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Energy From Waste
We produce too much waste and consume too much energy. Gasification and pyrolysis won’t change this, but can they help us deal with both issues? In the final instalment of our renewable energy series, Libby Peake gets to grips with advanced thermal treatments
The new government has vaguely promised to ‘work towards a zero waste economy’, aims ‘to promote a huge increase in energy from waste through anaerobic digestion’, and unequivocally supports ‘paying people to recycle’. Apart from these three things, however, little is known about the coalition’s stance on all things waste and this is certainly true in relation to the energy-from-waste technologies gasification and pyrolysis.
Under the last administration, a New Technologies Demonstrator Programme (NTDP) provided £30 million to new waste treatment technology projects to overcome ‘the real and perceived risks of introducing alternative technologies in England’. Before the programme concluded, however, two projects – Novera Energy’s gasification plant and the Avonmouth Renewable Energy Plant trialling pyrolysis and gasification – withdrew from the programme. Consequently, the Energos gasifier on the Isle of Wight and the Scarborough Power pyrolyser were the only advanced thermal treatment projects to complete the programme and many would say the technologies remain unproven.
Our new leaders are surely not alone in their indecision about pyrolysis and gasification – technologies of exotic names and predominantly little-understood processes. It’s difficult to take a stance on something you don’t understand, so Resource decided to look into the processes to determine their strengths and weaknesses.
Despite its space-age name, pyrolysis is an ancient process and has long been used to produce charcoal and, like gasification, to produce town gas for lighting and cooking in the 1800s and – more recently – to recrack waste oils in the petrochemical industry. It is, however, new to the waste sector and has yet to be proven on a large scale.
Pyrolysis entails the heating of carbon-based material in the absence of oxygen; it can be performed at varying speeds and temperatures, resulting in varying combinations of syngas, pyrolysis oils and char (making describing the technology rather difficult).
The one example of pyrolysis of municipal waste in this country is the Scarborough Power plant that took part in the NTDP. Andrew Bower of GEM, the company that supplied the technology, explains the ‘flash pyrolysis’ process as it is used there: “We take 25,000 tonnes of black bag waste per year. We take out all the oversized and inert materials – recyclate, aluminium cans and whatnot – and then we dry it from about 30 per cent moisture content down to about five per cent – leaving 12,500-13,000 tonnes of ‘fuel’ for the converter.
“We process waste that comes to us to two millimetres in one dimension – you want to make it very small so the heat can penetrate the material and turn it into a gas very quickly. Also, it should be as free of oxygen as possible. You put the material into exceptional heat, about 800°C. It’s kept there for about 50 seconds and you should get instantaneous conversion from a solid into a gas containing methane, hydrogen and other compounds. The gas cools and is cleaned very quickly and taken straight into an engine to create power as a substitute for natural gas.”
This syngas does not have the exact same properties as natural gas, though, and requires a specially-designed engine. According to Bower, the syngas has a calorific value of about 22 megajoules/kilogramme (MJ/kg) – around half that of natural gas (over 40MJ/kg). The syngas’s calorific value depends on the feedstock’s energy content, though, and so is not
Bower claims the plant achieves conversion efficiencies of 87 per cent going from waste to gas. Of course, the engine then loses more of the energy – 50 to 60 per cent – as it converts the syngas into electricity, so the process’s overall efficiency is around 35-44 per cent. Incinerators, by comparison, typically achieve just 15 to 25 per cent efficiency – in part because the gases generate steam that drives turbines – an additional, energy-sapping step.
Not all pyrolysis processes end with this final stage of combustion in an engine – the technology comes in many forms. Pyrolysis temperatures vary from 300°C to over a thousand and residence times of waste in converters vary from a couple of seconds to several hours. According to AEA, high temperatures and fast heating (as in the GEM model) favour the formation of syngas, but low temperatures and fast heating more often result in pyrolysis oils, and low temperatures and long residency more likely produce char. To make matters more complicated, syngas itself can come in two varieties: formed at low temperatures, it consists mainly of methane and CO2, but as temperatures increase, more of the carbon is ‘cracked’ until hydrogen and carbon monoxide mixes are produced.
Pyrolysis oils are often cited as the most common output of the pyrolysis process. These oils, however, pose problems in that they are highly complex and variable, as well as corrosive, meaning they can’t be used in combustion engines, but only in the less efficient process of driving steam turbines. The other likely output from the process, char, on the other hand, is produced at the expense of energy creation, but, crucially, sequesters carbon in solid form (for more, see ‘Biomagic’ in Resource’s Mar-Apr edition).
Gasification technology is generally considered to be more proven than pyrolysis as far as its application to waste is concerned, but the UK’s only operational plant – the Energos facility on the Isle of Wight – closed at the end of May after failing emissions testing for dioxins (independent testing recorded a reading of 0.86 nanogrammes per cubic metre, nearly nine times the legal limit). Nonetheless, eight additional UK facilities have received planning permission and at least four more have applications in. All these will be new builds, though, unlike the Isle of Wight plant, where gasification technology was retrofitted on an older incinerator, and many parts (including the malfunctioning bag house filter, identified as the problem’s source) were resused.
Like pyrolysis, gasification can come in various forms with varying results but in essence involves the application of extreme heat to carbon-based material in the presence of limited oxygen. The chemical processes result in a syngas as well as an ash (up to five per cent by volume of the incoming waste) and a waste slag of inorganic material.
Energos’s gasifier on the Isle of Wight employs a two-stage process and ultimately combusts syngas to generate steam and/or heat. Prepared waste is fed into a chamber where it is gasified at 1,000°C to produce syngas and ash, the latter of which is removed continuously. The syngas moves on to a high-temperature oxidation chamber before going into a heat recovery steam generator to – as the name suggests – drive a steam turbine. When operational, the plant can process 30,000 tonnes of waste a year, which Energos claims results in 1.8 megawatts of electricity.
As with pyrolysis, though, not all gasification units function in the same way – notably, syngas from some processes can be burned directly in an engine rather than used less efficiently to produce steam in a boiler to drive a turbine. This can only be done if the gas is clean enough, though, as syngas produced at lower temperatures is ‘dirty’ – contaminated by tars, soot, chars and ash. At least two businesses in the UK are advocating plasma gasification to produce cleaner syngas.
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