DE2643732A1 - Purification of waste gases by combustion - at widely varying rates and concentrations without backward flame propagation risk - Google Patents

Abstract

Methods of, and appts. for purifying by decomposition at 800-1000 degrees C., waste gases contg. organic or mineral substances, in which the flow of gas varies between wide limits is claimed. The concn. of O2 and of combustible substances varies so that the explosive limit is sometimes exceeded; supplementary combustible is added to maintain the decomposition temp. According to the invention, air or waste gas which contains no combustible substances, is added to the waste gases, the flow regime is made laminar and the gas mixt. is sent to the combustion chamber, with no boundary layer on the wall of the inlet duct. The quantity or combustible-free waste gas is added in a constant proportion which maintains the rate at a value defined on a curve determined experimentally, between 3 and 20% of the max. flow rate of the waste gases. The invention ensures direct combustion of flammable waste gases in a burner which prevents propagation of a backward flame and reduced explosion risk, to a greater extent than is possible with existing methods, and without incurring the high capital and running costs associated with existing methods.

Description

Process and device for the combustion of exhaust air

Exhaust gases can occur in various industrial companies, at times beyond the lower explosion limit with pollutants, e.g. solvent vapors, such as hexane, toluene, benzene, xylene, kerosene, ether, alcohol and / or flammable Gases such as

Hydrogen, acetylene, methane, carbon disulfide are loaded. These Exhaust gases must be fed to an exhaust gas cleaning system.

Processes are known in which the exhaust gases sorption via Ad, condensation or oxidation.

Of these known processes, only the oxidation processes represent a real destruction of pollutants, since all other processes the exhaust air problems just shift to subsequent problems. Because of the susceptibility to failure of a catalytic Exhaust gas oxidation is more and more used for oxidation in thermal exhaust air purification about, in which the pollutant-laden exhaust gas in a burner with additional fuel is burned or the pollutants are oxidized.

C-qWe currently do not know any operationally reliable burns that are ignitable Fuel-air mixtures can burn without this in practical operation with avoidable fluctuations, a flashback in the supply line up to a flame arrester or up to the point of origin of the exhaust gas. After this The state of the art must therefore treat ignitable exhaust gases with a dilution air supply be sure that they are no longer ignitable when they are in the burner reach; otherwise there would be a risk of an explosion or detonation in the supply lines Pipelines or reaction vessels. A flame arrester, such as

a flame sieve or an immersion fuse are only suitable to stop a brief flashback, they do not allow a continuous one Operation of a burner with ignitable exhaust gases.

The disadvantages of the known combustion methods for exhaust gases are especially in their high investment and operating costs, which are caused by the complex security measures.

It is therefore an aim of the present invention to direct ignitable exhaust gases to be burned in a flame-proof burner without the addition of dilution air. According to the present invention, it should be possible to meet practical requirements the amount of exhaust gas in accordance with exhaust gas in quantities that fluctuate greatly over time, backfire-proof to be able to render harmless. Finally, according to the present invention for Combustion of ignitable exhaust gases in the interaction of a backfire-proof burner, the necessary control technology and safety equipment a higher level of security against explosions as it is after the state of the art is possible through the measures already outlined.

The present invention thus relates to a method, exhaust gases, that are loaded with organic or inorganic combustible substances, by thermal Decomposes completely and safely at temperatures between 8000C and 10000C clean, with the amount of exhaust air fluctuating over time within wide limits and also the Concentrations of oxygen and flammable substances also fluctuate so that the lower explosion limit in the exhaust gas is temporarily exceeded and furthermore an auxiliary fuel, if necessary, to maintain the decomposition temperature is supplied, characterized in that a) a constant amount of fresh air is added to the exhaust gas or amount of exhaust gas that does not contain any flammable substances to comply with the figure 4 defined inflow velocity W is added, this constant Amount is approximately in the range between 3 and 20% by volume of the maximum exhaust gas amount, b) the flow of this exhaust gas mixture is largely laminarized and c) this largely Laminarized flowing exhaust gas mixture is fed directly to the combustion chamber is, if necessary with destruction of the wall boundary layer directly to the end the exhaust gas introduction.

The present invention also relates to a device for thermal exhaust air purification in a combustion chamber with gas supply and possibly Auxiliary fuel supply line, with a flow straightener in the gas supply line (2) is attached to which one in the direction of the combustion chamber (6) contracting The nozzle (1) is connected, with tear-off edges at the nozzle outlet (3) in the combustion chamber (6) (4) (numbers referring to Figure 1) are attached.

The method according to the invention enables backfire-free combustion explosive exhaust gases, the exhaust gases in their quantities in the ratio of about 1:10, preferably 1: can fluctuate. Any combustible pollutants, for example optionally substituted, aromatic and / or aliphatic hydrocarbons, such as.

Hexane, kerosene, ethylene, toluene, benzene, xylene, chlorobenzene, alcohols, such as methanol, ethanol, ethers such as diethyl ether, carbon disulfide compounds, such as CS2, hydrogen, acetylene, propylene oxide, ethylene oxide. The temperature, with which the exhaust gas is introduced into the combustion device is approximately 400 to 1000C, preferably 0 to 400C.

The method according to the invention and the method according to the invention are described below Device based on two preferred embodiments, illustrated in the figures 1 and 2, explained in more detail.

The first embodiment allows exhaust gases whose quantities are between 10 to 100% of the maximum amount fluctuates, burns backfire-proof.

In FIG. 1, the numbers have the following meaning: 1. nozzle; 2. flow straightener, preferably flame screen; 3. nozzle outlet; 4. tear-off edge, 5. auxiliary fuel supply; 6. Combustion chamber.

In detail, the pollutant-laden exhaust gas is passed through the flow straightener 2 inserted into the contracting nozzle 1. Already in the flow straightener 2 there is a partial laminarization of the flow of the exhaust gas in the contracting Part of the nozzle is reinforced. In the contracting part of the nozzle takes place a steady acceleration of the flow rate of the exhaust gas. That in the Nozzle with regard to its flow is largely swirled and eddy-free exhaust gas past a sharp tear-off edge 4 (at the nozzle outlet) into the combustion chamber 6 introduced. The one for reaching the pollutant decomposition temperature, if applicable required additional fuel, such as natural gas, can be via line 5 outside the nozzle outlet point are fed to the combustion chamber. Task of the tear-off edge 4 is the laminar flow boundary layer created on the wall of the nozzle largely to destroy, so that a backfire in this relatively slow-flowing Boundary layer from the combustion chamber into the nozzle 1 is avoided.

The tear-off edge 4 is about 0.2 to 10 mm, preferably 1 to 3 mm perpendicular to the main flow direction of the exhaust gas in the nozzle.

The principle according to the invention of this dynamic flashback protection device is based on the fact that in the narrowest nozzle cross-section, at every point, i.e. also near the wall the flow rate must be as greater as possible than the respective flame re-ignition rate. For this purpose, the slow laminar flow, which is in the immediate vicinity of the wall runs, destroyed by at least one tear-off edge.

In another embodiment, not shown, it is possible to do without the tear-off edge and the exhaust gas in the contracting nozzle to accelerate and laminarize so strongly that those in the immediate vicinity of the Wall running slower flow has such a small thickness that the at the heat generated by the re-ignition in this boundary layer immediately via the heat-dissipating Wall (if possible metals) is removed and thus interrupted the flashback will. While in the presence of a tear-off edge at a nozzle of 70 mm ~ the flow velocities of the exhaust gases at the nozzle outlet for exhaust gases that contain hydrocarbons approximately in of the order of 2 to 8 m per second, preferably 4 to 5 m per second, for exhaust gases, which contain hydrogen, acetylene, about 20 to 50 m per sec., preferably 25 should be up to 30 m per second, the flow velocities for exhaust gases must be of the first-mentioned type can be set about 10-30% higher at the nozzle outlet, if a tear-off edge is dispensed with.

The purpose of the contracting nozzle is in addition to accelerating the exhaust gas the generation of a flow profile that is as uniform as possible over the entire nozzle cross-section without any indentation in the middle of the nozzle.

Nozzles according to the invention have a diameter at the nozzle inlet from about 2 to 30 cm, preferably 5-8 cm, at the nozzle outlet about a third of the Nozzle cross-section at the nozzle inlet.

The nozzle width is based on the minimum and maximum flow rate. In any case, it must be prevented that the backfire rate with minimal throughput the flow velocity depending on the type of pollutant and concentration in the direction of the combustion chamber. The maximum amount determines the maximum pressure loss. In the described embodiment of the invention with rectified and accelerated flow, for example, the reignition speed is for an almost stoichiometric hexane-air mixture at - <3 m / sec.

The flow straightener is preferably used at the same time as a flame arrester designed, for example, as a heat-dissipating flame screen, and preferably sits at a distance that corresponds to 1 to 10 times the nozzle inlet diameter the nozzle.

The flame arrester is used for safety reasons to prevent a Flashback in the exhaust gas supply line in the event of incorrect operation. The gap width of the flame arrester must be adjusted in a known manner to the ignitability of the mixture.

When maintaining combustion temperatures of about 800 to 1000 0C in the combustion chamber 6, the organic pollutants are practically completely oxidized.

In the embodiment shown in Figure 2 is an inventive Cleaning of ignitable exhaust air or exhaust gas possible, with which the accumulating pollutant-laden Exhaust gases between 0 and 100, ~ the maximum amount with sudden changes can fluctuate.

In FIG. 2, the numbers have the following meaning: 7. exhaust gas supply line; 8. contracting nozzle; 9. combustion chamber; 10th control loop; 11. amount of fresh air; 12. auxiliary fuel supply; 13. Control circuit 14. Nozzle inlet 15. Tear-off edge Im The method according to the invention is carried out individually according to FIG. 2 as follows: The ignitable exhaust air enters the combustion chamber via line 7 through nozzle 8 9 a. A fresh air flow 11, which is kept constant, is generated via a control circuit 10 (the determination of the required fresh air flow depends on the specific Harmful gas shown in Figure 3) added, so that the also in Figure 3 below also defined speed WO at the nozzle outlet into the combustion chamber is adhered to when the exhaust air flow drops to 0. Here it is then without Exhaust air analysis possible by simply regulating the quantity ratio of Exhaust air and fresh air (7 and 11) to the auxiliary fuel quantity (12) have a minimum temperature from about 800 to 10000C in the combustion chamber 9 even at random and sudden Fluctuation of the exhaust air pollution to be adhered to, leading to the thermal decomposition of all organic Pollutants in the exhaust gas is sufficient.

This implementation of the method is as shown in FIG Relationships. In Figure 3, the flow rate or reignition rate of the exhaust gas in m / s (ordinate) depending on the pollutant concentration (vol % of combustible substances in the exhaust air or in the fresh air / exhaust air mixture) (abscissa) shown as an example for a hexane / air mixture (hexane pollutant).

The ignition speed in the nozzle flow is between the lower and upper explosion limit except for the concentration and the nozzle diameter strongly dependent on the laminar or turbulent flow condition. To as much as possible Achieving low flow-related ignition speeds is achieved with the nozzle according to the invention aimed at a rectified, calmed and accelerated flow. This minimizes the boundary layer on the nozzle wall. By the sharp Edge (15) at the nozzle mouth may also tear off the boundary layer d. h the boundary layer becomes infinitely thin at the edge. These measures have the purpose that To reduce back ignition possibilities in the boundary layer as possible, so that for the Overall nozzle the lowest possible reignition speed results.

To the actual sparkback properties of one according to Fig. 1 constructively To determine the optimized nozzle for a given pollutant, the dependency must be determined the re-ignition speed as a function of the pollutant concentration experimentally be determined.

For this purpose, a nozzle according to FIG. 1 is provided with a pollutant-air mixture to be produced with variation of the pollutant-air ratio within the explosion limits operated. After setting a stable, backfire-free combustion state on the Nozzle, the throughput flow is reduced until a flashback to the flame screen entry.

The re-ignition can be optically, acoustically or by temperature sensing can be determined on the flame side of the flame sieve. The flow velocities in the nozzle at the time of re-ignition are dependent on the pollutant concentration entered in a diagram analogous to FIG. 3 and result in a curve analogous to curve 1.

The dependence of the flow velocity W in the nozzle as a function of the pollutant concentration (curve 2, Fig. 3) results from the following relationships: 1) Air exhaust air mixture (o '+ n') X + (o '+ n' + s' ) Y = Vmixture o 'parts by volume oxygen> o volume concentration oxygen n' volume parts nitrogen> n volume concentration nitrogen s 'volume parts pollutant # s volume concentration pollutant X fresh air volume flow y exhaust air volume flow V mixture volume flow 2) For air is o' + n '= 0.79 + 0.21 = 1 3) For exhaust air, o' + n '+ s' = 0.79 + 0.21 + s '= 1 + s' o '+ 1 + s' 1 + s '1 + s' o + n + s = 1 4) s = 5' ~~~ 5 '= s 1 + s' 1-s By inserting it results from equation 1): 5) 1 X + (1 + 1 -S) Y #) Y = Vmix 6) Vmix = mix = W Flow velocity in the nozzle F nozzle 2 (F = nozzle cross-section in m (Cs = max. Concentration of the pollutant in the air-exhaust air mixture).

If you use s to represent the maximum expected pollutant concentration in Exhaust gas, you can create a curve 2 for the nozzle by varying the X, Y and F nozzle Calculate the flow velocity that has a desired safety distance (3) to the curve of the re-ignition rate (1). Appropriately you go here so that you have different values for F nozzle (nozzle cross section) and X (fresh air volume flow assumes and Y (exhaust air volume flow) varies from 0 # 100%.

In the diagram represents the ratio of the ordinate heights to the curve 2, point 4 and curve 1, point 5 at a certain pollutant concentration in the fresh air / exhaust air mixture represents the ratio of flow velocity to reignition velocity. It becomes according to the invention a ratio of 1.5 or higher is aimed for.

The curve 2 determined in this way intersects the ordinate at WO, from which by multiplying by the nozzle area nozzle (equation 6) the required fresh air flow results. This is usually far below the fresh air flow, which for itself cause a higher flow rate than the flashback rate in the nozzle would. This results in a considerable saving in operating costs.

With the method according to the invention it is possible with relatively little Pressure loss of up to about 20 m bar in the combustion chamber in explosive pollutants Exhaust gases completely and safely with a permissible volume fluctuation range for to burn the exhaust air from 0 j 100%.

The method according to the invention is explained below by way of example: An arrangement according to FIG. 1 was used, in which the flame screen (2) has a Diameter of 150 mm with 0.7 mm sieve channel width and the nozzle outlet 4 one 67 mm in diameter.

A hexane-air mixture was used as the explosive exhaust air. First, curve 1, FIG. 3 for the The flashback speed of this nozzle is determined. It was found that the greatest flashback rate occurred at a rate of 3.3 m / s at a hexane concentration of about 2.7% by volume. at A flow velocity of more than 3.3 m / s could be achieved at any concentration no re-ignition can be detected. If the flow velocity was reduced, so that the values were at or below the values of curve 1, backfiring took place and the flame burned on the flame sieve. By burning for about 3 minutes the sieve on the flame sieve was not damaged.

The curve 2, Fig. 3 was then with variation of the values X, Y and F nozzle and specifying the maximum pollutant content in the exhaust gas with s = 2.3 % determined. It was found that with X = 0.1 m addition of fresh air and a nozzle cross-section of 0.1 m2 s an exhaust air flow Y between 0 and 3 m3 backfire-proof through this nozzle out sn 3 can be. Due to the constant fresh air flow 0.1m 5 becomes without Exhaust air # reached a speed of WO = 1 at the nozzle. The curves 1 and 2 have their smallest distance or the ratio of the flow velocity at points 4 and 5 (4 to 6) to the re-ignition rate (5 to 6) is 3.1 to 1.75 equals 1.77. The resulting flow velocity of 3.1 m / s for this exhaust air flow is thus below the maximum Ignition speed of 3.3 m / s; despite this a backfire-proof operation is guaranteed, since the to this exhaust air flow The resulting flashback speed of 1.75 m / s is smaller with a safety margin.

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Claims (5)

Patent claims (1) Processes, exhaust gases with organic or inorganic combustible substances are loaded by thermal decomposition at temperatures between 8000C and 10000C can be cleaned completely and safely, with the amount of exhaust air being timed fluctuate within wide limits and so do the concentrations of oxygen and combustibles Substances also fluctuate so that the lower explosion limit in the exhaust gas temporarily is exceeded and furthermore to maintain the decomposition temperature an auxiliary fuel is optionally supplied, characterized in that a) the exhaust gas a constant amount of fresh air or exhaust gas that does not contain any flammable substances contains, in order to maintain the inflow velocity Wo defined in Figure 4 is admixed, this constant amount being approximately in the range between 3 and 20 vol maximum amount of exhaust gas is, b) the flow of this exhaust gas mixture is largely laminarized and c) this largely laminarized flowing exhaust gas mixture directly to the Combustion chamber is fed, possibly with destruction of the wall boundary layer directly at the end of the exhaust gas inlet. 2) Method according to claim 1, characterized in that for exhaust gases, whose temporal fluctuations between the minimum and maximum amount are only so great that the safety distance (distance 4-5) defined in Figure 4 between the flow velocity of Exhaust gas in the outflow opening and the backfire speed in every operating state is still maintained, no constant fresh air or unloaded exhaust gas volume is fed in will. 3) Method according to claim 1, characterized in that for the In the event that the oxygen content of the exhaust gas is temporarily (i0%, the constant feed Amount of fresh air with ^ z21% oxygen. 4) Device for thermal exhaust air purification in a combustion chamber with gas supply line and optionally trilfsbrcnnstoffzufuhrleitung, wherein in the gas supply line a flow straightener (2) is attached, the one in the direction of a Combustion chamber (n! Contracting nozzle (1) is connected, whereby at the nozzle outlet in the combustion chamber tear-off edges (4) (number based on Fig. 1) attached are. 5) Device according to claim 4, characterized in that the Abrcißkanten 0.2 to 10 mm wide perpendicular to the main flow direction of the exhaust gas are.