After heating and igniting the preheating and drying stages, under the action of radiant heat flow, when the temperature reaches about 100°C, the water between the fuel surface and the fuel gap gradually evaporates, gradually changing from liquid to liquid.
In the pyrolysis combustion stage, as the combustion temperature increases, the components with small molecular weight in the fuel are pyrolyzed and vaporized, and gas phase combustion occurs after reaching the ignition point. The main products obtained by pyrolysis gasification are CO and H2.
During the fixed carbon combustion stage, the fixed carbon remaining in the fuel is surrounded by the initial combustion of volatiles. Oxygen cannot touch the surface of the carbon. After a while, the coke starts to burn. During this stage, the fixed carbon mainly reacts with oxygen, carbon dioxide and water vapor, respectively. During this phase, the burn rate increases and the second burn rate peaks.
In the burnout stage, as the combustion progresses, the combustion rate gradually decreases, the ash content increases, and the remaining coke is wrapped in ash, which hinders the diffusion of volatiles, thereby hindering the continuous combustion of coke, thereby reducing the combustion rate. until burnt out. At the same time, residual carbon appears in the ashes.
After the above four stages, the combustion process of the fuel produced by the wood pellet machine manufacturer is actually the combustion process of volatile substances and coke. Although the former accounts for 15% of the burn time, the heat released accounts for 65% of the total heat.
Granular fuel is a solid fuel of shape and density obtained by mechanical pressing in a molding device after pretreatment such as drying and pulverization, and its energy density is not lower than that of coal. For example, a particulate fuel with a water content of 10% and a density of 1.25 g/cm3 can have an energy density ratio of 0.72 and contain about 1/3 the amount of organic compounds as the fixed carbon in the fuel. But the volatile content is high, so the pellets burn more easily than coal.