Pyrolysis reaction, heat requirement

The previous discussion on chemistry showed that oil components are intermediates that are susceptible to consecutive decomposition reactions, i.e., cracking and condensation. It is therefore essential to remove them from the reaction environment as soon as they are formed. In other words, the volatile components should have a very short residence time. The Pyrolysis process also requires the supply of heat to drive all these cracking and decomposition reactions. An elegant answer to these two challenges has been proposed and developed by van Swaaij et al. [30] and forms the basis of the Pyrolysis process that is commercialized by BTG, the Biomass Technology Group BV [31]. 

Pyrolysis of biomass is defined as the chemical degradation of the biopolymers (cellulose, lignin and hemicellulose) constituting the wood fuel which initially requires heat. As can be seen in Figure 51, all reaction pathways making up the pyrolysis are not endothermic, which implies that some of the pyrolysis reactions generate heat. However, overall the pyrolysis process is endothermic. 

One way to produce a rapid heat transfer to the sample is to diminish the sample size [5]. This implies that the amount of heat required by the sample to reach a certain temperature is small and that the heat can be transferred rapidly. Typical sample sizes in analytical pyrolysis vary from a few ng to a few mg. A small sample size is, however, related to other effects, some advantageous and some not. Secondary reactions during pyrolysis are diminished for a small sample, but the contact with metal surfaces may... 

With but one exception, past studies of the flash pyrolysis of cellulose using laboratory equipment have relied on radiation to achieve the high heating rates required. Lincoln (11) used pulsed carbon arcs and xenon flash tubes to achieve the complete volatilization of cellulose by pyrolysis reactions. Berkowitz-Mattuck... 

To examine the pyrolysis reaction of the scrap tires, pulverized scrap tires were heated in a quartz tube by an electric furnace (h). Then the continuous pyrolysis test plants in Table I were constructed to get the engineering data required for the design of an actual plant,... 

Much like thermal blankets, thermal well systems do not require costly excavation and they also offer additional benefits. They have been used to treat contaminants at depths up to 5.5m below the surface and much of the contaminants are destroyed in situ through oxidation or pyrolysis reactions.Furthermore, thermal well systems offer uniform heating and consequent treatment of contaminants is effective across a wide range of soil types. The long residence time favors desorption mechanisms that may be time dependent. ... 

In a dense-bed fluidized bed the preheated shale is further heated to and held at the retorting temperature for sufficient time to complete the pyrolysis reactions (Figure 3). The total inventory of shale in the retorting vessel is determined by the required residence time for complete kerogen conversion and the shale throughput. The retort heat requirements are supplied by ceramic balls which circulate in the inner loop. They are reheated in a separate vessel which may operate as a moving bed, raining pellet bed, or entrained flow heater. 

The test begins by slowly introducing the solid fuel feed into the bed via the screw feeder. The bed temperature immediately drops because of the sensible heat required to heat the solid to the reaction temperature plus the heat of pyrolysis. The solid feed rate is carefully adjusted so that the bed temperature does not drop below the desired 1400-1500°F range. The reaction system is allowed to come to a new steady-state condition with a constant splids feed rate, and the feed rate of the solids is determined by weight difference. 

Heat Requirement for the Pyrolysis Reaction. Another engineering parameter to be considered when designing a full scale pyrolysis... 

In Table V, the maximum conversion which has been reached is similar for the four reported experiments. It can be concluded that the heat required for vacuum pyrolysis of aspen wood between 1.3 and 3.9 kPa is about 733 kJ kg" of organic wood converted. Overall, the reaction is slightly endothermic. 

Heat required for the pyrolysis of wood is slightly endothermic and was determined to be 733 kJ kg of organic wood converted. The standard heat of reaction at 323 K and 1.6 kPa was found to be approximately 92 kJ kg" of air-dry wood. The thermal efficiency of the process is high at 82%. The mean residence time of the gas and vapour phase product in the reactor was empirically found to be about 40 s. Heat is mainly transferred by radiation in the type of reactor used. 

Pyrolyses For reactions that require high temperatures but which take place at atmospheric pressure (e.g., acetate pyrolysis, ketene formation), it is usually convenient to use a pyrolysis apparatus (Fig. 1-5). An electrically heated cylindrical furnace (cfl. 70) fitted with a thermocouple is clamped in a vertical position. The pyrolysis tube, which is made of Pyrex (usable up to 600°), Vycor (to 1200°), or quartz (to 1300°), is inserted then an addition funnel, a three-necked flask, and a condenser are assembled as shown. It is usually advantageous to pack the pyrolysis tube with glass beads, glass helices, or porcelain chips to increase the surface area within the tube. 

By contrast, pyrolysis of tranSytrans-, 5-cyc odtcdid txvt gave ra -l,2-divinyl-cyclohexane upon heating to 40°C with a half-life of 144 min. This reaction too requires a chair-like transition state (Scheme 11.102). 

In the process, the coal undergoes three processes in its conversation to synthesis gas (syngas) the first two processes, pyrolysis and combustion, occur very rapidly. In pyrolysis, char is produced as the coal heats up and volatiles are released. In the combustion process, the volatile products and some of the char react with oxygen to produce various products (primarily carbon dioxide and carbon monoxide) and the heat required for subsequent gasification reactions. Finally, in the gasification process, the coal char reacts with steam to produce hydrogen (Hj) and carbon monoxide (CO). 

Phosphorus is known as an effective char promoter. Charring limits the release of fuels to the flame and therefore reduces the heat released. Moreover, the char accumulates on the surface of the material and can act as a protective layer which limits the heat transfer from the flame to the condensed phase and the gas transfer from the pyrolysis zone to the flame. This phenomenon is called the barrier effect. Nevertheless, it should be noted that the presence of char is not a sufficient condition to observe an effective barrier effect. Its structure (cohesion, porosity, thickness) is also veiy important but rarely studied. Thermal stability is another important parameter to assess the reaction to fire of a material. Indeed, the higher is the degradation temperature of a polymer, the greater is the heat required to start its pyrolysis. Table 12.2 lists the effects of phosphorus-containing groups on the thermal stability and charring for a variety of polymers. 

Flame retardants can act chemically and/or physically in the condensed phase and/or in the gas phase. In reality, combustion is a complex process occurring through simultaneous multiple paths that involve competing chemical reactions. Heat produces flammable gases from the pyrolysis of polymers and if the required ratio between these gases and oxygen is attained, ignition and combustion of the polymer will take place. 

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