Power plants – Motive fluid energized by externally applied heat – Process of power production or system operation
Reexamination Certificate
2000-01-13
2001-05-29
Nguyen, Hoang (Department: 3748)
Power plants
Motive fluid energized by externally applied heat
Process of power production or system operation
C060S651000, C060S671000
Reexamination Certificate
active
06237339
ABSTRACT:
The present invention relates to a process for generating heat and/or power comprising a membrane reactor where a fuel is oxidised and further comprising an improved method for reducing emission of carbon dioxide and emission of oxides of nitrogen from said process.
Due to the environmental aspects of CO
2
and NO
x
and taxes on the emissions to the national authorities the possibilities for reducing the emissions of these compounds to the atmosphere from combustion processes, in particular from flue gas from gas turbines offshore, has been widely discussed.
Conventional combustion processes, used for carbon containing fuels and where the oxygen source is air, produce carbon dioxide concentrations of 3-15% in the exhaust gas dependent on the fuel and the applied combustion—and heat recovery process. The reason the concentration is this low is because air is made up of about 78% by volume of nitrogen.
Thus, a reduction in the emission of carbon dioxide makes it necessary to separate the carbon dioxide from the exhaust gas, or raise the concentration to levels suitable for use in different processes or for injection and deposition.
CO
2
can be removed from exhaust gas by means of several separation processes e.g. chemical active separation processes, physical absorption processes, adsorption by molecular sieves, membrane separation and cryogenic techniques. Chemical absorption by means of alkanole amines is considered as the most practical and economical method to separate CO
2
from power plant exhaust gas.
But this method does require heavy and voluminous equipment and will reduce the power output by about 10% or more. These known methods are however considered as not being very suited to practical implementation in a power generation process. In e.g. natural gas based power plants the fuel cost comprises a substantial part of the total cost of electric power. A high efficiency is therefore very important in order to reduce the cost of electric power.
In the mono ethanol ammine (MEA) process CO
2
from the cooled power plant exhaust gas reacts with aqueous solution of MEA in an absorption tower. Most of the CO
2
is thus removed from the exhaust gas that is released to the atmosphere. MEA will be degraded and e.g. a 350 MW combined cycle power plant will produce about 4000 tons MEA degration products per year which has to be destroyed or stored.
In order to meet national NO
x
control requirements different methods can be used for instance burner modifications, applications of catalytic bumers, steam additions or selective catalytic reduction (SCR) of the NOx in the exhaust gas. When air is used in combustion processes some of the nitrogen will be oxidised during the combustion to NO, NO
2
and N
2
O (referred to as thermal NO
x
). At least 80-98% of the NO
x
formed arises from the said oxidation of nitrogen in air. The rest arises from oxidation of the nitrogen content in the fuel.
A method to both increase the concentration of CO
2
in an exhaust gas and to reduce the NO
x
formation is to add pure oxygen to the combustion process instead of air.
However, commercial air separation methods (cryogenic separation and PSA) will require 250 to 300 KWh/ton oxygen produced. Supplying oxygen e.g. to a gas turbine by this method will decrease the net power output of the gas turbine cycle by at least 20%. The cost of producing oxygen in a cryogenic unit will increase the cost of electric power substantially and may constitute as much as 50% of the cost of the electric power.
The main object of this invention was to arrive at a more efficient heat and power generating process comprising a combustion process which produce an exhaust gas with a high concentration of CO
2
and a low concentration of NO
x
that makes the exhaust gas stream suitable for direct use in different processes or for injection in a geological formation for long term deposition or for enhanced oil or natural gas recovery.
Another object of the invention was to supply oxygen to the combustion process which implies reduced energy demands compared to other known methods.
A further object was to utilise existing process streams in the power generation plant in obtaining improved oxygen supply to the combustion process.
The problem mentioned above concerning reduced fuel efficiency and high costs can partly be solved by application of mixed conducting membranes which is defined as a membrane made from material with both ion and electronic conductivity.
Such a membrane can be a mixed oxygen ion and electron conducting membrane, for instance capable of separating oxygen from oxygen-containing gaseous mixtures at 400-1300° C. An oxygen partial pressure difference causes oxygen ions to be transported through the membrane by reduction of oxygen on the high oxygen partial pressure side (feed side) and oxidation of the oxygen ions to gas on the low oxygen partial pressure side (the permeate side). In the bulk of the membrane oxygen ions are transported by a diffusive process. Simultaneously the electrons flow from the permeate side back to the feed side of the membrane.
The application of these membranes is a rather new technique and is generally known from the European patent application 0658 367 A2 which describe separation of oxygen from air by means of a mixed conducting membrane which is integrated with a gas turbine system. Pure oxygen near atmospheric pressure or below and at high temperature is recovered from the permeate side of the conducting membrane. This method, however, entails that the oxygen has to be cooled to below approximately 50° C. and recompressed to required process pressure before being added to the oxidation reactor or burner in a combustion process.
The inventors have applied a mixed oxygen ion and electron conducting membrane reactor, hereafter called a membrane reactor, to combine the supply of oxygen and burning of a fuel giving a hot gas mixture consisting of CO
2
and water and minor amounts of CO and H
2
.
The principle of the electropox process as described in European patent application 0 438 902 A3 could be adopted for this membrane burner or the principle of the electrochemical reactor described in U.S. Pat. No. 5,356,728. Complete combustion of the fuel in the membrane burner is, probably, not possible. However, minor amounts of unconverted partially oxidised fuel in the CO
2
-containing purge gas leaving the gas turbine process, can be oxidised separately in a small catalytic or non-catalytic combustion chamber by mixing the CO
2
-containing purge gas with an oxygen-containing gas or pure oxygen. The CO
2
-containing purge gas could also be injected to a geological formation without further treatment. If the CO
2
-containing exhaust gas is applied for enhanced oil recovery the nearly zero oxygen content in the exhaust gas would be an advantage.
Further the inventors have used the recycled carbon dioxide or a mixture of carbon dioxide and water, e.g. part of the exhaust gas, from the combustion process as a coolant in the membrane reactor. Carbon deposition on the second surface (the oxidation side) can be avoided by properly selecting of catalyst material and by properly adjusting the ratio between fuel and recycled CO
2
and H
2
O containing exhaust gas. By applying the membrane reactor, oxygen could be recovered and reacted with a fuel without intermediate cooling and recompression of the oxygen as required in the European application 0658367. An additional advantage is that the operation pressure on the feed side of the membrane reactor can be lower or much lower than the operation pressure on the oxidation side of the membrane burner because the partial pressure of oxygen on the oxidation side will be less than about 10
−15
bar due to the oxidation reactions. This implies that oxygen can be supplied to a high pressure oxidation process without a first compression of air and the result of that is increased efficiency of the power production. In a conventional gas turbine power generator the compression of oxygen consumed in the combustion process constitute about 6 to 10% of
Åsen Knut Ingvar
Longva Petter Inge
Sandvold Erik
Nguyen Hoang
Norsk Hydro ASA
Wenderoth , Lind & Ponack, L.L.P.
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