Experimental Investigation of the Decomposition and Calcination of

Ind. Eng. Chem. Res. 2003, 42, 2245-2255

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Experimental Investigation of the Decomposition and Calcination of Calcium-Enriched Bio-Oil Stratis V. Sotirchos*,†,‡ and Adam R. Smith‡ Institute of Chemical Engineering and High-Temperature Chemical Processes, Foundation of Research and Technology Hellas (ICEHT-FORTH), P.O. Box 1414, 265 00 Patras, Greece, and Department of Chemical Engineering, University of Rochester, Rochester, New York 14627

The subject of this study is the experimental investigation of the decomposition of calciumenriched bio-oil (CEB), the product of the reaction of bio-oil with calcium hydroxide, at high temperatures and of the calcination of the CaCO3 material that is obtained when the decomposition is carried out in the presence of CO2. Decomposition and calcination experiments are conducted in a thermogravimetric analysis system, and the pore structures of dried, decomposed, and calcined samples are characterized using nitrogen adsorption-desorption, mercury intrusion-extrusion, and photomicrographic examination. The calcination results for CEB-derived CaCO3 are compared with those for two naturally occurring calcitic solids of high CaCO3 content. The pore structure characterization results show that the decomposition and calcination products of CEB have very high porosities, higher than 80-90%. Because of its high porosity, the calcination product exhibits a much higher calcination rate than particles of limestones and calcites with a characteristic size similar to that of the CEB layer. This indicates that calcium-enriched bio-oil might perform as a more efficient sorbent than limestones for SO2 capture in fossil fuel combustors. 1. Introduction Calcium-containing materials, mostly based on calcium carbonate and calcium hydroxide, are the most commonly used sorbents for in situ removal of SO2 in coal combustors. In the high-temperature environment of these reactors, these materials undergo decomposition to form particles of porous CaO, which reacts with SO2 to form CaSO4. If the partial pressure of CO2 in the reactor is higher than the value that corresponds to chemical equilibrium of the CaCO3 decomposition reactionsa situation occurring in high-pressure reactorss decomposition of CaCO3 cannot take place, and thus, the removal of SO2 occurs through reaction with nonporous or low-porosity CaCO3 particles. Because Ca and S occur in the solid product at a 1:1 atomic ratio, complete desulfuration of the gases produced in the reactor (combustor or gasifier) is theoretically possible at a 1:1 feed ratio of Ca and S (the sulfur contained in the coal) in the reactor. However, the reactions of CaO or CaCO3 particles with SO2 are very complex processes, and the interplay of the various subprocesses that are encountered in them lead to ultimate conversions that are much lower than 100%. In the case of the reaction of CaO particles with SO2 in the presence of oxygen, the low utilization of calcium is caused by the plugging of the pores with CaSO4, the solid product of the desulfurization reaction. Under significant diffusional limitations in the pore space, the conversion of CaO to CaSO4 proceeds faster in the vicinity of the external surface. The pores are plugged earlier there, and as a result, the ultimate average * To whom correspondence should be addressed (at the ICEHT-FORTH address). Tel.: +30-610-965-202. Fax: +30610-965-223. E-mail: [email protected]. † ICEHT-FORTH. ‡ University of Rochester.

conversion of CaO to CaSO4 in the particles can be much lower than the maximum allowable conversion for uniform plugging of all pores with solid product. It can be shown that, for constant particle size, the porosity at conversion ξ is given for initial porosity 0 by

) 0 - ξ(Z - 1)(1 - 0)

(1)

where Z is the ratio of stoichiometrically equivalent volumes of solid product and solid reactant. From eq 1, it is found that the initial porosity must be greater than (Z - 1)/Z to reach complete conversion (ξ ) 1) before complete pore plugging takes place. This translates into initial porosities greater than about 65% for the CaO to CaSO4 reaction, whereas the initial porosities that are typically exhibited by CaO particles obtained from the decomposition of nonporous CaCO3 or Ca(OH)2 are about 50-55%. The sulfation of partially sulfided CaO (resulting from the use of CaO for high-temperature capture of H2S in gasifiers) is also plagued by poreplugging phenomena.1 High initial porosities in CaO particles that result from the decomposition of a calcium-containing solid can be obtained by employing solids that are characterized by much higher molar volumes (per atom of Ca contained in them) than CaCO3 and do not undergo shrinkage during decomposition or solids whose decomposition to CaO is accompanied by an increase of their physical dimensions. The high-temperature decomposition of organic calcium salts, such as calcium acetate and calcium magnesium acetate, in particle form or as sprays of aqueous solutions, yields calcium oxide particles that are characterized by very high porosities, well above the value that is needed for 100% conversion before all pores are filled with solid product.2,3 The main reason for this is that the gases that are formed during decomposition of the organic salts and the vapors formed from the violent (explosive) boiling of the molten mate-

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rial or of the spray droplets lead to the formation of bubbles inside the decomposing structures. The bubbles can coalesce under some conditions into a single bubble, and when this happens, 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. Experimental studies have shown2,4,5 that the CaO particles that are obtained from the decomposition of calcium acetate, calcium magnesium acetate, and other organic salts of calcium can reach much higher sulfation conversions than the CaO particles that are obtained from the decomposition of CaCO3 or other inorganic calcium solids [e.g., Ca(OH)2]. The availability of inexpensive sources of organic acids, such as acetic acid, is a prerequisite for the application of organic calcium salts as agents of sulfur removal in coal combustion and in coal gasification. The cost of acetic acid and of other organic acids obtained from petrochemical sources is not low, but acetic acid suitable for the use in acetate production can be obtained from the fermentation of low-quality or negative-value waste products, such as sewage sludge.6 A much less expensive source of organic acids for the production of organic salts of calcium is provided by biooil, the product of the flash (fast) pyrolysis of biomass.7,8 In general, bio-oil consists of water (10-20%), a mixture of carboxylic acids, several aldehydes and alcohols, and pyrolytic lignin. Formic and acetic acids are the main acids contained in bio-oil, with their total weight content being about 10%.9,10 Calcium-enriched bio-oil (CEB) is obtained by reacting lime [CaO or Ca(OH)2] with bio-oil. Studies in pilot-scale units have shown11,12 that, as in the case of powders of pure organic calcium salts or sprays of aqueous solutions of carboxylic salts, the CaO particles that result from the decomposition of CEB exhibit very high porosities, a consequence of the formation of bubbles within the decomposing structure because of the release of volatile species from the decomposition or evaporation of the components of calcium-enriched bio-oil. The high porosity can lead, during desulfurization, to very high utilization of the calcium contained in the sorbent, well above the values observed for sorbents derived from CaCO3 or Ca(OH)2. As a first step in the direction of investigating the fundamental aspects of the performance of CaO particles obtained from the decomposition of calciumenriched bio-oil as agents for SO2 removal in coal combustors, the processes of the decomposition of calcium-enriched bio-oil and of the calcination of the CaCO3 material that is obtained in the presence of CO2 are examined in the present study. Experiments are carried out in a thermogravimetric analysis system using small samples of CEB, and the pore structure properties of decomposed and calcined samples are measured using nitrogen adsorption and mercury penetration porosimetry. The calcination results are compared with results obtained at the same conditions using two naturally occurring calcitic solids of very high calcium carbonate contents but different petrographic textures. 2. Experimental Apparatus and Procedures The decomposition and calcination experiments were carried out using a thermogravimetric analysis (TGA) system. This system is based on a Cahn D101 electronic microbalance that can recognize weight changes as

small as 1 µg and can handle as much as 100 g. The reacting gases enter from a side port at the top of the hangdown tube and flow down toward the pan. To protect the sensitive electronic weighing unit from contamination, a stream of inert gas (N2) is sent continuously through the chamber that houses it. A four-port valve is used to switch the stream that enters the hangdown tube through the side port from an inert mixture to the reactive mixture or to switch between reactive mixtures. A system of switching valves, rotameters, and T-junctions is employed to prepare the reactive mixtures used in the experiments. The samples used in the experiments were placed on a quartz or gold pan that had an area of about 10 mm2. The pan was suspended using platinum wire from the sample arm of the microbalance in the thermogravimetric arrangement. A very small amount of sample, enough to give 0.3-1 mg of CaO, was employed in the experiments. This was done because the calcination reaction is extremely fast, and large samples can lead to bulk concentrations of CO2 in the stream flowing around the sample that are much different from those in the feed mixture. The amount of sample was varied in some experiments to examine how sample size affected the conversion vs time results. The CEB samples were placed as a thin coating of the homogenized material on the bottom of the pan. The pan was suspended from the sample arm of the microbalance in the thermogravimetric arrangement and left exposed in the air until its weight did not change significantly. A furnace placed around the tube in which the pan was hanging was then used to raise the temperature of the sample from the ambient value to the value where the sulfation reaction was to be studied. Heating was done as a mixture of 70% CO2 in N2 was sent through the reactor. At the two temperatures examined in this study, 750 and 850 °C, the equilibrium partial pressure of CO2 is about 0.1 and 0.5 atm, respectively, and as a result, the material on the pan remained in the form of CaCO3. Before heat treatment was started, the dried sample was first dried further by raising its temperature to about 100-110 °C. The sample was then heated to the reaction temperature, and after the desired temperature had been reached, the CO2 stream was substituted with a stream of N2 to allow the calcination to occur. When calcium-enriched bio-oil droplets are injected into a combustor, the decomposition of the components of CEB and the calcination of CaCO3 and Ca(OH)2 occur simultaneously. For this reason, in some experiments, decomposition and calcination occurred simultaneously by heating the material in the presence of N2 (with small amounts of O2) or air only. In all cases, the total flow rate in the reactor was 200 mL/min at standard conditions. 3. Materials Samples obtained from two batches of calciumenriched bio-oil (CEB) were used in our experiments. Both batches were produced by reacting wood-derived bio-oil with a Ca(OH)2 suspension at about 60 °C, and they were provided to us by BTG (Biomass Technology Group) B.V. (Enschede, The Netherlands). They were prepared using bio-oil produced through flash pyrolysis of wood using the rotating cone reactor technology that BTG B.V. has developed.8,13 The calcium contained in the CEB samples was about 13-14 wt %. Water was

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present at about 45 wt %, including water contained in bio-oil and water contained in the Ca(OH)2 suspension added to it for the preparation of CEB. The addition of Ca(OH)2 to the bio-oil and the ensuing formation of organic calcium led to a pH of about 11.5, whereas the pH of bio-oil was about 3. It should be noted that 13-14% of Ca by weight requires about 40 wt % of acetic acid for its complete conversion to calcium acetate. Therefore, all of the organic content of bio-oil should be in the form of acidic units with an equivalent molecular weight of acetic acid in order for all calcium introduced into the bio-oil through the addition of Ca(OH)2 to be converted to organic calcium salts. The acidic content of bio-oil is considerably lowersits content in acetic and formic acid (the major acids found in it) is about 10 wt %9,10sand therefore, a major potion of calcium should exist in the form of calcium hydroxide, mostly in particle form because of the low solubility of Ca(OH)2 in water. The microscopic examination of CEB droplets spread on glass plates clearly showed the presence of solid particles. The acid titration of CEB gave a titration curve similar to that of a Ca(OH)2 suspension of the same loading in calcium. This was interpreted as further evidence of the presence of a large part of Ca in CEB in the form of Ca(OH)2. To compare the behavior of CEB-derived sorbents with that of calcium carbonate materials at the same conditions, calcination experiments were carried out using two calcium carbonate solids of very high CaCO3 content: a microcrystalline limestone distributed by Greer Limestone Co. (Greer Limestone) and a calcite (Iceland Spar) distributed by Wards Inc. in the form of large single crystals. The calcium carbonate content in Iceland Spar is 99.2% and in Greer Limestone 97.9%. The other major component is magnesium carbonate, but the product of its decomposition is not reactive at the typical conditions of sulfation or sulfidation in desulfurization applications. A detailed description of the chemical analysis of the calcium carbonate solids is given by Zarkanitis and Sotirchos.14 The limestone samples were crushed, and sieving was used to isolate samples in the 297-350 and 53-62 µm size ranges. 4. Heating and Decomposition of CEB Figures 1 and 2 show the variation of the temperature of the reactor and the variation of the weight of the sample with time during heating of CEB samples applied as thin coatings on the bottom of the pan at 750 and 850 °C, respectively, with 70% CO2 present in the gas phase. Before the experiments were started, the CEB samples were heat-treated at 110 °C until they showed insignificant weight change. Results on the variation of the weight of the sample are shown for two independent experiments (runs 1 and 2) at 850 °C (Figure 2). It is seen in this figure that the weight of the sample decreases continuously up to about 450 °C, increases slightly up to about 650 °C, and then starts to decrease again. The total weight loss is about 24% of the initial (dry) weight of the sample after drying (baking) at 100-110 °C. The rise in the weight of the sample that is seen to occur in Figure 2 between 450 and 650 °C was absent or not as marked in other decomposition curves (see Figure 1). A possible explanation for its occurrence is that it might result from the interaction of the products of the decomposition of the organic calcium salts and of

Figure 1. Variation of weight and temperature during decomposition of calcium-enriched bio-oil dried at 110 °C while heating to 750 °C in 70% CO2.

Figure 2. Variation of weight and temperature during decomposition of calcium-enriched bio-oil dried at 110 °C while heating to 850 °C in 70% CO2.

Ca(OH)2 with the CO2 present in the gas mixture. Decomposition curves for the anhydrous Ca(OH)2 that was used to produce the CEB samples studied here, with and without CO2 present in the gas phase, are given in Figure 3. About 5 mg of material was used to determine these decomposition curves, and the heating rate was the same as that used in Figure 1 for the decomposition of CEB at 750 °C. The total flow rate was 200 mL/min, and 70% CO2 was used in the decomposition experiment that was conducted in the presence of that gas. The balance of the gas mixture was N2 in both cases. The results of Figure 3 show that, in the absence of CO2, the decomposition of Ca(OH)2 starts at about 300 °C and is practically completed at about 450 °C. In the presence of CO2, the weight of the sample increases continuously within the same temperature range, an indication that the CaO formed from the decomposition of Ca(OH)2 is converted to CaCO3. The broken line curve gives the weight vs temperature curve that would be obtained from the decomposition curve in an inert atmosphere (without CO2 present) if it were assumed that the CaO formed from the decomposition of Ca(OH)2 is instantaneously converted to CaCO3. This curve is not much

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Figure 3. Weight change vs temperature during decomposition of Ca(OH)2 under N2 or N2 and 70% CO2. The temperature variation with time is the same as that in Figure 1. The dashed curve gives the weight vs time curve during decomposition under N2 with conversion of CaO to CaCO3.

Figure 4. Variations of weight and temperature during decomposition of calcium-enriched bio-oil dried at 110 °C while heating to 750 °C under N2 in the presence of small amounts of O2.

Table 1. Weight and Volume Percentages of the Remaining Material at Various Stages of Thermal Treatment of CEB material remaining state

wt %

vol %

as received air drying at 20 °C drying (baking) at 110 °C thermal treatment at 750 °C under CO2

100 62 44 33

100 64 49 -

different from that measured in the decomposition experiment in the presence of CO2, and therefore, one is led to conclude that the carbonation of the CaO formed from the decomposition of Ca(OH)2 is a fast reaction. With the exception of the differences that they exhibit in the 450-650 °C range, the decomposition curves of Figures 1 and 2 are in general qualitative agreement with each other. From a comparison of the results of the two figures, one concludes that the decomposition in CEB in the presence of CO2 at concentrations high enough to prevent calcination is completed between 700 and 750 °C. The differences between the weight loss curves of the two runs of Figure 2 are rather small considering the complexity of the combined drying, decomposition, and carbonation processes that the CEB material undergoes. Table 1 gives weight and volume data for the remaining material, as percentages of the initial weight and volume of CEB, at various stages of thermal treatment. These data were collected working with small amounts of homogenized CEB in test tubes. The sample was first allowed to dry at room temperature, and then it was brought slowly to 110 °C, where it was treated until its weight stopped varying. As the results of Table 1 show, the material undergoes considerable shrinkage as it loses material both at room temperature and at 110 °C that is comparable, on a percentage basis, to the extent of weight loss. Even though the density of the solid components of CEB [unreacted Ca(OH)2, organic calcium salts, and other organic compounds] is higher than that of the volatile compounds (mainly water), the comparable levels of weight loss and shrinkage indicate

Figure 5. Variations of weight and temperature during decomposition of calcium-enriched bio-oil dried at 110 °C while heating to 750 °C under N2 in the presence of small amounts of O2.

that the porosity developed in the solid material during these two steps is not very high. The observed weight loss from 110 to 750 °C is in agreement with that seen in the thermogravimetric analysis curves of Figure 2. The weight of the material that is obtained after treatment under CO2 at high temperature corresponds to about 13% Ca in the as-received CEB if this material is assumed to consist of only CaCO3. This is in agreement with the stoichiometry used in the preparation of the samples. Heating and decomposition of calcium-enriched biooil samples was also carried out in the absence of carbon dioxide. Figures 4 and 5 give the variation of the temperature of the reactor and the variation of the weight of the sample with time that were observed during those experiments. A comparison of the results of these figures with the corresponding results of Figures 1 and 2 indicates that the weight loss in the absence of CO2 from the gas phase is larger not only in the region of high temperature (above 500 °C), where decomposition of CaCO3 takes place, but also at lower temperatures. This is an indication that the decomposi-


tion of the organic carbon salts and of the other components of CEB is affected by CO2. The results presented in Figure 3 for the decomposition of Ca(OH)2 clearly show that this is definitely the case for this species. As in the case of heating under CO2, the results of Figures 4 and 5 show that the decomposition of CEB is completed at temperatures between 700 and 750 °C. The final weight of the material left on the pan in Figures 4 and 5 is about 42-43% of the weight of the sample that has been treated at 110 °C. The difference between this value and the final weight of the heattreated material in Figures 1 and 2 is consistent with the weight change that would be expected for complete calcination of CaCO3 (conversion to CaO). Even when organic calcium salts are decomposed in the absence of CO2, the first step in their decomposition leads to their conversion to CaCO3.3-5,15,16 For instance, in the case of calcium acetate, the first step is

Ca(CH3COO)2 f CaCO3 + CH3COCH3 Analogous reactions leading to ketone and calcium carbonate formation can be written for other calcium carboxylates. Studies on the decomposition of calcium acetate and calcium formate5,15,16 have shown that these salts are decomposed completely to calcium carbonate at temperatures below 400 °C and that CaCO3 starts to decompose with significant rate above 600 °C. In the CEB decomposition curves in CO2-free atmosphere of Figures 4 and 5, the weight loss above 600 °C is rather small. This is an additional indication that the content of the CEB in calcium organic salts is not very high. Oxygen was not present in the gas phase in the heating experiments that gave the results shown in Figures 1 and 2. Small amounts of oxygen (less than 1%) were added to the gaseous stream used in the decomposition experiments of Figures 4 and 5. Heating and decomposition experiments were also conducted in the absence of O2 at the conditions of Figures 4 and 5 and in the presence of that gas at the conditions of Figures 1 and 2 to examine whether combustion of CEB took place during the heating process. No effects of the addition of O2 were evident in the decomposition curves when decomposition took place in the presence of CO2. The variation of the weight with time when decomposition took place under a stream of pure N2 was similar to that displayed in Figures 4 and 5, but the final weight of the sample was higher. A visual examination of the samples that resulted from the various decomposition experiments revealed that those processed in the absence of oxygen and carbon dioxide had a dark (almost black) color, an indication of the presence of residual carbon in them. To remove the carbon, oxygen was introduced into the mixture flowing through the microbalance after the decomposition process was completed. A small weight change was observed, and the color of the samples became almost white. The weight of the samples decomposed under pure N2 also decreased when CO2 was sent through the reactor at partial pressures lower than the partial pressure for equilibration of the calcination reaction. At higher pressures of CO2, the weight of the sample initially decreased, apparently because of the gasification of the residual carbon, and subsequently, it started to increase as the conversion of CaO to CaCO3 progressed.

Figure 6. N2 adsorption-desorption curves at 77 K for porous samples obtained by drying calcium-enriched bio-oil (CEB) at 110 °C, heating the resulting structure to the decomposition temperature under CO2, and decomposing (calcining) it under 2% O2 in N2.

Figure 7. Mercury intrusion-extrusion curves at 77 K for porous samples obtained by drying calcium-enriched bio-oil (CEB) at 110 °C, heating the resulting structure to the decomposition temperature under CO2, and decomposing (calcining) it under 2% O2 in N2.

5. Characterization of Pore Structure The pore structure of the porous material that results from the decomposition of CEB was characterized using mercury porosimetry and nitrogen sorption. Figures 6 and 7 present the results obtained using these two methods. The internal surface area values that were obtained using BET analysis of the data in Figure 6 and of analogous data for other porous solids used in this study are given in Table 2. The relatively large quantities of material needed for mercury penetration and gas sorption analyses cannot be prepared using the proce-

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Table 2. Internal Surface Areas of Calcium-Enriched Bio-Oil (CEB) and Calcium Carbonate Solids after Various Treatments material and treatment CEB after forced drying at 110 °C CEB after drying at 110 °C and decomposition and calcination at 750 °C CEB after drying at 110 °C and decomposition and calcination at 850 °C Greer Limestone after calcination at 750 °C Greer Limestone after calcination at 850 °C Iceland Spar after calcination at 750 °C Iceland Spar after calcination at 850 °C

surface area (m2/g) 2.1 15 24 48 34 84 65

dure followed in the thermogravimetric experiments. Instead, the following procedure was used: A small quartz container was filled with a quantity of CEB (about 1 g) and placed in an oven kept at 110 °C. The CEB was allowed to completely dry, and the extent of drying was monitored by periodically weighing the container and its contents. It was decided to forcibly dry the material at a high temperature because preliminary experiments indicated that the drying of the material proceeded at a very low rate at room temperature. Once the drying of CEB was completed, the quartz tube was placed in the furnace used in the thermogravimetric experiments, and the material was heated to 750 or 850 °C under 70% CO2, as was done in the thermogravimetric analysis experiments. Calcination was carried out by sending either a mixture of N2 and O2 or pure N2 through the process tube in which the quartz container was placed. The results in Figure 6 for nitrogen adsorptiondesorption indicate that the volume of pores with size below 1000 Å (the practical upper limit for the application of the nitrogen sorption method) is very low for both temperatures of decomposition. The material produced at 750 °C has smaller surface area (see Table 2) and volume of small pores than the 850 °C material, but the latter has a smaller pore volume of pores close to the upper limit of the range covered by nitrogen sorption experiments. The main difference between the mercury intrusion-extrusion graphs of the two solids, shown in Figure 7, is that the material produced at 750 °C has a lower total porosity for pores below 3 µm in radius. A comparison of the two sets of curves indicates that this difference is mainly due to pores in the 300-2000 Å range. On the basis of the nitrogen adsorption and mercury penetration data, one would expect the 850 °C material to exhibit a higher overall reactivity because it is characterized not only by a higher total porosity but also by a larger fraction of small pores and a larger surface area. However, the preliminary experimental results on the sulfation of the CaO material obtained from CEB decomposition and calcination showed that the material produced at 750 °C can reach much higher conversions than that produced at 850 °C. (The performance of CaO obtained from CEB decomposition during sulfation and sulfidation will be addressed in detail in a future study.) The performance of a porous solid in a gas-solid reaction is determined not only by its apparent pore size distribution, as revealed by mercury penetration or gas sorption experiments, but also by the way in which pores of different sizes are interconnected within its structure, that is, by its connectivity. The mercury porosimetry results show that a large fraction of mercury remains trapped within the pore structure, indicat-

ing that the large pores are fed by pores of smaller size. Relatively broad hysteresis loops are also encountered in the nitrogen sorption results, and this points to the conclusion that, even in the size range below 100 nm, the large pores are fed by much smaller pores. It was pointed out in the introductory section of this paper that the characterization of the particles obtained from the decomposition of organic calcium salts showed that the structure of these particles is of cenospheric form, with large cavities (cenospheres) fed through smaller openings (blow holes).2,4,5,17 Such a structure is consistent with the results shown in Figures 6 and 7 for the mercury porosimetry and nitrogen sorption experiments, and it can also explain the lower ultimate sulfation conversion observed in the preliminary experiments for the material produced at 850 °C despite its higher porosity and higher surface area. If pores with a size around 100 nm are the feeder pores of the large cavities (cenospheres) of the pore structure, the 750 °C material, which has a higher volume fraction of pores of that size, would be capable of reaching higher conversions (and possibly complete conversion) before these pores would be plugged with solid product. When the quartz container introduced into the 110 °C oven was filled with CEB to a relatively large height (a few times its diameter), the CEB material started to expand, flowed out of the tube, and eventually formed a structure of very high porosity, estimated to be over 90-95%. This material exhibited pores on the order of a few hundred microns. An optical microscopy image of a section of the dried material is shown in Figure 8. The calcines that resulted from this sample did not present much different pore structure characteristics, as suggested by the gas adsorption and mercury porosimetry results, from calcines obtained from dried samples produced in two steps (starting at 25 °C and proceeding to 110 °C) or from placing small quantities of CEB in the tube. The calcined sample was converted to a form of small particles before it was used for pore structure characterization experiments. Therefore, the large pores of Figure 8 were not present in the material that was used in gas adsorption or mercury penetration experiments. In some experiments, the expanded material was crushed and converted to a form of small particles before it was heated to 750 or 850 °C. Even in that case, the pore structure characterization results obtained from gas adsorption or mercury porosimetry were similar to those shown in Figures 6 and 7. Figure 9 shows the gas adsorption-desorption results obtained using the material of Figure 8 before it was converted to CaCO3 or CaO through treatment at high temperatures. From a comparison of Figures 6 and 9, it can be concluded that,despite the great expansion of volume that occurred during the preparation of this material, the volume of pores with sizes below 1000 Å is rather small, an indication that most of the porosity in this pore size range is developed during decomposition and calcination. Assuming that the true density (skeletal density) of the decomposed material is that of CaO, one finds from the results of Figure 7 that the porosity of the decomposed material for pores below the cutoff size (3 µm) is above 75%. The total porosity of the material must be higher than this value because not all pores above 3 µm would correspond to voids among the particles of the powder sample. The decomposed material (CaCO3 or


Figure 8. Optical microscope image of the structure of the porous material that results after forced drying of calcium-enriched bio-oil at 110 °C.

Figure 9. N2 adsorption-desorption curve at 77 K for the porous material obtained after forced drying of calcium-enriched bio-oil (CEB) at 110 °C.

CaO) exhibits a high porosity even when the drying of CEB is carried out slowly in two steps (preliminary drying at room temperature followed by slow heating to 110 °C) and, hence, is not accompanied by expansion of volume. Using the relative volume and weight data of Table 1, the densities of CaCO3 and CaO, the total content of the CEB in calcium, and the density of CEB (about 1.5-1.6 g/cm3), it is found that, if no volume

change occurs after baking at 110 °C, the porosity of the carbonated solid should be 62% and that of CaO 82%. These values are consistent with the data obtained from the mercury penetration results. Because the plugging of the pores is the main cause of the incomplete utilization of CaO-based material as agents of SOx removal, the high porosity of the decomposed material is a clear indication of the great potential that CEB has as an efficient desulfurization sorbent. In view of the dramatic expansion that was observed when the CEB was forcibly dried at 110 °C, the CaO particles that result from the decomposition of CEB droplets should exhibit a much higher porosity than the samples used to obtain the results of Figures 6 and 7, in particular, porosity due to pores of relative large size (above a few microns in size). Figure 10 presents schematically the various stages of a physical model for the decomposition of a CEB droplet to CaO that is consistent with the observations made in the decomposition and pore structure characterization experiments. A CEB droplet is assumed to consist of a mixture (slurry) of organic salts (calcium salts) in solution with other components of bio-oil and water and Ca(OH)2 particles. As the temperature is raised and evaporation of water and other components of the liquid mixture starts to occur, a renewable crust of precipitated material (consisting of organic salts and other compounds dissolved in the liquid) is formed at the external surface of the droplet. The presence of this crust inhibits evaporation, and thus, the droplet starts to be superheated, that is, to reach temperatures above the evaporation temperature of the liquid mixture. This leads to the formation of bubbles within the structure

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Figure 10. Schematic representation of the CaO particle formation process from calcium-enriched bio-oil.

of the droplet, which, because of the relatively high viscosity of the liquid, tend to remain and grow at the point of their formation. The structure keeps expanding because of the formation and growth of bubbles until a network of interconnected bubbles is formed. Further evaporation of the volatile components of the liquid mixture and the decomposition of the solid salts leads to the formation of a rigid (noncompliant) spherical structure, whose size remains constant. Eventually, as all volatile components of the liquid phase evaporate and the organic calcium salts and the Ca(OH)2 microparticles decompose to form CaO, a porous particle of CaO is obtained. This particle is characterized by a bimodal pore size distribution, with the small pores resulting from the decomposition of the organic calcium salts and the Ca(OH)2 particles and the large pores corresponding to the interconnected network of cavities that resulted from the formation of bubbles in the superheated droplet. It must be noted that the presence of a bimodal pore structure in CaO particles resulting from CEB decomposition is consistent with the results of pore structure characterization. 6. Calcination of Decomposed Samples The decomposition of calcium carbonate to calcium oxide

CaCO3 f CaO + CO2

(2)

begins to occur as the mixture flowing over the decomposed CEB sample is changed to one containing CO2 at concentrations lower than the equilibrium concentration at the prevailing temperature. Past studies on the calcination of calcium carbonate materials have shown that the calcination temperature and the carbon dioxide

(CO2) concentration of the surrounding gas can significantly influence the decomposition reaction, producing calcines of different properties, such as porosity, internal surface area, and pore size distribution. In general, the product of the calcination of CaCO3 is porous because the resulting product occupies less space than the solid it replaces. The carbon dioxide molecules that are produced by the decomposition of CaCO3 are transported by diffusion and convection from the reaction interface to the surrounding gas phase. At the same time, because of the highly endothermic nature of the decomposition, significant temperature gradients can exist in the gas phase in the vicinity of the external surface of the particles and in their interior. These gradients can introduce heat-transfer limitations into the calcination process in addition to the mass-transfer resistance for carbon dioxide transport. Many studies have been carried out on the calcination of limestones. They have shown that the parameters that most influence the calcination reaction are sample size, calcination temperature, heating rate (for nonisothermal reaction), gas flow rate, and composition of the gas phase. Gallagher and Johnson,18 for example, found that the calcination rate depended on the sample size and composition of the reactive mixture. Higher reaction rates were observed in their study when the thermal conductivity of the reactive gas was increased, an indication of the importance of heat-transfer limitations. No effects of sample size or of the thermal conductivity were observed by Borgwardt,19 who used samples of about 7-50 mg dispersed in a differential packed-bed reactor and calcined them under a flow of inert gas with a velocity of about 9 m/s (24-44 L/min). Such calcination conditions cannot be applied in TGA systems, where the flow rate of the gas must be kept within limits and the


Figure 11. Weight change vs time for calcium carbonate obtained from a thin coating of calcium-enriched bio-oil and 53-62 µm particles of Greer Limestone and Iceland Spar calcined at 750 °C under N2.

Figure 12. Weight change vs time for calcium carbonate obtained from a thin coating of calcium-enriched bio-oil and 53-62 µm particles of Greer Limestone and Iceland Spar calcined at 850 °C under N2.

material must be placed in a container hanging from the sample arm of the microbalance. Many studies have been carried out on the characterization of the pore structure of calcined limestones.14,20-27 It was found that the total porosity of limestones did not change if the calcination temperature was below 950 °C, but it decreased with increasing temperature in the range of 950-1200 °C, possibly because of the occurrence of sintering. Dogu24 observed that the internal surface area of the calcined products decreased as the temperature increased from 750 to 950 °C, but remained almost constant above 950 °C. Hills22 and O’Neill et al.23 found that the rate of calcination and the overall conversion depended on the partial pressure of CO2 and the calcination temperature. Zarkanitis and Sotirchos,14 Efthimiadis and Sotirchos,25 and Krishnan and Sotirchos26,27 worked with the limestones that are used as reference solids to evaluate the performance of CaO samples derived from CEB samples in the present study. They carried out calcination, sulfation, and sulfidation experiments on these solids at the two temperatures used in the present study, 750 and 850 °C, but the TGA system they employed was different from that used here. For this reason, it was decided to carry out experiments in our own system on the two limestone solids under conditions at which a comparison with the results obtained for the CEB sample was to be made. Figures 11 and 12 compare the calcination behavior of the calcium carbonate material produced from CEB using the heating schedules of Figures 1 and 2 with those of 53-62 µm Greer Limestone and Iceland Spar particles at 750 and 850 °C, respectively. The experimental data are presented as 1 + ∆W/W0 vs time, where ∆W is the weight change and W0 is the initial weight of the sample. It is easy to show that, if the reaction goes to 100% completion, a pure calcium carbonate sample should attain a value of 1 + ∆W/W0 equal to 0.56. All of the conversion vs time curves approximately attain this expected value.

Significant differences exist among the calcination rates for the different sorbent types. At both temperatures, the CaCO3 sample obtained from the CEB material presents a larger calcination rate than the two calcium carbonate solids (Greer Limestone and Iceland Spar), whereas the Greer Limestone sample exhibits a much higher calcination rate than the Iceland Spar. The results of Figures 11 and 12 for the calcium carbonate solids are in good agreement with those obtained in past studies14,25 in a different TGA system. Studies done with different sample sizes and different particle sizes (for the limestone samples) showed a very strong dependence of the evolution of the weight of the sample on these parameters and on the way in which the sample was placed (spread) on the pan. Larger sample weights and larger particle sizes gave much slower evolutions of the weight of the sample during calcination, and the same observation was also made for samples not spread uniformly on the pan. These observations suggest that the calcination of calcium carbonate is a very fast reaction. In agreement with the results of past studies (e.g., Khinast et al.28), we found that the calcination rate of limestone and of CaCO3 obtained from CEB exhibited a highly nonlinear dependence on the concentration of CO2 in the bulk, with its value dropping exponentially as the equilibrium concentration of CO2 at the calcination temperature was approached. This implies that the kinetics of CaCO3 decomposition play an important role in the process and that the rate of this reaction is a highly nonlinear function of the concentration of CO2. For a finite rate of reaction, one expects the formation of three zones in the interior of the calcining particle: an outer region of fully converted material, an inner unreacted core, and an intermediate region of partially calcined material. With the kinetics of calcium carbonate decomposition being important, the overall rate of calcination should depend, for given temperature, pressure, and bulk gasphase composition, on the intrinsic kinetic parameters of calcination, the surface area available for reaction in the partially calcined zone, the effective mass-transport

2254


coefficient of CO2 in the pore space, the diffusivity of CO2 in the product (CaO) layer that covers the unreacted material, and the particle size. For a 10-mm2 area of pan, it can be found using the data of Table 1 and the density of the calcium-enriched bio-oil (about 1.5-1.6 g/cm3) that the thickness of the layer that is formed after drying at 110 °C a uniform coat of CEB with a 1-mg weight is about 30 µm. This value is very close to the half-size of the particles of the limestones. It can therefore be argued that the differences among the calcination curves of the three materials in Figures 11 and 12 are mainly a reflection of differences in the kinetic and mass-transport parameters and the surface area of the reaction zone. For a fixed concentration profile of CO2 in the reaction zone, thickness of reaction zone, intrinsic kinetic parameters, and product layer diffusivity of CO2, the reaction rate (calcination rate) is expected to increase with increasing surface area of the developing porous structure. Sotirchos and Zarkanitis29 analyzed sulfation data for the calcines of several limestones (including Greer Limestone and Iceland Spar) and found that the differences in their intrinsic kinetic parameters and their SO2 diffusivities in the product layer were rather small. If this is also the case for the differences in the kinetic parameters of the calcination reaction and the CO2 product layer diffusivity, different calcination rates must be a reflection of differences in the size of the reaction zone and the CO2 concentration profile there, which, in turn, must be caused by differences in the diffusional limitations of CO2 in the pore space. A smaller resistance to intraparticle diffusion should lead to a large reaction zone and a lower CO2 concentration there and, therefore, to higher calcination rates for similar values of the other process parameters. On the basis of the above discussion, the results of Figures 11 and 12 suggest that, at both temperatures, the diffusion coefficient through the CaO material formed from the decomposition of CEB must be higher than that through porous CaO formed from the decomposition of CaCO3 solids. CaCO3 obtained from CEB decomposition exhibits much higher calcination rates than Greer Limestone and Iceland Spar, even though the surface area of its calcined product is much lower than the surface areas of these two solids. The differences between the weight vs time curve for the calcination of the decomposed and carbonated CEB sample and those of the calcium carbonate solids are much smaller at 850 °C. This suggests that the difference between the diffusion coefficient of CO2 in the CaO obtained from CEB and those in the CaO samples obtained from the two carbonate solids is smaller at this temperature. Preliminary results have shown that this result is consistent with the behavior of the three CaO samples in sulfation and sulfidation experiments. The higher calcination rate of Greer Limestone, in comparison to that of Iceland Spar, is consistent with observations made in past studies on the behavior of these two solids during calcination, sulfation, and sulfidation.25,29 Specifically, the analysis of the sulfation and sulfidation data using a detailed model for gas-solid reactions with solid product showed that Iceland Spar is characterized by much higher resistance for transport of gases through its pore space. 7. Summary and Conclusions Past studies have shown that the decomposition of carboxylic salts of calcium oxide at high temperatures

yields calcium oxide particles of very high porosity, which might be free of the problems that arise from pore plugging phenomena during their use as sorbents for SO2 or H2S removal. Carboxylic salts of calcium and other organic compounds of calcium are present in significant quantities in calcium-enriched bio-oil (CEB), the material produced by reacting bio-oil, the product of the flash pyrolysis of biomass, with calcium hydroxide. The tendency of these compounds to yield CaO particles of high porosity upon decomposition makes CEB attractive for use as a means for in-reactor control of SO2 or H2S in fossil fuel combustors or gasifiers. As a first step of a fundamental study of the behavior of CEB in the high-temperature reactive environment of fossil fuel utilization units, the decomposition of calciumenriched bio-oil and the calcination of the CaCO3 material that is obtained in the presence of CO2 were investigated in the present study. Decomposition and calcination experiments were carried out in a thermogravimetric analysis system, operating at atmospheric pressure, using samples of calciumenriched bio-oil applied as thin coatings on the bottom of a quartz pan. After drying, these samples were decomposed to CaO under conditions of slow heating, and the resulting CaO was reacted with SO2 or H2S. In most experiments, the conversion of CEB to CaO was achieved in two steps: first converting the material to CaCO3 by heating it in the presence of CO2 and then calcining it by switching to a CO2-free mixture. Mercury penetration porosimetry and gas (N2) adsorption at liquid nitrogen temperature were used to characterize the pore structure of the decomposed samples. The structure of the dried material was examined visually using optical microscopy. The calcination results were compared with results obtained from two naturally occurring calcitic solids of very high content in CaCO3 (over 97%) at the same conditions. From the volume and weight changes that were observed during the drying of calcium-enriched bio-oil, it was concluded that the porous solid that is obtained after decomposition and calcination should exhibit a very high porosity, greater than 80%. From the mercury penetration results, it was determined that the porosity of the CEB-derived CaO samples due to pores with radii greater than 3 µm was greater than 75%. When drying of CEB was carried out under conditions of forced heating by exposing material contained in a quartz tube to an environment of 110 °C, bubbles formed from the evaporation of water and other volatile compounds led to swelling of CEB, and this, in turn, caused formation of a pore structure of very high porosity, over 90%. This is a very important observation for the use of CEB as a sulfur capture agent in gasification and combustion units because the droplets of CEB injected into the coal utilization unit are exposed instantly to the hightemperature environment, unlike the samples used in our thermogravimetric experiments, which go through successive stages of slow drying and slow decomposition. The calcination rate of CEB material that has been decomposed under CO2 (to be converted into CaCO3) was higher than those of the naturally occurring calcium carbonates. This behavior is in agreement with the much higher porosities of the CEB-derived calcines. It also suggests that the CaO material that results from the decomposition of calcium-enriched bio-oil has the potential of being a much more efficient SOx removal sorbent than calcined limestones. However, for a defi-


nite conclusion on this matter, the behavior of CEBderived CaO particles during reaction with SO2 must be investigated. The behavior of a gas-solid reaction involving a porous solid is influenced not only by the porosity of that solid but also by its pore size distribution and the connectivity of the pores (how pores of different sizes interact with each other). The mercury porosimetry results for the CEB-derived calcines showed that a large fraction of mercury remained trapped within the pore structure, an indication that large pores are fed by pores of smaller size. Moreover, the difference in the calcination rates of the CEB-derived CaCO3 and the naturally occurring calcitic solids decreased with increasing temperature, and this could signify decreasing connectivity of the pores with increasing temperature of operation. Acknowledgment This research was carried out under partial support by the European Commission within the framework of the Non-Nuclear Energy Program through Contract JOR3-CT97-0179. BTG B.V. is thanked for the supply of the CEB samples used in the study. The help provided by members of the research group of S. V. Sotirchos at ICEHT-FORTH in carrying out some of the experiments described in this study is gratefully acknowledged. Literature Cited (1) Abbasian, J.; Rehmat, A.; Banerjee, D. D. Sulfation of Partially Sulfided Calcium-Based Sorbents. Ind. Eng. Chem. Res. 1991, 30, 1990. (2) Levendis, Y. A.; Zhu, W. Q.; Wise, D. L.; Simons, G. A. Effectiveness of Calcium Magnesium Acetate as an SOx Sorbent in Coal Combustion. AIChE J. 1993, 39, 761. (3) Adanez, J.; de Diego, L. F.; Garcia-Labiano, F. Calcination of Calcium Acetate and Calcium Magnesium Acetate: Effect of the Reacting Atmosphere. Fuel 1999, 78, 583. (4) Steciak, J.; Levendis, Y. A.; Wise, D. L. Effectiveness of Calcium-Magnesium Acetate as Dual SO2-NOx Emission Control Agent. AIChE J. 1995, 41, 712. (5) Steciak, J.; Levendis, Y. A.; Wise, D. L.; Simons, G. A. Dual SO2-NOx Concentration Reduction by Calcium Salts of Carboxylic Acids. J. Environ. Eng. Div. (Am. Soc. Civ. Eng.) 1995, 121, 595. (6) Badawi, M. A.; Elshinnawi, M. M.; Blanc, F. C.; Wise, D. L.; Elshimi, S. A. Production of Acetic Acid from Thermally Treated Sewage Sludge in an Upflow Anaerobic Reactor. Resour. Conserv. Recycl. 1992, 7, 201. (7) Czernik, S.; Scahill, J.; Diebold, J. The Production of Liquid Fuel by Fast Pyrolysis of Biomass. J. Sol. Energy Eng. 1995, 117, 2. (8) Wagenaar, B. M.; Kuipers, J.; Van Swaaij, W. P. M. Fluoroptic Measurements of the Local Heat-Transfer Coefficient inside the Rotating Cone Reactor. Chem. Eng. Sci. 1994, 49, 3791. (9) Wang, D.; Czernik, S.; Montane, D.; Mann, M.; Chornet, E. Biomass to Hydrogen via Fast Pyrolysis and Catalytic Steam Reforming of the Pyrolysis Oil or Its Fractions. Ind. Eng. Chem. Res. 1997, 36, 1507. (10) Sipila¨, K.; Kuoppala, E.; Fagerna¨s, L.; Oasmaa, A. Characterization of Biomass-Based Flash Pyrolysis Oils. Biomass Bioenergy 1998, 14, 103.

(11) Oehr, K. H. Acid Emission Control. U.S. Patent 5,458,803, 1995. (12) Simons, G. A.; Oehr, K. H.; Zhou, J.; Pisupati, S. V.; Wojtowitz, M.A; Bassilakis, R. Simultaneous NOx/SOx Control Using BioLime in PCC and CFBC. In Proceedings of the 13th Annual International Pittsburgh Coal Conference; University of Pittsburgh: Pittsburgh, PA. 1996; p 1400. (13) Westerhout, R. W. J.; Waanders, J.; Kuipers, J. A. M.; Van Swaaij, W. P. M. Recycling of Polyethene and Polypropene in a Novel Bench-Scale Rotating Cone Reactor by High-Temperature Pyrolysis. Ind. Eng. Chem. Res. 1998, 37, 2293. (14) Zarkanitis, S.; Sotirchos, S. V. Pore Structure and Particle Size Effects on Limestone Capacity for SO2 Removal. AIChE J. 1989, 35, 821. (15) Deptula, A.; Lada, W.; Olczak, T.; Borello, A.; Alvani, C.; DiBartolomeo, A. Preparation of Spherical Powders of Hydroxyapatite by Sol-Gel Process. J. Non-Cryst. Solids 1992, 147, 537. (16) Silaban, A.; Narcida, M.; Harrison, D. P. Calcium Acetate as a Sorbent Precursor for the Removal of Carbon Dioxide from Gas Streams at High-Temperature. Resour. Conserv. Recycl. 1992, 7, 139. (17) Shemwell, B.; Atal, A.; Levendis, Y. A.; Simons G A. A Laboratory Investigation on Combined In-Furnace Sorbent Injection and Hot Flue-Gas Filtration to Simultaneously Capture SO2, NOx, HCl, and Particulate Emissions. Environ. Sci. Technol. 2000, 34, 4855. (18) Gallanger, P. K.; Johnson, D. W. The Effects of Sample Size and Heating Rate on the Kinetics of the Thermal Decomposition of CaCO3. Thermochim. Acta 1973, 6, 67. (19) Borgwardt, R. H. Calcination Kinetics and Surface Area of Dispersed Limestone Particles. AIChE J. 1985, 31, 103. (20) Fischer, H. C. Calcination of Calcite: I. Effect of Heating Rate and Temperature on Bulk Density of Calcium Oxide. J. Am. Chem. Soc. 1955, 38, 245. (21) Fischer, H. C. Calcination of Calcite: II, Size and Growth Rate of Calcium Oxide Crystallites. J. Am. Chem. Soc. 1955, 38, 284. (22) Hills, A. W. D. The Mechanism of the Thermal Decomposition of Calcium Carbonate. J. Chromatogr. 1968, 19, 237. (23) O’Neill, E. P., Keairns, D. L.; Kittle, W. F. A Thermogravimetric Study of the Sulfation of Limestone and DolomitesThe Effect of Calcination Conditions. Thermochim. Acta 1976, 14, 209. (24) Dogu, T. The Importance of Pore Structure and Diffusion in the Kinetics of Gas-Solid Non-Catalytic Reactions: Reaction of Calcined Limestone with SO2. Chem. Eng. J. 1981, 21, 213. (25) Efthimiadis, E. A.; Sotirchos, S. V. Sulfidation of LimestoneDerived Calcines. Ind. Eng. Chem. Res. 1992, 31, 2311. (26) Krishnan, S. V.; Sotirchos, S. V. Effective Diffusivity Changes during Calcination, Carbonation, Recalcination, and Sulfation of Limestones. Chem. Eng. Sci. 1994, 49, 1195. (27) Krishnan, S. V.; Sotirchos, S. V. Experimental and Theoretical Investigation of Factors Affecting the Direct LimestoneH2S Reaction. Ind. Eng. Chem. Res. 1994, 33, 1444. (28) Khinast, J.; Krammer, G. F.; Brunner, C.; Staudinger, G. Decomposition of Limestone: The Influence of CO2 and Particle Size on the Reaction Rate. Chem. Eng. Sci. 1996, 51, 623. (29) Sotirchos, S. V.; Zarkanitis, S. Inaccessible Pore Volume Formation during Sulfation of Limestone Calcines. AIChE J. 1992, 38, 1536.

Received for review May 30, 2002 Revised manuscript received February 20, 2003 Accepted February 20, 2003 IE020400Q

Experimental Investigation of the Decomposition and Calcination of

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