Mercury analysis in coal, ash and sorbents
Validation studies confirmed that the CV-AAS method using mercury analyser MA-3000 (Nippon Instruments Corporation) is accurate for coal samples with mercury content ranging from 25 to 600 μg kg−1 in air dried basis. The detection limit is 0.07 ng, and the quantification limit is 0.20 ng. The method is highly linear (r = 0.998), and uncertainty of results at 95% confidence level ranges from 3 to 10%, depending on measurement. The CV-ASS method has acceptable repeatability and reproducibility in the whole measurement range.
In other materials, including ashes and organic sorbents, the CV-AAS method was accurate for mercury content ranging from 5 to 1200 μg kg−1 with relative uncertainty from 2 to 20%. The highest values of relative uncertainty were noted for ash samples and AC, CD and LDC samples with mercury content below 10 μg kg−1.
LD contained 68.3 μg Hg kg−1, and its char (LDC) only 3.3 μg kg−1. Likewise, CD and AC, which are products of the coal carbonisation process, contained relatively small quantities of mercury: 10.5 and 5.4 μg kg−1, respectively (Table 3). CD, LDC and AC are materials obtained in the carbonisation process. Therefore, they also contain minimal quantities of volatiles: approximately 3 wt% for CD and LDC and approximately 15% for AC while LD contained close to 50%. Commercial activated carbon (AC) had more than 2.5 times higher ash content than other examined sorbents. Sulphur content in AC of 2.11 wt% was more than double than in the other sorbents. RC and RGC have high mercury content, at 158 and 73 μg kg−1, high sulphur content (2.26 and 2.45 wt%) and ash (19.4 and 23.2%), for RC and RGC, respectively.
Coke and lignite dust are macroporous materials with a moderately developed mesoporous and poor microporous structure (Table 4). Specific surface (SBET) of CD, LD and LDC amounts to several dozen square meters per·gram, but in AC this parameter is 670.5 m2 g−1. Due to these differences in structure, the equilibrium sorptive capacity of AC is more than 50 times higher for mercury than in those of the three other sorbents (Table 4). The specific surface area of rubber waste chars was approximately 70 m2 g−1.
Properties of sub-bituminous coal and lignite
Sub-bituminous coal contained an average of 103.7 μg kg−1 of mercury, while average mercury content in lignite was 443.5 μg kg−1 (Table 5). The average content of chlorine, which is a supporting factor in oxidation of mercury from Hg0 to Hg2+ in analysed sub-bituminous coal was equal to 0.3%, while lignite contained nearly seven times less chlorine. The average sulphur content for coal and lignite was 0.6 and 1.8 wt%. Lignite contained more than three times more ash, twice as many volatiles and ten times more moisture than sub-bituminous coal.
There was a noticeable amount of iron in lignite (18.5 wt%.), a factor proven to catalyse oxidation of mercury, whereas the iron content in sub-bituminous coal was equal to 4.8 wt% (Table 6). The content of calcium, which competes with mercury in binding of chlorine accounted for 2.3 wt% for sub-bituminous coal and 14.3 wt% for lignite (Table 6).
Assessment of the effectiveness of mercury sorption during combustion of lignite and sub-bituminous coals
Table 7 shows mercury sorption values on tested sorbents and effectiveness of mercury removal from flue gas. Commercial activated carbon, currently used in active flue gas mercury removal methods, was the most efficient, removing the mercury almost entirely. CD also shows high mercury sorption efficiency from both sub-bituminous coal and lignite—at 80 and 63%, respectively. Observations have shown lignite dust to be the worst sorptive material during coal combustion, decreasing the concentration of mercury in flue gas by only 30%, and by 23% in the combustion of lignite. LDC was more efficient in mercury removal than LD.
In the combustion process of lignite with mercury content of 443.5 μg kg−1, raw flue gas concentration of mercury amounted to 17.6 μg m−3—more than three times higher than the concentration of mercury in flue gas from combustion of sub-bituminous coals (Fig. 2).
The high concentration of mercury in flue gas from combustion of lignite is due to its high initial mercury content and low chlorine content (0.05 wt%, for sub-bituminous coal—3.3 wt%).
Most of the mercury from the combustion of sub-bituminous coal and lignite was removed by AC sorbent. After completing the process of coal combustion, the mercury content in sorbent was 96.3 and 356.1 μg kg−1, respectively for combustion of sub-bituminous coal and lignite (Table 7). The concentration of mercury in flue gas from coal and lignite were reduced to a value of 0.4 and 3.5 μg kg−1 (see Fig. 2).
AHg for CD was 83.2 and 277.0 μg Hg kg−1, which enabled a reduction of mercury concentration in flue gas to a value of 0.8 μg m−3 for coal and 6.9 μg m−3 for lignite. A comparable value of AHg was obtained for LDC, as mercury content determined in the flue did not exceed 91.3 for sub-bituminous coal and 221.8 μg kg−1 for lignite. The result was a reduction in the concentration of mercury in flue gas to 0.4 and 8.6 μg m−3.
Application of LD sorbent resulted in poor mercury removal. AHg for the combustion of coal was 31.7 and 101.0 μg kg−1 for lignite, resulting in 1.35 times the reduction of atmospheric mercury emission (see Table 7).
RC and RGC sorbents provided better mercury removal during combustion of lignite, where mercury flue gas concentration after using RC and RGC was 2.7 and 2.4 μg kg−1, respectively (see Fig. 2). Each of the sorbents used adsorbed more mercury from flue gases from lignite combustion than from sub-bituminous coal. As already mentioned, such adsorption was the consequence of mercury concentration that was three times higher in lignite than in sub-bituminous coal.
The best sorbent was AC, removing 92.9% of mercury from sub-bituminous coal and 80.3% of mercury from lignite flue gas (Table 7). The poorest results were obtained with LD sorbent, capturing up to 30.6 and 22.8% of the mercury in an equivalent experiment. Results comparable to MR were obtained for CD and LDC. In sub-bituminous coal flue gas, CD and LDC removed 80.3 and 88.1% of mercury. In flue gas from lignite, this parameter reached approximately 62.5 and 50.1%.
All tested sorbents were more effective in removing mercury from flue gas from sub-bituminous coal (30.6 to 92.9%) than from lignite (MR from 22.8 to 80.3%). This was most likely due to a larger share of the more easily adsorbed Hg2+ during coal combustion, whereas lignite flue gas had a much greater content of the difficult to adsorb Hg0.
A greater share of Hg2+ in flue gas from sub-bituminous coal was the consequence of a greater content of chlorine and lower amount of calcium in coal than in lignite. This resulted in different mercury oxidation potentials during both combustion processes.
RC and RGC proved to be highly capable of mercury sorption from coal combustion flue gas (79%, 81%), showing even better efficiency in the removal of mercury during combustion of lignite (85 and 87%). The concentration of mercury in emitted flue gas, with RC and RGC sorbents decreased by more than 80% (see Fig. 3).
Effect of sorbent particle size on mercury sorption ability
To investigate the effect of particle size on the effectiveness of mercury removal, sorbent CD was screened to form the following fractions: (i) > 0.2 mm, (ii) < 0.2 mm, and (iii) < 0.063 mm. The properties of individual grain fractions are shown in Table 8. The mercury content in individual size fractions differed only slightly. The finer grains contained slightly more ash and volatiles, and less carbon. The < 0.063-mm fraction contained 14.8 μg Hg kg−1, while the > 0.2-mm fraction—11.2 μg kg−1.
Significant differences in sulphur content in individual fractions were also observed: from 0.65% in the coarsest to more than 1% in the finest.
The finer the CD fraction, the better the mercury adsorption (AHg) and effectiveness of removal of mercury from flue gas (Table 9). A particularly strong impact of particle size on mercury adsorption was found in sorption of flue gas from lignite. The finest fraction of CD adsorbed 243–277 μg Hg kg−1 CD, while the coarsest fraction adsorbed only 95 μg Hg kg−1.
Observed regularity was reported by other researchers for other sorbents, i.e. activated carbons (McKay et al. 2015). Authors have demonstrated that a stepwise decrease of activated carbon particle size produced resulted in an increase in mercury adsorption (Kadirvelu et al. 2004).
The < 0.063-mm fraction of CD removed almost 92% of mercury from the flue gas of sub-bituminous coal, reducing mercury concentration from 5.3 to 0.4 μg Hg m−3. The same fraction of CD removed 93% of mercury from lignite flue gas, reducing the concentration of mercury in flue gas from 17.6 to 1.2 μg Hg m−3 (Fig. 3).
Influence of photochemical oxidation on gas phase mercury
After physical oxidation of mercury in the flue gas, the sorption of mercury from the flue gas from sub-bituminous coal and lignite was conducted with the use of LD and LDC (only for lignite emissions; however, because LDC reduced the concentration of mercury in flue gas of sub-bituminous coal to 0.4 μg kg−1 without photochemical oxidation of Hg, so it was not used in this experiment for this fuel). The results obtained were better than in trials carried out without UV radiation (see Fig. 4 and Fig. 5).
The method of photochemical oxidation of Hg0 to Hg2+, most likely by the mechanism described in Liu et al. (2014) allowed for 1.5- to 2-fold increase in the quantity of mercury adsorbed. LD during combustion of coal and lignite decreased the concentration of mercury in flue gas by 30 and 23%, respectively. After physical oxidation of Hg in the flue gas, this effectiveness increased to 60 and 50% (Fig. 4). A similar effect was obtained for LDC sorbent—after photochemical oxidation of mercury, the efficiency of mercury removal from flue gas from the combustion of sub-bituminous coal increased from 40 to 60%. (Fig. 5).