JPH0131560B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a steam reforming furnace having a structure suitable for reducing the amount of nitrogen oxides (hereinafter abbreviated as NOx) discharged from the steam reforming furnace.

Reforming furnaces, which add steam to hydrocarbon gas to reform this gas, burn fuel as a heating source in order to heat the mixture of hydrocarbon gas and steam, and for this reason, other types of reformers are used. Similar to furnaces, combustion exhaust gas
It is necessary to use equipment that reduces NOx.

Conventional methods to reduce NOx emitted from various types of furnaces include improving the burner structure, limiting the amount of combustion air, and recirculating exhaust gas at the combustion stage.
How to reduce the amount of NOx generated and NOx generated
The most commonly used method is to remove this by adding a reducing agent or the like. In the case of a steam reforming furnace, a radiant short flame burner is used as the heating element in order to accurately maintain the reaction temperature and protect the tube that forms the reaction part. is precisely controlled. For this reason, there is little room to control combustion from the perspective of reducing NOx, and catalytic denitrification equipment is used exclusively to reduce NOx.
However, this method is uneconomical because the catalyst itself is expensive, and if the temperature range at which the reducing agent is injected is incorrect, the reducing agent may thermally decompose and cause damage.
There are problems such as increasing the amount of NOx.

An object of the present invention is to solve the above-mentioned problems and to obtain a high denitrification rate with a small amount of catalyst.
Our objective is to provide a steam reforming furnace.

In short, this invention takes into account the characteristics of the catalytic reaction temperature in the downflow exhaust gas passage, places the catalyst layer in the exhaust gas passage in a temperature range of about 300 to 400°C, and places the reducing agent upstream near the catalyst layer. The device is characterized in that it is a device that sprays water.

Embodiments of the present invention will be described below with reference to the drawings.

Figure 1 shows a reforming furnace. A mixed gas of hydrocarbon-carbon gas and steam flows through the re-formation tube 3 and is heated and reformed by the radiant heat of the radiant short flame burner 2 (hereinafter simply referred to as "burner"), and the reformed gas is installed below. The processed gas flows to the boiler. On the other hand, exhaust gas generated by combustion in the burner 2 reaches an exhaust gas outlet 4 and flows into an exhaust gas duct 5. A plurality of heat exchangers are arranged in the exhaust gas duct 5 for the purpose of recovering sensible heat contained in the exhaust gas.
As shown in the figure, the heat exchangers are normally arranged in the following order from the top: a steam superheater 6, a waste heat recovery boiler 7, a process fluid (gas to be treated) preheater 8 divided into a plurality of tube groups, and a feed water preheater 9. It has been placed.

The selective catalytic denitrification device that removes NOx from exhaust gas is comprised of a denitrification catalyst layer and a reducing agent spray section located upstream of this catalyst layer. Looking at the reaction characteristics of the catalyst layer, temperature has a very large effect on the reactivity of the catalyst, as shown in a graph as shown in FIG. As is clear from this graph, the activity of the catalyst is high in the temperature range of 300 to 400°C, which is a suitable range for the installation of the catalyst layer. especially
The denitrification rate of the catalyst reaches its maximum in the temperature range of 350 to 400°C. On the other hand, if a reducing agent such as NH ₃ that is sprayed upstream of the catalyst layer is heated to too high a temperature, the reducing agent itself will be thermally decomposed, resulting in an increase in the amount of NOx produced. In other words, since the temperature increases as the distance from the installation position of the catalyst layer increases toward the upstream side, it is preferable to spray the reducing agent at a position close to the catalyst layer as long as uniform dispersion of the reducing agent is possible.

FIG. 3 shows the temperature distribution of the exhaust gas duct 5 of the reformer shown in FIG. 1, which is an embodiment of the present invention.
Approximately 1000 to 1100°C at the exhaust gas duct inlet A, approximately 800 to 850°C at the steam heater outlet B, approximately 600 to 650°C at the waste heat recovery boiler outlet C, and approximately 250 to 300°C at the process fluid preheater outlet D. Approximately 150 to 200℃ at water preheater outlet E
It is becoming. Therefore, from the reaction characteristics shown in FIG. 2, the catalyst layer is preferably installed at or near the process fluid preheater 8. In FIG. 1, a catalyst layer (designated 10) is located within a process fluid preheater 8 that is divided into a plurality of heat transfer tube groups. Next, a denitration agent such as NH ₃ is sprayed close to the catalyst layer 8 at a position where the reducing agent can be uniformly dispersed. In the illustrated case, in order to realize this, the reducing agent spraying device 11 is disposed upstream of the heat transfer tube group of the process fluid preheater 8 located upstream of the catalyst layer 10.

By implementing the present invention, it is possible to effectively reduce NOx in the exhaust gas discharged from a combustion device, such as a steam reformer, in which it is difficult to reduce NOx through combustion control, with a small amount of catalyst.

In addition, the inside of the exhaust gas passage is a vertical downward flow, and multiple devices for exchanging heat are arranged in this passage as described above, and there are multiple unit steam reforming furnaces, so the amount of exhaust gas is also downward. Since the exhaust gas flows through the flow path, the flow rate of the exhaust gas can be increased and the effect of heat exchange can be achieved, and the reducing agent spraying device is provided at a position where the exhaust gas temperature is suitable for denitrification.

On the other hand, with this structure, the floor area occupied by the steam reforming furnace shown in Figure 1 is much smaller than that of one in which the heat exchanger is arranged horizontally, and in the event of a suspension of operation, the steam reforming furnace will remain at the lower end of the exhaust gas passage. When the specific gravity of air is 1, the specific gravity of ammonia is 0.596, so this air acts as a gas seal against harmful leaked ammonia gas.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a steam reforming furnace that is an embodiment of the present invention, and FIG. 2 is a diagram showing the temperature characteristics of the denitrification catalyst.
FIG. 3 is a diagram showing the temperature distribution in each part of the boiler duct. 5...Exhaust gas duct, 8...Process fluid preheater, 10...Catalyst layer, 11...Reducing agent spray device.

Claims (1)

[Claims] 1 The top combustion gas passages of multiple furnaces of a unit steam reforming furnace in which a re-former tube is located in the furnace and a plurality of radiant short flame burners are installed on both sides of the furnace are assembled to form one downward flow exhaust gas passage. In order from upstream to downstream, the downflow exhaust gas passage includes a steam superheater 6, a waste heat recovery boiler 7, a first process fluid preheater 8, a reducing agent spray device 11, and a second process fluid preheater 8, 350.
The third catalyst layer 10 contacts the exhaust gas at a temperature of ℃ to 400℃.
A steam reforming furnace characterized in that a process fluid preheater 8 and a feed water preheater 9 are located.