Guideline for Estimation of Biomass Potentials, Barriers and Effects.
Unused Forest Energy Potential & Fuelwood
Most commercial forests in Europe have an
unused energy potential, which can be used without endangering their role in the
natural eco-systems. Beside this, most forests already have a production of
firewood. Mountain forests and other less commercial forests can in certain
cases also deliver wood for energy, but only after due environmental
consideration.
The available forest residues are generally branches with diameters
smaller than 7 cm. Generally, leaves and roots should be left in the forest to
preserve a healthy forest environment. They are also more difficult to use for
energy than branches.
It is not enough to use more firewood, the efficiency needs to be
increased as well: Traditional ovens and furnaces have in many cases
efficiencies as low as 30%, compared with about 80% for efficient furnaces.
Increased efficiency can thus more than double the energy outcome of wood
burning, without using more wood. For larger installations, flue-gas
condensation can raise efficiency further. For larger applications, wood
furnaces can be replaced with wood gasifiers + gas motors or steam boilers +
turbines, for cogeneration of electricity and heat.
Energy content
The energy content in
totally dry wood is apr. 5.2 kWh/kg. In normally dry firewood (20% humidity) the
energy content is apr. 4.2 kWh/kg (lower heating value). In most statistics,
wood is measured in cubic meter solid wood (with or without bark). The density
of dry wood varies from 800 kg/m3 for hard leafy wood (e.g. beech) to 600 kg/m3
for coniferous (e.g. pine). This gives energy contents of respectively 3400 and
2500 kWh/m3 for beech and pine (lower heating value, 20% humidity).
For furnaces with flue-gas condensers, the energy output can be 80-90% of
the higher heating value, which is respectively apr. 4% and 10% above lower
heating values for wood with 20% and 40% humidity.
Resource estimation
The available
amount of wood can be estimated from forest statistics as the difference between
annual growth (in m3, including bark) and the annual wood extraction for timber
and other non-energy purposes. Bark can be estimated to 20% of wood exclusive
bark. Often the statistics provide only commercial extraction, to which should
be added an estimate of non- commercial use. The non-commercial use is often in
the form of firewood-gathering by local inhabitants, and could thus be included
in the energy potential. In reality the resource might be lower than this
estimate due to problems of extracting all branches and/or due to the need of
leaving some branches in the forest for ecological reasons. These two factors
can reduce the resource with as much as 50% even in commercial forests.
If forest statistics are incomplete or unreliable, simplified estimates
can be made:
 if only figures for commercial use is available, the potential for
wood residues can be estimated as a fraction of the commercial use. Danish
experience is that wood for wood-chips (branches smaller 7 cm in diameter) is
equivalent to 25% of the timber production including bark or 31% of the timber
exclusive bark.
if only forest area is known, a first estimate can be made based on
area of commercial forest. An estimate from Germany (BUND) gives an annual
growth of forests of 10-15 tonnes/ha with an energy content of 150 - 225 GJ/ha
(42 - 63 MWh/ha). If 3/4 of this is used for timber, the available residues has
an energy content of 40-60 GJ/ha (11 - 16 MWh/ha). An estimation of residues
from forests on the Danish island Bornholm gives practical usable residues
smaller than 7 cm in diameter of 1.7 tons/ha, equivalent to 18 GJ/ha (5 MWh/ha)
with 40% humidity or 25 GJ/ha (7 MWh/ha) with 20% humidity. These estimates do
not take into account the important factors of climate and soil for the actual
wood production.
Barriers
Use of firewood
for heating does not in general pose barriers. The efficient use of firewood,
however, requires efficient ovens and basic knowledge of the users. Using
wood-chips requires equipment for producing the wood- chips, storaging, drying,
and feeding into an appropriate boiler. This production-chain should be set up
locally for successful use of wood-chips for heating. Wood-chips are most
suitable in larger boilers, above 100 kW. Often wood-chips have high humidity
(40 - 60%), and boilers with flue-gas condensation should be preferred.
Effects on economy, environment and employment
Economy
Use of firewood and
wood-chips are based on a local resource, requires minimal transport/import and
is therefore quite inexpensive in comparison to fossil fuels.
Price
estimates, excluding transport & profits (of leafy trees, density 760
kg/m3):
Denmark: 240 DKK/m3 equal to 0.11 DKK/kWh (0.0203 $/kWh)
Danish
example with Czech wages: 513 Csk/m3 equal to 0.24 CsK/kWh (0.011 $/kWh)
Of the Danish price 2/3 is wages, while the rest is fuel and machine
costs. Of the Czech price 1/3 is wages.
Environment
Use of wood replacing fossil fuels reduces net CO2
emissions, because the forest absorbs the same quantity of CO2, which is
released in the later combustion of the wood. The energy to process the wood is
in the order of a few percent of its heating value.
Wood combustion
emits very little sulphur (SO2) compared with coal and oil. NOx emissions depend
on the combustion process and often the lower combustion temperature leads to
lower emissions than for coal and oil combustion. Emissions of particulate and
unburned hydrocarbons are totally dependent on the combustion processes, and can
be a problem in small and badly designed furnaces. Ashes from the combustion can
often be used as fertilizer.
It is important that the extraction of
wood is done in a sustainable manner, with adequate re-planting etc.
Employment
According to French experience, utilizing of excess
energy from forests requires 450 jobs/TWh with the degree of mechanization that
is normal for Western Europe.
Hand-rules
Each ha of forest on good soil in Central Europe
grows 10 tons/ha of wood. If 25% of this is available as waste-wood for energy,
the output for energy is 2.5 tons or 11 MWh (20% humidity).
Residues from wood industry
In saw-mills, pulp mills and all wood
processing industries, residues are made that can be used for energy purposes.
From saw-mills is mainly bark and saw-dust. From pulp-mills (cellulose and paper
production) is black and sulphite liquors as well as wood and bark residues.
From sawmills comes edgings, chips, sawdust, bark and other residues. Some of
these residues are used for pulping, and particle-and fibreboard. Analysis of 7
countries shows that 30-70% of wood industry residues are used for these
non-energy purposes.
The residues in forms of larger pieces can be
made into wood- chips for wood-chip boilers, while sawdust can be burned in
special furnaces or compressed into wood pellets of brickets, that can be used
in smaller furnaces and ovens.
Often wood industry uses their wood
residues to meet own energy demands for heating, steam and eventually
electricity.
Energy content
The energy content for wood residues are about
4.2 kWh/kg (lower heating value, 20% humidity), equivalent to 3400 and 2500
kWh/m3 for beech and pine respectively. See also previous chapter.
Resource Estimation
Evaluation of wood residues can be based on trade-statistics of
non-energy wood and wood-products compared with total extraction from forests.
The difference is available for energy purposes, and is probably to some extent
already used as such in wood industries.
As a simple estimate can be
used that residues in general are 25-35% of total forest removals (e.g. Poland
29%, Canada 29%, Finland 33%, Sweden 36%, USA 37% from Biofuels). If a larger
part of forest removals are exported without processing, the figure will be
lower.
Barriers
This resource has
in general the fewest barriers of all renewable energies. An efficient
utilization requires, however, investments in new boilers, or at least in a
pre-combustion furnace, that can be attached to an existing (good) boiler.
Effect on economy, environment and employment
When the residues from industry are treated
as waste without commercial value, the economy of using them for energy is
almost always cost-effective, and has a better economy than wood residues from
forests.
Environmental effects are equal to wood residues from forests, as long as
combustion of chemically treated and painted wood residues is avoided. Such
wood-residues should be treated as municipal waste or chemical waste depending
on the treatment.
The direct employment of using industrial wood waste is low because the
waste has to be handled anyway. Indirectly it gives considerable employment
because it turns unused materials into a valuable product (energy).
Combustible waste from agriculture
Straw, prunings of fruit trees and wine and
olive oil residues are all residues from agriculture that can be used for energy
purposes. Straw harvest is depending on weather conditions and vary considerably
from year to year. The straw surplus has also large variations from year to
year. If a large part of the surplus is used, an alternative fuel should be
considered for years with little surplus straw. Such an alternative fuel could
be wood-chips forest residues, that can be used alternatively with straw in many
boilers. The forest residues can stay several years in the forests before usage.
Straw surplus can be ploughed into the field for enriching the humus layer of
the field. When this is needed for a sustainable agriculture, the surplus straw
for energy will be lower.
Energy Content
The energy content of straw is 4.9 kWh/kg of dry matter (high heating
value). With a typical of 15% humidity the lower heating value is 4.1
kWh/kg.
The energy in 1 m3 of densely compressed straw bales is 500
kWh (density 120 kg/m3).
The average efficiency for 22 straw-fired
heating stations in operation in Denmark is 80-85%, not including flue-gas
condensation.
Resource Estimation
Estimations of straw production can be obtained from agricultural
statistics. This value should be reduced with agricultural consumption of straw
for animal fodder and bedding. The agricultural consumption is very dependent on
the type of stables used. In Denmark the average available surplus for energy is
estimated to 59% of which 1/5 is already used, mainly for heating (Straw). In
Eastern Bohemia, this surplus is estimated to about 35%. As a general,
conservative estimate for Europe 25% of the straw production can be used for
energy. The straw production varies +/- 30% from average years to years with
high respectively low straw harvest.
 If
straw production is not available from statistics, relatively good estimates can
be made from statistics of grain production. As a rough estimate the amount in
tons of straw can be equalled to the amount of grain in tons. In the Czech
Republic the average ratio between straw and grain is found to:
wheat 1.3 tons
straw/tons grain
barley 0.8 tons straw/tons grain
rye
1.4 tons straw/tons grain
oat 1.1 tons straw/tons grain
A rough estimate can be made based on agricultural area and a straw
harvest of 4-7 tons/ha depending on soil, type of grain and weather.
Barriers
Limited experience
and funds for the necessary investments are often the largest barriers to use
straw for energy. Other barriers can be:
the need to develop a market for
straw with attractive prices for users as well as suppliers,
pesticides can
in certain situations give unwanted chlorine compounds in the straw. This can be
reduced by leving the straw for a period at the field before collection, so
called wilting.
use of straw in inadequate and polluting boilers can give straw a
bad reputation.
Effect on economy, environment and employment
Economy
In Denmark, straw-prices
vary from 0.085 DKK/kWh (1.2 EURO cent or 1.2 US cent) to 0.12 DKK/kWh for
baled straw delivered at a straw-firing station. In Czech Republic the prices
for straw collected at the farm has been quoted at 0.043 Csk/kWh (0,15 EURO
cent) for loose straw and 0.054 Csk/kWh (0.19 EURO cent) for baled straw.
Costs, average for 16 straw-fired installations in Denmark are per kWh
heat produced:
|
Danish
average |
Estimate for Czech
Republic |
| Fuel |
1,9 EURO cent |
0,26 EURO cent |
| Electricity* |
0,12 EURO cent |
0,12 EURO cent |
| O&M, administr. |
1,3 EURO cent |
0,26 EURO cent |
| Capital costs |
1,5 EURO cent |
1,5 EURO cent |
| TOTAL |
4,8 EURO cent |
2,14 EURO cent | * Electricity
consumption is in average 2.3% of heat produced
The environmental impact of using agricultural residues are, as for wood,
reduced CO2-emission, reduced sulphur emissions, compared with coal and
oil. Emissions of particulate, NOx and volatile organic compounds (VOC)
depend on furnaces and flue-gas treatment. Chlorine components in straw gives
emission of HCl as mentioned above. Danish experience from 13 straw-fires
heating stations shows the following emissions (all plants have particulate
filters):
|
Emission |
Average
Emission
g/kWh straw |
Variation of
emissions
g/kWh straw |
|
Particulate |
0,14 |
0,01-0,3 |
|
CO |
2,2 |
0,4-4 |
|
NOx |
0,32 |
0,14-0,5 |
|
SO2 |
0,47 |
0,4-0,6 |
|
HCl |
0,14 |
0,05-0,3 |
|
PAH* |
0,6 |
0,4-1 |
|
Dioxin** |
|
1-10 ng | * PAH = Polyaromatic
Hydro-Carbons. This is the carcinogenic part of VOCs.
** Dioxin
figures are based on only two measurements, figures given in nanogram,
10-9 g.
Employment
The direct employment of harvesting straw in a fully
mechanized agriculture in Denmark is estimated to 350 jobs/TWh. This is for
technologies with large straw-bales (500 kg each). For a system based on smaller
bales (10-20 kg), the employment is larger.
ENERGY CROPS
It
is estimated that 20-40 million hectares of land in the EU will be surplus to
conventional agricultural requirement. The same situation (agricultural
overproduction and setting the land aside) can be expected in Central Europe as
well. This set aside land can be used for different purposes, one of them is
energy crop production.
Promising crops which can be planted for energy purposes in Europe are
short rotation trees (coppice of various willows and poplars), Miscanthus and
Sweet Sorghum. These crops can be utilized by direct combustion for heat and
electricity production. Other promising energy crops are plants for liquid fuels
as rape seeds for bio-oil.
  Willows.
Energy
Contents and Yields
The following table gives an
overview of the expected yields and energy contents for three of the promising
plants for solid fuel production.
| |
Yields
(tonnes/ha/year) |
Energy
content
(GJ/dry tonne) |
Energy
Yields
(GJ/ha/year) |
|
Salix (Willow)* |
15 |
16 |
240 |
|
Miscanthus (Elephant grass) |
20 |
17 |
340 |
|
Sweet Sorghum |
25 |
18 |
450 | *Increment of Salix is
2-3 meters in one year (2-3 cm per day in the summer), harvest every third
year.
Miscanthus.
Another promising plant is hemp, which has yields up to 24 tonnes/hectare
in approximately 4 month. Hemp plantation is illegal in many countries, even
though some variants has very little content of cannabis.
Resource Estimation
The
energy potentials can be estimated from the area of land which is set aside in
the country/region and can be used for energy plantation and the expected
outcome of the above crops under the actual climate and soil conditions. In most
countries, national estimates exists of the different yields of the plants.
Using excess farm land and ecologically degraded land should be the
priority.
Important feature in estimation of potential is input : output ratio. If
the bagasse of Sweet Sorghum (2/3 of its energy content) and the sugar (1/3 of
its energy content) are utilised for energy purposes the input : output (I/O)
energy ratio will reach 1:5 . This means that five times more energy is
recovered from crop (on fuel basis) in comparison with energy utilised for
the seeding, fertilisers and pesticides treatment, harvesting, transport and
conversion into useable fuels. Usually the input : output ratio is larger than
1:5 for trees and smaller for plants for liquid biofuels.
Barriers
Short
rotation crops may require as much fertilization as traditional crops and
degraded land must be regenerated before cultivation using fertilization. For
tree crops these drawbacks may be offset by the fact that they retain an active
root system throughout the year. Wood ash would be an effective fertilizer for
biofuels plantation, reducing the problems caused by the leaching of fertilizers
into ground water.
Effect on Economy, Environment and Employment Economy, Costs:
Production costs for Sweet Sorghum are
50 Euros per dry tonne.
Production cost of Salix are 70 Euros (500 DKK) / tonne of dry matter in
Denmark (Hvidsed).
Electricity generation cost for biomass (Sweet sorghum ) fuelled system
(1992) and improved systems (2000):
Small facility : 0,16 EURO/kWh
Large facility : 0,08 EURO/kWh
Small improved : 0,07
EURO/kWh
Large
improved : 0,05 EURO/kWh
Environment
An
important feature for Salix is that it can be used for water purification - it
is possible to grow Salix in purification systems and in the same time harvest
the Salix for energy (10-20 tonnes of sludge can be used on each hectare every
year). Other benefits of biomass for energy plantation includes forest fire
control, improved erosion control, dust absorption, and used as replacement for
fossil fuels: no sulphur emission and lower NOx emissions.
Employment
For
Sweet Sorghum production cost 50% is manpower cost. Production of about 500
tonnes of dry biomass per year justifies the creation of one new job. Other new
jobs could be created in related industries such as composting, pulp for paper,
service organisation etc.
Hand Rule
Sweet Sorghum output for trials in different locations of
Central and Southern Europe:
Annually 90 tonnes of fresh
material = 25 tonnes of dry matter per hectare = 450 GJ or 11 tonnes of
oil equivalent can be produced. 1/3 as ethanol from sugars and 2/3 of fuel
from bagasse. This corresponds to the absorption of 30-45 tonnes of
CO2 per hectare and per year.Average yearly electricity
consumption of a West European person can be met by growing poplar on 0.25
hectare. |
BIOGAS
The
largest potential for biogas is in manure from agriculture. Other potential
raw-materials for biogas are:
sludge from mechanical and
biological waste-water treatment (sludge from chemical waste-water treatment has
often low biogas potential)
organic household waste
organic,
bio-degradable waste from industries, in particular slaughter-houses and
food-processing industries
Care should be taken not to include waste with heavy metals or harmful
chemical substances when the resulting sludge is to be used as fertilizer. These
kinds of polluted sludge can be used in biogas plants, where the resulting
sludge is treated as waste and e.g. incinerated.
Another biogas source is landfills with large amounts of organic waste,
where the gas can be extracted directly from drillings in the landfill, so
called landfill gas. Such drillings will reduce uncontrolled methane emission
from landfills.
Energy Content
The biogas-production will normally be in the range of 0.3 - 0.45 m3 of
biogas (60% methane) per kg of solid (total solid, TS) for a well functioning
process with a typical retention time of 20-30 days at 32oC. The lower heating
value of this gas is about 6.6 kWh/m3. Often is given the production per kg of
volatile solid (VS), which for manure without straw, sand or others is about 80%
of total solids (TS).
A biogas plant have a self-consumption of
energy to keep the manure warm. This is typically 20% of the energy production
for a well designed biogas plant. If the gas is used for co-generation, the
available electricity will be 30-40% of the energy in the gas, the heat will be
40-50% and the remaining 20% will be self-consumption.
Resource Estimation
For
manure, the available data is often the numbers of livestock. From this can be
made an estimation of available manure. While the amount of manure produced from
animals depends on amount and type of fodder, some average figures are made for
most countries.
The following table shows the figures for Denmark
:
|
Kind of
animal |
Manure
type |
Amount>
(kg/day) |
Solid amount
(kg/day) |
Biogas per animal
(m3/day)* |
Energy per
animal
(kWh/yr) |
|
Cow |
Slurry |
51 |
5,4 |
1,6 |
3400 |
|
Cow |
Dry |
32 |
5,6 |
1,6 |
3400 |
|
Sow |
Slurry |
16,7 |
1,3 |
0,46 |
970 |
|
Sow |
Dry |
9,9 |
2,9 |
0,46 |
970 |
|
Hen |
Dry |
0,66 |
0,047 |
0,017 |
36 | *biogas with 65%
methane.
Yearly energy output is for biogas plant with 20% average
self-consumption and 360 working days. When animals are not in stables around
the year, the figure will be smaller. The figures are for milking cows and for
sows with breeding pigs under 5 kg.
To make an estimation of the yearly production, it should be evaluated how
many days per year the animals are in stables. For large poultry farms and
pig-farms it is often the whole year, while cows are in stables from a few
months a year to the whole year.
To estimate amount of manure from calfs, pigs and chicken, the following
estimates can be used:
calfs 1-6 month: 25% of milking
cows
other cattle ( calfs > 6 months, cattle for meet, pregnant
cows): 60% of milking cows
small pigs, 5-15 kg: 28% of sows
with pigs
fattening pigs > 15 kg: 52% of sows with pigs
fattening
chicken: 75% of hens
Barriers
A
number of barriers hold back a large scale development of biogas plants in
CEEC:
commercial technology for agriculture (the largest resource base)
is not available and have to be developed from existing prototypes or
imported.
it is difficult to make biogas plants cost-effective with sale of
energy as the only income. The most likely applications are when other effects
of the sludge-treatment has a value. This can e.g. be better hygiene, easier
handling, reduced smell, and treatment of industrial waste.
little
knowledge on biogas technology among planners and decision-makers.
Effect on economy, environment and employment
Economy
The economy of a biogas
plant consists of large investments costs, some operation and maintenance costs,
mostly free raw materials, and income from sale of biogas or electricity and
heat. Sometimes can be added other values e.g. for improved value of sludge as a
fertilizer.
In an example from Czech Republic the price for a Czech
plant is estimated to about 70,000 US $ for a plant for treatment of manure from
100 cows. This plant will produce about 220 MWh/year + energy for its own
heating. This gives an investment of 0.32 US $ per kWh/year. New Danish biogas
plants have similar investment figures. It is estimated that a joint-venture of
Czech and Danish technology could reduce prices by about 40% (to about 0.2 US $
per kWh/year); but this has not been shown in practice.
Operating and
maintenance (O&M) will normally per year be 10-20% of investment costs, but
it vary much with organization, wages, type of plant and eventual transport of
sludge. If O&M is 10% of investment costs, simple pay-back requirement is 10
years and no price can be set to increased value of the sludge, the resulting
energy price will be 0.04-0.06 US $/kWh or 0.03 - 0.045 Euros/kWh (based on the
above examples from Czech Republic).
The environmental effects of biogas plants are:
production of energy that
can replace fossil fuels, reducing CO2 emissions
reduce smell and hygiene
problems of sludge and manure
treatment of certain kinds of
organic waste that would otherwise pose an environmental problem
reduce
potential methane emissions from uncontrolled anaerobic degradation of the
sludge.
easier handling of sludge, which can increase the fraction used as
fertilizer and facilitate a more accurate use as fertilizer
Employment
The
direct employment of biogas plants are for Denmark estimated to 560 jobs/TWh, of
which 420 jobs/TWh are operating and maintenance, while 140 job/TWh are
construction (2000 man-years to construct plants producing 1 TWh and with
lifetime of 14 years). This estimate will be valid for mechanized systems with
some degree of centralization: some of the manure is transported to the biogas
plant from nearby farms.
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