DE4014606A1 - REDUCTION OF NO (DOWN ARROW) X (DOWN ARROW) BY STAGE FUEL COMBUSTION WITH ACETYLENE-LIKE FUELS - Google Patents

Abstract

Acetylene-like gases are injected into furnaces for the purpose of reducing nitrogen oxides in the flue gases to nitrogen. These acetylene-like gases are much more effective than methane, which is presently used for this purpose. Preferred acetylene-like gases are propadiene or propylene for fuel staging (reburning) NOx control in combustion applications.

Description

Emissions of nitrogen and sulfur oxides generated in coal combustion plants are responsible for the precipitation of acid rain. Many different methods have been investigated to reduce these oxide emissions. To date, the most effective method of NO _x emission is post-combustion by gradually introducing methane outside the main coal-fired combustion zone. This approach is appealing because it represents a retrofit technology that can be applied to all types of heated boilers. However, this technology has not been easily implemented because the amount of methane introduced for post-combustion corresponds to 20% of the equivalent carbon fuel combustion in the main combustion zone of the boiler. Further research is not aimed at developing the first generation of this technology, but rather to demonstrate the use of combined technologies that will improve cost-effectiveness.

The present invention is directed to other fuels for the gradual post-combustion to reduce NO _x and heating such fuels with an atmospheric plasma source to increase the post-combustion, if necessary.

NO _x -Nachverbrennungschemie

The NO _x reduction efficiency in the afterburning zone is influenced both by the nitrogen dioxide formation and by the destruction of chemical reactions. The Zeldovich NO formation mechanisms involve the reaction of nitrogen atoms and molecules with OH, O and O ₂ :

N₂ + O = NO + N (1)
N + O₂ = NO + O (2)
N + OH = NO + H (3)

NO can also be formed by nitrogen molecules reacting with hydrogen residues such as CH and CH ₂ . Subsequent reactions with O, OH, O ₂ will provide NO.

NO can be destroyed either by reaction with ammonia cards to form molecular nitrogen, or by reaction with hydrocarbon residues such as CH and CH ₂ to form hydrogen cyanide, which in turn is converted to ammonia cards, which reduce NO to N ₂ :

CH + NO = HCN + O (4)
CH₂ + NO = CH₂O + N (5)
CH₂ + NO = HCN + OH (6)
CH₃ + NO = HCN + H₂O (7)

The invention becomes more complete with reference to the attached drawings described, wherein

FIG. 1 provides an overview of the effects of methane and Ace lene injection to the nitrogen oxide concentration;

Fig. 2a explains the structure of the chemical bond of acetylene;

FIG. 2b illustrates the structure of the chemical bond of methane;

Fig. 2c, the structure of the chemical binding of MAPP he explained; and

Fig. 2d explains the structure of the chemical bond of Propy len.

However, other hydrocarbon fuels can form of long-lived radicals, which can destroy NO, in Be  traditional costume. In experiments by Helfritch et al. has been found that acetylene forms long-lived radicals which NO destroy in an effective manner. The acetylene as well as others Hydrocarbons are at temperatures close to 4000 ° C in a vapor atmosphere plasma pressure heated. Spectroscopic measurements showed that the vapor plasma has large concentrations (1. OE17 / cc) of Contained OH and H radicals. The OH and H radicals in com combination with injected hydrocarbons, such as Wise methane and acetylene formed long-lived radical types:

CH₄ + OH (or H) = CH₃ + H₂O (or H) (8)
CH₃ + OH (or H) = CH₂ + H₂O (or H₂) (9)
CH₂ + OH (or H) = CH + H₂O (or H₂) (10)
C₂H₂ + OH (or H) = C₂H + H₂O (or H₂) (11)
C₂H₂ + O = CH₂ + CO (12)

The following NO reduction is:

CH + NO = N + HCO
CH + NO = O + HCN (13)
NH + NO = N₂ + OH (14)
C₂ + NO = CN + CO (15)
CH₂ + NO = CN + H₂O (16)
C₂H + H = C₂ + H₂
C₂H + O = CH + CO (17)

Helfritch found that acetylene is six (6) times more effective for NO reduction than methane, as can be seen in FIG. 1. Further information on the destruction of NO is given in the literature. Q. Le and M. Vanpee, Combustion and Flame, 62, pages 193 to 210 (1985) stabilized an acetylene-nitrogen oxide low-pressure burner. This flame is not easily ignited because of the very high temperature (T <1900 ° K) required to initiate the C ₂ H ₂ -NO reaction. In Helfritch's experiments, these high temperature conditions were achieved using a microwave plasma source. The stoichiometric equilibrium for this reaction is 17% C ₂ H ₂ and 83% NO. More abundant fuel burns were also considered by the above authors Le and Vanpee (27% C ₂ H ₂ and 73% NO). Their flames generated long-lived free radicals of OH, CN, C ₂ and NH. This C ₂ H ₂ NO flame is an effective means of destroying NO.

The combustion rates of different hydrocarbons with NO and NO ₂ were measured and compared. The combustion rate is an indication of the relative reaction rates for each combustion. These rates and the quench diameters are compared in Table I below. It should be noted that the acetylene reactions have the highest rates for all hydrocarbon reactions with NO and NO ₂ . In addition, the acetylene reactions have the smallest quench diameters, which suggests that the temperature at which the reaction starts in the flame front is very high. Therefore, contrary to the general view, flames with NO are easily obtained, provided that the conditions take into account the large quench diameters and the high ignition energies involved.

Table I

It was found that CH and CH _{2 are} important radical intermediates in the destruction of NO with acetylene. In addition, it has been found that the reaction of acetylene and NO requires a temperature of at least 1900 ° K. The highly reactive CH is generated during the combustion of acetylene and oxygen.

All the research in the literature, including the Work by Helfritch et al., Indicate that Acety len forms reactive radicals which destroy NO when the Ambient temperature is at least 1900 ° K. These studies point out that acetylene for gas afterburning Emission control can be used. The afterburn  can be set near the main coal burner in the heat main zone, or in the exhaust line for flue gas, the reheating with an atmospheric Plasma source is facilitated. It was also shown that acetylene more effective for the destruction of nitrogen oxides than is methane.

The problem with the use of acetylene in the verver Burning instead of methane is that acetylene only in small steel cylinders with a capacity of 300 liters assets because of its inherent volatility. To get large amounts of the gas, you have to get a lot Use steel cylinders, which drives the gas price very high and is not commercially attractive. This is because acetylene primarily as an operating material for cutting and welding burner is used, and not for any application when burning.

Fortunately, it has been found that the industrial gas industry has developed synthesis gases that are kinetically similar to acetylene but do not have its volatility. This enables the gas to be transported in tankers. They tend to have a price similar to that for propane gas, which could be very attractive for gas post-combustion if there is an advantage of post-combustion with these synthesis gases. These gases are currently sold as an acetylene substitute for use in welding technology. Two of the most common synthesis gases are MAPP gas, which is generally 66% methylacetylene-Allen (propadiene) with 34% butadiene and B-plus, which is generally propylene. These gases are manufactured and marketed by companies such as Airco, MG Industries, Matheson and Monsanto. These gases are complex hydrocarbons, which are very similar to acetylene in their chemical bonding structure (see Fig. 2a to 2d). It should be noted that acetylene has a triple bond which makes it very reactive, whereas methane only has single bonds. MAPP gas or methylacetylene allen (propadiene) is now an isomeric compound which has two molecular structures, namely methylacetylene allen and propadiene ( Fig. 2c). The triple bond is shown in the methylacetylene structure and is very similar to acetylene. B-plus or propylene is a long chain hydrocarbon with a double bond as shown in Figure 2d.

The chemical-kinetic properties of these gases are also similar if From a safety point of view, the impact sensitivity of acetylene is very high, whereas the other gas is considerably more stable. The reaction with copper and silver alloys is a common property of both acetylene and MAPP gas, suggesting a similarity in their reactive nature. The boiling point range at atmospheric pressure for MAPP gas, B-plus and propane is similar, so that MAPP gas and B-plus can be stored as liquids under pressure, as is usual for propane, and what for safety of these fuels. In terms of combustion properties, B-plus, acetylene and MAPP gas have high flame temperatures similar to those of methane. The formation reaction is endothermic for B-plus, acetylene and MAPP gas, but exothermic for methane. Finally, the ratio of secondary to primary combustion calorific values for B-plus, acetylene and MAPP gas is similar, but not for methane. The further information available from the gas suppliers is that B-plus reacts violently with the oxides of nitrogen (NO and NO ₂ ), which is a characteristic which makes B-plus a possible afterburning fuel. Because B-plus and MAPP gas are safe enough to be distributed in large volumes, their cost is only 10% more than that of propane, whereas the cost of acetylene in large quantities is a hindrance.

In practicing the invention, B-plus or MAPP, or mixtures thereof, in the range of about 10 to about 100 molecules for each molecule of NO _{x are} injected into the main combustion zone of coal-burning plants, or the B-plus or MAPP molecules heated to temperatures in the range of T <4000 ° C with an atmospheric plasma source and then injected into a post-combustion zone or into the exhaust gas chimney to reduce the emission of NO _x .

In conclusion, acetylene has been more effective than methane for the destruction of oxides of nitrogen in many laboratories proven rium investigations. However, the volatility and the Acetylene costs its use in afterburns projects. Although methane pilot projects After burning worked, they weren't coming done because the methane 20 to 30% of the energy represents released in the coal-fired burner and is therefore considered to be too expensive. The registration have a number of other fuels (MAPP Gas and B-plus) identified that are reactive and reactive to acetylene are kinetically very similar, but are much more stable and that according to be sold in large volumes. This lowers it at a price close to the amount of propane. It will assumed that MAPP gas and B-plus are fuels which for the afterburning of oxides of nitrogen and welding fel should be used. These gases can be close to the Main combustion zone can be applied or they can be applied to  very high temperatures (T <4000 ° C) with an atmospheric Plasma source heated and then inji in the exhaust stack be decorated.

From the foregoing, those skilled in the art can be seen that a main method tion to reduced copy of the NO _x emission has been disclosed and that such a method is of considerable industrial applicability.

Claims (8)

1. A method for reducing NO _x emissions in flue gas, characterized in that it comprises the after-burning of the NO _x -containing flue gas by gradually introducing acetylene-like gases into a combustion zone. 2. The method according to claim 1, characterized records that the acetylene-like gases in the main combustion chamber are introduced. 3. The method according to claim 1, characterized records that the acetylene-like gases are heated and on finally be introduced into a post-combustion zone. 4. The method according to claim 3, characterized records that the acetylene-like gases to a tem temperature of about 4000 ° before the introduction to the afterburn zone. 5. The method according to claim 4, characterized records that the acetylene-like gases in a plas maquelle are heated. 6. The method according to claim 1, characterized records that the acetylene-like gases from the group consisting of MAPP and B-plus, and mixtures thereof are selected. 7. The method according to claim 6, characterized records that the acetylene-like gas is MAPP. 8. The method according to claim 6, characterized records that the acetylene-like gas is B-plus.