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Desulphurization of coal-derived char using mixtures of steam and methane
Desulphurization of coal-derived mixtures of steam and methane M. Alam,
T. DebRoy,
A. K. Kulkarni”,
char using
C. I. Kim* and H. R. Jacobs*
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 7 6802, USA * Department of Mechanical Engineering, The Pennsylvania State University, University Park, PA 76802, USA (Received 9 November 7987; revised 19 February 7988)
Results of a laboratory scale experimental study on the desulphurization of coal char by steam-methane mixtures are presented. The char was prepared from a high sulphur, medium volatile, bituminous coal. The variables studied were temperature and reacting gas composition. The concentration of sulphur, specific surface area, pore size distribution and char reactivity were determined before and after the desulphurization reaction. A significant amount of sulphur was removed when coal char was reacted with 3-20 mol% steam in steam-methane mixtures in the temperature range 703323 K. Carbon gasification was effectively eliminated by this technique. Although small increases in both the specific surface area and pore volume occurred, the reactivity
of the char was practically
(Keywords: desulphurization;
unaffected
by the desulphurization
char; methane)
Desulphurization of carbonaceous fuels, such as coal or coke, prior to combustion, offers potential cost effective and technical advantages over flue gas desulphurization, coal liquefaction and gasification. At present only a few desulphurization methods are commercially feasible, all of which depend on the physical separation of coarse particles that contain sulphur-bearing minerals from coal. This paper presents results of a laboratory-scale study of a chemical process aimed at reducing sulphur in carbonaceous fuels to levels that will allow subsequent combustion offuels to be environmentally acceptable and cost effective. The process involves low temperature pyrolysis of coal using a mixture of steam and methane. The steam reacts with carbon to produce hydrogen and carbon monoxide according to the reaction C+H,O=H,+CO
(I)
The hydrogen then reacts with sulphur sulphide as follows:
to form hydrogen
S+H,=H$
(2)
Gasification of carbon by hydrogen introducing methane in the reactant reaction can be represented as
is suppressed by gas stream. This
2H,+C=CH,
(3)
The laboratory experiments reported here were conducted with the goal of determining optimum conditions that would allow sulphur removal by reacting with hydrogen, while suppressing any significant carbonhydrogen reaction. Sulphur may be removed prior to combustion, either by physical separation or by chemical processing. Physical methods (flotation, magnetic cleaning) are limited to removing pyritic sulphur only’. Chemical methods have a potential for the reduction of sulphur by removing both the organic and inorganic sulphur’. The 0016-2361/88/111542~653.00 0 1988 Butterworth & Co. (Publishers)
1542
FUEL,
1988,
reaction.
Ltd.
Vol 67, November
process discussed in the present paper is a chemical coal desulphurization method. Several different chemical methods of desulphurization have been studied before. Air oxidation of coal at 623 K to 723 K removed the pyritic sulphur by forming sulphur dioxide, however the calorific value and volatile matter content of the coal was reduced significantly during the proces?. Treatment by hydrogen, steam, nitrogen, oxygen and the pyrolysis gases themselves (in mixtures and separately) also helped desulphurization, and the sulphur removal rate depended on pressure and temperature up to certain limits4-‘. Sinha and Walker’ experimented with air, N,, CO and CO/steam mixtures. They found desulphurizing ability in decreasing order to be: air> steam/CO mixture>CO, N,. However, in all these cases of treatment, significant gasification and reduction in heating value of coal were associated with desulphurization. Oxydesulphurization of coal is one of the promising chemical techniques in which coal is treated with NO,, peroxygen compounds, or usually air in the presence of water at an elevated temperature (up to 793 K) and pressure (up to lo4 kPa). The steam-air treatment of coal has been claimed to remove up to 40 % of organic sulphur and more than 90 % of pyritic sulphur. Effects of steamto-air ratio, pressure, temperature and particle size on desulphurization have been reported in the literature9~‘0. In addition to the treatment by relatively common and inexpensive gases discussed above, more expensive, complex and rather exotic methods of chemical desulphurization have also been tried. They include the use of hot solutions of alkali containing dissolved oxygen aqueous hypochlorite oxidation and under pressure”, nitric acid oxidation’*, ozonization’ 3, use of electrophilic oxidant with subsequent treatment by a molten caustic14, hydroheating in the presence of Co-Mo/A120, catalyst’ 5 and microwave irradiation with caustic treatment16.
Coal-derived
char desulphurization
using
steam
THERMODYNAMICS
and methane:
M. Alam
OF THE C-H-O
et al.
SYSTEM
According to the proposed scheme of desulphurization, the hydrogen produced by the reaction of steam with hot carbon is prevented from reacting with carbon by the addition of methane in the inlet gas. Examination of the thermodynamics of the C-H-O system is important to determine the content of methane in the inlet gas mixture which is necessary to prevent hydrogenation of carbon. In addition to Equations (1) and (3), the following chemical reaction is to be considered to understand the C-O-H equilibrium. c+co,=2co
IO
Hz0
t
1
co 01
cop1 703
763
Temperature
823
(K
)
Figure 1 Equilibrium pressure of species as a function (PH>o= 13 kPa; P,-~*=87 kPa)
of temperature
Recently, desulphurization of very small amounts of finely divided coal samples by leaching with cupric ion solution at high temperatures has received some interest’ ‘. Electrolysis of acidic coal slurries using platinum or graphite electrodes has been found to cause pyrite to oxidize and dissolve and thus be removed”. Chemical degradation of coal using a variety of agents (such as potassium hydroxide, hydrochloric acid and N, Ndimethyl acetamide) before using ethanol and methanol to extract sulphur has been found to improve the desulphurization rate I9 . The amount of sulphur extracted in the form of H,S by hydropyrolysis was shown to be more effective for acid-leached char compared with untreated char ‘O. Also, certain typ es of bacteria are found to remove pyritic sulphur from coal’l. Kor22 conducted small scale desulphurization experiments on 10m4 to 2.5x 10m4 kg samples of dried coals and chars to study effects of various mixtures of hydrogen and methane at different pressures and temperatures. In his study, methane was seen to inhibit desulphurization, but it was also instrumental in suppressing the gasification rate; hydrogen had the opposite effect. The proportion of H, and CH,, and the process conditions, thus play important roles. Although the desulphurization of coal by steam and hydrogen can be accelerated by alkali metal salts, significant coal gasification occurs in the absence of methane23. The chemical desulphurization method discussed in this paper, which involves processing of coal using steam/methane mixtures, is a new contribution to coal desulphurization research. It is a potentially attractive technique because it utilizes process ingredients (steam and methane or natural gas) that are inexpensive and abundant, and it is not based upon processing coal at excessive pressure and temperature conditions.
(4)
Therefore, five gaseous species namely, H,O, CH,, CO, H, and CO, are involved in the system. To determine the equilibrium composition of these species, use was made of the three equilibrium constantsz4 for Equations (l), (3) and (4), and two mass balance equations based on hydrogen and oxygen. The five equations were solved numerically for the equilibrium pressure of the live species. The results of the calculation are presented in Figures I and 2. Figure I represents the variation of the equilibrium composition of the species as a function of temperature, for a given PHIo/PCHI = 0.15 in the inlet gas and at a total pressure of 101.3 kPa. The figure indicates that equilibrium moisture and methane content decrease while the equilibrium CO and H, contents increase with increasing temperature. Gasification of carbon by H, is prevented if methane pressure in the inlet gas lies above the equilibrium pressure for methane indicated in the figure. At higher temperatures, a relatively lower methane content is required to prevent gasification. Figure 2 represents the variation of the equilibrium composition of the species as a function of PH,o/PCH, in
7o-l-
60
t-
E z z
40
ci E
.z
-
30
C’-‘4
{:I,11 0.03
0.10
0.20 ‘Hz0
’
0.30
'CH4
Figure 2 Equilibrium pressure of species as a function ratio in the inlet gas (T = 823 K)
of PH&PCH,
FUEL, 1988, Vol 67, November
1543
Coal-derived
char desulphurization
using steam and methane: M. Alam et al.
Table 1 Ultimate and proximate analysis of coal and char on dry basis Coal
Char
Proximate analysis (%) Volatile Ash Fixed carbon
25.9 10.1 64.0
5.4 13.8 80.7
Ultimate analysis (%) Carbon Hydrogen Nitrogen Total sulphur oxygen
77.2 4.8 1.4 2.9 3.7
78.7 1.0 1.1 1.9 3.5
Sulphur forms (%) Pyritic Sulphatic Organic
1. Thermogravimetry, in which char samples were exposed isothermally to streams of steam/methane mixtures; 2. analysis of sulphur content in the samples; 3. specific surface area measurements; 4. pore size distribution in the samples; and 5. determination of reactivity in the char.
2.2 0.0 0.6 -.__
the inlet gas stream at 823 K and 10 1.3 kPa total pressure. The figure indicates that at 823 K, the equilibrium while hydrogen and methane contents decrease equilibrium moisture and CO contents increase with increasing PH,o/PCH, ratio in the inlet gas. Again gasification of carbon by Hz will not occur if the methane pressure in the inlet gas exceeds the pressure given by the equilibrium methane curve. The equilibrium curves in Figures 1 and 2 were used to select the steam-methane mixtures in the temperature range 703 to 823 K that would ensure prevention of Equation (3). THERMODYNAMICS
OF SULPHUR
REMOVAL
The hydrogen produced by the reaction of hot carbon with moisture may react with either pyrite (FeS,) or pyrrhotite (FeS) as follows: FeS,+H,=FeS+H,S FeS+H,=Fe+H,S
;:;
The free-energy change equations Equations (5) and (6) are”:
associated
In the temperature range 703-823 K the thermodynamic efficiency of utilization of hydrogen according to Equation (5) varies from 76 to 97% while that of Equation (6) varies from 0.005 to 0.023%. The thermodynamic efficiency of utilization is taken as the ratio of the moles of reactant gas consumed to the moles of reactant gas introduced in the reactor, multiplied by 100, and is calculated theoretically from the knowledge of the standard free energy change of the reaction at the reaction temperature. The efficiency values indicate that desulphurization of pyrrhotite (FeS) by hydrogen according to Equation (6) is extremely unfavourable, while desulphurization of pyrite to pyrrhotite is theoretically feasible. EXPERIMENTAL
Starting material Coal char was used as the starting material in most experiments because it eliminated the problem of handling devolatilization products in the reactor, thus
FUEL,
1988,
Vol 67, November
The samples were characterized both before and after the treatment with steam/methane mixtures. The experimental techniques are described below.
Thermogruvimetry The thermogravimetric set-up consisted of a Cahn microbalance (Model 1000,0.5 pg sensitivity) and a high temperature silicon carbide vertical tube furnace with a 0.025 m long equitemperature zone at the centre. The diameter of the reaction tube was 0.0475 m. The temperature was regulated by a Eurotherm controller to an accuracy of +2 K. About 0.5 g of char sample was placed in a platinum crucible and was suspended from one arm of the Cahn balance with a platinum wire. The system was first purged with argon flowing at a rate of 0.0041s-’ at 298 K and 101.3 kPa pressure for 1800 s. The furnace heating rate was kept constant for all the runs. After reaching the desired temperature a mixture of steam and methane was introduced in the reaction chamber.
with
AG” = 91086 - 138.9T kJ/kg mole AGo= 59957 - 3.2T kJ/kg mole
1544
allowing the study to be focused on the evaluation of the desulphurization scheme. A high sulphur, medium volatile, bituminous coal sample mined in Frenchville Township, Clearfield, Pennsylvania from the lower Kittanning seam was used. The coal was heated in air at 403 K for 24 h to avoid swelling during char formation. Subsequently, the coal was heated in N, at 1073 K for 3 h. The heating rate was kept at 0.05 K s-l to attain this temperature. The particle size of the char was between 270 and 325 mesh. Table 1 shows the composition of the coal and the char before steam/methane treatment. The experimental work consisted of:
Determination of gas composition In all experiments, an argon flow rate of 0.0041s-’ was maintained. Methane was passed at a rate of 0.0081s-’ through a column of water before being introduced into the reaction chamber. The water column was maintained at a constant temperature during the course of the experiment. The flow rate of moisture was determined from the knowledge of the initial and final weight of the water column and the bubbling time. The flow rate of moisture was varied by changing the temperature of the water column. Condensation of moisture in various parts of the set-up was prevented by using a heating tape.
Char characterization The char was characterized with respect to total sulphur content, specific surface area and pore size distribution. The total sulphurcontent in the samples was determined by a combustion iodometric procedure using the LECO 5 19 titrator. The specific surface areas of char samples were measured by the nitrogen adsorption technique using a Quantasorb Sorption System and the subsequent treatment of the data by the BET method. The pore size distribution in char samples was investigated by an Aminco mercury porosimeter using a mercury intrusion technique.
Coal-derived char desulphurization using steam and methane:
5.01 0
I
I
3600
7200
Time
10800
(s)
0.0
L 7200
Time
The data indicate that the gasification can be suppressed effectively with high methane fraction in the gas stream. Under none of the conditions in the present series of experiments did the weight loss exceed 1Y0of the initial sample weight. The significance of the data must be viewed in perspective with the data obtained by other workers. Block, Sharp and Darlage3 observed significant loss in weight when coal was treated with steam. In the first 15min of reaction, the weight loss was 14.4% at 723 K and 70.8% at 1173 K. Similarly, Kor22 demonstrated that the extent of gasification of carbon in coal decreases significantly with increasing methane content in H,-CH, mixtures in the temperature range 873-1073 K. The data presented in Figure 3 clearly demonstrate the importance of methane in steammethane mixtures in suppressing the gasification of carbon via Equation (3). Sulphur removal
Fire 3 Gasification of char as a function of time at 823 K and 101.3 kPa total pressure (PH,&PcH,=O.O~ (2); 0.09 (1); 0.13 (3); 0.20 (4))
I 3600
M. Alam et al.
10800
(s)
Figure4 Gasification ofchar as a function of time at PH,~‘PcH, =O. 14 and 101.3 kPa total pressure (T= 823 (3); 763 K (5); 703 K (6))
Char reactivity
Reactivity of the original and treated char samples was measured by subjecting the samples to pure CO2 at 101.3 kPa pressure and 1273 K for a period of 3600 s. The per cent weight loss sustained by the sample during this interval of time was taken as an indicator of the reactivity of the sample.
The char contained about 1.85 % total sulphur (Table 1). During preheating for about 1200 s in argon, when the sample lost about 6% of its initial weight some sulphur was also lost. During this period, the sulphur content dropped to 1.82%. Results of char desulphurization at fixed gas composition and temperature are shown in Figure 5 as a function of time. The figure indicates that initially desulphurization is rapid. Kor22 attributed the initial rapid desulphurization partly to the reduction of pyrite (FeS,) to pyrrhotite (FeS) and partly to the loss of less stable organic sulphur. The slow desulphurization rate during the later stages may be due to the reduction of pyrrhotite (FeS) to iron (Fe) and loss of organic sulphur. The figure also indicates that a significant amount of sulphur can be removed at PH20/PCHd = 0.2 and 823 K in 10800 s. The effects of relative gas composition and temperature on the sulphur removal of char and coal samples are shown in Figures 6 and 7 respectively. The sulphur removal rate increases with increasing steam fraction in the gas and the process temperature. Several coal samples were also processed in various steam-methane mixtures at 823 K for 10800 s. These results are also presented in Figure 6. A significant amount of sulphur was removed in
2.05 I
RESULTS AND DISCUSSION Gasification
Results of gasification (weight loss) are shown as a function of time at various gas compositions and temperatures in Figures 3 and 4. The data correspond to heating in steam-methane mixture at constant temperature and does not include the weight loss sustained by the char samples during the preheating stage in argon. During the preheating time of roughly 1200 s, the samples lost about 6% of the initial sample weight. This was due to the loss of the volatile matter and moisture content of the char. The figures indicate that the amount of gasification in the steam-methane mixtures almost reach a plateau after about 10800 s of reaction.
0.00 0
I 7200
I 3600
Time
(s.1
of char as a function Figure 5 Desulphurization PH,&‘cH,=~.~; PT= 101.3 kPa)
FUEL,
1988,
10800
of time (T= 823 K;
Vol 67, November
1545
Coal-derived
char desulphurization
of 82 mol % methane
10800 s with a mixture
and 18 mol %
steam. Structural
characterization
The specific surface area (SSA) and the porosity of the starting char were found to be 7500 m’/kg and 47.1% respectively. Table 2 gives the variation of the SSA and porosity of the char samples treated at various temperatures and gas compositions for 10800 s. The table indicates that the SSA increases moderately with increasing moisture content in methane and with temperature. This is in agreement with the fact that the extent of gasification and desulphurization increases with increasing moisture content in the gas phase and the temperature. The change in SSA is not dramatic, indicating that a complete breakdown of coal structure does not occur during the steam/methane treatment. The table also shows that porosity is relatively insensitive to temperature but it increases slightly with increasing moisture content in the reactant gas. In general, the comparison of treated and untreated samples shows that after the treatment pores are ‘opened up’, indicating an easier access for the hydrogen to the interior of the sample which is desirable for sulphur removal. Char reactivity Results of char reactivity as a function of relative gas composition and temperature are presented in Table 2. A small increase in the reactivity is seen as the moisture content of the mixture and temperature increase. The process
studied
here involves
pyrolysis
of char
*,*-l
using a mixture of superheated steam and methane. The basic, overall mechanism of sulphur removal and carbon gasification/suppression can be explained as follows. The superheated steam reacts with the carbon to produce hydrogen and carbon monoxide according to Equation (1). This reaction is usually accompanied by opening of the pore structure of coal, allowing an improved access of hydrogen to both the organic and inorganic sulphur in the fuel. The H, then reacts with sulphur to form H,S gas. Gasification of carbon by H, is suppressed according to Equation (3), by deliberately maintaining a high partial pressure of methane (CH,) in the reactant gas stream. If a commercial process is based on the concept presented in this paper, the H,S can be removed from the product gases by a suitable technique. Remaining product gases, rich in CH,, can be partially recycled into the reactor and also used to produce steam. It is important to note here that a significant amount of CH, can be recycled, H, can be generated in situ, and the energy required for the process can be derived from the reactor gas stream. It is a relatively simple process that uses commonly available ingredients and process conditions that are not excessive. However, verification of the concept cannot be complete unless larger bench scale tests are conducted under a wide range of conditions. Further research should also involve the effect of pressure on sulphur removal, types of sulphur influenced by the process, and a complete analytical model of the process. CONCLUSIONS Results of a laboratory scale experimental study are presented on a coal-char desulphurization process. The process uses mixtures of superheated steam and methane
0.0
0.24
0.16
0.00
0.00
&I. Alam et al.
using steam and methane:
643
I 703
PH*O ’ PCH, Figure 6
ff=823
Table 2
of specific surface area, porosity T=823K,
h,O/PCH,
Temperature
and reactivity
823
1
Desulphuriration of char as a function (Pu,cJPcu,=0.14; Pr= 101.3 kPa; time= 10800s)
of char as a function
of gas composition
of temperature
and temperature
Pu,0/P~u,=0.1,3,
time=108OOs
time=
10800 s
SSA
Porosity
Reactivity
T
SSA
Porosity
Reactivity
W/kg)
(%I
(%)
WI
W/kg)
(%I
(%)
703 763 823
8100 9300 9500
52.9 52.8
27.2 27.5 33.0
0.03 0.09 0.13
8900 9500 9500
52.8
28.0 31.0 33.0
0.20
101Ofl
55.3
30.8
1546
(K
Figure 7
Sulphur (wt %) in: 0, char; 0, coal versus gas composition K; Pr= 101.3 kPa; time= 10800s)
Variation
I 763
FUEL, 1988, Vol 67, November
Coal-derived
char desulphurization
of various compositions in the temperature range 703823 K. A well-characterized char was used as a starting material and measurements were made for sulphur removal, gasification, specific surface area, pore size distribution and reactivity as a function of temperature, gas composition and, in certain cases, time of exposure. From the results, it can be concluded that using commonly available steam and methane, a significant amount of sulphur can be removed without significant gasification and structural change of carbon.
10 11 12
ACKNOWLEDGEMENT This research was Pennsylvania Energy grant No. 85005.
partially supported by the Development Authority under
REFERENCES ‘Coal Desulfurization: Chemical and Physical Methods’ (Ed. T. D. Wheelock), Am. Chem. Sot. Symp. Ser., 1977, 64 ‘Coal Desulfurization Prior to Combustion’ (Ed. R. C. Elliot). Chem. Tech. Rev. No. 113/Pollu, Tech. Rev. No. 45, Noyes Data Corp., Park Ridge, NJ, USA, 1978 Block, S. S., Sharp, J. B. and Darlage, L. J. Fuel 1975,54, 113 Joshi, J. B., Shah, Y. T., Ruether, J. A. and Ritz, H. J. Fuel Proc. Technol. 1983, 7, 173 Maa, P. S., Lewis, C. R. and Hamrin, C. E., Jr. Fuel 1975,54,62 Joshi, J. B. and Shah, Y. T. Fuel 1981,60,612 Soo, S. L. and Cheung, K. C., ‘Steam Desulfurization of Coal’, Proc. 20th Intersociety Energy Conversion Engr. Cong., paper no. SAE P-164, August 1985 Sinha, R. K. and Walker, P. L., Jr. Fuel 1972,51, 329 Friedman, S., Lacount, R. B. and Warzinski, R. P. in ‘Coal Desulfurization: Chemical and Physical Methods’ (Ed. T. D. Wheelock), Am. Chem. Sot. Symp. Ser., 1977, pp. 164-172
17
18
19
20 21 22
23
24 25
using steam and methane:
M. Alam et al.
Tsai, S. C. Ind. Eng. Chem. Proc. Des. Deu. 1986, 25(l), 126 Chuang, K. C., Markuszewski, R. and Wheelock, T. D. Fuel Proc. Technol. 1983,7,43 Taylor, S. R., Miller, K. J., Hucko, R. E. and Deurbrouck, A. W., ‘New Methods for Coal Desulfurization’, Eight Int’l. Coal Prenaration Congress. USSR. 1979 Steinberg, M., Y&g, R. T., Horn, T. K. and Berlad, A. L. Fuel 1977,56,227 Aida, T., Squires, T. G. and Venier, C. G., US Patent 4 497 636, 1985 Inaba, A., Miki, K., Osafune, K., Saitoh, 1. et al. Nenryo Kyokaishi 1985,&l, 36 Bluhm, D. D., Marakuszewski, R., Richardson, C. K., Fanslow, G. E. et al., ‘British National Committee for Electroheat, Heating and Processing Conference’, NTIS, PC AOZ/MF AOl, CONF-860984-1, 1986 Chen, H. L. in ‘Study of Desulfurization by High Temperature Leaching’, Southern Illinois University report, NTIS, PC A02/MF AOl, DE85018428, 1985 Lalvani, S. B. and Ramaswami, K. in ‘Synfuels and Coal Energy Symposium’ (Ed. J. B. Dicks), ASME, New York, 1985, pp. 134 146 Chen, J. W., Muchmore,C. B. and Kent, A. C. in ‘Extraction and Desulfurization of Chemically Degraded Coal with Supercritical Southern Illinois University report, NTIS, PC Fluids’, A02/MF/AOl, DE85018431, 1985 Chou, M. M. and Loffredo, D. M. Fuel 1985,&l, 731 Chandra, D., Roy, P., Mishra, A. K., Chakrabarti, J. M. and Sengupta, B. Fuel 1979, 58, 549 Kor, G. J. W., ‘Desulfurization and Sulfidation of Coal and Coal Char’ in ‘Coal Desulfurization: Chemical and Physical Methods’ (Ed. T. D. Wheelock), Am. Chem. Sot. Symp. Ser., 1977, p. 221 Krauss, W. in ‘Catalytic Influence of Inorganic Coal Compounds on the Gasification of Coals with Hydrogen and Water Vapor’, Karlsruhe, Univ. report number NP-4770305; available from NTIS, PC A07/ME AOl, 1, 1982 ‘Janaf Thermochemical Tables’, 2nd Edn., US Government Printing Ofice, Washington DC, 1971 Turkdogan, E. T. in ‘Physical Chemistry of High Temperature Technology’, Academic Press, New York, 1980
FUEL,
1988,
Vol 67, November
1547
T. DebRoy,
A. K. Kulkarni”,
char using
C. I. Kim* and H. R. Jacobs*
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 7 6802, USA * Department of Mechanical Engineering, The Pennsylvania State University, University Park, PA 76802, USA (Received 9 November 7987; revised 19 February 7988)
Results of a laboratory scale experimental study on the desulphurization of coal char by steam-methane mixtures are presented. The char was prepared from a high sulphur, medium volatile, bituminous coal. The variables studied were temperature and reacting gas composition. The concentration of sulphur, specific surface area, pore size distribution and char reactivity were determined before and after the desulphurization reaction. A significant amount of sulphur was removed when coal char was reacted with 3-20 mol% steam in steam-methane mixtures in the temperature range 703323 K. Carbon gasification was effectively eliminated by this technique. Although small increases in both the specific surface area and pore volume occurred, the reactivity
of the char was practically
(Keywords: desulphurization;
unaffected
by the desulphurization
char; methane)
Desulphurization of carbonaceous fuels, such as coal or coke, prior to combustion, offers potential cost effective and technical advantages over flue gas desulphurization, coal liquefaction and gasification. At present only a few desulphurization methods are commercially feasible, all of which depend on the physical separation of coarse particles that contain sulphur-bearing minerals from coal. This paper presents results of a laboratory-scale study of a chemical process aimed at reducing sulphur in carbonaceous fuels to levels that will allow subsequent combustion offuels to be environmentally acceptable and cost effective. The process involves low temperature pyrolysis of coal using a mixture of steam and methane. The steam reacts with carbon to produce hydrogen and carbon monoxide according to the reaction C+H,O=H,+CO
(I)
The hydrogen then reacts with sulphur sulphide as follows:
to form hydrogen
S+H,=H$
(2)
Gasification of carbon by hydrogen introducing methane in the reactant reaction can be represented as
is suppressed by gas stream. This
2H,+C=CH,
(3)
The laboratory experiments reported here were conducted with the goal of determining optimum conditions that would allow sulphur removal by reacting with hydrogen, while suppressing any significant carbonhydrogen reaction. Sulphur may be removed prior to combustion, either by physical separation or by chemical processing. Physical methods (flotation, magnetic cleaning) are limited to removing pyritic sulphur only’. Chemical methods have a potential for the reduction of sulphur by removing both the organic and inorganic sulphur’. The 0016-2361/88/111542~653.00 0 1988 Butterworth & Co. (Publishers)
1542
FUEL,
1988,
reaction.
Ltd.
Vol 67, November
process discussed in the present paper is a chemical coal desulphurization method. Several different chemical methods of desulphurization have been studied before. Air oxidation of coal at 623 K to 723 K removed the pyritic sulphur by forming sulphur dioxide, however the calorific value and volatile matter content of the coal was reduced significantly during the proces?. Treatment by hydrogen, steam, nitrogen, oxygen and the pyrolysis gases themselves (in mixtures and separately) also helped desulphurization, and the sulphur removal rate depended on pressure and temperature up to certain limits4-‘. Sinha and Walker’ experimented with air, N,, CO and CO/steam mixtures. They found desulphurizing ability in decreasing order to be: air> steam/CO mixture>CO, N,. However, in all these cases of treatment, significant gasification and reduction in heating value of coal were associated with desulphurization. Oxydesulphurization of coal is one of the promising chemical techniques in which coal is treated with NO,, peroxygen compounds, or usually air in the presence of water at an elevated temperature (up to 793 K) and pressure (up to lo4 kPa). The steam-air treatment of coal has been claimed to remove up to 40 % of organic sulphur and more than 90 % of pyritic sulphur. Effects of steamto-air ratio, pressure, temperature and particle size on desulphurization have been reported in the literature9~‘0. In addition to the treatment by relatively common and inexpensive gases discussed above, more expensive, complex and rather exotic methods of chemical desulphurization have also been tried. They include the use of hot solutions of alkali containing dissolved oxygen aqueous hypochlorite oxidation and under pressure”, nitric acid oxidation’*, ozonization’ 3, use of electrophilic oxidant with subsequent treatment by a molten caustic14, hydroheating in the presence of Co-Mo/A120, catalyst’ 5 and microwave irradiation with caustic treatment16.
Coal-derived
char desulphurization
using
steam
THERMODYNAMICS
and methane:
M. Alam
OF THE C-H-O
et al.
SYSTEM
According to the proposed scheme of desulphurization, the hydrogen produced by the reaction of steam with hot carbon is prevented from reacting with carbon by the addition of methane in the inlet gas. Examination of the thermodynamics of the C-H-O system is important to determine the content of methane in the inlet gas mixture which is necessary to prevent hydrogenation of carbon. In addition to Equations (1) and (3), the following chemical reaction is to be considered to understand the C-O-H equilibrium. c+co,=2co
IO
Hz0
t
1
co 01
cop1 703
763
Temperature
823
(K
)
Figure 1 Equilibrium pressure of species as a function (PH>o= 13 kPa; P,-~*=87 kPa)
of temperature
Recently, desulphurization of very small amounts of finely divided coal samples by leaching with cupric ion solution at high temperatures has received some interest’ ‘. Electrolysis of acidic coal slurries using platinum or graphite electrodes has been found to cause pyrite to oxidize and dissolve and thus be removed”. Chemical degradation of coal using a variety of agents (such as potassium hydroxide, hydrochloric acid and N, Ndimethyl acetamide) before using ethanol and methanol to extract sulphur has been found to improve the desulphurization rate I9 . The amount of sulphur extracted in the form of H,S by hydropyrolysis was shown to be more effective for acid-leached char compared with untreated char ‘O. Also, certain typ es of bacteria are found to remove pyritic sulphur from coal’l. Kor22 conducted small scale desulphurization experiments on 10m4 to 2.5x 10m4 kg samples of dried coals and chars to study effects of various mixtures of hydrogen and methane at different pressures and temperatures. In his study, methane was seen to inhibit desulphurization, but it was also instrumental in suppressing the gasification rate; hydrogen had the opposite effect. The proportion of H, and CH,, and the process conditions, thus play important roles. Although the desulphurization of coal by steam and hydrogen can be accelerated by alkali metal salts, significant coal gasification occurs in the absence of methane23. The chemical desulphurization method discussed in this paper, which involves processing of coal using steam/methane mixtures, is a new contribution to coal desulphurization research. It is a potentially attractive technique because it utilizes process ingredients (steam and methane or natural gas) that are inexpensive and abundant, and it is not based upon processing coal at excessive pressure and temperature conditions.
(4)
Therefore, five gaseous species namely, H,O, CH,, CO, H, and CO, are involved in the system. To determine the equilibrium composition of these species, use was made of the three equilibrium constantsz4 for Equations (l), (3) and (4), and two mass balance equations based on hydrogen and oxygen. The five equations were solved numerically for the equilibrium pressure of the live species. The results of the calculation are presented in Figures I and 2. Figure I represents the variation of the equilibrium composition of the species as a function of temperature, for a given PHIo/PCHI = 0.15 in the inlet gas and at a total pressure of 101.3 kPa. The figure indicates that equilibrium moisture and methane content decrease while the equilibrium CO and H, contents increase with increasing temperature. Gasification of carbon by H, is prevented if methane pressure in the inlet gas lies above the equilibrium pressure for methane indicated in the figure. At higher temperatures, a relatively lower methane content is required to prevent gasification. Figure 2 represents the variation of the equilibrium composition of the species as a function of PH,o/PCH, in
7o-l-
60
t-
E z z
40
ci E
.z
-
30
C’-‘4
{:I,11 0.03
0.10
0.20 ‘Hz0
’
0.30
'CH4
Figure 2 Equilibrium pressure of species as a function ratio in the inlet gas (T = 823 K)
of PH&PCH,
FUEL, 1988, Vol 67, November
1543
Coal-derived
char desulphurization
using steam and methane: M. Alam et al.
Table 1 Ultimate and proximate analysis of coal and char on dry basis Coal
Char
Proximate analysis (%) Volatile Ash Fixed carbon
25.9 10.1 64.0
5.4 13.8 80.7
Ultimate analysis (%) Carbon Hydrogen Nitrogen Total sulphur oxygen
77.2 4.8 1.4 2.9 3.7
78.7 1.0 1.1 1.9 3.5
Sulphur forms (%) Pyritic Sulphatic Organic
1. Thermogravimetry, in which char samples were exposed isothermally to streams of steam/methane mixtures; 2. analysis of sulphur content in the samples; 3. specific surface area measurements; 4. pore size distribution in the samples; and 5. determination of reactivity in the char.
2.2 0.0 0.6 -.__
the inlet gas stream at 823 K and 10 1.3 kPa total pressure. The figure indicates that at 823 K, the equilibrium while hydrogen and methane contents decrease equilibrium moisture and CO contents increase with increasing PH,o/PCH, ratio in the inlet gas. Again gasification of carbon by Hz will not occur if the methane pressure in the inlet gas exceeds the pressure given by the equilibrium methane curve. The equilibrium curves in Figures 1 and 2 were used to select the steam-methane mixtures in the temperature range 703 to 823 K that would ensure prevention of Equation (3). THERMODYNAMICS
OF SULPHUR
REMOVAL
The hydrogen produced by the reaction of hot carbon with moisture may react with either pyrite (FeS,) or pyrrhotite (FeS) as follows: FeS,+H,=FeS+H,S FeS+H,=Fe+H,S
;:;
The free-energy change equations Equations (5) and (6) are”:
associated
In the temperature range 703-823 K the thermodynamic efficiency of utilization of hydrogen according to Equation (5) varies from 76 to 97% while that of Equation (6) varies from 0.005 to 0.023%. The thermodynamic efficiency of utilization is taken as the ratio of the moles of reactant gas consumed to the moles of reactant gas introduced in the reactor, multiplied by 100, and is calculated theoretically from the knowledge of the standard free energy change of the reaction at the reaction temperature. The efficiency values indicate that desulphurization of pyrrhotite (FeS) by hydrogen according to Equation (6) is extremely unfavourable, while desulphurization of pyrite to pyrrhotite is theoretically feasible. EXPERIMENTAL
Starting material Coal char was used as the starting material in most experiments because it eliminated the problem of handling devolatilization products in the reactor, thus
FUEL,
1988,
Vol 67, November
The samples were characterized both before and after the treatment with steam/methane mixtures. The experimental techniques are described below.
Thermogruvimetry The thermogravimetric set-up consisted of a Cahn microbalance (Model 1000,0.5 pg sensitivity) and a high temperature silicon carbide vertical tube furnace with a 0.025 m long equitemperature zone at the centre. The diameter of the reaction tube was 0.0475 m. The temperature was regulated by a Eurotherm controller to an accuracy of +2 K. About 0.5 g of char sample was placed in a platinum crucible and was suspended from one arm of the Cahn balance with a platinum wire. The system was first purged with argon flowing at a rate of 0.0041s-’ at 298 K and 101.3 kPa pressure for 1800 s. The furnace heating rate was kept constant for all the runs. After reaching the desired temperature a mixture of steam and methane was introduced in the reaction chamber.
with
AG” = 91086 - 138.9T kJ/kg mole AGo= 59957 - 3.2T kJ/kg mole
1544
allowing the study to be focused on the evaluation of the desulphurization scheme. A high sulphur, medium volatile, bituminous coal sample mined in Frenchville Township, Clearfield, Pennsylvania from the lower Kittanning seam was used. The coal was heated in air at 403 K for 24 h to avoid swelling during char formation. Subsequently, the coal was heated in N, at 1073 K for 3 h. The heating rate was kept at 0.05 K s-l to attain this temperature. The particle size of the char was between 270 and 325 mesh. Table 1 shows the composition of the coal and the char before steam/methane treatment. The experimental work consisted of:
Determination of gas composition In all experiments, an argon flow rate of 0.0041s-’ was maintained. Methane was passed at a rate of 0.0081s-’ through a column of water before being introduced into the reaction chamber. The water column was maintained at a constant temperature during the course of the experiment. The flow rate of moisture was determined from the knowledge of the initial and final weight of the water column and the bubbling time. The flow rate of moisture was varied by changing the temperature of the water column. Condensation of moisture in various parts of the set-up was prevented by using a heating tape.
Char characterization The char was characterized with respect to total sulphur content, specific surface area and pore size distribution. The total sulphurcontent in the samples was determined by a combustion iodometric procedure using the LECO 5 19 titrator. The specific surface areas of char samples were measured by the nitrogen adsorption technique using a Quantasorb Sorption System and the subsequent treatment of the data by the BET method. The pore size distribution in char samples was investigated by an Aminco mercury porosimeter using a mercury intrusion technique.
Coal-derived char desulphurization using steam and methane:
5.01 0
I
I
3600
7200
Time
10800
(s)
0.0
L 7200
Time
The data indicate that the gasification can be suppressed effectively with high methane fraction in the gas stream. Under none of the conditions in the present series of experiments did the weight loss exceed 1Y0of the initial sample weight. The significance of the data must be viewed in perspective with the data obtained by other workers. Block, Sharp and Darlage3 observed significant loss in weight when coal was treated with steam. In the first 15min of reaction, the weight loss was 14.4% at 723 K and 70.8% at 1173 K. Similarly, Kor22 demonstrated that the extent of gasification of carbon in coal decreases significantly with increasing methane content in H,-CH, mixtures in the temperature range 873-1073 K. The data presented in Figure 3 clearly demonstrate the importance of methane in steammethane mixtures in suppressing the gasification of carbon via Equation (3). Sulphur removal
Fire 3 Gasification of char as a function of time at 823 K and 101.3 kPa total pressure (PH,&PcH,=O.O~ (2); 0.09 (1); 0.13 (3); 0.20 (4))
I 3600
M. Alam et al.
10800
(s)
Figure4 Gasification ofchar as a function of time at PH,~‘PcH, =O. 14 and 101.3 kPa total pressure (T= 823 (3); 763 K (5); 703 K (6))
Char reactivity
Reactivity of the original and treated char samples was measured by subjecting the samples to pure CO2 at 101.3 kPa pressure and 1273 K for a period of 3600 s. The per cent weight loss sustained by the sample during this interval of time was taken as an indicator of the reactivity of the sample.
The char contained about 1.85 % total sulphur (Table 1). During preheating for about 1200 s in argon, when the sample lost about 6% of its initial weight some sulphur was also lost. During this period, the sulphur content dropped to 1.82%. Results of char desulphurization at fixed gas composition and temperature are shown in Figure 5 as a function of time. The figure indicates that initially desulphurization is rapid. Kor22 attributed the initial rapid desulphurization partly to the reduction of pyrite (FeS,) to pyrrhotite (FeS) and partly to the loss of less stable organic sulphur. The slow desulphurization rate during the later stages may be due to the reduction of pyrrhotite (FeS) to iron (Fe) and loss of organic sulphur. The figure also indicates that a significant amount of sulphur can be removed at PH20/PCHd = 0.2 and 823 K in 10800 s. The effects of relative gas composition and temperature on the sulphur removal of char and coal samples are shown in Figures 6 and 7 respectively. The sulphur removal rate increases with increasing steam fraction in the gas and the process temperature. Several coal samples were also processed in various steam-methane mixtures at 823 K for 10800 s. These results are also presented in Figure 6. A significant amount of sulphur was removed in
2.05 I
RESULTS AND DISCUSSION Gasification
Results of gasification (weight loss) are shown as a function of time at various gas compositions and temperatures in Figures 3 and 4. The data correspond to heating in steam-methane mixture at constant temperature and does not include the weight loss sustained by the char samples during the preheating stage in argon. During the preheating time of roughly 1200 s, the samples lost about 6% of the initial sample weight. This was due to the loss of the volatile matter and moisture content of the char. The figures indicate that the amount of gasification in the steam-methane mixtures almost reach a plateau after about 10800 s of reaction.
0.00 0
I 7200
I 3600
Time
(s.1
of char as a function Figure 5 Desulphurization PH,&‘cH,=~.~; PT= 101.3 kPa)
FUEL,
1988,
10800
of time (T= 823 K;
Vol 67, November
1545
Coal-derived
char desulphurization
of 82 mol % methane
10800 s with a mixture
and 18 mol %
steam. Structural
characterization
The specific surface area (SSA) and the porosity of the starting char were found to be 7500 m’/kg and 47.1% respectively. Table 2 gives the variation of the SSA and porosity of the char samples treated at various temperatures and gas compositions for 10800 s. The table indicates that the SSA increases moderately with increasing moisture content in methane and with temperature. This is in agreement with the fact that the extent of gasification and desulphurization increases with increasing moisture content in the gas phase and the temperature. The change in SSA is not dramatic, indicating that a complete breakdown of coal structure does not occur during the steam/methane treatment. The table also shows that porosity is relatively insensitive to temperature but it increases slightly with increasing moisture content in the reactant gas. In general, the comparison of treated and untreated samples shows that after the treatment pores are ‘opened up’, indicating an easier access for the hydrogen to the interior of the sample which is desirable for sulphur removal. Char reactivity Results of char reactivity as a function of relative gas composition and temperature are presented in Table 2. A small increase in the reactivity is seen as the moisture content of the mixture and temperature increase. The process
studied
here involves
pyrolysis
of char
*,*-l
using a mixture of superheated steam and methane. The basic, overall mechanism of sulphur removal and carbon gasification/suppression can be explained as follows. The superheated steam reacts with the carbon to produce hydrogen and carbon monoxide according to Equation (1). This reaction is usually accompanied by opening of the pore structure of coal, allowing an improved access of hydrogen to both the organic and inorganic sulphur in the fuel. The H, then reacts with sulphur to form H,S gas. Gasification of carbon by H, is suppressed according to Equation (3), by deliberately maintaining a high partial pressure of methane (CH,) in the reactant gas stream. If a commercial process is based on the concept presented in this paper, the H,S can be removed from the product gases by a suitable technique. Remaining product gases, rich in CH,, can be partially recycled into the reactor and also used to produce steam. It is important to note here that a significant amount of CH, can be recycled, H, can be generated in situ, and the energy required for the process can be derived from the reactor gas stream. It is a relatively simple process that uses commonly available ingredients and process conditions that are not excessive. However, verification of the concept cannot be complete unless larger bench scale tests are conducted under a wide range of conditions. Further research should also involve the effect of pressure on sulphur removal, types of sulphur influenced by the process, and a complete analytical model of the process. CONCLUSIONS Results of a laboratory scale experimental study are presented on a coal-char desulphurization process. The process uses mixtures of superheated steam and methane
0.0
0.24
0.16
0.00
0.00
&I. Alam et al.
using steam and methane:
643
I 703
PH*O ’ PCH, Figure 6
ff=823
Table 2
of specific surface area, porosity T=823K,
h,O/PCH,
Temperature
and reactivity
823
1
Desulphuriration of char as a function (Pu,cJPcu,=0.14; Pr= 101.3 kPa; time= 10800s)
of char as a function
of gas composition
of temperature
and temperature
Pu,0/P~u,=0.1,3,
time=108OOs
time=
10800 s
SSA
Porosity
Reactivity
T
SSA
Porosity
Reactivity
W/kg)
(%I
(%)
WI
W/kg)
(%I
(%)
703 763 823
8100 9300 9500
52.9 52.8
27.2 27.5 33.0
0.03 0.09 0.13
8900 9500 9500
52.8
28.0 31.0 33.0
0.20
101Ofl
55.3
30.8
1546
(K
Figure 7
Sulphur (wt %) in: 0, char; 0, coal versus gas composition K; Pr= 101.3 kPa; time= 10800s)
Variation
I 763
FUEL, 1988, Vol 67, November
Coal-derived
char desulphurization
of various compositions in the temperature range 703823 K. A well-characterized char was used as a starting material and measurements were made for sulphur removal, gasification, specific surface area, pore size distribution and reactivity as a function of temperature, gas composition and, in certain cases, time of exposure. From the results, it can be concluded that using commonly available steam and methane, a significant amount of sulphur can be removed without significant gasification and structural change of carbon.
10 11 12
ACKNOWLEDGEMENT This research was Pennsylvania Energy grant No. 85005.
partially supported by the Development Authority under
REFERENCES ‘Coal Desulfurization: Chemical and Physical Methods’ (Ed. T. D. Wheelock), Am. Chem. Sot. Symp. Ser., 1977, 64 ‘Coal Desulfurization Prior to Combustion’ (Ed. R. C. Elliot). Chem. Tech. Rev. No. 113/Pollu, Tech. Rev. No. 45, Noyes Data Corp., Park Ridge, NJ, USA, 1978 Block, S. S., Sharp, J. B. and Darlage, L. J. Fuel 1975,54, 113 Joshi, J. B., Shah, Y. T., Ruether, J. A. and Ritz, H. J. Fuel Proc. Technol. 1983, 7, 173 Maa, P. S., Lewis, C. R. and Hamrin, C. E., Jr. Fuel 1975,54,62 Joshi, J. B. and Shah, Y. T. Fuel 1981,60,612 Soo, S. L. and Cheung, K. C., ‘Steam Desulfurization of Coal’, Proc. 20th Intersociety Energy Conversion Engr. Cong., paper no. SAE P-164, August 1985 Sinha, R. K. and Walker, P. L., Jr. Fuel 1972,51, 329 Friedman, S., Lacount, R. B. and Warzinski, R. P. in ‘Coal Desulfurization: Chemical and Physical Methods’ (Ed. T. D. Wheelock), Am. Chem. Sot. Symp. Ser., 1977, pp. 164-172
17
18
19
20 21 22
23
24 25
using steam and methane:
M. Alam et al.
Tsai, S. C. Ind. Eng. Chem. Proc. Des. Deu. 1986, 25(l), 126 Chuang, K. C., Markuszewski, R. and Wheelock, T. D. Fuel Proc. Technol. 1983,7,43 Taylor, S. R., Miller, K. J., Hucko, R. E. and Deurbrouck, A. W., ‘New Methods for Coal Desulfurization’, Eight Int’l. Coal Prenaration Congress. USSR. 1979 Steinberg, M., Y&g, R. T., Horn, T. K. and Berlad, A. L. Fuel 1977,56,227 Aida, T., Squires, T. G. and Venier, C. G., US Patent 4 497 636, 1985 Inaba, A., Miki, K., Osafune, K., Saitoh, 1. et al. Nenryo Kyokaishi 1985,&l, 36 Bluhm, D. D., Marakuszewski, R., Richardson, C. K., Fanslow, G. E. et al., ‘British National Committee for Electroheat, Heating and Processing Conference’, NTIS, PC AOZ/MF AOl, CONF-860984-1, 1986 Chen, H. L. in ‘Study of Desulfurization by High Temperature Leaching’, Southern Illinois University report, NTIS, PC A02/MF AOl, DE85018428, 1985 Lalvani, S. B. and Ramaswami, K. in ‘Synfuels and Coal Energy Symposium’ (Ed. J. B. Dicks), ASME, New York, 1985, pp. 134 146 Chen, J. W., Muchmore,C. B. and Kent, A. C. in ‘Extraction and Desulfurization of Chemically Degraded Coal with Supercritical Southern Illinois University report, NTIS, PC Fluids’, A02/MF/AOl, DE85018431, 1985 Chou, M. M. and Loffredo, D. M. Fuel 1985,&l, 731 Chandra, D., Roy, P., Mishra, A. K., Chakrabarti, J. M. and Sengupta, B. Fuel 1979, 58, 549 Kor, G. J. W., ‘Desulfurization and Sulfidation of Coal and Coal Char’ in ‘Coal Desulfurization: Chemical and Physical Methods’ (Ed. T. D. Wheelock), Am. Chem. Sot. Symp. Ser., 1977, p. 221 Krauss, W. in ‘Catalytic Influence of Inorganic Coal Compounds on the Gasification of Coals with Hydrogen and Water Vapor’, Karlsruhe, Univ. report number NP-4770305; available from NTIS, PC A07/ME AOl, 1, 1982 ‘Janaf Thermochemical Tables’, 2nd Edn., US Government Printing Ofice, Washington DC, 1971 Turkdogan, E. T. in ‘Physical Chemistry of High Temperature Technology’, Academic Press, New York, 1980
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1988,
Vol 67, November
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