Algal Biofuels: Closer to Sustainability?Yasmin Pascual Khalil | December 16, 2014
To tackle concerns driven by carbon dioxide (CO2) emissions, chemical majors are using bio-process engineering technologies to convert carbon sources into new energy forms.
Presently, organic materials like plants, agricultural waste, forest residues, microbes and other sources otherwise known as biomass are being transformed into useful biofuels such as methane or transportation fuels: ethanol and biodiesel to meet environmental sustainability goals. To encourage those efforts, the U.S. Department of Energy recently announced a $25 million fund for biofuel research with one target: to drive down the cost of algal-derived biofuels to below $5 per gasoline gallon equivalent (GGE) over the next five years.
Biofuels are split into four generations based on their biomass source. The first generation is produced from food crops such as sugars and vegetable oils. The second one (also known as advanced biofuels) is generated from non-food crops like agricultural waste and lignocellulosic biomass.
Primarily, second generation biofuels were developed to end the food-versus-fuel dilemma. Third-generation biofuels are algae based and the fourth generation uses biomass materials that have absorbed CO2 while growing. This generation is aimed at producing sustainable energy while capturing and storing CO2.
Spotlight on Algae
Algae, or more correctly, microalgae, have been getting a lot of attention from the industry, academia and governments because of their production potential; specifically, their ability to multiply several times a day under the right environmental conditions.
In comparison to plant feedstock, microalgae offer several advantages in CO2 capture and bio-oil generation: high photosynthesis and solar conversion efficiencies, quick biomass production rates, non-competitiveness with the food market, capacity to produce a diverse range of biofuel feedstock and the ability to bloom in a variety of ecosystems.
Microalgae can be cultivated in open or closed systems, with open systems like lakes or pond raceways more easily scalable for production than closed systems, such as photobioreactors (PBRs). Large-scale open ponds are relatively cheaper to build and easy to operate, but there are always issues with water temperature, CO2 diffusion to the atmosphere, vapor losses and contamination risk. Due to these drawbacks, much attention has been given to closed PBRs for providing a regulated and controlled cultivation environment and reduced contamination risk.
Besides PBRs and pond raceways, heterotrophic cultivation in closed fermentors is another viable technology according to a 2011 IHS report on the “Economics of Commercial Scale Biofuels from Algae” by IHS Senior Analyst Sudeep Vaswani. In the study, all three processes were compared while taking variable and fixed costs along with capital investment costs into account. The heterotrophic method was described as “quite promising” by Vaswani, due to having high productivity and the lowest capital cost.
Aside from microalgae, cyanobacteria can also convert carbon dioxide to energy. Recently, engineered cyanobacteria have been sparking interest as catalysts for the direct conversion of CO2 into reduced fuel compounds. However, to play it safe, microalgae seem like the better option according to researcher Michael K. Danquah, a bio-process technologies expert from Curtin University.
“Microalgae have a better photosynthetic efficiency and are more resilient to different growth environments than cyanobacteria,” he says. “Cyanobacteria can also produce toxic biochemicals which could be hazardous to the environment.”
No Clear Advantage?
In a 2013 IHS Chemical Week cover story on industrial biotechnology, BASF remained neutral regarding the position of fossil and renewable energy sources, saying: “There are no clear-cut advantages or disadvantages to using fossil-based or renewable raw materials per se. It is best to decide on a case-by-case basis, taking environmental concerns, cost effectiveness and social impact over the entire product life cycle into account.”
Many share BASF’s perspective on energy sources. A variety of research initiatives are in progress to test the viability of biofuels not just environmentally, but economically and socially as well. For example, the Synthetic Genomics-ExxonMobil (XOM) collaboration is aimed at identifying and developing strains of microalgae that can be used to produce transport fuels in large volumes with promising economic returns. XOM allocated $600 million to this research project. Another pioneering alliance is by the Linde Group and Sapphire Energy for the development of a cost-efficient CO2 management system in support of commercial-scale, open-pond microalgae cultivation activities.
Aside from joint ventures and collaborations, prototype microalgae cultivation systems are in use today. The Offshore Membrane Enclosures for Growing Algae (OMEGA) initiative by NASA scientist Jonathon Trent uses an innovative method with a simple concept: let microalgae use solar energy and get the desired nutrients from wastewater and CO2 to produce oil-rich biomass that can be transformed into biofuels.
In 2013, the California Energy Commission (CEC) issued a report on the OMEGA project outlining findings from research facilities in San Francisco and Santa Cruz. According to Trent, more results on the techno-economic analysis and wastewater recovery as potable water (Desalgae) will be published soon.
Based on available data, the OMEGA system measures up well to other microalgae cultivation systems as it diminishes land use, utilizes wastewater, takes part in carbon capture and sequestration (CCS) and provides a multifunctional offshore platform.
“The production economics of biofuels is by far the major factor hindering its commercial viability,” Danquah says. “The production technologies of current processes, though may be environmentally sustainable, are still not economically competitive in comparison with petroleum fuels.”
According to the U.S. Department of Energy, algal biofuels produced at scale with current technology would be priced at $8 or more per gallon, higher than the $4 per gallon price for soybean oil. Lowering the cost of algal biofuels is vital to economic feasibility, and that may be accomplished with coordinated research and development (R&D) activities over the next decade.
Metabolic engineering-related roadblocks exist as well. For instance, carbon fluxes from the substrate (matter acted upon by an enzyme) can disperse into a complex metabolic system. Another issue is the shortage of adenosine triphosphate (ATP) supply. Usually, microbial hosts oxidize a large segment of the substrate to create ATP and nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) for biofuel synthesis activation. Both, ATP and NADPH are needed for cell maintenance. Moreover, mass transfer restrictions in large bioreactors can lead to heterogeneous growth conditions and micro-environmental fluctuations that will bring on metabolic stresses and genetic instability.
The stability of metabolically engineered cell lines under changing growth environments is also a major issue according to Danquah.
The U.S. Energy Department is prioritizing R&D needs toward achieving microalgae commercialization by focusing on four research areas. First, is basic algal biology to cover topics related to genetics, strain improvement tools, solar conversion, biochemistry and lipid productivity. Second, is process research to explore system design and engineering options. Third, is production and integrated process scale up to address CO2 supply, harvesting and extraction technologies, compliance with ASTM standards and long-term maintenance. The fourth area of research focuses on economic analysis to evaluate commercial feasibility.
Another algal biology research topic could focus on microbial biosequestration. As Danquah says, “recent advances in carbon recycling for biofuel development mostly relate to microbial biosequestration of carbon dioxide and further processing to convert the biomass to biofuels.”
To increase the viability of the OMEGA system, Trent and his co-researchers are addressing the triple bottom line (3BL) impact. This will cover environmental issues like biofuel production as an alternative to fossil fuels, global warming reduction via carbon capture, wastewater reuse and treatment and increased coastal biodiversity; social issues like improved employment opportunities and non-competitiveness with other industries such as agricultural and alternative energy generation; and economic issues like energy production and economic returns on investments based on the techno-feasibility analysis.
Danquah agrees with OMEGA’s 3BL analysis, saying “3BL-focused innovation through rigorous R&D is needed for more economically competitive technologies to be developed.”