Estimating excess heat from exhaust gases: why corrosion matters

Industry accounts for about 30% of the final energy demand in Germany. Of this, 75% is used to provide heat, but a considerable proportion of the heat is unused. A recent bottom-up estimate shows that up to 13% of the fuel consumption of industry is lost as excess heat in exhaust gases. However, this estimate only quantifies a theoretical potential, as it does not consider the technical aspects of usability. In this paper, we also estimate the excess heat potentials of industry using a bottom-up method. Compared to previous estimates, however, we go one step further by including the corrosiveness of the exhaust gases and thus an important aspect of the technical usability of the excess heat contained in them. We use the emission declarations for about 300 production sites in Baden-Württemberg as a data basis for our calculations. For these sites, we calculate a theoretical excess heat potential of 2.2 TWh, which corresponds to 12% of the fuel consumption at these sites. We then analyse how much this theoretical potential is reduced if we assume that the energy content of sulphur-containing exhaust gases is only used up to the sulphuric acid dew point in order to prevent corrosion. Our results show that 40% of the analysed excess heat potential is corrosive, which reduces the usable potential to 1.3 TWh or 7% of fuel consumption. In principle, it is possible to use the energy of the excess heat from sulphur-containing exhaust gases even below the dew point, but this is likely to involve higher costs. This therefore represents an obstacle to the full utilisation of the available excess heat. Our analysis shows that considering corrosion is important when estimating industrial excess heat potentials.