What are biorefineries and why should I care about them?
Due to climate change, there is an increasing demand for sustainable production of both products and energy. Ideally, the systems we should use to produce desirable compounds and products, along with the way we generate energy, should be “clean”. In an environmental context, clean refers to low or zero production of greenhouse gases, with the ideal situation being that a process is “carbon negative” i.e. it removes more carbon from the atmosphere than it contributes. Additionally, processes should not introduce pollutants such as heavy metals and other by-products that can contaminate land and water. In recent decades, there has been a significant shift towards “bioproduction” – employing living organisms to make products and energy. In this blog post, we will explore how this is done along with the challenges and opportunities provided by bioproduction.
What is biorefinery?
You can think of a biorefinery as a factory that turns “biomass” into products or energy. Biomass is a broad term and encompasses everything from pig manure to sugarcane. Ideally, a biorefinery will use a waste product to make something high value. A simple, and classic, example of a biorefinery would be something like a brewery, which uses yeast and malted barley to make beer, in which the desirable compound is ultimately ethanol. In the modern world, there has been a focus on the production of bioethanol using yeast and waste materials, as a green alternative to petrochemical routes. This process nicely highlights some of the key considerations of a biorefinery – what is the biomass that goes in, what organism will you use and, finally, what product(s) will you make and how much of them?
Making your biorefinery
If we ignore, here, the considerable engineering challenges that can be associated with creating any large-scale industrial plant, we still need to consider a number of elements.
What are you trying to make?
Replacing every chemical process with a bioprocess may be theoretically possible, but some are easier to access than others. Ideally, the chemical, compound or product you are trying to make will be made naturally at high levels by a microorganism such as a bacterium or yeast. This is because then, “all” you have to go is feed the microbe and it will make your products. Again, using ethanol as an example, yeast can make about 18% alcohol which is a good start to commercial production. Other compounds may be made at much lower levels, or not have established biological routes. Lots of modern research goes into designing pathways of enzymes (biological molecules that turn one compound into another) to make attractive products. Regardless, once you have selected your target, you need a cell to make it.
What organism should I use?
It’s fair to say that most biorefineries use microbial organisms. Mammalian cells can be used but these are much more complicated. You want to select an organism that either makes your compound naturally or can be engineered or selected to make it. Ideally, the organism you use will be non-pathogenic (i.e. not cause disease), easy to grow large amounts of and hardy – you want the cells to be able to survive relatively harsh conditions. The reason that bioethanol can only be produced up to 18% is because the ethanol kills the cells, just as alcohol hand gels kill bacteria. Generally, the choice comes down to bacteria or yeast and is determined by the type of molecule you want to make. Whatever your cell choice, you need something to feed them.
What feedstock should I use?
The food that you give to your cells is called the feedstock. This could either be a really pure molecule such as refined glucose, or a more complex mixture of materials such as grape juice. Ideally, what you feed your cells should be otherwise waste. This is sometimes referred to as the “food vs fuel” debate in which things like corn, which can be used to feed people, are sometimes diverted to make biofuels. There is an increasing trend to move towards waste products such as corn husks, wood chippings and rice straw.
What does the future hold?
Scientists are constantly working to increase the number of products that can be made biologically. However, a significant challenge remains in finding suitable feedstocks. Not all cells grow well on those and can suffer toxicity from some of the other compounds present. As well as increasing tolerance to these toxic compounds, a key area of research is considering ways to treat feedstocks, such as lignocellulosic biomass, to release sugars and allow cells to grow and produce their products. The future for biorefineries is bright – we are able to make more products using microbes with the added benefit of being better for the environment. Perhaps the ultimate aim is to use microbes that grow on carbon dioxide – actively cleaning the air whilst making products for people to use.
Read more about what lignocellulosic biomass is and how much value it contains in our other articles: Green chemistry and Bioknot, Bioethanol: a carbon dioxide neutral energy source; Rice straw, a worthless by-product?
Mongkhonsiri, G., Charoensuppanimit, P., Anantpinijwatna, A., Gani, R. & Assabumrungrat, S. Process development of sustainable biorefinery system integrated into the existing pulping process. Journal of Cleaner Production 255, (2020).
Sganzerla, W. G., Buller, L. S., Mussatto, S. I. & Forster-Carneiro, T. Techno-economic assessment of bioenergy and fertilizer production by anaerobic digestion of brewer’s spent grains in a biorefinery concept. Journal of Cleaner Production 297, (2021).