As the main equipment for converting coal from solid energy into gaseous and clean energy, the gasifier's choice of gasification coal type is crucial and is directly related to its overall gasification effect. The two-stage gasifier is suitable for gasifying bituminous coals such as non-caking coal, weakly caking coal, long bituminous coal, etc. It can also gasify lignite with good quality. Compared with the one-stage furnace,Two-stage gas generatorThe selection of coal types is wide, but the specific index requirements for gasification coal are strict.

1. Coal particle size

Any type of fixed-bed gasification device requires uniform particle size of coal entering the furnace to meet the needs of fluid mechanics and heat and mass transfer in the furnace. The reason why the two-stage gasifier has strict coal particle size requirements is because the total height of the material layer in the retort section and gasification section of the furnace is about 6-8 meters. If the coal particle size is very different, on the one hand, it will reduce the gap in the bed and increase the resistance in the furnace, resulting in a decrease in gasification intensity, a decrease in gas production, and an increase in the carbon content of the ash. On the other hand, it is impossible to ensure that the coal is evenly and fully retorted, which affects the gasification effect of the two-stage gasifier.

During the operation of the two-stage gas generator, users often only pay attention to whether the furnace structure is perfect and the equipment is operating normally, but they do not pay enough attention to the particle size of the gasification coal. It is precisely this that will directly affect the gasification effect of the two-stage gas station. In some gas stations, the particle size of the coal entering the furnace is even greater than 100mm, resulting in poor operating results. Some data show that when more than 20% of the lump coal entering the furnace has a particle size less than 10mm, the gasification efficiency of the two-stage furnace will drop by 25%.

After analyzing the successful experience at home and abroad, combined with the actual situation in my country, the particle size requirements of the lump coal fed into the two-stage furnace are best controlled within the following range:

Grain size range: 15～30mm; 20～40mm; 25～50mm; 30～60mm

Particle size composition:

>60mm should be <5%

60~40mm is 10%

40~20mm is 70%

20~15mm is 10%

<15mm should be <5%

The two-stage gas generator has corresponding requirements for the cohesiveness, ash melting point, volatile matter, thermal stability and moisture of the coal entering the furnace.

（1）    Adhesion

The cohesiveness of coal is a very important indicator that determines whether the coal can be gasified and used in the two-stage furnace. The reason is: when the coal is heated in the retort section, it will expand and stick. If the coal has strong cohesiveness, it will stick here and gather into large clumps or coal cakes, destroying the uniform distribution of the rising heat-carrying gas and affecting the retorting effect. It will also hinder or even block the uniform downward movement of the material layer, leading to the deterioration of the gasification process in the entire furnace.

（2）    The test items for the cohesiveness and expansion of reactive coal in the heated state include: free expansion number (CSN), colloid layer thickness (Y) value, Roga index and coke slag characteristics (1-8) in coal industry analysis, etc.

The ash melting point of coal is an indicator of coal ash melting characteristics. It is commonly expressed by the softening temperature ST of coal ash. The ash melting point is an important parameter for judging whether coal slags during the gasification process in the furnace. Single-stage furnaces usually require ST>1250℃; the ash melting point of coal used for gasification in two-stage furnaces should be higher, which is considered from the operating temperature distribution of the entire furnace. In order to meet the optimal reaction temperature of 450-600°C in the retort section, the temperature of the airflow rising from the gasification section should be between 600-700°C. Correspondingly, the temperature of the combustion layer will be higher than that of a single-stage furnace. For example, if the ST of the selected raw coal is >1300°C, it can meet the heat requirements of the reactions at all levels in the furnace without causing slag dissolution or slag hanging during the gasification process.

(3) Volatile matter

The two-stage gas generator structure can reduce the amount of dust carried by the coal gas; in the carbonization section, the volatile matter in the coal is converted into small molecular hydrocarbons and precipitated, and the calorific value of the coal gas can be recovered or increased. Therefore, we hope that the volatile matter content in coal will be correspondingly higher to highlight the above advantages. Generally speaking, the dry ash-free volatile matter content in coal should be no less than 25%, and the 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 the thermal stability of the coal is poor, the coal will crack and break due to heat after being put into the furnace, which will increase the resistance in the furnace and increase the amount of material brought out. In a single-stage gas furnace, the coal suddenly encounters high temperature and decomposes rapidly after entering the furnace. The volatile matter in the coal precipitates rapidly, which will cause the coal to burst and shatter. The two-stage furnace structure has a high retort section and a thick material layer. The coal after entering the furnace is slowly heated and the temperature of the moisture and volatilization in the coal is correspondingly slow, which will not cause too much impact on the coal seam. Therefore, the thermal stability requirements of the two-stage furnace for coal can be appropriately relaxed. For example, lignite with poor thermal stability can also be used in a two-stage furnace for gasification.

(5) Moisture

The high moisture content of coal is very unfavorable for gasification operations. Not only does evaporating the water in it require a certain amount of heat, which reduces the calorific value of the gas, but it also causes great inconvenience in storage, transportation and operation, especially in winter. Statistics show that the calorific value of the gas in the upper section of the two-stage furnace will significantly decrease as the moisture in the gasified coal increases, which is very sensitive.

Foreign data report that the maximum moisture content of the coal entering the two-stage furnace should be controlled within the range of 15 to 25%. Coal higher than this value must be dried before use. The FW-Stoic two-stage furnace in the United States stipulates that the moisture content in the coal entering the furnace is 12 to 15%; the W in the United Kingdom·The two-stage furnace requires that the moisture content in the coal entering the furnace is <15%; it has also been reported that the moisture content in the coal entering the furnace is <6%. According to the actual situation in my country, it is appropriate to control the moisture content in coal entering the furnace to <15%.

The moisture content of the coal used for gasification in the two-stage furnace will also affect the height design of the retort section. If the moisture content is high, the dry distillation section will be high, and vice versa.

In order to control the moisture content of the coal entering the furnace, a coal shed should be set up at the two-stage furnace gas station, which is especially important in the rainy southern region. Otherwise, after the wet coal enters the vibrating screen, the pulverized coal will block the screen and reduce the screening effect. It will be difficult to ensure the particle size requirements of the coal entering the furnace, thereby affecting the gasification effect.

It is worth noting that certain moisture is one of the leading factors causing poor thermal stability of coal. In particular, the bound water in the internal moisture is prone to sudden precipitation and vaporization, resulting in coal cracking. In the two-stage furnace, due to the higher carbonization section, the coal is heated slowly, and the water in the coal slowly evaporates and precipitates, which can produce a good drying effect and improve the thermal stability of the coal. Therefore, the requirements of the two-stage furnace on the moisture in coal can be relaxed appropriately. For example, lignite with a relatively high moisture content can also be used for gasification in the two-stage furnace.

(6) Mechanical strength

Two-stage gas generatorThe coal seam inside the furnace is relatively thick, generally more than 6 meters. When the coal moves downward in the furnace, extrusion and friction will occur. Therefore, the coal used in the two-stage gas generator is required to have higher resistance to crushing, falling and wear than the coal used in the single-stage gas generator.