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THERE ARE THREE SOURCES
OF RENEWABLE ENERGY
AVAILABLE TO US:

SOLAR ENERGY 

Energy from the sun (directly, or
indirectly via wind, wave and
biomass energy).

GEOTHERMAL ENERGY 
Energy from hot rocks underground.

TIDAL ENERGY 
Energy from gravitational effects in
the solar system.

 

 

is biomass?

Biomass is organic matter, mostly from plants such as wood,
which can be used as a source of energy. The term includes
animal waste (digested plant food) such a cow or pig dung.
The size of the biomass energy resource is enormous. Every
year, growing plants store enough energy to fuel the world
for five years. The majority of this energy, about 90% of it,
is stored in the wood of growing trees.

Biomass and #114?:

There are two natural carbon cycles. One is the organic
carbon cycle involving plant (see diagram)
which cycles aboUt 60 giga tonnes of carbon a year to and
from the atmosphere. (One giga tonne is 109, one thousand
million tonnes.) The other is the inorganic carbon cycle in
which carbon dioxide, for example from v0lcanic activity,
dissolves in rainwater. This solution reacts with rocks to
form carbonates which are washed down to the sea by
rivers. The shells of sea creatures contain calcium
carbonate. When the creatures die, the remains fall to the
seabed. In time, under the action of temperature and
pressure, this calcium carbonate forms rock such as
limestone. The process of rock formation and disintegration
repeats itself a cycle that takes millions of years to
complete. It is estimated that geophysical actions cycle
about 100 giga tonnes of carbon a year in this way.

Since the Industrial Revolution, the large scale use of fossil
fuels has caused an accelerated increase in the amount of
carbon dioxide entering the atmosphere. The result is an
imbalance in the natural carbon cycles. Carbon dioxide is
one of the so-called greenhouse gases. Increasing the
amounts of these gases in the air raises the temperature
near the earth?s surface. This is called the enhanced
greenhouse effect. Some scientists believe that this could be
affecting the climate on earth. The combustion of fossil
fuels, for example, adds about
six giga tonnes of carbon a year
to the atmosphere.

Mature trees/forests absorb
and release roughly equal
quantities of carbon dioxide,
making them COZ neutral

Decomposing vegetation

releases carbon dioxide

from the carbon stored
during growth

  
  
 
 
 
 
  
   

haw 

One (terawatt hour) is the energy supplied by a large
nuclear power station running at full power for two months.

The Earth receives 178,000 TW of energy from sunlight
every year. Out of this 178,000 TW, plants use only 100 TW
to make all the world?s biomass each year. This still adds up
to a lot of biomass energy. It is about 120,000 million
tonnes of dry biomass each year. Of course, we use only
part of this total for fuels or there would be no plants left.

All over the world, people use firewood and other kinds of
biomass fuel. Biomass is already the fourth largest energy
source worldwide providing more primary energy than we
get from hydroelectric or nuclear power. Since plants grow
almost everywhere, there is great potential for us to increase
our use of biomass in the future.

Biomass fuels are generally available to us as solids from a

wide variety of sources but we can convert them into liquid
and gaseous fuels.

Wood. Wood was the major source of energy from the
earliest times until the Industrial Revolution when coal very
largely replaced it. In the developing world, wood often
remains the major source of energy for heating and
cooking. Dry fuels contain more energy per tonne than
freshly cut material.

Charcoal. We make charcoal by heating wood in the
absence of air (called pyrolysis). In Europe, the soil or
turf?covered charcoal kilns date back over 5500 years.
Charcoal was the fuel that bronze and iron age peoples
used to smelt metals. They needed the high temperatures
that burning charcoal produces. Nowadays,
charcoal is familiar as the fuel used in
garden barbecues. The wood contains

Young developing trees
absorb more carbon
dioxide than they release

mt?- Ar; ..



 
  
 

A carbon cycle showing
how carbon dioxide is
absorbed and released.

substances that are volatile, both gases and liquids. These
are lost during traditional charcoal burning. Since these
volatiles in biofuels can account for 75% of their energy
content, charcoal burning is a very wasteful and polluting
way to make fuel.

Cutaway View of a traditional
charcoal kiln.

Smoke escapes through
a small slit in top

  
  
 
  
 
   

Earth/turf covering
to restrict the
amount of air that

can get in

The wood burns
without much air to
create the charcoal

Stacked
wood

Farm wastes. Farm wastes have been used as fuels since the
earliest times. The wastes include dried animal dung, crop
residues, husks from milling
food crops, straw and forestry
wastes from thinning and

been cutting and drying turfs

 
   

 

b. n: 
Drying dung in the sun in Bangladesh
to replace ?rewood as fuel.

of peat for thousands of years
to make into fuel.

Industrial processing. This can produce large amounts of
waste concentrated at one location. Such material can then
be used as a fuel.

The Varnamo Biomass Plant in'Sweden.

Agricultural waste. The waste from processing sugar cane is
called bagasse. This fibrous material is suitable for firing
boilers to raise steam and to generate electricity. Other
agricultural wastes include coconut waste and rice husks,
both produced in large tonnages but only available during

particular seasons. Environmental concerns about straw

stubble burning has meant that much more straw is now
available as a potential fuel.

Animal waste.
We can also use
very wet wastes
such as animal
slurries from a
cowshed. By
processing them
for a few days in

    

The ?bropower power station at in Suffolk, burns
a dry mixture of poultry litter and wood shavings to
generate electricity.

large tanks called
anaerobic
digesters, we can
obtain a useful fuel gas. The residue can be dried to give a
fertiliser.

Forestry waste. Much forestry waste was left to rot in the
past. The development of machines to harvest the waste
and convert it into wood chips has changed this. The chips
can be transported and dried for use in firing boilers.

Household waste. In industrialised countries this represents
a major source of fuel. At present in the UK, most
municipal waste is buried in landfill sites. These can be
disused clay pits or quarries. This is an increasingly
expensive and wasteful option.

There are two major ways to extract energy from household
waste. One is to separate the combustible material before
burial of the remainder. The other is to use the landfill site
as a fuel gas producer.

Landfill waste. The raw waste can be incinerated directly,

reducing the volume of the waste and so lowering disposal
costs. The energy recovered can be used for district heating
through a combined heat and power scheme, (called CHP).

  
 
  

Cutaway of land?ll Land fill?gas
gas extraction. . (methane and
carbon dioxide)
drawn off

Cement plug
to prevent gas
from escaping

Pipe with holes 
in to let in gas 
Impermeable
clay covering

 
 
 

stops gas
escaping

.23 Stone chippings
Waste material decays let gas pass
and releases gases through

If the raw waste is sorted first to remove things which
cannot burn, such as metals and glass, the combustion is
more efficient. Alternatively, the materials which will burn

 

can be processed to give fuel pellets, known as refuse?
derived fuel (or RDF for short).

When landfill sites are full they are capped to seal them.
The absence of a supply of oxygen from the air creates
anaerobic conditions. Under anaerobic conditions, natural
micro?organisms convert wastes to a mixture of gases
known as landfill gas. The two major components are
carbon dioxide, which does not burn, and methane, the
major component of natural gas. Originally the landfill gas
was allowed to vent naturally to the atmosphere but this
represented an explosion hazard. The next development
was to collect and flare off the gas to reduce the danger.
We now see landfill gas as a valuable resource. We can
collect it and burn it to heat boilers or to generate
electricity. The variable quality of landfill gas and the
presence of impurities make it unsuitable for addition to the
domestic gas distribution system.

In Europe some farmland has
been taken out of use to
reduce food production.This
provides opportunities to
grow energy crops
specifically for their use as
fuels. Fast?growing trees such
as willow and poplar,

or certain varieties of
perennial grass, can all be
grown as energy crops. For
trees, the technique of
coppicing is used. New
growth is cut every three or

Coppice showing newly cut stumps
and more mature trees in background.

four years for use as an
energy crop. The trees sprout
again and replace the cut material with new growth.

Most biomass fuels are used for heating and power but
liquid fuels can also be produced for use in transportation.
Ethanol is an alcohol which burns well and can be blended
with conventional fuels such as petrol. Sugar cane and
maize can both be used as the source of bio?ethanol. The
crops are fermented to give a mixture of water and alcohol.
This can be concentrated by distillation. In Brazil, millions
of vehicles are powered by ethanol or ethanol?petrol
mixtures. Vegetable oils can be used directly as fuels but
they perform better after chemical treatment. We call this
esterification. It involves chemically combining the oils with
an alcohol, either methanol or ethanol. The resulting fuel
can be blended with diesel for commercial use.

 

Environmental of using

i a r;

The growing and combustion of biomass fuels simply cycles

carbon through the environment. If we can operate on a

sustainable basis it will be carbon dioxide neutral, the

atmospheric concentration will remain stable. There is a

potential environmental gain that comes from the

substitution of biomass fuels for fossil fuels. However, the
substitution is not necessarily sustainable on a large scale.

Biomass fuels currently cost more to produce than fossil
fuels.

Liquid biomass fuels produce some different combustion
products to petrol or diesel. This does not mean,
however, they are significantly ?cleaner? fuels. The annual
sustainable yield of liquid biofuels from good agricultural
land is very low. In reality, there is a limit to how much
land could be allocated for this purpose. Every hectare set
aside for biofuel production is a hectare less for growing
food.

The large scale growth of energy crops will change the
landscape and can also have more subtle effects. Unless
care is taken, the crops can cause changes to the local
water table and increase the demand for nutrients from
the soil.

In the UK, biomass fuels currently provide less than 1% of
the supply of primary parts of the developing
world the proportion approaches 100%.

aspects of biomass
energy:

0 Biomass fuels are best suited for heat and power.

0 Although production costs are not falling as rapidly as
for other renewables such as photovoltaics, they seem
certain to fall further in the future because of the
economies of scale.

if we use the same amount of land for energy crops as
we use to grow softwood for papermaking, we could
provide up to 30% of worldwide electricity demand.

0 Liquid biofuels are not cost?competitive with petrol or
diesel. The production cost of biofuels is at least three
times higher than the production cost of fuels derived
from crude oil.

0 Some countries do not have large oil resources of their
own but can still produce biofuels.

0 Only 2% of the world?s biomass generated each year is
currently being used in fuels. Estimates suggest this could
increase to 8% by the middle of the 21 st century.

 

 

The 51177 is the source of biomass energy

Plants use the energy from the sun to convert carbon dioxide and water into carbohydrates (such as glucose)
and oxygen. This process is called Carbohydrates provide the plant with an energy store.
Carbohydrates in our diet supply us with energy as well. The energy stored in the plant is released on burning
(imagine the warmth of a log fire) or when the plant dies - and carbon dioxide and water return once more to
the atmosphere.

We still use ancient biomass energy. The fossil fuels of oil, natural gas and coal all have their origins in living
plant organisms and like living plant organisms, fossil fuels also release carbon dioxide when they burn.

The important step in looks like this:

is an energy converting process.
In energy from the sun is
harnessed by the plant and combined with
carbon dioxide and water to make food (such as
sugar). gives the green
The molecule traps
to use.

 
 
 
 
   
  
 
 
 
 
   
 
   

Energy from sunlight
absorbed and 
converted into sugars

and starches .
Water evaporation

Carbon dioxide l' 
absorbed from the air 7

Water and minerals
from the soil

   

 

 

 

Moisture content and combustion of biofuels

Compare the energy content of freshly cut wood with that of a similar
sample that has been air dried. You must make it a fair test by using:
same mass of wood each time
same size pieces (surface area effects). Note that wood shavings are
easier to light than large pieces.
same procedure to measure the heat output on combustion.

Place 200ml of cold water in a glass or metal container, supported on
a tripod.

Weigh out 10g of the fuel being tested into a fireproof dish.

Measure the water temperature at the start and the highest temperature Time (mins)
reached.

Ignite the fuel below the container of water.
Measure the time taken to reach the maximum temperature.
How does the moisture content affect the performance of the fuel?

What are the problems with making accurate measurements, how could
you solve them?

 

Making a liquid biofuel, ethanol

Fermentation requires days or even a few weeks to reach completion. Anaerobic respiration of
yeast produces the alcohol called ethanol, together with carbon dioxide gas. The rate of
production of the gas gives a measure of the progress of the fermentation.

Using a commercial wine kit, or simply a mixture of water, sugar and yeast, set up a
fermentation jar (1). This works best under warm conditions, about 40? Celsius. You must stop
air getting into the fermentation jar by using a water-filled airlock or a simple bunsen valve.

When fermentation ends, distil a sample of the mixture to recover the ethanol (boiling point
78? Celsius) (2).

Burn a 5ml sample of ethanol in a saucer (3).

Observe the flame colour, the ease of lighting and evidence of complete combustion no
smoke or soot.

What residue remains?

  
 

Cold water

sample