Antti Heikkilä, from Gasmet Technologies, explains how a new continuous mercury monitoring system provides cement process operators with an opportunity to improve environmental performance and demonstrate compliance with legislation.

The production of cement klinker and lime in rotary kilns is responsible for 10.7% of mercury emissions to air, which has resulted in steps being taken in several countries to impose emission limits. In the European Union BREF guidance for cement kilns (CLM BREF), mercury has a BAT-associated emission level of 0.05 mg/Nm3 for the half-hour average.

Gasmet Technologies has introduced a continuous mercury emission monitoring system (CMM) based on the cold vapour atomic fluorescence (CVAF) measurement principle. The analyser is integrated into a cabinet with a vacuum pump, an automatic calibrator and a nitrogen gas generator. The sample gas is extracted from the process duct with a dilution probe and heated sample line, specially designed for sampling mercury from process conditions.

The main source of measurement uncertainty that needs to be addressed by the analyser and the system design is the quenching effect – where other gases present in the sample, such as O2 and H2O, lower the fluorescence signal due to mercury atoms. In order to avoid these adverse effects, a dilution sampling approach is used and the dilution gas is synthetic nitrogen formed in a nitrogen generator inside the analyser cabinet. As the detection limit of the analyser is much lower than would be needed to monitor mercury in low µg/Nm3 ranges, dilution does not compromise the sensitivity of the instrument. On the other hand, dilution lowers the quenching effect by lowering the concentration of interfering gases by a factor of 50. Measuring mercury in a gas consisting of 98% nitrogen guarantees consistent measurement regardless of the fuel or emission abatement techniques.

The CVAF spectrometer measures atomic mercury vapour with a thermal catalytic converter which is used to convert all forms of mercury into atomic mercury.

The system has been field tested on various types of industrial plants to characterise the suitability and long-term stability of the sample probe and dilution system in various processes. Given the reactive nature of mercury, special care has been taken to ensure that mercury in the flue gas is not absorbed into dust accumulating in the sample probe filters. Mercury reacts readily with limestone dust, resulting in analyte loss and increased response time of the analyser.

The Gasmet CMM solution includes a smaller filter element, which minimises the amount of dust deposition on the filter, and a two-stage blowback mechanism which first removes dust from the filter element and then in the second stage expels the dust from the probe tube back into the process.

The CMM was installed on the emission stack of a rotary kiln cement plant with an electrostatic precipitator (ESP) for particulate emission control. The test period lasted 30 days with a variety of fuels being used. The raw mill was periodically stopped and the variation in mercury levels monitored together with changes in other process parameters. Average mercury concentration when the mill was running was 6 to 8 µg/Nm3 and when it was stopped, the concentrations could increase to between 20 and 40 µg/Nm3.

Test goals

The main goal of the test was to ensure the stability and repeatability of mercury measurement in demanding process conditions and to determine whether cement dust causes analyte loss and increased response time in the sample extraction probe.

The only process variable that clearly correlates with mercury concentration is the raw mill on/off state. When the raw mill is on, the variation in dust loading or other gas concentrations does not correlate with variation observed in mercury concentration. When the raw mill is switched off, all gases undergo a change in concentration, but this is clearly brought about by the raw mill state.

To estimate the repeatability of the Hg measurement at zero and span levels, the CMM analyser was configured to perform zero tests with synthetic nitrogen and span tests with Hg0 test gas generated by the mercury calibrator in the CMM system at four-hourly intervals. All test gases are injected into the probe upstream of particle filters so that the test gas has to pass through potentially contaminated filters.

The target level for the span check was 6.5 µg/Nm3 and the average span level was 6.60±0.036 µg/Nm3. The average result for the zero check was -0.006 ± 0.036964 µg/Nm3. If the dust accumulating in the sample extraction probe were to cause analyte loss during span tests, the later tests would show a decrease from the span check target value, but this was not observed. If the dust in the probe were to make the response time longer, the later tests would show a slower response than the first tests. Again, there was no systematic change in the test results and the tests 1-6 exhibited consistent results. The span and zero checks also provided an opportunity to characterise the response time of the analyser when the span test at a known concentration is followed by a zero check with a zero concentration.

The data from all six tests were combined into one dataset by synchronising the moment when the span/zero check cycle was started. Response times were evaluated as T90-10, the time interval between a reading representing 90% of the span check value and a reading representing 10% of the span check value. The response time from this calculation was 10.15 minutes or just over two measurement cycles (measurement data is obtained as five minute rolling averages of the mercury concentration). The live data from the emissions shows peaks of comparable sharpness, but these were not subjected to the same analysis as the span/zero check data.