Combustion control systems for solid fuels such as coal and bagasse (the fibrous material remaining after the juice is pressed from sugar cane) differ from those used on gas and oil fired boilers. MicroMod has designed and installed combustion control systems for many types of solid fuels. They can be used in conjunction with SteamPAK series drum level (DrumPAK) and Plant Master (PlantPAK) systems for total boiler control.

Solid fuels are normally fired on a grate. Variable speed feeders or stokers supply the fuel to maintain steam pressure in the header. Total air is regulated to burn the available fuel, and draft is controlled by modulating furnace pressure with the ID fan controls. Small particles will burn in suspension using over-fire air while the larger particles burn on the grate using air supplied through the grate.

The single-point positioning (jackshaft) type control found on older boilers and still supplied with many package boilers have no provision to compensate for the variable heating values of solid fuels. It must also be calibrated with maximum excess air for safety reasons and to ensure complete combustion. Fully-metered systems require fuel flow measurement, which is difficult and expensive.

The best combustion control system for fuels such as coal and bagasse is parallel positioning with O2 trim. The demand signal for fuel is sent to the fuel feeders, and an over-fire air fan is added to the system. The speed of the OFA fan is adjusted in relation to the main air flow. This strategy requires less excess air and can result in significant savings in fuel consumption and maintenance. A typical system is shown below:

Many plants which use biofuels also use oil or gas as a secondary fuel, to preheat the boilers on startup, to stabilize the combustion process when the solid fuel is damp or clogging the feeders, or when there is insufficient primary fuel to produce the required amount of steam. The addition of a second fuel requires the splitting of the combustion air in the proper amount to each fuel. This is done by using the FD Fan damper to control the air duct pressure. Separate dampers for air to oil and air to grate are needed. As the firing rate modulates, a one point positioning jackshaft modulating the air damper and fuel valve together with a pre-set relationship controls the oil fuel/air ratio. The air to the grate is modulated with boiler demand as before. The air duct pressure control assures that both dampers have sufficient air for both fuels.