Babcock has come full circle at a major power station where it will be upgrading electrostatic precipitators (ESPs) on six boiler units that Babcock designed and built in the 1980s.
Using technology not available at the time, Babcock will bring the boiler units into the 2020s, enabling the power station to comply with latest emission regulations
of below 50mg of dust per normal cubic metre (Nm3) of flue gas, essentially halving the impurities. This power station has provided reliable power to the utility for more than four decades, ranking as the best-performing and lowest-cost generating station in the utility’s fleet. The boiler was designed to burn coal with a low calorific value and an ash content above 40%. Producing fly-ash with a low carbon content proved beneficial for the cement industry, which relies on these properties to improve water resistance in cement. To meet the latest emission regulations, a three-part programme is required to enhance the total precipitator capability in order to achieve levels of below 50mg:
- Upgrade and refurbish the SO3 conditioning plant.
- Change the technology on the ESP’s transformers to convert them into high-frequency sets.
- Upgrade and refurbish the ESPs to maximise dust collection efficiency to comply with new legislation.
Babcock has been awarded the third portion of the programme and will refurbish four precipitator cases in each boiler beginning in 2023.
Removal of impurities from the air
Dave Brook, Technical Director at Babcock, explains that ESPs use an electric charge to remove impurities from air or gases through a series of collector and discharge electrodes. “Fly ash is a very fine dust found in hot flue gases from coal combustion. It travels through the heat transfer surfaces of the boiler and the air heater, and eventually emerges through the ducting and through electrostatic precipitators. After it is removed and deposited in the hoppers below, the fly ash is either removed or sold to the cement industry.” “The fine dust in the gas travels a torturous route in the boiler, resulting in areas where there is more dust, and others with less dust. In order for the ESPs to function optimally, the gas distribution should be as homogenous as possible,” added Brook. He explains that an ESP is a box with a series of electrodes or plates strung vertically inside of it. The dust particles are charged creating a flow to the collector electrodes upon which the dust is deposited and collected. When these plates get full, they are rapped (struck), and all the dust slides down into the hopper. Babcock will use computational fluid dynamics modelling, which was not available in the 1980s, to model the gases from the outlet of the boiler into the precipitator cases, to better understand and manage what is happening with the gases under different load conditions and improve dust distribution, thereby improving dust collection and removal. Also included in Babcock’s scope of work is the replacement of the current wire-type discharge electrodes with modern and more reliable rigid discharge electrodes. After time, the 12-metre long weighted discharge electrode wires have started to perish, diminishing the electric, or corona field, causing the collector electrodes to lose efficiency and collect less dust. The new rigid discharge electrodes, which will be manufactured in South Africa, cannot break and will create a more stable corona field, thereby maximising efficiency of the ESPs.