WO2011054803A1

WO2011054803A1 - Centrifugal separation of condensed co2 from a flue gas

Info

Publication number: WO2011054803A1
Application number: PCT/EP2010/066602
Authority: WO
Inventors: Jozef Johannes Hubertus Brouwers; Michael Zvi Golombok; Jeroen Adrianus Van Ooijen; Antonius Joseph Theunissen
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2009-11-03
Filing date: 2010-11-02
Publication date: 2011-05-12

Abstract

A method for centrifugal separation of condensed CO₂ from a flue gas comprises: - combusting a mixture of fuel and oxygen enriched air in a combustion chamber (26) to generate heat and flue gas (27); - splitting the flue gas (27) into a first and a second flue gas fraction (32,33); -recycling the first flue gas fraction (32) into the combustion chamber (26); - cooling the second flue gas fraction (33) to a temperature at which CO₂ condenses; - separating condensed CO₂ (35) from a CO₂ depleted second flue gas fraction (36) by centrifugal separation; and - discharging the CO₂ depleted second flue gas fraction (36) into the atmosphere. The replacement of normal air by oxygen enriched air (23) in the combustion chamber (26) enables to increase the CO₂ recovery rate from less than 50 mol% to more than 80 mol%.

Description

CENTRIFUGAL SEPARATION OF CONDENSED C0₂ FROM A FLUE GAS

BACKGROUND OF THE INVENTION

The invention relates to a method and system for separation of condensed CO2 from a flue gas.

Such a method and system are known from US patent applications US2009 / 0173073 ; US2005/028529 and

2001/000863; European patent application 2085587 and International patent application WO2008/153379.

The method disclosed in WO2008/153379 comprises:

- combusting a mixture of fuel and air to generate heat and heated flue gas;

- cooling the flue gas to a temperature at which CO2 condenses; and

- separating condensed CO2 from gaseous components of the flue gas by centrifugal separation, which may involve inducing the cooled flue gas to rotate in a swirl tube and feeding the swirling cooled flue gas stream into a rotator comprising a plurality of tubes which are

oriented substantially parallel to an axis of rotation of the rotator and in which the condensed CO2 droplets are induced to coalesce;

- the coalesced CO2 droplets are discharged from the tubes into a gas-liquid separation chamber in which liquid CO2 droplets and other condensed liquid components are separated from gaseous flue gas components.

A limitation of the known method is that due to the relatively low percentage of CO2 in the flue gas mixture only up to about 43 mol% of the CO2 fraction of the flue gas can be recovered by condensation and centrifugal separation from the flue gas.

US 2009/260585 discloses an oxyfuel combustion process in which the oxidant gas is a mixture of

substantially pure oxygen and recycled flue gas. A disadvantage of this known oxyfuel combustion process is that generation of substantially pure oxygen is expensive and hazardeous, which makes this process economically and environmentally unattractive.

It is an object of the present invention to increase the percentage of condensed CO2 that can be recovered by centrifugal separation from a flue gas in an economical and efficient manner without requiring an expensive oxygen generation plant.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a method for centrifugal separation of condensed CO2 from a flue gas, the method comprising:

- combusting a mixture of oxygen enriched air comprising between 40 and 60 mol% oxygen and fuel in a combustion chamber to generate heat and flue gas;

- splitting the flue gas into a first flue gas fraction and a second flue gas fraction;

-recycling the first flue gas fraction into the

combustion chamber;

- cooling the second flue gas fraction to a temperature at which CO2 condenses;

- separating condensed CO2 from a CO2 depleted second flue gas fraction by centrifugal separation, wherein the second flue gas fraction is induced to swirl in a swirl tube and fed into a rotator comprising a plurality of tubes which are oriented substantially parallel to an axis of rotation of the rotator and in which the

condensed CO2 droplets are induced to coalesce, and the coalesced CO2 droplets are discharged from the tubes into a gas-liquid separation chamber in which the coalesced CO2 droplets are separated from the CO2 depleted second flue gas fraction; and

- discharging the CO2 depleted second flue gas fraction into the atmosphere.

The fuel may comprise a fossil, biomass and/or other hydrocarbon fuel and the combustion may take place in a combustion chamber of an electrical power plant, such as a coal fired power plant.

The combustion chamber may be a combustion chamber, which is designed to combust fuel with normal air, and the level of recycling of the first flue gas fraction may be adjusted to maintain the temperature in the combustion chamber substantially at the combustion temperature of a combustion mixture comprising fuel with normal air, so that the method according to the invention may be used to retrofit a conventional combustion chamber which cannot withstand higher combustion temperatures than that of a combustion mixture comprising fuel and normal air, which comprises about 79 mol% nitrogen (N₂) and about 21 mol% oxygen ( O2 ) ·

The method according to the invention may be used to remove at least 50 mol% of the CO2 content of the second flue gas fraction, preferably at least 80 mol% of the CO2 content of the second flue gas fraction.

In accordance with the invention there is

furthermore provided a system for centrifugal separation of condensed CO2 from a flue gas, the system comprising:

- a combustion chamber for combusting a mixture of oxygen enriched air comprising between 40 and 60 mol% oxygen and fuel to generate heat and flue gas;

- means for splitting the flue gas into a first flue gas fraction and a second flue gas fraction; - a recycling conduit for recycling the first flue gas fraction into the combustion chamber;

- means for cooling the second flue gas fraction to a temperature at which CO₂ condenses;

- a centrifugal separator for separating condensed CO₂ from a CO₂ depleted second flue gas fraction, which separator comprises a swirl tube, a rotator comprising a plurality of tubes which are oriented substantially parallel to an axis of rotation of the rotator and in which the condensed CO₂ droplets of the second flue gas fraction are induced to coalesce, and a gas-liquid separation chamber in which the coalesced CO₂ droplets are separated from the CO₂ depleted second flue gas fraction; and

- means for discharging the CO₂ depleted second flue gas fraction into the atmosphere.

These and other features, embodiments and advantages of the method and system according to the invention are described in the accompanying claims, abstract and the following detailed description of non-limiting

embodiments depicted in the accompanying drawings, in which description reference numerals are used which refer to corresponding reference numerals that are depicted in the drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically shows an Oxygen-Enriched Coal Combustion (OECC) power plant with C0₂~capture using a Rotational Particle Separator (RPS).

Figure 2 is a schematic longitudinal sectional view of the RPS.

Figure 2A show at a larger scale than Figure 2 a detail of the RPS shown in Figure 2. Figure 3 shows a phase diagram of a 79/21 mol% 2/CO₂ mixture, where G = Gas, L = Liquid, S = Solid CO₂.

Figure 4 shows a phase diagram of a 50/50 mol% N₂/CO₂ mixture, where G = Gas, L = Liquid, S = Solid CO₂.

Figure 5 shows possible combinations of r_Co₂ and x_L in gas-liquid region of N₂/C0₂ mixture with x_F = 0.21 (dark grey) and x_F = 0.5 (light grey).

Figure 6 shows maximum recovery of CO₂ for different values of x₀2 ·

Figure 7 shows a comparison of pressure dependence of r_Co₂ for temperatures on solid CC>₂-line for flue gases from combustion with x₀₂ = 0.21 and 0.50.

Figure 8 shows a comparison r_Co₂ for highest with maximum r_Co₂ possible.

Figure 9 shows xi for highest Φ-values.

Figure 10 shows pressure for highest Φ-values.

Figure 11 shows r_Co₂ for x₀₂= 0.6.

Figure 12 is a flowscheme showing the steps of the method according to the invention.

DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS

The method and system according to the invention are particularly useful for CO₂ separation of coal

combustion, as this is a large CO₂ producer.

The method and system according to the invention are also referred to as C5-sep: "Condensed Contaminant

Centrifugal separation applied to Coal Combustion".

In accordance with the invention the separation efficiency of CO₂ from a coal combustion flue gas is enhanced by using oxygen enriched air for combustion.

Figure 1 shows that the whole process for CO₂ separation from an oxygen-enriched coal combustion power plant would consist of four main steps: 1. Oxygen enrichment of air;

2. Energy conversion in a power plant;

3. Preparation of the non-recycled second flue gas

fraction to the thermodynamic conditions suitable for separation;

4. Separation of the second flue gas fraction with a Rotational Particle Separator (RPS ) into a liquefied CO2 stream and a CO2 depleted gas stream, which is discharged into the atmosphere.

In the C 5-sep CO2 separation method according to the invention a centrifugal separation technology concept called "condensed contaminant centrifugal separation" - C3-sep, may be applied. The C3-sep concept is described in US patent 7,550,032 and has proven promising for cleaning natural gas contaminated with carbon dioxide ( CO2 ) or hydrogen sulfide (H₂S). International patent application WO2008/153379 describes a modification of the C3-concept, wherein the C3-concept is used for

centrifugal separation of condensed CO2 from a flue gas.

Figure 2 shows that C3-sep concept involves a two- step method, which first induces a phase change in an expansion turbine 5 to condense the contaminant and then separates the contaminant using centrifugal separation in a Rotational Particle Separator (RPS) 6.

The C5-sep technology is particularly attractive for separating flue gas mixtures with CO2 concentrations between 30 and 70%.

In the expansion turbine 5 the contaminant, which in this case CO2 , is condensed by cooling the flue gas to conditions at which the gaseous CO2 becomes liquid in the form of a micron-sized droplet mist. An advantage of an expansion turbine 5 is that the power it withdraws from the flue gas can be used to drive a compressor, which can bring the flue gas back to system pressure.

With expansion cooling a condition is created in which a mixture of fine CC>2-rich liquid droplets forms in a nitrogen rich flue gas. After the initiation of phase separation by expansion an induction period is present for the micro droplets to increase in size sufficiently for separation. Subsequently, spatial separation of the CC>2-rich liquid and the nitrogen-rich flue gas takes place in the rotational particle separator (RPS ) 6.

Figures 2 and 2A show that the RPS 6 consists of a cylindrical body 7 that is mounted on a shaft 8, so that the cylindrical body 7 and central shaft 8 rotate about a central axis 9. Rotation of the RPS 6 is induced by a rotating gas flow 11 in the flue gas conduit 12, which is also referred to as coagulation pipe.

Figure 2A shows that the central body 7 has a large number of axially oriented channels 10, each with a 1-2 mm diameter. In the channels 10, the centrifugal force will move the condensed CO2 droplets 13 in the gas radially to the outer walls during a short residence time. There the CO2 droplets 13 are squeezed to liquid films which leave the RPS 6 at the end as large CO2 droplets .

It is known from International patent application

WO2008/143379 that the C3-sep method may be used to separate CO2 from a combustion flue gas consisting mainly of CO2 , H₂0 and N₂. The concentration of CO2 in combustion flue gases is though relative low. Flue gas from methane combustion consists of only about 12% CO2 and from coal combustion of about 21%.

The C5-sep method according to the invention is based on the insight that with higher concentrations of CO₂, condensation characteristics of CO₂ in the mixture become more advantageous for higher CO₂ recovery.

Effective application of the C5-sep technology therefore only becomes possible by increasing the concentration of CO₂ in the flue gas mixture. In accordance with the invention the CO₂ concentration in a flue gas mixture of a coal fired power plant or other combustion process is increased by using oxygen-enriched combustion.

The influence of changing the composition of the feed gas mixture by oxygen enriched combustion on the thermodynamic properties of the flue mixture will be explained on the basis of phase diagrams.

Figures 3 and 4 show the p-T phase diagrams for respectively a normal combustion 79/21 mol% N₂/CO₂ mixture, which is also referred to as the mixture with

Xf= 0.21, and for an oxygen-enriched combustion 50/50 mol% N₂/CO₂ mixture, which is also referred to as the mixture with x_f= 0.5.

In this specification and accompanying drawings and claims the abbreviation x_F is the CO₂ concentration in

Comparison of the phase diagrams shown in Figures 3 and 4 indicates that a larger gas-liquid region (G+L)is present for the mixture with x_F = 0.5 as depicted in Figure 4.

The dew point line 15 in the p-T phase diagram shown in Figure 4 for the mixture with x_f=0.5 is, namely, positioned at higher temperatures (T = -9°C) than the dewpoint line 14 for the mixture with x_F = 0.21 (T=- 47°C) shown in Figure 3, while the CO₂ freeze-out line 16,

17 in the gas-liquid region is positioned at the same place for both compositions. Also, the gas-liquid region (G+L) is extended to both lower and higher pressures for the mixture with x_F = 0.5.

The extension of the gas-liquid region (G+L) to lower pressures will give the possibility to go to higher values for x_L . Initiation of condensation at higher temperatures causes higher values for r_Co2 near the solid CO₂ line (G+S). Furthermore, the larger gas-liquid

(G+L) region causes a larger variety in combinations of r_C02 and x_L .

Figure 5 shows the difference in possible

combinations between a N₂/CO₂ mixture with x_F = 0.21 and a N₂/CO₂ mixture with x_F = 0.5. It is shown that

significantly higher recovery levels are possible for the mixture with a higher x_F . Furthermore, higher recovery levels are possible at higher levels of x_L .

So, the maximal recovery of CO2 (r_Co2 ) depends on the concentration of CO₂ in the feed (x_F) .

x_F, which is the CO₂ concentration in the flue gas ( XC02 ) , is about equal to the concentration of oxygen in the air used for combustion ( X02 ) .The dependence of the maxima of r_Co2 on x₀2 is investigated by using the same method, as described in the preceding paragraph for a 79/21 N2/CO2 mixture, for different x₀2 ·

Figure 6 shows the calculated maxima of r_Co2 versus X02 · There is a considerable increase from r_Co2 = 0.43 at

X02 = 0.21 to r_Co2 = 0.90 at x₀2 = 0.50 which is about twice as high. So, relatively small increases in air enrichment yield most of the recovery improvements.

The temperature of the mixture at which the maximum r_Co2 value is found, lies again on the continuous (CO2)- line. The pressure at which this maximum is present can be found by looking at Figure 7 where r_Co2 is plotted versus pressure for temperatures on the solid CO₂ line for those specific pressures. The maximum recovery of r_Co2 = 0.87 is found at p = 109 bar with a corresponding CO2 concentration of xi = 0.83.

Figure 7 furthermore shows that for the oxygen enriched mixture, the recovery values do not decrease that much for lower or higher pressure values.

Especially the small decrease of recovery going to lower pressures (but still > 40 bar) is of interest, as then higher xi and lower energy costs are involved in the separation process for lower pressures.

So, for example for the enriched mixture with x_f = 0.5, at only 40 bar instead of 109 bar, still a recovery is present of r_Co2 0.80 with a higher xi of 0.94. These differences between the maximum recovery specifics and the specifics of a more useful point are shown in

Table 1:

Table 1: Comparison between separation conditions for a 50/50 N2 / CO2 mixture.

In Figure 5 the values of r_Co2 and xi, for each grid point were shown for both the gas-liquid regime of a standard coal combustion mixture and an enriched coal combustion mixture. The more optimal points of operation lie in the upper right corner of the cloud with points.

These points will have a relative lower pressure. To optimize the system for both the r_Co2 and xi, an

optimization factor is introduced in Equation(l):

<P=^rco2 ^{+ x}i Equation (1) As a starting point, both parameters r_Co2 and xi are taken equally important. Nevertheless, if it would appear that one of both is, for example, more economically viable, weighting factors could be introduced to make one of both parameters more important.

By calculating the optimization factor for all grid points in the gas-liquid region, an optimum can be found.

At these points still a relative high recovery is present while xi is improved. As a higher xi value means a lower pressure also the energy costs of separation will be lower .

Figure 8 shows the difference in r_Co2 for the optimal points in comparison with the maximum r_Co2 values found for a certain x₀2 · It is shown that the r_Co2 values of the optimum points are only slightly smaller than the maximum r_Co2 values.

In Figure 9 the corresponding xi values of the optimal points are plotted and just slightly increase from xi = 0.91 at x₀2 = 0.21 to xi = 0.92 at x₀2 = 0.5. Figure 10 shows the pressures at which these optimum values can be found.

It can be seen in Figure 10 that for higher x₀2, lower pressures are needed to separate at the optimum points, which implies that savings are accomplished resulting from reduced wall thicknesses of compressed gas containing equipment and/or reduced compression power requirements.

Figure 11 shows r_Co2 for x₀2= 0.6 and the fractional recovery of CO2 from the flue gas, when that flue gas results from combustion of coal with oxygen which has been enriched from 21% to 60%. The recovery is typically 90 -95%. However this is only a few percent better than that obtained with flue gas which results from the use of air enriched to 50%. In a "law of diminishing returns" type of behavior, the costs of the air enrichment remain higher for going from 50 to 60% than for 40 to 50%. On balance therefore the gain on going from 50 to 60% oxygen are not as high as for going from 40 to 50%.

Figure 12 is a flow scheme that shows the sequence of steps of the method according to the invention.

The first step is that a stream of air 20 is

separated in an air separation unit 21 into a first stream 22 of nitrogen (N₂ ) and a second stream 23 of oxygen enriched air, which preferably comprises 40 to 70 mol% oxygen and more preferably about 50 mol% Oxygen (O2) .

The second stream 23 of oxygen enriched air 23 is mixed with fuel 24 into a combustion mixture 25 and fed into a combustion chamber 26, in which the mixture 25 is combusted. The flue gas stream 27 discharged by the combustion chamber 26 is cooled in a heat exchanger 28, which is cooled by a stream of water 29, which is

converted into pressurized steam 30 that may be used to power a turbine (not shown) that may coupled to an electric generator (not shown) .

The cooled flue gas stream 31 discharged by the heat exchanger 28 is subsequently split into a first flue gas fraction 32 which is mixed with the combustion mixture 25 and recycled into the combustion chamber 26 in order to control the combustion temperature and a second flue gas fraction 33, which is supplied to a rotating particle separator (RPS)34 of which details are shown in Figure 2.

In the rotating particle separator (RPS) the second flue gas fraction 33 has a temperature and pressure at which CO₂ is in the liquid phase and a stream 35 of liquid CO₂ is separated by centrifugal separation from a stream 36 of a CO₂ depleted second flue gas fraction, which is discharged via a chimney 37 into the atmosphere 38. The stream 36B of CO₂ depleted second flue gas fraction has a low CO₂ content, which may be below 30 mol% CO₂ and preferably less than 20 mol% CO₂.

Claims

C L A I M S

1. A method for centrifugal separation of condensed CO2 from a flue gas, the method comprising:

- combusting a mixture oxygen enriched air comprising between 40 and 60 mol% oxygen and fuel in a combustion chamber to generate heat and flue gas;

-recycling the first flue gas fraction into the

combustion chamber;

- cooling the second flue gas fraction to a temperature at which CO2 condenses;

condensed CO2 droplets are induced to coalesce, and the coalesced CO2 droplets are discharged from the tubes into a gas-liquid separation chamber in which the coalesced

CO2 droplets are separated from the CO2 depleted second flue gas fraction; and

- discharging the CO2 depleted second flue gas fraction into the atmosphere.

2. The method of claim 1, wherein the fuel comprises a fossil, biomass and/or other hydrocarbon fuel.

3. The method of claim 1, wherein the combustion takes place in a combustion chamber of an electrical power plant .

4. The method of claim 2 and 3, wherein the electrical power plant is a coal fired power plant.

5. The method of claim 1, wherein the combustion chamber is a combustion chamber, which is designed to combust fuel with normal air, and the level of recycling of the first flue gas fraction is adjusted to maintain the temperature in the combustion chamber substantially at the combustion temperature of a combustion mixture comprising fuel with normal air.

6. The method of any one of claims 1-5, wherein the method is used to remove at least 50 mol% of the CO2 content of the second flue gas fraction.

7. The method of claim 6, wherein the method is used to remove at least 80 mol% of the CO2 content of the second flue gas fraction.

8. A system for centrifugal separation of condensed CO2 from a flue gas, the system comprising:

- means for enriching air to generate the oxygen enriched air comprising between 40 and 60 mol% oxygen;

- a combustion chamber for combusting a mixture of fuel and the oxygen enriched air to generate heat and flue gas ;

- means for splitting the flue gas into a first flue gas fraction and a second flue gas fraction;

- a recycling conduit for recycling the first flue gas fraction into the combustion chamber;

- means for cooling the second flue gas fraction to a temperature at which CO2 condenses;

- a centrifugal separator for separating condensed CO2 from a CO2 depleted second flue gas fraction, which separator comprises a swirl tube, a rotator comprising a plurality of tubes which are oriented substantially parallel to an axis of rotation of the rotator and in which the condensed CO₂ droplets of the second flue gas fraction are induced to coalesce, and a gas-liquid separation chamber in which the coalesced CO₂ droplets are separated from the CO₂ depleted second flue gas fraction; and