JP2013091045A - Exhaust gas treating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To treat a nitrogen oxide contained in exhaust gas that contains components usually called poisoning substance.SOLUTION: An exhaust gas treating method is provided for treating, in the presence of a denitration catalyst, the nitrogen oxide in exhaust gas containing the nitrogen oxide that contains at least one element selected from a group consisting of alkali metal, alkali earth metal, phosphorus, arsenic, silicon, zinc, lead, iron, antimony and vanadium, and/or its compound. The denitration catalyst contains an active component such as vanadium, niobium or tantalum, and an titanium oxide, and the titanium oxide is a composite oxide of titanium and a base material addition component such as aluminum, silicon or zirconium.

Description

  The present invention relates to treatment of exhaust gas containing nitrogen oxides, and in particular, technology relating to treatment of exhaust gas containing alkali metals, alkaline earth metals, phosphorus, arsenic, silicon, zinc, lead, iron, antimony, and vanadium. It is.

  As a method for removing nitrogen oxides in exhaust gas that is currently in practical use, a selective catalyst that catalytically reduces nitrogen oxides in exhaust gas using a reducing agent such as ammonia or urea and decomposes it into nitrogen and water. The reduction method (SCR method) is common. In recent years, as environmental pollution caused by nitrogen oxides has become serious worldwide, as represented by acid rain, a high-performance catalyst has been demanded.

  For example, as a conventional technique related to a denitration catalyst, as a catalyst effective for removing nitrogen oxides, a metal component composed of titanium, vanadium and tungsten or a compound group containing a metal component composed of titanium, vanadium, tungsten and molybdenum is mixed. A catalyst prepared using a heat-treated raw material is disclosed (Patent Document 1).

On the other hand, there are alkali metals, alkaline earth metals, phosphorus, arsenic, silicon, zinc, lead, iron in the exhaust gas discharged from combustion furnaces and boilers fueled with heavy oil and coal, exhaust gas from marine diesel engines, and waste incinerators Antimony and the like are contained, and these accumulate in the catalyst and cause poisoning of the catalyst active sites and coating of the catalyst surface, which causes a reduction in denitration activity. Furthermore, in combustion furnaces and boilers that use heavy oil as fuel, vanadium may be contained in the exhaust gas. When this accumulates in the catalyst, the oxidation activity of SO ₂ to SO ₃ increases with time, and the catalytic reactor is Problems such as corrosion of pipes and equipment on the flow side occur.

Therefore, in addition to being excellent in nitrogen oxide removal performance as a characteristic required for the catalyst, there is a case where it is required to suppress poisoning resistance to components contained in exhaust gas and increase in SO ₂ oxidation rate. Technology has not always been sufficient in these respects.

JP-A-10-323570

The object of the present invention is to solve the above-mentioned problems of the prior art, and to reduce the denitration activity due to the components contained in the exhaust gas such as alkali metal, alkaline earth metal, phosphorus, arsenic, silicon, zinc, lead, iron, antimony, vanadium, and An object of the present invention is to provide an exhaust gas treatment method capable of suppressing an increase in the SO ₂ oxidation rate.

  As a result of intensive studies to solve the above problems, the present inventor has found that it is effective to use the following denitration catalyst. The denitration catalyst used in the present invention contains at least one element selected from the group consisting of vanadium, niobium, tantalum, molybdenum and tungsten or a compound thereof (active component) and titanium oxide, and the titanium oxide is aluminum, silicon In addition, at least one element selected from the group consisting of zirconium, molybdenum and tungsten (base material addition component) and one or more oxides of a composite oxide and / or mixed oxide of titanium are used.

  The present invention relates to nitrogen oxide (NOx) contained in exhaust gas discharged from heavy oil fired boilers, coal fired boilers, diesel engines, thermal power plants, incineration facilities for treating industrial waste and municipal waste, and various industrial processes. The present invention relates to an exhaust gas treatment method for catalytic reduction using a reducing agent such as ammonia or urea.

By using the present invention, nitrogen oxides contained in exhaust gas can be effectively treated. Particularly treating nitrogen oxide-containing exhaust gas containing at least one element selected from the group consisting of alkali metals, alkaline earth metals, phosphorus, arsenic, silicon, zinc, lead, iron, antimony and vanadium and / or compounds thereof Even when this is done, a high nitrogen oxide treatment capacity can be maintained for a long time. In addition, an increase in the SO ₂ oxidation rate due to the accumulation of vanadium can be suppressed.

The denitration catalyst used in the present invention contains at least one element selected from the group consisting of vanadium, niobium, tantalum, molybdenum and tungsten or a compound thereof as an active component. Among these, the use of at least one element selected from the group consisting of vanadium, molybdenum and tungsten or a compound thereof as an active component is particularly preferable because it can maintain high NOx removal performance. The proportion of the active component in the total catalyst is preferably 0.1 to 40% by mass (as oxide), more preferably 0.2 to 20% by mass, and still more preferably 0.4 to 10% by mass. It is. If the content of the active ingredient is less than 0.1% by mass, sufficient NOx removal performance cannot be obtained, and it becomes susceptible to poisonous substances contained in the exhaust gas such as alkali metals, and exceeds 40% by mass. This is because if the amount is increased, the SO ₂ oxidation rate becomes high, which may cause problems such as corrosion of piping and equipment on the downstream side of the catalytic reactor. When the active ingredient contains at least two elements selected from the group consisting of vanadium, niobium, tantalum, molybdenum and tungsten, or a compound thereof, the content is within the above range for each element or the compound. Is good.

  The denitration catalyst used in the present invention is one in which the active component is supported on a base material component made of titanium oxide. The ratio of the base material component to the entire catalyst is preferably 60 to 99.9% by mass (as oxide), more preferably 80 to 99.8% by mass, and still more preferably 90 to 99.6% by mass. %. By making the ratio of the base component within this range, high NOx removal performance can be maintained.

  The main component of the base material component is titanium oxide, but in addition to titanium, as a base material addition component, at least one element selected from the group consisting of aluminum, silicon, zirconium, molybdenum, and tungsten and a composite oxide or mixture of titanium An oxide is used. Two or more complex oxides or mixed oxides may be mixed and used.

  The titanium content in the composite oxide or mixed oxide is preferably 40 to 99 mol%, more preferably 60 to 98 mol%, still more preferably 70 to 97 mol%. By setting the titanium content in the above range, high NOx removal performance can be maintained. In particular, in order to sufficiently exhibit the effects of the present invention, it is preferable that the complex oxide or mixed oxide contains silicon. In this case, the content of silicon in the composite oxide or mixed oxide is preferably 2 to 50 mol%, more preferably 4 to 40 mol%, still more preferably 5 to 30 mol%. There should be. This is because if the silicon content in the composite oxide or mixed oxide is less than 2 mol%, a sufficient effect may not be obtained, and if it exceeds 50 mol%, the NOx removal performance decreases.

The specific surface area of the denitration catalyst used in the present invention should be in the range of 50 to ²⁰⁰ m ² / g, more preferably 60 to 150 m ² / g, and still more preferably in the range of 65 to 130 m ² / g. Good. If the specific surface area of the catalyst is too low, sufficient catalyst performance cannot be obtained, and active component sintering tends to occur. If it is too high, the catalyst performance will not improve much, but the amount of poisonous substances accumulated will increase. This is because the durability may be lowered.

  The pore volume of the denitration catalyst used in the present invention is such that the total pore volume is in the range of 0.2 to 0.7 mL / g, more preferably 0.3 to 0.6 mL / g, Preferably it is in the range of 0.35 to 0.5 mL / g. If the pore volume of the catalyst is too small, sufficient catalyst performance will not be obtained, and if it is too large, the catalyst performance will not improve so much, but the mechanical strength of the catalyst will be reduced and handling will be hindered and abrasion resistance This is not preferable because there is a risk of adverse effects such as lowering.

  As a preparation method of the denitration catalyst used in the present invention, a general preparation method using various metal compounds can be used. For example, an impregnation method, a coprecipitation method, a kneading method, an alkoxide method and the like are used.

  As starting materials for each catalyst component, oxides, hydroxides, inorganic salts, organic salts, and the like of each element are used. Specific examples include ammonium salts, oxalates, sulfates, nitrates, halides, etc. For example, titanium sources include titanyl sulfate, titanium tetrachloride, tetraisopropyl titanate, etc. Silica sol, water glass, silicon tetrachloride and the like can be used. As the vanadium source, ammonium metatungsten vanadate is used, as the tungsten source, ammonium metatungstate, ammonium paratungstate, etc. are used, and as the molybdenum source, ammonium paramolybdate, molybdic acid, etc. are used.

  The denitration catalyst used in the present invention can be formed into a saddle shape, a pellet, a sphere, or a honeycomb shape by extrusion molding, tableting molding, rolling granulation, or the like. Further, a saddle-shaped, pellet, sphere, or honeycomb-shaped carrier can be used by coating the components of the denitration catalyst. A honeycomb shape is preferable for reducing the pressure loss of the exhaust gas treatment apparatus.

The exhaust gas treatment method of the present invention contains at least one element selected from the group consisting of alkali metals, alkaline earth metals, phosphorus, arsenic, silicon, zinc, lead, iron, antimony and vanadium and / or compounds thereof. An exhaust gas treatment method for removing nitrogen oxides in exhaust gas, selected from the group consisting of alkali metals, alkaline earth metals, phosphorus, arsenic, silicon, zinc, lead, iron, antimony and vanadium contained in the exhaust gas The concentration of the at least one element and / or compound thereof may be 100 g / m ³ (Normal) or less, preferably 50 g / m ³ (Normal) or less, more preferably 30 g / m ³ (Normal). It should be:

The concentration of at least one element selected from the group consisting of alkali metal, alkaline earth metal, phosphorus, arsenic, silicon, zinc, lead, iron, antimony and vanadium and / or its compound is 100 g / m ³ (Normal) This is because if it exceeds, the decrease in the activity of the catalyst increases, and in some cases, the increase in the SO ₂ oxidation rate also increases.

The treatment temperature of the exhaust gas in the exhaust gas treatment method of the present invention is in the range of 150 to 600 ° C, preferably 170 to 550 ° C, more preferably 200 to 500 ° C. If the exhaust gas treatment temperature is less than 150 ° C., sufficient NOx removal efficiency cannot be obtained, and the catalyst activity decreases greatly. If it exceeds 600 ° C., thermal deterioration of the catalyst increases, and the SO ₂ oxidation rate increases. It is. The space velocity during the exhaust gas treatment of the present invention is 500 to 100,000 hr ⁻¹ (STP), preferably 1,000 to 50,000 hr ⁻¹ (STP), more preferably 1,500 to 30,000 hr ^{−. It} should be in the range of ¹ (STP). The space velocity is not sufficient NOx removal efficiency can be obtained exceeds 100,000hr ^-1 (STP), although NOx removal efficiency does not change greatly increases the pressure loss of the exhaust gas treatment device is less than ^500hr -1 (STP), Also, the device itself is large and inefficient. Furthermore, the linear velocity of the gas passing through the catalyst layer in the exhaust gas treatment of the present invention is 0.1 to 10 m / sec (Normal), preferably 0.5 to 7 m / sec (Normal), more preferably 0.7. It should be in the range of ˜4 m / sec (Normal). If the linear velocity is less than 0.1 m / sec (Normal), sufficient NOx removal efficiency cannot be obtained, and if it exceeds 10 m / sec (Normal), the NOx removal efficiency does not change greatly, but the pressure loss of the exhaust gas treatment device increases. Because.

The denitration catalyst used in the present invention is suitably used under conditions where oxygen is present in the exhaust gas. In this case, the oxygen concentration is preferably in the range of 0.1 to 50% by volume, more preferably 0. It should be in the range of 3 to 20% by volume, more preferably 0.5 to 16% by volume. This is because if the oxygen concentration is less than 0.1% by volume, the NOx removal efficiency decreases, and if it exceeds 50% by volume, the SO ₂ oxidation rate increases. When the exhaust gas contains moisture, the concentration is preferably 50% by volume or less, more preferably 40% by volume or less, and further preferably 30% by volume or less. This is because NOx removal efficiency decreases when the moisture concentration in the exhaust gas exceeds 50% by volume. Furthermore, the SOx concentration in the exhaust gas is preferably 1 to 10000 ppm (volume basis), more preferably 10 to 7000 ppm, and still more preferably 20 to 5000 ppm. If the SOx concentration in the exhaust gas is 1 ppm or more, it is preferable because a decrease in activity due to poisonous substances in the exhaust gas such as alkali metals is reduced. On the other hand, if the SOx concentration in the exhaust gas exceeds 10,000 ppm, there may be a problem that the activity is reduced by SOx.

  The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the following examples as long as the effects of the present invention are achieved.

Example 1
<Preparation of denitration catalyst>
While mixing 20 liters of silica sol (containing 20 mass% as SiO ₂ ) and 225 kg of 25 mass% ammonia water, 240 L of sulfuric acid solution of titanyl sulfate (125 g / L as TiO ₂ , sulfuric acid concentration 550 g / L) is gradually stirred. After adding dropwise to form a precipitate, an appropriate amount of aqueous ammonia was added to adjust the pH to 8. The slurry was aged, filtered, washed and dried at 150 ° C. for 10 hours. This was fired at 550 ° C. for 6 hours in an air atmosphere, and further pulverized using a hammer mill to obtain a Ti—Si composite oxide powder. The composition of the Ti—Si composite oxide powder thus prepared was Ti: Si = 85: 15 (molar ratio).

Next, 0.6 kg of ammonium metavanadate, 0.7 kg of oxalic acid, and 0.2 kg of monoethanolamine were mixed and dissolved in 2 L of water to prepare a uniform solution. 4.4 L of this vanadium-containing solution and 10 mass% aqueous solution of ammonium paratungstate methylamine (400 g / L as WO ₃ ) are added to 20 kg of the Ti-Si composite oxide prepared above together with a molding aid and an appropriate amount of water. After kneading with a kneader, extrusion molding was performed into a honeycomb shape having an outer shape of 80 mm square, a length of 500 mm, an aperture of 3.2 mm, and a wall thickness of 0.5 mm. The honeycomb formed body was dried at 80 ° C. and then calcined at 450 ° C. for 3 hours in an air atmosphere to obtain a denitration catalyst A.

The composition of the denitration catalyst A is (Ti—Si composite oxide): V ₂ O ₅ : WO ₃ = 90: 2: 8 (mass ratio), the BET specific surface area is 116 m ² / g, and the total pore volume is It was 0.46 mL / g.

<NOx decomposition test>
The reaction tube was filled with the denitration catalyst A, and synthesis gas having the following composition was introduced under the following processing conditions. The catalyst filling amount was determined in accordance with the space velocity (SV) described in the following processing conditions.

[Syngas composition]
NOx: 250 ppm, NH ₃ : 200 ppm, SO ₂ : 2000 ppm, O ₂ : 2% by volume, H ₂ O: 10% by volume, N ₂ : balance
[Processing conditions]
Processing temperature: 350 ° C., space velocity: 10,000 hr ⁻¹ (STP), gas linear velocity: 2.0 m / sec (Normal)
Next, the NOx concentration at the denitration catalyst inlet and the denitration catalyst outlet was measured, and the denitration rate was calculated according to the following equation. The results are shown in Table 1.
Denitration rate (%) = {(Denitration catalyst inlet NOx concentration) − (Denitration catalyst outlet NOx concentration)} / (Denitration catalyst inlet NOx concentration) × 100
(Example 2)
<Impregnation of Na>
This is an example in which the catalyst performance with respect to the poisoning substance (Na) is evaluated by previously including in the catalyst Na that will adhere to the catalyst when Na is present as an exhaust gas component.

The denitration catalyst A prepared in Example 1 was impregnated with an aqueous sodium nitrate solution, dried at 80 ° C., and calcined at 400 ° C. for 3 hours in an air atmosphere. The Na content of this catalyst was 2% by mass (converted to Na ₂ O) with respect to the entire catalyst.

<NOx decomposition test>
In the NOx decomposition test of Example 1, the NOx decomposition test was performed in the same manner as in Example 1 except that the Na-impregnated catalyst was used instead of the denitration catalyst A. The results are shown in Table 1.

(Example 3)
<K impregnation>
This is an example in which the catalyst performance with respect to the poisoning substance (K) is evaluated by previously including in the catalyst K that will adhere to the catalyst when K is present as an exhaust gas component.

The denitration catalyst A prepared in Example 1 was impregnated with an aqueous potassium nitrate solution, dried at 80 ° C., and then calcined at 400 ° C. for 3 hours in an air atmosphere. The K content of this catalyst was 2% by mass (converted to K ₂ O) with respect to the entire catalyst.

<NOx decomposition test>
In the NOx decomposition test of Example 1, the NOx decomposition test was performed in the same manner as in Example 1 except that the K impregnation catalyst was used instead of the denitration catalyst A. The results are shown in Table 1.

Example 4
<Impregnation of Mg>
This is an example in which the catalyst performance with respect to the poisoning substance (Mg) is evaluated by preliminarily containing Mg that will adhere to the catalyst when Mg is present as an exhaust gas component.

  The denitration catalyst A prepared in Example 1 was impregnated with an aqueous magnesium nitrate solution, dried at 80 ° C., and then calcined at 400 ° C. for 3 hours in an air atmosphere. The Mg content of this catalyst was 2% by mass (MgO conversion) with respect to the whole catalyst.

<NOx decomposition test>
In the NOx decomposition test of Example 1, the NOx decomposition test was performed in the same manner as in Example 1 except that the Mg impregnated catalyst was used instead of the denitration catalyst A. The results are shown in Table 1.

(Example 5)
<Ca impregnation>
This is an example in which the catalyst performance with respect to the poisoning substance (Ca) is evaluated by previously including in the catalyst Ca that will adhere to the catalyst when Ca is present as an exhaust gas component.

  The denitration catalyst A prepared in Example 1 was impregnated with an aqueous calcium nitrate solution, dried at 80 ° C., and then calcined at 400 ° C. for 3 hours in an air atmosphere. The Ca content of this catalyst was 2% by mass (CaO conversion) with respect to the whole catalyst.

<NOx decomposition test>
In the NOx decomposition test of Example 1, the NOx decomposition test was performed in the same manner as in Example 1 except that the above Ca-impregnated catalyst was used instead of the denitration catalyst A. The results are shown in Table 1.

(Example 6)
<P impregnation>
This is an example in which the catalyst performance with respect to the poisoning substance (P) is evaluated by previously including in the catalyst P that will adhere to the catalyst when P is present as an exhaust gas component.

The denitration catalyst A prepared in Example 1 was impregnated with an aqueous phosphoric acid solution, dried at 80 ° C., and calcined at 400 ° C. for 3 hours in an air atmosphere. The P content of this catalyst was 2% by mass (converted to P ₂ O ₅ ) with respect to the whole catalyst.

<NOx decomposition test>
In the NOx decomposition test of Example 1, the NOx decomposition test was performed in the same manner as in Example 1 except that the P impregnation catalyst was used instead of the denitration catalyst A. The results are shown in Table 1.

(Example 7)
<As impregnation>
This is an example in which the catalyst performance with respect to the poisoning substance (As) is evaluated by previously including in the catalyst As that will adhere to the catalyst when As is present as the exhaust gas component.

The denitration catalyst A prepared in Example 1 was impregnated with an aqueous arsenic acid solution, dried at 80 ° C., and then calcined at 400 ° C. for 3 hours in an air atmosphere. As content of this catalyst was 2 mass% (As ₂ O ₃ conversion) with respect to the whole catalyst.

<NOx decomposition test>
In the NOx decomposition test of Example 1, the NOx decomposition test was performed in the same manner as in Example 1 except that the As impregnated catalyst was used instead of the denitration catalyst A. The results are shown in Table 1.

(Example 8)
<SO ₂ oxidation test>
The reaction tube was filled with the denitration catalyst A prepared in Example 1, and a synthesis gas having the following composition was introduced under the following conditions. The catalyst filling amount was determined in accordance with the space velocity (SV) described in the following processing conditions.

[Syngas composition]
NOx: 250 ppm, NH ₃ : 200 ppm, SO ₂ : 2000 ppm, O ₂ : 2% by volume, H ₂ O: 10% by volume, N ₂ : balance
[Processing conditions]
Processing temperature: 350 ° C., space velocity: 10,000 hr ⁻¹ (STP), gas linear velocity: 2.0 m / sec (Normal)
Next, the SO ₂ concentration at the denitration catalyst inlet and the SO ₃ concentration at the denitration catalyst outlet were measured, and the SO ₂ oxidation rate was calculated according to the following equation. The results are shown in Table 2.
SO ₂ oxidation rate (%) = (denitration catalyst outlet SO ₃ concentration) / (denitration catalyst inlet SO ₂ concentration) × 100
Example 9
<V impregnation>
In this example, the change in the SO ₂ oxidation rate due to the increase in V in the catalyst is evaluated by previously including in the catalyst V that will adhere to the catalyst when V is present as an exhaust gas component.

The denitration catalyst A prepared in Example 1 was impregnated with the ammonium metavanadate solution used in Example 1, dried at 80 ° C., and then calcined at 400 ° C. for 3 hours in an air atmosphere. V content of this catalyst was 5 mass% (V ₂ O ₅ conversion) with respect to the whole catalyst.

<SO ₂ oxidation test>
In the SO ₂ oxidation test of Example 8, an SO ₂ oxidation test was performed in the same manner as in Example 8 except that the V impregnation catalyst was used instead of the denitration catalyst A. The results are shown in Table 2.

(Comparative Example 1)
<Preparation of denitration catalyst>
A homogeneous solution was prepared by mixing and dissolving 0.6 kg of ammonium metavanadate, 0.7 kg of oxalic acid and 0.2 kg of monoethanolamine in 2 L of water. This vanadium-containing solution and 4.4 L of 10% by weight methylamine aqueous solution of ammonium paratungstate (400 g / L as WO ₃ ) together with a molding aid and an appropriate amount of water are mixed with TiO ₂ powder (manufactured by Crystal Global, DT-51 (Product Name) In addition to 20 kg, after kneading with a kneader, it was extruded into a honeycomb shape having an outer shape of 80 mm square, a length of 500 mm, an aperture of 2.9 mm, and a wall thickness of 0.4 mm. The honeycomb formed body was dried at 80 ° C. and then calcined at 450 ° C. for 3 hours in an air atmosphere to obtain a denitration catalyst B.

The composition of the denitration catalyst B is TiO ₂ : V ₂ O ₅ : WO ₃ = 90: 2: 8 (mass ratio), the BET specific surface area is 71 m ² / g, and the total pore volume is 0.30 mL / g. there were.

<SO ₂ oxidation test>
In the SO ₂ oxidation test of Example 8, the SO ₂ oxidation test was performed in the same manner as in Example 8 except that the above-described denitration catalyst B was used instead of the denitration catalyst A. The results are shown in Table 2.

(Comparative Example 2)
<V impregnation>
This is a comparative example for evaluating the change in the SO ₂ oxidation rate due to the increase of V in the catalyst by previously including in the catalyst V that will adhere to the catalyst when V is present as an exhaust gas component.

The denitration catalyst B prepared in Comparative Example 1 was impregnated with the ammonium metavanadate solution used in Comparative Example 1, dried at 80 ° C., and then calcined at 400 ° C. for 3 hours in an air atmosphere. V content of this catalyst was 5 mass% (V ₂ O ₅ conversion) with respect to the whole catalyst.

<SO ₂ oxidation test>
In the SO ₂ oxidation test of Example 8, an SO ₂ oxidation test was performed in the same manner as in Example 8 except that the V impregnation catalyst was used instead of the denitration catalyst A. The results are shown in Table 2.

  The present invention can be used for technology relating to industrial exhaust gas treatment and can be used for treatment of nitrogen oxides. Particularly effective is when treating nitrogen oxides contained in exhaust gas containing components usually called poisonous substances.

Claims (4)

Nitrogen in a nitrogen oxide-containing exhaust gas containing at least one element selected from the group consisting of alkali metals, alkaline earth metals, phosphorus, arsenic, silicon, zinc, lead, iron, antimony and vanadium and / or compounds thereof An exhaust gas treatment method for treating an oxide in the presence of a denitration catalyst, wherein the denitration catalyst is at least one element selected from the group consisting of vanadium, niobium, tantalum, molybdenum and tungsten or a compound thereof (active component) and titanium 1 of complex oxide and / or mixed oxide of titanium containing at least one element (base material addition component) selected from the group consisting of aluminum, silicon, zirconium, molybdenum and tungsten. An exhaust gas treatment method characterized by being an oxide of more than species. The exhaust gas treatment method according to claim 1, wherein the titanium oxide contains silicon. The exhaust gas treatment method according to claim 1 or 2, wherein the exhaust gas contains sulfur oxide (SOx). The exhaust gas treatment method according to claim 1, wherein the exhaust gas is exhaust gas from a marine diesel engine.