ABSTRACT
CONTENTS 15.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 607 15.2 Physical and Mathematical Description of Combustion in a PIM . . . 610
15.2.1 Physical Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 610 15.2.2 Mathematical Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613
15.3 Heat Transfer in Porous Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615 15.3.1 Effective Thermal Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615
15.3.1.1 Packed beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615 15.3.1.2 Consolidated porous media . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616
15.3.2 Convective Heat Transfer Coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . 617 15.3.2.1 Packed beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 617 15.3.2.2 Consolidated porous media . . . . . . . . . . . . . . . . . . . . . . . . . . . . 618
15.3.3 Radiation Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 619 15.3.3.1 Radiation characteristics of packed beds . . . . . . . . . . . . . 620 15.3.3.2 Radiation characteristics of open-celled materials . . 621
15.4 Overview of Porous Medium Based Combustors . . . . . . . . . . . . . . . . . . . . . . 622 15.5 Premixed Porous Medium Combustors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 624 15.6 Porous Medium Combustor-Radiant Heater . . . . . . . . . . . . . . . . . . . . . . . . . . . 628 15.7 Premixed Porous Medium Combustors-Heaters. . . . . . . . . . . . . . . . . . . . . . . 634 15.8 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 637 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 639 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 640
Combustion processes in porous media are of great practical importance and are encountered in numerous technological applications and systems such
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as VOCs oxidation, packed bed incinerators, regenerative-type combustors, porous radiant burners, catalytic reactors and converters, direct energy gas conversion devices and systems, in situ coal gasification, high-temperature materials synthesis processing, smoldering of foam and cellulosic materials, combustion of wood and agricultural waste, cigarette burning, and many others. Numerous applications of theporousburner technology in energy and thermal-engineering and processing industries have been identified which are based on stabilized combustion in porous media [1-3]. When exothermic chemical reactions release sufficient energy, continuous chemical reactions can be sustained in porous media. Depending on the physical and chemical nature of the porous materials, combustion in porous media can be classified into three main types: (a) inert, (b) catalytic, and (c) combustible [4]. The classification is somewhat arbitrary but it reflects the wide range of current technological applications. The discussion in this chapter of the handbook focuses exclusively on combustion in porous inert media (PIM). Combustion of a gasmixturewithin the voids of a porousmediumhas char-
acteristics that are different from those observed in other (i.e., gas phase only) systems. This is owing to the fact that the thermophysical properties of the solid and gas phases are vastly different, and there is enhanced conduction heat transfer in the solid matrix. The “long range” radiation heat exchange between the surface elements of the solid phase and the large interfacial surface area per unit volume contribute to effective heat transfer between the gas and the solid phases. The energy release during the chemical reactions is intimately coupled to heat transfer (i.e., extraction or addition to the flame) as well as advective energy transport, and flammability limits aswell as stability ranges that are different from those encountered in conventional designs. Combustion in a PIM-based system can be characterized as a heat recircu-
lating device in which the reactants or combustion air alone are preheated using heat “borrowed” from beyond the flame zone without mixing the two streams [5,6]. The concept of heat recirculation is illustrated schematically in Figure 15.1 for an adiabatic combustion system. A variety of such systems has been identified by Weinberg [5] and the comprehensive review has been updated [6]. Combustion systems of this kind which take advantage of heat recirculation are sometimes being referred as “excesses enthalpy,” “super-adiabatic flame temperature” or “filtration” combustion. Although the principle of heat recirculation is straightforward, the consequences of its application can be far reaching concerning the process efficiency, fuel conservation, combustion intensity, and pollutant emissions. In the absence of conclusive observations, the consensus of opinion is that
four types of combustion are possible in inert porous media: (1) free combustion takes place when a flame forms (say, above the porous burner surface) that consists of small multiple flames; (2) surface combustion occurs when the flame is “anchored” at the surface with some chemical reactions occurring within the pores, and the combustion occurs when the flow rate of the reactant mixture is set such that the gases reach their ignition temperature inside the medium and the mixture burns just under the surface; (3) buried
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(embedded) combustion occurs within the medium in a stable fashion when the mixture velocity is equal to the flame speed for the local temperature and heat loss conditions; and (4) unstable combustion (i.e., flashback) occurs when the flame speed exceeds the mixture velocity. The difference between a surface and embedded (buried) porous burners is highlighted schematically in Figure 15.2. As illustrated in Figure 15.2(a), the fuel-oxidant mixture passes through the PIM and then combusts partly near/or entirely in the downstream gas phase in the vicinity of the PIM. Actually, the buried flame
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combustion shown in Figure 15.2(b) is also corrugated and is discontinuous like the surface flame in Figure 15.2(a). Porous burners operate on the combustion stabilization principle, which
allows stable operation of the premixed combustion process in the porous matrix. The most important criterion which determines whether or not combustion can take place inside the porous structure is the critical pore size. If the size of the pores is smaller than this critical dimension, flame propagation inside the porous structure cannot be sustained; the flame is always quenched. The experiments of Babkin et al. [7] established the limiting condition in terms of the modified Péclet number, Pe = SLdmρcp/k > 65, where SL is the laminar flame speed, dm is the equivalent pore diameter, and cp, ρ, and k are the specific heat, density, and thermal conductivity of the gas mixture, respectively. If Pe ≤ 65 flame quenching occurs since heat is transferred to the porous matrix at a higher rate than is generated due to the chemical reactions. Premixed combustionwith the flame stabilized in aPIM is a newand innov-
ative technology that is promising for a variety of applications but which has not been discussed in textbooks [8] or reference books [9]. Recent accounts [1,4,10] provide excellent overviews of combustion in porous media, along with extensive citations to the current literature. It is difficult, in a limited space, to provide the reader with a fair and complete account of fundamentals and applications of combustion in porous media, particularly when the field is developing actively around the world. The best that can be hoped for is that this chapter will serve as a useful source of references and background information for both the students and practicing engineers working in the fields of combustion and thermal engineering. As already alluded to, the field of combustion in porous media is very broad and wide ranging; therefore, the discussion and scope in the chapter is exclusively focused only on stable combustion with the reaction zone embedded in the PIM.
