Gasification and Fischer Tropsch Synthesis
The influx of heavy feedstocks into refineries creates challenges but at the same time creates
opportunities by improving the ability of refineries to handle heavy feedstocks thereby
enhancing refinery flexibility to meet the increasingly stringent product specifications for
refined fuels. Upgrading heavy feedstocks essentially means extracting the maximum amount of
liquid fuels from each barrel of crude oil that enters the refinery.
There are some processes such as Solvent deasphalting and coking processes that are used in
refineries to upgrade heavy feedstocks to intermediate products that may be processed to
produce transportation fuels. However, the integration of gasification presents some unique
synergies that will enhance the future refinery.
As known, liquid fuels, including gasoline, diesel, naphtha, and jet fuel, are usually processed via
refining of crude oil; Due to the direct distillation, and therfore crude oil is the most suited raw
material for liquid fuel production. However, with fluctuating and rising prices of petroleum,
coal-to-liquid (CTL) and biomass-to-liquid (BTL) processes are currently starting to be considered
as an alternative route used for liquid fuel production. Both feedstocks are converted to syngas
(a mixture of carbon monoxide and hydrogen), which are subsequently converted into a mixture
of liquid products by Fischer–Tropsch (FT) processes. The liquid fuel obtained after FT synthesis
is eventually upgraded/processed using known petroleum refinery technologies to produce
gasoline, naphtha, diesel fuel, and jet fuel.
Gasification:
Gasification is a technological process that can convert any carbonaceous (carbon-based) raw
material such as coal, petroleum residua, biomass, or other carbonaceous feedstocks into fuel
gas, also known as synthesis gas (syngas for short). Gasification can be also used to convert
petroleum coke (petcoke) and other refinery waste streams and residuals into power, steam,
and hydrogen for use in the production of cleaner transportation fuels.
Gasification occurs in a gasifier, generally, a high temperature/pressure vessel where oxygen/air
and steam are directly contacted with the carbonaceous material causing a series of chemical
reactions to occur that convert the feed to syngas and ash/slag (mineral residues). This type of
reaction is known as partial oxidation process.
Syngas is composed primarily of the colorless, odorless, highly flammable gases carbon
monoxide (CO) and hydrogen (H2), syngas has a variety of uses. The syngas can be further
converted (or shifted) to hydrogen and carbon dioxide (CO2) by adding steam and reacting over
a catalyst in a water-gas-shift reactor. When hydrogen is burned, it creates heat and water,
resulting in the ability to create electricity with no carbon dioxide in the exhaust gases. Carbon
dioxide can be efficiently captured from syngas, preventing its greenhouse gas emission to the
atmosphere and enabling its utilization (such as for Enhanced Oil Recovery) or safe storage.
A number of reasons have increased the interest in gasification applications in petroleum
refinery operations:
• (1) Coking capacity has increased with the shift to heavier, more sour crudes being supplied
refiners.
• (2) Hazardous waste disposal has become a major issue for refiners in many countries.
• (3) There is strong emphasis on the reduction of emissions of criteria pollutants and
greenhouse gases.
• (4) Requirements to produce ultralow sulfur fuels are increasing the hydrogen needs of the
refineries.
Fischer–Tropsch synthesis (FTS):
The Fischer–Tropsch synthesis (FTS) represents technology that provides synthetic hydrocarbon
fuels and chemicals from initially coal, natural gas, and also biomass. The overall process from
original carbon source for the syngas to the FTS product is named after the feedstock employed,
hence the terminology ‘coal-to-liquids’ (CTL), ‘gas-to-liquids’ (GTL) and ‘biomass-to-liquids’
(BTL), collectively known as XTL (‘anything’-to-liquids).
Virtually any source of (hydro)carbon feedstock can be converted to a mixture of or syngas (CO
and H2), which is a key factor for the entire chemical industry, in which it could be based as
chemical building blocks to produce a broad range of chemicals using processes well established
in the chemical and petrochemical industry.
FTS stands for the reaction(s) of synthesis gas to aliphatic hydrocarbons, predominantly straightchain hydrocarbons, which can be paraffins from CH4 to waxes (CnH2n+2 with n from 1 to over
100), olefins from ethylene to much longer molecules (CnH2n, with n < or = 2 to over that), and
to a lesser extent oxygenated products such as alcohols. The main step in the XTL process is to
catalytically convert the syngas to a range of hydrocarbons via the FT synthesis, which mainly
yields linear alkanes and 1-alkenes. The last step is usually the workup of the hydrocarbons to
final products, which are typically fuels, but optionally also chemicals.
At the chemistry level, the FT synthesis is both a CO hydrogenation reaction and a
polymerization reaction. The former is reflected by the fact that the C–O bond must be broken
and new C–H bonds formed. Additionally, C–C bonds must be formed in order to effect
hydrocarbon chain growth. It also produces as main by-products water and/or carbon dioxide,
due to the water-gas shift (WGS) reaction. Being a highly exothermic reaction, it generates large
amounts of heat.