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J. Peter Hall 1
Renewable energy in the form of biomass is becoming increasingly important as countries realize that bioenergy offers a solution to international commitments to reduce carbon dioxide emissions. Bioenergy systems reduce greenhouse gas emissions by replacing fossil fuel in energy production. Increases in utilization of bioenergy from forests or agricultural land are not limited by technology, but rather by policy constraints. This can be addressed by a combination of targets and incentives. Targets include percentages of renewables in the energy mix to be attained, trading of energy certificates/ carbon and rewards for carbon sequestration investments. Incentives include carbon taxes, emissions trading and grants or subsidies that are used to `level the playing field'.
Forests and woody crops are a source of energy through the conversion of woody biomass into solid, liquid or gaseous fuels to provide energy for industrial, commercial or domestic use. Already biomass provides about 14% of world primary energy supplies. About 55% of the 4 billion m3 of wood used annually by the world's population is used as fuel wood or charcoal directly to meet daily energy needs of heating and cooking, mainly in developing countries. Bioenergy systems often use biomass that would otherwise be unmerchantable and the conversion of biomass may involve biochemical, thermochemical, or physical/chemical processes. (IEA 2002)
Energy and bioenergy are becoming more important subjects for the public, policymakers and decision-makers as a result of rises in the prices of fossil-derived energy coupled with concerns over nuclear energy. Enhanced environmental concerns encourage the use of alternative and renewable sources of energy particularly in developed countries. Governments are beginning to respond to the Kyoto Protocol (1997) of the United Nations Framework Convention on Climate Change as bioenergy is increasingly seen as one of the solutions to these issues. (IPCC 2001)
Sustainability, which combines economics, environment, and social/cultural considerations in relation to the use of forests involves ensuring that forest management and the benefits from forests do not compromise the opportunities for future generations to benefit from forests. Woody biomass can be a sustainable source of energy; a valuable renewable alternative to finite fossil fuels, but there still remains a lack of awareness and knowledge by policymakers, and community planners of the potential of bioenergy.
Forest harvesting is a major source of biomass for energy. The life cycle of forests includes reforestation/regeneration involving a sapling stage of rapid height growth, an intermediate stage of growth in diameter/height/volume, finally reaching maturity after 30-80 years. Harvesting may occur as thinning in young stands, or cutting in older stands for timber or pulp that also yields tops and branches usable for bioenergy. Stands damaged by insects, disease or fire are additional sources of biomass. Harvesting operations usually remove about 25-50% of the volume, leaving the residues available as biomass for energy. Additional benefits from harvesting include access for site preparation and planting, reduction of fire risk and damage from insects and diseases. In assessing the economic sustainability of biomass harvesting systems, all costs and benefits need to be considered. (Hall 2002)
Forest residues normally have low density and fuel values that keeps transport costs high, and so it is most economic to reduce the biomass density in the forest. This is done by comminution (reducing residues to small pieces with a chipper, grinder or a flail), or by compaction into bundles, termed compact residue logs which can be handled efficiently. Where biomass is used for heating, it can be stored/dried at roadside, a central terminal or conversion facility and distributed during peak periods of demand. Costs are variable and depend on type of cutting, stacking, chipping, forwarding, and transport to conversion facilities. Considerable success has been achieved where there is large-scale use of mill wastes and forest residues to produce heat and electricity, to the point in some areas where nearly all available fuels are being thusly used.
Dedicated energy crops are another source of woody biomass for energy. Short-rotation (3-15 years) techniques for growing poplar (Populus), or willow (Salix), eucalyptus, or gum trees, (Eucalyptus), or perennial grasses (Miscanthus) have been developed over the past 2-3 decades. Crops have been improved by selecting for rapid growth and tolerance to pests, and matching them to the best soil and site conditions; many clones for production have been so identified. Operational yields in the northern hemisphere approach 10-15 tonnes/ha annually, greater yields are possible where biological limitations are less. A 20 MW steam cycle power station using enrgy crops would require a land area of around 8,000 ha to supply energy sustainably in rotation. Hence, at any one time much of the area would be in growing biomass while adding an additional carbon sink. (Coombs,2002, Garten,2002)
Large-scale production of biomass uses silvicultural treatments that resemble agricultural ones; including site preparation by ploughing, discing, harrowing, followed by machine or hand planting of cuttings or rootstocks. The application of fertilizers and herbicides ensures sufficient nutrient levels and weed control. Stems may be cut back after the first growing season to stimulate sprouting and the crop is usually harvested after 3 years for willow, or 8-15 years for poplar. Harvesting usually occurs in the winter using purpose-built harvesting equipment. Harvested stems are often converted to chips on the site and then transported to the conversion plant. After harvesting the stumps are left to coppice and another crop (willow) is grown every three years. Poplars can also be coppiced but are generally grown as single stem crops and replanted after each harvest with new and improved clones. (Hall and Richardson, 2001)
The growing diversity of uses and public expectations related to forests has led to the concept of sustainable forest management as a central purpose in managing forests. Sustainable forest management is yet to be defined, however, governments and other organizations have developed systems of Criteria and Indicators to define and decribe sustainable forest management so that the range of forest activities can be assessed and their management adapted accordingly. These Criteria (values) and Indicators (measurements of values) are designed to be implemented on regional, national and international scales. Economic criteria consider levels of employment, price of wood and other forest products. Environmental criteria evaluate the health, productive capacity, biodiversity, soil, water, and carbon budgets. Social criteria take into account public participation in forest management decisions, the use of forests for spiritual and aesthetic uses, and other related activities. All combine to enable assessments of sustainability. Biomass production for energy is a product of forests, thus its benefits and impacts on forests are monitored to ensure sustainability. (Anon.,1995, Richardson et al, 2002)
Certification, an independent attestation that the products of forests are generated from sustainably managed lands, is another consideration which may soon affect biomass production. Certification is done to secure continued access to public forest lands through improved public acceptance of forest management activities. In some countries, certification that a power production system benefits the environment is being used as a marketing tool; as this gains acceptance the call for certification will become more widespread. (Hall, 2002).
The development of bioenergy markets can have many positive economic benefits including:
In most biomass production systems the cost and value of the biomass is low, though this may discourage initiatives in biomass production, it may also stimulate the use of bioenergy if the cost is competitive with fossil fuels. Biomass production on agricultural land will become more economically attractive as agricultural policies reduce subsidies for food crops and the opportunity is enhanced for growing bioenergy crops on now-surplus land. Some farmers are already planting trees for timber or biomass, rather than leaving the land idle. The potential for mixed forestry and biomass, coupled with carbon-sequestration plantations may make tree farming an increasingly attractive economic proposition. The prospect of carbon emissions trading will further enhance this process.
Biomass from integrated harvesting operations can improve the financial return from harvesting and make operations in previously marginal areas economically viable. Here, harvesting operations target stands where conditions are best for recovery and the scale of operations allows full utilization of equipment. In many areas there is a growing trend towards joint ventures, agro-forestry, or land leasing arrangements with farmers, so that rural communities are not displaced by expansion of planted forests. Already on some farms, willow or spruce (Picea) are being planted and economic opportunities for non-food crops is showing promise. Future energy markets are likely to be less regulated than today and the forest energy industry more subject to market forces, resulting in higher proces for carbon in response to demands from international protocols and treaties. Effective policies will need to be transparent, cost-effective in achieving objectives, and `fair' as regards renewable versus non-renewable energy systems. These factors must be considered in justifying the use of biomass for energy.
There are environmental impacts arising from the production of biomass, as there are in managing many natural resources. Three of the more important ones are site productivity, biodiversity, and greenhouse gas balances.
A commonly-expressed environmental concern about harvesting biomass for energy is that soil nutrients, organic matter and moisture-holding capacity may be depleted by intensive harvesting methods. (Hall and Richardson, 2001) Protection of soil relies on careful harvesting practices to reduce physical soil disturbance and compaction or removal of organic matter layers on the soil surface. Where roads and extraction tracks disturb organic layers, there is a need to manage water flows and runoff to reduce contamination of streams and waterbodies by soil and silt. Soil compaction, which reduces the extent and time of root growth, can be minimized by operating when soils are dry or frozen and by avoiding repeated passes of heavy equipment. Regulations governing harvesting practices are well-established and can be readily implemented in natural or planted forests.
Nitrogen and other elements are abundant in twigs and foliage so that harvesting all above-ground biomass could theoretically remove a large proportion of nutrients. In practice this does not occur since harvesting practices remove a small portion of the branches and tops and leave sufficient biomass to conserve organic matter and nutrients. Furthermore, if nutrients are returned to the forest through ash from combustion of the residues, this ash fertilization compensates for most nutrient losses. On nutrient-poor sites the ash should be recycled once per forest rotation. Thus forest residues can be utilised much more than they are today without significant negative environmental impacts. Short-rotation crops are relatively more demanding in terms of nutrient and cultural treatments than crops from natural forests. Nevertheless, the same principles apply and productivity can be mainteined by the appropriate silvicultural practices. (Hall and Richardson, 2001)
Science-based studies of site productivity and harvesting are now able to indicate which areas should not be harvested for biomass. Sites where nutrients are the primary limitation to tree growth should not be harvested, or harvest should be limited to removal of stemwood. Avoiding harvesting on drought-stressed sites, and limiting removals to once per rotation largely avoid the environmental impacts on harvesting. (Hall, 2002)
Biodiversity conservation is a central issue to forest management and is a significant public policy issue. Management of natural forests emphasises conservation of extant biodiversity by protecting critical habitat and balancing the vegetation structure, growth stages and forest ecosystems over time. In managing planted forests there is emphasis on retaining patches or riparian corridors of natural vegetation, and in some cases re-establishing native vegetation as part of plantation establishment. Natural or non-planted forests have traditionally had greater role in biodiversity conservation than plantations which are prized for production of wood fibre over other products. However, careful forest management in natural and/or planted forests can contribute to the conservation of biodiversity and to water regulation, carbon sequestration and recreational benefits. (Richardson et al, 2002)
Harvesting practices may be more intensive and so change wildlife habitats compared with conventional harvesting. Silvicultural techniques can overcome most of these effects through connection of fragmented habitats by reforestation, alteration of the size of harvested areas, elimination of pesticides, encouragement of ground vegetation, and the creation of a multi-aged, multi-species forest which provides a diverse habitat for wildlife. Experience in biomass production has shown that a normal utilisation of residues after forest operations has little negative impact on biodiversity, while the use of forest residues is environmentally beneficial because it replaces fossil fuels as an energy source. When energy crops are planted on abandoned agricultural land, species diversity may increase since diversity is low where single agricultural crops have been grown. Short-rotation crops have much higher yields than forests so smaller areas are needed to produce biomass thus reducing the area under intensive forest management. The creation of structurally and species diverse forests also helps to reduce the impacts of insects, diseases and weeds. Similarly the artificial creation of diversity is essential when genetically modified or genetically identical species are being planted. Finally, it is through an intelligent approach to managing forests that will ensure that sustainability goals are met.
Bioenergy systems offer significant possibilities for reducing greenhouse gas emissions when bioenergy replaces fossil fuel in energy production. (Matthews and Robertson, 2001. Schlamadinger et al, 1997.) The greenhouse gas balance of producing bio-energy is positive so replacement of fossil fuels with biomass both reduces emissions and enhances carbon sequestration since short-rotation crops or forests established on former agricultural land accumulate carbon in the soil (sinks). Bioenergy usually provides an irreversible mitigation effect by reducing carbon dioxide at source, but it may emit more carbon per unit of energy than fossil fuels unless biomass fuels are produced unsustainably. This is why bioenergy is so strongly promoted as a contributor to the problem of global change.
The Kyoto Protocol is stimulating policies directed towards the limitation of greenhouse gas emissions, particularly carbon dioxide (CO2). The two main causes of rising CO2 concentrations are the burning of fossil fuels and land-use changes, particularly deforestation. The reduction of CO2 emissions and reversal of deforestation by afforestation are opportunities to address climate change. Substitution of biomass energy for fossil fuel-based energy will also require an effective accounting of carbon. The United Nations Framework Convention on Climate Change and the Kyoto Protocol emphasise carbon and emissions accounting in all areas of the economy (IPCC, 2001).
The essence of social sustainability is how different societies benefit from biomass production. Biomass production systems require people to operate them thus creating jobs. As well, public perceptions of bioenergy systems may place different values on forests and landscapes. Most residue-harvesting operations are conducted by contractors who might supply biomass for a small district heating plant, or who collectively supply larger plants. The impact on employment is primarily in rural areas. In many countries, there remains a strong cultural tradition for the place of fuel wood biomass in the energy supply. However, as the efficiencies of scale increase, or as integrated harvesting systems are used, fewer people tend to be employed per volume of biomass harvested.
In some places cultural traditions are being revived by the increased use of woody biomass for bioenergy. In the boreal forest, many aboriginal communities have no year-round road or connections to the national electricity grid, and are dependent on diesel generators supplied by fuel flown or barged in at high cost. These communities are often surrounded by forest that could provide the necessary biomass for energy generation. This would make the community more self-sufficient, reduce costs, provide employment, and integrate well with a forest-based culture. There are examples where a shift to locally-produced bioenergy has been successful and these successes need to be encouraged.
These is a need for a radical shift and understanding by local planners and communities to take advantage of the opportunities presented by the increased use of bioenergy. Urban attitudes to biomass production are equivocal; here, we need better communication so that all citizens appreciate the use of biological systems to provide our needs sustainably. Biomass energy developments will be strongly influenced by policies and incentives to reduce greenhouse gas emissions. These developments can ensure a positive climate for investment in biomass energy to address the larger needs of society. (Richardson et al, 2002)
There is increasing recognition of the local and global environmental advantages of bioenergy. It is the largest renewable energy source in use representing nearly a billion tonnes of oil equivalent; consumption levels comparable to natural gas, coal and electricity. The trend towards cleaner, greener, smaller and more decentralized energy production facilities has a positive effect on demand for biomass. The issues of sustaining forest cover, slowing deforestation, regenerating natural forests, engaging in intensive forest management, and improving the management of agricultural and rangeland soils, can all be addressed through bioenergy production. Well-managed short-rotation forests are a sustainable resource that can be renewed in perpetuity, are carbon dioxide neutral, can be a good crop for unused agricultural lands while providing a source of employment and social stability. Land used for biomass production may be able to support a greater diversity of flora and fauna than agricultural lands. There are no technical reasons to prevent a major increase in utilisation of bioenergy from forests or agricultural land and there are clear environmental benefits if this were to occur.
It is evident that without a supportive policy environment for bioenergy, little progress will be made on a sustainable solution to future energy demands and climate change issues. An activist policy environment will include a combination of targets and incentives. Targets include certain percentages of renewables to be attained by a specified date, the trading of renewable energy certificates, rewards for carbon sequestration investments, and, of course, public promotion by governments. Incentives include climate change carbon levies (carbon tax), carbon trading, tariffs, grants, subsidies and increased depreciation rates; all used to `level the playing field' for increased use of bioenergy. (Sims, 2002)
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1 Canadian Forest Service, Ottawa, Canada, K1A 0E4. [email protected]