Energy-from-Waste technologies – Advanced Thermal Treatment: Pyrolysis and Gasification
Post AuthorPhilip Sharman
Every day the news on the TV or in the papers or on our PCs/tablets seems to be full of rubbish – and I’m not referring to ‘fake news’ or the trite diatribe that ‘modern society’ seems to regard as news! I’m talking about rubbish – waste materials, and particularly the current concerns about plastics waste overrunning our streets, countryside, water courses and oceans. Here in the UK, broadcaster and naturalist Sir David Attenborough’s series ‘Blue Planet 2’ seems to have pricked the conscience of the public, which, in turn, is increasing the pressure on politicians, local authorities, industry, researchers, etc. to come up with solutions – urgently. I agree.
Over the last two months, we have brought you a couple of (hopefully) thought-provoking pieces in our Monday Night Mail e-newsletter on one particular aspect of the waste problem that is directly relevant to IFRF Members, namely ‘energy-from-waste’ (EfW).
In the MNM of 5th February, I (Philip Sharman) contributed a piece titled ‘Waste-to-Energy – a significant growth area in the energy mix’, which reported on a recent report valuing the global EfW market in 2015 at $25 billion and increasing to around $45 billion by the middle of the next decade. I concluded that piece by commenting: “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…”.
In a follow-up MNM piece on 5th March titled ‘Energy-from-Waste technologies – thermal treatment: incineration’, we started to ‘unpack’ the EfW topic a little bit more from a technology and R&D perspective, by looking at incineration – the main ‘thermal treatment’ process currently deployed in EfW plants.
In this third piece in the series, we consider so-called ‘advanced thermal treatment’ (ATT) technologies for residual wastes – the principle ones being based on the gasification and/or pyrolysis. ATT technologies are designed to recover energy in the form of heat, electricity, fuels or chemical feedstock, and potentially can contribute significantly to the diversion of biodegradable waste from landfill.
A final piece next month will take a look at ‘biological treatment’ methods (i.e. anaerobic digestion).
Gasification and pyrolysis technologies for EfW
Certain ATT technologies, as distinct from incineration technologies, are at a relatively early stage of deployment for EfW plants handling residual wastes such a municipal solid waste (MSW), and hence generally have relatively limited track record (mainly in Asia, North America and some European countries). Several ATT processes are, however, well established, viable and ‘bankable’ for various waste streams (e.g. biomass, industrial wastes, tyres, etc.), and the expectation is that they will play an increasing role for other specific waste streams (e.g. plastics) and MSW – this anticipated expanded use is explicit in a number of recently published reports including the one published by US-based market research and consultancy company Grand View Research (GVR) in November and referred to above (see report here). Other ATT technology configurations are still at the pilot/early development stages. ATT plants are generally at a smaller scale than incinerators due, in part, to the technology maturity and the often specific waste streams that they are used to process (although, conversely, smaller-scale incineration facilities and larger-scale ATT facilities are not uncommon).
As is the case for incineration-based EfW applications, ATT-based EfW generally involves a number of discrete stages, although actual plant design and configuration of facilities differ considerably between technology providers.
Waste reception, handling and pre-treatment: Gasification and/or pyrolysis processes are focussed on treating the biodegradable fractions present in MSW (e.g. paper, card, putrescible waste, green waste, wood, etc.) as well as plastics. As a result, non-combustible materials and recyclables (e.g. metals, glass, etc.) are generally removed prior to the thermal treatment stage. In addition, depending on the ATT technology deployed, the waste feed may require processing to remove excess moisture and shredding to reduce size. As a result of the requirement for pre-treatment in ATT-based EfW plants, such units often form part of a wider residual waste management strategy, with ATT processes used in conjunction with other waste treatment technologies such as mechanical biological treatment (MBT) and mechanical heat treatment (MHT). Many MBT/MHT plants are configured to produce a ‘fuel stream’ (primarily paper, card and plastics) as one of the products from the process – commonly referred to as refuse-derived fuel (RDF) or solid recovered fuel (SRF): This may be more amenable to processing in an ATT plant than ‘raw’ MSW.
Thermal treatment reactor: Whether pyrolysis- or gasification-based (and often ATT technologies employ both – and indeed combustion-based reactors too – sequentially), the thermal treatment reactor produces a synthesis gas – ‘syngas’ – and solid residue. The composition of these products depends on the process conditions employed, including temperature, oxygen level, heating rate, residence time in the reactor, direction of gas flow, etc. The key types of ATT reactors available include: rotating kilns (pyrolysis, 300-850°C, up to large-size waste); heated tube reactors (pyrolysis, up to 800°C, up to large-size waste); surface contact reactors (pyrolysis, 600-850°C, small-size waste only); fluidised bed reactors (gasification, above 650°C) ; fixed bed reactors (gasification, above 650°C); plasma arc reactors (gasification, 1000-8000°C); and combination processes (pyrolysis-incineration, pyrolysis-gasification, gasification-combustion). IFRF readers can find detailed reviews of these different ATT reactor technologies on either the UK Defra website or the European IPPC Bureau website (see section 2.3.4).
Syngas and solid residue treatment plant: Whichever ATT technology is utilised, solids are inevitably produced during the reactor(s), including volatile metals (lead, tin, cadmium, mercury, etc.) and carbon. In gasification-based ATTs, relatively little carbon is produced, but in pyrolysis-based ATTs the level of carbon is significant. The larger particles of solids produced are usually discharged as bottom ash, char (from pyrolysis) and slag (from gasification), although in ATT technologies deploying higher temperature gasification regimes (e.g. plasma arc reactors), the solid residue may be a vitreous (glassy), inert slag with minimum leaching properties which may have recycling potential as an aggregate. The lighter fly ash is usually collected when the syngas is separated, using cyclones and filters. The volatile metals are carried in the syngas until such point as the gas cools sufficiently for them to condense out.
Pollution control in ATT-based plants is typically on a smaller scale than for incineration-based plants due to the smaller volume of process air required, however, compliance with appropriate legislation/regulations (e.g. European ATT plants are covered under the Industrial Emissions Directive – ‘IED’ – 2010/75/EU – when the syngas resulting from the treatment is subsequently combusted) is generally mandatory. The key requirements in the IED for the operation of an ATT facility (EfW or not) include: a minimum combustion temperature and residence time of the resulting combustion products (850°C and 2 seconds for MSW); specific emission limits for the release to atmosphere of SO2, NO, NO2, HCl, HF, gaseous and vaporous organics (expressed as TOC), CO, fly ash (dust), heavy metals, and dioxins and furans; and a requirement that the bottom ash and slag produced has a total organic content of less than 3%.
Energy recovery stage/syngas utilisation stage: Compared with the energy conversion/recovery stage for incineration-based EfW plants (see MNM of 5th March for details), ATT plant based on pyrolysis and/or gasification technologies offer a number of options due to the nature of the syngases produced.
For pyrolysis-based ATTs, the syngas is a mixture of combustible gases including CO, H2, CH4 and a broad range of VOCs, and typically has a net calorific value (NCV) of 10-20MJ/Nm3.
For gasification-based ATTs, the syngas is primarily CO, H2 and CH4, and typically has a NCV of 4-10MJ/Nm3.
In either case, the most common configuration in EfW applications is to combust the syngas in a boiler to generate steam, with this being used to generate power via standard turbo-generator equipment, or for heat supply to industrial/commercial/residential users, or, increasingly (as in the case of incineration-based EfW plants), for both i.e. in combined heat and power (CHP) mode – the most efficient option for EfW via a steam boiler. An ATT EfW plant producing exclusively electricity will have a net electrical efficiency (i.e. taking account of the parasitic load of the plant) in the range 10-20% (versus the commonly cited 14-27% for an incinerator-based EfW producing exclusively electricity). As for an incinerator 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 ATT plant and will vary seasonally and during a day.
However pyrolysis and gasification syngases can also be combusted in gas engines or combined cycle gas turbines, or co-fired in existing power plant, with net cycle efficiencies of 13-28%, ~30% and up to 27% respectively (dependant on the NCV), presenting a wide range of options not open to incinerator-based EfW plants.
One key issue associated with the use of syngas in energy recovery at ATT facilities is the problem of tarring: The deposition of tars can cause blockages and other operational challenges, and has been associated with plant failures and inefficiencies at a number of pilot- and commercial-scale facilities. Tarring issues can be overcome by utilising higher temperature secondary processing to ‘crack’ the tars and clean up the syngas prior to the energy recovery stage (referred to as ‘syngas clean-up’ or ‘polishing’).
In addition to, or as an alternative to, combusting the syngas from an ATT process to generate heat and/or power, a proportion of the constituent gases can be condensed by cooling the syngas, to produce chemical feedstock (i.e. oils, waxes and tars) or potentially used as liquid fuel. This offers further options for utilising the syngas, but would generally require the ATT plant to be local to the end user in order to offer a practical solution. It would also require high levels of gas cleanliness; pollutants (notably sulphur and halogens) may need to be removed prior to combustion of the syngas. Furthermore, both pyrolysis liquids and pyrolysis/gasification syngases can suffer from a wide variability in composition, limiting their use in ‘high-end’ markets.
Syngas from waste streams is also a potential source of hydrogen, which could have applications in both stationary power generation and as a transport fuel. Predicted high energy recovery efficiency benefits, coupled with as CO2 reduction benefits associated with hydrogen from waste, make this an attractive proposition compared with hydrogen sourced from steam methane reforming of natural gas or water electrolysis. Significant purification and reforming would be required to meet the appropriate hydrogen quality standards. With this potential in mind, considerable research is underway in Europe, North America and Asia to further these opportunities.
Emissions clean-up and waste water treatment: The clean-up of other emissions (e.g. fly ash and bottom ash), together with waste water treatment and control, are generally the same as for incineration-based EfW plants (see MNM piece of 5th March for details).
R&D challenges for ATT-based EfW plant
As was stated in the MNM piece of 5th 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 pyrolysis- and/or gasification-based ATT EfW specifically, R&D (and in some cases, demonstration) activities are focussing on:
- More full-scale operational plants to demonstrate the uses of the ATT technologies in order to increase confidence.
- Extracting value products from the pyrolysis/gasification syngas using membrane separation technologies.
- ‘Real’ gasification versus two-stage combustion.
- Developing modular plants; sizing of plants will be challenging in the future.
- Better technology transfer and secondary research on the application of ATTs in order to identify best practice in plant optimisation.
- Adaptability of ATT plants for varying calorific value of inputs – including plant design software.
- Carbon ‘foot-printing’ of different waste treatment options for EfW plants.
- Developing more rapid sampling tests and techniques to determine waste composition, and detailed waste characterisation including chemical analyses, calorific value and biodegradability.
- Development of innovative techniques and technologies to enable integrated treatment of MSW and commercial & industrial wastes (e.g. the ability to handle the heterogeneity associated with multiple waste streams).
Two interesting ATT developments (one gasification-based, the other pyrolysis-based) that I have come across recently that hold great promise for the future of EfW plants are:
Stopford Energy and Environment Ltd have developed a novel, modular, high-temperature ATT EfW technology – ‘Plasmergy’ microwave-induced plasma (MIP) gasification – which can be coupled with a gas engine, gas turbine or fuel cell to generate electricity and heat. Through investment of over £2m to date, the Plasmergy technology has now developed to pilot-scale, with a 50 tonnes (dry waste) per year facility operational.
With the current widespread interest in addressing the problem of plastic wastes that are difficult to recycle, Recycling Technologies Ltd has developed a modular ATT technology – the ‘RT7000’ – based on relatively mild pyrolysis conditions that can chemically recycle heterogeneous plastic waste back to an oil (named ‘Plaxx’). Plaxx can be used as a feedstock for making new, virgin-quality polymers, industrial waxes and as marine and process fuels. The RT7000 – capable of processing 7000t of dry waste into 5250t of Plaxx – will be mass produced for distribution internationally, allowing capacity to be added into the plastics recycling system very quickly and profitably.