FEATURE: Bioenergy will play a vital part in the future energy mix – here's why

Dr Jody Muelaner

Wood from pine trees can be used to make biofuel pellets (Credit: Shutterstock)
Wood from pine trees can be used to make biofuel pellets (Credit: Shutterstock)

The role of bioenergy in a decarbonised energy system is often poorly understood.

Bioenergy is not just another source of renewable energy, it can also provide storage, carbon capture and frequency stability. This unique combination of capabilities will make it a critical component of the future decarbonised energy mix. 

Bioenergy will allow a very significant amount of buffering between supply and demand. Wind and solar are intermittent, while bioenergy can provide power when it is needed. Large quantities of energy can be stored cheaply in simple wood piles, allowing seasonal variations in energy supply and demand to be matched. 

Another vital characteristic of bioenergy is that it allows us to start capturing carbon from the atmosphere cheaply and effectively right now, by planting trees, and then to cost-effectively recover that carbon when the biomass is used to produce energy in the next few decades. This carbon-negative generation will allow us to reach net-zero emissions without stopping all positive emissions. Biomass can also provide the inertia needed to maintain frequency stability, a real challenge in a decentralised renewables-intensive grid.

Dispatchable bioenergy

Dispatchable generation can be turned on and off in response to demand for electricity. It is highly likely that some dispatchable generation capacity will be required in the grid during periods of very high demand and/or low renewable power. Daily fluctuations and mismatch in supply and demand are likely to be resolved through a combination of smart charging and devices providing demand-side response, and grid energy storage using batteries and pumped-storage hydroelectricity. 

However, these solutions are unlikely to cope with a prolonged period with little wind during a cold dark winter. For this, dispatchable power provided by chemically stored energy is likely to be required. There are two major options here – hydrogen and bioenergy.

Power stations cost around the same to build, regardless of whether they will be operated continuously as baseload power, or used occasionally for dispatchable generation. The cost of dispatchable power is, therefore, dominated by capital costs, with fuel prices of less importance. The less often the dispatchable power is needed, the more true this becomes. Capital cost will be eased by the conversion of coal-fired power stations to biomass.

Although conventional coal power stations are in theory dispatchable, they are generally designed for baseload power and can take considerable time to cycle on and off. Despite this, such plants converted to bioenergy can play an important role in managing seasonal variations and meeting peak demand. 

Negative emissions without carbon capture

As trees grow they capture carbon from the atmosphere, eventually releasing it back into the atmosphere owing to decomposition and fires. Traditionally, it was thought that mature forests reach an equilibrium, releasing as much carbon as they capture. It is now known that even ancient primary forests continue to capture more carbon than they release, with some carbon accumulating in soil. 

Biomass can support reforestation and afforestation of land, which the Intergovernmental Panel on Climate Change (IPCC) estimates could capture 0.5 to 3.6Gt of CO2 annually, at a cost of between £3.60 and £36 per tonne. Bioenergy therefore usually has negative emissions, although not as much carbon is removed as when carbon capture and storage is combined with bioenergy.

Carbon intensity graph

The carbon intensity of bioenergy depends on how the biomass is produced. If mature forests are intensively harvested, it can lead to release of soil carbon as well as the carbon released when vegetation is burned. Even if the trees are replanted, this can result in net positive emissions since the soil carbon continues to be depleted. Traditional harvesting, in which logging residues are not harvested as fuel, preserves soil carbon. 

Replacing mature forests with fast-growing energy crops can release the carbon sink from the trees and the soil, with additional emissions owing to fertiliser use. Carbon intensity is highly sensitive to the biomass supply.

Particulate pollution

Particulate pollution has a huge impact on public health. The air pollution associated with coal-fired power stations kills more than 300,000 people a year in China alone. With stricter controls this can be reduced significantly. For example, in the US the deaths caused by burning a tonne of coal are about 10 times lower, yet coal-fired power stations still kill 15,000 people a year in that country. For every person killed by coal, there are many more living with chronic health conditions. The annual cost of producing electricity from coal in the US is $200bn but the hidden healthcare costs are estimated at between $350bn and $880bn. 

The toxicity of particulates from biomass is likely to be similar to those from fossil fuels. Technologies are available to reduce the emissions from both coal and biomass-fired power stations. 

Bioenergy with carbon capture and storage

It is estimated that, between 1750 and 2011, human activity added 2,040Gt of CO2 to the atmosphere. Approximately 40% of this remained in the atmosphere, 30% was absorbed by the oceans, resulting in acidification, and the rest was absorbed by other natural sinks such as soil and vegetation. This has now resulted in a 46% increase in the concentration of CO2 in the atmosphere. The total amount of carbon stored in all the organic matter on land is equivalent to 8,300Gt of CO2, so the vegetation and soil carbon would need to be increased by about 25% to remove all of the CO2 that humans have added. 

It is generally accepted that this is not feasible, so, even if we had net zero tomorrow, planting trees couldn’t undo the damage already done. It might seem discouraging that the natural carbon sinks are relatively small in relation to the CO2 that humans have already added to the atmosphere. However, the annual flows of carbon in and out of these natural sinks are many times greater than the emissions generated by human activity.

There are many areas where achieving zero emissions will be very difficult – such as process emissions from cement and aluminium production, long-haul aviation, and nitrous oxides given off by fertilisers. Carbon capture and storage will, therefore, be required just to achieve net zero, let alone to restore the atmosphere. 

Of the 116 scenarios considered in the IPCC’s special report Global Warming of 1.5°C, 101 included negative emissions. Bioenergy with carbon capture and storage (BECCS) currently appears to be the most mature and cost-effective technology for negative emissions on a large scale. The IPCC estimates for electricity generation in 2050 assume an annual generation from BECCS of 2,800TWh and potentially more than 8,500TWh. With just 2% of global biomass growth going into BECCS, 9Gt of CO2 could be captured, enabling net zero.

CO2 flow

There are various technologies that could be used to capture carbon from bioenergy generation. These can be classified as pre-combustion, oxy-fuel combustion or post-combustion. Pre-combustion processes use a gasifier to turn the biomass into streams of syngas and CO2. Oxy-fuel combustion recirculates the flue gas so that the concentration of CO2 increases. Nitrogen is removed from air using a non-cryogenic air-separation unit, and the oxygen-rich stream is then mixed with the recirculated flue gas to oxidise the combustion. The resulting flue gas is primarily water vapour and CO2. The water vapour is removed by condensation and the CO2 can then be stored.

Post-combustion carbon capture can enable power plants to be retrofitted to capture carbon. There are various post-combustion technologies being developed, based on the use of solvents, sorbents and membranes. Solvent-based carbon capture uses a liquid that absorbs CO2 from the flue gas stream and then releases it when heated or depressurised. Commercially available solvent-based carbon-capture systems achieve high levels of carbon capture but are energy intensive. Sorbent-based methods work in the same way, absorbing CO2 and regenerating when heated or depressurised; the solid materials may have lower regeneration energy but are a less mature technology. Membrane-based carbon capture is being developed which promises to provide energy efficiency while avoiding hazardous substances.

Once the CO2 has been captured, it must then be stored. The primary method, geologic sequestration, involves injecting CO2 into underground geological formations. In some cases, these would need to be capped and maintained indefinitely, while other sites would result in the CO2 becoming geochemically trapped in the rock relatively quickly. It is predicted that 98% of injected CO2 will remain trapped for at least 10,000 years. 

Unfortunately, BECCS will not be cheap. Although BECCS has the greatest potential to capture large quantities of CO2 from the atmosphere, it is also the most expensive of the available technologies at £73 to £145 per tonne of CO2. To put this in perspective, offsetting the UK’s net emissions of 451Mt CO2e would cost around £50bn. Of course, nobody is proposing to offset all our emissions. As far as possible electrification will be powered by the cheapest sources, namely wind and solar. But, for the most difficult to decarbonise industries, offsetting with negative emissions will be a cost-effective option.

Frequency stability

The frequency of the AC supply results from the speed at which generators rotate. As each pole of a generator’s rotor bar passes the stator windings, a current is induced, which reverses when the opposite pole passes. When electrical demand drops, the magnetic field weakens, causing the generator to speed up and the AC frequency to increase. 

To ensure that devices function correctly, the AC frequency and phase must be synchronised across the entire national grid. Frequency response can be achieved by turning generators on and off. The control becomes much easier, however, if the rate of changes is damped out by having large generators with significant inertia rotating within the system. Biomass can provide this inertia to support frequency stability in the grid.

The future of bioenergy

Unusually for a renewable energy source, bioenergy can provide both dispatchable power and inertia. Perhaps uniquely, it can also enable carbon capture and storage. These properties will make it a vital component of a decarbonised energy system. 

Bioenergy can, however, only play a niche role in decarbonisation. There isn’t enough land to generate all of our energy from biomass, and this would have a huge impact on both air quality and ecology. So bioenergy should be seen as an important last resort to be used when needed to provide dispatchable power, inertia and offsetting of non-energy emissions.

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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers. 


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