Energy Fuels 2009, 23, 4659–4667 Published on Web 08/19/2009
: DOI:10.1021/ef900623n
Experimental Investigation on Tar Formation and Destruction in a Lab-Scale Two-Stage Reactor Yi Chen, Yong-hao Luo,* Wen-guang Wu, and Yi Su Institute of Thermal Energy Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Received June 19, 2009. Revised Manuscript Received July 27, 2009
A lab-scale two-stage reactor has been constructed for studying the release and destruction of tars in the twostage gasifier. First, the pyrolysis characteristics of three fuel samples are investigated only using the single stage reactor. The results show that the maximum value of tar yield is: rice straw 25%, corn straw 22%, and fir sawdust 31% of the initial fuel. Then, the experimental program is extended to investigate the effect of operating conditions in the second stage of the reactor on tar removal. The effects of temperature, residence time, char particle size, char type, fuel type, and diluted air feeding to throat on tar emission has been studied. The results show that the tar decreased with increasing temperature and residence time and with decreasing char particle size. The char type has little effect on tar reduction. Tar emission with limited diluted air feeding is obviously less than that with empty second stage due to the more reactive radicals produced in oxidative conditions. The straw tars appear to have a different suite of compounds than the other two samples of derived material and presumably have different cracking pathways. The tars collected from first stage and second stage have been characterized by gas chromatography/mass spectrometry (GC/MS) and gel permeation chromatography (GPC). The results indicate that tar after pyrolysis contains a large amount of oxygenated constituents. With the increasing of reaction severity (from the empty heated second stage to heated second stage with char bed), the tar compounds reacted further (polymerized) to form larger molecular mass material. It is clear that the material characterized by GC/MS represents a very small part of the total tar. The results have shown that the tar emission from two-stage gasifier can reduce to low levels using optimized operating conditions, but complete tar removal is difficult to realize due to manipulation of operating parameters and fuel type. The methods for removing the tar from gasified gas can be categorized into two types depending on the location where tar is removed, either in the gasifier or outside the gasifier (secondary method).3 The former method consists of optimization of operating parameters, utilization of additive/catalyst, and modifications of gasifier. The latter comprise the chemical treatments of tar, such as thermal decomposition, catalytically cracking, and a combination of them, and mechanical separation of tar.3 But the efficient removal of tar still remains the main technical barrier to the successful commercialization of biomass gasification technologies.4 Until now relatively little attention has been paid to partial oxidation as a way to remove tars.5-10 Brandt and Henriksen5,6
1. Introduction Biomass is one of the main renewable energies for mitigating the shortage of fossil fuel and pollution. The primary energy structure in China is mainly dependent on fossil fuels, among which coal shares almost 70%.1 China produces a large amount of agricultural wastes annually, including 600 million tons of straw and 389 million tons of agricultural processing wastes.2 However, most of the agricultural residues are wasted by corrosion and burning in farmland in China. Therefore, it is of great importance for the stability and sustainable development of society to find a way that can effectively make use of biomass. The distributed power system by gasification for a village is a promising approach to utilize the agricultural residues. However, the tar, which is the byproduct produced in the biomass gasification process, often brings forth many problems. It will condense at reduced temperature, leading to blocking, fouling in duct, valve, and engine, and polymerizing to form more complex construction in combustion, which decreases the burnout ratio of tar and causes pollution. Therefore, the removal of tar in syngas has always been considered as one of the main bottlenecks for industrializing the technology of biomass gasification.
(3) Lopamudra, D.; Frans, K. J. P.; Janssen, J. J. G. Biomass Bioenergy 2003, 24 (2), 125–140. (4) Maniatis, K. Progress in biomass gasification: an overview. In Progress in Thermochemical Biomass Conversion; AV, B., Ed.; Blackwell Science Ltd: UK, 2001; Vol 1, pp 1-31. (5) Brandt, P.; Henriksen, U. In Decomposition of Tar in Pyrolysis Gas by Partial Oxidation and Thermal Craking, Proceedings of the Ninth European Bio-energy Conference, Kopenhagen, June,1996; Kopenhagen, 1996; pp 1336-1340. (6) Brandt, P; Henriksen, U. In Decomposition of Tar in Pyrolysis Gas by Partial Oxidation and Thermal Cracking. Part 2, Proceedings of the International Conference: 10th European Conference and Technology Exhibition, Wurzburg, 1998; H., K., Ed.; Wurzburg, 1998; pp 1616-1619. (7) Jensen, P. A.; Larsen, E.; Joergensen, K. H. In Tar Reduction by Partial Oxidation, Proceedings of the 9th European Bioenergy Conference, 1996; Chartier, P., F. G., Ed. 1996; pp 1371-1375. (8) Beenackers, G.; Beenackers, A. A. C. M. In Effects of temperature and gas composition on model tar compounds decomposition kinetics, Proceedings of the 1st World Conference on Biomass for Energy and Industry, Sevilla, 2000; Kyritsis, S.; A., B.; Helm, P.; Grassi, A.; Chiaramonti, D., Eds.; Sevilla: 2000; pp 2052-2055.
*To whom correspondence should be addressed. Institute of Thermal Engineering, No. 800 Dongchuan Road, Minhang District, Shanghai, China. Telephone: 86-21-34206047. E-mail:
[email protected]. (1) Chang, J. L.; Leung, D. Y. C.; Wu, C. Z.; Yuan, Z. H. Renew. Sustainable Energy Rev. 2003, 7, 453–468. (2) Yuan, Z.-H.; Wu., C.-Z.; Ma, L.-L. Utilization Principle and Technology of Biomass Energy; Chemical Industry Press: Beijing, 2005. r 2009 American Chemical Society
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investigated thermal cracking and partial oxidation of pyrolysis gas in a reactor at 800, 900, and 1000 °C. Experiments are performed with an excess air ratio varying from 0 (i.e., thermal cracking) to 0.7. The minimum tar content was measured at 900 °C together with an excess air ratio of 0.5. It was also shown that the temperature in the reactor only had an influence at small excess air ratios in the temperature regions investigated.6 Houben et al.10 examine the effect of partial combustion of the fuel gas mixture on the naphthalene using a partial combustion burner. It is found that, for fuel gases representative of biogasification products and at a λ of 0.2, the presented burner reduces the tar content of the gas with over 90% by cracking. Also few researchers have studied the tar reduction by biomass char. Biomass char can be produced inside the gasifier, and some experts have utilized biomass char as catalyst to reduce tar.11-16 The advantage of using a char bed as cracker is that the char is available in the reactor, and a separate tar-cracking reactor can thus be avoided.16 Boroson14 has studied the effect of thermal treatment with or without a wood char bed in a two-stage reactor system. The results show that the char-induced tar conversion was about 14 ( 7 wt % of tar using temperatures of 400, 500, and 600 °C and space time of 2.5-100 ms. Iversen15 indicates the char bed has selectivity in reducing the higher tar components. The total reduction factor is close to the reduction factor for naphthalene, because naphthalene is a dominating component. Biomass gasifiers are usually divided into updraft, downdraft, and fluid bed gasifiers. There is general agreement that updraft gasifiers are the dirtiest, downdraft gasifiers the cleanest, and fluid bed gasifiers intermediate. Milne uses the crude generalization that places updraft at 100 g/(N m3), fluid beds at 10 g/(N m3), and downdraft at 1 g/(N m3) in tar level.17 At Denmark Technical University (DTU) Iversen and Henriksen developed a two-stage downdraft gasifier fuelled with wood chips; the gasifier is particularly favored because of the low-tar content in syngas. The total tar reduction can be reduced from a level of 50 000 mg/(N m3) from the pyrolyzer to less than 1 mg/(N m3), and tar reduction is mainly through partial oxidation in throat and catalytically decomposition in the char-bed.15 However, the gasifier cannot be fuelled with straw due to the feeding system, which is only suitable for wood chips or shaped fuel. On the basis of the concept of the two-stage gasifier, Shanghai Jiao Tong University has developed a novel 60 kWe two-stage gasifier fuelled with straw. The
Figure 1. Schematic of lab-scale two-stage reactor.
new gasifier not only has the advantage of low tar in product gas, but also increases the adaptability for feed type.18 In the present study, a lab-scale two-stage reactor has been constructed to simulate conditions in a two-stage gasifier. Experiments are being conducted to study tar destruction and how the process operating conditions affect the tar yield from different feedstocks. The tars collected from several configurations of the reactor and sets of reactor conditions have been characterized by gas chromatography/mass spectrometry (GC/MS) and gel permeation chromatography (GPC) to identify how the nature of residual tar is affected by secondary reactions in the second stage. 2. Experimental Section 2.1. Apparatus. Early developments of the fixed-bed hot-rod reactor configuration have been reviewed in a recent publication,19 and the evolution of the hot-rod reactor design at Imperial College has been outlined in several papers.20-23 The design of the lab-scale two-stage reactor is intended to simulated conditions in two-stage gasifier.24 The schematic of two-stage fixed-bed reactor is shown in Figure 1, it consists of the first stage reactor (simulate pyrolysis stage) and the second stage reactor (simulate throat and char reduction zone). This design aims to simulate the behavior of pyrolysis stage, throat zone, and gasification zone of a real twostage gasifier. The two stages are constructed of 316S stainless steel and connected by one pair of flanges. Both sections are
(9) Hoeven, T. A.; H., C. d. L.; van Steenhoven, A. A. Fuel 2006, 85, 1101–1110. (10) Houben, M. P.; De Lange, L. H. C.; Van Steenhoven, A. A. Fuel 2005, 84, 817–824. (11) Bridgwater P., Anthony, V. Thermochemical Processing of Biomass; Butterworth: Markham, ON, Canada, 1984. (12) Mudge, L. K., Baker, E. Brown M. D. Research in Thermochemical Biomass Conversion; Elsevier Applied Science: London and New York, 1988. (13) Strom, E.; Linanki, L.; Sjostrom, K. Biomass Conversion; Elsevier Applied Science: London, 1985. (14) Boroson, M. L.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Energy Fuels 1989, 3 (6), 735–740. (15) Laudal, H.; Iversen, B. G. Update on Gas Cleaning Technologies for Biomass Gasification Gas for Different Applications; Biomass Gasification Group, Department of Mechanical Engineering, Technical University of Denmark: 2004. (16) Bentzen, J. D.; Bentzen, P. B.; Benny, G.; Hansen, C. H.; Henriksen, U. Optimering af 100 kW totrinsforgasningsanlæg pa˚ DTU(in Danish); Biomass Gasification Group, Department of Mechanical Engineering, Technical University of Denmark: 1999. (17) Milne, T. A.; Evans, R. J.; Abatzoglou, N. Biomass Gasifier “Tars”: Their Nature, Formation, and Conversion; National Renewable Energy Laboratory; 1998; NREL/TP-570-25357.
(18) Su, Y.; Luo, Y. H. In Experiment on Rice Straw Gasification in a Two-Stage Gasifier; Asia-Pacific Power and Energy Engineering Conference, Wuhan, 2009; Wuhan, 2009; pp 507-510. (19) Kandiyoti, R.; Herod, A. A.; Bartle, K. D. Solid Fuels and Heavy Hydrocarbon Liquids: Thermal Characterization and Analysis; Elsevier Science Pub.: Amsterdam, Oxford, London, New York, 2006. (20) Bolton, C.; Snape, C. E.; O Brien, R. J.; Kandiyoti, R. Fuel 1987, 66 (10), 1413–1417. (21) Gonenc, Z. S.; Gibbns, J. R.; Katheklakis, I. E.; Kandiyoti, R. Fuel 1990, 69 (3), 383–390. (22) Pindoria, R. V.; Chatzakis, I. N.; Lim, J.-Y.; Herod, A. A.; Dugwell, A. A.; Kandiyoti, R. Fuel 1999, 78 (1), 55–63. (23) O’Brien, R. J. Tar Production in Coal Pyrolysis ; The Effect of Catalysts, Pressure and Extraction; University of London: 1986. (24) Brandt, P.; Larsen, E.; Henriksen, U. Energy Fuels 2000, 14, 816– 819.
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direct resistance heated. The tar trap connected to the outlet of the second stage is constructed of 8 mm (i.d.) 316S stainless steel and immersed in liquid nitrogen for capturing volatiles exiting from the two-stage reactor. The maximum heating rate in two stages is limited to 10 °C/s in order to keep the radial temperature uniform. The first stage reactor represents the pyrolysis stage of the two-stage gasifier; its temperature is limited to 500 °C, and the pressure is atmospheric. The top of the first stage was fitted with a T piece, connecting the gas inlet supply and allowing an internal reactor temperature measurement using a thermocouple. The reactor body was 250 mm long with a 12 mm internal diameter. The heating rate is set as 30 °C/min based on the heating rate in pyrolysis stage of two-stage gasifier. A sample weight of 1 g is used. A low, downward flow rate of inert gas (N2) is used to sweep the products of pyrolysis into the throat. Diluted air (1% O2 and 99% N2) is added to the three uniformly distributed horizontal throat nozzles. The addition of air simulates the throat zone of a real two-stage gasifier. Undiluted air cannot be used in the lab-scale simulation due to the rapid combustion of the char bed and rapid overheating in the second stage. The throat is heated externally to 400 °C using heating tape, thus the condensable components under low temperature can avoid condensation. The second stage reactor simulates the high temperature reducing zone immediately below the air nozzles of a two-stage gasifier. It had a 12 mm (i.d.) and was 200 mm in length with flanges at both ends. It houses a static char bed where tar destruction reactions are studied. 2.2. Experimental Procedure. The experiment consists of two parts: tar formation and tar destruction. A single-stage reactor (only the first stage reactor) is used for tar formation experiment, and the two-stage reactor is used for the tar destruction experiment. The following description introduces the tar destruction experiment using a two-stage reactor. The singlereactor experiment has the similar procedure with exception to the second stage operation procedure. A preweighed wire mesh plug made of a strip of wire mesh was placed inside the first stage of the reactor, approximately 125 mm above the base. A known quantity of biomass fuel was then charged into the reactor and sat on top of the plug. A preweighed biomass char was added to the second stage reactor, again supported on a preweighed wire mesh plug. The two stages were then bolted together, and the thermocouples and electrodes were assembled. The tar trap was attached below the second stage, and the lower part of the trap was immersed in liquid nitrogen. The pressure was controlled using the control valve placed after the tar trap. The required gas flow rate to the top of the first stage and to the throat section were controlled by mass flow controllers (MFC) and fixed at 350 mL/min and 1 L/min, respectively. The heating program was controlled by computer, and the heating rate was fixed at 30 °C/min. The first stage was operated from ambient temperature to 600 °C at 30 °C/min with 10 min holding. By contrast, the second stage was set at a temperature of 700-1000 °C before the start of each test. After the run, the reactor system was allowed to cool to room temperature, while N2 flowed through the system to prevent oxidation of the products. Once the reactor was cooled to room temperature, the gas flow was turned off and the reactor depressurized. The inlet piece was removed, and the tar and char were recovered to determine the yields. 2.3. Biomass Fuels. The biomass materials used in this work are rice straw from Shanghai Chongming Island, corn straw from Henan province of China, and fir sawdust from Shanghai Minhang District. Before studied, the samples were dried at vacuum oven under 80 °C, then ground to a small particle size of less than 200 μm by miniature type coal grinding machine, and finally sieved to 100-150 μm for experiments. The analytical data are shown in Table 1. 2.4. Char Samples. Char samples derived from rice straw, corn straw, and fir sawdust were produced at 1000 °C under N2
Table 1. Proximate and Ultimate Analysis of Three Biomass Materials samples
rice straw
corn straw
fir sawdust
moisture volatiles matter fixed carbon ash
Proximate Analysis (wt %) 13.45 8.89 62.79 36.39 15.92 7.37 7.84 47.35
8.19 76.52 14.35 0.94
C H O N S
Ultimate Analysis (wt %) 35.58 29.16 4.63 2.87 37.36 10.81 0.94 0.80 0.20 0.12
47.57 5.64 37.46 0.20 rice straw>corn straw, while char yield is on the contrary. The maximum value of tar yield is: rice straw 25%, corn straw 22%, and fir sawdust 31%. All the char samples yields decreased rapidly over the 200-320 °C range and reached a limiting value of 34% for rice straw, 53% for corn straw, and 27% for fir sawdust by 440 °C. There was a significant increase in tar yields of rice straw and corn straw up to 260 °C and for fir sawdust up to 320 °C. The tar yields reached maximum value for rice straw by 440 °C and for corn straw and fir sawdust by 380 °C. There appeared to be little gas production at 200 °C, and the sum of the char and tar yields was close to 100%. It is noted that, as defined above, the gas yield will include low boiling point organic compounds that have been lost during the solvent removal stage. It is clear that at up to about 200 °C, negligible pyrolysis occurs, with only limited release of some low boiling point material from the fuel, which was collected as tar. The high char yields, measured at the low temperatures in the range studied, would have been mostly relatively unreacted feedstock. 3.2. Tar Destruction in the Two-Stage Reactor. In Section 3.1, tar formation results were presented for the single-stage reactor. The following studies will include: (1) first stage followed by heated empty second stage, (2) first stage plus diluted air feeding to the throat nozzles, and (3) first stage plus heated second stage packed with char. In what follows, the tar conversion mechanism under these three conditions will be described. The parameters investigated were temperature, residence time, char particle size, char type, and diluted air feeding to the throat. 3.2.1. Effect of Temperature. Figure 3 shows the effect of temperature on the tar destruction from heated second stage for test with the empty second stage (700-1000 °C), the empty second stage with diluted air feeding to throat nozzles (700-1000 °C) and the second stage packed with char (700-1000 °C). With the increasing of temperature, the amount of tar decreases under above three conditions; what’s more, the curve slope gradually decreases, which indicates that the second stage may not be sufficient to remove all tar at the existing residence time. As abovementioned in Section 3.1, for rice straw sample, the initial tar quantity is 25% from the outlet of first stage; after the heated empty second stage with 1000 °C, the amount of tar is rapidly reduced to 1.81% of the initial weight of sample. The other two conditions, including air feeding and char bed, can further reduce tar content to 1.58 and 0.73%, respectively.
Figure 2. Tar, char, and gas yield of three biomass samples.
The experiments results described above are the same as the trends reported in refs 25 and 26. 3.2.2. Effect of Residence Time. Figure 4 shows the effect of the gas residence time in the second stage on the emission of tar. The time was varied between 0.04 and 0.12 s by a combination of varying the quantity of char that was added (25) Houben, M. P. Analysis of Tar Removal in a Partial Oxidation Burner; Eindhoven University of Technology:Netherlands, 2004. (26) Brandt P.; Henriksen U., Decomposition of Tar in Gas from Updraft Gasifier by Thermal Cracking. In Proceedings of the First World Conference on Biomass for Energy and Industry; 2000.
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Figure 3. Effect of second-stage temperature on tar reduction. First stage: 440 °C, rice straw, 1 g; heating rate in the first stage: 30 °C/ min; particle size of second stage rice straw char bed: 100-150 μm.
Figure 5. Effect of particle size on tar reduction. First stage: 440 °C, rice straw, 1 g; gas residence time in second stage: 0.04s.
Figure 6. Effect of char type on tar destruction. First stage: 440 °C, rice straw, 1 g; heating rate in the first stage: 30 °C/min.
Figure 4. Effect of second-stage residence time on tar reduction. First stage: 440 °C, rice straw, 1 g; straw char bed: 100-150 μm; second stage temperature: 800 °C.
3.2.4. Effect of Char Type. Three char samples with size range of 100-150 μm have been used in the second stage with the temperature range of 700-1000 °C. The results obtained are shown in Figure 6. In the adopted temperature range the tar emission for rice straw is slightly smaller than other two char samples; however, the difference of tar emission is within ( 1%, which suggests that the type of char may have little effect on the tar emission. With the increasing of temperature up to 1000 °C, the amount of tar is rapidly reduced to as low as 0.73% of the initial sample weight for rice straw char. This suggests that in a two-stage gasifier fueled by straw or wood chip the char type is not a sensitive parameter in terms of tar reduction. Other parameters such as temperature, residence time, and particle size, may be more important. 3.2.5. Effect of Air Feeding via the Throat Nozzles. As described in Section 2.2, considering that undiluted air rapidly consumed the char bed and caused overheating, t was therefore necessary to use air diluted by N2 (1% O2 was used). The high level of dilution was chosen to minimize the decrease in the quantity of bed char in the second stage during a test. This is a limitation of the apparatus associated with the static nature of the char bed in the second stage; it is likely to have restricted the extent of tar destruction by combustion.
to the second stage and by using a longer reactor (300 mm). It can be seen that a 3-fold increase in the residence time caused a decrease in the amount of tar leaving the reactor to about 40% of the initial value. 3.2.3. Effect of Particle Size. Three particle size ranges of rice straw char have been conducted in the second stage, heated between 700 and 1000 °C, and the results are shown in Figure 5. The size ranges were 100-150, 335-600, and 1000-2000 μm. At the lower end of temperature range, the tar yield increases with the increasing particle size range, this effect is probably related with the decrease in the surface area of larger particles. However, the effect decreased with the increasing temperature, until there was no obvious effect at 1000 °C. At the higher temperatures, the kinetics of the reaction appears sufficiently fast to overcome the surface area effect and achieve near complete destruction of the tar. However, the fuel particle size used in the two-stage gasifier (typically about 5-10 cm length for rice straw or 30 30 5 mm for wood chips) is at least an order of magnitude greater than the char particle sizes used in the second stage of our reactor. The smaller particles might be expected to have enhanced the extent of tar destruction compared with the two-stage gasifier. 4663
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Chen et al. Table 4. Tar Destruction under Three Conditionsa
Table 2. Effect of Start Temperature and Duration Time on Tar Destructiona first stage temperature before start of diluted air feeding (°C)
air feeding time (min)
400
5 10 5 12 5 13 5 6 15 23 0
350 300 250 air feeding for whole test heated empty second stage a
tar emission (%)
tar emission (%)
second-stage temperature /°C
2.66% 2.46% 2.32% 2.33% 2.23% 2.14% 1.76% 1.78% 1.75% 1.71% 2.74%
700 800 900 1000 a
heated empty second stage
dilute air feeding
rice straw char bed
8.6 5.0 2.74 1.81
6.13 3.03 1.71 1.58
5.53 2.6 0.93 0.73
First stage: 440°C, rice straw.
Table 5. Effect of Biomass Type on Tar Destructiona tar yield (%)
first stage, rice straw; heated empty second stage, 900 °C. reactor configuration
Table 3. Tar Destruction Using Air Feeding and Char Beda first stage temperature second-stage before start of diluted temperature air feeding (°C) (°C)
char tar particle air feeding emission size (μm) time (min) (%)
250 250 250 250 250a
100-150 100-150 100-150 100-150
b a
900 900 1000 1000 1000 1000
100-150
5 15 5 15 15
first stage only (440 °C) second stage empty, 1000 °C second stage empty, air feeding, 1000 °C second stage empty, char bed, 1000 °C second stage empty, air feeding with packed char bed, 1000 °C
0.56 0.55 0.45 0.43 1.55 0.73
a
rice corn fir straw straw sawdust 25 1.81 1.58 0.73 0.43
22 2.07 1.78 1.03 0.61
31 3.37 2.59 1.85 1.15
Second stage: rice straw char; Char particle size: 100-150 μm.
methyl (CH3) radicals are the active radicals. When oxygen is present in the reacting mixture, hydrogen (H), oxygen (O), hydroxyl (OH), and hydroperoxy (HO2) radicals become dominant. The latter are capable of accelerating the hydrocarbon decomposition path by increased and faster H-atom abstraction. Oxygen is thus an excellent initiator of free radicals and plays an outstanding role in initiating reactions.27,29,30 Besides, oxygen (partially) oxidizes relevant hydrocarbons to exothermically form carbon monoxide and water, releasing the heat necessary for propagation reactions of the remaining hydrocarbon.30 3.2.6. Effect of Fuel Type. Tests have also been done with three different feedstocks in the first stage of the reactor. The results are shown in Table 5. All three sets of results show the progressive decrease in tar emission as reaction conditions were optimized. The data suggests that it is difficult to totally remove tar for all three samples. On the other hand, under the optimized condition (Second stage empty, air feeding with packed char bed, 1000 °C), the tar conversion ratio of three samples were 98.28% for rice straw, 97.23% for corn straw, and 96.29% for fir sawdust, respectively. The rice straw tar appears to have a different suite of compounds than the other two samples derived material and presumably have different cracking pathways. 3.3. Analysis of Tar Components. GC/MS and GPC have been used to characterize the tars collected at the outlet of the single-stage and two-stage reactors. The aim was to identify how the nature of the tars formed in the first stage changed with the conditions in the second stage. The following tar samples were collected during tests with three samples in the first stage: (sample 1) tar from outlet of first stage (rice straw, 440 °C, and N2 as sweep gas); (sample 2) tar from outlet of empty second stage (900 °C, with N2 as sweep gas); (sample 3) tar from outlet of second stage with diluted air (900 °C, with N2 as sweep gas with diluted air feeding to the nozzles); (sample 4) tar from outlet of second stage with char bed (100-150 μm particle size, 900 °C), and (sample 5) tar from
b
: air feeding without char bed; : only char bed. a First stage: rice straw; second stage: rice straw char.
Table 2 presents results obtained using different durations of diluted air feeding, with a second-stage temperature of 900 °C, without a char bed. The start temperature for air feeding plays an important role in tar removal. When the start temperature is 250 °C, the tar removal efficiency approaches the tar quantity of air feeding for the whole test. The extent of tar decrease increased with the duration of air feeding. However, when air feeding time is more than 5 min with the start temperature of 250 °C, the amount of tar almost keeps constant, which indicates that most tar after pyrolysis is released within 5 min after 250 °C; subsequently, only a small amount of tar is released. To minimize the decrease in the quantity of bed char in the second stage, although the tar removal efficiency for 15 min of air feeding is slightly better than 5 min, the optimum first stage temperature to commence the addition of the diluted air was 250 °C with 5 min of air feeding. Table 3 shows data obtained with a char bed (100-150 μm) in the second stage. Tests were done using air feeding for the whole test and with a 250 °C start for air feeding. The results show that a further reduction in tar had been achieved. Table 4 show the tar emission under three conditions; for the same second stage temperature, tar removal efficiency decreased in the following sequence: packed char bed > diluted air feeding > heated empty second stage. Tar emission with air feeding is obviously less than that with empty second stage. Literature generally agrees that oxidative treatment gives higher reactivity and has lower apparent activation energy than thermal treatment. The differences between thermal and oxidative reactions rely on the relative reactivity of the different radicals produced.27,28 Under thermal treatment conditions hydrogen (H) and
(29) Liu, X.; Li, W.; Xu, H.; Chen, Y. Fuel Process. Technol. 2004, 86, 151–167. (30) Liu, X.; Li, W.; Xu, H.; Chen, Y. React. Kinet. Catal. Lett. 2004, 81 (2), 203–209.
(27) Beretta, A.; Dente, M.; Ranzi., E. J. Catal. 1999, 184, 469–478. (28) Ranzi, E.; Forzatti, P.; Goldaniga, A.; Bozzano, G.; Faravelli., T. Prog. Energy Combust. Sci. 2001, 27, 99–139.
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Figure 7. GC/MS chromatogram of tar samples. Table 6. The Identified Tar Components in Five Samples relative concentration % M.W.
compound
name
1
60 98 94 124 128 128 154 178 178 202 202 202 256 282 284 390 400 412 414
C2H4O2 C5H6O2 C6H6O C7H8O2 C7H12O2 C10H8 C8H10O3 C14H10 C14H10 C16H10 C16H10 C16H10 C16H32O2 C18H34O2 C18H36O2 C24H38O4 C28H48O C29H48O C29H50O
acetic acid 2-furanmethanol phenol mequinol butanoic,2-propenyl ester naphthalene phenol,2,6-dimethoxyphenanthrene anthracene fluoranthene acephenanthrylene pyrene N-hexadecanoic acid oleic acid octadecanoic acid 1,2-benzenedicarboxylic acid, diisooctyl ester campesterol stigmasterol beta-sitosterol
4.91 1.38 10.37 7.89 32.21
outlet of second stage with diluted air feeding to the nozzles and char bed (100-150 μm particle size, 900 °C, and with N2 as sweep gas). 3.3.1. Characterization of Tar by GC/MS. Figure 7 shows the GC/MS chromatogram of four samples. It is obvious that the number of peaks and peak strength in the chromatograms declined with the presence and severity of conditions in the second stage. Some tar components with too low concentration may not show big peak value in GC/MS chromatogram. Due to the complexity of tar components, some compounds cannot be identified. Table 6 shows relative concentration of the main identified tar components. Sample 1 contains a large amount of oxygenated constituents (phenols and acids) and small oneor two-ring aromatic hydrocarbons. These components are easily decomposed into noncondensable gases and thermally
2
3
4
5
12.04
7.76
5.92
5.98
48.65 20.52 6.47 7.68 4.65
51.22 18.33 7.76 8.66 6.26
47.75 18.56 9.91 9.49 8.38
48.49 19.43 9.90 8.72 7.47
12.65
11.62 6.76 12.52 3.24 1.61 3.77 2.33
more stable compounds under high temperature conditions. Besides, some components with 100-200 molecular weight cannot be identified. In our previous research,31 a large amount of benzene and toluene can be detected in the TG/MS analyzer. However, due to the low boiling point of these compounds, they are lost during solvent evaporation in Rotovap and drying oven. Samples 2-5 are formed at 900 °C with the second-stage reactor. They all contain a large amount of polycyclic aromatic hydrocarbons (mainly 2-ring naphthalene, 3-ring fluorene /phenanthrene/anthracene, 4-ring pyrene) which are polymerized from low molecular tar components. With the increasing of reaction severity (from the empty heated (31) Chen, Y.; Duan, J.; Luo, Y. H. J. Anal. Appl. Pyrolysis 2008, 83 (2), 165–174.
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Figure 8. GPC chromatogram of tar samples. Table 7. Tar after Pyrolysis of Three Samples (wt %) sample
MW < 400
MW > 400
pyrolytic tar of rice straw pyrolytic tar of corn straw pyrolytic tar of fir sawdust
44.44 71.40 66.38
55.56 28.60 33.62
Table 8. Corresponding Time of GPC Peaks
sample 1 2 3 4 5
second stage to heated second stage with char bed), the concentration of 2-ring naphthalene decreased from 12.04 to 5.92% and that of 4-ring pyrene increased from 4.65 to 8.38%, which indicated that the tar compounds reacted further (polymerized) to form larger molecular material. These results are consistent with previous research.32-34 These works all show that aromaticity and chemical stability of the tars increases with increasing temperature. 3.3.2. Characterization of Tar by GPC. GPC chromatograms of the seven tar samples are shown in Figure 8, the last two peaks belong to the releasing peak of solvent tetrahydrofuran (THF). Tar constituents are released from retention time of 25 min. The peak area of GPC can relatively represent the mass ratio of certain compounds. Table 7 shows the details of biomass tar after pyrolysis. The mass ratio of tar constituents less than MW 400 decreased in the following sequence corn straw > fir sawdust > rice straw. Generally, the molecular weight less than 400 can be characterized by GC/MS analyzer. Therefore, the material characterized by GC/MS represents very small part of the total tar. As shown in Figure 8, the retention time corresponding to the last two peaks are compared. The former peak mainly contains phenols, acids, and 1-4-ring aromatic hydrocarbons. The latter peak is related with four-ring or above aromatic hydrocarbons or high molecular weight acids.
retention time corresponding to former peak,min
retention time corresponding to latter peak, min
tar emission %
31.930 31.917 31.915 31.899 31.903
33.555 33.533 33.536 33.535 33.519
25 2.74 1.71 0.93 0.58
Table 8 clearly shows that the retention time corresponding to the two peaks in the GPC chromatograms basically declined with the presence and severity of conditions in the second stage. This indicates that through the destruction in the second stage, the majority of low molecular weight tar constituents from the first stage were cracked to form gases, and simultaneously more volatile organic compounds were pylomerized and formed into soot precursors such as 2-ring naphthalene, etc. Comparing samples 1-5, it is obvious that the retention time for samples 4-5 was shifted to a shorter time when the tars were passed through char bed in the second stage, which suggests a small severity condition (heated empty second stage or diluted air feeding to throat nozzles); however, under more severe conditions (char bed), the tar compounds reacted further (polymerized) to form larger molecular material, which is consistent with the analysis results of GC/MS. 4. Summary and Conclusions The tar problem is considered as one of the main bottlenecks for industrializing the technology of biomass gasification. The two-stage downdraft gasifier is particularly favored because of the low-tar content in syngas. This work presents a lab-scale two-stage reactor used for simulating the two-stage gasifier, and experiments are being conducted to study tar destruction and how the process operating conditions affect the tar yield from different feedstocks.
(32) Kinoshita, C. M.; Wang, Y.; Zhou, J. J. Anal. Appl. Pyrolysis 1994, 29 (2), 169–181. (33) Stiles, H. N.; Kandiyoti, R. Fuel 1989, 68 (3), 275–282. (34) Yu, Q.; Brage, C.; Chen, G. X.; Sj€ ostr€ om, K. J. Anal. Appl. Pyrolysis 1997, 40-41, 481–489.
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The results of single stage reactor showed that both tar and gas yields decreased in the following sequence fir sawdust> rice straw > corn straw, whereas char yield was contrary to this trend. The maximum value of tar yield is: rice straw 25%, corn straw 22%, fir sawdust 31% of the initial fuel. The results using a two-stage reactor show that the tar decreased with increasing temperature and residence time and with decreasing char particle size. The char type is not a sensitive parameter in terms of tar reduction. Tar emission with limited diluted air feeding is obviously less than that with empty second stage due to the more reactive radicals produced in oxidative condition. The effect of fuel type on tar emission has also been done. The straw tars appear to have a different suite of compounds than the other two samples derived material and presumably have different cracking pathways. The tars collected from first and second stages have been characterized by gas GC/MS and GPC. The results indicate that tar after pyrolysis contain a large amount of oxygenated constituents. With the increasing of reaction severity (from the empty heated second stage to heated second stage with char
bed), the tar compounds reacted further (polymerized) to form larger molecular material. It is clear that the material characterized by GC/MS represents a very small part of the total tar. The results have shown that the tar emission from two-stage gasifier can be reduced to low levels using optimized operating conditions, but complete tar removal is difficult to realize due to manipulation of operating parameters and fuel type. The present work investigates the effects of temperature, of the presence char bed, type of char, and air feeding via the throat on tar emission. However, under the existing conditions, it is difficult to remove all the tars produce in the pyrolysis stage. It may due to the high complexity of tar compounds and the limitations of existing two-stage reactors, such as inadequate residence time in the second stage and char particles that are too large. Acknowledgment. The authors are grateful to the Science and Technology Commission of Shanghai Municipality for its financial support (under Grant 05dz12010).
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