Basic knowledge of coal gasification

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Basic Knowledge of Coal Gasification – Coal Gasification Technology I. Principle of Coal Gasification The gasification process is a thermochemical treatment process for coal. It is a process in which coal or coal coke serves as the raw material, with oxygen (air, oxygen-enriched air, or industrially pure oxygen) and water vapor acting as gasifying agents; under high temperature and pressure, chemical reactions are used to convert the combustible components in coal or coal coke into combustible gases. The combustible gas produced during gasification becomes gas; gas used as a raw material in the chemical industry is generally referred to as syngas (syngas can be produced using not only coal but also natural gas, heavy petroleum fractions, etc.). The equipment used for gasification is called a gas generator or gasifier. Coal gasification involves a series of physical and chemical changes. It generally includes four stages: pyrolysis, gasification, and combustion. Drying is a physical change; as the temperature rises, the moisture in coal evaporates due to heat. The others are chemical changes, and combustion can also be considered a part of vaporization. After the coal is dried in the gasifier, as the temperature rises further, the coal molecules undergo thermal decomposition, producing a large amount of volatile substances (including carbonized gas, tar, and pyrolytic water, etc.), while the coal transforms into semi-coke. The semi-coke formed after coal pyrolysis undergoes a chemical reaction at higher temperatures with the gasifying agent introduced into the gasifier, producing gaseous products that mainly consist of carbon monoxide, hydrogen, methane, as well as carbon dioxide, nitrogen, hydrogen sulfide, water, etc., namely crude gas. The gasification reaction involves many chemical reactions, primarily those between carbon, water, oxygen, hydrogen, carbon monoxide, and carbon dioxide. Among these, the reaction between carbon and oxygen is also known as a combustion reaction, and it provides the heat necessary for the gasification process. 　　 The main reactions are: 1. Water vapor conversion reaction: C + H2O = CO + H2 – 131 KJ/mol; 2. Water-gas shift reaction: CO + H2O = CO2 + H2 + 42 KJ/mol; 3. Partial oxidation reaction: C + 0.5 O2 = CO + 111 KJ/mol; 4. Complete oxidation (combustion) reaction: C + O2 = CO2 + 394 KJ/mol; 5. Methanation reaction: CO + 2H2 = CH4 + 74 KJ/mol; 6. Boudouard reaction: C + CO2 = 2CO – 172 KJ/mol. II. Coal gasification process: Although there are various ways to classify coal gasification technologies, the common approach is to classify them based on the chemical engineering characteristics of the production facilities, or in other words, according to the type of reactor used. The gasification process has a significant impact on the cost and efficiency of coal chemical products. Adopting efficient, low-consumption, and pollution-free gasification processes (technologies) is an important prerequisite for the development of coal chemistry. The reactor is the core of such processes; it can be said that the advancement of gasification technology goes hand in hand with the development of reactors. In order to increase the gasification rate and the efficiency of gasifiers while also improving the environmental conditions, the general trend in the development of new generation gasification technologies is to shift the gasification pressure from atmospheric pressure to medium to high pressures (8.5 MPa) ； The gasification temperature is moving towards higher values (1500–1600°C) ； The raw materials for gasification are becoming more diversified ； Solid slag discharge is evolving towards liquid slag discharge. 　　 1. Fixed-bed gasification: Fixed-bed gasification is also known as moving-bed gasification. Fixed-bed reactors generally use lump coal or coking coal as raw materials. Coal is fed from the top of the gasifier, while the gasifying agent is fed from the bottom. The upward force of the flowing gas does not cause any change in the relative positions of the solid particles; in other words, the solid particles remain in a relatively fixed state, and the bed height also stays essentially unchanged. Hence, this is referred to as fixed-bed gasification. Furthermore, from a macroscopic perspective, since coal is added from the top of the furnace and slag containing char is discharged from the bottom, during the gasification process the coal particles gradually and slowly move downward within the gasifier; hence it is also known as moving bed gasification. The characteristic of fixed-bed gasification is its simplicity and reliability. At the same time, since the gasifying agent comes into countercurrent contact with the coal, the gasification process proceeds more completely, and heat is utilized efficiently, resulting in a high thermal efficiency. Fixed-bed gasifiers commonly include two types: batch gasification (UGI) and continuous gasification (Lurgi). For the former, bituminous coal (anthracite) or coke must be used as raw materials in the production of syngas in order to reduce the CH4 content in the syngas. There are thousands of such gasification units in China, but they have many drawbacks ； The latter has over 20 furnaces in China, which are mainly used for producing city gas ； The preliminary gas purification system included in this technology is extremely complex, and it is not considered the preferred technology. (1) The fixed-bed batch gasifier (UGI) uses lumpful anthracite or coke as raw materials, and air and steam as gasifying agents to produce synthetic feed gas or fuel gas at atmospheric pressure. This technology was developed in the 1930s; it requires low investment and is easy to operate. However, it is now considered an outdated technology due to its low gasification rate, reliance on a single type of raw material, and high energy consumption. During the intermittent gas production process, large amounts of make-up air are vented – up to 5,000 m3 of make-up air per ton of synthetic ammonia produced. The vented gases contain CO, CO2, H2, H2S, SO2, NOx, and dust ； The wastewater discharged from the gas cooling and washing tower contains tar, phenols, and cyanides, causing environmental pollution. There are over 900 small and medium-sized fertilizer plants in our country, and most of them still use this technology to produce synthetic feed gas. As energy policies and environmental requirements become increasingly stringent, it will be gradually replaced by new gasification technologies in the near future. (2) Lurgi gasifier: In the 1930s, the German company Lurgi developed a fixed-bed continuous batch gasification technology. Due to its good adaptability to various feedstocks and high production capacity per unit, it has been widely used both domestically and internationally. The pressure in the gasification furnace is (2.5–4.0) MPa, while the temperature of the gasification reaction is (800–900) °C. Solid slag is discharged from the furnace. Several models of such furnaces have been developed (MK-1 to MK-5); among them, the MK-5 model has an inner diameter of 4.8 meters, a coal feeding rate of (75–84) tons per hour, and a production rate of pulverized coal gas of (10–14) ten thousand cubic meters per hour. In addition to CO and H2, gas contains up to 10%–12% CH4, which can be used as city gas, artificial natural gas, or syngas. The disadvantage is that the gasification furnace has a complex structure, with rotating components such as viscosity-breaking and coal distribution devices as well as grates inside, resulting in high manufacturing and maintenance costs ； The coal fed into the furnace must be lump coal ； The source of raw materials is subject to certain restrictions ； The off-gas from the furnace contains tar, phenols, etc. The wastewater treatment and gas purification processes are complex, involve long procedures, require many pieces of equipment, and the slag contains about 5% carbon. To address the aforementioned issues, in 1984, Lurgi and British Gas jointly developed the Liquid Slag Gasifier (BGL). It features a high gasification temperature, molten slag discharge, a high carbon conversion rate, good quality syngas, low wastewater generation during gasification with easy treatment. The production capacity per unit gasifier has increased by 3 to 5 times compared to previous models, making it a gasifier with great potential for development. 2. Fluidized bed gasification Fluidized bed gasification is also known as bubbling bed gasification. It uses fine coal particles as the gasification feedstock; under the action of the gasifying agent flowing from bottom to top, these fine particles remain in a state of continuous and chaotic bubbling and suspension, undergoing rapid mixing and heat exchange, which results in uniformity of temperature and composition throughout the entire bed. The main reason for the rapid development of fluidized-bed gasification is: (1) the production intensity is higher than that of fixed-bed gasification. (2) It directly uses small particle coal dust as raw material, adapting to the development of coal mining technology and avoiding the supply-demand contradiction for lump coal. (3) It has strong adaptability to different types and qualities of coal, and can use low-quality coals with high ash content such as lignite as raw materials. Common types of fluidized bed gasifiers include Winkler, U-Gas, circulating fluidized bed (CFB), and pressurized fluidized bed (PFB, which is the gasification part of PFBC), among others. (1) Circulating Fluidized Bed Gasifier CFB: The circulating fluidized bed gasifier (CFB) developed by Lurgi can gasify various types of coal; it can also use wood chips, bark, and municipal combustible waste as feed materials for gasification. Steam and oxygen are used as the gasification agents. The gasification process is quite complete, with a high gasification rate – twice that of a moving bed system. The carbon conversion rate is high (97%), and the ash discharged from the bottom of the furnace contains 2%–3% carbon. The amount of material that is recycled back into the gasifier during the feeding process is 40 times that of the newly added material. The airflow velocity inside the furnace ranges from 5 to 7 m/s, resulting in very high rates of heat and mass transfer. Vaporization pressure: 0.15 MPa. The gasification temperature is controlled depending on the raw materials; generally, the temperature of the circulating cyclone dust collector is maintained between (800–1050) °C. Luchi Company’s CFB gasification technology has been adopted in over 60 plants worldwide, with another 30+ plants under design or construction, giving it a leading position in the global market. CFB gasifiers operate at near-atmospheric pressure. When producing syngas from coal as the raw material, 1.2 kg of vaporized water and 0.4 kg of oxygen are consumed per kilogram of coal, yielding 1.9–2.0 m3 of gas. The composition of the gas is such that CO + H2 exceeds 75%, with a CH4 content of around 2.5% and a CO2 content of 15%; this CO2 level is lower than that in the gas produced by Texaco furnaces and Rucho MK-type furnaces, which is advantageous for ammonia synthesis. (2) Ash fusion fluidized bed pulverized coal gasification technology: This technology uses dry pulverized coal with a particle size of less than 6 mm as raw material, and air, oxygen-enriched air, or water vapor as the gasifying agent. The pulverized coal and gasifying agent are continuously fed in from the bottom of the gasifier, where they undergo rapid gasification reactions at high temperatures of (1050–1100)°C. The unreacted carbon and fly ash carried away by the raw gas are recovered using two-stage cyclone separators and then returned to the furnace for further gasification. This approach improves the carbon conversion rate, reduces the phosphorus content in the ash to below 10%, and simplifies the ash disposal system. Raw gas contains almost no harmful substances such as tar and phenols, making it easy to purify. China has successfully developed this advanced gasification technology on its own. This technology can be used for producing fuel gas, syngas, and for combined-cycle power generation. It is particularly suitable for replacing intermittent fixed-bed gasifiers in small and medium-sized nitrogen fertilizer plants; by using bituminous coal in place of anthracite to produce the raw material gas for synthetic ammonia, the cost of synthetic ammonia can be reduced by 15% to 20%, offering broad prospects for development. The U-Gas unit at the Shanghai Coking Plant (120 tons of coal per day) was put into operation in November 1994; it operated abnormally for a long time and was shut down at the beginning of 2002 ； The ICC ash fusion gasification furnace developed by the Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences, was tested in 2001 at Shaanxi Chenghua Co., Ltd. as an industrial demonstration unit capable of producing 100 tons of syngas per day. CFB and PFB can produce fuel gas, but there are no international precedents for producing syngas ； Winkler has been used in syngas production, but it has strict requirements regarding particle size and coal type; it also features a high methane content (0.7%–2.5%), and its operational efficiency is low, so it no longer represents the direction of development. 3. Fluidized bed gasification: Fluidized bed gasification is a type of co-current gasification. Based on the form of the raw material, there are two types: slurry coal and dry coal powder ； In terms of patents, Texaco and Shell are the most representative. In the former method, coal powder is first converted into a coal slurry, which is then pumped into the gasification furnace; the gasification temperature ranges from 1350 to 1500℃ ； In the latter case, a gasifying agent carries coal powder into the gasification furnace, where it is gasified at high temperatures of 1500–1900°C, and the residue is discharged in the form of slag. Inside the gasifier, fine coal particles enter the reaction chamber through special nozzles, where they catch fire instantly and undergo a flame reaction. Due to the insufficient oxidation conditions, their pyrolysis, combustion, and endothermic gasification reactions occur almost simultaneously. As the airflow moves, the unreacted gasifying agents, pyrolytic volatiles, and combustion products carry coal char particles at high speeds; during this movement, gasification reactions of the coal char particles take place. This state of motion is equivalent to the \"air-flow transportation\" of solid particles in the field of fluidization technology, and is commonly referred to as gas-bed gasification. Fluidized bed reactors exhibit high compatibility with various types of coal (bituminous coal, lignite), as well as with different particle sizes, sulfur contents, and ash contents. Internationally, there are already many large-capacity, pressurized plants of this type in operation, and their cleanliness and efficiency represent the current trend in technological development. The main types of dry powder feeding systems include the K-T (Koppres-Totzek) furnace, Shell-Koppres furnace, Prenflo furnace, Shell furnace, GSP furnace, and ABB-CE furnace, while the main types of wet coal slurry feeding systems are the Texaco gasifier and Destec furnace. (1) Texaco gasifier: The coal-water slurry gasification process developed by Texaco of the United States (which became part of Chevron at the beginning of 2002 and was acquired by GE in May 2004) involves grinding coal with water to produce a coal-water slurry with a concentration of 60–65%. Pure oxygen is used as the gasifying agent, and the gasification reaction takes place under high temperature and pressure; the gasification pressure ranges from 3.0 to 8.5 MPa, while the temperature is 1400°C. Liquid slag is produced as a by-product, and the composition of the gas consists of about 80% CO + H2. This process does not produce any organic substances such as tar or phenols, thus causing no environmental pollution. The carbon conversion rate is 96–99%, the gasification efficiency is high, the furnace structure is simple, energy consumption is low, operational efficiency is high, and it can handle a wide range of coal types. Currently, Texaco’s largest commercial facility is the Tampa Power Plant, which falls under the DOE’s CCT-3 program. It was approved for development in 1989, came online in July 1996, and verification operation was announced in December of the same year. The unit is a single furnace with a daily coal processing capacity of 2,000–2,400 tons; the gasification pressure is 2.8 MPa, the oxygen purity is 95%, the slurry concentration is 68%, the cold gas efficiency is around 76%, and the net power output is 250 MW. The Texaco gasifier consists of a nozzle, a gasification chamber, and a quenching chamber (or waste heat boiler). The nozzle is three-channel: process oxygen flows through channels 1 and 3, while the water-coal slurry flows through channel 2, located between the two oxygen jets. Water-coal slurry gasification nozzles often suffer from nozzle wear, primarily due to the erosive action of the water-coal slurry on metal materials at high linear velocities (around 30 m/s). Nozzles, gasifiers, quenching rings, etc., are the key technologies for Texaco water-coal slurry gasification. Since the late 1980s, China has introduced multiple Texaco water-coal slurry gasification units for the production of syngas, and the country has accumulated extensive experience and knowledge in design, installation, commissioning, as well as research and development of new technologies in the field of water-coal slurry gasification. Based on the operation of the already put into use water-coal slurry pressurized gasification units, the main advantages are: simple preparation, transportation, and metering control of the water-coal slurry, as well as safety and reliability ； The local production rate of equipment is high, resulting in lower investment costs. Due to imperfect engineering design and operational experience, it has not yet reached an optimal state of long-term, high-load, stable operation, and there are still many existing problems. The main drawbacks include a short lifespan for the nozzles, a lifespan of only one year for the quenching rings, and a pulping concentration for lignite of around 59%–61% ； The pulping concentration for bituminous coal is 65% ； Since the water in vaporized coal slurry accounts for 8% of the coal, the oxygen consumption is 12% to 20% higher than that when using dry coal powder as a raw material, resulting in lower efficiency. (2) Destec (Global E-Gas) gasifiers: Two commercial units of the Destec gasifier have been built, both in the United States: LGT1 (with a gasification capacity of 2,200 tons per day, at 2.8 MPa; put into operation in 1987) and Wabash River (two units, one in operation and one as a backup, with each unit having a capacity of 2,500 tons per day, at 2.8 MPa; put into operation in 1995). These gasifiers are similar to the K-T type, consisting of a first stage (horizontal section) and a second stage (vertical section). In the first stage, two nozzles are positioned 180 degrees apart from each other; the impact flow generated helps to enhance mixing, thereby overcoming the drawback of the Texaco type gasifiers, where the velocity distribution is bell-shaped (normal). The maximum reaction temperature is approximately 1,400°C. To improve the efficiency of cold gas, in the second stage, 10% to 20% of the total coal slurry volume is used for cold quenching (this differs from the gas-cooled quenching in the cycles of Shell and Prenflo); the reaction temperature here is around 1040°C, and the exhaust gas is used to recover heat in the fire-tube boiler. The slag flows down from the middle of the first section of the gasification furnace, is solidified by water cooling, and is then discharged as a slurry of slag and water. The E-Gas gasification furnace uses a pressure screw-type continuous slag discharge system. The disadvantages of the Global E-Gas gasification technology are: a short residence time for the secondary water-coal slurry, and a low carbon conversion rate ； A large separator is installed to separate the ash and slag, as well as the carbon residue, contained in the primary coal gas from the secondary coal slurry. This type of furnace is suitable for producing fuel gas but not for producing syngas. (3) Shell gasifiers: The first dry-powder fed gasifier to be industrialized was the K-T furnace; all others were developed on its basis. In the early 1950s, Shell succeeded in developing slag oil gasification. Building on this achievement, three stages were followed: in 1976, more than 30 different types of coal were tested ； In 1978, in collaboration with Germany’s Krupp-Koppers (the predecessor of Krupp-Uhde), a plant capable of processing 150 tons of coal per day was built in Harburg ； After the two companies parted ways, a plant was built in Deer Park, Houston, USA, in 1978 to process 250 tons per day of high-sulfur bituminous coal, or 400 tons per day of lignite with high ash and moisture content. It took a total of 16 years, until 1988, for Shell’s coal technology to be applied at the Buggenum IGCC power plant in the Netherlands. The design work for this facility took 1.6 years; construction began in October 1990, it was put into operation in 1993, and entered a 3-year verification period in January 1994. It is now in commercial operation. One furnace can process 2,000 tons of coal per day. The shell of the Shell gasifier has a diameter of approximately 4.5 m. Four nozzles are located on the same horizontal level at the bottom of the furnace, arranged evenly around the circumference. Impinging flow is utilized to enhance the heat and mass transfer process, thereby making the gas velocity across the cross-section of the furnace relatively uniform. The furnace lining is a membrane wall, with a total weight of 500 tons. There is a gap of about 0.5m between the water-cooled tube rows in the furnace shell, which is provided for installation and maintenance purposes. 20% to 30% of the total amount of ash carried by the gas moves upward along the axis of the gasifier; at a position near the top of the gasifier, recycled gas is introduced for cooling. The volume of this cooling gas accounts for about 60% to 70% of the total gas produced, and this cooling process reduces the temperature to 900°C, causing the slag to solidify. The resulting gas then exits the gasifier and moves upward through inclined pipes into a tubular waste heat boiler. 70% to 80% of the total coal ash flows into the bottom of the gasifier in a molten state, where it solidifies upon cooling and is then discharged from the bottom of the furnace. The pulverized coal is carried by N2 and conveyed in a dense phase into the nozzle. Process oxygen (with a purity of 95%) and steam also enter through nozzles, at a pressure of 3.3–3.5 MPa. The gasification temperature is 1500–1700°C, and the gasification pressure is 3.0 MPa. The efficiency of cold gas is 79%–81% ； 13% of the calorific value of the raw coal is converted into steam in the boiler ； 6% is lost to the atmosphere and cooling water as sensible heat of gas from the equipment and outlet coolers. The Shell gasification technology has the following advantages: it uses dry coal powder as feed, resulting in an oxygen consumption that is 15% lower than that of water-coal slurry ； High carbon conversion rate, up to 99%, with coal consumption 8% lower than that of water-coal slurry ； The load for mediation is easy to manage: by shutting off a pair of nozzles, the load is reduced by 50% ； The furnace lining is a water-cooled wall; its lifespan is said to be 20 years, while the lifespan of the nozzles is 1 year. Main drawback: Higher equipment investment compared to coal-water slurry gasification technology ； The structure of gasifiers and waste boilers is too complex, increasing the difficulty of processing. (4) GSP gasification furnace: GSP (GAS Schwarze Pumpe), known as the “black pump gasification technology,” was developed in 1956 by the German Fuel Research Institute (abbreviated as DBI) in former East Germany. Currently, this technology belongs to FUTURE ENERGY GmbH, a company founded in 2002 and a subsidiary of Sustec Holding AG. The GSP gasifier is a bottom-fed pressurized fluidized-bed gasifier with liquid slag discharge, whose coal feeding method is similar to that of the Shell gasifier, while its furnace structure is similar to that of the Texaco gasifier. The first industrial unit was built at the Heihe Pump Plant in December 1983; each gasification furnace was capable of handling 720 tons of coal per day, and it came online in 1985. The GSP gasification furnace is currently used very rarely, with only 5 plants employing it; none are in use in China yet. Ningmei Group (which is controlled by our company) plans to introduce this technology for use in coal chemical projects. In general, in terms of pressure, large capacity, and compatibility with different coal types, fluidized-bed gasification technology represents the future direction of gasification technology. Both slurry and dry pulverized coal feeding methods have their advantages and disadvantages, and the boundaries between them are not very clear; there are also diverse opinions among domestic technical experts. 3. Progress in coal gasification technology in China. Coal gasification technology has a history of nearly a century in China, but it remains relatively backward and develops slowly. Overall, coal gasification in China relies on traditional technologies; the processes used are outdated, environmental protection facilities are insufficient, the efficiency of coal utilization is low, and pollution is severe. At present, the only technology that is relatively mature in China is still atmospheric pressure fixed-bed gasification. It is widely used in industrial sectors such as metallurgy, chemicals, building materials, and machinery, as well as for domestic gas use, with fixed-bed gasification technologies such as UGI, two-stage water-gas furnace, and two-stage generator furnace being the main types. The advantage of atmospheric pressure fixed-bed gasification technology is its simple operation and low investment ； However, with outdated technology, low capabilities and efficiency, as well as severe pollution, technical upgrades are urgently needed. If the status quo is not changed, it will affect the coordinated development of the economy, energy, and the environment. Over the past 40 years, with **’s support, China has undertaken extensive work in research and development, as well as in adapting and adopting imported technologies. Our country has introduced a variety of coal gasification technologies from abroad over the years. Through the digestion and absorption of technologies introduced for coal gasification, and especially through **focused scientific and technological efforts to technically upgrade the imported equipment and localize it, significant progress has been made in the research and development of coal gasification technology in China. Research and development on K-T gasification simulation technology were carried out from the late 1950s to the 1980s ； In the 1980s, the Shanxi Coal Chemistry Institute of the Chinese Academy of Sciences developed a ash-polymerized fluidized-bed coal gasification process and obtained a patent for it ； “During the Ninth Five-Year Plan period, East China University of Science and Technology, Yankuang Lunan Fertilizer Plant, and China Tianchen Chemical Engineering Company undertook the key scientific and technological research project titled “Development of a new type of (multi-nozzle opposed) water-coal slurry gasifier” (a plant with a capacity of 22 tons of coal per day). The results from the pilot plant showed that the effective gas content was around 83%, which is 1.5–2 percentage points higher than that of Texaco’s production plants under similar conditions ； Carbon conversion rate >98%, 2–3 percentage points higher than that of Texaco ； Both specific coal consumption and specific oxygen consumption are 7% lower than those of Texaco. “During the 15th Five-Year Plan period, the multi-nozzle opposed water-coal slurry gasification technology has entered the commercial demonstration phase. “The \"new type water-coal slurry gasification technology\" was approved under the 10th Five-Year Plan’s High-Tech Research and Development Program (863 Program). It was undertaken by Yankuang Group Co., Ltd. and East China University of Science and Technology; a multi-nozzle opposed water-coal slurry gasifier along with related facilities were built at Yankuang Lunan Fertilizer Plant. Using two multi-nozzle opposed water-coal slurry gasifiers with a capacity of 1,150 tons of coal per day each (operating at 4.0 MPa), 240,000 tons of methanol were produced, while 71.8 MW of electricity was generated as a by-product. The total investment amounted to approximately 1.6 billion yuan. The device achieved a successful start-up on July 21, 2005, and operated continuously and stably for 80 hours. The preliminary operation results of the device show that the effective gas CO+H2 exceeds 82%, and the carbon conversion rate is above 98%. It marks a significant breakthrough in our country’s development of gasification technology that possesses independent intellectual property rights and is suited to **the energy structure**; this technology fills a gap in domestic capabilities and has reached international advanced levels.