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Energy & Fuels 1991,5,3Q-34
Athabasca Petroleum Coke Utilization: Coagglomeration with Sulfur Sorbents for Control of Sulfur Dioxide Emissions during Combustion J. U. Otaigbe and N. 0. Egiebor* Department of Mining, Metallurgical and Petroleum Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received February 16, 1990. Revised Manuscript Received September 1,1990
The high sulfur content (6-9 wt 5%) of Athabasca petroleum coke, produced as a byproduct in the fluid and delayed coking of Athabasca oil sands bitumen, precludes their possible utilization as a fuel. The effect of coagglomerating the coke with small amounts of conventional sulfur sorbents [Ca(OH)2and NaHC03], using bitumen as binder, on the sulfur dioxide emission of the coke samples during combustion was investigated. Up to 90% sulfur capture was observed for Syncrude coke and 80% for Suncor coke coagglomerated with Ca(OH)2at a Ca/S mole ratio of about 2.0 and burnt at 800 "C. At equivalent metal/sulfur (M/S) mole ratio, the coke samples coagglomerated with NaHC03 were observed to have lower sulfur dioxide emission (i.e., higher percentage of sulfur capture) than the coke/Ca(OH), agglomerates during combustion at 800 "C. For both sorbent types, the sulfur capture efficiency increased with a decrease in the combustion temperature. The ash content and calorific values of the agglomerates were determined with a view to obtaining the optimum M/S ratio of the agglomerates.
Introduction During the production of synthetic crude from Athabasca oil sands bitumen by thermal carbon rejection (delayed and fluid coking), large quantities of petroleum coke (over 3500 metric tons per day) are generated by the Suncor and Syncrude plants in Alberta. The coke produced by both plants is currently being stockpiled due to their high ash and sulfur contents (8 wt % ash; 5-9 wt % S). The calorific content of the coke is reported as 31.9 M J kg-'.' A possible use of these cokes is as solid fuel for industrial steam raising and process energy generation. However, direct combustion of the coke would result in SO2 emissions that are well in excess of environmentally acceptable levels. Consequently, the reduction of SO2 emission during combustion is an important area of research interest. Various desulfurization processes such as the combustion of the coke followed by flue gas desulfurization,2 fluidized-bed combustion of the coke with pulverized limestone or dolomite: coke gasification and subsequent removal of the sulfur compounds by conventional sour gas technology,' or desulfurization of the coke prior to combustion"' have been reported in the literature. These processes are not considered to be economically attractive because of the current low economic value of the coke to allow for their use as a fuel on a commercial scale. Consequently, incorporation of highly active sulfur sorbents (obtained from cheap and readily available sources) into the coke before combustion would be highly desirable. Recently, Majid et a1.8 reported on the wet coagglomeration of Athabasca petroleum cokes with conventional sorbents (i.e., limestone, lime, and hydrated lime) as a means of reducing sulfur emissions during combustion. They observed about 80-90% sulfur capture when Syncrude coke, which was coagglomerated with hydrated lime at a calcium/sulfur ratio greater than 1.2, was burnt in a muffle furnace at 1000 "C. The observed sulfur capture
* Author to whom correspondence should be addressed.
Table I. Prowrties of Syncrude and Suncore Cokes property sulfur content, % calorific value, MJ kg-' ash content, % a Values
Syncrude 7.61 29.8 (12897)' 8.6
Suncor 6.76 31.9 (13760)" 5.0
in parentheses are in units of Btu lb-'.
was significantly less than 90% when Suncor coke was used. This work forms part of a larger research program concerned with the production of smokeless solid fuels with high calorific value and low SO2emission for domestic and industrial energy generation. The objective of the work described here was to develop a cost-effective method for upgrading petroleum coke produced as a byproduct from Athabasca oil sands bitumen so that the coke may be used as a fuel without causing serious environmental damage. This objective was sought by coagglomerating petroleum coke with conventional sulfur sorbents (Ca(OH)2 and NaHC03) using a dry agglomeration method with small amounts of bitumen as binder, followed by combustion in an oxidizing atmosphere. The sulfur capture activities of the sorbents were studied together with the effects of the combustion temperatures. The variation of the ash content and calorific values with composition were also in(1) George, 2. M.; Schneider, L.; Kessick, M. A. Alberta Research Council, Contribution No. 1068, 1981. (2) Both, W. V.; Gehri, D. C. Paper presented at the 167th National Meeting of the American Chemical Society, California, April 1974. (3) Lee, D. C.; Georgakis, C. AIChE J. 1981,27(3), 472-481. (4) Choy, E. T.; Meisen, A. Paper presented at the Canadian Natural Gas Processing Association, Calgary, Alberta, November 15, 1977. (5) Parmar, B. S.; Tollefson, E. L. Can. J. Chem. Eng. 1977, 55, 185-191. (6)George, Z. M.; Schneider, L.; Hall, E. S.; Tollefson, E. L. Can. J. Chem. Eng. 1982,60,418-424. (7) Hall,E. S.; Tollefson, E. L. Proc. 35th Can. Chem. Eng. Conf., C. S . Ch. E., Calgary, October 1985 1985, 296-301. (8) Majid, A.; Clancy, V. D.; Sparks, B. D. Energy Fuels 1988, 2, 651-653.
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Athabasca Petroleum Coke Utilization
vestigated. The mechanism of sulfur capture of the sorbents in the coke will be discussed in a later communication.
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Experimental Section Materials. Suncor delayed and Syncrude fluid coke samples were obtained from Suncor Limited and Syncrude Limited, respectively. The relevant properties of the coke samples were determined and are summarized in Table I. Reagent grade Ca(OH)2and NaHCOSwere chosen for this investigation because of their high sulfur capture, ease of handling, and availability. The coke samples and sorbents used were in the form of powder, with particle sizes in the range 180-212 pm. Cold lake bitumen was used as the binder. Preparation of Agglomerates. Initially, a wet agglomeration method8 was used. This technique involves mixing of coke with a known amount of the sorbent and subsequent dispersion and vigorous agitation in water by means of a Waring blender, using bitumen as a binder. This was followed by filtration and drying. It was observed that large quantities of bitumen (40-60 w t 9%) were required to form good agglomerates of coke and sorbent in the form of semisolids which presented severe filtration and drying problems. Because of these difficulties associated with the wet agglomeration method, a new and more effective dry agglomeration method was used throughout for the preparation of the coke agglomerates in this study. In this method, the coke sample 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 3. Variation of ash content and calorific values with Ca/S ratio for the agglomerates: (a) Syncrude coke; (b) Suncor coke.
Figure 4. Variation of ash content and calorific values with Na/S ratio for the agglomerates: (a) Syncrude coke; (b) Suncor coke.
an increase in Ca/S ratio. At a Ca/S ratio of 3.0, the percent ash content in Syncrude coke was 38% compared with a value of 31% for Suncor coke. The higher ash content with higher Ca:S ratio is due to the sulfur fixing during combustion (eq 1). At this mole ratio (i.e., Ca/S = 3.0) the corresponding calorific values were approximately 9ooo Btu/lb (20.9 MJ kg') and 10940 Btu/lb (23.3 MJ kg-') for Syncrude and Suncor cokes, respectively. This represents a decrease of 27-30% relative to the calorific value of the original pure coke samples. It is pertinent to note that, even at this high Ca/S ratio (with high ash content), the calorific values of the agglomerates are comparable to the calorific values of typical Western Canadian thermal coals ( ~ l O O 0Btu/lb). 0 This implies that the present agglomerates may be used directly as a boiler fuel provided that the materials meet other existing conditions in commercial utility boilers. The ash generated can be upgraded for use in other applications. For exam-
ple, Schneider and George'O have shown that the presence of calcium compounds has a beneficial effect on the acid leaching of nickel and vanadium from coke ash. Calcium sulfate, one of the major components of the ash produced on combustion of the agglomerates, has been reported to be suitable for making gypsum board.1° Hence, coagglomeration of coke with Ca(OH)2also will have the added advantage that the ash from the burnt agglomerates will be more suitable for heavy metal recovery and/or CaSO, recovery as byproducts of the combustion process. The silicates present in the ash are readily volatilized by leaching with HF. Figure 4 shows the effect of NaHC03 sorbent on the ash content and calorific value of the coke samples. The (10) Schneider, L.G.; George, S.M. Paper presented at 'Extraction Metallurgy '81",Institute of Mining and Metallurgy, London, September 21, 1981.
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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 calorific values 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 at 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 at 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 at 800 "C and at 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
