Biomass for thermochemical conversion: targets and challenges

Abstract

Bioenergy will be one component of a suite of alternatives to fossil fuels. Effective conversion of biomass to energy will require the careful pairing of advanced conversion technologies with biomass feedstocks optimized for the purpose. Lignocellulosic biomass can be converted to useful energy products via two distinct pathways: enzymatic or thermochemical conversion. The thermochemical pathways are reviewed and potential biotechnology or breeding targets to improve feedstocks for pyrolysis, gasification, and combustion are identified. Biomass traits influencing the effectiveness of the thermochemical process (cell wall composition, mineral and moisture content) differ from those important for enzymatic conversion and so properties are discussed in the language of biologists (biochemical analysis) as well as that of engineers (proximate and ultimate analysis). We discuss the genetic control, potential environmental influence, and consequences of modification of these traits. Improving feedstocks for thermochemical conversion can be accomplished by the optimization of lignin levels, and the reduction of ash and moisture content. We suggest that ultimate analysis and associated properties such as H:C, O:C, and heating value might be more amenable than traditional biochemical analysis to the high-throughput necessary for the phenotyping of large plant populations. Expanding our knowledge of these biomass traits will play a critical role in the utilization of biomass for energy production globally, and add to our understanding of how plants tailor their composition with their environment.

Relationships between Biomass Composition and Liquid Products Formed via Pyrolysis

“…Advances in understanding cell wall biosynthesis, including genes responsible for synthesizing the major polymer classes (Bonawitz and Chapple, 2010;Scheller and Ulvskov, 2010;Pauly et al, 2013) and covalent interactions among them (Chiniquy et al, 2012;Bartley et al, 2013;Schultink et al, 2015); regulation of expression of the cell wall biosynthesis genes (Zhao and Dixon, 2011); and metal ion transport proteins that determine the abundance and distribution of plant mineral content (Ma et al, 2006;Yamaji and Ma, 2009;Zhong and Ye, 2015), lay the foundation for genetically engineering bioenergy crop cell wall content and structure. For example, lignin is an important target for genetic engineering for pyrolysis since the major lignin-derived products have a lower O:C ratio, a higher energy value, and are more stable than sugar-derived products (Tanger et al, 2013;Mante et al, 2014). Some important genes that participate in or regulate lignin synthesis have already been modified in energy crops without major interference with plant biomass yield (Baxter et al, 2014(Baxter et al, , 2015reviewed in Bartley et al, 2014).…”

“…f Galactoglucomannan is only abundant in gymnosperm woods. Dicots and grasses possess <8% of mannan and galactoglucomannan (Scheller and Ulvskov, 2010 (Pauly and Keegstra, 2008 increasing the content of phenolics relative to carbohydrates to reduce the oxygen content of bio-oil (Tanger et al, 2013). Here, we provide a more detailed description of the chemical structure and interactions among major cell wall components to aid in understanding more subtle relationships between biomass and bio-oil content.…”

“…Biomass consists of cell walls that establish the structure of the plant and, to a lesser extent, non-structural components ( Table 2). Cell walls determine the shape of leaves and stems and the cells that compose them and consist of cellulose, hemicellulose, lignin, as well as structural proteins and wall-associated mineral components (O'Neill and York, 2009;Vogel et al, 2011;Tanger et al, 2013). Non-structural components include sugars, proteins, and additional minerals (O'Neill and York, 2009;Vogel et al, 2011;Tanger et al, 2013).…”

“…Cell walls determine the shape of leaves and stems and the cells that compose them and consist of cellulose, hemicellulose, lignin, as well as structural proteins and wall-associated mineral components (O'Neill and York, 2009;Vogel et al, 2011;Tanger et al, 2013). Non-structural components include sugars, proteins, and additional minerals (O'Neill and York, 2009;Vogel et al, 2011;Tanger et al, 2013). For example, in switchgrass, an important potential bioenergy crop, dry biomass consists of ~70% cell walls, 9% intrinsic water, 8% minerals, 6% proteins, and 5% non-structural sugars (Vogel et al, 2011).…”

Selection of optimal conversion path for willow biomass assisted by near infrared spectroscopy

“…It is required to characterize large populations of plants in order to phenotype biomass to distinguish between genetic and environmental controls over individual bioenergy traits (Tanger et al 2013). Several crossing strategies have been recently employed within breeding programs in order to increase yields, enhance disease and pests' resistance, improve growth form and resistance to the frost or dry stresses (Tognetti et al 2013).…”