3. Results and discussion
The hydrogen mole fraction and temperature contour plots are illustrated
in Figure 1 in the thermally coupled reactor for conducting simultaneous
endothermic and exothermic reactions. Heating with part of the
combustion heat may be carried out by direct heating or radiation
heating. Accordingly, the waste heat can efficiently be recovered in a
low-temperature region, although the reactivity of a steam reforming
reaction is low in such a low-temperature region, because a reforming
catalyst effectively acts. The waste heat can also efficiently be
recovered even in the absence of a steam reforming catalyst in a
high-temperature region because the reactivity of a steam reforming
reaction is high in such a high-temperature region. In this connection,
metal tubular reactors have insufficient thermal stability. For example,
a metal material has significantly decreased strength at temperatures
higher than 800 °C and has strength of about 100 MPa at 1000 °C.
Accordingly, an actual steam reforming apparatus must operate at a
controlled operation temperature of about 200 °C to about 300 °C. The
temperature of 300 °C refers to temperature at which such a metal
tubular reactor can be used, and, if a metal tubular reactor is
continuously operated at temperatures above 300 °C, it may possibly be
broken due to decreased strength thereof. At a temperature equal to or
lower than the temperature where a metal tubular reactor can be used,
the conversion decreases when no catalyst is used, and therefore a
catalyst should essentially be used to compensate the decrease in
conversion. Accordingly, ceramic materials can be used as tubular
reactors even at temperatures of 1000 °C or higher where it is
impossible to use metal materials. In contrast, there is no need of
using a steam reforming catalyst at temperatures of 1000 °C or higher,
because the conversion is complete even in the absence of a catalyst at
temperatures of 1000 °C or higher. In addition, it is difficult to use a
steam reforming catalyst at such high temperatures. A steam reforming
catalyst generally includes catalytic metal nickel particles dispersed
on an alumina carrier, and the nickel particles gradually undergo
sintering and become coarse at temperatures of 1000 °C or higher. Such
coarse nickel particles have reduced specific surface areas and show
decreased reactivity. That is, steam reforming catalysts generally have
heat resistance up to at a temperature of about 1000° C, and it is
impossible to use such catalysts at temperatures of 1000 °C or higher,
while the heat resistance somewhat varies depending on the type of a
metal. Thus, steam reforming can be carried out even at temperatures of
1000 °C or higher by using a ceramic tube, whereas metal tubular
reactors and catalysts cannot be used at such high temperatures. The
thermally coupled reactor can thereby recover waste heat effectively,
and can carry out a steam reforming reaction even in a region at high
temperatures.