CaO Pellets with High-Pressure

1014

Energy & Fuels 2004, 18, 1014-1020

Gasification of Organic Material/CaO Pellets with High-Pressure Steam 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 January 28, 2004. Revised Manuscript Received April 7, 2004

Pellets made from subbituminous coal, lignite and wood mixed with CaO were steam gasified with high-pressure steam (4.2 MPa) at various temperatures. The product gas was composed mainly of H2 with a few other gases (CH4, CO2). Pellet reaction rates decreased in the following order: wood > lignite > subbituminous coal; however, the amount of H2 produced decreased in the order: subbituminous coal > lignite > wood. Pellet size and morphology did not change significantly during pyrolysis and gasification at 923 K, but they did change significantly during gasification at 973 K. Eutectically melted calcium compounds were expelled from the pellets. The compressive strengths of the subbituminous coal pellets increased after pyrolysis and gasification, and the increase in pellet strength was promoted by increased gasification temperature. H2 production seemed unaffected by CaO reuse.

Introduction Gasification of organic materials produces fuel gas containing H2 and CO2. Fixing the CO2 during the gasification can alter the reaction equilibrium so as to increase H2 production. Several early studies1,2 have attempted to use CaO to separate CO2 during some gasphase reactions, such as the methane reforming reaction and the water-gas shift reaction of CO to H2. During solid (coal) gasification, a CO2 acceptor process 3 has been used to first eliminate part of the CO2. These studies all use CaO in the form of dolomite (CaO‚MgO) to absorb CO2, since the reactivity of pure CaO (lime) is reduced quickly upon reuse.

CaO + CO2 f CaCO3

∆H298 ) -178 kJ/mol (1)

In our previous studies,4-6 we proposed a method for completely capturing the CO2 with pure CaO (lime) during gasification of organic materials to produce H2 (the HyPr-RING method). To recover the reactivity of CaO, CaO was hydrated to Ca(OH)2 in the gasifier. In the gasifier, CaO first reacts with H2O under high pressure to form Ca(OH)2, and this reaction supplies heat for the pyrolysis of organic material. Pyrolysis and char gasification generate CO2 and H2. Then Ca(OH)2 * Corresponding author. Tel: +81-29-861-8224. Fax: +81-29-8618209. E-mail: [email protected]. † Center for Coal Utilization. ‡ National Institute of Advanced Industrial Science and Technology. (1) Brun-Tsekhovoi, A. R.; Zadorin, A. N.; Katsobashvili, Y. R.; Kourdyumov, S. S. Proceedings of the 7th World Hydrogen Energy Conference, Moscow, 1988; pp 885-900. (2) Han C.; Harrison, D. P. Chem. Eng. Sci. 1994, 49, 5875. (3) Fink, C.; Curran, G.; Sudbury, J. D. Proceedings of 6th Synthetic Pipeline Gas Symposium, Chicago, 1974. (4) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada, M. Kagaku Kogaku Ronbunshu 1999, 25, 498. (5) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada, M. Energy Fuels 2001, 15, 339. (6) The Hydrogen Production Reaction Integrated Novel Gasification (HyPr-RING) Process, IEA report, Prospects for Hydrogen from Coal, ISBN 92-9029-393-4, 12/2003.

absorbs CO2 to form CaCO3, and the absorption process supplies heat for char gasification.

CaO + H2O f Ca(OH)2

∆H298° ) -109 kJ/mol (2)

C + H2O f CO + H2

∆H298° ) 132 kJ/mol (3)

CO + H2O f CO2 + H2 ∆H298° ) -41.5 kJ/mol (4) Ca(OH)2 + CO2 f CaCO3 + H2O ∆H298° ) -69 kJ/mol (5) A previous experiment7 confirmed that a high concentration (about 80%) of H2 with a scant amount of methane can be produced from powdered coal using this method. CO could be converted to H2 and CO2, which could be fixed as CaCO3. However, reaction rates were comparatively slow: 0.14/min and 0.49/min at 923 and 973 K, respectively. Moving-bed and fluidized bed reactors which are easy to control for a long solid residence time are available for these reactions. For the moving-bed reactor, particles of a certain size and strength of materials are required to maintain the bed height and the gas flow. In the present study, we prepared pellets using subbituminous coal, lignite, and wood mixed with CaO powder. The pellets were gasified with high-pressure steam using a flow-type fixed-bed reactor. We examined the product gases, the reactivity of the pellets, and their morphology before and after reaction. Experimental Section Pellet Preparation. Subbituminous coal (Taiheiyo coal, Japan), lignite (Wyoming coal, U.S.A.), and wood were used as the organic materials. They were crushed and sieved to a diameter of 0.1 mm. Table 1 shows the proximate and ultimate (7) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Fuel 2002, 81, 2079.

10.1021/ef040017t CCC: $27.50 © 2004 American Chemical Society Published on Web 05/15/2004

Gasification of Organic Material/CaO Pellets

Energy & Fuels, Vol. 18, No. 4, 2004 1015

Table 1. Analyses of the Organic Materials proximate anal. (wt %) organic materials subbituminous lignite wood

vol

moist

FC

ultimate anal. (wt %, daf)

ash

C

H

N

O

48.8 4.8 38.8 7.6 77.3 6.6 1.2 14.5 46.6 14.9 31.1 7.3 69.0 5.1 1.0 23.2 79.5 8.62 11.7 0.2 51.0 6.4 0.11 42.5

analyses for these materials. CaO was made by calcination of Ca(OH)2 powder (325 mesh under, Wako Pure Chemical Industries, Osaka, Japan). The organic material was shaken with CaO in a 1/4.2 ratio (wt/wt) until the organic particles were uniformly disposed in the CaO powders. The mixture was then placed in a metal mold and pelletized at a pressure of 200 kg/cm2. The resulting pellet was 6 mm in diameter and 15 mm long. Pressurized Reactor. The reactor was made with a Hastelloy tube coated with alumina on the inside (length 600 mm, inner diameter 20 mm). A fixed bed was located 150 mm from the bottom of the reactor. High-pressure, high-temperature steam, which was introduced from the top of the reactor, passed through the fixed bed, and then flowed out through the bottom of the reactor to a cooling section. The details of the reactor system have been described elsewhere.7 Experimental Procedure. Three to four pellets containing 0.5 g of raw organic material were uniformly dispersed in the fixed bed. The pressure of the reactor was increased by means of a nitrogen gas cylinder, and the temperature was raised with an electric furnace at a rate of about 7 K/min. A part of the N2 (4 L/min) was introduced into the reactor during gasification. Some of the organic material in the pellet was pyrolyzed as the temperature rose. When the pressure and temperature stabilized at set values, high-pressure and hightemperature steam was injected into the reactor to start the reaction. Steam partial pressure combining with N2 and steam supplied and gas superficial velocity in the reactor are shown in Table 2. The concentrations of H2, CH4, C2H6, CO, and CO2 in the product gas were continuously measured using a microGC (Chrompack CP2002, Netherlands). To check carbon conversion in the pellets during reaction, we performed pellet residue sampling experiments under the same conditions used for the gas analysis experiments. At a set temperature or time, the furnace was opened and the reactor was rapidly cooled with a fan. The cooling rate at the first 200 K was faster at 70 K/min. After the reactor cooled, pellet residue was sampled. Carbon, CaCO3, and Ca(OH)2 contained in the pellets before and after pyrolysis and gasification were quantified using thermogravimetric analysis (TGA). The heating rate for TGA was about 20 K/min, and the final temperature was 1123 K. Ca(OH)2 and CaCO3 decomposed in N2 atmosphere as the temperature rose. At 1123 K, air was injected to burn up the

Figure 1. Procedure for reusing CaO. carbon contained in the sample. The porosity of the pellets was measured using mercury porosimetry. We measured the compressive strength of the pellets before and after pyrolysis and gasification by placing a pellet under a plate and then loading force on the plate at a speed of about 0.2 kg/s. The compressive strength [kg/cm2] was defined as the loading force per cross-section area of the pellet at the point when the pellet broke. Measurements were performed three times for pellets obtained under the same experiment conditions, and an average value was taken. Another experiment was conducted to determine the effectiveness of reusing CaO. After gasification, pellets were burned at 1173 K with air at 0.1 MPa. The carbon contained in the pellets was converted to CO2, and the CaCO3 contained in the pellets was calcined back to CaO. The CaO was then crushed into 325 mesh (under) and mixed with coal, to make a pellet again using the method described above. Figure 1 shows the procedure for reusing CaO.

Results and Discussion Product Composition and Reaction Rate. Figure 2 shows gas products obtained from subbituminous coal pellets at various temperatures under a steam partial pressure of 4.2 MPa (total pressure 6.0 MPa), as well as the gas products obtained during the 40-min temperature increase before steam injection. When steam was injected into the reactor, the main product gas was H2, CH4, C2H6, and CO2 were also obtained. CO was completely converted to H2, and most of the CO2 was fixed by CaO. The amount of H2 produced increased quickly as the temperature was increased; the H2 levels

Table 2. Steam Partial Pressure and Superficial Gas Velocity in Reactor (6.0 MPa) temperature K

water supply [g min-1]

N2 supply [L (STP) min-1]

steam pressure [MPa]

superficial gas velocity [m s-1, at 6.0 MPa]

873 923 973 1023

7.7 7.7 7.7 7.7

4 4 4 4

4.2 4.2 4.2 4.2

0.038 0.041 0.043 0.047

Table 3. Product Gases Composition after Pellets Gasification for 30 Minutes gas composition [%] original materials

temperature [K]

H2

CH4

C2H6

CO

CO2

subbituminous

873 923 973 1023 973 973

75.8 81.6 82.9 80.5 81.9 81.5

11.6 4.8 3.5 2.3 5.0 4.2

0.14 0.09 0.08 0.08 0.1 0.1

0.3 0.3 0.5 0.6 0.5 0.5

12.2 13.2 12.9 16.6 12.5 13.6

lignite wood

1016 Energy & Fuels, Vol. 18, No. 4, 2004

Lin et al.

Figure 2. Product gas composition from subbituminous coal pellets at various gasification temperatures.

Figure 3. Product gas composition from lignite and wood pellets at 973 K. Table 4. Carbon Conversion in Pellets after Pyrolysis and Gasification subbituminous

temperature [K]

carbon conversion Xpr [-]

carbon conversion Xg,30min[-]

873 923 973 1023

0.10 0.12 0.16 0.20

0.21 0.38 0.54 0.70

at 923, 973, and 1023 K were, respectively, 1.5, 2, and 2.5 times the level at 873 K. However, the amount of CO2 product also increased with temperature (Figure 2d), since CO2 absorption by CaO or Ca(OH)2 is not favorable at higher temperatures. CO2 production doubled when the temperature was changed from 923 to 1023 K (Table 3).

We compared these results with those from our previous study,7 in which the subbituminous coal and CaO powders reacted directly with steam. We found that the composition of gas products from the pellets was not significantly changed from that obtained with the powders, but the gas product amounts from pellets were smaller than those from powders.



The carbon conversion, X, of the pellets was calculated from the TGA results:

X)

Figure 4. Reaction rates of pellets with high-pressure steam.

The amount of product gases obtained from lignite and wood were smaller than the amount obtained from subbituminous coal (Figure 3). For example, after 30 min of gasification, the amounts of total product gases from lignite and wood were 0.67 and 0.54 times the amount obtained from subbituminous coal, respectively. The reason for the decrease is that lignite and wood contain less carbon than subbituminous coal. However, the composition of product gases was not significantly different (Table 3).

r(C/Ca),b - r(C/Ca),a ;[-] r(C/Ca),b

(6)

where r(C/Ca),b and r(C/Ca),a are C/Ca molar ratios in the pellets before and after reaction, respectively. The carbon conversion during temperature raising before steam injection, Xpr , and the carbon conversion of steam gasification after steam injection, Xg, were shown in Table 4. Total carbon conversion X ) Xpr + Xg at steam gasification 30 min for subbituminous coal were 0.3, 0.5, 0.7, and 0.9 at temperatures 873, 923, 973, and 1023 K, respectively. The gasification rate, R, was defined as

R)

dX dt(1 - X)

|

Xg)0.1

(7)

Figure 4 is an Arrhenius plot of initial gasification rate at Xg ) 0.1 (carbon conversion of steam gasification) for the pellets of subbituminous coal, lignite, and wood. Initial gasification rates of subbituminous coal pellets

Figure 5. Photographs of subbituminous coal pellets before and after pyrolysis and gasification.


Lin et al.

Figure 6. TGA results of solid residue after subbituminous coal pellets gasification at 973 K.

Figure 7. Pore structure change of subbituminous coal pellets before and after pyrolysis and gasification.

increased with temperature; the apparent activation energy was about 224 kJ/mol. The gasification rate for the pellets at 973 K was lower by about half than the rate for the powdered samples (Lin et al.7) (Ea ) 182 kJ/mol). The initial gasification rates for lignite and wood shown in Figure 4 were 1.45 times and 3.31 times the rate for subbituminous coal. Morphology and Physical Properties of Pellets. The morphology and physical properties (size and porosity during reaction) of the pellets are important information not only for examining reaction kinetics, but also for operating a reactor. After pyrolysis, the pellets increased slightly in volume, and their surfaces were cracked (Figures 5a and 5b). The release of volatile material from the coal may have caused the cracking. Pellet size was not significantly changed by gasfication at 923 K (Figure 5c). The pellet was slightly whiter after gasification, probably because of the conversion of carbon to CaCO3. However, after gaification at 973 K,

the pellet consisted of two parts, a white part and a black part (Figure 5d). The black part, the parent pellet, contained carbon; the white part that grew from the pellet presumably contained calcium compounds. Since the pellets did not show this separation phenomenon after pyrolysis at 973 K (Figure 5b), the reaction of the pellet with H2O may lead to this phenomenon. TGA revealed that the white part contained primarily CaCO3, Ca(OH)2 and CaO, in a weight ratio of about 1.8/1.5/1 (Figure 6a). Curran et al.8 reported that eutectic melting occurs at certain composition ratios of calcium compounds. They reported that the lowest melting temperature is about 893 K when CaCO3 and Ca(OH)2 are combined in a molar ratio of about 3:2, which is relatively close to the ratio in the white part. During gasification at 973 K, melted calcium compounds (8) Curran, G.; Fink, C.; Corin, W. Proceedings of the 8th Synthetic Pipeline Gas Symposium, Chicago, 1976.



Figure 9. H2 production with recycled CaO pellets. Figure 8. Compressive strength of subbituminous coal pellets before and after pyrolysis and gasification.

may have been expelled from the pellet along with product gases. The black part contained calcium compounds and carbon (Figure 6b). The CaCO3/Ca(OH)2/CaO/C weight composition ratio in the black part was 1.6/0.32/1/1.6. Carbon comprised slightly over 1/3 of the weight of this part. The porosity of the pellet clearly increased after pyrolysis (Figure 7a and 7b). Most opened pores fell in the 2 × 104 to 2 × 106 Å range. The Chemical Engineers Handbook9 divides pores in solid materials into three size ranges: micropores (diameter < 100 Å), macropores (diameter > 1000 Å), and mesopores (diameter ) 1001000 Å). The pores in our pellets are thus macropores, which is helpful for gas transitions in and out of the pellets. After gasification at 923 and 973 K, the porosity of the pellets decreased absolutely (Figure 7c and 7d). The closed pores fell into all three pore size ranges. The closing of the pores increased with increasing temperature. It is possible that, during gasification, eutectically melted calcium compounds blocked the pore inlets, that the increased solid volume of reactions 2 and 5 occupied the pore space, or both. The compressive strengths of the pellets increased after pyrolysis and gasification (Figure 8). The strengths increased in the order raw pellet (7.7 kg/cm2) < pellet after pyrolysis (13 kg/cm2) < pellet after gasification (29 kg/cm2). Higher gasification temperatures increased the compressive strength of the pellets. In a moving-bed reactor, solid material needs a certain compressive strength to withstand the weigh pressure and abrasion of the upper material, so that the reactant gas can pass through. The compressive strength of the raw pellets (7.7 kg/cm2) is roughly equal to the weight pressure of 60 m of pellets (density about 1.2 g/cm3 for subbituminous/CaO pellets). The most important finding in this study is that compressive strength of the pellets was increased by its pyrolysis and gasification. This result indicates that if raw pellets are injected from the top of the moving-bed reactor and then move down, the pellets at the bottom have already been gasified, and thus have a higher compressive strength to better support the upper pellets. (9) Chemical Engineers Handbook, 5th ed.; The Society of Chemical Engineers, Japan, 1988; p 1111.

Figure 10. Variation of carbon conversion, CaCO3, and Ca(OH)2 with CaO recycling.

Effect of Reusing CaO. In the HyPr-RING method, it has been suggested that the pellet residue could be converted back into CaO for reuse. We tested this possibility by carrying out repeated gasfication reactions with recycled CaO pellets and measuring H2 production (Figure 9). We found that H2 production remained nearly constant over four reuse cycles. Figure 10 shows the variation in CaCO3 and Ca(OH)2 content in solid residues after gasification. Pellet residues contained CaCO3, Ca(OH)2, and some other components (probably ash and CaO). In subsequent cycles of CaO, the Ca(OH)2 amount decreased, and the levels of the other components increased. It may be that CaO reactivity decreased as the ash content in the reused CaO increased. However, the CaCO3 content in the pellet residue did not change significantly when CaO was recycled. Conclusion Pellets were made from CaO and subbituminous coal, lignite, and wood, and then steam gasified under 4.2 MPa steam pressure at various temperatures. The pellets reacted more slowly than the powdered material that we used in previous studies. However, the product gas from the pellets contained mainly H2 with few other gases. The reaction rates of the pellets decreased in the order: wood > lignite > subbituminous coal, whereas


the H2 product volumes decreased in the order subbituminous coal > lignite > wood. The pellets’ size and morphology were not significantly changed during pyrolysis and 923 K gasification, but they did change significantly during 973 K gasification. A white part material containing CaCO3, Ca(OH)2, and CaO were expelled from the pellet at higher temperature, probably because of eutectic melting of calcium compounds.

Lin et al.

The compressive strengths of the subbituminous coal pellets increased after pyrolysis and gasification, and the increase was promoted by higher gasification temperature. H2 production seemed unaffected by the reuse of CaO. However, CaO reactivity showed a decrease with the number of CaO cycles. EF040017T

CaO Pellets with High-Pressure

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