Reactive atmosphere pyrolysis

Atmospheric Conditions. In addition to complete combustion, wastes may be destroyed by treatment at high temperatures either without oxygen (qv) (pyrolysis), usiag limited oxygea (partial combustioa), or ia reactive atmospheres (gasiftcatioa), such as those containing steam (qv), hydrogea (qv), or carboa dioxide (qv). 

A variety of other ceramics are prepared by pyrolysis of preceramic polymers.32,38 Some examples are silicon carbide, SC, tungsten carbide, WC, aluminum nitride, AIN, and titanium nitride, TiN. In some cases, these materials are obtained by simple pyrolysis in an inert atmosphere or under vacuum. In other cases a reactive atmosphere such as ammonia is needed to introduce some of the atoms required in the final product. Additional details are given in Chapter 9. 

Other Factors. Several other factors influence, at least to some extent, the course of the pyrolysis process. These include particle size, bed configuration, pressure/vacuum during pyrolysis, nature of the coal ash, secondary reactions, etc.37 It is beyond the scope of this chapter to consider these items, but the interested reader can find additional information in the literature, including reports on pressure effects,21,38 effect of vacuum,23 effect of inorganics,26,39 and effect of a reactive atmosphere.23,40... 

Finally, we will show that this innovative processing route can be extended to the production of bulk ceramic components by pyrolysis in reactive atmosphere, such as for example, production of SiAlON ceramics by nitridation of PAIC in ammonia flow [75]. 

Preparation and Characterization of SiAlON Ceramics by Pyrolysis in Reactive Atmosphere... 

A major advantage of plasma processing is that the heat input may be accomplished in an atmosphere of any desired composition and reactivity. In practice there are only a few variations of chemical strategies available for thermal processing i.e. pyrolysis, oxidation, reactions with hydrogen and water. They were already reported elsewhere [5]. The most cost effective and friendly to the environment are the approaches of plasma employing for zero-waste fuel generation or for zero-waste incineration. 

The aqueous-phase pyrolysis can also be assisted by catalysts and reactive gases. For instance, the PERC process [3, 33] produces a pyrolysis oil upon dissolving wood chips in a recycle pyrolysis oil, mixing the slurry with a water-Na2C03 solution and treating the resulting mixture at 370 °C and 275 bar under synthesis gas atmosphere. 

H. R. Linden High temperature pyrolysis of coal with high energy sources seems to follow readily predictable paths similar to hydrocarbon pyrolysis. The effects of pressure, gas atmosphere, reaction time, and the volatile matter" content of the coal bear the same relationship to yields of methane, ethane, ethylene, acetylene, and hydrogen as for simple hydrocarbons. Effective reaction temperature, although not directly measurable, could be estimated by means of a suitable chemical thermometer, such as the C-. H-. -C. H4-H. system which approaches equilibrium very rapidly. As Dr. Given also noted, equating the volatile matter" to the reactive portion of the coal is an oversimplification but adequate for empirical purposes the C H ratio of the coal would probably be more suitable. 

Pyrolysis of the silylcarbodiimides at 1 000 °C in argon atmosphere leads to ceramic materials in the ternary system Si-C-N. The ceramic yield strongly depends on the substituents at the silicon atoms. Reactive groups such as Si-H and Si-vinyl can be cross-linked during pyrolysis which results in higher ceramic yields (Si-H substituted silylcarbodiimides 70 % Si-vinyl substituted silylcarbodiimides 64 %). In contrast to these results cross-linking of the dimethyl substituted silylcarbodiimide occurs to a lower extent and therefore a lower ceramic yield (28 %) is obtained. 

Polymeric silylcarbodiimides 1 and 2 react with dimethylsulfldborane forming a gel which leads to highly cross-linked, insoluble polymers IHB and 2HB, respectively. Whereas 1 can only be cross-linked by the carbodiimide group, the addition of borane in 2 mainly takes place at the vinylic sites due to their higher reactivity. Pyrolysis of IHB and 2HB at 1200 °C in argon atmosphere results in new SiBCN ceramics (ceramic yields IHB 61 %, 2HB 64 %). 

Pyrolysis can be conducted at various temperature levels, reaction times, pressures, and in the presence or absence of reactive gases or liquids, and of catalysts. Plastics pyrolysis proceeds at low (<400°C), medium (400-600°C) or high temperature (>600°C). The pressure is generally atmospheric. Subatmospheric operation, whether using vacuum or diluents, e.g. steam, may be selected if the most desirable products are thermally unstable, e.g. easily repolymerizing, as in the pyrolysis of rubber or styrenics. 

Figure 14.4 Schematic representation of an apparatus for FVP As with all high vacuum work, care must be taken. After all of the substrate has passed through the hot tube, turn off the furnace and allow to cool to room temperature (still under vacuum). Then turn off the pump and admit nitrogen to atmospheric pressure. Remove the traps to a fume cupboard and allow to warm to room temperature, and work up in the usual way. If the desired product is unstable towards air, water, or is simply very reactive, then a more sophisticated pyrolysis system might be required, and more elaborate work up procedures used.

Sealed vessel pyrolysis is another pyrolysis type that is performed in furnace type pyrolysers. In this type of pyrolysis, the sample is heated for a relatively long period of time, in a sealed vessel, generally at relatively low temperature (below 350° C). The pyrolysis products are further analyzed, commonly by off-line procedures (GC, GC/MS, FTIR, etc). The technique allows the pyrolysis to be performed for as long as months and to use different atmospheres (inert or reactive) [17a]. The procedure is not used only as an analytical tool, and it can be seen as a preparative pyrolysis technique. 

---