REPOSITORY > RESULTS

Abstract

Mercury is present as a trace pollutant in coal. During coal burning, it is volatized and emitted through the stack gases into the atmosphere, where it becomes a subject of long-range transport. With respect to Minamata convention, it is necessary to reduce those emissions by the installation of different air pollution control devices (APCD), such as electrostatic precipitators (ESP), selective catalytic reduction (SCR) and wet flue gas desulphurization (WFGD). WFGD system with forced oxidation is very efficient in SO2 removal and therefore the most commonly used technology in modern coal-fired power plants. It is known that in these systems elemental mercury, Hg(0), is not retained due to its low solubility in water, whereas the oxidized mercury is almost completely dissolved and accumulates in the WFGD scrubber. The biggest power plant in Slovenia, Šoštanj thermal power plant (Šoštanj TPP), burns lignite which is extracted from the nearby Velenje Coal Mine. In WFGD scrubber of Šoštanj TPP all forms of mercury effectively retain, with an average of 89% (99% for oxidized mercury, 98% for particulate mercury and 76% for Hg(0)). The removal of mercury from the solution by the reactions with the solids produced during desulfurization process represents a significant part of the chemistry. As a result, mercury concentration is elevated in the resulting gypsum.
Flue gas containing volatile elements, fine fly ash particulates not retained by particle control devices, and limestone are the most important sources of trace and major elements (TMEs) in WFGD gypsum. In this study, samples of gypsum slurry from block 4, 5 and 6 of Šoštanj TPP were separated into fine and coarse fractions. Since the separation procedure may significantly change the concentration of Hg in the samples, in the present doctoral dissertation, the differences in particle size distribution and the concentrations of TMEs in gypsum fractions after separation were studied. Multi-elemental analysis of 45 elements in the different size fractions of gypsum, slurry waters and lignite were performed by k0-INAA (instrumental neutron activation analyses). The study found that the volatile elements (Hg, Se and halogens) in the flue gas accumulate in the fine fractions of gypsum. Moreover, the concentrations of most TMEs are considerably higher in the fine fractions compared to the coarse fractions. The exceptions are Ca and Sr that primarily originate from the limestone. Variations of TMEs in the fine fractions are dependent on the presence of CaSO4·2H2O that is the main constituent of the coarse fraction. Consequently, the content of TMEs in the fine fraction is highly dependent on the efficiency of separating the fine fraction from the coarse fraction.
In addition to characterization of the gypsum samples, the main aim of this study was the identification of mercury compounds in gypsum samples. Temperature-programmed desorption/decomposition (TPD) technique with an atomic absorption spectrometric (AAS) detection is frequently used for determination of stability of mercury compounds present in solid substrates and different solid samples. Because the sensitivity of TPD method with AAS detection was not sufficient to detect low concentrations of Hg in coarse solid gypsum fractions, we turned to mass spectrometry. We developed a new method using mass spectrometer for studying mercury decomposition reaction and stability of mercury species in the solid sample at higher temperatures. In particular, the method was calibrated by using self-prepared mercury compounds. Identification of mercury compounds was performed by using actual gypsum samples. We hypothesise that better sensitivity and the ability to measure other volatile species and their fractions would allow us to identify the Hg species present in the gypsum samples.
Two different mass spectrometers were used, a magnetic sector and quadrupole mass spectrometer (QMS). Samples were heated and ions were produced by electron ionization. Results were recorded as thermograms (TPD spectra), representing the relative intensity/count per second of Hg concentration versus temperature. Two mercury isotopes were simultaneously measured, 200Hg and 202Hg. Important advantage compared to AAS is the ability to observe other volatile compounds (gases), not only Hg. However, we were not able to study the decomposition of mercury compounds in all gypsum samples, and predict the exact desorption/decomposition reactions for Hg-species present. This was possible only in case of pure mercury species such as HgS mixed with SiO2 or CaSO4·2H2O and in the samples with an elevated concentration of mercury. However, in actual samples high uncertainty exists since volatile compounds are present in much higher concentrations compared to Hg. Big advantage of QMS is also the possibility for continuous detection of evaporated Hg(0) at very small concentration, even at room temperature. Due to the differences in peak positions, we can distinct between weakly adsorbed mercury (Hg(0) adsorbed to the surface) and mercury adsorbed (bounded) within the sample matrix.
TPD method using mass spectrometer has been applied on gypsum samples from block 4, 5 and 6 from the lignite burning Šoštanj TPP. We examined the effect of particle size on mercury accumulation in WFGD gypsum. The mercury concentration in different gypsum samples varies from 0.22 mg kg-1 (coarse particles) to 20.6 mg kg-1 (fine particles). For comparison, samples from ten Chinese TPPs were analysed as well. Since the gypsum represents the mixture of finer and coarse fraction, so-called bulk gypsum, the concentration is significantly lower (0.04 – 1.66 mg kg-1). All the samples from block 6 have a similar, symmetrical, single-peak in the TPD spectra. On the contrary, gypsum fractions from block 5 show at least two peaks that differ from those from block 6. Block 4 has similar TPD spectra to samples from block 6, but the concentrations are much lower and do not exceed 4.05 mg kg-1 in a fine fraction. Chinese TPD spectra are similar to samples from block 4 and 6, and normally contain one peak. However, due to the presence of a lower Hg concentration in the sample, few TPD mass spectra have a high background that distorted the peaks. Decreasing the background and better sensitivity were also important issues in the present work.
Different mercury standards were prepared to identify mercury species present in various gypsum samples. The use of ‘wet’ methods for preparing mercury compounds is introduced in addition to the mercury standards prepared using the ‘dry’ method, as commonly found in the TPD approach. The study showed that selected metals, such as Fe enriched in gypsum samples, could significantly influence the shape and the maximum temperature of the Hg TPD curves and that during mercury compound preparation and the TPD process, Hg species undergo transformations that prevent the identification of their original identity. Moreover, the sample might change over time and part of Hg-compounds can be changed into other less stable mercury species or Hg(0). Literature data suggest HgCl2 and HgS as the most common mercury compounds present in the gypsum. In the present doctoral dissertation, the inability to detect directly low concentrations of HgCl2 in gypsum by mass spectrometry is discussed in detail. The hydrolysis of HgCl2 vapours under specific experimental conditions in the mass spectrometer was considered theoretically and proved experimentally. Also HgS has not been proved in gypsum samples of Šoštanj TPP when dry mercury standards were used for comparison. However, H2S and SO2 were found as a volatile species released simultaneously with Hg (as found with pure HgS mixed with CaSO4·2H2O) and thus we proposed the presence of HgS in the samples. Finally, X-ray Absorption Spectroscopy (XAS) method indicates the presence of Hg-S and Hg-O as a species present in the gypsum samples RECY from block 6.