Which gasification technology is more suitable for large-scale coal-to-gas projects?

Thread Content

Which gasification technology is more suitable for large-scale coal-to-gas projects? Author/Source: Huahua Net Coal Chemicals Date: 2018-05-22 Clicks: 242 Coal-to-gas is an abbreviation for coal-to-natural gas; it refers to the process in which coal is gasified to produce syngas, which is then converted and purified using carbon monoxide, before being transformed into natural gas through methanation. Our country is rich in coal resources; it is a country with little oil and gas but abundant coal, and its energy structure is dominated by coal. As demand for natural gas is growing rapidly, there is a severe shortage of natural gas for industrial and domestic use in our country. Given that China has relatively abundant coal resources, it has become an inevitable trend to develop coal-to-natural gas technology in order to alleviate the country’s tight demand for natural gas. Currently, the technology for producing natural gas from coal primarily relies on the coal gasification and methanation process. Due to differences in coal types, there are some variations in the process choices for coal-to-gas conversion. Methanization technology is now quite mature, so the main differences in the process lie in the upstream gasification step. Currently, large domestic coal chemical enterprises employ various gasification process technologies, which can be primarily categorized into three types: coal crushing under pressure gasification technology, pulverized coal under pressure gasification technology, and coal slurry gasification technology. Slurry coal technology is already well-developed in China, but it requires raw coal with low ash content and low ash melting point, and there are also certain requirements regarding its slurry-forming properties; therefore, strict standards are imposed on the type of coal used. In production, a large amount of water is involved in the gasification reaction and temperature rise, consuming substantial amounts of oxygen and carbon, resulting in relatively high energy consumption. With the rapid development of the coal chemical industry, and as dry powder gasification technology becomes increasingly mature, water-coal slurry gasification technology is facing severe challenges. Large-scale coal chemical industries are also gradually adopting dry powder gasification as the primary gasification technology. As dry powder gasification technologies, coal crushing under pressure and pulverized coal under pressure – we will compare these two technologies to see how they will develop in the future. I. Investment in auxiliary equipment: The coal pulverization pressurized gasification technology, of which the Lurgi technology is a typical example. Currently, coal-to-gas projects in China such as those operated by Datang, Qinghua, and Guanghui all use the Lurgi technology. Since the temperature in the gasification furnace under this technology is below 900°C, the volatiles in the raw coal are not converted into syngas (CO, H2) but instead evaporate. It is therefore necessary to employ subsequent processing steps to recover these volatiles (whose main components include ammonia, phenols, tar, naphtha, etc.). To recover these organic substances, more recovery and treatment facilities must be installed, which complicates the process flow and leads to a significant increase in additional investment costs. In the coal powder pressurization technology, the gasification temperature in the coal powder furnace reaches 1400°C; over 99% of the organic compounds in the raw coal are converted into syngas. This makes the subsequent processing steps simpler and easier to carry out. More importantly, it reduces the complexity of the process. The syngas produced as a result has a simple composition, which facilitates further processing such as conversion and purification, and thus reduces the required investment. II. Black water treatment: In dry powder coal gasification technology, whether coal crushing or coal powder techniques are used, the syngas produced by coal gasification requires dust removal through a water bath. Due to the presence of organic components in the syngas produced by the Lurgi process, it is difficult to completely separate these organic substances during the treatment of black water, which poses significant challenges to this treatment process and indirectly increases the costs associated with water treatment. Some companies often find it extremely difficult to meet environmental standards even after investing hundreds of millions, which becomes a fatal flaw. The wastewater treatment for powder coal gasification technology is relatively simple, as the syngas does not contain complex organic substances; after a water bath, only ash remains. Through sedimentation, water and ash can be separated, and the separated water can be reused. This is determined by the gasification characteristics of the furnace. III. Production capacity of gasifiers: The investment cost for a single semi-coke gasifier is low, but due to the characteristics of such reactors, the maximum amount of coal that can be fed into each unit per day is typically 1000 tons, which limits its production capacity. In contrast, powder coal gasifiers utilize a continuous coal feeding system, allowing up to 3000 tons of coal to be fed into each unit per day. Moreover, thanks to the use of water-cooled walls and slag-resistant technologies in the gasification chamber, it is even possible to meet production demands without the need for backup units. It can be seen that for coal-to-gas projects with the same production capacity, the number of pulverized coal boilers has decreased significantly, resulting in lower overall investment costs. IV. The main features of the powder coal gasification technology are as follows: 1. The water wall has a long service life; the gasifier adopts a water wall structure without a refractory lining. The design life of the water wall is considered to be 25 years. Moreover, the start-up and shutdown of the gasification furnace are relatively flexible and require little time, which provides a foundation for stable production. 2. The range of coal types suitable for gasifying raw coal is wide. Due to the high temperatures in gasification furnaces, a broader variety of coal types can be used – from lower-quality lignite, subbituminous coal, and bituminous coal to petroleum coke – and it is also possible to use a mixture of two different types of coal. It has a wider range of applicability regarding the ash fusion point of coal compared to other gasification processes; it can be used even with coal types that have high moisture content, high ash content, high sulfur content, and high ash fusion points. 3. It can achieve high gasification efficiency and carbon conversion rates; these values are high, with low oxygen consumption – 10~15% less compared to coal water slurry gasification. 4. The gasification furnace offers great operational flexibility and convenient load adjustment. Dry pulverized coal is used as the feedstock for gasification, and depending on the requirements of the subsequent chemical products, the coal powder can be transported using nitrogen or carbon dioxide, thereby ensuring safe and flexible operation. Load adjustment is relatively flexible. 5. The synthesis gas is purified using a process that combines a venturi with a scrubber tower, resulting in an excellent dust removal effect. In summary, pressurized coal gasification technology will play a crucial role in large-scale coal chemical industries, and large-scale coal gasifiers will be widely used. This is especially true for coal-to-natural gas production, as the process flow is simple and the project timeline is short. The prospects are bright. Of course, the use of large-scale pulverized coal gasifiers for coal-to-gas production also has its disadvantages: 1. The investment required for large-scale coal-to-gas projects is enormous, with each project costing hundreds of billions, which is beyond the means of ordinary companies; multiple parties are needed to provide funding. 2. Large domestic coal mines often lack water sources. Data shows that approximately 7 tons of water are required per cubic meter in the coal-to-gas production process; therefore, when establishing large-scale coal-to-gas projects, water resource issues must be given due consideration. 3. Coal gasification is carried out using a powder coal gasifier; the syngas contains no methane, which therefore increases the workload associated with methanation in subsequent processes. However, the equipment for methanation is not complex, the process technology is relatively mature, and both the processes and the equipment have been localized in China. Furthermore, methanation is an exothermic reaction that can generate large amounts of steam, providing effective power for the entire plant. In summary, based on this investigation, the author believes that powder coal gasifiers are more suitable for coal-to-gas projects, and large-scale coal-to-gas projects hold great potential in the future.