JP3556728B2

JP3556728B2 - Exhaust gas denitration method and apparatus

Info

Publication number: JP3556728B2
Application number: JP08563595A
Authority: JP
Inventors: 泰良加藤; 尚美今田
Original assignee: Babcock Hitachi KK
Current assignee: Mitsubishi Power Ltd
Priority date: 1995-04-11
Filing date: 1995-04-11
Publication date: 2004-08-25
Anticipated expiration: 2019-08-25
Also published as: JPH08281061A

Description

【０００１】
【産業上の利用分野】
本発明は、排ガス脱硝方法および装置に係り、特に発電プラント用ボイラ、ガスタービン、ゴミ焼却炉などの各種燃焼炉から排出される排ガス中の窒素酸化物（ＮＯｘ）のうち一酸化窒素（ＮＯ）の一部を二酸化窒素（ＮＯ_２）にあらかじめ酸化することにより、低温から効率よくアンモニア（ＮＨ_３）で接触還元することが可能な排ガス脱硝方法および装置に関する。
【０００２】
【従来の技術】
排ガス中にアンモニア（ＮＨ_３）を注入した後、触媒と接触させることにより排ガス中の窒素酸化物を窒素に還元除去する接触アンモニア還元脱硝法において、排ガスにあらかじめ酸化剤を注入して含有される一酸化窒素（ＮＯ）の一部を二酸化窒素（ＮＯ_２）にすることによりＮＨ_３との反応性を向上させ、より低温から脱硝できるようにする試みは数多く知られている（例えば特開昭５２−９４８６３号公報、特公昭５６−５０６１３号公報、特開昭５４−２３０６８号公報など）。
【０００３】
上記方法は、（１）式で示される一酸化窒素１モルおよび二酸化窒素１モルとＮＨ_３２モルの反応が（２）式で示される通常の脱硝反応に較べてきわめて速いため、予めＮＯ_２源を注入するか、オゾン（Ｏ_３）、過酸化水素（Ｈ_２Ｏ_２）、硝酸（ＨＮＯ_３）などの酸化剤を注入してＮＯの一部をＮＯ_２に酸化（（３）式〜（５）式）して運転温度の低温化を図ろうというものである。
脱硝反応
【０００４】
【化１】
ＮＯ＋ＮＯ_２＋２ＮＨ_３ → ２Ｎ_２＋３Ｈ_２Ｏ（１）式
【０００５】
【化２】
ＮＯ＋ＮＨ_３＋１／４Ｏ_２ → Ｎ_２＋３／２Ｈ_２Ｏ（２）式
ＮＯの酸化反応
【０００６】
【化３】
ＮＯ＋Ｏ_３ → ＮＯ_２＋Ｏ_２（３）式
【０００７】
【化４】
ＮＯ＋Ｈ_２Ｏ_２ → ＮＯ_２＋Ｈ_２Ｏ（４）式
【０００８】
【化５】
ＮＯ＋２ＨＮＯ_３ → ３ＮＯ_２＋Ｈ_２Ｏ（５）式
上記のＮＯとＮＯ_２を共存させる効果は１００〜３００℃で顕著であるため、古くから各種焼却炉排ガス、廃熱回収ボイラ、ガスタービンなどの燃焼器を初めとする低温脱硝への応用が試みられてきたが、広く実用されるには到っていない。
【０００９】
【発明が解決しようとする課題】
上記従来技術を実用化することの妨げになっている理由としては、脱硝率の制御が難しいことが第一にあげられる。図５はＮＯ単独、ＮＯ_２単独、ＮＯとＮＯ_２とを当モルで含む各排ガスをチタン（Ｔｉ）−タングステン（Ｗ）−バナジウム（Ｖ）系触媒を用いて脱硝する場合の温度特性を示したものであるが、ＮＯとＮＯ_２が当モルの場合には効率よくＮＯｘが除去されるが、ＮＯまたはＮＯ_２単独では除去性能が著しく低下する。このため、ＮＯｘの変動に対し酸化剤の注入量とＮＨ_３の注入量を個別に制御する従来技術では、次のような多くの問題を生じる。
（Ａ）酸化剤が不足し、排ガス中のＮＯ_２の含有割合が小さいと脱硝反応速度が低下するだけでなく多量の未反応アンモニアの流出が生じる。
（Ｂ）排ガス中のＮＯ_２濃度がＮＯ濃度を超えると脱硝反応はアンモニア４モルが二酸化窒素３モルに対し反応（（６）式）するようになり、ＮＨ_３の注入量制御が困難である。そればかりか、ＮＯ_２がＮＯを超えるとＮＯ_２とＮＨ_３から硝安が生成して触媒が徐々に劣化するとともに、Ｎ_２Ｏを副生するようになる。
【００１０】
【化６】
３ＮＯ_２＋４ＮＨ_３ → ７／２Ｎ_２＋６Ｈ_２Ｏ（６）式
（Ｃ）ＮＨ_３がＮＯ_２含有比率に見合わないと、ＮＯより公害を引き起こし易い
ＮＯ_２が煙突から排出されるのみならず、煙色が黄変する。
本発明はこのような従来技術の問題点に鑑み、現在広く実用化されているＮＯを主体とする排ガスのアンモニア接触還元法脱硝装置と同様に、容易でかつ精度の高い脱硝率のコントロールが可能な脱硝装置を提供するとともに、上記した問題を生じない脱硝方法を提供しようとするものである。
【００１１】
【課題を解決するための手段】
上記目的を達成するため本願で特許請求される発明は以下のとおりである。
（１）窒素酸化物の大部分を一酸化窒素（ＮＯ）として含有する排ガスにアンモニアと酸化剤としてオゾンまたは過酸化水素を注入して一酸化窒素の一部を二酸化窒素（ＮＯ₂）に転化させ、この排ガスを脱硝触媒と接触させて排ガス中の窒素酸化物を還元除去する排ガス脱硝方法において、排ガス流量情報と排ガス中のＮＯｘ濃度情報から必要アンモニア注入量を算出するとともに、アンモニアと酸化剤の注入量を連動させ、これらのモル比（アンモニア注入量／オゾンまたは過酸化水素注入量）が２．０±０．２モル／モルの範囲になるように制御することを特徴とする排ガス脱硝方法。
（２）窒素酸化物の大部分を一酸化窒素（ＮＯ）として含有する排ガスにアンモニアと酸化剤として硝酸を注入して一酸化窒素の一部を二酸化窒素（ＮＯ ₂ ）に転化させ、この排ガスを脱硝触媒と接触させて排ガス中の窒素酸化物を還元除去する排ガス脱硝方法において、排ガス流量情報と排ガス中のＮＯｘ濃度情報から必要アンモニア注入量を算出するとともに、アンモニアと酸化剤の注入量を連動させ、これらのモル比（アンモニア注入量／硝酸注入量）が３．０±０．３モル／モルの範囲になるように制御することを特徴とする排ガス脱硝方法。
（３）一酸化窒素含有排ガスにアンモニアと酸化剤としてオゾンまたは過酸化水素を注入して一酸化窒素の一部を二酸化窒素に転化させる手段と、転化処理した排ガスを脱硝触媒と接触させて窒素酸化物を還元除去する手段とを備えた排ガス脱硝装置において、前記アンモニアと前記酸化剤の注入量の制御手段を連動させ、アンモニア注入量／酸化剤注入量のモル比を２．０±０．２モル／モルの範囲に制御する手段を設けたことを特徴とする排ガス脱硝装置。
（４）一酸化窒素含有排ガスにアンモニアと酸化剤として硝酸を注入して一酸化窒素の一部を二酸化窒素に転化させる手段と、転化処理した排ガスを脱硝触媒と接触させて窒素酸化物を還元除去する手段とを備えた排ガス脱硝装置において、前記アンモニアと前記酸化剤の注入量の制御手段を連動させ、アンモニア注入量／酸化剤注入量のモル比を３．０±０．３モル／モルの範囲に制御する手段を設けたことを特徴とする排ガス脱硝装置。
【００１２】
【作用】
本発明のようにＮＨ_３と酸化剤注入量を上記したような範囲に制御すると前述の（１）式の反応がきわめて速いため、総反応として以下に示す（７）式〜（９）式の反応により酸化剤とＮＨ_３が過不足なく、かつ速やかに消費される。
【００１３】
【化７】
２ＮＨ_３＋Ｏ_３＋２ＮＯ → ２Ｎ_２＋３Ｈ_２Ｏ＋Ｏ_２（７）式
【００１４】
【化８】
２ＮＨ_３＋Ｈ_２Ｏ_２＋２ＮＯ → ２Ｎ_２＋４Ｈ_２Ｏ（８）式
【００１５】
【化９】
３ＮＨ_３＋ＨＮＯ_３＋２ＮＯ → ３／２Ｎ_２＋５Ｈ_２Ｏ（９）式
このため、従来技術で問題となった排ガス中のＮＯまたはＮＯ_２濃度がどちらかに偏った場合に生じる脱硝率の低下、ＮＯ_２のリーク、Ｎ_２Ｏの副生、または触媒劣化を起こすことがない。もし排ガス中のＮＯを除去するに必要な量以上にＮＨ_３と酸化剤が注入されたとしても、Ｏ_３とＨ_２Ｏ_２の場合にはＮＨ_３のリークと無害な酸素がリークするのみであり、これは通常の排煙脱硝装置と同様である。ＨＮＯ_３を酸化剤とする場合にはＮＯ_２がリークするもののＮＨ_３注入量の１／３以下の注入量であるためきわめてわずかであり、その弊害は大幅に軽減されることになる。
【００１６】
したがって、運転に当たっては酸化反応を意識することなくアンモニアの注入量を変化させることにより脱硝活性を設定できる。さらに、常に（７）式〜（９）式の速い反応のみを進行させることができるため、触媒使用量の低減または反応温度の低下を図ることが可能になる。
以上に示したように、本発明では常にアンモニア注入量と酸化剤注入量を一定比率で制御することにより、現在一般に用いられている脱硝装置と同様の簡便さで装置を運転することが可能になる。
【００１７】
本発明で採用した手段は、排ガス中のＮＯｘ濃度に関する情報を得て、その信号によりＮＯ酸化剤とＮＨ_３とを一定比率で添加できるようにした点に特徴がある。
【００１８】
【実施例】
図１に本発明の一実施例である排ガス脱硝装置の基本系統図を示す。この脱硝装置はＮＯｘ濃度測定装置９、酸化剤注入装置１０、ＮＨ_３注入装置１１、触媒反応器２とからなり、酸化剤注入装置１０とＮＨ_３注入装置１１はＮＯｘ測定装置９からの濃度信号１２と流量信号１３とにより、両者の注入量のモル比が常時所定の一定割合になるように制御される。
【００１９】
ここで、酸化剤とＮＨ_３の注入モル比の一定量とは酸化剤の種類により、次のような範囲のものを意味する。
（Ａ）酸化剤がＯ_３およびＨ_２Ｏ_２の場合
ＮＨ_３注入量／酸化剤注入量＝２．０±０．２モル／モル
（Ｂ）酸化剤がＨＮＯ_３の場合
ＮＨ_３注入量／酸化剤注入量＝３．０±０．３モル／モル
また、酸化剤注入装置１０とはオゾン発生器、過酸化水素または硝酸吹込み装置など酸化剤を排ガスに注入するための手段のほか、それら酸化剤の発生装置からの流量を制御するための制御弁、放電によるオゾン発生器の加電電圧の制御による発生量の制御などの、いわゆる制御手段をも含むものである。
【００２０】
本発明の実施に必要な基本的装置構成は図１に示したとおりである。
脱硝装置はＮＯｘ測定装置９、酸化剤注入装置１０、ＮＨ_３注入装置１１、触媒反応器２とからなり、酸化剤注入装置１０とＮＨ_３注入装置１１はＮＯｘ測定装置９からの濃度信号１２と流量信号１３とにより、酸化剤およびアンモニアの注入量のモル比が常時所定の一定割合になるように注入制御するように構成されている。
【００２１】
ここでＮＯｘ測定装置９は、赤外線式または化学発光式などの通常のＮＯｘ濃度測定装置が用いられ、排ガス中のＮＯｘ濃度に比例した信号を発生する。一方、流量信号１３は排ガス源からもらってもよいし、ピートー管、オリフィスなどで独自に計測して発生したものであってもよい。
両信号は酸化剤注入装置１０およびアンモニア注入量装置１１とに送られ、注入量制御信号に変換されて両者の注入量がガス量、ＮＯｘ濃度の変化に追従し、かつアンモニア注入量／酸化剤注入量モル比が常に一定割合になるように制御するために用いられる。本発明のポイントは、前述したようにアンモニア注入量／酸化剤注入量のモル比が常に一定割合になるように制御することにあり、したがって図２のように流量信号１３とＮＯｘ濃度信号１２をあらかじめ演算器１４で演算処理し、アンモニア注入量／酸化剤注入量のモル比が常に一定割合になるような制御信号１５を発生して制御するようにしてもよいことはいうまでもない。
【００２２】
アンモニア注入装置１１は、アンモニアガス、液化アンモニアなどのアンモニアの供給装置と流量制御弁などの制御手段とからなり、前記制御信号に比例したアンモニアを供給する。これは酸化剤注入装置においても同様であり、異なるのはアンモニア注入量／酸化剤注入量のモル比が常に一定割合になるようアンモニア注入量と連動されて制御されていることである。
【００２３】
酸化剤にはオゾン、硝酸、過酸化水素など排ガス中のＮＯをＮＯ_２に酸化できる能力を有するものが用いられ、その注入量は酸化剤の種類により請求範囲に記載したアンモニア注入量／酸化剤注入量のモル比になるように制御される。
また、触媒３には公知の酸化チタン系触媒、ゼオライト系触媒など通常の脱硝用触媒が用いられる。
【００２４】
排ガス中に注入された酸化剤によりＮＯの一部は前記（３）式〜（５）式のようにＮＯ_２に酸化された後、アンモニアが加えられ触媒反応器に導かれる。このときアンモニアと酸化剤とはアンモニア注入量／酸化剤注入量のモル比が常に一定割合になるように制御されているため、反応速度の非常に大きい（７）式〜（９）式の反応が選択的に進行し、アンモニアと酸化剤が過不足なく消費される。
【００２５】
２ＮＨ_３＋Ｏ_３＋２ＮＯ → ２Ｎ_２＋３Ｈ_２Ｏ＋Ｏ_２（７）式
２ＮＨ_３＋Ｈ_２Ｏ_２＋２ＮＯ → ２Ｎ_２＋４Ｈ_２Ｏ（８）式
３ＮＨ_３＋ＨＮＯ_３＋２ＮＯ → ３／２Ｎ_２＋５Ｈ_２Ｏ（９）式
このように、非常に速い反応のみを選択的に生じさせるため、運転温度を著しく低下させることが可能になり、１００〜３００℃、通常１５０〜２５０℃で触媒量の大幅な低減が図れる。また、未反応のアンモニアの流出や酸化剤により生成したＮＯ_２の発生がほとんどなく、従来技術で問題となった種々の問題を生じないことは前述したとおりである。
【００２６】
アンモニア注入量／酸化剤注入量のモル比は、Ｏ_３とＨ_２Ｏ_２を用いる場合には２、ＨＮＯ_３を用いる場合には３にした場合に最も効率よく脱硝できるが、特許請求範囲に記載した範囲内であれば実用上充分高い効率が得られ、問題も生じない。上記モル比が大きすぎても、低すぎても脱硝率の低下や触媒劣化、Ｎ_２Ｏの副生を生じるので好ましくない。
【００２７】
また、排ガス中のＮＯに対するＮＨ_３注入モル比はＯ_３、Ｈ_２Ｏ_２では１以下、ＨＮＯ_３では１．５以下に制御することが好ましいが、排ガス中に当初からＮＯ_２を含有する場合にはそれに応じて増減してもよい。
本発明は特に低温度において顕著な効果を発揮するが、装置の腐食等の理由で排ガスを余熱する設備を有してもよい。また、ガスタービンの起動時における低温排ガスの浄化方法として、本発明の思想に基づくアンモニア注入量／酸化剤注入量を一定値に制御する方法を不定期に採用する場合にも触媒量を低減できる効果が得られる。
【００２８】
本発明の思想によれば、図１および図２に示した範囲にとどまらず、図３〜４のように必要に応じて他の排ガス浄化用機器、もしくは余熱装置を配したもの、または酸化剤、アンモニアの注入を燃焼器出口の高温度ゾーンに配するなどの変更があっても同様の効果が得られ、本発明の範囲内である。
図３は燃焼器１と触媒反応器２の間に集塵装置１６を設けた場合の例であり、ゴミ焼却炉排ガスなどの煤塵の多い排ガス処理に適する。さらに、図４は排ガスラインに熱交換器または熱回収器１７を配する場合の一例であり、酸化剤を熱回収器１７の前流または熱回収伝熱管群の間に注入して、酸化剤によるＮＯの酸化を促進するよう工夫したものである。なお、図１〜４の実施例では酸化剤を注入した後の下流域にてアンモニアを注入する例を示しているが、酸化剤による一酸化窒素の二酸化窒素への酸化反応は急速に行われるので、アンモニアと酸化剤を同一個所で同時に注入しても差し支えない。
【００２９】
以下、本発明の実施例による具体的データを従来の脱硝装置のデータと対比してさらに詳細に説明する。
実施例１
図１の系統を有する小型反応装置に表１の組成のガスを流し、脱硝反応を行わせた。酸化剤にはオゾン生成器から発生させたＯ_３濃度として１０００ｐｐｍのガスを、また脱硝用還元剤として２％のアンモニアガスを用い、各々流量コントロール弁に接続し、両者の信号がＮＨ_３／Ｏ_３モル比が常に２になるように連動した。また、脱硝反応器にはＴｉ−Ｗ−Ｖ系触媒（Ｔｉ／Ｗ／Ｖ原子比＝９５／４／５、厚さ１ｍｍ）の板状触媒を４ｍｍピッチで配した触媒構造体を用い、表２のような条件に維持した。
【００３０】
【表１】

【００３１】
【表２】

本条件下で、ＮＨ_３／ＮＯ比を０．４〜１．４ｍｏｌ／ｍｏｌの間で変化させ、そのときのＮＯｘの減少率、触媒層出口のＮ_２Ｏ濃度を測定するとともに煙色の変化を調べた。
比較例１および比較例２
実施例１と同様の装置を用い、Ｏ_３／ＮＯモル比が各々０．４と０．６になるようにオゾンを一定値で注入した以外は実施例１と同様にして、ＮＨ_３／ＮＯモル比を０．４〜１．４の間で変化させてＮＯｘの減少率、触媒層出口のＮ_２Ｏ濃度を測定するとともに煙色の変化を調べた。
【００３２】
実施例１と比較例１および比較例２から得られた結果を対比して表３にまとめて示す。
【００３３】
【表３】

実施例１では、ＮＨ_３／ＮＯモル比に対応した脱硝率が得られ、ＮＨ_３／ＮＯ比が０．８以上では高い高率が得られた。また、いずれのＮＨ_３／ＮＯ比でもＮ_２Ｏの副生はなく、煙色の変化もなかった。
【００３４】
一方、比較例１では脱硝率が一定以上高くならないだけでなく、ＮＨ_３／ＮＯ比の小さい条件ではＮ_２Ｏの副生と煙色が黄色に変化した。
また、比較例２の場合にも同様に高い脱硝率が得られず、すべてのＮＨ_３／ＮＯ比でＮ_２Ｏの副生と排ガスの着色が見られた。さらに、高ＮＨ_３／ＮＯ比領域に長く保持すると脱硝性能が徐々に低下する現象も観察された。
実施例２
実施例１のオゾン生成器および流量コントロール弁に代えて１５％硝酸を流量制御可能な定量ポンプとその出口に設けた外熱式蒸発器を用いて酸化剤として注入し、ＮＨ_３／ＨＮＯ_３注入モル比が常に３になるように制御した。本装置を用いたほかは実施例１と同様にして、ＮＨ_３／ＮＯモル比を０．４〜１．４の間で変化させた場合のＮＯｘの減少率、触媒層出口のＮ_２Ｏ濃度を測定するとともに煙色の変化を調べた。
比較例３および比較例４
実施例２と同様の装置を用い、ＨＮＯ_３／ＮＯモル比が各々０．２と１．０になるように硝酸水溶液を一定値で注入した以外は実施例２と同様にして、ＮＨ_３／ＮＯモル比を０．４〜１．４の間で変化させてＮＯｘの減少率、触媒層出口のＮ_２Ｏ濃度を測定するとともに煙色の変化を調べた。
【００３５】
実施例２と比較例３および比較例４の結果を表４にまとめて示す。
【００３６】
【表４】

実施例２の場合にも実施例１同様高い脱硝率が得られただげでなく、Ｎ_２Ｏの副生や排ガスの着色現象は全く見られなかった。
【００３７】
これに対し、比較例３および比較例４では高い脱硝率が得られないだけでなく、Ｎ_２Ｏ副生および排ガスが黄色に強く着色する現象が見られた。さらにＮＨ_３／ＮＯ比の高い条件では反応管壁に硝安が析出し、実用上大きな問題になることが明らかになった。
実施例３
実施例１のオゾン生成器および流量コントロール弁に代えて１０％過酸化水素水を流量制御可能な定量ポンプとその出口に設けた外熱式蒸発器を用いて酸化剤として注入し、ＮＨ_３／Ｈ_２Ｏ_２注入モル比が常に２になるように制御した。本装置を用いたほかは実施例１と同様にして、ＮＨ_３／ＮＯモル比を０．４〜１．４の間で変化させた場合のＮＯｘの減少率、触媒層出口のＮ_２Ｏ濃度を測定するとともに煙色の変化を調べた。本例の場合にも実施例１および２と同様、高い脱硝率が得られただけでなく、脱硝装置の運用上問題になるＮ_２Ｏ副生や排ガスの着色現象は見られない優れた特性を示した。
【００３８】
【発明の効果】
本発明によればＮ_２Ｏ副生やＮＯ_２の残存による排ガスの着色といった実用の妨げとなる問題を生じることなく低温排ガスを効率よく脱硝することが可能になる。さらにＮＨ_３／ＮＯモル比の大きな変化に対しても問題なく対応できるため、負荷変動やＮＯｘ濃度の変化にも容易に追従できる。
【００３９】
これにより近年需要の増大しているゴミ焼却炉排ガスを初めとする各種低温排ガスを余熱することなく効率よく脱硝することができるようになるので、産業的、社会的価値も著しく高い。
【図面の簡単な説明】
【図１】本発明の実施例を示す脱硝装置の構成図。
【図２】実施例の他の実施例を示す脱硝装置の構成図。
【図３】本発明の他の実施態様を説明する図。
【図４】本発明の他の実施態様を説明する図。
【図５】従来技術の問題点を説明するための図。
【符号の説明】
１…排ガス源、２…触媒反応器、３…触媒、４、５…排気管、６…ＮＯｘサンプリング管、７…酸化剤注入ライン、８…アンモニア注入ライン、９…ＮＯｘ濃度計、１０…酸化剤注入装置、１１…アンモニア注入装置、１２…ＮＯｘ濃度信号、１３…流量信号、１４…演算器、１５…注入量制御信号。[0001]
[Industrial applications]
The present invention relates to an exhaust gas denitration method and apparatus, and particularly to nitric oxide (NO) among nitrogen oxides (NOx) in exhaust gas discharged from various combustion furnaces such as boilers for power plants, gas turbines, and waste incinerators. The present invention relates to an exhaust gas denitration method and apparatus capable of efficiently performing catalytic reduction with ammonia (NH ₃ ) from a low temperature by previously oxidizing a part of the exhaust gas to nitrogen dioxide (NO ₂ ).
[0002]
[Prior art]
In a contact ammonia reduction denitration method in which ammonia (NH ₃ ) is injected into the exhaust gas and then the catalyst is brought into contact with the catalyst to reduce and remove nitrogen oxides in the exhaust gas into nitrogen, an oxidizing agent is previously injected into the exhaust gas and contained. Many attempts have been made to improve the reactivity with NH ₃ by converting part of nitric oxide (NO) into nitrogen dioxide (NO ₂ ) so that denitration can be carried out at a lower temperature (for example, see Japanese Unexamined Patent Publication No. 52-94863, JP-B-56-50613, JP-A-54-23068, and the like.
[0003]
In the above method, since the reaction between 1 mol of nitric oxide and 1 mol of nitrogen dioxide represented by the formula (1) and 2 mol of NH ₃ is much faster than the ordinary denitration reaction represented by the formula (2), NO _{2 is} used in advance. A source is injected or an oxidizing agent such as ozone (O ₃ ), hydrogen peroxide (H ₂ O ₂ ), nitric acid (HNO ₃ ) is injected, and a part of NO is oxidized to NO ₂ (formula (3)). (Equation (5)) to reduce the operating temperature.
Denitration reaction [0004]
Embedded image
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O Formula (1)
Embedded image
NO + NH ₃ + ／ O ₂ → N ₂ + 3 / 2H ₂ O (2) Oxidation reaction of formula NO
Embedded image
NO + O ₃ → NO ₂ + O ₂ Formula (3)
Embedded image
NO + H ₂ O ₂ → NO ₂ + H ₂ O Formula (4)
Embedded image
NO + 2HNO ₃ → 3NO ₂ + H ₂ O (5) Since the effect of coexistence of NO and NO ₂ is remarkable at 100 to 300 ° C., combustion of various incinerator exhaust gas, waste heat recovery boiler, gas turbine, etc. has been used since ancient times. Attempts have been made to apply it to low-temperature denitration such as a vessel, but it has not been widely used.
[0009]
[Problems to be solved by the invention]
The first reason that hinders the practical use of the above-mentioned conventional technology is that it is difficult to control the denitration rate. FIG. 5 shows the temperature characteristics when each exhaust gas containing NO alone, NO ₂ alone, and NO and NO ₂ in equimolar amounts is denitrated using a titanium (Ti) -tungsten (W) -vanadium (V) -based catalyst. However, when NO and NO ₂ are equimolar, NOx is efficiently removed, but when NO or NO ₂ alone is used, the removal performance is significantly reduced. For this reason, the prior art in which the injection amount of the oxidizing agent and the injection amount of NH ₃ are individually controlled with respect to the fluctuation of NOx has many problems as follows.
(A) If the oxidizing agent is insufficient and the content of NO ₂ in the exhaust gas is small, not only does the denitration reaction rate decrease, but also a large amount of unreacted ammonia flows out.
(B) If the NO ₂ concentration in the exhaust gas exceeds the NO concentration, in the denitration reaction, 4 moles of ammonia react with 3 moles of nitrogen dioxide (formula (6)), and it is difficult to control the injection amount of NH _3. . In addition, when NO ₂ exceeds NO, nitrate is generated from NO ₂ and NH _3, and the catalyst gradually deteriorates, and N ₂ O is produced as a by-product.
[0010]
Embedded image
3NO ₂ + 4NH ₃ → 7 / 2N ₂ + 6H ₂ O (6) If NH ₃ does not satisfy the NO ₂ content ratio, not only NO _{2 that} is more likely to cause pollution than NO is discharged from the chimney, The smoke turns yellow.
In view of the problems of the prior art, the present invention enables easy and highly accurate control of the denitration rate, similarly to the NOx denitration apparatus for exhaust gas mainly composed of NO, which is currently widely used in practice. It is an object of the present invention to provide a simple denitration apparatus and a denitration method which does not cause the above-mentioned problems.
[0011]
[Means for Solving the Problems]
The invention claimed in the present application to achieve the above object is as follows.
(1) Most of the nitric oxide (NO) nitrogen dioxide a part of the exhaust gas as the ammonia oxidizer by injecting ozone or hydrogen peroxide in nitric oxide contained as nitrogen oxides (NO ₂₎ In the exhaust gas denitration method of reducing and removing nitrogen oxides in the exhaust gas by contacting the exhaust gas with a denitration catalyst, the required ammonia injection amount is calculated from the exhaust gas flow rate information and the NOx concentration information in the exhaust gas, The injection amount of the oxidizing agent is linked to control the molar ratio (amount of ammonia injected / amount of ozone or hydrogen peroxide) to be in a range of 2.0 ± 0.2 mol / mol. Exhaust gas denitration method.
(2) Ammonia and nitric acid as an oxidizing agent are injected into an exhaust gas containing most of nitrogen oxides as nitric oxide (NO ) to convert a part of nitric oxide to nitrogen dioxide (NO ₂ ). In an exhaust gas denitration method for reducing and removing nitrogen oxides in exhaust gas by contacting with a denitration catalyst, the required amount of ammonia to be injected is calculated from exhaust gas flow rate information and NOx concentration information in the exhaust gas, and the injection amount of ammonia and oxidizing agent is determined. An exhaust gas denitration method characterized by controlling the molar ratio (amount of ammonia injected / amount of nitric acid) to be in a range of 3.0 ± 0.3 mol / mol .
(3) A means for injecting ozone or hydrogen peroxide as an oxidizing agent into an exhaust gas containing nitrogen monoxide to convert a part of the nitrogen monoxide to nitrogen dioxide, and contacting the converted exhaust gas with a denitration catalyst to produce nitrogen. in the exhaust gas denitration apparatus and means for reducing and removing the oxide, in conjunction with the control means of the injection amount of the ammonia and the oxidizing agent, 2.0 ± a molar ratio of ammonia injection amount / oxidant injection volume 0. An exhaust gas denitration apparatus characterized in that a means for controlling the concentration within a range of 2 mol / mol is provided.
(4) A means for injecting ammonia and nitric acid as an oxidizing agent into a nitric oxide-containing exhaust gas to convert a part of the nitric oxide to nitrogen dioxide, and reducing the nitrogen oxide by bringing the converted exhaust gas into contact with a denitration catalyst. In the exhaust gas denitration apparatus provided with means for removing, the control means for controlling the injection amount of the ammonia and the oxidizing agent is linked, and the molar ratio of ammonia injection amount / oxidizing agent injection amount is 3.0 ± 0.3 mol / mol. Exhaust gas denitration apparatus, characterized in that a means for controlling the exhaust gas is provided in the range of.
[0012]
[Action]
When the injection amounts of NH ₃ and the oxidizing agent are controlled within the above-mentioned ranges as in the present invention, the reaction of the above-mentioned formula (1) is extremely fast, so that the total reaction is expressed by the following formulas (7) to (9). Due to the reaction, the oxidizing agent and NH ₃ are consumed without excess or shortage and quickly.
[0013]
Embedded image
2NH ₃ + O ₃ + 2NO → 2N ₂ + 3H ₂ O + O ₂ (7)
Embedded image
2NH ₃ + H ₂ O ₂ + 2NO → 2N ₂ + 4H ₂ O (8)
Embedded image
3NH ₃ + HNO ₃ + 2NO → 3 / 2N ₂ + 5H ₂ O (9) For this reason, the problem of the prior art is that the NO or NO ₂ concentration in the exhaust gas is deviated to one of the _two , and the NOx reduction and NO reduction occur. ₂ does not occur, by-product of N ₂ O, or catalyst deterioration. Even if NH ₃ and the oxidizing agent are injected in an amount larger than that required for removing NO in the exhaust gas, in the case of O ₃ and H ₂ O ₂ , only the leakage of NH ₃ and the harmless oxygen leak. Yes, this is similar to a normal flue gas denitration system. When HNO ₃ is used as the oxidizing agent, although NO ₂ leaks, the injection amount is 1/3 or less of the NH ₃ injection amount, so that the injection amount is extremely small, and the adverse effect is greatly reduced.
[0016]
Therefore, in operation, the denitration activity can be set by changing the injection amount of ammonia without being conscious of the oxidation reaction. Furthermore, since only the fast reactions of the equations (7) to (9) can always proceed, it is possible to reduce the amount of catalyst used or the reaction temperature.
As described above, in the present invention, by always controlling the injection amount of ammonia and the injection amount of the oxidizing agent at a fixed ratio, it is possible to operate the device with the same simplicity as that of the currently generally used denitration device. Become.
[0017]
The means adopted in the present invention is characterized in that information on the NOx concentration in the exhaust gas is obtained, and the NO oxidant and NH ₃ can be added at a constant ratio based on the signal.
[0018]
【Example】
FIG. 1 shows a basic system diagram of an exhaust gas denitration apparatus according to one embodiment of the present invention. This denitration apparatus comprises a NOx concentration measuring device 9, an oxidizing agent injecting device 10, an NH ₃ injecting device 11, and a catalytic reactor 2. The oxidizing agent injecting device 10 and the NH ₃ injecting device 11 provide a concentration signal from the NOx measuring device 9. Based on the flow rate signal 12 and the flow rate signal 13, control is performed such that the molar ratio of the injection amounts of the two always becomes a predetermined constant ratio.
[0019]
Here, the fixed amount of the injection molar ratio of the oxidizing agent and NH ₃ means the following range depending on the type of the oxidizing agent.
(A) Injection amount of NH ₃ / Oxidant injection amount = 2.0 ± 0.2 mol / mol when the oxidizing agent is O ₃ and H ₂ O ₂ (B) Injection amount of NH ₃ / Oxidizing agent is HNO ₃ / Oxidant injection amount = 3.0 ± 0.3 mol / mol The oxidant injection device 10 is a means for injecting an oxidant into exhaust gas, such as an ozone generator, a hydrogen peroxide or nitric acid blowing device, It also includes a control valve for controlling the flow rate of the oxidant from the generator, and so-called control means such as controlling the amount of generation by controlling the applied voltage of the ozone generator by discharge.
[0020]
The basic device configuration necessary for implementing the present invention is as shown in FIG.
Denitration apparatus NOx measuring device 9, the oxidant injection device 10, NH ₃ injection device 11 consists of the catalytic reactor 2 which, oxidizer injection device 10 and the NH ₃ injection device 11 of the concentration signal 12 from the NOx measuring device 9 The flow rate signal 13 is used to control the injection so that the molar ratio of the injection amounts of the oxidizing agent and ammonia always becomes a predetermined constant ratio.
[0021]
Here, the NOx measuring device 9 is a normal NOx concentration measuring device such as an infrared type or a chemiluminescent type, and generates a signal proportional to the NOx concentration in the exhaust gas. On the other hand, the flow signal 13 may be obtained from an exhaust gas source, or may be a signal generated by independently measuring with a Pitot tube, an orifice, or the like.
Both signals are sent to the oxidizing agent injection device 10 and the ammonia injection amount device 11, and are converted into injection amount control signals so that the injection amounts of both follow the changes in the gas amount and the NOx concentration. It is used for controlling the injection molar ratio to be always a constant ratio. The point of the present invention is to control the molar ratio of the amount of injected ammonia / the amount of oxidizing agent to be always constant as described above. Therefore, the flow rate signal 13 and the NOx concentration signal 12 are controlled as shown in FIG. It goes without saying that arithmetic processing may be performed in advance by the arithmetic unit 14, and the control signal 15 may be generated and controlled so that the molar ratio of the ammonia injection amount / oxidant injection amount always becomes a constant ratio.
[0022]
The ammonia injection device 11 includes a supply device of ammonia such as ammonia gas and liquefied ammonia and a control means such as a flow control valve, and supplies ammonia proportional to the control signal. This is the same in the oxidizing agent injection device, except that the molar ratio of the injected amount of ammonia / the injected amount of oxidizing agent is controlled in conjunction with the injected amount of ammonia so that the ratio always becomes a constant ratio.
[0023]
As the oxidizing agent, one having an ability to oxidize NO in exhaust gas to NO ₂ , such as ozone, nitric acid, and hydrogen peroxide, is used. The injection amount depends on the type of the oxidizing agent. It is controlled so as to be a molar ratio of the injection amount.
As the catalyst 3, a known denitration catalyst such as a known titanium oxide catalyst or zeolite catalyst is used.
[0024]
After a part of NO is oxidized to NO ₂ as described above (3) to (5) by the injected oxidant in the exhaust gas is led to the ammonia is added catalytic reactor. At this time, since the ammonia and the oxidizing agent are controlled so that the molar ratio of the amount of the injected ammonia / the amount of the oxidizing agent always becomes a constant ratio, the reaction rates of the equations (7) to (9) are extremely high. Proceeds selectively, and ammonia and oxidizing agent are consumed without excess and deficiency.
[0025]
_{_{2NH 3 + O 3 + 2NO →}} 2N 2 + 3H 2 O + O 2 (7) equation _{_{_{2NH 3 + H 2 O 2 +}}} 2NO → 2N 2 + 4H 2 O (8) Equation _{_{3NH 3 + HNO 3 + 2NO →}} 3 / 2N 2 + 5H 2 O (9) formula As described above, since only a very fast reaction is selectively caused, the operating temperature can be remarkably lowered, and the amount of the catalyst can be significantly reduced at 100 to 300 ° C, usually 150 to 250 ° C. Also, as described above, there is almost no outflow of unreacted ammonia or generation of NO ₂ generated by the oxidizing agent, which does not cause various problems which have been a problem in the related art.
[0026]
The most efficient denitration can be achieved when the molar ratio of the injection amount of ammonia / the injection amount of the oxidizing agent is 2 when O ₃ and H ₂ O ₂ are used, and ₃ when HNO ₃ is used. Within the stated range, a practically high efficiency can be obtained without any problem. If the molar ratio is too high or too low, the denitration rate decreases, the catalyst deteriorates, and N ₂ O is by-produced.
[0027]
Further, NH ₃ injection molar ratio NO in the exhaust gas _O _3, H 2 _O in ₂ 1 or less, it is preferable to control the HNO At ₃ 1.5 or less, when containing NO ₂ originally in the exhaust gas May be increased or decreased accordingly.
The present invention exerts a remarkable effect particularly at low temperatures, but may have equipment for preheating the exhaust gas due to corrosion of the apparatus or the like. Further, as a method of purifying low-temperature exhaust gas at the time of starting the gas turbine, the amount of catalyst can be reduced even when a method of controlling the injection amount of ammonia / the injection amount of the oxidizing agent to a constant value based on the idea of the present invention is employed irregularly. The effect is obtained.
[0028]
According to the idea of the present invention, the apparatus is not limited to the range shown in FIGS. 1 and 2, and another apparatus for purifying exhaust gas or a preheating apparatus is arranged as necessary as shown in FIGS. Even if there is a change such as distributing the injection of ammonia in the high temperature zone at the outlet of the combustor, the same effect can be obtained and is within the scope of the present invention.
FIG. 3 shows an example in which a dust collecting device 16 is provided between the combustor 1 and the catalytic reactor 2, and is suitable for treating exhaust gas containing much dust, such as exhaust gas from a garbage incinerator. Further, FIG. 4 shows an example in which a heat exchanger or a heat recovery unit 17 is arranged in an exhaust gas line. In this case, an oxidizing agent is injected before the heat recovery unit 17 or between the heat recovery heat transfer tube groups. To promote the oxidation of NO. Although the embodiment of FIGS. 1 to 4 shows an example in which ammonia is injected in the downstream region after the injection of the oxidizing agent, the oxidation reaction of nitrogen monoxide to nitrogen dioxide by the oxidizing agent is rapidly performed. Therefore, the ammonia and the oxidizing agent may be simultaneously injected at the same place.
[0029]
Hereinafter, specific data according to the embodiment of the present invention will be described in more detail in comparison with data of a conventional denitration apparatus.
Example 1
A gas having the composition shown in Table 1 was passed through a small reactor having the system shown in FIG. 1 to cause a denitration reaction. As an oxidizing agent, a gas having an O ₃ concentration of 1000 ppm generated from an ozone generator and a 2% ammonia gas as a denitrifying reducing agent are used, each of which is connected to a flow control valve, and the signals of both are NH ₃ / O _{The three} molar ratios were linked so that they were always 2. The denitration reactor uses a catalyst structure in which plate catalysts of Ti-WV-based catalyst (Ti / W / V atomic ratio = 95/4/5, thickness 1 mm) are arranged at a pitch of 4 mm. Conditions such as 2 were maintained.
[0030]
[Table 1]

[0031]
[Table 2]

Under these conditions, the NH ₃ / NO ratio was changed between 0.4 and 1.4 mol / mol, and the NO x reduction rate and the N ₂ O concentration at the outlet of the catalyst layer at that time were measured and the smoke color changed. Was examined.
Comparative Example 1 and Comparative Example 2
NH ₃ / NO was used in the same manner as in Example 1 except that ozone was injected at a constant value so that the O ₃ / NO molar ratio was 0.4 and 0.6, respectively, using the same apparatus as in Example 1. The reduction ratio of NOx and the concentration of N ₂ O at the outlet of the catalyst layer were measured while changing the molar ratio between 0.4 and 1.4, and the change in smoke color was examined.
[0032]
Table 3 collectively shows the results obtained from Example 1 and Comparative Examples 1 and 2.
[0033]
[Table 3]

In Example 1, the denitration rate corresponding to the NH ₃ / NO molar ratio was obtained, and a high rate was obtained when the NH ₃ / NO ratio was 0.8 or more. In addition, there was no N ₂ O by-product and no change in smoke color at any NH ₃ / NO ratio.
[0034]
On the other hand, in Comparative Example 1, not only did the denitration rate not increase beyond a certain level, but also the by-product of N ₂ O and the smoke color changed to yellow under the condition of a small NH ₃ / NO ratio.
Similarly, in the case of Comparative Example 2, a high denitration rate was not obtained, and by-products of N ₂ O and coloring of exhaust gas were observed at all NH ₃ / NO ratios. Further, a phenomenon was observed in which the denitration performance gradually decreased when the temperature was maintained in the high NH ₃ / NO ratio region for a long time.
Example 2
Instead of the ozone generator and the flow rate control valve of Example 1, 15% nitric acid was injected as an oxidant using a quantitative pump capable of controlling the flow rate and an externally heated evaporator provided at the outlet thereof, and NH ₃ / HNO _{3 was} injected. The molar ratio was controlled to always be 3. Except that this apparatus was used, the reduction rate of NOx and the N ₂ O concentration at the catalyst layer outlet when the NH ₃ / NO molar ratio was changed between 0.4 and 1.4 in the same manner as in Example 1 Was measured and the change in smoke color was examined.
Comparative Example 3 and Comparative Example 4
The same apparatus as in Example 2 was used, except that a nitric acid aqueous solution was injected at a constant value so that the HNO ₃ / NO molar ratio was 0.2 and 1.0, respectively, in the same manner as in Example 2 and NH ₃ / NO was used. By changing the NO molar ratio between 0.4 and 1.4, the reduction rate of NOx and the concentration of N ₂ O at the outlet of the catalyst layer were measured, and the change in smoke color was examined.
[0035]
Table 4 summarizes the results of Example 2, Comparative Example 3 and Comparative Example 4.
[0036]
[Table 4]

In the case of Example 2, as in Example 1, not only a high denitration rate was obtained, but also no by-product of N ₂ O or coloring of exhaust gas was observed at all.
[0037]
On the other hand, in Comparative Examples 3 and 4, not only a high denitration ratio was not obtained, but also a phenomenon was observed in which N ₂ O by-products and exhaust gas were strongly colored yellow. Further, it was found that under conditions with a high NH ₃ / NO ratio, ammonium nitrate was deposited on the reaction tube wall, which became a serious problem in practical use.
Example 3
In place of the ozone generator and the flow control valve of Example 1, 10% hydrogen peroxide water was injected as an oxidant using a metering pump capable of controlling the flow rate and an external heat evaporator provided at the outlet thereof, and NH ₃ / The H ₂ O ₂ injection molar ratio was controlled to always be 2. Except that this apparatus was used, the reduction rate of NOx and the N ₂ O concentration at the catalyst layer outlet when the NH ₃ / NO molar ratio was changed between 0.4 and 1.4 in the same manner as in Example 1 Was measured and the change in smoke color was examined. In the case of this example, as in Examples 1 and 2, not only a high denitrification rate was obtained, but also excellent characteristics in which N ₂ O by-products and exhaust gas coloring phenomena which are problematic in the operation of the denitration apparatus were not observed. showed that.
[0038]
【The invention's effect】
According to the present invention makes it possible to efficiently denitration low temperature exhaust gas without causing the problem that hinders practical such coloration of the exhaust gas due to residual N _{2 O} by-product or NO _2. Furthermore, since it is possible to cope with a large change in the NH ₃ / NO molar ratio without any problem, it is possible to easily follow a load change and a change in the NOx concentration.
[0039]
This makes it possible to efficiently denitrate various low-temperature exhaust gases such as exhaust gas from garbage incinerators, which have been increasing in demand in recent years, without any residual heat, so that the industrial and social value is remarkably high.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a denitration apparatus showing an embodiment of the present invention.
FIG. 2 is a configuration diagram of a denitration apparatus showing another embodiment of the present invention.
FIG. 3 is a diagram illustrating another embodiment of the present invention.
FIG. 4 is a diagram illustrating another embodiment of the present invention.
FIG. 5 is a diagram for explaining a problem of the related art.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Exhaust gas source, 2 ... Catalyst reactor, 3 ... Catalyst, 4/5 ... Exhaust pipe, 6 ... NOx sampling pipe, 7 ... Oxidant injection line, 8 ... Ammonia injection line, 9 ... NOx concentration meter, 10 ... Oxidation Agent injection device, 11: ammonia injection device, 12: NOx concentration signal, 13: flow rate signal, 14: arithmetic unit, 15: injection amount control signal.

Claims

The majority of the nitrogen oxides with ammonia and an oxidizing agent into the exhaust gas containing a nitrogen monoxide (NO) by injecting ozone or hydrogen peroxide is converted to a portion of the nitrogen monoxide to nitrogen dioxide (NO ₂₎ In an exhaust gas denitration method for reducing and removing nitrogen oxides in the exhaust gas by contacting the exhaust gas with a denitration catalyst, a necessary ammonia injection amount is calculated from exhaust gas flow rate information and NOx concentration information in the exhaust gas, Exhaust gas denitration method characterized by controlling the molar ratio (ammonia injection amount / ozone or hydrogen peroxide injection amount) in the range of 2.0 ± 0.2 mol / mol by linking the injection amounts. .

Ammonia and nitric acid as an oxidant are injected into an exhaust gas containing most of nitrogen oxides as nitric oxide (NO ) to convert a portion of the nitric oxide to nitrogen dioxide (NO ₂ ), and this exhaust gas is subjected to a denitration catalyst. In the exhaust gas denitration method of reducing and removing nitrogen oxides in the exhaust gas by contacting with the exhaust gas flow rate information and the NOx concentration information in the exhaust gas, the required ammonia injection amount is calculated, and the injection amounts of ammonia and the oxidizing agent are linked, An exhaust gas denitration method characterized by controlling such a molar ratio (amount of ammonia injected / amount of nitric acid) to be in a range of 3.0 ± 0.3 mol / mol .

A means for injecting ozone or hydrogen peroxide as ammonia and an oxidizing agent into a nitrogen monoxide-containing exhaust gas to convert a portion of the nitrogen monoxide to nitrogen dioxide, and contacting the converted exhaust gas with a denitration catalyst to remove nitrogen oxides. in the exhaust gas denitration apparatus and means for reducing and removing, by linking the control means of the injection amount of the ammonia and the oxidizing agent, the molar ratio of the ammonia injection amount / oxidant injection volume 2.0 ± 0.2 mol / An exhaust gas denitration apparatus comprising means for controlling a molar range .

A means for injecting ammonia and nitric acid as an oxidizing agent into a nitric oxide-containing exhaust gas to convert a portion of the nitric oxide to nitrogen dioxide, and a means for bringing the converted exhaust gas into contact with a denitration catalyst to reduce and remove nitrogen oxides In the exhaust gas denitration apparatus provided with the above, the control means for controlling the injection amount of the ammonia and the oxidizing agent is linked, and the molar ratio of the ammonia injection amount / the oxidizing agent injection amount is in a range of 3.0 ± 0.3 mol / mol. An exhaust gas denitration apparatus comprising a control means.