05

Research Feature “Micro Review”

１．IntroductionThe operation of an integrated coal gasification

combined cycle (IGCC) utilizes the gas produced from coal gasification to drive a gas turbine and then a steam turbine to efficiently generate electricity from coal. In this process, an amine solution is often used to remove the sulfur from the coal gasification gas (desulfurization), requiring that the gas be cooled to ambient temperature. Thus, dry desulfurization technology using Fe-Zn related sorbents1), 2), which can be used in the high temperature ranges of the desulfurization process of 723–823 K, should improve the efficiency of electricity generation.

The O2-CO2 blown gasifier is being developed to recover the CO2 more easily3). The system can capture CO2 by means of cooling the exhaust gas, and the recycled CO2 is used as the main gasification agent with a dry desulfurizer. In this novel O2-CO2 blown IGCC technology, the CO and CO2 partial pressures in the syngas are increased, while H2O partial pressure is reduced, relative to earlier methodologies. Therefore, the possibility of carbon deposition may be increased4). The increased pressure drop caused by carbon deposition in the desulfurizer could prevent continuous operation of the system. Thus, technology innovation is necessary to avoid carbon deposition in the desulfurizer.

The carbon deposition mechanism can be approximated as shown below. Reaction equations (1)–(3) show the three main reaction steps in carbon deposition. Equation (1) is the decomposition reaction of methane. It is an endothermic reaction that usually proceeds positively above 820 K. Equation (2) is the reverse of the Boudouard reaction. In both equations (2) and (3), CO decomposes in a similar temperature range to that in the dry desulfurizer. It is important to note that carbon deposition is not only related to the syngas composition, but is also sensitive to the properties of the sorbents. The sorbents contain Fe, Zn, Mn, and Si, of which Fe has a significant catalytic effect.

CH4 → C+ 2H2 (1)2CO → C+ CO2 (2)CO + H2 → C+ H2O (3)

The CO, H2, CO2, and H2O compositions change with the progress of reactions (1), (2), and (3), and the water shift reaction (4) also causes instantaneous change of the gaseous compositions of CO, H2, CO2, and H2O. We can measure the total compositional change brought about by reactions (1), (2), (3), and (4), however, we cannot monitor the progress of each reaction separately. Thus, the accurate analysis of which reactions are occurring is difficult.

CO + H2O → CO2 + H2 (4)

It is important to consider chemical equilibrium, catalytic reactions at the solid-gas interface, and quantitative carbon deposition rate to determine the best operating conditions in the desulfurizer. In this micro review, by reference to past studies related to carbon deposition, method of preventing carbon deposition in the desulfurizer using Fe-related sorbents for O2-CO2 blown IGCC are summarized.

2. Equilibrium of carbon depositionTable 1 shows the typical gaseous components in

the O2-CO2 blown gasifier2). There are some reaction routes leading to carbon deposition caused by gas species and solid catalysts. The chemical equilibrium shows the approximate reaction route of carbon deposition. Chemical equilibrium shows whether the gaseous compositions is in range of carbon deposition. If it is in range of carbon deposition, chemical equilibrium also shows to change the gaseous compositions by adding CO2 and H2O to avoid of carbon deposition.

Carbon deposition on the surface of desulfurization sorbent in an advanced IGCC can induce the blocking of gas flow. There are two prevention methods for this kind of carbon deposition. The first relies on the equilibrium of carbon deposition and the other on the kinetics. The ternary diagram of the C-H-O equilibrium clearly shows excesses of CO2 and H2O. However, these excesses may decrease both the efficiency of desulfurization and the energy conversion of the system. Thus, it is important to control the chemical reaction rate of carbon deposition by the addition of small amounts of CO2 and H2O. This micro review collates the results of previous studies on the effects of gas composition and catalytic behavior on the rate of carbon deposition in order to prevent carbon deposition to the desulfurizer.

Prevention of Carbon Deposition During Dry Desulfurization in a Coal GasifierYu Yu, Yuichi FujiokaFukuoka Women’s University

06NOVEL CARBON RESOURCE SCIENCES NEWSLETTER

is seen to be small. Carbon deposition occurs in the region below the boundary. At both 0.1 MPa and 2 MPa, when the hydrogen content is greater than 0.12, the increase in temperature moves the carbon deposition boundary higher, and the region in which there is no carbon deposition is enlarged. On the other hand, when the hydrogen content is less than 0.12, the carbon deposition boundary is lower. The typical gas composition, listed in Table 1, is located in the zone with carbon, hydrogen, and oxygen compositions of 0.36, 0.23, and 0.41, respectively. This point lies in the carbon deposition zone in which decreasing the temperature increases carbon deposition. Calculating the chemical equilibrium with added solid phase Fe2O3 enlarges the region with no carbon deposition, but the effect is small.

2.1 Reactants forming compounds of Fe and C From research into the phenomenon of the reduction

of iron in the blast furnace, the iron phase diagram associated with carbon deposition was elucidated. In 1920, it was reported that iron reacted with gaseous components in the temperature range 550 to 650ºC, and the products Fe3C and FeS were observed in the upper region of the blast furnace.

Figure 1 shows the relationship between Fe-C compounds and temperature. Cementite (Fe3C) is a composite material and not a single material. Perlite contains ferrite (α-Fe) and cementite, and ledeburite contains austenite (γ-Fe) and cementite. They are produced across a wide temperature range, from ambient temperature to 1000ºC5).

2.2 Carbon deposition boundary by Gibbs free energy method

The Gibbs free energy minimization method is applied to the C-H-O system to discuss equilibrium conditions. In 1964, Tevebaugh et al.6) determined the ternary diagram of the C-H-O equilibrium for the carbon deposition boundary, shown in Figures 2 and 3. From the temperature 298 to 1500 K and the pressure 0.1 to 2 MPa, the related carbon deposition can be calculated.

Comparing the carbon deposition boundaries at pressures of 0.1 and 2 MPa, the effect of pressure

CO CO2 H2 H2O N2 CH4 H2S

66.4 8.6 18.8 2.2 1.5 1.5 0.08

perlite(eutectoid)

0 2 4 6

1,400

1,600

1,200

1,000

800

600

400

200

Temperature (°C)

ledeburite(eutectic)

Percentcarbon(by mass)

cementiteFe3C

Fe3C+ ledeburite

Fe3C+ ledeburite

γ + Fe3C+ ledeburite

γ+ liquid

δ + liquid

austenite γ

δ + γ

δ

α + γ

liquid

ferrite α

α +

per

lite

Fe3C+liquid

Fe3C+ ledeburite+ perlite

C

CO

CO2

OHH2O

CH4

P = 1 Atmos.

ATOM PERCENT H

0 10 20 30 40 50 60 70 80 90 100

90

80

70

60

20

10

10

20

30

40

50

60

70

1500 °K

298 °K

1300110010501000950900850800

700500400

298

500700800

850900950

10001050

110013001500

500400

298

400

ATOM PERCENT C

ATOM PERCENT O

C

CO

CO2

OH

H2O

CH4

P = 20 Atmos.

ATOM PERCENT H

0 10 20 30 40 50 60 70 80 90 100

90

80

70

60

20

10

10

20

30

40

50

60

70

1300 °K

298 °K

110010501000950900850700

500400

298

500700850

900950

1000105011001300

500

400298

400

ATOM PERCENT C

ATOM PERCENT O

Table 1 Typical gas composition in O2-CO2 blown gasifi er (%).

Fig. 1 Iron at different temperatures and carbon contents.

Fig. 2 Carbon deposition boundary in C-H-O ternary diagram (0.1 MPa). 6)

Fig. 3 Carbon deposition boundary in C-H-O ternary diagram (2 MPa). 6)

07

Research Feature “Micro Review”

In the ternary diagram of C-H-O chemical equilibrium, a change in the gas composition perpendicular to the carbon deposition boundary minimizes the amount of additional gas. We must determine how much CO2 and H2O should be added by considering the effectiveness of carbon-deposition prevention. We should also conduct experiments to fi nd the point at which no carbon deposition occurs with the minimum addition of CO2 and H2O, i.e., along the line at some angle to the line of the minimum addition.

3. Carbon deposition reaction rate In the high-temperature dry desulfurizer, the

desulfurization agent fi xes H2S and COS as a form of FeS. The desulfurizer operates at a temperature of about 773 K. FeS is regenerated by O2-containing gas to form FeO and Fe2O3. The desulfurization cycle promotes pulverization of the iron particle. Other materials should be added to prevent powdering of the desulfurization agent. Recently, there has been much study of zinc ferrite as the main component for the desulfurization agent7). Zinc increases the strength and the internal reaction surface area of the desulfurization agent.

Hochman et al.8) showed the relationship between carbon deposition and the catalytic activity of the metal by evaluation of past operational data of petroleum reformers, as shown in Figure 4. Although the specifi c temperature is not shown in Figure 4, the temperature range in which Fe-alloy has a high catalytic performance is nearly the same as that in which the desulfurizer operates.

Olsson et al.9) performed the TG experiment using iron as a catalyst for carbon deposition and reported the carbon deposition properties along with previous data, as follows.

Approximate temperature

Rel

ativ

e re

activ

ity Fe

CoNi

(ALLOYING)

(ALLOYING)

(ALLOYING)

FOR REACTION2CO → C + CO2

600 C

400 C

800 C

600 C

400 C

800 C

0 20 40 60 80 1000

1

2

3

4

5

0

1

2

3

H2 , percent

RATE

, mol

e C/

min

ute

x 10

3

FOR REACTIONCO + H2 → C + H2O

In the CO/CO2 gas mixture, the carbon deposition rate decreased when the CO2 concentration was greater than 80%. At low H2 concentrations, increasing the H2 concentration caused a large increase in carbon deposition. However, at high H2 concentrations, increasing the H2 concentration had little effect on the carbon deposition under the CO/H2 mixture. In the case of CO/H2 = 50/50, both at a temperature of 673 and 873 K, the addition of steam suppressed carbon deposition. At higher temperatures, up to 1073 K, the effect of inhibiting carbon deposition was greater than at 673 and 873 K. The carbon deposition rate under a CO/H2/H2O mixture was lower than that for the H2/CO mixture. However, increasing the steam concentration in the CO/H2O mixture in the absence of H2 increased carbon deposition. Figure 5, from Olsson et al., shows the estimated reaction rate constants of reactions (2) and (3) using their data and previously reported data.

Olsson et al. also reported that because of the the production of FeS, H2S prevented carbon deposition, as shown in Figure 6. Iron is the only catalyst in which carbon deposition occurs by CO decomposition. Carbon, cementite, iron oxide, and iron sulfi de did not exhibit a catalytic effect on carbon deposition. It was conjectured that fi rst iron absorbed CO and then was converted to cementite, and fi nally the cementite decomposed to Fe and carbon, and the proposed carbon deposition mechanism and reactions for the formation and decomposition of cementite are as follows.

2CO(ads) + 3Fe = [Fe3(CO)2]*→ Fe3C + CO2(g) (5)　　　　　　　* reactive intermediate

Fe3C = 3Fe + C (graphite) 　(6)

Fig. 4 Catalysic activities of different metals in carbon deposition at different temperatures.

Fig. 5 Carbon deposition rate on Fe compound for H2 and CO. (The two carbon deposition reaction rates are calculated from experimental and reference data).

08NOVEL CARBON RESOURCE SCIENCES NEWSLETTER

TIME, minutes

WE

IGH

T G

AIN

, g

0 80 160 240

0.4

0.3

0.2

0.1

0

ab

c

d

H2S TURNED OFF

ppm H2S % SIN GAS IN SAMPLE

050100250

00.100.250.68

abcd

Car

bon

Dep

ositi

ong/

0.2g

Ox

for 9

0 m

in

PH

2O (o

pt.),

mm

Hg

Temperature, °C

0.1

0.2

0.4

0.6

0.8

0

10

20

30

40

400 450 500 550 650

Fig. 7 Maximum rate of carbon deposition and optimal H2O content at different temperatures. (The maximum carbon deposition rate is shown in the plot of water steam pressure).

Fig. 6 Effect of H2S concentration on carbon deposition for an Fe compound.

Fig. 9 Change in state of iron particle in the experiment of Kolesnik et al. (The left-hand panel is under an atmosphere of dry CO, the right is Fe2O3 after reaction with wet CO. Approximate magnifi cation is 37,840 times).

Fig. 8 Relationship between layer thickness of Fe compounds and H2/CO concentration ratio at 700 K.

Kolesnik et al.10) carried out carbon deposition experiments with a tubular reactor using Fe reduced from high purity Fe2O3.

Kolesnik et al. suggested that previous experiments conducted with CO contained small amounts H2O, and that even a little H2 or H2O could enhance carbon deposition. Hence, in their experiment, they prepared gases without H2O. Figure 7 shows the maximum rate of carbon deposition and H2O partial pressure that maximized the carbon deposition rate at each temperature. The maximum carbon deposition rate was found to be at a temperature of 773 K, and the maximum steam partial pressure decreased from 0.053 to 0.013 bar when the temperature rose from 673 to 773 K. Above 773 K, the steam partial pressure at the maximum carbon deposition rate was constant at a value of 0.013 bar.

They also showed the effect of CO and H2 concentration on the shape of the solids. As shown in Figure 8, unreacted Fe2O3 in the left-hand panel was powdered under CO without H2O present. In the right-hand panel, unreacted Fe2O3 was transformed to a rod shape under CO with H2O.

Zhang et al.11) evaluated the relationship between cementite formation and the H2/CO concentration ratio at a temperature of 973 K. Figure 9 shows that both layers had a similar structure, so the H2/CO concentration ratio had no effect on the cross section of the layer structure, in which carbon was in the top layer, followed by iron, cementite, and iron. They proposed that the Fe between the deposited carbon and cementite became so thin that the Fe3C formation rate increased greatly at low H2/CO ratio. An increase in CO caused a decrease in the thickness of Fe3C at that temperature. While an experimental temperature of 973 K is higher than the optimal temperatures for desulfurization, which is in the range 723–923 K, Figure 9 supported the carbon deposition mechanism through cementite by equations (5) and (6), and that the maximum rate of carbon deposition is not closely related to the CO concentration.

09

Research Feature “Micro Review”

Geng et al.12) reported transformations of the shapes of iron pellets reduced by CO, H2, and the CO/H2 mixture, as shown in Figure 10. The BET surface area after reduction by the CO/H2 mixture in (d) is 55 times larger than that of the original iron ore in (a), and is 8.6 times larger than that of the reduction by CO gas in (b). However, as shown in (c), the iron ore particles looked like a fusion material after reduction by H2, and the surface area of the iron ore powder decreased to 1/8 of the original area. The gas composition affected various properties, such as shape of iron oxide, surface area, and diffusivity of H2 and carbon into the inside of iron in the iron reducing process.

In the coal gasification gas, typical sulfur compounds are H2S and COS and the total concentration ranges from 0 to 1000 ppm. Olsson suggested that the formation of the metal sulfide suppressed carbon deposition caused by the decrease in the iron surface area. However, Ando et al. pointed out that the increase in the surface area, caused by powdering during formation of the metal sulfide, was one of the reasons for increased carbon deposition, so that the increase in iron surface area by powdering was larger than the decrease in surface area by desulfurization. FeS formed by H2S reduction did not have a catalytic effect on carbon deposition. They showed experimentally that when H2S and H2 partial pressures were extremely low, H2S could promote carbon deposition significantly. As shown in Figure 11, it was observed that the flat iron surface was converted to a rough surface composed almost completely of fine particles. The increase in surface area between iron and the gas resulted in the increase in carbon deposition.

Ando13) reported that there was an equilibrium between the adsorbed carbon on the iron surface and cementite, so the amount of carbon deposition was proportional to the amount of adsorbed carbon, not to the amount of cementite, and the following carbon deposition reactions were given.

C(ad) + 3Fe = Fe3C (7)

C(ad) → C(solid) (8)

Devan et al.14) investigated the relationship between conditions for carbon deposition and metal compounds from performance data collected from operations of a coal gasifier. As shown in Table 2, they reported that FeO and FeS could suppress the formation of carbon from CO, while FeS and Fe3C could avoid carbon deposition from the decomposition of CH4.

Kobayashi et al.15) investigated the effect of steam, CO2, and a mixture of steam and CO2, which simulated the recycled gas, on carbon deposition. They reported that steam was the most effective inhibitor of carbon deposition, but the addition of a large amount of steam decreased the desulfurization efficiency. The effectiveness of preventing carbon deposition by CO2 addition was smaller than that of H2O. However, the recycled gas, which contained steam and CO2 being fed upstream into the desulfurizer, was better than steam alone in preventing carbon deposition, from the points of view of both desulfurization efficiency and economic benefit.

4. Summary

The C-H-O ternary equilibrium diagram shows that additional amounts of CO2 and H2O can prevent carbon deposition in a desulfurizer. The addition of steam not only decreases carbon deposition caused by changes in equilibrium, but also results in low efficiencies of both desulfurization and the system. A practical method for the prevention of

Fig. 11 Iron nanoparticle produced by the reduction of FeS.

Fig. 10 Shape transformations of iron ore powder. (a) Iron ore. (b) Reduced in CO. (c) Reduced in H2. (d) Reduced in 50:50 H2-CO mixture.

10NOVEL CARBON RESOURCE SCIENCES NEWSLETTER

Gasifier system Temperature Fullyequilibrated

Carbondepositionsuppressed

Carbondeposition and CH4 formation

suppressed(°C)

Predominant phases

Piñon Pine(M. W. Kellogg)

Foster Wheeler/Wilsonville

Shell(oxygen-blown)

600

600

600

800

800

800

Graphite, FeO, FeSGraphite, FeO

Graphite, FeS

FeO FeO

FeO, FeS

FeO, FeS

FeO, FeS

FeS, Fe3C

FeS, Fe3C

Fe3C

aFe3C is also stable if H2S < 0.2 vol %.bFe3C is also stable if H2S < 0.32 vol %.

FeSa

FeSb

FeO

FeS

FeSGraphite, FeS

Graphite, FeO, FeS

carbon deposition is by combining changes in the carbon deposition boundary estimated by chemical equilibrium and the rate control of carbon deposition. The effect of gaseous compounds and iron catalysts on carbon deposition should be studied further, as well as the reaction rate of carbon deposition in an O2-CO2 blown gasifier.

References1) G. D. Focht, P. V. Randae, D. P. Harrison, “High tempe-

rature desulfurization using zinc ferrite: Reduction and sulfidation kinetics”, Chem. Eng. Sci., 43, 3005-3013 (1988).

2) M. Kobayashi, H. Shirai, M. Nunokawa, “Measurment of Sulfur Capacity Propotional to Zinc Ferrite-Silica Composite Powder in Pressurized Coal Gas”, Ind. Eng. Chm. Res., 41, 2903-2909 (2002).

3) S. Umemoto, S. Kajitani, Y. Zhang, S. Hara, “Modeling of Char Gasification Reaction for O2-CO2 Blown Gasifier - Proposal and Verification of Char Gasification Model for Coexistence of CO2 and H2O -”, Energy Engineering Research Laboratory Rep. NO. M09014 (2010).

4) M. Kobayashi, M. Nunokawa, “Optimization of Dry Desulfurization Process for IGCC Power Generation Capable of Carbon Dioxide Capture. Determination of Carbon Deposition Boundary and Examination of Countermeasure”, Energy Engineering Research Laboratory Rep. NO. M09015 (2011).

5) Maruzen, Japan Institute of Metals, “Kouza, Gendai no Kinzokukagakuzairyou-hen, (Vol.4) Tekkou Zairyou”, (1997).

Table 2 Metal compounds with the ability to suppress carbon deposition.

6) A. D. Tevebaugh, E. J. Cairns, “Carbon Deposition Boundaries in the CHO System at Several Pressures”, J. Chem. Eng. Data, 10, 359-362(1965).

7) G. N. Krishnan, V. K. Wong, B. J. Wood, “A Prelimi-nary Study of Carbon Depositin on Zinc Ferrite Sorbrn-ts”, U.S. Department of Energy, Office of Fossil Energy. DOE/MC/25006-3057 (1991).

8) R. F. Hochman, J. H. Burson, “The Fundamentals of Metal Dusting”, API Division of Refining Proc., 46, 331-44 (1966).

9) R. G. Olsson, E. T. Turkdogan, “Catalytic Effect of Iron on Decomposition of Carbon Monoxide: II. Effect of Ad-ditions of H2, H2O, CO2, SO2 and H2S”, Metall. Trans., 5, 21-25, (1974).

10) N. F. Kolesnik, G. R. S. T. Pierre, “The Influence of Wa-ter Vapor on the Decomposition of Carbon Monoxide on Reduced Iron Oxide”, Metall. Trans. B, 11B, 285-289 (1984).

11) J. Q. Zhang, A. Schneider, G. Inden, “Effect of gas com-position on cementite decomposition and coke formation on iron”, Corros. Sci., 45, 281–299 (2003).

12) S. H. Geng, W. Z. Ding, S. Q. Guo, Z. Fang, X. G. Lu, “The study on the carbon deposition in H2-CO mixture”, Adv. Mat. Res., 239, 445-449 (2011).

13) S. Ando, H. Kimura, “Effect of Sulfur in Iron on Carbon Deposition in CO-H2 Mixture at 923K”, J. Japan Inst. Metals, 8, 785-791 (1989).

14) J. H. DeVan, P. F. Tortorelli, P. R. Judkins, I. G. Wright, Oak Ridge National Laboratory, ORNL/TM-13014, 1 (1997).

15) M. Kobayashi, Y. Nakao, “Optimization of Dry Desulfurization Process for IGCC Power Generation Capable of Carbon Dioxide Capture”, Energy Engineering Research Laboratory Rep. NO. M10012 (2011).