Online Investigation of Steam Gasification Kinetics of Biomass Chars

Article pubs.acs.org/EF

Online Investigation of Steam Gasification Kinetics of Biomass Chars up to High Temperatures Leteng Lin* and Michael Strand Building and Energy Technology, Linnaeus University, S-351 95 Växjö, Sweden ABSTRACT: In this study, a novel aerosol-based method has been further developed and applied for online investigation of steam gasification kinetics of suspended biomass char particles. When the aerosol method is combined with thermogravimetric analysis, the gasification kinetics were established in a steam (33 vol %)−N2 atmosphere from 800 to 1300 °C for char samples produced from pelletized wood, straw, and miscanthus. The aerosol method includes steps for generating, suspending, and gasifying char particles. The conversion of the char particles was established by measuring the change in particle size distributions and mass concentrations using an aerodynamic particle sizer (APS) spectrometer and a tapered element oscillating microbalance (TEOM), respectively. The reactivity of three char samples could be ranked as wood > miscanthus > straw. The activation energy was 155 kJ mol−1 for wood char, 199 kJ mol−1 for miscanthus char, and 222 kJ mol−1 for straw char. Results interpreted from TEOM and APS measurements indicated that the effective density of char particles initially decreased until a certain level of conversion was reached and then remained constant. previous papers,6,7 the aerosol-based method can be used for online analysis of the gasification kinetics of char particles by measuring the changes of the particle mass concentration and particle aerodynamic diameter. The aerosol method presents a set of benefits that are advantageous compared to previously established techniques: no mass-transfer limitation at high temperatures because of the usage of fine particles, the flexibility to switch to different gas agents combined with continuous feeding of char particles, the online measurement of particle mass and size, and the potential of measuring online the reactivity of char particles extracted from the running gasifier. In this paper, the aerosol-based method has been further developed and used for online investigation of steam gasification kinetics of biomass chars at high temperatures. Three chars produced from wood, straw, and miscanthus pellets were investigated in the temperature range of 800−1300 °C. TGA was used in the temperature range of 800−900 °C, and the aerosol method was used in the temperature range of 1200−1300 °C.

1. INTRODUCTION Gasifying biomass to produce an energy-rich synthesis gas is a route to produce renewable transport fuel to reduce the consumption of fossil fuels.1 In gasification, the solid fuel undergoes sequential drying, devolatilization, and char gasification. Because char gasification is the rate-limiting step, reliable data on conversion kinetics of char are of importance for appropriate design, simulation, and optimization of an industrial gasification process. Several low-temperature gasification processes have been developed and commercialized for gasification of biomass, e.g., in fixed-bed and fluidized-bed gasifiers. The gasification rates of biomass char at low temperatures ( wood (Figure 3a); at 50% conversion, the order was wood ≈ miscanthus > straw (Figure 3b); and at 80% conversion, the order was wood > miscanthus > straw (Figure 3c). From all TGA results in this study, the gasification rates from all three char samples increase first with conversion up to a maximum and then start decreasing. Dependent upon the temperature, the maximum was reached at a conversion of 30−77% for wood char, 17−44% for miscanthus char, and 12−40% for straw char. If comparisons were made before the occurrence of the maximum rate, the ranking orders of the rates at different conversions would vary (as shown in Figure 3). In the profile of the reaction rate, the initial increase to the maximum was

char samples

apparent pre-exponential factor, Aa (s−1)

activation energy, E (kJ mol−1)

wood miscanthus straw

3.56 × 104 9.30 × 105 3.37 × 106

155 199 222

proven because of intraparticle diffusion.16 It is recommended that the intrinsic rate parameters should be determined from the slope in the region after the occurrence of the maximum.16,17 Therefore, the kinetic parameters for three types of chars were estimated on the basis of the reaction rates at 80% conversion (Figure 3c), as shown in Table 2. As shown in Figure 3c, at a conversion of 80%, the char reactivity varied in the low-temperature range then became similar at high temperatures. At 800 °C, the reactivity of wood 800-15 char was higher by a factor of about 7 than that of miscanthus 800-15 char. However, both showed similar reactivity at 1200 and 1300 °C. The alkali content in the ash 610

dx.doi.org/10.1021/ef402343x | Energy Fuels 2014, 28, 607−613

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Figure 4. Change of the particle effective density during char gasification. The points corresponded to the results from the aerosol method in Figure 2, measured at 1200 and 1300 °C. The lines were added as a visual guide.

may act as a catalyst in the char gasification reaction.18 Other inorganic constituents, such as silicon, on the other hand, could inhibit the char reaction rate.19,20 Silicon has been observed to react with potassium to form silicate that blocked the catalytic effect of potassium on the char gasification reaction.21 Dupont et al.19 reported a correlation between the K/Si ratio in biomass chars and char gasification reactivity, which highlighted the catalytic effect of potassium and the inhibiting effect of silicon on the char gasification. For straw and miscanthus chars, the high silicon content (Table 1) may explain their relatively lower reactivity.19,20 It was found that the TGA ash residues from wood char were only pure grayish ash, while some black matter existed in the residues from straw and miscanthus chars. That implies that the blockage built up from a high ash content might limit gas diffusion internally, which made chars not fully converted. Extrapolation should be applied with care to predict the hightemperature kinetics from the low-temperature TGA results. A comparison of the two extrapolations plotted in Figure 3c indicates that the extrapolated reactivity for wood and straw chars at 1300 °C from TGA results is about 5 and 9 times higher than that established by the aerosol method, respectively. Therefore, methods capable of measuring reaction rates in the high-temperature range need to be established, to contribute more to reactor design and process controlling. 4.3. Particle Density Variations. In the aerosol method, the APS instrument can count the number of particles, which are classified on the basis of the aerodynamic diameter. By assuming a spherical particle, the mass (m) can be estimated for a particle with an aerodynamic diameter dae, according to eq 2, as m=

⎛ C (d ) ⎞3/2 π π ρe d p3 = dae 3ρe−1/2 ρ0 3/2 ⎜⎜ c ae ⎟⎟ 6 6 ⎝ Cc(d p) ⎠

Figure 5. SEM images (magnification of 4000×) of wood char particles at different conversions: (a) X = 0, in N2 at 1300 °C; (b) X = 19%, in 33% steam at 1200 °C; and (c) X = 62%, in 33% steam at 1300 °C. The scale at the bottom of the images is 10 μm.

(9)

where ρe represents the effective density of particles. This equation can be further simplified if neglecting the slip correction factors, which are approximately equal to 1 for particles larger than 1 μm. π m = dae 3ρe−1/2 (10) 6

Assuming that the particle population has the same effective density, the total mass of the particles can be expressed on the basis of APS results as j

M= 611

π −1/2 ρ ∑ (nidaei 3) = ρe−1/2 MAPS 6 e i=1

(11)

dx.doi.org/10.1021/ef402343x | Energy Fuels 2014, 28, 607−613

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Figure 6. Examples of variations of number (left) and mass (right) size distributions of different char particles before and after gasification from APS measurements: (a) wood char, X = 32% (1300 °C and τ = 1.2 s); (b) miscanthus char, X = 64% (1300 °C and τ = 2.1 s); and (c) straw char, X = 43% (1300 °C and τ = 1.2 s). The mass size distributions were established on the basis of the estimated effective density, assuming spherical particles.

in which ni represents the number of particles measured by APS in channel i and j is the total channel number of diameter intervals in APS results. Because the true total mass of the particles, M, can be directly measured by the TEOM, it becomes possible to estimate the particle effective density (ρe) by comparing the TEOM mass to the APS mass (MAPS) that assumes spherical particles with the standard density 1 g cm−3. This type of density estimation is just the first trial in this paper according to the principle explained above (eq 11). Errors may exist in the estimated density, because of the assumption made for the idealized particles (having spherical shape, uniform density, and constant composition for different sizes), the neglect of the slip correction factors, and the particle losses, as well as the instrument limitations. In the experimental setup, particles will travel a longer distance in the sampling line to reach the APS instrument than the TEOM. Possible particle losses will cause fewer particles to enter the APS instrument compared to the TEOM. The APS instrument normally underestimates the particles in the sub-micrometer range.6 According to eq 11, the estimated effective density will probably be lower than the reality, because particle mass detected by the APS is lower. Therefore, further investigation is highly recommended to prove the reliability of the method concerning those absolute values of the estimated density. In this section, discussion mainly focuses on the relative change of the particle effective density versus conversion during gasification (see Figure 4). After the gasification reaction started, the effective density of the char particles decreased with conversion until it reached a certain level and then stabilized. For the straw char, the effective density stabilized around 35% conversion. For the miscanthus char and wood char, the effective density kept decreasing until

about 70% conversion. This trend implied that pore growth and intraparticle reaction are dominant in the initial stage of gasification. It agrees with the assumption of the RPM and is also consistent with the scanning electron microscopy (SEM) results discussed in the next section. In the later stage of gasification, where the effective density becomes constant, the particle diameter most likely starts decreasing because of shrinkage. 4.4. Char Morphology Variations during Gasification. To investigate the morphological changes of char particles during gasification at high temperatures, a series of wood char particles has been collected on the greased aluminum substrates by a one-stage impactor at different conversions and analyzed by SEM (Figure 5). At 19% conversion (Figure 5b), the char particles are quite similar to those unreacted in Figure 5a, considering the particle size and shape. Because of the difference of particle concentrations and sample time, the quantity of collected particles varied on each substrate. This might explain why the smaller particles in Figure 5b seem fewer in number than those in Figure 5a. At 62% conversion (Figure 5c), not many particles were collected on the substrate because of very low concentrations at high conversion. It seems that most particles had less sharp edges than those in panels a and b of Figure 5, and the quantity of large particles became much less. There are few large particles extant, which were more like agglomerates. That is to say, after gasification started, the size of particles did not apparently decrease until a certain conversion level. Kajitani et al.14 reported similar phenomenon from coal char gasification in steam or CO2, using a laser diffraction method; that is, particle size showed no changes until the conversion reached about 50%. It is reasonable to infer that, in the beginning of char gasification, pore growth is dominant up 612

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Svenskt Förgasningscentrum (SFC, Swedish Gasification Centre) for the financial support.

to a certain conversion. This also agreed with the trend of decrease of the effective density of particles over conversion, discussed in Figure 4. 4.5. Changes of Particle Size Distributions during Gasification and Stability of the Aerosol Method. Figure 6 presents the APS results about the number and mass size distributions of the char particle, comparing unreacted char particles (in pure N2) and partially reacted char particles (in H2O−N2). Only one example was picked up for each char under relevant conditions. Error bars are standard deviations derived from 5 to 10 scans within a period between 4 and 10 min, which were shown only on a few points, to reduce clutter on the plot. A decreasing tendency was observed for the total number concentrations of char particles. One possible explanation is due to the limited accuracy of the APS instrument in the sub-micrometer range.6 Leaving aside the part in the sub-micrometer range, the particle number size distributions for the three char samples seem to shift toward smaller aerodynamic diameter during gasification reaction. The particle mass size distributions were calculated according to the number size distribution using the particle effective density, which is estimated on the basis of APS and TEOM mass measurements (eq 11). In Figure 6a, the mass difference between mass size distributions of unreacted and reacted wood char particles represents a conversion of 32%, but a similar (or even smaller) mass difference from straw char particles in Figure 6c represents 43%. According to eq 5, this is a result of a much higher ash content in straw char than that in wood char (Table 2). The miscanthus char particles present in Figure 6b experienced longer residence time than both wood and straw char particles and had a higher conversion about 64% that was corresponding to a relatively larger gap between the unreacted and reacted mass size distributions. From the standard deviations, it can be seen that concentrations of the smaller particles (dae < 1 μm) fluctuate slightly more than concentrations of the larger particles. In general, the particle distributions from all three tested chars are under reasonably good control, which reveals the stability of the aerosol method.

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REFERENCES

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5. CONCLUSION The aerosol method was demonstrated to be successful for fragmenting, transporting, and gasifying the suspended char particles (0.5−10 μm) made from wood, straw, and miscanthus in the steam atmosphere. The intrinsic steam gasification kinetics for three types of biomass chars were established in a wide temperature range from 800 to 1300 °C by combining TGA with the aerosol-based method. The reactivity order from three chars was wood > miscanthus > straw. Results revealed that the effective density of char particles first decreased over conversion until a certain level was reached and then stabilized. This suggested that pore growth is dominant in the initial stage of char gasification.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +46-470-70-81-04. Fax: +46-470-70-87-56. Email: [email protected]. Notes

The authors declare no competing financial interest. 613

dx.doi.org/10.1021/ef402343x | Energy Fuels 2014, 28, 607−613