Process for generating power including a combustion process

Details Process for generating power including a combustion process Process for generating power including a combustion process

1999-12-21

2001-10-09

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

06298664

ABSTRACT:

The present invention relates to a process for generating power and/or heat in a combustion process where a fuel is combusted with an oxidant and further comprising an improved method for reducing the emissions of CO
2
and oxides of nitrogen to the atmosphere from said combustion 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 exhaust gas from gas turbines offshore, have 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. The recovered CO
2
may for instance be injected to a geological formation for long term deposition or for enhanced recovery of oil from an oil and natural gas reservoir. CO
2
can be removed from exhaust gas by means of several separation processes e.g. chemical separation processes, physical absorption processes, adsorption by molecular sieves, membrane separation and cryogenic techniques. Chemical absorption for instance by means of alkanole amines, is e.g. considered as the most practical and economical method to separate CO
2
from power plant exhaust gas. The method, however, require heavy and voluminous equipment and will reduce the power output with about 10% or more. Therefore this method is not very suited to practical implementation in a power generation process. In 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 MEA (mono ethanol amine) process CO
2
from the cooled power plant exhaust gas reacts with aqueous solution of MEA in a absorption tower. Most of the CO
2
is thus removed from the exhaust gas before the exhaust gas is released to the atmosphere. The aqueous solution containing MEA-CO
2
compounds is pumped to a stripper section where the reaction is reversed through heating with steam. The CO
2
and water vapour leaving the stripper is then cooled and CO
2
leaves the separation plant for further treatment. The MEA process will generate waste materials. E.g. a 350 MW combined cycle power plant will produce about 4000 tons MEA degradation products per year which has to be destructed or stored.
In order to meet national NO
x
emission requirements different methods can be used, for instance burner modification, applications of catalytic burners, steam additions or selective catalytic reduction (SCR) of the NO
x
in the exhaust gas when air used in combustion processes some of the nitrogen is 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 oxidation of nitrogen in said air. The rest arises from oxidation of the nitrogen content in the fuel. Lowering the peak combustion temperature is a very effective method of reducing the amount of NO
x
formed. Unfortunately this causes a substantial efficiency drop due to poor combustion or due to reduced temperature in the combustion chamber in a gas turbine system. SCR is an efficient method in reducing the NO
x
but require a reduction agent such as ammonia and an expensive catalyst installed downstream the combustion process.
One 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.
Commercial air separation methods (e.g. cryogenic separation or pressure swing absorption (PSA)) will require 250 to 300 KWh/ton oxygen produced. Supplying oxygen e.g. to a gas turbine by this methods will decrease the net power output of the gas turbine cycles 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/or 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
making 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 and 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 or heat generation plant in obtaining improved oxygen supply to the combustion process.
The problems mentioned above concerning reduced fuel efficiency and high costs can partly be solved by application of mixed conducting membranes which are defined as membranes made from materials with both ionic 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 to be transported through the membrane by reduction of oxygen on the high oxygen partial pressure side (the feed side) and oxidation of oxygen ions to oxygen 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.
Application of these membranes is rather new technique and is generally known from European patent application no. 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, however, entail 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.
Supplying oxygen to a natural gas based power plant by means of mixed conducting membranes as described in said European application could reduce the energy consumption to about 120-150 KWh/ton oxygen produced and the loss in the power output from a gas turbine to about 10% compared to the use of a cryogenic unit as described above. The energy penalty, however, is still high and at the same level as in e.g. the CO
2
absorption recovery process because oxygen has to be cooled and recompressed before being added to the combustion process.
The inventors found to applicate the exhaust gas or a part of the exhaust gas, from the combustion process as sweep gas in a mixed conducting membrane unit to pick up oxygen from the membrane surface. By feeding the exhaust gas containing CO
2
- and H
2
O to the permeate side of the membrane, oxygen could be picked up and supplied to a combustion process without intermediate cooling and recompression of the oxygen as required in the European application 0658367. This is possible because the recycled part of the exhaust gas is acting as a coolant in the combustion process instead of nitrogen which is acting as a coolant in conventional processes. The method will give and O
2
/CO
2
/H
2
O gaseous mixture for combustion rather than ordinary air. Thus, the said method comprises a first air-based power and/or heat generating process that generates an exhaust gas with a high concentration of nitrogen a

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