Pyrolysis reaction, heat

The importance of these concepts can be illustrated by the extent to which the pyrolysis reactions contribute to gas produdion. In a moving-bed gasifier (e.g., producer-gas gasifier), the particle is heated through several distinct thermal zones. At the initial heat-up zone, coal carbonization or devolatilization dominates. In the successively hotter zones, char devolatihzation, char gasification, and fixed carbon... 

Gas phase thermal cracking of the volatiles occurs, reducing the levels of tar. Char (fixed carbon) and ash are the pyrolysis byproducts that are not vaporized. In the second step, the char is gasified through reactions with oxygen, steam, and hydrogen. Some of the unbumed char may be combusted to release the heat needed for the endothermic pyrolysis reactions. 

Figure 4.2 presents a simplified flow diagram of the ENCOAL Liquid from Coal (LFC) process. The process upgrades low-rank coals to two fuels, Process-Derived Coal (PDF ) and Coal-Derived Liquid (CDL ). Coal is first crushed and screened to about 50 mm by 3 mm and conveyed to a rotary grate dryer, where it is heated and dried by a hot gas stream under controlled conditions. The gas temperature and solids residence time are controlled so that the moisture content of the coal is reduced but pyrolysis reactions are not initiated. Under the drier operating conditions most of the coal moisture content is released however, releases of methane, carbon dioxide, and monoxide are minimal. The dried coal is then transferred to a pyrolysis reactor, where hot recycled gas heats the coal to about 540°C. The solids residence time... 

Metal hydrides generate hydrogen gas via reversible pyrolysis reactions, i.e., 2M + nH, 5 2MH + Heat (5.48)... 

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. 

Steam cracking is a complex, highly endothermic pyrolysis reaction. During the reaction, a hydrocarbon feedstock is heated to approximately 8()()°C and 34 kPa for less... 

Under pyrolytic conditions at temperatures above 300°C, generally within 500-800°C, the pyrolysis reaction forms alkenes by carbon-hydrogen bond scissions. An early experiment, where propane was heated to 575°C for 4 min in a silica flask, yielded propylene by dehydrogenation [Eqs. (2.19)-(2.21)] at a somewhat slower rate than it yielded methane and ethylene by cracking 54... 

The pyrolysis reaction is strongly endothermic so that one of the main problems in designing the reactor is to provide for sufficiently high rates of heat transfer. The volume calculated above would be made up of a series of tubes, probably in the range 50-150 mm diameter, arranged in a furnace similar to that in Fig. 1.5c. (For further details of ethylene plants see Miller ".)... 

Browne (5) described the pyrolysis reactions and events which occur in each of four temperature zones or ranges when solid wood of appreciable thickness is exposed to heat in absence of air. 

The most widely accepted theory of the mechanism of fire-retardant chemicals in reducing flaming combustion of wood is that the chemicals alter the pyrolysis reactions with formation of less flammable gases and tars and more char and water (4,5,8,21,24-29). Some fire retardants start and end the chemical decomposition at lower temperatures. Heat of combustion of the volatiles is reduced. Shafizadeh (21) suggests that a primary function of fire retardants is to promote dehydration and charring of cellulose. 

Temperature is the most important operating variable in pyrolysis reactions the large amounts of heat must be... 

The basic assumption inherent to heat transfer limited pyrolysis models is that heat transfer rates, rather than decomposition kinetics, control the pyrolysis rate. Consequently, thermal decomposition kinetics do not come into play, other than indirectly through specification of Tp. This approximation is often justified on the basis of high activation energies typical of condensed-phase pyrolysis reactions, i.e., the reaction rate is very small below Tj, but then increases rapidly with temperature in the vicinity of Tp owing to the Arrhenius nature, and the high activation energy, of the pyrolysis reaction. 

In kinetically limited models, the pyrolysis rate is no longer calculated solely from a heat balance at the pyrolysis front. Instead, the rate at which the condensed-phase is volatilized depends on its temperature. This gives a local volumetric reaction rate (kg/m3-s) by assuming that all volatiles escape instantaneously to the exterior gas-phase with no internal resistance, the fuel mass flux is obtained by integrating this volumetric reaction rate in depth. One consequence is that the pyrolysis reaction is distributed spatially rather than confined to a thin front as with heat transfer limited models and the thickness of the pyrolysis front is controlled by decomposition kinetics and heat transfer rates. For a pyrolysis reaction with high activation energy or for very high heat transfer rates, the pyrolysis zone becomes thin, and kinetically limited models tend toward heat transfer limited models. 

Thermolysis (pyrolysis) reaction (Section 22.1) A reaction caused by heat. 

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