Page 226 - Synthetic Fuels Handbook

212 CHAPTER SEVEN

In many instances catalyst deactivation occurs during steam-methane reforming. There
can be many reasons for catalyst deactivation (van Beurden, 2004). In addition, there is a
distinction between poisoning and thermal deactivation. For example, if, on continued use,
the activity decreases more rapidly than surface area, poisoning may be suspected whereas,
if a decrease in surface area is concomitant with a decrease in activity, thermal deactivation
is indicated.
Carbon formation depends on the kinetic balance between the surface reaction of the
adsorbed hydrocarbons with oxygen species and the further dissociation of the hydrocarbon
into adsorbed carbon atoms. In fact, for a given hydrocarbon feed, temperature, and pres-
sure, carbon will be formed below a critical steam-to-carbon ratio (Twigg, 1989; Rostrup-
Nielsen et al., 2002). This critical steam-to-carbon ratio increases with temperature and
is dictated by thermodynamics. In practice however, carbon formation generally occurs
before the thermodynamic limit is reached (e.g., by poisons, temperature and concentration
gradients, etc.). By promotion of the catalyst it is possible to push the carbon formation
limit to the thermodynamic limit. For instance, controlled passivation of the catalyst surface
by sulfur, carbon deposition is inhibited (Udengaard et al., 1992). By using noble metal
catalysts, it is possible to push the carbon limit even beyond the thermodynamic limit.

7.4 GASIFICATION OF OTHER FEEDSTOCKS

Gasification offers more scope for recovering products from waste than incineration. When
waste is burnt in a modern incinerator the only practical product is energy, whereas the
gases, oils, and solid char from pyrolysis and gasification can not only be used as a fuel
but also purified and used as a feedstock for petrochemicals and other applications. Many
processes also produce a stable granulate instead of an ash which can be more easily and
safely utilized. In addition, some processes are targeted at producing specific recyclables
such as metal alloys and carbon black. From waste gasification, in particular, it is feasible
to produce hydrogen, which many see as an increasingly valuable resource.
Gasification can be used in conjunction with gas engines (and potentially gas turbines)
to obtain higher conversion efficiency than conventional fossil fuel energy generation. By
displacing fossil fuels, waste pyrolysis and gasification can help meet renewable energy tar-
gets, address concerns about global warming, and contribute to achieving Kyoto Protocol
commitments. Conventional incineration, used in conjunction with steam-cycle boilers and
turbine generators, achieves lower efficiency.
Many of the processes fit well into a modern integrated approach to waste management.
They can be designed to handle the waste residues and are fully compatible with an active
program of composting for the waste fraction that is subject to decay and putrefaction.
This, by analogy with coal, the high-temperature conversion of waste (Chap. 11) is a
downdraft gasification process which gasifies the feed material within a controlled and
limited oxygen supply. Combustion of the feed material is prevented by the limited oxygen
supply. The temperature within the reactor reaches 2700°C, at which point molecular disso-
ciation takes place. The pollutants that were contained within the feed waste material such
as dioxins, furans, as well as pathogens are completely cracked into harmless compounds.
All metal components in the waste stream are converted into a castable iron alloy/pig iron
for use in steel foundries. The mineral fraction is reduced to a nonleaching vitrified glass,
used for road construction and/or further processed into a mineral wool for insulation. All of
the organic material is fully converted to a fuel quality synthesis gas which can be used to
produce electrical energy, heat, methanol, or used in the production of various other chemical
compounds. The resultant syngas, with a hydrogen-to-carbon-monoxide ratio approximately
equal to 1, is also capable of being used for the production of Fischer-Tropsch fuels. Under