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Delivering speedy coal plant emission reductions

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Electrostatic precipitators are the most important pollution control devices to remove suspended particulates from the flue gas in coal plants, write Thompson Tsai and Ken Parker

The Taichung power plant in Taiwan is one of many coal-fired plants employing electrostatic precipitators (ESP) as a control device to collect dust from the flue gas.

With an installed capacity of 5500 MW, Taichung is also regarded as one of the largest coal-fired power stations in the world.

There are a total of ten ESP units. Most have been operated for more than 20 years and the reliability of the units' internal components has decreased, leading to higher emissions being recorded. Moreover, the tightening of stack emission regulations has also motivated the plant operators to make further improvements on ESP-collecting efficiency.

In order for the plant to be more environmentally friendly and respond to more stringent emissions regulations in Taiwan, it is proposed to enhance the ESP efficiencies phase by phase, in which ESP Unit 6 was chosen as the first unit for upgrading in order to increase equipment reliability and collection efficiency.

Taichung coal power plant, Taiwan
Taichung coal power plant, Taiwan
Credit: Tai & Chyun Associates Industries

Upgrade strategy

Based on the fact that the first and second field are contributing to 90 per cent of particulate collection, the ESP performance enhancement strategy was designed to upgrade both fields in each chamber by mechanical design changes and an electrical retrofit.

A series of performance tests were carried out beforehand to establish full operating conditions including electrical readings, fuel, gas volume, gas temperature, etc in order to evaluate what enhancement scenarios could be applied to improve ESP efficiency.

Since the ESP is generous in terms of design margins, extension by either series or parallel fields was discounted on cost and time outage reasons. Also, since the precipitator already has split fields, doubling the number of rectifiers was also discounted.

As a result, to increase reliability, the first and second fields in all chambers were upgraded with rigid discharge electrodes (RDEs) and the T/R set was changed to switch mode power supply (SMPS) units.

Assembling the discharge electrode frame
Assembling the discharge electrode frame
Credit: Tai & Chyun Associates Industries

The ESP is comprised of four separate chambers, giving a total of eight units of T/R sets to be upgraded with SMPS units at first and second field each chamber. Mechanically, there are 16,128 pieces of serrated strip type of discharge electrode (DE), also known as 'saw-band type DE', to be upgraded with RDEs.

The project was executed within 52 days including both mechanical and electrical upgrades. The result of post-modification testing showed that the unit successfully meets the customer's expectation and environmental regulations.

Benefits of an upgrade

The RDE electrode is more breakage-resistant and provides better durability compared with serrated type DE. Its spike and main body are integrated in a single piece of fabricated metal without any joints; therefore, after operation covering an appreciable time, the electrodes are usually found deformation-free. The aggressive spike design also generates a higher corona current with a lower onset voltage. This, together with SMPS operation, offers the advantage of producing an almost DC waveform, giving a higher field voltage, while modern electronics precisely detect and respond to any spark/arc situations more rapidly, resulting in a significant increase in the electrical operating conditions. Hence the retrofit enhanced the collection efficiency compared with the original design.

Installing switch mode power supplies
Installing switch mode power supplies
Credit: Tai & Chyun Associates Industries

ESP improvement

The performance of the ESP is expected to be below 15 mg/Nm3 at 6 per cent O2 after the retrofit. Accordingly, a baseline emission test was carried out before the project commenced, in which the test shows that the emission before the project commenced as 19.3 mg/Nm3 at 6 per cent O2. On the other hand, an acceptance test was carried out following the project completion and the result demonstrated an emission of 10.9 mg/Nm3 at 6 per cent O2.

According to the above comparison, the calorific value of the mixed coal for both tests is very similar, the calculated deviation rate is within ±0.2 per cent.

Therefore, it can be assumed as a fixed value, which means that at the same generation output the fuel consumption in both tests would be quite close, as would the gas flow.

For the acceptance test, the ash ratio has increased by 14.5 per cent over the baseline test, so one could assume that the inlet fly ash concentration has also increased by the same percentage.

Both the sulphur and moisture contents in the acceptance test are lower than the baseline test, which could mean that the dust resistivity is higher than that previously encountered. Taking these into consideration, it can be concluded that the retrofit project has reduced the outlet emission by 43.5 per cent under the baseline conditions of higher ash carryover and lower sulphur and moisture content of the fuel.

Faced with deteriorating mechanical reliability of the ESPs and the need to satisfy upcoming legislative demands to reduce emissions, utilizing RDEs and SMPS energization in only the first two fields of the existing ESP has proved successful in increased ESP collecting efficiency and performance, while the measured emissions were reduced by 43 per cent.

Implementation of this approach has proven to be a useful technology in the retrofitting of aging ESPs in terms of cost, time and space constraints.

In this instance, the project was brought to fruition within the scheduled 52-day outage.

Thompson Tsai is a vice-president of Tai & Chyun Associates Industries, which provides air pollution control for the power industry in Taiwan and Southeast Asia to ensure emission compliance with government regulation.

Ken Parker is chief consultant at Ken Parker Consultant APC and has over 40 years' experience in the air pollution control field covering both dry and wet forms of control.

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