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.

Pyrolysis

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
entirely constant.

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

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.

Peter Jones, chairman of Waste2Tricity, one of these two companies, describes the ‘plasma arc’ gasification technology as: “a bolt of lightning that transmits the electricity between an anode and a cathode. The plasma arc takes the temperature over the 2,000-degree level, and then you blast those tars into oblivion. You’ve got as near pure hydrogen and carbon monoxide as possible.” Plasma arc torches can carry out the gasification themselves or the electric arc can be applied after conventional gasification, which requires less initial energy input.

In the long term, Waste2Tricity plans to move beyond producing syngas for combustion, instead creating hydrogen for fuel cells. The proposed technique involves a ‘water shift’: “When you get to carbon monoxide and hydrogen, you pass the gas through high-temperature steam. The oxygen in the steam combines with the CO and you wind up with CO2 and more free hydrogen. You then pass that mix through a molecular sieve in the form of a membrane separator that allows the hydrogen to pass through while the CO2 goes another way and what you’ve got is a pure stream of hydrogen to be used in fuel cells and a pure stream of CO2 to be used for industrial applications or as a refrigerant.” The hydrogen fuel cells could then be used to power energy-intensive installations like factories. According to Jones, this final phase, which could increase output of electricity by 60 per cent over an internal combustion engine system or 130 per cent over a steam system, is six months to a year away
from commercialisation.

Strengths

Both gasification and pyrolysis are modular technologies, meaning they can be built on a small scale – starting at about 10 kilotonnes per annum (ktpa) – but can be scaled up to process up to 150-225ktpa (compared with incinerators, which need to be larger – 100-600ktpa – to be financially viable). Consequently, they are more likely to serve local communities, meaning waste can be treated with minimal transportation in accordance with the proximity principle.

Also, despite the recent performance of the Isle of Wight plant, these technologies are reputed to have low dioxin and VOCs (volatile organic compounds) emissions (because of the high operating temperatures) and low NOx (nitrogen oxides) and SOx (sulphur oxides) emissions (because of the lack of oxygen). Pyrolysis and gasification also achieve better volume reduction than standard combustion, especially when higher temperatures are employed.

Weaknesses

Some unforeseeable (or at least unforeseen) consequences are always possible with new technologies, but a few drawbacks are already apparent. For one thing, gasification and pyrolysis require a prepared feedstock that cannot vary too much in terms of moisture content and calorific value. Previously, these technologies have been used on fossil fuels with consistent energy values and a variable waste feedstock would obviously impact negatively on performance. (Jones, for his part, predicts that we may see a day when “instead of operating on a disaggregation basis, which is what a lot of MRFs are doing, you will actually have a blending process to produce orders for fuel feedstocks based on calorific value, particle size and moisture”.)

The ideal feedstock for these processes must contain a large proportion of organic waste and plastics (Jones has cited an ideal mix of 35 per cent organics, 35 per cent paper and cardboard, 25 per cent plastic and five per cent other materials). This is, of course, a cause for concern for resource campaigners who are worried that new energy-from-waste installations could crowd out recycling just as efficiently as old school incinerators do, that limited resources will be destroyed through the processes.

Uncertainties

While proponents of these technologies say two of their main strengths lie in their ability to produce energy with reduced CO2 emissions, the actual energy efficiency of these plants remains unclear. For one thing, they’re not actually closed processes, as emissions result when the gas or oils are combusted. Moreover, the 2007 Waste Strategy estimates that both processes have conversion efficiencies of just 30 per cent for electricity only, increasing to 70 per cent with combined heat and power (CHP). CHP from these plants – as with incinerators – faces difficulties as it requires a local pipe network to transport the heat and so is only really viable near new housing/industrial developments.

Also, it’s still unclear what the overall energy balance of these plants will be when the energy required to sort and prepare the feedstock and to run the machines is taken into account. The Environment Agency assured us that all the processes were ‘energy positive’, but with so few plants operational in the UK, was unable to provide specific figures.

It’s also still difficult to say how much these plants will cost to build and operate. Bower claims pyrolysis beats out incineration for price as it “needs lower gate fees... and then gets electricity sales with the ROC [Renewable Obligation Certificate] payments on top of the back end”. This financial situation might not hold true for all advanced thermal treatment installations, though. As Adam Read of AEA explains: “You could be looking at 20 or 30 per cent extra, maybe more, to build these plants compared to an equivalent-sized incinerator. It’s difficult to tell, though, because there’s very few of them in operation, so we’re not seeing a lot of comparative pricing.”

Interest in these technologies hasn’t disappeared even in these difficult financial times. Bower insists “the market is about to open up”, and if that’s the case, if these plants are built and either stand up to or wither under scrutiny, the new government and the rest of us will soon be better placed to pass judgement. Let’s just hope our current indecision doesn’t mean we miss meeting our landfill diversion targets…