Pyrolysis Time

FIGURE 3.3 Energy flows at the surface of the solid fuel sample. 

The classical analysis corresponding to the ignition process assumes a linear approximation for the surface re-radiation. The radiative term is then defined as 

This simplification allows an analytical solution of the one-dimensional heat conduction energy equation. By substituting Equation 3.11 into Equation 3.10, and assuming that the total heat-transfer coefficient (hT) is equal to the sum of the convective heat-transfer coefficient (hc) and the radiative heat-transfer coefficient (hT), the following expression (Equation 3.12) defines the net heat flux (q ) at the surface of the solid fuel sample. 

This can be solved (e.g., by means of a Laplace transformation) to provide a general solution for the temperature distribution in the sample as a function of the location (x) and time (/)  

FIGURE 3.4 Characteristic surface temperature histories (thermocouple at the center of ignition test specimen) for several heat fluxes. The material used is PMMA. 

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Figure 11.1 Py/methylation GC/MS chromatograms of lead white pigmented linseed oil paint after 610 °C Curie point pyrolysis assisted with on line methylation using 2.5% methanolic TMAH (the sample and TMAH solution was applied onto a rotating Curie point wire pyrolysis time 6 s, interface 180°C). 1, heptenoic acid, methyl ester 2, heptanoic acid, methyl ester 3, butenedioic acid, dimethyl ester 4, butanedioic acid, dimethyl ester 5, octenoic acid, methyl ester 6, octanoic acid, methyl ester 7, pentenedioic acid, dimethyl ester 8, pentanedioic acid, dimethyl ester 9, nonanoic acid, methyl ester 10, hexanedioic acid, dimethyl ester 11, decanoic acid, methyl ester 12, heptanedioic acid, dimethyl ester 13, octanedioic acid, dimethyl ester 14, 1,2 benzenedicarboxylic acid, dimethyl ester 15, a methyl octanedioic acid, dimethyl ester 16, nonanedioic acid, dimethyl ester 17, a methoxy octanedioic acid, dimethyl ester 18, a methyl nonanedioic acid, dimethyl ester 19, a,a dimethyl nonenedioic acid, dimethyl ester 20a, a methyl nonenedioic acid, dimethyl ester 20b, a,a dimethyl nonanedioic acid, dimethyl ester 21, decanedioic acid, dimethyl ester 22, a methoxy nonanedioic acid, dimethyl ester 23, a methyl decan edioic acid, dimethyl ester 24, undecanedioic acid, dimethyl ester 25, a methoxy decan edioic acid, dimethyl ester 26, pentadecanoic acid, methyl ester 27, dodecanedioic acid, dimethyl ester 28, hexadecanoic acid, methyl ester 29, heptadecanoic acid, methyl ester 30, octadecanoic acid, methyl ester 31,8 methoxy 9 octadecenoic acid, methyl ester 32, 11 methoxy 9 octadecenoic acid, methyl ester 33, 9 methoxy 10 octadecenoic acid and 10 methoxy 8 octadecenoic acid 34, 9 oxo octadecanoic acid, 10 oxo octadecanoic acid 35, 9 epoxy octadecanoic acid 36, eicosanoic acid, methyl ester 37, 9,10 dimethoxy octadecanoic acid, methyl ester 38, docosanoic acid, methyl ester. Reprinted from J. Anal. Appl. Pyrol., 61, 1 2, van den Berg and Boon, 19, Copyright 2001, with permission from Elsevier...

It has been recognized that best results are obtained when the temperature of the sample is raised rapidly and reproducibly to the pyrolysis temperature and then held closely at that temperature for the desired pyrolysis time. One obvious way of achieving this aim is by the use of an electrically heated microfumace. Considerable difficulties were encountered in the development of such furnaces with suitable characteristics, and although pyrolysers of this type are now readily available and in use, they still suffer from the relative disadvantage of rise times of several seconds. A design for a modem furnace is shown in Figure 11.23. 

Comparison of pyrolysis times for Curie-point pyrolysis and furnace pyrolysis. 

The structure of this compound has been confirmed by X-ray analysis 172>. The first s-diazadiborines were obtained by pyrolysis of tris(t-butyl-amino)borane 166 167). Other less hindered tris(amino) boranes form aminoborazines or poly(aminoborazines) the nature of the product depends on pyrolysis time and utilized temperature 175>. Exocyclic B—N bonding appears to contribute to the stability of 1,3-diamino-diazaborines thereby explaining their chemical stability as compared to that of the corresponding tetraorganocompounds. 

D. Price, F. Gao, GJ. Milnes, B. Ehng, C.I. Lindsay, and T.P. McGrail, Laser pyrolysis/time-of-fhght mass spectrometry studies pertinent to the behavior of flame-retarded polymers in real fire situations. Polym. Degrad. Stab., 64, 403 110 (1999). 

Equation 3.16 is the general solution to the surface temperature at all levels of incident heat flux. To obtain the pyrolysis time t, the surface temperature Ts is substituted by /j, and Equation 3.16 can be rewritten as... 

The first domain corresponds to high-incident heat fluxes, where the pyrolysis temperature (TP) is attained very fast, thus t Application of the first-order Taylor Series expansion to Equation 3.13 around tp/tc —> 0 yields the following formulation for the pyrolysis time (lp) ... 

As can be seen from Equation 3.20, the short-time solution for the pyrolysis time, tPi is independent of the total heat-transfer coefficient term, hT = (h, + h,). Thus, the pyrolysis time tp is only a function of the energy absorbed aq" due to radiation from the radiant panel and the properties (k, p, Cp) of the solid fuel sample. 

Solving Equations 3.20 and 3.21 for the pyrolysis time t will yield a theoretical value for the time at which the solid fuel sample begins to pyrolyze and produce fuel vapors. The use of the appropriate simplified solution will allow the evaluation of the pyrolysis time t over the entire domain of the imposed incident heat fluxes. 

The use of phosphorus-based flame retardants in combination with other, better established, flame retardants is most effective in situations in which the combination proves synergistic. However, as yet our understanding of such synergistic effects is far from complete and more fundamental work is required in this area Work in which the gaseous and solid products of combustion, with and without the presence of flame retardants, are carefully analyzed. Such analyses can now be undertaken more readily than in the past, owing to the relatively recent development of techniques such as gas-phase FT-infrared spectroscopy and laser-pyrolysis time-of-flight mass spectrometry for the identification of volatiles, and solid-state NMR spectroscopy and x-ray photoelectron spectroscopy for the analysis of chars. 

Figure 6. Top Mass increase of stainless steel wires as a function of pyrolysis time. Bottom Luminosity as a function of time. Arbitrary units are used.

Calcined temperature. Based on the degrading reaction of the component in dipping solution, temperatures of 400°C, 500°C, 600°C, 700°C, and 800°C were tested at the same pyrolysis time. From the results for phenol degradation (Fig. 14.2), a similar degradation results obtained at the pyrolysis temperature 500°C and 600°C. 

Fig. 4.7.1. Temperature-time profile of a Pyroprobe instrument A ideal pyrolysis with the ribbon probe B coil probe with quartz boat and 200°C interface temperature. Explanation of terms Py-T pyrolysis temperature Py-t pyrolysis time T rise t temperature-rise time...

Heated-filament pyrolyzers are often used to analyze lignins (Kratzl et al. 1965, Lindberg et al. 1982, Obst 1983, Gardner et al. 1985, Faix et al. 1987, 1991, Funazukuri et al. 1987, Salo et al. 1989). In this type of analyzer, electric current is passed through a resistance ribbon or coiled wire, both made of platinum. The dissipation of power increases the temperature of the conductor. Heat-up and pyrolysis times are selected from an instrument control. Characteristic parameters of this type of pyrolyzer have been described by Wells et al. (1980) and Wampler and Levy (1987). 

The characteristics of water layers adsorbed on the carbosils and the parent silica gel are summarized in Table 7. It should be noted that for porous materials the thickness of an adsorbed layer cannot be much greater than the radius of the adsorbent pores. Therefore, as the pyrolysis time increases, the total pore volume decreases, which results from carbon depositing in the pores, and so there is a tendency for the adsorbed water concentration to decrease when going from SG sample to the carbosils. If the extent of carbonization is small, then C and AG ... 

Because of the broad scope of direct biomass pyrolysis, the basic technologies and principal products are tabulated in Table 8.12 to facilitate easy comparison. The conversion conditions and major products shown in this table are typical, but subject to considerable variation. There are several commonalities among the different pyrolysis methods. Pyrolysis time and temperature are clearly the key operating parameters that have the most influence on product yields and distributions. Moderate but optimized temperatures are needed at short residence times to maximize liquid yields, whereas long residence times and... 

Heterogeneous kinetics of straw pyrolysis and straw gasification are essentia data for reactor design. Pyrolysis is a relatively fast process. In view of the poor heat conduction of straw and straw char, the pyrolysis time is the time which is required to heat the center of the particle to the decomposition temperature. This is a rather simplified model, but allows a reasonable time estimate in view of the order of magnitude. 

For an experimental determination of the pyrolysis time, a fluidised bed of sand and a large volume hot vessel have been used [20]. The method for reaction rate measurements in the fluidised bed is partly explained in Fig. 7. The pyrolysis gases in the fluidising nitrogen gas stream are combusted after 02 addition downstream from the vessel and the CO2 plus some CO are monitored with NDIR-analysers. Thus, the course of a slow pyrolysis has been followed with a time resolution of several seconds. After pyrolysis, the O2 is added upstream from the fluidised bed and the combustion of the pyrolysis char can be followed in the same way. The equipment is calibrated by injection of a known C02-volume to the bed. Area and time response of the resulting calibration signal are needed for data analysis. 

Pyrolysis kinetics for cylindrical 12 mm diameter straw pellets in a fluidised bed of sand at different temperatures are shown in Fig. 8. The long pyrolysis times around 100 s are a consequence of the large particle dimension. This demonstrates, that pelletisation is not an advantage it is expensive and destroys the high reactivity of untreated straw with thin ca. 0.5 mm thick walls. 

At comparable temperatures, the pyrolysis of straw chops proceeds more than 10 times faster than straw pellet pyrolysis. This is demonstrated in Fig. 9. Small cm-sized single walled straw chops have been added to a large preheated steel vessel with a sand layer at the bottom. The pyrolysis gases quickly displace a certain amount of inert gas in the thermostated vessel. The released inert gas is collected in a burette, whose level is observed with a TV-camera. A time resolution of < 0.1 s has been obtained in this way. Fig. 9 shows that the pyrolysis times at > 600 C are less than 10 s. Nodes in the straw stem halm must be squeezed to maintain short reaction times. 

In the case of 44 % moisture there is liquid water present in the structure and also the permeability of liquid influences the conversion time of the sample. Figure 6 shows the measured temperature profiles of Figure 1 and the two simulated Cases 6 and 7 in Table 1, simulated for liquid axial permeabilities of 10 and 10 respectively. In both cases the radial permeability is assumed to be 10 lower. It is seen from Figure 6 that the intrinsic permeability of liquid has a large influence on the pyrolysis time. A higher permeability leads to a larger transport of water through the wood and less water evaporates inside the sample, which reduces the time of conversion. 

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