Shell gasification technology

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1 Trends in the structure of energy consumption at home and abroad. Coal, oil, and natural gas are the three main sources of primary energy in the world today. Based on various factors such as the order of discovery, availability of resources, difficulty of extraction, conditions for transportation and storage, advancements in post-processing technologies, and the impact on the environment, the proportion of these 3 fuels in total energy consumption has been constantly changing throughout different historical periods. Before the 20th century, coal was the main source of energy consumption. In the developed countries of the 20th century, energy consumption underwent a transition from coal to oil. In the 21st century, as oil consumption declines and natural gas consumption rises, coal consumption will remain stable, with each accounting for 27% to 28%. After that, there will be a brief period dominated by natural gas, followed by a shift back to coal as the dominant fuel. This trend is now widely recognized. To adapt to such changes, energy diversification has attracted attention from the international community, and diverse options have also emerged in terms of chemical raw material sources. Fair competition will emerge in the technologies related to the 3 raw materials. The pace of development in the coal chemical industry depends not only on the prices and supply and demand of oil and natural gas, but also to a large extent on the progress of related technologies, such as clean coal technology, coal-fired power generation technology, gasification technology, carbon-one chemistry technology, as well as environmental protection technologies. Clean coal will become the main energy source in the latter half of the 21st century. Our country has little oil and gas, but is rich in coal resources, which account for a very large share of the total energy consumption in the national economy. The chemical industry began with coal chemistry, while petrochemicals started later. Although a policy of vigorously developing the petrochemical industry has been adopted, due to constraints in funding and raw materials, it remains difficult to change the situation dominated by coal. The current situation facing the chemical industry is that petrochemical products are far from sufficient to meet societal demands, while the modernization of coal chemical enterprises fails to keep up with these developments. Therefore, the chemical industry must necessarily adopt a diversified raw material strategy that relies on oil, gas, and coal. While vigorously developing petrochemicals, it is necessary to systematically and gradually advance modern coal chemical processing, with a focus on developing gasification technologies that are efficient and low in energy consumption. 2 Development trends in coal chemical technology. Although progress has been made in coal liquefaction technology over the past 20 years, due to its high energy consumption, difficulties in obtaining hydrogen, and high costs, industrial-scale production of chemicals from coal has not yet been achieved. To produce chemicals from coal, a gasification process is still required. Traditional gasification technologies use coke ovens, gas generators, and water gasifiers. In the 20th century, hundreds of gasification methods were developed for different types of coal and gas applications, among which the Ruhr coal gasifier under pressure, the atmospheric-pressure K–T furnace, and the Winkler gasifier are the most widely used. Since the 1970s, technical challenges such as improving the thermal efficiency of coal-fired power plants and reducing environmental pollution have led to the development of a new generation of gasification processes. Among the 45 clean coal technology demonstration projects in the United States, 7 are gasification combined cycle power generation projects, utilizing 6 different gasification technologies. At present, China’s energy supply is dominated by coal, accounting for about 72% of total consumption, with the majority of it being used directly through combustion in the power and transportation industries. The proportion of coal used to produce syngas through gasification is very low, and gasification technology remains quite backward. In syngas production, atmospheric pressure fixed-bed gasifiers are commonly used in China. Many years ago, domestic research institutions also developed K-T furnaces and slag furnaces using coal dust as raw material, as well as an oxygen-enriched continuous gasification process in atmospheric-pressure fixed-bed gasifiers; however, for various reasons, these technologies have not yet reached the stage of industrial application and widespread use. Gasification technology abroad was industrialized as early as the 1950s, but its development came to a halt due to the extensive exploitation of natural gas and oil. In the early 1970s, an energy crisis arose internationally. Fearing shortages in oil and gas supplies, developed countries brought coal gasification back onto the agenda as an alternative energy source, accelerating research into new coal gasification technologies. Over the past decade, many foreign companies have invested considerable effort in gasification combined cycle power generation technology in order to improve the thermal efficiency of coal-fired power plants and reduce environmental pollution, thereby promoting the development of gasification technology. A new generation of coal gasification process has been successfully developed, featuring wide adaptability to different coal types, high gasification pressure, high gasification efficiency, and low pollution. Representative ones among them include Shell’s SCGP process in the Netherlands, Texaco’s water-coal slurry gasification process in the United States, DOW Chemical’s DOW gasification process in the United States, Germany’s GSP process, the “Prenfio” process (pressurized K–T method), and the Lurgi process. The DOW gasification process is similar to the Texaco process, but its performance and experience are inferior to those of Texaco ； Although the GSP process and the “Prenfio” process have good technical parameters, there is currently only one demonstration unit available, resulting in limited operational experience ； Although the Luchi process features a large number of industrial installations and substantial operational experience, it suffers from a high content of CH4 in the gas, a low content of useful components (CO + H2), as well as high levels of tar and phenols in the gas. This makes wastewater treatment complex, thereby limiting its development and application. 3 Existing Major Coal Gasification Technologies in China 3.1 Pulsating Coal Gasification This method uses bituminous coal or coke with a particle size of 25–75 mm as raw material, and alternately introduces air (for blowing) and steam (for gas production) into the gasifier. Heat the bed while sending air in, and vent the generated purge gas. The gas generated during steam injection is sent to the gas holder. The gas furnace series includes models with diameters of φ2,600 mm, φ3,000 mm, φ3,200 mm, and φ3,600 mm. Each furnace has a gas production capacity of 6,000 to 13,000 m3/h, and the volume fraction of (CO + H2) in the gas ranges from 68% to 72%. The atmospheric pressure fixed-bed batch gasification process, although mature and reliable in terms of technology, with all equipment available domestically and lower investment requirements, has high energy consumption, high demands on coal quality – requiring smokeless lump coal – and low resource utilization efficiency. Due to operation at atmospheric pressure, the production intensity is low, resulting in high emissions of waste substances, which causes certain environmental pollution. 3.2 Lurgi pressure gasification: This method uses bituminous coal or lignite with a particle size of 8–50 mm, which is highly active and non-caking, as raw material; gas is produced through continuous gasification with oxygen and steam in a fixed-bed reactor. The vaporization pressure is 3.0 MPa, the vaporization temperature ranges from 900 to 1,050°C, and it operates with solid slag discharge. One furnace with a diameter of φ3,800 mm produces 35,000–55,000 m3/h of gas; the volume fraction of (CO+H2) in this gas is 65%, while CH4 accounts for 9%, and it also contains C2 compounds and tar. The 1,000 t/d ammonia synthesis plant at Shanxi Fertilizer Factory (Lucheng, Shanxi) in our country introduced this process technology from Lurgi in 1979. Since gas contains a high amount of methane, it is suitable only as city gas, or as a by-product in the production of syngas. Converting the methane within it into syngas would complicate the production process. Coal gasification wastewater contains large amounts of tar, phenols, ammonia, and other substances; therefore, extensive wastewater treatment facilities are required to meet environmental emission standards. 3.3 Texaco’s water-coal slurry gasification: Texaco developed technology for producing syngas from natural gas and heavy oil at an early stage. The oil crisis of the 1970s spurred the search for alternative energy sources and cleaner gasification technologies. After years of research, the company developed a water-coal slurry gasification process, which is a pressurized gasification method that involves adding coal in a wet manner and discharging slag in liquid form, all taking place in a fluidized bed. Water-coal slurry reacts with pure oxygen under high temperature and pressure to produce gas. 3.3.1 Characteristics of the water-coal slurry gasification process (1) Wide range of suitable raw coals: In addition to lignite with high moisture content, various types of bituminous coal, petroleum coke, and residues from coal hydroliquefaction can all be used as feedstocks for gasification, with younger bituminous coals being the most common. Coal dust can be used as the raw material, with no strict requirements regarding the particle size, cohesion, or sulfur content of the coal. However, practice has shown that for gasification, a ash fusion temperature t3 of the coal used is favorable only when it is below 1,350°C. The mass fraction of ash in the coal should not exceed 13%–15%, and the mass fraction of moisture in the coal must be below 8% in order to produce a water-coal slurry with a concentration of 60%–65%, thereby ensuring more stable and economical operation of the facility. (2) High vaporization pressure: Industrial units use a vaporization pressure ranging from 2.8 to 6.5 MPa, which can be selected according to the requirements for the gas to be used. Obviously, within a furnace of a certain volume, increasing the gasification pressure is directly proportional to an increase in production capacity; therefore, a higher gasification pressure allows for smaller furnace sizes and also helps to reduce the power consumption required to raise the gas pressure. (3) The gasifier uses a specially designed hot-wall furnace; to maintain the reaction temperature at 1,350–1,400°C, the combustion chamber is constructed from multiple layers of special refractory bricks. Heat recovery comes in two types: quenching and waste boiler, which can be selected based on the intended use of the gas. (4) The syngas has good quality; among the volume fractions of its active components, CO + H2 account for 80%, while the amount of methane