The Irish forestry sector harvests around four million tonnes of biomass annually, a figure that is projected to rise to six million tonnes by 2030.
Of this, only around 50 – 60% actually makes its way into useful products, such as fence posts or pulp for board.
What remains needs to be disposed of in one way or another. Forest thinnings may be left to rot down on the ground, while bark can be used as mulch. However, that still leaves a large amount of material to be disposed of by sawmills and tree surgeons.
One obvious solution would be to extract the potential energy still contained within it through the generation of electricity by burning. This is not without its problems; the major one being that it tends to retain a high moisture content.
Yet, Ireland does have one distinct advantage, in that its last remaining peat-fired power station is also ideally suited to the combustion of forestry and sawmill by-products.
The boiler at Edenderry is currently co-fuelled by remaining peat stocks and surplus biomass from the forestry and timber industries.
Currently, the plant consumes 300,000 metric tonnes (Mt) of biomass annually, 80% of which is indigenous and the rest is imported. There is also a 4% inclusion of willow grown under contract to Bord na Móna.
The imported material includes the residue from ethanol production and residue from paper pulp production.
No virgin wood is used and the pricing level is kept low, so as to prevent timber being directed straight to electricity generation rather than just the material, which would otherwise have no commercial value.
With the plant now prohibited from mining peat, it is seeking to continue increasing its intake of biomass. In its latest planning application, it is seeking to increase its intake of biomass to 500,000Mt.
This will enable the plant to run at its present output without relying on peat. It supplies around 2.5% of the country’s electricity requirements, so the switch to biomass will reduce Ireland’s carbon emissions by at least 1%.
The combustion process
The peat that is still being consumed is from stock previously mined rather than being new cut. It is brought to the plant by a narrow gauge railway and tipped onto a conveyor which takes it to the storage shed.
Biomass is brought to the site by truck and is tipped in a holding area where it is stacked by tracked excavators.
At this point it is sampled for moisture content and tested for impurities that may cause excessive internal chemical erosion of the furnace.
It is then transferred to a pit by wheeled loader, from which it is taken by auger and belt to the same holding shed as the peat.
It should be noted that all the biomass delivered to the site is already chipped, no reduction of wood is carried out on site. Payment is by the energy content rather than the actual weight delivered.
Moisture content usually lies between 25- 60%.
As the load from each belt is deposited into the shed it becomes mixed, resulting in a consistent material to be fed to the furnaces via a further pair of belts running along the base of the pile.
From the belts, the mix is elevated to above the boiler structure; from here it is fed by gravity into a stream of air which blows it into the furnace where it burns with a flame temperature of around 1,100°C .
The steam produced is fed to the single turbine at a pressure of 150 bar. It is circulated twice more through the furnace before being condensed back into water.
The ash from the burnt fuel is removed to a landfill site.
The boiler has been engineered to cope with feedstock of a high moisture content and so is ideally suited to biomass. It is considered one of the most efficient in Europe in coping with this form of material.
As a back up to the main plant, there are four gas turbines on site with the same generating capacity as the main plant. These are Pratt and Whitney units as found on the Boeing 373.
The turbines can reach full running capacity in less than six minutes; the main plant requires a day and a half at least, to return to full capacity after a shutdown. A stock of fuel sufficient for 72 hours running time is kept for both.
Biomass is key to Edenderry future
One of the main arguments put forward for continuing with the operation of Edenderry is that it ‘provides a price floor for lower value biomass product’.
This can only add to the overall profitability of the forestry and timber industry, in addition to reducing dependence upon fossil fuels.
The actual economic justification for a business to supply the plant with material is difficult to judge, as the price is subject to commercial confidentiality clauses.
However, we are assured that it is one price irrespective of quantities delivered, or size of contract entered into with suppliers.
Should the plan to run purely on biomass go ahead, it is estimated that it will have an output of 650,000MWh in 2030.
The overall capacity is 850,000MWhs, but it will throttle back during periods of high wind-energy generation. This is the equivalent of 75 wind turbines and its availability is independent of the weather.
No incentive for large-scale plants
The plant has every intention of continuing to operate, but the decision as to whether it can or cannot is now out of its hands, as it awaits planning permission from the local authority.
Should it close, then it is unlikely that anything of the same scale or capacity will be built elsewhere. EirGrid, which is charged with ensuring Ireland’s electricity supply, is presently only offering 10-year contracts.
A power station such as Edenderry will need a 25-year commitment at least, if it is to attract investment, and that is on top of a seven-year lead time.
The climate action bill has set ambitious targets. If they are to be met then Ireland’s biomass surplus is one resource that is ready and waiting to be utilised.
However, to fully realise its potential, it requires looking ahead a lot longer than a decade, which is considered rather short-term thinking in the energy sector.