Biomass Boiler Overfire Air Systems

Biomass is a challenging class of fuel. Its variable moisture content, heating value, size distribution and the inconsistent flow of material like wood bark and chips, agricultural waste, paper mill fines and sludge, and plywood/OSB sanding dust often make efficient combustion difficult. A poor combustion environment can cause high CO emissions, insufficient char burnout, and excessive fly ash carryover with resultant erosion issues. These deficiencies often require co-firing fossil fuel to sustain sufficient combustion.

Biomass derived fuels contain a large amount of volatile matter. To achieve more complete fuel burnout, a large amount of air needs to be delivered above the fuel bed by an overfire air (OFA) system. But to be effective, the OFA must be capable maintaining cross boiler penetration and turbulent mixing at all expected loads. These requirements are often unmet by existing OFA systems.

An OFA system with Jansen’s patented High-energy Multi-range Combustion Air Nozzle™ meets these operating criteria. By placing these nozzles on the furnace sidewalls and interlacing their flows, turbulent mixing is maintained across the full width of the boiler. This characteristic eliminates centralized vertical flow and keeps combustion and radiant heat evenly dispersed in the furnace cavity. This in turn ensures complete combustion of fuel fines and volatiles and increases operational efficiency.

The Jansen OFA nozzle design has several key features with installation and operational benefits:

  • Large damper box and nozzle volume – The resultant pressure drop is very low across the unit. Many installations make use of existing combustion air fans without modification.
  • Split nozzle tip – The air flow is divided into two flows of 66% and 34% of the volume. This allows for effective turndown to optimize the OFA air flow across a wide load range.
  • Internal dampers – Isolated from direct contact with the furnace and its radiant heat, the dampers remain cooler and, unlike conventional velocity dampers, will not warp or jam due to distortion.
  • Separate damper control – The dampers are controlled independently. Adjustments are made manually or via a system modulated actuator as determined by the user’s needs.

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