CA2391710C

CA2391710C - Method and device for catalytically treating exhaust gas containing dust and oxygen

Info

Publication number: CA2391710C
Application number: CA002391710A
Authority: CA
Inventors: Gurudas Samant; Gerd Sauter
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-03-10
Filing date: 2001-03-08
Publication date: 2007-05-22
Anticipated expiration: 2021-03-08
Also published as: EP1244511A1; WO2001066233A1; CA2391710A1; AU5217001A

Abstract

The invention relates to a method and a device for catalytically treating exhaust gas containing sulphur oxide and nitrogen oxides, dust and oxygen and emanating from chemical processes or the combustion of fossil secondary fuels or fossil fuels from power plants, such as waste or slurry, or from glass and cement works.
The exhaust gas is freed from sulphur oxides and nitrogen oxides in a reactor having a solid catalyst and at a temperature ranging from 200 °C to 500 °C and in the presence of and/or by adding one or more mediums selected from free oxides, carbonates and hydroxides of calcium, magnesium, sodium and potassium.

Description

= , METHOD AND DEVICE FOR CATALYTICALLY TREATING
EXHAUST GAS CONTAINING DUST AND OXYGEN
Description In chemical processes or during the combustion of fossil or secondary fuels -such as garbage or processed garbage - exhaust gases are produced, which apart from other pollutants also contain sulfur oxides and nitrogen oxides. Among experts, the sulfur oxides (SO2 and S03) are referred to as SOX, and the nitrogen oxides (NO, N02 and N20) are referred to as NOX. Sulfur oxides and nitrogen oxides are gase-ous pollutants which act as toxins to the environment and must therefore be re-moved from the exhaust gases, before the same get into the atmosphere. In the preceding years, considerable efforts were made to reduce the emissions of sulfur oxides and nitrogen oxides. In connection with the denitrification of exhaust gases several processes are being employed. The process most frequently used at pre-sent is the SCR process (SCR = Selective Catalytic Reduction). In this process, ammonia or ammonium-containing compounds are introduced into the catalyst-containing reaction chamber, and the nitrogen oxides in the flue gas are reacted to obtain nitrogen and steam. In connection with the SCR process it is reported that in the case of S02-containing exhaust gases sulfuric acid and ammonium hydro-gen sulfate are formed. The formation of sulfuric acid and ammonium hydrogen sulfate is undesired, as in those parts of the plant which are disposed behind the reactor they lead to considerable corrosion problems. In the case of S02-containing exhaust gases, separate desulfurization plants are therefore generally provided before the SCR process, which desulfurization plants operate according to the principle of the dry or wet flue gas desulfurization plant (REA). In the case of wet processes, the exhaust gas is cooled and reheated for the subsequent SCR
process, which is the case in most power plants and garbage incineration plants.
Such processes involve high costs, and the formation of CaS03 corresponding to the reaction CaO + SO2 > CaSO3 cannot be avoided. The presence of CaS03 in landfill materials is a hazard to the environment.

EP-A-0 671 201 describes a process for separating sulfur trioxide and for denitrifi-cation - in particular in garbage incineration plarits - ammonia or ammonium-containing compounds being introduced into the flue gas stream before a heat ex-changer package, preferably before the last heat exchanger package, or before the flue gas cleaning, so that the catalytic denitrification of the dedusted flue gases is then effected in the low-temperature range, in particular between 100 C and 280 C. The object is to reduce the SO3 concentration before the SCR reactor by forming ammonium sulfate. The disadvantage of this process consists in that not only ammonium sulfate aerosols are formed, but also ammonium hydrogen sul-fate, which later on is precipitated on the catalysts. The ammonium sulfate aero-sols can hardly be dedusted in succeeding filter means, so that they represent a considerable burden to the environment. Moreover, a separate gas washer is nec-essary for the removal of SO2. The flue gas must be reheated behind the gas washer, which is not achieved by heat exchange alone. Thus, an additional firing means, e.g. a surface burner with natural gas, is reiquired. Disadvantages include high investment and operating costs.

It is the object underlying the invention to develop a process for the simultaneous desulfurization and denitrification without the formation of ammonium sulfate or ammonium hydrogen sulfate, wherein NOx is decomposed to obtain N2 and N20.

This object is solved in that the treatment of the extiaust gases containing sulfur oxides and nitrogen oxides is performed by a process, at temperatures in the range from 200 C to 500 C by means of reducing agents in a reactor which is equipped with solid catalyst with flow passages, in which the free opening surface of the catalyst is more than 50% and in which the passages of the catalyst have a hydraulic diameter of more than 2mrn, wherein:
- the treatment is performed in the piresence of and/or with the addition of one or more substances selected from the group including free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium, - said substances being present in said exhaust gases prior to contact of said exhaust gases with said catalyst; and 2a - that during the treatment the operating conditions of the gas flow in the free reaction space corresponding to the Froude numbers lie in the range of I :53 / 4 = p = 9 < 100 with 2 Fr' g=dk Pk Pg g=dk in which:

p = the relative gas speed in m/s Fr = the Froude number p9 = the density of the gas in kg/m3 Pk = the density of the solid particle in kg/m3 dk = the diameter of the spherical dust particle in m g = the gravitational constant in m/s2 Surprisingly, it was noted that despite the approximately stoichiometric operation of the NH3/NOX ratios, a degree of denitrification of 95 % to 98 % and a degree of desulfurization of 80 % to 90 % can be achieved with the inventive process, the formation of ammonium sulfate, ammonium hydrogen sulfate and sulfuric acid be-ing avoided. This advantage is based on the fact that in the catalytic treatment not only NOX is converted to nitrogen and steam, but also SO2 is converted to SO3 and incorporated in the presence of free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium. The formation of ammonium sulfates, ammo-nium bisulfates and sulfuric acid is suppressed. These incorporated sulfates of calcium, magnesium, sodium and potassium can very easily be separated and utilized in a succeeding filter plant, e.g. a bag collector or electrostatic precipitator.
A preferred aspect of the invention is the use of honeycomb or plate catalysts, which apart from titanium dioxide and tungsten contain more than 0.5 wt-% vana-dium pentoxide. The catalytic conversion is increased. In accordance with a par-ticularly preferred aspect of the invention, the catalysts preferably contain 2 % to 8 % vanadium pentoxide. With this operation, degrees of denitrification and desulfu-rization of more than 95 % are achieved.

Another preferred aspect is the treatment in the presence of and/or with the addi-tion of one or more substances selected from the group including free oxides, car-bonates, hydroxides of calcium, magnesium, sodium and potassium, with an aver-age particle size d50 between 5 pm and 100 pm. The removal of the sulfur oxides is effected very quickly with little consumption of additives.

Furthermore, costs are preferably minimized by the treatment in the presence of and/or with the addition of one or more substances selected from the group in-cluding free oxides, carbonates, hydroxides of calcium, as calcium compounds are more economic as compared to alkali compounds.

As reducing agent, NH3-releasing compounds such as (NH4)2SO4, (NHa)2CO3, (NH4)HCO3, (COONH3)2H20, HCOONH4, NH3, NH4OH, H20-CO-NH2, NH2CN, Ca(CN)2, NaOCN, C2H4N4, C3H6N6 and NH3-containing waste waters from photo-chemical plants, singly or several of them, are introduced into the flue gas stream at several points in the gaseous, liquid or solid condition at temperatures in the range between 20000 and 1000 C before entering the catalytic reactor.

A preferred aspect consists in that as reducing agent NH3-releasing compounds in the form of dilute aqueous solutions are introduced into the flue gas stream, pref-erably at temperatures in the range between 300 C and 550 C. The partial steam pressure in the reaction space is increased and thus the improvement of the in-corporation of sulfur is achieved.

A particularly preferred aspect of the invention is the presence or addition of one or more substances selected from the group including free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium to the flue gas stream before the addition of NH3-releasing compounds. The formation of ammmonium hydrogen sulfate, ammonium sulfate and sulfuric acid is suppressed completely.
Flow to the reactor can be effected from above or from below. A particularly pre-ferred aspect of the invention consists in that the flow to the reactor equipped with the catalysts can alternately be effected from above and from below. By means of this alternate flow, the reactor can easily be kept clean of dust-laden exhaust gases, and blockage of the passages by dust can be avoided. Furthermore, the service life of the catalysts can be increased by alternating the flow to the reactor.
Another preferred aspect of the invention consists in that beside the breakdown of sulfur oxide and nitrogen oxide, the reactor equipped with catalyst can at the same time be used for the breakdown of halogen compounds, halogenated organic compounds, hydrocarbons and CO.

A preferred aspect of the invention consists in that the reactor equipped with cata-lyst can be used for the breakdown of sulfur oxides and nitrogen oxides in dust-laden exhaust gases in the chemical and metallurgical industries as well as in the cement and lime industries, in power plants and in garbage incineration plants in the process flow at temperatures in the range between 200 C and 500 C without additional preheating of the exhaust gas.

The object of the invention is also solved by means of an apparatus for the treatment of dust- and oxygen-containing exhaust gases of a cement factory, which exhaust gases in an exhaust gas strearrr contain sulfur oxides and nitrogen oxides, comprising:
- a reactor (19) equipped with a catalyst (20), said reactor (19) being disposed in the exhaust gas stream behind a cyclone heat exchanger (13) (and before a raw material grinder (21) and before a by-pass I to an evaporative cooler (22), - the treatment being performed in the presence of one or more substances selected from the group consisting of free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium, said substances being preserit in said exhaust gases prior to contact of said exhaust gases with said catalyst.

The drawings represent examples of the apparatus for performing above mentioned the process, which are explained in deta'il below. In the drawings:

Fig. 1 schematically shows an arrangement of the apparatus in the cement industry;

Fig. 2 schematically shows an apparatus for the cement industry;

Fig. 3 schematically shows an arrangement of the apparatus for power plants;

Fig. 4 schematically shows an apparatus for power plants.

Fig. 1 shows the arrangement of the inventive apparatus in a cement factory with rotary kiln 16 for the production of clinker. The SCR reactor 19 with catalyst mod-ules 20 and dust blowers 18 is arranged in flow direction behind the suspension-type cyclone heat exchanger 13 comprising the cyclones Zl to Z4 which are con-5a nected with each other. For metering Ni-l3-releaslrig uompounds, a plurality of, points A, B, C, D and E are provided at temperatures in the range from 300 C
to 1000 C. For metering ammonia, ammonia solution or urea solution, the metering points A, B and C are preferred. For metering NH3-containing waste water from phototechnical plants and other compounds of 'NH3, the metering points D and E
are preferably used. The calcium-containing raw meal 12 is charged between the cyclones Z1 and Z2. After the treatment in the SCR reactor 19, the exhaust gas is either supplied to the chimney 24 via the raw material grinder 21 and the dedust-ing means 23 in the case of a combined operation, or is supplied to the chimney 24 via the evaporative cooler 22 and the dedusting means 23 in the case of a di-rect operation.

Fig. 2 shows an apparatus with gas conduit from the bottom to the top, from the top to the bottom and alternately from below and from above.

For an alternate gas conduit from below and from above in operation, some addi-tional lines and flaps are provided, which are shown in Fig. 2. When alternately switching the exhaust gases from the bottom to the top, calcium-containing com-pounds and NH3-containing exhaust gas are introduced into the reactor 19 from below via line F and are withdrawn via line G. The flaps M1, M4, M6 and M8 re-main closed, and the flaps M2, M3, M5 and M7 remain open. In the case of a combined operation, the exhaust gas is then passed via the WT blower 25 and line H to the raw material grinder 21 and the dedusting means 23 to the chimney 24, or in the case of a direct operation via line I to the evaporative cooler 22 and the de-dusting means 23 to the chimney 24. The flaps M9 and M10 mutually act to block the combined operation or the direct operation. When alternately switching the gas conduit from above, calcium compounds and NH3-containing exhaust gas are in-troduced into the SCR reactor 19 from above via lines J and G behind the cyclone heat exchanger 13, and are discharged from below via lines F and K to the WT
blower 25. The flaps Ml, M8, M5, M3, M4 and M6 remain open, and the flaps M2 and M7 remain closed. In the case of a combined operation, the exhaust gas is then passed through the WT blower 25 via line H to the raw material grinder 21 and the dedusting means 23 to the chimney 24, or in the case of a direct operation via line I to the evaporative cooler 22 and the dedusting means 23 to the chimney 24.

In the case of an accident or shut-down of the SCR reactor 19, the addition of NH3-releasing compounds is stopped and discharged via a bypass, i.e. via line K
through the WT blower 25 either to the raw material grinder 21 or to the evapora-tive cooler 22. The flaps M2, M4, M6 remain open and the flaps M3, Ml, M8, M7 and M5 remain closed. The cold-air flap M11 is provided to control the exhaust gas temperature before the SCR reactor 19.

In the case of a design with gas conduit only from below, line J and the flaps Ml, M8 and M7 are superfluous and thus the apparatus is only provided with an SCR
reactor 19, bypass line K and the flaps M3, M4, M5, M6. In connection with space and cost savings, two individual flaps may be equipped with a switching flap.
In addition, the WT blower 25 may be installed shortly behind the cyclone heat ex-changer 13, depending on space requirements and design.

For instance, the SCR reactor 19 is provided with five catalyst layers with modules for the breakdown of SOX and NO,, and one catalyst layer with modules for the breakdown of hydrocarbons and carbon monoxide. Depending on the content of SO,, NOX, hydrocarbons and carbon monoxide, the number of catalyst layers may be changed. On the gas side, the catalyst elements or catalyst modules 20 are provided with a protection against wear or with antiwear grids made of hard metal or ceramics against the erosion of dust-laden exhaust gases. In the case of an alternate gas conduit from above and from below, a protection against wear of about 5-20 mm is mounted on both sides.

For cleaning the catalyst surface, dust blowers 18 are furthermore provided for each catalyst layer on the gas side. In the case of an alternate gas conduit in op-eration from above and from below, the dust blowers 18 are provided on both sides. Before entering the reactor 19, the air for the dust blowers 18 is heated to about 250 C.

Fig. 3 shows the arrangement of the inventive apparatus for power plants between boiler 27 and air preheater 26. Additives 28, e.g. Ca(OH)2, are added behind the boiler 27 and before the NHOH dosage 29.

Fig. 4 shows the gas conduit from below or from above or in an alternate operation from below and from above analogous to the description given in Fig. 2 for cement factories. In power plants, as compared to cement factories, the exhaust gas is supplied behind the SCR reactor 19 via the air preheater 26 and the dedusting means 23 to the chimney 24.

The process in accordance with the invention will be explained below with refer-ence to embodiments.

In a cement factory with an exhaust gas volume of 100000 m3N.tr/h a system is in-stalled as it is shown in Fig. 2. Experiments are made with partial gas streams of 3000-10000 m3N.tr. Before being introduced into the reactor, the raw gas has the following composition:

NOX content (calculated as NO2) = 1500 mg/m3N,tr SOz content = 500 mg/m3N.tr dust content = 8000 mg/m3N.t, O2 content = 3.2 vol-%
Temperature in the reactor = 320 C

The density of the gas is calculated with reference to the gas composition.
The dust content before entrance into the reactor (chiefly CaO and Ca(OH)2) is mg/m3N.tr. The determined particle density of the dust is about 3.1 kg/m3.
With ref-erence to these data and operating conditions, a gas speed of 6.5 m/s is deter-mined corresponding to the Froude numbers.

In the experiments, there were used honeycomb catalysts with different contents of active components and with the following specifications:

free opening surface = 85 %
pitch = 11 mm clear width of the passages = 10 mm wall thickness = 1 mm.

The content of active component (e.g. V205) in the catalysts is 0.1 %, 0.3 %, 1%, 3 % and 5 %. As reducing agent, gaseous NH3 with a stoichiometry, i.e. a molar ratio NH3/NO, of 0.85 is added before entry in the reactor.

For the experiments, a steel grid made of stainless steel is mounted on the module of the catalyst as a protection against wear before entry of the dust-laden exhaust gas.

The gas components NOX, SOX, NH3, CO, CO2 and H20 are measured continu-ously before and behind the reactor by means of a multicomponent analyzer MCS-100.

The breakdown of the most important components in dependence on the content of active component V205 is represented in the following Table:

Active component of Breakdown of Breakdown of Breakdown of the catalyst NOX SO, hydrocarbons V205 content 0.1 % 34 3 10 V205 content 0.3 % 42 5 15 V2O5 content 1.0 % 56 22 30 V2O5 content 3.0 % 75 70 55 V2O5 content 5.0 % 95 90 70 The results show that with the inventive process NOX and SOX are decomposed when suitable operating conditions - gas flow and selection of the active compo-nents - are adjusted.

In the experiments with 5 % V205, the NH3 content of the exhaust gas is about < 1 mg/m3N,tr. The analyses of the dust behind the catalyst exhibit no formation of am-monium sulfate, ammonium hydrogen sulfate or CaSO3. The SOx content is bound as CaSO4. Moreover, these experiments exhibit not dust deposits in the reactor or in the catalyst passages.

In another series of experiments, the operating conditions of the gas flow in the free reaction space are changed outside the inventive Froude numbers with the same gas composition, the same dust content and the same catalysts. It is noted that at gas speeds below 4 m/s the breakdown of NOX decreased considerably and the differential pressure at the reactor 19 increased. The result is a complete blockage of the catalyst passages with dust.

Claims

WHAT IS CLAIMED IS:

1. A process for the treatment of dust- and oxygen-containing exhaust gases, which contain sulfur oxides and nitrogen oxides, at temperatures in the range from 200°C to 500°C by means of reducing agents in a reactor (19) which is equipped with solid catalyst (20) with flow passages, in which the free opening surface of the catalyst (20) is more than 50 % and in which the passages of the catalyst (20) have a hydraulic diameter of more than 2 mm, wherein:
a) the treatment in the reactor (19) is performed in the presence of one or more substances selected from the group consisting of free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium, said substances being present in said exhaust gases prior to contact of said exhaust gases with said catalyst; and b) during the treatment, the operating conditions of the gas flow in the free reaction space are adjusted corresponding to the Froude numbers in the range of

2- The process as claimed in claim 1, wherein in the reactor (19) at least one type of catalyst chosen from the group consisting of honeycomb and plate catalysts (20) are used, which beside titanium dioxide and tungsten contain more than 0.5 wt-% vanadium pentoxide.

3- The process as claimed in claim 2, wherein said solid catalyst comprises 2.8 wt-% vanadium pentoxide.

4- The process as claimed in any one of claims 1 to 3, wherein the treatment is performed in the presence of one or more substances selected from free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium with an average particle size d50 between 5 µm and 100 µm.

5- The process as claimed in any one of claims 1 to 4, wherein as reducing agent there are used NH3-releasing compounds such as (NH4)2SO4, (NH4)2CO3, (NH4)2HCO3, (COONH3)2H2O, HCOONH4, NH3, NH4OH, H2O-CO-NH2, NH2CN, Ca(CN)2, NaOCN, C2H4N4, C3H6N6 and NH3-containing waste waters from photochemical plants, singly or several of them.

6- The process as claimed in claim 5, wherein before entry of the exhaust gases in the reactor (19), the NH3-releasing compounds are incorporated in the flue gas stream in the gaseous, liquid or solid condition at temperatures in the range between 200°C and 1000°C.

7- The process as claimed in claim 5 or 6, wherein the NH3-releasing compounds are incorporated in the flue gas stream in the form of dilute aqueous solutions at temperatures in the range between 300°C and 550°C.

8- The process as claimed in any one of claims 5 to 7, wherein the presence or the addition of one or more substances selected from free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium to the flue gas stream is effected before the use of NH3-releasing compounds.

9- The process as claimed in any one of claims 1 to 8, wherein the flow to the reactor (19) equipped with the catalyst (20) is effected from above or from below.

10- The process as claimed in any one of claims 1 to 9, wherein the flow to the reactor (19) equipped with the catalyst (20) is effected alternately from above and from below.

11- The process as claimed in any of claims 1 to 1o , wherein be-side the breakdown of sulfur oxides and nitrogen oxides, the reactor (19) equipped with the catalyst (20) is at the same time used for the breakdown of halogen compounds, halogenated organic compounds, hydrocarbons and CO.

12- The process as claimed in any of claims 1 to 11, wherein the reactor (19) equipped with the catalyst (20) is used for the breakdown of sulfur oxides and nitrogen oxides in dust-laden exhaust gases in the chemi-cal and metallurgical industries as well as in the cement and lime industries, in power plants and in garbage incineration plants in the process flow at temperatures in the range between 200°C and 500°C without additional preheating of the exhaust gas.

13- The process as claimed in any one of claims 1 to 12, wherein the treatment of the exhaust gas is performed in the presence of one or more substances selected from free oxides, carbonates, and hydroxides of calcium.