Detaillierte experimentelle Untersuchung der Schadstoffbildung bei der Holzverbrennung (English shop)

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Wood is an almost CO2-neutral fuel. In Germany alone, its energy potential is estimated at 10 to 17 million tonnes of hard coal equivalent per year, depending on how much fallow land is used for cultivation. It is therefore a renewable energy source with room for expansion. On the other hand, when wood is heated, combustible gas is formed to an even greater extent than with coal. The rate of devolatilisation depends strongly on parameters that mostly fluctuate, such as temperature and flow conditions, which makes wood combustion a complicated process. Because wood combustion is often not operated with the necessary process engineering effort, high concentrations of unburnt hydrocarbons and tar particles are produced. In densely populated areas this leads to conflicts of interest. In order to investigate the process of pollutant formation more precisely, small spherical beech wood samples were placed above a laminar premixed flat flame. The temperature and residual oxygen content of the burner exhaust gas were varied and the temporal evolution of three quantities was measured:
Mass loss as an indicator of combustion progress.
Composition of the pyrolysis gas by means of gas chromatography.
Particle size distribution by means of a differential mobility analyser.
With this measurement setup, approximately 200 individual substances could be detected. However, the largest part of the pyrolysis gas is formed by a few substances, such as carbon monoxide, propanone, butene, methane, furfural and oxygenated aromatics such as vanillin. At early experimental times or at temperatures below 900 K, the pyrolysis gas consists mainly of low-molecular substances. At later times or elevated temperatures, more complex substances are detected in increasing quantities. The emitted particles consist mainly of tar and to a small extent of soot. The measurements show a bimodal distribution: an accumulation of particles in the size range around 15 nm and a further accumulation at ≈ 100 nm. Medium-sized particles of about 35 nm hardly occur. Furthermore, there is a pronounced relationship between the number of particles and their volume.