JPS60212973A - Fuel cell system - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a fuel cell, and particularly to a fuel cell system with high power conversion efficiency.

[Background of the invention]

Operating a fuel cell under high pressure increases the electromotive force, improving power generation efficiency and volumetric efficiency.

To achieve this, a fuel cell system that compresses raw material gas by rationally utilizing the waste heat of fuel cells has been developed.
It is described in Japanese Patent Publication No. B-56230 and Japanese Patent Publication No. 58-56231. Figure 1 shows an example of a fuel cell system with high power generation efficiency and volumetric efficiency, in which waste heat from the fuel cell is recovered by a turbo compressor and raw material gas is compressed.

Hydrocarbon fuels such as natural gas and naphtha are first sent to a desulfurizer 1, where sulfur compounds are converted to hydrogen sulfide using hydrogen produced by a steam reforming reaction, and hydrogen sulfide is adsorbed in a zinc oxide bed 2. removed.

The desulfurized fuel is sent together with steam to an external thermal steam reformer 3 containing a nickel catalyst at a temperature of about 900C, where it is reformed into hydrogen and carbon monoxide, and then sent to a high temperature shift reactor 4 and a low temperature shift reaction. In vessel 5, carbon monoxide is converted into hydrogen and carbon dioxide gas.

- The reformed I gas reacts in the fuel cell body 6. The waste gas containing unreacted hydrogen has its moisture removed in a condenser 7 and is combusted in a steam reformer 3. The waste heat from the steam reformer 3 and the fuel cell main body 6 is used to operate the turbo compressor 8 after being heated by an auxiliary burner 9 if necessary. It is sent to the anode, and a portion is supplied to the steam reformer 3.

Fuel cells using phosphoric acid as an electrolyte typically operate at about 190C under pressure. Water generated on the anode side and water contained in the waste gas on the fuel electrode side is removed by a condenser and sent to the water treatment device 10 to hydrate the trace amount of phosphoric acid contained therein. A water cooling pipe is provided within the fuel cell main body 6. The waste heat recovered here is sent to the steam reformer 3 in the form of steam.

The DC output from the fuel cell main body 6 is transmitted to the power conversion adjustment device 11.
is converted into AC output and sent outside the system.

A simplified flowchart of the conventionally known fuel cell system described above is shown in FIG. 2. This flow sheet describes the fuel pretreatment devices, heat exchangers, and gas-liquid separator in the fuel cell system shown in FIG.

Power conversion adjustment equipment and other supplementary equipment are omitted.

[Purpose of the invention]

An object of the present invention is to provide a b-phosphate fuel cell system that converts the chemical energy of hydrocarbon fuel into electric power at a high rate, that is, has high electric power conversion efficiency.

[Summary of the invention]

The present invention compresses air using thermal energy generated within the system as a driving force, leads the compressed air to an air separation device to separate high concentration oxygen, and uses it as an oxidizing agent for the high concentration oxygen full cell. , relating to a phosphate fuel cell system.

The present invention originates from the discovery of the following fact after many case studies. In other words, the method of the present invention, in which compressed air is subjected to a separation operation to produce oxygen, and this oxygen is used as an oxidizing agent in a fuel cell, is superior to the conventional method in which compressed air is directly used as an oxidizing agent in a fuel cell. This is due to the fact that it has been found to have the following advantages (1) and (11) in comparison, and is industrially advantageous.

(1) The electromotive force of the battery can be increased.

(11) It is easy to maintain a high oxygen utilization rate at the battery anode, thus requiring less power.

In addition, the present invention also proposes the adoption of a simple internal heating type hydrocarbon reformer made of 1IlI, which is particularly advantageous when using high concentration oxygen.

The electromotive force of a fuel cell is greatly influenced by gas partial pressure. Document 1 “Handbook of Fuel Ce1l p
In erf□rmanceJ, the empirical formula (1) is reported as the amount of change in electromotive force ΔVo at the anode (oxygen electrode) when the oxygen partial pressure PG is different from the standard oxygen partial pressure P♂, and the hydrogen partial pressure Pa is the standard Empirical formula (2) has been reported as the amount of change in electromotive force ΔVa at the cathode (hydrogen electrode) when the hydrogen partial pressure Pit′ is different.

ΔVo=0.103 Log (Po /Po days) −
(1) ΔVi=0.077tog(Pg/Pg”) −
(2) What should be noted in these two equations is that an increase in oxygen partial pressure or hydrogen partial pressure brings about an increase in electromotive force.

The gas composition in industrially used fuel cells is
As shown in Table 1, the inlet oxygen concentration at the anode is 21 C, and the outlet oxygen concentration is about 9.5 C, and the inlet hydrogen concentration at the cathode is about 74 C, and the outlet hydrogen concentration is about 37 C. If these are standard concentrations and the total pressure is P, the standard value of the average oxygen partial pressure Pos is 0.15
3P, the standard value of the average hydrogen partial pressure P♂ can be expressed as 0.556 F.

Table 1 Gas composition at the inlet and outlet of the fuel cell (H) If pure oxygen is used at the anode, the oxygen partial pressure PO will be equal to the total pressure P, so according to equation (1), the electromotive force change ΔVo at the anode will be +
It is 0.084 volt. If pure hydrogen is used for the cathode, the hydrogen partial pressure Pg will be equal to the total pressure, so the electromotive force variation ΔVa at the cathode will be +0.020 volts according to equation (2). As described above, the room for increase in electromotive force due to the use of purified gas is large at the anode and small at the cathode. Therefore, if the intention is to increase the electromotive force by purifying gas using limited power, it is a good idea to refine the anode gas first.

The present invention aims to improve power generation efficiency by using purified anode gas, that is, highly concentrated oxygen.The present invention aims to improve power generation efficiency by using purified anode gas, that is, highly concentrated oxygen.The energy contained in the combustion exhaust gas in a fuel cell system is converted into rotational power by a gas turbine, and this power is used to drive a compressor. This project proposes a phosphate-type fuel cell system in which the compressed air is led to an air separation device to separate high-concentration oxygen, and this high-concentration oxygen is used for the anode of the fuel cell. be.

The main conditions for this system to be commercially viable are:
The power required to produce the highly concentrated oxygen consumed by the anode of the fuel cell can be covered by the energy retained in the combustion exhaust gas within the fuel cell system.

It is shown below that the above conditions are easily met.

Now, we will consider a fuel cell system that uses methane at 1,100 Nm''/h as fuel and has a power generation capacity of 5,000 kW.The power generation efficiency of this system is 41.2%.

One molecule of methane and two molecules of oxygen react to produce one molecule of carbon dioxide gas and two molecules of water as shown in equation G3) below, so the theoretical amount of oxygen Go corresponding to the methane flow rate Gy = 1,100 Nm''/h is 2,200 Nm”/h.

Approximately 80% of this theoretical oxygen amount Go is consumed at the anode and directly contributes to power generation, and the remaining approximately 20% is used to compensate for heat loss due to methane reforming reaction and carbon monoxide conversion reaction. used.

CH4+20*=C0z+2 HzO-=- (3) When oxygen is used in the anode, a high oxygen utilization rate can be achieved.

In currently known fuel cell systems, an oxygen utilization rate of about 50 cm is used at the anode. The oxygen concentration in the air at the anode inlet is 20.9%, but if we take an oxygen utilization rate of 50-, the oxygen concentration in the exhaust air at the anode outlet is 9.5%. The concentration is 15.3F, and the average oxygen partial pressure is 0.153F. Therefore, the standard oxygen partial pressure P
/ is 0.290F, the electromotive force variation 'rs' at the anode due to the use of air is -0,011 volts from equation (1).

On the other hand, when using oxygen, even if an oxygen utilization rate of 90% or more is adopted, the amount of reduction in electromotive force is small. Assume now that the oxygen concentration at the anode inlet is 99 cm, and the remainder is nitrogen or other inert gas. Assuming an oxygen utilization rate of 90%, the oxygen concentration in the exhaust oxygen at the anode outlet is 90.8%, the average oxygen concentration inside the anode is 94.5%, and the average oxygen partial pressure is 0.94.
It will be 5P. Therefore, the standard oxygen partial pressure Po8 is 0.2
09P, the amount of change in electromotive force at the anode due to the use of oxygen is +0.067 volts from equation (1). As described above, when oxygen is used, a sufficiently large increase in electromotive force of about +0.078 volts can be expected compared to the air method even if an oxygen utilization rate of 90 or more is adopted.

If the oxygen utilization rate is set to 90, the flow rate of high concentration oxygen Gol required at the anode inlet is Gol = 2.200 ÷
0.9 = 2,444 Nm"/h. Furthermore, if the oxygen yield in one air separation device is set to 99 cm, the flow rate 1 of compressed air required at the air separation device inlet is Gm = 2 ，
444÷0.209÷0.99=11,81 ONm”
/h.

Conventional air separation equipment provides high concentration oxygen at low pressure, which must be recompressed to the operating pressure of the fuel cell (several atmospheres). Therefore, the gas flow rate Gc to be compressed is the sum of the air flow rate G++ supplied to the air separation device and the high concentration oxygen flow rate Go+. That is, Gc=Gs+Go' = 14.254 Nm"/h
On the other hand, when air is used as the anode gas, the compressed air flow rate GA required is GA = 2,200 ÷ 0.5 ÷ 0.209 when setting an oxygen utilization rate of 50 cm.
=21,05ONm''/h.

As shown above, the required compressed gas flow rate Gc when oxygen is used as the oxidizing agent for the fuel cell is about 30 inches smaller than the compressed gas flow rate Gm when air is used. In this trial calculation example, Gm /GA = 0.68, which is 32 inches lower. A reduction in compressed gas flow rate is approximately proportional to a reduction in compression power.

As already explained in Fig. 1, the energy retained in the combustion exhaust gas within the fuel cell is recovered and air is compressed to supply the air required for the fuel cell (for the fuel cell anode and for fuel reformation)1. This is possible due to the energy balance of the fuel cell system, and the air oxidation method shown in FIG. 1 has been proven as a conventionally known technology. The oxygen oxidation method, which requires less air compression power than the air oxidation method, does not cause any contradiction in the energy balance of the fuel cell system.

As mentioned above, in the oxygen oxidation method, the air compression power can be sufficiently provided by recovering the energy retained in the combustion exhaust gas in the system, and it is expected to improve the power generation efficiency due to the increase in electromotive force. be able to.

When the battery pressure is set to 58 ia, when the battery voltage in the air oxidation system is 0.7 volts, the voltage in the oxygen oxidation system is 0.7'18 volts. At this time, the power generation efficiency is improved from 41.2% in the air oxidation method to 45.8% in the oxygen oxidation method, an improvement of 4.6%.

The above trial calculation results for 1k are summarized in Table 2.

Table 2 Trial calculation conditions [Embodiment of the invention] A simplified flow sheet of an oxygen oxidation fuel cell power generation system is shown in FIG.

A structural feature of the system of the present invention is that an air separation device is newly added as a component to the conventionally known system shown in FIGS. 1 and 2. Part a of the compressed air is led to an air separation device where it is oxygenated (oxygen concentration 99.5
The remainder is separated into nitrogen (inert gas such as nitrogen) and nitrogen,
Oxygen is pressurized by an oxygen compressor and sent to the anode of the fuel cell, and nitrogen is released into the atmosphere. Another portion of the compressed air is sent to the external thermal reformer as a combustion supporting gas, similar to the conventional system of FIGS. 1 and 2. At the anode, 9 of oxygen
0 reacts, and the concentration of exhaust gas from the anode is oxygen 90.8
m, and 9.2% of other inert gases.

Compared to the conventional systems shown in Figures 1 and 2, in this system, the oxygen partial pressure at the anode is higher, so the electromotive force of the battery is +
It increases by 0.078 volts, and the power generation efficiency becomes 45.8 cm. In addition, since oxygen is used as the oxidizing agent, the exhaust gas flow rate is reduced by 35 cm compared to the conventional system, but by setting the oxygen utilization rate at the fuel cell anode as high as 90%, the gas flow rate that requires compression ( (Total of raw material air and battery oxygen)
decreased by 33 inches, and the exhaust heat recovery system was almost in balance.

A flow sheet of a second embodiment of the oxygen oxidation fuel cell power generation system is shown in FIG.

The structural feature of this system is that an internal heat steam reformer is employed for reforming hydrocarbon fuel, and the other equipment structure is the same as that of the first embodiment shown in FIG. The compressed air is led to an air separation device where it is oxygenated (oxygen concentration 99.5 cm,
The rest is separated into nitrogen (inert gas, mainly nitrogen) and nitrogen.
Oxygen is pressurized by an oxygen compressor, and nitrogen is
1 Released into the atmosphere. A part of the pressurized oxygen C is sent to the anode of the fuel cell, and the remaining oxygen d is mixed with fuel and steam and then sent to the internal heat steam reformer.

The composition of the gas exiting an internally heated steam reformer is slightly less hydrogen and slightly more carbon dioxide than the gas composition from a conventional externally heated steam reformer. This gas is divided into two types: high temperature and low temperature.
Processed in a stage shift reactor - carbon oxide is converted to hydrogen and carbon dioxide. Therefore, the composition of the gas entering the cathode of the fuel cell is as shown in Table 3.

Table 3: Gas composition at the inlet and outlet of the fuel cell 80 g of hydrogen reacts at the cathode, and the concentration of the exhaust gas from the cathode is 36 g of hydrogen. Further, at the anode, 50 g of oxygen reacts as in the first embodiment, and the concentration of exhaust gas from the anode is 9.5 g of oxygen and 71.5 g of other inert gas.

Since this embodiment employs an internal heating type steam reformer with a simple structure, the hydrogen concentration in the fuel gas is lower than that of the first embodiment, and the power generation efficiency is not as high as that of the first embodiment. However, it is possible to achieve a sufficiently higher power generation efficiency than the conventionally known fuel cell system described above.

〔Effect of the invention〕

The present invention is based on separating oxygen from air and using the separated oxygen as an oxidizing agent in a fuel cell. According to the method of the present invention, the electromotive force at the anode can be increased compared to the conventional method using air as an oxidizing agent, and the electromotive force of the entire battery is improved by about 10 cm, and the power generation efficiency is increased by about 4%. Improvements are achieved.

Furthermore, by using highly concentrated oxygen, it is possible to employ an internal heat steam reformer with a simple structure, making it possible to construct a highly reliable power generation system.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional phosphoric acid electrolyte fuel cell system, and FIG. 2 is a simplified flow sheet of the system shown in FIG. FIG. 3 is a flow sheet showing one embodiment of the present invention, and FIG. 4 is a flow sheet showing another embodiment of the present invention. 3...Steam reformer, 6...Fuel cell main body, 8...
・Turbo compressor. Agent Patent Attorney Akio Takahashi 1 ``Fl'i!Usu I 2nd mouth tit-*-t electric n ¥:13 mouth

Claims (1)

[Scope of Claims] 1. A fuel cell main body having a fuel electrode, an oxidizer electrode, and an electrolyte, means for supplying air to the fuel cell main body, means for supplying fuel gas to the fuel cell main body, and a method for supplying the air to the fuel cell. What is claimed is: 1. A fuel cell system comprising a compression means for compressing the air before supplying it to the main body, the system comprising means for oxidizing the air compressed by the compressor. 2. The fuel cell system according to claim 1, wherein the compressed air oxidizing means comprises an air separation device. 3. A fuel cell body having a fuel electrode, an oxidizer electrode, and an electrolyte, means for supplying air to the fuel cell body, means for supplying fuel gas to the fuel cell body, and before supplying the air to the fuel cell body In a fuel cell system having means for compressing using system exhaust gas energy, an air separation means for oxidizing a part of the compressed air;
The remainder of the compressed air is brought into contact with a portion of the feedstock hydrocarbons or the waste water from the fuel cell main body and combusted, and this heat is used to convert the remainder of the feedstock hydrocarbons into water vapor. External heating type hydrocarbon reforming means for producing a mixed gas consisting of hydrogen, carbon oxide and carbon dioxide gas from a mixed gas mixture, and a means for producing hydrogen and carbon dioxide gas from carbon monoxide and water vapor in the mixed gas. 1. A fuel cell system comprising a carbon oxide converting means and supplying the gas that has passed through the carbon oxide converting means to the fuel cell main body as fuel gas. 4. A fuel cell body having a fuel electrode, an oxidizer electrode, and an electrolyte, means for supplying air to the fuel cell body, means for supplying fuel gas to the fuel cell body, and before supplying the air to the fuel cell body In a fuel cell system having a means for compressing using system exhaust gas energy, an air separation means oxidizes the compressed air, and a part of the oxygen that has passed through the air separation means is converted into raw material carbon without being supplied to the fuel cell main body. Internal combustion hydrocarbon reforming means for partially combusting a mixture of hydrogen and water vapor by contacting it and simultaneously reforming the remainder of the hydrocarbon to produce a mixed gas consisting of hydrogen, carbon oxide, and carbon dioxide; and carbon monoxide generating means for generating hydrogen and carbon dioxide gas from carbon monoxide and water vapor in the mixed gas, and supplying the gas that has passed through the carbon oxide transforming means to the fuel cell main body as a fuel gas. A fuel cell system characterized by: