Introduction
The use of wood for electricity generation and heat in modern (non-traditional) technologies has grown rapidly in recent years. For its supporters, it represents a relatively cheap and flexible way of supplying renewable energy, with benefits to the global climate and to forest industries. To its critics, it can release more greenhouse gas emissions into the atmosphere than the fossil fuels it replaces, and threatens the maintenance of natural forests and the biodiversity that depends on them. Just like the debate around transport biofuels a few years ago, this has become a highly contested subject with very few areas of consensus.
This paper aims to provide an overview of the debate around the impact of wood energy on the global climate, and to reach conclusions for policymakers on the appropriate way forward.
Global demand and supply
In energy policy terms, wood is one form of solid biomass, with other forms being agricultural crops and residues, herbaceous and energy crops, and organic wastes such as food waste or manure. Biomass-based energy is the oldest source of consumer energy known to humans, and is still the largest source of renewable energy worldwide, accounting for an estimated 8.9 per cent of world total primary energy supply in 2014.1 Most of this is consumed in rural areas of non-industrialized or less industrialized parts of the world for cooking and heating, usually on open fires or in simple cookstoves. Together with the use of wood charcoal, these are categorized as ‘traditional’ uses and are not covered in this paper or its companion papers.
The focus here is on the combustion of woody biomass to produce electricity or heat, or both, through modern, non-traditional technologies: power stations, combined heat and power facilities, industrial processes such as pulp and paper mills, modern biomass burners, and so on. Biomass can also be co-fired with coal; coal plants do not need to be modified up to a mix of about 5 per cent biomass, making this the cheapest way of using biomass for power.
Taken together with bioliquids (which are mainly used for transport fuel) and biogas, these forms of biomass are the largest source of modern renewable energy used worldwide, accounting for an estimated 5.1 per cent of total final energy consumption in 2014. Heating for industry and buildings accounts for the bulk of this, while combustion for electricity is comparatively small, though it has grown rapidly in recent years (see Figure 1).
Figure 1: Shares of traditional and modern biomass (solid, liquid and gaseous) in total final energy consumption and in final energy consumption by end-use sector, 2014
— Source: United Nations Environment Programme (2016), Renewables 2016: Global Status Report, Nairobi: United Nations Environment Programme, p. 43.
Figure 1: Shares of traditional and modern biomass (solid, liquid and gaseous) in total final energy consumption and in final energy consumption by end-use sector, 2014
The growth of biomass energy has the potential to continue as countries increasingly adopt support policies for these uses of biomass, primarily in response to climate and energy security concerns. In the EU – the largest global consumer of modern biomass energy – a major driver has been the 2020 targets set for member states under the 2009 Renewable Energy Directive. In 2012, of the over $7 billion invested in biomass-based power worldwide, Europe was the leader, accounting for about one-third.2 While the EU has the largest share of biomass-fired electricity generation, the US, China, Japan, India and Brazil are all also significant consumers (see Figure 2).
Figure 2: Bio-power global generation, by country/region, 2005–15
— Source: United Nations Environment Programme (2016), Renewables 2016: Global Status Report, Nairobi: United Nations Environment Programme, p. 45.
Figure 2: Bio-power global generation, by country/region, 2005–15
Most analyses assuming expansion in renewable energy envisage significant growth in the use of biomass, at least to 2030 and often beyond. In 2012, for example, the International Energy Agency (IEA) estimated that, as long as appropriate policies were in place by 2050, bioenergy (wood and other forms of biomass) could provide 3,100 terawatt hours (TWh) of electricity (7.5 per cent of total world electricity generation, an eight-fold increase from 2011), 22 exajoules (EJ) of final heat consumption in industry (15 per cent of the total, a tripling of the total) and 24 EJ in the buildings sector (20 per cent of the total, though this represented a fall from 35 EJ in 2009 as inefficient traditional forms of heating were gradually replaced).3
These estimates may be revised downwards, however, particularly for electricity generation, as the cost of other forms of renewable energy – mainly solar photovoltaic (PV) and wind – have fallen significantly in recent years and seem likely to reach grid parity with fossil fuel-sourced electricity very soon without subsidy. However, biomass energy has the advantage over solar and wind of being ‘dispatchable’; i.e. the electricity it generates can be dispatched at the request of power grid operators or of the plant owner. Biomass plants can be turned on or off, or can adjust their power output according to need, whereas solar, wind and hydroelectric power are present or not depending on the conditions (apart from pumped-storage hydroelectricity).4
In addition, there is growing interest in the combination of bioenergy and carbon capture and storage technology (BECCS) with the aim of providing energy supply with net negative emissions. The latest assessment report of the Intergovernmental Panel on Climate Change (IPCC) relies heavily on bioenergy for heat and power, and specifically on BECCS, in most of its scenarios of future mitigation options (see Chapter 1).5 Despite the falling price and growing share of other forms of renewable energy, biomass accordingly retains some potential for future growth.
Wood for power and heat
There are alternatives to the use of wood in biomass power and heat, including organic wastes, agricultural residues such as sugarcane bagasse or palm kernels, and energy crops such as miscanthus (elephant grass) or switchgrass. Agricultural wastes and residues are, or are planned to be, important sources of biomass energy in China, India and Brazil, and energy crops may become more significant in the EU, though there is considerable uncertainty over the likely availability of land for their cultivation, among other factors.6 However, all these forms of biomass tend to be less energy dense and more expensive to grow, collect and transport than wood. Wood is therefore likely to remain overwhelmingly the biomass fuel of choice for electricity generation and heat, at least in the short and medium term, as it is now in Europe, North America and Japan.
Wood in various forms can be used for electricity generation and heat. Primary end-products that are used for this purpose include:
- Fuelwood (or firewood): Simple logs, branches, twigs and so on, produced from logging, or thinnings and coppicings from managed forests. This is the simplest form of wood for fuel and requires no processing, but it is bulky and contains high levels of moisture. It can therefore be relatively difficult and costly to collect and transport.
- Wood chips: Medium-sized solid material (typically 30–60 mm in size) made by cutting, or chipping, larger pieces of wood. Wood chips are easier than fuelwood to transport and store but can contain just as much moisture. Globally, most high-quality chips are used for composite-board products such as oriented strandboard or the production of pulp and paper; lower-quality wood chips may be used for energy, particularly where the transport distances to the installation are relatively low.
- Wood pellets: These are produced by compressing wood material and extruding it through a die into cylinders (normally 6–12 mm in diameter and 10–30 mm in length). This process, together with the necessary drying of the wood, requires energy input. Compared to wood chips, pellets are more dense and have a lower moisture content, and are therefore better suited to transport and storage. They are now the favoured form of wood for biomass power generation, particularly where transport distances are great. Pellets can be made from any organic material, including agricultural wastes, sawdust or other wastes from sawmilling and wood product manufacturing, but many power stations, particularly those co-firing wood pellets with coal, can only use clean wood mainly sourced from whole trees (see Chapter 1).
- Wastes and residues: Bark, shavings, sawdust, trim ends, offcuts and so on can be burned for energy on-site in sawmills where they are produced or made into pellets. Residues from forest operations – stumps, tops, small branches and pieces too short or defective to be used for other purposes – can also be made into chips or pellets, but, as noted earlier, their quality is sometimes too low to be used in power stations.
- Black liquor: A waste product from pulp and paper mills, this is generally burnt in recovery boilers on-site to generate energy for the mill and often also for export to the local electricity grid. Although it is a liquid, black liquor is generally classified as solid biomass, and forms a substantial share of the wood-based fuel consumed in some EU member states and the US (see Chapter 1).
Several new technologies for using wood for energy are under development. So-called ‘torrefied pellets’, ‘black pellets’ or ‘biocoal’ are normal (‘white’) pellets heated in the absence of oxygen to further reduce moisture and sugar content. Compared to white pellets, they have a higher energy density (though also require more energy to produce) and are water-resistant and more robust in handling, and they can be more easily burned in coal stations.7 Wood (and other organic material) can also be gasified and the gas produced then used directly for electricity generation or fed into gas networks for heating or adapted for transport; though this technology has not been extensively commercialized so far.
About this paper
In national policy frameworks, biomass is always classified as a source of renewable energy, alongside other technologies such as solar PV, wind or tidal power. It benefits from the same kind of financial and regulatory support as those technologies on the grounds that, like other renewables, it is a carbon-neutral energy source. However, at the point of combustion, biomass is not carbon-neutral – if wood or other organic material is burnt in the presence of oxygen, it produces carbon dioxide – and the argument is increasingly being made that its use can have negative impacts on the global climate.
This classification of biomass as carbon-neutral derives from either one of two assumptions. The first is that biomass emissions are part of a natural cycle in which, over time, forest growth balances the carbon emitted by burning wood for energy. Chapter 1 examines this assumption.
The second assumption derives from IPCC reporting rules intended to avoid the double-counting of carbon emissions, which determine that emissions from wood energy are accounted for in the land-use sector and not in the energy sector. In effect, emissions are assumed to occur at the point of harvest, not at the point of burning, and thus biomass energy is carbon-neutral from the energy-sector perspective. Chapter 2 examines the framework for reporting and accounting of biomass emissions.
Governments, particularly those in the EU, have not been immune to the growing concerns over the impacts of the use of biomass for power and heat explored in this paper, and some have introduced or are planning to introduce sustainability criteria designed to minimize the environmental impact of biomass: biomass feedstocks must meet these requirements if they are to receive financial and regulatory support. Some private schemes are also being developed. Chapter 3 examines this development and considers the likely impact of the criteria currently in use or development.
This is the first of four papers to be published by Chatham House on this topic. Two more – Woody Biomass for Power and Heat: Global Patterns of Demand and Supply and Woody Biomass for Power and Heat: Demand and Supply in Selected EU Member States will review the recent and anticipated growth of demand for wood for electricity generation and heat in modern technologies on a global scale and in specific countries, and assess the likely sources of supply, in recent years and in the future. The fourth paper, Woody Biomass for Power and Heat: Impacts on the Local Environment and Forest Users, will consider the impacts of the use of woody biomass for energy on forest ecosystems and on other forest users.
1 United Nations Environment Programme (2016), Renewables 2016: Global Status Report, Nairobi: United Nations Environment Programme, p. 28.
2 Roberts, D. G. (2013), ‘International Wood Fibre Markets (and Emerging Shocks)’, presentation at Megaflorestais conference, Bali, October 2013.
3 International Energy Agency (IEA) (2012), Technology Roadmap: Bioenergy for Heat and Power, Paris: IEA, http://www.iea.org/publications/freepublications/publication/2012_Bioenergy_Roadmap_2nd_Edition_WEB.pdf (accessed 28 Dec. 2016).
4 These issues will be discussed at more length in the companion paper, Woody Biomass for Power and Heat: Global Patterns of Demand and Supply.
5 Intergovernmental Panel on Climate Change IPCC (2014), Climate Change 2014: Mitigation of Climate Change, Cambridge: Cambridge University Press, http://www.ipcc.ch/pdf/assessment-report/ar5/wg3/ipcc_wg3_ar5_full.pdf (accessed 28 Dec. 2016).
6 See, for example, Allen, B. et al. (2014), Space for energy crops – assessing the potential contribution to Europe’s energy future, London: Institute for European Environmental Policy, http://www.birdlife.org/sites/default/files/attachments/IEEP_2014_Space_for_Energy_Crops_0.pdf (accessed 30 Dec. 2016).
7 Several slightly different processes can be used to produce torrefied or black pellets, including thermal roasting and steam explosion. While technically these are not the same, the end products are similar and the terms ‘black pellet’ or, less commonly, ‘biocoal’, are often used to describe them all.