JP4316901B2

JP4316901B2 - Diesel exhaust gas treatment method and treatment apparatus

Info

Publication number: JP4316901B2
Application number: JP2003061385A
Authority: JP
Inventors: 尚美今田; 幾久浜田; 泰良加藤
Original assignee: Babcock Hitachi KK
Current assignee: Mitsubishi Power Ltd
Priority date: 2003-03-07
Filing date: 2003-03-07
Publication date: 2009-08-19
Anticipated expiration: 2023-03-07
Also published as: JP2004270520A

Description

【０００１】
【発明の属する技術分野】
本発明は、ディーゼル排ガス処理方法および処理装置に係り、特に、ディーゼルエンジンから排出されるディーゼル排ガス中の窒素酸化物を低温域でも効率よく分解、除去することができるディーゼル排ガス処理方法および処理装置に関する。
【０００２】
【従来の技術】
ディーゼルエンジンは、コジェネレーションシステムや、自動車、発電機などに多く用いられているが、近年地球環境保全に対する関心の高まりから、これらエンジンから排出される排ガスの規制が強化されている。
【０００３】
自動車排ガスの浄化用触媒としては、一酸化炭素（ＣＯ）および炭化水素（ＨＣ）の酸化とＮＯｘの還元とを同時に行うことができる三元触媒（特公昭５６−２７２９５号公報）が好適に用いられるが、リーンバーンエンジンやディーゼルエンジンの排ガスは酸素過剰雰囲気であるため、一般の三元触媒ではＮＯｘが分解されにくいという問題がある。そこで、このような排ガスの浄化技術として、例えば（１）フィルタにより粒子状汚染物質（以下、単にＰＭともいう）を除去した後、窒素酸化物（ＮＯｘ）を除去する方法、（２）白金（Ｐｔ）などの貴金属を担時した触媒上で炭化水素を還元剤として用いてＮＯｘを還元する方法（特開平５−１０３９８５号公報）、（３）ＮＯｘ吸蔵触媒を用いる方法など多くの技術が提案されている。
【０００４】
上記技術のうち、フィルタによりＰＭを除去した後、窒素酸化物を除去する方法は、図５に示したように、ディーゼルエンジン１から排出された排ガス中のＰＭをまず酸化触媒付きディーゼルパティキュレートフィルタ３（以下、酸化触媒付きＤＰＦという）で捕集し、該ＤＰＦに担時された酸化触媒成分によってNOの一部をNO₂に酸化させ（(1)式）、煤との反応性が酸素よりも高い、前記NO₂によって煤を燃焼させて二酸化炭素に無害化した後（(2)式）、尿素注入ノズル４から還元剤としてＮＨ₃または尿素などのＮＨ₃前駆体を添加し、脱硝触媒層５上で窒素酸化物を無害な窒素と水に還元、分解するものであり、ＰＭとＮＯｘの両方を確実に除去できる技術として実用化を目指した検討が行われている。
ＮＯ＋1/2Ｏ₂ → ＮＯ₂ ・・・（１）
Ｃ＋２ＮＯ₂ → ＣＯ₂ ＋２ＮＯ・・・（２）
【特許文献１】
特開平０６−１２３２１８号公報
【特許文献２】
特開平０８−１２１１４５号公報
【０００５】
【発明が解決しようとする課題】
上記従来技術における脱硝方法は、従来から発電所などのボイラ排ガス処理に広く採用されている方法であり、触媒としては、酸化チタン系の脱硝触媒のほか、ゼオライトに活性成分をイオン交換させた触媒などが用いられている。
【０００６】
しかしながら、上記脱硝触媒は、通常300〜450℃の温度範囲で高い脱硝活性を示すのに対し、ディーゼル自動車におけるエンジンの始動停止は頻繁であるために、その排ガス温度は、例えば100〜300℃の範囲である時間が長く、従来の脱硝触媒では、高い脱硝率が得られないという問題があった。特にディーゼルエンジンを搭載したトラックやバスなどの排ガスを処理する場合、起動停止が頻繁で排ガス温度が200〜300℃を超えることがほとんどないことから、脱硝率が低く、ＮＯｘが未処理状態で排出されるだけでなく、還元剤としてのＮＨ₃が多量にリークして二次公害の原因になるという問題があった。
【０００７】
本発明の課題は、上記従来技術の問題点を解決し、エンジン始動時などの排ガス温度が低い場合であっても高い脱硝性能を得ることができるディーゼル排ガス処理方法および処理装置を提供することにある。
【０００８】
【課題を解決するための手段】
上記課題を解決するため、本願で特許請求する発明は以下のとおりである。
（１）ディーゼルエンジンから排出される排ガスを酸化触媒付きディーゼルパティキュレートフィルタに導入して排ガス中の粒子状物質を除去した後、脱硝触媒層に導入して窒素酸化物を接触還元除去するディーゼル排ガス処理方法において、前記酸化触媒は、白金、パラジウム、イリジウムおよびロジウムから選ばれた少なくとも１種類をチタニア、ジルコニア、アルミナ、シリカおよびセリアから選ばれた少なくとも１種類の酸化物に担時した触媒であり、かつ、該酸化触媒付きディーゼルパティキュレートフィルタの前段および脱硝触媒層の前段にそれぞれ窒素酸化物の還元剤を注入する手段を設け、前記排ガス温度に基づいて前記還元剤の注入位置を切り替えることを特徴とするディーゼル排ガス処理方法。
【０００９】
（２）前記排ガス温度が所定温度未満の場合に酸化触媒付きディーゼルパティキュレートフィルタの前流に前記還元剤を注入し、排ガス温度が前記の温度以上の場合に前記脱硝触媒層の前流に前記還元剤を注入することを特徴とする上記（１）に記載のディーゼル排ガス処理方法。
（３）前記酸化触媒付きディーゼルパティキュレートフィルタの前段に前記酸化触媒層を設け、排ガス温度が所定温度未満の場合に該酸化触媒層の前流または酸化触媒層と前記酸化触媒付きディーゼルパティキュレートフィルタとの間に前記還元剤を注入することを特徴とする上記（１）または（２）に記載のディーゼル排ガス処理方法。
（４）前記所定温度は、２００〜３００℃であることを特徴とする上記（２）または（３）に記載のディーゼル排ガス処理方法。
（５）前記還元剤としてアンモニアまたはアンモニア前駆体を用い、アンモニア前駆体を用いる際は、ディーゼルエンジンの出口に近い排ガス煙道内に注入してアンモニアへの分解反応時間を確保することを特徴とする上記（１）〜（４）の何れかに記載のディーゼル排ガス処理方法。
【００１０】
（６）ディーゼルエンジンの排ガス煙道に、排ガスの流れに沿って順次酸化触媒付きディーゼルパティキュレートフィルタおよび脱硝触媒層を設け、該酸化触媒は、白金、パラジウム、イリジウムおよびロジウムから選ばれた少なくとも１種類をチタニア、ジルコニア、アルミナ、シリカおよびセリアから選ばれた少なくとも１種類の酸化物に担時した触媒であり、かつ、該酸化触媒付きディーゼルパティキュレートフィルタおよび脱硝触媒層の前段にそれぞれ還元剤注入手段を設け、かつ両還元剤注入手段の切り替え手段を設けたことを特徴とするディーゼル排ガス処理装置。
（７）前記酸化触媒付きディーゼルパティキュレートフィルタの前段に前記酸化触媒層を設け、前記酸化触媒付きディーゼルパティキュレートフィルタの前段の還元剤注入手段を、前記酸化触媒層の前段または酸化触媒層と前記酸化触媒付きディーゼルパティキュレートフィルタとの間に設けたことを特徴とする上記（６）に記載のディーゼル排ガス処理装置。
【００１１】
図４は、本発明の原理を示す説明図である。図４において、酸化触媒付きDPFとSCR触媒（脱硝触媒）のNO₂とNOの還元性能には温度依存性があることが分かる。すなわち、脱硝触媒において安定した脱硝活性が得られる温度は、２５０〜３００℃以上であるのに対し、酸化触媒付きＤＰＦにおける脱硝活性は、３００℃以下、特に２５０℃以下にピークがあることが分かる。このため、排ガス温度が低い場合には還元剤を酸化触媒付きDPFの前流側に添加することにより、酸化触媒付きDPFはNO、NO₂を低温で還元、除去する脱硝触媒として機能し、排ガス中のＮＯｘを低温で効率よく還元、分解することができる。一方、排ガス温度が高い場合には、還元剤を通常のように脱硝触媒層の前段に注入することにより、該脱硝触媒の存在下、排ガス中のＮＯｘが前記還元剤によって効率よく還元、分解される。
【００１２】
このように、本発明においては、ＰＭの除去手段である酸化触媒付きＤＰＦの前段にも還元剤であるアンモニアまたは尿素などのアンモニア前駆体を注入する手段を設け、被処理排ガス温度に基づいて前記還元剤の注入位置を、通常の脱硝触媒層前流と前記酸化触媒付きＤＰＦの前流との間で切り替えるものである。すなわち、排ガス温度が低温、例えば２００〜３００℃未満、好ましくは２５０〜３００℃未満の場合は、還元剤は酸化触媒付きＤＰＦの前流に注入され、２００〜３００℃以上、好ましくは２５０〜３００℃以上であるときは脱硝触媒層の前流に注入される。なお、酸化触媒付きＤＰＦにおいて、排ガス中の窒素酸化物は、還元剤であるアンモニアまたはその前駆体の酸化過程で還元、分解される。
【００１３】
ところで、自動車エンジンの排ガス処理では、脱硝触媒層の前流に尿素などの水溶液を注入する場合、排ガス温度が低いと尿素の加水分解反応が進行しにくいだけでなく、還元剤注入位置と触媒との距離が短いために排ガスと還元剤の混合が不十分となり、還元剤濃度に分布が生じてリークが発生し、脱硝率が低くなるという問題が生じやすいが、本発明方法では、排ガス温度が低いときにはＤＰＦの前流から還元剤が注入されるので、エンジン出口により近い場所に注入することができ、これにより尿素等のアンモニア前駆体の加水分解が促進され、分散度が向上して高いＮＯｘ除去率を得ることができるという利点がある。
【００１４】
本発明において、酸化触媒付きＤＰＦは、ウォールフロー型のディーゼルパティキュレートフィルタ（ＤＰＦ）上に通常のいわゆる貴金属触媒、例えば、白金、パラジウム、イリジウム、ロジウムなどの貴金属の少なくとも１種類をチタニア、ジルコニア、アルミナ、シリカ、セリアなどの少なくとも１種類の酸化物に担時した触媒成分を担持したものが使用される。
【００１５】
脱硝触媒としては、通常の、アンモニアを還元剤とした脱硝処理に用いられている触媒であればどのような物でもよく、例えば酸化チタンに活性成分を担時した触媒や、銅、鉄、セリウムなどの遷移金属をイオン交換したゼオライトを、コージェライトハニカム構造体などに担時した触媒などが好適に用いられる。
【００１６】
本発明において、還元剤としては、アンモニアまたはアンモニア前駆体が使用される。アンモニア前駆体としては、例えば尿素が好適に用いられる。還元剤注入手段としては、アンモニアまたは尿素水溶液を排ガス煙道内に噴霧するノズルが好適に用いられる。
【００１７】
本発明において、酸化触媒付きＤＰＦの前段に酸化触媒層を設けることもできる。この場合、酸化触媒としては、通常の酸化触媒、例えば上記ＤＰＦ上に担持される触媒成分と同様の組成からなる触媒成分をフルースロー型のコージェライトハニカム構造体に担持させたものが好適に用いられる。またこのとき、還元剤注入手段は、前記酸化触媒の前段または酸化触媒付きＤＰＦの前段（酸化触媒と酸化触媒付きＤＰＦの間）の何れか一方に設けられる。
【００１８】
【発明の実施の形態】
次に、本発明を図面を用いて詳細に説明する。
図１は、本発明の一実施例であるディーゼル排ガス処理装置の系統を示す説明図である。図１において、この装置は、ディーゼルエンジン１の排ガス煙道としての排気管１０に、前記排ガスの流れに沿って順次酸化触媒付きディーゼルパティキュレートフィルタ３および脱硝触媒層５を設け、前記酸化触媒付きＤＰＦ３および脱硝触媒層５の前段にそれぞれ還元剤注入手段としての尿素注入ノズル２および４を設け、かつその切り替え手段としての切替弁７を設けたものである。８は、ポンプ、９は、尿素水タンクである。
【００１９】
このような構成において、ディーゼルエンジン１から排出された排ガスは、排気管１０を通って酸化触媒付きＤＰＦ３に流入する。このとき排ガス温度が、例えば３００℃未満であれば、尿素注入ノズル２から排気管１０内に尿素水溶液が注入され、排ガス中のＰＭ、ＮＯおよびＮＯ₂が酸化触媒付きＤＰＦ３上で除去される。ＰＭ、ＮＯおよびＮＯ₂が除去された排ガスは、後流の脱硝触媒層５を通過して系外に排出される。一方、排ガス温度が、例えば３００℃以上であれば、注入ノズル２からの尿素水溶液の注入は行われず、排ガスは酸化触媒付きＤＰＦ３を流通し、ここで排ガス中のＰＭが除去された後、尿素注入ノズル４から注入された尿素水溶液と混合して後流の脱硝触媒層５に流入し、ここで、排ガス中のＮＯおよびＮＯ₂が前記尿素が分解したＮＨ₃によって還元、分解されたのち系外に排出される。
【００２０】
本実施例によれば、被処理ガスであるディーゼル排ガス温度に基づいてＮＯｘの還元剤であるアンモニアまたはアンモニア前駆体の注入位置を、酸化触媒付きＤＰＦ３の前流と脱硝触媒層５の前流との間で切り替えることにより、３００℃以下の低温域から３００℃以上の高温域に至るまで、ディーゼル排ガス中の窒素酸化物を効率よく分解除去することができる。
【００２１】
図２は、本発明の他の実施例を示す装置系統図である。図２において、この装置が図１の装置と異なるところは、酸化触媒付きＤＰＦ３の前段に酸化触媒層６を設け、該酸化触媒層６と前記酸化触媒付きＤＰＦ３との間に尿素注入ノズル２を設けた点である。
【００２２】
本実施例においても、排ガス温度が、例えば３００℃未満のときは尿素注入ノズル２から還元剤としての尿素水を注入することにより、実施例１と同様、排ガス中のＰＭ、ＮＯおよびＮＯ₂が酸化触媒付きＤＰＦ３上で除去されるので、起動停止が頻繁に生じるディーゼル自動車などから排出される比較的低温の排ガス中の窒素酸化物でも十分分解、除去することができる。なお、排ガス温度が、例えば３００℃以上のときは尿素注入ノズル４から尿素水溶液が注入されるよにう切り替えられるので、窒素酸化物は、脱硝触媒層５で分解除去される。
【００２３】
図３は、本発明の別の実施例を示す装置系統図である。図３において、この装置が図１の装置と異なるところは、酸化触媒付きＤＰＦ３の前流に酸化触媒層６を設け、該酸化触媒層６の前流側に尿素注入ノズル２を設けた点である。
本実施例においても、上記実施例１および２と同様、排ガス温度が、例えば３００℃未満のときは尿素注入ノズル２から還元剤としての尿素水を注入することにより、実施例１および２と同様、排ガス中のＰＭ、ＮＯおよびＮＯ₂が酸化触媒付きＤＰＦ３上で除去されるので、低温での脱硝性能を十分確保することができるうえ、排ガス温度が、例えば３００℃以上のときは尿素注入ノズル４から尿素水溶液が注入されるように切り替えられるので、上記と同様に脱硝触媒層５で窒素酸化物が分解、除去される。
【００２４】
【実施例】
次に本発明の具体的実施例を説明する。
実施例１
アルミナゾル（アルミナゾル520，日産化学社製）とセリアゾル（エコライトＣＥ、多木化学社製）と水とを混合し、アルミナおよびセリアをそれぞれ15%含有する水溶液を1リットル調製し、この液にウォールフロー型の担体（12mil/300cpsi、日立金属社製）100×100mm（50mm長さ）を浸漬させたのち遠心分離装置によって液切りする操作を２回繰り返し、大気中150℃で5時間乾燥したのち、500℃で2時間焼成して触媒担体とし、得られた触媒担体にジニトロジアンミン白金の水溶液を含浸させ、遠心分離装置により液切りした後、大気中150℃で5時間乾燥し、550℃で2時間焼成して体積当たりの白金の担持量が２g/リットルである酸化触媒付きＤＰＦを得た。
【００２５】
得られた酸化触媒付きＤＰＦ（５セル×５セル）を、流通式の反応装置に設置し、酸化触媒付きＤＰＦの前段に尿素水溶液を注入して下記表１の条件でＮＯおよびＮＯ₂の除去率（脱硝率）を測定したところ、脱硝率は８０％であった。
【００２６】
【表１】

【００２７】
比較例１
酸化チタン、酸化タングステン、メタバナジン酸アンモニウム、シュウ酸および水をニーダで混練してペースト状とし、これを押出し造粒した後乾燥し、500℃で2時間通気焼成し、得られた造粒物を粒径150μm以下に粉砕して脱硝触媒粉末を得た（Ti/W/V=89/5/6）。得られた脱硝触媒粉末と水とを攪拌機で混合してスラリ濃度３５％のスラリを調製し、これにフロースルー型のコージェライト担体(600cpsi) 100×100mm（50mm長さ）を浸漬し、エアブローで液切りした後乾燥する工程を３回繰り返した後、500℃で2時間焼成して脱硝触媒を得た。
【００２８】
実施例１の酸化触媒付きＤＰＦを上記脱硝触媒に替えた以外は上記実施例１と同様にしてＮＯおよびＮＯ₂の除去率を測定したところ、脱硝率は、４０％であった。
実施例１および比較例１の結果から、被処理排ガス温度が３００℃未満の場合、特に２５０℃未満の場合は、酸化触媒付きＤＰＦの前段に還元剤を注入し、該酸化触媒付きＤＰＦによって排ガス中のＰＭ、ＮＯ、およびＮＯ₂を除去することにより、脱硝率が著しく向上することが分かる。
【００２９】
【発明の効果】
本願の請求項１に記載の発明によれば、エンジン始動時などの排ガス温度が低い場合であってもディーゼル排ガス中の窒素酸化物を効率よく分解、除去することができるので、低温から高温の幅広い温度域で高い脱硝率を得ることができる。
本願の請求項２に記載の発明によれば、上記発明の効果に加え、排ガス温度が所定温度未満であっても高い脱硝率を得ることができる。
【００３０】
本願の請求項３に記載の発明によれば、上記発明と同様、低温から高温まで幅広い温度域で効率よくディーゼル排ガス中の窒素酸化物を分解、除去することができる。
本願の請求項４に記載の発明によれば、上記発明と同様、低温から高温まで幅広い温度域で脱硝処理が可能となり、排ガス温度が２００〜３００℃以下であっても高い脱硝率を得ることができる。
【００３１】
本願の請求項５に記載の発明によれば、上記発明の効果に加え、アンモニア前駆体の分解反応時間を確保して効率よく窒素酸化物を還元、除去することができる。
本願の請求項６に記載の発明によれば、低温から高温まで幅広い温度域でディーゼル排ガス中の窒素酸化物を効率よく分解、除去することができる。
本願の請求項７に記載の発明によれば、上記発明の効果と同様、低温から高温まで幅広い温度域でディーゼル排ガス中の窒素酸化物を効率よく分解、除去することができる。
【図面の簡単な説明】
【図１】本発明の一実施例であるディーゼル排ガス処理装置のフローを示す説明図。
【図２】本発明の他の実施例を示す説明図。
【図３】本発明の別の実施例を示す説明図。
【図４】本発明の原理を示す説明図。
【図５】従来技術を示す説明図。
【符号の説明】
１…ディーゼルエンジン、２…尿素注入ノズル、３…酸化触媒付きＤＰＦ、４…尿素注入ノズル、５…脱硝触媒層、６…酸化触媒層、７…切替弁、８…ポンプ、９…尿素水タンク、１０…排気管。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a diesel exhaust gas processing method and a processing apparatus, and more particularly to a diesel exhaust gas processing method and a processing apparatus capable of efficiently decomposing and removing nitrogen oxides in diesel exhaust discharged from a diesel engine even in a low temperature range. .
[0002]
[Prior art]
Diesel engines are widely used in cogeneration systems, automobiles, generators, and the like. However, in recent years, regulations on exhaust gas emitted from these engines have been strengthened due to increasing interest in global environmental conservation.
[0003]
As a catalyst for purifying automobile exhaust gas, a three-way catalyst (Japanese Patent Publication No. 56-27295) capable of simultaneously performing oxidation of carbon monoxide (CO) and hydrocarbon (HC) and reduction of NOx is suitably used. However, since the exhaust gas of a lean burn engine or a diesel engine has an oxygen-excess atmosphere, there is a problem that NOx is hardly decomposed by a general three-way catalyst. Therefore, as such exhaust gas purification technology, for example, (1) a method of removing particulate pollutants (hereinafter also simply referred to as PM) with a filter and then removing nitrogen oxides (NOx), (2) platinum ( Many techniques have been proposed, such as a method of reducing NOx using a hydrocarbon as a reducing agent on a catalyst carrying a noble metal such as Pt) (Japanese Patent Laid-Open No. 5-103985), and a method of using a NOx storage catalyst. Has been.
[0004]
Among the above techniques, after removing PM by a filter, the method of removing nitrogen oxides is as follows. As shown in FIG. 5, the PM in the exhaust gas discharged from the diesel engine 1 is first filtered with a diesel particulate filter with an oxidation catalyst. 3 (hereinafter referred to as DPF with an oxidation catalyst), a part of NO is oxidized to NO ₂ by the oxidation catalyst component carried by the DPF (Equation (1)), and the reactivity with soot is oxygen After the soot is burned with NO _{2 to} be rendered harmless by carbon dioxide (equation (2)), NH ₃ precursor such as NH ₃ or urea is added as a reducing agent from the urea injection nozzle 4 and denitration is performed. On the catalyst layer 5, nitrogen oxides are reduced and decomposed into harmless nitrogen and water, and studies aiming at practical use are being carried out as a technique that can reliably remove both PM and NOx.
NO + 1 / 2O ₂ → NO ₂ (1)
C + 2NO ₂ → CO ₂ + 2NO (2)
[Patent Document 1]
Japanese Patent Laid-Open No. 06-123218 [Patent Document 2]
Japanese Patent Laid-Open No. 08-121145
[Problems to be solved by the invention]
The above-mentioned conventional denitration method is a method widely used for boiler exhaust gas treatment in power plants and the like, and as a catalyst, in addition to a titanium oxide-based denitration catalyst, a catalyst in which an active component is ion-exchanged with zeolite. Etc. are used.
[0006]
However, while the above-mentioned denitration catalyst usually shows high denitration activity in the temperature range of 300 to 450 ° C, the start and stop of the engine in diesel automobiles is frequent, so the exhaust gas temperature is, for example, 100 to 300 ° C. The range of time is long, and the conventional denitration catalyst has a problem that a high denitration rate cannot be obtained. Especially when processing exhaust gas from trucks and buses equipped with diesel engines, the start-stop is frequent and the exhaust gas temperature rarely exceeds 200 to 300 ° C, so the NOx removal rate is low and NOx is discharged in an untreated state. In addition, there is a problem that a large amount of NH ₃ as a reducing agent leaks and causes secondary pollution.
[0007]
An object of the present invention is to provide a diesel exhaust gas processing method and a processing apparatus that can solve the above-described problems of the prior art and obtain high denitration performance even when the exhaust gas temperature is low, such as at the time of engine start. is there.
[0008]
[Means for Solving the Problems]
In order to solve the above problems, the invention claimed in the present application is as follows.
(1) Diesel exhaust gas that introduces exhaust gas discharged from a diesel engine into a diesel particulate filter with an oxidation catalyst to remove particulate matter in the exhaust gas, and then introduces it into a denitration catalyst layer to catalytically reduce and remove nitrogen oxides In the treatment method, the oxidation catalyst is a catalyst in which at least one selected from platinum, palladium, iridium and rhodium is supported by at least one oxide selected from titania, zirconia, alumina, silica and ceria. In addition, a means for injecting a nitrogen oxide reducing agent is provided at the front stage of the diesel particulate filter with an oxidation catalyst and the front stage of the denitration catalyst layer, respectively, and the injection position of the reducing agent is switched based on the exhaust gas temperature. A method for treating diesel exhaust gas.
[0009]
(2) When the exhaust gas temperature is lower than a predetermined temperature, the reducing agent is injected into the upstream of the diesel particulate filter with an oxidation catalyst, and when the exhaust gas temperature is equal to or higher than the above temperature, the reducing agent is introduced into the upstream of the denitration catalyst layer. The diesel exhaust gas treatment method as described in (1) above, wherein a reducing agent is injected.
(3) the said oxidation catalyst layer in front of the oxidation catalytic diesel particulate filter provided, diesel particulate filters the conditioned oxidation catalyst and the upstream side or the oxidation catalyst layer of the oxidation catalyst layer when the exhaust gas temperature is lower than a predetermined temperature The diesel exhaust gas treatment method according to (1) or (2), wherein the reducing agent is injected between the two.
(4) The diesel exhaust gas treatment method according to (2) or (3), wherein the predetermined temperature is 200 to 300 ° C.
(5) Ammonia or an ammonia precursor is used as the reducing agent, and when the ammonia precursor is used, it is injected into an exhaust gas flue close to the outlet of the diesel engine to ensure a decomposition reaction time into ammonia. The diesel exhaust gas treatment method according to any one of (1) to (4) above.
[0010]
(6) A diesel particulate filter with an oxidation catalyst and a denitration catalyst layer are sequentially provided in the exhaust gas flue of the diesel engine along the flow of the exhaust gas, and the oxidation catalyst is at least one selected from platinum, palladium, iridium and rhodium It is a catalyst that is supported by at least one oxide selected from titania, zirconia, alumina, silica, and ceria, and a reducing agent is injected before the diesel particulate filter with the oxidation catalyst and the denitration catalyst layer, respectively. And a diesel exhaust gas treatment apparatus characterized in that a means for switching between both reducing agent injection means is provided.
(7) the said oxidation catalyst layer in front of the oxidation catalytic diesel particulate filter provided, the front stage of the reducing agent injection means of the diesel particulate filter with an oxidation catalyst, wherein a pre-stage or oxidation catalyst layer of the oxidation catalyst layer The diesel exhaust gas treatment apparatus as described in (6) above, which is provided between the diesel particulate filter with an oxidation catalyst.
[0011]
FIG. 4 is an explanatory diagram showing the principle of the present invention. In FIG. 4, it can be seen that the NO ₂ and NO reduction performance of the DPF with an oxidation catalyst and the SCR catalyst (denitration catalyst) have temperature dependence. That is, the temperature at which stable denitration activity is obtained in the denitration catalyst is 250 to 300 ° C. or higher, whereas the denitration activity in the DPF with an oxidation catalyst has a peak at 300 ° C. or less, particularly 250 ° C. or less. . For this reason, when the exhaust gas temperature is low, by adding a reducing agent to the upstream side of the DPF with an oxidation catalyst, the DPF with an oxidation catalyst functions as a denitration catalyst that reduces and removes NO and NO ₂ at a low temperature. NOx inside can be efficiently reduced and decomposed at low temperatures. On the other hand, when the exhaust gas temperature is high, the NOx in the exhaust gas is efficiently reduced and decomposed by the reducing agent in the presence of the NOx removal catalyst by injecting the reducing agent into the previous stage of the NOx removal catalyst layer as usual. The
[0012]
Thus, in the present invention, a means for injecting an ammonia precursor such as ammonia or urea as a reducing agent is also provided in the preceding stage of the DPF with an oxidation catalyst, which is a means for removing PM, and based on the exhaust gas temperature to be treated. The injection position of the reducing agent is switched between the normal upstream of the denitration catalyst layer and the upstream of the oxidation catalyst-attached DPF. That is, when the exhaust gas temperature is low, for example, less than 200 to 300 ° C., preferably less than 250 to 300 ° C., the reducing agent is injected into the upstream of the DPF with an oxidation catalyst, and is 200 to 300 ° C. or more, preferably 250 to 300 ° C. When the temperature is higher than ° C., it is injected upstream of the denitration catalyst layer. In the DPF with an oxidation catalyst, nitrogen oxides in the exhaust gas are reduced and decomposed during the oxidation process of ammonia as a reducing agent or its precursor.
[0013]
By the way, in the exhaust gas treatment of an automobile engine, when an aqueous solution such as urea is injected into the upstream of the denitration catalyst layer, if the exhaust gas temperature is low, not only the hydrolysis reaction of urea is difficult to proceed, but also the reducing agent injection position and the catalyst However, the mixing of the exhaust gas and the reducing agent becomes insufficient, the distribution of the reducing agent concentration is generated, leaks occur, and the denitration rate tends to decrease. When it is low, the reducing agent is injected from the upstream of the DPF, so that it can be injected closer to the engine outlet, which promotes hydrolysis of ammonia precursors such as urea and improves the degree of dispersion and high NOx. There is an advantage that a removal rate can be obtained.
[0014]
In the present invention, the oxidation catalyst-attached DPF is a so-called noble metal catalyst such as platinum, palladium, iridium, rhodium, titania, zirconia, or the like, usually on a wall flow type diesel particulate filter (DPF). alumina, silica, those obtained by supporting the catalyst ingredients担時at least one oxide such as ceria is used.
[0015]
As the denitration catalyst, any catalyst can be used as long as it is a catalyst that is used in a conventional denitration treatment using ammonia as a reducing agent. For example, a catalyst in which an active component is supported on titanium oxide, copper, iron, cerium, or the like. A catalyst in which a zeolite obtained by ion exchange of a transition metal such as a cordierite honeycomb structure is used is preferably used.
[0016]
In the present invention, ammonia or an ammonia precursor is used as the reducing agent. For example, urea is preferably used as the ammonia precursor. As the reducing agent injection means, a nozzle for spraying ammonia or urea aqueous solution into the flue gas flue is preferably used.
[0017]
In the present invention, an oxidation catalyst layer may be provided in the front stage of the DPF with an oxidation catalyst. In this case, as the oxidation catalyst, a normal oxidation catalyst, for example, a catalyst component having the same composition as the catalyst component supported on the DPF is supported on a full throw type cordierite honeycomb structure is preferably used. It is done. Further, at this time, the reducing agent injection means is provided at either the preceding stage of the oxidation catalyst or the preceding stage of the DPF with the oxidation catalyst (between the oxidation catalyst and the DPF with the oxidation catalyst).
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be described in detail with reference to the drawings.
FIG. 1 is an explanatory view showing a system of a diesel exhaust gas treatment apparatus according to an embodiment of the present invention. In FIG. 1, this apparatus is provided with a diesel particulate filter 3 with an oxidation catalyst and a denitration catalyst layer 5 in an exhaust pipe 10 as an exhaust gas flue of a diesel engine 1 along the flow of the exhaust gas, and with the oxidation catalyst. The urea injection nozzles 2 and 4 as the reducing agent injection means are provided in front of the DPF 3 and the denitration catalyst layer 5, respectively, and the switching valve 7 as the switching means is provided. 8 is a pump, and 9 is a urea water tank.
[0019]
In such a configuration, the exhaust gas discharged from the diesel engine 1 flows into the DPF 3 with the oxidation catalyst through the exhaust pipe 10. At this time, if the exhaust gas temperature is lower than 300 ° C., for example, the urea aqueous solution is injected into the exhaust pipe 10 from the urea injection nozzle 2, and PM, NO, and NO ₂ in the exhaust gas are removed on the DPF 3 with an oxidation catalyst. The exhaust gas from which PM, NO and NO ₂ have been removed passes through the downstream denitration catalyst layer 5 and is discharged out of the system. On the other hand, if the exhaust gas temperature is, for example, 300 ° C. or higher, the urea aqueous solution is not injected from the injection nozzle 2, and the exhaust gas flows through the DPF 3 with an oxidation catalyst. It is mixed with the urea aqueous solution injected from the injection nozzle 4 and flows into the downstream denitration catalyst layer 5, where NO and NO ₂ in the exhaust gas are reduced and decomposed by NH ₃ decomposed by the urea. Discharged outside.
[0020]
According to the present embodiment, the injection position of ammonia or ammonia precursor, which is a NOx reducing agent, based on the temperature of the diesel exhaust gas that is the gas to be treated, the upstream of the DPF 3 with oxidation catalyst and the upstream of the denitration catalyst layer 5 By switching between these, nitrogen oxides in diesel exhaust gas can be efficiently decomposed and removed from a low temperature range of 300 ° C. or lower to a high temperature range of 300 ° C. or higher.
[0021]
FIG. 2 is an apparatus system diagram showing another embodiment of the present invention. In FIG. 2, this apparatus differs from the apparatus of FIG. 1 in that an oxidation catalyst layer 6 is provided in front of the DPF 3 with an oxidation catalyst, and a urea injection nozzle 2 is provided between the oxidation catalyst layer 6 and the DPF 3 with the oxidation catalyst. It is a point provided.
[0022]
Also in the present embodiment, when the exhaust gas temperature is lower than 300 ° C., for example, urea water as a reducing agent is injected from the urea injection nozzle 2, so that PM, NO, and NO ₂ in the exhaust gas are discharged as in the first embodiment. Since it is removed on the DPF 3 with the oxidation catalyst, it is possible to sufficiently decompose and remove even the nitrogen oxides in the relatively low temperature exhaust gas discharged from diesel vehicles or the like that frequently start and stop. When the exhaust gas temperature is, for example, 300 ° C. or higher, the urea aqueous solution is injected from the urea injection nozzle 4 so that the nitrogen oxide is decomposed and removed by the denitration catalyst layer 5.
[0023]
FIG. 3 is an apparatus system diagram showing another embodiment of the present invention. In FIG. 3, this apparatus differs from the apparatus of FIG. 1 in that an oxidation catalyst layer 6 is provided upstream of the oxidation catalyst-attached DPF 3 and a urea injection nozzle 2 is provided upstream of the oxidation catalyst layer 6. is there.
Also in the present embodiment, as in the first and second embodiments, when the exhaust gas temperature is less than 300 ° C., for example, urea water as a reducing agent is injected from the urea injection nozzle 2, thereby the same as in the first and second embodiments. Since PM, NO and NO ₂ in the exhaust gas are removed on the DPF 3 with the oxidation catalyst, it is possible to sufficiently secure the denitration performance at low temperature, and when the exhaust gas temperature is, for example, 300 ° C. or higher, a urea injection nozzle Since the urea aqueous solution is switched from 4 to 4 so as to be injected, nitrogen oxides are decomposed and removed by the denitration catalyst layer 5 as described above.
[0024]
【Example】
Next, specific examples of the present invention will be described.
Example 1
Alumina sol (Alumina sol 520, manufactured by Nissan Chemical Co., Ltd.), ceria sol (Ecolite CE, manufactured by Taki Chemical Co., Ltd.) and water are mixed, and 1 liter of an aqueous solution containing 15% each of alumina and ceria is prepared. After immersing a flow-type carrier (12mil / 300cpsi, manufactured by Hitachi Metals) 100 x 100mm (50mm length) and draining it with a centrifuge twice, after drying in air at 150 ° C for 5 hours The catalyst support was calcined at 500 ° C. for 2 hours, impregnated with an aqueous solution of dinitrodiammine platinum into the obtained catalyst support, drained by a centrifugal separator, dried in air at 150 ° C. for 5 hours, and at 550 ° C. The DPF with an oxidation catalyst having a platinum loading per volume of 2 g / liter was obtained by calcination for 2 hours.
[0025]
The obtained oxidation catalyst-attached DPF (5 cells x 5 cells) was placed in a flow reactor, and an aqueous urea solution was injected before the oxidation catalyst-attached DPF to remove NO and NO ₂ under the conditions shown in Table 1 below. When the rate (denitration rate) was measured, the denitration rate was 80%.
[0026]
[Table 1]

[0027]
Comparative Example 1
Titanium oxide, tungsten oxide, ammonium metavanadate, oxalic acid and water were kneaded with a kneader to form a paste, which was extruded and granulated, dried, ventilated and fired at 500 ° C for 2 hours, and the resulting granulated product was Denitration catalyst powder was obtained by grinding to a particle size of 150 μm or less (Ti / W / V = 89/5/6). The resulting denitration catalyst powder and water are mixed with a stirrer to prepare a slurry with a slurry concentration of 35%. A flow-through cordierite carrier (600 cpsi) 100 x 100 mm (50 mm length) is immersed in the slurry and air blown. The process of draining and drying after 3 times was repeated 3 times, followed by calcination at 500 ° C. for 2 hours to obtain a denitration catalyst.
[0028]
When the removal rate of NO and NO ₂ was measured in the same manner as in Example 1 except that the DPF with an oxidation catalyst in Example 1 was replaced with the above denitration catalyst, the denitration rate was 40%.
From the results of Example 1 and Comparative Example 1, when the temperature of the exhaust gas to be treated is less than 300 ° C., particularly less than 250 ° C., a reducing agent is injected before the DPF with an oxidation catalyst, and the exhaust gas is exhausted by the DPF with an oxidation catalyst. It can be seen that removal of PM, NO, and NO ₂ in the medium significantly improves the denitration rate.
[0029]
【The invention's effect】
According to the invention described in claim 1 of the present application, nitrogen oxides in diesel exhaust gas can be efficiently decomposed and removed even when the exhaust gas temperature is low such as when the engine is started. High denitration rate can be obtained in a wide temperature range.
According to the invention described in claim 2 of the present application, in addition to the effects of the above invention, a high denitration rate can be obtained even if the exhaust gas temperature is lower than a predetermined temperature.
[0030]
According to the invention described in claim 3 of the present application, similarly to the above-described invention, nitrogen oxides in diesel exhaust gas can be efficiently decomposed and removed in a wide temperature range from a low temperature to a high temperature.
According to the invention described in claim 4 of the present application, similarly to the above-described invention, it is possible to perform a denitration treatment in a wide temperature range from a low temperature to a high temperature, and to obtain a high denitration rate even when the exhaust gas temperature is 200 to 300 ° C. or less. Can do.
[0031]
According to the invention described in claim 5 of the present application, in addition to the effects of the above-described invention, it is possible to efficiently reduce and remove nitrogen oxides while securing the decomposition reaction time of the ammonia precursor.
According to the invention described in claim 6 of the present application, nitrogen oxides in diesel exhaust gas can be efficiently decomposed and removed in a wide temperature range from low temperature to high temperature.
According to the invention described in claim 7 of the present application, similarly to the effect of the invention, nitrogen oxides in diesel exhaust gas can be efficiently decomposed and removed in a wide temperature range from low temperature to high temperature.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing a flow of a diesel exhaust gas processing apparatus according to an embodiment of the present invention.
FIG. 2 is an explanatory view showing another embodiment of the present invention.
FIG. 3 is an explanatory diagram showing another embodiment of the present invention.
FIG. 4 is an explanatory diagram showing the principle of the present invention.
FIG. 5 is an explanatory diagram showing a conventional technique.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Diesel engine, 2 ... Urea injection nozzle, 3 ... DPF with oxidation catalyst, 4 ... Urea injection nozzle, 5 ... Denitration catalyst layer, 6 ... Oxidation catalyst layer, 7 ... Switching valve, 8 ... Pump, 9 ... Urea water tank 10 ... exhaust pipe.

Claims

In a diesel exhaust gas treatment method in which exhaust gas discharged from a diesel engine is introduced into a diesel particulate filter with an oxidation catalyst to remove particulate matter in the exhaust gas, and then introduced into a denitration catalyst layer to remove nitrogen oxides by catalytic reduction. The oxidation catalyst is a catalyst in which at least one selected from platinum, palladium, iridium and rhodium is supported by at least one oxide selected from titania, zirconia, alumina, silica and ceria, and the means for injecting respectively a reducing agent of nitrogen oxides in front of the oxidation catalytic diesel particulate previous and the denitration catalyst layer of the filter is provided, and switches the injection position of the reducing agent based on the exhaust gas temperature Diesel exhaust gas treatment method.

When the exhaust gas temperature is lower than a predetermined temperature, the reducing agent is injected into the upstream of the diesel particulate filter with an oxidation catalyst, and when the exhaust gas temperature is equal to or higher than the temperature, the reducing agent is introduced into the upstream of the denitration catalyst layer. It inject | pours, The diesel exhaust gas processing method of Claim 1 characterized by the above-mentioned.

Between the said oxidation catalyst layer in front of the oxidation catalytic diesel particulate filter provided, the oxidation catalytic diesel particulate filter and the upstream side or the oxidation catalyst layer of the oxidation catalyst layer when the exhaust gas temperature is lower than a predetermined temperature The diesel exhaust gas treatment method according to claim 1 or 2, wherein the reducing agent is injected into the exhaust gas.

The diesel exhaust gas treatment method according to claim 2 or 3, wherein the predetermined temperature is 200 to 300 ° C.

The ammonia or ammonia precursor is used as the reducing agent, and when the ammonia precursor is used, it is injected into an exhaust gas flue close to the outlet of the diesel engine to ensure a decomposition reaction time into ammonia. The diesel exhaust gas treatment method according to any one of?

A diesel particulate filter with an oxidation catalyst and a denitration catalyst layer are sequentially provided in the exhaust gas flue of the diesel engine along the flow of the exhaust gas, and the oxidation catalyst is at least one selected from platinum, palladium, iridium and rhodium. , A catalyst deposited on at least one oxide selected from zirconia, alumina, silica and ceria, and provided with a reducing agent injection means in front of the diesel particulate filter with oxidation catalyst and the denitration catalyst layer, respectively. And a diesel exhaust gas treatment apparatus provided with switching means for both reducing agent injection means.

Wherein said oxidation catalyst layer in front of the oxidation catalytic diesel particulate filter provided, wherein the front of the reducing agent injection means diesel particulate filter with an oxidation catalyst, wherein with an oxide catalyst and a previous or oxidation catalyst layer of the oxidation catalyst layer The diesel exhaust gas treatment apparatus according to claim 6, wherein the diesel exhaust gas treatment apparatus is provided between the diesel particulate filter and the diesel particulate filter.