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Utilization of chemical looping strategy in coal gasification processes
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Particuology 6 (2008) 131–142
Utilization of chemical looping strategy in coal gasification processes Liangshih Fan ∗ , Fanxing Li, Shwetha Ramkumar Department of Chemical and Biomolecular Engineering, The Ohio State University, United States Received 5 February 2008; accepted 11 March 2008
Abstract Three chemical looping gasification processes, i.e. Syngas Chemical Looping (SCL) process, Coal Direct Chemical Looping (CDCL) process, and Calcium Looping process (CLP), are being developed at the Ohio State University (OSU). These processes utilize simple reaction schemes to convert carbonaceous fuels into products such as hydrogen, electricity, and synthetic fuels through the transformation of a highly reactive, highly recyclable chemical intermediate. In this paper, these novel chemical looping gasification processes are described and their advantages and potential challenges for commercialization are discussed. © 2008 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. Keywords: Chemical looping; Gasification process; Combustion; Hydrogen
1. Introduction Coal, as a vital energy source for the modern society, accounts for 40% of the electricity generated world wide. At present, more than 90% of the electricity generated from coal is produced through coal-fired power plants, which emit nearly 2 gigatons of carbon into the atmosphere every year (World Coal Institute, 2005). In a typical coal-fired power plant, the heat released from burning coal is used to generate high-pressure steam, which drives a steam turbine-generator. Due to the highly corrosive nature of steam, the turbine operating temperature is limited, thereby curbing the efficiency of the process (Shinada, Yamada, & Koyama, 2002). In most cases, traditional combustion power plants convert only a third of the energy value of coal into electricity. Moreover, coal combustion with air produces a flue gas stream at atmospheric pressure that consists of ∼80% N2 (by volume) and 10–15% CO2 . The low concentration of carbon dioxide in the flue gas stream makes carbon capture from traditional coal combustion power plants inefficient and uneconomical. The global on-going R&D efforts on coal combustion encompass enhancement of electricity generation efficiency to above 40% by the use of “super critical” and “ultra super critical” steam boilers (Kitto, 1996). However, enforcement of future
carbon regulations would nevertheless impose a severe energy penalty to the ultra super critical power plants. It is estimated by the USDOE that the incorporation of CO2 capture in ultra super critical power plants would increase the cost of electricity by 84% (NETL, 2007). Other efforts such as chilled ammonia process and oxyfuel combustion process focus on the cost reduction for CO2 capture in coal combustion plants. These carbon capture techniques, however, would consume 25–28% of the total electricity generated from the power plant (Chˆatel-P´elage et al., 2005). Another approach to utilize coal is through gasification. The gasification processes, unlike traditional combustion processes which fully oxidize carbonaceous fuels to generate heat, convert the fuels such as coal or biomass into syngas via partial oxidation reactions using oxygen and/or steam. In the coal gasification process, coal first reacts with steam and/or oxygen to generate raw syngas. The raw syngas, with pollutants such as H2 S, ammonia, and mercury, is then purified and sent to a gas turbine-steam turbine combined cycle system for electricity generation and this is known as the integrated gasification combined cycle (IGCC) process. The electricity generation efficiency of the IGCC process can be higher than 45% without CO2 capture. Alternatively, the CO in the syngas stream can be further converted to H2 through the water gas shift (WGS) reaction: CO + H2 O → CO2 + H2
∗
Corresponding author. Tel.: +1 614 688 3262. E-mail address: [email protected] (L.S. Fan).
Thus, the resulting gas stream contains a high CO2 concentration (up to ∼40% by 1 volume, dry basis). The CO2 in the
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doi:10.1016/j.partic.2008.03.005
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Fig. 1. Schematic diagram of coal gasification processes for electricity, hydrogen, liquid fuel production.
gas mixture can be captured with ease to yield high-purity H2 . The H2 can be used to generate electricity through a combined cycle system or a fuel cell system without carbon emissions. This represents a typical coal gasification scheme for hydrogen production or electricity generation when CO2 capture is mandatory. Besides electricity and H2 generation, syngas can also be converted to chemicals and liquid fuels such as diesel and gasoline through the Fischer–Tropsch (F–T) reactions given below: (2n + 1)H2 + nCO → Cn H2n+2 + nH2 O Fig. 1 shows a schematic diagram of coal gasification processes for production of hydrogen, electricity and/or liquid fuels. As can be seen in the figure, the major advantages of gasification over combustion include product flexibility and ease in pollutant control. Therefore, gasification is considered a favorable candidate for use in next-generation coal-based plants (NETL, 2003, 2004, 2007; Stiegel & Maxwell, 2001). It is also considered to be a key enabling technology to the U.S. Department of Energy (USDOE) Vision 21 program (National Research Council, 2000). Despite its various advantages over direct combustion, gasification faces several challenges that impede its broad applications. Such challenges include intensive capital investment and large operating cost, both of which result from the elaborate oxygen production and syngas cleaning procedures. It has been estimated by Lewandowski and Gray (2001) that the capital investment for an oxygen-blown IGCC power plant is about $1241/kW, compared to $1170/kW for a pulverized coal combustion power plant with an ultra-supercritical boiler. Both technologies have similar energy conversion efficiencies. Additionally, although CO2 can be captured from a coal gasification system with ease compared to that from a coal combustion system, CO2 capture would nevertheless impose
a significant penalty for the system economics. For example, the state-of-the-art CO2 capture technique would increase the cost of electricity by 25% in a coal based IGCC plant (NETL, 2007). Thus, future coal gasification processes should be simplified through process intensification and coupled with more efficient carbon capture schemes in order to reduce the capital investment and to further increase the energy conversion efficiency. The implementation of the chemical looping concept in coal gasification processes represents a promising method to achieve these goals. The chemical looping concept has been widely applied to combustion processes for electricity generation from gaseous fuels (Ishida, Jin, & Olamoto, 1998; Mattisson, Johansson, & Lyngfelt, 2006; Ryu, Jin, & Yi, 2004). During the last decade, the chemical looping gasification strategy has been applied to produce hydrogen from coal and coal derived syngas. Among them, the GE fuel-flexible process (Rizeq et al., 2002) and the ALSTOM hybrid combustion–gasification process (Andrus et al., 2006) have been extensively studied. Both technologies use two different types of particles to convert coal into hydrogen: one type of particle is used to capture CO2 while the other serves as an oxygen carrier. The mixing between the two types of particles results in a decrease in the energy conversion efficiency. In this paper, three novel chemical looping gasification processes developed at OSU, i.e. the syngas chemical looping (SCL) process, calcium looping process (CLP), and coal/biomass direct chemical looping (CDCL) process, are described (Fan, Gupta, & Iyer, 2007; Fan, Li, Velazquez-Vargas, & Ramkumar, in press; Fan, Gupta, Velazquez-Vargas, & Li, 2007; Gupta, 2006; Gupta, VelazquezVargas, & Fan, 2007; Gupta, Velazquez-Vargas, Thomas, & Fan, 2004; Iyer, Fan, & Ramkumar, 2007; Iyer, Ramkumar, Wong, & Fan, 2006; Thomas, Fan, Gupta, & Velazquez-Vargas,
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2005; Velazquez-Vargas, Thomas, Gupta, & Fan, 2004 ). Unlike the GE and ALSTOM processes, each of these three processes utilizes a single type of particle optimized for its specific energy conversion scheme. The advantages and potential challenges of these processes are discussed in the succeeding sections. 2. Syngas chemical looping (SCL) process 2.1. SCL Process for Hydrogen Generation The SCL process co-produces hydrogen and electricity from syngas based on the cyclic reduction and oxidation of specially developed metal oxide composite particles (Gupta, VelazquezVargas, Li, & Fan, 2006; Gupta et al., 2007). The SCL process produces a pure hydrogen stream and a concentrated carbon dioxide stream in two separate reactors. Therefore, additional CO2 separation cost is avoided. Moreover, electricity can be efficiently co-generated with hydrogen in the SCL process, making it more flexible as well as economically attractive. 2.1.1. Process overview The SCL process consists of five major components: an air separation unit (ASU), a gasifier, a gas clean-up system, a reducer and an oxidizer. Fig. 2 shows a simplified block diagram of the SCL process. In the SCL process, a high-purity oxygen stream (oxygen concentration >95%) from an ASU is sent to the gasifier. The gasifier utilizes the oxygen from the ASU to partially oxidize coal forming high temperature raw syngas with contaminants such as particulates, sulfur, mercury, and halogen compounds. After the high temperature raw syngas quenching and particu-
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late removal steps, the pollutants in the gas stream are removed from the raw syngas using gas cleanup units that include acid gas removal (AGR) units and mercury removal units. These steps are identical to the modern coal gasification to hydrogen process shown in Fig. 1. The difference between SCL process and the state-of-the-art coal-to-hydrogen process lies in the H2 generation strategy. Compared to the coal-to-hydrogen process which requires a set of water–gas-shift reactors, CO2 separation units, and pressure swing adsorption units for the production of high-purity H2 from syngas, the SCL process utilizes only three major units to convert syngas into pure hydrogen (purity >99.95%) and electricity. The simplified overall energy conversion scheme results from the utilization of chemical looping strategy in modern coal gasification processes. In a typical SCL configuration, the syngas is converted to hydrogen in three major units, i.e. a reducer, an oxidizer, and a combustion train. 2.1.2. Reducer The purified syngas from the gas cleanup units is introduced into the reducer, which is a moving bed of iron oxide composite particles, 2–10 mm in diameter, at 750–900 ◦ C and 30 atm. In this reactor, the syngas is completely converted into carbon dioxide and water while the iron oxide composite particles are reduced to a mixture of Fe and FeO (reactions (1)–(4)). Fe2 O3 + CO → 2FeO + CO2
(1)
FeO + CO → Fe + CO2
(2)
Fe2 O3 + H2 → 2FeO + H2 O
(3)
FeO + H2 → Fe + H2 O
(4)
Fig. 2. Simplified schematic of the syngas chemical looping process for hydrogen production from coal.
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The overall reaction occurring in the reducer can be either slightly endothermic or slightly exothermic depending on the syngas composition, reaction temperature, as well as the particle reduction rate. The mild endothermic to mild exothermic nature of the reducer simplifies the heat integration of the reducer reactor, since heat can be easily carried in or out of the reactor by the particles. Since the syngas is completely oxidized by the Fe2 O3 composite particles, the exhaust gas from the reducer contains only CO2 and steam. Steam can be condensed out by extracting heat from the high temperature exhaust gas. This would result in a concentrated high-pressure CO2 stream that can be directly transported for sequestration. 2.1.3. Oxidizer Having been reduced in the reducer, the particles are then introduced into the oxidizer. In the oxidizer, the reduced particles react with steam to produce a gas stream that contains solely H2 and unconverted steam. Once the steam is condensed out from the gas stream, a H2 stream with a very high purity (>99.7%) can be obtained. The reactions involved in the oxidizer include: Fe + H2 O(g) → FeO + H2
(5)
3FeO + H2 O(g) → Fe3 O4 + H2
(6)
The steam used in the oxidizer is produced from the syngas cooling units and reducer/oxidizer exhaust gas cooling units. The oxidizer reactor operates at 30 atm and 500–750 ◦ C. Under such reaction conditions, both reactions (5) and (6) are slightly exothermic. The heat released from the oxidation of the particles to Fe3 O4 is used in the same reactor to preheat the steam feedstock. By introducing pressurized steam with a temperature lower than the oxidizer operating temperature, the oxidizer can be adjusted to operate under thermally neutral conditions. The reactor design is thus simplified. In the SCL process, hydrogen is produced using the chemical looping reforming concept, i.e. the syngas is indirectly converted to hydrogen with the assistance of iron oxide particles. This is fundamentally different from the traditional coal-tohydrogen process where the syngas is shifted to make hydrogen and followed by a CO2 removal operation. By using the syngas chemical looping process, the CO2 and hydrogen are produced from two different reactors, thereby eliminating the energy consuming CO2 separation and compression step. Since a significant portion of the carbon management cost is associated with the separation and compression of CO2 , the SCL process offers a major advantage over the traditional coal-to-hydrogen process. 2.1.4. Combustion train The Fe3 O4 formed in the reducer reactor is regenerated to Fe2 O3 in a unit called the combustion train. The combustion train is a riser driven by compressed air with a pressure slightly higher than 30 atm in order to compensate the pressure drop throughout the train. This unit serves as a pneumatic conveyor to transport solid discharged from the oxidizer reactor to the inlet of the reducer reactor. It also serves as a heat generator since significant amount of heat is produced during the oxidation of
Fe3 O4 to Fe2 O3 : 4Fe3 O4 + O2 → 6Fe2 O3
(7)
The high pressure high temperature spent air (mainly N2 ) produced from the combustion train can be used to drive a gas turbine-steam turbine combined cycle system to generate electricity for parasitic energy consumptions. In yet another configuration, a portion or all of the reduced particles from the reducer reactor can be directly sent to the combustion train without reacting with steam in the oxidizer reactor. By doing this, more heat would be available for power generation at the cost of decreased hydrogen production. In a typical SCL configuration, the reducer is installed on top of the oxidizer and the particles move from the inlet of the reducer all the way to the outlet of the oxidizer under gravity. The particles discharged from the oxidizer are then conveyed back to the reducer inlet by the combustion train. As can be seen, the SCL system can be considered as a special type of circulating fluidized bed in which the reducer and the oxidizer serve as the downer while the combustion train is the riser. The step of combusting the reduced particles is known as chemical looping combustion, which differs from the chemical looping reforming step in that the particle regeneration is performed with steam rather than oxygen. Hence, the utilization of both chemical looping reforming and chemical looping combustion concepts in the SCL system makes it flexible to adjust the ratio between H2 and electricity product. Moreover, only one type of highly reactive particle is circulating
Fig. 3. Bench scale demonstration unit (2.5 kWth ) for SCL process.
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among the three units, thus minimizing the solid circulation rate within the system. The operations of all three major reactors in the SCL process have been demonstrated individually with satisfactory results. The reducer and oxidizer were individually tested in a bench scale chemical looping demonstration unit while the combustion train was simulated in an entrained bed reactor assembly; both units were constructed at OSU. Fig. 3 shows the chemical looping demonstration unit, which is a moving bed reactor with a capacity of 2.5 kW thermo. The reactor allows for a large degree of flexibility in reaction conditions such as temperature, particle size, gas–solid contacting pattern and solid and gas flow rates. Gas and solid composition profile can be
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obtained over the length of the reactor bed once steady state operation is established. Useful information such as the reaction rates and gas–solid conversions can be elucidated by analyzing the demonstration results. Current demonstration results (Gupta et al., 2007) show >99.9% syngas conversion in the reducer and >99.95% purity hydrogen stream from the oxidizer. Nearly full conversion of gaseous hydrocarbons such as CH4 is also obtained in the SCL demonstration. Based on the bench scale demonstration results, an ASPEN® simulation model was constructed in order to evaluate the performance of the SCL process (Fig. 4b). The simulation outcomes for the SCL process were compared to those obtained from an ASPEN® model for the state-of-the-art
Fig. 4. ASPEN® simulation model. (a) State-of-the-art coal-to-hydrogen process; (b) SCL process.
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coal-to-hydrogen process (Fig. 4a) based on identical assumptions. The simulation results show that the overall efficiency for SCL process to be 64% (HHV) with 100% carbon capture as compared to 57% (HHV) for state-of-the-art coal-to-hydrogen process (Stiegel & Ramezan, 2006) (Fig. 4a). 2.2. Application of SCL process in coal-to-liquids processes Other than serving as a stand alone hydrogen/electricity producer, the SCL process can be integrated into other processes to improve the overall energy conversion scheme. The integration of the SCL process in the sate-of-the-art coal-to-liquids (CTL) process (Fan, Gupta, Velazquez-Vargas et al., 2007) exemplifies such novel application of the SCL process. There are several different configurations to incorporate the SCL system into the CTL process. In one conservative configuration, the SCL process is used as a retrofit to the traditional CTL plant. The role of SCL system is to compensate for the hydrogen deficit in the syngas from the modern gasifier. The schematic flow diagram of such configuration is illustrated in Fig. 5. In a state-of-the-art CTL plant using cobalt based catalyst, the syngas generated from the gasifier has a hydrogen concentration (30–40%) much lower than the optimum H2 content for liquid
fuel synthesis (67%). This deficit in hydrogen is usually met by converting a significant portion of the syngas from the gasifier into hydrogen using the traditional coal-to-hydrogen approach. Meanwhile, the Fischer–Tropsch (F–T) reactor converts only part of the syngas (60–85%) into a wide variety of hydrocarbons ranging from methane to hard wax. The gaseous hydrocarbon fuels and unconverted syngas are considered to be by-products from the F–T reactor. In a typical CTL configuration, a large portion of these gaseous by-products from the F–T reactor are combusted to generate electricity. In the SCL–CTL configuration shown in Fig. 5, the gaseous fuels and unconverted syngas from the Fischer–Tropsch reactor are introduced into the reducer of the SCL process. These gaseous fuels are converted to carbon dioxide and water through the following reaction. Cx Hy Oz + (2x + y/2 − z)MO → (2x + y/2 − z)M + xCO2 + y/2H2 O
(8)
Here MO and M refer to the different iron oxide phases. Reaction (8) reduces the iron oxide from higher oxidation states to lower oxidation states. The reduced iron particles are then introduced into the oxidizer where they are reacted with steam to produce pure hydrogen and regenerate the iron oxide
Fig. 5. Syngas chemical looping enhanced coal-to-liquids (SCL–CTL) process.
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(reaction (9)). M + H2 O → MO + H2
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that operates all three major SCL units is currently under construction. (9)
As can be seen, the major feedstock for the SCL reducer is the by-products from the F–T reactor. Meanwhile, the large amount of medium pressure (∼25 atm) steam generated by the waste heat from the F–T reactor provides ample supply for the steam required in the SCL oxidizer. Therefore, through the utilization of the SCL process, hydrogen, an essential feedstock for the CTL process, is generated from the by-products of the F–T synthesis. The liquid fuel yield of the CTL process is thus improved. Furthermore, the integrated carbon capture capability of the SCL process makes the SCL–CTL configuration even more attractive under a carbon constrained scenario. The SCL–CTL process has been independently evaluated by Noblis System under the contract with National Energy Technology Laboratory (NETL), USDOE. The analysis report (Tomlinson & Gray, 2007) states that “the (syngas) chemical looping systems such as that proposed by OSU have the potential to significantly (∼10%) increase the yield of the conventional cobalt based F–T process and allow more efficient heat recovery and much lower (∼19%) carbon emissions”. To generalize, the syngas chemical looping process has been successfully demonstrated in a 2.5 kWth bench scale unit. The demonstration results show that the SCL process can distinctively improve the energy conversion efficiency and product yield of the state-of-the-art coal gasification processes. Although no significant impediment for the industrial applications of the SCL process has been identified to date, high temperature solid handling is recognized as the major challenge for SCL commercialization. This issue can be addressed through scale up demonstrations of the process. A 25 kWth sub-pilot scale unit
3. Iron based chemical looping process for direct coal gasification—coal direct chemical looping (CDCL) process Unlike the SCL process which employs a conventional gasification system to generate syngas feedstock, the iron based coal direct chemical looping (CDCL) process takes coal directly as feedstock rather than syngas. This offers several advantages such as the reduction in O2 consumption and process intensification. Fig. 6 shows a simplified flow diagram of the CDCL process. The CDCL process has three main reactors, i.e., the coal reactor, the hydrogen reactor, and a combustion reactor. The coal reactor converts the coal into CO2 and H2 O while reducing Fe2 O3 to a mixture of Fe and FeO, the hydrogen reactor oxidizes the reduced Fe/FeO particle to Fe3 O4 using steam while producing a H2 rich gas stream. The combustion reactor pneumatically transports the Fe3 O4 particles from the H2 reactor outlet to the fuel reactor inlet using air, the Fe3 O4 particles are reoxidized to Fe2 O3 during the conveying process. 3.1. Fuel reactor The fuel reactor is a moving bed reactor operating at 750–900 ◦ C, 1–30 atm. The desirable reaction in the fuel reactor is: C11 H10 O(coal) + 26/3Fe2 O3 → 11CO2 + 5H2 O + 52/3Fe
Fig. 6. Simplified schematic of the coal direct chemical looping (CDCL) system.
(10)
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reactor is recycled to the bottom of the fuel reactor. By doing this, the steam formed from the reaction between hydrogen and FeO at the bottom of the fuel reactor would enhance char gasification. As opposed to other chemical looping processes which utilize fluidized bed reactor to convert fuels (Andrus et al., 2006; Mattisson, Lyngfelt, & Cho, 2001; Rizeq et al., 2002), the SCL and CDCL processes use countercurrent moving bed to increase gas–solid conversion. Such gas–solid contacting pattern is thermodynamically advantageous. The utilization of such patented countercurrent moving bed configuration represents a key enabling factor that makes the SCL and CDCL processes technically and economically attractive (Fan, Gupta, Velazquez-Vargas et al., 2007; Thomas, Fan, Gupta, and Velazquez-Vargas (2004)). 3.2. Hydrogen reactor Fig. 7. Gas–solid contacting pattern of the fuel reactor.
Here, the coal considered is Pittsburgh #8 and is represented by C11 H10 O given the elemental composition (Stultz & Kitto, 1992). This reaction is highly endothermic with a heat of reaction of 1794 kJ at 900 ◦ C. Therefore, heat needs to be provided to the first reactor. In order to balance the heat, coal is partially combusted in situ by sending substoichiometric amounts of O2 (according to coal) into the fuel reactor. The overall reaction, with negligible heat of reaction, is given as
3.3. Combustion reactor
C11 H10 O + 6.44Fe2 O3 + 3.34O2 → 11CO2 + 5H2 O + 12.88Fe
The hydrogen reactor is a moving bed reactor that operates at 700–900 ◦ C, and 1–30 atm. In the hydrogen reactor, the Fe and FeO mixture from the fuel reactor reacts with steam in a countercurrent mode to achieve maximum solid–gas conversion. The product of the reactor is Fe3 O4 particles as well as H2 . The reactions in the hydrogen reactor are the same as those in the oxidizer in the SCL process (reactions (5) and (6)). This reaction is slightly exothermic and therefore, steam with a lower temperature is introduced into the reactor to moderate the temperature of the reactor.
(11)
Since the amount of oxygen required for reaction (11) is significantly less than that for the coal gasification reactions, the size of the air separation unit is smaller than those used in traditional gasification processes. This corresponds to savings in both operation cost and capital investment of the coal to hydrogen plant. Fig. 7 shows the gas–solid contacting pattern inside the fuel reactor. Fresh Fe2 O3 composite particles are fed from the top of the fuel reactor while coal is introduced in the middle section of the reactor. This configuration allows the volatiles from the coal to react with Fe2 O3 particles to form CO2 and H2 O as they move upward. The devolatilized coal char, however, move downwards along with the partially reduced Fe2 O3 particles. The coal char is gradually gasified by the CO2 and H2 O formed at the lower part of the reactor. Provided enough residence time, which is estimated to be 30–90 min depending on the operating temperature and pressure, coal char can be converted fully, while the Fe2 O3 reduces to a mixture of metallic iron and FeO. Coal ash will come out along with the particles. Thus, the product from the fuel reactor is a solid stream with Fe, FeO, and coal–ash and a gas stream with mainly CO2 and H2 O. By condensing out the H2 O in the gas stream, a ready-to-sequester CO2 stream is obtained from the fuel reactor without the inherent separation costs. In order to promote the reaction rate between solids, a small quantity of hydrogen gas (<5%) produced from the hydrogen
In order to fully convert the CO and H2 from the char gasification into CO2 and H2 O, Fe2 O3 have to be used as the feedstock for the fuel reactor. Therefore, the Fe3 O4 from the hydrogen reactor needs to be oxidized. This regeneration step is done in the combustion reactor. The combustion reactor is a fluidized bed reactor operated at a pressure similar to that of both the fuel reactor and the hydrogen reactor in an adiabatic mode. Air is used to pneumatically convey the Fe3 O4 particles from the outlet of the hydrogen reactor to the fuel reactor inlet. The Fe3 O4 particles are oxidized to Fe2 O3 in this step (reaction (7)). This highly exothermic reaction will heat up the solid as well as the gas. The hot solid would be subsequently fed into the fuel reactor to partially compensate for the heat needed to convert the coal. The coal ash, with significantly smaller size than the Fe2 O3 composite particles, will be separated out from the cyclone on the top of the fuel reactor; makeup particles will also be introduced to the fuel reactor along with the regenerated particles. The CDCL process is currently under demonstration in a 2.5 kWth moving bed unit with encouraging results. ASPEN® simulation studies (Gupta et al., 2006) showed that a maximum coal to hydrogen conversion efficiency of above 80% (or 0.18 kg H2 /kg coal) can be achieved in the CDCL process. Potential challenges regarding this process include demonstration of high temperature solid handling and minimization of reactor volumes through further enhancement of reaction rates. Such challenges will be addressed through particle optimization and larger scale demonstrations.
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4. Calcium looping process (CLP) Commercially high-purity hydrogen is produced from syngas obtained by coal gasification by the water gas shift reaction (WGSR) in high- and low-temperature shift reactors using iron oxide and copper catalysts respectively. However for hydrogen production at high temperatures, the WGSR is thermodynamically limited which necessitates the addition of excess steam. Further, the commercial iron oxide and copper catalysts have a low tolerance to sulfur and chloride impurities and are prone to deactivation. Hence syngas clean up is required upstream of the shift reactors, which is conventionally achieved by using low-temperature scrubbing, which requires additional equipment and is highly energy-intensive due to the cooling and heating of the gas streams. In contrast, the calcium looping technology obviates the need for a catalyst and achieves simultaneous removal of carbon dioxide, sulfur, and chloride impurities at high temperatures during the production of high-purity hydrogen and yields a sequestration-ready CO2 stream during the regeneration phase. Thus, the calcium looping process not only improves the hydrogen yield and purity but also integrates a CO2 management scheme in the hydrogen production process. In addition to the production of hydrogen from syngas, the calcium looping process can also be applied to the production of liquid fuels. It is capable of producing a stream with a 2:1 ratio of H2 to CO while removing sulfur and chloride impurities to ppb levels which is required for liquid fuel synthesis by the Fischer–Tropsch reaction. Fig. 8 illustrates the schematic integration of the calcium looping process in a typical coal gasification system and shows
Fig. 9. Calcium looping process for the production of hydrogen.
that it is versatile and is capable of producing hydrogen for fuel cells, liquid fuel synthesis by Fisher–Tropsch reaction and electricity generation in a gas turbine. As shown in Fig. 9, the calcium looping process comprises of two reactors; the carbonation reactor where high-purity hydrogen is produced while contaminant removal is achieved and the calciner where the calcium sorbent is regenerated and a sequestration-ready CO2 stream is produced. A detailed explanation of the hydrogen production and sorbent regeneration sections of the calcium looping process are provided below.
Fig. 8. Schematic flow diagram of the calcium looping process.
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4.1. Carbonation reactor The carbonation reactor comprises of either a fixed fluidized bed or an entrained flow reactor that operates at high pressures ranging from 20 to 30 atm and temperatures of 550–650 ◦ C. The exothermic heat released from the carbonation reactor can be used to generate electricity in order to meet the parasitic energy requirement of the overall process. In the carbonation reactor, the thermodynamic constraint of the water–gas-shift reaction is overcome by the incessant removal of the CO2 product from the reaction mixture which enhances H2 production. This is achieved by concurrent water–gas-shift reaction and carbonation reaction of calcium oxide to form calcium carbonate thereby removing the CO2 product from the reaction mixture. In order to attain high CO2 capture capacity, the morphological properties of the calcium sorbent are tailored using surface modifiers (Agnihotri, Chauk, Mahuli, & Fan, 1999) The calcium oxide obtained from the Ohio State University patented mesoporous high reactivity precipitated calcium carbonate (PCC) sorbent attains an initial capture capacity of 70% by weight and a capture capacity of 40–36 wt.% over 50–100 cycles, which is significantly higher than most of the high temperature sorbents reported in literature (Iyer, 2006; Iyer, Gupta, Sakadjian, & Fan, 2004). In addition, the PCC-CaO sorbent is also capable of reducing the concentration of sulfur and halides in the outlet stream to ppb levels. Favorable conditions are maintained within the carbonation reactor for the reaction between the calcium sorbent and the sulfide and halide impurity. Since the water–gas-shift reaction is conducted in the presence of the PCC-CaO sorbent, equilibrium favors the formation of H2 . Hence, stoichiometric quantity of steam is sufficient to produce high-purity hydrogen. Thermodynamics predicts that the removal of H2 S and HCl impurities using calcium oxide is inhibited by the presence of steam and the removal of COS is inhibited by the presence of CO2 . Since almost all the steam is consumed in the enhanced WGSR and all the CO2 is removed by the sorbent, the removal of H2 S, HCl, and COS is also favored in the system. In addition to the production of hydrogen from syngas, it is also possible to reform hydrocarbons in the same system, thus making the system fuel flexible. When the calcium looping process is applied to the production of liquid fuels, as can be seen in Fig. 10, the unreacted syngas and lighter hydrocarbons at the exit of the Fischer–Tropsch
Fig. 10. Calcium looping process for the production of liquid fuels.
reactor are recycled back to the carbonation reactor where the reforming of the hydrocarbons and the water–gas-shift reaction result in the production of a stream with a 2:1 ratio of H2 /CO which is the optimum feed for the Fisher–Tropsch reaction. The reactions occurring in the carbonation reactor are as follows: WGSR : CO + H2 O → H2 + CO2
(12)
Carbonation : CaO + CO2 → CaCO3
(13)
Sulfur capture(H2 S) : CaO + H2 S → CaS + H2 O
(14)
Sulfur capture(COS) : CaO + COS → CaS + CO2
(15)
Halide capture(HCl) : CaO + 2HCl → CaCl2 + H2 O
(16)
4.2. Calcination reactor The spent sorbent, consisting mainly of CaCO3 , is regenerated back to CaO in the calciner. The calciner is operated at atmospheric pressure and at a temperature of 800–1000 ◦ C in a rotary calciner or a fluidized bed system. The regenerated sorbent produced from the calciner is then conveyed back into the high-pressure carbonation reactor through a lock hopper system. A mixture of CO2 and steam is used as the medium of calcination with the steam being condensed out at the exit of the calciner to produce a sequestration-ready CO2 stream. The heat can be supplied directly or indirectly where a mixture of fuel and oxidant can be supplied to the calciner. The reactions occurring in the calciner are: Calcination : CaCO3 → CaO + CO2
(17)
The hydrogen production as well as the sorbent regeneration steps of the calcium looping process have been studied extensively in an integrated fixed bed reactor to determine the kinetics as well as the optimum reaction condition of the process. Fig. 11 shows the integrated experimental setup used for the proof of concept studies of the calcium looping process. The bench scale reactor is coupled with a set of continuous gas analyzers which detect concentrations of CO, CO2 , H2 , H2 S, CH4 , and higher
Fig. 11. Integrated fixed bed reactor for proof of concept studies of the calcium looping process.
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Fig. 12. Process analysis of the calcium looping process using ASPEN® Plus simulator.
hydrocarbons in the product stream. The reactor setup is capable of handling very high pressures and temperatures of up to 20 atm and 900 ◦ C respectively, which are representative of the conditions in a commercial coal to hydrogen system. Experimental analyses have shown that very high-purity hydrogen can be produced by the calcium looping process with simultaneous removal of CO2 and sulfur impurities to ppm levels (Iyer et al., 2007; Iyer et al., 2006) The calcium looping process for the production of hydrogen from coal has also been modelled using ASPEN® Plus software (see Fig. 12). The simulations were conducted for the hydrogen production process in combination with a pressure swing adsorption (PSA) system for the production of fuel cell grade hydrogen. It has found that the over all efficiency of the process for the production of 99.999% pure hydrogen is 63% (HHV) when compared to the state-of-the-art process for the production of hydrogen from coal which has an efficiency of 57% (HHV). Hence, this technology provides an efficient “one box” mode of operation for the production of high-purity hydrogen with CO2 , sulfur and chloride capture that integrates the WGSR, CO2 capture, sulfur removal and hydrogen separation in one consolidated unit. This “one box” process depicts the potential to achieve higher system efficiencies with lower overall footprint by combining different process units in one stage. 5. Conclusions The three novel gasification looping strategies discussed in this paper provide integrated CO2 separation and efficient product generation with high energy conversion efficiency. These advantages make the hydrogen, electricity, and liquid fuel pro-
duction from coal more affordable even in a carbon constrained scenario. Thus, with the rising global energy demand and crude oil/natural gas prices, coupled with environmental concerns, the application of the chemical looping technologies presents a promising option for optimum coal conversion. Acknowledgement This work has been supported by the Ohio Coal Development Office of the Ohio Air Quality Development Authority, U.S. Department of Energy, Ohio State University and an industrial consortium. References Agnihotri, R., Chauk, S., Mahuli, S., & Fan, L.-S. (1999). Influence of surface modifiers on structure of precipitated calcium carbonate. Industrial and Engineering Chemistry Research, 38, 2283–2291. Andrus, H. E., Burns, G., Chiu, J. H., Liljedahl, G. N., Stromberg, P. T., & Thibeault, P. R. (2006). ALSTOM technical report for project DE-FC2603NT41866. Chˆatel-P´elage, F., Varagani, R., Pranda, P., Perrin, N., Farzan, H., Vecci, S. J., et al. (2005). Applications of oxygen for NOx control and CO2 capture in coal-fired power plants. In Proceedings of Second International Conference on Clean Coal Technologies for our Future. Fan, L.-S., Gupta, H., & Iyer, M. V. (2007). Separation of carbon dioxide (CO2 ) from gas mixtures by calcium based reaction separation (CaRS-CO2 ) process. PCT Int. Appl. WO 2007002792. Fan, L.-S., Gupta, P., Velazquez-Vargas, L. G., & Li, F. (2007b). Systems and methods of converting fuels. PCT Int. Appl. WO 2007082089. Fan, L.-S., Li, F., Velazquez-Vargas, L.G., & Ramkumar, S. (in press). Chemical looping gasification. Proceedings of 9th International Conference on Circulating Fluidized Beds, Hamburg, Germany, May.
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Gupta, P. (2006). Regenerable metal oxide composite particles and their use in novel chemical processes. Doctoral dissertation, The Ohio State University. Gupta, P., Velazquez-Vargas, L. G., & Fan, L.-S. (2007). Syngas redox (SGR) process to produce hydrogen from coal derived syngas. Energy and fuels, 21(5), 2900–2908. Gupta, P., Velazquez-Vargas, L. G., Li, F., & Fan, L.-S. (2006). Chemical looping reforming process for the production of hydrogen from coal. In Proceedings of 23rd Annual International. Pittsburgh Coal Conference. Gupta, P., Velazquez-Vargas, L. G., Thomas, T., & Fan, L.-S. (2004). Hydrogen production from combustion looping (solids-coal). In Proceedings of Clear Water Coal Conference. Ishida, M., Jin, H., & Olamoto, T. (1998). Kinetic behavior of solid particle in chemical-looping combustion: Suppressing carbon deposition in reduction. Energy and Fuels, 12(2), 223–229. Iyer, M. (2006). High temperature reactive separation process for combined carbon dioxide and sulfur dioxide capture from flue gas and enhanced hydrogen production with insitu carbon dioxide capture using high reactivity calcium and biomineral sorbents. Doctoral dissertation, The Ohio State University. Iyer, M., Fan, L. S., & Ramkumar, S. (2007). Calcium looping process for high purity hydrogen production. PCT Int. Appl. WO US2007079432. Iyer, M. V., Gupta, H., Sakadjian, B. S., & Fan, L.-S. (2004). Multicyclic study on the simultaneous carbonation and sulfation of high reactivity CaO. Industrial and Engineering Chemistry Research, 43, 3939–3947. Iyer, M., Ramkumar, S., Wong, D., & Fan, L.-S. (2006). Proceedings of 23rd Annual International Pittsburgh Coal Conference. Kitto, J. B. (1996). Developments in pulverized coal-fired boiler technology. Missouri Valley Electric Association Engineering Conference, April 10–12. Retrieved January 20, 2008, from http://www.babcock.com/pgg/tt/pdf/BR1610.pdf. Lewandowski, D., & Gray, D. (2001). Potential market penetration of IGCC in the East central North American reliability council region of the U.S. Paper presented at Gasification Technologies Conference Mattisson, T., Johansson, M., & Lyngfelt, A. (2006). The use of NiO as an oxygen carrier in chemical-looping combustion. Fuel, 85, 736–747. Mattisson, T., Lyngfelt, A., & Cho, P. (2001). The use of iron oxide as an oxygen carrier in chemical-looping combustion of methane with inherent separation of CO2 . Fuel, 80, 1953–1962. National Energy Technology Laboratory (NETL). (2003). Pi˜non Pine IGCC Power Plant A DOE Assessment. National Energy Technology Laboratory (NETL). (2004). Clean coal technology: Tampa electric integrated gasification combined-cycle project.
National Energy Technology Laboratory (NETL). (2007). CO2 sequestration – CO2 capture. Retrieved February 01, 2008, http://www.netl.doe.gov/technologies/carbon seq/core rd/co2capture.html. National Research Council. (2000). Vision 21: Fossil fuel options for the future. Washington, DC: National Academy Press. Rizeq, R. G., West, J., Frydman, A., Subia, R., Kumar, R., Zamansky, V. et al. (2002). Fuel-flexible gasification-combustion technology for production of H2 and sequestration-ready CO2 . (2002 Annual Technical Progress Report to DOE). Ryu, H.-J., Jin, G. T., & Yi, C. K. (2004). Demonstration of inherent CO2 separation and no NOx emission in a 50 kW chemical-looping combustor: Continuous reduction and oxidation experiment. In Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies Shinada, O., Yamada, A., & Koyama, Y. (2002). The development of advanced energy technologies in Japan IGCC: A key technology for the 21st century. Energy Conversion and Management, 43, 1221–1233. Stiegel, G. J., & Maxwell, R. C. (2001). Gasification technologies: The parth to clean, affordable energy in the 21st century. Fuel Processing Technology, 71, 79–97. Stiegel, G. J., & Ramezan, M. (2006). Hydrogen from coal gasification: An economical pathway to a sustainable energy future. International Journal of Coal Geology, 65, 173–190. Stultz, S. C., & Kitto, J. B. (1992). Steam – its generation and use (40th ed.). Barberton: Babcock and Wilcox Company. Thomas, T., Fan, L.-S., Gupta, P., & Velazquez-Vargas, L. G. (2004). Combustion looping using composite oxygen carriers. U.S. Patent Application Serial No. 11/010,648. Thomas, T. J., Fan, L.-S., Gupta, P., & Velazquez-Vargas, L. G. (2005). Combustion looping using composite oxygen carriers. U.S. Patent Application for Publication US 2005175533. Tomlinson, G., & Gray, D. (2007). Chemical looping process in a coal-to-liquids configuration. In USDOE/NETL-2008/1307 (Ed.), Retrieved February 01, 2008, from http://www.netl.doe.gov/energyanalyses/pubs/DOE%20Report%20on%20OSU%20Looping%20final.pdf. Velazquez-Vargas, L. G., Thomas, T., Gupta, P., & Fan, L.-S. (2004). Hydrogen production via redox reaction of syngas with metal oxide composite particles. In Proceedings of AIChE Annual Techincal Meeting World Coal Institute. (2005). The Coal resource—A comprehensive review of coal. Retrieved February 01, 2008, from http://www.worldcoal.org/assets cm/files/PDF/thecoalre source.pdf.
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Utilization of chemical looping strategy in coal gasification processes Liangshih Fan ∗ , Fanxing Li, Shwetha Ramkumar Department of Chemical and Biomolecular Engineering, The Ohio State University, United States Received 5 February 2008; accepted 11 March 2008
Abstract Three chemical looping gasification processes, i.e. Syngas Chemical Looping (SCL) process, Coal Direct Chemical Looping (CDCL) process, and Calcium Looping process (CLP), are being developed at the Ohio State University (OSU). These processes utilize simple reaction schemes to convert carbonaceous fuels into products such as hydrogen, electricity, and synthetic fuels through the transformation of a highly reactive, highly recyclable chemical intermediate. In this paper, these novel chemical looping gasification processes are described and their advantages and potential challenges for commercialization are discussed. © 2008 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. Keywords: Chemical looping; Gasification process; Combustion; Hydrogen
1. Introduction Coal, as a vital energy source for the modern society, accounts for 40% of the electricity generated world wide. At present, more than 90% of the electricity generated from coal is produced through coal-fired power plants, which emit nearly 2 gigatons of carbon into the atmosphere every year (World Coal Institute, 2005). In a typical coal-fired power plant, the heat released from burning coal is used to generate high-pressure steam, which drives a steam turbine-generator. Due to the highly corrosive nature of steam, the turbine operating temperature is limited, thereby curbing the efficiency of the process (Shinada, Yamada, & Koyama, 2002). In most cases, traditional combustion power plants convert only a third of the energy value of coal into electricity. Moreover, coal combustion with air produces a flue gas stream at atmospheric pressure that consists of ∼80% N2 (by volume) and 10–15% CO2 . The low concentration of carbon dioxide in the flue gas stream makes carbon capture from traditional coal combustion power plants inefficient and uneconomical. The global on-going R&D efforts on coal combustion encompass enhancement of electricity generation efficiency to above 40% by the use of “super critical” and “ultra super critical” steam boilers (Kitto, 1996). However, enforcement of future
carbon regulations would nevertheless impose a severe energy penalty to the ultra super critical power plants. It is estimated by the USDOE that the incorporation of CO2 capture in ultra super critical power plants would increase the cost of electricity by 84% (NETL, 2007). Other efforts such as chilled ammonia process and oxyfuel combustion process focus on the cost reduction for CO2 capture in coal combustion plants. These carbon capture techniques, however, would consume 25–28% of the total electricity generated from the power plant (Chˆatel-P´elage et al., 2005). Another approach to utilize coal is through gasification. The gasification processes, unlike traditional combustion processes which fully oxidize carbonaceous fuels to generate heat, convert the fuels such as coal or biomass into syngas via partial oxidation reactions using oxygen and/or steam. In the coal gasification process, coal first reacts with steam and/or oxygen to generate raw syngas. The raw syngas, with pollutants such as H2 S, ammonia, and mercury, is then purified and sent to a gas turbine-steam turbine combined cycle system for electricity generation and this is known as the integrated gasification combined cycle (IGCC) process. The electricity generation efficiency of the IGCC process can be higher than 45% without CO2 capture. Alternatively, the CO in the syngas stream can be further converted to H2 through the water gas shift (WGS) reaction: CO + H2 O → CO2 + H2
∗
Corresponding author. Tel.: +1 614 688 3262. E-mail address: [email protected] (L.S. Fan).
Thus, the resulting gas stream contains a high CO2 concentration (up to ∼40% by 1 volume, dry basis). The CO2 in the
1674-2001/$ – see inside back cover © 2008 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.partic.2008.03.005
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Fig. 1. Schematic diagram of coal gasification processes for electricity, hydrogen, liquid fuel production.
gas mixture can be captured with ease to yield high-purity H2 . The H2 can be used to generate electricity through a combined cycle system or a fuel cell system without carbon emissions. This represents a typical coal gasification scheme for hydrogen production or electricity generation when CO2 capture is mandatory. Besides electricity and H2 generation, syngas can also be converted to chemicals and liquid fuels such as diesel and gasoline through the Fischer–Tropsch (F–T) reactions given below: (2n + 1)H2 + nCO → Cn H2n+2 + nH2 O Fig. 1 shows a schematic diagram of coal gasification processes for production of hydrogen, electricity and/or liquid fuels. As can be seen in the figure, the major advantages of gasification over combustion include product flexibility and ease in pollutant control. Therefore, gasification is considered a favorable candidate for use in next-generation coal-based plants (NETL, 2003, 2004, 2007; Stiegel & Maxwell, 2001). It is also considered to be a key enabling technology to the U.S. Department of Energy (USDOE) Vision 21 program (National Research Council, 2000). Despite its various advantages over direct combustion, gasification faces several challenges that impede its broad applications. Such challenges include intensive capital investment and large operating cost, both of which result from the elaborate oxygen production and syngas cleaning procedures. It has been estimated by Lewandowski and Gray (2001) that the capital investment for an oxygen-blown IGCC power plant is about $1241/kW, compared to $1170/kW for a pulverized coal combustion power plant with an ultra-supercritical boiler. Both technologies have similar energy conversion efficiencies. Additionally, although CO2 can be captured from a coal gasification system with ease compared to that from a coal combustion system, CO2 capture would nevertheless impose
a significant penalty for the system economics. For example, the state-of-the-art CO2 capture technique would increase the cost of electricity by 25% in a coal based IGCC plant (NETL, 2007). Thus, future coal gasification processes should be simplified through process intensification and coupled with more efficient carbon capture schemes in order to reduce the capital investment and to further increase the energy conversion efficiency. The implementation of the chemical looping concept in coal gasification processes represents a promising method to achieve these goals. The chemical looping concept has been widely applied to combustion processes for electricity generation from gaseous fuels (Ishida, Jin, & Olamoto, 1998; Mattisson, Johansson, & Lyngfelt, 2006; Ryu, Jin, & Yi, 2004). During the last decade, the chemical looping gasification strategy has been applied to produce hydrogen from coal and coal derived syngas. Among them, the GE fuel-flexible process (Rizeq et al., 2002) and the ALSTOM hybrid combustion–gasification process (Andrus et al., 2006) have been extensively studied. Both technologies use two different types of particles to convert coal into hydrogen: one type of particle is used to capture CO2 while the other serves as an oxygen carrier. The mixing between the two types of particles results in a decrease in the energy conversion efficiency. In this paper, three novel chemical looping gasification processes developed at OSU, i.e. the syngas chemical looping (SCL) process, calcium looping process (CLP), and coal/biomass direct chemical looping (CDCL) process, are described (Fan, Gupta, & Iyer, 2007; Fan, Li, Velazquez-Vargas, & Ramkumar, in press; Fan, Gupta, Velazquez-Vargas, & Li, 2007; Gupta, 2006; Gupta, VelazquezVargas, & Fan, 2007; Gupta, Velazquez-Vargas, Thomas, & Fan, 2004; Iyer, Fan, & Ramkumar, 2007; Iyer, Ramkumar, Wong, & Fan, 2006; Thomas, Fan, Gupta, & Velazquez-Vargas,
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2005; Velazquez-Vargas, Thomas, Gupta, & Fan, 2004 ). Unlike the GE and ALSTOM processes, each of these three processes utilizes a single type of particle optimized for its specific energy conversion scheme. The advantages and potential challenges of these processes are discussed in the succeeding sections. 2. Syngas chemical looping (SCL) process 2.1. SCL Process for Hydrogen Generation The SCL process co-produces hydrogen and electricity from syngas based on the cyclic reduction and oxidation of specially developed metal oxide composite particles (Gupta, VelazquezVargas, Li, & Fan, 2006; Gupta et al., 2007). The SCL process produces a pure hydrogen stream and a concentrated carbon dioxide stream in two separate reactors. Therefore, additional CO2 separation cost is avoided. Moreover, electricity can be efficiently co-generated with hydrogen in the SCL process, making it more flexible as well as economically attractive. 2.1.1. Process overview The SCL process consists of five major components: an air separation unit (ASU), a gasifier, a gas clean-up system, a reducer and an oxidizer. Fig. 2 shows a simplified block diagram of the SCL process. In the SCL process, a high-purity oxygen stream (oxygen concentration >95%) from an ASU is sent to the gasifier. The gasifier utilizes the oxygen from the ASU to partially oxidize coal forming high temperature raw syngas with contaminants such as particulates, sulfur, mercury, and halogen compounds. After the high temperature raw syngas quenching and particu-
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late removal steps, the pollutants in the gas stream are removed from the raw syngas using gas cleanup units that include acid gas removal (AGR) units and mercury removal units. These steps are identical to the modern coal gasification to hydrogen process shown in Fig. 1. The difference between SCL process and the state-of-the-art coal-to-hydrogen process lies in the H2 generation strategy. Compared to the coal-to-hydrogen process which requires a set of water–gas-shift reactors, CO2 separation units, and pressure swing adsorption units for the production of high-purity H2 from syngas, the SCL process utilizes only three major units to convert syngas into pure hydrogen (purity >99.95%) and electricity. The simplified overall energy conversion scheme results from the utilization of chemical looping strategy in modern coal gasification processes. In a typical SCL configuration, the syngas is converted to hydrogen in three major units, i.e. a reducer, an oxidizer, and a combustion train. 2.1.2. Reducer The purified syngas from the gas cleanup units is introduced into the reducer, which is a moving bed of iron oxide composite particles, 2–10 mm in diameter, at 750–900 ◦ C and 30 atm. In this reactor, the syngas is completely converted into carbon dioxide and water while the iron oxide composite particles are reduced to a mixture of Fe and FeO (reactions (1)–(4)). Fe2 O3 + CO → 2FeO + CO2
(1)
FeO + CO → Fe + CO2
(2)
Fe2 O3 + H2 → 2FeO + H2 O
(3)
FeO + H2 → Fe + H2 O
(4)
Fig. 2. Simplified schematic of the syngas chemical looping process for hydrogen production from coal.
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The overall reaction occurring in the reducer can be either slightly endothermic or slightly exothermic depending on the syngas composition, reaction temperature, as well as the particle reduction rate. The mild endothermic to mild exothermic nature of the reducer simplifies the heat integration of the reducer reactor, since heat can be easily carried in or out of the reactor by the particles. Since the syngas is completely oxidized by the Fe2 O3 composite particles, the exhaust gas from the reducer contains only CO2 and steam. Steam can be condensed out by extracting heat from the high temperature exhaust gas. This would result in a concentrated high-pressure CO2 stream that can be directly transported for sequestration. 2.1.3. Oxidizer Having been reduced in the reducer, the particles are then introduced into the oxidizer. In the oxidizer, the reduced particles react with steam to produce a gas stream that contains solely H2 and unconverted steam. Once the steam is condensed out from the gas stream, a H2 stream with a very high purity (>99.7%) can be obtained. The reactions involved in the oxidizer include: Fe + H2 O(g) → FeO + H2
(5)
3FeO + H2 O(g) → Fe3 O4 + H2
(6)
The steam used in the oxidizer is produced from the syngas cooling units and reducer/oxidizer exhaust gas cooling units. The oxidizer reactor operates at 30 atm and 500–750 ◦ C. Under such reaction conditions, both reactions (5) and (6) are slightly exothermic. The heat released from the oxidation of the particles to Fe3 O4 is used in the same reactor to preheat the steam feedstock. By introducing pressurized steam with a temperature lower than the oxidizer operating temperature, the oxidizer can be adjusted to operate under thermally neutral conditions. The reactor design is thus simplified. In the SCL process, hydrogen is produced using the chemical looping reforming concept, i.e. the syngas is indirectly converted to hydrogen with the assistance of iron oxide particles. This is fundamentally different from the traditional coal-tohydrogen process where the syngas is shifted to make hydrogen and followed by a CO2 removal operation. By using the syngas chemical looping process, the CO2 and hydrogen are produced from two different reactors, thereby eliminating the energy consuming CO2 separation and compression step. Since a significant portion of the carbon management cost is associated with the separation and compression of CO2 , the SCL process offers a major advantage over the traditional coal-to-hydrogen process. 2.1.4. Combustion train The Fe3 O4 formed in the reducer reactor is regenerated to Fe2 O3 in a unit called the combustion train. The combustion train is a riser driven by compressed air with a pressure slightly higher than 30 atm in order to compensate the pressure drop throughout the train. This unit serves as a pneumatic conveyor to transport solid discharged from the oxidizer reactor to the inlet of the reducer reactor. It also serves as a heat generator since significant amount of heat is produced during the oxidation of
Fe3 O4 to Fe2 O3 : 4Fe3 O4 + O2 → 6Fe2 O3
(7)
The high pressure high temperature spent air (mainly N2 ) produced from the combustion train can be used to drive a gas turbine-steam turbine combined cycle system to generate electricity for parasitic energy consumptions. In yet another configuration, a portion or all of the reduced particles from the reducer reactor can be directly sent to the combustion train without reacting with steam in the oxidizer reactor. By doing this, more heat would be available for power generation at the cost of decreased hydrogen production. In a typical SCL configuration, the reducer is installed on top of the oxidizer and the particles move from the inlet of the reducer all the way to the outlet of the oxidizer under gravity. The particles discharged from the oxidizer are then conveyed back to the reducer inlet by the combustion train. As can be seen, the SCL system can be considered as a special type of circulating fluidized bed in which the reducer and the oxidizer serve as the downer while the combustion train is the riser. The step of combusting the reduced particles is known as chemical looping combustion, which differs from the chemical looping reforming step in that the particle regeneration is performed with steam rather than oxygen. Hence, the utilization of both chemical looping reforming and chemical looping combustion concepts in the SCL system makes it flexible to adjust the ratio between H2 and electricity product. Moreover, only one type of highly reactive particle is circulating
Fig. 3. Bench scale demonstration unit (2.5 kWth ) for SCL process.
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among the three units, thus minimizing the solid circulation rate within the system. The operations of all three major reactors in the SCL process have been demonstrated individually with satisfactory results. The reducer and oxidizer were individually tested in a bench scale chemical looping demonstration unit while the combustion train was simulated in an entrained bed reactor assembly; both units were constructed at OSU. Fig. 3 shows the chemical looping demonstration unit, which is a moving bed reactor with a capacity of 2.5 kW thermo. The reactor allows for a large degree of flexibility in reaction conditions such as temperature, particle size, gas–solid contacting pattern and solid and gas flow rates. Gas and solid composition profile can be
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obtained over the length of the reactor bed once steady state operation is established. Useful information such as the reaction rates and gas–solid conversions can be elucidated by analyzing the demonstration results. Current demonstration results (Gupta et al., 2007) show >99.9% syngas conversion in the reducer and >99.95% purity hydrogen stream from the oxidizer. Nearly full conversion of gaseous hydrocarbons such as CH4 is also obtained in the SCL demonstration. Based on the bench scale demonstration results, an ASPEN® simulation model was constructed in order to evaluate the performance of the SCL process (Fig. 4b). The simulation outcomes for the SCL process were compared to those obtained from an ASPEN® model for the state-of-the-art
Fig. 4. ASPEN® simulation model. (a) State-of-the-art coal-to-hydrogen process; (b) SCL process.
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coal-to-hydrogen process (Fig. 4a) based on identical assumptions. The simulation results show that the overall efficiency for SCL process to be 64% (HHV) with 100% carbon capture as compared to 57% (HHV) for state-of-the-art coal-to-hydrogen process (Stiegel & Ramezan, 2006) (Fig. 4a). 2.2. Application of SCL process in coal-to-liquids processes Other than serving as a stand alone hydrogen/electricity producer, the SCL process can be integrated into other processes to improve the overall energy conversion scheme. The integration of the SCL process in the sate-of-the-art coal-to-liquids (CTL) process (Fan, Gupta, Velazquez-Vargas et al., 2007) exemplifies such novel application of the SCL process. There are several different configurations to incorporate the SCL system into the CTL process. In one conservative configuration, the SCL process is used as a retrofit to the traditional CTL plant. The role of SCL system is to compensate for the hydrogen deficit in the syngas from the modern gasifier. The schematic flow diagram of such configuration is illustrated in Fig. 5. In a state-of-the-art CTL plant using cobalt based catalyst, the syngas generated from the gasifier has a hydrogen concentration (30–40%) much lower than the optimum H2 content for liquid
fuel synthesis (67%). This deficit in hydrogen is usually met by converting a significant portion of the syngas from the gasifier into hydrogen using the traditional coal-to-hydrogen approach. Meanwhile, the Fischer–Tropsch (F–T) reactor converts only part of the syngas (60–85%) into a wide variety of hydrocarbons ranging from methane to hard wax. The gaseous hydrocarbon fuels and unconverted syngas are considered to be by-products from the F–T reactor. In a typical CTL configuration, a large portion of these gaseous by-products from the F–T reactor are combusted to generate electricity. In the SCL–CTL configuration shown in Fig. 5, the gaseous fuels and unconverted syngas from the Fischer–Tropsch reactor are introduced into the reducer of the SCL process. These gaseous fuels are converted to carbon dioxide and water through the following reaction. Cx Hy Oz + (2x + y/2 − z)MO → (2x + y/2 − z)M + xCO2 + y/2H2 O
(8)
Here MO and M refer to the different iron oxide phases. Reaction (8) reduces the iron oxide from higher oxidation states to lower oxidation states. The reduced iron particles are then introduced into the oxidizer where they are reacted with steam to produce pure hydrogen and regenerate the iron oxide
Fig. 5. Syngas chemical looping enhanced coal-to-liquids (SCL–CTL) process.
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(reaction (9)). M + H2 O → MO + H2
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that operates all three major SCL units is currently under construction. (9)
As can be seen, the major feedstock for the SCL reducer is the by-products from the F–T reactor. Meanwhile, the large amount of medium pressure (∼25 atm) steam generated by the waste heat from the F–T reactor provides ample supply for the steam required in the SCL oxidizer. Therefore, through the utilization of the SCL process, hydrogen, an essential feedstock for the CTL process, is generated from the by-products of the F–T synthesis. The liquid fuel yield of the CTL process is thus improved. Furthermore, the integrated carbon capture capability of the SCL process makes the SCL–CTL configuration even more attractive under a carbon constrained scenario. The SCL–CTL process has been independently evaluated by Noblis System under the contract with National Energy Technology Laboratory (NETL), USDOE. The analysis report (Tomlinson & Gray, 2007) states that “the (syngas) chemical looping systems such as that proposed by OSU have the potential to significantly (∼10%) increase the yield of the conventional cobalt based F–T process and allow more efficient heat recovery and much lower (∼19%) carbon emissions”. To generalize, the syngas chemical looping process has been successfully demonstrated in a 2.5 kWth bench scale unit. The demonstration results show that the SCL process can distinctively improve the energy conversion efficiency and product yield of the state-of-the-art coal gasification processes. Although no significant impediment for the industrial applications of the SCL process has been identified to date, high temperature solid handling is recognized as the major challenge for SCL commercialization. This issue can be addressed through scale up demonstrations of the process. A 25 kWth sub-pilot scale unit
3. Iron based chemical looping process for direct coal gasification—coal direct chemical looping (CDCL) process Unlike the SCL process which employs a conventional gasification system to generate syngas feedstock, the iron based coal direct chemical looping (CDCL) process takes coal directly as feedstock rather than syngas. This offers several advantages such as the reduction in O2 consumption and process intensification. Fig. 6 shows a simplified flow diagram of the CDCL process. The CDCL process has three main reactors, i.e., the coal reactor, the hydrogen reactor, and a combustion reactor. The coal reactor converts the coal into CO2 and H2 O while reducing Fe2 O3 to a mixture of Fe and FeO, the hydrogen reactor oxidizes the reduced Fe/FeO particle to Fe3 O4 using steam while producing a H2 rich gas stream. The combustion reactor pneumatically transports the Fe3 O4 particles from the H2 reactor outlet to the fuel reactor inlet using air, the Fe3 O4 particles are reoxidized to Fe2 O3 during the conveying process. 3.1. Fuel reactor The fuel reactor is a moving bed reactor operating at 750–900 ◦ C, 1–30 atm. The desirable reaction in the fuel reactor is: C11 H10 O(coal) + 26/3Fe2 O3 → 11CO2 + 5H2 O + 52/3Fe
Fig. 6. Simplified schematic of the coal direct chemical looping (CDCL) system.
(10)
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reactor is recycled to the bottom of the fuel reactor. By doing this, the steam formed from the reaction between hydrogen and FeO at the bottom of the fuel reactor would enhance char gasification. As opposed to other chemical looping processes which utilize fluidized bed reactor to convert fuels (Andrus et al., 2006; Mattisson, Lyngfelt, & Cho, 2001; Rizeq et al., 2002), the SCL and CDCL processes use countercurrent moving bed to increase gas–solid conversion. Such gas–solid contacting pattern is thermodynamically advantageous. The utilization of such patented countercurrent moving bed configuration represents a key enabling factor that makes the SCL and CDCL processes technically and economically attractive (Fan, Gupta, Velazquez-Vargas et al., 2007; Thomas, Fan, Gupta, and Velazquez-Vargas (2004)). 3.2. Hydrogen reactor Fig. 7. Gas–solid contacting pattern of the fuel reactor.
Here, the coal considered is Pittsburgh #8 and is represented by C11 H10 O given the elemental composition (Stultz & Kitto, 1992). This reaction is highly endothermic with a heat of reaction of 1794 kJ at 900 ◦ C. Therefore, heat needs to be provided to the first reactor. In order to balance the heat, coal is partially combusted in situ by sending substoichiometric amounts of O2 (according to coal) into the fuel reactor. The overall reaction, with negligible heat of reaction, is given as
3.3. Combustion reactor
C11 H10 O + 6.44Fe2 O3 + 3.34O2 → 11CO2 + 5H2 O + 12.88Fe
The hydrogen reactor is a moving bed reactor that operates at 700–900 ◦ C, and 1–30 atm. In the hydrogen reactor, the Fe and FeO mixture from the fuel reactor reacts with steam in a countercurrent mode to achieve maximum solid–gas conversion. The product of the reactor is Fe3 O4 particles as well as H2 . The reactions in the hydrogen reactor are the same as those in the oxidizer in the SCL process (reactions (5) and (6)). This reaction is slightly exothermic and therefore, steam with a lower temperature is introduced into the reactor to moderate the temperature of the reactor.
(11)
Since the amount of oxygen required for reaction (11) is significantly less than that for the coal gasification reactions, the size of the air separation unit is smaller than those used in traditional gasification processes. This corresponds to savings in both operation cost and capital investment of the coal to hydrogen plant. Fig. 7 shows the gas–solid contacting pattern inside the fuel reactor. Fresh Fe2 O3 composite particles are fed from the top of the fuel reactor while coal is introduced in the middle section of the reactor. This configuration allows the volatiles from the coal to react with Fe2 O3 particles to form CO2 and H2 O as they move upward. The devolatilized coal char, however, move downwards along with the partially reduced Fe2 O3 particles. The coal char is gradually gasified by the CO2 and H2 O formed at the lower part of the reactor. Provided enough residence time, which is estimated to be 30–90 min depending on the operating temperature and pressure, coal char can be converted fully, while the Fe2 O3 reduces to a mixture of metallic iron and FeO. Coal ash will come out along with the particles. Thus, the product from the fuel reactor is a solid stream with Fe, FeO, and coal–ash and a gas stream with mainly CO2 and H2 O. By condensing out the H2 O in the gas stream, a ready-to-sequester CO2 stream is obtained from the fuel reactor without the inherent separation costs. In order to promote the reaction rate between solids, a small quantity of hydrogen gas (<5%) produced from the hydrogen
In order to fully convert the CO and H2 from the char gasification into CO2 and H2 O, Fe2 O3 have to be used as the feedstock for the fuel reactor. Therefore, the Fe3 O4 from the hydrogen reactor needs to be oxidized. This regeneration step is done in the combustion reactor. The combustion reactor is a fluidized bed reactor operated at a pressure similar to that of both the fuel reactor and the hydrogen reactor in an adiabatic mode. Air is used to pneumatically convey the Fe3 O4 particles from the outlet of the hydrogen reactor to the fuel reactor inlet. The Fe3 O4 particles are oxidized to Fe2 O3 in this step (reaction (7)). This highly exothermic reaction will heat up the solid as well as the gas. The hot solid would be subsequently fed into the fuel reactor to partially compensate for the heat needed to convert the coal. The coal ash, with significantly smaller size than the Fe2 O3 composite particles, will be separated out from the cyclone on the top of the fuel reactor; makeup particles will also be introduced to the fuel reactor along with the regenerated particles. The CDCL process is currently under demonstration in a 2.5 kWth moving bed unit with encouraging results. ASPEN® simulation studies (Gupta et al., 2006) showed that a maximum coal to hydrogen conversion efficiency of above 80% (or 0.18 kg H2 /kg coal) can be achieved in the CDCL process. Potential challenges regarding this process include demonstration of high temperature solid handling and minimization of reactor volumes through further enhancement of reaction rates. Such challenges will be addressed through particle optimization and larger scale demonstrations.
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4. Calcium looping process (CLP) Commercially high-purity hydrogen is produced from syngas obtained by coal gasification by the water gas shift reaction (WGSR) in high- and low-temperature shift reactors using iron oxide and copper catalysts respectively. However for hydrogen production at high temperatures, the WGSR is thermodynamically limited which necessitates the addition of excess steam. Further, the commercial iron oxide and copper catalysts have a low tolerance to sulfur and chloride impurities and are prone to deactivation. Hence syngas clean up is required upstream of the shift reactors, which is conventionally achieved by using low-temperature scrubbing, which requires additional equipment and is highly energy-intensive due to the cooling and heating of the gas streams. In contrast, the calcium looping technology obviates the need for a catalyst and achieves simultaneous removal of carbon dioxide, sulfur, and chloride impurities at high temperatures during the production of high-purity hydrogen and yields a sequestration-ready CO2 stream during the regeneration phase. Thus, the calcium looping process not only improves the hydrogen yield and purity but also integrates a CO2 management scheme in the hydrogen production process. In addition to the production of hydrogen from syngas, the calcium looping process can also be applied to the production of liquid fuels. It is capable of producing a stream with a 2:1 ratio of H2 to CO while removing sulfur and chloride impurities to ppb levels which is required for liquid fuel synthesis by the Fischer–Tropsch reaction. Fig. 8 illustrates the schematic integration of the calcium looping process in a typical coal gasification system and shows
Fig. 9. Calcium looping process for the production of hydrogen.
that it is versatile and is capable of producing hydrogen for fuel cells, liquid fuel synthesis by Fisher–Tropsch reaction and electricity generation in a gas turbine. As shown in Fig. 9, the calcium looping process comprises of two reactors; the carbonation reactor where high-purity hydrogen is produced while contaminant removal is achieved and the calciner where the calcium sorbent is regenerated and a sequestration-ready CO2 stream is produced. A detailed explanation of the hydrogen production and sorbent regeneration sections of the calcium looping process are provided below.
Fig. 8. Schematic flow diagram of the calcium looping process.
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4.1. Carbonation reactor The carbonation reactor comprises of either a fixed fluidized bed or an entrained flow reactor that operates at high pressures ranging from 20 to 30 atm and temperatures of 550–650 ◦ C. The exothermic heat released from the carbonation reactor can be used to generate electricity in order to meet the parasitic energy requirement of the overall process. In the carbonation reactor, the thermodynamic constraint of the water–gas-shift reaction is overcome by the incessant removal of the CO2 product from the reaction mixture which enhances H2 production. This is achieved by concurrent water–gas-shift reaction and carbonation reaction of calcium oxide to form calcium carbonate thereby removing the CO2 product from the reaction mixture. In order to attain high CO2 capture capacity, the morphological properties of the calcium sorbent are tailored using surface modifiers (Agnihotri, Chauk, Mahuli, & Fan, 1999) The calcium oxide obtained from the Ohio State University patented mesoporous high reactivity precipitated calcium carbonate (PCC) sorbent attains an initial capture capacity of 70% by weight and a capture capacity of 40–36 wt.% over 50–100 cycles, which is significantly higher than most of the high temperature sorbents reported in literature (Iyer, 2006; Iyer, Gupta, Sakadjian, & Fan, 2004). In addition, the PCC-CaO sorbent is also capable of reducing the concentration of sulfur and halides in the outlet stream to ppb levels. Favorable conditions are maintained within the carbonation reactor for the reaction between the calcium sorbent and the sulfide and halide impurity. Since the water–gas-shift reaction is conducted in the presence of the PCC-CaO sorbent, equilibrium favors the formation of H2 . Hence, stoichiometric quantity of steam is sufficient to produce high-purity hydrogen. Thermodynamics predicts that the removal of H2 S and HCl impurities using calcium oxide is inhibited by the presence of steam and the removal of COS is inhibited by the presence of CO2 . Since almost all the steam is consumed in the enhanced WGSR and all the CO2 is removed by the sorbent, the removal of H2 S, HCl, and COS is also favored in the system. In addition to the production of hydrogen from syngas, it is also possible to reform hydrocarbons in the same system, thus making the system fuel flexible. When the calcium looping process is applied to the production of liquid fuels, as can be seen in Fig. 10, the unreacted syngas and lighter hydrocarbons at the exit of the Fischer–Tropsch
Fig. 10. Calcium looping process for the production of liquid fuels.
reactor are recycled back to the carbonation reactor where the reforming of the hydrocarbons and the water–gas-shift reaction result in the production of a stream with a 2:1 ratio of H2 /CO which is the optimum feed for the Fisher–Tropsch reaction. The reactions occurring in the carbonation reactor are as follows: WGSR : CO + H2 O → H2 + CO2
(12)
Carbonation : CaO + CO2 → CaCO3
(13)
Sulfur capture(H2 S) : CaO + H2 S → CaS + H2 O
(14)
Sulfur capture(COS) : CaO + COS → CaS + CO2
(15)
Halide capture(HCl) : CaO + 2HCl → CaCl2 + H2 O
(16)
4.2. Calcination reactor The spent sorbent, consisting mainly of CaCO3 , is regenerated back to CaO in the calciner. The calciner is operated at atmospheric pressure and at a temperature of 800–1000 ◦ C in a rotary calciner or a fluidized bed system. The regenerated sorbent produced from the calciner is then conveyed back into the high-pressure carbonation reactor through a lock hopper system. A mixture of CO2 and steam is used as the medium of calcination with the steam being condensed out at the exit of the calciner to produce a sequestration-ready CO2 stream. The heat can be supplied directly or indirectly where a mixture of fuel and oxidant can be supplied to the calciner. The reactions occurring in the calciner are: Calcination : CaCO3 → CaO + CO2
(17)
The hydrogen production as well as the sorbent regeneration steps of the calcium looping process have been studied extensively in an integrated fixed bed reactor to determine the kinetics as well as the optimum reaction condition of the process. Fig. 11 shows the integrated experimental setup used for the proof of concept studies of the calcium looping process. The bench scale reactor is coupled with a set of continuous gas analyzers which detect concentrations of CO, CO2 , H2 , H2 S, CH4 , and higher
Fig. 11. Integrated fixed bed reactor for proof of concept studies of the calcium looping process.
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Fig. 12. Process analysis of the calcium looping process using ASPEN® Plus simulator.
hydrocarbons in the product stream. The reactor setup is capable of handling very high pressures and temperatures of up to 20 atm and 900 ◦ C respectively, which are representative of the conditions in a commercial coal to hydrogen system. Experimental analyses have shown that very high-purity hydrogen can be produced by the calcium looping process with simultaneous removal of CO2 and sulfur impurities to ppm levels (Iyer et al., 2007; Iyer et al., 2006) The calcium looping process for the production of hydrogen from coal has also been modelled using ASPEN® Plus software (see Fig. 12). The simulations were conducted for the hydrogen production process in combination with a pressure swing adsorption (PSA) system for the production of fuel cell grade hydrogen. It has found that the over all efficiency of the process for the production of 99.999% pure hydrogen is 63% (HHV) when compared to the state-of-the-art process for the production of hydrogen from coal which has an efficiency of 57% (HHV). Hence, this technology provides an efficient “one box” mode of operation for the production of high-purity hydrogen with CO2 , sulfur and chloride capture that integrates the WGSR, CO2 capture, sulfur removal and hydrogen separation in one consolidated unit. This “one box” process depicts the potential to achieve higher system efficiencies with lower overall footprint by combining different process units in one stage. 5. Conclusions The three novel gasification looping strategies discussed in this paper provide integrated CO2 separation and efficient product generation with high energy conversion efficiency. These advantages make the hydrogen, electricity, and liquid fuel pro-
duction from coal more affordable even in a carbon constrained scenario. Thus, with the rising global energy demand and crude oil/natural gas prices, coupled with environmental concerns, the application of the chemical looping technologies presents a promising option for optimum coal conversion. Acknowledgement This work has been supported by the Ohio Coal Development Office of the Ohio Air Quality Development Authority, U.S. Department of Energy, Ohio State University and an industrial consortium. References Agnihotri, R., Chauk, S., Mahuli, S., & Fan, L.-S. (1999). Influence of surface modifiers on structure of precipitated calcium carbonate. Industrial and Engineering Chemistry Research, 38, 2283–2291. Andrus, H. E., Burns, G., Chiu, J. H., Liljedahl, G. N., Stromberg, P. T., & Thibeault, P. R. (2006). ALSTOM technical report for project DE-FC2603NT41866. Chˆatel-P´elage, F., Varagani, R., Pranda, P., Perrin, N., Farzan, H., Vecci, S. J., et al. (2005). Applications of oxygen for NOx control and CO2 capture in coal-fired power plants. In Proceedings of Second International Conference on Clean Coal Technologies for our Future. Fan, L.-S., Gupta, H., & Iyer, M. V. (2007). Separation of carbon dioxide (CO2 ) from gas mixtures by calcium based reaction separation (CaRS-CO2 ) process. PCT Int. Appl. WO 2007002792. Fan, L.-S., Gupta, P., Velazquez-Vargas, L. G., & Li, F. (2007b). Systems and methods of converting fuels. PCT Int. Appl. WO 2007082089. Fan, L.-S., Li, F., Velazquez-Vargas, L.G., & Ramkumar, S. (in press). Chemical looping gasification. Proceedings of 9th International Conference on Circulating Fluidized Beds, Hamburg, Germany, May.
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