Catalytic Hot Gas Cleaning with Monoliths in Biomass Gasification in

NH3 would generate NOx contents above the legally accepted limits; this is the reason why very often NH3 has to be removed from a gasification gas tha...
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Ind. Eng. Chem. Res. 2005, 44, 2036-2045

Catalytic Hot Gas Cleaning with Monoliths in Biomass Gasification in Fluidized Beds. 3. Their Effectiveness for Ammonia Elimination Jose´ Corella,* Jose´ M. Toledo, and Rita Padilla Chemical Engineering Department (Faculty of Chemistry), University Complutense of Madrid (UCM), 28040 Madrid, Spain

Biomasses with relatively high N contents generate a gasification gas with a NH3 content between 500 and 6000 ppm, with 2000 ppm of NH3 as the value that can be selected as a reference. This NH3 would generate NOx contents above the legally accepted limits; this is the reason why very often NH3 has to be removed from a gasification gas that also contains tar and particulates. The present paper only focuses on the performance of Ni-based monoliths for NH3 elimination from a realistic gasification gas coming from a bubbling-fluidized-bed biomass gasifier, at small pilot-plant scale. Besides using NH3 conversions, analysis of the results was also made using effective kinetic constants (keff) not only for NH3 but also for the simultaneous and competitive tar removal reaction. Correlations were found between the keff,NH3 and keff,tar values, included in effective Sherwood numbers, and the Reynolds and Schmidt numbers, the pitch of the channels of the monolith, and the temperature. The effect of the temperature is of the potential type, not Arrhenius, with an exponent of 2.75 ( 1.75, because of the control by the external mass transfer. keff,NH3 is also somewhat smaller (0.8 ( 0.3) than the equivalent one for tar elimination (keff,tar). Data on the deactivation of the monoliths, their causes, and their possible solutions are also provided. A final discussion on the poor performance of these monoliths in biomass gasification in a fluidized bed is also included in this paper. Introduction Gasification of solid fuels generates a useful fuel gas (a mixture of H2, CO, N2, CH4, CO2, H2O, light hydrocarbons, etc., ...) containing some impurities that have to be eliminated for its most advanced or promising applications. In biomass gasification in a fluidized bed, besides the most studied impurities such as tar and particulates, which are not within the scope of this paper, there are nitrogen compounds, of which the most significant species are NH3 and HCN. They have to be eliminated from the gasification gas to avoid the formation of NOx in downstream burners, gas engines, or gas turbines. Formation of the NH3 and HCN species has to be avoided, or if formed, they have to be eliminated from the gasification gas. Of the two species NH3 and HCN, NH3 is the most abundant and problematic, and this paper will therefore only address this species. The formation of NH3 (and HCN) has been widely studied in the gasification not only of coal1-4 but also of model compounds5 and biomass.6-13 Today the parameters that affect the NH3 (and HCN) content(s) in the gasification gas, such as the gasifier temperature, the equivalence ratio (ER), and the N content in the feedstock (biomass in this paper), are very well-known. Also well-known is how to produce a gasification gas with a relatively low tar content; some cheap additives, such as dolomites and limestones, are typically used in the bed of the gasifier. These additives, once calcined (OCa‚OMg and OCa, respectively), have shown to have some catalytic influence on the NH3 formation and on its in situ (in-bed) partial elimination.14-19 It is known, therefore, how the NH3 (and HCN) content(s) in the gasification-produced gas strongly depend(s) on the * To whom correspondence should be addressed. Fax:+3491-394 41 64. E-mail: [email protected].

operating conditions of the gasifier and of the type of biomass used. Biomasses with relatively high N contents generate a gasification gas with NH3 contents between 500 and 6000 ppm, with 2000 ppm of NH3 as the value that can be selected as a reference. These NH3 contents would generate NOx contents above legal limits; this is the reason why NH3 very often has to be removed from the gasification gas. NH3, tar, and other impurities in the gasification gas can be removed by wet processes, like scrubbing, which are out of the scope of this paper. Catalytic dry and hot gas cleaning methods are preferred because they really destroy NH3 and the tar instead of transferring them into a liquid phase, which is difficult to dispose of. Leaving catalytic tar elimination aside, which has been the subject of numerous studies, this paper will be devoted only to the catalytic elimination of NH3 in a biomass gasification gas. This process has already been studied by some authors. The most abundant data come from the University of Lund in Sweden.20-22 Using “a commercial methanation Ni-based catalyst in a (4 mm) pellet form”, they obtained 65-95% NH3 removal at 800-900 °C and 3 s of space time.20,21 Their gasification gas had 10 g of tar/mn3, and they did not observe carbon formation on the catalyst probably because of the fact that in their tests the steam/tar ratio was between 4 and 11,22 which is quite a high value for a typical biomass gasification process. Leppa¨lahti et al.13 also studied the catalytic elimination of NH3 from a gasification gas produced in a fluidized-bed gasifier and, using a space velocity of 3680 h-1, “a commercial nickel catalyst”, which seemed to be the Ni-0301 from Engelhard, “decomposed the ammonia almost completely (over 99%) at a high temperature (above 850 °C)”.13 The tar content in the gasification gas in these tests was 4.7-5.4 g/mn3. Two more references on the catalytic

10.1021/ie040244i CCC: $30.25 © 2005 American Chemical Society Published on Web 03/02/2005

Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 2037

elimination of NH3 in the gasification gas have also been found. One of them comes from TPS AB23 and briefly indicates how, with “a nickel catalyst”, the 1500 ppm of NH3 in their gasification gas is reduced to less than 100 ppm of NH3 at temperatures above 850 °C. The second reference is a U.S. Patent from General Electric Co.,24 who used the MCR2X high-temperature methanation catalyst from Haldor Topsøe A/S and demonstrated its capacity for NH3 decomposition. There is, therefore, some previous information on the catalytic NH3 elimination in the gasification gas. Nevertheless, all of the above-mentioned data were obtained with commercial Ni-based catalysts. These have a particulate shape (rings, extrudates, pellets, or spheres) and need a filter before the catalytic bed to eliminate the particulates that are always present in relatively high amounts in the gas coming from a fluidized-bed gasifier. Although there are recent studies25 to improve the performance of hot ceramic filters in biomass fluidized-bed gasification processes, the use of these hot ceramic filters is still problematic and can be avoided by using monolithic catalysts, honeycomb structure, that may operate under a gasification gas containing particulates. The performance of these monoliths for tar abatement in the gasification gas, part 1 of this series of papers, was recently published26 by the authors. Because of their novelty for this process and their complex problems, the modeling of these monolithic reactors, part 2 of this series of papers, was presented in another separate paper.27 Now, the present paper only focuses on the performance of these Ni-based monoliths for NH3 elimination from a gasification gas coming from a bubbling-fluidized-bed (BFB) biomass gasifier. Commercial (ring- and pellet-shaped) Ni-based steam reforming catalysts have also been tested for NH3 elimination under a realistic gasification gas generated in a small circulating-fluidized-bed (CFB) biomass gasifier, but results obtained with those catalysts will be presented elsewhere. This paper is devoted only to the performance of monoliths for NH3 elimination from a gasification gas obtained in an atmospheric fluidizedbed biomass gasifier. In all practical applications or industrial uses, the above-indicated NH3 catalytic removal has to be carried out simultaneously with the tar removal process. The active sites in the catalyst surface have to adsorb, by different mechanisms, the tar species and NH3 [besides H2O and CO2, which are the molecules that, once adsorbed and dissociated, destroy the tar species by steam and dry (CO2) reforming mechanisms, such as those shown in ref 28, for example]. So, tar and NH3 are competing against each other with respect to the active sites of the catalyst. There are two, at least, competitive reactions, as the following scheme shows: keff,NH

3

2NH3 (+ ?) 98 N2 + H2 + ... keff,tar

tar + H2O + CO2 98 H2 + CO + ...

(1)

A fuel gas with a high tar content not only would quickly deactivate the catalyst but would render the catalyst very little active for the simultaneous NH3 removal reaction. Tar would cover most of the catalyst surface. Basically, it has to be remembered that, to get a relatively high activity and life for the downstream catalyst, the tar content in the fuel gas has to be “as

low as possible”. Corella and co-workers fixed this limit to around 2 g of tar/Nm3.29 It is not easy to get very low tar contents in the fuel or gasification gas. It depends on the gasifier design and its operation parameters. It can be stated that, to get a good catalytic reactor, for tar and NH3 removal, the upstream gasifier has to be optimized in design, operation parameters, and feedstock composition. Because of this basic idea, a big effort was devoted not only to the downstream catalytic reactor but also to the upstream gasifier. This means that important modifications, improvements, or revampings had to be introduced periodically in the upstream gasifier. The positive final result was that the tar content in the gasification or fuel gas was (on average) progressively reduced during the research work, therefore increasing the activity and life of the monolith located downstream from the gasifier. Experimental Section Gasifier and Facility Used. The gasifier used in our experiments was described in detail elsewhere.26,30 There are three relevant aspects of the gasifier: (1) It is an atmospheric BFB of 15 cm diameter, at the bottom zone, and 5.2 m height. (2) It was continuously fed with biomass directly into the bed, near the gas distributor. (3) Besides the primary air, there was a second air flow in the upper part of the gasifier. The main experimental conditions in the gasifier were the temperatures in the bed and in the upper part (after the addition of the second air flow), the superficial gas (air) velocity at the inlet of the gasifier, and the type and flow rate of the feedstock. This flow rate was usually expressed as weight hourly space velocity (WHSV), defined as [kg of biomass fed/h‚kg of in-gasifier-bed material (S + D or S + O)]. Details of the experimental conditions are shown in the upper part of Table 1 in part 1 of this series of papers.26 After two in-series cyclones, the main flow was sent, without filtration, to the catalytic (monolithic) reactor. There is also a slip flow, and with valves and rotameters at the end of these two exit lines, the gas flow in each line can be varied from test to test. Hence, the gas flow to the monolith is made independent of the total gasification gas flow produced. The gas hourly space velocity (GHSV) in the monolith becomes, in this way, relatively independent of the upstream gasifier operating conditions. Feedstock. The standard or most often used feedstock to date by Corella and co-workers was pine wood chips, but this material has a very low content of nitrogen (0.20 wt %, on average), generating a fuel gas with only a few (1-10) ppm of NH3. NH3 conversion by the downstream catalytic reactor has a big margin of error under this circumstance; besides, this type of operation was of no interest for this research. After several trials to find an “appropriate” feedstock, it was decided to use a mixture of “orujillo”, a residue from olive oil production of which about 2 million tons are generated every year in Spain, and pine wood chips. A detailed characterization of the “orujillo” and pine wood used was shown in Table 3 in ref 26. The mixture of pine wood chips and “orujillo” was 50:50 (wt %) in most of the tests, but because “orujillo” has not only a high (1.1-1.5 wt %) N content but also a high K2O content (see Table 3 in ref 26), it was further modified to 60 wt % pine wood + 40 wt % “orujillo”, thus decreasing the total amount of K2O in the feedstock. The exact com-

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Table 1. In-Bed Material Used in the Gasifier, Partitioning of the Air in the Whole Plant, and H2O/C* Ratio in the Fuel Gas test no. N-78 silica sand + dolomite silica sand + Ni/olivine silica sand + raw olivine ERtotal air partitioning (%) primary secondary tertiary H2O/C* ratio (mol/at-g) monolithic reactor inlet monolithic reactor exit

x

N-79 x

N-80

N-81

N-83

In-Bed Material x x

N-85

x

x

Partitioning of the Air 0.39 0.40 0.48 0.43

0.40

0.30

0.34

90 10

80 10 10

80 5 15

80

90

20

1.3 1.8

0.6 1.2

2.2 3.6

0.6 0.8

N-84

x

N-86

N-87

N-88

N-89

x

x

0.59

0.44

0.44

N-90

N-91

x

x x

x

0.50

0.41

0.46

10

79 9 12

78 10 12

78 9.5 12.5

80 10 10

80 0 20

80 0 20

80 0 20

78 0 22

1.3 2.6

1.1 1.6

3.4 3.7

1.9 3.8

2.7 2.5

2.7 2.8

5.3 5.0

3.7 4.6

2.7 5.4

position of the feedstock in each test run was shown in Table 1 of ref 26. In the feedstock, together or mixed with the biomass, there was always some (2-4 wt %) dolomite or olivine to replace the one being lost by elutriation or carry over from the gasifier bed. In-Bed Material. To get a tar elimination as high as possible in the same gasifier, a second and cheap catalytic material, which can also be called additive, was always used and mixed with the silica sand. Calcined dolomite was used in the first tests, but because it is soft and generates an important amount of particulates in the gasification gas, olivine was used instead of dolomite in later tests. The composition of the in-bed material used in each test is shown in Table 1. Physicochemical properties of the olivine used were reported previously.30 It may be remembered that dolomite is somewhat more active for tar destruction than olivine30 but olivine generates fewer particulates in the fuel gas than dolomite, which has an effect on the monolith downstream from the gasifier. Monolithic Reactor. The monolithic reactor was shown in detail previously.26 This reactor had two very different zones: (1) the preheating and gas reheating zone; (2) the monolith itself. Preheating and reheating of the fuel gas was made in two ways: with a big (10 kW) external oven and by using a third (or reheating) air flow, which burns a part of the fuel gas, heating it up to 1050 °C if needed. This heated gas then enters directly into the monolith. The reheating or third air flow is introduced by a special nozzle to get a good mix between the two flows, thus avoiding zones without O2. The monolithic reactor had five thermocouples. Two were on the face of the monolith, at the axis and near the wall on the inner side. Two were just at the exit of the monolith, at the axis and near the wall. The fifth thermocouple entered from the top of the reactor and was movable to measure the axial profile of temperature from the third air-feeding point to the face or front of the monolith. The vessel for the monolith was designed to obtain an adiabatic operation in it. There were two different insulating zones: one inside the reactor and another one, 15 cm wide, outside the reactor. Despite this, subsequent measurements of the temperature and heat balances indicated that the reactor was finally not adiabatic. A relatively high percentage of the heat released in the process was being lost through the walls and by the big (70 × 70 × 8 cm) flange located at the bottom of the reactor, next to the exit of the monolith. For this reason, the temperature at the monolith exit

was always lower than that corresponding to an adiabatic operation. This was analyzed in detail in part 2 of this series of papers (see Figure 2 of ref 27) and was solved in a “second-generation” or more advanced monolithic reactor, with two layers of monoliths, which was manufactured recently and is being tested at the time that this paper was being written. Air Partitioning. Temperature Profiles in the Whole Plant. The total air fed to the plant (ERtotal was between 0.30 and 0.59) was divided into three different flows: primary air at the bottom of the bed, secondary air at the gasifier freeboard, and tertiary or reheating air in the monolithic reactor, 60 cm above the face of the monolith. This air partitioning was different in each test and is shown in Table 1. The intervals of these air flows or percentages with respect to the total air flow were as follows:

primary air secondary air tertiary air

78-100% of total air 0-10% of total air 0-20% of total air

The partitioning of the air determines the temperature profile along the whole plant, as indicated in ref 26. This profile of temperature plays an important role in the tar content at the gasifier exit and in the softening of the ash in the fuel gas just before the monolith. These two important parameters, the tar content at the gasifier exit and ash softening, can and have to be controlled to some extent then by the air partitioning. H2O/C* in the Fuel Gas. H2O/C* is another important variable29 and may be modified in two ways: (1) by varying the moisture of the biomass and (2) by adding some H2O at the entrance of the monolithic reactor (as shown in part 1 of this series of papers26). H2O/C* values at the inlet and exit of the monolithic reactor are shown in Table 1 for each test. As a summary, H2O/C* at the monolith inlet ranged from 0.4 to 5.4. Monolith Used. Several monoliths were initially tested, but the only one that provided results that were good enough was the one supplied by BASF AG and coded V1693-BV0170. Results obtained with this Nibased monolith will be the only ones shown and analyzed in this paper. The monolith located in the catalytic reactor always had its full size (15 × 15 × 30 cm). Except in tests N-79 and N-81, each monolith was new (one different monolith) for each test run. Usually all channels of the

Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 2039 Table 2. NH3 Conversions in Test N-89a sample

first

second

average

time on stream (h) gasifier temperature (°C) bed top (after second air addition point) catalytic reactor temperature (°C) after third air add. point monolith face, axis monolith outlet, axis NH3 content (ppmv) after gasifier, inlet monolithic reactor monolith exit NH3 conversion (%) k′eff,NH3 [s-1] [rc] k′eff,NH3 [s-1] [nc, wet] keff,NH3 [m3(rc)/kgcat‚h] ln keff,NH3 [m3(rc)/kgcat‚h]

2.53-2.78

3.46-3.62

804 866

839 903

822 885

988 829 660

990 843 678

989 836 669

2630

2345

2488

1400 46.8 1.8 0.58 10 2.3

1060 54.8 2.2 0.73 13 2.6

1230 50.6 2.0 0.64 11 2.4

a ER total ) 0.50; ERbottom (80% ERtotal) ) 0.40; ERsecond air (0% ERtotal) ) 0. ERthird air (20% ERtotal) ) 0.10. GHSV ) 3290 [m3(nc, wet/h‚mcat.3].

monolith were open and used in each test except in later tests, in which some peripheral rows (2-4) of channels were blocked with a mask and with small (100-500 µm) particles of SiC. In this way, the face velocity of the gas (uf) and the Reynolds number in the remaining open channels were increased. The main experimental conditions in the monolithic reactor were already given in Table 2 of part 1 of this series of papers.26 These conditions include the temperatures just at the inlet and exit of the monolith, gas face velocity at the monolith inlet, gas flow rates, space times (τ), space velocities (SV), and area velocities (AV). These very important parameters were shown in detail in ref 26 with different units and for different uses. Sampling and Analysis. For measurement of the NH3 content in the gasification gas, sampling was made before and after the monolithic reactor, as indicated in ref 26. Sampling of the gasification gas was made under the well-known and standardized method already used and described, for example, in refs 12, 31, and 32. This sampling was made at different times on stream. That is to say, several (two to four) samples were taken before and after the monolith in each test. To give just one example, Table 2 shows the NH3 contents in the gasification gas before and after the catalytic reactor in experiment N-89. Operation Limits. This research on NH3 elimination was carried out simultaneously, in the same gasification tests, as the one on tar elimination described in part 1 of this series of papers.26 The operation limits both for the upstream gasifier and for the downstream monolithic reactor were therefore the same as those published previously.26

NH3 Conversions. NH3 conversions (XNH3) by the monolithic reactor, averaged with respect to the time on stream of the monolith, are shown for each test run in Table 3. These NH3 conversions for the Ni-containing BASF monolith ranged from 44 to 96.5 vol %. Apparent Kinetic Constants for the NH3 Elimination Reaction. NH3 conversion is not an index good enough to compare results because it depends, for example, on the space time and temperatures in the monolith. For this reason, to use kinetic constants is a more rational way to compare the results. To calculate and correctly use these kinetic constants, a model was developed for the monolithic reactor used here. That model was previously published in ref 27 and considers two different zones existing in the monolithic reactor: the gas reheating zone (zone 1), in which the gasification gas is reheated by means of a third air flow, which burns a part of the NH3 and of the tar, H2, CO, CH4, etc., present in the fuel gas, and a second zone or the monolith itself. The model for the whole monolithic reactor also included a microkinetic model for the overall NH3 elimination, which is based on a first-order reaction with respect to NH3:

(-rNH3)overall ) keff,NH3CNH3

(2)

in which the overall, observable or effective kinetic constant for NH3 elimination, keff,NH3, includes several and different contributions:

keff,NH3 ) kzone 1 + kNi cat. + kth,monolith + knot impregnated wall of the monolith + kcoke on the monolith (3) The model for the whole monolithic reactor includes mass and heat balances,27 besides microkinetic models for the reactions of the overall NH3 and tar eliminations. With this model, the effective kinetic constant for NH3, and also for the tar, elimination may be calculated for each sampling at the inlet and exit of the monolithic reactor. Two different approaches were used to calculate and handle keff,NH3. The first approach was based on using an averaged value of keff,NH3 (k h eff,NH3) with respect to the interval of temperatures in the monolith (from Tinlet to Texit). The second and more accurate approach took into account the important axial profiles of temperature in the monolith and used a keff,NH3 value for a given (900 °C) temperature of reference. The kinetic constant derived in this way will be called keff,NH3,900°C, k h eff,NH3, and keff,NH3,900°C values for all of the test runs and with different units are shown at the bottom of Table 3. These k values are also shown in Figure 1, under Arrhenius coordinates.

Results Gas composition, lowest heating value (LHV) of the gas, and tar content in the gasification gas before and after the monolithic reactor have already been discussed in part 1 of this series of papers (see Tables 4 and 5 of ref 26). We will now focus on the NH3 contents. These NH3 contents in the gasification gas before and after the monolith are shown in Table 3. Values shown in this table are the averaged ones with respect to the time on stream, that is to say, the average of the two to four different samplings and analyses made during the averaged 3-6 h of each test run under steady-state conditions with the start-up and shutdown periods excluded.

Analysis of the Data Comparison of Results. A comparison of the keff,NH3 values obtained in this work was made in two different ways: (1) In the same way as was made in part 1 of this series of papers26 for the keff,tar values obtained with monoliths, the k h eff,NH3 and keff,NH3,900°C values are compared in Figure 1 with existing values for catalytic tar abatement obtained with commercial (rings) Ni-based catalysts and also for dolomites. From this comparison, it may be concluded that the monoliths show an activity for NH3 abatement, under the experimental conditions used in this work, similar to that of cheap dolomites

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Table 3. Main Experimental Results: NH3 Contents in the Gasification Gas at the Inlet and Exit of the Monolithic Reactor, NH3 Conversions, and Effective Kinetic Constants for NH3 Removala test no. ERgasifier [first and second air flows] CNH3 at the inlet (averaged) ERthird air CNH3 at the exit (averaged) NH3 conversion k′eff,NH3 [Q(Tmon,wet)] k′eff,NH3 [Q(nc,wet)] k′′eff,NH3 keff,NH3,900°C

N-78

N-79

N-80

N-81

N-83

N-84

N-85

N-86

N-87

N-88

N-89b

N-90b

N-91c

0.40

0.27

0.29

0.31

0.36

0.42

0.37

0.52

0.40

0.35

0.40

0.33

0.36

ppmv

1300

3350

4780

2225

1770

425

200

2490

660

915

ppmv

0 560

0.030 2180

0.052 1340

0.077 1300

0.040 600

0.058 15

0.051 10

0.10 1390

0.082 230

0.10 410

57.0 2.4 0.59 0.13 13.0

35.0 1.1 0.27 0.06 6.10

72.0 3.6 0.86 0.18 19.6

41.6 1.3 0.31 0.07 7.20

66.1 2.2 0.46 0.10 12.0

96.5 11.6 2.5 0.57 62.5

95.0 3.9 0.89 0.19 20.9

44.0 4.3 1.4 0.25 23.0

65.2 5.3 1.5 0.29 28.4

55.2 4.9 1.4 0.23 26.7

% s-1 (rc) s-1 (nc) cm/s m3(Tcat.,wet)/ kg‚h)

1975 0.074

0.045 530 73.0 4.0 1.0 0.22 21.6

0.089

a V 3 3 2 b 3 2 c 3 mon ) 0.006 75 m ; Vused ) 0.005 63 m ; Smon used ) 2.65 m . Vused ) 0.003 63 m ; Smon used ) 1.81 m . Vused ) 0.002 98 m ; Smon used ) 1.44 m2.

Figure 2. Activity for NH3 removal of the BASF monolith for different face gas velocities.

Figure 1. Comparison (under Arrhenius representation) of catalyst activities for NH3 elimination (indicated here with O points) with some preexisting or former data for tar elimination with Ni-based catalysts and with dolomites (indicated here with lines and with other symbols).

for tar abatement. The reason for this low activity of the monoliths in this research is only because its first half is active or useful. When this fact, based on reality, is taken into account, the keff,NH3,900°C values (also presented in Figure 1) provide a more accurate comparison. (2) The keff,NH3,900°C values calculated from the model presented in ref 27 are given in the last line of Table 3. For the BASF monolith, these values can be summarized as follows:

Not deactivated (N-84)

62.5 [m3(rc)/kg‚h]

velocity (uf) of the gas in Figure 2. It can also be seen how this effective kinetic constant increases with uf. To correlate the overall monolith catalyst activity, some authors, for example, Uberoi and Pereira,33 Votruba et al.,37 and Ising,38 suggested using relationships between the Sherwood number (Sh) and (dH/L)RenScn′, with dH being the hydraulic diameter of the channels of the monolith, L the length of the monolith, and Re and Sc the Reynolds and Schmidt numbers, fully defined in part 2 of this series of papers.27 Because the overall study on gas cleaning was made to further predict not only NH3 conversions but also tar conversions, two different Sherwood numbers were used:

Sheff,NH3 ) βdH/DNH3-m ) keff,NH3dH/AvDNH3-m ) keff,NH3dH2/4DNH3-m (4) Sheff,tar ) βdH/Dnaph-m ) keff,tardH/AvDnaph-m ) keff,tardH2/4Dnaph-m (5)

Partially deactivated (+zone 1) 12-37 [m3(rc)/kg‚h] Not-impregnated + zone 1 (N-85) 20.9 [m3(rc)/kg‚h] Fully deactivated + zone 1 (N-79 and N-81) 6.1-7.2 [m3(rc)/kg‚h] Correlation of the keff,NH3 (and also keff,tar) Values. The keff,NH3,900°C values are correlated with the face

These Sherwood numbers correlate with (dH/L)Re(ScNH3) and (dH/L)Re(Scnaph) numbers as shown in Figure 3. After a careful analysis of the data, the following two correlations are proposed:

Sheff,NH3 ) CT,NH3[(dH/L)Re(ScNH3)]0.5(0.5 and

(6)

Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 2041

Sheff,tar ) CT,tar[(dH/L)Re(Scnaph)]0.5(0.5

In these correlations, there is some noticeable error in the exponents of the Re and Sc numbers. This is attributed to the important nonisothermicity of the monolith, which introduced some uncertainty in the analysis of the data. The CT,NH3 and CT,tar values were calculated for each test run. They were further correlated with the temperatures in the monolith in each test. Although there are not many test runs and these correlations have an important error, it may be proposed that

CT,i ) Bi(T/1000)2(1

(8)

where T is the temperature in the monolith, and

BNH3 ) 0.13 ( 0.03

(10)

for tar elimination. With eqs 6-10, it is concluded that for NH3 elimination with the Ni-based BASF monolith

4DNH3-m

( ) [( ) 2(1

dH Re(ScNH3) L

]

keff,tardH2 ) Sheff,tar ) 4Dnaph-m

( ) [( ) T 1000

(11)

2(1

dH Re(Scnaph) L

]

0.5

(12)

These two correlations are two major conclusions of this paper. Once uf and Tinlet of the monolith are fixed for a new situation and for gas composition and with dH and L of the monolith known, the Sherwood values can be calculated. From these, the keff,NH3 and keff,tar values, at the said Tinlet value, are further calculated. With these values, for a given value of the space time (τ), the NH3 and tar conversions at the monolith exit are easily calculated by the equations given in part 2 of this series of papers.27 If, on the other hand, XNH3 and Xtar are fixed, the required space time is easily calculated by using equations of the model shown in part 2 of this series of papers.27 Concluding Remarks on the Activity of the Monolith Equations 11 and 12 generate or provide two interesting conclusions: (1) Dividing eq 11 by eq 12 results in

Sheff,NH3 Sheff,tar

≡

keff,NH3Dtar-m keff,tarDNH3-m

from which is derived

which is an important conclusion of this work. (2) Using eq 11 at two different temperatures, T and 900 °C (1173 K) as the temperature of reference, it can be deduced that

( )( ) ( )( )

(keff,NH3)T

T ) keff,NH3,900°C 1173

2(1

ScNH3,T

0.5(0.5

ScNH3,1173

T 1173

DNH3-m,T

DNH3-m,1173

DNH3-m,T

2(1

≈

0.5(0.5

DNH3-m,1173

keff,NH3,T keff,NH3,900°C

)

T T (1173 ) (1173 ) 2(1

0.75(0.75

(16)

)

T (1173 )

keff,NH3,T ) keff,NH3,900°C

0.5

and for tar elimination

0.25

(15)

2.75(1.75

(17)

from which

)

T 0.13 1000

keff,NH3 ) (0.8 ( 0.3)keff,tar

and because DNH3-m ∝ T3/2,34 eq 16 becomes

Btar ) 0.25 ( 0.10

Sheff,NH3 )

(14)

That is, taking into account the interval of error in some calculations, it may be concluded that for these monoliths

(9)

for NH3 elimination, and

keff,NH3dH2

keff,NH3/keff,tar ≈ 0.8 ( 0.3

(7)

)

( )

0.13 ScNH3 0.25 Scnaph

0.5

(13)

T (1173 )

2.75(1.75

(18)

which is another important conclusion of this paper that was already used in parts 1 and 2 of this series of papers.26,27 This indicates how the overall NH3 elimination process is controlled by the external mass transfer in the channels of the monolith, which is a well-known fact for most of the processes carried out with monoliths.33 Data on the Deactivation of the Monolith. After each test, the monolithic reactor was opened and the “big square-cross section capsule” containing the monolith was carefully taken out of the reactor from its bottom part. The front or face of the monolith was analyzed in the following ways: (i) The samples were visually observed, with photographic records. (ii) Samples of the carbonaceous deposit on the monolith face were sent for analysis for the potassium content at the UCM’s Center for Atomic Spectroscopy. (iii) Some physical analysis were made to these deposits such as its stickiness character (determined by hand) and its elimination by different frontal air blows. Quantitative data on this matter are not yet available, and these studies are being continued at UCM. Soon there will hopefully be quantitative data on the blowing off of these carbonaceous deposits. After the analyses of the front of the monolith, the monolith was taken out of its “capsule” and its bottom part or exit side was also inspected and photographed. The exit sides or exit faces of the monoliths always appeared very different from their front sides. The two cokes or deposits appearing in each face of the monolith were different. At the exit, there were filaments of a whisker-type coke (a photograph of this was shown in part 1 of this series of papers26) and its formation was attributed to the relatively low (below 820 °C) temperatures at the exit of the monolith.27

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Figure 3. Correlation of the Sherwood numbers for NH3 and tar elimination based on the results from the BASF monolith.

From the analysis on the appearance of the monolith at the end of the test runs, it was concluded that there were the following two main, at least, causes of deactivation: (1) Deposits at the monolith front that very often were not capable of being eliminated by frontal air blows. The mechanism of the formation of these deposits is attributed to the fact that when the ash particles in the produced gasification gas impinge on the front of the monolith, a fraction of these ashes adhere to and/or stick to the front of the monolith. Although quantitative data can still not be provided, it can be said that the stickiness (determined by hand) of the deposits on the front of the monolith depends on the K2O content, and probably also on the Na2O content, of the ash, which, in turn, depends on the alkali content of the feedstock. As can be seen in Table 3 in part 1 of this series of papers,26 the ash from the “orujillo” used in the feedstock had a 27-36 wt % content in K2O, which is a very high value. When the “orujillo” content in the feedstock was increased, the deposits on the front of the monolith at the end of the test run were more sticky. That is to say, there is a stickiness-K2O content correlation in the feedstock. This correlation depends on the in-bed material used in the gasifier and on the profile of the temperature in the gasification plant: bed and freeboard of the gasifier and temperatures at the inlet of the monolithic reactor.

To solve the problem of the stickiness of the ash, three solutions are suggested: (i) to avoid the use of feedstocks with a high alkali content when there are monoliths downstream from the gasifier; (ii) to develop other types of monoliths, instead of Ni-based, that can operate at temperatures lower than the ones (ca. 900 °C) required by the Ni-based monoliths; (iii) to use some in-bed K2Ocapturing material. The K2O content in the gasification raw gas would then decrease, and the stickiness of the ash on the monolith front would decrease. This remains to be studied in further experiments in order to develop this process, before further commercial application of these monoliths to fluidized-bed biomass gasifiers can be made. (2) The whisker-type (filamentous) coke formation at the exit of the monolith. This is a well studied cause of deactivation, and it appears when Ni-based steam reforming catalysts operate at temperatures below approximately 750 °C. This cause of deactivation has to be eliminated by one of the following solutions: (i) to use other types of monoliths, not Ni-based or (ii) to work in the monolith exit zone always at temperatures above 750 °C. As demonstrated in part 2 of this series of papers,27 this may be achieved by using two layers of monoliths, instead of one as in this case, with some reheating of the gasification gas by means of a fourth air flow. This latest solution is already being used at

Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 2043

UCM with positive results and will be demonstrated in a future paper. Concerning the deactivation of the monolith, another fact was observed: its selective deactivation. As reviewed recently by one of the present authors,39 the deactivation of a catalyst may affect in a different way each one of the reactions involved in a complex network, as is the case of the reactions involved in the catalytic cleaning of the gasification gas. When the variation with time on stream (t) of the activity of the monolith for the two main reactions involved in the reaction scheme shown by eq 1 is analyzed carefully, it is seen how the deactivation of the monolith somewhat affects more the tar elimination reaction than the simultaneous NH3 elimination. Data obtained about this selective deactivation, in this work, are not very precise because only three samplings for tar and three for NH3 were typically made in each experiment. These deactivation data are therefore not very accurate, and that is the reason why they are not presented here. It can, however, be said that the NH3 conversion by the monolith does not decrease with t as fast as the tar elimination does, as shown in part 1 of this series of papers.26 In other words, the deactivation function ψ h , averaged over the entire monolith and defined in part 1 of this series of papers,26 for the NH3 elimination reaction, ψ h NH3, is smaller than the corresponding one, ψ h tar, for the tar elimination reaction. Final Conclusions. The Ni-based monoliths used in this work, the results of which are shown not only in this paper (part 3) but also in parts 1 and 2 of this series of papers,26,27 have proved not to have a performance very good for tar and NH3 elimination from the gasification gas. Monoliths were shown to have an activity, under the experimental conditions used in this work, similar, that is to say, not too high, to that of the cheap and competing dolomites for tar abatement. This not very good performance is attributed to several different types of factors: one is inherent of these monoliths; others may have a solution. The first main and inherent problem of these monoliths is the following: given that they have to work with a high content of particulates in the gasification gas, they have to have a big pitch, big size of channels, which results in the external mass-transfer process in these channel controls. This significantly lowers the overall reaction rates for the tar and NH3 elimination reactions. This problem, which is due to the big pitch of these monoliths, has a difficult solution, if any. The research carried out has proved how the not very good performance shown by these monoliths was also due to the experimental conditions used, which were not the optimal ones and which can and must be improved for future applications at bigger and commercial scales. Some experimental conditions that were not good and have to be solved in further developments are as follows: (i) The monolithic reactor did not have an optimal design. A high percentage of the heat released in the process was lost through the walls and flange, obtaining a too high ∆T across the monolith. At a bigger scale of work, the percentage of heat lost is smaller, decreasing therefore ∆T, with beneficial effects, (ii) Feedstock used had a too high K2O content. (iii) In-bed material was not the optimal one. It generated a too high particulates content in the gasification gas.

(iv) A further improvement or revamping of the gasifier, increasing its height, would decrease both the particulates and the tar content in the gasification gas. These possible improvements in the process would make much better the performance of the monoliths. The main problem of the monoliths that remains to be solved is their deactivation. From the experience gained in this research, it is believed that the life of the monoliths can be increased with the following solutions: (1) To avoid the stickiness of the ash particles in the front of the monoliths, the alkali content, mainly the K2O content in the feedstock, must be known and always kept low. Some types of biomasses have to be avoided in the gasifier, or they have to be diluted with other easier feedstocks, of low K2O content, such as pine wood chips. (2) The stickiness of the ash particles in the front of the monolith can also be reduced by lowering the operation temperature at the front of the monolith. It would imply developing and using another type of monolith, not Ni-based. (3) The deactivation by the filamentous whisker-type coke at the exit zone of the monolith must be solved, as mentioned above, by increasing the temperature in that zone. It can be obtained by using two layers of monoliths and some reheating between these two layers. An alternative would be to use another type of catalytic material that is capable of working at temperatures of 600-700 °C, for example, without the formation of this type of coke. Some research has still to be done on the deactivation of the monolith in long-term operation. The use of monoliths cannot be discarded as effective gas cleaning devices in biomass gasification, but this work has proved that both the monolith itself and the design of the monolithic reactors have to and can be improved for industrial application of this gas cleaning technology in biomass gasification. This paper, together with parts 1 and 2 of this series of papers, has identified some weak points of this technology that remain to be solved in the future. Acknowledgment The UCM team expresses their gratitude to the monolith manufacturer BASF AG by providing samples of their monoliths. Scientific discussions with Dr. Markus Ising from Fraunhofer UMSICHT in Oberhausen, Germany, and with Dr. Pekka Simell from VTT (Finland) helped to analyze the results. Alicia Laguna, Eduardo Torcal, and Jorge Ruiz-Peinado, students of chemical reactors (Chemical Engineering Department) at UCM, helped in the mathematical calculations. The staff at UCM’s Center of Atomic Spectrometry is acknowledged its their chemical analysis of the ashes. This work was carried out under the Ammonia Removal Project ERK5CT99-0020 of the EC, DG Research, Directorate J. The authors thank the European Commission for its financial support. Nomenclature AV ) area velocity in the monolith, defined as Q (nc, wet)/ Smon used (m/h) B ) coefficient in the CT vs T relationship, eq 9 BFB ) bubbling fluidized bed CNH3 ) concentration of NH3 in the fuel gas [mgNH3/m3(nc)]

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C* ) amount of atoms of carbon in hydrocarbons of different species which that react with the steam in the fuel gas (at‚g/h) CT,i ) fitting parameter in eqs 6 and 7, dependent on the temperature and on the i (NH3 or tar) species, dimensionless Di-m, Dnaph-m ) effective coefficients of diffusion of the ith species and of the naphthalene, respectively, in the gasification gas whose standard composition was shown in Table 2 of ref 27 (cm2/s) D ) calcined dolomite dH ) hydraulic diameter of the channels of the monolith (mm) ERtotal ) total equivalence ratio, defined as the total airto-fuel ratio used in the gasifier divided by the air-tofuel ratio for the stoichiometric combustion GHSV ) gas hourly space velocity in the monolith, [m3(nc)/h‚mcat.3] (h-1) kth, kcat. ) kinetic constants in eq 3 for the NH3 elimination by thermal and catalytic reactions, respectively [(m3(Tcat.,wet)/kgcat.‚h] keff,tar ) effective, overall or apparent kinetic constant for the tar elimination [(m3(Tcat.,wet or rc)/kgcat.‚h] keff,tar,900°C ) effective, overall or apparent kinetic constant for the tar elimination at 900 °C [(m3(Tcat.,wet or rc)/kgcat.‚ h] keff,NH3 effective, overall or apparent kinetic constant for the tar elimination, defined by eq 2 [(m3(Tcat.,wet or rc)/ kgcat.‚h] keff,NH3,900°C ) effective, overall or apparent kinetic constant for the tar elimination at 900 °C [(m3(Tcat.,wet or rc)/kgcat.‚ h] k′eff,NH3, k′′eff,NH3 ) effective, overall or apparent kinetic constant for the tar elimination at 900 °C, with units of s-1 and cm/s, respectively k h eff,NH3 ) averaged value of the kinetic constant for the whole (from inlet to exit) monolith, [(m3(Tcat.,wet)/kgcat.‚ h] L ) length of the monolith (mm) LHV ) lowest heating value of the fuel gas [MJ/m3(nc)] n, n′ ) exponents of the Re and Sc numbers nc ) “normal conditions” (273 K and 1 atm or 1.013 × 105 Pa) for the gas O ) olivine Q ) gas flow rate at the exit of the monolithic reactor [m3(nc, wet)/h] rNH3 ) reaction rate of NH3 elimination; according to eq 2, its units are (mgNH3/kgcat.‚h)[m3(Tcat.,wet)/m3(nc)] rc ) reactor conditions (at the face of the monolith) S ) silica sand SV ) space velocity of the monolith, different units (see Table 3 in ref 26) Smon used ) surface of the channels of the monolith open or available to the gas flow (m2) Scnaph ) Schmidt number for the naphthalene in the gasification gas, defined in ref 27 Sheff ) Sherwood number based on the effective or apparent kinetic constant, defined by eqs 4 or 5 T ) temperature (K) Tinlet, Texit ) temperature at the inlet and exit of the monolith (K) ∆T ) difference of temperature between the inlet and the exit of the monolith (K) t ) time on stream of the monolith (h) u0 ) superficial gas (air) velocity at the inlet (bottom) of the gasifier (cm/s) uf ) gas velocity at the face of the monolith (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 gasifier [kgbiomass/kgin-bed gasifier material‚h] XNH3 ) conversion of the ammonia Xtar ) conversion of tar Greek Symbols τ ) space time of the gas in the monolith, defined as W/Q [kg‚h/m3(Tcat.,wet)] ψ h ) deactivation function, as defined in ref 26, averaged for the whole monolith (h-1) η ) effectiveness factor

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Received for review September 16, 2004 Revised manuscript received January 14, 2005 Accepted January 14, 2005 IE040244I