2 BIOENERGY
2.1 Introduction
2.1.1 What is Bioenergy?
In order to understand what the term “bioenergy” means we first have to be familiar with
the terms “biomass” and “biofuel”. Definitions vary depending on the source.
Biomass is organic material which has stored sunlight in the form of chemical energy.
Biomasses include any organic matter that is available on a renewable basis. This means
mostly masses produced through photosynthesis, but also organic municipal and industrial
wastes. It is essential to understand that the origin of the chemical energy in biomasses is
solar power. Biomass is an important source of energy and the most important fuel
worldwide after coal, oil and natural gas.
Biofuels are solid, liquid or gaseous fuels produced from biomasses, although there are
exceptions. For example peat is in most cases referred to as a biomass, but is usually not
considered a biofuel because it renews so slowly. Biofuel technologies can efficiently
transform the energy in biomass into transportation, heating, and electricity generating
fuels.
Bioenergy means any usable energy obtained from biofuels. These relationships are
illustrated in Figure 2.1.
Figure 2.1: Relationships and processes between biomass, biofuel and bioenergy
2.1.2 Advantages and drawbacks of bioenergy
The main advantage of bioenergy today is that biofuel production and utilization has less
environmental impact than fossil fuel production and utilization. The atmospheric
emissions are usually smaller, most importantly carbon dioxide emissions. In some cases
producing biomass (energy plantations) can improve biodiversity and soil quality.
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In addition to bioenergy’s environmental benefits, bioenergy is also renewable. The fossil
fuel reserves will decline during this or next century. Therefore it is essential to develop
and start using substitutive renewable fuels, like biofuels. There are also some socioeconomic benefits associated with the use of biomasses, such as diversifying and securing
indigenous energy sources, employment especially in remote areas, suitability for
increasingly decentralised energy markets and improvement of energy supply for
developing countries.
Compared to coal, the level of ash-forming matter in biomass is typically much lower, but
instead the ash usually contains toxic metals and other contaminants. The ash has, unlike
fossil fuel ash, fertilizer value. Therefore, it can be used in forestry and agriculture to
replenish the nutrients removed by harvesting.
Bioenergy has also definite drawbacks. First of all, in most cases, especially when biomass
is harvested solely for energy purposes, it is still quite expensive. Biomass is expensive and
difficult to transport and the sources are often in rural areas. The heating values of
biomasses are low. It is possible to produce more easily transportable biofuels with higher
heating values, but the processes can be complicated and expensive. Biomass will also
decay during the time required for transport and storage. This reduces the already low
heating values of non-processed biomasses. Furthermore, the quality of biofuels varies
very much. This may cause problems, for example when the biofuel is combusted.
Bioenergy can also have negative environmental impacts. The emissions can be substantial
and may contain toxics such as dioxins. Annual energy crops sometimes have the same
problems as agriculture in general. Similarly, the constructions of power plants, harvesting
etc. have environmental impacts typical of any engineering project of similar scale.
Clearing residues may cause nutrient loss and erosion in forests.
2.1.3 History
Biofuels have been the fuels of necessity, particularly for heating, cooking, and distilling,
for most of mankind's short history. The late 19th century marked a change in fuel sources
away from biofuels toward coal and petroleum-based fuels. A prime mover in this change
was the gasoline-burning, internal combustion engine. Economic development in the
United States and Europe during most of the 20th century has been based on cheap energy
from non-biofuel sources, and much of the economic development in the Third World is
following this path. As we stand on the threshold of the 21st century, it is interesting to
note that we are again looking toward biofuels to supply a major portion of our energy
needs. However, our energy needs have changed, and so have the economic conditions
under which biomass is produced.
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2.2 Biomass
Biomass is a very poorly documented energy source. Lack of data has made decisionmaking regarding biomass energy difficult. The inability to fully appreciate the indigenous
biomass resource capacity and its potential contribution to energy and development is still
a serious constraint to the full realisation of this energy potential, despite a number of
efforts to improve biomass energy statistics. Still, as biomass can be found nearly
everywhere, biomass resources are potentially the world's largest and most sustainable
energy source - a renewable resource comprising 220 billion oven-dry tonnes (about 4 500
EJ) of annual primary energy production.
2.2.1 Characteristics
In the context of energy, biomass refers to all forms of plant-derived material that can be
used for energy: wood, herbaceous plant matter, crop and forest residues, dung etc. On a
dry-weight basis, heating values range from about 17.5 GJ/t (herbaceous masses) to about
20 GJ/t (woody masses). This is low compared to coal (30-35GJ/t). At harvest, biomass
contains considerable moisture, ranging from 8-20% (for wheat straw) to 95% (for water
hyacinth). Moisture content of coal is typically much lower (2-12%). Therefore, energy
densities for biomass are significantly lower than for coal, which results in need of either
drying or different combustion technologies. Combined with the dispersed nature of
biomass this implies that in order to be able to avoid high transportation costs, the
production of modern energy carriers (electricity or processed biofuels) from biomass
should usually be done in decentralised and relatively small installations.
2.2.2 Classification and definition of biomasses and biofuels
Biomass can be defined as:
• Plant and other growing species capable of being used as fuel.
• Organic material mainly composed of carbohydrate and lignin compounds, the
building blocks of which are the elements carbon, hydrogen and oxygen.
• Stored form of solar energy relying on the process of photosynthesis, by which
chlorophyll containing organisms capture energy in the form of light and convert
it to chemical energy.
Depending on the geographical and political context, the term “biomass” is
sometimes defined differently. This is further complicated by the fact that the
definition of “renewable” is flexible.
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2.2.3 Woody biomasses
The most important structural materials in wood are cellulose, hemi cellulose and lignin.
Lignin binds the wood’s fibres together and gives the wood its mechanical strength. In
wood there are also small amounts of extractives, for example resin consists of them. There
are lots of vaporizing substances in wood, so its flame is long and needs a large space for
combustion.
Wood consists mostly of carbon, hydrogen and oxygen, whose portion of the dry part of
wood is about 99% of the mass. The contents of elements in different parts of wood and
bark are seen in Table 2.1. It can be seen that the amounts of sulphur and chloride are very
low.
Table 2.1: Elements in woody biofuels
The humidity of live wood is usually high, 40% – 60%. This makes drying an essential part
of producing wood-based biofuels. Large power plants can use quite humid fuel, but
smaller scale burning requires that humidity is less than 25%. When wood is dried, “free
water” evaporates first in cavities. The physical characteristics of wood change when the
bound water in the cell walls starts to evaporate.
The net calorific heating value of the dry matter in wood is usually 18.3-20 MJ/kg.
Branches, tops and small trees usually give slightly higher values than whole trees.
Compared to other solid fuels like coal, the heating values of wood are quite low, which
sets limits to wood burning and handling equipment. Additionally, the need of storage
space is greater. The density of wood also has its impact on storage and it varies strongly
from species to species.
Wood fuels (meaning fuel wood, charcoal and black liquor) come from a variety of supply
sources, such as forests, non-forest lands and forest industry by-products. In 1998, 3.2
billion m3 of wood were harvested worldwide, more than 50% of which was used for wood
fuel. It has often been said that most wood fuels are obtained from forests, contributing to
deforestation in a major way. However, it is now estimated that considerable amounts of
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wood fuels come from non-forest areas, such as village lands, agricultural land, agricultural
crop plantations (rubber, coconut, etc.), homesteads and trees along roadsides.
Asia is by far the largest producer and consumer of fuel wood, accounting for 46% of
world production. Africa has the second highest share at 30%, followed by South America
and North America, both at around 8%. Africa relies heavily on wood fuel to cover for its
energy requirements.
Woody residues
Residues are currently the main sources of bioenergy and this will continue to be the case
in the short to medium term, with dedicated energy forestry/crops playing an increasing
role in the longer term. They are a quite under-exploited potential energy resource, and
present many opportunities for better utilisation. Estimates of global woody residues
production vary, but for instance Woods & Hall (1994) estimated these residues at 93 EJ a
year.
However, there are a number of important factors that need to be addressed when
considering the use of residues for energy. Firstly, there are many other alternative uses,
e.g. animal feed, erosion control, use as animal bedding, use as fertilizers (dung), etc.
Secondly, there is the problem of agreeing on a common methodology for determining
what is and what is not a recoverable residue. This is mainly due to variations in the
amount of residue assumed necessary for maintaining soil organic matter, soil erosion
control, efficiency in harvesting, losses, non-energy uses, disagreement about animal
manure production, etc. Nonetheless, many of these residues are readily available and
represent a good opportunity at low cost.
Forestry residues are obtained from forest management, and can increase the future
productivity of forests (Figure 2.2). Recoverable residues from forests have been estimated
to have an energy potential of about 35 EJ/yr. A considerable advantage of these residues is
that a large part is generated by the pulp and paper industry and sawmills and thus could be
readily available. Currently, a high proportion of such residues is used to generate energy
in these industries, but there is no doubt that the potential is considerably greater. For
example, Brazil's pulp and paper industry generates almost 5 Mtoe of residues that is
currently largely wasted. The estimated global generation capacity of forestry residues is
about 10 000 MWt. Forestry residues include many subclasses of biomass.
Fig. 2.2 Forestry residues are by-products of the pulp and paper industry and sawmills
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Bark consists of outer and inner bark (phloem). About 10-20 % of bole tree consists of
bark while its part of smaller branches can be even 60 %. As there are great amounts of
lignin in bark, its heating values are high, about 20 MJ/kg for inner and 20-32 MJ/kg for
outer bark. Generally, heating values of hardwood bark are higher than those of softwood.
As such, bark’s high humidity and ash content (1.5-3%) make it quite difficult as a fuel.
Combusting qualities can be improved by different kinds of treatments like drying
Sawdust is a by-product of timber sawing. It is often very humid and airy, although its
humidity can vary greatly. Sawdust is usually co-fired in industrial boilers.
Black liquor is a by-product of the pulp and paper industry. Black liquor supplied about
72 Mtoe of energy in 1997. The production and consumption of black liquor are
concentrated in developed countries with large pulp and paper industries. Therefore, about
50% of black liquor consumption is in North America, followed by Europe with 19% and
Asia with 12%. In North America and Europe, black liquors are widely used for fuelling
the heat and power plants of the large pulp and paper mills. Almost all of their energy
needs are met by black liquors and, in some cases, surplus electricity is sold to the public
grid.
2.2.4 Agricultural residues
Agricultural waste is a potentially huge source of biomass. Waste from agriculture
includes: the portions of crop plants discarded like straw, weather damaged or surplus
supplies, and animal dung. Every year, millions tonnes of straw are produced world-wide
with usually half of it being surplus to the need. In many countries this is still being burned
in the field or ploughed back into the soil, but in some developed countries environmental
legislation, which restricts field burning, has drawn attention to its potential as an energy
resource.
Smil (1999) has calculated that in the mid-1990's the amount of crop residues amounted to
about 3.5 to 4 billion tonnes annually, with an energy content representing 65 EJ, or 1.5
billion tonnes oil equivalent. Hall et al. (1993) estimated that by only using the world's
major crops (e.g. wheat, rice, maize, barley, and sugar cane), a 25% residue recovery rate
could generate 38 EJ and offset between 350 and 460 million tonnes of carbon per year.
The worldwide generation capacity from agricultural residues (straw, animal slurries, green
agricultural wastes) is estimated to be about 4 500 MWt.
Effort to remove crop residues from soils and to use them for energy purposes leads to a
central question: how much residue should be left and recycled into soil to sustain
production of biomass? According to the experience from developed countries around 35%
of crop residues can be removed from soil without adverse effects on future plant
production.
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The most important agricultural residues are straw and sugar production residue from sugar
cane, i.e. bagasse. On a dry-weight basis, heating values of wheat straw, sugarcane bagasse
are about 17.5 MJ/kg. Some heating values are given in Table 2.2. Straw is not a very good
fuel due to its high ash, alkali and chlorine content.
Table 2. 2: Heating values of straw and grain
2.2.5 Livestock residues
The potential of energy from dung alone has been estimated at about 20 EJ worldwide.
However, it is questionable whether animal manure should be used as an energy source on
a large scale, except in specific circumstances. This is because of:




Its greater potential value for non-energy purposes, e.g. if used as a fertiliser it may
bring greater benefits to the farmer;
It is a poor fuel and people tend to shift to other better quality biofuels whenever
possible;
The use of manure may be more acceptable when there are other environmental
benefits, e.g. the production of biogas and fertiliser, given large surpluses of manure
which can, if applied in large quantities to the soil, represent a danger for agriculture
and the environment;
Environmental and health hazards, which are much higher than for other biofuels.
On the other hand, biogas derived from manure is a much better fuel and environmentally
its production is sound, as greenhouse gases are collected, which otherwise would be
released into the atmosphere.
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2.3 From biomass to biofuel
Almost all biomasses are also biofuels as such, since they can usually be combusted
without further processing. The most basic processes, like chipping or drying, can usually
be done at least to some extent at the site of harvest. The products are the most basic
processed biofuels. As the process becomes more extensive and complicated, the quality of
the biofuel becomes better and its usability increases. Biomasses, processes, biofuels and
their uses can be seen in Figure 2.3.
Fig. 2.3: Biomass-based raw materials for energy conversion, their processing and end-product biofuels
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2.3.1 Processed Solids
(A) Charcoal
Charcoal is the oldest processed biofuel. Today charcoal means mainly charcoal used for
domestic purposes, but it is still widely used in industries, especially steel end silicon
production.
Charcoal can be produced from all tree species and parts of plants. Characteristics of the
product depend on both the raw material and the production process. Hardwood is usually
considered the best raw material for producing charcoal.
During pyrolysis or carbonisation the wood is heated in a closed vessel, isolated from the
oxygen of the air which otherwise would allow it to ignite and burn to ashes. Without
oxygen we force the wood substance to decompose into a variety of substances the main
one of which is charcoal, a black porous solid consisting mainly of elemental carbon. Other
constituents are the ash from the original wood amounting to 0.5 to 6% depending on the
type of wood, the amount of bark, contamination with earth and sand, etc. and tarry
substances which are distributed through the porous structure of the charcoal. As well as
charcoal, liquid and gaseous products are produced, which may be collected from the
vapours driven off. The liquids are condensed when the hot vapours pass through a watercooled condenser. The non-condensable gases pass on and are usually burned to recover
the heat energy they contain. This wood gas, as it is called, is of low calorific value
(around 10% of that of natural gas).
As the wood is heated in the retort it passes through definite stages during its conversion
into charcoal. The formation of charcoal under laboratory conditions has been extensively
studied and the following stages in the conversion process have been recognised.

At 20 to 110°C: The water in the wood absorbs the heat and is transformed into steam.
The temperature remains at or slightly above 100°C until the wood is bone dry.

At 110 to 270°C: The final traces of water are extracted and the wood starts to
decompose by releasing carbon monoxide, carbon dioxide, acetic acid and methanol.
Heat is absorbed.

At 270 to 290°C: This is the point at which exothermic decomposition of the wood
starts. The breakdown continues spontaneously providing that the wood is not cooled
below this decomposition temperature. Mixed gases and vapours continue to be
released together with some tar.

At 290 to 400°C: As breakdown of the wood structure continues, the vapours given
off comprise the combustible gases carbon monoxide, hydrogen and methane together
with carbon dioxide gas and the condensable vapours: water, acetic acid, methanol,
acetone, etc. and tars which begin to predominate as the temperature rises.

At 400 to 500°C: At 400°C the transformation of the wood to charcoal is practically
complete. The charcoal at this temperature still contains appreciable amounts of tar,
perhaps 30% by weight trapped in the structure. This soft burned charcoal needs further
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heating to drive off more of the tar and thus raise the fixed carbon content of the
charcoal to about 75%, which is normal for good quality commercial charcoal. To drive
off this tar the charcoal is subject to further heat inputs to raise its temperature to about
500°C, thus completing the carbonisation stage.
(B) Pellets
Pellets are usually made out of woody residues (sawdust and wood shavings), and are used
in large quantities by district heating systems (Figure 2.4). Pellets are cylindrical or
sometimes cubic granules that are produced by first drying and possibly grinding and then
compressing (usually woody) biomass. Their diameter is 8-12 mm and length 10-30 mm.
They are mainly used as such for small-scale heating. In larger power plants the pellets are
ground before burning in for example pulverized fuel boilers. Wood pellets have a low
moisture content (under 10% by weight), giving them a higher combustion value than other
wood fuels. The fact that they are pressed means they take up less space, so they contain
much energy per cubic meter. The burning process is highly combustible and produces
little residue. A major disadvantage of pellets is their weak humidity endurance. The net
calorific heating value is 14-17.5 MJ/kg.
Fig. 2.4 Wood pellets
(C) Briquettes
Briquettes are produced from dry sawdust, grinding dust or cutter chips into a cylindrical
form by compressing. The diameter of a briquette is between 50 and 80 mm. Compared to
other fuels, they are a heavy and dry fuel. Net calorific heating values are around 17
MJ/kg.
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(D) Wood chips
Wood-chips (Figure 2.5) are made of waste wood from the forests. Trees have to be thinned to
make room for commercial timber (beams, flooring, furniture). Wood-chips are thus a waste
product of normal forestry operations. Wood is cut up in mechanical chippers. The size and
shape of the chips depends on the machine, but they are typically about a centimetre thick and 2
to 5 cm long. The water content of newly felled chips is usually about 50% by weight, but this
drops considerably on drying. The heating value is 14.3 MJ/kg (25% moisture content on wet
basis). In many countries like in Denmark wood-chips currently produced are burnt in wood-chip
fired district heating stations. They are usually delivered by road, so there must be facilities for
storing at least 20 m3 of chips under cover if they are to be used in an automatic burner.
Fig. 2.5 Wood chips
2.3.2 Liquid Biofuels
Liquid fuels from biomass are generated by gasification, fermentation, and pyrolysis
technologies. Liquid fuels from biomass are methanol, ethanol, pyrolysis oils, synthetic
petroleum, vegetable oils, hydrocarbons and liquid hydrogen. Mainly methanol, ethanol and
vegetable oil production processes are more established from yields and cost point of view.
Liquid fuels from biomass can be used in transportation, in heat and power generation in fuel
cells and internal combustion engines.
(A) Ethanol Fuel
Ethanol can be produced by biologically catalysed reactions. Sugars can be extracted from
sugar crops, such as sugar cane and fermented into ethanol. Basic reaction of fermentation
that needs a catalyst (yeast) is:
For starch crops, such as corn, starch is first broken down into simple glucose sugars by
acids or enzymes known as amylases.
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Current world production of ethanol fuel is about 20 to 21 billion litres annually. Ethanol is
mostly used blended with gasoline in various proportions, and in a small percentage with
diesel. Ethanol fuel is a growing market, as it has a considerable potential for substituting
oil given the right conditions. Predictions vary enormously depending on when cellulose,
the most abundant raw material, can be used to produce ethanol commercially. For
example, it has been estimated that by 2020 over 30 billion litres (8 billion US gallons)
could be obtained from cellulose-based material in the USA alone. The environmental
benefits could be enormous, since about 2.3 tonnes of CO2 are saved for each tonne of
ethanol fuel, although this may be debatable. The market for ethanol is not confined to road
transport: it has many other applications, e.g. co-generation, domestic appliances, chemical
applications, aviation fuel.
(B) Methanol
Methanol from biomass is produced using gasification. Biomass must be dried and sized
prior to methanol synthesis. Gasification occurs in an atmosphere of steam and (or) oxygen
at moderately high temperatures (>1000K) and short residence times (0.5-20 seconds).
Pressure is 0.1-2.5 MPa. First synthesis gas composed mainly of H2 and CO is produced.
Then methanol is produced from clean synthesis gas by catalytically recombining CO and
hydrogen:
If excess hydrogen is present, CO2 reacts catalytically with hydrogen forming additional
methanol:
Similarly to ethanol, methanol can be used as a fuel as it is, or be reacted with isobutylene
to form methyl tertiary butyl ether (MTBE) for blending with gasoline.
(C) Vegetable oil
Vegetable oils are produced from plants using extraction technologies. Suitable sources of
vegetable oil are groundnuts, oil palms, coconuts and sunflowers. The basic extraction process is
relatively simple – the oil bearing part of the plant is separated and then squeezed, using a screw
press (expeller) to release the oil. The pretreatment steps vary depending on the crop.
One of potential advantages of vegetable oils as a fuel is that the processing steps can be
performed at almost any scale. Small units have a disadvantage of being relatively inefficient.
Oil extraction costs vary depending on plant size. The quantity of oil produced per tonne of
feedstock depends on the crop and the efficiency of extraction process.
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(D) Biodiesel
Biodiesel is a general name for vegetable oil-based diesel fuel. Biodiesel contains no
petroleum, but it can be blended at any level with petroleum diesel to create a biodiesel
blend. It can be used in compression-ignition (diesel) engines with little or no
modifications. Biodiesel is simple to use, biodegradable, non-toxic, and essentially free of
sulphur and aromatics. Biodiesel is made through a chemical process called
transesterification whereby the glycerin is separated with alcohol from the fat or vegetable
oil. The process leaves behind two products -- methyl or ethyl esters (the chemical names
for biodiesel) and glycerin. If the oil used was rapeseed oil, products are rapeseed methyl
or ethyl ester (RME or REE). The heating value of RME is 38.9 MJ/kg, which is a little
higher than that of rapeseed oil (36.9 MJ/kg). The greatest difference is between the
viscosities, which are 77.0 mm2/s for rapeseed oil, and 7.3 mm2/s for RME at 20°C. Same
kind of biodiesel can also be produced from tall oil, which is a by-product of sulphate pulp
process. Fatty acid is distilled out of tall oil and transesterified, producing the biodiesel,
which’ heating value is 39.5 MJ/kg.
(E) Pyrolysis bio-oil
Bio-oil produced by pyrolysis seems to be one of the most promising new biofuel products.
It is produced mainly from forest residue chips and sawdust, the maximum particle size
being less than 6 mm. Fast pyrolysis is a process in which organic materials are rapidly
heated to 450-600oC in absence of air. The process lasts about 0.3-0.7 seconds. Under
these conditions, organic vapours, pyrolysis gases and charcoal are produced. The vapours
are condensed to bio-oil. Pyrolysis oil consists of hundreds of chemical compounds, for
example organic acids, aldehydes, alcohols, esters etc. However, the amount of
hydrocarbons is quite small. Pyrolysis offers the possibility of easy handling of the liquids
and a more consistent quality compared to any solid biomass. With fast pyrolysis a clean
liquid is produced as an intermediate for a wide variety of applications.
When biomass decomposes at elevated temperatures, three primary products are formed: gas,
bio-oil and char.
2.3.3 Biogas
Solid biomass can be converted into gaseous fuels using biological and thermal gasification
technologies. Biogas consists mainly of methane and carbon dioxide. Both are colourless
and odourless gases. Smaller amounts of hydrogen sulphide, nitrogen, chloride- and
fluoride compounds are also found, depending on the source. The composition of biogas
depends also on the method of production. There are mainly three production methods that
can be used to produce gaseous fuels from biomass: gasification, anaerobic digestion, and
extraction of landfill gas. A major drawback of biogas is that it is difficult to store.
Economically it is possible to store only about one day’s biogas production. Therefore the
biogas should be constantly exploited. Biogas, especially landfill gas, contains also many
pollutants, as laughing gas (nitrous oxide), ammonia, and hydrochloric acid.
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(A) Gasification
In a gasification process, wood, charcoal and other biomass materials are gasified to
produce so called “producer gas” for power or electricity generation. The essence of a
gasification process is the conversion of solid carbon fuels into carbon monoxide gas by a
thermochemical process. A gasification system basically consists of a gasifier unit,
purification system and energy converters - burner or engine. The produced gas has a low
heating value of 2 – 5 MJ/m3.
(B) Anaerobic digestion
Biogas is a combustible gas derived from decomposing biological waste under anaerobic
conditions. The heating value of this biogas is 23 MJ/m3. As raw material for biogas, very
different kinds of biomasses can be used. Especially sludge is a good raw material, since
methane bacteria work best in wet surroundings. Such sludge is formed in agriculture, on
water purification plants etc. Biogas can be formed by controlled processes in different
kinds of reactors or by natural processes. Biogas production and gathering from liquid
manure on farms reduces the need of buying electricity and heat. One cubic meter of liquid
manure produces about 20 m3 of biogas a year, corresponding to 6.5 kWh of energy. The
manure residues after biogas production are good fertilizers without odour nuisances or
threat to ground or surface waters.
(C) Extraction of landfill gas
A large proportion of municipal solid waste is biological material, and its disposal in deep
landfills furnishes suitable conditions for anaerobic digestion. The landfill gas generated by
anaerobic digestion can be extracted from deep landfills. The heating value of this biogas is
also 23 MJ/m3. A medium sized (Finnish) waste disposal site produces methane 200-400
m3/h. Gathering of landfill gas, illustrated in Figure 2.6, is environmentally smart: Landfill
gas consists mainly of greenhouse gases, dissipates ozone layer and also often causes odour
nuisances.
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Fig. 2.6 Landfill gas gathering
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2.4 Energy from biofuels
Biomass materials are processed in various ways to produce heat, chemicals and other
types of fuels, as illustrated in Figure 2.7. Thermal conversion processes include
combustion, liquefaction, gasification and pyrolysis, while biochemical conversion
processes include anaerobic digestion and fermentation.
Fig. 2.7 Conversion of biomass into energy
At present, power generation from biomass is mostly done by means of direct combustion.
2.4.1 Basics of boilers
Definition
In a traditional context, a boiler is an enclosed container that provides a means for heat
from combustion to be transferred into the working media (usually water) until it becomes
heated or a gas (steam). One could simply say that a boiler is a heat exchanger between fire
and water. The boiler is the part of a steam power plant process, which produces the steam
and thus provides the heat. The steam or hot water under pressure can then be used for
transferring the heat to a process that consumes the heat in the steam and turns it into work.
A steam boiler fulfils the following statements:

It is part of a type of heat engine or process.

Heat is generated through combustion (burning).

It has a working fluid, a.k.a. heat carrier that transfers the generated heat away from
the boiler.

The heating media and working fluid are separated by walls.
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In an industrial/technical context, the concept “steam boiler” (also referred to as “steam
generator”) includes the whole complex system for producing steam for use e. g. in a
turbine or in industrial process. It includes all the different phases of heat transfer from
flames to water/steam mixture (economizer, boiler, superheater, reheater and air preheater).
It also includes different auxiliary systems (e. g. fuel feeding, water treatment, flue gas
channels including stack).
The heat is generated in the furnace part of the boiler, where fuel is combusted. The fuel
used in a boiler contains either chemically bonded energy (like coal, waste and biofuels) or
nuclear energy. Nuclear energy reactors will not be covered in this text. A boiler must be
designed to absorb the maximum amount of heat released in the process of combustion.
This heat is transferred to the boiler water through radiation, conduction and convection.
The relative percentage of each is dependent upon the type of boiler, the designed heat
transfer surface and the fuels that power the combustion.
A simple Boiler
In order to describe the principles of a steam boiler, one must consider a very simple case, where
the boiler simply is a container, partially filled with water (Figure 2.8). Combustion of fuel
produces heat, which is transferred to the container and makes the water evaporate. The vapour
or steam escapes through a pipe that is connected to the container and be transported elsewhere.
Another pipe brings water (called “feedwater”) to the container to replace the water that has
evaporated and escaped.
Fig. 2.8 Simplified boiler drawing
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Since the pressure level in the boiler should be kept constant (in order to have stable
process values), the mass of the steam that escapes has to be equal to the mass of the water
that is added. If steam leaves the boiler faster than water is added, the pressure in the boiler
falls. If water is added faster than it is evaporated, the pressure rises.
If more fuel is combusted, more heat is generated and transferred to the water. Thus, more steam
is generated and pressure rises inside the boiler. If less fuel is combusted, less steam is generated
and the pressure sinks.
Main types of a modern boiler
In a modern boiler, there are two main types of heat transfer means from flue gases to feed
water: Fire tube boilers and water tube boilers.
In a fire tube boiler the flue gases from the furnace are conducted to flue passages, which
consist of several parallel-connected tubes. The tubes run through the boiler vessel, which
contains the feedwater. The tubes are thus surrounded by water. The heat from the flue
gases is transferred from the tubes to the water in the container, thus the water is heated
into steam. An easy way to remember the principle is to say that a fire tube boiler has "fire
in the tubes".
In a water tube boiler, the conditions are the opposite of a fire tube boiler. The water circulates in
many parallel-connected tubes. The tubes are situated in the flue gas channel, and are heated by
the flue gases, which are led from the furnace through the flue gas passage. In a modern boiler,
the tubes, where water circulates, are welded together to form the furnace walls. Therefore the
water tubes are directly exposed to radiation and gases from the combustion. Similarly to the fire
tube boiler, the water tube boiler received its name from having "water in the tubes". A modern
utility boiler is usually a water tube boiler because a fire tube boiler is limited in capacity and
only feasible in small systems.
A simple power plant cycle
The steam boiler provides steam to a heat consumer, usually to power an engine. In a steam
power plant a steam turbine is used for extracting the heat from the steam and turning it into
work. The turbine usually drives a generator that turns the work from the turbine into electricity.
The steam, used by the turbine, can be recycled by cooling it until it condensates into water and
then return it as feedwater to the boiler. The condenser, where the steam is condensed, is a heat
exchanger that typically uses water from a nearby sea or a river to cool the steam. In a typical
power plant the pressure, at which the steam is produced, is high. But when the steam has been
used to drive the turbine, the pressure has dropped drastically. A pump is therefore needed to get
the pressure back up. The cycle that the described process forms is called a Rankine cycle and it
is the basis of most modern steam power plant processes (Figure 2.9).
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Fig. 2.9 Rankine cycle
2.4.2 Basics of combustion
Principles
Combustion is a high speed, high temperature chemical reaction. It is the rapid union of an
element or a compound with oxygen that results in the production of heat - essentially, it is
a controlled explosion. Combustion occurs when the elements in a fuel combine with
oxygen and produce heat. All fuels, whether they are solid, liquid or in gaseous form,
consist primarily of compounds of carbon and hydrogen called hydrocarbons.
Products of combustion
When the hydrogen and oxygen combine, intense heat and water vapor is formed. When
carbon and oxygen combine, intense heat and the compounds of carbon monoxide or
carbon dioxide are formed. When sulfur and oxygen combine, sulfur dioxide and heat are
formed. These chemical reactions take place in a furnace during the burning of fuel,
provided there is sufficient air (oxygen) to completely burn the fuel. Combustion can never
be 100% efficient. Usually fuels contain some moisture and non-combustibles, such as ashforming matter.
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Types of combustion
There are three types of combustion:
Perfect Combustion is achieved when all the fuel is burned using only the theoretical
amount of air, but as we said before perfect combustion cannot be achieved in a boiler.
Complete Combustion is achieved when all the fuel is burned using the minimal amount of
air above the theoretical amount of air needed to burn the fuel. Complete combustion is
always our goal. With complete combustion, the fuel is burned at the highest combustion
efficiency with low pollution.
Incomplete Combustion occurs when all the fuel is not burned, which results in the
formation of soot and smoke. The air supplied is usually lower than the theoretical amount.
Incomplete combustion as a combustion problem
During combustion, the biomass first loses its moisture at temperatures up to 100°C, using
heat from other particles that release their heat value. As the dried particle heats up,
volatile gases containing hydrocarbons, carbon monoxide, methane and other gaseous
components are released. In a combustion process, these gases contribute to about 70% of
the heating value of the biomass. Finally, char oxidizes and ash remains. The combustion
installation needs to be properly designed for a specific fuel type in order to guarantee
adequate combustion quality and low emissions. Distinct stages in the process of
combustion of a particle are: (1) heating and drying, (2) devolatilization and (3) char
oxidation.
Emissions caused by incomplete combustion are usually a result of either:

Poor mixing of combustion air and fuel in the combustion chamber, giving local fuelrich combustion zones

An overall lack of available oxygen

Combustion temperatures that are too low, often due to high moisture content

Residence times that are too short
Through experiments and modeling, new boiler geometries and combustion concepts have
been developed that result in significantly lower emissions. Examples of such
developments are reburning of fuel, air staging, air preheating, radiation shields, advanced
combustion control systems, application of novel materials, etc.
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2.5 Environmental issues
2.5.1 CO2 - Neutrality
The main advantage of bioenergy is the fact that the conversion from biofuel to usable
bioenergy doesn’t directly produce more carbon dioxide (CO2) emitted to the atmosphere.
As the biomass renews, carbon dioxide is bound from the atmosphere into the growing
biomass. The same amount of carbon dioxide is released when the biomass is combusted.
With non-renewable fuels the CO2 levels in the atmosphere increase, which strengthens the
greenhouse effect: greenhouse gases in the atmosphere trap energy from the sun, thus
causing a rise in temperature on the Earth. This is of course also true if the biomass used
doesn’t renew. Therefore, the removal of forests (especially in developing countries with
rainforests) is a huge, global problem. The greenhouse effect is illustrated in Figure 2.10.
Fig. 2.10 Illustration of greenhouse effect
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In Table 2.3 we can see life cycle emissions from renewables including energy crops
(biomass production) and conventional fossil fuels. The gap between fossil fuels’ and
energy crops’ CO2 emissions is huge. CO2 from biofuels is produced indirectly, for
example through transportation and harvesting. In some cases bioenergy even reduces
greenhouse gas emissions, for instance when other even worse greenhouse gases like
methane are used as a biofuel. Since global warming and climate change are major
environmental concerns, usage of renewable energy, of which bioenergy is a major part,
will become more and more important.
Table 2.3: Life Cycle Emissions from Renewables and Fossil Fuels
2.5.2 Other emissions
Other atmospheric emissions from bioenergy are small compared to fossil fuel energy, as
can be seen in Table 2.3. Emission reduction measures for biomass combustion are
available for virtually all harmful emission components; whether the emission reduction
measures are implemented or not is mainly a question of emission limits and costeffectiveness. Though scale-effects ensure that large installations (such as coal power
plants) can be equipped with flue gas cleaning more economically, local availability of the
biomass fuel and transportation costs will usually be a limiting factor for size. NOx and SOx
emissions from biomass combustion applications are in general low compared to those
from coal combustion, and secondary reduction measures are usually not required to meet
emission limits.
Emissions of NOx from biomass combustion applications originate mainly from the
nitrogen content in the fuel, in contrast to fossil fuel combustion applications where
nitrogen in the air to some extent also contributes to the NOx emission level. In most cases
the NOx emission level can be significantly lowered by the use of primary emission
reduction measures, and can be further decreased by implementing secondary emission
reduction measures.
2.5.3 Other environmental advantage
By substituting energy plantations for arable crops (particularly on areas that have an
overproduction of food crops), erosion rates will generally be reduced and lower levels of
agrochemicals be used. This can also lead to increased wildlife diversity. Energy crop
technologies and other bioenergy technologies can also be used to treat sewage sludge or
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municipal and industrial waste so that it no longer constitutes a pollution threat to
watercourses. Waste incineration, although very difficult in many cases, is also an
important way to deal with the growing waste problem.
2.5.4 Biomass ash as fertilizer
The long-term ecological effects of forest harvesting due to nutrient depletion from the
forests are reduced re-growth affecting the biodiversity and productivity of the forests. By
using the ash as a fertiliser, these problems can partly be coped with, since the ashes from
biomass combustion contain almost all the nutrients removed from the forest. In
experiments the growth response has been shown to be good. The alkaline ashes also have
a neutralising effect on acidified soils.
The composition of the ashes from biomass is more varying and the ashes often have
higher levels of heavy metals and other contaminants than ashes from fossil fuels. The
content of heavy metals and other contaminants, form an obstacle for the ash to be used for
the same applications as other ashes. Use as a fertiliser is the most suitable use and because
of its many benefits it should be encouraged. The ash should be pre-treated to achieve
desirable leaching properties and to make transportation and handling easier. The major
problems today include the rather unknown effects of the fertilisation on surrounding
watercourses and the control of the heavy metal content (especially cadmium). There are
limits set for the heavy metal contents allowed in ashes to be used in different applications.
Dioxins, furans and radioactive isotopes should be kept under observation as well. In most
countries proper legislation or standards for ash fertilisation do not yet exist.
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