Fluidised bed combustion provides numerous advantages in comparison with predominant combustion systems, such as high combustion efficiency, reduced coal crushing, ability to use low-grade coals and alternative fuels (wastes, biomass, sewage sludge) [1Raclavská, H.; Juchelková, D.V.; Roubíček, V.; Matysek, D. Energy utilization of biowaste-sunflower seed hulls for co-firing with coal. Fuel Process. Technol., 2011, 92, 13-20.
[http://dx.doi.org/10.1016/j.fuproc.2010.03.006] , 2Raclavská, H.; Juchelková, D.; Škrobánková, H.; Wiltowski, T.; Campen, A. Conditions for energy generation as an alternative approach to compost utilization. Environ. Technol., 2011, 32(3-4), 407-417.
[http://dx.doi.org/10.1080/09593330.2010.501089] [PMID: 21780708] ], etc. One of the most important advantages is low combustion temperature (below ash fusion temperature) facilitating easier ash removal and significantly reducing air pollution. Moreover, due to ground limestone/dolomite / CaO added during the combustion, the efficient desulphurization of emissions could be achieved.

The total amount of emissions from coal combustion power station is a complex phenomenon and depends on both operating conditions and the characteristics of the coal (e.g. trace elements in the coal) [3Wagner, N.J.; Hlatshwayo, B. The occurrence of potentially hazardous trace elements in five Highveld coals, South Africa. Int. J. Coal Geol., 2005, 63, 228-246.
[http://dx.doi.org/10.1016/j.coal.2005.02.014] , 4Wagner, N.J.; Tlotleng, M.T. Distribution of selected trace elements in density fractionated Waterberg coals from South Africa. Int. J. Coal Geol., 2012, 94, 225-237.
[http://dx.doi.org/10.1016/j.coal.2012.01.005] ]. Despite the progress in flue-gas cleaning technologies, fluidised bed combustion is still accompanied by emissions of numerous pollutants, such as heavy metals, toxic trace elements, or organic compounds etc. [5Klika, Z.; Bartoňová, L.; Spears, D.A. Effect of boiler output on trace element partitioning during coal combustion in two fluidised-bed power stations. Fuel, 2001, 80, 907-917.
[http://dx.doi.org/10.1016/S0016-2361(00)00164-2] , 6Wagner, N.J. The characterization of weathered discard coals and their behaviour during combustion. Fuel, 2008, 87, 1687-1697.
[http://dx.doi.org/10.1016/j.fuel.2007.09.009] ]. As the retention efficiency of fabric filters and electrostatic precipitators is relatively high, the capture of gaseous fractions of volatile elements, such as Hg, As, Cl, Br, etc [7Germani, M.S.; Zoller, W.H. Vapor-phase concentrations of arsenic, selenium, bromine, iodine, and mercury in the stack of a coal-fired power plant. Environ. Sci. Technol., 1988, 22(9), 1079-1085.
[http://dx.doi.org/10.1021/es00174a013] [PMID: 22148663] , 8Meij, R.; Janssen, L.H.J.M.; Van Der Kooij, J. Air pollutant emissions from coal-fired power stations. Kema Scientific & Technical Reports, 1986, 4, 51-69.] is more problematic (volatilized elements in gaseous form can easily pass through the particulate control devise).

To solve this problem, there is an effort to capture the volatile elements from flue gas onto solid adsorbent. Activated carbon is one of the most effective adsorbents for the retention of Hg [9Scala, F.; Chirone, R.; Lancia, A. Elemental mercury vapor capture by powdered activated carbon in a fluidised bed reactor. Fuel, 2011, 90, 2077-2082.
[http://dx.doi.org/10.1016/j.fuel.2011.02.042] , 10Vidic, R.D.; McLaughlin, J.B. Uptake of elemental mercury vapors by activated carbons. J. Air Waste Manag. Assoc., 1996, 46(3), 241-250.
[http://dx.doi.org/10.1080/10473289.1996.10467458] [PMID: 28065140] ] and it was tested for the retention of As and Se [11López-Antón, M.A.; Díaz-Somoano, M.; Fierro, J.L.G.; Martínez-Tarazona, M.R. Retention of arsenic and selenium compounds present in coal combustion and gasification flue gases using activated carbons. Fuel Process. Technol., 2007, 88, 799-805.
[http://dx.doi.org/10.1016/j.fuproc.2007.03.005] ] as well. However, to reduce the cost, cheaper alternatives are also being sought. Unburned carbon (in fly ash) is one of the most promising alternatives [12Hower, J.; Thomas, G.A.; Clifford, D.S.; Eady, J.D. Petrography and Chemistry of High-Carbon Fly Ash form the Shawnee Power Station, Kentucky. Energy Sources, 1996, 18, 107118.
[http://dx.doi.org/10.1080/00908319608908751] -14Raclavská, H.; Hlavsová, A.; Juchelková, D.; Zajonc, O. The geochemical properties of carbon black from pyrolysis of municipal waste. Inzynieria Mineralna, 2013, 1, 19-27.] and was studied in terms of possible flue gas cleaning from Hg [15Hower, J.C.; Maroto-Valer, M.; Taulbee, D.N.; Sakulpitakphon, T. Mercury capture by distinct fly ash carbon forms. Energy Fuels, 2000, 14, 224-226.
[http://dx.doi.org/10.1021/ef990192n] -17Hower, J.C.; Senior, C.L.; Suuberg, E.M.; Hurt, R.H.; Wilcox, J.L.; Olson, E.S. Mercury capture by native fly ash carbons in coal-fired power plants. Pror. Energy Combust. Sci., 2010, 36(4), 510-529.
[http://dx.doi.org/10.1016/j.pecs.2009.12.003] [PMID: 24223466] ] and some other trace elements [18Bartoňová, L. Unburned carbon from coal combustion: An overview. Fuel Process. Technol., 2015, 134, 136-158.
[http://dx.doi.org/10.1016/j.fuproc.2015.01.028] -21Bartoňová, L.; Klika, Z.; Spears, D.A. Characterization of unburned carbon from ash after bituminous coal and lignite combustion in CFBs. Fuel, 2007, 86, 455-463.
[http://dx.doi.org/10.1016/j.fuel.2006.07.024] ]. The major advantage of unburned carbon is its porosity and higher specific surface area [18Bartoňová, L. Unburned carbon from coal combustion: An overview. Fuel Process. Technol., 2015, 134, 136-158.
[http://dx.doi.org/10.1016/j.fuproc.2015.01.028] , 22Hower, J.C.; Groppo, J.G.; Graham, U.M.; Ward, C.R.; Kostova, I.J.; Maroto-Valer, M.M.; Dai, S. Coalderived unburned carbons in fly ash: A review. Int. J. Coal Geol., 2017, 179, 11-27.
[http://dx.doi.org/10.1016/j.coal.2017.05.007] ] in comparison with other ash grains; in combination with some functional groups it improves the retention of inorganic species [15Hower, J.C.; Maroto-Valer, M.; Taulbee, D.N.; Sakulpitakphon, T. Mercury capture by distinct fly ash carbon forms. Energy Fuels, 2000, 14, 224-226.
[http://dx.doi.org/10.1021/ef990192n] -17Hower, J.C.; Senior, C.L.; Suuberg, E.M.; Hurt, R.H.; Wilcox, J.L.; Olson, E.S. Mercury capture by native fly ash carbons in coal-fired power plants. Pror. Energy Combust. Sci., 2010, 36(4), 510-529.
[http://dx.doi.org/10.1016/j.pecs.2009.12.003] [PMID: 24223466] ] from flue gas or can be used for the retention of dyes from aqueous solutions [23Bartoňová, L.; Ruppenthalová, L.; Ritz, M. Adsorption of Naphthol Green B on unburned carbon: 2- and 3 parameter linear and non-linear equilibrium modelling. Chin. J. Chem. Eng., 2017, 25, 37-44.
[http://dx.doi.org/10.1016/j.cjche.2016.05.035] ]. In addition to carbonaceous materials, the retention of volatile elements was tested on kaolinite [24Wendt, J.O.L.; Lee, S.J. High-temperature sorbents for Hg, Cd, Pb and other trace metals: Mechanisms and applications. Fuel, 2010, 89, 894-903.
[http://dx.doi.org/10.1016/j.fuel.2009.01.028] ], MnO_x [25Scala, F.; Anacleria, C.; Cimino, S. Characterization of a regenerable sorbent for high temperature elemental mercury capture from flue gas. Fuel, 2013, 108, 13-18.
[http://dx.doi.org/10.1016/j.fuel.2010.12.028] , 26Scala, F.; Cimino, S. Elemental mercury capture and oxidation by a regenerable manganese-based sorbent: The effect of gas composition. Chem. Eng. J., 2015, 278, 134-139.
[http://dx.doi.org/10.1016/j.cej.2014.11.094] ] or desulphurization additives / calcareous adsorbents [27Scala, F.; Chirone, R.; Meloni, P.; Carcangiu, G.; Manca, M.; Mulas, G.; Mulas, A. Fluidised bed desulphurization using lime obtained after slow calcination of limestone particles. Fuel, 2013, 114, 99105.
[http://dx.doi.org/10.1016/j.fuel.2012.11.072] -29Bartoňová, L.; Klika, Z. Effect of CaO on retention of S, Cl, Br, As, Mn, V, Cr, Ni, Cu, Zn, W and Pb in bottom ashes from fluidized-bed coal combustion power station. J. Environ. Sci. (China), 2014, 26(7), 1429-1436.
[http://dx.doi.org/10.1016/j.jes.2014.05.008] [PMID: 25079991] ]. It is worth mentioning in this context that the percentage of CaO in fluidised-bed ashes is typically higher than that of unburned carbon (in some fractions it can reach even 50%) [29Bartoňová, L.; Klika, Z. Effect of CaO on retention of S, Cl, Br, As, Mn, V, Cr, Ni, Cu, Zn, W and Pb in bottom ashes from fluidized-bed coal combustion power station. J. Environ. Sci. (China), 2014, 26(7), 1429-1436.
[http://dx.doi.org/10.1016/j.jes.2014.05.008] [PMID: 25079991] ].

Desulphurization additives (lime, limestone etc.) are currently used for the retention of S. Except S, promising results on the possible retention of As on Ca-bearing compounds were reported as well [11López-Antón, M.A.; Díaz-Somoano, M.; Fierro, J.L.G.; Martínez-Tarazona, M.R. Retention of arsenic and selenium compounds present in coal combustion and gasification flue gases using activated carbons. Fuel Process. Technol., 2007, 88, 799-805.
[http://dx.doi.org/10.1016/j.fuproc.2007.03.005] , 29Bartoňová, L.; Klika, Z. Effect of CaO on retention of S, Cl, Br, As, Mn, V, Cr, Ni, Cu, Zn, W and Pb in bottom ashes from fluidized-bed coal combustion power station. J. Environ. Sci. (China), 2014, 26(7), 1429-1436.
[http://dx.doi.org/10.1016/j.jes.2014.05.008] [PMID: 25079991] , 30Clemens, A.H.; Damiano, L.F.; Gong, D.; Metheson, T.W. Partitioning behaviour of some toxic volatile elements during stoker and fluidised-bed combustion of alkaline sub-bituminous coal. Fuel, 1999, 78, 1379-1385.
[http://dx.doi.org/10.1016/S0016-2361(99)00066-6] ]. However, some recent studies concluded that not only Ca but also Fe could provide sites for As association [11López-Antón, M.A.; Díaz-Somoano, M.; Fierro, J.L.G.; Martínez-Tarazona, M.R. Retention of arsenic and selenium compounds present in coal combustion and gasification flue gases using activated carbons. Fuel Process. Technol., 2007, 88, 799-805.
[http://dx.doi.org/10.1016/j.fuproc.2007.03.005] , 31Seames, W.S.; Wendt, J.O.L. Regimes of association of arsenic and selenium during pulverized coal combustion. Proc. Combust. Inst., 2007, 31, 2839-2846.
[http://dx.doi.org/10.1016/j.proci.2006.08.066] -33Low, F.; Zhang, L. Arsenic emissions and speciation in the oxy-fuel fly ash collected from lab-scale droptube furnace. Proc. Combust. Inst., 2013, 34, 2877-2884.
[http://dx.doi.org/10.1016/j.proci.2012.05.026] ]. The conclusion that As can be captured from flue gas on active sites of both Ca and Fe was based on thermodynamic equilibrium calculations [32Contreras, M.L.; Arostegui, J.M.; Armesto, L. Arsenic interactions during co-combustion processes based on thermodynamic equilibrium calculations. Fuel, 2009, 88, 539-546.
[http://dx.doi.org/10.1016/j.fuel.2008.09.028] ] and laboratory-scale combustion [11López-Antón, M.A.; Díaz-Somoano, M.; Fierro, J.L.G.; Martínez-Tarazona, M.R. Retention of arsenic and selenium compounds present in coal combustion and gasification flue gases using activated carbons. Fuel Process. Technol., 2007, 88, 799-805.
[http://dx.doi.org/10.1016/j.fuproc.2007.03.005] , 31Seames, W.S.; Wendt, J.O.L. Regimes of association of arsenic and selenium during pulverized coal combustion. Proc. Combust. Inst., 2007, 31, 2839-2846.
[http://dx.doi.org/10.1016/j.proci.2006.08.066] , 33Low, F.; Zhang, L. Arsenic emissions and speciation in the oxy-fuel fly ash collected from lab-scale droptube furnace. Proc. Combust. Inst., 2013, 34, 2877-2884.
[http://dx.doi.org/10.1016/j.proci.2012.05.026] ]. Since As retention is a complex phenomenon, the results from real power stations are needed. Besides, fluidised-bed coal combustion ashes are advantageous for this study due to naturally high levels of Ca-bearing minerals (desulphurization additives) that can affect Fe₂O₃ interaction.

Possible association of volatilized As and S with CaO and Fe₂O₃ (creating solid product) could efficiently improve coal combustion flue gas cleaning. For this reason, herein, the relationships between S and As with CaO and Fe₂O₃ were evaluated in bottom ash and fly ash fractions from fluidised-bed co-combustion of coal and wastes (and limestone as desulphurization additive) through calculation of correlation coefficients and composition of magnetic concentrates.

The samples of Bottom Ash (BA) and Fly Ash (FA) were collected at atmospheric circulating fluidisedbed power station where lignite was co-combusted with wood, bark, sewage sludge and solid recovered fuel (blend of shredded paper, plastics, wood, textiles, carpets etc.). Desulphurization of emissions was achieved by adding limestone as (dry) desulphurization additive. Fluidised-bed combustion typically does not require powdered coal (combustion efficiency is achieved by circulating in fluidised-bed and longer dwell time); therefore, only limestone was finely ground for efficient interaction with S in flue gas. Efficient combustion can be documented by low unburned carbon content in both BA (< 3%) and FA (< 1%).

During the whole combustion test, fractional samples of bottom ash and fly ash were collected at regular time intervals and mixed properly. From these composite samples, representative subsamples were separated by quartering for further preparation and analyses in the laboratory.

From bulk BA and FA samples, the following particle size fractions were separated by sieving. BA fractions: >2 mm, 1-2 mm, 0.6-1.0 mm, 0.5-0.6 mm, 0.4-0.5 mm, 0.2-0.4 mm, 0.1-0.2 mm, 0.08-0.1 mm and <0.08 mm; FA fractions: >0.1 mm, 0.09-0.1 mm, 0.08-0.09 mm, 0.071-0.08 mm, 0.063-0.071 mm, 0.056-0.063 mm, 0.053-0.056 mm, 0.050-0.053 mm, 0.045-0.050 mm, 0.036-0.045 mm, 0.032-0.036 mm and <0.032 mm (percentage yields are given in Tables 1 and 2.

Table 1
Yields (%) and concentrations (in wt %) of target compounds and elements in BA fractions (mm).

Table 2
Yields (%) and concentrations (in wt %) of target compounds and elements in FA fractions (mm).

In order to distinguish between CaO and Fe₂O₃ associations, non-magnetic / magnetic fractions of BA and FA were analyzed for target compounds and elements. Due to quite different granulometry of BA (coarse grains) and FA (fine particles), similar particle-size fractions of BA (0.1-0.2 mm) and FA (> 0.1 mm) were used for the magnetic separation. Magnetic concentrates were manually separated (when dry) using a magnet.

The samples of BA and FA were milled using vibration mill Retsch MM400 and (using binding additive Boreox) 40 mm pellets were pressed on Vaneox automatic press. The elemental analysis was conducted on wavelength-disperzive X-ray fluorescence spectrometer ARL PERFORM´X 4200 W (Switzerland) equipped with 2 detectors, 9 crystals and 4 collimators.

Possible relationships between the concentrations of S and As and the contents of CaO and Fe₂O₃ were evaluated in BA and FA fractions from a fluidised-bed power station.

The dominant association of SO₃ was with CaO, which corresponds with efficient desulphurization using desulphurization additive, such as limestone. In contrast, As exhibited association with both CaO and Fe₂O₃ - CaO correlation was more significant in BA while Fe₂O₃ correlation slightly prevailed in FA. The difference in As behaviour in BA and FA could correspond (e.g.) with bulk ash composition (concentration of Fe₂O₃ in FA is twofold higher than that in BA) or grain-size effect.

The results document that not only (widely accepted) Ca-bearing additives but also Fe-based ash components can be used for efficient decreasing toxic As emissions from coal-fired power stations.