Abstract: 1 Introduction And Objectives

Transcript

DEVELOPMENT OF A BIOMASS HEATING DEVICE FOR LOW ENERGY AND PASSIVE HOUSES Abstract: Purpose - Decreasing energy demand due to improved building standards requires the development of new biomass combustion technologies to be able to provide individual biomass heating solutions. The objective of this work is therefore the development of a pellet water heating stove with minimal emission at high thermal efficiency. Design/methodology/approach - The single components of a 10 kW water heating pellet stove are analysed and partly redesigned considering the latest scientific findings and experimental know-how in combustion engineering. The outcome of this development is a 12 kW prototype which is subsequently down-scaled to a 6 kW prototype.. Finally the results of the development are evaluated by testing of an accredited institute. Findings – Based on an existing pellet water heating stove, the total excess air ratio was reduced, a strict air staging was implemented and the fuel supply was homogenized. All three measures improved the operating performance regarding emissions and thermal efficiency. The evaluation of the development process showed that the CO emissions are reduced by over 90% during full load and by 30-60% during minimum load conditions. Emissions of particulate matter are reduced by 70% and the thermal efficiency increased to 95%. Originality/value - The result represents a new state of technology in this sector for minimal emission and maximal thermal efficiency, which surpasses the directives of the Eco label “UZ37” in Austria and “Blauer Engel” in Germany, which are amongst the most stringent performance requirements in the European Union. Hence this design possesses a high potential as heating solution for low and passive energy houses. Keywords: biomass combustion, wood pellet, water heating stove, low emission, design criteria 1 Introduction and Objectives The implementation of the European Building Performance Directive (EBPD, Directive 2002/91/EC) will lead to a significant reduction of energy consumption in particular in the new building market. By 2020 the EBPD will require low energy and passive house standard for new buildings. In Austria, these houses are characterized by an energy benchmark lower than 50 kWh/m²a, which means that the nominal heat demand for the average private housing is less than 10 kW. Biomass combustion, however, is an attractive and highly sustainable solution to provide this energy for heating and hot water. Especially automatically fed systems, like pellet water heating stoves, are a potential technology group, which fit conveniently to this housing standard and additionally increase the living comfort due to its stove function. However, room heating appliances are mostly associated with higher emission of carbon monoxide (CO) and organic volatile compounds (VOC). This fact is also considered in emission directives by means of higher emission levels in comparison to boiler systems. Four main facts are held responsible for the higher emissions. Firstly, room heating appliances release a certain amount of energy by radiation and convection, which dissipates mainly from the combustion chamber. Due to that fact the temperature in the combustion chamber is lowered, so the reaction rate of the combustion is decreasing. Hence the emission of CO and VOC are at a higher level. Secondly, a nominal power output of less than 10 kW still poses a challenge in terms of grate design and fuel dosing. This is due to the fact that solid biomass is a demanding fuel. Due to the low heat demand the mass of the fuel bed on the grate is small, which makes the solid phase combustion susceptible for irregularities and subsequently increases the emissions. Regarding fuel dosing, pellets themselves are free-flowing. Nevertheless the dosing is at least limited to the mass of a single pellet, which is approximately 0.5 g. For a power output of 10 kW one pellet per 1.25 sec is necessary for a continuous fuel supply, at lower loads the dosing is even more challenging. Thirdly room heating appliances are mostly equipped with a glass door. To keep the glass door clean a airwash system is installed. The additional air supply increases the excess air ratio and reduces the thermal efficiency. Furthermore it may chill the combustion chamber and cause therefore higher emissions. Fourthly, the grate design of such stoves usually bases on a fixed grate. These grates hardly fulfil the requirements for an air staged combustion, which is state of technology for boiler systems. Considering these challenging facts, this development concentrates on the improved redesign of an existing pellet hydronic stove with glass door (TWIST 80/20 by calimax Energietechnik GmbH) with 10 kW nominal heat output. The redesigned product shall establish a new state of technology. The following target factors have been defined: • CO emissions less than 100 mg/Nm³ (ref. 13% O2). • Particle emissions (TSP) less than 20 mg/Nm³ (ref. 13% O2). • Thermal efficiency higher than 91%. The heat output shall be dissipated to 80% by the water circulation and to 20% by radiation and convection directly into the room. • 12 kW nominal load for first redesign; subsequent down-scaling to 6 kW nominal load As the stove is to be used as room heating appliance, the design and appearance during operation is another criterion for the development. Therefore the cleanness of the glass door is also a target. The final development results of the 12 kW and the 6 kW pellet water heating stove are evaluated by testing according to DIN EN 14785 by an accredited institute. 2 Methodology 2.1 Development methodology The development is carried out in 4 key phases, which are displayed in figure 1. Take in figure 1 Figure 1: Methodology of development phases. In the first phase (component analysis) the initial state of the existing pellet water heating stove is examined. The system is divided in three major components: combustion chamber, grate/burner pot and fuel supply. These components are analyzed and characterized. Secondly, the components are separately optimized by experimental development. Each modification in designengineering of the components is analysed in combustion tests and its improvement potential is evaluated. After the experimental development of the 12 kW prototype is finished, a scale down process is performed for a 6 kW design. Finally, the outcomes of the development process, a 12 kW and the 6 kW water heating pellet stove, are tested according to DIN EN 14785 (Residential space heating appliances fired by wood pellets - Requirements and test methods) by an external testing institute and the results are used for the evaluation of the overall development process. 2.2 Testing methods The combustion experiments for the component analysis and the experimental development are conducted on the basis of EN 14785. The test setup, which is shown in figure 2, states the base for energy balance and emission measurement. Take in figure 2 Figure 2: Testing setup of combustion experiments. The energy balance is calculated by the fuel consumption, the dissipated heat in water cycle and the energy loss to the chimney. The fuel consumption is measured by a scale. The dissipated heat in water circulation is measured by the water volume flow, the feed and return temperature. The energy loss to the chimney is determined by the flue gas temperature and by unburnt residues in flue gas (carbon monoxide). This is also the determination method for the thermal efficiency factor (heat loss method). Emission measurement of gaseous compounds is performed with a flue gas analyser (O2 paramagnetic, CO2, CO and NOx by NDIR). The determination of the particulate matter emission is conducted by gravimetrical dust measurement according to VDI 2066. 3 Component analysis 3.1 Initial state The development starts from an existing water heating stove TWIST 80/20 manufactured by calimax Energietechnik GmbH. The nominal heat output is 10 kW, whereas 80% is released to the water circulation and 20% dissipates by radiation and convection directly into the room. Take in figure 3 Figure 3: Water heating pellet stove TWIST 80/20 by calimax Energietechnik GmbH. Figure 3 shows the design of the stove. The pellets are transported from the hopper upwards by an inclined srew conveyor to a chute, which leads to the burner pot. The pellets drop into the burner pot that is equipped with a flat, fixed grate at the bottom, through which the primary air is supplied. The combustion chamber above the burner pot is made of steel. The back of the combustion chamber is made of fire bricks. The front door is made of glass ceramics, which is held clean by an airwash system through the bearing of the glass. For component analysis the operation of the initial state is surveyed in a preliminary test. Carbon monoxide was used as indicator for the gas phase burn out quality and for organic matter emission (gaseous and particulate), which correspond sufficiently (Johansson et.al 2004). The pellet stove was steadily operated at nominal load. The carbon monoxide emissions (CO) and the excess air ratio were measured. The correlation is shown in Figure 4. Take in figure 4 Figure 4: Correlation of carbon monoxide emissions and excess air ratio during full load combustion test. Overall the emission of CO is below the Austrian emission limit for automatically fed combustion systems (Art. 15a BVG 1998), which is at 500 mg/MJ or approx. 610 ppm13% O2. In the surveyed combustion test a mean CO emission of 170 ppm13% O2 was measured. Nevertheless, three unsatisfying performance characteristics could be observed, which limit a high efficient and lowest emission operation: a) Combustion system operates at a high excess air ratio. b) Range of CO emission is widely spread c) Operation at a wide range of excess air ratio. These characteristics are further analysed in the following chapters of component analysis 3.2 Combustion chamber design The combustion system is operated at a high excess air ratio (λ > 2.5) even at full load conditions. This fact is mainly ascribed to the airwash system of the glass door. To keep the glass door clean a high amount of air is introduced into the combustion chamber through the airwash system. Nevertheless this air contributes only minimally to the gas phase burn out. But it has a chilling effect on the combustion chamber and therefore increases the CO emission. Moreover the high excess air ratio decreases the thermal efficiency. Therefore an optimization of the airwash system by reducing the corresponding air supply and at the same time realizing a clean glass door during operation is necessary. 3.3 Burner pot and grate design The CO emissions at a specific excess air ratio are varying, which means that the gas phase combustion is unstable. This indicates that mixing of supply air and combustible gas is insufficient. As mentioned in the section before it is assumed that the airwash does not contribute to the combustion. Therefore the insufficient mixing of the secondary air and the combustible gases is responsible for the varying CO emission. The secondary air is inserted 2 cm above the grate. The primary air is inserted directly into the fuel bed. The amount of primary air is significantly higher than the amount of secondary air. Staged air combustion, however, needs a distinguishable separation of primary and secondary air. Primary air should mainly be responsible for devolatilization and char burnout and secondary air for gas phase burnout. Therefore a strict separation of primary and secondary combustion zone has to be realized and the distribution of primary to secondary air supply needs to be adapted (van Loo and Koopejan 2002, Kaltschmitt 2001, Nussbaumer 2003). In addition, by the means of an optimized air staging the emission of particulate matter may be reduced. Two crucial facts are well defined, which contribute to low particulate matter emission (Brunner et.al 2006, Obernberger et.al 2007). On the one hand the emission of organic particulate matter is reduced due to steady and complete combustion conditions. On the other hand air staging implies an understochiometric combustion in the primary zone, whereby the temperature is lowered and the emission of inorganic particulate matter is reduced. These considerations have to be implemented in a new burner pot and grate design. 3.4 Fuel supply The combustion system is operated at a wide range of excess air ratio (2.5 < λ < 4), which indicates that the combustion in the fuel bed is unsteady. Considering a constant air supply, the unsteady combustion can be traced back to the fluctuating fuel supply. The fluctuating fuel supply leads to two extreme conditions. If there are too high amounts of fuel in the fuel bed, the combustion is lean of oxygen. Too small amounts of fuel in the fuel bed lead to a high excess air ratio. Both cases result in high CO emission. Besides the grate design the fuel supply has strong influence on the steadiness of the fuel bed combustion. Especially at low load operation the continuous dosing of (pellets) fuel poses a challenge. At a nominal load of 10 kW, 0.6 g/s of fuel are needed, which equals one pellet per 1.25 seconds. The current design of the fuel supply is an inclined screw conveyor, which transports pellets from the fuel tank on the grate, at an inclination angle of 60°. The fuel is dropped in a chute. At this point pellets may pile as can be seen in figure 5. Take in Figure 5 Figure 5: Dropping point of screw conveyor shortly before dropping a fuel pile into the chute. This piling leads to fluctuations in the fuel supply and to fluctuations in the fuel bed combustion, respectively. Therefore various measures, which are suited to increase the homogeneity and to overcome the fluctuations in the fuel supply, are investigated. 4 Experimental development 4.1 Combustion chamber design The reduction of the excess air ratio is seen as a decisive factor for the overall development and improvement process. Therefore combustion experiments were conducted to optimize the airwash system. In first tests only small amounts of air were supplied at the top of the glass door for airwash. During these tests, inorganic salts deposited on the glass door (see figure 6). The fact that the deposits were inorganic indicates that a complete gas phase burn out has been achieved; the airwash system did not affect the combustion process negatively. In the next combustion experiments a variation of the airwash inlet design and the combustion chamber design was carried out. To achieve a laminar flow of the air along the glass door surface an inlet vent was constructed. Furthermore the combustion chamber was lined with Vermiculite, so that streams of combustion gas towards the glass door were reduced. A further effect of the lining was a raise in the combustion temperature. By this measure a clean look of the glass door was achieved even after 15 hours of operation (see figure 7) Take in Figure 6 Figure 6: Glass door with drastically reduced airwash after full load operation. Take in Figure 7 Figure 7: Glass door with laminar airwash and optimized combustion chamber design after full load operation. 4.2 Burner pot and grate design As already mentioned in chapter 3.3, steady and complete gas phase combustion at low excess air ratios can be reached by air staging. Therefore the initial design of the grate was completely redesigned. At first, the distance from fuel bed to secondary air inlet was expanded significantly to achieve a distinguishable separation of primary and secondary combustion zones. Furthermore the ratio of primary and secondary air supply was adjusted by adapting their cross section area. A ratio of 40% primary and 60% secondary air supply was realized. The primary air was supplied directly into the fuel bed from the bottom of the grate. Take in figure 8 Figure 8: View on the grate, primary and secondary air supply. The supply of the secondary air was performed by two lines of inlets (see figure 8). Consequently a slip of combustible gas between the through the secondary air inlet zones is minimized and the mixing with the secondary air supply is enhanced. The results of this development step are shown in figure 9, where the correlation of CO to excess air ratio is displayed. Overall the total CO emissions were reduced to 21 ppm13%O2 during full load and to 104 ppm13%O2 during minimal load conditions, which are 30% of the nominal load. Furthermore the range of the CO emissions is reduced compared to the initial state. During minimal load, nevertheless, the spreading of the CO emissions is still observable, because the smaller flow velocity during minimum load operation results in smaller mixing rates of combustible gases and secondary air. Take in Figure 9 Figure 9: Correlation of CO to excess air ratio during full and minimum load combustion test with optimized combustion chamber and grate design. A reduction of the primary air also increases the mass of the fuel bed, because the burning rate is reduced and therefore the retention time of each pellet is extended. The increase of the fuel bed mass also contributes to steady solid phase combustion by countervailing fuel supply fluctuations. This fact is depicted in figure 9. The range of excess air ratio during full load operation is significantly reduced (1.5 < λ < 2) Nevertheless the fluctuating fuel supply has still an influence on the excess air ratio during minimum load operation (2.5 < λ < 4), which shall be further reduced by optimizing the fuel supply. 4.3 Fuel supply The determination of the fuel supply fluctuations and its influence was performed by conveying tests. A screw conveyor with fuel tank was set up and the inclination angle of the auger was varied. All tests were conducted at the same auger speed. The transported fuel mass was measured continuously to gain information about the steadiness of the fuel supply. Take in Figure 10 Figure 10: Results of conveying tests at varying inclination angle. 1 rpm auger speed, 28 mm auger pitch. The results of the tests showed (as can be seen in figure 10) that the fuel mass rate is reduced for increasing inclination angle. This is due to the fact that gravity is working against the transportation direction and the slip of pellets in the screw conveyor is increased. A low inclination angle of the screw conveyor guarantees a higher efficiency of the fuel supply system overall. Furthermore the inclination angle has an effect on the piling of pellets at the dropping point, which is the crucial fact regarding fuel supply fluctuations. To compare of the degree of fluctuations the error sum for each linear regression was taken. The deviation of single mass flow values (each 0.5min) from the mean mass flow is summed up. The calculated error sums are presented in figure 11. Take in Figure 11 Figure 11: Error sum of conveying tests at varying inclination angle. The highest fluctuations were measured at an inclination angle of 80°, where also the highest piling was observed. With lower inclination angle the fluctuations are reducing. From approximately 40° inclination the error sum stays constant. Therefore an inclination of 40° was chosen for the prototype. The results of this development step (including the previous steps) are shown in figure 12. In comparison to the previous steps the overall CO emission level was decreased during full (5 ppm13%O2) and minimum load conditions (51 ppm13%O2). Furthermore the range of excess air ratio was reduced especially during minimum load conditions, which was the aim of this development step. The excess air ratio during minimum load ranges between 1.8 < λ < 2.6. Take in figure 12. Figure 12: Correlation of CO to excess air ratio during full and minimum load combustion test with optimized fuel supply. 5 Scale Down The 6 kW pellets stove is dimensioned based on the 12 kW prototype design criteria. The burner pot and the grate design were scaled down linearly. The same scale down was performed regarding primary and secondary combustion air supply. The combustion chamber was fully lined with Vermiculite to maintain high temperatures for a complete gas phase burn out. Regarding the fuel supply system special adjustments were needed. As the heat output is further reduced, fluctuations of the fuel supply have a higher influence on the combustion. At minimum load (1.8 kW) a fuel input of 0.1 g/s is needed, which means 1 pellet every 5 seconds. This low fuel mass flow can only be achieved by additional auxiliary means. Using a screw conveyor as feeding system a dropping assistance is needed, which is displayed in figure 13. Take in Figure 13 Figure 13: Dropping assistance at dropping point of screw conveyor. Four pins (in the prototype designed as bolts) are inserted at the dropping point of the screw conveyor, so that pellets are dropped each quarter rotation. By these means the fluctuations of the fuel supply could be reduced. Figure 14 shows the performance of the 6 kW prototype. During full load conditions the CO emissions are at a very low level (33 ppm13%O2). The range of CO emissions is rather small, which indicates that the mixing of secondary air and combustible gases is sufficient. Also the range of excess air ratio is small, which indicates that during full load conditions the fluctuations of the fuel supply are insignificant. During minimum load conditions, however, the CO emissions are slightly elevated (99 ppm13%O2). The range of the CO emissions also indicates that the mixing of secondary air and combustible gases is insufficient. Furthermore the excess air ratio is varying over a wider range. Hence at this very low load of 1.8 kW, the fuel supply and dosing are still challenging. Take in Figure 14 Figure 14: Correlation of CO to excess air ratio of the 6 kW prototype during full and minimum load combustion test. 6 Evaluation The design parameters for the 12 kW and 6 kW pellet water heating stove prototypes were transferred to marketable production models, which were tested by an external testing institute. The testing results are compared to the testing of the initial state 10 kW pellet water heating stove, displayed in table 1. The developed stoves showed high improvements in all surveyed parameters. Overall the excess air ratio could be reduced from λ = 3.0 to λ =1.6 and 1.9 during nominal load. Therefore, the heat efficiency was raised from 82% to more than 95% at all load conditions. The CO emissions were reduced by over 90% for both stoves. During minimum load operation the CO emissions were 259 mg/Nm³13%O2 for the 12 kW stove and 124 mg/Nm³13%O2 for the 6 kW stove. The particulate matter emissions were reduced by more than 60% in comparison to the full load initial state. The heat is still dissipated to 80% by the boiler and to 20% by direct room heating. Table 1: Results of testing (1 conducted internally, DIN EN 14785) 2 conducted by an accredited institute according to load Initial State1 12 kW stove2 6 kW stove2 nominal minimum nominal minimum nominal minimum λ [-] 3.0 1.6 2.6 1.9 3.0 CO TSP [mg/Nm³13%O2] 130 30 5 11 259 34 15 9 124 14 ηID [-] 81.9 94.9 97.7 96.4 97.7 Boiler heat room heat output dissipation [kW] 7.8 9.9 1.8 2.6 0.6 4.6 1.3 1.6 0.7 7 Conclusions Two highest efficiency and lowest emissions pellet water heating stove prototypes with nominal heat outputs of 12 kW and 6 kW were developed. Minimum CO emissions of 5 mg/Nm³13%O2 were achieved with the 12 kW pellet stove during full load. The particulate matter emission was constantly below 11 mg/Nm³13%O2 during full load operation and the thermal efficiency exceeded at all load stages 95%. The results of the testing show that the objectives of this work are achieved. Furthermore this state of technology surpasses the Eco-label directives “UZ37” and “Blue Angel” for automatically fed stove appliances, which are presented in table 2. Three crucial development steps were defined with the objective to minimize emissions and to maximize the thermal efficiency. In a first step the excess air ratio was minimized, by reducing the airwash-air of the glass door. A reduction from λ = 3 to λ = 1.6 was accomplished without deposits on the glass door after 15 h of operation. Secondly a strict separation of primary and secondary air supply was applied. Thereby the mixing of secondary air and combustible gas was enhanced and a steady combustion was reached during full and minimum load conditions. This leads to a significant reduction of the CO emissions and the emissions of organic compounds (gaseous and particulate). In addition, the air staging improved the fuel bed combustion. By reducing the primary air supply the fuel bed mass was increased, which enhanced the steadiness of the fuel bed combustion. Thirdly, the fuel supply and dosing was improved. To achieve steady combustion in the fuel bed a steady fuel supply is necessary. Especially fluctuations during minimum load conditions are improved by adapting the screw conveyor inclination angle and the design of the pellet dropping point. Therefore the stove operates at a steady excess air ratio and the occurrence probability of extreme conditions (very high or low excess air ratio), which produce CO emission, is reduced. Nevertheless the influence of the fuel supply is still existent. Future research is needed, if the nominal load of combustion units shall be further reduced. Table 2: Eco-label emission directives in 1 Austria (values are calculated from mg/MJ) and 2 Germany for automatically fed stove appliances and the benchmark values of the top 25% pellets stoves and boilers in Europe (Musil-Schläffer et.al 2010) load CO TSP ηID [mg/Nm³13%O2] [-] nominal 180 30 90 UZ371 minimum 410 nominal 180 25 90 2 Blue Angel minimum 400 90 Top 25% Pellet nominal 104 14 94 Water Stoves minimum 377 95 Top 25% Boiler nominal 38 16 94 < 15 kW The developed stoves presented in this work represent a new state of technology for automatically fed room heating appliances. The products exceed the TOP 25% performing pellet water heating stoves identified in a recent study by Musil-Schläffer et al. (2010) in all performance criteria by far, which are displayed in table 2. Moreover this novel generation of pellet water heating stoves are in the range of emissions and efficiencies reached by the TOP 25% performing pellet BOILERS, also shown in table 2, in the load range of < 15 kW identified in the aforementioned survey. Consequently, this paper describes the development of a highest performing pellet water heating stove that is in its 6 kW version the perfect solution for low energy and passive energy houses. To economically sensitive customers this product guarantees efficiency and emission performance equal to the TOP 25% performing pellet boilers while it increases at the same time the living comfort due to its direct radiation of heat in its place of installation. Notifications O2 CO VOC TSP NCV λ ηID Nm³ MJ kW kg ppm Oxygen Carbon monoxide Volatile organic carbon Total suspended particulate matter Net calorific value Excess air ratio Thermal efficiency factor determined by indirect method (heat loss method) standard cubic meter Mega Joule Kilo Watt Kilo Gramm Parts per million Acknowledgment: This study was mainly conducted within the project “low-emission-boiler” funded by the Austrian Kplus programm. The authors also would like to acknowledge calimax energietechnik GmbH for their effort to develop a zero emission combustion technology. References: Brunner, T., Bärnthaler, G. and Obernberger, I. (2006), “Feinstaubemissionen aus modernen PelletKleinfeuerungsanlagen – Charakterisierung, Bildung und Möglichkeiten zur Emissionsminderung”, Österreichische Ingenieur- und Architekten-Zeitschrift (ÖIAZ), Vol 151, pp 258-266. Johannson, L.S., Leckner, B., Gustavsson, L., Cooper, D., Tullin, C. and Potter, A. (2004), “Emission characteristic of modern and old-type residential boilers fired with wood logs and wood pellets”, Atmospheric Environment, Vol 38, pp 4183-4195. van Loo, S. and Koppejan, J. (2002), Handbook of Biomass Combustion and Co-Firing, Twenty University Press, Enschede, The Netherlands. Nussbaumer, T. (2003), “Combustion and co-combustion of biomass: Fundamentals, technologies, and primary measures for emission reduction”, Energy and Fuels, Vol. 17 No. 6, pp. 1510-1521. Musil-Schläffer, B., McCarry, A., Schmidl, C., Haslinger, W., (2010), European wood-heating technology survey: An overview of combustion principles and the energy and emissions performance characteristics of commercially available systems in Austria, Germany, Denmark, Norway and Sweden, New York State Energy Research and Development Authority: www.nyserda.org/programs/Research_Development/wood_heating.asp. Kaltschmitt, M., Hartmann, H. and Hofbauer, H. (2001), Energie aus Biomasse, Springer Verlag, Berlin, Germany. Obernberger, I., Brunner, T. and Bärnthaler, G. (2007), “Fine particulate emissions from modern Austrian smallscale biomass combustion plants” in 15th European Biomass Conference & Exhibition, Berlin, Germany, pp 1546-1557. Ohnmacht, R. (2007), Untersuchung der Fördercharakteristik Pelletabwurffeuerung, Diplomathesis, TU-Wien. von Schneckenförderern für eine