September 2007
PF 07-08
Profiles
Cofiring of coal with waste fuels
‘Some waste fuels can be
considered renewable
and to be CO2 neutral’
‘Cofiring with coal has
the potential to overcome
some of the drawbacks
of firing waste alone’
‘Sewage sludge, RDF,
tyres and MBM have
been cofired in all types
of coal-fired plant’
There has been considerable interest
in cofiring both biomass and waste
fuels in coal-fired power plants in
recent years. This is principally due to
concerns regarding the emissions of
greenhouse gases from these plants.
Biomass and some waste fuels can be
considered to be renewable and not to
produce net CO2 emissions when
combusted hence cofiring in a coalfired plant reduces overall greenhouse
gas emissions. Another reason for
cofiring is that as there is limited space
available for landfilling, it is desirable
to find alternative routes for waste
disposal. A further disadvantage of
landfilling is that landfilling can lead to
methane emissions which have an even
more potent greenhouse effect than
CO2. Waste fuels are also generally
much cheaper than coal or biomass.
Coal and waste fuels have been
cofired in stoker, cyclone, fluidised bed
combustion and pulverised coal
combustion boilers but many issues
need to be addressed when considering
the use of biomass or waste products
for power generation. The composition
of waste fuels is very different from
coal. Wastes can contain much higher
moisture levels which can adversely
affect combustion by absorbing heat
during evaporation. Their heating
values tend to be lower than coal.
Moreover, their bulk densities are also
lower hence greater volumes of fuel are
needed to be collected, handled,
transported and stored. There can also
be considerable public opposition to the
construction of waste-fired units.
Cofiring waste with coal has the
potential to overcome some of the
drawbacks of firing waste alone.
Cofiring does not entail the high costs
of building a new plant but the
significantly lower retrofitting costs at
an existing plant. Cofiring boilers can
fire waste when supplies are plentiful
but switch back to coal when supplies
are low. Cofiring increases the
efficiency of waste to energy
conversion by firing it in a larger plant
rather than in a smaller plant firing only
waste. Disadvantages can, however,
arise with cofiring as wastes can
contain higher concentrations of heavy
metals and chlorine which can lead to
problems with corrosion, effects on
SCR units and ash disposal.
The types of waste which could be
suitable for cofiring with coal include
sewage sludge, municipal solid waste
(including municipal sludge, refuse
100
90
80
Total waste, %
70
60
50
40
30
20
10
0
Greece Portugal United Ireland Finland
Kingdom
landfill
Waste disposal in Europe
Italy
Spain
France Austria
Germany
Belgium
Netherlands
Luxembourg
Sweden
Denmark
recycled/composted (and other)
incineration
derived fuel and plastic), RDF, used
tyres, MBM and specialised industrial
wastes. Stoker boilers are capable of
firing a wide range of fuels including
fairly large fuel pieces. The types of
wastes that have been cofired with coal
in stoker boilers include wood, tyres as
well as residential and commercial
refuse. FBC boilers have been
commonly utilised for the cocombustion of biofuels and coal
particularly in Scandinavia. This is due
to the high degree of fuel flexibility of
this technology in relation to the
particle size, density, moisture and ash
contents of the fuel. They achieve high
boiler efficiency even with challenging,
low grade fuels. It is most common to
cofire waste wood with coal in CFB
plant and the cofiring with RDF is less
common. Many fuels such as waste
wood and tyres have been cofired in
cyclone boilers. PCC is the dominant
method for burning coal for power
generation worldwide. As all the
constituents of the waste enter the coal
boiler, several technical issues arise
which need to be considered. Sewage
sludge, industrial waste and tyres have
been more commonly cofired in PCC
plants than RDF.
The types of waste fuels considered
in this report are RDF, sewage sludge,
tyres and meat and bone meal. The fuel
characteristics of these fuels are very
different from those of coal and there is
a greater variation in these properties
for a given fuel compared with typical
coals. The heating values of sewage
sludge and MBM are considerably
lower than coal. The ash contents can
be considerably higher. The moisture
contents of some waste fuels are lower
than that of coal but in others can be an
order of magnitude higher. The fuel
nitrogen and sulphur levels of wastes
can sometimes be higher and
sometimes lower than those of coal.
RDF can contain very high Cl levels.
All these properties need to be
considered when cofiring wastes with
coal. The issues regarding the delivery,
storage and preparation of wastes are
different from those for coal. They can
have a much lower bulk density than
coal and as they sometimes have lower
heating values, the overall fuel density
of waste fuels in MJ/m3 can be
considerably lower than coal. Hence,
cofiring wastes at even 10% of thermal
input may require comparable
volumetric flows of coal and wastes.
Some fuels such as poultry manure and
sewage sludge may require pre-drying.
On an as-received basis these fuels can
have moisture levels in the range 6080% or even greater. As such they have
little net-heating value and cofiring can
only be regarded as a method of waste
disposal. Sewage sludge is generally
mechanically dewatered at the sewage
treatment plant but further drying at the
power plant may be required to increase
its heating value.
Waste fuels such as sewage sludge,
RDF, tyres and MBM have been
cofired in all types of coal-fired plant
but to a much lesser extent than fuels
such as herbaceous and woody fuels.
Sewage sludge has been extensively
cofired in Germany in nearly 20 power
plants and in a few other countries.
Waste fuels have been extensively
cofired in all eight coal-fired plants in
the Netherlands. RDF has also been
cofired in a few plants capable of firing
coal in Austria, Germany and the UK.
Tyre derived fuels have mainly been
cofired in the United States. Similar
technical issues affecting the operation
of the plant arises with all these fuels
which need to be addressed. The
legislation applicable to plant cofiring
waste is invariably stricter than plant
cofiring biomass and more rigorous
emission controls will be needed.
Public opposition to cofiring waste will
also be greater. Nevertheless cofiring
waste in coal-fired plant may be more
environmentally acceptable than other
methods of waste disposal.
Each issue of Profiles is based on a
detailed study undertaken by IEA
Clean Coal Centre, the full report of
which is available separately. This
particular issue of Profiles is based on
the report:
Cofiring of coal with waste fuels
Rohan Fernando
CCC/126, ISBN 92-9029-445-0,
34 pp, September 2007,
£255*/£85†/£42.50‡
*
†
‡
non-member countries
member countries
educational establishments within member
countries
IEA Clean Coal Centre is a
collaborative project of member
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Energy Agency (IEA) to provide
information about and analysis of
coal technology, supply and use.
IEA Clean Coal Centre has
contracting parties and sponsors
from: Australia, Austria, Brazil,
Canada, China, Denmark, the
European Commission, Germany
India, Italy, Japan, Republic of
South Korea, the Netherlands,
New Zealand, Russia, South
Africa, Sweden, Spain, the UK
and the USA.
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