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ACHIEVING LOW EMISSIONS AND STABLE HEAT RELEASE FROM WOOD STOVES AND FIREPLACES FIRING AT LOW LOAD Edvard Karlsvik, Research Scientist/M.Sc., SINTEF Energy Research AS, Postboks 4761 Sluppen, NO 7465 Trondheim E-mail: [email protected], Phone: +47-735 92512, Mobile: +47-932 05504, Fax: +47-735 92889 Øyvind Skreiberg, Senior Research Scientist/Dr.ing., SINTEF Energy Research AS, Postboks 4761 Sluppen, NO 7465 Trondheim E-mail: [email protected], Phone: +47-735 93993, Mobile: +47-906 59751, Fax: +47-735 92889

This work aims at achieving low emissions and stable heat release from wood stoves and fireplaces designed for firing at low load, i.e. new technologies and solutions with an increased focus on the combustion process and its control, the combustion quality and optimum design to ensure low emissions and high energy efficiency. To minimize the negative effects of the batch combustion process (compared to a continuous combustion process) of wood logs in wood stoves and fireplaces, especially if low load and long burning time is the aim, a more stable heat generation and heat release is needed. Modern wooden buildings in Norway built according to new and much stricter standards with respect to building insulation demands wood stoves and fireplaces with much reduced effects (down to 1 kW) and these units do not exist in the market today. A substantial research effort is therefore needed in the development of this next generation of wood stoves and fireplaces, to ensure that the particle emission levels and emission levels of other unburnt compounds still can be kept low, and potentially also be reduced compared to today’s best available technology. At the same time, the energetic performance of the units should be high and the user comfort should be improved. This means that much effort must be put into innovative design aiming at stable heat release conditions. The StableWood project (New solutions and technologies for heating of buildings with low heating demand: Stable heat release and distribution from batch combustion of wood) is lead by SINTEF Energy Research and financed by the Research Council of Norway and four industry partners. StableWood focus on new solutions and technologies for heating of buildings with low heating demand and specifically batch combustion of wood and the increased need for stable heat release and distribution from wood log combustion in wood stoves and closed fireplaces. In this work the StableWood project, with a total budget of about 2 million Euro and running from 2011 to 2014, and the rationale behind it is presented. Keywords: Wood, stoves, fireplaces

1 INTRODUCTIONS

Several new national strategies points out the importance of bioenergy in the future energy supply for Norway, including KLIMAKUR 20201 and Strategy for increased expansion of bioenergy - 20082. The most important one is the latter, launched recently by the Ministry of Petroleum and Energy. This strategy states that Norway shall double the bioenergy production from 14 to 28 TWh by 2020 and that the major single contributor of new bioenergy production shall come from bioenergy use in small-scale heating appliances 1 http://www.klif.no/Aktuelt/Nyheter/2010/Februar/Klimakur2020-viser-hvordan-Norge-kan-redusere-utslippene/ 2 http://www.regjeringen.no/nb/dep/oed/dok/rapporter/2008/ strategi-for-okt-utbygging-av-bioenergi.html?id=505401

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for space heating, meaning in practise the use of wood log combustion in wood stoves and fireplaces (~50% or 7 TWh). Wood log combustion has long traditions in Norway and currently constitutes as much as 50% of the current use of biomass for energy purposes. A doubling of the use of wood log combustion will be a huge undertaking to implement. Increasingly higher electricity prices, extremely cold winters at times and an increasing awareness among the people of Norway to use renewable energy can to a certain extent encourage people to use wood log combustion more actively than today. However, this will not be enough and new solutions and technologies that will enable a more widespread use of wood log combustion are clearly necessary to reach these ambitious targets. New

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Figure 1: Towards stable heat release in wood stoves and fireplaces

Figure 2: Expected expansion of wood log heating season in Norway due to introduction of low load wood stoves and fireplaces with a long burning time

houses as well as retrofit/upgrading of old houses have increasingly focussed on improved insulation. This will set new demands on the heat source, which should be able to deliver a stable effect down to as low as 1 kW. Norwegian houses are mainly wooden houses, meaning two important things: • They can not support large heat accumulation stoves (due to weight restrictions on the wooden floors) • The building itself will not be able to accumulate much heat compared to a concrete/brick building • This means that we need to develop new solutions and technologies that can: - give a stable heat release and distribution - be able to deliver a heat release as low as 1 kW

Figure 1 shows three different scenarios. The horizontal flat curve can be considered to be an ideal curve for stable heat release from wood stoves and fireplaces, but it is not possible to obtain in practice. The green curve (Current technology) shows the current situation for heat release during one batch, while the orange curve (New technology) shows the target for the StableWood project. Through new solutions that will combine heat production, storage and distribution in an optimum way, it will be possible to achieve a substantially more stable heat release and distribution in wooden houses than the current solutions and technologies can offer, thus enabling Norway to reach its ambitious targets. These new solutions and technologies will open up for a groundbreaking shift in the wood log heating season

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in Norway and will give two major results, as illustrated in Figure 2: • It will enable a substantially longer wood log heating season due to solutions that can operate at low effect early and late in the heating season • It will give lower peak effect per combustion unit due to implementation into new and better insulated buildings with a lower heating demand, but higher overall peak effect due to a doubling of the total wood log use Using wood logs is important as a part of security of supply in Norway where we today rely to a very large extent on the electricity grid to be able to deliver the needed heating for our houses. Today 78% of the houses are heated by electricity and 15-20% by wood logs. Norway has only 1-2% heating of houses by district heating and nearly no heating by gas. This makes Norway quite special compared with other European countries. Hence, low load wood stoves and fireplaces in new buildings demands new technologies and solutions with an increased focus on the combustion process and its control, the combustion quality and optimum design to ensure low emissions and high energy efficiency. To minimize the negative effects of the batch combustion process (compared to a continuous combustion process) of wood logs in wood stoves and fireplaces, especially if low load and long burning time is the aim, a more stable heat generation and heat release is needed. The proposed project focus on new solutions and technologies for heating of buildings with low heating demand and specifically batch combustion of wood and the increased need for stable heat release and distribution from wood log combustion in wood stoves and closed fireplaces.

3 OBJECTIVES

The overall objective of this project is development of new strategies for improved heat production, storage and distribution from wood stoves and fireplaces through: • Improved combustion control by increased understanding of the batch combustion process • New heat storage solutions • New heat distribution solutions Sub-objectives: • Improved heat production concepts through improved combustion control (by increased understanding of the batch combustion process) • New or improved heat storage concepts by optimum material location and choice, including phase transition and change options, and through room integration • New or improved heat distribution concepts through optimum passive and active methods and through building integration

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• Education of highly skilled candidates within this area and training of industry partners • Monitoring of activities and state-of-the-art within this area and dissemination of knowledge to the industry partners, and other interested parties where applicable

4 FRONTIERS OF KNOWLEDGE AND TECHNOLOGY

Poor technology and/or operation results in excessive and harmful emission of various compounds3,4. The main reason for this is the batch combustion principle utilized in these rather small combustion units, with usually the aim of direct space heating in a single room. The combustion quality in wood stoves and fireplaces depends on a number of parameters5,6. Fuel properties, both physical and chemical properties, influence the batch combustion process heavily. Physical properties such as fuel size/shape, density and moisture content7, and chemical properties such as the fuel and ash elemental composition are all important for the energetic efficiency and the combustion quality5. The combustion process ongoing in a batch combustion unit is very different from continuously fed combustion units. In a batch combustion process the properties of the part of the fuel burning at any time will change, from moist virgin wood in the beginning to charcoal in the end of the batch combustion process. Being able to model the fuel composition throughout the combustion cycle will the transient greatly enhance the possibility to predict the effects of the fuel composition and variations in this on heat generation and the combustion quality5,8,9. The process conditions influence heavily on the combustion process and influence both the transient and average amount of heat generated (per time unit, kW) in the combustion process5,8,9. Design determines to some extent the process conditions and influences the combustion process. However, design also influences the heat storage and distribution abilities of the combustion unit5,8,9. Heat generation versus the heat release profile of the combustion units is heavily influencing the ability of the combustion unit to 3

van Loo S and Koppejan J (editors). IEA Task 32 Handbook Biomass Combustion and Cofiring. Second Edition, 2008, Earthscan, UK, ISBN: 978-1-84407-249-1. 4 Edvard Karlsvik and Heikki Oravainen. Intelligent Energy Europe, Quality Wood project – EIE/06/178/SI2.444403, Guidebook Effective and environmentally friendly firing of firewood. 5 Skreiberg Ø and Obernberger I. IEA Task 32 Handbook Biomass Combustion and Cofiring - Chapter 2. 6 Edvard Karlsvik. Intelligent Energy Europe, Quality Wood project – EIE/06/178/SI2.444403, Summary report of the workshop, “Current firewood firing technology” at SINTEF Energy Research, Trondheim, 13. Oct. 2008. 7 Saastamoinen, JJ, Impola, RK. Drying of solid fuel particles in hot gases. Drying Technology 13 (5-7), 1995, pp. 1305-1315. 8 Skreiberg Ø. Theoretical and experimental studies on emissions from wood combustion, PhD thesis, NTNU, 1997.

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achieve a stable heat release to a room. The heat release profile can to some extent be influenced and controlled by influencing the heat storage and heat distribution capabilities of the combustion unit5,8,9. To achieve the ultimate goal of stable heat release to a room, the batch combustion process needs to fit the process conditions applied and the design of the specific combustion unit. This requires a combination of deriving of fundamental knowledge, modelling and experimental work. Batch combustion models for wood logs were developed by Skreiberg9, who also carried out batch combustion modelling in wood stoves and fireplaces. These models and modelling approaches must be refined and expanded for the intended use in the proposed project. Detailed models for the thermal degradation and combustion of wood also exist10,11, but are too detailed/specific for the intended use in the proposed project. CFD modelling of wood stoves and fireplaces has been carried out by12,13, for stationary combustion conditions, and with considerable simplifications. CFD modelling gives opportunities with respect to design optimization as well as verification of experimental results. However, the quality of the input to the CFD modelling is crucial for the validity of the modelling results. Efficiencies in, and emissions from, wood stoves and fireplaces are greatly varying, for many reasons3,8,14,15,16,17. In addition to the ultimate goal of stable heat release, the goal is also to achieve a highest possible overall energetic efficiency18. Today the heat 9 Skreiberg Ø. PostDoc report - Part 3, Fuelsim-Transient: A mass, volume and energy balance spreadsheet for batch combustion applications, NTNU, Rapport ITEV 2002:03. 10 Grønli MG. A theoreticaland experimental study of the thermal degradation of biomass. PhD thesis, NTNU, 1996. 11 Porteiro J, Granada E, Collazo J, Patiño D and Morán JC. A Model for the Comb. of Large Partic. of Densif. Wood. Energy & Fuels 2007, 21, 3151–3159. 12 Huttunen M, Saastamoinen J, Kilpinen P, Kjäldman L, Oravainen H, Boström S. Emission formation during wood log combustion in fireplaces - Part I: Volatile combustion stage. Progress in Computational Fluid Dynamics 6 (4-5), 2006, pp. 200-208. 13 Saastamoinen J, Huttunen M, Kilpinen P, Kjäldman L, Oravainen H, Boström S. Emission formation during wood log combustion in fireplaces - Part II: Char combustion stage. Progress in Computational Fluid Dynamics 6 (4-5), 2006, pp. 209-216. 14 Skreiberg Ø, Karlsvik E, Hustad JE, Sønju OK. Round robin test of a wood stove: The influence of standards, test procedures and calculation procedures on the emission level. Biomass and Bioenergy 12 (6), 1997, pp. 439-452. 15 Gupta S, Saksena S, Shankar VR and Joshi V. Emission factors and thermal efficiencies of Cooking biofuels from five countries. Biomass and Bioenergy Vol. 14, Nos. 5/6, pp. 547-559, 1998. 16 Allen RW, Leckie S, Millar G, Brauer M. The impact of wood stove technology upgrades on indoor residential air quality. Atmospheric Environment 43 (2009) 5908–5915. 17 Johansson LS, Leckner B, Gustavsson L, Cooper D, Tullin, C Potter A. Emission characteristics of modern and old-type residential boilers fired with wood logs and wood pellets. Atmospheric Environment 38 (2004) 4183–4195. 18 MacCarty N, Still D, Ogle D. Fuel use and emissions

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storage is limited in wood stoves and fireplaces, and therefore far from stable heat release profiles results. The choice of materials typically used in the combustion unit is few. Dynamic modelling is needed, as done for e.g. heat storing stoves19. In addition to material choice and localization material phase transition and phase change properties20 can potentially be utilized in composite walls in the combustion unit. Experience from ZEB (Zero Emission Buildings, http://www. sintef.no/Projectweb/ZEB/) will be discussed and taken into consideration during this work. The aim is to store heat when the heat generation is high and release heat when it is low. A large number of such materials are available in any required temperature range from -5 up to 190°C21. Heat distribution options include different possibilities to transfer and distribute the heat generated in the combustion process to the room. Both passive and/or active22 heat distribution can be utilized. Neither heavy heat storing stoves nor boilers are very relevant options in Norway, due to typically wooden houses and a lack of water based heating systems.

4.1. Research tasks

The research tasks are organised within 5 subprojects (SPs), each with several work packages (WPs), as shown in Figure 3, while project links and information flow are shown in Figure 4. Subproject 1 (SP1) will address experimental and detailed modelling issues, while subproject 2 (SP2) and 3 (SP3) will focus on heat storage and heat distribution solutions and technologies, respectively. Subproject 4 (SP4) addresses education and training, while subproject 5 (SP5) addresses technology monitoring and dissemination. StableWood will have an interface with FME CenBio and BIP VILL (see Figure 4), i.e. adding complementary synergetic values through coordinated research efforts, without direct overlapping research activities.

SP1: Heat production

The major objectives of SP1 are to identify and quantify the fuel properties of relevance for batch combustion processes of wood and their environmental performance of fifty cooking stoves in the laboratory and related benchmarks of performance. Energy for Sustainable Development. In press. 19 Saastamoinen J, Tuomaala P, Paloposki T, Klobut K. Simplified dynamic model for heat input and output of heat storing stoves. Applied Thermal Engineering 25 (17-18), pp. 2878-2890. 20 A phase change material (PCM) is a substance with a high heat of fusion which, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa; thus, PCMs are classified as latent heat storage units. 21 M. Kenisarin and K. Mahkamov. Renewable & Sustainable Energy Reviews 11 (2007) 1913-1965. 22 Messerer A, Schmatloch V, Pöschl U, Niessner R. Combined particle emission reduction and heat recovery from combustion exhaust A novel approach for small wood-fired appliances. Biomass and Bioenergy 31 (2007) 512–521.

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Board and Project  Management

SP1: Heat  production

SP2: Heat  storage

SP3: Heat  distribution

SP4: Education  and training

SP5: Technology  monitoring and  dissemination

WP1.1 Fuel properties

WP2.1 Heat storage through  phase transition and  phase change 

WP3.1 Passive heat  distribution solutions

WP4.1 Education

WP5.1 Technology monitoring

WP1.2 Batch combustion  models

WP2.2 Heat storage through  material choice and  location

WP3.2 Active heat  distribution solutions

WP4.2 Industrial seminars

WP5.2 Dissemination

WP1.3 Batch combustion  process and CFD  modelling

WP2.3 Heat storage through  room integration

WP3.3 Heat distribution  through building  integration

WP1.4 Experiments and  emissions

Figure 3: Management and work break down structure of the project performance, derive batch combustion models, model the batch combustion process and thereby support the development of new solutions, technologies and strategies with high energy efficiency and low emissions.

objective is also to increase the competence level in the industry. The long-term goal is competence building and strengthening of the education within combustion of wood in wood stoves and fireplaces.

SP2: Heat storage

SP5: Technology monitoring and dissemination

The major objectives of SP2 are to develop new solutions, technologies and strategies for increased energy efficiency and reduced transient effects through the use of different heat storage concepts, involving both the combustion unit itself and room integration possibilities.

SP3: Heat distribution

The major objectives of SP3 are to develop new solutions, technologies and strategies for increased energy efficiency and reduced transient effects through the use of different heat distribution solutions, involving both the combustion unit itself through passive and active heat distribution solutions and building integration possibilities.

SP4: Education and training

The major objective of SP4 is to strengthen the education within this field through MSc and PhD students. The

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The major objectives of SP5 are to monitor the latest research and technological developments and to disseminate research results.

5 RESEARCH APPROACH, METHODS

The project includes experimental, computational and theoretical work. Initially, state-of-the-art reviews will be produced to establish an explicit science and technology platform for the project, as outlined in the WP descriptions. On this basis, and based on input from the industrial partners of the project, a framework for the experimental activity of the project will be developed. Statistical experimental planning methods will be used in order to obtain maximum useful information from the planned experiments. Literature studies will form a major part of the basis

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for the computational and theoretical activities. The computational models developed on this basis will be stationary state models and transient models. The basic stationary state models will include mass- and energy balances and thermodynamic equilibrium. These models will be extended with transient models developed in this project including drying, reactivity and global kinetics, and also sizing and costing correlations, of which most are expected to be available from experience and existing chemical and thermal engineering literature23. Experimental campaigns (wood stoves and fireplaces) based on theoretical studies and bench-scale and laboratory experiments and use of conventional and advanced measurements equipment 23

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(e.g. FTIR, impactor (ELPI)) will be performed. Equipment and infrastructure includes a wood stove testing stand with instrumentation, two continuousfeed multi-fuel reactors, and advanced measuring equipments (FTIR, GC, ELPI, SEM-EDX, TGA, DSC, ICP-OES and MS). The project has or will acquire access to the hardware and software resources required to carry out the theoretical and computational tasks in a timely manner (batch combustion modelling software (Fuelsim), CFD tools (ANSYS, Fluent, Spider), process simulators (ProII, Hysim), generic modelling and optimisation tools (MatLab and NLPQL), a network of high-capacity workstations (6.8 TeraFlops) and access to NTNU / SINTEF’s supercomputers).

And embedded in special purpose software

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