METHOD AND SYSTEM FOR THE REMOVAL OF IMPURITIES IN A FLUE GAS

FIELD OF THE INVENTION

The present invention relates to a method and system for the removal of impurities from a flue gas. In particular, the present invention relates to a method and system for the removal of impurities such as SO3 (sulphur trioxide), SO2 (sulphur dioxide) and/or NO2 (nitrogen dioxide) from a CO2 (carbon dioxide) rich flue gas.

BACKGROUND OF THE INVENTION

Flue gases from power plants and other industrial activities include pollutants, for example greenhouse gases. One such greenhouse gas is CO2 (carbon dioxide). Emissions of C02to the atmosphere from industrial activities are of increasing concern to society and are therefore becoming increasingly regulated.

To reduce the amount of CO2 released into the atmosphere, CO2 capture technology can be applied. The selective capture of CO2 allows C02to be re-used or geographically sequestered.

The selective capture from a flue gas is sometimes called post-combustion recovery. In postcombustion recovery, C02 from the flue gas is selectively separated from other gases present in the flue gas (such as nitrogen and oxygen) by contacting a flue gas with a suitable solvent (for example a carbon capture solvent). The contact typically occurs in an absorber.

Prior to post-combustion recovery, the concentration of impurities in the flue gas can be reduced. This step usually takes part in a part of the CO2 capture system called the pre-treatment section. Impurities present in the flue gas include SOx (sulphur oxides) and NOx (nitrogen oxides) gases such as SO3 (sulphur trioxide), SO2 (sulphur dioxide) and/or NO2 (nitrogen dioxide). Impurities are formed by the combustion of fuels, for example burning coals containing sulphur produces SO2 in a flue gas.

Ideally, the level of impurities is reduced to less than 50 ppmv (parts per million by volume), or less than 25 ppmv, or, less than 15 ppmv, or less than 12 ppmv (parts per million by volume). If the concentration of impurities in the flue gas is not reduced prior to contacting the flue gas with a carbon capture solvent, degradation, loss and/or damage of the carbon capture solvent is accelerated.

W02009003238A1 discloses a process for removing carbon dioxide from a flue gas, wherein the process can include cooling the flue gas to below 50°C by contacting the flue gas with a counter current stream of liquid water and removing the carbon dioxide by directly contacting the flue gas with a scrubbing agent, wherein the scrubbing agent can be an amine or methanol.

1

SUBSTITUTE SHEET (RULE 26) WO2020159868A1 discloses methods for sequestering CO2, NOx and SO2. The gases are then converted into products including sodium bicarbonate and sodium nitrate.

System 1: A known system used in the pre-treatment of a flue gas prior to CO2 removal from the flue gas

Figure 1 illustrates a known system 100 used in the pre-treatment of a flue gas prior to CO2 removal from the flue gas.

As shown in Figure 1 , a flue gas 101 enters the system 100. Typically, the flue gas is at a temperature of from 1 15 to 200°C, typically at ambient pressure of 101325 Pa (1.01325 bar).

The flue gas 101 can pass through a flue gas blower 102. The flue gas blower 102 increases the pressure of the flue gas 101 to compensate for the pressure drop through the CO2 removal system (i.e. system 100 and the downstream carbon capture system, not shown in Figure 1 ). This ensures that the pressure of the flue gas 101 is at the same, or a similar, pressure as the flue gas at the outlet of the downstream carbon capture system (not shown in Figure 1 ). Typically, the flue gas blower 102 is an induced draft fan provided at the battery limit. The flue gas leaving the flue gas blower 102 is flue gas 103. Alternatively, the flue gas blower 102 can be downstream of system 100.

System 100 comprises two packed beds on two separate columns, the two packed beds are called a first packed bed and a second packed bed. The packed beds can be both or individually a packed bed tower, or, a static packed bed, or, a rotating packed bed which enables efficient gas-liquid contact.

The flue gas 103 then passes to a first stage of a pre-treatment section. The first stage of the pretreatment section is a direct contact cooler 104, which comprises the first packed bed.

In the direct contact cooler 104, the flue gas 103 is cooled by a first cooling medium 112 flowing in an opposite direction. Usually, the first cooling medium 112 is water, or cool air, or, a cool CO2 capture solvent. In the direct contact cooler 104, the flue gas 103 and the first cooling medium 112 come into contact in a counter-current configuration (i.e., one fluid moves in the opposite direction to the other fluid). In the direct contact cooler 104, the flue gas 103 is typically cooled to a temperature of from 40 to 49°C. Upon cooling, flue gas 105 is formed. In the direct contact cooler 104, the first cooling medium 112 is heated to a temperature of from 40 to 60°C forming second cooling medium 107.

During cooling in the direct contact cooler 104, the flue gas 103 will lose water as it is cooled through contact with the first cooling medium 112. The second cooling medium 107 comprises this water. The second cooling medium 107 (which includes the water) passes to a pump 108. The pump 108 moves the second cooling medium 107 from the pump 108 back to the direct contact cooler 104, via a cooler 111.

Upon leaving the pump 108, the second cooling medium 107 is split into two, forming third cooling medium 109 and fourth cooling medium 110. The proportion of the split is dependent upon the amount of water which is lost from the flue gas 103 as it is cooled. The amount of water present in the cooling medium needs to be maintained at a constant level, and removal of water in this part of system 100 provides a means to control the amount of water present in the cooling medium. The more water that is lost from the flue gas 103 upon cooling, the more water there is present in the second cooling medium 107 and the greater the amount of the third cooling medium 109 that is formed. A valve (not shown in Figure 1 ) controls the proportion of second cooling medium 107 forming third cooling medium 109 and fourth cooling medium 110

The third cooling medium 109 passes to a sewer or is reused.

The fourth cooling medium 110 passes to the cooler 111 , wherein the temperature of the fourth cooling medium 110 is reduced to a temperature of 40°C. The cooler 1 11 cools the fourth cooling medium 110 by using another cooling medium comprising sea water, or, cooling water from a cooling tower, or, cool air present in the cooler 111. Once cooled, the first cooling medium 112 is reformed. The first cooling medium 112 is ready for reuse in the direct contact cooler 104.

Upon leaving the direct contact cooler 104, the flue gas 105 then passes to a second stage of the pretreatment section. The second stage of the pre-treatment section is a SOx and NOx removal section 106. The SOx and NOx removal section 106 comprises the second packed bed.

In the SOx and NOx removal section 106, the flue gas 105 is contacted with a first scrubbing solution 120. The first scrubbing solution 120 contains scrubbing agents which react with, and subsequently remove, impurities in the flue gas 105. The first scrubbing solution 120 is heated to a temperature of 41 °C as a result of the reaction between the scrubbing agents and flue gas 105. Typically, the scrubbing agents present in the first scrubbing solution 120 are caustic soda (NaOH) in water, or sodium bicarbonate in water, or sodium carbonate in water, or, sodium bicarbonate and sodium carbonate in water, or, caustic soda (NaOH) and sodium bicarbonate in water. The scrubbing agents are used for the removal of SO2 and NO2 from flue gases. Typically, the flue gas 105 is contacted with the scrubbing agents in the first scrubbing solution 120 so that the concentration of impurities within the flue gas 105 is reduced to 12 ppmv or less.

A consequence of the reaction between the scrubbing agents present in the first scrubbing solution 120 and the flue gas 105, is that salts are formed which need to be removed. To remove the salts, the reacted scrubbing solution is removed from the SOx and NOx removal section 106 as second scrubbing solution 1 14. The second scrubbing solution 1 14 passes to a pump 115, which moves the second scrubbing solution 114 from the pump 115 back to the SOx and NOx removal section 106 via a cooler 118.

Upon leaving the pump 115, the second scrubbing solution 114 is split into two streams, third scrubbing solution 117 and fourth scrubbing solution 116. The proportion of the split is dependent upon the concentration of salts which are formed when the flue gas 105 reacts with the scrubbing agents in the first scrubbing solution 120. The concentration of salts which are formed is dependent on the concentration of SOx and NOx gases present in the flue gas 101 . The concentration of salts present in the cooling medium needs to be maintained at a constant level, and removal of the scrubbing solution in this part of system 100 provides a means to control the concentration of salts present in the scrubbing solution. The more salts that are formed during the reaction, the more of the scrubbing solution that is removed. A valve (not shown in Figure 1 ) controls the proportion of second scrubbing solution 114 forming third scrubbing solution 117 and fourth scrubbing solution 116.

The third scrubbing solution 117 is sent to an Effluent Treatment Plant (ETP) for treatment before removal.

The fourth scrubbing solution 116 passes to the cooler 118. The cooler 118 reduces the temperature of the scrubbing solution 116 to 40°C to form fifth scrubbing solution 119. The cooler 118 cools the fourth scrubbing solution 116 to form fifth scrubbing solution 119 by using another cooling medium comprising sea water, or, cooling water from a cooling tower, or, cool air present in the cooler 118.

To make up the loss of the scrubbing agents in the third scrubbing solution 117, fresh scrubbing solution 121 is added to the fifth scrubbing solution 119 to reform the first scrubbing solution 120. The fresh scrubbing solution 121 is formed in scrubbing solution tank 122.

Upon reacting with the first scrubbing solution 120, the flue gas 105 has a reduced concentration of impurities and forms flue gas 123. The flue gas 123 then passes to the downstream carbon capture system (not shown in Figure 1) for removal of CO2.

In system 100, two packed beds and two coolers are required. Each of the packed beds and coolers require a large area footprint. Furthermore, the packed beds and coolers are expensive equipment, requiring large expenditure to purchase and maintain. There is therefore a need for a system that removes impurities in flue gases (prior to CO2 removal) which is inexpensive, and which requires a small area footprint. System 2: A known system used in the pre-treatment of a flue gas prior to CO2 removal from the flue gas

Figure 2 illustrates a known system 200 used in the pre-treatment of a flue gas prior to CO2 removal from the flue gas.

As shown in Figure 2, a flue gas 201 enters the system 200. Typically, the flue gas 201 is at a temperature of from 115 to 200°C, typically at an ambient pressure of from 101325 Pa (1.01325 bar).

The flue gas 201 can pass through a flue gas blower (not shown in Figure 2). The flue gas blower increases the pressure of the flue gas to compensate for the pressure drop through the CO2 removal system (i.e., system 200 and the downstream carbon capture system, not shown in Figure 2). This ensures that the pressure of the flue gas 201 is at the same, or a similar, pressure as the flue gas at the outlet of the downstream carbon capture system (not shown in Figure 2). Typically, the flue gas blower is an induced draft fan provided at the battery limit. Alternatively, the flue gas blower can be downstream of system 200.

System 200 comprises two packed beds on the same column, the two packed beds are called a first packed bed 202 and a second packed bed 210. The packed beds can be both or individually a packed bed tower, or, a static packed bed, or, a rotating packed bed which enables efficient gas-liquid contact.

The flue gas 201 passes to a first stage of a pre-treatment section. The first stage of the pre-treatment section is the first packed bed 202.

In the first packed bed 202, the flue gas 201 is cooled by a first cooling medium 209 flowing in an opposite direction. Typically, the first cooling medium 209 enters the first packed bed 202 at a temperature of 40 °C. Usually, the first cooling medium 209 is water, or cool air, or, a cool CO2 capture solvent. In the first packed bed 202, the flue gas 201 and the first cooling medium 209 come into contact in a counter-current configuration (i.e., one fluid moves in the opposite direction to the other fluid). In the first packed bed, the flue gas 201 is typically cooled to a temperature of from 40 to 49°C. Upon cooling, flue gas 201 passes the second packed bed 210. In the first packed bed 202, the first cooling medium 209 is heated to a temperature of greater than 40°C to 60°C forming second cooling medium 203.

During cooling in the first packed bed 202, the flue gas 201 will lose water as it is cooled through contact with the first cooling medium 209. The second cooling medium 203 comprises this water. The second cooling medium 203 (which includes the water) passes to a pump 204. The pump 204 moves the second cooling medium 203 from the pump 204 back to the first packed bed, via a cooler 208. Upon leaving the pump 204, a third cooling medium 205 is formed. The third cooling medium 205 is then split into two, forming fourth cooling medium 206 and fifth cooling medium 207. The proportion of the split is dependent upon the amount of water which is lost from the flue gas 201 as it is cooled. The amount of water present in the cooling medium needs to be maintained at a constant level, and removal of water in this part of system 200 provides a means to control the amount of water present in the cooling medium. The more water that is lost from the flue gas 201 upon cooling, the more water there is present in the second cooling medium 203 and the greater the amount of the proportion of fourth cooling medium 206 that is formed. A valve (not shown in Figure 2) controls the proportion of second cooling medium 203 forming the fourth cooling medium 206 and the fifth cooling medium 207

The fourth cooling medium 206 passes to a sewer or is reused.

The fifth cooling medium 207 passes to the cooler 208, wherein the temperature of the fifth cooling medium 207 is reduced to a temperature of 40°C. The cooler 208 cools the fifth cooling medium 207 by using another cooling medium comprising sea water, or, cooling water from a cooling tower, or, cool air present in the cooler 208. Once cooled, the first cooling medium 209 is reformed. The first cooling medium 209 is ready for reuse in the first packed bed 202.

Upon leaving the first packed bed 202, the flue gas 201 then passes to the second packed bed 210. The second packed bed 210 is a SOx and NOx removal section.

In the second packed bed 210, the flue gas 201 is contacted with a first scrubbing solution 220. The first scrubbing solution 220 contains scrubbing agents which react with, and subsequently remove, impurities in the flue gas 201 . The first scrubbing solution 220 is heated to a temperature of 41 °C as a result of the reaction between the scrubbing agents and flue gas 201. Typically, the scrubbing agents present in the first scrubbing solution 220 are caustic soda (NaOH) in water, or sodium bicarbonate in water, or sodium carbonate in water, or, sodium bicarbonate and sodium carbonate in water, or, caustic soda (NaOH) and sodium bicarbonate in water. The scrubbing agents are used for the removal of SO2 and NO2 from flue gases. Typically, the flue gas 201 is contacted with the scrubbing agents in the first scrubbing solution 220 so that the concentration of impurities within the flue gas is reduced to 12 ppmv or less.

A consequence of the reaction between the scrubbing agents present in the first scrubbing solution 220 and the flue gas 201 , is that salts are formed which need to be removed. To remove the salts, the reacted scrubbing solution is removed from second packed bed as second scrubbing solution 211. The second scrubbing solution 211 passes to a pump 212, which moves the second scrubbing solution 211 from the pump 212 back to the second packed bed 210 via a cooler 216.

Upon leaving the pump 212, a third scrubbing solution 213 is formed. The third scrubbing solution 213 is split into two streams, a fourth scrubbing solution 214 and a fifth scrubbing solution 215. The proportion of the split is dependent upon the concentration of salts which are formed when the flue gas 201 reacts with the scrubbing agents in the first scrubbing solution 220. The concentration of salts which are formed is dependent on the concentration of SOx and NOx gases present in the flue gas 201 . The concentration of salts present in the scrubbing solution needs to be maintained at a constant level, and removal of the scrubbing solution in this part of system 200 provides a means to control the concentration of salts present in the scrubbing solution. The more salts that are formed during the reaction, the more of the scrubbing solution that is removed. A valve (not shown in Figure 2) controls the proportion of third scrubbing solution 213 forming fourth scrubbing solution 214 and fifth scrubbing solution 215.

The fourth scrubbing solution 214 is sent to an Effluent Treatment Plant (ETP) for treatment before removal.

The fifth scrubbing solution 215 passes to the cooler 216. The cooler 216 reduces the temperature of the scrubbing solution 215 to 40°C to form sixth scrubbing solution 217. The cooler 216 cools the fifth scrubbing solution 215 to form sixth scrubbing solution 217 by using another cooling medium comprising sea water, or, cooling water from a cooling tower, or, cool air present in the cooler 216.

To make up the loss of the scrubbing agents in the sixth scrubbing solution 217, fresh scrubbing solution 219 is added to the sixth scrubbing solution 217 to reform the first scrubbing solution 220. The fresh scrubbing solution 219 is formed in scrubbing solution tank 218.

Upon reacting with the first scrubbing solution 220, the flue gas 201 has a reduced concentration of impurities and forms flue gas 221. The flue gas 221 then passes to the downstream carbon capture system (not shown in Figure 2) for removal of CO2.

In system 200, two packed beds and two coolers are required. Each of the packed beds and coolers require a large area footprint. Each of the packed beds and coolers are expensive equipment, requiring large expenditure to purchase and maintain. There is therefore a need for a system that removes impurities in flue gases (prior to CO2 removal) which is more efficient and more inexpensive, and which requires a small area footprint.

System 3: A known system used in the pre-treatment of a flue gas prior to CO? removal from the flue gas

Figure 3 illustrates a known system 300 used in the pre-treatment of a flue gas prior to CO2 removal from the flue gas. System 300 reduces the area footprint of a system used in the pre-treatment of a flue gas, however the loss of scrubbing agents in system 300 is greater than in systems 100 and 200. As shown in Figure 3, a flue gas 301 enters the system 300. Typically, the flue gas is at a temperature of from 115 to 200°C, and typically at ambient pressure of 101325 Pa (1.01325 bar).

The flue gas 301 can pass through a flue gas blower (not shown in Figure 3). The flue gas blower increases the pressure of the flue gas to compensate for the pressure drop through the CO2 removal system (i.e., system 300 and the downstream carbon capture system, not shown in Figure 3). This ensures that the pressure of the flue gas 301 is at the same, or a similar, pressure as the flue gas at the outlet of the downstream carbon capture system (not shown in Figure 3). Typically, the flue gas blower is an induced draft fan provided at the battery limit. Alternatively, the flue gas blower can be downstream of system 300.

System 300 comprises a single packed bed 302 on a single column. The packed bed can be a packed bed tower, or, a static packed bed, or, a rotating packed bed which enables efficient gas-liquid contact.

The flue gas 301 passes to the packed bed 302. In the packed bed 302, the flue gas 301 is contacted with a first scrubbing solution 312 flowing in an opposite direction. The first scrubbing solution 312 contains scrubbing agents which react with, and subsequently remove, impurities in the flue gas 301 and simultaneously cool the flue gas 301. The first scrubbing solution 312 comprises caustic soda (NaOH) in water, or sodium bicarbonate in water, or sodium carbonate in water, or, sodium bicarbonate and sodium carbonate in water, or, caustic soda (NaOH) and sodium bicarbonate in water and the scrubbing agents are used for the removal of SO2 and NO2 from flue gases.

In the packed bed 302, the flue gas 301 and the first scrubbing solution 312 come into contact in a counter-current configuration (i.e., one fluid moves in the opposite direction to the other fluid).

In the packed bed 302, the flue gas 301 is typically cooled to a temperature of from 40 to 49°C through the contact with the first scrubbing solution. The first scrubbing solution 312 is heated to a temperature of 60°C or less as a consequence of this reaction. During cooling in the packed bed 302, the flue gas 301 will lose water as it is cooled through contact with the first scrubbing solution 312. A second scrubbing solution 303 comprises this water.

At the same time as being cooled, the flue gas 301 reacts with the scrubbing agents present in the first scrubbing solution 312. Typically, the flue gas 301 is contacted with the scrubbing agents in the first scrubbing solution 312 until the concentration of impurities within the flue gas is reduced to 12 ppmv or less. A consequence of the reaction between the scrubbing agents present in the first scrubbing solution 312 and the flue gas 301 , is that salts are formed which need to be removed. To remove the salts, the reacted scrubbing solution is removed from the packed bed as second scrubbing solution 303. The second scrubbing solution 303 comprises the water and salts formed from the contact of the flue gas 301 with the first scrubbing solution 312.

The second scrubbing solution 303 passes to a pump 304, which moves the second scrubbing solution 303 from the pump 304 back to the packed bed 302 via a cooler 308.

Upon leaving the pump 304, a third scrubbing solution is formed 305. The third scrubbing solution 305 is split into two, forming a fourth scrubbing solution 306 and a fifth scrubbing solution 307. The proportion of the split is dependent upon the amount of water which is lost from the flue gas 301 as it is cooled and the proportion of salts formed when the flue gas 301 reacts with the scrubbing agents in the first scrubbing solution 312. The proportion of salts formed is proportional to the concentration of impurities in the flue gas 301. The amount of water and salts present in the scrubbing solution need to be maintained at a constant level, and removal of the water and salts in this part of the system 300 provides a means to control the amount of water and salts present in the scrubbing solution. The more water that is lost from the flue gas 301 upon cooling and the more salts that are formed upon the flue gas 301 reacting with the first scrubbing solution 312, the more water and salts that are present in the third scrubbing solution 305 and the greater the amount of fourth scrubbing solution 306 that is formed. A valve (not shown in Figure 3) can be used to control the proportion of third scrubbing solution 305 forming the fourth scrubbing solution 306.

The fourth scrubbing solution 306 is sent to an Effluent Treatment Plant (ETP) for treatment before removal.

The fifth scrubbing solution 307 passes to the cooler 308, The cooler 308 reduces the temperature of the fifth scrubbing solution 307 to 40°C to form sixth scrubbing solution 309. The cooler 308 cools the fifth scrubbing solution 307 to form sixth scrubbing solution 309 by using another cooling medium comprising sea water, or, cooling water from a cooling tower, or, cool air present in the cooler 308.

To make up the loss of the scrubbing agents in the sixth scrubbing solution 309, fresh scrubbing solution 311 is added to the sixth scrubbing solution 309 to reform the first scrubbing solution 312. The fresh scrubbing solution 311 is formed in scrubbing solution tank 310.

Upon reacting with the first scrubbing solution 312, the flue gas 301 has a reduced concentration of impurities and forms flue gas 313. The flue gas 313 then passes to the downstream carbon capture system (not shown in Figure 3) for removal of CO2.

In system 300, only one packed bed and only one cooler is required and thus the footprint area of system 300 is smaller than the footprint area of systems 100 and 200. However, system 300 loses a large proportion of the scrubbing solution as the fourth scrubbing solution 306, which requires costly makeup of the scrubbing solution prior to the first scrubbing solution 312 entering the packed bed

302.

There is therefore a need for an improved method and system for the pre-treatment of a flue gas prior to CO2 removal. In particular, there is a need for a more efficient and a more inexpensive method and system for the pre-treatment of a flue gas prior to CO2 removal which does not require a large area footprint, which is energy efficient (for example by reducing the amount of equipment required) and cost efficient (for example by reducing the amount of expensive equipment required and the amount of solution makeup required).

SUMMARY OF THE INVENTION

The present invention relates to a method and system for the removal of impurities from a flue gas. In particular, the present invention relates to a method and system for the removal of impurities such as SO3 (sulphur trioxide), SO2 (sulphur dioxide) and/or NO2 (nitrogen dioxide) from a CO2 (carbon dioxide) rich flue gas.

Representative features of the present invention are set out in the following clauses, which stand alone or may be combined, in any combination, with one or more features disclosed in the text and/or figures of the specification.

The present invention is as set out in the following clauses:

1. A process of pre-treating a flue gas prior to carbon dioxide (CO2) capture, the process comprising the steps of:

(i) cooling a flue gas comprising carbon dioxide (CO2) to form a cooled flue gas;

(ii) contacting the cooled flue gas with a fluid comprising a scrubbing solution such that scrubbing agents within the scrubbing solution remove impurities from the flue gas to form a flue gas with reduced impurity content and a fluid comprising the impurities;

(iii) passing the fluid comprising the impurities to at least one reverse osmosis membrane to form a fluid comprising concentrated impurities and purified water.

Another way of referring to the purified water is as a filtrate or a permeate. Another way of referring to the fluid comprising concentrated impurities is as a retentate or a concentrate.

2. The process of clause 1 , wherein the flue gas comprising carbon dioxide (CO2) has a temperature of from greater than 50 to 230°C and is cooled in step (i) to form the cooled flue gas having a temperature of from 25 to 50°C. The process of clause 1 or clause 2, wherein the flue gas comprising carbon dioxide (CO2) has a temperature of from greater than 50 to 230°C, or, from 70 to 230°C, or, from 110 to 230°C, or, from 105 to 220°C, or from 110 to 210°C, or from 115 to 200°C. The process of any one of clauses 1 to 3, wherein the flue gas comprising carbon dioxide (CO2) has a pressure of from -1000 to 300000 Pa (-0.01 to 3 bar), or, from 0 to 250000 Pa (0 to 2.5 bar), or, from (75000 to 200000 Pa (0.75 to 2 bar), or, 101325 Pa (1.01325 bar). The process of any one of clauses 1 to 4, wherein the cooled flue gas comprising carbon dioxide (CO2) has a temperature of from 25 to 50°C, or, from 30 to 50°C; or, from 35 to 50°C; or, from 37 to 50°C, or from 40 to 49°C. The process of any one of clauses 1 to 5, wherein the flue gas comprising carbon dioxide (CO2) is cooled by contacting the flue gas with a fluid comprising a cooling medium, optionally, wherein the cooling medium is water, or, water comprising a solvent; optionally, wherein the solvent is a CO2 capture solvent. The process of clause 6, wherein the flue gas is contacted with the fluid comprising the cooling medium in a counter current configuration. The process of any one of clauses 1 to 7, wherein the scrubbing agents within the scrubbing solution comprise, or consist of, sodium sulphite (Na2SOs), caustic soda (NaOH) in water, or sodium bicarbonate in water, or sodium carbonate in water, or, sodium bicarbonate and sodium carbonate in water, or, caustic soda (NaOH) and sodium bicarbonate in water. The process of clause 8, wherein the concentration of the scrubbing agents in the water is from 0.5 to 10 weight %, or, from 1 to 7.5 weight %; or, from 1 .5 to 5 weight %, or, 4 weight %; the balance being water. The process of any one of clauses 1 to 9, wherein the flue gas is contacted with the fluid comprising the scrubbing solution until the concentration of impurities within the flue gas is reduced to 50 ppmv or less, or, 25 ppmv or less, or, 15 ppmv or less, or, 12 ppmv or less. The process of any one of clauses 1 to 10, wherein the flue gas is contacted with the fluid comprising the scrubbing solution in a counter current configuration. The process of any one of clauses 6 to 11 , wherein the fluid comprising the cooling medium and the fluid comprising the scrubbing agents is the same fluid; optionally, wherein the cooling of step (i) and the removal of impurities of step (ii) in clause 1 are performed simultaneously. The process of any one of clauses 1 to 12, wherein the step of passing the fluid comprising the impurities to the reverse osmosis membrane(s) removes at least 90 weight % of salts from the fluid comprising the impurities to form a fluid comprising concentrated impurities and purified water, preferably, wherein the step of passing the fluid comprising the impurities to the reverse osmosis membrane(s) removes from 90 to 99.9 weight %, or, from 95 to 99.9 weight %, or from 97 to 99.9 weight % of salts from the fluid comprising the impurities to form a fluid comprising concentrated impurities and purified water. The process of any one of clauses 1 to 13, wherein the step of passing the fluid comprising the impurities to the reverse osmosis membrane(s) removes sodium sulphate (Na2SO4), sodium carbonate (Na2CC>3) and sodium bicarbonate (NaHCC ) solution. The process of any one of clauses 1 to 14, wherein the reverse osmosis membrane(s) operates under a pressure of from 200000 to 12000000 Pa (2 to 120 bar), or, 200000 to 11000000 (2 to 110 bar), or, from 200000 to 10000000 Pa (2 to 100 bar), or, from 200000 to 8000000 Pa (2 to 80 bar), or, from 200000 to 6000000 (2 to 60 bar), or, from 200000 to 1000000 (2 to 10 bar), or, 200000 to 500000 (2 to 5 bar). The process of any one of clauses 1 to 15, wherein the reverse osmosis membrane(s) operates at a temperature of from 20 to 45°C, or, 30 to 45°C, or, from 40 to 45°C. A system for pre-treating a flue gas prior to carbon dioxide (CO2) capture, the system comprising:

(i) a pre-treatment section for cooling a flue gas comprising carbon dioxide (CO2) to form a cooled flue gas, and, for contacting the flue gas comprising carbon dioxide (CO2) with a fluid comprising a scrubbing solution such that scrubbing agents in the scrubbing solution remove impurities from the flue gas comprising carbon dioxide (CO2) to form a flue gas with reduced impurity content and a fluid comprising the impurities;

(ii) at least one reverse osmosis membrane for forming purified water and a fluid comprising concentrated impurities from the scrubbing solution comprising the impurities. The system of clause 17, wherein the flue gas comprising carbon dioxide (CO2) has a starting temperature of from greater than 50 to 230°C and is cooled to form a cooled flue gas having a cooled temperature of from 25 to 50°C. The system of clause 17 or clause 18, wherein the pre-treatment section comprises, or consists of, a packed bed on a single column. The system of clause 19, wherein the packed bed is a packed bed tower, or, a static packed bed, or, a rotating packed bed. The system of any one of clauses 17 to 20, wherein the flue gas comprising carbon dioxide (CO2) has a starting temperature of from greater than 50 to 230°C, or, from 70 to 230°C, or, from 110 to 230°C, or, from 105 to 220°C, or from 110 to 210°C, or from 115 to 200°C. The system of any one of clauses 17 to 21 , wherein the flue gas comprising carbon dioxide (CO2) has a pressure of from -1000 to 300000 Pa (-0.01 to 3 bar), or, from 0 to 250000 Pa (0 to 2.5 bar), or, from (75000 to 200000 Pa (0.75 to 2 bar), or, 101325 Pa (1.01325 bar). The system of any one of clauses 17 to 22, wherein the flue gas is cooled to form a cooled flue gas having a temperature of from 25 to 50°C, or, from 30 to 50°C; or, from 35 to 50°C; or, from 37 to 50°C, or from 40 to 49°C. The system of any one of clauses 17 to 23, wherein the flue gas is cooled by contacting the flue gas with a fluid comprising a cooling medium, optionally, wherein the cooling medium is water, or, water comprising a solvent; optionally, wherein the solvent is a CO2 capture solvent; optionally, wherein the flue gas is contacting with the fluid comprising the cooling medium in a counter current configuration. The system of any one of clauses 17 to 24, wherein the scrubbing agents within the scrubbing solution comprise, or consist of, sodium sulphite (Na2SOs), caustic soda (NaOH) in water, or sodium bicarbonate in water, or sodium carbonate in water, or, sodium bicarbonate and sodium carbonate in water, or, caustic soda (NaOH) and sodium bicarbonate in water. The system of any one of clauses 17 to 25, wherein the concentration of the scrubbing agents in the water is from 0.5 to 10 weight %, or, from 1 to 7.5 weight %; or, from 1 .5 to 5 weight %, or, 4 weight %; the balance being water. The system of any one of clauses 17 to 26, wherein the flue gas is contacted with the fluid comprising the scrubbing solution until the concentration of impurities within the flue gas is reduced to 50 ppmv or less, or, 25 ppmv or less, or, 15 ppmv or less, or, 12 ppmv or less. The system of any one of clauses 17 to 27, wherein the flue gas is contacted with the fluid comprising the scrubbing solution in a counter current configuration. The system of any one of clauses 17 to 28, wherein the step of passing the fluid comprising the impurities to the reverse osmosis membrane(s) removes at least 90 weight % of salts from the fluid comprising the impurities to form a fluid comprising concentrated impurities and purified water. 30. The system of any one of clauses 17 to 29, wherein the step of passing the fluid comprising the impurities to the reverse osmosis membrane(s) removes from 90 to 99.9 weight %, or, from 95 to 99.9 wight %, or from 99 to 99.9 weight % of salts from the fluid comprising the impurities to form a fluid comprising concentrated impurities and purified water.

31. The system of any one of clauses 17 to 30, wherein the step of passing the fluid comprising the impurities to the reverse osmosis membrane(s) removes sodium sulphate (Na2SO4), sodium carbonate (Na2CC>3) and sodium bicarbonate (NaHCC ).

32. The system of any one of clauses 24 to 31 , wherein the fluid comprising the cooling medium and the fluid comprising the scrubbing solution is the same fluid; optionally, wherein cooling of the flue gas comprising carbon dioxide (CO2) and removal of impurities from the flue gas comprising carbon dioxide (CO2) in step (i) of clause 17 occurs simultaneously.

33. The system of any one of clauses 17 to 32, wherein the reverse osmosis membrane(s) operates under a pressure of from 200000 to 12000000 Pa (2 to 120 bar), or, 200000 to 11000000 (2 to 110 bar), or, from 200000 to 10000000 Pa (2 to 100 bar), or, from 200000 to 8000000 Pa (2 to 80 bar), or, from 200000 to 6000000 (2 to 60 bar), or, from 200000 to 1000000 (2 to 10 bar), or, 200000 to 500000 (2 to 5 bar).

34. The system of any one of clauses 17 to 33, wherein the reverse osmosis membrane(s) operates at a temperature of from 20 to 45°C, or, 30 to 45°C, or, from 40 to 45°C.

35. The system of any one of clauses 17 to 34, wherein the system comprises at least two, three, four, or more reverse osmosis membranes.

36. The system of clause 35, wherein the osmosis membranes are arranged in parallel, or, in series, or a portion of the reverse osmosis membranes are in a first series, a portion of reverse osmosis membranes are in a second series and the first series and second series are in parallel.

37. Use of the system according to any one of clauses 17 to 36, in a carbon dioxide (CO2) capture system.

DETAILED DESCRIPTION

Embodiments of the invention are described below with reference to the accompanying drawings. The accompanying drawings illustrate various embodiments of systems, methods, and embodiments of various other aspects of the disclosure. Any person with ordinary skills in the art will appreciate that the illustrated element boundaries (e.g., boxes, groups of boxes, or other shapes) in the figures represent one example of the boundaries. It may be that in some examples one element may be designed as multiple elements or that multiple elements may be designed as one element. In some examples, an element shown as an internal component of one element may be implemented as an external component in another and vice versa. Furthermore, elements may not be drawn to scale. Non-limiting and non-exhaustive descriptions are described with reference to the following drawings. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating principles.

Figure 1 is a block diagram of a prior art system 100 used to reduce the concentration of impurities in a flue gas.

Figure 2 is a block diagram of a prior art system 200 used to reduce the concentration of impurities in a flue gas.

Figure 3 is a block diagram of a prior art system 300 used to reduce the concentration of impurities in a flue gas.

Figure 4 is a block diagram of a reverse osmosis membrane.

Figure 5 is a block diagram of a system 500 according to the present invention, wherein the system 500 is used to reduce the concentration of impurities in a flue gas prior to CO2 removal.

Embodiments of the present disclosure will be described more fully hereinafter with reference to the accompanying drawings in which like numerals represent like elements throughout the several figures, and in which example embodiments are shown. Embodiments of the claims may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

The words "comprising”, “having”, “containing”, and “including”, and other forms thereof, are intended to be equivalent in meaning and be open ended in that an item or items following any one of these words is not meant to be an exhaustive listing of such item or items or meant to be limited to only the listed item or items. It must also be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. Although any systems and methods similar or equivalent to those described herein can be used in the practice or testing of embodiments of the present disclosure, the preferred systems and methods are now described.

Some of the terms used to describe the present invention are set out below: “Absorber” refers to a part of a carbon capture system where components of a solvent (CO2 lean solvent) uptake CC from the gas phase to the liquid phase to form a CO2 rich solvent. An absorber column contains trays or packing (random or structured), which provide a transfer area and intimate gas-liquid contact. The absorber column may be a static column or a Rotary Packed Bed (RPB). An absorber column typically functions, in use, for example at a pressure of from 100000 to 3000000 Pa (1 bar to 30 bar).

“Direct contact cooler” refers to a part of a system where a flue gas is cooled. Typically, the flue gas enters a direct contact cooler at a temperature of 50 to 230°C, or, from 70 to 230°C, or, from 100 to 230°C, or, from 110 to 230°C, or, from 105 to 220°C, or, from 110 to 210°C, or, from 115 to 200°C, and is cooled by contacting a recirculating loop of a cooling medium in a packed bed or tray or, a rotating packed bed which enables efficient gas-liquid contact. Typically, the gas stream is cooled to a temperature of from 25 to 70°C; or, from 30 to 60°C; or, from 35 to 55°C; or, from 37 to 50°C, or, from 40 to 49°C.

“Flue gas” refers to a gas exiting to the atmosphere via a pipe or channel that acts as an exhaust from a boiler, furnace or a similar environment, such as a cement kiln. For example, a flue gas may be the emissions from power plants and other industrial activities that burn hydrocarbon fuel such as coal, gas and oil fired power plants, combined cycle power plants, coal gasification, hydrogen plants, biogas plants and waste to energy plants. Typically, the flue gas contains carbon dioxide. A “carbon dioxide rich flue gas” refers to a flue gas comprising carbon dioxide from 2.5 volume % to 51 volume %. A “carbon dioxide lean flue gas” refers to a flue gas comprising carbon dioxide below 2.5 volume weight %.

“Osmosis” refers to the natural diffusion of water molecules across a semi-permeable membrane from a region where the water molecules are in higher concentration to a region where they are in lower concentration. The driving force is the chemical potential of water molecules, and no external driving force is needed.

“Reverse osmosis” refers to a process acting to the opposite of osmosis, where water molecules move from a region of lower water concentration to a region of higher water concentration. From this movement, a purified water stream and another stream which is concentrated in molecules other to water molecules (such as salts) are formed. Traditionally, a reverse osmosis membrane is used to effect reverse osmosis. Traditionally, reverse osmosis membranes have been used in wastewater treatment units and desalination plants where the membranes have been used to remove salts and other contaminants from the wastewater. Traditionally, reverse osmosis membranes are used to remove NaCI (sodium chloride) from water. In the present invention, reverse osmosis membranes are used to remove salts such as, but not limited to, sodium sulphate (Na2SC>4), sodium carbonate (Na2CC>3) and sodium bicarbonate (NaHCC ). Figure 4 shows a schematic of reverse osmosis, in which a liquid feed stream 401 is separated into two streams (a concentrated stream 402 and a purified stream 403) using a reverse osmosis membrane 404. The concentrated stream 402 contains the molecules (such as salts), and the purified stream 403 contains water only. The membrane used in reverse osmosis is typically impermeable to any salts and contaminants (i.e. the membrane is permeable to water only). Impermeable as used in this definition means 90 weight % or more, or, 95 weight % or more, or, 97.5 weight % or more, or, 99 weight % or more impermeable to salts. Preferably, the reverse osmosis membranes comprise a spiral-wound element with polyamide thin- film composite membrane and has a rigid glass-fiber composite outer wrap, or, comprises siloxane coated carbon steel. Preferably, the reverse osmosis membrane operates under a pressure of up to and including 12000000 Pa (120 bar). Preferably, the reverse osmosis membrane operates at a minimum pressure of 200000 Pa (2 bar). Preferably, the reverse osmosis membrane has a maximum operating temperature of 45°C, preferably from 35 to 45°C, or, from 40 to 45°C. Preferably, the reverse osmosis membrane can operate in a pH range of from 2 to 11. Preferably, the reverse osmosis membrane removes from 90 to 99.9 weight %, or, from 95 to 99.9 weight %, or, from 97 to 99.9 weight % of salts from a liquid. Examples of reverse osmosis membranes which can be used include SeaPRO™ and SeaPRO-E™ reverse osmosis membranes produced by Suez and XUS180808 reverse osmosis element produced by DuPont™.

“Rotary Packed Bed (RPB)” refers to a packed bed where the packing is housed in a rotatable disk (rather than in a static bed, as in a static column), which can be rotated at high speed to generate a high gravity centrifugal force within the RPB. The rotary packed bed can be used in an absorber, a direct contact cooler and/or a system used to remove SOx and NOx from a gas.

“Solvent” refers to an absorbent. The solvent may be liquid. The solvent may be an intensified solvent. Optionally, the intensified solvent comprises a tertiary amine, a secondary amine, or, a primary amine. Optionally, the intensified solvent may comprise a tertiary amine, a sterically hindered amine, a polyamine, a salt and water. Optionally, the tertiary amine in the intensified solvent is one or more of: N-methyl-diethanolamine (MDEA) or Triethanolamine (TEA). Optionally, the sterically hindered amines in the intensified solvent are one or more of: 2-amino-2-ethyl-1 ,3-propanediol (AEPD), 2-amino-2-hydroxymethyl-1 ,3-propanediol (AHPD) or 2-amino-2-methyl-1 -propanol (AMP). Optionally, the polyamine in the intensified solvent is one or more of: 2-piperazine-1-ethylamine (AEP) or 1-(2-hydroxyethyl)piperazine. Optionally, the salt in the intensified solvent is potassium carbonate. Optionally, water (for example, deionised water) is included in the solvent so that the solvent exhibits a single liquid phase. Optionally, the solvent is CDRMax as sold by Carbon Clean Solutions Limited. CDRMax, as sold by Carbon Clean Solutions Limited, has the following formulation: from 15 to 25 weight % 2-amino-2-methyl propanol (CAS number 124-68-5); from 15 to 25 weight % 1-(2- ethylamino)piperazine (CAS number 140-31-8); from 1 to 3 weight % 2-methylamino-2-methyl propanol (CAS number 27646-80-6); from 0.1 to 1 weight % potassium carbonate (584-529-3); and, the balance being deionised water (CAS number 7732-18-5). Optionally, the solvent is MEA (monoethanolamine). “Static column” refers to a part of a system used in a separation method. It is a hollow column with internal mass transfer devices (e.g., trays, structured packing, random packing). A packing bed may be structured or random packing which may contain catalysts or adsorbents.

System 500: Removal of SOx and NOx impurities in a flue gas by using reverse osmosis

According to a first aspect of the present disclosure, there is provided a system and a method for reducing the concentration of impurities in a flue gas. In particular, the system and method reduce the concentration of impurities in a flue gas by providing a system with a reduced capital cost and a reduced area footprint compared to traditional systems (systems 100 and 200), wherein the system has minimal scrubbing solution loss compared to traditional systems (system 300).

Advantageously, the system and method use reverse osmosis through the use of a reverse osmosis membrane. The reverse osmosis membrane retains the scrubbing solution in the system, whilst allowing any excess water produced to be removed.

Further advantageously, the system and method comprise a single packed bed and a single cooler.

Figure 5 illustrates a system 500 according to the present invention, which is used in the removal of SOx and NOX impurities from a flue gas.

As shown in Figure 5, a flue gas 501 enters the system 500. Typically, the flue gas 501 is at a temperature of from 50 to 230°C, or, from 70 to 230°C, or from 110 to 230°C, or, from 105 to 220°C, or from 110 to 210°C, or from 115 to 200°C, typically at ambient pressure of from -1000 to 300000 Pa (-0.01 to 3 bar), or, from 0 to 250000 Pa (0 to 2.5 bar), or, from (75000 to 200000 Pa (0.75 to 2 bar), or, 101325 Pa (1.01325 bar).

The flue gas 501 can pass through a flue gas blower (not shown in Figure 5). The flue gas blower increases the pressure of the flue gas 501 to compensate for the pressure drop through the CO2 removal system (i.e., system 500 and the downstream carbon capture system, not shown in Figure 5). This ensures that the pressure of the flue gas 501 is at the same, or a similar, pressure as the flue gas at the outlet of the downstream carbon capture system (not shown in Figure 5). Typically, the flue gas blower is an induced draft fan provided at the battery limit. Alternatively, the flue gas blower can be downstream of system 500.

System 500 comprises a single packed bed 502 on a single column. The packed bed can be a packed bed tower, or, a static packed bed, or, a rotating packed bed which enables efficient gas-liquid contact. The flue gas 501 passes to the packed bed 502. In the packed bed 502, the flue gas 501 is cooled by a first scrubbing solution 522 flowing in an opposite direction. Typically, the first scrubbing solution 522 has a temperature of from 30 to 50°C, or from 35 to 45°C, or 40°C.

The first scrubbing solution 522 contains scrubbing agents which react with, and subsequently remove, impurities in the flue gas 501 . For example, the first scrubbing solution 522 can comprise caustic soda (NaOH) in water, or sodium bicarbonate in water, or sodium carbonate in water, or, sodium bicarbonate and sodium carbonate in water, or, caustic soda (NaOH) and sodium bicarbonate in water. The scrubbing agents are used for the removal of SO2 and NO2 from flue gases.

When the scrubbing agents are sodium bicarbonate and sodium carbonate, the concentration of sodium bicarbonate and sodium carbonate in aqueous solution is each: from 0.5 to 10 weight %; or, from 1 to 7.5 weight %; or, from 1 .5 to 5 weight %, or, 4 weight %; the balance being water. Without wishing to be bound by theory, it is believed that impurities such as SC and NO2 react with the scrubbing agent to form salts, as follows:

2 NaHCO ₃ + SO ₂ ->• Na ₂ SO ₃ + 2CO ₂ + H ₂ O

2NO2 +4Na2SO ₃ — >N2+4Na2SO4

When the flue gas 303 has a high concentration of NO2, but a low concentration of SO2, additional Na2SO ₃ is added to a scrubbing solution tank (reference 520 in Figure 5) where the scrubbing solution is made, to ensure NC is sufficiently removed from the flue gas 501. Typically, when the concentration of NO2 is higher than 50 ppm, the concentration of NO2 is considered high. Typically, when the concentration of SO2 is 5 ppm or below, the concentration of SO2 is considered low.

In the packed bed, the flue gas 501 and the first scrubbing solution 522 come into contact in a counter-current configuration (i.e., one fluid moves in the opposite direction to the other fluid).

Through the contact between the flue gas 501 and the first scrubbing solution 522, the flue gas 501 is typically cooled to a temperature of from 25 to 50°C; or, from 30 to 50°C; or, from 35 to 50°C; or, from 37 to 50°C, or at 40 to 49°C. Through this contact, the first scrubbing solution 522 is heated to a temperature of from 30 to 60°C, or, from 35 to 50°C, or, from 40 to 48°C, or 46°C. During cooling in the packed bed 502, the flue gas 501 will lose water as it is cooled through contact with the first scrubbing solution 520. A second scrubbing solution 503 will comprise this water.

At the same time as being cooled, the flue gas 501 reacts with the scrubbing agents present in the first scrubbing solution 522. Typically, the flue gas 501 is contacted with the scrubbing agents in the first scrubbing solution 522 until the concentration of impurities within the flue gas is reduced to 12 ppmv or less. During the reaction, the first scrubbing solution 522 is heated to a temperature of from 30 to 60°C, or, from 35 to 50°C, or, from 40 to 48°C as a result of the reaction between the scrubbing agents and flue gas 501. A consequence of the reaction between the scrubbing agents present in the first scrubbing solution 522 and the flue gas 501 , is that salts are formed which need to be removed. To remove the salts, the reacted scrubbing solution is removed from the packed bed as second scrubbing solution 503.

The second scrubbing solution 503 comprises the water and salts formed from the contact of the flue gas 501 with the first scrubbing solution 522. Typically, the second scrubbing solution 503 has a temperature of from 30 to 60°C, or from 40 to 50°C, or 46°C.

The second scrubbing solution 503 passes to a pump 504, which moves the second scrubbing solution 503 from the pump 504 back to the packed bed 502 via a cooler 506. Upon leaving the pump, third scrubbing solution 505 is formed. The third scrubbing solution 505 passes through the cooler 506. The cooler 506 reduces the temperature of the third scrubbing solution 505 to from 30 to 50°C, or, from 35 to 45°C, or, 40°C to form fourth scrubbing solution 507. Preferably, the temperature of the third scrubbing solution is cooled to 40°C.

The fourth scrubbing solution is then split into two streams, to form a fifth scrubbing solution 509 and a sixth scrubbing solution 508. The proportion of the split is dependent upon the amount of water which is lost from the flue gas 501 as it is cooled. The amount of water present in the scrubbing solution needs to be maintained at a constant level, and removal of the water in this part of the system 500 provides a means to control the amount of water present in the scrubbing solution. The more water that is lost from the flue gas 501 upon cooling, the greater the amount of the proportion of fifth scrubbing solution 509 that is formed. A valve (not shown in Figure 5) controls the proportion of fifth scrubbing solution 509 that is formed.

Furthermore, the split of the fourth scrubbing solution controls the proportion of scrubbing solution which passes to a reverse osmosis membrane 514. The ratio of fifth scrubbing solution 509 and sixth scrubbing solution 508 formed can be varied, to provide this control.

The fifth scrubbing solution 509 passes to a booster pump 510. The booster pump 510 provides sufficient pressure to ensure the reverse osmosis membrane 514 is functional (i.e., sufficient pressure for water to pass through the reverse osmosis membrane against the concentration gradient). Typically, the booster pump increases the pressure to from 200000 to 12000000 Pa (2 to 120 bar), or, 2000000 to 10000000 Pa (20 to 100 bar), or, 4000000 to 5000000 Pa (40 to 50 bar). Seventh scrubbing solution 511 is formed upon leaving the booster pump 510. Preferably, the pressure of the seventh scrubbing solution 511 is 4600000 Pa (46 bar).

Seventh scrubbing solution 511 passes to a filter 512. The filter removes any particles which will cause clogging of the reverse osmosis membrane 514 such as, but not limited to, particulate matter and/or dust picked up from the flue gas 501. Eighth scrubbing solution 513 is formed upon leaving the filter 512. System 500 can comprise of one, two, three, four, five, six, seven, eight, nine, ten, or, more than ten filters.

The fifth, seventh and/or eight scrubbing solutions 509, 511 and 513 can pass to a cooler which will optimise the temperature of the scrubbing solution passing to the reverse osmosis membrane 514. A cooler can be present between the cooler 506 and the pump 510, between the pump 510 and the filter 512 and/or between the filter 512 and the reverse osmosis membrane 514.

The eighth scrubbing solution 513 passes to the reverse osmosis membrane 514, wherein water molecules from the eighth scrubbing solution 513 pass through the reverse osmosis membrane to form a water stream 515. The water stream 515 is either sent to a sewer or can be reused. Typically, the pressure under which the reverse osmosis membrane 514 operates is from 200000 to 12000000 Pa (2 to 120 bar), or, 200000 to 11000000 (2 to 110 bar), or, from 200000 to 10000000 Pa (2 to 100 bar), or, from 200000 to 8000000 Pa (2 to 80 bar), or, from 200000 to 6000000 (2 to 60 bar), or, from 200000 to 1000000 (2 to 10 bar), or, 200000 to 500000 (2 to 5 bar).Typically, the reverse osmosis membrane 514 has a maximum operating temperature of up to and including 45°C, preferably operating at a temperature of from 20 to 45°C, or, 30 to 45°C, or, from 40 to 45°C. Typically, the reverse osmosis membrane removes at least 90 weight % of salts from the water stream 515, preferably from 90 to 99.9 weight % of salts from the water stream, or, from 95 to 99.9 weight % of salts from the water stream, preferably, from 97 to 99.9 weight % of salts from the water stream. Typically, the reverse osmosis membrane removes salts such as sodium sulphate (Na2SC>4), sodium carbonate (Na2CC>3) and sodium bicarbonate (NaHCC ).

The scrubbing solution which cannot pass through the reverse osmosis membrane forms a ninth scrubbing solution 516. The ninth scrubbing solution is split into two streams: a tenth scrubbing solution 517 and an eleventh scrubbing solution 518. The proportion of the split is dependent upon the concentration of salts which are formed when the flue gas 501 reacts with the scrubbing agents in the first scrubbing solution 522. The concentration of salts which are formed is dependent on the concentration of SOx and NOx gases present in the flue gas 501 . The concentration of salts present in the scrubbing solution needs to be maintained at a constant level, and removal of the scrubbing solution in this part of system 500 provides a means to control the concentration of salts present in the scrubbing solution. The more salts that are formed during the reaction, the more of the scrubbing solution that is removed. A valve (not shown in Figure 5) can be used to control the proportion of tenth scrubbing solution 517 which is formed.

The tenth scrubbing solution 517 is sent to an Effluent Treatment Plant (ETP) for treatment before removal.

The eleventh scrubbing solution 518 is mixed with the sixth scrubbing solution 508 to form a twelfth scrubbing solution 519. The sixth scrubbing solution 508 bypassed the reverse osmosis membrane 514. The twelfth scrubbing solution 519 is supplemented with fresh scrubbing agents 521 , which are made in tank 520, to reform the first scrubbing solution 522. Typically, the scrubbing agents are at a temperature of from 15 to 35°C, or, from 20 to 35°C, or, at 25°C.

Upon reacting with the first scrubbing solution 522, the flue gas 501 has a reduced concentration of impurities and forms flue gas 523. The flue gas 523 then passes to the downstream carbon capture system (not shown in Figure 5) for removal of CO2.

At least one reverse osmosis membrane may be used to pre-treat a flue gas. Pre-treatment of the flue gas can use one, two, three, four, or more reverse osmosis membranes, wherein the osmosis membranes are arranged in parallel, or, in series, or a portion of the reverse osmosis membranes are in a first series, a portion of reverse osmosis membranes are in a second series and the first series and second series are in parallel. In an alternative embodiment, the reverse osmosis membrane may be located downstream in the carbon capture system (not shown in Figure 5). In an alternative embodiment, a reverse osmosis membrane may be used in the pre-treatment of a flue gas and/or used downstream in the carbon capture system.

Advantageously, by using a reverse osmosis membrane in a system used to remove SOx and NOx impurities from a flue gas, the components of the system can be reduced to one packed bed and one cooler wherein the system has the same efficiency at removing excess water and salts formed by the purifying process and cooling process as a system comprising two packed beds and two coolers (systems 100 and 200).

Advantageously, through pre-treating the flue gas 501 with a system that uses a reverse osmosis membrane, less scrubbing agents are lost from the system (when compared to system 300) thus increasing the overall efficiency of the pre-treatment of a flue gas compared to traditional pretreatment of flue gases which also have only one packed bed and only one cooler.

Advantageously, system 500 provides a method of removing impurities in a flue gas which has a minimal footprint area (compared to traditional systems 100 and 200) but which has the same efficiency at removing excess water and salts formed by the purifying process and cooling process as a system comprising two packed beds and two coolers (systems 100 and 200).

Advantageously, the reaction occurring in the packed bed 502 is operating at a higher temperature than the reaction occurring in the SOx and NOx removal section 106 of system 100 and the second packed bed (210) of system 200. The kinetics (i.e., speed of reaction) occurring between the scrubbing agents and the flue gas 501 in system 500 is thus improved compared to systems 100 and 200. Thus, the overall efficiency of the removal of impurities from the flue gas is improved. Advantageously, system 500 provides flexibility with regard to adjusting the proportion of the scrubbing solution that is removed from system 500 as the tenth scrubbing solution 517. This allows optimisation of the salt concentration present in the scrubbing solution.

Advantageously, the temperature of the scrubbing solution passing to the reverse osmosis membrane 514 can be separately adjusted through use of a cooler present between the cooler 506 and the pump 510, between the pump 510 and the filter 512 and/or between the filter 512 and the reverse osmosis membrane 514. This ensures that the performance of the reverse osmosis membrane is optimised.

Advantageously, the water stream 515 can be reused in another process and not only sent to a sewer. The quality of water recovered from the reverse osmosis membrane is high, and therefore minimal (if any) purification steps are required.

EXAMPLES

The following are non-limiting examples that discuss, with reference to tables and figures, the advantages of the present invention. The examples set forth herein are merely examples among other possible examples.

Example 1: Comparison of a traditional flue-gas pre-treating process to the flue-gas pre-treating process of the present invention

In this non-limiting example of the present invention, the operating costs of a traditional flue-gas pretreating process as described in system 300 was compared with the flue gas pre-treating process of the present invention as described in system 500.

To compare the traditional flue-gas pre-treating process as described in system 300 with the flue gas pre-treating process of the present invention as described in system 500, the scrubbing solution makeup required for each system was calculated. In this example, the scrubbing solution consisted of sodium bicarbonate in de-mineralised water. A unit cost of 0.2 euros per kg was assumed for sodium bicarbonate and a unit cost of 5 euros per metre cubed was assumed for demineralised water.

ProMax simulation was used for the calculations.

Table 1 sets out the results of the comparison. Table 1 : A traditional flue-gas pre-treating process as described in system 300 was compared to the flue gas pre-treating process of the present invention as described in system 500 in terms of operating cost.

As shown in Table 1 , the operating cost of pre-treating a flue gas according to the present invention is reduced compared to the operating cost of pre-treating a flue gas according to a traditional method. Thus, the efficiency of the pre-treatment of a flue gas is increased upon applying the present invention.

Example 2: Comparison of a traditional flue gas pre-treating process to the flue-gas pre-treating process of the present invention

In this non-limiting example of the present invention, the loss of scrubbing agent in a traditional flue gas pre-treating process as described in system 300 was compared with the flue gas pre-treating process of the present invention as described in system 500 by using ProMax software.

In this example, the scrubbing solution consisted of sodium bicarbonate in de-mineralised water.

Table 2 sets out the results of the comparison.

Table 2: A traditional flue gas pre-treating process as described in system 300 compared to the flue gas pre-treating process of the present invention as described in system 500 in terms of scrubbing agent and water lost from each respective system.

As shown in Table 2, system 500 has reduced loss for both scrubbing agent and water compared to system 300. As a result, system 500 requires less scrubbing agent and water makeup, making system 500 a more economical and more efficient system. Furthermore, by having less water lost, system 500 advantageously saves costs because less water is used, and, minimizes environmental impact because less water is required. Example 3: Conditions of the flue gas in system 500

In this non-limiting example of the present invention, the flue gas 501 entering the system 500 and the flue gas 523 exiting the system 500 was simulated in ProMax. The results are set out in Table 3.

Table 3: Temperature and composition of the flue gas 501 entering the system 500 and the flue gas 523 exiting the system 500.

Advantageously, the flue gas 523 (the flue gas which has passed through system 500) has a very minimal SO2 content and a reduced NO2 content.

Example 4: Chemical makeup of parts of system 500

In this non-limiting example of the present invention, the chemical makeup of the eighth scrubbing solution 513, water stream 515 and ninth scrubbing solution 516 of system 500 was simulated in ProMax. The results are set out in Table 4.

Table 4: The chemical makeup (in weight %) of the eighth scrubbing solution 513, water stream 515 and ninth scrubbing solution.

Advantageously, the water stream 515 produced is high quality water, with minimal impurities present.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilized for realizing the invention in diverse forms thereof.

Although certain example aspects of the invention have been described, the scope of the appended claims is not intended to be limited solely to these examples. The claims are to be construed literally, purposively, and/or to encompass equivalents