GB2494639A - Reducing emissions - Google Patents

Abstract

A system 100 for reducing emissions comprises: an absorber (20) arranged to mix pre-combustion gas with a gas remover fluid such that a proportion of the pre-combustion gas is removed. A holding tank (110) is arranged to store the gas remover fluid at an elevated pressure above ambient pressure following mixing in the absorber with the pre-combustion gas. A gas remover fluid regenerator (50) is arranged to regenerate the gas remover fluid. The pre-combustion gas may enter (30) the system (100) at a lower portion of the absorber and is scrubbed before leaving the system through an exit pipe (80). The system may include a heat generator when forming part of an industrial plant process such as an electricity generating or chemical producing facility. Advantageously the system is used to reduce the emission of carbon dioxide (CO2) or sulphur compounds into the atmosphere.

Description

REDUCING EMISSIONS

Field of the Invention

The present invention relates to a system and method for reducing emissions and in particular but not limited to reducing emissions during power generation and other industrial processes.

Background of the Invention

Reducing emissions during industrial processes may be reguired in order to reduce the impact of those processes on the environment. For example, reducing the emission of carbon dioxide and/or sulphur compounds into the atmosphere is desirable. However, capture of such gaseous emissions may require additional energy, which itself has an adverse effect on the environment requiring further resources to generate the additional energy.

Furthermore, where additional energy is needed then larger and more costly infrastructure may be reguired.

EP0537593 describes a method for recovering carbon dioxide gas from combustion exhaust gases using a morioethanolamine solution. The carbon dioxide absorbing solution is mixed with combustion exhaust gas produced by a steam generating power plant or other equipment producing carbon dioxide within flue gas. The carbon dioxide absorbing solution is regenerated by heating it usually by using some of the generated steam. During periods of high demand from the power plant, regeneration of the aqueous solution does not occur. Instead, unused or previously regenerated aqueous solution is drawn from a storage tank to be mixed with the combustion gases to remove carbon dioxide.

Used aqueous solution rich in carbon dioxide is stored in a further storage tank. A regenerator that uses steam (for example, from the power plant or other associated steam-raising equipment) regenerates the stored aqueous solution when demand for power is low.

However, this method is limited to post-combustion gas processing. Therefore, there is required a system or method that overcomes this problem.

Sununary of the Invention Against this background and in accordance with a first aspect there is provided a system for reducing emissions comprising: an absorber arranged to mix pre-combustion gas with a gas remover fluid such that a proportion of the pre-combustion gas is removed; a holding tank arranged to store the gas remover fluid at an elevated pressure above ambient pressure following mixing in the absorber with the pre-combustion gas; and a gas remover fluid regenerator arranged to regenerate the gas remover fluid. Pressurisation of the holding tank facilitates the use of deferred regeneration during periods of high system demand without affecting pre-combustion gas emission reduction. Pre-combustion gas emission reduction can Improve overall emissions especially from other gases such as sulphur compounds.

Preferably, the regenerator heats the gas remover fluid. The heat may be obtained from an industrial plant or another dedicated heat source. The regenerator may also comprise a gas collection device for collecting gas driven off the gas remover fluid during regeneration.

Preferably, the system cf may further comprise a reservoir tank arranged to store regenerated gas remover fluid or fresh (unused) gas remover fluid, and supply the regenerated gas remover fluid to the absorber. This allows the circulating volume or mass of gas remover fluid to remain substantially constant or exactly constant and provides a reservoir of lean fluid to build up during periods of lower demand to be used during periods of high demand.

Preferably, the gas remover fluid may be a 002 remover.

Various types of liquids may be used.

Preferably, the regenerator may use heat to regenerate the gas remover fluid. The regenerator may use excess heat produced by the system to regenerate the fluid. The heat may be in the form of steam or hot water and generated by a fuel fired boiler for example.

Preferably, the gas remover fluid may be a H2S remover.

Various types of liquids may be used, which preferably removes both 002 and H2S.

Preferably, the absorber may be pressurisable to a pressure above ambient pressure during absorption of gas.

This may improve gas removal especially of 002. The absorber may be pressurisable to the same or a higher pressure than the holding tank.

Optionally, the system may further comprise a flash vessel arranged between the absorber and the storage tank and arranged to reduce the pressure of the gas remover fluid before storage in the storage tank. This provides oontrolled 002 removal, whilst leaving H2S in solution. The flash vessel may then reduoe the pressure required in the holding tank.

Optionally, the elevated pressure of the holding tank may be above 10 x l0 Pa or 10 bar. Preferably, the pressure may be between 10 x l0 Pa and 70 x 10 Pa.

Optionally, the system may further comprise: a first valve arranged to allow the supply the holding tank with used gas remover fluid; and a second valve arranged to allow the reservoir tank to supply regenerated gas remover fluid or fresh (unused) gas remover fluid to the absorber. This arrangement of control valves allows the system to respond to demand. For example, during high demand, stored lean gas remover fluid or fresh (unused) gas remover fluid may be made available by opening the second valve, whilst rich or semi-rich gas remover fluid cnoe used may be admitted into the pressurised reservoir tank through the first valve. The valves may also be used to empty the holding tank or vessel of rioh or semi-rich gas remover fluid through the regenerator and into the holding vessel during periods of lower demand. When the reservoir tank is full and/or the holding tank is empty then both valves may be closed.

Optionally, the system may further comprise a bypass valve arranged to bypass the gas remover fluid regenerator.

Optionally, the system may further comprise a pump arranged to pump gas remover fluid to the regenerator from the holding tank. This may reduce the time taken to provide an adequate reserve of fluid.

Preferably, the fluid may be a liquid.

Optionally, the system may further comprise a pressurisation system for applying a pressurising fluid into the holding tank and/or the reservoir tank.

According to a second aspect there is provide a method for reducing emissions comprising the steps of: mixing pre-combustion gases with a gas remover fluid such that a proportion of the pre-combustion gas is removed; storing the gas remover fluid at an elevated pressure above ambient pressure after mixing; and regenerating the gas remover fluid.

Preferably, the method may be for reducing emissions during energy production such as for example, electricity production.

Preferably, the method may further comprise the step of: diverting the gas remover fluid to a storage tank for storing the gas remover fluid at the elevated pressure during periods of high energy demand.

Preferably, the method may further comprise the step of: using stored regenerated gas remover fluid or fresh (unused) gas remover fluid for mixing with the pre-combustion gases during pericds of high demand. The high demand may be high energy demand.

Preferably, regenerating the gas remover fluid may occur when energy demand is low. This may be low energy or electricity demand in a power station. The power station may be gas fired, oil fired, coal fired, biomass fired, waste fired, petroleum ooke fired, or other conventional fuel type.

According to a third aspect there is provided an industrial plant, the industrial plant comprising: a heat generator; and a system as described above. The tndustrial plant may be gas fired, oil fired, coal fired, biomass fired, waste fired, petroleum coke fired, or other conventional fuel type used to produce a product such as electricIty, or chemical product for example. The heat generator may be a boiler used to supply steam to a generator such as a steam turbine, for example. The industrial plant may take fuel such as carboniferous materials including gas, oil, coal, biomass, waste or petroleum coke as a part of an industrial procedure and process this to form the pre-combustion gas.

For example, a mixture of 00, 002 and In some circumstances H2 may be produced from carbon or coke (and H20 in some processes) using processes known in the art.

Sulphur containing species may also be present where the carbon or coke sources contained such impurities. Removal of the 002 from this pre-combustion gas (mixture) reduces overall emissions that may otherwise be emitted from a chimney or other exhaust device. Removing the 002 pre-combustion or before final oxidation of the gaseous mixture is advantageous for emission control.

Preferably, sulphur-containing species from the gas may be removed at the same time and stage of the process as the 002.

The sulphur-containing species, may otherwise be emitted through a chimney or other exhaust device, for example, as sulphur dioxide or other compounds of sulphur.

Where the process includes the production of a gas which includes sulphur-containing species (for instance hydrogenated sulphur, H25), it may be required to remcve these sulphur-containing species from the gas to reduce emissions.

Preferably the sulphur-containing species (for example as hydrogen suiphide) receive further treatment so that beneficial use can be made from the components therein.

The process to remove the sulphur-containing species and the process to provide further treatment of the sulphur-containing species may be maintained in equilibrium so that all of the sulphur-containing species that are removed from the gas remover fluid are dispatched for further treatment.

In the eventuality that equipment providing further treatment of the sulphur-containing species is not available to the process or industrial plant (for instance as a result of planned or unplanned maintenance) the potential exists for there to be an imbalance between the process to remove the sulphur containing species and the process to provide further treatment. In this instance it may be necessary to shut down the process to remove the sulphur-containing species from the gas. This would normally result in either a reduction in the capacity or a total shut-down of the facility or industrial plant employing carboniferous materials such as gas, oil, coal, biomass, waste or petroleum coke as a part of the process.

Therefore, a further benefit is that instead of shutting down or reducing the capacity of the facility or industrial plant it becomes possible to store the sulphur-containing species within the gas remover fluid in the holding tank and continue operations as normal.

Once the equipment providing further treatment of the sulphur-containing species has been repaired, or otherwise returned to service, at a convenient time the gas remover fluid could be regenerated and stored in the reservoir tank.

This aspect is effective in improving the availability of the facility employing carboniferous materials such as gas, oil, coal, biomass, waste or petroleum coke as a part of a process.

Preferably, the regenerator may be configured to use heat from the heat generator to regenerate the gas remover fluid.

The method described above may be implemented as a computer program comprising program instructions to operate a computer. The computer program may be stored on a computer-readable medium.

It should be noted that any feature described above may be used with any particular aspect or embodiment of the invention.

Brief description of the Figures

The present invention may be put into practice in a number of ways and embodiments will now be described by way of example only and with reference to the accompanying drawings, in which: FIG. 1 shows a schematic diagram of a prior art system for reducing emissions; and FIG. 2 shows a schematic diagram of an improved system for reducing emissions, given by way of example only.

It should be noted that the figures are illustrated for simplicity and are not necessarily drawn to scale.

Detailed description of the preferred embodiments

There are a number of methods for capturing carbon dioxide (CC2) and other gases from facilities that utilise the energy from fossil fuel and other materials. These may be broadly grouped Into four categories: * Post-combustion * Pre-combustion * Oxy-fuel * Others CO2 and other gases may be absorbed by a gas remover fluid or solvent. These other gases include for example hydrogen sulphlde (H2S) Both post-and pre-oombustion gas removal (in -10 -particular, CC2) may involve washing the gases (including 502) using a suitable solvent out of a flue or exhaust gas following oxidation of the fossil or other carboniferous material. Examples of this gas remover fluid or solvent include ammonia and amines. Other fluids may be used.

Solvent processes tend to perform best under steady state conditions and, as a consequence, rapid changes in gas conditions or throughput may result in changes to the composition of the product gases reducing emission control.

The solvent used in the removal of CO2 and other gases (in particular acid gases) may be regenerated so that it can be recycled and re-used within the process. Regeneration may be achieved by solvent pressure reduction and/or by heating the solvent. Heating may be provided by diverting generated steam from elsewhere in the facility, or by other means, such as a separate steam-raising boiler.

Pre-combustion capture typically has different operating requirements over post-combustion capture. Pre-combustion capture of gases may take place in four stages: Absorption Desorption Regeneration 25. Cooling and solvent return Two types of solvent processes (chemical and physical) may be used in pre-combustion capture of gases. She two process types involve the same four basic stages, but what happens is different for the two types.

Various chemical solvents may be used. Higher pressures may be used especially where two gases need to be -11 -separated out.

In an absorption stage, the fluid or solvent may fall under gravity down a tower or absorber against a rising pre-combustion gas stream. Packing may be provided within the tower to increase the contact time between the pre-combustion gas and the solvent. Some of the components within the rising gas may be absorbed by the solvent. The particular solvent may be chosen depending on what gases it is intended to absorb. This stage takes place under pressure, typically in the 10 x 10 Pa to 70 x io Pa (10-70 bar) range, but higher or lower pressures may be experienced depending on the specific application.

The absorption stage may take place in two parts.

Regenerated solvent (lean solvent) may be used to absorb CO2 in one part of the tower or absorber. EJ2S may then be absorbed into the C02-rich solvent (semi-lean solvent) in the other part of the absorber. Other gases may be dissolved in the solvent (i.e. rich solvent), but typically only in trace quantities.

In the desorption stage the gas or gases dissolved in the rich solvent may be released. In the example of a solvent in which both CO2 and H25 are dissolved in the rich solvent, the CO2 (together with some other gases, often only in trace quantities) can be released by reducing the pressure in a flash vessel. This leaves the solvent rich In HS.

The H2S may be released In the regeneration stage, which is achieved by heating the depressurised or partially depressurised H2S-rich solvent. This heating may be by -12 -steam or by other means.

Once the gases have been released from the solvent (lean solvent), the warmed solvent may be cooled and pumped to a suitable pressure to return it to the CO2 absorption tower for the cycle to repeat.

Figure 1 shows a schematic diagram illustrating the structure and arrangement of a system for implementing the removal of gases from pre-combustion gas that would otherwise contribute to emissions. The system 10 contains an absorber 20 incorporating a first tower portion 22 where 002 may be absorbed below which a second tower portion 24 absorbs H2S. Incoming pre-combustion gas 30 enters the lower tower portion 24 rising into the upper tower portion 22. Scrubbed pre-combustion gas leaves through an exit pipe 80. Cooled solvent or gas remover fluid enters at the top of first tower portion 22 and falls through the rising pre-combustion gas. Following mixing of the gas remover fluid with the pre-combustion gas, the gas remover fluid, rich in 002 and PbS, is drawn off from the bottom of second tower portion 24.

The mixing of the pre-combustion gas and gas remover fluid occurs at elevated pressures, as described above.

This improves the gas removal efficiency, especially relating to 002. 002 may be separated from the gas remover fluid by reducing its pressure and this occurs in flash vessel 40 as shown in figure 1. The 002 may be recovered and stored as appropriate (not shown in this figure) . The gas remover fluid, which is now rich in H2S is directed to a regenerator 50, which incorporates a heat source 60 that may utilise a steam coil or other heating devices. Heating the -13 -gas remover fluid within the regenerator 50 releases the H73, which again may be recovered and stored appropriately (not shown in this figure) A cooler 70 then cools the lean gas remover fluid before repeating the cycle by directing the cooled fluid into the top of the absorber 20.

A degree of operational flexibility may be provided from within a CO7 (or other gas) absorber facility by storing the solvent prior to the regeneration process.

Where the regeneration process is achieved by heating the solvent in a regenerator 50, storing the gas-containing solvent may enable the steam or heat to be utilised elsewhere. An example of this technique may be used in post-combustion CO7 removal from an electricity generating facility, where low pressure steam may be bled from the expansion turbine to regenerate the gas-containing solvent in a regenerator 50. Storing the gas-containing solvent allows the steam that would normally be diverted to instead provide energy for within the expansion turbine (not shown in the figures) and additional electricity produced (for example) as a result during periods of high electricity demand. When electricity or other heat demand falls, the flow of gas through the absorber 20 may be reduced.

Diverting heat to the regenerator 50 may then resume in order to regenerate the stored gas-containing solvent.

The solvent may be stored at intermediate stages of the pre-combustion capture process. This allows the heat (for example, in the form of steam) that may otherwise be used In either the desorption or regeneration stages to be utilised elsewhere, for instance in the pioduction of additional -14 -electricity during periods of high demand. Regeneration can then take place during a period of lower demand.

For a physical absorpticn system, storage tanks, pumps and valves may be used. Some types of chemical solvents, particularly those at higher pressures where two gases need to be separated out, may use a similar process.

Figure 2 is a schematic diagram of a system:00 incorporating features that provide this additional operational flexibility for storing gas remover fluid after It has been used within the absorber 20. Similar features have the same reference numerals as those described with reference to figure 1 and these features shall not be described again in detail, but operate in a similar way in the system 100 shown in figure 2.

A holding tank 110 may be in fluid communication with the output from the flash vessel 40 to receive gas removal fluid rich in ET2S. The holding tank may be pressurised to the same or substantially the same pressure as the fluid after it leaves the flash vessel 40. The pressure in the holding tank 110 may be achieved or maintained by introducing another fluid, preferably an inert gas such as nitrogen, acting as a pressurising medium. Other fluids including inert gases other then nitrogen may be used. This coupling may be achieved using a valve 120 that may open to allow the holding tank 110 to receive the gas removal fluid or close to allow the gas removal fluid to flow directly to the regenerator 50 instead. A valve 160 may be included to allow the exit of the pressurising medium simultaneously with the entrance of the gas removal fluid. A pump 130 may be used to draw off gas remover fluid rich in H2S (or other -15 -gases) and direct this fluid into the regenerator 50 when additional regeneration of the stored fluid is reguired and for removing the dissolved H2S.

A reservoir tank 120 may hold regenerated, lean gas remover fluid or fresh (unused) gas remover fluid to be used during periods of high heat demand. A valve 140 may be fitted in a regenerated gas remover fluid (lean solvent) return line after the regenerator 50 so that the flow through the regenerator 50 may be reduced or stopped during the period when the gas removal fluid is taken from the reservoir tank 120.

The reservoir tank 120 may be pressurised to the same or substantially the same pressure as the gas remover fluid as it enters the cooler 70. The pressure in the reservoir tank 120 may be achieved by introducing another fluid preferably at the top through a valve, 170. This pressurising fluid should preferably an inert gas such as nitrogen, acting as a pressurising medium. Other fluids including inert gases other than nitrogen may be used.

During these periods of high demand, a valve 150 is opened (i.e. when valves 120, 160 and 170 are open and valve 140 is fully or partially closed) allowing lean gas remover fluid to be cooled in cooler 70 and directed into the top of the absorber 20. During this period of high demand, heat or steam to the heater 60 within the regenerator 50 Is shut off or diverted to other uses such as power generation, for example. Such steam shutoff may utilise other valves or controls (not shown in this figure) Advantageously, the pressurising medium exiting the holding tank 110 as the gas remover fluid enters the holding -16 -tank 110 through valve 120 may be used as the pressurising fluid for the reservoir tank 120, allowing for a control valve 85 to balance and regulate the pressures in the two tanks.

In other words, during periods of high demand, the cycling nature of the gas remover fluid and regeneration ceases with the process instead relies on stored fluid being used up from a reservoir and used fluid rich in H2S being stored in the holding tank 110. Obviously, such a situation cannot continue indefinitely, but may continue until either or both the reservoir 120 is emptied and/or the holding tank becomes full. When the period of high demand comes to an end and spare heat capacity becomes available, then valve 120 and valve 150 close, and valve 140 partially or fully opens. Iherefore, excess heat or steam may be provided to heater 60 within the regenerator 50 to allow gas remover fluid within the circuit to be regenerated as it is used.

If enough spare heat capacity is available, then pump 130 may be used to empty the holding tank 110 and divert the stored gas remover fluid that is rich in H23 into the regenerator 50. Whilst the pump is operating and spare regenerated or lean gas remover fluid enters the system then valve 150 may be used in reverse to refill reservoir tank 120 with this excess lean fluid for use during a future period of high demand.

During this period of low demand the pressurising fluid in the reservoir tank 120 may be drained through valve 170 simultaneously with the gas removal fluid entering the reservoir tank 120 through valve 150. At the same time the pressure in the holding tank 110 may be maintained by adding additional pressurising fluid through valve 160.

-17 -Advantageously the same pressurising fluid exiting the reservoir tank through valve 170 may be used to maintain the pressure in the holding tank 110, allowing for the balancing of the pressure by the use of a compressor or similar device 90.

These valves, pump and other system controls may be operated manually or automatically following a suitable process ccntroi routine that may be electronically or computer controlled, for example.

During normal cperaticn (i.e. when demand for heat or power is normal or low) the tank or tanks 110 in the regeneration section may be wholly or partially empty and the lean solvent storage tank or tanks 120 may be wholly or partially filled. Regeneration of the solvent may be carried out at the rate at which it is being used or consumed in the absorber 20. In this state the system may be ready to react to higher demand without reducing emission control.

Holding tank 110 may be of an accumulator design allowing the solvent to remain at a pressure (i.e. elevated) that does not allow the H2S to be released. In other words, holding tank 110 may be pressurised above atmospheric pressure but may be below the pressure of the absorber 20 (i.e. between 10 x 10 and 70 x io Pa). Therefore, H25 may be retained without leakage into the atmosphere or wider environment and instead captured during periods of high and low demand.

The valves may be controlled to maintain a constant -18 -mass flow through the absorber 20. During periods of low demand the pre-combustion gas flow through the absorber 20 may be gradually reduced and the gas remover fluid from holding tank 110 can be pumped into the regeneration section 50 using the pump 130. The steam flow through the regenerator 50 may be maintained at a normal leve, providing additional solvent regeneration capacity. Surplus lean solvent or gas remover fluid may then be returned to storage in reservoir tank 120.

As will be appreciated by the skilled person, details of the above embodiment may be varied without departing from the scope of the present invention, as defined by the appended claims.

For example, the industrial process may be power generation (electricity) , hydrogen production, fertiliser, or steel production.

Many combinations, modifications, or alteratfons to the features of the above embodiments will be readily apparent to the skilled person and are intended to form part of the invention. Any of the features described specifically relating to one embodiment or example may be used in any other embodiment by making the appropriate changes.

Claims (1)

1. <claim-text>-19 -CLAIMS: 1. A system for reducing emissions comprising: an abscrber arranged to mix pre-combustion gas with a gas remover fluid such that a proportion of the pre-combustion gas is removed; a holding tank arranged to store the gas remover fluid at an elevated pressure above ambient pressure following mixing in the absorber with the pre-combustion gas; and a gas remover fluid regenerator arranged to regenerate the gas remover fluid.</claim-text> <claim-text>2. The system of claim 1 further comprising a reservoir tank arranged to store regenerated or unused gas remover fluid and supply the regenerated gas remover fluid to the absorber.</claim-text> <claim-text>3. The system of claim 1 or claim 2, wherein the gas remover fluid is a C02 remover.</claim-text> <claim-text>4. The system according to any of claims 1 to 3, wherein the regenerator uses heat to regenerate the gas remover fluid.</claim-text> <claim-text>5. The system according to any previous claim, wherein the gas remover fluid is a H2S remover.</claim-text> <claim-text>6. The system according to any previous claim, wherein the absorber is pressurisable to a pressure above ambIent pressure during absorption of gas.</claim-text> <claim-text>7. The system of claim 6 further comprising a flash vessel arranged between the absorber and the storage tank and 6?87 -20 -arranged to reduce the pressure of the gas remover fluid before storage in the storage tank.</claim-text> <claim-text>8. The system according to any previous claim, wherein the elevated pressure of the holding tank is above 10 x 10 Pa.</claim-text> <claim-text>9. The system of claim 2 further comprising: a first valve arranged to allow the supply the holding tank with used gas remover fluid; and a second valve arranged to allow the reservoir tank to supply regenerated and/or fresh gas remover fluid to the absorber.</claim-text> <claim-text>10. The system of claim 9 further comprising a pump arranged to pump gas remover fluid to the regenerator from the holding tank.</claim-text> <claim-text>11. The system according to any previous claim, wherein the fluid is a liquid.</claim-text> <claim-text>12. A method for reducing emissions comprising the steps of: mixing pre-combustion gases with a gas remover fluid suoh that a proportion of the pre-combustion gas is removed; storing the gas remover fluid at an elevated pressure above ambient pressure after mixing; and regenerating the gas remover fluid.</claim-text> <claim-text>13. The method of olaim 12 further comprising the step of: diverting the gas remover fluid to a storage tank for storing the gas remover fluid at the elevated pressure during periods of high demand. 6?87</claim-text> <claim-text>-21 - 14. The method of claim 12 or claim 13 further comprising the step of: using stored regenerated and/or fresh gas remover fluid for mixing with the pre-combustion gases during periods of high demand.</claim-text> <claim-text>15. The method acoording to any of claims 12 to 4, wherein regenerating the gas remover fluid occurs when demand is low.</claim-text> <claim-text>16. The method according to any of claims 12 to 4, wherein the fluid is a liquid.</claim-text> <claim-text>17. An industrial plant comprising: a heat generator; and a system according to any of claims 1 to 11.</claim-text> <claim-text>18. The industrial plant of claim 17, wherein the regenerator is configured to use heat from the heat generator to regenerate the gas remover fluid.</claim-text> <claim-text>19. A method substantially as described with reference to any of the accompanying drawings.</claim-text> <claim-text>20. An apparatus substantially as described and shown in any of the accompanying drawings. 6?87</claim-text>