Lignin pyrolysis

Complex pyrolysis chemistry takes place in the conversion system of any conventional solid-fuel combustion system. The pyrolytic properties of biomass are controlled by the chemical composition of its major components, namely cellulose, hemicellulose, and lignin. Pyrolysis of these biopolymers proceeds through a series of complex, concurrent and consecutive reactions and provides a variety of products which can be divided into char, volatile (non-condensible) organic compounds (VOC), condensible organic compounds (tar), and permanent gases (water vapour, nitrogen oxides, carbon dioxide). The pyrolysis products should finally be completely oxidised in the combustion system (Figure 14). Emission problems arise as a consequence of bad control over the combustion system. 

Simoneit, B. R. T., W. F. Rogge, M. A. Mazurek, L. J. Standley, L. M. Hildemann, and G. R. Cass, Lignin Pyrolysis Products, Lignans, and Resin Acids as Specific Tracers of Plant Classes in Emissions from Biomass Combustion, Environ. Sci, Technoi, 27, 2533-2541 (1993). 

The list of pyrolysis products of cottonwood shown in Table VII (llj reflects the summation of the pyrolysis products of its three major components. The higher yields of acetone, propenal, methanol, acetic acid, CO, water and char from cottonwood, as compared to those obtained from cellulose and xylan, are likely attributed to lignin pyrolysis. Other results are similar to those obtained from the pyrolysis of cell-wall polysaccharides. This further verifies that there is no significant interaction among the three major components during the thermal degradation of wood. 

Quantification of Pyrolysis Products. The pyrolysis must be performed separately ( off-line approach, see Fig. 4.7.5), and the products are trapped completely so that an internal standard can be added. Accurate quantification of the pyrolysis products is achieved using a gas chromatograph equipped with a flame ionization detector (Faix et al. 1987). Fluoranthene is a suitable internal standard as it elutes from the column after the last detectable lignin pyrolysis product (trans-sinapyl alcohol). 

Because of the outstanding performance of modern quartz capillary columns, the gas chromatographic separation of lignin pyrolysis products is easily achieved. Generally, nonpolar or medium polarity columns are used the selection of the stationary phase is less important than for conventional packed columns. For the separation of pyrolysis products from wood or lignin a modified dimethylpolysiloxane liquid phase (e.g., SE-54 or OV-1701) should be used. 

The most common ionization technique is electron impact (El) ionization at 70 eV. As applied to lignin pyrolysis products, this mode of ionization produces molecular and typical fragment ions in sufficient abundance to permit structure assignments to be made. 

Table 4.7.1. Retention times of phenolic lignin pyrolysis products relative to guaiacol (1 0) separated on columns with differing liquid phases...

Fig. 4.7.10. a-f (pp 191-196). Electron impact (El) mass spectra of lignin pyrolysis products. Numbers refer to Table 4.7.1. Spectra recorded at 70 eV, source temp. 170°C... 

Haw JF, Schultz TP (1985) Carbon-13 CP/MAS NMR and FT-IR study of low-temperature lignin pyrolysis. Holzforschung 39 289-296... 

To represent lignin pyrolysis under our experimental conditions, a formal reaction model with two parallel series reactions had to be set up ... 

However, we need further studies to confirmed the above said hypothesis and to reveal more details related to the mechanism of hydrolysis lignin pyrolysis. 

Benson S.W. (Ed) (1976) Thermochemical Kinetics, 2nd edn. Wiley, New York. Petrocelii P. F. Klein T. M. (1985) Simulation of Kraft Lignin Pyrolysis. In Fundamantals of Thermochemical Biomass Conversion. (Eds R.P. Overend, T.A. Milne L.K. Mudge), pp. 257-273. Elsevier Applied Science Publisher, London. 

Other ionization techniques were also utilized in LC/MS such as FAB, which puts the sample in a liquid matrix such as glycerol and uses fast atom bombardment as the source of energy for the ions. This technique practically does not heat the sample, allowing the formation of molecular ions for certain unstable molecules. An example of a FAB spectrum for syringoresinol detected in lignin pyrolysis is given in Figure 5.7.4. 

In these dimer units R, indicates the rest of the polymer, while the substituents R2 on the aromatic rings are H or O-CH3. These structures can be recognized as stable units and are also generated during lignin pyrolysis. 

A Py-IR Study of kraft lignin pyrolysis in a temperature gradient (FT-IR-EGA study) [4] showed the formation of different compounds evolving at certain temperature ranges as indicated in Figure 9.1.1. 

Numerous analyses of the tar generated during lignin pyrolysis were reported [6-8]. In one study [6] pyrolysis was used to determine the predominance of H, G, or S units in... 

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