A two mile stroll downhill from IFRF’s office in Sheffield, and surrounded by industrial, commercial and residential buildings, is a very modern-looking plant.  This impressive complex is Veolia’s ‘Energy Recovery Facility’ (ERF), providing up to 21MW of electricity to the UK’s National Grid and the hub of Sheffield’s award-winning City District Energy Network, supplying up to 45MW of heat to over 140 buildings (including the IFRF’s host – the  University of Sheffield, leisure centres, schools, colleges, hotels, etc.) as well as some 3,000 homes.  The ERF is based around a large, gas-fired incinerator, handling up to 225,000 tonnes of residual (mainly household) waste per year, feeding a combined heat and power (CHP) plant.  No smell, no noise, and only a wisp of flue gases coming out of its 75m chimney – impressive!

Last month (MNM, 5^th February edition), Philip Sharman contributed a thought-provoking piece on ‘Waste-to-Energy – a significant growth area in the energy mix’, which reported on a recent report from US consultants GVR valuing the global WtE (more correctly, ‘energy-from-waste’ – EfW) market at $25 billion in 2015 and increasing to some $45 billion by the middle of the next decade.  Philip concluded by saying:  “So, with the utilisation of fossil fuels for power generation and industrial processes in decline in many parts of the world, perhaps the IFRF should pay increased attention to this already substantial – and growing – market opportunity for combustion technology…”

With this still fresh in our minds, we thought it would be good to ‘unpack’ the EfW topic a little bit more from a technology and R&D perspective.  The MNM piece on 5^th February pointed out that around 80% of the global EfW sector uses ‘thermal treatment’ processes – primarily incineration but also so-called ‘advanced thermal treatment’ (ATT) technologies based on gasification and pyrolysis – with the remaining ~20% using ‘biological treatment’ methods (i.e. anaerobic digestion – AD).  Given this breakdown, the sensible place to start is with incineration.

Future MNM pieces over the next few months will examine ATTs and AD in terms of the technologies used in EfW applications and the R&D challenges to be addressed.

Incineration technologies for EfW

Incineration of residual waste (i.e. waste that is left over when all the ‘upper’ elements of the ‘hierarchy of waste’ (i.e. prevent – reduce – reuse – recycle) have been deployed, leaving just the ‘lower’ elements (i.e. recover – dispose) to deploy – see previous MNM piece) and the recovery of the energy content of the waste stream involves a number of discrete stages as follows.

Some authorities or waste treatment companies choose to use some form of pre-treatment stage for their residual wastes so as to extract more recyclables and produce a ‘fuel’ with a more specific calorific value (CV) and/or biogenic content.  Types of pre-treatment include: mechanical sorting; mechanical biological treatment (MBT); and mechanical heat treatment (MHT).  These processes generally use mechanical sorting and processing techniques to remove recyclates, remove moisture, and shred and/or homogenise the waste to form some kind of ‘refuse-derived fuel’ (RDF) or ‘solid recovered fuel’ (SRF).  The removal of moisture, recyclates and organic matter (where applicable) generally increases the CV of the RDF (i.e. mixed municipal waste normally has a CV in the range 8-11MJ/kg, while RDF will typically be in the range 11-17MJ/kg).  Of course, pre-treatment  facilities require energy and this needs to be taken into account when considering the overall energy balance.  The pre-treatment stage (if used) may be co-located with an EfW facility or may be at a separate location.

The EfW facility will generally have a reception area, where waste is delivered, handled and stored.  The storage of waste is different for different types of wastes – municipal solid waste (MSW), hazardous waste, sewage sludge, liquid waste and clinical waste all necessitate specific storage facilities.

From the reception area, the waste feed is transferred to the thermal treatment stage – in this case, the incineration stage.  Incinerators come in a wide range of types (and sizes), which can generally be broken down into: moving/fixed grate incinerators (rocking-, reciprocating-, travelling-, or roller-grate systems); hearth furnaces (stepped-, static-, stepped-inclined-, inclined-counter-rotating-, or multiple-hearth systems); fluidised bed (FB) combustors (bubbling FB, or circulating FB combustors); rotary kilns (fully-, partial-rotating-, or oscillating-kiln systems); hybrid systems (e.g. hearth-FB); or other specialist systems designed for specific waste streams (e.g. modular systems, cycloid incinerator chambers for sewage sludge, etc.).  IFRF readers can find detailed reviews of these different incinerator technologies on either the UK Defra website or  the European IPPC Bureau website.

Whichever incinerator technology is utilised, the waste feed (MSW or RDF) encounters a number of different combustion regimes during its incineration: drying and degassing to evolve the volatile content (generally at 100-300°C); pyrolysis, i.e. further decomposition of the organic matter (250-750°C); gasification of the residue with water vapour and CO2 (500-1,000°C); and oxidation of the combustible gases produced in the previous regimes (800-1,450°C).  Generally, these regimes do not follow sequentially, but overlap and influence each other.  However, by using in-furnace technical measures such as furnace design, air distribution and instrumentation and control technology, it is possible to influence these steps so as to reduce the polluting emissions that are produced.

In fully-oxidising incineration, the main constituents of the flue gases evolved are: water vapour, N₂, CO₂ and O₂.  Depending on the composition of the waste feed and the operational conditions of the incinerator, smaller quantities of CO, HCl, HF, HBr, HI, NOx, SO₂, VOCs, PCDD/F, PCBs and heavy metal (and other) compounds are formed or remain.  Depending on the combustion temperatures during the incineration, volatile heavy metals and inorganic compounds (e.g. salts) are totally or partially evaporated.  In order to ensure the proper breakdown of toxic organic substances within the incinerator, national or regional legislation/regulations (e.g., for Europe, the European Commission’s Waste Incineration Directive – ‘WID’ – 2000/76/EC and the Industrial Emissions Directive – ‘IED’ – 2010/75/EU) will require that the flue gases reach a minimum temperature (typically at least 850°C, 1,560°F) for a minimum time period (typically at least two seconds).

The substances evolved during incineration are transferred from the feed waste stream to the flue gases and the particulate matter they contain.  A mineral residue ‘fly ash’ (dust) and heavier ‘bottom ash’ (solid ash) are created: In MSW incinerators, bottom ash is approximately 20-30 wt-% (but only about 10 vol-%) of the solid waste input, although this varies greatly according to waste type and detailed process design; fly ash quantities are generally much lower (typically only a few percent of input volume/weight).

Clearly, for effective oxidative combustion to occur, sufficient supply of oxygen is necessary.  The air ratio number (‘n’) of the supplied incineration air to the stoichiometric (i.e. chemically required) incineration air is usually in the range 1.2-2.5, depending on the fuel type (gas, liquid or solid) and the incinerator type.

Following the incineration stage, the energy conversion/recovery stage generally involves using the combustion heat to generate steam in a boiler.  Up to 80% of the total available energy in the waste can be recovered as steam, with this being used to generate power (via standard turbo-generator equipment), or for heat supply to industrial/commercial/residential users, or, increasingly, for both i.e. CHP – the most efficient option for EfW via a steam boiler.  An incinerator producing exclusively steam for heat can have a thermal generating efficiency of around 80-90% (comparable with a boiler fired with natural gas or oil), whereas the net electrical efficiency (i.e. taking account of the parasitic load of the plant) of an incinerator producing exclusively electricity is often cited as in the range 14-27% (c.f. a typical coal-fired power station at 33-38% or a CCGT power station at over 50%).  For an incinerator-based EfW plant operating in CHP mode, the electrical and thermal efficiencies will vary depending on the split between heat and power: The actual electrical and/or heat output depends on establishing energy customers – electricity can easily be supplied to grid, whereas heat will need to be used locally to the incinerator and will vary seasonally and during a day.  Forming part of a district heating system – as is the case with Veolia’s ERF coupled with Sheffield’s City District Energy Network – has many advantages, and with increasing energy costs, such district heating schemes based around EfW may become more common.  In the UK, the Government has incentivised the use of heat through the development of the Renewable Heat Incentive (RHI), and qualified incineration with energy recovery for Renewable Obligation Certificates (ROCs) where ‘good quality’ CHP is in place.

Emissions control, bottom ash handling and waste water treatment are very important stages in EfW plant operations.  To meet national or regional emission limit legislation/regulations (e.g. in Europe, through environmental permitting regulations under IED/WID), the incineration process must be correctly controlled and the flue gases (typically 70-75 wt-% of the solid waste input) cleaned prior to their final release.  This will generally entail: ammonia or urea injection for the control of NO_xemissions; lime or sodium bicarbonate injection for control of SO₂ and HCl emissions; activated carbon injection for capturing heavy metal species; and filter systems for the removal of fly ash particulates and other solids (e.g. lime/bicarbonate and carbon).  As stated above, the control of CO, VOCs and dioxin concentrations is primarily achieved through maintaining the correct combustion conditions.

The clean-up of the flue gases from incinerators produces solid residues comprising fly ash, lime/bicarbonate and carbon.  These residues are usually combined (although some systems separate fly ash and other components) and are often referred to as ‘air pollution control residues’ (APCr) and classified as ‘hazardous waste’ – this means that their disposal (e.g. to specialist landfill sites) must be undertaken in compliance with the relevant legislation/regulations.  APCr produced is typically around 2-6 wt-% of the original feed.

The main residual material from the incineration of MSW or RDF is the bottom ash (often referred to as ‘incinerator bottom ash’ – IBA), which is discharged continuously from the incinerator’s combustion chamber, cooled and disposed of as non-biodegradeable, non-hazardous waste to conventional landfill sites (although it does have the potential to be used as an aggregate replacement).  The amount of IBA depends on the level of pre-treatment, but, as indicated above, is typically ~20-30 wt-% of the original waste feed to the incinerator for MSW (considerably less for RDF incinerators).  IBA contains ferrous and non-ferrous metals (typically 2-5 wt-% of feed), which can be recovered and sold for re-smelting.

Waste water treatment and control includes additional cooling prior to discharge, segregating different waste water streams (e.g. from wet flue gas scrubbers), physico-chemical treatment, recovery of Hg using ion exchange processes, HCl recovery, gypsum recovery from wet scrubbers, and crystallisation.

R&D challenges for incineration-based EfW plant

As was stated in the MNM piece of 5^th February, the main R&D challenges to address for EfW systems in the future generally relate to improving the health and environmental aspects, reducing the costs of installation of EfW systems and improving the efficiency of waste conversion (i.e. the energy recovery stage).

For incineration specifically, R&D (and in some cases, demonstration) activities are focussing on:

• Reducing dioxin emissions by adding inhibitors to the waste feed or using hot gas de-dusting methods;
• Reducing the concentration of polyaromatic hydrocarbons (PAHs) in the flue gases by using oil scrubbing methods;
• Reducing CO₂ emissions in the flue gases by injecting sodium hydroxide (caustic soda) to react with the CO₂ to produce sodium carbonate;
• Using membrane separation techniques for advanced waste water treatment;
• Using flameless, pressurised oxy-combustion techniques in the incinerator chamber;
• Using steam spraying instead of air injection in the post-combustion chamber;
• Reheating of turbine steam to increase electricity production; and
• New, hybrid thermal treatment/ATT techniques (e.g. the ‘PECK’ process – a two-stage process that combines a gasification stage, followed by an incineration stage).