The Coventry WtE plant will have an expected life longer than 60 years. Its story is one of a successful upgrade of combustion lines and control of a plant first commissioned in 1975. It has successfully operated as a waste solution and energy provider for hundreds of thousands of households and local businesses. Here are some facts about the Coventry story:
- Coventry is located in the English midlands.
- A Martin WtE combustion plant was commissioned in Coventry in 1975 and has been completely refurbished on the same site in 2012-13 after more than 30 years of successful operation.
- The plant is operated by a private company which is owned by the Coventry City Council (2/3) and the Solihull metropolitan Borough Council (1/3).
- The refurbishment provided three new Martin combustion lines, ancillary equipment and the most modern Martin control system (including Infrared analysis of combustion conditions).
- Refurbishment was completed progressively, on time and within the budget.
- The plant is located adjacent to residential housing and shops. The new plant will see a continuance of WtE for steam and electricity on the same site for a further anticipated 30 years.
- While valued for its utilitarian purpose, the facility also makes a significant annual profit for its shareholders.
- The gate fee for burnable refuse is in the order of £95 UK per tonne.
The Reasons For Refurbishment
- Although the old plant was still functioning adequately, Coventry decided on a replacement because the new systems provided a greater flexibility to deal with the changing nature of refuse and material types – particularly as the facility accepts household and trade waste and waste for other Councils and recycles 67% of this refuse before passing the balance to the WtE plant.
- The opportunity existed to attract greater residual refuse tonnes.
- Efficiency offered by later Martin technology enables greater efficiency and reduced consumables consumption throughout all components of the plant.
- The plant is a revenue producing enterprise for its Council owners.
- Emission factors were improved.
- The plant has three combustion lines, with a capacity for processing 315,000 tonnes per annum.
- Heat in the form of steam is produced for local consumption and electrical output is fed into the local grid.
- The new plant recorded 89% uptime in its commissioning year.
- There were only two complaint reports about the plant from the local community over the last 12-month reporting period.
- The entire operation including a separate recycling process pre transfer for WtE require just 67 staff over all shifts, 24/7- 365 days per annum.
- Electrical equivalent output net is 17MW continuous.
- Water use is sourced from the river and bores, and is cleaned for use as a function of the plant.
The Energy Recovery Process
- Waste delivery vehicles pass over a weighbridge where the waste is weighed. The waste is tipped into a concrete bunker which can hold 2500 tonnes.
- Overhead Cranes transfer the waste from the bunker to the feed chutes. The cranes are operated from a control room overlooking the bunker and CCTV monitors feed chutes, bunker and vehicle reception area.
- A hydraulic ram pushes the waste from the feed chute onto an inclined moving grate in the combustion chamber.
- The rate of passage down the grate is controlled to ensure that all material is fully burned before it reaches the discharge chute. Gas fired burners are used for start up and shutdown of the Unit and if necessary, to maintain incineration temperatures above 850°C.
- Residual ash falls from the end of the grate, quenched in water and transferred to a vibrating conveyor system.
- A Magnetic Separator removes ferrous metal from the ash and transfers it to a storage bunker for recycling. The ash is discharged into a separate bunker for disposal. An overhead crane is used to load the ash and metals into vehicles for transportation off site.
- The hot combustion gases (1300oC) from the combustion chamber pass through water tube boilers and economisers that generate steam and cool the gases to 140oC.
- The cooled combustion gases are directed into a Venturi Reactor Tower where hydrated lime and activated carbon are injected to remove pollutants. The gases then pass into a Fabric Filter to remove over 99.9% of particulate material (dust) from the exhaust gas stream. 80% of this material (fly ash) is re-circulated to optimise the treatment process, and 20% of the cleaned flue gases are returned to the combustion chamber in order to reduce the NOx emission levels.
- The cleaned flue gas goes to atmosphere through a 92 metre high chimney. Emissions are tightly controlled and continuously monitored as part of our Integrated Pollution Prevention and Control (IPPC) Permit, which is issued and regulated by the Environment Agency.
- Heat from burning waste produces steam in the boilers at 17.5 bar pressure and 208oC. The steam feeds two steam turbine generators, producing 12.5MW. and 4.8MW of electricity for internal power needs and export to the National Grid.
- Exhaust steam from the turbines is condensed before returning to the Hotwell in the main plant building for reuse.
Water is extracted from the nearby River Sherbourne for cooling and is supplemented as necessary with towns water.
- Emissions are monitored on a continual basis both by the plant operators (according to EU and UK regulations) and the operation is licensed by the UK Environment Agency.
- The company environment management system is independently audited.
- Local residents can have access to monthly reporting data.
- The Martin plant easily meets and improves on required minimum emission factors. The results for the last full 12-month reporting period are shown in the following table:
- The plant uses an SNCR (selective non catalytic reduction) system that enables effective control of NOx emissions.