Abstract
Because of regulatory requirements to reduce SO₂ emissions which became prevalent during the 1960s, utilities began looking to low-sulfur coal as a cost effective way to meet these requirements.  However, it became apparent that existing cold-side electrostatic precipitators were not going to perform well with these coals.  Engineers determined that “hot side” ESPs could be constructed at power plants upstream of air heaters where typical flue gas temperatures would be in the range of 550 to 850^OF.  It was believed that, in this temperature range, the ash from most coals would have low resistivity.  However, as the number of hot-side ESPs in service grew, there developed a realization that these ESPs were having significant problems with low sulfur coal ash.  One of these poor performance problems was related to high resistivity.

In a hot-side precipitator, high resistivity, caused by the process know as sodium depletion, is typically time dependent.  The clean hot-side ESP performs well initially, but its performance degrades over a period of weeks or months.  One technique to deal with the high resistivity problem caused by sodium depletion is the use of positive polarity.  A number of tests have been conducted in laboratory settings to investigate the use of positive polarity; however, tests at pilot scale using actual flue gas have not been performed.  This is the subject of the study reported here.

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Abstract
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Abstract
The paper details the investigation into the use of collecting electrode (CE) mass measurement to optimize a full scale electrostatic precipitator (ESP) unit, Initially, work was done at Eskom’s Hendrina Power Station, but due to problems experienced, further testing and experimentation had to be done at another test ESP unit.  The work at Hendrina Power Station showed that the research concept was viable and solutions were found to some of the problems experienced.  The present test unit is located at Eskom’s Lethabo Power Station situated in the Free State province of the Republic of South Africa (RSA).  This coal-fired power plant consists of six boiler units, each generating 620 MW. Each boiler is serviced by four rigid frame, Rothemuhle ESP casings in parallel, each consisting of seven fields in series, with a specific collecting area of 191.6 s/m and an aspect ration of 2.4.  The coal burnt at Lethabo can be considered as “difficult” coal – high ash content (up to 42%), highly resistive (in excess of 10¹³ ohm.cm) and low sulphur (0.6%).

One ESP casing was chosen for experimentation and load cells were installed in each field to provide an on-line, real-time, indication of the ESPs operational behavior.  An opacity monitor was installed on the outlet duct.  It was correlated against mass emission by iso-kinetic measurements.  ESP collection patterns were measured and trended against varying boiler conditions.  ESP collection rates and corresponding re-entrainment effects were also successfully measured.  Rapping optimization was approached on two fronts – emissions as well as wear and tear reductions.  The experimental set-up is being tested for total ESP optimization as well as automated operation and control.  Results to date show an emission reduction of 133 mg/Sm³ down to an operating level of 110 mg/Sm³. Rapping induced wear and tear has been potentially reduced by 90%.