Wood use for energy: Is the criticism justified?

According to the [German] Federal Environment Agency (UBA), fine particulate matter emissions (PM 2.5, i.e. airborne dust particles with a diameter < 2.5 µm) decreased from 202,000 tonnes in 1995 to 81,000 tonnes in 2020, i.e. by 60 %. Particulate matter emissions from households and small consumers also fell during this period from 40,900 to 21,800 tonnes, a decrease of 47 %. This value includes all domestic firing systems, i.e. including oil and gas boilers.

It is a fact that the air in Germany has become cleaner and cleaner over the past decades. What has changed are the required standards of air quality, which have become more and more stringent. Another fact, however, is that wood firing systems are responsible for almost one fifth of particulate matter emissions in Germany (Fig. 1).

Dust separators or other technical optimisations in new wood firing systems significantly reduce or virtually eliminate dust and fine particulate emissions. Operating errors with single-room log firing systems can on the other hand cause high particulate emissions, especially during the lighting process, or if the fuel is not sufficiently dry. Air flow control errors can nowadays be reduced to a minimum by technical means, through automatic combustion air regulation. The Technology and Support Centre (TFZ) has published a series of explanatory videos on the correct operation of log firing systems.

Modern central heating boilers fuelled with pellets or wood chips produce almost no particulate emissions. So when it comes to heating with wood, it all depends on the system being used, how it is operated, and whether the fuel being used is suitable. Fine particulate matter emissions are not a fundamental argument for excluding the use of wood-firing systems.

In the recent past, the set standards for emissions have been made considerably stricter by several ordinances on the implementation of the Federal Immission Control Act (BImSchV). The 1st BImSchV ordinance regulates replacement obligations, measurement procedures, type tests and emission threshold values for small and medium-sized firing installations with a rated heat output of 4 kW to 1000 kW. The 13th ordinance on the other hand regulates the use of large firing installations with a thermal output of more than 50 MW. The 44th ordinance sets the threshold values for installations with a thermal output of 1-50 MW, and took effect at the end of 2018. A survey of 1,000 private households in the course of the Energy Wood Market 2020 study showed that the regulations of the Immission Control Act are effective, and that the replacement of old wood-firing systems with modern systems is in full swing.

Figure 3 shows the climate impact of emissions from wood combustion in comparison with other heating systems: It is clear that the emissions per kilowatt hour of heat from all three types of wood firing are in the order of magnitude of solar energy, and significantly lower than those of oil or natural gas systems, and also than those of heat pumps. The emissions that occur upstream in the supply chain - i.e. during logging, transport and processing - are also included in the calculation for the wood combustion systems. As long as the quantity of carbon stored by the forest does not decrease, but maintains its level or continues to increase as before, the CO₂-emissions from wood combustion do not burden the carbon emissions balance.

In greenhouse gas reporting, no greenhouse gas emissions are calculated for the use of biomass and thus also wood in the "Energy" sector. This does not mean that the CO₂ emissions from the combustion of wood are not taken into account in the reporting, however, because the extraction of wood from the forest, including the extraction of wood for energy, is already counted as CO₂ emission. This use of biomass is accounted for in the "land use, land-use change and forestry" sector (LULUCF). In the LULUCF sector, the emissions generated by timber extraction can be offset by the growth of wood (= binding of atmospheric CO₂).

As long as the forest’s carbon stock does not shrink, timber extraction and with it the use of wood for energy is climate-neutral (bottom bar in Figure 5), so the atmosphere is not burdened with additional CO₂ through the burning of wood. If the amount of timber extracted is lower than the wood growth increment, the forest even acts as a CO₂ sink (middle bar in Figure 5). Conversely, however, the forest becomes a source of CO₂ (top bar in Figure 5) if more timber is removed than grows. The harvesting of timber for material use counts as an inflow to the carbon storage in wood products. If this inflow is greater than the outflow from the carbon storage of wood products, this storage also acts as a CO₂ sink.

Climate experts believe that the next few years are going to be crucial for the achieving of our climate goals. They say it is essential that we begin immediately to reduce CO₂ emissions. Critics of the use of wood for energy argue that for the same amount of energy, it is better to emit slightly less CO₂ from fossil fuels into the atmosphere, than to emit a slightly larger amount of CO₂ from the biosphere. They are thus of the opinion that the wood increment (which is shown as a carbon sink in Figure 5) should be left in the forest, and that fossil fuels should be used instead. In the short term, such considerations are entirely justified - but only if the CO₂ really does remain in the forest.

It must not be forgotten that both deadwood and forest residues left in the forest after harvesting operations are eventually largely decomposed again by microorganisms. The CO₂ originally bound in the wood is released again through the decomposition process. If forest residues are not used for energy and instead remain in the forest, most of the CO₂ bound in them will very soon end up in the atmosphere anyway. But it is not just forest residues that are a significant source of wood fuel.

When roundwood logs are processed into beams and planks at the sawmill, wood residues such as wood shavings and sawdust are also produced. If these are not used for energy but composted instead, the majority of the CO₂ bound in them is also released into the atmosphere. Consequently, a renunciation of the use of wood for energy does not increase the storage of CO₂ in the biosphere. On balance, the renunciation in fact leads to more CO₂ emissions: those from the burning of fossil fuels and those arising from the biodegradation of forest residues or the leftovers from wood processing.

A complete renunciation of wood use would not be expected to have a positive effect, either. It would mean that wood products would also have to be replaced by other materials - and this in turn would usually be associated with a considerably higher consumption of fossil fuels. The amount of CO₂ stored in harvested wood products (HWPs) such as roof trusses, furniture and wooden flooring would also gradually shrink if new wood products were no longer being continually produced as substitutes. The carbon stock in wood products would thus become a source of CO₂. A positive effect of usage stops on the forest carbon stock itself could only be expected if it were possible to expand the forest carbon stock.  In our opinion, this option is however unrealistic.

Climate-washing is the term used to describe the “greenwashing” of climate balances. In the Framework Convention on Climate Change, countries worldwide have agreed to account for their greenhouse gases separately by sector, and each state for itself (source and territory principle). The EU regulation that governs European reporting (the so-called Monitoring Regulation) states that: "The emission factor of biomass shall be zero". This statement has led to some European countries, especially coastal countries, converting their coal-fired power plants to pellets and importing the pellets from overseas (Fig. 7). In the accounting of CO₂ emissions from fossil fuels, this took pressure off their “Energy” sectors.

They do not have to take the effects of pellet use into account for their forest carbon storage. This happens at the expense of the countries supplying them from overseas, who ought to include the impact that the supplying of pellets has on forest carbon stocks in the accounting for their LULUCF sector. This is done on the basis of forest inventories and is costly. The concern of many scientists, however, is that the supplier countries do not account for their forest carbon stocks correctly. Their criticism is particularly strong when it is not residual wood that is being used there, but whole trees that are being harvested exclusively for pellet production.

By contrast, in Germany pellets are usually produced from wood residues left over from the processing of raw logs into harvested wood products. This procedure is not criticised by the scientific community. Germany is self-sufficient in pellets, and in fact exported a net total of 431,000 tonnes of wood pellets in 2021. The accusation of climate washing against Germany would thus be unjustified.

Another argument against the use of wood for energy is that wood is a scarce resource. A material use of the wood would keep the carbon bound in the harvested wood products for longer. The substitution of wood products for products such as steel or bricks, production of which is more energy-intensive, also helps to save fossil fuel energy (material substitution).

These statements are all correct. However, not all trees or tree parts are suitable for material use. Moreover, the volume demand is not sufficient everywhere for it all to be put to material use. Very small-diameter or crooked wood is not suitable for sawmills. This wood could be used for the production of chipboard or paper and pulp. Graphic paper consumption has decreased in recent years, so that the demand for raw wood for paper production has fallen in Bavaria. Raw wood consumption for chipboard production is also declining in Germany. It thus seems more appropriate to use the low-diameter wood for energy purposes close to the site rather than to transport it to sawmills some distance away. Tree parts such as spruce crowns are also often only chopped and used as energy for forest protection reasons, i.e. in order to minimise the breeding space for bark beetles and other harmful insects.

When comparing wood with other renewable energy sources, people often overlook the fact that wood represents stored energy that can be called up at any time. If not consumed immediately, electricity generated by wind or photovoltaics has to be stored in batteries or pumped-storage hydroelectricity reservoirs, or used to produce hydrogen. In all cases, considerable losses are made during the conversion of the power.