Boiler and Combustion Controls

Burning biomass in boilers has many challenges over and above the standard challenges from burning coal or oil:

• The energy density is considerably less, so more mass and volume of fuel must be burned to obtain an equivalent heat value.
• Water content is often significant, also resulting in more fuel being required to obtain the necessary heat value.
• A larger proportion of the fuel is typically inert ash, so together with the larger amount of fuel required to obtain the necessary heat, ash production can be far higher than with coal producing equivalent heat.
• Material handling is more challenging. Biomass is almost always fibrous, which is more challenging to handle than liquid, chunks of coal, or pulverized coal powder. Biomass also often comes in a very wide variety of sizes, and (especially if construction debris is part of the source stream) can contain foreign material such rebar and nails, or good material that is too large, such as whole lumber.
• Emission controls are challenging to a degree similar to coal, but with different specifics. This is especially true when burning waste streams such as MSW, TDF, and creosote-treated lumber, in which complete combustion must be assured.
• Biofuel boilers with grates are far more subject to piling, where unburned biofuel builds up on one section of the grate while burning well on other sections. This reduces efficiency, maximum load, and forms clinkers.

Biofuel combustion controls, therefore, have corresponding challenges over and above the standard challenges from controlling the combustion of coal or oil:

• There is typically far more fuel and ash handling equipment relative to what would be required for the same steam production from burning coal.
• More auxiliary equipment is often required.
• Combustion is often less consistent due to varying water content, fuel quality & size, so the amount of air needed at a particular load is much more variable.
• The flow of biomass into the boiler is typically not measured – and if it is, the heat value is still unknown – so traditional cross-limiting does not work, and flow must be assumed or estimated from belt scales and screw speeds.