The coal quality condition is an important basis for boiler design. Different coal qualities lead to significant differences in boiler design types and heating surfaces, resulting in different types of pulverization systems and parameters selected for auxiliary equipment. If the coal quality changes, it will have a significant impact on various systems of the boiler, and at the same time affect the safety and economy of the unit operation.
The influence of different components in coal quality on boiler operation
Boiler coal composition: moisture, ash, volatile matter, sulfur, fixed carbon, calorific value, ash melting point. Both moisture and ash are detrimental to combustion, reducing the combustion temperature of the fuel, hindering the contact between combustible substances and oxygen, and increasing smoke loss. In addition, ash content is also the root cause of furnace slagging and heating area ash. An increase in sulfur content will exacerbate the corrosion of the heating surface (resulting in an increase in the production of hydrogen sulfate ammonia, which will exacerbate the blockage and corrosion of the air preheater). The fixed carbon content reflects the degree of coal carbonization, and the higher the content, the more difficult it is to burn out. Volatile matter is an important characteristic of fuel combustion, and its low ignition temperature makes coal easy to ignite. Volatile matter is also an important basis for classifying coal. According to volatile components, it is divided into lignite, bituminous coal, and anthracite. Asphalt coal is classified into low, medium, medium high, and high volatile coal based on its volatile content>10-20%,>20-28%,>28-37%, and>37%.
1. The impact of volatile components in coal on boiler economy
Volatile matter is an important characteristic of coal, and its level has a significant impact on the ignition and combustion of fuel. Volatile matter is a combustible gas with a low ignition temperature, making coal easy to ignite. The flame is large and stable, but the flame temperature is lower. In addition, volatile matter precipitates from the inside of coal powder particles, making them porous and having a larger contact area with combustion air, making them easier to burn out. When the volatile content decreases, the situation is opposite, and the combustible content of boiler fly ash is relatively high; At the same time, as the flame center moves upwards, the heat absorption of the convective heating surface increases, and the exhaust temperature at the tail also rises, resulting in an increase in exhaust loss. Coal with low volatile matter is difficult to ignite, unstable in combustion, and increases heat loss due to incomplete chemical and mechanical combustion; In addition, the volatile matter of coal is closely related to the storage of coal and the safe operation of the pulverization system. When the volatile content of coal is high, the coal powder in the pulverization system is prone to ignition and spontaneous combustion. The probability of ignition in the pulverization system significantly increases when using high volatile Indonesian coal in our factory.
2. The impact of coal's calorific value on economy
When the calorific value of coal decreases, the actual amount of coal used for the same boiler load increases. For a direct blowing pulverization system, the primary air volume required to transport coal powder also increases, the power consumption of the coal mill will increase, and the resistance of the pulverization system will also increase. This leads to an increase in the required fan pressure and power consumption, resulting in an increase in the plant power consumption rate; At the same time, it leads to a decrease in the theoretical combustion temperature and the temperature level inside the furnace, causing a delay in the ignition of the coal powder airflow, deterioration of combustion stability, and affecting the burnout of the coal powder. The decrease in heat generation of coal will also increase the exhaust gas temperature of the boiler, increasing the heat loss of exhaust gas. The decrease in heat generation of coal can also lead to serious accidents such as boiler shutdown.
3. The impact of coal ash content on boiler operation
(1) Affects ignition and combustion processes. The ash content in coal plays a role in hindering the chemical reaction between oxygen and carbon in boiler combustion. An increase in ash content can easily lead to ignition delay, and at the same time, the combustion temperature in the furnace decreases, resulting in a decrease in the degree of coal combustion and causing significant incomplete combustion losses.
(2) The ash content in coal is the non combustible part. During the combustion process of coal, not only does it not generate heat, but the high-temperature slag discharged from the furnace loses a large amount of physical heat, and the mechanical incomplete loss increases.
(3) Affects safe operation. As the ash content increases, the wear and tear of boiler equipment and ash discharge system components almost increases proportionally, while also exacerbating the contamination and slagging of heating surfaces and corrosion of pipelines, thereby posing safety hazards.
4. The influence of coal moisture on boiler operation
The presence of moisture not only reduces the content of combustible substances in coal, reduces heat generation, but also consumes a large amount of heat due to thermal evaporation and vaporization, resulting in a decrease in furnace temperature, difficulty in ignition of coal powder, increased smoke exhaust, and increased plant power consumption. At the same time, it also increases the probability of blockage in coal and ash transportation systems, affecting normal power supply.
Burning high moisture coal results in high partial pressure of water vapor in the flue gas, which promotes the formation of sulfuric acid vapor from sulfur trioxide in the flue gas, increases the condensation and deposition of sulfuric acid vapor at the low temperature of the boiler tail, and causes corrosion, ash blockage, and peeling of the chimney lining of the air preheater.
5. The impact of coal sulfur content on boiler operation
The sulfur in coal can be divided into combustible sulfur and non combustible sulfur, and the sum of the two becomes total sulfur. Burning high sulfur coal in boilers will exacerbate the corrosion of the heating surfaces at high and low temperatures, especially the air preheater, which has a greater impact. When burning high sulfur coal, it is also necessary to consider the output limitation of desulfurization to avoid unit load limitation caused by substandard SO2 emissions. Generally, the sulfur content during coal blending should not exceed 0.8%.
6. The impact of ash fusibility on boiler operation
Ash fusibility is an important characteristic indicator that affects the safe and economic operation of boilers. If the melting temperature of coal ash is low, the boiler is prone to slagging, which has a significant impact on the operation of the boiler.
