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Power Engineering: Why Drax has a growing interest in full conversion.



Concrete manifestations that the 4000 MW Drax coal fuelled power station in the UK is well on the way to further increasing its biomass burn are now clearly visible on the power plant site in the form of new biomass storage domes. These are being built under a contract with Shepherd announced in March 2012. That announcement talked about spending £50 million in 2012 to secure full benefit from the site's existing co-firing facilities and producing "up to 20% of the power station's output from sustainable biomass."


In July, the UK government published its final decisions on support levels for the various renewables (following its review of "ROC banding"). This makes full conversion of coal units to biomass more attractive than mere co-firing (which Drax has been doing since around 2003) and enabled Drax to announce even more ambitious longer term plans for biomass: "As a result of the government's conclusions on support levels, and our excellent technical progress," said CEO Dorothy Thompson, "we can now move forward with our plan to become a predominantly biomass-fuelled generator. We expect to convert progressively three of our six generating units to biomass within the next five years, the first in the second quarter of 2013."


Drax has been able to move forward now that the regulatory framework is clear, with a generous level of support, namely 1 ROC (renewables obligation certificate) per MWh for a fully converted unit, effective from April 2013, grandfathered to 2027.


"Fully converted" allows the use of up to 10% of "additives" (including even some coal) in the interests of achieving high boiler reliability, efficiency and output. Drax has trialled a range of such additives as part of its programme to demonstrate the feasibility of converting the units fully to biomass.


Work is still going on to evaluate corrosion test results and to optimise NOx performance, but overall the trials appear to have been successful and provide confidence that the first converted unit will be available in the second quarter of 2013.


There has also been progress in establishing the all important, and very extensive, biomass supply logistics, where the plan is develop a sustainable biomass supply chain sufficient to provide the fuel for three converted units within about five years.


In the area of fuel contracting Drax has secured rights to 2 million t of biomass for 2013 and is working towards strategic partnerships in the forestry business, with accelerated activity following the positive outcome of the banding review decisions.


The planned biomass supply chain includes two pellet plants in the southern USA - total capacity about 1 million t/y - for which sites have been identified and first planning/permit applications submitted, with first investment decisions to be taken very shortly, in time to support conversion of the second unit.


There has also been substantial progress in port, shipping and rail logistics, with a number of facilities under development.


Capital investment in the biomass conversion project of around £650-£700 million is envisaged over the period 2012-2014, with about half of it being spent on equipment installations and modifications at Drax site, including biomass delivery, storage and distribution systems. This includes the addition of a rail unloading facility and enclosed conveyor equipment to transfer biomass from storage to the boilers, and the biomass storage domes, which are needed because, unlike coal, biomass cannot be stored outside as it biodegrades when it becomes wet.


The rest comprises upstream supply chain infrastructure, predominantly pelleting facilities, and pollution control measures (in particular deNOx, but precise plans (eg, SCR or SNCR?) have yet to be announced) to achieve EU Industrial Emissions Directive compliance. Modification to the mills and some parts of the boilers will also be needed as part of the biomass conversion, but these items will be a relatively small proportion of overall spend compared with that needed for fuel supply and handling.


The economic returns on the biomass conversion project would appear to be very attractive over a range of spark spread scenarios, with the Drax efficiency advantage (greatly helped by the recently completed major steam turbine retrofit carried out by Siemens) amplified when burning biomass.


For Drax, full conversion of its existing coal units looks like a much better commercial proposition than building new dedicated biomass plants.


A few years ago it had plans for three such plants, sized at 299 MW (to avoid the need to be "carbon capture ready"), which it was looking at developing with Siemens Power Ventures. The last remaining project, that proposed for Immingham, has now been cancelled.


Biomass burn, sustainable or not?


Drax and other power plant operators using, or planning to use, biomass, of course all argue that what they are doing results in reduced carbon dioxide emissions. But a recent report put out by RSPB (Royal Society for Protection of Birds), Friends of the Earth and Greenpeace, against the background of announcements by large UK coal plants such as Drax and Eggborough that they are converting to biomass, begs to differ, concluding that "burning whole trees in power stations can be dirtier than coal."


The report, Dirtier than Coal? Why government plans to subsidise burning trees are bad news for the planet, notes that the UK government's bioenergy strategy includes commitments to only support sustainable bioenergy that delivers genuine greenhouse gas emission reductions, but "these principles are now being undermined by government proposals to continue to subsidise large-scale power generation from wood."


The report claims the government's own analysis, provided to Princeton academic Timothy Searchinger, shows that the use of whole trees in power plants "would increase greenhouse gas emissions by at least 49% compared to using coal over 40 years." Yet the government's current proposals to continue to subsidise biomass power via the ROC scheme do not account for this by distinguishing between different sources of biomass and are "therefore likely to actually increase greenhouse gas emissions."


The NGOs call for, among other things: an immediate revision of the government's impact assessment to include emissions that arise as a result of the time delay between combustion and forest re-growth, and from taking wood out of existing industries that may have to use non-wood alternatives, such as plastic and concrete; withdrawal of public subsidy for power generation employing tree trunks (roundwood and sawlogs); and development of an accounting system for biomass that includes carbon debt and indirect emissions from product substitution.


In response, Drax has issued the following "fact sheet" on the carbon balance and biomass:


Key points


- Bioenergy can deliver significant carbon savings relative to fossil fuels even after taking account of energy consumed in harvesting, processing and transport


- Burning wood only releases the carbon removed from the atmosphere when the tree was growing.


- Provided bioenergy comes from sustainably managed forests where growth is in excess of harvest there is no 'carbon debt', even in the short term.


- Managed forests are more productive and absorb more carbon than undermanaged forests because younger trees, or trees able to thrive thanks to good management, absorb more carbon than mature trees where growth has plateaued.


- A 'carbon debt' is only incurred if there is an imbalance between the amount of carbon dioxide emitted from bioenergy production and the amount of carbon sequestrated in the forest from which the biomass was extracted.


Is carbon debt a problem?


A carbon debt can occur if the extraction of biomass from a forest results in a reduction in the carbon stock in that forest. However, the way that the UK power sector sources woody biomass means that there is no carbon debt since the sector ensures that it:


- only uses wood from sustainably managed forests; and


- uses wood from forests where, overall, at least as much (and preferably more) carbon is being absorbed than is taken out and used.


The debate around 'carbon debt' is often framed by assumptions which do not reflect the above reality. These assumptions deserve to be identified and proven erroneous.


Error 1: A tree burns in seconds but takes years to grow so there is a carbon debt while it re-grows


It is completely inaccurate to focus on a single tree. An assessment of the carbon stock of a forest has to look at the whole area because for every tree which is removed there will be many others at different stages of growth. Less mature trees will be growing fast and absorbing carbon at a corresponding fast rate, while those closer to maturity and ready for harvesting will be absorbing carbon at a slower rate. Provided the rate at which carbon is absorbed by the forest exceeds the rate at which it is being removed there is no carbon debt. In fact many of the world's forests are in 'carbon credit' because of improving forest management and reforestation.


Error 2: Bioenergy undermines industries, such as furniture and construction, which keep the wood intact and do not add to carbon debt


The forest industry does not exist purely to provide wood for energy. In fact the economics of forest management usually mean that energy is well down the list of potential uses for wood. Timber, the most valuable part of a tree, is used for furniture and building materials, while smaller diameter wood can be used for fence posts or by the paper and wood panel industries. In some geographic regions, decline of industries such as construction and paper and pulp means that energy producers may be able to afford fibre previously supplied to these, currently declining, markets. This is welcomed by the forest owners who need to maintain investment in forest management. However, it is typically the lower value branches, thinnings and by-products of other industries which are used to produce energy as they usually have no other commercial use.


By definition, if the energy industry is generally using by-products, there are often other parts of any given tree being used to make primary products which continue to store carbon. Carbon debt calculations often fail to acknowledge this. Neither do detractors recognise the efforts made by these other industries in developing best practice and forest management certification systems which have gained public acceptability of their performance.


Error 3: The bioenergy industry is contributing to an escalation in deforestation around the world


On the contrary, the bioenergy industry stimulates reforestation. Alternative demand for bioenergy, often met by wood that previously had little value, can underpin the investment case for better forest management and new forest plantation. FAO's Global Forest Resources Assessment 2010 shows forest cover has been steady in Canada and increasing in, for example, in the US, Russia and Europe between 1990 and 2010. Most biomass used in the UK comes from these areas.


Error 4: The carbon cycle starts when a tree is harvested and the debt is only re-paid when the tree is fully re-grown


The sustainable management of the trees in a production forest means they have been absorbing far more carbon than they would have done if the forest had not been managed. In effect this means such trees are building up a carbon credit as they grow. In carbon accounting terms, it makes sense to start the clock when the tree was planted and even more sense to look at the forest as a whole. By using aerial photography, satellite images and other techniques scientists can work out how much growth has occurred, how much carbon has been absorbed and how much has been released. They can then work out how much wood goes to industries which will not release the carbon and how much is used for bioenergy. This gives a much more accurate picture of the carbon balance than focusing on an individual tree and starting the clock on the day it is harvested.


Error 5: Wood produces less useful energy per tonne than fossil fuels and is therefore 'dirtier than coal'


It is absurd to suggest that bioenergy can be worse than fossil fuels when considering the carbon cycle. Carbon emissions and sequestration with biomass are part of a closed cycle, with exchanges between carbon pools in the atmosphere and biosphere. When a forest is growing, the carbon in the atmosphere decreases, and vice versa, with no net increase in carbon. With fossil fuels the situation is radically different. When fossil fuels are extracted and burnt the carbon that would otherwise have been stored in the earth's crust is released into the atmosphere producing a net increase in atmospheric carbon, creating a real carbon debt for future generations.


Error 6: We don't need bioenergy since other, zero carbon, renewables will meet our energy needs


Bioenergy has a special role in the energy mix because it is available when it is needed, it is not intermittent and it is reliable. Most other renewables fulfil a different role in the mix because they can't be relied upon to be available to meet demand at any given time. The choice is usually, therefore, between biomass and fossil fuels, not biomass and some unspecified zero carbon renewable.