08-21 dairy cows

[Image above] Farmers are in the midst of an economic crisis. Could production of electricity and methane from biogas, a natural byproduct of organic wastes such as animal manure, be a way to turn a profit? Credit: UGA CAES/Extension, Flickr (CC BY-NC 2.0)

Even before the COVID-19 pandemic, small farmers in the United States were in the midst of a crisis.

“Farm debt, at $416 billion, is at an all-time high. More than half of all farmers have lost money every year since 2013, and lost more than $1,644 this year [2019]. Farm loan delinquencies are rising,” a TIME article from last November states.

Numerous factors stemming largely from technology and globalization—and trade wars—led to the current crisis, and so far no cohesive plans to address the crisis exist. And now with the COVID-19 pandemic, the crisis is only exacerbated as small farmers are left behind in COVID-19 relief funding.

Some farmers are looking for alternative ways to make money beyond just the sale of their crops and animals. And one option that has generated debate in recent years is the sale of electricity and fuel from biogas and upgraded biogas, respectively.

Biogas is the mixture of gases produced by breakdown of organic matter in the absence of oxygen. It consists primarily of methane (~60%) and carbon dioxide (~40%) plus small amounts of possible impurities including H2S, NH3, H2O, O2, N2, siloxanes, and halocarbons.

Biogas can be burned to generate electricity, or it can be upgraded (purified) to create biomethane, which can be injected as fuel into the existing natural gas grid or converted into a high-value vehicular fuel.

The economic viability and environmental impact of biogas and upgraded biogas are debated, but profitability is especially questionable for small farmers. They face unique obstacles to selling biogas-generated electricity to consumers on the local power grid, so for them, upgraded biogas may be an easier pathway to a profitable market.

However, upgrading biogas is generally not possible for small-scale biogas sources like dairy farms. Many CO2 removal methods, such as chemical absorption and cryogenic separation, are too expensive for farmers. Water scrubbing is the least expensive, but it is a slow physical process that is moderately energy intensive.

If less expensive and more timely methods for upgrading biogas were developed, more farmers could potentially profit from this natural byproduct of their operation. And developing such a method is what a group of researchers led by Sarah E. Baker at Lawrence Livermore National Laboratory looked to do in their recent study published this May.

Sorbents: Upgrade biogas at a lower cost

The recent study is a continuation of the researchers’ previous experiments involving sorbents, or materials used to absorb or adsorb liquids or gases.

In 2015, Baker and her colleagues found composite sorbent materials consisting of sodium carbonate and silicone showed promise for absorbing CO2. In particular, their paper—which investigated sorbent in the shape of a microcapsule, with an aqueous sodium carbonate core and highly permeable silicone shell—found the carbonate solution reacted with absorbed CO2 to form bicarbonate.

In 2019, they investigated increasing the CO2 absorption rates of these sorbents by 3D printing the sorbents in new geometries. And in the paper describing these investigations, they showed CO2 absorption rates improved by an order-of-magnitude relative to aqueous carbonate in a conventional structured shape due to the new structures having increased surface area.

In the recent paper, the researchers again used the 3D printing technique of direct ink writing to shape composite sorbents in unique structures. But this time, they tested CO2 absorption rates using simulated and industrial biogas rather than pure CO2 gas, as was used in previous studies.

Tests showed the composite sorbent did a very good job removing CO2 from the biogas—it produced a methane gas stream of greater than 99% purity by volume %. Once the sorbents became saturated with CO2, they could easily be regenerated (desorbed) using a low-energy air stripping process.

Compared to other methods for CO2 removal, the researchers say their composite sorbent method is expected to cost noticeably less—only $700–$1,600 per m3·hr for capital expenses, compared to $2,700–$3,100 per m3·hr for water scrubbing.

There are two primary reasons for the lower cost:

  1. The system operates at ambient pressure, which allows for less expensive equipment than pressurized systems; and
  2. Regeneration (desorption) of the sorbent is driven by the natural concentration gradient between the biogas stream and ambient air rather than heat or compression, which makes the process more energy efficient.

A C&EN article on the research says the researchers have partnered with Southern California Gas Company to make larger prototype devices out of the sorbent and test them in the field.

The 2015 paper, published in Nature Communications, is “Encapsulated liquid sorbents for carbon dioxide capture” (DOI: 10.1038/ncomms7124).

The 2019 paper, published in Industrial & Engineering Chemistry Research, is “3D printed polymer composites for CO2 capture” (DOI: 10.1021/acs.iecr.9b04375).

The 2020 paper, published in Environmental Science & Technology, is “Three-dimensional printable sodium carbonate composite sorbents for efficient biogas upgrading” (DOI: 10.1021/acs.est.0c01755).