**4. Chemistry, partitioning and fate of inorganic trace pollutants during PCC-FGD**

According to the foregoing discussion, trace elements during combustion may get concentrated on the coarse residues BS or BA, partition equally between BS or BA and FA particulates, emitted with a different form of occurrence (e.g. from sulphide in coal to oxides and chlorides in flue gas), and/or condense onto the surface of smaller particles in flue-gas streams. Either way, most of trace metals are retained in particulate control devices and only specific high volatile metals may escape from ESP and reach FGD systems in a gaseous mode of occurrence.

In the FGD systems, under operational conditions of water re-circulation, inorganic trace pollutants in FGD waters may reach equilibrium and a subsequent saturation in the water stream after a number of water re-circulations in the scrubber. The gradual increase in the concentration of inorganic trace pollutants from the sub-saturation to equilibrium and/or saturation because of continuous water re-circulation in the scrubber, accounts for enriched inorganic trace pollutants in the re-circulated water. Other elements retained in high proportions by gypsum sludge and/or FGD-gypsum do not pose this problem because they are extracted from the system by the gypsum by-product that is used for different applications or for landfilling [4]. The general trends of the inorganic trace pollutants in through the PCC to FGD are reported below.

*Arsenic* is present as As-sulphides species in raw coals and it is mostly released as As<sup>2</sup> O3 (g) [47] during PCC. In the boiler, gaseous As<sup>2</sup> O3 can be chemisorbed on the FA surface and/ or remain in the gas phase. The chemisorption of As<sup>2</sup> O3 on FAs, which will depend on the temperature and gas composition, may occur via reaction with CaO to form Ca3 AsO<sup>4</sup> . The small fraction of FAs in the flue gas that escapes from the control would be the main route by which Ca3 AsO<sup>4</sup> -FA would enter the FGD. Accordingly, Ca3 AsO<sup>4</sup> -FA would pass through the sprayers and dissolve to AsO<sup>4</sup> 3− in the aqueous phase of the sorbent slurry. If As<sup>2</sup> O3 remains in the gas phase during the post-combustion atmosphere, a proportion of gaseous As<sup>2</sup> O3 could enter the FGD either by reacting with moisture in the flue gas to form H3 AsO<sup>4</sup> and condense in the scrubber as the flue gas undergoes a rapid quench (50–60°C), or by diffusing through the gas to the aqueous phase of the sorbent slurry where As<sup>2</sup> O3 would get hydrated to H3 AsO<sup>4</sup> . Depending on the operating FGD conditions, As partitioning and fate may differ. Generally, As is mostly partitioned in the FGD-gypsum with values comprised in the range 90–99%.

Coal *chlorine* is released primarily as HCl during combustion in the high temperature zone of a boiler [56]; as the combustion gases cool (430–475°C), a proportion of HCl can partially be oxi-

In the FGD gas-to-liquid contact zone, HCl may diffuse through the gas to the aqueous phase of the sorbent slurry when flue gas passes through the sprayers and react with cations such as

*Fluorine* is released as HF during combustion. The main route by which gaseous HF can reach the FGD is with the incoming FGD flue gas. Depending on the operating FGD conditions, F partitioning and fate may differ. HF contained in the flue gas may be dissolved in the aqueous phase of the sprayed droplets of limestone slurry, if a limestone-based FGD, giving rise to the

These compounds can coat the surfaces of the limestone particles and consequently cause a decrease the reactivity of limestone [58, 59]. In addition, if F is partitioned in the FGDgypsum, it can play a crucial role in the leaching potential of the FGD gypsum end-product as a consequence of the precipitation of F solid species on FGD-gypsum surface. In order to avoid this, the use of additives has been proposed as measure for the optimisation of the SO<sup>2</sup>

*Heavy metals such Zn, Cu, Cr, Ni, Mn and U* tend to form highly soluble aqueous complexes

fer. Acidic conditions, for example, contribute to the stabilisation of metals in solution, therefore, in the aqueous effluent. As a result, very low amounts of heavy metals are found in the leachates of the FGD-gypsum. Alkaline conditions, however, contribute to the precipitation of

Coal is currently a target to accomplish with the Paris climate agreement for both countries and companies. As a consequence, in 2016, world coal production fell by 6.2%, the largest decline on record. However, coal is the world's most abundant energy resource, meaning that despite the decline in coal production and consumption, coal is and will be a reliable source for power generation. The most negative consequence of coal combustion is the emis-

Pb, Mn, Ni, and Se, integral components of fine PM. A number of studies have shown that FGD chemistry allows also the capture of many pollutants other than S, such as F, As, B, Cl, Se or Hg both in a gaseous form and/or as PM. Most of specialised literature reports that most of trace elements in FGD systems are removed in the aqueous effluent (filtered water) and only a fraction of a few remain in the flue gases (such as B, Hg, and Se). According to these studies, it can be concluded that wet limestone FGD systems reach high retention efficiencies for trace elements (>90%). However, it is also important to note that the retention efficiency

2−. Depending on the operating FGD conditions, their partitioning and fate may dif-

aqueous effluent (filtered water) and only a fraction may remain in the flue gases.

with limestone to form Al-F compounds, typically represented by CaAlF3

removal efficiency and for reducing the precipitation of F solid species, respectively.

. The main route by which HCl can reach the FGD is with the incoming FGD flue gas.

Emissions of Inorganic Trace Pollutants from Coal Power Generation

http://dx.doi.org/10.5772/intechopen.79918

, Mg2+, etc. to form highly soluble salts. Generally, Cl in FGD systems is removed in the

. However, in the presence of aluminium compounds, HF may also react

(OH)<sup>2</sup>

, NOx, PM, HCl, HF, Hg and As, Be, Cd, Cr,


[57].

139

dised to Cl<sup>2</sup>

Ca2+, Na<sup>+</sup>

with SO<sup>4</sup>

**5. Conclusions**

these elements in the FGD-gypsum.

sions of a number of air pollutants including SO<sup>2</sup>

formation of CaF<sup>2</sup>

*Boron* is largely organically associated in coals although a fraction can also be associated to aluminium silicates. Boron is generally released as H3 BO3 and HBO<sup>2</sup> [48] during PCC. Since the chemisorption mechanism of H3 BO3 and HBO<sup>2</sup> on FAs has not been documented, the main route by which H3 BO3 can reach the FGD is with the incoming FGD flue gas. In the FGD gas-to-liquid contact zone, H3 BO3 may diffuse through the gas to the aqueous phase of the sorbent slurry when flue gas passes through the sprayers and remain in the FGD reaction tank. Depending on the operating FGD conditions, B partitioning and fate may differ but in general, B is removed in the aqueous effluent (filtered water) and only a fraction of B may remain in the flue gases. However, since B concentration in FAs is relatively high and largely leachable, we can assume that FA containing B species such as CaHBO3 can also contribute to increase B content in the in the FGD-gypsum [49].

*Selenium* casually present as selenide in coal is volatilised as elemental Se0 and SeO<sup>2</sup> . In the boiler, gaseous SeO<sup>2</sup> can be chemisorbed on the FA surface to form the stable CaSeO3 . The main route by which Se would reach the FGD is the small fraction of FAs escapes from the control and reaches the FGD. In such a case, Se chemisorbed in FAs could dissolve in the aqueous phase of the lime slurry to form an array of aqueous Se-complexes such as selenosulphate (SeSO3 2−), selenotrithionate (Se(SO3 )2 2−) and selenopentathionate (Se(S<sup>2</sup> O3 )2 2−) by reaction with polyoxysulphur donors [50]. Selenium can either be removed in the aqueous effluent (filtered water) or partitioned in the FGD-gypsum. However, a small fraction of Se can remain in the flue gases and be emitted into the atmosphere.

*Mercury* occurs in coals in mineral sulphide impurities, although other forms of occurrence, such as Hg-Se species, have been described [51]. During combustion, Hg is released as elemental Hg (Hg0 ). During post-combustion, and with decreasing temperature, Hg0 (g) may remain as a monatomic species or may oxidise to Hg<sup>2</sup> 2+ and Hg2+ compounds. The reaction of Hg0 (g) with HCl(g) or Cl2(g) to form HgCl2(g) is generally considered to be the dominant Hg transformation mechanism in coal combustion flue gas [51]. The main route by which gaseous Hg can reach the FGD is with the incoming FGD flue gas. Gaseous compounds of Hg2+ are generally water-soluble and can dissolve in the aqueous phase of the sorbent slurry of wet FGD systems [52]. However, gaseous Hg0 is insoluble in water and therefore does not dissolve. It is speculated that some of the absorbed Hg (HgCl<sup>2</sup> ) can be converted back to Hg0 and re-emitted, S (IV) being the main precursor of Hg0 re-emission [53–55].

Coal *chlorine* is released primarily as HCl during combustion in the high temperature zone of a boiler [56]; as the combustion gases cool (430–475°C), a proportion of HCl can partially be oxidised to Cl<sup>2</sup> . The main route by which HCl can reach the FGD is with the incoming FGD flue gas. In the FGD gas-to-liquid contact zone, HCl may diffuse through the gas to the aqueous phase of the sorbent slurry when flue gas passes through the sprayers and react with cations such as Ca2+, Na<sup>+</sup> , Mg2+, etc. to form highly soluble salts. Generally, Cl in FGD systems is removed in the aqueous effluent (filtered water) and only a fraction may remain in the flue gases.

*Fluorine* is released as HF during combustion. The main route by which gaseous HF can reach the FGD is with the incoming FGD flue gas. Depending on the operating FGD conditions, F partitioning and fate may differ. HF contained in the flue gas may be dissolved in the aqueous phase of the sprayed droplets of limestone slurry, if a limestone-based FGD, giving rise to the formation of CaF<sup>2</sup> . However, in the presence of aluminium compounds, HF may also react with limestone to form Al-F compounds, typically represented by CaAlF3 (OH)<sup>2</sup> -CaF<sup>2</sup> [57]. These compounds can coat the surfaces of the limestone particles and consequently cause a decrease the reactivity of limestone [58, 59]. In addition, if F is partitioned in the FGDgypsum, it can play a crucial role in the leaching potential of the FGD gypsum end-product as a consequence of the precipitation of F solid species on FGD-gypsum surface. In order to avoid this, the use of additives has been proposed as measure for the optimisation of the SO<sup>2</sup> removal efficiency and for reducing the precipitation of F solid species, respectively.

*Heavy metals such Zn, Cu, Cr, Ni, Mn and U* tend to form highly soluble aqueous complexes with SO<sup>4</sup> 2−. Depending on the operating FGD conditions, their partitioning and fate may differ. Acidic conditions, for example, contribute to the stabilisation of metals in solution, therefore, in the aqueous effluent. As a result, very low amounts of heavy metals are found in the leachates of the FGD-gypsum. Alkaline conditions, however, contribute to the precipitation of these elements in the FGD-gypsum.
