JPH0459022A - Desulfurizing method by spraying fine powder desulfurizing agent to waste combustion gas - Google Patents

Abstract

PURPOSE:To eliminate bad influence of molten desulfurizer in a boiler furnace on heat transfer tubes by spraying a fine powder desulfurizing agent to the flow passage of the waste combustion gas at the gas temp. ranging from 500 to 800 deg.C from the boiler furnace to the flue. CONSTITUTION:A fine powder desulfurizing agent is sprayed to the flow passage of the waste combustion as at the gas temp. ranging from 500 to 800 deg.C from the boiler furnace to the flue so as to desulfurize the waste combustion gas. This powder desulfurizing agent collects sulfur oxide in the waste gas. The combustion waste gas at high temp. in the boiler furnace is cooled in a first heat exchanger 7 and second heat exchanger 8. The exhaust gas is at 1500-1600 deg.C near the burner, though this depends on the boiler type, and cooled in the first heat exchanger 7 to 700-900 deg.C. By spraying the fine powder desulfurizing agent in the flow passage of the waste gas from the exit of the first heat exchanger 7, high desulfurizing effect can be obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a desulfurization method for removing sulfur oxides (hereinafter sometimes referred to as S○) from combustion exhaust gas.
In particular, the present invention relates to a desulfurization method in which a fine powder desulfurization agent, which requires simple equipment cost, is sprayed into the combustion gas flow path from the boiler furnace to the flue.

[Prior Art] Desulfurization equipment for thermal power generation boilers is mainly based on a desulfurization method called the limestone-gypsum method. The limestone-gypsum method is a wet desulfurization method in which water is added to pulverized limestone to form a slurry, and this slurry is brought into contact with combustion exhaust gas generated from a boiler to absorb sulfur dioxide gas and recover gypsum as a byproduct. Further, a typical example of a dry method is an activated carbon desulfurization method.

Although the desulfurization performance is high in both the wet method and the dry method, there is a problem in that the equipment is heavy and the initial cost is higher than 1. The former limestone-gypsum method requires wastewater treatment equipment, waste gas reheating equipment, etc. In contrast, a desulfurization method has been proposed in which a powdered desulfurizing agent is sprayed into a boiler furnace. Although this desulfurization method has simple equipment and reduces equipment costs, it has poor desulfurization performance and has not been adopted as a method for desulfurizing combustion exhaust gas from thermal power generation boilers in Japan, where emissions regulations are particularly strict. However, equipment cost I
・Started to attract attention because of its low value, and improvements have been made. In addition, limestone is mainly used as a fine powder desulfurization agent.

[Problems to be Solved by the Invention] In the desulfurization method in which a fine powder desulfurization agent is sprayed into the boiler furnace, the desulfurization reaction when limestone is sprayed into the high temperature furnace part of the boiler is as follows. First, limestone decomposes at high temperatures to produce quicklime, which reacts with SO and 02 to produce anhydrite. It is necessary to spray limestone into a high-temperature atmosphere because it is necessary to carry out a reaction that produces quicklime from limestone. However, if the spraying temperature range is too high, the sprayed limestone may sinterlink, reducing the desulfurization reaction activity, so it is necessary to choose the optimal spraying location. In oil-fired boilers, the temperature inside the furnace is higher than in coal-fired boilers, and limestone cannot be sprayed into the furnace. Furthermore, in coal-fired boilers, the melting temperature of coal ash varies depending on the type of coal being burned, and the limestone sprayed as a desulfurization agent has the problem of lowering the melting point of coal ash. Therefore,
In order to efficiently carry out the limestone quickliming reaction and desulfurization reaction,
Where in the boiler the limestone is sprayed is important.The location of the limestone spray is important. In addition, coal-fired boilers are operated with daily load changes, and when the load changes, the temperature distribution inside the boiler furnace changes, and it is necessary to change the spray position according to the load.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the possibility of 1 to 1 rub due to melting of coal ash and to improve desulfurization performance in a desulfurization method in which a fine powder desulfurization agent is sprayed into combustion exhaust gas.

1.Means to solve the problem] The above object of the present invention is achieved by the following configuration.

That is, in the desulfurization method in which a fine powder desulfurizing agent is sprayed onto the flue gas of a thermal power boiler, the fine powder desulfurizing agent is sprayed from the boiler furnace into the flue gas flow path in the flue where the temperature of the flue gas is in the range of 500°C to 800'C. This is a desulfurization method in which a fine powder desulfurization agent that collects sulfur oxides in the exhaust gas is sprayed onto the combustion exhaust gas.

The fine powder desulfurization agent uses alkali metals or alkaline earth metals including calcium, sodium, and magnesium as metals that fix sulfur oxides. In particular, hydroxides or hydrates of calcium, sodium, and magnesium can improve desulfurization performance in the combustion exhaust gas temperature range of 500°C to 800°C.

Typical examples of common calcium, sodium, and magnesium hydroxides or hydrates include calcium hydroxide, mono-lithium hydroxide, and magnesium hydroxide. In particular, it has been revealed that slaked lime can fix sulfur oxides in combustion exhaust gas to calcium sulfite within a residence time of several seconds in the above-mentioned combustion exhaust gas temperature range. The desulfurization reaction between the flue gas and slaked lime reaches its maximum value when the flue gas temperature is around 600°C. For other hydroxides or hydrates, in order to desulfurize them by contacting them with the combustion exhaust gas in the form of fine powder, the temperature must be 150°C higher than the decomposition temperature of the hydroxides or hydrates.
The optimum temperature range is between 200°C and 200°C. From this, the above-mentioned hydroxide or hydrate should be heated from 500°C to 800°C.
By spraying in the combustion exhaust gas temperature range, the equipment becomes simple and a high desulfurization rate can be achieved.

The sulfite collected in the fine powder desulfurization agent produced by the desulfurization method may be oxidized by an oxidation catalyst installed upstream of the collector to be collected as sulfate. In addition, after the fine powder desulfurization agent is heat-treated at a temperature of 100°C to 500°C,
A material impregnated with an oxidation promoter such as sulfur dioxide gas or calcium sulfite may also be used.

Furthermore, in the desulfurization method, when ammonia gas is supplied to the flue gas flow path accompanied by a fine powder desulfurization agent, nitrogen oxides and sulfur oxides coexisting in the flue gas are collected as ammonia compounds by the fine powder desulfurization agent, so the desulfurization At the same time, denitrification also occurs.

[Action 1 Sulfur oxides in the flue gas generated by coal combustion are combined with hydroxides or hydrates of calcium, sodium, and magnesium sprayed in the temperature range of 500"C to 800°C and the following (1 ) to <4), the reaction occurs and desulfurization is performed.

Ca (○l-1), -+CaO+H20・ ・ --C
a○+SO2→CaSO3・・CaO+S○2+1. /2SO2→Ca5O4 (part of the reaction)... CaSO3+1/2C)z→CaS○4 (part of the reaction) ・ (1) ・ (2) ・ (3) Also, sodium, magnesium hydroxide or water The reaction also occurs in the following manner.

2NaOH+502->Na2SO3+1-120...
(5) M g (OH> 2 +S O2 → MgS○
3+H20 (6) When using slaked lime, the high desulfurization rate at a flue gas temperature of 600°C is mainly due to the reactions of equations (1) and (2), and the byproduct recovered by the electrostatic precipitator is calcium sulfite. be.

This calcium sulfite needs to be oxidized to calcium sulfate.

Note that unreacted CaO in formulas (1) and (2) is reacted with 120 at a temperature of around 400° C. or lower in the flue gas flue to generate Ca (OH) 2.

This newly produced slaked lime reacts with sulfur oxide to produce calcium sulfite as shown in reaction formula (7).

Ca(OH)2+5O2-)CaSOG+1-120 Therefore, the desulfurization method that sprays slaked lime performs a high desulfurization reaction at a high combustion exhaust gas temperature of 500°C to 800°C, and even at a low temperature of 150°C to 60°C. This is a two-step method in which the desulfurization reaction is carried out, and a high desulfurization rate can be obtained.

On the other hand, by adding coal ash to slurry in the presence of metal salts of calcium, sodium, and magnesium, Aβ2o) and si○2Fe2O3 are eluted from the coal ash, and polyhydric hydroxide or water produce a sum product. Typical hydroxides or hydrates when using slaked lime are shown below.

Ca(OH)2+coal ash +3CaO-A#zO3-3CaSOt-mIH20
(8) Ca(OH)2+coal ash→3CaO-Pe203・3CaSOt・m2H20(
9) Ca(OH)2+coal ash→3Ca○・S], 02.3CaSOt・m3H20
(10) Even when the metal salt to be added is sodium or magnesium, hydroxides or hydrates similar to the products shown in formulas (8) to (10) are produced. Such similar hydroxides or hydrates produce needle-like crystals with a particle size of 4 μm or less, increasing the contact area with SO2 and achieving a high desulfurization rate.

The hydroxides or hydrates of (8) to (10) are 12 to 3
2. It has crystal water and the combustion exhaust gas temperature ranges from 500℃ to 800℃.
When it is atomized and sprayed in the temperature range of °C, pores effective for SO2 adsorption develop, and desulfurization performance can be improved.

As mentioned above, when using slaked lime, the desulfurization agent, which has been turned into quicklime (Cab) at a combustion exhaust gas temperature of 500°C to 800°C, has a carbon
The aqueous phase reaction of a (OH) 2 proceeds, and the desulfurization reaction of formula (7) occurs by direct reaction with S02.

A by-product accompanying the desulfurization reaction at a combustion exhaust gas temperature of 800°C to 60'C is calcium sulfite, which is stabilized by being oxidized to calcium sulfate after being recovered by an electrostatic precipitator.

[Example] An example of the present invention is shown in FIG. 1 below. In FIG. 1, pulverized coal 2 is supplied to a coal-fired power generation boiler 1,
It is combusted by air 3. The fine desulfurizing agent 4 passes through a conveyor 5 and is sprayed into the boiler 1 through a flow path 6 along with air introduced through a flow path 9. In the boiler furnace, high-temperature combustion exhaust gas is cooled by a first heat exchanger 7 and a second heat exchanger 8, as shown in FIG.

Depending on the boiler type, the temperature near the burner], 500
The combustion exhaust gas at a temperature of 1600°C to 700°C is cooled by the first heat exchanger 7, and the temperature of the combustion exhaust gas becomes 700°C to 900°C. By spraying the fine powder desulfurization agent into the flue gas flow path at the outlet of the first heat exchanger 7, high desulfurization performance can be achieved by the method of the present invention. The combustion exhaust gas 10 is transferred to the second heat exchanger 8.
It is cooled to around 00°C. As shown in FIG. 1, the cooled exhaust gas 10 is cooled through a flow path 11 through an air preheater 12, and then guided through a combustion exhaust gas flow path 13 to a desulfurization tower 14. In the fine powder desulfurization agent, unreacted CaO in the desulfurization agent is digested into Ca (OH) 2 from the outlet temperature region of the air preheater 12, and is directly converted into SO2.
A desulfurization reaction occurs and produces Ca SO3. In the desulfurization tower 14, water is actively sprayed and supplied from the channel 15 in order to advance the CaO digestion reaction. Coal ash and fine powder desulfurization agent in the combustion exhaust gas are introduced into the electrostatic precipitator 17 through the flow path 16 and recovered. The treated combustion exhaust gas is discharged from the chimney 18. It is also possible to supply the water supplied from the flow path 15 through the flow path 9, and in this case, the water is supplied after being atomized by spraying or as water vapor.

Further, instead of supplying the entire amount of the fine powder desulfurization agent through the flow path 6, it is also possible to supply it in parts to the desulfurization tower 14.

The desulfurizing agent that has collected the sulfur oxides in the combustion exhaust gas is collected by an electrostatic precipitator] 7, the recovered dust l~ is immersed in water, and the supernatant liquid is supplied again to the combustion exhaust gas system from the flow path], 5. It's okay.

In order to understand the desulfurization characteristics of the desulfurization method that involves spraying a fine powder desulfurization agent, we investigated using the experimental apparatus shown in Figure 3. Kanthal heating wires 2 are placed in the ceramic reaction tube 25 at a pitch of 1 mm.
6 to adjust the temperature inside the reaction tube 25. The reaction tube in Figure 3 is a ceramic tube with an inner diameter of 13 mm (effective length 12 mm).
00mm). The fine powder desulfurizing agent 27 was air-flow-transported from the feeder 28 to the reaction tube 25 via two dispersers.

Reaction gas 31 is mixed with carrier gas 30 and reacted in reaction tube 2.
5 and the desulfurizing agent 27 in parallel flow to carry out a desulfurization reaction.

At the outlet of the reaction tube 25, the processing gas 33 and the desulfurizing agent 32 are separated, and a portion of the gas is led to an SO2, N01C02, and O3 analyzer to measure the concentration.

The example shown in FIG. 4 does not show typical experimental results showing the effects of the present invention. The symbols △○ in Figure 4 indicate the temperature dependence of slaked lime, and Ca/S are 1°5, 20.2, and 7, respectively.
This is the result. On the other hand, ・mu■ma shows the temperature dependence of the desulfurization reaction of limestone, and shows the results of changing the particle size of limestone.

The average particle size of limestone is 3.3μm, 78μm, 10.1μm
m, 10.6 μm, and later correspond to the codes ・, mu, kaku, and ma, respectively. The sulfur dioxide gas concentration in the simulated combustion exhaust gas composition is 1500 ppm, 0.26%, CO2 10%
, H2O3%.

When limestone was used, the desulfurization performance was highest at around 1000°C, and as the reaction temperature rose further, the desulfurization performance decreased. The residence time in the effective reaction zone at 1000°C was from 0.9 seconds to 1.2 seconds. On the other hand, when slaked lime is used as a desulfurization agent, when the reaction temperature decreases from 1000℃, the desulfurization performance decreases slightly up to 800℃;
Desulfurization performance begins to increase when the temperature drops below 00°C, and reaches its maximum value at the reaction temperature of 600°C. reaction temperature is 60
Desulfurization performance decreased when the temperature decreased below 0°C. The residence time of slaked lime in the effective reaction region during the experiment varied slightly at each reaction temperature, but ranged from 0.9 seconds to 1.54 seconds. Reaction temperature 600°
Figure 5 shows the results of examining the residence time at C. The residence time was varied from 0.7 seconds to 27 seconds. Desulfurization performance is 0.
The longer the residence time is up to 9 seconds, the higher it becomes, but even if it is longer than that, the desulfurization performance is not affected much. This result shows that the residence time from the outlet of the first heat exchanger 7 shown in FIG.
Since 1 second can be taken from seconds, the present invention can be applied. The X-ray diffraction results of the desulfurizing agent after the reaction obtained for each reaction temperature are shown in FIGS. 6 and 7.

FIG. 6 shows the results of X-ray diffraction of the desulfurization agent, limestone, after the desulfurization reaction was carried out at a reaction temperature of 1000'C. Sulfur oxide is fixed as calcium sulfate, but the sulfur oxide in the by-product of desulfurizing slaked lime at a reaction temperature of 600° C. in FIG. 7 was calcium sulfite.

The example shown in FIG. 8 does not show the results of a desulfurization experiment using a vertical combustion furnace that burns 50 kg/h of pulverized coal in order to verify the effects of the present invention. The combustion furnace includes a main body 40, a pulverized coal supply burner 41, a desulfurization agent supply system 42, 43, and a combustion exhaust gas analysis system 44. The results of the desulfurization experiment were obtained when coal with a sulfur content of 0.38% was burned at an excess air ratio of 1.1 to 1.15. A comparative study was conducted on the desulfurization performance when limestone with a particle size of 325 μm or less or 95% or more was sprayed into the combustion furnace by air, and when slaked lime with an average particle size of 10.2 μm was sprayed into the combustion furnace. .

The example in FIG. 9 shows the relationship between Ca/S and desulfurization efficiency for fine powder desulfurization agents of limestone and slaked lime. When comparing limestone (○ mark) and slaked lime (△ mark), the desulfurization rate of slaked lime was higher even with the same Ca/S. The average flue gas temperature at the position where limestone was supplied into the combustion furnace was 1000°C to 1100°C. In addition, slaked lime was sprayed in the temperature range of the combustion exhaust gas in the combustion furnace from 700°C to 900°C. The residence time of the flue gas from the position where limestone and slaked lime were supplied to the sample position of the flue gas was 0.8 to 1.3 seconds.

In addition, a desulfurization experiment was conducted in the reaction tube 25 shown in Fig. 3 under conditions that optimized the desulfurization reactions of slaked lime and limestone, and a sample of the desulfurization agent was collected at that time, and the low temperature region of the main desulfurization process using the desulfurization agent sample was measured. A simulated desulfurization experiment was conducted.

Desulfurizing agent samples of slaked lime and limestone were used by uniformly mixing 10% of each with coal ash.

The experiment was conducted by spraying the desulfurizing agent sample described above into a reaction tube with a diameter of 50 mm and a length of 2.5 m, simulating a low temperature range of 150'C to 65°C, where the desulfurization reaction occurs actively. The residence time was 8 to 10 seconds. S○2 in exhaust gas
The concentration was 750 ppm, N.

was 350 ppm, CO2 was 10%, and the water content of H2O was adjusted so that the exhaust gas temperature at the outlet of the reaction tube was 65°C.

Figure 10 shows limestone (○) and slaked lime (△) as desulfurizing agents.
) The Ca/S and desulfurization rate are shown for the desulfurization agent sample.

Ca/S was calculated from the analytical value of unreacted CaO in the desulfurized sample, the spray supply amount, the supplied SO2 concentration, and the gas residue, and was expressed as a molar ratio of each. Same-Ca/
It was also found that the slaked lime desulfurization sample has higher desulfurization performance than the limestone desulfurization sample.

Therefore, slaked lime is
The same Ca/
Even S has the effect of providing higher desulfurization performance than limestone. As a result, the desulfurization performance in the range of combustion exhaust gas temperature from 800°C to 65°C is 59.8% for Ca/S = 2 in the spray desulfurization method using limestone, whereas the spray desulfurization method using slaked lime The desulfurization method results in 83.2%.

Note that slaked lime is used when the temperature of the combustion exhaust gas is from 500℃ to 800℃.
In the desulfurization method, which sprays in the temperature range of °C, most of the by-products are calcium sulfite, and what is recovered by an electrostatic precipitator is oxidized to form calcium sulfate.

For this purpose, sulfur dioxide gas in a dust collector or in combustion exhaust gas is oxidized to So, which can be recovered as calcium sulfate using an electrostatic precipitator. In addition, slaked lime impregnated with an oxidation promoter can be heated from 500℃ to 800℃.
Calcium sulfate can be recovered by spraying into the flue gas temperature range of °C. As an oxidation promoter, vanadium salt, iron oxide, mankan oxide, etc. are used at several pI.
) It is effective to impregnate it with slaked lime. Furthermore, in order to impregnate the oxidation promoter, it is effective to perform a heat treatment at a temperature of 100° C. to 500° C. to develop pores.

In addition, by spraying ammonia gas into the low-temperature region where the combustion exhaust gas temperature ranges from 500°C to 800°C, part of the sulfur oxides are collected as ammonium sulfate, and at the same time, nitrogen oxides are converted into calcium nitrite and calcium nitrate. It can be collected as

[Effects of the Invention] The desulfurization method of spraying the fine powder desulfurization agent of the present invention from a boiler furnace into the flue gas flow path in which the flue gas temperature ranges from 500°C to 800°C is a method for spraying the powdered desulfurization agent into a boiler furnace using limestone. Compared to the desulfurization method, the spray temperature range is lower, and there is no harm to the heat transfer tubes due to melting of the desulfurization agent in the boiler furnace.

Additionally, the efficiency of the boiler heat exchanger can be maintained at a high level. Furthermore, this method has the effect of increasing desulfurization performance compared to the method of spraying limestone into the boiler furnace.

Furthermore, the sulfur oxides in the exhaust gas can be recovered as calcium sulfate by spraying a fine powder desulfurizing agent impregnated with an oxidation promoter or by oxidizing the sulfites collected in the fine powder desulfurizing agent. I can do it.

Furthermore, by spraying ammonia gas together with a fine powder desulfurization agent, the exhaust gas can be desulfurized and denitrated.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the desulfurization process of spraying the pulverized desulfurization agent of the present invention, Figure 2 is a diagram showing a typical example of the spraying position of the present desulfurization agent, and Figure 3 is a diagram of the basic experimental equipment simulating the inside of a boiler furnace. Schematic diagram, Figure 4 is a diagram showing the combustion exhaust gas temperature dependence on desulfurization performance when slaked lime is used as the desulfurization agent of the present invention, and Figure 5 is a diagram showing the residence time and desulfurization performance of slaked lime at a temperature of 600'C. Figures 6 and 7 show the relationship between by-products
Linear diffraction diagram, Figure 8 is a diagram of the desulfurization experiment equipment using a pulverized coal combustion furnace, Figure 9 is a comparison diagram of the desulfurization performance of slaked lime and limestone, and Figure 10 is a diagram of the present invention method and a limestone furnace in the temperature range of 150 to 65°C. It is a figure which shows the comparison result of the desulfurization performance of the internal blowing method. Co... Boiler furnace, 2... Pulverized coal, 3... Combustion air, 4... Fine powder desulfurization agent, 5... Conveyor, 6... Desulfurization agent spray channel, 7 - First heat Exchanger, 9. Spray air flow path, 12. Air preheater, 14.. Desulfurization tower, 15.. Water supply flow path, 17.. Electrostatic precipitator Applicant Babcock Hitachi Co., Ltd. Agent Patent attorney Takayoshi Matsunaga Haka 1 person

Claims (10)

[Claims] (1) In the desulfurization method in which a fine powder desulfurizing agent is sprayed onto the flue gas of a thermal power generation boiler, the fine powder desulfurizing agent is sprayed from the boiler furnace into the flue gas flow path in the flue where the flue gas temperature is in the range of 500 to 800 degrees Celsius and then combusted. A desulfurization method that involves spraying a fine powder desulfurization agent onto combustion exhaust gas, which is characterized by the fact that the sulfur oxides in the exhaust gas are collected by the desulfurization agent. (2) The fine powder desulfurization agent is a compound containing at least one component of calcium, sodium, and magnesium,
A desulfurization method in which the fine powder desulfurization agent according to claim 1 is sprayed into combustion exhaust gas, wherein the desulfurization agent is a hydroxide or hydrate that releases water at a temperature of 500°C to 800°C to become a metal oxide. (3) The fine powder desulfurization agent is a slurry obtained by adding water to a hydroxide or hydrate containing at least one component of calcium, sodium, and magnesium. A desulfurization method in which fine powder desulfurization agent is sprayed into the combustion exhaust gas. (4) Combustion of the fine powder desulfurizing agent according to claim 3, wherein the fine powder desulfurizing agent is a mixture of hydroxide or hydrate containing at least one component of calcium, sodium, and magnesium to which water vapor is added. A desulfurization method that sprays into exhaust gas. (5) A desulfurization method in which the fine powder desulfurizing agent according to claim 1 is sprayed into combustion exhaust gas, characterized in that the fine powder desulfurizing agent is impregnated with an oxidation promoter. (6) A desulfurization method in which the fine powder desulfurizing agent is sprayed into the combustion exhaust gas according to claim 1, characterized in that water is sprayed downstream of the position where the fine powder desulfurizing agent is sprayed. (7) The fine powder desulfurizing agent according to claim 1, wherein the fine powder desulfurizing agent is impregnated with an oxidation promoter such as sulfur dioxide gas or calcium sulfite after being heat-treated at a temperature of 100°C to 500°C. A desulfurization method that sprays combustion exhaust gas. (8) The sulfite collected in the fine powder desulfurization agent produced by the desulfurization method according to claim 1 is oxidized to sulfate by an oxidation catalyst installed upstream of the collector, and then the collector A desulfurization method that sprays a fine powder desulfurization agent, which is recovered in the combustion exhaust gas, into the combustion exhaust gas. (9) Dust containing a fine desulfurizing agent that has collected sulfur oxides in combustion exhaust gas by the desulfurization method according to claim 1 is collected by a collector installed in a flue, the collected dust is immersed in water, and the supernatant is collected. A desulfurization method for spraying a fine powder desulfurizing agent into a combustion exhaust gas, characterized in that a liquid is sprayed into a flue gas flow path downstream of a position where the fine powder desulfurizing agent is sprayed. (10) In the desulfurization method according to claims 1 to 8, ammonia gas is supplied to the flue gas flow path in which the fine powder desulfurizing agent is accompanied, and nitrogen oxides and sulfur oxides coexisting in the flue gas are converted into ammonia compounds and converted into the fine powder desulfurizing agent. A desulfurization and denitrification method that sprays a fine powder desulfurization agent, which is characterized by collection, into combustion exhaust gas.