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Use of Nitrogen Stable Isotope Analysis To Understand Char Nitrogen Evolution during the Fluidized-Bed Co-combustion of Coal and Sewage Sludge Ana Arenillas, Roberto Garcı´a, Chenggong Sun, and Colin E. Snape* Nottingham Fuel and Energy Centre, School of Chemical, Environmental and Mining Engineering, University of Nottingham, NG7 2RD, United Kingdom
Angel H. Moreno, Fernando Rubiera, and Jose J. Pis Instituto Nacional del Carbo´ n, CSIC, Apartado 73, 33080 Oviedo, Spain Received June 16, 2004. Revised Manuscript Received November 19, 2004
NOX emissions from sewage sludge combustion are a concern, because of the usually high nitrogen content of this fuel. The interactions during co-combustion in a fluidized-bed reactor of sewage sludge and a bituminous coal were evaluated, in relation to the nitrogen evolution during the combustion process. The nitrogen stable isotope measurements provide novel results regarding the tracing of nitrogen during combustion. Our preliminary results show that the co-combustion chars retain more nitrogen than expected, with the additional nitrogen being mainly derived from the sludge. Additional measurements are planned on the resultant co-combustion gases, to aid source apportionment of the NOX arising from coal/sewage interactions.
Introduction The combustion of sewage sludge1 and its blends with coal combines several advantages, such as reduction of the volume of sludge waste or thermal destruction of toxic organic compounds and pathogens. Moreover, these methods allow recovery of the energy value of this material, while replacing a nonrenewable fuel such as coal. However, the environmental regulations have become increasingly severe, and the addition of sludge to a combustion plant designed for a particular base fuel may require expensive modifications to meet the legislative restrictions. The emissions of both heavy metals and harmful gases are of concern. Regarding the latter, the emission of nitrogen oxides (NOX) is an important problem that has not been extensively studied. Sewage sludge has very high nitrogen contents, typically 5%-8% (dry basis, db); hence, there is a potential for elevated NOX and N2O emissions. During the combustion of coal blends, interactions can occur between the coals, affecting the reactivity as well as other aspects (such as NOX emissions), although these interactions are dependent on the coal rank and the operating conditions.2,3 Similarly, some interactions during the co-combustion of coal and sewage sludge in fluidized bed combustion (FBC) plants have been already reported, with, for example, reductions in the expected NO emissions.4 However, other authors have observed * Author to whom correspondence should be addressed. Fax: +44 (0)115 95 14 115. E-mail address:
[email protected]. (1) Anthony, E. J. Prog. Energy Combust. Sci. 1995, 21, 239-268. (2) Arenillas, A.; Backreedy, R. I.; Jones, J. M.; Pis, J. J.; Pourkashanian, M.; Rubiera, F.; Williams, A. Fuel 2002, 81 (5), 627-636. (3) Rubiera, F.; Arenillas, A.; Arias, B.; Pis, J. J. Fuel Process. Technol. 2002, 77-78, 111-117.
different interactions between a high-volatile bituminous coal and three types of sewage sludge by thermogravimetric analysis.5,6 The nitrogen content, as determined by ultimate analysis, and the detection of gaseous nitrogen compounds evolved during combustion are commonly used, to be correlated with the interactions between blend components that have occurred during combustion. However, these analyses can only detect the total nitrogen content or the total nitrogen compounds evolution and cannot distinguish between their sources. The objective of this communication, on the co-combustion of sewage sludge and a bituminous coal in a benchscale fluidized-bed reactor, is to show the potential of the nitrogen stable isotope measurements for estimating the relative contributions from the nitrogen in both fuels to the nitrogen in the resulting chars. This is the very first time that nitrogen stable isotope measurements have been applied to establish the sources of the char nitrogen in the co-combustion of coal with another feedstock. Materials and Procedure In this work, a low-volatile bituminous coal and dry sewage sludge from an urban wastewater treatment plant located in Asturias (Spain) were used. The main chemical characteristics for both fuels are presented in Table 1. It can be observed that the sludge sample presents high ash content, as well as nitrogen and oxygen contents that are significantly higher than those of the bituminous coal. (4) Leckner, B.; Åmand, L. E.; Lu¨cke, K.; Werther, J. Fuel 2004, 83, 477-486. (5) Otero, M.; Dı´ez, C.; Calvo, L. F.; Garcı´a, A. I.; Mora´n, A. Biomass Bioenergy 2002, 22, 319-329. (6) Folgueras, M. B.; Diaz, R. M.; Xiberta, J.; Prieto, I. Fuel 2003, 82, 2051-2055.
10.1021/ef049857y CCC: $30.25 © 2005 American Chemical Society Published on Web 01/11/2005
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Table 1. Chemical Characterization of the Samples Studied Proximate Analysis (%, db)
Table 2. Carbon and Nitrogen Stable Isotope Ratios for the Samples Studied
Ultimate Analysis (%, daf)
sample
VM
FC
ash
coal sewage sludge
15.4 9.8
75.8 60.9
8.8 29.3
91.7 4.7 1.2 0.6 1.8 49.1 6.9 5.3 1.0 37.7
C-10 C-20 C-30
11.6 8.8 8.4
75.7 76.1 76.4
12.7 15.1 15.2
95.0 2.0 1.3 0.6 95.4 1.8 1.3 0.6 96.6 0.7 1.3 0.6
S-10 S-20 S-30
8.2 6.3 0.1
15.0 4.9 5.2
76.8 88.8 94.7
71.6 61.7 43.2
10.9 11.0 10.9
57.8 55.1 56.9
31.3 33.9 32.2
91.6 3.1 1.8 0.8 94.0 1.4 1.6 0.7 95.2 0.7 1.4 0.9
C/S-10 C/S-20 C/S-30
C
H
N
S
O
1.1 0.9 0.8
5.4 2.2 20.8 2.2 3.3 32.8 3.5 53.3 2.7 2.3 1.8
A series of chars with different degrees of burnoff were obtained in a bench-scale fluidized-bed reactor from the individual fuels (i.e., coal and sludge) and their 50 wt % blend. The chars were prepared from the 1.6-2.0 mm particle size fraction of the low-volatile bituminous coal and the sewage sludge. The experimental procedure involved the injection of batches of coal particles (0.5 g) into a 52-mm internal diameter stainless-steel fluidized-bed reactor that has been electrically heated at 850 °C. Silica sand (d ) 0.30 mm) was used as bed material and air was used as a fluidizing gas. The air was preheated before entering the reactor. Full details of the experimental setup can be found elsewhere.7,8 A basket made of No. 16 steel wire mesh was used to collect the chars from the bed at different burn-off levels. This method was based on that initially developed at MIT.9 The chars were retrieved from the reactor at different intervals, while N2 was injected at room temperature, to ensure the quenching of the reaction. Each run was repeated several times, to provide sufficient sample for analysis. The main chemical characteristics from these chars are also presented in Table 1. The char samples are identified with a letter denoting the raw material used (i.e., C for coal, S for sludge, and C/S in the case of the 50 wt % blend) and a number that indicates the delay time for the combustion (in seconds). The original materials (coal and sewage sludge), their combustion chars, and those for the blend were subjected to nitrogen and carbon stable isotope ratio measurements. These were conducted using a Carlo Erba model NA1500 elemental analyzer for on-line combustion (oxidation at 1020 °C, reduction at 650 °C). The evolving gases of interest (N2 for nitrogen and CO2 for carbon) were analyzed using a Thermo Finnigan Delta Plus XP gas source isotope ratio mass spectrometer. The 15N/14N isotope ratio of a sample is reported using the δ15N (‰) notation, relative to atmospheric nitrogen, which is the international standard for nitrogen isotope ratio measurements, according to eq 1. The δ-notation was also used in the case of the 13C/12C isotope ratio, using eq 2, with Vienna-Pee Dee Belemnite, VPDB, as the standard material.
δ15N (‰, air N2) )
δ15C (‰, VPDB) )
[ [
(15N/14N)sa
] ]
(15N/14N)st - 1 (15C/14C)sa
(15C/14C)st - 1
× 1000
(1)
× 1000
(2)
The results in both cases are the average values of three measurements. The subscript “sa” denotes data regarding the (7) Marba´n, G.; Pis, J. J.; Fuertes, A. B. Combust. Flame 1995, 103, 41-58. (8) Arenillas, A.; Rubiera, F.; Pevida, C.; Moreno, A. H.; Pis, J. J. 6th International Conference on Technologies and Combustion for a Clean Environment; Porto, Portugal, July 9-12, 2001; Vol. II, Paper No. 31.4, pp 967-972. (9) Andrei, M. A. Ph.D. Thesis, Chemical Engineering Department, Massachusetts Institute of Technology, Cambridge, MA, 1979.
sample
δ13C (‰)
δ15N (‰)
coal sewage sludge
-22.6 -25.5
3.2 8.4
C-10 C-20 C-30
-22.6 -22.7 -22.8
4.1 4.1 4.8
S-10 S-20 S-30
-25.9 -25.5 -24.7
11.6 17.5
C/S-10 C/S-20 C/S-30
-22.8 -22.4 -23.3
6.8 6.7 6.4
Table 3. Experimental Combustion Char Yields for the Coal (YC) and the Sludge (YS), and Experimental and Calculated Values for Their 50:50 Blend (YC/S and YC/S,calc, Respectively) residence time (s)
YC
YS
YC/S
YC/S,calca
10 20 30
78.9 76.3 75.3
6.7 2.9 1.3
64.2 53.4 46.8
47.4 44.2 43.0
a Y C/S,calc ) [YC(100 - AshC) + YS(100 - AshS)]/(200 - AshC AshS), where AshC is the ash content of the coal andAshS is the ash content of the sludge.
sample, whereas the subscript “st” denotes data regarding the standard material.
Results and Discussion According to the data presented in Table 1, the sewage sludge sample seems to be very reactive, because the chars display a large decrease in volatile matter (VM) content with increasing combustion time, being virtually negligible after 30 s (see sample S-30 in Table 1). Note the rapid decrease of the nitrogen content with the increasing delay time. The latter indicates that nitrogen is mainly contained in the volatiles of the sewage sludge. The proximate and ultimate analyses of the chars for the coal/sludge blend cannot be predicted linearly, indicating that some interactions occur when both fuels are processed together. The nitrogen stable isotope ratios for the original materials and the FBC chars are listed in Table 2. Clear differences can be observed between samples, because of their different origins. The coal possesses the lowest δ15N value (3.2‰), and its chars display similar values, slightly increasing with the residence time in the combustion reactor (4.1‰-4.8‰). This shift observed in δ15N values for FBC chars is similar to other values from previous work on pf coal combustion in a drop tube furnace.10 On the other hand, the δ15N value is much higher in the sludge sample (8.4‰) and increases up to 11.6‰ and 17.5‰ in the combustion chars. Therefore, the release of nitrogen compounds during combustion seems to involve preferentially the lighter 14N isotope, especially in the case of the sludge material. The char yields are higher than predicted (Table 3), together with the carbon and nitrogen contents in the combustion chars of the blend, with the differences decreasing with increasing residence time. Figures 1 and 2 respectively display the distribution of carbon and nitrogen in the three chars from the coal/sludge blend, in terms of the source (i.e., carbon and nitrogen contents being denoted as 100% in the starting material). From (10) Snape, C. E.; Sun, C.-G.; Fallick, A. E.; Irons, R.; Haskell, J. Prepr. Symp.sAm. Chem. Soc., Div. Fuel Chem. 2003, 48 (1), 3-5.
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Figure 1. Estimated carbon distribution between sludge and coal in the fluidized-bed co-combustion chars. (Cadd refers to the additional carbon obtained, calculated as the difference between the experimental and estimated carbon content.)
Figure 2. Estimated nitrogen distribution between sludge and coal in the fluidized-bed co-combustion chars. (Nadd refers to the additional nitrogen obtained, calculated as the difference between the experimental and estimated nitrogen content.)
the carbon and nitrogen removal observed in the combustion of the individual components, coal and sludge, the theoretical proportions of carbon and nitrogen remaining in the blend chars coming from the individual components can be estimated (see these contributions in Figures 1 and 2). The difference with the experimentally observed carbon and nitrogen contents in these chars is referenced as Cadd and Nadd, respectively, and these can also be observed in Figures 1 and 2. The different contributions to the carbon and nitrogen of the blend chars must account for the isotopic ratios observed. As Figure 1 shows, the main contribution to the carbon of the char comes from the coal and, accordingly, the δ13C values of the blend chars are similar to those of the coal (see Table 2), with the influence of the carbon of the sludge being almost negligible. Thus, the additional carbon (Cadd) resulting from the interaction between coal and sludge (Figure 1) does not exert any significant influence. In the case of the nitrogen in the blend chars, the main contribution also comes from the coal (see Figure 2); however, the δ15N values are intermediate between those of the coal and the sludge (see Table 2). Furthermore, Nadd is proportionally higher than Cadd (see Figures 1 and 2) and must have its own significant influence on the resultant δ15N values. Considering the additive character of the δ15N parameter and making the assumption that the experimentally obtained isotopic ratios of the coal and the sludge do not change when they are blended together, the value of δ15N was estimated for the blend chars considering different contributions to Nadd, and Figure 3 compares the estimated values with the experimental
Energy & Fuels, Vol. 19, No. 2, 2005 487
Figure 3. Experimental and estimated δ15N values for the fluidized-bed co-combustion chars. (Nadd refers to the additional nitrogen obtained, calculated as the difference between the experimental and estimated nitrogen content.)
ones (horizontal lines). In all the combustion experiments (residence times of 10, 20, and 30 s), the estimated δ15N line intersects the experimental δ15N line when the contribution of the sludge to Nadd is >50%, especially in the case of the 30-s experiment (∼90%). Thus, these results seem to indicate that much of the nitrogen retained as a result of the coal/sludge interaction in the co-combustion chars is derived from the sludge. Under FBC conditions, the heterogeneous catalyzed reactions and the gas/solid-phase reactions have an important role. An important factor in this is the char, which provides an active surface with active carbon (-C) sites that are able to react with the gaseous nitrogen compounds.11 Previous studies indicate that -CN and -CNO may be the main N-functionalities that are formed on the char surface. They can be produced, under FBC conditions, by reaction of NO with carbon sites (-C) and/ or oxygen surface functionalities, the latter being formed either from NO or O2 heterogeneous reactions with the char surface.11-13 These nitrogen complexes, although they are able to undergo further reactions to give N2O or N2 as nitrogen reduced species, can also contribute, in some way, to the incorporation and retention of nitrogen in the resultant char, especially if the combustion reaction is quenched at very short residence times, such as the chars obtained in this work. Although the role of the char in the heterogeneous reactions can be applicable to all types of combustion systems, under FBC conditions, other factors are involved. The presence of CaO, MgO, and Fe2O3 from the fuel can lead to the formation of a very active bed and, thus, favor the reaction with NO and N2O from the gas phase, considerably increasing the amount of nitrogen incorporated/retained in the char. In the case of sewage sludge combustion, the effect of the high ash content of this fuel must be considered.14 However, in the combustion of sewage sludge alone, the small amount of char present does not permit the catalytic effect to be effective. However, apparently in the co-combustion of coal/sewage (11) Hayhurst, A. N.; Lawrence, A. D. Combust. Flame 1996, 105, 341357. (12) de Soete, G. G. Proceedings of the 23rd International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; pp 1257-1264. (13) Pels, J. R.; Wo´jtowicz, M. A.; Kapteijn, F.; Moulijn, J. A. Energy Fuels 1995, 9, 743-752. (14) Werther, J.; Ogada. T. Prog. Energy Combust. Sci. 1999, 25, 55116.
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sludge, the catalytic property of the sludge ash can favor the retention of nitrogen in the char, being mainly from the coal. In conclusion, our preliminary results show that the co-combustion chars retain more nitrogen than expected with the isotope measurements, indicating that the additional nitrogen is mainly derived from the sludge. Additional measurements are planned on the resultant co-combustion gases, to aid source apportionment of the NOx arising from the coal/sewage interactions.
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Acknowledgment. A.A. and R.G. thank Secretaria de Estado de Educacio´n y Universidades (Direccio´n General de Universidades, Ministerio de Educacio´n Cultura y Deporte, Spain) and Fondo Social Europeo for financial support. The UK Department of Trade and Industry, DTI (Coal Program, Contract No. 231) is also acknowledged. EF049857Y
