GB2192624A - Denitrating device for high temperature waste gas - Google Patents

Abstract

A denitrating device of gas phase reduction type for high temperature waste gas including nitrogen compound in which ammonia is injected in the upstream of waste gas stream, and hydrogen peroxide is injected in the downstream. A plurality of nozzles 4 are provided to inject hydrogen peroxide solution into the gas stream 1, with the nozzles being arranged in a zigzag layout in the cross section of a duct of the waste gas stream to direct the hydrogen peroxide toward the upstream, whereby a uniform mist of fine grain hydrogen peroxide is formed in the duct and toward the upstream. <IMAGE>

Description

SPECIFICATION Denitrating device for high temperature waste gas The present invention relates to a denitrating device for waste gas and, particularly to a denitrating device of gas phase NO, reduction type with ammonia (referred hereinafter as NH3) and hydrogen peroxide (referred hereinafter as H202) being injected into the stream of waste gas.

Japanese Patent Publication 57-19700 shows a prior art denitrating device of gas phase NO, reduction type wherein H202 is passed through the central portion of a pipe with air being circulated through the peripheral portion of the pipe so asOto prevent the H202 passing through the central portion from overheating and the decomposition resulting therefrom. Further, it is shown that the injection should be performed in a plural stages so as to improve denitrating characteristics. However, when air is utilized to cool H202 it is required to use a large amount of air to effectively prevent the decomposition of H202 due to overheat since the heat capacity of air is small and the heat transfer thereof is insufficient. Thus, a large sized air compressor and large diameter air pipes should be provided.It is not described with respect to the arrangement of injection nozzles. Further, it is described that the amount of H202 being injected should have the mol ratio of less than 1 with respect to the concentration of nitrogen compound (referred hereinafter as NO,) included in the waste gas, and that the amount of H202 is controlled by, a flow control valve. The concentration of H202 being injected is not described in the publication.

Accordingly, there are problems to realize a denitrating device having improved denitrating characteristics and preventing the decomposition of H202 prior to the injection, with a stable composition of H202 being supplied to the denitrating device.

According to the present invention, there is provided a denitrating device of gas phase NO, reduction type for high temperature waste gas including NO, in which NH3 is injected in the upstream of waste gas stream, and H202 is injected in the downstream, wherein a plurality of H202 injecting nozzles are arranged to oppose the gas stream, and in a zigzag layout in the cross section of a duct of the waste gas stream thereby forming uniform mist of fine grain H202 in the duct and toward the upstream.

Preferably, H202 is conducted through an inner tube of a dual tube with cooling water being conducted through an outer tube of the; dual tube.

According to a preferred embodiment, the H202 is formed by providing a tank of undiluted solution of H202, a tank of diluting water, and pumps of adjustable discharge being connected respectively to the tanks, with the discharge of the pumps being adjusted in accordance with a desired rate of dilution.

Preferably, the pumps are started-and stopped simultaneously. Further, a tank of diluted H202 may be connected with outlets of the pumps of undiluted H202 and diluting water, and another pump may be provided to conduct the H202 from the tank of diluted H202 to the nozzles.

According to the invention, uniform mist of fine grain of H202 is formed in the duct and directed toward the upstream, thus, improved denitrating characteristics can be obtained.

Further, H202 can be cooled sufficiently so that decomposition of H202 in the pipeline can be prevented, which is advantageous in preventing thermal deformation of the pipeline and nozzles.

Further, according to the invention, H202 is diluted automatically to a desired concentration.

By way of example an embodiment of the invention is now described in detail, with reference to the drawings, in which Figure 1 is a perspective view showing the arrangement of H202 injecting nozzles of a denitrating device; Figure 2 is a view as viewed along line Il-Il in Fig. 1; Figure 3 is a view as viewed along line Ill-Ill in Fig. 1; Figure 4 is a sectional view of H302 injecting pipe in portion C in Fig. 1; Figure 5 is a longitudinal sectional view of H202 injecting pipe of Figs. 1 and 4, and Figure 6 is a schematic view of H202 diluting and injecting system.

Fig. 1 is a view showing the arrangement of H202 injecting nozzles of dehydrating device embodying the present invention. High temper- ature waste gas 1 which includes NOx is flowed through a gas phase NO, reduction duct 2 and is, firstly, contacted with NH3 3, then, contacted with H202 8 so as to reduce NOx.

To inject NH3, there are provided a plurality of pipes 5 each having uniformly spaced nozzles 4. Diluted H202 solution 8 is injected at one or more stages or longitudinal locations along the duct 2 (Fig. 1 shows two stages) through a plurality of injection pipes 9. Each pipe 9 is, as shown in Figs. 4 and 5, a dual pipe which includes a central H202 tube 15 and a peripheral water tube 16. A plurality of nozzles 10 are provided along each of injection pipe 9, and are directed toward the upstream of the flow of the gas 1. There are provided a plurality of small diameter branch tubes 17 on the H202 tube 15 to mount the nozzles 10 on the tip ends thereof by such as screw threads.The pipes 9 extend vertically and are spaced equally from each other, and the nozzles are also spaced equally from each other and are mounted horizontally so that the axis of each nozzle 10 is parallel to the axis of the duct 2.

The water tube 16 is circulated by cooling water which cools the H202 tube 15 and branch tubes 17 and, escapes from the injection pipe 9 through outlet piping 7. The cooling water can effectively maintain H202 at a low temperature thereby preventing the decomposition prior to the injection and, prevent thermal deformation of the injection pipe 9.

It will be noted that H202 is a chemically unstable material and, at high temperature, reduces into water and oxygen, thus, it is required to prevent decomposition prior to the injection by such as cooling and, that H202 should be injected evenly into waste gas so as to form uniform mist of fine grain of H202 to react effectively with NOx.

Fig. 2 shows the arrangement of the first stage nozzles and, Fig. 3 shows that of the second stage. At the first stage, injection pipes 9 extend vertically defining equal spaces 13 between each other and between inner walls of the duct 2. The nozzles 10 on each of the pipes 9 are arranged at equal spaces 14, and located at mid positions of the spaces 14 of adjacent pipes 9 so that the nozzles 10 in the duct 2 are arranged in a zigzag layout as shown in Fig. 2. The layout shown in Fig. 3 is similar to Fig. 2, but the nozzles 10 are located to avoid overlapping.

According to the embodiment, H202 injection pipes are cooled by water having large specific heat, thus, the diameter of the pipes 9 can be minimized which does not impede the flow of the gas. Further, the construction of the pipe 9 shown in Figs. 4 and 5 enables to cool H202 sufficiently in preventing the decomposition prior to the injection. The nozzles 10 are directed toward the upstream of the gas flow, thus, sprayed H202 expand into generally circular configuration which can improve the uniformity in the density of H202. The zigzag layout as shown in #Figs. 2 and 3 can further improve the uniformity in the density which minimize the waste gas passing through the duct 2 without reacting with H202.

Fig. 6 shows a H202 diluting system according to the present invention. A tank 21 receives concentrated or undiluted H202 20, and is connected to a diluting tank 41 through a pump 23, a flow meter 24 and a back pressure valve 25. The pump 23 is of variable discharge type such as reciprocating pump.

The back pressure valve 25 controls the discharge pressure of the pump 23 so as to improve the reading of the flow meter 24, and may be such as a spring loaded safety valve known per se.

A tank or a pool 31 receives diluting water 30 for H202 and is connected with the diluting tank 41 through a pump 33, a flow meter 34 and a back pressure valve 35. The pump 33 and the valve 35 are similar respectively to the pump 23 and the valve 25 in the undiluted H202 circuit and further description is omitted. The diluting water 30 is required to be pure water or water having no impurities.

The undiluted H202 20 and the diluting water 30 are mixed in the diluting tank 41, preferably by utilizing a mixer 26. A liquid level switch 27 having two, H (High) and L (Law) contact points is provided on the tank 41, such that the pumps 23 and 33 are started at level L and are stopped at level H.

The switch 27 generates electric signals 28 and 29 for respectively and simultaneously starting and stopping the pumps 23 and 33.

Another contact point such as LL (not shown) may be provided below the contact point L so as to control the mixer 26 and an injection pump 41 which wiJI be described hereinafter.

The diluted H202 8 is delivered to the injection pipes 9 by the pump 42, and is sprayed into the duct 2. Further, a cooling water supplying system 50 is provided to cool the diluted H202 8 in the pipes 9. The system 50 may include a pump, a tank or a pool, and a cooling tower and the like (not shown).

In diluting the undiluted H202 20 in the diluting tank 41 to a desired density, the discharge of pumps 23 and 33 are determined from following equation and affirmed by flow meters 24 and 34.

(The density % of diluted H202)=100 x (Discharge of undiluted H202) x (Density of undiluted H202) . [(Discharge of undiluted H202) x (Density of undiluted H202) + (Discharge of diluting water) ] Thus, the density of diluted H202 can be adjusted as desired. The pumps 23 and 33 simultaneously start and stop, thus, the density is maintained.

Preferably, flow meters 24 and 34 are graduated by weight flow to simplify the calculation of the density weight %, or by mass flow to simplify the calculation of the density of mass %.

It will be noted that, in denitrating the waste gas including NOx by H202, when the density of H202 is high the amount of H203 is too small it is difficult to make a uniform mist in the gas stream and high density H202 (over 36 wt%) is a dangerous article according to Japanese Fire Defence Law and is difficult to handle. Thus, equipments such as tank, pipeline, pump and the like for the high density H202 should be explosion proof construction.

According to the embodiment, only the parts receiving and handling the high density H202 are constructed to prevent explosion, thus, the system is less expensive and is safe. Further, the density of the diluted H202 can be adjusted as desired so as to match the condition of the waste gas thereby maximizing the denitrating characteristics. The backing valves 25 and 35 enable to control correctly the discharge of the pumps 23 and 33.

With the apparatus described, high tempera ture waste gas including NO, can effectively and reliably be denitrated by diluted H202 solution of desired density being diluted automatically and reliably, and which solution is sprayed into the stream of the waste gas uniformly making a mist of fine grain H202 in a novel manner.

Claims (6)

1. A denitrating device of gas phase reduction type for high temperature waste gas including nitrogen compound in which ammonia is injected in the upstream of waste gas stream, and hydrogen peroxide is injected in the downstream, characterized in that a plurality of hydrogen peroxide injecting nozzles are arranged to oppose the gas- stream, and in a zigzag layout in the cross section of a duct of the waste gas stream for forming a substantially uniform fine mist of hydrogen peroxide in the duct and toward the upstream.

2. A denitrating device according to claim 1, wherein the hydrogen peroxide is conducted through an inner tube of a dual tube with cooling water being conducted through an outer tube of the dual tube.

3. A denitrating device according to claim 1 or claim 2, wherein the hydrogen peroxide is formed by providing a tank of undiluted solution of hydrogen peroxide, a tank of diluting water, and pumps of adjustable discharge being connected respectively to said tanks, with the discharge of said pumps being adjusted in accordance with a desired rate of dilution.

4. A denitrating device according to claim 3, wherein said pumps are started and stopped simultaneously.

5. A denitrating device according to claim 3 3 or claim 4, wherein a tank of diluted hydrogen peroxide is connected with outlets of said pumps, and another pump is provided to conduct said hydrogen peroxide from the tank of diluted hydrogen peroxide to said nozzles.

6. A denitrating device substantially as herein described and shown with reference to the drawings.