Harvesting and processing forest biomass for energy production in Ireland
Tom Kent, Waterford Institute of Technology*
Pieter Kofman, Danish Forestry Extension (DFE)
Dr Eleanor Owens, Waterford Institute of Technology
Enda Coates, Waterford Insitute of Technology
Sarah Cooley, Waterford Institute of Technology
* Email: email@example.com
COMPLETION DATE: June 2009
Concerns over climate change, security of energy supplies and sustainable forest management have directed international policy towards supporting the development of renewable energy from wood fuel. The EU plans to produce 20% of energy requirements from renewable sources, with Ireland's overall target 16% by 2020. At a European level, it is expected that almost 65% of the renewable energy target will come from biomass, most in the form of wood. The Irish Energy White Paper and Bioenergy Action Plan for Ireland set out the framework for meeting these targets in Ireland. Sustainable Energy Ireland has mounted a successful campaign to encourage private individuals and commercial firms to install wood fuel boilers, thus creating a demand for wood fuel in the form of wood pellets and dry wood chip.
Three peat-fired power stations are gearing up to fulfill their obligation to co-fire peat with 30% biomass by the year 2015. The Forest Service has had two calls for grants for companies to buy wood chipping equipment.
Wood for energy is a relatively new assortment in Irish forestry and much knowledge remains to be gained. Ireland has a ready-made wood fuel resource in the large areas of farm forests planted over the last 25 years, which now require thinning to achieve production potential. Forest ownership is fragmented and knowledge of harvesting and storing wood for energy limited. The Forest Energy project, commenced in 2006, and renewed annually, aims to develop cost-effective supply chains by adapting commercially used methods from Europe to Irish conditions.
- Demonstration of harvesting, extraction and wood fuel processing equipment in Ireland;
- Production and quality assessment of both wood chip and firewood products;
- Assessment of optimum storage systems to promote maximum seasoning at lowest cost;
- Investigation into moisture content/climate relationships with the view to developing a moisture content reduction model based on simple climatic indicators;
- Chemical composition of wood samples;
- Organisation of dissemination activities including public demonstrations, articles, workshops, presentation of results and display of wood fuel sample materials.
Harvesting trials in softwood and broadleaf first thinnings
A range of harvesting methods was employed to carry out first thinning in broadleaf plantations, with both wood chip and firewood produced. The generally smaller size of broadleaf stands, the small average tree size, the close spacing and the threat of damage by large machines have led to the opinion that small scale harvesting methods are more appropriate for broadleaves. All broadleaf thinning was carried out in accordance with guidelines on line and selection around potential final crop trees. Felling was generally by chainsaw, though a harvester producing delimbed lengths was trialled at one site. All roundwood was produced to standard 3 m lengths to facilitate transport, and processing into firewood. Extraction by forwarder, grapple, ATV and horse were trialled. Different firewood processors were tried using the 3 m lengths to produce standard length firewood logs. Woodchip was produced on two sites by felling whole trees into lines and terrain chipping and chip forwarding using the Silvatec chipper and forwarder. All operations were time studied and production was monitored. Production costs to the forest road were calculated for each production system.
Results indicate that the lowest cost method of thinning broadleaves is the whole tree terrain chipping option at between €46/m3 and €67/m3. The cost of producing 3 m lengths by chainsaw and extracting to roadside by forwarder was similar, ranging from €44/m3 to €98/m3 on the four sites trialled, but this does not include the cost of processing the logs into firewood. The firewood processors trialled ranged in productivity from 0.53 m3/hr to 3.3 m3/hr, including adding production costs that ranged from €11/m3 to €49/m3. The harvester was more productive compared with chainsaw harvesting but not sufficiently so to offset the higher operating cost. Small-scale extraction methods, such as the tractor with grapple, quad and trailer and horse and arch were all less productive and more expensive than the forwarder. However, these methods are very sensitive to the operating cost assumed. A forest owner could employ these methods directly, with the time valued differently to a contractor. Finally, the productivity of chainsaw harvesting is very sensitive to the operator, as the production rate ranged between 0.38 m3/hr and 0.8 m3/hr under broadly similar site and mean tree size conditions.
Energy parameter characterisation of Irish softwoods and broadleaves
All woodfuels produced from Forest Energy first thinning sites were characterized for moisture content, ash content and gross calorific value. Wood chip was also tested for bulk density and particle size distribution. Sampling, sample preparation and test methods were all carried out according to CEN Solid Biofuel technical specifications.
Woodchip bulk density
Bulk density of wood chip, measured in kg/m3, is an important parameter, as energy content is quantified by weight, whereas the transportation and storage of wood chip is generally limited by volume, as wood chip is a relatively low density fuel compared with fossil fuel. Bulk density is determined by the basic density of wood, moisture content and mean and range of particle sizes produced by the chipper. A total of 1207 bulk density samples were measured on fourteen sites, moisture content was sub-sampled for each bulk density sample. This allowed the relationship of bulk density and moisture content to be investigated. The measured bulk density of woodchip from Sitka spruce roundwood against the sample moisture content - there is a strong non-linear relationship, described by a polynomial function with an R2 of 0.92.
Variations within species basic density and, to a lesser extent, the variations in particle size distribution have a confounding effect. However, the strength of the relationship between bulk density and moisture content, could allow for the development of a wood fuel quantification method for payment purposes.
The two critical variables for estimating the energy content of wood fuel are weight and moisture content. Where it is neither possible nor practical to measure weight and moisture content directly, the bulk density could be estimated easily by sampling and the total load weight be derived from the load volume and estimated bulk density. The moisture content could be estimated from a fixed relationship with bulk density described for individual species and assortments. Delivered energy content could then be estimated from load weight and moisture content estimates. There are obviously large potential errors associated with this method, but in the absence of a more accurate approach, it could facilitate trade in wood chip for energy purposes.
Variation in gross calorific value
Over 1,100 samples from all sites were assessed for calorific value using a Parr 5200 bomb calorimeter. The general trends, which emerge are that wood is relatively homogenous in calorific value, ranging from 19-20 MJ/kg on a dry matter basis. Sufficient samples of different assortments of ash and Sitka spruce were analysed to gain further insight into variation in calorific value. The mean calorific value of ash is significantly lower than that of Sitka spruce. There was no significant difference between the ash roundwood (RW) and whole tree (WT) mean calorific values. Similarly, there was no significant difference between Sitka spruce roundwood and energywood assortments. The spruce whole tree assortment was significantly higher, indicating the higher calorific value associated with bark compared with wood.
Gross calorific value of woodfuels derived from forest sources vary according to species and the relative proportions of bark and wood. Gross calorific value, however, may be used to indicate the purity of the wood fuel tested, as the natural range of variation is narrow. Substantially lower or higher gross calorific values will only be caused by the presence of non-wood material.
In-forest and in-yard storage trial results have been previously reported. Work is on-going in developing a climate-based model estimating seasoning period for in-yard storage. Meanwhile the storage bins and load cells were relocated to Redmondstown, Co Tipperary, and reconstructed on a site provided by Coillte Panel Products. In 2010, it is proposed to undertake a storage trial of compact residue bundles.
Investigation of moisture content variation in Irish softwood and broadleaves
Moisture content of eleven species on eight sites around Ireland was assessed over 2007, 2008 and early 2009. All trees were randomly selected from Forest Energy trial sites. Additional samples were collected from a mixed softwood first thinning stand at Lismore Estate, Co Waterford. A total of 902 trees were felled, extracted, chipped. Five representative moisture content samples were collected from each tree. It should be noted that moisture content is calculated as a percentage of the total weight. There was significant variation in mean moisture content between all species with the exception of alder and Douglas fir. Broadleaf moisture contents were all lower than those of softwoods. Ash is the species with lowest moisture content, averaging 40%; while Scots pine had the highest moisture content at 64.2%, albeit from a relatively small sample.
Sufficient samples of Sitka spruce, lodgepole pine, ash, oak and alder were sampled on a monthly basis to make a preliminary estimate of the variation in moisture content over the year. The bars associated with each data point represent the 95% confidence interval. All five species display a similar trend in seasonal moisture content variation. Moisture content peaks in the summer months of May to August. It falls in September and October and remains low over the winter, rising again in the spring. There is an obvious relationship between rising moisture content and the commencement of annual growth, and a similar relationship between falling moisture content and the cessation of growth in the autumn. The difference between peak month mean moisture content and the lowest mean moisture content was significant for all species. The moisture content of ash was 36.8% in April, rising to 45.6% in July. Sitka spruce moisture content in November was 55.3% and increased to 61.2% in May.
Planning a thinning to coincide with the season when moisture content is naturally low can bring significant advantages in reduced post-harvest drying, where energy is the target market for the thinning.
The current Forest Energy Programme is closed and reports and COFORD Connects Notes are in preparation. A further programme of joint research in wood energy, by WIT, UCD and DFE, is proposed for the period of 2010-2014.
Kent, T. and Kofman, P. 2009. Wood Energy Supply Chains for Softwood First Thinning in Irish Forests. Conference Proceedings: 17th European Biomass Conference and Exhibition: From Research to Industry and Markets, Hamburg.
Kofman, P. 2009. Evaluation of Moisture Content Changes in Sitka spruce Roundwood and Energywood Assortments in Ireland: The Bin Trial. Poster Presentation at 17th European Biomass Conference and Exhibition: From Research to Industry and Markets, Hamburg.
download 2008 report as pdf (pdf 309Kb)
Tom Kent, Waterford Institute of Technology*
Pieter Kofman, Danish Forestry Extension
Dr Eleanor Owens, Waterford Institute of Technology
Dr Dmitri Botvich, Waterford Institute of Technology
Enda Coates, Waterford Institute of Technology
Sarah Cooley, Waterford Institute of Technology
Sean Kelly, Waterford Institute of Technology
Meabh Kenneally, Waterford Institute of Technology
Cathy Walsh, Waterford Institute of Technology
Dan O'Mahony, Waterford Institute of Technology
Nicholas Mockler, Waterford Institute of Technology
Damien O'Donovan, Waterford Institute of Technology
Tom O'Dwyer, Waterford Institute of Technology
Alastair Dunnett, Waterford Institute of Technology
Seamus Forde, Waterford Institute of Technology
* Email: firstname.lastname@example.org
COMPLETION DATE: February 2009
Concerns over climate change, security of energy supplies and sustainable forest management have directed international policy supporting the development of renewable energy from wood fuel. The EU plans to produce 20% of energy requirements from renewable sources. It is expected that almost 65% of that goal is to come from biomass and most of that biomass would be in the form of wood.
The Irish Energy White Paper and Bioenergy Action Plan for Ireland set out the framework for meeting these targets in Ireland. Sustainable Energy Ireland has mounted a successful campaign to encourage private individuals and commercial firms to install wood fuel boilers, thus creating a demand for wood fuel in the form of wood pellets and dry wood chip. Three peat-fired power stations are gearing up to fulfill their obligation to co-fire 30% biomass with peat by the year 2015. The Forest Service has had two calls for grants for companies to buy relevant wood chipping equipment. These initiatives have created a supply and demand situation.
Wood for energy is a relatively new assortment in Irish forestry and much knowledge remains to be gained. Ireland has a ready-made wood fuel resource in the large areas of farm forests planted over the last 25 years, which now require thinning to achieve production potential. Forest ownership is fragmented and knowledge of harvesting and storing wood for energy limited. The ForestEnergy project, commenced in 2006, and renewed annually, aims to develop cost-effective supply chains by adapting commercially used methods from Europe to Irish conditions.
The main project objective was to secure marketable wood fuel of acceptable moisture content for sale from Irish first thinnings of softwoods and hardwoods.
Specific objectives were to:
- Demonstrate harvesting, extraction and wood fuel processing equipment on softwood and hardwood sites, representative of regional and site variation in Ireland;
- Produce and assess the quality of both wood chip and firewood products;
- Assess optimum storage systems to promote maximum seasoning at lowest cost;
- Investigate moisture content/climate relationships with the view to developing a moisture content reduction model based on simple climatic indicators;
- Analyse chemical composition of wood samples collected during the harvesting trials across the country, and subsequently analyse the change in chemical composition of wood fuel assortments during storage;
- Organise dissemination activities including public demonstrations, articles, workshops, presentation of results and display of wood fuel sample materials.
Forest harvesting and wood fuel processing trials
Between 2006 and 2008, nine softwood sites, totalling 130 ha, received first thinning treatments. A further 35 ha of first thinning was carried out over five hardwood sites. Harvesting and chipping trials were also carried out on forest plots on Bord na Móna cut-over peat. A range of methods was used for harvesting, extraction and chipping. All machinery and methods were studied to develop productive time per unit output for the range of site types and tree sizes encountered.
The whole tree method was the lowest cost method of producing woodchip from first thinnings in softwoods: the trees were felled by chainsaw in a line thinning, seasoned in the extraction rack and chipped by a Silvatec terrain chipper and extracted to roadside by a chips forwarder. The Silvatec terrain chipper was used on eight softwood sites, three hardwood sites and one cut-over peat site during the ForestEnergy project. On eight sites over three years, the average production cost by this method was in the range of €2.03 to €4.28/GJ. The next cheapest method, at €2.80/GJ, was also a whole tree method where a feller-buncher carried out a selective thinning and bunched the trees in the rack, for post-seasoning terrain chipping. Disadvantages of the whole tree terrain chipping system were no distinct brash mat and no machine infrastructure available in Ireland to efficiently transfer chips from the chip forwarder to a road transportation system.
In comparison, the production cost of wood chip using standard harvester and forwarder roundwood line and selective thinning, with logs stacked in the forest to season and chipped at roadside, was in the range of €6.53 to €7.59/GJ. A variation of this standard thinning system was to replace the pulp assortment with an energy wood assortment, with no minimum top diameter and branches only loosely delimbed leaving stubs, cut to a variable length up to 4.5 m. The production cost of chip with this energy wood system was between €5.82 and €6.81/GJ.
The substantial difference in production costs can be attributed to three main factors: harvesting whole trees yielded on average 50% more biomass and the energy wood assortment yielded 15% additional biomass per hectare in comparison to the round wood methods. Also, the whole tree harvesting operation is faster and cheaper as there is no delimbing and cross-cutting to length. Finally whole trees dried more in the stand compared to logs stacked at roadside in first thinnings.
In hardwoods, time studies were carried out on marking stands prior to thinning, in addition to studying various systems for thinning to produce wood chip and firewood assortments. Marking the crop, in accordance to the Forest Service/Teagasc guidelines, averaged 1 ha per man day. There was little variation between personnel studied or between sites.
As in the softwood trials, the lowest cost method of producing woodfuel was terrain chipping whole trees. Production costs ranged from €2.82/GJ for a line thinning only by chainsaw, to €7.09/GJ when the thinning included selection between lines. Using the feller buncher for line and selection thinning in hardwoods gave a production cost of €3.37 - €4.07/GJ.
Wood fuel storage trials
Softwood storage trials were carried out in the forests and were carried out at an open, exposed depot site for comparison. Hardwood trials were carried out in the forest and storage trials of 3 m length logs and firewood products stored under a variety of conditions were also carried out. All material was harvesting between March and June and all chipping was carried out in August ¿ September to benefit from summer seasoning. Sites harvested in 2006 were chipped after one summer only. Sites harvested in 2007 were partially chipped after one summer and the remainder chipped after two summers seasoning. In the forest, whole trees were seasoned in the extraction rack; roundwood and energy wood assortments were stacked at roadside. Some stacks were covered and some left uncovered. Moisture content was sampled at time of harvest and again post seasoning as the material was chipped.
Freshly felled Sitka spruce at first thinning varied in moisture content between sites and assortments but averaged 60% moisture content. There was a large variation between years, between sites and between assortments in the rate of drying. The hardwood results were interesting, in that ash had a mean initial moisture content of 40%. On the other hand, the ash assortments stored in the forest did not dry to any great extent.
General conclusions from the in-forest storage trials were that the forest environment at first thinning stage is not conducive to rapid, even, predictable drying of stored wood. Whole tree assortments generally performed better than roundwood and energy wood assortments. Covered stacks dried better than uncovered stacks, however the covers used degraded substantially in the second year and were not effective. Wood fuel stored in the forest did not dry sufficiently for use in commercial boilers requiring fuel of less than 35% moisture content. In-forest storage may be used to dry wood fuel to 45-50% moisture content.
A storage trial was constructed near Rochfortbridge on an open, exposed Bord na Mona site to assess drying potential of logs moved from the forest to a depot. Eight steel bins were constructed, placed on load cells and c. 25 tonnes of logs were placed in each to assess the loss of weight over time. The assumption was made that any loss of weight would represent a loss of moisture. Moisture content and other parameters were sampled intensively at the beginning and end of the trial. Bins were filled with freshly felled material in April, June, September and December 2007 to assess the variation in drying seasonally. All bins were emptied in August 2008.
All log stacks achieved an end moisture content in the range of 18-22%. The mean time length to achieve a 30% moisture content required for commercial wood fuel boilers was 19 weeks, for material stored in April this was 16 weeks, whereas material stored in December required 26 weeks. On average, the stacks lost 47% of the initial total weight, and 63% of initial total moisture. The energy content of the wood fuel increased on average by 17% over the storage period.
General conclusions from the storage trials were that logs and trees stored in the forest will not achieve a moisture content of less than 30% over either one or two summer seasons, due to the shelter effect and humid micro climate found. Covered stacks will dry 10% in one summer and an additional 5% in two summers. Uncovered stacks are not recommended. Seasoning roundwood to less than 30% moisture content is possible in ambient Irish climate in an open exposed depot. Logs that begin seasoning in spring will dry fastest, while logs beginning seasoning in winter will dry slowest. Covering stacks in an open depot will both aid faster drying and ensure more uniform moisture content in the stack.
The final project report is in preparation. Twenty-one COFORD Connects Notes covering all harvesting, storage, fuel quality and tree moisture content outputs are being written at present. Dissemination of project outputs is continuing through the woodenergy.ie website. A number of research papers are in preparation.
Kent, T. Wood Energy ¿ From Forest to Market. Presentation made at National Forestry Conference: New forestry initiatives - opportunities for the sector. 7 March 2008, Enfield, Co Meath.
Kofman, P. Fuelling your Boiler the Right Way ¿ Feedstock Quality for Commercial Boilers. Presentation made at Bioenergy 2008 Conference: A Growing Opportunity for Energy and the Environment. 19 June 2008, Teagasc Mellows Centre, Athenry, Co Galway.
Kent, T. Wood Fuel Quality Parameter Testing ¿ Summary of Current Activities. Presentation to NSAI CEN Solid Biofuel Mirror Group Meeting, 25 June 2008, Waterford Institute of Technology.
Kofman, P. Developing Quality Wood Fuel from Irish Forests, Avoiding the Pitfalls. Presentation made at SEI Wood Energy Conference: Fuelling Your Future. 10 September 2008, Westport, Co Mayo.
Kent, T. COFORD Forest Energy Programme Overview. Presentation to Inter-Departmental Bioenergy Working Group, hosted by Sustainable Energy Ireland, 11 September 2008, Dublin.
Kent, T. Development of Wood Energy Supply Chains. Presentation made at Forest Service Seminar - Wood Fuel: The Energy Solution for Kerry. 19 September 2008, Killarney, Co Kerry.
Kent, T. Producing Wood Chip for Energy ¿ Experiences from the COFORD Forest Energy Programme. Presentation to the Wicklow Uplands Private Forest Owners Group Seminar. 12 November 2008, Co Wicklow.
Kent, T. and Kofman, P. Moisture Content Management - The key to wood fuel quality. A result of COFORD Forest Energy Programme. Irish Bioenergy Association Wood Energy Information Day. 19 November 2008, Enniskillen Co Fermanagh.
Kent, T. and S. Kelly. Forest Energy Project: results and conclusions of the COFORD funded research programme. Society of Irish Foresters/Irish Timber Growers Association/Irish Farmers Association Field Day 23 May 2008, Ballybofey, Co Donegal.
Coates, E., Cooley, S., Kelly, S., Kent, T. Mockler, N. and Owens, E. Woodfuel Quality Testing Demonstrations and Talks and Wood Energy Assortment and Machinery Demonstrations and Talks. COFORD/SEI/Teagasc Bioenergy 2008, Teagasc, Athenry, Co Galway.
Coates, E., Kent, T., Kelly, S. and Kofman, P. Wood fuel supply chain and quality assessment on softwood first thinning site, Croaghrimcarra, Co Mayo. Public demonstration arranged for at SEI Wood Energy Conference: Fuelling Your Future. 10 September 2008, Westport, Co Mayo.
Coates, E. and Kent, T. Wood chipping methods and costs and wood chip quality demonstration and talk. Teagasc Out-wintering Pad Public Demonstration Day, 19 November, Moorepark Research Centre, Co Cork.