Research PaperPhysical and chemical properties of pellets from energy crops and cereal straws
Highlights
► Biomass pellets are compared in terms of their physical and chemical properties. ► The new EN 14961 fuel specifications give a much broader spectrum of quality levels. ► Straws chemical content makes them unsuitable for the majority of pellet boilers. ► Developments in boiler design necessary for future alternative biomass utilisation.
Introduction
The use as fuel of pellets made from sawdust and wood shavings has been rapidly increasing throughout Europe. Sweden and Denmark have already virtually exhausted this source. In Ireland, the estimated supply of sawdust is about 200,000 wet tonnes. Grant schemes for domestic and commercial biomass stoves and boilers are likely to stimulate a rapid increase in demand for pellets, which are already being imported from many countries. With several pelleting plant projects completed or at an advanced stage of planning, sawdust supplies in Ireland are likely to become depleted in the near future (Howley, O'Leary, & O'Gallachoir, 2006).
Lignocellulosic biomass (biomass from plants), in its original form usually have a bulk density which can be as low as 30 kg m−3 and a moisture content ranging from 10% to 70% (wet basis (wb)). Pelleting increases the specific density (gravity) of biomass to more than 1000 kg m−3 (Lehtikangas, 2001; Mani, Tabil, & Sokhansanj, 2004). Pelleted biomass is low and uniform in moisture content. It can be handled and stored cheaply and safely using well developed handling systems for grains (Fasina & Sokhansanj, 1996).
Forest and sawmill residues, agricultural crop residues, and energy crops can be densified into pellets. Pellets are cylindrical, 6–8 mm in diameter and 10–12 mm long. In North America, more than 1.2 million t of fuel pellets are produced annually. Most of the US pellets are bagged and marketed for domestic pellet stoves. In Canada, pellets produced from sawdust and wood shavings are exported to European countries – Sweden and Denmark.
Pelleting can upgrade the fuel and facilitates its utilisation in several ways:
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It reduces dust emission in the handling of the fuel
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It improves the flow properties, which simplifies conveying and storage
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It increases the bulk density, which eases storage and transportation
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It leads to a more stable, uniform product with more efficient combustion control
These advantages are likely to stimulate its use in small stoves and boilers, where its enhanced properties will outweigh its extra cost. However, several issues remain to be resolved before this can happen. Firstly, the ability to process various potential feedstocks into durable, stable pellets needs to be established. Secondly, the fuel properties of the pellets and any effects on boiler/stove performance and flue emissions have to be measured. Finally, all the costs of raw material production, assembly and processing, and the effect of plant scale on these costs, are necessary information for any potential investor.
Apart from animal feed, sawmill residues are the main biomass that are pelletized extensively worldwide. Pelleting processes consist of three major unit operations, drying, size reduction (grinding), and densification (pelleting). The biomass is dried to about 10% (wb) in the rotary drum dryer. Superheated steam dryers, flash dryers, spouted bed dryers, and belt dryers are also common in European countries (Stahl, Granstrom, Berghel, & Renstrom, 2004; Thek & Obernberger, 2004).
After drying, a hammer mill equipped with a screen size of 3–6 mm reduces the dried biomass to a particle size suitable for pelleting. The ground biomass is compacted in the press mill to form pellets. The individual pellet density ranges from 1000 to 1200 kg m−3. The bulk density of pellets ranges from 550 to 700 kg m−3 depending on size of pellets. Pellet density and durability are influenced by physical and chemical properties of the feedstock, temperature and applied pressure during the pelleting process (Mani, Tabil, & Sokhansanj, 2003). In some operations, the ground material is treated with superheated steam at temperatures above 100 °C before compaction. The superheated steam increases moisture and temperature of the mash causing the release and activation of the natural binders present in the biomass. Moisture also acts as a binder and lubricator (Robinson, 1984).
In some operations, binders or stabilising agents are used to reduce the pellet springiness and to increase the pellet density and durability. Most widely used binders for pelleting of animal feeds are calcium lignosulphonate, colloids, bentonite, starches, proteins and calcium hydroxide (Pfost, 1964; Tabil & Sokhansanj, 1996). Pfost and Young (1974) reported that there was a significant increase in pellet durability when using colloids and calcium lignosulphonate as additives in the range of 2.6% by weight. Biomass from woody plants contains higher percentages of resins and lignin compared to agricultural crop residues (straw and stover). When lignin-rich biomass is compacted under high pressure and temperature, lignin becomes soft exhibiting thermosetting properties (van Dam, van den Oever, Teunissen, Keijsers, & Peralta, 2004). The softened lignin acts as glue.
The temperature of pellets coming out of the pellet mill ranges from 70 °C to 90 °C. The elevated temperature is due to the frictional heat generated during extrusion and material pre-heating. Pellets are cooled to within 5 °C of the ambient temperature in a cooler. The hardened cooled pellets are then conveyed from the cooler to storage areas using mechanical or pneumatic conveying systems.
Biomass, by its nature is hygroscopic. Stable pellets can be formed with a range of moistures between 8 and 12 % (wb) depending on the biomass in question. Above these moistures, problems with mould formation and pellet degradation can occur. Biomass pellets are very susceptible to moisture uptake from the surrounding environment, so it is very important to ensure that watertight storage facilities are in place. Another factor to consider is that the higher the moisture content, the lower the net calorific value (NCV) of the biofuel pellet presented for combustion.
Bulk density is defined as the mass per unit volume of biomass. It is important in terms of transport and storage. The higher the bulk density, the greater the mass that can be transported or stored in a fixed volume container, thus minimising transport and storage costs. The bulk density is a function of the pellet density and size.
Durability is the main parameter used to describe the physical quality of densified solid biofuels such as pellets and briquettes. Pellets are very susceptible to physical wear and tear which leads to the production of fine particles or dust during transport and storage. Vinterback (2004) discussed how dust can be both a health and fire hazard as well as causing problems in some boiler handling and combustion systems. The European Committee for Standardisation (CEN) standard method CEN/TS 15210 describes physical durability as the ability of the pellet to remain intact when handled; i.e., the ability of the pellet to withstand vibrations and shock. Temmerman, Rabier, Daugbjerg, Hartmann, & Bohm (2006) investigated different methods of durability testing. It was shown that the tumbling method as described by American Society for Agricultural and Biological Engineering (ASABE) standard 269.4 reaches acceptable accuracy levels (1%) with a limited number of replications. As such it was decided that this method would be suitable for use in this experimental procedure.
These are the three main components of any solid biofuel. During combustion C and H are oxidised to form CO2 and H2O. The higher the C and H contents the higher the Gross Calorific Value (GCV), while a high O content decreases the GCV. The C content of wood fuels is reported to be slightly higher than those of herbaceous biomass, which would in theory lead to a slightly higher GCV.
In theory, N contents of agricultural residues is higher than those of woody biomass due to the large amounts of N fertiliser applied during crop growth. During combustion Nitrogen is almost entirely converted to gaseous N2 and oxides of nitrogen (NOx) (Obernberger, Brunner, & Barnthaler, 2006). The amount of N2O and Nitrogen incorporated in the ash tend to be very low in modern biofuel boilers. Obernberger et al. (2006) give a value of 0.6% N above which emission related problems could occur.
The chlorine content of herbaceous biomass tends to be much higher than that of wood. Chlorine contained in biofuels is mostly converted to gaseous HCl, Cl2 or alkali chlorides such as NaCl or KCl during combustion. When the flue gas cools, much of the chloride condenses as salts on the heat exchanger and flue surfaces. These chloride salts can have a very corrosive damaging effect on any metal parts they come in contact with. Obernberger (2003) described a level of O.1% Cl, above which, these damaging effects can occur.
Sulphur behaves very similarly to Cl during combustion with the formation of alkali salts and their subsequent corrosive nature being the main problem. Investigations by Obernberger (1997) showed that 40–70% of the S is incorporated in the ash. The remaining S is emitted in gaseous form as SO2. A value of 0.2% by weight was given by Obernberger et al. (2006) above which sulphur can have a very damaging effect.
GCV is defined as the specific energy of combustion in joules for unit mass of a solid biofuel, burned in oxygen in a calorimetric bomb under the conditions specified. NCV takes into account the moisture content and chemical composition of the biomass. The calorific value is an important value in determining the amount of energy contained in a set volume of biomass.
Ash content of herbaceous biomass tends to be significantly higher than that of wood. Knowledge of ash content is essential for choosing the correct combustion and ash cleaning technologies. For fuels with high ash content underfeed stokers are not suitable due to the risk of ash layer formation, which can lead to irregular air flow, causing incomplete combustion and increased emissions. For these types of fuels, grate, or fluidised bed combustion is more suitable. The type and melting point of ash also has a large effect on combustion. However, this is not considered in this research.
Sweden developed the first European pellet quality standard in 1998. Standards institutes in Austria (Österreichisches Normungsinstitut or Önorm), Germany (Deutsche Institut für Normung or DIN) and Italy (Comitato Termotecnico ItalianoorCTI) subsequently developed national standards while a Code of Good Practice exists in the UK. A European standards technical committee (335) has formulated standards for biomass fuels and a relatively new EN 14961 standard has been formulated (Fig. 1). Most countries rely on this technical standard and have not developed their own individual national standards.
The aim of this research was to show how the pellets from the six different biomass types, compared in terms of physical and chemical characteristics.
Section snippets
Materials and methods
Biomass pellets were produced using a biomass pellet mill (Farm Feed Systems, Cinderford, Gloucestershire, England) located at the Oak Park Research Centre (Fig. 2). The raw materials (miscanthus, rape straw, barley and wheat straw were sourced from the on site research farm) were firstly tested for moisture content and then chopped using a Tomahawk straw chopper (Teagle Machinery Ltd., Truro, Cornwall, England) with a 24 mm sieve. The pelleting process involves movement through a 5 kW hammer
Moisture content
All raw materials entered the pelleting process with <18% MC It was found that wetter material could not be readily milled and blockages were seen to occur in the hammer mill. Moisture contents ranging from 5 to 7% were lost through the pelleting line due to temperature increases at milling and also at the pelleting die.
Table 1(a) shows the stable pellet moisture condition tested after the pellet outlet as seen in Fig. 2. Wood pellets (9.39%) had significantly lower (p < 0.001) moisture content
Conclusion
As can be seen from this research, the biomass pellets investigated displayed a large degree of variability in terms of both their physical and chemical properties. The majority of pellet boilers currently on the market are designed specifically for wood pellets and thus, are not capable of adapting to the differing properties of these alternative pellets. Boilers which can also take miscanthus pellets are becoming more plentiful, while the expense required to build the more robust boilers
Acknowledgements
The authors would like to thank the Department of Agriculture and Food, Republic of Ireland for funding this research under the Research Stimulus Fund scheme.
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