It is important to ensure that both the resource and the process technology used as well as the products made are environmentally acceptable. The twentieth century saw the development of processes designed for the production of energy and organic chemicals based on the oil refinery. The twenty-first century must see the development of similar processes based on the biorefinery. The aim is to design an integrated process capable of generating a cost-effective source of energy and chemical feedstocks using biomass as a raw material. The key is to find alternative sources of carbon to oil, available in high quantities and process them using green chemical technologies, ensuring products obtained are truly green as well as sustainable. Technologies used should ideally be flexible enough to accommodate the natural variation of biomass associated with seasonal or variety change (Clark et al, 2009). The efficiency of the process needs to be maximal: ideally every output has to have a use and a value/market. We can no longer afford the luxury of waste. Practices based on industrial symbiosis looking at re-using the waste produced by one process to feed another, or converting waste into a useful by-product with a marketable value need to be developed. The aim would be to achieve a zero waste biorefinery able to compete economically with existing systems used to produce energy and chemicals, an objective increasingly pushed by EU regulations (see Section 1.3.1).

1.2

Scheme describing an integrated biorefinery as a mixed feedstock source of chemicals, energy, fuels and materials.

Adding value to every output of the biorefinery can be achieved by combining several technologies together, using a sequential approach to extract chemicals before biomass is converted to energy. The main green extraction processes used to extract valuable compounds from biomass include liquid and supercritical CO2, ultrasonic or microwave-assisted extraction and accelerated extraction. Microwave-assisted extraction is a commercial reality with Crodarom using this technique to extract purer and more degradation stable plant materials (Crodarom, n. d.). The extraction can be followed by biochemical or thermochemical processes and internal recycling of energy and waste gases. This approach ideally constitutes the basis of an economically sound starting point for the design of a biorefinery and is illustrated in Fig. 1.3. The integration of technologies for the biorefinery takes into account the complex nature of lignocellulosic biomass, in order to produce several products and render the biorefinery concept cost-effective.

Biomass contains an array of functionalized molecules, with many of them having a market value. Compounds such as natural dyes or colorants (e. g., carotenoids), polyphenols, sterols, waxes, nonacosanol or flavonoids (e. g., hesperidin), amino acids, and fatty acid derivatives can be extracted selectively using clean extraction techniques prior to the treatment of biomass by biochemical and thermochemical processes. These compounds have uses in cosmetics, as nutraceutical or semiochemicals (Clark et al., 2006; Deswarte et al., 2006). Often, secondary metabolites are extracted using volatile organic solvents, but clean extraction techniques such as liquid and supercritical CO2 are very selective, allowing fractionation of extracted mixtures and have the advantage of being allowed for processing raw materials, foodstuffs, food components and food ingredients (together with ethanol and water) according to Directive 2009/32/EC of the European Parliament on extraction solvents used in the production of foodstuffs
and food ingredients. This technique also does not leave any residues (Budarin et al., 2011), allowing it to be used for pharmaceutical, food and cosmetic applications and compensating for both high technology capital cost and energy consumption. The polarity of CO2 can also be fine-tuned using co-solvents such as methanol or ethanol (Sahena et al., 2009). As a matter of comparison, the polarity of supercritical CO2 can be compared to that of hexane (Deye et al., 1990). Although energy requirements of supercritical CO2 are high, the technology has been commercially used for hop extraction, decaffeination of coffee and dry cleaning (Arshadi et al., 2012).

Biochemical and thermochemical processes complement each other well, the former being very selective but slow compared to the latter. Biochemical processes require low temperatures but pre-treatments are often required (e. g., ammonia fibre expansion or AFEX, dilute acid hydrolysis) to open up biomass’s fibre structure and yield fuels and chemical intermediates used for further downstream processing (Eggeman and Elander, 2005; Tao et al., 2011). Processing times and space-time yields are high compared to thermochemical processes, but they are less energy intensive (Kamm and Kamm, 2004). Thermochemical processes, which include gasification, pyrolysis and direct combustion (see Table 1.2), usually operate above 500°C and are much less selective, yielding oils, gas, chars and ash (Fernandez et al., 2011).

Biomass with a high acid, alkali metal and water content can be difficult to use in conventional thermal treatments: the high water content can render pyrolysis or gasification processes very difficult and the acidity of the feedstock can limit the applications of the pyrolysis oil obtained, for example.

Microwave technology has been studied for the pyrolysis of straw. This technology was proven to improve the quality of bio-oils obtained at lower temperatures (typically under 200°C), yielding oils with properties outperforming commercial fuel additives: bio-oils produced have a lower oxygen, alkali, acid and sulphur content (Budarin et al., 2009).

The properties of the oil obtained could also be modified by using additives during the heating phase, showing how microwave technology is versatile and can offer an alternative to conventional thermal processes. Microwave technology has an added advantage compared to conventional thermal heating: it activates cellulose at a temperature of 180°C, helping the conversion process (Budarin et al., 2010). It has been reported that at this precise point and under microwave heating conditions, the rate of decomposition of the amorphous part of cellulose increases due to in-situ pseudo acid catalysis, yielding a char and bio-oil of superior properties compared to those produced when using conventional heating methods.