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Technical Market Support

Effect of Sodium Dosing on ESP Performance - Parts 1 and 2

Technical Market Support » Thermal Coal

Published: November 05Project Number: C13001

Get ReportAuthor: Allen Lowe, Tony Boorer | A&SJ Lowe Consulting Services, Delta Energy

PART 1 REPORT

Fly ash resistivity is an important characteristic in regard to collection of coal ash in an electrostatic precipitator (ESP). Where the resistivity is excessive the ESP can exhibit symptoms of back corona. This can lead to excessive dust emission, resulting in operational difficulties at the power plant and possible issues for the coal supplier.

Ash resistivity has been found to correlate with the content of sodium and lithium in the ash and those coals which exhibit high resistivity symptoms in ESPs often contain low sodium in ash. This raises the possibility that application of a sodium based additive to the coal may improve ESP performance with that coal.

A review of the literature yielded a number of reports where sodium compounds have previously been successfully applied to reduce high fly ash resistivity. However, these studies were all carried out at gas temperatures above that relevant to the low temperature ESPs commonly used in power stations in Australia and in customer countries for Australian coal. Further, at these higher temperatures ash resistivity is controlled by somewhat different mechanisms to that applying at low temperatures. This study was therefore undertaken to investigate the potential for use of sodium compounds to ameliorate high fly ash resistivity in ESPs operating with gas temperatures in the range 130 to 145 °C.

The method adopted was to perform measurements on the ACIRL pilot scale combustion test rig at Riverview. An Australian coal was selected as representative of a low sodium in ash coal and burnt at a rating of 150 kW. ESP efficiency was determined by isokinetic sampling of ESP inlet and outlet gas flows while burning both dosed and undosed coal.

A review of potential additives yielded a number of candidate materials. Trials were carried out with sodium sulphate, sodium carbonate and sodium silicate to test for differences in response to the form of sodium. In addition sodium sulphate was tested at three dosing rates to develop a response curve while one test with sodium sulphate at an intermediate dosing rate was carried out at a slightly increased temperature to identify temperature effects.

Sodium sulphate dosing yielded dramatic improvements in ESP performance as sodium oxide in ash was increased progressively from the raw coal value of 0.35% to a maximum of 1.03 %, a level that remains within the range of sodium in ash occurring naturally in Australian export black coals.

Dust emissions from the pilot scale ESP decreased by over an order of magnitude as sodium in ash was raised from the raw coal level to the maximum level of sodium addition. At the maximum rate (1.03 % total Na2O in ash) the pilot scale ESP achieved emission levels below 10 mg/Nm3 while operating with a relatively small SCA of 82 m2/m3/s.

The tests to distinguish differences between the various additives and the effect of temperature were inconclusive. These tests in part followed directly from raw coal tests and it appears that residual effects of low ESP collection efficiency on raw coal may have persisted through the changeover period allowed.

Sodium sulphate appears to be the most economical form of sodium containing additive. It is commercially available in bulk and is widely used in various industries. At current prices it is estimated that dosing to increase the Na2O in ash content of an export quality (15 % ash) coal by 0.5 % will cost of the order of $A0.60 per tonne of coal.

Sodium in coal has been associated with a number of problems within power station boilers including slagging, fouling and fireside corrosion. A review of the literature relating to these effects suggests that no problems will be encountered at dosing levels sufficient to improve ESP performance. Further, previous trials with sodium dosing have not identified any limitation related to boiler fireside problems at target dosing rates.

Environmental and ash marketing issues must be considered for a full-scale operation. However, a brief preliminary review did not identify any significant issues in relation to sodium dosing.

In view of the substantial benefit potentially available, but recognising the limitations inherent in extrapolating from the pilot scale tests a full scale trial was carried out to confirm the pilot results.

PART 2 REPORT

Electrostatic precipitators (ESP) are widely used for the control of dust emissions from coal fired power stations. Certain coals are found to produce ashes with high resistivity, these are difficult to capture in the ESP and can result in operating problems in the power stations.

18,000 tonnes of test coal were stockpiled and then burnt over three days at Vales Point Power Station. This coal had previously been found to produce high dust emissions at this station. The first of the three test days was intended to provide base line performance data for this coal in the ESP. For the second and third days sodium sulphate was added, sufficient to raise the sodium oxide in ash to approximately 1.0%. ESP performance and other plant data were monitored during the test burn.

The sodium sulphate appeared to produce only a relatively small reduction in dust emissions when used as the sole additive. The carrying out of an offline rap of the ESP and the simultaneous introduction of a relatively small amount of sulphur trioxide at the end of the test sequence resulted in a substantial reduction in emissions.

The small impact of sodium dosing on ESP performance was confirmed by video photography of the stack plume. It was difficult to distinguish photographs of the plume with sodium dosing from those without dosing.

Analysis of the resistivity of the test coal showed that at the normal operating temperature of this plant, resistivity is not in the range expected to cause ESP problems. However, resistivity is sensitive to small changes in ash composition. It is possible that other samples of coal from the same mine will display higher resistivity characteristics.

The analysis also showed that resistivity for the coal increases rapidly with temperature. It has previously been observed with this coal that gas temperature at the ESP rises after some days of continuous boiler operation. This is believed to be due to changes in boiler heat transfer characteristics. If this were the case then resistivity could well rise into the region associated with poor ESP performance.

Excessively fine particle size distribution is expected to adversely affect ESP performance. Analysis of the dust collected at the ESP outlet indicated that the test coal produced dust with a similar size distribution to normal station coal.

Sodium dosing is expected to improve ESP performance only with high resistivity ashes. In the present instance the resistivity appears to have been below the level of concern and where sodium dosing would be beneficial. This may have resulted in the minimal effect observed for sodium dosing. Further, the electrical performance of the ESP indicated that certain stages were fouled with dust. This may also have contributed to the minimal effect observed for sodium dosing

In the event that higher boiler exit temperatures are observed after a longer campaign with the test coal, sodium dosing may well result in a more substantial reduction in emissions than was observed during these tests. Also, the short and long term benefit of sodium dosing into a clean ESP remains to be tested.

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