Hur valet av förbränningsteknik påverkar utsläpp till luft vid närvärmecentraler vid eldning av spannmål

• The aim of this project was to take a closer look at problems connected to combustion of energy grain in a systematic way. The work was restricted to appliances of 100 - 400 kW. Eight users of energy grain were interviewed about there experiences. The majority had smaller problems, besides from relatively many running-in problems, presumably because the instructions for installation and operation were insufficient. Shutdowns were rare and caused by sintering. Burning other cereals than oat often led to sintering. Corrosion is avoided by keeping the flue gas temperature high. Measurements were performed on three boilers for energy grain. Measured carbon monoxide was 70 - 900 mg/Nm3, OGC 0 - 28 mg/Nm3, nitrogen oxides 490 - 945 mg/Nm3 and dust 160 - 440 mg/Nm3. All emissions were higher compared to pellets. Sulphur dioxide and hydrochloric acid were estimated from fuel analysis, and were higher than from combustion of wood. Energy grain does not contribute to global warming and have good properties in the form of high heating value, relatively high density and the grain has a size and form easy to combust. The Swedish Board of Housing, Building and Planning prescribes maximum allowed levels of OGC. The boilers studied in this project had emission well below these limits. For boilers < 500 kW, there are no legislations or recommendation that restrict emissions of dust, nitrogen oxides, sulphur dioxides or hydrochloric acid. The lack of legislation/recommendations makes it difficult to judge whether the emissions are acceptable or not. Today, boilers for energy grain are designed to give low emissions of unburned components and not optimised for low emissions of nitrogen oxides. By changing the design of the burner (it was equipped with three steps with air holes) the emissions were reduced by 27 % without an increase of unburned components. The objective with the steps was to divide the combustion into two zones: one with deficit of oxygen and one rich with oxygen. It is probable possible to reduce the emission even more by dividing the combustion in an even more sophisticated way. Measurements in three different boilers showed that dust emission from one of the boilers that used modulating power regulation was substantially lower than from the other two that were controlled by an on-off thermostat. This may be explained by higher degree of ash pulled by the gas flow at high power, or by lower temperature in the glow bed at low power. However, more data is required to draw general conclusions. Emissions of dust, nitrogen oxides, sulphur dioxide or hydrochloric acid constitute an obstacle for a considerable expansion of energy grain and similar fuels; because the emission will be paid attention to if they increase, especially if they increase in connection to densely populated areas. Therefore, a continuous increase of combustion of energy crops should be accompanied by continuous improvements of combustion technique to reduce the emissions.

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