VOC emissions of timber felled for fuel
The strong smell of freshly felled forest and resin is probably familiar to most Finns. It is rarer that anyone thinks where the smell comes from, and even rarer that the share of the timber’s heating value lost with the scent to the sky is considered. The matter has scarcely been studied, and a complete picture is very difficult to obtain, based on scattered written sources or experiences. However, it has been observed in practice that volatile organic compounds (VOC) that evaporate off timber are also significant in terms of burning and the heating value of fuel.
As a concept, the burning of fresh woodchips is new and rather heretical. The traditional way to burn woodchips, which many think as the ‘right’ way, is to store the felled timber for a long time on roadsides and allow them to dry before chipping and burning. Practical issues of logistics have also contributed to this model.
In the autumn of 2015, a new 10 MW grate boiler plant and a modern flue gas scrubber with a heat pump connection were commissioned at Kauhavan Kaukolämpö Oy’s Kauhava plant. The concept of the plant and operations is based on the burning of fresh woodchips – from harvesting timber through the logistical chain to burning and heat recovery. Based on the operational experience of the past winter, the concept of using fresh woodchips is working very efficiently, providing significant cost savings in the acquisition of fuel to the plant operator. Despite high moisture, the burning of woodchips can be controlled easily and specific emissions are low.
It can be assumed that the concept of fresh woodchips works efficiently in part because a significant portion of the volatile components contained in timber is included in combustion, rather than allowed to evaporate into air. This article discusses the significance of volatile organic compounds in the heating value of fuel.
Storage and VOC losses
According to research, there is a storage loss of 6–16 % of the heating value of timber as a result of slow combustion during storage, decay due to bacterial activities and fungi, and loss due to material dropping off (a VTT study in 2002). In addition, decaying produces methane, among other things, which is a greenhouse gas significantly more detrimental than carbon dioxide. The storage loss of timber has been studied e.g. at the University of Turku.
The VOC emissions of timber during drying and in its natural state have been studied at the Karlstad University in Sweden. There is significantly less material and actual research data on emissions right after felling the trees. Many kinds of compounds evaporate off trees during their lifetime already, but after the trees are felled, the emissions increase considerably.
According to a study, the monoterpene content of air at a logging site was 1.0–1.5 mg/m³, while the normal level above foliage is 0.1–0.5 mg/m³. This means that after felling trees, the terpene content of the surrounding air has been up to 15 times higher than the normal content. The elevated content is visible at logging sites for several weeks.
The chemical composition of wood
Wood consists of thousands of different chemical compounds, with the main groups of cellulose (40–45 %), hemicellulose (25–35 %), lignin (20–30 %) and extractives (2–10 %). Volatile organic compounds (VOC) are mainly contained in extractives. The extractives content varies between wood species. There are also differences between different parts of a tree, and even between individual trees. The bark, branches and foliage contain considerably more extractives than heartwood.
For trunk chips, the share of heartwood is particularly interesting. With spruce, the extractives content of the stem wood varies in the range of 0.8–3.8 % and with pine in the range of 3.0–6.0 %. The share of bark in a tree is approximately 10 %, so the bark also forms a significant source of extractives. For softwood trees, the extractives content of the bark varies in the range of 28–39 %. Some of the extractives are easily water-soluble and slow to evaporate. Some, on the other hand, are not water-soluble and evaporate quickly. The estimate on the volatile compounds in timber varies in the range of 4–10 %, depending on whether the canopy and bark are included.
Volatile organic compounds
The classification of extractives in timber is difficult due to the large number of compounds and compound groups. However, extractives can be roughly divided into three main groups:
- terpenes, terpenoids and sterols
- fats, waxes and their compounds
- phenolic compounds
Even though exact research information is missing, preliminary assessments can be made on the amount and significance of VOC losses. Let us assume that the amount of volatile compounds is 4 % of the mass of the dry matter in timber. The average heating value of volatile compounds is 40 MJ/kg. The heating value of ‘regular’ dry matter in timber is 19.5 MJ/kg. Calculated as a weighted average, the heating value of dry matter in fresh timber is, in fact, 20.32 MJ/kg, if the timber is taken quickly to burning. The extra energy from volatile organic compounds is therefore 0.82 MJ/kg.
In addition, the extra energy from volatile organic compounds goes directly into the effective heating value in the burning process. The heating value of woodchips with a moisture of 55 % in their arrival state is approximately 7.1 MJ/kg, but the benefit obtained from volatile compounds can be counted directly as net benefit, making the actual heating value in the arrival state 7.92 MJ/kg. Therefore, the increase in heating value is more than 11 %.
Prerequisites for the use of fresh woodchips
The use of fresh woodchips requires a whole new kind of thinking from the plant and logistics. The concept can also be applied to old plants if the structure of the boiler is suitable for burning moist fuel. Fresh fuel also sets some requirements for the fuel storage and fuel supply systems. An essential part of the system is, however, a flue gas scrubber system that efficiently recovers heat from flue gases, condensing the moisture of the fuel vaporised in the boiler. The scrubber should work reliably and efficiently under all conditions, regardless of the variation in the return temperature of district heating.
With a heat pump integrated in the scrubber, flue gases can be cooled efficiently to even below +30°C, making it possible to utilise a significant part of the condensation heat of the water contained in the flue gas. For the purpose of optimising and ensuring the profitability of heat production, a heat pump scrubber has, in practice, been established as the only potential recovery technology for lost heat.
Experiences from Kauhava
Based on the experiences of the past winter, Kauhavan Kaukolämpö Oy is convinced of the excel-lence of fresh woodchips, compared with regular dried woodchips or bark. According to Ari-Matti Mattila, managing director, combustion is even and steady. The flame is bluish, even though the moisture level of the fuel has been 50–60 %.
The CO content of combustion has also been low, clearly under 10 ppm, even though burning has taken place using a low lambda value, with residual oxygen of approximately 3.0 volume per-cent. Internal electricity consumption has also been lower with fresh woodchips than with those stored for a longer period of time. Some of the fuel has been felled the same day, and some has been at most a few weeks old. The fuel has also been of uniform quality. However, the system sets its own, new challenges for logistics.
Heat recovery at the Kauhava plant is top class. The average heat recovery during the winter has been more than 35 %, meaning that every third truck load of fuel has been left out, compared to a situation without the scrubber. The total plant efficiency has been over 120 % at its best.
When considering the entire fuel chain from harvesting to burning and heat recovery, there are several sections in the chain where the energy content of fuel can be saved. These include:
Decreasing VOC emissions by shortening the time from harvesting to burning. The impact on effective heating value can be expected to exceed 10 %.
Decreasing storage loss. According to research, the impact is 6–16 % of the energy content of the dry matter of fuel.
Efficient heat recovery, with the final flue gas temperature even below 30°C. Heat recovery can exceed 35 %.
In total, the burning of fresh woodchips and efficient heat recovery can be used to obtain cost savings of 25–45 % in fuel costs, compared to a traditional plant without efficient heat recovery. The savings potential is so large and complex that the subject deserves further scientific study.
Author: Mika Nummila