I pass on any responsibility for people getting bored whilst reading this to Wobs and Forfi (also any credit, to be fair, as I probably wouldn't have written all of this otherwise).
This is what I found out when I attended a presentation about and tour of Sheffield Energy Recovery Facility.
Background History
In the 1970s it was decided that Sheffield would integrate it's waste incinerator with a network of pressurised hot water pipelines under part of the city to use the heat from burning waste to warm homes and buildings. The heat was also used to heat water and power a steam turbine to produce electricity.
By the year 2000 the level of waste that the city generated was alot more than the current incinerator could handle. Plans to treble the waste recycling rate by 2005 would not be sufficient to prevent increasing amounts of rubbish being sent to landfill. Also, a tightening of the legislation governing waste incineration was looming. In the end Sheffield City Council agreed a contract with Veolia which included building a new 'Energy Recovery Facility'.
The man said that it was a perfect example of technology being started off in the UK, then exported to France, where it was improved, and then bought back again.
Interesting? bits of the Contract between Veloia and Sheffield City Council
The primary objective in the contract between Sheffield City Council and Veolia is to increase the rate of household recycling in the city. It means that if Veolia does anything to discourage recycling it is automatically in breach of its contract with the council. The person doing the presentation was asked if they are concerned with the potential for increased recycling to reduce the waste going into the incinerator (and thus the amount of energy that they can produce to sell on). The man said it was not an issue for them because they could take on more commercial contracts to make up the reduced tonnage from households.
The operation of the incinerator is very tightly regulated both by the Environment Agency and the council. Veolia get fined if they fail to notify the council of any correspondence they receive from the Environment Agency within two days of receipt. The subject of the correspondence is irrelevant. The condition applies even if it is a fax acknowledging receipt of a cheque. Apparently they didn't notice this bit in the contract for the first month or so and got fined a lot of money by the council, until they realised that they speed of the post meant that the council were receiving things a day late so they switched to faxing.
Any breaches of the contract between Veolia and the council can result in fines, and in the worst case complete stock transfer from Veolia to the council (the incinerator and all the staff included) so they have some pretty big incentives to do things right.
Emissions
The emissions from the Energy Recovery Facility are monitored continuously during operation. The data is freely available on their website. The Environment Agency requires a lot of readings, much more than one a day (I think it might be every half hour, I can't quite remember). The Environment Agency also set limits to the emissions of a number of different substances. As the whole of Sheffield is classed as a Clean Air Zone these are apparently tighter for some substances that the minimum regulation standards. The plant must be shut down if it exceeds the limits.
Emissions are reduced in a number of ways;
-The boiler must always be a minimum of 850 deg C before any waste can be fed into it. There are two gas jets the enter the boiler and are lit to preheat, or increase the temperature of the boiler if it drops. (This ensures complete combustion)
- Urea is used to react with combustion products and convert NOx (I think that they are converted to ammonia, does anyone know if this is correct? SOMETHING was converted to ammonia).
- Lime and 'activated carbon' (I'm not sure what this is) is used to neutralise the acidity of the flue gas and absorb other pollutants.
- The gases from the boiler are passed through 1760 filters to collect the dust. The dust is then taken for treatment in a process plant and then disposed of.
- Ash from the incineration process is kept in a bunker and disposed of separately.
If emissions look like they might get too high then they can stop transferring waste from the rubbish pit to the incinerator. In the worst case the people in the control room must turn the incinerator off.
They try to operate the incinerator at the maximum emissions levels permitted (taking spiking into account). When asked why they didn't try to minimise their emission as much as possible the man told us that it was a balancing act with many of the emissions. E.g. if you add lots of urea, you reduce the NOx but it will increase the ammonia (assuming I've got my chemical process right, hopefully it explains the principle anyway). Also, the more chemicals you have to add to process the boiler exhaust gas, the greater the cost and the greater the energy used (all of the chemicals are commercially manufactured and bought).
What Happens to the Heat Produced?
The energy from the incineration process is transferred to a district heating network. The network travels a long way from the incinerator. Weston Park Hospital is the furthest building that is currently on the network. I think that it is about 2000m away. It is a closed loop system with heat exchangers at each building to transfer the heat to the buildings' own systems. Apparently the equipment takes up less space than the usual building heating plant. The buildings have individual control of their own heating, so demand for heat must be balanced just as demand for electricity from the National Grid is.
Any excess energy that is not required for heating is converted to electricity by a steam turbine and sold to the National Grid (I think they meet the plant's electricity requirements first though).
All of the time the man was talking about economic justifications for their activities (It really does make a load of money for them in lots of ways) so he was asked if electricity of district heating was the most economic use for the heat. Despite electricity being sold at a higher price than heat, the reduced efficiency of the electricity generation compared to the heating meant that they could get more money for heat. They were trying to promote the connection of building near the district heating network to increase their revenue (and save a lot of carbon obviously
).
Recycling
An electromagnetic overband separator removes metal from the ash (I didn't get to see this bit
). The metal is then delivered to a local company for recycling, in the same way that cans etc are recycled. The man was quite miffed that the metal tonnage does not count as recycled if it has been through the incinerator first.
They are currently looking at getting the ash used in concrete manufacture.
Other Information
The legislation covering the operation and emissions from incinerators is The European Union Waste Incineration Directive 2000/76/EC and Municipal Solid Waste Incineration is subject to regulation under Pollution Prevention and Control (PPC sector 5.1)
If the incinerator has to be taken offline for any reason (e.g. for maintenance) then the waste is diverted to landfill. Currently about 15% of Sheffield's municipal waste is sent to landfill. Veolia aim to reduce this to 10% which would reflect the downtime of the incinerator.
Sheffield Energy Recovery Facility is the best in the country. That isn't just local pride, it's because it is the newest. When another one is built, it will be the best because with each one they learn more lessons and refine things a bit more. Apparently all of the bosses of the incinerators are quite competitive. They will all claim that theirs is the best but then have to add things like (apart from the efficiency) or whatever if they are being honest.
Anyone still awake :P? If you've managed to make it this far, CONGRATULATIONS
Any questions?
*edited for loads of spelling mistakes, maybe I should get Mozilla
*
Sheffield Energy Recovery Facility (Incinerator)
Linear Mode
