GB2571353A

GB2571353A - Method

Info

Publication number: GB2571353A
Application number: GB1803180.7A
Authority: GB
Inventors: Mainza De Koeijer Gelein; Johan Rørtveit Geir
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2018-02-27
Filing date: 2018-02-27
Publication date: 2019-08-28
Also published as: GB201803180D0

Abstract

A method of generating electricity and/or shaft power and capturing CO2, using a reciprocating engine comprises recycling a first portion 6 of exhaust gas into the reciprocating engine, oxidising a second fuel in the presence of a second portion 5 of the exhaust gas to produce a CO2-rich exhaust gas 7, cooling the CO2-rich exhaust gas and capturing CO2 contained in the CO2-rich exhaust gas 8 in a CO2 capture unit. The system may provide power for offshore operations. The high concentration of carbon dioxide in the CO2 rich exhaust gas achieved in this invention allows the use of a compact CO2 capture unit, e.g. with a height of 10 to 40 metres.

Description

Method

FIELD OF THE INVENTION

The present invention relates to a method of generating electricity and/or shaft power and capturing CO₂. The invention further relates to systems for carrying out the method of the invention.

BACKGROUND

Greenhouse gases, such as CO₂, in the earth’s atmosphere help to regulate global temperatures through the greenhouse effect. Greenhouse gases are therefore essential to maintaining the temperature of the earth so that it is habitable to humans, animals and plants alike. However, excess greenhouse gases in the atmosphere contribute to global warming by raising the temperature of the earth to harmful levels. The effects of global warming have already begun to be observed, e.g. in rising sea levels and in the melting of polar ice caps. According to simulation models, an increased CO₂ concentration in the atmosphere is suspected to cause further and potentially more dramatic changes in the climate in the future. As a result, scientists, environmentalists and politicians throughout the world are driving initiatives to reduce the amount of CO₂ discharged into the atmosphere by combustion of fossil fuel.

A primary cause of increased CO₂ levels in the atmosphere is the burning of fossil fuels such as coal, oil, or natural gas. One approach being adopted to minimise the environmental impact of facilities that burn large amounts of fossil fuels is to capture CO₂ (i.e. prevent the release of CO₂) from the exhaust gases, e.g. from thermal power plants, before they are released to the atmosphere. The captured CO₂ may be injected into subterranean formations such as aquifers, oil wells for enhanced oil recovery or in depleted oil and gas wells for storage since tests indicate that CO₂remains in the subterranean formation for thousands of years and is not released back into the atmosphere.

With offshore operations, such as offshore facilities for generating electricity and/or shaft power (via mechanically coupled drives), there is a desire to employ compact and low weight equipment in order to save space while still ensuring a good efficiency. In light of this, various technologies exist for compact power production, e.g. single cycle gas turbines, combined cycle gas turbines, etc.

In a typical single cycle gas turbine power plant, a fossil fuel is burnt in the presence of compressed air. The resultant high temperature and high pressure gas is used to drive a turbine, which in turn drives a shaft for shaft power or a generator to generate electricity. A combined cycle gas turbine power plant operates in a similar fashion to a single cycle gas turbine power plant, except that the operation makes use of the remaining thermal energy of the exhaust gas exiting the gas turbine to produce extra electricity (e.g. by using it to generate steam which can drive a steam turbine).

Whilst it is known to combine single cycle gas turbines or combined cycle gas turbine with a CO₂ capture unit, these combinations are heavy and demand large amounts of space. The combinations are also not particularly energy efficient because numerous utilities and pre-treatment steps must be carried out as part of the CO₂capture process. The combinations are therefore particularly problematic for use in offshore operations due to the lack of space and the limited energy supply available.

SUMMARY OF INVENTION

Thus viewed from a first aspect the present invention provides a method of generating electricity and/or shaft power and capturing CO₂, comprising:

(i) combusting a first fuel in a reciprocating engine to generate electricity and/or shaft power and an exhaust gas;

(ii) dividing said exhaust gas into at least a first portion and a second portion;

(iii) recycling said first portion of said exhaust gas into said reciprocating engine;

(iv) oxidising a second fuel in the presence of said second portion of said exhaust gas to produce a CO₂-rich exhaust gas;

(v) cooling the CO₂-rich exhaust gas; and (vi) capturing CO₂ contained in the CO₂-rich exhaust gas in a CO₂ capture unit.

Viewed from a further aspect the present invention provides a system for generating electricity and/or shaft power and capturing CO₂comprising:

(a) a reciprocating engine for generating electricity and/or shaft power having an inlet for fuel, an inlet for air, an inlet for recycled exhaust gas and an outlet for an exhaust gas;

(b) a means for recycling exhaust gas back into said reciprocating engine;

(c) an oxidation unit fluidly connected to said outlet for exhaust gas of said reciprocating engine and having an inlet for exhaust gas, an inlet for fuel and an outlet for CO₂-rich exhaust gas;

(d) a cooling unit fluidly connected to said outlet for CO₂-rich exhaust gas of said oxidation unit and having an inlet for CO₂-rich exhaust gas and an outlet for cooled CO₂-rich exhaust gas; and (e) a CO₂ capture unit fluidly connected to said outlet for cooled CO₂-rich exhaust gas of said cooling unit and having an inlet for cooled CO₂-rich exhaust gas, an outlet for CO₂-lean exhaust gas and an outlet for CO₂.

DEFINITIONS

As used herein, the term “compact CO₂ capture unit” refers to a CO₂ capture unit having a 3 to 50 MW electricity or equivalent shaft power output, more preferably a 10 to 20 MW electricity, or equivalent shaft power, output, when coupled to a reciprocating engine. The compact capture unit has a volume and/or weight which is less than a conventional CO₂ capture unit e.g. a combined cycle gas turbine of ~25 MW electricity with monoethanolamine (MEA) based post-combustion with an equipment weight of roughly 1000 tonnes.

As used herein, the term “means” includes a pipe.

DETAILED DESCRIPTION

The present invention provides a method of generating electricity and/or shaft power and capturing the CO₂ generated in the process, which is both more compact and efficient than the known technology. This is achieved in the method of the invention by employing a reciprocating engine to generate electricity and/or shaft power. In contrast, most new technologies emerging today employ single cycle gas turbines (SCGTs) or combined cycle gas turbines (CCGTs) because these are more efficient in terms of energy production. It has been found, however, that when CO₂capture is carried out, the total efficiency, both in terms of energy and space, of the method of the invention is improved over SCGT or CCGT technologies.

The method of the present invention improves space and energy efficiency by utilising a reciprocating engine and a catalytic oxidation unit to increase the CO₂concentration in the exhaust gas, from where CO₂ is ultimately captured, compared to the CO₂ concentration in the exhaust gas from SCGT or CCGT technologies. By increasing the CO₂concentration in the exhaust gas, the active surface area of the CO₂capture unit can be significantly smaller than in the units capturing C0₂from SCGT or CCGT engines, meaning the capture unit takes up less space, weighs less and requires less energy to operate. The latter means that the total energy efficiency of the process is increased, when the energy required to carry out CO₂ capture is taken into account.

A reciprocating internal combustion engine (or reciprocating engine or piston engine) converts chemical potential energy in fuel to mechanical shaft power using one or more reciprocating pistons. The reciprocating engines employed in the methods of the present invention can be two-stoke cycle engines, four-stroke cycle engines, or diesel engines. The reciprocating engines are able to tolerate back pressure. Preferred reciprocating engines for use in the methods and systems of the present invention include engines manufactured by GE Jenbacher, MAN Turbo engines and Wartsila engines, e.g. Wartsila 31DF or GE Jenbacher 920 Flextra.

The reciprocating engine employed in the methods of the present invention may drive a generator, which feeds the electricity generated into the consumer grid. The engine is started and accelerated to rated speed and synchronized with the grid. Alternatively, the reciprocating engine may drive a shaft for another reciprocating engine, e.g. for a compressor. Alternatively, the reciprocating engine may drive a generator and a shaft for another reciprocating engine, e.g. for a compressor.

A first fuel is supplied to the combustion chamber(s) of the reciprocating engine in such a way that optimum efficiency or maximum possible output is achieved. Preferred fuels for use in the reciprocating engine include diesel, oil, kerosene, gasoline, jet fuel, natural gas, condensates, propane vapour, and biogas. A particularly preferred fuel is natural gas.

The reciprocating engines used in the methods of the present invention are naturally aspirated, i.e. air enters the combustion chamber(s) under atmospheric pressure. The air that is fed into the reciprocating engine preferably has a temperature of -30 to 60 °C (e.g. 15 °C). Combustion of the first fuel in the combustion chamber(s) in the presence of air causes the reciprocating pistons to move and thereby drive a shaft for another reciprocating engine or a generator to generate electricity.

An exhaust gas is produced during combustion. The temperature of the exhaust gas exiting the reciprocating engine is preferably 200 to 600 °C, more preferably 250 to 500 °C (e.g. 350 °C). The amount of CO₂ in the exhaust gas is preferably 4.5 to 10 mol%, more preferably 5 to 9 %vol (e.g. 8.3 mol%). The exhaust gas may also contain an amount of oxygen, preferably 1 to 10 mol%, more preferably 2 to 9 mol% (e.g. 2.8 mol%).

In the methods of the present invention, at least part of the exhaust gas is recycled back into the reciprocating engine prior to oxidising in step (iv). Preferably at least 5 to 80%, more preferably 20 to 60% (e.g. 40%), of the exhaust gas is recycled back into the reciprocating engine. The effect of recycling at least part of the exhaust gas back into the reciprocating engine is to increase the amount of CO₂ in the exhaust gas which thereby increases the partial pressure of the CO₂ entering the catalytic oxidation unit. Recycling at least part of the exhaust gas back into the reciprocating engine also helps reduce the amount of noxious NO_X gases in the exhaust gas.

The mixed air entering the reciprocating engine, which comprises a combination of air and recycled exhaust gas, preferably has a temperature of -30 to 250 °C, more preferably 0 to 200 °C (e.g 163 °C). The amount of CO₂ in the mixed air entering the reciprocating engine, which comprises a combination of air and recycled exhaust gas, is preferably 0.5 to 5 mol%, more preferably 1 to 4 mol% (e.g. 3.5 mol%). The amount of oxygen in the mixed air entering the reciprocating engine, which comprises a combination of air and recycled exhaust gas, is preferably 10 to 30 mol%, more preferably 12 to 20 mol% (e.g. 13.2 mol%).

In step (iv), a second fuel is oxidised in the presence of the exhaust gas to produce a CO₂-rich exhaust gas. More specifically, oxygen present in the exhaust gas reacts with the second fuel to generate further CO₂. Preferred fuels include gasoline, natural gas, propane vapour, and biogas. A particularly preferred fuel is natural gas. When step (iv) is conducted in the presence of a catalyst, heavy fuels and sulphur containing fuels are less suitable due to catalyst poisoning.

In the methods of the present invention, step (iv) is preferably conducted in the presence of a catalyst. Preferred catalysts comprise platinum, palladium, rhodium, or mixtures thereof. More preferably, the catalyst comprises platinum. The catalyst may be unsupported or supported, but is preferably supported. Examples of suitable supported platinum catalysts include Pt/AI₂O₃, Pt/ZrO₂, Pt/TiO₂, Pt/SiO₂ and Pt/H-ZSM5 (where ZSM-5 is Zeolite Socony Mobil-5). The use of a catalytic oxidation in step (iv) means that combustion can occur at lower temperatures without the need for a flame. The temperature of the CO₂-rich exhaust gas is preferably 300 to 700 °C, more preferably 350 to 600 °C (e.g. 564 °C)

Incomplete combustion of the first fuel preferably occurs in the reciprocating engine such that the exhaust gas also contains unburned hydrocarbons and/or carbon monoxide. Oxygen present in exhaust gas can also react with the unburned hydrocarbons and/or carbon monoxide during step (iv) of the methods of the present invention to generate CO₂. This helps to further increase the CO₂ concentration and, resultantly, the CO₂ partial pressure.

In preferred methods of the invention, the mass flow of CO₂ in the gas stream preferably increases by 2 to 50%, more preferably by 5 to 20%, during step (iv) (e.g. 11%).

The amount of CO₂ in the CO₂-rich exhaust gas is preferably 6 to 15 mol%, more preferably 6 to 10 mol% (e.g. 9.2 mol%), i.e. step (iv) causes the concentration of CO₂ in the exhaust gas exiting the reciprocating engine to be increased. In comparison, the amount of CO₂ in the exhaust gas of a conventional single cycle gas turbine or combined cycle gas turbine is typically 3 to 4 mol%.

The amount of oxygen in the CO₂-rich exhaust gas is preferably 0 to 10 mol%, more preferably 0 to 7 mol% (e.g. 1 mol%).

In step (v), the CO₂-rich exhaust gas is cooled. Typically, the CO₂-rich exhaust gas is cooled to a temperature of 10 to 70 °C. However, where the CO₂ capture unit comprises membranes or employs amine technology, the CO₂-rich exhaust gas is preferably cooled to a temperature of 30 to 40 °C. Alternatively, where the CO₂ capture unit is able to tolerate high exhaust temperatures, the CO₂-rich exhaust gas is preferably cooled to a temperature of 70 to 250 °C, more preferably 100 to 150 °C (e.g. 150 °C).

In preferred methods of the invention, heat is recovered from the CO₂-rich exhaust gas during step (v), preferably using a heat recovery unit or a heat exchanger. The recovered heat can be used in numerous applications, e.g. as an energy source for other onsite processes, hot water production, heating of offices/buildings, , etc. Furthermore, any waste water or steam from the cooling or heat recovery process can be recycled back to the reciprocating engine.

In step (vi), CO₂ contained in the CO₂-rich exhaust gas is captured in a CO₂capture unit. In preferred methods of the invention, the CO₂ capture unit is a compact CO₂ capture unit, preferably having a height of 10 to 40 m. In contrast, conventional absorber towers are typically 40 to 60 m tall. The volume and weight of the compact CO₂ capture unit employed in the methods of the present invention are also significantly decreased compared to conventional CO₂ capture units. In preferred methods of the invention, the volume of the compact CO₂ capture unit is at least 30% less, more preferably at least 40% less, even more preferably at least 50% less, than conventional CO₂ capture units, e.g. CO₂ capture units which use a 30 wt% monoethanolamine (MEA) absorption/desorption tower to treat a natural gas-based flue gas (i.e. an atmospheric flue gas containing about 4 mol% CO₂). In preferred methods of the invention, the weight of the compact CO₂ capture unit is at least 30% less, more preferably at least 40% less, even more preferably at least 50% less, than conventional CO₂ capture units, e.g. CO₂ capture units which use a 30 wt% monoethanolamine (MEA) absorption/desorption tower to treat a natural gas-based flue gas (i.e. an atmospheric flue gas containing about 4 mol% CO₂). . As discussed above, the reduced dimensions of the CO₂ capture unit are a direct result of the increased CO₂concentration in the gas stream.

In preferred methods of the invention, the CO₂ capture unit does not require steam, thereby reducing the number of utilities required. More preferably, the CO₂capture unit comprises membranes and/or an absorber.

The CO₂ capture unit may be, for example, a CO₂ capture apparatus comprising an absorption tower and a regeneration tower. Such towers are conventional in the art. Preferably the CO₂-rich exhaust gas is contacted, typically in counter flow, with an aqueous absorbent in an absorber column. The gas leaving the absorber column is preferably CO₂ depleted and can be released to the atmosphere. The CO₂ preferably leaves the absorber column reacted to the absorbent. Typically the absorbent is subsequently regenerated in a regenerator column and returned to the absorber column. The CO₂ separated from the absorbent is preferably sent for storage, e.g. in a subterranean formation.

A particularly preferred absorber is a rotary system absorber such as that outlined in WO 2015/060723. For example, WO 2015/060723 describes a rotary system absorber comprising an absorber for absorbing CO₂ from a gas stream by use of an absorption liquid (e.g. amines, carbonates, amino acid salts) and a desorber for desorbing CO₂ from CO₂-rich absorption liquid. The absorber comprises a rotatable main cylinder having an absorption section provided with rotatable means for disintegration of droplets of absorption liquid and means for rotating the absorber, such that absorption liquid droplets are moved by aid of centrifugal force in a cross flowdirection in relation to the gas stream whereby CO₂ is absorbed from the gas stream by the absorption liquid droplets. The desorber is connected to the absorber to receive CO₂ rich absorption liquid, and the desorber is rotatable and comprises a desorption chamber provided with a rotatable heat exchanger and means for rotating the desorber, such that the absorption liquid droplets are moved by aid of centrifugal force through the heat exchanger whereby the absorption liquid droplets are heated and CO₂is desorbed and separated from the absorption liquid droplets, and lean absorption liquid is circulated to the absorber. A similar rotary system absorber is described in WO 01/45825.

Preferred absorbents for use in absorption apparatus are, for example, aliphatic or cycloaliphatic amines having from 4 to 12 carbons, alkanolamines having from 4 to 12 carbons, cyclic amines where 1 or 2 nitrogens together with 1 or 2 alkylene groups form 5-, 6- or 7-membered rings, mixtures of the above and aqueous solutions of the above amines and mixtures. Representative examples of amines that may be used include monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), diethylethanolamine (DEEA), diisopropylamine (DIPA), aminoethoxyethanol (AEE), methyldiethanolamine (MDEA), piperazine (PZ), 2-amino-2-methyl-1-propanol (AMP) and mixtures of the above and aqueous solutions of the above.

In some preferred methods of the invention, the CO₂ capture unit comprises membranes. Gas separation membranes work on the principle that one component (in this case CO₂) in a gas stream is allowed to pass through the membrane faster than the other components, thereby allowing separation to be achieved. Preferred membranes for use in the methods of the invention include porous inorganic membranes, carbon membranes, silica membranes, polymeric membranes (e.g. polyethylene, polyamides, polyimides, cellulose acetate, polysulphone, polydimethylsiloxane), metal-organic framework membranes, zeolite membranes and perovskite membranes.

In preferred methods of the invention, up to 90 %vol of the CO₂ contained in the cooled CO₂-rich exhaust gas is removed therefrom during step (vi) (e.g. 50 to 90 %vol). In further preferred methods of the invention, up to 98 %vol of the CO₂ contained in the cooled CO₂-rich exhaust gas is removed therefrom during step (vi) (e.g. 50 to 98 %vol). In particularly preferred methods of the invention, up to 99 %vol of the CO₂ contained in the cooled CO₂-rich exhaust gas is removed therefrom during step (vi) (e.g. 50 to 99 %vol).

In preferred methods of the invention, the CO₂-lean exhaust gas exiting the CO₂capture unit has a temperature of 50 to 350 °C, preferably a temperature of 80 to 300 °C (e.g. 150 °C). In preferred methods of the invention, the amount of CO₂ in the CO₂lean exhaust gas exiting the CO₂ capture unit is 0 to 6 mol%, preferably 0 to 4 mol% (e.g. 2 mol%). In preferred methods of the invention, the amount of oxygen in the CO₂9 lean exhaust gas exiting the CO₂ capture unit is 0 to 8 mol%, preferably 0 to 7 mol% (e.g. 1 mol%).

In preferred methods of the invention, the captured CO₂ exiting the CO₂ capture unit has a temperature of 10 to 130 °C, preferably 10 to 100 °C (e.g. 50 °C). In preferred methods of the invention, the amount of CO₂ in the captured CO₂ exiting the CO₂ capture unit is 50 to 100 mol%, preferably 65 to 100 mol% (e.g. 82 mol%). In preferred methods of the invention, the amount of oxygen in the captured CO₂ exiting the CO₂ capture unit is 0 to 15 mol%, preferably 0 to 10 mol% (e.g. 1 mol%).

In preferred methods of the invention, water obtained by cooling in the CO₂capture unit is recycled to the reciprocating engine. Water recycle to the engine helps to increase the mass flow through the engine and therefore also its power output, which in turn results in an increased CO₂ concentration in the exhaust gas. Some reciprocating engines and humidified air turbines (HATs) operate on a similar basis.

The methods of the present invention may comprise the further step of:

(vii) removing oxygen from the captured CO₂.

Oxygen can be removed from the captured CO₂ by way of either a catalytic combustion process or distillation. Preferably, step (vii) involves a catalytic combustion process as this provides the most compact solution.

The methods of the present invention may comprise the further step of:

(viii) storing the captured CO₂.

The captured CO₂ is stored in pure form or, alternatively, is mixed with water or natural gas before being stored.

The captured CO₂ may be injected into subterranean formations such as aquifers, oil wells for enhanced oil recovery or in depleted oil and gas wells for deposition.

The present invention also relates to a system for carrying out the method of the invention hereinbefore described. Preferred features of the method hereinbefore described are also preferred features of the system. The systems of the present invention can be used either onshore or offshore, but are preferably used offshore.

The systems of the present invention comprise a reciprocating engine for generating electricity and/or shaft power having an inlet for fuel, an inlet for air, an inlet for recycled exhaust gas and an outlet for an exhaust gas, and a means for recycling exhaust gas back into the reciprocating engine. The system further comprises an oxidation unit fluidly connected to the outlet for exhaust gas of the reciprocating engine and having an inlet for exhaust gas, an inlet for fuel and an outlet for CO₂-rich exhaust gas. The systems further comprise a cooling unit fluidly connected to the outlet for CO₂rich exhaust gas of the oxidation unit and having an inlet for CO₂-rich exhaust gas and an outlet for cooled CO₂-rich exhaust gas. The systems also comprise a CO₂ capture unit fluidly connected to the outlet for cooled CO₂-rich exhaust gas of the cooling unit and having an inlet for cooled CO₂-rich exhaust gas, an outlet for CO₂-lean exhaust gas and an outlet for CO₂.

As used herein the term “fluidly connected” refers to means to transport a fluid from a first unit to a second unit, optionally via one or more intervening units. The fluid connection may therefore be direct or indirect.

In preferred systems of the invention, the reciprocating engine tolerates back pressure and so the systems do not include a fan for blowing exhaust gas through the CO₂ capture unit.

In preferred systems of the invention, the CO₂ capture unit further comprises an outlet for water, the reciprocating engine further comprises an inlet for water and the reciprocating engine is fluidly connected to the outlet for water of the CO₂ capture unit.

In preferred systems of the invention, said oxidation unit is a catalytic oxidation unit. Preferred catalysts for use in the catalytic oxidation unit are as hereinbefore described.

In preferred systems of the invention, said cooling unit comprises a heat recovery unit or a heat exchanger.

In preferred systems of the invention, said CO₂ capture unit is a compact CO₂capture unit, preferably having a height of 10 to 40 m. In contrast, conventional absorber towers are typically 40 to 60 m tall.

In preferred systems of the invention, the CO₂ capture unit does not require steam, thereby reducing the number of utilities required. More preferably, the CO₂capture unit comprises membranes and/or an absorber. Still more preferably, the CO₂capture unit comprises a rotary system absorber as hereinbefore described.

In preferred systems of the invention, the CO₂-lean exhaust gas exiting the CO₂capture unit is released to the atmosphere.

Preferred systems of the invention further comprise means for removing oxygen from captured CO₂ wherein said means are fluidly connected to the compact CO₂capture unit.

Preferred systems of the invention further comprise means for storing captured CO₂ wherein said means are fluidly connected to the CO₂ capture unit. In a preferred embodiment, said means has an inlet for water. In a preferred embodiment, said means has an inlet for natural gas.

DESCRIPTION OF THE FIGURES

Figure 1 shows a system according to the present invention.

Figure 2 shows a preferred system according to the present invention.

DETAILED DESCRIPTION OF THE FIGURES

Referring to Figure 1, air 1 and fuel 3are fed into a reciprocating engine to produce electricity and/or shaft power. The exhaust gas typically comprises 4.5 to 10 mol% CO₂. The exhaust gas 4 from the reciprocating engine is divided into a first portion 6 and a second portion 5. The second portion 5, which constitutes approximately 20 to 95% of the exhaust 4 from the reciprocating engine and which has a high CO₂ content, is fed into a catalytic oxidation unit. The first portion 6, which constitutes approximately 5 to 80% of the exhaust 4 is recycled back to the reciprocating engine in order to increase the CO₂ concentration in the exhaust gas and reduce levels of NO_X. When exhaust gas recycle is employed, the gas fed to the reciprocating engine typically comprises 0.5 to 5 mol% CO₂

A second fuel is added to the catalytic oxidation unit (shown by the dotted arrow) where it is catalytically oxidised in the presence of the second portion 5 of the exhaust 4 to produce a CO₂-rich exhaust gas 7, which has an even higher concentration of CO₂ than the second portion 5 of exhaust 4. Typically, the concentration of CO₂ in exhaust gas 7 is 6 to 15 mol%. The CO₂-rich exhaust gas 7is then cooled in a cooling unit to a temperature suitable for use in a CO₂ capture unit. The cooled CO₂-rich exhaust gas 8 is then sent to a compact CO₂ capture unit where CO₂ is removed therefrom and sent for storage via line 10.

The typical and preferred amounts of CO₂ in the various gases of this process are summarised in the table below:

Gas	Typical mol% CO₂	Preferred mol% CO₂
Second portion of exhaust from reciprocating engine (5)	4.5 to 10	5 to 9
Air + hot exhaust recycle (2)	0.5 to 5	1 to 4
CO₂-rich exhaust (7)	6 to 15	6 to 10

The embodiment shown in Figure 2 is similar to that shown in Figure 1. The main difference is that instead of a cooling unit being present, the embodiment of Figure 2 employs a heat recovery unit to cool the CO₂-rich exhaust gas 7. Furthermore, the embodiment shown in Figure 2 also incorporates a water recycle in line 11 from the 5 CO₂ capture unit to the reciprocating engine. This feature helps to increase the CO₂concentration in the exhaust 4, and may also help to increase power output.

EXAMPLES

Table 1 shows a modelled case of a system according to the present invention with closed mass and energy balance (e.g. 7200 kJ LHV combustion energy from fuel per kWh electricity delivered at 100% load).

	1	2	5	7	9	10
Description	Air	Mixed air	Engine exhaust	Combustor exhaust	Exhaust to air	CO₂ product
T(°C)	15	163	350	564	150	50
CO₂ (mol%)	0	3.5	8.3	9.2	2	82
O₂ (mol%)	21	13.2	2.8	1	1	1

Table 1

Claims

CLAIMS:

1. A method of generating electricity and/or shaft power and capturing CO2, comprising:

(iv) oxidising a second fuel in the presence of said second portion of said exhaust gas to produce a CO2-rich exhaust gas;

(v) cooling the CO2-rich exhaust gas; and (vi) capturing CO2 contained in the CO2-rich exhaust gas in a CO2 capture unit.

2. A method as claimed in claim 1, wherein 5 to 80% of the exhaust gas is recycled back into the reciprocating engine.

3. A method as claimed in claim 1 or claim 2, wherein the temperature of the exhaust gas is 200 to 600 °C.

4. A method as claimed in any one of claims 1 to 3, wherein the amount of CO2 in the exhaust gas is 4.5 to 10 mol%.

5. A method as claimed in any one of claims 1 to 4, wherein the first fuel is selected from diesel, oil, kerosene, gasoline, jet fuel, natural gas, condensates, propane vapour, and biogas.

6. A method as claimed in any one of claims 1 to 5, wherein incomplete combustion occurs in the reciprocating engine.

7. A method as claimed in any one of claims 1 to 6, wherein the second fuel is selected from gasoline, natural gas, propane vapour, and biogas.

8. A method as claimed in any one of claims 1 to 7, wherein step (iv) is conducted in the presence of a catalyst.

9. A method as claimed in claim 8, wherein said catalyst comprises platinum, palladium, rhodium, or mixtures thereof.

10. A method as claimed in any one of claims 1 to 9, wherein the amount of CO2in the CO2-rich exhaust gas is 6 to 15 mol%.

11. A method as claimed in any one of claims 1 to 10, wherein the temperature of the CO2-rich exhaust gas is 300 to 700 °C.

12. A method as claimed in any one of claims 1 to 11, wherein during step (iv) oxygen present in the exhaust gas reacts with said second fuel to generate CO2.

13. A method as claimed in any one of claims 1 to 12, wherein during step (iv) oxygen present in exhaust gas reacts with unburned hydrocarbons and/or carbon monoxide present in the exhaust gas to generate CO2.

14. A method as claimed in any one of claims 1 to 13, wherein during step (v) heat is recovered from the CO2-rich exhaust gas.

15. A method as claimed in any one of claims 1 to 14, wherein the CO2-rich exhaust gas is cooled to a temperature of 10 to 70 °C.

16. A method as claimed in any one of claims 1 to 15, wherein the CO2 capture unit is a compact CO2 capture unit.

17. A method as claimed in claim 16, wherein the compact CO2 capture unit has a height of 10 to 40 m.

18. A method as claimed in any one of claims 1 to 17, wherein the CO2 capture unit does not require steam.

19. A method as claimed in any one of claims 1 to 18, wherein the CO2 capture unit comprises a membrane.

20. A method as claimed in any one of claims 1 to 18, wherein the CO2 capture unit comprises an absorber.

21. A method as claimed in any one of claims 1 to 20, wherein during step (vi) up to 90 vol% of the CO2contained in the cooled CO2-rich exhaust gas is removed therefrom.

22. A method as claimed in any one of claims 1 to 21, wherein water obtained by cooling in the CO2capture unit is recycled to the reciprocating engine.

23. A method as claimed in any one of claims 1 to 22, comprising the further step of:

(vii) removing oxygen from the captured CO2.

24. A method as claimed in claim 23, comprising the further step of:

(viii) storing the captured CO2.

25. A method as claimed in claim 24, wherein the captured CO2is stored in pure form.

26. A method as claimed in claim 24, wherein the captured CO2 is mixed with water before being stored.

27. A method as claimed in claim 24, wherein the captured CO2 is mixed with natural gas before being stored.

28. A system for generating electricity and/or shaft power and capturing CO2 comprising:

(b) a means for recycling exhaust gas back into said reciprocating engine;

(c) an oxidation unit fluidly connected to said outlet for exhaust gas of said reciprocating engine and having an inlet for exhaust gas, an inlet for fuel and an outlet for CO2-rich exhaust gas;

(d) a cooling unit fluidly connected to said outlet for CO2-rich exhaust gas of said oxidation unit and having an inlet for CO2-rich exhaust gas and an outlet for cooled CO2-rich exhaust gas; and (e) a CO2 capture unit fluidly connected to said outlet for cooled CO2-rich exhaust gas of said cooling unit and having an inlet for cooled CO2-rich exhaust gas, an outlet for CO2-lean exhaust gas and an outlet for CO2.

29. A system as claimed in claim 28 which does not include a fan for blowing exhaust gas through the CO2 capture unit.

30. A system as claimed in claim 28 or claim 29, wherein said CO2 capture unit further comprises an outlet for water, said reciprocating engine further comprises an inlet for water and said reciprocating engine is fluidly connected to said outlet for water of said CO2 capture unit.

31. A system as claimed in any one of claims 28 to 30, wherein said oxidation unit is a catalytic oxidation unit.

32. A system as claimed in any one of claims 28 to 31, wherein said cooling unit comprises a heat recovery unit or a heat exchanger.

33. A system as claimed in any one of claims 28 to 32, wherein said CO2 capture unit is a compact CO2 capture unit.

34. A system as claimed in claim 33, wherein said CO2 capture unit is as described in claim 17 or claim 18.

35. A system as claimed in any one of claims 28 to 34, further comprising means for removing oxygen from captured CO2 wherein said means are fluidly connected to the compact CO2 capture unit.

36. A system as claimed in claim 35, further comprising means for storing captured CO2 wherein said means are fluidly connected to the CO2 capture unit.

37. A system according to claim 36, wherein said means has an inlet for water.

38. A system according to claim 36, wherein said means has an inlet for natural gas.

39. A system as claimed in any one of claims 28 to 38, wherein said system operates

5 at atmospheric pressure.

40. A system as claimed in any one of claims 28 to 39 for use onshore.

41. A system as claimed in any one of claims 28 to 39 for use offshore.