This document summarizes the benefits of pre-treating municipal solid waste (MSW) to produce refuse derived fuel (RDF) or solid recovered fuel (SRF) before energy recovery. Pre-treating MSW through mechanical biological treatment (MBT) or mechanical heat treatment (MHT) can upgrade the waste to make it more homogeneous and suitable for gasification or pyrolysis technologies. This pre-treatment can reduce greenhouse gas emissions, improve energy conversion efficiency, lower ash content and acid gas emissions, and increase the potential for material recycling compared to direct incineration of raw MSW. However, the energy demands of MBT and MHT are higher than direct incineration. The overall environmental and economic impacts depend
3. Incineration or
plasma gasification
SORTED
WASTE WASTE
ANAEROBIC
DIGESTION,
Material Segregation
COMPOSTING
Sterilisation in
Autoclave Material Segregation
PYROLYSIS, GASIFICATION,
REMEDIATION, COMBUSTION
Waste management alternatives to landfill
biologically or thermally treating it. This MBT uses mechanical sorting to separate
fuel can then be used to create a range of recyclates from the MSW, this is typically
products at high conversion efficiencies. followed by one of three processes:
This briefing paper explores the benefits • Aerobic decomposition
of producing RDF and SRF for use in (Composting)
energy from waste facilities. • Anaerobic digestion
Pre-processing of waste • Bio-drying
A range of processes exist for upgrading Aerobic decomposition and anaerobic
MSW, from simple sorting and shredding digestion use organisms to break down
through to more complex mechanical the organic waste. This helps to stabilise
biological treatment (MBT) and the waste, reduce its volume and create
mechanical heat treatment (MHT). useful sorted fractions such as compost
Energy from Waste Briefing Page 2
4. MSW MSW
rejects Initial
Shredder rejects
3.6% scalping 3.6%
Co2 3.1%
Bio-drying
H2o 32.2% autoclave
Densimetric Densimetric fe metals
seperation seperation Metal 5.4%
Trommel
seperation non-fe
metals 1.0%
fe metals Magnetic Magnetic fe metals
1.4% seperation seperation 3.5% Heavy
Inerts Density Density
residues
6.4% seperation seperation
12.2%
non-fe non-fe
eddy current eddy current
metals metals light
seperation seperation
0.5% 0.5% fibre 61.5% autoclave residues
9.9%
pelletiser Inert residues
8.6%
Srf 46.6%
MBT mass balance example MHT mass balance example
in the case of aerobic decomposition, waste, but at present isn’t widely used in
and digestate and a combustible gas the treatment of MSW.
– known as biogas – in the case of
MHT processes produce dry recyclates,
anaerobic digestion.
a residual fraction and a fibre similar
Bio-drying – a variation of aerobic to SRF. A typical MHT mass balance is
decomposition that uses natural process illustrated above (right).
heat to dry rather than fully stabilise
waste – is an approach used to produce
Why pre-treat waste?
RDF and SRF. A typical mass balance for While pre-treating waste to RDF or
MBT with bio-drying is shown above SRF is usually not needed for mass
(left). burn incineration (MBI), it can add
considerable value to waste streams
In contrast, MHT processes use steam
and is often a requirement for
or direct heat to treat waste. MHT
more advanced energy conversion
processes typically incorporate the use
technologies, like gasification and
of autoclaves – devices used to sterilise
pyrolysis.
and break down organic matter.
This additional pre-processing is
Autoclaving is common in other
normally necessary because gasifiers
industries, like the treatment of medical
5. and pyrolysers need a fuel with a GHG balances of different MSW to
consistent particle size and a moisture energy technologies. They compare:
content below about 30 per cent.
• MBI with electricity generation or
However, this isn’t always the case, for
combined heat and power (CHP)
example ‘plasma’ gasifiers can process
production using untreated MSW
untreated MSW.
• MBT (bio-drying) with electricity
Pre-treating waste with MBT or MHT to generation in a co-fired power plant
make RDF or SRF can offer a number of or CHP production using SRF
benefits when used in conjunction with
• MHT with electricity generation
energy recovery. These include:
in a co-fired power plant or CHP
• Reduced greenhouse gas (GHG) production using SRF
emissions, as well as fewer heavy
The energy and fuel use for the three
metals and less dust in the fly ash2
pre-treatments are illustrated in the
• Improved downstream efficiency of table below. This shows that the energy
energy recovery demands of MBT and MHT processes are
• Increased recycling potential considerably higher than those of MBI.
However, the energy demands and However, to appreciate the overall pre-
outputs of MBT and MHT processes will treatment benefits, displaced energy
vary according to a number of factors, demand from recovering recycled
including the composition of the waste, materials and downstream energy
process design and the downstream user conversion of the pre-treated waste
requirements, such as biogenic content must be considered.
and calorific value.
According to Papageorgiou et al.3 if we
Papageorgiou et al.3 is one of the few take into account whole system or life
studies that compares the energy and cycle energy credits for each conversion
Pre-treatment energy demand and fuel use
Input (kWh/tonne MBI MBT MHT
of waste received)
Electricity 4 80 24
Diesel 1 10 10
Natural Gas 0 0 177
Total 5 90 211
Energy from Waste Briefing Page 4
6. technology, MHT returns up to 40 per
cent of the energy present in each tonne
of MSW, compared to around 33 per cent
MBI with electricity
for MBI. MBT with electricity (co-firing)
Greenhouse gas emissions
The GHG emissions saved by pre-
MHT with electricity (co-firing)
treating waste depend on what energy
process they are being used to displace.
MBI with CHp
To date the evidence is inconclusive as
MBT with CHp
to whether pre-treatment saves GHG
emissions compared to MBI, when the
MHT with CHp
energy is recovered in a CHP plant.
Although there may be some benefit
-350 -250 -150 -50 50
to using MHT as a pre-treatment. In
Total GHG emissions (kg CO2 eq
addition, there is very little information
per tonne of waste) for contrasting
available to compare MBI to advanced
energy from waste options3
conversion technologies, which is an
area in need of further research. potential emissions savings seen when
pre-treating waste by MBT or MHT
However, there is evidence3 to suggest followed by energy recovery will depend
that when displacing coal in a power on the market for recyclates and final
plant there are significant advantages use of the SRF.
to converting waste to RDF or SRF, as
shown above right. If we consider a realistic scenario for
the UK, where most MSW is incinerated,
If there is no market for the recovered with smaller but increasing quantities
materials such as ferrous metals, non- converted to SRF via MBT and MHT,
ferrous metals and inerts like glass and significant GHG savings can be realised
ash, waste pre-treatment can still reduce compared to straight incineration of
life cycle GHG emissions, although these untreated MSW3 when displacing coal
savings decrease by as much as 50 per and in some cases gas, assuming the SRF
cent3. is used for energy production and the
recovered materials are recycled.
In contrast, if the SRF is landfilled
rather than used for energy recovery, While GHG emissions are an important
GHG emissions for MBT or MHT will be social and political consideration, it
higher than MBI3. This shows that the is also valuable to consider the wider
10. per MWh, although this may not always
be the case since the lower heating value
Looking to the future
can vary. To date energy from waste has not
fulfilled its potential as a method for
If plant operators switch from MSW to material or energy recovery and has
SRF or RDF with a higher acid gas level been used more as a waste management
per tonne, care will need to be taken tool. But we are starting to see a shift
to ensure that emissions do not rise where the value of waste is being more
beyond permitted levels, regardless of widely recognised.
whether the plant produces more net
MWh of power per year. The end-of-life use of waste should
conform, where possible, to the waste
hierarchy, but where it cannot be
practically re-used or recycled, energy
Material consistency
MSW is a heterogeneous resource, both
recovery is preferable to landfilling.
in terms of its constituent materials and
particulate size; meaning it is composed The most appropriate technology to
of particles of different shapes and sizes. convert waste to energy will depend on
its intended end-use and the market for
Processing MSW into RDF or SRF will
recyclates, but MBT and MHT can offer
have the effect of homogenising the
significant environmental and economic
material and making it easier to burn or
benefits over incineration.
gasify consistently.
Background information
1. Evaluation of Opportunities for Converting Indigenous UK Wastes to Fuels and
Energy. Barker N and Evans L. 2009. NNFCC Report 09-012.
2. Analysis and comparison of municipal solid waste and reject fraction as fuels for
incineration plants. Montejo C, Costa C, Ramos P and del Carmen Márquez M. 2011.
Applied Thermal Engineering (31), pp. 2135-2140.
3. Assessment of the greenhouse effect impact of technologies used for energy
recovery from municipal waste: a case for England. Papageorgiou A, Barton J R and
Karagiannidis A. 2009. Journal of Environmental Management (90), pp. 2999-3012.
4. An integrated appraisal of energy recovery options in the United Kingdom using
solid recovered fuel derived from municipal solid waste. Garg A, Smith R, Hill D,
Longhurst PJ, Pollard SJ and Simms NJ. 2009. Waste Management (29), pp. 2289-2297.
5. UK jobs in the bioenergy sectors by 2020. McDermott F. 2012. NNFCC Report 11-025.
12. NNFCC is a leading international consultancy with expertise on the
conversion of biomass to bioenergy, biofuels and bio-based products.
NNFCC
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