JP2008188576A - Method of removing nitrogen oxide from exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve at once the problems that a process requiring a reaction temperature close to that immediately after exhaust and a precious rare noble metal catalyst to reduce NOx in an exhaust gas of a diesel engine is a present mainstream, further, a sulfur content of a fuel increases PM in the exhaust gas and obstructs a catalyst function, and that if the trend continues, a dead-end situation can be produced technically and in view of a resource aspect, further also, a cost aspect in the near future. <P>SOLUTION: The exhaust gas is introduced into a packed bed of a reductant in which a metal carbide, particularly, high carbon ferromanganese and calcium carbide are respectively individually or blend-mixed, by which 80% or more of NOx in the exhaust gas can be directly reduced and decomposed without receiving any obstruction of the sulfur content if the temperature is 200°C or more. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technique for reducing and removing nitrogen oxides in exhaust gas of an internal combustion engine and exhaust gas of a combustion furnace.

  It is not easy to reduce NOx in the presence of oxygen, such as diesel engine exhaust gas, and many proposals have been made for mixing hydrocarbons (HC) into exhaust gas or using platinum group metal catalysts. However, it has not reached the full practical level. Recently, a method of blowing urea water into exhaust gas to generate ammonia and reducing NOx by this ammonia has been advanced, but this method also requires a platinum group metal catalyst.

  The above-described method using a noble metal such as platinum as a catalyst is not only expensive, but these noble metals are indispensable for other important chemical reactions and should be used for the treatment of automobile exhaust gas. Some people think that it is not. Moreover, the shortage of these metals is predicted only for fuel cells that are expected to develop in the future. Considering such a shortage of platinum group metal catalysts in terms of earth resources, attention will be focused on NOx reduction methods that do not require a catalyst in the future.

  In contrast to conventional NOx treatment technology, a method has been proposed in which a metal carbide that acts as a reducing agent for NOx itself, in particular high carbon ferromanganese containing manganese or iron carbide, is brought into contact with exhaust gas to advance the reduction reaction. ing. (See Patent Document 1)

  JP 2005-125300 A

  Patent Document 1 proposes a direct reduction method of NOx with interstitial carbides such as Mn, Fe, Ni, etc., but the present invention was made in order to improve and advance this method, and the removal rate of NOx in exhaust gas. The purpose is to improve the reaction time and extend the duration of the reduction reaction.

When a new alkaline earth metal carbide such as calcium carbide (CaC ₂ ) is used as a reducing agent for the direct reduction reaction of NOx without using a platinum group metal catalyst, the NOx reduction reaction proceeds smoothly even at a temperature of 100 ° C. or lower. I found.

The NOx reduction reaction using a platinum group metal catalyst is a reaction start at a temperature significantly lower than the point where it is difficult to maintain the progress of a reliable reaction unless a temperature around 300 ° C. is secured. That is, it can be said that CaC ₂ is more reactive than FMnH than the platinum group metal catalyst as far as NOx reduction is concerned. Although the details of the mechanism of this reaction are unknown, it involves the presence of a calcium compound that easily reacts with NOx and that water vapor in the exhaust gas reacts with CaC ₂ to produce acetylene (C ₂ H ₂ ). It is thought that it is because.

When CaC ₂ reacts with NOx or water vapor, it produces powders such as CaO, Ca (OH) ₂ , Ca (NO ₃ ) ₂ and residual carbon, and deteriorates the flow of exhaust gas in the reducing agent packed bed, which causes the reaction to proceed. This may reduce the above-mentioned adverse effects when mixed with high carbon ferromanganese (FMnH) of Patent Document 1 rather than using CaC ₂ alone, because it may interfere with or result in discharging dust. . Further, if the reaction temperature is set to 200 ° C. or higher, the volume change is large and the powdered Ca (OH) ₂ does not appear, and CaC ₂ is apparently unchanged and CaO is unchanged, so the gas flow is not hindered and dust is discharged. The problem is also solved.

C ₂ H ₂ produced by reacting with water vapor and CaC ₂ in the exhaust gas not only reduces and decomposes NOx, but also reduces the oxide film on the surface of FMnH produced by reacting with NOx, so that the reduction ability of FMnH is restored. Helps maintain.

When diesel exhaust gas was fed into a reducing agent packed bed in which FMnH and CaC ₂ were mixed at a volume ratio of 1: 1 (weight ratio of 4: 1), NOx was reliably reduced and decomposed at 200 ° C., and more than 300 ° C. It was confirmed that the reducing ability of the packed bed did not decrease for a long time under the above conditions.

CaC ₂ has been confirmed to be excellent in reactivity with NOx, but it reacts easily with moisture in the atmosphere to generate C ₂ H ₂ gas for digestion and digestion, so a sealed container is required for its storage. This is accompanied by fateful inconvenience in use. In order to solve the problem of storability and digestion (pulverization) during the low temperature reaction, a method of coating the surface of CaC ₂ grains with asphalt, pitch or a thin film of plastics was devised.

Coating of heated and melted asphalt or pitch or plastics it like materials that do or CaC ₂ grains close contact with the pulling Ru and CaC ₂ grain surfaces after immersion in the melt is sprayed CaC ₂ grain surface is formed Is done. In addition, when asphalt, pitch, and plastics are cut back with an appropriate solvent such as kerosene and used as a solution having good fluidity, the workability of the coating is improved and a uniform and thin film can be formed. Defect-free coatings can prevent storage of moisture in the atmosphere, thus simplifying storage.

Since the CaC ₂ grains that have been subjected to the coating treatment are blocked from contact with the gas, it is necessary to perform the following treatment prior to the reaction with NOx. That is, the as-coated CaC ₂ particles are first heated in a container that is shielded from air. Asphalt and the like begin to polycondensate from about 250 ° C. after the low-boiling substances and solvents are volatilized, and when heated to 400 to 500 ° C., the decomposition progresses to a final carbonized film. Since this carbonized film is hard and strong, and many fine cracks are formed, the internal CaC ₂ can react with NOx in the exhaust gas, and the reaction products CaO and Ca (OH). ₂ is prevented from becoming dust. The CaC ₂ grains are protected in a gas permeable carbonaceous shell.

Of course, it is possible for CaC ₂ particles coated with a carbonized film to proceed with the NOx reduction and decomposition reaction in the exhaust gas alone. However, as described above, when mixed with FMnH particles, the shape of the packed bed becomes stable. _C 2 _{H 2} from CaC ₂ the NOx reduction ability of the packed bed so reducing the oxide film of Mn and Fe is maintained for a long time.

All of the conventional main technologies related to NOx reduction in exhaust gas are established for the first time by the presence of a noble metal catalyst, and even if these catalysts are used and the temperature close to immediately after engine exhaust is not maintained, metal carbide itself in the present invention as compared to reductive decomposition reaction does not proceed smoothly acts as a reducing agent for NOx, particularly in the case of CaC ₂ is NOx reduction reaction even at room temperature extremely speaking progresses. This is because water vapor in the exhaust gas reacts with CaC ₂ to produce acetylene (C ₂ H ₂ ) gas, and its chemical activity is greater than that of other hydrocarbons, reducing NOx.

The surface of the carbide of Mn and Fe is a set of lost binding partner atoms, whereas the activity of the surface is the driving force for NOx reduction, _C 2 _{H 2} from CaC ₂ when used as a mixture with CaC ₂ Reduces the oxide film of Mn and Fe, so that the NOx reduction potential of the entire reducing agent can be maintained for a long time.

  In the method of the present invention, it is not necessary to warm up the reaction vessel in advance, and NOx reduction proceeds smoothly from the initial stage of engine starting, which is a technical feature and advantageous effect.

The economic effect of the present invention is advantageous in terms of cost since the reducing agents are inexpensive and abundant in terms of resources. This can be easily understood Come to compare the prices of the precious metals and FMnH and CaC _2.

  One of the characteristics of the NOx reducing agent of the present invention is the effect of removing sulfur in the exhaust gas. Ca, Mn, and Fe are all highly reactive with sulfur compounds and easily form their respective sulfides and sulfates to immobilize the sulfur content. Therefore, it is possible to use fuel with a high sulfur content.

Since the present invention is a reaction between a solid and a gas, the reaction proceeds more smoothly as the opportunity for contact between the two increases. Normally, in this type of reaction, the exhaust gas is fed into a packed bed of metal carbide particles as a reducing agent. However, in order to reduce the flow resistance of exhaust gas, metal carbide is generally used for in-vehicle use. It is crushed to a size of 20 mm to 5 mm, CaC ₂ is protected with a carbonized shell, and these are blended and filled into a stainless steel wire cylinder and partitioned into two or more compartments by a partition plate or the like. It is good. The size of the packed bed is determined by the target engine, but it can be reduced to a size that can be accommodated for ordinary in-vehicle use.

The reducing agent is used by mixing CaC ₂ and FMnH. CaC ₂ is excellent in reactivity and can be used as a simple substance. However, it is desirable to use the mixture in order to maintain the shape stability of the packed bed and the reduction potential. Since CaC ₂ can reduce and decompose NOx even at room temperature, preheating of the packed bed is not necessary, but it is desirable to raise the temperature to 200 ° C. or more as soon as possible to avoid the formation of Ca (OH) ₂ .

CaC ₂ is gradually consumed as the reaction proceeds, leaving CaO, Ca (OH) ₂ . The carbon content of FMnH is discharged as CO ₂ and Mn and Fe form an oxide film. The oxide film lose C ₂ will be restored to the original is reduced with H ₂ gradually activity from CaC ₂ is unavoidable. If it is used for a certain period of time, it is necessary to replace it with a new one, but it is easiest to provide a spare packed bed and switch the exhaust gas flow path. If the used reducing agent is immersed in an acid with a stainless steel wire mesh cylinder, the calcium-based material is easily eluted, the oxide film on the surface of FMnH dissolves and peels off, and a new surface is regenerated. This part can be used repeatedly.

The NOx reduction experiment was conducted using the exhaust gas of a van type car equipped with a 2184cc diesel engine. Focusing only on NOx as a component in the exhaust gas, it was confirmed that the NOx concentration was kept at 120 ± 5 ppm when the engine was rotating at a constant 1500 rpm, so the exhaust gas NOx concentration reference was set to 120 ppm. The NOx concentration was measured using a NOx-O ₂ concentration meter MODEL5100-0 manufactured by Nippon Thermo.

As the reducing agent, FMnH and carbonized shell-treated CaC ₂ were mixed and used in a weight ratio of 4: 1. The particle size is on a 15-5 mm sieve with both crushed.

This mixed reducing agent was charged into three types of cylindrical rods made of 1 mm stainless wire mesh to form three types of packed beds. The packed bed has a diameter of 28 mm and lengths of A 150, B 100, and C 75 mm, respectively. Since the exhaust gas flow rate is constant at 2 L / min, the packed bed space velocity H ⁻¹ is calculated as A 1306 B 1935 C 2612 respectively.

The packed bed is set at the center of a magnetic tube (30φ × 600) in a nichrome wire heating electric furnace and heated to a predetermined temperature. The upstream side of the packed bed was filled with stainless fiber to make the exhaust gas pre-tropical. The experimental temperatures were 220 ° C. and 300 ° C. to avoid the formation of Ca (OH) ₂ . The NOx concentration of the exhaust gas after the reaction was measured for the gas collected inside the outlet of the magnetic pipe. The average value of 6 measurements every 10 minutes after the start of the experiment was used as a representative value.

The experimental results are shown in Table 1.

Compared to the denitration experiment with FMnH alone (Patent Document 1), the NOx concentration level of the exhaust gas after reaction is low in the FMnH-CaC ₂ mixing experiment, and the effect of CaC ₂ mixing is clear. Looking at the denitration rate, it seems that there is a factor effect of reaction temperature and space velocity, but considering the low NOx concentration level as a whole, it cannot be said that there is a significant difference between each factor within the scope of this experiment. However, the results of this experiment confirm the possibility of practical application of the present invention.

  The present invention is a proposal relating to a method for directly reducing NOx with a reducing agent made of abundant metal carbide without using an expensive rare precious metal catalyst. It can be applied to all devices that emit NOx, such as cars, generators, and various combustion furnaces.

In particular, ship diesel engines that are not subject to exhaust gas regulations emit a large amount of NOx around large cities, and this is one of the major causes of photochemical smog. The application of the present invention has a great effect on environmental purification. The action of CaC ₂ and FMnH constituting the reducing agent of the present invention is not hindered by the sulfur content in the exhaust gas, but rather can react with various sulfur compounds to fix the sulfur content, so that NOx is simultaneously used. It is necessary to pay attention to the potential for removing Sox in the exhaust gas. From the advantages described above, the spread of the present invention in a wide range of fields is expected.

Claims (2)

  A method for removing nitrogen oxides in exhaust gas, wherein exhaust gas is introduced into a packed bed of calcium carbide alone or a mixture of calcium carbide and high carbon ferromanganese to reduce nitrogen oxides in the exhaust gas .   Spraying or melting asphalt, pitch, plastics or asphalt, pitch, plastics cut back with solvent on calcium carbide grains, or immersing them in these liquids to coat the surface with these materials and shut off the air The exhaust gas is introduced into a mixed packed bed of calcium carbide grains or a mixed packed bed of high carbon ferromanganese grains coated with a carbonaceous shell having fine cracks, and nitrogen oxides in the exhaust gas. A method for removing nitrogen oxides from exhaust gas, characterized in that