Despite its complicated history, the Avonmouth pyrolysis plant in the UK has been brought back into operation and is approaching full operation capacity. Here the EPC contractor provides an account of the story and the technology process being used.
By Dr Robert Eden
Clinical and hazardous waste management has come under increasingly structured legislation, driven in large part from Europe. Currently, all clinical waste handling and disposal procedures must comply with the following regulations:The Environmental Protection Act 1990 (including the Duty of Care Regulations) The Controlled Waste Regulations 2012 The Hazardous Waste Directive 2011 The Carriage of Dangerous Goods Regulations.
However, the main legislation governing clinical waste disposal remains the Environmental Protection Act 1990. This stipulates that all producers of waste have a Duty of Care to ensure the correct and proper management of waste is performed and states that it is unlawful to deposit, recover or dispose of controlled clinical waste without a waste management licence, or in a way that causes pollution of the environment or harm to human health.
The key principles of clinical waste regulations relate to the correct segregation, storage, disposal and documentation of waste. The Safe Management of Healthcare Waste Memorandum (HTM 07-01) issued by the Department of Health provides guidance on the secure and legally compliant management of clinical waste.
This recommends that the segregation of clinical waste occurs at the point of production using colour coded waste receptacles and outlines a best practice waste segregation colour coding scheme for producers of waste to follow. This separation ensures clinical waste legislation is adhered to and waste is stored, transported and disposed on in the correct manner.
One method for treating medical waste is pyrolysis – the thermal degradation of a substance in the absence of any oxidising agent (other than that which forms part of the substance itself) to produce a char and one or both of gas or liquid. The technology in one particular plant in Avonmouth, a port and suburb of Bristol in England, UK is worth looking at in closer detail but first it's important to understand its history.
A prototype pyrolysis technology from Compact Power started out on a trial at the Finham Sewage Works near Coventry in central England in 1991, running on various feedstocks including sewage sludge and municipal waste.
The technology employed in the original Compact Power facility drew inspiration from the gas production technology dating from the mid to late 18th century. to produce gas, known at that time as "inflammable air".
The first use of public lighting in industrial England was by means of the pyrolysis of coal, the gas being reticulated through the streets and combusted in lampposts. At its inception, it was considered to be a true marvel, lighting factories, streets and homes which beforehand had been lit by glass oil-lamps. In 1813 the Chartered Gas-light and Coke Company was formed to commercialise the use of coal-gas produced by pyrolysers.
In the original systems, coal was driven through air tight tubes held at temperature by burning coal or oil to produce the syngas. The driving force was obtained either from the use of gravity, with vertical chambers, or by means of rods being pushed through by operators.
The late-twentieth century version of the coal-gas technology employed the same basic principles but was equipped with automation, so that operators pushing feedstock through the pyro-tubes was no longer a necessity.Avonmouth
Following the success in wastewater treatment, Nic Cooper, CEO of Compact Power went onto fund the construction of a facility in Avonmouth. Compact Power was then listed on the AIM stock market in 2001, raising the funds for the ongoing requirements of the business.
The technology that resulted involved a series of unit processes combined into the Compact Power clinical and hazardous waste process. All waste consignments were barcoded. Incoming waste was placed into a hopper, via a tippler system, feeding a compactor/shredder. Compacted waste ensured that air was not drawn into the pyrolyser tubes through the feed port and the plant was kept under negative pressure so that all gases were drawn through the plant and processed under appropriate conditions.
From the compactor, the feed was passed to a tube and screw pyrolyser. The outside of the tube was retained at a temperature of over 800 deg C (~1,500 deg F). The waste passing through the pyrolyser was effectively carbonised, with volatiles being driven off, leaving a carbon char.
Carbon char left the end of the pyrolyser tube entered into a gasification chamber, where it was reduced to ash by means of the water-gas reaction. The resulting syngas from gasification was combined with the pyrogas and passed directly into a thermal oxidiser, where it was combusted. As the gases were retained at high temperature, there was no issue with the condensation of tars, a perennial problem for gasifiers feeding gas engines and turbines. The thermal oxidiser was retained at a temperature of up to 1,250 deg C (~ 2,280 deg F), with a retention time of two seconds to meet prevailing technical standards.
The exhaust from the thermal oxidiser was then passed through a boiler to raise steam. The use of a boiler opened the door to conventional steam technologies. In the case of Compact Power, the first intention was to produce power, perhaps harking back to the objectives formed when looking at a clean-coal application. However, commercial opportunity took precedence and the steam was used in an autoclave-sterilisation facility for non-combustible clinical and hazardous wastes.
Emissions were managed by means of the Dry Solvay Process, using sodium bicarbonate to remove sulphur and chlorides to exceeding low levels. The pyrolysis process was inherently a low emission thermal process. The absence of combustion during the thermal degradation of waste streams meant that the by-products of combustion, the most notorious of which are dioxins and furans, did not have the opportunity to form.
In 2003 the UK Environment Agency, in a section entitled "Cleaner Air for Everyone", had described in its annual report that: "Monitoring at Avonmouth has shown that the plant easily meets the tough requirements of the new Waste Incineration Directive which will apply to existing plants from the end of 2005."
In January 2008, a year that marked the deepest recesses of the global financial crisis, Compact Power announced that it had gone into administration. Four days later and the business was acquired by Ethos Recycling, previously known as Sweeney Environmental. At that time Compact Power was planning the construction of a larger pyrolysis facility in Avonmouth. The project had been granted £5 million from the UK Department of Environment, Food and Rural Affairs (DEFRA) as a demonstration project for new technologies.
During the course of 2014, CliniPower secured ownership of the former Compact Power facility. It had been abandoned and vandalised with the removal of anything of value, including all copper cables and electronic control systems.
CliniPower retained engineering company Organics Ltd of Coventry as the EPC (engineering, procurement, construction) Contractor to help refurbish the facility. Through the summer and autumn of 2014 CliniPower took on the challenge of bringing the plant back into operation.
Backed by the original CEO Nic Cooper, many of the original Compact Power team have come together and have restored the
Clinipower facility to commercial operation. Waste contracts are increasing and the plant is approaching full operational capacity. At present heat from the refurbished facility is being dumped through a series of air blast coolers, but it is planned to move forwards with both a steriliser and a steam turbine generating renewable energy under the UK Government supported Feed in Tariff arrangements.
The steriliser will be a thermal screw arrangement, currently under design and development by Organics. The commercial operation of the existing unit at Avonmouth will be consolidated by the team.
Despite the passage of time and complicated history of the site, there remains a great deal of global interest in the use of pyrolysis for hazardous and clinical waste disposal.
It is, as the original company name intended to imply, compact, requiring less space than other thermal technologies. The footprint of the thermal treatment building is 20m x 30m, the height of the building is approximately 13m with the stack being 2 metres above this at 15m.
Despite some turbulent times and having to overcome great technical challenges, the Avonmouth pyrolysis plant is once again nearing full operational capacity. It goes to show what can be done with true grit and determination.
Dr Robert Eden is the managing director of Organics PLC.More Waste Management World Articles
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