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FUELS FROM SYNTHESIS GAS 203

future, these systems may be able to achieve efficiencies approaching 60 percent. A con-
ventional coal-based boiler plant, by contrast, employs only a steam turbine-generator and
is typically limited to 33 to 40 percent efficiency.
Higher efficiency means that less fuel is required to generate the rated power, resulting
in better economics (which can mean lower costs to the consumer) and the formation of
fewer greenhouse gases—a 60 percent-efficient gasification power plant can cut the forma-
tion of carbon dioxide by 40 percent compared to a typical coal combustion plant.

7.2 GASIFICATION OF PETROLEUM FRACTIONS

One of the important aspects of petroleum refining is the supply of adequate amounts of
hydrogen for the various hydrotreating processes, such as desulfurization and in hydrocon-
version processes, such as hydrocracking (Speight, 2007b).
As hydrogen use has become more widespread in refineries, hydrogen production has
moved from the status of a high-tech specialty operation to an integral feature of most refin-
eries. This has been made necessary by the increase in hydrotreating and hydrocracking,
including the treatment of progressively heavier feedstocks. In fact, the use of hydrogen
in thermal processes is perhaps the single most significant advance in refining technology
during the twentieth century (Speight, 2007b and references cited therein).
In some refineries, the hydrogen needs can be satisfied by hydrogen recovery from
catalytic reformer product gases, but other external sources are required. However, for the
most part, many refineries now require on-site hydrogen production facilities to supply the
gas for their own processes. Most of this non-reformer hydrogen is manufactured either by
steam-methane reforming or by oxidation processes. However, other processes, as refiner-
ies and refinery feedstocks evolved during the last four decades, the demand for hydrogen
has increased and reforming processes are no longer capable of providing the quantities of
hydrogen that are adequate for feedstock hydrogenation.
In conjunction with hydrogen production, usually by partial oxidation processes, there
is the concurrent production of carbon monoxide. Commonly, steam reforming of low
molecular-weight hydrocarbons is the main method of hydrogen production which also
produces synthesis gas.
The most common, and perhaps the best, feedstocks for steam reforming are low-boiling
saturated hydrocarbons that have a low sulfur content, including natural gas, refinery gas,
liquefied petroleum gas (LPG), and low-boiling naphtha.
Natural gas is the most common feedstock for hydrogen production since it meets all the
requirements for reformer feedstock. Natural gas typically contains more than 90 percent
methane and ethane with only a few percent of propane and higher-boiling hydrocarbons.
Natural gas may (or most likely will) contain traces of carbon dioxide with some nitrogen
and other impurities. Purification of natural gas, before reforming, is usually relatively
straightforward (Speight, 2007a). Traces of sulfur must be removed to avoid poisoning
the reformer catalyst; zinc oxide treatment in combination with hydrogenation is usually
adequate.
Light refinery gas, containing a substantial amount of hydrogen, can be an attractive
steam reformer feedstock since it is produced as a by-product. Processing of refinery gas
will depend on its composition, particularly the levels of olefins and of propane and heavier
hydrocarbons. Olefins, that can cause problems by forming coke in the reformer, are con-
verted to saturated compounds in the hydrogenation unit. Higher-boiling hydrocarbons in
refinery gas can also form coke, either on the primary reformer catalyst or in the preheater.
If there is more than a few percent of C and higher compounds, a promoted reformer cata-
3
lyst should be considered, in order to avoid carbon deposits.