Energy & Fuels 2002, 16, 915-919
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Repressing NOx and N2O Emissions in a Fluidized Bed Biomass Combustor Go¨ran Olofsson,* Wuyin Wang, Zhicheng Ye, Ingemar Bjerle, and Arne Andersson Department of Chemical Engineering II, Lund University, SE-221 00 Lund, Sweden Received November 20, 2001. Revised Manuscript Received April 12, 2002
The emission of NOx and N2O were studied experimentally in a fluidized bed biomass combustor. To investigate effectively the influence of different parameters on the NOx formation with a limited number of experiments, an orthogonal experimental design was adapted. Four biomass fuels, three NOx-repressing additives, four bed materials, and four alkali-capturing additives were tested in a temperature range of 670-900 °C and at two pressure levels, 1.0 and 1.5 MPa. The NOxrepressing additive was added to repress the formation of NOx and N2O and the alkali-capturing additive was added to prevent the bed from agglomeration. In the present study it was found that it was not possible to minimize both NO (NOx) and N2O with the same combination of parameters. Instead, either NO formation or N2O formation has to be minimized. The results also showed that the NOx-repressing additives were active in repressing NOx and N2O, except for NH4HCO3. The optimal combination of the parameters investigated in the present study for minimizing the emissions were: sawdust, bone ash, mullite, and urea for NO minimizations, and straw, bone ash, clay, and Na2CO3 for N2O minimizations.
Introduction The increased awareness of the need to reduce NOx/ SOx emissions from combustion systems has led to an interest in biomass as a fuel source. Biomass has been considered a good alternative to coal. Biomass is especially important for the reduction of CO2 emissions. The interest in biomass as fuel is continuously increasing and in combination with fluidized bed combustion technology NOx emissions to the atmosphere can be lowered. However, the nitrogen content in biomass fuel is too high to be neglected when the NOx and N2O emissions are of concern. Considerable research has been directed at techniques for the thermochemical conversion of biomass and at their application and commercialization. One of these techniques, fluidized bed combustion, has the advantage of more effective heat transfer, greater fuel flexibility, lower operational temperatures, and lower emissions of SO2/NOx as compared with other technologies. The low emission of NOx is partly due to the low combustion temperature. However, the relatively low combustion temperature also increases the emission of N2O relative to other combustion systems.1,2 Although fluidized bed combustion is a mature technique for use with coal, frequent operational problems have been encountered in its application to biomass. One of these is agglomeration of the bed material. Biomass ash is relatively rich in alkali and alkaline * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Johnsson, J. E. Fuel 1994, 73, 1398. (2) de Diego, L. F.; Londono, C. A.; Wang, X. S.; Gibbs, B. M. Fuel 1996, 75, 971.
earth metals, causing it to melt at relatively low temperatures. Fouling of the heat exchangers is an additional problem. While NOx limit values have existed for a long time, N2O has received more attention only recently, as its effect on greenhouse gas has become clearer.3 In fluidized bed combustion of biomass, the NOx and N2O emissions originate mainly from the fuel-bound nitrogen. Thermal NOx are of minor importance since 1300 °C is usually quoted as the beginning temperature of thermal NOx formation. Most of the reactions proceeding in which nitrogen oxides are formed or reduced are catalytic ones. NOx formation due to oxidation of Ncontaining components is favored by CaO, which acts as a catalyst under the oxidation conditions. On the other hand, NO reductions are catalytically accelerated as well by CO, with char and ash as a catalyst. N2O decomposition is enhanced over CaO, MgO, and iron oxide.3 Air staging is a technique being successfully used to reduce NOx emission, but its effect on N2O emission is unclear.4,5 Limestone addition to a fluidized bed combustor (FBC) has been shown to reduce the N2O emission and to increase the NOx emission.2 Staged combustion combined with NH3 injection has been shown to have potential for reducing NOx formation.6 A way to lower NOx and N2O emission is to add specific chemicals to the fuel. Those chemicals then react with (3) Ko¨psel, R. F. W.; Halang, S. Fuel 1997, 76, 345. (4) Lyngfelt A.; Leckner, B. Fuel 1993, 72, 1553. (5) Hosada, H.; Hirama, T. A Novel Technique for Simultaneous Reduction of Nitrous and Nitrogen Oxides Emissions from a Bubbling Fluidized-Bed Combustor. Proceedings of the International Conference on Fluidized Bed Combustion; 1995, 2, 1469-1475.
10.1021/ef0102768 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/29/2002
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Figure 1. A picture of the combustion test rig at LU.
NOx and produce N2 and water. The added chemicals will in most cases first be converted to NH3. NH3 is known to react selectively with NO in the gas phase to produce N2 and water through reactions 1 and 2. This technique is an example of a selective noncatalytic reduction (SNCR)7
4NH3 + 6NO f 5N2 + 6H2O
(Reaction 1)
4NH3 + 4NO + O2 f 4N2 + 6H2O (Reaction 2) Ammonia species are both initiators of NO formation and able to reduce the NO already formed. NO can also be transferred into molecular nitrogen by reaction with carbon monoxide. The reaction is catalyst by CaO, char, and ash.3 NO can react to molecular nitrogen with the nitrogen retained in char. The relative importance of oxidation and reduction reactions consequently determines the resultant nitrogen oxide emissions. Most of these reactions are catalytic.8 Some chemicals (CaO, MgO, and Na2CO3) added to the fuel will act as catalysts and enhance the degradation of NO by reaction with CO. Sodium carbonate will decompose to sodium oxide (Na2O) and CO2. It is believed that Na2O can act as a catalyst in the same way as CaO. In addition to coal, soot, and coke particles, the coal ash has catalytic effects. (6) Gibbs, B. M.; Salam, T. F.; Sibtain, S. F.; Pragnell, R. J.; Gauld, D. W. In Twenty-Second Symp. (Internation) on Combustion; The Combustion Institute: Pittsburgh, 1988; pp 1147-1154. (7) Wuyin, W. Study and Modeling of the Reduction of Sulfur Dioxide, Nitrogen Oxides and Hydrogen Chloride by Dry Injection Technologies. Ph.D. Thesis, Lund University, 1997. (8) Furusawa, T.; Koyama, M.; Tsujimura, M. Fuel 1985, 64, 413.
In a biomass fired fluidized bed combustor several operational problems relating to the fuel and bed material can occur. The bed material can agglomerate, leading to less fluidization. In the worst case it can result in total defluidization. An important parameter for the agglomeration tendency is the alkali content of the bed material. Relative little information on NOx emissions from biomass combustion can be found in the literature. Most of the work reported concerns fossil fuel; especially coal. This paper present a study of the influence of three fuel additives (NH4HCO3, (NH2)2CO, and Na2CO3) for repressing the formation of NOx and N2O during combustion of biomass in a pressurized fluidized bed combustor. The influence of bed materials, fuel, and alkali-capturing additive in combination with NOx-repressing additives on the NOx and N2O emission has also been studied. Experimental Section The experiments were carried out in pressurized fluidized bed combustor with a fuel feeding capacity of 90 kWth, shown in Figure 1. It consists of three main parts: a combustion reactor, a SiC-candle filter, and a catalytic reactor. Each of these three parts is placed in a separate cylindrical pressure vessel with an inner diameter of 0.5 m. The catalytic reactor was not in operation in the present study. The combustor can perform at a fuel-feeding rate of 10-20 kg/h. A detailed description of the facility and of the operating procedures was provided in an earlier publication.9 Four biomass or biomass-derived fuels and four types of bed materials were tested. The four types of fuel were sawdust, straw, willow, and meat and bone meal (MBM). The compo-
Repressing NOx and N2O Emissions in a Biomass Combustor
Energy & Fuels, Vol. 16, No. 4, 2002 917
Table 1. Fuel Characterizationa fuel
sawdust
MBM
straw
willow
HHV, MJ/kg LHV, MJ/kg volatile, % fixed carbon, % water, % wt % dry ash C H N O S Cl Na K
19.31 17.90 76.03 14.87 8.36
n.a. n.a. n.a. n.a. 2
16.9 n.a. 73.4 n.a. 6.9
18.3
0.76 51.27 6.21 0.25 41.56 0.01 0.01 0.01 0.76
43 n.a. n.a. 1.26 n.a. 0.2 0.12 0.59 0.19
8.2 45 6.1 0.54 n.a. n.a. 0.365 0.025 0.65
3.25 48.6 6 0.6 41.5 0.04 0.005 0.028 0.34
a
76 n.a. 10.1
n.a. - not analyzed. Table 2. Compositions (in %) of Bed Materials and Additives Fyle sand magnesite mullite bone ash calcite
Na2O MgO Al2O3 SiO2 P2O3 SO2 K2O CaO TiO2 Fe2O3
0.03 0.01 0.45 98.2 0.0015 0 0.06 0.04 0.49 0.21
0.0035 84.4 0.34 3.93 0.05 0.16 0.007 7.55 0.013 3.07
0.2 0.03 75.2 24.5 0.0003 0 0.01 0.02 0.01 0.03
3 1.7 0.7 8 28.8 0.6 1.67 44.82 0.03 4.8
clay
0.006 0.07 0.21 0.82 0.05 18.2 0.25 38.45 0.005 0.005 0.012 0.014 0.014 1.28 53.8 0.29 0.003 0.40 0.035 7.56
sitional analyses are shown in Tables 1 and 2. The temperatures were measured at 5 positions in the reactor, at the bottom, below and above the feeding point for the fuel, the upper part of the reactor, and at the free board, T1-T5 in Figure 1. The temperatures tested were in the range of 670900 °C. Two pressure levels were selected: 1.0 and 1.5 MPa. Both the flow rate of the carrier gas and the fuel-feeding rate were adjusted so that an oxygen concentration of approximately 6 vol % in the product gas could be maintained. The gas velocity in the reactor was 0.25 m/s for 1.5 MPa and 0.30 m/s for 1.0 MPa and the residence times for the gas were 13.2 and 11 s, respectively. Particles of the bed materials were nominally 200 µm in size. Na2CO3, urea, and NH4HCO3 were added to the fuel input as pure chemicals in a ratio of 5 wt % for suppressing NOx emissions The additives used for capturing the alkali were mullite, calcite, clay, and a mixture of clay and calcite. They were added in a ratio of 10 wt % and the compositional analyses are shown in Table 2. A test takes about 1 day (20 h) to perform, including the time for heating the reactor to the operational temperature. The reactor is then running at steady state for about 5 h. An orthogonal experimental design was adopted so as to be able to investigate the influence of different parameters on emission of NOx and N2O in an effective way with use of only a limited number of experiments. The fuel, the NOx additive, the bed material, and the alkali additive selected served as experimental parameters, each parameter having four levels. The emissions of NOx and N2O occurring were evaluated and were related to the parameters selected. The combustion gas was analyzed on-line by means of mass spectrometry (Balzers QMG 420 MS) and Fourier transform infrared spectrometry (Gasmet FT-IR gas analyzer). The used bed material from the bottom of the reactor and the fly ash from the filter were also sampled and analyzed by means of atomic absorption spectroscopy (GBC 908AA) and scanning electron microscopy (JEOL JSM-840A SEM). (9) Padban N.; Wang W.; Ye, Z.; Bjerle, I.; Odenbrand, I. Energy Fuels 2000, 14, 603-611.
Figure 2. Suggested7 reaction scheme for nitrogen reactions.
Results NOx is the sum of NO and NO2. The emission of NO2 in the present study was very low (a few ppm) so all the calculations for NOx were made on NO only. The results from the evaluation of the experiments are listed in matrix form in Tables 4 and 5. The impact of each parameter (j) at each level (i) was calculated as the average emission of NOx and N2O (in ppm), (ki,j) (i ) 1, 2, 3, 4, and j ) A, B, C, D, E), for all experiments in which the parameter level of concern was employed. The impact of urea on the emission of NO, for example, is the average emission of NO for all the experiments in which urea was employed as an NOx-repressing additive. The importance of each parameter (fuel, NOx additive, bed material, or alkali additive) on emissions were indicated by the maximum difference in the ki,j value for it in column Rj. The relative importance of different parameters can be quantified then by comparing their Rj value. As can be seen from Table 4, the most significant factor for NO emissions was the type of fuel, followed by the bed material and NOx-repressing additive. From Table 5 it can be seen that for the N2O emissions the fuel was the most significant factor, followed by the bed material and the NOx-repressing additive. Discussion At high temperatures, the ammonium bicarbonate (NH4HCO3) decomposes to ammonia, CO2, and water, while urea ((NH2)2CO) at 300 °C is converted mainly to ammonia and isocyanic acid (HNCO). Above 500 °C urea decomposes completely to volatile products.10 During the heating of the fuel, the fuel-bound nitrogen is partly released. A part of the nitrogen escapes as cyanic (CN) and amino (NH) compounds. These compounds can be transformed into nitrogen oxides by several complex reaction steps during the combustion. A suggested reaction scheme is given in Figure 2.7 The most significant factor for NO and N2O emissions was the type of fuel. From Tables 4 and 5 it can be seen that the R-factor is 312 and 246 for NO and N2O, respectively, which are the highest of the R-values. Since all biomass contains nitrogen compounds, see Table 1, the combustion of biomass always leads to emissions of NOx. However, MBM has a high content of proteins, and as a consequence a high nitrogen content (1.26%) compared to the other fuels (0.25-0.6%); (10) Wynne, A. M. J. Chem. Educ. 1987, 64, 180.
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Table 3. Values of Some Interesting Parameters in the Present Study fuel amount (g/min)
ERa
airflow (sl/min)
temperature range (°C)
fuel
1.0 MPa
1.5 MPa
1.0 MPa
1.5 MPa
1.0 MPa
1.5 MPa
below fuel inlet
above fuel inlet
sawdust straw willow MBM
51 56 50 66
63 58 74 n.a.
350 350 350 350
450 450 450 450
1.54 1.59 1.62 1.34
1.72 1.78 1.45
810-850 740-770 780-820 700-750
840-900
a
ER ) equivalence ratio (QO2 in/QO2 stoichiometric).
Table 4. Parameter Significance on NO Emission (ppm) j fuel A
bed material B
alkali additive C
NOx additive D
1 2 3 4
straw MBM willow sawdust
MgO sand bone Ash mullite
clay calcite mullite C1 + C2
Na2CO3 NH4HCO3 none OdC(NH2)2
10 15
k1 k2 k3 k4 R
200 396 160 84 312
271 161 55 251 216
198 227 135 214 92
154 250 220 121 129
185.5 197.6
i
pressure E
12.1
Table 5. Parameter Significance on N2O Emission (ppm) j
i
fuel A
bed material B
alkali additive C
NOx additive D
1 2 3 4
straw MBM willow sawdust
MgO sand bone Ash mullite
clay calcite mullite C1 + C2
Na2CO3 NH4HCO3 none OdC(NH2)2
10 15
k1 k2 k3 k4 R
26 271 84 34 246
89 53 21 167 146
22 89 115 73 93
37 141 96 41 104
97.9 69.7
pressure E
28.2
Table 6. Conversion of Fuel-N to NO and N2Oa N-conversion to NO (%)
N-conversion to N2O (%)
fuel
1.0 Mpa
1.5 MPa
1.0 MPa
1.5 MPa
sawdust straw willow MBM
15 13 16 9
3 19 8 n.a.
21 n.a. 15 18
2 8 10 n.a.
a
840-880
n.a. - not analyzed.
therefore, as can be seen from Tables 4 and 5, MBM greatly contributes to the emissions. As a consequence, if MBM is excluded as a fuel, the parameter fuel should not be so noticeable. The conversion of fuel-nitrogen to NOx and N2O are different for different fuels. In the present study the conversion to NO was found to be between 9 and 16% for the pressure level of 1.0 MPa. For the pressure level 1.5 MPa the conversion was between 3 and 19%. The conversion to N2O was found to be between 15 and 21% for the lower pressure and between 2 and 10% for the higher pressure. The conversion to N2O is rather high, which can be explained by the relatively low combustion temperatures (
