ITMI20111160A1 - SYSTEM AND METHOD TO SEPARATE CO2 FROM COMBUSTION FUMES CONTAINING SOX AND NOX THROUGH FUEL CELLS IN CARBONATI FUSI (MCFC) - Google Patents

Description

DESCRIPTION

"SYSTEM AND METHOD FOR SEPARATING C02 FROM COMBUSTION FUMES CONTAINING SOX AND NOX BY MEANS OF MELTED CARBONATE FUEL CELLS (MCFC)"

The present invention relates to a system and a method for separating C02 from combustion fumes containing SOX and NOX by means of molten carbonate fuel cells (MCFC).

In particular, the invention is placed in the field of systems for the separation of C02 from combustion fumes and more specifically in the field of processes and devices capable of selectively extracting C02 from the fumes in which it is diluted, to make it available concentrated in a flow gaseous from which it is easily separable. Even more specifically, the invention is part of the field of MCFC-CCS systems, i.e. systems in which, by means of molten carbonate fuel cells (MCFC), C02 is extracted from the fumes in which it is diluted and concentrated in a gaseous flow rich in H20 vapor and substantially free of N2 diluent, in order to facilitate its subsequent capture (CCS = "Carbon Capture and Storage").

Various installation diagrams for MCFC-SCC systems are known, as described for example in JP3106418, EP04188 64, US7396603.

In general, known configurations provide MCFC-CCS systems which have a single battery (stack) of cells, or a plurality of batteries in parallel (cathode side). In all cases, the system includes a manipulation device, in particular for the separation and recovery of C02, which is arranged at the final exit of the anode exhaust.

Also for solutions with batteries in series on the anode side, the teaching of the known art would similarly consist in providing a single manipulation / separation device at the anode output of the last battery of the series.

Ultimately, the known solutions all have a device for manipulating the anodic waste, disposed only at the final outlet of the system.

This solution suffers from a main drawback: some contaminants (for example, F) must be radically eliminated, regardless of the type of MCFC battery or the process conditions, because their accumulation in the cells inevitably leads to the progressive destruction of the carbonates. But for other contaminants, in particular for SOx and NOx, there is room for maneuver. For SOx and NOx the permitted thresholds change according to the type of battery (for example, with DIR type batteries with direct reformer, the SOx thresholds allowed are at least one order of magnitude more stringent than with type IIR or ER batteries) and, for a given type of battery, they can be raised if the anode gas is rich in H2, not only at the inlet but also at the outlet.

The MCFC-CCS solutions with exhaust handling device only on the final anodic outlet have been conceived assuming clean fumes.

In fact in such systems:

to have a high yield, it necessarily takes a high use of H2;

- if the use of H2 is high, the anode gas, at least at the outlet, is depleted in H2;

- if the anode gas (at least at the outlet) is poor in H2, the sensitivity to SOx and NOx is high;

- if the sensitivity to SOx and NOx is high, the pollutant thresholds allowed must be low.

Basically, for these systems there is incompatibility between the high efficiency of the MCFC cells and the raising of the tolerable thresholds of SOx and NOx.

Some of the solutions shown in US7396603 actually allow to increase the efficiency without depleting the anode gas at the outlet, in systems where the device for handling the anodic exhaust is only on the final outlet: in these solutions, the hydrogen present in the the anode output of the MCFC can be kept high because it is in any case suitable for use inside the system or outside; or a flow of H2 (or in any case rich in H2) recovered from the manipulation organ of the exhausted anode is recirculated at the anode inlet. These measures, which make it possible to achieve a high efficiency even with a high H2 content at the anode output of the MCFCs, however, have indirect effects which in any case prevent the thresholds on H2S and NOx from being raised.

In particular, in a solution that uses PEM cells ("proton exchange membrane"), the threshold on the input SOx becomes the very low threshold imposed by the catalysts of the PEM cell, onto which the MCFC would transfer in the form of H2S all the sulfur present in the fumes that had not been previously eliminated.

In another solution, which provides for a complete anodic recirculation of H2 after separation, an accumulation mechanism of N2 is triggered (so-called "build-up of N2) which inexorably ends up blocking the possibility of washing the electrolyte from the anode side. nitrites and / or nitrates that are formed at the cathode by reaction with the NOx present in the fumes (washing stops if and when the conversion of nitrites and / or nitrates into N2 and H20 is inhibited). This solution therefore imposes low thresholds on incoming NOx.

Ultimately, for known MCFC-CCS systems with the exhaust handling device only on the final anodic output, MCFC cells can have high yields only if stringent thresholds on SOx and NOx at the cathode input are respected. Indeed:

- either the anode gas leaving the MCFC cells is low in H2 (due to the incompatibility between high efficiency and high H2 content in the anodic output gas),

- o the H2 content in the anode gas leaving the MCFC cells is kept high with measures that produce retroactive effects that accentuate the sensitivity to SOx and NOx.

It is an object of the present invention to provide a system and a method for separating C02 from combustion fumes by means of molten carbonate fuel cells (MCFC) which is free from the drawbacks highlighted herein of the known art; in particular, it is an object of the invention to provide a system of the MCFC-CCS type which operates, with respect to known systems, with high electrical efficiency and at the same time with higher thresholds on SOx and NOx in the cathode gas.

The present invention therefore relates to a system and a method for separating C02 from combustion fumes by means of molten carbonate fuel cells (MCFC) as defined in essential terms in the annexed claims 1 and 13 respectively, as well as for the additional preferred characters, in the dependent claims.

Basically, the invention consists in raising the H2 content in the anode outputs of the cells without wasting fuel, with systems derived from the known art such as the recirculation at the anode inlet of a flow rich in H2 or the recovery for electrical generation purposes. of the high content of fuel left in the anodic exhaust by means of a combination "PEM shift reactor-pile", but taking care to add specific measures that allow both to remove H2S and to avoid N2 accumulation mechanisms.

The invention therefore relates to an MCFC system consisting of one or more batteries of cells and combined with devices for handling the anode waste; type of manipulation devices and organization modalities of the gaseous flows in the system are such as to allow the system as a whole, with limited need for additional cleaning of the treated fumes, to operate as a C02 separator from combustion fumes containing SOx and NOx, generating at the same time electricity with high efficiency.

Further characteristics and advantages of the present invention will appear clear from the following description of a non-limiting example of its implementation, with reference to the figures of the annexed drawings, in which:

Figure 1 is a schematic view of a system for separating C02 from combustion fumes by means of molten carbonate fuel cells (MCFC) made in accordance with a first embodiment of the invention;

Figures 2 to 4 are respective schematic views of further embodiments of the invention.

With reference to Figure 1, a system 1 for separating C02 from combustion fumes by means of fused carbonate fuel cells (MCFC) comprises one or more batteries 2 of MCFC cells, and a device 3 for handling the exhausted anode.

In the following, reference will be made, for simplicity, to a battery 2 of MCFC cells, schematically represented as a single block having an anode compartment 5 and a cathode compartment 6; but it is understood that the system 1 can comprise several batteries 2 of MCFC cells connected together.

Battery 2 is powered by an anode power supply line 7, which leads to an anode input 8 of the anode compartment 5 a flow of fuel, and by a cathode power supply line 9, which leads to a cathode input 10 of the cathode compartment 6 a flow of combustion fumes to be treated, optionally enriched with air.

The cathode compartment 6 has a cathode output 11 connected to a discharge line 12, while the anode compartment 5 has an anode output 13 connected, via an anode output line 14, to the device 3.

The device 3 comprises a plurality of functional units 15-18 arranged in series along the anode output line 14 to manipulate the flow of anode exhaust outgoing from the battery 2, and specifically to eliminate H2 and other residual fuels from the anode exhaust.

In particular, the device 3 comprises, starting from the anode output 13: an H2S removal unit 15, a shift reaction unit 16, a H20 separation unit 17A, at least one PEM stack unit 18, and a further H20 separation unit 17B.

The H2S removal unit 15 is located upstream of the other units of the device 3 and specifically of the shift reaction unit 16 and of the PEM stack unit 18.

The PEM battery unit 18 has an anode compartment 21 arranged in series with the previous units of the device 3 along the anode output line 14 and therefore fed with the flow that has passed through the previous units of the device 3, and a cathode compartment 22 which It is powered with air. A flow containing H20 and C02 exits from the anode compartment 21 of the PEM cell unit 18 from which, after removing the H20 in the unit 17B, a flow of C02 is recovered.

Optionally, the H2S removal unit 15 is also configured to remove a first portion of C02 present in the anode exhaust.

Conveniently, but not necessarily, in this plant configuration the MCFC cells of battery 2 are of the type with internal indirect reformer, or with external reformer, or of another type but not of the type with direct internal reformer (DIR).

Battery 2 operates with low H2 use: the H2 content in the anode exhaust stream can in fact be kept relatively high, approximately in the order of 30-50% vol., Because the high H2 content present in the exhaust anodic is recovered for the purpose of electricity generation; precisely, the exhausted anode has a high content of remaining fuel which is exploited by the combination of the shift reaction unit 16 and the PEM cell unit 18; however, the negative effects of the known solutions are avoided as H2S is removed upstream of the shift reaction unit 16.

In the embodiment of figure 2, in which (as also in the following figures) the details similar or identical to those already described are indicated with the same numbers, the system 1 always comprises a battery 2 of MCFC cells having an anode compartment 5 and a cathode compartment 6 (or, as already highlighted, several batteries of MCFC cells connected to each other), and a device 3 for handling the exhausted anode.

The battery 2 is still powered by means of an anode power supply line 7, which brings a flow of fuel to the anodic compartment 5, and from a cathodic power supply line 9, which leads to the cathode compartment 6 a flow of combustion fumes to be treated. optionally enriched with air.

The cathode compartment 6 has a cathode output 11 connected to a discharge line 12, while the anode compartment 5 has an anode output 13 connected, via an anode output line 14, to the device 3.

The device 3 comprises a selective separation unit 25 for C02, which separates from the anodic exhaust a flow of C02 containing H2S (ready to be sent to subsequent sequestration operations) and, separately, a flow of H20; the residual treated flux, rich in H2 and also containing the residues not removed by the separation unit 25, is recirculated to the anode inlet 8 by means of a recirculation line 26 which connects to the anode supply line 7; however, the treated flux is not entirely recirculated to the anodic compartment 5, but a portion of it is tapped through a branch line 27 which departs from the recirculation line 26 and is connected to a burner unit 28, in particular a catalytic burner unit .

In this way, the system 1 includes an exit way for N2, constituted by the branch line 27; it is thus possible to establish a limit to the N2 accumulation phenomenon, which is essential in order to guarantee washing on the anode side of the nitrites and / or nitrates that are formed at the cathode by reaction with NOx.

In order to reduce the penalty deriving from the fact that the branch line 27 also subtracts a portion of H2 from the recirculation, the system 1 optionally includes external reformers (not shown), arranged downstream of the burner unit 28 or integrated with it so that the heat generated in the burner unit 28 is exploited to reform at least a portion of the methane needed for the anode powering of the battery 2.

Also in this embodiment it is advisable, although not strictly necessary, that the MCFC cells are not of the direct internal reformer (DIR) type.

Also in this case, the battery operates with low H2 use: the recirculation of H2 at the anodic inlet makes it possible to maintain a relatively high H2 content in the exhausted anode; the device 3 ensures at the same time both to remove H2S and to avoid, thanks to the branch line, mechanisms of "build up" of N2 in the MCFC cells.

In the embodiment of Figure 3, the device 3 comprises two separation units in series: a first H2 selective separation unit 25A, which selectively extracts only H2 from the anode exhaust stream, and a second separation unit 25B which treats the residual flow to obtain C02 suitable for seizure.

In greater detail, the first separation unit 25A separates from the anode exhaust flow: a flow of H2, which is entirely recirculated at the anode inlet by means of a recirculation line 26 which engages on the anode power supply line 7; a stream of H2O; and a residual gas stream, containing C02, H2S and residues.

The residual gas flow outgoing from the first separation unit 25A cannot therefore be sent directly to the sequestration, but first passes through the second separation unit 25B, which separates C02 suitable for sequestration; preferably, the second separation unit 25B comprises an oxy-combustor 29 (of known type) which eliminates, by reaction with O2 and conversion into H20 and C02, the fuels remaining in the residual flow rich in C02 exited from the first separation unit 25A, and a final separator 30 of C02.

In the embodiment of Figure 4, as in the diagram of Figure 3, the device 3 comprises a first H2 selective separation unit 25A, and a second separation unit 25B.

The first separation unit 25A selectively extracts a flow of H2 from the flow of anodic exhaust, which is still recirculated (integrally) to the anode inlet 8 through the recirculation line 26, and a flow of H20; the residual gas flow, containing C02, H2S and residues, is sent to the second separation unit 25B, which is configured to obtain the C02 suitable for sequestration, separating it from H2 and H20.

As already highlighted, in all the illustrated configurations the system 1 can include a plurality of batteries 2 of MCFC cells.

In particular, the batteries can be connected in series on the anode side, and the system includes an anode line that connects the outputs of the anode compartments of the various batteries. In this case, advantageously, the system includes means for intermediate tapping along the anode line connected to respective manipulation devices having the characteristics described above, to selectively extract products from the anode exhaust of respective batteries.

Finally, it is understood that further modifications and variations may be made to the system and method described and illustrated here, which do not depart from the scope of the attached claims.

Claims (22)

CLAIMS 1. A system (1) for separating CO2 from combustion fumes using molten carbonate fuel cells (MCFC), comprising at least one battery (2) of MCFC cells having an anodic compartment (5) and a cathode compartment (6), and a manipulation device (3) for treating an anodic exhaust stream leaving the battery (2) of MCFC cells; the system (1) being characterized by the fact that the device (3) is configured in such a way as to remove H2S from the anode exhaust and separate from the anode exhaust a flow containing H2 and substantially H2S free, and to send the flow containing H2 to the anodic compartment (5; 21) of a fuel cell in a way that leaves an exit route for N2 originating from the conversion of NOx present in the fumes. System according to claim 1, wherein the device (3) comprises a plurality of functional units (15-18; 25, 28) arranged in series along an anode output line (14) to manipulate the outgoing anodic exhaust stream from the battery (2) of MCFC cells. System according to claim 1 or 2, wherein the H2-containing and substantially H2S-free stream is sent to the anode compartment (21) of a PEM cell unit (18). 4. System according to claim 1 or 2, in which the flow containing H2 and substantially H2S-free is sent to the anode compartment (5) of the battery (2) of MCFC cells, avoiding or limiting the accumulation of N2 in said anode compartment (5) of the battery (2) of MCFC cells. System according to claim 3, wherein the device (3) comprises, starting from an anode output (13) of the battery (2) of MCFC cells: an H2S removal unit (15), a unit (16) of shift reaction, an H2O separation unit (17A), and at least one PEM stack unit (18) having an anode compartment (21) connected in series to the previous units (15-17) of the device (3) and arranged downstream of them. System according to claim 5, wherein the H2S removal unit (15) is located upstream of the shift reaction unit (16) and the PEM stack unit (18). 7. System according to claim 5 or 6, wherein the H2S removal unit (15) is configured to remove a first portion of CO2 present in the anode exhaust; a second portion of CO2 present in the anode exhaust being separated downstream of the PEM cell unit (18), removing H2O through a further separation unit (17B). 8. System according to claim 4, wherein the device (3) comprises: a selective separation unit (25) for CO2, which separates from the anode exhaust a CO2 stream containing H2S and ready to be sent to subsequent units of seizure; a recirculation line (26) that connects the separation unit (25) to an anode power supply line (7) of the battery (2) of MCFC cells to recirculate to the anode compartment (5) of the battery (2) of cells MCFC a residual treated flux, rich in H2 and also containing residues not removed from the separation unit (25); and a branch line (27) which departs from the recirculation line (26) and is connected to a burner unit (28), in particular a catalytic burner unit, to tap a portion of the residual treated flow. System according to claim 8, wherein the system (1) includes one or more external reformers, arranged downstream of the burner unit (28) or integrated with it, so that the heat generated in the unit (28 ) burner is used to reform at least a portion of the fuel needed for the anodic power supply of the battery (2) of MCFC cells. System according to claim 4, wherein the device 3 comprises two series separation units (25); a first separation unit (25A) being a selective H2 separation unit, which selectively extracts only H2 from the anodic exhaust stream; a second separation unit (25B) being configured in such a way as to treat the residual flow outgoing from the first separation unit (25A) and obtain CO2 suitable for sequestration. 11. System according to claim 10, wherein the first separation unit (25A) separates from the anode exhaust stream a stream of H2, which is recirculated entirely to an anode inlet (8) of the battery (2) of MCFC cells by means of a recirculation line (26) which connects to an anode power supply line (7). System according to claim 10 or 11, wherein the second separation unit (25B) comprises an oxy-combustor (29) which eliminates, by reaction with O2 and conversion into H2O and CO2, the fuels remaining in the CO2-rich residual stream released from the first separation unit (25A). 13. Method for separating CO2 from combustion fumes by means of molten carbonate fuel cells (MCFC), comprising the steps of treating an anodic exhaust stream leaving an anodic compartment (5) of a battery (2) of MCFC cells in a such as to remove H2S from the anode exhaust and separate from the anode exhaust a flow containing H2 and substantially H2S-free, and to send the flow containing H2 to the anode compartment of a fuel cell in such a way as to leave a way of ™ output for N2 originating from the conversion of NOx present in the fumes. Method according to claim 13, wherein the H2-containing and substantially H2S-free stream is sent to the anode compartment (21) of a PEM cell unit (18). 15. Method according to claim 13, in which the stream containing H2 and substantially H2S-free is sent to the anode compartment (5) of the battery (2) of MCFC cells, avoiding or limiting the accumulation of N2 in said anode compartment (5) of the battery (2) of MCFC cells. 16. Method according to claim 13 or 14, comprising, in order, the steps of: removing H2S from the anodic exhaust; conduct a shift reaction on the H2S-free anodic exhaust; send the anode exhaust to an anode compartment (21) of a PEM battery unit (18). 17. Method according to claim 13, 14 or 16, in which together with H2S a first portion of CO2 is removed from the anode exhaust; a second portion of CO2 present in the anodic exhaust being separated downstream of the PEM cell unit (18), removing H2O through a separation unit (17B). 18. Method according to claim 13 or 15, comprising the steps of: selectively separating a stream of CO2 containing H2S from the anodic exhaust; recirculating a first portion of the residual treated flux to the anode compartment (5) of the battery (2) of MCFC cells; and sending a second portion of the residual treated flow to a burner unit (28), in particular a catalytic burner unit. Method according to claim 18, comprising a step of exploiting at least a part of the heat generated in the burner unit (28) to reform at least a portion of the fuel fed to the anode compartment (5) of the battery (2) of MCFC cells. Method according to claim 13 or 15, comprising a first step of selective H2 separation, in which only H2 is selectively extracted from the anodic exhaust stream; and a second separation phase, carried out in series with the first separation phase and in which CO2 suitable for sequestration is separated. Method according to claim 20, wherein the H2 stream separated in the first step of separation from the anode exhaust stream is entirely recirculated to the anode compartment (5) of the MCFC cell battery (2). Method according to claim 21, wherein the second separation step includes an oxy-combustion step which eliminates, by reaction with O2 and conversion to H2O and CO2, the fuels remaining in the CO2-rich residual stream coming out of the first separation step.