Wood residue as an energy source for the forest products industry

CONTENTS

Introduction

Energy Value of Wood

Conversion technologies

Combustion technologies

Future Prospects for Australian Forest Products Industry

Conclusion

References

Links to sites of interest

Introduction

Around one-half of timber arriving at mills for processing in Australia becomes a finished product, the remainder is residue or waste. There is potential in the forest products industry for utilisation of these wood residues to generate energy onsite. The utilisation of wood residues as an energy source has both economic and environmental benefits over traditional energy sources. This website will provide an insight into the various classes of systems available for wood energy conversion.

The utilisation of wood residues for energy production in the forest products industry is not a new concept and has been commonly practiced for a long time. With the growth of fossil fuel dependency for electricity and industrial uses the forest industries began seeing wood residues as no longer an energy source, but as a waste product. However, with the OPEC oil crises of the 1970's and growing social pressures to reduce fossil fuel dependencies, there has been growing interest in the forest industries to utilise renewable energy sources. Modern forest products industries including sawmills, plywood mills, veneer mills and the pulp and paper industries are large consumers of energy. These industries are unique however in that many of the residues they produce (sawdust, wood-off cuts and pulping residues) can be utilised to produce energy for their own use. Table 1 shows the energy requirements of four forest product industries in Australia, with sawmilling having the lowest energy requirement and the pulp and paper industry having the highest.

Table 1: Energy Requirements of Four Forest Products Industries (from Fung 1982)

	Energy Requirements Per Unit of Product
Industry	Electricity (kWh)	Heat (MJ)	*Equivalent wood burnt
Sawmilling	80	860-4000	0.16-0.34 t/m^3
Plywood	200-300	4600	0.39-0.59 t/m^3
Fibreboard	600-650	10000-12000	1.2-1.3 t/t
Pulp and Paper	400-1100	6800-18000	0.8-2.2 t/t

* Based on cogeneration using wood with a calorific value of 18200 MJ/t.

Wood residues can be combusted in solid, liquid or gaseous states in furnaces for three main end uses. Firstly wood residues can be used to heat air, oil or steam for use in regulating temperature in timber drying kilns [See Figure 1, see source]. Secondly the wood residues can produce pressurised steam to turn turbines to generate electricity. Thirdly, is a combination of the two, cogeneration, where steam is used for turning turbines for electricity and then exhausted for use in kilns (Evans and Zaradic 1996).

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Figure 1: Timber Drying kiln at CSR Timber Products Tumut, NSW, Australia

Energy value of Wood

The amount of heat wood residue gives off when it is burnt is largely dependent on its moisture content. Energy yield is usually expressed as its net calorific value, which will increase as the wood moisture content is reduced (Figure 2). Calorific values are important in determining energy outputs from furnaces and can be calculated using species specific formulas (Fordyce and Ensor 1982). Obviously the drier the wood, the greater will be its heat yield. As most timber in Australia is sawn in a green state, the moisture content of wood limits its energy yield. Moisture content of wood residues is an important factor in influencing the design of many of the conversion technologies used for energy production.
Carbon and lignin content also have an influence on the energy content of wood. Lignin is a valuable by-product from many pulping processes and is recovered and burned for heat recovery during the chemicals recovery phase of a pulp and paper mill operation. It has also been long recognised that charcoal can concentrate woods carbon content and hence its energy output.

Figure 2: Net calorific value of waste wood (Pinus radiata) As a function of moisture content (from Fordyce and Ensor 1982)

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Conversion Technologies

Wood residues can be burnt directly for heat and electricity or can be converted into solid, liquid or gaseous fuels using conversion technologies such as carbonization, gasification, fermentation and wood densification. When wood is dry and is completely combusted in the presence of oxygen, the following overall chemical reactions can take place:

C + O2 = CO2 + heat
H2 + 1/2O2 = H2O + heat

As mentioned above, carbonization or making of charcoal concentrates the carbon in wood. The basic process has been carried out for hundreds of years and involves the slow burning of wood in low oxygen environments. This process reduces the weight and volume and concentrates the energy into easily transportable and storable form (Satonaka 1982).
Gasification as with carbonization involves the partial combustion of wood in a low oxygen environment [Commercial gassification systems link], can lead to the production of combustible gases. The main gases formed are carbon monoxide and methane as shown in the following reactions:

C + 1/2O2 = CO2 + heat
CH4 + 2O2 = CO2 + 2H2O + heat

If these gases are ducted to a boiler and additional oxygen is added the reaction will be completed and more heat produced as follows:

CO + 1/2O2 = CO2 + heat
CH4 + 2O2 = CO2 + 2H2O + heat

Fermentation is the process of deriving ethanol from the sugars found in wood. It first involves removing the lignin from wood, in order to unlock the cellulose and hemicellulose for subsequent hydrolysis into fermentable sugars, either by acid-catalysis or using enzymes as follows:

(C6H10O5)n ------------------------> nC6H12O6 -------------------------> 3nC2H5OH

Densification of wood residues is another way to increase their heating value and make them efficient to transport and store. A number of methods exist for densifying wood fuels (Resch 1982), however they usually involve subjecting sawdust, wood chips and similar material, and subjecting them to heat and pressure in a mold to form briquettes or pellets.

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Combustion Technologies

For the conversion of wood fuels into energy, there are a variety of boiler designs that are employed (Cheremisinoff 1980). Five of the main types of boiler designs used in the forest industries will now be briefly outlined.
Dutch ovens are a basic system consisting of a two-stage furnace. The primary furnace evaporates moisture from the wood before the fuel is gasified and combusted in the secondary furnace. This system has been around for quite a while, however it has been superceded by more energy efficient systems.
Spreader ­ Stoker type systems introduce the fuel to the furnace from above by a penumatic or mechanical spreader system. A portion of the fuel is combusted in suspension before reaching the great at the base of the furnace where combustion is completed.
Fuel cell type designs consist of a two-stage furnace design and are a modification of the old Dutch oven design. The fuel is introduced from above onto a water-cooled grate in the primary furnace. Gasification takes place and these combustible gases pass into the secondary combustion chamber where burning continues until completion. These systems are quite efficient and a number have been utilised for generating steam for use in kilns for drying timber.
Inclined grate design introduces fuel to the furnace at the top part of the grate in a continuous stream. The fuel passes over the upper drying section where moisture is evaporated before it then descends into a lower burning section. These types of furnaces are designed to burn green wood wastes and are particularly suited to use for forest industries.
Fluidized bed incineration uses a heated bed of sand in constant motion i.e. fluidized, to burn wood wastes. This design ensures excellent contact between air and wood fuels, thus ensuring efficient heat transfer, thorough mixing and uniform temperature conditions. Trials in Western Australian sawmills drying Jarrah timber of shown that green sawdust with moisture contents of 60 to 74 per-cent can be utilised to generate temperatures in excess of 1000 degrees Celsius (Van Doornum and Sheadly 1986).

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Future Prospects for Australian Forest Products Industry

Whilst Australia's forest products industries have utilised a large proportion of their wood residues to provide energy onsite, there has been little attempt to generate surplus electricity to be sold into the grid. The sawmilling industries particularly have potential to generate surplus power as they fulfill their own energy needs from only 20 to 45% of their wood wastes (Fung 1982). Bioenergy production from wood residues has certain environmental advantages over more traditional energy sources and could reduce greenhouse emissions in Australia by 10% [(Fung 1998), go to article]. Wood residues from managed forests and plantations is a renewable resource and results in a carbon dioxide neutral fuel cycle (Gustavsson et al 1995). Wood also has advantages over the burning of coal in that it has a low sulfur content which minimizes the potential of emissions to form acid rain.
Globally bioenergy is an important energy source and has been utilised in many northern European countries which have large forest industries. Countries such as Finland derive 18% [Finland's bioenergy link] and Sweden derives 16% of their total energy from bioenergy sources (Hall 1997). However, Australia is not as well positioned as many traditional Northern Hemisphere industrialised countries to reduce fossil fuel dependencies. Recent global conference on climate change in Kyoto, Japan as forced many Northern Hemisphere countries to reduce fossil fuel emissions and enter carbon credit schemes for polluting industries by 2003 [(Brand 1998), go to article]. Australia on the other hand has been given exemption from this regulation and will be allowed to increase its greenhouse emissions [Australian Geenhouse Office link]. This does bode well for renewable energy sources such as bioenergy as there will need to be increases in the cost of fossil fuel energy for wood derived energy production to become economically viable (Gustavsson 1997).

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Conclusion

Forest products industries have long recognised the importance of wood residues as energy sources. The oil price rises of the 1970's made their utilisation again economically feasible for onsite energy production, particularly in boilers to produce steam for timber drying kilns. However, recent decisions by Australian government and Industry has halted their potential wider utilisation for electricity production. Fossil fuels will still be used to provide the bulk of Australia's energy needs until their full environmental cost is factored into their price.

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References

BRAND, D.G. (1998) Criteria and Indicators for the Conservation and Sustainable Management of Forests: The special case of biomass and energy from forests. Paper presented at the International Energy Agency, Bioenergy Task Group Meeting, Tampere, Finland, September 7-11, 1998. at:
http://www.forest.nsw.gov.au/Business%20Services/Carbon2/Papers_on_Carbon/biomassenergy.htm

CHEREMISINOFF, N.P. (1980) Wood for Energy Production. Energy Technology Series - Ann Arbor Science, Michigan.

EVANS, R.L. and ZARADIC, A.M. (1996) Optimisatation of a Wood-Waste-Fueled, Indirectly Fired Gas Turbine Cogeneration Plant. Bioresource Technology. 57(2):117-126.

FORDYCE and ENSOR (1982) Energy from Waste Wood Burning ­ Resource Conservation Series 9, Department of Trade and Industry New Zealand ­ September 1982.

FUNG, P.Y.H. (1982) Wood Energy Prospects, In: Smith, R.W. (ed.) Energy from Forest Biomass, XVII IUFRO World Congress Energy Group Proceedings, Academic Press, New York (pp. 155-170).

FUNG, P. (1998) Converting Tree Waste into Energy. CSIRO Media Release, 5th January 1998, Ref 98/04 at http://www.csiro.au/news/mediarel/mr1998/mr9804.html.

GUSTAVSSON, L.; BORJESSON, P.; JOHANSSON, B. and SVENNINGSSON, P. (1995) Reducing CO2 Emissions by Substituting Biomass for Fossil Fuels. Energy 20(11):1097-1113.

GUSTAVSSON, L. (1997) Energy Efficiency and Competitiveness of Biomass-Based Energy Systems. Energy. 22(10):959-967.

HALL, D.O. (1997) Biomass Energy in Industrialised Countries ­ A View of the Future. Forest Ecology & Management. 91(1):17-45.

RESCH, H. (1982) Densified Wood and Bark Fuels, In: Smith, R.W. (ed.) Energy from Forest Biomass, XVII IUFRO World Congress Energy Group Proceedings, Academic Press, New York (pp. 109-128).

SATONAKA, S. (1982) Carbonization and Gasification of Wood, , In: Smith, R.W. (ed.) Energy from Forest Biomass, XVII IUFRO World Congress Energy Group Proceedings, Academic Press, New York (pp. 155-170).

VAN DOORNUM, M. and SHEADLY, P. (1986) Wood Residue and Gasification in a Fluidized Bed. Technical Report No. 10, July 1986, Department of Conservation and Land Management, Western Australia.

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Links to sites of interest

Biomass taskforce of Australia - http://www.users.bigpond.com/Steve.Schuck/ABT/index.htm

Good links to bioenergy sites - http://www.users.bigpond.com/Steve.Schuck/ABT/Links.htm

Companies producing commercial waste wood energy conversion systems

- http://www.users.on.net/mec/tree.html

- http://www.industrialboilers.com/

- http://www.brightstarsynfuels.com/

Australian Greenhouse office - http://www.greenhouse.gov.au

FAO Forest energy forum - http://www.fao.org/waicent/faoinfo/forestry/energy/cont.htm

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[ANU Forest Products][Non-Wood Forest Products] [Resins, Oils & Chemicals]

Copyright 1999 The Australian National University

Author: Andrew Crisp

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Date last Modified: 18 th October 1999

URL : http://www.anu.edu.au/Forestry/wood/nwfp/woodres/woodres.html