JPS5856045B2 - Fluidized bed combustion method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a combustion method using a fluidized bed, and in particular to a combustion method using a fluidized bed, and in particular to a method for controlling nitrogen oxides (NOx), carbon oxides, etc. in exhaust gas after combustion when solid combustion containing a nitrogen valve is used. This invention relates to a fluidized bed combustion method that reduces the concentration of combustible gas components.

The conversion of nitrogen during combustion into nitrogen oxides tends to be promoted as the combustion temperature and residual oxygen concentration increase.

Nitrogen from the use of air as the oxygen source required for combustion can also be converted to nitrogen oxides, and this also progresses more easily at higher temperatures.

The former is fuel NOx, the latter is fuelNOx.
It is called NOx.

In combustion under atmospheric pressure, almost no thermal NOx is generated unless the temperature reaches 1200°C or higher.

On the other hand, carbon monoxide, which tends to coexist with oxygen at relatively high temperatures, is also relatively quickly converted to carbon dioxide at temperatures of 750° C. or higher.

Combustion methods using a fluidized bed can control the temperature of the combustion part almost uniformly and do not create localized high-temperature areas, so they produce less NOx than normal combustion methods, such as heavy oil boilers. It is possible to keep it fairly low.

This is due to extremely low generation of thermal NOx.

However, if a fuel containing a large amount of nitrogen, such as coal, is used, a non-negligible amount of N is produced even in the fluidized bed combustion method.
Ox is generated.

Most of this NOx is considered to be fuel NOx.

As a method for reducing this fuel NOx, it is conventionally known to reduce the amount of oxygen to be supplied, in other words, to reduce the concentration of oxygen gas remaining in the exhaust gas after combustion.

However, lowering the residual oxygen concentration causes disadvantages such as a decrease in fuel combustion efficiency and the start of generation of flammable gases such as carbon monoxide, hydrogen, and methane.

The present invention has been made in an attempt to improve the above-mentioned drawbacks.
The aim is to create a fluidized bed combustion method that generates extremely little NOx even when solid fuel with a high nitrogen content is used, has few combustible components in the exhaust gas, and does not require any additional exhaust gas after-treatment. is to provide.

That is, the feature of the present invention is that solid fuel containing nitrogen is
In a method of fluidized combustion in a combustion device, the combustion device has a main fluidized bed section and a sub fluidized bed section separated by a partition wall, and has a common freeboard section above both fluidized beds. For complete combustion of the solid fuel and the fuel in the main fluidized bed section, insufficient primary air is introduced to fluidize and burn most of the fuel, and then the combustion residue in the main fluidized bed section is The solid fuel containing the solid fuel is moved to a sub-fluidized bed section, and a sufficient amount of secondary air is introduced into the sub-fluidized bed section for combustion of the solid fuel remaining after combustion and the combustible gas component in the freeboard section. In this fluidized bed combustion method, gases generated from the upper and sub-fluidized bed sections are combined in a freeboard section, and combustible components in the gas are oxidized by oxygen contained in the gas.

In the method of the present invention, solid fuels containing a large amount of nitrogen, such as coal, asphaltenes produced by tax sources, household dust, wood, coke, and coke-like fuel adhering decomposers produced by heavy oil cracking reactions are used to remove NOx and CO. It is possible to suppress generation and achieve complete combustion.

The combustion apparatus used in this method consists of a main fluidized bed section, a subfluidized bed section, and a freeboard section, and the two fluidized bed sections are separated by a partition wall.

Moreover, a freeboard section is provided above both fluidized bed sections for allowing gases generated from both fluidized bed sections to join together and oxidize combustible components.

A solid fuel containing nitrogen is introduced into the main fluidized bed section, and primary air is introduced from below, where it is fluidized and combusted.

The amount of primary air introduced is slightly insufficient for complete combustion of the solid fuel.

That is, the chemical equivalent ratio of primary air (to theoretical air ratio) is 0.9
It is more preferably 0.91 to 0.97 than 0 to 0.98.

Since the main fluidized bed section and the sub-fluidized bed section are divided by partition walls, the bed heights are almost the same, and the cross-sectional area ratio of the main and sub-fluidized bed sections is 9:1 to 11:1.

Therefore, the volume of the fluidized bed is approximately 9:1 to 11:1.
It is.

That is, the volume of the main fluidized bed is 9 to 11 times that of the auxiliary fluidized bed.
Therefore, the main fluidized bed section is combusted with a slight lack of oxygen, but most of the solid fuel (approximately 90%) introduced into the combustion apparatus is combusted within the main fluidized bed section. It turns out.

Furthermore, since oxygen-deficient combustion is performed using air supplied from the bottom of the fluidized bed, the gas generated from the bed is rich in combustible gas components such as CO and methane ethylene in addition to water vapor and N2.

No generated locally becomes CO in the process of rising through the fluidized bed.
will be reduced by

The unburned solid fuel remaining or components in the main fluidized bed, the catalyst carrier components, and the solid unburnt portions in the main fluidized bed overflow the partition wall and are introduced into the auxiliary fluidized bed section.

Excess secondary air is introduced from below into the sub-fluidized bed section,
There, the remaining solid fuel is completely combusted.

The gas generated from the auxiliary fluidized bed section contains almost no combustible gas components, but contains an excessive amount of 02.

As mentioned above, combustion in the auxiliary fluidized bed is oxygen-excessive combustion, so NOx is generated, but only a small portion of the fuel introduced into the entire combustion apparatus is combusted; There is very little NOx generated from the layer.

On the other hand, in the main fluidized bed section, oxygen-deficient combustion occurs, so even if most of the fuel is combusted, the combustion temperature is low and NOx is not generated due to the generation of CO.

The gases generated from both fluidized beds meet at the freeboard section,
Combustible gas components such as CO, methane, and ethylene from the main fluidized bed part and 02 from the auxiliary fluidized bed part join together, and through an oxidation (combustion) reaction, CO, methane, ethylene, etc. are converted into CO2, H
Changes to 2O.

Therefore, a sufficient excess amount of secondary air should be introduced into the auxiliary fluidized bed section, but if necessary, additional air may be newly introduced into the freeboard section.

In order to cause combustion in the freeboard section, consideration must be given to gas mixture and residence time.

Combustible gases, especially carbon monoxide, which easily coexists with oxygen, must be heated at a temperature of 750°C or higher, more preferably 8°C.
If it is above 00℃, the residual oxygen concentration is low 0.5 to 1.0
It is possible to reduce carbon monoxide to a practically satisfactory low concentration within a reaction time that can be carried out even under conditions of about 10%.

Figure 3 shows this, which shows that the residual oxygen concentration is 0.
5 to 1.0%, and experimental results showing the relationship between reaction temperature and residual carbon oxide concentration using reaction time τ as a parameter.
The broken line is the carbon monoxide concentration before the reaction.

As can be seen from this result, the higher the reaction temperature, the shorter the time required for the reaction, but if the temperature is extremely high, the aforementioned thermal NOx will be generated, so air or a similar type of oxygen source for combustion is recommended. When using a gas containing nitrogen gas, the temperature is preferably below 1200°C.

Additionally, if the oxidation of the combustible gas described above is accompanied by a reaction that generates a new combustible gas, the concentration of the combustible gas cannot be sufficiently reduced, so a new source of combustible gas is introduced into the oxidation reaction region. It is necessary to prevent the introduction of particles containing unburned fuel.

The present invention will be explained in more detail below with reference to Examples.

Example 1 FIG. 1 shows an example in which the present invention is applied to the regeneration of a catalyst used for catalytic cracking of vacuum residue oil.

In Figure 1, 1 is the main fluidized bed section, 2 is the sub-fluidized bed section, and 3 is the main fluidized bed section.
is a freeboard part, 4 is an insert, 5 is a cyclone, 6 is a distributor, 7 is a primary air introduction pipe, 8 is a secondary air introduction pipe, 9
is an exhaust pipe, 10 is a catalyst supply pipe, 11 is a catalyst extraction pipe, 12
1 is a combustion device (regeneration tower), 13 is a partition wall, and 14 is a heat exchanger tube.

The spent catalyst with coke-like hydrocarbons produced as a by-product of catalytic cracking adhering to the surface is supplied to the main fluidized bed section 1 from the catalyst supply pipe 10.

Most of the air required for the combustion of the hydrocarbons is introduced from the primary air introduction pipe 7, and is uniformly supplied to the main fluidized bed section 1 by the disperser 6, and at the same time fluidizes the catalyst in the main fluidized bed section 1. , most of the hydrocarbons adhering to the surface of the catalyst are removed by combustion.

The temperature of the main fluidized bed section 1 can be controlled by providing heat transfer tubes 14 in the main fluidized bed section 1 as necessary.

The catalyst from which most of the hydrocarbons adhering to the surface have been removed overflows from the main fluidized bed section 1 to the auxiliary fluidized bed section 2 partitioned by partition walls 13.

Air in excess of the amount required to burn the hydrocarbons remaining on the catalyst and the combustible components in the combustion gas in the main fluidized bed section 1 is supplied to the auxiliary fluidized bed section 2 from a secondary air introduction pipe 8 via a disperser 6. The catalyst particles in the auxiliary fluidized bed section 2 are fluidized, and at the same time, the hydrocarbons remaining on the surface are completely burned off.

The combustion gas from the main fluidized bed section 1 containing a low concentration of combustible components and the combustion gas from the auxiliary fluidized bed section 2 containing a high residual oxygen concentration merge at the upper part of the fluidized bed and are oxidized while rising through the freeboard section 3. Slowly oxidizes combustible components such as carbon and hydrogen.

The inserts 4, 4', and A' promote the mixing of the combustion gases rising in the freeboard section 3, allowing the oxidation of combustible components to occur in a shorter time, and at the same time, inertial separation of particles scattered from the fluidized bed. Structure is preferred.

Among combustible gases, hydrogen can be oxidized and removed in a short time even at a relatively low temperature, but a relatively high temperature and long reaction time are required to sufficiently remove carbon monoxide.

By giving the inserts 4, 4', and A' a particle separation effect, new carbon monoxide generation within the freeboard section 3 is suppressed, and the remaining carbon monoxide concentration can be more easily reduced. enable.

The combustion gas is finally led to a cyclone 5 installed at the top of the freeboard section 3, and after separating the accompanying catalyst particles, is led out of the system through an exhaust pipe 9.

The cross-sectional area of the main fluidized bed section 1 is 1150 cyyt, and the sub-fluidized bed 2
Using a device with a cross-sectional area of 120 CIA, the concentrations of residual oxygen, carbon monoxide, and nitrogen monoxide in the exhaust gas when changing the amount of air supplied to the primary air introduction pipe 7 and the secondary air introduction pipe 8 are calculated as follows. Shown in the table.

Table 2 shows the main properties of the catalyst particles used and the elemental analysis results of the coke-like hydrocarbons attached to the surface.

Combustion was carried out under a pressure of approximately 1.5 kg/crltG, and the temperature of the main fluidized bed section 1 was within the range of 845-857°C.

7I7-The temperature inside the board section 3 gradually decreased with height, dropping at 810-823° Ct at the entrance of the cyclone 5.

The gas residence time in the freeboard section 3 is about 4.5 seconds at the above temperature.

The average catalyst supply rate is 33 kg/hr, and the coke-like hydrocarbon entrained and supplied to the main fluidized bed section 1 is 10 kg/hr.
, 7 kg/hr, whereas the theoretical air amount calculated from elemental analysis was 95.2 Nm''/hr.

*Comparative Example 1 Using the same equipment as in Example 1 shown in FIG. 1, nitrogen gas was supplied from the secondary air introduction pipe 8 for the purpose of fluidizing the auxiliary fluidized bed section 2.
When all the air necessary for combustion was supplied from the primary air introduction pipe 7, the results shown in Table 3 were obtained.

Among these, in two experimental examples (lower two in Table 3) where the air supply amount was insufficient, residual hydrocarbons were observed on the catalyst.

Comparative example 2 Insertion 4, 4' from the apparatus of Example 1 of FIG.

stomach' When , was removed, the results shown in Table 4 were obtained.

FIG. 2 illustrates the relationship between the amount of primary air fed to the device shown in FIG. 1 and the concentration of nitrogen monoxide in the exhaust gas.

The relative primary air amount expressed as a ratio to the theoretical air amount is 1.0
The concentration of nitric oxide decreases rapidly near the point where the

Since it has been confirmed that nitrogen dioxide is less than 1/10 of nitrogen monoxide, the total amount of nitrogen oxides can be approximately regarded as nitrogen monoxide.

Therefore, the generation of nitrogen oxides can be suppressed by operating the relative air supply amount at 1.0 or less, more preferably at 0.97 or less.

The reason why the nitrogen monoxide concentration increases when the relative air supply amount is 0.9 or less is because the nitrogen monoxide generated by combustion in the sub-fluidized bed section 2 can no longer be ignored.

Therefore, the amount of secondary air to the main fluidized bed section is 0.9 of the theoretical amount.
0 to 0.97, and reduce the combustion load on the auxiliary fluidized bed section.

That is, it is important to set the volume ratio to the main fluidized bed part to 9:1 to 11:1 in order to suppress NOx as a whole.

In a fluidized bed combustion apparatus, although there is a capacity range, it is not practical to greatly change the flow rate of the fluidizing gas, so the cross-sectional area ratio of the main fluidized bed section 1 and the sub fluidized bed section 2 is A button breast ratio of 9:1 to 11:1 is preferable for suppressing NOx and CO.

As shown in FIG. 1, it is preferable for the main fluidized bed section and the auxiliary fluidized bed section to be placed adjacent to each other by a partition wall in the combustion apparatus in terms of heat utilization and design.

The combustion device may be either circular or box-shaped, and in the case of a box-shape, the partition wall will be in the shape of a partition plate or an inner box.

The reduction in the concentration of combustible components is completely incompatible with the reduction in nitrogen oxides even in Comparative Example 1, which is not according to the present invention.

Furthermore, even in Comparative Example 2 in which only secondary air was supplied, even if nitrogen oxides were sufficiently reduced, a considerable amount of combustible gas components remained, and mixing of the secondary air was promoted. The effect of the present invention, which is accompanied by a reduction in entrained particles, is remarkable.

According to this embodiment, by controlling the secondary and secondary air supply amounts, it is possible to suppress the generation of nitrogen oxides even under high residual oxygen concentrations. This makes it possible to operate without increasing the amount of combustible gas produced.

Furthermore, in this example, by providing an auxiliary fluidized bed section,
It becomes possible to completely regenerate the catalyst even under conditions where the final residual oxygen concentration is quite low.

As described above, according to the present invention, even solid fuel containing nitrogen can be combusted while suppressing the emission of NOx and combustible components to extremely low concentrations, and therefore has great industrial value.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of one embodiment of the present invention, Fig. 2 is a graph showing the effect of the apparatus shown in Fig. 1, and Fig. 3 is a graph showing the result of reducing carbon monoxide depending on reaction temperature and time. It is. DESCRIPTION OF SYMBOLS 1...Main fluidized bed part, 2...Subfluidized bed part, 3...Freeboard part, 4, 4', 4'...Interpolate, 5...
Cyclone, 6... Distributor, 7... Primary air introduction pipe, 8... Secondary air introduction pipe, 9... Exhaust pipe, 14...
・Heat transfer tube.

Claims (1)

[Claims] 1. A method of fluidized combustion of a solid fuel containing a nitrogen valve in a combustion device, the combustion device having a main fluidized bed section and a sub fluidized bed section separated by a partition wall, and , has a common freeboard section above both fluidized beds, and introduces the solid fuel and primary air, which is insufficient for complete combustion of the fuel, into the main fluidized bed section to burn most of the fuel. Fluidized combustion and then the solid fuel containing the combustion residue in the main fluidized bed section,
A sufficient amount of secondary air is introduced into the secondary fluidized bed section to burn the solid fuel in the combustion residue and the combustible gas component in the freeboard section to cause fluidized combustion. A fluidized bed combustion method characterized in that gases generated from upper and sub fluidized bed sections are combined in a freeboard section, and combustible components in the gas are oxidized by oxygen contained in the gas. 2 The cross-sectional area of the main fluidized bed part and the sub fluidized bed part is 9=1 to 11
Claim 1 characterized in that the range is: 1.
Fluidized bed combustion method described in section.