The biomass feeding process
An ‘old boiler-man's motto is: “If you can get the fuel into the furnace and the ash out of the furnace most of your combustion problems are solved”. How true.
Babcock & Wilcox Vølund hold vital experience in the various processes, and we have developed feeding technologies suitable for a wide spectrum of bio-fuels.
Bio-fuels can be split into two main groups:
- Straw, grass and similar
- Wood and by-products from food processing
Two different feeding systems
For solid bio-fuels, two different ways of handling the feedstock during ‘harvest’ have lead to the development of two different feeding systems, each optimised for one main fuel type.
Feeding of straw, grass and similar
Our biomass rake (RBR)
These fuels are pressed into bales to facilitate transportation and handling. For fuels pressed into “Heston” bales we have developed a bale “disintegrator”, the biomass rake (RBR).
Bales are handled by crane with a number of grabs depending on the specific fuel consumption. The bales are brought to the boiler house from storage by chain conveyors.
Bales are then introduced to the rake through an air sluice to minimise air leakage. The rake splits the bales into its natural slices and loosens it to a lower density.
The straw falls down into a double-screw stoker, which presses it into the furnace through a water-cooled tunnel. It is then burned via a combination of surface combustion and traditional grate combustion while being transported through the furnace by the water-cooled vibrating grate (HVB).
Feeding of wood and by-products from food processing
The majority of by-products from food processing are delivered as bulk particles (nuts, shells, etc.). Wood, bark, pruning, etc. are also delivered predominantly in bulk particles in the form of chips.
Different bulk material handling equipment such as moving bottom, scraping or belt conveyors are used to bring the fuel to the day silo in the boiler house.
From the silo, the fuels are metered out by speed-controlled screw conveyors. Each conveyor takes the fuel to an air-swept spout (ASA) which casts the fuel into the furnace.
Small particles will dry, pyrolyse and burn instantly in suspension, whereas larger or more humid particles will be spread out over the grate surface and burn through a traditional grate combustion process.
Co-firing
Co-firing wood and straw is possible to a certain extent. If both fuel types are used as the main fuel in the same boiler, both systems should be installed.
Fuels not compatible with the main systems can be stoked into the furnace with a reciprocating feeding grate or a hydraulic bio-feeder (HF).
