Bioconversion of forest products industry waste cellulosics to fuel ethanol: A review

Biofuels are fossil fuel alternatives produced from agricultural biomass or other organic matter; considered sustainable, eco-friendly, and bioeconomic biofuels have come up as a topic of discussion for over a decade. Their practical use depends on the production methods, low cost-technology implementation, and substrate used. This review gives an insight into different generations of biofuels with their applications and implications. First-generation biofuels are produced from edible biomass, but even in highly efficient processes, their yield isn't enough to cast them as a better alternative to conventional fuels. The second-generation biofuels are produced from non-edible biomass, where the substrate is eco-friendly and provides a sustainable use of solid waste, but the pretreatment is overpriced and sophisticated technology is needed to carry out the process. Third-generation biofuels are produced from substrates like seaweed or microalgae for which no specific area or separate cultivation process is required. Biofuels such as biohydrogen produced through microbial dark fermentation, bio-syngas generated via gasification, and biodiesel obtained through transesterification, have emerged as promising and environmentally friendly alternatives to conventional fuels. These biofuels have the potential to pave the way for a bioeconomic system of fuel production, offering economic viability and efficiency. However, extensive research conducted in the field of bioenergy, several challenges persist, hindering their commercialization prospects. To overcome the problems of first, second and third-generation biofuels, fourth-generation biofuels are under development using techniques like co-culturing, nanotechnology, and genetically modified organisms. Future generations of biofuels would set a system for a circular bioeconomic pathway for sustainable development in the fuel industry.

By the start of the 21st century, biobutanol attracted interests as a drop-in liquid fuel that can be produced from renewable carbohydrate resources. A wide range of research studies dedicated to reviving the old acetone-butanol-ethanol (ABE) fermentation. ABE fermentation has been widely utilized all over the world at commercial scale during World War I and World Ward II for its acetone production but lost its economic vitality in the competition with petrochemical industry. In the search for a sustainable route from renewable resources to liquid biofuels, ABE fermentation attracted interests for n-butanol production. With such a unique industrial background, ABE fermentation has been targeted by some companies to be revived as an industrial process. Furthermore, the development of recombinant stains for the production of isobutanol had promising results and commercialized by two American companies. In this chapter, different aspects of microbial production of n-butanol and isobutanol are presented.

The drainage from closed and abandoned mines is often acidic with elevated heavy metal concentrations. The interaction between acid mine drainage (AMD) and the environment has a significant impact. While temporary remediation can lessen this impact, complete amelioration is only possible using active and/or passive treatment. Active technologies are often costly and can produce a secondary waste stream that may require additional treatment. In this project, we explored a passive option combining different technologies. We recognize AMD as a resource of sulfuric acid, which we used as a pretreatment on cellulosic feedstock. Following this, we enzymatically digested the partially hydrolyzed cellulose to produce fermentable sugars (which could be used to produce bioethanol). Concurrently, the pH of the AMD increased from 2.11 to 5.46, and 62% of the iron was removed from the solution. Previous research has shown that sulfate is remediated [primarily through dissimilatory sulfate reduction (DSR)]. In this research, we successfully combined two separate yet established technologies. We remediated AMD with a cellulosic feedstock through DSR and iron precipitation while using the cellulose residue as a feedstock to produce fermentable sugars which can be used for biofuel production. Besides producing important biofuels, providing water with improved qualities, and reducing the environmental impact associated with AMD, the treatment plants may employ local communities tasked with sourcing the lignocellulosic material. This technology may be important to developing countries where complex technologies for wastewater treatment are either unavailable or costly to implement. The production of biofuels from lignocellulose material would reduce the dependence on fossil fuels and the environmental crisis associated with the use of fossil fuels.