Basic Knowledge of Coal Gasification

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The equipment used for coal gasification is called a gasifier. Classifying according to the movement of the fuel within the gasifier is a common method; it is generally divided into moving bed (also known as fixed bed), bubbling bed (also known as fluidized bed), gas flow bed, and molten bed, among others. Depending on the gasifying agent used, the properties and applications of the produced gas vary. If air is used as the gasifying agent, the gas produced is called air gas ； Gas produced by using a mixture of air (oxygen-enriched air or pure oxygen) and water vapor as the gasifying agent is called mixed coal gas ； If air (oxygen-enriched air or pure oxygen) and water vapor are fed into the gasification furnace alternately, on a intermittent basis, the gas produced is called water gas ； After the gas composition is appropriately adjusted (mainly by adjusting the amount of nitrogen), the gas produced meets the requirements for raw material gas in ammonia synthesis; this type of gas is called semi-water gas. Furthermore, during the production process, gasifiers are divided into atmospheric pressure gasifiers and pressurized gasifiers, depending on the pressure used ； Based on different slag discharge methods, they can be divided into solid-slag discharge gasification furnaces and liquid-slag discharge gasification furnaces. Generally speaking, gasifiers with various structures are basically composed of three main parts, namely the coal feeding system, the gasification reaction section, and the ash discharge system. Due to the different furnace types, the specific structure of these three parts varies greatly. But in general, the coal feeding system needs to take into account the distribution of coal after it enters the furnace as well as the sealing issues during coal feeding. The gasification section is the main site where coal is gasified. The primary concern in this section is how to convert coal into high-quality gas that meets users’ requirements with minimal consumption. Of course, since the coal gasification process takes place at very high temperatures, it is also essential to install an inner lining or water jackets to protect the furnace body. On the one hand, the water jacket serves to protect the furnace body (including the coal distributor or mixing devices inside the furnace), and on the other hand, it can absorb heat from the gasification zone to produce steam; this steam can then be used as steam required for gasification and fed into the gasification furnace. The residue remaining after coal gasification is coal ash, which is regularly removed from the gasification furnace by the ash discharge system. This ensures a stable layer height of the material inside the furnace, as well as the continuous and stable progress of the gasification process. In the case of a moving bed, since the grating (the device for distributing the gasifying agent) and the ash discharge system are integrated together, the uniform distribution of the gasifying agent and the proper ash discharge are two important aspects in the production process. Regardless of the type of gasifier used or the type of gas produced, it remains the same that the fuel comes into direct contact with the gasifying agent at a certain particle size, allowing for physical and chemical changes that convert the combustible components in the fuel into gas. Meanwhile, the ash and slag generated are removed from the furnace. However, with different furnace types, various types and compositions of gasifying agents, and at different gasification pressures, there are significant differences in the composition, calorific value, and various economic indicators of the produced gas. The structure of the gasifier, the gas-solid phase reaction processes within it, and its various economic indicators are closely interconnected. I. Gas-solid phase reaction: In a gasification furnace, substances exist in essentially two phases. One is the gas phase, which includes air, oxygen, water vapor (referred to as gasifiers) and the gas produced during gasification. The other is the solid phase, which consists of the fuel and the solids formed after the fuel is gasified, such as ash and slag. In industry, this type of reaction is referred to as a gas-solid phase reaction, and it is a very important category of reactions in the coal gasification process. In industrial production, gas-solid phase reactions are usually carried out in a cylindrical vessel. At the bottom of the cylindrical container, there is a porous distribution plate; solid particles are piled on this porous plate to form a fixed layer known as the bed layer. When gas is blown in from the bottom of the bed at a certain velocity, uniform gas distribution through the perforated plate ensures that the gas passes evenly through the bed from bottom to top, and after reaction, it is discharged from the upper part of the container. Depending on the flow velocity of the gas, the entire bed formed by the solid particles will exhibit different patterns of movement, as shown in Figure 4-1. When the gasifying agent passes through the bed at a low velocity, the gas flows through the gaps formed by the accumulation of solid particles, while the solid particles in the bed remain stationary; such a bed is generally referred to as a fixed-bed. For a gasifier, since the gasification process is continuous, fuel is continuously fed in from the upper part of the gasifier while the resulting ash is continuously discharged from the bottom; as a result, the fuel moves downward at a slow pace, which is why it is appropriate to refer to it as a moving bed. As the gas flow velocity continues to increase, the gaps between the particles start to widen, causing the bed to expand and its height to rise. The particles at the upper part of the bed are lifted by the gas flow. When the fluid flow velocity reaches a certain level, all the particles are lifted; they move vigorously but remain within the bed and are not carried away by the fluid. This state of the bed is known as solid fluidization, meaning that the solid particles exhibit fluid-like properties. At this point, the bed is referred to as a fluidized bed. During the fluidized bed stage, if the flow rate is increased further, some of the particles with smaller particle sizes will be carried out of the fluidized bed. At this point, the bed acts like a pneumatic conveying device, which is why it is called a pneumatic bed. The pressure drop and heat transfer in the three types of bed layers are shown in Figure 4-2. The pressure drop in a fixed bed is primarily caused by friction between the fluid and the solid particles, as well as by the sudden expansions and contractions in the flow path as the fluid passes through the bed layer. It increases proportionally with the increase in flow velocity; after reaching a maximum value, the bed layer enters the fluidized state. In this fluidized state, the pressure drop remains constant, being roughly equal to the weight of the bed layer. This maximum value is known as the critical fluidization velocity. Upon entering the fluidized bed, the bed pressure drop drops sharply due to a large amount of particles being carried out of the bed. Regarding heat transfer in the three types of beds, in a fixed bed, the heat transfer rate is low at first; it increases in a linear manner as the flow velocity rises. When the bed starts to fluidize, the heat transfer rate increases rapidly, remains constant during the fluidized bed phase, and then drops sharply once entering the pneumatic bed phase. The gas-solid phase reactions occurring within the gasifier mainly involve reactions between carbon and the gasifying agents, such as the combustion of carbon and reactions between water vapor and hot carbon. Meanwhile, certain components in the gas produced by gasification can also react with carbon, such as the reaction between carbon dioxide generated by combustion and carbon. In addition, a series of reactions can also occur between certain components in the gas phase, such as the reaction between carbon monoxide and water vapor. Therefore, in terms of reaction types, the gasification process of coal involves both homogeneous and heterogeneous reactions. In gas-solid phase uncatalyzed reactions (where the fuel itself participates in the reaction), as the reaction proceeds, the particle size of the fuel gradually decreases, with gas products being continuously generated. As described in Chapter 2, its reaction process consists of five steps, namely the external diffusion of the gasifying agent toward the surface of the fuel particles ； The gasifying agent is adsorbed on the surface of the fuel particles ； Surface chemical reactions occur between the adsorbed vaporizer and fuel particles and the carbon on their surfaces ； The resulting product molecules desorb from the particle surface ； The product diffuses from the surface of the particles through the gas film to the main stream of gas. These processes often have different effects on the rate of coal gasification; the gasification rate determined by these processes together is generally referred to as the macroscopic reaction rate. Meanwhile, ignoring the influence of other processes and considering only the rate of the third step, namely the surface chemical reaction, is known as the intrinsic kinetic rate. However, for actual production, the macroscopic reaction rate is more meaningful. In these five processes, if one of the steps is carried out at the slowest pace while the other steps are much faster in comparison, then the overall reaction rate is determined by this step, and it is referred to as the controlling step. If the diffusion resistance of the reactants is the greatest, the reaction proceeds at the slowest rate; this is known as diffusion control. Increasing the rate of diffusion control is key to accelerating the overall reaction rate. If the chemical reaction rate is the slowest, it is referred to as chemical reaction control; increasing the speed of the chemical reaction is key to accelerating the overall reaction rate. Identifying the controlling steps for different types of reactions is an important basis for determining and adjusting process conditions. Most of the reactions in coal gasification are reversible reactions, with both forward and reverse reactions occurring. When the forward and reverse reaction rates are equal, the chemical reaction reaches dynamic equilibrium. For example, for the endothermic reaction: C + O2 → CO2, according to thermodynamic equilibrium theory, the equilibrium constant Kp, expressed in terms of the equilibrium partial pressures of the various components in the gas, is as follows: If the reaction is endothermic (such as the reaction between carbon and water vapor mentioned above), then according to equation (2-2) in Chapter 2, Kp increases as T rises ； Conversely, for exothermic reactions, Kp decreases as T increases. In other words, for endothermic reactions, increasing the temperature facilitates the chemical reaction to proceed in the direction of product formation ； For exothermic reactions, lowering the temperature is beneficial for proceeding in the direction of product formation. Regarding the effect of pressure on the equilibrium of the gasification process, the basic rule is that for reactions in which the volume increases after the reaction takes place (i.e., an increase in the number of molecules), the equilibrium concentration of the products decreases as pressure increases ； Conversely, for reactions involving volume reduction, increasing pressure facilitates the formation of the product. The above basic laws regarding gas-solid phase reactions also apply to the coal gasification process, and serve as the fundamental basis for determining and adjusting the process conditions in coal gasification.