Recent pellet stoves perform excellently under type test conditions. In contrast, typical real life emissions show significantly higher values under usual operational conditions. Consequently, type testing procedures may not account for real life stove operation and, thus, do not allow to distinguish between low- and high-tech appliances. The EU-project beReal aimed at the development of a testing method for pellet stoves that reflects real life operations better and to support innovative pellet stoves that perform well under typical operational conditions. Based on an online survey and field observations, an advanced real life testing procedure for pellet stoves was established reflecting real life user behavior, e.g. regarding different load levels and the ignition phase. A field test was designed at the end of the project to demonstrate the applicability and practical relevance. The field test proved that emission values for beReal at the test stand and in the field stay within the same range. A Round Robin test proved the repeatability and reproducibility of the beReal testing procedure. The beReal method can be reproduced with the same statistical variability or performed even better than the type testing method with exception of PM between different laboratories. © 2017, ETA-Florence Renewable Energies. All rights reserved.

The Wood Stove year 2020 - a part of the living environment

This project is a collaboration between Nibe AB / Contura and SP Technical Research Institute of Sweden. Based on earlier evaluations of six different technology tracks, three tracks were selected to further evaluate: CFD modeling, active control and catalyst technology, with the long term goals of particle emission, control of maximum output and long burning time. A literature review of available sensors for O2, CO and hydrocarbons have been conducted followed by a market research, focusing on developments during the last decade and experiences from biomass combustion. Sensors were long-term tested. The lambda probe works well and is affordable, while sensors for unburnt gases still need to be further optimized for accuracy and long-term stability and also become less expensive to be of practical use. Active control has been applied to a prototype and evaluated with good results. In further work with active control, emphasis should be on safety. A CFD method for stove combustion has led to an industrial application with significant cuts in time and resources. Advanced CFD simulation has been used to examine the impact of furnace temperature on wood conversion and how wood log size can be used to counteract an accelerating power output. Simulation results show that log size has good potential to contribute to an improved combustion, but should be combined with further design of the stove. A rig for evaluation of catalyst coatings has been built and experimental results are promising. CO was in principle eliminated from the flue gas and hydrocarbon concentration were reduced down to a level equivalent to 25-70% of the original content.

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.

Combustion processes in a biomass bed are investigated experimentally. Special attention is paid to the influence of primary airflow and particle properties on the ignition front, its temperature and on the composition of the gas leaving the front. Two test rigs have been built: a large rig in the same size as a boiler for domestic use and a small laboratory test rig. In both rigs the ignition front moves in opposite direction to the primary airflow. Three combustion regimes are identified: a sub-stoichiometric regime with incomplete consumption of oxygen, a sub-stoichiometric regime with complete consumption of oxygen and an over-stoichiometric regime. The results show that a fuel with higher density and thermal conductivity (but in other respects similar to other fuels) has a wider sub-stoichiometric regime where oxygen is cornpletely consumed. If the particle size is increased (for the same fuel quality) the airflow range of this regime becomes shorter and starts at higher airflow.

Rökgaskondensering vid förbränning av biobränslen motiveras av energivinst med kort återbetalningstid. Rökgaskondensering ger även en miljövinst, då gasformiga ämnen kan avskiljas med hög avskiljningsgrad. När föravskiljning i form av el- eller textilfilter saknas avskiljs även mycket stoft i kondensorn, varvid emissionerna till luft av stoft och stoftbundna ämnen som tungmetaller reduceras. Problem med igensättning och sänkt tillgänglighet förekommer, oavsett typen av föravskiljning. Vissa anläggningar fungerar väl, medan andra fungerar sämre. Det finns ett behov av att bättre veta hur kondensorns konstruktion samverkar med resten av systemet och med graden av föravskiljning. I detta projekt har telefonintervjuer genomförts med 27 anläggningar för fjärr- och när-värme och 2 pappersbruk, inkluderande 38 rökgaskondensorer. Bränslet var på fjärrvär-mesidan till största delen fuktigt biobränsle i form av blandningar av skogs- och såg-verksspill. Målsättningen var att samla information om samband mellan konstruktions-typ, föravskiljning och stoftproblem. Även övrig problematik noterades. Resultatet av rundringningen visade att typen av konstruktion inte var avgörande för funktionen. Däremot var en riktig dimensionering av rökgaskondensorn till pannan och pannans last avgörande för funktionen. Dimensionering av spoldysor är av stor betydel-se för att minska problem med igensättning, men även konstruktionstyp är avgörande. Problem av engångskaraktär förekom i samband med intrimning och anpassning till aktuell anläggning. Kringutrustningen var påfallande enkel, men detaljer vars funktion kan förbättras är uppfuktare och spoldysor. Cirka 80 % hade hög tillgänglighet. Stoftkrav från myndighet innehölls, med endast enstaka undantag. Ett el- eller textilfilter som föravskiljare minimerade stoftproblemen, eftersom kondensorn då endast fungerar som en värmeväxlare. I flera fall hade man löst tidigare problematik genom att installera något typ av filter. Men det fanns även ett fler-tal kondensorer med endast cyklon som föravskiljare som fungerade utmärkt med hög tillgänglighet och låga krav på skötsel, även om fler problem återfanns i denna grupp. Kondensorns förmåga att reducera stoft var sällan känd av användaren. I de fall stoft var uppmätt före och efter kondensorn, varierade avskiljningsgraden för fjärrvärmeverken från ingen alls till 10 gånger och för pappersbruken mellan 16 till 36 gånger. Detta visar att det finns potential för förbättring av stoftavskiljning hos många rökgaskondensorer. För att öka kunskapen kring hur mycket av ingående stoft som kondensorn kan avskilja och hur kondenseringen påverkar stoftets sammansättning och karaktär, föreslås mät-ningar av stoft före och efter kondensering, både på luft- och vattensidan. Mätningarna bör inkludera stoftets kemiska sammansättning och partikelstorleksfördelning. Viktiga parametrar är troligen stofthalt in till kondensorn, gashastighet, kondensorns dimensio-nering, quenchningens funktion och spraydropparnas storleksfördelning. I sin förläng-ning kan resultatet utmynna i konkreta råd till anläggningsägare om bra lösningar då kondensorn används både för energivinst och för stoftrening.

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

A burner for firing of grain was investigated for function, safety and emissions. Fuel, ash, particulates, gas and condensed flue gas from the inner surface of the chimney were analysed, and the stainless steel chimney were analysed with SEM/EDX. Three security systems should give a high safety against back-firing, provided the sensors are correctly placed and in function. Frequent cleaning of the boiler/burner was needed and some sintering noted. Emissions of CO and OGC were low, but of par-ticulates, NOx, SO2 and HCl higher compared with pellet. The condensed moisture was very sour and had high content of chlorine. After 800 cycles, the chimney was ruined by corrosion. Establishing of a list of near-accidents and fires in cooperation with insurance companies and an inventory and closer study of suitable materials for chimneys is recommended. If grain firing expands, techniques for re-duction of particulates, nitrogen oxides and acidifying substances should be developed.

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.