|Significant demand exists for sound methods to extract value from waste that would otherwise be sent to landfill Credit: Recycling Technologies|
A system is available that generates combustible gas from material that might otherwise go into landfill. Among other advantages, it can feed a CHP plant, says Arthur op den Brouw.
A plant developed by Recycling Technologies treats mixed plastic waste (MPW) and provides all the usual advantages of CHP. It is also small enough to be located in or alongside existing facilities, avoiding the need for specialist buildings. Furthermore, it can sit at a site where the waste is already located, overcoming the need for transport, and adjacent to processes that require heat, ensuring high efficiencies. Its low-capital cost also brings benefits concerning return on investment (ROI).
Considerable demand exists for affordable technology that can use environmentally-sound methods to extract value from waste that would otherwise be sent to landfill.
In the UK, for example, what drives this need are the fast-rising costs of landfill and the ever-increasing political pressure to reduce the amount of waste sent there. In the case of MPW the technology already exists to liberate its energy through combustion.
Energy from Waste (EfW) plants work well, particularly if preceded by a sorting plant that can remove most of the chlorine-producing PVC. They are better than the landfill solution but also have downsides. They tend to be large facilities that require material to be transported to them, which causes the old problem of creating a significant quantity of waste heat in a central location, heat which is difficult to use efficiently. The plants are also expensive to build, which means payback periods are long, and there are losses in distributing the power back from a central location to the point of use. Consequently the number of such facilities in comparison with the quantity of MPW created is wholly inadequate to deal with the volume of waste, at least in the European Union (EU).
But plastic recycling has come on a long way. Most consumers are in the habit of separating some plastics out of general waste for recycling. Yet the fact remains that the majority of plastic used every year, in Europe at least, ends up as landfill. Drinks bottles and milk bottles are made of PET and HDPE, respectively, which are widely recycled. Near-infrared scanners can detect these materials and remove them from a conveyor containing MPW. Once separated they are washed and flaked mechanically, ready for re-use.
This mechanical process does itself consume a significant amount of fossil fuel but is nonetheless a common process and the recycled material is widely used and traded. However, the material left on the conveyor is usually bailed and either used in refuse derived fuel (RDF) or disposed of.
The EU creates 47 million tonnes (Mt) per year of plastic each year, but only 6.3 Mt is recycled and 8.6 Mt used in EfW plants, according to Plastic Europe.
EfW plants that consume RDF across the EU do not have anywhere near the capacity that would be needed to process all the MPW, and they are not uniformly distributed either. The UK and many other countries have fallen behind the likes of Germany and The Netherlands, which have spent considerable amounts on such facilities.
In Britain companies and local authorities can end up paying around £45/tonne (US$69/tonne) to companies that will accept the RDF. To sort, dry and bale the material to RDF standards can cost as much as £20/tonne, so the total cost of disposing this material via the EfW route is £65/tonne.
This shipping of energy across borders may make sense in the short term when compared with the cost of landfilling in a country without sufficient EfW capacity. But in the long term the economic impact of such energy flows and the environmental impact of shipping make the practice unsustainable.
To change this behaviour a commercially attractive technology is required that challenges the categorisation of such material as waste. Instead of viewing, it needs to be seen as a valuable resource.
British company Warwick Ventures at the UK’s University of Warwick has commercialised such a technology. In 2010, a team at Warwick University, under the direction of Professor Jonathan Seville, was looking at ways of de-polymerising plastic via pyrolysis. Recycling Technologies was subsequently establised.
An increasing number of engineers and specialist suppliers have been developing the solution for, which aims to provide an environmentally sound and economically attractive system for turning this material into resource at source.
MPW’s true value
Known as WarwickFBR, a typical installation will use 1 tonne/hour of MPW to run a 3 MWe generator. The gap between the material created and that currently recycled or used in an EfW plant is around 32 Mt per year, according to Plastics Europe. If we hypothetically consider all this MPW being used in a fleet of Recycling Technologies’ systems, they could generate up to 98 TWh per year.
To put this in context, the UK’s power demand in 2010 was 310 TWh, according to DUKES 2012, so the plastic wasted across the EU last year could have supplied 32% of the UK electricity need. Of course, no one approach will be used universally to handle all the material in any waste stream, but this comparison does demonstrate how much energy is contained in the plastic manufactured and used each year in the EU alone.
Companies that have access to MPW could start viewing it as a valuable resource. Recognising the energy content of what is currently labelled waste is only one part of the picture; there have to be ways of turning it into something that is wanted at a cost that is attractive.
It is worth contemplating why countries without the capacity to use this fuel do not follow the lead of Germany and The Netherlands and build appropriate mass burning facilities. In our view the answer is a combination of cost and the use of the waste heat. Such facilities are very expensive and usually require government assistance to finance their construction. They have often been built to achieve landfill diversion targets rather than because of the ROI they achieve.
Achieving this level of government commitment to landfill diversion is difficult, particularly in the current economic climate. Even more difficult to emulate is the use of the waste heat in district heating schemes. Getting planning permission to build a power station at the heart of thousands of houses and pipe the heat out to them is far from straightforward. In some countries there is a history of constructing houses with district heating systems, but elsewhere, unless such infrastructure already exists, retrofitting is almost impossible to justify financially.
The WarwickFBR is a cost effective alternative to centralised mass burning facilities without compromising on the efficiencies achieved via district heating schemes. A typical installation costs £3–4 million and can provide an ROI in less than two years. This is based on the financial benefits of avoiding landfill costs, the value of electricity generated and the value of the heat that can be usefully captured. This level of investment falls well within the reach of many organizations and avoids the need for governments to finance the mass burning alternatives.
WarwickFBR plants are not only more cost effective per megawatt of installed capacity but, given the ability to position them alongside existing industrial or recycling processes that require the heat, the efficiency of the conversion from chemical to usable energy is as good as a mass-burning facility replete with district heating facilities. The result therefore is that MPW need no longer be regarded as waste but as a valuable resource.
Waste to fuel
WarwickFBR is designed to be positioned at the end of a plastics sorting line that already removes material such as PET and HDPE. It accepts the plastic that is not going to be economically recycled, using it instead to provide the electricity and heat needed to recycle the plastics that will be. This avoids the use of virgin fossil fuel in the recycling process, as well as the transport of plastic waste to landfill or as RDF to mass-burning facilities.
MPW is shredded, the tramp metal removed and the waste is then dried and stored in a bunker. This prepared material is then fed constantly into a pyrolysis reactor where, in the absence of oxygen, the long hydrocarbon chains that form the polymers are chemically cracked into compounds of shorter chain lengths, which exit the chamber as a hot gas.
The issue with using waste material as a fuel is, of course, that its constituents can vary dramatically, so the quality of the fuel produced also tends to be variable. To combat this problem the system uses a reactor management module to continually adjust the conditions in the reactor to even out such variations. By constantly monitoring the gas being produced the operating parameters can be adjusted to enhance the fuel properties.
Since the machine is designed to accept general plastic waste, chlorine will be present, having come from PVC, as will fluorine from PTFE and many other compounds in varying quantities that have to be dealt with appropriately. The bank of filtration and catalytic devices following the pyrolysis chamber ensures that this hot gas is fully filtered so that the resulting fuel can be combusted reliably and without harmful emissions. This hot, filtered gas is then condensed into a tank ready to be pumped into a combustion device that best suits the host facility.
The combustion device could be a steam plant if a lot of steam is used in the facility, a combination of a steam plant and steam turbine to produce electricity, a medium-speed diesel engine or a gas turbine generator. Clearly the latter needs great capital cost when compared with a diesel engine but, as is true for most CHP plants, the greatly increased service intervals can justify this if the generator is required to run constantly. In most installations it is envisaged that a diesel generator will be the optimal combustion device.
The relative merits of different power generation approaches raises an important facet of the system’s design. To maximise efficiency the pyrolysis process is designed to run 24/7, but since the fuel generated can be stored in an appropriate tank, its use in power generation can effectively be decoupled from the pyrolysis process and the production of the fuel. So the hours that the generator runs can be varied to maximise the value of the electricity and the quantity of heat recovered.
In other words, if this CHP system is installed in a factory that only runs during the day, either a large engine or multiple engines could be run during the day to consume the fuel when the electricity and heat is actually needed rather than simply to keep up with the pyrolysis process. For example a 6 MWe generator could be run for 12 hours per day rather than having a 3 MWe unit running constantly.
This ability to generate power on demand gives rise to the exciting prospect of using power generated from waste to balance other renewables.
Wind turbines produce electricity when the wind blows. Solar systems do so when the sun shines. Neither always does when the power is really needed. The ability to turn waste into a fuel that can be used to supply power on demand will enable some smoothing of this variability.
An interesting point about the fuel production is its effective independence from the use of the fuel to generate power. This means the two processes can be physically remote from each other. The energy density of MPW is significantly increased by turning what is often ‘fluffy’ material into a semi-solid fuel that can be transported more economically to a site where the heat from the engine can be used most efficiently.
A standard 1 tonne/hour installation will consume around 7000 tonnes in moisture-free weight of MPW per year. The energy content of different waste streams will vary depending on the plastics it contains, but frequently it exceeds 33,000 kJ/kg. The rate of energy conversion – chemical to electrical – of the WarwickFBR is around 34%, so a CHP plant of this size will produce around 21 GWh of electricity per year and a similar quantity of heat from the diesel engine.
The heat produced exists in two forms: hot air recovered from the exhaust and water at around 90oC from the cooling circuit. Clearly the quantity of heat that can be used will depend on the host facility but, in facilities where plastics are mechanically recycled, considerable volumes of hot water are needed for washing the material. This type of demand is an ideal way to use the heat produced by the engine, ensuring high overall efficiency.
The plant is not too large either. Space required by the shredders, separators and silos will vary from site to site depending on the nature and condition of the feedstock, but the pyrolysis plant itself has the footprint of a 12-metre shipping container, as does the typical engine and generator set. The fuel tanks are built into 6-metre shipping containers.
Looking to the future
Recycling Technologies aim is to continue R&D into pyrolysis as a mechanism for turning waste into fuels in a commercially attractive way, so the company has established relationships with Birmingham University’s Dr. Gary Leeke, an expert in fluidised bed reactors, and Dr. Athanasios Tsolakis, a specialist in combustion and internal combustion engines.
A well-known company has also entered into an agreement to purchase the first system, and investors have also recognised the potential of the approach. Initial funding has been raised from the Wroxall Investor Group, a syndicate of high net worth investors. This cash injection allowed the company to start recruiting the team that is needed to turn the system into reality by 2014.
In 2012, investor Peter Jones OBE joined the board as an advisor, providing a significant insight into how Recycling Technologies could contribute to the political and economic drive to turn the waste industry into an resources industry.
Arthur op den Brouw is Marketing Manager at Recycle Technologies. https://recyclingtechnologies.co.uk