Integrated Coa lGasification Combined Cycle IGCC

Page | 001

1.2
Integrated Coal
Gasifi cation Combined
Cycle (IGCC)
Gary J. Stiegel
NETL
626 Cochrans Mill Road,
P.O. Box 10940
Pittsburgh, PA 15236
email: gary.stiegel@netl.doe.gov
1.2-1 Introduction
Integrated Coal Gasifi cation Combined Cycle (IGCC) refers to the
technology of converting coal to a fuel gas by contacting it with a mixture
of oxygen (or air) and steam, burning the fuel gas in a combustion turbine/
generator, using the waste heat from the turbine to raise steam, and sending
the steam to a steam turbine for additional power generation. IGCC has a
number of technical advantages, but until recently, higher capital costs plus
the availability of cheap natural gas have limited its application. However,
as pollution limits become more stringent and natural gas prices increase, the
superior performance of IGCC will make it increasingly attractive, particularly
as technical advances reduce costs.
Gasifi cation is a well-proven technology that had its beginnings in
the late 1700s. In the 19th century, gasifi cation was used extensively for the
production of “town gas” for urban areas. Although this application has all
but vanished in the 20th century with the widespread availability of natural
gas, gasifi cation has found new applications in the production of fuels and
chemical feed stocks and in large-scale power generation. Today, gasifi cation
technology is being widely used throughout the world. A study conducted
in 2004 indicated that there were 156 gasifi cation projects worldwide. Total
capacity of the projects in operation was 45,000 MW (thermal) with another
25,000 MW (thermal) in various stages of development.
1.2-2 The Gasifi cation Process1
Massood Ramezan
phone: (412) 386-6451
email: massood.ramezan@sa.netl.
doe.gov
Howard G. McIlvried
phone: (412) 386-4825
email: howard.mcilvried@sa.netl.doe.
gov
SAIC
P.O. Box 10940
Pittsburgh, PA 15236
59
59
The major difference between combustion and gasifi cation from the
point of view of the chemistry involved is that combustion takes place under
oxidizing conditions, while gasifi cation occurs under reducing conditions. In
the gasifi cation process, a carbon-based feedstock in the presence of steam and
oxygen at high temperature and moderate pressure is converted in a reaction
vessel called a gasifi er to synthesis gas, a mixture of carbon monoxide and
hydrogen, generally referred to as syngas. The chemistry of gasifi cation is quite
complex and involves many chemical reactions, some of the more important
of which are:
C + O2  CO2 ΔH
r = -393.4 MJ/kmol (1)
C + ½ O2  CO ΔH
r = -111.4 MJ/kmol (2)
C + H2O  H2 + CO ΔH
r = 130.5 MJ/kmol (3)
C + CO2 ↔ 2CO ΔH
r = 170.7 MJ/kmol (4)
CO + H2O ↔ H2 + CO2 ΔH
r = -40.2 MJ/kmol (5)
C + 2H2  CH4 ΔH
r = -74.7 MJ/kmol (6)
Reactions (1) and (2) are exothermic oxidation reactions and provide
most of the energy required by the endothermic gasifi cation Reactions (3)
and (4). The oxidation reactions occur very rapidly, completely consuming
all of the oxygen present in the gasifi er, so that most of the gasifi er operates
under reducing conditions. Reaction (5) is the water-gas shift reaction, which
in essence converts CO into H2. The water-gas shift reaction alters the H2/
CO ratio in the fi nal mixture but does not greatly impact the heating value of
the synthesis gas, because the heats of combustion of H2 and CO on a molar
basis are almost identical. Methane formation, Reaction (6), is favored by
high pressures and low temperatures and is, thus, mainly important in lower-
temperature gasifi cation systems. Methane formation is an exothermic
reaction that does not consume oxygen and, therefore, increases the effi ciency