The experimental set-up in Niels Mendel’s lab looks deceivingly uncomplicated. A semi-transparent, plastic box partly hides what’s going on inside. But with a bit of effort, the vague contours can be seen of a 10 centimeters large Petri dish, placed in a small sealed container, with some transparent plastic tubes sticking out. The petri dish is filled with, 3 millimeter large grey-colored clay particles, and placed on top of a scale.
‘Here we test the efficiency of CO2 absorbance by these clay pellets’, Mendel explains. ‘Through one tube, we lead CO2 gas over the clay, they subsequently adsorb a certain amount, and we can quantify this by the weight increase, visualized on the scale.’ At some point, the weight remains constant, indicating that the clay is saturated with CO2. But scientists can regenerate the clay by flushing the grains with nitrogen gas: the clay-absorbed CO2, will be released and Mendel can follow that process by a subsequent weight decrease. This simple set-up is very effective to study an innovative and cost-effective way to remove CO2 from biogas, so that mostly methane, remains.
'Here they make that gas by fermenting pig manure. That provides enough energy for about 3000 households'
Harvesting biogas
Micro-organisms can produce biogas by fermenting organic streams, like food and plant waste, but also animal manure. This biogas, methane, can be used for heating and cooking, ‘In the Netherlands, biogas is commercially produced, for example at Twence in Zenderen’, Mendel says. ‘Here, biogas is made by fermenting pig manure, enough for about 3000 households.’ Current CO2 removal methods to purify methane often involve membranes. The smaller CO2 molecule is pressed through the membrane pores, that don’t allow the passage of the bigger methane molecule. However, this process is quite energy intensive, and, overall, more expensive than purification by clay grains, where the beauty and elegance lie in its simplicity.
'Since clay can only retain CO2 temporarily, the idea arose to use it as a material to be regenerated to remove CO2 from biogas'
Promising material
Mendel’s biogas purification project, happened more or less as a coincidence: it started as research to capture CO2 and permanently store it. ‘Clay was a promising material to absorb this gas, but we found out that it held the absorbed CO2 for only a limited amount of time. It proved to be unsuitable for long-term storage’, Mendel says. ‘Since clay can hold CO2 only temporarily, the idea came up to use it as a regenerable material to remove CO2 from biogas. Afterall, this is some kind of short-term storage.’
According to Mendel, clay has almost the perfect properties to store CO2 gas, and with just some minor modifications it’s a cheap and effective substance to temporarily capture this gas. The clay Mendel uses is build-up out of many thin layers. The distance between these layers is set by the size of the ions that are naturally present between them. In the natural clay material, these are typically small ions, such as sodium (Na+) or calcium (Ca2+). This distance between the layers is too small to fit CO2.
‘To create more space, we replaced the small sodium and calcium ions with larger ions, like cesium or tetra-methyl ammonium’, Mendel explains. ‘The resulting increase in the layer spacing is large enough to fit the CO2 molecules almost perfectly, capturing them effectively. The slightly larger methane molecules are simply too big and therefore can’t be stored.’ With the current set-up, the clay can hold about 30 times more CO2 than methane. In between the clay layers, hardly any methane is present, there’s only a bit adsorbed on the outer layers.
Worth the effort
To give the clay the right properties, and test the clay’s suitability to absorb CO2, the clay had to be modified. ‘To compare our results with other studies, we performed our first tests with relatively expensive clay of scientific grade’, Mendel says. ‘But soon we switched to commercially available clay, used in construction, with a substantially lower price. It worked similarly well.’ This clay came in bags containing small clay cylinders, about 2 centimeters long and half a centimeter wide.
Mendel first submerged the clay cylinders into a salt solution containing cesium, to replace the smaller sodium and calcium ions and increase the space between the clay layers. In a next step he manually ground the dried clay, and mixed the resulting clay powder with an adhesive glue, and reshaped it into small round pellets. These were eventually used to perform the CO2 adsorption tests. ‘I spent literally hours to role these small pellets by hand’, Mendel says with smile. ‘But it was worth the effort.’
Production of chemicals
The beauty of using clay as absorbent to purify biogas, is the possibility to regenerate the clay, by releasing the absorbed CO2. This can be done by leading a stream of nitrogen over the CO2-saturated clay pellets. But another method, where the clay is pulled vacuum in a closed chamber has the benefit that simultaneously relatively pure CO2 is produced. The resulting pure CO2 gas can now be used for the production of chemicals, for example biofuels. Another application is the use of CO2 in greenhouses to promote plant growth and boost production.
'Since clay can only retain CO2 temporarily, the idea arose to use it as a material to be regenerated to remove CO2 from biogas'
Enough information
Now the proof of principle has been shown, Mendel is ready for the next step: a scaled pilot plant at a farm. In February 2024 the test phase in a real-life situation is planned to take off. ‘We aim at a relatively small-scale farm, producing about a few cubic meters biogas per hour’, Mendel explains. ‘Our pilot plant will fit into a small sea container for easy transport. The magic will happen in a couple of, one- to two-meter-long cylinders, with a diameter of about 25 centimeters, filled with the clay pellets.’ Using the newly developed method the biogas will be purified into at least 90% methane. After several months, the researchers hope to have collected enough information to decide if the method will be tested on a large, industrial scale. With this cost-effective method, the harvest and use of biogas will be step closer, reducing emissions and closing cycles.
