World Student Community for Sustainable Development

by Maria Olsson

The increased global warming necessitates exchanging fossil fuels for biofuels. Wood pellets are a relatively new and increasingly popular biomass fuel. This fuel can both be used in residences and as an inexpensive and convenient alternative to fossil fuels in large-scale heat and power plants. However, in many forested countries like Sweden, the residential use of wood pellets is today increasing rapidly, and pellets are thus replacing more and more fossil oil and firewood. Today, the annual small-scale use of wood pellets amounts to almost 250 000 tonnes.

Wood pellets are made by compressed sawdust and shavings, that are by-products from sawmills and joinery mills. Fuel pellets are made in more than 20 production plants producing almost one million tonnes annually. Softwood pellets can be burnt in residences in burners, boilers or stoves with low emissions to air. This has been studied both in laboratory burnings and in chimney samples from stoves and burners.

As the demand for pellets increases other biomass wastes than wood chips and shavings will be considered as raw materials. Examples of these may be biofuels as logging residues, energy crops, agricultural waste, and the organic fraction of household waste. However, these new fuels might be chemically complex with pollutants and higher amounts of mineral nutrients. An incomplete combustion of these pellets could lead to uncontrolled emissions of environmental and health hazardous compounds. Therefore, the emissions and burning characteristics of pellets made of new raw materials must be studied before they possibly are introduced on the residential market. Pellets with high emissions to air will have to be burnt in large-scale combustion plants with advanced flue-gas purification.

The increased use is supported by environmental and ecotoxicological arguments in favour of reserving pellets of pure trunk wood for residential use. Pellets of other biomass and waste should be burnt in large-scale combustion plants with flue-gas purification.

1. Introduction

Sweden is one of the leading biofuel users in the European Union with biomass accounting for 16% of the primary energy supply [1]. The largest users of biofuel are the forest industry and the district heating sector. However, the small-scale use of biofuels in Sweden has a long history and a market for wood pellets has developed rapidly during the past few years. This article gives information about the production and use of wood pellets in Sweden (Section 2).

Wood pellets are an environmentally well adapted biomass fuel that is increasingly used as residential biofuel in Sweden. Pellets have replaced mainly petroleum oil and firewood. Due to the increased residential installation of pellet burners, stoves and boilers, it is important to study the emissions of different compounds. The environmental aspects and health impacts of the emissions have to be evaluated and compared to emissions from the fuel previously used in the residences. When considering the effects on health and the environment, it is very important to study not only the total emissions, but also the composition of different compounds in the smoke. A previously overlooked aspect of the emissions has been the organic compounds. They are all emitted in varying amounts from different kinds of combustion and have different effects and properties. A short overview of the results from the previous smoke studies is given (Section 3).

As the interest for pellet burning increases there will be a need for new raw materials. These biomass fuels have to be studied thoroughly before they might be introduced on the residential market in order to avoid unnecessary environmental impact. The possibilities for an environmentally sustainable use of wood pellets have been studied in relation to other biofuels (Section 4).

2. Production and use of wood pellets

Wood pellets are made by compressed sawdust and shavings from the wood industry in many forested countries like Sweden. Pellets are a processed biofuel, which offers many advantages compared to firewood and fossil oil. Pellets are therefore rapidly replacing petroleum oil and firewood for residential heating in burners, boilers and stoves in Sweden and other countries.

2.1. Wood pellets compared to other fuels

Processing of biomass fuels may give rise to many advantages, for example relatively low costs, low moisture content and convenient use and transport. Pellet fuel is the most suitable reformulated biofuel for small-scale use, as it is the easiest fuel to transport and feed into burners.

The properties of wood pellets differ a great deal from the properties of firewood. This is mainly due to the homogeneity and low moisture content of wood pellets, which is achieved by drying and pressing the sawdust and shavings. The high moisture content of unprocessed biomass is normally a drawback, leading to high transport costs and microbial degeneration during handling and storage. The variations in moisture content also give rise to an uneven combustion with unnecessary emissions and lower efficiency. Drying and compressing biofuels to pellets give a higher energy density and better combustion properties. The storage properties of wood pellets with low moisture content are favourable [2]. Pellets are also more convenient to transport [3]. The processing costs are offset by lower transport and storage costs due to the higher energy density. Another advantage of pellets compared to unprocessed biomass fuels is convenience to the customer. Pellets can be automatically fed into the burner, does not need splitting and have a lower ash content. There are, however, some disadvantages, as pellets are more expensive and the fine fraction may disturb the combustion and the feed of the pellets into the burner.

Pellets also have several advantages compared to fossil oil. Wood pellet fuel is a renewable fuel that does not provide any net contribution to the greenhouse effect. The burning of 5 m3 of oil releases 13.8 tonnes of carbon dioxide into the atmosphere. Thus, the change from fossil fuels to biofuels is a significant way to decrease the Swedish carbon dioxide emissions [4]. One way to encourage this change is through a carbon dioxide tax [5]. Furthermore, wood pellet fuel is cheaper than petroleum oil and electricity and has a more stable price development. No major increase is expected in the price of wood fuels in Sweden, since both the domestic supply and the possibilities for importing biofuels are large [6]. The price for oil and electricity has lately increased significantly. However, it is difficult to compare the environmental impact of a change from electricity to pellets as it depends on how the saved electricity is produced. As pellets are a native fuel, they provide employment opportunities in Sweden. Some drawbacks compared to petroleum oil are the need for larger storage facilities, regular control and removal of ashes.

2.2. Pellets in Sweden

At present, there are more than 20 production sites for wood pellets in Sweden, with an annual total production of almost one million tonnes. The production and use of fuel pellets has been investigated [7]. In Figure 1 all major producers are shown with their pellet production in 2003 in thousands of tonnes [8].

Figure 1: Production of wood pellets in Sweden (in thousands of tonnes) [8].

Swedish wood pellet production began at the beginning of the 1980s with an annual production of 45 000 tonnes [9] and has increased dramatically ever since (Figure 2).

Figure 2: Total Swedish wood pellet production and residential use in tonnes [9,10].

The total Swedish production of wood pellets would be sufficient to heat 300 000 homes, but the major proportion is still burnt in large-scale heating plants. However, approximately 30 percent is used for residential heating. According to the latest information from pellet producers, residential use amounts to almost 250 000 tonnes annually, for use in about 37 000 residences [11] (Figure 2). There are great possibilities for further increase in the shift to wood pellets [6].

Most of the residential heating appliances for pellets have replaced wood stoves. The reasons for the change from firewood to pellets are convenience, mainly due to the automatic feed, and the fact that pellet fuel is a cheap alternative. Today, the number of oil burners being replaced by pellet burners is increasing. This is due to pellets being an environmentally friendly alternative and oil being relatively expensive. The price of pellets for a small-scale customer is approximately 0.40 SEK/kWh.

Sweden is not the only country where the production and use of wood pellet fuel is significant. Pellets have been used for residential heating for 15 years in the US, and there are more than 70 producers [12]. Unlike the Swedish market, the American market has always been dominated by the use of pellets in stoves. This is because the residential heating using electricity or oil is convenient and rather cheap. The pellet stove is used as a complement during the winter. In Europe there is significant production and use in Denmark, Austria, Italy, the Netherlands and other countries. Pellets are imported to Sweden from Canada, the Baltic Countries and Finland and exported to Denmark [1]

2.3. Production of pellets

Pellets are at present produced from softwood sawdust and shavings at more than 20 production sites all over Sweden [9]. In order to produce pellets as economically as possible and in the most environmentally friendly way, the location of the production site is very important. When choosing where to produce pellets, access to cheap raw material, preferably close to a saw mill, and already existing fodder pellets production plant are factors often considered. Furthermore, the distance to customers and the possibilities to use surplus energy are important [13].

Figure 3: Example of flow sheet for the production of pellets from sawdust and shavings [9].

Figure 3 shows a flow sheet for pellet production in Sweden. The raw materials must be correctly stored to avoid mould growth and anaerobic decomposition with methane emissions [14]. The first step in the production is the drying of moist sawdust from 50-55 percent moisture content to 8-12 percent, which usually requires a lot of energy. It also leads to emissions of terpenes and other hydrocarbons [15,16]. Dry shavings are mixed with the dried sawdust. The mixture is ground and a binding agent is sometimes added. The use of binding agent is not necessary, but may give more stable pellets compared to the binding by the natural wood lignin. The binding agent may be Wafolin S or potato starch. Wafolin S contains hydrocarbons and calcium lignosulphonathe, which gives an unwanted increase in the formation of sulphur dioxide during combustion. Potato starch is a better alternative as it does not contain sulphur and is easier to handle.

After the grinding, the raw material is pressed through cylindrical holes in a pellet matrix (Figure 4). The friction gives a temperature rise to above 100°C. The pellet strings are cut off with a knife or automatically broken. The pellets are left to cool before sieving, storing and transporting to the customer.

Figure 4: Pellet production matrix.

2.4. Residential burning of pellets

Pellets can be burnt in residences in pellet burners, stoves and boilers. An old wood or oil boiler can easily be converted to pellet burning by installing a pellet burner or exchanged for a pellet boiler. A pellet stove is a suitable heating addition to for example electrical heating, district heating or a heat pump. Most pellets used in Swedish residences are burnt in burners installed in old wood or oil boilers.

3. Environmental aspects on the emissions from wood pellets

Even though pellet fuel is an environmentally friendly residential fuel with low emissions to air, it is of great importance to examine the emissions from the burning of wood pellets, both in order to get a view of the different compounds in the smoke and their relative amounts and to reach an understanding of how to minimise the emissions of health and environmentally hazardous compounds in the smoke. The organic content of the emissions from the combustion of wood pellets was therefore analysed.

3.1. Smoke analyses

Smoke from flaming and glowing pellets [17] was studied in laboratory experiments. Pellets from three different manufacturers were burnt in order to analyse the organic fraction of the flue gases with GC-MS. The experiments were designed to simulate incomplete combustion in a pellet burner or pellet stove.

Furthermore, chimney samples were taken from residential heating appliances [18]. The smoke was sampled in the chimney outlet during normal operation of the burning appliance. The organic content of the flue gases was analysed with GC-MS at the same time as other smoke characteristics were studied.

Volatile hydrocarbons in smoke from different combustion stages of softwood pellets were more thoroughly analysed in laboratory studies [19]. The different stages studied were initial smouldering, early flaming, late flaming, after-flame smouldering and final glowing.

3.2. Results

The emissions from residential burning of wood pellets are generally low and pellets are environmentally well suited to replace petroleum oil and firewood. Even though the emissions are low, they should be measured, examined and, when possible, also decreased. During temporary disturbance and start-up, the emissions are larger, but have a different chemical composition. It is therefore important to acquire knowledge of the emissions from different types of combustion.

Softwood pellets from different Swedish manufacturers were burnt in laboratory experiments, and the organic components of the smoke were analysed. It was clear that the combustion conditions greatly affected the content of the smoke, but also that different pellet brands gave rise to similar emissions. Flaming incomplete laboratory burning of wood pellets emitted significant amounts of 2-methoxyphenol antioxidants, whereas glowing burning emitted the carcinogenic benzene and polycyclic aromatic hydrocarbons in low concentrations [17]. The occurrence of phenolic antioxidants in the smoke is of great interest due to their ability to counteract the effect of free radicals.

Further laboratory studies were made on the volatile hydrocarbons emitted from different combustion stages of softwood pellets [19]. Early and late flaming burning was very efficient with low emissions. The emissions from final glowing were even lower. Smouldering burning emitted larger amounts of volatile hydrocarbons.

Chimney emissions were studied for residential burners and stoves. The smoke samples were collected in the chimney outlet during normal operation of the burning appliance. The burning of softwood pellets in residential pellet stoves gave rise to emissions of 2-methoxyphenol antioxidants together with aromatic hydrocarbons (Figure 5) [18]. The burning of wood pellets in residential pellet burners was more complete than the combustion in a pellet stove, but emitted benzene and other aromatic hydrocarbons in small amounts. The results emphasise the importance of measuring not only the total amount of emitted organic carbon (OGC), but also the specific organic substances emitted during the combustion phases. A large difference between emissions from different residential burning appliances for wood pellets was observed and makes it important to choose one of the best available equipments and to install it correctly.

Figure 5: Relative proportions (%) of aromatic hydrocarbons and antioxidant methoxyphenols (Gu) in smoke from pellet stoves. The first three chimney samples were taken from the same stove on two different occasions. The fourth sample was taken from another stove [9].

4. Ecological aspects on pellets

The ecological and ecotoxicological aspects are immensely important when discussing the potential role of pellets in a sustainable biofuel system. The general conclusion of this discussion is that wood pellets should be reserved for residential heating whereas other, more contaminated, biomass wastes must be burnt in large-scale combustion plants with flue-gas purification [9,18]. These conclusions are illustrated in Figure 6.

Figure 6: Sustainable use of biofuels from different biomass sources with respect to biomass resources and environmental pollution.

In order to discuss pellets in an ecologically and environmentally correct way they should be compared with other biomass fuels, as in this case logging residues and biomass waste.

4.1. Logging residues

Logging residues comprise twigs and foliage that can be collected as biofuels when timber and pulpwood is taken out from the forest. This gives rise to ecological problems in the forest and may lead to increased emissions during the later biomass combustion. In the forest, the outtake of logging residues leads to a loss of nutrients and an increased acidification. Logging residues contain most of the mineral nutrients of the trees, such as calcium, potassium, nitrogen and phosphorous [20]. The nutrients are removed from the forest with the logging residues and the forest becomes less fertile and, in long-term, less productive [21]. The logging residues may also have a long-lasting positive effect on nitrogen mineralization in the soil [22]. On the other hand, removal of felling residues has been found to decrease the leaching of nitrogen in the cleared area and to counteract nitrogen accumulation in the forest ecosystem [23].

The soil becomes more acid as the trees are taking up nutrient ions from the surrounding soil during the growth [20]. Most of these ions are cat ions. Thus, the trees must return H+ ions to the soil to keep the neutral ion balance. This process leads to an increased acidification of the soil. When a tree dies and is mineralised, the nutrition cat ions are returned to the soil, which once again becomes less acid. According to Lundborg, the most critical environmental aspect of whole-tree harvesting is that it leads to an unsustainable base cat ion balance [23].

The mineral nutrients are a problem during the combustion of the biofuels, giving rise to higher emissions of nitrogen oxides as well as more ash. Chopped logging residues can be pelletised with good results [24], but are more suitable for large combustion plants with flue gas purification, due to the high content of mineral nutrients. The ecological disturbances in the forest can possibly be managed by returning the ashes to the forest ecosystem. The ash must not be contaminated and must be returned in a suitable way. Spread, slowly-dissolving, stabilised ashes can compensate for the removal of forest fuels without observed undesirable environmental effects [25]. The circulation of the ashes is not an option with small-scale combustion.

4.2. Biomass waste

Waste consisting of wood, paper and other biomass is a potentially useful biofuel. Even if the material is recycled a few times, it finally ends up as waste. A total amount of almost 10 million tonnes of waste is produced in Sweden yearly [9] and European trade with biomass waste is increasing [26]. Large quantities of waste have previously been deposited on dumps, but new waste regulations and higher deposition taxes will decrease this amount. If the amount of biomass waste burnt will increase, the amount of forest biofuel used can be decreased and thus also the impact on the natural forest ecosystem.

Biomass waste is a complex fuel with hazardous chemical additives and contaminants [27]. It must therefore be handled in large-scale combustion plants with rigorously controlled combustion and advanced flue gas purification.

4.3. Wood pellets

Fuel pellets are made from wood shavings and sawdust. These raw materials are by-products of sawmills and the timber industry and are therefore regarded as waste. An important difference when comparing this sort of waste to other biomass waste is the high purity. Wood pellets are clean and homogeneous, with a low mineral nutrient and moisture content, and are thus a suitable fuel for combustion in small heating devices [24,28].

4.4. Future raw materials for pellets

The increased demand for wood pellets, both for residential and large-scale use, may lead to a future shortage of sawdust and shavings. If the price and demand continues to rise, other sources of biomass waste might be considered for pellets. Examples of such future raw materials are; logging residues, energy crops, farming residues, sewage sludge and the organic fraction of the household waste. For example, studies have been carried out on pellets produced from bark, logging residues [24], reed canary grass [29,30], lignite [31], wheat straw, hay [28], palm fibre and palm nut shell [32]. These new fuels must be thoroughly studied before they can be considered for the residential market. Not only do they have different technical burning characteristics, such as higher ash content, a larger tendency towards sintering and a lower energy content, but they are also more contaminated by pollutants and mineral nutrients. This will give rise to uncontrolled emissions of environmental and health hazardous compounds and may give pellets a bad reputation. It is therefore of great importance to study the emissions of different organic compounds from new pellet fuels before they are burnt in residences.

For example, the nitrogen content of the fuel directly influences the formation of nitrogen oxides [33,34]. Chlorine in the fuel is partly emitted to air and forms HCl and dioxins. Dioxins are formed in reactions on the surface of fly ash particles at temperatures of between 250 and 500°C in the presence of C, Cl and O [34]. Higher amounts of dioxins have been found in smoke from household waste and wood from demolished buildings than from natural wood [35].

Acknowledgement

Generous assistance and information was given by a wide range of Swedish burning appliance companies, wood pellet suppliers and other pellet specialists. Many thanks to Prof. Göran Petersson and Dr. Jennica Kjällstrand at Chemical Environmental Science, Chalmers University of Technology for valuable scientific help.

- Maria Olsson, Chalmers University of Technology, Sweden

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