1、 The Development of Coal Chemical Industry in China
The coal chemical process is the industrial process of converting coal into gas, liquid, and solid products or semi-finished products, and then further processing them into chemical and energy products. Including coking, coal gasification, coal liquefaction, etc.
Coking is the earliest and still the most important method in various chemical processing of coal. Its main purpose is to produce metallurgical coke, while producing by-products such as coal gas and aromatic hydrocarbons such as benzene, toluene, xylene, naphthalene, etc.
Coal gasification also plays an important role in coal chemical industry, used for the production of urban gas and various fuel gases (widely used in industries such as machinery and building materials). It is a clean energy source that is conducive to improving people's living standards and environmental protection; It is also used in the production of synthesis gas (as a raw material for the synthesis of ammonia, methanol, etc.), and is a raw material for the synthesis of various products such as liquid fuels.
Direct coal liquefaction, also known as high-pressure hydrogenation liquefaction of coal, can produce artificial petroleum and chemical products. In times of oil shortage, coal liquefaction products can replace current natural oil.
The characteristics of China's energy endowment are "lack of oil and gas, relatively abundant coal resources", and relatively low coal prices. The coal chemical industry in China is facing huge market demand and development opportunities.
The new coal chemical industry will play an important role in the sustainable utilization of energy in China and is an important development direction for the next 20 years. This is of great significance for China to reduce environmental pollution caused by coal combustion, reduce dependence on imported oil, and ensure energy security.
The new coal chemical industry mainly produces clean energy and products that can replace petrochemicals, such as natural gas, diesel, gasoline, aviation kerosene, liquefied petroleum gas, ethylene raw materials, polypropylene raw materials, alternative fuels (methanol, dimethyl ether), etc. When combined with energy and chemical technologies, it can form an emerging industry of coal energy chemical integration.
At present, new coal chemical projects in China are rapidly developing and blooming everywhere. In Xinjiang alone, there are 14 coal to natural gas projects under construction or planned. According to incomplete statistics, the under construction and planned production capacity of coal to olefin in China has reached 28 million tons, coal to oil has reached 40 million tons, coal to natural gas has approached 150 billion cubic meters, and coal to ethylene glycol has exceeded 5 million tons. After all these projects are completed, China will become the world's largest producer of new coal chemical industry.
2、 The significance of zero discharge of coal chemical wastewater
2.1 Water conservation
The new coal chemical industry consumes a huge amount of water. For large-scale coal chemical projects, the water consumption per ton of product is over ten tons, and the annual water consumption is usually as high as tens of millions of cubic meters. The rapid development of coal chemical industry has triggered an imbalance between regional water resource supply and demand. China's coal resources are mainly concentrated in the north and northwest, where water resources are severely lacking. At present, water rights disputes have emerged in these areas. If this situation continues to develop, it will affect the normal development of local industry and agriculture, and also bring many social problems.
Zero discharge of coal chemical wastewater and maximum reuse of wastewater can save water resources and alleviate the severe shortage of water resources.
2.2 Protecting the ecological environment and avoiding water and groundwater pollution
Coal chemical enterprises consume a large amount of water, and the wastewater they discharge mainly comes from processes such as coal coking, gas purification, and chemical product recycling and refining. This type of wastewater has a large volume and complex water quality, containing a large amount of organic pollutants such as phenols, sulfur, and ammonia, as well as toxic pollutants such as biphenyl, pyridine indole, and quinoline, which are highly toxic. In areas with abundant coal resources, such as Yili region in Xinjiang, Ningxia, Inner Mongolia and other coal chemical bases, implementing zero emissions can effectively protect the ecological environment and avoid water and groundwater pollution.
2.3 Significance of Zero Emissions
Zero emissions "refers to the treatment of production wastewater, sewage, and clean wastewater generated during the coal chemical industry, all of which are reused without discharging wastewater to the outside world, known as" zero emissions ". For the coal chemical projects currently under construction or planned in the northwest region, "zero emissions" are particularly important, which not only solves some water resource problems, but also does not cause pollution and damage to the local environment and ecology.
3、 Characteristics of Coal Gasification Wastewater
Source and characteristics of gasification wastewater: during coal gasification, some nitrogen, sulfur, chlorine and metals contained in coal are partially converted into ammonia, cyanide and metal compounds during gasification; Carbon monoxide reacts with water vapor to produce a small amount of formic acid, which then reacts with ammonia to produce formic acid ammonia. Most of these harmful substances are dissolved in the washing water, gas washing water, separated water after steam separation, and tank drainage during the gasification process, and some are vented during equipment pipeline cleaning.
For coal gasification technology, there are currently three main types: fixed bed, fluidized bed, and fluidized bed; For furnace types, there are various types such as fixed bed interval gasification furnaces, ash fusion furnaces, Texaco furnaces, and Ende furnaces. The drainage water quality of fixed bed, fluidized bed, and fluidized bed gasification processes is shown in the following table:
4、 Coal gasification wastewater treatment technology
4.1 Water quality of coal gasification wastewater after phenol ammonia recovery
The wastewater generated by the three gasification processes has a high ammonia content; The phenol content produced by the fixed bed process is high, while the other two are relatively low; The fixed bed process has a high tar content, while the other two have lower tar content; The formic acid compounds produced in the gas flow furnace process are relatively high, while the other two processes do not produce much; Cyanide is produced in all three processes; The fixed bed process produces the most organic pollutant COD and causes the most severe pollution, while the other two processes have less pollution.
The wastewater from the above three processes cannot be directly subjected to biochemical treatment without pretreatment, especially with high ammonia content and high phenol content in the Lurgi furnace.
For the wastewater from the Lurgi furnace, a phenol ammonia recovery device is required for pre-treatment and recovery; The gasification wastewater from fluidized bed and fluidized bed processes requires ammonia recovery pretreatment. The water quality of each wastewater after pretreatment is as follows:
4.2 Coal gasification (fixed bed process) wastewater biochemical treatment process
The CODcr concentration of gasification wastewater from fixed bed process is high, which belongs to organic wastewater and contains a large amount of ammonia nitrogen and phenol. It has a certain chromaticity and the following characteristics:
(1) The concentration of organic matter in sewage is high, with a B/C value of about 0.33, and biochemical treatment technology can be used.
(2) Wastewater contains recalcitrant organic compounds such as monophenols, polyphenols, and other substances containing benzene rings and heterocycles, which have certain biological toxicity. These substances are difficult to decompose in aerobic environments and require ring opening and degradation in anaerobic/facultative environments.
(3) The concentration of ammonia nitrogen in sewage is high, making it difficult to treat. Therefore, it is necessary to use treatment processes with strong nitrification and denitrification capabilities. Coal gasification wastewater treatment technology
(4) Wastewater contains floating oil, dispersed oil, emulsified oil, and dissolved oil substances, with the main components of dissolved oil being aromatic compounds such as phenols. Emulsified oil needs to be removed by air flotation, while soluble phenolic substances need to be removed by biochemical and adsorption methods.
(5) Containing toxic inhibitory substances such as phenols, polyphenols, and ammonia nitrogen in wastewater, it is necessary to improve the anti toxicity ability of microorganisms through domestication and select appropriate processes to enhance the system's impact resistance.
(6) The impact of abnormal sewage discharge, when there are problems in the production process, can lead to the discharge of high concentration of pollutants in abnormal sewage, which cannot directly enter the biochemical treatment system and requires measures such as accident regulation.
(7) The wastewater has a high chromaticity and contains some substances with color developing groups.
Therefore, in order to ensure the quality of the effluent from the process wastewater treatment, a biochemical treatment process with the main focus on removing CODcr, BOD5, ammonia nitrogen, etc. (mainly considering nitrification and denitrification) is selected for the process wastewater, a pretreatment process with the main purpose of oil removal and decolorization is selected, and a post-treatment enhancement process with the main focus on physical-chemical treatment is selected. The adopted process is as follows:
4.3 Biochemical treatment process for gasification (fluidized bed and fluidized bed) wastewater
The wastewater generated by fluidized bed and fluidized bed processes has low COD and good biochemical properties (especially the wastewater generated by fluidized bed processes). The main characteristic of these wastewater is high ammonia nitrogen, and treatment processes with good nitrification and denitrification effects should be selected.
However, biochemical treatment only removes organic pollutants, oil, ammonia, phenols, cyanides, etc. from wastewater, and cannot remove salts from the wastewater.
5、 Zero discharge of coal gasification wastewater
5.1 Classification of Coal Chemical Drainage
The drainage of coal chemical industry in production includes: production wastewater, domestic wastewater, clean sewage, initial rainwater, etc. The main production wastewater is gasification wastewater; Clean wastewater mainly comes from circulating water discharge and concentrated saltwater discharged from desalination stations; The initial rainwater is mainly collected in the first ten minutes of contaminated areas.
The larger amounts of water in the above-mentioned drainage are clean wastewater and production wastewater. Generally, it is considered to collect clean wastewater separately from production wastewater, domestic wastewater, initial rainwater, etc., which are divided into two categories: clean water and sewage.
5.2 Reuse of sewage
The coal chemical production process requires a large amount of circulating water, and the scale of the circulating water station is generally large, requiring a large amount of supplementary water. When considering the reuse of clean wastewater and sewage treatment effluent, it is generally considered to reuse it as supplementary water for circulating water stations.
Although the effluent from the sewage treatment plant removes a large amount of organic pollutants, ammonia, phenols, and other substances, its salt content has not decreased. The salt content in clean wastewater and concentrated saltwater from desalination stations is generally 4-5 times higher than that of raw water. Therefore, in order to reuse sewage, desalination treatment is required, otherwise salt will circulate and accumulate in the system.
5.3 Types of reclaimed water reuse processes
At present, the desalination processes for water that have been applied in China include chemical desalination (i.e. ion exchange desalination), membrane separation technology, distillation desalination water treatment, and desalination processes combining membrane and ion exchange methods.
(1) Ion exchange desalination process
The ion exchange water treatment technology is quite mature and suitable for applications with low salt content in water. However, when treating high chloride, high salt, high hardness water, brackish water, and seawater, this technology has the disadvantages of consuming a large amount of acid and alkali during resin regeneration, and polluting the environment with its discharged liquid.
(2) Membrane desalination process
With the progress of membrane research, membrane separation technology has rapidly developed, and the field of membrane use is becoming more and more extensive. It has become an industrialized high-tech, with the advantages of easy operation, compact equipment, safe working environment, energy saving and chemical saving. Its main separation process is reverse osmosis technology, and ultrafiltration and fine filtration technologies are used as pre-treatment processes for reverse osmosis. It can be combined into various processes based on the different water qualities of the raw water.
(3) Desalination process combining membrane method and ion exchange method
The desalination system composed of reverse osmosis membrane method and ion exchange method is currently a widely used desalination water treatment system. In this system, reverse osmosis serves as a pre desalination system for ion exchange, removing over 95% of the salt and the vast majority of other impurities such as colloids, organic matter, bacteria, etc. from the raw water; The remaining salt in the reverse osmosis produced water is removed through subsequent ion exchange systems.
5.4 Selection of Wastewater Reuse Process
The mixed water from sewage treatment plants and clean wastewater is reused, with a generally large water volume and a low salt content between 1000-3000mg/L. If distillation method is directly used, it requires a large amount of heat source and wastes energy, which is not suitable. Due to the presence of certain organic pollutants in wastewater, using ion exchange resin may clog the resin. Additionally, since the water quality requirements for recycled water are not high, ion exchange is not suitable; With the improvement of membrane separation technology and membrane production processes, the service life of membranes is constantly increasing, and the price of use is constantly decreasing. The use of membranes is becoming more and more popular. It is recommended to prioritize the use of dual membrane methods (ultrafiltration+reverse osmosis) in the main process of wastewater reuse, and to pretreat wastewater according to the different characteristics of water quality to meet the conditions for the use of dual membranes.
5.5 Concentrated saltwater membrane concentration
Many companies both domestically and internationally are researching the membrane re concentration of concentrated saltwater produced by the double membrane method to achieve a salt content of 60000 to 80000 mg/L. This aims to increase the salt content in wastewater as much as possible, reduce the scale of subsequent evaporators, decrease investment, and save energy.
The commonly used processes internationally include Aquatech's HERO membrane concentration process, GE's nanofiltration membrane concentration process, Veolia's OPUS membrane concentration process, and Maiwang's vibrating membrane concentration process. The above process has achieved success in salt concentration abroad. Some domestic companies are also researching membrane concentration processes, but there are currently no achievements or engineering examples of their use.
5.6 Evaporation
After reaching a salt concentration of 60000 to 80000 mg/L in concentrated saline water, evaporation is carried out. In foreign countries, the evaporation process for wastewater generally adopts the "falling film mechanical steam compression recirculation evaporation technology", which is currently the most reliable and effective technical solution for treating high salt wastewater in the world. When using mechanical compression recirculation evaporation technology to treat wastewater, the heat energy required for evaporating wastewater is mainly provided by the heat energy released or exchanged during steam condensation and condensate cooling. During operation, there is no loss of latent heat. The only energy consumed during operation is the water pump, steam compressor, and control system that drive the circulation and flow of wastewater, steam, and condensate in the evaporator.
When using steam as thermal energy, 554 kcal of thermal energy is required to evaporate every kilogram of water. When using mechanical compression evaporation technology, the typical energy consumption for treating one ton of saline wastewater is 20 to 30 kWh of electricity, which means that only 28 kcal or less of heat energy is needed to evaporate one kilogram of water. The efficiency of a single mechanical compression evaporator is theoretically equivalent to that of a 20 effect multi effect evaporation system. Adopting multi effect evaporation technology can improve efficiency, but it increases equipment investment and operational complexity. Evaporators can generally increase the salt content in wastewater to over 20%. Usually sent to an evaporation pond for natural evaporation and crystallization; Alternatively, it can be sent to a crystallizer for crystallization and drying into a solid, and then sent for disposal.
6、 Introduction to Domestic Zero Emission Project Cases
Yili Xintian's 2 billion cubic meter coal to natural gas project
Ø Tuke Fertilizer Project Phase I of middling coal Ordos Energy and Chemical Co., Ltd. with an annual output of 1 million tons of synthetic ammonia and 1.75 million tons of urea
Ø China Power Investment Corporation Yinan 3 × 2 billion Nm 3/a Coal to Natural Gas Project Phase I 2 billion Nm 3/a Project
Shenhua Coal Direct Liquefaction Project
Zero emission project performance
6.1 Yili Xintian Annual Production 2 billion cubic meters Coal to Natural Gas Project (General Contracting)
Ø Gasification process: Crushed coal pressurized fixed bed gasification technology (Luqi furnace)
Ø Project Product: Annual production of 2 billion Nm 3 of natural gas
Ø Sewage treatment system content:
Sewage treatment plant: 1200m3/h
Wastewater reuse:
① Biochemical wastewater reuse unit: 1200m3/h
② Salt containing wastewater reuse unit: 1200m3/h
③ Multi effect evaporation unit: 300m3/h
6.2 Tuke Fertilizer Project (EPC) of middling coal Ordos Energy and Chemical Co., Ltd
Ø Gasification process: Pressure gasification technology for crushed coal slag (BGL)
Ø Project products: 1 million tons/year of synthetic ammonia and 1.75 million tons/year of urea
Ø Sewage treatment system content:
Wastewater treatment plant: 360m3/h
Reclaimed water treatment device: 1200m3/h
Concentrated salt water treatment device: 200m3/h
Processing technology:
Sewage treatment process flow
6.3 China Power Investment Corporation Yinan 3 × 2 billion Nm 3/a Coal to Natural Gas Project Phase I 2 billion Nm 3/a Project (Overall Design+Basic Design)
Gasification process: Pure oxygen fluidized bed gasification technology (GSP furnace)
Ø Project Product: Annual production of 2 billion Nm 3 of natural gas
Ø Sewage treatment system content:
Wastewater treatment plant: 280m3/h
Reclaimed water treatment device: 900m3/h
Concentrated salt water treatment device: 120m 3/h
Ø Processing technology:
Sewage treatment device: pre-treatment+secondary biochemistry+advanced treatment
Reclaimed water treatment device: pretreatment+ultrafiltration+reverse osmosis
Concentrated salt water treatment device: membrane concentration+evaporation crystallization
6.4 Shenhua Coal Direct Liquefaction (Coal to Oil) Project
Ø Sewage treatment system content:
Biochemical treatment section: including oily wastewater system and high concentration wastewater system
Salt treatment section: including salt containing wastewater system, catalyst preparation wastewater system, evaporator concentrate treatment system
Ø Processing scale:
Oily wastewater system: 204m3/h
High concentration sewage system: 150m 3/h
Salt containing sewage system: 286m3/h
Catalyst preparation wastewater system: 103m3/h
Concentrated salt water treatment system: evaporator, crystallization, evaporation pond area of about 12 square meters
