Ind. Eng. Chem. Res. 1998, 37, 4617-4624
4617
Biomass Gasification with Air in Fluidized Bed: Reforming of the Gas Composition with Commercial Steam Reforming Catalysts Jose´ Corella,* Alberto Orı´o, and Pilar Aznar Department of Chemical Engineering, University “Complutense” of Madrid, 28040 Madrid, Spain
Four commercial catalysts for steam reforming of higher hydrocarbons (naphthas) and three for steam reforming of light hydrocarbons are tested for hot gas clean up and upgrading in biomass gasification with air in fluidized bed. The catalysts used originate from four manufacturers: BASF AG, ICI-Katalco, Haldor Topsoe a/s, and United Catalysts Inc. The work is performed in a small pilot plant (1-2 kg of biomass fed/h) with three reactors in series: gasifier, guard bed of dolomite, and full flow catalytic bed. Samples of gas are taken before and after the catalytic bed at different times-on-stream. It is shown how the H2, CO, CO2, CH4 and steam contents in the flue gas change because of the catalytic bed approaching contents near to the ones corresponding to the equilibrium state. Variations in the heating value of the gas and gas yield as a result of the catalytic bed are also reported. Introduction Biomass is a renewable source of energy with an important future in some scenarios (Hall, 1997). Among the thermochemical methods used for biomass processing, gasification offers important advantages over pyrolysis and combustion. Practitioners in this field know very well how biomass gasification in a fluidized bed, bubbling or circulating, produces a raw gas which has to be cleaned for most of its applications. Compounds in such raw flue gas that are considered to be impurities and have to be removed are tars, particulates, NH3, HCN, .... Particulates can be removed with several types of filters, but tar can crack into the pores of such filters forming coke and plugging them. Thus, tar has to be removed beforehand from the raw gas and not only because of plugging of filters but also for many other reasons such as (i) it condenses in exit pipes and plugs them, (ii) it is very dangerous because of its carcinogenic character, (iii) it contains a lot of energy which can be transferred to the flue gas as H2, CO, CH4, ..., and (iv) most gas engines and turbines do not accept tar in the incoming gas. Tar is a complex lump whose sampling, analysis, and characterization has been standarized by tobacco companies around the world (smoking is a gasification process). Nevertheless, this standarization was not agreed until March 1998 by institutions working on gasification of “other types of biomass”. Each institution working around the world in this field has been using its own method of tar sampling and analysis. So, when speaking of tar each one can have had a different measure of it (Narva´ez et al., 1996; Oesch et al., 1996; Milne et al., 1997). Tar, in biomass gasification, has been thus a relative or subjective concept and lump but, to advance in the field, we must use this term. For most people working on gasification, catalytic tar removal (destruction, elimination, ...) is the best method for raw gas clean up. With a catalyst placed downstream of the gasifier, the raw gas can be not only cleaned but also upgraded. This is to say: its composi* To whom correspondence should be addressed. Fax: + 3491-394 4164. E-mail:
[email protected].
tion can be tailored to fulfill different end uses as recently has been proved (Aznar et al., 1997, 1998). There are, at least, two methods for catalytic hot gas cleaning and upgrading: using calcined dolomites (or related materials) and using steam reforming, nickelbased, catalysts. Using dolomites has been exhaustively studied, and it is out the scope of this paper, which is devoted only to the use of commercial steam reforming nickel-based catalysts. The state-of-the-art of using these catalysts for this application can be found in some recent papers (Narva´ez et al., 1997; Caballero et al., 1997; Aznar et al., 1998). Gas and tar composition at the gasifier exit depend to a great extent on the gasifying agent used in it. Catalytic activity for tar removal reactions, downstream of the gasifier, thus depends on the gas and tar composition (Corella et al., 1996a, 1996b) which, in turn, depend on the gasifying agent used, air or steam. Tar produced in biomass gasification with steam and steamO2 mixtures is, for instance, easier to destroy with calcined dolomites than the tar produced in biomass gasification with air, the other gasification and tar cracking parameters being the same in both cases (Orı´o et al., 1997). In Spain there are two groups performing a simultaneous and similar research in two different gasification set ups using different gasifying agents: At Saragossa University, nickel catalysts have been tested in gasification with steam and steam-O2 mixtures (Aznar et al., 1993, 1998; Caballero et al., 1997). At the University “Complutense” of Madrid such research is concentrated on gasification with air. A “standard” steam reforming catalyst from BASF AG, the G1-25S one, was thoroughly tested there (Narva´ez et al., 1997). Due to the interest for this application, some years ago a number of other promising commercial nickel catalysts started to be the subject of study in biomass gasification. This work is thus devoted to a study of the usefulness of these “new” commercial catalysts in biomass gasification (in fluidized bed) with air. This paper is concentrated on the characterization of the catalysts and on the reforming and upgrading of the gas composition.
10.1021/ie980254h CCC: $15.00 © 1998 American Chemical Society Published on Web 11/11/1998
4618 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
Figure 1. Small pilot plant for advanced biomass gasification in bubbling fluidized bed with full flow reactors for hot gas cleaning.
Experimental Section Facility Used. The tests were made in a new facility, expressly set up for this research. The facility is shown in Figure 1 and it is similar to (but not the same as) the one previously used and shown in the paper of Narva´ez et al. (1996). Some problems were detected in the previous facility during the 3 years of using it. To overcome such problems a new facility (the one shown in Figure 1) was designed and built. For example, some novelties of the new facility are given as follows: (i) a more advanced design for the screw feeders; (ii) a gasifier freeboard bigger than the one in the previous facility, to obtain higher residence times of the gas in order to crack a larger amount of tars; (iii) a higher overall gasifier allowing higher gas superficial velocities and (for the same equivalence ratio, ER) throughput of biomass, where the increased height of the overall gasifier (bed + freeboard) generated an exit raw gas with somewhat lesser tar contents than the ones published by Narva´ez et al. (1996); (iv) a new high efficiency cyclone after the bed of dolomite (Figure 1) to avoid fine particles going to the catalytic bed; (v) easier-to-handle sampling devices for condensates; (vi) an improved design of the catalytic reactor (shown in Figure 2). In the small pilot plant there are three different reactors: biomass gasifier, guard bed (with a calcined dolomite in it), which is in fact also a catalytic reactor, and a last reactor for the nickel catalyst. The gasifier is an atmospheric and bubbling fluidized bed continuously fed near its bed bottom. In the gasifier there is always a stationary bed of silica sand. The biomass flow rate is about 1 kg/h. The gasifying agent is air. Some main operation variables are shown in Table 1 for selected tests. After the biomass gasifier there is a ceramic filter which operates at 500-600 °C followed by a bed of a
Figure 2. Details of the catalytic reactor used.
calcined dolomite. As is well-known, such bed of dolomite eliminates 90-95 wt % of the tars present in the flue gas (Orı´o et al., 1997). From 2 to 20 gtars/m3(n.c.) in the raw gas at the gasifier exit, the bed of
Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4619 Table 1. Some Main Experimental Conditions and Results from the Catalytic Bed Downstream of the Gasifier and the Guard Bed experiment no.
25
26
35
38
45
mB, kg/h hs, wt% Qair,L, L (NTP)/min QH2O(v), L (NTP)/min ER1 T1,b, °C T2,c,av, °C type of dolomite in guard bed
0.55 21.0 11.0 2.4 0.33 800 870 Norte
0.48 23.0 11.0 2.3 0.39 810 850 Norte
0.74 32.0 11.0 4.9 0.28 710 830 Chilches
0.72 18.0 12.8 2.7 0.28 800 850 Ma´laga
catalyst used W3, g (H2O/C)3, mol/(at g) T3,c,av, °C dp3, mm umf,3, cm/s u3,o, cm/s u3,c, cm/s τ3, s τ′3, kg/(m3/h) SV3, h-1 Qgas,3, L (NTP)/min
ICI 57-3 100 5.19 740 -1.6 + 1.0 65 100 105 0.058 0.022 16600 21.4
UCI 100 2.37 800 -1.6 + 1.0 65 107 117 0.054 0.020 17100 22.4
R-67 80 6.07 800 -1.6 + 1.0 65 118 128 0.049 0.014 18800 24.6
Catalytic Reactor ICI 46-1 G1-50 80 170 2.55 1.11 800 800 -1.6 + 1.0 -1.6 + 1.0 65 65 126 134 135 150 0.05 0.08 0.014 0.026 20000 11300 25.9 28.8
Figure 3. Temperature profile in the axis of the bed of the catalytic reactor in an experiment (run no. 62, fixed bed) with longitudinal gradient of temperature.
dolomite cleans up to about 0.5-1.0 gtars/m3(n.c.) depending, of course, on the experimental conditions (Orı´o et al., 1997). Since in this work the bed of dolomite was smoothly fluidized some erosion and carry over of fine particles occurred. A high efficiency cyclone was thus placed after the bed of dolomite. It retained most of these particles. Then, at the end of the process, was placed the catalytic reactor whose study is the objective of this paper. Catalysts and Catalytic Reactor. The catalytic reactor is shown in detail in Figure 2. Its internal diameter is 60 mm, and it has an external electrical oven to maintain the temperature at the desired level in each experiment. The reactor has two thermocouples, as indicated in Figure 2, in the center and in the wall (inner side). These two thermocouples can be moved along the bed height to measure longitudinal profiles of temperature like those shown in Figure 3. The amount of catalyst used in each test was about 100 g, corresponding to a height of the bed (bulk fixed conditions) of 4-7 cm. Some operating conditions for this catalytic reactor are shown in Table 1. The shape and particle size of the catalysts used are important variables in this work. Due to the fact that there are not yet commercial monoliths for steam reforming applications, only ring-shaped catalysts (with one or several holes) were obtained for this study. These rings, in their commercial size, are too big to be used in our small reactor without a big channeling. On the
0.75 14.0 12.8 2.2 0.26 800 836 Ma´laga
50
55
58
59
60
0.59 15.8 12.8 1.9 0.34 775 845 Chilches
0.69 23.7 10.7 3.4 0.26 800 805 Ma´laga
0.79 18.7 11.2 3.1 0.23 805 850 Chilches
0.94 39.9 11.2 7.7 0.26 815 850 Inert
0.87 19.2 12.7 3.5 0.23 805 825 Norte
RKS-1 100 2.28 800 -1.6 + 1.0 65 120 124 0.05 0.018 18600 21.8
G1-50 100 2.14 820 -1.6 + 1.0 65 120 128 0.052 0.018 17200 22.1
G1-25/1 100 1.22 805 -1.6 + 1.0 65 132 140 0.044 0.016 20700 24.4
ICI 46-1 100 4.17 810 -1.6 + 1.0 65 145 154 0.040 0.015 22600 25.2
Inert 100 0.76 815 -1.6 + 1.0 50 147 151 0.030 0.015 29800 24.5
other hand, some time ago, using for this same application the BASF G1-25-S catalyst, it was found that internal diffusion starts to play an effect for catalyst particle diameters higher than about 0.2-0.3 mm (Narva´ez et al., 1997). These two reasons caused the authors to use the catalyst with a relatively small particle size (1.6-1.0 mm; see Table 1). This particle size has been the most frequently used in this work so far. There was another important reason for selecting and using this particle diameter: although the catalytic bed is relatively small (60 mm diameter and 50 mm height, by average), given the endothermicity of the tar and methane reforming reactions, there could be important radial and longitudinal temperature gradients (Figure 3) which would make the analysis of results more difficult. Orı´o et al. (1997) have recently shown how important these temperature gradients can be in fixed beds of calcined dolomites for tar removal from the product gas. To avoid big temperature gradients, it was decided to plan the gasification tests in such a way that the catalytic reactor would be somewhat fluidized. But all commercial steam reforming catalysts are designed and manufactured for fixed bed operations. These catalysts can lose the active material, nickel, if fluidized. These simultaneous and contradictory arguments caused the authors to carefully design each gasification test to work in the catalytic reactor under very weak and smooth fluidization (1 < u3,0/umf,3 < 3). Longitudinal temperature gradients were therefore not large, but the isothermicity was not fully obtained. u3,0 in this “full flow” catalytic reactor depends on the gasifier operation, and umf depends on the particle diameter of the catalyst and on the experimental conditions (temperature, pressure, and gas composition) in the catalytic bed (Delgado et al., 1991). To get a smooth fluidization in the small pilot plant used here, a particle diameter of 1.0 to 1.6 mm was selected and used in most of the experiments. This obliged us to crush the catalyst, which is of course not the best solution (in particular, for catalysts made by impregnation), but the authors had no other choice. The selected particle diameter (i) is thus small enough to obtain the activity of the catalyst under a moderate influence of the internal diffusion, (ii) avoids the gas channeling in the bed (which would occur in our reactor if commercial
4620 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 Table 2. Chemical Characterization of the Different Commercial Steam Reforming Catalysts Used in This Work (from the Catalogs Provided by Manufacturers) composition of catalyst, wt % BASF component
G1-25/1
NiO MgO CaO Al2O3 SiO2 MgAl2O4 K2O a
ICI G1-50
46-1 P
8 66
