Page 225 - Synthetic Fuels Handbook

FUELS FROM SYNTHESIS GAS 211

heat release by the exothermic carbonation reaction supplies most of the heat required by
the endothermic reforming reactions. However, energy is required to regenerate the sorbent
to its oxide form by the energy-intensive calcination reaction, that is,
CaCO → CaO + CO
3 2
Use of a sorbent requires either that there be parallel reactors operated alternatively and
out of phase in reforming and sorbent regeneration modes, or that sorbent be continuously
transferred between the reformer/carbonator and regenerator/calciner.
In autothermal (or secondary) reformers, the oxidation of methane supplies the nec-
essary energy and is carried out either simultaneously or in advance of the reforming
reaction. The equilibrium of the methane-steam reaction and the water-gas shift reac-
tion determines the conditions for optimum hydrogen yields. The optimum conditions
for hydrogen production require: high temperature at the exit of the reforming reactor
[800–900°C (1472–1652°F)], high excess of steam (molar steam-to-carbon ratio of 2.5 to 3),
and relatively low pressures (below 450 psi). Most commercial plants employ sup-
ported nickel catalysts for the process.
At the operating temperatures, some of the methane may completely decompose and
deposit a thick layer of inactive carbon on the catalyst surface (coke). Especially with
nickel-based catalysts, steam reforming involves the risk of carbon formation, which may
cause serious operational problems and catalyst deactivation. Generally, higher hydrocar-
bons are more prone to carbon formation than methane because the initial surface carbon
intermediates are more readily formed. The concentration of these intermediates is an impor-
tant factor, and is critical in influencing the delicate balance between carbon-forming and
carbon-removing reactions. On nickel surfaces, carbon formation may take place mainly by
three routes (Table 7.1) (Rostrup-Nielsen, 1984; Rostrup-Nielsen et al., 2002).
At lower temperatures (say 500°C and below), adsorbed hydrocarbons may accumulate on
the surface and slowly may be transformed into a nonreactive polymer film (“gum”), block-
ing and deactivating the surface. This phenomenon can be retarded by hydrogen. Because of
the endothermic nature of the steam-reforming reaction, high catalyst activity leads to a low
temperature at the reaction site, resulting in a higher risk for carbon formation.

TABLE 7.1 Possible Routes to Deposition of Carbonaceous Products During Steam-Methane
Reforming

Carbon type Reactions involved Phenomena Critical parameters
Gum C n H → (CH ) → gum Blocking of surface Low S/C ratio, absence of
m 2 n
by polymerisation of H , low temperature
2
adsorbed C H (below ~500°C),
n m
radicals: progressive presence of aromatics
deactivation
Whisker CH → C + 2H 2 Break-up of catalyst Low S/C ratio, high
4
carbon, 2CO → C + CO pellet (whisker temperature (above
2
amorphous CO + H → C + H O carbon: no ~450°C), presence of olefins,
2
2
m
carbon C H → nC + / H deactivation of aromatics
n m 2 2
the surface)
Pyrolytic C H → olefins → coke Encapsulation of High temperature (above
n m
coke catalyst pellet ~600°C), high residence
(deactivation), time, presence of olefins,
deposits on tube wall sulfur poisoning