Cellular Ceramics / 5

494Part 5 Applications

5.5.4

Radiant Surface Burners

Radiant surface burners operate by stabilizing a premixed flame near or partially inside a noncombustible porous burner support. The enthalpy of combustion released in the gas phase heats the porous matrix, which then emits thermal radiation to a heat load. In small-scale applications radiant surface burners have already shown performance gains over conventional open-flame burners in the form of higher efficiencies, lower NOx emissions, and more uniform heating.

In most cases flat plate geometries with many small holes or with a volumetric foamlike porous structure are used as burner support. A flat flame sheet is produced on the surface of the ceramic plate, and depending on its position relative to the ceramic plate, it heats the ceramic plate, which radiates heat to the appliance.

The ceramic supports may be sinteror fiber-based, while the materials range from mullites and alumina up to silicon carbide fibers. Important properties of the structures are their effective thermal conductivity, optical thickness, and emissivity, which affect the flashback behavior and the radiation properties. Especially the effective conductivity (which describes the effective heat transport inside the porous structure due to conduction, dispersion, and radiation, see Chapter 4.3) is of special importance with regard to flashback safety. In general, flashback safety is increased if the heat load is increased and the ceramic burner support is cooled more effectively by convection of the fresh air/gas mixture. Ceramic supports with high effective conductivity transport a high heat flux through the burner support to the back side where the upstream air/gas mixture is located. As a result the temperature of the ceramic support increases on the back side and may ignite the incoming air/gas mixture (increased tendency for flashback). At the same time the temperature of the radiating surface decreases due to increased heat losses to the upstream direction. Thus, ceramic burner supports with very low effective conductivity are advantageous for this type of application and result in increased safety, a higher turn-down ratio,

and higher radiative operating temperatures. The lowest effective conductivity is achieved for fiber-based structures (ca. 0.1 W m–1 K–1), while the highest values are reached with sintered plates having low microporosity (ca. 2 W m–1 K–1).

Most radiant surface burners operate in the radiant mode at heat loads between 100 and 600 kW m–2 (for natural gas under atmospheric conditions and without air preheat-

ing). At higher heat loads the flame sheet lifts to a distance of several millimeters from the burner support and thus the burner support cools down. This mode of operation is also called the “blue-flame mode”, in which no significant radiation is emitted. In the heat load range between the two modes a transition from the radiant mode to the blue-

flame mode takes place. The maximum surface temperatures of the burner support are reached for heat loads of approximately 300–400 kW m–2 (depending on the actual

burner-support properties) and they are in the range of up to 1100 C.

Radiant surface burners can be also operated at significantly higher heat loads up to 3000 kW m–2 in the blue-flame mode. In this mode the lifted flame reaches sig-

nificantly higher temperatures, while the temperature of the ceramic burner plate decreases. Emissions are correspondingly higher, since no heat extraction through

5.5 Porous Burners 495

radiation of the “cold” burner surface takes place. One interesting concept to improve the performance of such burners in the blue-flame mode is to use ceramic structures with a bimodal pore size distribution. This delivers a corresponding bimodal velocity distribution through the ceramic structure. Thus, portions of the flame in the positions of lower velocity remain at the burner surface, while those in positions with higher velocities are lifted from the surface. Thus, even at higher heat loads a fraction of the heat is still extracted by radiation.

5.5.5

Volumetric Porous Burners with Flame Stabilization by Thermal Quenching

Volumetric combustion in porous media offers exceptional advantages compared to conventional combustion technologies with free flames. Unlike conventional premixed combustion processes, combustion in porous media does not operate with free flames. Rather, a flameless combustion takes place in the three-dimensionally arranged cavities of a porous medium. The superior heat-transfer properties of the porous matrix are superposed on the heat transfer in the gas phase and coupled through the intensive heat exchange between the gas and the porous matrix. Radiation, conduction, and heat transfer due to dispersion imposed by the porous matrix are the dominant heat-transfer mechanisms. The relatively new technology of po- rous-medium burners is characterized by higher burning rates, increased flame stability with low noise emissions, and controllable, homogeneous combustion-zone temperatures which lead to a reduction in NOx and CO emissions. Porous-medium burners also show a high power density that translates into compact designs and, depending on the flame stabilization principle used (Section 5.5.2.3), an extremely high power turndown ratio of up to 1:30. Additionally, complex combustion chamber geometries, adapted to the specific needs of the individual applications, which are not feasible with conventional state-of-the-art combustion techniques, are possible.

Figure 7 shows a photograph of a stabilized combustion process in a porous medium with the temperature distribution. The good heat-transfer properties keep the maximum temperature low, which also keeps NOx emissions low. A very homogeneous temperature field is also achieved, and this keeps the carbon monoxide concentration in the waste gas low, too. A considerable part of the heat from the combustion region is also transported upstream, and hence higher flame speeds are up to 30 times higher than with laminar free flames. This translates into a greater power density and better space utilization.

Compared to more conventional combustion processes with free premixed flames, properly stabilized combustion processes in porous media lead to the following advantages, which result mostly from the very intense heat transport inside the porous structure and the stabilization principle:

496 Part 5 Applications

Fig. 7 Porous burner with flame stabilization by thermal quenching.

Due to these outstanding properties, porous-burner technology is attractive for different fields of application.

5.5.5.1

Materials and Shapes for Porous-Medium Burners

A special feature of porous-burner technology is its dependency on special highly temperature resistant porous ceramic components. Therefore, a short overview of materials and shapes which are suitable for combustion in porous media is given. More details are reported in Ref. [2].

The most important materials and forms for porous burners are sintered silicon carbide (S-SiC) foams as well as silicon-infiltrated composite SiC-based foams, and mixerlike structures made of Al2O3 fibers, ZrO2 foams, or C/SiC structures. Table 2 lists the most important data for these materials. The maximum use temperatures are rather theoretical values that are not always reached in practice. For some applications also iron–chromium–aluminum alloys and nickel-base alloys can be used. All of the mentioned materials are substantially different with regard to manufacturing and properties. Al2O3 and ZrO2 materials can be used at temperatures above 1650 C. Metals and SiC materials do not meet this qualification; however, they show outstanding characteristics with regard to thermal shock resistance, mechanical strength, and conductive heat transport. Wire meshes and mixerlike structures

The overall performance of the porous body is always a combination of the base material itself and the actual porous structure. Therefore, for both the suitable materials and for the shapes of porous structures the basic properties are given below. However, the emissivity data are not very reliable and strongly depend on the actual surface structure.

Alumina structures (Fig. 8a) can be in principle used up to process temperatures in the range of 1900 C, although the technical temperature limit of these structures is nowadays at about 1700 C. Alumina-based materials show an intermediate thermal conductivity ranging from 5 W m–1 K–1 at 1000 C to about 30 W m–1 K–1 at 20 C. Alumina shows intermediate thermal expansion, intermediate resistance to thermal shock, and an overall emissivity at 2000 K of about 0.28.

High-quality silicon carbide materials (Fig. 8b) are characterized by a maximum usage temperature of about 1650 C, high thermal conductivity in the range of 20 W m–1 K–1 at 1000 C and 150 W m–1 K–1 at 20 C, very low thermal expansion, and very good resistance to thermal shock. The overall emissivity at 2000 K is about 0.8–0.9.

Temperature-resistant metal alloys (Fig. 8d) can be used for temperatures lower

than 1250 C. Their properties include high thermal conductivity ranging from 10 W m–1 K–1 at 20 C to about 28 W m–1 K–1 at 1000 C, high thermal expansion, and an

extremely good resistance to thermal shock. The emissivity of metals varies strongly with the surface finish and the surface itself (0.1–0.7).

Of all the presented materials, zirconia (Fig. 8c) shows the highest temperature resistance, up to 2300 C. The thermal conductivity of solid zirconia is not significantly temperature dependent and in the range of 2–5 W m–1 K–1. The emissivity at 2000 K is about 0.31. Porous zirconia-based mixer structures, as shown in Fig. 8c, combine the extremely high maximum temperature resistance of solid zirconia with a short start-up phase and an extremely good resistance to thermal shock, due to the high inner porosity of the ZrO2 lamellas.

498 Part 5 Applications

Fig. 8 Different materials in the combustion region. a) Al2O3 fiber-based lamellar structure; b) SiC foam; c) ZrO2 foam-based lamellar structure; (d) Fe–Cr–Al alloy wire mesh.

Static mixer structures (Fig. 8a and 8c) are characterized by a low conductive heat transport, a short start-up phase, excellent radiative heat transport, excellent dispersion properties, and very low pressure drop.

Wire meshes (Fig. 8d) show poor conductive heat-transport and dispersion properties, due to their high porosity. On the other hand, they have a short start-up phase, excellent radiative heat-transport properties, and very low pressure drop.

Ceramic foams (Fig. 8b) are widely used for porous burners. Foam structures feature good conductive heat transport, a rather long start-up phase due to lower macroporosity, intermediate radiative heat-transport properties, intermediate dispersion properties, and relatively high pressure drop.

5.5.5.2

Applications of Volumetric Porous Burners

Due to its outstanding properties, such as the wide operating range with regard to thermal power, small dimensions, and low pollutant emissions, porous-burner tech-

500 Part 5 Applications

Fig. 10 Heating time of a steel plate with different infrared burners.

Figure 9 shows the radiant heater and the numerically simulated temperature distribution in the porous burner. It consists of a 20 mm thick alumina fiber plate with 1 mm pore diameter in the preheating region and a 15 mm, 10 ppi SiC foam in the combustion region. The numerically and experimentally determined maximum temperatures are more than 1400 C, reaching up to 1550 C for this burner geometry, in comparison to 1100 C for conventional infrared radiant heaters. The thermal power density can be up to 1500 kW m–2, in contrast to the radiant-mode limit of about 400 kW m–2 of conventional systems.

In Fig. 10 an experimental comparison of the time for heating steel plate with different radiant burners is shown. The experimental arrangement with four porous burners can be seen on the left, while the diagram on the right shows the superior performance of the porous burner in this application in comparison to conventional radiant heaters having the same radiative surface area. The conventional radiant heaters were operated at their respective maximum radiant output, while the porous burner was operated at 70 % of its maximal power.

Application in Domestic Gas Boilers

An important aspect for the development of new gas burners for household applications is the fact that the power requirement for the heating operation decreases more and more, due to the improved insulation of the buildings. This can be related to the trend of lowering energy costs by energy-saving buildings. However, for hotwater production the energy requirement remains unchanged, since high comfort is strongly requested in this field. Taking into account this discrepancy, burners for heating systems are required to have a wide power-modulation range. This also relates directly to the number of burner starts. As the highest emissions occur during the warm-up phase, a higher power modulation automatically leads to a decrease in waste-gas emissions [35]. Also in stationary operation, emissions of porous media burners are minimized in comparison to many domestic gas appliances working with free flames because it is possible to control the combustion temperature with the porous material in the combustion region. Table 3 shows for a 30 kW domestic gas boiler operating with a porous burner that the NOx and CO emissions are clearly below the most stringent European emission standards.

Another aspect that makes the use of volumetric porous-medium combustion especially interesting for household applications is the compactness of porous-medi- um burner units. Power densities of 3500 kW m–2 (cf. 300–600 kW m–2 for conventional systems) can be realized with various gases and air ratios [42]. Because of the small burner sizes separate heating rooms are no longer necessary. Instead the porous burner units can be installed in small wall niches or even outside the house.

Figure 11 shows a sketch and a photograph of a complete 30 kW condensing gas boiler for domestic use based on porous-burner technology. It is about half as large as a conventional heating system for the same nominal power output and shows a power turndown ratio of 1:10 (3–30 kW).

flame trap region A Al2O3-hole plate

combustion region C alumina lamellas

Fig. 11 Porous burner and integrated heat exchanger unit for household applications.

5.5 Porous Burners 503

burner at a temperature of about 270 C. The exhaust gases enter the condensing boiler after the porous burner.

Figure 13 shows the emission characteristics of the first prototypes over the entire power-modulation range. Emissions for nonstaged operation of the cool-flame vaporizer with preheated air at 350 C are shown (1st version) for comparison purposes. Clearly, the staged vaporizer concept (2nd version) requires much less air preheating (only the secondary air is preheated at a much lower temperature of 200 C) and results in significantly less nitrogen oxide emissions, due to the resulting reduced combustion temperatures, although a lower excess-air ratio was set. Details on this ongoing development can be found in Ref. [43].

Power [kW]

Fig. 13 Emission characteristic of the BIOFLAM burner.

The excellent emissions characteristic of the BIOFLAM boiler unit operating with a staged cool-flame vaporizer and a porous burner is comparable to the emission levels of low-emission gas burners, which represents a breakthrough for oil burners.

Porous Burner as Chemical Reactor for HCl Synthesis

The advantages of porous burners can also be used for different gas-phase chemical reactions, for example, the synthesis of HCl from H2 and Cl2. Conventional reactors for HCl synthesis from H2 and Cl2 operate with free-flame diffusion burners at high hydrogen excess ratios due to the flashback safety aspects of premixed combustion with such mixtures. The diffusion burner mode leads to extremely long reactors (up to 6 m) operating with large excess of hydrogen (up to 50 %) to achieve a complete conversion without any unconverted chlorine and thus meet the extreme purity demands of the HCl product.