Biomass power stations can be fuelled from Short Rotation Coppice or from timber industry waste products, i.e. sawdust and bark. They are combusted using a fluidised bed and the heat generated is used to raise steam in a boiler. This steam runs a steam turbine driving an electric power generator.
Biomass power stations can be supplied from purpose grown Short Rotation Coppice (SRC) such as woody grasses or willow and poplar saplings and waste products from the timber industry.
This is mainly bark which is chipped before use and sawdust which can be combusted as delivered or formed into wood pellets. Whichever type of wood derivative used it is important to have the supply within 30 miles of the power plant as most wood wastes, coppices and grasses are low density and uneconomical to transport over large distances.
We shall look at wood pellets as a power supply. Once at the plant a fluidised bed boiler is the most efficient way to burn the wood pellets. These are ignited in the fluidised bed, the hot gasses producing steam which drives a steam turbine producing electricity. A typical wood pellet burning power plant could have an output of about 145MW.
This type of power generation can be classed as renewable energy - so long as replacement trees, coppice and grasses are planted to make up for the ones used for fuel in combustion.
Biomass Fuel – Wood Pellets
In this type of power plant, the sawdust is transported from nearby sawmills and stored in heaps under cover in a building much like a Dutch barn, having a high roof and ventilated sides.
The sawdust is conveyed under a magnet that removes any metals to a dryer where the moisture is removed before passing onto the pellet processing plant. Here the sawdust is passed into a hammer mill where the sawdust becomes fibrous strands which are then compressed in dies to form small dense pellets. These are about 6mm diameter and 20mm long and are fed into the fluidised bed boiler combustion chamber by a moving grate.
Fluidised Bed Combustors
There are two categories of fluidised beds,
- FBC operating at atmospheric pressure
- PFBC operating under high pressure
And within these a further two types,
- (BFB) Bubbling Fluidised bed
- (CFB) Circulating Fluidised Bed
In this article we shall examine a circulating fluidised bed boiler.
Circulating Fluidised Bed
The bed is located in the bottom of the combustion chamber and consists of a layer of non-combustible, low density material such as sand through which pressurised air is blown.
The initial firing is by gas burners and these bring the temperature of the fluidised bed up to 800ºC. Once this temperature is achieved the gas burners are shut off and the air pressure is increased. The wood pellets are now fed into the furnace under the bed by a moving grate, quickly igniting and maintaining the temperature of the gasses between 800 and 950ºC. Maintaining this temperature control is very important as it will prevent the formation of NOx and eliminate the need for post combustion gas nitrogen dioxide scrubbing in the fume treatment plant.
Some of the sand can be blown upwards out of the bed, this is collected by cyclones and recirculated into the combustor– hence the name recirculated fluidised bed.
Waste Heat Boiler and CHP
The boiler is a conventional three drum water tube waste heat boiler, except that it has larger than normal down-comer water tubes which run between the bottom water drums, usually outside the combustion area. They pass under the combusting wood pellets, having a cooling effect which in turn slows down the burning process ensuring complete combustion, thus helping further reduce the possibility of NOx and eliminating the formation large pieces of fly ash.
The superheated steam leaves the boiler and enters the steam turbine, HP, IP, and LP phases driving a power generator supplying electricity to the national grid via the plant transformers.
Meanwhile, the expanded steam passes from the LP section into a vacuum condenser where it is condensed and returned to the boiler feed system. The cooling water exiting from the condenser is very hot and is transferred to a community heating system which this makes this type of plant very efficient.
Fume Treatment Plant
Wood pellets contain very minute particles of sulphur and heavy metals, but current air pollution control does not require any scrubbing to remove heavy metals or SOx. Likewise the control of the temperature in the fluidised bed combustor prevents the formation of NOx. This leaves the removal of furnace ash, fly ash, and dust particles.
The safest way to remove the fly ash and dust particles from the exhaust fumes is to pass them through an electrostatic precipitator or a baghouse filter unit as described in an earlier article.
These both remove 99% of the minute, very harmful PMs and the fly ash. A vibrating hammer knocks the dust and ash gathered on the precipitator plates or air jetting the bags into a hopper, and this can be sold as aggregate or, after testing for cadmium, can also be used as land fertiliser.
Last in line is the induction fan which draws the exhaust gasses through the plant and discharges them up the flue.
Emissions to Atmosphere
Emissions of SOx and NOx and cadmium are negligible, and the fly ash and harmful PM’s are taken out by the dust control measures.
CO2 is still produced of course but this is offset by the fact that the carbon dioxide produced by the combustion of the wood pellets will be taken up in photosynthesis by the replacement trees planted in a sustainable manner.
However the transportation and drying of the sawdust in the production, the pellets, also the fertilisers used, harvesting and chipping machinery used in short rotation coppice must be taken into account when calculating CO2 emissions.
As this is the last in the present series on fossil fueled and biomass power plants in particular the fume extraction portion, I have included a table made up while researching the subject on the noted internet sites. The table compares Carbon emissions in tonnes by GWh from fossil fuel and wood pellet fired power stations and can be found in the sketches section.
Sketch of Wood Pellet Burning Power Plant and Fuel CO2 Emission Table