Biogas upgrading goes with a swing

A biogas purification system that is compact, efficient and cost effective could help to turn organic waste into valuable streams of methane and carbon dioxide at small scales.

Biogas is a renewable energy source, produced using microbes that break down sewage sludge, agricultural residues or food waste. Biogas is roughly 50% to 70% methane — which can replace natural gas used for cooking, heating, or generating electricity — while the remainder is mostly carbon dioxide.

A method called pressure swing adsorption (PSA) is used to separate these gases, by passing high-pressure biogas over a porous adsorbent material in a separation column. The adsorbent selectively traps carbon dioxide from the mixture, leaving relatively pure methane. Once this methane has been piped away, the pressure is lowered so that the adsorbent releases its carbon dioxide in a stream of ‘tail gas’. Several separation columns can be connected together to improve the purity of each gas stream over multiple adsorption cycles.

However, tail gas often contains traces of methane, which has up to 30 times greater global warming potential than carbon dioxide. This mixture is typically released into the atmosphere, where it contributes to climate change.

Carlos Grande at KAUST, along with his Ph.D. student, Saravanakumar Ganesan, and research scientist, Rafael Canevesi, aimed to design a PSA system that could economically upgrade biogas into purer product streams, even when operating at small scales^[1]. This would not only avoid methane emissions in the tail gas, but also deliver carbon dioxide that is sufficiently pure for industrial use, so that it is not merely vented.

“If small units could be made affordable, they could be implemented in farms or small communities, extending the capabilities of producing biomethane as a renewable fuel,” says Grande. “Our studies in this field are targeted to make a profitable case for farm-scale implementation of biogas upgrading while also being environmentally responsible.”

The team simulated different PSA configurations that used a commercially-available carbon molecular sieve as the adsorbent, and found that a dual-stage PSA was the most successful¹. In the first stage, four columns produce a high-purity stream of carbon dioxide. Methane-rich gas from this stage is then piped into the second stage, which uses two columns to maximize the methane’s purity. Tail gas from the second stage is recycled back into the first stage, giving any stray methane another opportunity to be extracted.

Then the researchers refined their design, by using industrial balloons to manage the flow of gases through the system. This enabled a simpler PSA configuration with only two columns that achieved more than 97 percent methane purity, and carbon dioxide purity over 99.5 percent — enough to satisfy the most stringent legislation in this area, Grande says. This system also had a lower energy demand than previous designs, and would be cheaper to set up^[2].

“The next stage is to reduce the cost by 30 percent in small units,” says Grande. “We can also tailor the PSA units to different biogas sources, adapting the technology to treat different streams available in Saudi Arabia.”