Rökgasbrunn vid spannmålseldning

The interest for cereal grain as a fuel is large today. It’s a domestic fuel, renewable, easy to combust and relatively cheap. Energy grain has, as similar fast growing energy crops, higher contents of ash, sulphur and chorine than virgin wood. During combustion of grain, sulphur dioxide (SO2), which can form sulphuric acid (H2SO4), and hydrochloric acid (HCl) are formed. If the moisture in the flue gas is allowed to condense, it is very aggressive with, and corrosion damages may appear in a short time, also on stainless steel. Sulphur dioxide and hydrochloric acid also contribute to acidification of the envi-ronment. High content of ash leads to high content of dust (fly ash) in the flue gas, which may cause health problems. The aim of this project has been to demonstrate and evaluate the possibility to avoid cor-rosion problems and to minimise emissions of acid species and dust during combustion of grain by the use of a flue gas well. A flue gas well means that the flue gases are lead through a flue gas channel in the ground to a well. The flue gas temperature is lowered, moisture is condensed and some of the acid species and the fly ash is also condensed and gathers in the well. The project consisted of an informatory part and of measurements at an existing flue gas well. The informatory part shows that a boiler for energy grain can be connected to a flue gas well after relatively simple modification of the flue gas wells that today are connected to driers heated with oil burners. Right choice of materials is important to avoid corro-sion. As flue gas channels, acid resistant plastic or glazed concrete channels may be used. An ejector fan may be used to avoid corrosion in the fan. The channels should be equipped with a cleaning device because of the high dust content in the flue gas. A pres-sure sensor is recommended to close the burner if the draught disappears, for example if the electricity disappears or if the channel is blocked. The cost to install a flue gas well is estimated to 5000 – 10 000 SEK plus the cost for digging and for the pressure sensor. The cost depends of the size of the appliance. The acid condensed moisture should be prevented from reaching the surrounding grounds. To this end, at smaller boilers, limestone can be put in the bottom of the well. For a smaller boiler, at least 75 kg/year is estimated theoretically, but more experience is needed to know how much limestone is needed. At larger boilers, large amounts of con-densed moisture must be infiltrated in the ground or taken care of in some other way. Today, there exists today no recommendations or rules from the authorities of how this could be done. When a flue gas well is used, limestone should not be added to the fuel to avoid the risk of hardening of the fly ash. The latent heat from condensation is considerable, and may deteriorate the separation of acid species and fly ash in the flue gas channel and the well. Measurements were performed at an existing flue gas well, both after the boiler and after the well. Results showed a reduction of chlorine with 40 %, of dust with 42 %, and of sulphur with 67 %. Continued evaluations are recommended of larger appliances, from 100 kW to a few MW, especially concerning possible solutions for increased reduction through the use of nozzles providing water droplets, recommendations for taki