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Effect of biomass on burnouts of Turkish lignites during co-firing
Energy Conversion and Management 50 (2009) 2422–2427
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Effect of biomass on burnouts of Turkish lignites during co-firing H. Haykiri-Acma, S. Yaman * Istanbul Technical University, Chemical and Metallurgical Engineering Faculty, Chemical Engineering Department, 34469 Maslak, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 23 October 2008 Accepted 30 May 2009 Available online 21 June 2009 Keywords: Coal Lignite Biomass Sunflower Co-combustion
a b s t r a c t Co-firing of some low quality Turkish lignites with woody shells of sunflower seed was investigated via non-isothermal thermogravimetric analysis method. For this purpose, Yozgat-Sorgun, Erzurum-Askale, Tuncbilek, Gediz, and Afsin-Elbistan lignites were selected, and burnouts of these lignites were compared with those of their blends. Biomass was blended as much as 10 and 20 wt.% of the lignites, and heating was performed up to 900 °C at a heating rate of 40 °C/min under dry air flow of 40 mL/min. This study revealed that the same biomass species may have different influences on the burnout yields of the lignites. Burnouts of Erzurum-Askale lignite increased at any temperature with the increasing ratio of biomass in the blend, whereas burnout yields of other lignites decreased to some extent. Nevertheless, the blends of Turkish lignites with sunflower seed shell did not behave in very different way, and it can be concluded that they are compatible in terms of burnouts for co-combustion in a combustion system. Although the presence of biomass in the lignite blends caused to some decreases in the final burnouts, the carbon dioxide neutral nature of biomass should be taken into account, and co-combustion is preferable for waste-to-energy-management. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The ‘‘White Paper Report” on energy for future-renewable sources of energy sets an indicative target of 12% of the EU total energy consumption from renewable energy sources by 2010 in all EU member countries [1]. At this point, the biomass fraction of Refuse Derived Fuel (RDF) and Solid Recovered Fuel (SRF) is crucial on greenhouse gas protocols, such as the European Union Emissions Trading Scheme and the Renewable Obligation Certificate program in the United Kingdom. The combusted biomass fraction of RDF/SRF is used by stationary combustion operators to reduce their overall reported CO2 emissions since biomass is regarded to be CO2-neutral. Considering the use of waste-derived fuels, the European directive 2001/77/EC on electricity from renewable energy sources promotes the co-utilization of SRF in large-scale energy production [2]. However, according to the definitions of the European initiative on SRF standardisation CEN/TC 343, such fuels have to be produced from non-hazardous bio-residues, mixed- and mono-waste streams [3]. 2000/76/EC European legislation set strict limits not only for the mono-combustion of waste in incineration plants but also for cocombustion in coal-fired power plants [4]. More stringent regulations came into force in Europe by The EU directive 2001/80/EC
* Corresponding author. Tel.: +90 212 2856873; fax: +90 212 2852925. E-mail address: [email protected] (S. Yaman). 0196-8904/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.05.026
with respect to the emission of certain atmospheric pollutants from power plants [5]. As a member country to EU, Turkey must take effective measures to decrease emissions from power stations in which low quality Turkish lignites are burned. In fact, this is a serious problem in Turkey since these lignites take place the most important national primary energy sources. Consequently, about 8.5% of world lignite consumption is carried out by Turkey alone. Co-combustion of biomass with coal usually helps to reduce the emissions such as SO2, NOx, CO and CO2 per unit energy generated [6]. On the other hand, some negative aspects coming from intrinsic properties of biomass may take place during co-combustion if unsuitable biomass species is handled. There has been concern that the impurities in some biomasses and wastes, particularly the alkali metals and halogens, could cause operational problems with regard to slagging, fouling or corrosion. Besides, some of the heavy metals can volatilize at temperatures generally applied during co-combustion [7]. Also in case of combustion of specific biomass species such as sewage sludge, high nitrogen content leads to high emissions of NO, NO2, and N2O. Deactivation of DeNOx-catalysts by phosphorus during co-combustion of sewage sludge or ‘‘meat and bone meal” is a common problem [8]. Moreover, the emissions of dioxins could be considerable when fuels with high chlorine content are used. Turkey has an important potential of agricultural waste materials which can be used for energetic purposes. Furthermore, most of these wastes are non-hazardous materials and they are sustainably-grown biomass. For instance, sunflower (Helianthus annus L.)
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seed shell can be regarded as a suitable biomass species with an annual amount of 850,000 tones. So a huge amount of woody shell is obtained after peeling of the seeds in the vegetable oil industry. Besides, the physical and chemical properties of this agricultural woody waste can be regarded to meet the conditions described by above EU directives. For example, concentrations of arsenic, cadmium, and mercury in this biomass are below 1 ppm, while concentrations of chromium, lead, nickel, and vanadium are approximately 2 ppm [9]. Chlorine content is also only 0.05% [10]. Low ash content of this biomass also makes it a favorable alternative fuel. However, one of the challenges of co-combustion is to manage different type of fuels in the same combustion system. Burnout of each fuel plays important role in determining not only the performance of individual fuel but also their compatibility under the same conditions. In this context, synergistic or antagonistic interactions often happen between the constituents of different type of fuels, and accordingly burnouts during co-combustion may be closely affected from these interactions. Experience on co-utilization of a given biomass species with low rank coals is rather limited, and most of such investigations have been conducted using higher rank coals such as bituminous or sub-bituminous coals. Thus, this study addresses the burnout characteristics of low rank Turkish lignites with woody shells of sunflower seed under non-isothermal co-combustion conditions.
2. Experimental Yozgat-Sorgun (YS), Erzurum-Askale (EA), Tuncbilek (T), Gediz (G), and Afsin-Elbistan (AE) lignites were selected for this study since they represent an important part of the Turkish lignites with high deposits. The ages of these coals are Eocene, Miocene, Miocene, Middle Miocene, and Pliocene for YS, EA, T, G, and AE lignites, respectively [11–13]. Air-dried lignite samples and sunflower seed shell (SSS) were grounded to particle size of <0.25 mm. Proximate analysis and the calorific value measurements were performed according to ASTM standards. Elementary analysis was conducted using a Eurovector EuroEA3000 model elemental analyzer. Major biomass ingredients of SSS such as holocellulosics (hemicellulosics + celluloses), lignin, extractives, and a-cellulose were determined by the following analytical procedures. Extractive components were determined according to ASTM D1105 (2001). The sample remaining after alcohol-benzene extractions was used to obtain the holocellulosics by means of NaClO2 extraction procedure. The lignin content of the sample was found according to the method of Van Soest [14]. a-cellulose content was determined according to TAPPI T203 om-88 standard (1988). Burning characteristics of the fuel samples were investigated by non-isothermal thermogravimetric analysis method using a Shimadzu TG 41 thermal analyzer with a cylindrical alumina crucible. Initial sample weight was fixed at 40 mg for mono-combustion experiments of either coals or biomass. Although there have been some studies in literature reporting that 10–30 wt.% is the optimum range for blending of biomass with coal in terms of minimum pollutant emissions per unit energy output, co-firing up to 20 wt.% biomass with coal in utility boilers gave the best results [5,6]. Thus, the biomass additions in this study were chosen as 10 and 20 wt.% of lignite so the initial sample weights in case of co-combustion tests were 44 and 48 mg, respectively. Heating up to 900 °C was performed at the heating rate of 40 °C/ min under dry air flow of 40 mL/min, and enough hold time was allowed to reach an unchanged weight. When temperature
reached to 105 °C, the sample was allowed to stay at this temperature for 10 min to eliminate the moisture. All the experiments were repeated three times to check the reproducibility, and the mean of the closest two results was used. 3. Results and discussion Tables 1 and 2 show the analysis results of biomass and lignite samples. It is seen from data on these tables that calorific value of biomass is lower than those of two lignite samples (YS and G), while it is higher than those of other two lignite samples (T and AE). Surprisingly, calorific values of biomass and EA lignite are exactly the same. Since the previous studies in literature have usually been conducted using coal samples that have obviously higher calorific values compared to biomass materials tested, in those studies biomass has been regarded as a factor that decreases the heat capacity of the system. Thus, burning of biomass is often considered not to give high energy output when compared with coal combustion due to the high volatile matter- and moisture-contents. In this study, however, calorific value of the biomass species is comparable to the Turkish lignites, offering a special occasion for co-processing. The moisture content of air-dried biomass is about 10 wt.%, which is noticeably lower than that for usual biomass species. It is widely known that high content of moisture in biomass leads to lower combustion temperatures as well as increasing CO emissions [15]. From this point of view, SSS is a favorable secondary fuel that can be added to a base fuel.
Table 1 Analyses results of sunflower seed shell. Proximate analysis (dry basis) Volatiles (%) Fixed carbon (%) Ash (%) Higher calorific value (MJ/kg)
84.7 11.7 3.6 17.6
Ultimate analysis (dry ash free basis) C (%) H (%) N (%) Oa (%)
51.7 6.2 1.0 41.1
Chemical analysis (dry basis) Extractives (%) Holocellulose (%) Lignin (%) a-cellulose (%)
13.8 62.5 31.4 28.5
a
Calculated by difference.
Table 2 Analyses results of lignites. Proximate analysis (%, dry)
YS
EA
T
G
AE
Volatiles Fixed carbon Ash
33.5 57.0 9.5
31.9 25.6 42.5
29.1 26.0 44.9
32.6 52.6 14.8
50.5 17.2 32.3
Ultimate analysis (%, daf) C H N S Oa
74.0 5.1 1.6 2.4 16.9
70.8 4.6 2.2 5.3 17.1
71.8 5.6 2.6 2.8 17.2
74.4 5.6 1.3 8.7 10.0
56.5 5.3 2.0 3.1 33.1
Higher calorific value (MJ/kg, dry)
29.0
17.6
13.4
30.1
10.1
a
Calculated by difference.
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H. Haykiri-Acma, S. Yaman / Energy Conversion and Management 50 (2009) 2422–2427
10
50
SSS
dm/dt (mg/min)
m (mg)
SSS
8
40
30
20
10
6
4
2
0
0 0
250
500
750
1000
0
250
500
750
1000
Temperature (°C)
Temperature (°C) Fig. 1. TGA and DTG curves of sunflower seed shell.
Fig. 1 indicates that combustion curve of SSS can be divided into two stages representing rapid devolatilization at relatively low temperatures resulting from thermally unstable components such as hemicellulosics and celluloses, and followed by slow burning of fixed carbon at higher temperatures up to 450 °C, and possibly accompanied by decomposition of inorganics. An important part
50
YS
EA
T
G
AE
m (mg)
40
30
20
10
0 0
250
500
750
1000
Temperature (°C) Fig. 2. Comparison of the thermal reactivities of lignite samples.
of the carbonaceous biomass structure quickly decomposed during the former stage, and the rate of the mass loss reached to 8.3 mg/ min at 275 °C during this stage. Whereas, the maximum burning rate was only 1.7 mg/min at 765 °C in case of char burning stage. The end temperature of combustion for SSS was determined as 900 °C so no hold time was in question under the working conditions. Compared to lignites used in this study, the highest content of volatiles, the lowest-fixed carbon and -ash contents belong to SSS. These inherent characteristics of SSS are the indication of its special structure and thermal reactivity that differentiates it from coals. In the design of fluidized bed combustor, this high content of volatiles must be considered since an important part of combustion may take place at the upper regions above the bed. But, such high contents of volatiles should not be always evaluated as a problem. For instance, some waste biomass materials such as textile residues have been reported to burn with so low burning rates that it often deteriorates the combustion performance of the whole system [16]. It is well-known that problematic deposit formations such as fouling owing to biomass materials during co-combustion systems are usually diminished if an interaction happens between the sulfur content of coal and some inorganics found in biomass. Considering the fact that SSS contains low ash, and Turkish lignites are rich in sulfur, so it is possible to predict that an appropriate condition is available for these binary mixtures to eliminate the likely fouling problem resulting mainly from biomass.
Table 3 Burnout characteristics. Temperatures (°C) Sample
200
300
400
500
600
700
800
900
Final
thold (min)
Rhold (mg/min)
Burnout at hold (%)
YS YS10% YS20% EA EA10% EA20% T T10% T20% G G10% G20% AE AE10% AE20%
12.1 11.6 11.1 6.5 6.8 9.2 13.6 15.5 14.6 4.5 12.0 9.4 26.0 21.0 26.7
15.1 19.3 19.2 8.7 13.1 16.6 20.0 24.7 24.3 7.3 19.2 18.0 35.3 38.2 39.5
25.5 29.4 30.2 13.8 18.3 22.3 29.4 32.7 32.0 17.6 30.7 26.6 45.8 47.5 46.2
33.2 35.1 34.4 20.0 24.3 27.1 37.1 39.8 38.5 25.0 35.0 31.5 50.5 53.3 52.1
39.2 40.4 39.7 25.0 28.9 31.7 44.3 46.6 42.9 31.5 38.9 36.2 57.5 56.8 56.2
44.6 44.6 44.2 30.0 33.4 36.6 47.1 49.5 46.3 37.7 44.5 40.1 64.6 62.8 61.5
48.5 49.8 47.4 35.0 37.5 39.4 51.2 54.3 49.8 43.9 50.7 45.0 71.0 67.6 67.7
55.8 54.5 51.7 39.2 43.2 42.5 57.5 60.7 55.0 48.8 56.8 49.0 80.0 71.6 71.2
98.0 90.2 82.1 60.0 56.1 53.1 90.0 83.6 77.1 86.2 92.0 73.5 85.0 83.6 87.5
25 17 22 16 9 9 15 13 13 23 16 15 14 7 10
0.68 0.92 0.66 0.52 0.63 0.57 0.87 0.78 0.82 0.65 0.97 0.79 0.14 0.76 0.78
42.2 35.7 30.4 18.9 13.0 10.6 32.5 23.0 22.1 37.5 35.2 24.6 5.0 12.0 16.2
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Fig. 2 displays a comparison among the thermal reactivities of the lignite samples. In contrast to burning profile of the biomass sample, all the lignite samples needed some hold times at 900 °C for complete combustion. According to Fig. 2, irrespective of temperature, under the process conditions lignite AE gave the maximum burnout values and basing on this it can be said that this lignite is thermally most reactive coal sample. Notice that lignite
80
AE contains the lowest-fixed carbon and highest volatiles contents, and a rapid devolatilization took place even at low temperatures, and it went on until the end temperature. Table 3 presents the variation of the burnout yields depending on temperature as well as the final burnouts, and hold time behaviors. On the other hand, the lowest thermal reactivity was detected for lignite EA from 400 °C to the end of the experiment, whereas at
80
YS YS10% YS20%
60
Burnout (%)
Burnout (%)
60
EA EA10% EA20%
40
20
40
20
0
0 0
250
500
750
1000
0
250
Temperature (C)
80
750
1000
750
1000
Temperature (C)
80
T T10% T20%
60
G G10% G20%
Burnout (%)
60
40
20
40
20
0
0 0
250
500
750
1000
0
250
Temperature (C)
500
Temperature (C)
80
60
Burnout (%)
Burnout (%)
500
40
20
AE AE10% AE20%
0 0
250
500
750
Temperature (C) Fig. 3. Burnouts of the lignites and their blends.
1000
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lower temperatures such as 200 and 300 °C lignite G showed the lowest burnouts. The low thermal reactivity of lignite G can be explained by its low content of oxygen, since the higher oxygen content the higher thermal reactivity. Conversely, lignite G is the sample that has the maximum content of carbon. As to lignite EA, it is rich in ash forming minerals, and some inorganic constituents may show inhibiting effect on burning, leading low thermal reactivity. Lignite samples can be arranged in the increasing order of reactivity as EA, G, YS, T, and AE. In addition to, the times elapsed during hold time and consequently burning at this stage designates significant variations. Although the coal samples used in this study were low rank and consequently they should have been thermally high reactive, the applied thermal process with a heating rate of 40 °C/min was not provide adequate residence time to assure the complete burning of the samples while it was being heated up to 900 °C. Burnouts at 900 °C were 55.8%, 39.2%, 57.5%, 48.8%, and 80.0% of the initial weight on original basis for YS, EA, T, G, and AE lignites, respectively. Then, a great deal of additional burnouts took place during hold time at 900 °C for most of the samples. That is, final burnouts reached to 98.0, 60.0, 90.0, 86.2, and 85.0 wt.% for YS, EA, T, G, and AE lignites, respectively. The increase in the burnout of lignite YS from 55.8% to 98.0% is very striking. Lignite YS has the highest fixed carbon content among the samples, and heterogeneous combustion of fixed carbon occurs mostly at high temperatures. So important part of combustion for this lignite took place during the hold time period. Similarly, lignite G, which is the other coal with high fixed carbon, also showed serious increase in burnout from 48.8 to 86.2 wt.%. Hold times (thold) of 25, 16, 15, 23, and 14 min were determined to reach the final burnout values for YS, EA, T, G, and AE lignites, respectively. The lengths of the hold times can be described by the fixed carbon contents of the lignites. As expected the longest hold time was determined for the lignite YS which contains the highest fixed carbon content so it necessitated the longest time for complete burning. In this context, the shortest hold time was detected for the sample AE having the minimum fixed carbon content. On the other hand, the ratios of the burnouts during hold times to the final burnouts indicated that considerable variations are available among the lignites. Moreover, except for lignite AE, addition of biomass to lignites played an apparent role on the contribution of hold time to the final burnouts in the favor of decrease. This circumstance may be resulted from two possible reasons. Either the presence of biomass makes the burning faster before entering hold time or it terminates the burning quickly. In this case, the latter is main reason for the decreasing contribution of hold time. Consequently, the extents of the final burnouts diminished in general after blending with biomass. Burnout rates (Rhold) during hold time were calculated basing on the burnouts during hold time and the extent of the corresponding time taken during this stage, and they were 0.68, 0.52, 0.87, 0.65, and 0.14 mg/min for YS, EA, T, G, and AE lignites, respectively. Lignite T has the highest burnout rate during hold time, and meanwhile this lignite is the richest sample with respect to ash. So it is possible to conclude that an important part of the mass losses during hold time may be resulted from the decomposition of the mineral phases found in the coal. On the other hand, lignite AE which has the highest volatiles content among the coal samples gave the lowest burnout rates during hold time. This is probably due to the fact that a significant part of this sample was burned or at least eliminated in the form of volatiles, causing relatively less solid structure left to burn. Fig. 3 presents the distribution of the burnout values before hold time in cases of with or without biomass additions, and at the first sight it is seen from the parallel curves that as if there were no important changes in the trends of the weight loss charac-
teristics from some lignites such as YS, T, and AE. However, final burnouts for these lignites affected negatively and serious decreases took place. Atimtay and Kaynak also reported that when co-combustion of some fruit stones was carried out with a lignite, efficiencies of the burning system decreases to some extent [17]. Another similar remark on the reduced system efficiency due to presence of biomass was proposed by McIlveen-Wright et al. [5]. On the other hand, lignite EA revealed a different status that biomass addition increased the burnout as proportional to the biomass added. So the highest burnout values were obtained in case of the blend 20% up to 900 °C. This is in good accordance with the findings of Chao et al. who reported that high volatile matter content of biomass plays a key role in improving the combustion performance in the system [6]. But, the final burnouts of the blends of lignite EA could not reach to that for this lignite alone. Besides, lignite G behaved in different way when it has blended with biomass. This sample showed a non-additive behavior that the presence of 10 wt.% of biomass increased the mass losses from this sample. However, when the addition of biomass was raised to 20 wt.%, not only the same effect could not be observed but also some decrease in burnout occurred. This predicts that the amount of the biomass in the blend is crucial to increase or decrease the burnout of this lignite. These results suggest that within the blending ratios tested in this study, addition of biomass to lignites did not seriously worsen the burning behavior of the lignites. The decreases in the burnout yields may be tolerated. In addition, some problematic features of these low quality coals can be compensated by blending with biomass. Besides, energy potential of this renewable waste biomass material would be evaluated, contributing to lower CO2 emissions. 4. Conclusions Sunflower seed shell is a potential fuel that can be utilized in lignite firing systems by the aim of waste-to-energy management. When low rank coals such as Turkish lignites are handled, sunflower seed shell is a favorable material in terms of calorific value and low ash in order to compensate the negative aspects of these coals. On the other hand, combustion of this biomass species ends in a shorter time compared to those for lignites. This study revealed that the same biomass material may be able to play different roles on the burnout yields of the Turkish lignites. Burnouts of EA lignite increased with the increasing ratio of biomass in the blend, whereas burnout yields of other lignites decreased to some extent. Nevertheless, the blends of Turkish lignites with sunflower seed shell did not behave in very different way, and they can be co-combusted in the existing combustion system in the blending ratios carried out in the present study. Although biomass addition to lignite led to some decrease in the burnout, the carbon dioxide neutral nature of biomass should be taken into account, and co-combustion should be considered. References [1] Report: white paper for a community strategy and action plan. Communication from the commission energy for the future: renewable sources of energy, COM (97) 599 final (26/11/1997). [2] Hilber Th, Thorwarth H, Stack-Lara V, Schneider V, Maier J, Scheffknecht G. Fate of mercury and chlorine during SRF co-combustion. Fuel 2007;86:1935–46. [3] Hilber Th, Martensen M, Maier J, Scheffknecht G. A method to characterize the volatile release of solid recovered fuels (SRF). Fuel 2007;86:303–8. [4] Werther J. Gaseous emissions from waste combustion. J Hazard Mater 2007;144:604–13. [5] McIlveen-Wright DR, Huang Y, Rezvani S, Wang Y. A technical and environmental analysis of co-combustion of coal and biomass in fluidised bed technologies. Fuel 2007;86:2032–42. [6] Chao CYH, Kwong PCW, Wang JH, Cheung CW, Kendall G. Co-firing coal with rice husk and bamboo and the impact on particulate matters and associated
H. Haykiri-Acma, S. Yaman / Energy Conversion and Management 50 (2009) 2422–2427
[7]
[8] [9]
[10] [11]
polycyclic aromatic hydrocarbon emissions. Bioresource Technol 2008;99:83–93. Lievens C, Yperman J, Cornelissen T, Carleer R. Study of the potential valorisation of heavy metal contaminated biomass via phytoremediation by fast pyrolysis: Part II: Characterisation of the liquid and gaseous fraction as a function of the temperature. Fuel 2008;87:1906–16. Jens B, Sven U. The behavior of particle bound phosphorus during the combustion of phosphate doped coal. Fuel 2007;86:632–40. Ooi TC, Aries E, Ewan BCR, Thompson D, Anderson DR, Fisher R, et al. The study of sunflower seed husks as a fuel in the iron ore sintering process. Miner Eng 2008;21:167–77. Demirbas A. Effect of temperature on pyrolysis products from four nut shells. J Anal Appl Pyrol 2006;76:285–9. Palmer CA, Tuncalı E, Dennen KO, Coburn TC, Finkelman RB. Characterization of Turkish coals: a nationwide perspective. Int J Coal Geol 2004;60:85–115.
2427
[12] Karayigit AI, Gayer RA, Querol X, Onacak T. Contents of major and trace elements in feed coals from Turkish coal-fired power plants. Int J Coal Geol 2000;44:169–84. [13] Karayigit AI, Spears DA, Booth CA. Antimony and arsenic anomalies in the coal seams from the Gokler coalfield, Gediz, Turkey. Int J Coal Geol 2000;44:1–17. [14] Van Soest PJ. Use of detergents in the analysis of fibrous feeds. II. A rapid method for the determination of fiber and lignin. J Assoc Offic Anal Chem 1963;46:829–35. [15] Kwong PCW, Chao CYH, Wang JH, Cheung CW, Kendall G. Co-combustion performance of coal with rice husks and bamboo. Atmos Environ 2007;41:7462–72. [16] Ryu C, Phan AN, Sharifi VN, Swithenbank J. Co-combustion of textile residues with cardboard and waste wood in a packed bed. Exp Therm Fluid Sci 2007;32:450–8. [17] Atimtay AT, Kaynak B. Co-combustion of peach and apricot stone with coal in a bubbling fluidized bed. Fuel Process Technol 2008;89:183–97.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Effect of biomass on burnouts of Turkish lignites during co-firing H. Haykiri-Acma, S. Yaman * Istanbul Technical University, Chemical and Metallurgical Engineering Faculty, Chemical Engineering Department, 34469 Maslak, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 23 October 2008 Accepted 30 May 2009 Available online 21 June 2009 Keywords: Coal Lignite Biomass Sunflower Co-combustion
a b s t r a c t Co-firing of some low quality Turkish lignites with woody shells of sunflower seed was investigated via non-isothermal thermogravimetric analysis method. For this purpose, Yozgat-Sorgun, Erzurum-Askale, Tuncbilek, Gediz, and Afsin-Elbistan lignites were selected, and burnouts of these lignites were compared with those of their blends. Biomass was blended as much as 10 and 20 wt.% of the lignites, and heating was performed up to 900 °C at a heating rate of 40 °C/min under dry air flow of 40 mL/min. This study revealed that the same biomass species may have different influences on the burnout yields of the lignites. Burnouts of Erzurum-Askale lignite increased at any temperature with the increasing ratio of biomass in the blend, whereas burnout yields of other lignites decreased to some extent. Nevertheless, the blends of Turkish lignites with sunflower seed shell did not behave in very different way, and it can be concluded that they are compatible in terms of burnouts for co-combustion in a combustion system. Although the presence of biomass in the lignite blends caused to some decreases in the final burnouts, the carbon dioxide neutral nature of biomass should be taken into account, and co-combustion is preferable for waste-to-energy-management. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The ‘‘White Paper Report” on energy for future-renewable sources of energy sets an indicative target of 12% of the EU total energy consumption from renewable energy sources by 2010 in all EU member countries [1]. At this point, the biomass fraction of Refuse Derived Fuel (RDF) and Solid Recovered Fuel (SRF) is crucial on greenhouse gas protocols, such as the European Union Emissions Trading Scheme and the Renewable Obligation Certificate program in the United Kingdom. The combusted biomass fraction of RDF/SRF is used by stationary combustion operators to reduce their overall reported CO2 emissions since biomass is regarded to be CO2-neutral. Considering the use of waste-derived fuels, the European directive 2001/77/EC on electricity from renewable energy sources promotes the co-utilization of SRF in large-scale energy production [2]. However, according to the definitions of the European initiative on SRF standardisation CEN/TC 343, such fuels have to be produced from non-hazardous bio-residues, mixed- and mono-waste streams [3]. 2000/76/EC European legislation set strict limits not only for the mono-combustion of waste in incineration plants but also for cocombustion in coal-fired power plants [4]. More stringent regulations came into force in Europe by The EU directive 2001/80/EC
* Corresponding author. Tel.: +90 212 2856873; fax: +90 212 2852925. E-mail address: [email protected] (S. Yaman). 0196-8904/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.05.026
with respect to the emission of certain atmospheric pollutants from power plants [5]. As a member country to EU, Turkey must take effective measures to decrease emissions from power stations in which low quality Turkish lignites are burned. In fact, this is a serious problem in Turkey since these lignites take place the most important national primary energy sources. Consequently, about 8.5% of world lignite consumption is carried out by Turkey alone. Co-combustion of biomass with coal usually helps to reduce the emissions such as SO2, NOx, CO and CO2 per unit energy generated [6]. On the other hand, some negative aspects coming from intrinsic properties of biomass may take place during co-combustion if unsuitable biomass species is handled. There has been concern that the impurities in some biomasses and wastes, particularly the alkali metals and halogens, could cause operational problems with regard to slagging, fouling or corrosion. Besides, some of the heavy metals can volatilize at temperatures generally applied during co-combustion [7]. Also in case of combustion of specific biomass species such as sewage sludge, high nitrogen content leads to high emissions of NO, NO2, and N2O. Deactivation of DeNOx-catalysts by phosphorus during co-combustion of sewage sludge or ‘‘meat and bone meal” is a common problem [8]. Moreover, the emissions of dioxins could be considerable when fuels with high chlorine content are used. Turkey has an important potential of agricultural waste materials which can be used for energetic purposes. Furthermore, most of these wastes are non-hazardous materials and they are sustainably-grown biomass. For instance, sunflower (Helianthus annus L.)
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seed shell can be regarded as a suitable biomass species with an annual amount of 850,000 tones. So a huge amount of woody shell is obtained after peeling of the seeds in the vegetable oil industry. Besides, the physical and chemical properties of this agricultural woody waste can be regarded to meet the conditions described by above EU directives. For example, concentrations of arsenic, cadmium, and mercury in this biomass are below 1 ppm, while concentrations of chromium, lead, nickel, and vanadium are approximately 2 ppm [9]. Chlorine content is also only 0.05% [10]. Low ash content of this biomass also makes it a favorable alternative fuel. However, one of the challenges of co-combustion is to manage different type of fuels in the same combustion system. Burnout of each fuel plays important role in determining not only the performance of individual fuel but also their compatibility under the same conditions. In this context, synergistic or antagonistic interactions often happen between the constituents of different type of fuels, and accordingly burnouts during co-combustion may be closely affected from these interactions. Experience on co-utilization of a given biomass species with low rank coals is rather limited, and most of such investigations have been conducted using higher rank coals such as bituminous or sub-bituminous coals. Thus, this study addresses the burnout characteristics of low rank Turkish lignites with woody shells of sunflower seed under non-isothermal co-combustion conditions.
2. Experimental Yozgat-Sorgun (YS), Erzurum-Askale (EA), Tuncbilek (T), Gediz (G), and Afsin-Elbistan (AE) lignites were selected for this study since they represent an important part of the Turkish lignites with high deposits. The ages of these coals are Eocene, Miocene, Miocene, Middle Miocene, and Pliocene for YS, EA, T, G, and AE lignites, respectively [11–13]. Air-dried lignite samples and sunflower seed shell (SSS) were grounded to particle size of <0.25 mm. Proximate analysis and the calorific value measurements were performed according to ASTM standards. Elementary analysis was conducted using a Eurovector EuroEA3000 model elemental analyzer. Major biomass ingredients of SSS such as holocellulosics (hemicellulosics + celluloses), lignin, extractives, and a-cellulose were determined by the following analytical procedures. Extractive components were determined according to ASTM D1105 (2001). The sample remaining after alcohol-benzene extractions was used to obtain the holocellulosics by means of NaClO2 extraction procedure. The lignin content of the sample was found according to the method of Van Soest [14]. a-cellulose content was determined according to TAPPI T203 om-88 standard (1988). Burning characteristics of the fuel samples were investigated by non-isothermal thermogravimetric analysis method using a Shimadzu TG 41 thermal analyzer with a cylindrical alumina crucible. Initial sample weight was fixed at 40 mg for mono-combustion experiments of either coals or biomass. Although there have been some studies in literature reporting that 10–30 wt.% is the optimum range for blending of biomass with coal in terms of minimum pollutant emissions per unit energy output, co-firing up to 20 wt.% biomass with coal in utility boilers gave the best results [5,6]. Thus, the biomass additions in this study were chosen as 10 and 20 wt.% of lignite so the initial sample weights in case of co-combustion tests were 44 and 48 mg, respectively. Heating up to 900 °C was performed at the heating rate of 40 °C/ min under dry air flow of 40 mL/min, and enough hold time was allowed to reach an unchanged weight. When temperature
reached to 105 °C, the sample was allowed to stay at this temperature for 10 min to eliminate the moisture. All the experiments were repeated three times to check the reproducibility, and the mean of the closest two results was used. 3. Results and discussion Tables 1 and 2 show the analysis results of biomass and lignite samples. It is seen from data on these tables that calorific value of biomass is lower than those of two lignite samples (YS and G), while it is higher than those of other two lignite samples (T and AE). Surprisingly, calorific values of biomass and EA lignite are exactly the same. Since the previous studies in literature have usually been conducted using coal samples that have obviously higher calorific values compared to biomass materials tested, in those studies biomass has been regarded as a factor that decreases the heat capacity of the system. Thus, burning of biomass is often considered not to give high energy output when compared with coal combustion due to the high volatile matter- and moisture-contents. In this study, however, calorific value of the biomass species is comparable to the Turkish lignites, offering a special occasion for co-processing. The moisture content of air-dried biomass is about 10 wt.%, which is noticeably lower than that for usual biomass species. It is widely known that high content of moisture in biomass leads to lower combustion temperatures as well as increasing CO emissions [15]. From this point of view, SSS is a favorable secondary fuel that can be added to a base fuel.
Table 1 Analyses results of sunflower seed shell. Proximate analysis (dry basis) Volatiles (%) Fixed carbon (%) Ash (%) Higher calorific value (MJ/kg)
84.7 11.7 3.6 17.6
Ultimate analysis (dry ash free basis) C (%) H (%) N (%) Oa (%)
51.7 6.2 1.0 41.1
Chemical analysis (dry basis) Extractives (%) Holocellulose (%) Lignin (%) a-cellulose (%)
13.8 62.5 31.4 28.5
a
Calculated by difference.
Table 2 Analyses results of lignites. Proximate analysis (%, dry)
YS
EA
T
G
AE
Volatiles Fixed carbon Ash
33.5 57.0 9.5
31.9 25.6 42.5
29.1 26.0 44.9
32.6 52.6 14.8
50.5 17.2 32.3
Ultimate analysis (%, daf) C H N S Oa
74.0 5.1 1.6 2.4 16.9
70.8 4.6 2.2 5.3 17.1
71.8 5.6 2.6 2.8 17.2
74.4 5.6 1.3 8.7 10.0
56.5 5.3 2.0 3.1 33.1
Higher calorific value (MJ/kg, dry)
29.0
17.6
13.4
30.1
10.1
a
Calculated by difference.
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10
50
SSS
dm/dt (mg/min)
m (mg)
SSS
8
40
30
20
10
6
4
2
0
0 0
250
500
750
1000
0
250
500
750
1000
Temperature (°C)
Temperature (°C) Fig. 1. TGA and DTG curves of sunflower seed shell.
Fig. 1 indicates that combustion curve of SSS can be divided into two stages representing rapid devolatilization at relatively low temperatures resulting from thermally unstable components such as hemicellulosics and celluloses, and followed by slow burning of fixed carbon at higher temperatures up to 450 °C, and possibly accompanied by decomposition of inorganics. An important part
50
YS
EA
T
G
AE
m (mg)
40
30
20
10
0 0
250
500
750
1000
Temperature (°C) Fig. 2. Comparison of the thermal reactivities of lignite samples.
of the carbonaceous biomass structure quickly decomposed during the former stage, and the rate of the mass loss reached to 8.3 mg/ min at 275 °C during this stage. Whereas, the maximum burning rate was only 1.7 mg/min at 765 °C in case of char burning stage. The end temperature of combustion for SSS was determined as 900 °C so no hold time was in question under the working conditions. Compared to lignites used in this study, the highest content of volatiles, the lowest-fixed carbon and -ash contents belong to SSS. These inherent characteristics of SSS are the indication of its special structure and thermal reactivity that differentiates it from coals. In the design of fluidized bed combustor, this high content of volatiles must be considered since an important part of combustion may take place at the upper regions above the bed. But, such high contents of volatiles should not be always evaluated as a problem. For instance, some waste biomass materials such as textile residues have been reported to burn with so low burning rates that it often deteriorates the combustion performance of the whole system [16]. It is well-known that problematic deposit formations such as fouling owing to biomass materials during co-combustion systems are usually diminished if an interaction happens between the sulfur content of coal and some inorganics found in biomass. Considering the fact that SSS contains low ash, and Turkish lignites are rich in sulfur, so it is possible to predict that an appropriate condition is available for these binary mixtures to eliminate the likely fouling problem resulting mainly from biomass.
Table 3 Burnout characteristics. Temperatures (°C) Sample
200
300
400
500
600
700
800
900
Final
thold (min)
Rhold (mg/min)
Burnout at hold (%)
YS YS10% YS20% EA EA10% EA20% T T10% T20% G G10% G20% AE AE10% AE20%
12.1 11.6 11.1 6.5 6.8 9.2 13.6 15.5 14.6 4.5 12.0 9.4 26.0 21.0 26.7
15.1 19.3 19.2 8.7 13.1 16.6 20.0 24.7 24.3 7.3 19.2 18.0 35.3 38.2 39.5
25.5 29.4 30.2 13.8 18.3 22.3 29.4 32.7 32.0 17.6 30.7 26.6 45.8 47.5 46.2
33.2 35.1 34.4 20.0 24.3 27.1 37.1 39.8 38.5 25.0 35.0 31.5 50.5 53.3 52.1
39.2 40.4 39.7 25.0 28.9 31.7 44.3 46.6 42.9 31.5 38.9 36.2 57.5 56.8 56.2
44.6 44.6 44.2 30.0 33.4 36.6 47.1 49.5 46.3 37.7 44.5 40.1 64.6 62.8 61.5
48.5 49.8 47.4 35.0 37.5 39.4 51.2 54.3 49.8 43.9 50.7 45.0 71.0 67.6 67.7
55.8 54.5 51.7 39.2 43.2 42.5 57.5 60.7 55.0 48.8 56.8 49.0 80.0 71.6 71.2
98.0 90.2 82.1 60.0 56.1 53.1 90.0 83.6 77.1 86.2 92.0 73.5 85.0 83.6 87.5
25 17 22 16 9 9 15 13 13 23 16 15 14 7 10
0.68 0.92 0.66 0.52 0.63 0.57 0.87 0.78 0.82 0.65 0.97 0.79 0.14 0.76 0.78
42.2 35.7 30.4 18.9 13.0 10.6 32.5 23.0 22.1 37.5 35.2 24.6 5.0 12.0 16.2
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Fig. 2 displays a comparison among the thermal reactivities of the lignite samples. In contrast to burning profile of the biomass sample, all the lignite samples needed some hold times at 900 °C for complete combustion. According to Fig. 2, irrespective of temperature, under the process conditions lignite AE gave the maximum burnout values and basing on this it can be said that this lignite is thermally most reactive coal sample. Notice that lignite
80
AE contains the lowest-fixed carbon and highest volatiles contents, and a rapid devolatilization took place even at low temperatures, and it went on until the end temperature. Table 3 presents the variation of the burnout yields depending on temperature as well as the final burnouts, and hold time behaviors. On the other hand, the lowest thermal reactivity was detected for lignite EA from 400 °C to the end of the experiment, whereas at
80
YS YS10% YS20%
60
Burnout (%)
Burnout (%)
60
EA EA10% EA20%
40
20
40
20
0
0 0
250
500
750
1000
0
250
Temperature (C)
80
750
1000
750
1000
Temperature (C)
80
T T10% T20%
60
G G10% G20%
Burnout (%)
60
40
20
40
20
0
0 0
250
500
750
1000
0
250
Temperature (C)
500
Temperature (C)
80
60
Burnout (%)
Burnout (%)
500
40
20
AE AE10% AE20%
0 0
250
500
750
Temperature (C) Fig. 3. Burnouts of the lignites and their blends.
1000
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lower temperatures such as 200 and 300 °C lignite G showed the lowest burnouts. The low thermal reactivity of lignite G can be explained by its low content of oxygen, since the higher oxygen content the higher thermal reactivity. Conversely, lignite G is the sample that has the maximum content of carbon. As to lignite EA, it is rich in ash forming minerals, and some inorganic constituents may show inhibiting effect on burning, leading low thermal reactivity. Lignite samples can be arranged in the increasing order of reactivity as EA, G, YS, T, and AE. In addition to, the times elapsed during hold time and consequently burning at this stage designates significant variations. Although the coal samples used in this study were low rank and consequently they should have been thermally high reactive, the applied thermal process with a heating rate of 40 °C/min was not provide adequate residence time to assure the complete burning of the samples while it was being heated up to 900 °C. Burnouts at 900 °C were 55.8%, 39.2%, 57.5%, 48.8%, and 80.0% of the initial weight on original basis for YS, EA, T, G, and AE lignites, respectively. Then, a great deal of additional burnouts took place during hold time at 900 °C for most of the samples. That is, final burnouts reached to 98.0, 60.0, 90.0, 86.2, and 85.0 wt.% for YS, EA, T, G, and AE lignites, respectively. The increase in the burnout of lignite YS from 55.8% to 98.0% is very striking. Lignite YS has the highest fixed carbon content among the samples, and heterogeneous combustion of fixed carbon occurs mostly at high temperatures. So important part of combustion for this lignite took place during the hold time period. Similarly, lignite G, which is the other coal with high fixed carbon, also showed serious increase in burnout from 48.8 to 86.2 wt.%. Hold times (thold) of 25, 16, 15, 23, and 14 min were determined to reach the final burnout values for YS, EA, T, G, and AE lignites, respectively. The lengths of the hold times can be described by the fixed carbon contents of the lignites. As expected the longest hold time was determined for the lignite YS which contains the highest fixed carbon content so it necessitated the longest time for complete burning. In this context, the shortest hold time was detected for the sample AE having the minimum fixed carbon content. On the other hand, the ratios of the burnouts during hold times to the final burnouts indicated that considerable variations are available among the lignites. Moreover, except for lignite AE, addition of biomass to lignites played an apparent role on the contribution of hold time to the final burnouts in the favor of decrease. This circumstance may be resulted from two possible reasons. Either the presence of biomass makes the burning faster before entering hold time or it terminates the burning quickly. In this case, the latter is main reason for the decreasing contribution of hold time. Consequently, the extents of the final burnouts diminished in general after blending with biomass. Burnout rates (Rhold) during hold time were calculated basing on the burnouts during hold time and the extent of the corresponding time taken during this stage, and they were 0.68, 0.52, 0.87, 0.65, and 0.14 mg/min for YS, EA, T, G, and AE lignites, respectively. Lignite T has the highest burnout rate during hold time, and meanwhile this lignite is the richest sample with respect to ash. So it is possible to conclude that an important part of the mass losses during hold time may be resulted from the decomposition of the mineral phases found in the coal. On the other hand, lignite AE which has the highest volatiles content among the coal samples gave the lowest burnout rates during hold time. This is probably due to the fact that a significant part of this sample was burned or at least eliminated in the form of volatiles, causing relatively less solid structure left to burn. Fig. 3 presents the distribution of the burnout values before hold time in cases of with or without biomass additions, and at the first sight it is seen from the parallel curves that as if there were no important changes in the trends of the weight loss charac-
teristics from some lignites such as YS, T, and AE. However, final burnouts for these lignites affected negatively and serious decreases took place. Atimtay and Kaynak also reported that when co-combustion of some fruit stones was carried out with a lignite, efficiencies of the burning system decreases to some extent [17]. Another similar remark on the reduced system efficiency due to presence of biomass was proposed by McIlveen-Wright et al. [5]. On the other hand, lignite EA revealed a different status that biomass addition increased the burnout as proportional to the biomass added. So the highest burnout values were obtained in case of the blend 20% up to 900 °C. This is in good accordance with the findings of Chao et al. who reported that high volatile matter content of biomass plays a key role in improving the combustion performance in the system [6]. But, the final burnouts of the blends of lignite EA could not reach to that for this lignite alone. Besides, lignite G behaved in different way when it has blended with biomass. This sample showed a non-additive behavior that the presence of 10 wt.% of biomass increased the mass losses from this sample. However, when the addition of biomass was raised to 20 wt.%, not only the same effect could not be observed but also some decrease in burnout occurred. This predicts that the amount of the biomass in the blend is crucial to increase or decrease the burnout of this lignite. These results suggest that within the blending ratios tested in this study, addition of biomass to lignites did not seriously worsen the burning behavior of the lignites. The decreases in the burnout yields may be tolerated. In addition, some problematic features of these low quality coals can be compensated by blending with biomass. Besides, energy potential of this renewable waste biomass material would be evaluated, contributing to lower CO2 emissions. 4. Conclusions Sunflower seed shell is a potential fuel that can be utilized in lignite firing systems by the aim of waste-to-energy management. When low rank coals such as Turkish lignites are handled, sunflower seed shell is a favorable material in terms of calorific value and low ash in order to compensate the negative aspects of these coals. On the other hand, combustion of this biomass species ends in a shorter time compared to those for lignites. This study revealed that the same biomass material may be able to play different roles on the burnout yields of the Turkish lignites. Burnouts of EA lignite increased with the increasing ratio of biomass in the blend, whereas burnout yields of other lignites decreased to some extent. Nevertheless, the blends of Turkish lignites with sunflower seed shell did not behave in very different way, and they can be co-combusted in the existing combustion system in the blending ratios carried out in the present study. Although biomass addition to lignite led to some decrease in the burnout, the carbon dioxide neutral nature of biomass should be taken into account, and co-combustion should be considered. References [1] Report: white paper for a community strategy and action plan. Communication from the commission energy for the future: renewable sources of energy, COM (97) 599 final (26/11/1997). [2] Hilber Th, Thorwarth H, Stack-Lara V, Schneider V, Maier J, Scheffknecht G. Fate of mercury and chlorine during SRF co-combustion. Fuel 2007;86:1935–46. [3] Hilber Th, Martensen M, Maier J, Scheffknecht G. A method to characterize the volatile release of solid recovered fuels (SRF). Fuel 2007;86:303–8. [4] Werther J. Gaseous emissions from waste combustion. J Hazard Mater 2007;144:604–13. [5] McIlveen-Wright DR, Huang Y, Rezvani S, Wang Y. A technical and environmental analysis of co-combustion of coal and biomass in fluidised bed technologies. Fuel 2007;86:2032–42. [6] Chao CYH, Kwong PCW, Wang JH, Cheung CW, Kendall G. Co-firing coal with rice husk and bamboo and the impact on particulate matters and associated
H. Haykiri-Acma, S. Yaman / Energy Conversion and Management 50 (2009) 2422–2427
[7]
[8] [9]
[10] [11]
polycyclic aromatic hydrocarbon emissions. Bioresource Technol 2008;99:83–93. Lievens C, Yperman J, Cornelissen T, Carleer R. Study of the potential valorisation of heavy metal contaminated biomass via phytoremediation by fast pyrolysis: Part II: Characterisation of the liquid and gaseous fraction as a function of the temperature. Fuel 2008;87:1906–16. Jens B, Sven U. The behavior of particle bound phosphorus during the combustion of phosphate doped coal. Fuel 2007;86:632–40. Ooi TC, Aries E, Ewan BCR, Thompson D, Anderson DR, Fisher R, et al. The study of sunflower seed husks as a fuel in the iron ore sintering process. Miner Eng 2008;21:167–77. Demirbas A. Effect of temperature on pyrolysis products from four nut shells. J Anal Appl Pyrol 2006;76:285–9. Palmer CA, Tuncalı E, Dennen KO, Coburn TC, Finkelman RB. Characterization of Turkish coals: a nationwide perspective. Int J Coal Geol 2004;60:85–115.
2427
[12] Karayigit AI, Gayer RA, Querol X, Onacak T. Contents of major and trace elements in feed coals from Turkish coal-fired power plants. Int J Coal Geol 2000;44:169–84. [13] Karayigit AI, Spears DA, Booth CA. Antimony and arsenic anomalies in the coal seams from the Gokler coalfield, Gediz, Turkey. Int J Coal Geol 2000;44:1–17. [14] Van Soest PJ. Use of detergents in the analysis of fibrous feeds. II. A rapid method for the determination of fiber and lignin. J Assoc Offic Anal Chem 1963;46:829–35. [15] Kwong PCW, Chao CYH, Wang JH, Cheung CW, Kendall G. Co-combustion performance of coal with rice husks and bamboo. Atmos Environ 2007;41:7462–72. [16] Ryu C, Phan AN, Sharifi VN, Swithenbank J. Co-combustion of textile residues with cardboard and waste wood in a packed bed. Exp Therm Fluid Sci 2007;32:450–8. [17] Atimtay AT, Kaynak B. Co-combustion of peach and apricot stone with coal in a bubbling fluidized bed. Fuel Process Technol 2008;89:183–97.