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Biomass and renewable gas

By Dave Elliott

Not everyone backs biomass, given the emission/biodiversity/land-use issues, but  biomass does offer a range of flexible green fuel options, biogas especially.  The World Bioenergy Association (WBA) says bioenergy already contributes over 14% to the global energy mix, and its use is bound to expand.  So what are the options?The “WBA Global Bioenergy Statistics 2014” report provides useful data on the current global situation, by area and source. Overall, solid biomass supplied 89% of the total bioenergy and more than 90% of bioenergy is used for heat – the rest for transport and electricity. 70% of global biofuel production comes from the Americas, while Europe led in biomass energy conversion plants, whereas Asia and Africa are the leading users of biomass for direct heat.

What next? Biomass has a promising future in the world’s supply of renewable energy, according to the roadmap developed by the International Renewable Energy Agency (IRENA). It foresees a major role for modern, sustainable biomass technologies in efforts to double the share of renewables in the global energy mix. A new IRENA report, “Global Bioenergy Supply and Demand Projections for the Year 2030”, examines the biomass potential in world regions and with different technologies for rapid and sustainable scale-up. If all the technology options envisaged in the REmap analysis are deployed, total biomass demand could reach 108 exajoules worldwide by 2030 (including biofuels for vehicles) representing 60% of total global renewable energy use. That would be equal to 20% of the total primary energy supply. IRENA says that renewables overall could supply up to 36% of global primary energy by 2030, so clearly biomass is seen as an important element.

The new IRENA report says that approximately 40% of the total global biomass supply potential could originate from agricultural residues and waste, with another 30% originating from sustainable forestry products. Some green NGOs will object to the use of wood from forests for power production, and will also object to large biofuel plantations for vehicle fuels, but IRENA says biomass sources need not compete with the resources that are required for food production, such as land and water, and can make a significant contribution to reducing the global CO2 emissions.  IRENA Director of Innovation and Technology Dolf Gielen said: “Sustainably sourced biomass, such as residues, and the use of more efficient technology and processes can shift biomass energy production from traditional to modern and sustainable forms, simultaneously reducing air pollution and saving lives.”

Focussing on perhaps less contentious areas than biofuels for vehicles and forestry biomass electricity generation, “Renewable Gas”, a Palgrave book by Jo Abbess, looks to the role of biomass and other sources in producing green gas. Gas is a useful fuel. As she makes clear, fossil gas use has boomed in recent years, in part since it is less carbon intense than coal, but of course burning it still produces carbon dioxide, and reserves, even with shale gas included, are not infinite. That is a worry, since unlike electricity, it is easily stored and can be transmitted over long distances with low energy losses. Gas-fired power plants can be quite flexible, so some see them as an important complement to variable renewables. Peak fossil gas may be some way off, and it may be possible for carbon dioxide emissions from gas-fired plants to be captured and stored – at a price. However, in terms of low carbon options, there is also another approach: using biomass stocks of various kinds, including wastes, to make “renewable gases”. If done right, with the right feed-stock and sustainably  managed sources, producing and then burning biogas can be almost carbon neutral over time. And even maybe carbon negative, if used with Carbon Capture and Storage.

Switching to renewable gas does not just involve a simple low carbon fuel change, it also opens up some interesting new possibilities, including an alternative to decarbonisation by electrification. One currently quite dominant view in the UK is that domestic heating should no longer be done using fossil gas, but by heat pumps using electricity from renewable sources and/or nuclear, instead. Renewable gas offers an alternative, which might make more sense, given that the UK gas grid currently carries 3-4 times more energy than the electricity grid. Switching from gas to electric heating would put a huge strain on the power transmission, distribution and supply system. One study suggested that it would add 60% of the cost of the system for the necessary local electricity grid reinforcement.

There are land-use and biodiversity limits to how much biomass is available for gas production, and also limits to how much biogas can be produced from farm and food wastes via Anaerobic Digestion (AD), but there may be a novel solution – the production of synthetic green gases, using renewable electricity and carbon dioxide captured from the air or from power plant exhausts. The simplest approach is just to make hydrogen gas by electrolysis of water, but this can also be converted into methane using captured CO2. The conversion processes will of course add to the costs, but that can be offset if use is made of surplus electricity from wind or PV, which would otherwise just be dumped. This so-called “power to gas” system, with power being stored as gas for use when wind or PV are less available, would provide a way to balance variable renewables. Like biogas, it can also  replace or augment fossil gas in the gas main and be used as a green vehicle fuel.

As the new book illustrates, as well as the fully renewable biogas and power to gas options, there are many other intriguing “green gas” options, including the decarbonisation of various industrial gases and fossil fuel sources with, in some cases, the carbon being recycled and used, with hydrogen, to make syngases. Although not strictly renewable, these sources offer substantial amounts of low-carbon energy for as long as fossil reserves last. They could be an important interim or transitional gas option, for use while fully renewable sources are developed.

The wide-scale adoption and use of these various “green gas” options would have significant implications for how energy systems were developed and for energy policy generally. As the book reports, some of the technologies are under development or being deployed (it reviews several), and although many of these options may still be relatively new, the policy issues raised need to be considered urgently. The technical and strategic analysis in this text provides a good starting point.

There are of course other biomass-based options. In a new report from the Smith School of Enterprise and the Environment, Oxford University researchers say that our best hope for dealing with climate change is planting trees and improving soil quality to sequester more CO2, i.e afforestation, planting trees where there were none before; and biochar, improving the soil by burying a layer of dense charcoal made from biomass. They claim that these options are more effective at delivering carbon negativity than using biomass with engineered carbon capture and storage (BECCS) or air capture of CO2 (using chemical absorption), although they admit that “attaining negative emissions is in no sense an easier option than reducing current emissions” and that “to remove CO2 on a comparable scale to the rate it is being emitted inevitably requires effort and infrastructure on a comparable scale to global energy or agricultural systems.” They calculate that an 11-13% carbon reduction might be achieved by 2050 mainly from tree and biochar sequestration, coupled with soil upgrades, whereas more advanced (and expensive) industrial approaches like BECCS and air capture might only achieve a 2% reduction by 2050:

It does seem clear that growing more trees would be helpful to sequester carbon dioxide gas, although of course trees do die and rot, and can burn, releasing the carbon back into the atmosphere. Improving soil quality would also be wise, and although sequestration in biochar makes sense, it reduces the amount of biomass available for energy production. We need low carbon fuels to reduce emissions at source, and done right, biomass, and green gases generally, can play a role.

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