Carbon-Neutral Synthetic Fuel

Fig. 1: Comparison of domestic crude oil production (blue) versus imported (red) crude oil in the U.S.

Imagine that there were a breakthrough in synthetic fuel production that was both economically viable and scalable. All fossil fuels (e.g. coal, oil, and natural gas) could then be replaced with a net-zero carbon dioxide (CO₂) emitting fuel. How could this seemingly far-fetched thought experiment become reality and what effect would it have on our lives? This paper will explore the potential effects just such a transition to carbon-neutral synthetic fuels might have in the United States (U.S.) and around the world.

Considerations

Currently, the global energy economy is heavily reliant on fossil fuels, which are limited in supply. [1] The complex web of production, supply, and consumption of these fuels can cause political instability from the local to the international level. [2] Fig. 1 illustrates the growing reliance on imported crude oil by the U.S. [3] Additionally, increased concern over the potential climatic effects of increased greenhouse gas concentrations in the atmosphere, notably CO₂, have spurred many politicians and scientists to look for alternative, "clean" sources of energy. [4] Technologies being explored to replace fossil fuels include converting biomass to biofuels and using photoelectrochemistry to derive fuels from incombustible, and plentiful reactants such as water. [5-7] Both processes can involve CO₂ capture from the atmosphere for fuel production, and both use sunlight as the primary energy source. [7]

The natural carbon cycle is complex and an extensively researched and debated topic. [4,7] The main players in this cycle are the biosphere, the ocean, and the atmosphere. On a basic level, one can think of the atmosphere as a medium of exchange for carbon sequestration by terrestrial biomass and by the ocean. The average lifetime of a CO₂ molecule in the atmosphere is estimated at 50 to 200 years. [8] Uptake of CO₂ from the atmosphere by biomass occurs through photosynthesis where it can then be reemitted though decay processes. [7] The ocean absorbs and reemits CO₂ based largely on temperature, salinity, and alkalinity, and in the long-term, this carbon can be sequestered in sedimentary rock. [4]

Fig. 2: CO2 emissions released from the combustion of fossil fuels (red) and atmospheric concentrations of CO2 obtained from ice core data (blue) and from Mauna Loa Observatory (black).

Historic atmospheric concentrations of CO₂ obtained through analysis of ice core samples in Antarctica show multiple cycles lasting tens of thousands of years with values between 180-300 ppmv (parts per million by volume). [9] This suggests long-term balance of the carbon cycle. However, since the industrial revolution began in the late 1700s, the concentration of CO₂ in the atmosphere has deviated from this pattern, growing very rapidly to current levels of nearly 390 ppmv. [10] This value is almost 38% higher than CO₂ estimates from the year 1800. [11] Further, the ice core data suggests it is the highest measured value in the last 650,000 years and some studies say the last 25 million years. [4,9] Fig. 2 shows a correlation between an increase in fossil fuel combustion and the rise of global CO₂ concentrations in the atmosphere over the last few centuries. [10-12]

Net-Zero Carbon Fuel and Landfills

In 2002, the U.S contributed nearly 22% to global CO₂ emissions, where 94% of this contribution came from the combustion of fossil fuels. [1,12] As the natural carbon cycle is roughly in balance - with the biosphere and oceans both functioning as a natural carbon source and sink - how can the man-made carbon cycle be balanced? Combustion of fossil fuels is a source of carbon for the atmosphere, but what can be considered a man-made sink?

Landfills are a significant anthropogenic carbon sink that sequester CO₂ in the form of harvested wood products (e.g. paper, lumber), yard trimmings, and food waste. [13,14] While some CO₂ is emitted during decay processes, the majority of the carbon remains onsite. If we consider only net-zero carbon fuel usage and that the non-fossil fuel carbon cycle is still roughly in balance, we can approximate the time it would take for U.S. landfills to sequester one year of CO₂ emissions produced in the U.S by fossil fuel combustion.

The U.S. emitted 5565 Tg (10¹² grams) CO₂ into the atmosphere in 2002 from the combustion of fossil fuels. [15] That same year, carbon sequestered by U.S. landfills was estimated at 45.1 Tg of carbon. [13,14] Converting Tg of CO₂ to carbon can be accomplished by multiplying the mass of CO₂ by an atomic weighting factor of 12/44, resulting in 1515 Tg C. It would take approximately 33 years for U.S. landfills to sequester one year's worth of U.S. CO₂ emissions that was produced from fossil fuel combustion.

U.S. CO₂ emissions not associated with fossil fuel combustion accounted for 344 Tg CO₂ or 6% of the total in 2002. [15] Weighting by the same atomic factor as before, gives a value of 94 Tg of carbon per year. This is roughly twice the value that landfills sequester in one year, resulting in net emission of CO₂. However, this value is now 3% of the original U.S. anthropogenic CO₂ emission value and only 0.7% of global emissions that year from fossil fuels. [1,15]

Implications

The natural carbon cycle is a dynamic system; even with elevated concentrations, models predict that the carbon cycle can naturally sequester CO₂ from the atmosphere so that it can reach a state of equilibrium again. [4,12,16] This will largely be accomplished through carbon exchange with the ocean. [15] How long this takes is dependent on the starting CO₂ concentration. Given current usage habits, if all known fossil fuel stores are combusted and the resulting CO₂ is released into the atmosphere, it may take thousands of years for the CO₂ concentration to decline back to the elevated level that it is today. [16]

Whether one's goal is climate change mitigation or national energy security or both, the realization of a viable net-zero synthetic hydrocarbon fuel that utilizes solar energy and atmospheric CO₂ has great advantages. For those concerned about climate change, synthetic fuels could significantly curb CO₂ emissions and even act as a carbon sink if production consistently exceeded consumption. This would generate a backup fuel supply in the process. For those concerned about energy security, sunlight, CO₂, and water are generally available in much of the world to create synthetic fuels; thus, domestic fuel and energy production could significantly outweigh the need for international trade. Additionally, the use of synthetic fuel fits in with current distribution and usage systems, easing the requirements on restructuring the energy infrastructure.

While future energy needs will likely be met through fossil fuel usage concomitant with a growing presence of renewable energy technologies - such as large-scale wind- and solar-to-grid - synthetic fuel production and consumption offer an appetizing addition to the energy mix.

© 2011 Michael Machala. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

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