Distribution of selected elements during the co-combustion of lignite with wood and wood wastes

Acta Geodyn. Geomater., Vol. 9, No. 4 (168), 491–501, 2012 DISTRIBUTION OF SELECTED ELEMENTS DURING THE CO-COMBUSTION OF LIGNITE WITH WOOD AND WOOD WASTES .Zdeněk KLIKA *, Lucie BARTOŇOVÁ and Jana SERENČÍŠOVÁ VŠB – Technical University of Ostrava, Tř. 17. listopadu 15, 708 33 Ostrava – Poruba, Czech Republic *Corresponding author‘s e-mail: [email protected] (Received January 2012, accepted May 2012) ABSTRACT Combustion of lignite with limestone, co-combustion of lignite with limestone and wood, and combustion of wood were performed in a circulating fluidised bed in 7 different combustion regimes. The inorganic matter composition and properties of all input and output materials were characterized. For each combustion regime the material streams and the boiler outputs were calculated. The total inorganic mass and the mass of selected trace elements in fuels (Cl, Zn, As, Se, Hg, and Pb) were based on the output of a 1 GW circulating fluidised-bed boiler; additionally, the concentrations of volatile elements (As, Se, Hg) in fly ash (FA) were evaluated in relation to the mass of FA. Element (Cl, Zn, As, Se, Hg, and Pb) enrichment in FA compared with BA was related to the ratio R between the combusted mass of organic wood wastes and total combusted fuel. Additionally, the unburned carbon in bottom ash (BA) and fly ash (FA) was determined, and selected elements were studie d in unburned materials separated from BA. The results show that combustion of wood and/or co-combustion of lignite with wood waste brings about significant environmental benefits. KEYWORDS: lignite, wood wastes, ash, co-combustion, trace elements distributions The compositions of different waste fuels vary greatly, and most biofuels are strongly prone to create fireside deposits during combustion. This is the reason why co-combustion of waste biofuels with lignite is usually preferred in order to diminish the unfavourable effects of simple biofuel combustion (Arvelakis and Frandsen, 2010). It was found tha t blending waste and fossil fuels resulted in changes o f fuel characteristics that are consequently reflected in the different behaviour of these fuels in the combustion chamber. The blending of waste and fossil fuels leads to different volatilization of toxic pollutants during co-combustion and their differen t redistribution between bottom ash, fly ash and emissions (e.g. Díaz-Somoano et al., 2007; Steenari and Lindquist, 1999). This is why the composition o f combustion ashes originating in coal combustion differs depending on whether or not waste materials were added. Of course, there are considerable differences in composition even among different coal ashes, as is presented e.g. by Asokan et al. (2005). The concentration range of As in Indian coal combustion ashes is 5-68 ppm; for Pb it is 10144 ppm; in the case of Se, Cu and B their content reaches 1-10 ppm, 39-1000 ppm and 100-1000 ppm respectively. The content of Pb, Se, As, Cu, and Hg in coal combustion ashes collected at two Czech power stations (Klika et al., 2001; Bartoňová et al., 2007) was: Pb 17-223 ppm, Se 0.65-16.8 ppm, As 9.445.2 ppm, Cu 90.9 – 675 ppm, and Hg 0.0041. INTRODUCTION Worldwide, there are many reasons in favour o f the utilization of biofuels for energy purposes. Above all, this material brings significant environmental benefits (Kajikawa et al., 2008; Haykiri-Acma and Yaman, 2008; Van Loo and Kopperjan, 2008; Ogaji and Probert, 2009) and a reduction of dependency on imported oil. Biofuels are typically low in sulphur and ash content, and therefore their use brings abou t reductions in acid rain, CO2 as a greenhouse gas, and hazardous elements that can enter the environment as a volatile species or can be dissolved from solid p roducts of lignite combustion. Moreover, biomass is a renewable fuel, available for heat and power production in relatively substantial amounts (Escoba r et al., 2009; Yilgin and Pehlivan, 2009; Pettersson e t al., 2009). In general, there are two approaches when using waste biofuels for energy production: a) combustion in current power stations (originally designed for lignite combustion), or b) the use of new combustion facilities specifically designed for biofuels. At present there is a strong tendency to prefer the first approach due to cost reasons. However, the different behaviour of waste fuel in the combustion chamber is connected with several unfavourable effects, such as fouling, slagging or corrosion. The p redisposition of waste fuel to fouling or slagging is caused mainly by the composition of its ash, i.e. by the content of alkali metals – Ca, P, Cl, Si, Al, etc. Z. Klika et al. 492 in mass flows of selected elements related to the unit of CFB output for regimes with different ratios between lignite and wood wastes. 2. EXPERIMENT At a power station situated at the Mondi Packaging paper mill in Štětí near Mělník (Czech Republic), lignite combustion and co-combustion o f lignite/wastes were performed in a circulating fluidised-bed boiler (K11) at about 850-860 °C. Fluidisation air is brought into the furnace by a burne r nozzle positioned at the bottom of the fire grate, while secondary air is introduced into the furnace from the side wall at different heights. This enables gradual combustion of fuel without the formation of a large quantity of redundant NOx. Flue gases go from the combustion chamber through a cyclone in which the coarse fractions of ash are separated and removed as b ottom ash. Finer ash fractions are separated in an electrostatic precipitator. A diagram of the combustion facility is given in Figure 1. In the everyday combustion regime, lignite is combusted with wood wastes from the production o f cellulose in a circulating fluidisedb ed boiler (K11) in the weight ratio 10:1. This paper describes 7 combustion regimes (I – VII) in which lignite is combusted and/or co-combusted with wood wastes from cellulose production in a wide variety o f lignite/biofuel weight ratios. In the referential regimes I and IV, the lignite was combusted only with limestone; in regimes II, V, VI, and VII lignite was combusted with wood wastes; and in regime III only wood wastes (wood, sawdust and wood chips) were combusted. 1.75 ppm. More information on coal combustion ashes, including their composition, can also be found in some other studies (e.g. Gal b reath et al., 2000; Ratafia-Brown, 1994; Meij, 1995; Hower et al., 1996; Fulekar et al., 1983). The comparison of elemental content in lignite ash and wood ash (in ppm) was reported e.g. by Skodras et al. (2002), who found the bigges t differences in element content (in ppm) between coal and wood ashes for As (coal ash 13, wood ash 0.42), Cr (coal ash 29, wood ash 0.6), Ni (coal ash 20.4, wood ash 0.88), Pb (coal ash 3.62, wood ash 0.48) and Co (coal ash 5.2, wood ash 0.18). Significant variability in the composition of coal ash, coal/wood ash and wood ash was also found e.g. by Klika et al. (2010). In contrast to Skodras et al. (2002), the content of Cl, As, Pb, Zn, and Se in these ashes falls within a larger interval: Cl (303-836 ppm), As (140438 ppm), Pb (41-65 ppm), Zn (25-268 ppm), and Se (< 1.3-12.5 ppm) but the quantity of ash from the combustion of coal/wood and wood is much less. Many studies have focused on evaluating the content of Hg and other toxic elements in emissions from the combustion of lignite with different biofuels. Some of these studies are based on the evaluation o f selected element mass flows (Åmand and Leckner, 2004; Klika et al., 2010; Elled et al., 2007); however, direct measurement of elemental content in emissions has been studied only sporadically (e.g. Skodras et al., 2002). This contribution studies the mass flows o f elements (Cl, Zn, As Se, Hg and Pb) during the combustion of lignite, lignite with wood waste, and/o r wood waste itself in a circulating fluidisedb ed boiler (CFB). The aim of the study is to show the differences Fig. 1 Diagram of the combustion facility. A – Simplified diagram of the CFB, B – detail of the combustion chamber. DISTRIBUTION OF SELECTED ELEMENTS DURING THE CO-COMBUSTION OF …. 493 formula   i djiji Wcc  10010 , 2 ,, and concentrations determined from biofuel ashes Aji c, were recalculated on ji c, (original moist basis) using the formula i Ajiji Acc  , 2 ,10 , where Aji c, is the concentration determined in the ash (450 oC). The concentrations in BA ( jAB c,, ) and FA ( jAF c,, ) were determined from original samples of ashes. Elements Pb, Cl, Zn, As, and Se were determined by X-ray fluorescence spectrometry (SPECTRO XEPOS) and mercury content was determined using an AMA-254 spectrometer. Unburned carbon (UC) from coal present in bottom ash (BA) and fly ash (FA) was determined by a method devised at our laboratory. This method is b ased on the removal of the carbonates by leaching with diluted HCl (1:4) at 90 oC. These samples are washed out with distilled water, dried at 105 oC, and the carbon content (unburned carbon) is determined using a CS-2000 analyzer (ELTRA). 3. RESULTS AND DISCUSSION 3.1. MASS BALANCE OF INORGANIC MATTER Chemical analyses of selected elements (Cl, Zn, As, Se, Hg, and Pb) were performed for lignite and Each combustion regime reached a steady state after 8-10 h. The samples of lignite, limestone, biofuels and ashes were collected at 2-hour intervals in quantities of approx. 2 kg. An average sample fo r each material was then prepared and used for analysis. The total sampling period was 12 hours. Moisture ( i W) of samples was determined gravimetrically by drying at 105-110 oC. The ash content ( d i A) of dried lignite and limestone was determined at 900 oC and the ash content of dried biofuels d i A (sawdust, treeb ark, wood, wood chips, sewage sludge, alternative solid fuel (ASF) and/or soap) at 450 oC. Recalculations of the ash content d i A (db, dry basis) on i A (original moist basis) were performed using the formula   i d ii WAA  10010 2; where i A, d i A and i W are given in wt. % and index i relates to the i-th input material (see Table 1). Chemical analyses of Pb, Cl, Zn, As, Se and Hg were p erformed from dried samples of lignite and limestone (105-110 oC) and from biofuel ashes prepared a t 450 oC. Concentrations of the j-th element determined in dried lignite and limestone dji c,(db) were recalculated on ji c, (original moist basis) using the Table 1 Mass flows of lignite, limestone and biofuels (mi) for combustion regimes I -VII. mi (kg.h-1) Fuel I II III IV V VI VII Lignite (C) 25920 11840 - 29980 34960 26750 28550 Limestone (L) 2630 970 - 3490 4320 3240 2630 Sawdust (S) - 5220 3114 - - - - Tree-bark (B) - 5620 - - - - - Wood (W) - - 14905 - 19728 15160 24260 Wood chips (WCh) - - 14870 - - - - Sewage sludge (SS) - - - - - 15160 - Alternative solid fuel (ASF) - - - - - 940 - Soap (So) - - - - - - 200 Table 2 Moisture ( i W) of lignite, limestone and other materials used for the combustion regimes I -VII. i W (%) Fuel I II III IV V VI VII Lignite (C) 14.70 16.40 19.70 25.60 25.60 20.80 Limestone (L) 0.45 0.45 0.79 0.91 0.63 0.79 Sawdust (S) 28.10 29.7 Tree-bark (B) 26.70 Wood (W) 12.8 58.00 58.00 29.90 Note: Moisture ( i W) relates to the original moist basis; moistures of other fuels are: WCh (26.8 %), SS (67.0 %), ASF (4.5 %) and So (28.5 %). Z. Klika et al. 494 Table 3 Ash content ( i A) of lignite, limestone and other materials used in the combustion regimes I -VII. i A (%) Fuel I II III IV V VI VII Lignite (C) 18.3 16.80 22.9 24.3 24.3 22.9 Limestone (L) a) 65.9 65.90 66.3 66.7 66.5 66.3 Sawdust (S) 0.64 0.47 Tree-bark (B) 4.80 Wood (W) 0.25 2.7 2.7 6.8 Note: Ash content ( i A) relates to the original moist basis; ash content of the other biofuels relates to the original moist basis: WCh (1.04 %), SS (0.9 %), ASF (8.1 %) and So (7.6 %). a) Ignited limestone at 850 OC Table 4 Mass of inorganic matter (mIN,i) present in lignite, limestone and other materials used in the combustion regimes. Input streams mIN,i (kg/h) Output streams mIN,i (kg/h) Combustion regime C L S B W WCh SS ASF So mIN BA FA I 4743 1733 - - - - 6476 3210 3266 II 1989 640 33 750 3412 2020 1392 III - - 14 - 37 155 206 5 203 IV 6865 2314 9179 3670 5510 V 8495 2881 - - 532 11909 4770 7140 VI 6500 2155 409 136 76 9277 3710 5570 VII 6538 1743 1649 15 9946 3720 6226 Note: Inorganic matter (mIN,i) relates to the original moist basis The mass of ash present in flue gas is negligible and therefore it is not included in the calculation. For example, the very fine solid particles determined in emissions reach about 0.42, 0.57 and 0.16 kg/h in combustion regimes I, II, and III, respectively. 3.2. BOILER OUTPUT The relation between the boiler output and other parameters is given by Eq. 2:     calCMout QmQ (2) where Qout (MW) is boiler output, Qcal (MJ.kg-1) is calorific value based on dry and ash-free fuel, mCM is combustible mass of fuels (kg.h-1) for the combustion regimes I – VII, and η is boiler efficiency. The combustible mass of lignite and other biofuels (mCM) was calculated for each of the combustion regimes I – VII from Eq. 3.     ii iiCM AWmm   2 101 (3) where mi is mass of the i-th fuel and/or biofuel (original moist basis) in a given regime (see Table 1). The calculated boiler output Qout and other p arameters are given in Table 5 for each of the 7 combustion regimes. The parameter (R) is defined as the mass ratio b etween the combusted mass of organic limestone from dried samples (110 oC) and chemical analyses of all biofuels from ash prepared at 450 oC. Using the data of moisture and ash content (Tables 2 and 3) these analytical data have been recalculated on original moist samples basis (Table 6) necessary for mass balances. Analyses BA and FA are given in Table 7A and Table 7B, respectively. The mass flows of non-dried lignite, wood wastes and other alternative fuels in combustion regimes I –VII (mi) are given in Table 1. The data relate to as-received non-dried fuels. Lignite with limestone was combusted in regimes I and IV, wood wastes in regime IV, and cocombustion of lignite with wood wastes and alternative fuels took place in regimes II, V, VI, and VII. The moisture and ash content of fuels are shown in Tables 2 and 3 respectively. The inorganic mass (mIN,i) present in lignite, limestone and other materials for various combustion regimes was calculated from Eq. (1); the results are given in Table 4. The mass flows of bottom ash (BA) and fly ash (FA) are also given in the same table. i ii iiININ Ammm     , (1) where: index i relates to lignite (C), limestone (L), sawdust (S), treeb ark (B), wood (W), wood chips (WCh), sewage sludge (SS), alternative solid fuel (ASF) and/or soap (So) in the input streams. DISTRIBUTION OF SELECTED ELEMENTS DURING THE CO-COMBUSTION OF …. 495 Table 5 Parameters related to boiler output Qout. Combustion regime mCM (kg/h) Qcal (MJ.kg-1) η Q out (MW) mF (kg.h-1) mC (kg.h-1) R (%) mIN/Qout (kg.h-1.MW-1) I 17366 15 0.89 64 25920 25920 0 101 II 15479 14 0.90 54 22680 11840 49 63 III 24410 12 0.90 73 32889 0 100 2.8 IV 17208 15 0.89 64 29980 29980 0 143 V 25268 12 0.90 76 54688 34960 36 157 VI 25047 12 0.90 75 58010 26750 54 123 VII 31558 12 0.90 95 53010 28550 46 105 b y the combustion of a large amount of wood (regime V and VI) and sewage sludge (regime VI) containing very high moisture (Tables 1 and 2). The higher moisture content in fuels of regimes V and VI is also reflected in Figure 3, which plots the ratio of total inorganic matter to boiler output (mIN/Qout) and the ratio between combusted mass o f organic wood wastes and total mass of combusted fuels (R). The decrease of mIN/Qout with increasing ratio R, and extremely low mIN/Qout for combustion o f wood with wood wastes (R = 100 %), are observed. 3.3. SELECTED TRACE ELEMENTS Chemical analyses of selected elements (Cl, Zn, As, Se, Hg, and Pb) were performed for all fuels (Table 6), for BA (Table 7A), and for FA (Table 7B). Samples of fuel, BA and/or FA were prepared as mean samples from approx. 7 sample portions collected during the measurement in each of the regimes I – VII. For regimes I – VII mean values and estimated standard deviations of element concentrations ( ji c,± 2s) were calculated for lignite, limestone and wood (Table 6), while for S, B, WCh, wood wastes and total mass of combusted fuel (Eq. 4). F CF m mm R 100 (4) where mF is the total mass of lignite (original moist basis) and biofuels and m C is the mass of lignite (original moist basis) in a related combustion regime. This ratio varies from 0 (for regimes I and IV) to 1 (for regime III). The other parameters related to b oiler output are also given in Table 5. The ratio between the mass of inorganic matter (mIN,i) present in lignite/limestone and boiler output (Qout) was calculated from data given in Tables 4 and 5, respectively. The boiler output (Qout) and the combustible mass of fuel are plotted in Figure 2. On the left side o f the diagram the combustion regimes I – VII are denoted. The plot between Qout and mCM is practically linear with almost no dependence on the type o f combustion regime. In contrast, the plot between boiler output Qout and mass of fuel mFshows two outsiders for regimes V and VI. The much lower boiler output Qout for both regimes can be explained Fig. 2 Plot of boiler output and mass of fuel. mF – total mass of fuels; mCM – total combustible mass of fuels. Z. Klika et al. 496 Fig. 3 Plot between ratio of total inorganic matter to boiler output and ratio R between combusted mass of organic wood wastes and total combusted fuel. Table 6 Concentrations and estimated standard deviations ( ji c,± 2s) of selected trace elements in lignite, limestone and other biofuels in regimes I - VII. ji c,± 2s (ppm) Fuel Cl Zn As Se Hg Pb Lignite 407 (±150) 026 (±7) 87 (±40) 1 (±0,5) 0.16 (±0.04) 11 (±0.5) Limestone 733 (±300) 034 (±3) 01.9 (±1) 0.6 (±0.25) 0.01 (±0.001) 008 (±4) Sawdust 004 005.6 00.24 0.013 0.05 000.19 Tree-bark 016 041 00.18 0.038 0.06 001.7 Wood 258 (±37) 059 (±8) 00.32 (±0.3) 0.05 (±0.01) 0.05 (±0.05) 004(±1) Wood chips 011 003 01.2 0.05 0.05 000.43 Sewage sludge 022 003.4 00.08 0.02 0.003 000.6 Alternative solid fuel 128 208 00.48 0.2 0.7 37 Soap 364 049 00.18 0.11 0.001 0 0.8 (R = 0 %), while for the co-combustion of lignite with wood wastes, and particularly for wood wastes only (R=100 %), the enrichment is significantly decreased. This element diversity is also dependent on the mass of FA (mFA) in which these volatile elements (Hg, Se, As) can be captured (Fig. 5). It is particularly visible for the most volatile Hg, which does not condense and remains in flue gas if the mass of FA (about 400 kg.h-1 in regime III) is very low. In contrast, the mass of fly ashes, as well as their surface area, is much higher, enabling highly efficient capture of volatile elements at approximately similar volumes of flue gas in all combustion regimes. No data on the plot cFA,j/cBA,j in relation to R ratio or mass of FA have been reported up to now. Pedersen et al. (2010) studied the release of trace elements from waste materials when combusted. The study found that Pb, Zn, Cl, S, Na, K, and As were released to a significant extent and that this release was fuel specific. Lind et al. (2007) studied enrichment / depletion of trace elements in SS, ASF, and So (for abbreviations see note to Table 1) mean concentrations were not calculated, because these fuels were used only for one or two regimes. The concentrations of elements in BA and FA differ more than those of fuels, and therefore they are presented for each regime (I – VII) separately. They are given for BA (cBA,j) in Table 7A and for FA (cFA,j) in Table 7B. The ratio of element concentrations in FA/BA against the ratio R is plotted in Figure 4. In FA elements such as Cl, Pb and Zn are hardly enriched at all in contrast to Se, As and Hg. These results are in agreement with the conclusions of Elled et al. (2007), who studied the distribution of As, Cd, Hg, Pb, Se, Sb, and Hg during the combustion o f sewage sludge and wood. They explain the higher ratio cFA,j/cBA,j of elements as a result of the higher specific surface area of fine-grained fly ash. Higher enrichment of FA with volatile Se, As, and Hg is observed for the combustion of lignite DISTRIBUTION OF SELECTED ELEMENTS DURING THE CO-COMBUSTION OF …. 497 Table 7A Concentrations of selected trace elements in BA for the combustion regimes I – VII. cBA,j (ppm) Combustion regime Cl Zn As Se Hg Pb I 569 188 248 1.4 0.002 41.2 II 836 251 140 1.3 0.005 56.0 III 566 223 298 1.0 0.003 48.5 IV 1568 151 97 0.6 0.002 33.4 V 1315 139 97 0.3 0.001 33.3 VI 1141 94 39 0.2 0.001 41.1 VII 490 170 158 0.4 0.002 32.9 Note: Concentrations , B Aj c of the j-th element present in BA Table 7B Concentrations of selected trace elements in FA for the combustion regimes I – VII. cFA,j (ppm) Combustion regime Cl Zn As Se Hg Pb I 303 172 375 12.5 0.605 58.9 II 375 243 438 7.9 0.280 65.0 III 440 268 291 7.8 0.010 57.4 IV 1163 225 535 7.4 0.779 36.1 V 1003 261 375 5.0 0.282 45.0 VI 975 196 268 5.1 0.672 46.6 VII 650 264 586 7.3 0.537 41.8 Note: Concentrations , F Aj c of the j-th element present in FA. Fig. 4 Plot of FA/BA ratio of element concentrations and ratio R between the combusted mass of organic wood wastes and total combusted fuel. Calculations of the mass of elements volatilized into flue gas from the mass balance are usually burdened by relatively high errors. These errors are b rought about above all by the imprecise determination of lignite, limestone and other materials used for combustion and also by the imprecise determination of the mass of bottom ash and fly ash. The mass of fuel is usually determined in relation to the vapour output of the boiler, and/or determined with a certain imprecision from the volume o f combusted fuels. The mass of fly and bottom ash is PM1.0 particles during waste combustion in grate firing and in fluidised bed combustion. It was found that the behaviour of As and Sb differed for these two combustion units. The above data shows that FA ashes are usually enriched with volatile elements, but the ratio cFA,j/cBA,j is very individual and depends on many parameters of fuel combustion (e.g. fuel rank, fuel composition and affinity of trace elements; combustion unit and condition of fuel combustion, temperature of combustion and FA separation, mass of FA, etc.). Z. Klika et al. 498 Fig. 5 Plot of FA/BA concentration ratio of Hg, Se and As concentrations versus mass flow of FA. Fig. 6 The plot between total input mass flows of elements (Cl, As, Zn, Pb, Se, Hg) and ratio R between the combusted mass of organic wood wastes and total combusted fuel. performed here, for various combustion regimes I – VII. Input mass streams were calculated (Eq. 5) fo r selected elements (Cl, As, Zn, Pb, Se, and Hg) using the data in Tables 1 and 6. ji iij cmm ,    (5) where: mj is the total mass flow of the j-th element in lignite, limestone and other biofuels; ci,j is the usually calculated from the mass and ash content o f the combusted fuel and from measured data related to BA and/or FA. Errors also arise from sampling and analyses of lignite, limestone and other biofuels used for combustion and also analyses of BA and FA. Because the calculation of the elements in flue gas based on the difference between input and outpu t mass streams is burdened by high errors, only the comparison of mass flows of selected elements is DISTRIBUTION OF SELECTED ELEMENTS DURING THE CO-COMBUSTION OF …. 499 Table 8B Percentages of unburned carbon (cUC) in FA and fractions from FA for combustion regimes I - III. Table 8A Percentages of unburned carbon (cUC) in BA and fractions from BA for combustion regimes I - III. cUC (%) sample I II III FA 0.32 0.53 0.27 fraction > 0.09 mm 0.08 4.66 0.05 fraction 0.04-0.09mm 0.21 0.27 0.14 fraction < 0.04 mm 0.48 0.11 0.42 cUC (%) sample I II III BA 1.60 0.72 0.28 fraction > 2.0 mm 4.66 2.42 1.11 fraction 0.4-2.0 mm 0.27 0.32 0.09 fraction < 0.4 mm 0.11 0.10 0.07 Table 9 Chemical analyses of elements in separated UM from BA and BA in combustion regimes I – III. cj (ppm) samples Cl Zn As Se Hg Pb I-UM 638 53 79 0.4 0.03 30 I-UM-rec 1960 163 243 1.23 0.09 92 I-BA 569 188 248 1.4 0.002 41.2 II-UM 716 62 165 2.4 0.03 36 II-UM-rec 1566 136 361 5.25 0.06 79 II-BA 836 251 140 1.3 0.005 56.0 III-UM 738 70 98 1.8 0.03 46 III-UM-rec 1180 112 157 2.9 0.05 74 III-BA 566 223 298 1.0 0.003 48.5 volatile elements will decrease with increasing ratio R. This assumption was recently proved (Klika et al., 2010) for sulphur, balanced for the regimes I (R = 0 %) and III (R =100 %). 3.4. UNBURNED MATERIAL Unburned material (UM) is the unburned part o f coal present in BA and FA after the combustion o f coal. The content of unburned material in BA and FA reflects the efficiency of coal combustion; the higher the quantity of unburned material, the lower the efficiency of the combustion. Unburned material was studied in BA and FA from regimes I – III. The samples of BA and FA were sieved, and from each ash 3 fractions were separated and unburned carbon (UC) determined. The percentages of UC for the symplex of BA and their 3 fractions are given in Table 8A, for FA and their 3 fractions in Table 8B. There is a higher content of UC in BA compared with FA. However, both in BA and FA the p ercentages of UC are low, so the combustion is very effective for all three combustion regimes I – III. The UM was mechanically separated from BA, while fro m FA it was not possible to perform a separation, eithe r mechanically or in heavy liquids. This is due to the very small FA grain sizes. Ash content in the mechanically separated unburned material of BA (AUC) is 32.5 %, 45.7 % and 62.5 % for regimes I, II and III respectively. Chemical analyses of Cl, Zn, As, Se Hg and Pb in separated UM samples from BA were concentration of the j-th element in the i-th input stream (lignite, limestone and other biofuels); miis mass in the i-th input fuel stream. The calculated total mass flows mj of the elements in various combustion regimes were based on boiler outputs Qout = 1 GW. For this, the real boiler outputs for the regimes I – VII given in Table 5 were used. The calculated mass flows mj of elements (Cl, Zn, As, Se, Hg, and Pb) are plotted versus the ratio R for each combustion regime (Fig. 6). The results show that the total input mass flows of all the studied elements (mj) decrease with an increase of R ranging from 0 to 100 % (Fig. 6). In a circulating fluidisedb ed boiler with an estimated output of 1 GW, the following decrease of total input mass of the elements (mj) was observed in regimes combusting lignite (regimes I and IV; R = 0 %) and regimes combusting wood wastes (regime III; R = 100 %): For Cl: from 219 to 36 kg/h For As: from 40 to 0.3 kg/h For Zn: from 13 to 11 kg/h For Pb: from 5 to 0.8 kg/h For Se: from 0.5 to 0.03 kg/h For Hg: from 0.10 to 0.02 kg/h It can be supposed that the mass of these elements present in flue gas will follow the quantity o f volatile elements in input fuel, i.e. the quantity o f