How the choice of technology influence on emissions to air during combustion of energy grain in small scale district heating boilers; Hur valet av foerbraenningsteknik paaverkar utslaepp till luft vid naervaermecentraler vid eldning av spannmaal
The interest for grain as a fuel has increased lately. Many farmers take economical advantage by heating their houses and farm buildings with energy grain. Furthermore, some farmers supply heat to schools and other enterprises. Yet, manufacturers and users of boilers are concerned about problems connected to combustion of grains, such as corrosion, sintering and emissions of nitrogen oxides, dust, sulphur oxide and hydrochloric acid. The aim of this project was to take a closer look at these problems and their extent in a systematic way. The work was restricted to appliances of 100 - 400 kW. Eight users of energy grain were interviewed about their experiences from combustion of grain. The majority had only had smaller problems with their appliances, besides from relatively many running-in problems, presumably because the instructions supplied for installation and operation were insufficient. Shutdowns were relatively rare and occurred only one or a few times per year. The most common cause for shutdown was sintering in the burner. When oat was fired, sintering was rare, but using barley, wheat or rye more often led to sintering. Some of the users fired only oat to avoid sintering. Problems with corrosion were reported to a small extent. Corrosion is avoided by keeping the flue gas temperature high enough to avoid condensation of moisture. Most of the appliances had only been used a shorter time; all but one was installed year 2004 or later. Slow corrosion damages may therefore show in a few years. Measurements were performed on three boilers for energy grain on carbon monoxide (CO), hydrocarbons (OGC, organic gaseous compounds), nitrogen oxides (NO{sub x}) and dust. Two of the boilers were on 150 kW and the third were on 95 kW, but because of warm weather they were fired on part load. Measured values of carbon monoxide were, for the three boilers, between 70 and 900 mg/Nm{sup 3} and emissions of OGC were between zero and 28 mg/Nm{sup 3} , which are in average higher compared with emissions from combustion of wood pellets, but lower compared with wood logs. Emissions of nitrogen oxides varied between the boilers between 490 and 945 mg/Nm{sup 3}, and this is considerably higher than emission from wood pellets or logs. Emissions of dust varied between 160 and 440 mg/Nm{sup 3}, which are higher than emissions from pellets. Since emissions of unburned components were low, the degree of ash particles was high in the dust. Emissions of sulphur dioxide and hydrochloric acid were estimated from the fuel analyses, and were considerably higher than from combustion of wood. The estimated levels were considerably higher than what is allowed from waste combustion. Energy grain is attractive as a fuel because it does not contribute to global warming. Furthermore, energy grain has good combustion 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 (organic gaseous compounds). The boilers studied in this project had emission well below these limits. The emissions of unburned components (soot, OGC and carbon monoxides) were low. A back-draw with energy grain is the high content of ash, nitrogen, sulphur and chlorine that leads to extensive emissions of dust, nitrogen oxides, and sulphur dioxide and hydrochloric acid. For smaller boilers, < 500 kW, there are no legislations or recommendation that restrict these emissions. The lack of legislation/recommendations makes it difficult to judge whether the emissions are acceptable or not. Today, boilers for energy grain are designed and optimised to give low emissions of unburned components (soot, carbon monoxide, OGC). Unfortunately, low levels of unburned components are commonly accompanied by high levels 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 was substantially lower than from the other two. The boiler with low dust emission used modulating power regulation, while the other two 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 about whether modulated power regulation may lead to reduced dust levels during part load energy grain combustion. Today, there is no adequate legislation or recommendations that limits the emissions of dust, nitrogen oxides, sulphur dioxide or hydrochloric acid from combustion of biofuels in boilers designed for smaller enterprises, small scale district heating or similar. Despite this, these emissions constitute an obstacle for a considerable expansion of energy grain and similar fuels; because these emissions 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. The possibility to reduce nitrogen oxides through dividing the combustion in zones should be investigated, as a suggestion through continued development and validation of the air supply in the burner. The influence of power control on dust formation is also very interesting to investigate, as a suggestion through measurements focused on formation and reduction on dust emission during power modulation and during on-off power control.
