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Straw Black Liquor Steam Reforming in a Fluidized Bed Reactor. Effect of Temperature and Bed Substitution at Pilot Scale Jose´ L. Sa´nchez,* Alberto Gonzalo, Gloria Gea, Rafael Bilbao, and Jesu´s Arauzo Thermochemical Processes Group, Arago´ n Institute of Engineering Research (I3A), University of Zaragoza, Marı´a de Luna, 3, 50018 Zaragoza, Spain Received January 17, 2005. Revised Manuscript Received April 20, 2005
Gasification via the steam reforming of straw black liquor (BL) was performed in a pilot-plant fluidized bed reactor. The effects of bed material replacement (from an initial bed of calcium carbonate to a bed composed of BL char) and bed temperature (575-640 °C) on the dry gas composition, the dry gas lower heating value (LHV), gas yield, fraction of carbon to gas, and cold gas efficiency have been investigated. Although bed defluidization problems were experienced under the operating conditions tested, a medium heating value gas (with a dry gas LHV of ∼8200 kJ/m3 STP) (where STP indicates standard temperature and pressure) was obtained, composed mainly of H2 (65 vol %) and CO2 (30 vol %). Because of the high ratio of steam to biomass used, little influence of the reaction temperature on the gas composition was observed at >550 °C. At 600 °C, which is a temperature lower than that usually utilized in biomass gasification, ∼2.2 m3 STP/kg daf of gas production, 75% carbon conversion to gas, and a cold gas efficiency of 90% were achieved, showing that the low-temperature steam reforming of BL could be a suitable process if bed defluidization can be overcome.
Introduction Black liquor (BL), which is the main biomass-containing waste from chemical pulp and paper production, is also an important fuel, supplying the major share of energy used in pulp mills.1 It consists of inorganic compounds from the chemicals used in the pulping process and organic compounds from the raw material used for paper production. Recovery of the chemicals used, as well as the energy contained in the organic compounds, is important for the economics of the paper industry. Traditionally, the Tomlinson recovery boiler technology has been used to recover chemicals and energy from BL. Since their introduction, Tomlinson boilers have been subject to many improvements; however, they continue to suffer from relatively low thermal efficiency and a low ratio of power output to total heat input when coupled to a steam power turbine. For this reason, the pulp and paper industry is investing money and effort into the development of new recovery systems. BL gasification promises to be a future alternative or a complement to conventional Tomlinson recovery boilers,2 as integrated gasification-combined-cycle power plants can produce electrical energy with higher efficiency than conventional steam-cycle power plants.3 It also offers the possibility of recovering sodium and sulfur separately, which would be very advantageous in the pulping process.4 * Author to whom correspondence should be addressed. Telephone: +34 976 76 22 24. Fax: +34 976 76 18 79. E-mail address:
[email protected]. (1) Berglin, N.; Berntsson, T. Appl. Therm. Eng. 1998, 18, 947961. (2) Na¨sholm, A. S.; Westermark, M. Energy Convers. Manage. 1997, 38, 1655-1663.
The gasification alternatives proposed fall roughly into two categories: high- and low-temperature processes. Both types have advantages and disadvantages. In high-temperature processes, higher carbon conversion can be reached in a shorter residence time, and tar production will be low. On the other hand, BL inorganics usually have a low melting point and, in a hightemperature process, will be recovered as a smelt that is very corrosive and can cause smelt-water explosions if improperly handled. Low-temperature processes avoid the presence of the smelt, but the carbon conversion rate is slower and the amount of tar produced can be significant. Several industrial processes have been proposed,5-8 but only two seem to be approaching full-scale commercial development at the moment. One of these, the Chemrec process, is a high-temperature air blown gasification process,9 in which the BL is processed in an entrained flow reactor to obtain a low calorific value gas. The Chemrec process has also been tested under pressurized conditions and using oxygen instead of air as the gasifying agent. The other process, the StoneChem (3) Larson, E. D.; McDonald, G. W.; Yang, W. R.; Frederick, W. J.; Iisa, K.; Kreutz, T. G.; Malcolm, E. W.; Brown, C. A. In Proceedings of the 1998 International Chemical Recovery Conference, June 1-4, 1998, Tampa, FL; pp 1-18. (4) Sadowski, R. S.; Parthasarathy, P.; Henderson, S.; Kinstrey, B. Tappi J. 1999, 82 (11), 59-62. (5) Grace, T. M.; Timmer, W. M. Proceedings of the 1995 International Chemical Recovery Conference, April 24-27, 1995, Toronto, Canada; pp 269-275. (6) Panda, A. Ippta J. 1992, 4 (3), 12-26. (7) Empie, H. J. Tappi J. 1991, 74 (5), 272-276. (8) Kulkarni, A. G.; Pant, R. Ippta J. 1990, 2 (3), 30-36. (9) Stigsson, L. Proceedings of the 1998 International Chemical Recovery Conference, June 1-4, 1998, Tampa, FL; pp 663-673.
10.1021/ef050020s CCC: $30.25 © 2005 American Chemical Society Published on Web 06/02/2005
Steam Reforming of Straw Black Liquor
process (formerly the MTCI process), is a low-temperature gasification, based on a fluidized bed steam reformer indirectly heated by pulse burners. Pulse burners offer a very high gas-side heat-transfer rate, thereby eliminating a major cause of heat-transfer resistance10,11 and improving the heat-transfer rate. Therefore, the heat-exchange surface can be smaller than that in other systems, which makes them very well-suited for indirect heating of the fluidized bed.7,10-12 Another advantage of indirect heating is that any change in the biomass fed, such as an increase in the water content or a decrease in the calorific value, would not affect the quality of the gas produced as much as it would in an air-blown gasification system, because the surplus of heat needed would be supplied by the pulse burner simply burning more fuel gas. However, in an air- or oxygen-blown process, because the heat is supplied by the partial combustion of the biomass, the produced gas will be more diluted with CO2 and N2 (from the air, if used). More details on pulse-enhanced gasification technology can be found elsewhere.13-15 The work presented in this paper is based on the MTCI technology that is applied to straw BL. Even though the main raw material for virgin pulp production is wood, ∼10% of the total chemical pulp produced in the world is made using nonwoody material (commonly straw). The high silica content in nonwood fibers makes the recovery of chemicals and energy from BL more difficult, because it provokes more deposits in the boiler and higher viscosity of the liquor when evaporated. The study of the gasification of alkaline BL from straw is of interest, because it can offer a reliable process for chemical and energy recovery that is not currently possible by means of conventional recovery boilers. Experimental Section Material. The straw BL used in this study was derived from the soda pulping of wheat straw. It was produced by an integrated paper mill that uses recovered paper and cereal straw as raw materials. In the pulping process, straw is cooked at ∼90 °C with a caustic soda solution. The ultimate and proximate analyses are listed in Table 1. The elemental analysis was obtained using a CHNS elemental analyzer (Carlo Erba, model EA1108). The proximate analysis was performed following the standard norms (ISO 1171 for ashes and ISO 5623 for volatiles). The metal analysis was performed by atomic absorption after previous alkaline fusion, in accordance with ASTM standard D368296. As can be observed, there were significant amounts of sodium, potassium, and chloride, which lower the melting point of the ashes. The lower heating value (LHV) of the BL solids was 15 160 kJ/kg. The BL used as raw material was composed of 90%-95% water in its original state but was concentrated to 37% solids (10) Mansour, M. N.; Durai-Swamy, K.-S.; Warren, D. W. Indirectly heated thermochemical reactor apparatus and processes, U.S. Patent No. 5,306,481, April 26, 1994. (11) Mansour, M. N.; Durai-Swamy K.-S.; Warren, D. W. Indirectly heated thermochemical reactor processes, U.S. Patent No. 5,536,488, July 16, 1996. (12) Black, P. N. Tappi J. 1991, 74 (2), 65-68. (13) Rockvam, L. N.; Tenore, F. Proceedings of the 1996 Engineering Conference, September 16-19, 1996, Chicago, IL; pp 269-273. (14) Mansour, M. N.; Steedman, W. G.; Durai-Swamy, K.; Kazares, R. E.; Raman, T. V. Proceedings of the 1992 International Chemical Recovery Conference, June 7-11, 1992, Seattle, WA; pp 473-478. (15) Durai-Swamy, K.; Warren, D. W.; Mansour, M. N. Proceedings of the 1989 International Chemical Recovery Conference, April 3-6, 1989, Ottawa, Canada; pp 217-221.
Energy & Fuels, Vol. 19, No. 5, 2005 2141 Table 1. Ultimate and Proximate Analysis (Dry Basis) for the Black Liquor under Study element or component
quantity of alkaline black liquor from straw (wt %) Ultimate Analysis
C H N S Cl K Na Si other
39.05 4.54 1.00 0.78 3.50 4.10 8.83 0.23 37.97 Proximate Analysis
ash volatile matter, VM fixed carbon, FC
20.63 65.98 13.39
(by weight) in a falling film evaporator prior to being fed into the fluidized bed reactor. Fluidized Bed Reactor. The gasification plant was designed and built by MTCI, initially for Kraft BL, and has a nominal capacity of 25 kg/h of BL. Figure 1 shows a diagram of the plant. The reactor is made of two sections of stainless steel, with an inner layer of refractory concrete. The lower section is 182 cm high and has an inner diameter of 21.6 cm; the upper one (freeboard) is 162 cm high and has an inner diameter of 31.7 cm. The distributor plate consists of eight sparger tubes. In the fluidized bed reactor, superheated steam is used as a fluidizing and reforming agent. The steam flow rate was measured by a differential manometer and a calibrated orifice meter. Concentrated BL was injected at the bottom of the reactor, near the distributor plate, by means of a peristaltic pump and a steam spray. The gas leaving the reactor flows into two cyclones in series. The first returns the solids that are removed from the gas stream to the reactor, whereas the solids removed by the second cyclone are collected in a charpot. During the operation, solids are periodically removed from the bed. The bed drainage consists of a sloping pipe that connects the bed to a small tank, which collects the solids. A differentialpressure manometer was used to provide a rough indication of the bed height. The bed temperature was measured by eight thermocouples located at different points. The maximum difference between them was ∼5 °C, indicating that the bed was well-fluidized. The pulse burner is located below the reactor, originally designed to work with natural gas, it was modified so that propane (LPG) could be used as fuel gas, providing the energy necessary to evaporate the water fed with the BL and to maintain the gasification reactions. Two U-shaped vertical resonance pipes, with a length of ∼3 m each and a diameter of 5 cm, are immersed in the bed and transfer the heat to the reactor. Exhaust gas from the burner was used to heat the fluidizing steam up to ∼400 °C. Immediately after leaving the cyclones, the gas was cleaned in a venturi scrubber, where the hot gas is put in contact with cool water to condense the steam and the tar present after the gasification process and remove small particles that escape from the second cyclone. The plant is provided with a closed circuit for the cleaning water. Gas samples were taken periodically and analyzed in a gas chromatograph, and the gas was burned in a flare. Experimental Procedure. For the startup of a run, ∼80 kg of calcium carbonate was introduced into the reactor. Calcium carbonate, with a mean particle diameter of 200 µm, was chosen as the initial bed, because it is less hygroscopic than sodium carbonate, which is the main component to be expected after BL gasification, and both have similar particle properties. Because, at the beginning of an experimental run,
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Figure 1. Schematic diagram of the pilot plant. Table 2. Runs Performed for Bed Substitution
Table 3. Runs Performed at Different Temperatures
Black Liquor (BL) fed bed temp run duration run (°C) (h) 1 2 3 4
550 560 555 560
82 72 53 78
Black Liquor (BL) fed
(kg)
(kg daf)
steam/BL ratio
868 892 645 911
249 255 187 263
6.7 5.4 4.7 5.6
the reactor and bed are at ambient temperature, nitrogen was used as a fluidizing agent. The pulse burner was started and, when the temperature downstream from the reactor exceeded 100 °C, the nitrogen was switched to steam. After the bed temperature reached the level desired, the steam flow rate was adjusted to obtain a superficial velocity in the bed of 30 cm/s (used for all the experiments performed) and the BL was fed into the reactor. The plant was run continuously for several days because one of the objectives of this work was to test the feasibility of BL steam reforming as a continuous process. Continuous operation was not possible for long periods because, after several hours (in the range of 50-120 h), bed defluidization was observed around the heating pipes. A steady-state composition of the reactor bed could not be attained in a single run because, due to the length of the experiments performed and the feeding rate used, the inorganics fed into the reactor were insufficient to completely replace the calcium carbonate used for the startup of the plant. For this reason, four runs were performed, at a bed temperature of ∼550 °C, to study the influence of the bed composition on the gasification. To complete the substitution of the initial calcium carbonate bed with inorganic material from BL, bed material from a previous run was used as the initial bed for the subsequent run. Table 2 shows the bed temperature, run duration, BL fed (on a wet, dry, and ash-free [daf] basis), and steam/BL ratio ((kg/h)/(kg daf/h)) for these runs. Five experiments were performed to study the influence of the reaction temperature, with an initial bed of calcium carbonate. Table 3 shows the bed temperature, run duration, BL fed (wet and daf basis), and steam/BL ratio ((kg/h)/(kg daf/ h)) for these runs. As can be observed in Tables 2 and 3, two important parameters for the gasification were used that substantially
bed temp run duration run (°C) (h) 5 6 7 8 9
475 550 575 600 640
54 120 110 87 6
(kg)
(kg daf)
steam/BL ratio
825 1153 873 491
236 330 254 141
4.9 8.1 9.2 11.4
differ from what is common in biomass gasification: the temperature used is rather low (475-640 °C) and the ratio of steam to biomass is quite high (5-11). The low temperature is due to the necessity of avoiding the melting of the BL inorganics. The BL ash melting point was measured following ASTM standard 1857-87. The initial deformation temperature was 610 °C under a reducing atmosphere (60% CO and 40% CO2) and 615 °C under an oxidizing (air) atmosphere. Total melting of the sample was observed at 675 °C in both atmospheres. Although at 640 °C, which was the highest temperature tested for BL gasification, the BL inorganics are not completely melted, the sintering rate seems to be enough to disallow operation at that temperature. In biomass steam gasification, the steam/biomass ratios used usually are in the range of 0.8-1.5. The high steam/BL ratios used in this work could be reduced either by increasing the BL fed to the reactor, increasing the solid content of the BL, or decreasing the superficial velocity in the fluidized bed. In the reactor used, the bottleneck in the BL feeding capacity is caused by the pulse burner, because it was not possible to increase the feeding rate of BL and keep the bed temperature at the desired level. On the other hand, it was not possible to handle the straw BL with a solid content greater than ∼37% (wt) without problems, because a small temperature drop made its viscosity too high for the pump available. In regard to superficial velocity, it was found that, with a lower velocity in the bed, bed defluidization occurred more easily, so it was decided to keep this parameter constant. The bed defluidization observed has also been observed on the laboratory scale in fluidized and spouted bed reactors16 using the same BL. For fluidized bed reactors at laboratory scale, defluidization (16) Olazar, M.; Aguado, R.; Sa´nchez, J. L.; Bilbao, R.; Arauzo, J. Energy Fuels 2002, 16, 1417-1424.
Steam Reforming of Straw Black Liquor
Figure 2. Evolution of bed composition (runs 1-4): (0) sodium, (O ) soluble matter, (4) organic matter, and (9) average particle diameter (dp)). is so severe that the operation of the reactor becomes impossible. The same problem has been observed in Kraft BL fluidized-bed air-blown gasification at bench scale.17,18 A high steam/biomass ratio is beneficial because it increases the hydrogen content and diminishes the tar content; however, on the other hand, the LHV of the dry gas is also diminished19 as the CO2 content increases and the CO and hydrocarbon content decreases because of the water gas shift and steam cracking reactions.
Results and Discussion Effect of Bed Substitution. As is explained in the Experimental Section, for the startup of an experiment, calcium carbonate was introduced into the reactor, and because the length of the experiments was insufficient to replace it with solids from the BL fed, the bed composition could not be considered to be able to attain steady state in a single run. To determine if the bed composition has an influence on the gasification performance, four runs were performed in each of which the bed from a previous experiment was used as the initial bed in the following experiment. The temperature selected for these experiments was 550 °C. To determine when the total replacement of the bed was attained, samples were taken during the runs and the sodium content, matter soluble in water (mainly sodium and potassium carbonates and chlorides), and organic matter (determined as weight loss in a muffle furnace at 500 °C; this material is likely to be formed mainly by fixed carbon, but also can contain hydrogen, oxygen, sulfur, and other minor compounds present in the initial BL organic composition) were analyzed. The results are shown in Figure 2, where the weight percentages of soluble matter, sodium and organic matter (left axis), and mean particle diameter (right axis) are shown versus the cumulative time for runs 1-4. The amount of soluble matter in the bed increases up to ∼70% (in weight), and the organic matter of the (17) Southards, W. T.; Blue, J. D.; Dickinson, J. A.; McIlroy, R. A.; Verrill, C. L. Final Report, U.S. DOE Contract No. DE-FC3694G010002, June 1997. (18) Verrill, C. L.; Dickinson, J. A.; Kitto, J. B. Proceedings of the 1998 International Chemical Recovery Conference, June 1-4, 1998, Tampa, FL; pp 1067-1078. (19) Franco, C.; Pinto, F.; Gulyurtlu, I.; Cabrita, I. Fuel 2003, 82 (7), 835-842.
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Figure 3. Effect of bed substitution on gas composition: (0) H2, (O) CO2, (4) CO, (3) CH4, and (]) C2. Plot of solid squares (9) represents the effect of bed substitution on the lower heating value (LHV).
bed up to ∼23%, indicating that the calcium carbonate is being substituted by char and inorganic matter from the BL fed. The sodium in the bed solids represents ∼30% of the material at the steady state. During the fourth experiment, it was confirmed that the bed composition no longer changed. The mean particle diameter of the bed was also analyzed, as can be observed in Figure 2. After 280 h of operation, the final value was ∼50% larger than that at the beginning of run 1. It can be observed that, during run 4, the particle diameter of the bed does not change noticeably. No fluidization problems were detected as a result of the bigger particle diameter. As can be observed in Figure 3, the effect of replacing the initial calcium carbonate bed with solids from BL on the dry gas composition is negligible: only the hydrogen percentage seems to be slightly higher after run 1, and the CO percentage slightly lower. Because of these changes, the dry gas LHV, which also is shown in Figure 3, undergoes a small decrease up to ∼8500 kJ/m3 STP (where STP indicates standard temperature (0 °C) and pressure (1 atm)). In Figure 4, the yields obtained to dry gas, expressed as m3 STP/kg daf, the carbon conversion to gas (%), and the cold gas efficiency (%) are shown for runs 1-4. The fraction of carbon to gas can be calculated from the dry gas composition (% of CO, CO2, C2, and CH4), the gas yield, and the carbon content of the biomass. The cold gas efficiency measures the efficiency of the gasification, expressed as the percentage of the energy in a biomass that is recovered in the fuel gas produced, only taking into account the LHV of both biomass and gas. It is calculated as shown in eq 1:
cold gas efficiency ) [LHV of dry gas (kJ/m3 STP) × dry gas yield (m3 STP/kg BL daf)/ LHV of BL (kJ/kg BL daf)] × 100 (1) As can be observed, the gas yield increases during the operation time, showing a more significant increase after 150 h of operation. Although, as can be observed in Table 2, the mean reaction temperature was not exactly the same for the four experimental runs, this
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Figure 4. Effect of bed substitution on (0) gas yield, (O) fraction of carbon to gas, and (4) cold gas efficiency.
Figure 5. Effect of bed substitution on the yield of dry gas components: (0) H2, (O) CO2, (4) CO, (3) CH4, and (]) C2.
tendency does not seem to be caused by these temperature changes, because, from run 1 to run 4, the mean temperature increased and decreased. As a result of the increase of the yield to gas, the fraction of carbon to gas and the cold gas efficiency also increase. In Figure 5, the yield to each gas component (H2, CO2, CO, CH4, and C2) is shown (in units of m3 STP/kg daf). It can be observed that the yields to H2 and CO2 increase over time, because of the change in the bed composition, whereas the minor dry gas components do not show any significant tendency. It is well-known that alkali metals are very effective catalysts for carbon gasification reactions and have been used to promote the carbon gasification of coal,20-23 increasing the gasification rate by a factor of 10-100, with respect to noncatalyzed reactions. It can also be added as a catalyst promoter, to diminish the coke deposition on tar cracking catalysts.24 Sodium is present in BL in large quantities, because it is a cooking chemical in the pulping process, (20) Wen, W. Y. Catal. Rev.-Sci. Eng. 1980, 22 (1), 1-30. (21) Kapteijn, F.; Peer, O.; Moulijn, J. A. Fuel 1986, 65, 1371-1376. (22) Sadman, F.; Sams, D. A.; Punjak, W. A. Fuel 1987, 66 (2), 16581663. (23) Nishiyama, Y. Fuel Process. Technol. 1991, 29, 31-42. (24) Zhang, R. Q.; Brown, R. C.; Suby, A.; Cummer K. Energy Convers. Manage. 2004, 47 (7-8), 995-1014.
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and, in the case of straw BL, also potassium, which is present in straw. Its effect in Kraft BL char gasification with CO2 has been studied by Li and van Heiningen25,26 and Frederick and co-workers.27,28 An interesting finding from these works is that the catalytic effect of sodium in the CO2 gasification of Kraft BL char is stronger than that in coal gasification when catalyzed by alkali metals, because the gasification rate is ∼1 order of magnitude faster. The Na/C optimum ratio is higher for BL. The reason for this higher activity found in BL could be that sodium is very well dispersed in its carbonaceous matrix. Steam gasification of Kraft BL char also has been studied by Li and van Heiningen29 and by Whitty et al.30 It was been found that the steam BL gasification rate is ∼2 orders of magnitude faster than that for alkali-catalyzed coal gasification and several times faster than the CO2 gasification rate of the same BL. van Heiningen et al.31 studied the influence of pyrolysis heating rate on the BL char reactivity. It was found that fast pyrolysis chars show a higher reactivity than slow pyrolysis chars, which could also be due to a finer dispersion of active metals in the char. The influence of sodium content in the CO2 and steam gasification of synthetic BL has been studied by Verrill et al.32 They found that, for both steam and CO2, the gasification rate increases with the sodium content, up to an optimum value of the Na/C ratio and that the gasification rate is slower when physically mixing Na2CO3 with sodium-free organic compounds, confirming that catalyst dispersion is important to enhance the carbon gasification rate. Similar results have been found in the CO2 gasification of the straw BL used in this work.33 Even though all the works cited relating to the catalytic activity of alkali metals have been performed at higher temperatures than those used in this study, the increase observed in the different yields could be due to the presence of a higher amount of sodium in the bed. Effect of Temperature. Despite the influence observed of the bed composition on the gasification yield, it is considered that, although the experiments performed to study the temperature influence do not have the same duration, they can be used for purposes of comparison, because the bed composition effect on the different yields is more noticeable after ∼150 h of continuous operation and the influence of the temperature is stronger than that of the bed composition. It must also be considered that, in runs 5-9, the steam/ biomass ratio, as shown in Table 3, could not be kept constant, so the results obtained can also be influenced by the change in this parameter. (25) Li, J.; Van Heiningen, A. R. P. Can. J. Chem. Eng. 1989, 67, 693-697. (26) Li, J.; van Heiningen, A. R. P. Ind. Eng. Chem. Res. 1990, 29, 1776-1785. (27) Frederick, W. J.; Hupa, M. Tappi J. 1991, 74 (7), 177-184. (28) Frederick, W. J.; Wåg, K.; Hupa, M. Ind. Eng. Chem. Res. 1993, 32, 1747-1753. (29) Li, J.; van Heiningen, A. R. P. Ind. Eng. Chem. Res. 1991, 30, 1594-1601. (30) Whitty, K.; Hupa, M.; Frederick, W. J. J. Pulp Paper Sci. 1995, 21 (6), 214-221. (31) van Heiningen, A. R. P.; Arpiainen, V. T.; Ale´n, R. Pulp Pap. Can. 1994, 95 (9), 55-60. (32) Verrill, C. L.; Whitty, K.; Backman, R.; Hupa, M. J. Pulp Paper Sci. 1998, 24 (3), 103-110. (33) Gea, G.; Sa´nchez, J. L.; Murillo, M. B.; Arauzo, J. Ind. Eng. Chem. Res. 2004, 43, 3233-3241.
Steam Reforming of Straw Black Liquor
Figure 6. Mean dry gas composition versus temperature: (0) H2, (O) CO2, (4) CO, (3) CH4, and (]) C2.
The mean dry gas composition for the different temperatures tested is shown in Figure 6. Two attempts were made to work at a temperature approaching 650 °C (run 9). In both cases, bed defluidization was observed a very short time after the startup of the run (3 and 6 h). This was due to the partial melting of the BL inorganics, which have a very low melting point, because of their composition, being rich in chloride, sodium, and potassium. For these short experiments, mass balances were not performed and only the mean dry gas composition is shown. For temperatures of >550 °C, there is no significant change in the gas composition, with H2 (65%) and CO2 (30%) being the most important components in volume. Only a small decrease in CH4 can be observed as the temperature is increased from 550 °C to 640 °C, which could be due to the enhancement of the steam reforming of methane. Similar gas composition and tendencies have been reported for Kraft BL steam gasification using the same technology at temperatures of 554-627 °C.15 As can be observed in Figure 6, only the gas produced at 475 °C shows a different composition, with 35% of CO2, ∼25% of both H2 and CH4, 10% of CO, and 3% C2. This gas composition is similar to that obtained in the pyrolysis of the same BL in a fixed bed at 500 °C,34 which suggests that this composition is mainly due to the pyrolysis of BL and the influence of the water-gas shift reaction is less important probably due to kinetic constraints, because of the low reaction temperature. The change observed in gas composition affects the LHV of the dry gas, which is a parameter that indicates the gas quality. Figure 7 shows the evolution of LHV for runs 5-8. As would be expected, the higher the temperature, the lower the LHV of the gas, this being due to the increase of the CO2 percentage (formed by water gas shift) and the decrease of CO (consumed in the water-gas shift reaction) and hydrocarbons (by steam reforming). Figure 8 shows the gas yield, obtained in runs 5-8. The significant effect of the temperature can be observed, as the gas yield increases by 1 order of magnitude when the temperature is increased from 475 °C to 600 °C. The gas yield obtained at 600 °C (∼2.2 m3 STP/ (34) Gea, G. Ph.D. dissertation, University of Zaragoza, Spain, 2001.
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Figure 7. Gas LHV (for runs 5-8): (0) 475 °C, (O) 550 °C, (4) 575 °C, and (3) 600 °C.
Figure 8. Gas yield (for runs 5-8): (0) 475 °C, (O) 550 °C, (4) 575 °C, and (3) 600 °C.
kg daf) is rather high, despite the low temperature used. The gas yield obtained with different biomasses in fluidized bed reactors (such as pine chips,35 eucalyptus,36 and almond shells37) at higher temperatures (∼800 °C) and lower steam/biomass ratios is ∼1.5 m3 STP/kg daf. The higher value obtained in this work could be due to the steam/BL ratio used, which is known to have an influence on the gas production.19,35-38 The evolution of the fraction of carbon to gas with time is shown in Figure 9. As can be seen, this parameter is strongly dependent on the reaction temperature, as well as on the steam/biomass ratio.19 The maximum value obtained, for the experiment at 600 °C, is ∼75%. Higher carbon conversion has been obtained at higher temperatures from steam gasification of biomass (∼83% in pine gasification at 800 °C and a steam/biomass ratio of 0.8),19 and also for air gasification of Kraft BL in a fluidized bed of TiO2 particles,39 where (35) Rapagna`, S.; Latif, A. Biomass Bioenergy 1997, 12, 281-288. (36) Rapagna`, S.; Jand, N.; Kiennemann, A.; Foscolo, P. U. Biomass Bioenergy 2000, 19, 187-197. (37) Encinar, J. M.; Gonza´lez, J. F.; Gonza´lez, J. Fuel Process. Technol. 2002, 75, 27-43. (38) Gil, J.; Corella, J.; Aznar, M. P.; Caballero, M. A. Biomass Bioenergy 1999, 17, 389-403. (39) Zeng, L.; van Heiningen, A. R. P. Energy Fuels 2000, 14, 8388.
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Figure 9. Fraction of carbon to gas (for runs 5-8): (0) 475 °C, (O) 550 °C, (4) 575 °C, and (3) 600 °C.
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Figure 11. Plot showing (0) the percentage of solids produced and (O) carbon content, each relative to temperature. Table 4. Estimation of the Carbon Content in Tar Carbon Content (%) run
temp (°C)
fraction in solids
fraction to gas
fraction to tar
5 6 7 8
475 550 575 600
38.1 17.3 13.3 15
14.1 30.1 52.2 72.1
47.8 52.6 34.5 12.9
Table 5. Sodium Distribution (for Runs 5-8) sodium content (%)
Figure 10. Cold gas efficiency (for runs 5-8): (0) 475 °C, (O) 550 °C, (4) 575 °C, and (3) 600 °C.
the carbon to gas conversion ranged from 82% at 706 °C to 90% at 896 °C. In Figure 10, cold gas efficiency is shown for runs 5-8. Once again, the higher the temperature, the more beneficial the effect on the gasification process. Here, the steam gasification process for alkaline BL shows a very good performance, because cold gas efficiency at 600 °C is almost 90%, which means that the gas produced contains most of the calorific power of the BL solids. As a consequence of the low temperatures used in the fluidized bed operation, significant amounts of char and tar are formed. These are undesirable products of the gasification process, because they diminish the yield to gas, which is the desired product. Moreover, tar causes problems downstream from the gasifier, because, when gas is cooled below the tar dew point, it condenses on cold surfaces, which causes fouling and plugging. In Figure 11, the percentage of solids produced, in relation to the BL solids fed, is shown versus the bed temperature for runs 5-8. These solids produced in the gasification of BL contain the inorganics present in BL solids and char, which is composed mainly of carbon. It can be observed that the solids produced decrease up to a value near the ash content of BL solids, and that the
run
temp (°C)
fraction in scrubbing water
5 6 7 8
475 550 575 600
1.2 4.7 5.1 13.5
fraction in solids 96.3 95.3 93.3 86.2
carbon content, also shown in Figure 11, is ∼15%. The solids that escape from the cyclone are not taken into account and should be recovered from the scrubber water. The tar produced could not be determined accurately. When a slip stream of gas was cooled, the sampling equipment (ice cooled condensers or cooled impingers with 2-propanol) very quickly filled with a mixture of tar and water, and the glass frits of the impingers became plugged with tar. An indirect estimation of the tar produced can be made from the carbon balance of the gas and solids. In Table 4, the percentage of carbon in the tar is calculated as the balance of the mean fraction of carbon to gas and the percentage of carbon in the solids produced (shown in Figure 11), accounting for 100% of the carbon fed in the BL solids. At 475 °C, ∼50% of the carbon contained in BL is converted into tar. This amount decreases to ∼10% at 600 °C. Another important issue in BL treatment is the sodium recovery. In Table 5, the sodium distribution between the solids produced and the scrubbing water is shown. As can be observed, the higher the temperature is, the lower the percentage of sodium recovered in the solids and, obviously, the higher the sodium percentage in the scrubbing water. It is not likely that sodium evaporation occurred in view of the low temperature used. It has been found in Kraft BL pyrolysis
Steam Reforming of Straw Black Liquor
and gasification experiments40 that sodium vapor is produced over 675 °C by a reaction of sodium carbonate with carbon. If no sodium evaporation occurs, the increase of sodium in the scrubbing water could be due to small particles escaping from the cyclones. It can be considered that the steam fluidized bed gasification of straw BL works well from the point of view of sodium recovery, but that it is probably necessary to recover sodium from the scrubbing water as well as from the solids removed from the bed or the cyclone. Conclusions Steam gasification of straw black liquor (BL) in a fluidized bed has been studied in a pilot-scale reactor. To avoid bed defluidization due to the melting of inorganics, the temperature of the bed must be kept at 550 °C. The (40) Li, J.; van Heiningen, A. R. P. Tappi J. 1990, 73 (12), 213219.
Energy & Fuels, Vol. 19, No. 5, 2005 2147
dry gas composition is not significantly dependent on the temperature, probably because of the high steam/ BL ratio used in the experimental apparatus. As would be expected, the higher the temperature, the higher the gas and the lower the char and tar production. Additional runs were performed to complete the replacement of the initial bed material (calcium carbonate) by solids from the gasification of the straw BL used. The dry gas composition is not significantly dependent on the bed substitution. However, the gas yield, the fraction of carbon to gas, and the cold gas efficiency showed a continuous increase, which could be due to the catalytic effect of sodium, which increased in percentage, in terms of the bed solids. Acknowledgment. The authors thank Miguel Pelayo, head of the R&D department of the SAICA Paper Company, and they also thank the Ministerio de Educacio´n y Ciencia for the research grants awarded to J.L.S. and G.G. EF050020S
