HIGH SOLIDS FLUX CIRCULATING CARBONATION REACTOR

Abstract

A system for capturing carbon dioxide CO.sub.2 by carbonation in a circulating fluidized bed (CFB) carbonation reactor wherein temperature profile is adjusted by recirculation of solid fractions of metal oxide MeO and metal carbonate MeCO.sub.3 to the CFB carbonation reactor.

Claims

1. A system for capturing carbon dioxide from a carbon dioxide CO.sub.2 rich flue gas stream; the system comprising: a circulating fluidized bed (CFB) carbonation reactor configured to capture carbon dioxide present in the flue gas by a carbonation reaction; a pipe for forwarding a metal oxide MeO solids stream to the CFB carbonation reactor; a pipe for forwarding the carbon dioxide CO.sub.2 rich flue gas stream to the CFB carbonation reactor; a separation device downstream the CFB carbonation reactor configured to separate the flue gas from a MeCO.sub.3 rich solids stream; a splitting device for dividing the MeCO.sub.3 rich stream in two or more portions downstream the separation device; a fluidized bed heat exchanger configured to cool the MeCO.sub.3 rich solids stream before redistribution to the CFB carbonation reactor; a pipe for recirculating a first portion of the MeCO.sub.3 rich solids stream to the CFB carbonation reactor; and at least one pipe for adding the cooled portions of said MeCO.sub.3 rich solids stream to the CFB carbonation reactor at various possible.

2. The system according to claim 1, further comprising a heat exchanger configured to cool a MeO rich solids stream forwarded via a pipe from a calcination system.

3. The system according to claim 1, further comprising a heat exchanger configured to heat a MeO rich solids stream forwarded via a pipe from a calcination system.

4. The system according to claim 1 wherein the pipes for adding the cooled portions of said MeCO.sub.3 rich stream to the CFB carbonation reactor are positioned for recirculation of the MeCO.sub.3 rich stream at a lower or intermediate part of the CFB carbonation reactor.

5. The system according to claim 1, further comprising a pipe for bypassing a portion of the MeCO.sub.3 rich stream to the CFB carbonation reactor.

6. The system according to claim 1, wherein the separator is a cyclone-type separator.

7. The system of claim 1 wherein the separator is a particle entrainment device.

8. The system according to claim 1 wherein the separator is located external to the CFB carbonation reactor.

9. The system according to claim 1 wherein the separator is located internal to the CFB carbonation reactor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The present invention is described in more detail below with reference to the appended drawings:

[0041] FIG. 1 is a schematic view of a system for carbonization via carbon dioxide CO.sub.2 rich flue gas and with cooling system connected thereto.

DETAILED DESCRIPTION

[0042] The carbonation reaction, thus the reaction between the CO.sub.2 in the flue gas and the sorbent material, for example selected from a metal oxide (MeO) forming MeCO.sub.3 according to the following reaction equation:

MeO+CO.sub.2.fwdarw.MeCO.sub.3+Heat

[0043] The reaction is an exothermic reaction which proceeds at a temperature depended on the metal oxide used. Controlling the temperature is important to ensure an efficient reaction system which balances kinetic and equilibrium requirements. The capturing of carbon dioxide CO.sub.2 may occur with different metal oxides MeO forming metal carbonates, for example limestone. The metal oxides may also be part of a synthetic solid particle. The metal oxides used for the invention may be selected from calcium oxide CaO, magnesium oxide MgO, aluminium oxide Al.sub.2O.sub.3, zink oxide ZnO, and calcium magnesium oxide CaMgO forming calcium carbonate (CaCO.sub.3), for example in form of calcite or aragonite; magnesium carbonate (MgCO.sub.3), for example in form of magnesite, alumina carbonate (Al.sub.2(CO.sub.3).sub.3); zinc carbonate (ZnCO.sub.3) or in form of calcium magnesium carbonate, such as dolomite (CaMg(CO.sub.3).sub.2), respectively. The list of metal oxides is not exhaustive and the form in which the oxides are present on the solids particles is not limited.

[0044] The carbonation reaction, thus the reaction between the CO.sub.2 in the flue gas and the metal oxide (MeO) is an exothermic reaction which proceeds at a temperature of, typically, between 600 C. and 850 C., preferably about 650 C., when the metal oxide is CaO. The carbonization is an exothermic reaction, thus heat is generated and shall be removed to optimize yield, thus to optimize the portion carbon dioxide CO.sub.2 captured by the metal oxide MeO.

[0045] Also the temperature profile present in the reactor, i.e. the circulating fluidized bed carbonation reactor is an important parameter for an efficient reaction. The energy and heat must be removed if a uniform temperature profile will be obtained. By optimizing the temperature profile present in the carbonation reactor the system can be made more efficient; smaller and less expensive.

[0046] Optimization of the reactor also must consider the concentration of solid particles, the mass fraction of solids in the reactor and the partial pressure of carbon dioxide CO.sub.2 over the height of the reactor. The modification of all parameters is considered with the ultimate goal to minimize plant costs (capital costs and energy consumption).

[0047] FIG. 1 is a schematic representation of the system 1 for capturing carbon dioxide CO.sub.2 from carbon dioxide rich flue gas by carbonization. The system comprises a circulating fluidized bed (CFB) carbonation reactor 10 wherein the bulk of the carbonization is taking place.

[0048] In the CFB carbonation reactor 10, the reaction between the CO.sub.2 present in the flue gas and the solid metal oxide MeO fed to the reactor occurs. The reactor is a so-called circulating fluidized bed wherein the solid particles are fluidized together with the flue gas. The flue gas is introduced in the bottom of the reactor via the duct 14 and the metal oxide MeO rich solids are forwarded via the pipe 13 to the CFB carbonation reactor 10.

[0049] The temperature profile within the reactor varies depending on the exothermic reaction. Due to the reaction taking place heat evolution shall be controlled and adjusted. In an optimized system the operating temperature profile should be far enough below the corresponding equilibrium temperature (according to the CO.sub.2 concentration profile) so as not to hinder or slow the overall reaction rate.

[0050] After reaction in CFB carbonation reactor 10, a stream rich in the metal carbonate MeCO.sub.3 entrained in the flue gas is forwarded from the CFB carbonation reactor 10 via pipe 15 to a separation device 30. (Remaining CO.sub.2 in the flue gas may undergo residual reaction in the solids separation device but this is small in comparison to that occurring in the CFB carbonation reactor 10. Thus, the temperature of this stream is close to the outlet temperature of the reactor and is preferably kept at about 650 C. when the metal carbonate is calcium carbonate CaCO.sub.3.

[0051] The separation device 30 separates CO.sub.2 lean flue gas from the stream of MeCO.sub.3 rich solid particles and any non-reacted metal oxide MeO. The separation device 30 may be external to the CFB carbonation reactor 10 (as shown), for example, a cyclone but may also be a device which is partially integrated into the CFB carbonation reactor 10 acting to lower particle entrainment. It is also possible to use a combination of both types of devices internal and external. The cleaned flue gas is forwarded to a flue gas cooler via the outlet 35. The remaining solid, material rich in MeCO.sub.3, is forwarded via the pipe 16 from the separation device 30. A device 50 splits the stream into several parts, this may be a type of solids-loop-seal.

[0052] The solid materials separated in the separation device 30 comprise the metal carbonate MeCO.sub.3 as the main part, and is herein denoted as a MeCO.sub.3 rich stream. When calcium oxide CaO is considered as the metal oxide for capturing carbon dioxde CO.sub.2 the stream has a temperature of about 650 C., when forwarded from the separation device 30, via pipe 16, to a split point 50 wherein the stream is divided into two or more portions, or streams (shown by streams 51, 53 and 18).

[0053] A portion of solids from the separation device 30 shall be forwarded to the fluidized bed heat exchanger 20. The solids present in this fluidized bed heat exchanger 20 are fluidized by a fluidizing gas forwarded into the fluidized bed heat exchanger 20 via duct 58, and leaving the heat exchanger via duct 81. The fluidized bed heat exchanger 20 is fed with fluidizing gas, the fluidizing gas, in duct 58, may be compressed air or compressed flue gas or steam. The metal carbonate MeCO.sub.3 rich stream may then be split into multiple streams, i.e. two or more streams and returned to different locations in the reactor. The stream rich in solid MeCO.sub.3 entering the heat exchanger 20 has a temperature of about 650 C. Depending on the solids circulation rate the temperature of the solids stream exiting the fluidized bed heat exchanger 20 must be selected to off-set the heat of reaction before being circulated back to the reactor. The point where the solids are removed from the exchanger may be used to influence the stream temperature and the point where the solids are introduced to the reactor shall be selected to ensure a suitable temperature profile over the height of the reactor. The CFB carbonation reactor 10 may use internal devices to improve the solids distribution and thus heat exchange and temperature profile.

[0054] The fluidized bed heat exchanger 20 may be one unit or may be several units operating in parallel at different temperatures. Either the stream 51 cooled before splitting (as shown) or the stream 51 is split before cooling. In any case the cooler streams of solids forwarded from the fluidized bed heat exchanger 20 are re-circulated to the CFB carbonation reactor 10 at a suitable position to improve the temperature profile. Stream 54 enters near the bottom, stream 55 near the mid-section of the riser and stream 56 near the top of the riser, as shown

[0055] Another portion of the stream 16 may be bypassed to the CFB carbonation reactor 10, via pipe 53. The bypass is used to control the temperature of the lower bed to avoid considerable inlet temperature drops during plant upsets or start-up. This portion has typically a temperature of about 650 C. but during start-up may also be somewhat cooler.

[0056] The first 51 and second 53 streams as described above are re-circulated to the carbonation reaction taking place in the CFB carbonation reactor 10. The position of the inlets along with the temperature and mass flow of the streams 56, 55 or 54 may be adjusted to optimize the temperature profile in the reactor.

[0057] Optionally, fluidized bed heat exchanger 20 may be split into parallel units so that stream 52 of solids obtained after cooling may by multiple streams flowing in parallel at various temperatures, herein shown by the two streams 54, 55, 56. A portion of the stream of solids 52 enters the CFB carbonation reactor 10 via the pipe 55. Another portion of the stream of the solids 52 enters via the pipe 56. Another portion of the solids may be lifted/transported to a higher level in the reactor height profile by a suitable device 59, for example a screw device for solid material or pneumatic transport using compressed air, compressed flue gas or steam as transport medium.

[0058] From the split point, split device 50, a portion of the stream 16 of solid materials rich in CaCO.sub.3 is also to be forwarded via pipe 18. The solid materials have preferably a temperature of about 650 C. This third stream is forwarded from the split point 50 via pipe 18 for further processing in a separate system. The metal carbonate MeCO.sub.3 rich stream may, for example, be forwarded to a unit for decarbonisation (not shown) to convert the metal carbonate MeCO.sub.3 into metal oxide and carbon dioxide CO.sub.2. This reaction or process (MeCO.sub.3+heat.fwdarw.MeO+CO.sub.2) may also be called calcination.

[0059] The system 1 is integrated together with a system for decarbonisation of MeCO.sub.3 to MeO, a process also called calcination, thus a system wherein CO.sub.2 is released from the metal carbonate leaving remaining metal oxide MeO rich solids. MeO rich solids are fed to system 1 via pipe 11 into the CFB carbonation reactor 10.

[0060] Optionally, the MeO rich stream forwarded from the calcination process may be cooled in a feed effluent fluidized bed heat exchanger 70, or in a fluidized bed cooler 60, or in a system including both.

[0061] Optionally, also stream 18 may be fed to a feed effluent heat exchanger 70 for transferring heat from the hot product MeO to the cold MeCO.sub.3 reducing total process heating and cooling requirements. Here, the metal carbonate is heated by a counter current stream of metal oxide MeO entering unit 70 via pipe 11. The cold MeCO.sub.3 is forwarded via pipe 19 for further processing in a separate system (not shown). The cooled MeO rich stream 12 is forwarded to a second heat exchanger which further reduces the temperature before entering the CFB carbonation reactor 10 via pipe 13. The metal oxide MeO rich stream returning from the calcination process may be further cooled by fluidized bed heat exchanger 60 in parallel to unit 20. Optionally stream 12 may be fed directly to unit 20 and cooled before redistribution via stream 52 to CFB carbonation reactor 10 (not shown in the figure).

[0062] The heat exchanger 60 may be a fluidized bed heat exchanger in which case fluidizing gas (air flue gas or stream) is fed via duct 62 and exits unit 60 via duct 82. Heat removed over unit 20 and unit 60 may be used for generating steam the heat streams are indicated schematically as stream number 61 and 63.

[0063] While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.