Cellular Ceramics / 5

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5.5

Porous Burners

Dimosthenis Trimis, Olaf Picken cker, and Klemens Wawrzinek

5.5.1

Introduction

Combustion processes dominate the entire energy and transport sectors. The different burner types mainly depend on the fuel type (gaseous, liquid, or solid) and the application requirements (operating conditions, size, and emission limits). Although all possible fuel types are presently being used, there are clear signs that natural gas is becoming of increasing importance, especially in fields where lowemission thermal energy is needed. This is the case in domestic appliances such as household heating and hot-water supply systems, where low cost is another important factor that burner must have to be of interest to the market. In industrial processes such as drying, coating, and preheating, more stringent emission requirements and increased process efficiency are the driving forces for further developments in burner technology.

Two types of burner emissions are of major importance: 1) emissions due to incomplete combustion such as carbon monoxide and unburned hydrocarbons, and 2) hazardous combustion byproducts such as nitrogen oxides and sulfur oxides (greenhouse gases and/or toxic). Concerning emissions due to incomplete combustion, several measures can be applied, such as premixed combustion instead of diffusion-type flame, longer residence time in the reaction region, better insulation of the combustion chamber, higher combustion chamber temperature, more homogeneous mixing of the reactants, etc. Concerning combustion byproducts, the amount of sulfur oxides is related only to the sulfur content of the applied fuel (which is highest for solid fuels and lowest for natural gas), while the amount of nitrogen oxides, which are the most important nowadays in environmental protection, basically depends on the combustion temperature and the residence time at high temperatures. Thus, all efforts dealing with the abatement of nitrogen oxide emissions focus on reduction of the combustion temperature while taking into account that a spatially homogeneous minimum combustion temperature is compulsory to avoid incomplete combustion.

Several concepts for low-NOx burners emerged in the last two decades, especially for burners fired with natural gas. A reduction in combustion temperature is common to all concepts, but it is realized in different ways: exhaust gas recirculation, internal recirculation regions, staged combustion with heat decoupling, and direct

Cellular Ceramics: Structure, Manufacturing, Properties and Applications.

Michael Scheffler, Paolo Colombo (Eds.)

Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-31320-6

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and flame stabilization are discussed. The case of volumetric porous burners stabilized by flame quenching is discussed in more detail. Also an overview on the different structures, their characteristic properties, and the suitable applications is given.

5.5.2

Flame Stabilization of Premixed Combustion Processes in Porous Burners

Flame propagation of premixed fuel/oxidant mixtures in porous inert media depends on the structure and physical properties of the solid matrix and on the properties of the combustible gas. The resulting flame propagation modes can be classified into different regimes, of which some important parameters are given in Table 1. This chapter discusses only regimes in which no pressure gradient occurs in the reaction zone, that is, the low-velocity regime (LVR) and the high-velocity regime (HVR). The solid matrix strongly influences the reaction conditions of combustion. Heat transport in the burner is dominated by the properties of the solid material, especially under real operating conditions, where the porous matrix reaches high temperatures. Ceramic spheres and other packing material can be used, but cellular ceramics are the most interesting structures for practical applications. In Ref. [2] an overview on applicable porous media for porous burners is given.

Table 1 Flame propagation regimes in porous media [3–6].

Flame stabilization of premixed combustion in porous media differs significantly from that of free premixed flames. Stabilization depends mainly on the heat-trans- port properties of the solid matrix. Heat transport in a porous medium is often described by an effective thermal conductivity which comprises radiation and heat conduction of both solid and gas phases and additionally gas convection and dispersive mechanisms (see Chapter 4.3). The effective heat transport inside of a porous medium is 2–3 orders of magnitude higher than in free flames and can be considered the dominant parameter for flame propagation in most cases. Compared with free premixed flames, the higher heat transport leads to 10–30 times faster flame

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Combustion in porous media

Premixed Non-premixed

Fig. 2 Overview of flame stabilization methods in porous media.

propagation [7, 8], which hampers flame stabilization in porous media. For solving the problem of flame stabilization in porous media, different approaches have been developed, which are summarized in Fig. 2.

In the LVR and HVR regimes most relevant to burner applications, flame propagation or extinction can be described by a modified P clet number Pe formulated with the laminar flame speed SL instead of a flow velocity

where dp,eff is an equivalent porous cavity diameter, cp,f the specific heat capacity, f the density, kf the thermal conductivity, and af the thermal diffusivity of the gas mixture. The modified P clet number describes the ratio between the heat release due to combustion in a pore and the heat removal on the walls of a pore. It also describes the ratio of the characteristic porous cavity diameter to the laminar flame front thickness. This ratio must exceed a critical value for flame propagation in a cold porous medium. According to Ref. [3] flame propagation in porous media is possible if:

If Pe < 65 then the flame structures extinguish (quenching), since heat is transferred to the porous matrix at a higher rate than it is produced. Flame propagation in porous media in the LVR and HVR regimes (Table 1) is mostly dominated by convection but the high conductivity and the radiation properties of the solid matrix also influence the flame speed, especially at higher temperatures. The quantity dp,eff represents an equivalent length scale of the pore size for heat transport. Whether or not flame propagation will occur in a porous inert medium can be decided by the choice of the pore size of the solid matrix. Thus, a critical pore size exists above which flame propagation and below which flame quenching occurs. Nevertheless, flame propagation may also occur in subcritical cavities if the temperature of the matrix is high enough that reactions are not quenched by the low wall temperature.

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Thus, combustion processes inside subcritical porous media need a special starting procedure to preheat the porous matrix to the operating temperature. This kind of operation is often called filtration combustion.

5.5.2.1

Flame Stabilization by Unsteady Operation

Combustion in subcritical porous media (low characteristic porous cavity size) will only take place if the temperature of the matrix is high enough to ignite the mixture. Flame propagation is driven mainly by the thermal conductivity of the solid material and interphase heat exchange, while the heat transport in the gas phase is negligible. The so-called combustion wave travels very slowly through the porous matrix with a speed of 10–5 to 10–4 m s–1 [9–11], either in or against the flow direction. In the former case superadiabatic and in the latter subadiabatic combustion temperatures occur (in comparison to the adiabatic flame temperature). The direction of the wave is mainly dependent on the heat capacities of the solid and gas phases and on interphase heat exchange. For a standing wave only one operational point is possible for a certain gas mixture. However, the very low speed of the combustion wave allows operation with varying gas velocities by changing the flow direction when the combustion wave reaches the end of the reactor. Figure 3 shows the temperature profiles in a reactor with a subcritical porous matrix. In this case the combustion wave travels in the flow direction. The heat of combustion heats the porous matrix downstream of the combustion wave.

When the flow direction is reversed, the hot porous matrix preheats the fresh gas mixture, and superadiabatic combustion occurs [10, 11]. With this principle gas mixtures with very low heat content can be burned. The flame speeds in such reactors are about 2–4 times higher than the laminar flame speed [12, 13].

shift of combustion wave with time

gas mixture

Fig. 3 Scheme of a subcritical porous reactor in alternating operation.

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5.5.2.2

Flame Stabilization under Steady Operation by Convection and Cooling

In porous media the same flame propagation mechanisms act as in free flames, but the higher effective thermal conductivity must be considered. The effective thermal conductivity and thermal diffusivity are 2–3 orders of magnitude higher than in a gas. Following the simplified theory of flame propagation [14] the flame speed S is

where af is the thermal diffusivity, and s a characteristic timescale of the reaction. From this it results that the flame speed in porous media is 10–30 times higher than the laminar flame speed. One possibility to stabilize combustion in porous media with supercritical pore sizes (flame propagation is possible) is to induce a change in flow speed by a stepwise or continuous change in the cross sectional area (convective stabilization). For methane/air mixtures in porous media the required flow speed to avoid flashback lies in the range of 4–12 m s–1, which is about 10–30 times higher than in free flames. The exact velocity depends on the heat-transport properties of the applied porous structures and the operating conditions. This type of flame stabilization was already used in the early 1900s [15–17]. The major drawback of this flame-stabilization principle is that the power modulation range is rather small, and for low powers with corresponding low flow velocities, flame flashback may occur.

Another possibility for convective stabilization is active control of the air and/or the fuel flow rate based on temperature measurements in the porous matrix. By changing the air and/or the fuel flow rate, the flow velocity and the flame speed changes and the flame front can be stabilized by active control in a desired position within the porous matrix [18]. This stabilization principle can be used for subcritical and for supercritical pore sizes with respect to the flame-propagation criterion according to Eq. (2), but the operational range is small, and for each configuration a single operating condition is found by active control.

Another possibility for flame stabilization is to cool the reaction zone, for example, by embedded water-cooled tubes in the main reaction zone, which was also already realized in the early 1900s [19]. In Fig. 4 (left) the principle of flame stabilization by cooling of the reaction zone is schematically shown. Combustion takes place inside the pores, and the heat of reaction is conducted to the embedded water tubes.

Cooling of the reaction zone can also alternatively be realized by intense radiation from it. This principle is used nowadays in surface radiant burners, which are widely used in domestic and industrial appliances. As shown in Fig. 4 (right) the combustion region must be located close to the surface of the porous matrix in order to extract heat by radiation. However, the combustion process may be completed outside of the porous matrix, depending on the operating conditions. The porous surface extracts heat from the flame and radiates it to the environment. Effective heat transport by radiation only occurs at high temperatures above the ignition limits. This means that for radiation-cooled burners the porous matrix must be subcriti-

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Fig. 4 Flame stabilization in porous media by cooling.

cal with respect the modified P clet number (Eq. (1)), because otherwise the flame could move into the porous matrix, which eventually would lead to flashback. A significant drawback of this stabilization principle is that by increasing the heat load and correspondingly the flow rate of the incoming gas/air mixture, the flame location moves completely outside of the porous matrix and the burner operates in the so called blue-flame mode with significantly increased emissions. Thus, the beneficial combustion process at least partly inside of the porous matrix is only possible for a small range of power modulation (see Section 5.5.4).

5.5.2.3

Flame Stabilization under Steady Operation by Thermal Quenching

Research and development on stationary combustion completely trapped in the cavities of porous inert media started in the early decades of the 1900s. Bone [20] designed the first boilers and muffle heaters, while Lucke [21] built radiant room heaters, crucible furnaces, and cooking stoves, operating with town gas. However, even earlier pioneers such as Welch [22], Mitchell [23], Ruby [24], and Schnabel [25] developed the first appliances for different fields, most probably independently of each other. The porous media used in the early stages were packings of aluminosilicate pebbles or spheres. Also in the 1930s [26] and up to the present time [18, 27–30], there was limited but continuous research and development activity in porous-medi- um combustion. Major drawbacks in porous-medium combustion were the deficiencies of the flame stabilization concepts for a stationary combustion process completely trapped inside a porous medium.

A novel combustion technique based on combustion in porous media was developed recently [2, 31–35]. The major novelty of this work is the combustion stabilization principle based on thermal quenching, which allows extremely stable operation of the premixed combustion process in the porous matrix for a wide range of power

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mainly heat-transport processes are considered and, if diffusive mass transport becomes dominant, incorrect critical pore diameters may be predicted.

5.5.2.4

Diffusive Mass-Transport Effects on Flame Stabilization

The influence of diffusive mass transport on the stabilization of a flame in a porous medium is not well known. In Refs. [3, 36] flame instabilities were observed for gas mixtures containing hydrogen. These instabilities were linked to the increased influence of diffusive mass transport, which decreased the dominance of the heat transport in the flame stabilization. The ratio between diffusive mass transport and heat transport can be described by the Lewis number Lec of a component c of the gas mixture:

which is the ratio between the temperature diffusivity af and the diffusion coefficient of the component Dc. According to Refs. [3, 37] the flame structure changes for Lewis numbers smaller than unity. For many widely used gases (e.g. methane/air mixtures), Le is close to unity and therefore its influence is often neglected. However, mixtures with Lewis numbers far below unity require the consideration of mass diffusion. For example hydrogen/air and hydrogen/chlorine mixtures have Lewis numbers of about 0.4. For such low values one can expect a strong influence of diffusion on flame stabilization in porous media. This influence is neglected in the previously described P clet number criterion, because only the laminar flame speed is considered, which does not account for deformations of the flame front. This means that the constant value of the critical P clet number is not sufficient for the design of a porous burner flame trap at low Lewis numbers.

Recent flame propagation experiments in porous media for mixtures with different Lewis numbers show the dependence of the critical P clet number on the Lewis number [38]. The flame propagation limits were experimentally determined in packed beds of spheres. For each packing the critical P clet number was calculated for the lean and rich flame propagation limits with the laminar flame speed at the corresponding equivalence ratio. The average of the critical P clet numbers of different packings was plotted against the Lewis number (Fig. 6). For Lewis numbers close to and larger than unity the criterion of Ref. [3] of Pecrit » 65 could be confirmed, but for mixtures with lower Lewis number the critical P clet number was found to lie far below this limit.

The results of this experimental study show that combustion gas mixtures with a high diffusivity compared with the thermal diffusivity of the mixture, that is, Lewis numbers less than unity, have smaller critical pore diameters or critical P clet numbers than mixtures with Lewis number equal to or larger than unity. The Lewis number must be calculated for the reaction component that is lacking for a stoichiometric mixture. The results of Fig. 6 can be used for better design of flame traps for highly diffusive mixtures.

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Fig. 6 Critical P clet number versus Lewis number [38].

5.5.3

Catalytic Radiant Surface Burners

Most catalytic surface burners operate with nonpremixed (i.e., diffusion) combustion. The gas flows through the flat burner support, which is coated with a catalyst (in most cases noble metals such as Pt and Pd distributed on a wash coat layer, e.g., c-alumina) on the combustion side. Ceramic burner supports may have the form of a porous plate or a flexible fiber mat. The oxygen for the combustion process is provided by the surrounding air and is transported to the catalytic combustion region by diffusion processes. The surface power load is very limited due to these diffusion processes, and typical values are in the range up to 50 kW m–2. The free radiating surface at such low heat loads leads to a very low surface and combustion temperatures in the range up to 600 C. Combustion at such low temperatures is only possible in the presence of a catalyst. According to Wien’s displacement law [39] the radiation maximum moves to shorter wavelength the higher the temperature of the emitting surface is. Thus, catalytic gas IR burners have a relatively long wavelength of the emitted radiation. Due to the relatively low temperatures and the flashbacksafe diffusion mode of operation, the material requirements for the burner support are relatively low, and the main focus of such developments lies on the catalyst technology.

Premixed catalytic radiant surface burners have not reached wide commercial usage yet and are mainly the subject of research activities. In most cases premixed catalytic supported combustion [40] is performed (in contrast to full catalytic combustion) at surface temperatures of about 1100 C.