Joshua Hoehne; Elizabeth Nelson
Joshua Hoehne; Elizabeth Nelson

Biofuels: A Third Act for Your Leftovers

by KeShonna Jackson ’24

Every year, Nazih Kassem sits down to sort through heaps of data detailing the amount of organic waste, in tons, discarded by every restaurant, brewery, distillery, grocery store, farm, and school in New York State. Food waste floods our landfills, while agricultural waste, such as manure, is typically applied to fields. In either case, the waste decomposes and produces methane, a powerful greenhouse gas that traps heat in the atmosphere and contributes to global warming.

Kassem, a PhD student in Biological and Environmental Engineering, believes that we can redirect organic waste and extract its untapped potential.

“I look at waste-to-energy systems. This is how to convert food and agricultural waste into energy products, reducing greenhouse gas emissions and the environmental impacts of fossil fuels,” Kassem says.

Biofuels from Scraps

Kassem designs computer simulations, known as models, that test various scenarios of waste-to-energy conversion, evaluating how feasible, effective, and costly they would be if implemented on a large scale.

Fossil fuels currently supply most of the world’s energy needs. It’s a 200-year-old energy system that relies on limited resources and has dire environmental outcomes. Kassem has recently devised a high-efficiency scenario to replace it: a three-step process that converts organic waste into a source of renewable energy.

Bacteria naturally break down food waste in a process called anaerobic digestion. This is where Kassem’s model begins.

In the first step, anaerobic digestion produces an organic fuel called biogas. Biogas can replace natural gas to generate electricity and heat, and it can be upgraded to make fuel for transportation. Other countries, such as Sweden, already use biogas to power many of their cars and buses.

But microbes can’t digest and convert all the materials found in food and agricultural waste. The undigested remainder, called digestate, is a byproduct rich in nutrients and carbon.

The second step in Kassem’s model, hydrothermal liquefaction (HTL), refines the digestate. “This process is operated at very high temperatures and pressures. It neutralizes microbes and pathogens—and at the same time converts the carbon that’s left over [in the digestate] into a useful fuel called bio-oil,” Kassem says.

Researchers are currently testing bio-oil’s potential as a precursor for liquid biofuels. As with biogas, bio-oil would reduce the effects of greenhouse gas emissions. Biofuels recycle carbon dioxide that is already present in the atmosphere and has been captured by plants through photosynthesis, whereas fossil fuels, such as gasoline and other petroleum products, release carbon that has been trapped in the Earth’s crust for millennia.

Lost Energy for Bio-Methanation

At this point, the biorefinery process is complete. But the first two steps of Kassem’s model both yield carbon dioxide with energy potential that he doesn’t want to waste. So Kassem takes it a step further.

“This combination [of hydrogen and carbon dioxide] produces a bio-methane molecule that is basically identical to natural gas. But it’s produced using renewable feedstocks.”

During anaerobic digestion, microorganisms produce carbon dioxide as they metabolize waste, much as humans do during respiration. And HTL produces a gaseous stream consisting of carbon dioxide. In both cases, the carbon dioxide, if combined with hydrogen molecules, could be used to create a third type of renewable fuel.

Kassem has devised a method to supply the needed hydrogen molecules from excess renewable electricity. Renewable energy sources, such as solar and wind power, sometimes capture more energy than can be stored at the utility-scale. This excess energy might otherwise be lost, but Kassem’s model uses it to split water molecules and collect pure hydrogen. The hydrogen would then be combined with the carbon dioxide from the initial two steps of the biorefinery process.

“This combination [of hydrogen and carbon dioxide] produces a bio-methane molecule that is basically identical to natural gas. But it’s produced using renewable feedstocks. We can inject this into the natural gas grid, contributing to its decarbonization,” Kassem explains.

Every step in Kassem’s work produces a beneficial and renewable alternative to nonrenewable energy sources. The system is efficient, resourceful, and regenerative.

The one thing it is not: cheap.

Funding Decarbonization

“The problem is the price of the bio-methane,” Kassem explains. “It costs a lot to build our system, so we need bio-methane to command a higher price than what conventional fuels like natural gas are currently selling for in today’s energy markets. It basically makes us uncompetitive. So I'm looking at policy, what level of government incentives would be needed to make a system like this profitable, or at least break even.”

Beginning in 2022, New York State will require businesses that produce more than two tons of food waste weekly to redirect that waste to a biorefinery or organic recycling facility, provided that such a facility is located within 25 miles. The law is called the Food Donation and Food Scraps Recycling Law.

Kassem believes this law is a step in the right direction, but more needs to be done to monetize the environmental benefits of the biorefinery process.

“We need streamlined waste collection and transportation infrastructures, more deployment of biorefinery plants for increased accessibility to wastes, and more awareness of the environmental situation and benefits in general,” he says.

Kassem’s work is contributing to a cross-disciplinary research group led by his adviser, Jefferson W. Tester, Chemical and Biomolecular Engineering. “We’re hoping that we can use our work here to show policymakers that we need incentives to mitigate the cost [of the biorefinery system]. There are so many benefits to this, for example, helping New York State achieve its decarbonization goals, improvement in air quality, and reduced water contamination from the current waste disposal methods. It's worth it to pursue this.”

On a smaller scale, Kassem’s models have already proved to be successful. “We met with a farmer who wanted to produce biodiesel [a renewable alternative to diesel fuel] using animal fats. He reached out to us and asked me to model his system to see whether it would make financial sense for him to do this and what the environmental impacts would be. We communicated the results to the farmer and now he has a fully functioning truck running on biodiesel,” says Kassem.

The input-output flow of waste and energy is key to restoring the planet and creating a more sustainable future, according to Kassem. In his work, he is always thinking about the bigger impacts, focused on making change.

“Being a part of this is really motivating for me. We're seeing all these things happening today—floods, droughts, forest fires—all caused by global warming. Knowing that what I'm doing will actually help decrease carbon emissions and contribute to mitigating the climate change impacts is very rewarding.”