Lignin char formation

Wood burns because the cell wall polymers undergo hydrolysis, oxidation, dehydration, and pyrolysis reactions with increasing temperature to give off volatile, flammable gases. The lignin component contributes more to char formation than do the cellulose components, and the charred layer helps insulate the wood from further thermal degradation see Chapter 13). 

Pyrolysis of cellulose at temperatures below 300 0 results mainly in char formation. Any lignin present in the MSW (Kraft paper, cardboard, and wood waste contain significant proportions) tends to char, even at higher temperatures. On the other hand, the cellulose and hemicelluloses readily decompose to volatile products at temperatures above 300 0. Most of the plastics present thermally degrade at a significantly higher temperature (400-450OC) (2). 

Formation of functionalized carbonaceous materials, such as saccharides, starch, sucrose, and glucose, has been observed by hydrothermal carbonization of cellulose at 220-250°C [115]. The formation of this material follows essentially the path of a hydrolysis followed by dehydration and fragmentation into soluble (monomer) products [115]. Shu-Hong Yu and Titirici extensively studied the hydrothermal carbonization of biomass and the mechanisms of char formation. The chemical reactions of biomass (e.g., saccharides, cellulose, lignins) involved in the hydrothermal process are complex in nature and most of the reports focus on initial steps of the reaction, such as hydrolysis, dehydration, and decarboxylation. Recently, formation of polymerization and aromatization in cellulose hydrothermal carbonization has been described, too [36,178,179]. 

Finally, the already mentioned investigations on the Mediterranean pine Pinus halepensis shall be cited again in which commercial cellulose as a model compound for studies of forest fuel pyrolysis, pine needles, pine needle lignin and extractives were analyzed with and without two ammonium salts as fire retardants [46]. Both salts provoked a lowered pyrolysis temperature of cellulose and a significant increase in char formation for cellulose (up to 2.4 times) and intact needles (up to 1.7 times), but they had negligible effects on lignin and extractives. 

The products of low temperature pyrolysis are char and low molecular components, see pathway 1 in Figure 53. At moderate temperature levels, the formation of a variety of lignin monomers (see Figure 49) occurs via pathway 2. And at high temperatures (> 500°C), fragmentation reactions take place, forming CO, H2, and reactive vapours. [67]... 

These very simple calculations give only first indications. A more sophisticated approach would need to take into account the fact that lignin gives rise to higher fractions of char than cellulose. Also, the existence of mass transfer resistances must not be ignored if the primary products are not efficiently removed from the reacting zone, they have more chances to take part in subsequent reactions, with formation of char. 

As shown in the preceding parts, kinetic parameters cannot be directly calculated when internal heat transfer limits pyrolysis. A model taking into account both kinetic scheme and heat- mass transfers becomes necessary, A one-dimension model has already been implemented and solved. It features a classical set of equations for heat and mass transfers in porous media, i.c. heat transfer through convection, conduction, radiation and mass transfer due to pressure gradient (Darcy s law) and binary diffusion. Different kinetic schemes from e literature arc and will be tested mass-loss as lumped first order reaction, formation of volatiles, tars and char from decomposition of cellulose, hcmicellulose and lignin [26], the Broido-Shafi2adeh model [30] and the one proposed by Di Blasi [31]. None of them can describe the composition of the volatiles and supplementary schemes have to be found. 

Ciganov E.A, Akhmina E.I.. Galaudina V.V (1978) Formation of porous stmcture chars of hydrolitic lignin by pyrolysis. Chimiya drevesini, 5,97. 

More work was carried out to study the effect of wood, cellulose, lignin, and activated charcoal on the thermal decomposition of polystyrene (PS) and polyethylaie (PE) occurring in municipal waste. Py-GC/MS revealed that these materials had a similar influence on PS and PE thermal decomposition under slow and fast heating conditions. The effect is related to the char-forming capability of the wood-derived additives thus cellulose had the least and pure charcoal had die greatest influence on the decomposition of the polymers studied. The yield of monomers, dimers, and trimers decreased, and the formation of other products (e.g., toluene, ethyl benzene, and a-methyl styrene) was detected in the obtained pyrograms. 

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