JP6119426B2 - Urea SCR system - Google Patents

Urea SCR system Download PDF

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JP6119426B2
JP6119426B2 JP2013111749A JP2013111749A JP6119426B2 JP 6119426 B2 JP6119426 B2 JP 6119426B2 JP 2013111749 A JP2013111749 A JP 2013111749A JP 2013111749 A JP2013111749 A JP 2013111749A JP 6119426 B2 JP6119426 B2 JP 6119426B2
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ammonia
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exhaust gas
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urea
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和生 大角
和生 大角
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Isuzu Motors Ltd
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Description

本発明は、ディーゼル排ガスの後処理としての尿素ＳＣＲシステムに係り、特に、高温域でのＮＯｘの浄化率を向上させることができる尿素ＳＣＲシステムに関するものである。   The present invention relates to a urea SCR system as a post-treatment of diesel exhaust gas, and more particularly to a urea SCR system capable of improving the NOx purification rate in a high temperature range.

ディーゼル排ガスに含まれるＮＯｘ（窒素酸化物）を無害化する装置の１つとして、尿素ＳＣＲ（Selective Catalytic Reduction；以下ＳＣＲと略記）が実用化されている。   As one device for detoxifying NOx (nitrogen oxide) contained in diesel exhaust gas, urea SCR (Selective Catalytic Reduction; hereinafter abbreviated as SCR) has been put into practical use.

図４は、ＳＣＲを用いたディーゼル排ガスの後処理システムを示したものである。ディーゼル排ガスの後処理システムは、ディーゼルエンジンの排気管１０に、ＤＯＣ（酸化触媒）１１、ＤＰＦ（ディーゼルパティキュレートフィルタ）１２、ＳＣＲ１３が順に接続され、そのＳＣＲ１３の上流側に尿素噴射ノズル１４を設けて構成される。   FIG. 4 shows an aftertreatment system for diesel exhaust gas using SCR. In a diesel exhaust gas aftertreatment system, a DOC (oxidation catalyst) 11, a DPF (diesel particulate filter) 12, and an SCR 13 are sequentially connected to an exhaust pipe 10 of a diesel engine, and a urea injection nozzle 14 is provided on the upstream side of the SCR 13. Configured.

ディーゼルエンジンからの排ガスは、ＤＯＣ１１内で、排ガス中の未燃焼燃料（ＨＣ）や、一酸化炭素（ＣＯ）等が酸化された後、排ガス中のＰＭ（パティキュレートマター）がＤＰＦ１２で捕集される。次に、排ガスがＳＣＲ１３に流入する際、ＳＣＲ１３の入口側の尿素噴射ノズル１４から噴射された尿素水が、排ガスの熱で加水分解されてアンモニアとなり、そのアンモニアと排ガス中のＮＯｘとが、ＳＣＲ１３内のＳＣＲ触媒で、反応して窒素と水とに還元されて無害化されてテールパイプから排気される。   The exhaust gas from the diesel engine is oxidized in the DOC 11 by unburned fuel (HC), carbon monoxide (CO), etc. in the exhaust gas, and then PM (particulate matter) in the exhaust gas is collected by the DPF 12. The Next, when the exhaust gas flows into the SCR 13, urea water injected from the urea injection nozzle 14 on the inlet side of the SCR 13 is hydrolyzed by the heat of the exhaust gas to become ammonia, and the ammonia and NOx in the exhaust gas are converted into SCR 13. It reacts with the SCR catalyst in the inside, is reduced to nitrogen and water, rendered harmless, and exhausted from the tail pipe.

またＤＰＦ１２で捕集したＰＭが一定量堆積したときは、堆積したＰＭを除去すべくＤＰＦ１２を強制再生することが行われている。ＤＰＦ１２の再生は、多段噴射、ポスト噴射、排気管噴射等を行い、排ガス中の燃料（ＨＣ）をＤＯＣ１１で酸化燃焼させて排ガス温度を５００℃以上に上げ、ＤＰＦ１２に堆積したＰＭを燃焼除去するものである。   When a certain amount of PM collected by the DPF 12 is deposited, the DPF 12 is forcibly regenerated to remove the accumulated PM. For regeneration of the DPF 12, multistage injection, post injection, exhaust pipe injection, etc. are performed, and the fuel (HC) in the exhaust gas is oxidized and burned by the DOC 11 to raise the exhaust gas temperature to 500 ° C. or more, and PM deposited on the DPF 12 is burned and removed. Is.

この尿素ＳＣＲシステムにおいては、排ガスの温度が、低温（１５０℃）から高温（６００℃）まで広範囲の温度でＮＯｘ浄化率の向上が求められている。   In this urea SCR system, it is required to improve the NOx purification rate in a wide range of exhaust gas temperatures from a low temperature (150 ° C.) to a high temperature (600 ° C.).

しかし、尿素の分解（融点）温度は、１３２．７℃であり、排ガス温度が低温域では、尿素の加水分解反応が促進しないため、ＳＣＲの前段に尿素加水分解触媒を配設し、排ガス温度が低温（１００〜１５０℃）でも尿素の加水分解反応を促進させて尿素からアンモニアの生成率を高めたり（特許文献１）、また特許文献２に提案されるように、ＳＣＲ触媒の担体に、直接尿素水を送液して浸透させることで、加水分解を促進させる（特許文献２）ことが提案されている。   However, since the decomposition (melting point) temperature of urea is 132.7 ° C., and the exhaust gas temperature is low, urea hydrolysis reaction is not accelerated. Therefore, a urea hydrolysis catalyst is disposed in front of the SCR, and the exhaust gas temperature is Promotes the hydrolysis reaction of urea to increase the production rate of ammonia from urea even at low temperatures (100 to 150 ° C.) (Patent Document 1), and as proposed in Patent Document 2, It has been proposed to promote hydrolysis by directly feeding and penetrating urea water (Patent Document 2).

また、ＤＰＦ再生時など排ガス温度が高温（５００℃以上）のときには、ＳＣＲ触媒へのアンモニアストレージ量が低下するためＮＯｘ浄化率が悪くなる。そこで、ＤＰＦ再生時のアンモニアスリップを防止するために、尿素水の噴射を制御する（特許文献３）ことが提案されている。   Further, when the exhaust gas temperature is high (500 ° C. or higher), such as during DPF regeneration, the amount of ammonia stored in the SCR catalyst decreases, so the NOx purification rate becomes worse. Therefore, in order to prevent ammonia slip during DPF regeneration, it has been proposed to control urea water injection (Patent Document 3).

特開２００６−２１２５９１号公報JP 2006-212591 A 特開２０１０−２６５８５６号公報JP 2010-265856 A 特開２０１２−２０６５号公報JP 2012-2065 A 特開２０１２−１０１１５７号公報JP 2012-101157 A

しかしながら、このＤＰＦ再生時の高温域では、ＮＯｘ浄化が十分に行われない問題がある。   However, there is a problem that NOx purification is not sufficiently performed in a high temperature region during the regeneration of the DPF.

そこで、高温域で触媒活性のあるＳＣＲ触媒の開発がなされているが、現状では、触媒種を選定しても５００℃以上でＮＯｘ浄化率を向上させるＳＣＲ触媒は得られていない。   Thus, although an SCR catalyst having catalytic activity in a high temperature range has been developed, at present, an SCR catalyst that improves the NOx purification rate at 500 ° C. or higher has not been obtained even if a catalyst type is selected.

従って従来は、ＳＣＲの断面積を大きくして、排ガスの線速度下げるなど、排ガスとＳＣＲ触媒の接触確率を向上させて対応しているが、高温域でのＮＯｘ浄化率を向上させるものではなく、装置の大型化が避けられない問題がある。   Therefore, conventionally, the SCR cross-sectional area is increased and the linear velocity of the exhaust gas is reduced to improve the contact probability between the exhaust gas and the SCR catalyst. However, this does not improve the NOx purification rate in the high temperature range. There is a problem that an increase in the size of the apparatus cannot be avoided.

そこで、本発明の目的は、上記課題を解決し、高温域でのＮＯｘ浄化率を向上できる尿素ＳＣＲシステムを提供することにある。   Accordingly, an object of the present invention is to provide a urea SCR system capable of solving the above-described problems and improving the NOx purification rate in a high temperature range.

上記目的を達成するために本発明は、エンジンの排気管に、ＤＯＣ、ＤＰＦ、ＳＣＲを接続し、ＳＣＲの上流側に尿素噴射ノズルを設けた尿素ＳＣＲシステムにおいて、ＳＣＲの下流側にアンモニアを改質するアンモニア改質触媒を設け、排ガスが低温のときに前記ＳＣＲ内でＮＯｘを浄化し、排ガスが高温のとき、アンモニア改質触媒で未反応アンモニアを改質して水素を生成し、その水素でＮＯｘを浄化することを特徴とする尿素ＳＣＲシステムである。   In order to achieve the above object, according to the present invention, in a urea SCR system in which a DOC, DPF, and SCR are connected to an exhaust pipe of an engine and a urea injection nozzle is provided on the upstream side of the SCR, ammonia is modified on the downstream side of the SCR. When the exhaust gas is cold, NOx is purified in the SCR. When the exhaust gas is hot, unreacted ammonia is reformed by the ammonia reforming catalyst to generate hydrogen. This is a urea SCR system characterized by purifying NOx.

前記アンモニア改質触媒は、担体に、アンモニアを窒素と水素に分解するアンモニア分解触媒と、排ガス中の酸素とＣＯとを反応させるＣＯ酸化触媒とが担持され、そのＣＯ酸化触媒で、排ガス中の酸素とＣＯとを反応させてＣＯ2とし、その雰囲気でアンモニア分解触媒が、未反応アンモニアを分解して水素を生成させるのが好ましい。 In the ammonia reforming catalyst, an ammonia decomposition catalyst for decomposing ammonia into nitrogen and hydrogen and a CO oxidation catalyst for reacting oxygen and CO in the exhaust gas are supported on the carrier. It is preferable that oxygen and CO are reacted to form CO 2, and in that atmosphere, an ammonia decomposition catalyst decomposes unreacted ammonia to generate hydrogen.

前記アンモニア改質触媒は、アルミナからなる担体に、アンモニア分解触媒としてのＮｉ又はＲｕイオンを含浸させた後、乾燥し、さらにその担体に、ＣＯ酸化触媒としての貴金属イオンを含浸させた後、これを乾燥し、焼成して形成されるのが好ましい。   The ammonia reforming catalyst is obtained by impregnating a support made of alumina with Ni or Ru ions as an ammonia decomposition catalyst, and then drying, and further impregnating the support with noble metal ions as a CO oxidation catalyst. Is preferably formed by drying and baking.

ＤＰＦにＰＭが堆積したとき、ポスト噴射又は排気管噴射を行って、排ガス温度を上昇させてＤＰＦを再生させ、その再生中に、酸素濃度に応じたＣＯをＤＰＦで生成し、ＣＯ酸化触媒で排ガス中の酸素とＣＯとを反応させ、アンモニア分解触媒で、未反応アンモニアを分解して水素を生成させてＮＯｘを浄化するのが好ましい。   When PM accumulates in the DPF, post injection or exhaust pipe injection is performed to raise the exhaust gas temperature to regenerate the DPF. During the regeneration, CO corresponding to the oxygen concentration is generated in the DPF, and the CO oxidation catalyst It is preferable to purify NOx by reacting oxygen in the exhaust gas with CO, and decomposing unreacted ammonia with an ammonia decomposition catalyst to generate hydrogen.

本発明は、ＳＣＲでＮＯｘを浄化するにおいて、ＤＰＦ再生時など排ガスが高温域になっても排ガス中のＮＯｘを除去できるという優れた効果を発揮する。   The present invention exhibits an excellent effect that, when purifying NOx with SCR, NOx in the exhaust gas can be removed even when the exhaust gas reaches a high temperature range such as during DPF regeneration.

本発明の尿素ＳＣＲシステムの装置の要部概略図を示す図である。It is a figure which shows the principal part schematic of the apparatus of the urea SCR system of this invention. 本発明の尿素ＳＣＲシステムにおける触媒構成の概略を示す図である。It is a figure which shows the outline of the catalyst structure in the urea SCR system of this invention. 本発明の尿素ＳＣＲシステムにおける排ガス温度に対するＮＯｘ除去率を示す図である。It is a figure which shows the NOx removal rate with respect to the exhaust gas temperature in the urea SCR system of this invention. 従来の尿素ＳＣＲシステムの装置構成を示す図である。It is a figure which shows the apparatus structure of the conventional urea SCR system.

以下、本発明の好適な一実施の形態を添付図面に基づいて詳述する。   A preferred embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

図１は、本発明の尿素ＳＣＲシステムの装置の要部概略図を示したもので、基本構成は図４で説明したディーゼル排ガスの後処理システムと同じであり、機能が同じものは同一符号を付して説明する。   FIG. 1 shows a schematic diagram of a main part of the apparatus of the urea SCR system of the present invention, the basic configuration is the same as the diesel exhaust gas aftertreatment system described in FIG. 4, and the same functions are denoted by the same reference numerals. A description will be given.

図１において、ディーゼルエンジンの排気管１０には、ＤＯＣ（酸化触媒）１１、ＤＰＦ（ディーゼルパティキュレートフィルタ）１２、ＳＣＲ１３が順に接続され、そのＳＣＲ１３の上流側の排気管１０内に尿素噴射ノズル１４が設けられてディーゼル排ガスの後処理システムが構成される。   In FIG. 1, a DOC (oxidation catalyst) 11, a DPF (diesel particulate filter) 12, and an SCR 13 are sequentially connected to an exhaust pipe 10 of a diesel engine, and a urea injection nozzle 14 is disposed in the exhaust pipe 10 upstream of the SCR 13. Is provided to constitute a diesel exhaust gas aftertreatment system.

本発明においては、ＳＣＲ１３の下流側にアンモニアを改質するアンモニア改質触媒１５を設けたものである。またアンモニア改質触媒１５の下流側には、排ガス中に残った未反応のアンモニアを酸化するためのアンモニアスリップ触媒（酸化触媒）１６が設けられる。   In the present invention, an ammonia reforming catalyst 15 for reforming ammonia is provided on the downstream side of the SCR 13. Further, an ammonia slip catalyst (oxidation catalyst) 16 for oxidizing unreacted ammonia remaining in the exhaust gas is provided on the downstream side of the ammonia reforming catalyst 15.

アンモニア改質触媒１５は、図２に示すように基材２０にアルミナ等の担体２１を保持し、その担体２１に、アンモニア分解触媒２２を担持させた後、そのアンモニア分解触媒２２上にＣＯ酸化触媒２３を担持させたものである。   As shown in FIG. 2, the ammonia reforming catalyst 15 holds a carrier 21 such as alumina on a base material 20, supports an ammonia decomposition catalyst 22 on the carrier 21, and then performs CO oxidation on the ammonia decomposition catalyst 22. The catalyst 23 is supported.

すなわち、セラミックスでハニカム状に形成した基材２０の流路１８となる面に担体２１としてのアルミナを保持し、そのアルミナに、Ｎｉ又はＲｕ等の金属イオンを含浸させた後、乾燥し、さらに、そのアルミナに貴金属イオンを含浸させた後、これを乾燥し、焼成することで、基材２０の各流路１８の内面に、アンモニア分解触媒２２とＣＯ酸化触媒２３が担持されアンモニア改質触媒１５とされる。   That is, alumina serving as the carrier 21 is held on the surface of the base material 20 that is formed into a honeycomb shape with ceramics, and the alumina is impregnated with metal ions such as Ni or Ru, and then dried. Then, after impregnating the alumina with noble metal ions, this is dried and calcined, whereby the ammonia decomposition catalyst 22 and the CO oxidation catalyst 23 are supported on the inner surface of each flow path 18 of the substrate 20, and the ammonia reforming catalyst. 15.

このように、本発明は、従来のＳＣＲに、アンモニアの改質を合わせた構造とすることで、排ガスが低温域のときには、ＳＣＲ１３により、排ガス中のＮＯｘを、尿素の加水分解で生成したアンモニアで除去し、ＤＰＦ１２の再生時など、排ガス温度が５００℃以上の高温域では、ＳＣＲ１３でのＮＯｘ浄化率が落ちるため、アンモニア改質触媒１５で、ＳＣＲ１３で未反応のアンモニアを水素と窒素に分解し、その水素で、ＮＯｘを除去するものである。これにより低温域から高温域まで高効率でＮＯｘを浄化することが可能となる。   As described above, the present invention has a structure in which the reforming of ammonia is combined with the conventional SCR, so that when the exhaust gas is in a low temperature range, the SCR 13 generates NOx in the exhaust gas by hydrolysis of urea. In the high temperature range where the exhaust gas temperature is 500 ° C. or higher, such as when the DPF 12 is regenerated, the NOx purification rate in the SCR 13 decreases, so the ammonia reforming catalyst 15 decomposes unreacted ammonia into hydrogen and nitrogen in the SCR 13. Then, the hydrogen removes NOx. This makes it possible to purify NOx with high efficiency from a low temperature range to a high temperature range.

このアンモニアの改質は、排ガス温度が５００℃以上から反応が活発になるので、従来の尿素ＳＣＲと反応が競合することはない。   In this ammonia reforming, the reaction becomes active from an exhaust gas temperature of 500 ° C. or higher, so that the reaction does not compete with the conventional urea SCR.

アンモニア分解触媒２２によるアンモニアの改質（分解）は、
２ＮＨ3→Ｎ2＋３Ｈ2
の反応である。 The reforming (decomposition) of ammonia by the ammonia decomposition catalyst 22 is
2NH 3 → N 2 + 3H 2
It is reaction of.

このＨ2を用いて、排ガス中のＮＯｘを、
ＮＯｘ＋Ｈ2→Ｎ2＋Ｈ2Ｏ
の反応で浄化する。 Using this H 2 , NOx in the exhaust gas is
NOx + H 2 → N 2 + H 2 O
Purify by the reaction.

排ガス中の酸素がリッチなると酸化雰囲気となるため、これらの反応が進みにくくなるので、ＥＧＲ（排気再循環）にて排ガス中の酸素濃度を低くする。しかし、それでも酸素濃度が高いときには、ＣＯ酸化触媒２３にて、
２ＣＯ＋Ｏ2→２ＣＯ2
の反応で、上記の反応の前にＯ2を、ＣＯと反応させてＣＯ2を生成させておく。 When oxygen in the exhaust gas becomes rich, an oxidizing atmosphere is created, and these reactions are difficult to proceed. Therefore, the oxygen concentration in the exhaust gas is lowered by EGR (exhaust gas recirculation). However, when the oxygen concentration is still high, the CO oxidation catalyst 23
2CO + O 2 → 2CO 2
In this reaction, before the above reaction, O 2 is reacted with CO to produce CO 2 .

なお、ポスト噴射や排気管噴射で、排ガス中に生じたＨＣは、すでに５００℃以上であり、これらの反応には影響しない。   In addition, HC generated in the exhaust gas by the post injection or the exhaust pipe injection is already 500 ° C. or higher and does not affect these reactions.

図４は、本発明の尿素ＳＣＲシステムと従来の尿素ＳＣＲシステムにおけるＳＣＲ入口温度に対するＮＯｘ浄化率を比較したものである。   FIG. 4 compares the NOx purification rate with respect to the SCR inlet temperature in the urea SCR system of the present invention and the conventional urea SCR system.

従来の尿素ＳＣＲシステムでは、点線の曲線ｂで示したように、ＮＯｘ浄化率は、温度が２００℃以上で、ＮＯｘ浄化率が１００％近くに上昇し、４５０℃以上となるとその浄化率が急減に落ちる特性となる。これに対して本発明の尿素ＳＣＲシステムでは、ＳＣＲで未反応で残ったアンモニアをアンモニア改質触媒が改質して、水素と窒素に分解し、その改質で生じた水素で、ＮＯｘを浄化するため８００℃の高温域でもＮＯｘ浄化率を略９０％に保つことができる。   In the conventional urea SCR system, as indicated by the dotted curve b, the NOx purification rate increases to nearly 100% when the temperature is 200 ° C. or higher, and when the temperature is 450 ° C. or higher, the purification rate sharply decreases. It becomes the characteristic that falls. On the other hand, in the urea SCR system of the present invention, ammonia remaining unreacted in the SCR is reformed by the ammonia reforming catalyst, decomposed into hydrogen and nitrogen, and NOx is purified by the hydrogen generated by the reforming. Therefore, the NOx purification rate can be maintained at approximately 90% even in a high temperature range of 800 ° C.

１０ 排気管
１１ ＤＯＣ
１２ ＤＰＦ
１３ ＳＣＲ
１４ 尿素噴射ノズル
１５ アンモニア改質触媒 10 Exhaust pipe 11 DOC
12 DPF
13 SCR
14 Urea injection nozzle 15 Ammonia reforming catalyst

Claims (4)

エンジンの排気管に、ＤＯＣ、ＤＰＦ、ＳＣＲを接続し、ＳＣＲの上流側に尿素噴射ノズルを設けた尿素ＳＣＲシステムにおいて、ＳＣＲの下流側にアンモニアを改質するアンモニア改質触媒を設け、排ガスが低温のときに前記ＳＣＲ内でＮＯｘを浄化し、排ガスが高温のとき、アンモニア改質触媒で未反応アンモニアを改質して水素を生成し、その水素でＮＯｘを浄化することを特徴とする尿素ＳＣＲシステム。   In a urea SCR system in which a DOC, DPF, and SCR are connected to the exhaust pipe of an engine and a urea injection nozzle is provided on the upstream side of the SCR, an ammonia reforming catalyst that reforms ammonia is provided on the downstream side of the SCR. NOx is purified in the SCR when the temperature is low, and when the exhaust gas is hot, unreacted ammonia is reformed with an ammonia reforming catalyst to generate hydrogen, and the NOx is purified with the hydrogen SCR system. 前記アンモニア改質触媒は、担体に、アンモニアを窒素と水素に分解するアンモニア分解触媒と、排ガス中の酸素とＣＯとを反応させるＣＯ酸化触媒とが担持され、そのＣＯ酸化触媒で、排ガス中の酸素とＣＯとを反応させてＣＯ2とし、その雰囲気でアンモニア分解触媒が、未反応アンモニアを分解して水素を生成させる請求項１記載の尿素ＳＣＲシステム。 In the ammonia reforming catalyst, an ammonia decomposition catalyst for decomposing ammonia into nitrogen and hydrogen and a CO oxidation catalyst for reacting oxygen and CO in the exhaust gas are supported on the carrier. and CO 2 by reacting oxygen and CO, ammonia decomposition catalyst in the atmosphere, a urea SCR system of claim 1, wherein to produce hydrogen by decomposing unreacted ammonia. 前記アンモニア改質触媒は、アルミナからなる担体に、アンモニア分解触媒としてのＮｉ又はＲｕイオンを含浸させた後、乾燥し、さらにその担体に、ＣＯ酸化触媒としての貴金属イオンを含浸させた後、これを乾燥し、焼成して形成される請求項２記載の尿素ＳＣＲシステム。   The ammonia reforming catalyst is obtained by impregnating a support made of alumina with Ni or Ru ions as an ammonia decomposition catalyst, and then drying, and further impregnating the support with noble metal ions as a CO oxidation catalyst. The urea SCR system according to claim 2, which is formed by drying and baking. ＤＰＦにＰＭが堆積したとき、ポスト噴射又は排気管噴射を行って、排ガス温度を上昇させてＤＰＦを再生させ、その再生中に、酸素濃度に応じたＣＯをＤＰＦで生成し、ＣＯ酸化触媒で排ガス中の酸素とＣＯとを反応させ、アンモニア分解触媒で、未反応アンモニアを分解して水素を生成させてＮＯｘを浄化する請求項２記載の尿素ＳＣＲシステム。   When PM accumulates in the DPF, post injection or exhaust pipe injection is performed to raise the exhaust gas temperature to regenerate the DPF. During the regeneration, CO corresponding to the oxygen concentration is generated in the DPF, and the CO oxidation catalyst The urea SCR system according to claim 2, wherein oxygen in the exhaust gas is reacted with CO, and an ammonia decomposition catalyst decomposes unreacted ammonia to generate hydrogen to purify NOx.
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