CaO, and Coal

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Energy & Fuels 2003, 17, 602-607

Comparison of Pyrolysis Products between Coal, Coal/CaO, and Coal/Ca(OH)2 Materials Shiying Lin,*,† Michiaki Harada,† Yoshizo Suzuki,‡ and Hiroyuki Hatano‡ Center for Coal Utilization, Japan, 24 Daikyocho, Shinjuku-ku Japan, and National Institute of Advanced Industrial Science and Technology Received September 17, 2002

Products from coal, coal/CaO, and coal/Ca(OH)2 pyrolysis under various pressures were examined and compared experimentally. A high-pressure flow-type reactor with a rapid cooling system was used for the pyrolysis experiment, and it was found that the amount of gas products increased in response to CaO and Ca(OH)2 addition. The total volume of gas product from pyrolysis was in the order of coal/Ca(OH)2 > coal/CaO > coal at extremely high pressure. H2 products in coal/CaO and coal/Ca(OH)2 pyrolysis peaked in the temperature range of 923-973 K. At this H2 peak temperature, the H2 content in product gas was approximately 80%, while there were little CO, CO2, and CH4. One reason for this is that the H2O contained in coal can be brought to a higher temperature by the reactions of Ca(OH)2 formation and decomposition. H2O reacted with coal at high temperature, producing H2 and CO. It was also found that CO can shift to CO2, and that CO2 can be fixed by CaO to CaCO3 in the temperature range of the H2 peak.

Introduction Fossil fuels (including coal) are the earth’s largest energy resource, with approximately 85% of human energy consumption now depending on fossil fuel and 22% depending on coal.1 However, coal use releases CO2, the green house gas, into the atmosphere, and reducing these emissions from coal utilization is considered to be urgent. Increasing coal energy conversion efficiency is helpful for the reduction of CO2 emission. Calcium oxide, CaO, can absorb CO2 to form calcium carbonate, CaCO3, which is a stable material in nature. Several early studies2,3 attempted to use CaO to remove some of the CO2 in coal gasification. In our previous studies,4,5 we proposed a method of completely capturing the CO2 with CaO (Ca(OH)2) during coal pyrolysis and char gasification to produce hydrogen (HyPr-RING method). That is, by injecting coal, CaO, and H2O into a gasifier, CaO would first react with H2O to form Ca(OH)2 under high pressure, and supplies heat for coal pyrolysis and char gasification. Coal pyrolysis and char gasification generates CO2 and H2. Ca(OH)2 absorbs CO2 to form CaCO3 and also supplies heat for char gasification. Thus, high H2 production efficiency is expected.6 * Author to whom correspondence should be addressed at Clean Fuel Research Group, Institute for Energy Utilization, AIST, 16-1, Onogawa, Tsukuba 305-8569, Japan. Tel: +81-298-61-8224. Fax:+81-298-618209. E-mail: [email protected]. † Center for Coal Utilization, Japan. ‡ National Institute of Advanced Industrial Science and Technology. (1) World Energy Outlook 2001; 2001, IEA, Oct. (2) Curran, G. P.; Clancey, J. T.; Scarpiello, D. A.; Fink, C. E.; Gorin, E. Chem. Eng. Prog. 1966, 62 (2), 80. (3) McCoy, D. C.; Curran, G. P.; Sudbury, J. D. Proceedings of 8th Synthetic Pipeline Gas Symposium, Chicago, 1976. (4) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada, M. Kagaku Kogaku Ronbunshu 1999, 25, 498-500. (5) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada, M. Energy Fuels 2002, 15, 339-343.

Coal pyrolysis is a much faster process than char gasification, strongly influencing the final gas product.7 Several factors such as the heating rate, temperature, pressure, and particle size influence coal pyrolysis.8,9 Recently, studies have found that catalysis of alkali or alkali earth metals also influences the product yields of coal pyrolysis.10-13 They established that more gaseous and liquid products are formed from coals impregnated with CaO and Ca(OH)2 than from parent coals under the same pyrolysis conditions. In our proposed HyPr-RING method, because CaO (Ca(OH)2) is added for CO2 absorption, the amount and mixture of addition, and the operating conditions (high pressure) are different from those used in previous catalysis coal-pyrolysis studies. Calcium compounds phase equilibrium may vary with pressure between the following equations:

CaO + H2O a Ca(OH)2 KC1 CaO + CO2 a CaCO3

KC2

(1) (2)

However, the calcium compounds vary with pressure, and their effect on product yields during coal pyrolysis are not yet clear. (6) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada, M. Energy Convers. Manage. 2002, 43, 1283-1290. (7) Elliott, M. A. Chemistry of Coal Utilization, Second Supplementary Volume; Elliott, M. A., Ed.; Wiley: New York, 1981; pp 665-784. (8) Misra, M. K.; Essenhigh, R. H. Energy Fuels 1988, 2, 371-285. (9) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Seiro, M. A.; Deshpande, G. V. Energy Fuels 1988, 2, 405-422. (10) Xu, W. C.; Tomita, A. Fuel 1989, 68, 673-676. (11) Longwell, J. P.; Chang, C-S.; Peters, W. A. Annual Progress Report, DOE/MC /16026-1611, 1984; pp 1-41. (12) Cazorla-Amoros, D.; Linares-Solano, A.; Salinas-Martinez, C.; Nomura, M.; Yamashita, H.; Tomita, A. Energy Fuels 1993, 7, 625631. (13) Shevkoplys, V. N.; Saranchuk, V. I. Fuel 2000, 79, 557-565.

10.1021/ef020204w CCC: $25.00 © 2003 American Chemical Society Published on Web 03/20/2003

Pyrolysis Products of Coal, Coal/CaO, and Coal/Ca(OH)2

Energy & Fuels, Vol. 17, No. 3, 2003 603

Table 1. Properties of Coal proximate anal. wt %

ultimate anal. wt %, daf

coal

vol.

moist.

FC

ash

C

H

N

O

Taiheiyo (dry base)

51.3 54.0

5.08

40.8 43.0

7.9 8.3

77.1

6.6

1.2

14.5

Figure 2. Temperature profile during experiment.

Figure 1. Schematic diagram of the experimental apparatus.

To statistically study coal-CaO(CaOH)2)-H2O reaction systems in the present study, the behaviors and products between coal, coal/CaO, and coal/Ca(OH)2 mixture pyrolysis under various pressure were investigated and compared experimentally. A high-pressure flow-type reactor with a fixed bed was used for the pyrolysis experiments under a pressure of 0.1-6.0 MPa. Experimental Section Sample Preparation. A sub-bituminous (Taiheiyo coal, Japan) was used in this work. The coal was crushed and sieved to approximately 0.05 mm. Table 1 shows the proximate and ultimate analyses for this coal. CaO was made by calcination of Ca(OH)2 powder (325 mesh under, Wako Pure Chemical Industries). Pressurized Reactor. Figure 1 shows a schematic diagram of the experimental apparatus. The reactor was made with a Hastelloy tube. The reactor with a fixed bed had a length of 100 mm and an inner diameter of 10 mm, and was held in a pressure vessel. The reactor pressure was increased with a nitrogen gas cylinder, and the temperature was raised by an electric furnace. Some of the N2 (1.0 L/min) was introduced during pyrolysis experiments. Gas was supplied from the top, passed through the fixed bed, and then flowed out the bottom of the reactor. By passing through a gas filter, product gas was introduced into the analyzer. The gas filter was connected beyond the reactor and cooled by ice water during the experiments in order to capture the liquid product tar. The gas filter was made by a cylindrical stainless steel tube (100 mm in length and with a 10 mm i.d.), and was filled with quartz wool. Under the reactor, there was a water tank for rapidly cooling the reactor after pyrolysis. Experimental Procedure. Approximately 0.5 g coal, 0.5 g coal mixed with 1.0 g CaO, or 0.5 g coal mixed with 1.5 g Ca(OH)2 were used as materials for the experiment. The material was uniformly dispersed in the fixed bed. The bed temperature was measured with a K-type thermocouple. With

Figure 3. Equilibrium constants. the material properly placed in the reactor, the reactor pressure was increased, and the temperature was raised at a rate of 20 K/min to the final temperature chosen. Pyrolysis gas accompanied by N2 gas was passed through the gas filter, then out of the reactor for analysis. After pyrolysis, the reactor was rapidly cooled by ice water at a cooling rate of approximately 200 K/min. Figure 2 shows the temperature profile of the reactor during rises and drops in temperature. Analysis. The concentrations of H2, CH4, C2H6, CO, and CO2 in the product gas were continuously measured by using a micro-GC (chrompack CP-2002). Tar was collected by THF. Solid products were analyzed by a TG technique. Reaction-Equilibrium Constants. Figure 3 shows the reaction equilibrium constants of KC1 and KC2 for reactions 1 and 2 as a function of temperature. KC1 and KC2 all decreased with increases in temperature, indicating that high temperature favors the CaO phase. On the other hand, Ca(OH)2 and CaCO3 are favored by the high pressure of H2O and CO2, respectively. At 923 K, it can be seen that Ca(OH)2 and CaCO3 formed at above 0.6 MPa of H2O and 0.0008 MPa of CO2, respectively.

Results and Discussion Coal, coal/CaO, and coal/Ca(OH)2 materials were pyrolyzed under pressures of 0.1, 1.0, 3.0, and 6.0 MPa,

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Figure 4. Gas products from coal pyrolysis.

respectively. Figure 4 shows that the gas products change with temperature for coal pyrolysis. It can been seen that at 0.1 MPa, CO and CO2 generation began from 500 K and peaked at approximately 700 K. CH4 generated from 600 K and peaked at approximately 800 K. H2 generation began from 700 K and rapidly increased from 800 to 900 K. With pressure rising, the temperature patterns of gas generation did not appear to change, but peak heights decreased. For example, when the pressure rose from 0.1 to 6.0 MPa, the peak heights of H2, CH4, CO, and CO2 were reduced to 0.58, 0.83, 0.33, and 0.57, respectively. These behaviors are in agreement with those of general coal-pressurized pyrolysis,7-9 such as those reported by Seebauer et al.,14 with the total volatile yield from coal pyrolysis being strongly influenced by pressure, decreasing by -20% when the pressure rose to 4.0 MPa. This behavior has been explained by Eatough et al.15 As pressure is increased, the transit time of volatiles within the particles increases, making it more likely that condensation and secondary repolymerization reactions of volatiles would occur on char surfaces. (14) Seebauer, V.; Petek, J.; Staudinger, G. Fuel 1997, 76, 12771282. (15) Eatough, C. N.; Smoot, L. D. Fuel 1996, 75, 1601-1605.

Gas products from coal/CaO mixture pyrolysis are shown in Figure 5. As can be seen, CO and H2 generated from the coal/CaO mixture pyrolysis were much different than the products of coal-only pyrolysis. At 0.1 MPa, both the CO and H2 peaks were shifted to a higher temperature at approximately 900 K. The peak heights for CO and H2 were approximately twice and 1.5 times higher than those from coal pyrolysis, respectively. In contrast, the patterns of CO2 and CH4 generation with temperature appeared to be similar to those in coal pyrolysis. The effects of alkali and alkaline earth metals on coal pyrolysis have been investigated in several studies.10-13 It has been found that with the addition of alkali or alkaline earth metals, the gas and liquid products are both increased. The alkali and alkaline earth metals also affect tar decomposition; with the secondary reaction of pyrolysis, as reported by Longwell et al.,11 CaO facilitates tar decomposition, leading to H2, CH4, and coke formation. In the present study, because the CaO solid was only physically mixed, added to the outsides of the coal particles, it is thought that CaO catalysis primarily affects tar decomposition outside the char, rather than the coal pyrolysis occurring inside the particles. Some of the increased gas products would be



Figure 5. Gas products from coal/CaO mixture pyrolysis.

caused by tar decomposition catalyzed by the CaO. Experimental confirmation in this study has shown that the amounts of tar were decreased by CaO addition during coal pyrolysis, as will be discussed in detail later in this report. However, the catalysis effects on pyrolysis do not explain the peak shifts of CO and H2 to higher temperature. New reactions related to CaO addition must occur during pyrolysis to produce these CO and H2 shifts. In the figure illustrating the Ca(OH)2-CaO phase equilibrium (Figure 3), it can be seen that Ca(OH)2 is favored at lower temperature and CaO is favored at higher. Accordingly, the H2O contained in coal may react with CaO first to form Ca(OH)2 at a relatively lowtemperature range,

H2O + CaO f Ca(OH)2

(3)

with the Ca(OH)2 then decomposing when the temperature rises, to release H2O. Thus, H2O reacts with coal to produce H2 and CO.

The H2 should peak at a temperature with the best generation rate by combining the H2O release (eq 4) and the char gasification (eq 5). In Figure 5, the optimum temperature for H2 generation seems to be in a range of 923-973 K. For CO generation, however, with pressure, the peak was shifted to a higher temperature than that of the H2 peak. For example, at 0.1, 1.0, and 3.0 MPa, the CO peaked at 973, 1014, and 1073 K, respectively. The reason for this can be explained as follows. When reaction 5 occurs to produce CO and H2, CO reacts with H2O again to form CO2, and CaO absorbs CO2 into CaCO3, as shown in reactions 6 and 7.

CO + H2O f CO2 + H2

(6)

CO2 + CaO f CaCO3

(7)

Pressure enhances reactions 6 and 7, pushing the equilibrium toward the products. The CaCO3 decomposition temperature (see Figure 3) is greater than that of Ca(OH)2, and the following two reactions would occur at a higher temperature:

Ca(OH)2 f CaO + H2O

(4)

CaCO3 f CaO + CO2

(8)

H2O + C f 1/2CO + 1/2H2

(5)

CO2 + C f 2CO

(9)

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Figure 6. Gas products from coal/Ca(OH)2 mixture pyrolysis.

It can be clearly seen in Figure 5 that at high pressures such as 6 MPa, CO and CO2 could not be detected at low-temperature ranges below 923 K, but were detectable above 1073 K. Calcium compounds contained in the solid residue after pyrolysis were confirmed by XRD analysis. Figure 7a shows the results of XRD analysis for the residue of coal/CaO pyrolysis under 6 MPa. The residue was sampled by rapidly cooling the pyrolysis reaction at 923 K. It can be seen that Ca(OH)2 and CaCO3 peaks were detected in this figure. Accordingly, it can be concluded that H2O can be carried by Ca(OH)2 formation and decomposition up to high temperatures, to react with coal to produce H2. In addition, CO and CO2 can be carried by CaCO3 up to higher temperatures over the optimal temperature for H2 generation. H2 production by the coal reaction with H2O, which is carried by Ca(OH)2 at high temperatures in pyrolysis, was further examined by coal/Ca(OH)2 mixture pyrolysis. Figure 6 shows the resulting gas products. It can be seen that, at 1.0 MPa, much more H2, CO, and even CO2 were generated than from coal pyrolysis. The peak heights compared with coal pyrolysis were 2.4, 5, and 7 times higher for H2, CO, and CO2, respectively, because the added Ca(OH)2 can carry more H2O for the coal

reaction. With increasing pressure, it can be seen that the H2 peak did not significantly change, but the CO peak was shifted to a higher temperature, such as 1000, 1050, and 1100 K for 1.0, 3.0, and 6.0 MPa, respectively. The CO2 peak also shifted to the higher temperature. From the XRD analysis of the solid residue, as shown in Figure 7b, we can see that at 6.0 MPa, a lot of Ca(OH)2 still remained in the residue. The CaCO3 peak was larger than that in the XRD analysis for the coal/ CaO pyrolysis residue (Figure 7a). The CaO, Ca(OH)2, and CaCO3 contents in the solid residue were then analyzed by the TG technique. Table 2 shows the TG analysis results. All solid residues were sampled by rapidly cooling the pyrolysis at 923 K. It can be seen that Ca(OH)2 and CaCO3 contents were increased with pressure, confirming that pressure enhances Ca(OH)2 and CaCO3 formation. However, in Figure 6, it is quite interesting that CO and CO2 can be fixed in the temperature range of 923973 K, which is the optimal temperature for H2 generation. Figure 8 shows the gas products (volume per gram coal) at 923 K by replotting data from Figures 5 and 6 for coal/CaO and coal/Ca(OH)2 pyrolysis under various pressures. With increasing pressure, the CO and CO2 decreased more rapidly than hydrocarbon gases in both the coal/CaO and coal/Ca(OH)2 pyrolysis results. For



Figure 7. XRD analysis results. Table 2. Calcium Compounds in Coal/Ca(OH)2 Pyrolysis Residues [mol %] CaO Ca(OH)2 CaCO3 0.1 MPa 87.0 1.0 MPa 80.2

6.2 9.3

6.8 10.5

CaO Ca(OH)2 CaCO3 3.0 MPa 75.4 6.0 MPa 73.6

10.3 11.8

Figure 8. Gas products at 923 K for coal/CaO and coal/Ca(OH)2 mixture pyrolysis.

14.3 14.6

example, for coal/CaO pyrolysis, CO and CO2 made up only 3.1 and 3.3% of the product gas at 6.0 MPa, respectively. For coal/Ca(OH)2 pyrolysis, CO and CO2 made up 2.1 and 2.2% of the product gas at 6.0 MPa, respectively. H2, the main product gas, made up approximately 80% of the product gas for both the pyrolysis of coal/CaO and coal/Ca(OH)2. However, the amount of H2 produced from coal/Ca(OH)2 pyrolysis was approximately twice that from coal/CaO pyrolysis. Figure 9 shows the tar product from pyrolysis with both coal and coal/CaO materials. With the addition of CaO, the tar product decreased to as little as half that produced by coal-only pyrolysis, which is considered to have been caused by the enhancement of tar decomposition by CaO catalysis. The amount of tar product also showed a slight decrease with increased pressure. Conclusion Coal, coal/CaO, and coal/Ca(OH)2 materials were pyrolyzed under various pressures. Gas products and tar were compared between pyrolysis of these materials, and the effects of CaO and Ca(OH)2 addition on pyrolysis were investigated. The following results were obtained. (1) Gas products increased with CaO and Ca(OH)2 addition. The total volume of gas product was in the

Figure 9. Tar products during coal and coal/CaO pyrolysis.

order of coal/Ca(OH)2 > coal/CaO > coal, especially at high pressure. (2) H2 products in coal/CaO and coal/Ca(OH)2 peaked at temperatures of approximately 923-973 K. One reason for this is that H2O can be carried in the form of Ca(OH)2 to a higher temperature to react with coal to produce H2. (3) CO and CO2 also can be fixed in the form of CaCO3 at a temperature over the H2 peak temperature range. EF020204W

CaO, and Coal

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