GB2246121A

GB2246121A - Desulphurization and denitration of furnace gases

Info

Publication number: GB2246121A
Application number: GB9115069A
Authority: GB
Inventors: Yoshimasa Miura; Etsuo Ogino; Michio Ito; Michio Ishida; Teruyuki Doi
Original assignee: Hitachi Zosen Corp; Hitachi Shipbuilding and Engineering Co Ltd
Current assignee: Kanadevia Corp
Priority date: 1990-07-13
Filing date: 1991-07-12
Publication date: 1992-01-22
Anticipated expiration: 2011-07-12
Also published as: IT1250274B; DE4123155A1; ITTO910547A1; GB2246121B; CN1061915A; GB9115069D0; KR930012041B1; KR920002210A; ITTO910547A0

Abstract

A process for simultaneously effecting desulfurization and denitration within a furnace by: supplying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate, and c) a powder or aqueous solution of urea and urea compound in one of the three modes of: i) applying one of the chemical agents to an upstream region within the furnace having a temperature of not higher than 1100 DEG C to not lower than 700 DEG C, an intermediate stream region within the furnace having a temperature of not higher than 900 DEG C to not lower than 500 DEG C and a region downstream from the outlet of the furnace and having a temperature of not higher than 500 DEG C to treat a gas in three steps, ii) applying one of the chemical agents singly and the other two chemical agents in mixture to two of the three regions to treat a gas in two steps, and iii) applying at least two of the chemical agents in mixture to one of the three regions to treat a gas in one step, and primarily effecting a denitration reaction in the upstream region and a desulfurization reaction and a second-step denitration reaction in the intermediate stream region and the downstream region in the case of the mode i) or ii), or simultaneously effecting a desulfurization reaction and a denitration reaction in the case of the mode iii). <IMAGE>

Description

1:2:2 -Q- C2, -1:2.1.

-I- TITLE OF THE INVENTION PROCESS FOR SIMULTANEOUSLY EFFECTING DESULFURIZATION AND DENITRATION WITHIN FURNACE

BACKGROUND OF THE INVENTION

The present invention relates to processes for simultaneously effecting desulfurization and denitration within furnaces for removing sulfur oxides (SOx) and nitrogen oxides (NOx) at the same time from combustion exhaust gases discharged from boilers, heating furnaces, refuse incinerators, etc.

DESCRIPTION OF THE PRIOR ART

The desulfurization and denitration processes generally employed in Japan at present inclue a denitration process which is practiced predominantly and in which ammonia is used as a reducing agent for selectively catalytically reducing NOx in the presence of a catalyst, and wet desulfurization processes such as the wet lime-gypsum process.

However, these processes require a large area for the installation of equipment and high initial and running costs, so that it is desired to provide processes which can be practiced at low costs by compact equipment.

On the other hand, the direct desulfurization process wherein limestone or like desulfurizing agent is placed directly into furnaces is considerably lower in initial cost and running cost, whereas the chemical' agent is utilized with not higher than one-fialf the efficiency achieved by the wet process to discharge unreacted CaO or like agent. For example, when the direct d&sulfurization process is employed for coalburning boilers, the fly ash discharged contains large quantities of CaSO 4 and CaO and must therefore be treated by a method which needs to be established.

An object of the present invention is to provide a process for simultaneously effecting desulfurization and denitration within furnaces which can be practiced at low costs and which nevertheles achieves high desulfurization and denitration efficiencies to meet the demand.

Another object of the present invention is to provide a process for capturing and recovering the unreacted ammonia released from the exhaust gas treating process or the ammonium sulfate or acidic ammonium sulfate resulting from the process for the reuse of the compound.

SUMMARY OF THE INVENTION

The present invention, which-has been accom- plished to fulfill the above objects, provides a first j i 1 1 i 1 1 1 1 process for simultaneously effecting desulfurization and denitration within a furnace by: supplying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate, and c) a powder or aqueous solution,of urea and urea compound in one of the three modes of:

i) applying one of the chemical agents to an upstream region within the furnace having a temperature of not higher than 1100 0 C to not lower than 7000 C, an intermediate stream region within the furnace having a temperature of not higher than 900 0 C to not lower than 5000 C and a region downstream from the outlet of the furance and having a temperature of not higher than 500 0 C to treat a gas in three steps, ii) applying one of the chemical agents singly and the other two chemical agents in mixture to two of the three regions to treat a gas in two steps, and iii) applying at least two of the chemical agents in mixture to one of the three regions to treat a gas in one step, and primarily effecting a denitration reaction in the upstream region and desulfurization reaction and a second-step denitration reaction in the intermediate stream region and the downstream region in the case of the mode i) or ii), or simultaneously effecting a desulfurization reaction and a denitration reaction in the case of the mode iii).

Preferably, the first process comprises the steps of: effecting a firststep denitration reaction and some desulfurization reaction by applying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, and b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate to the upstream region, effecting a desulfurization reaction and a secondstep denitration reaction by applying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate, and c) a powder or aqueous solution of urea and urea compound to the intermedi ate stream region, and effecting a second-step desulfurization reaction by applying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, and b) an aqueous solution of j 1 1 i i I ammonium sulfate and acidic ammonium sulfate to the downstream region.

The present invention provides a second process for simultaneously effecting desulfurization and denitra- tion within a furnace, the process comprising the steps of:

is treating an exhaust gas by supplying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate, and c) a powder or aqueous solution of urea and urea compound in one of the three modes of: i) applying one of the chemical agents t9 an upstream region within the furnace having a temperature of not higher than 1100 0 C to not lower than 700 0 C, a. n intermediate stream region within the furnace having a temperature of not higher than 900 0 C to not lower than 500 0 C and a region downstream from the outlet of the furnace and having a temperature of not higher than 500 0 C to treat the gas in three steps, ii) applying one of the chemical agents singly and the other two chemical agents in mixture to two of thb three regions to treat the gas'in two steps, and 1 -5 iii) applying at least two of the chemical agents in mixture to one of the three regions to treat the gas in one step, and primarily effecting a denitration reaction in the upstream region and a desulfurization reaction and a second-step denitration reaction in the intermediate stream region and the downstream region in the case of the mode i) or ii), or simultaneously effecting a desulfurization reaction and a denitration reaction in the case of the mode iii), and recovering the unreacted ammonia or the resulting ammonium sulfate or acidic ammonium sulfate discharged from the the exhaust gas treating step by an exhaust gas scrubber disposed in a flue downstream from the furnace.

Preferably, the second process comprises the steps of: treating the exhaust gas by: effecting a first-step denitration reaction and some desulfurization reaction by applying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, and b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate to the up-stream region, effecting a desulfurization reaction and a second- i 1 i 1 J 1 i i step denitration reaction by applying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate, and c) a powder or aqueous solution of urea and urea compound to the intermediate stream region, and effecting a second-step desulfurization reaction by applying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, and b) an aqueous solution of ammonium sufate and acidic ammonium sulfate to the downstream region, and recovering the unreacted ammonia or the resillting ammonium sulfate or acidic ammonium sulfate discharged from the the exhaust gas treating step by an exhaust gas scrubber disposed in a flue downstream from the furnace.

Further preferably, the second process comprises the ammonia recovery-gypsum crystallization step of reacting a slurry or powder of quick lime or slaked lime with the ammonia or aqueous solution of ammonium sulfate or acidic ammonium sulfate collected by the recovering step in a reactor-crystallizer to recover the ammonia in the form of a water vaporcontaining gas, and reacting the sulfate radical or the acidic sulfate radical with calcium ion to cause gypsum to separate out.

Preferably, the second process comprises the step of producing ammonia water by compressing and cooling the water vapor-containing ammonia gas recovered by the ammonia recovery-gypsum crystallization step.

Further preferably, the second process comprises the step of subjecting the gypsum slurry recovered by the ammonia recovery-gypsum crystallization step to solid-liquid separation to recover the gypsum as a solid fraction and ammonia water or unreacted aqueous solution of ammonium sulfate or acidic ammonium sulfate as a liquid fraction. 0 In practicing the second process, the treating chemical agent may be applied in the same manner as described for this process to a region having a temperature of not higher than 11000 C to not lower than 700 0 C and preceding the region within the furnace having a temperature of not higher than 9000 C to not lower than 500 0 C, and/or to a region downstream from the furnace outlet, having a temperature of not higher than 500 0 C and subsequent to the region having a temperature of not higher than 9000 C to not lower thdn 500 0 C. BRIEF DESCRIPTION OF THE DRAWINGS i i 1 1 1 1 i 1 i FIG. 1 is a flow chart showing a combustion test; FIG. 2 is a graph showing the relationship between the urea/S02 equivalent ratio and the desulfurization efficiency; FIG. 3 is a graph showing the relationship between the temperature and the denitration efficiency; FIG. 4 is a graph showing the relationship between the calculated NH 4 + value and the NH4 + value 10 found; FIG. 5 is a graph showing the relationship between the temperature and the desulfurization efficiency; PIG. 6 is a graph showing the relationship between the temperature and the denitration efficiency; FIG. 7 is a graph showing the relationship between the calculated NH 4 + value and the NH 4 + value found; FIG. 8 is a graph showing the relationship 20 between the amount of ammonia gas added and the desulfurization efficiency; FIG. 9 is a graph showing the relationship between the time and the reaction efficiency; and FIGS. 10 to 12 are flow charts showing 25 desulfurization-denitration processes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the present invention for simultaneously effecting desulfurization and denitration within furnaces comprises the following four steps. Step I Denitration and desulfurization within furnace (1) Step of applying a) ammonia gas or an aqueous solution thereof, and b) an aqueous solution of ammonium sulfate and/or acidic ammonium sulfate individually or in Mixture to an upstream region within a furnace having 0 a temperature of not higher than 1100 C to not lower than 900 0 C to effect a first-step denitration reaction and some desulfurization reaction. (2) Step of applying a) ammonia gas or an aqueous solutica thereof,-b) an aqueous solution of ammopium sulfate and/or acidic ammonia sulfate, and c) a powder or aqueous solution of urea and/or urea compound individually or in mixture to an intermediate stream region within 0 the furnace having a temperature of not higher than 900 C to not lower than 500 0 C to effect a desulfurization reaction and a second-step denitration reaction. (3) Step of applying a) ammonia gas or an aqueous solution thereof, and b) an aqueous solution of ammonium sulfate and/or acidic ammonium sulfate individually or in mixture into a flue downstream from-the outlet of the furnace and having a temperature of not higher than z 1 1 i 1 1 i i i 1 i 1 i i 1 1 i i 1 i i 500 0 C to effect a second-step desulfurization.

Step II Recovery of unreacted ammonia or vapor, and the resulting ammonium sulfate or acidic ammonium sulfate Step of recovering the ammonia gas or water vapor containing the gas, or gas or fume of ammonium sulfate or acidic ammonium sulfate discharged from Step I by a wet scrubber having water as an absorbing medium or other suitable absorbing device disposed in a flue downstream from a dust collector attached to a boiler or the like (or in a flue immediately preceding a smokestack when the dust collector is not provided).

Step III Reaction of quick lime or slaked lime with the compound recovered by Step II for recovery of ammonia and delivery of gypsum Reaction-crystallization step of feeding the ammonia water or aqueous solution of ammonium sulfate or acidic ammonium sulfate recovered by Step II to a reactor crystallizer, adding a powder or aqueous slurry of quick lime or slaked lime to the liquid to cause calcium ion to react with the sulfate radical or acidic sulfate radical forming the ammonium sulate or acidic ammonium sulfate and cause gypsum to separate out, and releasing the ammonia resulting from the reaction' in the form of a gas or vapor.

Step IV Recovery and recycling of ammonia, separation and recovery of gypsum, and recovery and recycling of aqueous solution of ammonium sulfate (1) Step of producing ammonia water by compressing and cooling the water vapor-containing ammonia gas recovered by Step III. (2) Step of subjecting the slurry of gypsum crystals from the reaction of Step III to solid-liquid separation by a centrifugal separator or the like to collect the crystalline gypsum as a product, and mixing the resulting liquid as an aqueous solution of ammonium sulfate with the aqueous solution of ammonia from the above step (1) to recycle and reuse the mixture as the desulfurizing- denitrating agent lor Step I.

According to the presernt invention, the foregoing steps are employed in different combinations in accordance with the desulfurization and denitration efficiencies to be achieved and depending on whether the by-product to be obtained is gypsum or aqueous solution of ammonium sulfate for other use. Examples of useful combinations are as follows.

(1) To absorb SO 2 from an exhaust gas and collect the compound as gypsum, and to achieve high desulfurization and denitrati6n efficiencies:

The combination of Step 1, (1) to (3) and Steps i 1 1 1 i f 1 1 1 1 1 1 1 II, III and IV.

The combination of Step 1, (1) and (2), or (2) and (3) and Steps II, III and IV.

(2) To obtain ammonium sulfate or acidic ammonium sulfate as the by-product for other use:

The combination of Step 1, (1) to (3), (1) and (2), or (2) and (3) and Step II. (In this case, the aqueous solution of ammonium sulfate or acidic ammonium sulfate recovered by Step II is immediately recycled to Step I for reuse, with a portion there of discharged from the system for other use.) Desulfurization and Denitration Tests The reactions involved in the steps of the present process were substantiated by a verti.cal combustion testing apparatus as shown in FIG. 1 and other glass testing device.

The apparatus for use in the tests chiefly comprises a pulverized coal burning combustion chamber 6, and a desulfurization-denitration reaction chamber 1 connected to the downstream side of the chamber 6. The apparatus has a maximum capacity to burn pulverized coal at a rate of 10 kg/hour and is adapted to control the cumbustion temperature by burning propane as an auxiliary fuel, control the amount of Nox to be produced and adjust the SO 2 concentration of the exhaust gas by injecting SO 2 gas. Propane only, a mixture of propane and pulverized coal or pulverized coal only was burned for testing. The combustion temperature was controlled to a specified level by controlling the supply of fuel and controlling the amount of air to be supplied for combustion.

The exhaust gas discharged from the reaction chamber is cooled by an air heater 4 and a gas cooler 5, passed through a bag filter 3 for the removal of dust and released to the atmosphere.

The reaction chamber 1 is formed by a stainless tube having an inside diameter of 330 mm, and a height of 4 m. Tubular electric heaters 2 are provided around the reaction chamber 1 for controlling the temperature of the combustion exhaust gas inside the chamber 1 to the specified level. Desulfurizing and denitrating agents are injected as entrained in an air stream into the reaction chamber 1 through a top inlet 11.

The 0 2' so 2 and NO x concentrations of the exhaust gas are measured by analyzers 7, 8 which are disposed at the outlet of the reaction chamber 1 and the outlet of the bag filter 3, respectively. In these analyzers 7, 8, the exhaust gas from which dust is removed by a sintered metal filter equipped with a self-cleaner is led to an infrared SO 2 analyzer and i 1 i i zirconia oxygen analyzer for the analysis of SO 2 and at the location concerned. NOx analysis is further conducted by a chemiluminescence NOx analyzer at the outlet of the bag filter 3, and the wet exhaust gas analysis method prescribed in JIS is resorted to for analyzing SOx, analyzing total S content absorbed by the absorbing solution and analyzing NH 4 concentration. The temperature of the portions shown in FIG. 1 is measured by thermocouple thermometers 9. The amount of fuel charged is measured from a reduction in the amount measured out by a device. The amount of propane fed is measured by a gas flowmeter. The amount of combustion air is measured by an orifice flowmeter. The amount of exhaust gas is measured by a vqnturi flowmeter 10. The theoretical amount of exhaust gas calculated from the amounts of fuel and air charged was well in match with the actual measurement with a difference of within several percent, indicating that the meters operated properly. so 2 gas for industrial use was added to the exhaust gas in an adjusted amount so as to give an SO 2 concentration of 500'ppm to the exhaust gas. (1) Desulfurization and denitration reactions of urea aqueous solution While urea undergoes a denitration reaction -is- as is already known, our experiments have revealed that urea also undergoes a desulfurization reaction.

It is thought that these reactions occur as represented by the following formulae. 1) Denitration reaction 2NO + (NH 2)2 CO + 1/20 2 - 2N 2 + Co 2 + 2H2_0 2) Desulfurization reaction (NH 2)2 CO + H 2 0 - NH 3 + Co 2 (2) NH 3 + so 2 + 1/20 2 + H 2 0 -1- (NH 4)HSO 4 (3) 2NH 3 + so 2 + 1/20 2 + H 2 0 - (NH 4)2 so 4 (4) The denitration reaction of the formula (1) is already known in literature, while the formulae (2) and (3) are based on our assumption. Although it still remains to be clarified whether urea produces ammonium sulfate or acidic ammonium sulfate through these reactions or whether urea directly reacts with SO, ammonium sulfate or acidic ammonium sulfate is undoubtedly produced as will be described below.

FIG. 2 shows the desulfurization efficiency achieved by injecting an atomized aqueous solution of urea into the reaction chamber 1 of the testing apparatus of FIG. 1 through the inlet 11. FIG. 3 shows the denitration efficiency then attained. In FIG. 2, the urea/So 2 mole equivalent ratio is plotted as abscissa vs. the desulfurization efficiency as ordinate, and -16 1:

i 1 i 1 i the temperature of combustion gas at the position of injection is taken as a parameter.

The graph shows that a higher desulfurization efficiency is achieved at a lower temperature, and that nearly 100% desulfurization can be achieved at a temperature of 750 0 C at an equivalent ratio of about 1.1.

The combustion exhaust gas conditions at this time are as follows.

Fuel Propane so 2 concentration: 500 ppm NOx concentration: 170 ppm-60 ppm Amount of exhaust gas: 90-100 Nm 3 /hr Concentration of urea aqueous solution: 25-50 ú/hr 1 Note The NOx concentration varies with the combustion temperature.

In FIG. 3, the temperature at the position of injection is plotted as abscissa vs. the denitration efficiency as ordinate.

Plotted in this graph is the data obtained when the urea/NOx mole equivalent ratio was fixed to 5. FIG. 3 reveals that a denitration efficiency of about 80% is achieved when the temperature at the injection position is not lower than 820 0 C, further showing the tendency that the denitration efficiency gradually decreases as the temperature lowers from this level. The denitration efficiency tends to decrease at temperatures over 1150 0 C although not shown.

Next, the exhaust gas passing through the sintered metal filter at the outlet of the bag filter 3 of FIG. 1 during the present test was washed with water by the method precribed in JIS, and the amount of sulfuric acid and the total amount of sulfur present in the washings as dissolved therein were determined respectively by volumetric analysis with 1/10N standard sodium hydroxide solution and by gravimetric analysis according to barium sulfate precipitation method. Further the ammonia concentration of the same washings was measured by an ammonia ion electrode analyzer. The amount of SO 4 (A) was calculated from the total amount of sulfur, and the amount of SO 4 (B) from the amount of sulfuric acid to calculate the amount of NH 4 + reactive with the amount (A-B). Incidentally, it had been ascertained before this procedure that the absorbing was free from any SO 3 and contained SO 4 only. FIG. 4 shows the correlation between the amount of NH 4 + thus calculated and the amount of NH 4 + measured by the ion electrode analyzer., 25 With reference to FIG. 4, spots are present on 1 1 1 i i i i 1 i 1 a line of gradient 1 which is obtained by calculation done on the assumption that the reaction product is ammonium sulfate, thus substantiating that the reaction product is ammonium sulfate. Further the area of FIG. 4 wherein the calculated NH 4 + values are small represents the test results obtained when the equivalent ratio of urea is low. In this case, the spots are present on a line with a gradient of 112 obtained by calculation on the assumption that the reaction product is acidic ammonium sulfate. This substantiates that the product is acidic ammonium sulfate. These results indicate the occurrence of the reactions of the formulae (2) to (4).

The reaction products such as ammonium sulfate are present in the exhaust gas passing through the bag filter 3 and the sintered metal filter. It is thought that the ammonium sulfate and like product of the present process are present in the form of a fume or gas. If these products were solid, a deposit of the reaction products would be observed on the fabric of the bag filter or on the surface of sintered filter, whereas noneof such deposit was found on the filter surfaces after the present test was conducted for a long period of time. Further when an aqueous solution of urea was added during the combustion of pulverized coal as a procedure of the present test, only traces is of NH 4 + were almost always found to be present in the fly ash trapped by the bag filter.

The foregoing results indicate that the substance produced by the desulfurization reaction of urea is ammonium sulfate or acidic ammonium sulfate, which is in the form of a gas or fume when present in 0 an atmosphere of at least 100 C and which can not be trapped by the bag filter or the like. (2) Desulfurization and denitration efficiencies of ammonium sulfate Ammonium sulfate has an SO 4 radical in its structure, is therefore likely to release SO 2 on decomposition when used as a desulfurizing and denitrat ing agent in furnaces and was not used as suqh generally However, our experiments have revealed that ammonium sulfate has desulfurizing and denitrating effects as will be described below.

1)Denitration reaction i) High-temperature region 2NO + (NH 4)2 so 4 - 2N 2 + so 2 + 4H 2 0 (5) ii) Medium-temperature region 2NO + (NH 4)2 so 4 + 1/20 2 _" 2N 2 + H 2 so 4 + 3H 2 0 (6) The reaction of the formula (5) occurs in a high-temperature region, apparently releasing SO 2 under this condition. The reaction of the formula (6) occurs i 1 1 1 1 1 i in a medium7temperature region, causing denitration but no desulfurization under this condition.

2) Desulfurization reaction (NH 4)2 so 4 + so 2 + 1/20 2 + H 2 0 + 2(NH 4)HSO 4 (7) In a region of relatively low temperature, even ammonium sulfate undergoes a desulfurization reac tion. It appears that desulfurization occurs when the ammonium sulfate, absorbing SO 2' is thereafter oxidized to acidic ammonium sulfate as represented by the formula (7).

The sulfuric acid produced by the reaction of the formula (6) presumably undergoes the reaction of the following formula (8) to become acidic ammonium sulfate.

(NH 4)2 so 4 + H 2 so 4 "' 2(NH 4)HSO 4 (8) To substantiate these reactions, an aqueous solution of ammonium sulfate having a concentration of 40 g/liters was supplied as atomized into the reaction chamber 1 of FIG. 1 through the inlet 11 under the same conditions as in the foregoing desulfurization and denitration test and checked for desulfurization and denitration characteristics.

FIG. 5 shows the relationship between the temperature and the desulfurization efficiency as determined when the mole equivalent ratio of the ammonium sulfate aqueous solution to the SO 2 in exhaust gas was set to about 1. In the graph, the exhaust gas temperature at the position of injection of the solution is plotted as abscissa vs. the desulfurization efficiency achieved in the furnace as ordinate. The negative efficiency values indicate that the decomposition of ammonium sulfate rele ased SO 2 FIG. 5 reveals that the aqueous solution of ammonium sulfate used as a desulfurizing and denitrating agent also caused some desulfurization reaction at a temperature of up to 800 0 C. This appears attributable to the reaction of the formula (7) as already stated.

In an atmosphere having a temperature of not lower than 900 0 C, release of So 2 due to the.reaction of the formula (5) was observed with a negative desulfurization efficiency. In a temperature region intermediate between the desulfurization reaction and the release of So 2' the reactions of the formulae (5) to (8) occur in an entangled mode, which appears responsible to great variations in data. Besides the desulfurization phenomena described, a denitration reaction was also observed.

FIG. 6 shows the denitration characteristics determined at this time.

FIG. 6 is similar to FIG. 5 except that the i i 1 i i 1 1 i j i i i denitration efficiency achieved within the furnace is plotted as ordinate.

FIG. 6 reveals that the denitration reaction attains a higher efficiency at a higher temperature, and that the efficiency tends to remain at a constant value of 60% at temperatures lower than about 800 0 C.

Further since release of SO 2 also occurs at the same time at temperatures of not lower than 8boo C, it is thought that the reaction is in accordance with the formula (5). At lower temperatures, the reaction presumably proceeds in accordance with the formula (6).

The denitration efficiency shown in the graph is based on the NOx produced when propane is burned without adding ammonium sulfate. For example, when the temperature of injection position is 1100 0 C, the amount of NOx produced is 170 ppm. If the temperature is 700 0 C, the value is 60 ppm. Thus, the NOx value providing the base differs at different temperatures. Next to identify the product of the desulfur- ization and dentration reactions conducted by injecting the aqueous solution of ammonium sulfate, the correlation between the calculated NH 4 + concentration value and the NH 4 + concentration value obtained by ion electrode analysis was determined by the same procedure as already described for the use of urea as shown in FIG. 4, which 1 manifestly indicates that the product is ammonium sulfate or acidic ammonium sulfate. Thus, it is apparent that the reactions of the formula (7) and (8) produce acidic ammonium sulfate in a larger amount than when urea is used.

The results described above indicate that the aqueous solution of ammonium sulafte undergoes desulfurization and denitration reactions depending on the reaction conditions to produce acidic ammonium sulfate.

The ammonium sulfate represented in FIG. 7 appears to be unreacted ammonium sulfate not partici pating in the reactions of the formulae (5) to (8) and _rapped when released from the system. I (3) Desulfurization and denitration efficiencies of ammonia It is well known that ammonia hasactivity to reduce NOx. When to be used as a denitrating agent, ammonia is used generally in combination with a catalyst so as to achieve an improved reaction efficiency.

It is also well known that when SO 2 gas is passed through ammonia water with an excess of air further passed therethrough, ammonia readily reacts with so 2 and 0 2 to form ammonium sulfate. - These reactions are represented by the follow- i i 1 1 j j ing formulae. 1) Denitration reaction 6NO + 4NH 2 - 5N 2 + 6H 2 0 (9) 2NH 3 + S02 + 1/20 2 + H 2 0 + (NH 4)2SO4 (10) (NH 4)2 so 4 + so 2 + 1/20 2 + H 2 0 -.1- 2(NH 4)HS04 (11) When injected into furnaces, ammonia serving as a desulfurizing and denitrating agent is expected to produce a considerable denitration effect although the reaction efficiency is lower than when a catalyst is used conjointly therewith.

FIG. 8 shows the denitration efficiency attained by ammonia gas introduced into the reaction chamber 1 shown in FIG. 1 through the inlet 11. In the graph, the conc?ntration of ammonia gas added to thq exhaust gas is plotted as abscissa vs. the denitration and desulfurization efficiencies achieved by the gas.

The conditions of the exhaust gas used for testing are as follows.

Fuel: Mixture of propane and pulverized coal Amount of combustion exhaust gas: 105 Nm 3 /hr Temperature of position of addition: 800 0 C so 2 concentration: 800 ppm NOx concentration: 200 ppm FIG. 8 indicates that ammonia-achieved a denitration efficiency of 70% when used in an amount of at least 600 ppm (NH 3 INO equivalent ratio = 3). The efficiency did not increase but levelled off if the amount was further increased.

However, no desulfurization reaction occurrc,_d at this time as seen in FIG. 8. Use of dry ammonia gas in the high- temperature region failed to produce any desulfurization effect. This is attributable to the fact that the reaction does not proceed since water required for the reaction of the formula (10) is absent and to the excessively high reaction temperature.

Apparently, the reaction of the formula (10) readily proceeds in the presence of a sufficient amount of water as already described. Accordingly, ammonia gas was made into ammonia water, which was test for desulfurization reaction in a low-temperature region of up to 500 0 C in a sufficiently wet state, i.e., semiwet state. The result is also shown in FIG. 8.

The graph shows that the addition of sufficient amount of water required for the reaction of the formula (10) readily permits ammonia to undergo the desulfurization reaction.

(4) Absorption of ammonium sulfate or acidic ammonium sulfate by water As already described, the desulfurization and denitration reactions conducted with use of urea, j i 1 i 1 i ammonium sulfate and ammonia water serving as desulfurizing and denitrating agents afford ammonium sulfate or acidic ammonium sulfate as a by-product, which is in the form of a fume or gas in the exhaust gas of 100 0 C.

These sulfates are highly soluble in water and can therefore be easily absorbed and trapped by a simple exhaust gas scrubber of the wet type using water as a medium.

When a test was conducted using a simple exhaust gas scrubber such as absorption bottles used for the foregoing wet analysis, NH 4 + was entirely trappeed in a firist absorption bottle, and little or no NH 4 + was detectable from a second absorption bottle, hence high absorbability. 1 (5) Crystallization of gypsum from ammonium sulfate or acidic ammonium sulfate We made investigations on a method of obtaining gypsum from ammonium sulfate or acidic ammonium sulfate as a by-product to recover ammonia.

Quick lime or slaked lime was placed into an aqueous solution of ammonium sulfate or acidic ammonium sulfate and checked for the resulting reactions.

Presumably, these reaction proceed as represented by the following formulae. ' 1) Quick lime prdouces slaked lime when placed into water.

CaO + H 2 0 - Ca (OH) 2 (12) 2) Slaked lime reacts with ammonium sulfate or acidic ammonium sulfate to cause gypsum to separate out.

(NH 4)HSO 4 + Ca(OH) 2 + H 2 0 CaSO 4" 2H 2 0 + NH 4 OH (13) (NH 4)2 so 4 + Ca(OH) 2; + 2H 2 0 CaSO 4 2H 2 0 + 2NH 4 OH (14) 3) When heated, ammonia water releases ammonia in the form of vapor or gas.

heating NH 4 OH 4 JO NH 3 + H 2 0 (15) cooling The gas is restored to ammonia water on cooling.

Formulae (12) and (15) are known reaction formulae, whereas it was necessary to substantiate the formulae (13) and (14).

To substantiate the reactions of the formulae (13) and (14), we conducted a reaction test using a simple glass testing device. The test was conducted by placing 500 ml of aqueous solution of ammonium sulfate into a 1000-ml flask, placing slaked lime into the flask, thereafter heating the solution for boiling and evaporation, cooling the resulting vapor by a Liebig condenser, and injecting the condensate into a dilute solution of sulfuric acid to cause the solution to absorb and react with NH 4 The absorption solution was replaced by a fresh dilue solution of sulfuric acid every 10 minutes, and this procedure was repeated 6 times i 1 I i i i 1 i 1 i i 1 i 1 1 1 C over a period of 60 minutes. The absorption solution thus obtained was analyzed by the ammonia ion electrode method to determine the amount of NH 4 + and check the progress of the reaction.

FIG. 9 shows the accumulated amount of NH 4 + thus released by the reaction with the lapse of time.

The test conditions for A and B shown in the graph are as follows.

Reaction conditions Amount of aqueous solution of ammonium sulfate (ml) Concentration of aqueous solu tion of ammonium sulfate (g/1) Amount of slaked lime added (g) Reaction temperature ( 0 C) Reaction time (min) Amount of 30% sulfuric acid solution for absorbing NH 4 + (ml) is A B 500 500 22 100-102' 100-102 60 50 The only difference between the conditions A 10 and B is the amount of slaked lime added.

FIG. 9 shows that slaked lime reacted with ammonium sulfate fairly easily. In the case of A, about 80% of the amount of NH 4 + in the ammonium sulfate used reacted during the period of 60 minutes. The correspond 15 ing amount in the case of B was 90%.

i The amount of NH 4 + remaining unreleased plus the amount of NH 4 + released was approximately equal to the amount of NH 4 + in the initial charge of ammonium sulfate, hence good balance.

The result indicates that the reaction of the formula (14) readily proceeds. It appears that the reaction is relevant to the amount of slaked lime used, i.e., the alkali concentration of the solution. The case B wherein 22 g of slaked lime was placed in was higher in reaction velocity than the case A wherein 15 g was used.

The reaction of the formula (13), although not substantiated, will presumably be further higher in reaction velocity than that of the formula (1A) theoretically. (6) Recovery of gypsum and ammonia The gypsum slurry resulting from the foregoing reaction was easily subjected to solid-liquid separa tion by an aspiration filtering test using filter paper. The filtrate was transparent, and no solid particles were found therein.

A large quantity of unreacted slake remains in the gypsum separated off. Further investigations need to be made in providing the gypsurri as a product by reducing the residual slaked lime content and 1 converting the remaining slaked lime to gypsum.

Ammonia gas can be easily recovered merely by cooling the vapor from the reaction to obtain a condensate as already described. The recovery loss can 5 be reduced to zero.without using any special device.

Although the filtrate contains unreacted ammonium sulfate, the filtrate can be admixed with the ammonia water obtained for reuse as the desulfurizing and denitrating agent mentioned in the section (1) above. (7) Summary of the test results

The results of the research will be summarized as follows. 1) Desulfurization and denitration reactions.in Step 1 i) The aqueous solution of urea exhibits excellent desulfurizing and denitrating effects and is expected to achieve 100% desulfurization at urea/SO 2 equivalent ratio of 1.1. These effects are influenced by the temperature at the position of injection of the solution. Denitration can be achieved with an efficiency of about 80% in a temperature region of not lower than 800 0 C. The reactions of the formula (1) to (4) are involved in this case.

ii) The aqueous solution of ammonium sulfate also has desulfurizing and denitrating effects, which 1 are greatly influenced by temperature. At temperatures of not lower than 800 0 C, an SO 2 re-releasing reaction occurs. The desulfurizing effect is in conflict with the denitration effect, such that an enhanced denitrat- ing effect leads to release of So 2' The reactions of the formulae (5) to (8) are involved in complex fashion in this case. The test results reveal that ammonium sulfate has an effect to achieve at least 60% denitra- tion.

iii) Ammonia gas attains an excellent denitration efficiency, e.g., 70% at 800 0 C at an ammonia/NOx equivalent ratio of 3. The reaction of the formula (9) is then involved.

When dry, ammonia fails to produce.any denitrating effect. When wet, however, ammonia exhibits a great denitrating effect through the reactions of the formula (10) and (11).

iv) The reaction by-product in the above cases i) to iii) is ammonium sulfate or acidic ammonium sulfate, which is present in the form of a fume or gas in the combustion exhaust gas after the reaction in the furnace. In an atmosphere having a temperature of at least 1000 C, the sulfate is not in the form of particles and can not therefore be capt:ured by a bag filter or like dust collector.

t 1 i 1 i i 1 i 1 Only traces of the sulfate is present in the fly ash resulting from the combustion of pulverized coal, and the dust collector fails to capture the sulfate. 2) Absorption of ammonium sulfate or acidic ammonium sulfate in Step II The product of Step I, i.e. ammonium sulfate or acidic ammonium sulfate, is highly soluble in Water and can be wholly recovered by a simple scrubber.

3) Gypsum crystallization reaction and recovery of ammonia in Step III Ammonium sulfate or acidic ammonium sulfate readily reacts with slaked lime, causing gypsum to separate out and releasing ammonia through the reactions of the formula (12) to (15).

The gypsum crystals separating out can be effectively filtered off and thus readily separated from the liquid. The filtrate is free from gypsum and transparent.

The ammonia gas released can be easily recovered by condensation on cooling. 4) The ammonia resulting from Step IV and the ammonia obtained from Step III of recovering and recycling the aqueous solution of ammonium sulfate can be admixed, 25 in the form of ammonia water, with the filtrate of ammonium sulfate aqueous solution for use as the desulfurizing and denitrating agent in Step I. EXAMPLES Examples of the invention will be described below in greater detail with reference to the drawings concerned. Example 1 Based on the results of the foregoing tests, a-process is practiced which comprises injecting urea, ammonium sulfate, acidic ammonium sulfate and ammonia singly or in mixture into a furnace as uniformly diffused therethrough to effect desulfurization and denitration reactions within the furance, recovering a gas or fume of ammonium sulfate or acidic ammonium sulfate as the by-product from the combustion exhaust gas by a scrubber, further adding quick lime or slaked lime to the resulting aqueous solution of by-product to effect a gypsum crystallization reaction, recovering gypsum and reusing the ammonium sulfate contained in the resulting filtrate and the ammonia produced by the reaction as desulfurizing and denitrating agents.

This process consumes the desulfurizing and denitrating agents in considerably reduced amounts and makes it possible to eventually recover'SO 2 in the form of gypsum.

i 1 i i j i i FIG. 10 shows an exemplary process having the above features and developed by the present inventors.

The flow chart of FIG. 10 will be described in detail. Pulverized coal is supplied as a fuel to a burner 22 for a boiler body 21. The combustion exhaust gas produced is passed through a group of boiler tubes 23 and an economizer 24, thereby fully deprived of heat, led into a dust collector 25 for the removal of ash, then passed through an exhaust gas scrubber 26, i.e., an absorption tower, guided into a smokestack 28 by an induced draft fan 27 and thereafter discharged from the apparatus.

To practice the present process, a mixture of ammonia water and aqueous solution of ammoniqm sulfate or acidic ammonium sulfate is uniformly diffused as atomized into an upstream region within the furance having a temperature of not higher than 1100 0 C to not lower than 700 0 C from a nozzle 40 installed in the boiler body 21 at a portion thereof having a relatively high temperature to primarily effect a first-step denitration reaction and an extent of desulfurization reaction. An aqueous solution of urea is uniformly diffused into an intermediate stream region within the furnace having a temperature of not: higher than 900 0 C to not lower than 500 0 C from a nozzle 41 installed 4 i in the boiler body 21 at a portion thereof having a relatively low temperature to primarily effect desulfurization and a second-step denitration reaction. The ammonium sulfate or acidic ammonium sulfate produced as a by-product by these desulfurization and denitration reactions within the furnace is present in the form of a fume or gas in the hot combustion exhaust gas, therefore passed through the group of boiler tubes 23, economizer 24 and dust collector 25 and collected by the scrubber 26 in the form of an aqueous solution.

If the white fume present in the exhaust gas poses a problem, a heater is disposed downstream from the gas scrubber 26.

Process water is supplied to the sQrubber as a cooling water replenishment to compensate for evaporation. The water is sprayed into the scrubber by a sprinkler provided at an upper level to inhibit a mist of absorption liquid. The fume or gas of ammonium sulfate or the like is absorbed by the absorption liquid which is recycled by a recycling pump 29 and sprayed into the tower through nozzles at a lower level.

The absorption liquid, i.e., aqueous solution of ammonium sulfate or acidic ammonium sulfate, is partly introduced into a reactor-crystallizer 30. Quick lime or slaked lime is supplied to the crystallizer 30 and thoroughly 1 1 1 i j i i i i mixed with the solution by the impeller 32 of a stirrer 31. Inside the reactor-crystallizer 30, the sulfate reacts with slaked lime, causing gypsum to separate out. To accelerate the reaction inside the crystallizer 30, it is desirable to heat the mixture and therefore it is necessary to inject steam into the unit or provide a heater for this purpose.

The gypsum r esulting from the reaction is then separated off by a solidliquid separator 33 and dis- charged from the system. The filtrate from the separator 33 is pressurized by a pump 34 and thereafter recycled for use.

On the other hand, the water vapor-containing ammonia gas produced by the reaction is pressurized by a compressor 35, then passed through a cooling condenser 36 and made into ammonia water, which is stored in a reservoir 37. The ammonia water is further pressurized by a pump 38 and admixed with the filtrate delivered from the separator 33. The mixture is uniformly diffused into the furnace through the nozzle 40 or 41 and used again as a desulfurizing and denitrating agent. When required, the mixture is supplied as uniformly diffused into a region downstream from the outlet of the furnace and having a temperature of up to 500 0 C through a nozzle 44 downstream from the economizer 24 and is subjected to a second-step desulfurization reaction.

The urea to be used primarily for desulfurization is dissolved in process water within a solution preparing container 42 equipped with a stirrer, and the solution is pressurized by a pump 43 and uniformly diffused into the furnace via the nozzle 41 to effect the above-mentioned desulfurization and second-step denitration reactions within the furnace.

The mixture of recycled ammonia water and aqueous solution of ammonium sulfate to be supplied to the nozzle 40 is partly admixed with the urea solution to reduce the consumption of urea. Since the degrees of desulfurization and denitration vary with the proportions of the portions of the recycled mixture to be fed to the nozzles 40 and 41, the proportions must be determined optimally.

As a specific example of application, the desulfurization and denitration efficiencies achieved, amounts of chemical agents used, etc. will be described with reference to FIG. 10.

The values in parentheses show the material balance at the locationsconcerned. From the particulars given below, it will be apparent that the present process achieved a desulfurization efficiency of 97.5% and a denitration efficiency of 70%, produced gypsum at a i 1 i i i 1 1 1 j 1 1 1 1 1 rate of 1297 kg/hr and attained desulfurization and denitration effectively within the furnace. (1) Specifications of exhaust gas produced by combustion

0 2 of coal Kind of coal: Australian coal Rate of combustion of coal: 21.57 tons/hr Rate of production of exhaust gas: 212,000 Composition of exhau st gas CO 2 14.5 vol.

3.3 vol.

H 2 0 8.4 vol.

so 2 800 Nox 200 (2) Sp.cifications gas (at inlet of smokestack) so 2 20 ppm NOx 60 ppm (3) Specifications of agents for first-step desulfuriza tion and denitration (of solution diffused from nozzle 40) Rate of supply of solution: 1.2 m 3 /hr Composition of solution Ammonium sulfate: 3.5 wt. % Ammonia water: 14.8 wt. % Water: 81.7 wt. % PPM ppm (after Nox-inhibited combustion) of desulfurized and denitrated exhaust (4) Specifications of agents for second-step desulfuri zation and denitration (of solution diffused from nozzle 41) Rate of supply of solution: 2.5 m 3 /hr Composition of solution Urea: 7.0 wt. % (192 kg/hr) Ammonium sulfate: 2.8 wt. % Ammonia water: 11.6 wt. % Water: 81.7 wt. % (5) Rate of supply of quick lime: 434 kg/hr (6) Rate of supply of steam: About 1000 kg/hr (7) Supply of absorption liquid to reactor-crystallizer Rate of supply: 5.2 m 3 Ar Composition of solution Ammonium sulfate: 30.8 wt. % 69.3 wt. % Water:

4 (8) Rate of production of gypsum: 1297 kg/hr (containing 2.2% of slaked lime) Example 2 FIG. 11 shows another exemplary process embodying the present invention. The flow chart of the drawing will be described in detail. Pulverized coal is supplied as a fuel to a burner 52 for a boiler body 51. The combustion exhaust gas produced is assed through a 25 group of boiler tubes 53 and an economizer 54, thereby i j j 1 i i i j i 1 fully deprived of heat, led into a dust collector 55 for the removal of ash, then passed through an exhaust gas scrubber 56, i.e., an absorption tower, guided into a smokestack 58 by an induction draft fan 57 and there- after discharged from the apparatus. The ash trapped by the economizer 54 and the dust collector 55 is fly ash containing anhydrous gypsum.

In the exhaust gas flow of the present process, a powder of calcium carbonate is supplied as entrained in 10 air from a storage tank 64 and uniformly diffused as atomized into an upstream region within the furnace having a temperature of not higher than 1100 0 C to not lower than 700 0 C from a nozzle 61 installed in the boiler body 51 at a portion thereof having a,relatively is high temperature to primarily effect a first-step denitration reaction and an extent of desulfurization reaction. An aqueous solution of urea is uniformly diffused into an intermediate stream region within the furnace having a temperature of not higher than 9000 C to not lower than 500 0 C from a nozzle 61 installed in the boiler body 51 at a portion thereof having a relatively low temperature to primarily effect desulfurization reaction and a second-step denitration reaction. The urea aqueous solution, which comprises urea singly, is prepared in a solution preparing container 62 having a. stirrer 69 and pressurized by a pump 63. The ammonium sulfate or acidic ammonium sulfate formed as a byproduct by these desulfurizati.on and denitration reactions within the furnace is present -in the form of a fume or gas in the hot combustion exhaust gas, therefore passed through the group of boiler tubes 53, economizer 54 and dust collector 55 and collected by the scrubber 56 in the form of an aqueous solution.

If the white fume present in the exhaust gas poses a problem, a heater is disposed downstream from the gas scrubber 56.

Process water is supplied to the scrubber 56 as a cooling water replenishment to compensate for evaporation. The water is sprayed into the t;ower by a sprinkler provided at an upper level to inhibit a mist of absorption liquid. The fume or gas of ammonium sulfate or the like is absorbed by the absorption liquid which is recycled by a recycling pump 59 and sprayed into the tower through nozzles at a lower level. The absorption liquid, i.e., aqueous solution of ammonium sulfate or acidic ammonium sulfate, is partly sent by a pump 65 to the nozzle 60 or to a nozzle 66 disposed above the nozzle 60, from which the solution is uniformly diffused into the upstream region within the furnace having a temperature of not higher than 1100 0 C 1 i j i i 1 i i i 1 i 1 j i i to not lower than 700 0 C. Further when required, the solution is supplied as uniformly diffused into a region downstream from the outlet of the furnace and having a temperature of up to 500 0 C through a nozzle 74 downstream from the economizer 54 and is subjected to a second-step desulfurization reaction. Example 1 2 FIG. 12 shows another exemplary process embodying the present invention. The flow chart of the drawing will be described in detail. Pulverized coal is supplied as a fuel to a burner 82 for a boiler body 81. The combustion exhaust gas produced is passed through a group of boiler tubes 83 and an economizer 84, thereby fully deprived of heat, led into a dust collector 85 for the removal of ash, then passed through an exhaust gas scrubber 86, i.e., an absorption tower, guided into a smokestack 88 by an induced draft fan 87 and thereafter discharged from the apparatus. The fly ash collected by the economizer 84 and the dust collector 85 is sent to the gypsum reactor 95 to be described below.

In the exhaust gas flow of the present process, a powder of calcium carbonate is supplied as entrained in air stream from a storage tank 94 and uniformly diffused as atomized into an upstream region within the furnace having a temperature of not higher than 1100 0 C to not lower than 7000 C from a nozzle 90 installed in the boiler body 81 at a portion thereof having a relatively high temperature to primarily effect a first-step denitration reaction and an extent of desulfurization reaction. An aqueous solution of urea is uniformly diffused into an intermediate stream region within the furnace having a temperature of not higher than 900 0 C 0 to not lower than 500 C from a nozzle 91 installed in the boiler body 81 at a portion thereof having a relatively low temperature to primarily effect desulfurization reaction and a second-step denitration reaction. The urea aqueous solution is a solution comprising urea singly, prepared in a solution preparing container 92 equipped with a stirrer 99 and pressurized by a pump 93, or is a mixture of this solution and the solution to be applied to the furnace via the nozzle 116 to be mentioned below. The ammonium sulfate or acidic ammonium sulfate formed as a by-product by these desulfurization and denitration reactions within the furnace is present in the form of a fume or gas in the hot combustion exhaust gas, therefore passed through the group of boiler tubes 83, economizer 84 and dust collector 85 and collected by the scrubber 86 in the form of an aqueous solution.

If the white fume present in the exhaust gas poses a problem, a heater is diposed downstream from the 1 t 1 1 i 1 1 1 1 i i i 1 1 i 1 1 i i i gas scrubber 86.

Process water is supplied to the scrubber 86 as a cooling water replenishment to compensate for evaporation. The water is sprayed into the tower by a sprinkler provided at an upper level to inhibit a mist of absorption liquid. The fume or gas of ammonium sulfate or the like is absorbed by the absorption liquid which is recycled by a recycling pump 89 and sprayed into the tower through nozzles at a lower level. The absorption liquid, i.e., aqueous solution of ammonium sulfate or acidic ammonium sulfate, is partly sent to the gypsum reactor 95 equipped with a stirrer 101, in which the solution is reacted with the collected fly ash. The fly ash containing the gypsum dihy4rate separating out through the reaction is separated off by a solid-liquid separator 103 and discharged from the apparatus. The filtrate sent out from the separator 103 is led into a storage tank 96, then pressurized by a pump 104 and thereafter recycled for use.

On the other hand, the water vapor-containing ammonia gas produced by the reaction is passed through a cooling condenser 106 and made into ammonia water, which is stored in a reservoir 107. The ammonia water is further pressurized by a pump 108 and admixed with 25 the filtrate delivered from the separator 103. The -4 5- mixture is sent to the nozzle 116 above the nozzle 90, from which the mixture is supplied as uniformly diffused into the upstream region within the furnace having a temperature of not higher than 1100 0 C to not lower than 700 0 C and is reused as a desulfurizing and denitrating agent. When required, the mixture is supplied as uniformly'diffused into a region downstream from the out- 0 let of the furnace and having a temperature of up to 500 C through a nozzle 114 downstream from the economizer 84 10 and is used for a second- step desulfurization reaction.

The process of the present invention has the features described above and can therefore be practiced at reduced costs to simultaneously effect desulfurization and denitration in furnaces with high eficiencies. Moreover, the unreacted ammonia released from the exhaust gas treating process or the ammonium sulfate or acidic ammonium sulfate resulting from the process can be trapped and recovered for reuse.

I i 1 J i 1 1 i 1 1 i f i 1 i i 1 1 1 1 1 1 i 1 - i i i 1 i

Claims

What is claimed is:

1. A process for simultaneously effecting desulfurization and denitration within a furnace by: supplying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate, and c) a powder or aqueous solution of urea and urea compound in one of the three modes of:

i) applying one of the chemical agents to an upstream region within the furnace having a temperature of not higher than 1100 0 C to not lower than 700 0 C, an intermediate stream region within the furnace having a temperature of not higher than 9,00 0 C to not lower than 500 0 C and a region downstream from the outlet of the furance and having a temperature of not higher than 500 0 C to treat a gas in three steps, ii) applying one of the chemical agents singly and the other two chemical agents in mixture to two of the three regions to treat a gas in two steps, and i) applying at least two of the chemical agents in mixture to one of the three regions to treat a gas in one step, and primarily effecting a denitration reaction in the ii - 7 0 upstream region and a desulfurization reaction and a second-step denitration reaction in the intermediate stream region and the downstream region in the case of the mode i) or ii), or simultaneously effecting a desulfurization reaction and a denitration reaction in 30 the case of the mode iii).

2. A process as defined in claim 1 which comprises the steps Of: effecting a first-step denitration reaction and some desulfurization reaction by applying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, and b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate to the upstream region, effecting a desulfurization reaction and a secondstep denitration reaction by applying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate, and c) a powder or aqueous solution of urea and urea compound to the intermediate stream region, and effecting a second-step desulfurization reaction by applying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous 1 1 M - solution thereof, and b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate to the downstream region.

3. A process for simultaneously effecting desulfurization and denitration within a furnace, the process comprising the steps of:

treating an exhaust gas by supplying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, b) an aqueous solution of ammonium sulfate and acidic ammonium sulf ate, and c) a powder or aqueous solution of urea and urea compound in one of the three modes of:

i) applying one of the chemical agents tg an upstream region within the furnace having a temperature of not higher than 1100 0 C to not lower than 700 0 C, an intermediate stream region within the furnace having a temperature of not higher than 900 0 C to not lower than 500 0 C and a region downstream from the outlet of the furnace and having a temperature of not higher than 500 0 C to treat the gas in three steps, ii) applying one of the chemical agents singly and the other two chemical agents in mixture to two of the three regions to treat the gas in two steps, and C.

1 TID iii) applying at least two of the chemical agents in mixture to one of the three regions to treat the gas in one step, and primarily effecting a denitration reaction in the upstream region and a desulfurization reaction and a second-step denitration reaction in the intermediate stream region and the downstream region in the case of the mode i) or ii), or simultaneously effecting a desulfurization reaction and a denitration reaction in the case of the mode iii), and recovering the unreacted ammonia or the resulting ammonium sulfate or acidic ammonium sulfate discharged from the the exhaust gas treating step by an exhaust gas scrubber disposed in a flue downstre,m from the furnace.

4. A process as defined in claim 3 which compiises the steps of: treating the exhaust gas by: effecting a -first- step denitration reaction and some desulfurization reaction by applying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, and b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate to the upstream region, effecting a desulfurization reaction and a second- 11 1 1 1 1 i i i i 1 i 1 step denitration reaction by applying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, b) an aqueous solution of ammonium sulfate and acidic ammonium sulfate, and c) a powder or aqueous solution of urea and urea compound to the intermediate stream region, and effecting a second-step desulfurization reaction by applying at least one chemical agent selected from the group consisting of a) ammonia gas or an aqueous solution thereof, and b) an aqueous solution of ammonium sufate and acidic ammonium sulfate to the downstream region, and recovering the unreacted ammonia or the resulting ammonium sulfate or acidic ammonium sulfate discharged from the the exhaust gas treating step by an exhaust gas scrubber disposed in a flue downstream from the furnace.

5. A process as defined in claim 3 or 4 which comprises the ammonia recovery-gypsum crystallization step of reacting a slurry or powder of quick lime or slaked lime with the ammonia or aquoeus solution of ammonium sulfate or acidic ammonium sulfate collected by the recovering step in a reactor-crystallizer to recover the ammonia in the form of a water vapor5;- - containing gas, and reacting the sulfate radical or the acidic sulfate radical with calcium ion to cause gypsum to separate out.

6. A process as defined in claim 5 which comprises the step of producing ammonia water by compressing and cooling the water vapor-containing ammonia gas recovered by the ammonia recovery-gypsum crystal- lization step.

7. A process as defined in claim 5 which comprises the step of subjecting the gypsum slurry recovered by the ammonia recovery-gypsum crystallization step to solid-liquid separation to recover the gypsum as a solid fraction and ammonia water or unreacted aqueous solution of ammonium sulfate or acidic ammonium sulfate as a liquid fraction.

8. A process substantially as hereinbefore described with reference to Figs. 1 to 10, Figs. 1 to 9 and 11, or Figs. 1 to 9 and 12 of the accompanying drawings.

9. Any novel subject matter or combination including novel subject matter disclosed in the foregoing specification or claims and/or shown in the drawings, whether or not within the scope of or relating to the same invention as any of the preceding claims.

Published 1992 at The Patent Office. Concept House. Cardiff Road. Newport. Gwent NP9 I RH. Further copies may be obtained front Sales Branch. Unit 6. Nine Mile Point. Cwmifelinfach. Cross Keys, Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Marv Cray. Kent.

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