Solutions to Atmospheric Carbon Maintain Advantages of Petroleum
The traditional advantages of petroleum-based transport fuels are challenged by the need to lower atmospheric carbon. Despite significant research, development, and investment during the past few decades, humanity still seeks commercially viable carbon-neutral alternatives to petroleum. This paper presents approaches to carbon abatement using petroleum that have a strong chance to succeed in fulfilling technological and economic goals.
Comparing Renewable Fuel With a Fossil-Fuel/Carbon-Offset Combination
Typically, renewable liquid fuels are more expensive than their fossil fuel counterparts on a unit-energy basis, but they result in lower carbon dioxide (CO2) addition to the atmosphere. Fossil fuels could also be used in such a way that produced CO2 is sequestered at a cost. With carbon-offset costs added to fossil fuel, both can be considered to provide carbon-neutral energy and can be compared on bases of cost and availability.
Commodity prices fluctuate because of a variety of factors. Aside from the higher costs of renewables, reading too much into exact crossover points is not useful, Nonetheless, a general statement could be made that, at carbon-abatement costs of approximately $80–100/tonne of CO2, a renewable energy source can compete with fossil fuels. The cited costs of carbon capture and storage also have varied in the range of $100–150/tonne of CO2. The costs depend on emission sources and locations and the technology used for carbon capture. Even at reduced carbon costs, one will still be better off using gasoline—that is, if both commodities (renewable fuels and fossil fuels) were equally available at needed amounts.
Technology for food-based biofuels (e.g., corn-based ethanol) has advanced sufficiently and mostly is used commercially in the US for making bioethanol for blending with gasoline. However, food-based biofuels compete with the availability of food and result in rising food prices. Data suggest that food-based biofuels are neither cost-competitive nor quantitatively sufficient to supplant fossil fuels. While they can play a role in carbon abatement to an extent if the carbon offset costs rise above $100/tonne of CO2, at present, they do not appear to be the solution to reduce emissions on their own.
Can Useful Products Be Monetized From CO2 Conversion?
Little other than energy is consumed in proportions greater than 4 billion tonnes/year (e.g., cement). Even if produced CO2 is converted to something that can replace cement, for example, the supply of that substance will be far greater than the amount of cement the world needs and, hence, it will not be able to retain its market—or any—price. If CO2 is converted to other substances, the quantity of products or byproducts will be on the order of 10-plus billion tonnes. This means that no prospective revenues from the useful byproducts can be relied upon, and, thus, it is prudent to only include costs involved in carbon abatement.
Conversion of CO2 to Liquid Fuels
As indicated previously, the commodities that human society uses in the greatest amounts that could compare with the size of annual emissions are various energy products. Of oil alone, the world consumes approximately 5 billion tonnes annually [Note: This paper was authored prior to the COVID-19 effects on oil demand]. Potentially, CO2 could be converted back to liquid or other fuels using solar energy; thus, in principle, the amount of CO2 in the atmosphere could be stabilized.
Use of fossil fuel combined with a low-cost carbon-offset method holds promise to solve the problem in an optimal way. But using CO2 produced from fossil fuels and converting it to liquid fuels, as has been suggested by some, does not help. This is because the cost of the offset rises because of high costs of conversion and leads to replacing petroleum and losing its cost advantages.
Electrochemically Mediated Biomass Production
The complete paper discusses approaches that can sequester carbon produced from oil and gas. The first of these, a lower-oxidation (or “loxidation” concept, as the author describes it), is detailed in the complete paper. The concept of loxidation is aimed at arresting the process of energy production (combustion) before production of a gaseous waste. But, once carbon has been completely oxidized to gaseous CO2, it can still be “fixed,” a concept discussed in the complete paper, as nature already does in large amounts using the sun’s energy or photosynthesis. The basic unit of solid structure into which photosynthesis fixes gaseous carbon is the glyceraldehyde-triphosphate (G3P) molecule containing three carbons and a phosphate group. The task of carbon fixation is essentially achieved with the production of G3P, but, typically, nature produces glucose or polysaccharides, starch, or cellulose using these G3Ps.
Photosynthesis has two types of reactions: light-dependent reactions and light-independent reactions. These reactions are essentially redox reactions. In theory, any part of this complex reaction system could be carried out artificially, but, because of advances in photovoltaic (PV)-cell technology and a dramatic reduction in PV-cell costs during the past 50 years, photo energy can now be harnessed and converted to electrical energy in a relatively inexpensive manner. That power can then be used to split water molecules with electrolysis, the same task achieved by the light-dependent reactions in photosynthesis. The inclusion of PV-cell-assisted water electrolysis is mentioned to suggest that the technology for such a process already exists. The lower-energy molecules involved in photosynthesis [adenosine diphosphate (ADP) and nicotinamide adenine dinucleotide phosphate (NADP+)] could also be potentially reduced using solar power. This subprocess allows decoupling of the two parts of photosynthesis. In another reactor, CO2 could be reduced to G3P or other nongaseous molecules using different systems of catalysts. Fig. 1 describes this idea schematically.
The amount of carbon in fossil emissions over what is fixed by nature is relatively small (approximately 7 billion tonnes), but it is accumulative and builds up in the atmosphere. Natural fixation of carbon in biomass lasts only on an average 12 to 15 years (called a “biostorage period”) because other natural processes—insect and microbial activity—cause it to decay and release the stored carbon back to atmosphere. If this period could be extended to more than 200 years by pyrolyzing part of the produced biomass, lasting sequestration can be achieved. While biochar production from a portion of produced biomass at large scale is an effective solution, other approaches using natural biomass production should not be ignored.
Pyrolysis of the biomass leaves pure carbon in the solid, which is immune to microbial and insect activity, more or less, and, hence, lasts for centuries. But, if the average biostorage period is increased to even 50 or 75 years, a significant dent in atmospheric CO2 concentration can be made. This is achieved naturally in some plant life that is resistant to insects and microbial activity. Planting such species of trees as part of a massive afforestation plan can increase the average biostorage period. Moreover, in comparison with conversion to charcoal, this process sequesters twice as much carbon because no immediate loss of carbon associated is involved with the pyrolysis process.
Regardless of how the biomass is produced—naturally by afforestation or with assisted agriculture—another method exists other than addition of chemicals or pyrolysis to secure a longer biostorage period. The produced biomass could be shipped and stored in areas where insect and microbial activity are naturally diminished, such as the great deserts of the world. This does not require a new technology to be developed. Produced logs could be secured as part of the sequestration effort and towed in the ocean to the nearest geographically accessible desert locations where land is dry and barren and relatively inexpensive. This, obviously, will entail varied costs in transport and storage that must be assessed against other available methods for determining practicality.
Any method that prevents CO2 from entering the atmosphere for prolonged periods effectively amounts to carbon sequestration.
Given the state of technology and experience, petroleum fuels offer a more-affordable solution to providing carbon-neutral energy compared with renewable fuels such as bioethanol.
Three routes for future development are presented as concepts for carbon abatement conforming to the framework:
Assisted biomass production
Preservation of biomass
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