Tar Emission during Pyrolysis of Low Rank Coal in a Circulating

Jan 18, 2018 - National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan. Energy Fuels ...
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Tar Emission during Pyrolysis of Low Rank Coal in a Circulating Fluidized Bed Reactor Yasumasa Kawabata,† Hideki Nakagome,† Takaaki Wajima,† Sou Hosokai,‡ Hiroaki Sato,‡ and Koichi Matsuoka*,‡ †

Chiba University, 1-33 Yayoicho, Inage-ku, Chiba 263-8522, Japan National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan

‡

ABSTRACT: A circulating fluidized bed (CFB) reactor composed of a pyrolyzer and combustor was developed to observe tar emission during pyrolysis of low rank coal. Tar emission in the CFB pyrolyzer was examined under a wide range of operating conditions. Emissions of light tar substances (e.g., benzene, toluene, naphthalene, etc.) could be suppressed at 973 K by enhancement of contact between tar and resultant char in the pyrolyzer (i.e., enhancement of the volatile−char interaction (VCI)). It was also confirmed that about 50% of the heavy tar fraction emissions could be suppressed by the enhancement of VCI at 973 K. These trends were also observed at higher temperature (1173 K). A certain amount of heavy tar was emitted even after enhancement of VCI, so the mechanism of tar elimination was qualitatively determined using Spiral-type TOF-MS. The heavy tar was homogeneously deposited on the char and then was cracked to form lighter fractions by enhanced contact between tar and resultant char during pyrolysis.

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INTRODUCTION Coal gasification is widely applied for power generation and for production of chemicals, liquid fuels, gaseous fuels, and so on. To increase the thermal efficiency of endothermic coal gasification, lowering the gasification temperature is preferable. So far, several types of gasifiers (entrained bed, fixed bed, moving bed, and fluidized bed) have been examined for coal gasifcation.1 Among these types, the fluidized bed reactor is promising for realizing low temperature gasification because the residence time of the coal in the reactor can easily be controlled. To realize low temperature gasification using a fluidized bed reactor, the following two issues have to be solved. First is promotion of the rate-determining char gasification step, and the other is suppression of tar emission. It is known that char gasification is strongly influenced by the presence of volatiles (i.e., hydrogen gas2 and tar3) formed by pyrolysis prior to subsequent char gasification. That is to say, volatiles adsorb to the char surface to cover the active sites needed for gasification (volatile−char interaction, VCI), after which char gasification is inhibited. On the basis of such insights, we developed a triple bed circulating fluidized bed (TBCFB) reactor for enhancement of char gasification in the absence of voilatiles.4,5 In the TBCFB reactor, three zones (pyrolysis, gasification, and combustion) were physically isolated. After pyrolysis was terminated within the pyrolysis zone, the volatiles were evacuated. Therefore, volatiles were not present during char gasification in the gasification zone, and the char gasification was successfully enhanced.5 After the gasification, the resultant char was transported to the combustor to be combusted. As for the tar emission issue, several kinds of metal catalysts including natural minerals have been examined to suppress tar emission. However, coke deposition on the catalyst is a severe problem, and the cost for catalytic operation is also high for commercial plants. Furthermore, as an alternative to the use of © XXXX American Chemical Society

metal catalysts, there are some reports that gasification residue char can be utilized to capture tar or for cracking.6−8 For example, Zhang et al. examined tar behavior during pyrolysis of Australian lignite by adding ex situ char to a drop tube reactor.8 They reported that tar could be significantly decreased when cofeeding the reactor with raw lignite and ex situ char, indicating that the tar was decomposed and/or deposited on the cofeeding char. In the TBCFB gasification4 process described above, we also found that emission of members of the “light tar” fraction (benzene, toluene, naphthalene, etc.) from the pyrolysis zone could be suppressed if a certain amount of coal char was recycled from the combustor to the pyrolysis zone in the TBCFB. However, details of the behavior of heavy tar with high molecular weight (>300) are still unknown, and the key parameters for tar elimination are also unclear. Therefore, in the present study, we used a circulating fluidized bed reactor consisting of a bubbling pyrolyzer and a bubbling combustor to simulate the behavior of tar in the pyrolysis zone of a TBCFB reactor. The tar emission behavior then was examined when coal char that was partially combusted in the combustor was forced to accumulate in the pyrolyzer under a wide range of operating conditions. In other words, the tar behavior was examined during enhancement of the volatile− char interaction. In addition, an attempt was made to clarify the fate of heavy tar through detailed analysis of it by applying a Kendrick diagram,9−11 a powerful methodology used to analyze natural organic matter like that in crude oil, soil, and so on. Received: November 6, 2017 Revised: January 5, 2018

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DOI: 10.1021/acs.energyfuels.7b03419 Energy Fuels XXXX, XXX, XXX−XXX

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enhanced. Outlet gases from the pyrolyzer and combustor were independently analyzed with micro gas chromatographs. A portion of the outlet gas (80 cm3) from the pyrolyzer was sampled with a syringe during fixed intervals. The syringe was washed with methanol, and its soluble fraction was analyzed using a gas chromatography mass spectrometer (GC−MS). The amounts of benzene, toluene, xylene, phenol, cresol, naphthalene, phenanthrene, anthracene, and pyrene were determined, and these species are together defined as the light tar fraction. On the other hand, the heavy tar fraction was captured using a quartz thimble filter heated to about 393 K and then washed with tetrahydrofuran (THF) under ultrasonic irradiation at ambient temperature. The heavy tar fraction was obtained by evaporation of THF soluble fraction under vacuum. Carbon content in the heavy tar was determined by ultimate analysis. Note that some species of light tar (boiling point >393 K) would be captured in the quartz thimble filter with the heavy tar. Therefore, the accurate amount of heavy tar was determined by subtracting the amounts of light tar (boiling point >393 K) from the total amount of tar after the evaporation. Furthermore, the molecular weight distribution of the heavy tar was determined by Spiral-type time of flight mass spectrometry (Spiral TOF-MS) analysis.

EXPERIMENTAL SECTION

Coal Samples. Australian low rank coal (Loy Yang) was used here. The sample properties are listed in Table 1. Prior to use, the pulverized coal was sieved (0.5−1.0 mm) and then dried at 353 K for 8 h under vacuum.

Table 1. Sample Properties of Loy Yang Coal [wt %, daf] Loy Yang (LY)

[wt %, dry]

C

H

N

S

O(diff)

ash

66.7

4.8

0.6

0.2

27.7

0.7

Experimental Procedure. A circulating dual bubbling fluidized bed reactor (CDBFB) was used in the present study. A schematic diagram of the apparatus is shown in Figure 1. A bubbling bed

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RESULTS AND DISCUSSION Product Gas Yields under Different Conditions. In Figure 2, an example of the variation of gas yield in the

Figure 1. Schematic diagram of circulating dual bubbling fluidized bed reactor. pyrolyzer was connected with a bubbling bed combustor. Silica sand, which is inactive for tar cracking/reforming, was used as the bed material. The static bed height in the pyrolyzer was 15 cm. The bubbling bed pyrolyzer was heated using a gold-coated transparent furnace at the temperature from 973 to 1173 K, and the combustor was heated at 1223 K using another transparent furnace. Because both reactors were made of quartz (i.e., transparent), the behavior of the bed materials in each reactor was directly observed during pyrolysis and combustion. The coal particles were fed at about 0.4 g/min into the pyrolyzer over the bubbling bed (defined as top feeding) or near the distributor (defined as bottom feeding) by a screw feeder. The coal was pyrolyzed to evolve tar and gas as volatiles, and also formed char in the pyrolyzer. The resultant char and bed material were circulated to the bubbling combustor through a downer. The char was combusted in the combustor, and then the bed materials were circulated back to the pyrolyzer. The extent of combustion in the combustor could be controlled by controlling the O2 concentration there. The char could be completely combusted by feeding a sufficient amount of O2 for the amount of char in the combustor, after which only the silica sand would be circulated back to the pyrolyzer. On the other hand, the char could be recirculated from the combustor to the pyrolyzer by feeding insufficient O2 for the amount of char in the combustor (defined as char recycling). The char was gradually accumulated by extending the char recycling time, meaning that the contact of char with tar was

Figure 2. Yield of product gases during pyrolysis of Loy Yang coal in pyrolyzer at 973 K (a) and during combustion of resultant char in combustor at 1223 K (b).

pyrolyzer and combustor is shown. In this case, the coal feeding mode was top feeding, and the feeding was continued for about 5 h. The yield of outlet gases from the pyrolyzer was almost stable, suggesting that stable circulation of solids could be achieved. Excess O2 for the amount of char in the combustor was fed in, and complete combustion of the char was sustained for 1.5 h, in this case. Only CO2 was detected until 1.5 h, and the yield of CO2 from the combustor was stable. The yield of char during pyrolysis could be determined from the yield of CO2 during complete combustion, as shown in Figure 2b. The concentration of O2 fed into the combustor was quickly decreased at 1.5 h to recirculate a certain amount of char from the combustor to the pyrolyzer (char recycling). After changing the O2 concentration at 1.5 h, CO as well as CO2 was observed, and the sum of these yields was leveled off. Trends seen here were similar under other operating conditions. B

DOI: 10.1021/acs.energyfuels.7b03419 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels The yields of products (i.e., gas, light and heavy tar fractions, and char) at different pyrolysis temperatures are summarized in Figure 3. Although small, coke deposited on the pyrolyzer wall

Figure 4. Heavy tar yield during pyrolysis in case of top feeding of LY coal at 973 K (a) and 1173 K (b). Figure 3. Carbon material balance during LY coal pyrolysis at different temperatures.

could not be measured during the experiment. This amount was not accounted for in the carbon material balance. This is the main reason for the observed lower carbon material balance. However, it can be said that a good material balance on a carbon weight basis was generally obtained at different pyrolysis temperatures, as determined by the carbonaceous gas yields and tar analyses. The yield of gas increased with increasing pyrolyzer temperature, while the heavy tar yield decreased with increasing pyrolysis temperature. There have been some studies on the fate of heavy tar during pyrolysis.12−14 These studies suggested that the heavy tar was secondarily cracked to form the lighter fraction at higher temperature.12−14 The results in Figure 3 also show a similar trend. Details of the fate of the heavy tar will be discussed in a later section. Heavy Tar Behavior during Volatile−Char Interaction. As shown in Figure 2, char recycling from the combustor to the pyrolyzer was commenced by decreasing the O2 concentration in the combustor. Figure 4a and b represents LY heavy tar yield during char recycling at different temperatures with top feeding. Time elapsed after decreasing the concentration of O2 loaded into the combustor is defined as char recycling time in Figure 4. Heavy tar yield decreased with increasing char recycling time because volatiles can easily contact recycled char given the increasing char accumulation in the pyrolyzer. The yield of heavy tar at 1173 K was relatively smaller than that at 973 K, because the secondary cracking was enhanced at higher temperature.12,13 In the case of bottom feeding, the amount of heavy tar was generally smaller than that for top feeding during char recycling (Figure 5a and b). From direct observation of the pyrolyzer during heating, we confirmed that volatile formation occurred in the upper part of the bubbling bed during top feeding. On the other hand, volatile formation occurred near the distributor during bottom feeding. As compared to top feeding, the contact of volatiles with recycled char was greater with bottom feeding. Furthermore, the secondary cracking of tar was also enhanced in the bubbling bed with bottom feeding. This was because the residence time

Figure 5. Heavy tar yield during pyrolysis in case of bottom feeding of LY coal at 973 K (a) and 1173 K (b).

of the volatiles in the bed was longer. If the heavy tar emission for the top feeding before char recycling is defined as the base case, about 80% of heavy tar emission at 1173 K could be suppressed by bottom feeding char recycling (4 h). To minimize the heavy tar emission, bottom feeding at higher temperature with char recycling is preferable because the secondary cracking of heavy tar would be enhanced in comparison with top feeding. In this experimental setup as shown in Figure 1, solids (bed material and char) were overflowed from the pyrolyzer to the downer. Residence time of volatiles in the bubbling bed was almost fixed, because the height in the bubbling bed was independent of the char recycling time. Char concentration in the bubbling bed was increased by extending char recycling time. Char/coal ratio as a weight basis was changed from about 6 (before char recycling) to 15 (char recycling after 4 h). Therefore, heavy tar emission was suppressed by the enhancement of contact of volatiles with char in the bubbling bed. C

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Figure 6. Molecular weight distribution of heavy tar during pyrolysis at 1173 K: (a) top feeding before char recycling, (b) top feeding after char recycling for 4 h, (c) bottom feeding before char recycling, and (d) bottom feeding after char recycling for 4 h.

Figure 7. Molecular weight distribution of heavy tar during pyrolysis at 973 K: (a) top feeding before char recycling, (b) top feeding after char recycling for 4 h, (c) bottom feeding before char recycling, and (d) bottom feeding after char recycling for 4 h.

Even though the operating condition was optimized within the present experimental conditions, heavy tar emission could not be completely suppressed. Detailed analysis of the heavy tar fraction was done using TOF-MS to determine what parts of the heavy tar could not be eliminated by the volatile−char interaction. The results follow. Detailed Analysis of Heavy Tar. Figure 6 shows the molecular weight distribution of the LY heavy tar fraction formed at 1173 K for different feeding modes before and after char recycling for 4 h. Several hundred peaks were resolved and identified in each spectrum. With top feeding at 1173 K before char recycling (Figure 6a), the molecular weight distribution ranged from ∼150 to 550. From the comparison of Figure 6a and b, after the char recycling, the distribution was somewhat narrower than that before char recycling, suggesting that the heavier fraction was reduced. This trend was also observed in the bottom feeding case, as shown in Figure 6c and d. In Figure 7, the molecular weight distributions of the heavy tar at 973 K are also shown. The distributions are broader than those at 1173 K, because secondary cracking of the heavy tar was not as pronounced as with 1173 K. As is seen in Figure 6, the heavier fraction is reduced by char recycling at 1173 K, as well.

In Figure 8, typical spectra of heavy tar at 1173 and 973 K after char recycling are shown. In Figure 8a is an example of detailed spectra for char recycling at 1173 K after 4 h (Figure 6b). At nominal mass of 306 (Figure 8a), two distinct spectra were found: one was assigned to C24H18 and the other to C23H14O. Also, in Figure 8b, typical spectra at 973 K for the nominal mass of 296 are shown. As is seen, the three spectra could be assigned to C23H20, C22H16O, and C21H12O2. From such detailed analysis of each spectrum, the elemental composition of heavy tar was obtained. However, it was not easy to analyze all of the spectra shown in Figures 6 and 7. Therefore, we introduced here a Kendrick diagram to determine what parts of the heavy tar fraction could be eliminated by the volatile−char interaction. Briefly, the results from the Kendrick diagram could be summarized follows.9−11 The Kendrick mass assumes the mass of CH2 to be 14.0000 with no decimal fraction, even though the IUPAC mass of CH2 is 14.01565; thus: Kendrick mass = IUPAC mass × 14.0000/14.01565

(1)

If a heavy tar molecule is represented as CnHmOl (n, m, l: integer): D

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For example, the “Kendrick mass”, “nominal Kendrick mass”, and “Kendrick mass defect” of coronene (C24H12) are 299.7588, 300, and 0.2412 (=300 − 299.7588), respectively. From the plot of the Kendrick mass defect against the nominal Kendrick mass on the basis of hundreds of peaks in heavy tar (Kendrick diagram), the composition of the heavy tar fraction can be speculated. Figure 9a and b shows Kendrick diagrams for heavy tar formed at 1173 K before and after char recycling for 4 h with top feeding. In this figure, many lines are also drawn to make it easier to understand the composition of heavy tar. Straight lines and dotted lines mean carbon atomic number and hydrogen atomic number, respectively. Substances composed of carbon and hydrogen atoms can be plotted at the intersections of the diagram (this corresponds to l = 0 in eq 4). For example, coronene (C24H12) should be plotted at the intersection of C24 and H12. As shown in Figure 8, heavy tar was composed of not only hydrogen and carbon, but hydrogen, carbon, and oxygen. If a heavy tar molecule includes heteroatoms like oxygen, that is, l ≠ 0 in eq 4, points cannot be plotted at the intersections. Figure 9c and d shows Kendrick diagrams for heavy tar formed at 1173 K before and after char recycling for 4 h with bottom feeding. The effect of bottom feeding on heavy tar decomposition can be observed by comparing Figure 9a with c or Figure 9b with d. Almost all of the points are plotted at the intersections in Figure 9c and d, while there are many points that are not at the intersections in Figure 9a and b. On the basis of the detailed analysis, most of the points that are not plotted at the intersections mean one or two oxygen atom-including heavy tar molecules (Figure 9a and b). Points at the intersections in Figure 9c and d mean that heavy tar was composed of only two elements (i.e., carbon and hydrogen). Heavier fraction (nominal Kendrick mass >350) with bottom feeding after char recycling was reduced in comparison with

Figure 8. Typical spectra of heavy tar after char recycling for 4 h at 1173 K in top feeding (a) and after char recycling for 4 h at 973 K in bottom feeding (b).

Kendrick mass = (12n + 1.007825m + 15.994915l) × 14/14.01565

(2)

Because “nominal Kendrick mass” is equal to “Kendrick mass” rounded off to integer value: “nominal Kendrick mass” in the above case = 12n + 1m + 16l

(3)

The difference between “Kendrick mass” and “nominal Kendrick mass” is defined as the “Kendrick mass defect”; thus: Kendrick mass defect = 0.0067(2n − m) + 0.0226l

(4)

Figure 9. Kendrick diagram of heavy tar during pyrolysis at 1173 K: (a) top feeding before char recycling, (b) top feeding after char recycling for 4 h, (c) bottom feeding before char recycling, and (d) bottom feeding after char recycling for 4 h. E

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Figure 10. Kendrick diagram of heavy tar during pyrolysis at 973 K: (a) top feeding before char recycling, (b) top feeding after char recycling for 4 h, (c) bottom feeding before char recycling, and (d) bottom feeding after char recycling for 4 h.

that before char recycling, while this trend was not observed in case of top feeding. These results suggest that heavy tars formed during bottom feeding are substantially subjected to secondary cracking in the bubbling bed, while such secondary cracking was not promoted during top feeding because the contact of heavy tar with char in the bubbling bed was limited. Secondary cracking of the heavy tar caused progressive deoxygenation. Figure 10 shows Kendrick diagrams for heavy tar formed at 973 K before and after char recycling for 4 h. As shown in this figure, there are many points that are not at the intersections. As described in Figure 9, most of the points that are not plotted at the intersections are one or two oxygen atom-including heavy tar molecules. This indicates that oxygen-rich heavy tar was formed by pyrolysis at 973 K in comparison to at 1173 K. This is because deoxygenation is not substantial at lower temperature. Heavier fraction was eliminated in case of bottom feeding case with char recycling, while the trend was not observed in case of top feeding. This trend was similar to Figure 9. The Fate of Light Tar. Figure 11 depicts the yield of light tar during char recycling for different feeding modes. The light tar was analyzed using GC−MS. In the GC−MS analyses, benzene, toluene, xylene, phenol, naphthalene, and phenanthrene were detected, and other species were not detected. As is seen, the most abundant light tar component was benzene under all of the conditions. The yields of benzene and toluene slightly decreased with increasing char recycling time, suggesting deposition of these species onto the char.15 The yields of naphthalene and phenanthrene at 1173 K were slightly higher than at 973 K, indicating that heavy tar was secondarily cracked to lighter components. As shown in Figures 4 and 5, heavy tar is deposited on the recycled char, and then cracked/ reformed to form light tar. This trend is promoted at higher temperatures.

Figure 11. Comparison of light tar yields during pyrolysis of LY coal in case of top feeding at 973 K (a), 1173 K (b) and in case of bottom feeding at 973 K (c), 1173 K (d).

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Energy & Fuels Mechanism of Suppression of Tar Emission. The above results indicate that tar would deposit on the surface and/or in the pores of the char. To examine the behavior of the deposited tar during char recycling, change of gas yield during char recycling was examined in detail. Figure 12 shows the gas yield (H2, CO, CO2) before and after the char recycling for 4 h with top feeding and bottom

Table 2. Specific Surface Area before and after Char Recycling [m2/g] pyrolyzer temp 973 K

pyrolyzer temp 1173 K

394

436

315

394

455

457

without char recycling (sampling: downer) after char recycling for 4 h (sampling: downer) after char recycling for 4 h (sampling: loop seal valve)

Alkali and alkaline earth metallic (AAEM) species are rich in LY coal.16 Inherent AAEM species would be dispersed on the char surface during the pyrolysis and then act as catalysts for cracking and/or reforming of deposited tar. The role of AAEM species on tar cracking and/or reforming to clarify the detailed mechanism of suppression of tar emission utilizing VCI is one of the subjects for future study.

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CONCLUSIONS Emission of light tar and heavy tar during enhancement of the volatile−char interaction in the CFB reactor was examined here. The coal feeding mode was quite an important parameter for suppressing tar emission. With bottom feeding, the contact of tar with char was substantial (volatile−char interaction was promoted), and secondary tar cracking was also enhanced in comparison with top feeding. Therefore, the bottom feeding was more effective for reducing tar emission. Heavy tar evacuated before and after the volatile−char interaction was accurately analyzed using Spiral TOF-MS. Kendrick diagrams were applied to analyze the composition of the heavy tar fraction. The deoxygenation of the heavy tar was clearly enhanced at higher pyrolysis temperature during the bottom feeding. The composition of heavy tar, even after the volatile− char interaction, was similar to that before the volatile−char interaction in case of top feeding, while the heavier fraction was preferentially eliminated in the case of bottom feeding. From the quantitative analysis and accurate analysis, it was determined that the tar deposited on the char and then reformed to create components of the lighter fraction. We can demonstrate that char is a plausible material for suppressing tar emission by enhancement of the contact of char with tar in the pyrolysis reactor. Critical operating conditions to reduce tar emission more effectively will be elucidated in a future study.

Figure 12. Gas yield during pyrolysis of LY coal before and after char recycling for 4 h in case of top feeding (a) and bottom feeding (b).

feeding case. In both cases, CO2 yield was independent of the char recycling. On the other hand, H2 yield after the char recycling was higher. A similar trend was also observed in the CO case. Roughly speaking, at 973 K, the difference in H2 yield before and after the char recycling was higher than that of CO yield. On the other hand, at 1173 K, the difference in H2 yield was similar to that of CO yield. This indicates that tar is deposited onto the char and thermally cracked or reformed with steam formed by pyrolysis. At 973 K, decomposition of deposited tar by thermal cracking would be dominant in comparison with steam reforming. Decomposition of deposited tar by thermal cracking would be enhanced at higher temperature. From the comparison of decrease in tar yield (Figures 4, 5, and 11) with increase in gas yield (Figure 12), it can be inferred that most of the deposited tar was not reformed and remained on the char. Therefore, the char structure would be changed due to the deposition. The char was independently collected from the downer before and after char recycling for 4 h, and some of the char from the loop seal was sampled after partial combustion. Table 2 shows the BET surface area of the char before and after char recycling. The surface area of the char after it was recycled was less than that before char recycling, suggesting that light and/or heavy tar was deposited on the char. The surface area of the char after combustion (sampling point: loop seal valve) was largest among the three samples, because the deposited tar was removed by partial combustion. During operation of the CFB, tar was captured by the recycled char in the pyrolyzer and the pore structure of the char was destroyed. The char then was partially combusted to remove the deposited tar and regenerate the developed structure.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Koichi Matsuoka: 0000-0001-9844-1629 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the Strategic International Collaborative Research Program (SICORP) of The Japan Science and Technology Agency (JST).

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REFERENCES

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