Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
2001-07-31
2003-12-30
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C048S077000, C048S101000, C048S210000, C422S142000, C422S198000, C423S648100
Reexamination Certificate
active
06669917
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to methods and apparatus for converting coal, air and high temperature steam into three separate gas streams—one consisting of wet, substantially pure hydrogen, a second containing “sequestration-ready” carbon dioxide, i.e., CO
2
that is relatively pure and is at an elevated pressure thereby rendering its disposal less difficult, and a third stream consisting of oxygen depleted air.
More particularly, the invention relates to a process in which mixtures of coal, calcium and iron compounds are circulated among multiple reactors charged with either high temperature steam or compressed air that produce essentially pure hydrogen for use in fuel cells as a product of a controlled gasification reaction. The process according to the invention results in a separable and substantially pure carbon dioxide waste stream having residual amounts of sulfur dioxide, and an oxygen depleted air stream having high temperature heat that can be used, for example, in downstream power generation subsystems. The oxidation/reduction reactions of the present invention are much more thermodynamically efficient than conventional fossil fuel mixed combustion systems and offer significant environmental advantages over prior art processes using coal or other fossil fuels or biomass fuels to generate heat and combustion gases for use in gas turbine engines.
During the 21
st
century, the United States will continue to rely heavily on fossil fuels, such as natural gas, oil and petroleum distillates, as the primary source of fuel for gas turbine engines used to generate electrical power. Recently, the use of substantially pure hydrogen in fuel cells has been found to be more efficient and virtually pollution-free as compared to other conventional fossil fuel/air combustion technologies. Hydrogen fuel cells would be an ideal solution to many of the nation's energy needs as a clean-burning fuel source. However, the need exists for a thermodynamically-efficient and economical process capable of producing large amounts of pure hydrogen from a readily available and inexpensive energy resource such as coal.
Various conventional systems exist for oxidizing (burning) coal to generate free hydrogen in addition to producing heat for generating steam. Invariably, such systems pose significant environmental problems because of the potential release of oxidized carbon and sulfur compounds into the atmosphere from burning coal. Conventional hydrogen generating methods also involve high equipment costs due to the inefficiencies inherent in attempting to recover and isolate hydrogen from the other products of fossil fuel/air combustion.
It is al so well known that the carbon dioxide resulting from coal-fired systems contributes to the greenhouse effect in the atmosphere and potential global warming. Other types of air pollution produced by coal combustion include particulate emissions, such as fine particles of ash resulting from pulverized coal firing, as well as the release of undesirable oxides of nitrogen, chiefly NO and NO
2
.
Thus, a significant need exists to produce relatively pure free hydrogen for use in electrical power generation in an economical and thermodynamically efficient manner, but without polluting the atmosphere. The need also exists to control the nature and extent of any carbon dioxide, and sulfur dioxide emissions created during coal combustion by isolating and disposing of the oxidized contaminants without releasing them into the atmosphere. Ideally, coal and other fossil fuels could be used to generate heat in a manner that allows the by-products of combustion, particularly CO
2
, to be readily and economically recovered at elevated pressure and in a relatively pure state, i.e., making the CO
2
“sequestration-ready.”
In the past, a number of different CO
2
disposal methods have been proposed such as pumping liquid CO
2
into deep parts of the ocean. However, one recurring problem in the disposal of CO
2
concerns the purity of the waste stream itself. Since most disposal options involve liquid CO
2
, it is generally accepted that for CO
2
to be “sequestration-ready,” it cannot contain more than small amounts of impurities or other gases that do not liquefy under pressure.
In addition to air pollution problems, the combustion of coal to drive gas turbine engines suffers from the same limitations in thermodynamic efficiency inherent in all systems that rely on mixed (air) combustion of coal as the primary heat source. Gas turbines are considered to be among the lowest capital cost systems available for generating electrical power. However, their thermodynamic efficiency is notably lower than other systems. Although the efficiency increases with increasing turbine inlet temperature, the hot gases produced by coal firing contain fly ash which can be erosive to turbine blades. The higher temperature exhaust vapors can also be corrosive because of the acidic by-products of coal combustion, such as sulfur dioxide and HCl. Consequently, the maximum turbine inlet temperature that can be tolerated for coal firing is considerably lower than that associated with a “clean” fuel, such as oil or natural gas.
Over the years, some improvements in gas turbine metallurgy have increased the inlet temperatures that could be tolerated with coal-fired systems. By definition, the same technological advances serve to increase the inlet temperatures for cleaner fuels such as natural gas. Thus, the disadvantages of coal relative to cleaner fuels remain regardless of the gas turbine metallurgy involved and prevent coal despite its lower cost from being considered an attractive gas turbine fuel. The gas turbine industry has long recognized that if a process could be developed for burning coal in a manner that produced large quantities of relatively “clean” hot gases that were not erosive or corrosive, coal could become a much more economically viable fuel source for use in electrical power generation.
One proposed solution to the problem of using coal to power gas turbines is a process known as “gasification” in which coal and steam are fed to a high temperature reactor vessel and react to form a mixture of H
2
, CO and CO
2
. Because the gasification reaction is endothermic, heat must be supplied in some manner. Thus, in most gasification designs, air is mixed with the high temperature steam so that a portion of the coal burns while the remainder reacts with steam to form H
2
, CO and CO
2
. In other designs, a portion of the fuel solids are heated by combustion and then mixed with coal and steam to supply the heat needed to drive the gasification reaction forward.
The literature describes a coal gasification process in which a CO
2
acceptor (either limestone or dolomite) circulates between a pair of fluid beds, one fluidized with steam and the other with air. See G. P. Curran, C. E. Fink, and E. Gorin (Chapter 10 in FUEL GASIFICATION, ACS Advances in Chemistry series 69, 1967). The temperature in the steam-fluidized bed remains low enough so that the CaO+CO
2
═CaCO
3
reaction gasifies coal to virtually pure hydrogen. Only part of the carbon in the coal, however, becomes gasified in the steam fluidized reactor. The remainder moves to an air fluidized bed where it is oxidized (“burned”), liberating heat and decomposing the CaCO
3
back into CaO. Since the CO is in equilibrium with the CO
2
via the well-known water gas shift reaction, removal of the latter removes the former. The basic gasification process has the advantage of producing relatively pure hydrogen, but suffers from a disadvantage in that the CO
2
is released directly into the atmosphere along with air and other oxidized by products of coal combustion such as sulfur dioxide.
U.S. Pat. Nos. 5,339,754; 5,509,362; and 5,827,496 (incorporated herein by reference) disclose a method for burning fuels using a catalyst that can be readily reduced when in an oxidized state, and then readily oxidized when in a reduced state. The fuel and air are alternately contacted with the catalyst. The fuel
General Electric Co.
Langel Wayne A.
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