Biomass for heat – the sustainability debate
Our Principal Engineer, Andrew McGhee, shares the second part of his article on carbon neutral distilling, addressing the common question “is biomass really sustainable?”
There has been a debate around this question which has gone on and on for years. Those arguing that wood is not sustainable point out that there may actually be more CO2 coming out of the chimney of a wood fuelled boiler than one running on oil or gas. They may dismiss the argument about re-absorption of CO2 as trees grow by saying that it takes decades for this to happen, and that this is too long in the current climate emergency. In this equation, the CO2 in the atmosphere for the time period between release and absorption by growing trees is termed the carbon debt. All of this misses one critical fact – that UK forests are growing faster than we are felling them. So in actual fact, the carbon storage in UK forests is increasing, year on year, by about 20-30 million tonnes of stored CO2 equivalent per annum. [https://www.forestresearch.gov.uk/documents/7435/Complete_FS2019_zIuGIog.pdf].
So, while we are nibbling at one edge of the UK managed plantation forest, felling about 2% per year, the forest is growing in other areas faster than we are felling it. The CO2 released when wood is burnt in 2030 is not absorbed 20 years later – it’s being absorbed right now by UK forests. The fact that the UK managed forest sector is putting on weight is not – of course – just because of people using wood fuel. But wood fuel is a part of the overall market for forestry products and helps to sustain a healthy and growing forestry industry in the UK. These facts are recognised and supported by governments across the world, and by supra-national bodies such as the EU and UN.
There is another argument which is often made – that we shouldn’t grow trees just to burn them. Well the truth is – we generally don’t. Wood fuels’ place in the overall forest products market is widely mis-understood. The main product of almost all forestry is saw log – big diameter logs which are sent to sawmills to be made into timber for construction, fencing, furniture etc. This is where foresters want their felled trees to go – it’s the highest value, most lucrative destination for felled timber. Next on the pecking order for timber is wood destined for pallet manufacture and fencing. Then it’s packaging products, panel board manufacture and paper pulp.
Only when local markets for these products have been supplied do the leftovers go for fuel. The material typically destined for fuel is rejected by the sawmills as unsuitable, too small for sawmills to bother with, or offcuts and sawdust from the sawmills. There are exceptions which depend on local market conditions, but generally the only whole trees that go for fuel are from forestry thinnings – when small and weak trees are removed from a growing forest to make space for the strong to grow. Stands of low-quality timber – unsuitable for sawmilling – may also be harvested for fuel in particular circumstances, such as during the Coronavirus pandemic when demand for saw logs has been very low, or when no alternative local market exists.
As the recent (2018) UNECE report into wood energy highlights, “wood fuel removals can offer important ecological, economic and social benefits if forests are well managed”, which are in addition to the carbon benefits which accrue from not burning fossil fuels. This should be particularly reassuring for those sourcing their fuel from the UK, as we have one of the best established national forest management frameworks anywhere in the world.
The EU also supports the use of wood as a low carbon fuel via the Renewable Energy Directive, but the WWF were critical of this position, focusing on the actual CO2 coming out of the stack of a boiler house and ignoring the bigger picture. Read deeper into the WWF position paper though, and they acknowledge that the use of wood for fuel is appropriate as long as it is not wood that could have been used in solid form (and so locking up the carbon it contains). They recommend using wood for construction, and only using material unsuitable for a higher purpose as fuel. As I have discussed, this is basically what the forestry products industry does anyway.
I have only discussed the domestic UK timber market here because this is the material generally used to feed industrial boilers of the type applicable to distilleries. There is also significant import of fuel wood into the UK in the form of wood pellets from the USA and the Baltic States. This import supplies the Drax power station and some of the pellet demand for UK domestic and small commercial properties. For distilleries, wood chip is usually the preferred fuel, though in particular circumstances a pellet boiler may be chosen.
Part three will cover the practicalities of using biomass to raise steam for a distillery or any other process load.