SYSTEM FOR CAPTURING OF CO2 FROM PROCESS GAS

The system includes a first reactor configured to discharge CO2 depleted process gas. The first reactor having a first and a second portion of particulate sorbent material having captured CO2. A second reactor is arranged to receive the first portion of particulate sorbent material and is configured to release CO2 from the particulate sorbent material by decarbonation, return the first portion of particulate sorbent material to the first reactor, and discharge a CO2 rich gas stream. A third reactor is arranged to receive the second portion of particulate sorbent material and is configured to supply water to the second portion of particulate sorbent material to hydrate at least a part of a remaining portion of calcium oxide of the second portion of particulate sorbent material to form calcium hydroxide, and return the second portion of particulate sorbent material to the first reactor.

It is understood that the detailed description below is intended to improve the understanding of the invention, and should not be interpreted as limiting the scope of the invention.

DETAILED DESCRIPTION

Reactions Taking Place in Embodiments:

The calcium oxide of the sorbent material may react with CO2under formation of calcium carbonate. When calcium oxide, CaO, and CO2is brought in contact, calcium carbonate, CaCO3may be formed according to carbonation reaction 1 (R1) under release of energy as heat. In the system of embodiments of the invention, R1 may take place, for example, in the first reactor, such as in the carbonator reactor.

In the first reactor, R1 results in lowering of the CO2concentration in the process gas, such as flue gas, thus resulting in a gas effluent from the first reactor having a considerable lower concentration of CO2than the inlet concentration. In the case with flue gas, the concentration of CO2before capturing or reaction according to reaction (1), may be for example 15%. Reaction 1 may take place, for example, at or below 650° C., and at pressures of approximately 1 atmosphere, which may exist in the first reactor.

In addition to reacting with CO2, CaO may react with sulfur dioxide, or SO2, under formation of calcium sulfate and energy in the form of heat, according to reaction 2 (R2), for example if SO2is present in the process gas.

R2 may, for example, take place in the calciner reactor. Particularly if in-situ oxyfired coal combustion takes place in the second reactor R2 may take place in the calciner reactor.

In addition SO2may react with CaCO3under formation of calcium sulfate and energy in the form of heat, according to reaction 3 (R3).

R3 may, for example, take place in the calciner reactor where partial pressures of CO2are expected.

CaO may react with water in a hydration reaction taking place in the third reactor, such as a hydrator reactor, according to reaction 4 (R4).

The reversed R4 describes dehydration of Ca(OH)2, an endothermic reaction. Dehydration may efficiently be performed, for example, at conditions of atmospheric pressure, at a partial pressure of water at 0.1 atm, and at temperatures above approximately 400° C., such as above 410° C. Thus, dehydration may take place in the first reactor, such as the carbonator reactor, and/or in pipings and/or solids separation devices, such as for example cyclones, of the system comprising hot flue gas downstream the first reactor.

Reactions with sulphur dioxide, for example as described by R2 and R3, have a negative effect on the capacity of the sorbent material for capturing CO2. Reactions according to R2 and R3, may occur if sulphur dioxide is present in the process gas, for example in the case of the process gas being flue gas for example resulting from burning of fuels such as coal, or other sulphur containing fuels. CaSO4may result in blockage of pores in the sorbent material, thus reducing the efficiency of CO2capturing by the sorbent material, for example due to blocking of the CaO present in the core of the sorbent particles. Further, a layer of CaCO3formed on the sorbent material particles reduces the efficiency of the sorbent material in capturing CO2. Sintering of sorbent material which may occur also reduces the efficiency of the sorbent material in capturing CO2. Since H2O is a small molecule it is capable of penetrating product layers of CaSO4and/or CaCO3forming Ca(OH)2in less accessible regions of the particle. The molar volume of Ca(OH)2is larger than the molar volume for CaO. Thus, particles comprising CaO may swell when hydrated according to R4, resulting in crack formations in any present layer of CaSO4and/or CaCO3, thus hydration according to R4 may improve the efficiency of the sorbent material and regenerate the sorbent material. Thus, the system according to embodiments are efficient for capturing of CO2from process gas such as flue gas, which may contain sulphur.

The third reactor, such as for example a hydrator reactor, arranged to hydrate solid material received from the first reactor and recycle hydrated solid material to the first reactor is an efficient way of increasing surface area of the sorbent material.

With reference toFIG. 1, a system for capturing CO2from flue gas which may or may not comprise sulfur, is described. The system comprises a carbonator arrangement1, receiving flue gas via piping2. The carbonator arrangement1comprises a circulating fluidized bed carbonator reactor1′ optionally with internal heat transfer area in addition to solids separation device1″ removing solids from the gas stream before the gas stream leaves the system through piping3. The circulating fluidized bed carbonator reactor1′ contains particulate sorbent material, in this particular example the sorbent material essentially consists of CaO. The sorbent reacts with CO2and CO2depleted flue gas leaves the carbonator reaction system1via piping3, having undergone bulk solids removal.

Reacted sorbent is regenerated by decarbonation in the calciner arrangement11forwarded from the carbonator arrangement1by piping90, which regeneration process can be described by endothermic reversed R1. The calciner arrangement11comprises a circulating fluidized bed calciner reactor11′ with solids separation device11″ removing solids in the gas stream before the gas stream leaves the system via piping12. Thus, in the calciner arrangement11, CaCO3is converted to CaO and CO2, and CO2exits the calciner arrangement11by means of piping12, having undergone bulk solids removal. Means for energy input into the calciner arrangement11is indicated by piping13, which forwards for example a carbon source, such as coal, and an oxygen stream, such as oxygen diluted with CO2. Sorbent, predominantly in the form of CaO particles exits the calciner arrangement11by means of piping14and is recycled to the carbonator arrangement1. Optionally, a heat exchanger15is used to reduce the temperature of sorbent being recycled back to the first reactor. Optionally, a heat exchanger95is used to heat sorbent before entering the calciner arrangement11. As an additional option these two heat exchangers may be combined so that heat is transferred from heat exchanger15to heat exchanger95. Sorbent make-up flow, for example in the form of limestone, may be added through piping75to the stream of sorbent being recycled back to the carbonator arrangement1from the calciner arrangement11.

The sorbent may be detoriated by sulfatization if sulfur is present in the flue gas, for example if the flue gas is resulting from burning of coal, as described by R2 and R3, and/or by sintering, for example during calcination. Sorbent particles, comprising CaCO3, CaO, and possible CaSO4, leave the carbonator arrangement1via piping4and enter hydrator reactor5. The hydrator reactor5is equipped with heat transfer surface so that heat released during the hydration reaction can be removed from the hydrator reactor5. Optionally, the sorbent particles may in addition or alternatively be cooled before entering the hydrator reactor5, such as by means of optional heat exchanger6. Reactivation medium comprising gaseous H2O is fed to the hydrator reactor5via piping7. Inside the hydrator reactor the sorbent is reacting according to R4 such that CaO is transferred to Ca(OH)2. Under this reaction sorbent particles are regenerated. Possible excess gas, such as steam may leave the hydrator reactor by means of piping8or may be returned to carbonator arrangement1. Regenerated sorbent particles are transferred back to the carbonator arrangement1via piping9. Optionally, hydrated sorbent particles will be heated and dehydrated before entering the carbonator arrangement1, such as by means of optional heat exchanger10.

It will be understood that in addition to any heat exchangers discussed, heat may be removed from for example the carbonator arrangement1and the hydrator reactor5by suitable means.

With reference toFIG. 2, a system is illustrated which in addition to what is described with reference to the system ofFIG. 1, discloses means for pelletizing fine sorbent particles into a size suitable for the system. Particulate sorbent hydration may act, in conjunction with a number of particle size reduction mechanisms, to reduce the average particle size of the circulating particulate sorbent material by producing fine material what is hereafter referred to as fines. Re-processing of fine material or fines into larger particles decouples sorbent fines losses from process make-up requirements increasing sorbent utilization and reducing operational costs. The fines may be agglomerated to particles of a suitable size by means of the pelletizer19. CO2depleted flue gas leaves the carbonator arrangement1through piping3containing a residue of particulate sorbent material, having undergone solids separation, and enters a sorting device16such as for example an electrostatic precipitator or bag filter, which removes most of the residual solids particulate material, such as fine sorbent particles, or fines, from the CO2depleted flue gas. Flue gas leaves the sorting device16by means of piping17, while the particles leaves the sorting device16through piping18, and enters the pelletizer19. In addition, fines may be fed to the pelletizer19from the calciner arrangement11and separator93, particularly for example in the case where ash free or indirect heating method is used to bring heat into the calciner arrangement11. In the pelletizer19the fines are mixed with a liquid such as water or a mixture of water and binding agent fed from piping91, resulting in wet agglomerates of the fine sorbent particles. Thus, agglomeration takes place inside the pelletizer19. The agglomerates leave the pelletizer19by means of piping20. The agglomerates may either be forwarded to the hydrator reactor5, or to the carbonator arrangement1. Since the hydration reaction in hydrator reactor5takes place at a higher temperature and requires water, the agglomerates may be forwarded and introduced into the hydrator reactor5where any water from the agglomerates will provide water for the hydration reaction. Thus, fine sorbent particles may be converted to larger sorbent particles. In addition to agglomeration taking place inside the pelletizer19, or agglomerator, hydration may take place in the pelletizer19. Thus, pelletizer19may function as a hydrator. It is realized that the pelletizer may be positioned elsewhere in a system than what is described inFIG. 3, and that pelletizer19in addition to receiving fines from separators16and93, may receive fines from other suitable sources. The embodiment with reference toFIG. 2may also comprise a calciner arrangement11as previously described from which calciner arrangement11CO2rich gas is discharged via residual dust separator93and piping94.

With reference toFIG. 3, a carbonator reactor and pelletization system according to one embodiment is illustrated. The system illustrated inFIG. 3comprises a plurality of gas-solids separators22,23,24which act in separating solids from gas and further act in heating solids while cooling gas, thus resulting in particles with a temperature suitable for the system while cooling the flue gas that is leaving the system, thus minimising energy input or heat transfer surface requirements. The separators22,23,24may, for example, be of cyclon type, or any other solids separator suitable for the purpose. Flue gas enters the carbonator reactor1through piping2where it optionally may be heated via heat exchanger50. The carbonator reactor may be comprised of one or more sections in which the particulate sorbent material is contacted with CO2rich flue gas and may contain heat transfer surface to remove the heat released through reaction. A mixture of CO2depleted flue gas and sorbent particles leaves the carbonator reactor1at a temperature T1 through piping3and is forwarded to a point25where the gas and the sorbent particles are combined with a flow of sorbent particles from separator23at a lower temperature T2 and the combined flow is forwarded to separator22. By means of separator22, sorbent particles are separated from the gas and fines, which gas and fines are leaving the separator22via piping26. Larger sorbent particles, having a temperature T3 between T1 and T2, are leaving the separator22and at least a part of the larger particles are forwarded to the hydrator reactor5in which hydration takes place as described above while at least a part of the sorbent particles are recycled back to the carbonator reactor1. The gas and fines from separator22, having a temperature T3 being between T1 and T2 are forwarded to a point27where they are combined with a flow of sorbent particles from separator24and with hydrated sorbent particles from the hydrator reactor5, having temperatures T4 and T5 respectively both preferably lower than T3 before the combined flow is forwarded to separator23via pipe28. It is realised that the flows may not be combined in the same point27, but that one of the flows may enter downstream of the other flow, or vice versa. From the separator23, sorbent particles are forwarded through piping29to point25, as previously described at temperature T2, between T3 and T4 or T5, while gas and fines, at temperature T2, are forwarded through piping85to a connection point30where the fines and the gas is combined with a make-up flow comprising sorbent material from piping86. A heat exchanger51may be positioned downstream separation device23to cool the flue gas stream before it is mixed with the cool sorbent make-up stream, the heat removed from heat exchanger51may optionally be coupled to the heating of flue gas in heat exchanger50so that heat flows from heat exchanger51to heat exchanger50. The flow is forwarded to separator24, from which sorbent particles, at temperature T4 lower than T2, are forwarded to point27, as previously described while fines and gas are forwarded through piping31, and leaving the system, for example towards a chimney (not illustrated). Before leaving the system, the gas and fines are optionally cooled in heat exchanger52before entering separator32, separating the gas from fines, and forwarding fines to pelletizer19. The pelletizer is fed liquid for agglomeration of the fines, such as water or water and binding agent through piping33. In addition, the pelletizer19may be fed with fines from the calciner reactor11, (not illustrated) through piping34if the operational mode restricts ash content from being too high. Agglomerated fines from the pelletizer are forwarded to hydrator reactor5, wherein the agglomerated fines are transformed into dry sorbent particles under release of free water. The hydrator reactor5is fed liquid comprising water through piping7. Piping90feed sorbent material from the carbonator reactor1towards the calciner system11(not illustrated) and piping14from the calciner reaction system11to carbonator reactor1.

The system described above with reference toFIG. 3, thus describes efficient transformation of heat from flue gas to sorbent particles, such that flue gas is cooled and sorbent particles are heated to a suitable temperature before the particles enters the carbonator reactor1. The embodiment is suitable for systems utilizing primarily steam hydration at higher temperatures, such as a hydration temperature about 510° C. or less. For example, such a system and for hydrating around 20% of the sorbent stream, the following temperatures may, for example, apply: T1 being around 650° C., T2 being between 350 and 550° C., T3 being between 540 and 610° C., T4 being between 200 and 300° C. and T5 being between 300 and 510° C.

With reference toFIG. 4, an embodiment of the invention is illustrated. The embodiment only differs from the embodiment discussed with reference toFIG. 3in that the stream of sorbent material from separator23is forwarded by means of piping directly into carbonator reactor1as an additional solids feed with the purpose of direct cooling and to improve the solids distribution and equilibrium driving forces in the upper section of the carbonation reactor. Thus, according to the embodiment illustrated byFIG. 4, the flow of sorbent material from separator23is not combined with a flow of gas and sorbent material from carbonator reactor1before entering the carbonator reactor1. It is realised that the temperature of the sorbent material which is forwarded to the hydrator reactor5from separator22has a temperature essentially identical to the temperature of the sorbent material leaving the carbonator reactor1through pipings3, for example around 650° C.

With regard to the embodiments, conditions for reactions R1 and R4, including for example temperatures and pressures, may be selected and/or maintained to favour desired reactants and/or products. Further the conditions may be selected to be suitable for the treated gas, the reactors and the system. Equilibrium pressures for gaseous reactants at different temperatures may be considered for selecting suitable conditions. For example, and with reference to R1, the fraction of CO2in flue gas may, for example, vary between 10 and 15 percent by volume for flue gas from power production and be as high as 30 percent by volume of CO2for flue gas from a conventional cement plant. For operating pressures of around 1 atm in the carbonator reactor, the temperature may be selected to be suitable for removal of 90% of the CO2. For the case of power production, 650° C. is acceptable in order to reduce the concentration of CO2in the treated flue gas to 1 percent by volume. For example for the case of cement plants flue gas, increased temperatures would still allow a similar removal efficiency. Since diffusions processes proceed at an increased rate with increasing temperature, a staged temperature profile in the carbonation reactor may also be an advantage. According to one embodiment, the temperature of an upper section of the carbonator reactor may be below 650° C. and a lower section of the carbonator reactor above 650° C. It is realised that in analogy with the discussions above regarding suitable temperatures and pressures of the carbonator reactor, suitable conditions, such as for example temperatures and pressures, for other parts of the system, such as for example the calciner reactor and/or the hydrator reactor, or other parts, may be selected.

For example, CO2depleted flue gas having 10 percent by volume of water could be used to dehydrate sorbent around or slightly above 410° C. According to one embodiment the dehydration takes place above 410° C.

The position of the hydrator reactor5downstream of the carbonator reactor1according to the embodiments, such that sorbent material is forwarded to the hydrator reactor from the carbonator reactor without passing through the calciner reactor, results in efficient operating conditions for the hydrator reactor reducing the amount of cooling required for the material entering the hydrator reactor. For example, the temperature of the hydrator reactor may be maintained at or below 510° C., and the partial pressure of water may be selected at or below 1 atm.

Herein before it has been described that the carbonator reactor and the calcinator reactor are fluidized bed type of reactors, it is also appreciated that other types of carbonator reactors and calcinator reactors can be used.

To summarize, the present disclosure relates to a system for capturing CO2from a process gas. The system comprises a first reactor arranged to receive a stream of process gas and a particulate sorbent material comprising calcium oxide able to capture the CO2present in the process gas such that calcium carbonate is formed, the first reactor comprising means for discharging CO2depleted process gas, a first portion of particulate sorbent material having captured CO2, and a second portion of particulate sorbent material having captured CO2, a second reactor arranged to receive the first portion of particulate sorbent material from the first reactor, the second reactor comprising heating means arranged to cause release of CO2from the particulate sorbent material by decarbonation of the calcium carbonate to form calcium oxide, the second reactor further comprising means for returning the first portion of particulate sorbent material to the first reactor and means for discharging a CO2rich gas stream, and a third reactor arranged to receive the second portion of particulate sorbent material from the first reactor, the third reactor comprising means for supplying water to the second portion of particulate sorbent material to hydrate the calcium oxide to form calcium hydroxide, the third reactor further comprising means for returning the second portion of particulate sorbent material to the first reactor.