Electricity and Control August 2022

ENERGY MANAGEMENT + THE INDUSTRIAL ENVIRONMENT

Upgrading ESPs to meet new emissions regulations Babcock has come full circle at a major power station where it will be upgrading electrostatic precipitators (ESPs) on six boiler units that it designed and built in the 1980s. Using technology not available at the time of the original construction, Babcock will bring the boiler units into the 2020s, enabling the power station to comply with current emissions regulations.These limit dust to below 50 mg per normal cubic metre (Nm 3 ) of flue gas, essentially halving the impurities.

T his 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 bene ficial for the cement industry, which relies on this product property to improve water resistance in cement. Looking to the latest emissions regulations, the upgrade of the precipitators’ capability required to achieve dust lev els of below 50 mg/Nm 3 will entail a three-part programme: ƒ The upgrade and refurbishment of the SO 3 condition ing plant ƒ Changing the technology on the ESPs’ transformers to convert them into high-frequency sets ƒ The upgrade and refurbishment of the ESPs to max imise dust collection efficiency to comply with new legislation. Babcock has been awarded the third portion of the pro gramme and will refurbish four precipitator cases in each boiler, starting in 2023. 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 sur faces of the boiler and the air heater, and emerges through the ducting and through elec trostatic precipitators. Af ter it is removed and deposited in the

hoppers below, the fly ash is either removed from site or sold to the cement industry,” Brook says. “The fine dust in the gas travels a tortuous route in the boiler, resulting in areas where there is more dust, and oth ers with less dust. For the ESPs to function optimally, the gas distribution should be as homogenous as possible,” he adds. Brook describes an ESP as a box with a series of elec trodes or plates strung vertically inside it. The dust parti cles are charged, creating a flow to the collector electrodes on 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 model ling, which was not available in the 1980s, to model the gases from the outlet of the boiler into the precipitator cas es, to better understand and manage what is happening with the gases under different load conditions and improve dust distribution. This, in turn, will improve dust collection and removal. Babcock’s scope of work also includes the replacement of the current wire-type discharge electrodes with modern and more reliable rigid discharge electrodes. Over time, the 12-metre long weighted discharge electrode wires have started to perish, diminishing the electric, or corona field which means the collector electrodes 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, maximising the efficiency of the ESPs. Babcock is the lead contractor responsible for all project management, integration, delivery and construction for the project, and will be working in partnership with USA-based Babcock & Wilcox (B&W). B&W owns the technology of the original design of the precipitators and will be undertaking the process and detailed engineering. As one of the most established and experienced steam generator and industrial plant suppliers in Africa, Babcock has delivered solutions to many of the power stations in the country. It is currently working at several sites undertaking projects that include high-pressure piping, coal burner overhauls and mill maintenance. □ For more information visit: www.babcock.co.za

Babcock will upgrade the electrostatic precipitators on six boiler units at the power station to reduce dust in the flue gas.

Electricity + Control AUGUST 2022 14