Ash from coal combustion

Boiler coking is a common problem in the operation of coal-fired boilers. It refers to the phenomenon where molten slag particles in coal-fired boilers stick to the heated surfaces. Under normal circumstances, the temperature of the flame in the furnace is very high; at such temperatures, the ash resulting from fuel combustion is usually in a melted or softened state. The ash particles that move along with the flue gas are cooled together with the flue gas due to the heat absorption by the heated surfaces of the furnace’s water-cooled walls. If the liquid slag particles solidify due to the drop in temperature before reaching the water wall or furnace lining, they will form a loose layer of ash when attached to the tubes of the heated surface; this layer can be easily removed during operation by soot blowing. If slag particles adhere to the wall of the heated tubes or the furnace wall in a liquid or semi-liquid state, a dense layer of ash and slag is formed, a condition known as coking. After coking occurs on the heating surface, the thermal resistance of the coked layer becomes very high, which reduces the heat transfer capacity of the heating surface. As a result, less heat is absorbed inside the furnace, leading to an increase in flue gas temperature and greater exhaust losses for the boiler. At the same time, this will cause the steam temperature to rise; in order to maintain the rated parameters during operation, it becomes necessary to increase the amount of water used for temperature reduction, and in some cases, the output capacity has to be reduced. An increase in the furnace exit temperature, which leads to coking at the furnace exit, increases the flue gas resistance and also results in reduced operational efficiency of the boiler. After coking occurs in the water wall, its heat transfer capacity decreases, and the heating is uneven in the coked and uncoked areas, which may lead to tube failure in the water wall. After coking occurs inside the furnace, the smoke temperature at the furnace outlet rises, which leads to an increase in the superheated steam temperature. Coking in the superheaters and reheaters further increases the thermal deviation, resulting in over-temperature damage and explosion of the high-temperature superheaters and high-temperature reheaters. When the boiler is severely clogged and large pieces of slag suddenly fall, it can also lead to the extinguishment of the fire; in addition, it may even damage the water-cooling wall tubes, resulting in serious accidents.

This is easy to understand. For example, if the melting point of gray material is 2000 degrees, then coking occurs at 2000 degrees; since the temperature in the furnace never reaches such a high level, coking certainly won’t happen.

When designing circulating fluidized bed boilers, the bed temperature is generally controlled to be about 200 degrees lower than the ash softening temperature℃

Ash is the substance in coal that does not burn and produces no heat. The melting point of the ash is high; therefore, when the combustible components of coal burn, they are not affected by the ash

Coking and slagging are two different concepts; the poster is referring to slagging properties, I guess

The higher the ash fusion temperature, the easier it is to form ash.