Maximising the returns from growing biomass

by Michael Doran, business development manager of Rural Generation Limited

Farmers should not expect to find that simply growing energy crops will prove very profitable. They must also use such crops to meet needs besides those for heat and power which people will pay them to fill.

Rural Generation is a small private limited company formed in 1996 by John Gilliland, the current President of the Ulster Farmers Union, to commercialise an R & D project undertaken by the Department of Agriculture in Northern Ireland involving gasifying willow chip to produce heat and power. Mr. Gilliland had been looking at alternative land uses from the 1990s and had planted some willow coppice in 1994 in association with the Horticulture and Plant Breeding Station at Loughgall, Co. Armagh, on his Brook Hall Estate in Co Derry. Further plantings have taken place since and we now have 45 hectares of which 15 hectares are harvested each year.

Willows may be planted from March to June, and planting can be by hand or by automatic planter, which is normal on areas greater than one acre.
Planting Willows
The planter shown is capable of planting four rows at a time. Willows are usually planted in double rows, with cuttings 70cm apart in each row, with a space of 1.4m between double rows. A cutting 20cm long is taken from a rod approximately 3m long. The cutting is put into the ground vertically without an established root system. Cuttings are usually planted at 14,000 to the hectare.

Willows will grow in most soils, except highly organic. Ground preparation is important, and includes ploughing, power harrow and rolling. Rabbit fencing is critical for the first six months of growth, until the willows are established. The only pest that is likely to cause problems at the early stage is the leatherjacket, which will eat the cuttings unless treated.

Typically in the first year the willows will grow to a height of approximately 1.5m. They will grow as a single stem. It is normal to mix 5 or 6 different clonal types when planting. Willows are susceptible to attack by rust, which will affect the leaves. While rust will not actually kill the plant it will effectively destroy the willows' ability to transpire through the leaf and will severely stunt growth. The use of polyclonal types will completely eradicate damage from rust.

In the winter at the end of the first year of growth the willows are cut back to ground level, usually using a finger bar mower. When the plant re-grows in spring of the second year it will "multi-shoot" and 5 or 6 stems will be reproduced from the original single stem plant.
Willows on Brook Hall Estate
This coppicing is done to increase the yield. After cut back the willow will usually grow 3- 4m in the first year. It will continue to grow for another two years after which time it is harvested. Further harvesting usually takes place every three years. This has been found to provide the best yield because if crops are harvested every one or two years their bulk yield is small while past year three, the percentage increase in bulk yield is marginal.

At Brook Hall Estate we use a modified Claas forage harvester that makes chip. This is fed directly into trailers and taken back to the farmyard. Our willow normally yields approximately 30 dry tonnes of chip per hectare per harvest. As the harvest is only done every three years this equates to 10 dry tonnes per hectare per annum. The harvester can cut up to 8 hectares per day in good conditions. Even though the harvesting is usually carried out in January or February the ground tends to be dry as a result of the willow establishment.

Traditionally willow is harvested by rod. This can be carried out by using a strimmer, or by a mechanical rod harvester. The main benefit of rod harvesting is that is avoids the necessity for drying the willow before combustion. At harvest time willow is usually about 55% moisture content. If chipped, the chips will have to be dried or the biomass will effectively create a compost heap, with a considerable loss of combustible material. If rod harvested, the rods can be stacked at the end of the field, and by early summer they will have dried naturally to approximately 35% moisture content without any degradation of the fuel.

Rod harvesting is labour intensive and is impractical for large areas. However, for smaller plots, and where drying facilities are not available, rod harvesting is far more cost effective.

Figure 6D1 gives an indication of the cost of planting willow coppice per hectare.

The total indicative cost is approximately €3,000 per hectare. This includes for mechanical planting, cuttings, fencing and weed control which is important in the first year of growth. Planting costs in Scandinavia are approximately €2,000 per hectare because their industry has established a "critical mass" which ensures economies of scale particularly with regard to the cost of cuttings and the hire of mechanical planters. As the industry develops in this country we expect these establishment costs to fall.

Figure 6D2 shows the typical cost of maintaining and harvesting the willows per rotation. The harvesting cost depends on whether it is by hand or by machine and on the volume i.e. a large area, in excess of 10 hectares, harvested by machine is likely to be around €600 per hectare.


Rural Generation Limited has developed the use of willows as a bio-filter to treat dirty water or effluent. In 1998 an EU research project was set up a Culmore in Co. Derry to monitor the effects of disposing of dirty water and sludge onto willow coppice. Regulations for the disposal of wastewater and sludge are becoming stricter and environmentally acceptable options must be found for dealing with these wastes.

Four hectares of short rotation coppice were planted in May 1998 adjacent to Culmore Water Treatment Works. This is a relatively large treatment plant servicing the equivalent of 120,000 people. A ground water irrigation system was installed consisting of 28mm diameter pipes with holes at 8m centres. The flow of effluent through the pipes is controlled by pumps and motorised valves.

The site is divided into separate plots, one control plot receives no irrigation, a second plot receives irrigation with pure water, a third plot receives the sludge, a fourth plot receives irrigation with waste water at 1 times the potential evapotransperation (PE), a fifth plot at 2 x PE and a sixth plot 3 x PE The site was harvested early in 2002 and yields were measured, as well as chemical and microbiological ground water, pests, disease and weed problems.

During harvesting the irrigation pipework was only damaged twice and this was repaired easily. Figure 6D3 summarises the results. The volumes of NP & K were recorded as well as the total volume of effluent, which was only applied during the growing season.

Figure 6D4 indicates the yields from the various trial plots. It is interesting to note that the control plot which received no irrigation has in fact yielded greater mass than the plots which received 1 x PE waste water, 2 x PE waste water and 1 x PE clear water. We do not have an explanation for this.

One of the reasons why the site at Culmore was chosen was because the water table in the ground is only 60cm below ground level. When monitoring the capability of the willows to absorb nutrient we were particularly concerned that effluent was not flowing through the willows and entering the ground water. Therefore, had we selected a site with a water table deep below the surface we would have been unsure if it was the willows or the ground filtering that was absorbing the nutrients. By selecting a site with a high water table we were able to establish that no detectable levels of nitrate, phosphate or heavy metals were entering the ground water.

Generally there was a trend towards greater biomass yields as the volume of wastewater irrigated rose. There are practical limits as to how much effluent can be absorbed by willow and potassium (P) is normally the limiting factor. It is also likely that effluent with a Biological Oxygen Demand (BOD) in excess of 500 can be adequately treated using willow as a bio-filter.


This section is concerned with the combustion of willow chip and wood chip in solid fuel boilers, and also with the combustion of waste agricultural products such as chicken litter and spent mushroom compost.

Rural Generation Limited has taken on the Irish agency for Farm 2000 boilers. There are several different boiler types within the range including Big Bale boilers, High Temperature boilers and automatic feed Swebo boilers.
Big Bale, Multi fuel, Farm 2000 Boiler
Big Bale boilers, as illustrated, were originally designed to accommodate full size round bales. It is now more usual to burn waste products such as timber, cardboard or paper than to burn materials that have a value such as straw.

The automatic feed Swebo boilers consist of a full storage hopper, a combustion chamber and a modified boiler. In automatic feed mode the wood chips or willow chips within the hopper are delivered to the round combustion chamber by a screw auger. The rate of combustion and the amount of heat produced can be regulated by the auger speed. Full combustion takes place within the ceramic pot, and a flame is then fed into the boiler chamber, where the water is heated.

The automatic feed systems can also be operated in manual mode, where the fuel is fed directly into the boiler through the usual door. This allows the operator to use a range of waste fuels such as cardboard, paper, logs etc. While it is labour intensive it can be cost effective if the fuel has little or no cost. Figure 6D5 shows the comparative cost of wood chips versus oil. For example, if wood chips are being purchased at €73 per ton, and heating oil is costing 34.5 cents per litre then the heating bill for a system running on wood chips will be 54% of the comparative bill for an oil fired systems.

It is worth noting that these costs are based on wood chips with 30% moisture content. However, the willow chips produced by Rural Generation typically have moisture content of approximately 10%. Therefore, the savings will be greater because the calorific value of timber rises as the moisture content falls. Taking the €73/34.5 cents per litre oil scenario, for wood chips at 10% moisture the comparative cost would be 34% i.e. heating by wood chip would be 34% of the cost of heating by oil.

Rural Generation Limited has recently installed one of these boilers into an alcohol rehabilitation unit in Co. Donegal. They run the system in both automatic feed and manual feed mode depending on the availability of labour.Rural Generation also supplies willow chip to a community centre in Derry. Another manufacturer supplied the boiler in Creggan Community Centre, but the quality of chip provided by Rural Generation proved to be a better fuel than anything else they could obtain.

A gasifer was built under a Non Fossil Fuel Obligation (NFFO) contract at Brook Hall Estate in Derry with a capacity of 95kWe and 200kWth. It has been producing heat and power for the past six years.
Gasifier and Generator Building with Grid Connection, Brook Hall Estate
The gasifier has been modified continuously as part of a managed development programme and is now capable of producing 200 cubic metres of wood gas per hour. This wood gas is used to drive a diesel engine that in turn is linked to a generator. The generator produces electricity, which is exported to the Northern Ireland grid via the transformer. The gasifier comprises a 9cubic metre stainless steel hopper, which holds wood chip or willow chip. This feeds a down draft gasifier that is linked to an Iveco diesel engine. The system runs in batch mode and typically operates for 12/14 hours per day producing electricity and thermal energy. The thermal energy (hot water) is used to dry cereals on the 400-hectare Brook Hall Farm. The heat is also used in the winter months to dry willow chip that will subsequently be used in the gasifer.

The capital cost of the gasifier is approximately €2,000 per kW. This includes containerised units, hopper, control system, electricity generating system, safety mechanisms, heat exchangers on the engine exhaust and the engine cooling system and all installation and commissioning. The cost does not include for structural modifications to accommodate the CHP unit, compressed air supply, fuel storage silo or grid connection costs.


I have already discussed the work that Rural Generation has done with regard to bio-filtration, heat and power generation from willows and heat only applications. We are however continuing to push forwards to find holistic solutions to other problems.

Northern Ireland produces approximately 200,000 tonnes of spent mushroom compost every year. This is becoming increasingly difficult to dispose of, as the traditional route of land spread becomes less viable due to legislative changes. Rural Generation has developed a system for co-combusting spent mushroom compost with dried willow chip. The heat generated can then be recycled to heat the mushroom house.

Spent mushroom compost typically has a 80% moisture content. Virtually all other attempts to reduce this moisture content to a level at which the compost will satisfactorily burn, (usually about 40%), have required a greater energy input than is subsequently derived from the combustion. I.e. you have to expend more energy to reduce the moisture content than the compost subsequently produces on combustion. Rural Generation has found a solution to this problem.

Rural Generation has also successfully combusted chicken litter in their boiler systems. Tests are now going on to ensure that emission levels, particularly of ammonia, are reduced to an acceptable level. Rural Generation has also been involved in trials using "fine" willow chip as a bedding material in chicken houses. While this is still at an experimental stage early indications are that the salicylic acid in the willow can substantially reduce the amount of ammonia in the chicken litter. This is always a prime concern for chicken farmers who are trying to reduce "hock burn" by keeping levels of ammonia as low as possible. If our tests are successful this will open up a new market for willow chip. The chicken industry in Northern Ireland currently relies on imported wood shavings for the bulk of its litter.

The future lies in integrated environmental solutions and not in energy alone. Rural Generation believes that while the production of willow as an energy crop can generate profits for the farmer of the order of €250 per hectare, the profitability of the venture is enhanced significantly when the coppice has supplemental use.
Willows ready for harvest
If short rotation coppice is used as a bio-filter for sewage sludge the profit margins are likely to be increased significantly. Similarly if willow chip is used as chicken litter the value of the chip is likely to be greater than the market value for chip as a fuel which is currently around €65 per ton.

We believe that farmers and commercial growers should look at all the options which willows offer before committing to a course of action. Traditionally farmers have produced a product. They deliver the product to the farm gate and by the time that product has reached the consumer it will have increased in value or cost by a factor of around 4. If farmers commit the same sin when producing willow, by stopping at the first interface in the supply chain they will prevent themselves from maximising value from their product. This is probably the most difficult aspect of willow cultivation for farmers to grasp. They are comfortable with production only scenarios. They need to increase their involvement in the supply chain to maximise returns and to add value to what is a relatively simple product.

This is one of almost 50 chapters and articles in the 336-page large format book, Before the Wells Run Dry. Copies of the book are available for £9.95 from Green Books.

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