JP2006104003A - Fuel reforming system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel reforming system having a mechanism for preventing clogging of the gas flow caused by the condensation of water vapor contained in the reformed gas, in a system supplying hydrogen gas required for hydrodesulfurization by recycling the reformed gas. <P>SOLUTION: By arranging a return gas pipeline of the system supplying hydrogen gas by recycling the reformed gas to receive exhaust heat from at least one reactor selected from the group consisting of a desulfurizer, a reformer, a CO converter, and a CO selective oxidizer, the temperature of the reformed recycling gas in the return gas pipeline is prevented from being lowered to a dew point or below. Preferably, the flow path of the reformed recycling gas flowing through the return gas pipeline is arranged at least not to be up-gradient toward a horizontal direction, so as to move water drain present in the return gas pipeline along a flow of the gas with the aid of gravity and discharge from the return gas pipeline. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a fuel reforming system, and more particularly to a fuel reforming system that reforms a raw fuel gas mainly composed of a hydrocarbon compound to produce a hydrogen rich gas.

  A fuel cell power generation device that can extract electrical energy directly from the chemical bond energy of a substance by electrochemically reacting hydrogen and oxygen via an electrolyte. Power generation in a wide range of electrical output from 1 kW to 1000 kW The system is expected to be used as a clean energy supply method because it has high power generation efficiency and emits less air pollutants such as nitrogen oxides NOx and sulfur oxides SOx.

  A fuel cell needs to be supplied with a hydrogen-rich gas having a high hydrogen gas concentration as a fuel, and the production and supply method of the hydrogen-rich gas varies depending on the system, but hydrocarbon compounds such as methanol, methane, propane, and butane are used. The raw fuel gas as a main component can be obtained by utilizing a steam reforming reaction in which the raw fuel gas is reacted with steam at a high temperature in the presence of a reforming catalyst. For example, a hydrogen gas generation reaction system when methane gas is steam-reformed is typically described by a chemical reaction formula shown below.

_{CH 4 + H 2 O → CO} + 3H 2
Actually, a fuel gas mainly composed of a hydrocarbon compound flows through a reformer, which is a reactor, and a catalyst holding carrier that constitutes the reformer at an optimum reaction temperature of usually 600 to 700 ° C. The reaction proceeds by contacting the reforming catalyst supported on the catalyst.

  In home on-site power generation systems to meet household electricity and heat demand, city gas and LP gas (liquefied petroleum gas) mainly composed of methane, which is widely supplied to general households, is used as is for hydrogen-rich gas. Since it can be used as a raw material for manufacture and supply, it is not necessary to develop an infrastructure infrastructure and it can be used at low cost socially, which is convenient, but as an odorant for city gas and LP gas. A small amount of sulfur compound added in the order of several ppm is in contact with the reforming catalyst during the reaction and causes a problem of the function of the reforming catalyst due to its adhesion.

  In order to prevent the problem of poisoning of the reforming catalyst due to such sulfur content, a desulfurizer is installed upstream of the reformer to desulfurize the raw fuel gas, and sulfur compounds are removed from the raw fuel gas in advance. However, as a desulfurization method for that purpose, a method of adsorbing a sulfur compound on an adsorbent, a method of adding hydrogen to hydrogen sulfide, and removing hydrogen sulfide are known. Among these, hydrodesulfurization performed by adding hydrogen is in principle explained by the chemical reaction formula shown below. In the following formula, R represents a hydrocarbon group.

_{R-CH 2 SH + H 2} → R-CH 3 + H 2 S (1)
H ₂ S + ZnO → ZnS + H ₂ O (2)
In practice, the sulfur compound (R—CH ₂ SH) in the raw fuel gas is usually supported at a catalyst holding carrier constituting the desulfurizer at an optimum reaction temperature of 200 to 300 ° C. Catalytically reacts with hydrogen to convert to hydrogen sulfide (H ₂ S). The generated hydrogen sulfide reacts with the adsorbent, zinc oxide (ZnO), which is supported on the adsorbent holding carrier constituting the desulfurizer, and is retained on the carrier constituting the desulfurizer as zinc sulfide (ZnS), and the fuel Removed from the gas.

  Here, as is clear from the above equation (1), since hydrogen is required for this reaction, it is necessary to add hydrogen to city gas, LP gas, or the like as raw fuel gas. In the site power generation system and the like, unlike the large hydrogen production plant, there is no process of purifying hydrogen. Therefore, in order to supply the hydrogen necessary for hydrodesulfurization, the fuel gas reformed into hydrogen rich gas by the reformer is used. In general, a system in which a part is recycled to the desulfurizer as reformed recycle gas is employed.

  As an example of adopting a system for recirculating reformed recycle gas to a desulfurizer, for example, the following Patent Document 1 describes a desulfurizer filled with a hydrodesulfurization catalyst and a reformation obtained by reforming a hydrocarbon fuel. In the fuel gas reformer having a line for returning a part of the gas to the inlet of the desulfurizer, a moisture removing means for removing moisture in the reformed gas is provided in the part of the return line of the reformed gas. There is disclosed a fuel gas reformer characterized by being provided.

Further, in Patent Document 2 below, a reforming reactor for reforming raw fuel to convert it into hydrogen-rich gas, and a reformer-reacted fuel gas provided on the downstream side of the reforming reactor. A fuel reforming system comprising at least a CO selective oxidation reactor that selectively oxidizes carbon oxide (CO) to carbon dioxide (CO ₂ ), and reforming raw fuel to produce a hydrogen-rich gas having a low CO concentration. Steam condensing / separating means for condensing and separating water vapor from at least a part of the reformed and reacted fuel gas between the downstream side of the reforming reactor and the upstream side of the CO selective oxidation reactor; There is disclosed a fuel reforming system comprising a recycle line for mixing, as a recycle gas, a gas obtained by condensing and removing water vapor by a condensing / separating means with a raw fuel supplied to the reforming reactor. .
JP 2002-97001 A JP 2002-356308 A

  The amount of hydrogen to be mixed with the raw fuel gas for hydrodesulfurization is about 5 to 15% as a hydrogen gas concentration. Usually, in a small fuel cell power generation system of 1 to several kW class, a hydrogen gas concentration of 60 to About 70% of the reformed recycle gas is set to be supplied to the raw fuel gas at a flow rate of about 1.4 to 4 L / min. At that time, the inner diameter of the reflux gas pipe for returning the reformed recycle gas to the desulfurizer is set to about 3 to 6 mm in order to facilitate the handling.

  However, under such conditions that the inner diameter of the reflux gas pipe must be reduced in this way, moisture mixed in the reformed recycle gas is condensed and plugged in the reflux gas pipe, and stable hydrogenation is performed. There was a problem that I could not.

  In order to deal with such problems, Patent Document 1 or Patent Document 2 proposes a method of previously removing water vapor contained in the reformed recycle gas. That is, moisture is removed from the inflowing reformed recycle gas by the steam condenser / separator installed on the upstream side of the reflux gas pipe, and water condensation and drain generation can be prevented.

  However, this method requires a steam condenser / separator composed of a cooler and a condenser and a structure for draining water removed from the reformed recycle gas, and the system becomes complicated. In the fuel cell power generation system, downsizing and cost reduction have been hindered.

  Accordingly, an object of the present invention is to provide a new system for preventing plugging of a gas flow path due to condensation of water vapor contained in the reformed gas in a system for supplying hydrogen gas necessary for hydrodesulfurization by recycling the reformed gas. An object of the present invention is to provide a fuel reforming system having a mechanism.

  The fuel reforming system of the present invention includes a desulfurizer that reduces the concentration of a sulfur compound contained in a raw fuel gas, a reformer that reforms the fuel gas processed by the desulfurizer into a hydrogen-rich gas, and the reformer. CO concentration reducer for reducing the carbon monoxide concentration contained in the fuel gas processed by the above, and the desulfurizer, the reformer, the CO concentration reducer, and the equipment disposed downstream of the CO concentration reducer A fuel gas line for circulating the fuel gas through the fuel gas line, and a gas line branched from the middle of the fuel gas line, at least of the fuel gas reformed into hydrogen-rich gas through the reformer In a fuel reforming system comprising at least a recirculation gas line arranged to recirculate a part as reformed recycle gas to the desulfurizer, the recirculation gas line is heated by heating the recirculation gas line Recycling reforming road Characterized in that to keep the above the dew point temperature of the gas.

  According to the present invention, when the reformed recycle gas is circulated through the reflux gas line, the recycle gas line maintained above the dew point temperature of the reformed recycle gas can be circulated. It is possible to prevent the gas from dropping below its dew point temperature in the reflux gas line.

  In the fuel reforming system of the present invention, the CO concentration reducer is composed of a CO converter that converts carbon monoxide contained in the fuel gas processed by the reformer, and the reflux gas pipe is A part of the fuel gas reformed into hydrogen rich gas by the reformer and further processed by the CO converter may be arranged to be recirculated to the desulfurizer as reformed recycle gas. .

  According to this aspect, a part of the reformed gas whose CO concentration is reduced to about 1% by the CO converter can be recirculated to the desulfurizer as the reformed recycle gas.

  Further, in the fuel reforming system of the present invention, the CO concentration reducer is processed by a CO converter that converts carbon monoxide contained in the fuel gas processed by the reformer, and the CO converter. A CO selective oxidizer that selectively oxidizes carbon monoxide contained in the fuel gas, and a fuel gas conduit that is connected between the CO converter and the CO selective oxidizer to circulate the fuel gas. The reflux gas line is branched from the middle of the fuel gas line between the CO converter and the CO selective oxidizer, and a part of the fuel gas processed by the CO converter is modified. It can also be set as the structure arrange | positioned so that it may recirculate | reflux to the said desulfurizer as quality recycle gas.

  According to this aspect, the CO concentration in the reformed gas that flows into the fuel cell and becomes the fuel can be lowered to about 10 ppm or less by the CO selective oxidizer, and at the same time, it is necessary for the selective oxidation of carbon monoxide. It is possible to avoid nitrogen from being mixed into the reformed recycle gas from fresh air.

  In the fuel reforming system of the present invention, at least one reaction selected from the group consisting of the desulfurizer, the reformer, or the CO concentration reducer is provided as a heating means for heating the reflux gas line. It is possible that the exhaust heat from the vessel is received by the reflux gas pipe.

  According to this aspect, it is possible to provide a system that effectively uses exhaust heat from the desulfurizer, reformer, and / or CO concentration reducer.

  Further, in the fuel reforming system of the present invention, the recirculation gas pipe is disposed so that the reformed recycle gas flow path flowing through the recirculation gas pipe does not have an upward gradient at least in the horizontal direction. It is preferable that

  According to this aspect, the drain of water existing in the reflux gas pipeline can be moved along the gas flow with the help of the earth's gravity and discharged from the reflux gas pipeline.

  According to the present invention, heat radiation from the reformed recycle gas flowing through the reflux gas pipeline can be prevented, and water condensation and drainage can be prevented from occurring due to condensation of water vapor contained in the reformed recycle gas. Can do.

  Further, according to a preferred aspect of the present invention, the reformed recycle gas is circulated while being at least not ascending with respect to the flow direction and mixed with the raw fuel gas upstream of the desulfurizer. Even if condensation or drainage of water occurs in the reflux gas line, it is carried along the flow with the help of gravity, so the flow path in the reflux gas line is temporarily or It can be prevented that the flow becomes unstable due to permanent blockage.

  Although there is no restriction | limiting in particular in the raw fuel gas utilized in this invention, The city gas, LP gas, etc. which are supplied to a general household can be utilized preferably.

  Although there is no restriction | limiting in particular in the fuel cell utilized in this invention, A solid polymer type fuel cell and a phosphoric acid type fuel cell can be used preferably. In that case, if a fuel cell power generation system with a power generation output of 0.5 to 3 kW is used, the fuel reforming system of the present invention is more effectively configured.

  The water vapor partial pressure of the reformed recycle gas flowing into the reflux gas pipe is about 20 kPa, and the dew point is 55 to 65 ° C. Therefore, in the present invention, the temperature for maintaining the reflux gas line through which the reformed recycle gas flows above the dew point temperature of the reformed recycle gas may be set as appropriate, but is preferably set to 70 ° C. or higher.

  In the present invention, in order to allow the reformed recycle gas for desulfurization to flow into the gas pipe to which the raw fuel gas is supplied through the reflux gas pipe, passive inflow due to the difference in back pressure in the gas pipe is used. However, if a fuel gas compressor is arranged upstream of the reformer so that the fuel gas mixed with the reformed recycle gas flows through the fuel gas compressor and then flows into the desulfurizer, the supply The required amount of reforming recycle gas for desulfurization can be caused to flow into the intake side of the fuel gas compressor due to the difference in gas pressure between the raw fuel gas and the reformed recycle gas in the reflux gas pipe.

  Carbon monoxide (CO) generated in the reforming reaction can be reduced by circulating it through the CO concentration reducer. However, when the reformed fuel gas is supplied to the phosphoric acid fuel cell and used. As the CO concentration reducer, a CO converter that reduces the CO concentration to about 1% can be used. In addition, when the fuel gas is supplied to a polymer electrolyte fuel cell and used, CO in the reformed gas serving as fuel causes a decrease in the function of the electrolyte membrane of the polymer electrolyte fuel cell. Needs to be lowered to about 10 ppm or less. Therefore, a CO selective oxidizer that selectively oxidizes carbon monoxide by oxygen in the air is connected through the fuel gas line, and further to the downstream side of the CO transformer, It is preferable to use the CO concentration reducer together with a CO selective oxidizer.

  Nitrogen is mixed in the reformed gas that has passed through the CO selective oxidizer, but nitrogen becomes ammonia in the reformer and deteriorates the characteristics of the fuel cell. Therefore, such reformed gas containing nitrogen is used as fuel gas. Refluxing is not preferred. Therefore, when a CO selective oxidizer is provided to perform a CO selective oxidation reaction, the reflux gas line is preferably configured to branch downstream from the CO converter and upstream from the CO selective oxidizer.

  In the present invention, the hydrodesulfurization catalyst used for hydrodesulfurization in the desulfurizer is not particularly limited, and a hydrodesulfurization catalyst such as a Co-Mo type or Ni-Mo type can be used. The optimum reaction temperature is preferably 200 to 300 ° C.

  In the present invention, the reforming catalyst used for steam reforming in the reformer is not particularly limited, and a reforming catalyst such as a noble metal catalyst or a nickel catalyst can be used. Is preferably 600 to 700 ° C. The heating means for the reaction is not particularly limited, but the burner installed in the reformer burns off-gas from the fuel cell, and the catalyst filled in the reformer is heated from the outside with the combustion gas. This method is preferable because off gas from the fuel cell can be used effectively.

In the present invention, in the CO converter, the carbon monoxide (CO) contained in the reformed fuel gas by the catalytic action of the CO conversion catalyst supported on the catalyst holding carrier constituting the CO converter. Reacts with water vapor (so-called CO shift reaction) and is transformed into carbon dioxide (CO ₂ ) and hydrogen. Here, the CO conversion catalyst used for CO conversion in the CO converter is not particularly limited, and a CO conversion catalyst such as a copper-zinc catalyst can be used, but the optimum reaction temperature is It is preferable that it is 180-350 degreeC. In addition, temperature control means for the reaction may be provided, but it is necessary to control the temperature particularly when the reformed fuel gas sent from the reformer is kept in the optimum reaction temperature range. There is no.

In the present invention, in the CO selective oxidizer, oxygen in the air introduced into the CO selective oxidation reaction is catalyzed by the catalyst for CO selective oxidation supported on the catalyst holding carrier constituting the CO selective oxidizer. Then, it reacts only with CO without reacting with hydrogen contained in the reformed fuel gas (so-called CO selective oxidation), and as a result, the CO remaining in the CO gas subjected to CO conversion treatment is carbon dioxide (CO ₂ ) To be selectively oxidized. Here, the CO selective oxidation catalyst used for the selective CO oxidation in the CO selective oxidizer is not particularly limited, and a CO selective oxidation catalyst such as a zeolite-supported catalyst can be used. The reaction temperature is preferably 120 to 170 ° C. In addition, a temperature control means for the reaction may be provided. However, when the reformed fuel gas sent from the CO converter is kept in the optimum reaction temperature range, it is particularly necessary to control the temperature. There is no.

  In the present invention, a series of reactions such as hydrodesulfurization, steam reforming, CO shift reaction, and CO selective oxidation are performed under a high temperature condition of 120 to 700 ° C., so that the reforming that flows into the reflux gas line is performed. The gas is at a high temperature of about 120 to 150 ° C., but in order to prevent heat dissipation during circulation in the reflux gas pipeline, the reflux gas pipeline is made of a heat insulating material such as rock wool, glass wool, calcium silicate, etc. , May cover all or part thereof.

  In the present invention, each of the desulfurizer, reformer, CO converter, and CO selective oxidizer is configured such that the catalyst reaction temperature therein is thermally conducted to the surface of one or more reactors. It is sufficient to use a reactor that is widely used. For example, the surface temperature of each reactor at the operating temperature of the above preferred embodiment is about 150 to 250 ° C. for the desulfurizer, about 400 to 600 ° C. for the reformer, about 150 to 300 ° C. for the CO converter, and CO selection. It is about 100-150 degreeC with an oxidizer.

  In the present invention, there is no particular limitation on the means for heating the reflux gas line, but the reflux gas line receives the exhaust heat from the desulfurizer, reformer, CO converter, and CO selective oxidizer. It can be a method. In this case, it is preferable to arrange the reflux gas pipeline so as to contact the surfaces of the reactors, because the exhaust heat can be received directly and efficiently. At that time, in order to prevent excessive heating of the reflux gas pipe, heat may be received through a heat insulating material made of the above material.

  The material of the gas pipe used in the present invention is not particularly limited, but stainless steel and polypropylene are preferable from the viewpoints of durability, heat resistance, pressure resistance, thermal conductivity, ease of molding, and the like.

  Hereinafter, the present invention will be described in more detail with reference to the drawings, but the present invention is not limited thereto.

  FIG. 1 is a configuration diagram schematically showing one embodiment of a fuel reforming system of the present invention. In this embodiment, the fuel reforming system of the present invention is applied to a fuel cell power generator that uses city gas as raw fuel gas.

  In this fuel cell power generator, city gas, which is the raw fuel gas, passes through gas lines indicated by straight lines and arrows in the figure through a desulfurizer 3, a gas compressor 9, a reformer 4, a CO converter 5, and CO selective oxidation. The fuel gas that has been reformed into the hydrogen-rich gas is supplied to the anode electrode side 21 of the fuel cell 7 of the fuel cell. In addition, 22 is a cathode electrode side and 23 is an electrolyte.

  The reformer 4 includes a reforming catalyst support 4a and a combustion gas generator 4b, and the fuel gas desulfurized by the desulfurizer 3 contacts the reforming catalyst supported on the reforming catalyst support 4a. Is configured to do. The combustion gas generator 4b is connected to an offgas return gas line 2 for recirculating a part of the offgas containing hydrogen gas discharged from the anode electrode 21 and not used to the reformer. The temperature of the reforming catalyst support 4a is adjusted to a temperature suitable for steam reforming by using combustion gas generated by air combustion of the off gas.

  In the fuel reforming system shown in FIG. 1, the raw fuel gas is supplied again to the gas pipe branched from the upper branch point A located in the middle of the gas pipe of the CO converter 5 and the CO selective oxidizer 6. The return gas pipe 1 is formed by being connected to the fuel gas pipe at the lower branch point B so as to be returned to the gas pipe. Here, the upper branch point A is a branch point of the gas pipe disposed above the lower branch point B with respect to the direction of gravity, and as a result, reforming and recycling that flows through the reflux gas pipe 1. The gas flow path is not at least ascending with respect to the horizontal direction. The reflux gas pipe 1 is disposed so as to be in contact with the surface of the reformer 4, and the exhaust heat from the reformer 4 is received at the heat receiving portion 1a of the reflux gas pipe 1 indicated by a dotted line in the figure. Yes.

  According to this aspect, the fuel reforming system of the present invention can be obtained by changing the piping arrangement of the return flow path of the desulfurization reforming recycle gas in the configuration of the conventional fuel cell power generator.

  FIG. 2 is a schematic diagram showing an embodiment of a means for heating the reflux gas line in the fuel reforming system of the present invention.

  Here, a cylindrical reformer 4 schematically shown in FIG. 2 a is covered with a heat insulating material 10, and the reflux gas pipe 1 is spirally wound on the outer peripheral surface thereof, so that an upper branch point A The heat receiving portion 1a (FIG. 2b) of the reflux gas pipe located between the lower branch point B and the lower branch point B receives the exhaust heat from the reformer 4. Although not shown in the drawing, the heat insulating material 10 is disposed so as to be physically in contact with the surface of the reformer 4, and the reflux gas pipe 1 is placed on the outer peripheral surface of the reformer 4 with the heat insulating material 10 interposed therebetween. It comes to wrap around directly.

  According to this aspect, the exhaust heat from the reformer 4 can be received by the reflux gas pipeline, while excessive heat of the reflux gas pipeline can be prevented by receiving heat through the heat insulating material. it can. Further, since water condensation and drain existing in the reflux gas pipeline move to the lower branch point B by the action of gravity, it can be passively excreted from the reflux gas pipeline.

  Note that the partial pressure of water vapor in the fuel gas once the reformed recycle gas is refluxed and mixed with the city gas is 2 to 6 kPa, and the dew point is about 17 to 36 ° C. There is little need to worry about the condensation of water vapor in the fuel gas line for circulating gas.

  ADVANTAGE OF THE INVENTION According to this invention, the fuel reforming system provided with the new mechanism for preventing the water vapor | steam condensation of the reforming recycle gas for deflowing can be provided. Further, by adopting the fuel reforming system of the present invention in a fuel cell power generator, it is possible to achieve cost reduction and compactness by simplifying the configuration of the fuel cell power generator.

It is a lineblock diagram showing typically one embodiment of a fuel reforming system of the present invention. It is a schematic diagram which shows one Embodiment of the means to heat the recirculation | reflux gas pipe line of the fuel reforming system of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Recirculation gas pipe 1a Heat-receiving part 2 of recirculation gas pipe 2 Return gas pipe 3 for off gas 3 Desulfurizer 4 Reformer 4a Reformation catalyst support part 4b Combustion gas generation part 5 CO converter 6 CO selective oxidizer 7 Fuel cell Cell 9 Gas compressor 10 Insulating material 21 Anode electrode side 22 Cathode electrode side 23 Electrolyte A Upper branch point B Lower branch point

Claims (5)

A desulfurizer that reduces the concentration of sulfur compounds contained in the raw fuel gas;
A reformer for reforming the fuel gas treated by the desulfurizer into a hydrogen-rich gas;
A CO concentration reducer that reduces the concentration of carbon monoxide contained in the fuel gas treated by the reformer;
A fuel gas conduit for flowing fuel gas to the desulfurizer, the reformer, the CO concentration reducer, and a device disposed downstream of the CO concentration reducer;
A gas line branched from the middle of the fuel gas line, and at least a part of the fuel gas that has been reformed to hydrogen-rich gas after passing through the reformer is recirculated to the desulfurizer as a reformed recycle gas. In a fuel reforming system comprising at least a reflux gas pipe arranged as described above,
A fuel reforming system, wherein the reflux gas line is heated to keep the reflux gas line at or above the dew point temperature of the reformed recycle gas.   The CO concentration reducer is composed of a CO converter that converts carbon monoxide contained in the fuel gas processed by the reformer, and the reflux gas line is reformed to a hydrogen-rich gas by the reformer. 2. The fuel reforming system according to claim 1, wherein a part of the fuel gas treated by the CO converter is further returned to the desulfurizer as a reformed recycle gas.   The CO concentration reducer selectively converts the carbon monoxide contained in the fuel gas processed by the reformer and the carbon monoxide contained in the fuel gas processed by the CO converter. A CO selective oxidizer that oxidizes the fuel gas, and a fuel gas conduit that is connected between the CO converter and the CO selective oxidizer to circulate the fuel gas. A branch is made from the middle of the fuel gas line between the converter and the CO selective oxidizer, and a part of the fuel gas processed by the CO converter is returned to the desulfurizer as a reformed recycle gas. The fuel reforming system according to claim 1, wherein the fuel reforming system is disposed.   The heating means for heating the reflux gas line is configured to supply exhaust heat from at least one reactor selected from the group consisting of the desulfurizer, the reformer, or the CO concentration reducer to the reflux gas line. The fuel reforming system according to claim 1, wherein the fuel reforming system receives heat.   5. The fuel reformer according to claim 1, wherein the recirculation gas pipe is disposed so that a flow path of the reformed recycle gas flowing through the recirculation gas pipe does not have an ascending gradient at least in the horizontal direction. Quality system.

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