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This post was last edited by ryn on 2010-3-1 20:42. A brief analysis of the coal requirements for gasification in two-stage gas generators. As the main equipment used to convert coal, a solid fuel, into gaseous and clean energy, the choice of coal suitable for gasification is crucial, as it directly affects the overall efficiency of the gasification process. A two-stage gas generator is suitable for gasifying bituminous coals such as non-sticky coal, weakly sticky coal, and long-bituminous coal, as well as lignites of better quality. Compared to single-stage generators, two-stage generators allow for a wider range of coal types to be used, but they have stricter requirements regarding the specific properties of the coal used for gasification. 1. Coal particle size: Any type of fixed-bed gasification unit requires that the particle size of the coal fed into the furnace be uniform, in order to meet the requirements of fluid dynamics as well as heat and mass transfer within the furnace. The reason why two-stage furnaces have strict requirements regarding the particle size of coal is that the total height of the material layer in the drying and gasification sections of the furnace is approximately 6–8 meters. If the particle sizes of the coal vary greatly, it will reduce the gaps within the bed layer, increase the resistance inside the furnace, leading to a decrease in gasification efficiency and a lower yield of gas. Additionally, the carbon content in the ash and slag will also increase as a result ； On the other hand, it is not possible to ensure uniform and thorough carbonization of the coal, which affects the gasification efficiency of the two-stage gas generator. During the operation of two-stage furnaces, users often focus only on whether the furnace structure is sound and whether the equipment is operating properly, but they pay insufficient attention to the particle size of the coal used for gasification; it is precisely this factor that directly affects the gasification efficiency of two-stage gasification plants. In some gas stations, the particle size of the coal fed into the furnace is even greater than 100 mm, resulting in poor operational performance. Data shows that when more than 20% of the lump coal fed into the furnace has a particle size of less than 10 mm, the gasification efficiency of the two-stage furnace decreases by 25%. By analyzing successful experiences both domestically and internationally, and taking into account the actual conditions in our country, the particle size requirements for lump coal fed into the two-stage furnace should ideally fall within the following range: Particle size range: 15–30 mm ； 20–40mm ； 25～50mm ； Particle size distribution: >60mm should be <5%; 60–40mm accounts for 10%; 40–20mm accounts for 70%; 20–15mm accounts for 10%; <15mm should also be <5%. 2. Coal quality requirements: Two-stage furnaces have specific requirements regarding the cohesion, ash fusion point, volatile matter, thermal stability, and moisture content of the coal fed into them. (1) Cohesiveness The cohesion of coal is a very important indicator that determines whether it can be used for gasification in a two-stage furnace. This is because, during the carbonization stage, the coal expands and sticks together when heated; if its cohesion is high, it will clump together to form large lumps or coal cakes, which disrupts the uniform distribution of the rising hot gases and affects the efficiency of carbonization. Moreover, it hinders or even blocks the uniform downward movement of the material layer, thereby deteriorating the gasification process throughout the furnace. The test parameters for the cohesion and expansibility of reactive coal when heated include: the free expansion number (CSN), the vitrinite thickness (Y) value, the Rogge index, and the cinder characteristics in coal industrial analysis (1-8), among others. Experience shows that the cohesion of the coal fed into the furnace should preferably be selected according to the parameters recommended in Table 1. Table 1 Requirements for the caking property of coal in two-stage furnaces. Item, Recommended values: Free expansion number (CSN) < 2; Roga index (R.I), % < 20; Gel layer thickness (Y value, mm) < 10; Clinker characteristics (1-8) < 3. The ash fusion point of coal is an indicator of its melting properties, commonly expressed by the softening temperature of the ash. It is an important parameter for determining whether coal will form clinker during gasification in the furnace. Single-section furnaces generally require ST＞1250℃ ； The ash fusion point of the coal used for gasification in the two sections should be higher, taking into account the temperature distribution throughout the furnace. To achieve the optimal reaction temperature of 450–600°C in the dry distillation section, the temperature of the airflow rising from the gasification section should be between 600–700°C; accordingly, the temperature in the combustion layer will be higher than that in a single-stage furnace. For example: the ST value of the selected raw coal is greater than 1300°C, which allows the gasification process to meet the heat requirements of various reaction layers within the furnace without the occurrence of slag formation or sticking of slag. (3) Volatiles: The two-stage furnace structure can reduce the dust content in the gas ； In the dry distillation section, the volatiles in coal are converted into small-molecule hydrocarbons and separated out, enabling their centralized recovery or increasing the calorific value of the gas. Therefore, we hope that the volatile matter content in coal is relatively high, so as to highlight the aforementioned advantages. Generally, the content of dry ash-free basis volatile matter in coal should be at least 25%, while the ratio of fixed carbon to volatile matter should be greater than 1. (4) Thermal stability: For fixed-bed gasifiers, the thermal stability of coal is an important factor affecting normal gasification operations. If coal has poor thermal stability, it will crack and break upon being heated in the furnace, which in turn increases the resistance inside the furnace and the amount of material carried away. In a single-stage gas furnace, once coal enters the furnace it is exposed to high temperatures immediately, causing it to undergo rapid thermal decomposition; the volatiles in the coal are released quickly, which leads to the coal breaking apart and shattering ； In a two-stage furnace with a high retorting section and a thick coal layer, the coal particles are heated slowly after entering the furnace; as a result, the water and volatiles in the coal evaporate at a slower pace, which does not cause excessive stress on the coal layer. Therefore, the requirements regarding the thermal stability of the coal in such a two-stage furnace can be relaxed to some extent. For example, lignite with poor thermal stability can also be used for gasification in a two-stage furnace. (5) Moisture: A high moisture content in coal is very detrimental to gasification processes. Not only does the evaporation of this moisture require heat, thereby reducing the calorific value of the gas, but it also causes significant difficulties in storage, transportation, and operation, especially during winter. Statistics show that the calorific value of the gas in the upper section of a two-stage furnace decreases significantly as the moisture content in the coal used for gasification increases, showing high sensitivity. According to foreign sources, the moisture content in the coal fed into a two-stage furnace should be kept within the range of 15–25%; coal with a higher moisture level must first be dried before it can be used. For the FW-Stoic two-stage furnace in the United States, the moisture content in the coal fed into the furnace is specified to be 12–15% ； The W-I two-stage furnace in the UK requires that the moisture content in the coal fed into the furnace be <15% ； It has also been reported that a moisture content of <6% in the coal fed into the furnace is optimal. Given the actual conditions in our country, it is advisable to keep the moisture content in the coal fed into the furnace at less than 15%. The moisture content of the coal used in the two-stage gasification process also affects the design of the retorting section height. The moisture content is high; it is also high during the carbonization stage, and low otherwise. To control the moisture content of the coal fed into the furnace, coal storage areas should be installed at the two-stage furnace gas stations, especially in the rainy regions of the south. Otherwise, when wet coal enters the vibrating screen, the coal particles will clog the screen mesh, reducing the screening efficiency and making it difficult to meet the required particle size of the coal fed into the furnace, which in turn affects the gasification process. It is worth noting that certain types of moisture are among the main factors contributing to poor thermal stability of coal; in particular, the bound water within the inherent moisture can suddenly release and vaporize, thereby causing the coal lumps to break apart. In two-stage furnaces, due to the presence of a higher retorting section, the coal is heated slowly, allowing the moisture in the coal to evaporate gradually. This results in an effective drying effect, which in turn improves the thermal stability of the coal. Therefore, the requirements for moisture content in coal for two-stage furnaces can be relaxed to some extent; for example, lignite with a higher moisture content can also be used for two-stage furnace gasification. (6) Mechanical strength: The coal layers in the two-section furnace are relatively thick, usually over 6 meters; as the coal moves downward within the furnace, compression and friction occur. Therefore, coal used for two-stage furnaces is required to have higher resistance to crushing, falling, and wear compared to coal used for single-stage furnaces. In summary, the quality of the coal fed into the furnace has a crucial impact on the proper gasification in a two-stage furnace. Quality standards for coal used in atmospheric pressure fixed-bed gas generators (GB9143-88) are shown in Table 2 ； The technical specifications for two types of coal gasified by gas generator (GB50195-94) are shown in Table 3. For the readers' reference. Project technical requirements Test methods Particle size classification Bituminous coal: 13–25, 25–50, 50–100 mm Anthracite: 6–13, 13–25 ； For 25–50 mm GB189 coal, the maximum allowable size fraction is 50–100 mm; the particle size distribution should be ≤15% for 25–50 mm, ≤18% for 25–80 mm. MTI gangue content, grade 1: <2.0% ； Level 2: 2.0%–3.0%. For MTI, the ash content at Grade A is ≤18.0%, while at Grade B it is >18–24%. According to GB212, the total sulfur content shall be ≤2%. Per GB214, when the ash content (on a dry basis) is ≤18%, the coal ash softening temperature (ST) must be ≥1150℃ ； When ash content (on a dry basis) > 18%, ST ≥ 1250°C. GB219: Thermal stability (TS+6) > 60%. GB1573: Crushing strength (>25 mm) > 60%. GB7561: Thickness of the gum layer (Y); when there is no stirring device in the furnace, Y < 12 mm, and when there is a stirring device, Y < 16 mm. GB497: Calorific value Qnet.v,ar – for anthracite, it is > 23 MJ/kg (>5500 Kcal/kg); for bituminous coal, it is > 21 MJ/kg (>5000 Kcal/kg). GB213 Table 3 Technical specifications for coal types used in two-stage gas generator furnaces (GB50195-94) Item Technical specification Particle size (mm) 20–40 ； 25－50 ； 30–60; Ratio of maximum particle size to smallest particle size ≤ 2; Percentage of coal particles below the specified size limit ≤ 10; Percentage of gangue ≤ 2; Volatiles on a dry basis, Vd (%) ≥ 20; Ash content on a dry basis, Ad (%) ≤ 18; Sulfur content on a dry basis, Std (%) ≤ 2; Softening temperature for ash fusion, ST (°C) ≥ 1250°C; Thermal stability, TS+6 (%) > 60; Crushing strength (>25 mm), % > 60; Rogge index, R·I ≤ 20; Free expansion order, CSN ≤ 2