Constituents, biomass

The subsequent fate of the assimilated carbon depends on which biomass constituent the atom enters. Leaves, twigs, and the like enter litterfall, and decompose and recycle the carbon to the atmosphere within a few years, whereas carbon in stemwood has a turnover time counted in decades. In a steady-state ecosystem the net primary production is balanced by the total heterotrophic respiration plus other outputs. Non-respiratory outputs to be considered are fires and transport of organic material to the oceans. Fires mobilize about 5 Pg C/yr (Baes et ai, 1976 Crutzen and Andreae, 1990), most of which is converted to CO2. Since bacterial het-erotrophs are unable to oxidize elemental carbon, the production rate of pyroligneous graphite, a product of incomplete combustion (like forest fires), is an interesting quantity to assess. The inability of the biota to degrade elemental carbon puts carbon into a reservoir that is effectively isolated from the atmosphere and oceans. Seiler and Crutzen (1980) estimate the production rate of graphite to be 1 Pg C/yr. 

The molecular structures of the main biomass constituents are given in Figures 10.8 and 10.9. These structures induce a physicochemical behavior that is markedly different from the behavior of the hydrocarbons contained in crude oil (see Table 10.2). Note that the relative thermal fragility of the biomass molecular structures encourages the chemist to prefer thermally mild and therefore low-energy-consuming conversion techniques such as fermentation or hydrothermolysis to exploit biomass (see Figure 10.10). 

Products from biomass by supercritical water (SCW) depend on the nature and stracture of the biomass. The effects of SCW on the biomass constituents should be separately studied. For example SCW affects unsatmated compounds, and unsaturated fatty and resin acids, rather than those of saturated ones under different reaction conditions (Watanabe et al., 2006). The diffusion or mass transfer rate of SCW into the individual component of biomass has been studied separately (Antal et al., 2000 Feng et al., 2004). 

Table 3 Degradation products of fast pyrolysis of biomass constituents (12)...

The pyrolysis of wheat straw could be expected to be described by a superposition model, i.e. described by a number of parallel first order reactions, representing the decomposition of the biomass constituents (cellulose, hemicellulose and lignin) [4,5]. In this work, a superposition model has been used, in which the pyrolysis is assumed to be described by N independent first order reactions (for i=l, 2,..., N) ... 

The basic interpretation of equation (13.1.25) is that it describes the distribution of substrate consumption into three portions assimilation of substrate into biomass constituents, synthesis of products, and provision of the energy necessary to survive. Aspects of viability incorporated in the term for maintenance activities include facilitating transport of ions or other solutes across cell membranes and synthesizing cell constituents as replacements for damaged or malfunctioning constituents of the metabolic network of the cell, thereby repairing those pathways essential to the life of the microorganism. 

This chapter gives a general introduction to the book and describes briefly the context for which the editors established its contents and explains why certain topics were excluded from it. It covers the main raw materials based on vegetable resources, namely (i) wood and its main components cellulose, lignin, hemicelluloses, tannins, rosins and terpenes, as well as species-speciflc constituents, like natural rubber and suberin and (ii) annual plants as sources of starch, vegetable oils, hemicelluloses, mono and disaccharides and algae. Then, the main animal biomass constituents are briefly described, with particular emphasis on chitin, chitosan, proteins and cellulose whiskers from molluscs. Finally, bacterial polymers such as poly(hydroxyalkanoates) and bacterial cellulose are evoked. For each relevant renewable source, this survey alerts the reader to the corresponding chapter in the book. 

The different thermal stabilities of hemiceUulose, cellulose, and lignin provide an opportunity to use pyrolysis for the thermal fractionation of biomass. The bar graph in Figure 8.1 presents a schematic overview of the different thermal stabilities of each of the main biomass fractions. The height of the bars corresponds to the approximate temperature level at which the thermal degradation rate of the biomass constituent under isothermal conditions and in an inert atmosphere reaches a maximum as can be measured by thermogravimetric analysis. 

Figure 8.1 indicates the potential for thermal fractionation of the biomass. The order of thermochemical stability of the individual biomass constituents ranges from hemicel-lulose (fast degassing/decomposition from 200 to 300 °C) as the least stable polymer to... 

Biomass constituent (thermal degradation range) Pyrolysis products (major value-added chemicals underlined) Market application examples of the underlined chemicals... 

Biomass constituent (thermal value-added chemicals Market application examples... 

Biomass constituent Biomass constituent Biomass constituent Protein constituent (from nitrogen-fixing organisms) DNA/RNA constituent pollutants... 

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