Ind. Eng. Chem. Res. 2004, 43, 2433-2445
2433
Catalytic Hot Gas Cleaning with Monoliths in Biomass Gasification in Fluidized Beds. 1. Their Effectiveness for Tar Elimination Jose´ Corella,* Jose´ M. Toledo, and Rita Padilla Chemical Engineering Department (Faculty of Chemistry), University Complutense of Madrid (UCM), 28040 Madrid, Spain
Full-size 15 × 15 × 30 cm nickel-based monoliths are tested for tar elimination in fuel gas produced by biomass gasification in a fluidized bed at a small pilot-plant scale. The feedstocks used were mixtures of pine wood chips and “orujillo”, the residue from olive oil production. Temperatures at the front or face of the monolith ranged from 820 to 956 °C, gas hourly space velocities in the monolith ranged from 1280 to 4550 h-1 (normal conditions, nc), area velocities ranged from 2.7 to 7.1 m/h, and superficial or face gas velocities at the inlet of the monolith ranged from 0.34 to 1.3 m/s. Samples of gas and tar were taken before and after the monolith reactor, and variations in gas composition and tar content were determined. Using a macrokinetic model presented elsewhere, some key kinetic constants for the tar-removal reaction are calculated for the monolith and used as indexes of its activity. The effects of some important operating conditions on the activity and sometimes on the deactivation of the monolith are presented here. The intrinsic activity of the monolith for tar abatement is finally compared with those of other competing catalysts such as dolomites and commercial steam-reforming catalysts. It is concluded that these monoliths are not very high in activity, but they can operate with a fuel gas containing particulates, thereby avoiding the use of hot filters, which are problematic when used in biomass gasification. Introduction It is a well-known fact that biomass gasification with air in fluidized-bed reactors produces a fuel gas that is a mixture of N2, H2, CO, CO2, H2O, CH4, and light hydrocarbons whose use in advanced applications is hindered by the presence of contaminants such as tar, ammonia, and particulates. These contaminants have to be eliminated from the gasification gas for use in its most promising applications. Therefore, to use this gas in gas engines and gas turbines, in addition to having an optimized design and optimized operation of the biomass gasifier, there is a need for cleaning and upgrading the gasification gas. Among the possible gas cleaning methods, catalytic hot gas cleaning is the most recommended because it completely destroys the tar, as well as the ammonia, rather than transferring them to a waste liquid stream that is very difficult to dispose. Two different types of catalysts have been studied up to now for hot gas cleaning: The first type includes dolomites, limestones, and related materials. Such catalysts are inexpensive, but their activity is not very high. They have been studied extensively (ref 1, for instance) and are not of concern for this paper, at least for catalysts located downstream from the gasifier. The second type of catalysts for tar, and ammonia, elimination from the gasification gas are nickel-based. These catalysts have the same or similar composition as the existing steam-reforming catalysts for naphthas and natural gas. As the Pacific Northwest Laboratory (PNL) in Richland, WA, demonstrated in the mid-1980s, these nickel catalysts are not recommended for use in gasifier beds because they quickly become deactivated by coke (formed from the tar) and by the char existing in the * To whom correspondence should be addressed. Fax: +3491-394 41 64. E-mail:
[email protected].
gasifier. In the authors’ opinion, which concurs with the PNL findings, nickel-based catalysts are useful for this process only when they are used downstream from the gasifier. It is this approach that is studied in this paper. Nickel-based catalysts for hot gas cleanup can be classified, in turn, into two very different classes: commercial particulate-shaped catalysts (rings, spheres, pellets, and extrudates), and as-yet-uncommercialized monoliths. Commercial nickel-based catalysts have been extensively studied during the past 20 years for use in biomass gasification with pure steam,2 with steam-O2 mixtures,3,4 and with air5-8 as gasifying agents. Previous work with nickel-based catalysts carried out worldwide, including that of the pioneers, has been analyzed and compared in the references already cited. For this reason, no further bibliographical analysis is made in this paper. Most of the commercial nickel-based catalysts (rings) have proven to be very effective and useful for tar elimination, but they require a fuel gas without particulates. However, fluidized-bed biomass gasifiers generate gases containing a great deal of solids, and even three in-series cyclones3,4 are not good enough to eliminate the particulates to an acceptable level for a downstream fixed-bed reactor with rings. For this reason, a hot filter is needed before the catalytic bed. Because some coke might be formed (by thermal tar cracking) in its pores, the hot filter can become plugged, which causes a problem in the overall gasification process. For this reason, another approach or solution could be employed: the use of monolithic catalysts, honeycomb structures that can operate with gases containing particulates.9 The study of the performance and usefulness of these monoliths for hot fuel gas cleaning is the subject of this paper.
10.1021/ie0307683 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/08/2004
2434 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004
Figure 1. Small pilot plant used to study the monoliths under a realistic gasification gas.
As for nickel-based monoliths for hot gas cleaning in biomass gasification in fluidized bed, to the authors’ knowledge, apart from University Complutense of Madrid (UCM), there are only two organizations in the world, VTT in Finland and the Fraunhofer UMSICHT Institute in Oberhausen, Germany, who are studying the use of these monoliths. However, mainly because of their novelty and possible commercial application, only a few papers (refs 10-13) have been published on this subject. Data on conversions, yields, or gas compositions are sometimes sufficient for comparing solid catalysts, but in the authors’ opinion, such is not the case for this process. For instance, most of the reactions occurring in these monoliths are highly endothermic, and because these monoliths have to operate in adiabatic form, significant longitudinal gradients of temperature in the monolith can be generated. Under typical operating conditions, therefore, these monoliths are not isothermal at all. To analyze the data obtained with these monoliths and to further compare these data with other those of competing approaches for hot gas cleaning, such as the use of dolomites or commercial (ring) catalysts, a good data analysis method is needed. For this reason, a mathematical model for the monolith itself and for the overall full catalytic reactor had to be developed. This model is being presented in another paper,14 and it will be used here to obtain some kinetic parameters of the monolith for further comparison with values from other catalysts (e.g., dolomites, nickel-based rings) for the same application. The key kinetic parameter used here as an index of the activity of the monolith is the apparent, effective kinetic constant for the overall tar-elimination reaction at 900 °C (keff,tar,900°C), which is the temperature of reference. keff,tar,900°C is obtained in this work at several
experimental conditions, always operating the monolith under a real biomass gasification gas. keff,tar,900°C values obtained for the monolith are further compared with those obtained with commercial nickel-based catalysts (rings) and with dolomites. Facility Used The facility used to study the monolith and the overall (gasifier + gas cleaning unit) process is new and is depicted in Figure 1. The gasifier used was an atmospheric bubbling fluidized bed (BFB) with an internal diameter of 15 cm and a total height of about 5.0 m. The reactor was continuously fed by biomass at the bottom, near the gas distributor plate. Some features of this BFB gasifier are as follows: (i) Feedstock characteristics and flow rate can be varied from test to test. (ii) Both primary and secondary air flows can be used. (iii) There is a throat near the secondary air feeding point; the internal diameter of the gasifier at this point is reduced to 6.0 cm to get a good mixture of the rising gas and the secondary air flow. (iv) There are two external ovens to facilitate the gasification at relatively high temperatures with a biomass of relatively high moisture. (v) A temperature of 1000 °C can be achieved in the freeboard without much difficulty, which helps decrease the tar content at the gasifier exit. There are two cyclones in series, instead of the typical single cyclone, to decrease the particulate content in the produced gas. After the second cyclone, the produced gas is split into two different flows. The main flow of produced gas is directly sent without filtration to the monolith reactor, as shown in Figure 1. The secondary flow is filtered, cleaned and flared afterward.
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2435
Figure 2. Scheme of the catalytic reactor used to test full-size monoliths.
Monolith Reactor. The monolith reactor, directly connected to the gasifier, was specifically designed and manufactured for this work. This reactor is shown in Figure 2. Some of the main features of the monolith reactor are as follows: (i) There is a tertiary air flow, as shown in Figure 2, to reheat the fuel gas to the desired operating temperature in the monolith. (ii) The monolith reactor has two different zones, as shown in Figure 2. In the first zone, the fuel gas is partially burned, increasing its temperature from around 400-500 to 920-1050 °C. The second zone is the monolith itself. (iii) A small flow of H2O can be introduced to increase the H2O/C* ratio in the gas entering the monolith. This might increase its performance by decreasing its deactivation rate. (iv) The temperature can be measured at four different points: two at the monolith front/face and two at its exit. (v) The monolith reactor can accommodate one or two pieces of full-size (15 × 15 × 30 cm) monoliths. A typical axial profile of temperature in the gasification plant, including the whole monolith reactor, is shown in Figure 3. Because the predominant reactions in the monolith are endothermic, the temperature of the fuel gas decreases along the axis or length of the monolith, as is shown in Figure 3. This axial profile of temperature is of paramount importance for the performance of the monolith. The catalytic reactor was designed to reach adiabatic operation in the zone of the monolith. Nevertheless, it was soon realized that the monolith did not operate adiabatically, and that it would not, even if more than 20 cm of isolation were provided in this zone. The temperatures measured at the exit of the monolith were less than those initially foreseen (calculated in ref 14). Some loss of heat also occurred in the zone where the monolith was located, mainly because of the 60 × 60 × 4 cm flange at the bottom of the reactor.
Figure 3. Profile of temperature along the entire plant (top) and location of the sampling points for tar (bottom).
Sampling. Samples for main gas components, tar, particulates, and NH3 were taken periodically in each test before and after the monolith reactor. Sampling and analysis for tar and particulates were made according to the tar protocol recently established by many institutions.15 On the other hand, sampling was not possible just before the monolith because of the very high temperatures (near 1000 °C) in this zone. For this reason, sampling at the inlet was done just before the monolith reactor, rather than before the monolith itself. Tar conversion in the monolith reactor is due to both thermal and catalytic mechanisms. The third air flow burns part of the tar in the fuel gas. For this reason, at the face of the monolith, some tar conversion, denoted Xtar,inlet, already occurs. To determine Xtar,inlet, some tests were performed under the same conditions without using monoliths or using unimpregnated monoliths (monoliths having no nickel at all). Information was obtained from these tests about the relative importance of the thermal contribution to overall tar elimination. Tar sampling was carried out at different times on stream. Tar conversion and variations in the H2, CO, CH4, etc., contents in the fuel gas were measured at different times on stream and for different operating conditions. Figure 4, which corresponds to test N-90, shows an example of the sampling performed in each experiment. Monoliths Used. The core of this work was done with monoliths supplied by BASF AG in Ludwigshafen, Germany, as shown at the bottom of Table 1. These monoliths were provided to UCM under a confidential agreement, so that no technical data about them can be provided here. Nevertheless, other researchers can also obtain these monoliths from BASF and reproduce
2436 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004
Figure 4. One example (test N-90) of start-up under combustion conditions throughout the gasifier and monolith gas cleaning unit. Evolution with time of the temperatures at different points of the plant and sampling periods.
the work presented here. It can only be said that these monoliths are for gases with high dust contents. Some monoliths were fully impregnated with nickel, and others had no impregnation. The monoliths that were not impregnated with nickel were used to obtain information about the thermal activity of the overall catalytic reactor for the tar-elimination reaction. Some other monoliths were also obtained from a Polish institution and from two other German companies (one of which is Su¨d-Chemie AG). These monoliths were also tested, but they were not specifically designed for this process, and they did not provide good results, so no results about their use are reported here. Operating Conditions The main experimental conditions concerning both the gasifier and the monolith reactor are listed in Table 1. An important operating parameter is the air partitioning among the first, second, and third air flows. This air partitioning, together with the total air flow, determines the temperature profile throughout the entire plant. This parameter, the air partitioning, was a very important independent operating variable that is further discussed in more detail below. Some other important variables in monolith reactor operation are (i) the gas flow rate in the monolith reactor (Q), (ii) the space-time of the monolith (τmon), (iii) the space velocities of the monolith (GHSV and WHSV), and (iv) the area velocity of the monolith (AV). These variables are reported for each test in Table 2. They are given in different units to facilitate further comparison with results obtained by different authors. Although this point might be obvious, one must be sure that, when using any of these variables, one first checks the units and how they were calculated. Omission of this basic practice leads to incorrect conclusions.
Feedstocks. The main type of biomass used was small (1.0-5.0 mm) chips of pine (Pinus pinaster) wood. Its composition is given in Table 3. It can be considered as an easy, unproblematic feedstock, as its ash contains only a very small amount of alkali (K and Na) materials. Problems such as ash agglomeration and melting do not arise with this feedstock. It also has a very low nitrogen content. For this reason, only a few parts per million of NH3 appear in the gasification gas. Additionally, the effectiveness of these monoliths for NH3 elimination has been studied.16 For this reason, another feedstock with a higher N content was used. After several trials, a residue (“orujillo”) from olive oil production, of which a few millions of tons is generated every year in Spain, was selected as the second type of biomass to be used. The particle size used was the interval from 1.0 to 6.0 mm. Results of the analysis of this material are also presented in Table 3. Because the content of K2O in the ash from this biomass is very high (∼30 wt %), some significant problems (such as agglomeration and sintering) occurred in the gasification plant. For this reason, a mixture of pine wood chips and orujillo was used as the feedstock. The amounts of these two materials in the mixture were varied from test to test. The overall K2O content in the feedstock was also varied from test to test to investigate the effect of this important variable. The detailed composition of the feedstock used in each test is reported at the top of Table 1. An inexpensive catalytic material (dolomite or olivine, as indicated in Table 1) was also continuously introduced in the gasifier. It was mixed (catalytic material: 2.0 to 4.0 wt %) with the biomass in the feedstock. It replaced the same and existing in-bed material, which was eroded by its fluidization and carried out of the gasifier by elutriation. Biomass Moisture and H2O in the Fuel Gas. The H2O content in the fuel (gasification) gas is an important parameter for at least three reasons: (1) Tar elimination
g/Nm3
b
°C °C °C mol of H2O/ atom‚gC*
m3 (nc, wet)/ h‚m3cat
1.3 1.8 3.7 DV
V
938 865 680
1750
0.6 0.8 -
880 820 650
1490
S+D
S+D
V
0.6 1.2 1.3
945 890 700
1690
S+D
842 920 46.1 0.70
80 5 15
0.34
48.5 48.5 3.0 10 4.5
N-80
DV
2.2 3.6 7.5
835 665
1400
S+D
785 840 33.3 0.66
80 20
0.39
48.5 48.5 3.0 19 4.2
N-81
0.48
Air 0.40
S+D
859 870 31 0.70
Gasifier
S+D
850 965 38 0.54
79 9 12
48.5 48.5 3.0 20 4.4
Feedstock 48.5 48.5 3.0 10 4.5
90 10
N-84
N-83
d
0.9 1.0 3.6
925 855 615
V
1.3 2.6 5.0
1044 956 678
V
1.1 1.6 4.1
1003 912 714
Catalytic Reactor 1320 1390 1630
S+D
795 865 35.3 0.96
90 10
0.30
48.5 48.5 3.0 11 6.1
N-82
N
3.4 3.7 -
1045 937 720
1530
S+D
814 932 32 0.51
78 10 12
0.43
97.0 3.0 25 4.1
V
1.9 3.8 -
1050 961 682
1280
S+N
827 936 25 0.33
78 9.5 12.5
0.59
100.0 22 2.7
N-86
test no. N-85
V
2.7 2.5 -
910 854 668
1740
S+N
879 941 39 0.42
80 10 10
0.44
50.0 50.0 16 3.5
N-87
V
2.7 2.8 -
c
1015 910 710
2790
S+D
850 850 52 0.72
80 0 20
0.44
48.5 48.5 3.0 20 5.9
N-88
B
5.3 5.0 4.0
985 832 660
3290
S+D
820 885 51 0.47
80 0 20
0.50
38.8 58.2 3.0 30 4.2
N-89
B
3.7 4.6 1.2
950 875 715
3580
S+O
852 900 48 0.48
80 0 20
0.41
38.4 57.6 4.0 15 4.8
N-90
B
2.7 5.4 0.3
970 880 720
4240
S+O
850 902 56 0.43
78 0 22
0.46
38.4 57.6 4.0 18 4.2
N-91
N
4.8 6.2 0.9
935 820 700
4550
S+O
852 896 56 0.46
78 0 22
0.39
28.8 67.2 4.0 18 4.2
N-92a
N
3.0 3.9 0.9
1005 840 710
4040
S+O
866 900 56 0.46
78 0 22
0.39
28.8 67.2 4.0 18 4.2
N-92b
a
Residue from olive oil production. S + D ) silica sand + dolomite, S + N ) silica sand + Ni/olivine, S + O ) silica sand + raw olivine. V ) V1693, DV ) used/deactivated V1693, B ) BV0170, N ) not impregnated (without nickel). d Made with a monolith from Su¨d-Chemie.
at the inlet at the exit dust loading at the inlet of the monolith BASF monolith usedc
temperatures after third air addition monolith face, axis monolith outlet, axis H2O/C* ratio in the fuel gas
space velocity (GHSV)
in-bed materialb
785 960 43.7 0.69
80 10 10
0.30
48.5 48.5 3.0 10 4.4
N-79
840 950 43.7 0.61
90 10 -
% % %
°C °C cm/s kgbiomass/ h‚kg(S+D)
0.40
-
ERtotal air partitioning primary air secondary air tertiary air
temperatures bed after second air addition u0 (bed, inlet) WHSV of biomass
48.5 48.5 3.0 8.3 3.9
N-78
wt % wt % wt % wt % wt %, ar kg/h
units
orujilloa pine wood chips raw olivine dolomite moisture feed flow rate
parameter
Table 1. Main Experimental Conditions in the Gasification Plant in Tests Carried out with a Monolithic Reactor
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2437
2438 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 Table 2. Some Main Experimental Conditions in the Monolithic Reactora test no. parameter
units
temperature axis, inlet axis, outlet u0 (monolith inlet) Q (nc, wet)
°C °C cm/s m3 (nc, wet)/ h Q (Tcat, wet) m3 (Tcat, wet)/ h τ [Vcat/Q (nc, wet)] m3cat‚h/ × 104 m3 (nc, wet) τ′′′′ (τFmonolith) kg‚h/ m3 (nc, wet) τ′′′′′ [Vcat/Q (Tcat, wet)] kg‚h/ × Fmonolith m3 (Tcat, wet) τ′ (L/u0) s (reactor conditions) τ′′ [S/Q (Tcat, wet)] m2‚h/ m3 (Tcat, wet) τ′′′ [S/Q (nc, wet)] m2‚h/ m3 (nc, wet) space velocity GHSV ) 1/τ m3 (nc, wet)/ h‚m3cat WHSV ) 1/τ′′′′ m3 (nc, wet)/ kg‚h area velocity m3 (nc, wet)/ m2‚h ) m/h (AV ) 1/τ′′′) cm/s
N-78
N-79
N-80
N-81 N-82b N-83
N-84
N-87
N-88 N-89c N-90c N-91d N-92ad
N-92bd
820 650 40 8.5
865 680 46 9.9
890 700 46 9.5
835 665 34 8.1
855 615 35 7.8
956 678 37 8.1
912 714 46 9.6
855 670 56 9.7
910 710 84 13.5
832 660 82 10.7
875 715 102 12.3
880 720 131 9.1
820 700 125 12
840 710 110 12
35
41
40
33
32
37
42
38
51.9
36
45
47
45
41
6.7
5.7
5.9
6.9
7.3
6.9
5.9
5.7
3.6
3.0
2.8
2.4
2.4
2.4
0.45
0.38
0.39
0.46
0.49
0.46
0.39
0.38
0.24
0.20
0.19
0.16
0.15
0.16
0.11
0.091 0.094 0.11
0.12
0.10
0.090 0.10
0.072 0.067 0.054 0.044
0.044
0.049
0.74
0.66
0.86
0.80
0.63
0.35
0.24
0.27
0.076 0.064 0.066 0.081 0.083 0.072 0.064 0.070 0.051 0.050 0.040 0.031
0.032
0.036
0.31
0.27
0.28
0.33
0.34
0.33
0.28
0.27
0.20
0.17
0.15
0.16
0.12
0.12
1490
1750
1690
1400
1320
1390
1630
1740
2790
3290
3580
4240
4550
2.2
2.6
2.5
2.2
2.1
2.2
2.5
2.6
4.2
5.0
5.4
6.
6.7
6.
3.2
3.7
3.6
3.1
2.9
3.1
3.6
3.7
5.1
5.9
6.8
6.
8.4
8.
0.10
0.085 0.082 0.085 0.10
0.10
0.14
0.16
0.19
0.8
0.3
0.3
0.089 0.10
0.66
0.89
0.54
0.36
0.29
0.24
4040
V1693 from BASF; Sused ) 2.65 m2, F ) 666 kg/m3, Vmon ) 0.006 75 m3, Vused ) 0.005 63 m3. b Monolith from Sued-Chemie AG. c Monolith BV0170 from BASF; V 3 2 d Monolith BV0170 from BASF; V 3 2 used ) 0.00 363 m , Sused ) 1.81 m . used ) 0.002 88m , Sused ) 1.44 m . a
Table 3. Characterization of the Feedstock Used in the Gasification Test Runs parameter proximate analysis moisture ash volatile matter fixed carbon ultimate analysis C H O S N Cl LHVb ash analysis K2O Na2O Al2O3 SiO2 Fe2O3 CaO MgO a
units wt %, dry basis
wt %, dry basis
MJ/kg, daf wt %
small pine wood chips
orujilloa
20 0.7 81 16
13 2.9-5.8 74-84 19.4-19.6
49-50 5.2-5.7 44-45 0 0.10-0.30 18.0-18.4
52-54 5.0-6.7 34-39 0-0.10 1.1-1.5 20-21
6.9 0.30 4.3 25 3.6 25 2.8
27-36 0.4-4.0 2.5-2.9 18-32 1.9-2.3 10-12 3.8-7.0
Residue from olive oil production. b Lowest heating value.
with these catalysts occurs mainly by steam-reforming reactions. H2O becomes a reactant influencing the kinetics of the overall tar-elimination reaction. (2) The H2O content determines the H2O/C* [or (H2O + CO2)/ C*] ratio in the produced gas, which plays an important role in simultaneous coke removal from the catalyst surface and, as a consequence, in the life of the monolith. (3) When the H2O content is high, the heating value of the produced gas is low. In fact, the heating value can be as low as 2.2 MJ/Nm3, as in the case of the Lahti plant.17 H2O is fed to the plant as biomass moisture, apart from the small amount also entering with the air. It can also be fed independently, as a small flow at the top of the monolith reactor.
Biomass moisture was varied from test to test by adding some H2O to the feedstock before it was placed in the feeding hopper. Moisture of the feedstock in each test is indicated at the top of Table 1. A high moisture content increases the H2O/C* ratio in the produced gas, which has a positive effect on the life of the monolith, but it also decreases the temperature in the gasifier bed and increases the tar content in the gasification gas.18 Therefore, high moisture content in the biomass is not desired in this process. Nevertheless, H2O can be introduced not only as biomass moisture at the bottom of the gasifier but also as an independent flow at the top of the monolith reactor, as shown in Figure 2. Depending on where the H2O is added, the overall effect on the process is different. In the last tests in this work, H2O was added not only with the feedstock but also at the top of the monolith reactor. The H2O/C* ratio in the fuel gas at both the inlet and the exit of the monolith reactor is reported in Table 1 for each test. In-Gasifier-Bed Material. In addition to silica sand, a typical material used in biomass gasification in fluidized beds, the following solids were used as additives in the gasifier bed: raw dolomite, raw olivine, sintered olivine, and nickel on olivine. These additives were asuumed to have some catalytic effect on the ingasifier tar elimination process. The exact mixture of solids used in the gasifier bed in each test is shown in Table 1. The dolomite used was provided by a Spanish quarry (Calcinor, in Bilbao, Northern Spain), and all of its physicochemical properties have already been published in detail.1 The Ni/olivine catalyst was provided by the University of Strasbourg (Strasbourg, France) and has been described by Kiennemann and co-workers.19 The olivine was supplied by an Austrian quarry (Magnolithe) and was used at UCM in two different forms: raw and sintered at 1500 °C. These two forms have very different pore structures and surface areas. The sintered olivine
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2439
is quite hard and does not generate many particulates in the gasification gas; however, because it has essentially no internal surface area, it proved20 to have no catalytic activity for tar elimination. The raw (uncalcined) olivine and the olivine calcined at 900 °C had BET surface areas of 7.7 and 7.6 m2/g, micropore areas of 0.97 and 1.04 m2/g, external surface areas of 6.7 and 6.6 m2/g, and average pore diameters (by BET) of 93 and 220 Å, respectively. The hardnesses and particle densities of these catalytic materials are different. For this reason, the particle contents in the produced or gasification gas are also different for each one of these materials. These particle or dust contents in the gasification gas are reported at the bottom of Table 1. Air Partitioning. The total air fed into the gasification plant was partitioned into three different flows, as indicated in Table 1. These three air flows were as follows: (i) Primary air was fed at the bottom of the bed and constituted between 78 and 100% of the total equivalence ratio (ERtotal). (ii) Secondary air was fed in the gasifier freeboard and constituted 0-10% of ERtotal. (iii) Tertiary air was fed at the top of the monolith reactor and constituted 0-22% of ERtotal This partitioning is very important because the temperature profile, and therefore the tar content in the fuel gas throughout the plant, depends on it. A scheme of the temperature profile for the whole plant is shown in Figure 3. Depending on the secondary and tertiary air flows, the T′′, T iv, and T v values indicated in Figure 3 vary a great deal. Because the tar content at the inlet of the monolith depends on these temperatures, as a result of the thermal reactions involved in tar elimination, this tar content depends on the air partitioning being used. Some other important process parameters, such as the stickiness of the particulates in the fuel gas at the monolith front, also depend on the temperature profile. For instance, depending on the tertiary air flow used, the temperature at the front of the monolith (T iv in Figure 3) varies. When the T iv value increases above the softening point of the ash, which is relatively low when the ash contains a high amount of K2O, the ash becomes partially melted and can stick on the front of the monolith, irreversibly deactivating it, which is of paramount importance in this process. Thus, the temperature profile becomes an important variable of operation that can be controlled by the air partitioning. It should also be noted that the temperature profile can also vary from plant to plant. For example, depending on the distance from the gasifier to the catalytic reactor and on the isolation of the corresponding connecting pipe, there will be a greater or lesser need for the third air flow. Operation Limits The process operating variables that were studied and their intervals can be obtained from Tables 1 and 2 together with the preceding paragraphs. Nevertheless, because of its importance, this information is clearly repeated here. The temperature at the front/face of the monolith ranges from 820 to 956 °C. The total equivalence ratio (ERtotal) ranges from 0.30 to 0.59. The partitioning of the air among the first, second, and third flows ranges from 100-0-0 to 78-0-22.
The in-bed material in the gasifier consists of four types of solids. The feedstock characteristics are as follows: (i) The type of biomass used is pine wood and orujillo, a residue from olive oil production. (ii) The particle size distribution ranges from 1 to 6 mm. (iii) K2O content ranges from the absence of K2O to a significant amount of K2O (introduced by the orujillo). The gas velocity at the inlet of the monolith (uo) ranges from 34 to 131 cm/s. The H2O/C* ratio at the inlet of the monolith ranges from 0.4 to 5.3. The methods of feeding H2O are as moisture with biomass and as an independent flow at the top of the monolith reactor. The characteristics of the monolith are as follows: (i) The gas hourly space velocity (GHSV) ranges from 1280 to 4240 h-1 (normal conditions, nc). (ii) The area velocity (AV) ranges from 2.7 to 7.1 m/h. (iii) The gas residence time ranges from 0.24 to 0.89 s (subject to reactor conditions). (iv) The gas flow rate ranges from 7.2 to 13.5 Nm3/h. Finally, the tar content at the monolith inlet ranges from 960 to 15 400 mg of tar/Nm3. Results Gas Composition. The gas compositions measured before and after the monolith reactor are reported in Table 4 for all of the tests carried out. Among the gas components (H2, CO, CO2, CH4, etc.), the H2 content is a good index of the gas composition. It can be used for a quick estimate of the quality of the gasification gas. The H2 contents before and after the monolith reactor are included in Table 4. One might note that, in some tests, the H2 content increases in the monolith reactor whereas, in some other tests, it decreases. The overall variation is the result of two different types of reactions: (a) generation of H2 by the steam reforming of tar and light hydrocarbons, as well as the simultaneous CO-shift reaction and (b) elimination of some H2 from the gas by the partial combustion of the fuel gas by the third air flow. The overall result comes from a balance of these two opposite types of reactions. Lowest Heating Value (LHV) of the Gas. Once the gas composition is known, the LHV is easily calculated. The LHVs of the fuel gas before and after the monolith reactor are two important pieces of data, and for this reason, they are reported separately in Table 5. Note that the LHV at the exit of the monolith reactor was never below 3.5 MJ/Nm3. The gas after the monolith is therefore a useful gas and can be used for several applications. The compositions and LHVs of the gas at the exit of the gasifier reported in Tables 4 and 5 can be compared with the corresponding values obtained in different gasification plants. The heating value and the H2 content, for instance, obtained in the Lahti plant17 are considerably lower than the values reported in this paper. These parameters are interesting indexes of the quality of gasification and gas cleaning among the different institutions. Tar Content and Tar Conversion by the Monolith Reactor. The tar contents in the fuel gas before and after the monolith reactor are reported in Table 4. From these values, the tar conversion is easily calcu-
2440 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 Table 4. Gas Compositions at the Inlet and Exit of the Monolithic Reactor test no. parameter ERgasifiera avg gas composition H2 CO CO2 CH4 C2Hn N2 avg tar content
units
N-79
N-80
N-81
0.40
0.27
0.29
0.31
13.6 16.8 16.1 3.6 1.1 48.7 6300
17.4 17.1 14.0 3.6 1.1 46.7 1090
18.1 17.4 14.2 3.8 1.1 45.4 4550
18.2 15.6 16.5 4.8 1.1 43.8 3150
0 15.7 16.0 16.7 3.1 0.6 47.9 2700 57.1
N-82
N-83
N-84
N-85
N-86
N-87
N-88
N-89
N-90
N-91 N-92a N-92b
Inlet of the Catalytic Reactor 0.27 0.36 0.42 0.37 0.52
0.40
0.35
0.40
0.33
0.36
0.30
0.30
10.0 11.9 11.1 2.8 0.5 54.9 15 400
8.9 12.0 10.9 1.9 0.7 65.7 5000
11.5 13.5 15.0 3.8 1.3 54.9 930
14.2 12.5 16.1 3.5 0.9 52.8 1185
9.5 12.8 13.7 3.5 1.0 59.5 4280
13.2 12.1 13.6 4.0 1.8 55.3 1860
15.4 14.6 13.2 4.6 1.1 51.1 2230
2250
Exit of the Catalytic Reactor 0.030 0.052 0.077 0.030 0.040 0.058 0.051 0.074 0.045 0.089 0.10
0.082 0.10
0.085
0.085
17.5 16.7 13.2 3.0 0.6 49.0 500 54.8
10.9 13.8 10.3 3.1 0.2 61.8 1310 69.4
11.2 13.1 10.0 4.0 0.4 61.4 1970 12.0
1390 38.0
%, dry basis
mg/Nm3
ERthird air avg gas %, dry composition basis H2 CO CO2 CH4 C2Hn N2 avg tar content mg/Nm3 tar conversionb % a
N-78
18.6 17.7 13.0 2.8 0.5 47.4 2550 56.0
15.3 13.2 13.9 2.7 0.4 54.5 1500 52.4
20.9 16.9 14.1 3.5 0.4 44.2 2800 81.8
12.8 18.7 13.7 2.9 0.7 50.5 5700
18.8 20.0 12.8 2.0 0.2 46.2 3100 72.0
9.4 13.3 12.6 4.0 1.2 59.5 5230
16.5 17.2 13.3 3.6 0.2 49.2 270 94.8
12.4 15.0 14.0 3.9 1.2 53.4 2230
13.6 14.8 13.5 3.3 0.4 54.4 1200 46.2
15.3 14.1 9.5 3.3 0.4 59.6 3720
15.9 12.6 11.7 1.8 0.4 57.6 350 90.6
10.3 13.1 14.5 2.2 0.5 59.5 1245 75.0
13.3 13.5 13.8 2.0 0.3 57.1 200 78.5
11.3 11.3 15.6 2.6 0.3 59.0 935 26.0
9.6 11.0 12.8 2.8 0.2 63.6 570 69.0
First and second air flows. b At t ) 0.
Table 5. Lowest Heating Values (LHV, MJ/Nm3, Dry Basis) of the Fuel Gas before and after the Monolith Reactor test no. ERgasifiera LHV at inlet LHV at exit ERthird air a
N-78
N-79
N-80
N-81
N-82
N-83
N-84
N-85
N-86
N-87
N-88
N-89
N-90
N-91
N-92a
N-92b
0.40 5.6 5.3 0
0.27 6.0 5.4 0.030
0.29 6.2 5.4 0.052
0.31 6.3 4.5 0.077
0.27 4.3* 5.9* 0.030
0.36 5.5 5.0 0.040
0.42 4.8 5.4 0.058
0.37 5.3 4.8 0.051
0.52 4.6 4.2 0.074
0.40 3.6 3.9 0.045
0.35 5.1 4.0 0.089
0.40 4.9 3.8 0.10
0.33 4.6 4.1 0.082
0.36 5.5 3.5 0.10
0.30 6.0 4.5 0.085
0.30 0.085
First and second air flows.
lated. Tar conversion in each test is listed at the bottom of Table 4. It ranged from 21 to 96%, depending on the operating conditions. Analysis of Results Simply to use tar conversion as a means of conversion among reactors is insufficient because it depends on a number of variables such as temperature and gas residence time. Moreover, to compare results obtained with monoliths and with particular-shaped (rings or spheres) catalysts that have a different unit of reference (cubic meters and kilograms, respectively), the same unit has to be used for all types of catalysts. The best way to compare results is probably by using a correct kinetic analysis, but because kinetic constants depend on the reaction network used and on the macrokinetic model selected, there are different possible approaches. For the “catalytic tar-elimination problem” four different approaches have been developed to date.21-23 They consider the tar as composed of one, two, six, and infinite (continuous mixture) lumps and they use 1, 4, or 11 kinetic constants or two parameters (mean and variance of the molecular weight distribution of tar), respectively. Although any of these approaches could be used as a basis of comparison, for now, only the first approach will be used in this paper. The catalytic tarelimination reaction is considered here as an irreversible, single and one-lump reaction, with the following kinetics (more details are given in ref 14): (c) Kinetic Equation. The reaction rate for the irreversible, single, first-order, one-lump tar-removal
reaction is given by
(-rtar) ) (kthCtar + kcatCtar)a ) [(kth + kcat)Ctar]a ) j (1) keff,tarCtar,0(1 - Xtar)a where a j is the average activity with respect to the length of the monolith (regarding its deactivation). (b) Deactivation of the Monolith. Deactivation of the monolith might be due to different and simultaneous causes. For instance, at the exit of the monolith some whisker-type coke formation was detected, as Figure 5 clearly shows, when the temperature at this point was below 750 °C. The kinetic equation in this case, accounting for the different causes of deactivation, can be very complex, as demonstrated by Corella and Monzo´n.24 Nevertheless, because a detailed study of the life of the monolith will probably require several years of further research, only the following deactivation kinetic equation will be considered here
-
da j )ψ ha jd dt
(2)
Equation 2 is the simplest and easiest possible deactivation kinetic equation. ψ h is the so-called deactivation function, averaged over the length of the monolith. For the simplest case, d ) 1, an estimate that is checked further below, eq 2 can be integrated to give
a j)e
-ψ ht
(3)
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2441
According to the model described in ref 14, a heat balance in the monolith leads to the relation
T ) Tinlet - ∆Texp.,tarXtar
(8)
where
∆Texp,tar )
Tinlet - Texit Xtar,exit
(9)
This parameter (∆Texp,tar), to be used in eq 8, was calculated for each experiment by eq 9, which includes only well-known (measured) parameters. (f) Macrokinetic Model. Combining eqns. 4, 7, and 8, the result becomes
τmon ) Figure 5. Example (test N-87) of whisker-type (filamentous) coke formation at the exit of the monolith when this zone was operating below 750 °C (see Figure 3).
(c) Mass Balance for Tar. With a piston-flow 1-D model, the mass balance for the monolith, together with eqs 1 and 3, leads (ref 14) to
τmon )
∫XX
dXtar
tar,exit
tar,inlet
keff,tar(1 - Xtar)e-ψt
∫XX
) dXtar
tar,exit
tar,inlet
keff,tar,t(1 - Xtar)
(4)
where
j ) keff,tare-ψh t keff,tar,t ) keff,tara
(5)
Consider now an average kinetic constant (k h eff,tar,t), averaged between the inlet and exit, for the whole monolith. This k h eff,tar corresponds to the whole interval of temperatures in the monolith, which was around 150-200 °C. Using this average k value, eq 4, upon integration, becomes
-ln
(1 - Xtar,exit) (1 - Xtar,inlet)
)k h eff,tar,tτmon
(6)
With this very easy-to-handle equation, the k h eff,tar,t values were calculated for each test at different times on stream in the monolith. (d) Dependence of the Kinetic Constants on the Temperature (T). After a careful analysis, to be shown elsewhere20 in detail, it was observed that the dependence of the kinetic constants on the temperature did not follow Arrhenius’ law because the overall tarelimination process in the channels of the monolith is mass-transfer- or external-diffusion-controlled. The following dependence was obtained
keff,tar,t ) keff,tar,900°C
T (1173 )
2.75(1
(7)
The temperature of 900 °C was targeted as the reference temperature for the front of the monolith. keff,tar,900°C is the kinetic constant for tar elimination at 900 °C, and it is used as a reference in further comparisons of the results. (e) Heat Balance. To calculate keff,tar,900°C from eqs 4 and 7, a heat balance in the monolith is needed.
∫XX
tar,exit
tar,inlet
keff,tar,900°C
dXtar Tinlet - ∆Texp,tarXtar 1173
(
)
2.75(1
(1 - Xtar) (10)
This equation is the basic equation of the model/ approach used here.14 All parameters in it, except for keff,tar,900°C, are known for each experiment. Values of the Effective or Apparent Kinetic Constant. Equation 10 was used to calculate the keff,tar,900°C values for use as an index of the activity of the monolith. keff,tar,900°C values determined in this way are reported in Table 6. They were calculated in different units for further comparison purposes. The units for the k values correspond to those of the space times listed in Table 2. Effects of Some Important Operating Variables. keff,tar,900°C is plotted against the gas face velocity (uo), or the velocity at the front of the monolith, in Figure 6. Notice that each point in the figure (each keff,tar,900°C value) corresponds to a different degree of deactivation. The points fit different lines, one line for each time on stream. Some important conclusions can drawn from Figure 6, as follows: (1) The gas face velocity (uo) plays an important role. This is because the overall process is mass-transfercontrolled, as will be shown in more detail elsewhere (ref 20). For values of u0 ) 1 m/s and above, the mass transfer starts to have less influence on the overall process; therefore, all further uses of these monoliths, at both the pilot and commercial scales, should be made with uo values higher than 1 m/s. (2) The deactivation of the monolith determines the value of the kinetic constant for tar removal. Deactivation is absolutely the key determining factor in this process. Although the data in Figure 6 are not yet complete (more data will be recorded soon), it can be said that, under the present experimental conditions, the deactivation of the monolith in 10 h decreased its initial activity for tar abatement by approximately 5 times. The problem of the life of the monolith becomes more important than its activity. (3) Taking into account the above analysis and the data shown in Figures 3 and 5, it is concluded that the temperature at the exit of the monolith (T vi in Figure 3) has to be above the limit for whisker-type coke formation, which is around 750 °C. (4) The present problem of the monolith in this process is its life or its possible deactivation. The main
2442 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 Table 6. Effective Kinetic Constants for Tar Elimination Calculated for Each Test test no. parameter Ctar (inlet) tar conversion k′eff,tar [Q (Tcat, wet)] k′eff,tar [Q (nc, wet)] k′′′eff,tar k′′′′′eff,tar,900°C
units
N-89
N-90 N-91 N-92a N-92b
15 400 5690 5230 2230 3720 5000 960 81 72 95 46 91 75 79 1.9 1.6 4.7 1.1 3.8 2.6 4.5
1260 26 0.80
4280 1860 2230 69 69 12 4.0 4.9 0.50
2250 38 1.8
0.60
0.49
1.3
0.26
0.84
0.67
1.2
0.28
1.2
cm/s 0.075 0.082 0.082 0.063 0.13 m3(Tcat, wet)/ 12 13 12 9.8 24 kg‚h
0.11 17
0.30 46
0.06 7.6
0.18 29
0.14 21
0.22 29
0.050 0.22 6.7 29
mg/Nm3 % s-1 (reactor conditions) s-1
N-78
N-79
N-80
N-81
N-82*
9300 57 1.1
1100 55 1.2
4550 56 1.2
3210 52 0.84
0.35
0.38
0.38
0.29
N-83 N-84 N-85 N-86 N-87 N-88
1.4
0.16
0.54
0.21 36
0.030 4.2
0.11 14
Figure 6. Activity for tar removal from the BASF monolith for different face gas velocities and times on stream.
interest is to generate a new gas-cleaning process with monoliths in biomass gasification. The kinetic equation for monolith deactivation becomes of secondary interest for the authors. Even then, a complementary study was made, and from the data obtained at uo ) 46 cm/s, the activity (a j ) of the monolith was calculated by eq 5 and is shown in Figure 7a at different times on stream. According to the fitting shown in Figure 7b, under the experimental conditions used in these tests, this deactivation follows eq 3 (d ) 1) with ψ h ) 0.27 h-1. Similar values for ψ h are obtained if the calculation is performed at other values of uo. The values found for ψ h were always considered to be unacceptably high. Therefore, the rates of deactivation have to be decreased in future work. That is, for a future commercial application of monoliths in biomass gasification, their life span has to be increased. (5) One key factor in the good use (long life) of monoliths in this process is that the gasification gas entering it has to have a tar content below 2 g/Nm3. This limit was initially established by Corella et al.4,5 for commercial nickel-based catalysts (rings). Now, the same generalization can also be applied to monoliths. However, this limit of 2 g of tar/Nm3 can only be achieved by the optimized design and operation of the upstream gasifier. In this research, the produced (gasification) gas contained 1-15 g of tar/Nm3, as shown in Table 4. This tar content, considered as an index on the “quality” of the gasifier used, was too high for the monoliths. Therefore, the upstream gasifier has to be optimized to generate a gas with a tar content below 2 g/Nm3. (6) Another key factor in the good use (long life) of the monolith is the K2O content of the feedstock. This parameter has to be low to avoid the problem of ash particles sticking on the front of the monolith.
Figure 7. Variation of the average overall activity of the monolith with time on stream (uo ) 46 cm/s). (a) Raw a j -t data. (b) Checking of d ) 1.
Comparison of Results with Other Types of Tar-Elimination Catalysts The activity for tar elimination of the monoliths investigated in this work, expressed as k h eff,tar and/or keff,tar,900°C , was compared with the values measured for the following catalysts: (i) commercial nickel-based steam-reforming catalysts (rings) and (ii) calcined (at 900 °C) dolomites. For these comparisons the same units for all of the k values had to be used. For this reason, keff,tar,900°C and the space time (τ) for the monoliths were referred to the mass (in kilograms) of the catalyst and not to some other unit typically used for monoliths. The commercial nickel-based catalysts used were of commercial size, cut in half,8 with effectiveness factors (η) well below 1.0 and ground to 1.0-1.6 mm,7 with η )
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2443
Figure 8. Comparison (in an Arrhenius plot) of the activities for tar elimination (+, O, b) with those for commercial nickel-based catalysts (rings) and for dolomites (see figure legend).
1.0. These commercial nickel-based catalysts were from three different manufacturers: BASF, ICI-Katalco, and TOPSOE. The dolomites were obtained from four different quarries in Spain. Their pore structures and BET surface areas were similar, but their contents in Fe3O4 were different. Their detailed chemical and physical characteristics were published previously by Orio et al.1 Regarding the comparison of the results obtained with different and competitive approaches for catalytic gas cleanup, both the commercial steam-reforming catalysts and the dolomites were used under a fuel gas with a composition and tar content similar to those used with the monoliths in this paper. In addition, the kinetic constants used in this comparison for the commercial catalysts and the dolomites were calculated using the same macrokinetic approach as the one used in this paper. The keff,tar values calculated in this way for monoliths, rings (full size and crushed), and dolomites can be directly compared. They are shown together, in an Arrhenius plot, in Figure 8. The k values obtained in this paper for the monolith are presented in Figure 8 in two different ways: as k h eff,tar values (O points) and as keff,tar,900°C values (+ and O points). The data in Figure 8, which are the results of more than 10 years of continuous work on the subject, are conclusive. From these data, it can be concluded that, unfortunately, the activity of the expensive monoliths for tar abatement is nearly the same as, and even somewhat less than, the activities of the competitive and cheaper dolomites. This is not a good conclusion for the future use of monoliths at the commercial scale in
biomass gasification plants. Their relatively low performance for tar elimination will probably not compensate for their high cost and difficult technology. Some points for monoliths in Figure 8 could be further refined by taking into account the different degrees of deactivation of each monolith. For instance, + points in Figure 8, obtained at t ) 0 without monolith deactivation, are much better than O points obtained with partially deactivated monoliths. Nevertheless, even the + points do not surpass the activity of the dolomites, as shown on the left side of Figure 8. One must also consider that some countries or locations do not have dolomites nearby. Moreover, dolomites erode quite a lot in this process and have to be continuously fed (to the gasifier or to a downstream catalytic reactor) in amounts of around 3 wt % of the total biomass flow rate. Taking into account these two problems with dolomites, the use of monoliths in biomass gasification could be a better solution if the life span of the monoliths were longer. One conclusion from this work is that future research on monoliths, for this application, has to focus on finding operating conditions under which such severe deactivation of the monoliths does not occur. Concerning only the activity for tar abatement, according to Figure 8, the best solution/process would be to use commercial (ring) steam-reforming catalysts. Their performance is clearly higher than that of monoliths, but they require a preceding hot (400-550 °C) gas-filtration unit that has not yet been proven for long-term operations, to the best of the authors’ knowledge.
2444 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004
Acknowledgment The UCM team express their gratitude to the catalyst manufacturers BASF AG, Su¨d-Chemie AG, and Siemens AG for providing samples of their monoliths. Scientific discussions with Dr. Markus Ising from Fraunhofer UMSICHT in Oberhausen, Germany, and with Dr. P. Simell from VTT, Finland, helped in the analysis of the results. Alicia Laguna, Eduardo Torcal ,and Jorge Ruiz-Peinado, students of Chemical Reactors (Chemical Engineering Department) at UCM, helped in mathematical calculations. This work was carried out under the NOVACAT Project NNE5-2000-0098 of the EC, DG Research, Directorate J. The authors thank the European Commission for its financial support. Nomenclature a j ) activity of the monolith, regarding its deactivation, averaged overfor its length and for the different causes of deactivation AV ) area velocity in the monolith, defined as Q (nc, wet)/ Sm (m/h) BFB ) bubbling fluidized bed Ctar ) concentration of tar in the fuel gas [mg of tar/m3 (nc)] C* ) number of atoms of carbon in hydrocarbons of different species that react with the steam in the fuel gas (atom‚g/h) D ) calcined dolomite d ) deactivation order, defined by eq 2 ERtotal ) total equivalence ratio, defined as the total airto-fuel ratio used in the gasifier divided by the air-tofuel ratio for stoichiometric combustion GHSV ) gas hourly space velocity in the monolith [m3 (nc)/ h‚m3cat, h-1] kth, kcat ) kinetic constants for tar elimination by thermal and catalytic reactions, respectively [m3 (Tcat, wet)/kgcat‚ h] ktar ) intrinsic (for η ) 1) kinetic constant for the overall tar-elimination reaction [m3 (Tcat, wet)/kgcat‚h] keff,tar ) effective, overall, or apparent kinetic constant for tar elimination at t ) 0, defined by eq 1 [m3 (Tcat, wet)/ kgcat‚h] keff,tar,900°C ) effective kinetic constant for tar elimination at t ) t and at 900 °C keff,tar,t ) effective kinetic constant for tar elimination at any time (t) on stream k′eff,tar, k′′′eff,tar, k′′′′′eff,tar ) effective kinetic constant for tar elimination at t ) 0 with units of s-1, cm/s and m3 (Tcat, wet)/kg‚h, respectively k h eff,tar ) average value of the kinetic constant for the whole (from inlet to exit) monolith, defined by eq 6 [m3 (Tcat, wet)/kgcat‚h] nc ) normal conditions (273 K and 1 atm or 1.013 105 Pa) for the gas Q ) gas flow rate at the exit of the monolith reactor [m3 (nc, wet)/h] rtar ) tar-elimination reaction rate, according to eq 1{(mgtar/ kgcat‚h)[m3 (Tcat, wet)/m3 (nc)} S ) silica sand Sused ) surface area of the channels of the monolith open or available to the gas flow (m2) T ) temperature (K) Tinlet, Texit ) temperatures at the inlet and exit, respectively, of the monolith (K) T′, T′′, T′′′, T iv, etc. ) temperatures at different points in the gasification plant, as indicated in Figure 3 (K) Tcat ) temperature of the monolith, averaged between the inlet and exit (K)
∆T ) difference of temperature between the inlet and exit of the monolith (K) ∆Texp,tar ) parameter defined by eq 8 (K) u0 ) superficial gas (air) velocity at the inlet (bottom) of the gasifier (cm/s) Vmon ) volume of the monolith (m3) Vused ) volume of the monolith used in the test (m3) W ) weight of the monolith (kg) WHSV ) weight hourly space velocity in the monolith [m3 (nc, wet)/kgmon‚h] Xtar ) conversion of tar Xtar,inlet, Xtar,exit ) conversion of tar at the inlet and exit of the monolith, respectively Greek Symbols F ) overall density of the monolith (kg/m3) τmon ) space time of the gas in the monolith, defined as W/Qo [kg‚h/m3 (Tcat, wet)] τ, τ′, τ′′, τ′′′, τ′′′′, τ′′′′′ ) space time of the gas in the monolith, with different units, as indicated in Table 2 ψ h ) deactivation function, defined by eq 2, averaged over the entire monolith (h-1) η ) effectiveness factor
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Received for review October 15, 2003 Revised manuscript received February 25, 2004 Accepted March 1, 2004 IE0307683
