The great hope of biofuels
Lively, R.P., P. Sharma, B.A. McCool, J. Beaudry-Losique, D. Luo, V.M. Thomas, M. Realff, R.R. Chance. 2015. "Anthropogenic CO2 as a feedstock for the production of algal-based biofuels." Biofuels, Bioproducts and Biorefining. 9 (2015): 72-81. DOI: http://dx.doi.org/10.1002/bbb.1505
Climate change represents a central challenge for humankind and will for generations. A critical factor in the response to that challenge the portfolio of fuels used for transportation globally and the related carbon emissions. In this matter, biofuels appear as a great alternative because of several benefits. These include the fact that they are nearly carbon-neutral fuel; the carbon content of the organic matter used — corn, vegetal oil, wood, and algae — is "recycled" with the solar energy stored in that fuel, displacing fossil fuel derived energy. Also biofuels can be considered as "drop-in" fuels, since they are fully infrastructure-compatible fuels. This means they can be used in vehicles without engine modifications, utilizing existing petroleum distribution systems.
A major challenge is the difficulty of producing the volume of renewable fuels necessary to meet existing demands. And for that, advanced biofuels such as algae-based biofuels are expected to provide important contributions. The highlight here is the production of this type of biofuel using carbon dioxide generated by co-located power plants as input.
In a recent study published in the journal Biofuels, Bioproducts and Biorefining (BBB), a group of researchers from Georgia Institute of Technology, along with engineers from Algenol Biofuels, contrasted the main concerns associated with algae-based biofuel production — such as the costs of CO2 deliveryto the process, as well as the overall lifecycle of CO2 consumed — from extraction to use in vehicles. The analysis also includes the comparison of different power plants as source for the CO2 delivery, and compares the carbon footprint of different transportation fuel options. The authors say their goal is not to make a perfectly accurate techno-economic and life-cycle analysis, but instead to demonstrate the overall carbon footprint performance of more advanced biofuels and biorefineries that are expected to become reality in the future, and make comparisons on a consistent basis.
The authors focused on the analysis of CO2 capture from both natural gas and coal power plants, and the consequences of integration with biorefineries that produce algae-based biofuels. As they point out, integrated biorefineries have several requirements for CO2 sources, such as the available feed rate, the source location, the ease of CO2 recovering, and the overall cost. It is important to note that the CO2 provided by natural gas power plants presents many advantages due to its characteristics when compared with coal power plants. The flue gas resulting from natural gas plants is much more "clean," meaning that the gas is mostly free of problematic contaminants, which are very common in the gas generated in coal plants. All the clean-up processes that are necessary in the latter add costs to the operation; these processes also create large amounts of sludge wastewater that also need treatment. These kinds of requirements must be considered to meet standards for the quality of the gas that will feed the biorefinery.
On the other hand, coal power plants have the capacity to deliver post-combustion CO2 at higher concentration than natural gas plants (940 versus 367 kilograms of CO2 per megawatt-hour generated). Consequently, recovering CO2 from coal plants is generally lower cost than recovery from natural gas plants due to its carbon intensity. However, those costs usually do not consider all the cleaning steps that must be included. Still, the alternative of co-located biorefineries together with the CO2 producers also avoids costs of transportation and delivery of the CO2.
Turning to the carbon footprint analysis, the researchers note that in the integrated biorefinery the production of ethanol from the blue-green algae (cyanobacteria) can yield a biofuel with a carbon footprint that can be less than 20 percent that of gasoline. They compared the biorefinery, a carbon capture and utilization (CCU) option, against the carbon capture and sequestration (CCS) alternative — where the carbon usually is stored underground in geological formations — as well as the no-capture option. In terms of overall carbon footprint, the algal-based biorefinery or CCU option presents major advantages over no capture and is highly competitive to CCS. Also important is the comparison of the algae-based fuels to common fuels, such as gasoline, diesel, corn ethanol, and grid electricity (for plug-in electric vehicles).
The authors considered the fuels' production cost and the carbon footprint of the production on a "well-to-wheels" basis Results show that the algal ethanol production is cost competitive with fossil fuels and corn ethanol, with substantial improvement in carbon footprint. Electric vehicles provide a low-cost option, but have a carbon footprint that is highly dependent on their electricity source. For the average U.S. grid, the carbon footprint for electric vehicles achieves only a modest improvement over fossil fuels. If the electricity comes from coal, they are worse.
The integrated biorefineries proposed in the study still need to be deployed and validated commercially, and thus present uncertainties such as carbon utilization efficiencies, CO2 capture costs, system integration, and the economics compared to fossil fuels. Either way, the potential for biofuels to reduce the carbon footprint of transportation fuel consumption is evident. Further research on specific sites, CO2 sources, and purity must be done. For now, the evidence suggests that the integration of natural gas power plants to algal biorefineries is a positive step toward improving transportation fuels production.