**5. Conclusions**

temperature and gas composition, may occur via reaction with CaO to form Ca3


diffusing through the gas to the aqueous phase of the sorbent slurry where As<sup>2</sup>

BO3

BO3

leachable, we can assume that FA containing B species such as CaHBO3

*Selenium* casually present as selenide in coal is volatilised as elemental Se0

2−), selenotrithionate (Se(SO3

of Se can remain in the flue gases and be emitted into the atmosphere.

remain as a monatomic species or may oxidise to Hg<sup>2</sup>

solve. It is speculated that some of the absorbed Hg (HgCl<sup>2</sup>

re-emitted, S (IV) being the main precursor of Hg0

FGD systems [52]. However, gaseous Hg0

by which Ca3

hydrated to H3

in the range 90–99%.

main route by which H3

boiler, gaseous SeO<sup>2</sup>

selenosulphate (SeSO3

elemental Hg (Hg0

of Hg0

As<sup>2</sup> O3 AsO<sup>4</sup>

the sprayers and dissolve to AsO<sup>4</sup>

AsO<sup>4</sup>

the chemisorption mechanism of H3

FGD gas-to-liquid contact zone, H3

aluminium silicates. Boron is generally released as H3

138 Air Pollution - Monitoring, Quantification and Removal of Gases and Particles

BO3

increase B content in the in the FGD-gypsum [49].

small fraction of FAs in the flue gas that escapes from the control would be the main route

remains in the gas phase during the post-combustion atmosphere, a proportion of gaseous

could enter the FGD either by reacting with moisture in the flue gas to form H3

and condense in the scrubber as the flue gas undergoes a rapid quench (50–60°C), or by

may differ. Generally, As is mostly partitioned in the FGD-gypsum with values comprised

*Boron* is largely organically associated in coals although a fraction can also be associated to

and HBO<sup>2</sup>

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

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

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

*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

AsO<sup>4</sup>

3− in the aqueous phase of the sorbent slurry. If As<sup>2</sup>

and HBO<sup>2</sup>

may diffuse through the gas to the aqueous phase of

2−) and selenopentathionate (Se(S<sup>2</sup>

2+ and Hg2+ compounds. The reaction

) can be converted back to Hg0

is insoluble in water and therefore does not dis-

re-emission [53–55].

can reach the FGD is with the incoming FGD flue gas. In the

. Depending on the operating FGD conditions, As partitioning and fate

BO3

can be chemisorbed on the FA surface to form the stable CaSeO3

)2

). During post-combustion, and with decreasing temperature, 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

AsO<sup>4</sup>


O3

[48] during PCC. Since

can also contribute to

. In the

O3 )2 2−)

(g) may

and

. The

and SeO<sup>2</sup>

on FAs has not been documented, the

. The

O3

AsO<sup>4</sup>

would get

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 emissions of a number of air pollutants including SO<sup>2</sup> , NOx, PM, HCl, HF, Hg and As, Be, Cd, Cr, 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 of FGD system for trace elements may be reduced because of the emission of fly dust in evaporate droplets saturated with gypsum. In view of the discussion exposed in the review, we can conclude that the emissions of inorganic trace pollutants from coal power generation depend on each facility including the flue gas depuration train as well as the operating conditions associated to it.

[7] Reference Document on Best Available Techniques for Large Combustion Plants (BREFs). [Internet]. Available from: http://eippcb.jrc.ec.europa.eu/reference/BREF/LCP/JRC107769\_

Emissions of Inorganic Trace Pollutants from Coal Power Generation

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

141

[8] American Society for Testing and Materials (ASTM). Annual book of ASTM standards. Section 5: Petroleum Products, Lubricants, and Fossil Fuels. 5.05: Gaseous Fuels: Coal

[9] World Coal Association. [Internet]. Available from: https://www.worldcoal.org/coal/

[10] US Energy Information Association. [Internet]. Available from: https://www.eia.gov/

[12] World Energy Council. [Internet]. Available from: www.worldenergy.org/data/resources/

[13] International Energy Agency (IEA). Clean Coal Centre. [Internet]. 2010. Available from: http://www.iea-coal.org.uk/site/2010/database-section/ccts/pulverised-coal-combus-

[14] Ground Truth Trekking. [Internet]. Available from: http://www.groundtruthtrekking. org/Issues/AlaskaCoal/CoalCombustionMethods.html [Accessed: 2018-06-21]

[15] TuDelft. [Internet]. Available from: https://repository.tudelft.nl/islandora/object/uuid%

[16] Co-combustion a summary of technology. [Internet]. 2007. Available from: http://www.

[17] Koffi M, da Silva RJ. Control of the SOx emissions during the petroleum coke combustion in fluidized bed. In: 19th International Congress of Mechanical Engineering; November

[18] Cenni R, Frandsen F, Gerhardt T, Spliethoff H, Hein KRG. Study on trace metal partitioning in pulverized combustion of bituminous coal and dry sewage sludge. Waste

[19] Bartoňová L, Klika Z, Milčák P. Determination of organic carbon content in ash from

[20] Leckner B. Co-combustion—A summary of technology. Thermal Science. 2007;**11**(4):5-40 [21] Stangera R, Walla T, Spörl R, Paneru M, Grathwohl S, Weidmann M, Scheffknecht G,

[22] Córdoba P, Diego R. Mass balance and partitioning of trace elements under oxy-coal

capture in power plants. International

fluidised-bed coal combustion. Odpadové Fórum. 2008. pp. 22-24

[11] Sxcoal. [Internet]. Available from: http://www.sxcoal.com/news/4567443/info/en

LCP\_bref2017.pdf [Accessed: 2018-05-010]

uses-coal/coal-electricity [Accessed: 2018-06-22]

resource/coal/ [Accessed: 2018-06-17]

3Ac2b16256-45f1-419c-854a-7b24ad29d770

energy-pathways.org/pdf/R5\_co-combustion.pdf

McDonald D, et al. Oxyfuel combustion for CO<sup>2</sup>

Journal of Greenhouse Gas Control. 2015;**40**:55-125

combustion: First experiences. Fuel. 2018;**215**:892-903

tion-pcc [Accessed: 2018-06-17]

5-9, 2007; Brasília, DF

Management. 1998;**18**:433-444

todayinenergy/detail.php?id=34992 [Accessed: 2018-06-22]

and Coke; 1991
