Energy & Fuels 2003, 17, 1541-1548
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Emissions of Carbon Dioxide from Tar Sands Plants in Canada Edward Furimsky IMAF Group, 184 Marlborough Avenue, Ottawa, Ontario K1N 8G4, Canada Received May 6, 2003. Revised Manuscript Received July 28, 2003
The CO2 emissions from the Canadian tar sands plants approach 0.09 and 0.16 tonne per barrel of synthetic crude produced in the plants employing fluid coking and delayed coking processes, respectively. The total CO2 emissions from the utilization of liquid fuels by combustion approach 0.4 tonne per barrel. When the CO2 emissions from the production of synthetic crude, refining, and utilization of fuels are combined, the emissions from utilization account for about 80 and about 70% of the emitted CO2 when fluid coking and delayed coking processes are considered, respectively. Then, there is the much greater potential for the reduction of CO2 emissions on the fuel utilization side than that on the synthetic crude production side. The amount of CO2 emitted from the expanded production of synthetic crude depends on the coking process chosen for expansion. A plant producing ∼500 000 bbl/d of synthetic crude using the fluid coking process may emit about ∼16 million tonnes of CO2 annually, whereas the same daily production in the plant employing a delayed coking process would emit ∼30 million tonnes of CO2 annually. The combined production of 1 million barrels a day of synthetic crude would emit ∼46 million tonnes of CO2 annually, which accounts for less than 8% of the Canadian CO2 emissions. At the same time, the combined production would contribute almost 50% to the liquid fuels pool in Canada. Definitely, the reduction of CO2 emissions can be achieved more readily by implementing proper actions and regulations on the liquid fuels utilization side than those on the production side.
I. Introduction The tar sands reserves in the province of Alberta in Western Canada (Figure 1) represent an important resource of hydrocarbon fuels. Synthetic crude has been produced by two companies using coking technology, i.e., Syncrude Canada and Suncor, which employ fluid coking and delayed coking processes, respectively. The heavy feed for coking is obtained from tar sands using the Clark process, the essential part of which is the hot water separation step. The sands and clay are returned to the open pit mine and used for the land reclamation. The primary products from coking are subjected to hydrotreatment to meet the synthetic crude specifications. Synthetic crude is delivered by pipeline to petroleum refineries in Canada and the United States. There is a volume of information available in the literature on the properties of heavy feed used for coking1,2 as well as on the details of the fluid coking and delayed coking processes used in the Canadian tar sands plants.3-7 From the energy supply point of view, the tar sands reserves have been considered as an important resource of hydrocarbon fuels which can decrease the dependence (1) Speight, J. G. Chemistry and Technology of Petroleum, 2nd ed.; Marcel Dekker: New York, 1991. (2) Speight, J. G. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1987, 32, 413. (3) McKetta, J. J. Petroleum Processing Handbook; Marcel Dekker: New York, 1992. (4) Furimsky, E. Fuel Process. Technol. 2000, 67, 205. (5) Busch, R. A.; Kociscin, J. J.; Schroeder, H. F.; Shah, G. N. Hydrocarbon Process. 1979, September, 136. (6) Corbett, R. A. Oil Gas J. 1989, June 26, 42. (7) DeBiase, R.; Elliott, J. D. Hydrocarbon Process. 1982, 5, 99.
of North America on imported crude. The expansion of synthetic crude production from tar sands is required for ensuring the security of fuel supply. The tar sands plants have also been identified as the contributor to the overall emissions of CO2 in Canada. It should be pointed out that the CO2 emissions were a nonissue when the major decisions regarding the development of tar sands were made. The CO2 emissions will gradually increase as the result of the present and anticipated expansion of the synthetic crude production. In the spirit of Kyoto protocol, it is necessary that the amount of the additional CO2 emitted from tar sands plants is monitored. It is, however, important that this is done in the context with other sources of CO2. For example, during utilization the CO2 emissions per barrel of liquid fuels produced from synthetic crude obtained from tar sands are significantly greater than those from the production of one barrel of synthetic crude including a hydrotreating step. For the purpose of the present study, both plants, i.e., one using the fluid coking process and the other using the delayed coking process, were evaluated. Because plant data were not available, the cases were defined on the basis of information in the literature.1-7 This enabled a reasonably accurate estimate to be made of the yields of synthetic crude and byproducts such as coke and gases from coking as well as the amount of natural gas used for the H2 production required for the hydrotreatment of primary liquids from coking. With such data, the emissions of CO2 from fluid coking relative to delayed coking as well as those from antici-
10.1021/ef0301102 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/16/2003
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Figure 1. Location of tar sands plants in Western Canada.
pated expansions could be estimated. The plants upgrading the feeds obtained from heavy oil reservoirs were not included in the estimate. II. Sources of CO2 Emissions from Tar Sands Plants The first step in estimating the overall CO2 emissions from the tar sands plants was the identification of its primary sources. In this regard, coke is one of the important contributors, particularly if most of its daily production is burned as fuel. The gaseous byproducts from coking are utilized on the site to generate electricity and heat for the plant requirements, particularly for tar sands mining and separation. There is also a possibility of using part of the gaseous byproducts either as petrochemical feedstock or for the conversion to liquid fractions by oligomerization. Flaring of the coking gas is either not practiced at all or its contribution to the overall CO2 emissions from the plants is negligible. The upgrading of the primary liquids from coking to synthetic crude requires H2 which is produced by the steam reforming of natural gas. This generates high concentration CO2 as byproduct. There are other sources of CO2 in the plants; however, their contribution to the overall emissions is considered to be rather small. The estimate of CO2 emissions from the combustion of coke and gas requires data on their chemical composition. For coke, the database is rather extensive and readily available,4,8 contrary to the limited information on the composition of the gaseous byproducts. For the purpose of this study, the yield of gas (in m3/d and/or t/d) was expressed on the natural gas equivalent basis. The chemical composition of the clean natural gas used (8) Furimsky, E. Fuel Process. Technol. 1998, 56, 263.
Table 1. Composition of Natural Gas component
vol %
H2 CH4 C2 C3 C4 + C5
2.1 96.8 0.7 0.4 tr
Table 2. Composition of Coke (wt %) carbon hydrogen sulfur nitrogen ash moisture
fluid coke
delayed coke
83.7 1.8 6.5 2.0 0.3 4.8
73.5 3.0 6.2 1.8 3.4 12.1
for calculations is given in Table 1. This approaches the composition of the desulfurized natural gas produced in Western Canada. The combustion of one cubic meter of such gas would yield about 1.95 kg of CO2 compared with about 2.0 kg produced by the combustion of pure methane. The same composition was used for estimating the CO2 emissions from the H2 plant. The composition of fluid and delayed cokes used for the calculations are shown in Table 2.4,8 Information on the yield of synthetic crude, coke, and gaseous byproducts from both the fluid and delayed coking processes is available in the literature.1-9 This includes the heavy feeds from Western Canada as well as those from various parts of the world. This information was the basis for estimating the yields of coke and fuel gas, the combustion of which are among the primary sources of the CO2 emissions from tar sands plants. These results are shown in Table 3. The CO2 (9) Furimsky, E. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 637.
Emissions of CO2 from Tar Sands Plants in Canada
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Table 3. Yield of Synthetic Crude and By-products from Tar Sands Plants Employing Fluid Coking and Delayed Coking Process feed rate synthetic crude coke to stockpile coke combusted fuel gas
fluid coking
delayed coking
∼ 200 000 bbl/d (∼30 000 t/d) ∼ 170 000 bbl/d (∼22 500 t/d) 3500 t/d 1200 t/d 2500 t/d (3.5 × 106 m3/d)
∼85 000 bbl/d (13 000 t/d) ∼65 000 bbl/d (8500 t/d)
emissions estimate was based on the daily production of synthetic crude of 235 000 bbl/d from two tar sands plantssone employing fluid bed coking and the other delayed coking producing 170 000 bbl/d and 65 000 bbl/d of synthetic crude, respectively. Attempts were made to prorate the CO2 emissions to reflect the expansion of the synthetic crude production to 500 000 bbl/d for each plant, i.e., 1 million bbl/d combined production. II.1. Upgrading of Bitumen. The bitumen separated from tar sands is upgraded to synthetic crude using either fluid coking or delayed coking technology. In the former plant, part of the bitumen is upgraded using a hydrocracking technology. II.1.1. Fluid Coking. The simplified flow diagram of the fluid bed coking process is shown in Figure 2. It consists of the reactor employing the fluidized bed of hot coke which provides all the heat necessary for coking. The coking is accomplished by the transfer of heat from the hot coke particles to the bitumen which is sprayed into the fluid bed of the former via nozzles. The bitumen is pyrolyzed to gas, liquid products, and coke which is deposited on the surface of the fluidizing coke particles. The gaseous and liquid products comprising the naphtha and gas oil fractions are withdrawn at the top. After desulfurization, the gaseous products are transferred to the utility plant for combustion to generate electricity and steam. A part of the gaseous products is used as the fuel for plant furnaces. Cold coke is withdrawn at the bottom of the reactor and transferred to the burner where its heat is increased by partial combustion before it is recirculated to the reactor. The low-heating-value gas from the burner is transferred to a boiler. A portion of the coke is continuously withdrawn from the burner to maintain a constant coke inventory. The coke withdrawn from the burner is stockpiled in an open pit mine together with the sands from the separation process.
Figure 2. Flowsheet of the fluid coking reactor-coker assembly.
3000 t/d 1500 t/d (2.1 × 106 m3/d)
The simplified flowsheet of the tar sands plant employing the fluid coking process is shown in Figure 3. It shows the integration of the fluid coker with hydrocracker and hydrotreaters as it is presently used in the Syncrude plant. This integrated system was presented and described by Corbett.6 As previously stated, the CO2 emissions estimate is based on the conditions prevailing when the plant was producing about 170 000 bbl/d of synthetic crude from bitumen (spec. gr. ) 0.96) obtained from tar sands. The primary liquids (spec. gr. ) 0.85) from coking, i.e., naphtha and gas oil fractions, are combined and hydrotreated to meet the specifications of synthetic crude (spec. gr. ) 0.83). For the purpose of this study, the estimate of the CO2 emissions is based on the reactor-burner-hydrocracker assembly processing ∼ 200 000 bbl/d of bitumen. This includes about 46 000 bbl/d of bitumen processed in hydrocracker. Two such assemblies would be required if their capacity was about 100 000 bbl/d of bitumen each. The bitumen processed in the fluid coker includes about 20 000 bbl/d of the vacuum bottoms derived from the distillation of the hydrocracker product. The total production of synthetic crude from one such assembly may approach ∼170 000 bbl/d. The approximate yields of the products and byproducts from such system are given in Table 3.3 Three such integrated systems would be required in order to produce ∼500 000 bbl/d of synthetic crude. The amount of tar sands to be mined for this amount of bitumen may approach one million tonnes per day, indicating significant energy requirements for mining operations. As was indicated above, there are three main sources of CO2 emissions associated with the upgrading of bitumen, i.e., the combustion of the burner off gas in a boiler (from partial combustion of coke), the combustion of fuel gas from the coking reactor (utility, plant furnaces, etc.), and the water gas shift process in the H2 plant. An estimate was made for each source independently. It is noted that the amount of CO2 emitted from the utility plant generating electricity for the consumption during the mining operation is included in this portion of the overall CO2 emitted from the plant. CO2 from Burner. According to Table 3 about 1200 t/d of coke is combusted in the burner. The O2 starving conditions ensure less than 1% conversion of the coke inventory in the burner.4 The topmost layer formed on the surface of the coke particles during the last trip to the reactor will play a predominant role during combustion in the burner. This layer is about 5 µm thick and its carbon content is lower than that of the bulk coke.3 Therefore, the calculation based on the carbon content of the bulk coke (∼86 wt %) would give an overestimate of the CO2 emissions from this source. Experimental results of the pyrolysis of cold fluid coke under helium at 600 °C showed that on the weight basis, the amount
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Figure 3. Flowsheet of the tar sands plant with the integrated fluid coking reactor-coker and hydrocracking processes.
of carbon released in the gas (as CO, CO2, CH4, and COS) to achieve less than 1% conversion varied between two extremes, i.e., from 30% to about 60% with H2 and H2S accounting for the balance.10 This suggests that the partial combustion of the pyrolysis gases is the important contributor to the overall heat release in the burner. The partial combustion of the gas and coke surface in the burner occur in parallel. With these uncertainties in mind, the estimate of the daily CO2 release from the burner was based on the carbon content of 70% rather than 83.7% reported in Table 2. For one reactor-burner-hydrocracker assembly producing 170 000 bbl/d of synthetic crude, the total daily amount of CO2 released from the burner can be estimated from the data in Table 3 as follows: 1200 × 0.7 × 3.67 ) ∼3100 t/d. Then, the total CO2 emissions from the burner after scaling up the production to 500 000 bbl/d of synthetic crude will approach 9300 t/d. CO2 from Fuel Gas. For case A, it was assumed that the entire daily production of the fluid coker reactor off gas was combusted in the utility plant to generate electricity and steam as well as the fuel for the plant furnaces. The yield of fuel gas in Table 3 is reported on the natural gas equivalent basis. The composition of natural gas used for the calculations is given in Table 1. The weight of fuel gas given in Table 3 corresponds to about 3.5 × 106 m3/d. It was shown above that after combustion, each m3 of the fuel gas will release about 1.95 kg of CO2. For one reactor-burner-hydrocracker (10) Furimsky, E.; Ohtsuka, Y. Energy Fuels 1997, 11, 1074.
assembly, the total CO2 emissions from the combustion of fuel gas are estimated in the following way: 1.95 × 3.5 × 106 ) ∼7000 t/d. Source #2 would emit about 21 000 t/d of CO2 after achieving the expanded production of crude from ∼170 000 bbl/d to ∼500 000 bbl/d. Another estimate was performed assuming that half of the of fuel gas produced was oligomerized to liquids. This option is feasible only if the combustion of the other half of fuel gas and partial combustion of coke can supply enough energy for the operation of entire plant. Nevertheless, the estimate of the CO2 emissions will be made assuming that half of the fuel gas produced is converted to liquid fuels. For this estimate, the formation of octane from methane was used as an example. Theoretically, one cubic meter of methane would yield about 0.636 kg of octane. Then, the total yield of octane from ∼1.8 × 106 m3 of the gas for the base case B would approach 10 000 bbl/d. Case B results in the reduction of the CO2 emissions from the combustion of fuel gas by about 10 500 t/d and at the same time the yield of liquid products is increased by ∼30 000 bbl/d for the plant producing ∼500 000 bbl/d of synthetic crude. In real situation, oligomerization involves C2 to C4 hydrocarbons which have to be separated from coking gas. CO2 from H2 Plant. It was given by Corbett6 that the amount of H2 required for the secondary upgrading of the primary liquids to produce ∼170 000 bbl/d of synthetic crude approaches ∼7 × 106 m3 of H2 per day. Further, all this H2 was produced from the steam reforming of natural gas (Table 1).
Emissions of CO2 from Tar Sands Plants in Canada
Figure 4. Flowsheet of the delayed coking twin reactors system.
This is shown on the following general reaction involving CH4 as the main component:
CH4 + 2H2O ) 4H2 + CO2 The complete conversion of each CH4 molecule to four moles of H2 will produce one mole of CO2, i.e., each cubic meter of the converted CH4 would release one cubic meter of CO2. Therefore, the amount of H2 required for upgrading correspond to less than 2 × 106 m3 of CO2 released. It was estimated above that the weight of each cubic meter of CO2 is about 2 kg (1000/22.4 × 44). This will give the total CO2 emissions of about 4000 t/d from the utilization of natural gas for the one reactorburner-hydrocracker assembly producing ∼170 000 bbl/d and about 12 000 t/d for the scaled-up plant producing ∼500 000 bbl/d of synthetic crude. II.1.2. Delayed Coking. The simplified flowsheet of the delayed coking process is shown in Figure 4. It consists of the twin reactor system operating simultaneously in the coking (operative) and decoking (nonoperative) mode. The yields of the products and byproducts used for the estimate of the CO2 emissions from the one twin reactor system processing about ∼85 000 bbl/d of bitumen are shown in Table 3.1,3,7,9 The plant will produce about 65 000 bbl/d of synthetic crude (spec. gr. ) 0.83). The production scaled up to the level of about 500 000 bbl/d of synthetic crude would require almost eight twinreactor systems. CO2 from Combustion of Coke. The composition of delayed coke used for the estimate is shown in Table 2.4,8 The relatively high content of moisture compared with that of the fluid coke results from the method used for decoking which involves the use of high-pressure water jets to cut the coke out of the coke drums. Complete combustion of the coke will produce about 8000 t/d of CO2 (e.g., 3,000 × 0.735 × 3.67). Eight twin reactor systems required for the production of ∼500 000 bbl/d will emit about 64 000 t/d of CO2. This assumes that all coke is combusted in plant. Therefore, either finding other outlets for the utilization of coke or stockpiling at least its part can decrease the overall CO2 emissions from this source. CO2 from Combustion of Fuel Gas. The volume of fuel gas in Table 3 was estimated in a manner similar to that for the fluid coking process. For case A, it was
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assumed that the total volume of this gas (2.1 × 106 m3/d) was completely combusted in the utility plant to generate electricity and heat for other plant requirements. The excess of electricity produced in the plant may be sold to the grid. Then, the CO2 emissions can be estimated as for the fluid coking case, i.e., 2.1 × 106 × 1.95 ) ∼4100 t/d for one twin reactor system. Scaling up the plant production to about 500 000 bbl/d of synthetic crude, this source would add ∼33 000 t/d of CO2 to the overall plant emissions in the case in which all gaseous byproducts are combusted. Cases B and C are the alternatives to the complete combustion of fuel gas and coke. For Case B it is assumed that 1 × 106 m3/d of the fuel gas was converted to liquid products. This will increase the yield of liquid products by about 5000 bbl/d for one twin reactor system. The remaining 1.1 × 106 m3/d of the fuel gas could be used in the H2 plant to displace a similar amount of natural gas. Therefore, in case B of the delayed coking plant, the CO2 emissions from the combustion of fuel gas could approach nil. At the same time, the scale-up to 500 000 bbl/d of synthetic crude would require only seven twin reactor systems. Case C will involve the complete combustion of fuel gas and the stockpiling of 2000 t/d of coke. As it was determined above, the former would generate ∼33 000 t/d of CO2 for a plant producing 500 000 bbl/d of synthetic crude. CO2 from H2 Plant. In this case, the estimate of the CO2 emissions was based on the H2 consumption used for hydrotreatment of the primary products from the fluid coking process. Thus, the steam reforming of the natural gas to H2 required for hydrotreatment to produce ∼65 000 bbl/d of synthetic crude will release ∼1500 t/d of CO2 for one twin reactor system and ∼12 000 t/d for the plant producing about 500 000 bbl/d of synthetic crude. CO2 from SO2 Scrubbers. The Ca-containing agent (lime) used for the SOx removal from flue gas produced during the combustion of delayed coke (practiced currently) is obtained by the decomposition of limestone. This is a rather minor contributor to the overall plant emission of CO2 compared with the other sources. If all of the 3000 t/d of coke containing 6.2 wt % of sulfur (Table 2) are burnt, this would involve the combustion of 186 t/d (or 5.8 × 106 moles) of sulfur requiring at least ∼12 × 106 moles of Ca (Ca/S ) ∼2) to be converted to CaSO4. This would add the same number of moles of CO2 to the overall emissions, i.e., ∼520 t/d per one reactor pair and a total of ∼4200 t/d for the plant scaled up to produce 500 000 bbl/d of synthetic crude. This amount of CO2 is within the accuracy of the current emissions estimate. II.2. Mining and Separation. With respect to the energy consumption, the mining of tar sands is the most intensive operation in the plants. The technology employed is common for both plants in Fort McMurray. Most of the equipment used for mining operation and transportation of tar sands is driven by electricity which is supplied from the utility plant. Therefore, most of the CO2 emissions associated with the mining operation were already accounted for in the estimate of CO2 emissions from the combustion of coke and fuel gas. A mobile fleet carries on several tasks during the tar sands mining and transportation as well. The equipment
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Table 4. Sources of CO2 Emissions (t/d) from the Scaled-up Plants Producing 500 000 bbl/d of Synthetic Crude Using Fluid Coking and Delayed Coking Process fluid coking production of synthetic crude coke combustion fuel gas combustion flue gas desulfurization hydrotreating mining & separation total CO2 from production, t/d million t/a CO2/synth. crude, t/bbl refining of synthetic crude CO2 from H2 plant, t/d million, t/a CO2/liquid fuel, t/bbl utilization of liquid fuels CO2/liquid fuel, t/bbl million, t/a
delayed coking
case A
case B
9300 21 000 0 12 000
9300 10 500 0 12 000
2000
2000
case A 64 000 33 000 4200 12 000 2000
case B
case C
64 000 nil 4200 12 000
21 000 33 000 4200 12 000
2000
2000
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44 300 ∼16 0.09
33 800 ∼12 0.07
115 000 ∼42 0.23
82 200 ∼30 0.16
72 000 ∼26 0.14
12 000 ∼4.4 0.02
12 000 ∼4.4 0.02
12 000 ∼4.4 0.02
12 000 ∼4.4 0.02
12 000 ∼0.5 0.02
0.4 ∼73
0.4 ∼73
0.4 ∼73
0.4 ∼73
0.4 ∼73
involved is fueled by diesel oil produced on the site. According to the Annual Report 2002 published by Syncrude, the total consumption of diesel oil in 2002 approached 1 million barrels (2500 bbl/d). At the same time, the daily production of synthetic crude approached 250 000 bbl/d. This translates into about 2000 t/d of CO2 emitted from the utilization of diesel oil on the site for a plant producing 500 000 bbl/d of synthetic crude. This amount of CO2 emitted is within the accuracy of the estimate. Because of the similarity of the operations, the amount of CO2 emitted will be similar in both plants, i.e., one employing the fluid coking process and the other delayed coking process. Nevertheless, the total CO2 emissions which are directly associated with the mining operation are rather small compared with the other sources. Little CO2 emissions from the tar sands separation plant can be identified. Thus, most of the units which are part of the operation run on electricity supplied by the utility plant. III. Refining of Synthetic Crude and Utilization of Fuels The final refining of synthetic crude to marketable fuels is performed by petroleum refineries rather than by tar sands plants. The commercial fuels are utilized in various sectors of the economy. The CO2 emissions which are occurring during these activities were compared with those from the production of synthetic crude. It is believed that such a comparison is necessary for identifying the sectors with a great potential for the reduction of CO2 emissions. The synthetic crude produced in the tar sands plants consists predominantly of distillates. The last step of the production involved hydrotreatment to ensure its stability during pipelining to refinery. The CO2 emissions associated with the refining of synthetic crude should be lower than those from the processing of conventional crudes because the former contains little residue. The residue is present in almost every case of a conventional crude. Most of the techniques used for utilization and/or disposal of the refinery residues are the source of CO2 emissions which add to those produced
during the H2 production required for refining. At the same time, in the refinery processing synthetic crude the only CO2 emissions are those from the H2 plant, i.e., ∼0.02 t of CO2 per barrel of fuels, i.e., ∼12 000 t/d for a plant producing ∼500 000 bbl/d of synthetic crude (Table 4). Therefore, refining adds another 4.4 million t/a of CO2. The utilization of fuels produced from synthetic crude, i.e., as the transportation fuels, heating fuels, etc., will release about 0.4 tonne of CO2 per barrel of the commercial fuel produced from synthetic crude (e.g., 0.159 × 0.8 × 0.86 × 3.67 ) 0.4). In this case, the specific gravity and carbon content of the fuel was assumed to be 0.8 kg/L and 86 wt %, respectively. Therefore, the utilization of 500 000 bbl/d of commercial fuels will add ∼200 000 t/d (73 million t/a) of CO2. A similar amount of CO2 is released during the utilization of fuels derived from conventional crudes. It is essential that the amount of emitted CO2 from refining and utilization (e.g., ∼77 million t/a for ∼500 000 bbl/d of fuel) is not added to the total amount emitted during the production of synthetic crude (e.g., 16 million t/a for case A and 30 million t/a for case B for ∼500 000 bbl/d of fuels produced by the fluid coking and delayed coking, respectively). Thus, in the case that synthetic crude would be unavailable, the same amount of liquid fuels would have to be produced, most likely from the imported crude, to meet demand. This suggests that actions taken on the fuel utilization side may have a much greater impact on the reduction of the overall CO2 emissions than the production of synthetic crude and its refining. Thus, significant potential for the improvement in the liquid fuels combustion efficiency during transportation and home heating has been already identified as one of the way to tackle this problem IV. Discussion The present analysis was based on the extensive information on the coking of heavy feeds using both fluid and delayed coking processes which is available in the scientific literature1-9 rather than on plant data. This includes the information on the bitumen obtained from
Emissions of CO2 from Tar Sands Plants in Canada
the Canadian tar sands. It is believed that the developed cases met the objective of the study, i.e., the identification of the sources of the CO2 emissions in tar sands plants, the estimate of the daily and annual emissions of CO2 from the fluid coking plant relative to that from the delayed coking plant and determination of the ratio of the amount of CO2 emitted per barrel of the synthetic crude produced as well as that emitted during the refining of synthetic crude to commercial fuels and their utilization as transportation fuels, heating fuels, etc. The summary of these results is given in Table 4. In the absence of plant data, the estimate of CO2 emissions will be affected by a number of uncertainties. First of all, an accurate estimate of the amount of CO2 from the combustion of fuel gas would require measurements of the yield of the latter and the detailed analysis of its chemical composition, although the natural gas equivalent used for the calculations should give similar results. An uncertainty exists in determining the amount of emitted CO2 from the burner in the fluid coking process. These and other uncertainties can only be eliminated by using the plant data. Despite this, the established database correctly identifies the sources and the ranges of the CO2 emissions from the Canadian tar sands plants. Once the plant data are available, an accurate estimated of the CO2 emitted from tar sands plants can be readily obtained using the methodology developed in the present study. The summary of the results obtained by the calculations is shown in Table 4. Case A was based on the assumption that the entire daily production of fuel gas in both plants was combusted. The CO2 emissions were scaled up to consider the expansion of the production of synthetic crude using the fluid coking and delayed coking process from about 170 000 bbl/d and 65 000 bbl/ d, respectively, to about 500 000 bbl/d. This expansion would result in the combined daily production of ∼1 000 000 bbl/d. By simple prorating the case A, the increased production will result in the total CO2 emissions of ∼42 000 t/d and ∼115 000 t/d for the plants employing the fluid coking and delayed coking processes, respectively. This translates into the annual emissions of 16 million tonnes and ∼44 million tonnes of CO2, respectively. For case A, the amount of CO2 released per one barrel of synthetic crude produced is 0.09 and 0.23 t/bbl for the fluid coking and delayed coking plants, respectively. The obvious advantage of the fluid coking process results from the fact that most of the produced coke is sequestrated and/or stockpiled on the site. It is noted that the suitability of fluid coke for stockpiling was confirmed by the extensive evaluations of its oxidation stability8 and leachability.11 Tests were conducted in accordance with the toxicity characteristics leaching procedure standardized by the US Environmental Protection Agency.12 They confirmed that with respect to the trace elements and volatile organics, the fluid coke is virtually nonleachable and nonflammable. It is believed that delayed coke is benign as well although this would have to be confirmed by tests. Then, together with the sands and clay obtained from the tar (11) Chung, K. H.; Janke, L. C. G.; Dureau, R.; Furimsky, E. Environ. Sci. Eng. 1996, 50. (12) U.S. Environmental Protection Agency Federal Register, 1991.
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sands separation step, the cokes can be used for the land reclamation which has been successfully practiced on the site of both plants. Cases B and C are used to illustrate that there is a potential for the reduction of CO2 emissions by modifying the plant operation. Apparently, case B approaches the current mode of operation in the delayed coking plant. This case assumes that half of the daily production of fuel gas in the fluid coking plant and delayed coking plant was converted to liquid products and the other half to H2. This will reduce the CO2 emissions per barrel of synthetic crude, particularly in the delayed coking plant, i.e., the amount of CO2 released per barrel of the synthetic crude produced decreased from 0.23 t/bbl to 0.16 t/bbl (Table 4). It is again stressed that case B approaches the current mode of the operation in the delayed coking plant. Case C assumes that 2000 t/d of the coke produced in the delayed coking plant will be stockpiled and most of the energy requirements will be supplied by the complete combustion of fuel gas and that of about 1000 tonne of coke. This may result in the further reduction in the overall CO2 emissions. The CO2 emissions from the delayed coking plant would approach those from the fluid coking plant in the case that the entire daily coke production in the former plant is stockpiled. In such a case, CO2 emissions would decrease from 0.23 t/bbl for case A to 0.09 t/bbl for case C. However, the combustion of fuel gas would eliminate oligomerization of hydrocarbons as the additional source of liquid fuels. In such a case, the number of twin reactor assemblies required for the scaling up the production of synthetic crude in the delayed coking plant from 68 000 bbl/d of to 500 000 bbl/d would have to increase from seven to eight. The open pit mining of tar sands is the highly energy intensive operation. Most, if not all energy required for mining and other units operation in tar sands plants can be supplied by the combustion of gaseous byproducts from fluid and delayed coking. For the fluid coking and delayed coking plants producing ∼500 000 bbl/d of synthetic crude each, about 11 × 106 m3/d and 17 × 106 m3/d of fuel gas, respectively, may be produced. It was extrapolated from the available data13 that the normal plant load requirement in the former plant of this size could approach 600 MW.14 This amount of electricity together with about 260 MW of steam can be produced by the combustion of 6.2 × 106 m3/d of fuel gas15 suggesting that some fuel gas is still available to feed plant furnaces and for other needs. The combustion of ∼6.2 × 106 m3 of fuel gas would release ∼12 000 t/d of CO2. Depending on the quality of coal, the same amount of electricity obtained from the coal burning utility using a conventional combustion technology would release about twice more CO2 not including the emissions generated during the mining and transportation of coal. Less of either fluid or delayed coke than a coal would be required to generate this amount of electricity and steam using a conventional combustion technique. However, because the carbon content of coke is generally higher than that of coal, the amount of the CO2 released would be similar.16,17 (13) Syncrude Canada Ltd. Public Affairs Report, 1993. (14) Zheng, L.; Furimsky, E. Energy Convers. Manage., in press. (15) Geosits, R. F.; Mohammed-Zadeh, Y. Power Gen., Americas 93, Dallas, TX, 1993.
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With respect to the reduction of the CO2 emission from the Canadian tar sands plants, the efficient utilization of fuel gas may play an important role. In this regard, the expansion of plants provides an opportunity to further decrease the CO2 emissions by improving the efficiency via integration and cogeneration of electricity and process heat within the plant or even between the fluid and delayed coking plants. Therefore, an independent scaling up of the plants, as it was done above, may in fact represent an overestimate of the overall CO2 emissions. The improved efficiency in the plants may increase the availability of electricity for public consumption. Consequently, this would decrease the total amount of CO2 released in the province providing that the electricity displaced from the market was generated by the combustion of solid fuels. It was indicated earlier that the CO2 emissions from combustion of coke and/or coal are more than twice those from the combustion of fuel gas required to generate the same amount of electricity. The database generated in this study clearly indicates that the choice of process for the expanded production may influence the incremental increase in CO2 emissions. For example, the expansion from ∼170 000 bbl/d to ∼500 000 bbl/d using the fluid coking process using case A (Table 4) would result in the increase of CO2 emissions from ∼5 million tonne annually (t/a) to about 16 million t/a. The expansion from ∼65 000 bbl/d to ∼500 000 bbl/d using the delayed coking process as shown in case B (approaching the current mode of operation) would increase the overall CO2 emissions from ∼4 million t/a to about 30 million t/a, whereas for case C of the delayed coking process, the CO2 emissions would only increase to 23 million t/a. Then, the total CO2 emissions from the expanded plants, producing about 1 million bbl/d of synthetic crude would approach 46 million t/a of CO2 for case A and case B of the fluid coking and delayed coking plant, respectively. This accounts for less than 8% of the total CO2 emissions in Canada. Thus, in 2001, CO2 emissions in Canada approached 600 million t/a. There is little reason to believe that these emissions decreased since that time. The total CO2 emissions can be further reduced by improving overall efficiency, which can be realized from the expanded production. At the same time, almost 50% of the liquid fuels requirement in Canada, which would have to be otherwise imported, can be secured from the expanded operations. The complete combustion of ∼2 million bbl/d of liquid fuels during the annual utilization in Canada will release almost 300 million t/a of CO2 compared with ∼46 million t/a from the tar sands plants producing 50% of this fuel requirement. This again suggests that potential for the reduction of CO2 emissions in Canada is much greater on the fuel utilization side than during the production of synthetic crude from (16) Furimsky, E. Rev. Inst. Fr. Pet. 1999, 54, 597. (17) Furimsky, E. Fuel Proc. Technol., accepted.
Furimsky
tar sands including its refining. For example, about 15% reduction in the consumption of liquid fuels in Canada can account for all CO2 emissions from tar sands plants with the combined production of synthetic crude of 1 million bbl/d. Then, in an attempt to decrease overall CO2 emissions, the densely populated regions of the country should be the focus of attention. The CO2 emissions from the production of synthetic crude from heavy oils in Western Canada were not estimated. It is noted that presently there are two plants which employ hydrocracking technology for the heavy oil upgrading. The pitch obtained after the distillation of hydrocracked products is further converted in the delayed coker. The produced coke may be suitable for the industrial utilization because of the much lower ash content than that of the coke produced in the tar sands plants. The gasification of coke to synthesis gas which after the water-gas shift reaction can be converted to a H2 + CO2 mixture is the potential option to be considered. In this case, H2 can be used in the plant and the associated production of CO2 may be employed in the oil fields for enhanced oil recovery. It is believed that the overall CO2 emissions per barrel of crude produced in these plants are lower than those from the fluid and delayed coking plants. Conclusions The results generated in this study are based on the cases which were developed using the published data. However, the developed methodology can be used for identifying the sources and estimating the ranges of CO2 emissions from both tar sands plants in Canada. On the basis of the same methodology, determination of the exact CO2 emissions from tar sands plants can be readily performed whenever the plant data are available. In 2001, the annual CO2 emissions in Canada approached 600 million metric tonnes. On the basis of the present calculations, a further scaled-up plant to 1 million bbl/d of synthetic crude employing the fluid coking process would emit about 32 million t/a of CO2. This would account for only about 5% of the total national emissions of CO2 while contributing almost 50% to the liquid fuel pool in Canada. The same production plant employing the delayed coking process would release ∼60 million t/a of CO2. This amount would account for about 10% of the Canadian CO2 emissions. However, without any doubt, the scaled-up CO2 emissions represent an overestimate. Thus, a significant reduction of the CO2 emissions per barrel of the synthetic crude produced can be realized from the scaled-up operation. Definitely, such CO2 emissions may be decreased by the choice of coking process as well as the technology (e.g., coking versus hydrocracking) for expansion. EF0301102
