JP3562551B2 - Activated carbon catalyst and flue gas desulfurization method - Google Patents
Activated carbon catalyst and flue gas desulfurization method Download PDFInfo
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- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 title claims description 271
- 239000003054 catalyst Substances 0.000 title claims description 61
- 238000000034 method Methods 0.000 title claims description 30
- 238000006477 desulfuration reaction Methods 0.000 title claims description 23
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- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 title claims description 15
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- 229910052815 sulfur oxide Inorganic materials 0.000 claims description 7
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Description
【０００１】
【発明の属する技術分野】
本発明は、排ガス中に含まれる硫黄酸化物を、接触硫酸化反応によって硫酸として回収除去するための活性炭触媒およびこれを用いた排煙脱硫方法に関するものである。
【０００２】
【従来の技術】
従来より、排ガス中に含まれる亜硫酸ガス等の硫黄酸化物を、低温で共存する酸素によって酸化することにより最終的に硫酸とし、これをそのまま硫酸として、或いはこれとカルシウム化合物とを反応させることにより石膏として回収するプロセスは周知である。
このような、排ガス中の亜硫酸ガス等を酸化させる触媒としては、活性炭が最もこのましいとされている。すなわち、上記触媒として、例えばアルミナ、シリカ、チタニア、ゼオライトのようなセラミックス系担体を用いた場合には、それだけでは活性が不足するために、これに触媒種として金属或いは金属酸化物を加える必要がある。ところが、これらの触媒種は、生成する硫酸の攻撃を受け、溶解または変質してしまうために、長時間にわたって安定した活性を維持することができないという欠点がある。これに対して、上記活性炭にあっては、何の触媒種も担持することなく活性が発現し、かつそれが長時間劣化することなく持続するという特長を有するからである。
【０００３】
しかしながら、排煙脱硫装置としての工業的使用にあたって、市販の活性炭をそのまま用いた場合には、接触硫酸化反応における触媒活性が低いために、所望の脱硫効果を得るためには触媒充填量が極端に大きくなってしまい、よって湿式排煙脱硫プロセス等の他の脱硫プロセスと比較して経済的に太刀打ちすることができないという問題点がある。
そこで、従来このような排煙脱硫プロセスに用いられる活性炭の触媒活性を高める方策として、大別して２つのアプローチがある。その一方は、亜硫酸ガスの吸着、酸化反応活性点を増大させようとするものであり、他方は、活性炭の細孔内に生成する硫酸を出来る限り早く細孔外に排出させようとするものである。
【０００４】
上記前者のアプローチにおいては、前提として活性炭の亜硫酸ガスに対する吸着、酸化反応の活性点が何であるかを決定する必要がある。この点に付き、例えば日本化学会誌（１９７５年）Ｎｏ．１０，Ｐ１７０５〜１７１２においては塩基性表面化合物であるとの示唆があり、またＡｐｐｌｉｅｄ Ｃａｔａｌｙｓｉｓ Ｂ：Ｅｎｖｉｒｏｎｍｅｎｔａｌ ３（１９９４）Ｐ２２９〜２３８においては、活性炭上の強塩基性点の数が亜硫酸ガスの吸着、酸化活性と比例関係にあり、かつこの塩基性点は含酸素官能基であることが示唆されている。そして、特に前者の文献においては、活性炭をアンモニアガスにより８００℃で賦活処理することにより、亜硫酸ガスの吸着、酸化活性を大きくし得ることが述べられている。
しかしながら、両文献とも、どのような活性炭を選択すれば高活性を発現し得るかについては全く触れられておらず、よって当該アプローチは、現実的な適用までには未だ到達していない。
【０００５】
また、上記他方のアプローチについてみると、本来活性炭の亜硫酸ガス吸着、酸化活性（以下、単に活性と略す。）は、排ガス中に水分がなければ非常に大きい。しかしながら、生成物である硫酸は、吸湿性が非常に大きいため、水蒸気の存在下では活性炭表面上で水分を吸収して希硫酸を生成し、これが活性炭の細孔内に充満して亜硫酸ガスの拡散および接触を妨害する結果、活性炭の表面活性が充分に発揮されないことになる。
そこで、活性炭に撥水性を付与して、生成した硫酸を速やかに活性炭の細孔から排出することにより、当該活性炭の高活性を維持させようとする各種の方法が提案されている。
【０００６】
例えば、Ｃｈｅｍ． Ｅｎｇ． Ｃｏｍｍ． Ｖｏｌ．６０（１９８７）、Ｐ２５３には、平均直径０．７８ｍｍの活性炭にポリテトラフルオロエチレン（以下、ＰＴＦＥと略す。）の分散液を吹掛けることにより、ＰＴＦＥの添加量８〜２０％の領域において亜硫酸ガスの吸着、酸化反応の速度定数が３倍に上昇したとの事例が開示されている。
また、特開昭５９−３６５３１号公報には、活性炭に撥水化処理を施すことで亜硫酸ガスの吸着、酸化活性が上昇すること、具体的には５〜１０ｍｍの粒状活性炭にＰＴＦＥ分散液を含浸させ、２００℃で２時間加熱処理することにより、活性炭単味の触媒に比べて遥かに高い活性を示すことが開示されている。
【０００７】
【発明が解決しようとする課題】
本発明者等は、このような従来の触媒としての活性炭の活性を高める２方向のアプローチについて検証すべく、以下の確認実験を行なった。
先ず、一方のアプローチである活性炭上の亜硫酸ガス吸着、酸化点を増大させる試みとして、塩基性点を付与するといった限定的な考えに囚われることなく、活性炭に何等かの表面変性をもたらすような種々の表面処理を試みてみた。ここで、表面処理剤として、酸またはアルカリを用いた場合には、それと活性炭とを煮沸処理した。また、高温のＨＢｒ、ＨＣｌ、アンモニア等によるガス処理も試みた。さらに、スチーム、ＣＯ２ のようなガス中に、少量の還元性ガス（水素）や酸化性ガス（弗素、塩素、酸素）を混ぜて、８００℃の高温処理を行なう方法などを検討した。
この結果、アンモニアガスによる高温処理が最も高活性を発揮したが、数百時間の運転により、その活性は未処理の活性炭と同等の程度まで低下してしまった。これに対して、スチーム、ＣＯ２ 等の賦活ガス中に酸素を少量混ぜて高温処理する方法は、活性向上効果が長時間持続したが、活性の向上幅が小さく、当初の目標に到達し得なかった。
【０００８】
次に、本発明者等は、他方のアプローチである活性炭の撥水化についての検証を行なった。
先ず、上述した従来の撥水化技術に基づいて、吸着、酸化活性点の多い活性炭を撥水化処理することが最も接触硫酸化反応の活性の向上に効果的であると考え、２．８〜４．０ｍｍφの粒径範囲にある各種市販の活性炭に、スプレー法或いは含浸法によってＰＴＦＥを担持させ、その活性を測定したところ、活性炭単味の触媒と比較してある程度の活性の向上とその長時間の持続性が認められた。
しかしながら、大規模な工業的実施を考慮した場合には、他の競合する排煙脱硫プロセスに勝るためには、依然として触媒としてこの程度の活性では充分とは言えず、より一層の触媒活性の向上が必要であるとの認識に達した。
【０００９】
ところで、この種の触媒として用いられる活性炭としては、木材、褐炭などを活性化剤としての薬品で処理したり、或いは木炭などを水蒸気で活性化処理したもの、石炭を原料とするもの、椰子殻を用いたもの、さらにはピートや石油系ピッチを原料とするもの等各種の活性炭が知られている。
そこで、本発明者等は、上記アプローチとは視点を変えて、前提となる活性炭自体が、その原料の相違によって撥水化処理した場合の活性化効果に相違が生じるか否かを確認すべく、１８種類の活性炭について撥水化処理前後における活性の変化を比較してみた。
その結果、上記撥水化処理により、特に石炭を主原料とする活性炭触媒が、のきなみ高活性を発現したのに対して、他の椰子殻あるいはピート、石油系ピッチなどを原料とする活性炭触媒は、それに比べてかなり低い活性しか示さないという事実が判明した。このように、石炭を原料とする活性炭が、撥水化処理した場合に優位性を示す理由については定かではないが、現段階においては以下のように推定することができる。
【００１０】
すなわち、石炭系の活性炭は、本来他の原料からなる活性炭と比較して、亜硫酸ガスの吸着、酸化活性点の数が多いと考えられる。ところが、石炭系の活性炭は、他の原料からなる活性炭よりも疎水性において劣るため、生成された硫酸の排出が遅く、この結果総括活性がそれ程高い値を示さないものと思われる。したがって、これに撥水化処理を施した場合には、活性点の数のみによって活性炭の優劣が決まることになり、よって本来の石炭系の活性炭における優れた活性が顕著に現れたものと考えられる。
以上のことから、石炭を主原料とする活性炭に撥水処理を施した活性炭触媒を用いることにより、排ガス中の硫黄酸化物の酸化除去性能に優れた排煙脱硫プロセスを開発することができるという本発明を完成するに至ったのである。
【００１１】
そこで次に、本発明者等は、石炭を主原料とする活性炭に限らず一般的な各種の活性炭において、撥水化により上記活性炭の活性向上をより一層発揮させるには、当該活性炭のどの部分を撥水化すれば効果的であるかを調べてみた。
先ず、活性炭にＰＴＦＥ分散液をスプレー担持、或いは含浸担持する従来の方法で作成した活性炭触媒における弗素の分布をＥＰＭＡで面分析した。その結果、ＰＴＦＥ粒子は、活性炭の粒子内部には全く侵入しておらず、すべて粒子外表面に付着していることが判明した。これは、市販の活性炭には、１μｍ以上の細孔が殆ど存在しないため、直径が０．２〜０．４μｍの範囲にあるＰＴＦＥ粒子が上記細孔内に侵入するには、抵抗が大き過ぎるためと考えられる。ちなみに、ＰＴＦＥ分散液に代えて、平均粒径が０．３μｍのポリスチレン粒子の分散液を用いた場合についても、同様の実験結果を得た。そして、これら２種類の撥水性粒子を担持した活性炭触媒について活性試験を行なったところ、ＰＴＦＥを担持したものの方が、ポリスチレン粒子を担持した活性炭触媒よりも僅かに活性が高いという知見は得られたものの、いずれも期待するほどの高活性を発現することはなかった。
【００１２】
本発明者等はそこで、活性点近傍を含めた活性炭の全表面を一様に撥水化することにより、生成硫酸の排出が大幅に促進されることを期待し、活性炭における５ｎｍ以下の細孔直径を有する孔（以下、ミクロポアと略称する。）を含めた全表面の撥水化処理を行なうことにした。
第１の方法として、活性炭に、１００〜４００℃の弗素ガスを適量流すことにより、表面弗素化度の異なる種々の活性炭触媒を調製した。
また、第２の方法として、分子量の小さい撥水性物質であるステアリン酸、スチレンオリゴマー（平均分子量約３２０）、弗素含有油（平均分子量約５００）等を適当な低沸点溶媒に溶解させた後、これに活性炭を減圧下で浸漬し、細孔内にこれらの溶液を充分浸透させた後に減圧乾燥して溶媒を飛ばすことにより、撥水性物質で活性炭の細孔内をコーティングした。
このようにして調製した活性炭触媒の比表面積は、撥水性物質の担持に伴う重量増加によるみかけ上の減少範囲内に収まっており、これらの担持物が細孔を閉塞したり、破壊したりしていないことが確認された。
【００１３】
次いで、第１の方法によって得られた１〜２０％の弗素化率を有する種々の活性炭触媒を用いて、亜硫酸ガス反応活性試験を行なったところ、弗素化率が上昇するのに伴って、水をはじく性質が徐々に大きくなることが水面浮遊時間テスト（これは、撥水化処理した活性炭粒子を水面に静かに浮かべ、その沈降開始時間と沈降終了時間の平均値をとるもので、撥水性の相対比較のための簡便法である。）から明らかになったものの、亜硫酸ガスの吸着、酸化活性は、むしろ弗素化率の上昇と逆比例して、低下して行くことが判った。
また、第２の方法によって得られたステアリン酸、スチレンオリゴマー、弗素化油等を担持した活性炭触媒にあっては、いずれも担持量が増加するのに伴って、同様に水面浮遊時間テストによる撥水性の増大が認められるものの、亜硫酸ガスの吸着、酸化活性については、０．５〜２％の添加領域において活性炭単味の触媒活性を僅かに上回るのみで、添加量の増大とともに活性が急速に低下して行くことが判った。
【００１４】
以上のことから、活性炭のミクロポアを含めた全面的な撥水化は、活性炭の吸着、酸化活性点を被覆或いは破壊するために、充分な活性向上の効果が得られなくなるものと推定した。
そこで、本発明者等は、活性炭のミクロポアは撥水化せずに、マクロポア（５ｎｍを超える細孔直径を有する孔）のみを撥水化することを試みた。先ず、分子量が１０万以上のポリエチレン、ポリスチレン粉末を６０〜７０℃に加熱したトルエンに数％溶解させ、これに活性炭粒子を減圧下で浸漬した後に、加熱しながら減圧乾燥してトルエンを徹底的に飛散させた。このようにして得られた活性炭触媒は、撥水物質が原料活性炭に対して０．３〜１．５ｗｔ％の担持範囲において、かなり活性の向上を示した。
これは、分子量が１０万以上のポリエチレン、ポリスチレンが仮に球状でトルエン溶媒中に分散しているとすると、その直径は溶媒に膨潤していないとしても７ｎｍ以上となり、これは到底活性炭のミクロポアに侵入できるサイズではない。したがって、この活性炭触媒は、活性炭粒子のマクロポアと外表面とを撥水化していると考えるべきである。そして、上述したように、本反応系では、活性炭の外表面の撥水化は、反応活性の向上にそれ程大きく寄与していないことから、結局活性炭のマクロポアの撥水化こそが、最も活性向上に寄与するものであることが推論される。
【００１５】
したがって、上述した石炭を主原料とする活性炭に撥水化処理を施した活性炭触媒を用いた場合においても、さらに当該撥水化処理として、原料活性炭におけるミクロポアを除くマクロポアを撥水化することにより、より一層硫黄酸化物の酸化除去性能に優れた排煙脱硫プロセスを開発することができるとの知見を得るに至った。
本発明は、かかる知見に基づいてなされたもので、他の排煙脱硫プロセスと比べて脱硫効率において遜色が無く、よって経済性に優れる排煙脱硫を可能にする活性炭触媒およびこれを用いた排煙脱硫方法を提供することを目的とするものである。
【００１６】
【課題を解決するための手段】
請求項１に記載の本発明に係る活性炭触媒は、硫黄酸化物を含む排ガスと接触させることにより、上記硫黄酸化物を吸着、酸化させて硫酸として回収除去するための活性炭触媒であって、石炭を主原料とする活性炭に、５ｎｍ以下の細孔直径を有する孔を除いて撥水化処理を施してなることを特徴とするものである。
【００１７】
また、請求項２〜４に記載の発明は、それぞれ請求項１に記載の発明における撥水化処理の実施形態であり、請求項２に記載の発明は、上記撥水化処理が、分子量が１万以上の高分子撥水性物質を有機溶媒に溶解して、上記活性炭に含浸担持させてなることを特徴とするものであり、また請求項３に記載の発明は、上記撥水化処理が、平均粒径１５〜１００ｎｍの撥水性物質の分散液を、上記活性炭に含浸担持させてなることを特徴とするものである。さらに、請求項４に記載の発明は、活性炭粉末と、平均粒径０．１〜１．０μｍの弗素樹脂の粒子またはその分散液とを混練成形してなることを特徴とするものである。
そして、請求項５に記載の本発明に係る排煙脱硫方法は、上記請求項１〜４のいずれかに記載の活性炭触媒に、硫黄酸化物を含む排ガスを接触させることにより、当該排ガス中の上記硫黄酸化物を上記活性炭触媒に吸着、酸化させて硫酸として回収除去することを特徴とするものである。
【００１８】
【発明の実施の形態】
以下、本発明に係る活性炭触媒の実施形態について具体的に説明する。
本発明に係る活性炭触媒は、排ガス中の亜硫酸ガスを共存する酸素によって酸化して硫酸として回収除去するためのものであって、石炭を主原料とする活性炭に、撥水化処理を施したものである。ここで、上記撥水化処理を行なう場合には、高い活性を発現させるために、上述したように当該活性炭のミクロポアを修飾しないような高分子の撥水性物質を選定することが好ましい。このような撥水化物質としては、ポリエチレン、ポリプロピレン、ポリスチレン、弗素樹脂等を挙げることができる。
また、石炭系の活性炭に対する撥水化処理としては、原料活性炭の形状をそのまま活かし、上記撥水性物質を有機溶媒に溶解して、上記石炭系の活性炭に含浸担持させる方法や、平均粒径が１５〜１００ｎｍの上記撥水性物質の分散液を、上記石炭系の活性炭に含浸担持させる方法がある。上記原料活性炭の形状をそのまま活かして撥水化処理する場合には、特に後者の方法が最も高活性を得ることができ、その際の撥水化物質の好ましい添加量は、原料活性炭に対して、０．２〜３ｗｔ％である。
【００１９】
さらに、原料活性炭に対して直接撥水化処理を行なわずに、活性炭粉末を用い、この活性炭粉末に、平均粒径０．１〜１．０μｍの弗素樹脂の粒子またはその分散液とを混練成形して撥水化された活性炭触媒を得ることもできる。
この場合に、使用する弗素樹脂としては、上述したＰＴＦＥの他、パーフルオロアルコキシ樹脂（ＰＦＡ）、４弗化エチレン６弗化プロピレン共重合体（ＦＥＰ）、３弗化塩化エチレン樹脂（ＰＣＴＥＦ）等が好適である。これらの弗素樹脂は、ポリスチレンやポリエチレン等よりも撥水性が大きく、しかも分散液中におけるこれらの弗素樹脂の平均粒径は、０．２〜０．４μｍと比較的大きいために活性炭粉末のミクロポア内に侵入することがなく、よってこれらを混練成形することにより、マクロポアまでが撥水化された所望の活性炭触媒を得ることができる。また、本方法によれば、活性炭粉末を原料として任意形状のものを作ることができ、上述した活性の向上と併せて製造コストの観点からも好適である。なお、上記触媒形状としては、ハニカム状、板状、球状、円柱状、サドル状などを挙げられ、触媒層の耐閉塞性が大きく、かつ触媒層の圧力損失が小さいものが好ましい。
【００２０】
【実施例】
次に、本発明を実施例により更に具体的に説明する。
（実施例１）
先ず、市販の石炭、椰子殻、ピート、木材、石油ピッチ等原料を異にする活性炭１８種を粗砕機にかけて粉砕し、ステンレス製の篩にて２．８〜４．０ｍｍφの粒状触媒各５０ｇを得た。但し、石油ピッチを原料とする活性炭は、市販品として入手できるものが一炭種しかなかったため、粒子径の異なる０．７１〜１ｍｍφの粒状触媒を用いた。
次に、この１８種の活性炭を、それぞれ窒素気流中、８００℃で１時間焼成した。以下、これを未処理触媒と呼ぶ。次に、市販の球状ポリスチレン（平均粒子径２８ｎｍ）の水分散液（１０ｗｔ％）に脱イオン水を加えて５０倍に希釈し、得られた球状ポリスチレン分散液各１００ｃｃに、ポリスチレン未担持触媒各２０ｇを浸漬して、ロータリーエバボレーターで減圧乾燥した後に、さらに４５〜５０℃の乾燥機中にて１２ｈｒ乾燥した。なお、球状ポリスチレンの担持量は、ポリスチレン担持前後の活性炭の乾燥重量の差より求めた。その値は、いずれも約１ｗｔ％程度となった。
【００２１】
次に、未処理活性炭及び球状ポリスチレン担持活性炭を、接触硫酸化反応試験装置にかけてその活性試験を行った。各触媒とも各々内径１６ｍｍφのジャケット付き硝子製反応器に４０ｍｌ充填し、ＳＯ２ ；１０００ ｖｏｌ ｐｐｍ、Ｏ２ ；４ ｖｏｌ％、ＣＯ２ ；１０ ｖｏｌ％、Ｎ２ ；ｂａｌａｎｃｅ、相対湿度１００％の組成のガスをこの反応器に５０℃、１６５ｄｍ３／ｈｒで流し、ＳＯ２ 計（紫外式・赤外式）により出口ＳＯ２ 濃度を測定し触媒活性を評価した。
図２は、各触媒の試験開始後１００ｈｒにおけるの脱硫性能を示すものである（試料名：Ａ１〜Ｇ１）。同図から、撥水処理を行った石炭を原料とする活性炭が比較的高い脱硫性能を示し、接触硫酸化反応に適当であることを見い出した。
【００２２】
（実施例２）
市販されている６種の活性炭（石炭系：Ａ１，Ｆ２／椰子殻系：Ａ２／ピート系：Ｃ２／木材系：Ａ４／石油ピッチ系：Ｇ１）を、それぞれ窒素気流中、８００℃で１時間焼成した。
次に、上記活性炭をそれぞれ市販されている粉砕器にて粉砕した後、活性炭（約１００ｇ／１回）をステンレス製の篩（１０６μｍ以上２１２μｍ以下）を用いて、篩振盪器にて２時間の分級操作を行った。この様な操作を繰り返すことにより得た活性炭を以下微粉活性炭と呼び、得られた微粉活性炭粒子の代表径（以下粉砕粒子径と呼ぶ）は、組合せた各篩のメッシュの平均値とした。すなわち、上記操作の場合には、得られた微粉活性炭の粉砕粒子径は１５９μｍとなる。
次に、市販のＰＴＦＥ分散液（６０ｗｔ％）に水を加えて６倍に希釈し、上記微粉活性炭をそれぞれ該ＰＴＦＥ分散液と混練した後、圧縮成形機にて成形し（成形圧５００ｋｇｆ／ｃｍ２ ）、ＰＴＦＥを１０ｗｔ％含有する触媒を得た。次いで、この混合成形触媒を４５〜５０℃にて、１２ｈｒ乾燥した後、粗砕・分級して、２．８〜４．０ｍｍφの粒状触媒を得た。
【００２３】
（実施例３）
また、上記実施例２で取り上げた６種の活性炭を、それぞれ窒素気流中、８００℃で１時間焼成した。
次に、市販のポリエチレン０．５ｇを各々６０〜７０℃に温めたトルエン約１００ｃｃに溶解させ、上記活性炭５０ｇを該ポリエチレン溶液に浸漬し、ロータリーエバボレーターで減圧含浸および乾燥を行った。その後、１００〜１１０℃の乾燥機中にて１２ｈｒ減圧乾燥を行い、ポリエチレンの担持量が約１ｗｔ％の触媒を調製した。
【００２４】
そして、上記実施例２および実施例３において調製した触媒を、実施例１に記載された反応試験装置を用いて同じ条件で活性試験を行い、それぞれの触媒活性を評価した。
図１は、このようにして得られた各触媒における試験開始後１００ｈｒの脱硫性能を、実施例１の結果も含めて示したものである。図１により、原料活性炭のうちでは、特に石炭系活性炭が椰子殻系活性炭やピッチ系活性炭よりも一段と高い脱硫性能を示すことが明らかであり、さらに市販されている炭種（石炭、椰子殻、ピート、木材、石油ピッチ）の異なる活性炭に対し実施した上記実施例１〜実施例３のミクロポアを除いた撥水化方法は、いずれも非常に有効であったことが判る。
【００２５】
【発明の効果】
以上説明したように、請求項１〜４のいずれかに記載の活性炭触媒にあっては、原料活性炭として石炭を主原料とする活性炭を用い、これに撥水化処理を施しているので、石炭系の活性炭が有する吸着、酸化活性点の数が多いという特性を活かし、かつ生成硫酸の排出性能を向上させることにより、他の原料からなる活性炭触媒と比較して、より優れた硫黄酸化物に対する活性を得ることができる。特に、最も接触硫酸化反応に寄与するミクロポアを除いて、生成硫酸の流路となるマクロポアの撥水化処理を行なうことにより、一層顕著な活性の向上効果を得ることができる。
したがって、請求項１〜４のいずれかに記載の活性炭触媒を用いた請求項５に記載の排煙脱硫方法によれば、他の排煙脱硫プロセスと比べて脱硫効率において遜色が無く、よって経済性に優れる排煙脱硫が可能になる。
【図面の簡単な説明】
【図１】本発明の実施例における活性試験の結果を示すグラフである。
【図２】実施例１の活性試験の結果を示す図表である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an activated carbon catalyst for recovering and removing sulfur oxides contained in exhaust gas as sulfuric acid by a catalytic sulfation reaction, and a flue gas desulfurization method using the same.
[0002]
[Prior art]
Conventionally, sulfur oxides such as sulfurous acid gas contained in exhaust gas are finally oxidized by coexisting oxygen at a low temperature to finally produce sulfuric acid, which is directly used as sulfuric acid, or by reacting it with a calcium compound. Gypsum recovery processes are well known.
Activated carbon is considered to be the most preferable as a catalyst for oxidizing sulfurous acid gas and the like in exhaust gas. That is, when a ceramic-based carrier such as alumina, silica, titania, or zeolite is used as the catalyst, the activity alone is insufficient, so that it is necessary to add a metal or metal oxide as a catalyst species to this. is there. However, these catalyst species have a drawback that they cannot maintain stable activity for a long time because they are attacked by the generated sulfuric acid and are dissolved or deteriorated. On the other hand, the activated carbon has a feature that the activity is exhibited without carrying any catalyst species and the activity is maintained without deterioration for a long time.
[0003]
However, in commercial use as a flue gas desulfurization device, when commercially available activated carbon is used as it is, the catalytic activity in the catalytic sulfation reaction is low. Therefore, there is a problem that compared with other desulfurization processes such as a wet flue gas desulfurization process, it cannot be economically competitive.
Therefore, there are roughly two approaches for increasing the catalytic activity of activated carbon used in such a flue gas desulfurization process. One is to increase the active sites of adsorption and oxidation reaction of sulfurous acid gas, and the other is to discharge sulfuric acid generated in the pores of activated carbon out of the pores as soon as possible. is there.
[0004]
In the former approach, it is necessary to determine what the active sites of adsorption and oxidation of activated carbon to sulfur dioxide gas are. Regarding this point, for example, the journal of the Chemical Society of Japan (1975) No. 10, P1705-1712 suggest that the compound is a basic surface compound, and in Applied Catalysis B: Environmental 3 (1994) P229-238, the number of strong basic points on activated carbon is determined by adsorption of sulfurous acid gas. It is proportional to the oxidizing activity, and it is suggested that this basic point is an oxygen-containing functional group. In particular, the former document describes that activation and activation of sulfurous acid gas can be increased by activating activated carbon with ammonia gas at 800 ° C.
However, neither of these documents describes at all what type of activated carbon can be selected to achieve high activity, and thus this approach has not yet reached a practical application.
[0005]
As for the other approach, the sulfur dioxide adsorption and oxidation activity (hereinafter simply referred to as activity) of activated carbon is very large unless the exhaust gas contains moisture. However, the product sulfuric acid has a very high hygroscopicity, so in the presence of water vapor, it absorbs water on the activated carbon surface to generate dilute sulfuric acid, which fills the pores of the activated carbon to form sulfurous acid gas. As a result of preventing diffusion and contact, the surface activity of the activated carbon is not sufficiently exerted.
Accordingly, various methods have been proposed for imparting water repellency to activated carbon and quickly discharging the generated sulfuric acid from the pores of the activated carbon to maintain the high activity of the activated carbon.
[0006]
For example, Chem. Eng. Comm. Vol. 60 (1987) and P253, a dispersion of polytetrafluoroethylene (hereinafter abbreviated as PTFE) was sprayed on activated carbon having an average diameter of 0.78 mm, so that sulfite was added in a region where the amount of PTFE added was 8 to 20%. There is disclosed an example in which the rate constants of gas adsorption and oxidation reactions have increased three-fold.
Japanese Unexamined Patent Publication (Kokai) No. 59-36531 discloses that a water-repellent treatment is applied to activated carbon to increase the activity of adsorbing and oxidizing sulfurous acid gas. Specifically, a PTFE dispersion liquid is applied to granular activated carbon of 5 to 10 mm. It is disclosed that by impregnating and heat-treating at 200 ° C. for 2 hours, the catalyst exhibits a much higher activity than the activated carbon-only catalyst.
[0007]
[Problems to be solved by the invention]
The present inventors conducted the following confirmation experiments to verify such a two-way approach to enhance the activity of activated carbon as a conventional catalyst.
First, as one approach, adsorption of sulfurous acid gas on activated carbon, an attempt to increase the oxidation point, various methods that cause some surface modification to activated carbon without being limited by the limited idea of adding a basic point. I tried surface treatment. Here, when an acid or an alkali was used as the surface treatment agent, it was boiled with activated carbon. Further, gas treatment with high-temperature HBr, HCl, ammonia or the like was also attempted. Further, a method of mixing a small amount of a reducing gas (hydrogen) or an oxidizing gas (fluorine, chlorine, oxygen) into a gas such as steam or CO 2 and performing a high-temperature treatment at 800 ° C. was examined.
As a result, high-temperature treatment with ammonia gas exhibited the highest activity, but after several hundred hours of operation, the activity was reduced to a level equivalent to that of untreated activated carbon. On the other hand, in the method in which a small amount of oxygen is mixed in an activation gas such as steam or CO 2 to perform high-temperature treatment, the activity improvement effect is maintained for a long time, but the activity improvement range is small, and the initial target may not be achieved. Did not.
[0008]
Next, the present inventors verified the other approach, that is, water repellency of activated carbon.
First, based on the above-mentioned conventional water-repellent technology, it is considered that it is most effective to improve the activity of the catalytic sulfation reaction by performing water-repellent treatment on activated carbon having many active sites for adsorption and oxidation. PTFE was supported on various types of commercially available activated carbon having a particle size range of ~ 4.0 mmφ by a spraying method or an impregnation method, and the activity was measured. Prolonged persistence was observed.
However, considering large-scale industrial practice, this level of activity is still not sufficient as a catalyst to surpass other competing flue gas desulfurization processes, and further improvement in catalytic activity is required. Recognized that it is necessary.
[0009]
By the way, activated carbon used as a catalyst of this type includes wood, lignite, etc., treated with a chemical as an activator, or charcoal, etc., activated with steam, coal as a raw material, coconut shell, and the like. Various activated carbons are known, such as those using carbon dioxide, and those using peat or petroleum pitch as a raw material.
Therefore, the present inventors changed the viewpoint from the above approach, and tried to confirm whether the activated carbon itself, which is the premise, had a difference in the activation effect when the water-repellent treatment was performed due to the difference in the raw materials. , 18 types of activated carbon were compared before and after the water-repellent treatment.
As a result, the activated carbon catalyst using coal as a main raw material exhibited high activity as usual as a result of the water-repellent treatment, whereas activated carbon using other coconut shells, peat, or petroleum pitch as a raw material. It has been found that the catalyst exhibits a much lower activity. As described above, it is not clear why activated carbon made of coal is superior when water-repellent treatment is performed, but it can be estimated as follows at this stage.
[0010]
That is, it is considered that activated carbon based on coal has a larger number of active sites for adsorption and oxidation of sulfurous acid gas than activated carbon originally consisting of other raw materials. However, coal-based activated carbon is inferior in hydrophobicity to activated carbon made of other raw materials, so that the produced sulfuric acid is discharged slowly, and as a result, the overall activity does not seem to be so high. Therefore, when water-repellent treatment is applied to this, the superiority of activated carbon is determined only by the number of active points, and it is considered that the excellent activity in the original coal-based activated carbon was remarkably exhibited. .
From the above, it can be said that by using an activated carbon catalyst obtained by subjecting activated carbon made of coal as a main raw material to a water-repellent treatment, it is possible to develop a flue gas desulfurization process excellent in oxidizing and removing sulfur oxides in exhaust gas. The present invention has been completed.
[0011]
Then, the present inventors, in order to further enhance the activity of the activated carbon by water repellency, not only in activated carbon using coal as a main raw material, but also in various types of activated carbon, I tried to see if it would be effective to make it water-repellent.
First, the distribution of fluorine in the activated carbon catalyst prepared by a conventional method of spray-supporting or impregnating the PTFE dispersion on activated carbon was analyzed by EPMA. As a result, it was found that the PTFE particles did not penetrate into the activated carbon particles at all, and were all attached to the outer surfaces of the particles. This is because commercially available activated carbon hardly has pores of 1 μm or more, so that the resistance is too high for PTFE particles having a diameter in the range of 0.2 to 0.4 μm to enter the pores. It is thought that it is. Incidentally, similar experimental results were obtained when a dispersion of polystyrene particles having an average particle diameter of 0.3 μm was used instead of the PTFE dispersion. An activity test was performed on the activated carbon catalyst supporting these two types of water-repellent particles. As a result, it was found that the activated carbon catalyst supporting PTFE had slightly higher activity than the activated carbon catalyst supporting polystyrene particles. However, none of them exhibited the expected high activity.
[0012]
The inventors of the present invention expect that the uniform surface of the activated carbon including the vicinity of the active point will be made water-repellent, thereby greatly promoting the discharge of the generated sulfuric acid. Water-repellent treatment was performed on the entire surface including holes having a diameter (hereinafter abbreviated as micropores).
As a first method, various activated carbon catalysts having different degrees of surface fluorination were prepared by flowing an appropriate amount of fluorine gas at 100 to 400 ° C. through activated carbon.
As a second method, stearic acid, a styrene oligomer (average molecular weight of about 320), a fluorine-containing oil (average molecular weight of about 500) and the like, which are small water-repellent substances, are dissolved in a suitable low-boiling solvent. Activated carbon was immersed under reduced pressure, the solution was sufficiently penetrated into the pores, and then dried under reduced pressure to remove the solvent, thereby coating the inside of the activated carbon pores with a water-repellent substance.
The specific surface area of the activated carbon catalyst thus prepared is within an apparent decrease range due to an increase in weight due to the support of the water-repellent substance, and these supports may block or destroy pores. Not confirmed.
[0013]
Next, a sulfurous acid gas reaction activity test was performed using various activated carbon catalysts having a fluorination ratio of 1 to 20% obtained by the first method. As the fluorination ratio increased, water was found to increase. The water repellency test is to gradually increase the water repellency (this is the average value of the sedimentation start time and sedimentation end time of the water-repellent treated activated carbon particles gently floating on the water surface. This is a simple method for the relative comparison of the above.), It was found that the adsorption and oxidation activities of sulfurous acid gas decrease rather in inverse proportion to the increase of the fluorination rate.
In the case of the activated carbon catalyst supporting stearic acid, styrene oligomer, fluorinated oil and the like obtained by the second method, the repellency was similarly measured by the water surface floating time test as the supported amount increased. Although an increase in aqueous activity is observed, the activity of adsorbing and oxidizing sulfurous acid gas slightly exceeds the catalytic activity of activated carbon alone in the addition region of 0.5 to 2%, and the activity rapidly increases with an increase in the addition amount. It turned out to go down.
[0014]
From the above, it was presumed that the entire water repellency including the micropores of the activated carbon covered or destroyed the active sites for adsorption and oxidation of the activated carbon, so that a sufficient effect of improving the activity could not be obtained.
Then, the present inventors tried to make only the macropores (pores having pore diameters exceeding 5 nm) water-repellent without making the micropores of activated carbon water-repellent. First, a polyethylene or polystyrene powder having a molecular weight of 100,000 or more is dissolved in toluene heated to 60 to 70 ° C. by several percent, and activated carbon particles are immersed under reduced pressure. Scattered. The activated carbon catalyst thus obtained showed a considerable improvement in activity when the water-repellent substance was loaded in the range of 0.3 to 1.5 wt% relative to the raw material activated carbon.
This is because if polyethylene or polystyrene having a molecular weight of 100,000 or more is spherical and dispersed in a toluene solvent, its diameter will be 7 nm or more even if it does not swell in the solvent, and this will penetrate into the activated carbon micropores. Not the size you can do. Therefore, it should be considered that the activated carbon catalyst makes the macropores and the outer surface of the activated carbon particles water-repellent. And, as described above, in this reaction system, the water repellency of the outer surface of the activated carbon does not significantly contribute to the improvement of the reaction activity. It is inferred that it contributes to
[0015]
Therefore, even in the case of using an activated carbon catalyst obtained by performing a water-repellent treatment on activated carbon using the above-described coal as a main raw material, the water-repellent treatment is further performed by repelling macropores excluding micropores in the raw material activated carbon. It has been found that it is possible to develop a flue gas desulfurization process which is more excellent in the performance of removing and oxidizing sulfur oxides.
The present invention has been made based on such knowledge, and has an activated carbon catalyst which has a comparable desulfurization efficiency as compared with other flue gas desulfurization processes, and thus enables flue gas desulfurization which is excellent in economic efficiency, and a flue gas using the same. It is an object of the present invention to provide a smoke desulfurization method.
[0016]
[Means for Solving the Problems]
The activated carbon catalyst according to the present invention according to
[0017]
The inventions according to
And the flue gas desulfurization method according to the present invention described in claim 5 is characterized in that the activated carbon catalyst according to any one of
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the activated carbon catalyst according to the present invention will be specifically described.
The activated carbon catalyst according to the present invention is for oxidizing sulfur dioxide gas in exhaust gas by coexisting oxygen to recover and remove it as sulfuric acid. Activated carbon mainly made of coal is subjected to a water-repellent treatment. It is. Here, when performing the above-mentioned water-repellent treatment, it is preferable to select a high-molecular-weight water-repellent substance that does not modify the micropores of the activated carbon as described above in order to exhibit high activity. Examples of such a water-repellent substance include polyethylene, polypropylene, polystyrene, and fluorine resin.
Further, as the water-repellent treatment for the coal-based activated carbon, a method of dissolving the water-repellent substance in an organic solvent, making use of the shape of the raw activated carbon as it is, and impregnating and supporting the coal-based activated carbon, or an average particle diameter is as follows. There is a method in which a dispersion liquid of the water-repellent substance having a thickness of 15 to 100 nm is impregnated and supported on the coal-based activated carbon. In the case of performing the water-repellent treatment while keeping the shape of the raw material activated carbon as it is, the latter method can obtain the highest activity, and the preferable addition amount of the water-repellent substance at that time is based on the raw material activated carbon. , 0.2 to 3 wt%.
[0019]
Further, the raw activated carbon is not directly subjected to the water-repellent treatment, but the activated carbon powder is used, and the activated carbon powder is kneaded with fluororesin particles having an average particle diameter of 0.1 to 1.0 μm or a dispersion thereof. Thus, a water-repellent activated carbon catalyst can be obtained.
In this case, as the fluororesin to be used, in addition to the above-mentioned PTFE, a perfluoroalkoxy resin (PFA), a tetrafluoroethylene hexafluoropropylene copolymer (FEP), a trifluoroethylene chloride resin (PCTEF), or the like is used. Is preferred. These fluororesins have higher water repellency than polystyrene, polyethylene, etc., and the average particle size of these fluororesins in the dispersion is relatively large, 0.2 to 0.4 μm. Thus, by kneading and molding these, it is possible to obtain a desired activated carbon catalyst in which even macropores are made water-repellent. Further, according to the present method, an activated carbon powder can be used as a raw material to produce an arbitrary shape, which is suitable from the viewpoint of production cost in addition to the above-mentioned improvement in activity. Examples of the shape of the catalyst include a honeycomb shape, a plate shape, a spherical shape, a columnar shape, a saddle shape, and the like. A catalyst shape having a high blocking resistance and a small pressure loss of the catalyst layer is preferable.
[0020]
【Example】
Next, the present invention will be described more specifically with reference to examples.
(Example 1)
First, 18 kinds of activated carbons having different raw materials such as commercially available coal, coconut shell, peat, wood, and petroleum pitch are crushed by a coarse crusher, and 50 g of each of 2.8 to 4.0 mmφ granular catalyst is passed through a stainless steel sieve. Obtained. However, since only one type of activated carbon using petroleum pitch as a raw material was available as a commercial product, a granular catalyst of 0.71 to 1 mmφ having a different particle diameter was used.
Next, each of the 18 types of activated carbon was fired at 800 ° C. for 1 hour in a nitrogen stream. Hereinafter, this is referred to as an untreated catalyst. Next, deionized water was added to an aqueous dispersion (10 wt%) of a commercially available spherical polystyrene (average particle size: 28 nm) to dilute it 50-fold. After immersing 20 g and drying under reduced pressure with a rotary evaporator, it was further dried in a dryer at 45 to 50 ° C. for 12 hours. The amount of spherical polystyrene carried was determined from the difference in dry weight of activated carbon before and after carrying polystyrene. The values were all about 1 wt%.
[0021]
Next, the untreated activated carbon and the activated carbon carrying spherical polystyrene were subjected to an activity test by using a catalytic sulfation reaction test apparatus. And 40ml packed in a jacketed glass reactor made of each inner diameter 16mmφ in each catalyst, SO 2; 1000 vol ppm,
FIG. 2 shows the desulfurization performance of each
[0022]
(Example 2)
Six types of commercially available activated carbon (coal system: A1, F2 / coconut shell system: A2 / peat system: C2 / wood system: A4 / oil pitch system: G1) are each placed in a nitrogen stream at 800 ° C. for 1 hour. Fired.
Next, after each of the activated carbons was pulverized with a commercially available pulverizer, the activated carbon (about 100 g / time) was passed through a sieve made of stainless steel (106 μm or more and 212 μm or less) for 2 hours using a sieve shaker. A classification operation was performed. The activated carbon obtained by repeating such an operation is hereinafter referred to as fine activated carbon, and the representative diameter of the obtained fine activated carbon particles (hereinafter referred to as a pulverized particle diameter) is an average value of the mesh of each combined sieve. That is, in the case of the above operation, the pulverized particle size of the obtained fine powdered activated carbon is 159 μm.
Next, water was added to a commercially available PTFE dispersion (60 wt%) to dilute it 6-fold, and the fine powdered activated carbon was kneaded with each of the PTFE dispersions, followed by molding with a compression molding machine (forming pressure 500 kgf / cm). 2 ) A catalyst containing 10% by weight of PTFE was obtained. Next, the mixed catalyst was dried at 45 to 50 ° C. for 12 hours, and then crushed and classified to obtain a granular catalyst of 2.8 to 4.0 mmφ.
[0023]
(Example 3)
The six types of activated carbons taken up in Example 2 were fired at 800 ° C. for 1 hour in a nitrogen stream.
Next, 0.5 g of commercially available polyethylene was dissolved in about 100 cc of toluene heated to 60 to 70 ° C., and 50 g of the activated carbon was immersed in the polyethylene solution, and impregnated with a rotary evaporator under reduced pressure and dried. Thereafter, the catalyst was dried under reduced pressure in a dryer at 100 to 110 ° C. for 12 hours to prepare a catalyst having a polyethylene loading of about 1 wt%.
[0024]
The catalysts prepared in Examples 2 and 3 were subjected to an activity test using the reaction test apparatus described in Example 1 under the same conditions, and the respective catalyst activities were evaluated.
FIG. 1 shows the desulfurization performance of each of the catalysts thus obtained 100 hours after the start of the test, including the results of Example 1. From FIG. 1, it is clear that among the activated carbon raw materials, coal-based activated carbon in particular exhibits a much higher desulfurization performance than coconut shell-based activated carbon and pitch-based activated carbon. It can be seen that the water-repellent methods of Examples 1 to 3 except for the micropores performed on activated carbons having different peats, woods, and oil pitches were all very effective.
[0025]
【The invention's effect】
As described above, in the activated carbon catalyst according to any one of
Therefore, according to the flue gas desulfurization method according to claim 5 using the activated carbon catalyst according to any one of
[Brief description of the drawings]
FIG. 1 is a graph showing the results of an activity test in Examples of the present invention.
FIG. 2 is a table showing the results of an activity test in Example 1.
Claims (5)
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| JP14720397A JP3562551B2 (en) | 1997-05-21 | 1997-05-21 | Activated carbon catalyst and flue gas desulfurization method |
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2017174472A1 (en) * | 2016-04-04 | 2017-10-12 | Cppe Carbon Process & Plant Engineering S.A. | Sulfur dioxide removal from waste gas |
| US10478776B2 (en) | 2016-04-04 | 2019-11-19 | Cppe Carbon Process & Plant Engineering S.A. | Process for the removal of heavy metals from fluids |
| US11369922B2 (en) | 2016-04-04 | 2022-06-28 | Cppe Carbon Process & Plant Engineering S.A. | Catalyst mixture for the treatment of waste gas |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| US7967899B2 (en) | 2006-01-06 | 2011-06-28 | Cataler Corporation | Activated carbon and canister using the same |
| JP5553966B2 (en) | 2008-03-19 | 2014-07-23 | 千代田化工建設株式会社 | Mercury adsorbent and smoke treatment method using the adsorbent |
| JP5076471B2 (en) * | 2006-12-05 | 2012-11-21 | 千代田化工建設株式会社 | Method for producing carbon-based catalyst for flue gas desulfurization |
| CN103551117A (en) * | 2013-09-25 | 2014-02-05 | 蚌埠首创滤清器有限公司 | N-capric acid modified cocoanut active charcoal adsorbent and preparation method thereof |
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Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2017174472A1 (en) * | 2016-04-04 | 2017-10-12 | Cppe Carbon Process & Plant Engineering S.A. | Sulfur dioxide removal from waste gas |
| LU93012B1 (en) * | 2016-04-04 | 2017-11-08 | Cppe Carbon Process & Plant Eng S A En Abrege Cppe S A | Sulfur dioxide removal from waste gas |
| US10471388B2 (en) | 2016-04-04 | 2019-11-12 | Cppe Carbon Process & Plant Engineering S.A. | Sulfur dioxide removal from waste gas |
| US10478776B2 (en) | 2016-04-04 | 2019-11-19 | Cppe Carbon Process & Plant Engineering S.A. | Process for the removal of heavy metals from fluids |
| US11369922B2 (en) | 2016-04-04 | 2022-06-28 | Cppe Carbon Process & Plant Engineering S.A. | Catalyst mixture for the treatment of waste gas |
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