Biomass
Biomass
Biomass
Energy Source
.
Peter Stuart
Loughborough University
Loughborough, Leics LE11 3TU
1.
INTRODUCTION
2. ADVANTAGES
The primary use of anaerobic digestion is waste
stabilisation. There are however many ancillary benefits
which must be fully exploited in order to ensure the
economic viability of the process.
2.1 Available Resources
The starting material for anaerobic digestion is normally
some form of waste unless biomass has been produced
specifically for use as a feedstock. A vast amount of
waste is available for processing from a variety of
sources:
Domestic / Municipal Waste:
The UK
produces nearly 30 million tonnes of domestic
waste each year. 60% of household waste is
biodegradable of which 25% is kitchen waste
and other organics and 30-35% is paper [1].
Percentage
Composition
60-80%
20-40%
0-5%
0-3%
Traces
(3)
(4)
Eqns. 3 and 4 show that nearly all the energy from the
glucose is transferred to a simple gaseous hydrocarbon
where is resides at a higher mass density. The methane
produced can easily be separated from the aqueous
system, and if so desired from the co-product CO2.
Anaerobic digestion therefore offers the opportunity to
efficiently produce a premium hydrocarbon fuel from
waste materials of a potentially negative value.
Up to one third of the biogas is required to sustain the
reactor. The remaining two thirds can be used to generate
heat, electricity or both. Combined heat and power (CHP)
production is by far the most efficient use of the available
energy. If such a system is put in place the whole digester
heat requirement is normally recovered from the engine
and exhaust gas. Heat and electricity can be used to meet
the energy requirements of the facilities at which they are
generated (farms, sewage treatment plants etc.) or can be
sold to external consumers.
Growth Of B iogas Plants In Germany
700
617
600
500
450
370
400
300
b a
C n H a Ob ( aq ) (n ) H 2 O
2 4
n b a
n a b
( )CH 4 ( g ) ( )CO2 ( g )
2 4 8
2 8 4
139
159
168
1992
1993
1994
100
0
(1)
C 6 H 12 O6 ( aq ) 3CH 4 ( g ) 3CO2 ( g )
200
274
(2)
1995
Y ear
1996
1997
1998
3. Disadvantages
Unfortunately implementation of the anaerobic digestion
process has substantial drawbacks, largely responsible for
its limited uptake.
3.1 Environmental Sensitivities
The anaerobic digestion process can be viewed as
inefficient in that most of the energy of the organic
material ends up in the methane contained in the biogas.
The anaerobic bacteria in comparison to their aerobic
counterparts are very bad at extracting energy from the
organic feed. Only about 5-10% of the energy transferred
from the complex organics to methane is available for the
bacteria to use for growing, compared to 50% for the
equivalent aerobic process [14]. As a result anaerobic
bacteria tend to be slower growing and more sensitive to
changes in conditions. Maintaining a suitable
4. CONCLUSIONS
The advantages and disadvantages of the anaerobic
digestion have been assessed. The process is primary of
importance in terms of waste stabilisation and pollution
control but its many fringe benefits improve its viability.
The methane produced by anaerobic digestion is an
important renewable energy source but does not justify
implementation solely for this purpose. The inherent
complexities and high start-up costs of anaerobic
digestion continue to hamper its development.
5. ACKNOWEDGEMENTS
I would like to thank Prof. Andrew Wheatley for all his
advice on the subject in question. I would also like to
thank Severn Trent Water Ltd. for the opportunity to visit
their biogas facility which is featured in the photographs
in this report. Great appreciation is owed to David
Longden who took the pictures.
6. REFERENCES
[1] Thames Waste Management Limited web-page,
http://www.twm.co.uk/anaerobic_digestion.html
(Accessed November 2002)