Energy & Fuels 1996,9, 743-752
743
Trade-off between NO, and N20 in Fluidized-Bed Combustion of Coals? J. R. Pels,* M. A. W6jtowicz,* F. Kapteijn, and J. A. Moulijn Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Received September 7, 1994@
NO and Nz0 formation and destruction was studied in a laboratory-scale fluidized-bed combustor for a set of coals ranging from lignite to anthracite. For a given coal, the sum of fuel-N conversions to Nz0 and NO was found to be remarkably constant over a range of temperatures, although emission levels of individual species were strongly temperature-dependent (NO increases and NzO decreases with increasing temperature). A similar behavior was observed for the formation of NO and NzO during combustion of chars. This trade-off between NzO and NO emissions can be regarded as a constant conversion of fuel-bound nitrogen to Nz. The influence of NO addition on the formation of NzO and Nz was investigated in packed-bed gasification experiments using char prepared from a bituminous coal. The inlet concentration of NO was at a level typical for fluidized-bed combustion. It was found that NO reduction on char surface was strongly enhanced in the presence of oxygen. The majority of the added NO was converted to Nz, but some NzO was also formed. The temperature-governed trade-off between emissions of NO and NzO, and the formation of NzO and Nz from NO reduction, are qualitatively explained on the basis of known heterogeneous and homogeneous NO/NzO chemistry.
Introduction The role which nitric oxide (NO) plays in the formation of photochemical smog and acid rain is well-known. In addition to causing these hazards, NO exhibits a strong catalytic activity in the ozone destruction cycle. NO is too short-lived, however, to penetrate into the stratosphere. It is formed there mainly as a product of nitrous oxide (NzO)decomposition,the latter gas having a sufficiently long lifetime to diffise into the stratosphere. The exact contribution of thus formed NO to ozone layer depletion is subject to debate.1-3 In addition t o being a powerful, albeit indirect, perpetrator of ozone layer depletion, NzO has been recognized as a strong greenhouse gas. The radiative forcing of N2O is 270 times stronger than that of COz, and its atmospheric lifetime is comparable with that of CFC's (NzO: 150 years; CFC-11,60 years; CFC-12,130 years2). This makes Nz0 an extremely potent and longlived greenhouse gas. The current NzO contribution to the anthropogenic enhancement of the greenhouse effect has been estimated t o be about 6%.lJ +This paper was presented a t the 6th International Workshop on Nitrous Oxide Emissions, 7-9 June 1994, Turku, Finland. * Author to whom correspondence should be addressed. E-mail:
[email protected]. Current address: Dr. Jack McKenzie Limerick Pulp and Paper Research and Education Centre, Incutech Centre, University of New Brunswick, P.O. Box 69000, Fredericton, N.B., Canada E3B 6C2. E-mail address:
[email protected]. Current address: Advanced Fuel Research, 87 Church Street,P.O. Box 380379, East-Hartford, CT 06138-0379. Abstract published in Advance ACS Abstracts, May 15, 1995. '(1)Takeshita, M.; Sloss, L. L..; Smith, I. M. NzO emissions from coal use; IEA Perspectives IEAPEWO6, IEA Coal Research: London, U.K, November 1993. (2) Houghton, J . T.; Jenkins, G. J., Ephraums, J. J., (Eds.) Climate change: the IPCC scientific assessment; IPCC: Cambridge University Press: Cambridge, U.K., 1990. (3)Houghton, J. T., Callander, B. A., Varney, S. K., Eds. Climate change 1992: the supplementary report to the IPCC scientific assessment; IPCC: Cambridge University Press: Cambridge, U.K., 1992. @
Fluidized-bed combustion has emerged as an environmentally attractive method of burning carbonaceous fuels, mainly because of its low combustion temperatures (typically 1000-1200 K)which result in low NO, emissions and advantageous conditions for efficient sulfur removal. Sulfur capture is conveniently implemented by in situ limestone injection, which eliminates the need for flue-gas treatment. Unfortunately, these merits are offset by NzO levels of up to 250 ppm in the flue gas, high when compared with 5-20 ppm levels which are characteristic of conventional combu~tors.l~~-~ At the moment, the contribution of coal-fired power stations to the total NzO release is moderate and comparable with other anthropogenic Nz0 sources. Nevertheless, an increase in the proportion of coal combusted in fluidized systems raises a serious concern about high levels of future NzO emission^.^^^ It is well-known that NO emissions increase and NzO emissions decrease as combustion temperature increases, temperature being the most important single parameter governing the magnitude of NO,/N20 level^.^^^-^^ It has been reported in several studies that the combined conversion of coal-bound nitrogen (coal(4) Hulgaard, T. Ph.D. thesis, Department of Chemical Engineering, Technical University of Denmark, Lyngby, Denmark, 1991. (5) Andersson, C.; Brannstrom-Norberg, B. M., Hanell, B. "Nitrous oxide emissions from different combustion sources"; Report No. U(V) 1989131, Vattenfall, Sweden, 1989. (6) De Soete, G. G. P m . LNETIIEPAIIFPEur. Workshop Emission of Nitrous Oxide, LNETI, Lisbon, Portugal 1990,41-45. (7) W6jtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel Process. Technol. 1993,34, 1-71. (8)Amand, L.-E.; Andersson, S. Proceedings of the 10th International Conference FBC; ASME: New York, 1989; Vol. 1,pp 49-56. (9) Aho, M. J.; Rantanen, J. T. Fuel 1989,68, 586-590. (10) Hiltunen, M.; Kilpinen, P.; Hupa, M.; Lee, Y. Y. Proceedings of the 11th International Conference on FBC; ASME: New York, 1991;
0887-0624/95/2509-0743$09.00/00 1995 American Chemical Society
Pels et al.
744 Energy & Fuels, Vol. 9, No. 5, 1995 Table 1. Characterizationof Coals and Chars Used in FBC Eweriments mateial
rank
volatile elemental analysis mattera ashb C H N S
thermomuple filter
analysis co
DE53 coal lignite DE53 char
53.9
AU52 coal subbituminous AU52 char
53.3 11.6 75.0 5.1 0.9 0.3 21.2 95.0 0.7 1.2 0.0
DE38 coal high volatile bituminous DE38 char
40.4
12.0 81.2 4.8 1.7 1.1 9.6 94.3 0.7 2.2 1.1
BE30 coal high volatile bituminous BE30 char
31.9
2.3 88.2 4.9 1.7 0.9 7.4 95.8 0.7 2.3 1.1
DE10 coal semianthracite DE10 char
11.6
8.3 92.4 3.5 1.4 0.5 12.9 94.5 0.7 1.5 0.7
4.5
2.1 94.1 2.7 0.9 0.6 3.2 94.1 2.7 0.9 0.6
Figure 1. A schematic diagram of the laboratory-scale fluidized-bed combustor.
7.0 92.8 0.8 2.0 1.6
observed emission levels was attributed to the combustion of volatiles.l* The chars were prepared from their parent coals by devolatilization in nitrogen, which was carried out in a tube furnace. Heat-treatment temperature was 1073-1173 K, soak time 1-3 h, and the heating rate 5-10 Wmin.18 Combustion experiments were carried out in a laboratoryscale quartz fluidized-bed combustor (FBC), operating under atmospheric pressure, with coal fed in continuously from a coal feeder, and dry air supplied at the bottom. A schematic diagram of the combustor section of the experimental setup is shown in Figure 1. The reactor internal diameter was 29 mm and the freeboard height was 275 mm. A small stream of air was introduced to the coal feeder in order t o entrain coal particles and feed them into the fluidized bed of S i c (particle size d, = 115-150 pm). The static-bed height used in the experiments was 30 mm, and the total air flow rate was 0.74mmol/s. The fluidization velocity was -0.1 d s , 5 times the minimum fluidization velocity at 1023 K. Coal, ground t o 150-212 pm particle size, was supplied at such a rate that the oxygen concentration in flue gas was about 6%. In the case of deviation from this value, the data were corrected to correspond to 6% 0 2 excess. The system operated under steady-state conditions, in the temperature range 923-1273 K. Ash and the entrained fines were collected in a stainless steel cyclone and on a filter. A small fraction of ash remained in the bed; it is believed that its presence there did not substantially alter the operating conditions of the system. The runs were relatively short (approximately 3 h) and the entire set-up was cleaned after each run. Blank tests were carried out with calibration gas to ensure that no Nom20 conversion occurred between the FBC and gas analyzers. A mixture of argon and oxygen was used in a few runs t o make sure that thermal and prompt Nom20 formation was negligible. The flue gas was dried before its composition was determined. Paramagnetic, nondispersive infrared (NDIR) and chemiluminescent gas analyzers were used for 0 2 , CO/CO2, and NO determination, respectively. NzO analysis was performed using a gas chromatograph equipped with an electron capture detector (ECD, Carlo Erba). Separation was effected at 508 K on a 5 A,molecular sieve column (2 m long), with nitrogen as a carrier gas. The laboratory-scale FBC in which the combustion experiments were carried out was designed in such a way that the gaseous reaction products were quickly quenched to room temperature. The temperature profile was uniform within the fluidized bed but dropped rapidly in the freeboard (see Figure 2). The idea behind this design was to give prominence to NO/ NzO formation mechanisms with destruction pathways playing a negligible role in the overall Nom20 chemistry. For gasphase reactions, this seems to be a fair assumption due to a short residence time of the gas in the bed. Additional support for this comes from a previous paper,18in which experimental results are discussed in terms of a coal-N evolution model. Data analysis shows that, under the experimental conditions
GB04 coal anthracite GB04 char Daw Mill high volatile bituminous char a
4.1 65.9 4.5 0.6 0.3 7.8 94.2 0.8 1.0 0.4
Dry and ash-free basis. Dry basis.
N) to NO and NzO was remarkably constant over a wide range of temperatures.18-20 Clarification of this point is important because the trade-off between NzO and NO has been reported not only in laboratory but also in some large-scale systems.20j21It is the purpose of this study to examine the inverse relationship between NzO and NO emissions as a function of temperature as well as the effect of temperature on the combined conversion of coal-N to NO and N20. The results are interpreted against the background of known Nom20 formation mechanisms. The practical significance of this work is related to the effectiveness of NOJ"0 control that is based on optimization of combustion temperature. Insights into the underlying mechanism of pollutant formation are expected to be helpful in combustor design and in the formulation of abatement strategies.
Materials and Experimental Procedure To factor out any peculiarities in the combustion behavior of a single material, a number of coals were used in this study. They represent various coal ranks, from lignite t o anthracitic materials. The coals were obtained from the European Centre for Coal Specimens (SBN), and the characteristics of these materials are presented in Table 1. To distinguish between gas-phase and heterogeneous Nom20 formation (Le., from volatiles and chars, respectively), emissions arising from the combustion of coals were compared with those resulting from the combustion of corresponding chars; the difference in the (12) Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. Proceedings of the l l t h International Conference on FBC; ASME: New York, 1991; pp i-n- w i -. .-i - i-n-i (13)Lu, Y.; Jahkola, A.; Hippinen, I.; Jalovaara, J. Fuel 1992, 71, 693-699. ..- -. -.
(14) Bramer, E. A.; Valk, M. Proceedings of the 11th International Conference on FBC; ASME: New York, 1991; pp 701-707. (15) Collings, M. E.; Mann, M. D.; Young, B. C. Energy Fuels 1993, 7, 554-558. (16) Pels, J . R.; W6jtowicz, M. A,; Moulijn, J. A. Fuel 1993,72,373379. (17) W6jtowicz, M. A.; Oude Lohuis, J . A.; Tromp, P. J . J.; Moulijn, J. A.Pr0ceeding.s of the l l t h International Conference on FBC; ASME: New York, 1991; pp 1013-1020. (18)Oude Lohuis, J . A.; Tromp, P. J. J.; Moulijn, J. A. Fuel 1992, 71, 9-14. (19) Gavin, D. G.; Dorrington, M. A. Fuel 1993,72, 381-388. (BOIBoemer, A.; Braun, A.; Renz, U. Proceedings of the 12th International Conference on FBC; ASME: New York, 1993; Vol. 1, pp 585-598. (21) Braun, A. Proc. 21st IEA - AFBC Meeting, Belgrade, Yugoslavia 1990.
ice bath
t air
Fluidized-Bed Combustion of Coals 270
240
E
180
E
Energy & Fuels, Vol. 9,No. 5, 1995 745
I \
1.0
I
0.0
‘
t \
1 \
L
900
\
30t -30 400
600
800
1000
1200
Temperature [K]
Figure 2. Temperature profile along the reactor (bed temperature 1223 K, static bed height 35 mm). used in this work, N2 formed as a result of Nom20 reduction in the gas-phase is negligible. The carbon load of the bed was found to be very low, typically less than 0.1 wt%. Therefore, secondary heterogeneous reduction of NO and N2O on char surface should also be negligible. Char intraparticle reduction of NO and N20 plays a role in the overall emission, however. It is difficult, if not impossible, to carry out experiments excluding this effect. Although it is unlikely that Nom20 reduction was completely eliminated, it was certainly severely limited in the FBC tests reported in this study. That this effort was at least partly successful can be concluded from the exceptionally high NO/ N2O emissions recorded for all the coals (up to 1000 ppm NO and up to 400 ppm N2O). The corresponding emissions found in industrial FBC’s, in which both formation and destruction mechanisms take place, are typically 100 ppm NO and 150 ppm N2O. Three additional packed-bed gasification experiments were carried out at conditions relevant t o FBC combustion. Char derived from Daw Mill bituminous coal was used in these runs, and the char-preparation procedure is described below. A coal sample has been ground and sieved to 150-212pm. The chars were prepared by devolatilization in a flow of nitrogen (1 h at 1173 K, 10 Wmin). Thereafter, the char was ground and sieved to 106-212 pm. Its characteristics can be found in Table 1. A sample of 50 mg of Daw Mill char, mixed with 450 mg of S i c (106-150 pm), was placed in a packed-bed reactor of 9 mm in diameter. A description of the setup can be found elsewhere.22 Samples were pretreated to remove oxygen groups from the surface by a 15 min heating period at 1073 K in helium. This was followed by lowering the temperature t o the desired level and switching to the gas mixture used in each particular experiment. The gas flow rate (+”), temperature (T), and pressure ( P ) were identical in all the runs (& = 250 mumin; T = 1000 K and P = 2.5 bar). The evolution of COZ, CO, N20, NO, N2, and 0 2 was monitored until the gasification of the sample was completed. In one experiment (no. I), a mixture of 488 ppm NO and 4 ppm N2O in He was fed into the reactor to determine the NO and NzO reduction rates over (22) W6jtowicz, M. A.; Pels, J. R.; Moulijn, J. A. The Fate of Nitrogen Functionalities in Coal during Pyrolysis and Combustion: Fuel, in press; presented at the conference Coal Utilization and the Environment, Orlando, Florida, 1993, in press.
I 1000
1100
1200
1300
temperature [K]
Figure 3. Conversion of coal-N to NO + N2O in a laboratoryscale FBC. Materials: ).( = GB04 (anthracite), (+) = DE10 (semianthracite), (0)= BE30 (high-volatile bituminous), ( 0 ) = DE38 (high-volatile bituminous), (0) = AU52 (subbituminous) and (+) = DE53 (lignite). The dotted lines represent conversions of coal-N t o NO and NzO for GB04. Such curves have a similar shape for other coals but for the sake of clarity are not shown. char in the absence of oxygen. The NzO detection limit was 0.1 ppm, which made it possible to determine N2O conversion with adequate accuracy. In another experiment (no. 111,a char sample was gasified in 0.5% 0 2 in He. In the third experiment (no. 1111, the char was gasified under the same conditions as in experiment 11, but 488 ppm NO and 4 ppm NzO were also added to the flow.
Results Conversions of fuel-N to NO, N20, and N2 are defined as the fraction of fuel-bound nitrogen that reacts to NO, N20, and N2 (i.e., N20 and N2 emissions are multiplied by a factor 2). For a given coal, the sum of fuel-N conversions to N2O and NO was found t o be remarkably constant over a range of temperatures as demonstrated in Figure 3. (The dotted lines represent conversions of coal-N t o NO and N20 for the GB04 anthracite (such curves have a similar shape for other coals and for the sake of clarity are not shown in the figure). This occurred despite the fact that the individual conversions to NO and to NzO showed a strong variation with temperature (NO rose and N2O decreased as temperature increased). Thus, there appears to be a conservation of the total emission of nitrogen oxides. Accordingly, the observed temperature dependence can be described as a temperature-governed trade-off between NO and N2O emission levels, lower temperatures favoring N2O formation a t the expense of NO. In the laboratory-scaleFBC described above, complete burn-out of the coal and char particles was achieved in almost every case. This means that all the fuel-N was converted to some gaseous species. It is also fair to assume that, with an oxygen level of about 6%, the emission of hydrocarbons and nitrohydrocarbons is negligible. Emissions of NO2 have been monitored for some of the combustion experiments and have been found to be very small (NOZ/”O < 0.1). Therefore, it can be assumed that of all the nitrogenous species produced during fluidized-bed combustion, only NO, N20, and N2 are present at appreciable levels. Accordingly, the temperature-governed trade-off between NO and N20 can also be interpreted as a constant release of N2 over the whole temperature range. The fact that the N2 level remains constant over a wide temperature
746 Energy & Fuels, Vol. 9,No. 5, 1995
Pels et al. ,
1.00
.-:0.8
1
0.80
EP)
I
0 . 8 2 ~ -
20 0.6 0
'
0.0 900
.U-..
1000
1100
1200
---o
1300 20
0
40
temperature [K]
Figure 4. Conversion of char-N to NO + N2O in a laboratoryscale FBC. Materials prepared from the following coals:).( = GB04 (anthracite), (+) = DE10 (semi-anthracite), (0) = BE30 (high-volatile bituminous), ( 0 )= DE38 (high-volatile bituminous), (0) = AU52 (subbituminous), and (+) = DE53 (lignite). The dotted lines represent conversions of char-N to NO and N2O for GB04.Such curves have a similar shape for other coals but for the sake of clarity are not shown.
range indicates that either the mechanism for the formation of N2 is temperature-independent or there is a change in the mechanism, with a compensating effect of one pathway for the other. The above-mentioned trade-off between N2O and NO was observed in both coal and char combustion (Figures 3 and 41, and thus it should also occur in the combustion of volatiles. More careful examination of the results published earlierls showed that char-bound nitrogen (char-N) is converted t o NO, N20, and N2, while volatile-N forms mainly NO and N2O. This supports the assumption of the fast quenching of reaction products in the freeboard. Under the experimental conditions used in this work, the formation of N2 can be attributed entirely t o char combustion, and explaining the constant release of N2 from the combustion of coal reduces to explaining the constant release of N2 during char combustion. In the case of volatiles, where NO and N2O are presumed to be the only products, a trade-off between them is self-explanatory. Nevertheless, it is important to elucidate the temperature-dependent shift in the product yields in terms of changes in the reaction mechanism. This needs t o be done for the combustion of both volatiles and char. Three packed-bed experiments have been performed to study the heterogeneous mechanism involved in nitrogen evolution during char oxidation. The reduction of NO and N2O was measured for a sample of Daw Mill char in a packed-bed reactor (experiment I). Initially, the NO reduction was found to be high, but it decreased slowly. The steady state was reached afier approximately 2 h and then the NO conversion was very low: 2%. Simultaneously, the N2O reduction was found to be 82%. In experiment 11, the char was gasified with 0.5% oxygen in helium; in experiment 111,488ppm NO and 4 ppm N2O were added t o the 0.5% 0 2 in He. The latter experiment was carried out to see if NO addition would enhance the formation of N2O in the presence of 0 2 , with experiment I used as a background. The results of experiments I, 11, and I11 are presented in Figure 5 , 6 and 7, respectively. (The detection limit for a reliable Nz measurement is 25 ppm.) In experiment I, no N2O formation was found from NO reduction experiments; even if N20 was formed it
60
80
120
100
time [minutes]
Figure 5. Conversion of NO and N2O over a fresh sample of Daw Mill char, with 488 ppm NO and 4 ppm N2O in He at 1000 K; without 0 2 (experiment I):).( = NO, (+) = N2O; with 0.5% 0 2 (experiment 111): ( 0 )= NO. 6000
g
40
R
5000
hn
-0"
4000
P
0
0 3000
0
20
8 .-sU .-
E
z" z C
2000 10
.-n
.n E
w
1000
0
0 0
20
40
60
80
100
time [minutes]
Figure 6. Emissions of COz. CO, 02, N2, NO, and N2O durting the gasification of Daw Mill char with 0.5%02 in He at 1000 K and 2.5 bar (experiment 11): (B) C02, (+) = CO, (+) = 0 2 , ( 0 )= N2, (A)= N20, and ( A ) = NO. The detection limit for a reliable N2 measurement is 25 ppm.
7
5000
500
En
0"
>
En
I
I
B
4000
400
0.
3000
300
s
2000
200
sz"
100
.-U .-
0
0" 0
C
U
.-
e
4x
1000
0 0
20
40
60
80
e
0 100
time [mlnutes]
Figure 7. Emissions of COz, CO, 0 2 , Nz, NO and N2O durting the gasificaion of Daw Mill char with 0.5% 0 2 in He at 1000 K and 2.5 bar, with addition of 488 ppm NO and 4 ppm N2O (experiment 111): (m) = COZ,(+I = CO, (+I = 0 2 , ( 0 )= N2, (A) = 10N20, and (A) = NO.
did not escape from the char particle in measurable amounts due to intraparticle reduction. Similar results have been reported by other^.^^-^' In experiment 11, (23) Mochizuki, M.; Koike, J.; Horio, M. Proc. 5th Int. Workshop Nitrous Oxide Emissions Tsukuba, Jpn. 1992,NIREAFP/EPA/SCEJ, p 5-3-1.
Fluidized-Bed Combustion of Coals small amounts of N2O were found in the effluent, mainly a t the beginning of the run. Since N2O can only be formed from char-N, and the char has a strong N2O reduction capacity at that temperature, this means that much larger amounts must have been formed during gasification. Addition of NO (experiment 111)enhances the emission of N20 significantly. A concentration of 8 ppm was found over h long time of the run: 4 ppm above the 4 ppm that was present in the supply gas a t the inlet. This implies that much larger amounts of N2O are formed, even more than during the gasification without NO addition (experiment 11). Experiment I11 shows that addition of NO does not influence the emissions of CO2 and CO. On the other hand, the NO reduction is strongly enhanced in the presence of oxygen, when compared with experiment I. During a gasification experiment, NO reduction gradually decreases t o zero due to the char burn-out.
Discussion An attempt is made to provide a qualitative explanation of the experimental data. First, the inverse nature of N2O and NO formation in the gas phase is addressed on the basis of homogeneous chemistry postulated by Kramlich et aL2*and discussed further by Kilpinen and H ~ p a Subsequently, . ~ ~ a heterogeneous mechanism, based on surface reactions given by De Soete,25v30-32 will be discussed to explain the trade-off between N2O and NO formation from char combustion. A Gas-PhaseReaction Mechanism for the Combustion of Volatile-N. NH3- and HCN-based species are the main nitrogenous products of coal devolatilization. Only 6-8% of the coal-N is directly converted to NH3 and HCN during devolatilization; the rest being retained in tar and char.33 There is no or very little N2 formed directly during devolatilization. The nitrogen in the tar (tar-N) is present mainly in pyridinic and pyrrolic functionalities. From the break-up of tar, HCN is the primary nitrogenous product. There are indications that NH3 is formed from HCN, when hydrogen is r e l e a ~ e d . ~This ~ - ~means ~ that NH3 and HCN are the only nitrogen-containing intermediates in the combustion of volatile-nitrogen (volatile-N). They can act as gas-phase precursors of both NO and N2O. N H 3 reacts mainly to NO, while HCN can give rise t o either NO or NzO, depending on combustion temperature. N2O can further react to N2, as explained by the following set of reaction^^^,^^^^^ for homogeneous Nom20 formation and (24)Rodriguez Mirasol, J.; Ooms, A. C.; Pels, J. R.; Kapteijn, F.; Moulijn, J. A. Combust. Flame 1994,99,495-507. (25)De Soete,G.G. Proc.5th Int. Workshp Nitrous Oxide Emissions Tsukuba. Jon. 1992. NIREAFPIEPAISCEJ. 199. (26)Baimann, H: Private communication, 1992. (27)Gulyurtlu, I.; Esparteiro, H.; Cabrita, I. Proc. 5th Int. Workshop Nitrous Oxide Emissions Tsukuba, Jpn. 1992, NIREAFPIEPAISCEJ, p 5-5-1. (28)Kramlich, J. C.; Cole, J. A.; McCarthy, J. M.; Lanier, W. S.; McSorley, J. A. Combust. Flame 1989,77,375-384. (29)Kilpinen, P.; Hupa, M. Combust. Flame 1991,85,94-104. (30)De Soete, G. G. Proceedings of the 23rd Symposium (International) Combustion; The Combustion Institute: Pittsburgh, PA, 1990;pp 1257-1264. (31)De Soete, G. G. Proceedings EPAIIFP Eur. Workshop Emission Nitrous Oxide Fossil Fuel Combust., Rueil-Malmaison, Fr. 1988. (32)De Soete, G.G. Rev. de Z'Inst. Fr. Pkt. 1993,48,413-451. (33)Baumann, H.; Mtiller, P. Er&l Kohle 1991,44,29-33. (34)Bassilakis, R.; Zhao, Y.; Solomon, P. R.; Serio, M. A. Energy Fuels 1993,7,710-720. (35)Aho, M. J.; Hamalthen, J. P.; Tummavuori, J . L. Fuel 1993, 72,837-841.
Energy & Fuels, Vol. 9, No. 5, 1995 747 reduction (activation energy, E,, from Miller and Bowman37~38):
HCN + 0 NCO + H
E , = 21 kJ/mol
(1)
NCO + OH
E , = 63 kJ/mol
(2)
E , = -1.7 kJ/mol
(3)
E , = 0 kJ/mol
(4)
E, = 64 kJ/mol
(5)
---c
-NO + HCO NCO + NO - N20+ CO NO + NH, - N2+ H20 N20+ H-",
+ OH
It should be noted that both NO and N2O are formed from HCN via the common gas-phase precursor (NCO), which constitutes the essence of the NO vs N2O tradeoff. At relatively high temperatures, reactions 1and 2 are responsible for NO being the main formation product, whereas a t lower temperatures, reaction 3 successfully competes with reaction 2 for the available pool of NCO. The activation energy of reaction 3 is small (practically zero) compared to the activation energy of reaction 2. The effect is that with increasing temperature, reaction 2 is strongly enhanced, while reaction 3 is slightly inhibited. There is a complication, because NO, a product of reaction 2, is reactant in reaction 3, and hence it is a series-parallel reaction mechanism. Therefore, an elevated concentration of NO increases the importance of reaction 3. This happens at the expense of reaction 2, thus lowering the formation rate of NO. In brief, the NO formation is subjected to a negative feedback an increase in temperature results in a shift from N2O to NO, but this shift is damped by the feedback mechanism. The shift is only damped and cannot be compensated completely by the feedback mechanism, because the damping is caused by the shift itself. Thus, at high temperatures, NO is the preferred reaction product, which is consistent with experimental data. In this way, for the gas phase, the trends in temperature as well as the NO vs N20 trade-off are explained by the above gas-phase reactions. In the laboratory-scale FBC experiments, the reduction reactions 4 and 5 play a minor role. In large-scale FBC, however, they can be an important factor in the overall NO and N2O emissions. Reactions 4 and 5 may lead to deviations from the constancy of NO N2O levels in large-scale combustors. A schematic overview of the reactions is given in Figure 8. Depending on the coal rank and the combustion conditions, considerable amounts Of N H 3 can be formed. The HCN/NH3 ratio is an uncertainty in the description of the reaction scheme and its value is difficult t o estimate. However, for most coals the formation of N H 3 from coal-N during devolatilization is small compared to the formation of HCN. It is generally accepted that most NH3 is formed from hydrogenation of HCN.7p33-35 Kramlich et aZ.28showed that in the gas phase, NH3 reacts mainly to NO, with only small amounts of N2O formed. The effect of the NO formation from N H 3 can result in an offset in the NO formation, which is partly
+
(36)Houser, T.J.; McCarville, M. E.; Zhuo-Ying, G. Fuel 1988,67, 642-650. (37)Miller, J. A.; Bowman, C. T. Prog. Energy Combust. 1989,15, 287-388. (38)Miller, J. A.; Bowman, C. T. Int. J.Chem. Kinet. 1991,23,289313.
748 Energy & Fuels, Vol. 9, No. 5, 1995 e - - - - - - - -
*.
I
'
-
TAR-N
-
XNZO 0.5 XNO= 0.1
VO LAT I LE- N
''\(*
I
Pels et al. XNPO 0.9 XNO= 0.3
100
\
fuel-N +. N,O+
t
N,
.-0
E
B 0
0
fuel-N +.NO 0 900
CHAR-N
Aetlvrllon energy E.
Temperature [K]
in kJ mol '
Figure 8. Homogeneous reaction mechanism of the reactions occurring during gas-phase combustion of volatile-N.
/ \
COAL-N
CHAR-N
-
/.--*
NH3
_______
-
?*
NO-
N,
..i/l
,,7;
/I HCN
E.-l08
NCO
E.-.l
7
N,O
AoItva11011energy E, in kJ mol
Figure 9. Homogeneous reaction mechanism of the reactions occurring during gasification and gas-phase combustion of char-N compensated by the reaction of NO with NHi or NCO, forming N2 and N20, respectively. There is no evidence of N2O reacting to NO in the gasphase. Houser et who studied NO/N20 formation from HCN in stirred- and plug-flow reactors, found that once N20 is formed a t milder temperature conditions, it will not oxidize to NO, even if the temperature should be raised a t a later stage. A Homogeneous Reaction Mechanism for the Combustion of Char-N. Mechanisms describing the conversion of char-N to NO, N20, and N2 are important in the combustion of coals. Experiments revealed that the proportion of coal-N reacting to NO/N20 along heterogeneous pathways was far from negligible (2070%, depending on the coal18). Furthermore, elemental analysis of coals and chars used in this study indicates that a large percentage of coal-N is retained in char during pyrolysis (usually more than 60%). It is possible to postulate a reaction mechanism for the formation of NO and N2O during the combustion of char based on homogeneous reactions: char-N
-
HCN and NH,
-
N 2 0 and NO
-
N2
The char is gasified in the presence of oxygen, but still under relatively oxygen-lean conditions. Char-N is converted to HCN and NH3, and subsequently NO/ N2O formation proceeds in the gas phase, as discussed before. A schematic overview of the occurring reactions is given in Figure 9. A few remarks should be made. (1) As with the combustion of the volatiles, the HCN/ NH3 ratio is an unknown factor. NH3 is supposed to be formed during devolatilization mainly from hydrogenation of HCN.7!33-35Since the gasification takes
-
I 1200
Figure 10. Qualitative representation of temperature-dependent NO/NzO formation and reduction to Nz. place in an oxidizing environment, H2 required for the formation of NH3 is almost not present, and the formation of N H 3 is severely limited. (2) It is unlikely that only HCN (and a small amount of NH3) are formed during oxidative g a s i f i ~ a t i o n . In ~ ~ addition to HCN other, larger cyanide derivatives like CH3CN and HCCCN are probably involved, as proposed by Ah0 et ~ 1 ( 3 ) It could very well be that char gasification results in the direct release of nitrogenous molecules including an oxygen atom, e.g., HNCO and NCO. This does not alter the mechanism si@cantly, since such species are the intermediates in the HCN oxidation scheme. The gasification according to this mechanism takes place in the pores of the char with NO and N20 formed close to the char surface. Therefore, it is expected that the overall emission of NO and N2O is strongly influenced by intraparticle NO and N20 reduction. In our experiments the heterogeneous NO and N2O reduction was found to be important, because almost all the N2 formed from coal combustion results from char combustion, and only insignificant amounts of N2 can be formed directly from the char. As compared to the combustion of volatiles, where a cloud of devolatilization products is present, there are smaller amounts of radicals present and this leads to a relative shift toward reaction 3, and subsequently to a larger formation of N2O. It should be noted that the activity of a char with respect to N20 reduction is an order of magnitude higher than for NO reduction.30 The constant conversion of char-N to Nz and the temperature dependence of the trade-off between N2O and NO can be explained as follows. At low temperatures, large amounts of N2O are formed and a large part of this N2O is reduced at the char surface. With increasing temperature, less N20 is formed, with its majority still reduced to N2. NO formation increases with increasing temperature, but also the reduction of NO is enhanced. This results in an almost constant overall N2 formation. A qualitative representation is given in Figure 10. The difference between this mechanism and the combustion of the volatiles is that almost all of the N2O and small amounts of NO formed from the gasification of char are reduced by the char itself, resulting in the observation that char-N converts mainly to NO and N2. Preliminary results of modeling work shows that this is a realistic possibility. The trade-off between N20 and NO in FBC of coals is correctly predicted, with and (39) Amand, L.-E. Personal communication, 1994.
.
~
~
Fluidized-Bed Combustion of Coals
Energy & Fuels, Vol. 9, No. 5, 1995 749
without heterogeneous reduction by char. It also is more realistic to separate it into two reactions (6a) and (6b), with reaction 6a as rate determining:30 shown that in the latter case Nz is mainly formed from N2O reduction, with an increasing contribution from NO NO 2(C) (CN) CO (64 reduction with increasing temperature. However, more work is needed t o establish a model that correctly 2(CN) N2 2(C) (6b) describes the formation of NzO, NO and NZfrom coalN. Other reaction steps can also be taken into considerThe gas-phase reactions according to the homogeation: neous reaction mechanism can also easily explain the enhanced NzO formation during the gasification of char NO (CN) (C) N, (C) (CO) (7) in experiment 111, in which extra NO was added: the reaction of NCO with NO according to reaction 3 was NO (CN) NzO (C) (8) enhanced and more N2O is formed. However, only a of NO contributed to the overall formation small fraction NO (CO) (C) CO, (CN) (C*> (9) of N20, the rest was converted to Nz. A Heterogeneous Reaction Mechanism for the NzO reduction: N 2 0 (C) N2 (CO) (10) Combustion of Char-N. A similar analysis regarding heterogeneous Nom20 formation pathways is more complex, mainly because N2 formation was also found NzO (CO) N, CO, (C*) (11) to be important in the heterogeneous route. Before one can proceed with a description and consideration of where (C*) denotes a newly formed site. various pathways of heterogeneous NzO and NO formaThe above scheme can explain the results of NO tion in order to explain the temperature-dependent reduction over char as observed in experiment I. Before trade-off between NzO an NO, it is necessary t o discuss NO is added the char surface is cleaned from oxidized mechanisms for the formation of N2. W6jtowicz et aL7 groups by heat treatment. During the initial phase of provide a review of reactions in which NZis formed from experiment I, when NO is added, NO reduction is high, NO or NzO, both in the gas phase and on the char but steadily decreases. (CO) groups are formed on the surface. surface by reactions 6a and 7,while reaction 6b is fast. Under the reaction conditions of the laboratory-scale NO reduction is strongly inhibited by the presence of FBC experiments, formation of NZin the gas-phase is those (CO) groups on the char surface. At 1000 K, the strongly inhibited. Formation of N2 (and also NzO) desorption of (CO)and the reaction of (CO) with NO directly from char-N is supposed to be unimportant, (reaction 9) are not fast enough to clear the surface and because it requires two adjacent nitrogen atoms. Esreactions 6a and 7 are gradually inhibited. Reaction timations show that a random distribution of nitrogen 6b and reaction 8 may take place but are also inhibited atoms in a char containing 3 wt % nitrogen, results in when the char surface is covered with large amounts of about 10% of the nitrogen in adjacent positions. At first stable (CO) groups. Even when NzO is formed it is sight this is consistent with the results of FBC runs, easily reduced, reactions 10 and 11,and cannot escape which show that typically only 1-10% of char-N is from the char particle in sufficient amounts to be converted to N ~ 0 . lSimilar ~ numbers were also reported detected. by De Soete, who carried out packed-bed experiments The stable (CO) groups on the char surface can be in which only 1-6% of char-N reacted toward N20.30 removed, enhancing the NO reduction, when a scavengHowever, our data show that the total of N2O and NZ ing agent is present, like CO, NzO, and 0 2 . Rodriguez formed from char combustion is large, being a t least Mirasol et aLZ4have studied the NO and N2O reduction 40%. Reaction of only char-bound nitrogen atoms can on a number of coal chars, and the influence of CO, 0 2 , never explain the total conversion of char-N t o NzO NO, and N2O addition. They report that upon the N2. Therefore, it is presumed that NZ (and possibly addition of CO, NzO, or 0 2 , the NO reduction is N20) is formed from heterogeneous reactions of NO and enhanced due to the removal of stable (CO) groups from N20, previously formed from char-N. This happens the char surface. even at low temperatures where, in the absence of The scavenging effect of CO is explained by the oxygen, NO reduction over chars is with following reactions: oxygen present the NO reduction is much enhanced, (see (CO) co (C*) Figure 5). (12) For the reduction of NO and N2O on the char surface, co (CO) CO2 (C) (13) the following mechanisms can be proposed:7~24~30~40-42
+
+
- +
+ +
-
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-
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+
+
-
+
-
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NO reduction:
NO + (C)
-
(1/2)Nz
+ (CO)
(6)
Parentheses denote surface-bound species. Authors often refer to reaction 6 as a single reaction, but it is (40)Yamashita, H.; Tomita, A.; Yamada, H.; Kyotani, T.; Radovic, L. R. Energy Fuels 1993,7 , 85-89. (41)Suuberg, E.M.;Teng, H.; Calo, J. M. Proceedings of the 23rd Symposium (International) Combustion; The Combustion Institute: Pittsburgh, PA, 1990;p 1190. (42)Smith, R.N.;Swinehart, J.; Lesnini, D. J. Phys. Chem. 1959, 63,544-547
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+
At lower temperatures, reaction 12 is too slow t o be important. However, (CO) groups can be removed by CO according t o reaction 13. A free surface site is generated and NO reduction can take place according to reaction 6 or 9. The enhancement of NO reduction by NzO addition is explained by the occurrence of reaction 11,removing the stable CO groups forming COS and creating a new free surface site. The scavenging effect of NzO is comparable to that of CO. Stable (CO) groups are removed by reaction 11, freeing surface sites available for NO reduction.
760 Energy & Fuels, Vol. 9, No. 5, 1995
Pels et al.
The effect of 0.2 is not directly clear. It can be explained in several ways. Rodriguez Mirasol et aLZ4 give a simple explanation for the enhancement of NO reduction by 0 2 , following Chan et al.43 The reaction of 0 2 with char is fast compared to the reaction of NO with char and transport of 0 2 in the pores of the char is limited. In the pores, the oxidation of the char under oxygen-lean conditions results in an increased concentration of CO, which leads to the (C0)-scavenging effect described by reaction 13. An alternative explanation can be the following: the presence of oxygen strongly enhances char oxidation and surface intermediates are continuously renewed. During gasification, the char surface is largely covered with (CO) complexes, due to the reaction of 0 2 with free carbon sites: (1/2)0,
+ (C) - (CO)
(14)
According to Yamashita et aZ.,40such surface complexes can be divided into active and stable (CO) groups. The stable (CO) groups play an unimportant role in the reduction of NO, but the active (CO) groups can easily react with NO, e.g., via reaction 9. This reaction creates also a free site, where NO can be reduced according to reaction 6. Although the newly formed free sites would be attacked mainly by 0 2 , the probability of a reaction with NO is also increased. Yet another possibility is considered by Yamashita et a1.40,44 Molecular oxygen facilitates NO oxidation t o the more reactive NOz, which then easily reacts with carbon to form Nz and NO. Chu and Schmidt45found that NOz, produced from NO and 0 2 , reacts with carbon at a higher rate than 0 2 . In batch combustion of coals, Tullin et a1.46947 found a decrease in NO formation with increasing oxygen partial pressure, accompanied by an increase in Nz0 formation. They explain this effect by an enhanced oxidation of char-N, followed by a heterogeneous conversion of NO to NzO:
+ (CN) - (CNO) NO + (CNO) - NzO + (CO) (1/2)0,
(15) (16)
In their hypothesis, reaction of NO with char-N is the main path for NzO formation and the role of oxygen is t o consume the carbon, releasing firmly bound char-N. Furthermore, Tullin et al.46found an increased formation of NzO when adding NO during the combustion (as compared to combustion without NO). All these explanations are consistent with the enhanced NO reduction (and NzO formation) in the presence of oxygen, found in experiment 111. However, it is difficult t o indicate which reaction mechanism is predominant. (43) Chan, L. K.; Sarofim, A. F.; BeBr, J. M. Combust. Flame 1983, 52, 37-45 (44) Yamashita, H.; Yamada, H.; Tomita, A. J.Appl. Catal. 1991,
78, Ll-L6.
(45)Chu, X.; Schmidt, D. Ind. Eng. Chem. Res. 1993, 32, 13591366. (46) Tullin, C. J.; Sarofim, A. F.; Be&, J. M. Proceedings ofthe 12th International Conference on FBC, ASME: New York, 1993; Vol. 1,pp 599-609. (47) Tullin, C. J.; Goel, S.; Morihari, A.; Sarofim, A. F.; Be&, J. M. Energy Fuels 1993, 7 , 796.
Table 2. Heterogeneous Reaction Mechanism for NO/N20 Formation and Reduction; (Activation Energy, E,, from W6jtowicz et aL7) (A) The oxidation of char-N to NO/N20/N2 ' / 2 0 2 + (C) (CO) E, = 84 kJ/mol (14) VzOz + (CN) (CNO) E, = 84 kJ/mol (15) (CNO) - N O (C) E, = 14 kJ/mol (17) (CN) + (CO) NO + 2 (C) (18) (CN) + (CNO) N2O + 2 (C) E, = 28 kJ/mol (19)
--
-+2(CN)-N2+2(C)
+ char or char-N - N20/N2 NO + 2 (C) - (CN) + (CO) E, = 139 kJ/mol NO + (CN) + (C) - N2 + (C)+ (CO) NO + (CN) - NzO + (C) NO + (CO) + (C) - C02 + (CN) + (*) NO + (C) - (CNO) (C) N2O + char - NZ NzO + (C) - N2 + (CO) E, = 108 kJ/mol
(6b)
(B)NO
NzO + (CO)
- N2 + C02 +
(*)
(6a) (7) (8) (9) (20)
(10) (11)
A heterogeneous reaction scheme is examined below to see if it may explain the competitive emissions of NzO and NO during fluidized-bed combustion of chars. Sets of reactions are given in Table 2. Surface reactions are mainly based on mechanisms postulated by De Soete.25~30-32 Parentheses denote surface-bound intermediates. Reaction scheme A describes the major heterogeneous reactions of char-bound nitrogen to NO, NzO, and Nz. It is apparent that in scheme A, oxidized surface sites (CNO)may play the role of a common NzO and NO precursor, like the radical NCO for the gas phase. If reaction 17 rather than 18 is the predominant NO formation pathway, then the NO vs NzO trade-off is quite obvious: (CNO)can react either to NO (reaction 17) or to NzO (reaction 19). Even if reaction 18 plays a role in the scheme, it should be noted that the nitrogencontaining site (CN) is also a common substrate in NzO and NO formation routes 18 and 19, and the sum of (CN) and (CNO) can be regarded as the common precursor. The fact that this balance exists is not so surprising, taking into account that fuel-N (in this case char-N) is the only source of nitrogen for NO/NzO formation. The temperature-dependent shift in the NO/ NzO balance may be explained by the existence of a competition between NzO and NO formation routes for the precursor (CNO). At low temperatures, (CNO) is formed from (CN) at a relatively low rate (reaction 15), and NzO and NO formation occurs mainly via reactions 17 and 19. NO is expected to be a more abundant product of (CNO) desorption since Nz0 formation involves a reaction between two neighboring sites. It is possible that NZis formed directly from (CN)by reaction 6b, but it is believed to be unimportant because it requires two neighbouring sites of unoxidized char-N atoms. At higher temperatures, the rate of (CN) oxidation to (CNO) increases and so does the rate of NO production. NzO formation is hampered due to a decreasing inventory of (CN), despite the fact that the activation energy of reaction 19 is higher than that of reaction 17. The constancy of the char-N to Nz conversion is achieved in a way analogous to the homogeneous mechanism: at low temperatures Nz0 is reduced to form Nz, while the bulk of NO remains unreacted. This interpretation is supported by the fact that the NzO reduction rate is an order of magnitude higher than the corresponding rate for NO.30 With increasing temperature, less NzO is formed and, although the reduction is enhanced, the overall formation of Nz from NzO is
Fluidized-Bed Combustion of Coals
Energy &Fuels, Vol. 9, No. 5, 1995 751
scheme A valid, and the trade-off between HO and N2O for fluidized-bed combustion of char can be explained accordingly. With inclusion of NO reduction reactions, the temperature dependency of the NO and N20 formation can be described as follows. At low temperatures, char oxidation is relatively slow; NO reacts with free carbon sites on the char surface in competition with oxygen, while 0 2 is also able to remove stable (CO) groups from the surface, generating new free sites. The dissociative adsorption of NO creates (CN) groups at the surface Figure 11. Heterogeneous reaction mechanism for the com(reaction 6 4 . Two adjacent (CN) groups form rapidly bustion of char-N. Nz (reaction 6b), but part of it is oxidized forming (CNO) (reaction 15); (CNO) groups can also be formed from decreasing; simultaneously, the reduction of NO is adsorption of NO (reaction 20). N20 is formed from a enhanced and the formation of Nz from NO becomes combination of (CN) and (CNO) (reaction 19), and NO important. Apparently, both pathways balance each is formed from a single (CNO)(reaction 17). Part of the other. The above scheme can qualitatively account for thus formed NO dissociates again on the char surface the constancy of the sum of NzO and NO levels over a via reaction 6a. In this way a part of the nitrogen atoms range of temperatures as well as for decreasing NzO and are recycled: (CN) (CNO) NO (CN) etc. Part of increasing NO formation rates with temperature. the NzO is subsequently reduced to Nz (reactions 10 and The major difficulty with reaction scheme A is that 11). for the formation of NzO and Nz, two adjacent nitrogen atoms on the surface are required. Since N2 is supposed With increasing temperature, the combustion of char t o be formed mainly from NzO (reactions 10 and 111,or becomes faster and the oxidation of carbon and nitrogen possibly directly from the char (reaction 6b), it should atoms a t the surface is enhanced (reactions 14 and 15). be concluded that there are simply not enough adjacent With increasing temperature, the dissociative NO adnitrogen atoms available to account for the amounts of sorption forming (CN) is enhanced, but it is not inN2 and N20 observed. In this way, reaction scheme A creased much, despite the high activation energy of alone can in principle explain the trade-off between NO reaction 6a, due to a decreasing inventory of free carbon and N2O but it cannot explain the total amounts of N2O sites. Also the oxidation of (CN) t o (CNO)is enhanced. and N2 that are formed. As a result, the formation of Nz and NzO is inhibited In order to explain the observed amounts of NzO and due to a decreasing inventory of (CN),and the formation Nz, the reduction of NO on the char surface, according of NO is increased. The increase in NO formation, to reaction scheme B must be taken into account. however, is damped by the increase of the dissociative Reaction scheme B itself represents an obvious tradeNO reduction. Due t o the enhanced oxidation of (CN) off between NO and NzO (a combination of reaction 6a to (CNO) with increasing temperature, it is also likely or 9 with reaction 81, but it cannot explain the temperthat the direct formation of N2 from 2(CN) decreases ature shift. The opposite of the observed trend would relative to the formation of Nz0, while the formation of be expected since at higher temperatures the reduction Nz from Nz0 reduction increases. Apparently both of NO will be enhanced. It is thus necessary to combine pathways t o form N:! balance each other and the total the reduction reactions of reaction scheme B with the conversion of char-N to Nz is constant. The overall oxidation reactions of reaction scheme A. An attempt result of an increase in the temperature is that more t o represent these reactions is given in Figure 11. NO is formed and less N20, and that the formation of In the presence of oxygen, NO reduction takes place NZremains the same. a t temperatures a t which in the absence of 0 2 , NO The packed-bed char combustion experiments can also reduction is very small (compare experiments I and 111). be explained by the proposed scheme. In experiment As can be seen from Figure 5 , the conversion of NO in I1 (gasification without additional NO), the NO concenthe presence of oxygen is comparable to the conversion tration is low, resulting in a very low production of NzO. of NO over a fresh char sample, before it has been The small maximum in NzO formation at the beginning covered with inactive (CO) complexes. The decrease in of this experiment and the steadily increasing NO NO conversion in time in experiment I11 is due to the formation can be explained as follows. All of the consumption of the char during gasification. The NO available oxygen is reacting with the char surface, with adsorption on the char surface can be dissociative, only part of the surface covered by oxidized species. This forming (CN) by reactions 6a and 9 or nondissociative, leaves more (CN) groups unoxidized at the beginning forming (CNO) by reaction 20. The reaction of an NO of the run than at the end, when the total char surface molecule from the gas phase with a (CN) group forming of the sample is smaller. This results in a larger N20 and Nz is possible, but reaction 7 can be seen as formation of NzO at the beginning of the run (reaction reaction 6a or 9 followed by reaction 6b, and reaction 8 19). Nz0 is reduced easily at the char surface (82% as reaction 20 followed by reaction 19. De S ~ e t e ~ conversion ~ at initial conditions), and this means that reports the direct reaction of NO with (CN) via reaction much larger amounts of Nz0 must have been formed. 8, but concludes that it is only a minor pathway. The The peak in NZoccurs also a t the beginning when most adsorption of NO at the char surface provides the of the Nz0 is formed. As the gasification advances, less adjacent nitrogen atoms required to make reaction (CN) groups are available and (CNO)reacts more to NO (reaction 17). NO is reduced on the char by reaction (48)De Soete, G. G. Internal IFP Report No. 36752, July 1989; 6a, up to 40% conversion in the presence of oxygen. The quoted in ref 32.
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Pels et al.
752 Energy & Fuels, Vol. 9,No. 5, 1995
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sum of nitrogen oxides (NO 2N20) increases during the experiment relative to sum of carbon oxides (CO2 CO). This indicates that the reduction of nitrogen oxides decreases during the experiment. This happens because the release of char-N is proportional to the release of carbon from the char,30while the reduction of NO and N2O is proportional to the remaining char surface. In experiment I11 (gasification with addition of NO), NO was added in a concentration typical for fluidizedbed combustion (20 times higher than the NO emission in experiment 11). The presence of N2O in the gas supply makes the interpretation of the results somewhat difficult, but since this N2O is also reduced (proportional t o the char surface), it is clear that larger amounts of N2O are formed than without NO addition. This is not only at the beginning but throughout the whole run. The explanation is that NO competes with 0 2 for the vacant (C) sites. As a result, much more (CN) and (CNO) sites are created than in experiment 11, and the formation of N2O is promoted during the whole run. The formation of N2 is also higher than without NO addition. It is formed either directly from NO reduction (reactions 6a and 6b) or from the reduction of N2O (reactions 6a, 15, and 19 followed by reactions 10 or 11). The above discussion shows that it is possible to qualitatively explain the NO vs N2O trade-off on the basis of homogeneous and heterogeneous NO/N20 chemistry. However, it still remains difficult to define precisely which pathways are important and how they change with temperature. Not only the activation energy of a reaction step is important but also other kinetic data and the concentration of active surface sites. Those data are very scarce in the literature, and more research still needs to be done.
+
Conclusions 1. NO and N2O formation was studied in a laboratory-scale FBC. For a given coal, the sum of fuel-N conversions t o N20 and NO was found to be remarkably constant over a range of temperatures although emission levels of individual species showed strong variation with temperature. A similar behavior was observed for formation of NO and NzO during combustion of chars. 2. The temperature-governed trade-off between emissions of NO and N2O can be qualitatively explained on the basis of known Nom20 chemistry in fluidized-bed combustion for combustion of both the char and the volatiles. For the combustion of volatiles, a mechanism based on gas-phase reactions is given. The key in this mech-
anism is the reaction intermediate NCO, originating from HCN. At relatively high temperatures NCO reacts mainly to NO, while at lower temperatures NCO reacts with NO to form N2O. For the combustion of char, two mechanisms are proposed. One is based on gasification of the charbound nitrogen to HCN, its derivatives, and NH3. The trade-off between NO and N20 is explained by a mechanism similar to the gas-phase combustion of the volatiles. The other reaction mechanism involves surface reactions, where the char-bound complexes (CN) and (CNO) play the key roles. At low temperatures, they preferentially react to N2 and N20, while at higher temperatures more NO is formed at the expense of N2O. The (dissociative) adsorption of NO on the char surface provides the required adjacent nitrogen atoms for N20 formation. In both mechanisms, the reduction of NO and N2O to N2 on the char surface is a complicating factor. At relatively low temperatures, N2O is the preferred reaction product, which undergoes appreciable reduction to N2; NO is reduced to a lesser extent. With increasing temperature NO becomes the predominant reaction product, but also the reduction of NO is enhanced. The result it that the overall conversion of char-N to N2 is almost constant over a large temperature range. 3. It seems impossible to use temperature as the only control measure to reduce formation of (NO N2O). Temperature variation results in an increase of the emission of one pollutant at the expense of the other. NOJN20 pollution control should thus focus on NO and N2O destruction mechanisms. In practice, it may prove advantageous to deal with only one nitrogenous pollutant. To this end, temperature could be utilized to increase the selectivity toward either NO or N20 (NO high temperature, N20 low temperature), and this should be combined with appropriate control measures directed against the selected pollutant (see else-
+
here'^,^^,^^).
Acknowledgment. The authors gratefully acknowledge the support for this work which was provided by the Commission of the European Communities under contracts No. JOUF-00474 and JOU2-0229, and by the Netherlands Organisation for Scientific Research (NWO). EF940171N (49)Leckner, B.Int. J.Energy Res. 1992,16, 351-363. (50)W6jtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel 1994,73,14161422.
