A wide range of possibilities
The amount of biomass available to provide energy depends a lot on the amount of land available to grow energy crops, and how much that land can yield. Different assumptions on these variables produce quite different estimates of the total resource, and numerous studies over the years have produced a wide range of results. The amount of waste biomass available also matters, but potential availability from this source is smaller.
A comprehensive review of estimates of the biomass resource was carried out two years ago by researchers at Imperial College[i] (see chart). It showed a variation in estimates of a factor of around 40, from of the order of 30 EJ to over 1000 EJ (1EJ =1018 J, or a billion GJ, or 278 TWh). This compares with total world primary energy demand of just under 600 EJ, transport demand of around 100 EJ, and at least 250 EJ to produce present levels of electricity, assuming biomass combustion to remain relatively inefficient[ii].
Estimates of available biomass resource
Source: Slade et. al. (2014)
The authors examine reasons for differences in estimates, which I’ve summarised in the table below. The differences are largely assumption driven, because the small scale of commercial bioenergy at present provides little empirical evidence about the potential for very large scale bioenergy, and future developments in food demand and other factors are inevitably uncertain.
Reasons for variation in estimates of total biomass supply
|Up to 100EJ||Limited land available for energy crops, high demand for food, limited productivity gains in food production, and existing trends for meat consumption. Some degraded or abandoned land is available.|
|100-300 EJ||Increasing crop yields keep pace with population growth and food demand, some good quality agricultural land is made available for energy crop production, along with 100-500Mha of grassland, marginal, degraded and deforested land|
|300-600EJ||Optimistic assumptions on energy crop availability, agricultural productivity outpaces demand, and vegetarian diet|
|600 EJ +||Regarded as extreme scenarios to test limits of theoretical availability|
Reasons for caution
In practice there seem to me to be grounds for caution about the scale of the available resource, although all of these propositions require testing, including through implementation of early projects.
- There will rightly be emphasis on protection of primary forest on both carbon management and biodiversity grounds, with some reforestation and rewilding.
- There is little evidence of a shift away from meat consumption. With the exception of India, less than 10% of people in most countries are vegetarian despite many years of campaigning on various grounds[iii]. In China meat consumption is associated with rising living standards.
- Demand for land for solar PV will be significant, although a good deal of this will be on rooftops and in deserts
- The nitrogen cycle is already beyond its limit, constraining the role of fertiliser, and water stress is a serious issue in many places (agriculture accounts for 70% of current fresh water use). The UN Food and Agriculture Organisation has projected fairly modest increases in future yields.
- Difficulties in limiting lifecycle emissions from biofuels are likely to lead to caution about widespread deployment.
- Concerns about food security may limit growth of biofuels.
Small scale to date, despite many years of interest
- There has been little progress to date compared with other low carbon technologies. Though traditional biofuels remain widely used, modern biofuels account for a very small proportion of demand at present. World biofuels consumption currently accounts for only 0.2% of world oil consumption[iv] . Many biofuels programmes have had subsidies cut and there is still limited private sector investment.
In this context some estimates of the potential for biomass to contribute to energy supply seem optimistic. For example, Shell’s long-term scenarios (Oceans and Mountains) show biomass of 74 EJ and 87 EJ respectively for commercial biomass, 97-133 EJ including traditional biomass by 2060[v]. These totals are towards or above the more cautious estimates for the resource that might ultimately be available (see table above). A recent review article[vi] suggested that by 2100 up to 3.3 GtCp.a. (around 12 billion tonnes of CO2) could be being removed, and producing around 170EJ of energy. However the land requirements for this are very large at about 10% of current agricultural land. The authors suggest instead a mean value for biomass potential of about a third of that, or 60EJ.
On balance it seems that biomass is likely to account for at most less than 10% of commercial global energy (likely to be around 800-900EJ by mid-century), and potentially much less if land availability and difficulties with lifecycle emissions prove intractable.
It thus seems likely that biomass energy will be relatively scarce, and so potentially of high value. This in turn suggests it is likely to be mainly used in applications where other low carbon alternatives are unavailable. These are not likely to be the same everywhere, but they are likely often to include transport applications, especially aviation and likely heavy trucking, and perhaps to meet seasonal heat demand in northern latitudes. For example, according to Shell’s scenarios aviation (passengers + freight) is expected to account for perhaps 20-25EJ by 2050, and biomass could likely make a useful contribution to decarbonisation in this sector.
None of this implies that biomass is unimportant, or has no role to play. It does imply that policies focussing on deploying other renewable energy sources at large scale, including production of low carbon electricity for transport, will be essential to meeting decarbonisation targets. And the optimum use of biomass will require careful monitoring and management.
Implications for the role of negative emissions technologies
There appears to be increasing likelihood that atmospheric concentrations of greenhouse gases will grow to exceed levels consistent with the target specified in the recent UNFCCC Paris agreement of limiting temperature rises to “well below” two degrees. Such an outcome would require CO2 to be removed from the atmosphere faster than natural sinks allow, in order to restore concentrations to safe levels. Near zero net emissions in the latter part of this century will also be needed to stabilise concentrations.
Many models of future emissions pathways now show negative emissions technologies (technologies that result in a net decrease in CO2 in the atmosphere) needing to play a major role for in meeting climate goals. They have the potential to remove carbon dioxide from the atmosphere in the event of “overshoot” of target atmospheric concentrations, and to balance remaining emissions from sectors where abatement is difficult with a view to achieving net total emissions of close to zero. But the application of such technologies should not be considered in isolation.
Bio Energy with CCS (BECCS)
Bio energy with CCS (BECCS ) is the most widely discussed approach to negative emissions. BECCS is not a single technology but a combination of two major types of CO2 removal. Bioenergy from burning biomass to generate electricity or heat emits a substantial amount of CO2 on combustion – around as much as a coal plant. However much of this can be captured using CCS. More carbon dioxide is reabsorbed over time by the regrowth of the plants, trees – or perhaps algae – that have been harvested to produce the bioenergy. Together, the capture of the CO2 from the flue gas and the subsequent absorption of CO2 from the atmosphere by regrowth of biomass can lead to net removal of CO2 from the atmosphere, although this takes time. (The net amount of CO2, that is emitted on a lifecycle basis from burning biomass without CCS depends on the type of biomass and is subject to considerable variation and uncertainty – a controversial topic that would need another post to review.)
However at an energy system level, where the CO2 is captured – biomass plant or fossil fuel plant – is less relevant than the total amount that’s captured. Broadly speaking, if there is a biomass plant and fossil fuel plant both running unabated the same benefit can be achieved by capturing a tonne of CO2 from either.
Indeed there may be advantages to putting CCS on a conventional plant rather than biomass plant. It may be technically more tractable. Furthermore, CCS require a lot of energy to capture the CO2 and then to compress and pump it for permanent storage. Biomass is likely to be supply constrained (again, this is an issue that requires a post in itself), so using biomass rather than fossil fuels to provide this energy may limit other applications. Only when there are no fossil fuel plants from which to capture CO2 does biomass plant become unambiguously a priority for CCS. This is clearly some way off.
Furthermore biomass with CCS for electricity generation may not be the best use of bioenergy. Converting sunlight to bioenergy then turning that into electricity is a very inefficient process. Typically only 1-2% of the sunlight falling on an area of cropland ends up as useful chemical energy in the form of biofuels. There are various reasons for this, not least of which is that photosynthesis is a highly inefficient process. Burning biomass to make electricity adds a further layer of inefficiency, with only perhaps a third or less of the energy in the biomass turned into electricity. Sunlight therefore gets converted to electricity with an efficiency of perhaps 0.5%. This compares with around 15- 20% for solar cells, implying that scarce land is often likely to be best used for solar PV. (The calculation is closer if solar PV is used to create storable energy e.g. in the form of hydrogen). For similar reasons the use of biofuels rather than electricity in transport is inefficient, in part because internal combustion engines are less efficient than electric motors.
Limited available biomass may be better used for those applications where it is the only available lower carbon energy source, notably in aviation and (likely) heavy trucks, or for other applications such as district heating with CCS, where the ability to store energy seasonally is especially valuable.
At the very least, detailed system modelling will be required to determine the optimal use of biomass. Suggesting that BECCS should have a major role to play simply because in isolation it has negative emissions may lead to suboptimal choices.
Direct Air Capture (DAC)
Direct Air Capture (DAC), where carbon dioxide is chemically absorbed from the atmosphere and permanently sequestered, can also reduce the stock of CO2 in the atmosphere. However the typical concentration of CO2 in a flue gas of a power plant or industrial plant is several percent, about a hundred times as great as the concentration of 0.04% (400 ppm) found in the atmosphere. This makes the capture much easier. Furthermore, millions of tonnes can be captured and piped from a single compact site using CCS, generating economies of scale on transport and storage. In contrast DAC technology tends to be more diffuse. These considerations imply that CCS from power plants and industrial facilities is always likely to be preferable to direct air capture until almost all the opportunities for CCS have been implemented, which again is a very long way off.
Uncertainties and optionality
There are many uncertainties around negative emissions technologies, including the availability of biomass, cost, and the feasibility of reducing emissions in other sectors. For these reasons developing optionality remains valuable, and research to continue to develop these options, including early trial deployment, is needed, as others have argued[i].
One of the best ways of developing optionality is to deploy CCS at scale on fossil fuel plants. This will reduce the costs and enable the development of improved technologies through learning on projects. It will also help build infrastructure which can in turn benefit BECCS. This needs to run in parallel with ensuring that the lifecycle emissions from the bioenergy production chain are reduced and biodiversity is safeguarded.
Negative emissions technologies may well have a role to play in the latter part of the century. But they seem likely to make more sense when the economy is already largely decarbonised. In the meantime deployment of CCS, whether on industrial facilities or power plants, needs to be a much greater priority.
Updated 5th August 2016
[i] Slade et.al., Global Bioenergy Resources, Nature Climate Change February 2014
[ii] Data on final consumption and electricity production from Shell and IEA data. 35% efficiency for biomass in electricity is assumed, which is likely to be somewhat optimistic, especially if CCS is employed.
[iv] BP Statistical Review of World Energy
[v] http://www.shell.com/energy-and-innovation/the-energy-future/shell-scenarios.html These totals include biofuels, gasified biomass and biomass waste solids, and traditional biomass.
[vi] Smith et. al., Biophysical and economic limits to negative CO2 emissions, Nature Climate Change, January 2016. The paper estimates land requirement for 170 EJ of 380-700 Mha, around 10% of total agricultural land area in 2000 of 4960Mha.