Owing to people’s growing awareness of air pollution and recent studies of the hazardous effects on human health posed by Particulate Matter (PM2.5), public pressure has urged coal-fired power plants to adopt technologies that can reduce particulate emission more effectively.
Electrostatic Precipitator (ESP) is one of the most common and important pollution control devices that removes suspended particulates (dust) from the flue gas in coal-fired power stations. For many old power stations built in the 1980s ~ 1990s, their ESPs were designed with dust collection efficiency of 98% ~ 99.5% and outlet emission at 25mg/ Nm3 ~ 200mg/Nm3, in compliance with emission regulation at the time.
Nevertheless, after more than three decades, air pollution regulations have become more stringent while ESP internal parts have also deteriorated over decades of long operation. The existing ESP dust collection efficiency is no longer sufficient and so major upgrade improvement is required.
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Upgrade Strategy
There are 2 common strategies in upgrading ESP dust collection efficiency; one is to increase the dust collecting area, the other is to increase the strength of electrical fields inside the ESP.
While the former requires either additional fields or ESP structure modification that demand space availability, long outage time and are cost consuming, the latter can be achieved by retrofitting Discharge Electrode (DE) to Rigid Discharge Electrodes (RDE) and upgrading a conventional transformer rectifier (T/R) unit to a Switch Mode Power Supply (SMPS) unit, which is more cost effective and can be completed within the time-constraint annual shutdown period.
By optimising the strength of electrical field to the most extent, ESP outlet emission can be reduced to as low as under 10mg/Nm3.
Case Study: Coal-Fired Power Plant, Taichung, Taiwan
Taichung Power Plant is one of the largest power stations in the world, built for a generation capacity of 5,500 MW, accounting for 15% of total power generation in Taiwan. There is a total of 10 ESP units, with unit 1 through unit 8 of identical design. Commissioned in the 1990s, ESP units were designed for outlet emission of 28mg/Nm3 with 99.29% dust collection efficiency.
Being the largest among the state-owned power stations, it serves as a paradigm of dedication to continuous improvement in environmental protection. Thus, the objective led to the launch of the ESP efficiency upgrade project to reduce emission and respond to more stringent emission regulation.
Each ESP consists of 4 chambers with 6 fields per chamber, forming a 4*6 matrix. Based on the theory that the first and second fields collect 90% of the particulate matter and an actual series of performance test carried out beforehand, full operating conditions were established including but not limited to electrical readings, fuel type, gas volume and temperature.
The ESP upgrade project had then been concluded to retrofit the original serrated type DE (also called saw band type) in the first and second fields (phase I) to RDE (total 8 fields) and replace conventional T/R unit by SMPS unit.
The pilot project was first executed on unit 6, followed by unit 5, 7, and 8, respectively during successive scheduled shutdown periods. Each unit took around 52 days with 2 cranes working in parallel in both chambers to complete. The results of the performance test showed the project was a success, meeting not only environmental standards but also customer expectations.
Benefits of an Upgrade
RDEs are break-resistant and much more durable than serrated type DE. Its spikes and main body are integrally fabricated from a single piece of steel plate (jointless); therefore, after an operation covering an appreciable time, the RDEs are usually found with spikes intact and deformation-free. The aggressive spike design also generates a higher corona current with a lower onset voltage.
SMPS, a 3-phase power source, produces an almost DC waveform, giving a higher field voltage, while its modern electronics precisely detect and respond to any spark/ arc more rapidly and thus minimize the setback time, providing a significant improvement in electrical operating conditions. The combination of RDE retrofit and SMPS upgrade largely enhances the ESP dust collection efficiency in comparison with its original design.
ESP Improvement
The expectations from the upgrade project were to have ESP outlet emission below 15mg/Nm3 at 6% O2 and emission reduction rate of not less than 10%. Before the upgrade, ESP outlet emissions were measured at each unit and the results were: unit 5 at 10.98 mg/Nm3, unit 6 at 19.38 mg/Nm3, unit 7 at 10.4 mg/Nm3, and unit 8 at 10.86 mg/Nm3 at 6% O2, respectively.
After the upgrade, performance tests were conducted under similar operating conditions i.e. boiler loading at 500MW±10%, while using mixed coal with calorific value and ash content less than ±10% deviation. The results were: unit 5 at 9.77 mg/Nm3, unit 6 at 10.98 mg/Nm3, unit 7 at 9.27 mg/Nm3, and unit 8 at 9.55 mg/Nm3 at 6% O2, respectively.
In summary, the implementation RDE retrofit and SMPS upgrade combined has been proven to be a preferable technology for ageing ESPs that require dust collection efficiency improvement in terms of cost and time saving and unavailability of space in order to achieve single-digit emission under certain conditions.
Further Challenges
Upon the completion of the above upgrade project, Taichung Power Plant was further stressed by the public and government to reduce the usage of coal. The current coal blending ratio of Australian coal to Indonesian coal was then adjusted from 2:3 to 3:2 in order to maintain the same calorific value needed for the same amount of electricity generation.
Unfortunately, the new formula of coal mixture contains higher ash content and thus inevitably caused higher ESP inlet dust loading. As a consequence, ESP outlet emission on unit 8 increased from 9.55 mg/Nm3 to 11.56 mg/Nm3 from the baseline test results. Therefore, applying the same method of ESP upgrade as abovementioned to the 3rd and 4th fields (phase II) was carried out on unit 8 as a trial project.
Phase II trial project was executed 2 years after phase I upgrade project. The ESP outlet emission was expected to be under 11.5 mg/Nm3 at 6% O2 and an emission reduction rate of not less than 8%. The performance test results showed that the emission had again been successfully reduced from 11.56 mg/Nm3 to 9.47 mg/Nm3 with 18.1% reduction. Since phase II upgrade was done in the 3rd and 4th fields where finer dust particles were more difficult to collect, SMPS played a vital role in increasing dust collection efficiency while still achieving this remarkable reduction percentage.
Maximising dust collection efficiency of aging ESPs by RDE retrofit and SMPS upgrade has once again been proven to be a feasible and economical approach that ultimately allows power plants to minimise their negative impacts on the environment.
About the authors
Thompson Tsai and Wendy Hsu – Vice President and Project Coordinator of Tai & Chyun Associates Industries, Inc., a provider of optimized solutions of parts and services for Electrostatic Precipitator (ESP) to ensure emission compliance with standard regulations.
Alex Tsai – Maintenance Manager of Taichung Power Station, one of the largest coalfired power plants under a state-owned Taiwan Power Company (Taipower), providing electricity to Taiwan and its off-shore islands.
