Page 221 - Synthetic Fuels Handbook

FUELS FROM SYNTHESIS GAS 207

The exhaust gas from the gas turbine having a temperature of about 550 to 600°C
(1022–1112°F) flows into the heat recovery steam generator consisting of five heat
exchange elements. The first element is a superheater in which the combined stream of
the high-pressure saturated steam generated in the waste heat boiler and in the second
element (high-pressure steam evaporator) is super heated. The third element is an econo-
mizer, the fourth element is a low pressure steam evaporator and the final or the fifth
element is a deaerator heater. The off gas from heat recovery steam generator having a
temperature of about 130°C is emitted into the air via stack.
In order to decrease the nitrogen oxide (NO ) content in the flue gas, two methods can
x
be applied. The first method is the injection of water into the gas turbine combustor. The
second method is to selectively reduce the nitrogen oxide content by injecting ammonia gas
in the presence of de-NO catalyst that is packed in a proper position of the heat recovery
x
steam generator. The latter is more effective than the former to lower the nitrogen oxide
emissions to the air.
Hybrid Gasification Process. In the hybrid gasification process, a slurry of coal and
residual oil is injected into the gasifier where it is pyrolyzed in the upper part of the reactor
to produce gas and chars. The chars produced are then partially oxidized to ash. The ash is
removed continuously from the bottom of the reactor.
In this process, coal and vacuum residue are mixed together into slurry to produce clean
fuel gas. The slurry fed into the pressurized gasifier is thermally cracked at a temperature of
850 to 950°C (1562–1742°F) and is converted into gas, tar, and char. The mixture oxygen
and steam in the lower zone of the gasifier gasify the char. The gas leaving the gasifier is
quenched to a temperature of 450°C (842°F) in the fluidized bed heat exchanger, and is then
scrubbed to remove tar, dust, and steam at around 200°C (392°F).
The coal and residual oil slurry is gasified in the fluidized bed gasifier. The charged
slurry is converted to gas and char by thermal cracking reactions in the upper zone of the
fluidized bed. The produced char is further gasified with steam and oxygen that enter the
gasifier just below the fluidizing gas distributor. Ash is discharged from the gasifier and
indirectly cooled with steam and then discharged into the ash hopper. It is burned with an
incinerator to produce process steam. Coke deposited on the silica sand is removed in the
incinerator.

Hydrocarbon Gasification. The gasification of hydrocarbons to produce hydrogen is
a continuous, noncatalytic process that involves partial oxidation of the hydrocarbon.
Air or oxygen (with steam or carbon dioxide) is used as the oxidant at 1095 to 1480°C
(2003–2696°F). Any carbon produced (2–3 percent by weight of the feedstock) during
the process is removed as a slurry in a carbon separator and pelletized for use either as a
fuel or as raw material for carbon-based products.
Hypro Process. The hypro process is a continuous catalytic method for hydrogen manu-
facture from natural gas or from refinery effluent gases. The process is designed to convert
natural gas:
CH → C + 2H 2
4
Hydrogen is recovered by phase separation to yield hydrogen of about 93 percent purity;
the principal contaminant is methane.

Pyrolysis Processes. There has been recent interest in the use of pyrolysis processes to
produce hydrogen. Specifically the interest has focused on the pyrolysis of methane (natural
gas) and hydrogen sulfide.