What is the combustion method of solid fuel?
Fuel combustion generates combustion products (smoke) and releases heat. A portion of the heat is consumed by heating the new fuel, which quickly reaches the ignition temperature, while the rest of the heat is absorbed by the working fluid (water, gas-water mixture, saturated steam) flowing in the heating surface of the furnace. The ability of fuel to fully burn and release all heat is related to the structural characteristics of the combustion furnace, the combustion method used, as well as the air volume regulation and operation mode during the combustion process.
Combustion of solid fuels:
The combustion methods of solid fuel (coal) in industry can generally be divided into three types: laminar combustion and chamber combustion (also known as over limit combustion). Some furnaces have micro firing methods that fall between the three types of firing methods. When the fine coal particles are thrown into the empty enclosure above the fuel by a large mechanical coal spreader, the combustion method of the furnace belongs to suspension combustion or torch layer combustion.
Layer burning method
In a layer burning furnace, only solid fuel is burned. Figure 1-1 shows the working principle of a layer burning furnace. The fuel is sent to a fixed or moving grate to form a fuel layer with uniform thickness. Air flows from bottom to top through the hole gaps on the grate, and there is no relative movement between air and fuel. Most of the fuel is burned in the fuel layer on the grate. The fine coal particles blown into the combustion chamber space, the volatile matter released from the fuel layer, and the unburned combustible gas formed around the coke during combustion are burned in the upper furnace space of the fuel layer. According to the structural characteristics of the grate of a layer fired furnace, it can be divided into three types: the fuel layer is stationary (fixed grate), the fuel layer moves on the grate (vibrating grate and flipping grate), and the fuel layer moves together with the grate (chain grate).
The solid fuel used in the various layer combustion furnaces mentioned above is in small pieces of 0-30mm shape, and the air required for combustion is calculated by the furnace and sent to people. Therefore, the combustion rate of fuel in the layer combustion furnace depends on the surface area of the fuel and the speed of sending air. The smaller the fuel block, the larger its surface area, and the faster the combustion reaction proceeds. However, excessive fine coal particles not only increase air flow resistance and affect air supply, but also are easily carried away by flue gas, increasing combustion heat loss. Therefore, the size of fuel particle size has a significant impact on the combustion of a layer combustion furnace. In addition, the flow rate of air also has an impact on the combustion rate. For example, increasing the air flow rate can accelerate the combustion rate of fuel, but excessive wind speed can blow up some of the fuel, causing damage to the stability of the fuel layer. The steam load of industrial boilers often changes, and the amount of fuel required for combustion also changes accordingly. Therefore, the air supply speed and the amount of coal sent to the grate can be changed to meet the needs of the load.
The layer combustion method plays an important role in small and medium-sized power plants and industrial boilers. Its main advantage is that it is enthusiastic and can be supported by increasing the blower. It is not suitable for large-scale power generation. It is also impossible to achieve full mechanization and automation.