JP7742898B2 - Cement kiln firing device and method for denitrifying cement kiln exhaust gas - Google Patents

Description

The present invention relates to a technology for reducing the concentration of nitrogen oxides (hereinafter referred to as "NOx") in combustion gases emitted from cement kilns.

NOx is generated in the high-temperature region of the burning zone of a cement kiln. When the NOx concentration in cement kiln exhaust gas becomes high, denitrification agents such as urea or ammonia are added, or the NOx concentration is reduced by the reduction effect of combustion in the calciner. However, using urea as a denitrification agent increases operating costs due to its high price. Furthermore, adding urea alone does not achieve the desired denitrification efficiency, and there is a risk that excess ammonia that does not react with NOx will be emitted directly from the system.

Patent document 1 describes an invention in which organic waste such as waste plastics is gasified to produce reducing gas, which is then injected into the combustion gas flow path in the preheater from the raw material inlet end of the cement kiln to the calciner, into the calciner itself, or into the combustion gas flow path from the calciner to the cyclone two stages above, and used as a reducing agent for nitrogen oxides.

JP 2008-18371 A

The invention described in Patent Document 1 is effective, but there was a problem in that even if the reducing gas obtained by gasifying organic waste was injected into the bottom of a cement kiln, if the mixture with the cement kiln exhaust gas was poor, the desired NOx reduction effect could not be achieved.

The present invention has been made in consideration of the problems in the above-mentioned conventional technology, and aims to efficiently reduce the NOx concentration in cement kiln exhaust gas using a reducing gas.

To achieve the above-mentioned objectives, the cement firing apparatus of the present invention comprises a separation device that separates preheated raw materials from a preheater cyclone; a gasification device that uses the preheated raw materials separated by the separation device as a heat source to gasify organic waste and produce reducing gas; an injection device that injects the reducing gas produced by the gasification device and the preheated raw materials discharged from the gasification device into a combustion gas flow path at the end of the cement kiln or from the end of the kiln to a calciner; and a denitration burner that injects fuel and a portion of the combustion air into the combustion gas flow path to partially combust the fuel, the denitration burner comprising a swirling means for swirling a mixed flow of the fuel and a portion of the combustion air, and a pair of denitration burners with opposite swirling directions that are located below the injection point into the combustion gas flow path of the reducing gas produced by the gasification device and/or the preheated raw materials discharged from the gasification device.

In the cement firing apparatus of the present invention, by installing a denitration burner in the combustion gas flow path at the end of the cement kiln or from the end of the kiln to the calciner, mixing of the cement kiln exhaust gas with reducing gas is promoted, and the synergistic effect of installing the denitration burner and introducing reducing gas can efficiently reduce the NOx concentration in the cement kiln exhaust gas. Furthermore, the swirling means improves the mixing efficiency of the cement kiln exhaust gas with reducing gas, further enhancing the NOx reduction effect. Furthermore, by installing a pair of denitration burners with opposite swirling directions below the introduction section into the combustion gas flow path of the reducing gas generated by the gasification system and/or the preheated raw material discharged from the gasification system, mixing of the cement kiln exhaust gas with reducing gas can be further promoted, thereby increasing denitration efficiency.

In the above-mentioned method for denitrifying cement kiln exhaust gas, the reducing gas introduced into the combustion gas flow path at the end of the cement kiln or from the end of the kiln to the calciner can be set to 300°C or higher and 600°C or lower. By introducing a reducing gas with a relatively low temperature, the formation of coating in the combustion gas flow path can be suppressed, and the amount of fuel used in the calciner can be increased.

As described above, according to the present invention, the NOx concentration in cement kiln exhaust gas can be efficiently reduced using a reducing gas.

1 is a schematic diagram showing an embodiment of a cement kiln burning apparatus according to the present invention. 1A to 1D are schematic diagrams showing an example of the arrangement of denitration burners in a cement kiln firing apparatus according to the present invention, in which (a) is a partially cutaway front view, (b) is a partially cutaway left side view, (c) is a partially cutaway right side view, and (d) is a top view. 2A to 2C are diagrams showing the denitration burner of FIG. 1, in which (a) is a cross-sectional view, (b) is a side view, and (c) is a developed view of the tip of the burner.

Figure 1 shows one embodiment of a cement firing apparatus according to the present invention. This cement firing apparatus 1 comprises a preheater 2 that preheats cement raw material (hereinafter referred to as "raw material") R, a calciner 3 that calcines raw material R1 from the second-lowest cyclone 2b of the preheater 2 using fuel F2 blown in from a calciner burner 11, a cement kiln 5 that calcines raw material R2 from the lowest cyclone 2a of the preheater 2 using fuel F1 blown in from a main burner 10, a clinker cooler 6 that cools cement clinker fired in the cement kiln 5, and a separator 7 that separates the preheated raw material from the lowest cyclone 2a. the preheated raw material R5 discharged from the gasification apparatus 14 into the combustion gas flow path from the kiln end 4 to the calciner 3; a denitrification burner 9 that injects fuel F3 and a portion A of the combustion air into the combustion gas flow path, and partially combusts the fuel F3 at a low air ratio. The preheater 2, calciner 3, cement kiln 5, and clinker cooler 6 are configured in the same manner as conventional ones.

The gasification device 14 gasifies organic waste, such as waste plastic, wood chips, waste tires, and sludge, to produce reducing gas RG containing gases such as hydrogen, carbon monoxide, methane, and carbon dioxide. A pyrolysis furnace can be used. The introduction device 15 can be a pipe if the gas can be sucked in by the negative pressure inside the cement kiln 5, or a fan or similar if not. The reducing gas RG from the introduction device 15 can be introduced into the combustion gas flow path through the chute 16 that introduces the preheated raw material R5, or it can be introduced through a pipe separate from the chute 16.

As shown in Figure 2, a total of four denitration burners 9 (9A-9D) are provided, and fuel F3 and a portion of the combustion air A are blown in from each of the denitration burners 9 (9A-9D). Of the four denitration burners 9, two (9B, 9C) are located below the chute 16 into which reducing gas RG and preheated raw material R5 are introduced. As shown in Figure 2(c), the swirling flow of denitration burner 9B is set to be clockwise, and the swirling flow of denitration burner 9C is set to be counterclockwise, thereby effectively promoting the mixing of cement kiln exhaust gas G and reducing gas RG. If reducing gas RG and preheated raw material R5 are not introduced through the same chute 16, the two denitration burners (9B, 9C) are located below either the piping into which reducing gas RG is introduced or the chute 16 into which preheated raw material R5 is introduced.

As shown in Figure 3, the denitration burner 9 comprises a cylindrical member 9h with six to ten swirl vanes (swirl means) 9b fixed at the tip of the cylindrical body 9a at a swirl angle of 10 to 50 degrees, a rod-shaped member 9j located at the center of the cylindrical member 9h, and a ring-shaped member 9f located outside the tip of the cylindrical member 9h. The tip of the denitration burner 9 is covered with refractory material 9e, and the mounting member 9c at the rear end of the body 9a is connected to the surface 4a of the kiln bottom 4 by a bracket 9d, and the burner is attached to the kiln bottom 4, etc. The kiln bottom 4, etc. is also protected by refractory material 4b. The swirl angle is the angle θ between the axial direction 9m of the denitration burner 9 and the center line 9g of the swirl vanes 9b when the cylindrical member 9h is unfolded in a plane as shown in Figure 3(c). This angle corresponds to the swirl angle of the solid powdered fuel flow F at the tip 9k of the flow path. The number of swirl blades 9b and the swirl angle can be changed as appropriate.

Next, the method for denitrifying cement kiln exhaust gas according to the present invention will be explained with reference to Figures 1 to 3.

The preheater 2 of the cement firing apparatus 1 preheats raw material R using exhaust gas G from the cement kiln 5, and raw material R1 from the cyclone 2b of the preheater 2 is calcined in the calciner 3. During this process, tertiary air G1 from the clinker cooler 6 is blown in together with fuel F2 from the calciner burner 11 to combust the fuel F2. Pulverized coal, heavy oil, combustible waste, combustible gas, etc. may be used as fuel F2, and combustible waste may be used as part or all of the fuel F2.

Meanwhile, preheated raw material is collected from the raw material chute 12 of the lowest cyclone 2a by a collection device 13, and the collected preheated raw material R4 is used as a heat source to gasify waste plastic as organic waste at approximately 10 t/h in a gasification device 14. The generated reducing gas RG, which is between 300°C and 600°C, is injected by an injection device 15 via a chute 16 into the kiln end 4 or into the combustion gas flow path from the kiln end 4 to the calciner 3. Meanwhile, preheated raw material R5 discharged from the gasification device 14 is injected via a chute 16 into the kiln end 4 or into the combustion gas flow path from the kiln end 4 to the calciner 3.

If more than 50% of the fuel F2 supplied to the calciner 3 is introduced into the combustion gas flow path without using reducing gas RG, coating will form and operation will not be possible. However, by introducing reducing gas RG with a relatively low temperature of 300°C or higher and 600°C or lower and preheated raw material R5 discharged from the gasification device 14, coating deposition can be suppressed and the amount of fuel F2 introduced into the calciner 3 can be increased.

Fuel F3 is also injected from a denitration burner 9 installed at the kiln end 4 of the cement kiln 5, along with a portion of the combustion air A, to promote mixing of the cement kiln exhaust gas G and the reducing gas RG. The synergistic effect of installing the denitration burner 9 and introducing the reducing gas RG can efficiently reduce the NOx concentration in the cement kiln exhaust gas G. As with fuel F2 for the calciner 3, fuel F3 can be pulverized coal, heavy oil, combustible waste, combustible gas, etc., and any one or more of these may be used simultaneously.

By injecting fuel F3 from the denitration burner 9, NOx contained in the exhaust gas G from the cement kiln 5 can be reduced in the low-oxygen region within the kiln end. Furthermore, by partially combusting fuel F3 using a portion of the combustion air A and completely combusting the unburned portion with tertiary air G1 in the calciner 3, raw materials R2 and R3 can be decarbonated, improving the decarbonation efficiency of the entire cement firing apparatus 1. In this case, the swirl blades 9b provided on the denitration burner 9 swirl the mixed flow of fuel F3 and a portion of the combustion air A and inject it between the kiln end 4 and the calciner 3, improving the combustion efficiency of fuel F3.

Next, fuel F1 such as pulverized coal is injected from the main burner 10 of the cement kiln 5 to burn the raw material R2 from the cyclone 2a of the preheater 2, and the resulting clinker is cooled in the clinker cooler 6 to obtain cement clinker CL.

Conventionally, when targeting cement kiln exhaust gas G, the injection of reducing gas RG alone was effective in reducing NOx by approximately 50 ppm, and the installation of a denitration burner 9 (two-stage combustion) alone was effective in reducing NOx by approximately 50 ppm. However, according to the present invention, the synergistic effect of installing a denitration burner 9 and the injection of reducing gas RG achieves a further NOx reduction effect of approximately 20 ppm or more.

In the above embodiment, preheated raw material was taken from the lowest cyclone 2a and used as a heat source for the gasification device 14, but a preheater cyclone 2b above the lowest cyclone 2a can also be used.

1 Cement burning device 2 Preheater 3 Calciner 4 Kiln bottom 5 Cement kiln 6 Clinker cooler 9 (9A to 9D) Denitrification burner 10 Main burner 11 Calciner burner 12 Raw material chute 13 Separation device 14 Gasification device 15 Feeding device 16 Chute

Claims (1)

a fractionation device that fractionates the preheated raw material from the preheater cyclone;
a gasification device that gasifies organic waste using the preheated raw material separated by the separation device as a heat source to generate reducing gas;
an input device that inputs the reducing gas generated by the gasification device and the preheated raw material discharged from the gasification device into a combustion gas flow path at the end of the cement kiln or from the end of the kiln to a calciner;
a denitration burner that blows fuel and a portion of combustion air into the combustion gas flow path to partially combust the fuel,
the denitration burner is provided with a swirling means for swirling a mixed flow of the fuel and a portion of the combustion air,
The cement burning apparatus is characterized in that the denitration burners are a pair of denitration burners having different rotation directions, and are provided below an input portion into the combustion gas flow path of the reducing gas generated by the gasification apparatus and/or the preheated raw material discharged from the gasification apparatus.