Carbon in Residual
Household Waste
Both landfill and combustion (which includes energy recovery
facilities, energy from waste facilities and combined heat and
power plants) of residual household waste will result in the release
of carbon into the atmosphere. However they do so in different
ways, in different amounts and with different potential impacts.
Carbon in Residual Household Waste
Carbon
Carbon is released to the atmosphere in the form of
carbon dioxide (CO2) and methane (CH4). Methane
is around 25 times as damaging as carbon dioxide
based on latest (2013) integrated pollution
prevention and control (IPPC) guidance. CO2
equivalents are used as a way of comparing the
effect of different gases. CO2 is given a global
warming potential of one while a given unit of CH4
will be 25 CO2 equivalents.
Biogenic and fossil carbon
There are two types of carbon:
• Waste from biological sources i.e. plants
= biogenic carbon
• Waste made from fossil fuels e.g. plastics
= fossil carbon
Some wastes e.g. clothes may be a mix of
biogenic and fossil carbon (e.g.
cotton/polyester mixes) and some wastes e.g.
metals contain little or no carbon.
The refuse derived fuel recovered fuel (RDF)
produced by mechanical biological treatment
(MBT) is classed as a pre-treatment. The RDF is then
subjected to combustion usually in a combined heat
and power plant (CHP).
It is important to know what type of carbon is
in the waste because:
a) They behave differently in landfill – plastic
does not decompose
b) They are counted differently in terms of
how they are calculated to contribute to
global warming. Although the
atmosphere cannot distinguish between
CO2 released from a biogenic or a fossil
source when considering the overall
climate impacts it is important they are
accounted for and treated differently to
avoid double counting.
The intergovernmental panel on climate
change (IPCC) have agreed conventions
for doing this which are applied here.
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e.g. When calculating the amount of carbon
released, biogenic CO2 is ignored as it is
considered “short cycle”. Fossil CO2 is
counted as it was absorbed millions of years
ago and only recently released.
Of the waste in a typical residual bin (2013)
somewhere between one half and two thirds
will contain biogenic carbon.
Carbon in Residual Household Waste
Comparison between Carbon Released
by Landfill and Combustion
Landfill
Combustion (which includes Thermal Recovery Facilities, Energy from
Waste Facilities and Combined Heat and Power Plants)
CO2 and CH4 are produced in
equal proportions
Only CO2 is produced
All of the fossil carbon stays in the
ground and does not break down.
Some of the biogenic material
does break down with the carbon
converted to a mixture of CO2
and CH4 know as landfill gas.
Some of the CH4 would escape
into the atmosphere.
Nearly all of the carbon is released immediately in the form of CO2
A large proportion of the landfill
gas would be captured and burnt.
Burning landfill gas produces,
“short cycle”, biogenic CO2 .
When the waste is combusted any biogenic CO2 released is ignored as it is considered
“short cycle”.Fossil CO2 is counted as it was absorbed millions of years ago and only
recently released.
A large proportion of the landfill
gas would be captured and burnt,
generating energy and offsetting
power station emissions.
More energy is generated than is used to run the plant. This energy substitutes for energy
generated from a fossil fuel fired power station1 which saves the fossil carbon dioxide which
would have been released by the power station. This means that some of the fossil carbon
dioxide released by the combustion can be offset by the saving from the power station.
The more efficiently the combustions plant converts the waste to useful energy, the greater
amount of carbon dioxide which can be offset.
1 A gas fired power station (Combined Cycle Gas Turbine – CCGT) is the current standard comparator as this is the “marginal” technology if you wanted to build a new power station.
Once the energy offset is taken into account,
the net carbon dioxide equivalents from the
methane release from landfill would be greater
than the net carbon dioxide released by
combustion.
This means that when comparing like for like
energy recovery by combustion from residual
household waste has a lower greenhouse gas
impact than landfill.
This is a simplified argument other factors
which should be taken into account are:
• The changing biogenic content of residual
waste over time
• How the biogenic carbon dioxide is
counted
• The fact that not all biogenic material
breaks down in landfill
• The level of landfill gas capture
• The impact of recycling metals from ash
generated by combustion plants
• The impact of pre-treatments such as
MBT-AD
• How to allow for the fact that the landfill
gas is released over a number of years.
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(taken from DEFRA (2013) Energy from Waste A
guide to the debate. Available at:
:https://www.gov.uk/government/uploads/syste
m/uploads/attachment_data/file/221042/pb13
892-energy-from-waste.pdf
Carbon in Residual Household Waste
Glossary of Terms
Term or Abbreviation
Definition
CHP
Combined Heat and Power. The combined production of electricity and usable heat. Steam or
hot water, which would otherwise be rejected when electricity alone is produced, is used for
space or process heating.
MBT-AD
Mechanical Biological Treatment - Anaerobic Digestion. MBT is the name given to several
processes that are used to treat the waste that arrives at the plant. As the name suggests, MBT
involves both mechanical and biological methods. The ‘mechanical’ part refers to the processes
that are used for preparing and separating the waste when it arrives at the plant such as
shredding and sieving. The ‘biological’ part of MBT refers to either composting or digesting of
the slurry using aerobic bio-drying, aerobic in-vessel composting or anaerobic digestion (AD).
RDF/SRF
Refuse Derived Fuel (RDF). The combustible fraction of municipal waste can be mechanically
and/or thermally separated into RDF (sometimes referred to as Solid Recovered Fuel (SRF).
This may be pelletised or utilised as a loose flock product in a controlled combustion process
(either a dedicated incineration/gasification process or co-combusted in an industrial type
application, for example a cement kiln or power station). RDF is produced in Mechanical
Biological Treatment – Anaerobic Digestion (MBT-AD) facilities. In Greater Manchester it is
burned at Runcorn Energy Recovery Facililty (ERF) to produce electricity.
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www.recycleforgreatermanchester.com
E : [email protected]