The Technology

SHREWS Ltd promotes Energy from Waste projects based solely on Advanced Thermal Technologies (ATT). These technologies are different from the mass burn incineration in that they rely on pyrolysis and gasification.

The technology basically converts the combustible materials into a gas as the first stage and then oxidises the gas. By breaking the oxidation into carefully controlled stages rather than trying to burn everything in a grate, projects can operate on a smaller scale and more easily achieve the very high emission standards required under EU regulations. To put the emissions into context, the air coming out of an ATT is often cleaner than the air going in.

The advantages of ATT are set out below.

OPERATION AND CONSTRUCTION

Note	Traditional Incineration	ATT
Waste types accepted	Large or bulky items may need to be excluded. Items containing wire, metals or inert materials may cause problems with waste loading and ash removal systems	All materials can be processed but normally inert materials and useful recyclates are selected out from the waste stream before the reside is fed to the gasifier
Process complexity	Shredders / pelletizers, moving grates / hearths and sophisticated de-ashing systems and controls mean that equipment failure is commonplace, and often expensive to rectify.	Virtually no moving parts, except for slowly rotating gasifier. Other parts, such as pumps and fans easily replaceable. Controls are simple and, as a result, equipment failure is rare.
Build / Installation	Long-term build and installation periods due to system complexity. 2 years + is commonplace for the development of this technology	Modular design and simple, robust construction provides for speedy build and installation. 12-18 months is the average development period for larger installations
Repair and Maintenance	Generally requires an in-house engineering team to provide constant support for maintenance	One, on-site general engineer is required, due to design simplicity.
Downtime	Total plant shut-down required for maintenance and repairs	The modular nature of the plant means that only parts of the plant may need to be shut down for maintenance and repairs

ENVIRONMENTAL

Note	Traditional Incineration	Planet BOS Gasification
General Compliance with anti-pollution legislation	Relies heavily upon the down-stream emissions abatement systems, due to problems with incomplete combustion	Able to comply with USA and Canadian emissions standards without sophisticated emissions abatement systems. With abatement systems European standards (strictest worldwide) easily achieved due to the inherent quality and completeness of combustion.
Control of particulate matter	Highly turbulent combustion process produces large amounts of fly-ash (airborne particulate) which requires clean-up via a large-scale particulate arrestor.	The segregation in the gasifier of the waste into gas (the volatile hydrocarbons) and residual carbon and inert material precludes the creation of any fly ash. The carbon and inert materials are passed through a char burner which again precludes the creation of fly ash.
Control of volatile materials	In high temperature primary combustion, volatile materials gasify immediately, and these gases may be too intense to be dealt with efficiently by proper combustion, causing problems with emissions	In the Stein gasifier the gasification is closely controlled so the gas is released from the waste at the required rate and the long chain hydrocarbons are broken down into short chain hydrocarbons. Even tyres and plastics can be processed efficiently, generating a controlled release of gases to match the demands of the gas engines or turbines.
Acid Gases	These are produced in all thermal treatment systems, where the waste contains Sulphur or Chlorine. Acid gases are easily neutralised with traditional, wet or dry scrubbers, using lime or sodium bicarbonate as the reactant.	These are produced in all thermal treatment systems, where the waste contains Sulphur or Chlorine. Acid gases are easily removed through the water bath which acts to clean the gases and cool them. Exhaust gases from the engines and the char burner are cleaned with traditional, wet or dry scrubbers, using lime or sodium bicarbonate as the reactant.
NOx	Inability to accurately control combustion temperatures may result in periods of excessive temperature, leading to the formation of Nox. In such cases, additional scrubbing equipment will need to be incorporated, adding to cost and complexity of the system.	The process allows for the accurate maintenance of controlled combustion temperatures, resulting in minimal Nox production.
Dioxins and Furans	Accelerated production of dioxins and furans can take place, where systems are unable to properly control temperature, during and post combustion. Incomplete combustion, and the production of fly-ash also add to this problem, making traditional incinerators more reliant upon the use of scrubbers (active carbon) and filters to minimise the effect.	The process precludes the formation of dioxins and furans.
Ash quality	Excessive carbon remains in the ash, due to incomplete combustion. Many, established incinerators will struggle to meet the high ash standards required under European anti-pollution legislation, as a result.	The residue is a vitreous slag which is carbon free. Not only does this easily surpass the European standards, but it is also capable of being recycled for use in road surfacing or building materials.
Light Alloys and metals	In high temperature incineration, light metals (e.g. aluminium) are likely to oxidise and break down, resulting in additional problems with ash quality and emissions.	Light metals and alloys would normally be recovered from the waste for recycling before processing but any metals which are passed through the plant and captured in the residual slag.

OTHER ISSUES

Note	Traditional Incineration	Planet BOS Gasification
Capital and operating costs	Significantly higher than ATT	Simple design and robust engineering means lower costs.
Scale of operations	Normally, due to high production costs, plant of 300,000 tonnes per annum capacity or above, may be justified	Modular units from 10,000 – 60,000 tonnes per annum are achievable and cost effective. It is widely recognised, throughout Europe, that smaller, localised plants, located around a region, are more beneficial than operating major plant, which will require waste to be collected and transported over large distances. This ‘Proximity Principle’ is a major focus for European waste producers, as it minimises the environmental impact of collection and transport.
Planning Permission	Because of the large scale of plant and the very large number of lorry movements associated with it, planning permission can be difficult to obtain because of public antagonism	Small scale of plant means minimal visual impact and smaller number of lorry movements making it more acceptable to the public
Integration with Waste Strategy	Large scale regional solutions no longer fit with small scale initiatives to encourage re-use, re-cycling and composting at local level. Large plants depend, over their lifetime, on large volumes of a homogeneous waste and cannot be scaled back retrospectively if recycling initiatives reduce the volume of residual waste available. These plants are simply not flexible enough for modern day requirements	Small scale solutions fit well with an integrated waste strategy that emphasises local participation and increases public buy-in. The modular nature of small scale plants that are not dependent on a single waste stream but can take a variety of industrial, commercial and municipal waste are capable of re-sourcing waste if one waste stream reduces, or can scale down by moving a module elsewhere. Equally, plants can scale up on the same site if necessary by adding another module.

Concept | EU Legislation | Project Examples | Future Projects | Technology