The soil microbial biomass plays an important role in ecosystem function primarily through the regulation of nutrient cycles. In their search for energy to grow, the soil microbial biomass relies on nutrients in the plant litter and soil. Because of its competitive ability, the soil microbial biomass is often considered the source and sink of essential plant nutrients. The wide diversity and function of the soil microbial biomass exemplifies its role in a wide variety of processes, including parasitism, pathogenesis, and symbiosis. This wide functionality has made the study of soil microbial biomass problematic and often leads to misinterpretation of its importance in ecosystem function.

Biorefinery has been suggested to provide relevant substitutes to a number of fossil products. Feedstocks and conversion technologies have, however, been the bottleneck to the realization of this concept. Herein, feedstocks and bioconversion technologies under biorefinery have been reviewed. Over the last decade, research has shown possibilities of generating tens of new products but only few industrial implementations. This is partly associated with low production yields and poor cost‐competitiveness. This review addresses the technical barriers associated with the conversion of emerging feedstocks into chemicals and bioenergy platforms and summarizes the developed biotechnological approaches including advances in metabolic engineering. This summary further suggests possible future advances that would expand the portfolio of biorefinery and speed up the realization of biofuels and biochemicals.

Energy from biomass plays a large and growing role in the global energy system. Energy from biomass can make significant contributions to reducing carbon emissions, especially from difficult‐to‐decarbonize sectors like aviation, heavy transport, and manufacturing. But land‐intensive bioenergy often entails substantial carbon emissions from land‐use change as well as production, harvesting, and transportation. In addition, land‐intensive bioenergy scales only with the utilization of vast amounts of land, a resource that is fundamentally limited in supply. Because of the land constraint, the intrinsically low yields of energy per unit of land area, and rapid technological progress in competing technologies, land intensive bioenergy makes the most sense as a transitional element of the global energy mix, playing an important role over the next few decades and then fading, probably after mid‐century. Managing an effective trajectory for land‐intensive bioenergy will require an unusual mix of policies and incentives that encourage appropriate utilization in the short term but minimize lock‐in in the longer term.

From ancient times, plant biomass has been combusted to produce heat. Depending on the bioenergy feedstock and the energy processing platform, there are three major biofuel products: bioethanol (1G and 2G), biogas (1G and 2G), and biodiesel (3G). The basic steps involved in operating a biomass based biorefinery are similar regardless of the feedstock. Biomass first needs transformation, which involves separation or extraction of plant components by grinding, followed by fractionation or cracking by biological or physical–chemical technologies. The key steps in bioconversion of lignocellulose to fuels are size reduction, pretreatment, hydrolysis, and fuel production. Life cycle analysis or assessment (LCA) is an internationally recognized methodology for evaluating the global environmental performance of a product, process, or pathway along its partial or whole life cycle, considering the effects generated from “cradle‐to‐grave”.

The torrefaction of biomass is a thermochemical decomposition process in which hemicellulose degradation is the dominant reaction, with the cellulose and lignin fractions largely unaffected. The primary product is a solid material that retains 75–95% of the original energy content. Properties of the torrefied solid include improved grindability, hydrophobicity, and energy density. Torrefied biomass has been processed successfully in batch‐mode and continuous process devices; net thermal efficiencies of the process as high as 90% have been reported. Torrefied biomass has been proposed as a feedstock for coal co‐combustion, as well as for gasification‐combustion and Fischer‐Tropsch fuel production. Analyses of supply chain impacts indicate that, in some scenarios, torrefaction can be the lowest cost and most energy efficient option for supplying fuel, especially when combined with pelletization of the material.

Significant gaps still exist in our understanding of torrefaction; there is need to further study this important process for its potential benefits to bioenergy production. Some of the more pressing needs include characterization of chemical pathways of the torrefaction reaction, investigation of equipment performance and equipment‐related influences on the process, and elucidation of supply chain impacts. © 2011 Society of Chemical Industry and John Wiley & Sons, Ltd