Saturday, 4 August 2012

Does it make sense to import biomass?

We have heard in the news recently that Drax, the UK's largest coal-fired power station, is going to convert half of its boilers to use biomass instead of coal [1] [2]. Sourcing enough biomass in the UK to supply half of Drax will be impossible in the short term at least so a lot of the biofuel will be imported, which means more energy needed for transport which generates more greenhouse gas emissions. We are told there is still a net saving but not how much, at least not in the newspapers, so I went looking and found a life cycle analysis report from the Environment Agency [3] for using forest biomass in power stations. It makes interesting reading. There certainly is a net saving in GHG emissions even allowing for the extra transport for imports. However, the net savings are barely enough to meet the standard requirement for subsidy by renewable obligation certificates (ROCs) - this is a 60% cut in lifecycle carbon emissions or a limit of 285 gCO2/kWh generated.

The ROC requirement may be 60% cut but the overall UK target for total carbon emissions is an 80% cut by 2050. Using the ROC standard as a base, if 285 gCO2/kWh is a 60% cut then an 80% cut would be 143 gCO2/kWh so I will use that.  UK forest biomass meets this target fairly comfortably. Imported wood does not. Wind power, by comparison, qualifies easily.

Sources forest products [3] (but ignoring carbon uptake by the soil), wind [4]


I have adapted figures from the Environment Agencies report to create the chart above.
  • I have selected only scenarios based on sustainable forest management by periodic thinning and felling
  • I have ignored carbon absorbed into the soil, since this only applies for the first few generations of felling
In all these scenarios the EA has assumed
  • Valuable timber is sold separately, so only a portion of the GHG emissions from forest management are allocated to the biofuel
  • The wood is transported 45km (30 miles)  from the forest into storage and then to port or straight to the power station by road. International transport is by sea - which is much more efficient than by road.
  • The conversion to electricity in the power station is 25% efficient. This is lower than you would expect for a gas or coal power station (last year 48% for gas and 38% for coal [4]). However, biomass has a moisture content which doesn't help.
Points to note:
  • The UK forest biomass emissions do meet the 80% cut level, though for conifer it is close.
  • The chart above is for roundwood: branches and small trunks. Woodchip has higher processing costs but this is offset by lower growing costs because there is less waste. Making the wood into pellets takes another 160 gCO2/kWh and doesn't quite meet the 60% standard, never mind the 80% target.
  • The distance travelled for broadleaf roundwood or woodchip could be increased up to 80 miles and still meet the 80% target.
  • For wood imported from the USA transport alone accounts for 119 gCO2/kWh so it is difficult to see how the 80% target 143 gCO2/kWh can ever be achieved.
  • Even imported wood gives carbon emissions well below the current level from gas power stations which are 398 gCO2/kWh [5]
This analysis is for wood and there are lots of other biomass options which vary in their requirement for growing, harvesting and processing,  but the emissions from transport depend only on the distance and the energy density of the product. Miscanthus, a high yielding bamboo-like grass which is being promoted in this country by Natural England, has a similar energy density to wood both by weight and volume; waste straw is similar by weight but more bulky.  Bagasse from sugar cane and palm kernels are also possibilities, but the energy density by weight from all are much the same, as long as the moisture content is similar. This means that no matter what material is being imported, if it comes from as far away as North America the GHG emissions will be higher than the target 80% cut for electricity.

Of course the EA analysis is based on the present day situation where lorries and ships, and also tractors and chain saws all run on diesel fuel. If they used renewable energy instead that would also reduce the carbon emissions but you still need a reasonable return in terms of energy balance. Assuming that all the carbon emissions are due to burning diesel then it takes 0.45 kWh biodiesel to make 1 kWh electricity from UK wood, and with wood imported from USA it would be 1.1 kWh biodiesel, of which 0.44 kWh is transport, to make 1 kWh electricity. That is not as bad as it sounds because you lose a lot of energy converting to electricity. The EA have assumed it takes 4 kWh of wood to produce 1 kWh electricity; the USA case is then 1.1 kWh biodiesel to make 4 kWh of wood energy which is still a reasonable win.

Overall, running power stations on imported wood fuel won't achieve the target 80% cut in carbon emissions today, though it can qualify at the 60% level for ROC subsidy. This makes sense if you consider the biomass power stations as part of a diverse mix including other renewable sources like wind as well. You can use the biomass  to supply some of the base load only, and/or use it as a backup for when wind is not available. You can't use imported biomass to replace wind, or other low carbon sources because it just isn't low carbon enough.


[1] Drax aims to go coal-free after biomass subsidy review www.businessgreen.com 26/July/2012
[2] Farming Today This Week BBC Radio  4/8/2012 (will be available for 7 days only)
[3] Including UK and international forestry in Biomass Environmental Assessment Tool (BEAT2) Environment Agency July 2011
[4] Energy and Carbon Emissions: the way we Live today Nicola Terry 2011
[5] Digest UK Energy Statistics 2011 DECC

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