Char combustion

In die presence of oxygen, more complex thermo-oxidative processes occur in polyesters containing aliphatic moieties. They result in crosslinked products and in the formation of compounds such as aldehydes, carboxylic acids and vinyl esters, as reported in the case of PET.93,94 On the other hand, the presence of oxygen has little effect on the thermal resistance of wholly aromatic polyesters below 550°C. Above this temperature a char combustion process takes place.85... 

The high temperatures of coal char oxidation lead to a partial vaporization of the mineral or ash inclusions. Compounds of the alkali metals, the alkaline earth metals, silicon, and iron are volatilized during char combustion. The volatilization of silicon, magnesium, calcium, and iron can be greatly enhanced by reduction of their refractory oxides to more volatile forms (e.g., metal suboxides or elemental metals) in the locally reducing environment of the coal particle. The volatilized suboxides and elemental metals are then reoxidized in the boundary layer around the burning particle, where they subsequently nucleate to form a submicron aerosol. 

Moors, J.H.J. Pulverised char combustion and gasification at high temperatures and pressures, Ph.D. thesis, Eindhoven University of Technology, The Netherlands, (1999). 

This review defines the thermochemical conversion processes of solid fuels in general and biofuels in particular that is, what they are (drying, pyrolysis, char combustion and char gasification) and where they take place (in the conversion zone of the packed bed) in the context of the three-step model. 

The conversion process occurs both on macro- and micro-scale, that is, on single particle level and on bed level. In other words, the conversion process has both a macroscopic and microscopic propagation front. One example of the macroscopic process structure is shown in Figure 10. The conversion front is defined by the process front closest to the preheat zone, whereas the ignition front is synonymous with the char combustion front. 

Appendix B includes a review and a classification of conversion concepts. It also investigates the potentials to develop an all-round bed model or CFSD code simulating the conversion system. This review also contains a great deal of information on the heat and mass transport phenomena taking place inside a packed bed in the context of PBC of biomass. The phenomena include conversion regimes, pyrolysis chemistry, char combustion chemistry, and wood fuel chemistry. The main conclusions from this review are ... 

The thermochemical conversion of solid fuels is a complex process consisting of drying, pyrolysis, char combustion, and char gasification. 

The fuel bed (packed bed) is a two-phase system, also referred to as a porous medium [20]. Thermochemical conversion processes, such as drying, pyrolysis, char combustion and char gasification, take place simultaneously in the conversion zone of the fuel bed (Figure 16). They are extremely complex, and are reviewed more in detail in section B. 4. Review of thermochemical conversion processes. 

To approach the analysis of, and to be able to comprehend, the complex phenomena of thermochemical conversion of solid fuels some idealization has to be made. For a simplified one-dimensional analysis, there is an analogy between gas-phase combustion and thermochemichal conversion of solid fuels, which is illustrated in Figure 41. Both the gas-phase combustion and the thermochemical conversion is governed by a exothermic reaction which causes a propagating reaction front to move towards the gas fuel and solid fuel, respectively. However, there are also some major differences between the conversion zone and the combustion zone. The conversion front is defined by the thermochemical process closest to the preheat zone, which is not necessarily the char combustion zone, whereas for the flame front is defined by the ignition front. In practice, many times the conversion zone is so thin that the ignition front and the conversion front can not be separated. 

The conceptual model of the conversion of a single particle is usually assumed to be divided into two phases, namely the flame phase and the glowing phase, see Figure 42. Due to the evolution and combustion of volatiles during the flame phase, oxygen is prevented from reaching the particle surface [57], Consequently, char combustion cannot start until the pyrolysis has finished. 

The char combustion phenomenology has been reviewed by many researchers [11,26,73]. It is a very complex process and is usually divided into three char combustion regimes, namely (I), (II) and (III) [23,54,74,75]. The combustion regimes are consequences of the initial size and temperature of the char particle, see Figure 55. 

Figure 56 above describes the phenomenology of the char combustion regime (III). The concept of the shrinking core or shrinking particle model is usually applied in mathematical modelling of char combustion in regime (III). 

The oxygen diffuses through the boundary layer to the particle surface and countercurrent diffusion of char combustion products (carbon monoxide and carbon dioxide), see Figure 56. [73,77]... 

Step one is, oxygen diffusion in the porous system of the particle inwards to the char combustion front and the reaction site, (2) adsorption of oxygen to the active sites on the intraparticle char phase, (3) oxidation reaction with carbon, and (4) desorption of... 

The char combustion is sustained by its own heat release. The heat release and heat transport is thereby coupled with the oxygen transport, which is usually the controlling factor. The heat evolved from reaction is transported by heat conduction and convection out of the particle. [73]... 

The heat and mass transport phenomena of the char gasification is not described in the literature as much as for the char combustion [11,28,78]. There are good reasons to believe that it is quite analogous to the char combustion phenomenology [79]. However, the heterogeneous gasification reactions are overall endothermic which results in some differences with respect to the intraparticle heat transport [79]. 

Figure 58A-C shows that the heat source promoting the drying process changes during the course of the batch combustion. At times to and ti there is an artificial heat source located in the over-bed section. At time t2 the flaming combustion has started. The flames feed back heat to the bed by means of radiation. At time t3 the basic heat flow comes from the char combustion and the ignition front, by means of conduction and radiation. [12,24]...

Pyrolysis commences at bed surface temperatures in the range of 150-300°C [22,23]. Almost simultaneously, flaming combustion takes place in the combustion system above the fuel bed (see Figure 58C). At t2 the pyrolysis is sustained by heat from over-bed flames. The heat is transported by radiation. At times ts to t4 the dominant heat source has changed to the char combustion zone (ignition front) instead. The heat from the ignition front is also transported by means of conduction and radiation. 

The heat and mass transport on the small scale during char combustion is similar to the single particle behaviour. The char combustion products generated in the intraparticle phase (see Figure 56) enter the interstitial gas flow, which transport it out of the bed by convection. 

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