Energy recovery within a waste strategy
May 16, 2018
May 16, 2018
This article first appeared in the May edition of Energy World magazine.
Waste-to-energy plants, or waste-fuelled energy recovery facilities, have come a long way since early incinerators, and now take their place firmly within the national resources and waste strategy, writes Viridor’s Lisa Templeton.
Modern energy recovery facilities (ERFs – often also referred to as energy from waste technologies) play a crucial role in offering sustainable resource and waste management solutions, working effectively alongside recycling services and enabling the UK to divert non-recyclable waste from landfill while generating vital base-load energy and heat.
The crucial factor in setting this modern technology apart from previous incarnations, which saw waste burnt under controlled conditions, is the efficient power plant component which is central to the energy recovery process. It is not enough to simply combust waste as part of a means of processing materials that cannot be recycled – the UK’s modern fleet of ERFs make a vital contribution to the UK’s renewable energy supply.
Government figures show that approximately two-thirds of ERF inputs, that is waste material utilised as fuel, are deemed to be biogenic, or short-cycle, carbon sources and, therefore, a strong indigenous renewable fuel supply.
Regulated by the Environment Agency, the Scottish Environment Protection Agency and National Resource Wales, and part of a process which sees even its output residues – incinerator bottom ash and air pollution control residues – given a new purpose and recycled for use in construction, these sophisticated waste-fuelled power plants are an efficient means of putting societies’ waste to work. They do this as the important fourth step of the Waste Hierarchy – reduce, reuse, recycle and recover.
The sector, however, still occasionally suffers from a lingering and decades-old misunderstanding of the combustion or incineration component of this process, with its sophisticated engineering and control systems and its power and heat generation capacity downplayed or, in some cases, completely excluded from the conversation. In addition, energy recovery is often characterised as being somehow ‘in competition’ with recycling.
To enable better understanding of the technology, Viridor, with one of the UK’s largest networks of 300+ recycling and waste treatment facilities, including 20 large scale recycling plants and 12 energy recovery facilities (ERFs), including four either in commission or under construction, has invested in education centres at its ERFs. These centres host tours and programmes, including school and community visits and academic and industry partnerships, to foster a greater understanding of the waste hierarchy and how individuals and organisations can contribute to a more ‘circular economy’. The centres also help to demonstrate that the waste fuel which is required at ERFs does not reduce recycling efforts because the fuel is that material which cannot be recycled.
Simply put, recycling comes first, while residual waste is put to work through the generation of largely renewable energy and heat. Education can be the key to ensuring valuable recyclate is not part of the energy recovery process, with this important waste segregation taking place in homes, council collections and in businesses. Increasingly this is being better understood with businesses requesting visits to energy recovery facilities and assistance in helping staff to segregate waste forming part of many total waste management contracts.
Energy recovery is the use of technology which sees pre-treated non-recyclable waste burned at high temperatures under carefully controlled conditions.
ERFs generally comprise the following: waste reception hall, waste bunker, boiler hall, waste combustion grate and boiler, steam turbine, control room, flue gas treatment plant, stack, bottom ash bunker, auxiliary energy centre, air-cooled condenser and staff facilities and offices.
The process begins with residual waste being delivered to the ERF tipping halls by road and/or rail. Post-recycling and non-hazardous waste from households and businesses is generally bulked at transfer stations before final delivery to an ERF, and every delivery has a transfer note checked at the weighbridge, showing composition and provenance. In the tipping hall, material is transferred from waste vehicles to the bunker for storage and mixing. Waste bunkers can typically hold in the region of 3,000 – 7,500 tonnes of waste with this value based on four to five days worth of throughput.
Grab cranes take the waste from the bunker, having first mixed it to ensure a steady burn, and load it into hoppers where hydraulic rams guide it into the combustion chamber. It is at this point that waste becomes fuel.
Overseeing operations are skilled and experienced control room staff, with the crane operator’s station strategically positioned with a view into the tipping bays, the bunker and the refuse feeding hoppers. A closed circuit camera over the feeding hoppers allows ERF staff to view the hopper conditions and the grab when it is unloading, to prevent disruptive blockages.
The waste is combusted at between 850-950C for at least two seconds to release the energy which is used to produce power, and the heat exchange between the gases and pre-heated water results in steam production.
Waste fed into the furnace for combustion passes over the grate in approximately 30 minutes during which time the energy contained in the feedstock is released by combustion. The residual gases and by products are held at temperatures in excess of 850Cto ensure complete combustion before being passed into the boiler for energy recovery. The boiler serves two purposes in such applications being the primary source of energy recovery and in the process quenching the flue gases down to 140C in two to three seconds.
Superheated (400C) steam raised in the boiler is then used to power a high-pressure turbine. With a close coupled high voltage generation set. Turbine power output is used both for internal (parasitic) distribution and all remaining power is then made available for export to the national grid as base load supply. Turbine exhaust steam then passes to air or water cooled condensers so that the condensate can be returned to the boiler within a closed system, and the steam generation process can continue.
Metals which remain at the end of the process are removed by magnets and eddy current separators or recycling and the incinerator bottom ash is also fully recycled to be used as high quality aggregate material in the construction industry.
The Flue Gas Treatment (FGT) system which forms an important part of ERFs comprises proven and reliable techniques developed to enable controlled combustion of waste-derived fuels while meeting all requirements of the Industrial Emissions Directiveand maintaining local air quality standards.
Around a third of the plant is dedicated to clean up with hot gases treated to remove pollutants – ammonia/urea can be injected into the combustion chamber to prevent oxides of nitrogen, activated carbon and lime is introduced in a reactor loop to fix combustion by-products and to neutralise acid gases, and advanced filtering systems then capture these injected materials, fine ash and small particles.
Even air pollution control residues can now be neutralised (the material is classed as hazardous waste by virtue of the residual alkalinity resulting from lime injection) and recycled for use in block making or as loose aggregate. The result is that only cleaned gases (mainly CO2) and water vapour are released through the stack. Each plant operates to strict emissions standards, with half-hourly, daily and annual compliance limits set out in its Environmental Permit. Advanced monitoring systems enable operators to refine performance on a continual basis and regulators have 24/7 access to the plant and online monitoring data.
Viridor’s Runcorn Energy Recovery (Combined Heat and Power) Facility is an excellent example of how this technology can be maximised to not only safely process waste but to efficiently produce vital heat and power for a regionally important industrial facility
The Invoyn Runcorn site is a major chemical manufacturing site, employing around 1,600 people, and is one of the largest single site energy users in Europe. The ERF operates as a combined heat and power (CHP) facility, producing much-needed steam and electricity which can then be used within the chlorine and PVC manufacturing process at the Runcorn site. It achieves this by burning fuels derived (after the removal of recyclables) from solid waste produced by homes and businesses across Greater Manchester. A good proportion of inputs comes via a well-established contractual partnership between Viridor and the Greater Manchester Waste Disposal Authority (now the GM Combined Authority), aiming for high recycling and landfill diversion rates. The facility is accessible by both road and rail making it an economic and sustainable solution for its customers.
Runcorn treats up to 850,000 tonnes of refused derived fuel each year and generates up to 70MW of electricity and up to 51MW of heat for exclusive use by INOVYN (INEOS ChlorVinyls).
By reducing the amount of waste sent to landfill and using it as a resource from which to recover energy, the Runcorn facility helps reduce the North West’s reliance on fossil fuels, improving sustainability and achieving significant reductions in greenhouse gas emissions, which in turn helps reduce the UK’s carbon footprint.
And, moving south to London, at Beddington, the construction of a £205m ERF at the existing landfill and recycling site in Sutton, began in 2015. The latest facility in Viridor’s ERF fleet, now in the commissioning phase, will provide the South London Waste Partnership and local businesses with a safe, robust and cost-effective alternative to landfill.
The ERF has been designed to process around 275,000 tonnes of non-hazardous residual waste a year. Wider environmental benefits will include the landfill diversion of over 95 per cent of waste delivered to the facility and a generation capacity of up to 26MW of electricity, which will power the facility itself and supply more than 22MW to the National Grid. A network of underground pipes is now being installed as part of a scheme to provide sustainable, low-carbon heating to homes and businesses in Sutton.
The Sutton Decentralised Energy Network (SDEN) will capture heat from the newly built ERF and existing landfill gas engines in Beddington, which will then be transported in highly insulated pipes to efficiently provide heat and hot water to properties. The first phase is set to go live later this year and will initially supply energy to around 725 homes, a care home and a supermarket in Hackbridge.
Sutton Council views this a big step towards its ambition of creating a zero-carbon borough, enabling residents to access a renewable energy source and points to the fact that, unlike other decentralised energy networks, it is not using fossil-fuels powering high-efficiency energy plants, but instead capturing low carbon heat from an independent source. For Viridor, the partnership forms part of its ethos that all waste should be valued as a resource and given new life as recycled raw materials or as vital and largely renewable energy, rather than being seen as rubbish.
There are now over 40 ERFs in the UK (and 400 across Europe), and their role and further potential has been recognised in the Government’s Clean Growth Plan and Industrial Strategy. The challenge is to ensure that the fossil-fuel (low-grade plastics) element of the fuel input is further minimised where possible and that heat and efficiency opportunities are maximised, to ensure that ERFs are central to the resource efficiency ambitions of the forthcoming Resources and Waste Strategy for England. It’s clear that there is still a capacity gap in the provision of this important infrastructure across the UK. Taking non-recyclable waste and using it to create electricity and heating for UK homes and businesses is a goal well worth pursuing and something the resource and waste management sector is seeking to replicate around the UK.