Heating with wood - how to do it today?

If a solar irradiation of 1000 kWh/(m²∙a) and a combustion enthalpy per m³ of hard wood of 3 MWh is taken as a basis, a typically low conversion efficiency for biomass is calculated

η = (117 million m³∙3MWh/m³)/(1MWh/m²∙114∙109m²) = 350TWh/114000 TWh ≈ 0.3 %.

If one subtracts deadwood and bark quantities (together about 30 million m³) as well as an annual storage quantity of 15 million m³ necessary for sustainability from the annual timber yield, a possible sustainable harvest quantity of 75 million m³ would remain. In Germany, about 56 million tons are harvested annually, which corresponds to an enthalpy of 170 TWh (1 terawatt hour = 1 billion kWh). Approximately 37 million tons of solid fuels are burned each year in 700,000 boilers and almost 12 million individual room furnaces. The heat generated in this way amounts to about 110 TWh. By comparison, the total annual heating demand in Germany is about 560 TWh. The largest contribution, 300 TWh, supply for this 13 million gas burners.

The highest timber growth is achieved by spruce (Fig. 1), followed by beech and oak. However, in Germany, spruce is acutely threatened by climate change and is to be partially replaced by other tree species in favor of a climate-resistant mixed forest. Since the wood growth of all three tree species mentioned decreases significantly after about 80 years (Fig.1), it is appropriate with regard to an optimal yield to start felling after this time (which corresponds to about 3 human generations).

It is gladly conceded that more than 100-year-old trees are often of special aesthetics. And that an ancient forest serves the preservation of biodiversity is undisputed.

If wood for burning is increasingly required, the use of so-called fast rotation plantations with significantly higher growth rates is appropriate (Fig.1 dashed course). These can be realized with poplars and willows. Here, harvesting is possible after only 5 to 10 years and re-growth occurs from the stumps. However, such plantations are established on arable land, which is thus withdrawn from the agricultural economy.

The wood turnover in Germany is with 240 Mill. m³ substantially higher than the impact of 56 Mio.t. In addition come 130 Mio.t wood import, 44 Mio.t waste paper and 11 Mio.t waste wood. On the other hand, about 120 million tons are exported and also about 120 million tons are consumed in the country, mainly for heat generation and paper production.

For the combustion enthalpy contained in the wood to be used efficiently, the moisture content must be as low as possible. As can be seen in Fig.2, the water content should not be higher than 10% in order to achieve an enthalpy of about 17 MJ/kg ≈ 4.7 kWh/kg ≈ 3 kWh/m³. This is achieved when wood is stored in the air - protected from rain - for about 2 years. In the process, a large part of the water contained in the wood is expelled, desorbed. Our air has an average moisture content of about 50% over the year, and according to the adsorption isotherms in Fig. 2, this corresponds to a moisture content of about 10%.

In wood chip applications, usually in larger heating or cogeneration plants, the fuel is usually dried by the waste heat from the plant. In pellet systems, drying is not required. After all, the pellets are formed at a pressure of 100 bar and a temperature of 150°C, during which the lignin in the wood melts and glues the wood chips together. The foreign energy for the pellet production amounts to approx. 15% of the calorific value.

The burning of wood is considered as by-and-through pollution free, since primeval times for the heat production used technology. However, it also has a "CO2 footprint": for planting, care, harvesting, transport, sawing and splitting of the wood are about 15-20% of the combustion enthalpy of the wood to apply.

When burning wood also unburned hydrocarbons, CO, NOx and fine dusts are emitted - in large quantities mainly by the mass use of single-room furnaces. Who lives like me, the author, surrounded by enthusiastic wood-heaters, can hardly open windows in winter.

According to the Federal Environment Agency, about 21,000 tons of fine dust came from wood-burning furnaces in 2015. Politicians have reacted to this problem and issued regulations for heating with wood (Federal Immission Control Ordinance, BImSchV for short): For single room fireplaces that went into operation before the ordinance came into force on March 22, 2010, the limits are 4 g CO and 0.15 g dust per m³ of air. Stoves installed from March 2010 to the end of 2014 may emit a maximum of 2 g CO and 0.075 g dust m³. Furnaces put into operation after 2015 must comply with limits of 1.25 g CO and 0.04 g dust per m³. The following deadlines apply in the event of non-compliance with the limit values: stoves built up to 1984 already had to be retrofitted/replaced at the end of 2017. The deadline for models built between 1985 and 1994 expires at the end of 2020. Retrofits to comply with the limit values are possible with catalytic converter systems, in the future probably also with electrostatic precipitation

Historic models manufactured before 1950, open fireplaces and cooking stoves are exempt from the BImSch regulation. Also tiled stoves as sole heating systems do not have to be shut down.

Sources:

Thünen Working Paper 57 (2016), Dr. Holger Weimar, Thünen Institute of International Forestry and Forest Economics, Leuschnerstraße 91, 21031 Hamburg (Germany)

http://www.schornsteinfegermeister.de, Homepage Andreas Gärtner

R.H.Ahrens, VDI Nachrichten,24.11.2017,p.18, A catalyst for wood-burning stoves

Th.Nussbaumer, A.Lauber, BWK 63(5)63(2011) Separating fine dust from wood-burning stoves electrostatically

Images:

J.Fricke and W.L.Borst, Essentials of Energy Technology, Wiley-VCH, Weinheim 2013.