BIOMASS ENERGY APPLICATIONS
Teaching Note 2020/21
BSE4412 – RENEWABLE ENERGY APPLICATION
Prepared by:
Prof. Vivien Lin Lu
The Hong Kong Polytechnic University
October 2020
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1.1 Introduction
Biomass
All the earth’s living matter, its biomass, exists in the thin surface layer called the biosphere. It
represents only a tiny fraction of the total mass of the earth, but in human terms it is an enormous
store of energy. More significantly, it is a store which is being replenished continually. The source
which supplies the energy is of course the sun. Although only a small fraction of the solar energy
reaching the earth each year is fixed by organic matter on land, it is nevertheless equivalent to some
eight times our total primary energy consumption. This energy stored in plants is recycled naturally
through a series of conversions involving chemical and physical processes in the plant, the soil, the
surrounding atmosphere and other living matter, until it is eventually radiated away from the earth
as low-temperature heat - except for a small fraction which may remain in peat and a tiny
proportion which may slowly become fossil fuel energy.
As shown in Figure 1, the routes by which the energy in a plant is ultimately recycled into the
atmosphere.
Figure 1 Some routes for the solar energy absorbed by a plant
The importance of this cyclic process for us is that if we can intervene and ‘capture’ some of the
biomass at the stage where it is acting as a store of chemical energy, we have a fuel. Moreover, two
facts of great environmental significance are that, provided our consumption does not exceed the
natural level of recycling, in burning biofuels we generate no more heat and create no more carbon
dioxide than would have been produced in any case by natural processes. So it seems that we have
here an energy source whose use should have no deleterious environmental effect at all.
Biofuels
The term ‘biofuels’ covers a very wide range of energy sources, from a simple wood fire to a
thousand tones of urban waste feeding a multi-megawatt power station. For a more precise
definition, we can quote a recent publication of the UK Energy Technology Support Unit (ETSU,
1991):
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The ‘biofuels’ are any solid, liquid or gaseous fuels produced from organic materials, either
directly from plants or indirectly from industrial, commercial, domestic or agricultural wastes.
They can be derived from a wide range of raw materials and produced in a variety of ways.
The inclusion of all the energy-from-wastes processes under this heading may seem a little strange,
until we notice that most present systems for deriving energy from biomass are in fact using wastes-
residues associated with plant or animal products cultivated for other purposes. Urban and industrial
wastes are not so evidently biomass (plastic bags and tin cans come to mind), but nevertheless much
of the content of the average household dustbin is of biological origin.
1.2 Case study: wood as fuel
It is difficult to think of a more suitable case for using wood: a country estate with some 120
hectares of semi-derelict woodland and a large house with an inadequate, inefficient and
increasingly expensive oil-fired heating system.
A decade ago, faced with the need to replace the boiler, the managers of the Drayton estate in East
Anglia looked at the options. Installing new oil-fired plant would be simplest, but fuel costs would
be prohibitive - and uncertain. Gas was not available, but two biofuels were: straw and wood. The
system finally chosen was a boiler with 250 kW heat output, designed to burn wood in the form of
chips a few centimeters long. There were three possible fuel sources:
l thinnings and scrub clearance of the existing woodland;
l wastes from a local sawmill;
l arable coppice.
Figure 2 The Drayton wood-burning system: (Left) wood chip hopper; (Right) the 250 kW boiler
In the early years, the first two were the main sources. In the future, coppice wood is expected to
play an increasing role. Overall, the result of this use of biomass energy has been a much more
satisfactory heating system and a halving of heating costs. It has also improved derelict woodland,
dealt with a disposal problem of sawmill wastes and, for the future, will provide a use for set-aside
land.
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1.3 Biomass past and present
From wood to coal
The replacement of wood by coal during the early Industrial Revolution provides an interesting case
study in technological change. There is general agreement that an increasingly serious shortage of
wood was a major factor, but opinions then diverge. Three contrasting views of early
industrialization and its causes might be summarized as follows.
l Growing prosperity brought conditions which favored technical innovation. This led to the
increasing use of machines, for which coal was a more suitable fuel than wood.
l Scientific inventiveness led to widespread technological change, with energy from coal
replacing wood, wind and water. Increasing prosperity was one consequence of this growing
industrialization.
l Population growth, poverty and the rising price of wood forced the use of coal, which was
perceived as a less desirable fuel. Surface coal was soon exhausted, deep mines became
necessary, and with them the need to pump flood water from great depths. This led to the first
machines of the Industrial Revolution.
Present biomass contributions
Obtaining a reliable estimate of the total world-wide energy contribution from the many biomass
sources is a task fraught with difficulties. There is no large scale organization like OPEC keeping
track of the consumption of biofuels; indeed, much of the trade in biomass is local and unrecorded,
often involving no financial transaction at all. It is not perhaps surprising then that many studies of
‘world energy consumption’ have chosen to ignore this source altogether. Unfortunately, this gives
the misleading impression that its contribution is small and relatively unimportant, which is
certainly not the case.
Figure 3 Biomass contributions to primary energy. (a) World consumption. Total energy 400 EJ/y.
Per capita energy 80 GJ/y. (b) Industrialized countries. Total energy 250 EJ/y. Per capita energy
210 GJ/y. (c) Developing countries. Total energy 140 EJ/y. Per capita energy 36 GJ/y.
There may be uncertainty about the detailed figures, but there is little doubt that biomass is a major
energy provider over much of the world.
1.4 Biomass as a fuel
What are fuels?
A fuel is a substance which interacts with oxygen and in doing so changes chemically and releases
energy. The release of energy must mean that in some way the original fuel and oxygen contain
more energy than the products formed in the combustion.
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We know enough about the composition of the most common fuels to be able to predict what these
products will be. Consider, for instance, methane -a biofuel and also the principal component of
natural gas. It consists entirely of carbon and hydrogen, each methane molecule containing one
carbon and four hydrogen atoms: CH4. Oxygen gas consists of molecules with two atoms each (O2)
and in combustion each methane molecule reacts with two of these:
CH4 + 2O2 → CO2 + 2H2O + energy
Although this particular reaction represents the combustion of methane, it contains the essential
features of the burning of any common fuel: a compound containing carbon and hydrogen interacts
with oxygen from the air to produce carbon dioxide and water. (The latter of course usually appears
in the form of water vapor or steam.) Indeed, if we know the relative masses of the elements, we
can predict how much carbon dioxide and water vapor will be produced in burning a given amount
of fuel.
Example 1: Burning natural gas
An average UK household uses about 60 GJ a year for space heating and hot water. If this is
produced by burning natural gas, and the efficiency of the system is 60%, how much CO2 is emitted
by the household in a year? The masses of the atoms of carbon, hydrogen and oxygen are in the
ratio 12:1:16, respectively. The energy content of natural gas is 55 GJ per tone.
Solution
Natural gas is essentially methane (CH4), and we can use the data above to associate masses with
the items in the equation for the burning process:
CH4 + 2O2 → CO2 + 2H2O
12 + (4×1) + 2×(2×16) → 12 + (2×16) + 2×(2×1+16)
We see, therefore, that burning 16 tones of CH4 produces 44 tones of CO2. But burning 16 tones of
natural gas releases 16×55 = 880 GJ of heat. So for each 20 GJ of heat, we produce one tone of CO2.
At 60% efficiency, the system requires a fuel input of 100 GJ to produce the required 60 GJ of
useful heat. The household therefore emits five tones of CO2 a year into the atmosphere in meeting
its needs for warmth and hot water.
Oil and coal, our other main fuels, are more complex than methane, but their combustion is a
similar process. The heat produced per tone is rather less, however, and as the ratio of carbon to
hydrogen atoms is greater they produce more CO2 per unit of heat output (Table 1). These fossil
fuels, the result of hundreds of millions of years of slow geological change acting on plant or animal
matter, are examples of hydrocarbons and consist almost entirely of carbon and hydrogen.
Table 1 Proportions of carbon, hydrogen and oxygen in fuels
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Most of the biofuels, derived from living or recently dead biomass, contain oxygen as well. The
molecules of biological materials are also much larger and more complex than methane, but we can
represent their combustion in a much simplified way by considering the carbohydrates as an
example. In these the ratio of the constituents is approximately one oxygen and two hydrogens to
each carbon, so [CH2O] can stand for a typical sub-unit of a carbohydrate molecule. The burning
process is then:
[CH2O] + O2 → CO2 + H2O + energy
The details will be specific to each type of biomass, but this shows the general idea, and the data in
Table 2 are some examples of the energy which can be obtained in burning various biological
materials, with the main fossil fuels for comparison.
Table 2 Average energy content of fuels
Biomass as a solar energy store
Carbon, hydrogen and oxygen are the main constituents of all our conventional fuels. That C, H and
O are also the main constituents of living matter is no coincidence. As we have seen, the fuel-
oxygen combination is an energy store, and the energy is dissipated as heat when the fuel bums.
Natural decomposition is a similar oxidation process, also leading to carbon dioxide and water. But
the process does not finish there. Nature completes the cycle, putting energy back into these final
products to create more fuel and oxygen. The mechanism is photosynthesis (from photo: to do with
light and synthesis: putting together). This is the process by which plants take in carbon dioxide and
water from their surroundings and use energy from sunlight to convert these into the sugars,
starches, cellulose, etc. which make up ‘vegetable matter’.
The essential features of the process can be represented:
CO2 + H2O + energy → [CH2O] + O2
Notice that the first item on the right, the [CH2O], again indicates a sub-unit of a carbohydrate
molecule. This is not necessarily the final ‘vegetable matter’, but as before it will serve for our
simple example. The second item is of course oxygen, and it will by now be obvious that this
process is exactly the reverse of the decomposition/combustion discussed above. The plant grows
by using solar energy to convert carbon dioxide and water into carbohydrate or similar material,
with a release of oxygen. When it decays - or we bum it - oxygen is used and energy is released as
heat.
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1.5 Extracting the energy
If biofuels, and energy from wastes in particular, are to compete with our present fuels, they must
be able to meet the demand for appropriate forms of energy at competitive prices. Two important
criteria are the availability and the ‘transportability’ of the supply. The premium fuels - oil and
natural gas - are valued because their energy can be stored with little loss and made available when
we need it. And these fuels, together with electric power, offer the further advantage of energy that
is easily transferred from place to place.
The biomass resource comes in a variety of forms: wood, sawdust, straw, rape seed, dung, waste
paper, household refuse, sewage and many others. Nearly all types of raw biomass decompose
rather quickly, so few are very good long-term energy stores; and because of their relatively low
energy densities, they are likely to be rather expensive to transport over appreciable distances.
Recent years have therefore seen considerable effort devoted to the search for the best ways to use
these potentially valuable sources of energy.
The answer may be to burn the raw biomass in situ to produce heat - for direct use or for power
generation. Alternatively, it may be that relatively simple treatment (physical processing) can
produce a more durable and transportable fuel to be used elsewhere for these purposes. At the next
level of technology, there is chemical processing. In some cases heat is the means to bring about
this change - simple heating or, in the more extreme examples, using the full technology of the
modern chemical industry to take apart the biomass material and rebuild its elements into the fuels
we need. In other cases the changes occur by biological means, requiring only time and suitable
conditions. We’ll look at each of these options in turn.
Direct Combustion
Boiling a pan of water over a wood fire is a simple process. Unfortunately, it is also very inefficient.
Example 2: Boiling water
How much wood should you need to bring a liter of water to the boil from room temperature?
A liter of water requires 4.2 kJ of heat to increase its temperature by one degree Celsius.
Solution
If the water is initially at 20°C, the energy needed to bring one liter to the boil (100°C) is about 340
kJ.
The heat content per cubic meter of air-dry wood (Table 2) is about 10 GJ, or 10 million kJ. So the
required quantity of wood is 34 millionths of a cubic meter (34 cm3). One stick about a centimeter
thick and a third of meter long should be enough!
Example 3: Removing the moisture
How much of the heat produced in burning air-dried wood is used in evaporating the moisture it
contains?
Use the information in Table 2 and the fact that 2 MJ is needed to evaporate one kilogram of water.
Solution
Table 2 shows that air-dried wood contains 20% moisture, so one tone contains 200 kg, requiring
400 MJ (0.4 GJ) for evaporation.
One tone of air-dried wood produces 15 GJ of heat, so evaporation accounts for just under 3%.
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In the combustion process itself there are always two stages, because any solid fuel contains two
combustible constituents. The volatile matter is released as a mixture of vapors or vaporized tars
and oils by the fuel as its temperature rises. The combustion of these produces the little spurts of
flame seen around burning wood or coal. The solid which remains consists of the char together with
any inert matter. The char is mainly carbon, and bums to produce CO2, whilst the inert matter
becomes clinker, slag or ashes.
A feature of the biofuels is that three-quarters or more of their energy is in the volatile matter
(unlike coal, where the fraction is usually less than half). It is important therefore that the design of
any stove, furnace or boiler should ensure that these vapors bum, and don’t just disappear up the
chimney. For complete combustion, air must also reach all the char, which is best achieved by
burning the fuel in small particles. This can raise a problem, because finely-divided fuel means
finely divided ash - particulates which must be removed from the flue gases. The air flow should
also be controlled: too little oxygen means incomplete combustion and also leads to the production
of carbon monoxide, which is a poison.
Example 4: Electricity from refuse
A proposed 38 MW power station will burn 400,000 tones of household wastes a year, delivering its
full power for three-quarters of the time (i.e. an annual plant load factor of 75%). What is its overall
conversion efficiency?
Solution
The average energy content of household wastes is 9 GJ per tone (Table 2) so 400,000 ty-1 means an
input of 3.6 million GJ y-1. One watt is one joule per second, so 38 MW is 38 million J S-1. If the
plant runs for 75% of the time, its annual output will be 900,000 GJ.
The efficiency is the energy output divided by the energy input, expressed as a percentage: in this
case (0.9/3.6)×100, or 25%.
Figure 4 The composition of Municipal Solid Wastes in the UK
Pyrolysis
Pyrolysis is the simplest and almost certainly the oldest method of processing one fuel in order to
produce a better one. Conventional pyrolysis involves heating the original material in the near
absence of air, typically at 300- 500 °C, until the volatile matter has been driven off. The residue is
then the char -more commonly known as charcoal -a fuel which has about twice the energy density
of the original and bums at a much higher temperature. For many centuries, and in much of the
world still today, charcoal is produced by pyrolysis of wood. Depending on the moisture content
and the efficiency of the process, 4-10 tones of wood are required to produce one tone of charcoal,
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and if no attempt is made to collect the volatile matter, the charcoal is obtained at the cost of
perhaps two-thirds of the original energy content. (The use of wood in developing countries to
produce high-quality ‘barbecue charcoal’ for export may earn valuable foreign currency, but is not
perhaps the best way to treat a depleting resource.)
With more sophisticated pyrolysis techniques, the volatiles can be collected, and careful choice of
the temperature at which the process takes place allows control of their composition. The liquid
product has potential as fuel oil, but is contaminated with acids and must be treated before use. Fast
pyrolysis of plant material, such as wood or nutshells, at temperatures of 800-900 °C leaves as little
as 10% of the material as solid char and converts some 60% into a gas rich in hydrogen and carbon
monoxide. This makes fast pyrolysis a competitor with conventional gasification methods, but like
the latter, it has yet to be developed as a treatment for biomass on a commercial scale.
At present, conventional pyrolysis is considered the more attractive technology. The relatively low
temperatures mean that fewer potential pollutants are emitted than in full combustion, giving
pyrolysis an environmental advantage in dealing with certain wastes.
Gasification
The term gasification covers a range of processes in which a solid fuel is reacted with hot steam and
air or oxygen to produce a gaseous fuel. Figure 5 shows the processes in outline. There are several
types of gasifier, with operating temperatures varying from a few hundred to over a thousand
degrees Celsius, and pressures from near atmospheric to as much as 30 atmospheres. The resulting
gas is a mixture whose main constituents are carbon monoxide, hydrogen and methane, together
with carbon dioxide and nitrogen, in proportions which depend on the’ processing conditions and
whether air or oxygen is used. There are several reasons for interest in this rather complex way of
treating the original material. Firstly, it can result in a fuel which is much cleaner than the original
biomass, as undesirable chemical pollutants can be removed during the processing, together with
the inert matter which produces particulates (smoke) when the fuel is burned. Then a gas is a much
more versatile fuel. And finally, gasification under suitable conditions can produce synthesis gas, a
mixture of carbon monoxide and hydrogen which can be used to synthesize almost any hydrocarbon.
Figure 5 Gasification processes
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Gas Turbines
One potential use for the clean fuel gas from biomass gasifiers is to run gas turbines for local power
generation. A gas-turbine power station is similar to conventional steam plant except that instead of
using heat from the burning fuel to produce steam to drive the turbine, it is driven directly by the
hot combustion gases (Figure 6). Increasing the temperature in this way improves the
thermodynamic efficiency, but in order not to corrode or foul the turbine blades the gases must be
very clean - which is why nearly all present gas-turbine plants bum natural gas.
Figure 6 Types of generating system (a) conventional steam turbine; (b) simple gas turbine; (c)
steam-injected gas turbine
Substitution of gas from biomass gasifiers would serve a double purpose, conserving a premium
fuel and reducing the emission of greenhouse gases - provided the biomass cycle is CO2-neutral, of
course. In situations where the wastes from processing of vegetable material provide the input, the
requirement is likely to be for process heat (hot steam) as well as electric power, with the heat
requirement often taking precedence. For this purpose the steam-injected gas turbine (STIG) is very
suitable. As the name suggests the turbine is driven by a combination of combustion gases and
high-pressure steam, so the plant incorporates steam generators and can be operated much more
flexibly in response to varying demands for heat.
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Synthesizing fuels
A gasifier which uses oxygen rather than air can produce a gas consisting mainly of H2, CO and
CO2, and the interesting potential of this lies in the fact that removal of the CO2 leaves the mixture
called synthesis gas, from which almost any hydrocarbon compound may be synthesized. Reacting
the H2 and CO is one way to produce pure methane, for instance:
2CO + 2H2 → CH4 + CO2
Another possible product is methanol (CH3OH), a liquid hydrocarbon with an energy density of 23
GJ per tone. Producing methanol in this way involves a series of sophisticated chemical processes
with high temperatures and pressures and expensive plant, and one might wonder why it is of
interest. The answer lies in the product: methanol is that valuable commodity, a liquid fuel which is
a direct substitute for gasoline. At present the production of methanol using synthesis gas from
biomass is not a commercial proposition, but the technology already exists, having been developed
for use with coal as feedstock - as a precaution by coal-rich countries at times when their oil
supplies were threatened.
Anaerobic digestion
As its name suggests, anaerobic digestion, like pyrolysis, occurs in the absence of air; but in this
case the decomposition is caused by bacterial action rather than high temperatures. It is a process
which takes place in almost any biological material, but is favored by warm, wet and of course
airless conditions. It occurs naturally in decaying vegetation on the bottom of ponds, producing the
marsh gas which bubbles to the surface and can catch fire -giving rise to the myths of the ‘will o’
the wisp’.
Anaerobic digestion also occurs in situations created by human activities, and in two major
instances these have been developed as energy sources. One is the biogas which is generated in
concentrations of sewage or animal manure, and the other is the landfill gas produced by domestic
refuse buried in landfill sites. In both cases the resulting gas is a mixture consisting mainly of
methane and carbon dioxide; but major differences in the nature of the input, the scale of the plant
and the time-scale for gas production lead to very different technologies for dealing with the two
sources.
The detailed chemistry of the production of biogas and landfill gas is complex, but it appears that a
mixed population of bacteria breaks down the organic material into sugars and then into various
acids which are decomposed to produce the final gas, leaving an inert residue whose composition
depends on the type of system and the original feedstock.
Biogas
The dung or sewage which is the feedstock for biogas is fed into a purpose-built digester as a slurry
with up to 95% water. Digesters range in size from perhaps one cubic meter for a small ‘household’
unit (roughly 200 gallons) to some ten times this for a typical farm plant (Figure 7) and as much as
2,000 m3 for a large commercial installation. The input may be continuous or in batches, and
digestion is allowed to continue for a period of from ten days to a few weeks. The bacterial action
itself generates heat, but in cold climates additional heat is normally required to maintain the ideal
process temperature of at least 35°C, and this must be provided from the biogas. In extreme cases
all the gas may be used for this purpose, but although the net energy output is then zero, the plant
may still pay for itself through the saving in fossil fuel which would have been needed to process
the wastes
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Figure 7 A farm biogas digester
A well-run digester will produce 200-400 m3 of biogas with a methane content of 50% to 75% for
each dry tone of input - at best about 11 GJ of useful energy. Comparison with Table 2 shows that
this is about two-thirds of the fuel energy of the original dung, and probably represents the best
which can be achieved. However, even at lower conversion efficiencies, the process may be
worthwhile in order to obtain a clean fuel and at the same time dispose of unpleasant wastes. The
effluent which remains when digestion is complete can also be of considerable value as a fertilizer.
Example 5: Energy content of biogas
A particular sample of biogas contains 60% methane, 35% carbon dioxide and 5% other inert gases
by volume. What is its energy content per cubic meter at normal pressure? The energy content of
CH4 at normal pressure is 38 MJ m-3. The CO2 has no fuel value.
Solution
One cubic meter of the biogas contains 0.60 m3 of CH4, giving an energy of about 23 MJ m-3.
Landfill Gas
A large proportion of ordinary domestic refuse -municipal solid wastes, or MSW - is biological
material, and its disposal in deep landfills furnishes suitable conditions for anaerobic digestion. That
landfill sites produce methane has been known for decades, and recognition of the potential hazard
led to the fitting of systems for burning it off; however, it was only in the 1970s that serious
attention was paid to the idea of using this ‘undesirable’ product.
The waste matter is more miscellaneous in a landfill than in a biogas digester, and the conditions
neither as warm nor as wet, so the process is much slower, taking place over years rather than
weeks (Figure 8). The end product, known as landfill gas (LFG), is again a mixture consisting
mainly of CH4 and CO2, In theory, the life-time yield of a good site should lie in the range 150-300
m3 of gas per tone of wastes (as collected), with between 50% and 60% by volume of methane. This
suggests a total energy of 5-6 GJ per tone of refuse, but in practice yields are much less.
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Figure 8 The changing gas composition in a landfill site
Example 6: Power from a landfill site
A large landfill contains 1 million tones of MSW. It produces 6 m3 of LFG a year per tone of refuse,
with an average methane content of 55%. There is a proposal to use this LFG for electric power
generation. The overall energy conversion efficiency of the power plant is expected to be 28% and
it will run with an annual plant load factor of 80%. What should be the installed generating capacity?
Solution
With 55% methane content, and assuming the remainder to be CO2, the energy content of the gas is
0.55×38 = 21 MJ m-3.
The million tones of refuse will produce 6 million cubic meters of gas a year, with an energy of
6×21 = 126 million MJ.
The annual electrical output will be 28% of this: 35 million MJ. This is produced in 80% of a year,
which is 25 million seconds. The required capacity is therefore 35/25 = 1.4 MJ S-1, or 1.4 MW.
Fermentation
In considering the uses of synthesis gas, we have seen how an alcohol (methanol) could be
produced from biomass by a series of sophisticated chemical processes. There are of course other
ways to produce alcohols from biomass - as vintners and brewers have known for centuries.
Fermentation is an anaerobic biological process in which sugars are converted to alcohol by the
action of microorganisms, usually yeast. The resulting alcohol is ethanol (C2H5OH) rather than
methanol (CH3OH), but it too can be used in internal combustion engines, either directly in suitably
modified engines or as a gasoline extender in gasohol: gasoline (petrol) containing up to 20%
ethanol.
The value of any particular type of biomass as feedstock for fermentation depends on the ease with
which it can be converted to sugars. The best-known source of ethanol is sugar-cane - or the
molasses remaining after the cane juice has been extracted. Other plants whose main carbohydrate
is starch (potatoes, corn and other grains) require processing to convert the starch to sugar. This is
commonly carried out, as in the production of some alcoholic drinks, by enzymes in malts. Even
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wood can act as feedstock, but its carbohydrate, cellulose, is resistant to breakdown into sugars by
acid or enzymes (even in finely divided forms such as sawdust), adding further complication to the
process.
The liquid resulting from fermentation contains only about 10% ethanol, which must be distilled off
before it can be used as fuel. The energy content of the final product is about 30 GJ t-1, or 24 GJ m-3.
The complete process requires a considerable amount of heat, which is usually supplied by crop
residues (e.g. sugar cane bagasse or maize stalks and cobs). Table 3 shows the yields of ethanol
obtainable per tone of raw material and per hectare of land for several crops. The energy loss in
fermentation is substantial, but this may be compensated for by the convenience and transportability
of the liquid fuel, and by the comparatively low cost and familiarity of the technology.
Table 3 Ethanol yields
Example 7.9: Ethanol from sugar cane
If the energy content of ethanol is 24 GJ m-3 and the yield from one tone of sugar cane is 70 liters of
ethanol, what percentage of the original energy is lost in the process?
Solution
A liter is one thousandth of a cubic meter, so the energy content of 70 liters of ethanol is 1.7 GJ.
The energy content of air-dry sugar cane is about 14 GJ per tone (Table 7.2). Thus about 12%
appears in the ethanol and 88% is ‘lost’ through processing, unused residues, etc.
1.6 Agricultural residues
Although wood remains the predominant biomass fuel worldwide, crop and animal wastes provide
significant amounts of energy in many countries.
Wood Residues
Operations such as thinning of plantations and trimming of felled trees generate large volumes of
forestry residues. At present these are often left to rot on site - even in countries with fuelwood
shortages. They can be collected, dried and used as fuel by nearby rural industry and domestic
consumers, but their bulk and high water content makes transporting them for wider use
uneconomic. In developing countries where charcoal is an important fuel, on-site kilns can reduce
transport costs. Mechanical harvesters and chippers have been developed in Europe and North
America over the last 15 years to produce uniform 30-40 mm wood chips which can be handled,
dried and burned easily in chip-fired boilers. The use of forest residues to produce steam for heating
and/or power generation is now a growing business in many countries.
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Tropical crop wastes
Bagasse (sugar cane fiber) has significant potential as a biomass fuel since it mainly arises at sugar
factories where flows of materials and energy are already well organized. Most sugar factories use
bagasse as a source of heat for raising steam, but deliberately burn it inefficiently in order not to
accumulate surplus wastes. Many sugar factories also produce electricity for their own needs, but
only a few at present export electricity because of operational and contractual difficulties with
selling power only during the cane growing season.
Animal wastes
The combination of intensive animal rearing and stricter environmental controls on odor and water
pollution by manures is leading farmers to invest in anaerobic digestion as a means of waste
management. The biogas produced by a digester can be used to produce heat or electric power as
required -or in many cases both. One system uses the gas to run large internal combustion engines
which drive electric generators, while their cooling water and exhaust gases are used to provide heat
to the digester (Figure 9).
Figure 9 Power from anaerobic digestion of pig slurry.
1.7 Energy from Municipal solid waste
Our city produces large amount household wastes each year, and a similar quantity of combustible
commercial and industrial wastes.
The technology for MSW combustion has been well tried and tested, and collection networks
already exist to ensure a continuous supply of municipal wastes.
Landfill gas developments
Anaerobic digesters for MSW
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Figure 10 A large MSW combustion plant
Figure 11 Extraction of landfill gas
Figure 11 Anaerobic digestion of municipal sold wastes. The flow diagram shows the processes and
resulting products for an average tone of MSW in a large (2000 m3) high-solids system. As can be
seen, the total useful output is 5 GJ per tone
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1.8 Energy crops
‘Energy crops’, plants grown specifically as energy sources, have attracted increasing attention in
recent years. The reduction in net CO2 emissions may be sufficient reason for substituting biomass
for fossil fuels, but in many countries the more immediate motives for change have been the
problem of surplus agricultural land and the search for indigenous alternatives to imported oil. The
preferred crops, depending on the relative importance of these factors and on local conditions,
include wood for burning, plants for fermenting to ethanol and crops whose seeds are particularly
rich in oils (Table 3).
Table 3 Annual yields of various crops
Woody Crops
Wood remains a major energy source in much of Asia, most of Africa and several South American
countries. Wood (or charcoal) is often the main household fuel, and can also contribute appreciably
to industrial energy consumption - in Brazil, for instance, where the steel industry uses over two
million tones of charcoal a year. The wood resource is accordingly of great importance. Where a
forestry industry exists, wood residues can be used, but deforestation is leading to local shortages in
many areas.
Ethanol from sugar cane
Ethanol from sugar cane or maize is perhaps the best-known of all energy crops, with many
examples of its use around the world.
Maize, sorghum and miscanthus
Bio-ethanol production has developed around conversion of surplus maize into alcohol for blending
into gasoline, at up to 10% by volume. Miscanthus is a temperate climate grass adapted to moist
soils, which has some of the photosynthetic characteristics of sugar cane.
Vegetable oils
Seeds of many plants can be crushed ‘on the farm’ to yield a range of vegetable oils, most of which
are compounds of hydrocarbon-like fatty acids and glycerol. Many vegetable oils can be burned
directly in diesel engines, either pure or blended with diesel fuel. However, blends containing a high
proportion of vegetable oil tend to clog the diesel injectors and form deposits in other parts of the
engine. Simple chemical processing of vegetable oil, by ‘esterifying’, i.e. combining it with ethanol
or methanol, leads to a superior diesel substitute which does not foul engines. Vegetable oils have
an energy content of about 37-39 GJ t-1, similar to that of diesel (about 42 GJ t-l) and superior in this
respect to ethanol (30 GJ t-l). Blends of up to 30% vegetable oil with diesel are the main
applications at present.
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