What are the key technical challenges in coal pyrolysis? How to break through?

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This post was last edited by liaifeng on 2018-7-27 16:31. What are the key technical challenges in coal pyrolysis? How to break through? Author/Source: Huahua Network Coal Chemicals Date: 2018-07-04 Clicks: 3 In recent years, although new technologies for coal pyrolysis in China have seen rapid development, key technical challenges such as low tar yield and difficulties in dust treatment remain unresolved. “To address these issues, in addition to some conventional solutions, combined production with technologies such as combined cycle power generation is also a good option. ”At the 2018 China Lignite Industry Green Development and Application Innovation Conference held in Shenmu, Shaanxi from June 27 to 28, experts present put forward their suggestions. Larger scale operation is becoming a trend. Coal pyrolysis is a process in which coal is heated at moderate to low temperatures in an air-free environment, causing the tar and gas contained within the coal to evaporate, thereby producing tar, gas, and blue carbon (semi-coke). It represents a key step in the selective utilization of coal. “Low-rank coal accounts for more than half of China’s total coal volume, reserves, and production. By using medium- and low-temperature pyrolysis to produce semi-coke, it is possible to obtain coal tar and raw gas as by-products, representing a direction for the clean and efficient utilization of coal. ”Shang Jianxuan, deputy general manager of Shaanxi Coal and Chemical Industry Group and director of the **Key Laboratory for High-Quality Clean Conversion of Energy Coal, said that they have successfully developed a number of technologies for coal pyrolysis and the utilization of semi-coke, and are currently working on industrial demonstrations of the application of semi-coke of different particle sizes. Ma Baoqi, the lead scientist at the Yulin Coal Chemical Industry Development Promotion Research Center, also said that after nearly 40 years of rapid development, a wide range of technologies for the pyrolysis of low-grade coal has emerged in China. There are more than 10 types of pyrolysis reactors currently in use for pilot testing and industrial production, including vertical furnaces, fluidized bed reactors, gas-flow bed reactors, moving-bed reactors, rotary furnaces, chain grate furnaces, and microwave furnaces. The annual processing capacity per reactor has increased from tens of thousands of tons, to 100,000 tons, to 300,000 tons, and now to over 500,000 tons. Among them, the 500,000 tons per year regenerative hydrogen-enriched gas heat carrier moving bed coal pyrolysis industrial test facility built by Shaanxi Coal Group in Yulin is the largest vertical furnace system in China capable of long-term stable operation. At present, the technical process package is being revised and improved to accelerate the development of industrial projects on a million-ton scale. The rotating bed low-temperature pyrolysis technology developed by Henan Longcheng Group has been put into use at the coal clean utilization production line in Caofeidian, Hebei, with each unit having a processing capacity of 800,000 tons per year. Shaanxi Shuangyi Coal Chemical Technology Industry Co., Ltd. also constructed a 2 million tons per year coal pyrolysis and diversified utilization project in Shenmu, Yulin, last year; the capacity of each unit for dry distillation of coal dust is 200,000 tons per year, and it is expected to be operational by October this year. In addition, the coal value-added utilization project for producing chemical new materials that has already been launched by Shaanxi Coal Group, the 10 million tons per year integrated project for the clean and efficient comprehensive utilization of pulverized coal that Longcheng Group is building in Yulin, and the large-scale coal clean comprehensive utilization project centered on coal pyrolysis that Yan’an Petroleum plans to establish in the Yushen Industrial Park are all included in the **13th Five-Year Plan**. Breaking through bottlenecks and overcoming challenges: Experts believe that although China’s coal pyrolysis technology has developed rapidly, there are still some bottlenecks. For example, the added value of the products is low, environmental protection measures need to be improved, and resources are not being fully utilized. The coal dust pyrolysis process suffers from shortcomings such as immature technology, and the need to make breakthroughs in key technologies and major equipment; among these, low tar yield and dust treatment techniques are the main constraints. Pei Xianfeng, director of the Coal Coking Quality Testing and Process Technology Research Institute under the Coal Chemical Engineering Division of the Coal Science and Technology Research Institute, told reporters that the new low-rank coal pyrolysis technology using a rotating moving bed developed by this institute follows the principle of \"controlling dust at the source, reducing dust inside the furnace, and removing dust precisely.\" By combining moving-bed pyrolysis with dust removal techniques, this approach addresses the dust problem at its source, thereby reducing the difficulties associated with subsequent separation processes. At the same time, it utilizes a unique external indirect heating method in a horizontal pyrolysis reactor; the material moves while being mixed, which ensures uniform heating and pyrolysis, thereby enabling effective control over the quality of coal tar products. The resulting tar has a clear appearance, with a tar yield of 7.6% and a dust content of less than 1%. This technology is now set to be put into industrial use in Ordos. Yang Shengzhi, manager of Beijing Guodian Futong Technology Development Co., Ltd., explained that in late June, the China Petroleum and Chemical Industry Federation organized experts to conduct on-site calibration of the Guofu Furnace (GF) regenerative hydrogen-rich gas heat carrier moving bed coal blending pyrolysis industrial demonstration unit. The results showed that the tar yield reached 87.15% of the tar yield produced from the type of coal used, while the proportion of useful gases in the gas reached 84.4%. Gas can be used as a raw material for hydrogen production or LNG processing, effectively improving the tar yield and the quality of semi-coke. The reporter learned that Shaanxi Coal Group is accelerating the construction of a 1.2 million tons per year pilot plant for rapid thermal pyrolysis of pulverized coal gas, with the goal of achieving a tar yield that is over 150% higher than that obtained through dry distillation. In the field of advanced processing of coal tar, a pilot plant for the hydrogenation of coal tar using a suspended-bed reactor with a capacity of 300,000 tons per year to produce aromatics, as well as a demonstration project for the hydrogenation of the entire fraction of coal tar to produce naphthenic oils with a capacity of 500,000 tons per year, are also under construction simultaneously. Cogeneration improves utilization efficiency. \"The pyrolysis gas (raw gas) produced as a by-product of coal pyrolysis is actually a secondary energy source, and some of its valuable components should be fully utilized to produce chemicals with higher added value.\" ”Jin Yong, an academician of the Chinese Academy of Engineering and a professor at Tsinghua University, said so. Gas typically contains 54%–59% hydrogen and 24%–28% methane, and hydrogen can be produced directly through high-temperature pyrolysis to further develop new energy sources such as hydrogen fuel cells. At the same time, the temperature of the gas escaping from the carbonization chamber is between 650°C and 800°C, and the heat carried by this gas accounts for 36%; cooling is required using circulating ammonia water, and this heat is wasted needlessly. He suggested that the escaped gas could be fed directly into the pyrolyzer without cooling, where the organic materials are pyrolyzed into useful synthetic gases, which can then be further processed into chemical products. Furthermore, Jin Yong suggested that coal pyrolysis could also be combined with combined cycle power generation: semi-coke is used as fuel in boilers, and the steam generated is utilized to drive steam turbines for power production; after oil removal, the gas is fed into gas turbines to generate electricity, which can increase energy efficiency by 55% to 60%. Based on the calculation that 320 standard cubic meters of raw gas are produced per ton of dry coal during pyrolysis, China’s annual production of raw gas is estimated to be 36 million standard cubic meters. “Conventional low-temperature pyrolysis at 600°C is not reasonable. ”In the view of Cheng Xiangkui, chairman of Shanxi Changxiang Technology Co., Ltd., low-grade coals contain large amounts of hydrocarbon compounds. In low-rank coals, these hydrocarbons account for 30% to 35% of the total mass of the anhydrous coal; therefore, a staged pyrolysis approach should be adopted to increase the yield of hydrocarbons. Moreover, the hydrocarbon recovery efficiency is related to the number of sections; the more sections there are, the higher the retention rate of hydrocarbons. Cheng Xiangkui explained that 380°C to 450°C is the optimal temperature range for maintaining the integrity of hydrocarbons. The medium- and low-temperature staged pyrolysis unit developed by this company is capable of indirectly heating coal at different temperatures; it carries out staged pyrolysis to produce gas, and then purifies the gas of various compositions separately, thereby making subsequent processing steps more efficient and cost-effective. The hydrocarbon-poor gas obtained in the high-temperature stage has a CO content of around 30% and a H2 content of over 50%, making it suitable for hydrogen extraction or use as synthesis gas; the hydrocarbon-rich gas already reaches an economically viable level of gaseous hydrocarbons without the need for further enrichment. After separation, about 40% of the gaseous hydrocarbons can be used to produce natural gas and liquefied gas, while cryogenic processing can be employed to produce liquid methane (LNG) ； About 50% of the remaining components are light tar and other light hydrocarbon oils, which can serve as high-quality raw materials for the production of gasoline and diesel.