Fluidised Bed Combustion is a technology developed for low value, variable moisture content fuels. It’s typically used in large power plants, but BHSL have developed it for on farm applications.
How It Works
A normal diesel burner raises the combustion chamber up to operating temperature, then the manure is dropped onto a bed of sand and combustion starts. The diesel is no longer needed after start-up – the plant can operate using only manure.
The BHSL fluidised bed has been proven to accept a wide range of forest and agricultural biomass fuels. To aid combustion maximum air recirculation is used.
The combustion chamber (furnace) is equipped with a gas oil burner used for start-up purposes and to support combustion in the event of abnormal or upset conditions.
The furnace has been designed to ensure that the fuel conversion to heat energy is complete and with a minimum of undesirable emissions. The furnace is lined with refractory designed to withstand the operating temperatures.
Primary air is released into the sand bed through a system of nozzles. In order to achieve clean combustion and ensure minimal harmful flue gas emissions BHSL’s furnace and bed design targets an energy release that ensures bed temperature stability and good flue gas residence times.
Poultry manure is directed from the buffer hopper down a chute and falls by gravity onto the fluidised bed. The larger and heavier particles of the fuel are absorbed into the bed and combusted. Lighter fractions tend to dry and combust above the bed.
Combustion temperatures above the bed and in the upper furnace zone (freeboard) are constantly monitored and the results are utilised for combustion control programming inputs as well as ensuring that minimum combustion temperatures are maintained for destruction of pathogens that may arise from the biomass fuel.
The ‘seething’ mass of hot sand has excellent thermal properties. If manure is presented which is particularly wet, then this can be broken up and the ‘thermal shock’ can be spread within the sand very quickly. The combustion is reliable, even when the quality of the fuel is not.
There are limits to every process of course, and broiler manure must typically have a Moisture Content of less than 45% (it can be higher for short periods) to be reliably processed. Improved growing conditions on BHSL sites result in average moisture
content lower that 45%.