What Is the Applicable Fuel Humidity Range for Biomass Reciprocating Grates?

The fuel moisture range for a biomass reciprocating grate is set separately for loose material and pellets. Moisture affects drying speed, ignition stability and thermal efficiency, and must be held in a reasonable band.

Optimum moisture

For conventional loose biomass such as wood chips and chopped straw, the as-received moisture is best at 15%-25%, balancing drying speed, stable combustion and thermal efficiency, and matching the upstream drying zone of the reciprocating grate. Biomass pellets should have as-received moisture no higher than 15%, ideally 8%-15%, where combustion is stable and efficiency high.

Permissible upper limit

A reciprocating grate with an optimised drying zone can take high-moisture agricultural and forestry waste such as bark and wet sawdust at up to 40% for short periods; thermal efficiency then drops clearly, flue gas heat loss rises and furnace temperature suffers, so continuous full-load operation is not possible. Above 40% for long periods, ignition is difficult, the furnace runs too cold, and smouldering, slagging and accelerated grate corrosion become likely.

Operating control

Below 8% moisture the fuel burns too fast, flames flash back and local overheating raises the risk of grate burnout, so primary air should be reduced, grate speed raised and feed rate controlled. In practice the drying zone length, primary air temperature and feed rate are adjusted to the incoming moisture so the fuel entering the furnace sits near the optimum band.