Replacing coal with biomass: a viable option?

Are we poised to see the world’s coal power plants transformed into giant wood burners?

They call it Megawatt Valley. From the green floodplain of the rivers Ouse and Aire in North Yorkshire rise the concave cooling towers of some of the biggest names in British coal power: Ferrybridge, Eggborough, Drax. The last of these is the UK's largest single generator: with its dozen cooling towers and nearly 4GW output, it provides a whacking 7% of the country's electricity – and emits the most CO2 from any single source.

Drax intends to change the last part. It has already invested in turbine upgrades that provide for 'co-firing' – mixing coal with biomass. As a result, up to 12.5% of the fuel it burns today comes from forest and crop waste, some local, some imported, as well as specially grown crops of Miscanthus, or elephant grass, provided under contract by around 100 local farmers. It has plans to build three new biomass-only plants, each with a capacity of 290MW.

Now it is talking about an even bigger leap: converting one of its six 660MW coal-fired units to run entirely on biomass. In the words of Melanie Wedgbury, Head of External Affairs, Drax is planning to "take coal out of the mix, and replace it with renewables". It is, she adds, "an exciting prospect".

But it's a pricy one, too. Conversions of this scale cost money: industrial quantities of it. The most obvious source of such largesse would be the Renewables Obligation, the government support mechanism for large-scale renewable generation in the UK. For the moment, it leaves co-fired biomass a poor relation of other renewables. Offshore wind, for example, qualifies for two renewable obligation certificates (ROCs) per MWh; non-energy crop biomass is awarded just half of one. That supposedly reflects the relative expense of offshore wind as a technology, but as Wedgbury points out, that still doesn't make biomass economical. "In many ways we'd rather use it than coal, but we can't afford to", she says.

At present, Drax is just cautiously ramping up the biomass proportion of co-firing. But if the support regime shifts in favour of biomass, says Wedgbury, then it would start work on the full conversion as a demonstration project.

So could this be the start of a mass conversion from coal to biomass? Are we poised to see the world's coal power plants transformed into giant wood burners?

Mark Candlish, Director of the Renewable Energy Association and Chair of its heating and cooling group, has already been through the conversion process, albeit on a smaller scale. Back in 2004 he was Commercial Director of Slough Heat and Power, overseeing its 80MW combined heat and power (CHP) plant shift to burning locally sourced woodchip and waste paper. The scheme was a commercial success, and was later sold to power utility Scottish and Southern.

But getting it up and running was no picnic. Everything from the fuel storage system to the conveyor needed to be replaced, explains Candlish. "They were all built for coal, and wood chips blocked them up. For the first year we could only run because the engineers operating the plant were prepared to continually free blockages using poles and air lances."

One of Candlish's main headaches was sourcing the fuel. "We had to create whole new supply chains from the forestry and waste industries to get sufficient volumes", he says. Given that this process was difficult for a small-scale unit like Slough, the challenge is an order of magnitude greater for Drax.

So, is there enough of the stuff out there? And can we source it sustainably? Recent scare stories have painted a picture of ancient forests being incinerated in a 'rush to biomass', and of Britain being forced to rely on imports from halfway across the world – making a nonsense of energy balances.

All wildly exaggerated, says Candlish. "Forestry ... has never been in a more depressed state – there are vast resources being wasted right now. Trees are worth virtually nothing at the moment", which means woodland managers have no incentive to manage their estates properly, thinning them out to allow healthy growth and let light in for wild flowers. "I live near a wood, and every 20th tree is lying on the ground. It's not even harvesting sunlight…" Biomass power could help reinvigorate the forestry sector, he says. "In a healthy market, you'd have a balance between supply and demand. At the moment there's a huge abundance of supply. There is no shortage of biomass in the UK, Europe or globally", concludes Candlish. "There's enough to fuel the world's primary energy needs five times over."

Fighting talk. But an ample supply is one thing: an ample and sustainable supply quite another.

Drax says that any biomass it burns is sourced sustainably, and Candlish points out that biomass energy ROCs won't in any case be awarded unless this can be proved to be the case. But Rupert Fausset, Principal Sustainability Adviser at Forum for the Future, sounds a note of caution: "There's only a limited amount of certified timber [available]. If you buy a load and burn it, someone else may have to go to other, uncertified, forests to get hold of their supply." Such 'displaced demand' issues, particularly in developing countries, can be hard to estimate, or control.

And even if it is sustainably sourced, what if that source is across the Atlantic – or further? Won't transporting it halfway across the globe wipe out the benefits? No, says Candlish. "Even if you use woodchips imported from 4,500 miles away, you'll still only be producing about 10% of the emissions that would be generated if you sourced the same amount of electricity from burning natural gas… People talk as though biomass has to come from the next village", he adds, "while at the same time they're quite happy importing oil from Venezuela or coal from Australia…"

Wedgbury agrees. "The idea that importing biomass [negates the carbon advantages] is just a big myth. We calculate the total carbon footprint from field to furnace, whether imported or domestic", she says, and the results show that Drax is comfortably meeting its commitment that burning biomass will achieve emissions reductions of 70% compared to coal.

Which is just as well, given that the domestic market alone can't at present meet demand.

That need not always be the case, however. As part of the Supergen Bioenergy Project, Patricia Thornley of the Tyndall Centre for Climate Change Research leads a team which has analysed the UK's potential for sustainable biomass. They concluded that a minimum of 5% of the country's total energy demand – for electricity, heat and transport fuels – could be met entirely sustainably through biomass. It sounds a relatively small proportion, but their definition of sustainable is deliberately cautious, ruling out, for example, any imports of wood. Instead, the biomass will be drawn from sources such as waste wood from forest management, short-rotation coppice, straw and even poultry litter, among others.

All of which, however, begs the question: is burning biomass to produce electricity the most effective way to use it? Paul Ekins, Professor of Energy and Environment Policy at the UCL Energy Institute, thinks not. "I'm not convinced that it is a good use of biomass, to burn it in large power stations at a maximum efficiency of 40%", he says. "We need to be looking for a way of making much more energy bang from the biomass buck than those kinds of efficiencies."

Instead, he recommends burning the biomass in CHP plants and using the waste heat in district heating systems, significantly increasing efficiency. It makes more sense to subsidise mid-scale biomass-fired CHP, to make a "real push" as a country to get CHP systems installed. "It's a proven technology in other countries," he says. "We know it can work."

Consultant Dick Bradford agrees. He led Barnsley's pioneering drive to convert its council heating systems from coal to wood, winning an Ashden Award in the process.

"Power stations chuck lots of heat into the sky [which is] such a waste", he says. "And super power stations are out in the middle of nowhere, with twice as much dumped in the sky, because there's nowhere around to use that heat. We have to replace power stations anyway. Why don't we have smaller power stations serving cities, providing both heat and power?"

Given the option between using the heat and wasting it, the choice seems clear, but of course there are complications.

At the moment, there's no guarantee that anyone would buy heat, so there's no incentive to capture it, though that could change later this year with the launch of the Renewable Heat Incentive. This would guarantee payment for every unit of heat generated for a fixed term (such as 5 pence per kWh over 10 years).

But small-scale CHP systems and district heating aren't common in the UK, says Ekins: "We don't have any tradition of municipal energy supply… The best known example is Copenhagen, where practically the whole city has evolved into a district heating scheme. But in order for that to work, you've got to have strong regulatory structures, because people have to be obliged to take the heat." Many buildings have their own central heating systems – gas or electric – so it would take a significant infrastructural and regulatory upheaval to shift to district heating.

Candlish agrees, warning that the technology is difficult to implement in Britain, because customers don't tend to sign up and commit to heat for the life of a plant. Local authorities are an exception to that, he says, but then you're waiting for the "natural replacement cycle" of public facilities. "The loss of heat really is a big deal", adds Thornley. "It means power stations are chucking out 50% of useful energy as waste. You can pipe heat a long way – 20 to 30km. It costs money but it's not impossible. But in the UK we just don't have the infrastructure." Over time, she hopes there will be a strategy to address this, perhaps involving incentive payments for recovering heat, and long-term partnerships between generators and, say, social or private housing developers.

"We need CHP", says Candlish, "but it's not currently that efficient at a small scale, and it's hard enough building a power station as well as finding someone to promise to take your heat for 10 years. So while we're building up capacity on the heat side, it's legitimate to still have large-scale generation."

So however promising the prospects of CHP long term, simply converting existing coal-fired plants to biomass may be a quicker, cheaper, easier and more practical way of bringing down emissions. And looming regulatory changes might act as a spur, too.

Several of the UK's coal-fired plants are expected to close by 2016 because of the Large Combustion Plant Directive, which regulates air quality emissions from coal plants, including sulphates, nitrates and dust. These plants have decided not to reduce emissions and will have to close instead, unlike Drax, which has fitted flue gas desulphurisation (FGD) so it can continue generating.

There also remains the possibility of an emissions performance standard, which would limit the allowed CO2 emissions of new coal plants. The Energy Bill of 2008 contained wording to the effect that no new power plant may emit more than 400kg/MWh of CO2. Compare that with Drax at just over 1400kg/MWh in 2009), though it didn't survive to the final legislation.

So, old dirty plants will close, while new plants will need to prove they can dramatically reduce emissions – and/or ensure the carbon is captured – if they are to be built at all. Those which remain will be faced with a business-as-usual future where they will be penalised for their carbon under the EU emissions trading scheme. Given the choice of fitting carbon capture and sequestration technology – a comparatively unproven and expensive route – or using alternative fuels like biomass, coal generators may find themselves looking increasingly to the latter option as the path of least resistance.

It's not a case of 'either/or', says Thornley. Co-firing biomass with coal – with CCS in place – could be a very effective method of carbon reduction. Get it right, she adds, and "you could even be getting negative emissions".

Wedgbury is keen to make the case. "We're saying to Government: 'Look, you have these carbon targets, so why not achieve them in the least costly way?' Using biomass for electricity generation will allow you to do that. Even if you have to increase the level of support, it will still cost less than putting everything into offshore wind."

In a world where we have to cut CO2 emissions but keep the lights on, this relatively low-hanging fruit may prove difficult to resist. As Thornley puts it: "While aspiring to get the best possible savings in the long term, we shouldn't rule out something which can help us now in terms of the global climate. We don't have a lot of time on our hands: we need to act now."

Rupert Fausset, meanwhile, offers Drax some advice in sustainability. "They need to be open about where they're getting the biomass from, and they need to hold it up to scrutiny," he says. "It would be really great to see an organisation tackle this, to do it in the open – share the learning – and then everyone can move forward."

– Chris Alden, Michael Ashcroft and Martin Wright

Image credits: Mgov/istock; Heatherwick Studio