Synthetic Fuels by a Limited CO2 Emission Process Which Uses Both

Sep 25, 2007 - where fossil fuels are used as feedstock and solar energy as heat source. Gasification of coal and steam reforming of natural gas have ...
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Energy & Fuels 2007, 21, 3668–3675

Synthetic Fuels by a Limited CO2 Emission Process Which Uses Both Fossil and Solar Energy Maria Sudiro* and Alberto Bertucco Department of Chemical Engineering (DIPIC), UniVersity of PadoVa, Via Marzolo 9, 35131 PadoVa, Italy ReceiVed June 7, 2007. ReVised Manuscript ReceiVed July 26, 2007

A new process combining fossil fuels and solar energy to obtain synthetic liquid fuels from coal and natural gas is investigated. The aim of the work is to develop a hybrid process for the production of liquid fuels, where fossil fuels are used as feedstock and solar energy as heat source. Gasification of coal and steam reforming of natural gas have been coupled, examined, and simulated with respect to material and energy balances. The technical feasibility of the process is discussed in two different cases: a plant located in Italy (Sicily) and one in the USA (Texas). A comparison in terms of CO2 emissions is performed among the processes presently proposed, a gas to liquid (GTL) process, a coal to liquid (CTL) process, and another hybrid configuration coupling GTL and CTL, called the modified CTL process. It is shown that the CO2 produced per unit weight of liquid fuel is the minimum among all the alternatives considered, except for the GTL one, where the value is comparable. Also, the thermal efficiency of the hybrid process is shown to be higher than those of the other cases examined, i.e., 67% more than the CTL one. The total mirrors area needed for case 1 (Italy) is 5.70 km2 and 3.37 km2 for case 2 (USA), for a production of 100 t/h of liquid fuels.

1. Introduction Coal is the world’s most plentiful fossil fuel resource, with known reserves estimated to last for several centuries ahead, at the present consumption rate. Although it is one of the major sources of energy and, as predicted, it will continue to play an important role in meeting the world’s increasing energy demands in the future, its use requires us to face several challenges. The major one is the considerable emissions of CO2, SOx, and NOx, which cause pollution and climate change, so that using coal as energy source can be a real perspective only when a final solution about CO2 sequestration will be found.1 These emissions can be significantly reduced or even completely eliminated by substituting coal with cleaner fuels, obtained by converting solar energy into chemical energy carriers (called solar fuels) such as solar hydrogen and solar methanol, the latter of which can be long-term stored and longrange transported.2 The thermochemical conversion of concentrated solar heat at high temperature to chemical fuels has the advantage of producing long-term storable energy carriers from solar energy and also enables solar energy transportation from the Sunbelt to remote population centers. However, solar radiation reaching the earth is diluited, intermittent, and unequally distributed.3 On the other hand, the substitution of fossil fuels with solar ones is a long-term goal requiring the development of novel technologies.1 In the 20th century the use of petroleum dominated the process industries and the transportation energy demand. * Corresponding author: e-mail [email protected]; Tel +39-0498275472; fax +39-0498275461. (1) Zedtwitz, P. v.; Steinfeld, A. The solar thermal gasification of coalenergy conversion efficiency and CO2 mitigation potential. Energy 2003, 28, 441–456. (2) Hirsch, D. The solar thermal decarbonization of natural gas. Int. J. Hydrogen Energy 2001, 26, 1023–1033. (3) Steinfeld, A.; Epstein, M. Light years ahead. Chem. Br. 2001, 37, 30–32.

Nowadays, fuels from crude oil supply about 96–98% of the worldwide energy demand for transportation (cars, ships, airplanes) and more than 50% of oil extracted is refined to produce fuels. Estimates of the oil availability span from 40 to 60 years at the present rate of consumption.4 It is important to realize that the world’s petroleum reserves are not infinite. Because they are rapaciously being consumed, their costs are rising, and sooner or later we will have to look for a replacement of petroleum. On the other hand, the currently known reserves of methane and of coal exceed those of crude oil by factors of about 1.5 and 25, respectively.5 Production of syngas from methane or coal and subsequent conversion of syngas to a variety of fuels and chemicals could become increasingly of interest as the reserves of crude oil are depleted and/or the price of crude rises. It is most likely that alternative technologies will be soon developed to produce liquid synthetic fuels with high volumetric density of energy. Alternatives to fossil fuels are biofuels (bioethanol and biodiesel), hydrogen (note that hydrogen is currently obtained from hydrocarbons), and synthetic liquid fuels (from processes known as gas to liquid, GTL, and coal to liquid, CTL); however, any processes using fossil fuels, or synthetic liquid fuels, suffer of the great problem of high CO2 emissions. So our interest in alternative solutions is headed to reduce considerably these emissions. Our goal is to consider midterm solutions such as, for example, hybrid solar/fossil endothermic processes, in which fossil fuels are used mainly as reactants and solar energy is the source of the process heat. An example of such a hybrid process is solar coal gasification; the mixing of coal and solar energy creates a link between today’s coal-based technology and tomorrow’s solar chemical technology.1 Solar coal gasification (4) www.ifp.fr. (5) Dry, M. E. The Fischer–Tropsch Process: 1950–2000. Catal. Today 2002, 71, 227–241.

10.1021/ef7003255 CCC: $37.00  2007 American Chemical Society Published on Web 09/25/2007

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has been already investigated in the USA at the end of 1970 to allow a solar energy storage in solar fuels.6 The aim of this work is develop and simulate a hybrid process that combines the use of coal and natural gas in a single solar reactor to carry out coal gasification and steam reforming simultaneously. As both of these are endothermic processes that produce synthesis gas with different H2/CO molar ratio, they have been coupled to obtain in a single solar reactor a syngas with a H2/CO molar ratio equal to 2; that is exactly the value required by Fischer–Tropsch synthesis (FT). This process has been analyzed in two different scenarios—USA (Texas) and Italy (Sicily), which are typical regions greatly insolated—in order to evaluate how large the area to supply the heat needed in the gasifier-reforming reactor should be. In any case, natural gas is also employed to supplement solar radiation when it is not sufficient to run the reactor, and its amount has been taken into account. The main advantage of this hybrid process is that no oxygen, and its related production cost, is needed. In addition, in the conventional processes for producing synthetic fuels (gas to liquid and coal to liquid) a large amount of CO2 is emitted due to the consumption of coal or natural gas for the endothermic reactions, whereas the hybrid process presently studied reduces CO2 emissions to a minimum value. It will also be shown that the discharge of pollutants is avoided, the gaseous products are not contaminated by combustion byproducts, and the heat value of the fuel is upgraded.

Table 1. Main Elemental Chemical Composition (in wt %), Low Heating Value in (MJ/kg), and Elemental Molar Ratios of H/C and O/C for Two Types of Coala coal type

bituminuos (USA)

anthracite (Germany)

C H O N S LHV (MJ/kg) H/C (mol/mol) O/C (mol/mol)

78.4 5.4 9.9 1.4 4.9 33.7 0.83 0.09

91.8 3.6 2.5 1.4 0.7 36.2 0.47 0.02

a Higman, C.; Van Der Burgt, M. Gasification; Gulf Professional Publishing (Elsevier): Burlington, 2003.

Figure 1. Equilibrium composition of the system bituminous coal–CH4–H2O as a function of temperature at 30 bar (20.08 kg/h of coal, which corresponds to about 1 kmol/h of C, 1 kmol/h of methane, and 2 kmol/h of steam). At 1500 K the H2/CO ratio is 2.19.

2. Process Development We are interested in the steam-gasification of coal, coupled with steam-reforming of natural gas in order to produce syngas, by using concentrated solar radiation and methane combustion as integrated sources of high-temperature energy. A simplified block flow diagram of this process includes a solar gasification reactor, followed by a Fischer–Tropsch (FT) reactor where synthetic fuels are obtained and a section of products separation. It is noteworthy that by coupling coal and methane in a suitable ratio a syngas characterized by a H2/CO molar ratio of 2 can be obtained, which is the precise value of the feedstock composition needed to produce liquid fuels. 2.1. Solar Coal Gasification Steam Reforming and Fischer–Tropsch Synthesis. Steam gasification of coal is a highly endothermic reaction, which is greatly dependent upon energy and high temperature. The energy to drive such reaction is usually supplied by partial coal combustion with oxygen or air, but this mode of operation releases large amounts of CO2 to the atmosphere. Clean high-temperature energy from concentrated solar radiation in the Sunbelt may be utilized to supply the process heat requirements. The use of high-temperature solar heat to drive the endothermic reactions associated with coal gasification has been suggested and investigated for 20 years.7 This process, called “solar coal gasification”, not only produces a highly useful and transportable end product but also results in the storage of a significant fraction of solar energy in the chemical bonds of the fuel molecules. Because of the highly endothermic reactions involved, the syngas obtained has greater calorific value, ideally higher than initial coal by 44–45%.7 Thus, solar energy is transformed into a form that (6) Gregg, D. W. Solar Coal Gasification. Sol. Energy 1980, 24, 313– 321. (7) Kodama, T. High-temperature solar chemistry for converting solar heat to chemical fuels. Prog. Energy Combust. Sci. 2003, 29, 567–597.

is both storable and transportable and can be used in existing equipment and engines. Thermochemical reforming of natural gas is a catalytic reaction between low hydrocarbons, such as methane, with either steam or carbon dioxide. The product is a gaseous mixture of primarily CO and H2. Steam- or CO2-reforming of methane is highly endothermic and is the basis for upgrading the calorific value of the hydrocarbons feed. Steam-reforming of methane and coal gasification are both attractive candidates for converting solar high-temperature heat to syngas and then to liquid chemical fuels; both of them have been investigated and demonstrated to be suitable for about 20 years.7 The steam gasification of coal is a complex system, but the overall chemical conversion can be represented by a simplified stoichiometry (eq 1): C1HxOy + (1 - y)H2O T [(x ⁄ 2) + 1 - y]H2 + CO

(1)

where x and y are the elemental molar ratios of H/C and O/C in coal, respectively. The quality of the chemical product (synthesis gas) depends on both x and y. Table 1 shows chemical composition, heating value, and elemental molar ratios for two different types of coal used in the simulation discussed below. They have been chosen according to the different locations of the plant: anthracite coal for a plant located in Italy and bituminous coal in the USA. Natural gas reforming has been carried out by assuming methane as raw material. In order to develop the final configuration of the hybrid process here presented, we have simplified the steam gasification of coal with the following reaction (eq 2): C(s) + H2O T CO + H2

(2)

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Figure 2. Equilibrium composition of the system anthracite coal–CH4–H2O as a function of temperature at 30 bar (14.63 kg/h of coal, which corresponds to 1 kmol/h of C, 1 kmol/h of methane, and 2 kmol/h of steam). At 1500 K the H2/CO ratio is 2.10. Table 2. Parameter Values for Different Values of Temperature Approach, Where Molar Conversions Are in % (for Bituminuos Coal) T approach (K)

XCH4

XH2O

XC

kmol/h H2S

0 (equilibrium) -5 -10 -15 -20 -30 -40 -50 -100 -150 -200 -300 -400

94.69 94.43 94.16 93.87 93.57 92.94 92.24 91.49 86.66 79.73 70.27 43.40 12.55

87.48 87.39 87.28 87.18 87.07 86.83 86.57 86.29 84.48 81.92 78.50 69.33 57.86

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 95.43

2.31 × 10-2 2.31 × 10-2 2.31 × 10-2 2.31 × 10-2 2.31 × 10-2 2.31 × 10-2 2.31 × 10-2 2.31 × 10-2 2.31 × 10-2 2.31 × 10-2 2.31 × 10-2 2.31 × 10-2 2.32 × 10-2

Table 3. Relationship between Temperature Approach Concept and Equilibrium Distance, in Terms of Conversion % of Methane, Steam, C, and Increasing % of Molar Flow Rate of H2S (for Bituminuos Coal) T approach (K)

XCH4/XCH4 (*)

XH2O/XH2O (*)

XC/XC (*)

kmol/h H2S increase %

0 (equilibrium) -5 -10 -15 -20 -30 -40 -50 -100 -150 -200 -300 -400

100.0 99.72 99.44 99.13 98.82 98.15 97.41 96.61 91.52 84.20 74.21 45.83 13.26

100.0 99.89 99.77 99.65 99.52 99.25 98.96 98.63 96.57 93.64 89.73 79.25 66.14

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 95.43

0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.02 0.03 0.04 0.08 0.24

If we combine in the same reactor the steam-reforming of methane (eq 3) CH4 + H2O T CO + 3H2

(3)

C(s) + CH4 + 2H2O T 2CO + 4H2

(4)

we obtain eq 4:

From this reaction syngas with a H2/CO molar ratio of 2 is obtained, without the need of a water-gas shift section to adjust it. Note that the H2/CO ratio value varies between 1.14 and 1.27 in the case of coal gasification, depending on coal type,1 whereas in the steam-reforming of methane it is around 3. Fischer–Tropsch synthesis has been very well known for long times.5 Reactions occurring in this case can be represented in a simplified way by the following (eqs 5 and 6): nCO + (2n + 1)H2 f CnH2n+2 + nH2O

(5)

nCO + 2nH2 f CnH2n+1OH + (n - 1)H2O

(6)

where n is an integer number. 2.2. Gasification at Equilibrium Conditions. A chemical equilibrium approach has been applied for simulating the solar reactor in the hybrid process. The gasifier–reformer was represented by two units: a yield reactor and an equilibrium reactor at fixed temperature. In the first one coal is broken down into its elements H2, N2, O2, S, C (solid), ash, and water as moisture, whereas in the second unit the equilibrium compositions are calculated, assuming as products the following compounds: H2, CO, CO2, H2O, CH4, C(solid), H2S, COS, and N2 (as an inert), and introducing the following four independent reactions (eqs 7–10): CH4 + H2O f CO + 3H2

(7)

C(s) + H2O f CO + H2

(8)

CO + H2O f CO2 + H2

(9)

COS + H2O f CO2 + H2S

(10)

The last reaction occurs because the compounds H2S and COS are inevitably produced and must be removed before FT synthesis. Equilibrium composition are computed using Aspen Plus as a process simulator. Figures 1 and 2 show equilibrium compositions for the system coal–CH4–steam respectively for bituminous coal and anthracite coal as a function of temperature at 30 bar; these compositions are calculated downstream of ash, H2S, and COS removal sections. In the case of bituminous coal a higher mass flow rate is needed to ensure a molar ratio C:CH4:H2O ) 1:1:2, depending on the lower carbon content, as shown in Table 1.

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Figure 3. Block flow diagram of the hybrid process producing liquid fuels.

From Figures 1 and 2 it is shown that the equilibrium CO2 molar fraction increases up to 900 K, whereas at higher temperatures it decreases down to values of 0.66% and 0.38% at 1500 K for bituminous coal and anthracite coal, respectively. In fact, at lower temperatures the effect of the water-gas shift reaction prevails, whereas at higher temperatures reforming and gasification reactions are dominant, with more H2 and CO produced. Note that at T > 1100 K all the C (solid) reacts whereas high methane conversion requires higher temperature. For example, at 1500 K methane molar equilibrium conversion are 94.69% and 90.38% for case 2 (bituminous coal) and 1 (anthracite coal), respectively. In the final configuration of the reactor the small amount of CO2 produced is separated from gases in a separation step and recycled to the top of the solar reactor to adjust the H2/CO molar ratio to the exact value of 2. 2.3. Gasification at Nonequilibrium Conditions. In real operating conditions lower values than those at equilibrium are achieved at the reactor outlet. It is important to quantify the effect of the equilibrium distance on the flow rate and syngas composition and the corresponding final mass flow rate of liquid fuels produced. In a simplified, nonetheless significant way, this is currently done by specifying the temperature approach to equilibrium; this is defined as the difference between the temperature at which the chemical equilibrium is calculated and the reactor temperature. For all individual reactions (i.e., eqs 7–10), the same temperature approach has been imposed. Another simplification applied to this study is that the extent of carbon conversion is assumed to be independent of kinetic effects (coal particle size distribution, residence time in reactor, etc.). Although this may drive to an apparent better process performance, the improvement acts in the same way for the present hybrid process under study as well as for the base one. The comparison between the two remains thus acceptably valid. To quantify the equilibrium distance, we have chosen methane, steam, and C (solid) molar conversions. Molar conversion (Xi) is defined by eq 11: Xi )

molar flow ratei,in - molar flow ratei,out molar flow ratei,in

(11)

Molar conversions have been related to those corresponding to equilibrium thermodynamic conditions and then summarized in Tables 2 and 3 for the case of bituminous coal, as an example. From Table 2 it is clear that the higher is the distance from equilibrium, the lower is the molar conversion of methane and

Figure 4. Matrix of NRTL binary parameters (A ) Aspen Plus, B ) regressed, C ) assumed, 0 ) zero; 1 ) methanol, 2 ) ethanol, 3 ) 1-propanol, 4 ) 1-butanol, 5 ) 1-pentanol, 6 ) 1-hexanol, 7 ) acetic acid, 8 ) propionic acid, 9 ) butyric acid, and 10 ) water).

steam, whereas for C (solid) a conversion of 100% is obtained even if a temperature approach of 300 K is imposed. 3. Process Simulation The complete process from coal and methane to synthetic fuels has been simulated: it includes the sections of syngas production and purification, FT synthesis, hydrocracking reactor, and separation of products. The process models were developed using Aspen Plus. Material and energy balances were accounted for and solved for every process unit; the temperature approach technique was applied, so that no chemical kinetic models were included in reactor simulation. A plant capacity of 100 t/h of liquid fuel (summing up gasoline, diesel, and LPG) was set, starting from natural gas and coal as feedstocks; two different types of coal were considered (Table 1). A simplified block diagram of the entire process is shown in Figure 3. 3.1. Components and Thermodynamic Model. The following compounds have been selected from the Aspen Plus databank: O2, N2, CO, CO2, H2, H2O, H2S, ethanol, methane, ethylene, ethane, propylene, propane, butene, butane, and all the linear and saturated hydrocarbons from C5H11 to C30H62, C32H66, and C36H74. Besides, other compounds between C37 and C60 have been added to describe rigorously the products of FT synthesis: C37H76, C38H78, C39H80, C40H82, C45H92, C50H102, C55H112, and C60H122. For them a minimum number of properties were introduced into the simulator: vapor pressure (API tables), density, molecular weight, normal boiling point, and critical constants (Joback model). Finally, nonconventional solids have been defined to describe coal and ash. For these components two models were defined: one for density (DCOALIGT) and one for enthalpy (HCOALGEN), which requires to specify proximate analysis and ultimate analysis of the solids.

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Figure 5. Product distribution (calculated and experimental) at the output of the FT reactor.

For both reactors and separators the selection of a suitable thermodynamic model is essential. For the mixtures involved in the three processes both equations of state and gE models were used. The Peng–Robinson equation of state with Boston– Mathias R-function was applied in the main units (reactors, distillation columns, and two-phase separators). In separation sections involving also liquid–liquid–vapor three-phase systems, equilibra have been represented by the NRTL equation. In particular, this detailed thermodynamic model has been useful in the section of water treatment: the water produced from FT synthesis contains oxygenated compounds, which must be removed before discharge. Ten components have been chosen to describe this mixture: methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, acetic acid, propionic acid, butyric acid, and water; alcohols higher than C6 and acids higher than C4 were neglected. For the pairs of components with missing parameters in Aspen Plus we have assumed that the system 1-pentanol–acetic acid has same the parameters as the system 1-butanol–acetic acid; the systems methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, and 1-hexanol with propionic acid and butyric acid have the same parameters as the corresponding systems methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, and 1-hexanol with acetic acid; for the system propionic acid–butyric acid the binary parameters have been set to zero. In Figure 4 the matrix of all pairs of components is shown with the indication on how binary parameters were obtained. Experimental data were retrieved from literature for the systems ethanol–acetic acid and butanol–acetic acid,8 hexanol–acetic acid,9 water–acetic acid and water–propionic acid,10 and acetic acid and propionic acid.11 3.2. Blocks. As far as syngas formation is concerned, in the hybrid process they are produced by a single solar reactor fed by coal, natural gas, and steam. This has been represented by two units: a RYield reactor and a RGibbs equilibrium model. In the first one coal is broken into its elements (hydrogen, nitrogen, oxygen, sulfur, solid carbon, ash, and water as (8) Rius, A. Equilibres liquide-vapeur de mélanges binaires donnant une réaction chimique: systèmes méthanol-acide acétique; éthanol-acide acétique; n-propanol-acide acétique; n-butanol-acide acétique. Chem. Eng. Sci. 1959, 10, 105–111, 288–290. (9) Apelblat, A. Association in Carboxylic Acid-Aliphatic Alcohol Mixtures: The Binary Mixtures of Acid Acetic with n-Butanol, n-Hexanol, n-Octanol and n-Dodecanol. Z. Phys. Chem. (Wiesbaden) 1983, 137, 129– 137. (10) Ito, T.; Yoshida, F. Vapor-Liquid Equilibra of Water-Lower Fatty Acid Systems: Water-Formic Acid, Water-Acetic Acid and Water-Propionic Acid. Chem. Eng. Data 1963, 8, 315–320. (11) Tamir, A.; Wisniak, J. Vapour-Liquid Equilibra in Associating Solutions. Chem. Eng. Sci. 1975, 30, 335–341.

moisture, whereas in the second unit natural gas and steam are added: here the same independent four reactions as before occur (eq 7–10). FT synthesis was modeled by using a RYield reactor. The experimental products distribution is known,12 and syngas conversion is assumed equal to 87%.13 Overall, 44 reactions of the types displayed in eqs 5 and 6 were written for all compounds from CH4 to C60H122 and ethanol. Figure 5 shows the simulated products distribution, which is in acceptable agreement with the experimental profile. Product distribution on a weight basis is: gasoline (C5 to H11) 25.6%, diesel (C12 to H18) 40.3%, waxes (C19 to H60) 31.6%, light gases 1.6%, and oxygenated compounds 1%. Hydrocracking was modeled using a RYield reactor as well. Product yields have been calculated assuming a conversion of the heavy feed (waxes) of 80% while the unreacted 20% is recycled to the reactor, after products separation. The hydrogen flow rate used in this section is 0.65% with respect to the heavy feed.14 Waxes are converted to diesel (80 wt %), gasoline (15 wt %), and gaseous compounds such as methane, ethane, propane, and butane (5 wt %).15 Products separation from FT reactor outlet was achieved by four distillation/stripping columns. The first one uses direct steam injection and has a lateral stripper to recover the diesel fraction; here, the bottom product contains waxes while gaseous compounds and gasoline are extracted from the top. The bottom is sent to the hydrocracking reactor, and the products of the reactor are sent to a second column similar to the first one, with direct injection of steam and a lateral stripper to recover the diesel fraction. The bottom is recycled to hydrocracking reactor, and the top is sent, with the top of the other column, to two columns in series, in order to recover GPL and gasoline products. 3.3. Results. Complete block flow diagrams of the two cases considered are shown in Figure 6 (a, for bituminous coal; 6, for anthracite coal), with indication of all main flow rates. Note that water and CO2 are removed from the syngas before the FT reactor. The CO2 is recycled to the gasifier-reforming reactor to adjust at 2 the value of the H2/CO molar ratio. Downstream (12) Oukaci, R. Fischer–Tropsch Synthesis. In Proceedings of 5th Annual World GTL Conference, London, 2005. (13) Moulijn, J. A.; et al. Chemical Process Technology; John Wiley & Sons, Ltd.: Chirchester, 2005. (14) Sudiro, M. Alternatives fuels production for automotive purposes by synthesis processes: a technical and economical analysis. Chemical Engineering Thesis, DIPIC, University of Padova, 2005. (15) Tijmensen, M. J. A. Exploration of the Possibilities for Production of Fischer–Tropsch Liquids and Power via Biomass Gasification. Biomass & Bioenergy 2002, 23, 129–152.

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Figure 6. (a) Block flow diagram of the hybrid process obtained coupling anthracite coal and natural gas in a solar reactor for producing liquid fuels (case 1: Italy). (b) Block flow diagram of the hybrid process obtained coupling bituminous coal and natural gas in a solar reactor for producing liquid fuels (case 2: USA).

of the FT reactor we have a first section of separation, a hydrocraking reactor to convert heavy hydrocarbons to gasoline and diesel, and finally a second separation section to obtain the synthetic fuels. In order to consider a nonequilibrium model for the solar reactor, a temperature approach of -100 K was reasonably used for all four reactions; accordingly, molar conversions of methane, steam, and coal with respect to equilibrium conversion were calculated. They resulted in 91.52%, 96.57%, 100.0% and 89.93%, 96.19%, 100.0% for the case of bituminous and anthracite coal, respectively, so that the mass flow rates of liquid fuel produced decrease by 6.90% when using bituminous coal and by 7.36% with anthracite coal, with respect to the process

carried out at equilibrium, due to a lower syngas production when operating under nonequilibrium conditions. 4. Discussion: Energy Duties and CO2 Emissions The amount of surface of an heliostat field to provide the heat needed to run the gasifier-reforming reactor previously described was calculated. Experimental data of average daily solar radiation are required to this purpose. Among other possibilities, two different cases have been examined: a plant located in Italy (Sicily) and one in the USA (Texas). Because the process must be run in steady-state conditions, an additional external energy source is needed. In fact, solar plant input is

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limited by diurnal (from 6 A.M. to 6 P.M.), seasonal, and weather-related insolation changes. In order to cope with these fluctuations, the solar plant input must be backed up by fossil fuels. Methane has been selected for these calculations. 4.1. Solar Reactor. Solar chemical reactors for highly concentrated solar systems usually feature the use of a cavity-receiver type configuration. This reflection and concentration of direct irradiation is achieved by sun-tracking mirrors (“heliostats”), which can be of four types: parabolic trough, central power tower, parabolic dish, and the double-concentration systems.7 The solar energy absorption efficiency ηabs of a solar reactor is defined as the net rate at which energy is being absorbed (Qr,net) divided by the solar power coming from the concentrator, i.e., the solar energy input (Qsun) (eq 12): ηabs )

Qr,net Qsun

(12)

The efficiency is expressed by the relationship (eq 13) ηabs ) 1 -

( ) σT4 IC

Qsun Qfield

reactor duty Qreactor (MW) ηabs Qsun (MW) ηfield solar energy (MW) average daily solar radiation per month (kW h/(m2 day)) irradiation time (h) minimum area of mirrors (m2) minimum area of mirrors (km2) noninsolation time (h) fuel consumption (kg/h)c CO2 emissions using solar energy (kg/h) CO2 emissions using fossil fuel (kg/h)

case 1 (Sicily, Italy)

case 2 (Texas, USA)

581.98 0.856 679.51 0.7 970.73 4.09a

547.19 0.856 638.89 0.7 912.70 6.5b

12 5696000 5.70 12 41879 0 115170

12 3370000 3.37 12 39376 0 108280

a Solar Database, available to energy.caeds.eng.uml.edu/index.htm. www.oksolar.com/technical/daiy_solar_radiation.html. c Methane LHV ) 50028 kJ/kg. Perry, R. H.; Green, D. W. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill Book Co.: New York, 1998. b

(13)

where I is the normal beam irradiation (I ) 1 kW/m2) and C is the flux concentration ratio of the solar concentrating system, defined as the ratio of the solar flux intensity achieved after concentration to the incident beam normal irradiation (C ) 2000, dimensionless). T is the nominal cavity-receiver temperature (K), and σ is the Stefan–Boltzmann constant (5.6705 × 10-8 W/(m2 K4)). The value of ηabs expresses the capability of a solar reactor to absorb incoming concentrated solar energy, without including the losses incurred in collecting and concentrating solar energy.2 Crucial to the solar thermochemical path are the parabolicshaped reflectors that concentrate the solar radiation by up to 5000 times. Solar receivers absorb this concentrated solar radiation and deliver it in the form of high-temperature heat.3 We assumed the solar reactor to be a cavity receiver operating at Treactor, with solar energy absorption efficiency, expressed by eq 13. The reactants (coal, steam, and methane) enter the solar reactor and are heated up to Treactor; chemical equilibrium at Treactor is assumed to be achieved inside the reactor, so the net power absorbed in the solar reactor is given by the difference between the enthalpy of products evaluated at Treactor and the enthalpy of reactants (coal, steam, and methane). To find the necessary area of an heliostat field, the optical efficiency ηfield of the solar concentrating system must be known; this is defined as (eq 14) ηfield )

Table 4. Results in Terms of Total Area of Mirrors When Using Solar Energy and Fuel Consumption without Sun for Case 1 (Italy Anthracite Coal) and Case 2 (Texas Bituminous Coal)

(14)

where Qfield is the solar energy incident on the heliostat field.16 From literature an efficiency of the mirrors field of about 0.7017 has been assumed. 4.2. Results. On the basis of the thermal duty of the solar reactor, calculated from process simulation, and of the availability of solar radiation data (from the literature), the total area of mirrors needed for providing the energy required during the day and the flow rate of fossil fuel during the night were evaluated. The results are summarized in Table 4, where also (16) Steinfeld, A. Personal communications, 2006. (17) Kolb, G. J. Recommendations for improvements in the design and operation of future solar central receiver power plants based on experience gained from the solar one pilot plant. Report 899526,performed at Sandia National Laboratories, which is operated for the U.S. Department of Energy, 1985.

Table 5. CO2 Emissions Comparison between the New Process Developed and the Other Conventional Processa kg CO2/kg liquid fuel new hybrid process developed (case 1: Italy) new hybrid process developed (case 2: USA) gas to liquid (GTL) process coal to liquid (CTL) process CTL modified process

0.67 0.64 0.63 4.66 2.70

a We remember that when the fuel is used for automotive purposes, other 3 kg CO2/kg fuel are released to the atmosphere.

Table 6. Comparison between the Entire Process Thermal Efficiency Values for the Four Processes Considered: the Two Conventional Process GTL and CTL, a CTL Modified Process, and the New Hybrid Process Developed (for Case 2: Plant Located in Texas, USA) process GTL process CTL process CTL modified process new hybrid process developed (case 2: USA)

process thermal efficiency: LHV product (MW)/LHV feedstock (MW) 0.629 0.574 0.598 0.957

the CO2 produced when natural gas is used as energy source is reported. Irradiation time has been chosen as 12 h per day.18 In case 1 (Sicily) an area of 5.70 km2 of mirrors is needed, whereas in case 2 (Texas) this is reduced to 3.37 km2. Table 5 summarizes a comparison of the total CO2 production per kg of liquid fuel for the processes considered: the currently developed hybrid process, the conventional GTL and CTL ones, and also a modified CTL process.19 In this last case the CO2 produced from coal gasification is sent to a dry reforming reactor to convert the carbon dioxide to H2 and CO using natural gas. The modified CTL process previously investigated19 allowed to reduce the CO2 produced to about one half with respect to the CTL one; with the newly developed process the amount of carbon dioxide is further reduced by about 85.62% (case 1) and (18) www.sesec.fsu.edu/documents/lectures/ECS2005/SolarThermal.pdf. (19) Sudiro, M.; Bertucco, A. Production of synthetic gasoline and diesel fuel by alternative processes using natural gas and coal: process simulation and optimization. In proceedings of ECOS07, 20th International Conference, Padova, Italy, June 25–28, 2007; Volume II, pp1361–1368.

Synthetic Fuels

Energy & Fuels, Vol. 21, No. 6, 2007 3675

Table 7. Values of Energy Inputs and Outputs for the New Hybrid Process Developed (Case 2: Usa) Used To Calculate the Overall Process Thermal Efficiency input

kg/year

LHV (MJ/kg)

MW

H2 CH4 (during the night) CH4 (for reaction) bituminous coal

2.08 × 106 3.21 × 108 5.73 × 108 7.18 × 108

119.95 50.03 50.03 33.70

8 547 978 629

output

kg/year

LHV (MJ/kg)

MW

GPL gasoline diesel excess steam unreacted syngas

5.71 × 106 2.75 × 108 5.58 × 108 5.59 × 109 2.57 × 108

46.3 44 42.7 –a –b

9 413 812 356 218

a Steam in excess is available at 15 bar (2.90 × 109 kg/year) and at 30 bar (2.69 × 109 kg/year). b Lower heating value of the syngas is calculated from the LHV of single compounds in the mixture.

Table 8. Thermal Efficiency Values for the Syngas Generation Unit of the New Hybrid Process Developed (for Case 2: Plant Located in Texas, USA)a

7; they refer to an annual production (8150 h). From this table the value of the thermal efficiency of the process reported in Table 6 can be immediately calculated. It is clear that the CTL process has the lowest thermal efficiency, and the new hybrid process developed (case 2) yields the highest efficiency increase (66.87%) with respect to CTL. It is finally interesting to evaluate the thermal efficiency of the syngas generation unit alone, which can be calculated as the ratio between the energy content in the syngas and that in the feedstocks: coal and CH4 as reactants and CH4 as heat source (12 h only). This syngas production thermal efficiency is shown in Table 8 (case A), where it is compared with other two values: B accounts also for the steam as a feedstock (only for 12 daylight hours because overnight steam is produced with the flue gas from methane combustion), and in case C steam is taken into account for all 24 h, without recovering any heat from combustion gases. The advantage of using solar energy is evident, as at least 10% of the chemical energy stored in the syngas is provided by solar radiation.

configuration

process thermal efficiency: LHV product (MW)/LHV feedstock (MW)

5. Conclusions

A B C

1.166 1.134 1.103

The production of synthetic fuels by a process combining steam gasification of coal, steam-reforming of natural gas in a solar reactor to produce syngas, and Fischer–Tropsch synthesis with hydrocracking downstream was developed and simulated with respect to material and energy balances. It was shown that such a process, in terms of CO2 emissions, is much better than other processes investigated (coal to liquid and modified coal to liquid) and reduces the CO2 produced per unit mass of liquid fuel to a value typical of a gas to liquid process (0.67 kg/kg fuel). The minimum mirrors surface needed to run the solar reactor is 5.70 km2 in case 1 (Sicily), whereas in case 2 (Texas) it is 3.37 km2, for a production of about 100 t/h of liquid fuels. The calculated thermal efficiency of the new process developed is not far from 100% and is much higher (i.e., 67% more) than that of a carbon to liquid process. It is suggested that solar gasification of coal supported by simultaneous methane reforming could play an important role in the near future for production of liquid fuels from coal.

a A is calculated without steam energy content as feedstock; B and C are with steam for only 12 h and for 24 h, respectively.

86.27% (case 2) with respect to reference CTL process and is quite close to that coming out of a GTL plant. These are mean values because they were calculated with an average daily solar irradiation; in real operating conditions there are periods where solar radiation is less, thus requiring an external heat source is needed, and periods where it is higher. In this last case a system of energy storage is needed. Another result is given by the thermal efficiency of the entire process, which can be calculated on the basis of material and energy balances from simulation, as the ratio between the energy contents in the products and in the feedstocks, based on lower heating values (note that products are not only liquid fuels but also the steam in excess and the unreacted syngas). The efficiency values of the four processes considered are compared in Table 6. To calculate those overall thermal efficiencies of the process, values of the energy inputs (both solar and from fuels) and of the energy outputs used were taken from the simulation. In the case of the new hybrid process developed (case 2: USA) these values are summarized in Table

Acknowledgment. We thank Ing. Marco Fontana very much for helpful discussions and appreciated suggestions. EF7003255