【Ten Years of Great Progress in Chemical Engineering Technology】155-2018 Industrialized Technology for Segmented, Multi-Layer Pyrolysis of Low-Rank Coal Using Gas as a Heat Carrier (SM-GF)
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Our country is rich in low-rank coal resources, which account for more than half of the total coal supply. Low-rank coal is characterized by high volatiles, low ash content, and good chemical reactivity; medium- and low-temperature pyrolysis is the best method for its clean and diversified utilization based on its different quality characteristics. Although pyrolysis technology has a long history in China and new technologies continue to emerge, technical challenges such as coal powder pyrolysis and environmental protection treatments still need to be overcome. In particular, there are very few large-scale industrial facilities with a capacity of 500,000 tons per year or more. The SM-GF industrialized technology for the pyrolysis of low-rank coal using gas as a heat carrier, developed by Shaanxi Coal and Chemical Industry Group and Beijing Guodian Futong Technology Development Co., Ltd., has opened up new paths for the large-scale processing of low-rank coal in China, and won the First Prize for Scientific and Technological Progress from the China Petroleum and Chemical Industry Federation. In July 2018, the SM-GF technology passed an evaluation of scientific and technological achievements organized by the Petrochemical Federation in Beijing. Experts believed that this technology enabled the industrial-scale low-temperature pyrolysis of low-grade coal; the yields of gas, tar, and semi-coke were high, their quality was good, and their value was significant. The process could be adjusted flexibly, and the installation could operate for extended periods. Its overall performance and indicators were at the international leading level, prompting recommendations to accelerate its promotion and application. It is reported that this technology focuses on the pyrolysis of mixed coals with different particle sizes below 30 millimeters. It integrates key technologies such as segmented multi-layer processing, uniform heat and mass transfer, efficient separation of oil, dust, and gas, use of self-produced hydrogen-rich gas as a thermal carrier for regenerative heating, staged pyrolysis at medium and low temperatures, and dry quenching. The developed segmented multi-layer vertical rectangular pyrolyzer has a processing capacity of 500,000 tons per year; it features a multi-layer structure that enables dryening, dry distillation, and cooling in sequential stages, thereby achieving self-dust removal and uniform distribution of material and gas within the furnace. This approach addresses various technical challenges associated with the industrialization of mixed coal pyrolysis, including limited processing scale per unit facility and low efficiency in separating oil, dust, and gas. The main innovations of this technology lie in the use of mixed coal for staged, multi-layer pyrolysis in the furnace, which results in lower raw material costs; precise temperature control enables adjustable product quality; the adoption of medium- and low-temperature coupled pyrolysis leads to high tar yields and a higher content of light components; the pyrolysis temperature can be controlled flexibly, reducing the amount of heat carrier needed and thus lowering equipment investment; the quality of the gas produced is good, making it suitable as a raw material for hydrogen production or LNG processing.
Low-grade coal mainly includes lignite and poorly metamorphosed bituminous coal. Lignite contains high levels of internal water and humic acid, has high moisture and ash content, low calorific value, and is prone to weathering and fragmentation in air. Lignite in our country is mainly found in Inner Mongolia, Xinjiang, Yunnan and other regions, while low-metamorphic bituminous coal is primarily located in Shaanxi, Inner Mongolia, Xinjiang and other places. The ash content of the coal from northern Shaanxi and Shendong areas ranges from 5% to 10%, with a sulfur content of less than 0.7%, making it high-quality low-rank coal. He Yongde, a member of the Coal Chemicals Committee of the Petrochemical Federation and honorary president of the Shaanxi Chemical Society, told reporters that low-grade coal generally has a high oil content. Through medium- and low-temperature pyrolysis, the coal is heated to 500°C–800°C in an air-free or oxygen-deficient environment, which allows it to be broken down into three components: pyrolysis gas, coal tar, and semi-coke. Pyrolysis is the best method for the clean and selective utilization of low-rank coal; it not only improves resource utilization efficiency by extracting tar from the coal to produce gas and semi-coke, but also features mild production conditions with minimal environmental impact, as well as lower water consumption compared to gasification. In recent years, breakthroughs have been made in the development and demonstration of low-rank coal pyrolysis technologies in our country. There are now over 20 new generations of pyrolysis technologies, half of which have been put into industrial production. Industry experts generally believe that low-grade coal in China is suitable for comprehensive utilization through pyrolysis technologies such as SM-GF, offering broad prospects for promotion and application, as well as significant economic benefits and social importance. If 1 billion tons of low-grade coal are converted on-site in the western region, it can be roughly estimated that 650 million tons of semi-coke, 80 million tons of tar, and 160 billion cubic meters of gas can be produced annually. Improvement of the demonstration plant: In September 2016, Shaanxi Shanbei Qianyuan Energy Chemical Co., Ltd., which is part of Shaanxi Coal Group, collaborated with Guodian Futong to build an industrial demonstration plant with an annual capacity of 500,000 tons in Yulin, Shaanxi. Utilizing SM-GF technology, this plant operated using long-flame coal with a particle size of less than 30 millimeters as raw material, enabling long-term operation. In June 2018, the Petrochemical Federation organized experts to conduct a 72-hour on-site calibration assessment of the unit. The calibration results show that the device operates stably, with a processing capacity of 102% of the designed value. The yield of semi-coke per ton of coal is 65.69%, while the tar yield is 8.09% of that from the raw coal; this figure corresponds to 87.2% of the tar yield obtained using the Kiggen process. The gas production is 167 standard cubic meters per ton of coal, with the proportion of useful gases in the gas reaching 84.4%. The energy conversion efficiency is 91.9%, resulting in low overall energy consumption. This is also the only industrialized facility for SM-GF technology at present. The person in charge of Qianyuan Nenghua Company said that they are modifying the equipment to address some technical issues identified during its initial operation, with completion expected in the first half of 2020.
The SM-GF technology mainly includes a Guofu furnace pyrolysis unit, a gas cooling and purification unit, a heating furnace unit, a flue gas purification unit, and a wastewater treatment unit. Process description: The raw coal enters the drying section of the Guofu furnace through the coal storage bin on top of the furnace, where it exchanges heat thoroughly with the high-temperature flue gas and all its moisture is removed. After this heat exchange, the flue gas passes through dust removal and desulfurization systems before entering the cooling section of the Guofu furnace. The dried raw coal enters the pyrolysis section of the Guofu furnace, where it exchanges heat with the high-temperature gas carriers coming from the pyrolysis chamber. This process results in the formation of semi-coke, tar, gas, and pyrolysis water. The high-temperature semi-coke then enters the cooling section of the Guofu furnace, where it exchanges heat with the low-temperature flue gas from the desulfurization system. As a result, the temperature of the semi-coke drops below 100°C, while the temperature of the flue gas rises; this flue gas is then recycled to the drying section to continue heating the raw coal. The oil-water-gas mixture produced after pyrolysis is thoroughly purified by a gas purification system to yield clean gas. Part of this gas is returned and heated in a heater before being fed into the pyrolysis section of the Guofu furnace as a heat source, while the remaining part is sent outside. The oil-water mixture resulting from purification enters the tar recovery system, enabling efficient separation of oil and water.
Technical advantages of SM-GF: The SM-GF technology integrates various techniques such as segmented multi-layer processing, uniform mass and heat transfer, regenerative heating using self-generated hydrogen-rich gas as a heat carrier, medium- and low-temperature staged pyrolysis, dry quenching of coke, efficient separation of oil dust and gases, and medium- and low-temperature staged pyrolysis. The combination of these various techniques helps to address common problems in the industry, including poor air permeability, coking inside the furnace, limited controllability over product quality, poor quality of tar, low processing capacity per furnace, severe pollution, and difficulties in treating wastewater. Compared with other processes in the industry, SM-GF offers significant technical advantages and strong competitiveness. It has the following technical advantages: (1) It has strong adaptability to raw materials, and can process mixed coals of all particle sizes up to 30 mm. The Guofu furnace, with its unique gas distribution structure and internal design, can overcome uneven heat exchange caused by the accumulation of fine-grained material. It has a wider range of compatibility with different particle sizes of material, allowing raw coal with particle sizes of up to 30 mm to be fed directly into the furnace. This effectively solves the problem posed by the strict requirements regarding coal particle size imposed by the mainstream furnace types in the semi-coke market. It helps to break the deadlock resulting from a shortage of lump coal and an excess of crushed coal, not only improving the utilization rate of raw coal but also reducing the production costs of semi-coke. (2) The processing capacity per unit is high, up to 500,000 tons per year. The Guofu furnace adopts a segmented approach of drying, carbonization, and cooling, overcoming the structural defects of conventional furnace types. Multiple layers of internal components are installed within each segment to ensure uniform material distribution and gas flow, as well as automatic dust removal inside the furnace. This reduces the resistance of the bed layer and the amount of dust carried by the gases, thereby increasing the annual processing capacity per furnace. (3) High tar yield. By using self-produced hydrogen-rich gas as a circulating heat carrier, the polymerization of oil-forming active fragments and the cracking of tar inside the furnace are effectively suppressed. The internal components of the Guofu furnace are designed to allow tar to be removed from the furnace in a timely manner, thereby reducing tar cracking and increasing the yield of tar. (4) The quality of tar, semi-coke, and gas is good. The Guofu furnace features a moving bed design for its process system, as well as a sedimentation design within the furnace itself, which significantly reduces the dust content at the gas outlet. The dust removal efficiency of cyclone dust collectors can exceed 90%, ensuring efficient separation of oil and dust from the gas and resulting in a low content of tar and mechanical impurities. Dry quenching of coke can prevent the decrease in the fixed carbon content of semi-coke that occurs in wet quenching due to the water-gas reaction; therefore, the SM-GF technology offers the distinct advantage of yielding semi-coke products with a high fixed carbon content. The effective components of the gas account for over 80%, giving it a high calorific value of more than 17 MJ/m3.
(5) The product quality is stable. Within the Guofu furnace, specialized air passage channels are created by designing multiple layers of air distribution and collection umbrellas, which enables a uniform distribution of the temperature and pressure fields within the furnace. This results in uniform mass and heat transfer, leading to high product consistency as well as stable quality of gas, tar, and semi-coke. (6) Low wastewater generation and high system energy efficiency. The drying section of the Guofu furnace removes the free water from coal in advance, thereby reducing the amount of wastewater generated. The cooling section employs a dry quenching process, in which the flue gas that has been heated in the drying section is used to directly exchange heat with the hot semi-coke, cooling the semi-coke to below 100°C. This approach makes full use of the residual heat in the semi-coke, improving the energy efficiency of the system. It also prevents the generation of phenol- and ammonia-containing wastewater associated with wet quenching, thus offering environmental and energy-saving benefits. (7) Flexible manufacturing processes, with a wide variety of products. The multi-layer pyrolysis version of the SM-GF technology enables staged, coupled pyrolysis at medium and low temperatures. By adjusting the temperature of the heat carriers in each layer, it is possible to control the final pyrolysis temperature of the coal, thereby producing products with various specifications and properties to meet the needs of different industries. (8) The autonomous sewage treatment process is more efficient and cleaner. The wastewater generated by the system is treated using the proprietary LAB technology (Active Carbon Adsorption and Biochemical Degradation Coupling Technology), resulting in effluent that meets the Class A standard specified in the Discharge Standards for Urban Wastewater Treatment Plants (GB18918-2002). (9) The process is mature and reliable, with low investment and operating costs. Compared with existing pyrolysis technologies, the SM-GF technology features reliable processes, lower investment costs, reduced operating expenses, and smaller land requirements, making it suitable for the clean and efficient conversion as well as diversified utilization of low-grade coal.
Such projects have a particularly significant impact on the environment; there are issues such as VOCs (odors), dust, wastewater, waste residues, etc., in the area surrounding them, and there are also many carcinogenic factors, so it’s best to stay as far away as possible.