Cellulose char-residue

Both 1st- and 2nd-order rate expressions gave statistically good fits for the control samples, while the treated samples were statistically best analyzed by 2nd-order kinetics. The rate constants, lst-order activation parameters, and char/residue yields for the untreated samples were related to cellulose crystallinity. In addition, AS+ values for the control samples suggested that the pyrolytic reaction proceeds through an ordered transition state. The mass loss rates and activation parameters for the phosphoric acid-treated samples implied that the mass loss mechanism was different from that for the control untreated samples. The higher rates of mass loss and... 

At temperatures above 250°, vacuum pyrolysis of cellulose provides, in addition to depolymerization and decomposition (with evolution of water, carbon dioxide, and carbon monoxide), a volatile tar which contains mainly levoglucosan and leaves a charred residue. Under these conditions, the oxidation reactions are eliminated or minimized, and the levoglucosan is removed from the heated area where it could undergo further decomposition. 

This mechanism implies that a portion of the charred residue from the pyrolysis of cellulose could be derived from the secondary reactions involved in the decomposition of levoglucosan. In the production of levulinic acid, about 25 % of the initial D-glucose residues is converted into the humin that separates as an insoluble, charred residue. This humin could be formed from 5-(hydroxymethyl)-2-furaldehyde, which is readily polymerized and decomposed on heating under acid conditions. The kinetic investigations indicate that the humin could also be formed from the reaction of 5-(hydroxy-methyl)-2-furaldehyde with one of its precursors. ... 

The interest in the properties of the chars derived from cellulosic or biomass solid.s extends beyond those associated with thermal transport in the char. Insofar as the char residue from a pyrolysis process must typically be burned, gasified, or put to use as an activated carbon product, there is also a need to examine the porous nature of the char, bi acbvated carbons, the pore structure is key to adsorption performance. In combustion or gasification, the porosity can play a role in determining conversion kinetics in the intrinsic rate controlled or pore diffusion controlled regimes. 

Lewellen et al. (9) at M.I.T. studied flash pyrolysis of cellulose using a bench scale, electrically heated screen. They found that for heating rates ranging between 400 and 10,000°C/sec in an inert helium atmosphere the cellulose completely vaporized by pyrolysis reactions , leaving no char residue. Only by extended heating of the cellulose at 250°C was the M.I.T. group able to produce some char (2% by weight of the initial cellulose). 

Phosphorus and Chlorine Contents. Phosphorus and chlorine contents of the samples in char residue at kOO C are shown in Table IV. Phosphorus content did not change by thermal decomposition in all samples. But chlorine content decreased by pyrolysis. Chlorine introduced by stannic chloride treatment hardly decreased, but that by grafting decreased easily by pyrolysis. Stannic chloride is not introduced in cotton in the form of stannic chloride, because the chlorine content of stannic chloride treated cellulose is very lower than that calculated from weight increase. X-ray diffraction trace of burned sample... 

Figure 14.32 The flame resistance of polymeric materials, indicated by the oxygen index. 1, polyformaldehyde 2, polyethylene, polypropylene 3, polystyrene, poiyisoptene 4, polyamide 5, cellulose 6, poly(vinyl alcohol) 7, poly(ethylene terephthalate) 8, polyactylonitrile 9, poly(phenylene oxide) 10, polycarbonate 11, aromatic nylon 12, polysulfone 13, Kynol 14, polylmide 15, carbon. Polymers producing large values of char residue are more fire resistant.

The most widely studied reaction involves the effect of phosphorus flame retardants on cellulose materials. In the presence of phosphorus-containing compounds, the decomposition of cellulose occurs in such a way that a decrease is observed in the amount of fuel gases and an increase in the amount of char residue. The amount of char residue can attain a value of almost 80% of a polymer s... 

This scheme does not reflect the real mechanism of cellulose decomposition, Nevertheless, a tendency can clearly be traced towards a reduction in the combustibility of cellulose materials with an increase in the yield of char residue during combustion. 

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). 

Analysis of cellulose TGA data involves obtaining a corrected mass (Me), which is dimensionless, to account for residual char and is calculated by ... 

Fig. 9. —Rate of Pyrolysis of Cellulose at 288°C. in Nitrogen. [(A) Weight of the residual char (B) remaining D-glucose residues.]...

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