Integrated Biochemical and Thermochemical Routes to Produce Advanced Biofuels
The production of biofuels and other bioproducts from lignocellulosic materials – composed of cellulose, hemicellulose and lignin – has traditionally been done through two distinct technological platforms.
The first, based on biochemical processes, in which biomass fractions are converted into soluble sugars through pretreatment and enzymatic hydrolysis. These solubilized fractions are then transformed into biofuels such as ethanol and butanol by fermentative processes using microorganisms, which are responsible for converting the sugars into biofuels through a sequence of metabolic reactions.
The second platform consists of thermochemical processes, in which biomass is transformed into low-molecular-weight intermediates through pyrolysis or gasification, producing bio-oil or syngas, respectively. These intermediates are subsequently transformed into biofuels by Fischer-Tropisch synthesis, property matching, upgrading, and fractionation processes. Both platforms have advantages and disadvantages that influence their sustainability indicators.
Biochemical processes have the advantage of requiring simpler equipment, since the enzymatic hydrolysis and fermentation reactions normally occur at ambient temperature and pressure. An exception is the pretreatment reactors, which usually represent a critical point in terms of investment in biochemical processes. Biochemical processes also require large amounts of thermal energy to separate the products from the feed media, which occur in dilute media, and produce large amounts of liquid effluent. They also have relatively long reaction times, and the conversion of some materials can be low, as is the case of lignin and its degradation products. This fraction of the biomass may remain in solid state, depending on the pretreatment used, and partial solubilization products may act as inhibitors for the following reactions of hydrolysis and fermentation, reducing, in addition to productivity, the yields of the process.
Thermochemical processes, on the other hand, usually require more complex equipment, since the reactions occur at high temperatures and pressures, but this disadvantage is partly offset by significantly shorter reaction times. Another characteristic of thermochemical processes is that all fractions of lignocellulosic biomass – cellulose, hemicellulose and lignin – are converted. Despite having high conversion, these processes also have some inefficiencies in the transformation of carbon into products of interest, such as the formation of tar and carbon dioxide as unwanted products. Thermal deconstruction of biomass through pyrolysis and gasification processes produces large amounts of thermal and electrical energy, and the process is usually self-sufficient in terms of energy.
As an alternative to these conventional biomass conversion processes, the combination of thermochemical and biochemical processes creates a new conversion platform: hybrid processes that can leverage the synergies between the processes and provide high carbon conversion efficiency for desired products.
Several proposals have been put forward for the integration of these two technological platforms.Some of these proposals are based on using thermochemical processes as an initial processing step of the biomass (Shen et al., 2015; Kumar et al., 2019). This strategy aims to overcome the recalcitrance of biomass, eliminating the need for pretreatment reactors and the use of enzymatic cocktails from biochemical processes. Both pyrolysis and gasification can be used as the first process step, which follows with fermentation of the bio-oil or syngas. The pyrolysis bio-oil presents in its composition hydrocarbons that can be used as drop-in fuels and oxygenated compounds such as sugars and carboxylic acids, normally undesired in a standalone thermochemical process. With the proper separation process, these oxygenated compounds can serve as substrate for biofuel production by fermentative process.
Fermentation can also be applied to the synthesis gas produced in the gasification process, with the advantage of being less restrictive in terms of carbon monoxide:hydrogen ratio and the presence of contaminants than the Fischer-Tropsch synthesis process, as well as having greater selectivity. It is worth remembering, however, that in both cases additional challenges for the fermentation process need to be overcome, such as the high presence of inhibitors in the pyrolysis bio-oil and mass transfer problems for fermentation of gaseous compounds in the case of syngas fermentation.
Another process integration strategy is based on using thermochemical processes to process the unconverted biomass coming from biochemical processes (Klein et al., 2018). This is the strategy that is being studied in the BioValue project. In a purely biochemical process, the unsolubilized biomass, composed primarily of lignin and the most recalcitrant fraction of cellulose, is used as fuel for the cogeneration process, responsible for providing the energy required for the conversion process employed. Alternatively, this residual biomass from the biochemical process can serve as feedstock for additional biofuel production from thermochemical processes. One of the advantages is that this biomass not converted in the biochemical process is rich in lignin, the fraction of lignocellulosic biomass with lower oxygen content, which favors thermochemical processes, increasing their conversion efficiency. Another advantage lies in the thermal and electrical energy surpluses of the thermochemical processes, which can supply the energy needed for the biochemical process, eliminating the need for cogeneration units.
The integration of the biochemical and thermochemical processes will result in a greater production of biofuels from the same amount of biomass and certainly represents an important step towards increasing the sustainability of the processes, always seeking lower environmental impacts and better economic and social performance.
Fundamental Bibliographical References