by Dr. Philip Pienkos, Strategic Project Lead at the National Bioenergy Center, NREL Source: rdmag.com
t the National Renewable Energy Laboratory (NREL), we have used techno-economic analysis (TEA) to build a model for the production of algal biomass in large open ponds. This model allows us to determine the economies of scale that can reduce cost as the size of cultivation facilities (basically microalgae farms) increase. Based on this analysis, we have determined that production costs can drop significantly until a farm reaches about 5000 acres of open ponds, at which point additional area leads to diminishing returns.
At the 5000-acre farm scale, we believe that it is possible to produce biomass based on verified growth rates for approximately $1000 per dry ton, but operating at the smaller scales currently available, the cost would be much higher. Thus, even economic production of high value products like omega-3 fatty acids is challenging, and economic production of low value products like biofuels is impossible.
In addition to the expected cost saving from driving towards economy of scale, it is expected that improvements in strain characteristics and cultivation processes will also lead to reduced costs.
Our TEA modeling suggests that achieving a target of $300 per dry ton can be achieved. Reaching this cost target would enable economic production of a number of higher value products, but biofuel production would still not be able to compete with crude oil. In the classical approach to using algal biomass as a feedstock for biofuel production, the lipids are extracted and upgraded to biodiesel or renewable diesel/jet fuel. Microalgal biomass is well suited for this sort of process, because under the right cultivation conditions, it can contain more than 50% lipids by weight. However, even at this high lipid content, 50% of the biomass (largely protein and carbohydrate) is wasted.
In our TEA models, this residual biomass is sent to anaerobic digesters for conversion to biogas which can be used to generate heat and power for the biorefinery and to provide a means to recycle nutrients back into the ponds. Although this can improve the sustainability of the process by reducing the amount of fossil fuels needed for heat and power, biogas adds little to the overall economics due to low value in the face of cheap, readily available natural gas.
Researchers at NREL have taken a page from the petroleum refinery (and the meat packing industry) to propose a multi-product algal biorefinery concept as a means to let no component of the microalgal biomass go to waste. Using our expertise at compositional analysis and thermochemical and biochemical conversion technologies, we have identified a number of new opportunities for production of fuels and higher value products from the lipids and the residual biomass. This led us to develop what we call the Combined Algal Processing (CAP) scheme (Figure 1).
Using sulfuric acid and elevated temperatures, we disrupt the algal biomass, hydrolyzing the carbohydrates to monomeric sugars (largely glucose and mannose). This slurry is pumped into fermenters for conversion of the sugars to fuels and chemicals. To date we have produced ethanol, succinic acid, and butyric acid using both yeast and bacteria and we are confident that the microalgal sugars can be substituted for corn, cane or cellulosic sugars for any fermentation process.
After fermentation, the product is recovered and the remaining liquor is extracted with hexane to recover the algal lipids. We have shown that the fermentative organisms are unable to utilize lipids, and that they can be recovered in high yield from the liquor.
We have also explored additional products from algal protein including bioplastics, microbial culture media, and conversion to biofuels through multiple pathways. It is reasonable to ask why we would want to use algal proteins for low value products when increasing amounts of protein are needed to feed a growing population. This is certainly an important role that microalgae could play, but the challenges of producing food or feed grade algal biomass using non-potable water sources such as waste water or produced water from oil or natural gas extraction suggests that alternative uses for algal protein could remain an important priority.
Up to this point, we have only been discussing products based on natural strains of microalgae. Additional products can also be made through metabolic engineering. One area that NREL researchers have explored is the engineering of cyanobacteria to overexpress the efe gene from a bacterium which codes for the ethylene forming enzyme. Expression of this single gene provides the cyanobacterium with the ability to convert light and CO2 into ethylene, released into the gas phase of the culture. Although this concept requires a special cultivation system to facilitate ethylene recovery, the potential addition of this commodity chemical to our biorefinery concept is important.
Taken together, this approach allows us not only to take advantage of all the major natural components of microalgae but to manipulate the organism to produce novel molecules. The various options are summarized in Figure 2.
Our TEA modeling is now focused on identifying the best suite of products to maximize profitability for an algal biorefinery with a primary focus on biofuels. We believe that this concept could lead to biofuel production at a price competitive with petroleum through the added revenues of the higher value coproducts. Ultimately we hope to use everything about the microalgae, period.