The Green Chemistry Centre of Excellence has been involved for a number of years in research on the effects of microwave radiation on chemical compounds. This has provided new insight and understanding on how microwaves can accelerate chemical reactions.
We have discovered that microwaves may be used to selectively activate components of biomass (trees, grasses, crops, agricultural and food waste, macro and micro-algae, etc.) leading to a much more controlled decomposition process than can normally be achieved, e.g. using acid treatment. In this way, we are able to make a range of valuable products including liquid and solid fuels, and chemicals from sustainable sources using green chemical technology. We have proven this at scales from grams to tens of kilograms.
Microwave technology for heating has been shown to be more energy efficient than conventional methods in many applications. Microwave irradiation is rapid and volumetric with the whole material heated simultaneously. In contrast, conventional heating is slow and the heat is introduced into the sample from the surface. This feature of microwaves is very important for processing poor thermal conducting materials such as wood.
Microwave heating can be controlled instantly and the power applied can be accurately regulated. This allows safe and precise control, even when applying very rapid heating rates. Microwaves also promote novel reaction pathways and can greatly accelerate reaction rates.
The use of microwaves for the conversion of biomass to valuable products offers a number of very important advantages:
- Lower temperatures
- Mobile processors that can be located next to large concentrations of biomass, such as farms
- Flexible processing enabling the production of liquid fuels, solid fuels and chemical products
- Reduced CO2 burden and low product carbon footprints
- Rapid, continuous processing and high energy efficiency
Our microwave facilities
We have created a dedicated laboratory scale microwave facility where we can take reactions from 1mL to 1000mL scale under hydrolysis or pyrolysis conditions using our suite of eight microwaves. These include the Milestone SynthWAVE capable of carrying out extraction at high (300⁰C) temperatures and high pressures (199bar); the Sairem Miniflow configured to run reactions in continuous flow or batch mode and a Milestone RotoSynth fitted with a 2L reactor and specifically designed distillation apparatus to enable biomass pyrolysis to be carried out followed by bio-oil fractionation.
Our lab-scale microwaves are also used to investigate cleaner synthetic techniques in research carried out by, and during, the manufacturing process of Starbons®, a novel family of carbonaceous mesoporous materials derived from polysaccharides, which were developed by the GCCE and are now part of the spin-out company Starbons® Ltd.
We also have access to scale-up facilities at the Biorenewables Development Centre which is equipped with a microwave pyrolysis unit capable of processing 30 kg/hour of feedstock and a bespoke designed, semi-continuous microwave hydrolysis reactor with a 30L capacity.
Our laboratory and pilot-scale facilities enable us to:
- Conduct our world leading research on the controlled microwave decomposition of biomass
- Optimise process conditions for the preparation of liquid fuel (intermediate), solid fuel and chemical products
- Design associated microwave processing equipment
- Prove processes in continuous mode and in production of multi Kgs quantities of products
- Produce samples and products to allow for commercial decisions to be made
InnovateUK, BBSRC, EPSRC IB Cat: Round 3 Early stage translation
Integrated energy efficient microwave and unique fermentation processes for pilot scale production of high value chemicals from lignocellulosic waste (2016-2020, £3.2 M). In collaboration with University of Bath and C-Tech Innovation Ltd, Croda, AB Agri.
Recently, we reported an innovative one-step microwave (MW) process for the depolymerisation of bio-wastes. This key enabling technology achieves high sugar yields despite low energy inputs. Though a range of inhibitors are also formed in the process which limit the growth of most yeasts, we have shown that the robust yeast Metschnikowia pulcherrima (Mp) thrives on this feedstock to produce valuable 2-phenylethanol, arabinitol and a microbial oil akin to palm oil. Therefore this project aims to develop a pilot scale multi-product process by coupling these breakthroughs in low energy waste treatment and unique fermentation to produce high value chemicals.
Bilberry press-cake is a by-product from juice production and contains abundant polysaccharides. This research investigated the possibility of applying microwave hydrolysis for multiple chemical recovery.
Trials were carried out at lab and pilot-scale and compared to conventional (Soxhlet) extraction methods where microwaves showed a significant advantage with over 55 per cent conversion achieved. Extraction products included mono and di-saccharides alongside anthocyanins and pectin. A model was proposed for the extraction of bilberry press-cake where conditions (mild to intense) could be selected based on the target products.
Citrus essential oils are used as fragrances in many skin, bath and beauty products. Citrus peel can contain up to four per cent of these essential oils and D-limonene constitutes up to 90 per cent of the oils from orange peel waste and 75 per cent in lemon peel waste. D-limonene can be used in the flavour and fragrance industry but has also been used as a non-toxic solvent or cleaning agent.
This project used wet orange peel and microwave technology to propose a more scalable and flexible process with lower energy consumption and reduced environmental impact in comparison to conventional methods.
The liquid and solid products obtained following microwave irradiation of microcrystalline cellulose was carried out over a range of temperatures. A mechanism for cellulose interaction with microwaves was proposed with higher temperatures (250⁰C) and higher microwave density resulting in high glucose yields (>20 per cent).
Microwave hydrolysis of waste softwood lignin was carried out in air, at atmospheric pressure without additives. This process converted 40 per cent of the residual saccharides in the lignin with high selectivity (~90 per cent) which represents 8 per cent of the total lignin. Levoglucosenone is a valuable intermediate for the production of fuel and new solvents such as Cyrene™. The lignin remained intact structurally so could be depolymerised further to aromatics.
This process facilitated the efficient dissolution of hemicellulose in bamboo at 200⁰C while obtaining hemicellulose free residue which could be used further as a starting material within many industrial processes. A new route to produce hemicellulose-based films that could potentially be used for food packaging was proposed. The overall approach opens up an opportunity for a low cost and sustainable bamboo-based bio-refinery.
A systematic kinetic study was carried out investigating the effect of NaCl on the cellulose hydrolysis process. It was found that the presence of NaCl simultaneously enhances the generation of acidic products from cellulose decomposition and pushes the generated protons to the surface of cellulose, dramatically increasing surface acidity and facilitating the autocatalytic hydrolysis of cellulose.