Cofiring Lignite with Hazelnut Shell and Cotton Residue in a Pilot

Mar 26, 2008 - ... cotton residue was investigated in 0.3 MWt METU Atmospheric Bubbling Fluidized Bed Combustion (ABFBC) Test Rig in terms of combusti...
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Energy & Fuels 2008, 22, 1620–1627

Cofiring Lignite with Hazelnut Shell and Cotton Residue in a Pilot-Scale Fluidized Bed Combustor Zuhal Gogebakan and Nevin Selçuk* Department of Chemical Engineering, Middle East Technical UniVersity, 06531 Ankara, Turkey ReceiVed NoVember 1, 2007. ReVised Manuscript ReceiVed February 9, 2008

In this study, cofiring of high ash and sulfur content lignite with hazelnut shell and cotton residue was investigated in 0.3 MWt METU Atmospheric Bubbling Fluidized Bed Combustion (ABFBC) Test Rig in terms of combustion and emission performance of different fuel blends. The results reveal that cofiring of hazelnut shell and cotton residue with lignite increases the combustion efficiency and freeboard temperatures compared to those of lignite firing with limestone addition only. CO2 emission is not found sensitive to increase in hazelnut shell and cotton residue share in fuel blend. Cofiring lowers SO2 emissions considerably. Cofiring of hazelnut shell reduces NO and N2O emissions; on the contrary, cofiring cotton residue results in higher NO and N2O emissions. Higher share of biomass in the fuel blend results in coarser cyclone ash particles. Hazelnut shell and cotton residue can be cofired with high ash and sulfur-containing lignite without operational problems.

Introduction Applications of fluidized bed combustion technology developed for burning coal with high efficiency and within acceptable levels of gaseous pollutant emissions have been steadily increasing in both capacity and number over the past decade. However, gradual introduction of increasingly restrictive legislations on emissions from combustion sources has been increasing the interest in biomass combustion. Biomass is a renewable energy source due to the fact that it can be considered as CO2neutral fuel and contributes to the reduction of SO2 and NOx emissions due to its low sulfur and nitrogen contents. Cofiring biomass with coal in fluidized bed combustors is a promising alternative which leads to an economical and environmentally friendly use of coals by reducing pollutant emissions and at the same time provides utilization of biomass residues.1–7 Cofiring of biomass with coal has extensively been studied in fluidized bed combustion systems with various types of agricultural and woody biomass.8–13 However, cofiring of hazelnut shells and cotton residues has not been carried out to * To whom correspondence should be addressed. E-mail: selcuk@ metu.edu.tr. Telephone: + 90 (312) 210 26 03. Fax: + 90 (312) 210 26 00. (1) Anthony, E. J. Prog. Energy Combust. Sci. 1995, 21, 239–268. (2) Bapat, D. W.; Kulkarni, S. V.; Bhandarkar, V. P. Proc. 14th Int. Conf. Fluidized Bed Combust. (ASME) 1997, 1, 165–174. (3) Hein, K. R. G.; Bemtgen, J. M. Fuel Process. Technol. 1998, 54, 159–169. (4) Hughes, E. Biomass Bioenergy 2000, 19, 457–465. (5) Sami, M.; Annamalai, K.; Wooldridge, M. Prog. Energ. Combust. Sci. 2001, 27, 171–214. (6) Sonderal, E. A.; Benson, S. A.; Hurley, J. P.; Mann, M. D.; Pavlish, J. H.; Swanson, M. L.; Weber, G. F.; Zygarlicke, C. J. Fuel Process. Technol. 2001, 71, 7–38. (7) Laursen, K.; Grace, J. R. Fuel Process. Technol. 2002, 76, 77–89. (8) Shen, B. X.; Mi, T.; Liu, D. C.; Feng, B.; Yao, Q.; Winter, F. Fuel Process. Technol. 2003, 84, 13–21. (9) Lin, W.; Johansen, K. D. Proc. 15th Int. Conf. Fluidized Bed Combust. 1999;ASME, Paper no. FBC99-0120 (in CD-ROM). (10) Armesto, L.; Veijonen, K.; Bahillo, A.; Cabanillas, A.; Plumed, A.; Salvador, L. Proc. of 16th Int. Conf. on Fluidized Bed Combustion 2001, ASME, Paper no. FBC01–0044 (in CD-ROM) (11) Adànez, J.; Diego, L. F.; Gayàn, P.; Garcio-Labiano, F.; Cabanillas, A.; Bahillo, A. Proc. 17th Int. Conf. Fluidized Bed Combust. 2003; ASME, Paper no. FBC2003-064 (in CD-ROM).

date. Hazelnut shells are residues from hazelnut crushing plants. Hazelnut production is prevalent in Turkey and about 70% of the worlds’ total production takes place in Turkey with 584 000 ha of plantation and 530 000 tons of annual production.14 Cotton residue is also a specific type of biomass produced from cotton oil production process. It is the remaining part of cottonseeds after extraction of cotton oil. Turkey is one of the leading producers of cotton in the world with 546 880 ha of plantation, 1 291 180 tons of cotton lint and 863 700 tons of cottonseed production.14 Therefore, significant amounts of hazelnut shells and cotton residues are available to be used in cofiring applications. The absence of studies on cofiring of indigenous lignite with hazelnut shell/cotton residue blends in fluid bed combustors on one hand and the recent trend in utilization of biomass with local reserves in industry and utility boilers on the other necessitate investigation of combustion and emission characteristics of these fuel blends. In an attempt to achieve this objective, a typical indigenous lignite is cofired with hazelnut shells and cotton residues in the METU (Middle East Technical University) 0.3 MWt Atmospheric Bubbling Fluidized Bed Combustion (ABFBC) test rig with limestone addition at several shares of biomass in the fuel mixture. Experimental Section This study is based on experimental data collected as part of a research project for the investigation of combustion and gaseous emission characteristics of biomass cofired with a typical low-quality lignite with high ash and sulfur contents. Tests were carried out on the 0.3 MWt ABFBC test rig located in the Chemical Engineering Department of Middle East Technical University. Figure 1 shows the flow sheet of the 0.3 MWt ABFBC test rig. As can be seen from the figure, the test rig basically consists (12) Leckner, B.; Karlsson, M. Proc. 12th Int. Conf. Fluidized Bed Combust. (ASME) 1993, 1, 109–115. (13) Gayàn, P.; Adànez, J.; Diego, L. F.; Garcia-Labino, F.; Cabanillas, A.; Bahillo, A.; Aho, M.; Veijonen, K. Fuel 2004, 83, 277–286. (14) Statistical Year Book of Turkey 2006.

10.1021/ef700650x CCC: $40.75  2008 American Chemical Society Published on Web 03/26/2008

Cofiring Lignite with Hazelnut Shell and Cotton Residue

Energy & Fuels, Vol. 22, No. 3, 2008 1621

Figure 1. 0.3 MWt ABFBC test rig.

of a forced draft (FD) fan, a windbox with an ash removal system, a modular combustor, a cyclone with a recycle leg, a baghouse filter, an induced draft (ID) fan, and a coal and limestone feeding system. The main body of the test rig is the modular combustor formed by five modules of internal cross section 0.45 m × 0.45 and 1 m height. Inner walls of the modules are refractory lined and insulated. The first and fifth modules from the bottom refer to bed and cooler, respectively, and the ones in between are the freeboard modules. There exist two cooling surfaces in the modular combustor, one in the bed and the other in the cooler, providing 0.35 and 4.3 m2 of cooling surfaces, respectively. There are 14 ports for thermocouples and 10 ports for gas sampling probes along the combustor. Two ports for feeding coal/biomass/limestone mixture are provided in the bed module, one 0.22 m and the other 0.85 m above the distributor plate. In order to measure the concentrations of O2, CO, CO2, SO2, NO, and N2O along the combustor and also downstream of cyclone at steady state, combustion gas is sampled by gassampling probes and transferred to gas conditioning system through a heated line, where the sample is filtered, dried, and cooled. Sampled gas then passes through two analyzers in series, ABB Advanced Optima 2000 and Siemens Ultramat 6, respectively. In ABB Advanced Optima 2000, O2 concentration is measured by a magnetomechanical analyzer module Magnos 106, whereas CO, CO2, NO, and N2O concentrations are measured by an infrared analyzer module Uras 14. In Siemens Ultramat 6, SO2 concentration is measured by nondispersive infrared module.

The output signals from analyzers and process values such as temperatures, air and water flow rates, pressures, and speed of screw conveyors are logged to a PC by means of a data acquisition and control system, Bailey INFI 90. Further details of the test rig are given elsewhere.15 In order to investigate the effect of biomass share on emission performance of the test rig, a total of seven tests without and with limestone addition were carried out at several biomass shares. In test 1, coal is burned without limestone and biomass addition whereas in test 2, coal is burned with limestone addition. In tests 3-5, coal is burned with limestone addition at several hazelnut shell shares, i.e., 11, 30, and 42 wt %, respectively. In the last two tests, tests 6 and 7, coal is burned with limestone addition at cotton residue shares of 30 and 41 wt %, respectively. The characteristics of Can lignite and biomass burned in tests are summarized in Tables 1 and 2, respectively. As can be seen from these tables, lignite is characterized by high ash content (∼30%) and high total sulfur content (∼4%) whereas biomasses are characterized by high volatile matter (VM)/fixed carbon (FC) ratio (∼4 and ∼6 for hazelnut shell and cotton residue, respectively) and low ash content (∼1.5 and ∼5 for hazelnut shell and cotton residue, respectively). Ash constituents of the fuels are shown in Table 3. With regard to ash composition, lignite ash is mainly composed of acidic oxides whereas hazelnut shell and cotton residue ashes are mainly composed of basic oxides. For the tests (15) Degirmenci, E.; Selçuk, N. Proc. 15th Int. Conf. Fluidized Bed Combust. 1999; ASME, Paper no. 100 (in CD-ROM).

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Table 1. Characteristics of Can lignite, Hazelnut Shell, and Cotton Residue lignite test 1

test 2

16.35 28.78 29.79 25.17 905

16.48 26.74 31.05 25.74

carbon hydrogen nitrogen oxygen combustible sulfur ash total sulfur

44.60 3.95 1.09 11.97 3.98 34.41 4.17

LHV

12.3

moisture ash volatile matter fixed carbon bulk density (kg/m3)

test 3

test 4

test 5

test 6

test 7

hazelnut shell

cotton residue

Proximate Analysis (as received basis, wt %) 17.19 16.05 17.14 17.05 25.29 24.36 27.46 27.06 31.22 32.17 30.36 30.97 26.30 27.42 25.04 24.93

17.47 24.29 31.44 26.80

7.62 1.46 73.04 17.89 320

6.93 5.38 75.57 12.14 364

44.93 4.09 1.14 13.96 3.86 32.02 4.07

Ultimate Analysis (dry basis, wt %) 46.47 42.22 41.92 39.87 4.26 4.23 4.01 3.89 1.15 1.01 0.96 0.88 13.95 19.35 15.64 18.28 3.63 4.17 4.33 4.46 30.54 29.02 33.14 32.62 3.63 4.17 4.33 4.46

40.04 3.84 0.98 21.42 4.29 29.43 4.35

49.77 5.86 0.56 42.15 0.08 1.58 0.11

46.79 6.48 4.40 36.23 0.32 5.78 0.32

13.3

Calorific Value (as received basis, MJ/kg) 13.4 13.9 12.5 12.5

13.4

17.5

17.4

Table 2. Size Distribution of Can Lignite, Hazelnut Shell, and Cotton Residue lignite (wt %) size (mm)

test 1

test 2

test 3

test 4

test 5

test 6

test 7

hazelnut shell (wt %)

cotton residue (wt %)

16.000–19.000 12.700–16.000 8.000–12.700 6.300–8.000 4.750–6.300 3.350–4.750 2.000–3.350 1.000–2.000 0.500–1.000 0.355–0.500 0.180–0.355 0.106–0.180 0.000–0.106 d50 (mm)

0.00 1.08 3.04 4.90 6.10 16.86 13.56 21.48 10.85 5.82 5.55 4.09 6.66 1.79

0.00 2.03 8.43 9.24 10.41 23.38 13.26 14.16 5.55 2.69 2.55 2.68 5.61 3.56

0.00 0.95 8.14 11.76 13.42 27.00 13.12 11.47 3.95 1.93 1.92 2.12 4.20 3.42

0.00 1.29 8.14 8.86 9.68 23.41 12.98 14.53 6.16 3.17 3.26 2.93 5.59 3.43

0.00 1.27 5.06 7.10 7.81 18.75 13.20 17.78 9.07 4.93 5.17 3.49 6.37 2.33

0.00 0.88 7.19 6.40 5.93 13.78 10.69 18.02 11.10 6.58 7.28 4.63 7.51 1.72

0.00 1.09 3.79 5.15 6.15 16.31 13.34 21.00 11.41 6.18 6.44 3.72 5.42 1.80

0.00 9.04 64.45 13.87 5.84 5.16 0.63 0.31 0.19

1.67 6.35 31.65 12.52 7.60 14.56 7.90 7.90 2.99

0.05

5.50

0.45 9.71

1.37 6.6

Table 3. Ash Analyses of Can Lignite, Hazelnut Shell, and Cotton Residue lignite SiO2, wt % Al2O3, wt % Fe2O3, wt % CaO, wt % MgO, wt % SO3, wt % Na2O, wt % K2O, wt % TiO2, wt %

test 1

test 2

test 3

test 4

test 5

test 6

test 7

hazelnut shell

cotton residue

57.29 19.67 12.05 4.85 0.82 2.00 1.58 0.21 1.53

56.56 17.49 10.99 9.21 0.57 2.05 1.45 0.31 1.38

50.02 23.81 12.03 8.19 0.63 2.43 1.05 0.24 1.59

51.43 22.80 12.28 7.26 0.52 2.23 1.59 0.26 1.63

51.91 21.83 12.15 7.92 0.58 2.31 1.60 0.33 1.37

55.55 21.35 11.71 5.42 0.53 2.71 1.05 0.20 1.48

50.11 22.57 11.46 7.79 0.55 4.24 1.51 0.18 1.58

2.28 2.59 7.11 38.84 6.60 5.50 7.40 27.86 1.81

0.00 0.81 4.95 10.83 14.77 0.00 10.29 57.51 0.85

Table 4. Characteristics of Beypazari Limestone size distribution size (mm)

weight (%)

1.000–1.180 0.850–1.000 0.710–0.850 0.600–0.710 0.500–0.600 0.425–0.500 0.355–0.425 0.180–0.355 0.106–0.180 0.000–0.106

13.01 5.09 6.01 10.85 3.95 10.07 6.45 16.82 10.13 17.63

chemical analysis (wet) component

weight (%)

moisture 0.69 88.92 CaCO3 MgCO3 6.44 SiO2 2.91 0.15 Na2O 0.08 K2O Al2O3 0.39 Fe2O3 0.43 LOI 42.43 d50 ) 0.41 mm

with limestone additions, limestone with the physicochemical properties shown in Table 4 was utilized. Table 5 lists the operating conditions of the tests. Lignite was first burned without and with limestone addition, and then cofired with hazelnut shells at 11, 30, and 42 wt % shares and

with cotton residues at 30 and 41 wt % shares in their own ashes. The operating parameters other than biomass share were tried to be maintained constant from one test to another. Feed point location was 0.22 m above the distributor plate for all tests. Results and Discussion Combustion Efficiencies. The effects of operating parameters upon the magnitude of combustibles loss were investigated by analyzing all solid streams in terms of their carbon contents and CO emission in the flue gas. The fractional combustibles loss for each run was calculated as the ratio of the heat lost due to CO emission in the flue gas and unburned combustibles in bed drain, carryover, and bag filter ashes to the potential heat of combustion of fuel mix. Combustion efficiencies calculated from fractional combustibles losses are shown in Table 6. Inspection of the table reveals

Cofiring Lignite with Hazelnut Shell and Cotton Residue

Energy & Fuels, Vol. 22, No. 3, 2008 1623

Table 5. Operating Conditions of the Tests lignite firing tests

hazelnut shell cofiring tests

cotton residue cofiring tests

parameter

test 1

test 2

test 3

test 4

test 5

test 6

test 7

coal flow rate, kg/h biomass flow rate, kg/h limestone flow rate, kg/h Ca/S molar ratio (added) bottom ash flow rate, kg/h cyclone ash flow rate, kg/h baghouse ash flow rate, kg/h air flow rate, kmol/h excess air, % superficial velocity, m/s av bed temp, °C av freeboard temp, °C bed height, m bed pressure drop, cm H2O

77 0 0 0 7 14 0.4 16 23 2.2 894 866 1.0 54

69 0 22 3 8 19 1.2 14 21 1.9 848 817 1.1 63

54 7 19 3 6 17 1.0 14 20 1.9 857 831 1.2 66

41 17 14 3 3 15 1.4 14 21 1.9 853 832 1.2 63

32 23 14 3 2 12 1.9 14 22 1.9 854 835 1.1 60

46 20 17 3 6 17 0 14 10 1.9 860 849 1.2 65

36 25 13 3 0 18 0 15 21 2.0 857 843 1.2 63

Table 6. Combustion Efficiencies combustion efficiency, % test test test test test test test

1 2 3 4 5 6 7

97 96 97 97 98 97 97

that combustion efficiencies are very high (∼97%) for the reactive lignite under consideration despite the absence of cyclone ash recycle. Combustion efficiency reduces from 97% (test 1) to 96% (test 2) with addition of limestone. The reduction in combustion efficiency of about 1% is in agreement with the findings of a previous study burning similar lignite under similar conditions in the same test rig.16 Decrease in efficiency with limestone addition is due to the combined effect of introduction of a cooler solid which leads to loss of sensible energy from the system and the net energy loss resulting from endothermic decomposition reaction of CaCO3 and exothermic formation reaction of CaSO4 at Ca/S molar ratio of 3. Comparisons of efficiencies of biomass cofiring runs (tests 3-7) with that of base run (test 2) reveal that combustion efficiency increases with biomass addition. This increase results from the high volatile matter content of hazelnut shell and cotton residue (∼73 and 76% on as-received basis, respectively) as high-volatile matter of biomass rapidly burns and results in highly porous char accelerating the char combustion as well.5,17 The increase in combustion efficiency with biomass addition is in accordance with previous studies.11,13,19,20 During cofiring of hazelnut shell, combustion efficiency remains constant at 97% up to 30 wt % hazelnut shell in the fuel blend and increases to 98% with increase of hazelnut shell share to 42 wt % in the fuel blend. Cofiring of cotton residue with lignite resulted in 97% combustion efficiency irrespective of cotton residue share in the fuel mixture. Temperature Profiles. Temperature measurements taken during experiments carried out with lignite combustion with and (16) Selcuk, N.; Gogebakan, Y.; Harmandar, H.; Altindag, H. Combust. Sci. Technol. 2004, 176, 959–975. (17) Senneca, O. Fuel Process. Technol. 2007, 88, 87–97. (18) Armesto, L.; Bahillo, A.; Cabanillas, A.; Veijonen, K.; Otero, J.; Plumed, A.; Salvador, L. Fuel 2003, 82, 993–1000. (19) Kakaras, E.; Vourliotis, P.; Grammelis, P. Proc. 16th Int. Conf. Fluidized Bed Combust. 2001; ASME, Paper no. FBC2001-004 (in CDROM). (20) Liu, D. C.; Wu, Z. S.; Shen, B. X.; Lin, Z. J. Energy Fuels 1999, 13, 1252–1254. (21) Leckner, B.; Karlsson, M. Biomass Bioenergy 1993, 4, 379–389.

without limestone addition are displayed in Figure 2. Inspection of the temperature profiles for the experiments without and with limestone addition shows that temperature decreases considerably in both bed and freeboard with addition of limestone due to introduction of cooler solid. The fall in gas temperature toward the exit is due to the presence of the cooler in the final module. The effect of biomass share on temperature profiles is illustrated in Figures 3 and 4. Comparisons show that temperatures slightly increase especially in the freeboard region with increasing biomass share. This is considered to be due to the high volatile content of hazelnut shell and cotton residue (∼73

Figure 2. Temperature profiles of lignite firing tests.

Figure 3. Temperature profiles of hazelnut shell cofiring tests.

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Gogebakan and Selçuk

Figure 4. Temperature profiles of cotton residue cofiring tests.

Figure 7. Measured SO2 emissions.

Figure 5. Measured CO emissions.

Figure 8. Measured NO emissions.

Figure 6. Measured CO2 emissions.

and 76% on as-received basis, respectively) and is in accordance with the findings of previous studies.21,22 Emissions. Emissions measured downstream of cyclone are shown in Figures 5-9. Points in the figures represent mean measured values during steady state period. The standard deviation values were found to be 18–40 ppm for CO, 0.15–0.50% for CO2, 15-40 ppm for SO2, 5–10 ppm for NO, and 1–3 ppm for N2O. Inspection of the Figures 5 and 6 reveals that CO and CO2 emissions of tests 1 and 2 are similar to each (22) Cliffe, K. R.; Patumsawad, S. Waste Manage. 2001, 21, 49–53. (23) Baxter, L. Fuel 2005, 84, 1295–1302.

Figure 9. Measured N2O emissions.

other so that limestone addition has almost no effect on emissions of these species. Hazelnut shell cofiring shows no significant effect on CO. Cotton residue addition on the other hand, leads to higher CO emissions compared to those of tests 1–5. This is attributed to the combined effect of high volatile matter content of cotton residues (∼76% on as-received basis) and lignite combustion with low amount of limestone addition before cotton residue cofiring runs due to problems in the feeding system. CO2 emissions are not affected at all by biomass cofiring. However, as biomass fuels are CO2 neutral, their contribution to CO2 emission can be considered to be negligible.

Cofiring Lignite with Hazelnut Shell and Cotton Residue

Therefore, CO2 emissions can be reduced by the proportion of biomass in fuel feed for cofiring runs.13,23 SO2 emission is reduced drastically by addition of limestone to lignite in test 2 (Figure 7). Biomass addition leads to further decrease in SO2 emissions due to negligible sulfur contents of biomass. SO2 emissions from hazelnut shell cofiring runs are measured to be lower than that of cotton residue cofiring runs. This may be due to lower sulfur content of hazelnut shells compared to that of cotton residue (0.08 and 0.32 wt %, on dry basis, respectively) and segregation of cotton residue particles which leads to poor contact between limestone particles and SO2. Increase in biomass share leads to lower SO2 emission due to lower amount sulfur in the fuel feed. Increasing the hazelnut shell share in the fuel feed leads to slightly lower NO emissions due to low nitrogen content of hazelnut shell (Figure 8). Reduction in NO emissions by cofiring is also reported in previous studies cofiring coals with woody biomass.24,25 Cofiring cotton residue with 30 wt % in the fuel blend leads to lower NO emissions, however, increasing the share to 41 wt % results in higher NO emissions due to increase in nitrogen content of the fuel blend. Fuel nitrogen to NO conversion is an important parameter for estimation of NO emissions. Two different approaches are followed in the literature for determination of this parameter. The first approach relates the fuel nitrogen to NO emission and fuel char combustion,20,26,27 the second is just based on NO emission relative to fuel nitrogen.12 Both definitions are given below. fuel-N to NO conversion, % )

(

CNO × 100 NFuelMC × (CCO + CCO2) CFuelMN (1)

)

where CNO, CCO, and CCO2 are the concentrations of NO, CO, and CO2 in the flue gas, and CFuel and NFuel are the carbon and nitrogen contents, respectively. MN and MC are the atomic masses of nitrogen and carbon, respectively. Fuel-N to NO conversion, % ) nitrogen in the emitted NO × 100 (2) nitrogen in the fuel Previous studies have shown that fuel nitrogen to NO conversion is a function of H/N weight ratio28–30 as well as the volatile matter content of the fuels18,31,32 in addition to the operating conditions such as bed temperature, excess air ratio, etc. In an attempt to see the validity of these correlations for the results obtained in this study, fuel nitrogen to NO conversions calculated by using (eqs 1 and 2) together with volatile matter (24) Kokko, A.; Nylund, M. Proc. 18th Int. Conf. Fluidized Bed Combust. 2005; ASME, Paper no. FBC2005-78035 (in CD-ROM). (25) Wischnewski, R.; Ratschow, L.; Redemann, K.; Hartge, E.-U.; Werther, J.; Heidenhof, N. Proc. 19th Int. Conf. Fluidized Bed Combust. 2006;Paper no. FBC2006-90 (in CD-ROM). (26) Konttinen, J.; Hupa, M.; Kallio, S.; Winter, F.; Samuelsson, J. Proc. 18th Int. Conf. Fluidized Bed Combust. 2005; ASME, Paper no. FBC200578025 (in CD-ROM). (27) Goel, S. K.; Morihara, A.; Tullin, C. J.; Sarofim, A. F. Proc. 25th Int. Symp. Combust. 1994, 1051–1059. (28) Chyang, C. S.; Wu, K. T.; Lin, C. S. Fuel 2007, 86, 234–243. (29) Hämäläinen, J. P.; Aho, M. J.; Tummavuori, J. L. Fuel 1994, 73, 1894–1898. (30) Aho, M. J.; Rantanen, J. T. Fuel 1989, 68, 586–590. (31) Shimizu, T.; Tachiyama, Y.; Souma, M.; Inagaki, M. Proc. 11th Int. Conf. Fluidized Bed Combustion (ASME) 1991, 695–700. (32) Selçuk, N.; Batu, A.; Oymak, O. Proc. of 17th Int. Conf. on Fluidized Bed Combustion 2003; ASME, Paper no. FBC2003-109 (in CDROM).

Energy & Fuels, Vol. 22, No. 3, 2008 1625 Table 7. Fuel Nitrogen to NO Conversion

test test test test test test test

1 2 3 4 5 6 7

volatile matter of fuel feed, % as received

H/N wt ratio of the fuel feed

conversion, % (eq 1)

conversion, % (eq 2)

29.8 31.1 35.8 44.4 48.3 44.3 49.5

3.6 3.6 4.1 5.5 6.2 2.3 2.0

7.4 7.1 7.2 7.8 8.2 2.4 3.3

6.8 6.8 7.4 8.9 10.1 2.9 4.0

Table 8. Sulfur Retention Efficiencies sulfur retention efficiency, % test test test test test test test

1 2 3 4 5 6 7

8 84 92 92 92 76 83

content and H/N ratio the fuel blends are tabulated in Table 7. As can be seen from the table, hazelnut shell cofiring runs (test 3-5) reveal that conversion increases with H/N ratio despite increasing volatile matter content of the fuel blend. Cotton residue cofiring results show no dependency on both parameters. It is worth noting that cotton residue yields very small fuel nitrogen to NO conversion. These results reveal that predictive accuracy of fuel nitrogen to NO conversion relationship with volatile matter content and H/N ratio of the fuels is very poor for systems cofiring fuels with different characteristics. Inspection of N2O emissions shown in Figure 9 shows that N2O emissions decrease with the addition of limestone. Addition of hazelnut shells leads to further reduction in N2O emissions; however, cotton residue addition results in higher emissions due to higher nitrogen content of cotton residue (4.4 wt % on dry basis). Sulfur Retention Efficiencies. Sulfur retention efficiencies obtained in all tests are tabulated in Table 8. As can be seen from the table, as high as 84% retention efficiency is obtained when high sulfur content lignite is burned with limestone addition despite the absence of cyclone ash recycle. This is attributed to increased residence time resulting from underbed feeding rather than the reactivity of the limestone utilized as the same limestone resulted in 69% retention efficiency when the fuel/limestone mixture was fed 85 cm above the grid and burned under similar conditions.16 Cofiring of hazelnut shells results in 92% sulfur retention efficiency irrespective of the share of hazelnut shell. Increasing the share of cotton residue from 30 to 41 wt % leads to increase sulfur retention efficiency from 76 to 83%. Particle Size Distributions of Ashes. Particle size distributions of the inlet and outlet streams for tests burning lignite without and with limestone addition are shown in Figure 10. As can be seen from the figure, particle size decreases in the following order: bottom ash, cyclone ash, and baghouse filter ash, as expected. Addition of fine limestone in test 2 is seen to decrease the particle size of cyclone ash compared to cyclone ash of test 1. This can be attributed to the fine particles of limestone elutriated with the gas. Particle size distributions of inlet and outlet streams in biomass cofiring runs are illustrated together with those of lignite firing with limestone run in Figures 11 and 12. In the figures, test 2 has no biomass and hence can be taken as a reference case. As can be seen from Figure 11, the effect is only noticeable in

1626 Energy & Fuels, Vol. 22, No. 3, 2008

Gogebakan and Selçuk

Figure 10. Ash particle size distributions of lignite firing tests.

Figure 11. Ash particle size distributions of hazelnut shell cofiring tests.

Figure 12. Ash particle size distributions of cotton residue cofiring tests.

cyclone ash for the largest share of hazelnut shell in the fuel blend. This may be considered to be due to the combined effect of lower bulk density and coarser particle size of hazelnut shells compared to those of lignite. Figure 12 shows particle size distributions of inlet and outlet solid streams for cofiring tests carried out with cotton residues. Trends are similar to those of hazelnut shells.

Ash Compositions. Compositions of bottom, cyclone, and baghouse ashes are given in Tables 9-11. As can be seen from the tables, order of magnitudes of concentrations of all ash components remain the same with increasing share of biomass. Na2O and K2O concentrations in all ash streams were found insensitive to the share of hazelnut shell and cotton residue. This is indicative of the absence of problems

Cofiring Lignite with Hazelnut Shell and Cotton Residue

Energy & Fuels, Vol. 22, No. 3, 2008 1627

Table 9. Chemical Analyses of Bottom Ashes SiO2, wt % Al2O3, wt % Fe2O3, wt % CaO, wt % MgO, wt % SO3, wt % Na2O, wt % K2O, wt % TiO2, wt %

test 1

test 2

test 3

test 4

test 5

test 6

test7

47.88 23.69 8.85 8.97 1.17 4.83 1.83 1.24 1.54

32.74 16.09 5.68 28.30 1.60 11.99 1.23 1.17 1.20

14.65 7.59 3.83 46.14 2.76 21.71 1.70 0.76 0.87

26.14 9.17 4.45 37.88 1.94 17.09 1.20 1.08 1.06

27.52 12.42 4.88 37.93 1.34 12.58 1.01 1.10 1.22

21.23 8.01 3.69 45.60 1.35 17.53 0.94 0.78 0.86

24.62 7.94 4.30 39.95 1.93 18.08 1.15 0.89 1.13

Table 10. Chemical Analyses of Cyclone Ashes SiO2, wt % Al2O3, wt % Fe2O3, wt % CaO, wt % MgO, wt % SO3, wt % Na2O, wt % K2O, wt % TiO2, wt %

test 1

test 2

test 3

test 4

test 5

test 6

test7

51.61 20.92 10.48 7.41 0.69 4.05 2.34 0.77 1.73

23.27 8.04 7.40 39.96 4.61 13.47 1.36 0.97 0.91

24.83 6.77 7.64 40.60 2.92 13.66 1.72 1.00 0.86

21.91 9.40 8.01 38.71 2.12 15.64 1.82 1.17 1.22

31.51 7.96 6.98 31.48 2.06 15.94 1.46 1.31 1.30

22.93 6.28 7.54 43.47 2.01 14.81 0.61 1.37 0.98

17.52 2.45 6.89 53.10 1.89 15.93 0.36 1.10 0.76

Table 11. Chemical Analyses of Filter Ashes SiO2, wt % Al2O3, wt % Fe2O3, wt % CaO, wt % MgO, wt % SO3, wt % Na2O, wt % K2O, wt % TiO2, wt %

test 1

test 2

test 3

test 4

test 5

test 6

test7

45.82 14.68 17.36 9.49 0.80 7.93 1.57 0.47 1.88

32.19 9.69 12.61 22.95 1.40 17.80 1.14 0.51 1.72

22.34 10.29 15.95 27.58 3.24 15.69 1.98 1.26 1.66

29.55 8.78 14.80 25.24 2.54 14.75 1.67 1.12 1.56

27.60 8.42 14.68 27.46 1.95 15.27 1.55 1.02 2.04

31.15 6.72 14.61 28.35 1.40 13.52 1.86 0.97 1.41

32.16 5.98 17.52 22.91 1.54 15.81 1.97 1.07 1.05

associated with basic oxides of biomass during cofiring. There occurred a satisfactory synergy between coal and biomass

that impact of cofiring minimized the negative effect of high alkali content biomass ash. Conclusions Combustion and emission performance of typical Turkish lignite cofired with hazelnut shell and cotton residue at several shares were investigated by burning them in their own ashes in 0.3 MWt ABFBC test rig. The following conclusions were reached under the observations of this study: 1. Cofiring of cotton residue and hazelnut shell with lignite leads to higher combustion efficiencies and freeboard temperatures. 2. Cofiring of hazelnut shell and cotton residue shows no significant influence on total CO2 emissions; however, it reduces net CO2 emissions. 3. SO2 emissions reduce with increasing biomass share in the fuel blends. 4. Hazelnut shell cofiring results in lower NO and N2O emissions with increasing hazelnut shell share. Cotton residue cofiring, on the other hand, leads to higher NO and N2O emissions with increasing cotton residue share. 5. Compared to lignite firing, cofiring of lignite with hazelnut shell and cotton residue results in formation of coarser particles in cyclone ash. 6. Hazelnut shell and cotton residue can be cofired with high ash and sulfur-containing lignite without any ash-related operational problems. Acknowledgment. Financial support provided by The Scientific and Technical Research Council of Turkey (TUBITAK) through a research project MAG 104M200 in aid of this research is gratefully acknowledged. EF700650X