JP3637894B2 - Fuel cell system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel cell system, and more particularly to a fuel cell system equipped with a NOx purification reactor.
[0002]
[Prior art]
As a conventional fuel cell system equipped with a NOx purification reactor, for example, there is a technique described in Japanese Patent Application Laid-Open No. 09-180744. In a fuel cell system including a start-up combustor such as a burner, a reforming reactor, and a CO converter (Water-gas-shift reactor), NOx for removing NOx in the outside air introduced into the start-up combustor is removed. A configuration comprising a purification reactor is disclosed.
[0003]
[Problems to be solved by the invention]
However, the configuration described in Japanese Patent Laid-Open No. 9-180744 has a problem that NOx generated in the start-up combustor cannot be removed. In particular, the unburned gas such as CO generated by rich combustion in the start-up combustor at the start of the fuel cell system and the air supplied to the reforming reactor and the CO converter are burned in the reforming reactor and the CO converter. As a result, when raising the temperature of the reforming reactor or the like by heating with the reaction heat in addition to the sensible heat of the combustion gas, it becomes a problem that NOx generated in the start-up combustor cannot be removed.
[0004]
In order to solve this problem, in the configuration provided with the NOx purification reactor downstream of the startup combustor, it is possible to purify CO and the like produced by rich combustion and NOx. However, the air-fuel ratio (λ) of the air-fuel mixture supplied to the start-up combustor that can obtain CO or the like for generating the combustion heat necessary for the above-described start-up heating in the reforming reactor and the CO converter Is about 0.3, and under this rich air-fuel ratio state, the NOx purification reactor causes CO adsorption poisoning or the like on the catalyst surface and cannot sufficiently produce a NOx purification reaction by CO or the like.
[0005]
If the air-fuel ratio λ is close to 1.0,
・ The combustion temperature in the start-up combustor rises, the amount of NOx generated increases, and the load on the NOx purification reactor increases. ・ The CO required for heating at the start-up of the reforming reactor and the CO converter cannot be obtained. It will be.
[0006]
In view of such problems, the present invention proposes a fuel cell system that effectively removes NOx at the time of startup and enables long-term operation.
[0007]
[Means for Solving the Problems]
A first invention is a reforming reactor for reforming a hydrogen-containing fuel, and a shift reactor for generating hydrogen by oxidizing carbon monoxide in reformed gas discharged from the reforming reactor with steam. A carbon monoxide remover that oxidizes carbon monoxide in the reformed gas from the shift reactor with oxygen, a reformed gas whose carbon monoxide concentration is reduced to a predetermined concentration, and a supplied oxidizing agent. In a fuel cell system including a fuel cell stack for generating power, a start-up combustor that supplies combustion gas by rich combustion to warm up the reforming reactor at the start-up, and nitrogen in the combustion gas or the reformed gas A NOx purification reactor for purifying the oxide, and temporarily increasing the air-fuel ratio of the combustion gas of the combustion gas supplied to the NOx purification reactor during operation of the start-up combustor. NOx removal performance To recovery.
[0008]
In a second aspect based on the first aspect, the air-fuel ratio of the combustion gas supplied to the NOx purification reactor is adjusted by mixing air into the combustion gas.
[0009]
According to a third invention, in the first or second invention, the air-fuel ratio of the combustion gas supplied to the NOx purification reactor is increased according to the NOx removal efficiency of the NOx purification reactor.
[0010]
According to a fourth aspect, in the second aspect, the air-fuel ratio during normal operation of the startup combustor is less than 0.5, and the air-fuel ratio of the combustion gas supplied to the NOx purification reactor after air supply is 0. 5 to 1.0.
[0011]
In a fifth aspect based on the first aspect, the NOx purification reactor is installed between the startup combustor and the reforming reactor.
[0012]
In a sixth aspect based on the first or fifth aspect, the NOx purification reactor is a three-way catalyst.
[0013]
According to a seventh invention, in any one of the first to fourth inventions, the air-fuel ratio of the combustion gas supplied to the NOx purification reactor is increased at least once during the operation of the startup combustor.
[0014]
In an eighth aspect based on the seventh aspect, the air-fuel ratio of the combustion gas supplied to the NOx purification reactor is changed when the start-up combustor is stopped.
[0015]
【The invention's effect】
In the first invention, the startup combustor that supplies the combustion gas to the reforming reactor or the like when the fuel cell system including the NOx purification reactor is started supplies the combustion gas in a rich combustion state. The air-fuel ratio of the combustion gas supplied to the NOx purification reactor installed between the reforming reactor and the start-up combustor is temporarily controlled so that the air-fuel ratio becomes larger than that in the normal operation. By performing the control to increase the air-fuel ratio in this way, the amount of oxidant contained in the combustion gas is increased, the poisonous substances adsorbed on the NOx purification reactor are purified, and the purification reaction efficiency of the NOx purification reactor is increased. Can be improved.
[0016]
In the second invention, since the air-fuel ratio of the combustion gas supplied to the NOx purification reactor is adjusted by mixing air into the combustion gas, the adsorption poisoning of the NOx purification reactor is more effectively removed. be able to.
[0017]
In the third invention, since the air-fuel ratio of the combustion gas supplied to the NOx purification reactor is increased in accordance with the NOx removal efficiency of the NOx purification reactor, the consumption of reducing substances such as CO in the NOx purification reactor is reduced. The temperature can be kept to a minimum, and the amount of oxidation reaction in the reforming reactor or the like can be secured at the time of start-up, so that warming up can be performed efficiently. In addition, consumption of supplied hydrogen can be suppressed, and the efficiency of supplying hydrogen to the fuel cell stack can be improved.
[0018]
In the fourth invention, the air-fuel ratio during normal operation of the startup combustor is less than 0.5, and the air-fuel ratio of the combustion gas supplied to the NOx purification reactor after the air supply is 0.5 to 1.0. By doing so, the reducing substance adsorbed in the NOx purification reactor can be efficiently removed.
[0019]
In the fifth aspect of the invention, the NOx purification reactor is installed between the startup combustor and the reforming reactor, so that NOx can be prevented from being supplied to the reforming reactor or the like during startup.
[0020]
In the sixth invention, the NOx purification reactor uses a three-way catalyst.
[0021]
In the seventh aspect of the present invention, the purification of the NOx purification reactor, which has decreased due to the adsorption of the reducing substance, is increased at least once during the operation of the startup combustor to increase the air-fuel ratio of the combustion gas supplied to the NOx purification reactor. Efficiency can be restored.
[0022]
In the eighth invention, since the air-fuel ratio is changed when the startup combustor is stopped, the catalyst of the NOx purification reactor can be stopped with a high purification efficiency, and the NOx can be quickly purified at the next startup. .
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the fuel cell of the present invention will be described with reference to the drawings. FIG. 1 is a diagram for explaining a configuration of an embodiment of a fuel cell system of the present invention.
[0024]
The fuel cell system of the present invention includes a reforming reactor 1 that generates a hydrogen-rich reformed gas from fuel gas and steam, and CO in the reformed gas discharged from the reforming reactor 1 is oxidized by steam. A shift reactor 2 that generates hydrogen, a CO remover 3 that oxidizes CO in the reformed gas discharged from the shift reactor 2 with oxygen to reduce the CO concentration in the reformed gas, and a predetermined CO concentration or less The reformed gas thus obtained is supplied to the fuel electrode, and the fuel cell stack 4 is configured to generate electric power by supplying air to the air electrode. This is a system that supplies power to a load such as a motor that drives a wheel.
[0025]
Exhaust gas (exhaust hydrogen gas, exhaust air) that has not been used for power generation in the fuel cell stack 4 is supplied to the combustor 5 and combusted, and fuel and water are vaporized in the evaporator 6 by the heat of the combustion gas, The vaporized fuel and water vapor are supplied to the reforming reactor 1. In this way, normal operation of the fuel cell system is performed.
[0026]
Furthermore, in the fuel cell system of the present invention, a startup combustor 7 for warming up (heating) the reforming reactor 1 and the like when the fuel cell system is started is installed upstream of the reforming reactor 1. . Air and fuel are supplied to the start-up combustor 7 and combusted. The high-temperature combustion gas from the start-up combustor 7 is supplied to the reforming reactor 1, and the reforming reactor 1 is warmed up. The combustion gas at this time contains a large amount of NOx, and if it is released into the atmosphere as it is, there is a risk of adversely affecting the environment.
[0027]
Therefore, in the present invention, the NOx purification reactor 8 is installed between the start-up combustor 7 and the reforming reactor 1 to remove NOx in the combustion gas, and the low NOx combustion gas is supplied to the reforming reactor 1. Supply. The NOx purification reactor 8 is a three-way catalyst, and any one of Pt, Rh, and Pd is selected as a noble metal to be supported. In the present embodiment, the NOx purification reactor 8 is installed between the start-up combustor 7 and the reforming reactor 1 so that the reformed gas does not flow during normal operation. For example, it may be installed downstream of the reforming reactor 1.
[0028]
Further, air is supplied to each reformer from directly upstream of the reforming reactor 1, the shift reactor 2, the CO remover 3, and the NOx purification reactor 8.
[0029]
In this configuration, in addition to the sensible heat of the combustion gas, the combustion reaction heat of the unburned gas is also used for heating. The start-up combustor 7 is empty to supply sufficient unburned gas components to each reactor. Combustion is performed in a rich atmosphere with a fuel ratio λ of about 0.3 (for example, a range of 0.25 to 0.33). However, in the combustion gas in this atmosphere, the downstream NOx purification reactor 8 purifies NOx, and the catalyst surface causes adsorption poisoning by poisonous substances such as CO and HC, so that the NOx purification efficiency gradually decreases. Come on. Therefore, in the present invention, at the time of start-up, at least once at a predetermined interval, air is supplied immediately upstream of the NOx purification reactor 8 and mixed with the combustion gas, and the air-fuel ratio of the combustion gas supplied to the NOx purification reactor 8 λ is temporarily increased from 0.3 at normal time to 0.5 to 1.0. The timing of supplying air immediately upstream of the NOx purification reactor 8 may be supplied in accordance with the adsorption poisoning of the catalyst. For example, the time until the lower limit of the purification efficiency is measured in advance by experiment or the like, and this time Air may be supplied every time, or the poisoning state of the catalyst may be directly measured and compared with the reference value of the purification efficiency, and air may be supplied when the reference value is below the reference value. By performing control to increase the air-fuel ratio λ in this way, the amount of oxidant contained in the combustion gas supplied to the NOx purification reactor 8 increases, and the purification reaction efficiency of the catalyst of the NOx purification reactor 8 is restored. can do. Note that these components are controlled by a controller (not shown).
[0030]
It will be described with reference to FIGS. 2 and 3 that the purification efficiency of the catalyst is improved by supplying air. O ₂ in the NOx purification reactor 8 increased by temporarily introducing air into the NOx purification reactor 8 and increasing the air-fuel ratio λ, such as CO and NO in the combustion gas, as shown in FIG. It does not react with the reducing gas, but selectively reacts with poisonous substances such as CO adsorbed on the catalyst due to rich combustion at the time of startup. Therefore, the poisoning substance adsorbed on the catalyst is removed, and the removal capability of the NOx purification reactor 8 is restored.
[0031]
In a rich atmosphere with the air-fuel ratio λ = 0.3, which is the normal operating state of the startup combustor 7, NOx in the NOx purification reactor 8 exists mainly as NO, and the air-fuel ratio λ becomes 0. On the surface of the catalyst from which poisoning has been removed by setting it to 5 to 1.0, NO first changes to highly reactive NO ₂ , and then a reduction reaction is caused by CO and H ₂ in the combustion gas, so that NOx is reduced. Purified.
[0032]
The effect of setting the air-fuel ratio λ of the NOx purification reactor 8 to 0.5 to 1.0 will be described with reference to FIG. 3. FIG. 3 shows the case where the air-fuel ratio λ in the start-up combustor 5 is controlled and air is mixed. The removal efficiency of both NOx purification reactors 8 when the air-fuel ratio λ in the NOx purification reactor 8 is controlled is shown. Compared with adjusting the combustion air-fuel ratio of the start-up combustor, when the air-fuel ratio is adjusted by introducing air, the NOx purification efficiency is better recovered in the rich combustion region. It is preferable to do this. When the air-fuel ratio was increased, the air-fuel ratio was in the range of 0.5 or more, and the effect of recovering the purification efficiency was great. On the other hand, when the air-fuel ratio λ is set to exceed 1.0, the reducing gas in the combustion gas is consumed by the NOx purification reactor 8, and the temperature rise of the reforming reactor 1 and the like located downstream is efficiently performed. I can't do it. Therefore, by setting the air-fuel ratio of the combustion gas temporarily supplied to the NOx purification reactor 7 to 0.5 to 1.0, both the NOx purification efficiency of the catalyst and the warm-up operation of each component can be achieved. .
[0033]
Next, the control content performed by the controller when the fuel cell system is started will be described with reference to the flowchart of FIG.
[0034]
First, in step 1, the activation state of the system is determined by, for example, turning on and off the main switch. If it can be activated (ON state), the process proceeds to step 2. If it is not in the activated state (OFF state), the control is terminated. In step 2, fuel and air are supplied to the start-up combustor 7, and the fuel is ignited by a spark plug and burned. The air-fuel ratio λ of the air-fuel mixture combusted at this time is controlled to 0.3. By controlling in this way, CO and H ₂ in the combustion gas can be supplied to the reforming reactor 1 and the like located downstream of the NOx purification reactor 8, and at the same time, air is supplied to the reforming reactor 1 and the like, respectively. By introducing, the temperature of the reforming reactor 1 and the like can be raised by reaction heat and warming up can be started.
[0035]
In the subsequent step 3, a counter number T = 1 indicating that the unit time has elapsed is added to the counter of the controller. Therefore, the elapsed time can be detected by the counter number T of the counter.
[0036]
In step 4, it is determined whether the startup operation time T0 has elapsed. The start reference time T0 is a time from the start to the time when the lower limit removal efficiency of the NOx purification reactor 8 is reached. The start reference time T0 is calculated in advance by experiment, and the start operation time T0 is compared with the elapsed time obtained from the counter number T. To do. When the elapsed time is longer than the startup operation time T0, the process proceeds to Step 5, and when it is shorter, the process returns to Step 3.
[0037]
In step 5, air is introduced from directly upstream of the NOx purification reactor 8, and the amount of air is adjusted so that the air-fuel ratio λ of the fuel gas mixed with air is between 0.5 and 1.0, for example, 0.7. Control.
[0038]
In step 6, 1 is further added to the counter number T. In subsequent step 7, the air introduction time upstream of the NOx purification reactor 8 is determined. The elapsed time calculated from the counter number T is obtained by experimentally obtaining a time T1 for the catalyst of the NOx purification reactor 8 under the combustion conditions set in Step 5 to recover from the lower limit removal efficiency of NOx to a predetermined efficiency. If the elapsed time is longer than the time T1, the process proceeds to step 8, and if it is shorter, the process returns to step 3. In this way, air is temporarily introduced in response to a decrease in the NOx removal efficiency of the catalyst, and the NOx removal efficiency of the catalyst is restored, so that the reduction of reducing substances such as CO is suppressed to a minimum. The amount of oxidation reaction in the quality reactor 1, the CO remover 2, etc. can be ensured, and the temperature of the reactor can be increased efficiently. Moreover, consumption of unburned gas can be suppressed and the temperature rising efficiency of the reactor can be increased. An example showing the relationship between the times T0 and T1 and the purification efficiency is shown in FIG.
[0039]
In step 8, the counter number is reset to zero.
[0040]
In step 9, the warm-up state of the fuel cell system is determined. The warm-up state is determined based on the temperature state in the CO remover 3 disposed upstream of the fuel cell stack 4. The temperature in the CO remover 3 is detected using a temperature sensor or the like, and if the detected temperature R is equal to or higher than a preset CO removable temperature R1, it is determined that the warm-up has been completed, and the operation is started in step 10 The supply of fuel, air, and water to the combustor 7 and the supply of air to the NOx purification reactor 8 are stopped, and the start operation is shifted to the normal operation. When the detected temperature R is lower than the temperature R1, the process returns to step 3. Note that the NOx purification reaction is performed by setting the air-fuel ratio λ of the combustion gas supplied to the startup combustor 7 from 0.5 to 1.0 when the startup combustor 7 is stopped during the transition from the startup operation to the normal operation. The NOx purification reactor 8 can be stopped in a state where the NOx removal efficiency of the catalyst of the vessel 8 is recovered, and NOx removal can be reliably performed at the next start-up.
[0041]
In this control content, the time until the lower limit of the NOx removal efficiency and the time to return to the predetermined efficiency are set in advance. For example, a NOx concentration detection means is installed to detect and judge the removal efficiency. It is also possible.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a fuel cell system showing an embodiment of the present invention.
FIG. 2 is a view for explaining CO adsorption poisoning and a NOx purification state of a NOx purification reactor catalyst.
FIG. 3 is a diagram showing a relationship between an air-fuel ratio λ and NOx removal performance.
FIG. 4 is a flowchart for explaining control contents executed by a controller;
FIG. 5 is a diagram for explaining the timing of introducing air and its effect.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Reforming reactor 2 Shift reactor 3 CO remover 4 Fuel cell stack 5 Combustor 6 Evaporator 7 Startup combustor 8 NOx purification reactor

Claims (8)

A reforming reactor for reforming the hydrogen-containing fuel;
A shift reactor that oxidizes carbon monoxide in the reformed gas discharged from the reforming reactor with steam to generate hydrogen;
A carbon monoxide remover that oxidizes carbon monoxide in the reformed gas from the shift reactor with oxygen;
In a fuel cell system comprising a reformed gas whose carbon monoxide concentration has been reduced to a predetermined concentration and a fuel cell stack for generating power using the supplied oxidant,
A startup combustor for supplying combustion gas by rich combustion to warm up the reforming reactor at startup;
A NOx purification reactor for purifying nitrogen oxides in the combustion gas or reformed gas,
A fuel cell system that recovers the NOx removal performance of the NOx purification reactor by temporarily increasing the air-fuel ratio of the combustion gas supplied to the NOx purification reactor during operation of the startup combustor. The fuel cell system according to claim 1, wherein the air-fuel ratio of the combustion gas supplied to the NOx purification reactor is adjusted by mixing air into the combustion gas. The fuel cell system according to claim 1 or 2, wherein the air-fuel ratio of the combustion gas supplied to the NOx purification reactor is increased in accordance with the NOx removal efficiency of the NOx purification reactor. The air-fuel ratio during normal operation of the start-up combustor is less than 0.5, and the air-fuel ratio of the combustion gas supplied to the NOx purification reactor after air supply is 0.5 to 1.0. The fuel cell system according to claim 2. The fuel cell system according to claim 1, wherein the NOx purification reactor is installed between the startup combustor and the reforming reactor. The fuel cell system according to claim 1 or 5, wherein the NOx purification reactor is a three-way catalyst. The fuel cell system according to any one of claims 1 to 4, wherein the air-fuel ratio of the combustion gas supplied to the NOx purification reactor is increased at least once during the operation of the start-up combustor. The fuel cell system according to claim 7, wherein the air-fuel ratio of the combustion gas supplied to the NOx purification reactor is changed when the startup combustor is stopped.

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