Furnaces – Process – Treating fuel constituent or combustion product
Reexamination Certificate
2001-07-31
2002-12-17
Lazarus, Ira S. (Department: 3749)
Furnaces
Process
Treating fuel constituent or combustion product
C110S347000, C110S245000, C110S233000, C060S039120
Reexamination Certificate
active
06494153
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an improved method of burning coal in order to power gas turbines by providing virtually complete oxidation of the coal using unmixed combustion, but without the adverse environmental consequences associated with conventional coal-fired systems.
The types of air pollution produced by coal combustion are well known and include particulate emissions such as fine particles of ash from pulverized coal firing, the oxides of sulfur, SO
2
and SO
3
, carbon monoxide emissions, and the emission of two of the oxides of nitrogen, NO and NO
2
. More recently, the problem of global warming due to Greenhouse gas emissions has been recognized, and the emissions of CO
2
from power plants and other combustion systems have become a matter of serious environmental concern.
The problem of global warming due to CO
2
emissions from coal and other fossil fuel combustion processes involves two basic issues. First, because of the significant amount of carbon dioxide released by conventional coal-fired systems, a need exists to modify the processes so that the CO
2
leaves the process in a form that allows it to be readily disposed, i.e., producing “sequestration ready” CO
2
that can be isolated and disposed of without release into the atmosphere. Second, the need exists to find improved methods for disposing of CO
2
and other waste gases that are technically feasible and economically viable. Since many proposed disposal options all involve liquid CO
2
(such as pumping liquid CO
2
into deep parts of the ocean), it is generally accepted that for the CO
2
to be “sequestration ready,” it need not be free of all impurities, but must not contain more than small amounts of other gas components that do not liquefy under pressure at ambient temperature.
In addition to known air pollution/CO
2
emission problems, the combustion of coal has certain disadvantages relative to other fuels in powering gas turbine engines. Gas turbines are the lowest capital cost systems available for generating electrical power on a large scale. However, the thermodynamic efficiency of gas turbines normally is lower than other higher capital cost systems. As a general proposition, the thermodynamic efficiency improves by increasing the inlet temperature of the gas working fluid. Thus, recent efforts to maximize turbine efficiency have focussed on increasing the turbine inlet temperature. As a result, turbine blades and other key components of gas turbines exposed to the working fluid have been engineered to tolerate higher first stage inlet gas temperatures.
Despite the possibility for increased operating temperatures in gas turbines, a known disadvantage of coal-fired systems concerns the composition of hot gases produced by coal oxidation. The exhaust gases from coal contain fly ash that can be erosive to and corrosive to most metals used for conventional gas turbine blades. Consequently, the maximum turbine inlet temperature for coal firing typically is lower than that for a “clean” fuel. Although improvements in gas turbine design and metallurgy have resulted in higher inlet gas temperatures for coal-fired systems, the maximum temperatures for clean fuels also increased. Thus, the efficiency disadvantage of coal relative to clean fuels remained. Over the years, this disadvantage has prevented the lower cost of coal from being considered as the principal gas turbine fuel in this country. Thus, if a process were developed whereby coal could be burned in a manner that produced hot gases that were not erosive or corrosive, particularly at higher inlet temperatures, coal could become a much more attractive gas turbine fuel.
With respect to global warming, coal has the further disadvantage that its CO
2
emissions per BTU produced are significantly higher than those of “cleaner” fossil fuels. Again, if the means existed whereby coal could be burned in a manner that did not result in the emission of large amounts of CO
2
and/or other pollutants, this disadvantage would disappear.
U.S. Pat. Nos. 5,339,754 and 5,509,362; and 5,827,496 (incorporated herein by reference) disclose a new method of burning solid fuels such as coal known as “unmixed combustion” which involves the use of a catalyst that is readily reduced when in an oxidized state and readily oxidized when in a reduced state. In essence, fuel and air are alternatively contacted with the catalyst. The fuel reduces the catalyst and is oxidized to CO
2
and water vapor. The air in turn oxidizes the catalyst and becomes depleted of oxygen. Thus, combustion can be effected without the need of mixing the fuel and air prior to or during the combustion process. If means are provided whereby the CO
2
and water vapor and the oxygen depleted air can be directed in different directions as they leave the combustion process, then mixing can be completely avoided.
The total volume of combustion gases produced by unmixed combustion is comparable to that produced in conventional combustion. However, the volume of the CO
2
+water vapor steam represents only small part of the total. Those skilled in the art recognize that the cost of removing acid gases such as SO
2
, HCl and HF from combustion effluents by scrubbing increases with the volume of gas being scrubbed. Thus, if unmixed combustion can be done in such a manner that the acid gases leave the combustion process in the CO
2
+water vapor steam, the volume of gas which must be scrubbed is greatly reduced. Likewise, the cost of scrubbing becomes significantly lower.
The '362 patent notes that operating unmixed combustion in a manner such that the acid gases leave the combustor in the CO
2
+water vapor steam requires an appropriate choice of catalyst. The patent is discussed in greater detail in Paper 98F-36, presented at the October 1998 meeting of the Western States Section of the Combustion Institute. The authors of the paper (hereafter referred to as the “Combustion Institute paper”) include R. K. Lyon, a named inventor of the '362 patent, and J. A. Cole. The present invention involves an improvement to the basic combustion process described in the patent and 1998 paper.
The Combustion Institute paper discloses a conceptual process for using coal to power a gas turbine and reports on a series of experiments illustrating certain specific aspects of the process. One such experiment uses a fluid bed of powdered chemically pure iron oxide (i.e., FeO/Fe
2
O
3
) operated at atmospheric pressure in which the gas used to fluidize the bed is switched from air to a mixture of 5% SO
2
+95% N
2
and back again.
Various two-step experiments were done with this setup. In the first step, a bed fully oxidized to Fe
2
O
3
was fluidized with the 5% SO
2
+95% N
2
at a temperature of 857° C. A small amount of Illinois coal was then injected into the bed while the gases coming out of the bed were continuously analyzed. In the second step, the fluidizing gas was switched to air while the gases coming out of the bed were continuously analyzed. Based on this data, the Combustion Institute paper concludes that in the presence of SO
2
coal is readily oxidized and that the chief carbon-containing product of this oxidation is CO
2
with little or no CO being produced.
The paper attributes the ability of the solid particles of Fe
2
O
3
to rapidly oxidize the solid particles of coal to a catalytic action by the SO
2
used in the fluidizing gas. In this catalytic process, the SO
2
reacts with the coal, converting it into to CO
2
, CO, CS
2
, COS, and sulfur vapor. The CO, CS
2
, COS and sulfur vapor are oxidized by the Fe to CO
2
and SO
2
. The gases exiting the bed when fluidized with air contained little or no SO
2
and little or no CO and CO
2
. From this, the Combustion Institute paper concludes that the Fe
2
O
3
oxidized the coal to completion during the first step, i.e., while the bed was fluidized with 5% SO
2
+95% N
2
. The oxidation converted all the sulfur in the coal to SO
2
and other volatile species which exited the bed during the first step of t
General Electric Co.
Rinehart K. B.
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