Long before there was gas, there was wood. And though the modern, Western world has been largely dependent on fossil fuels like gas and coal to meet our demands for electricity and heat, the humble tree is making a comeback, along with its consorts – offcuts and crops. As supplies of other energy sources begin to run out and governments throughout the world scramble to cut carbon, biomass – touted by many as a ‘zero carbon fuel’ – is becoming an increasingly attractive energy source.

Since the Renewables Obligation was introduced in 2002, licensed electricity producers in the UK have been obligated to source an increasing proportion of their electricity from renewable sources – wind, hydro, tidal, solar, hydroelectric, but also biomass. Consequently, interest in biomass, especially from large electricity producers, has been rising dramatically over the past few years.

‘Biomass’ is perhaps a harder fuel to define than some of the other renewables, as it comes in many forms, whereas a wind turbine is a wind turbine is a wind turbine. It’s largely not a case of chopping down trees and feeding them into incinerators as you might imagine, though – many bits suitable for combustion are the scraps and dust left over from saw mills (up to half of what goes in, surprisingly), as well as forestry residues – branches and whatnot taken off during felling operations or tree surgeon activity – and reclaimed wood, predominantly from the packaging, construction and demolition sectors. You can also combust agricultural residues like straw and dedicated bioenergy crops, predominantly short rotation coppice willow and miscanthus, or elephant grass, in the UK. Dr Geoff Hogan of the Forestry Commission’s Biomass Energy Centre notes these types of fuels are less common, though, as they have to “pay their way” and tend to be prohibitively expensive when residues are available for free. Basically, any organic material with a moisture content below 50 per cent (wet material takes longer to burn or requires energy to dry) is a prime candidate for combustion as biomass fuel.

And just as biomass can come from many sources, so biomass burners come in many shapes and sizes. Domestic users can burn logs or pellets (typically compacted sawdust) in wood-burning stoves, or can install full boiler systems that produce 10-40 kilowatts of energy, though burning a steady stream of solid fuel at a high temperature necessitates a much more substantial and expensive bit of kit than, say, a gas boiler. Hogan says that, at the moment, the majority of biomass boilers are at the 100-500 kilowatt scale, situated in places like stately homes, hotels, leisure centres and district heating plants – as at this level, the equipment is not prohibitively expensive and such boilers can run on wood chips, a cheaper fuel than pellets. Biomass combustion plants can get much, much bigger, though: there are some plants already operating at the 10-60 megawatt level and – encouraged along by the Renewables Obligation – plans are afoot to build mass-burn biomass incinerators that generate hundreds of megawatts of electricity.

Indeed, the £400 million, 350-megawatt plant being developed at Port Talbot in South Wales will be the largest biomass energy plant in the world. Guy Watt of John Clegg Consultancy says: “As of December 2009/10, there were 16 large-scale biomass energy plants going and there were 47 proposed... It’s a rapidly changing scene.”

Last year, John Clegg Consultancy produced a report, ‘Wood fibre availability and demand in Britain 2007 to 2025’, which forecast that demand could exceed potential availability as soon as 2012. Stuart Goodall, CEO of the Confederation of Forest Industries (ConFor), which co-commissioned the study, notes: “The report demonstrated potential demand for 30 million tonnes of wood in the UK. Now, the production of wood in the UK is about 12 million tonnes a year, so this is new demand of nearly three times what we produce. Our concern is that these companies, with the subsidies provided by the Renewables Obligation, would wish to buy all this wood to put into a plant. By doing that, they would starve existing businesses that use that wood – sawmills, principally.” (At the moment, at least, Hogan claims: “It’s not economically viable to burn large-diameter, high-quality saw wood because they’re worth far more in a sawmill.”)

But it’s not just sawmills that use this UK-produced wood – fibre also goes into wood panels and paper products or is exported to feed already established biomass plants on the Continent. According to ConFor, much, though not all, wood fibre currently has an end use and the supply and demand of the material is very finely balanced. The entry of more large electricity producers could upset this equilibrium; Goodall notes that the Port Talbot plant alone “would require four million tonnes of wood a year, equivalent to a third of the UK’s production of wood”.

Concerns about supply and demand extend to the recovered wood market, as well. Recovered wood is very attractive to energy producers because it tends to be very dry (though if it has been treated with halogens or heavy metals, it must be burnt in a Waste Incineration Directive compliant plant), but even most recovered wood already has a market. Sixty per cent of the 4.5 million tonnes produced annually goes to the wood panelling industry and more than 10 per cent is exported. A 2009 WRAP report on the wood waste market in the UK noted that because of the recession, waste wood generation could fall by 14 per cent, and that this, coupled with rising demand from energy companies, could result in shortages.

Now, it should be noted that most of the proposed large electricity plants plan to source the bulk of their feedstock from abroad. Hogan points out that “the sawmills particularly of Canada, but also of the US, South America and the Baltic States have huge quantities of residues” that could be used as feedstock for UK plants. Asked whether transporting fuel such a distance would damage the claim that biomass is ‘zero carbon’, Hogan notes that because emissions from shipping are so low: “You can transport wood 3,000 miles from Canada with the same environmental impact as shipping it perhaps 300 miles within the UK by lorry.” The Americas are thousands of miles across, though, and whether or not the wood would come exclusively from east coast mills “remains to be seen”.

The claim that biomass is a zero carbon fuel is an interesting one – based on the fact that the biomass absorbs as much carbon in its life as it does in its combustion – but slightly misleading. After all, until that fibre is combusted it’s a carbon sink and even fossil fuel ultimately comes from organic matter that absorbed carbon when it was living. Keiran Allen, Technology Acceleration Manager at the Carbon Trust, explains: “We would never describe biomass as a ‘zero carbon fuel’, but as a ‘lower carbon fuel’ or a ‘very low carbon fuel’.”

In fact, biomass can never be considered truly zero carbon, no matter how you figure the numbers, because it takes energy to harvest and transport the material, and to convert it into useable form, which rarely, though sometimes, involves energy-intensive drying. Biomass does seem to compare well with other fuels in most lifecycle assessments, though this too is misleading as these omit biogenic carbon released in the combustion process, since it is considered to be offset by that absorbed in the growing process. One study by the World Energy Council found that biomass emits between 15-49 kilogrammes of CO2 per megawatt hour (kg CO2/MWh) when used to generate electricity (compared to 369-398 kg CO2/MWh for natural gas) and between 10-23 kg CO2/MWh when used for space heating (compared with 263-302 kg CO2/MWh for natural gas).

Of course, the process has to be managed properly to achieve these impressive results. As Hogan notes: “Biomass is a potentially genuinely beneficial technology. But like any energy-generating mechanism – whether it’s coal burning, gas burning, wind farms, photovoltaics, or biomass – it will have impacts. All of them can be done badly or they can be done as well as possible. There certainly are potentially negative impacts of doing biomass badly... you could cut down large swathes of Amazon rainforest and convert it all to wood pellets and that would have an entirely negative impact in many, many ways.” Yet active management – the felling of certain trees and culling of select branches – as opposed to whole-scale deforestation is actually good for a forest’s biodiversity. Hogan adds: “If you just leave your woodlands, the canopy covers over and you start to lose all the spring flowers and the understory, the brambles, and the butterflies that come with it.”

But ConFor is not alone in worrying that the UK is not necessarily using biomass in the most efficient, sustainable way possible, and not just because of concerns over deforestation or supply and demand. Goodall is of the opinion that much of the wood being eyed up by the electricity sector would be put to better use in buildings or furniture (where its carbon content would be fixed), and that the biomass residues unsuited for such purposes would be best used for heat, not electricity. He says: “If the objective of all this is to reduce carbon in the atmosphere and as a secondary objective to create green jobs, then you get far more carbon reductions and jobs out of putting it into a house than you do from creating energy. However, if you are then going to create energy either from virgin wood which has not currently got a market or from recycled wood, then it’s far more efficient to create heat than to create electricity.” Indeed, burning wood to create electricity alone means that most of the energy in the biomass is lost as waste heat; such power plants (like waste incinerators) have efficiencies that top out at around 30 per cent. By comparison, basic combustion for heating purposes can achieve efficiencies of 85 per cent.

All this begs the question: why has government been incentivising biomass combustion for large-scale electricity generation, and not for small-scale heat generation? While electricity, which can come from a myriad of renewable sources, accounts for only 18 per cent of energy use in the UK, nearly half goes into heating, and renewable fuels that can be used for this purpose are few and far between. Hogan admits: “Up until recently the government has only supported renewable electricity and there has been no explicit support for renewable heat generation, mainly because of the difficulty in incentivsing heat production. Electricity is relatively straightforward to meter, but to try and meter all the homes in the UK if they started using renewable heat would be well nigh impossible.” Still, it looks like renewable heat is about to get a boost. The Renewable Heat Incentive, originally announced under the Labour administration has been re-announced by the coalition, and those interested in sustainable biomass combustion are waiting for it with baited breath.