Cellular Ceramics / 5

504 Part 5 Applications

For this application of volumetric porous burners, three main parameters differ greatly compared with CH4/air combustion: The stoichiometric adiabatic temperature of an H2/Cl2 flame of 2400 C is significantly higher than that of CH4/air combustion. The maximum temperature and the chemical resistance of the solid material are limiting parameters of the porous burner. Additionally, the laminar flame speed of the stoichiometric H2/Cl2 system of 2.2 m s–1 is five times higher than that of the CH4/air system [45]. Finally the Lewis number of H2/Cl2 with hydrogen excess, in the range of 0.3–0.4, is significantly lower than unity and results in a significantly lower critical P clet number for flame stabilization. In Table 4 the major differences between methane/air and chlorine/hydrogen combustion are summarized.

Table 4 Differences between methane/air, methane/oxygen, and chlorine/hydrogen combustion [38].

By reducing the combustion temperature and the flame speed, the critical pore diameter for flame stabilization based on thermal quenching can be increased to realistic values for a large-scale industrial reactor despite the very low critical P clet number of approximately 7 resulting from the low Lewis number of the H2/Cl2 mixture. HCl recirculation can be applied to reach these goals [46]. As indicated in Table 4, a recirculation ratio of 3 between HCl and Cl2 must be applied to be able to realize a flame trap as a packed bed of ceramic spheres with a diameter of 5 mm showing an effective pore size of about 1.8 mm. Alternatively, other inert components such as H2O can be added to the mixture of reactants to reach the same target.

In Fig. 14 a laboratory porous burner (top) built of graphite is shown. Alumina sphere packings were used as porous structure, due to their robustness and chemical stability against the aggressive radicals in this application. Stable operation was achieved as expected at an HCl recirculation rate of 3, leading to an extremely pure HCl product with only 5 % excess hydrogen and a reactor height of 20 cm. The lower part of Fig. 14 shows a pilot reactor for the production of 30 t d–1 of HCl having a cross section of 1 0 1 m and an overall height of 0.5 m. A conventional reactor for the same load and product quality requirements would have 2 m diameter and a height of 6 m and would operating at 30–50 % excess hydrogen. Details on this ongoing development can be found in Ref. [38].

5.5 Porous Burners 505

Fig. 14 Laboratory porous burner for HCl sythesis built from graphite housing with alumina packings and pilot chemical reactor for 30 t d–1 production of HCl.

Oxy-Fuel Radiant Porous Burner

Oxy-fuel burners are increasingly applied in high-temperature applications because of the primary high efficiency without recuperation (low investment costs) and the reduced NOx emissions due to the very low nitrogen concentration in the furnace environment. However, due to the relatively low flow rates in comparison to conventional air–fuel burners and the higher combustion temperature, a significant part of the heat has to be transferred by radiation from the gas phase, which makes large gas volumes necessary. However, the extremely high combustion temperature of oxy-fuel burners does not allow the construction of oxy-fuel radiators in a conventional way.

The above considerations led to the design of an annular-gap-shaped combustion chamber, which is shown in Fig. 15. Only a very lean methane–air mixture enters

506 Part 5 Applications

Fig. 15 a) Schematic diagram of the oxy-fuel radiant burner design and b) partly mounted oxy-fuel radiant burner with SiC porous body.

the actual combustion chamber in the tangential direction. The rest of the methane is pressed through the permeable inner tube, thus leading to a continuously staged combustion process. The annular gap is primarily fed with a lean methane–oxygen mixture (excess oxygen ratio of about 5, Fig. 15a). Alternatively oxygen staging can be applied, as shown in Fig. 15b.

Details of this development can be found in Ref. [47]. Development work on the improvement of the applied ceramic components is still going on for this application.

5.5.6

Summary

In the present chapter an overview of the different porous burners using cellular ceramics as burner supports was given, and the major properties affecting flame stabilization and the overall combustion process were discussed. It was indicated that the flame-stabilization concept may cause significant drawbacks in the operational characteristics. Conventional radiant surface burners, for example, are limited to a maximum surface temperature of about 1100 C due to the applied flame stabilization concept.

The utilization of volumetric porous-medium combustion with flame stabilization based on thermal quenching is a good solution that overcomes most of the drawbacks of previous flame-stabilization concepts for combustion in porous media. However, volumetric porous burners require cellular ceramics which are stable in the long term at high temperatures in aggressive atmospheres. Such cellular ceramics are already reality but not yet widely available.

5.5 Porous Burners 507

Selected application fields of porous-medium combustion technology were outlined. Taking advantage of its outstanding benefits like compact design, high power turndown ratio, and load-independent and minimal waste-gas emissions, porousburner technology is predestined to find use in many different industrial branches.

In addition to the applications described in Section 5.5.5.2, porous burners have been successfully developed and tested in the following application fields:

The described basic principles and design criteria for flame stabilization allow the transfer of porous-burner technology to different applications with a wide range of significantly different requirements.

References