September 2012

Energy from Waste: The benefits of upgrading household

waste before energy recovery

NNFCCThe Bioeconomy Consultants

As we move towards a zero waste economy, waste managers are having to look for alternatives to landfilling waste. Where waste cannot realistically be prevented, re-used, or recycled, recovering the energy stored in waste starts to become an environmentally and economically attractive option.

Energy recovery can be particularly useful in treating mixed waste streams like municipal solid waste (MSW). The UK alone creates around 30 million tonnes of MSW every year1, and thanks to its high levels of organic matter – like paper and food waste – it could have enormous value as a source of low carbon energy.

We can recover energy from waste by simply incinerating it at high

temperatures to produce heat and power. But this can be inefficient, particularly if the feedstock has a high moisture content.

Alternatively, we can “partially” upgrade MSW to refuse derived fuel (RDF) or “fully” upgrade it to solid recovered fuel (SRF) by mechanically separating the recyclates from the waste and

Waste has become a valuable resource for producing energy but which end-of-life energy recovery option offers

the best returns for the environment and for the economy?

Useful definitionsMBT - Mechanical Biological Treatment; a mechanical sorting process followed or preceded by a biological treatment, such as composting or anaerobic digestion

MHT - Mechanical Heat Treatment; a thermal treatment using autoclaving followed by mechanical sorting

INEOS Bio plan to generate low carbon electricity and fuel from pre-treated MSW

in the Tees Valley, UKImage: © INEOS Bio

biologically or thermally treating it. This fuel can then be used to create a range of products at high conversion efficiencies.

This briefing paper explores the benefits of producing RDF and SRF for use in energy from waste facilities.

Pre-processing of waste

A range of processes exist for upgrading MSW, from simple sorting and shredding through to more complex mechanical biological treatment (MBT) and mechanical heat treatment (MHT).

MBT uses mechanical sorting to separate recyclates from the MSW, this is typically followed by one of three processes:

• Aerobic decomposition (Composting)

• Anaerobic digestion

• Bio-drying

Aerobic decomposition and anaerobic digestion use organisms to break down the organic waste. This helps to stabilise the waste, reduce its volume and create useful sorted fractions such as compost

InCIneraTIon orplaSMa gaSIfICaTIon

anaeroBICDIgeSTIon,

CoMpoSTIng

pYrolYSIS, gaSIfICaTIon, reMeDIaTIon, CoMBUSTIon

WaSTeSorTeDWaSTe

Sterilisation in Autoclave

Material Segregation

Waste management alternatives to landfill

Material Segregation

Energy from Waste Briefing Page 2

in the case of aerobic decomposition, and digestate and a combustible gas – known as biogas – in the case of anaerobic digestion.

Bio-drying – a variation of aerobic decomposition that uses natural process heat to dry rather than fully stabilise waste – is an approach used to produce RDF and SRF. A typical mass balance for MBT with bio-drying is shown above (left).

In contrast, MHT processes use steam or direct heat to treat waste. MHT processes typically incorporate the use of autoclaves – devices used to sterilise and break down organic matter.

Autoclaving is common in other industries, like the treatment of medical

waste, but at present isn’t widely used in the treatment of MSW.

MHT processes produce dry recyclates, a residual fraction and a fibre similar to SRF. A typical MHT mass balance is illustrated above (right).

Why pre-treat waste?

While pre-treating waste to RDF or SRF is usually not needed for mass burn incineration (MBI), it can add considerable value to waste streams and is often a requirement for more advanced energy conversion technologies, like gasification and pyrolysis.

This additional pre-processing is normally necessary because gasifiers

MHT mass balance example MBT mass balance example

MSW

fe metals1.4%

rejects3.6%

Srf 46.6%

pelletiser

eddy currentseperation

Magneticseperation

Densimetricseperation

Bio-drying

Shredder

eddy currentseperation

Magneticseperation

Densimetricseperation

non-femetals0.5%

Co2 3.1%H2o 32.2%

fe metals3.5%

non-femetals0.5%

Inert residues8.6%

MSW

Inerts6.4%

rejects3.6%

fibre

61.5%

Densityseperation

autoclave

Initialscalping

Densityseperation

Metalseperation

fe metals 5.4%non-fe

metals 1.0%Heavy

residues12.2%

lightresidues

9.9%

Trommel

autoclave

and pyrolysers need a fuel with a consistent particle size and a moisture content below about 30 per cent. However, this isn’t always the case, for example ‘plasma’ gasifiers can process untreated MSW.

Pre-treating waste with MBT or MHT to make RDF or SRF can offer a number of benefits when used in conjunction with energy recovery. These include:

• Reduced greenhouse gas (GHG) emissions, as well as fewer heavy metals and less dust in the fly ash2

• Improved downstream efficiency of energy recovery

• Increased recycling potential

However, the energy demands and outputs of MBT and MHT processes will vary according to a number of factors, including the composition of the waste, process design and the downstream user requirements, such as biogenic content and calorific value.

Papageorgiou et al.3 is one of the few studies that compares the energy and

GHG balances of different MSW to energy technologies. They compare:

• MBI with electricity generation or combined heat and power (CHP) production using untreated MSW

• MBT (bio-drying) with electricity generation in a co-fired power plant or CHP production using SRF

• MHT with electricity generation in a co-fired power plant or CHP production using SRF

The energy and fuel use for the three pre-treatments are illustrated in the table below. This shows that the energy demands of MBT and MHT processes are considerably higher than those of MBI.

However, to appreciate the overall pre-treatment benefits, displaced energy demand from recovering recycled materials and downstream energy conversion of the pre-treated waste must be considered.

According to Papageorgiou et al.3 if we take into account whole system or life cycle energy credits for each conversion

Pre-treatment energy demand and fuel use

Input (kWh/tonne of waste received)

MBI MBT MHT

electricity 4 80 24Diesel 1 10 10natural gas 0 0 177Total 5 90 211

Energy from Waste Briefing Page 4

technology, MHT returns up to 40 per cent of the energy present in each tonne of MSW, compared to around 33 per cent for MBI.

Greenhouse gas emissions

The GHG emissions saved by pre-treating waste depend on what energy process they are being used to displace.

To date the evidence is inconclusive as to whether pre-treatment saves GHG emissions compared to MBI, when the energy is recovered in a CHP plant. Although there may be some benefit to using MHT as a pre-treatment. In addition, there is very little information available to compare MBI to advanced conversion technologies, which is an area in need of further research.

However, there is evidence3 to suggest that when displacing coal in a power plant there are significant advantages to converting waste to RDF or SRF, as shown above right.

If there is no market for the recovered materials such as ferrous metals, non-ferrous metals and inerts like glass and ash, waste pre-treatment can still reduce life cycle GHG emissions, although these savings decrease by as much as 50 per cent3.

In contrast, if the SRF is landfilled rather than used for energy recovery, GHG emissions for MBT or MHT will be higher than MBI3. This shows that the

potential emissions savings seen when pre-treating waste by MBT or MHT followed by energy recovery will depend on the market for recyclates and final use of the SRF.

If we consider a realistic scenario for the UK, where most MSW is incinerated, with smaller but increasing quantities converted to SRF via MBT and MHT, significant GHG savings can be realised compared to straight incineration of untreated MSW3 when displacing coal and in some cases gas, assuming the SRF is used for energy production and the recovered materials are recycled.

While GHG emissions are an important social and political consideration, it is also valuable to consider the wider

Total GHG emissions (kg CO2 eq per tonne of waste) for contrasting

energy from waste options3

MBI with electricity

MHT with CHp

MBT with CHp

MBI with CHp

MHT with electricity (co-firing)

MBT with electricity (co-firing)

-350 50-50-150-250

implications of waste pre-treatment. Processing waste into SRF or RDF can also improve the combustibility of wastes in a number of other ways.

Organic matterSometimes it is desirable to control the amount of organic matter present in the fuel. Organic matter has one of the lowest calorific values and highest moisture contents in the slate of materials making up MSW. If we reduce the percentage of organic matter in the waste stream, this will make energy conversion more efficient.

We can achieve this by processing MSW into SRF or RDF. The organic matter in RDF is typically less than 25 per cent by mass, while in untreated MSW it can be as high as 50 or 60 per cent2.

The improved calorific value allows smaller waste fuelled power stations to produce more electrical output from the

same tonnage of material, which may be desirable.

Alternatively, if it is more important to have a high organic matter content – perhaps for those looking to claim renewable energy subsidies – the process can be modified accordingly.

Moisture content

Before a fuel can be converted to energy, all remaining moisture must be driven off inside the combustion or gasification reaction chamber.

Hence, increased moisture levels represent increased efficiency losses. In the case of gasification, some of this drying may need to be carried out before the waste is introduced to the gasifier, depending on the gasifier type.

MSW typically has a moisture content of between 30 and 40 per cent, compared

Lakeside energy from waste plant Image: © Viridor

Energy from Waste Briefing Page 6

By 2020 energy from waste could generate more than 3TWh of electricity in the UK5

to a moisture content of just 15 to 18 per cent for SRF3,4.

However, the fibre produced by MHT can contain as much as 50 per cent moisture, resulting in a low heating value. To use MHT fibre as a fuel it normally needs to be dried, preferably using waste heat.

Ash content

Removing non-combustibles from MSW means that SRF and RDF have reduced ash contents compared to untreated MSW. MSW can contain around 20 to 40 per cent ash content by mass2,4 while SRF and RDF contain around 10 to 20 per cent by mass2,3,4.

This compares more favourably to coal and woody biomass which have ash contents of 5 to 10 per cent by mass and 1 to 2 per cent by mass, respectively4.

The reduced ash content of SRF compared to MSW means that less heat is lost heating up the ash in the combustion or gasification chamber.

As a result more of the available chemical energy is converted into useful product such as electricity, resulting in increased conversion efficiency.

The reduced ash content of SRF compared to MSW also provides the following environmental benefits:

• Lower fly ash emissions from the combustion process, which reduces smog potential and removal costs.

• Smaller ash handling and holding system is needed at incineration plants. This decreases capital costs, albeit not greatly.

• Where batteries are removed by upstream MSW processing to produce RDF or SRF, heavy metals levels (e.g. zinc, cadmium, mercury) in the recovered ashes and any fly ashes will be reduced3.

Acidic flue gases

Per tonne, SRF contains more sulphur and chlorine than MSW. Hence, more sulphur dioxide and hydrochloric acid will be emitted when one tonne of SRF is burnt compared to one tonne of MSW.

However, more energy will be produced from the SRF so on an acidic gases per MWh basis the SRF is cleaner. This means that SRF will typically have a lower acidification potential than MSW

Air Products are building the worlds largest advanced gasification energy

from waste facility in Teesside, UK - due to be completed in 2014

Image: © Air Products

Energy from Waste Briefing Page 8

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.

per MWh, although this may not always be the case since the lower heating value can vary.

If plant operators switch from MSW to SRF or RDF with a higher acid gas level per tonne, care will need to be taken to ensure that emissions do not rise beyond permitted levels, regardless of whether the plant produces more net MWh of power per year.

Material consistency

MSW is a heterogeneous resource, both in terms of its constituent materials and particulate size; meaning it is composed of particles of different shapes and sizes.

Processing MSW into RDF or SRF will have the effect of homogenising the material and making it easier to burn or gasify consistently.

Looking to the future

To date energy from waste has not fulfilled its potential as a method for material or energy recovery and has been used more as a waste management tool. But we are starting to see a shift where the value of waste is being more widely recognised.

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 recovery is preferable to landfilling.

The most appropriate technology to convert waste to energy will depend on its intended end-use and the market for recyclates, but MBT and MHT can offer significant environmental and economic benefits over incineration.

Energy from Waste Briefing Page 10

nnfCC is a leading international consultancy with expertise on the conversion of biomass to bioenergy, biofuels and bio-based products.

nnfCCBiocentreYork Science parkInnovation WayYorkYo10 5DgUnited Kingdom

Telephone: +44 (0)1904 435182email: enquiries@nnfcc.co.ukWebsite: www.nnfcc.co.uk