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Influence of oxidation in aqueous medium on the structure and properties of lignites
Fuel 79 (2000) 777–783 www.elsevier.com/locate/fuel
Influence of oxidation in aqueous medium on the structure and properties of lignites S. Yaman*, S. Ku¨c¸u¨kbayrak Istanbul Technical University, Chemical and Metallurgical Engineering Faculty, 80626, Maslak, Istanbul, Turkey Received 18 February 1999; received in revised form 10 September 1999; accepted 12 September 1999
Abstract In this paper, the effect of oxygen uptake on the structure and properties of lignites was investigated using five different Turkish lignites. Lignite samples were oxidised in aqueous medium at 423 K under 1.5 MPa partial pressure of oxygen for 60 min. Relations between the oxygen uptake and the elemental compositions of the lignite samples were investigated. FT-i.r. technique was applied to the original and oxidised lignite samples to investigate functional group analyses. Mercury porosimetry analyses of the samples were carried out up to about 220 MPa. Effect of oxygen uptake on the bulk density, apparent density, porosity, and pore radius of the lignite samples were compared. Differential Thermogravimetry (DTG) technique was performed to compare the thermal reactivity of the original and oxidised lignite samples. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: Lignite; Oxidation; Structure
1. Introduction Interaction of coal with oxygen takes place thermodynamically more easily and more rapidly, compared with other gases. Therefore, a number of desulphurization methods based on oxidation have been developed to apply to coal in either solid phase or liquid phase [1–6]. Oxidation by means of dissolved oxygen is one of the influential methods, in which oxygen is applied to a suspension of coal, and dissolved oxygen under pressure in the liquid phase, converts sulphur compounds into soluble sulphates. In order to increase the effectiveness of this process, some additional chemicals such as sodium carbonate, ammonia, calcium carbonate, borax, sodium hydroxide, etc., and some natural minerals and residues such as trona mineral and fly ash were used as promoters [7–10]. Since the electronegativity of carbon is the same as that of sulphur, oxidation leads to important variations in the organic structure and consequently in the properties of coal. Physical and chemical properties of coals except anthracite are highly sensitive to oxidative treatments. Oxidation of coal changes elemental composition, density, specific heat, mechanical strength, floatability, functional * Corresponding author. Tel.: ⫹90-212-285-6873; fax: ⫹90-212-2852925. E-mail address: [email protected] (S. Yaman).
groups, surface properties, spontaneous combustion, water holding capacity, pyrolytic behaviour, coking property, etc. [11–14]. These variations are proportional to the severity of the processes applied. Important changes take place in the properties of coal not only under strongly oxidative conditions, but also under weakly oxidative conditions these changes still occur to some extent. For example, when moist coal is exposed to air, it loses its moisture until reaching an equilibrium with the humidity of air. On the other hand, coal shrinks during water desorption and swells during re-adsorption of water. The level of the shrinking or swelling depends on the rank of the coal. Shrinking of the bituminous coals is almost negligible, whereas it is more noticeable for the low rank coals. Also, chemisorption of oxygen occurs when coal is subjected to oxygen. The extent of the chemisorption depends on the rank of coal and particle size, petrographic composition, surface area, pore size distribution, etc. There has been detailed information about structural changes resulting from oxidation of bituminous coals, but for lower rank coals information is limited. The aim of this paper is to present some information about changes in the chemical composition and some properties such as thermal reactivity, functional groups, porosity of lignites as a result of oxidation with dissolved oxygen in aqueous medium.
0016-2361/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(99)00207-0
S. Yaman, S. Ku¨c¸u¨kbayrak / Fuel 79 (2000) 777–783
778
Table 1 Proximate analysis and gross calorific value results of the lignite samples Sample
Moisture (%)
Agacli Bolluca Tavsanli Malkara Yenikoy
Dry basis
28.6 39.7 13.9 31.1 30.2
Volatile matter (%)
Ash (%)
Fixed carbon (%)
Gross calorific value (MJ/kg)
54.9 55.3 40.1 46.5 52.1
11.8 9.9 15.5 13.7 9.6
33.3 34.8 44.4 39.8 38.3
24.5 22.6 26.8 23.6 22.8
CVR % calorific value of recovered coal=calorific
2. Experimental In the experiments, five different Turkish lignite samples, Agacli, Bolluca, Tavsanli, Malkara, and Yenikoy, were used. The proximate and ultimate analyses were performed according to ASTM standards. Results are given in Tables 1 and 2. Oxidation of the lignite samples was carried out in a 1 l, magnetically stirred Parr autoclave made of 316 stainless steel. Inner surfaces of the autoclave were protected using a removable nickel liner. For each experiment, 10 g lignite having 250 mm particle size was thoroughly wetted with 300 ml distilled water. After sealing, 1.5 MPa partial pressure of oxygen was established and the autoclave was heated to 423 K. Stirring at 500 rpm was initiated when the temperature reached 423 K. The autoclave was held at the fixed temperature with the stirring rate for 60 min. For this period, suspended coal in the water was interacted with dissolved oxygen. Oxidation of pyrite during this process changed the pH of the medium from neutral to acidic. 4FeS2 ⫹ 15O2 ⫹ 8H2 O ! 2Fe2 O3 ⫹ 8H2 SO4
1
Some reductions were observed in the partial pressure of oxygen due to consumption of oxygen during this period. At the end of this batch process, the autoclave was rapidly cooled and the pressure was released. The suspension was filtered and washed several times with hot distilled water. The treated lignite was dried under nitrogen atmosphere in a vacuum oven at 383 K for 24 h. “Coal Recovery” (CR), “Calorific Value Recovery” (CVR) and “Ash Removal” (AR) were calculated using the following equations:
3
value of feed coal100 AR % ash content of feed coal-ash content of recovered coal=ash content of feed coal100
4
FT-i.r. spectra of the lignite samples were obtained using a Mattson 1000 Series FT-i.r. spectrometer. 300 mg of KBr pellets containing 1 wt% lignite were dried at 383 K for 36 h and spectra were obtained at a resolution of 8 cm ⫺1. Thermogravimetric analyses were performed using a Shimadzu TG 41 thermal analyser. 20 mg samples (after grinding to pass a 250 mm sieve) were spread uniformly on the bottom of an alumina crucible. The samples were oxidised in a dynamic dry air atmosphere of 40 ml min ⫺1. The temperature was raised from ambient to 1223 K at a heating rate of 10 K min ⫺1. Mercury porosimetry analyses of the lignite samples having 250 mm particle size were carried out using a Quantachrome Autoscan-33 mercury porosimeter, pressurising up to about 220 MPa. 3. Results and discussion
CR % weight of recovered coal=weight of feed coal100 2
Ultimate analysis results of the oxidised lignite samples are shown in Table 3. Since oxygen combines with carbon and hydrogen in the lignite samples, the extent of oxygen uptake on the lignite samples varied depending primarily on the abundance of carbon and hydrogen content. Relations between oxygen uptake, which was calculated from the difference of oxygen content of oxidised and original samples, and content of carbon and hydrogen are shown
Table 2 Ultimate analysis results of the original lignite samples (dry ash-free basis)
Table 3 Ultimate analysis results of the oxidised lignite samples (dry ash-free basis)
Sample
C (%)
H (%)
N (%)
S (%)
O a (%)
Sample
C (%)
H (%)
N (%)
S (%)
O a (%)
Agacli Bolluca Tavsanli Malkara Yenikoy
58.7 63.2 73.1 71.8 76.9
4.8 4.9 5.6 5.7 5.7
3.8 3.6 4.2 5.0 3.7
1.8 1.8 2.4 2.9 2.3
30.9 26.5 14.7 14.6 11.4
Agacli Bolluca Tavsanli Malkara Yenikoy
53.5 61.4 57.7 65.9 65.5
4.0 4.1 3.2 4.3 4.7
2.4 2.4 4.3 4.3 2.9
1.0 0.8 1.1 1.3 1.0
39.1 31.3 33.7 24.2 25.9
a
Calculated by difference.
a
Calculated by difference.
S. Yaman, S. Ku¨c¸u¨kbayrak / Fuel 79 (2000) 777–783
Fig. 1. Relation between carbon content (daf) of the original lignite samples and oxygen uptake.
in Figs. 1 and 2. More oxygen uptake occurred when the lignite samples contained high content of carbon and hydrogen. The carbon and hydrogen content of the oxidised lignite samples decreased to the values at which it can be said that they reached about their equilibrium values under investigated conditions. The range of the carbon content of the oxidised lignite samples was 53.5–65.9% (daf), whereas that of the original lignite samples was 58.7– 76.9% (daf).
Fig. 2. Relation between hydrogen content (daf) of the original samples and oxygen uptake.
779
Fig. 3. Relation between oxygen uptake and loss of carbon content.
Likewise, the hydrogen content of the oxidised lignite samples was between 3.2 and 4.7%, the hydrogen content of the original lignite samples was in the range of 4.8–5.7% (daf). In the initial step of oxidation, oxygen diffuses into pores of lignites and fills the apertures by physical means. Then, as oxidation proceeds, this adsorbed oxygen is combined with the coal matrix chemically. Further oxidation converts chemisorbed oxygen into surface oxides. Formation of surface oxides accelerates the formation of some decomposition products such as CO2, H2O, CO, CXHY, H2 from the organic matrix. Thus, considerable losses took place in the carbon and hydrogen content of the oxidised lignite samples. These findings are in good agreement with the
Fig. 4. Relation between oxygen uptake and loss of hydrogen content.
S. Yaman, S. Ku¨c¸u¨kbayrak / Fuel 79 (2000) 777–783
780 Table 4 Coal and calorific value recoveries Sample
Coal recovery (%)
Calorific value recovery (%)
Agacli Bolluca Tavsanli Malkara Yenikoy
85.1 86.3 97.1 78.3 82.4
83.4 85.9 83.9 76.4 86.3
Table 5 Ash content of the oxidised lignite samples and ash removals (dry basis) Sample
Ash content (%)
Ash removal (%)
Agacli Bolluca Tavsanli Malkara Yenikoy
8.6 6.0 13.3 9.4 6.7
27.1 39.4 14.2 31.4 30.2
results presented in literature [11]. Figs. 3 and 4 illustrate the relation between oxygen uptake and losses of carbon and hydrogen content. It can be seen from these figures that increasing oxygen uptake on the lignite samples led to marked losses in the carbon and hydrogen content. Table 4 presents coal and calorific value recoveries. Coal Fig. 6. FT-i.r. spectra of the oxidised lignite samples.
Fig. 5. FT-i.r. spectra of the original lignite samples.
recovery results changed between 78.3 and 97.1%, depending on the extent of the removal of some ash forming mineral matter, and decomposition of the organic structure of the lignite samples. Since the oxidation process was performed under acidic conditions, some acid-soluble mineral species were eliminated from the lignite samples, and consequently ashes of the oxidised lignite samples were reduced in the range of 14.2–39.4%. Results are tabulated in Table 5. When coal is exposed to an oxidising atmosphere in an aqueous medium, first its pyrite content is rapidly converted to sulphate. Previous works on this subject show that most of the pyrite content is removed at the very beginning of the reaction. Since the reaction between dissolved oxygen and pyrite is a heterogeneous reaction controlled by the diffusion of dissolved oxygen through the solid layer, reaction rate is slowed down as time proceeds [15]. On the other hand, further oxidation causes important changes in the organic matrix [16–18]. Decrease in the amount of the recovered lignite samples due to ash removal is accompanied by the decomposition of the organic coal matrix as a result of oxidation. In the initial stages of organic matrix oxidation, coals may show some weight gain due to incorporation of oxygen. However, this is soon replaced by progressive weight losses as some of the chemisorbed oxygen leaves the coal with carbon and hydrogen (mainly in the form of carbon oxides and water). In a cyclical process this is
S. Yaman, S. Ku¨c¸u¨kbayrak / Fuel 79 (2000) 777–783
781
Table 6 Comparison of mercury porosimetry measurements of the original and oxidised lignite samples Sample
Bulk density (g/ml)
Apparent density (g/ml)
Porosity (%)
Pore radius (mm)
Agacli-Original Agacli-Oxidised Bolluca-Original Bolluca-Oxidised Tavsanli-Original Tavsanli-Oxidised Malkara-Original Malkara-Oxidised Yenikoy-Original Yenikoy-Oxidised
1.15 1.01 1.02 1.01 1.12 1.02 0.86 0.95 1.00 0.99
1.33 1.33 1.36 1.33 1.41 1.29 1.22 1.37 1.38 1.25
10.3 23.4 24.7 23.0 16.3 20.0 39.3 33.8 27.2 20.9
1.40 0.88 1.62 0.99 0.64 0.70 1.23 0.84 1.42 1.00
Calorific value of the lignite samples tended to fall with increasing formation of oxygen bearing functional groups. Bond contributions to the heat of combustion are 49.3, 54.0,
and 117.4 kcal/mol for C–C, C–H, and CyC, respectively. However, C–O, O–H, and CyO bonds have combustion heats of 10.0, 7.5, and 13.5, respectively [19]. Figs. 5 and 6 illustrate FT-i.r. spectra of the original and oxidised lignite samples, respectively. Bands at 470 and 540 cm ⫺1 represent the mineral matter content of the lignite samples. 1030 and 1265 cm ⫺1 bands are due to C–O stretching. 1450 and 1610 cm ⫺1 bands are aromatic CyC stretching and CyO stretching bands, respectively. Aliphatic C–H groups have absorbance at 2854 cm ⫺1. Absorbance at 2923 cm ⫺1 shows the presence of –CH3 and –CH2 groups. Bands at about 3300 cm ⫺1 belong to O–H stretching. Beyond 3500 cm ⫺1, it is possible to expect the existence of the clay minerals. Effects of oxidation on the functional groups of the lignite samples are obvious. Absorbances of the bands representing C–O, CyO, and O–H increased as a
Fig. 7. Burning profiles of the original lignite samples.
Fig. 8. Burning profiles of the oxidised lignite samples.
replaced by newly sorbed oxygen that is subsequently similarly eliminated. Losses of carbon and hydrogen content due to oxygen uptake, and collapse of the organic matrix caused further reductions in the coal recovery results. Calorific value recoveries were between 76.4 and 86.3%. Calorific value of the lignite samples was effected from the variation of the elemental composition. Following Eq. (5) represents the dependence of loss of calorific value on losses of carbon and hydrogen contents and oxygen uptake. Loss of Calorific Value ⫺1636 ⫺ 906DC ⫺ 60DH ⫹ 892DO r2 0:864
5
S. Yaman, S. Ku¨c¸u¨kbayrak / Fuel 79 (2000) 777–783
782
Table 7 Comparison of some thermal data of the original and oxidised lignite samples Sample
Ignition temperature (K)
Peak temperature (K)
Maximum combustion rate (mg/min)
Burn-out temperature (K)
Agacli-Original Agacli-Oxidised Bolluca-Original Bolluca-Oxidised Tavsanli-Original Tavsanli-Oxidised Malkara-Original Malkara-Oxidised Yenikoy-Original Yenikoy-Oxidised
531 438 491 501 546 500 524 500 480 449
650 663 630 630 694 688 633 636 618 617
0.56 0.58 0.52 0.62 0.54 0.52 0.46 0.66 0.58 0.72
908 981 879 927 1011 1044 871 926 875 925
result of oxidation. On the other hand, intensities of C–H bands reduced. These spectra are the evidence of strong oxygen uptake on the lignite samples. Decrease of carbon and hydrogen content, and augmentation of oxygen content can be explained as the changes of the functional group distribution. Chemisorbed oxygen on the surface of the lignite samples led to formation of acidic functional groups such as –OH, –COOH, and CyO. Additionally, peroxide and hydroperoxide complexes might be formed as a result of oxidation. Since the pH of the oxidation medium was not alkaline, further degradation of the lignitic structures resulting from alkaline-soluble humic acids was prevented. Nevertheless, only 76.4% of the calorific value of Malkara lignite could be recovered. Mercury porosimetry measurements of the original and oxidised lignite samples are shown in Table 6. Bulk density and apparent density generally decreased after oxidation, except for Malkara lignite sample, which had the highest porosity among the samples. Pore radius of the oxidised lignite samples was generally lower than that of the original lignite samples. This indicates that pore structure of the lignite samples were extremely sensitive to oxidation. Pores of the lignite samples were plugged physically by oxygen. Moreover, formation of the oxygen bearing functional groups on the surfaces of the lignite samples caused chemical filling of the pores. These effects decreased the pore radius of the oxidised lignite samples. On the other hand, elimination of some mineral matter from the samples may lead to increase in the porosity to some extent. It can be concluded that the effect of oxidation on porosity is highly complex. Burning profiles of the original and oxidised lignite samples are presented in Figs. 7 and 8, and some thermal data are given in Table 7. In the burning profiles, the peaks originating from moisture release were not included. The temperatures at which dm/dt began to increase was allowed as the ignition temperature. The rate of the loss in mass of the lignite samples reached a maximum point at which the combustion rate was maximum and beyond that the rate slowed down and finally became zero. Temperature at the
zero rate point was recorded as the burn-out temperature. Burn-out temperatures for all of the samples increased as a result of oxidation. Ignition temperatures usually decreased in connection with oxidation. This can be explained by the increasing reactivity of the samples owing to oxidation and thermal treatment. Moreover, formation of the functional groups containing oxygen and structures of aldehydes and ketones on the organic coal matrix, resulting from oxidation, were reported to contribute to the decrease in the ignition temperature of the lignite samples [20]. Although some increase was observed in the ignition temperature of Bolluca lignite sample, the difference was not considerable. Likewise, the increase in the maximum combustion rate is the evidence of the increasing thermal reactivity. 4. Conclusion Lignitic structures are extremely sensitive to oxidation even under non-severe conditions. Very high concentrations of oxygen were chemically sorbed on the lignite samples leading to lower carbon and hydrogen content. On the other hand, proportional relations were determined between the extent of the oxygen uptake on the lignites, and carbon and hydrogen content of the original lignite samples. Functional group characteristics of the lignite samples changed regarding oxidation. Concentrations of C–O, CyO, and O–H groups increased and that of C–H bands decreased after oxidation. These changes affected the calorific value of the lignite samples negatively. Because of the acidic conditions, some mineral species were removed from the lignite samples. Thermal reactivity of the lignite samples increased after oxidation. Ignition temperature of lignite samples became lower, and maximum combustion rate became higher. Bulk and apparent densities, and pore radius of lignite samples reduced related to oxidation. References [1] Meyers RA. Coal desulfurization, New York: Marcel Dekker, 1977.
S. Yaman, S. Ku¨c¸u¨kbayrak / Fuel 79 (2000) 777–783 [2] Ku¨c¸u¨kbayrak S, Kadioglu E. Thermochim Acta 1989;132:289. [3] Chuang KC, Markuszewski R, Wheelock TD. Fuel Process Technol 1983;7:43. [4] Akhtar SS, Cliffe KR. In: Markuszewski R, Wheelock TD, editors. Processing and utilisation of high sulfur coals, 3. Amsterdam: Elsevier, 1990. p. 405. [5] Sareen SR. ACS Symp Ser 1977;64:173. [6] Palmer SR, Hippo EJ, Dorai XA. Fuel 1994;73:161. [7] Yaman S, Ku¨c¸u¨kbayrak S. Energy Sources 1996;18:461. [8] Yaman S, Ku¨c¸u¨kbayrak S. Energy Sources 1996;18:677. [9] Yaman S, Ku¨c¸u¨kbayrak S. Proceedings of the 13th Annual International Pittsburgh Coal Conference, 1996. p. 216–20. [10] Yaman S, Ku¨c¸u¨kbayrak S. Fuel 1997;76(1):73. [11] Klein R, Wellek R. Sample selection, aging and reactivity of coal, New York: Wiley, 1989.
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[12] Garcia AB, Moinelo SR, Martinez-Tarazona R, Tascon MD. Fuel 1991;70:1391. [13] Gorbaty ML, Kelemen SR, George GN, Kwiatek PJ. Fuel 1992;71:1255. [14] Wu MM, Baltrus JP, Winschel RA. In: Markuszewski R, Wheelock TD, editors. Processing and utilisation of high sulfur coals, 3. Amsterdam: Elsevier, 1990. [15] Yaman S, Ku¨c¸u¨kbayrak S. Fuel Process Technol 1996;47:195. [16] Azik M, Yu¨ru¨m Y, Gaines AF. Energy & Fuels 1993;7(3):367. [17] Azik M, Yu¨ru¨m Y, Gaines AF. Energy & Fuels 1994;8(3):798. [18] Azik M, Yu¨ru¨m Y, Gaines AF. Energy & Fuels 1994;8(1):188. [19] Berkowitz N. The chemistry of coal, coal science and technology, 7. Amsterdam: Elsevier, 1985. [20] Yaman S, Ku¨c¸u¨kbayrak S. Thermochim Acta 1997;293:109.
Influence of oxidation in aqueous medium on the structure and properties of lignites S. Yaman*, S. Ku¨c¸u¨kbayrak Istanbul Technical University, Chemical and Metallurgical Engineering Faculty, 80626, Maslak, Istanbul, Turkey Received 18 February 1999; received in revised form 10 September 1999; accepted 12 September 1999
Abstract In this paper, the effect of oxygen uptake on the structure and properties of lignites was investigated using five different Turkish lignites. Lignite samples were oxidised in aqueous medium at 423 K under 1.5 MPa partial pressure of oxygen for 60 min. Relations between the oxygen uptake and the elemental compositions of the lignite samples were investigated. FT-i.r. technique was applied to the original and oxidised lignite samples to investigate functional group analyses. Mercury porosimetry analyses of the samples were carried out up to about 220 MPa. Effect of oxygen uptake on the bulk density, apparent density, porosity, and pore radius of the lignite samples were compared. Differential Thermogravimetry (DTG) technique was performed to compare the thermal reactivity of the original and oxidised lignite samples. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: Lignite; Oxidation; Structure
1. Introduction Interaction of coal with oxygen takes place thermodynamically more easily and more rapidly, compared with other gases. Therefore, a number of desulphurization methods based on oxidation have been developed to apply to coal in either solid phase or liquid phase [1–6]. Oxidation by means of dissolved oxygen is one of the influential methods, in which oxygen is applied to a suspension of coal, and dissolved oxygen under pressure in the liquid phase, converts sulphur compounds into soluble sulphates. In order to increase the effectiveness of this process, some additional chemicals such as sodium carbonate, ammonia, calcium carbonate, borax, sodium hydroxide, etc., and some natural minerals and residues such as trona mineral and fly ash were used as promoters [7–10]. Since the electronegativity of carbon is the same as that of sulphur, oxidation leads to important variations in the organic structure and consequently in the properties of coal. Physical and chemical properties of coals except anthracite are highly sensitive to oxidative treatments. Oxidation of coal changes elemental composition, density, specific heat, mechanical strength, floatability, functional * Corresponding author. Tel.: ⫹90-212-285-6873; fax: ⫹90-212-2852925. E-mail address: [email protected] (S. Yaman).
groups, surface properties, spontaneous combustion, water holding capacity, pyrolytic behaviour, coking property, etc. [11–14]. These variations are proportional to the severity of the processes applied. Important changes take place in the properties of coal not only under strongly oxidative conditions, but also under weakly oxidative conditions these changes still occur to some extent. For example, when moist coal is exposed to air, it loses its moisture until reaching an equilibrium with the humidity of air. On the other hand, coal shrinks during water desorption and swells during re-adsorption of water. The level of the shrinking or swelling depends on the rank of the coal. Shrinking of the bituminous coals is almost negligible, whereas it is more noticeable for the low rank coals. Also, chemisorption of oxygen occurs when coal is subjected to oxygen. The extent of the chemisorption depends on the rank of coal and particle size, petrographic composition, surface area, pore size distribution, etc. There has been detailed information about structural changes resulting from oxidation of bituminous coals, but for lower rank coals information is limited. The aim of this paper is to present some information about changes in the chemical composition and some properties such as thermal reactivity, functional groups, porosity of lignites as a result of oxidation with dissolved oxygen in aqueous medium.
0016-2361/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(99)00207-0
S. Yaman, S. Ku¨c¸u¨kbayrak / Fuel 79 (2000) 777–783
778
Table 1 Proximate analysis and gross calorific value results of the lignite samples Sample
Moisture (%)
Agacli Bolluca Tavsanli Malkara Yenikoy
Dry basis
28.6 39.7 13.9 31.1 30.2
Volatile matter (%)
Ash (%)
Fixed carbon (%)
Gross calorific value (MJ/kg)
54.9 55.3 40.1 46.5 52.1
11.8 9.9 15.5 13.7 9.6
33.3 34.8 44.4 39.8 38.3
24.5 22.6 26.8 23.6 22.8
CVR % calorific value of recovered coal=calorific
2. Experimental In the experiments, five different Turkish lignite samples, Agacli, Bolluca, Tavsanli, Malkara, and Yenikoy, were used. The proximate and ultimate analyses were performed according to ASTM standards. Results are given in Tables 1 and 2. Oxidation of the lignite samples was carried out in a 1 l, magnetically stirred Parr autoclave made of 316 stainless steel. Inner surfaces of the autoclave were protected using a removable nickel liner. For each experiment, 10 g lignite having 250 mm particle size was thoroughly wetted with 300 ml distilled water. After sealing, 1.5 MPa partial pressure of oxygen was established and the autoclave was heated to 423 K. Stirring at 500 rpm was initiated when the temperature reached 423 K. The autoclave was held at the fixed temperature with the stirring rate for 60 min. For this period, suspended coal in the water was interacted with dissolved oxygen. Oxidation of pyrite during this process changed the pH of the medium from neutral to acidic. 4FeS2 ⫹ 15O2 ⫹ 8H2 O ! 2Fe2 O3 ⫹ 8H2 SO4
1
Some reductions were observed in the partial pressure of oxygen due to consumption of oxygen during this period. At the end of this batch process, the autoclave was rapidly cooled and the pressure was released. The suspension was filtered and washed several times with hot distilled water. The treated lignite was dried under nitrogen atmosphere in a vacuum oven at 383 K for 24 h. “Coal Recovery” (CR), “Calorific Value Recovery” (CVR) and “Ash Removal” (AR) were calculated using the following equations:
3
value of feed coal100 AR % ash content of feed coal-ash content of recovered coal=ash content of feed coal100
4
FT-i.r. spectra of the lignite samples were obtained using a Mattson 1000 Series FT-i.r. spectrometer. 300 mg of KBr pellets containing 1 wt% lignite were dried at 383 K for 36 h and spectra were obtained at a resolution of 8 cm ⫺1. Thermogravimetric analyses were performed using a Shimadzu TG 41 thermal analyser. 20 mg samples (after grinding to pass a 250 mm sieve) were spread uniformly on the bottom of an alumina crucible. The samples were oxidised in a dynamic dry air atmosphere of 40 ml min ⫺1. The temperature was raised from ambient to 1223 K at a heating rate of 10 K min ⫺1. Mercury porosimetry analyses of the lignite samples having 250 mm particle size were carried out using a Quantachrome Autoscan-33 mercury porosimeter, pressurising up to about 220 MPa. 3. Results and discussion
CR % weight of recovered coal=weight of feed coal100 2
Ultimate analysis results of the oxidised lignite samples are shown in Table 3. Since oxygen combines with carbon and hydrogen in the lignite samples, the extent of oxygen uptake on the lignite samples varied depending primarily on the abundance of carbon and hydrogen content. Relations between oxygen uptake, which was calculated from the difference of oxygen content of oxidised and original samples, and content of carbon and hydrogen are shown
Table 2 Ultimate analysis results of the original lignite samples (dry ash-free basis)
Table 3 Ultimate analysis results of the oxidised lignite samples (dry ash-free basis)
Sample
C (%)
H (%)
N (%)
S (%)
O a (%)
Sample
C (%)
H (%)
N (%)
S (%)
O a (%)
Agacli Bolluca Tavsanli Malkara Yenikoy
58.7 63.2 73.1 71.8 76.9
4.8 4.9 5.6 5.7 5.7
3.8 3.6 4.2 5.0 3.7
1.8 1.8 2.4 2.9 2.3
30.9 26.5 14.7 14.6 11.4
Agacli Bolluca Tavsanli Malkara Yenikoy
53.5 61.4 57.7 65.9 65.5
4.0 4.1 3.2 4.3 4.7
2.4 2.4 4.3 4.3 2.9
1.0 0.8 1.1 1.3 1.0
39.1 31.3 33.7 24.2 25.9
a
Calculated by difference.
a
Calculated by difference.
S. Yaman, S. Ku¨c¸u¨kbayrak / Fuel 79 (2000) 777–783
Fig. 1. Relation between carbon content (daf) of the original lignite samples and oxygen uptake.
in Figs. 1 and 2. More oxygen uptake occurred when the lignite samples contained high content of carbon and hydrogen. The carbon and hydrogen content of the oxidised lignite samples decreased to the values at which it can be said that they reached about their equilibrium values under investigated conditions. The range of the carbon content of the oxidised lignite samples was 53.5–65.9% (daf), whereas that of the original lignite samples was 58.7– 76.9% (daf).
Fig. 2. Relation between hydrogen content (daf) of the original samples and oxygen uptake.
779
Fig. 3. Relation between oxygen uptake and loss of carbon content.
Likewise, the hydrogen content of the oxidised lignite samples was between 3.2 and 4.7%, the hydrogen content of the original lignite samples was in the range of 4.8–5.7% (daf). In the initial step of oxidation, oxygen diffuses into pores of lignites and fills the apertures by physical means. Then, as oxidation proceeds, this adsorbed oxygen is combined with the coal matrix chemically. Further oxidation converts chemisorbed oxygen into surface oxides. Formation of surface oxides accelerates the formation of some decomposition products such as CO2, H2O, CO, CXHY, H2 from the organic matrix. Thus, considerable losses took place in the carbon and hydrogen content of the oxidised lignite samples. These findings are in good agreement with the
Fig. 4. Relation between oxygen uptake and loss of hydrogen content.
S. Yaman, S. Ku¨c¸u¨kbayrak / Fuel 79 (2000) 777–783
780 Table 4 Coal and calorific value recoveries Sample
Coal recovery (%)
Calorific value recovery (%)
Agacli Bolluca Tavsanli Malkara Yenikoy
85.1 86.3 97.1 78.3 82.4
83.4 85.9 83.9 76.4 86.3
Table 5 Ash content of the oxidised lignite samples and ash removals (dry basis) Sample
Ash content (%)
Ash removal (%)
Agacli Bolluca Tavsanli Malkara Yenikoy
8.6 6.0 13.3 9.4 6.7
27.1 39.4 14.2 31.4 30.2
results presented in literature [11]. Figs. 3 and 4 illustrate the relation between oxygen uptake and losses of carbon and hydrogen content. It can be seen from these figures that increasing oxygen uptake on the lignite samples led to marked losses in the carbon and hydrogen content. Table 4 presents coal and calorific value recoveries. Coal Fig. 6. FT-i.r. spectra of the oxidised lignite samples.
Fig. 5. FT-i.r. spectra of the original lignite samples.
recovery results changed between 78.3 and 97.1%, depending on the extent of the removal of some ash forming mineral matter, and decomposition of the organic structure of the lignite samples. Since the oxidation process was performed under acidic conditions, some acid-soluble mineral species were eliminated from the lignite samples, and consequently ashes of the oxidised lignite samples were reduced in the range of 14.2–39.4%. Results are tabulated in Table 5. When coal is exposed to an oxidising atmosphere in an aqueous medium, first its pyrite content is rapidly converted to sulphate. Previous works on this subject show that most of the pyrite content is removed at the very beginning of the reaction. Since the reaction between dissolved oxygen and pyrite is a heterogeneous reaction controlled by the diffusion of dissolved oxygen through the solid layer, reaction rate is slowed down as time proceeds [15]. On the other hand, further oxidation causes important changes in the organic matrix [16–18]. Decrease in the amount of the recovered lignite samples due to ash removal is accompanied by the decomposition of the organic coal matrix as a result of oxidation. In the initial stages of organic matrix oxidation, coals may show some weight gain due to incorporation of oxygen. However, this is soon replaced by progressive weight losses as some of the chemisorbed oxygen leaves the coal with carbon and hydrogen (mainly in the form of carbon oxides and water). In a cyclical process this is
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Table 6 Comparison of mercury porosimetry measurements of the original and oxidised lignite samples Sample
Bulk density (g/ml)
Apparent density (g/ml)
Porosity (%)
Pore radius (mm)
Agacli-Original Agacli-Oxidised Bolluca-Original Bolluca-Oxidised Tavsanli-Original Tavsanli-Oxidised Malkara-Original Malkara-Oxidised Yenikoy-Original Yenikoy-Oxidised
1.15 1.01 1.02 1.01 1.12 1.02 0.86 0.95 1.00 0.99
1.33 1.33 1.36 1.33 1.41 1.29 1.22 1.37 1.38 1.25
10.3 23.4 24.7 23.0 16.3 20.0 39.3 33.8 27.2 20.9
1.40 0.88 1.62 0.99 0.64 0.70 1.23 0.84 1.42 1.00
Calorific value of the lignite samples tended to fall with increasing formation of oxygen bearing functional groups. Bond contributions to the heat of combustion are 49.3, 54.0,
and 117.4 kcal/mol for C–C, C–H, and CyC, respectively. However, C–O, O–H, and CyO bonds have combustion heats of 10.0, 7.5, and 13.5, respectively [19]. Figs. 5 and 6 illustrate FT-i.r. spectra of the original and oxidised lignite samples, respectively. Bands at 470 and 540 cm ⫺1 represent the mineral matter content of the lignite samples. 1030 and 1265 cm ⫺1 bands are due to C–O stretching. 1450 and 1610 cm ⫺1 bands are aromatic CyC stretching and CyO stretching bands, respectively. Aliphatic C–H groups have absorbance at 2854 cm ⫺1. Absorbance at 2923 cm ⫺1 shows the presence of –CH3 and –CH2 groups. Bands at about 3300 cm ⫺1 belong to O–H stretching. Beyond 3500 cm ⫺1, it is possible to expect the existence of the clay minerals. Effects of oxidation on the functional groups of the lignite samples are obvious. Absorbances of the bands representing C–O, CyO, and O–H increased as a
Fig. 7. Burning profiles of the original lignite samples.
Fig. 8. Burning profiles of the oxidised lignite samples.
replaced by newly sorbed oxygen that is subsequently similarly eliminated. Losses of carbon and hydrogen content due to oxygen uptake, and collapse of the organic matrix caused further reductions in the coal recovery results. Calorific value recoveries were between 76.4 and 86.3%. Calorific value of the lignite samples was effected from the variation of the elemental composition. Following Eq. (5) represents the dependence of loss of calorific value on losses of carbon and hydrogen contents and oxygen uptake. Loss of Calorific Value ⫺1636 ⫺ 906DC ⫺ 60DH ⫹ 892DO r2 0:864
5
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Table 7 Comparison of some thermal data of the original and oxidised lignite samples Sample
Ignition temperature (K)
Peak temperature (K)
Maximum combustion rate (mg/min)
Burn-out temperature (K)
Agacli-Original Agacli-Oxidised Bolluca-Original Bolluca-Oxidised Tavsanli-Original Tavsanli-Oxidised Malkara-Original Malkara-Oxidised Yenikoy-Original Yenikoy-Oxidised
531 438 491 501 546 500 524 500 480 449
650 663 630 630 694 688 633 636 618 617
0.56 0.58 0.52 0.62 0.54 0.52 0.46 0.66 0.58 0.72
908 981 879 927 1011 1044 871 926 875 925
result of oxidation. On the other hand, intensities of C–H bands reduced. These spectra are the evidence of strong oxygen uptake on the lignite samples. Decrease of carbon and hydrogen content, and augmentation of oxygen content can be explained as the changes of the functional group distribution. Chemisorbed oxygen on the surface of the lignite samples led to formation of acidic functional groups such as –OH, –COOH, and CyO. Additionally, peroxide and hydroperoxide complexes might be formed as a result of oxidation. Since the pH of the oxidation medium was not alkaline, further degradation of the lignitic structures resulting from alkaline-soluble humic acids was prevented. Nevertheless, only 76.4% of the calorific value of Malkara lignite could be recovered. Mercury porosimetry measurements of the original and oxidised lignite samples are shown in Table 6. Bulk density and apparent density generally decreased after oxidation, except for Malkara lignite sample, which had the highest porosity among the samples. Pore radius of the oxidised lignite samples was generally lower than that of the original lignite samples. This indicates that pore structure of the lignite samples were extremely sensitive to oxidation. Pores of the lignite samples were plugged physically by oxygen. Moreover, formation of the oxygen bearing functional groups on the surfaces of the lignite samples caused chemical filling of the pores. These effects decreased the pore radius of the oxidised lignite samples. On the other hand, elimination of some mineral matter from the samples may lead to increase in the porosity to some extent. It can be concluded that the effect of oxidation on porosity is highly complex. Burning profiles of the original and oxidised lignite samples are presented in Figs. 7 and 8, and some thermal data are given in Table 7. In the burning profiles, the peaks originating from moisture release were not included. The temperatures at which dm/dt began to increase was allowed as the ignition temperature. The rate of the loss in mass of the lignite samples reached a maximum point at which the combustion rate was maximum and beyond that the rate slowed down and finally became zero. Temperature at the
zero rate point was recorded as the burn-out temperature. Burn-out temperatures for all of the samples increased as a result of oxidation. Ignition temperatures usually decreased in connection with oxidation. This can be explained by the increasing reactivity of the samples owing to oxidation and thermal treatment. Moreover, formation of the functional groups containing oxygen and structures of aldehydes and ketones on the organic coal matrix, resulting from oxidation, were reported to contribute to the decrease in the ignition temperature of the lignite samples [20]. Although some increase was observed in the ignition temperature of Bolluca lignite sample, the difference was not considerable. Likewise, the increase in the maximum combustion rate is the evidence of the increasing thermal reactivity. 4. Conclusion Lignitic structures are extremely sensitive to oxidation even under non-severe conditions. Very high concentrations of oxygen were chemically sorbed on the lignite samples leading to lower carbon and hydrogen content. On the other hand, proportional relations were determined between the extent of the oxygen uptake on the lignites, and carbon and hydrogen content of the original lignite samples. Functional group characteristics of the lignite samples changed regarding oxidation. Concentrations of C–O, CyO, and O–H groups increased and that of C–H bands decreased after oxidation. These changes affected the calorific value of the lignite samples negatively. Because of the acidic conditions, some mineral species were removed from the lignite samples. Thermal reactivity of the lignite samples increased after oxidation. Ignition temperature of lignite samples became lower, and maximum combustion rate became higher. Bulk and apparent densities, and pore radius of lignite samples reduced related to oxidation. References [1] Meyers RA. Coal desulfurization, New York: Marcel Dekker, 1977.
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