Energy Recovery
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Introduction
The Energy Recovery Plant in Lerwick generates hot water by burning waste for the Lerwick District Heating Scheme which is operated by Shetland Heat Energy and Power Ltd (SHEAP).
The Plant burns 22,000 tonnes of waste per year and generates 7MW of energy. The waste comes from Shetland, Orkney and Offshore.
Waste is tipped into the waste bunker before it is fed into the furnace feed chute by an overhead crane/grab at a rate of approximately 3 tonnes per hour.
The waste is then pushed into the furnace where it starts burning with the radiant heat from the waste already burning in the furnace. The waste burns at 1,100 to 1,200 degrees centigrade. Gas oil burners are in standby to ensure that temperatures in the after combustion chamber do not come below the minimum temperature of 850 degrees centigrade.
Situated above the after combustion chamber is the 50 tonne water boiler. The hot gases pass through the three pass boiler to heat the water to 115 degrees centigrade to be distributed to the district heating scheme.
The after combustion chamber is designed to produce a cyclone which removes most of the larger particulate before it enters the boiler tubes. Urea is sprayed into the after combustion chamber to reduce NOx emissions if required.
Further gas cleaning is carried out by an electrostatic precipitator, a wet scrubber and a bag filter.
The electrostatic precipitator removes all the fine particulate and the wet scrubber reduces the acid gases HCl and SO2. The bag filters are coated with a carbon/lime mix which filters out the dioxins should they be regenerated after the incineration process.
Further treatment is required to treat the strong acids generated by the wet scrubber. This is done in the water treatment plant where it is neutralised and metals are removed in the filter press.
Residues from the flue gas cleaning and the water treatment plant is shipped away from Shetland as special waste.
A magnetic separator screens bottom ash from the furnace before it is transported to the nearby landfill site to be used as cover material. The ferrous metal from the magnetic separator is recycled.
Emission Monitoring
With the introduction of the Waste Incineration Directive in 2005, stringent legislation is enforced on the incineration of waste.
Emissions from the Plant's 46 metre high stack are continuously monitored to ensure compliance with the legislation.
See attached emission data for 2007 and 2008 to date.
Daily Average Emission Limits
Oxides of Nitrogen (NOx) |
300 mg/Nm3 |
Carbon Monoxide (CO) |
50 mg/Nm3 |
Sulphur Dioxide (SO2) |
50 mg/Nm3 |
Hydrogen Chloride (HCl) |
10 mg/Nm3 |
Total Organic Carbon (TOC) |
10 mg/Nm3 |
Dust (Particulate) |
10 mg/Nm3 |