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Energy & Fuels 2008, 22, 2–8
Combustion Characteristics of Sewage Sludge in a Bench-Scale Fluidized Bed Reactor† Dong Ho Lee,* Rong Yan, Jingai Shao, and David Tee Liang Institute of EnVironmental Science and Engineering, Nanyang Technological UniVersity, InnoVation Center, Block 2, Unit 237, 18 Nanyang DriVe, Singapore 637723, Singapore ReceiVed May 25, 2007. ReVised Manuscript ReceiVed September 3, 2007
A total of 20 representative sludge samples (6 original and 14 composites) were collected from 6 wastewater treatment plants, for investigating the feasibility of sludge thermal treatment. After a thorough identification of the sludge samples, characteristics of sewage sludge combustion were studied using a bench-scale fluidized bed reactor. To obtain a suitable operating condition, precombustion analysis was made based on both calculation and cold/hot bed testing, to determine the gas flow rate and minimum/maximum fluidized velocities for maintaining a steady fluidization. Critical operational parameters of the fluidized bed reactor in the study were evaluated, including temperature, flow rate of bottom gas supply and total flue gas, resistance time, fuel/air ratio, and sludge-feeding manner and rate. The optimum operation condition was selected to evaluate the combustion efficiencies of 20 sludge samples from analyzing the collected flue gas and ash compositions. Meanwhile, the emission of SOx, NOx, and CO in flue gas was determined, and mass balance analyses of carbon, nitrogen, and sulfur were carried out. Finally, sludge combustion characteristics were summarized, in terms of fuel quality, combustion quality, gas emission, fluidization quality, and material balance.
1. Introduction Fluidized bed combustion (FBC) technology has emerged recently as the new, flexible, multifuel boiler for waste combustion and energy recovery from low-grade fuels.1,2 It offers such advantages as efficient combustion, ease of control, the ability to handle variable feeds, the ability to operate intermittently, and reasonable-to-low capital and operating costs. Moreover, FBC is particularly useful because of the good mixing and relatively low operation temperature, preventing many of the ash-related problems, which might occur frequently in other types of boiler. Nevertheless, FBC is a complex technology, which basically requires the user to first-of-all understand well both the combustion process and the fuel quality prior to operation. If improper conditions were applied, ash-related problems can still lead to operational failures with FBC.3,4 A major potential problem encountered in fluidized beds is bed sintering or agglomeration, which in the worst case, may result in total defluidization, often leading to unscheduled downtime. Another major concern related to waste fuel incineration is the high emission of pollutants, such as SOx, NOx, HCl, polycyclic aromatic hydrocarbons (PAHs), heavy metals, and dioxin/furan. From our previous experiences,5,6 it was observed that the † Presented at the International Conference on Bioenergy Outlook 2007, Singapore, April 26–27, 2007. * To whom correspondence should be addressed. Telephone: (65)67906806. Fax: (65)67921291. E-mail:
[email protected]. (1) Oppelt, E. T. J. Air Pollut. Control Assoc. 1987, 37 (5), 558–586. (2) Stevenson, E. M. EnViron. Sci. Technol. 1991, 25 (11), 1808–1814. (3) Anthony, E. J.; Jia, L.; Preto, F.; Iribarne, J. V. Proc. Int. Conf. Fluid. Bed Combust. 1997, 12, 839–846. (4) Compo, P.; Pfeffer, R.; Tardos, G. I. Powder Technol. 1987, 51, 85–101. (5) Yan, R.; Liang, D. T.; Laursen, K.; Li, Y.; Tsen, L.; Tay, J. H. Fuel 2003, 82 (7), 843–851. (6) Yan, R.; Liang, D. T.; Tsen, L. Energy ConVers. Manage. 2005, 46 (7–8), 1165–1178.
problems encountered during fluidized bed waste fuel combustion are often relative to the wastes themselves. A better performance of the incinerator can be achieved through the reactor design based on a full investigation of the waste fuels and an optimization of the combustion process. Sludge is a rich source of organic matters, and incineration is one of the best options for sludge treatment.7,8 In this study, 20 representative sludge samples (6 original and 14 composites) were collected from 6 wastewater treatment plants (WWTPs), for investigating the feasibility of sludge thermal treatment. However, the above-mentioned problems might also occur in sludge combustion, regarding both operational and emission concerns. To prevent potential problems, such as low thermal efficiency, high emission, and ash agglomeration, a thorough identification of waste fuel and conducting bench test of sludge combustion characteristics are recommended. The objective of this work is to gain a clear insight of sludge combustion through a thorough characterization of 20 sludge samples and evaluating their combustion characteristics in a bench-scale FB reactor. The combustion characteristics of the 20 sludge samples were obtained from a number of combustion trials carried out, including (1) preliminary combustion trials and basic calculations, (2) screening of the operation parameter, (3) combustion efficiency and emission evaluation of 20 sludge samples, and (4) mass balance analysis of 14 composites. The emission of the pollutant in this study is emphasized on SOx and NOx, excluding HCl, PAHs, heavy metals, and dioxin/furan. The information obtained would be highly valuable in assisting (7) Glendinning, S.; Lamont-Black, J.; Jones, C. J. F. P. J. Hazard. Mater. 2007, 139 (3), 491–499. (8) Wong, W. Y.; Lu, Y.; Nasserzadeh, V. S.; Swithenbank, J.; Shaw, T.; Madden, M. J. Hazard. Mater. 2000, 73 (2), 143–160.
10.1021/ef700266q CCC: $40.75 2008 American Chemical Society Published on Web 10/06/2007
Combustion Characteristics of Sewage Sludge
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Table 1. Proximate Analysis of Sewage Sludge Samples by Thermogravimetric Analysis (TGA, wt %) under Nitrogen sample ID S1 S2 S3 S4 S5 S6 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 average value of 14 composites
fixed carbon fixed ash moisture volatiles plus ash carbon (fixed solid) 64.11 77.66 39.29 76.71 76.92 64.04 59.63 59.06 61.87 63.29 64.81 71.14 65.31 60.01 63.36 59.62 74.95 64.67 61.49 65.33 63.90
27.01 14.79 10.97 15.46 14.40 17.12 31.89 29.08 29.62 25.25 24.25 19.22 16.66 32.91 28.82 28.65 14.83 27.95 24.04 23.18 25.45
8.88 7.55 49.74 7.83 8.68 18.84 8.48 11.86 8.51 11.46 10.94 9.64 18.03 7.08 7.82 11.73 10.22 7.38 14.47 11.49 10.65
1.72 3.08 11.83 3.04 2.49 2.29 0.12 1.68 0.00 1.20 0.00 0.00 5.77 0.93 0.00 3.49 4.10 0.00 3.13 2.61 1.65
7.41 4.47 37.91 4.79 6.19 16.55 8.36 10.19 10.17a 10.25 14.90a 10.69a 12.26 6.15 7.85a 8.24 6.12 7.95a 11.34 8.88 9.53
Table 2. Ultimate Analysis Results (Dry Basis) sample ID S1 S2 S3 S4 S5 S6 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 average of 14 composites a
nitrogen carbon hydrogen sulfur oxygen weight (wt %) (wt %) (wt %) (wt %) (wt %)a (mg) 2.02 5.91 3.19 5.69 5.65 4.69 3.63 3.05 3.48 3.42 3.07 2.72 2.29 3.69 4.27 3.20 4.08 3.85 3.23 3.20 3.37
33.00 38.09 20.31 32.27 35.83 28.27 39.17 28.24 39.63 32.86 35.55 37.53 29.76 39.55 44.03 38.21 38.44 43.65 36.61 40.18 37.39
4.35 8.94 4.11 7.20 8.39 6.83 7.50 5.19 8.00 5.47 6.73 6.30 5.04 7.75 8.40 7.59 8.12 8.52 7.45 9.51 7.26
0.50 2.03 0.74 0.92 1.60 1.76 1.29 1.08 1.44 1.10 1.04 0.98 0.80 1.27 1.45 1.10 1.28 1.28 1.16 1.45 1.19
60.13 45.03 71.65 53.92 48.53 58.45 48.41 62.44 47.45 57.15 53.61 52.47 62.11 47.74 41.85 49.90 48.08 42.70 51.55 45.66 50.79
1.0 1.3 1.3 1.2 1.4 1.1 1.0 1.2 1.3 1.1 1.1 1.1 1.2 1.1 1.0 1.1 1.2 1.0 1.0 1.5 1.14
The content of oxygen was determined by difference.
a
The ash (fixed solid) content measured by method 2540G is slightly higher than that remaining from the proximate test using TGA; thus, the fixed carbon content is taken as 0.
the design of a suitable FB burner for industrial-scale sludge thermal treatment. 2. Experimental Section 2.1. Sludge Samples. In this study, 20 representative sludge samples (6 original and 14 composites) were studied. The 6 original sludge samples were labeled as S1, S2, ..., and S6, while 14 composites were C1, C2, ..., and C14. The 14 composites were purposely prepared from the 6 original sludge samples in 14 consecutive days, on the basis of the preset percentage ratio. About 100 kg of sludge each from the 6 WWTPs was collected, and a “size reduction” was carried out to form a composite sample in ∼2 kg by a “Cone and Quarter” method.9 This method can ensure that the collected sample is homogenous and representative for subsequent tests. The sludge characterization included (1) basic tests, such as chloride, total organic carbon (TOC), total polychlorinated biphenyl (PCB), fixed and volatile solids, heating value, and ash fusion temperature, (2) proximate and ultimate analysis, and (3) trace metal and mineral matter analyses. The overall observations from this sludge characterization could enable us to make a preliminary judgment on the quality of sludge as a waste fuel and to estimate the potential concerns of pollutant emissions and bed agglomeration in sludge combustion. However, because of the space limitation, this paper focuses only on the combustion characteristics; while the details of the full identification of 20 sludge samples are not present here, the main results are given for the purpose of a clear interpretation. The results of proximate and ultimate analyses of the sludge samples are summarized in Tables 1 and 2, respectively. 2.2. Fluidized Bed Reactor. The schematic diagram of our fluidized bed is given in Figure 1. The reactor consisted mainly of a furnace and a quartz tube (I.D. of 5 cm, O.D. of 5.5 cm, and bed height of 41 cm) with a continuous feeding system at the top and a gas distributing plate at the bottom of the tube. The gas (air or a mixture of N2 and O2 at the desired air excess numbers) was introduced from the bottom of the reactor, with the control of the flow rate by mass flow meters. The reactor also included a gascleaning system consisting of a cyclone and fabric fiber, a cooling (9) Gerlach, R. W.; Dobb, D. E.; Raab, G. A.; Nocerlno, J. M. Gy sampling theory in environmental studies. 1. Assessing soil splitting protocols. J. Chemom. 2002, 16, 321–328.
Figure 1. Schematic diagram of the bench-scale FB reactor.
system for the separation of water and tar, and gas measurement devices [Horiba, Fourier transform infrared spectroscopy (FTIR), and MicroGC]. The operating temperature in the bed was up to 1000 °C. The ash or residues after combustion were collected from the cyclone by removing the bottom plug for further identifications. Prior to loading the bed materials, a thin layer of quartz wool was first packed over the gas-distributing plate to prevent the dropping of fine sands. Then, Raschig rings (made of quartz) were laid to enhance the gas distribution. Above the rings, 0.5 mm diameter average sands were layered to make the depth of the bed 2 times that of the tube diameter (i.e., 100 mm). 2.3. Precombustion Testing. 2.3.1. Cold Bed Calculation and Test. To form and maintain a stable fluidization bed, the velocity (minimum and terminal velocities) of the gas flow and pressure drop of the bed must be understood. Theoretical calculations of the characteristics of the fluidized bed were carried out, following the equations given in ref 10. Assuming that sand particles with the following property: sand particle diameter (dp) ) 0.5 mm, sand density (Fs) ) 2.0 g/cm3, and sand particle sphericity (φs) ) 1 (dimensionless), the minimal fluidizing velocity was calculated using the following equations: umf ) [dp2(Fs - Fg)g]/(1650µ) umf2 ) [dp(Fs - Fg)g]/(24.5Fg)
Rep < 20
(1)
Rep > 1000
(2)
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Lee et al.
Table 3. Bed Height Gained at Different Gauge Pressures and Flow Rates pressure 1 bar
air flow rate 30 L/min 40 L/min minimum flow rate required for fluidization
pressure 2 bar
pressure 3 bar
Fluidization Bed Height Gained 1.5 cm 2.0 cm 2.5 cm 3.5 cm 4.0 cm 4.5 cm 30 L/min 28 L/min 26 L/min
where umf is the minimal fluidizing velocity (m/s), Fg is the gas density (kg/m3), g is the acceleration of gravity (9.81 m/s2), µ is the air viscosity (kg m–1 s–1), and Rep is the Reynolds number based on the particle diameter (dimensionless). The Reynolds number (Rep) was calculated using the following equation: Rep ) Fgu0dp/µ
(3)
where u0 is the superficial gas flow velocity (measured on an empty tube basis) through the bed, u0 ) Q/A (m/s). Q is the air flow rate (L/min), and A is the cross-sectional area of the bed (cm2). The air properties at 30 °C are Fg ) 1.1649 kg/m3 and µ ) 1.862 × 10-5 kg m–1 s–1. When Q ) 30 L/min and u0 ) 0.255 m/s, the Reynolds number (Rep) ) Fgu0dp/µ ) 7.98 < 20. Therefore, eq 1 was used to calculate the minimum fluidizing velocity (umf); it was found that umf ) 0.160 m/s and the minimum flow rate (Qmf) ) umfA ) 18.8 L/min. To recheck the Rep using the calculated umf (0.160 m/s), Rep changed to 5.01, still 100 ppmv).
the bench-scale FB reactor at the following conditions to reach high combustion efficiency for all samples: temperature of 900 °C, total and bottom air supplies at 16.4 and 9.0 L/min, respectively, sample feeding rate of 1 g/min, residence time for 3 s, and fuel feeding ratio (FR) at 6 (i.e., the molar ratio of air/fuel is 6). A higher temperature (900 °C) and FR (6) can guarantee the full combustion of all samples, and feeding of 1 g/min of sample can satisfy practically the need of FR at 6 and also help the maintaining of a stable fluidization status. 3.1. Combustion Quality. The combustion characteristics of 20 sludge samples and the efficiency of combustion (ηcomb.) evaluated by gas components (the CO and CO2 concentrations in combustion flue gas) are listed in Table 4. Overall, the combustion efficiency (ηcomb.) for the 20 sludge samples in the FB reactor and under the preselected operating parameters is high (g99.5%), except for sample S1 and C8, whose combustion efficiencies attained only 98.7 and 99.2%, respectively. The high oxygen percentage in gas indicated the oxygen-rich combustion of sludge in the FB reactor, because of the high FR used. The fly ashes generated from the combustion of 20 sludge samples were collected in the cyclone and sent to the elemental analyzer (Perkin Elmer 2400 series II) for ultimate analysis. The results are listed in Table 5. The carbon burnout was calculated, and the efficiency (ηcarbon) is also provided, which is generally higher than 99.9%. The collection of bottom ash after combustion was conducted selectively, and the result of ultimate analysis of bottom ash from C4 is shown in Table 6, as an example. Bottom ashes generated from other sludge combustion were not collected separately because of the continuous use of sand as a bed material. After it, two types of bottom ashes were generally observed in the mixture of bed materials: one is light (red) color, and another is dark-color. Both types of bottom ashes were sent for ultimate analysis to see any difference in carbon burnout, and the results are also given in Table 6. It was found that, although the colors of the two types of bottom ash are very different, their compositions in terms of the combustible C, H,
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Table 5. Ultimate Analysis Results of Fly Ash from Sludge Combustion
sample ID S1 S2 S3 S4 S5 S6 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 average of 14 composites a
N (%) C (%) H (%) S (%) O 0 0.03 0.03 0 0 0 2.12 1.98 3.18 3.04 2.76 4.85 3.71 5.99 4.65 4.86 5.83 0.00 0.00 0.00 3.07
0.16 0.27 0.32 0.27 0.23 0.18 0.92 0.44 0.61 0.43 0.24 0.3 0.17 0.29 0.25 0.17 0.24 0.36 0.3 0.33 0.36
0 0 0 0 0 0 2.12 0.23 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.17
0 0 0 0 0 0 0.34 0.08 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03
(%)a
99.84 99.70 99.65 99.73 99.77 99.82 94.5 97.27 96.21 96.53 97 94.85 96.12 93.72 95.1 94.97 93.93 99.64 99.7 99.67 96.37
carbon weight burnout, (mg) ηcarbon (%) 1.2 1.3 1.0 1.3 1.1 1.4 1.1 1.3 1.0 1.1 1.4 1.0 1.5 1.0 1.4 1.5 1.4 1.0 1.1 1.3
99.98 99.98 99.98 99.99 99.98 99.95 99.95 99.91 99.92 99.93 99.97 99.98 99.97 99.95 99.96 99.99 99.99 99.98 99.99 99.99 99.96
Figure 3. Concentrations of N and S in 20 sludge samples (dry basis).
Oxygen was determined by difference.
Table 6. Ultimate Analysis Results of Bottom Ash from Sludge Combustion sample bottom ash from C4 combustion light-color bottom ash dark-color bottom ash a
Figure 2. Concentration of NOx and SO2 in the flue gases of 20 sludge combustions.
N (%) C (%) H (%) S (%) O (%)a weight (mg) 61.30
0.19
0.00
0.00
38.51
1.4
71.96
0.18
0.00
0.00
27.86
1.4
78.23
0.16
0.00
0.00
21.61
1.5
The content of oxygen was determined by difference.
N, O, and S are quite comparable. There is no concern on the unburnt carbon in the black dark-color bottom ash. 3.2. Basic Emission Evaluation. Table 4 also gives the concentration of gas pollutants (SO2, NOx, and CO) in the flue gas. The CO emission in the flue gas from combustion of 14 composites is averaged at 69 ppmv, well below the Environmental Protection Agency (EPA) emission limit (∼100 ppmv) for a waste fuel incineration plant. In the industrial application with improved gas/solid mixing and a longer residence time, the sludge combustion efficiency should be better and CO emission will be further decreased. Although the emission of NOx is averaged at ∼204 ppmv for 14 composites, which is slightly higher than the emission standard (195 ppmv), the measured data of NOx emission for most of trials still obey the requirement in the basic combustion trials. However, high emissions of SO2 (∼130 ppmv) from the sludge combustion in our FB reactor were found to be in conflict with the emission standard (70 ppmv); feeding limestone for sulfur capture is thus recommended for industrial-scale sludge combustion to remove SO2. A chart of the two pollutants emission (SO2 and NO) in the flue gas from 20 sludge combustion is plotted in Figure 2, which can be compared with Figure 3 showing the ultimate analysis result of each original sample, in terms of the concentrations of N and S. When Figure 2 is compared to Figure 3, not all of the N present in the sludge is converted to NOx. The NOx formation is also due to the oxidation of N2 in combustion air at high temperatures. However, it is clear that the NO/SO2 emissions in the flue gases from 20 sludge sample combustions
Table 7. Mass Flow Rate in Combustion of 14 Composites in
out
sample ID
sludge (g/min)
bottom ash (g/min)
fly ash (g/min)
flue gas (L/min)
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 average
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
0.121 200 0.046 333 0.112 857 0.171 333 0.222 500 0.127 750 0.161 000 0.182 000 0.177 167 0.175 634 0.176 667 0.135 750 0.282 167 0.177 455 0.162
0.062 400 0.116 333 0.046 190 0.022 667 0.037 500 0.031 500 0.051 000 0.028 833 0.027 667 0.020 278 0.027 111 0.079 750 0.023 167 0.023 455 0.0427
16.4 16.4 16.4 16.4 16.4 16.4 16.4 16.4 16.4 16.4 16.4 16.4 16.4 16.4 16.4
demonstrate a changing tendency, which is quite consistent with the distribution of combustible nitrogen/sulfur contents in the 20 sludge samples. In other words, the SO2 and NOx emissions from sludge combustion are most likely attributable to the high presence of combustible S and N in the sludge samples. 3.3. Mass Balance in Sludge Combustion. In another series of combustion trials, the mass balance analysis was conducted for the 14 composites under the same conditions: temperature of 900 °C, sample feeding rate of 1 g/min, residence time for 3 s, and fuel feeding ratio at 6. The combustion trial lasted for 1–2 h each, to get enough ash samples for further analysis. After combustion, the generated fly ash was collected by cyclone and weighed, the bottom ash was separated from the bed material, and the weight of the bottom ash was obtained by weighing the bed material before and after the combustion. Both fly and bottom ashe combustions of 14 sludge composites were sent for ultimate analysis. Together with the gas composition recorded by the Horiba gas analyzer (PG-250, Japan), the mass balance of C, H, N, and S can therefore be estimated. The material flow (rate and amount) in the combustion of 14 composites is given in Tables 7 and 8, respectively. The mass flow rate of the bottom ash is generally higher than that of the
Combustion Characteristics of Sewage Sludge
Energy & Fuels, Vol. 22, No. 1, 2008 7
Table 8. Mass Flow Balance in Combustion of 14 Composites in
in (g)
sample ID
sludge (g)
bottom ash (g)
fly ash (g)
percentage of the gas product from total sludge feeding (%)
C1 C2 C3a C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 average
25.0 60.0 31.0 75.0 60.0 40.0 50.0 60.0 60.0 61.2 45.0 40.0 60.0 55.0 51.6
3.03 2.78 2.37 12.85 13.35 5.11 8.05 10.92 10.63 10.74 7.95 5.43 16.93 9.76 8.56
1.56 6.98 0.97 1.70 2.25 1.26 2.55 1.73 1.66 1.24 1.22 3.19 1.39 1.29 2.07
81.64 83.73 89.23 80.60 74.00 84.08 78.80 78.92 79.52 80.41 79.62 78.45 69.47 79.91 79.38
a Sand sprayed out during the test, causing the combustion trial to stop at a total feeding of 31 g of C3.
Table 9. Mass Balance of C in the Combustion of Sludge in (g) sample ID C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 average
Table 10. Mass Balance of N in the Combustion of Sludge
out
out (g)
sludge
bottom ash
fly ash
flue gas
total
recovery (%)
9.79 16.94 7.93 24.65 21.33 13.14 14.88 23.73 26.42 23.31 17.30 17.46 21.97 22.10 18.64
0.013 94 0.008 62 0.009 24 0.029 56 0.034 71 0.009 71 0.012 88 0.015 29 0.020 20 0.011 81 0.011 13 0.008 15 0.027 09 0.018 54 0.016 49
0.117 78 0.104 00 0.005 92 0.007 31 0.005 40 0.003 78 0.004 34 0.005 02 0.004 15 0.002 11 0.002 93 0.011 48 0.004 17 0.004 26 0.020 19
7.468 34 15.143 47 6.067 18 22.017 56 19.372 09 9.731 43 12.783 86 22.177 79 19.756 42 19.278 03 15.058 73 13.121 13 16.758 43 17.880 09 15.472 47
7.600 06 15.256 09 6.082 34 22.054 43 19.412 20 9.744 91 12.801 08 22.198 09 19.780 76 19.291 96 15.072 79 13.140 76 16.789 69 17.902 89 15.509 15
77.61 90.04 76.74 89.49 91.01 74.19 86.03 93.54 74.88 82.77 87.14 75.26 76.43 81.01 82.58
fly ash, except for the case of C2 combustion (Table 7). During the preparation of C2 pellets, it was found that the disk of C2 is so brittle that the cutting of the disk caused the generation of lots of finer particles of C2. The feeding of these finer particles leads to the collection of much more fly ash in the cyclone. The total mass flow balance (see Table 8) indicates that ∼80% of the fed sludge was converted to gas products during combustion. The results of mass balance analysis for C, N, and S in combustion are listed respectively in Tables 9–11, where the material “in” from sludge and material “out” from fly/bottom ashes and flue gas are iterated for the 14 composites. The total recovery of carbon is in the range of 74–93% and averaged at 82.58%. It is noteworthy that carbon contents in sludge and fly/bottom ashes were measured by the elemental analyzer. Therefore, they are only combustible (organic) carbon contents. There might also exist some inorganic carbon components in ashes as (for example) carbonates, but they are not included in this mass balance analysis, possibly attributed to the lower carbon closure in its mass balance analysis. The inorganic C-bearing compounds in ash (both fly and bottom ashes) are at low quantity and might be present in different forms of organic salts. There is no direct method available to obtain their total amount without a series of very complicated chemical pretreatments; thus, the fraction of C-bearing inorganic species in ashes are neglected in this mass balance analysis. The recoveries of N in its mass balance analysis (Table 10) are poor, averaged at
sample ID C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 average
out (g)
sludge
bottom ash
fly ash
flue gas
total
recovery (%)
0.91 1.83 0.70 2.39 1.84 0.95 1.15 2.21 2.56 1.95 1.84 1.54 1.94 1.76 1.68
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0.033 07 0.138 20 0.030 85 0.051 68 0.062 10 0.061 11 0.094 61 0.103 63 0.077 19 0.060 26 0.071 13 0.327 93 0.164 44 0.171 05 0.103 37
0.097 50 0.210 78 0.077 33 0.250 88 0.252 72 0.146 40 0.224 85 0.244 14 0.234 92 0.221 95 0.160 41 0.139 15 0.244 20 0.213 12 0.194 17
0.130 58 0.348 98 0.108 17 0.302 56 0.314 82 0.207 51 0.319 46 0.347 77 0.312 11 0.282 22 0.231 53 0.467 08 0.408 64 0.384 17 0.297 54
14.39 19.07 15.54 12.64 17.09 21.80 27.90 15.71 12.18 14.46 12.61 30.33 21.09 21.83 18.33
Table 11. Mass Balance of S in the Combustion of Sludge in (g)
out (g)
sample ID
sludge
bottom ash
fly ash
flue gas
total
recovery (%)
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 average
0.32 0.65 0.29 0.83 0.62 0.39 0.44 0.76 0.87 0.67 0.58 0.45 0.70 0.80 0.60
0 0.006 39 0 0.011 95 0.017 36 0.004 09 0.009 66 0.010 92 0.020 20 0.010 74 0.014 31 0 0.011 85 0.019 52 0.009 78
0.005 30 0.005 58 0 0 0 0 0 0 0 0 0 0 0 0 0.000 78
0.341 18 0.788 65 0.213 51 0.824 28 0.757 06 0.410 00 0.536 94 0.887 12 0.803 61 0.658 22 0.552 17 0.403 02 0.626 74 0.745 43 0.611 81
0.346 49 0.800 62 0.213 51 0.836 23 0.774 42 0.414 09 0.546 60 0.898 04 0.823 81 0.668 96 0.566 48 0.403 02 0.638 59 0.764 95 0.622 37
107.44 123.55 74.13 101.36 124.11 105.63 124.23 117.85 94.69 99.70 98.35 89.96 91.75 95.92 103.48
18.33%. The poor recovery is expected because combustion N2 in combustion air is not taken into account and, in combustion processes, not all nitrogen-containing compounds in the sludge are converted into NOx. Similar to the carbon mass balance, some inorganic nitrogen in ashes, which could not be easily tested using available instruments, accounts for this deviation. The recovery of sulfur is averaged at 103.48% (>100%), most likely because of the very high SO2 emission detected from flue gas during long-time combustion trials. Much higher SOx emission (averaged at 842 ppmv for 14 composites) was observed in the flue gas from the continuous combustion (1–2 h) of composites. Concentrations of SOx are sometimes in the range of 1000–1500 ppmv, with rare occasions reaching 2000 ppmv, which is significantly higher than our previous observations with the basic combustion tests, where combustion trials lasted for only a few minutes and the averaged SOx emission is at 130 ppmv. Nevertheless, the NOx emission level increased slightly compared to the previous findings (from 204 to 328 ppmv). The preliminary explanation to the sharp increase of SOx goes to the lower O2 content occurring in the case of continuous combustion. However, further investigation is needed to understand it better. The emissions of SOx and NOx from full-scale incinerators burning the studied sludge can also be estimated on the basis of the concentrations of combustible S and N in the fuel and the mass flow balance observed from the bench-scale reactor, assuming that the sludge fed in the industrial reactor will be fully burnt out. Nevertheless, operating parameters can still
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Energy & Fuels, Vol. 22, No. 1, 2008
affect to a certain extent the real emission levels of SOx and NOx in the flue gas from an industrial incinerator. 4. Conclusions It is clear from this study that incineration is a suitable measure to treat the studied sludge, although there still exist emissions of SOx and NOx in flue gas and a few clinkers formed in several combustion trials of this bench study. Technology of applying a FB incinerator to treat sludge is already mature, and problems such as emission and bed agglomeration can be eventually avoided on the basis of a proper design of the FB burner and a careful selection of suitable operation conditions. On the basis of the thorough characterization and a number of combustion trials for the 20 sludge samples carried out in the bench-scale FB reactor under various conditions, the following conclusions can be drawn: On Fuel Quality. (1) The measured sludge calorific value (LHV, dry basis) is averaged at 24.97 MJ/kg for 14 composites, which is sufficiently high for combustion to be used as a suitable measure of sludge disposal. The GHV (“as-received”, wet basis) of the 14 composites, 7.47 MJ/kg, however, is somewhat low. Nevertheless, natural drying of sludge in open air (normal sunlight day, slightly windy conditions) was able to remove ∼37–57% of the moisture content from “as-received” sludge. This has increased its GHV favorably to 19–23 MJ/kg, suitable for raw sludge to be used as a fuel by itself. (2) The total solids of the 14 composites are averaged at 33.53% by weight. About 24.66% is volatile solid, and 8.87% is fixed solid. The high volatile fraction of solid in sludge will generally benefit its combustion. (3) The average values of the 14 composites (“asreceived” basis) for the total, inorganic, and organic sulfurs are 0.90, 0.51, and 0.39%, respectively. The combustible sulfur and nitrogen in composites are 1.19 and 3.37%, respectively, which are corresponding to the high emissions of SO2 and NOx in flue gas from sludge combustion. On Combustion Quality. (1) High combustion efficiency (>99.9%) was achieved for most of the combustion trials with the feeding of predried (at 150 °C) sludge pellets, while suitable operational parameters of the temperature, residence time, and feeding ratio are chosen. (2) Direct feeding of the raw sludge
Lee et al.
sample into our FB reactor was once tested after open-air drying of sludge. A comparison of the combustion efficiency of predried and raw sludge combustion shows comparable results. However, because of certain constraints of the feeding system in our FB reactor, sludge pellets were used for all of the other combustion trials. (3) The best conditions of sludge combustion in the laboratory-scale FB reactor are shown as: temperature of 850-1000 °C (upper limit to avoid the clinker formation because of ash fusion), air/fuel ratio > 5, and residence time > 2 s. (4) There is little residual carbon in all of the ash samples tested (fly ash plus bottom ash). On Flue Gas Emissions. (1) The flue gas CO emission levels were found to be well below the discharge standards, and they are not expected to be an issue. (2) However, the SOx and NOx emission levels exceeded the emission standards (∼70 ppmv for SOx and 195 ppmv for NOx) in most the combustion tests. An additional flue-gas-cleaning device for SOx and NOx removal (such as lime addition) is recommended if sludge treatment using FBC is considered. On Fluidization Quality. (1) The used FB allows a wide range of flow rates for maintaining fluidization. The selected flow rate for combustion trials also considered the reasonable air/fuel ratio. (2) The sand used is suitable bed material for fluidization in the FB reactor. (3) The fluidization status is wellmaintained during the combustion trial in the FB reactor. Although the accumulated bottom ash in the bed is bigger in size, it is lighter in density compared to sand and, thus, located always at the top of the bed material in the course of combustion. On Material Balance. (1) Roughly 80% of the fed sludge was converted to gas products during combustion in the FB reactor. (2) The mass balance of C, N, and S was calculated from material balance, sludge composition, and monitoring gas and (bottom and fly) ash streams. The closure of C, N, and S is at 82.6, 18.3, and 103.5%, respectively. The lower recovery of N might be due to the inorganic nitrates occurring in ash samples, which is not tested and, thus, excluded in the mass balance. A high emission of SOx in flue gas attributes to the over-recovery of S in its mass balance analysis. EF700266Q
