When it comes to burning fossil fuels to produce energy, the main forms that usually come to mind are natural gas, oil, and coal. However, there are other materials and technologies that can also make a useful contribution – one of these is harvesting energy from various waste materials, or ‘waste-to-energy’ (WTE).
This form of energy production tends to be overshadowed by its bigger siblings such as gas and coal-fired power stations, but nevertheless, WTE plants can fulfill the useful dual role of generating heat and/or electricity, at the same time disposing of huge quantities of unwanted, low-value materials.
It has been estimated that global waste generation will double in the decade from 2015 to 2025, increasing from 3 to over 6 Mt/d. And further significant rises are expected in the coming years. There are a multitude of different wastes generated by commercial and industrial processes, but there is also one form that practically every individual on the planet contributes to, namely municipal or household waste, and this provides much of the input to the growing number of WTE plants in operation.
Most WTE projects are developed on a case-by-case basis, and their appearance is not universally welcomed – such was the case with the plant that I visited recently. This is the Javelin Park WTE plant near the City of Gloucester in the UK, which opened a few years ago. Although the technology adopted can differ, Javelin Park is typical of the type being embraced by many newer facilities springing up across much of the developed world.
There are mixed opinions on the plant’s ‘modernist’ appearance, and early in the planning process, the project caused much local controversy over concerns of possible pollution and visual blight. The planning application was initially rejected in 2013; however, following an appeal by the developers, approval was granted by the UK government in 2015. The plant was built and now manages much of the county of Gloucestershire’s waste left after recycling and reuse.
The process
The Gloucester facility takes in around 130 kt of municipal waste each year. It has a capacity of 190 kt, so some commercial waste is also accepted. Wastes are delivered by via refuse collection lorries or HGVs, each carrying up to 25 t, and are first deposited in a reception bunker that can hold approximately 10,000 t. From here, overhead cranes lift the required amount to the top of an inclined moving grate (this is the bit where the ‘magic’ happens). Waste travels slowly down the grate – the first few metres essentially dry the incoming materials, followed by the actual combustion process that takes place at around 1000oC, encouraged by pressurised air from above and below (underfire and overfire air). Air for the combustion process is pre-heated through a heat exchanger, using steam that has passed through the site’s steam turbine. Finally, the last section of the grate handles the residual ash, before feeding it into a quench bath. The whole journey takes about an hour.
The heat from the combustion process is used to generate steam in a boiler at 282oC – this is superheated to 427oC using flue gas before being fed to the turbine at a pressure of 6 MPa (870 psi). The turbine drives an electricity generator rated at 16.5 MW, although it regularly produces up to 19 MW; around 2.9 MW is consumed running the plant, with at least 14.5 MW of electricity exported to the UK national grid. This is stepped up from 11 kV to 33 kV via an on-site transformer, providing enough energy to power the equivalent of around 25,000 homes. Once the steam has passed through the turbine, it is condensed back to water using large A-frame air-cooled condensers, before circulating back to the boiler for reuse.
How clean is the plant – what does it emit?
This is a question often asked, as some earlier designs of WTE plants had a poor reputation in terms of their emissions to air. In the case of Javelin Park, apart from the occasional whiff of steam from the plant’s stack, there never appear to be any visible emissions. The flue gases generated from burning the waste are quenched in a conditioning tower, after which they are scrubbed using a mixture of lime and activated carbon – this neutralises any acidic species present; for example, burning PVC contained within the waste stream can create hydrochloric acid.
The gases then pass through a series of long bag filters to remove particulates. There are 900 individual bags arranged in six banks of 150 – they have a lifetime of around 5 years. As the gases pass through the bags, a crust of particulates, lime and activated carbon (‘filter cake’) forms on the outside. Sensors measure the pressure drop as the cake thickens, and when too low, a jet of air is pulsed into the individual bag, causing the cake to detach and fall to the bottom for collection.
After cleaning, flue gas is finally exhausted through the stack. Emissions must meet stringent regulated levels imposed by the UK Environment Agency, in accordance with the EU Industrial Emissions Directive. Levels are monitored constantly using a Continuous Emissions Monitoring system. This measures levels of nitrogen oxides, carbon monoxide, sulphur dioxide, hydrochloric acid, and particulates. It also checks for volatile organic compounds, which can be formed during the combustion of a range of materials such as paints, disinfectants, adhesives, carpets, and upholstery.
What’s left at the end?
Given the range of materials that can find their way into the incoming waste stream, it would be surprising if there weren’t some solids left at the end of the process. There are two main streams, one of which is known as incinerator bottom ash (IBA). This is made up of inert aggregates, ceramics, glass, clinker, and metals – the latter are recovered for recycling, and the remainder used as construction aggregate, suitable for road bases for example.
The other solid stream comprises the residues from the flue gas cleaning process, mainly spent lime and particulates, or Air Pollution Control residues (APCr). Because of their alkaline nature, this material is classed as a hazardous waste and is disposed of in specialised landfills. Each year, the plant recovers around 37,000 t of IBA aggregates, 3000 t of metals, and produces 5000 t of APCr.
Overall, only around 2.5% of what arrives at the Javelin Park facility ends up in landfill; a good thing as most local landfills have been closed and capped. But even here, historical buried waste is making a useful contribution, as landfill gas (LFG) is increasingly being recovered and used to produce electricity. LFG is composed of roughly 50% methane and 50% carbon dioxide.
Final thoughts
When it comes to the provision of affordable, sustainable energy, there are undoubtedly difficult days ahead, and any process that can generate electricity and feed it into the national grid is to be welcomed. Waste-to-energy plants take unwanted, troublesome materials and turn them into something useful; in the process, eliminating much of the need for landfills with their attendant problems. In the case of the Gloucestershire facility, because of the organic content of the incoming waste, around half of the energy produced is classified as renewable; this is checked and certified by Ofgem, the UK energy regulator.
One subject I haven’t mentioned is CO2 emissions. There are various technologies commercially available capable of capturing CO2 from plant flue gas, although most efforts have focused on larger point sources such as fossil fuel-fired power stations and industrial applications. However, CO2 capture might become a necessity for WTE plants in the future. There are several WTE projects underway around the world examining this, including Twence in the Netherlands, Klemetsrud in Norway, and Saga City in Japan.
Around much of the developed world, volumes of municipal solid waste continue to rise. Historically, much of this has ended up in landfill, an option that is becoming increasingly environmentally unsustainable and uneconomic. Although WTE plants are not universally loved, they nevertheless play a vital role in transforming the world’s growing mountains of detritus into useful products such as electricity, heat, and building materials. Fortunately, modern facilities are much cleaner and more efficient than some of their predecessors.
It was encouraging to see just how effective plants such as Javelin Park can be in minimising environmental impacts and helping meet the world’s ever-growing demand for energy.
