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Emissions of NOx and N2O during co-combustion of dried sewage sludge with coal in a circulating fluidized bed combustor
Fuel 86 (2007) 2308–2315 www.fuelfirst.com
Emissions of NOx and N2O during co-combustion of dried sewage sludge with coal in a circulating fluidized bed combustor Tadaaki Shimizu *, Masanori Toyono Department of Chemistry and Chemical Engineering, Niigata University, 2-8050 Ikarashi, Niigata 950-2181, Japan Received 15 October 2006; received in revised form 19 January 2007; accepted 19 January 2007 Available online 28 February 2007
Abstract Emissions of NOx and N2O were measured during mono-combustion of dried sewage sludge and co-combustion with coal in a benchscale circulating fluidized bed combustor (CFBC). The results were compared with previous results obtained using a bubbling fluidized bed combustor (BFBC). The increase in NOx with sludge ash accumulation in the combustor was less for the CFBC than the BFBC, partly because of the higher attrition rate of sludge ash in CFBC resulting from the higher gas velocity. The influence of sludge ash on the formation of NOx in CFBC was less than that in BFBC during sludge combustion. The effects of fuel type on NOx and N2O emissions were also evaluated. 2007 Elsevier Ltd. All rights reserved. Keywords: Fluidized bed combustion; Sludge; Nitric oxide; Nitrous oxide; Attrition
1. Introduction Fluidized bed combustors (FBCs), both circulating (CFBCs) and bubbling (BFBCs), have been developed as combustion technologies that are useful for widely different fuels such as coal, wastes, and biomass. Municipal sewage sludge is a product of wastewater treatment. Increasing amounts of sewage sludge pose a problem for waste management in many developed countries [1]. Dried sewage sludge has moderate heating value. Therefore, the dried sludge can be burned in FBCs as a partial substitute for fossil fuels. A salient problem of sewage sludge combustion is its high nitrogen content, which might engender high emissions of NOx (NO + NO2) and N2O, the latter of which is known to be a greenhouse gas. High NOx emissions from sludge combustion are also anticipated not only because of their high nitrogen content but also their high contents of
*
Corresponding author. Tel./fax: +81 25 262 6783. E-mail address: [email protected] (T. Shimizu).
0016-2361/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.01.033
metal oxides such as iron oxides and magnesium oxide in the ash; solids containing such metal oxides are known to boost NOx emissions during coal combustion through oxidation of HCN and NH3 to NOx [2–6]. Indeed, the authors conducted dried sludge combustion in a BFBC and found that the sludge ash accumulation in the reactor significantly increased NOx emissions [7]. Although emissions of NOx and N2O from co-combustion of sludge and coal or plastics have been reported in the literature for CFBCs [5,8–10], the effect of ash accumulation on the emissions of NOx and N2O has not yet been fully elucidated: it remains uncertain whether ash accumulation influences NOx emissions from CFBCs. The ash attrition rate is known to increase with increasing gas velocity [11]. Therefore, the ash attrition rate in CFBCs is expected to be much higher than that in BFBCs. In the present work, mono-combustion of dried sewage sludge and co-combustion with coal were carried out and emissions of NOx and N2O were measured. A bench-scale circulating fluidized bed combustor was used. The effects of ash accumulation and fuel type on emissions of NOx and N2O were observed. The present results are compared with
T. Shimizu, M. Toyono / Fuel 86 (2007) 2308–2315
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Nomenclature a
mass ratio of sludge in mixed fuel (including ash and moisture) [CO2+CO] concentration of CO2 and CO in flue gas (ppm) Dp particle diameter (m) F.R. (fixed carbon content)/(volatile matter content) (–) [FC]c fixed carbon content of coal (value is given in Table 1) (–) [FC]m,daf fixed carbon content of mixed fuel (in dry ashfree basis) (–) [FC]s fixed carbon content of dried sludge (value is given in Table 1) (–) FGas total feed rate of air (mol s1) Fs feed rate of dried sludge (kg s1) kA ash removal rate constant (s1) kb attrition rate constant given by Eq. (4) (–) MAsh amount of sludge ash in the reactor (kg) MAsh,st amount of sludge ash in the reactor at steady state (kg) Nfuel/Cfuel N/C molar ratio of fuel (–) [NOx] concentration of NOx in flue gas (ppm)
those in the previous work using the same fuels, but employing a different reactor type (BFBC). 2. Experimental 2.1. Circulating fluidized bed combustor A bench-scale (1.9 m high, 2.2 cm i.d.) circulating fluidized bed combustor (CFBC) was used. The top of the riser was connected to a cyclone, in which particles were separated from the gas stream. The captured particles were recycled to the bottom of the riser through a bubbling fluidized bed (loopseal). The riser and the loopseal were heated by electric heaters; the temperature in the riser was maintained at 1123 K. Quartz sand of 80–150 mesh (average size 0.15 mm), which is known to be inert for the reactions of NOx and N2O [12], was used as the bed material. The minimum fluidizing velocity (Umf) of the sand was approximately 0.008 m s1. The total pressure drop across the riser was fixed at 1.5 kPa. Primary air was fed through a gas distributor at the bottom of the riser. Secondary air was injected through a nozzle located at 0.63 m above the distributor. The secondary air feed rate was half of the total air feed rate. The total air feed rate including the air for pneumatic transportation for the fuel feed was fixed at 11.3 · 103 mol s1 (= 15.24 Nl min1), i.e., the superficial gas velocity above the secondary air inlet (U) was 2.70 m s1 at 1123 K. Fuel was fed continuously through a rotary feeder, conveyed pneumatically in the air stream, and injected into the bottom of the riser
[N2O] t tf ts
concentration of N2O in flue gas (ppm) time (s) time when sludge feed was started (s) time when fuel feed was switched from sludge to coal (s) U superficial gas velocity (m s1) Umf minimum fluidizing gas velocity (m s1) V volume of particle (m3) [VM]c volatile matter content of coal (value is given in Table 1) (–) [VM]m,daf volatile matter content of mixed fuel (in dry ash-free basis) (–) [VM]s volatile matter content of dried sludge (value is given in Table 1) (–) wAsh ash content of dried sludge (–)
Greek symbols gNOx conversion of fuel-N to NOx gNOx ;inert conversion of fuel-N to NOx without catalytic effect of sludge ash gN2 O conversion of fuel-N to N2O
at 0.08 m above the distributor. The fuel feed rate was controlled so that the desired O2 concentration (3–5 vol.%) was attained in the flue gas. Flue gas from the combustor was filtered. Then it was cooled using an ice bath to remove water vapour. Concentrations of O2, total NOx (NO and NO2), CO2, CO, and SO2 in the dried flue gas were measured continuously using a magnetic oxygen analyzer for O2, chemical luminescence for NOx, and NDIR absorption for CO2, CO, and SO2. Some sample gas was stored in Tedler gas bags. Concentrations of O2, N2, CO2, and N2O in the gas were measured using gas chromatography with a thermal conductivity detector. Concentrations of NOx and N2O described below are corrected to the concentrations of dry flue gas containing 6% oxygen. Details of the gas analyzers are given elsewhere [7]. 2.2. Fuels Dried sewage sludge containing edible oil (rapeseed oil) was used. The mass ratio of the dried sludge to the oil was 7:3. The dried sludge was sieved and the fraction from 0.5 mm to 1.68 mm was employed. Characteristics of the fuel are shown in Tables 1 and 2. Details of the sludge sample preparation are described elsewhere [7]. Three kinds of coal with different volatile matter content, high volatile bituminous coal (HVB), medium volatile bituminous coal (MVB), and semi-anthracite (SA) were employed as fuels for co-combustion. Results of analyses of those fuels are shown in Tables 1 and 2. The fuel size was 0.3–1.0 mm. For co-combustion, the dried sludge with
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Table 1 Analyses of fuel Ultimate analysis (daf%) C c
Dry sludge with oil (ds:o = 7:3) High-volatile bituminous coal (HVB) Medium-volatile bituminous coal (MVB) Semi-anthracite (SA) a b c d
H
62.8 78.1 85.9 92.3
O
6.9 6.3 4.9 3.5
16.8 13.4 7.0 3.5
Proximate analysis (wt.%) N 5.4 1.3 1.7 1.5
S 1.7 1.0 0.5 0.2
Ash 21.6 14.3 15.0 12.8
V.M.a d
72.9 39.2 26.3 12.0
F.C.b
Moisture
5.5d 41.2 56.2 73.1
0 5.3 2.5 2.1
Volatile matter. Fixed carbon. Calculated from the composition of dry sludge after kerosene removal and rapeseed oil. Added rapeseed oil was assumed to be volatile matter.
Table 2 Analysis of fuel ash (as oxides) Al2O3
Fe2O3
CaO
MgO
P2O5
a
– 54.6
3.0 35.7
27.5 5.1
13.3 1.1
19.1 0.3
15.0 0.2
46.3
32.6
5.1
7.7
1.3
1.2
46.6
36.3
2.5
5.0
0.8
0.4
SiO2 Sludge High-volatile bituminous coal Medium-volatile bituminous coal Semi-anthraciteb a
Not measured. Sample from the same coal mine but different lot from the present sample. b
edible oil was mixed with the coal at a mass ratio of coal:sludge = 7:3 before it was stored in the fuel hopper. 3. Results and discussion Fig. 1 shows results of mono-combustion of mediumvolatile bituminous coal and dried sludge. First, coal combustion was conduced to attain the steady state. Then the fuel was switched to the dried sludge at t = tf. Finally, the fuel was switched to the coal again at t = ts. During the initial coal combustion, NOx and N2O emissions were almost constant. The ash from the present coal is considered to be inert or to be so fragile that the accumulation of coal ash did not affect the emissions of NOx and N2O.
NOx, N2O [ppm](6%O2)
600
NOx N2O
MVB coal
500 400 300
Coal100%
Sludge100%
2000
tf ts 4000 6000 Time, t [s]
Coal100%
200 100 0 0
8000
10000
Fig. 1. Dynamic change in emissions of NOx and N2O from monocombustion of coal and dried sludge (coal 100% (t < tf = 3720 s) ! sludge 100% (tf < t < ts = 6240 s) ! coal 100% (ts < t); flue gas oxygen concentration = 3–5 vol.% (dry)).
After starting sludge combustion, the NOx concentration became higher than those of coal. This increased concentration is attributable to the higher nitrogen content of dried sludge than coal. During the sludge feed, however, an increase in NOx with time because of ash accumulation was not observed. To the contrary, a significant increase in NOx was observed in the previous study using a bubbling fluidized bed combustor burning the same sludge. After switching the fuel from sludge to coal (t > ts), the NOx emissions decreased. However, the NOx emissions from coal after sludge combustion were higher than that before the sludge feed. Subsequently, the NOx emissions decreased gradually with time. These results suggest that the sludge ash that remained in the reactor catalyzed formation of NOx, and that the sludge ash amount decreased gradually with time, possibly because of attrition and elutriation. In contrast to high NOx emissions during sludge combustion, the emissions of N2O were slightly lower than those from coal combustion. In contrast to NOx, the emissions of N2O from coal combustion after the sludge feed were nearly equal to those before sludge feed. This rough equivalence also suggests that the accumulation of sludge ash in the reactor played only a minor role in N2O emissions. Fig. 2 shows results of co-combustion of sludge with three different coals. At first, coal alone was fed to stabilize the bed temperature. Then the dried sludge mixed with the coal was fed into the reactor. After starting co-combustion of sewage sludge with coal, the NOx concentration became higher than that from coal combustion. However, the increase in NOx emissions with time afterwards was not great. Emissions of N2O from co-combustion were nearly equal to or slightly higher than those from coal combustion. The present sludge ash was reported to impart considerable activity to increase NOx emissions from BFBC; the NOx emissions increased with time during sludge feed [7]. Nevertheless, the emissions of NOx from mono-combustion and co-combustion of the sludge were nearly constant with time for the present CFBC. Two possible explanations of the difference exist between BFBC and CFBC. One is the difference in the amount of ash: the attrition rate is higher in CFBC because of the higher gas velocity. Consequently, the ash is readily removed from the combustor through attrition and elutriation. Another explanation is that the
T. Shimizu, M. Toyono / Fuel 86 (2007) 2308–2315
NOx,N2O [ppm] (6%O2)
500
NOx N2O
HVB coal
400 300 Coal100%
Sludge30%+Coal70 %
200 100 0
0
2000
4000
6000
Time, t [s] 300 NOx,N2O [ppm] (6%O2)
M Ash ðtÞ ¼ M Ash ðts Þ expfk A ðt ts Þg:
Coal100% 0
300
Sludge30%+Coal70 %
2000
4000 Time, t [s]
6000
NOx,N2O [ppm] (6%O2)
SA coal
8000
NOx N2O
100
0
Sludge30%+Coal70%
2000
4000
6000
Time, t [s] Fig. 2. Change in emissions of NOx and N2O during co-combustion of dried sludge with coal (fuel: coal 100% ! dried sludge 30 wt.% + coal 70 wt.%; flue gas oxygen concentration = 3–5 vol.% (dry)).
½NOx ðtÞ ½NOx;inert ¼ ð½NOx ðts Þ ½NOx;inert Þ ð3Þ
Fig. 3 shows the change in the increase in NOx emissions ([NOx] [NOx,inert]) from coal combustion after stopping the sludge feed. The change is well approximated by Eq. (3) given a removal rate constant of kA = 8.0 · 104 s1. This value is about seven times larger than the removal rate constant for BFBC (1.2 · 104 s1) [7]. Fig. 4 shows that these values of ash removal rate constants for BFBC and CFBC are very close to the attrition rate that is estimated using the rate expression proposed by Cammarota et al. for granulated sewage sludge [11]: dDp =dt ¼ ðk b =3ÞðU U mf Þ;
ash had less influence on formation of NOx in dilute CFBC than the dense bed in BFBC. In dilute CFBC, nitrogen compounds in the volatile matter (e.g., NH3 and HCN) are likely to be oxidized in the gas phase before they reach the ash surface to be oxidized to form NOx. The selectivity to NOx during oxidation catalyzed by ash components such as Fe2O3 is known to be much higher than that of homogeneous oxidation [3]. For BFBC, because of high solid concentration in the dense bed, nitrogen compounds have greater likelihood to be oxidized by the ash to form more NOx. Accumulation of the sludge ash in the reactor was estimated using a model of ash attrition proposed by the authors in the previous study [7]. The assumptions of this model are: (1) the increase in NOx emission attributable to sludge ash accumulation is proportional to the amount of ash in the reactor; (2) the ash is removed from the reac-
ð2Þ
Fig. 1 shows that NOx emissions from coal combustion after finishing sludge combustion (t > ts = 6240 s) were higher than the emissions before sludge combustion (t < tf). The increase in NOx caused by ash accumulation is given as the difference between NOx emissions before the sludge feed ([NOx,inert]) and after the sludge feed. The change in NOx emissions caused by the solid removal is given as
expfk A ðt ts Þg:
200
Coal100%
ð1Þ
After stopping the sludge feed at t = ts, the amount of ash decreases with time as
NOx N2O
MVB coal
100
0
tor by elutriation of fines formed by attrition; and (3) the removal rate is proportional to the amount of sludge ash. Primary fragmentation of sludge ash was not taken into account since Cammarota et al. [11] reported that the primary fragmentation was insignificant for dry granulated sludge. In this model, coal ash is not taken into consideration since it had no effect on emissions of NOx and N2O as shown in Fig. 1 in the period of t < tf. The balance of feed and removal by attrition determines the amount of sludge ash as follows: dM Ash =dt ¼ F s wAsh k A M Ash :
200
0
2311
ð4Þ
where kb for granulated sludge was reported to be 4.39 · 107 (–). The change in particle volume is given as follows: ð1=V ÞðdV =dtÞ ¼ ð3=Dp ÞðdDp =dtÞ ¼ ðk b =Dp ÞðU U mf Þ:
ð5Þ
Under the present conditions of two-stage combustion in CFBC, two different gas velocities exist that might affect attrition: gas velocity in the lower part below the secondary air inlet and that above the secondary air inlet. Consequently, the experimentally obtained kA was plotted against both velocities. The calculated ash attrition rate of a particle of typical size (1 mm) was nearly the same as experimentally obtained kA. The difference in kA between BFBC and CFBC is explainable by the difference in the attrition rate because of the difference in gas velocity. This rough similarity between estimation and experimental
T. Shimizu, M. Toyono / Fuel 86 (2007) 2308–2315
[NOx(t)] - [NOx,inert] [ppm]
2312 200
results, for both BFBC and CFBC, suggests that ash attrition followed by carry-over of fines was the controlling factor of ash removal from the combustors examined here. Effects of ash accumulation on NOx emissions were analyzed based on the present ash accumulation model. When the dried sludge is fed continuously into the combustor at a constant rate, the change in the ash amount in the reactor with time after starting sludge feed is given as
MVB coal Line: [NOx(t)] - [NOx,inert] = 157exp{-0.00080(t - t s)}
100
M Ash ¼ M Ash;st ½1 expfk A ðt tf Þg;
0
[NOx(t)] - [NOx,inert] [ppm]
0
500
1000
1500 2000 t - t s [s]
2500
3000
3500
200
Line: [NOx (t)] - [NOx,inert] = 180exp{-0.00080(t - t s )} 100
0 500
1000
1500
2000
2500
3000
3500
t - t s [s]
[NOx(t)] - [NOx,inert] [ppm]
100
SA coal
80
Line: [NOx (t)] - [NOx,inert] = 50exp{-0.00080(t - t s)}
60 40 20 0 -20 0
500
1000
1500 2000 t - t s [s]
2500
3000
3500
Fig. 3. Change in the increase in NOx with time during coal combustion after stopping sludge combustion.
CFBC, 1mm CFBC, Exper. BFBC, 1mm
CFBC, 1.68mm CFBC, 0.5mm BFBC, Exper.
0.001
1 Gas velocity, U [m/s]
10
Fig. 4. Calculated ash attrition rate compared with experimental values of kA for a circulating fluidized bed (present work) and a bubbling fluidized bed [7] (parameter of calculation: sludge particle diameter).
ð7Þ
Fig. 5 shows the estimated ash accumulation in the reactors. Because the gas feed rate, which is proportional to the fuel feed rate, was different between CFBC and BFBC, the amount of ash was normalized by the total air feed rate (FGas). The accumulation of ash in CFBC was estimated as less than that in BFBC. For CFBC, the amount of ash became nearly steady at 3000 s after starting sludge feed, although it continued to increase even after 10 000 s for BFBC. One explanation for the nearly steady NOx emissions during co-combustion in CFBC (Fig. 2) is that the amount of ash became steady soon after starting the sludge feed. The relationship between the ash accumulation and NOx emissions for CFBC was also different from that for BFBC. Fig. 6 shows the effect of ash accumulation on emissions of NOx during mono-combustion of sludge and co-combustion of sludge with coal. Estimated amounts of ash are used for the discussion because it was difficult to determine the amount of ash experimentally during combustion experiments. For BFBC, a clear increase in NOx emissions was observed [7]. For CFBC, however, the influence of ash accumulation on NOx emissions was within limits of the scattering of the data, suggesting that the catalytic effect of ash plays only a minor role in NOx formation during sludge combustion. An explanation of the minor influence of ash on NOx formation in CFBC is that the solid concentration in CFBC is lower than that in BFBC. For that reason, homogeneous oxidation plays a major role; volatile nitrogen compounds
M Ash /F Gas [kg/(mol/s)]
(1/V )(dV /dt ) (Calculation) k A (experimental) [1/s]
0.01
0.0001 0.1
where MAsh,st is the amount of ash in the reactor at a steady state given as M Ash;st ¼ F s wAsh =k A :
HVB coal
0
ð6Þ
Fuel: Sludge (0.3) + SA (0.7)
1.0 0.8 0.6
BFBC CFBC
0.4 0.2 0.0
0
2000 4000 6000 8000 10000 12000 Time after starting sludge feed, t - t f [s]
Fig. 5. Estimated ash accumulation in CFBC and BFBC (BFBC result: [7]).
T. Shimizu, M. Toyono / Fuel 86 (2007) 2308–2315
Fuel: Sludge
1000 800 600 400 0
1000 NOx [ppm] (6%O2)
BFBC, linear CFBC
200 0
Fuel: Sludge (0.3) + MVB (0.7) BFBC, linear CFBC
600 400 200 0
600 400 200
1000
0.2 0.4 0.6 0.8 1 M Ash/F Gas [kg/(mol/s)]
Fuel: Sludge (0.3) + HVB (0.7)
800
0
0.5 1 1.5 M Ash/F Gas [kg/(mol/s)]
800
0
NOx [ppm] (6%O2)
1200
1000
NOx [ppm] (6%O2)
NOx [ppm] (6%O2)
1400
2313
BFBC, linear CFBC 0
0.2 0.4 0.6 0.8 1 M Ash/F Gas [kg/(mol/s)]
Fuel: Sludge (0.3) + SA (0.7)
800 600 BFBC, linear CFBC
400 200 0
0
0.2 0.4 0.6 0.8 1 M Ash/F Gas [kg/(mol/s)]
Fig. 6. Change in NOx emissions with ash accumulation for both co-combustion and mono-combustion of dried sludge (CFBC results, this work; BFBC results, linear approximation of the previous work [7]).
NOx [ppm] (6%O2)
released from the fuel are considered to be consumed by the homogeneous oxidation reactions before they reach the ash surface. In contrast, for BFBCs, volatile nitrogen compounds are considered to reach ash particles in the neighborhood before being oxidized by homogeneous reactions. Although the influence of the sludge ash on formation of NOx from the sludge in CFBC is minor, the sludge ash had activity to increase NOx formation from coal. Fig. 7 shows the effect of sludge ash accumulation on NOx emissions from coal combustion; the NOx emissions before sludge combustion were compared to those after sludge combustion in the bed with sludge ash. A possible mechanism of the difference in the influence of sludge ash on NOx formation between coals and sludge is the difference in nitrogen species in the fuel. Because the sewage sludge consists
mainly of microbes that contain protein or amino acid, nitrogen is considered to be contained as amino (–NH2) and peptide bond (–(CO)–(NH)–) forms. In coals, nitrogen contained as amino was reported to be minor; the major part of nitrogen was in the form of pyrrole (five-membered rings) or pyridine (six-membered rings) [13]. Intermediates formed by pyrolysis are known to be dependent on the type of chemical bond in the fuel [13]. Therefore, the difference in the nitrogen species in the form is considered to be a reason for the different influence of sludge ash on NOx formation. However, the search for the detailed mechanism of the difference in behaviour of fuel-bound nitrogen between coal and sludge is beyond the scope of this work. Instead, this is left as a subject for future studies. The effect of fuel type on the conversions of fuel-N to NOx (gNOx ) and N2O (gN2 O ) was also evaluated. Conver-
1000 900 800 700 600 500 400 300 200 100 0
CFBC, MVB CFBC, SA CFBC, HVB BFBC, MVB BFBC, SA BFBC, HVB 0
0.5 1 1.5 M Ash/F Gas [kg/(mol/s)]
2
Fig. 7. Increase in NOx emissions with sludge ash accumulation for mono-combustion of coal (CFBC results: this work; BFBC results: [7]).
T. Shimizu, M. Toyono / Fuel 86 (2007) 2308–2315
sions of fuel-N to NOx (gNOx ) and N2O (gN2 O ) are given respectively as gNOx ¼ ð½NOx =½CO2 þ COÞ=ðNfuel =Cfuel Þ;
ð8Þ
and
gN2 O ¼ 2ð½N2 O=½CO2 þ COÞ=ðNfuel =Cfuel Þ:
ð9Þ
The influence of ash accumulation to the emissions of NOx and N2O during co-combustion of sludge with coal and mono-combustion of sludge was only minor. Therefore, the conversions observed under a steady state were considered to be intrinsic conversions. For coal, the results obtained in the sand bed before the sludge feed are employed. As an index of the fuel type, F.R. defined as (fixed carbon content)/(volatile matter content) was employed. For mixed fuel, fixed carbon content and volatile matter, contents were calculated respectively as follows: ½FCm;daf ¼ ða½FCs þ ð1 aÞ½FCc Þ=fað½FCs þ ½VMs Þ þ ð1 aÞð½FCc þ ½VMc Þg;
ð10Þ
½VMm;daf ¼ ða½VMs þ ð1 aÞ½VMc Þ=fað½FCs þ ½VMs Þ þ ð1 aÞð½FCc þ ½VMc Þg:
ð11Þ
0.15
Conv. to NOx [-].
Mono-combustion Co-combustion 0.1
0.05
0
0.01
0.1
1
10
F.R. [-] Fig. 8. Effect of fuel properties on conversion of fuel-N to NOx.
a
Sludge ratio (Mass basis) = 0.3
0.1
Conv. to NOx (co-combst) [-]
This work
0.05
0
Fig. 8 shows that no clear relationship between the conversion to NOx and F.R. was inferred. Among the different coal types, no systematic tendency of conversion to NOx under mono-combustion condition was apparent. Thus, the conversion to NOx cannot be predicted by fixed carbon content of fuel alone. Instead, conversion to NOx under co-combustion conditions increased with increasing conversion to NOx during mono-combustion of base fuel (coal), as shown in Fig. 9a. A similar tendency, increase in conversion during co-combustion with increasing conversion during monocombustion of base fuel, was found in literature [5], as shown in Fig. 9b. These results suggest that the reduction of NOx in the upper part of the combustion chamber plays an important role in determining NOx emissions; it is well known that NOx concentration is high in the bottom section of CFBCs and that it decreases with the combustor height during coal combustion [14]. Consequently, the conversion to NOx is determined by the balance between the formation in the bottom and the reduction in the upper part. The reductant of NOx in the upper part is considered to be char, which is known to reduce NOx under FBC conditions. Conversion to NOx is expected to be high if the reduction in the upper part is insufficient. For co-combustion of sewage sludge with coal, volatile matter from the fuel is released in the bottom section immediately after being introduced in the reactor. It forms NOx. Then, the part of NOx is considered to be reduced by co-existing char particles, which are formed mainly from coal because the fixed carbon content of coal is much higher than that of sludge. Therefore, the reduction of NOx within the combustor is considered to be dependent on the capability of NOx reduction by the coal char in the upper part. In contrast to NOx, the conversion to N2O increased clearly with increasing F.R., i.e., with decreasing volatile matter content, as shown in Fig. 10. Similar results for conversion were reported in the literature [10]. The increase in conversion of N2O with increasing fixed carbon content was also found in BFBCs [7].
b Conv. to NOx (co-combust.) [-].
2314
0.04
Åmand L-E. et al. 0.03
0.02
0.01
0.05 0.1 Conv. to NOx (base fuel, mono) [-]
Base fuel: coal Base fuel: wood
0 0
Sludge ratio (Energy basis) = 0.12-0.15
0
0.1
0.2
Conv. to NOx (base fuel, mono) [-]
Fig. 9. Relationship between conversion of fuel-N to NOx during mono-combustion of base fuel and that during co-combustion of sludge with base fuel ˚ mand et al. [5]). (a: this work and b: A
T. Shimizu, M. Toyono / Fuel 86 (2007) 2308–2315
This work (Mono-) This work (Co-) Leckner, Mono, Normal Leckner, CTH Leckner, TUHH This work, Approx.
Conv. to N2O [-].
1
0.1
0.01
0.001 0.01
0.1
1
10
F.R [-] Fig. 10. Effect of fuel properties on conversion of fuel-N to N2O. (Leckner [10]).
4. Conclusions Mono-combustion of dried sludge and co-combustion of dried sludge with coal were carried out in a bench-scale circulating fluidized bed combustor. The accumulation of sludge ash did not increase the NOx emissions from mono-combustion of sludge and co-combustion of sludge with coal, but ash accumulation increased NOx emissions from mono-combustion of coal. Conversion of fuel-N to NOx during co-combustion showed no clear dependence on fixed carbon contents of fuel, but it had clear dependency on the conversion to NOx during mono-combustion of the base fuel (coal). Conversion of fuel-N to N2O increased with increasing fixed carbon content of fuel. Acknowledgement This work was conducted in cooperation with Kobelco Eco-Solutions Co., Ltd. References [1] Werther J, Ogada T. Sewage sludge combustion. Progr Energy Combust Sci 1999;25:55–116.
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[2] Hirama T, Kochiyama Y, Chiba T, Kobayashi H. Effects of NH3 on NOx reduction in homogeneous and heterogeneous reaction systems. Nenryo-Kyokai-Shi (J Fuel Soc Jpn) 1982;61:268–75. [3] Shimizu T, Togashi T, Miura M, Karahashi E, Yamaguchi T, Tonsho M, et al. Simultaneous reduction of emissions of N2O, NOx and SO2 from fluidized bed combustors. In: Proc of 6th Int Workshop on N2O ˚ AU/HUT/VTT/NIRE/IFP/US EPA, emissions, Turku, Finland, A 1994, 115–24. [4] Hirama T, Hosoda H, Moritomi H, Suzuki Y, Harada M, Shimizu T, et al. Formation and decomposition of nitrous oxide from a circulating fluidized-bed coal combustor. Nihon-Energy-Gakkai-Shi (J Jpn Inst Energy) 1993;72:252–62. ˚ mand LE, Miettinen-Westberg H, Karlsson M, Leckner B, Lu¨cke [5] A K, Bundinger S, et al. Co-combustion of dried sewage sludge and coal/wood in CFB-A search for factors influencing emissions. In: Proc 16th Int Conf on Fluidized Bed Combustion, Reno, NV, USA, ed. by Geiling DW, Am. Soc. Mech. Eng. (ASME), New York, USA; 2001, CD-ROM, Paper No. FBC01-0176. [6] Klein M, Rotzoll G. Formation of N2O and NO during fluidized bed combustion of single coal and char particles. In: Proc 6th Int ˚ AU/HUT/VTT/ Workshop on N2O emissions, Turku, Finland, A NIRE/IFP/US EPA, 1994, 239–53. [7] Shimizu T, Toyono T, Ohsawa T. Emissions of NOx and N2O during co-combustion of dried sewage sludge with coal in a bubbling fluidized bed combustor. Fuel 2007;86:957–64. [8] Werther J, Ogada T, Philippek C. Sewage sludge combustion in the fluidized bed-comparison of stationary and circulating fluidized bed techniques. In: Heinschel E, editor. Proc 13th Int Conf on Fluidized Bed Combustion (Reno, NV, USA). New York, USA: Am Soc Mech Eng (ASME); 1995. p. 951–62. [9] Sa¨nger M, Werther J, Ogada T. NOx and N2O emission characteristics from fluidised bed combustion of semi-dried municipal sewage sludge. Fuel 2001;80:167–77. ˚ mand LE, Lu¨cke K, Werther J. Gaseous emissions from [10] Leckner B, A co-combustion of sewage sludge and coal/wood in a fluidized bed. Fuel 2004;83:477–86. [11] Cammarota A, Chirone R, Salatino P, Scala F, Urciuolo M. Attrition phenomena during fluidized bed combustion of granulated and mechanically dewatered sewage sludges. Proc Combust Inst 2005;30:3017–24. [12] Moritomi H, Shimizu T, Suzuki Y, Ninomiya Y, Naruse I, Ono N, et al. Measurement of N2O emission from commercial scale and Bench-scale coal-fired fluidized bed combustors. In: Reuther RB, editor. Proc 15th Int Conf on Fluidized Bed Combustion, (Savannah, GA, USA). New York, USA: Am Soc Mech Eng (ASME); 1999. paper no. FBC0099-36. [13] Kanbara S, Takarada T, Yamamoto Y, Kato K. Relation between functional forms coal nitrogen and formation of NOx precursors during rapid pyrolysis. Energy Fuels 1993;7:1013–20. ˚ mand LE, Leckner B, Andersson S. Formation of N2O in [14] A circulating fluidized bed boilers. Energy Fuels 1991;5:815–23.
Emissions of NOx and N2O during co-combustion of dried sewage sludge with coal in a circulating fluidized bed combustor Tadaaki Shimizu *, Masanori Toyono Department of Chemistry and Chemical Engineering, Niigata University, 2-8050 Ikarashi, Niigata 950-2181, Japan Received 15 October 2006; received in revised form 19 January 2007; accepted 19 January 2007 Available online 28 February 2007
Abstract Emissions of NOx and N2O were measured during mono-combustion of dried sewage sludge and co-combustion with coal in a benchscale circulating fluidized bed combustor (CFBC). The results were compared with previous results obtained using a bubbling fluidized bed combustor (BFBC). The increase in NOx with sludge ash accumulation in the combustor was less for the CFBC than the BFBC, partly because of the higher attrition rate of sludge ash in CFBC resulting from the higher gas velocity. The influence of sludge ash on the formation of NOx in CFBC was less than that in BFBC during sludge combustion. The effects of fuel type on NOx and N2O emissions were also evaluated. 2007 Elsevier Ltd. All rights reserved. Keywords: Fluidized bed combustion; Sludge; Nitric oxide; Nitrous oxide; Attrition
1. Introduction Fluidized bed combustors (FBCs), both circulating (CFBCs) and bubbling (BFBCs), have been developed as combustion technologies that are useful for widely different fuels such as coal, wastes, and biomass. Municipal sewage sludge is a product of wastewater treatment. Increasing amounts of sewage sludge pose a problem for waste management in many developed countries [1]. Dried sewage sludge has moderate heating value. Therefore, the dried sludge can be burned in FBCs as a partial substitute for fossil fuels. A salient problem of sewage sludge combustion is its high nitrogen content, which might engender high emissions of NOx (NO + NO2) and N2O, the latter of which is known to be a greenhouse gas. High NOx emissions from sludge combustion are also anticipated not only because of their high nitrogen content but also their high contents of
*
Corresponding author. Tel./fax: +81 25 262 6783. E-mail address: [email protected] (T. Shimizu).
0016-2361/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.01.033
metal oxides such as iron oxides and magnesium oxide in the ash; solids containing such metal oxides are known to boost NOx emissions during coal combustion through oxidation of HCN and NH3 to NOx [2–6]. Indeed, the authors conducted dried sludge combustion in a BFBC and found that the sludge ash accumulation in the reactor significantly increased NOx emissions [7]. Although emissions of NOx and N2O from co-combustion of sludge and coal or plastics have been reported in the literature for CFBCs [5,8–10], the effect of ash accumulation on the emissions of NOx and N2O has not yet been fully elucidated: it remains uncertain whether ash accumulation influences NOx emissions from CFBCs. The ash attrition rate is known to increase with increasing gas velocity [11]. Therefore, the ash attrition rate in CFBCs is expected to be much higher than that in BFBCs. In the present work, mono-combustion of dried sewage sludge and co-combustion with coal were carried out and emissions of NOx and N2O were measured. A bench-scale circulating fluidized bed combustor was used. The effects of ash accumulation and fuel type on emissions of NOx and N2O were observed. The present results are compared with
T. Shimizu, M. Toyono / Fuel 86 (2007) 2308–2315
2309
Nomenclature a
mass ratio of sludge in mixed fuel (including ash and moisture) [CO2+CO] concentration of CO2 and CO in flue gas (ppm) Dp particle diameter (m) F.R. (fixed carbon content)/(volatile matter content) (–) [FC]c fixed carbon content of coal (value is given in Table 1) (–) [FC]m,daf fixed carbon content of mixed fuel (in dry ashfree basis) (–) [FC]s fixed carbon content of dried sludge (value is given in Table 1) (–) FGas total feed rate of air (mol s1) Fs feed rate of dried sludge (kg s1) kA ash removal rate constant (s1) kb attrition rate constant given by Eq. (4) (–) MAsh amount of sludge ash in the reactor (kg) MAsh,st amount of sludge ash in the reactor at steady state (kg) Nfuel/Cfuel N/C molar ratio of fuel (–) [NOx] concentration of NOx in flue gas (ppm)
those in the previous work using the same fuels, but employing a different reactor type (BFBC). 2. Experimental 2.1. Circulating fluidized bed combustor A bench-scale (1.9 m high, 2.2 cm i.d.) circulating fluidized bed combustor (CFBC) was used. The top of the riser was connected to a cyclone, in which particles were separated from the gas stream. The captured particles were recycled to the bottom of the riser through a bubbling fluidized bed (loopseal). The riser and the loopseal were heated by electric heaters; the temperature in the riser was maintained at 1123 K. Quartz sand of 80–150 mesh (average size 0.15 mm), which is known to be inert for the reactions of NOx and N2O [12], was used as the bed material. The minimum fluidizing velocity (Umf) of the sand was approximately 0.008 m s1. The total pressure drop across the riser was fixed at 1.5 kPa. Primary air was fed through a gas distributor at the bottom of the riser. Secondary air was injected through a nozzle located at 0.63 m above the distributor. The secondary air feed rate was half of the total air feed rate. The total air feed rate including the air for pneumatic transportation for the fuel feed was fixed at 11.3 · 103 mol s1 (= 15.24 Nl min1), i.e., the superficial gas velocity above the secondary air inlet (U) was 2.70 m s1 at 1123 K. Fuel was fed continuously through a rotary feeder, conveyed pneumatically in the air stream, and injected into the bottom of the riser
[N2O] t tf ts
concentration of N2O in flue gas (ppm) time (s) time when sludge feed was started (s) time when fuel feed was switched from sludge to coal (s) U superficial gas velocity (m s1) Umf minimum fluidizing gas velocity (m s1) V volume of particle (m3) [VM]c volatile matter content of coal (value is given in Table 1) (–) [VM]m,daf volatile matter content of mixed fuel (in dry ash-free basis) (–) [VM]s volatile matter content of dried sludge (value is given in Table 1) (–) wAsh ash content of dried sludge (–)
Greek symbols gNOx conversion of fuel-N to NOx gNOx ;inert conversion of fuel-N to NOx without catalytic effect of sludge ash gN2 O conversion of fuel-N to N2O
at 0.08 m above the distributor. The fuel feed rate was controlled so that the desired O2 concentration (3–5 vol.%) was attained in the flue gas. Flue gas from the combustor was filtered. Then it was cooled using an ice bath to remove water vapour. Concentrations of O2, total NOx (NO and NO2), CO2, CO, and SO2 in the dried flue gas were measured continuously using a magnetic oxygen analyzer for O2, chemical luminescence for NOx, and NDIR absorption for CO2, CO, and SO2. Some sample gas was stored in Tedler gas bags. Concentrations of O2, N2, CO2, and N2O in the gas were measured using gas chromatography with a thermal conductivity detector. Concentrations of NOx and N2O described below are corrected to the concentrations of dry flue gas containing 6% oxygen. Details of the gas analyzers are given elsewhere [7]. 2.2. Fuels Dried sewage sludge containing edible oil (rapeseed oil) was used. The mass ratio of the dried sludge to the oil was 7:3. The dried sludge was sieved and the fraction from 0.5 mm to 1.68 mm was employed. Characteristics of the fuel are shown in Tables 1 and 2. Details of the sludge sample preparation are described elsewhere [7]. Three kinds of coal with different volatile matter content, high volatile bituminous coal (HVB), medium volatile bituminous coal (MVB), and semi-anthracite (SA) were employed as fuels for co-combustion. Results of analyses of those fuels are shown in Tables 1 and 2. The fuel size was 0.3–1.0 mm. For co-combustion, the dried sludge with
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Table 1 Analyses of fuel Ultimate analysis (daf%) C c
Dry sludge with oil (ds:o = 7:3) High-volatile bituminous coal (HVB) Medium-volatile bituminous coal (MVB) Semi-anthracite (SA) a b c d
H
62.8 78.1 85.9 92.3
O
6.9 6.3 4.9 3.5
16.8 13.4 7.0 3.5
Proximate analysis (wt.%) N 5.4 1.3 1.7 1.5
S 1.7 1.0 0.5 0.2
Ash 21.6 14.3 15.0 12.8
V.M.a d
72.9 39.2 26.3 12.0
F.C.b
Moisture
5.5d 41.2 56.2 73.1
0 5.3 2.5 2.1
Volatile matter. Fixed carbon. Calculated from the composition of dry sludge after kerosene removal and rapeseed oil. Added rapeseed oil was assumed to be volatile matter.
Table 2 Analysis of fuel ash (as oxides) Al2O3
Fe2O3
CaO
MgO
P2O5
a
– 54.6
3.0 35.7
27.5 5.1
13.3 1.1
19.1 0.3
15.0 0.2
46.3
32.6
5.1
7.7
1.3
1.2
46.6
36.3
2.5
5.0
0.8
0.4
SiO2 Sludge High-volatile bituminous coal Medium-volatile bituminous coal Semi-anthraciteb a
Not measured. Sample from the same coal mine but different lot from the present sample. b
edible oil was mixed with the coal at a mass ratio of coal:sludge = 7:3 before it was stored in the fuel hopper. 3. Results and discussion Fig. 1 shows results of mono-combustion of mediumvolatile bituminous coal and dried sludge. First, coal combustion was conduced to attain the steady state. Then the fuel was switched to the dried sludge at t = tf. Finally, the fuel was switched to the coal again at t = ts. During the initial coal combustion, NOx and N2O emissions were almost constant. The ash from the present coal is considered to be inert or to be so fragile that the accumulation of coal ash did not affect the emissions of NOx and N2O.
NOx, N2O [ppm](6%O2)
600
NOx N2O
MVB coal
500 400 300
Coal100%
Sludge100%
2000
tf ts 4000 6000 Time, t [s]
Coal100%
200 100 0 0
8000
10000
Fig. 1. Dynamic change in emissions of NOx and N2O from monocombustion of coal and dried sludge (coal 100% (t < tf = 3720 s) ! sludge 100% (tf < t < ts = 6240 s) ! coal 100% (ts < t); flue gas oxygen concentration = 3–5 vol.% (dry)).
After starting sludge combustion, the NOx concentration became higher than those of coal. This increased concentration is attributable to the higher nitrogen content of dried sludge than coal. During the sludge feed, however, an increase in NOx with time because of ash accumulation was not observed. To the contrary, a significant increase in NOx was observed in the previous study using a bubbling fluidized bed combustor burning the same sludge. After switching the fuel from sludge to coal (t > ts), the NOx emissions decreased. However, the NOx emissions from coal after sludge combustion were higher than that before the sludge feed. Subsequently, the NOx emissions decreased gradually with time. These results suggest that the sludge ash that remained in the reactor catalyzed formation of NOx, and that the sludge ash amount decreased gradually with time, possibly because of attrition and elutriation. In contrast to high NOx emissions during sludge combustion, the emissions of N2O were slightly lower than those from coal combustion. In contrast to NOx, the emissions of N2O from coal combustion after the sludge feed were nearly equal to those before sludge feed. This rough equivalence also suggests that the accumulation of sludge ash in the reactor played only a minor role in N2O emissions. Fig. 2 shows results of co-combustion of sludge with three different coals. At first, coal alone was fed to stabilize the bed temperature. Then the dried sludge mixed with the coal was fed into the reactor. After starting co-combustion of sewage sludge with coal, the NOx concentration became higher than that from coal combustion. However, the increase in NOx emissions with time afterwards was not great. Emissions of N2O from co-combustion were nearly equal to or slightly higher than those from coal combustion. The present sludge ash was reported to impart considerable activity to increase NOx emissions from BFBC; the NOx emissions increased with time during sludge feed [7]. Nevertheless, the emissions of NOx from mono-combustion and co-combustion of the sludge were nearly constant with time for the present CFBC. Two possible explanations of the difference exist between BFBC and CFBC. One is the difference in the amount of ash: the attrition rate is higher in CFBC because of the higher gas velocity. Consequently, the ash is readily removed from the combustor through attrition and elutriation. Another explanation is that the
T. Shimizu, M. Toyono / Fuel 86 (2007) 2308–2315
NOx,N2O [ppm] (6%O2)
500
NOx N2O
HVB coal
400 300 Coal100%
Sludge30%+Coal70 %
200 100 0
0
2000
4000
6000
Time, t [s] 300 NOx,N2O [ppm] (6%O2)
M Ash ðtÞ ¼ M Ash ðts Þ expfk A ðt ts Þg:
Coal100% 0
300
Sludge30%+Coal70 %
2000
4000 Time, t [s]
6000
NOx,N2O [ppm] (6%O2)
SA coal
8000
NOx N2O
100
0
Sludge30%+Coal70%
2000
4000
6000
Time, t [s] Fig. 2. Change in emissions of NOx and N2O during co-combustion of dried sludge with coal (fuel: coal 100% ! dried sludge 30 wt.% + coal 70 wt.%; flue gas oxygen concentration = 3–5 vol.% (dry)).
½NOx ðtÞ ½NOx;inert ¼ ð½NOx ðts Þ ½NOx;inert Þ ð3Þ
Fig. 3 shows the change in the increase in NOx emissions ([NOx] [NOx,inert]) from coal combustion after stopping the sludge feed. The change is well approximated by Eq. (3) given a removal rate constant of kA = 8.0 · 104 s1. This value is about seven times larger than the removal rate constant for BFBC (1.2 · 104 s1) [7]. Fig. 4 shows that these values of ash removal rate constants for BFBC and CFBC are very close to the attrition rate that is estimated using the rate expression proposed by Cammarota et al. for granulated sewage sludge [11]: dDp =dt ¼ ðk b =3ÞðU U mf Þ;
ash had less influence on formation of NOx in dilute CFBC than the dense bed in BFBC. In dilute CFBC, nitrogen compounds in the volatile matter (e.g., NH3 and HCN) are likely to be oxidized in the gas phase before they reach the ash surface to be oxidized to form NOx. The selectivity to NOx during oxidation catalyzed by ash components such as Fe2O3 is known to be much higher than that of homogeneous oxidation [3]. For BFBC, because of high solid concentration in the dense bed, nitrogen compounds have greater likelihood to be oxidized by the ash to form more NOx. Accumulation of the sludge ash in the reactor was estimated using a model of ash attrition proposed by the authors in the previous study [7]. The assumptions of this model are: (1) the increase in NOx emission attributable to sludge ash accumulation is proportional to the amount of ash in the reactor; (2) the ash is removed from the reac-
ð2Þ
Fig. 1 shows that NOx emissions from coal combustion after finishing sludge combustion (t > ts = 6240 s) were higher than the emissions before sludge combustion (t < tf). The increase in NOx caused by ash accumulation is given as the difference between NOx emissions before the sludge feed ([NOx,inert]) and after the sludge feed. The change in NOx emissions caused by the solid removal is given as
expfk A ðt ts Þg:
200
Coal100%
ð1Þ
After stopping the sludge feed at t = ts, the amount of ash decreases with time as
NOx N2O
MVB coal
100
0
tor by elutriation of fines formed by attrition; and (3) the removal rate is proportional to the amount of sludge ash. Primary fragmentation of sludge ash was not taken into account since Cammarota et al. [11] reported that the primary fragmentation was insignificant for dry granulated sludge. In this model, coal ash is not taken into consideration since it had no effect on emissions of NOx and N2O as shown in Fig. 1 in the period of t < tf. The balance of feed and removal by attrition determines the amount of sludge ash as follows: dM Ash =dt ¼ F s wAsh k A M Ash :
200
0
2311
ð4Þ
where kb for granulated sludge was reported to be 4.39 · 107 (–). The change in particle volume is given as follows: ð1=V ÞðdV =dtÞ ¼ ð3=Dp ÞðdDp =dtÞ ¼ ðk b =Dp ÞðU U mf Þ:
ð5Þ
Under the present conditions of two-stage combustion in CFBC, two different gas velocities exist that might affect attrition: gas velocity in the lower part below the secondary air inlet and that above the secondary air inlet. Consequently, the experimentally obtained kA was plotted against both velocities. The calculated ash attrition rate of a particle of typical size (1 mm) was nearly the same as experimentally obtained kA. The difference in kA between BFBC and CFBC is explainable by the difference in the attrition rate because of the difference in gas velocity. This rough similarity between estimation and experimental
T. Shimizu, M. Toyono / Fuel 86 (2007) 2308–2315
[NOx(t)] - [NOx,inert] [ppm]
2312 200
results, for both BFBC and CFBC, suggests that ash attrition followed by carry-over of fines was the controlling factor of ash removal from the combustors examined here. Effects of ash accumulation on NOx emissions were analyzed based on the present ash accumulation model. When the dried sludge is fed continuously into the combustor at a constant rate, the change in the ash amount in the reactor with time after starting sludge feed is given as
MVB coal Line: [NOx(t)] - [NOx,inert] = 157exp{-0.00080(t - t s)}
100
M Ash ¼ M Ash;st ½1 expfk A ðt tf Þg;
0
[NOx(t)] - [NOx,inert] [ppm]
0
500
1000
1500 2000 t - t s [s]
2500
3000
3500
200
Line: [NOx (t)] - [NOx,inert] = 180exp{-0.00080(t - t s )} 100
0 500
1000
1500
2000
2500
3000
3500
t - t s [s]
[NOx(t)] - [NOx,inert] [ppm]
100
SA coal
80
Line: [NOx (t)] - [NOx,inert] = 50exp{-0.00080(t - t s)}
60 40 20 0 -20 0
500
1000
1500 2000 t - t s [s]
2500
3000
3500
Fig. 3. Change in the increase in NOx with time during coal combustion after stopping sludge combustion.
CFBC, 1mm CFBC, Exper. BFBC, 1mm
CFBC, 1.68mm CFBC, 0.5mm BFBC, Exper.
0.001
1 Gas velocity, U [m/s]
10
Fig. 4. Calculated ash attrition rate compared with experimental values of kA for a circulating fluidized bed (present work) and a bubbling fluidized bed [7] (parameter of calculation: sludge particle diameter).
ð7Þ
Fig. 5 shows the estimated ash accumulation in the reactors. Because the gas feed rate, which is proportional to the fuel feed rate, was different between CFBC and BFBC, the amount of ash was normalized by the total air feed rate (FGas). The accumulation of ash in CFBC was estimated as less than that in BFBC. For CFBC, the amount of ash became nearly steady at 3000 s after starting sludge feed, although it continued to increase even after 10 000 s for BFBC. One explanation for the nearly steady NOx emissions during co-combustion in CFBC (Fig. 2) is that the amount of ash became steady soon after starting the sludge feed. The relationship between the ash accumulation and NOx emissions for CFBC was also different from that for BFBC. Fig. 6 shows the effect of ash accumulation on emissions of NOx during mono-combustion of sludge and co-combustion of sludge with coal. Estimated amounts of ash are used for the discussion because it was difficult to determine the amount of ash experimentally during combustion experiments. For BFBC, a clear increase in NOx emissions was observed [7]. For CFBC, however, the influence of ash accumulation on NOx emissions was within limits of the scattering of the data, suggesting that the catalytic effect of ash plays only a minor role in NOx formation during sludge combustion. An explanation of the minor influence of ash on NOx formation in CFBC is that the solid concentration in CFBC is lower than that in BFBC. For that reason, homogeneous oxidation plays a major role; volatile nitrogen compounds
M Ash /F Gas [kg/(mol/s)]
(1/V )(dV /dt ) (Calculation) k A (experimental) [1/s]
0.01
0.0001 0.1
where MAsh,st is the amount of ash in the reactor at a steady state given as M Ash;st ¼ F s wAsh =k A :
HVB coal
0
ð6Þ
Fuel: Sludge (0.3) + SA (0.7)
1.0 0.8 0.6
BFBC CFBC
0.4 0.2 0.0
0
2000 4000 6000 8000 10000 12000 Time after starting sludge feed, t - t f [s]
Fig. 5. Estimated ash accumulation in CFBC and BFBC (BFBC result: [7]).
T. Shimizu, M. Toyono / Fuel 86 (2007) 2308–2315
Fuel: Sludge
1000 800 600 400 0
1000 NOx [ppm] (6%O2)
BFBC, linear CFBC
200 0
Fuel: Sludge (0.3) + MVB (0.7) BFBC, linear CFBC
600 400 200 0
600 400 200
1000
0.2 0.4 0.6 0.8 1 M Ash/F Gas [kg/(mol/s)]
Fuel: Sludge (0.3) + HVB (0.7)
800
0
0.5 1 1.5 M Ash/F Gas [kg/(mol/s)]
800
0
NOx [ppm] (6%O2)
1200
1000
NOx [ppm] (6%O2)
NOx [ppm] (6%O2)
1400
2313
BFBC, linear CFBC 0
0.2 0.4 0.6 0.8 1 M Ash/F Gas [kg/(mol/s)]
Fuel: Sludge (0.3) + SA (0.7)
800 600 BFBC, linear CFBC
400 200 0
0
0.2 0.4 0.6 0.8 1 M Ash/F Gas [kg/(mol/s)]
Fig. 6. Change in NOx emissions with ash accumulation for both co-combustion and mono-combustion of dried sludge (CFBC results, this work; BFBC results, linear approximation of the previous work [7]).
NOx [ppm] (6%O2)
released from the fuel are considered to be consumed by the homogeneous oxidation reactions before they reach the ash surface. In contrast, for BFBCs, volatile nitrogen compounds are considered to reach ash particles in the neighborhood before being oxidized by homogeneous reactions. Although the influence of the sludge ash on formation of NOx from the sludge in CFBC is minor, the sludge ash had activity to increase NOx formation from coal. Fig. 7 shows the effect of sludge ash accumulation on NOx emissions from coal combustion; the NOx emissions before sludge combustion were compared to those after sludge combustion in the bed with sludge ash. A possible mechanism of the difference in the influence of sludge ash on NOx formation between coals and sludge is the difference in nitrogen species in the fuel. Because the sewage sludge consists
mainly of microbes that contain protein or amino acid, nitrogen is considered to be contained as amino (–NH2) and peptide bond (–(CO)–(NH)–) forms. In coals, nitrogen contained as amino was reported to be minor; the major part of nitrogen was in the form of pyrrole (five-membered rings) or pyridine (six-membered rings) [13]. Intermediates formed by pyrolysis are known to be dependent on the type of chemical bond in the fuel [13]. Therefore, the difference in the nitrogen species in the form is considered to be a reason for the different influence of sludge ash on NOx formation. However, the search for the detailed mechanism of the difference in behaviour of fuel-bound nitrogen between coal and sludge is beyond the scope of this work. Instead, this is left as a subject for future studies. The effect of fuel type on the conversions of fuel-N to NOx (gNOx ) and N2O (gN2 O ) was also evaluated. Conver-
1000 900 800 700 600 500 400 300 200 100 0
CFBC, MVB CFBC, SA CFBC, HVB BFBC, MVB BFBC, SA BFBC, HVB 0
0.5 1 1.5 M Ash/F Gas [kg/(mol/s)]
2
Fig. 7. Increase in NOx emissions with sludge ash accumulation for mono-combustion of coal (CFBC results: this work; BFBC results: [7]).
T. Shimizu, M. Toyono / Fuel 86 (2007) 2308–2315
sions of fuel-N to NOx (gNOx ) and N2O (gN2 O ) are given respectively as gNOx ¼ ð½NOx =½CO2 þ COÞ=ðNfuel =Cfuel Þ;
ð8Þ
and
gN2 O ¼ 2ð½N2 O=½CO2 þ COÞ=ðNfuel =Cfuel Þ:
ð9Þ
The influence of ash accumulation to the emissions of NOx and N2O during co-combustion of sludge with coal and mono-combustion of sludge was only minor. Therefore, the conversions observed under a steady state were considered to be intrinsic conversions. For coal, the results obtained in the sand bed before the sludge feed are employed. As an index of the fuel type, F.R. defined as (fixed carbon content)/(volatile matter content) was employed. For mixed fuel, fixed carbon content and volatile matter, contents were calculated respectively as follows: ½FCm;daf ¼ ða½FCs þ ð1 aÞ½FCc Þ=fað½FCs þ ½VMs Þ þ ð1 aÞð½FCc þ ½VMc Þg;
ð10Þ
½VMm;daf ¼ ða½VMs þ ð1 aÞ½VMc Þ=fað½FCs þ ½VMs Þ þ ð1 aÞð½FCc þ ½VMc Þg:
ð11Þ
0.15
Conv. to NOx [-].
Mono-combustion Co-combustion 0.1
0.05
0
0.01
0.1
1
10
F.R. [-] Fig. 8. Effect of fuel properties on conversion of fuel-N to NOx.
a
Sludge ratio (Mass basis) = 0.3
0.1
Conv. to NOx (co-combst) [-]
This work
0.05
0
Fig. 8 shows that no clear relationship between the conversion to NOx and F.R. was inferred. Among the different coal types, no systematic tendency of conversion to NOx under mono-combustion condition was apparent. Thus, the conversion to NOx cannot be predicted by fixed carbon content of fuel alone. Instead, conversion to NOx under co-combustion conditions increased with increasing conversion to NOx during mono-combustion of base fuel (coal), as shown in Fig. 9a. A similar tendency, increase in conversion during co-combustion with increasing conversion during monocombustion of base fuel, was found in literature [5], as shown in Fig. 9b. These results suggest that the reduction of NOx in the upper part of the combustion chamber plays an important role in determining NOx emissions; it is well known that NOx concentration is high in the bottom section of CFBCs and that it decreases with the combustor height during coal combustion [14]. Consequently, the conversion to NOx is determined by the balance between the formation in the bottom and the reduction in the upper part. The reductant of NOx in the upper part is considered to be char, which is known to reduce NOx under FBC conditions. Conversion to NOx is expected to be high if the reduction in the upper part is insufficient. For co-combustion of sewage sludge with coal, volatile matter from the fuel is released in the bottom section immediately after being introduced in the reactor. It forms NOx. Then, the part of NOx is considered to be reduced by co-existing char particles, which are formed mainly from coal because the fixed carbon content of coal is much higher than that of sludge. Therefore, the reduction of NOx within the combustor is considered to be dependent on the capability of NOx reduction by the coal char in the upper part. In contrast to NOx, the conversion to N2O increased clearly with increasing F.R., i.e., with decreasing volatile matter content, as shown in Fig. 10. Similar results for conversion were reported in the literature [10]. The increase in conversion of N2O with increasing fixed carbon content was also found in BFBCs [7].
b Conv. to NOx (co-combust.) [-].
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0.04
Åmand L-E. et al. 0.03
0.02
0.01
0.05 0.1 Conv. to NOx (base fuel, mono) [-]
Base fuel: coal Base fuel: wood
0 0
Sludge ratio (Energy basis) = 0.12-0.15
0
0.1
0.2
Conv. to NOx (base fuel, mono) [-]
Fig. 9. Relationship between conversion of fuel-N to NOx during mono-combustion of base fuel and that during co-combustion of sludge with base fuel ˚ mand et al. [5]). (a: this work and b: A
T. Shimizu, M. Toyono / Fuel 86 (2007) 2308–2315
This work (Mono-) This work (Co-) Leckner, Mono, Normal Leckner, CTH Leckner, TUHH This work, Approx.
Conv. to N2O [-].
1
0.1
0.01
0.001 0.01
0.1
1
10
F.R [-] Fig. 10. Effect of fuel properties on conversion of fuel-N to N2O. (Leckner [10]).
4. Conclusions Mono-combustion of dried sludge and co-combustion of dried sludge with coal were carried out in a bench-scale circulating fluidized bed combustor. The accumulation of sludge ash did not increase the NOx emissions from mono-combustion of sludge and co-combustion of sludge with coal, but ash accumulation increased NOx emissions from mono-combustion of coal. Conversion of fuel-N to NOx during co-combustion showed no clear dependence on fixed carbon contents of fuel, but it had clear dependency on the conversion to NOx during mono-combustion of the base fuel (coal). Conversion of fuel-N to N2O increased with increasing fixed carbon content of fuel. Acknowledgement This work was conducted in cooperation with Kobelco Eco-Solutions Co., Ltd. References [1] Werther J, Ogada T. Sewage sludge combustion. Progr Energy Combust Sci 1999;25:55–116.
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[2] Hirama T, Kochiyama Y, Chiba T, Kobayashi H. Effects of NH3 on NOx reduction in homogeneous and heterogeneous reaction systems. Nenryo-Kyokai-Shi (J Fuel Soc Jpn) 1982;61:268–75. [3] Shimizu T, Togashi T, Miura M, Karahashi E, Yamaguchi T, Tonsho M, et al. Simultaneous reduction of emissions of N2O, NOx and SO2 from fluidized bed combustors. In: Proc of 6th Int Workshop on N2O ˚ AU/HUT/VTT/NIRE/IFP/US EPA, emissions, Turku, Finland, A 1994, 115–24. [4] Hirama T, Hosoda H, Moritomi H, Suzuki Y, Harada M, Shimizu T, et al. Formation and decomposition of nitrous oxide from a circulating fluidized-bed coal combustor. Nihon-Energy-Gakkai-Shi (J Jpn Inst Energy) 1993;72:252–62. ˚ mand LE, Miettinen-Westberg H, Karlsson M, Leckner B, Lu¨cke [5] A K, Bundinger S, et al. Co-combustion of dried sewage sludge and coal/wood in CFB-A search for factors influencing emissions. In: Proc 16th Int Conf on Fluidized Bed Combustion, Reno, NV, USA, ed. by Geiling DW, Am. Soc. Mech. Eng. (ASME), New York, USA; 2001, CD-ROM, Paper No. FBC01-0176. [6] Klein M, Rotzoll G. Formation of N2O and NO during fluidized bed combustion of single coal and char particles. In: Proc 6th Int ˚ AU/HUT/VTT/ Workshop on N2O emissions, Turku, Finland, A NIRE/IFP/US EPA, 1994, 239–53. [7] Shimizu T, Toyono T, Ohsawa T. Emissions of NOx and N2O during co-combustion of dried sewage sludge with coal in a bubbling fluidized bed combustor. Fuel 2007;86:957–64. [8] Werther J, Ogada T, Philippek C. Sewage sludge combustion in the fluidized bed-comparison of stationary and circulating fluidized bed techniques. In: Heinschel E, editor. Proc 13th Int Conf on Fluidized Bed Combustion (Reno, NV, USA). New York, USA: Am Soc Mech Eng (ASME); 1995. p. 951–62. [9] Sa¨nger M, Werther J, Ogada T. NOx and N2O emission characteristics from fluidised bed combustion of semi-dried municipal sewage sludge. Fuel 2001;80:167–77. ˚ mand LE, Lu¨cke K, Werther J. Gaseous emissions from [10] Leckner B, A co-combustion of sewage sludge and coal/wood in a fluidized bed. Fuel 2004;83:477–86. [11] Cammarota A, Chirone R, Salatino P, Scala F, Urciuolo M. Attrition phenomena during fluidized bed combustion of granulated and mechanically dewatered sewage sludges. Proc Combust Inst 2005;30:3017–24. [12] Moritomi H, Shimizu T, Suzuki Y, Ninomiya Y, Naruse I, Ono N, et al. Measurement of N2O emission from commercial scale and Bench-scale coal-fired fluidized bed combustors. In: Reuther RB, editor. Proc 15th Int Conf on Fluidized Bed Combustion, (Savannah, GA, USA). New York, USA: Am Soc Mech Eng (ASME); 1999. paper no. FBC0099-36. [13] Kanbara S, Takarada T, Yamamoto Y, Kato K. Relation between functional forms coal nitrogen and formation of NOx precursors during rapid pyrolysis. Energy Fuels 1993;7:1013–20. ˚ mand LE, Leckner B, Andersson S. Formation of N2O in [14] A circulating fluidized bed boilers. Energy Fuels 1991;5:815–23.