Engineers suggest new ways for refining industries to cut carbon

A bio-refining plant for producing bio-fuel

Reducing carbon dioxide emissions from bio-refineries and chemical manufacturing are the focus of two new studies at Imperial College London.

Researchers from Imperial have carried out a study that demonstrates how to reduce harmful carbon dioxide emissions from bio-refineries. They have also shown how manufacturers of Olefins, an essential chemical used to make plastics, can reduce carbon dioxide emissions without affecting their competitiveness.

The research, led by Dr Mahdi Sharifzadeh in collaboration with Dr Lei Wang and Professor Nilay Shah, is part of wider efforts by academics at the Centre for Process Systems Engineering to refine manufacturing processes, so that they are more efficient, sustainable and able to adapt to the challenges of the 21st Century, such as global climate change.

Dr Mahdi said: “Imperial has a long tradition of working closely with industry to improve manufacturing processes and to educate the next generation of process systems engineers. The challenge of global climate change means that now more than ever we need to find solutions that enable industries to manufacture the products our modern societies depend on, but in a sustainable way. However, for these solutions to be adopted they need to make economic sense. We’ve produced two studies that show how bio-refineries and chemical manufacturers can be more environmentally friendly, while still maintaining their bottom line.”

Improving bio-refineries

The Imperial team have developed a new bio-refinery process, which they suggest could reduce emissions.

Conventional bio-refineries convert biomass such as wood and grasses into a renewable fuel, called biofuel. Bio-refining technology is relatively immature, with only a handful of plants currently in operation around the world. 

The process for making biofuel involves heating biomass at very high rates. The produced liquids, called biomass pyrolysis oil or simply bio-oil is then mixed with hydrogen and substances that increase the rate of chemical reactions, converting it into bio-fuels that are compatible with automobile engines, which normally run on fossil-based fuels. However, conventional bio-refineries can emit high amounts of harmful carbon dioxide.

The team have calculated that only six per cent of carbon atoms in the original biomass feedstock would be emitted into the environment as carbon dioxide, using their new process. A further 19 per cent of the carbon atoms would end up as biomass waste, which could be used as fertilizer, while the remaining 75 per cent is converted into bio-fuel.

The new process, which was published in the journal Renewable and Sustainable Energy Reviews, involves capturing carbon dioxide emissions using a solution called an amine solvent. The captured carbon dioxide is then pumped into a vat of microscopic algae, which converts it into oxygen via photosynthesis. The excess micro-algae would then be harvested and used in the bio-refining process, reducing the need for external sources of biomass feedstock.

Dr Sharifzadeh and his colleagues have calculated that the cultivated micro-algae would provide 20 per cent more fuel than the conventional bio-refinery process. This extra biofuel would compensate for the increased energy needed to capture the carbon dioxide.

Reducing harmful emissions from olefin manufacturing

The team have also developed a new process for producing chemicals called Olefins, which they suggest could decarbonise the process by up to 44 per cent.

Olefins such as ethylene and propylene are chemicals used in large quantities by industry to make plastics – from Tupperware to toys. The conventional method for producing olefins involves a device called a hydrocracker. In this process, hydrogen breaks down the chemical naphtha into olefins under extremely high temperatures. The drawback of this process is that it is energy intensive and it emits high levels of carbon dioxide.

The team’s proposal, which was published in the journal Applied Energy, involves retro-fitting an existing olefin plant so that it can use bio-oil, instead of naphtha, to produce olefins. The researchers were able to show that converting the plant’s infrastructure to bio-oil is cost effective. Furthermore, they were able to demonstrate that olefins produced from bio-oil are financially competitive for chemical companies to manufacture and sell.

As the retrofits and modifications suggested by the team are relatively modest by comparison to the initial cost of establishing olefin refining, which is a relatively mature industry, the researchers believe their proposal could be more acceptable for companies to adopt.