Energy & Fuels 1991,5,34-40
34
general trends in these figures are similar to that observed for the coke samples coagglomerated with Ca(OH)2(Figure 3), except that the coke/NaHC03 agglomerates had relatively lower ash content and lower calorificvalues at similar meta1:sulfur ratios. This trend is valid regardless of the type of coke sample used.
Conclusions 1. Dry agglomeration method, as used here, is more effective than the wet agglomeration method for the production of the coke-sorbent agglomerates in terms of the amount of bitumen binder required and ease of handling. 2. With Ca(OH), sorbent, up to 90% sulfur capture was observed a t a Ca/S mole ratio of 2.0 or higher and up to 84% at a Na/S mole ratio of 2.0 with NaHC03 sorbent at a combustion temperature of 800 OC. 3. Temperature effects on the sulfur capture efficiencies of the cokes coagglomerated with the sorbents suggest that the agglomerates with lower M/S ratio may be burnt at lower temperatures (C700 "C) without causing serious environmental damage. However, at this temperature, the rate of combustion (combustibility) of the agglomerates
may be reduced. This is being investigated. 4. Compared with Alberta thermal coals the agglomerates showed comparable calorific values despite ash contents as high as 40 wt 5%. 5. The combustion data show that incorporation of sufficient amount of sorbent material (Ca(OH), or NaHCOB)in Syncrude and Suncor coke gives coke/sorbent agglomerate that may be burnt for process energy generation with significantly lower SO2emissions compared with that of the original coke. The major disadvantage of the above desulfurization process is the relatively larger amounts of coke ash to be handled. However, the presence of the sorbents may facilitate the extraction of valuable metals (vanadium and nickel) from the product ash..
Acknowledgment. Financial support from Alberta Oil Sands Technology and Research Authority (AOSTRA) and the technical assistance provided by Mr. Bashir Mohammedhai (University of Alberta) are gratefully acknowledged. Registry No. Ca(OH)2,1305-62-0; NaHC03,144-55-8;SOz, 7446-09-5.
Petroleum Coke Utilization: Effect of Coagglomeration with Novel Silica-Enhanced Sulfur Sorbents J. U.Otaigbe and N. 0. Egiebor* Department of Mining, Metallurgical and Petroleum Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received March 12, 1990. Revised Manuscript Received September 1, 1990
Novel silica-enhanced hydrated lime sulfur sorbents were prepared by pressure hydrating lime and silica mixtures a t a Ca(OH)2/Si02mole ratio of 1.0, and tested for their sulfur capture efficiency when incorporated into Syncrude and Suncor coke. These activated sorbents were observed to be more active than the conventional sorbent [Ca(OH)2]in the removal of sulfur dioxide emission during combustion a t 800 "C and a t equivalent Ca/S ratio. Coagglomeration of the coke with the silicaenhanced sorbents resulted in higher ash content and higher calorific values relative to the coke/ Ca(OH), agglomerates. The sulfur capture efficiencies of the activated sorbents in the coke were found to depend mainly upon meta1:sulfur (M/S) ratio, combustion temperature, type of sorbent, and coke. These results will provide a basis for the production of smokeless solid fuels from the coke and sorbent materials via carbonization.
Introduction The high sulfur and ash content of Athabasca petroleum coke precludes their possible utilization as a solid fuel for industrial steam raising and process energy generation. Various processes14 for the control of sulfur dioxide emissions during petroleum coke combustion have been reported in the literature. These desulfurization processes are not considered to be sufficiently economically attractive presently to allow the use of the coke as a boiler fuel on a commercial scale. In a previous c o m m ~ n i c a t i o nwe ~ ~reported ~ the possibility of reducing SO2emissions during the combustion of Athabasca petroleum coke by coagglomerating the coke with conventional sorbents (Ca(OHI2and NaHC03) prior *Author to whom correspondence should be addressed.
to combustion. It was observed that this approach required relatively higher Ca/S ratios to produce acceptable reductions in SO2. Various sorbent alternatives to the (1) Botta, W. V.; Gehri, D. C. Paper presented at the 167th American Chemical Society National Meeting, California, April 1974. (2) Lee, D. C.; Georgakis, C. AIChE. J. 1981,27(3), 472-481. (3) Choy, E. T.; Meiaen, A. Paper presented at the Canadian Natural Gas Processing Association, Calgary, Alberta, November 15, 1977. (4) Parmar, B. S.; Tollefson, E. L. Can. J. Chem. Eng. 1977, 55, 185-191. (5) George, Z. M., Schneider, L., Hall, E. S.; Tollefson, E. L. Can. J. Chem. Eng. 1982,60,418-424. (6) Hall, E. S.; Tollefson, E. L. Proc. 35th Can. Chem. Eng. Conf., C. S.Ch. E., Calgary, Oct. 1985 1985,296-301. (7) Otaigbe, J. U.; Egiebor, N. 0. Petroleum Coke Utilization Paper presented at the 14th Annual AOSTRA/Industry/University,Technical Seminar, Banff, Alberta, October 22,1989. (8) Otaigbe, J. U.; Egiebor, N. 0.Submitted for publication in Energy Fuels.
0887-0624/91/ 2505-0034$02.50/0 0 1991 American Chemical Society
Petroleum Coke Utilization
above sorbents have been wed with success, to a lesser or greater extent, and information on the sorbents is well d o ~ u m e n t e d .More ~ recently, Jozewicz et a1.I0 reported that the sulfur capture activity of conventional sorbent (hydrated lime) may be enhanced by pressure hydrating it with silica before exposure to SO2in a dry injection bed. This paper is concerned with the development of costeffective combined coke-sorbent pellets or briquettes to give enhanced SOz emission control, at lower Ca/S ratios, during combustion. It focuses mainly on the pressure hydration of pure silica (Syncrude tailings sand) and coal fly-ash with Ca(OH)z to produce silica-enhanced sulfur sorbents, and their subsequent incorporation in the coke matrix to produce cokeorbent agglomerates with superior SO2emission control during combustion at relatively lower Ca/S mole ratios. The silica-enhanced sorbents were prepared by pressure hydrating Ca(OH), with Syncrude tailings sand(99.6 wt % Si02) and/or coal fly-ash (64 wt % Si02),at a Ca(OH)z/SiOzmole ratio of 1.0, in a pressure reactor and coagglomerated with the coke by using a dry agglomeration method?18J1 The sulfur capture activities, ash content, and calorific values of the agglomerates were determined and compared with some base-line data for coke-conventional sorbent agglomerates reported previous1y,8,11 with the aim of establishing optimum process conditions for SOz removal from the coke during combustion. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to study the morphology and/or composition of the sorbents. The ultimate aim of this project is to determine the feasibility of the production of smokeless solid fuels with high calorific value and low SO2 emission for domestic and industrial energy generation. Experimental Section Materials. Suncor delayed and Syncrude fluid coke samples were obtained from Suncor Limited and Syncrude Limited, respectively. Reagent grade Ca(OH)2was used. The tailings sand (Lane Mountain silica sand, 99.6 wt % Si02)and coal fly-ash (64 wt % Si02)were obtained from Lane Mountain Silica Co., WA, and TransAlta Utilities, Alberta, respectively. The coke and sorbent materials were used in the form of powder, with particle sizes in the range 180-212 pm. Cold lake bitumen was used as binder. Preparation of Silica-Enhanced Sorbents. The silica-enhanced sorbents were prepared by batch pressure hydration of the siliceous material (Lane Mountain silica sand or coal fly-ash) and Ca(OH)2and drying to constant weight in a vacuum oven maintained at 85 O C . A stainless steel pressure reador of 3WmL capacity was used as the hydrator. Desired amounts of the dry siliceous material and Ca(OH)2were placed into the cold pressure hydrator, and the desired amount of water was poured into a 300-mL high-pressure sampling cylinder. The injection valve in the line connecting the sampling cylinder with the hydrator was closed, and the sampling cylinder was pressurized to 100 psi higher than the vapor pressure of water at the experimental temperature (150O C ) . The pressure hydrator was then heated electrically, controlled by a thermocouple inside the reactor. When the temperature reached 150 OC, the injection valve was opened and water was instantaneously injected from the sampling cylinder into the hydrator. Throughout the hydration period, the contents of the hydrator were vigorously stirred. After the water injection was completed (monitored by the pressure gauge), the injection
valve was closed and the sampling cylinder disconnected. The water to solids ratio in the hydrator was maintained at 15:l (9) Slack, A. V. Sulfur Dioxide Removal from Waste Gases, 2nd ed.; Noyes Data Corp: London, 1975. (IO) Jozewin, W.; Chang, J. C. S.; Sedman, C. B.; Bma, T. G. JAPCA
1988,38,1027-1034.
(11)Otaigbe, J. U.; Egiebor, N. 0. Petroleum Coke Utilization Technical Report to AOSTRA, January, 1990.
Energy & Fuels, Vol. 5, No. 1, 1991 35
throughout. On expiration of the hydration time, the hydrator was debressurized via steam release through the injection valve. The powdery sample was then filtered by means of a Buchner funnel and dried to constant weight in a vacuum oven at 85 O C . The morphology and composition of the sorbents were characterized by using scanning electron microscopy (SEM)coupled with an energy dispersion system and X-ray diffractometer (XRD). Preparation of Agglomerates. The coke and a known amount of the sorbent material were dry-mixed with a wrist shaker (Spex Mixer/mill) until a homogeneous mixture was obtained. A minimal amount of bitumen binder (
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Figure 7. Variation of ash content and calorific values with Ca/S ratio for Suncor coke with silica-enhanced sorbent agglomerate
Figure 8. Variation of ash content and calorific valuea with Ca/S ratio for Suncor coke with silica-enhanced sorbent agglomerate [F-Ash-CSH].
Comparison of the Properties of Silica-Enhanced Sorbents and Conventional Sorbents. The 90S capture activities and ash content of the present silica-en-
Table 11. Combustion Property Data for the Coke and Sorbents Used at the Metallsulfur (M/S) Mole Ration Shown M/S calorific combustn Z ash value: temp, mole 3'% S agglomerate ratio capture content Btu lb-' OC 9000 800 3.0 89.0 38.1 Syncrude/Ca(OH), 1.0 11 613 3.0 84.6 31.6 10990 800 Suncor/Ca(OH), 1.0 13OB0 8535 800 3.0 91.6 27.3 Syncrude/NaHCOS 2.0b 9 648 9838 800 3.0 85.1 23.6 Suncor/NaHCOS 2.0b 11093 40.6 9493 800 Syncrude/L-MT1.4 85.5 CSH' 1.0 10081 38.8 9775 800 Svncrude/ F-Ash1.1 76.6 -CSHd ' 34.9 11042 800 Suncor/L-MT-CSH 1.4 73.3 1.0 11 584 33.3 11124 800 Suncor/F-Ash-CSH 1.1 62.3
[L-MT-CSH].
hanced sorbent and the coke/conventional sorbent8J1agglomerates (at a M/Satom ratio of 1.0) are compared in Figure 11. This plot reveals that the NaHCOs sorbent was the most active sorbent closely followed by the silica-enhanced sorbents. The pure Ca(OH)2sorbent was the least active among the sorbents studied. This observed limitation of the sulfur capture activity of Ca(OH), sorbent has been ascribed to the expansion of the crystal structure from calcium oxide to calcium sulfate which leads to the closing of the fine p~res.~~J"'@ Inasmuch as the sizes of the unit cells of the calcium silica& are coneiderably larger than that of calcium oxide, it is reasonable to speculate that the crystal size expansion upon reaction with SO2 is smaller with the silicates than with calcium oxide. Based on the same chemical reactivity and pore structure, the silicates should be more capable of absorption of SO2. It is important to note that the coke/conventional sorbent agglomerates, at the above combustion temperatures, required relatively higher Ca/S ratios for sulfur capture activities (not reported here but may be found elsewhere8J1) comparable to the above values. The ash content of the coke coagglomerated with silica-enhanced sorbent was observed to be higher than that of the coke/conventional sorbent agglomerates. The (16)Borgwardt, R. H.; Harvey, R.D. Enuron. Sci. Technol. 1972,6, 350-360. (17)Wen, C. Y.;Ishida, M. Enuiron. Sci. Technol. 1973, 7,703-708. (18) Hartman, M.;Coughlin, R. W. AIChE J. 1976,22,490-498. (19)Ramachandran, P.A.; Smith, J. M. AZChE J. 1977,23,353-361.
'To convert to MJ kg-' multiply by 2.3 X lo-$. *2.0Na/S ratio = 1.0 Ca/S ratio. CL-MT-CSH = calcium silicate hydrate prepared from Ca(OH), and Lane Mountain silica sand. dF-Ash-CSH = calcium silicate hydrate prepared from Ca(OH), and coal fly-ash.
combustion and SO2analysis data for all the materials, at their respective maximum M/S mole ratios studied, are compared in Table 11. An interesting feature from this table is that the coke coagglomerated with the silica-enhanced sorbents had higher calorific values in spite of their generally higher ash content. This unexpected observation may be due to the more favorable thermodynamic conditions for the silica-enhanced sorbent-S02-02 reaction reported by Yang et al.14 Using X-ray and IR analyses, they observed that the sulfate and silica were chemically bonded in the product of the above reaction. This bond
Otaigbe and Egiebor
40 Energy & Fuels, Vol. 5, No. 1, 1991 100
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formation will increase the free energy of formation of the reaction product (hence more negative) which leads to higher heat of combustion at constant entropy and temperature. Further work in this area is in progress. The ash generated by these agglomerates mainly contain hydrated metal silicates and calcium sulfate. The former is the main constituent of cement and may therefore find some use in the building industry. Also, hydrated calcium sulfate (CaS04*2H20)has been shown to be suitable for making gypsum board by Schneider and Georgemand may also be useful in the cement industry.
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Figure 10. Effect of combustion temperature on sulfur capture activity of F-Ash-CSHsorbent in the coke samples (Ca/S = 1.1). Suncor/F-Ash-CSH
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Conclusions It has been shown that Syncrude and Suncor coke coagglomerated with the silica-enhanced sorbents [L-MTCSH and F-Ash-CSH], at Ca/S ratios of of 1.1-1.4, may be burnt at temperatures 1800 "C for process energy generation without causing serious environmentaldamage. The agglomerates showed comparable calorific values to Western Canadian thermal coals even with ash content of 40%. Compared with calcium hydroxide, the silica-enhanced sorbents were found to be more active in terms of control of SO2emissions during the combustion of Athabasca petroleum coke combustion. Pressure hydration of lime/coal fly-ash mixtures leads to the formation of complex calcium aluminum silicate hydroxide [Ca3A12(SiO4)(OH)J.
Figure 11. Comparison of % sulfur capture and ash content for all the coke/sorbent agglomerates at 800 "C [M/S atom ratio = 1.01.
(20) Schneider, L. G.;George, S. M. Proceedings of "ExtractionMetallurgy '81";Institute of Mining and Metallurgy: London, 1981; pp 413-420.
Registry No. Si02, 7631-86-9; Ca(OH)2, 1305-62-0; SOZ, 7446-09-5.
SuncorlCa(0H)t
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The results suggest t b feasibility of the utilization of petroleum coke with high sulfur content for process energy generation with limited sulfur dioxide emission. Acknowledgment. Financial support from Alberta Oil Sands Technology and Research Authority (AOSTRA) is gratefully acknowledged.
