Tar Formation in Pressurized Fluidized Bed Air Gasification of Woody

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Tar Formation in Pressurized Fluidized Bed Air Gasification of Woody Biomass Nader Padban,* Wuyin Wang, Zhicheng Ye, Ingemar Bjerle, and Ingemar Odenbrand Department of Chemical Engineering II, Chemical Center, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden Received August 26, 1999

The tars from an air-blown pressurized bubbling fluidized bed 90 kW (thermal) pilot biomass gasifier and also from an 18 MW IGCC demonstration plant were analyzed. The accuracy of the sampling method and its advantage/disadvantage was compared with other methods.

Two fractions of tars were identified: (a) one GC-MS detectable tar fraction consisting of PAHs ranging from naphthalene to benzo(g,h,i)perylene, (b) an unknown fraction that, despite its solubility in dichloromethane, could not be identified by GC-MS, but instead was analyzed by TGA/DTA. A study of parameters such as gasification temperature, equivalence ratio (ER), and particle size showed relationships that could explain the behavior in the two different gasifiers. For large particles (>1.5 mm), the GC-MS detectable fraction was more than 90 wt % of the total tars. Increased temperature decreased the conversion of fuels to the tars. For small fuel particles (∼0.7 mm), the proportion of the GCMS detectable tars was low but increased with an increase in temperature. The unknown tar fraction lost 46 wt % at 1 atm in an air atmosphere between the temperatures 220 and 480 °C. The final weight was achieved at 620 °C. Increased ER slightly decreased the tar formation. The most profound effect of the ER was on benzene formation, which increased drastically with an increase in ER. The proportion of the heavier polyaromatic hydrocarbons (PAHs) declined with an increase in ER. The mechanism of tar formation based on the fundamentals of the pyrolysis reaction is discussed. Introduction During gasification of woody biomass more tar (defined as organic compounds having boiling point above that of benzene) is produced in comparison to the gasification of coal.1 In processes designed for conversion of the fuel to producer gas, tars are undesirable products whose content in the product gas must be decreased. The extent to which the reduction must be driven is a matter of process and end-user demand for gas quality. The tolerance level regarding the total tar amount in the gas is much lower for the processes designed for cold gas filtration in comparison to those based on hot gas * Corresponding author. (1) Simell, P.; Kurkela, S.; Ståhlberg, P. In Advances in Thermochemical Biomass Conversion; Bridgwater, A., Ed.; Blackie Academic & Professional: London, 1994; pp 1103-1116.

cleaning. The majority of biomass gasification research work is focused on cold gas cleaning systems, which aim to reduce the tar content in the gas, to levels below some hundreds mg/Nm3 by means of catalytic tar cracking.2-6 For systems based on hot gas cleaning, the tars are considered problematic only if they condense during the transport from the gasifier to the end-user equipment, e.g., the gas turbines. At the same time other important properties of the tars should not be neglected, i.e., their sooting tendency. Understanding both the tar formation mechanism and the effects of different parameters on formation would provide a useful tool in the prediction and control of the tars in gasification processes. It is evident that some polyaromatic hydrocarbons (PAHs) in the tars can have boiling points as high as 510 °C. The tars become problematic in the process of hot gas cleaning only if the fraction of the high-boiling compounds in the tars becomes considerable. In this case undesired condensation of the tars at different places would occur. The condensation phenomenon is dependent on two different parameters: the composition of the tar and the total amount of the tar. These two parameters are therefore of vital importance when tars from gasification are discussed. The separation of the tars into phenolic and aromatic parts is shown in the literature under conditions of pyrolysis and low gasifier temperature.7,8 Very little (2) Bangala, D. N.; Abatzoglou, N.; Martin, J. P.; Chornet, E. Ind. Eng. Chem. Res. 1997, 36 (10), 4184. (3) Caballero, M. A.; Aznar, M. P.; Gil, J.; Martin, J. A.; Frances, E.; Corella, J. Ind. Eng. Chem. Res. 1997, 36 (12), 5227. (4) Olivares, A.; Aznar, M. P.; Caballero, M. A.; Gil, J.; Frances, E.; Corella, J. Ind. Eng. Chem. Res. 1997, 36 (12), 5220. (5) Orio, A.; Corella, J.; Narvaez, I. Ind. Eng. Chem. Res. 1997, 36 (9), 3800. (6) Simell, P.; Stahlberg, P.; Solantausta, Y.; Hepola, J.; Kurkela, E. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professionals: London, 1997; pp 1103-1116. (7) Roseń, C.; Bjo¨rnbom, E.; Yu, Q.; Sjo¨stro¨m, K. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professionals: London, 1997; pp 817-827. (8) Evans, R. J.; Milne, T. A. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds., Blackie Academic & Professionals: London, 1997; pp 803-816.

10.1021/ef990185z CCC: $19.00 © 2000 American Chemical Society Published on Web 04/29/2000

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Figure 1. Schematic description of LU-PFB gasifier.

research has focused on detailed studies of the tars from high-temperature biomass gasification. Our research from parallel gravimetric and GC-MS tar measurements from two gasifiers operating at high temperatures and high pressures has shown that the tars consist of two different fractions: (1) one GC-MS detectable tar fraction (GC-dtf) consisting of PAHs ranging from naphthalene to benzo(g,h,i)perylene, (2) a possibly heavier or high-temperature boiling fraction that despite its solubility in dichloromethane, cannot be identified by GC-MS (GC-utf). This fraction was quantified by calculating the difference in mass between total tar and the GC-dtf. Depending on parameters such as fuel properties, gasification temperature, and ER, the proportion of the GC-utf in the tars can vary between 3 and 85 wt %. The objective of this work was to study the formation and characteristics of the tars from different fuels in a pressurized bubbling fluidized bed reactor. The sampling, characterization, and quantification of the tars were carried out with the aim of finding the mechanism of tar formation during gasification of fuels having different properties. The discussions are based on the effects of ER, temperature, and particle size on tar formation and its composition.

Equipment LU Pressurized Fluidized Bed Air Blown. The gasification test rig (Figure 1) consists of four main units: a bubbling, pressurized fluidized bed gasifier, a hot gas filtration unit, a catalytic conversion unit, and a pressurized feeding unit. The test rig is designed for operation at temperatures between 850 and 950 °C and pressures between 5 and 20 bar. The gasification reactor, the hot gas filter unit, and the catalytic conversion unit are placed inside three cylindrical pressure vessels of 0.5 m inner diameter. The fluidized bed reactor consists of a tube 102 mm in inner diameter having a total length of about 3.3 m. Air as a fluidizing agent is fed into the reactor bottom through a funnelshaped inlet. The feedstock is introduced by a screw feeder about 300 mm from the bottom end. The reactor tube is surrounded by six electrically heated furnaces. The furnaces supply the desired heat to the reactor to achieve the ignition temperature during the start-up phase. During gasification the reactor temperature can, to some extent, be adjusted by these heaters. The hot product gas enters at the top of the filtration unit. The gas is then cooled and subsequently filtered through a candle filter made from sintered SiC. The filtering unit is provided with both the electric heating

Tar Formation in Air Gasification of Woody Biomass

Energy & Fuels, Vol. 14, No. 3, 2000 605

Figure 2. Schematic drawings of tar samplers: (a) LU condensation method; (b) cold trapping method; (c) combined LU-cold trapping, (d) SPA.

and the steam cooling systems (175 °C, 10 bar) to maintain a constant temperature during the experiments. Gas filtration temperature is normally around 650 °C. The filter is cleaned from dust and solid particles by back-blow pulsation. Dust-free product gas continues downstream to the catalytic reactor. This unit at the moment consists of an empty tube. The temperature of the product gas is reduced gradually inside the catalytic reactor. When leaving the gasifier the product gas has a temperature of about 220 °C. A slipstream from the main gas outlet provides the gas for analyses instrument and tar sampler. Precalcined magnesite with a mean particle size of about 180 µm was used as bed material during this set of experiments. The total weight of the bed was about 6 kg. To keep the bed in a bubbling fluidized mode, a minimum gas velocity of 0.2 m/s through the bed was required. The space time for the gas, calculated on the basis of fluidizing air flow, was 15 s but in reality it is much shorter due to the additional volumetric flow contributed from the product gas. The gasifier is equipped with a continuous fuel feeding system. Biomass fuel is fed into a compression unit and compressed by a hydraulic piston to half its original volume before entering a reservoir that is connected to the gasification reactor. The pressure inside this reservoir is the same as that of the gasifier. There are two screw feeders placed at the bottom of the reservoir and these transport the fuel to the reactor. The larger screw has a variable speed that can be adjusted

between 0 and 10 rpm. The feeding rate is determined by the rotational speed of this screw. A smaller screw is placed in front of the larger one and has the same center of rotation. It picks up the fuel from the large screw and transports it rapidly (200 rpm) through the heated reactor wall into the reactor. The feeding operates very smoothly. Only at low feeding rates (less than 4 kg/h) nonuniform delivery of solids to the reactor can be observed. It is vital that pyrolysis is avoided during the transport phase and therefore cooling of the small screw is necessary. A cooling jacket with water at 175 °C and 10 bar is used as the cooling medium. The capacity of the feeding system is dependent on the fuel bulk density and is for the woody biomass around 300 g/min. A more detailed description of the gasifier can be found elsewhere.9,10 Tar Samplers. Condensation Method: LU Tar Sampler. The product gas left the gasifier at a temperature between 210 and 220 °C. The gas to the tar sampler was taken via a slipstream from the main product gas line after the filter. The slip sream line had a length of about 500 mm and was heat traced and kept at 200 °C. The tar sampling device is shown in Figure 2a and (9) Hallgren, A.; Bjerle, I.; Chambert, L. A. PCFB gasification of biomass. In Proceedings of the 206th American Chemical Society National Meeting, Fuel Chemistry Division, Chicago, IL, 1993. (10) Hellgren, R. Thermochemical conversion of biomass: A technical feasibility study concerning wood pyrolysis at high temperatures and biomass gasification at elevated pressures. Licentiate Thesis. Department of Chemical Engineering II, Lund University, Lund, Sweden, 1995.

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Table 1. Fuel Analysis bark particle size (mm) volatile fraction (%) fixed carbon (%) ash

sawdust A

sawdust B

dry [wt %] dry [wt %] dry [wt %] 1-2.5 1-2 0-1 (dpmean)0.6) 70.89 83.39 82.73 25.48 15.93 16.55 3.63 0.68 0.72

C: H: O: N: S:

50.51 5.84 36.51 0.52 0.03

50.77 6.13 42.12 0.30 0.00

51.33 6.21 41.41 0.33 0.00

gross calorific value: (MJ/kg) net calorific value (MJ/kg)

19.2 17.8

20.4 19.2

19.4 18.0

consists of three stainless steel vessels in series. The first vessel is a cooler submerged in an ice bath and used to condense the water and the PAHs. The second vessel (aerosol catcher) consists of several sections that are separated by fine-mesh metal nets. The aerosols leaving the cooler are captured on these metal nets. The third vessel, a so-called dryer, is used for the drying and the final capturing of the condensable compounds from the gas. The dryer is filled with a fixed layer of sawdust followed by a layer of silica gel. Before entering the first vessel, the pressure of the product gas was reduced to 1.5-2.0 bar (abs). The outlet of the sampling device had atmospheric pressure. The pressure difference between the inlet and outlet forces the gas forward through the sampling device. An accumulative volume flow meter was used to measure the amount of the gas passed through the system. Cold Trapping. The method was used to approve the reliability of the LU-condensation method. The gas was led through a scrubbing system consisting of six glass flasks in series and containing dichloromethane (DCM) as the solvent. Flasks 1-4 were placed in an ice bath. The last flasks were immersed in a cooling bath containing a mixture of dry ice and acetone (∼-50 °C) (Figure 2b). The same procedure of pressure control and flow measurements that was used for the LU sampler was also applied to this procedure. The concentrations of the PAHs were then determined by GC-MS. Combined LU-Cold Trapping. The outlet of the LU cooler was connected to three scrubbing flasks in series, the first was empty and immersed in an ice bath while the second and the third flasks each contained 50 mL DCM, immersed in a cooling bath (∼-50 °) (Figure 2c). Light aromatics and PAHs from each system were analyzed separately by GC-MS. Solid-Phase Absorption (SPA). Standard SPA sampling procedure was applied to collect the tars from the gas stream. Product gas (100 mL) was drawn out through a syringe containing an active sorbent phase (Figure 2d). The reason behind applying the method was its proposed simplicity. A detailed description of the method development is described by Brage et al.11 Fuels Bark and sawdust with two different particle sizes were used during the gasification experiments. All fuels originated from forestry residues. Elementary analysis of the fuels is shown at Table 1. (11) Brage, C.; Yu, Q.; Chen, G.; Sjo¨stro¨m, K. Fuel 1997, 76 (2), 137.

Bark consisted of particles of between 0.5 and 2 mm. Sawdust A consisted mainly of hardwood from conifers with a particle size between 1 and 2 mm. Sawdust B was also hardwood particles from conifers but with a very wide particle size distribution between 0 and 1 mm. Experimental Section Gasification. The gasification experiments were carried out at 12 ( 0.5 bar (abs) with a constant air input of 21 Nm3 per hour. The changes in ER were made by changing the fuel feeding rate. The temperature along the gasifier was set by varying the efficiency of the furnaces surrounding it. Tar Sampling Procedure. The tar sampling was started when the gasifier had reached the desired steady state at which the product gas composition, the temperature profile along the reactor, and the operation pressure were constant. The prepared and weighed sampling vessels were then connected to the sampling line. The volume of the gas varied during different samplings but was normally between 0.2 and 0.4 Nm3. Tar Quantification. Gravimetric Measurements (LU sampler). The three vessels were disconnected and weighed separately. The weight gain for each vessel was then noted. The condensed water in the cooler is decanted and weighed. The gravimetrically determined tar weight was the sum of the weight increase in the cooler and the aerosol filter minus the decanted water. The gravimetrically determined tar fraction (Gr-dtf) was defined as

(Gr - dtf) )

gravimetrically determined tar weight gas volume (g/Nm3)

For experiments with small amounts of tar the residual water remaining after decanting can introduce a large error in the gravimetric analysis. It was assumed that all PAHs were condensed before reaching the dryer and that the weight gain of the dryer was due to the absorption of water and light aromatics. GC-MS Analysis (LU Sampler). The organic residues condensed inside the cooler were washed thoroughly with proanalytic dichloromethane (DCM). Depending on the amount of tar, the volume of solvent used can vary between 50 and 200 mL. This solution was then filtered and its volume adjusted to 250 mL. Filtration separates all solid particles (mainly soot and dust) from the solution. The weight gain of the filter was measured at several occasions. The contribution of the particles captured on the filter to the total tar amount was less than 0.1%. Quantification of PAHs captured in dryer has shown that their contribution to the total PAH is less than 2 wt %. A portion of the 250 mL solution was diluted 2-10 times depending on the tar concentration. A Varian GC/MS equipped with an on-column injector, together with a 30m × 0.252 mm capillary column coated with DB-Wax (DB-5), was used for the PAH analysis. A total of 21 polyaromatic compounds were quantified by GC-MS. The lightest compound was naphthalene (MW: 128) and the heaviest was benzo(g,h,i)perylene (MW: 276). A more detailed description of the GC-MS analysis method can be found in previous work.12 Table 2 shows three examples of detailed tar analysis from different fuels. Comparison of the Methods. By collecting the tars from the same gas stream, the LU-condensation method was compared with the cold trapping and the SPA methods. Combined LU-cold trapping was applied to check if the lighter (12) Padban, N.; Odenbrand, I. Energy Fuels 1999, 13 (5), 1063.



Table 2. Concentration of PAHs in the Gas (mg/Nm3) fuel

bark

temp. (top) temp. (bottom) gasifier mean temp. ER

840 930 847 0.25

2-rings naphthalene 7160 2-methyl-naphthalene 320 1-methyl-naphthalene 22 biphenyl 210 3-rings acenaphthylene 1000 acenaphthene 110 dibenzofuran 95 fluorene 200 phenanthrene 1500 anthracene 240 fluoranthene 810 4-rings pyrene 860 2,3-benzofluorene 75 chrysene 230 benzo[a]anthracene 40 5-rings benzo[k+b]fluoranthene 490 benzo[a]pyrene 650 perylene 80 6-rings indeno(1,2,3-cd)pyrene 250 dibenz(a,h)anthracene 25 benzo(g,h,i)perylene 170 g/Nm3 (GC-MS) g/Nm3 (gravimetric) recovery

835 870 848 0.25

850 936 804 0.23

5500 130 10 90 720 90 70 140 1000 160 540 580 30 250 12 310 430 60 200 18 170

14.5 15.4 0.9

3900 40 3 80 420 60 30 50 440 60 400 250 15 35 7 190 110 10 0 0 80

10.5 10.8 1.0

6.2 9.7 0.6

Table 3. Comparison between Condensation, SPA and Cold Trapping Methoda Condensation naphthalene 2-methyl-naphthlene 1-methyl-naphthalene biphenyl acenaphthylene acenaphthene dibenzofuran fluorene phenanthrene anthracene fluoranthene pyrene 2,3-benzoflueren chrysene benzo[a]anthracene benzo[k+b]fluoranthene benzo[a]pyrene perylene indeno(1,2,3-cd)pyrene dibenz(a,h)anthracene benzo(g,h,i)perylene

Table 4. Water-Soluble Organic Carbon from Gasifier

sawdust A sawdust B

SPA

I

II

III

I

II

0.99 0.90 1.02 1.02 1.02 1.06 1.05 1.01 0.97 1.08 1.04 1.05 1.01 0.98 1.15 0.95 0.96 1.16 1.06 1.03

1.03 0.97 0.92 1.00 0.92 1.02 1.07 1.09 1.04 1.04 0.88 0.87 0.77 0.75 0.81 0.86 0.91 0.89 0.99 1.06 0.99

1.02 1.02 1.05 1.02 0.94 0.99 1.19 0.93 1.03 0.95 0.99 0.95 0.91 1.03 1.09 1.02 0.81 1.17 0.86 1.24 1.19

0.90 0.73 0.85 0.50 0.69 1.13 0.69 0.94 0.48 0.60 0.38 0.74 0.17 0.74 0.87 0.41 0.22 0.55 0.06 0.05 0.09

0.68 0.52 0.60 0.49 0.84 0.73 0.45 0.63 0.44 0.72 0.83 0.64 0.78 0.65 0.75 0.35 0.15 0.35 0.03 0.02 0.01

a The values are normalized on the basis of results from cold trapping method.

PAHs escaped the LU tar sampler. Three different comparison tests were carried out for each method.

Results and Discussions Reliability of the LU Sampling Method. Comparison of the cold trapping and the LU-condensation methods shows good agreement between the methods for PAHs (naphthalene and heavier). The observed difference is less than 5% of the total. The comparative values for PAHs measured by these methods are presented in Table 3. As seen in the table the agreement between the methods is relatively good. The most notable disadvantage of the condensation method is that the light aromatics go through the system without

sample

ER

in decanted water (mg/L)

in product gas (mg/Nm3)

conversion (mg/kg-fuel)

I II

0.4 0.3

0.25 59.85

0.02 0.44

0.1 1.7

getting trapped (less than 0.1% of the benzene was found in the cooler). On the other hand the condensation method gives an acceptable value for the gravimetric measurements of the tars. Compared to the cold trapping and the condensation method, the SPA was the less attractive method. This method, despite its proved advantage of separating and sampling the phenolic fraction of the tars,11 showed itself not to be a suitable method for collection of the heavy PAHs. Likewise the deviations in the amounts of lighter compounds were larger for this method. Deviations in the amounts of lighter compounds might be due to inaccuracies in the volume of the gas taken into syringe. This volume is strongly affected by the temperature of the gas and environment. Deviations for heavier compounds can be related to their possible condensation before reaching the sampler. Additionally the concentrations of PAHs in SPA solutions are much lower compared to the solutions from the other methods. This in turn decreases the accuracy of the GC-MS measurements. In the literature the occurrence of the phenolic tar fractions is related to low gasification temperatures. Indeed the analysis of the condensates on the feeding screw, where the conversion temperature is low, confirms this result. However, at high-temperature gasification the impact of the heavier PAHs is more important than the phenolic compounds. Total organic carbon (TOC) analyses of the water decanted from LU sampler (Table 4) showed that the contents of the oxygenated or other water-soluble tar components are negligible. The results from the combined condensation-cold trapping method showed that, at maximum, 4 wt % of the PAHs escape the cooler and of these naphthalene is the predominant compound. GC-MS Detectable Tar Compounds. In the literature the tars from gasifiers are usually presented in mg/ Nm3 unit. Presenting the data in this way can give a false picture of the phenomenon of tar formation for the air-blown fluidized bed gasification process. For these systems the minimum or terminal fluidization velocity of the bed material particles sets the required airflow to the gasifier. There is a narrow window within which the airflow is allowed to be varied and therefore the changes in ER are normally achieved by increasing or decreasing the fuel flow. The nitrogen from the air is not involved in any gasification but does dilute the product gas. The dilution effect of the nitrogen is dependent on the product gas/ air input ratio. Lower ERs correspond to higher fuel input and consequently higher gas production. For these cases the nitrogen dilution effect becomes less pronounced. At higher ERs when the amount of the gas produced is less, the dilution effect is more considerable. The observed sharp decrease of the gas tar content in these cases is largely a direct consequence of the dilution effect. To eliminate the dilution effect we decided to present the data in g/kg-fuel unit. To simplify the presentation

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Table 5. Fuel Conversion to PAHs and Benzene (g/kg-fuel) fuel(task nr.)

2 rings

3 rings

4 rings

5 rings

6 rings

GC-dtf

Gr-dtf

RDa

benzene

bark (1) bark (2) bark (3) sawdust A(1) sawdust A(2) sawdust A(3) sawdust A(4) sawdust B(1) sawdust B(2) sawdust B(3) sawdust B(4) sawdust B(5) sawdust B(6) sawdust B(7) sawdust B(8)

26.8 26.1 36.2 15.8 18.0 18.5 23.2 10.7 12.4 9.8 11.8 12.9 8.9 6.2 4.9

12.5 10.7 14.8 6.4 7.7 9.7 13.5 3.4 3.2 4.1 5.9 3.5 2.4 1.7 1.0

5.9 3.5 9.4 4.6 4.9 5.5 7.7 0.5 2.2 3.8 7.7 4.0 2.8 2.2 0.6

4.4 1.6 5.7 2.6 2.8 4.0 8.1 0.2 1.0 1.9 3.0 2.8 1.3 1.4 0.2

1.7 0.7 2.1 0.8 1.4 2.6 3.6 0.0 0.2 1.0 1.4 1.4 1.2 0.7 0.1

51.2 42.5 68.3 30.2 34.8 38.6 54.5 14.8 19.0 20.6 29.8 24.6 16.7 12.2 6.8

54.8 44.8 72.4 34.6 35.7 52.5 76.9 20.5 29.8 42.4 41.0 36.3 47.4 43.2 47.1

0.93 0.95 0.94 0.87 0.97 0.74 0.71 0.72 0.64 0.49 0.73 0.68 0.35 0.28 0.15

19.2 21.5 24.0 45.6 34.2 31.1 15.9 15.9 19.8 24.3 25.0 56.5 34.0 26.6 27.0

a

Dimensionless. Table 6. Results from TGA/DTA Analyses of the Evaporation Residues

Figure 3. Recovery Degree for Tars from Biomass B.

of the data the PAHs are divided into 5 different groups on the basis of their ring number. A summary of the results is presented in Table 5. Tar Recovery Degree. For some samples there is a large difference between the results from gravimetric determined tar fraction (Gr-dtf) and GC-MS detectable tar fraction (GC-dtf). The recovery degree (RD) is defined as the ratio between GC-dtf and Gr-dtf. Analysis of the results showed that RD is a function of the fuel particle size, the bed temperature, and the ER. For the same fuel higher RD is achieved at higher ER and higher gasification temperature. For bark the RD was greater than 0.9, and for sawdust A the RD was between 0.7 and 0.95. The lowest RD corresponds to the lowest ER and the lowest temperature. For sawdust B the RD varied between 0.15 and 0.7. If the ER is held constant, the RD increases linearly with increasing temperature (Figure 3). GC-MS Undetected Tar Fraction (GC-utf). As mentioned above the RD was very low for some samples, and these deviations may be accounted for by the following three explanations: (a) inaccuracy of the GC-MS analysis, (b) inaccuracy of the gravimetric measurements, (c) different properties of the tars. (a) Using standard calibration PAH solutions the accuracy of the GC-MS was checked. The results showed a maximum average error of (10%, indicating that the reason could not be related to the analytical method. (b) To determine the inaccuracies of the gravimetric method, 50 mL volumes from four different tar solutions were taken into the weighed glass beakers. The contents

section

temperature (°C)

weight loss (%)

rate (mg/°C)

1 2 3 4 total

20-220 220-480 480-520 520-620 25-1000

0 46 12 37 ∑ ) 95

0.018 0.008 0.026

of the beakers were allowed to stand in a well-ventilated box and the solvent was evaporated. The residues were weighed and dissolved in 50 mL DCM again and 1 mL of each solution analyzed by GC-MS. Evaporation, dissolution, and analyzing procedures were repeated until a stable residue weight was achieved. Calculations showed that the final weights of the residues were in good agreement with the original results from gravimetric measurements. (c) Two samples of the residues were analyzed by a temperature-programmed Thermogravimetric analyzer (TGA). TGA analyses were preformed in an air atmosphere with a heating rate of 10 °C/min. Both samples behaved similarly and showed a total weight loss greater than 95% at the temperature interval 25-1000 °C. The results from the experiments are presented in Table 6. The weight losses in sections 2 and 3 are due to evaporation of the tar compounds and therefore endothermic processes. The weight loss in section 4 is an exothermic reaction probably due to combustion of the residue. It is possible that at 520 °C, which is the start temperature of the section, the residue reaches ignition or glowing temperature. The residue can be either hightemperature boiling macromolecules or char and char, which are evolved during this slow heating rate TGA analyses. The high evaporation temperature indicates that a large fraction of the residues consists of compounds with higher boiling temperature than those which were detected by GC-MS. The reason for the higher boiling temperature of the GC-utf could be either the higher molecular weight of the subspecies or their polarity. It is also more likely that the fraction consists of unreacted fragments of cellulose, hemicellulose, or lignin. However the part of the GC-utf was largest in the tars from small particles (Sawdust B). Parameter Studies on Tar Formation. Assumed Tar Formation Mechanism. According to the fundamen-


tals of the pyrolysis and gasification theories, lowtemperature slow heating rate pyrolysis enhances the conversion to char and permanent gases, while hightemperature fast pyrolysis results in a higher yield of oils and heavy tars.13,14 Studies by Evans and Milne on direct molecular beam mass spectrometry analysis on pyrolysis of wood particles showed that tertiary products (PAHs) were the major tar constitutes at high temperatures and at the absence of oxygen.8 At 700 °C the effect of oxygen at levels below 10% was to increase the naphthalene abundance. Higher oxygen concentrations led to destruction of naphthalene. Aleń et al. in their studies on the thermochemical behavior of sawdust from Scots pine, that was conducted by using a pyrolysis-gas chromatography with atomic emission detection (Py-GC/ AED), concluded that the gradual degradation of organic polymers in feedstock apparently proceeded via highmolecular-mass tars rather than directly to a multitude of low-molecular-mass gases.15 Based on the results achieved from different experiments conducted in TGA and ablative reactors and during pyrolytic conditions, Lede et al. discussed the possibility of the existence of an intermediate “active” species or state. According to their conclusion this intermediate exists and consists of low viscosity liquid at ablative pyrolysis temperature, is a solid at room temperatures, and appears to have a higher molecular weight than the condensates made from pyrolysis products.16 The above-mentioned points give an understanding about the initial steps of tar formation during pyrolysis of biomass particles. However, the conditions within gasification processes which are optimized to maximize the gas yield are far from pyrolitic conditions. Gasification is normally performed at a high temperature and at the presence of an oxidizing agent to maximize the total carbon conversion and minimize char and tar yields. In a fluidized bed gasifier, after entering the reactor the fuel is simultaneously exposed to a very high temperature and a heating rate of several thousand degrees per second.17 During this period the volatile matter, which produces the permanent gases and light aromatics, leaves the particle. The rest is a high-density residue that moves down toward the reactor bottom. Polycyclic compounds probably make the structural basis of this residue. We have assumed that the tars are mainly produced from this residue. The residue from the large particles falls down to the bottom of the reactor where the environment is oxidizing. At this reactor level the surface of the particles becomes exposed to oxygen and burning of the outer shell gives a higher outer temperature. Inside the particle devolatilization of the heavy compounds take place. The released volatiles have a possibility to be cracked to smaller PAHs and light aromatics. The larger (13) Reed, T. B. Biomass Gasification: Principals and Technology; Noyes Data Corporation: New Jersey, 1981; p 103. (14) Chemistry of coal utilisation, 2nd supplementary volume; Elliot, M. A., Ed.; John Wiley & Sons: New York, 1981; pp 718-719. (15) Aleń, R.; Oesch, P.; Kuoppala, E. J. Anal. Appl. Pyrolysis 1995, 35, 259. (16) Lede, J.; Diebold, J. P.; Peacocke, G. V. C.; Piscorz, J. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professionals: London, 1997; pp 27-42. (17) Chemistry of coal utilisation, 2nd supplementary volume; Elliot, M. A., Ed.; John Wiley & Sons: New York, 1981; pp 1160-1162.


the particle size of the residues, the longer the time required for the tars to diffuse to the surface. Consequently the cracking reactions will be more thorough, resulting in a higher yield of lighter PAHs. The residue from the small particles never reaches the bottom of the reactor, but undergoes the abovementioned reactions at a higher reactor level, where the availability of oxidant is less. The diffusion of the volatiles from these particles is faster and therefore the cracking is less severe. For any particle size the inner part has a lower temperature than the surface and the volatiles contained within it are in a condensed form. The gas-phase volatiles are formed by evaporation of the liquid volatiles inside the particles. High operating pressure will increase the partial pressure of the volatiles in the gas phase, while the liquid inside the particle will not be affected by the external pressure. Therefore, high pressure decreases the devolatilization rate and consequently enhances the cracking reactions. On the basis of the above-mentioned statements the results achieved are discussed below. ER Effect. The higher availability to the oxidizing agent at higher ERs can promote decomposition of the tars to lighter compounds such as benzene or the permanent gases. Moeresch et al.18 studied the effects of ER and temperature on the formation of different tar compounds in an atmospheric bubbling fluidized bed reactor. They reported that compared to the temperature, ER had a less pronounced effect. Lammers et al.19 studies of catalytic tar cracking in a microreactor showed that the secondary air injection could result in a significant tar decomposition both with and without catalyst. This work confirms these results to some extent. Figure 4 illustrates the yield of PAHs from the experiments carried out with sawdust A. Four different ER values were tested. For the ER values 0.39 and 0.33, the gasifier temperature was around 850 °C while for the lower ER values (0.2 and 0.3) the temperature was kept at around 800 °C. As shown in the figure an increase in ER slightly decreases the conversion to PAHs. However, the most obvious effect of the ER can be seen in the distribution of the PAHs. The amount of the heavy fractions decreases with increased ER, while the conversion to benzene increases drastically. At higher ER values more oxygen is available for the fuel. It is more likely that the higher availability to the oxygen enhances the cracking of the heavier compounds. The observed increase in the benzene content can be related to the decomposition of the heavier compounds. Temperature and Particle Size Effect. The gasification reactor is equipped with five thermocouples located at different reactor levels. Table 7 shows the location of these thermocouples along the reactor. Higher temperatures, in an environment rich in hydrogen and water, can favor the thermal cracking of the PAHs via hydrogenation and hydrocracking reac(18) Moersch, O.; Spliethoff, H.; Hein, K. R. G. In Biomass for Energy and the Environment; Chartier, P., Ferrero, G. L., Henius, U. M., Hultberg, S., Sachau, J., Wiimblad, M., Eds.; Pergamon: London, 1996; pp 1398-1403. (19) Lammers, G.; Orio, A.; Beenackers, A. A. C. M. In: Biomass for Energy and the Environment; Chartier, P., Ferrero, G. L., Henius, U. M., Hultberg, S., Sachau, J., Wiimblad, M., Eds.; Pergamon: London, 1996; pp 1416-1422.

610


Padban et al.

Figure 4. Conversion of biomass A to benzene and PAHs. Table 7. Location of the Thermocouples RMT1 RMT2 distance from the bottom (cm) a

5

(feeding point)a RMT3 RMT4 RMT5

30

(37)

50

170

350

No temperature measurement.

tions. We believe that the effect of the temperature on tar formation is very important and we therefore investigated the relationship between tar formation and the gasifier temperature. The tar conversion of each fuel was plotted against the measured gasifier temperatures. The results showed that for bark and sawdust A, a relationship could be found only when the conversion was plotted against RMT1, i.e., the bottom temperature (Figures 4 and 5). For sawdust B, which had a smaller particle size than bark and biomass A, a linear relationship was found if the conversion was plotted against RMT2 (30 cm above the bottom) (Figure 6). These results could indicate that the tar conversion in large particles takes place at the reactor bottom,

Figure 5. Plot of bark conversion versus temperature.

while smaller particles undergo adequate reactions at a higher level in the reactor. For large particle sizes, the amounts of the GC-utf are much less than the GC-dtf. Increased bottom temperature results in decreased conversion of GC-dtf as shown in Figure 5. For small particles, the GC-utf is a considerable proportion of the tars. Increasing the temperature decreases the total amount of tars while resulting in an increase in the GC-dtf. Figure 6 clearly illustrates this statement, indicating that the GC-dtf is produced from the heavier tar fractions. The data presented in Figure 6 correspond to the same ER value (0.37-0.38). For biomass B, the lowest (total) tar formation was achieved at an ER value of 0.2 and a bed temperature of about 860 °C. Increased top temperature resulted in increasing amounts of the GC-dtf. These results seem to be in contradiction to previous statements that higher temperatures and ER values result in declining tar formation. However, the results are not surprising since at lower ER values, the gasification environment is more reducing and cracking reactions are suppressed and a large proportion of the residues leave the gasifier without getting fully converted. In these experiments the total carbon conversion was about 10% less. Tar Formation in an 18 MW Circulating Fluidized Bed IGCC Biomass Gasifier. Results from tar measurement at Va¨rnamo20 IGCC demonstration plant, which is a 18 MW air-blown, pressurized circulating fluidized bed gasifier, show that for the same fuel the conversion to tars is lower by one magnitude compared to in the LU gasifier. There is also a clear difference in the composition of the tars. Naphthalene is the dominating compound in Va¨rnamo PAHs, and the fraction of the GC-utf is negligible. There are several reasons that may explain this difference. (a) Reactor construction. In a circulating fluidized bed the residues are reintroduced to the oxidizing parts of the bed. This in turn promotes the formation of the lighter compounds by the mechanism described previously. (b) Higher gasification temperature. The operation temperature for Va¨rnamo plant is higher than the LU


Figure 6. Conversion of biomass B versus temperature. (ER ) 0.37 - 0.38).

gasifier and is normally between 950 and 1000 °C. Higher temperatures enhance the cracking of the heavier fractions. (c) Higher pressure. Va¨rnamo plant operates at higher pressure (20 bar). Higher pressure results in slower devolatilization and consequently a lower amount of tars produced. The pressure effect in reducing the tar production has been reported previously.21


very large for the tars produced from small particles. For large particles, conversion to tars occurs at the oxidizing environment of the reactor bottom where the presence of oxygen enhances tar cracking. Increased temperature increases the amounts of GC-dtf. Tar formation from small particles takes place at higher reactor levels and in a moderately reducing environment. Increased temperature decreases the total tar amount while at same time increasing the proportion of the GC-dtf. At the same temperature, higher availability to oxygen slightly decreases the total amount of the tars; however, the most pronounced effect is on the conversion to benzene which increases drastically. Acknowledgment. The gasification tests were performed within the framework of the Multiclient-Project supported by the following Swedish companies: Helsingborg Energi, Plastkretsen AB, Svenska Kartongåtervinning, Sydkraft AB, SYSAV Utveckling AB, Tetra Pak, and Vattenfall Utveckling. This particular part of the research was financially supported by the Research Foundation of the Swedish energy company Sydkraft AB.

Conclusions The tars from the gasifier can be divided into two fractions: relatively light PAHs that can be detected by GC-MS, and one unknown fraction that consists of very high-temperature boiling components. The conversion of fuel to tars is a function of fuel composition, ER, temperature, and particle size. Tars from large particles consist mainly of compounds that could be identified by GC-MS. The proportion of the unknown fraction can be (20) Ståhl, K.; Neergaard, M. In Power production from Biomass III, Gasification and Pyrolysis R&DD for Industry; Sipila¨, K., Korhonen, M., Eds.; Espoo: VTT, 1999; pp 7378. (21) Aldeń, A.; Hagstro¨m, P.; Hallgren, A.; Waldheim, L. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professionals: London, 1997; pp 1131-1143.

Abbreviations: DCM: dichloromethane DTA: differential thermal analyses ER: equivalence ratio, defined as the ratio between the actual air input and the air input that is needed for stoichiometric combustion of the fuel GC-dtf: GC-MS detectable tar fraction GC-utf: GC-MS undetectable tar fraction Gr-dtf: gravimetric determined tar fraction LU: Lund University PAH: polynuclear aromatic hydrocarbon RD: recovery degree RMT: reactor middle temperature TGA: thermogravimetric analyses EF990185Z

Tar Formation in Pressurized Fluidized Bed Air Gasification of Woody

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