Energy &Fuels 1992, 6 , 16-21
16
Flash Pyrolysis of Coal-following Thermal Pretreatment at Low Temperature Kouichi Miura,* Kazuhiro Mae, Kiyoyasu Sakurada, and Kenji Hashimotof Research Laboratory of Carbonaceous Resources Conversion Technology and Department of Chemical Engineering, Kyoto University, Kyoto, 606 Japan Received July 1, 1991. Revised Manuscript Received October 10, 1991
A new and simple pyrolysis method is presented in which the coal preheated a t 150-200 "C for 1 h is pyrolyzed successively without exposure to air. This method was developed based on the experimental fact that the hydrogen bond of coal is broken thermally at 150-200 "C. When eight coals of different rank were pyrolyzed in an inert atmosphere using a Curie-point pyrolyzer, the conversion and the tar yield increased by 3-470 for several lower rank coals as compared with the nontreated coals. These increases were found to be brought about by the suppression of water forming cross-linking reactions, which were effected by the breakage of the hydrogen bond prior to the pyrolysis. A Japanese coal was also pyrolyzed by use of a free-fall pyrolyzer in an inert atmosphere, in which the coal preheated a t 200 OC while traveling a screw feeder was continuously fed to the pyrolyzer. The total volatile matter as well as the tar yield increased by 2-570 at the pyrolysis temperatures of 650-850 "C as compared with the pyrolysis yields of the nontreated coal. Thus the proposed method was found to be a simple and effective pyrolysis method. Introduction Nowadays coals are believed to be complex macromolecular networks containing many covalent and noncovalent cross-links based on accumulated works. For lower rank coals, the amount of noncovalent cross-links represented by the hydrogen bond is much larger than that of covalent cross-links. For example, the ratio of the hydrogen-bonded cross-links to covalent cross-links of Illinois No. 6 coal is reported to be more than 5.' Therefore, noncovalent cross-links are expected to play an important role in coal conversion technologies. Several attempts have been performed to improve the coal conversion efficiency by modifying the noncovalent cross-links of coal. The so called 0-alkylation2 completely breaks the hydrogen bond of coal by replacing H of -OH groups by paraffins. Pyrolysis of the 0-alkylated coal In brown enhanced the coal conversion ~ignificantly.~,~ coal Ca forms ionic cross-links between -COOH groups. Removing the Ca prior to liquefaction brought about the increase of liquefaction yield.5 Solvent swelling of coal is believed to be realized through the breakage of the hydrogen bond. We have recently reported that pyrolysis of the coal swollen by hydrogen donor solvents is an effective method tQ increase the total volatile matter as well as the tar yield.6 These methods all succeeded in improving the conversion efficiency by breaking the noncovalent cross-links of coal beforehand. However, it is questionable if these methods are directly utilized in practical coal conversion processes because of rather complicated procedures involved in the methods. More simplified methods are desirable in practical processes. In this paper we clarify that the hydrogen bond of coal is thermally broken or released a t 100-200 "C by use of the differential scanning calorimetry (DSC) and the in situ FTIR technique. Then we propose a new and simple pyrolysis method in which the coal preheated at 100-200 OC is successively pyrolyzed in a flash mode. We will show that this method is effective to increase the total volatile matter as well as the tar yield based on the detailed ex-
* To whom correspondence should be addressed. 'Department of Chemical Engineering. 0887-0624/92/2506-0016$03.00/0
perimenh utilizing a Curie-point pyrolyzer and a free-fall pyrolyzer.
Experimental Section Samples. Nine kinds of coal listed in Table I were used. The coal was ground into fine particles of 74-120 Fm in diameter and was dried in vacuo at 110 "C for 24 h before use. Characterization of Coal. To examine the change of noncovalent bonds in coal with heat treatment, the heat effecta were measured using a differential scanning calorimeter (Shimazu Co., DSC 50) during the heat up of coal at a heating rate of 5 OC/min. The change of hydrogen bondings in a coal, TC, with the increase in the temperature was also measured using an in situ Fourier transform infrared spectrometer (Shimazu Co., FTIR 4300). Flash Pyrolysis of Coal Using a Curie-Point Pyrolyzer. To clarify the effect of the low-temperature heating of coal on the product yields of the pyrolysis, eight coals were pyrolyzed using a Curie-point pyrolyzer (Japan Analytical Ind., JHP-2s) with and without the pretreatment. The experimental apparatus was described in detail in a previous papere6 About 2 mg of coal particles wrapped up by a ferromagnetic foil was placed in a reactor at room temperature. The pyrolysis without the pretreatment was performed by heating the coal particles in the ferromagnetic foil from room temperature to a pyrolysis temperature between 385 and 920 OC (Py-1). On the other hand, for the pyrolysis with the pretreatment, the coal particles in the ferromagnetic foil were kept at 70-150 "C for 1h, and then they were heated from the pretreatment temperature to a pyrolysis temperature (Py-2). In both experimentsthe heating rate effectad by an induction heating coil is 3000 OC/s. The tar produced was trapped completely by the quartz wool placed just below the foil. Gaseous products were all led to a gas chromatograph equipped with a Porapak Q column to analyze inorganic gases and hydrocarbon gases. The yields of char and tar were measured from the weight changes of the foil and the reactor. We performed the same experiments several times. The (1) Larsen, J. W.; Green, T.
K.; Kovac, J. J. Org. Chem.
1985, 50,
4729-4735.
(2) Liotta, R. Fuel 1979,58,724. Liotta, R.;Rose, K.; Hippo, E. J. Org. Chem. 1981, 46, 277-283. (3) Chu, C. J.; Cannon, S. A.; Hauge, R. H.; Margrave, J. L. Fuel 1986, 65, 174C-1746.
K.: Stock, L. M.: Zabranskv. R. F. Fuel 1989. 68. ( 5 ) Mochida, I.; Sakata, R.; Sakanishi, K. Fuel 1989, 68, 306-310. (6) Miura, K.; Mae, K.; Asaoka, S.; Yoshimura, T.; Hashimoto, K.
(4) Ofosu-Asante. 567-572.
Energy Fuels 1991, 5, 340-346.
0 1992 American Chemical Society
Flash Pyrolysis of Coal
Energy & Fuels, Vol.6,No.1, 1992 17 Table I. Analysis of Coals proximate analysis, wt % FC VM Ash C
coal (abbrev) Morwell (MW) 48.2 Jacobsranch (JR) 48.8 Highvale (HV) 52.6 Taiheiyo (TC) 43.2 48.6 Baiduri (BA) Coal Valley (CV) 50.7 Datong (DC) 59.0 Liddel (LD) 57.4 Beatrice (BR) 76.5 Taiheiyo (TC*)a 42.0 "TC* comes from a seam different from TC.
50.3 39.4 35.4 45.8 49.7 38.2 33.7 34.5 18.4 47.2
1.5 11.8 12.0 11.0 1.7 11.1 7.3 8.1 5.1 10.9
67.1 68.3 74.11 74.5 74.9 76.4 81.5 83.4 89.8 73.9
ultimate analysis, wt olo (dafl H
N
4.9 5.5 4.6 6.0 4.1 4.9 4.8 5.5 4.7 6.2
0.6 0.9 1.0 1.3 1.6 1.0 0.9 2.2 1.0 1.3
0 27.1 24.8 20.2 18.0
S 0.3 0.5 0.2 0.4 0.3 0.2 1.0 0.6 0.7
19.1 17.5 11.8 8.3 3.8
(18.6)b
This was used for the flash pyrolysis in a free-fall pyrolyzer. bSum of S and 0 values. I
,
I
O.O83"C/s
T~ -0.05
1
Ice trap
Char collector
6
50
Gas bag
Figure 1. Schematic diagram of a free-fall pyrolyzer.
standard deviation of each yield was obtained as follows: total volatile matter, tar,H20,CO, and C02are within f0.5 %, and the hydrocarbon gas is *0.05%. Flash Pyrolysis of Coal Using a Free-Fall Pyrolyzer. To confirm the validity of the proposed method, flash pyrolysis of a coal, TC*, was also performed by using a continuous free-fall pyrolyzer with and without pretreatment. Figure 1 shows the schematic diagram of the apparatus. The reactor was a straight tube of stainless steel of 3/4 in. outer diameter and 1.1m length. The electric furnace of 0.060 m diameter and 1 m total length was divided into five sections. The reactor was kept at a reaction temperature by controlling each section independently. The coal particles were fed via a screw feeder to the reactor at the rate of 0.10 kg/h with nitrogen gas of 1.5 L/min. The residence time of the coal particles in the feeder was about 1h. For the pyrolysis with the pretreatment (Py-2),the screw feeder was kept at 200 OC. On the other hand, for the pyrolysis without the pretreatment (Py-1), the screw feeder was kept at room temperature. The coal particles fed to the reactor were pyrolyzed under a high heating rate while they fall freely to the bottom. The coal residence time was estimated to be around 2 s. The solid product, char, was separated from the tar and gaseous products at the bottom of the reactor and collected in a char collector. The tar and the condensable gases were completely collected by ice cooled and dry ice-methanol cooled traps placed in series. The flow rates of the noncondensable gases were measured by a soap meter; then a part of the gases was collected in a gas bag before being discharged for the analysis of the composition. The yields of tar and char were calculated from the changes in weight of the char collector and the tar traps, respectively. In all experiments, the material balances were obtained within 98-103%. Characterization of Product. The noncondesable gaseous products sampled by the gas bag were analyzed by a gas chromatograph equipped with a Porapak Q column to determine the composition of inorganic gases (H2,CO, C02,H20)and hydrocarbon gases (CHI, C2H4,C2He,C&, C~HS, C4Hs,C4H10,C5and CBgaseous compounds, benzene, toluene, and xylene). Tar components were analyzed by a gas chromatograph equipped with an OV-17 or an OV-101 column to determine the amount of the main products (methanol, 2-methyl-1-propanol,benzene, toluene,
100 150 Temperature ('C 1
200
Figure 2. DSC profdes of the nine coals measured at the heating rate of 5 OC/min.
xylene, ethylbenzene, indene, tetralin, naphthalene, alkylnaphthalene, phenanthrene, and alkylphenanthrene). From the tar analysis the distillation curve of the fractions whose boiling points are below 300 O C was constructed. The fractions of the tar components whose boiling points are above 300 "C were represented by the total weight, since we could not identify the components.
Results and Discussion Change of Noncovalent Bonds by Low-Temperature Preheating. Figure 2 shows the DSC profiles measured in a nitrogen atmosphere at the rate of 5 OC/min for the nine coals listed in Table I. The weight change curves measured simultaneously showed that only MW and HV decomposed slightly at above 180 "C. No weight decrease due to the evaporation of water was detected for all the coals, because they were dried completely just before the experiments. Therefore, the endothermic rates except the high-temperature region of MW and HV are totally due to heat effects other than the evaporation of water and the breakage of covalent bonds in coal. For all the coals except BR, the endothermic rate started to increase abruptly a t a certain temperature, and the temperature tended to be lower with the decrease of coal rank. The endothermic rate of BR, the highest rank coal, did not show such an abrupt change, but increased gradually with increasing temperature. This is a typical DSC curve observed for solid crystals in the temperature region where the solid crystals show no transitions. Then the abrupt increase of the endothermic rate observed for the other coals must be caused by some transition in the coal. We are expecting that this transition is the breakage of the hydrogen bond in the coal. Using the FTIR technique, Painter et aL7pointed aut that the hydrogen bondv of a pyridine extract of a Polish (7) Painter, P.C.; Sobkowiak, M.; Youtcheff, J. Fuel 1987,66,973-978.
Miura et al.
18 Energy & Fuels, Vol. 6, No. 1, 1992 1
I I
'
1
'
I
"
1
1
"
' I
50 40
TC
I 25'C il0OT
fb
I
,
I
30 20 10 0
30
20 10
0
3500
3000
2600
Wave number (cm-')
Figure 3. Effect of heat treatment temperature on the FTIR spectrum of Taiheiyo coal. coal are significantly broken when it was heated to 130 "C. Solomon8also stated that the hydrogen bonds of a coal tar were found to be broken when it was heated to 250 "C. These may also happen for the coal when it is heated up to 100-200 "C. Actually, Lynch et al.9Joclarified that the hydrogen mobility, which represents the change of noncovalent bonds, of low-rank coals increases significantly even a t 100-200 "C by use of the 'H NMR technique. Then to examine if the hydrogen bond of the coal is broken upon heating, the FTIR spectrum of TC was measured by use of the in situ method as was done by Painter et al. Figure 3 shows the spectra measured at three temperatures of 25,100, and 170 "C. No difference was found between the spectra a t 25 and 100 "C (I). However, the spectrum a t 170 "C (11) showed slight changes a t high-wavenumber region. To show the changes more clearly, the difference spectrum, I1 - I, was calculated as was done by Painter et al. and Solomon. The difference spectrum shows the negative bands a t 3000 to 3400 cm-' and the positive bands a t above 3400 cm-l. I t was confirmed that this change is not due to water, because the spectrum a t 170 "C (11) returned exactly to the spectrum a t 25 " C (I) when the sample was cooled down to room temperature again in a dry air stream. The difference spectrum shows overlapping of many groups. Painter et al. assigned several absorption bands to OH groups as follows: free OH groups (3611 cm-'), OH-?r bonds (3516 cm-'), self-associated OH groups (3400 cm-l), OH-ether hydrogen bonds (3300 cm-'1, and OH-N hydrogen bonds (3100-2800 ~ m - 9 .In~ light of these assignments, the absorption bands deriving from self-associated OH groups (3400 cm-') and OH-ether hydrogen bonds (3300 cm-') were judged to be weakened, and the absorption bands representing free OH groups (3611 cm-') and OH- bonds (3516 cm-') were strengthened with increasing temperature. On the basis of this observation, we can conclude that the abrupt increase in the endothermic rate in Figure 2 for all the coals except for BR is due to the release or the breakage of the hydrogen bonds of the coals. Flash Pyrolysis of the Coal Preheated at Low Temperature. Above results showed that some of the (8) Solomon, P. R.In Coal Structure; Gorbaty, M. L., Ouchi, K., Eds.; Adv. Chem. Ser. 192; American Chemical Society: Washington, DC,1981; pp 95-112. (9) Lynch, L. J.; Sakurovs, R.; Webster, P. J. Fuel 1988,67,1036-1041. (10) Lynch, L.J.: Webster. P. J.: Sakurovs. R.: Barton. W. A. Fuel 1988,67, 579-583.
0.4
1
0.2
-
-4
Coal Type
Figure 4. Effect of preheating at 150 "C on the product yields during the flash pyrolysis of the eight coals. hydrogen bonds are broken and/or released when the eight coals except for BR were simply heated up to 150 "C or so. Then there is a possibility to change the pyrolysis behavior of coal by simply preheating the coal before the pyrolysis. Figure 4 compares the product yields obtained by pyrolyzing the eight coals at 764 O C by use of the Curie-point pyrolyzer with (Py-2) and without (Py-1) the heat pretreatment. The pretreatment was performed a t 150 "C which is judged to be high enough to break hydrogen bonds from Figure 2. All yields were represented on the basis of 100 kg of dry and ash free (daf) coal. The total volatile matter increased when the coal was preheated for all the coals except for CV and DC, and it increased by 3-4% for TC and BA. The tar yield also increased by 3% or so for MW, BA, and TC. These results show that the heat pretreatment, in other words the breakage and/or the release of the hydrogen bond of coal, is an effective pretreatment method to increase both the total volatile matter and the tar yield during the pyrolysis especially for lower rank coals which will contain many hydrogen bonds. Then the mechanism by which the total volatile matter and the tar yield increase was examined by referring to other products. The yields of hydrocarbon gases and inorganic gases (CO, C02, H2) changed little by the preheating. On the contrary the H20 yield decreased by 1-3% for MW, BA, and TC. This means that the increase in the tar yield was compensated by the decrease in the H20yield as well as the char yield for these coals. Since H 2 0 is mainly produced by the cross-linking of OH groups, the release and/or breakage of the hydrogen bonds through the heat treatment was judged to have suppressed the cross-linking reaction. Suppression of this cross-linking reaction is expected to contribute to convert the coal fragments which would be stabilized as char to tar.
Flash Pyrolysis of Coal
Energy & Fuels, Vol. 6, No. 1, 1992 19 60tTotal volatile matter
3
20 10
-
5w 0
8 i?
0
, --L
4 -
00.6
0.21 0.01
n n n I I
I I
I I
25
70
100
. n-
n1
I I
385
150
Preheating temperature ('C)
485 590 670 764 Pyrolysis temperature ('C)
Figure 5. Effect of the preheating temperature on the product yields during the flash pyrolysis of TC coal.
Figure 6. Difference of the flash pyrolysis characteristic between nontreated TC coal and preheated TC coal.
Effect of the Preheating Temperature on the Product Yield of TC Coal. Preheating of coal was effective to increase both the coal conversion and the tar yield especially for TC and BA as stated above. Then the effect of the preheating on the pyrolysis yield of TC was examined in mme detail. Figure 5 shows the effect of the preheating temperature on the product distribution during the flash pyrolysis of TC a t 764 "C. The preheating temperature was chosen to be 70,100, and 150 "C. The result obtained without preheating (this corresponds to the preheating at 25 "C)is also shown for comparison in Figure 5. The product yields are exactly same a t the preheating temperatures of 25 "C (no preheating), 70 and 100 OC. Preheating a t around 150 "C is necessary for the preheating to affect the pyrolysis yields. This result coincides well with the change of the hydrogen bond upon heat treatment. Heating up to 170 "C was necessary to detect the breakage of the hydrogen bond by the FTlR technique as shown in Figure 3, and the abrupt increase in the endothermic rate showing the breakage of the hydrogen bond occurred a t around 150 "C for TC as shown in Figure 2. This coincidence again shows that the increase in the total volatile matter and the tar yield brought about by the heat treatment was due to the breakage and/or the release of the hydrogen bond prior to the pyrolysis. Change of the Pyrolysis Mechanism by the Heat Treatment. To examine how the breakage and/or the release of the hydrogen bond prior to the pyrolysis affect the progress of the pyrolysis, the TC coal preheated at 150 "C was pyrolyzed a t different temperatures. Figure 6 compares the pyrolysis yields obtained with and without the pretreatment. Both the heat-treated and nontreated coals started to decompose at around 400 "C. The tar yield of the heat-treated coal is about 3% larger than that of the nontreated coal at all temperatures above 600 "C. The
increase in the tar yield is well compensated by the decrease in the yields of char and H 2 0 a t all the temperatures. This indicates that the preheating of the coal affected the pyrolysis reaction at 500-600 "C exclusively. At these temperatures cross-linking reactions proceed by consuming functional groups such as -OH, XOOH, -CzH5, and so on. Therefore, the preheating of coal is judged to have suppressed the cross-linking reaction forming HzO from -OH groups at these temperatures. This contributed to increase the tar yield, although we cannot clarify the mechanism now. Pyrolysis of the Heat-Treated Coal in a Free-Fall Pyrolyzer. The preceding experiments were all performed by use of the Curie-point pyrolyzer in which the secondary gas-phase reaction was suppressed as stated earlier. The experimental technique is suitable to examine the primary decomposition exclusively. In practical operations, however, the secondary gas-phase reaction inevitably occurs. Then pyrolysis of the preheated TC* was performed by use of the free-fall pyrolyzer shown in Figure 1to examine the effect of the secondary gas-phase reaction. Preheating was performed a t 200 "C while the coal particles traveled the screw feeder in about 1 h. Figure 7 compares the pyrolysis yields obtained with and without the heat treatment. The total volatile matter and the tar yield of the preheated coal were larger than those of the nontreated coal, indicating that the preheating of coal is effective in practical operations also. However, product composition in the free-fall reactor is different from that obtained by use of the Curie-point pyrolyzer. The hydrocarbon gas yield of the heat treated coal was larger than that of the nontreated coal at all the pyrolysis temperatures. The HzO yield of the heat treated coal was smaller a t 650 OC,but was almost same at 750 and 850 O C as that of the nontreated coal. The inorganic gases (CO COP H,) yield of the heat-treated coal was also larger
+
+
Miura et al.
20 Energy & Fuels, Vol. 6,No. 1, 1992 60 40 20
0
30
20
10
0
I
0
I
100 200 300 0
I
I
I
I
l
l
100 200 300 0 100 200 300
Boiling temperature('C1
Figure 8. Comparison of the distillation cwes construded from the tar analysis between the preheated and nontreated TC*coals. Tar amount is accumulated from the lower boiling temperature component. (A) benzene, (B)toluene, (C) xylene, (D)indene, (E) naphthalene, (F)1-methylnaphthalene, (G)Cz-naphthalene.
75Q 850 Pyrolysis temperature ('C)
650
Figure 7. Effect of preheating at 200 "C on the product yield during the flash pyrolysis of TC* coal in a free-fall pyrolyzer. than that of the nontreated coal a t higher temperatures. This increase was due to the increase in the amount of CO. These results are different from those obtained by use of the Curie-point pyrolyzer (see Figure 6). These differences were judged to be brought about by the secondary gasphase reaction, because the increase in the yields of the HCG and CO with the increase in pyrolysis temperature is compensated partly by the decrease in the tar yield for both the heat-treated and the nontreated coals. Since the amount of tar produced by the primary decomposition of the heat-treated coal is larger than that of the nontreated coal as was clarified by the Curie-point experiments, the secondary decomposition of the tar will be more significant for the heat-treated coal, and more hydrocarbon gases and CO will surely be produced from the heat-treated coal especially at high temperatures. H20 will also be produced at high temperatures by the secondary gas-phase reaction of the tar produced by the primary decomposition, although we cannot clarify the mechanism. This will be the reason why the H20yield of the heat-treated coal is almost the same as that of the nontreated coal a t the pyrolysis temperatures of 750 and 850 "C. Qualityof the Tar Produced from the Heat-Treated Coal. The above discussions clarified that the heat treatment of coal prior to the pyrolysis increases the total volatile matter by mainly increasing the tar yield in both the Curie-point pyrolyzer and the free-fall pyrolyzer. In practical operations coal will be surely dried prior to the pyrolysis. Then the heat pretreatment is just an extension of the drying and is easily performed during coal particles travel the feeder as was done in this work. However, there is one concern which should be considered to judge if the heat treatment is really valid in practical operations. The increase in the tar yield was brought about because larger coal fragments which would be stabilized as char were converted to tar through the heat
treatment. This means that the fraction of higher molecular substances increased in the tar produced from the heat-treated coal. If the molecular weights of the substances are too large, the substances would not be utilized practically. Then the composition of the tar produced from the heat-treated TC* coal were compared with that produced from the nontreated TC* coal in Figure 8. The curves in this figure are similar to distillation curves. That is, the amounts of tar accumulated from smaller molecules are shown against the boiling temperatures, Tb. Exact distillation curves were constructed only up to Tb = 300 "C, and the fractions of Tb > 300 "C were equated to the difference between the total amounts and the fractions of Tb 1300 "c. At the pyrolysis temperatures of 650 and 750 "C the heat treatment increased exclusively the higher boiling temperature fractions (Tb> 300 "C). So, pyrolysis of the heat-treated coal a t these temperatures may not be so attractive if the high-boiling fractions have little value. At 850 "C, however, the fractions corresponding to the derivatives of one or two aromatic rings increased significantly through the heat treatment. These fractions are of great use in chemical industries. This shows that the pyrolysis of the heat-treated coal a t higer temperatures is really an attractive and simple method to increase the yield of valuable chemicals.
Conclusion Hydrogen bonds of lower rank coals were found to be broken thermally at as low as 100-200 "C by the DSC and FTIR measurements. Then the flash pyrolysis of the coal preheated at 156200 OC was performed to examine if the thermal breakage of the hydrogen bond prior to the pyrolysis contributes to increase the total volatile matter. From the basic experimenta using a Curie-point pyrolyzer, it was found that this method was effective for lower rank coals which had many noncovalent bonds. For Taiheiyo and Baiduri coals, the total volatile matter and the tar yield increased by 3 4 % as compared with those of nontreated coals. The increase in the total volatile matter was compensated by the decrease in the H20 yield in part. This indicates that the breakage of noncovalent bonds by the heat treatment suppresses the cross-linking reactions forming H20. The flash pyrolysis of Taiheiyo coal was also performed using a continuous free-fall pyrolyzer to examine the effect
Energy & Fuels 1992,6, 21-27 of secondary gas-phase reaction on this method. The coal was preheated a t 200 "C for 1h while traveling through a screw feeder. The total volatile matter and the tar yield increased at d temperatures between 650 and 850 "C. At 850 "C the yield of valuable chemicals such as naphthalene as well as the yield of total tar significantly increased. Thus it was clarified that the flash pyrolysis of coal pre-
21
heated at low temperature is a simple and effective method to increase the total volatile matter and the tar yield.
Acknowledgment. This work was financially supported by the Ministry of Education, Culture and Science of Japan through the Grant-in-Aid on Priority-Area Research (Grant No. 63603014, 01603012, and 02203112).
Development of Zinc Ferrite Sorbents for Desulfurization of Hot Coal Gas in a Fluid-Bed Reactor R. Gupta* and S. K. Gangwal Research Triangle Institute, P.O. Box 12194, Research Triangle Park, North Carolina 27709
S. C . Jain Morgantown Energy Technology Center, U.S. Department of Energy, Morgantown, West Virginia 26507 Received July 15, 1991. Revised Manuscript Received September 30, 1991
Advanced integrated gasification combined cycle (IGCC) power generation systems require the development of high-temperature, regenerable, desulfurization sorbents capable of removing hydrogen sulfide from coal gasifier gas to very low levels. Fluidized-bed desulfurization reactors offer significant potential advantages in IGCC systems compared to fixed- and moving-bed reactors because of their ability to control the highly exothermic regeneration involved. However, a durable, attrition-resistant sorbent in the 100- to 300-pm-size range is needed. Thus, the objective of this investigation was to identify and demonstrate methods for enhancing the long-term chemical reactivity and mechanical strength of zinc ferrite, a leading regenerable sorbent, for fluidized-bed applications. A bench-scale, high-temperature, high-pressure, fluidized-bed reactor (3-in. i.d.) system capable of operating up to 350 psig and 850 "C was designed and built. A total of 175 sulfidation-regeneration cycles were carried out using KRW-type coal gas with various zinc ferrite formulations. A number of sorbent manufacturing techniques, including spray drying, impregnation, crushing and screening, and granulation, were investigated. While fluidizable sorbents prepared by crushing the durable pellets and screening had acceptable sulfur capacity, they underwent excessive attrition during multicycle testing. The sorbent formulations prepared by a proprietary granulation technique were found to have excellent attrition resistance and chemical reactivity during multicycle testing. However, application of zinc ferrite was limited to a maximum temperature of about 550 "C, above which excessive sorbent weakening was observed possibly due to chemical transformations.
Introduction An integrated gasification combined cycle (IGCC) power system employing hot-gas desulfurization of the coal-derived gas is the most promising advanced technology for producing electric power from coal. Compared to conventional pulverized coal combustion power plants,' the IGCC system has the potential of greater reductions in pollutant emissions and improvements in electrical generation efficiencies. Recent developments in hot-gas desulfurization technologies have focused on regenerable, solid mixed metal oxide sorbents, such as zinc ferrite, to remove reduced sulfur species (H2S,COS,etc.) from coal gas.2 The sulfided sorbents are reused after regeneration with air. These sflidation-regeneration steps form a cycle. A commercially viable sorbent should withstand numerous (1) Williams, M. C.; Bedick, R. C. 'Gas Stream Cleanup"; Technology Status Report, DOE/METC-89/0263, 1988. ( 2 ) Jalan, V. High-Temperature Desulfurization of Coal-Gases. Acid and Sour Gas Treating Processes; Gulf Publishing Co.: Houston, TX, 1985; pp 466-491.
(at least 100) cycles before deactivating to an unacceptable level of reactivity, e.g., sorbent sulfur absorption capacity dropping below 10-20% of its theoretical value. Zinc ferrite, ZnFe204,originally developed at the U.S. Department of Energy/Morgantown Energy Technology Center (DOE/METC) by Grindley and Steinfield,3 contains an equimolar mixture of zinc oxide and iron oxide calcined at 800-850 "C with a suitable binder. It combines the high desulfurization efficiency of zinc oxide and the high capacity and reactivity of iron oxide. It can be easily regenerated using air/diluent mixtures and has consistently demonstrated hydrogen sulfide (H3) removals to less than 10 parts per million (ppm) by volume over multiple cycle~.~3~ The chemistry of H2Sremoval by zinc ferrite and regeneration of sulfided sorbent is discussed e l s e ~ h e r e . ~ , ~ The theoretical maximum sulfur absorption capacity of zinc ferrite is close to 40 g of sulfur per 100 g of sorbent (3) Grindley, T.; Steinfeld, G. "Development and Testing of Regenerable Hot Coal-Gas Desulfurization Sorbents", DOE/MC/16545-1125, 1981.
0887-062419212506-0021$03.00/ 0 0 1992 American Chemical Society
