Wood for Energy Production
Technology - Environment - Economy
The Centre for Biomass Technology
Wood for Energy production was prepared in 2002 by the Centre for Biomass Technology (www.videncenter.dk) on behalf of the
Danish Energy Agency. The first edition was named “Wood Chips for Energy Production”. The publication can be found on the web
site: www.ens.dk. The paper edition can be ordered through the National Energy Information Centre or the Centre for Biomass Tech-
nology at the following addresses:
National Energy Danish Technological dk-TEKNIK ENERGY The Danish Forest and Landscape
Information Centre Institute & ENVIRONMENT Research Institute
EnergiOplysningen Teknologisk Institut dk-TEKNIK ENERGI & MILJØ Forskningscentret for Skov & Landskab
Teknikerbyen 45 Kongsvang Allé 29 Gladsaxe Møllevej 15 Hørsholm Kongevej 11
DK-2830 Virum DK-8000 Århus C DK-2860 Søborg DK-2970 Hørsholm
Tel. +45 70 21 80 10 Tel. +45 72 20 10 00 Tel. +45 39 55 59 99 Tel. +45 45 76 32 00
Fax +45 70 21 80 11 Fax +45 72 20 12 12 Fax +45 39 69 60 02 Fax +45 45 76 32 33
www.energioplysningen.dk www.teknologisk.dk www.dk-teknik.dk www.fsl.dk
Authors: Helle Serup (Editor), The Danish Forest and Landscape Research Institute
Hans Falster, dk-TEKNIK ENERGY& ENVIRONMENT
Christian Gamborg, The Danish Forest and Landscape Research Institute
Per Gundersen, The Danish Forest and Landscape Research Institute
Leif Hansen, dk-TEKNIK ENERGY & ENVIRONMENT
Niels Heding, The Danish Forest and Landscape Research Institute
Henrik Houmann Jakobsen, dk-TEKNIK ENERGY & ENVIRONMENT
Pieter Kofman, The Danish Forest and Landscape Research Institute
Lars Nikolaisen, Danish Technological Institute
Iben M. Thomsen, The Danish Forest and Landscape Research Institute
Cover: The cover shows “Energiplan 21", Klaus Holsting and Torben Zenths Tegnestue
Harboøre Varmeværk, Ansaldo Vølund A/S
Chipper in operation, BioPress/Torben Skøtt
Front-end loader on a wood chip pile at Måbjergværket, BioPress/Torben Skøtt
Printed by: Trøjborg Bogtryk. Printed on 100% recycled paper
Wood for Energy Production
Technology - Environment - Economy
Second Revised Edition - 2002
The Centre for Bio-
The emission of CO2 and other greenhouse gases is one of the greatest environmental problems of our
time. At the United Nations Climate Change Conference in 1997 in Japan, it was agreed that total world-
wide emissions should be reduced by 5.2% by the year 2012. The European Union has undertaken the
major reduction of 8% compared to the 1990 level.
Today only 6% of the European Union’s consumption of energy is covered by renewable energy, but the EU
Commission Renewable Energy White Paper, published in December 1997, prescribes a doubling of the
proportion of renewable energy by the end of the year 2010.
Biomass is the sector that must be developed most and fastest. It is estimated that in 2010 it should amount
to 74% of the European Union’s total consumption of renewable energy.
Danish experiences acquired in the field of biomass are already now significant. We have achieved much in
the field of both the individual and the collective energy supply. Denmark’s strongholds are in the field of col-
lective heating supply and decentralised CHP (combined heat and power) generation based on biomass,
and cost-effective fuel production, in particular.
This publication illustrates how Denmark has succeeded in utilising its wood resources in an environmentally
desirable and CO2-neutral energy production. It provides an introduction to the most recent Danish develop-
ments in the field of wood for energy production, both with regard to technology, environment, and economy.
At present more than 10% of Denmark is covered with forests, and the intention is a doubling of the area
within the next century. The forest trees are used for timber and for manufacturing in the wood industry. The
forest also provides thinning wood and other wood waste that can all be used for energy production.
The long-term perspective of the Government’s plan for a sustainable energy development in Denmark, En-
ergy 21 (Energi 21), is to develop an energy system where the proportion of renewable energy continuously
increases. This preconditions a continuous and gradual fitting in of renewable energy concurrent with the
technological and financial possibilities.
The enlargement will primarily take place by means of an increased application of bioenergy and wind power.
Therefore, biomass will contribute considerably to Denmark’s and the European Union’s energy production
in the next decades.
At the same time, biomass is an area of great potential for the Danish energy industry - also on the export
Minister for the Environment and Energy
Danish Energy Policy
1. Danish Energy Policy
Danish energy policy is in a constant fixed at 8% in 2012 compared to the
process of change. The Government’s 1990 level.
Energy Action Plan of 1996, Energi 21, Denmark’s CO2 aim shall be achieved
is the fourth in a series of plans that by both improving the energy intensity by
all have or have had as their aim to 50% up to the year 2030 and by renew-
optimise the Danish energy sector to able energy contributing by 35% of the
the present national and international gross energy consumption in 2030.
conditions in the field of energy. Energy 21 assumes that renewable
energy covers 12-14% of the country’s
total energy consumption in 2005. By far
The Four Energy Plans the most significant renewable energy
The aim of the first energy plan, Danish source is and will continue to be bio-
Energy Policy 1976 (Dansk Energipolitik mass. Biomass contributed with 61 PJ in
1976), was to safeguard Denmark 1996, which should increase to 85 PJ in
against supply crises like the energy cri- 2005 and 145 PJ in 2030. The increase
sis in 1973/74. up to 2005 will primarily be achieved by
The second energy plan, Energy the centralised power plants’ increased
Plan 81 (Energiplan 81), gave added use of straw and wood chips (see the
weight to socio-economic and environ- section on the Biomass Agreement). An
mental considerations, thus continuing Energy 21(Energi 21) shall contribute to increased use of biomass and landfill gas
the efforts of reducing the dependence a sustainable development of the Danish also contributes to achieving the aim of
on the import of fuels. society. The energy sector shall continue 85 PJ. In connection with Energy 21, the
The third energy plan in the series is to be a financially, vigorously, and tech- Danish island Samsø has been declared
the action plan Energy 2000 (Energi nologically efficient sector that forms part a renewable energy island, and the is-
2000) /ref. 1/ of 1990. This plan is an am- of a dynamic development of society. land shall thus function as display win-
bitious attempt to increase the use of en- dow for Danish renewable energy tech-
vironmentally desirable fuels. At the same The aims are achieved by means of a nology.
time, the aim of a sustainable develop- wide range of activities: Energy savings, Thus the initiatives in the field of
ment of the energy sector is introduced. In tax on CO2 emission, conversion to the biomass are directed at the following par-
Energy 2000, the environmentally desir- use of environmentally desirable fuels by tial aims of Energy 21:
able fuels are defined as natural gas, means of CHP generation, subsidised
• Increased use of straw and wood chips
solar energy, wind, and biomass (straw, construction and operation of district
at centralised power plants.
wood, liquid manure, and household heating systems, subsidised establishing
• Increased CHP generation based on
waste). The use of biomass is based on of biofuel boilers in rural districts etc.
straw, wood chips, biogas, and landfill
the facts that it is CO2 neutral, that it The fourth and last energy plan is
saves foreign currency, that it creates Energy 21 (Energi 21) /ref. 2/ that was
• Conversion to the greatest possible ex-
Danish jobs, that it utilises waste products introduced in 1996. The intention of this
tent of block heating units above 250
from agriculture, forestry, households, plan is that the administration of our
kW in rural districts from fossil fuels to
trade and industry. The ambitious aim of resources shall have a central role. Our
Energy 2000 is that Denmark compared consumption of depletable, fossil energy
• Permission to establish biofuel systems
to the year 1988 shall achieve the follow- sources, and emissions resulting from
and biogas production from collective
ing aims by the end of 2005: the consumption and energy production
systems, industrial systems, and landfill
shall be further reduced. A significant
sites etc. in areas previously reserved
• Reduce the energy consumption by aspect of Energy 21 is thus that the ex-
for natural gas.
15%. isting aim of Energy 2000, i.e., that Den-
• Increase the consumption of natural mark should reduce its CO2 emission by Figure 2 shows the distribution of the in-
gas by 170%. 20% in 2005 compared to the 1988 level, dividual renewable energy sources.
• Increase the consumption of renewable is supplemented with a long-term aim.
energy by 100%. The CO2 emission should be halved in
• Reduce the consumption of coal by 2030 compared to 1998. In addition, in-
45%. ternational climate change negotiators EU Commission Renewable Energy
• Reduce the consumption of oil by 40%. will advocate that the industrialised White Paper 1997/ref. 3/ fixes an in-
• Reduce the CO2 emission by at least countries by 2030 halve their emissions crease in the EU use of renewable en-
20%. of CO2 compared to the 1990 level. At ergy from 6% to 12% up to the year
• Reduce the SO2 emission by 60%. the UN Climate Change Conference in 2010. It is estimated that the biomass
• Reduce the NOx emission by 50%. Kyoto in 1997, the EU reduction was sector will be the fastest growing sector
Page 6 Wood for Energy Production
Danish Energy Policy
in the field of renewable energy technolo- gas-fired CHP plants and the remaining able energy receive a total subsidy of
gies. The use of agricultural land is district heating plants to biofuels. See DKK 0.27/kWh.
closely connected with the EU agricul- also the section on the Biomass Agree- • “State-Subsidised Completion of Dis-
tural policy. The most recent EU draft ment on the adjustment of the progress trict Heating Nets”. Under this act, up to
proposal for future agricultural policies of the phase. 50% of the construction costs could be
suggests that the legal obligation to fal- subsidised. The scheme expired at the
low land shall be abolished, and that end of 1997.
there shall be one rate for subsidies no
The CO2 Acts
matter the choice of crop. This will affect The Heat Supply Act was followed by The present subsidies of DKK 0.10/kWh
the farmers’ managements also with re- three new acts offering the prospective of and DKK 0.17/kWh respectively in con-
gard to growing energy crops on land, vol- subsidising the process of conversion to nection with the electrical power reform
untarily left fallow. Energy 21 mentions ex- environmentally more desirable fuels. will be financed via the consumption
plicitly that the aim of 45 PJ energy crops The purpose was that the Minister of En- price in a transitional period. In the fu-
in 2030 can be achieved by other biomass ergy could then counteract consumers ture, the electrical power generation sub-
use subject to EU modifying its agricul- being charged higher heating prices as a sidies and the DKK 0.10/kWh from the
tural policy and subsidy schemes so as to result of the conversion. CO2 tax will be replaced by “green” re-
encourage this. The three acts are Acts Nos. 2, 3, newable certificates with the minimum
and 4, 1992 and the titles are: price being DKK 0.10/kWh. The organi-
sation and function of the “green” market
The Heat Supply Act • “State-Subsidised Promotion of Decen- will be clarified during 1999.
For the purpose of implementing the ac- tralised Combined Heat and Power and
tivities suggested in Energy 2000 /ref. 1/, Utilisation of Biomass Fuels Act”. Un-
Development of Renewable
the Heat Supply Act June 3,1990 was der this act, it is possible to receive
passed by the Danish parliament “Fol- subsidies of up to 50% of the construc-
ketinget”. This Act gave the Minister of tion costs. In practice, subsidies have A 3-year bioenergy development
Energy wide powers to control the choice been in the range of 20-30% of the programme for 1995-97 (BUP-95) /ref. 6/
of fuel in block heating units, district heat- construction costs. has had the aim to encourage the tech-
ing plants, and decentralised CHP plants. • “State-Subsidised Electrical Power nological development in the field of bio-
This was accomplished by the so-called Generation Act”. A subsidy of DKK 0.10 mass-based systems. The programme
“Letters of Specific and General Precon- /kWh is granted for electrical power recommends the following activities:
ditions” /ref. 5/ that are circulated to mu- generation based on natural gas, and a
nicipalities and owners of plants in three subsidy of DKK 0.17/kWh for electrical • The development of CHP technologies
staggered phases. The “Letters of Spe- power generation based on straw and based on straw and wood chips as fu-
cific and General Preconditions” describe wood chips. On January 1, 1997, an els. The technologies are steam, gasifi-
in details the conversion to environmen- executive order was put into force re- cation, and the Stirling engine.
tally desirable fuels to selected munici- quiring e.g. a biomass plant overall effi- • District heating systems should focus
palities and owners of plants. In addition, ciency of 80% in order for the plant to on fuel flexibility and an environmen-
“Letters of General Preconditions” that receive the max. subsidy. In addition, tally desirable handling of fuels.
describe the prospects of voluntary con- the CO2 tax of DKK 0.10/kWh is re- • Environmentally desirable and
version from coal and oil to more envi- funded in the case of renewable en- user-friendly boiler systems should be
ronmentally desirable fuels are circulated ergy. Thus private producers of renew- developed for private dwellings.
to all Danish municipalities.
The conversion was immediately 30 Figure 1: Fuel
implemented. Phase 1 took place from
prices at the be-
1990-1994 and included the conversion 25 ginning of 1999
of a number of coal and natural gas-fired
for district heating
district heating plants that should be con-
20 purposes includ-
verted to natural gas-fired, decentralised
ing taxes but ex-
CHP. Phase 2 took place from 1994-
15 cluding VAT /ref.
1996 and included the remaining coal
and natural gas-fired district heating
plants that are converted to natural
gas-fired, decentralised CHP. In addition,
small district heating plants outside the 5
large district heating systems should be
converted to biofuels. Phase 3 began in 0
Gas oil Fuel oil Natural Coal Wood Wood Straw
1996 and is not finished yet. The aim gas pellets chips
was that small, gas-fired district heating
plants should be converted to natural Price excluding taxes Energy tax CO2 tax Sulphur tax
Wood for Energy Production Page 7
Danish Energy Policy
• Energy crops should be investigated PJ/per annum
with a view to the growing, handling, 250 Wind energy
and use of them.
The Danish Energy Agency’s scheme,
the “Development Scheme for Renew- 200
able Energy”, subsidises projects for the Ambient heat
promotion of biomass in the energy sup-
ply and uses e.g. the Bioenergy Develop- Solar heat
ment Programme (BUP)-95 as the basis 150
of their decisions when considering appli- Biogas
cations for subsidies.
The Plant Pool
The Government subsidises the promo-
tion of decentralised CHP generation
and the utilisation of biofuels. The
scheme includes subsidies for the con-
version of district heating plants to CHP
plants based on biofuels and for the pro-
motion of an increased use of biofuels in 0
1975 1985 1995 2005 2015 2025
areas without collective heating supply.
Under this scheme, subsidies amount- Figure 2: Energy 21(Energi 21) proposal for the use of renewable energy sources up
ing to DKK 25 million can be granted to the year 2030 /ref. 2/.
1998 if they choose biomass-based • The centralised power plants are al-
CHP. lowed a freer hand when choosing
The Biomass Agreement • Six towns in Phase 3 may postpone the among straw, wood chips, and willow
In order to ensure the achievement of conversion to biomass-based CHP until chips, since the consumption should in-
the aims of Energy 2000, the Govern- 2000. clude 1.0 million tonnes of straw and 0.2
ment, the Conservative Party, the Lib- • Approx. 60 small towns in Phase 3 million tonnes of wood chips but with the
eral Party, and the Socialist People’s should be converted to biomass-based remaining part being optional, but so as
Party entered into an agreement on district heating by the end of 1998. to make out a total of 19.5 PJ.
June 14, 1993, on an increased use of • Biomass-based CHP generation will be
biomass in the energy supply with a The agreement has resulted in Sønder- permitted in natural gas areas.
special view to use at centralised power jyllands Højspændingsværk (electricity • The municipalities shall give priority to
plants. The main points of the agree- utility) having constructed a biomass- CHP generation based on biogas,
ment are as follows: based power plant in Aabenraa with a landfill gas, and other gasified biomass.
consumption of 120,000 tonnes of straw • Seven towns in Phase 3 may continue
• A gradual increase in the use of bio- and 30,000 tonnes of wood chips per the present district heating supply until
mass at power plants shall take place year. Sjællandske Kraftværker (elec- a conversion to biomass-based CHP
so that the consumption by the year tricity utility group) has constructed a generation is technically and financially
2000 amounts to 1.2 million tonnes of straw and wood chip-fired CHP plant in appropriate.
straw and 0.2 million tonnes of wood Masnedø with an annual consumption
chips per year equal to 19.5 PJ. of 40,000 tonnes of straw and 5-10,000
• Eleven towns in natural gas districts tonnes of wood chips, and is presently
that have not converted to natural also constructing plants in Maribo- It is characteristic that since the middle of
gas-fired CHP generation within Phase Sakskøbing and in Avedøre near Co- the 1980s, changing governments, par-
1 or Phase 2 may choose between penhagen. liamentary majorities, and ministers of
biofuels and natural gas as fuels. It is On July 1, 1997 the political parties energy have persisted in the importance
possible to wait until 2000 in order to to the Biomass Agreement drafted a sup- of an active energy policy thereby adding
e.g. await the development and com- plementary agreement with the intention weight to the resource-based and envi-
mercialisation of technologies in the of improving the prospects of integrating ronmentally responsible policy. Denmark
field of biomass. biomass in the energy supply. In princi- has a leading position in several fields of
• Phase 2 towns outside natural gas ar- ple, the supplementary agreement renewable energy, and Energy 21 will
eas can postpone the conversion until means that: maintain this leading position.
Page 8 Wood for Energy Production
Wood as Energy Resource
2. Wood as Energy Resource
2.1 Amount of Consumption Annual harvesting, million m3 solid volume
4 Figure 3: Wood Har-
vest 1950-1996 dis-
Wood is an important energy source tributed on commer-
all over the world. In Denmark energy 3 cial timber, fuelwood,
wood is available in the form of forest and wood chips. The
chips, fuelwood, wood waste, wood wind breakages in
pellets, and also it is produced to a 2
1967 and 1981, in
very limited extent from willow crops particular, resulted in
in short rotation forestry. The major increased harvesting
part of wood harvested on the forest 1
area of approx. 460,000 ha ends up as
energy wood directly or after having
been applied for other purposes first. 0
1950 1960 1970 1980 1990 2000
In the light of the Government’s aim to Year
increase the forest area by doubling it
during a rotation, Denmark’s total Commercial timber Wood chips
wood fuel resources will increase over Fuelwood Total
Wood chips result from first and second is used primarily in the industry’s own
thinnings in spruce stands, from harvest- boiler furnaces. Approx. 640,000 m3 s.
Consumption of Energy Wood ing overmature and partly dying pine vol is used per year of which part of it is
According to the Danish Energy Agency’s plantations, from harvesting in climate- used for the production of wood pellets
survey of the energy production in 1997, and insect damaged stands, from the and wood briquettes, a rather new pro-
wood covers approx. 21,000 TJ which is harvesting of nurse trees (species that duction in Denmark. In addition to that, a
equal to 28% of the total production of re- are planted at the same time of the pri- huge amount of wood waste is imported
newable energy and equal to approx. mary tree species in order to protect for the purpose of this production. The
500,000 tonnes of oil. Table 1 illustrates the them against e.g. frost and weeds), and consumption of wood pellets and wood
distribution among the individual wood fuels. from tops by clear-cutting (timber har- briquettes amounts to approx. 200,000
Since 1950, Statistics Denmark has vesting of the whole stand at the end of tonnes and approx. 20,000 tonnes re-
made detailed statistics classifying the the rotation) in spruce stands. Wood spectively per year.
wood harvest in Danish forests, and it chips have become a still more important Energy willow is grown in short ro-
amounts to approx. 2 million m3 s. vol fuel over the two most recent decades, tations (3-4 years) on farmland, but the
(solid volume) with fluctuations around and the production amounts to approx. production is not yet so widespread in
the wind breakage disasters in 1967 200,000 m3 s. vol per year. Denmark, where willow covers an area
and 1981. In 1996, an amount of Fuelwood is obtained primarily in of only approx. 500 ha. The amount of
approx. 620,000 m3 s. vol, equal to hardwood stands by thinning and by fuel produced from willow is therefore
approx. 108,000 tonnes of oil, was used clear-cutting in the form of tops, branches not so important compared to other
for direct energy production, which is and butt ends. Earlier, fuelwood was the wood fuels.
approx. 33% of the total harvest. most important product of the forest, but
around the turn of the century, wood as a
source of energy was substituted by coal
Fuel Consumed Proportion
1997 (%) and later by oil. The oil crisis in the 1970s The Danish Forest and Landscape Re-
(TJ) and the increase in taxes imposed on oil search Institute has calculated the
and coal in the middle of the 1980s re- amount of available wood fuel resources
Forest chips 2,703 13 sulted in an increased interest in wood (fuelwood and wood chips) from Danish
Fuelwood 9,603 46 for the purpose of energy production. forests above 0.5 ha /ref. 10/. The re-
According to statistics, forestry pro- sources have been calculated on the ba-
Wood waste 5,879 28
duces 420,000 m3 s. vol of fuel, but the sis of information provided by the forest
Wood pellets 2,828 13 consumption of fuelwood from gardens, inventory in 1990 of tree species, age-
parks, hedges/fringes etc. is not regis- class determination, and wood production
Totalling 21,013 100
tered. The total consumption is estimated of the individual forests. The calculations
Table 1: Consumption of wood fuels. By at approx. 700,000 m3 s. vol per year have been made in the form of annual av-
way of comparison, it may be mentioned /ref. 9/. erages from 1990-1999, 2000-2009, and
that the energy content of 1000 tonnes Wood waste consisting of bark, 2010-2019 based on hypotheses that are
of oil is 42 TJ /ref. 7/. sawdust, shavings, demolition wood etc. deemed to be realistic under the prevail-
Wood for Energy Production Page 9
Wood as Energy Resource
Annual harvesting, million m3 solid volume tion supply the necessary amount of year in order to achieve the aim, of this
3.5 wood, i.e., 200,000 tonnes of wood chips 2,000-2,500 ha by private forest owners.
3.0 per year, which is equal to approx. Since 1989, only approx. 50% of the
250,000 m3 s. vol, which the power plants has been planted.
plants according to the Biomass Agree- In the Danish Energy Agency’s sur-
2.0 ment shall use as from the year 2004. vey of 1996 on the wood chip amounts
1.5 from Danish forests up to the year 2025,
1.0 which is based on /ref. 10/, an increase
2.2 Afforestation and Wood
of the forest area of 5,000 ha per year
0.5 for Energy has been included. Energy wood produc-
0 Afforestation includes the planting of tion in the form of wood chips from affor-
1990-99 2000-09 2010-19 new forests on agricultural land. The estation is estimated at 4 PJ per year out
Commercial timber Wood fuel future supply of energy wood should of a total energy contribution of almost 10
be ensured partly through afforesta- PJ per year from wood chips. Thus affor-
Figure 4: Forecast from 1994 of the po- tion. Here, the energy wood produc- estation is expected to contribute consid-
tential annual harvesting of commercial tion can be increased by increasing erably to the total consumption of energy
timber and wood fuel in the periods the number of plants compared to the wood in future /ref. 14/.
1990-99, 2000-09, and 2010-19. Har- number of plants in normal stands,
vesting is expected to rise in a good two and by using nurse trees.
Energy Wood from Future
decades /ref. 10/.
ing outlets for cellulose wood and other
The Energy Political Aim The energy wood production by future af-
competing products for wood fuel. It says in the preamble to the Danish For- forestation can be increased in propor-
Total annual harvesting is expected estry Act that in addition to protect and tion to the energy wood production in the
to increase in the next two decades to preserve the Danish forests and improve existing forests by, e.g., increasing the
approx. 3.2 million m3 s. vol due to, the stability of forestry, ownership struc- number of plants in proportion to normal
among other things, afforestation (Figure ture, and productivity, the aim is to"... practice, and by using nurse trees. An in-
4). Note that the total harvesting (Figure contribute to increasing the forest area" crease in yield should not be at the ex-
3) according to Statistics Denmark is /ref.12/. It is the aim of the Government pense of the all-round forestry where the
approx. 500,000 m3 s. vol lower per year to double the forest area over the next production of quality wood, preservation of
compared to the forecast for 1990-99. rotation (80-100 years). This aim is also nature, protection of the cultural heritage,
This apparent divergence is due to the in relation to the energy policy of political and recreation are given high priority.
fact that forestry does not have sufficient interest, and it should be seen in connec- A high stocking percentage results in
outlets for wood for energy. The annual tion with the Biomass Agreement of 1993 a faster plant cover of the area and thus a
commercial timber harvest is expected to and the Government’s action plan, En- larger production. Calculations show that
increase in both periods after the year ergy 21, of which it appears that the use the prospective spruce wood chip produc-
2000, while the harvesting of fuelwood of biomass in the energy sector should tion can be increased by 30-50 % by in-
and wood chips is predicted to decrease be increased, including wood chips /ref. creasing the number of plants from approx.
from approx. 950,000 m3 s. vol to approx. 2/. In the Danish strategy for sustainable 4,500 to 6,500 plants per ha. As the cost
800,000 m3 s. vol, and then again in- forestry, it is clearly stated that this dou- of planting increases with the larger num-
crease to approx. 900,000 m3 s. vol in bling of the forest area should be ber of plants, and the increased yield of
the last period (Figure 4). The change in achieved by “... aiming at a regular plant- wood chips does not cover the cost of
harvesting is due to an unequal ing intervals” /ref. 13/. This means that more plants, the method of large numbers
age-class distribution of the spruce area, approx. 5,000 ha should be planted per of plants will only be of interest if in addi-
the finishing of mountain- and contorta
pine wood stands, and an increase in the Afforestation on agri-
harvesting of wood fuel in hardwood cultural land. With the
stands /ref. 11/. present planting pro-
While the total potential annual har- gram of 2,000-2,500
vesting can be forecast with great cer- ha per year, it is nec-
tainty, the distribution among fuel and essary to increase the
other products will be subject to a range afforestation or in-
of outward circumstances. If the develop- crease the energy
ment of the most recent years continues, wood production from
photo: søren fodgaard
the fuel proportion will increase. the existing forest ar-
Based on the figures of the survey, eas in order to comply
the forests are capable of currently sup- with the aim of the
plying the present chip-fired heating and Danish Energy
CHP plants with wood chips and in addi- Agency.
Page 10 Wood for Energy Production
Wood as Energy Resource
tion to the increased yield of wood chips, Wood chips production, cubic metre loose volume per ha.
the added benefit of improved wood qual- 600 Figure 5: Production
ity, improved stand stability and reduced of wood chips in m3 l.
cost of weed control etc. can also be 500 vol per ha for an un-
achieved. mixed Norway spruce
Traditionally, nurse trees are planted 400 stand and a stand
at the same time of the primary tree spe- consisting of Norway
cies, which are normally more sensitive 300 spruce with larch as
species, in order to protect against frost, nurse trees with vari-
weeds etc. As nurse trees are trees that 200 ations in the number
are fast growing in their youth, the wood of plants in the East-
production increases resulting in larger 100 ern part of Denmark.
quantities of wood chips produced from The wood chip pro-
the thinnings in immature stands that are 0 duction increases
performed by harvesting the nurse trees 0 2,000 4,000 6,000 8,000 10,000 12,000
considerably by using
row by row. Relevant nurse tree species Number of plants per ha (stand density)
nurse trees. /ref. 15/.
are e.g. hybrid larch, alder, poplar, Unmixed Norway spruce Norway spruce with larch
Scotch pine and birch. By using hybrid
larch as nurse trees in a spruce stand,
the yield of wood chips can be increased an improvement of the wood quality. The experiments are currently inspected
by approx. 35% with a number of plants The demo - field experiments include and measurements are taken, and the
of 6,400 per ha distributed on 4,200 nine different planting models using the actual energy wood yield figures will be
spruce and 2,200 hybrid larch compared following mixture of species: available in connection with thinning in
to an unmixed Norway spruce plantation immature stands in approx. 15-20 years.
• Mixed softwoods (Sitka spruce/Norway
(Figure 5) /ref. 15/. The results form the basis of the plan-
spruce, and Douglas fir with or without
Normally, wood chips are only har- ning of future afforestation.
larch as nurse trees).
vested in softwood stands, but by pro-
• Pure hardwood stands and mixed
ducing wood chips from hardwood, such
as beech, the yield of wood chips can be
hardwood stands (beech, oak, and oak Legislation and Subsidies
greatly increased when using nurse In connection with afforestation, the
• Mixed hardwood- and softwood stands
trees. By planting hybrid larch also, the planting plans must be approved by the
(beech with Douglas fir and beech with
yield of wood chips could be tripled in Directorate for Agricultural Development,
proportion to a pure beech stand. and the afforestation must be shown to
The calculations of the yield of wood A standard number of plants is chosen, be in conformity with the counties’ desig-
chips are based on existing research data which is doubled either with the primary nations in their regional land use plans of
on spruce, but new requirements for the tree species (beech, Norway plus and minus land for afforestation, i.e.,
forests in respect of increased diversity and spruce/Sitka spruce, oak, Douglas fir) or the areas where afforestation is wanted
flexible stands may mean that more mixed by using nurse tree species (alder, larch). or not.
stands will be established in the future.
The effect of increased stand den-
sity is investigated by research.
Demo - Field Experiments
photo: the danish land development service/bert wiklund
In co-operation with The National Forest
and Nature Agency, the Danish Forest
and Landscape Research Institute estab-
lished in 1998-99 demo - field experi-
ments on three afforestation areas in
Denmark. The purpose of the experi-
ments is, among other things, to investi-
gate the energy wood production in
mixed stands on various soil types. The
experiments are aiming at demonstrating
the additional expenditure involved in in-
creasing the number of plants, prospec-
tive gains in the form of reduced need for
weed control and replanting (replanting The Danish forest area will be doubled over the next 80-100 years. Many of the new fo-
after dead plants), and in the long term rests will be hardwood forests with oak and beech being the primary species.
Wood for Energy Production Page 11
Wood as Energy Resource
The public authorities try to encourage
private forest owners to carry out affores-
tation via various subsidy schemes, but
so far they have only succeeded partly.
The major part of the afforestation takes
place on the National Forest and Nature
Agency’s own areas or on privately
owned properties without subsidies. At
the turn of the year 1996-97, a new sub-
sidy scheme under the Danish Forestry
Act came into force which has intensified
the interest in afforestation, e.g., due to
income compensation and increased
photo: biopress/torben skøtt
possibilities of being subsidised. This has
resulted in applications exceeding the
means available within the schemes.
The framework of afforestation and
the possibilities of being granted subsi-
dies are laid down in a range of acts and
executive orders. A precondition for be-
ing granted subsidies is, that the area is The area has been carefully cleaned before planting the willow cuttings. The planting
designated as a forest reserve in order to takes place by a two-furrow planting machine, and a tractor marking arm ensures quite
secure the existence of the forest in fu- parallel rows. The dual wheels of the tractor distribute the ground pressure so that the
ture. In addition to that, there are certain soil is not unnecessarily compressed.
requirements for the structural design
and the size of the forest. The subsidy According to the Energy Action Plan of winter months when harvesting takes
schemes include among other things 1996 (Energi 21), it is the intention that place /ref. 17/.
subsidies for preparatory investigations the contribution of energy crops or other When establishing energy planta-
like locality mapping (investigations of biomass, excluding straw, to the energy tions in Denmark, cloned withy cuttings
soil) and land plotting, planting, and care supply shall be increased from 0 in the have so far proven to have the best pro-
of stands, establishing of hedges and in- year 2005 to approx. 45 PJ in the year duction potential. When planting, which
come compensation for a period of 20 2030. If not supplemented with other bio- takes place in spring, traditionally approx.
years /ref. 16/. Further information can mass, this is equal to the yield of approx. 15,000-20,000 cuttings taken from one
be obtained by contacting the State For- 500,000 ha willow. However, the growing year old shoots are planted per ha. The
est Service. of energy crops will to a high extent de- cuttings are inserted in the ground by
pend on the EU agricultural policy and machine, and the 20 cm long cuttings are
2.3 Energy Plantation subsidy schemes. In order to estimate forced straight into the ground so that
the potential of the energy crops, a demo only a few cm stand up. By way of com-
(Short Rotation Coppice) and development programme has been parison, it may be mentioned that a new
Willow has been used as a cultivar for implemented in order to analyse future method has proven that the cost of plan-
centuries for the purpose of tools, use of energy crops. ting can be reduced by 50% by horizon-
barrel hoops, basketry, and wattles. In Denmark, willow is only grown on tally spreading the material, cut in lengths
For the purpose of the production of 500 ha agricultural land /ref. 15/, while it is of approx. 20 cm, and hence grooving it
wood chips for energy, willow has estimated that willow is grown on approx. down into the ground /ref. 18/. The first
only been cultivated for a few years in 17,000 ha land in Sweden. Willow is an winter after planting, the shoots can be
Denmark, and at present willow wood agricultural crop, which means that it is cut off at a height of 5-8 cm in order to
chips are only used to a limited extent possible to stop growing willow and encourage more sprouting. Cutting down
at heating plants in Denmark. change to another crop if so desired. is considered advantageous in thin
stands and where there are only 1-2
shoots per cutting /ref. 19/.
Energy Plantations in Denmark Willow Growing The worst enemy during the initial
The term energy plantations applies to Willow can be grown on various soil phase is weeds, particularly grasses,
hardwood plantations (generally willow) types. Soil types ensuring a good supply and the area should therefore be thor-
that are growing fast in their juvenile of water are suitable. Light soil types oughly cleaned before planting e.g. by
phase and capable of multiplication by without irrigation will result in unstable subsoil ploughing. Weed control is easi-
cuttings and stump shooting. Through in- yield. Willow roots may block drain sys- est and best performed by means of
tensive cultivation, these properties are tems. The area should be suitable for herbicides combined with mechanical
utilised for the production of biomass that mechanical equipment including being weeding. At the time of harvesting,
can be used for energy production. capable of bearing machines during the which is done at a few years interval,
Page 12 Wood for Energy Production
Wood as Energy Resource
everything is removed except leaves N P K Table 2: Recom-
and roots, and that makes the applica- mended applica-
tion of fertiliser necessary in order to Planting year - 0-30 80-130 tion of fertiliser to
maintain the level of production. st
1 prod. year 45-60 - - energy willow be-
Table 2 illustrates the application of fore and after first
2nd prod. year 100-150 - -
fertiliser to a willow cultivation over the harvesting (kg per
individual years. 3rd prod. year 90-120 - - ha). - means no
The application of nutrients to en- st fertiliser applied.
1 year after harv. 60-80 0-30 80-160
ergy willow with waste water, sewage The amount of fer-
sludge or liquid manure is an alternative 2 year after harv. 90-110 - - tiliser varies with
to the application of fertiliser. The dense, 3rd year after harv. 60-80 - - the soil character-
deep striking willow root system is suit- istic /ref. 19/.
able for capturing the plant nutrients and
heavy metal content of the sludge. Thus storage means that willow wood chips
The Production of
compared to wood chips, the fuel will are normally hauled directly to the heat-
contain relatively large quantities of ni- ing plant.
trogen and cadmium. Under ideal com- In plantations, the entire cost of produc-
bustion conditions, the major part of the tion should be paid by a low value prod-
nitrogen will be released in the form of
Fuel Characteristics uct, i.e. willow chips. This makes the pro-
N2, and the heavy metals will remain in Willow chips do not differ very much from duction of energy willow chips vulnerable
the ash. This is an important precondi- other types of wood chips, but may con- compared to the production of straw or
tion for stating that using sludge for en- tain more bark and more water. The forest chips. By the production of straw
ergy willow will be environmentally ben- lower calorific value of bone dry willow for energy, the cereal production carries
eficial /ref. 20/. does not differ from that of other wood all the costs including combine harvest-
species, but is approx. 18 GJ per tonne ing, and the straw will only have to pay
of bone dry material. But compared to for the collection, transport and storage.
Harvesting and Storage most other wood species, willow wood is Similarly, the production of sawmill timber
The first harvesting on the area takes relatively light. This means that one m3 l. pays for tree growth, while the wood
place 3-4 years after planting when the vol (loose volume) of willow chips con- chips pay for chipping, storage, and
willow shoots are approx. 6 metres high. tains less dry matter (approx. 120 kg/m3 transport to heating plant. Willow growing
It is done in winter, and the following l. vol) than e.g. one m3 l. vol of beech is therefore financially risky and depends
spring the plants start growing from the chips (approx. 225 kg/m3 l. vol) This is of to a high extent on the harvesting yield.
stumps, and after another 3-4 years, har- importance to the amounts by volume a Therefore, the calculation of the pro-
vesting can take place again. It is ex- heating plant must be capable of han- duction level for willow plantations in
pected that the willows can grow for at dling in order to achieve the same gener- Denmark has received much attention.
least 20 years without any reduction in ation of heat. The high moisture content Occasionally, high yield figures of 10-12
the plant yield, and that means that har- makes the wood chips particularly suit- tonnes of dry matter per ha per year or
vesting can take place 4-5 times before able at plants equipped with a flue gas more are recorded, but they have often
new planting will be necessary. condensation unit. If so, the evaporation been achieved in individual, small and
Research has shown that long-time heat is recovered. very intensively cultivated willow stands
storage of willow chips is difficult to han- and are thus not a realistic estimate for
dle. This is due to the fact that the mois- yields in commercial stands. Yield mea-
ture content is approx. 55 - 58% of the surements, carried out in Danish culti-
total weight of green willow, and that vated willow stands from 1989 to 1994,
young willow shoots contain a large pro- show that the average yield is approx.
portion of bark and nutrients. In piles of 7.5 tonnes of dry matter per ha per year,
photo: biopress/torben skøtt
willow chips, a fast temperature develop- which is not as much as previously esti-
ment typically takes place resulting in a mated. The results of the yield measure-
considerable loss of dry matter. This de- ments have not been able to unambigu-
velopment depends on the size of the ously explain the influence of the stand
chips. The larger the chips are, the lesser factors on the production level, but this
is the decomposition. Long-term storage average yield has been achieved in wil-
is best if the willow has not been chipped low stands with fertiliser being intensively
but is stored in the form of whole shoots, By harvesting of whole shoots which applied and with half of the stands being
which is expensive. A different method takes place by specially designed ma- irrigated. Measurements of the yield have
that has proven successful during experi- chines during the winter, everything, ex- been carried out on clones, that were
ments is airtight sealing of willow chips. cept leaves and roots, is removed. The common at the beginning of the 1990s
Without oxygen, no decomposition takes willow shoots are harvested close to the /ref. 22/. Danish measurements on new
place /ref. 21/. The difficult long-time soil surface. clones form part of an EU project. Prelim-
Wood for Energy Production Page 13
Wood as Energy Resource
inary results indicate that the additional Fraction unit Table 3: Require-
yield of the new clones is modest in com- (%) ments for the size
parison with the old clones. classification of fine
Name Screen tray Fine Coar. and coarse fuel
Overlarge 45 mm round holes <5 < 15 chips according to
Willow Growing in the Future the old Standard
Overthick 8 mm slats < 25 < 40
For the time being, there is good reason No. 1 which is cur-
to follow the development of willow grow- Accept 7 mm round holes > 40 > 23 rently being revised
ing in Sweden, who has taken the lead. /ref. 26/.
Pin chips 3 mm round holes < 20 < 15
More and more information is obtained * Diameter > 10 mm.
about cloning developments, harvesting Fines < 10 <7
yields, cost of harvesting, and soil types Hereof:
preferred by willow. It may be possible for
agriculture to take up a niche production Slivers 100-200* 100-200 mm length <2 < 12
of willow on soils suitable for the growing Slivers > 200* > 200 mm length < 0,5 <6
of willow, but less suitable for cereals.
Finally, willow may conquer a niche
where it can contribute to solving some suitable for the majority of wood stoves. wood chips. The new quality description
environmental problems in the form of Firewood consists of wood and bark. is therefore based on five types of wood
waste water and soil purification. The moisture content in green chips, i.e., fine, coarse, extra coarse, air
spruce is approx. 55-60% of the total spout and gassifier. Note that the names
2.4 Physical Characteri- weight and correspondingly approx. 45% refer to the size-grading only and not to
for beech /ref. 24/. After drying during the the quality.
sation of Wood Fuels summer season, the moisture content is Concurrently with the preparation of
In Denmark, wood from forestry and from reduced to approx. 15% of the total weight a new Danish quality description, a Euro-
wood industry is used in the form of fire- - depending on weather, stacking and pean standardisation work in respect of
wood, wood chips, bark, shavings, bri- covering - which is the recommended solid biofuels has been implemented.
quettes, pellets, and demolition wood for moisture content for use in wood stoves The purpose of this work is to standard-
firing in, e.g., wood stoves, wood pel- /ref. 25/. The ash content is often below ise measuring methods and to arrive at
let-fired boilers, district heating plants, 2% of the dry matter. common quality descriptions.
and CHP plants. The technologies used Screen analyses indicate the weight
at these plants stipulate various require- distribution among various size catego-
ments in respect of the physical proper-
Wood Chips ries of wood chips. In the old standard,
ties of the wood i.e. size, size distribu- Wood chips are comminuted wood in these size categories were based on a
tion, moisture content, ash content, and lengths of 5-50 mm in the fibre direction, shaking screen that is also used for cellu-
pollutants (stones, soil, and sand). longer twigs (slivers), and a fine fraction lose- and chipboard chips. The new qual-
A physical characterisation of wood (fines). Whole-tree chips are chipped ity description is based on a new rotating
fuels is important when choosing fuels for from whole trees including branches in screen unit that is more capable of
various boiler systems and technologies. the first thinning of spruce stands or in size-grading the wood chips.
In addition, information on the physical connection with converting old mountain The five types of wood chips are
properties of the wood fuels can be used pine and contorta pine plantations. Wood aimed at different types of consumers.
when drafting contracts for future deliver- chips are also produced from top ends Fine chips are suitable for small do-
ies, specifying the fuel in relation to cer- and other residues in clear-cuttings. mestic boilers where the chips are trans-
tain types of boiler systems, and the Sawmill wood chips are a by-product of ported from the silo to the boiler with a
drafting of quality descriptions of the the sawing of logs. Furthermore willow screw conveyor. The screws are of a
wood fuel. Knowledge of these proper- wood chips are produced from short rota- smaller dimension and very sensitive to
ties in relation to various types of wood tion coppice grown on agricultural land. large particles and slivers.
fuels thus contributes to a promotion of The required type of wood chips Coarse chips are suitable for larger
an environmentally and economically op- depends on the type of heating system. boilers that are able to handle a coarser
timal application of the fuel /ref. 23/. A new system for the quality description chip.
of wood chips based on size classifica- Extra coarse chips with a limited
tion is currently underway because the amount of fine material are suitable for
Fuelwood old standard from 1987 no longer covers heating plants with grates where the
Fuelwood is split, round or chopped the kind of wood chips produced and chips normally are forced into the boiler.
wood from delimbed stems, cut-off root used today. The old standard divided Air spout chips are suitable for in-
ends, and tops and branches of hard- wood chips into fine and coarse wood stallations throwing the chips into the
wood or softwood. Ready-to-use fire- chips (Table 3). combustion chamber. These installations
wood is normally split to 15-35 cm. The wood chips delivered to the need a certain amount of “dust” and are
Chunks of 6-8 cm thickness are most heating plants are coarser than coarse sensitive to slivers.
Page 14 Wood for Energy Production
Wood as Energy Resource
Gassifier chip is an extra coarse type of Name Hole size Fine Medium Coarse Air spout Gassifier
chips with a very limited amount of “dust”
and other fine particles. This type of chip Dust £ 3.15 mm <10 % <8 % <8 % >2 % <4 %
is particularly suitable for smaller Small 3.15< ´ £ 8 mm <35 % <30 % <20 % >5 % <8 %
Medium 8< ´ £ 16 mm * * * >60 %** <25 %
A detailed description of the various
quality classes can be found in Table 4. Large 16< ´ £ 45 mm <60 % * * >60 %** >60 %***
All size-distributions are measured with a
Extra Large 45< ´ £ 63 mm <2.5 % <6 % * <15 % >60 %***
rotating screen that is developed with
support from the Danish Energy Agency. Overlarge >63 mm <0.25 % <0.6 % <3 % <3 % >60 %***
The screen sorts out the so-called over- Overlong 10 100-200 mm <1.5 % <3 % <6 % <4.5 % <6 %
long particles before the remaining parti-
cles are distributed into the six classes Overlong 20 >200 mm**** 0% <0.5 % <1.5 % <0.8 % <1.5 %
by means of five screens with round * No demands
holes of 3.15, 8, 16, 45 and 63 mm diam- ** These two classes shall make up for minimum 60 %
eter respectively. *** These three classes shall make up for minimum 60 %
**** Particles with the following dimensions are not allowed
These holes are in accordance with
- longer than 500 mm with a diameter >10 mm
the ISO Standard 3310/2. Particles larger - larger than 30 ´ 50 ´ 200 mm
than 63 mm and smaller than 100 mm
are discharged from the end of the sieve. Table 4. The new quality description includes five types of chips. The table states the
The overlong particles are sorted by demands for size distribution in percentages of the total weight.
hand into two classes: 100 to 200 mm
length and over 200 mm length. needles may exceed 5% of the dry mat- bark cannot be regarded as wood chips,
According to the old standard, sliv- ter weight, in branches and bark approx. but size analyses of bark - based on
ers were defined as particles longer than 3%, and in stemwood approx. 0.6% /ref. wood chip standard - show that bark has
10 cm and at the same time thicker than 27/. Wood fuel for small boilers and dis- a very heterogeneous size distribution
1 cm. These particles can be very trou- trict heating plants has an ash content of with a large proportion of fines /ref. 28/.
blesome in screw conveyors. In the new 1-2% of the dry matter weight. Bark is very moist, approx. 55-60 % of
quality description, the term overlong the total weight, and single firing with
covers all particles longer than 10 cm, ir- bark normally takes place in special boil-
respective of diameter. These particles
Bark ers because of problems with the high
are problematic during feed stock han- Bark for energy production is produced moisture content. Bark is the outermost
dling. The proportion of particles above by peeling of bark at softwood sawmills layer of the tree, where pollutants are of-
10 cm length is of great importance to and by the cutting of slabs at hardwood ten found in the form of soil, sand, and to
the wood chip bridging propensity. sawmills. Strictly speaking, comminuted a certain extent lead from cartriges.
The moisture content in whole-tree
chips depends on the production method.
The moisture content of wood chips pro-
duced from green trees is approx. 50-
60% of total weight, but after summer
drying of the trees for 3-6 months, the
moisture content is reduced to approx.
35-45% of the total weight. Chip-fired
boilers with stoker for detached houses
etc. can manage wood chips with a mois-
ture content between 20 and 50% of the
total weight, while district heating plants
normally accept wood chips with a mois-
ture content of 30-55%. District heating
photo: finn jensen
plants with flue gas condensation nor-
mally want wood chips with a high mois-
ture content in order to utilise the con-
Wood chips may be polluted with The prototype of a new rotating classifier. Wood chips are filled into the hopper from
stones, soil, and sand which increase the the top, lengthwise orientated on a shaking table, and passed to the funnel tube (on
ash content. The ash content in whole the left), where the chips fall into the rotating drum. The round holes in the drum in-
trees depends on the wood species and crease in size from left to right. The content of the drawers is weighed. From left: Over-
the quantity of needles, branches, and long, fines, small, medium, large, extra large and overlarge.
stemwood. The natural ash content in
Wood for Energy Production Page 15
Wood as Energy Resource
Sawdust and Shavings
Sawdust and shavings that are pro-
duced by planing, milling etc. are a
by-product or residue from wood indus-
tries. Sawdust and shavings are be-
tween 1 and 5 mm in diameter and
photo: the danish forest and landscape research institute/flemming rune
length. The moisture content in sawdust
varies with the material that has been
sawed, originating from wood industries
that manufacture rafters and windows
etc., and may have a moisture content
of 6-10% of the dry matter weight, but
45-65% of the total weight if the wood
was green, recently harvested.
Shavings are very dry with a mois-
ture content between 5 and 15% of the
total weight. Therefore, they are normally
used for the production of wood pellets
and wood briquettes. They contain few
pollutants, since it is normally stemwood
that is used, and the ash content is
therefore less than 0.5% of dry weight.
Forest chips, sawdust, and fresh bark from spruce, and wood pellets.
Wood Briquettes and Wood
approx. 8-10 % of the total weight /ref. fore burning varies very much in size.
Pellets 29/. Slagging problems are very limited Demolition wood is often relatively dry
Wood briquettes are square or cylindrical when burning briquettes and pellets, and with a moisture content of approx. 10-
fuels in lengths of 10-30 cm and a diame- the amount of ash is low, approx. 0.5-1% 20% of the total weight. The burning of
ter/width of 6-12 cm. Wood pellets are of the dry matter weight /ref. 30/. demolition wood and other industrial
cylindrical in lengths of 5-40 mm and a wood waste may be problematic, since
diameter of 8-12 mm. the wood may be polluted with residues
Briquettes and pellets consist of dry,
Wood Waste from paint, glue, wood preservatives,
comminuted wood, primarily consisting of Wood waste is wood that has been used metal, rubber, and plastic material de-
shavings and sawdust compressed at for other purposes e.g. constructions, pending on the previous use. If the wood
high pressure. The size distribution is residues from new buildings or recon- waste contains glue (more than 1% of
very uniform which makes the fuel easy structed buildings before being used as the dry matter weight), paint etc., a waste
to handle. Pellets from the same con- fuelwood. Other types of recycling wood tax should be paid, and the wood waste
signment will be of the same diameter. include disposable pallets and wood con- cannot be burnt in conventional boilers
Moreover the moisture content is low, tainers. The wood that is comminuted be- /ref. 31/.
Page 16 Wood for Energy Production