JP2004174371A - Method for treating gas and system therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently remove carbon dioxide stably at a lower cost and make effective use of the removed carbon dioxide. <P>SOLUTION: This system is provided with a gas treatment apparatus 1, a regeneration tank 3 and an adjustment tank 4. The apparatus 1 is provided with a sodium ion-permeable electrolytic membrane 10, an anode 11 to which an aqueous solution of a sodium compound is supplied and a cathode 12 to which the gas to be treated is supplied and is used for separating/removing the carbon dioxide in the gas as the form of a carbonate on the side of the cathode 12 by impressing DC voltage between the anode 11 and the cathode 12. The tank 3 is provided with another sodium ion-permeable electrolytic membrane, an anode chamber and a cathode chamber which are formed by arranging the electrolytic membrane, a route for supplying the carbonate aqueous solution produced in the apparatus 1 to the anode chamber and another route for supplying water to the cathode chamber and is used for producing sodium hydroxide in the cathode chamber by impressing DC voltage between the liquid in the anode chamber and that in the cathode chamber. The tank 4 is used for controlling the sodium ion concentration of a sodium hydroxide aqueous solution to be produced in the cathode chamber and the amount of the sodium hydroxide aqueous solution to be supplied to the anode 11 of the apparatus 1 to be constant. <P>COPYRIGHT: (C)2004,JPO

Description

２Ｎａ^＋と２Ｈ^＋は、イオン導電体６４を介してカソード側へ移動する。
【００６８】

また、ガス処理装置１のカソード１２において、十分な水（Ｈ_２Ｏ）が存在した場合、温度が低いときは完全に炭酸ナトリウム水溶液とならず、炭酸水素ナトリウム水溶液となる場合がある。
【００６９】
図５（ｂ）は、この炭酸水素ナトリウムから水酸化ナトリウム水溶液を再生する場合の実施形態である。水酸化ナトリウム水溶液の再生の原理は図５（ａ）と同様である。すなわち、アノード５１側の炭酸水素ナトリウム水溶液はナトリウムイオンと炭酸イオンとに電離し電極へ電子を放出して二酸化炭素及び酸素を放出する。分離したナトリウムイオンと水素イオンは、イオン導電体５４を通って、カソード５２側に移動し、水と反応して水素を発生しながら、水酸化ナトリウムとなる。得られた水素ガスは燃料ガスとして利用できる。このようにして得られた水酸化ナトリウム水溶液は、図示省略されたポンプ等の移送手段によって、調整槽４に供給され、ナトリウムイオンの濃度が所定濃度（例えば５〜１０ｍｏｌ／ｌ）に調整された後に、ガス処理装置１のアノード１１に適宜供給される。ＮａＯＨ再生に係る反応を以下にまとめた。
【００７０】

２Ｎａ^＋と２Ｈ^＋は、イオン導電体６４を介してカソード側へ移動する。
【００７１】

以上のように、本実施形態に係るガス処理システムによれば、ナトリウムイオン源の回収・再利用を行え、低廉に二酸化炭素を除去することができる。
【００７２】
また、被処理ガスがメタンを含んだガス例えば汚泥消化ガスのようなガスである場合、このガスを本実施形態のガス処理システムに供することで、効率的に濃縮メタンガスを得ることができる。この濃縮メタンガスは、燃料電池やガスタービン等の燃料またはその他の燃料として売却するなどの有効利用が可能となる。
【００７３】
下水処理場で発生する汚泥の消化ガスや家畜糞尿設備で発生するバイオガスは有用なエネルギー源として活用が可能であるが、従来の技術においては特に中小容量の処理場において熱源としての用途しか見出すことができなかった。その一つの理由としては、メタンガス濃度が低いことが挙げられる。そこで、汚泥消化ガス中の二酸化炭素成分を前記ガス処理システムで除去することで、前記ガス中のメタンガス濃度を高めることができ、汚泥消化ガスを高発熱量ガスに転換させることができる。
【００７４】
そして、特に図２に示した実施形態例のような下水処理システムにおいて、本発明のガス処理システムを具備させれば、汚泥消化ガスを有効利用することができる共に、水処理システムにおける硝化脱窒機能を安定化させることができる。
【００７５】
図２に示された水処理システムは、汚泥消化ガスが得られる設備は主に下水処理施設で硝化工程さらには脱窒工程を有する。硝化工程及び脱窒工程の液相は最終沈殿池にて固液分離処理される。最終沈殿池で得られた上澄水は、さらに高度処理に供されるか若しくは放流される。ここで、最終沈殿池にて分離された汚泥の一部は活性汚泥として再利用するために硝化工程に返送される一方で、一部の汚泥は消化させるために汚泥消化工程に供される。汚泥消化工程では、汚泥を好気的または嫌気的に処理している。特に、嫌気処理の過程において、メタンガスを含んだガスが発生する。
【００７６】
一般的な下水処理では先ず、硝化工程で下水中の窒素化合物（アンモニア性窒素）を硝化菌の作用で硝酸イオンなど（ＮＯ_２ ⁻、ＮＯ_３ ⁻）に変換し、次いで脱窒工程で脱窒菌の作用により窒素ガス（Ｎ_２）へ還元する必要がある。硝化工程における下水のｐＨは８〜９の範囲であるが、硝化反応が進行していくとアルカリ度が消費されてｐＨ値が低下する。ｐＨ値の低下が生じると処理効率が低下する落ちるばかりでなく、硝化菌が死滅する恐れがある。したがって、ｐＨ値を８〜９に調整するために、消石灰（Ｃａ（ＯＨ）_２）、ソーダ灰（Ｎａ_２Ｃｏ_３）、水酸化ナトリウム（ＮａＯＨ）といったアルカリ剤（ｐＨ調整剤）を投入しなければならない。
【００７７】
前述のガス処理装置１のカソード１２側から排出される炭酸塩水溶液（廃液）は、そのｐＨ値は８〜１０程度であり（尚、二酸化炭素を完全に吸収していない場合は、ＮａＯＨ成分が高くなるためそれ以上のｐＨ値となる）、アルカリ剤として利用できる。そこで、ガス処理装置１を備えたガス処理システムを下水処理システムに具備させることで、高発熱量ガスの生産と下水処理における硝化脱窒機能を安定化させることができる。
【００７８】
以下に、本実施形態におけるガス処理システムの実施例を示した。
【００７９】
図８は、電流密度と二酸化炭素濃度及び水素濃度との関係を示した特性図である。
【００８０】
当該実験に係るガス処理システムは、図１記載のシステムにおいて図４記載のガス処理装置を用いたものを採用した。ここでは、セルサイズ２００ｍｍ×２００ｍｍの電解質を二枚積層（有効電極面積：６００ｃｍ^２）したガス処理装置に、被処理ガス（ガス成分 二酸化炭素：４０％、窒素：６０％）を１ｌ／ｍｉｎの流量で供給した。このとき、被処理ガスはスクラバーを介さないでガス処理装置のカソードに供給したまた、印加直流電圧は２．５Ｖに設定した。さらに、電解質はフッ素樹脂系のものを、ナトリウム化合物の水溶液には水酸化ナトリウム水溶液を、電極にはニッケルからなるものを採用した。
【００８１】
図示された結果から明らかなように、電流密度の上昇に伴い、二酸化炭素濃度が減少していくことが確認できる。また、当該実施例においては、電流密度１００ｍＡ／ｃｍ^２時において二酸化炭素は完全に除去できたことも確認された。
【００８２】
ここで、当該実施例の結果に基づき算出した二酸化炭素除去に必要な電力、使用電力による二酸化炭素発生量（１分間）、二酸化炭素固定量（１分間）、水素発生量（１分間）を以下に示した。
【００８３】
二酸化炭素除去に必要な電力：２．５［Ｖ］×６０［Ａ］＝１５０［Ｗ］
使用電力による二酸化炭素発生量（１分間）：１５０×１／１０００×１／６０×０．１２／４４＝０．００７［ｍｏｌ］ 尚、０．１２は、石油火力を電源として用いた時の二酸化炭素排出量を示す。
【００８４】
二酸化炭素固定量（１分間）：０．４［ｌ／分］×１［分］／２２．４［ｌ］＝０．０１８［ｍｏｌ］
水素発生量（１分間）：０．０１８［ｍｏｌ］
かかる計算結果から明らかなように、わずかな電力で二酸化炭素を効率的に除去でき、かつ固定した二酸化炭素と等モルの水素が発生することが確認できる。
【００８５】
また、表１に、被処理ガスをスクラバーに供した後にガス処理装置に供給した場合の各処理工程から排出されたガス、すなわちスクラバーを通過したガス（以下、第一処理ガス）及びガス処理装置から排出されたガス（以下、第二処理ガス）の組成を被処理ガスのガス組成と共に開示した。ここでは、被処理ガスの組成をメタン：二酸化炭素＝６０：４０としたこと以外は図５記載の実施例と同様の条件でガスを処理した。
【００８６】
【表１】

【００８７】
表に示された通り、第一処理ガスのメタンガス濃度は８５％、また二酸化炭素濃度は１５％となっていることが確認された。さらに、第二処理ガスにおいては、二酸化炭素が検出されず、可燃性ガスの濃度は１００％となっていることが確認された。このことは、スクラバーを本発明のガス処理装置と組み合わせることで、ガス処理装置に対する二酸化炭素負荷量が軽減され、二酸化炭素除去における省電力化及び装置システムの小型化が図れることを示唆するものである。尚、第二処理ガスは、そのメタンガス濃度から明らかなように、燃料電池、ガスタービン等の燃料や、燃料として売却するなどの有効利用に供することができるがわかる。
【００８８】
図９は、本発明の実施例によるＩ−Ｖ特性を示した特性図である。
【００８９】
実施例１に係るガス処理装置は電解質に塩化ビニル系の樹脂からなる電解質膜を、比較例に係るガス処理装置はセラミックス系の電解質を採用した。実施例１及び比較例は、ナトリウム化合物水溶液に５ｍｏｌ／ｌの水酸化ナトリウム水溶液を採用し、動作温度は室温の２０℃とした。図示された結果から明らかなように、電解質に塩化ビニル系樹脂の電解質膜を採用した実施例１においては、反応が始まってから電流の立ち上がりがよくなり、セラミックス系の電解質よりも、大幅な省電力化が可能であることが確認できる。
【００９０】
ここで、図１０に実施例１による二酸化炭素の除去特性を示した。当該特性試験は、１００ｍＡ／ｃｍ^２の定格電流方式で行った。電極面積は３００ｃｍ^２に設定した。図から明らかなように、初期濃度約２０％の二酸化炭素は電流が流れ始めると同時に除去が始まり、約２０分で０．６％程度までに低減させることができることが確認できる。このとき、電流を停止すると、二酸化炭素濃度は上昇し始めることが確認された。そして、当該試験においては、二酸化炭素の除去率は９７％となることが確認された。また、二酸化炭素の除去率は電流が流れている間はほぼ一定に保持することも確認された。さらに、ナトリウムイオン供給源を供給続ければ、連続的に二酸化炭素を吸蔵することも確認されている。
【００９１】
図１１は、本発明の実施例によるＩ−Ｖ特性を示した特性図である。
【００９２】
実施例２に係るガス処理装置は電解質にフッ素樹脂系の電解質膜を、比較例に係るガス処理装置はセラミックス系の電解質を採用した。実施例２及び比較例は、ナトリウム化合物水溶液に５ｍｏｌ／ｌの水酸化ナトリウム水溶液を採用し、動作温度は８６℃とした。図示された結果から明らかなように、電解質にフッ素樹脂系電解質膜を採用した実施例２においては、反応が始まってから電流の立ち上がりがよくなり、セラミックス系の電解質よりも、大幅な省電力化が可能であることが確認できる。
【００９３】
（実施形態２）
図６（ａ）は、本実施形態のガス処理システムを示した概略図である。また、図６（ｂ）は、当該システムが備えた再生槽６の概略構成図である。
【００９４】
本実施形態に係るガス処理システムは、二酸化炭素除去する工程でカソード１２に吸蔵した二酸化炭素成分を強酸によって遊離し放出する工程と、この工程で生成した塩から水酸化ナトリウムを再生する工程とを有する。強酸としては例えば塩酸が用いられる。
【００９５】
ガス処理装置１は、実施形態１に係るガス処理装置と同様の構成である。その構成の説明は当該実施形態に譲る。本実施形態においては、被処理ガスを導入するための経路には、三方バルブＶ１（以下Ｖ１とする）を介して、強酸水溶液を供給するための経路が接続されている。また、ガス処理装置１と気液分離槽３とを連絡する経路には、三方バルブＶ２（以下Ｖ２とする）を介して、カソード１２側で生成した塩化ナトリウム水溶液を再生槽６に移送するための経路が接続されている。
【００９６】
再生槽６は、図８（ｂ）に示されたように、塩化ナトリウム水溶液が供給される槽６０がイオン導電体６４で仕切られた２槽構造となっている。そして、塩化ナトリウム水溶液が供給される槽には直流電源６３の陽極と導通可能なアノード６１が設置され、また他方の槽には、水が供給され、同電源６３の陰極と導通可能なカソード６２が設置されている。ナトリウムイオンを通過させることができるものであれば公知のものでよい。例えば、実施形態１で述べたフッ素樹脂、塩化ビニル樹脂またはスルホン酸樹脂等を含ませて形成した電解質膜がある。
【００９７】
本実施形態のガス処理システムの動作例について図６及び図７に基づき説明する。
【００９８】
二酸化炭素除去工程においては、図７（ａ）に示されたように、Ｖ１は、被処理ガスをガス処理装置１に供給するように制御される。Ｖ２は、処理ガスを気液分離槽３に供給するように制御される。当該工程におけるカソード１２での作用の説明は実施形態１に譲る。
【００９９】
二酸化炭素放出工程においては、図７（ｂ）に示されたように、Ｖ１は、強酸水溶液をガス処理装置１に供給するように制御される。Ｖ２は、処理ガスを気液分離槽３に供給するように制御される。当該工程は、二酸化炭素除去工程において被処理ガス中に含まれる二酸化炭素成分を捕獲して得た炭酸ナトリウムは弱酸と強塩基の化合物であることに着目し、弱酸の二酸化炭素の取出しを行っている。すなわち、当該炭酸塩に強酸ここでは塩酸水溶液を噴霧することで以下の反応を起こさせ、二酸化炭素の取出し系外に移送させている。
【０１００】
Ｎａ_２ＣＯ_３ ＋ ２ＨＣｌ → ２ＮａＣｌ ＋ Ｈ_２Ｏ ＋ ＣＯ_２
また、炭酸水素ナトリウムを得ている場合には、これに強酸（例えば塩酸水溶液等）を噴霧することで以下の反応を起こさせ、二酸化炭素を取り出し系外に移送させる。
【０１０１】
ＮａＨＣＯ_３ ＋ ＨＣｌ → ＮａＣｌ ＋ Ｈ_２Ｏ ＋ ＣＯ_２
かかる反応により上記炭酸塩は直ちに分解し、二酸化炭素が遊離される。このとき、副生されたナトリウム塩であるところの塩化ナトリウム水溶液はナトリウムイオン源再生工程に供される。図１２に本実施形態における二酸化炭素の吸蔵と放出示した実験結果例を開示した。該図によると、二酸化炭素の吸蔵量と放出量が一致していない。これは、二酸化炭素除去工程においてカソード１２表面に得られた化合物が、炭酸ナトリウムではなく、炭酸水素ナトリウムであり、これが水溶液の形態で洗い流されているためであると考えられる。
【０１０２】
次いで、水酸化ナトリウムの再生工程では、図６（ｂ）に示したように、アノード６１には直流電源６３の陽極が導通し、カソード６２には同電源６３の陰極と導通し、直流電圧が印加される。このとき、電気分解が進行し、カソード６２側の液相において水成分は水酸化物イオンと水素ガスに分解される。ここで、副生された水素ガスは、燃料ガスとして利用が可能となるばかりでなく、共に生成した塩素ガス共に塩酸の原料となり得るので、系外に移送され二酸化炭素放出工程に用いられる塩酸の生成に供することができる。一方、ナトリウムイオンはイオン導電体６４を経て、カソード５２側の水槽に移行し、該水槽内の液相は水酸化ナトリウムの水溶液となる。このようにして得られた水酸化ナトリウム水溶液は、図示省略されたポンプ等の移送手段によって、調整槽４に供給され、ナトリウムイオンの濃度が所定濃度（例えば５〜１０ｍｏｌ／ｌ）に調整された後に、ガス処理装置１のアノード１１に適時供給される。当該工程における反応を以下にまとめた。
【０１０３】

【０１０４】

尚、本実施形態において、ガス処理装置１への被処理ガス供給ラインには、実施形態１と同様の趣旨で、被処理ガスに液滴を接触させるスクラバー１５を備えるとなおよい。さらに、実施形態１と同様に、下水処理システムにおいて、本実施形態のガス処理システムを具備させれば、汚泥消化ガスを有効利用することができる共に、炭酸塩水溶液をｐＨ調整剤して水処理システムに供してやれば、水処理における硝化脱窒機能を安定化させることができる。
【０１０５】
【発明の効果】
以上の説明から明らかなように、本発明の請求項記載のガス処理方法とそのシステムによれば、より低廉かつ効率的さらに安定した二酸化炭素の除去を実現すると共に、除去した二酸化炭素成分を有効的に利用できる。
【０１０６】
特に、本発明においては、アノード側におけるナトリウム化合物水溶液のナトリウムイオン濃度を一定に保持させているので、二酸化炭素の除去性能は安定する。
【０１０７】
また、二酸化炭素成分の除去に伴い吸蔵した炭酸塩を水酸化ナトリウムの水溶液に変換し、これを二酸化炭素除去に供するナトリウム化合物水溶液として利用しているので、大幅なランニングコストの削減が可能となる。
【０１０８】
さらに、被処理ガスをアソードに供する前に、スクラバーにおいて、水若しくは水酸化ナトリウム水溶液の液滴と接触させることで、ガス中のおおまかな二酸化炭素成分は前記微細液体粒子に捕捉されるので、カソードに対する負荷が軽減され、二酸化炭素除去時及び水素ガス生成時の省電力化が可能となり、これにより装置システムの小型化が図れる。また、スクラバーから供給された被処理ガスはより水分を含んでいるので、より多くの水素ガスの生成が可能となると同時に、吸蔵させた炭酸塩を容易に水溶液化させることができるので、連続的な二酸化炭素の除去及び水素ガスの生成も可能となる。
【０１０９】
そして、本発明から排出された炭酸塩水溶液は、ｐＨ調整剤として使用できるので、水処理システムに供することで、このシステムにおける硝化脱窒作用を安定させることができる。
【０１１０】
また、被処理ガスが例えば汚泥消化ガスのようなメタンを含んだガスである場合には、本発明によって、該ガスから効率的に濃縮メタンガス（高熱量ガス）を生産することができ、このガスは燃料電池やガスタービンまたはその他の燃料として売却及び利用ができるので、温室効果ガスの有効利用が図れる。
【図面の簡単な説明】
【図１】本発明のガス処理システムの実施形態例を示す概略図。
【図２】本発明のガス処理システムの実施形態例を示す概略図。
【図３】ガス処理装置の実施形態例を示す概略図。
【図４】ガス処理装置の実施形態例を示す概略図。
【図５】（ａ）炭酸ナトリウム水溶液が供給される再生槽の概略説明図、（ｂ）炭酸水素ナトリウム水溶液が供給される再生槽の概略説明図。
【図６】（ａ）本発明に係るガス処理システムの実施形態例を示す概略図、（ｂ）塩化ナトリウム水溶液が供給される再生槽の概略説明図。
【図７】（ａ）二酸化炭素除去工程の概略説明図、（ｂ）二酸化炭素放出の概略説明図。
【図８】電流密度と二酸化炭素濃度及び水素濃度との関係を示した特性図。
【図９】本発明（実施例１）と比較例におけるＩ−Ｖ特性図。
【図１０】本発明による二酸化炭素の除去特性図。
【図１１】本発明（実施例２）と比較例におけるＩ−Ｖ特性図。
【図１２】二酸化炭素の吸蔵と放出を示した実験結果例。
【符号の説明】
１…ガス処理装置、１０…電解質膜、１１…アノード、１２…カソード、１３…電源
１５…スクラバー
２…水供給手段
３…気液分離槽
４…調整槽、１４…槽、１４３…イオン濃度測定手段
５，６…再生槽
７…ガス処理装置
５０，６０，７０…槽、５１，６１，７１…アノード、５２，６２，７２…カソード、５３，６３，７３…電源、５４，６４，７４…イオン導電体
８，９…炭酸ガス吸収槽[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a gas treatment method and a system therefor, and more particularly to a method for removing carbon dioxide in a gas inexpensively and efficiently, and further using the removed carbon dioxide component.
[0002]
[Prior art]
In December 1998, the 3rd Conference of the Parties to the United Nations Framework Convention on Climate Change (COP3) was held in Kyoto City to determine a new international framework for preventing global warming since 2000. Was. At the Kyoto Conference, the Kyoto Protocol was adopted, including a numerical target to reduce greenhouse gases in developed countries as a whole by more than 5% from 2008 to 2012, among which the numerical targets for Japan were It is said to be reduced by 6%.
[0003]
Carbon dioxide (CO ₂ ), which is generated as a result of burning energy, accounts for most of the anthropogenic emissions of greenhouse gases. As a result of Japan's proactive energy conservation efforts since the oil crisis, the energy use efficiency of the industrial sector is at the highest level in the world, and primary energy consumption per GDP is about one-third of that of the United States and about one-half of that of Germany. 1 and low levels compared to other developed countries.
[0004]
However, with the remarkable growth of energy consumption mainly in the transportation and consumer sectors in recent years, the amount of carbon dioxide emissions caused by energy has increased by more than 8% in 1995 compared to 1990. For this reason, it is necessary to immediately start maximal measures to achieve the goals of the Kyoto Protocol.
[0005]
In the Kyoto Protocol, greenhouse gases include carbon dioxide (CO ₂ ), methane gas (CH ₄ ), nitrous oxide (N ₂ O), hydrofluorocarbon (HFC), perfluorocarbon (PFC), and sulfur hexafluoride (SF ₆ ) The emission of greenhouse gases in Japan has increased by 8.6% in FY1995 compared to FY1990. Of these, carbon dioxide generated by the burning of energy accounts for nearly 90% of the total and has a close relationship with overall economic and social activities.
[0006]
In Japan, these energy-related carbon dioxide emissions have been increasing rapidly since the latter half of the 1980s. In particular, the transport and consumer sectors have been growing remarkably in recent years, and in 1995, increased by 8.1% compared to 1990. It has become. Therefore, in Japan, the central issue when considering domestic greenhouse gas countermeasures is measures to reduce energy-related carbon dioxide, including strengthening measures in the transport and civilian sectors.
[0007]
Although it is difficult to accurately determine the amount of carbon dioxide, it is said that it is absorbed by the action of forests and the like, and research and development on carbon dioxide fixation technology are also being advanced. In addition, as for alternative chlorofluorocarbons (HFC, PFC and SF ₆ ), they are used as semiconductor cleaning and as refrigerants in products, and technologies for recovering and decomposing those remaining in the city are being developed. I have.
[0008]
In this way, while taking measures to reduce gas emissions as a whole in greenhouse gas measures, gas absorption, fixation, recovery and separation, such as the creation of systems for preservation and maintenance of forests and agricultural land and the collection of alternative CFCs, are being promoted. It is also extremely important to take various measures related to such factors.
[0009]
The CO ₂ emission concentration of the exhaust gas is about 10% for the diesel engine in the exhaust gas, about 13% for the boiler, and about 40% for the digested gas in the sewage treatment plant and the food factory.
[0010]
At present, methods and apparatuses for decomposing and recovering carbon dioxide are being developed in many fields. In particular, at around room temperature, a pressure swing method for adsorption and desorption using a zeolite catalyst or the like (Non-Patent Document 1), a membrane separation method using a polymer membrane (Patent Document 1), a method of dissolving in water, an aqueous alkaline solution, or the like (Non-Patent Document 2) and the like have been developed, and these have become mainstream.
[0011]
In the pressure swing method, carbon dioxide is adsorbed by filling a carbon dioxide-adsorbing substance such as zeolite. Then, after the adsorption is saturated, the pressure is maintained at a low pressure or a high temperature to release the adsorbed carbon dioxide. However, when this method is used, since a water component is also adsorbed in addition to carbon dioxide, it is necessary to provide a dehydration device in a preceding stage. Further, continuous processing is not possible, and at least two processing towers are required, and additional equipment such as heating equipment and vacuum equipment is required, which is a large-scale equipment.
[0012]
In a membrane separation method using a polymer membrane, a pressurized gas is introduced into one of carbon dioxide selective permeable membranes (poly ([1-trimethylsilyl]) propylene, polydimethylsiloxane, etc.) to selectively remove carbon dioxide. This is a method of increasing the concentration of carbon dioxide by permeating it. However, in such a method, it is difficult and expensive to optimize the gas pressure and carbon dioxide permeability applied to the membrane itself. In addition, in general, gases such as nitrogen and oxygen as well as carbon dioxide permeate, so that complete separation becomes difficult. Further, additional equipment such as a gas pressurizing device also becomes large.
[0013]
There are two major methods for dissolving in water, an aqueous alkaline solution, and the like. The first method uses a phenomenon in which carbon dioxide is absorbed into an aqueous sodium hydroxide solution to form a carbonate (Na ₂ CO ₃ ).
[0014]
2NaOH + CO ₂ → Na ₂ CO ₃ + H ₂ O
Although such a method is a very simple method, it is difficult to post-process the generated carbonate (Na ₂ CO ₃ ) (neutralization with hydrochloric acid), and until now it has been discarded after heating and drying. It is the current situation.
[0015]
The second method uses a thermal decomposition / absorption method of sodium hydrogen carbonate (NaHCO ₃ ).
[0016]
2NaHCO ₃ → Na ₂ CO ₃ + CO ₂ + H ₂ O (65 ° C. or higher)
Na ₂ CO ₃ + CO ₂ + H ₂ O → 2NaHCO ₃ (room temperature to 50 ° C.)
In such a method, the absorption amount and the absorption rate of sodium hydrogencarbonate for carbon dioxide are extremely slow for an aqueous sodium hydroxide solution, resulting in inefficiency.
[0017]
Therefore, the inventors have created a method for removing carbon dioxide described in Patent Document 2. In this method, an ion source material is brought into contact with a positive electrode (anode) of a solid electrolyte provided with a positive electrode (anode) and a negative electrode (cathode), and carbon dioxide and oxygen are supplied to a negative electrode (cathode) of a fixed electrolyte. By contacting the contained gas and applying a voltage between the positive and negative electrodes, carbon dioxide in the gas is separated and fixed on the negative electrode side as a carbonate. The carbonate can be reused as the ion source material at the anode.
[0018]
[Non-patent document 1]
Saburo Nagakura et al. “Dictionary of physics and chemistry”, Iwanami Shoten [0019]
[Patent Document 1]
Japanese Patent No. 2521884 (pages 2 to 4)
[0020]
[Non-patent document 2]
"Chemical Dictionary" edited by the Chemical Dictionary Editor, Kyoritsu Publishing Co., Ltd. [0021]
[Patent Document 2]
JP-A-2002-239373 (pages 2 to 4, FIG. 1)
[0022]
[Problems to be solved by the invention]
In Patent Literature 2, carbonate (mainly sodium hydrogen carbonate) discharged from the cathode side by carbon dioxide occlusion dissolves in water only at about 1 mol / l at room temperature. For this reason, the discharged aqueous solution of carbonate cannot be concentrated to the concentration of the aqueous sodium hydroxide used on the anode side even if the water is evaporated. Therefore, if this is used as it is as the ion source material, the sodium ion concentration on the anode side will decrease. This results in an increase in the dosage of the high concentration sodium source. In addition, the carbonate may precipitate without being completely dissolved and may cause clogging of the piping. These things result in the lack of stability of the carbon dioxide removing function.
[0023]
Further, with the removal of carbon dioxide, the amount of stored carbonate increases, but how to handle the carbonic acid component becomes a problem.
[0024]
The present invention has been made in view of such circumstances, and an object of the present invention is to realize a more inexpensive, efficient, and more stable removal of carbon dioxide, and a gas treatment method and a gas treatment method capable of effectively utilizing the removed carbon dioxide component. In providing the system.
[0025]
[Means for Solving the Problems]
In order to solve the above problems, the present invention is characterized by the following.
[0026]
The invention according to claim 1 is a gas processing method,
An anode and a cathode are provided on an electrolyte membrane that transmits sodium ions. An aqueous solution of a sodium compound is brought into contact with the anode, a gas to be treated is supplied to the cathode, and a DC voltage is applied between the two electrodes. In the step of separating and removing carbon dioxide in the gas in the form of a carbonate,
An anode liquid phase and a cathode liquid phase are formed through an electrolyte membrane that transmits sodium ions, and an aqueous solution of the carbonate is supplied to the anode liquid phase and water is supplied to the cathode liquid phase. Generating sodium hydroxide in the cathode liquid phase by applying a DC voltage;
Adjusting the sodium ion concentration of the aqueous solution of sodium hydroxide obtained in this step, and then supplying the aqueous solution to the anode.
[0027]
The invention according to claim 2 is a gas processing method,
An anode and a cathode are provided on an electrolyte membrane that transmits sodium ions. An aqueous solution of a sodium compound is brought into contact with the anode, a gas to be treated is supplied to the cathode, and a DC voltage is applied between the two electrodes. In the step of separating and removing carbon dioxide in the gas in the form of a carbonate,
Reacting the carbonate with an aqueous solution of hydrochloric acid to liberate the carbonate as carbon dioxide and transfer it out of the system;
An anode liquid phase and a cathode liquid phase are formed through an electrolyte membrane that transmits sodium ions. An aqueous solution of a salt generated by the reaction between a carbonate and an aqueous hydrochloric acid solution is supplied to the anode liquid phase, and water is supplied to the cathode liquid phase. Supplying a direct current voltage between the liquid phases to generate sodium hydroxide in the cathode liquid phase;
Adjusting the sodium ion concentration of the aqueous solution of sodium hydroxide obtained in this step, and then supplying the aqueous solution to the anode.
[0028]
According to a third aspect of the present invention, in the gas treatment method according to the first or second aspect, the gas to be treated is brought into contact with a droplet of water or an aqueous solution of sodium hydroxide in a scrubber and then supplied to the cathode. Features.
[0029]
According to a fourth aspect of the present invention, in the gas processing method according to the first to third aspects,
The aqueous solution of carbonate discharged from the cathode side is used as a pH adjuster in a nitrification step of water treatment.
[0030]
The invention according to claim 5 is a gas processing system,
An electrolyte membrane permeable to sodium ions, an anode provided on the electrolyte membrane to which an aqueous solution of a sodium compound is provided, and a cathode provided on the electrolyte membrane and provided with a gas to be treated; A gas treatment device for applying a voltage to separate and remove carbon dioxide in the gas in the form of carbonate on the cathode side,
An anode membrane and a cathode chamber are formed by installing an electrolyte membrane that transmits sodium ions, and a path for supplying an aqueous solution of carbonate to the anode chamber and a path for supplying water to the cathode chamber are provided. A regeneration tank that applies a DC voltage between the liquid phases to generate sodium hydroxide in the cathode chamber;
An aqueous solution of sodium hydroxide generated in the cathode chamber is supplied, and an adjustment tank is provided for controlling the concentration of sodium ion in the aqueous solution and the supply amount to the anode of the gas processing device to be constant.
[0031]
The invention according to claim 6 is a gas processing system,
An electrolyte membrane permeable to sodium ions, an anode provided on the electrolyte membrane to which an aqueous solution of a sodium compound is provided, and a cathode provided on the electrolyte membrane and provided with a gas to be treated; A gas processing device for applying a voltage to separate and remove carbon dioxide in the gas in the form of carbonate on the cathode side,
An acid supply path for providing an aqueous solution of a strong acid to the carbonate generated at the cathode,
An anode membrane and a cathode chamber are formed by installing an electrolyte membrane that allows sodium ions to pass through, and a path for supplying an aqueous solution of a salt generated by a reaction between a carbonate and a hydrochloric acid aqueous solution to the anode chamber, and supplying water to the cathode chamber. And a regeneration tank that applies a DC voltage between the liquid phases of the two chambers to generate sodium hydroxide in the cathode chamber.
An aqueous solution of sodium hydroxide generated in the cathode chamber is supplied, and an adjustment tank is provided for controlling the concentration of sodium ion in the aqueous solution and the supply amount to the anode of the gas processing device to be constant.
[0032]
According to a seventh aspect of the present invention, in the gas processing system according to any one of the fifth to sixth aspects, the gas to be processed supplied to the anode of the gas processing apparatus is brought into contact with water or an aqueous solution of sodium hydroxide. And a scrubber.
[0033]
According to an eighth aspect of the present invention, in the gas treatment system according to any one of the fifth to seventh aspects, the aqueous carbonate solution discharged from the gas treatment device is provided as a pH adjuster to the nitrification step in the water treatment system. It is characterized by.
[0034]
According to a ninth aspect of the present invention, in the gas processing method and the system according to any one of the fifth to eighth aspects, the gas to be treated is a sludge digestion gas.
[0035]
In the inventions according to claims 1 to 9, since the carbonate occluded with the removal of the carbon dioxide component is converted into an aqueous solution of sodium hydroxide, and this is used as an aqueous solution of a sodium compound to be used for carbon dioxide removal, the present invention has a significant effect. Running costs can be reduced. At this time, since the sodium ion concentration of the aqueous sodium compound solution on the anode side is kept constant, the performance of removing carbon dioxide is stabilized.
[0036]
Further, if the gas to be treated is brought into contact with a droplet of water or an aqueous solution of sodium hydroxide in a scrubber before the gas to be treated is supplied to the asode as in the invention according to claims 3 and 7, rough carbon dioxide in the gas can be obtained. Since the components are captured by the fine liquid particles, the load on the cathode is reduced, and power saving and reduction in the size of the device system can be achieved during carbon dioxide removal and hydrogen gas generation. Since the gas to be treated supplied from the scrubber contains more moisture, it is possible to generate more hydrogen gas and, at the same time, easily convert the occluded carbonate into an aqueous solution. This also enables continuous removal of carbon dioxide and generation of hydrogen gas.
[0037]
Furthermore, since the carbonate can be used as a pH adjuster, if it is applied to a water treatment system as in the inventions of claims 4 and 8, the nitrification and denitrification action in the system can be stabilized.
[0038]
If the sludge digestion gas is used as the gas to be treated as in the ninth aspect of the present invention, the methane gas in the gas can be concentrated to a high concentration. , For gas turbines and the like, and for effective use such as sale as fuel.
[0039]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0040]
(Embodiment 1)
FIG. 1 is a schematic diagram of the gas processing system of the present embodiment.
[0041]
The gas treatment apparatus 1 includes an electrolyte membrane 10 provided with an anode 11 to which a sodium compound aqueous solution (referred to as a Na ⁺ source in the figure) is supplied and a cathode 12 to which a gas to be treated is supplied. A power supply 13 for applying a voltage.
[0042]
The electrolyte membrane 10 is made of a sodium ion conductor. As the sodium ion conductor, for example, a polymer electrolyte membrane having a sodium ion exchange function is employed. Examples of the electrolyte membrane include those based on a fluororesin or a vinyl chloride resin. Furthermore, a sulfonic acid resin having a copolymer of styrene and divinylbenzene as a base, a perfluorocarbon sulfonic acid resin, a polytrifluorostyrene sulfonic acid resin, a polystyrene sulfonic acid resin, or a phenol sulfonic acid resin was formed. There is something.
[0043]
As the aqueous sodium compound solution, an aqueous sodium hydroxide (NaOH) solution, an aqueous sodium carbonate (Na ₂ CO ₃ ) solution, an aqueous sodium hydrogen carbonate (NaHCO ₃ ) solution, or the like is employed.
[0044]
Examples of the anode 11 and the cathode 12 include Pt, Au, Cr, Cu and Ni, and oxides thereof, and a porous electrode is used. The electrodes are formed on the electrolyte membrane 10 by screen printing, brushing, vapor deposition, thermal spraying, dip coating, or the like.
[0045]
The power supply 13 has a function of applying a DC voltage, and may be a known power supply. As a power supply for applying a voltage, a general constant-potential power supply (such as a potentiostat) can be used, but a power supply having a variable current density is preferably used. As one means for adjusting the current density, there is adjustment of the applied voltage.
[0046]
Here, FIG. 3 and FIG. 4 show an embodiment of the gas processing apparatus 1.
[0047]
The gas processing apparatus shown in FIG. 3A is a cell 30 configured by sandwiching an electrolyte membrane 10 having an anode 11 and a cathode 12 between an anode-side frame 31 and a cathode-side frame 32. . At this time, the anode side frame 31 is provided with a flow path for temporarily retaining a sodium compound aqueous solution (referred to as a Na ⁺ source in the figure), a supply pipe 311 for supplying the aqueous solution, and a supply pipe 311 for supplying the aqueous solution. And a discharge pipe 312 for discharging the water. Similarly, the cathode side frame 32 is provided with a flow path for temporarily storing the gas to be processed, and a supply pipe 321 for supplying the gas to be processed and a discharge pipe 322 for discharging the processing gas. , Are connected. In addition, a bypass plate 323 is provided in the flow path of the anode frame 51 and the cathode frame 32. In the cathode side frame body disclosed in FIG. 3A, one detour plate 323 is provided, but the present invention is not limited to this, and is similar to the frame bodies 31 and 32 shown in FIG. 3B. May be provided in plurality. With this configuration, the gas to be treated and the aqueous solution of the sodium compound can be uniformly supplied to the anode surface and the cathode surface of the electrolyte membrane 10, so that the carbon dioxide removal efficiency increases.
[0048]
Further, as shown in FIG. 3 (c), a plurality of gas treatment apparatuses having a cell structure are provided according to the load of the gas to be treated. The gas processing apparatus according to this embodiment has a configuration in which three cells 30 are installed. At this time, the manifolds 33, 34, 35, 36 are installed on the side surfaces of the device. A supply pipe 121 for supplying the gas to be processed is connected to the manifold 33. A discharge pipe 122 for discharging the processing gas is connected to the manifold 34. A supply pipe 111 for supplying an aqueous solution of a sodium compound is connected to the manifold 35. A discharge pipe 121 for discharging the aqueous sodium compound solution is connected to the manifold 36.
[0049]
FIG. 4A is a configuration example in a case where two electrolyte membranes 10 are stacked. The number of layers is determined by the load of the gas to be processed and the like. As shown in the figure, two electrolyte membranes 10 are stacked with a separator 42 interposed therebetween, and plates 41 and 43 are provided from above and below, respectively. At this time, the plate 41 is provided with a flow path for a sodium compound aqueous solution (a Na ⁺ source in the figure). A flow path for the gas to be treated is provided on the upper surface of the separator 42, and a flow path for the aqueous solution of the sodium compound is provided on the lower surface. The separator 43 is provided with a passage for the gas to be treated. As shown in FIG. 4C, a manifold 44 provided with a supply pipe 121 is provided on the inflow side of the gas to be processed, and a manifold 45 provided with a discharge pipe 122 is provided on the discharge side of the processing gas. You. A manifold provided with an inflow pipe 111 is provided on the inflow side of the aqueous sodium compound solution, and a manifold provided with a discharge pipe 112 is provided on the discharge side. As shown in FIG. 4B, a plurality of partition plates 421 may be provided in the channels provided in the plates 41 and 43 and the separator 42. With this configuration, the gas to be treated and the aqueous solution of the sodium compound can be uniformly supplied to the anode surface and the cathode surface of the electrolyte membrane 10, so that the carbon dioxide removal efficiency increases.
[0050]
In FIG. 1, the aqueous sodium compound solution (Na ⁺ source) is supplied from the supply tank 111 via the path 142 by the circulation pump from the regulating tank 4. Excess sodium compound aqueous solution is returned from the discharge pipe 112 via the path 140.
[0051]
The adjustment tank 4 includes a tank 14 for temporarily storing a sodium compound aqueous solution to be supplied to the anode 11. At this time, the sodium ion concentration in the liquid phase in the tank 14 is adjusted to be constant. To this end, the adjustment tank 4 includes an ion concentration measuring unit 143 for measuring the sodium ion concentration, and a path 141 for supplying a high-concentration sodium ion source and water. Valve means for opening and closing based on the ion concentration of the inner liquid phase is provided. The ion concentration measuring means has, for example, a pH measuring function, and calculates the concentration of sodium ions based on the pH value.
[0052]
The gas to be processed is supplied into the gas processing apparatus 1 from the supply pipe 121 via the path 100 by a transfer means such as a blower or a fan (not shown). The gas from which carbon dioxide has been removed passes through the gas-liquid separation tank 3 from the discharge pipe 122 via the path 31 and is then transferred out of the system via the path 32. The gas-liquid separation tank 3 cleans the gas introduced by retaining the aqueous solution of carbonate.
[0053]
In the present embodiment, the path 100 is provided with the water supply means 2 for supplying water (or steam) to the gas to be treated. The water supply means 2 may be a known one as long as it can supply moisture to the gas to be treated. For example, a spray type is used. Since the carbonate generated at the cathode 12 can be easily converted to an aqueous solution by the water supply means 2, the carbonate can be easily removed from the system, and continuous carbon dioxide can be removed. The aqueous carbonate solution is collected in the gas-liquid separation tank 3 and can be reused as a sodium ion source.
[0054]
Further, in this embodiment, a regeneration tank 5 for obtaining an aqueous solution of sodium hydroxide from the stored carbonate is provided.
[0055]
FIG. 5 schematically shows an embodiment of the regeneration tank 5. FIG. 5A shows an example of a case where a carbonate aqueous solution is provided, and FIG. 5B shows an example of a case where a hydrogen carbonate aqueous solution is provided.
[0056]
The regeneration tank 5 has a two-tank structure in which a tank 50 to which a carbonate aqueous solution is supplied is partitioned by an ion conductor 54. An anode 51 capable of conducting with the anode of the DC power supply 53 is installed in a tank to which the aqueous carbonate solution is supplied, and a cathode capable of supplying pure water to the other tank and conducting with the cathode of the power supply 63. 52 are provided. The ion conductor 54 may be a known one as long as it can pass sodium ions and hydrogen ions. Further, as the ionic conductor 54, one that simultaneously transmits sodium ions (Na ⁺ ) and hydrogen ions (H ⁺ ) or one that combines two ionic conductors, each of which transmits ions, is employed. Examples of the ion conductor include a sodium ion conductive ceramic such as NASICON and β-alumina, and a sodium ion permeable electrolyte membrane represented by Nafion. Other electrolyte membranes include those formed by incorporating the above-mentioned fluorine resin, vinyl chloride resin, sulfonic acid resin, or the like.
[0057]
Next, an operation example of the gas processing system of the present embodiment will be described with reference to FIGS. Here, a case where the sodium ion source is an aqueous sodium hydroxide solution (hereinafter, NaOH aqueous solution) and the gas to be treated is a gas containing CH ₄ and CO ₂ will be described.
[0058]
The carbon dioxide gas removal in the gas processing system is performed by applying a DC voltage between the anode 11 and the cathode 12.
[0059]
That is, in FIG. 1, an NaOH aqueous solution is supplied from the supply pipe 111 to the anode 11 side, and the anode 11 is immersed in this sodium ion source. A gas to be processed is supplied from the supply pipe 121 to the cathode 12 side. Here, water or steam is injected from the water supply means 2 into the gas to be treated. Then, a DC voltage is applied from the power supply 13 between the anode 11 and the cathode 12. At this time, first, the following reaction occurs on the surface of the anode 11 contacted with NaOH.
[0060]
^{NaOH → Na + + OH - ...} (1)
2OH ⁻ → 1 / 2O ₂ + H ₂ O + 2e ⁻ (2)
Then, sodium ions (Na ⁺ ) pass through the anode 11, migrate through the electrolyte membrane 10, and reach the cathode 12. On the other hand, the electrons (e ⁻ ) released on the anode 11 side reach the cathode 12 via the power supply 13.
[0061]
Here, when carbon dioxide (CO ₂ ) and moisture (H ₂ O) in the gas phase come into contact with the surface of the cathode 12, the following reaction occurs.
[0062]
Na ⁺ + e ⁻ + CO ₂ + H ₂ O → NaHCO ₃ + 1 / 2H ₂ (3)
2Na ⁺ + CO ₂ + H ₂ O + 2e ⁻ → Na ₂ CO ₃ + H ₂ (4)
In this manner, carbon dioxide contained in the gas to be treated is separated and removed by being fixed on the surface of the cathode 12 in the form of carbonate. Then, hydrogen gas is generated with this reaction. At this time, the generated carbonate exists in a state of being dissolved in water, and is washed off from the electrode surface at the same time as the generation. Then, the generated aqueous carbonate solution is separated from the gas in the gas-liquid separation tank 3. In this way, carbon dioxide can be continuously removed.
[0063]
Here, as shown in the figure, it is more preferable that the path 100 includes a scrubber 15 for bringing the droplet into contact with the gas to be treated. The scrubber 15 employs, for example, a spray method. Examples of the washing water that becomes droplets include water and an aqueous sodium hydroxide solution.
[0064]
When the gas to be treated is supplied to the scrubber 15, the carbon dioxide component in the gas comes into contact with the droplets and is captured by the droplets. Thereby, a rough carbon dioxide component in the gas to be treated is removed. Further, along with this, other components contained in the gas, such as hydrogen sulfide, sulfur oxides, and fine dust components, are removed, so that when the gas to be treated is introduced into the gas treatment apparatus 1, The contact efficiency between the carbon component and the cathode 12 increases. The gas that has passed through the scrubber 15 is immediately supplied to the gas processing device 1 to remove the remaining carbon dioxide component. Since the gas treated by the scrubber 15 further contains a water component, the carbonate generated at the cathode 12 can be easily converted to an aqueous solution, and the reactions (3) and (4) can be promoted. By providing the scrubber 15 in this way, not only can the carbon dioxide load on the gas treatment apparatus 1 be reduced, but also not only can the carbon dioxide dioxide be removed more efficiently, but also hydrogen can be generated.
[0065]
On the other hand, when the carbonate aqueous solution is supplied to the regeneration tank 5, it is converted into a sodium hydroxide aqueous solution as shown in FIGS. 5 (a) and 5 (b).
[0066]
When sodium carbonate is provided, as shown in FIG. 5A, sodium ions (Na +) and carbonate ions (CO ₃ ²⁻ ), and water contains some hydrogen ions (H ⁺ ) and hydroxide ions. (OH ⁻ ). In this state, when a potential is applied with the sodium carbonate aqueous solution side as the anode (+) and the pure water side as the cathode (-), sodium ions and hydrogen ions permeate the ionic conductor membrane. At this time, on the anode (+) side, carbonate ions and hydroxide ions emit electrons under the influence of equilibrium partial pressure and potential, and carbonate ions become carbon dioxide and oxygen, and hydroxide ions become oxygen. Released into the atmosphere. Conversely, sodium ions transmitted to the cathode (−) side receive electrons from the electrode and become metallic sodium, but act with water (H ₂ O) to produce NaOH and H ₂ . The permeated hydrogen ions also receive electrons from the electrodes and become hydrogen gas. After a sufficient time has passed, the cathode (-) side is a high-concentration aqueous sodium hydroxide solution, and the aqueous sodium carbonate solution can be separated and recovered as an aqueous sodium hydroxide solution. In addition, since this method also has electrolysis of water, it can be used as a hydrogen supply source. The sodium hydroxide solution thus obtained is supplied to the adjustment tank 4 by a transfer means such as a pump (not shown), and the concentration of sodium ions is adjusted to a predetermined concentration (for example, 5 to 10 mol / l). Later, it is supplied to the anode 11 of the gas processing device 1 as appropriate. Reactions relating to NaOH regeneration are summarized below.
[0067]

2Na ⁺ and 2H ⁺ move to the cathode side via the ionic conductor 64.
[0068]

Further, when sufficient water (H ₂ O) is present in the cathode 12 of the gas processing apparatus 1, when the temperature is low, the aqueous solution may not be completely converted to an aqueous sodium carbonate solution but may be changed to an aqueous sodium hydrogen carbonate solution.
[0069]
FIG. 5B shows an embodiment in the case of regenerating an aqueous sodium hydroxide solution from this sodium hydrogen carbonate. The principle of regeneration of the aqueous sodium hydroxide solution is the same as in FIG. That is, the aqueous sodium hydrogen carbonate solution on the anode 51 side is ionized into sodium ions and carbonate ions, and emits electrons to the electrode, thereby releasing carbon dioxide and oxygen. The separated sodium ions and hydrogen ions move to the cathode 52 side through the ionic conductor 54, react with water to generate hydrogen, and become sodium hydroxide. The obtained hydrogen gas can be used as fuel gas. The aqueous sodium hydroxide solution thus obtained is supplied to the adjustment tank 4 by a transfer means such as a pump (not shown), and the concentration of sodium ions is adjusted to a predetermined concentration (for example, 5 to 10 mol / l). Later, it is supplied to the anode 11 of the gas processing device 1 as appropriate. Reactions relating to NaOH regeneration are summarized below.
[0070]

2Na ⁺ and 2H ⁺ move to the cathode side via the ionic conductor 64.
[0071]

As described above, according to the gas processing system of the present embodiment, the sodium ion source can be recovered and reused, and carbon dioxide can be removed at low cost.
[0072]
When the gas to be treated is a gas containing methane, for example, a gas such as a sludge digestion gas, by supplying this gas to the gas treatment system of the present embodiment, a concentrated methane gas can be obtained efficiently. This concentrated methane gas can be used effectively, for example, sold as fuel for a fuel cell or a gas turbine or other fuel.
[0073]
The digestive gas of sludge generated in sewage treatment plants and the biogas generated in livestock manure can be used as a useful energy source, but conventional technology finds use only as a heat source especially in small and medium capacity treatment plants I couldn't do that. One reason is that the methane gas concentration is low. Then, by removing the carbon dioxide component in the sludge digestion gas by the gas treatment system, the methane gas concentration in the gas can be increased, and the sludge digestion gas can be converted to a high calorific value gas.
[0074]
In particular, if the gas treatment system of the present invention is provided in the sewage treatment system such as the embodiment shown in FIG. 2, the sludge digestion gas can be effectively used and the nitrification and denitrification Function can be stabilized.
[0075]
In the water treatment system shown in FIG. 2, the equipment for obtaining sludge digestion gas is mainly a sewage treatment facility and has a nitrification step and a denitrification step. The liquid phase in the nitrification step and the denitrification step is subjected to solid-liquid separation in the final sedimentation basin. The supernatant water obtained in the final sedimentation basin is subjected to further advanced treatment or discharged. Here, a part of the sludge separated in the final sedimentation basin is returned to the nitrification step for reuse as activated sludge, while a part of the sludge is subjected to a sludge digestion step for digestion. In the sludge digestion process, sludge is treated aerobically or anaerobically. Particularly, in the process of anaerobic treatment, gas containing methane gas is generated.
[0076]
In general sewage treatment, first, in a nitrification step, a nitrogen compound (ammoniacal nitrogen) in the sewage is converted into nitrate ions or the like (NO ₂ ⁻ , NO ₃ ⁻ ) by the action of a nitrifying bacterium. Needs to be reduced to nitrogen gas (N ₂ ). The pH of the sewage in the nitrification step is in the range of 8 to 9, but as the nitrification reaction proceeds, alkalinity is consumed and the pH value decreases. When the pH value decreases, not only does the treatment efficiency decrease, but also the nitrifying bacteria may die. Therefore, in order to adjust the pH value to 8-9, slaked lime (Ca _(OH) 2), soda ash _(Na 2 _Co 3), no alkaline agent such as sodium hydroxide (NaOH) and (pH adjusting agent) were charged Must.
[0077]
The pH value of the aqueous carbonate solution (waste solution) discharged from the cathode 12 side of the gas processing apparatus 1 is about 8 to 10 (when the carbon dioxide is not completely absorbed, the NaOH component is not contained). The pH value becomes higher because of a higher pH), and can be used as an alkali agent. Therefore, by providing the sewage treatment system with the gas treatment system provided with the gas treatment device 1, it is possible to stabilize the production of a high calorific value gas and the nitrification and denitrification function in the sewage treatment.
[0078]
An example of the gas processing system according to the present embodiment is described below.
[0079]
FIG. 8 is a characteristic diagram showing the relationship between the current density and the concentrations of carbon dioxide and hydrogen.
[0080]
As the gas treatment system according to the experiment, a system using the gas treatment device shown in FIG. 4 in the system shown in FIG. 1 was adopted. Here, a gas to be treated (gas component: carbon dioxide: 40%, nitrogen: 60%) was supplied at a rate of 1 l / min to a gas treatment apparatus in which two electrolytes each having a cell size of 200 mm × 200 mm were laminated (effective electrode area: 600 cm ² ). It was supplied at a flow rate. At this time, the gas to be processed was supplied to the cathode of the gas processing apparatus without passing through a scrubber. The applied DC voltage was set to 2.5V. Further, a fluorine resin-based electrolyte was used, an aqueous sodium hydroxide solution was used for the aqueous solution of the sodium compound, and a nickel electrode was used for the electrode.
[0081]
As is clear from the results shown in the figure, it can be confirmed that the carbon dioxide concentration decreases as the current density increases. In this example, it was also confirmed that carbon dioxide was completely removed at a current density of 100 mA / cm ² .
[0082]
Here, the power required for carbon dioxide removal, the amount of carbon dioxide generated by the power used (1 minute), the fixed amount of carbon dioxide (1 minute), and the amount of hydrogen generated (1 minute) calculated based on the results of the example are as follows. It was shown to.
[0083]
Electric power required for carbon dioxide removal: 2.5 [V] × 60 [A] = 150 [W]
Amount of carbon dioxide generated by electric power used (1 minute): 150 × 1/1000 × 1/60 × 0.12 / 44 = 0.007 [mol] Note that 0.12 represents the case where oil-fired power was used as a power source. Shows carbon dioxide emissions.
[0084]
Fixed amount of carbon dioxide (1 minute): 0.4 [l / min] x 1 [min] /22.4 [l] = 0.018 [mol]
Hydrogen generation amount (1 minute): 0.018 [mol]
As is apparent from the calculation results, it can be confirmed that carbon dioxide can be efficiently removed with a small amount of electric power, and hydrogen in an amount equivalent to the fixed carbon dioxide is generated.
[0085]
Further, Table 1 shows that the gas discharged from each processing step when the gas to be processed is supplied to the gas processing device after being supplied to the scrubber, that is, the gas that has passed through the scrubber (hereinafter, the first processing gas) and the gas processing device The composition of the gas (hereinafter, referred to as the second processing gas) discharged from the apparatus together with the gas composition of the gas to be processed is disclosed. Here, the gas was processed under the same conditions as in the example shown in FIG. 5 except that the composition of the gas to be processed was methane: carbon dioxide = 60: 40.
[0086]
[Table 1]

[0087]
As shown in the table, it was confirmed that the methane gas concentration of the first processing gas was 85% and the carbon dioxide concentration was 15%. Furthermore, in the second processing gas, no carbon dioxide was detected, and it was confirmed that the concentration of the combustible gas was 100%. This suggests that by combining the scrubber with the gas treatment device of the present invention, the carbon dioxide load on the gas treatment device can be reduced, thereby achieving power saving in carbon dioxide removal and downsizing of the device system. is there. As is apparent from the methane gas concentration, the second processing gas can be used for fuel such as a fuel cell, a gas turbine, and the like, and can be provided for effective use such as being sold as a fuel.
[0088]
FIG. 9 is a characteristic diagram showing an IV characteristic according to the embodiment of the present invention.
[0089]
The gas processing apparatus according to the first embodiment employs an electrolyte membrane made of a vinyl chloride resin as an electrolyte, and the gas processing apparatus according to the comparative example employs a ceramic electrolyte. In Example 1 and Comparative Example, a 5 mol / l aqueous solution of sodium hydroxide was used as the aqueous solution of the sodium compound, and the operating temperature was room temperature of 20 ° C. As is clear from the results shown in the figure, in Example 1 in which the electrolyte membrane of the vinyl chloride resin was used as the electrolyte, the rise of the current was improved after the reaction started, and the efficiency was significantly reduced as compared with the ceramic electrolyte. It can be confirmed that electric power is possible.
[0090]
Here, FIG. 10 shows the carbon dioxide removal characteristics according to the first embodiment. The characteristic test was performed by a rated current method of 100 mA / cm ² . The electrode area was set to 300 cm ² . As is apparent from the figure, it can be confirmed that carbon dioxide having an initial concentration of about 20% starts to be removed at the same time as the current starts flowing, and can be reduced to about 0.6% in about 20 minutes. At this time, it was confirmed that when the current was stopped, the carbon dioxide concentration started to increase. And in this test, it was confirmed that the removal rate of carbon dioxide was 97%. It was also confirmed that the removal rate of carbon dioxide was kept almost constant while the current was flowing. Further, it has been confirmed that if the supply of the sodium ion supply source is continued, carbon dioxide is continuously stored.
[0091]
FIG. 11 is a characteristic diagram showing an IV characteristic according to the embodiment of the present invention.
[0092]
The gas treatment apparatus according to Example 2 employed a fluororesin-based electrolyte membrane as an electrolyte, and the gas treatment apparatus according to Comparative Example employed a ceramics-based electrolyte. In Example 2 and Comparative Example, a 5 mol / l sodium hydroxide aqueous solution was employed as the sodium compound aqueous solution, and the operating temperature was 86 ° C. As is evident from the results shown, in Example 2 in which the fluororesin-based electrolyte membrane was employed as the electrolyte, the rise of current was improved after the reaction started, and the power consumption was significantly reduced compared to the ceramic-based electrolyte. It can be confirmed that is possible.
[0093]
(Embodiment 2)
FIG. 6A is a schematic diagram illustrating the gas processing system of the present embodiment. FIG. 6B is a schematic configuration diagram of a regeneration tank 6 provided in the system.
[0094]
The gas processing system according to the present embodiment includes a step of releasing and releasing the carbon dioxide component absorbed in the cathode 12 by a strong acid in the step of removing carbon dioxide, and a step of regenerating sodium hydroxide from the salt generated in this step. Have. For example, hydrochloric acid is used as the strong acid.
[0095]
The gas processing device 1 has the same configuration as the gas processing device according to the first embodiment. The description of the configuration is left to the embodiment. In the present embodiment, a path for supplying a strong acid aqueous solution is connected to a path for introducing the gas to be treated via a three-way valve V1 (hereinafter referred to as V1). Further, a path connecting the gas processing apparatus 1 and the gas-liquid separation tank 3 is provided for transferring the aqueous sodium chloride solution generated on the cathode 12 side to the regeneration tank 6 via a three-way valve V2 (hereinafter referred to as V2). Are connected.
[0096]
As shown in FIG. 8B, the regenerating tank 6 has a two-tank structure in which a tank 60 to which an aqueous solution of sodium chloride is supplied is partitioned by an ionic conductor 64. An anode 61 capable of conducting with the anode of the DC power supply 63 is installed in a tank to which the aqueous sodium chloride solution is supplied, and a cathode 62 capable of supplying water and conducting with the cathode of the power supply 63 in the other tank. Is installed. Any known materials can be used as long as they can pass sodium ions. For example, there is an electrolyte membrane formed by including the fluorine resin, vinyl chloride resin, sulfonic acid resin, or the like described in the first embodiment.
[0097]
An operation example of the gas processing system of the present embodiment will be described with reference to FIGS.
[0098]
In the carbon dioxide removing step, as shown in FIG. 7A, V1 is controlled so as to supply the gas to be processed to the gas processing apparatus 1. V2 is controlled so as to supply the processing gas to the gas-liquid separation tank 3. The operation of the cathode 12 in this step will be described in the first embodiment.
[0099]
In the carbon dioxide releasing step, V1 is controlled so as to supply the strong acid aqueous solution to the gas treatment device 1 as shown in FIG. V2 is controlled so as to supply the processing gas to the gas-liquid separation tank 3. The process focuses on the fact that sodium carbonate obtained by capturing the carbon dioxide component contained in the gas to be treated in the carbon dioxide removal process is a compound of a weak acid and a strong base, and the removal of the weak acid carbon dioxide is performed. I have. That is, the following reaction is caused by spraying a strong acid, here a hydrochloric acid aqueous solution, on the carbonate to transfer the carbon dioxide to the outside of the system for taking out carbon dioxide.
[0100]
Na ₂ CO ₃ + 2HCl → 2NaCl + H ₂ O + CO ₂
When sodium bicarbonate is obtained, the following reaction is caused by spraying a strong acid (for example, aqueous hydrochloric acid) to remove carbon dioxide and transfer it to the outside of the system.
[0101]
NaHCO ₃ + HCl → NaCl + H ₂ O + CO ₂
The carbonate is immediately decomposed by this reaction, and carbon dioxide is liberated. At this time, the aqueous sodium chloride solution which is a by-produced sodium salt is subjected to a sodium ion source regeneration step. FIG. 12 discloses an example of experimental results showing occlusion and release of carbon dioxide in the present embodiment. According to the figure, the occluded amount and the released amount of carbon dioxide do not match. This is considered to be because the compound obtained on the surface of the cathode 12 in the carbon dioxide removing step is not sodium carbonate but sodium bicarbonate, which has been washed away in the form of an aqueous solution.
[0102]
Next, in the step of regenerating sodium hydroxide, as shown in FIG. 6B, the anode of the DC power supply 63 is connected to the anode 61, and the cathode of the power supply 63 is connected to the cathode 62. Applied. At this time, electrolysis proceeds, and the water component is decomposed into hydroxide ions and hydrogen gas in the liquid phase on the cathode 62 side. Here, the by-produced hydrogen gas can be used not only as a fuel gas but also as a raw material for hydrochloric acid together with the chlorine gas generated together, so that the hydrochloric acid used for the carbon dioxide releasing step is transported out of the system and used in the carbon dioxide releasing step. Can be subjected to generation. On the other hand, sodium ions move to the water tank on the cathode 52 side via the ionic conductor 64, and the liquid phase in the water tank becomes an aqueous solution of sodium hydroxide. The aqueous sodium hydroxide solution thus obtained is supplied to the adjustment tank 4 by a transfer means such as a pump (not shown), and the concentration of sodium ions is adjusted to a predetermined concentration (for example, 5 to 10 mol / l). Later, the gas is supplied to the anode 11 of the gas processing device 1 as appropriate. The reactions in this step are summarized below.
[0103]

[0104]

In the present embodiment, it is more preferable that the processing gas supply line to the gas processing apparatus 1 be provided with a scrubber 15 for bringing droplets into contact with the processing gas in the same manner as in the first embodiment. Further, as in the first embodiment, if the gas treatment system of the present embodiment is provided in the sewage treatment system, the sludge digestion gas can be effectively used, and the pH of the carbonate aqueous solution can be adjusted by the water treatment. If provided for the system, the nitrification and denitrification function in water treatment can be stabilized.
[0105]
【The invention's effect】
As is apparent from the above description, according to the gas treatment method and the system according to the present invention, it is possible to realize more inexpensive, efficient and stable removal of carbon dioxide, and to effectively use the removed carbon dioxide component. Can be used
[0106]
In particular, in the present invention, since the sodium ion concentration of the aqueous solution of the sodium compound on the anode side is kept constant, the performance of removing carbon dioxide is stable.
[0107]
Further, since the occluded carbonate is converted into an aqueous solution of sodium hydroxide along with the removal of the carbon dioxide component and this is used as an aqueous solution of a sodium compound to be used for carbon dioxide removal, a significant reduction in running costs can be achieved. .
[0108]
Furthermore, before supplying the gas to be treated to the asode, the rough carbon dioxide component in the gas is captured by the fine liquid particles by being brought into contact with a droplet of water or an aqueous solution of sodium hydroxide in a scrubber. , The power consumption during the removal of carbon dioxide and the generation of hydrogen gas can be reduced, and the size of the device system can be reduced. Further, since the gas to be treated supplied from the scrubber contains more moisture, it is possible to generate more hydrogen gas, and at the same time, it is possible to easily convert the occluded carbonate into an aqueous solution, so that the It is also possible to remove carbon dioxide and generate hydrogen gas.
[0109]
Since the aqueous carbonate solution discharged from the present invention can be used as a pH adjuster, by supplying it to a water treatment system, the nitrification and denitrification action in this system can be stabilized.
[0110]
Further, when the gas to be treated is a gas containing methane such as a sludge digestion gas, the present invention can efficiently produce a concentrated methane gas (high calorific gas) from the gas. Can be sold and used as fuel cells, gas turbines, or other fuels, thus enabling effective use of greenhouse gases.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an embodiment of a gas processing system of the present invention.
FIG. 2 is a schematic diagram showing an embodiment of a gas processing system of the present invention.
FIG. 3 is a schematic diagram showing an embodiment of a gas processing apparatus.
FIG. 4 is a schematic view showing an embodiment of a gas processing apparatus.
5A is a schematic explanatory view of a regeneration tank to which an aqueous sodium carbonate solution is supplied, and FIG. 5B is a schematic explanatory view of a regeneration tank to which an aqueous sodium hydrogen carbonate solution is supplied.
6A is a schematic diagram showing an embodiment of a gas processing system according to the present invention, and FIG. 6B is a schematic explanatory diagram of a regeneration tank to which an aqueous sodium chloride solution is supplied.
FIG. 7A is a schematic explanatory view of a carbon dioxide removing step, and FIG. 7B is a schematic explanatory view of carbon dioxide release.
FIG. 8 is a characteristic diagram showing a relationship between a current density, a carbon dioxide concentration, and a hydrogen concentration.
FIG. 9 is an IV characteristic diagram in the present invention (Example 1) and a comparative example.
FIG. 10 is a characteristic diagram of carbon dioxide removal according to the present invention.
FIG. 11 is an IV characteristic diagram of the present invention (Example 2) and a comparative example.
FIG. 12 is an example of an experimental result showing occlusion and release of carbon dioxide.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Gas processing apparatus, 10 ... Electrolyte membrane, 11 ... Anode, 12 ... Cathode, 13 ... Power supply 15 ... Scrubber 2 ... Water supply means 3 ... Gas-liquid separation tank 4 ... Adjustment tank, 14 ... Tank, 143 ... Ion concentration measurement Means 5, 6 Regeneration tank 7 Gas processing apparatus 50, 60, 70 Tank, 51, 61, 71 Anode, 52, 62, 72 Cathode, 53, 63, 73 Power supply, 54, 64, 74 Ion conductors 8, 9 ... carbon dioxide absorption tank

Claims (9)

An anode and a cathode are provided on an electrolyte membrane that transmits sodium ions. An aqueous solution of a sodium compound is brought into contact with the anode, a gas to be treated is supplied to the cathode, and a DC voltage is applied between the two electrodes. In the step of separating and removing carbon dioxide in the gas in the form of a carbonate,
An anode liquid phase and a cathode liquid phase are formed through an electrolyte membrane that transmits sodium ions, and an aqueous solution of the carbonate is supplied to the anode liquid phase and water is supplied to the cathode liquid phase. Generating sodium hydroxide in the cathode liquid phase by applying a DC voltage;
Adjusting the sodium ion concentration of the aqueous solution of sodium hydroxide obtained in this step, and then supplying the aqueous solution to the anode. An anode and a cathode are provided on an electrolyte membrane that transmits sodium ions. An aqueous solution of a sodium compound is brought into contact with the anode, a gas to be treated is supplied to the cathode, and a DC voltage is applied between the two electrodes. In the step of separating and removing carbon dioxide in the gas in the form of a carbonate,
Reacting the carbonate with an aqueous solution of hydrochloric acid to liberate the carbonate as carbon dioxide and transfer it out of the system;
An anode liquid phase and a cathode liquid phase are formed through an electrolyte membrane that transmits sodium ions. An aqueous solution of a salt generated by the reaction between a carbonate and an aqueous hydrochloric acid solution is supplied to the anode liquid phase, and water is supplied to the cathode liquid phase. Supplying a direct current voltage between the liquid phases to generate sodium hydroxide in the cathode liquid phase;
Adjusting the sodium ion concentration of the aqueous solution of sodium hydroxide obtained in this step, and then supplying the aqueous solution to the anode. The gas treatment method according to claim 1, wherein the gas to be treated is supplied to the cathode after being brought into contact with water or a droplet of an aqueous solution of sodium hydroxide in a scrubber. The gas treatment method according to any one of claims 1 to 3, wherein the aqueous solution of carbonate discharged from the cathode side is used as a pH adjuster in a nitrification step of water treatment. An electrolyte membrane permeable to sodium ions, an anode provided on the electrolyte membrane to which an aqueous solution of a sodium compound is provided, and a cathode provided on the electrolyte membrane and provided with a gas to be treated; A gas treatment device for applying a voltage to separate and remove carbon dioxide in the gas in the form of carbonate on the cathode side,
An anode membrane and a cathode chamber are formed by installing an electrolyte membrane that transmits sodium ions, and a path for supplying an aqueous solution of carbonate to the anode chamber and a path for supplying water to the cathode chamber are provided. A regeneration tank that applies a DC voltage between the liquid phases to generate sodium hydroxide in the cathode chamber;
An aqueous solution of sodium hydroxide generated in the cathode chamber is supplied, and a regulating tank for controlling the sodium ion concentration of the aqueous solution and the supply amount to the anode of the gas treatment device to a constant level is provided. system. An electrolyte membrane permeable to sodium ions, an anode provided on the electrolyte membrane to which an aqueous solution of a sodium compound is provided, and a cathode provided on the electrolyte membrane and provided with a gas to be treated; A gas processing device for applying a voltage to separate and remove carbon dioxide in the gas in the form of carbonate on the cathode side,
An acid supply path for providing an aqueous solution of a strong acid to the carbonate generated at the cathode,
An anode membrane and a cathode chamber are formed by installing an electrolyte membrane that allows sodium ions to pass through, and a path for supplying an aqueous solution of a salt generated by a reaction between a carbonate and a hydrochloric acid aqueous solution to the anode chamber, and supplying water to the cathode chamber. And a regeneration tank that applies a DC voltage between the liquid phases of the two chambers to generate sodium hydroxide in the cathode chamber.
An aqueous solution of sodium hydroxide generated in the cathode chamber is supplied, and a regulating tank for controlling the sodium ion concentration of the aqueous solution and the supply amount to the anode of the gas treatment device to a constant level is provided. system. The gas treatment according to any one of claims 1 to 6, further comprising a scrubber for bringing a gas to be treated supplied to an anode of the gas treatment device into contact with water or a droplet of an aqueous solution of sodium hydroxide. system. The gas treatment system according to any one of claims 1 to 7, wherein the aqueous carbonate solution discharged from the gas treatment device is used as a pH adjuster in a nitrification step in the water treatment system. The gas processing method and system according to any one of claims 1 to 8, wherein the gas to be processed is a sludge digestion gas.

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