Could algae and biowaste be the next black gold? Yuanhui Zhang believes it very well could be. Since 1996, the professor in agricultural and biological engineering at the University of Illinois at Urbana-Champaign, has successfully converted materials such as swine manure and food processing waste using a thermochemical conversion route, called hydrothermal liquefaction, into biocrude oil. It wasn’t until the past few years, however, that Zhang and his team focused research on utilizing algae cultivated on wastewater to be used as a feedstock for conversion into biocrude oil. “We find that algae is nicer to work with,” Zhang tells Algae Technology & Business.
Currently, the majority of algae-to-biofuel research is almost exclusively focused on strains with high-lipid content and extracting those oils for biodiesel production. While that area of research may be gaining traction as a viable route for producing biofuel, algae with high-lipid content typically have low-biomass productivity because high-lipid production can be associated with stress conditions, such as nutrient deprivation, which can reduce photosynthetic efficiency and biomass growth.
To that end, Zhang and his team of researchers theorize that algae-to-biofuel strategies that can utilize low-lipid algae hold significant advantages in large-scale production and can better facilitate synergistic combinations with wastewater treatment and carbon capture. In addition to low-lipid algae and swine manure, other feedstocks tested by Zhang and his team via the HTL process include sawdust, garbage and sewage sludge.
“We’re converting, not extracting, and therefore utilizing the low-lipid algae,” Zhang says. “That’s very promising because usually low-lipid algae can grow faster.”
He adds that while cultivating algae for its high-lipid content can be achieved, the process is typically time-consuming and can, at times, be costly. “This is a huge bottleneck,” he says.
According to Zhang, the crux of his approach is to mimic nature’s thermochemical process by producing a biobased version of fossil-based crude oil that naturally forms beneath the Earth’s crust over hundreds of millions years and compress that time into hours or minutes.
“Crude fossil-based oil is derived from algae and diatoms,” Zhang explains. “My research is moving towards the engineering of a process that can reproduce this process and make our fossil fuels renewable using biowaste and algae. Now, our big challenge is, can we produce enough?”
According to data produced by Zhang and his team, the entire process looks promising.
How it Works, and Advantages
Algal biomass presents the opportunity to employ many potential routes for the conversion into biofuels, including hydrothermal liquefaction. During HTL, high-moisture biomass is typically subjected to elevated temperatures, 482 to 662 degrees Fahrenheit, and pressures of 1,450 to 2,900 pounds per square inch (psi), and breaks down and reforms the chemical building blocks into a biocrude oil. At these temperatures and pressures, water becomes a highly reactive medium that promotes the breakdown and cleavage of chemical bonds that facilitate the reformation of biological molecules. As the HTL process continues, the monomer units are further cleaved and broken into smaller fragment molecules. During fragmentation, the goal is to remove oxygen and other heteroatoms such as nitrogen, sulphur and phosphorous that leave behind the initial carbon and hydrogen atoms in the form of low-molecular-weight compounds. This process maximizes the energy content of the biocrude oil and increases the value and ability to refine the final biocrude oil product.
The primary advantage of using an HTL process on wet algal biomass for conversion into biocrude oil, according to Zhang, is that since water acts as a beneficial aqueous reaction medium, the process is capable of bypassing the energy-intensive and costly step of drying incoming feedstock. In fact, he says the process works better if water is present in the biomass.
“With the HTL process, we can directly process the wet feedstock that still contains 80 percent water with 20 percent solids,” Zhang says. “Many of the solid-to-liquid separation technologies commonly used today can’t compete with that.”
According to lab-scale experiments conducted by Zhang and his team, they’ve successfully developed an HTL technology that converts 70 percent of volatile solids in swine manure on a dry mass basis into biocrude oil, with heating values between 32 and 38 megajules per kilogram, which is equal to 75 and 90 percent of the heating value of petroleum crude oil heating value. Depending on the feedstock, the resulting biocrude oil is shown to have a heating value comparable to that of bunker crude oil and can be burned in boilers or upgraded and refined into higher value fuel or chemical intermediates.
“It has a low quality at this point, but it has high heating values—higher than ethanol—because of how we treat it,” Zhang says. “There are lots of impurities. We need a lot more research to deal with how to upgrade this biocrude oil into more useful end products like gasoline, diesel or biojet fuels. If I produce a lot of biocrude oil, the oil industry has the capabilities to upgrade it, even if it’s low quality.”
The energy recovery ratio, as defined by the energy output of the HTL biocrude oil compared to the process energy input, is 3-to-1 at lab scale and 11-to-1 when heat exchangers are included in a pilot-scale HTL reactor scenario. Zhang has also performed HTL conversion of algae and cyanobacteria to biocrude oil without the use of catalysts; however, Zhang says he’d consider integrating catalysts if they can improve the process to run more efficiently.
“It all depends how good the reaction is,” he says. “We can do it without catalysts, but maybe a catalyst can make the process better.”
Existing algal species such as Chlorella and Spirulina were mostly used in lab experiments and were shown to efficiently be converted into biocrude oil, according to Zhang, but he says the use of a mixture of algal species, including cyanobacteria, are in consideration to be integrated in future production trials.
“We hope some new species will be further developed, like genetically modified algal species,” Zhang says. “We found quite a spectrum of mixtures collected from a wastewater treatment plant works well.”
Zhang’s work using the HTL pathway on algal and other wastes for the production of biocrude oil is simply one spoke within a wheel of sustainability that aims to integrate sound principles of waste treatment, water cleaning and carbon dioxide sequestration into one closed-loop system; a synergistic process he calls “E2 Energy” (Environment-Enhancing Energy) that brings two rival components—energy production and environmental protection—together to complement rather than compete with each other.
“That’s my vision,” Zhang says, “to use our wastewater from animal farms, cities, municipalities and so forth because we need to treat used wastewater anyway and algae is a good medium to uptake the nutrients from used wastewater. It’s also good at capturing carbon, and then we can convert the carbon later into hydrocarbons, which is the biocrude oil we need and, in the meantime, clean and recycle the wastewater back into the system.”
With help from different sources of grants by the state of Illinois, the federal government and the National Pork Producers Council, Zhang says he intends to seek out the many unanswered scientific questions that linger in his research. The university has filed patents on the technology.
“We can get the oil but we need to understand how the oil forms and the exact pathway,” he says. “We know what to use and we know what we got and how to do it, but why? We don’t know yet.”
Further studies include conducting a life-cycle analysis and examining algal coproducts, such as minerals, that could be used for fertilizer, according to Zhang, adding that he’s open to the right partnership that could economically and efficiently scale up his technology, and eventually deploy it on a commercial scale.
“This work is very exciting,” Zhang says. “I think I’m on the right direction.”
Author: Bryan Sims
Associate Editor, Algae Technology & Business