Autoclave Technology for F1 and aerospace production further developed for waste disposal
The aerospace and automotive business continues under separate ownership, but AeroThermal’s main focus is now the treatment of waste. Mr Toll says: ‘‘There are people who have tried to apply autoclave technology to waste treatment in the past without any great success. We are coming at it from a slightly different engineering perspective and believe that we have overcome the problems. Furthermore, we have taken that technology and married it with a range of complementary technologies from the same industry; and because we can include or leave these out depending on the customer’s specific needs, what we now have is a complete waste-to-energy solution.’’
The autoclave acts like a giant pressure cooker, using steam to sterilise and break down the waste to give a thick ‘soup’ that is rich in hydrolised organic matter and from which it is easy to separate recyclable materials such as steel, aluminium and plastics. This ‘soup’ can also be the starting point for a number of additional recycling and reclamation processes, including reclaiming the cellulose fibres in the mixture (mostly derived from ‘organics’, primarily food and paper waste), which then can be used in the production of materials such as cardboard. These fibres, know as floc, can also be composted, or burnt as a bio-mass for generating heat and power.
Autoclaving can handle unsorted municipal waste, removing the need for separate collections for recyclables, as well as waste from supermarkets – including food waste. The process also accommodates clinical waste and animal by-products, plus it allows for the easy separation of clean, sterile recyclables.
FURTHER BENEFITS
However, Mr Toll says there is an added benefit to putting waste through an autoclave, as the resulting ‘soup’ can be subjected to a second process called anaerobic digestion. This breaks down the organic matter to produce methane gas, which cab be used to drive a generator to produce ‘green’ electricity.
Because this is electricity produced from a renewable source, it attracts extra allowances under the Renewable Obligation Credits (ROCs) scheme, making it worth around 15p per kWhr – and most of this electricity can be sold to the National Grid. The process of generating electricity also generates waste heat, which is used to produce the steam for the autoclaves. In addition, surplus heat can be used for schemes such as community swimming pools and local heating, or in other recycling processes, such as drying floc briquettes.
‘’We call it the perfect circle,’’ says Mr Toll. ‘’First of all, you are starting off with a material that nobody wants – municipal standard waste. You put that through the autoclave, which doesn’t destroy anything of value, just sterilises and cleans up the metals and plastics, which can be recycled. Anything with a high organic fraction and a high water content goes into the ‘soup’, which contains a tremendous amount of volatile solids.
‘’Once you have removed the metals and plastics, you can either remove the cellulose floc, or leave it in the mixture that goes into the anaerobic digester. The bio-gas that comes off the digester is then used to generate electricity. The generator is only about 35% efficient, and the rest of the energy is released as heat. We then use this exhaust heat to generate steam for the autoclave. So the whole process is self-sustaining in terms of the energy it requires; it also brings in cash for the recycled materials and the electricity, while the sludge we get out of the digester can be burnt as a bio-mass, put into a gasifier to produce ‘syngas’, composted or even into a building material. There are lots of options’’.
Processing the organic materials in the autoclave results in them breaking down much more quickly in the anaerobic digester; the lignin (a complex chemical compound) in the organic matter starts to break down, so more gas is produced more quickly. The gas yield can be double that form non-autoclave waste; furthermore, the peak gas flow rate is produced in four days rather than four weeks.
TIMELY DEVELOPMENT
Mr Toll says that the current political, economic, social and technological landscape makes this approach particularly attractive. The EU landfill directive calls for the amount of organic waste sent to be halved by 2013, and this requirement is backed up by an escalating tax regime. EU Landfill Tax is rising at a rate of £8 per tonne per year (it is currently at £28 per tonne) and is expected to reach £70 per tonne within 5 years. Including tax, the cost of disposing of waste to landfill is currently around £50 a tonne.
The social climate is also in favour of sustainable waste solutions; there is a general desire to show more concern for the environment, but at the same time, people do not like the idea of being fined for putting out to much rubbish or mixing up recyclable products. AeroThermal’s system not only removes the need to separate out different types of waste; it can also offer local authorities the chance to profit form their waste, rather then paying to get rid if it.
The system (a pilot plant is currently in use at the group’s Poole facilities) uses an inclined bottle-shaped rotating-drum autoclave that has an internal Archimedes screw welded to the vessel. This is rotated in one direction during loading to facilitate the loading of the autoclave, and rotated in the other direction during operation to break up the waste and ensure that the load is evenly processed.
Once the vessel is fully loaded, all the air is extracted to create a vacuum. This vacuum bursts open any packaging or unopened containers and also helps to ensure that, when the steam is let into the vessel, it completely penetrates the load. When the chamber has reached its optimal operating conditions (160°C and several atmospheres of pressure), the mixture is allowed to cook for about 40 min.
AeroThermal has standardised on three sizes of vessel (all scaled from the same basic design), which are supplied in pairs to allow the steam to be recycled from one vessel to the other to save energy. The smallest vessel has a seven-tonne capacity and is primarily aimed at processing food waste, while the 15-tonne and 30-tonne vessels are suitable for local-authorities and large scale treatment of municipal standard waste.
A pair of the 30-tonne autoclaves can process around 600 tonnes a day (200,000 tonnes a year), which equates to the waste disposal needs of about 400,000 people. Based on recognised prices for the components of a standard tonne of waste from the borough of Tower Hamlets in London, this will produce over £5.5 million worth of recyclable material, including over £1.5 million worth of floc, and generate £10 million of gate fees for a commercial operator (or save the same amount for a local authority).
If the organic matter, including the cellulose floc, is instead processed in anaerobic digesters and used to produce electricity, this will generate an additional £3.2 million worth of ‘green’ electricity and cover all the heat and energy needs of a plant.