Thermal Processing of Straw Black Liquor in Fluidized and Spouted

Energy & Fuels 2002, 16, 1417-1424

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Thermal Processing of Straw Black Liquor in Fluidized and Spouted Bed M. Olazar,† R. Aguado,† J. L. Sa´nchez,‡ R. Bilbao,‡ and J. Arauzo*,‡ Chemical Engineering Department, UPV, Lejona, Spain, and Chemical & Environmental Engineering Department, University of Zaragoza, Spain Received February 15, 2002

Nowadays, black liquor recovery is important for the economics of the pulp and paper production industry. As a result of the high capital cost of the recovery unit, the corrosive nature of the smelt, and the risk of smeltwater explosions, alternatives to the conventional recovery cycle are under research in order to achieve more efficient and environmentally cleaner processes. These alternatives fall into two categories: high-temperature and low-temperature, according to whether or not the melting point of the black liquor inorganics is reached. The advantage of lowtemperature processes is to avoid the formation of smelt. In this work, the feasibility of the thermal processing at low temperature of straw black liquor in two different bench scale reactors has been tested. In the fluidized bed, the loss of fluidization due to bed agglomeration was found to be the main problem in the reactor used. The second reactor used, a spouted bed, presents different characteristics from the fluidized bed, and has been tested in order to overcome the agglomeration observed. Experiments in different operating conditions were carried out in order to get a basic knowledge about the behavior of this residue during pyrolysis, gasification, and combustion processes. To work below the melting point of the black liquor inorganics, reaction temperature was kept under 600 °C. Liquid black liquor and dry black liquor were used as feedstocks. Nitrogen, air, and nitrogen-oxygen mixtures were considered as reaction atmospheres.

Introduction Black liquor (BL) recovery is an important issue for the pulp and paper industry, as BL constitutes a residue that must be treated because of environmental aspects, but also as it is a source of energy and chemicals for the cooking process. Nowadays black liquor is burned in recovery boilers to produce steam and to recover a smelt, which contains chemicals in a suitable form for its re-utilization in the cooking process. Because of the high capital cost of the recovery unit, the corrosive nature of the smelt, and the risk of smeltwater explosions, alternatives to the conventional recovery cycle are under research. These alternatives fall in two categories: high-temperature and low-temperature, according to whether or not the melting point of the BL inorganics is reached. One of the most promising alternatives is gasification, which allows energy recovery as a combustible gas. To convert the fixed carbon present in any organic material as BL, high temperatures are required. Thus, temperatures of 800-900 °C are common in the gasification of biomass and coal. The disadvantage of working at high temperature is that inorganics present in BL are usually in the form of smelt. * Author to whom correspondence should be addressed at Chemical & Environmental Engineering Department, University of Zaragoza, Marı´a de Luna, 3, 50018 Zaragoza, Spain. Fax: +34 976 76 18 79. E-mail: [email protected]. † Chemical Engineering Department, University of the Basque Country (UPV). ‡ Chemical and Environmental Engineering Department, University of Zaragoza.

The advantage of low-temperature processes is to avoid the formation of smelt. One kind of reactor that can be suitable for work below the melting point of BL inorganics is the fluidized bed, as has been proposed earlier by several authors.1,2,3 Anyhow, there are few works available in the open literature on fluidized bed gasification of BL on a bench scale, and they were carried out with kraft BL.4,5 In this work, BL from soda cooking of straw has been used, which presents some characteristics different from kraft BL. The most noticeable one is a lower sulfur content, as no sodium sulfide is used for cooking. However, to process this straw BL in a fluidized bed, its higher Cl and K content seems to be more critical because of the lower melting point of these compounds. A matter of significant importance can also be its different swelling behavior6 and a higher viscosity. As an alternative to the fluidized bed reactor, thermal processing of straw BL has been tested in a spouted bed reactor. There are no references in the open literature (1) Fallavollita, J. A.; Avedesian, M. M.; Mujumdar, A. S. Can. J. Chem. Eng. 1987, 65, 812. (2) Durai-Swamy, K.; Warren, D.; Mansour, M. M. TAPPI Proceedings of the 1990 Engineering Conference, TAPPI Press: Atlanta, GA, 1990. (3) Dahlquist, E.; Jacobs, R. Pulp Pap. Can. 1994, 95 (2), 73. (4) Southards, W. T.; Blue, J. D.; Dickinson, J. A.; McIlroy, R. A.; Verril, C. L. Final Report, US DOE Contract No. DE-FC36-94G010002 (June 1997). (5) Verril, C. L.; Kitto, J. B.; Dickinson, J. A. 1998 International Chemical Recovery Conference, 1067. (6) Gea, G. Thermochemical Behaviour of Alkaline Black Liquor from Straw. Ph.D. Dissertation, University of Zaragoza, Spain, 2000.

10.1021/ef020034n CCC: $22.00 © 2002 American Chemical Society Published on Web 10/12/2002

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Figure 1. Fluidized bed plant scheme.

on the use of a spouted bed to process BL of any kind, so a preliminary study is presented in this work. The conical spouted bed reactor operates in a transition regime between the hydrodynamically well-defined regimes of spouting and jet spouting. This type of reactor has been successfully used in operations in which the particle is sticky and its size changes with residence time, such as in bituminous coal gasification7 and catalytic polymerizations.8 In previous papers, fundamental aspects of these extreme regimes have been studied, as are hydrodynamics with solids of regular texture,9 hydrodynamics with biomass,10 geometric design of these contactors,11 solid flow,12 gas flow,13 and segregation.14 Their great versatility as far as gas flow is concerned makes this equipment especially suitable for solids that are difficult to handle in a fluidized bed or in a conventional spouted bed (cylindrical instead of conical bottom). Experimental Section Black Liquor. The black liquor used in this study was supplied by a Spanish paper mill where cereal straw is cooked with soda as the only chemical. The straw black liquor, as produced, has a solid content ranging between 5 and 10% in weight. The ultimate analysis of the black liquor solids, obtained in a CHNS Carlo Erba elemental analyzer (model EA 1108), is: N 1.29% (in weight), C 45.85%, H 4.49%, and S 0.55%. The ash weight content of the BL solids is 22%, with the main components being 33.6% carbonate, 8.0% chloride, 28.0% sodium, 7.2% potassium, and 8.8% silica. The chloride comes from the water used for cooking, while K, N, SiO2, and sulfur come from the straw and were dissolved while cooking. Experiments were performed in a fluidized bed and in a spouted bed. Black liquor (BL) has been fed as a liquid (in the (7) Uemaki, O.; Tsuji, T. Proc. Eng. Found. Conf. Fluid. 5th 1986, 497. (8) Olazar, M.; San Jose, M. J.; Zabala, G.; Bilbao, J. Chem. Eng. Sci. 1994, 49, 4579. (9) San Jose´, M. J.; Olazar, M.; Aguayo, A. T.; Arandes, J. M.; Bilbao, J. Chem. Eng. J. 1993, 51, 45. (10) Olazar, M.; San Jose´, M. J.; Llamosas, R.; Bilbao, J. Ind. Eng. Chem. Res. 1994, 33, 993. (11) Olazar, M.; San Jose´, M. J.; Aguayo, A. T.; Arandes, J. M.; Bilbao, J. Ind. Eng. Chem. Res. 1993, 32, 1245. (12) San Jose´, M. J.; Olazar, M.; Alvarez, S.; Izquierdo, M. A.; Bilbao, J. Chem. Eng. Sci. 1998, 53, 3561. (13) San Jose´, M. J.; Olazar, M.; Pen˜as, F. J.; Arandes, J. M.; Bilbao, J. Chem. Eng. Sci. 1995, 50, 2161. (14) San Jose´, M. J.; Olazar, M.; Pen˜as, F. J.; Bilbao, J. Ind. Eng. Chem. Res. 1994, 33, 1838.

fluidized bed) and as a solid (in the fluidized and spouted beds). Liquid BL was not fed as received, but the solid content was increased through evaporation by up to 25% in weight and kept hot (60-70 °C) in a reservoir. The liquid was fed into the reactor using a peristaltic pump at a mass flow rate of 10 g/min. It was not possible to feed liquid BL with a higher solid content than 25% of solids, because viscosity increases exponentially with solid content and the feeding pump was unable to force the flow. Solid BL was obtained drying the as-received BL in an oven at 105 °C until completely dry. Solids obtained are smashed and sieved to a particle diameter between 250 and 350 µm. The feeding system used for solid BL has been described earlier.15,16 Fluidized Bed Plant. The fluidized bed reactor is 41.2 mm in inner diameter and is externally heated by an electric oven. Experiments were performed at 550 and 600 °C. Sand was used as bed, with a particle diameter of +150-350 µm, and nitrogen and air were used as fluidizing media. The minimum fluidization velocity of the sand at 550 °C is 2.7 cm/s for nitrogen and 2.6 cm/s for air, and at 600 °C is 2.6 cm/s for nitrogen and 2.5 cm/s for air (calculated using the Wen-Yu correlation). A superficial velocity of 20 cm/s was kept in all the experiments, which gives a residence time for the gas in the bed of 0.3 s; entrainment of bed particles was not observed. Liquid and solid BL were used as feed. A stream of nitrogen is fed together with BL to facilitate its inlet into the reactor. BL enters the reactor by its upper part, through a 4 mm diameter pipe, immersed in the bed which can be externally cooled in order to avoid the pyrolysis of BL before reaching the bed. Gas exiting the reactor goes through a small cyclone, where entrained solid particles can be recovered into a charpot. Product gas is cooled by a water jacket, and the tar is collected in a tar pot. A cotton filter cleans the gas before its analysis in a gas chromatograph. Figure 1 shows a scheme of the experimental plant. Spouted Bed Plant. The laboratory unit, Figure 2, consists of the following components: feeder; device for feeding nitrogen and oxygen; the spouted bed reactor; condensation system; device for char quenching and extraction; and a section for on-line gas analysis by gas chromatography. The geometric factors of the reactor, Figure 3, are the following: total reactor height, HT ) 34 cm; height of the conical section, Hc ) 20.5 cm; angle of the conical section, γ ) 28°; diameter of the cylindrical section, Dc ) 12.3 cm; base diameter, Di ) 2 cm; inlet diameter, Do ) 1 cm. Nitrogen or mixtures of nitrogen with oxygen are fed through the bottom of the reactor once it has been heated by direct contact with resistances inserted into ceramic material (15) Scott, D. S.; Piskorz, J. Ind. Eng. Chem. Fundam. 1982, 21 (3), 322. (16) Scott, D. S.; Piskorz, J. Can. J. Chem. Eng. 1982, 60, 666.

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Figure 4. Minimum spouting velocity vs temperature.

Figure 2. Spouted bed plant scheme.

Solid and not liquid BL has been used in the spouted bed reactor for two reasons. In the spouted bed reactor the feeding inlet is located in a relatively low temperature zone, so pyrolysis at the inlet is not so likely to occur as in the fluidized bed reactor, and it is easier to handle solid BL than liquid BL, that must be kept hot in order to avoid a viscosity increase due to a temperature drop. In the experimental unit used in this study, feeding can only be done from the upper part of the reactor, and drying of BL is a step that takes several seconds.17 In order to feed liquid BL into the spouted bed, drying must happen during the falling of a BL droplet, so a dry or partially pyrolyzed BL particle reaches the bed of sand particles in motion, and the spouted bed used does not have an HT long enough to fulfill this. The average residence time of the gas in the bed is 50 ms and for the gas that circulates along the spout zone (approximately 60% of the total flow) the residence time is of the order of 20 ms. To ensure that the spouting regime was correctly attained, a hydrodynamic study was carried out to ascertain the validity at high temperature (600 °C) of the equation determined for calculation of the minimum spouting velocity at room temperature:7

Figure 3. Geometric factors of a conical spouted bed.

Reoms ) 0.126 Ar0.50 (Db/Do)1.68 (tan (γ/2)) - 0.57 and covered with a metallic case. Another resistance, inserted into ceramic material, covers the conical section of the reactor and maintains the wall temperature. The reactor and the upper cylindrical section are insulated. The gas inlet temperature and the wall temperature are controlled by means of two fixed thermocouples. Furthermore, there are three additional thermocouples, which are located at three radial locations and are provided with free vertical movement. The gases generated, together with the inert gas, leave the reactor and, once they pass through a cyclone where fine solid particles are retained, go to a cooling and condensing system. A bubble flow-meter is placed at the exit of the filter in order to measure the gaseous stream flow rate. The runs have been carried out by continuously feeding solid BL under the following conditions: particle size between 250 and 350 µm; flow rate between 0.8 and 3.0 g min-1; sand (520 g) of two particle sizes, 1600-2000 µm and 1000-1600 µm; temperature, 600 °C; N2 flow rate between 2 and 3 times that corresponding to the minimum spouting velocity, within the 10-23 L(STP) min-1 range, depending on sand quantity and turbulence required.

(1)

where Reoms is the Reynolds modulus referred to the inlet diameter (Do), Db is the upper diameter of the bed, and Ar is the Archimedes modulus. Figure 4 shows the experimental results (points) obtained for the minimum spouting velocity and those calculated with eq 1 (lines). As is observed, eq 1 is also useful for predicting the minimum spouting velocity at 600 °C. This is due to the fact that, unlike the equations proposed for conventional spouted beds (cylindrical), this equation includes the density and viscosity (in the Reynolds and Archimedes modulus) and, consequently, it takes into account the change of these parameters with temperature.

Results Experiments in Fluidized Bed. Four different kinds of experiments have been performed: experiments using air as fluidizing medium feeding liquid BL and (17) Frederick, W. J. Report One Contract DOE/CE/40637-T8 (March 1990).

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Table 1. Results of Fluidized Bed Experiments (N2, liquid BL)

a

run no.

15

27

temperature (°C) sand bed (g) black Liquor fed (g) solids in BL (wt %) run time (min) mean gas composition (vol %)a H2 CH4 CO CO2 C2+

550 120.2 653.3 26.2 64.5

600 120.6 559.2 24.7 58

n.d. 7.3 8.6 84.1 n.d

4.4 12.6 15.2 59.6 8.3

Nitrogen- and moisture-free.

solid BL and experiments using nitrogen as fluidizing medium feeding liquid BL and solid BL. a-1. Experiments in Air and Nitrogen Atmospheres Feeding Solid BL. All the experiments performed in air atmosphere using solid BL as feed ended in a few minutes (from 2 to 5 after feeding started). Pyrolysis and swelling of solid BL inside the feeding pipe happened readily although it was cooled by a stream of compressed air, and only small amounts of swelled char could be observed in the sand bed. Different strategies were tried (changing pipe size, solid BL size and flow rate, nitrogen flow through feeding pipe) were tested but without success, so results cannot be given. Experiments in nitrogen atmosphere yielded the same problem and could not be performed either. a-2. Experiments in Air Atmosphere Feeding Liquid BL. It was easier to feed liquid BL in the fluidized bed reactor, as water prevents the pyrolysis from happening in the feeding pipe. Experiments were performed at 550 and 600 °C and, in this case also ended after a short period (2 to 9 min), not due to plugging of the feeding pipe but to defluidization of the bed. To check if inorganics melting was happening, char from BL was prepared in a fixed bed reactor at 600 °C under a controlled heating (10 °C/min) and in nitrogen atmosphere, smashed and sieved to a particle size between 250 and 350 µm. Char was mixed with sand and put into the fluidized bed reactor. The reactor was heated at 10 °C/min up to 550 °C and 600 °C; after 30 min the reactor was cooled and the bed was examined. At both temperatures the bed was agglomerated. As BL swelling did not occur in these tests (swelling occurs during pyrolysis, which was performed in a different reactor), it is likely that partial melting of inorganics was the cause for defluidization due to hot spots on char particles as reaction with oxygen took place. a-3. Experiments in Nitrogen Atmosphere Feeding Liquid BL. Due to the results obtained, it was decided not to continue using air in the fluidization medium in fluidized bed, but to use nitrogen instead. The only reactive agent present in these experiments is provided by evaporation of the water contained in the BL. Even though the feeding system worked satisfactorily and feeding pipe plugging happened in just a few cases, in all the experiments performed bed defluidization due to agglomeration of bed particles was observed and was the cause of experiment termination. Table 1 shows the conditions of the two longer experiments performed at 550 and 600 °C, as well as

Figure 5. Gas composition. Run 15.

Figure 6. Gas composition. Run 27.

the mean composition of the gas obtained free of nitrogen and moisture. Figures 5 and 6 show gas composition along time obtained in experiments no. 15 and no. 27, free of nitrogen and moisture. It can be observed that the most important product of the reaction is CO2, which constitutes 84% by volume at 550 °C and 60% at 600 °C (see Table 1). CO constitutes 9% at 550 °C and 15% at 600 °C. Hydrogen and C2 species are not detected at 550 °C, being 4% and 8% at 600 °C, respectively. Furthermore, gas yield was very low: 9.1% of the organic weight fed to the reactor was recovered as gas at 550 °C, and 16.7% at 600 °C. Char obtained was 52% of the organic weight fed at 550 °C, and 40% at 600 °C. Tar collected in the cooling and condenser system of the lab plant represents 30% of the organic weight fed at 550 °C and 33% at 600 °C. Mass balance closure is not very good because tar content could not be evaluated accurately due to the huge amount of water recovered in the cooling system. To find out the origin of the bed agglomeration, samples from a bed were analyzed by scanning electronic microscopy. The apparatus used (JEOL JSM 6400) is also provided with a qualitative microanalysis system by X-rays, which allowed the identification of some elements on a specific place of the sample being photographed. Si emission lines were used to identify sand and Na and K lines to identify char from BL.

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Figure 7. SEM microphotography of bed from experiment 15 (×80).

Figure 8. SEM microphotography of bed from experiment 15 (×120).

Figures 7 and 8 show bed samples microphotographs from experiment no. 15. In Figure 7, particles of sand (smooth) and of char-covered sand (rough) can be observed from nonagglomerated bed. In Figure 8, a portion of agglomerated bed can be seen and the shape of sand particles can be noticed. The rough material which covers the particles and binds them together is char from BL. Ultimate analyses were performed on bed samples. Table 2 shows the results obtained. Sample A-1 is solid BL, sample A-2 is nonagglomerated bed. The difference between samples A-3 and A-4 is the position where the sample was taken: sample A-3 from the inner part of an agglomerated part and sample A-4 from the outer part.

Table 2. Ultimate Analysis of Different Bed Samples sample no. material N, % C, % H, % S, %

A-1

A-2

solid BL nonagglomerated bed 1.26 0.78 39.66 4.50 4.65 0.20 0.50 0.11

A-3

A-4

agglom erated bed 0.66 34.32 1.20 0.86

agglom erated bed 0.35 18.40 0.60 0.52

It can be observed that, in sample A-2, percentages of N, C, H, and S are very low, so it can be assumed that the nonagglomerated bed is mainly sand covered with some BL char. Sample A-3 shows a carbon percentage very high, only 5% below solid BL, and sample A-4 shows also a high C percentage, so both samples are mainly char from BL.

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Table 3. Combustion Experiments Performed in Spouted Bed run no. sand bed, g sand Dp, µm air flow rate, L(STP) min-1 solid BL flow rate, g min-1

1 8 +1600-2000 23.2

2 8 +1600-2000 15.6

3 6 +1600-2000 10

4 10 +1000-1600 22

5 10 +1000-1600 22

6 10 +1000-1600 22

7 15 +1000-1600 22

8 20 +1000-1600 22

0.89

0.71

0.90

0.72

2.0

0.65

0.75

0.86

Experiments in Spouted Bed. Fluidized bed reactors have a high particle density, with a bed porosity that can be around 0.4-0.6. This is one of the reasons why particle agglomeration and, consequently, defluidization occurs in these type of beds. Spouted bed reactors are well-known because of the vigorous movement or high turbulence that can be achieved by increasing air velocity. Furthermore, conical spouted beds do not have entrainment problems (for a sufficiently long upper section) because cross section increases as bed level is higher. Particle attrition is especially important in the spout or central zone, which means that once the agglomerates enter the spout, they rise along this zone because of the high air velocity and, at the same time, they can break due to collisions between particles. Another important fact is that heat exchange is very efficient in spouted beds due to the counter-current movement of particles, descending along the annular zone and rising in the spout. From this point of view, spouted beds act like counter-current heat exchangers for the solid. To check if spouted bed characteristics and performance can overcome the bed agglomeration observed in the fluidized bed, three sets of experiments were carried out in the spouted bed reactor using solid BL as feed. In the first set, combustion of BL with air as spouting agent was carried out; in the second, pyrolysis of BL with nitrogen as inert gas; and, in the third, combustion of BL with mixtures of nitrogen and air (5, 10, and 15% O2). b-1. Combustion with Air as Spouting Medium. Preliminary experiments have been carried out using different combinations of sand amount in the bed, particle size of the sand, and flow rates of air and solid BL, to know the range where operation is feasible without agglomeration, Table 3. The temperature used in the experiments was 600 °C, which is below the melting point of BL inorganics. By carrying out experiments feeding the solid through different injection points, the maximum combustion efficiency is achieved when the feed is introduced into the annular zone (at an intermediate point between the spout and the wall). The first important conclusion drawn is that, when coarse particles are used, high air velocities are required for vigorous movement and this gives way to entrainment of BL particles (first run in Table 3). When velocity is lowered with these particles (second and third runs), agglomeration occurs. On the other hand, clearly sand is needed for a good performance of combustion. Consequently, if a bed is made up of smaller particles, a greater amount of sand is required. Runs 4 to 8 of Table 3 were carried out with a smaller particle size, 10001600 µm, and the bed amount was increased from 10 to 20 g. When a bed of 10 g is used, combustion is performed well up to solid BL flow rates of 3 g/min. As the bed amount is increased without increasing air flow rate, the maximum BL flow rate that can be burnt

Figure 9. Operation map for BL combustion at different flow rates.

without agglomeration is lower. In fact, a 20 g bed spouted with 22 L(STP)/min of air (run 8 of Table 3), which is near that corresponding to the minimum spouting velocity, only permits the operation up to 0.86 g/min of BL flow rate, as higher flow rates of solid BL lead to bed agglomeration. On the basis of the information obtained in these experiments, the ranges for operation below the melting point of BL inorganics are given by combinations of bed weight (stagnant bed height), air velocity, and BL flow rate. Thus, an operation map has been obtained for the combustion of BL in a bed of 10 g of sand. Figure 9 shows a graph of solid BL flowrate (QBL) plotted against air flowrate (Qair). The theoretical combustion zone corresponds to the right of the dashed diagonal. The lines correspond to different combustion temperatures calculated by means of a heat balance (which means they are theoretical) assuming ideal combustion (100% efficiency). The points of Figure 9 are given combinations of air and solid flow rate that have been obtained experimentally under stable operating conditions without agglomeration. b-2. Pyrolysis. On the basis of the information obtained in the combustion experiments, a sand particle size of 1000-1600 µm, an nitrogen flow rate of 22 L(STP)/min, a BL flow rate of 1 g/min, three bed heights (three sand bed amounts of 6, 10, and 15 g), and a temperature of 600 °C were chosen as the most suitable conditions for carrying out pyrolysis experiments. Unfortunately the maximum operation time was 15 min for the highest bed amount of 15 g, as char accumulated at the bottom of the reactor causing agglomeration of the sand bed. This may be due to the fact that reaction rate is lower in the pyrolysis process than in combustion

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Table 4. Results Obtained in the Spouted Bed Using N2-O2 Mixtures as Spouting Agent % oxygen in spouting gas run time (min) yield of fractions (wt %)a gas liquid char mean gas compositionb H2 CH4 CO CO2 C2+

0 15

5 16

10 18

15 48

21 56

23.9 36.0 37.0

30.5 32.1 34.1

38.6 25.5 32.1

65.2 24.9 9.2

72.6 21.4 5.1

3.4 8.4 25.5 61.2 1.3

2.3 5.6 21.3 68.2 1.3

2.8 4.2 16.3 73.3 1.5

2.6 3.8 10.8 79.9 1.7

2.7 2.0 8.0 84.8 1.6

a Referred to organic matter fed. b Nitrogen-, oxygen-, and moisture-free.

and, consequently, particles have a sticky nature for a longer time than in combustion. Nitrogen velocity was increased in order to obtain a higher turbulence; however, as the linear velocity in the upper cylindrical section was too high, particle entrainment occurred. To avoid this problem requires building a new reactor with a higher diameter. Another alternative is to increase the inlet which, as has been proven, allows us to obtain higher turbulence with the same or lower gas flow rates.9 In view of the impossibility of carrying out pyrolysis with the present configuration of the reactor, combustion with different oxygen concentrations was tried. b-3. Combustion with Mixtures N2-O2 as Spouting Medium. Experiments were carried out with a bed of 15 g of sand, a gas flow rate of 22 L(STP)/min, a BL flowrate of 1 g/min, three percentages of oxygen in the feed, 5, 10 and 15%, at a temperature of 600 °C. Using those flowrates, at 5% O2 in the spouting gas oxygen amount is approximately the stoichometric for combustion of 1 g/min solid BL, where as at 10 and 15% O2, the oxygen fed to the reactor is 1.5 and 3 times the stoichometric oxygen for complete combustion, respectively. To achieve gasification conditions, it would be necessary either to decrease gas flow rate, keeping solid BL flow rate or, keeping the gas flow rate at 22 L(STP)/ min, to increase the BL flow rate. None of those conditions are possible in the experimental system used, as decreasing gas flow rate leads to a decrease in turbulence and bed agglomeration, whereas BL flow rate cannot be increased at one’s discretion without agglomerating the bed. Operation was not satisfactory for oxygen percentages equal to or lower than 10% for this BL flow rate, as at those O2 concentrations, agglomeration of the bed was observed after some minutes. Table 4 shows product distribution and mean gas composition obtained in these experiments as well as in a pyrolysis and an air combustion run. In all the experiments performed, gas composition remained approximately constant (but when the bed agglomerated and the experiment had to be terminated). Mean gas composition is shown free of nitrogen and excess oxygen, so gas composition from different runs can be compared. General Discussion of the Results For the straw BL used and the experimental conditions tested, gasification in a fluidized bed reactor does not seem possible for two reasons:(1)Reaction temper-

ature must be kept below approximately 650 °C to avoid BL inorganics melting, and at low temperature, char consuming reactions proceed very slowly. (2)Defluidification of the bed happened in all the experiments. It is believed to be caused by the swelling properties of BL, and, as swelling is a characteristic stage of the thermal behavior of BL, it seems an unavoidable problem. Due to bed agglomeration, experiments are not repetitive, and the gas-solid contact is bad, so conclusions drawn from the fluidized bed experiments performed must be taken carefully. Gas compositions observed in the experiments are more similar to pyrolysis conditions than to gasification ones, as CO2 percentage in the exit gas is very high, and CO and H2, typical products from char gasification, are very low, showing that steam reforming reactions of BL char are not taking place. Bed agglomeration was also observed by Southards et al. 4 and Verrill et al.5 Anyhow, they used a bigger reactor and their experiments lasted longer that the ones performed in this study, up to 240 min. Southards et al. also used liquid kraft BL, with a solid content of about 50% in weight, and also had problems trying to feed solid BL due to feeding system plugging. The main difference between both feeding systems is that they fed BL from the top of the reactor, dropping BL on the surface of the fluidized bed with a spray, and in our system, BL is fed inside of the fluidized bed. Probably their system is more appropriate for feeding liquid BL, because the longer time allows BL droplets to dry or even partially pyrolyze before coming in contact with bed particles. Temperature in both systems is similar, around 600 °C. Although BL composition differs, besides sulfur content, their liquor has a lower Cl content, so presumably the melting point of the inorganic fraction is higher than in the straw BL used in this study. Anyhow, they used a mixture of oxygen and nitrogen to fluidize the bed and carry out the reaction and did not observe melting, whereas the use of air was not possible in our system, even with BL char, because partial melting caused fluidization loss. To find out the origin of the agglomeration observed in the fluidized bed experiments, ultimate analyses were performed on different samples, solid BL, nonagglomerated bed ,and agglomerated bed. Results of the analyses can be explained if the behavior of BL during its thermal decomposition is taken into account. Swelling is a well-known deeply studied phenomenon of BL during pyrolysis.18-21 BL droplets entering into the reactor swell as they are heated, and capture sand particles from the bed. As more BL is fed into the reactor, the initial agglomerate grows, thus isolating the inner part and avoiding further pyrolysis, and as a result having a higher content in C, H, N, and S as is observed in the ultimate analyses. The conical spouted bed reactor can be an alternative contactor to fluidized bed in order to avoid the agglomeration resulting from BL swelling, but, in the bench reactor tested, operation does not seem possible (18) Hupa, M.; Solin, P.; Hyo¨ty, P. J. Pulp Pap. Sci. 1987, 13 (2), 67. (19) Frederick, W. J.; Noopila, T.; Hupa, M. J. Pulp Pap. Sci. 1991, 17 (5), 164. (20) Ale´n, R. Bioresour. Technol. 1994, 49, 99. (21) Frederick, W. J.; Hupa, M. J. Pulp Paper Sci. 1994, 20 (10), 274.

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without the presence of more than 10% oxygen in the spouting medium. As was observed in the fluidized bed, spouted bed agglomeration could be a result of BL swelling during its pyrolysis. The effect of oxygen on kraft BL swelling has been studied by Frederick and Hupa:21 experiments performed in a laboratory furnace showed that a higher oxygen concentration in the reaction atmosphere leads to less swelling on BL droplets, but this effect is essentially a temperature one, since oxygen increases the temperature of the burning volatiles around a BL particle. Even though in this work the authors do not discuss the effect of oxygen content on swelling time, presumably the higher the oxygen content, the shorter the time BL droplets take to complete swelling. This effect could be causing bed collapse in the spouted bed at low oxygen percentages. Increasing the oxygen concentration in the spouting agent makes the product distribution change: char and liquid fractions diminish and the gas fraction increases. In the experiments with 0, 5, and 10% O2, where the bed collapsed, char fraction obtained is 30-40%. In conditions of higher oxygen concentration, the char fraction obtained falls to 5-9%. The decrease observed in the liquid fraction obtained could be due to acceleration of gas-phase oxidation reactions as the oxygen partial pressure increases.

Olazar et al.

In what concerns gas composition, CO2 is the main product, its percentage increasing as oxygen concentration increases. On the other hand, methane and CO decrease as oxygen concentration increases. Hydrogen and C2+ species seem to be insensitive to oxygen concentration. The combustion of straw BL seems to be feasible in a spouted bed reactor, provided that the right experimental conditions are achieved (bed amount, air and BL flowrates, reactor configuration and dimensions). This could be useful for soda BL, where no sodium sulfide is used in the cooking process. The use of spouted bed combustion cannot be considered an alternative for the thermal processing and recovery of kraft BL, where sulfur must be recovered in the form of sulfide in order to be readily used in the cooking process. This means that future work on spouted bed should be directed to achieve a steady operation under gasification (reducing) conditions. Acknowledgment. The research team at the University of Zaragoza express their gratitude to the CICYT (Project AMB95-0575) for providing frame support for this work and to MEC for research grants awarded to J. L. Sa´nchez. EF020034N