Elsevier

Biomass and Bioenergy

Volume 27, Issue 6, December 2004, Pages 621-628
Biomass and Bioenergy

Industrial processes for biomass drying and their effects on the quality properties of wood pellets

https://doi.org/10.1016/j.biombioe.2003.08.019Get rights and content

Abstract

This paper contributes to the discussion of how different kinds of industrial scale dryers for biomass influence the quality properties of wood pellets. It also discusses how the drying technique can affect the environment. The most common biomass drying processes in use, i.e., convection dryers are discussed. The discussion of drying techniques is based on advantages and disadvantages with a focus on the drying medium, temperature and residence time. The choice of drying technique is particularly important if the end-user’s choice of pellets is made due to the specific requirements for the heating system used. Some specific parameters were tested in order to investigate how the choice of drying technique affects the pellet quality. The parameters tested were moisture content and the emissions of volatile hydrocarbons. Pellets available on the market were chosen for the tests. The amount of volatile hydrocarbons left in sawdust after drying vary with drying technique, as emissions of terpenes are larger in dryers with long residence times. Low emissions of volatile hydrocarbons would improve the energy content of the sawdust, and by decreasing air pollution improve the work environment and the environment in the surroundings of the dryers.

Introduction

The high taxes on emissions of carbon dioxide from fossil fuels and the high price of fuel oil has made the residential heating market demand for pellets in Sweden grow very rapidly the last few years, and last year especially. In March this year (2002) the environmental ambassadors in the European Union accepted the commission’s proposal to approve the Kyoto Protocol. That decision implies that the European Union pledges to decrease the emissions of greenhouse gases by 8% until 2010 in comparison with the level of emissions in 1990. This could increase the pellet market in Sweden even further [1]. From the year 2000 to 2001 the pellets burner market for the Swedish domestic market has gone from 5000 burners sold to almost 15 000 burners sold [2]. The small-scale customers used about 250 000 tonnes of pellets in 2001 and that market is still growing [2]. Last year 22 pellet factories in Sweden produced about 715 000 tonnes of pellets although the production capacity is more than 1 million tonnes a year [3]. Sweden also imported about 180 000 tonnes of pellets and exported about 60 000 tonnes of pellets [4].

Cutter shavings and dry sawdust are preferred materials for production of wood pellets. Today most pellets are made from sawdust or shavings and those materials are used to the maximum extent. As the demand for pellets increases the supply of dry sawdust will be insufficient and other raw materials will be used. At first more wet sawdust will be dried and used and if the use of pellets continues to grow the raw materials bark, branches and crowns (grot in Swedish), stem wood and recycled fuel should come into use in that specific order [5]. The drying of biomass has increased during the last decades and with a growing pellet market it will increase further. The wet raw material used today for pellet production contains about 50–55% water. For production of pellets the Swedish manufacturers normally dry the raw material to water content 8–12% before the pelleting process begins [6].

Drying of wood-based biofuels is important, since wet wood results in low combustion temperatures, low energy efficiency and high emissions of hydrocarbons and particles compared to for example pellets. If biofuels are dried and compressed to pellets or briquettes, the fuel will have controlled moisture content (MC), have a higher energy density and be easier to transport. It will also take up less room and be less susceptible to mould and insect attacks during storage. The shape of the pellet, diameter 6–8 mm and a length of 4 or 5 times the diameter according to the Swedish standard [7] also make it easier to use in fully automatic operation. Pellets have an additional advantage in being a standardised fuel, simplifying construction and operation of burners. It is important to the pellet producers to have quality standardisation. It increases the customer’s confidence since the customer wants to know what product he or she buys [8]. This is particularly important when dealing with home heating customers, as they are not as tolerant as large-scale users for differences in pellet quality. Small-scale users now demand a high and even quality of pellets. Therefore, it is very important to have good control over drying parameters like temperature and residence time.

Dryers can be classified according to the medium used in the drying process. In Sweden flue gas dryers and superheated steam dryers are used for commercial drying of sawdust. Rotary-drum dryers using combustion gases as the heating medium are the most common technique for drying sawdust in Sweden. Dryers investigated in this paper work as convection dryers where a gas both supplies the necessary energy and transports away the emitted steam. According to Mujumdar [9] the superheated steam dryers have some key advantages compared to air dryers. No oxidation or combustion reactions are possible. Steam dryers have higher drying rates than air and gas dryers. Steam drying also avoids the danger of fire or explosions and allows toxic or valuable liquids to be separated in condensers. However, the systems are more complex and even a small steam leakage is devastating to the energy efficiency of the steam dryer [10].

In a steam drying process, Johansson et al. [11] investigated the temperature in a small wood piece dried in superheated steam. They found that the temperature of the material stays at the boiling temperature until the MC decreases to the critical level at 10% (water wet basis, wb). Below this the material will gradually approach steam temperature. In flue gas dryers the dried material is heated to the wet bulb temperature until the MC reaches the critical MC, below this the material temperature will approach the gas temperature. Flue gas rotary dryers combine high inlet temperature with long residence time. According to Wimmerstedt et al. [12] this can result in pyrolysis and partial gasification. In lumber drying a kiln uses heated air as the drying medium. According to Rosen [13] the conventional inlet temperature is 60–82 °C. The temperature of the wood is near the wet temperature.

In the presence of nitrogen oxides and sunlight the volatile organic compounds (VOC) emitted to air during drying contribute to the formation of harmful photo-oxidants. Elevated levels of photo-oxidants are an important cause of widespread forest and crop damage in Europe. Photo-oxidants are also harmful to humans, as they cause irritation in the respiratory tract and in sensitive parts of the lungs. Monoterpenes (C5H8)2 are the most volatile group of the components present in wood, and is of particular interest in studies of emissions during drying. Due to the short atmospheric lifespan of monoterpenes, the highest photo-oxidant concentrations can be expected within 5 h after the emission takes place, and within a distance of 50 km [14].

When emissions during drying of wood fuel are considered in relation to the type of dryer used, the parameters residence time, temperature and drying medium are most significant. The release of VOC (g/s kg oven dried weight (odw)) is rapid early in the drying process, with a small second emission peak at about 10% MC [15], [16]. The total amount of emitted VOC (g/kg odw) from wood will increase rapidly at the beginning of the drying process, and then level out [16]. Increased drying medium temperature increases the total amount of emitted VOC (g/kg odw) [16], [17], [18]. The MC of the drying medium does not affect the reduction of terpene content in sawdust during drying [16].

In the Swedish Standard Institutes test of pellet quality the dimensions, MC, density, bulk density, ash content, heating value, durability, sulphur and chlorine content and additives are specified. In Lehtikangas [19] paper all the pellet quality parameters according to the Swedish standard [7] are tested. The purpose of this paper is to discuss the drying media, the residence time and the temperature used in the drying processes. Furthermore, the aim is to discuss how those parameters influence the MC and the heating value with a special focus on the emissions of VOC from the dryers but also from the pellet production.

Section snippets

Test materials

Samples from six different producers (A–F) were tested. Three pellet producers supplied samples of wet and dry sawdust as well as pellets. All the pellet assortments had 8 mm diameter. Samples of sawdust before and after drying in a laboratory steam dryer were also obtained. From the pellets producers using raw material from lumber dried wood (E and F), dried material and pellets were examined from E and pellets from F (Table 1).

Four producers uses sawdust as raw material, one uses dried sawdust

Moisture content

The raw material had MC between 41.2 (D) and 53.5% (B) (Table 3). After drying the material MC between 5.9 (B) and 11.7% (A) are reached and the pellet has MC between 6.0 (E) and 7.8% (D). According to the Swedish standard SS 187120, the MC of pellets should be less then 10% or less than 12% depending on which standard group it belongs to [7].

Terpene content

Material A contains much more terpenes than the other materials (Table 4). The terpene content in pellets is more than 100 mg/kg odw for A and F, somewhat

Discussion

The difference in emissions between the dryers investigated is believed to be due to different residence times. As the terpene release rate from wood particles is large at the beginning of the drying process and then levels out [16], most of the terpenes will be emitted during the first minutes of the drying process and the rest diffuse out of the particle over time. The residence time is shortest for the spouted bed dryer (B), longer for the flash dryer (A), and longest for the rotary drum

Acknowledgements

We thank our supervisor Professor Bengt Månsson for valuable discussions and comments. We are most grateful to the suppliers for delivering the test materials.

References (23)

  • A. Johansson et al.

    High temperature convective drying of wood chips with air and superheated steam

    International Journal for Heat and Mass Transfer

    (1997)
  • P. Lehtikangas

    Quality properties of pelletised sawdust, logging residues and bark

    Biomass & Bioenergy

    (2001)
  • Bioenerginytt 3 (Bioenergy News 3). Swedish Bioenergy Association (SVEBIO),...
  • Östergren A. (Swedish Heating Boilers and Burners Association, SBBA), Installationer av villabrännare slår record....
  • Lagergren F. SVEBIO, Personal Contact...
  • Fernström I. (Swedish Pellets Producers Association, PIR), Hur skall pelletsproducenterna möta den ökande efterfrågan?...
  • Hillring B. (Swedish University of Agricultural Sciences), Råvarutillgångar för pelletstillverkningen. (Raw material...
  • Olsson M. Träpellets som småskaligt biobränsle. Report Chalmers University of Technology, Göteborg and STEM,...
  • Swedish Standard. Biobränslen och torv—Bränslepellets—Klassificering (Biofuels and peat—Fuel pellets—Classification)....
  • OPET Sweden 2001. Refined biomass—a source for climate change and business opportunities,...
  • A.S. Mujumdar

    Superheated steam drying

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