Thermochemical degradation

Due to the very differentiated and complex phenomenology inside a thermochemically degrading fuel bed, it is very important to have a clear and differentiated terminology for the physical structure of the fuel bed, herein referred to as fuel-bed structure, so that the many specific thermochemical conversion phenomena can be more precisely adressed inside the packed bed. 

Carbohydrates would be the predominant raw materials for future biorefineries. The major polysaccharides found in nature are cellulose, hemicellulose and starch (see Chapter 1). These molecules would be mainly utilised after they are broken down to their respective monomers via enzymatic hydrolysis, thermochemical degradation or a combination of these two. Cellulose and hemicellulose, together with lignin, constitute the main structural components of biomass. Starch is the major constituent of cereal crops. This section would focus on the potential utilisation of carbohydrates and lignocellulosic biomass for chemical production. 

Krochta, J.M. Hudson, J.S. Tillin, S.J. "Kinetics of alkaline thermochemical degradation of polysaccharides to organic acids." In This Volume. 

Kinetics of Alkaline Thermochemical Degradation of Polysaccharides to Organic Acids... 

Figure 1. Effect of time on thermochemical degradation of starch in alkaline solution according to Equation 3.

Figure 4. Effect of time on organic acid production from thermochemical degradation of starch and cellulose in alkaline solution.

Starch and cellulose can both be thermochemically degraded in alkaline solution to water-soluble compounds of relatively low molecular weight. (A parallel study examines more completely the nature of these compounds (15).) Both starch and cellulose degradation processes can be described by second-order kinetics, with the hydroxide ion concentration determined by the stoichiometry of polysaccharide conversion to organic acids. The thermochemical degradation activation energy in alkaline solution for both starch and cellulose is 39,500 calories/mole. 

The effect of chemical constituents in the environment or the presence of solvents can cause polymers to undergo chemical reactions and/or change in their chemical make-up. Usually, this is accompanied by some amoxmt of heat since the bond-making and bondbreaking process involves evolution and absorption of heat. Thermochemical degradation in commonly observed in polymers such as polyethylene (PE) and polypropylene... 

From the examples given earlier, a few important points can be noted regarding thermochemical degradation of polymers the overall degradation is affected greatly by the chemical structure of the polymer (bond type, bond strength, branching) and the chemical environs where it is used. 

Another important aspect which affects thermochemical degradation is the extent of branching. Disproportionation and chain scissions may selectively occur at sites requiring low amoimts of energy. This can lead to formation of lower-molecular-weight products before further decomposition. This particular effect is illustrated by data shown in Scheme 6.8(a-b) [31]. With greater the amount of oxygen present in the side chain of the polymeric units, especially attached to tertiary or quaternary carbons, the thermal... 

BruchmuUer J, van Wachem BGM, Gu S, Luo KH, Brown RC Modeling the thermochemical degradation of biomass inside a fast pyrolysis fluidized bed reactor, AIChE J 58 3030-3042, 2012. 

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