From first generation waste re-use to second generation waste re-use

The example given in Section 1.3.2 shows how one step change in a process can avoid further fossil resource deletion by recycling waste. But there are smarter ways of using food supply chain waste: this type of co-product is rich in chemical compounds and it is important to take advantage of that resource before using it for energy generation. The food supply chain generates a high amount of waste, even at a pre-consumer stage. Around 89 million tons of food waste is generated every year in the EU-27 (Bio Intelligence Service, 2010). Some 38% is generated by the manufacturing sector, 42% by the household sector (other sectors: 19%). First generation food supply chain waste re-use such as anaerobic digestion, composting, or conversion to animal feed only has marginal economic value compared to the revenue that could be generated from the production of pectin (10-12 £/kg) from citrus peels, for example.

In addition, when using waste, several criteria need to be considered in order to make sure the feedstock chosen is going to be used over the long term. Volumes available, occurrence in several geographical locations, guaranteeing a regular supply throughout the year, chemical functionalities present, extractables recoverable and their value as well as fitting the feedstock with appropriate green chemical technologies are all important parameters to consider when selecting a waste by-product for valorization.

Wheat straw is a major by-product of the agricultural sector. It is estimated that in the UK alone, 6.3 million tonnes of wheat straw was generated in 2007 (NNFCC, 2008), with a net surplus over livestock demand of 5.7 million tonnes in 2007. In the context of the UK Government’s new targets on biomass generated heat and power (5% by 2020) (HM Government, 2009), wheat straw represents a good choice of feedstock for combustion for heat and power generation. However, available valuable chemical functionalities should be recovered before any thermo — or biochemical processes are applied to wheat straw for conversion to energy. Two valorization routes have been demonstrated (see Fig. 1.8): the combustion of wheat straw and subsequent valorization of the slag and fly ash produced and supercritical CO2 extraction of waxes followed by char production from wheat straw by microwave pyrolysis. Both approaches are aiming for the development of a close to zero integrated wheat straw biorefinery. The first one valorizes the by-product of the combustion of wheat straw: the high


1.8 A wheat-straw based biorefinery: comparison between two possible routes.

content of alkalis (chloride, K2O and SiO2) can be extracted by water at room temperature. Up to 30% of the silica present in the ash (wheat straw ash contains 44.25% silica on a dry weight basis) can be extracted at room temperature in the form of a bio-silicate solution by using wheat straw’s own alkali content. Silicates are studied as an alternative to formaldehyde — based adhesives in entirely bio-derived, fire resistant, moisture resistant construction boards and have the potential to improve the cost-effectiveness of energy producing technologies such as combustion and help the direct production of materials from agricultural biomass (Dodson et al, 2011).

The second approach takes advantage of the combination of two green technologies: supercritical CO2 extraction and low temperature microwave pyrolysis, benefiting from the financial return offered by the extraction of phytochemicals prior to the production of char by microwave pyrolysis at 180°C. The first step is the extraction of the wax coating the wheat straw: between 0.9 and 1.1 wt% at 32°C and 100°C, respectively, which is comparable to hexane. The added advantage associated with using supercritical CO2 over hexane is that unwanted components such as pigments, free sugars and polar lipids are less soluble in supercritical CO2 than in hexane. Compounds found in the extracted wax range from 6,10,14-trimethyl 2-pentadecanone used in detergents, to nonacosane, a bio-derived type of paraffin wax, and octadecanal, an aldehyde used as a flavouring additive in foods. The de-waxed wheat straw is then pyrolysed using microwaves as a heating method, producing five fractions. They are described as follows:

1. A char (29 wt%) of a calorific value of 27.2 kJ/g, which can be demineralized to avoid alkali corrosion during combustion due to the formation of alkali ash.

2. Bio-oil (21 wt%) with a reduced water (1%) and acid content (pH 7) compared to oils obtained by fast pyrolysis at temperature above 350°C, requiring less downstream processing to be used in blends with crude oil for chemical and fuel production.

3. An aqueous solution (36 wt% together with the second aqueous fraction) made of formic acid, formaldehyde, acetic acid and acetaldehyde, all of which represent interesting starting materials for further downstream chemistry. Formaldehyde has an existing market as a disinfectant.

4. An aqueous solution of sugars which can be fermented to higher volume chemicals or biofuels.

5. A gaseous fraction (14 wt%) composed of CO and CH4 that could be used to fuel the process and CO2 which could be used for the wax extraction.

It should be noted that both technologies are scalable and are commercially used by the food industry and yield several useful marketable products in the context of a wheat straw biorefinery. Microwave technology is less sensitive to water content than conventional convection heating. The use of biomass with a high water content can prove to be advantageous as water can dissociate at higher temperatures under microwave conditions (Vaks et al., 1994) and can generate an in-situ acidic pseudo catalysis process benefiting the targeted process (extraction, chemical reaction). Furthermore it is portable, tuneable (additive, temperature, pressure, power) and fast, proving to be applicable to a variety of feedstocks, or feedstock agnostic. In terms of energy consumption, the described process only requires 1.8 kJ/g of energy compared to 2.7 kJ/g when using convection heating for the pyrolysis stage (Budarin et al., 2011). Supercritical CO2 may require a very high capital investment, but on a large scale, it has been proven to be more cost-competitive than using hexane (List et al., 1989), as this technique is virtually residue-free, requiring less downstream separation to achieve high purity of the extracted compounds.

Straw represents 50% of the yield of a cereal crop (Clynes, 2009) and with 650,881,002 tonnes of wheat produced in the world in 2010, wheat straw represents an important agricultural by-product occurring on every continent on the planet, with Europe, Asia and North America being the largest wheat producers. In conclusion, by integrating just two green processes, several products can be obtained starting from a unique feedstock available worldwide.

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