Which Parameters of a Grate Incinerator Should You Adjust to Strengthen In-Furnace Airflow Disturbance?
Adjusting secondary air, primary air, grate movement and furnace geometry in a grate incinerator all changes the strength of gas turbulence in the furnace. Secondary air has the more direct effect, primary air and grate movement act together with it, and furnace geometry acts indirectly.
Secondary air parameters
Increasing secondary air volume can markedly strengthen turbulence in the furnace, so that secondary air mixes more strongly with primary air and the combustion gas and the degree of turbulence rises. Unburned particles and gases then spread better through the furnace, their contact area with oxygen increases and combustion becomes more complete. Too much air, however, makes the furnace temperature fall too quickly or even blows the flame out. Raising the velocity strengthens the penetration of secondary air into the furnace gas, so that it reaches deeper into the combustion zone and breaks up the existing stratification.
The direction and angle must also be adjusted to the combustion conditions and flame shape; injecting obliquely upward from the side, for example, pushes the gas and unburned matter of the combustion zone upward and sideways and forms a circulating flow in the furnace. Injection points are usually placed in the upper furnace or near the throat, where they work with the shape of the furnace arch to form a recirculation zone more easily.
Primary air parameters
Primary air mainly supplies the oxygen for the early stage of waste combustion. Distributing primary air sensibly between the different grate zones changes the gas distribution inside the waste bed. Adding air at the feed end dries and starts burning the waste quickly, and the gas produced then works better with secondary air in the later zones. Too low a primary velocity cannot penetrate a thick waste bed, while too high a velocity may blow away unburned waste particles, so a suitable velocity has to be found through tests and operating experience so that good air channels form in the bed.
Grate movement parameters
Faster grate movement raises the turning frequency of the waste in the furnace; the bed is continuously turned and mixed, gas in the bed escapes more easily and mixes better with the furnace air. For wetter or coarser waste, a suitable increase in speed prevents the bed from piling too thick and keeps it permeable. The type of movement matters as well: a reciprocating grate turns the waste back and forth, a rotary grate rolls it around the circumference, and choosing or optimising the combination for the waste characteristics and combustion requirements increases the degree of mixing between waste and air.
Furnace geometry parameters
Furnace height and shape have an indirect effect on gas turbulence. A taller furnace gives the gas a longer path, so that it has more time and space to mix. A converging furnace, or one with internal guide structures, directs the gas flow and promotes circulation, and a converging furnace reduces the cross-section as the gas rises, raising gas velocity and helping turbulence. Inlet and outlet positions and sizes also need optimisation: sensible arrangement lets primary and secondary air enter the furnace more evenly, while the size and position of the outlet affect the negative pressure state and the gas velocity in the furnace.
