Catalytic Decomposition of Biomass Tars with Dolomites - Energy

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Energy & Fuels 2009, 23, 2264–2272

Catalytic Decomposition of Biomass Tars with Dolomites Elizabeth Gusta,† Ajay K. Dalai,*,† Md. Azhar Uddin,‡ and Eiji Sasaoka‡ Department of Chemical Engineering, UniVersity of Saskatchewan, 57 Campus DriVe, Saskatoon, Saskatchewan S7N 5A9, Canada, and Department of EnVironmental Chemistry and Materials, Faculty of EnVironmental Science and Technology, Okayama UniVersity, 3-1-1 Tsushima Naka, Okayama 700-8530, Japan ReceiVed NoVember 14, 2008. ReVised Manuscript ReceiVed January 7, 2009

Catalytic gasification of wood biomass was carried out using a double-bed microreactor in a two-stage process. Temperature-programmed steam gasification of biomass was performed in the first bed at 200-850 °C. Following in series was isothermal catalytic decomposition and gasification of volatile compounds (including tars) in the second bed containing various dolomites. Dolomites from Canada, Australia, and Japan were examined for their effects on tar conversion and the overall gaseous product. A total of 74% of biomass carbon was emitted as volatile matter during tar gasification (200-500 °C biomass bed temperature). Dolomites improved tar conversion to gaseous products by an average of 21% over noncatalytic results at a 750 °C isothermal catalyst bed temperature using 1.6 cm3 dolomite/g of biomass. The iron content in dolomite was found to promote tar conversion and the water-gas shift reaction, but the effectiveness reached a plateau at 0.9 wt % Fe in Canadian dolomites. The maximum tar conversion of 66% was achieved at 750 °C using a Canadian dolomite with 0.9 wt % Fe (1.6 cm3/g of biomass). Carbon conversion to gaseous products increased to 97% using 3.2 cm3 dolomite/g of biomass at the same temperature. The dolomite seemed stable after 15 h of cyclic use at 800 °C.

1. Introduction Biofuel technologies are gaining much interest in light of concerns over climate change and record high prices of fossil fuels. Biomass is predominantly composed of carbon, oxygen, and hydrogen in the forms of cellulose, hemicellulose, and lignin. Fuels produced from biomass are considered CO2-neutral and renewable. Using biomass sources, such as forestry and agricultural wastes, is an ethical solution the food versus fuel debate, but they require a robust conversion technology for efficient and economical fuel production. Gasification technology is capable of processing whole biomass into syngas (H2 and CO), which can then be further converted to biofuels via gas-to-liquid conversion technology. During the overall process of gasification, a particle of biomass undergoes drying, pyrolysis, and subsequent gasification of the remaining char. Between 25 and 120 °C, the biomass is dried. From ∼200 to 500 °C, biomass transforms into volatile matter and charcoal, via pyrolysis. The condensable organic fraction of the volatile matter is referred to as tar. Gasification occurs in a controlled oxidizing atmosphere, such as H2O or CO2 at higher temperatures, typically ∼700-1300 °C.1 The char reacts with an oxidizer to produce CO, H2, CO2, and ash. The effects of temperature, residence time, and pressure on product compositions were examined during steam gasification of biomass in a fixed-bed reactor in the temperature range of * To whom correspondence should be addressed. Telephone: +1-306966-4771. Fax: +1-306-966-4777. E-mail: [email protected]. † University of Saskatchewan. ‡ Okayama University. (1) Mermoud, F.; Salvador, S.; Van de Steene, L.; Golfier, F. Influence of the pyrolysis heating rate on the steam gasification rate of large wood char particles. Fuel 2006, 85, 1437–1482.

500-750 °C.2 Increasing temperatures and residence times resulted in increased CH4 formation. The maximum H2 production (18 mol %) was observed at 750 °C. At temperatures above 650 °C, the steam gasification reaction proceeded quickly and generated increasing amounts of hydrocarbons. Increased pressure inhibited the gasification process. Tar in the product gas is undesirable because it leads to many problems in the process equipment and rapid deactivation of catalysts downstream. The tar content of biomass can be as high as 80 wt %,3 and its composition and quantity are dependent upon a variety of factors, including the composition of the biomass and the process conditions. Managing tar formation may be achieved using optimized process conditions and catalysis. Different feedstocks were compared during noncatalytic air-steam gasification in a downdraft fixed-bed reactor at 900 °C.4 Individual biomass components cellulose and lignin were gasified, as well as whole biomass samples of Japanese oak and red pine bark. It was shown that higher lignin content increases difficulty in processing, resulting in higher tar production, lower conversion to gaseous product, and lower CO production. Catalysts for tar conversion during biomass gasification have been under rigorous investigation since the 1980s. Devi et al. (2) Klass, D., Ed. Biomass as a non-fossil fuel source: Based on a symposium sponsored by the Division of Petroleum Chemistry at the ACS/ CSJ Chemical Congress. Presented at the 177th ACS National Meeting, Honolulu, Hawaii, April 2, 1979, ACS Symposium Series, Washington, D.C., 1981. (3) Quaak, P.; Knoef, H.; Stassen, H. Energy from biomass: A review of combustion and gasification technologies. World Bank Technical Paper 422, Energy Series, 1999. (4) Hanaoka, T.; Inoue, S.; Uno, S.; Ogi, T.; Minowa, T. Effect of woody biomass components on air-steam gasification. Biomass Bioenergy 2005, 28, 69–76.

10.1021/ef8009958 CCC: $40.75  2009 American Chemical Society Published on Web 02/20/2009

Decomposition of Biomass Tars with Dolomites

and Sutton et al. have written extensive reviews on the various catalysts used for gasification, including natural catalysts, alkali metal catalysts, and nickel-based catalysts. Nickel is effective for steam reforming of methane but is expensive and experiences rapid deactivation when reforming hydrocarbons larger than CH4.5,6 Nickel catalysts must be employed in a downstream reactor; as an additive in a fluidized-bed gasifier, nickel experiences rapid deactivation.7 Earth alkaline metals (Mg and Ca) are frequently used in Ni/ Al2O3 catalyst formulations to improve stability. The effect of Mg and Ca as additives in Ni/Al2O3 is attributed to the increase of the steam-carbon reaction and to the neutralization of the acidity of the support, suppressing cracking and polymerization reactions.8 Calcined dolomites and limestone are frequently used for tar elimination prior to final gas cleaning by supported nickel catalysts.9 Dolomite, CaO · MgO(CO3)2, is well-known as an active catalyst for tar conversion when it is in a calcined state as CaO · MgO. Dolomite is inactive for the steam reforming of methane; thus, dolomites are suitable for tar conversion but not for the purification of synthesis gas.6 Trace minerals found in dolomites include potassium and iron oxides, which are also active for tar-removal reactions, including steam/dry-reforming reactions and steam/thermal-cracking reactions.7 It is widely stated that maximum tar conversion with dolomites reaches a ceiling of 95-98%. A majority of the published research on biomass gasification with dolomites for tar conversion comes from Corella and coworkers in Spain and primarily involves fluidized gasification systems. Dolomite (CaO · MgO), calcite (CaO), and magnesite (MgO) from Spain were observed to have very low surface areas, at 11.2, 5.4, and 9.5 m2/g, respectively.10 Dolomites performed better than calcites (CaO · CO3) or magnesites (MgO · CO3). Sutton et al. reported that the activity for tar conversion was in the order of dolomite > magnesite > calcite.6 The most common operating temperature range for dolomites is 800-900 °C,6 and deactivation because of carbon deposition was reported by several groups. The coking of dolomites, limestones, and magnesites occurrs in three intervals: (i) an initial deactivation by coke formation, (ii) subsequent regeneration via coke removal by steam and dry (CO2) gasification, (iii) a final period depending upon the coke formed and gasified.10 The lifespans of calcite and limestone are shorter than that of dolomites. Impregnation of 5.5 wt % CaO into biomass (cellulose, cedar, and aspen) was sufficient for significantly improving the total gaseous product yield, as well as H2 and carbon yields.11 Dolomite was found to prevent or reduce agglomeration when used as an additive in a fluidized-bed biomass gasifier.5,6 Oliviares et al. examined gasification of pine chips in a fluidized (5) Devi, L.; Ptasinski, K.; Janssen, F. A review of the primary measures for tar elimination in biomass gasification processes. Biomass Bioenergy 2003, 24 (2), 125–140. (6) Sutton, D.; Kelleher, B.; Ross, J. Review of literature on catalysts for biomass gasification. Fuel Process. Technol. 2001, 73, 155–173. (7) Caballero, M.; Aznar, M.; Gil, J.; Martin, J.; Frances, E.; Corella., J. Commercial steam reforming catalysts to improve biomass gasification with steam-oxygen mixtures. Ind. Eng. Chem. Res. 1997, 36 (12), 5227– 5239. (8) Lisboa, J.; Santos, D.; Passos, F.; Noronha, F. Effect of the addition of promoters to steam reforming catalysts. Catal. Today 2005, 101, 15–21. (9) Aznar, M.; Corella, J.; Delgaldo, J.; Lahoz, J. Improved steam gasification of lignocellulosic residues in a fluidized bed with commercial steam reforming catalysts. Ind. Eng. Chem. Res. 1993, 32, 1–10. (10) Delgado, J.; Aznar, M.; Corella, J. Calcined dolomite, magnesite, and calcite for cleaning hot gas from a fluidized bed. Ind. Eng. Chem. Res. 1996, 35 (10), 3637–3643.

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bed, with a downstream fixed bed of dolomite at different concentrations, whose temperature was varied from 795 to 835 °C.12 A significant increase in H2 production was achieved with an increase of dolomite from 0 to 30 wt %. Similarly, CO decreased. This was attributed to both the in situ tar destruction and the water-gas shift (WGS) reaction, which they considered to be promoted by CaO · MgO in the bed. Increasing the dolomite content resulted in a leveling off of H2 production. A 15-30 wt % of (calcined) dolomite in the fluidized bed can result in tar contents of the gaseous product to reach below 1 g/m3 during air gasification of pine wood chips.13 It is generally suggested that trace amounts of iron improve the tar conversion abilities of the dolomites. There was some small improvement in using dolomites with larger pore diameters and iron contents.14 Iron is also known to be a catalyst for the WGS reaction and the Fischer-Tropsch synthesis reaction.15 Metallic iron was found to be active for decreasing tar content in gasification product gas but less effective than dolomite as evidenced by lower total gas production.16 In one study, Fe3O4 was used in tar conversion and achieved over 90% conversion of tars.17 In that study, it was determined that surface area was an important factor in tar conversion. Tar conversion was stable in cyclic use of the catalyst, but the activity for promotion of the WGS reaction decreased, possibly because of carbon deposition. The redox pair magnetite (Fe3O4) and wustite (Fe1-δO, δ e 0.13) were employed to convert methane in gasifier product gas but found that conversion was low because H2 and CO react much faster with iron oxide than CH4.18 Tar conversion in a packed-bed reactor was investigated with Ni/ modified dolomite, modified dolomite with increased Fe2O3 content, unmodified dolomite, and nickel catalysts. In that study, modified dolomite (5 wt % Fe2O3) performed marginally better than natural dolomite (0.1 wt % Fe2O3) for tar conversion during steam gasification of biomass tar.19 Olivine, FeMg(SiO4)2, has a much higher resistance to attrition than dolomite, which prolongs its life in fluidized beds. Dolomite was ∼1.40 times more effective than raw olivine ((Mg, Fe)2SiO4); however, there were ∼4-6 times more particulates generated in the gasification gas than olivine.20 The exact mechanism behind the catalytic activity of olivine is not clearly understood yet, but it is closely related to the presence of Fe at (11) Dalai, A.; Sasaoka, E.; Hikita, H.; Ferdous, D. Catalytic gasification of sawdust derived from various biomass. Energy Fuels 2003, 17, 1456– 1463. (12) Oliviares, A.; Aznar, P.; Caballero, A.; Gil, J.; Frances, E.; Corella, J. Biomass gasification: Produced gas upgrading by in-bed use of dolomite. Ind. Eng. Chem. Res. 1997, 36 (12), 5220–5226. (13) Gil, J.; Caballero, M.; Martin, J.; Aznar, M.; Corella, J. Biomass gasification with air in a fluidized bed: Effect of the in-bed use of dolomite under different operation conditions. Ind. Eng. Chem. Res. 1999, 38 (11), 4226–4235. (14) Orio, A.; Corella, J.; Narva´ez, I. Performance of different dolomites on hot raw gas cleaning from biomass gasification with air. Ind. Eng. Chem. Res. 1997, 36, 3800–3808. (15) Satterfield, C. Heterogeneous catalysis in industrial practice, 2nd ed.; McGraw-Hill: New York, 1991. (16) Nordgreen, T.; Liliedahl, T.; Sjostrom, K. Metallic iron as a tar breakdown catalyst related to atmospheric, fluidized bed gasification of biomass. Fuel 2006, 85, 689–694. (17) Uddin, M. A.; Tsuda, H.; Wu, S.; Sasaoka, E. Catalytic decomposition of biomass tars with iron oxide catalysts. Fuel 2008, 87, 451–459. (18) Sturzenegger, M.; D’Souza, L.; Struis, R.; Stucki, S. Oxygen transfer and catalytic properties of nickel iron oxides for steam reforming of methane. Fuel 2006, 85, 1599–1602. (19) Wang, T.; Chang, J.; Lu, P.; Zhu, J. Novel catalyst for cracking of biomass tar. Energy Fuels 2005, 19 (1), 22–27. (20) Corella, J.; Toledo, J.; Padilla, R. Olivine or dolomite as in-bed additive in biomass gasification with air in a fluidized bed: Which is better? Energy Fuels 2004, 18, 713–720.

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Gusta et al. Table 1. Elemental Composition of Dolomites

(wt %)

Canada 1 (NE SK)

Canada 2 (central MB)

Canada 3 (S ON)

Canada 4 (S MB)

Australia

Japan (Tochigi)

Ca Mg Fe Oa totalb

28.2 15.9 0.9 21.7 67

34.0 25.1 2.4 30.1 92

36.2 23.1 0.5 29.7 89

67.6 3.4 0.2 29.3 100

41.2 25.5 0.5 33.3 100

42.8 23.3 0.1 32.5 99

a Theoretical oxygen content calculated from Ca and Mg to represent CaO and MgO species. b Total wt % accounted for including theoretical oxygen content.

Table 2. Porosimetry, Surface Area, and Specific Gravity of Dolomite Samplesa

N2 adsorptionb

Hg porosimetryc gravimetry

BET SA pore vol. (1.2-30 nm) micropore vol. avg. pore d. (4V/A)

m2/g cm3/g cm3/g nm

pore area total pore vol. avg. pore d. (4V/A) specific gravityd

m2/g cm3/g nm cm3/g

Canada 1 (NE SK)

Canada 2 (central MB)

Canada 3 (S ON)

Canada 4 (S MB)

Australia

Japan (Tochigi)

8.0 0.021 0.004 9 3.8 1.07 107 1.3

11.5 0.024 0.003 7.2 4.0 1.03 111 1.4

14.0 0.044 0.006 12.3 4.6 1.06 95 1.4

13.2 0.053 0.010 15.3 4.7 0.99 96 1.3

22.0 0.13 0.010 9.8 3.5 1.03 84 1.4

22.3 0.15 0.008 12.2 2.7 1.06 89 1.3

a

BET SA, N2 BET surface area; avg. pore d., average pore diameter; vol., volume. b N2 adsorption gives porosimetry from 1.2 to 30 nm pore diameter (micropores < 2 nm) and is also used for surface area measurements. c Hg porosimetry measures pores from 6 to 32 400 nm (macropores > 50 nm). d Specific gravity ) volume per unit mass.

the surface of the catalyst.21 Olivine is also not as abundant geographically as dolomite. In this study, the focus is on tar conversion during gasification of biomass, with steam as an oxidizing agent. The catalytic tar conversion activities of four dolomites from different regions of Canada were compared to dolomite samples from Japan and Australia. Catalytic gasification was carried out in a two-stage packed-bed reactor. Semibatch gasification of the sawdust (200 f 850 °C) occurred in the first stage. In the second stage, the catalysts were assessed at three isothermal bed temperatures, where the noncatalytic conversion of tars would be minimized - 650, 700, and 750 °C. The best catalyst was further assessed for stability (repeated use at 800 °C) and effect of space velocity (0.5× and 2× volume of catalyst per run, at 750 °C). The factors held constant in this experiment are the reaction atmosphere and biomass packed-bed stage factors, including the maximum operating temperature of 850 °C. The product gas composition was analyzed at regular intervals of 15 min throughout the experiment after the biomass temperature reached 200 °C. The reliability of the results (and repeatability) was assessed by repeating one of the experiments and observing the proximity of the two sets of results. 2. Experimental Section 2.1. Biomass Preparation and Characterization. The feedstock biomass was obtained as shavings of Jack pine from the Iroquois Lake area of Saskatchewan. The shavings were ground to sawdust using a mill grinder and then sieved to particles 0.15-0.30 mm in diameter to improve processability. The sieved sawdust was then spread out on paper sheets and left in ambient conditions for 3 days to equalize the moisture content before storage. The carbon, hydrogen, nitrogen, and sulfur content of the sawdust were analyzed using an Elementar Vario EL III (Elementar Analysensysteme, Hanau, Germany) CHNS combustion analyzer located in the Department of Chemical Engineering, University of Saskatchewan. 2.2. Catalyst Preparation and Characterization. Natural dolomites from Canada were obtained from the Department of (21) Corella, J.; Aznar, M.; Gil, J.; Caballero, M. Biomass gasification in a fluidized bed: Where to locate the dolomite to improve gasification? Energy Fuels 1999, 13, 1122–1127.

Geology at the University of Saskatchewan, Canada. Dolomites from Japan and Australia were obtained from the Department of Environmental Chemistry and Materials at the University of Okayama, Japan. The dolomite samples were obtained as whole ore samples and required preprocessing, which included crushing with a ball mill and sieving to obtain granules 1-2 mm in diameter. The dolomites were activated via calcination at 800 °C for 1 h under 300 cc/min N2 flow. The dolomite samples were then crushed and sieved to particles sized 0.30-0.45 mm and stored in an airtight container. Inductively coupled plasma-mass spectrometry (ICP-MS) analysis of Ca, Mg, and Fe content in the dolomites were analyzed with a Perkin-Elmer ELAN 5000, and the results are shown in Table 1. The approximate oxygen content of the samples was calculated estimating CaO · MgO species present. The powder X-ray diffraction (XRD) pattern of the calcined dolomites was recorded using a Shimadzu XRD-6100 diffractometer with a Cu KR irradiation (30 kV, 30 mA). The specific surface area of the calcined dolomites was measured by a conventional N2 adsorption Brunauer-EmmettTeller (BET) method (Micromeritics Gemini 2375). N2 BET adsorption, mercury porosimetry, and gravimetry results for the dolomites are shown in Table 2. 2.3. Apparatus and Procedure. Gasification of biomass is carried out using a down-flow dual-stage quartz microreactor under atmospheric pressure (see Figure 1). The microreactor consisted of a quartz tube 800 mm long, 6 mm inner diameter. The two stages are biomass and catalyst fixed beds (270 mm from the top and 230 mm from the bottom), each equipped with its own separate heating zone via programmable temperature controllers. The experimental procedure was as follows: in a typical run, biomass (0.04 g, particle size of 0.15-0.3 mm) was placed in the top reactor bed on a quartz wool pad and the desired amount (0.033-0.13 cc) of dolomite (catalyst) was placed on another pad in the bottom bed. The reactor system was purged with N2 flow for 30 min, while a mixture of 30% H2O/N2 (42 cm3/min) was passed through the reactor bypass. At the same time, the catalyst bed was heated to the isothermal reaction temperature (650-800 °C) at a rate of 10 °C/min while under N2 flow. When the catalyst bed temperature reached the target temperature, the heating program of the biomass bed was started at a heating rate of 3 °C/min to the maximum temperature of 850 °C (and maintained at that temperature for 1 h), while a mixture of 30% H2O-N2 was passed through the reactor. In this reaction system, the volatile matter (including tar) generated from biomass at 200-500 °C is directly transported to



Figure 1. Dual-stage apparatus for semibatch packed-bed steam gasification. Table 3. Elemental Composition of Jack Pine Sawdust carbon hydrogen oxygena nitrogen sulfur a

wt %

mol %

47.9 6.5 45.4 0.0 0.2

30.1 48.5 21.4 0.0 0.0

Estimated as 100% - (C + H + N + S).

the catalyst bed, which is preheated to the target isothermal temperature (650-800 °C). The tar remaining in the product stream of the reactor is removed by the ice trap, as shown in Figure 1. The char component produced at the top bed is then gasified with steam at 500-850 °C. Once the biomass bed temperature reached 200 °C, the reactor outlet was analyzed via online gas chromatography every 15 min until the reaction was completed. The product gas was collected in Tedlar sampling bags throughout the experiment (200-850 °C biomass bed temperature) to analyze the complete gas samples, with the tar evolution (200-500 °C) of gaseous products in one set of Tedlar bags and char gasification (500-850 °C) phase in another set of bags. The product gases were analyzed with two online gas chromatographs equipped with a thermal conductivity detector (TCD) and three columns: a Molecular Sieve 13× column was used to analyze H2 with Ar carrier, and O2, CO, and CH4 were analyzed with He carrier. A Porapak QS column with He carrier was used to analyze CO2, CH4, and C2H4. All gas volumes are reported at standard temperature and pressure (STP). The amount of biomass used is too small (0.04 g per experimental run) to collect enough tar for analysis; therefore, catalytic activity was assessed via the gaseous products only.

3. Results and Discussion 3.1. Biomass Properties. It was determined via CHNS analysis that dried Jack Pine sawdust is 30.1 mol % carbon, 48.5 mol % hydrogen, and 21.4 mol % oxygen, with no distinguishable quantities of sulfur or nitrogen. The oxygen content of the biomass was assumed to comprise the remaining material unaccounted for in CHNS analysis and was calculated as such. The results are shown in Table 3. The moisture content of the sawdust was determined using thermogravimetric analysis (TGA). The flow-type thermogravimetric apparatus was equipped with a quartz tubular reactor

(1.5 cm inner diameter). From TGA analysis, the moisture content of the homogenized sample was determined to be 2.04 wt % and the ash content was 0.32 wt %. 3.2. Dolomite Properties. Catalyst characterization was performed on each dolomite sample after calcination and sieving, as described in the catalyst preparation section. Results from ICP analysis of the calcined dolomite samples, converted to wt %, are shown in Table 1. The predominant components are Ca and Mg, present in oxide forms (CaO and MgO) after calcination. The oxygen wt % of the samples was theoretically approximated using a 1:1 ratio for both Ca/O and Mg/O. As a result, the total wt % accounted for in Table 1 is only a theoretical approximation used to indicate which samples may have additional compounds unaccounted for, which may impact gasification results. Canada 1 ICP results indicate the probability that other elements are present in significant quantities in this sample. Likely candidates include silicon and aluminum, resulting in silica (SiO4), alumina (Al2O3), and possibly combined silicaalumina phases. There is a possibility of promoting cracking, polymerization, and isomerization reactions with these crystalline phases.15 However, concentrations of Si and Al were not targeted, and their presence was not confirmed via ICP analysis in this set of experiments. Si and Al were not targeted during the initial phase of dolomite analysis because they were considered to be insignificant as compared to Ca, Mg, and Fe, in both the presence and contribution to the activity of dolomites for tar conversion. However, after analysis of the catalytic performance of the dolomites as described later on in this section, it is recommended that additional ICP tests including Si and Al analysis should be performed on the dolomites in future tests. University of Saskatchewan Geologist Dr. Brian Pratt, who donated the Canadian dolomite samples, indicated that Canada 4 is not a dolomite but rather a dolomitic limestone. This was confirmed via ICP analysis, which revealed that Canada 4 has a much lower Mg content (3.4 wt %) than the other samples. The other dolomite samples (excluding Canada 4) have an average Ca/Mg ratio of 1.6. The iron content is well-varied in the samples, ranging from 0.1 to 2.4 wt % Fe. Canada 1 and 2 have the highest iron


Figure 2. Repeatability tests on Australian dolomite, 700 °C catalyst bed temperature (0.065 cm3 of catalyst, 0.040 g of biomass, 42 cm3/ min N2, and 18 cm3/h H2O).

contents, not surprising considering their reddish color in an uncalcined state (because of the red pigment of Fe3O4). After calcination under N2 flow, the iron would be present as hematite (Fe2O3). Porosimetry and gravimetry results in Table 2 indicated that the surface areas of calcined dolomites are very low (∼8-22 m2/g). The Canada dolomites have much lower surface areas than the ones from Japan and Australia, by about 10 m2/g. It would appear that the dolomites with higher iron content also have lower pore volumes, indicating a slight decrease in microporosity (pores < 2 nm). Observing the properties in Tables 1 and 2, the dolomites can be grouped together for comparative analysis. The Canada dolomites have a wide distribution of Fe concentrations (which are suggested to have an effect on conversion) and fairly similar surface areas. Canada 1 and 2 have the lowest surface areas (indicating the most macropores). Canada 2 and 3 have fairly similar composition and properties, but Canada 2 has a much higher iron content and larger macropores. Canada 3 and 4 have similar porosimetry properties, but the composition is significantly different. Japan and Australia dolomites have differing iron contents and slightly differing pore sizes but are otherwise very similar in properties. XRD analysis confirmed the presence of CaO and MgO phases via a comparison to a database of standard peaks. Peaks for iron and other trace compounds (such as alumina, silica, and potassium oxide) were not apparent. Because ICP confirmed the presence of trace amounts of iron in these dolomites, the iron must be well-dispersed in the sample and therefore beyond the detection limit of XRD equipment used for this analysis. This is also true for silica, alumina, and silica-alumina crystalline species, whose presence were suspected in dolomite 1 but could not be confirmed. 3.3. Repeatability of Results. Repeatability was assessed via two gasification runs performed at an isothermal catalyst bed temperature of 700 °C with Australian dolomite (0.065 cm3 of catalyst, 0.040 g of biomass, 42 cm3/min N2, and 18 cm3/h H2O). The experiments are continuously monitored over a 7 h period, including 5 h of sampling every 15 min on two separate GCs. The continuously monitored gaseous products are H2, CO, and CH4. The gaseous product evolution profiles over the course of the semibatch reaction for the repeated experiments are shown in Figure 2. A comparison of the gaseous products from the two repeat experiments indicated that there is good repeatability

Gusta et al.

of results, with a maximum difference of 3.6% occurring between results for the yield of carbonaceous gases during tar conversion (products evolving at 200-500 °C). 3.4. Assessment of Influences on Noncatalytic Gasification. The quartz wool catalytic support was inactive as a catalyst material but may act as a surface on which the tars can crack and deposit coke at high temperatures. The noncatalytic runs, including the effects of quartz plugs, are used as a baseline for a comparison to catalytic results. It was important to know the extent of the influence of quartz, to ensure that it was insignificant. The effect of the support material on the experiments was assessed by comparing noncatalytic runs with and without the quartz support material included (0.40 g of biomass, 42 cm3/ min N2, and 18 cm3/h H2O). The difference of including versus excluding the quartz plug during noncatalytic gasification was determined to be insignificant at 700 °C, displaying a maximum difference of 4.2% between results. Between 650, 700, and 750 °C catalyst bed temperature, noncatalytic results do not vary greatly with regard to tar conversion and overall carbon yield. However, at 800 °C, tar conversion and gaseous products from tar increased dramatically; noncatalytic tar conversion increased from 28% at 650 °C to 55% at 800 °C catalyst bed temperature (excluding catalyst). Hydrogen production also increased from 36 to 41%, and carbon production increased from 45 to 61%. This indicates that, at 800 °C, coke deposits from thermal cracking are reacting with the steam on the quartz surface. Noncatalytic experiments excluding the quartz wool were not conducted at 800 °C; however, it is anticipated that the results would be closer to those in the 650-750 °C range because of the decreased surface area (no quartz wool present) for thermal cracking. This rise in coking serves to contribute to the total char yield. 3.4. Tar Conversion. Tar conversion using various dolomites is determined via the carbonaceous gas product emitted between 200 and 500 °C (biomass bed temperature). The product emitted during this phase is a mixture of gases and condensable organic compounds. The objective is to minimize the amount of condensable materials via catalytic means, thereby increasing production of gases. The amount of carbonaceous gases collected was determined from GC analysis and included CO, CH4, CO2, and C2H2 gases captured from 200 to 500 °C (biomass bed temperature). The maximum amount of carbon emitted between 200 and 500 °C was determined to be 74% of the total amount of carbon in the biomass sample. This was calculated from the carbon gas products of the char phase (500-850 °C) during noncatalytic gasification runs and the carbon content of the biomass as determined by CHNS analysis results. Tar conversion was calculated as follows: % tar conversion ) carbon gases collected in catalytic run (200-500 °C) × 100% maximum amount of carbon emitted (200-500 °C) The fraction of carbon emitted as tar was calculated from the average amount of carbon produced during the char gasification segment (500-850 °C) of noncatalytic gasification and the total known carbon in the biomass sample as determined from ICP results. Tar conversion results are given in Figure 3. Significant improvements were achieved by the use of dolomites in a downstream packed bed, for all temperatures examined. During catalyst screening at 650, 700, and 750 °C, the average tar conversion was improved by 14, 17.5, and 21% over noncata-


Energy & Fuels, Vol. 23, 2009 2269 Table 4. Assessment of Influential Dolomite Properties on Tar Conversion (0.065 cm3 of Catalyst, 0.040 g of Biomass, 42 cm3/min N2, and 18 cm3/h H2O)

Figure 3. Tar conversion results for dolomites over several temperatures (0.065 cm3 of catalyst, 0.040 g of biomass, 42 cm3/min N2, and 18 cm3/h H2O).

Figure 4. Percent tar conversion versus iron content for Canadian dolomites (0.065 cm3 of catalyst, 0.040 g of biomass, 42 cm3/min N2, and 18 cm3/h H2O).

lytic results, respectively. Maximum tar conversion of 66% was achieved by Canada 1 at 750 °C using 1.6 cm3 of catalyst/g of biomass. This was an improvement of 31% over noncatalytic results at the same temperature. Another study reported that, when using dolomites in a fluidized bed for bed temperatures of 790-830 °C, tar conversion was maximized at the highest temperature.13 Canada 1 yielded best results for tar conversion at 700 and 750 °C, achieving 29 and 31%, respectively. This dolomite was selected for extended tests, including assessment at a catalyst bed temperature of 800 °C. The other extended tests are discussed later on in this paper, but experimental results from a run at 800 °C with 0.065 cm3 of catalyst (standard amount) was also included in Figure 3 for further demonstration of tar conversion. Tar conversion at 800 °C reached 80%, an improvement of 25% over noncatalytic results. The dolomites with higher iron content were observed to have higher activity for tar conversion. However, a plot of Fe wt % versus tar conversion for all catalysts did not yield a clear trend, indicating that other catalyst properties are significant factors. When considering the measured properties of the dolomites, Australian and Japanese dolomites were observed to be somewhat different from the Canadian dolomites. Therefore, the relation between iron content and tar conversion was plotted using Canadian dolomites only, in Figure 4. It is apparent that the effect of iron content on tar conversion is positive, which corresponds to the results of another study on artificial modification of dolomites with iron.19 The effect reaches a plateau at 0.9 wt % Fe for Canadian dolomites, however. At this point, the average improvement in tar conversion was 25% above noncatalytic results. An iron

Fe (wt %) Ca (wt %) Mg (wt %) BET SA (m2/g) pore volume (12-300 area) (cm3/g) average increase in tar conversiona (%) a

Japan

Australia

Canada 4

Canada 3

0.14 42.8 23.3 22.3 0.15

0.53 41.2 25.5 22.0 0.13

0.17 67.6 3.4 13.2 0.05

0.51 42.8 23.1 14.0 0.04

11.5

13.8

13.0

22.2

Increase ) higher than noncatalytic results.

concentration of 0.5 wt % provides a noticeable improvement in tar conversion, by approximately 22% over noncatalytic results. It should be noted that the Australian dolomite, with Fe content of 0.5 wt %, performed better than Japanese dolomite at 0.2 wt %. The deficiency of Mg in Canada 4 did not appear to be a significant influencing factor on its activity, having an improved performance as compared to Japanese dolomite, which has nearly the same amount of iron. The performances of Australian and Japanese dolomites during tar conversion were noticeably lower than Canada 3 and 4, although they have approximately the same iron contents (respectively). Table 4 suggests that, for the dolomites observed, those with lower microporosity and smaller average surface areas will perform better. Possible reasons include deposition of coke in the fine pores10 or agglomeration of iron active sites at high temperatures.22 Quantifiable confirmation of these trends would ideally require a comparison to additional dolomitic ores. Because of the small amount of catalyst used, quantification of coking behavior had to be assessed qualitatively and rated in severity based on the average darkness of the spent catalyst collected after gasification was completed. It must be remembered that coke is deposited on the catalysts during tar conversion and is removed via a reaction with steam. Coking was experienced by all catalysts at 650 °C, diminished at 700 °C, and all but disappeared at 750 °C. Australian dolomite was the only one observed to have coking at 750 °C. Canada 3, closest to Australian dolomite regarding composition, encountered very little coking comparatively. No correlation between any catalyst properties and coking behavior could be found. 3.5. Effects of Iron on CO Production and CO2/CO Ratio in Product Gas. Canadian dolomites were used to demonstrate the influence of iron content on CO production and the CO2/CO ratio, which indicates promotion of the WGS reaction. In the WGS reaction, CO and H2O are consumed to produce CO2 and H2. Keeping in mind that the optimal H2/CO ratio for ethanol synthesis is 3:1, promotion of H2 production may be desirable. However, the sensitivity of downstream processes to the concentration of CO2 in the product gas may be an issue. Figure 5 indicates overall CO production as a function of the iron content in Canadian dolomites. CO production while implementing catalysis seems to fare best at 700 °C on average. Noncatalytic gasification at 750 °C yields the largest amount of CO, which then decreased with dolomites and iron content, indicating that the use of dolomites at 750 °C promotes the consumption of CO. Figure 6 shows the changes in CO2/CO molar ratio for (a) tar conversion and (b) overall gaseous product, respectively. (22) Galvita, V.; Hempel, T.; Lorenz, H.; Rihko-Struckmann, L.; Sundmacher, K. Deactivation of modified iron oxide materials in the cyclic water-gas shift process for CO-free hydrogen production. Ind. Eng. Chem. Res. 2008, 47 (2), 303-310.


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Figure 5. Overall CO production as a function of Fe wt % in Canadian dolomites (0.065 cm3 of catalyst, 0.040 g of biomass, 42 cm3/min N2, and 18 cm3/h H2O).

Figure 7. (a) Overall H2 production and (b) H2/CO ratio for dolomites (0.065 cm3 of catalyst, 0.040 g of biomass, 42 cm3/min N2, and 18 cm3/h H2O).

Figure 6. Effect of the Fe content in Canadian dolomites on the CO2/ CO ratio for (a) tar conversion (200-500 °C biomass bed temperature), and (b) overall gaseous product (200-850 °C biomass bed temperature) (0.065 cm3 of catalyst, 0.040 g of biomass, 42 cm3/min N2, and 18 cm3/h H2O).

An increase in the molar ratio indicates promotion of the WGS reaction, most prominent when using dolomites with high iron content at 750 °C. CO consumption increases with Fe wt % of the Canadian dolomites and reaches a plateau at 0.9 wt % Fe. In another study, it was reported that, below 700 °C, the reaction rate of WGS drops to the point at which the gas composition remains unchanged.3 The CO2/CO ratio sharply increased from 1.2 to 5.7, with Fe varying from 0.2 to 0.9% at 750 °C, and then reached a plateau at 5.7 between 0.9 and 2.4 wt %. This along with the CO results shown in Figure 6 indicates that the promotion of the WGS reaction of Fe peaks at 1.0%. The

increase in the CO2/CO ratio at 750 °C between parts a and b of Figure 6 indicates that the dolomites convert CO to CO2 during char gasification as well. Gasification at 700 and 650 °C yielded fairly stable ratios, regardless of the iron content of the catalyst. At 650 °C, there is a small increase in the average CO2 concentration; however, overall CO produced was lower than at 700 °C. At 700 °C, there is an average decrease of 0.079 in the ratio, indicating a slight increase in CO production during the char gasification stage. If CO production is targeted or CO2 production is to be minimized during catalytic tar conversion, it is recommended to use dolomites with 0.5 wt % Fe or less. 3.6. Hydrogen Production and H2/CO Ratio. Dolomites significantly improve the production of hydrogen as compared to noncatalytic gasification process. Figure 7a shows the total volume of hydrogen produced for different dolomites at three temperatures. At 750 °C, the improvement in H2 production is an average of 415 cm3/g of biomass or 616% increase over noncatalytic results via the use of dolomites. Maximum H2 production at 750 °C is achieved by the dolomites with 0.5 wt % Fe. Canada 3 and Australian dolomite achieved 916 and 1053 cm3 of H2/g of biomass, respectively. The H2/CO ratio may be significantly increased by catalytic promotion of the WGS reaction. For industrial use, it is important to consider the purpose of the overall gas product when deciding the optimum ratio to be targeted. For instance, as previously stated, synthesis of ethanol ideally requires a stoichiometric ratio of 3:1 H2/CO. Figure 7b shows the molar ratios of H2/CO using dolomites at 650, 700, and 750 °C. Low H2 production and H2/CO ratios at 650 °C indicate that operation at higher temperatures is necessary for producing syngas suitable for ethanol synthesis. At 700 °C, higher H2 production is met with H2/CO ratios at around 3 for Canada 1, 3, and 4. At 750 °C, H2 production moderately increases slightly but the H2/CO ratio climbs steeply for many of the catalysts, indicating significance of the WGS reaction. Only Canada 4 achieves a ratio of ∼3 at 750 °C. This correlates to another



Figure 8. Overall H2/CO ratio as a function of Fe wt % for Canadian dolomites (0.065 cm3 of catalyst, 0.040 g of biomass, 42 cm3/min N2, and 18 cm3/h H2O).

study that compared fluidized gasification without and with dolomite in the bed (from 790 to 830 °C bed temperature), where the H2 content increased from 6-10 to 12-17 vol % and CO content increased from 9-16 to 16-22 vol %.13 Figure 8 indicates the relationship between the overall H2/ CO ratio and Fe wt % for Canadian dolomites. The Fe content of the dolomites has a positive effect on the production of H2, because of the promotion of the WGS reaction of Fe at high temperatures. This effect was previously demonstrated when observing CO2/CO ratios with respect to the Fe content of the dolomites. When implementing dolomites for tar conversion, the H2/CO ratio is not greatly affected by the iron content of the catalyst at 650 and 700 °C. The WGS reaction becomes significant at 750 °C, indicating that an excess of hydrogen is produced when the dolomite contains 0.5 wt % Fe or more. If hydrogen production is to be targeted in gasification, then the WGS reaction should be promoted. Dolomites with iron contents of ∼1 wt % or more and high operating temperatures are recommended for H2 production. If CO content of the product gas is of significant concern, then it is recommended to minimize the Fe content of the dolomites, to 0.5 wt % or less. 3.7. Extended Tests on Optimum Dolomite. Canada 1 was determined to be the optimal catalyst in the series of dolomites during catalyst screening, having achieved 66% tar conversion at 750 °C using 0.065 cm3 of catalyst (see Figure 3). Thus, extended tests were performed on this catalyst, including activity at increased temperature (800 °C), effect of space velocity at 750 °C, and repeat use of the same material at 800 °C. Conversion of materials to gaseous products improved at 800 °C using Canada 1, with an increase in tar conversion from 66 to 80% between 750 and 800 °C, as indicated in parts a and b of Figure 9. Increases in H2, CO, and CO2 concentrations at 800 °C were calculated to be 11, -3, and 2%, respectively, indicating a rather insignificant change from 750 °C. A noticeable change in the gasification product between 750 and 800 °C was that C2H4 production increased from 2 to 25 cm3/g of biomass. This could be attributed to an increase in steam reforming on the catalyst; i.e., the hydrocarbon chains of tar were broken up into smaller molecules. This reasoning is supported by evidence that C2H4 was not present in significant quantities during noncatalytic runs at the same temperatures. Steam reforming was indicated to be a significant factor in tar conversion in another study, where the reaction conditions were varied from 790-830 °C.13 Between 750 and 800 °C, production of CH4 also increased in catalytic and noncatalytic runs, by 21 and 33 cm3/g of biomass, respectively. This serves to indicate

Figure 9. Performance of Canada 1 (a) during tar conversion and (b) overall, with a catalyst bed temperature of 650-800 °C (0.065 cm3 of catalyst, 0.040 g of biomass, 42 cm3/min N2, and 18 cm3/h H20).

that cracking increases at 800 °C, which is largely responsible for the improvement in tar conversion. A sharp increase in noncatalytic tar conversion was observed during noncatalytic gasification as shown in Figure 3, which supports this conclusion. In comparison of parts a and b of Figure 9, it is somewhat apparent that the majority of CO production occurs during tar conversion and char gasification leads to CO2 production. It is possible that the CO production occurring during tar conversion is largely from the gasification of coke deposited on the catalysts, which then has minimal opportunity to be further reacted as it exits the reactor. CO produced during gasification of the char residues may be converted in the catalytic stage to CO2 during the WGS reaction. The effect of space velocity at 750 °C on the conversion of tars and product distribution is shown in Figure 10. The amount of catalyst was varied as 0.5×, 1×, and 2× the standard amount of catalyst used in previous experiments, which was 0.065 cm3 of catalyst per run. At 750 °C, increasing the volume of catalyst used had a significant effect: 97% conversion of tar was achieved with 3.2 cm3 of catalyst/g of biomass. The concentrations of H2 and CO2 increased dramatically from 0.5× to 2× catalyst, while the CO concentration fluctuated. The most likely source of CO is the reaction of H2O with Cs (coke), which was deposited on the surface of the catalyst from tar cracking. It would appear that the increase in the space velocity allows for both an increase in tar cracking (and Cs deposition) and further conversion of the CO with the WGS reaction. The improvement of tar conversion for 2× catalyst at 750 °C is far greater than 1× catalyst at 800 °C (97 versus 66% conversion). This is a positive result, because it serves to indicate that high conversions may be obtained at lower temperatures.


Gusta et al.

second and third use of the dolomites. Concentrations of the product gases were not found to vary to a significant degree; on average, the deviation of the second and third use was 2 and 2.4%, respectively. Thus, with over 95% certainty, it can be assumed that the dolomites are not deactivated over approximately 15 h of use when implemented in a packed-bed system. 4. Conclusions

Figure 10. Effect of space velocity on Canada dolomite 1 at 750 °C (0.065 cm3 of catalyst, 0.040 g of biomass, 42 cm3/min N2, and 18 cm3/h H20).

Figure 11. Effect of reusing Canada 1 dolomite in subsequent runs at 800 °C (0.065 cm3 of catalyst, 18 cm3/h H2O, and 42 cm3/min N2).

Further optimization of catalyst/biomass ratios and reaction temperatures are advisable in a fluidized-bed system, however. Repeat usage of Canada 1 at 800 °C catalyst bed temperature is shown in Figure 11. The dolomites were not subjected to any treatment prior to reuse. In repeat runs, the reactor tube had a fresh sample of biomass added and the tube was purged under N2 flow until the following morning when the next run was performed. Tar conversion decreased from 80 to 77% during

During noncatalytic temperature-programmed gasification, 74% of Jack pine sawdust is committed to volatile matter (including condensable materials referred to as tars) between 200 and 500 °C biomass bed temperatures using 0.04 g of biomass, 42 cm3/min N2, and 18 cm3/h H2O. Calcined dolomites have low surface areas, from ∼8 to 22 m2/g for the samples studied. The surface areas of Canadian dolomites were an average of 10 m2/g lower than those from Japan and Australia, attributed to a lower concentration of micropores. Dolomites significantly improved tar conversion by an average of 21% over noncatalytic results (from 35 to an average of 56%) at an isothermal catalyst bed temperature of 750 °C using 1.6 cm3 of catalyst/g of biomass. For Canadian dolomites, the effect of iron content on tar conversion was positive but reached a plateau of 66% tar conversion at 0.9 wt % Fe (750 °C). At this point, the average improvement in tar conversion was 25% above noncatalytic results. Iron oxide content in Canadian dolomites also promoted the WGS reaction, where the H2/CO ratio reached a plateau of 9.0 at 0.9 wt % Fe and 750 °C. Canada 1 dolomite achieved 66% tar conversion and H2 + CO production of 60 mol % at 750 °C catalyst bed temperature using 1.6 cm3 of catalyst/g of biomass. Tar conversion increased to 97% and H2 + CO production was 55 mol % using 3.2 cm3/g of biomass at 750 °C. Operating at 800 °C using 1.6 cm3/g of biomass, Canada 1 seemed stable after 3 repeated runs, equivalent to 15 h of cyclic use. Acknowledgment. We thank Dr. Brian Pratt from the Geology Department of the University of Saskatchewan for providing the Canadian dolomite samples used in this study. EF8009958

Catalytic Decomposition of Biomass Tars with Dolomites - Energy

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