Managing Our Environmental Impact
Technology
SP Generation is adopting various innovative technologies to improve the environmental performance of our power stations in line with emerging European and UK legislation on air quality and climate change. These technologies are outlined below:
Low NOx Burners
NOx is a by-product of coal combustion and originates both from the coal-bound nitrogen and the nitrogen from the air, used in the combustion process. There are three formation mechanisms for NOx:
- Thermal NOx – the reaction between oxygen and nitrogen in the combustion air at temperatures >1300ºC in oxidising atmospheres; ;
- Fuel NOx – the oxidation of coal-bound nitrogen compounds at temperatures >750ºC; and
- Prompt NOx – the fixation of atmospheric (molecular) nitrogen by hydrocarbon fragments in reducing atmospheres.
Low NOx burners reduce emissions of oxides of nitrogen (NOx) from coal or gas-fired power stations by staging the mixing of fuel and air in the burner to reduce the formation of thermal NOx. There are three stages to fuel combustion in a typical low NOx burner. In the first stage, combustion takes place in a fuel-rich, oxygen-deficient zone where NOx and hydrocarbon fragments are formed. During stage two, the hydrocarbon fragments then react with the NOx to form elemental nitrogen. In the final stage of combustion, further air is added to complete the burnout of the carbonaceous matter. The formation of NOx in the final stage can be minimised by completing the combustion in an air-lean environment.
Low NOx burners can be combined with other primary measures of NOx reduction, such as Boosted Overfire Air techniques.
Boosted Overfire Air (BOFA)
BOFA reduces the formation of oxides of nitrogen (thermal NOx ) during combustion and is an emissions reduction measure over and above Low NOx burners
Thermal NOx is produced when oxygen mixes with coal in the hottest part of the coal flame. NOx production can therefore be reduced by controlling the fuel and air mixing. The BOFA system is an extension of this principle.
BOFA works by further staging where the fuel and air meet. Air is taken from the normal, secondary air supply to the burners and is added back into the boiler between the upper row of the burners and the top of the furnace section of the boiler.
Additional fans forcefully inject this 'over fire' air to ensure adequate mixing between the air and any remaining unburned coal particles. Since this final burn out of the fuel occurs at a lower temperature, nitrogen, rather than NOx, tends to be formed. BOFA will typically reduce NOx levels by between 20 and 25%.
Flue Gas Desulphurisation
ScottishPower is investing in excess of £170 million to implement Flue Gas Desulphurisation (FGD) on three of the four units at Longannet Power Station.
FGD is regarded as 'Best Available Technique' (BAT) for abatement of oxides of sulphur (SO2 ) and will cut our SO2 emissions by over 90%.
The Seawater Scrubbing FGD technology now being commissioned at Longannet, works by using seawater’s inherent alkaline properties to absorb and neutralise acidic SO2. Longannet uses large quantities of water from the River Forth as cooling water for its condensers.
This condenser cooling water will be reused by bringing it into close contact with the station’s flue gases in purpose-built absorber towers to absorb the SO2, resulting in a harmless soluble sulphate (SO4) that can be discharged back to the river.
The process does not require additional chemicals – only seawater and air are used – and the discharged seawater will comply fully with environmental regulations. The adoption of Seawater Scrubbing FGD at Longannet represents the largest deployment of the technology in the UK aimed at capturing more than 90% of all SO2 emissions.
Selective Catalytic Reduction
ScottishPower is investigating the use of Selective Catalytic Reduction (SCR) technology to control NOx emissions at Longannet. SCR is a post-combustion method of NOx abatement that has been proven in Japan and the US and more recently in European power stations.
It involves taking the exhaust gas from the boiler at a temperature of up to 400oC, injecting aqueous ammonia and passing it over a catalyst (usually vanadium pentoxide). This facilitates a chemical reduction between the ammonia and NOx to create harmless atmospheric nitrogen and water vapour.
The decision to investigate SCR as a solution for Longannet follows a feasibility study in 2008 that thoroughly reviewed available options. Longannet is now commencing an engineering design study to tackle the technical challenge of installing SCR in the limited physical space available - further information can be found in "our projects" section.