Environ. Sci. Technol. 2008, 42, 2723–2727
An Investigation of Synthetic Fuel Production via Chemical Looping FRANK ZEMAN* AND MARCO CASTALDI Department of Earth and Environmental Engineering, Columbia University, 918 Mudd, MC 4711, 500 West 120th Street, New York, New York 10027
Received November 2, 2007. Revised manuscript received January 3, 2008. Accepted January 8, 2008.
Producing liquid hydrocarbon fuels with a reduced greenhouse gas emissions profile would ease the transition to a carbonneutral energy sector with the transportation industry being the immediate beneficiary followed by the power industry. Revolutionary solutions in transportation, such as electricity and hydrogen, depend on the deployment of carbon capture and storage technologies and/or renewable energy systems. Additionally, high oil prices may increase the development of unconventional sources, such as tar sands, that have a higher emissions profile. One process that is gaining interest is a system for producing reduced carbon fuels though chemical looping technologies. An investigation of the implications of such a process using methane and carbon dioxide that is reformed to yield methanol has been done. An important aspect of the investigation is the use of off-the-shelf technologies to achieve the results. The ability of the process to yield reduced emissions fuels depends on the source for the feed and process heat. For the range of conditions considered, the emissions profile of methanol produced in this method varies from 0.475 to 1.645 moles carbon dioxide per mole methanol. The upper bound can be lowered to 0.750 by applying CCS and/ or using nonfossil heat sources for the reforming. The process provides an initial pathway to incorporate CO2 into fuels independent of electrolytic hydrogen or developments in other sectors of the economy.
1. Introduction The desire to reduce emissions of greenhouse gases to the atmosphere led to the development of carbon capture and storage (CCS) technologies that target large, point source emitters of carbon dioxide (CO2) (1). While CCS promises to have a big impact in reducing carbon emissions, even when fully deployed, CCS can only capture about 50% of current CO2 emissions. The remainder originate from small, dispersed and mobile sources. In turn, the majority (65%) of these are associated with the transportation sector, which is also experiencing the largest growth in emissions (2). Emissions could also rise if the amount of oil from unconventional sources, such as tar sands, increases dramatically (3). Proposed solutions to effectively decarbonize the sector, include electricity and hydrogen (H2) (4, 5). Decarbonized transportation fuels reduce anthropogenic CO2 emissions only if the source of the fuel and the overall production process is carbon neutral. For hydrogen and electricity, the key development is either the full deployment * Corresponding author phone: (212) 854-7441; fax: (212) 8547081; e-mail:
[email protected]. 10.1021/es702761z CCC: $40.75
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2008 American Chemical Society
of conventional CCS or a paradigm shift to an electricity grid completely dominated by renewable power systems such as wind, solar, and biomass. Even with these developments, both CCS and renewable power have associated fugitive emissions. The CO2 profile of CCS (40–152 g of CO2/kWh) (1) is on a similar scale to renewable systems (12–63 g of CO2/ kWh) (6, 7). Nonhydro renewable power accounts for 18% of current electricity generation, including nuclear (8). Hydropower is considered renewable although some controversy exists over the release of soil carbon because of reservoir flooding. There is currently one power plant with oxy-fuel systems (Vattenfall) planned and one application at a coal gasification facility (Dakota Gasification) that employs CCS for enhanced oil recovery (EOR) purposes (1). The necessary changes to the energy infrastructure, both for CCS or renewable power, will take time and suggests that “bridge” fuels may be necessary to reduce emissions. The decarbonized solutions also depend on the separate development of infrastructure to transport and deliver the hydrogen or electricity.
2. Chemical Looping Systems Alternative methods for fuel production from synthesis gas (syngas) exist such as coal to liquid, gas to liquids, and biomass gasification among others (9). We have investigated a method for synthesizing carbonaceous fuels with a reduced emissions profile by thermal methods using methane and CO2. The synthesis method is based on chemical looping combustion (CLC) technology suggested as a method for CO2 capture in the power industry (1, 10). The process begins with the thermal reforming of methane and carbon dioxide then adjusts the feed mixture to match the stoichiometry required for the target fuel. A metal oxide removes oxygen from the syngas by reducing carbon monoxide and produces hydrogen when it is reduced by methane. Between the two reactions, steam is used to gasify the carbon (soot) deposited during the reduction of the syngas. In CLC, a metal oxide (MO) is used as an oxygen carrier to transfer oxygen from air to the fossil fuel while avoiding the mixing of CO2 and nitrogen (N2) (1). The reaction pair is shown schematically in reactions 1 and 2. The objective is to generate a stream of CO2 that does not contain nitrogen and therefore is easier to purify. M + air T MO + O2 depleted air (oxidation)
(1)
MO + fuel T M + CO2 + H2O
(2)
(reduction)
Here we take this concept and use it to remove oxygen from carbon monoxide (CO) in the oxidation reaction and produce hydrogen (H2) in the reduction reaction using methane (CH4). The modified chemical loop is shown schematically in reactions 3 and 4. M + CO T MO + C 2MO + CH4 T 2M + CO2+ 2H2
(oxidation)
(3)
(reduction)
(4)
The use of this technique allows for the optimization of the CO to H2 ratio necessary for fuel synthesis. Gas conditioning is currently done by reacting steam with the excess CO to produce H2 and CO2. Known as the water- gas shift (WGS) reaction, the result is that CO2 is vented taking the carbon with it. The objective of the chemical looping technique is to produce “reduced carbon” synthetic fuels by thermal methods while (1) incorporating the carbon derived from VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Differences in Molar Constituents between Syngas and Fuels
FIGURE 1. Overview of fuel synthesis via chemical looping process. CO2 into the fuel, (2) avoiding the use of electrolytic hydrogen, and (3) avoiding the water gas shift reaction. We define fuels as “reduced carbon” when they incorporate recycled or biogenic carbon so that the emissions profile (the amount of fossil carbon per unit carbon in the fuel) is less than one. Chemical looping has been considered for hydrogen production through steam reduction (11, 12). The work on hydrogen focused on using syngas to reduce the metal oxide, which is the opposite intention of this work. Svoboda et al. noted that iron was not ideal for H2 production because soot and iron carbide (Fe3C) were formed in the reducing phase and released in the oxidation phase. Soot (C) formation was enhanced under pressurized conditions. Research has been performed on the production of carbon from CO2 using magnetite (Fe3O4) and wustite (FeO) (13, 14). The work focused on active forms of the compounds that were oxygen deficient (up to 3%). The active compounds were produced by reducing stable iron oxides with H2 at 300 °C. The kinetics of the decomposition improved with the degree of oxygen deficiency. It was found that the reduction of CO2 to CO was the rate-limiting step. Yamasue et al. (15) found that CO2 could be effectively decomposed using stable FeO if it was mechanically milled to a particle size of less than 10 µm. The milled FeO has the further advantage of producing less Fe3C than active iron compounds.
3. Process Overview The process, shown in Figure 1, involves three components: the reformer, the chemical loop, and the fuel synthesis reactor. The process is intended for syngas mixtures containing equal parts CO and H2 derived from an equal mixture of CO2 and CH4. The feed material was chosen because of its wide application in the energy sector, thus is readily available, and both are the dominant greenhouse gases. In addition, such a mixture can be obtained from biogas produced from anaerobic digestion, natural gas fields with high CO2 contents, and combining stranded methane with CO2 from CCS or biofuel production. We investigate this process with regards to its thermodynamic efficiency and energy balance. It should be noted that the origin of the CO2 affects the carbon footprint of the fuel. For example, if the CO2 were derived from natural deposits then any produced fuel would be similar to gasoline, when considering greenhouse gas emissions. The use of CO2 captured from power plants would have a reduced, but not zero, emissions profile owing to it being used twice. Biogenic CO2 can be considered carbon neutral because once emitted, it is recaptured by photosynthesis. Given these options, the CO2 can have an emissions factor ranging from 0 to 1.
4. Gas Conditioning The overall process will depend on the fuel targeted for production. In this work, we are aiming to produce carbon2724
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component
syngas
2x CH3OH
C2H4
C2H6 O
oxygen hydrogen
1 2
0 +2
-1 0
-0.5 +1
aceous fuels that contain all of the carbon found in the feed. We consider fuels with increasing number of carbon molecules starting with methanol (CH3OH) then higher order fuels such as dimethyl ether (C2H6O). The relative ease of producing these various fuels can be gauged by comparing the ratios of carbon, hydrogen, and oxygen with those present in the syngas mixture. The starting point for all of these fuels is the reforming of methane with CO2 according to reaction 5. CH4 + CO2 T 2CO + 2H2 ∆H ° ) +259 kJ/mol CO2, T ) 650 °C (5) The reforming with CO2 provides an initial thermal requirement for the proposed process. The main issue is that another process needs to be coupled to this to provide the needed energy to drive the reaction. However, the GHG emissions from that energy producing process must ensure that the net outcome achieves a reduction in carbon emissions. In molar terms, reaction 5 produces two moles of carbon (C) and one mole of oxygen (O2) with two moles of H2. A comparison of this mixture with the stoichiometric equivalent in the fuel helps ascertain the amount of gas conditioning required. By comparison, two moles of methanol contain two moles of C, one mole of O2 and four moles of H2. The task of the chemical loop is to add two moles of H2 to the process to produce a stoichiometric mixture suitable for methanol synthesis. The required gas conditioning for several fuels is presented in Table 1; it includes ethylene (C2H4), as it is a precursor to other, higher order molecules. We will first examine methanol production using chemical looping and then consider other fuels. 4.1. Methanol Production and Greenhouse Gas Emissions. Current methanol production is dominated by the steam reforming of methane (SRM), accounting for 75% of global production (8). As SRM is endothermic (224 kJ/mol), it is usually combined with autothermal reforming of methane where the gas is partially oxidized using oxygen and the heat released drives the SRM reaction. Conventional stream reforming using methane as fuel has a thermal efficiency of 63.6% (16). A lifecycle analysis of methanol production via SRM suggests that 1.16 moles of CO2 are produced for every mole of methanol (8). Methanol can also be produced from the hydrogenation of CO2 according to reaction 6. Such a system has been previously considered as the basis of a renewable, carbonaceous fuel system (17). CO2(g) + 3H2(g) T CH3OH(l)+ H2O(l) ∆H ° ) -130 kJ/mol, T ) 50 °C (6) While the reaction itself is exothermic, it relies on the availability of hydrogen. This is where the energy penalty is found and thus a hydrogen production process needs to be identified. Producing hydrogen (285.8 kJ/mol H2 from water splitting) from fossil fuels with CCS, via gas shifting, would produce more CO2 than is used in fuel production, as shown in reaction 7. C + 2H2O(g) T CO2 + 2H2 ∆H ° ) +100 kJ/mol, T ) 700 °C (7)
The gasification of fossil fuels can be considered a conventional technology as it is practiced on an industrial scale. Therefore, renewable energy systems will be used to produce the electrolytic H2 required for synthesis based on reaction 6. On the basis of a 75% efficient electrolyzer (18), we note that each mole of hydrogen required 381 kJ of electrical energy. The total endothermic load for CO2 hydrogenation is therefore 1143 kJe/mol CH3OH (3 × 381 kJe/mol H2). This value can be compared to the heat of combustion of methanol, which is 726 kJ/mol CH3OH and results in a thermal efficiency of 63.5%, not including CO2 capture. The thermal efficiency does not include the heat released by synthesis. This heat, while it conceptually figures into the overall energy balance, is released at low temperatures (250 °C) (8) and cannot provide the heat required for high temperature gasification. It is likely that uses for the heat, for example, steam generation, will be found in a complicated process such as fuel synthesis. 4.2. Gas Conditioning via Chemical Looping. An alternative method for methanol synthesis based on chemical looping technologies, as shown in reactions 3 and 4, might be a more feasible approach. The objective of the chemical loop is to change the stoichiometry of the feed stream prior to entering the synthesis reaction. Known as gas conditioning, this step is located between the reformer and synthesis reactors as shown in Figure 1. Gas conditioning is necessary to allow any CO2 formed due to the water gas shift (WGS) reaction to be recycled back to the reformer for eventual synthesis into methanol. Considering iron, there are three possible oxidation reactions shown sequentially as reactions 8 though 10. Each reaction results in the reduction of one mole of CO and the production of one mole of carbon. 2Fe3O4 + CO(g) T 3Fe2O3 + C ∆H ° ) -124 kJ/mol, T ) 250 °C (8) 3FeO + CO(g) T Fe3O4 + C ∆H ° ) -185 kJ/mol, T ) 675 °C (9) 2FeO + CO(g) T Fe2O3 + C ∆H ° ) -170 kJ/mol, T ) 525 °C (10) The products of the reduction reactions are solid and it is necessary to separate the carbon for use in the synthesis reaction. One method for such a separation process is gasification using steam according to reaction 11. C + H2O(g) T CO(g) + H2(g) ∆H ° ) +136 kJ/mol, T ) 650 °C (11) The inclusion of the steam gasification results in the addition of one mole of H2 through thermal means. The second mole of H2 is produced in the reduction reactions for the iron compounds. The reactions 12 through 14 are the complementary reduction reactions to reactions 8 though 10. The reduction reactions are balanced to produce a mixture of CO2 and H2. In this manner, the second mole of H2 is produced for synthesis. 3Fe2O3 + 0.5CH4 T 2Fe3O4 + 0.5CO2+ H2 ∆H ° ) +77 kJ/mol,
T ) 200 °C (12)
Fe3O4 + 0.5CH4(g) T 3FeO + 0.5CO2(g) + H2(g) ∆H ° ) +144 kj ⁄ mol, T ) 650 °C (13) Fe2O3 + 0.5CH4(g) T 2FeO + 0.5CO2(g) + H2(g) ∆H ° ) +130 kJ/mol, T ) 500 °C (14) Reviewing the appropriate reactions pairs, we notice that the combined reactions are exothermic (-40 to -47 kJ/mol) with the exothermic reaction able to transfer heat to the endothermic reaction. From a thermodynamic point of view,
this part of the process will not add to the energy demand of the overall process. The chemical loop does generate 0.5 moles of CO2 for reaction 1 or 0.25 moles of CO2 for each mole CH3OH. Gas conditioning for methanol synthesis can effectively be done using chemical looping technologies based on iron. The process produces at least 20 kJ per mole of CH3OH and 0.25 moles of CO2 from methane oxidation. While the production of CO2 is not desirable, it is produced in a manner highly suitable to CCS and could be captured. This would not interfere with the objectives of the process; however, it would increase the emissions profile. Reacting methane with a metal amounts to the controlled oxidation of methane, which is not always a “clean” reaction and may result in the deposition of carbon and release of two moles of H2. Experiments have shown that at temperatures below 700 °C less than 5% of the methane decomposes (19). According to Figure 1, the deposited carbon would be sent along with the reduced metal to the oxidation reaction, eventually gasified using steam according to reaction 11 and sent to the synthesis reactor to be converted to methanol. As such, the methane would not be wasted, however, it would increase the fossil content of the fuel produced proportionately. 4.3. Higher Order Carbon Fuels. Changing the target fuel from methanol to a higher order carbon chain, such as DME, results in the need to remove oxygen from the syngas as shown in Table 1. The oxygen can be removed through the metal oxidation reaction, reactions 8-10, resulting in carbon deposits on the catalyst. The subsequent reduction of the metal, reactions 12-14, produces the hydrogen required to produce the stoichiometric mixture for DME. The steam gasification of carbon, reaction 11, is not necessary, which reduces the thermal load by 33%. The heat of combustion of DME is also marginally higher than CH3OH, which should result in a more efficient process. The challenge of a net removal of oxygen from the process is recovery of the carbon deposited on the metal. At this point, steam gasification is not feasible as it introduces oxygen in the form of CO back into the syngas, as does Boudouard (CO2) gasification. Similarly oxidizing the carbon using air or pure oxygen would produce CO2 that could be vented at a cost of the carbon, which is needed for synthesis. The only option is the gasification of the carbon using the available hydrogen. In a practical system, however, hydrogen gasification forms methane. Once methane is formed it can either be combusted or, ideally, processed to form higher order hydrocarbons. Unfortunately, there has been no method identified which converts methane to liquids.
5. Carbon Balance for Methanol Production One of the stated objectives of the chemical looping process was to produce fuels with a reduced greenhouse gas footprint. The total emissions will depend on the sources of the methane and CO2 as well as the emissions generated by the process. The fugitive emissions, discussed below, will be shared between the two moles of methanol produced in each “pass”. We treat the case of biogenic methane from anaerobic digestion as a special case as the CO2 is generated simultaneously and the carbon is considered neutral. Here the only fugitive emissions are from the process. The fugitive CO2 emissions from the process are associated with providing the heat for the endothermic reactions and the CO2 produced during the reduction of the metal oxide. As mentioned earlier, the reduction reaction produces 0.5 moles of CO2 in each “pass” or 0.25 moles of CO2 for each mole of methanol produced. The endothermic reactions require 395 kJ of heat for each “pass”. The amount of CO2 produced will depend on the source of the heat. We consider the case of nonfossil fuels, including nuclear thermal, solar VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Emissions Profile for Various Methods of Chemical Looping Methanol Production (mole CO2/mol CH3OH) feed
CH4
CO2
chemical looping
thermal methane
thermal coal
total
biogas neutral CO2 captured CO2 fossil CO2
0.000 0.500 0.500 0.500
0.000 0.000 0.250 0.500
0.250 0.250 0.250 0.250
0.225 0.225 0.225 0.225
0.395 0.395 0.395 0.395
0.250–0.645 0.750–1.145 1.000–1.395 1.250–1.645
TABLE 3. Thermal Efficiencies of Various Methanol Synthesis Processes
process
methane reforming (SRM)
CO2 hydrogenationa
chemical loopinga
thermal efficiency (%)
75
64
88
a
Does not include capture of the feed CO2.
thermal, and biomass, as well as fossil fuels such as coal and methane. In the case of nonfossil fuels, such as biomass or nuclear or solar thermal, we assume the heat generation does not affect greenhouse gas emissions. In the case of fossil fuels, we consider representative heating values for methane (880 kJ/mol) and coal (500 kJ/mol) (20). The use of methane would produce 0.45 moles of CO2 per pass or 0.225 moles per mole of methanol. Coal would produce 0.79 moles of CO2 per pass or 0.395 moles per mole of methanol. The source of the CO2 is also an important component of the emissions profile. It has been suggested that CO2 captured from fossil power plants is a method of reducing emissions based on a “two for one” concept. Rather than extracting fossil carbon from the ground, fossil CO2 is converted into a fuel, which is then oxidized back to CO2. Assuming the fugitive or life cycle emissions are the same then such a system could be said to reduce emissions by 50%. For this work, we assign an emissions profile of 0.5 to CO2 from fossil CCS. Biogenic sources for the CO2, such as fermentation or biomass power, would have an emissions profile of 0. Capture of CO2 directly from the atmosphere using renewable energy (17) would also have a zero emissions profile for the CO2. Thermal air capture systems (21, 22) depend on CCS to ensure that there are no fugitive emissions. At this point, we can summarize the various emissions from the process, as shown in Table 2. Aside from the biogas case, we attribute 0.5 moles of CO2 per mole of fuel from the feed methane. This is therefore the minimum emissions profile of a fuel produced in the manner proposed. The chemical looping portion of the process also produces 0.25 mol of CO2. The emissions profile of the CO2 in the feed is taken as 0.25 for fossil CO2 from CCS and 0 for biogenic CO2. The thermal load can be provided by methane, coal, or carbon neutral fuels. These options result in a range of emissions resulting from the process. Consider the case of neutral CO2, the emissions tabulated in the first three columns are process emissions and when combined with a carbon neutral fuel, such as biomass, establish an emissions profile of 0.75 mol CO2/mol CH3OH (0.50 + 0.25). The use of coal to provide the thermal energy would result in an emissions profile of 1.145 (0.75 + 0.395) with methane use resulting in an intermediate value of 0.975 (0.75 + 0.225). The total value can be further reduced if CCS is applied to the chemical looping step. Aside from using biogas, Table 2 shows that using coal to provide the thermal load would result in an emissions profile higher than SMR methanol (1.16 mol CO2/mol CH3OH). The overall emissions profile can be significantly reduced through the application of CCS technologies and/or renewable energy 2726
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sources such as biomass or solar thermal. The chemical looping exhaust gases can be readily separated to produce a high purity stream of CO2, ready for CCS. Similarly, the thermal load can be provided using fossil fuel combustion in oxygen, which would increase the energy penalty but allow for CCS. The overall efficiency of the process can be estimated by comparing the thermal inputs to the energy content of the methanol. The process consumes 375 kJ of heat, 1 mol of methane (880 kJ) and generates one mole of CO2 (393 kJ) while producing two moles of methanol (726 kJ each). Dividing the energy of methanol by the inputs produces a thermal efficiency of 88%. The maximum thermal efficiency of chemical looping methanol synthesis is compared to alternatives in Table 3. While this value is higher than the thermal efficiency of SRM (75%) (8), it does not include the capture of CO2 or thermal efficiencies and assumes all reactions go to completion. While electrolysis is possible today, the carbon free energy to power the device is currently unavailable and wind power is costly. Chemical looping synthesis can be implemented with off the shelf technologies that allow for the introduction of CO2 in the fuel making process. Eventually, as greenhouse gas restrictions increase, using neutral CO2 and CCS can reduce the emissions profile of the methanol.
6. Discussion In this work, we outline a process for making methanol from CO2 by thermal means. The objective was to develop a method to produce a liquid carbonaceous fuel with a reduced greenhouse gas footprint. The objective is possible but not automatic. The process is amenable to CCS technologies, which would reduce the greenhouse gas footprint. The initial thermal efficiency is high enough to warrant further research. The larger need to introduce CO2 into the fuel making process can be explored and gradually phased in with this technology. The emissions profile of the methanol can vary significantly depending on the details of the process heat and feed sources. This flexibility can be exploited to allow for a gradual implementation as carbon markets and regulations are imposed. The use of off the shelf technologies provides a starting point for immediate implementation but also suggests that process specific innovations are possible.
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