Final pyrolysis temperature

Figures 6.5 to 6.7 show the effect of temperature on yields of + paraffins obtained from biomass samples by pyrolysis. As can be seen in Figs. 6.5 to 6.7, the percentage of + paraffins in gaseous products obtained from the samples of hazelnut shell, tea waste and spmce wood increased, while the final pyrolysis temperature was increased from 700 to 950 K.

Figure 6.23 shows the plots for yields of hydrogen in gaseous products from the samples by pyrolysis. The percentage of hydrogen in gaseous products from the samples of Polytrichum commune, Thuidium tamarascinum, Cladophora fracta and Chlorella protothecoides increased from 21.3 to 38.7%, 23.0 to 41.3% and 25.8 to 44.4% and 27.6 to 48.7% by volume, respectively, while the final pyrolysis temperature was increased from 650 to 875 K. 

During pyrolysis, the yield of gaseous and liquid products can vary from 25 to 70 percent by weight, depending on a number of variables, such as coal type, type and composition of the atmosphere present, final pyrolysis temperature, time-temperature path, and pressure. Although certain operating conditions may lead to increased product yield, achieving these conditions may result in increased costs. 

A. M. Li et al., Pyrolysis of solid waste in a rotary kiln influence of final pyrolysis temperature on the pyrolysis products, Journal of Analytical and Applied Pyrolysis, 50, 149-162 (1999). 

Another subtractive pre-column which shows potential utility is molecular sieve 5A. This material is known to selectively subtract straight chain organic species, although compounds other than alkanes are removed. Table II shows results obtained at different final pyrolysis temperatures for oil shale samples pyrolyzed in the thermal chromatograph. 

The studied variables were the heating rate and the final pyrolysis temperature. Their effects on the product yields, gas composition and energy recovery were analysed. The effect of the two cited variables on the specific surface area of the resulting char is also analysed. 

Fig. 3 also shows an interesting result, the percentage of CO increases considerably as final pyrolysis temperature does it as well, which is specially remarkable at 600 In the case of CO , its percentage remains practically independent of the final pyrolysis temperature up to around 750 C. For temperatures over 750 C, a certain increasing tendency is observed. The conqiarison of these results with those cited before [3] offers a new difference between them. In the referred case it was noticed that as final pyrolysis temperature increased, the percentage of each of the analysed compounds decreased (specially marked between 450 and 500 C) except for H ,... 

Fig. 5. shows both the percentage of energy recovery and the gas yield obtained for the different Tpj, studied. It can be noticed that both, the gas yield and the energy recovery increase as final pyrolysis temperature increases too. So the energy yield varies from a 3% in the case of Tpj, = 250 C up to 50% in the case of Tpi, = 900 "C. The main contribution to the energy recovery conies from the lower yield of pyrolysis gases such as H2 and hydrocarbons. 

The registered CO and CO2 profiles versus time obtained by l.R. equipment during the process corroborates what it has just been explained. Fig. 7 shows these profiles measured as percentage in volume of CO and CO2 in the exit gas for two different final pyrolysis temperature experiments, 650 and 750 C respectively. These two experiments have been selected for their graphical representation due to the fact that both correspond with the first leap in Fig. 6. Fig. 7 is expected to allow us to explain this leap. 

At 650 C some differences can be found at Tpf, of 650°C and 750 C. In the first case, during the isotherm stage it is observed a CO peak at the same time that a CO2 profile valley appears too. Later, at 10300 s a lower COj peak appears. For a final pyrolysis temperature of 750 C the CO rises rapidly up to 5,5%. Meanwhile the CO2 does not reach the 2%. 

Commonly, analytical pyrolysis is performed as flash pyrolysis. This is defined as a pyrolysis that is carried out with a fast rate of temperature increase, of the order of 10,000° K/s. After the final pyrolysis temperature is attained, the temperature is maintained essentially constant (isothermal pyrolysis). Special types of analytical pyrolysis are also known. One example is fractionated pyrolysis in which the same sample is pyrolysed at different temperatures for different times in order to study special fractions of the sample. Another special type is stepwise pyrolysis in which the sample temperature is raised stepwise and the pyrolysis products are analyzed between each step. Temperature-programmed pyrolysis in which the sample is heated at a controlled rate within a temperature range is another special type. 

The humic substances and soils were pyrolyzed in a type 0316 Curie-point pyrolyzer (Fischer, 53340 Meckenheim, Germany). The samples were not pretreated except drying and milling. The final pyrolysis temperatures employed were 573 K, 773 K and 973 K, respectively. The total heating time was varied between 3 and 9. 9 s. 

Figure 1. Per cent weight loss vs. initial weight of sample in crucible for different rates of heating and different final temperatures (Tf is final pyrolysis temperature achieved)...

Results to date strongly imply that, depending on the residence time, the tar yields for final pyrolysis temperatures above 300 0 will be independent of heating rates. This observation is strengthened by the finding from TGA data analysis that the apparent activation energy for cellulose decomposition is similar to that for tar formation. 

Since the temperature and the rate of heating the filament are completely variable, the instrument may control these parameters independently, as single steps or multiple steps. This gives the analyst the control to select any final pyrolysis temperature and to heat the sample to this temperature at any desired rate. Instruments are commercially available that heat as slowly as 0.01°C/minute and as rapidly as 30,000°C/second. 

As with all pyrolytic reactions, CARBONIZATION is a complex process in which many reactions take place concurrently, such as dehydrogenation, condensation, hydrogen transfer and isomerization. The final pyrolysis temperature applied controls the degree of CARBONIZATION and the residual content of foreign elements (e.g. at 1200K, the carbon content of the residue exceeds a mass fraction 90 wt%, whereas at 1600K, more than 99 wt.% carbon is found). 

Sample Origin Final Pyrolysis Temperature (460°C) Final Pyrolysis Temperature (750°C) ... 

Micro)furnace pyrolysers, which are preheated to the desired final pyrolysis temperature before introduction of the sample, are categorised as continuousmode pyrolysers. In such devices, the sample is either moved into a preheated pyrolysis chamber (isothermal mode) or heated rapidly from ambient to pyrolysis temperature (programmable mode). However, furnace pyrolysers are generally held isother-maUy at the desired pyrolysis temperature, and the samples are introduced into the hot volume. 

The thermal soak time can be different depending on the final pyrolysis temperature [24]. This parameter may be used to fine-tune the transport properties of a ear-bon membrane using a particular final pyrolysis temperature [91], Previous studies showed [41,87, 89, 91,92] that increments in thermal soak time would inerease the selectivity of carbon membranes. It is beheved that only mierostmctural rearrangement occurs during the thermal soak time, thus affeeting the pore size distribution and average porosity of carbon membranes [34]. 

In principle, one can determine which pyrolysis parameters are important and contribnte most significantly to the stmctural changes of the material. In that case, it wonld be possible to predict the trends of transport properties for a given carbon material more effectively. Recently, the precnrsors were pyrolyzed in a step-by-step fashion (i.e. after reaching a certain temperatnre, holding the membrane at that temperature for a period of time and then heating it again) imtil the final pyrolysis temperature is reached [34, 44, 51, 92, 93]. 

Alternatively, arbitrary pyrolysis conditions may be selected and the sample pyrolysed. The amount of residual organic material remaining can be determined by repyrolysing the sample at 980 C for 1 second for the ribbon probe and 10 seconds for the coil probe. A large residual peak under these conditions indicates that only a small part of the sample was initially pyrolysed. A higher final pyrolysis temperature should flien be used. 

Figure 47. Devolatilisation rate versus temperature at different heating rates of lignocellnlose at a final pyrolysis temperature of 800 "C...

Pyrolysers can be divided into two main categories on the basis of their mode of operation, i.e. the continuous type, where the sample is supplied to a furnace preheated to the final temperature, and pulse mode reactors in which the sample is introduced into a cold furnace which is then heated to the final pyrolysis temperature. In the analytical pyrolysis of solid and some liquid materials mainly pulse mode pyrolysers are used and the following sections will focus on a few of the most popular pyrolysis techniques utilizing this mode of operation. However, for pyrolytic studies of liquid and gaseous samples continuous pyrolysers are applied. 

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