Pollution Prevention and Treatment in the Coal Chemical Industry     

By Pan Liansheng  

Coal is a major energy source and raw material of the chemical industry and a complex material composed of organic and inorganic chemical substances. Deriving energy from coal will inevitably produce some pollutants while converting such a complex, unclean material into clean energy and chemicals through physical and chemical processes. In general, coal can generate much more pollution than hydrocarbons (such as oil, natural gas). For the healthy development of the coal chemical industry, the pollution problem must be solved.

The coal chemical industry must address its pollution problem for healthy development.  

The coal chemical industry can produce a variety of chemical products and energy chemicals. The type and level of pollution caused by production varies with the processes. In recent years, the development of the coal chemical industry has mainly focused on gasifying coal into syngas and then turning the syngas into a variety of chemicals by deep processing. Coal gasification is a key technology for the production of various coal-based chemicals (ammonia, methanol and dimethyl ether, etc.), coal-based liquid fuels and coal-based low-carbon olefins, as well as for hydrogen generation, advanced IGCC power generation and the polygeneration system. So it is the base of the coal chemical industry.
   Different coal gasification processes cause different levels of pollution. For example, the Lurgi gasification process can generate much more pollution than the Texaco gasification process. The gasification of lignite and soft coal can produce much more pollution than the gasification of anthracite and coke. Different gasification processes leave different pollutants in the wastewater. Compared with the fixed bed process, the fluidized-bed and entrained bed processes can produce less polluted wastewater.
   Large-scale coal chemical enterprises mainly adopt the following coal gasification processes: the GE -Texaco coal-water slurry pressurized gasification process (chilling process), the GE multi-nozzle opposed coal-water slurry pressurized gasification process (chilling process), Shell's dry coal powder pressurized gasification process which is now undergoing an adaptability test, the GSP dry coal gasification process (chilling process) which is now adopted in a demonstration project, the GSP two-stage dry coal gasification process (chilling process), and the GSP multi-nozzle opposed dry coal powder gasification process (chilling process) that has passed a pilot-scale test. These gasification processes each have led several pollutants. The first pollutant is the crude slag discharged by the process of chilling molten slag. For example, 60% of the total ash content of coal is in the slag discharged by the Shell dry coal powder pressurized gasification process. The slag discharged in the GE coal-water slurry pressurized gasification process or the GSP process contains 85% of the total ash content of the coal. If the crude slag can be properly stacked or utilized (used as raw materials for road building and cement production, for example), environmental pollutant can be avoided. The second pollutant is the fly ash discharged from the high-temperature and high-pressure fly ash filter of the Shell coal gasification unit. The fly ash discharged by this unit includes about 34% of the total ash content of the coal. For example, a coal gasification plant with a capacity of processing 2000 t/d dry coal powder that has an ash content of 20% can discharge 136 tons of fly ash a day. Using the fly ash comprehensively is a major issue worthy of attention. If fly ash is not consumed promptly but is casually stacked, it can pollute the surrounding environment. The third pollutant is black water discharged by the coal gasification unit. The black water can be recycled after being treated with flocculants. The remaining gray water can be discharged after ammonia and cyanides are removed from it. After pressurized filtering, the ash that had been deposited in the black water is pressed into cakes which can be used as a fuel for the coal gasification unit. Hydrogen sulfide (H2S) in syngas can be removed and recycled in the follow-up acidic gas removal process. A small amount of hydrogen sulfide-containing exhaust gases from the flash evaporation of the black water can be recycled and used or sent to the combustion chamber.
   Industrial demonstration tests are being done for the direct or indirect liquefaction of coal for oil and MTO (methanol-to-oil). It is believed that the formal assessment reports of the tests will come up with specific pollution treatment approaches. According to related literature, the pollution of these processes is controllable and treatable. Of course, this needs advanced technologies and more investment.

In general, four issues should be considered for pollution prevention and treatment in the coal chemical industry.  

1. As long as the coal chemical industry pays adequate attention to preventing and treating pollution, adopts various advanced technologies and equipment and applies sufficient investment in infrastructure construction and regular operations, most of the pollution can be treated and controlled.
   International experience shows that a large-scale modern petrochemical plant needs to put over 20% of its total infrastructural investment into its environmental and safety facilities. For a large-scale modern chemical plant, this figure is about 16%. The coal chemical industry is a heavy polluter, so it needs to invest more in environmental safety provision.

2. The focal and difficult point of pollution treatment in the coal chemical industry is wastewater.

Coking wastewater contains phenol, ammonia, cyanides, hydrogen sulfide, and hydrocarbons. Wastewater from the synthesis of ammonia, alcohols and hydrocarbons by coal gasification contains alcohols, acids, ketones, aldehydes, esters and other organic substances. The wastewater contains much chemical oxygen demand (COD) and biological oxygen demand (BOD). Some organic substances in the wastewater are not readily biodegradable. Much effort including scientific research must be made for the harmless treatment of the wastewater.
   China has only 6% of the world's freshwater. Freshwater shortage and water pollution have become two major problems faced by China. Already, China's surface water has been severely polluted. Its seven major river systems have been moderately polluted, and lake eutrophication has become a prominent problem. About 40% of the total water volume of the rivers is industrial wastewater. In China, the chemical industry discharges about 30% of the country's total wastewater - more than any other industry.
   Due to rapid economic and social development, irrational geographic distribution of industries and production accidents, pollution cases have frequently occurred in the middle and upper reaches of the Yellow River basin in recent years. In 2005, the water there was seriously polluted, mainly by organic pollutants. Sixty one percent of the 18 national controlling water cross-sections did not meet the national Grade III water standard. The surface water of the Wei and Fen rivers in Shaanxi province is scarce, and their pollution is very serious. In 2005, the discharged wastewater and COD in the middle and upper reaches of the Yellow River basin were 2.91 billion tons and 1.235 million tons, respectively, and the average COD content in the discharged wastewater was 60% higher than the national average. According to the socio-economic development forecast, the basin-wide waste emissions in 2010 will be over 25% more than in 2005, so the water environment pressur