Cellulose decomposition pathway

Fig. 3 Simplified cellulose thermal decomposition pathways at atmospheric pressure...

Some clues may be available from studies of the decomposition of lignin. Lignin constitutes the second most abundant carbon polymer on earth after cellulose (46). The understanding of biodegradative pathways of lignin and lignin-cellulosic polymers may elucidate the problems of reduced plant productivity associated with surface residues in conservation production systems. 

Carbohydrates ranging from cellulose to simple sugars are subject to thermal alteration. Factors such as temperature, pH, compound concentration, and other reactants present can alter both the rate and complexity of decomposition reactions. Carbohydrate types are reviewed relative to degradation/carameli-zation pathways and endproducts. Some of the resulting typical food flavors produced are also discussed. 

Pyrolysis as defined is a process of thermal decomposition occurring in the absence of oxygen. Pyrolysis of biomass is a complicated multistage reaction for which many pathways and mechanisms have been proposed.551-558 The best known is the model developed by Broido and Shafizadeh559,560 for pyrolysis of cellulose that can be also applied, at least qualitatively, to whole biomass (Fig. 33.30). 

To draw the reaction model, separate experiments were performed with the decomposition products of cellulose. Cellulose was reacted in 300 C of hot-compressed water with 5 wt% of Na2C03 (Run 4) to obtain the water-soluble products and the oily products, which were easily separated by decantation. Then both were gasified with the nickel catalyst. As shown in Table 2, the water-soluble products could be gasified to H2, CO2 and CH4, while the oily, polymerized products could not be gasified. This result supports the competition of gasification pathway to polymerization pathway. The following total reaction model can be drawn. 

Figure 21.11 The hypothetical chemical pathways for thermal decomposition of cellulose, lignin and hemicellulose. Adapted from Refs. [13,15,96,97,100].

The fast pyrolysis decomposition of cellulose starts at temperatures as low as 150°C. Pyrolysis of cellulose below 300°C results in the formation of carboxyl, carbonyl, and hydro peroxide groups, elimination of water and production of carbon monoxide and carbon dioxide as well as char residue (Evans and Milne, 1987). Therefore low pyrolysis temperatures will produce low yields of organic liquid yields. Fast pyrolysis of cellulose, above 300°C, results in liquid yields up to 80 wt.%. Cellulose initially decomposes to form activated cellulose (Bradbury et al., 1979). Activated cellulose has two parallel reaction pathways, depolymerization and fragmentation (ring scission). The main products from each reaction pathway are rather different as ring scission produces hydroxyacetaldehyde, linear carbonyls, linear alcohols, esters, and other related products (Bradbury et al., 1979 Zhu and Lu, 2010 Lin et al., 2009) and depolymerization produces monomeric anhydrosugars, furans, cyclopentanones, and pyrans and other related products (Bradbury et al., 1979 Zhu and Lu, 2010 Lin et al., 2009). Each reaction pathway is independent and is influenced by pyrolysis temperature and residence time (Bradbury et al., 1979). 

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