34 Environmental Laboratory
LOW CARBON, BIG IMPACT: MAKING THE MOST OF BIOWASTE Making useful materials from biological waste streams using pyrolysis
Pyrolysis of bio-derived waste has the potential to provide a lower-carbon, more sustainable way of producing chemicals that are currently made from petrochemicals. But with biowaste feedstocks being so diverse, how can we determine the best use of a particular resource? We talk to Dr Daniel Nowakowski at Aston University’s Energy and Bioproducts Research Institute (EBRI) about the challenges faced when thermally processing biowaste, and how collaborating with Shimadzu is helping him and his team to maximise the potential benefi ts of these feedstocks.
What is biowaste?
The term ‘biowaste’ covers a wide range of materials – from crop stalks to woodchip, and from spent hops to seaweed. But what it all has in common is that it was recently living material that was sequestering carbon from the atmosphere, and so inherently avoids the large CO2
footprint of fossil fuels.
Society has long used biowaste in the form of compost for improving or fertilising soils, or for releasing heat by burning. But in the last 20 years or so, there has been growing realisation that biowaste can also be used to generate useful chemicals, by subjecting it to high temperatures in the absence of air (known as pyrolysis).
These products can be very varied, says Dr Daniel Nowakowski, an expert in pyrolysis at Aston University, UK: “By heating up biowaste in a carefully controlled fashion, we can generate solids, liquids and gases with a wide range of interesting uses”.
Deciding what to do with a feedstock
As Lecturer in Chemical Engineering and Applied Chemistry at Aston’s Energy and Bioproducts Research Institute (EBRI), Dr Nowakowski has fi rst-hand knowledge about the types of biowaste sources and the ways in which the resulting products can be used – from carbon-storing soil conditioners to sustainable fuels.
This knowledge is put to use every day, not just through the team’s fundamental research, but through their project work for government agencies, organisations and companies looking to do more with their biowaste. When they’re given a biowaste sample to investigate, he says, we ask one question: “How can we tailor the pyrolysis process to derive products with the greatest value or benefi t?”.
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For example, he says, they might want to maximise yields of a particular product or eliminate an unwanted contaminant. “Working out the processing conditions that will do this requires experience and research, but also a lot of experimentation: a single Ph.D. student can work for three years on optimising the yield of a single component from a pyrolysed feedstock!”.
Pyrolysis – Process and products
Before going further, it’s worth fi rst understanding what pyrolysis is. Otherwise known as ‘heat treatment’, pyrolysis involves raising the temperature of a material in a reactor to 400–600 °C in the absence of oxygen. The material needs to be fairly dry, with typically less than 20% water, to avoid compromising energy effi ciency and causing phase separation of pyrolysis liquids.
This closely mirrors the age-old process of making charcoal, says Dr Nowakowski, but with much greater control over the conditions. Also, he says, nothing goes to waste: “Unlike in conventional charcoal production, we’re not just interested in the solid matter left behind – we also capture all the vapours and gases emitted, in order to extract value from them too”, he explains.
During the pyrolysis process, there is extensive rearrangement and degradation of the bonds in the lignin, cellulose and hemicellulose and other compounds that make up a typical biowaste feedstock. This leads to the production of three classes of materials:
• Biochar is a highly porous form of carbon, and typically takes the form of a black powder. It’s primarily of interest as a soil conditioner that can store carbon in the soil for extended periods, in a process known as carbon sequestration and storage (CCS). There are numerous standards for biochar, and biochar classes have also been recognised on the basis of the carbon content, particle size, and presence of plant nutrients such as potassium and nitrogen. Biochar can also be ‘engineered’ further into catalyst supports, polymer fi llers, and absorbents for heavy metals.
• Bioliquids contain a complex mix of carboxylic acids, phenols (and its derivatives), esters, ketones, anhydrides, ethers, and oxygen-containing heterocycles. During pyrolysis, they’re released as vapours, but are subsequently condensed into liquids. The water-miscible (aqueous) fractions, because of their acid content, are also known as ‘wood vinegar’, which has attracted interest as a fungicide and plant growth enhancer. The water-immiscible (‘organic’) fractions are rich in aromatic compounds, which can either be separated from each other and used as chemical ‘building blocks’, or catalytically upgraded to remove the oxygen atoms and give hydrocarbon fuels.
• Pyrolysis gas (also called ‘pyrogas’) includes methane, ethane and other hydrocarbons up to C5, hydrogen, carbon monoxide and traces of carbon dioxide. Gaseous hydrocarbons can be used to fuel the pyrolysis reactor, while hydrogen and carbon
Adam Elaradi (Undergraduate Researcher) and Dr Sarah Asplin (Post- doctoral Research Associate) operate the laboratory-scale intermediate pyrolysis system at EBRI. The system can take up to 300 g/h of biomass, and has a maximum processing temperature of 600 °C.
monoxide (syngas) can be used to produce ammonia or methanol. Hydrogen is also of interest as a ‘green’ fuel for transport, particularly for heavy goods vehicles (for which electrifi cation is not currently practical).
Tailoring the pyrolysis process
The amounts of biochar, bioliquids and biogases produced vary greatly depending upon the volatile content of the original material and the processing conditions, says Dr Sarah Asplin, a Research Associate at EBRI who is using her knowledge and skills from her Ph.D. in the fi eld of biomass pyrolysis. She explains that slow, relatively low-temperature pyrolysis of biomass in a batch reactor might yield up to 50% biochar and 25% each of bioliquid and biogas. In contrast, quick, high- temperature pyrolysis of high-quality woodchip in a fl uidised- bed reactor could result in up to 75% bioliquid, with most of the remainder being gases, and very little biochar.
Along with feedstock quality, all these factors feed in to decisions about what to do with a particular feedstock, says Dr Asplin: “For example, clean feedstocks, especially those that are lignin-rich,
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