Graphene addition enables the fast recovery of gas conversion in biological CO₂ methanation

Authors: Dr Xihui Kang, Benteng Wu and Dr Richen Lin

Affiliation: MaREI, ERI, University College Cork

Do you know that the sunshine we enjoy and the breeze we feel every day can be used as sources of renewable energy? Emerging technologies with photovoltaics and wind turbines that use sunlight and wind are termed respective solar energy and wind power. As the world aims to cut anthropogenic greenhouse gas emissions, tremendous efforts are being made at present to use these renewable energy resources for electric power as alternatives for fossil fuels. However, due to their intermittent nature, these renewable energy sources cannot provide baseload electric power. To overcome this problem, one solution is to integrate them with the engineered energy storage system.

Among different storage strategies for excess power, power to gas technology bears an elevated storage capacity with high charge/discharge periods; the primary product–green hydrogen–combined with CO₂ can be converted to produce infrastructure-compatible transportation fuel–methane, which is called the methanation process. The methanation of CO₂ in a biological way with anaerobic digestion technology has the advantage of producing high-quality methane under moderate conditions without expensive catalysts. “There are two types of biological methanation pathways: one is termed in-situ bio-methanation, during which hydrogen is injected into an anaerobic digester to react with the CO₂ in biogas. Another is termed ex-situ bio-methanation, during which the biogas or CO₂ and hydrogen are injected into a different digester to produce methane.” Dr. Richen Lin explains, who leads the EPA-funded project on “Advanced gaseous biomethane” together with Prof Jerry Murphy and Prof Alan Dobson in University College Cork. “Our previous work demonstrated that ex-situ bio-methanation performs better than the in-situ one as there is no dramatical pH drop when injecting high hydrogen flowrates into the ex-situ digester. However, one major problem in the ex-situ system is the decreased gas conversion efficiency due to the intermittent hydrogen supply. This encouraged us to find a possible solution to recover the gas conversion efficiency and overcome the deterioration in bio-methanation performance.”

Conductive materials to the rescue

Carbon-based materials such as pyro-char and graphene have been successfully applied in enhancing the stability of anaerobic digestion of many organic wastes such as food and agricultural waste. “The reason why these materials can enhance the digestion performance is that they can firstly facilitate the communication between diffident functional microorganisms, which increases the methane conversion, and secondly they can help alleviate several adverse stresses such as pH drop due to acid accumulation during the process.” As Dr. Lin states.

“We applied both pyro-char and graphene into our ex-situ bio-methanation system in the lab. We observed that without hydrogen supply for only one day, the gas conversion efficiency significantly decreased by over 20%; however, with the addition of 1 g/L graphene, the gas conversion efficiency can almost maintain the same level without any decrease in performance.” Benteng Wu enthuses, who is a Ph.D. student funded by EPA. “These up-and-coming results may ensure a robust biological CO₂ conversion with the potential for intermittent energy storage.”

This EPA-funded project is still underway. The research team is carrying out continuous experiments in a large-scale digester with the same concept to achieve more reliable and practical applications.

Decarbonizing Ireland’s distilleries with advanced fuel: making Irish whiskey sustainable

Authors: Dr Xihui Kang and Dr Richen Lin

Affiliation: MaREI, ERI, University College Cork

Have you ever, while drinking a pint of savory Irish whiskey, breathed in the lovely smell and wondered, “besides the after-morning headache, what damage it may bring to our mother planet in the long run?”

You would not be the first one to ask this question. The severe environmental issues that alcohol production may possess include the generation of the huge volume of spent wash, which can potentially cause eutrophication of water resources if improperly managed. In addition, the massive carbon dioxide emissions from the consumption of natural gas for distillation will definitely contribute to climate change. You may wonder if there is a silver bullet to these problems? Fortunately, this EPA funded project on “advanced gaseous biomethane” finds a way to simultaneously treat the distillery by-products and provide a low-carbon renewable energy resource “biogas” –using anaerobic digestion in a circular bioeconomy approach.

Decarbonization of distillers’ heat

Anaerobic digestion (AD) exists in nature for a very long history during which a group of microorganisms break down organic matters into biogas. Distillery by-products (such as spent grains and spent wash) contain many organic components such as proteins, cellulose, and hemicellulose (a form of sugars), which are suitable feedstocks for AD. Through CO2 removal, biogas can be upgraded to green gas (containing 97% methane) , which is an alternative to natural gas. The use of green gas instead of natural gas for distillation can reduce the carbon footprint of whiskey production.

However, the efficiency of AD can be affected by many factors such as temperature, nutrients balance, and the degradability of the feedstock. “We compared the digestion performance of distillery by-products in two temperature ranges and found that mesophilic condition (37 ℃) is more suitable for digesting distillery by-products as the digestion process is more stable and can avoid ammonia inhibition.” Dr Lin states.

“Another problem when using distillery by-products as feedstocks is that the spent grain (such as draff) is hard to degrade because of the complex natural plant structure.” Dr Lin explains. To increase the digestion efficiency of spent grain and reduce the size of the digester, Dr Xihui Kang (a postdoctoral researcher fudned by EPA) in the group developed a pretreatment technology to loosen the rigid structure. “Pretreatment is a necessary step when treating grass-like lignocellulosic materials. We increased the digestion efficiency of distillery by-products by 14% by employing acid pretreatment technology, which means that the digestion time required and the size of the digester can be significantly reduced.” Dr Kang adds.

When asked by how these results from lab can be applied in distillery, Dr Lin comments that “The distillery uses a large amount of heat energy provided by fuel oil and natural gas. The use of these fossil fuels will emit CO₂ into the atmosphere, contributing to climate change. Biogas can replace the consumption of natural gas. Dr Kang calculated that up to 64% of the natural gas consumption can be replaced by the biogas from the AD of distillery by-products (including draff and spent wash (thin stillage and thick stillage)) for a modeled distillery, reducing the direct CO₂ emissions by 54%. We believe that these exciting results will aid the distillery industry transiting to a low-carbon and sustainable alcohol production process.”