Gasification of oil sand coke: Review

Fuel Processing Technology 56 Ž1998. 263–290

Review

Gasification of oil sand coke: Review Edward Furimsky

)

IMAF Group, 184 Marlborough AÕenue, Ottawa, Ontario, Canada K1N 8G4 Received 21 October 1997; revised 13 January 1998; accepted 5 February 1998

Abstract The production of synthetic crude from the tar sands in Western Canada has been steadily increasing. Most of the delayed coke produced by Suncor is combusted on site, whereas all fluid coke produced by Syncrude is stockpiled. The database on the chemical and physical properties of the oil sand coke, including the composition and fusion properties of the mineral matter, has been established. The reactivity of the coke was determined by oxygen chemisorption, fixed bed and fluid bed bench scale gasification and pilot plant gasification. The reactivity of the oil sand coke for gasification is rather low and comparable to high rank coals, such as anthracite. Slurrability tests revealed that a solid concentration in water, approaching 70 wt.%, can be achieved. Gasification is the front runner among clean technologies for the conversion of carbonaceous solids to useful products. Several commercial gasifiers are available to cover the wide range of severity. Because of the low reactivity of oil sands coke, high severity conditions are required to achieve high gasification conversion. Such conditions can be attained in entrained bed gasifiers. Gasifiers employing both dry and slurry feeding systems are suitable. A high efficiency, low SO x and NO x emissions, as well as a low solid waste production are among the key advantages of the gasification technology compared with the competing technologies. Commercial gasification of oil sands coke is delayed because of the availability of natural gas on the site of the upgrading plants. Potential for the transportation of the oil sand coke to USA for electricity generation using the integrated gasification combined-cycle ŽIGCC. technology was evaluated. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Oil sand coke; Gasification; Synthetic crude; Tar sands; Oxygen chemisorption; Slurrability test

1. Introduction Vast oil sand and heavy oil reserves in Western Canada ŽFig. 1. could account for almost 50% of Canada’s production of hydrocarbon fuels in a foreseeable future. )

Corresponding author. E-mail: [email protected].

0378-3820r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 3 8 2 0 Ž 9 8 . 0 0 0 4 8 - 4

264

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

Fig. 1. Location of oil sand deposits in Western Canada.

Syncrude Canada and Suncor mine more than 350 000 brd of bitumen and the total production of synthetic crude exceeds 300 000 brd. A number of other companies are starting new projects w1x. Currently, the combined daily production of oil sand coke by Suncor and Syncrude Canada plants in Fort McMurray Alberta exceeds 6000 t. This may represent almost 1000 MW of electricity, if all production is utilized as the feedstock in an integrated gasification-combined cycle ŽIGCC. plant. In the case of the Syncrude plant, all produced coke is stockpiled on the site of the plant, whereas most of Suncor’s production is burnt in the boiler to raise steam and electricity. The boiler was recently upgraded by adding an advanced flue gas desulfurization unit. Currently, the total amount of Syncrude coke in the stockpiles may exceed 30 million t. Considering the properties of the Syncrude coke, such as high heating value, low ash content, suitable particle size, etc., these stockpiles represent perhaps the most unique case of an energy reserve in the world. There are two other plants in Western Canada upgrading heavy oils. In this case, no sands separation is required. The upgrading technology is based on an ebullated bed and fixed bed catalytic reactors. Vacuum pitch produced is the feed for delayed coker. Unfortunately, no database on the oxidation andror gasification characteristics of these cokes is available. The commercial utilization of Syncrude coke for electricity generation using IGCC technology cannot compete with the current prices of natural gas in the region. Moreover, the combined amount of electricity from the combustion of coke in Suncor and combustion of the gaseous by-products in Syncrude can satisfy needs of both plants

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

265

and nearby communities. Only some significant expansion of production in both plants and other plants in the region, as well as the expansion of the communities around would create an additional market for electricity. But, this can still be satisfied by readily available natural gas. Thus, the price of natural gas appears to be the main factor influencing the timing for commercial utilization of the stockpiled coke. Besides electricity, the synthesis gas produced by gasification of the coke can be used for the production of hydrogen required for hydroprocessing units in both plants. However, also in this case, the cost of hydrogen produced via gasification would have to compete with that from the natural gas. Therefore, trends in natural gas prices should be monitored and once a more favorable level is approached, the utilization of coke via gasification should be thoroughly reevaluated. In the meantime, Syncrude Canada has been taking all precautions to ensure the environmental acceptability of the stockpiles. The study on leachability of coke samples taken from the stockpiles, according to the methods prescribed by the US Environmental Protection Agency ŽEPA., confirmed that the Syncrude coke is virtually non-leachable with respect to the regulated elements, as well as the volatile organics w2x. The same was confirmed under conditions designed to simulate high severity leaching, i.e., under very low pH. Adding to this reclamation efforts, such as adapting stockpiles to the conditions of the regional environment, the coke stockpiles can be regarded, as a well preserved energy resource for future generations. Nevertheless, the oil sand coke has been attracting the attention of researchers as the feedstock for both combustion and gasification. During the past several years, studies addressing utilization options for oil sand coke were appearing in the literature. The review and update of all information relevant to coke gasification, taking into consideration current and future developments in the region, is the primary objective of this study. This will include some fundamental research, as well as feasibility studies focussing on commercial gasification of oil sand coke. 1.1. Why gasification? Gasification is a partial combustion of carbonaceous feeds in the presence of a controlled amount of oxygen, i.e., oxygenrcarbon ratio is adjusted to ensure that most of the carbon in the feed is converted to CO and most of the hydrogen to H 2 . All aspects of the gasification technology are well documented in the literature. It is evident that the environmental performance of gasification is unmatched by competing means of dealing with the solid feeds such as coke. Typical emissions from competing technologies for electricity generation were published by Bechtel and are shown in Table 1 w3x. In this case, the sulfur content of the coke was assumed to be 5 wt.%, which is lower than that of the oil sand coke, i.e., between 5.5 and 7.5 wt.%. This information shows that the gaseous emissions from an IGCC plant are significantly lower than those from the circulating fluid bed boiler, as well as pulverized coke fired boiler. Thus, the gaseous emissions from an IGCC plant approach those from the natural gas burning boiler. In addition, the amount of solid waste from IGCC represents only a fraction of that from the competing technologies. Moreover, there are some uncertainties regarding the environmental acceptability of the solid wastes from combustion of oils sands coke.

266

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

Table 1 Comparison of typical emissions from power generating technologies using petroleum coke ŽlbrMW h, 100% capacity. w3x Emissions

Natural gas comb. cycle

Gasification comb. cycle

Circulating fluid bed

PC boiler FGD&SCR

SO 2 NO x CO VOC Particulates CO 2 Solid waste

0.0 0.3 0.2 0.02 0.05 820 0

0.6 0.4 0.4 0.07 0.07 1930 9.1a

3.7 0.9 1.5 0.08 0.2 2170 350

3.6 1.5 NrA NrA 0.2 2120 190

a

The solid from a coke gasifier contains only the feed metals and some carbon.

Thus, because of high contents of vanadium and nickel, such solids could be classified as hazardous wastes in the case that these metals are included among the EPA regulated elements. The available information confirms that solids from gasification are virtually non-leachable. The pressurized fluid bed combustion ŽPFBC. technology may emerge as a competitor for IGCC. In this case, sulfur content of the feedstock is the determining parameter. As shown in Fig. 2 IGCC technology is preferred for high sulfur feedstocks such as oil sand coke w4x. Among feedstocks containing the same amount of sulfur but varying in reactivity, those of the low reactivity will approach the region favorable for IGCC sooner. It is believed that the same argument applies to the circulating fluidized

Fig. 2. Effect of sulfur content on selection of technology w4x.

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

267

Fig. 3. Gasification complex. Air Sep.: air separation. Prep&del.: preparation and delivery. BFW: boiler feed water. Desulf.: desulfurization w3x.

bed combustion ŽCFBC.. Thus, as the flowsheet of a plant in Fig. 3 shows w3x, during gasification, all sulfur is removed as elemental sulfur, which is a saleable product. A low reactivity of oil sand coke compared with that of coals is another parameter favoring

Fig. 4. Slate of potential final products from gasification Žadapted from Ref. w4x..

268

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

IGCC over CFBC and PFBC. Thus, high conversions of oil sand coke may not be achieved in the fluidized beds because of relatively low temperatures employed, i.e., less than 10008C compared with about 15008C, which can be attained in entrained bed gasifiers. Moreover, more than 90% of sulfur is of an organic origin and is buried in the coke matrix as the result of prolonged tempering in the coking process. Release of such sulfur for its capture by calcium compounds in fluidized bed may be more difficult than that from the inorganic sulfur-containing compounds Že.g., pyrite., which are predominant sulfur-containing species in coal. Gasification is the first step in the production of a slate of final products ŽFig. 4.. In this regard, steam, hydrogen and electricity are the most common products, although organic chemicals can also be produced. Choice of the final product depends on the market requirements. A high pressure steam can be produced by recovering heat from the hot gasification products directly in the gasifier andror in the syngas cooler. An intermediate and low pressure steam are produced in the heat recovery systems downstream from the syngas cooler. Hydrogen is the product of the water–gas shift reaction of the clean synthesis gas. If electricity is the final product, most of the steam and synthesis gas are utilized in the steam turbine and gas turbine, respectively. In a site specific case, gasification can be part of a district heating systems.

2. Properties of oil sand coke The cokes produced by Suncor and Syncrude are referred to as delayed coke and fluid coke, respectively. This reflects type of the technology used for primary upgrading, i.e., delayed coking and fluid coking processes. The details of these processes can be found in the scientific literature w5–8x. The difference in the technology employed determines the difference in some properties of the produced cokes. Perhaps a prolonged exposure to temperatures higher than 5008C is the most common fact for both cokes. Thus, it is well known that graphite-like structures, which always result from such prolonged exposures can be detected in both cokes, i.e., using the X-ray diffraction technique. With respect to the reactivity during gasification, both the chemical composition and physical properties of the coke are important. In the case of the entrained bed gasifiers, the properties of the organic and inorganic parts of the coke may be equally important. As it was indicated earlier, at least in the case of Syncrude coke, an entrained bed gasifier appears to be the gasifier of the choice. 2.1. Chemical composition Based on the processing conditions, namely heating the Syncrude coke above 6008C during the oxidation stage in the burner would suggest a lower content of volatiles compared with the Suncor coke. For the same reasons, a higher HrC ratio of the Suncor coke can be anticipated. It is generally known that the type of sulfur in both cokes is similar, i.e., for both cokes, more than 95% of the total sulfur is of an organic type. Also, the proximity of the production mines of the Suncor and Syncrude operations would suggest a similar composition of the mineral matter.

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

269

2.1.1. Syncrude coke The basic compositional data obtained from the proximate, ultimate and ash composition analyses can be used for predicting the behavior of the coke particles during gasification. For Syncrude coke, such results are shown in Table 2 w2x. The dates in brackets indicate years of stockpiling. These samples were taken from the stockpiles in 1995. The small differences in the composition indicate on resistance of the coke to weathering during prolonged exposure to air. Thus, the differences in oxygen content were within the accumulated analytical error. This may be the best available information on the uniformity of the coke. The results in Table 2 indicate an uniform composition of the coke samples over more than 15 years of operation. For example, key parameters such as ash, carbon and sulfur contents have exhibited little variation. Of particular importance is the very low ash content compared with many coals used in commercial units, i.e., the ash content is Table 2 Analysis of Syncrude coke from stockpiles Žwt.%. w2x Cokes Žyears.

2 Ž80r82.

3 Ž82r83.

4 Ž83r85.

5 Ž85r95.

Property proximate Moisture 0.44 Ash 5.40 Volatiles 4.85 Fixed carbon 89.31

0.60 7.21 5.11 87.08

0.50 5.18 6.23 88.09

0.69 7.52 6.10 85.69

0.25 4.83 4.99 89.95

Ultimate Carbon Hydrogen HrC Nitrogen Sulfur Oxygen

82.73 1.72 0.25 1.75 6.78 1.18

80.73 1.63 0.24 1.70 6.63 1.50

81.80 1.66 0.24 1.98 6.84 2.04

80.94 1.56 0.23 1.73 6.15 1.41

83.74 1.77 0.25 2.03 6.52 0.88

Ash composition SiO 2 Al 2 O 3 Fe 2 O 3 TiO 2 P2 O5 CaO MgO SO 3 Na 2 O K 2O BaO SrO V2 O5 MnO Cr2 O 3 LOF Total

38.80 24.35 9.72 3.64 0.25 4.26 1.62 3.59 1.51 1.83 0.20 0.11 4.46 0.26 0.08 2.90 98.66

50.06 20.94 8.18 2.86 0.21 2.58 1.29 2.73 1.17 1.78 0.15 0.06 3.20 0.21 0.05 2.30 98.58

41.60 24.22 9.26 3.25 0.23 4.20 1.44 2.65 1.57 1.83 0.07 0.09 4.86 0.25 0.08 1.82 98.57

41.26 24.94 12.14 4.84 0.35 1.63 1.40 1.87 1.16 1.93 0.14 0.06 3.21 0.29 0.08 2.50 98.63

37.64 24.33 11.42 4.63 0.40 2.94 1.46 2.88 1.67 1.72 0.09 0.11 4.94 0.27 0.09 2.62 98.35

LOF: lost on fusion.

1 Ž79r80.

270

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

usually well above 10 wt.%, sometimes approaching 25 wt.%. The carbon content is in the range of high rank coals such as anthracites. This results in a high heating value of the feedstock and a high production of electricity and other final products per unit weight of the coke. Also, this has a favorable impact on the sizing of the gasifier. Little change in sulfur content over more than 15 years ensures a smooth operation of the gas clean up and sulfur recovery systems. From the processing point of view, the most important difference between the oil sand coke and coals is the significantly higher content of sulfur in the former similarly as between the oil sand coke and petroleum coke derived from conventional crudes. In the latter case, the sulfur content approaches about 2 wt.%. If sample a2 is excluded, the sum of acidic oxides such as SiO 2 and Al 2 O 3 also exhibits little variation. This sum is important for gasification because it determines the slagging properties of the ash, if an entrained bed gasifier is employed. In this regard, the fluctuation in the content of basic oxides such as Fe 2 O 3 , CaO, MgO, Na 2 O and K 2 O is important as well. Also, some species may corrodererode refractory, if unsuitable material chosen for the gasifier walls. The presence of the oxidic and basic oxides in the ash represents a major difference between the oil sand coke and a typical petroleum coke, in which case the content of such oxides is very small compared with V2 O5 and NiO. The content of V2 O5 and NiO is important because it may determine the potential of the slags and ashes from gasification for recovery of the metals. The content of regulated elements such as arsenic, barium, cadmium, chromium, lead, mercury, selenium and silver, as well as halogens such as chlorine and fluorine was also determined and published elsewhere w2x. It was quite evident that the contents of these elements were significantly below that of typical coals used for gasification. 2.1.2. Suncor coke The information on the chemical composition of the Suncor coke is less extensive than that on Syncrude coke. Moreover, the published reports do not provide sufficient information on the origin of the investigated samples, e.g., the date of sampling, etc. The information shown in Table 3 was taken from three independent references w9,12,13x. To make the comparison more meaningful, all results are expressed on a moisture-free basis. As anticipated, parameters such as content of volatiles, hydrogen, carbon and ash differ from those of Syncrude coke ŽTable 2.. It is emphasized that these differences are relatively small when compared with similar parameters of coals of different ranks. Thus, in the latter case, such small differences would be considered less meaningful. However, a more extensive database, if available, would almost certainly confirm the consistency of these differences between the two cokes. The ash composition of the Suncor samples is almost identical to that of some Syncrude samples. This is certainly not surprising, as indicated above. The advantages, such as high yields of the final products per unit weight of the Suncor coke, as well as gasifier sizing are at least similar, if not better, than those in the case of Syncrude coke, if used as gasification feedstock. The chemical composition of organic matter and, if pyrite is present, also inorganic matter determine the gross heating value of the coke. Both the feedstock composition and heating value are used for determining the efficiency of gasification and product gas

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

271

Table 3 Analysis of Suncor cokes Sample

1 ŽRef. w9x.

2 ŽRef. w10x.

3 ŽRef. w11x.

Property proximate Ash Volatiles Fixed carbon

3.99 12.48 83.53

3.8 12.5 83.7

3.09 12.16 84.75

Ultimate Carbon Hydrogen HrC Nitrogen Sulfur Oxygen

84.02 3.67 0.50 1.38 5.73 1.21

83.7 3.7 0.53 1.8 5.7 1.3

84.90 3.93 0.55 1.32 6.02 0.73

Ash composition SiO 2 Al 2 O 3 Fe 2 O 3 TiO 2 P2 O5 CaO MgO SO 3 Na 2 O K 2O BaO SrO V2 O5 NiO Total

37.00 21.31 11.47 3.34 NrA 4.91 2.91 5.39 0.68 0.91 NrA NrA NrA 1.28 94.55

42.18 22.72 11.85 3.28 0.29 3.45 1.59 2.53 0.75 1.93 0.03 0.02 4.40 1.21 96.23

yield. Ash fusion temperature is another important parameter influencing the gasification behavior of the coke. This parameter depends on the chemical composition of mineral matter. The slagging index, determined from the ratio of basic oxides to acidic oxides, indicates the slagging potential during gasification. The gross heating value and ash fusion temperature determined in a reducing atmosphere for the ash derived from Suncor coke and Syncrude coke are shown in Table 4. As the similar ash compositions suggest, Table 4 Heating values Ždb. and ash fusion temperatures of cokes w9x Syncrude coke Gross heating value ŽBturlb. Ash fusion temperature Ž8C. Initial Softening Hemispherical Fluid

13 923 1082 1293 1335 1418

Suncor coke 15.115 1074 1285 1338 1446

272

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

the fusion temperatures are similar for both cokes. It is believed that slagging can be readily achieved during entrained bed gasification using a dry fed gasifier Že.g., Shell and Prenflo.. However, for a slurry fed gasifier such as Texaco and Destec, the addition of a fluxing agent to decrease the slagging temperature may be required because the temperature of the gasification flame in the slurry fed gasifiers is lower than that in the dry fed gasifiers. 2.2. Physical properties The particle size distribution and grindability index are important parameters for the feedstock preparation. In this regard, the Suncor coke differs significantly from the Syncrude coke. Thus, the former is produced in the form of large lumps, whereas the produced coke from the Syncrude operation is in a powder form. The typical particle size distribution of a sample of Syncrude coke is shown in Fig. 5. An extensive grinding of Suncor coke is required to attain similar particle size distribution. The different form of cokes determines the grinding requirements which, for given reasons, are significantly greater for Suncor coke. This was a major conclusion after evaluation of both cokes in the Texaco Laboratory w9x, i.e., 40% of the Syncrude coke did not require any grinding. This may result in a significant saving in the design of the grinding system for a commercial plant. In the case of the Texaco gasifier, parameters such as slurrability,

Fig. 5. Particle size distribution of a Syncrude coke sample.

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

273

slurry viscosity and pumpability are important to maintain steady feeding. The pH of the slurry is also important in the case that the presence of a viscosity modifier is required. 2.3. ReactiÕity of cokes With respect to gasification, the reactivity of cokes for oxidation is of primary importance. Although some results of the ultimate and proximate analyses ŽTables 2 and 3., e.g., content of volatiles and HrC ratio, are already an indication of reactivity, it is felt that the reactivity of the cokes deserves a separate section. The exposure of cokes to air, such as in stockpiles, represents an extreme case of slow oxidation. For Syncrude coke, this is supported by the ultimate analysis ŽTable 2., i.e., a negligible uptake of oxygen after several years of exposure to air compared with the significant uptake of oxygen by some coals under similar conditions. For Suncor coke, similar results are not available. It is, however, anticipated that Suncor coke will be more susceptible to air. This is supported by the significantly higher HrC ratio of the Suncor coke compared with that of the Syncrude coke ŽTables 2 and 3.. In the former case, more aliphatic C–H groups should be present. Such groups are active sites for the oxygen chemisorption w11x. These expectations were confirmed during a temperature programmed oxygen chemisorption of various carbonaceous solids. The results of these tests are shown in Fig. 6 w14x. In this case, the materials having HrC ratio of 0.55 and 0.23 represent Suncor and Syncrude coke, respectively, whereas 0.43 and 0.38 is a semi-anthracite and

Fig. 6. Temperature programmed oxidation of coals, cokes and chars in air w14x.

274

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

anthracite, respectively. The other curves represent coals of varying rank. For chars A to E, the number indicates the temperature of charring of coals over the HrC ratio of char. Thus, the charring was conducted under more severe conditions than that applied during the preparation of the cokes Žabout 6008C.. These results show that carbonaceous solids will chemisorb oxygen during their exposure to air. The oxygen surface complexes will rapidly decompose at certain temperatures, as indicated by the sudden weight loss. This decomposition causes particles to ignite. The results for Suncor and Syncrude coke show that the former will ignite at lower temperatures, as can be predicted from the results in Tables 2 and 3, i.e., higher volatility and HrC ratio of the former. With respect to the reactivity during oxidation, particle size can play an important role as well. A detailed analysis of the effect of mean particle diameter on the oxygen chemisorption was conducted for Syncrude coke w15x. The results in Fig. 7 support an increased chemisorption with a decreasing mean particle diameter. The oxygen chemisorption rates, estimated from such curves, also increased with decreasing diameter, as shown in Fig. 8. In this case, little change in HrC ratio in the wide range of particle size was observed. This ensures oxygen chemisorption was dominated by surface factors, whereas chemical factors were less important. Rather low meso- and microporosities of the Syncrude sample suggest that chemisorption was occurring mostly on the external surface of the particles. The oxygen chemisorption results for a similar particle size range of the Suncor coke are not available. However, the higher HrC ratio of Suncor coke would indicate a higher chemisorption for the same particle size. The effect of surface area and porosity on the oxygen chemisorption of Suncor particles is difficult to predict. Thus, the particle size fraction similar to those of the

Fig. 7. Effect of particle size on oxygen chemisorption in air at 2008C for Syncrude coke w15x.

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

275

Fig. 8. Effect of particle size on the rate of oxygen chemisorption in air at 2008C for Syncrude coke w15x.

Syncrude coke would have to be prepared by crushing the Suncor coke. Nevertheless, a comparison of both cokes on the same particle size basis could yield new knowledge on the surface properties of the cokes. 3. Gasification studies An extensive review of the gasification processes with respect to gasification of oil sand coke was initiated by the Alberta Oil Sand Technology Authority ŽAOSTRA. some 15 years ago w16x. However, little attention was paid in this review to the selection of the most suitable gasifier. Also, at the time of publication of this review, the database on properties of the oil sand coke was rather limited. Since that time, gasification technology has undergone significant changes. Also, a number of gasification studies, dealing specifically with oil sand coke, have appeared in the literature. This includes studies on a laboratory-, bench- and pilot-plant scales. New information on the processes and properties of the oil sand coke under various gasification conditions has significantly enhanced the understanding of the issue. It will be shown later that, based on the new information, the preselection of gasifiers can be narrowed down to the entrained bed systems. 3.1. Laboratory and bench-scale studies The first attempts to compare the gasification reactivities of the Suncor and Syncrude cokes were made in the 1980s w17x. This work was performed in a fixed bed reactor

276

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

using about a 50-g sample load and steam as the gasification medium. The catalytic effect of a lignite ash on the coke gasification was evaluated as well. The results in Fig. 9 show the conversion of carbon with time at 830 and 9308C with and without ash. The different shapes of the curves are attributed to the difference in the volatile matter of the cokes. Thus, the initial rapid increase of the carbon conversion for Suncor coke ŽDC. almost certainly results from the devolatilization. The catalytic effect of the ash is evident only at 9308C. A similar conclusion can be drawn from the results shown in Fig. 10. Also in this case, the origin of the initial formation of H 2 was devolatilization rather than gasification. For the Suncor coke, the H 2 formed during the devolatilization accounts for about half of the total H 2 formed. The estimate of gasification rates, i.e., as dCrdt and dH 2rdt from Figs. 9 and 10, respectively, in the later stages of the experiments, i.e., after the devolatilization stage, indicates a higher reactivity of the

Fig. 9. Effect of temperature and ash on conversion of carbon for Syncrude ŽFC. and Suncor ŽDC. cokes w18x.

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

277

Fig. 10. Effect on temperature and ash on production of hydrogen Syncrude ŽFC. and Suncor ŽDC. cokes w18x.

Syncrude ŽFC. coke when compared to the Suncor coke. It is believed that during the later stages most of the CO, CO 2 and H 2 originated from the gasification reactions. It was proposed that the catalytic gasification resulted from the combined effect of calcium and iron oxides present in the ash which was added to the coke w18x. The work on steam gasification was extended to include gasification using a mixture of air q steam w12x. The carbon conversions of Suncor and Syncrude cokes are shown in Figs. 11 and 12, and H 2 yields in Figs. 13 and 14, respectively. For the Syncrude coke, the final carbon conversions were consistently higher by about 15% than that of the Suncor coke. However, this is a rather small difference to indicate the different reactivities under more severe gasification conditions, such as those encountered in commercial entrained bed gasifiers, in which case temperatures approach 15008C. Therefore, one may conclude from theses studies that when a similar particle size is used, the gasification reactivities of both cokes should be similar unless an evaluation of wide ranges of samples of both cokes confirm otherwise. Overall, both cokes can be classified as having a low gasification reactivity. This can be concluded from the comparison of the results of the carbon conversions of the cokes with that of a high reactivity lignite and a medium

278

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

Fig. 11. Effect of temperature on conversion of carbon for Syncrude coke w12x.

reactivity bituminous coal. After 30 min of gasification under the same conditions as in Figs. 11 and 12, the carbon conversion of the lignite was almost 100% w18x and that of the bituminous coal about 40% w10x. This reflects different gasification rates ŽdCrdt ., i.e., five times and two times greater, respectively, than that of the oil sand coke. Also, the results in Fig. 6 show that the cokes are less reactive than coals of varying rank, however, slightly more reactive than anthracite and semi-anthracite. It is noted that the results shown in Figs. 9–14 were obtained using the Syncrude coke and Suncor coke samples taken from the stockpile a3 ŽTable 2. and the sample a3 ŽTable 3., respectively.

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

279

Fig. 12. Effect of temperature on conversion of carbon for Suncor coke w12x.

The bench-scale fluidized bed batch reactor was used to compare the gasification reactivities of the Suncor and Syncrude cokes using air q steam as the gasification medium w12x. The results of these tests are summarized in Table 5. Also in this case, the Syncrude coke was more reactive than the Suncor coke. Although the difference in reactivity was small, it was quite consistent. The decreased carbon conversions at higher temperatures are attributed to the increased linear velocity of the gasification medium, which could have resulted in the shorter contact time andror entrainment of fines. The reactivities of Suncor and Syncrude cokes in steam were compared by Nguyen and Watkinson w13x. This is perhaps the most detailed kinetic study involving these cokes. As shown in Fig. 15 using Suncor coke, at 9308C the reactivity increased with

280

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

Fig. 13. Effect of temperature on production of hydrogen during gasification of Syncrude coke w12x.

decreasing mean particle diameter. The effect of temperature on the coke gasification is shown in Fig. 16, using a mean particle diameter of 2 and 0.14 mm for Suncor and Syncrude coke, respectively. Several kinetic models for gasification were tested with the experimental data. Rate expressions were presented for carbon conversion and for the release of sulfur. The extrapolation of the results from Fig. 15 for 0.14 mm particles of Suncor coke gives about 55% carbon conversion at 240 min, which is little higher than that of Syncrude coke for the same mean particle diameter at 9308C. However, the difference is considered to be within the combined experimental errors. Therefore, a similar conclusion may be drawn from this study as from the other studies, i.e., little difference between the reactivities of the Suncor and Syncrude cokes in steam should be observed when a similar particle size is used at the same temperature. The evolution of porosity and surface area in the course of gasification observed by Nguyen and Watkinson w13x deserves special attention. As the results in Fig. 17 for Suncor coke show, the surface area increases with carbon conversion, reaches a maximum and then declines. Similar trends were also confirmed for Syncrude coke. The results were interpreted in terms of a gradual increase in the number of pores until the

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

281

Fig. 14. Effect of temperature on production of hydrogen for Suncor coke w12x.

pores began to grow into each other. In this regard, the results published by DiPanfilio and Egiebor w19x should be noted. These authors made an attempt to prepare an activated carbon from Syncrude coke. For this purpose they designed special procedures for a

Table 5 Carbon conversion Žwt.%. and yield of products Žmolr100 g. from fluidized bed gasification w12x Temperature Ž8C.

Carbon conversion

Yield H2

CO

CO 2

CH 4

Suncor coke 815 856 927 970

37.0 50.6 48.0 40.1

0.50 0.80 1.16 1.06

0.86 1.04 1.24 1.02

1.54 1.84 2.02 1.54

0.22 0.26 0.22 0.40

Syncrude coke 805 880 930 1020

48.6 55.3 52.9 43.9

1.04 1.38 1.20 0.72

1.22 1.70 1.86 1.26

2.06 2.04 1.70 1.70

Tr Tr Tr Tr

282

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

Fig. 15. Effect of particle size on gasification rate for Suncor coke w13x.

mild pyrolysis followed by activation using steam at 8508C. The result of this activation was a remarkable increase in surface area and porosity. Thus, as the data in Table 6 show, the development of porosity and surface area increased with the time of activation and overall carbon conversion. After 6 h of activation, the surface area increased almost 60 times. The activated carbons were tested for adsorption of iodine and methylene blue dye according to the standard methods. The potential for the preparation of activated carbon was demonstrated. Similar results for Suncor coke are not available. 3.2. Pilot plant studies An important study on gasification of oil sand cokes was conducted at the University of British Columbia under the direction of Prof. Paul Watkinson w20x. In this case, testing was conducted in a continuous pilot plant gasifier using at least 100 times more coke than it was used in the previous studies. At first, the Suncor coke was gasified in a spouted bed reactor and Syncrude coke in a fluid bed reactor using a steam q oxygen mixture w13x. The reactor had about a 20-kgrh feeding rate. The choice of the systems was influenced by the particle size of the cokes, i.e., the mean particle diameter of the Suncor coke was about 10 times greater than that of Syncrude coke. This gave a different residence time of particles in the reaction zone. Nevertheless, at about 9508C, more than 90% and about 80% carbon conversions were achieved for Suncor and Syncrude coke, respectively. The produced gas consisted of 33–38 vol.% H 2 , 22–28 vol.% CO and 38–44 vol.% CO 2 and had a heating value of about 8.8 MJrm3. Some

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

283

Fig. 16. Effect of temperature on gasification rate of Ža. Suncor coke Ž d p s 2 mm. and Žb. Syncrude coke Ž d p s 0.14 mm. w13x.

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

284

Fig. 17. Relationship between carbon conversion and surface area for Suncor coke w13x.

tests were conducted in the presence of dolomite as a sorbent for scavenging the sulfur. Over 95% of H 2 S reduction was achieved when dolomite was used as bed material and also fed with the coke, compared to runs in the absence of sorbent. All results support rather low reactivity of the cokes when compared with the bituminous and lower rank coals. Also, the results obtained in the fixed bed reactor may differ from those obtained in the fluidized bed reactors, e.g., in terms of the reaction controlled process. Moreover, predominantly organic sulfur is buried in the coke as the result of the prolonged tempering. This may lower the sulfur reactivity during gasification. Then, temperatures approaching 15008C may be required to achieve high coke conversions. At such temperatures, the gasification of pulverized coke approaches equilibrium. Such conditions are attained in the entrained bed gasifiers. Yet, the

Table 6 Properties of activated carbons prepared from Syncrude coke w19x Sample Activation % Burnoff BET surface Micropore Methylene blue dye Iodine adsorption time Žh. area Žm2 rg. volume Žcm3 rg. adsorption Žmgrg. Žmgrg. Raw coke 1 y 2 y

y y

ActiÕated coke 2 4 6

22.7 53.8 60.0

4.6 5.5

0.004 0.004

118.8 172.3 318.6

0.079 0.143 0.244

8.2

80–100

28.0

200

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

285

experimental database was established below 10008C. It is believed that even under these conditions the relative reactivities of the cokes and coals will be similar although an estimate by the direct extrapolation cannot be made. From the operational point of view, the low reactivity of the coke will translate into the lower reactor throughputs to attain longer reaction times. 3.3. Commercially related studies Technical literature on commercial gasification systems is quite extensive and it indicates that there is a number of commercial projects on gasification of various solid and semi-solid feedstocks, including petroleum coke w21,22x. Some major commercial units, currently in operation andror in different stages of construction and commissioning are listed in Table 7 w23x. It is apparent that the entrained bed gasifiers are gasifiers of the choice. In view of the similarity between properties of the oil sand coke and petroleum coke, the entrained bed gasifiers are preferred for gasification of the former as well. Two types of the entrained bed gasifiers are available commercially, i.e., dry fed and slurry fed. Shell and Prenflo gasifiers are the best known dry fed gasifiers, whereas Texaco and Destec gasifiers employ a slurry feeding system. The gasification temperature in dry fed gasifiers may exceed 15008C, whereas in the slurry fed gasifier approaches about 13008C. This ensures that most of the mineral matter in coke is fused into the non-leachable slag. A slagging cannot be attained in the fluidized bed gasifiers because of the lower temperature employed. The licensors of commercial entrained bed gasifiers, such as Texaco and Shell claim to have an experience with oil sand coke w24,25x even though no experimental data are given on testing, e.g., in demonstration andror commercial units. However, the information on gasification of petroleum coke is quite extensive. Little information was found on the gasification of coke in the Destec gasifier. It is believed that, in view of the similarities between the oil sand coke and petroleum coke, such information can be used to determine suitability of the former as the feedstock for a particular gasifier. No similar information could be found to indicate utilization of the oil sand coke by the licensors of the fluid bed and moving bed gasifiers. Based on many years experience, gasification performance of the cokes can be quite accurately predicted from the basic compositional data. Thus, the operators of the commercial entrained bed units have established a significant database on the testing of the wide range of gasification feedstocks. This database was used to develop mathematical models describing the behavior of various feedstocks during gasification. It is believed that an evaluation Že.g., on a demonstration scale. may be required only after a

Table 7 Gasification plants utilizing petroleum coke w23x Plant location

Process

Feedstock

Final product

Ube, Japan Thermie, Spain El Dorado, USA

Texaco Prenflo Texaco

Petcoke 50r50 petcokerlignite petcoke and waste oil

Ammonia Electricity Electricity and steam

286

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

major decision is made on commercial utilization of the cokes to confirm the results generated by the models. Also, it may be necessary to obtain samples of by-products Žslags, sulfur, waste water, etc.. to determine their quantity, composition and environmental acceptability, as well as the type and quantity of the emissions to determine gas cleaning requirements. It may be necessary to obtain samples of slags and fly slag for a preliminary evaluation of their utilization and disposal options as well. 3.3.1. Texaco gasifier Texaco gasifier is the refractory lined cylindrical vessel. The slurry of the pulverized Žabout 40 m m mean particle diameter. solid coke and water as well as oxygen enter at the top. The amount of solids in the slurry can approach 70 wt.%. Gasifier products and slag exit at the bottom to radiant cooler before entering syngas cooler if the full heat recovery mode is used. In the partial heat recovery mode, gasifier products and slag are quenched directly with water. This route is preferred when chemicals are the anticipated products. A full heat recovery mode is preferred for the electricity generation as part of the IGCC technology. Several commercial units are in operation in various parts of the world. The evaluation of feedstocks for the Texaco gasifier involves three stages of tests termed as ‘Type I’, ‘Type II’ and ‘Type III’ analysis. The latest involves testing on a demonstration scale Ž15 trd. using the facility in Montebello, CA. The ‘Type I’ analysis of Syncrude and Suncor cokes confirmed that both cokes appear to be good candidate feedstocks for the Texaco gasifier w12x. Both cokes slurry well in that high slurry concentrations, i.e., high 60s on the weight percent solids basis, can be achieved even without the use of viscosity reducing additives. For this purpose, a commercially available ammonium lignin sulfonate was successfully used for comparison w9x. Accordingly, both slurried samples have high heating values and would be expected to produce a high product gas yield. Neither feedstock would appear to require an ash fusion temperature moderator. It was, however, noted that sample variability should be determined to ensure that the fusion temperatures do not approach the point at which the fluxing agent should be considered. Regarding the Syncrude coke, it was also noted that because of the large amount of the fine material, about 40% of the solids was mixed directly into the slurry without grinding. If this characteristic is representative of the commercial production, significant savings in the design of the grinding system of a commercial plant would be achieved. 3.3.2. Shell and Prenflo gasifiers The Shell gasifier consists of a vertical vessel equipped with a water-cooled membrane wall covered with a protective refractory and a layer of the frozen slag. The membrane wall serves as a radiant cooler, generating high pressure steam. Hot gaseous products exit the gasifier and enter the syngas cooler for the additional production of the high pressure steam. The gasification part of the Prenflo gasifier is similar to that of the Shell gasifier. However, the first stage heat recovery occurs in the radiant cooler on the top of the gasifier. Dried and pulverized solids used as the feedstock have a mean particle diameter of about 40 m m. A blend of petroleum coke and a lignite is the

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

287

Table 8 Summary of indicators used for the prefeasibility study w25x Capacity: 4150 trd Two gasifiers trains Two advanced design gas turbines Capex gasification: US$450 MM Capex combined cycle: 286 MW Location: US Pacific Northwest or Upper Midwest

Delivered coke price: US$24rt Two high pressure air separation units Plant design: 18 months Construction: 36 months Power output: 550 MW Gas output: 3220 MM Bturh ŽLHV.

feedstock for the IGCC plant Ž350 MW. employing Prenflo gasifier, now in the commissioning stage in Spain w26x. Shell has claimed to have an experience with both Syncrude and Suncor cokes and used the former for a prefeasibility study on production of electricity using the dry fed Shell gasifier which was published by Hauser w25x. In spite of a greater hardness of Syncrude coke compared to that of typical petroleum cokes, the former was considered to be an ideal feedstock for the Shell gasification process. Thus, the greater hardness can be overcome by the equipment choice and facilities design. The summary of the economic and technical parameters included in the preliminary feasibility study is given in Table 8. The design takes into account the unique properties of the Syncrude coke, e.g., high heat content results in more steam make in the gasifier, low chlorine levels result in a low cost chloride removal option and high sulfur content requiring an optimal sizing of the gas cleaning system. The coke would be transported from Fort McMurray to the project site by rail. The total capital cost of the plant ŽUS$740. is based on 1Q 1994 US$ or US$1340rkW on an US Gulf coast basis. The heat rate is 8270 BturkW h and is based on 33% of the air separation unit requirements being extracted from the gas turbine air compressor to the air separation unit. These parameters suggest that the IGCC plant could be cost competitive with a pulverized coal burning facility with the installed flue gas desulfurization facility. The plant would need to be able to compete against a two trains natural gas based combined cycle plant generating 480 MW, at a heat rate of 6900 BturkW h.

Table 9 Levelization factors w25x Consumable costs Sulfur Limestone Fuels

Project finance Cost of capital Inflation Plant life Book life Local taxes and ins.

US$25rt credit US$20rt Syncrude coke: US$24rt Ž$0.87 MM Btu. Natural gas: US$2.65rMM Btu Escalation and inflation q1%

11% Ž75r25 debtrequity. 4.0% 25 years 25 years 2% capital

288

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

Capital cost would be US$260 MM or US$550rkW ŽUS Gulf Coast basis.. The delivered natural gas price would be US$2.65rMM Btu escalating at inflation plus 1% over the life of the project. Levelized cost of electricity would be 3.8 centsrkW h for the Syncrude coke and would be competitive with the cost of electricity generated with natural gas. The summary of the levelization factors is given in Table 9.

4. Why entrained bed gasifiers? All commercially related studies on the gasification of oil sands coke published in the literature indicate the use of the entrained bed gasifiers suggesting that they are the gasifiers of the choice. Both dry fed and slurry fed entrained bed gasifiers appear to be suitable. There are several determining factors for the selection of entrained bed gasifiers andror exclusion of the other gasifiers. In this regard, the particle size distribution and reactivity of the coke are essential parameters. The environmental acceptability of the solid byproducts must also be considered. For Syncrude coke, the particle size distribution is controlled by the process conditions. Thus, fewer options are available for the additional change of the particle size compared with that for the Suncor coke for which different particle size fractions, including those suitable for both the moving bed and fluidized bed gasifiers, can be prepared by selecting grinding conditions. With respect to the gasification reactivity, both the Syncrude and Suncor cokes can be classified as low reactivity feedstocks. In the case of Syncrude coke, the presence of fines would suggest that moving bed gasifiers are unsuitable. The potential entrainment of fines suggests that fluidized bed gasifiers could also be excluded for gasification of Syncrude coke. Moreover, because of a relatively low reactivity of Syncrude coke, much higher temperatures are required than those attained in the fluidized bed gasifiers for achieving high gasification rates. For Suncor coke, the particle size suitable for the moving bed and fluidized bed gasifiers may be prepared. However, some modifications to the typical moving bed gasifiers may be needed to accommodate Suncor coke because of the low reactivity. Petroleum coke was successfully gasified in the KRW fluidized bed gasifier w27x. In this case, limestone was used in the fluidized bed for the in situ sulfur capture. However, in the case of oil sands coke, about four times more limestone would be required than that for petroleum coke. However, it is not certain that the CarS ratio of about 2, usually used for the gasification of coals would be sufficient for that of the oil sands coke. It is believed that a higher ratio Žthan 2. may be required to achieve a high level of sulfur capture. This would require some modifications of the KRW gasifier. Moreover, a low reactivity of the oil sands coke could be an additional factor due to relatively low temperatures employed. Last but not least, it is uncertain that the solids produced could meet the requirements for non-hazardous wastes. Thus, although the suitability of a fluidized bed gasifier for the gasification of petroleum coke was suggested, its use for the gasification of Syncrude and Suncor cokes should be thoroughly evaluated. It is believed that entrained bed gasifiers are the gasifiers of choice for the gasification of Syncrude and Suncor cokes. High temperatures attained in these gasifiers Žapproaching 15008C. ensure high gasification rates. The licensors of the most advanced

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

289

entrained bed gasifiers, e.g., Texaco and Shell indicated that both the Syncrude and Suncor cokes are the ideal feedstocks. Thus, the required particle size can be readily achieved by properly designed grinding equipment. The coke can be fed either dry ŽShell and Prenflo gasifiers. or slurried with water ŽTexaco gasifier.. In the latter case, a high solid concentration in the slurry can be achieved ensuring efficient gasification. The gas cleaning facilities can be sized to accommodate high sulfur contents of the cokes. Further, the slurry feeding option offers an opportunity to co-gasify refinery sludges and other wastes which may be added during the slurry preparation andror injected separately into the gasifier. There are some indication that dry fed gasifiers will also be able to co-gasify sludges and other wastes.

References w1x Anonymous, Oil Gas J., May 20, 1996, 25. w2x K.H. Chung, L.C.G. Janke, R. Dureau, E. Furimsky, Leachability of cokes from Syncrude stockpiles, Environ. Sci. Eng., March 1996, 50. w3x R.F. Geosits, Y. Mohammad-zadeh, Coke gasification for power generation: options and economics, Power-Gen. Americas ‘93, Dallas, TX, Nov. 1993. w4x D.H. Scott, A.M. Carpenter, Advanced power systems and coal quality, IEA Coal Research, IEACRr87, London, 1996. w5x J.D. Elliot, R.L. Godino, M.J. McGrath, Non-catalytic heavy crude upgrading, Proc. Third UNITAR Conf. on Heavy Crudes and Tar Sands, Long Beach, CA, July 2–31, 1985, p. 1698. w6x G.L. Hamilton, M. Fitzgerald, V. D’Amico, Retrofitting heavy oil processes, Proc. 1986 AIChE Meeting, Miami, FL, Nov. 2–4. w7x J.G. Speight, Chemistry and Technology of Petroleum, 2nd edn., Marcel Dekker, New York, 1991. w8x J.J. McKetta, Petroleum Processing Handbook, Marcel Dekker, New York, 1992. w9x Entrained bed gasification of residues using the Texaco gasification process, Texaco Report to CANMET, ERLrERP-85, 1985. w10x A. Palmer, F. Furimsky, Steam gasification of Balmer coal in the presence of lignite ash, Fuel Sci. Technol. Int. 4 Ž1986. 433. w11x J.A. Howard, Adv. Free Radic. Chem. 14 Ž1973. 72. w12x E. Furimsky, Gasification reactivities of cokes derived from Athabasca bitumen, Fuel Processing Technol. 11 Ž1985. 167. w13x Q.T. Nguyen, A.P. Watkinson, Gasification kinetics of oil sand coke, Can. J. Chem. Eng. 68 Ž1990. 814. w14x E. Furimsky, Effect of HrC ratio on coal ignition, Fuel Processing Technol. 19 Ž1988. 203. w15x C. Fairbridge, A.D. Palmer, S.H. Ng, E. Furimsky, Surface structure and oxidation reactivities of oil sand coke particles, Fuel 66 Ž1987. 688. w16x Review of gasification technology, Monenco Report for AOSTRA. w17x E. Furimsky, Gasification of Onakawana lignite, Can. J. Chem. Eng. 62 Ž1984. 257. w18x E. Furimsky, A. Palmer, Catalytic effect of lignite ash on steam gasification of oil sand coke, Appl. Catal. 23 Ž1986. 355. w19x R. DiPanfilio, N.O. Egiebor, Activated carbon production from synthetic crude coke, Fuel Processing Technol. 46 Ž3. Ž1996. 157. w20x A.P. Watkinson, G. Chang, D.P.C. Fung, Gasification of oil sand coke, Fuel 68 Ž1989. 4. w21x T. Takematsu, C. Maude, IEA Coal Res. Rep. IEACRr37, London, March 1991. w22x A. Kristiansen, Understanding coal gasification, IEA Coal Research, IEACRr86, London, March 1996. w23x D.L. Heaven, Oil Gas J., May 27, 1996, 49. w24x J.S. Falsetti, R.L. Skarbek, Petroleum coke utilization with Texaco gasification process, 1993 NPRA Annual Meeting, San Antonio, TX, March 21–23, 1993.

290

E. Furimskyr Fuel Processing Technology 56 (1998) 263–290

w25x N. Hauser, The use of the Shell coal gasification process in heavy crude and tar sands derived petroleum coke gasification, Sixth UNITARrUNDP Int. Conf. on Heavy Oil and Tar Sands, Section 10, Houston, TX, Feb. 1995. w26x W. Schellenberg, M. Hooper, The Puertollano IGCC plant with Prenflo technology, Proc. 13th Intern. Pittsburgh Conf., Vol. 1, Sept. 3–7, 1996, p. 427. w27x W.M. Campbell, Kellogg’s KRW fluid bed process for gasification of petroleum coke, Handbook of Petroleum Refining Processes, Marcel Dekker, New York, December 1994.