2598
Energy & Fuels 2008, 22, 2598–2603
Decomposition and Calcination Characteristics of Calcium-Enriched Bio-oil Yang Xulai, Zhang Jian, and Zhu Xifeng* Key Laboratory for Biomass Clean Energy of Anhui ProVince, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China ReceiVed December 4, 2007. ReVised Manuscript ReceiVed March 27, 2008
The decomposition and calcination characteristics at high temperatures of calcium-enriched bio-oil (CEB) were investigated from 900 to 1100 °C in the study. The CEB were produced by reacting bio-oil with calcium hydroxide, which combined the functions of desulfurization and providing heat during the decomposition and calcination processes. The decomposition of CEB consisted of four steps, partly similar to the case of pure calcium acetate (CA). The rate of the final step, i.e., the calcination rate to derive CaO, was higher in the case of CEB than in the case of CA. The alkali metal migrated from bio-oil might accelerate the calcination rate of CEB-derived CaCO3 and sintering rate of CaO. The porosities of CEB-derived CaO particles were higher than that from CA, but the Brunauer-Emmett-Teller (BET) surface areas were lower. Dried CEB of pH 10.0 (CEB10) were typical amorphous solids, which should be the optimum bifunctional material with a medium heating value. The decomposition of CEB10 yielded porous CaO particles of moderate surface area and sintering rate for potential SO2 capture.
1. Introduction Calcium-containing materials, mostly based on calcium carbonate and calcium hydroxide, are the most commonly used adsorbents for in situ removal of SO2 and are considered being a simpler and cheaper method to control SO2 in coal combustions at the power plant site. Recently, organic calcium salts, such as calcium acetate (CA), represent a new kind of adsorbent used for in situ removal of SO2. Experimental studies have shown that the unique pore structure and larger surface area make the cenospheric CaO particles that are obtained from the decomposition of organic salts of calcium reach much higher sulfation conversions than the CaO particles that are obtained from the decomposition of CaCO3 or other inorganic calcium solids.1,2 Moreover, the acetone releasing from decomposition of calcium acetate further decomposes at higher temperatures to allene or to hydrocarbon radicals, which can react with other pollutant gas compounds as NOx or can even be burned with oxygen.3–5 However, producing the organically bonded calcium by the reactions of lime or calcium hydroxide with the conventional acetic acid6,7 renders these adsorbents more expensive than the natural ones and commercial uses impossible. Therefore, special attention was paid on biomass pyrolysis liquors * To whom correspondence should be addressed. Telephone: +86-5513600040. Fax: +86-551-3606689. E-mail:
[email protected]. (1) Levendis, Y. A.; Zhu, W. Q.; Wise, D. L.; Simons, G. A. AIChE J. 1993, 39, 761–733. (2) Steciak, J.; Levendis, Y. A.; Wise, D. L. AIChE J. 1995, 41, 712– 722. (3) Steciak, J.; Levendis, Y. A.; Wise, D. L. Combust. Sci. Technol. 1994, 102 (1-6), 193–211. (4) Adanez, J.; de Diego, L. F.; Garcıa-Labiano, F. Fuel 1999, 78, 583– 592. (5) Nimmo, W.; Patsias, A. A.; Williams, P. T. Energy Fuels 2006, 20, 1879–1885. (6) Wu, S.; Sumie, N.; Su, C.; Sasaoka, E.; Uddin, M. A. Ind. Eng. Chem. Res. 2002, 41, 1352–1356. (7) Sasaoka, E.; Sada, N.; Uddin, M. A. Ind. Eng. Chem. Res. 1998, 37, 3943–3949.
(usually called bio-oil and considered as the substitute for fossil liquid fuels). The bio-oil can be obtained easily by rapid pyrolysis of any kind of biomass in the absence of oxygen,8–10 whereby subsequent condensation of the product vapors yields 70 wt % bio-oil, which contain about 10 wt % acidic content (the major acids found in it are acetate acid and formic acid).11–13 Herewith, bio-oil is acidic and can be used as a low-cost acidic resources. DynaMotive Technologies Corporation have produced Biolime by reacting acidic fractions of bio-oil with a Ca(OH)2 suspension.14,15 The calcinations of calcium-enriched bio-oil yields finely dispersed, microscale-sized CaO that is highly reactive toward SOx.16,17 It must be taken into account that biooil is a perfect substitute of conventional acetic acid used in the synthesis of adsorbent. In our present work, bio-oil derived from rice husk was used as a swelling reagent. The calcium-enriched bio-oil was produced by reacting calcium hydroxide with bio-oil. Decomposition and calcination characteristics of calcium-enriched biooil were investigated. The reaction atmosphere had a great influence on the calcination conversion and the pore structure (8) Ingram, L.; Mohan, D.; Bricka, M.; Steele, P.; Strobel, D.; Crocker, D.; Mitchell, B.; Mohammad, J.; Cantrell, K.; Pittman, C. U. Energy Fuels 2008, 22 (1), 614–625. (9) Zheng, J.; Zhu, X.; Guo, Q.; Zhu, Q. Waste Manage. 2006, 26, 1430– 1435. (10) Mohan, D.; Pittman, C. U., Jr.; Steele, P. H. Energy Fuels 2006, 20 (3), 848–889. (11) Zhang, J.; Toghiani, H.; Mohan, D.; Pittman, C. U., Jr.; Toghiani, R. K. Energy Fuels 2007, 21 (4), 2373–2385. (12) Branca, C.; Giudicianni, P.; Di Blasi, C. Ind. Eng. Chem. Res. 2003, 42, 3190–3202. (13) Diebold, J. P. A Review of the Chemical and Physical Mechanisms of the Storage Stability of Fast Pyrolysis Bio-oils. Report NREL/SR-57027613; National Renewable Energy Laboratory, Golden, CO, 2000. (14) Oeher, K. H.; Scott, D. S.; Czernik, S. U.S. Patent 5,264,263, 1993. (15) Oeher, K. H. U.S. Patent 5,458,803, 1995. (16) Sotirchos, S. V.; Smith, A. R. Ind. Eng. Chem. Res. 2003, 42, 2245– 2255. (17) Sotirchos, S. V.; Smith, A. R. Ind. Eng. Chem. Res. 2004, 43, 1340– 1348.
10.1021/ef700728e CCC: $40.75 2008 American Chemical Society Published on Web 05/17/2008
Calcium-Enriched Bio-oil
Energy & Fuels, Vol. 22, No. 4, 2008 2599
Table 1. Physical Characteristic of Bio-oil Used in the Present Study physical property
value
physical property
water content (wt %) LHV (MJ/kg) elemental analysis (wt %) C H O N
28 16.5
pH ICP-AES analysis (ppm) K Ca Na Mg Fe
39.92 8.15 51.07 0.61
value 3.2 94.4 56.2 6.8 21.5 597
developed by the adsorbent during CaCO3 calcination. According to Adanez et al.,4 the highest surface area values were obtained under nitrogen and later followed the sequence from reducing, oxidizing, and noncalcining atmospheres. However, this work was only concerned with the effect of the oxidizing (air) atmosphere on the pore structure developed during calcination. To compare the behavior of CEB-derived CaO with that of organic calcium salts at the same conditions, calcination experiments were carried out using CA. Degradation and calcination experiments of CEB were carried out in a DTG60H thermogravimetric analyzer and a temperature-programmed muffle furnace under air flow, respectively. 2. Experimental Section 2.1. Materials. Analytically pure calcium hydroxide used in this study is produced by China National Medicines Corporation Ltd. Bio-oil derived from rice husk was produced by a fast pyrolysis operation in an autothermal fluidized-bed reactor with a capacity of 120 kg/h in our laboratory. The characteristics of the pyrolysis reactor have been described elsewhere;9 therefore, only the essential information was provided here. Tables 1 and 2 opened up the physical property and composition of bio-oil used in the present study. The inorganic materials in bio-oil contained some alkaline metals and iron at low concentration. The organic acids mainly comprising acetic acid accounted for 6.97 wt % of whole bio-oil. Esters and aldehydes, which were potential sources of acids through hydrolyzation and oxidization, made up about 1.15 wt % of the whole bio-oil. 2.2. Synthesis of CEB. Calcium hydroxide powders were added to the bio-oil ultrasonically at 60 °C for 20 min to adjust the pH to an alkaline level sufficient to hydrolyze the esters, causing at least partial oxidation of the formaldehyde. The pH is preferably above 7 but preferably sufficiently low as to avoid residual unreacted calcium hydroxide in the calcium salt. Therefore, the pH of 8, 10, and 12 was practiced in the study, whereas the pH of bio-oil was about 3.2. The synthesized samples were named CEBX, where X is the pH value of synthesis medium. Then, CEB were evaporated under vacuum to partly remove the majority of the water and volatile materials, and then, a semisolid CEB was obtained. The CEB were dried at 110 °C and powdered to particles with a size of 120 mesh, through sieving with a mechanical sieve. 2.3. Quality Assurance/Quality Control. Because of the low solubility of Ca(OH)2 in water, the processes of synthesis have a great effect upon the characteristics of CEB powders (CEBs). Ultrasonic processing can accelerate the dissolution of Ca(OH)2 in bio-oil to reach the target pH value rapidly. Ca(OH)2 were ovendried for 2 h at 80 °C before use. CA and all CEBs were dried at 110 °C until nearly no mass loss before thermogravimetric analysis (TGA) experiments. The small amount of sample and the slow heating rate ensures that the heat-transfer limitations can be ignored.18 An initial sample amount of all specimens was about 8 mg for TGA. 2.4. Characterization. 2.4.1. Heating Value Study. Bio-oil is a renewable and clear substitute for fossil liquid fuels, with a (18) Junqing, C.; Yiping, W.; Limin, Z.; Qunwu, H. Fuel Process. Technol. 2008, 89, 21–27.
medium-heating value of 16.5 MJ/kg. Moreover, the CEBs can also provide heat in coal combustions at the power plant site; they can be considered as the bifunctional fuels. Therefore, we determined the heating values of CEBs. A total of 1 g of CEBs was pressed into a pallet, and then, the heating value was determined on an oxygen bomb calorimeter (XRY-B, Shanghai, China). 2.4.2. X-ray Diffraction Analysis. The CEBs obtained respectively in pH 8, 10, and 12 were analyzed by X-ray diffraction (XRD) analysis on a Rigaku (Japan) D/max-γA X-ray diffractometer to investigate the chemical species and mode of occurrence of calcium within the CEBs. The XRD analysis conditions were the same as that used for calcium acetate compared to CEBs. 2.4.3. Decomposition and Calcination of the CEBs. The decomposition experiments were carried out using a TGA system under air flow on a DTG-60H detector. The samples were evenly and loosely distributed in an open sample pan of 6.4 mm diameter and 3.2 mm deep, with an initial sample amount of about 8 mg. The temperature change was controlled from room temperature to 1000 °C, at a heating rate of 10 °C/min. An air stream was continuously passed into the furnace at a flow rate of 50 mL/min at atmospheric pressure during devolatilization and to carry away the decomposition products from the reaction zones. The analytical reagent of calcium acetate was used to be a contrast for CEBs in all operations. According to the limitations that TGA can handle, the relatively large quantities of material needed for gas sorption analyses cannot be prepared using the procedure followed in the thermogravimetric experiments. Instead, calcination experiments for analysis of physicochemical characteristics were carried out using a temperature-programmed muffle furnace. The CEBs were heated from 50 °C to the reactive temperatures with a ramp rate of 10 °C/min and kept at the temperature for 30 min for the purpose of full calcination to calcium oxide. According to the conclusions of some literature,19–21 almost all of the emitted sulfur was captured fast at about 1000 °C by CaO adsorbents, in the order of milliseconds. Below 900 °C, the sulfation reaction is too slow; above 1200 °C, the sulfation reaction is unfavorable because the product CaSO4 becomes unstable. Therefore, in the present study, three reactive temperatures, 900, 1000, and 1100 °C, were investigated for calcination of specimen. The CaO particles obtained from decomposition and calcination of CEBs were analyzed by different methods to determine their physicochemical characteristics. The specific surface area of the particles was measured by N2 physisorption at 77 K in a Micromeritics ASAP 2020 M+C analyzer and using the Brunauer-EmmettTeller (BET) multipoint method over a P/P0 range of 0-1.0. The external area was determined by t-plot method. The pore size distributions were also obtained using the Barrett, Joyner, and Halenda (BJH) method. Prior to the experiments, the samples were degassed at 300 °C for 10 h under vacuum.
3. Results and Discussion 3.1. Physical Characteristics of CEBs. As shown in Table 3, the heating value of the CEBs increased with the increase of the bio-oil content. CEB8 and CEB10 had higher heating values than the value of pure cellulose or hemicellulose (17 MJ/kg)22 but lower than the value of pure lignin (25 MJ/kg).22 Therefore, the CEBs cofiring with coal can provide additional heat in coal combustors before they become adsorbents for SOx or H2S. CEB8 and CEB10 were homogeneous liquid before drying at 110 °C and could be sprayed into combustors, but CEB12 exhibited the rapid phase separation. The chemical species and mode of occurrence of calcium within CEBs were analyzed by (19) Zhang, L.; Sato, A.; Ninomiya, Y.; Sasaoka, E. Fuel 2004, 83, 1039–1053. (20) Sohn, H. Y.; Han, D. H. AIChE J. 2002, 48 (12), 2978–2984. (21) Wang, W.; Bjerle, I. Chem. Eng. Sci. 1998, 53 (11), 1973–1989. (22) Stanford University, Global Climate and Energy Project. An assessment of biomass feedstock and conversion research opportunities. GCEP energy assessment analysis. Spring, 2005 (http://gcep.stanford.edu).
2600 Energy & Fuels, Vol. 22, No. 4, 2008
Xulai et al.
Table 2. Main Composition of Bio-oil Used in the Present Study acids
esters
formic acid methyl formate acetic acid methyl acetate hydroxy-acetic acid methyl propionate propanoic acid valerolactone valeric acid
ketones
sugars
aldehydes
furans
phenols
3-methyl-cyclopent-2-enone 1-hydroxy-butan-2-one hydroxyl-propanone 1,3-dihydroxy-propanone 3-hydroxy-cyclohexanone
levoglucosan xylitol fructose D-xylose lactose
glycolal formaldehyde acetaldehyde ethanedial pentanal
2-methyl-furan 2-furanone furfural 5H-furan-2-one 2,5-dimethoxy-tetrahydrofuran
phenol o-cresol m-cresol catechol syringol isoeugenol 4-methoxy-catechol 3-ethoxy-phenol 4-methyl-guaiacol 4-methoxy-phenol 3-methyl-catechol
Table 3. Heating Values with Different Calcium Contents of Specimens materials
CEB8
CEB10
CEB12
CA
LHV (MJ/kg) bio-oil content (wt %)
20.36 85.26
18.65 80.78
7.23 53.64
6.87
XRD (Figure 1), which included that of pure calcium acetate for comparison. For the CEB12, all of its peaks matched those of the raw calcium hydroxide. The existence of calcium hydroxide in CEB12 should be due to the impurity caused during the reaction, which is the raw material having a high crystalline degree compared to the organically bonded calcium. On the other hand, the broad XRD patterns of CEB8 and CEB10, which were typical for amorphous solids, confirmed the absence of any ordered crystalline structure and did not match the peaks of pure calcium acetate or raw calcium hydroxide at all. At least we can consider that calcium hydroxide in CEB8 and CEB10 were reacted with some components in bio-oil to form amorphous organic calcium salts. Carboxylic salts of calcium and other organic compounds of calcium were present in significant quantities in the calcium-enriched bio-oil. 3.2. Thermal Decomposition Characteristics of CEBs. Figure 2 showed the patterns of the decomposition of all specimens; the decomposition of CEBs consisted of four steps,
Figure 1. XRD patterns of specimens.
similar to that of pure CA partly. Table 4 listed the fractions of volatile products released along the temperature zones. The initial decomposition of CEBs started at the temperature around 90 °C; this was due to the loss of water (free and hydration). The second mass loss started at about 200 °C; devolatilization and carbonization of unreacted organic materials occurred in this step. Almost all of the organic materials in CA were in the form of organic calcium salts. Consequently, there was almost no mass loss in the relative temperature zone. The third step and the quick decomposition of organic calcium salts or heavy organic compounds in CEBs took place initially at about 430 °C, and the greatest loss during the decomposition of organic calcium salts to CaCO3 took place at this stage, indicating some progress of pyrolysis and/or gasification of the carbon and organic compounds remaining in the adsorbents. At this stage, CA had a lower pyrolysis temperature than all of the CEBs; the decomposition took place starting at 363 °C. The fourth and final decomposition step took place starting at about 550 °C, and the weight loss was due to the decomposition of CaCO3 to CaO. At this stage, CA-derived CaCO3 was degraded starting at 607 °C. The two temperatures were far lower than that of limestone at about 700 °C.23 Moreover, the termination decomposition temperatures of CaCO3 were 703 °C for CEB8, 726 °C for CEB10, 744 °C for CA, and 756 °C for CEB12. It could be hypothesized that more rapid decomposition would allow the adsorbent a longer period time in the CaO form to react with SO2. In addition, because of the much faster calcination of the CEB-derived CaCO3, there should be minimal interference between its calcination and sulfidation in the simultaneous process, while this is not the case for calcium carbonate solids.17 3.3. Differential Thermal Analysis (DTA) Investigation. As shown in Figure 3, decomposition reactions at the main step of the samples were exothermic reactions. The residues were
Figure 2. TG results of all specimens.
Calcium-Enriched Bio-oil
Energy & Fuels, Vol. 22, No. 4, 2008 2601
Table 4. Fractions (wt %) of Volatile Products Released along the Experimental Temperature Zones (TZs) (°C) CEB8
CEB10
CEB12
CA
TZs
fractions
TZs
fractions
TZs
fractions
TZs
fractions
25-183 225-382 433-556 575-703
6.898 29.183 46.997 8.841
25-169 238-375 435-510 556-726
2.306 30.189 44.061 11.795
25-161 217-382 448-501 533-756
4.312 16.965 22.342 23.927
25-207
11.436
363-483 607-744
32.298 25.666
white under the experimental conditions, indicating complete combustion of the carbon and organic compounds. The shallow exothermic peak started at the onset of 220 °C in CEB decomposition processes, and a sharp exothermic peak at about 490 °C indicated the decomposition of organic calcium salts and combustion of organic compounds with oxygen. The heat outputs of decomposition of CEBs were higher than CA, especially for CEB8 and CEB10. This was interpreted as further evidence that CEBs could be used as the low-cost medium
Table 5. Surface Area (m2 g-1) of Specimens under Different Experimental Temperatures 900 °C 1000 °C 1100 °C
Figure 4. Nitrogen adsorption-desorption isotherms for CEBs calcined at 1000 °C.
CEB8
CEB10
CEB12
23.8 18.4 10.1
23.2 17.6 10.8
19.7 15.4 9.6
Table 6. Porosity (%) of Specimens under Different Experimental Temperatures 900 °C 1000 °C 1100 °C
Figure 3. DTA curves of the specimens.
CA 31.5 30.0 24.9
CA
CEB8
CEB10
CEB12
79.3 76.4 74.1
88.6 84.2 80.6
88.2 86.3 82.1
82.3 83.4 77.6
heating value fuels during the decomposition process. The property might make CEBs superior to other calcium-containing materials, such as limestone as adsorbents, although the most likely phases formed during the decomposition of almost all of calcium-containing materials were CaCO3 and CaO. 3.4. Microstructure Characteristics of Calcined CEBs. The nitrogen adsorption-desorption isotherms of CEBs after calcination at 1000 °C were presented in Figure 4. The isotherms of particles from CEBs could be classified as being of type H3 according to International Union of Pure and Applied Chemistry (IUPAC) classification,24 which were characteristic of mesoporous material. Hysteresis is usually attributed to a different size of pore mouth and pore body.25 The observed hysteresis loops presented in the range of 0.55-0.9 of P/P0 for particles from CEB8 and CEB10 but approached P/P0 ) 1 for particles from CEB12. Tables 5 and 6 listed the surface areas and total porosities of specimens calcined at different temperatures, respectively. Interestingly, the increase in temperature from 900 to 1100 °C caused only a slight decrease in porosity and surface area of particles from CA. However, there were significant decreases in the surface areas of particles from CEBs, especially for particles from CEB8. However, the total porosities of CEBderived particles were larger than that of CA all of the time and were decreased slightly from 900 to 1100 °C. These results suggest that some micrometer-size pores are transformed to mesoporous pores with an increasing sintering temperature, thereby decreasing the total porosity. As shown in Figure 5, it is interesting that the distribution of the mesoporous pores was increased to larger pore diameters with an increasing sintering temperature, as the peak of the distribution shifted from 3.7 nm at 900 °C to 12.5 nm at 1100 °C. On the other hand, with an increasing calcination temperature, the hysteresis loops shifted to the region of higher relative pressure and the areas of the hysteresis loops gradually became small (see Figure 6). It was also interesting to note, from the contents of Tables 5 and 6, that whereas the porosities of all three CEBs-derived particles were a little higher than those of CA, their BET surface areas were a lot lower. This suggests that CA-derived particles contain a larger number of finer pores than that from CEBs, which contribute the most of the reactive surface area to heterogeneous sulfation reactions, if of course pore plugging is not widespread.
2602 Energy & Fuels, Vol. 22, No. 4, 2008
Figure 5. Pore size distributions of CEB10 calcined from 900 to 1100 °C.
Figure 6. Nitrogen adsorption-desorption isotherms of CEB10 calcined from 900 to 1100 °C.
The difference of calcination characteristics between CA and CEBs or CEB-selves was believed to be associated with elemental composition of adsorbent precursors. According to Adanez et al.,26 alkali metal salts not only catalyze the surface chemical reaction and enhance crystal lattice defect concentration but also accelerate the sintering of adsorbents. Therefore, the alkali metals in the bio-oil (Table 1) were transferred into CEBs during the synthesis process. The metals from bio-oil accelerated the sintering of the particles from CEBs. At temperatures above 900 °C, the sintering process was quite important and clearly dominated over surface area generation, producing a net decrease in specific surface area. The highest sintering rate from 900 to 1100 °C was obtained from calcination of CEB8; the surface area was decreased significantly from 23.8 m2 g-1 at 900 °C to 10.1 m2 g-1 at 1100 °C and later followed the sequence of CEB10 > CEB12 > CA. The sequence was the same with the sequence of the bio-oil contents and also with the decomposition temperatures of CEB-derived CaCO3. However, alkali metal salts with an optimum content in the (23) Ninomiya, Y.; Zhang, L.; Nagashima, T.; Koketsu, J.; Sato, A. Fuel 2004, 83, 2123–2131. (24) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, U.K., 1982. (25) Leofanti, G.; Padovan, M.; Tozzola, G.; Venturelli, B. Catal. Today 1998, 41 (1-3), 207–219. (26) Adanez, J.; Fierro, V.; Garcia-Labiano, F.; Palacios, J. M. Fuel 1997, 76 (3), 257–265.
Xulai et al.
adsorbent within a certain range of sulfation temperatures can help the adsorbent to form a better microstructure and obtain higher reactive activity;27 iron can also play a role as a sulfur transfer in the desulfurization at high temperatures, and calcium-iron complex desulfurizers increase sulfur-capture efficiency.28 However, the optimum content of alkali metal salts in CEBs for desulfurization was not determined here; in other words, the best weight ratio of bio-oil/Ca(OH)2 during the CEB synthesis process should be investigated in the future work. Although the particles from CEB12 had a lower sintering rate than others, they had a smaller surface area and lower total porosity. This is believed to be associated with the weight percent of organic compounds in the adsorbent molecule; as the weight percent increases, pore surface area and total pore volume of the cenospheres also increase.2,29 Consequently, the particles from CEB10 were the preferable adsorbents of moderate sintering rate and surface area and the porosity of which was higher than 80% at the experimental condition. The high porosity of CEB-derived particles is due to the fact that the gases formed during decomposition of the organic compounds or organic salts and the vapors formed from the violent boiling of the molten materials or the droplets lead to the formation of bubbles inside the decomposing structures, and then, the bubbles can coalesce under some conditions into a single bubble; while this happenes, the resulting particles have the form of cenospheres surrounded by a thin shell of CaO perforated with a number of blow holes through which the decomposition gases escaped.16 In addition, the average pore size of particles from CEB10 increased from 8.2 to 20.2 nm with the sintering temperature from 900 to 1100 °C; the pore size distribution should be within the optimum pore size range (10-20 nm), which provided a medium surface area for the sulfation reaction without causing rapid pore filling and pore blockage, as Gullet and Bruce stated.30 4. Conclusions Bio-oil is a renewable resource that is usually used as a potential substitute for fossil liquid fuels. The property of a low pH value of bio-oil is a disadvantage for use as a fuel but is the merit for use as an acidic swelling reagent of desulfurizers. Calcium-enriched bio-oil was produced by reacting bio-oil with calcium hydroxide in different pH values, which can be used as SO2 absorbents after calcination and also as medium heating value fuels. Carboxylic salts of calcium and other organic compounds of calcium were present in significant quantities and existed in the form of amorphous structures. Decomposition experiments were carried out in a TGA system, operating at atmospheric pressure. The decomposition of CEB was an exothermic process consisting of four steps, partly similar to that of pure CA. The initial decomposition was due to the loss of water (free and hydration). Devolatilization and carbonization of unreacted organic materials occurred in the second mass loss step. The third step was the decomposition of organic calcium salts to CaCO3, including the progress of pyrolysis and/or gasification of the carbon and organic compounds remaining in the adsorbents. The exothermic reactions mainly took place at this stage. The final decomposition step (27) Kuihua, H.; Chunmei, L.; Shiqing, C.; Gaiju, Z.; Yongzheng, W.; Jianli, Z. Fuel 2005, 84, 1933–1939. (28) Zhenxue, L.; Mei, L.; Shuyi, T. Coal ConVers. 1997, 20 (3), 56– 62. (in Chinese), Fuel Energy Abstr. 1999, 40 (2), 101-102. (29) Shemwell, B.; Levendis, Y. A.; Simons, G. A. Chemosphere 2001, 42, 785–796. (30) Gullett, B. K.; Bruce, K. R. AIChE J. 1987, 33 (10), 1719–1726.
Calcium-Enriched Bio-oil
was due to the decomposition of CaCO3 to CaO. The CaCO3 derived from CEBs had a higher calcination rate than that from CA. Calcination experiments for the analysis of CEB-derived CaO particles were carried out using a temperature-programmed muffle furnace. The calcination results were compared to results obtained from CA at the same conditions. The porous CaO obtained from calcined CEBs exhibited very high porosities, higher than that from CA. The sintering rate and surface area of CaO particles at experimental temperatures were associated closely with the content of alkali metals migrated from bio-oil. The highest sintering rate was obtained from calcination of CEB8 and later followed the sequence of CEB10, CEB12, and CA. The increase in temperature from 900 to 1100 °C caused
Energy & Fuels, Vol. 22, No. 4, 2008 2603
a significant decrease in surface areas and an increase in pore sizes of CaO particles from CEBs. The decomposition of amorphous CEB10 yielded calcium oxide particles of very high porosity and moderate surface area and sintering rate at the experimental conditions, which might solve the problems that arise from pore-plugging phenomena during their use as adsorbents for SO2 or H2S removal. Acknowledgment. The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (50576091), Chinese Academy of Sciences Innovation Program (KGCX2-YW-306-4), and National Basic Research Program of China (2007CB210203). EF700728E
