JPS5813802B2 - NOx suppression combustion method using hydrogen and hydrocarbon gas co-combustion - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for suppressing the generation of nitrogen oxides (hereinafter referred to as NOx) during hydrogen combustion.

Hydrogen is expected to be a fuel of the future, but this is based on the fact that hydrogen is a clean fuel.

However, in reality, hydrogen combustion generates soot, dust, sulfur oxides, and nitrogen oxides, which are said to be the three major combustion pollutions, and although the first two are guaranteed to be harmless, there is no
There is no guarantee for x at all.

Especially when hydrogen is combusted at a high premixed air ratio, NO is generated.
x concentration is extremely low, and hydrogen can be said to be an extremely low NOx fuel. However, when it is combusted at a premixed air ratio of less than 0.4, the NOx concentration generated is lower than that of other fuels such as hydrocarbon gas, as shown in Figure 1. It is extremely expensive and cannot be called a clean fuel.

Furthermore, hydrogen is a fuel that is extremely prone to flashback, and it is very dangerous to operate at a high premixed air ratio.

This is due to the fact that hydrogen has the highest combustion speed at a low air ratio (approximately 0.5 to 0.7), a characteristic not found in other fuels (the maximum combustion speed of hydrocarbon gas appears at an air ratio of approximately 1). ) and is an inevitable phenomenon.

In this way, if hydrogen is to suppress the generation of NOx, it is necessary to use a high premixed air ratio, which increases the risk of flashback, and to avoid flashback, it is necessary to use a low premixed air ratio. However, the problem is that the generation of NOx greatly increases.

This phenomenon has recently become a problem as the "hydrogen flame dilemma" (for example, "Research on Hydrogen Combustion Technology, 1977 Report" edited by the Japan Gas Association, Chapter 1, Section 2, Section 2, 1977).
.

In the present situation where no definitive measure against flashback has been found, hydrogen combustion must be used at a low premixed air ratio, and therefore a high degree of NOx generation is unavoidable.

Therefore, suppression of NOx in a hydrogen flame can be regarded as one of the most important technical issues in the utilization of hydrogen fuel.

On the other hand, in the combustion of hydrocarbon gas, the premixed air ratio is generally 0.
5 to 1 are used.

It is known that if a portion of the hydrocarbon gas is replaced with an equal caloric amount of hydrogen (or an equivalent amount of oxygen consumed) and co-combusted under these conditions, the amount of NOx generated will increase.

Therefore, there is a general impression that hydrogen co-firing is disadvantageous in suppressing NOx.

As a result of investigating the suppression of NOx generation in a hydrogen diffusion flame, the present inventor surprisingly found that when a small amount of hydrocarbon gas is added under the condition of a premixed air ratio of less than 0.4, NOx The present invention was completed by discovering scales that drastically reduce the amount of scales generated.

That is, in the present invention, hydrogen is preliminarily mixed with hydrocarbon gas having an oxygen consumption of 30 equivalent% or less at an air ratio of 0.4 or less,
This provides a NOx suppression combustion method that involves burning this.

The hydrocarbon gas added to the system can be either saturated or unsaturated as long as it is a gas at room temperature.

Specific examples include methane, ethane, ethylene, acetylene, propylene, fluoropylene, and butane, and hydrocarbon gases having 3 or more carbon atoms are particularly preferably used.

The premixed air ratio (λ,) of the hydrogen flame may be any value as long as it is 0.4 or less, and from λ1 = 0.4 (perfect diffusion flame) to λ1
-0 is considered, but λ1-0~λ, =
0. A range of 3 is particularly preferred.

The amount of hydrocarbon gas added is preferably 30 equivalent % or less, particularly 5 to 20 equivalent %, based on the equivalent amount of hydrogen.

However, here, the equivalent amount is indicated by the amount of oxygen consumed during combustion of hydrocarbons and hydrogen, and the equivalent volume of each gas is the following value.

H2-1, CH4: 4, C2H2:5, C3H6:9
, C3H8:10.

The present invention will be explained below based on experimental results.

FIG. 1 is a diagram showing the relationship between the premixed air ratio of various gases and the generated NOx concentration.
This is a measured value for a flame that was supplied and burned to the same equivalent as 0 l/min (25 Kcal/min), and is an experimental value of approximately the same calorific value/min.

Generally, at high premixed air ratios, the hydrogen flame itself already has significantly low NOx, as seen in FIG. 1, so the addition of hydrocarbon gas is an unnecessary measure.

Therefore, considering only the low air premixing condition, the NOx level of H2 in FIG. 1 is almost constant at a high level when λ1<0.4, but requires the help of NOx suppression technology.

Regarding other hydrocarbon gases, with the exception of a portion of C2H2, when λ1<0.4, the NOx level is low and there is no significant fluctuation.

Therefore, when a small amount of hydrocarbon gas is co-combusted with hydrogen, it is necessary to verify whether or not there is an advantage of co-combustion, depending on whether the NOx level of the exhaust gas from the combustion of both gases alone is higher or lower than the weighted average value.

Figure 2 shows a comparison between the actually measured NOx concentration and the theoretical average value (the weighted average value of the NOx level of the individual combustion flue gas of each gas) for a mixed system of hydrogen and saturated hydrocarbon gas, using solid lines and dotted lines. be.

In both cases, the measured line is below the theoretical line, clearly indicating that mixed firing reduces NOx more than the sum of single firing.

FIG. 3 shows a similar comparison for a mixed system of hydrogen and unsaturated hydrocarbon gas, and also clearly shows that co-firing results in a better NOx reduction.

The downward shift of the measured NOx line shown in FIGS. 2 and 3 indicates the following.

1) For each line, a large downward deviation of the solid line appears when X≧70, especially around X=80 to X-95 (range where 5 to 20 equivalent percent of hydrocarbon gas is added to hydrogen).

2) There is no significant difference between saturated and unsaturated hydrocarbons.

The co-firing benefit (the downward shift of the solid line from the dotted line) tends to be greater for hydrocarbons with a larger carbon number.

As described above, the present invention is characterized by mixing hydrogen with a hydrocarbon gas having a premixed air ratio of 0.4 or less and an oxygen consumption of 30 equivalent % or less, and combusting the mixture.

As a result, according to the present invention, NO in flue gas is unexpectedly reduced.
The x concentration can be kept significantly lower than the weighted average value of the NOx concentration in the flue gas when hydrogen and hydrocarbon gases are each combusted alone.

Further, in the present invention, since the premix air ratio is set to 0.4 or less, steady combustion operation can be performed while preventing hydrogen flashback.

Therefore, according to the present invention, it is possible to solve all the problems during hydrogen combustion, where conventionally it was impossible to prevent flashback and reduce NOx concentration at the same time, and to improve the status of hydrogen as a clean fuel. can be established.

Furthermore, in the present invention, the amount of hydrocarbon gas mixed with hydrogen is 30 equivalent % or less as oxygen consumption, that is, volume %.
Since the amount is about 3%, it is required in a very small amount and does not increase the hydrogen cost much, further increasing the practical value of the present invention.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the dependence of NOx generation of various gases on the premixed air ratio. λ1: Premixed air ratio, NOx: NOx concentration in flue gas (air ratio converted to 1) FIG. 2 is a diagram showing NOx generation in hydrogen-methane system and hydrogen-propane system co-combustion. X: Oxygen consumption equivalent rate in mixed gas of hydrocarbon gas, NO
x: Same as Figure 1, -: Actual NOx value, ...
・: Weighted average value of exhaust NOx for each individual gas combustion. FIG. 3 is a diagram showing NOx generation in hydrogen-acetylene system and hydrogen-propylene system co-firing. x, NOx: According to Fig. 1 and Fig. 2. , 5...: Same as Figure 2.

Claims (1)

[Claims] 1 At an air to hydrogen ratio of 0.4 or less, oxygen consumption is 3
A NOx suppression combustion method by co-firing hydrogen and hydrocarbon gas, which is characterized by premixing 0 equivalent % or less of hydrocarbon gas and combusting it.