results indicate that the energy consumption via these integrations is decreased by approximately 18%compared with the conventional SCUT process.The cost of wastewater treatment can be reduced to 0.2 CNY/ton of wastewater.