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Steam Gasification and Reforming Technologies
4.6.2 STeAm reForming reACTorS
While hydrogen production can be achieved by a number of commercially proven
technologies such as gasification of coal, biomass, and residue (waste); methanol
decomposition; and steam reforming of methane, renewable materials, and liquid
hydrocarbons, it is the last technology that produces the largest portion of hydrogen
production [5,33,126,128,175–190]. With the considerable advances in unconventional
production of natural gas that includes shale gas, deep gas, tight gas, coal bed methane,
gas from geopressurized zones, and gas hydrates, the steam reforming of natural gas is
likely to become even more important. An increase in natural gas production is likely
to make the steam reforming of methane the choice of significant hydrogen production.
The design of a steam reforming plant requires the considerations that
1. The economics of the process is very scale dependent [33]. For example,
for 5 ×  10 6  Nm 3 /day plant, the operating cost can be as low as $80/kW of
H 2 , whereas for 2300 Nm 3 /day plant, the same cost would be $4000/kW for
hydrogen.
2. The capital cost can be large due to large size of the plant (i.e., in large
plants, reformer tubes can be as long as 12 m) and the need for expensive
alloy materials for high-temperature and high-pressure operations.
3. The small-scale operation, while expensive, is often used for niche application such as FC technology and hydrogen refueling station, and this requires
small and compact reformers at low cost.
Due to these considerations, both large- and small-scale reformers have been
developed.
In normal commercial reformers, the steam-to-hydrocarbon ratio is kept high
enough to prevent coking but to avoid overloading the reformer duty. Generally, the
ratio of 3 is used. The inlet temperature of 760°C is used, and because reforming
reaction is endothermic, additional heat is added as mixture flows down the catalystfilled reformer tubes. A critical factor in the reformer heater design is keeping the
tube wall temperature uniform and hot enough to promote reforming reaction. For
this purpose, two types of heater design, side-fired reforming furnace and roof-fired
heater design, have been employed [33].
In side-firing furnace, two parallel rectangular boxes are connected at the top
with horizontal ductwork into the vertical convection stack. Several rows (typically
four) are used to directly fire the tubes. A typical reformer furnace has 300 burners.
Reformer tubes are 5 inch in diameter with a wall thickness of 0.5 inch and about
34 ft of wall is exposed to the burners. The tubes are generally 25% chrome, 20%
nickel, or a high nickel steel such as HL40 [33].
The top-fired reformer is a rectangular box, the tubes are still vertical, and the
inlet and outlet are pigtails to the pigtail inlet header and the outlet transfer line.
The burners have a pencil-shaped flame design. All burners are located above the
inlet manifold. Hydrogen plants with single reformer heaters and a capacity of up
to 100,000 ft 3 /day are used in the vertical down-firing approach. The outlet transfer line from the reformer is used to generate high-pressure (650 psig) steam. The
reformer effluent gas exits through the transfer line at about 760°C [33].