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Chemistry
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Biowastes: a new feedstock for the chemicals industry
Biowastes: a new feedstock for the chemicals industryValuable WasteThe chemicals industry has been wedded to fossil resources for the past hundred and fifty years, but times are a-changing. As the price of oil increases inexorably and environmental impact rises on the policy and regulatory agenda, industry leaders are starting to exploit new, more sustainable feedstocks for the manufacture of fuel and chemicals. Squarely in the frame as potential feedstocks are the vast tonnages of biowastes produced each year from agriculture, brewing and food processing. 
Examples of biowaste resources
Living on a current account The world’s fossil reserves represent stores of “old sunlight” accumulated over millions of years by plants photosynthesising, fixing carbon and deliquescing. We can view these reserves as a deposit account that the humanity has been plundering like teenagers let loose with a parental credit card. The reserves are diminishing and we need to start living on our current account: energy and materials that are renewable year on year. Whilst there are a range of potential sources for renewable energy, for instance wind, wave, sunlight, geothermal, the resources for chemical manufacture are restricted to carbon fixed by living organisms. The biowastes arising from production of food for the 9 billion people that are anticipated to inhabit our planet by 2050, look an attractive option for producing the chemicals of the future. Market drivers The production of chemicals from biorenewable resources is not new: the International Energy Agency (IEA) estimates current annual production of bio-based chemicals and polymers at around 50 million tons [1]. Biorenewables could represent nearly 40% of bulk chemicals by 2050 under favourable market conditions. The use of biowastes rather than agricultural crops as feedstock both reduces the costs of manufacture of biorenewable chemicals and enhances their sustainability because they are not competing with food production for agricultural resources. Furthermore, it simply seems more sensible to extract molecules which nature has assembled rather than rebuilding chemical structures. Companies are looking for ways to improve their environmental footprint for reasons of corporate responsibility and compliance with regulations on emissions. Equally important, as the environment moves up the agenda of consumers, green credentials can add value to their brand. As oil prices rise, these issues come together to motivate the search for alternative, cheaper and more sustainable feedstocks for bio-renewable chemicals – bio-wastes lie squarely in this frame.
Fig. 1 Arabidopsis plant seedlings
Fig. 2 Hemp oil
Fig. 3 Hemp seed
Biowaste resources The volumes of waste arising from agriculture and food production are significant (see table 1 for examples) and they represent a huge reservoir of valuable compounds. For instance the skins of fruit and vegetables are usually discarded, but they are rich in flavour, fragrance and bioactive compounds. Although many biowastes are mixed, the food industry also produces a number of single-component waste streams. These are the low hanging fruit for processing into high value chemicals. Examples include coffee grounds which are being converted into boards and soil improvers, and brewery wastes which can be transformed into energy products or used as a source of food additives. Disposal of straw from small grains is a major source of land and air pollution globally. However wheat straw, like many agricultural products, contains a range of valuable compounds including natural waxes. Waxes have uses in products ranging from surface coatings to cosmetics and the compounds found on the surface of wheat straw have properties comparable to a number of waxes currently in widespread use. Technology is already being commercialised to convert the cellulose in straw into ethanol but there is also potential to use the five carbon pentose sugars in the hemicellulose fractions for the manufacture of bulk chemicals such as citric acid. The first generation biofuel installations produce significant quantities of waste. For instance ten tons of biodiesel makes over a ton of glycerol by-product; bioethanol fermentation from grain creates seven kilograms of dried distillers grains with solubles (DDGS) for every ten litres of ethanol. These waste materials often go to relatively low value applications such as anaerobic digestion for energy but they have potential as feedstocks for the chemicals industry. Companies are already investing in facilities to exploit waste streams for chemical production. For instance, Archer Daniels Midland is developing a portfolio of chemicals fed by renewable raw materials. They have commissioned a 100,000 ton propylene glycol plant in Illinois which will use glycerol from soybean and canola oil production as feedstock. There are also thermochemical options for conversion of lignocellulosic wastes into chemicals or energy products. Torrefaction offers an attractive route to manufacture of a solid energy carrier, biochar. This has a higher energy density than the unmodified biomass and is easier to handle and burn. At higher temperatures (ca. 500°C) pyrolysis can be applied to form liquid energy products such as heavy heating oil or chemicals such as phenols, furans or anhydro- sugars. Research is underway on methods to upgrade these so called “bio-oils” into liquid transport fuels for road and aviation. New chemical technologies
New technology is needed for reliable extraction, separation and transformation of chemicals from biowastes. Substances of interest are often present at low concentrations in aqueous mixtures or are immobilised in complex bio-polymers. Ideally, we would like to access compounds through “natural” solvents such as water, ethanol, and carbon dioxide using minimal energy. CO2 is particularly useful as a selective extractant for a range of high value natural compounds. It is also a highly tuneable solvent for chemical reactions, particularly those that are enzyme-mediated, as it offers excellent mass transfer with simple product work-up. Software is now available for modelling which “green” and bio-derived solvents are most favourable for particular reactions. Microwave technology is also finding a place in valorising biowastes. It offers a rapid, flexible, energy-efficient heating method, well adapted for continuous processing to produce liquid and solid fuels as well as chemical products. The microwave system offers a more flexible and lower temperature thermochemical conversion option than those described above (ca. 200°C). The equipment can be installed on mobile units enabling transport between different waste producers. Microwaves can promote novel reaction pathways and accelerate the rate of chemical reactions. White biotechnology New industrial biotechnology techniques are critical for the successful exploitation of bio-wastes for high value chemicals. Scientists now have high throughput “omics” tools that allow them to select strains of micro-organisms that catalyse reactions of choice. For instance Aspergillus is a versatile filamentous fungus that is capable of metabolising a range of substrates commonly found in waste streams from bioprocessing industries. Researchers from the Centre for Novel Agricultural Products at the University of York together with the BDC and a small technology company, Citration Technology Ltd, have been using genomics to work up potential routes for the production of commercially attractive industrial chemicals using Aspergillus. Most agricultural crops have been optimised for production of food or feed. Scientists are now using molecular breeding approaches to develop crops with improved bio-wastes – for instance straw that is more easily digested to allow its use for production of biofuel and chemicals. Innovation Whilst imaginative science has been applied to develop ingenious technology in the laboratory, too often the ideas remain in academic journals rather than being implemented in the real world. Facilities like the Biorenewables Development Centre, adjacent to the University of York in the UK, aim to bridge this gap between laboratory research and commercial application – a gap known as the “valley of death” amongst innovation specialists. The BDC’s open-access facilities are designed for scale up novel technologies to produce an amount of material that industry can test in their own products. Equipment is arranged in a modular fashion and can take crude raw material and refine it through a diverse set of processes. It offers access to analytical and processing technology that industry, especially SMEs, could not otherwise access.
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L&M int. 2 / 2013
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