Reactor catalytic burner

Figure 17.23. Representative temperature profiles in reaction systems (see also Figs. 17.20, 17.21(d), 17.22(d), 17.30(c), 17.34, and 17.35). (a) A jacketed tubular reactor, (b) Burner and reactor for high temperature pyrolysis of hydrocarbons (Ullmann, 1973, Vol. 3, p. 355) (c) A catalytic reactor system in which the feed is preheated to starting temperature and product is properly adjusted exo- and endothermic profiles, (d) Reactor with built-in heat exchange between feed and product and with external temperature adjustment exo- and endothermic profiles.

This section will provide information about micro structured reformer reactors, gas purification devices and catalytic burners, the last also in combination with an evaporator, for fuel processors. However, the specific problems related to the peripheral equipment will not be discussed in depth. 

For reformate flow rates up to 400 Ndm3 min-1, the CO output was determined as < 12 ppm for simulated methanol. The reactors were operated at full load (20 kW equivalent power output) for -100 h without deactivation. In connection with the 20 kW methanol reformer, the CO output of the two final reactors was < 10 ppm for more than 2 h at a feed concentration of 1.6% carbon monoxide. Because the reformer was realized as a combination of steam reformer and catalytic burner in the plate and fin design as well, this may be regarded as an impressive demonstration of the capabilities of the integrated heat exchanger design for fuel processors in the kilowatt range. 

R 20] The fuel processing system consists of a fuel evaporator, a reformer, a reactor for the preferential oxidation of carbon monoxide and a catalytic burner (Figure 4.48) [95],... 

Under normal operating conditions, in which the combustor is sufficiently warm and operated under fuel rich conditions, virtually no NOx is formed, although the formation of ammonia is possible. Most hydrocarbons are converted to carbon dioxide (or methane if the reaction is incomplete) however, trace levels of hydrocarbons can pass through the fuel processor and fuel cell. The shift reactors and the preferential oxidation (PrOx) reactor reduce CO in the product gas, with further reduction in the fuel cell. Thus, of the criteria pollutants (NOx, CO, and non-methane hydrocarbons [NMHC]), NOx CO levels are generally well below the most aggressive standards. NMOG concentrations, however, can exceed emission goals if these are not efficiently eliminated in the catalytic burner. 

These calculations show that the heat transfer characteristics of the reactor will sigitificantly change as the catalyst deactivates. This will effect the design and control of the heating mechanism (i.e. catalytic burner) that provides the heat to the reactor. 

Delsman et al. investigated the advantages of a microstructured methanol reformer coupled with a catalytic burner for anode off-gas over a conventional fixed-bed system [36]. Two ranges of electrical power output of the corresponding fuel processor-fuel cell system were considered, namely 100 W and 5kW. The calculations revealed a more than 50% lower reactor size and more than 30% less catalyst mass required for the microreactor in case of the 100 W system. For the 5 kW system, the reactor volume was only 30% lower, but the catalyst savings were up to 50%. 

The heat integration system consists of a catalytic burner reactor (BUR) and three heat exchangers (HXl, HX2, HX3). In the catalytic burner reactor, the unconverted hydrogen is combusted at 370 °C to deliver the heat necessary for the vaporizer and the reformer. The whole FP-FC system operates autothermally. 

Good thermal insulation and integration is important in order to minimize heat losses in the reactors. It is best if the catalytic burner is made as part of the reformer (such as in a concentric reformer and burner configuration) in order to maximize the heat utilization produced by the burner and to achieve more even temperature distribution within the reformer. Since the... 

The mixture composition was chosen as H2O C = 1.9 and O2 C = 0.47 to achieve complete conversion and to avoid carbon deposition. The WGS reaction was performed in two adiabatic reactor stages with inlet temperatures of 673 and 573 K. An intermediate heat exchanger was not required as cold water was injected into the system. The catalytic burner and reformer were both required for water evaporation and superheating to cover the complete heat demand and to guarantee the inlet temperature for the first WGS reactor stage. When the operation temperature of... 

Many of the models can be used to describe not only the behavior of single cells, but also that of a whole stack. This extension of the model equations has not been discussed here but, as indicated in Table 28.1, this has been reahzed with many models. Other extensions and modifications, such as the application ofequihbrium assumptions with regard to the reforming reactions, the modeling of a catalytic burner between the anode exhaust and the cathode inlet, or the addition of model equations describing an indirect internal reforming reactor, are frequently apphed in MCFC models. For the sake of brevity, they have not been discussed in detail in this chapter. 

FigureS.IO Comparison oftemperature profiles along the reactor length axis as calculated for a microchannel steam reformer with integrated catalytic burner for isooctane combustion (case A) and a microchannel steam reformer supplied with energy from hot combustion gases fed into heating channels (case B) [383] S/C ratio was 3.0 in both cases the air excess was 20% in case A and 94% in case B.

A similar concept study was performed by Seo et al. [449] for natural gas reforming. However, fixed-bed reactors were used and the heat supply originated not from autothermal reforming, but from a catalytic burner in the centre of the fuel processor. Two fuel processors of this type were then built with 1- and 2-kW electrical power equivalents [450]. The smaller system was operated at a S/C ratio 3.0 and 89% methane conversion was achieved, while other hydrocarbons present in the natural gas feed were completely converted. The fuel processor efficiency was calculated as the ratio of the lower heating value of the hydrogen produced to the lower heating value of natural gas fed to the reformer and the burner. It was in the range between... 

Figure 6.7 Conventional heat exchanger (top), cooled catalytic burner (middle) and coupling of exothermic and endothermic reaction at the wall of a heat-exchanger reactor coated with...

Subsequently, Peters et al. continued these investigations. Their natural gas feed contained, besides methane, ethane, propane and butane, also 0.11 vol.% pentane and 0.1 vol.% higher hydrocarbons. Feed pre-heating, water evaporation and superheating to a temperature between 350 and 400 ° C was supplied with energy from the hot off-gas of a catalytic burner. The pre-reformer was then operated at temperatures between 536 and 785 °C. Around 20% methane conversion was achieved in the reactor, while the ethane conversion ranged between 40 and 50%. 

A substoichiometric amount of air or oxygen is used. The reaction can be carried out in the presence or the absence of a catalyst. In the non-catalytic process, a mixture of oxygen and natural gas is preheated, mixed and ignited in a burner the reactor temperature must be high enough to reach complete CH4 conversion (typically 1200-1500 °C). Combustion products such as CO2 and H2O are also formed to a certain extent. 

A further consequence of the upstream diffusion to the burner face could be heterogeneous reaction at the burner. Such reaction is likely on metal faces that may have catalytic activity. In this case the mass balance as stated in Eq. 16.99 must be altered by the incorporation of the surface reaction rate. In addition to the burner face in a flame configuration, an analogous situation is encountered in a stagnation-flow chemical-vapor-deposition reactor (as illustrated in Fig. 17.1). Here again, as flow rates are decreased or pressure is lowered, the enhanced diffusion tends to promote species to diffuse upstream toward the inlet manifold. 

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