JPH0773893A - Low temperature heat recovery system in fuel cell power generating system - Google Patents

Abstract

PURPOSE:To effectively recover low temperature heat generated in a fuel cell by adding heat energy to a gaseous component obtained by utilizing low temperature heat produced in a fuel cell facility and heat of exhaust gas from an expansion turbine, to be utilized as a power source for another expansion turbine. CONSTITUTION:A part of pressurized air to be supplied from an air compressor 3 to a fuel cell unit 1 is branched to passages 12-16, to be fed to heat exchangers 17-21 to atomize pressurized water. Meanwhile, exhaust heat (Q1+Q2) is supplied from the fuel cell unit 1 to the heat exchangers 17, 18 via the passages 12, 13. Mixture fluid composed of air, water and steam in the heat exchangers 17-21 is supplied to an air-liquid separator 25, where a gaseous component (air and steam) is separated from water. Exhaust from an expansion turbine 9 is mixed with gas discharged from the heat exchanger 29, to be supplied to a combustor 7 together with the gaseous component from the air-liquid separator 25, and then, supplys heat energy, which is utilized in another expansion turbine 10. Consequently, low temperature heat generated in the fuel cell unit 1 can be effectively recovered.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a system for recovering low temperature heat generated from a fuel cell power generation system, and more particularly to a low temperature heat recovery system for improving power generation efficiency of a fuel cell.

[0002]

2. Description of the Related Art Hydrogen is converted into hydrogen ions by the catalytic action of a fuel electrode (anode), oxygen in the air is converted into oxygen ions by the catalytic action of an air electrode (cathode), and direct current power is obtained between both electrodes by a chemical reaction of a cell. Various fuel cells have been developed. As a power generation system using this fuel cell, a cogeneration system capable of efficiently extracting electricity and heat at the same time is known. FIG. 4 shows an example of this cogeneration system.

Natural gas, which is a fuel, is mixed with water vapor supplied from the steam pipe 32 after removing the odorant containing sulfur added therein by a hydrodesulfurizer (not shown). After that, it is converted into hydrogen or the like by the catalytic action in the reformer 33 as shown in the following reaction formula. CH ₄ + H ₂ O = CO + 3H ₂ (reforming reaction) CO + H ₂ O = CO ₂ + H ₂ (metamorphic reaction) CH ₄ + 2H ₂ O = CO ₂ + 4H ₂ (overall reaction) Next, hydrogen-rich reformed gas Is carbon monoxide (C
O) The CO concentration contained in the reformed gas in the shift converter 35 is 1.
After the water content is reduced to 75% or less, excess water is removed by the steam condenser 36, and then the fuel electrode (anode) 38 of the fuel cell 37.
Is supplied to.

On the other hand, the air is pressurized by the first-stage air compressor 39 and the second-stage air compressor 40 and supplied to the air electrode (cathode) 41 of the fuel cell 37. In the main body of the fuel cell 37, the hydrogen of the fuel electrode 38 and the oxygen of the air electrode 41 are filled with the electrolyte 4
Direct-current electricity is generated through the electrochemical reaction of the following formula via 2. H ₂ = 2H ⁺ + 2e ⁻ (anode) 1 / 2O ₂ + 2H ⁺ + 2e ⁻ = H ₂ O (cathode) H ₂ + 1 / 2O ₂ = H ₂ O (overall reaction) It is converted to and used in the power load device.

Unreacted hydrogen in the fuel electrode 38 (exhaust fuel gas)
Is used as a fuel for heating the reformer 33. The air electrode 4 is used for the combustion of unreacted hydrogen in the reformer 33.
Exhaust air from 1 is used as the oxygen source. The heat generated by the power generation in the fuel cell 37 is cooled by the cooling water circulated and supplied from the pipe 45 through the cell cooling plate 44. Then, the heat removed by the cooling water is recovered by the heat recovery device 46. Further, heat generated when the steam in the exhaust air discharged from the air electrode 41 is condensed by the steam condenser 47, and excess water in the gas discharged from the CO shift converter 35 is removed by the steam condenser 3
The heat obtained by cooling in 6 is recovered by the heat recovery devices 49 and 50 provided in the respective flow paths and used for cooling and heating, hot water supply, and the like. The condensed water generated in each of the steam condensers 36 and 47 is used as cooling water for the cooling plate 44 for the fuel cell after increasing the purity in a water treatment device (not shown). Further, the water used for cooling the fuel cell 37 is separated into steam by the steam separator 52 and then circulated again by the pump 53 as cooling water for the fuel cell 37, and the separated steam is used for reforming natural gas. It is supplied to the steam pipe 32 as water and used. The exhaust gas generated in the reformer 33 is heated by the combustor 51, and then, as rotational energy of the rotation shaft of the first-stage expansion turbine 54 and the rotation shaft of the second-stage expansion turbine 55, the second-stage air compressor 40 and After being collected by the first stage air compressor 39, it is discharged into the atmosphere. In addition, the heat recovery device 4
The heat recovered in the cooling waters 6, 49 and 50 is cooled by makeup water in the cold water tower 57 and reused.

[0006]

In the power generation system using the fuel cell 37, the heat generated in the system is recovered by the heat recovery devices 46, 49 and 50. That is, the heat of the circulating cooling water for cooling the fuel cell 37 is transferred to the heat recovery device 46.
And the heat of condensation of the excess moisture of the exhaust air from the air electrode 41 of the fuel cell 37 is recovered by the heat recovery device 49,
The heat of condensation of excess water in the reformed gas before being supplied to the heat recovery device 50 was recovered by the heat recovery device 50. This heat recovery device 46, 49,
The energy supply and demand in the fuel cell power generation system including 50 can be conceptually expressed as shown in FIG. That is, when the fuel cell power generation system is divided into the fuel cell unit 1 and the heat recovery and pressurized air supply unit 2 of the fuel cell unit 1, the fuel cell fuel FG ₁ and the air compressor 3 are considered.
The air AIR ₁ pressurized by 9, 40 is the fuel cell unit 1.
And the electric energy E ₁ is extracted from the fuel cell unit 1. In addition, an expansion turbine 5 used for pressurizing air from the fuel cell unit 1 to the heat recovery and pressurized air supply unit 2.
Exhaust fuel FG ₂ that supplements the heat energy of 4, 55 and the respective exhaust heat Q ₁ recovered by the heat recovery devices 49, 50, 46,
Q ₂ and Q ₃ (exhaust heat Q ₁ and Q ₂ and exhaust heat Q ₃ have different exhaust heat temperature levels, so separate heat recovery systems are used.) And reformer combustion exhaust gas heat and pressure energy EXH with
₁ is released. On the other hand, the heat recovery and pressurized air supply unit 2
Therefore, the air AIR ₁ pressurized by the air compressors 39 and 40 is supplied to the fuel cell unit 1.

The heat recovery and pressurized air supply unit 2 is for cooling the air AIR ₂ supplied to the air electrode 41 of the fuel cell unit 1 and the cooling water collected by the heat recovery devices 46, 49 and 50. Make-up water W and heat recovery devices 46, 49
50 is supplied with auxiliary power E ₂ (not shown in FIG. 4) such as a power pump (not shown) for circulating cooling water or the like. Further, the exhaust gas EXH from the heat recovery and pressurized air supply unit 2
₂ is discharged into the atmosphere. In the energy supply and demand system of FIG. 5, the temperatures of the exhaust heat Q ₁ , Q ₂ , Q ₃ and the exhaust fuel FG ₂ discharged from the fuel cell unit 1 to the pressurized air supply unit 2 are low, so The recovery rate of was not good. Therefore, an object of the present invention is to improve the heat recovery efficiency of the heat recovery from the fuel cell section and the compressed air supply section, and finally to improve the energy efficiency of the fuel cell power generation system.

[0008]

The above objects of the present invention can be achieved by the following constitutions. That is, in a fuel cell power generation system that generates electricity by an electrochemical reaction between an air electrode and a fuel electrode via an electrolyte, the low temperature heat generated from the fuel cell facility and the heat of exhaust gas from the expansion turbine are used to add heat. A fuel that sprays water on compressed air to generate a mixed fluid of water, water vapor and compressed air, adds thermal energy to a gas component obtained from the mixed fluid, and uses this as a power source of the expansion turbine. It is a low temperature heat recovery system in a battery power generation system. In the present invention, the compressed air is obtained by branching a part of the generated compressed air to supply it to the air electrode of the fuel cell, and from a mixed fluid of water, steam and compressed air. Exhaust vapor generated from the fuel cell body can be used as part of the thermal energy applied to the gas component.

[0009]

In the past, heat recovery from low temperature heat of about 100 ° C. generated from a fuel cell could not be efficiently performed because of poor heat exchange efficiency. However, in the present invention,
A part of the pressurized air supplied to the air electrode of the fuel cell is branched, and the branched pressurized air and the low temperature exhaust heat from the fuel cell are heat-exchanged for heating. The water is sprayed on the obtained pressurized and warmed air to lower the partial pressure of water to vaporize the water, and at a temperature below the boiling point of water under the partial pressure, a state where water and steam coexist is created. . Thus, by utilizing the low temperature heat from the fuel cell for vaporizing water, not only the low temperature heat is recovered as sensible heat of water but also as latent heat of vaporization of water.

Further, by evaporating water in this way, the volume of the pressurized fluid (fluid consisting of steam and air) is expanded,
A pressurized fluid available for the expansion turbine can be obtained. Moreover, a system that can be practically used can be obtained by further adding thermal energy to this pressurized fluid to make it into a high-temperature gas and increasing the operating efficiency of the expansion turbine. At this time, by adding exhaust heat of steam separated from the circulating water for cooling the fuel cell or circulating cooling water as heat to be supplied to the fluid whose volume has been expanded, all low temperature heat generated from the fuel cell section is effective. The fuel cell power generation system is further improved in power generation efficiency by adding a low temperature heat recovery system to the fuel cell power generation system.

The energy balance between the fuel cell unit 1 and the heat recovery / pressurized air supply unit 2 is schematically illustrated by comparing the present invention with the conceptual diagram of the prior art of FIG. can get. This FIG. 1 corresponds to the concept of the prior art of FIG. 5 in which the fuel FG ₃ and AC power generation energy E ₃ are added to the heat recovery and pressurized air supply unit 2. Moreover, as compared with the prior art, only the heat exchanger is newly installed and the capacities of the air compressor and the expansion turbine are increased, and there is almost no increase in operability and maintenance load. Thus, according to the present invention, not only the low-temperature heat that has been conventionally released to the atmosphere via the cooling tower or the like is recovered as sensible heat and latent heat of water, but also the volume expanded by water becoming steam is increased. By utilizing it as the rotational energy of the expansion turbine, the low temperature heat generated from the fuel cell unit 1 can be efficiently recovered. Further, when the operation efficiency of the expansion turbine is increased, it can be used to extract the AC power generation energy E ₃ by the surplus rotational energy.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below in detail with reference to the drawings. An outline of the low temperature heat recovery system in the fuel cell power generation system of the present embodiment is shown in FIG. Figure 2
Since the configuration of the fuel cell unit 1 shown in FIG. 3 is the same as that of the conventional technique, the devices having the same functions as those in FIG. Fuel cell unit 1 and the fuel cell unit 1
Two-stage air compressor (first-stage air compressor 3, second-stage air compressor 4) of air to be supplied to the two-stage combustor 6, 7 and two-stage using exhaust gas from the fuel cell unit 1. Expansion turbine (first stage expansion turbine 9, second stage expansion turbine 10),
The compressed air compressed by the first-stage air compressor 3 is divided into a plurality of compressed air flow passages 12, 13, 14, 15, 16 and heat exchange of the compressed air is performed in each of the flow passages 12 to 16. The heat exchangers 17 to 21 and the pressurizing pump 26 having a spray nozzle (not shown) opened toward the pressurized air flow passages 12 to 16 immediately before the heat exchangers 17 to 21, respectively. The water supply path 23 (in FIG. 2, the pressurized air flow path 12 ...
The same flow path as 16 is shown. ), A gas-liquid separator 25 for combining the mixed fluids of water, steam, and air discharged from the heat exchangers 17 to 21 to separate water from the mixed fluids, and separated from the gas-liquid separator 25. The pressurized water supply path 2
3, a water supply path 27 for supplying the gas component to the No. 3 via a pump 26, and a gas component separated from the gas-liquid separator 25 into a heat exchanger 29.
A combustor 7 that serves as an oxygen source for combustion via the second stage expansion turbine 10 that uses the gas obtained in the combustor 7;
It comprises two-stage heat exchangers 29, 21 and the like for exchanging heat with the exhaust gas from the expansion turbine 10.

The feature of this embodiment is that, in addition to the system for supplying the compressed air compressed by the air compressors 3 and 4 to the fuel cell unit 1 to the conventional fuel cell power generation system, this compressed air is branched. And a system for spraying water on it (pressurized air supply channel 12
˜16, heat exchangers 17 to 21, pressurized water supply passage 23, etc.) and exhaust steam heat Q ₃ from the cooling water for the cooling plate 44 of the fuel cell unit 1 for heating the gas for supplying the expansion turbine 9 Is used as a part of the heat source of the combustor 6 used for. The efficiency of recovering the axial energy recovered by adding this Q ₃ is very large.

A detailed description will be given below step by step with reference to FIG. 3, which is rewritten based on the energy balance and the like corresponding to the low temperature heat recovery system of FIG. First, 288K
Air at (15 ° C) is introduced into the air compressor 3 in the first stage,
Add 227 × 10 ⁴ kcal / hr to this and add 1
Compressed air at 54 ° C. and 3.36 atm is obtained. In order to effectively perform the compression of the pressurized air in the next stage, the air cooled by the spray water in the heat exchanger 20 and the mixed fluid of water and water vapor 12
After cooling to 0 ° C., a part of this pressurized air is further pressurized in the second stage air compressor 4 to a pressure of 8.6 atm by applying a power of 59 × 10 ⁴ kcal / hr. The obtained pressurized air is cooled to 190 ° C. by the heat exchanger 19 and introduced to the air electrode of the fuel cell unit 1.

In the fuel cell section 1, electric energy is generated by an electrochemical reaction between an air electrode and a fuel electrode (not shown),
After subtracting auxiliary machinery and DC-AC conversion loss necessary for battery operation, 430 × 10 ⁴ kcal / hr of electric energy is AC-transmitted. The energy supplied together with the reforming raw material and the heating fuel to the reformer 33 of the fuel cell unit 1 is 1019 × 10 ⁴ kcal / hr. From the fuel cell unit 1, exhaust fuel FG ₂ having thermal energy of 9 × 10 ⁴ kcal / hr and 170 kg at 170 ° C. and 8.0 atm are used.
・ Exhaust vapor Q _{3 at} a flow rate of mol / hr and 389 ° C., 8.0
Exhaust gas EXH _{1 of} atmospheric pressure is discharged. In addition, the fuel cell unit 1
From the heat exchangers 17 and 18 for a total of 137 × 10 ⁴ kca
Exhaust heat Q ₁ + Q _{2 of 1} / hr is discharged. Conventionally, this 1
Since the exhaust heat Q ₁ + Q ₂ discharged from the fuel cell unit 1 of 37 × 10 ⁴ kcal / hr is low temperature heat of about 100 ° C., the conventional heat recovery devices 46, 49, 50 shown in FIG.
It is either used for showers, water heaters or the like after being recovered by heat or the like, or is simply radiated to the atmosphere by a cooling tower or the like.

The present embodiment is characterized by the following parts. That is, 2371 kg which is a part of the air that has been compressed by the first-stage air compressor 3 to 3.36 atm and has undergone heat exchange.
-Mol / hr pressurized air is supplied to the branched pressurized air supply channels 12 to 16, and the respective air supply channels 12 to 16 are supplied.
In the flow passages 12 to 16 immediately before the heat exchangers 17 to 21, pressurized water is supplied from the supply passage 23 to 571 kg · mol / hr.
And is sprayed from each heat exchanger 17-21. Here, the reason why the heat exchangers 17 to 21 are divided into a plurality is that heat is generated from the flow paths of fluids different from each other (pressurized air with different pressurization degree, mixed fluid of steam and compressed air, expansion turbine exhaust gas). This is for the purpose of recovery, and is designed so that the pressure can be maintained in a state where water and water vapor coexist in each heat exchanger 17-21. Note that due to changes in operating conditions in the low temperature heat recovery system and the fuel cell unit 1 and other disturbances, the pressure may not be maintained in a state where water and steam coexist in the heat exchangers 17 to 21. Therefore, although not shown, flow rate control valves may be provided in the pressurized air supply channels 12 to 16.

In each of the heat exchangers 17 to 21 shown in FIG. 3, the mixed fluid is heat-exchanged with the fluid in the fluid flow path represented by the same reference numeral as that shown in each heat exchanger 17 to 21. Will be exchanged. That is, the second-stage air compressor 4
The heat of the compressed air at the outlet of the air is recovered by the heat exchanger 19, and the heat of the compressed air at the outlet of the first-stage air compressor 3 is recovered by the heat exchanger 2.
The heat of the exhaust gas, which is recovered in 0, is discharged from the second-stage expansion turbine 10 in the heat exchanger 21, and is cooled in the heat exchanger 29, is recovered, and a mixed fluid of air, water vapor, and moisture is obtained. The respective mixed fluids join together to obtain a mixed fluid of air, steam and water at 100 ° C. The gas component (air and water vapor) and water in this fluid are separated by the gas-liquid separator 25.
Then, only the gas mixture fluid exchanges heat with the high temperature exhaust gas from the second-stage expansion turbine 10 in the heat exchanger 29.
It becomes a gas at 5 ° C. This gas contains 9 from the fuel cell unit 1.
Exhaust fuel F with thermal energy of × 10 ⁴ kcal / hr
G ₂ and 164 × 10 ⁴ kcal / hr The heat amount from the auxiliary heat source having the heat energy is replenished to the combustor 6 for combustion, and the first expansion turbine 9 expands it to 93 ×
The power of 10 ⁴ kcal / hr is taken out, and a part of the power is linked to the expansion turbine 9 by an air compressor 3,
It is used as the rotational energy of 4. Further, the exhaust gas from the expansion turbine 9 is mixed with the gas discharged from the heat exchanger 29 and is obtained in the combustor 7 from the auxiliary heat source 96.
9 by replenishing heat energy of 1 × 10 ⁴ kcal / hrs
A gas of 26 ° C. is obtained. This gas is used in the expansion turbine 10 of the second stage and 752 × 10 ⁴ kcal / hr
Of the air compressor 3,
It is used as the rotational energy of 4. Further, the surplus rotational energy activates the AC generator 11 to generate 559 ×
Energy of 10 ⁴ kcal / hr can be taken out.

The reason why the two expansion turbines 9 and 10 are provided is to expand the gas from the fuel cell part 1 and the gas of the heat-recovered gas mixture fluid at two different pressure levels, and to combust the combustor. The reason why the two units 6 and 7 are provided is that the turbine-introduced gas matching the respective energy levels must be heated in order to operate the expansion turbines 9 and 10 efficiently. 625 ° C. discharged from the second-stage expansion turbine 10
The exhaust gas is heated to 280 ° C. in the heat exchanger 29 and to 150 ° C. in the heat exchanger 20, and is discharged into the atmosphere.

In the above-mentioned low temperature recovery system, the additionally supplied heat energy (FG ₃ ) is the energy supplied to the combustors 6 and 7 by the auxiliary fuel FG ₃ = (164 + 961) × 10 ⁴ kcal / hr = 1125 × 10 ⁴ kcal / hr, and the surplus recovered power energy (PW) is the energy of the difference between the power output of the expansion turbines 9 and 10 and the power consumption of the air compressors 3 and 4 PW = {(752 + 93)-(59 + 227)} It is x10 < ⁴ > kcal / hr = 559 * 10 < ⁴ > kcal / hr. Therefore, according to the low temperature heat recovery system of the fuel cell power generation system of the present embodiment, the energy recovery efficiency η
_TN is η _TN = PW / FG = 0.497.

The total energy efficiency η _TOTAL of this system is η _TOTAL = (430 + 559) / (1019 + 961 + 164) = 0.461. As described above, in this embodiment, even if additional energy is supplied, extra power is recovered, so not only the energy recovery efficiency by these added systems is high, but also the low temperature heat recovery system of this embodiment is used. The energy efficiency η _FC in the fuel cell unit 1 not equipped with η _FC is η _FC = 430/1019 = 0.422, but the overall heat recovery efficiency (η _TOTAL ) is improved.

Moreover, in the case of the conventional atmospheric pressure type fuel cell in this embodiment, the compressor for the air supplied to the fuel electrode does not extract the air in the intermediate stage, but in the present embodiment, it is a two-stage air compressor. 3 and 4 are used to secure the temperature and pressure of the air supplied to the fuel electrode, and a relatively low pressure air obtained from the air compressor 3 in the first stage is used to mix the partial pressure of water with the water mixed refrigerant system. Is kept low, vaporization of water is ensured, and the exhaust heat Q ₁ and Q ₂ from the fuel cell unit 1 is effectively used. Therefore, when this embodiment is applied to a power generation system using a high-pressure fuel cell, it is not necessary to newly install an air compressor, which is advantageous in terms of cost. Moreover, as compared with the prior art, the auxiliary combustor 7 and the heat exchangers 19 to 21 and 29 are newly provided, and the capacities of the air compressors 3 and 4 and the expansion turbines 9 and 10 are simply increased. There is almost no burden on maintenance, etc.

[0022]

EFFECTS OF THE INVENTION According to the present invention, not only the low-temperature heat that has been conventionally released to the atmosphere via a cooling tower or the like is recovered as sensible heat and latent heat of water, but also water becomes steam. By utilizing the expanded volume as the rotational energy of the expansion turbine, the low temperature heat generated from the fuel cell can be efficiently recovered.

[Brief description of drawings]

FIG. 1 is a diagram illustrating an energy balance of a low temperature recovery system in a fuel cell power generation system of the present invention.

FIG. 2 is a diagram showing a low temperature recovery system in a fuel cell power generation system according to an embodiment of the present invention.

FIG. 3 is a diagram for explaining the energy balance and the like of a low temperature recovery system corresponding to the fuel cell power generation system of FIG.

FIG. 4 is a diagram illustrating an energy balance of a fuel cell power generation system including a conventional low temperature recovery system.

FIG. 5 is a diagram showing a concept of a low temperature recovery system in a conventional fuel cell power generation system.

[Explanation of symbols]

1 ... Fuel cell unit, 2 ... Heat recovery and pressurized air supply unit,
3, 4 ... Air compressor, 6, 7 ... Combustor, 9, 10 ... Expansion turbine, 11 ... Alternator, 12-16 ... Pressurized air supply passage, 17-21, 29 ... Heat exchanger, 23 ... pressurized water supply path, 25 ... gas-liquid separator, 26 ... pump, 27 ... water supply path, 33 ... reformer, 37 ... fuel cell, 38 ... fuel electrode (anode), 41 ... air electrode (cathode), 42 …Electrolytes,
43 ... Orthogonal transformation device, 44 ... Battery cooling plate

Claims (3)

[Claims] 1. In a fuel cell power generation system for generating power by an electrochemical reaction between an air electrode and a fuel electrode via an electrolyte, low temperature heat generated from a fuel cell facility and heat of exhaust gas from an expansion turbine are used. Then, water is sprayed on the compressed air to generate a mixed fluid of water, steam and compressed air, and thermal energy is added to a gas component obtained from the mixed fluid, which is used as a power source of the expansion turbine. A low temperature heat recovery system in a fuel cell power generation system characterized by being used. 2. The low temperature in the fuel cell power generation system according to claim 1, wherein the pressurized air is used by branching a part of the generated pressurized air for supplying to the air electrode of the fuel cell. Heat recovery system. 3. The exhaust vapor generated from the fuel cell main body is used as a part of thermal energy applied to a gas component obtained from a mixed fluid of water, water vapor, and pressurized air. Low-temperature heat recovery system for the fuel cell power generation system in Japan.