WO1996025223A1

WO1996025223A1 - Process for the catalytic treatment of flue gases

Info

Publication number: WO1996025223A1
Application number: PCT/DE1996/000287
Authority: WO
Inventors: Siegfried Nowak; Heinrich Klare; Dagmar Sittig; Freimut J. Marold; Manfred Voigt; Karl H. Schugarcz
Original assignee: Uve Institut Für Technische Chemie Und Umweltschutz Gmbh - Private Forschungseinrichtung
Priority date: 1995-02-16
Filing date: 1996-02-15
Publication date: 1996-08-22
Also published as: DE19506864A1; EP0809531A1

Abstract

The invention concerns a process for the catalytic oxidation of organic pollutants in flue gases of various origins and compositions. A disadvantage of pyrolysis and high-temperature combustion as a way of eliminating solid, liquid and gaseous waste products is the creation of incompletely broken down and partially oxidised hydrocarbons. Complete oxidation has hitherto been possible only with vehicle exhaust gases using noble metal catalytic converters. The object of the invention is thus to develop a simple and technically applicable process for converting hydrocarbons contained in untreated flue gas using a catalyst and creating advantageous reaction energy conditions. The proposed novel process is characterised by the fact that a honeycomb element with a sieve structure and honeycombs consisting of a modified or unmodified calcium aluminate as the catalyst is mounted immediately downstream of the combustion chamber in a flow of unprocessed flue gas with a temperature in the range 400 to 1300 °C and a sojourn time in the honeycomb element of 0.2 to 3 seconds.

Description

Process for the catalytic treatment of flue gases

The invention relates to a process for the catalytic oxidation of organic pollutants in flue gases of various origins and compositions in the presence of water vapor.

Thermal, physico-chemical and biological processes are used to remove organic pollutants and products of different compositions and origins.

Thermal processes such as pyrolysis and high temperature combustion currently make the greatest contribution to the removal of solid, liquid and gaseous waste products.

Notwithstanding the progress made in conventional combustion, smoldering, selectoforming, and the like. a. have these shortcomings and weaknesses. In particular, the reliable compliance with the limit values required by law causes difficulties or can only be achieved by complex and expensive multifunctional cleaning stages which not only increase the operating costs, but also lead to instabilities in the trial operation when the composition of the waste products / pollutants varies greatly . In particular, the De Novo synthesis, which is caused by incompletely degraded and partially oxidized hydrocarbons, puts a strain on incineration plants, has in some cases led to the decommissioning of smaller incineration plants and is also increasingly leading to acceptance problems among the population.

There are procedural measures, so-called primary, for flue gas cleaning and to avoid emissions Measures such as B. the optimal design of the combustion chamber, better mixing of the reactants, the inhibition of dioxin formation by adding amines as well as secondary measures such as the separation / removal of pollutants by means of activated coke or sorbalite mixture consisting of calcium hydroxide and lignite activated coke, recommended and enforced. These measures are not only expensive, they also harbor the risk of overheating and spontaneous combustion due to hot spots in activated carbon filters and must also be supplemented by catalytic afterburning of hydrocarbons still contained in the flue gas.

The total catalytic oxidation is already being carried out on a large scale with precious metal catalysts for motor vehicle exhaust gases; The precious metal catalysts used for this require a temperature level below 600 ^* C as well as sulfur and metal-free combustion gases.

In furnaces catalysts are used essentially for the denitrification namely to accelerate the reaction of Ammo¬ niak with nitrogen oxides to nitrogen and water, which are usually used C after the dust removal, and partly also to the flue gas scrubber in the temperature range of 400 ^*.

These are honeycomb bodies, e.g. made of titanium dioxide with a coating of vanadium or tungsten oxides. The use of oxides of cobalt and nickel is also known.

For the purification of exhaust air and exhaust gases which contain hydrocarbons, as they occur in paint shops, in leather and plastics processing, in syntheses of all kinds and in many other industrial activities, metal oxides are successfully used which have a good cleaning effect for defined pollutants guarantee a long service life. The catalytic post-oxidation of combustion gases to destroy residual hydrocarbons is increasingly being sought in waste incineration plants and in special waste disposal plants. Above all, high sulfur contents and the presence of higher hydrocarbons as well as tars, soot and dusts as aerosols in the combustion gases lead to a shortening of the service life of known catalysts, which has a disadvantageous effect on the economy of the entire process.

These reasons have catalysts, the search for efficient Kata¬ and additives, such as H ₂ 0 _2, at relatively low reaction temperatures - implement (200 250 ^* C), dioxins and other toxi¬ Pollutants reliably into simple hydrocarbons triggered.

The invention is based on the object of developing a simple and technically applicable process for converting hydrocarbons contained in the uncleaned flue gas with the aid of a catalyst and with an energetically favorable reaction procedure.

The process according to the invention for the catalytic treatment of flue gases is characterized in that a honeycomb shaped body with a sieve structure, the honeycombs of which consist of a modified or unmodified calcium aluminate as a catalyst, is arranged immediately after the combustion chamber in an uncleaned flue gas stream, the flue gas being at a temperature of in the range of 400 to 1300 ^* C and the residence time in the honeycomb molding is 0.2 to 3 seconds.

This arrangement of the catalyst avoids the formation of dioxins and realizes a more technically and energetically more favorable reaction procedure. The direct use of the catalyst directly in the flue gas stream after combustion processes and in the presence of water vapor requires a robust, thermostable, compared to hetero element compounds and Aggressive substances arising from the combustion process are resistant to catalysts which maintain their catalytic action even in the presence of dust particles contained in flue gases.

For the conversion of CO and other hydrocarbons, cement-containing catalysts have also been recommended at high temperatures for cleaning various exhaust gases, but these are only used in "pure" oxidation processes which are not accompanied by any dust formation. The previously known application of these catalysts is carried out solely in the form of strands or spheres which do not permit the conversion of dust-containing flue gases due to the risk of growth and clogging which lead to the shutdown of the processes.

The use of a honeycomb molded body with a defined sieve structure, while tolerating a certain dust content in the flue gas, ensures the catalytic effectiveness of the molded body and the direct conversion of residual hydrocarbons into unpurified flue gases.

The essence of the invention therefore consists in the use of a highly temperature-resistant, steam-resistant and catalytically active honeycomb molded body which is resistant to hetero-element compounds, which is arranged directly in the flue gas stream using the temperature conditions prevailing there and a total conversion of those still contained in the flue gas burned hydrocarbons are realized in such a way that the material basis is removed from the De Novo synthesis.

According to the invention, in a preferred embodiment, the catalyst or honeycomb shaped body consists of a calcium aluminate modified with different promoters, which preferably contains the phases C ₁₂ A ₇ and C ₃ A active for the intended reaction, and via the catalytic, thermal described and physico-chemical properties disposes.

The production of a honeycomb body made of calcium aluminates not previously known with this catalytic effect and its use for the direct conversion of a flue gas loaded with dust particles leads to technologically and economically more advantageous solutions than with known conventional combustion plants (saving of process steps or simpler design of the same).

According to the invention, the catalyst consists of a weight ratio of CaO: γ-Al ₂ 03 of preferably 30:70. By promoting the calcium aluminate support with elements from sub-groups V to VIII of the periodic system of the elements, in particular V, Ni, Mn and Mo , in a concentration between 0.2 - 10%, preferably up to 5%, in particular with 1% by weight, the bifunctional effect of the catalyst - the total oxidation or the thermal-catalytic cleavage of the hydrocarbons and the acceleration of the water gas crack tion - varied and greatly increased.

The water gas reaction which preferably takes place under certain conditions also leads to complete cleavage of aliphatic and aromatic hydrocarbon structures. In addition, the gasification of coke and gum deposits on the catalytic converter is promoted, thereby achieving high long-term activity of the catalytic converter.

The method according to the invention can be used for the purification of waste gases from waste oil, the special waste incineration of hospital waste, solvents, chlorine and metal-containing waste from the metalworking industry and others where there is a risk of dioxin regression if combustion is incomplete;

• from smokers, roasters and related businesses to reduce odor nuisance and to comply with immis sion laws;

From the incineration in the removal of waste, environmental pollutants, such as in contaminated soils, contaminated solid residues, waste plastics, electronic waste, sewage sludge, printed circuit board material, etc.

The honeycomb molded body is produced using customary techniques for the production of honeycomb shaped bodies, it being possible, for example, to use calcium carbonate and aluminum oxide as starting products for the calcium aluminate, which are formed, dried, and fired after slurrying with the addition of conventional auxiliaries to the shaped bodies if in compliance with intermediate temperature levels.

The passage openings in the honeycomb shaped body are l x l to 10 x 10 mm. Passage openings smaller than 1 mm are not expedient because of the possibility of clogging by dust components in the flue gas, but in the case of a particularly low-dust flue gas it is also possible to work with smaller openings. Openings larger than 10 mm are not practical due to the insufficient catalytic reaction given the short residence times. The openings can be rectangular, square or hexagonal.

Residence times below 0.2 seconds are generally not sufficient for the desired catalytic reaction; Times over 3 seconds are generally uneconomical because of the reduced throughput. In this invention, "flue gases" are understood to mean all exhaust gases from combustion processes.

The honeycomb molded body can be arranged as a battery of several molded bodies both horizontally and vertically and in inclined directions according to the combustion chamber in accordance with the respective spatial conditions. It is expedient if a plurality of shaped bodies are arranged in a form-fitting manner horizontally and / or vertically and / or inclined to Arrange the flue gas direction one above the other, side by side and / or one behind the other in a size or number sufficient for catalytic cleaning. The actual configuration can easily be determined for the person skilled in this field depending on the amount and type of the harmful components to be removed from the fuel / flue gas and the selection of the corresponding catalyst (doping).

The invention is illustrated by the accompanying drawings. In it shows

Fig. 1 shows the honeycomb shaped body in section

The results presented in the following examples come from laboratory apparatus, which essentially consists of a preheating zone and the subsequent combustion zone / catalyst zone. The gaseous reaction product obtained after cooling and condensation was examined by gas chromatography and by coupling GC and mass spectrometry.

In the exemplary embodiments, catalysts of the following composition were used

CaO: A1 ₂ 0 ₃ = 1: 3; Promotion with 1% Ni or V or Mn at reaction temperatures between 750 and 900 ^* C in the form of Raschig rings and honeycomb moldings. In order to simulate the conversion of pollutants, model hydrocarbon mixtures which are representative of real systems were used.

example 1

Oxidation of hydrocarbon mixtures on calcium aluminate TZ70

Test conditions:

Catalyst: TZ 70 (phases: C ₁₂ A ₇ , C ₃ A; / 0% Al ₂ 0 ₃ , 30% CaO)

Average reactor temperature: 750 - 850 ° C

Dwell time: 0.6 s

Load:: 0.25 g / g catalyst Passing the hydrocarbons mixed with air through a preheater into the reactor; Residual oxygen content around 10%.

Table 1 Oxidation of a mixture of acetonitrile, benzene, chlorobenzene, n-hexane (mass ratio 4/59/8/29)

Catalyst: TZ70; increasing operating time, dwell time 0.4 seconds. Data in% sales

BI = Federal Immission Control Ordinance; Sp = traces <1 ppb [A] = mg / m ³

The oxidative degradation of the acetonitrile is always already at 750 ^* C voll¬. Who examined hydrocarbons in the exhaust gas are only in traces at 850 ^* C. The conversion of benzene and chlorobenzene is practically complete; the concentration of the two compounds in the exhaust gas drops by three orders of magnitude. To a lesser extent, naphthalene is broken down oxidatively. In addition to small amounts of unreacted naphtha, the liquid condensate of the exhaust gas contains halens traces of acetic acid, maleic anhydride and benzene. The amount of HCl found corresponds to approx. 90% of the chlorobenzene used.

Examples 2 to 4

Table 2 shows the results and shows that with increasing temperature over time (Example 2: 750-800 ^* C, 4 hours; Example 3: 850 ^* C, 10 hours; Example 4: 850 'C 20 hours) the oxidation products also largely disappear, the concentration of these substances being in the range of ng / 1.

At reaction temperatures up to 800 ° C, a wide spectrum of by-products was found by means of GC / MS. At 850 ^* C, most of these compounds are no longer contained in the exhaust gas. The test conducted over 20 h also showed the stability of the oxidative effect of the catalyst under these test conditions

Table 2 By-products of the total oxidation of the substrate mixture

Organ. Compound concentration [ng / m ³ ]

Temperature ["C] 750-800 850 850

Test duration [h] 4 10 20

Benzoic acid 10 41 19

2-methylpropylcyclo- 105 77 9 pentanone

4,6-di (1,1-dimethyl-20 43 3 ethyl) -2-methylphenol

2-ethyl-4,5-dihydro-26 - - oxazole

Methyl indole 1 - -

4-chlorophenol (contains 2 - - saturated Cl-benzene)

Methylbenzofuran 60 - -

Isoquinoline 1 - -

1,4-hydroquinone 3 - -

2-chloro-l, 4-hydrochi- 26 - - non

Chloromethylbenzofuran 4 - -

Benzopyran-4-one 3 - -

3-phenyl-2-propenal 12 - -

Benzopyran-2-one 2 - -

5-chloro-2-methylbenzo-2 - - furan

1- (4-chlorophenyl) pen-1 - - tenone

4,6-dichloro-l, 3-hydro-6 - - quinone

2, 2-dichloro-l, 1 ^'bi- 7 7 - phenyl Example 5

Table 3: Oxidation of a mixture of acetonitrile, benzene, chlorobenzene, n-hexane (mass ratio 4/59/8/29) Catalyst: TZ70 doped with 1% Ni, residence time 0.4 seconds. Figures in conversion [% ] or mg / m ³

Tem. [ ^* C] 750 800 850 [mg / m ³ ] BImSchV [mg / m ³ ]

Acetonitrile 99.6 100.0 100.0 _ 5

Benzene 95.9 100.0 100.0 Sp 5

Cl-Benzene 79.6 99.2 99.9 3.6 100 n-hexane 100.0 100.0 100.0 150

Acetonitrile, benzene, naphthalene and n-hexane is almost completely umge¬ on a doped catalyst already at 750 ^* C. Chlorobenzene achieves a conversion of approx. 80% at this temperature. At 850 ^* C, this value also increases to 100%. The oxygen conversion and the formation of carbon dioxide correspond to the complete oxidative conversion at a reaction temperature of 800-850 ^* C.

The last column of Table 3 shows that the values required by the BimSchV are undercut.

Example 6

Sales of benzene

Table 4: Oxidation of benzene with air at different stoichiometric ratios

Catalyst: calcium aluminate, C ₃ A, C ₁₂ A ₇ with 1% Ni, 70 g

Air volume: 42.54 1 / h; Dwell time: approx. 1 s; Temperature: 800 ° C Table 4

ml / h benzene 1.2 2.4 3.6 3.9 4.2 4.8

Vol%

Oxygen 16.21 9.36 2.93 0 0 0

Nitrogen 78.53 79.4 78.3 75.46 75.23 72.0

Carbon dioxide 9.5 14.6 15.8 15.8 13.9

Hydrogen nn nn nn 0.083 0.816 5.85

Carbon monoxide nn nn nn nn 0 4.82

Benzene free <0.000% <0.000% 0.2% 0.41% 0.3% 0.10%

nn = undetectable

Benzene was chosen as the substrate because it is the most difficult to oxidize of all hydrocarbons.

This example shows the complete degradation of the benzene with excess air and longer residence times than in the previous examples; in the event of overdosing (λ <l), another property of the catalyst comes into play: the water gas shift reaction that occurs converts the rest of the unburned benzene to hydrogen and carbon oxides and thus also removes it from the exhaust gas.

Example 7

Systematic further investigations to increase the activity of the catalyst system led to the results in Table 5. The table clearly shows the influence of the doping elements on the working temperature of the catalyst. Table 5

Summary results from the combustion of benzene with differently doped catalysts based on calcium aluminate. Doping in each case 1% of the element based on calcium aluminate; the temperatures at which no traces of benzene were found in the exhaust gas are indicated (<5 mg / m ³ ); Zero value = 15x10 ^δ counts from the FID in 200 μl air.

Kataly¬ Tempe¬ Luft Benzen Dwell- ° ₂ Counts sator ratur time = Rest¬

['C] [1 /] [ml / h] [sec] [vol%] quantity

Quartz¬ 800 60 3.6 1.26 10.8 1.58 grit XlO *

TZ70 700 26.4 1.5 2.6 8.4 4.18 X 10 ⁶

800 26.7 1.5 2.3 10.4

C3AC12A7 + V 700 26.7 2.0 2.5 8.1 3.81 XlO ⁶

800 26.7 2.0 2.5 7.5

C3AC12A7 + CU 780 60 3.6 1.26 9.9 22987

800 60 3.6 1.26 10

C3AC12A7 + CO 700 32 1.5 2.1 12.2

C3AC12A7 + Ni 700 32 1.5 2.1 12.5

TZ70 + Ni 600 27.5 1.5 2.7 9.0

TZ70 + Fe 675 60 3.6 1.26 9.9 34434

700 60 3.6 1.26 9.9

TZ70 + Cr 600 60 3.6 1.26 9.4

TZ70 + Mn 545 60 3.6 1.26 9.9

Claims

claims

1. A process for the catalytic treatment of flue gases, characterized in that a honeycomb shaped body with a sieve structure, the honeycombs of which consist of a modified or unmodified calcium aluminate as a catalyst, is arranged immediately after the combustion chamber in an unpurified flue gas stream, the flue gas being at a temperature of in the range of 400 to 1300 ^* C and the residence time in the honeycomb molding is 0.2 to 3 seconds.

2. The method according to claim 1, characterized in that the passage openings in the honeycomb molded body l x l to 10 x 10 mm be¬.

3. The method according to claim 1, characterized in that the calcium aluminate catalyst from 10 to 90% by weight calcium oxide or 90 to 10% by weight aluminum oxide, preferably 30: 70% by weight CaO: A1 ₂ 0 ₃ consists.

4. The method according to claim 2, characterized in that the calcium aluminate catalyst preferably contains the phases C ₁₂ A ₇ and C ₃ A.

5. The method according to claim 3 or 4, characterized in that the calcium aluminate catalyst is doped with elements of subgroup V. to VII or subgroup VIII. Of the periodic system of the elements, in particular in amounts of 0.1-10 %.

6. The method according to claim 5, characterized in that the calcium aluminate catalyst is doped with elements of nickel, vanadium, manganese or molybdenum, preferably in amounts of

1%, based on the respective element, C ₁₂ A ₇ and C ₃ A preferably being present as active phases.

7. The method according to claim 1, characterized in that the honeycomb molded body in a plurality of molded bodies is arranged horizontally and / or vertically and / or inclined to Rauchgasrich¬ direction one above the other, side by side and / or one behind the other is arranged in a sufficient for catalytic cleaning ¬ the size or number.

8. The method according to claim 1, characterized in that the flue gas temperature is in the range from 700 to 900 ^* C, in particular in the range from 800 to 870 "C.