Synfuel Cycle Efficiency

Introduction

A number of studies on the world oil and coal reserves have stated that the world will use up all of the easily accessible and reliable carbon-based fossil fuel sources in the next 150 years. [1-3] Carbon-based fuels support a majority of the current energy economy, and it will likely take considerable time and money to build up an infrastructure based on alternative energy sources to replace oil and coal. However, even though the world will need to rethink where it gets its energy, the way portable energy is stored and distributed in the future for certain purposes does not necessarily need a complete overhaul.

In particular, current car and truck transportation is almost completely dependent upon gasoline and diesel fuels, and there are serious obstacles that still need to be overcome for the leading carbon-free alternatives, such as electric and hydrogen powered cars. While these technologies are constantly being improved, gasoline still has significant advantages over these alternate fuel sources in terms of energy density, engine complexity, cost, safety, refuel times, and sometimes even environmental impact. [4-6] It is possible that alternative fuel sources will not surpass the performance of gasoline before the world's oil resources are exhausted. Fortunately, the technology already exists to allow the continued use of gasoline as an energy storage medium for transportation, even after carbon-based fossil fuels are exhausted.

In this paper, I will outline a renewable cycle for creating synthetic gasoline out of CO₂ extracted from the atmosphere, and will discuss the technology and energy efficiency associated with each of the four steps of the cycle. The first is the extraction of carbon dioxide from the atmosphere. [7] The second and third steps are the conversion of CO₂ into gasoline using methanol synthesis followed by the Exxon Mobil process. [8-10]. The final step is combustion of the gasoline within a vehicle, releasing the CO₂ back into the atmosphere. The entire cycle is carbon neutral since the carbon was initially extracted from the atmosphere or the ocean. A similar process has been proposed in a white paper by scientists at Los Alamos National Laboratory, where they call it Green Freedom. [11]

CO₂ Extraction

The basic idea of the CO₂ extraction presented in [7] is to first absorb CO₂ from the atmosphere into an aqueous solution, next to remove that solution from contact with atmosphere into an enclosed chamber, and finally to extract the CO₂ from the solution and store it in a holding tank or pump it directly to the methanol synthesis plant. The first step is made possibly by using a chemical reaction that is strongly exothermic,

This reaction pulls CO₂ from the air at a rate roughly proportional to the concentration of carbon dioxide in the air, and stores it in a liquid form where it can be then transported out of the atmosphere. Next, within a separate enclosed chamber another exothermic reaction forms a precipitate, removing the carbon from the solution,

Next, this precipitate is dried and then heated in a kiln in order to provide enough energy to drive the endothermic reaction to release the CO₂,

Once the CO₂ is released, it is pumped into either a holding tank, or directly into a methanol synthesis plant. Following the release of all the CO₂, the calcium is converted back into a hydroxide for reuse by the application of steam in a final reaction,

The sum of all the enthalpy changes is exactly zero, however the process still requires energy. This is due to the thermodynamic requirement that energy is used in order to reduce local entropy by pulling the CO₂ from the mixed atmosphere. According to thermodynamics, the bare minimum energy requirement to extract CO₂ from the atmosphere is 19.5 kJ/mol. [7] The physical process discussed in [7] actually requires much more energy (442 kJ/mol) due to many necessary steps, such as actively blowing the atmosphere across the solution in the first step, drying the lime precipitate, and compressing the CO₂. Even without compressing or blowing the atmosphere, an energy consumption of 328 kJ/mol leads to a very low thermodynamic efficiency of 6%. However, more CO₂ is extracted from the atmosphere than is used to produce the energy required to perform the extraction, and future improvements could move the efficiency closer to the fundamental limit.

Conversion to Methanol

The next step in the process is to take the compressed CO₂ and convert it into Methanol. The net reaction here is

While this reaction is exothermic, it is reversible, and the opposite reaction will run at the same time and be more likely at higher temperature. [12] Furthermore, the reaction rate is larger at higher temperature, so an optimization of both high rate and low yield is difficult. For this reason, a large amount of research has focused on Cu/ZnO-based multicomponent catalysts [8,9] which can both increase the yield and reaction rate. However, these catalysts can require temperatures over 500 K to achieve substantial reaction rates. Lower-temperature catalyst processes are being studied, but the problem is far from optimized. [13] In order for this process to occur fast enough and efficiently enough, the carbon dioxide conversion process will require new catalysts pathways.

Neglecting the energy required to obtain the hydrogen gas, we can approximate the energy cost of this process by assuming it is required that the gas is heated to 500 K. The energy cost per mole can be estimated as the energy required to heat the gas up from 300 K to 500 K at constant volume (0.028 kJ/(mol K) and 0.021 kJ/(mol K) for CO₂ and H₂ , respectively) minus the enthalpy of reaction. [14] Fortunately, the heat capacities of the reactants is so small that this estimation results in a value of -31.3 kJ/mol, meaning that the reaction produces more heat than is required to heat the gas to 500 K. If the system is designed to be continuous flow, no additional heat input will be required to maintain a sufficiently high temperature. We will include the energy cost for things during this conversion such as pumping along with the conversion of methanol to gasoline in the next step, assuming that both steps occur at the same plant.

Conversion to Gasoline

Purification of the resulting gas mixture before feeding it into the Exxon Mobil methanol-to-gasoline (MTG) process is an important step. However, the MTG process takes in a liquid mixture of methanol and water, so a simple condensation step after the above reaction would allow methanol and water to be removed from the gas mixture. [10] The liquid mixture can then be pumped into the MTG process while the remaining gas mixture is cycled for further CO₂ conversion. We can assume that this condensation can be passive, requiring no additional energy input.

In the MTG process, the main reaction is conversion from methanol to hydrocarbon chains,

The length of the hydrocarbon chains produced is dependent upon the actual process, but this process has been optimized by Exxon Mobil and realized in a plant in New Zealand. [10]

The conversion to gasoline is exothermic, releasing 55.7 kJ/mol (1740 kJ/kg), and this energy is reused for pumping and refining with a thermal efficiency of 54%. [10,15] Information about the total energy required for the MTG processes is not easily accessible, but we can attempt to estimate this energy requirement by looking at the energy consumption of petroleum refineries in the US and comparing it to the gasoline output of these refineries. In 2006, US refineries produced 3 billion barrels of finished motor gasoline. [16] At 42 gallons per barrel and an average gasoline density of 720 kg/m³, this corresponds to 3.4 × 10¹¹ kg of gasoline produced. The amount of energy consumed as a fuel (excluding feedstock) by US refineries in 2006 was 3000 trillion btu, or 3.2 ×10¹⁵ kJ. [17,18] Dividing these values, we get 9400 kJ/kg as the average energy requirement per kg of gasoline produced in a US refinery. This is a very rough estimate, and is probably an over-estimate since US petroleum refineries use energy to produce many other fuels besides gasoline, while the MTG plant is streamlined exclusively for gasoline. Therefore, it seems reasonable to assume that this estimate also covers the energy for additional pumping and filtering required during the CO2 to methanol conversion.

If approximately half of the thermal heat produced by the MTG process reactions is reused for refining as stated, then that only accounts for approximately one tenth of the total energy requirement. An additional 8500 kJ are required to convert CO₂ into gasoline for every kg of gasoline produced.

Combustion

The final step of the cycle is the combustion of the gasoline within a vehicle, where the hydrocarbons react with oxygen to create CO₂ and water. The hydrocarbons in gasoline have an average combustion energy of around 42000 kJ/kg. [19] This is the energy content of the synthetic gasoline.

After combustion, the thermal energy released is converted into motion. Engine thermal efficiencies are typically quoted at around 35%, but researchers have obtained thermal efficiencies in a lab averaging over 55%. [20] However, this efficiency will not be included as part of the total synfuel conversion efficiency since it is not related to the creation of the fuel.

Energy Conversion Efficiency

The total energy cost of extracting CO₂ from the atmosphere and then converting it into gasoline can now be computed. The extraction step takes 442 kJ/mol = 10000 kJ/kg, and the conversion from CO₂ to methanol to gasoline is estimated to require 8500 kJ/kg. Comparing the energy stored in the gasoline, E_stored to the energy used during conversion, E_conv, we can compute the energy conversion efficiency &\eta;,

Thus 31% of the energy used to create the gasoline is used during the conversion, placing a majority of the energy in the gasoline. This efficiency does not take into account the H₂ gas required in the conversion to methanol, which will require additional energy to create or obtain. Note that the only net materials consumed throughout the cycle are H₂ gas during the methanol step and O₂ gas during the combustion, and the net product is H₂0. Both of these can be obtained by electrolysis of water.

Conclusion

Gasoline usage will continue in the near future, and possibly even after all the profitable carbon fossil fuels have been used up. Gasoline is a very good portable storage medium for energy due to its high energy density, ease of storage, and safety, among other things. Here, we have considered a scenario where CO₂ is extracted from the air, and then converted into gasoline. We have estimated the efficiency of the energy conversion to be 69%, which is quite reasonable. However, the cost of gasoline obtained this way and the throughput of the CO₂ extraction or conversion steps have not been considered here. The first will limit whether or not this process is profitable, and the second will limit how widely used this gasoline will be.

Furthermore, competing fuels and processes have not been considered. Ethanol, diesel, methanol, syngas, and other hydrocarbon fuels can all be manufactured with similar processes. Carbon can also be extracted from plant material, such as corn stover, which could prove to be a more efficient source of carbon. However, the technology to create gasoline from CO₂ is already available, and if no other process or fuel is proven to be more reliable or cheaper, gasoline fuel can be manufactured for transportation uses, allowing the cars to continue running well after fossil fuels are exhausted.

© Darin Sleiter. The author grants permission to copy, distribute and display this work in unaltered form, with the attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

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