Towards a carbon-negative sustainable bio-based economy

Abstract

The bio-based economy relies on sustainable, plant-derived resources for fuels, chemicals, materials, food and feed rather than on the evanescent usage of fossil resources. The cornerstone of this economy is the biorefinery, in which renewable resources are intelligently converted to a plethora of products, maximizing the valorization of the feedstocks. Innovation is a prerequisite to move a fossil-based economy toward sustainable alternatives, and the viability of the bio-based economy depends on the integration between plant (green) and industrial (white) biotechnology. Green biotechnology deals with primary production through the improvement of biomass crops, while white biotechnology deals with the conversion of biomass into products and energy. Waste streams are minimized during these processes or partly converted to biogas, which can be used to power the processing pipeline. The sustainability of this economy is guaranteed by a third technology pillar that uses thermochemical conversion to valorize waste streams and fix residual carbon as biochar in the soil, hence creating a carbon-negative cycle. These three different multidisciplinary pillars interact through the value chain of the bio-based economy.

Keywords: anaerobic digestion; biochar; biomass; fermentation; lignocellulose; pyrolysis; saccharification.

FIGURE 1

Recycling of energy and nutrients within the carbon-negative bio-based economy. The carbon-negative bio-based economy is based on three integrated processes: green biotechnology (top), white biotechnology (middle), and thermochemical conversion (bottom). Plants use solar energy to convert carbon dioxide into biomass, mainly plant cell walls with cellulose as most abundant polymer. This polymer is enzymatically converted to glucose monomers (saccharification) which are used as carbon source by microorganisms to produce chemical compounds, among which bioethanol. The efficiency of this process is mainly dependent on the recalcitrance of the cell wall that is considerably reduced by physical, thermal, or chemical pretreatments of the biomass. Waste streams are minimized or concentrated to feed anaerobic digesters for the production of biogas that can be integrated in the system. Rest fractions are converted into added value compounds, energy, or biochar by pyrolysis. The latter is used as a long acting soil amendment on the field. During passage over the different segments, specific fractions of the initial biomass can leave the processing treadmill for valorization into bio-based products (purple arrows), while carbon and nutrient waste streams are recycled as soil additive (blue arrows), or as energy (red arrows).

FIGURE 2

Structure of cellulose within the plant cell wall. The plant cell is surrounded by a recalcitrant cell wall composed of cellulose (β-1,4-coupled glucose monomers) aggregated into microfibrils (black). Microfibrils are crosslinked by hemicelluloses (red) and the resulting polysaccharide network is embedded in lignin (brown).

FIGURE 3

The synergistic function of different enzymes to degrade crystalline cellulose. Complete depolymerization of celluloses is obtained by the synergistic action of several enzymes. Endoglucanases (EG) cleave internal bonds at amorphous sites and create new chain ends. Exocellulases or cellobiohydrolases (CBH) cleave two units from the ends of the cellulose polymer, releasing disaccharides (cellobiose). There are two types of CBH: one working from the reducing end (CBH I), and another working from the non-reducing end of cellulose (CBH II). β-glucosidases (BG) hydrolyze the oligosaccharide products into individual monosaccharides. In turn, cellobiose dehydrogenases (CDH) use an acceptor molecule to oxidize cellobiose by a radical reaction. The released electrons can be used by polysaccharide monooxygenases, such as those from glycosyl hydrolase 61 (GH61), to depolymerize crystalline cellulose through reductive elimination. Most of the enzymes are coupled to carbohydrate-binding modules (CBMs), which have no catalytic activity, but secure the binding of the catalytic domain to the polysaccharide (based on Horn et al., 2012).