In the field of humidity quantities, the first CIPM key comparison, CCT-K6 is at its end. The corresponding European regional key comparison, EUROMET.T-K6, was completed in early 2008, about 4 years after the starting initial measurements in the project. In total, 24 NMIs from different countries took part in the comparison. This number includes 22 EURAMET countries, and Russia and South Africa. The comparison covered the dew-point temperature range from -50 C to +20 C. It was carried out in three parallel loops, each with two chilled mirror hygrometers as transfer standards in each loop. The comparison scheme was designed to ensure high quality results with evenly spread workload for the participants. It is shown that the standard uncertainty due to the long-term instability was smaller than 0.008 C in all loops. The standard uncertainties due to links between the loops were found to be smaller than 0.025 C at -50 C and 0.010 C elsewhere. Conclusions on the equivalence of the dew-point temperature standards are drawn on the basis of calculated bilateral degrees of equivalence and deviations from the EURAMET comparison reference values (ERV). Taking into account 16 different primary dew-point realizations and 8 secondary realizations, the results demonstrate the equivalence of a large number of laboratories at an uncertainty level that is better than achieved in other multilateral comparisons so far in the humidity field.

There is always some risk of incident associated with the combustion of solid fuels in automatically stoked boilers or stoves. These incidents are of essentially three types: smoke from the burner back into the boiler room (or back into the living room from pellet stoves), gas explosions in the combustion chamber and back burning in the stoker. There can be a number of reasons for these problems, and in many cases such incidents have not been investigated. The Swedish Building Regulations stipulate that firing equipment shall have adequate safety against fire. They also recommend that stokers/burners should have at least two independent safety systems against back burning. The objective of this project has been to analyse such incidents and to test safety systems for pellet and wood chip burners, in order better to understand how they work. It has also included evaluation of the“fireguard” water sprinkler system. The results from the tests with pellet burners showed that the burners that were tested provided relatively good protection against back burning. However, it was found that it was very important how and where the systems (temperature limiter, water sprinkler etc.) were fitted on the burners in order to ensure correct and effective operation of the safety system. One of the key question was also where to position electronic equipment on the burners in order to ensure that it is not exposed to temperatures higher than those for which it is designed. Over the years, there have been major problems with back burning in older designs of wood chip burners using old technology. The chip burner used in this project was of modern type. The test with simulated back burning showed that the water sprinkler stopped the fire. However, for greater safety the water sprinklers should have included two separate water container. The test with the pellets stove showed that the temperature sensor was of crucial importance for safety against back burning in the stove. Back burn occurred in one of the tests, but only after all the stove’s safety systems had been rendered inoperable. The “Fireguard” water sprinkler system performed well in our tests. However, the position of the temperature sensors, and the integrity of their thermal contact, was of crucial importance for how quickly the sprinklers responded. Another water sprinkler (Syr) showed itself to be very slow to respond when its sensor were in poor thermal contact with the surface of the fuel feed chute. Experience from the tests in this project has shown that it is relatively difficult to simulate back burning in boilers and pellet stoves, despite setting up very favourable conditions for such back burning. Although modern burners and pellet stoves are equipped with all the safety systems that can be expected, several incidents still happen every year. This means that continued analysis of incidents and development of safety systems is essential. The results from this project will be included when SP’s P-marking rules for pellet burners, pellet stoves and chip wood burners are next reviewed

Most appliances for energy grain are found on farms and are between 15-30 kW. A continued increase of use of energy grain and similar fuels are expected also in appliances somewhat larger. Back draws are high ash content and high content of sulphur and chlorine compared to wood based fuels. Sulphur is during combustion formed into sulphur dioxide that can form sulphuric acid, and chlorine forms hydrochloric acid. Severe corrosion damages have been reported when these acids are allowed to condense. Corrosion has also been noticed in on roofs etc and the species contribute to acidification of the environment. To avoid corrosion during combustion of grain it is recommended to keep the temperature high in all parts of the boiler and in the chimney, and/or to dilute the flue gas through or to choose a corrosion resistant chimney. The aim of this project has been to investigate whether it is possible to absorb the acid species through additives added in the fuel or in the supplied air. Measures were performed with the addition of 1-2 weight % of additives in a 20 kW boiler. Lime stone (calcium carbonate) and sodium bicarbonate were chosen. They are not dangerous to handle and are ready available. In larger quantities, the price for lime stone is 1850 SEK per metric ton and for bicarbonate 3000 SEK per metric ton. If the price of oat is 0.85 SEK, the price increase for each percentage of addition is 2.2 or 3.5 % respectively. The results show reductions between 20 and 40 % of sulphur dioxide and hydrochloric acid. Additives mixed in the fuel affected the combustion and the emissions of unburned species increased and the efficiency of the boiler was decreased. When bicarbonate was used, sintered lumps were found in the burner after the test. Lime stone added in the supplied air did not affect the combustion. When bicarbonate was added in the supplied air increased sintering was noticed in the burner. Measured dust emissions indicate that addition of bicarbonate in the fuel may lead to an increase in dust emission. Lime stone added in the fuel decreased dust emissions. Lime stone added in supplied air increased dust emission because calcium particles that followed the gas flow without reacting. A more exact interpretation of how the additives influence on dust emission demands increased knowledge and experiences from the area. The influence of additives on dust emissions is an interesting lead that should be followed. Today, boilers larger than 500 kW in urban areas often have a maximum allowed emission of 100 mg/Nm3, or even lower, which is difficult to reach when combusting energy grain. A reduction of 20-40 % of sulphur and chlorine is hardly enough to guarantee that corrosion is avoided. A burner of this size is built with small margins and a disturbance, in this case, ”junk in the fuel” easily influence on the combustion. It is possible that in a larger boiler a higher percentage of additives could be used without influencing the combustion.

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 takin

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.