Sustainable Transportation Fuels From Atmospheric Carbon Dioxide

Reducing carbon dioxide (CO2) emissions from the transportation sector may be the hardest part of the carbon challenge. The sector is not amenable to the Carbon Capture and Storage (CCS) technologies as summarized in the recent IPCC special report on CCS [1]. In the U.S., the transportation sector was responsible for 27% of total GHG emissions in 2005 [2]. It is therefore responsible for over 50% of the emissions that cannot be handled by conventional CCS or conversion to nuclear or renewable power. Transportation also experienced the largest growth (24%) in direct use emissions over the last decade [2]. While there are many opportunities for continued improvements in vehicle energy efficiency, achieving the drastic reductions needed for climate protection will require a more deep-rooted approach. We propose adding carbon neutral, carbonaceous fuels as an option to the collection of technologies capable of solving the transportation challenge. The transportation sector encompasses a wide variety of applications, which suggests a portfolio approach as opposed to seeking a ?silver bullet?.

Proposed solutions for the transportation sector sometimes assume that changes will be spurred by the depletion of conventional fuels. The rapid depletion of oil reserves is indeed the subject of much speculation [3], however, there is little doubt that known fossil reserves, especially coal, are sufficient for several centuries [4]. External factors, such as energy security and population growth, or consistently high oil prices may lead to rapid development of unconventional oil sources. Sources such as oil sands, heavy crude or coal are generally more energy intensive and can increase upstream CO2 emissions anywhere from 10% to 90%, depending on the technology [5].

Given the magnitude of the carbon challenge and the limitations of existing solutions, such as corn ethanol, it is crucial to understand if other options exist for de-carbonizing the transportation sector. Here we examine several alternatives for near-zero emission transportation systems that are linked by the use of carbon-based transportation fuels. The concept is based on collecting an equal amount of CO2 from the atmosphere as is contained in the fuels. Following is a discussion of the various ways such a fuel cycle can be attained?which depend to varying degrees on CCS and/or non-fossil energy sources?and a first order comparison of the costs involved in hydrogen and electrical fuel cycles.

Fuels from the Atmosphere

Carbon neutral fuels refer to fuels containing carbon that do not produce a net increase in atmospheric CO2 content upon use. Such fuels can be achieved by capturing CO2 from the atmosphere, either as biomass or directly by industrial means (air capture). Air Capture refers to the industrial removal of CO2 directly from the atmosphere at concentrations on the order of 400 ppm [6]. The alternative is the formation of biomass via photosynthesis, the natural analogue to air capture. The ability to capture CO2 from the atmosphere opens new routes towards sustainable transportation fuels. Once captured, the CO2 may be sequestered to offset emissions from fossil transportation fuel combustion elsewhere (referred to as indirect methods). Alternatively, it may be recycled in the fuel cycle by its incorporation into a synthetic fuel, conventional biofuels, or a hybrid of the two (referred to as direct methods). The synthetic fuel pathway, for instance, depends on a source of primary energy to drive the required chemical reactions including the supply of hydrogen. As with hydrogen and electricity, these synthetic hydrocarbons are an energy carrier produced from a primary energy source such as wind, nuclear power or fossil fuels with CO¬2 capture. Unlike hydrogen and electricity, they are carbonaceous fuels that are nevertheless carbon neutral as they were derived from the atmosphere. The relationship between all of the options is presented in Figure 1.

The use of biomass systems for fuel production on an industrial scale requires a certain degree of sustainability. As noted by the IPCC [7], ?biomass production and use imply the resolution of issues relating to competition for land and food, water resources, biodiversity and socio-economic impact.? Furthermore, climate change itself may alter current patterns of water usage and arable land putting more pressure on food production and water resources, in addition to land allocated to energy production. Natural systems are very complex and subject to the law of unintended consequences. Biomass based systems may still be viable; however, they inherently contain a larger amount of risk. The risk can be summarized as the challenge of solving a climate change problem with a technology totally dependent on the climate.