Basic Knowledge of Coal Chemical Industry

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Coal chemical industry is an industry that uses coal as raw material and employs chemical processing to achieve the efficient and clean comprehensive utilization of coal. The coal chemical reaction units are shown in the figure: The main products of coal carbonization include gaseous substances (coal gas), liquid substances (tar), and solid substances (semi-coke or coke). The dry distillation of coal involves unit processes such as low-temperature dry distillation of coal, high-temperature dry distillation of coal for coke production, and the recovery and processing of coking products. The main active components of coal gasification include carbon monoxide, hydrogen, methane, etc. The coal gasification process involves moving-bed gasification, pulverized coal fluidized-bed gasification, and gas-flow bed gasification of coal. Other methods include unit processes such as fluidized-bed gasification, catalytic gasification of coal, hydrogenation gasification of coal, and underground gasification of coal. The main product of coal-to-oil (coal liquefaction) is a liquid product in which the large organic molecules in coal are converted into medium-sized molecules. Its production process includes two different process units: direct coal hydrogenation liquefaction and indirect coal liquefaction. Coal-based chemical products include process units such as coal-based carbon products, calcium carbide production, lignite wax production, and coal-based methanol to olefins technology. Coal cogeneration technologies include gasification combined cycle power generation, gasification–liquid products–hydrogen production–power generation. The specific unit processes are as follows: 1. Carbonization of coal. Carbonization of coal is a complex process involving a series of physical and chemical changes that occur when coal is heated to high temperatures in an oxygen-free environment; this process is known as coal pyrolysis, or thermal decomposition and carbonization. To date, the main process for coal processing remains thermal processing, with the coal coking industry being a typical example. Coal thermal decomposition refers to the changes that occur in coal during the heating process. It can be seen that the process of coal pyrolysis is roughly divided into three stages: The first stage (room temperature to 300°C): the shape of the coal remains unchanged. The second stage (300°C to 600°C): the coal binds together to form semi-coke. The third stage (600°C to 1000°C): coke is formed. Factors affecting coal pyrolysis include the degree of coalification: as the degree of coalification increases, the starting temperature for pyrolysis rises gradually. Final heating temperature: As the final temperature increases, the yields of coke and coal tar decrease, while the gas yield increases; however, the calorific value of the hot gas decreases. Heating rate: The heating rate has a significant impact on the caking property of coal, and can increase the yields of gas and tar. Pyrolysis pressure: The amount of liquid products and the residence time increase as pressure increases. Pyrolysis atmosphere: The total amount of gaseous and liquid products resulting from pyrolysis under hydrogen is much higher than at atmospheric pressure. Low-temperature carbonization of coal: Low-temperature carbonization refers primarily to the process of carbonizing coal at an ultimate temperature of 500°C to 700°C. Coal suitable for low-temperature dry distillation is non-caking coal not intended for coking, such as lignite or high-volatility bituminous coal. Our country has abundant reserves of this type of coal, which is currently used mainly for direct combustion. If gas and tar can be recovered through low-temperature carbonization, the coal can be utilized more effectively. Properties of products from low-temperature carbonization: 1. The ratio of reactivity to resistivity of semi-coke is much higher than that of high-temperature coke, and the lower the degree of coal transformation, the higher its reactivity and specific resistivity. 2. The high resistivity of semi-coke makes it an excellent raw material for ferroalloy production. 3. Semi-coke has a lower sulfur content than raw coal, high reactivity, and a low ignition point (around 250°C); it is an excellent fuel and is also suitable for producing activated carbon, carbon molecular sieves, reducing agents, or for hydrogen production via gasification. The process of low-temperature carbonization of coal: There are many methods and types of low-temperature carbonization. Based on the heating method, they can be classified as external heating, internal heating, or a combination of both. Depending on the form of the coal, it can be lump coal, briquetted coal, or pulverized coal ； Depending on the heating medium, there are gas heat carriers and solid heat carriers ； Based on the movement state of the coal, they are further classified into fixed-bed, moving-bed, fluidized-bed, and gas-flow-bed types, etc. 1 A continuous external-heating vertical furnace, the Wood furnace which is commonly used for producing city gas, is shown in Figure 6-1-02. Bituminous coal is continuously added to the carbonization chamber from an auxiliary coal tank located at the top of the chamber; the heat-generated semi-coke is discharged into a discharge tank at the bottom. During the carbonization process, steam is introduced at the bottom to cool the semi-coke, resulting in the formation of some water gas. This water gas, along with the dry distillation gas, is carried out through upward pipes. The total length of the carbonization chamber is 2080 mm, and each carbonization chamber in the wood furnace can process about 8 tons of coal per day. The heating gas is producer gas generated on the furnace side using self-produced semi-coke. 2 Continuous internal-heating vertical furnaces: The Lurgi low-temperature retorting furnace developed in Germany is shown in Figure 6-1-03. The movement of coal inside the furnace is divided into three stages: the drying stage, the carbonization stage, and the coke cooling stage; hence it is also known as a three-stage furnace. The hot exhaust gas used for heating is supplied by clean gas burned in two separate combustion chambers, one at the top and one at the bottom. Coal is heated to 500–850°C in a dry distillation furnace. A plant equipped with a Lurgi three-stage furnace capable of processing 300–500 t/d of lignite can produce 150–250 t/d of shaped coke, 10–60 t/d of tar, and 180–220 m3/t (of coal) of residual gas. The continuous vertical furnace with internal and external heating is the Koppers furnace, developed by the German company Koppers; it consists of a carbonization chamber, a combustion chamber, and upper and lower regenerative chambers located on one side. The coal material is fed into the carbonization chamber from above, and the heat required for the carbonization process is mainly supplied through the furnace walls. The fuel for heating is producer gas or gas from dry distillation in the furnace; the exhaust gases resulting from the combustion of this gas in the vertical flue enter the upper and lower regenerative chambers alternately. Blast furnace gas is blown in at the bottom of the carbonization chamber, which not only recovers the heat from the hot semi-coke but also ensures uniform heating of the coal material. The heat consumption for coal carbonization in this furnace is low, at 2400 kJ/kg (coal), whereas that of the aforementioned furnace is 3320 kJ/kg (coal). 4 Solid heat carrier dry distillation method: External heat dry distillation devices have slow heat transfer and low production capacity. The combustion exhaust gas with internal air heating dilutes the gaseous products of carbonization. Solid heat carriers are used for coal carbonization, enabling a fast heating rate; for example, the Toscoal method in the United States uses heated ceramic balls as heat carriers to carry out low-temperature carbonization of subbituminous coal at 500°C. The Lurgi-Ruhrgas (LR) process in Germany uses hot semi-coke as a heat carrier; production facilities have been established with a capacity of 1,600 tons of semi-coke per day. The semi-coke produced is used as a raw material in coking coal blending, and its carbonization process is shown in the diagram. 5. Hydrogenation dry distillation process: Hydrogenation pyrolysis can significantly increase the yield of hydrocarbon gases and light oils; processes that have been developed for this purpose include the Coalcon hydrogenation dry distillation process and the CS-SRT hydrogenation dry distillation process. The CS-SRT hydrocarbonization pyrolysis process is designed to produce synthetic natural gas with a high calorific value; it also enables the production of light aromatics (BTX), while the residue from the pyrolysis is used for hydrogen production. The coal conversion rate of the CS-SRT process can reach 60%–65%, of which (methane, ethane) ≈ 30%, (BTX) = 8%–10%, and (light oil) = 1%–3%. High-temperature carbonization of coal – coking. Coal is heated to around 1000°C in a coking furnace in the absence of air, and after going through a series of carbonization stages, coke is produced; this process is known as high-temperature carbonization, or high-temperature coking, or simply coking. The main purpose of coking is to produce coke, which is a raw material for iron production. The gas and chemical products produced as by-products during coking, especially aromatic compounds, are widely used in the chemical industry. Functions and quality requirements of coke: Coke serves three functions in a blast furnace: ① It acts as a framework to maintain the permeability of the blast furnace ； ②Provide a heat source ； ③As a reducing agent for iron ore. To this end, the requirements for coke used in blast furnaces are: low ash content, low sulfur and phosphorus levels, high strength, uniform reactivity, density, low reactivity, and high strength after reaction. With the increasing size of blast furnaces today, the requirements for coke quality are as follows: the process of coal transforming into coke in the carbonization chamber, and the recovery and processing of coal coking products. During coal coking, about 75% turns into coke, while 25% converts into various chemical products and gas. Recycling these chemical products enables the comprehensive utilization of coal resources and promotes the development of the national economy. Some** have produced over 500 different types of coking products. China also extracts hundreds of products from coke oven gas, crude benzene, and coal tar. The recycling and processing processes at home and abroad are divided into positive pressure operation and negative pressure operation. 1 Positive-pressure operating coke oven gas treatment system: The blower is located behind the primary cooler, and the entire system downstream of the blower operates under positive pressure. This process is widely used in China. After compression, the gas temperature rises by 50°C; therefore, it is particularly suitable for systems that use the saturator method for producing ammonium sulfate (which requires 55°C) and the Fussam method for ammonia recovery. A negative-pressure operated coke oven gas treatment system places the blower at the end of the system; it raises the pressure of the coke oven gas from -7 to -10 KPa to 15 to 17 KPa before sending it to the users. The advantage of this process is that it eliminates the need for a gas-based final cooling system, which reduces the amount of cold water used, lowers overall energy consumption, and reduces pipe corrosion. Its drawback is that during negative pressure operation, the volume of coal gas increases, which in turn leads to an increase in the volume of the coal gas pipelines and equipment, thereby reducing the absorption driving force ； All equipment pipes must be sealed more tightly to prevent air from leaking in. The negative pressure process is suitable for the water-washing ammonia process. Coal gasification The coal gasification process is a thermochemical process. It is a process in which coal or coal coke (semi-coke) serves as the raw material, and oxygen (air, oxygen-enriched air, or pure oxygen), water vapor, or hydrogen are used as gasifying agents (or gasification media); under high-temperature conditions, chemical reactions are employed to convert the combustible components in coal or coal coke into gases. The goal is to convert coal into combustible gases, and the coal gasification process includes steps such as the pyrolysis of coal and the gasification of semi-coke. The main components of gas are CO, CO2, H2, CH4, and H2O; the gas obtained through gasification is referred to as gas, and its active components include hydrocarbons, hydrogen, and methane. Among various coal conversion technologies, particularly in the development of clean coal technologies, coal gasification is one of the most promising techniques. The table below lists the coal pyrolysis reactions that occur during gasification, the homogeneous and heterogeneous reactions, as well as their heat effects. The gas involved in the reaction may be the initial vaporizer or a product of the vaporization process. Among these reactions, R3, namely the reaction between water vapor and carbon, is of the greatest significance; it is involved in various coal gasification processes, and this reaction is highly endothermic. In the reaction, R4 is also an important gasification reaction. The endothermic R1 and R2 reactions, combined with the exothermic R3 and R4 reactions, play an important role in the self-heating gasification process. The hydrogenation gasification reaction R5 is important for producing synthetic natural gas (SNG). The production of hydrogen or syngas is achieved through a combination of reactions R1, R2, and R3. The small amounts of elemental nitrogen and sulfur present in coal yield nitrogenous and sulfurous compounds during the gasification process; the main sulfides are H2S, COS, CS2, etc., while the main nitrogenous compounds are NH3, HCN, NO, etc. Coal gasification methods: 1. Moving-bed (fixed-bed) coal gasification; 2. Pulverized coal fluidized-bed gasification; 3. Gas-flow bed coal gasification. Moving-bed coal gasification uses lump coal as the raw material – the coal is fed in from the top of the gasifier, while the gasifying agent is introduced from the bottom. ✦The gasifying agent comes into counter-current contact with the coal, allowing the gasification reaction to proceed more completely, resulting in less residual carbon in the ash. ✦A considerable portion of the sensible heat in the product gas is used for drying and carbonization prior to gasification; the low temperature of the gas outlet, combined with the sensible heat of the ash, preheats the gasifying agent fed into the furnace, thereby resulting in high gasification efficiency. ✦This is an ideal method of complete gasification. Mixed producer gas is obtained by using a mixture of steam and air as the gasification agent; the gas produced in this way is known as mixed producer gas. 3M13 type gas generator. It is characterized by a double-drum continuous feeding method. It uses a rotary furnace grate for continuous ash discharge, and is equipped with stirring rods to break up clumps; it is suitable for weakly caking coals such as long-flame coal and gas coal. The inner diameter of the furnace is 3 m, the diameter of the air inlet is 500 mm, the diameter of the gas outlet is 900 mm, and the maximum inlet pressure is 4000–6000 Pa. Coal consumption is 1,700–2,500 kg/h, with a gas production of 5,500–8,000 m3/(h·unit). The amounts of steam and air required are 0.3–0.5 kg (steam)/kg (coal) and 1.5–2.5 m3 (air)/kg (coal), respectively. 2 Water gas: Water gas is a type of gas produced by the reaction of hot carbon with water vapor; it is primarily composed of CO and H2. Compared to producer gas, it contains very little nitrogen and has a higher calorific value. It produces a blue flame when burned, which is why it is also known as blue water gas. 3. Moving-bed pressurized gasification: The most mature furnace design for moving-bed pressurized gasification is the Lurgi type. Similar to atmospheric-pressure moving beds, it is also a self-heating counter-current reactor; the difference lies in the use of oxygen-water vapor or air-water vapor as the gasifying agent, with continuous gasification taking place at pressures of 2.0–3.0 MPa and temperatures of 900–1100°C. Coal particle fluidized bed gasification. The reason for developing the fluidized bed gasification method is to increase the production capacity of individual furnaces and to adapt to the advancements in coal mining technology; by using small coal particles as raw material, it is possible to utilize low-quality coals with high ash content such as lignite. It is also known as fluidized bed gasification, in which a gasifying agent (water vapor and oxygen-enriched air or oxygen) is introduced into the gasifier to cause the coal particles to be in a fluidized state for the gasification reaction. Inside the reaction bed, when the gas flow rate is below the fluidization critical velocity, it is a moving-bed system; when the gas flow rate is above the maximum settling velocity of the particles, it is a gas-flow-bed system. When the gas flow rate is between these two values, it is a fluidized bed. 1 Fluidized bed gasification process: The fluidized bed is different from the moving bed, but it still has an oxidation zone and a reduction zone. The thickness of the oxide layer is approximately 80–100 mm; the reduction layer lies above the oxide layer and extends all the way to the upper boundary of the entire material layer. 1 Winkler gas furnace: It is a vertical cylindrical furnace made of refractory materials from Uchimura, with a conical shape at its lower part. Water vapor and oxygen (air) are introduced through several rows of nozzles located at different heights in the fluidized bed. Its lower section is a conical fluidized bed, while the upper section, whose height is about 6 to 10 times that of the fluidized bed, serves as the solid separation zone. 3. High-temperature Winkler (HTW) method: To address the shortcomings of the Winkler furnace, the main improvements made to the HTW furnace include: ① Raising the gasification pressure to 1 MPa; ② Increasing the gasification temperature; ③ Recycling the coarse powder carried out of the fluidized bed back into the fluidized-bed gasification process, thereby improving the carbon conversion rate. 4. Ash-agglomerated fluidized-bed coal gasification method: Gasified coal in a fluidized bed. A fluidized bed refers to a system in which the gasifying agents (water vapor and oxygen) carry pulverized coal into the gasification furnace for co-current gasification. The pulverized coal is carried by the gasifying agent through a special nozzle into the reactor, where it ignites instantly to form a flame with a temperature of up to 2000°C. Coal powder and the gasifying agent flow parallel to each other within the flame; the coal powder burns and gasifies rapidly, with the reaction time being only a few seconds. It can be assumed that the exothermic reactions and the endothermic reactions occur almost simultaneously. At the tip of the flame, that is, before the gas leaves the gasifier, all of the carbon has been consumed. The caking property of coal has no impact on the coal gasification process. Dry-feed fluidized bed gasification methods: ① K-T type gasifier ② Shell method ③ Prenflo method. A IGCC demonstration plant has been established in Puerto Ilano, Spain, using these methods; it generates 300,000 kWh of electricity, the gasifier has a capacity of 2,600 t/day, and it produces 180,000 m3/h of syngas. ④The GSP gasifier is a down-flow pressurized fluidized bed liquid slag discharge gasifier that was developed in 1976 by the Black Water Pump Company of VEB Gaskombinant in East Germany. It operates at a pressure of 2.5–3.0 MPa, uses pulverized coal and oxygen for combustion, and its structure and working principle combine features of both the Texaco and Shell gasifiers. Gas flow bed gasification method with wet feed: ① Texaco gasification method; ② Destec gasification method (originally known as the DOW method); ③ Multi-nozzle opposed gasification method