Air Preheater Enhancement – West Burton Power Station, UK
Modifications to incorporate the Howden VN sealing system on the air heaters of West Burton Power Station halved the air leakage, which allowed the station to generate an additional 20 MW from each unit.
West Burton is a 2000MW coal fired station and each of the four units is fitted with two main and two mill rotary regenerative heat exchangers that preheat the secondary and primary air respectively. A gradual increase in air leakage through the mill heater had reduced the flow of air to the coal mills and this had reduced their capacity to effectively dry and transport the pulverised fuel to the burners.
Similar difficulties on the main air preheaters meant that the combined flow of flue gas plus air leakage eventually exceeded the maximum capacity of the induced draught fans, which reduced their capacity to remove gas from the boiler resulting in the output from each unit being 20MW below the nominal 500MW rating.
Inspection by Howden engineers identified the moving seals at the hot end of the rotor as the main source of increased leakage. These had been intended to compensate for downward deflection of the rotor as boiler load increased. In addition, the seals behind the adjustable axial sector plates had deteriorated, further increasing the leakage.
It was decided that the heaters would be converted to the Howden VN design. This involved firstly eliminating seals that could be adjusted while the boiler was online, and secondly doubling the number of radial and axial seals on the rotors.
Before the modifications, typical leakage in the main air preheaters had been greater than 8%.
This was reduced to 4.3%. Similarly, leakage in the mill heaters, (which had a higher pressure differential over their seals than the main heaters), was reduced from 35% to just over 17%. This permitted the electrical output from each unit to increase by 20MW to the design value of 500MW.