JP2004311168A - Power generation method by fuel cell and fuel cell power generation system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance fuel utilization factor of a fuel cell power generation system. <P>SOLUTION: Fuel gas containing hydrogen is produced by reacting hydrocarbon base fuel with steam in a reforming reactor 2, the fuel gas is circulated, a fuel electrode 31 of a first fuel cell 3 using an oxide ion conductor 30 as an electrolyte and a fuel electrode 41 of a second fuel cell 4 using a proton conductor 40 as the electrolyte are arranged so as to contact with the fuel gas, an air electrode 32 of the first fuel cell 3 and an air electrode 42 of the second fuel cell 4 are arranged so as to contact with oxidizing agent gas flowing through the outside of the reforming reactor 2. Hydrogen produced with the reforming rector 2 by power generation with the first fuel cell 3 is consumed and steam is supplied to the inside of the reforming reactor 2, hydrogen produced with the reforming reactor 2 by power generation with the second fuel cell 4 is consumed and steam is exhausted to the outside of the reforming reactor 2. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【数２】

【数３】

【００２５】
以下に、第１燃料電池３に用いる材料の好適例について説明する。第１燃料電池３の電解質３０に用いる材料としては、例えばイットリア安定化ジルコニア（Ｚｒ_０．９Ｙ_０．１Ｏ_１．９５）、ガドリアドープセリア（Ｃｅ_０．８Ｇｄ_０．２Ｏ_１．９）、ランタンガレート（Ｌａ_０．８Ｓｒ_０．２Ｇａ_０．８５Ｍｇ_０．１５Ｏ_{２．８２５}）などが、５００〜１０００℃付近の温度域で使用可能であり、酸化物イオン導電性を良好に示すことから、好適例として挙げられる。
【００２６】
また、第１燃料電池３の空気極３２に用いる材料としては、例えばＬａ_１−ｘＡＥ_ｘＣｏ_１−ｙＦｅ_ｙＯ_３やＬａ_１−ｘＡＥ_ｘＭｎ_１−ｙＣｏ_ｙＯ_３など（ＡＥ＝Ｃａ，Ｓｒ等のアルカリ土類金属元素）のペロブスカイト酸化物が、好適例として挙げられる。尚、Ｌａ_１−ｘＡＥ_ｘＣｏ_１−ｙＦｅ_ｙＯ_３の場合、一般に０．１≦ｘ≦０．４、０．１≦ｙ≦０．４の組成範囲が、酸素を酸素イオン化する触媒反応を良好に示すので好ましい。また、Ｌａ_１−ｘＡＥ_ｘＭｎ_１−ｙＣｏ_ｙＯ_３の場合は、０．１≦ｘ≦０．４、０≦ｙ≦０．４の組成範囲が一般に好ましい。特に、Ｍｎ系ペロブスカイト酸化物は、（Ｌａ_１−ｘＡＥ_ｘ）_１−ａＭｎ_１−ｙＣｏ_ｙＯ_３も、化学的に安定な電極材料として存在でき、０≦ａ≦０．１が好ましい組成範囲として考えられる。空気極３２の作製法は特に限定されず、例えば固相反応法、クエン酸法、共沈法などの既知の方法によって作製して良い。出発物質としては、例えば酸化ランタン（Ｌａ_２Ｏ_３）、アルカリ土類金属炭酸塩（ＣａＣＯ_３、ＳｒＣＯ_３等）、酸化マンガン（Ｍｎ_２Ｏ_３）、酸化コバルト（Ｃｏ_３Ｏ_４）、酸化鉄（Ｆｅ_２Ｏ_３）等を用いることができる。一例としてＬａ_０．８Ｓｒ_０．２Ｍｎ_０．９Ｃｏ_０．１Ｏ_３よりなる空気極３２を固相反応法により作製する場合について説明すると、出発物質である酸化ランタン、炭酸ストロンチウム、酸化マンガン、酸化コバルトのモル比が、０．８：０．２：０．９：０．１となるように秤量して混合し、１２００℃で１０時間程度仮焼した後、もう一度粉砕し混合し焼成を行って、ペロブスカイト形構造を有する酸化物の粉体とする。当該粉体をテレピン油などの有機溶剤を用いて電解質３０上にコーティングし、乾燥後、１２００℃程度かつ４時間程度で焼結させて、空気極３２とする。
【００２７】
また、第１燃料電池３の燃料極３１に用いる材料としては、ＴＥ（ＴＥ＝Ｎｉ，Ｆｅ，Ｃｏ，Ｃｕ）金属とセラミックスの混合物（ＴＥ系セリアサーメットやＴＥ系ジルコニアサーメットなど）が、好適例として挙げられる。尚、ＴＥ金属とセラミックスのサーメットは多くの実験結果とｐｅｒｃｏｌａｔｉｏｎ理論から、一般的に、ＴＥ金属が約４０体積％程度含まれると充分な導電率が得られ、４０体積％のＴＥ金属と６０体積％のセラミックスから構成されるサーメットの電極特性が特に優れたものが多いことが知られている。燃料極３１の作製法は特に限定されず、例えば固相混合法、噴霧熱分解法、燃焼合成法などの既知の方法によって作製して良い。一例として、ニッケルジルコニアサーメットよりなる燃料極３１を固相混合法により作製する場合について説明すると、イットリア安定化ジルコニア（ＹＳＺ）の粗い粒子と細かい粒子、および酸化ニッケルを混合し、その後、テレピン油などの有機溶剤を用いて電解質３０上にコーティングし、乾燥後、１４００℃程度かつ５時間程度で焼結させて、燃料極３１とする。この燃料極３１は、燃料（水素）が供給されると酸化ニッケルが還元され、ニッケルジルコニアサーメットとなる。
【００２８】
プロトン導電体４０を電解質に用いた第２燃料電池４の原理を図４を用いて説明する。この第２燃料電池４では、燃料極４１に水素を含有した燃料ガスを供給し（図４中の符号１１は水素分子を示す）、空気極４２に酸化剤ガス（例えば空気）を供給することにより（図４中の符号１２は酸素分子を示し、符号１３は窒素分子を示す）、燃料ガス中の水素が水素イオン（プロトン）に変化して、プロトン導電体４０を通って空気極４２側に移動する。空気極４２側では、空気中の酸素が酸素イオンに変化しており、この酸素イオンが電解質４０を移動してきたプロトンと結合し、空気極４２側に水を発生させる。換言すれば、第２燃料電池４では、改質反応器２内の水素を改質反応器２の外側に移動させ、改質反応器２の外側の酸素と反応させて水に変化させる。このときの空気極４２および燃料極４１における電気化学的な反応により第２燃料電池４に電圧が発生する。空気極４２と燃料極４１とを電子を流す導体で接続して外部回路を組めば、燃料極４１から空気極４２へ外部回路を通って電子が流れ、外部回路の中で負荷１５をとれば、電子流はその電位差によって仕事をする（即ち発電が行われる）。尚、第２燃料電池４の部材に電子が流れることでジュール熱が発生する。このジュール熱は、例えば空気極４２側で発生した水を媒体とし、熱交換器などを利用して、供給する炭化水素系燃料の予熱や炭化水素系燃料の水蒸気改質用の熱源として利用することができる。また、空気極４２側で発生する高温水蒸気をコジェネレーション用熱源として再利用することができる。尚、燃料極４１における化学反応式は数式４で表され、空気極４２における化学反応式は数式５で表され、全体の反応は数式６で表される。
【００２９】
【数４】

【数５】

【数６】

【００３０】
以下に、第２燃料電池４に用いる材料の好適例について説明する。第２燃料電池４の電解質４０に用いる材料としては、例えば希土類元素をドープしたバリウム系セレート酸化物（例えばＢａＣｅ_１−ｘＮｄ_ｘＯ_３−ｚ、ＢａＣｅ_０．９Ｎｄ_０．１Ｏ_２．９５）、希土類元素をドープしたストロンチウムジルコネート（例えばＳｒＺｒ_１−ｘＹ_ｘＯ_３−ｚ、ＳｒＺｒ_０．９Ｙ_０．１Ｏ_２．９５）、希土類元素をドープしたストロンチウム系セレート酸化物（例えばＳｒＣｅ_１−ｘＹｂ_ｘＯ_３−ｚ、ＳｒＣｅ_０．９５Ｙｂ_０．０５Ｏ_{２．９７５}）などが、５００〜１０００℃付近の温度域で使用可能であり、プロトン導電性を良好に示すことから、好適例として挙げられる。尚、ｚは置換される元素の原子価に合わせて生成される酸素空孔量を示している。電解質４０の作製法は特に限定されず、例えば固相反応法、クエン酸法、共沈法などの既知の方法によって作製して良い。出発物質としては、例えば酸化セリウム（ＣｅＯ_２）、酸化ジルコニウム（ＺｒＯ_２）、アルカリ土類金属炭酸塩（ＳｒＣＯ_３、ＢａＣＯ_３等）、希土類酸化物（Ｎｄ_２Ｏ_３、Ｙ_２Ｏ_３、Ｙｂ_２Ｏ_３等）などを用いることができる。一例としてＢａＣｅ_０．９Ｎｄ_０．１Ｏ_３よりなる電解質４０を固相反応法により作製する場合について説明すると、出発物質である炭酸バリウム、酸化セリウム、酸化ネオジムのモル比が、１：０．９：０．１となるように秤量して混合し、１２００℃で１０時間程度仮焼きした後、もう一度粉砕し混合し加圧成形して、１５００〜１６００℃で１０時間程度焼成を行って電解質４０とする。
【００３１】
第２燃料電池４の空気極４２に用いる材料としては、Ｌａ_１−ｘＡＥ_ｘＣｏ_１−ｙＦｅ_ｙＯ_３やＬａ_１−ｘＡＥ_ｘＭｎ_１−ｙＣｏ_ｙＯ_３など（ＡＥ＝Ｃａ，Ｓｒ等のアルカリ土類金属元素）のペロブスカイト酸化物が、好適例として挙げられる。尚、Ｌａ_１−ｘＡＥ_ｘＣｏ_１−ｙＦｅ_ｙＯ_３の場合、一般に０．１≦ｘ≦０．４、０．１≦ｙ≦０．４の組成範囲が、酸素を酸素イオン化する触媒反応を良好に示すので好ましい。また、Ｌａ_１−ｘＡＥ_ｘＭｎ_１−ｙＣｏ_ｙＯ_３の場合は、０．１≦ｘ≦０．４、０≦ｙ≦０．４の組成範囲が一般に好ましい。特に、Ｍｎ系ペロブスカイト酸化物は、（Ｌａ_１−ｘＡＥ_ｘ）_１−ａＭｎ_１−ｙＣｏ_ｙＯ_３も、化学的に安定な電極材料として存在でき、０≦ａ≦０．１が好ましい組成範囲として考えられる。空気極４２の作製法は特に限定されず、例えば固相反応法、クエン酸法、共沈法などの既知の方法によって作製して良い。出発物質としては、例えば酸化ランタン（Ｌａ_２Ｏ_３）、アルカリ土類金属炭酸塩（ＣａＣＯ_３、ＳｒＣＯ_３等）、酸化マンガン（Ｍｎ_２Ｏ_３）、酸化コバルト（Ｃｏ_３Ｏ_４）、酸化鉄（Ｆｅ_２Ｏ_３）等を用いることができる。一例としてＬａ_０．８Ｓｒ_０．２Ｍｎ_０．９Ｃｏ_０．１Ｏ_３よりなる空気極４２を固相反応法により作製する場合について説明すると、出発物質である酸化ランタン、炭酸ストロンチウム、酸化マンガン、酸化コバルトのモル比が、０．８：０．２：０．９：０．１となるように秤量して混合し、１２００℃で１０時間程度仮焼した後、もう一度粉砕し混合し焼成を行って、ペロブスカイト形構造を有する酸化物の粉体とする。当該粉体をテレピン油などの有機溶剤を用いて電解質４０上にコーティングし、乾燥後、１２００℃程度かつ４時間程度で焼結させて、空気極４２とする。
【００３２】
第２燃料電池４の燃料極４１に用いる材料としては、ＴＥ（ＴＥ＝Ｎｉ（ニッケル），Ｆｅ（鉄），Ｃｏ（コバルト），Ｃｕ（銅））金属とセラミックス（例えばプロトン導電体酸化物）の混合物（サーメット）が、好適例として挙げられる。尚、ＴＥ金属とセラミックスのサーメットは多くの実験結果とｐｅｒｃｏｌａｔｉｏｎ理論から、一般的に、ＴＥ金属が約４０体積％程度含まれると充分な導電率が得られ、４０体積％のＴＥ金属と６０体積％のセラミックスから構成されるサーメットの電極特性が特に優れたものが多いことが知られている。燃料極４１の作製法は特に限定されず、例えば固相混合法、噴霧熱分解法、燃焼合成法などの既知の方法によって作製して良い。一例として、ニッケル−ストロンチウムセレートサーメットよりなる燃料極４１を固相混合法により作製する場合について説明すると、ＳｒＣｅ_０．９５Ｙｂ_０．０５Ｏ_{２．９７５}の粗い粒子と細かい粒子、および酸化ニッケルを混合し、その後、テレピン油などの有機溶剤を用いて電解質４０上にコーティングし、乾燥後、１４００℃程度かつ５時間程度で焼結させて、燃料極４１とする。この燃料極４１は、燃料（水素）が供給されると酸化ニッケルが還元されて、ニッケル−ストロンチウムセレートサーメットとなる。
【００３３】
本実施形態の改質反応器２は、炭化水素系燃料（例えばメタン（ＣＨ_４）を主成分とする天然ガス）と水蒸気（Ｈ_２Ｏ）とを反応させて、主として水素（Ｈ_２）と二酸化炭素（ＣＯ_２）を生成する下記の数式７の反応を起こさせるものとしている。
【００３４】
【数７】

【００３５】
尚、改質反応器２内のガスは、実際には、水蒸気（Ｈ_２Ｏ）、一酸化炭素（ＣＯ）、二酸化炭素（ＣＯ_２）、メタン（ＣＨ_４）、水素（Ｈ_２）等の混合気体となるため、「主として水素を生成する」と表現している。改質反応器２には、数式７の反応を起こさせるための改質触媒９が設けられている。この改質触媒９としては、例えば金属ニッケルのようなニッケル系触媒、あるいはニッケル系触媒におけるガス流方向の後部に酸化鉄や鉄−クロム複合酸化物のような鉄系酸化物を混合したものなど、数式７の反応を起こさせる場合に用いられる既知の触媒を利用して良い。
【００３６】
反応速度論から、数式７の平衡反応定数（Ｋ）は、次式で表すことができる。
【００３７】
【数８】

但し、
Ｋ：平衡反応定数
Ｐ_ＣＯ２：二酸化炭素の分圧
Ｐ_Ｈ２：水素の分圧
Ｐ_ＣＨ４：メタンの分圧
Ｐ_Ｈ２Ｏ：水蒸気の分圧
【００３８】
平衡反応定数（Ｋ）は各温度に対して一定の値をとる。ル・シャトリエ−ブラウンの法則で知られているように、数式７の平衡反応では、改質反応器２内の圧力が高圧になるに従い、式の左側方向に反応が進行するため、発生する水素の分圧が小さくなる。
【００３９】
水蒸気改質が進む場合の反応、即ち数式７の平衡反応が右側方向に進行する場合の反応は、次式で表すことができる。
【００４０】
【数９】

【００４１】
数式９から明らかなように、改質反応器２における水蒸気改質反応ではメタン分子１モルと水分子２モルとから４モルの水素分子が生成される。一方、酸化物イオン導電体３０を電解質に用いた第１燃料電池３では、数式３に示したように、水素分子１モルに対して水分子１モルを生成する原理となっている。従って、第１燃料電池３のみで発電を行い、尚且つ当該発電により生じた水蒸気を用いて数式７の水蒸気改質を行う場合、水蒸気改質による水素生成量が第１燃料電池３の発電による水素消費量と比較して２倍も多く、改質反応器２内の物質収支が一致せず、改質反応器２内の圧力が増加し続けてしまうといった問題が生じる。改質反応器２内の圧力を低下させるために、改質反応器２内のガスを排気ガスとして抜いてしまうと、燃料電池発電システム１の燃料利用率が低下し、排気ガスを燃焼させて当該燃焼により生じた熱を供給する炭化水素系燃料の予熱や炭化水素系燃料の水蒸気改質用の熱源として利用したとしても、燃料電池発電システム１の熱効率は大きく低下してしまう。
【００４２】
これに対し、本実施形態の燃料電池発電システム１では第１燃料電池３と第２燃料電池４を併用するようにしている。従って、改質反応器２における水蒸気改質で得られる水素は、第１燃料電池３と第２燃料電池４における発電に使用される。第１燃料電池３では、改質反応器２内の水素を消費して改質反応器２内に水蒸気を供給するが、第２燃料電池４では、改質反応器２内の水素を消費し且つ改質反応器２の外側に水蒸気を排出する。つまり、改質反応器２内における水蒸気改質と第１燃料電池３の発電との相互作用によって改質反応器２内に発生する過剰な水素が、第２燃料電池４の発電によって、改質反応器２の外部に水蒸気の形で放出される。このため、改質反応器２における水素および水蒸気に関する物質収支は、第１燃料電池３における発電量と、第２燃料電池４における発電量によって、制御することが可能となる。
【００４３】
ここで、本実施形態の改質反応器２では、炭化水素系燃料と水蒸気との反応により、水素と二酸化炭素とが生成されるが（数式７参照）、改質反応器２内に二酸化炭素が残留し続けると、数式７の平衡反応が右側方向へ進行し難くなってしまう。そこで、例えば本実施形態の燃料電池発電システム１では、二酸化炭素分離手段としての二酸化炭素分離膜５を改質反応器２に備えるようにしている。
【００４４】
二酸化炭素分離膜５は、改質反応器２と二酸化炭素分離室６との仕切り（隔壁）となる。例えば本実施形態では、二酸化炭素分離膜５の材料に、アルカリ金属系複合酸化物Ａ_２ＢＯ_３（Ａ＝Ｌｉ，Ｎａ，Ｋ等のアルカリ金属元素、Ｂ＝Ｔｉ，Ｚｒ等のチタン族元素、但し、ＡまたはＢに該当する元素は一種類だけでなく複数種の元素であっても良く、例えば、ＬｉＫＺｒＯ_３、Ｌｉ_２Ｚｒ_０．５Ｔｉ_０．５Ｏ_３でも良い。）またはアルカリ金属系オルソシリケートＡ_４ＳｉＯ_４（Ａ＝Ｌｉ，Ｎａ，Ｋ等のアルカリ金属元素、但し、Ａに該当する元素は一種類だけでなく複数種の元素であっても良く、例えば、Ｌｉ_２Ｋ_２ＳｉＯ_４でも良い。）の一方または双方を少なくとも用いるようにしている。そして、二酸化炭素分離室６の二酸化炭素分圧を、改質反応器２内の二酸化炭素分圧よりも低くなるように、圧力差を設けるようにしている。このため、例えば吸気装置などを用いて二酸化炭素分離室６の二酸化炭素分圧を負圧（例えば大気圧以下）の状態となるようにしている。
【００４５】
二酸化炭素分離膜５の作用を、Ａ_２ＢＯ_３を例に挙げ、図８を用いて説明する。改質反応器２側の二酸化炭素分離膜５の表面では、Ａ_２ＢＯ_３が二酸化炭素（ＣＯ_２）と反応してＡ_２ＣＯ_３とＢＯ_２となる（即ち、数式１０の反応が進む）。
【００４６】
【数１０】

【数１１】

【数１２】

【００４７】
当該反応により生じたＡ_２ＣＯ_３またはＣＯ_３ ^２−は溶融状態となり、二酸化炭素分離膜５中を二酸化炭素分離室６側に拡散する。そして、二酸化炭素分離室６側の二酸化炭素分離膜５の表面では、Ａ_２ＣＯ_３が解離して二酸化炭素（ＣＯ_２）を二酸化炭素分離室６に放出する（即ち、数式１１の反応が進む）。またはＣＯ_３ ^２−が解離して二酸化炭素（ＣＯ_２）を二酸化炭素分離室６に放出する。当該解離により生じたＡ_２Ｏは二酸化炭素分離膜５中を改質反応器２側に逆拡散する。当該逆拡散したＡ_２Ｏは、ＢＯ_２と反応してＡ_２ＢＯ_３に戻り（即ち、数式１２の反応が進む）、改質反応器２側の二酸化炭素分離膜５の表面で二酸化炭素（ＣＯ_２）と反応して、再び溶融状態のＡ_２ＣＯ_３またはＣＯ_３ ^２−となって、二酸化炭素分離膜５中を二酸化炭素分離室６側に拡散する。これにより、物質移動のサイクルが実現され、改質反応器２側での二酸化炭素の吸収および二酸化炭素分離室６側での二酸化炭素の放出が連続的に行なわれる。
【００４８】
Ａ_４ＳｉＯ_４の場合も同様の原理で改質反応器２から二酸化炭素分離室６へ二酸化炭素を分離できる。即ち、改質反応器２側の二酸化炭素分離膜５の表面では、Ａ_４ＳｉＯ_４が二酸化炭素（ＣＯ_２）と反応してＡ_２ＣＯ_３とＡ_２ＳｉＯ_３となる（即ち、数式１３の反応が進む）。
【００４９】
【数１３】

【数１４】

【数１５】

【００５０】
当該反応により生じたＡ_２ＣＯ_３またはＣＯ_３ ^２−は溶融状態となり、二酸化炭素分離膜５中を二酸化炭素分離室６側に拡散する。そして、二酸化炭素分離室６側の二酸化炭素分離膜５の表面では、Ａ_２ＣＯ_３が解離して二酸化炭素（ＣＯ_２）を二酸化炭素分離室６に放出する（即ち、数式１４の反応が進む）。またはＣＯ_３ ^２−が解離して二酸化炭素（ＣＯ_２）を二酸化炭素分離室６に放出する。当該解離により生じたＡ_２Ｏは二酸化炭素分離膜５中を改質反応器２側に逆拡散する。当該逆拡散したＡ_２Ｏは、Ａ_２ＳｉＯ_３と反応してＡ_４ＳｉＯ_４に戻り（即ち、数式１５の反応が進む）、改質反応器２側の二酸化炭素分離膜５の表面で二酸化炭素（ＣＯ_２）と反応して、再び溶融状態のＡ_２ＣＯ_３またはＣＯ_３ ^２−となって、二酸化炭素分離膜５中を二酸化炭素分離室６側に拡散する。これにより、物質移動のサイクルが実現され、改質反応器２側での二酸化炭素の吸収および二酸化炭素分離室６側での二酸化炭素の放出が連続的に行なわれる。
【００５１】
Ｌｉ_２ＺｒＯ_３を例に挙げて、二酸化炭素分離膜５の作用を更に詳細に説明する。図９にＬｉ_２ＺｒＯ_３のＣＯ_２吸収および放出特性を示す。図９の縦軸は、ＣＯ_２雰囲気下にあるＬｉ_２ＺｒＯ_３のＣＯ_２吸収による重量変化率（１００×（同図中横軸の各温度で測定した重量−測定前の初期重量）／（測定前の初期重量）［％］）、つまりＬｉ_２ＺｒＯ_３のＣＯ_２吸収量を示す。同図中の符号Ａで示すグラフは二酸化炭素分圧が０．７ＭＰａの場合を、図中の符号Ｂで示すグラフは二酸化炭素分圧が０．５ＭＰａの場合を、図中の符号Ｃで示すグラフは二酸化炭素分圧が０．３ＭＰａの場合を 、図中の符号Ｄで示すグラフは二酸化炭素分圧が０．１ＭＰａの場合を示す。ＣＯ_２雰囲気下、約４６０℃程度以上となると、Ｌｉ_２ＺｒＯ_３はＣＯ_２を吸収して、Ｌｉ_２ＣＯ_３とＺｒＯ_２に変化する（即ち、数式１６の反応が進む）。さらに高温（例えば約９００℃程度）となると、Ｌｉ_２ＣＯ_３はＣＯ_２を放出して、Ｌｉ_２ＺｒＯ_３に戻る（即ち、数式１７および数式１８の反応が進む）。
【００５２】
【数１６】

【数１７】

【数１８】

【００５３】
図９に示すように、二酸化炭素分圧が高い方が、二酸化炭素分圧が低い場合と比べて、ＣＯ_２の吸収量が多く、また吸収したＣＯ_２を放出するようになるまでの温度が高くなる。このため、ある一定の温度範囲（図９の例では６００℃〜８００℃程度、より好ましくは７５０℃付近）において、二酸化炭素分離膜５を挟んで二酸化炭素の分圧差を設ければ、二酸化炭素分離膜５は高圧側でＣＯ_２を吸収し低圧側でＣＯ_２を放出するようになる。上記一定の温度範囲は、二酸化炭素分離膜５の作動温度となる。
【００５４】
従って、高圧側（改質反応器２側）では、数式１６の反応が進行し、Ｌｉ_２ＺｒＯ_３がＣＯ_２と反応して、溶融状態のＬｉ_２ＣＯ_３と固体のＺｒＯ_２が生じる。溶融状態のＬｉ_２ＣＯ_３またはＣＯ_３ ^２−は二酸化炭素分離膜５中を低圧側（二酸化炭素分離室６側）に拡散する。低圧側では、数式１７の反応が進行し、Ｌｉ_２ＣＯ_３またはＣＯ_３ ^２−がＣＯ_２を放出して、Ｌｉ_２Ｏとなり、逆に高圧側に拡散する。高圧側に拡散したＬｉ_２Ｏは、ＺｒＯ_２と反応してＬｉ_２ＺｒＯ_３に戻り、再びＣＯ_２と反応して溶融状態のＬｉ_２ＣＯ_３またはＣＯ_３ ^２−となり、二酸化炭素分離膜５中を低圧側に拡散する。これにより、ＣＯ_２を連続的に改質反応器２から二酸化炭素分離室６へと分離することが可能となる。
【００５５】
尚、例えば本実施形態では、薄膜状の二酸化炭素分離膜５を多孔質膜１９で支持するようにしている。この多孔質膜１９の材料としては、例えばアルミナやイットリアなどの希土類元素で安定化したジルコニアなどが好適である。また、例えば本実施形態では、Ａ_２ＢＯ_３やＢＯ_２などの粒成長を妨げる粒子であって粒径がナノオーダの粒子（不活性ナノ粒子）１８を、二酸化炭素分離膜５の材料に添加するようにしている。不活性ナノ粒子１８としては、例えばアルミナなどが好適である。尚、図８中の符号１７はＡ_２ＢＯ_３（例えばＬｉ_２ＺｒＯ_３）粒子を示し、符号１６はＢＯ_２（例えばＺｒＯ_２）粒子を示し、符号２０は溶融状態のＡ_２ＣＯ_３（例えばＬｉ_２ＣＯ_３）を示す。
【００５６】
ここで、ＣＯ_２分離を連続的に行なうためには、二酸化炭素分離膜５中の円滑な物質移動が必要となるため、高圧側のＣＯ_２吸収反応により生成されて低圧側でＣＯ_２を放出する機能を担う物質（例えばＬｉ_２ＣＯ_３など）は、溶融状態であることが望ましい。そこで、上記物質の融点を調整することで、二酸化炭素分離膜５の作動温度（即ち、高圧側でＣＯ_２を吸収し尚且つ低圧側でＣＯ_２を放出する温度）を制御できると考えられる。例えばアルカリ金属系炭酸塩を膜材料にドープ（添加）することで、上記物質の融点を下げることができる。
【００５７】
Ｌｉ_２ＣＯ_３の融点は７３２℃であるが、Ｌｉ_２ＣＯ_３とＫ_２ＣＯ_３の混合物の融点は５００℃以下となることが知られている（例えば図１０（Ａ）参照）。さらに、Ｌｉ_２ＣＯ_３とＫ_２ＣＯ_３とＮａ_２ＣＯ_３の混合物の融点は４００℃以下となることが知られている（例えば図１０（Ｂ）参照）。従って、例えばＫ_２ＣＯ_３とＮａ_２ＣＯ_３の一方または双方を膜材料Ｌｉ_２ＺｒＯ_３に添加し且つその添加量を調整することで、二酸化炭素分離膜５の作動温度を約４００〜７００℃の範囲内で制御することが可能となる。例えばＫ_２ＣＯ_３とＮａ_２ＣＯ_３の一方または双方を、膜材料Ｌｉ_２ＺｒＯ_３に添加し、Ｌｉ_２−ｘＮａ_ｘＺｒＯ_３またはＬｉ_２−ｘＫ_ｘＺｒＯ_３あるいはＬｉ_{２−ｘ−ｙ}Ｎａ_ｘＫ_ｙＺｒＯ_３とする。ＣＯ_２吸収反応によりＬｉ_２ＣＯ_３またはＬｉ_２−ｘＮａ_ｘＣＯ_３またはＬｉ_２−ｘＫ_ｘＣＯ_３またはＬｉ_{２−ｘ−ｙ}Ｎａ_ｘＫ_ｙＣＯ_３が生成されるが、これら生成物が目的とする温度域で溶融状態となるようにする。これにより、これらの物質が二酸化炭素分離膜５中で良好に拡散し、ＣＯ_２分離が連続的に行なわれる。
【００５８】
二酸化炭素分離膜５の作動温度は、燃料電池発電システム１における温度調整を容易とする観点から、改質触媒９、第１燃料電池３、第２燃料電池４の作動温度に合わせることが好ましい。また、改質反応器２内の温度は、例えば５００℃〜１０００℃程度であることが好ましい。これは、本実施形態の第１燃料電池３および第２燃料電池４の作動温度が５００℃〜１０００℃程度であること、１０００℃を超えるとＬｉ_２ＺｒＯ_３が二酸化炭素を吸収しなくなることから、二酸化炭素分離膜５の作動が困難と考えられること、等の理由によるものである。
【００５９】
この二酸化炭素分離膜５は、数式７の反応場のような高温で機能するので、改質反応器２内の混合ガスを冷却する工程は不要である。さらに、この二酸化炭素分離膜５では、二酸化炭素の吸収と放出が連続的に行なわれるので、吸収剤などの再生工程は不要である。このため、多くのエネルギーが消費される冷却工程や再生工程が不要となり、エネルギー効率を大幅に向上させることができる。また、原理としては、二酸化炭素しか二酸化炭素分離膜５を透過できないため、純度の高い二酸化炭素が回収でき、二酸化炭素の回収により地球温暖化防止にも寄与できる。
【００６０】
また、改質反応器２から分離された二酸化炭素は高い熱を有している。そこで、当該二酸化炭素の有する熱を有効に利用することで、燃料電池発電システム１の熱効率をさらに高めることができる。例えば、本実施形態では、熱交換器２１を用いて二酸化炭素が有する熱を炭化水素系燃料および水の供給管２２に伝えて、炭化水素系燃料や水などの予熱に利用するようにしている（図７参照）。その他、熱交換器などを用いて二酸化炭素が有する熱を改質反応器２や予備水蒸気改質用の反応器等に伝えて、水蒸気改質の吸熱反応に利用しても良い。また、改質反応器２から分離された高温の二酸化炭素をコジェネレーション用熱源として再利用することができる。
【００６１】
以上のように構成される燃料電池発電システム１の動作の一例について、主に図７を用いて説明する。数式７の反応に必要な炭化水素系燃料は燃料供給手段（例えばメタンを主成分とする天然ガスが充填されたタンク）２３より供給される。運転初期段階に必要な水は水供給手段（例えば給水タンク）２４より供給され、運転初期段階に必要な熱は熱供給手段（例えば熱交換器や加熱装置）２５により供給される。例えば、燃料供給手段２３より供給される高圧の天然ガスと水供給手段２４より供給される水とは、混合され、共に熱供給手段２５により例えば５００℃〜１０００℃の高温とされた後に、改質反応器２に供給される。
【００６２】
改質反応器２では、改質触媒９のもとで炭化水素系燃料と水蒸気が反応し主として水素と二酸化炭素が生成される。生成された水素は、第１燃料電池３と第２燃料電池４における発電に使用される。第１燃料電池３の発電によって、改質反応器２内の水素が消費され、且つ改質反応器２内に水蒸気と熱が供給される。数式７で示される水蒸気改質を進行させるためには水が必要であり、また同改質反応は吸熱反応であるため熱も必要とするが、これら必要な水と熱が第１燃料電池３の発電によって得ることができる。一方、水蒸気改質による水素生成量は第１燃料電池３の発電による水素消費量よりも多く、改質反応器２内に過剰な水素が発生するが、当該過剰な水素は、第２燃料電池４の発電によって消費され、且つ改質反応器２の外部に水蒸気の形で放出される。即ち、改質反応器２内の水素に関する物質収支が、第１燃料電池３の発電量と第２燃料電池４の発電量によって制御される。さらに、本実施形態の燃料電池発電システム１では、二酸化炭素分離膜５により改質反応器２内で生成された二酸化炭素が改質反応器２の外側に放出される。即ち、改質反応器２内の炭素成分に関する物質収支が二酸化炭素分離膜５によって制御される。改質反応器２内の水素が第１燃料電池３および第２燃料電池４により消費されると同時に、改質反応器２内の二酸化炭素が二酸化炭素分離膜５により改質反応器２の外側に放出されるので、改質反応器２内における数式７の平衡反応を右側方向へ速やかに進行させることができる。炭化水素系燃料は、以上の反応に関与するまで、改質反応器２の中を循環する。尚、第２燃料電池４の発電によって改質反応器２の外部に放出された高温水蒸気は、熱交換器などを用いて、供給する炭化水素系燃料の予熱や水蒸気改質用の熱源として利用することができる。また、改質反応器２から分離された高温の二酸化炭素は、熱交換器２１を用いて、供給する炭化水素系燃料の予熱用の熱源として利用することができる。
【００６３】
この燃料電池発電システム１によれば、２種類の燃料電池３，４と二酸化炭素分離膜５を組み合わせることで、燃料電池発電システム１の内外を出入りする物質収支を一致させ（換言すれば物質移動のバランスを取り）、供給する炭化水素系燃料および生成される水素を、第１燃料電池３および第２燃料電池４の発電に１００％利用することができる。そして、当初必要な水と熱を水供給手段２４や熱供給手段２５により改質反応器２に供給しておけば、以後、水については第１燃料電池３の発電によって自然に供給され、熱については第１燃料電池３および第２燃料電池４の発電並びに改質反応器２から分離された高温二酸化炭素によって自然に供給されるので、改質反応器２には燃料供給手段２３により炭化水素系燃料のみを供給すれば良い。従って、水および熱についてのクローズドサイクルが実現され、燃料電池発電システム１のエネルギー効率、省エネルギー性を高めることができる。また、本発明によって燃料利用率１００％を実現する技術が確立されることで、ＳＯＦＣに関するコスト的な問題も解決できる。また、本発明では、燃料電池発電システム１の内外を出入りする物質収支を一致させることで、燃料利用率１００％を達成するので、燃料ガスの出口側の水素濃度を０とするような必要はなく、燃料ガス出口側の水素濃度が極端に低いために起きる過電圧増加や迷走電流発生といった現象は起きない。この燃料電池発電システム１によれば、燃料電池本体で７０％以上の熱効率を達成できると考えられる。
【００６４】
なお、上述の実施形態は本発明の好適な実施の一例ではあるがこれに限定されるものではなく、本発明の要旨を逸脱しない範囲において種々変形実施可能である。例えば、運転条件等によって改質反応器２内の水や熱が不足する場合には、水供給手段２４や熱供給手段２５によって不足分の水や熱を適宜供給しても良いのは勿論である。
【００６５】
【発明の効果】
以上の説明から明らかなように、請求項１記載の燃料電池による発電方法および請求項２記載の燃料電池発電システムによれば、改質反応器における物質収支を一致させ、換言すれば物質移動のバランスを取り、供給する炭化水素系燃料および生成される水素を、第１燃料電池および第２燃料電池の発電に１００％利用することができる。そして、当初必要な水と熱を改質反応器に供給しておけば、以後、必要な水や熱が第１燃料電池および第２燃料電池の発電によって自然に供給されるので、改質反応器には炭化水素系燃料のみを供給すれば良い。従って、水および熱についてのクローズドサイクルが実現され、燃料電池発電システムのエネルギー効率、省エネルギー性を高めることができる。また、本発明により燃料利用率１００％を実現する技術が確立されることで、ＳＯＦＣに関するコスト的な問題も解決できる。また、本発明では、物質収支を一致させることで燃料利用率１００％を達成し、燃料ガスの出口側の水素濃度を０とするような必要はないので、過電圧の増加や迷走電流の発生といった弊害は生じない。
【００６６】
さらに、請求項３記載の燃料電池発電システムによれば、改質反応器内の炭素成分に関する物質収支も制御することが可能となり、改質反応器内の水素が第１燃料電池および第２燃料電池により消費されると同時に、改質反応器内の二酸化炭素が二酸化炭素分離手段により改質反応器の外側に放出されるので、改質反応器内における水素生成反応を速やかに進行させることができ、燃料電池発電を効率的に行なうことが可能となる。
【図面の簡単な説明】
【図１】本発明の燃料電池発電システムの実施の一形態を示す概略構成図である。
【図２】図１のＩＩ−ＩＩ線で切った断面を示す概略構成図である。
【図３】電解質が酸化物イオン導電体である第１燃料電池の原理を示す概念図である。
【図４】電解質がプロトン導電体である第２燃料電池の原理を示す概念図である。
【図５】本発明の燃料電池発電システムの他の構成例（燃料電池の単セルを複数スタックした場合）を示す概略構成図である。
【図６】図５のＶＩ−ＶＩ線で切った断面を示す概略構成図である。
【図７】本発明の燃料電池発電システムの実施の一形態を示す概略構成図である。
【図８】本発明の二酸化炭素分離手段の原理を示す概念図である。
【図９】Ｌｉ_２ＺｒＯ_３のＣＯ_２吸収および放出特性を示すグラフであり、横軸は温度［℃］を、縦軸はＣＯ_２吸収によるＬｉ_２ＺｒＯ_３の重量変化率（１００×（横軸の各温度で測定した重量−測定前の初期重量）／（測定前の初期重量）［％］）を示す。
【図１０】（Ａ）はＬｉ_２ＣＯ_３とＫ_２ＣＯ_３の相図を示し、（Ｂ）はＬｉ_２ＣＯ_３とＫ_２ＣＯ_３とＮａ_２ＣＯ_３の相図を示す。
【符号の説明】
１ 燃料電池発電システム
２ 改質反応器
３ 第１燃料電池
４ 第２燃料電池
５ 二酸化炭素分離膜（二酸化炭素分離手段）
３０ 酸化物イオン導電体
３１ 第１燃料電池の燃料極
３２ 第１燃料電池の空気極
４０ プロトン導電体
４１ 第２燃料電池の燃料極
４２ 第２燃料電池の空気極[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a power generation method and system using a fuel cell. More specifically, the present invention relates to a method and a system for generating power from a solid oxide fuel cell using a hydrocarbon-based fuel as a raw material.
[0002]
[Prior art]
2. Description of the Related Art Polymer Electrolyte Fuel Cells (hereinafter abbreviated as PEFCs) are establishing technologies that can sufficiently withstand practical use, and applications to automotive power sources and stationary power sources are being actively made. (For example, see Non-Patent Document 1). PEFC is a fuel cell using a polymer ion exchange membrane as an electrolyte and platinum as an electrode (also referred to as a catalyst), and has features such as a low operating temperature (for example, about 60 to 100 ° C.). PEFC uses high-purity hydrogen as fuel. For this reason, PEFC power generation systems that use hydrocarbon fuels such as city gas as raw materials are equipped with auxiliary equipment such as a reformer for producing high-purity hydrogen required for cell reactions from hydrocarbon fuels. . As a standard power supply capacity for home use, for example, 4 to 5 kW for a family of five, PEFC for stationary (home) currently being prototyped is designed according to the amount of heat used at home. The power generation capacity is set to a considerably low value (for example, about 0.8 kW).
[0003]
On the other hand, as a fuel cell which is less mature than a PEFC at a technical level, it operates at a high temperature (for example, about 500 to 1000 ° C.) as a solid oxide fuel cell (Solid Oxide Fuel Cell, a solid electrolyte fuel cell). (Hereinafter abbreviated as SOFC) (see Non-Patent Document 2, for example). The SOFC operates at a high temperature (for example, about 500 to 1000 ° C.) and does not require an expensive catalyst, and can directly use a hydrocarbon-based fuel. Combined with the attractiveness of high-output power generation, it is likely that the technology will be widely used when the technology is established.
[0004]
[Non-patent document 1]
"Polymer Fuel Cell / Hydrogen Energy Utilization Achievement Proceedings", New Energy and Industrial Technology Development Organization, Hydrogen Energy Technology Development Office, March 12, 2003
[Non-patent document 2]
"Solid oxide fuel cells and the global environment": Hiroaki Tagawa, Agune Shofusha, 1998
[0005]
[Problems to be solved by the invention]
The research and development of PEFC is aiming at introduction of about 5 million vehicles and about 1000 kW for stationary in 2020. It is said that there is a limit to the amount of stationary fuel cells that can be introduced. Furthermore, since PEFC can use only extremely high-purity hydrogen as fuel, high-purity hydrogen production technology and improvement of the social environment of hydrogen supply infrastructure are essential. In the case of a PEFC power generation system using a hydrocarbon-based fuel as a raw material, a reformer or the like for producing high-purity hydrogen is required as an accessory facility, which increases costs. Further, the features of the reformer and the fuel cell include the following. The reaction of the reformer is a thermochemical reaction process in which hydrogen is produced by reforming city gas containing methane or the like as a main component. Utilizing the produced hydrogen, the fuel cell generates power by an electrochemical process. That is, the hydrogen production process in the reformer is a thermochemical reaction, and the consumption of hydrogen in the fuel cell is an electrochemical reaction, so that the reaction speed between the reformer and the fuel cell is different. The thermochemical reaction process is inferior in instantaneous load response, and the amount of hydrogen used is small when the fuel cell is under low load, so that the fuel reforming efficiency (hydrogen production efficiency) by the thermochemical reaction process deteriorates. On the other hand, since the fuel cell is an electrochemical process, its responsiveness is excellent when the load fluctuates, and the efficiency of the fuel cell (the efficiency of converting the chemical energy of the fuel into electric energy) is improved when the load is low. There is a tendency. As described above, the load fluctuation and the low load characteristic of the fuel cell and the reformer are contradictory, and the PEFC that can use only high-purity hydrogen as a fuel has a large energy loss, and the energy efficiency is low. Some observers do not expect a dramatic increase in efficiency.
[0006]
On the other hand, SOFCs operate at high temperatures, so that they do not require expensive catalysts and can directly use hydrocarbon fuels, so there is no need for a reformer that produces high-purity hydrogen. Is considered to provide more heat than PEFC, and when designed according to the amount of heat used at home, the power generation capacity becomes even lower than PEFC (the fuel cell is designed to meet the heat demand for home use). For example, if the power generation capacity of PEFC is 0.8 kW and the power generation capacity of 0.4 kW in SOFC satisfies the heat demand sufficiently, the SOFC system purchases extra 0.4 kW more expensive electricity from the power company. Need to be done). Therefore, even if it can be manufactured at a lower cost than PEFC, the merit of cost reduction by power generation becomes thin, and the feeling of high equipment cost cannot be denied.
[0007]
Further, common to all types of fuel cells, fuel cells have the limitation that the fuel utilization cannot be 100%. The fuel utilization rate is, for example, when hydrogen is supplied to a fuel cell as fuel, the hydrogen is electrochemically used in the fuel cell to be converted into a current, and the higher the fuel utilization rate, the more the supplied fuel becomes. It is an index indicating that chemical energy has been efficiently converted to electric energy. When flowing hydrogen gas into the fuel cell, the hydrogen concentration is high at the power generation section on the inlet side of the hydrogen gas, and low at the outlet side of the hydrogen gas. Since the voltage of the fuel cell is determined by the concentration of hydrogen and the concentration of oxygen, a voltage difference occurs between the power generation unit at the inlet and the outlet of the hydrogen gas in one or one fuel cell. The voltage flows from the higher side to the lower side (stray current), resulting in energy loss. In an extreme case where the hydrogen concentration in the power generation unit on the outlet side of the hydrogen gas is 0, power generation is not performed in the power generation unit on the outlet side. In addition, when the hydrogen concentration is low, the concentration diffusion resistance is increased, so that the overvoltage of the fuel electrode is increased and the power generation efficiency is reduced. Therefore, it is necessary to leave some hydrogen (about 20 to 40% of the supplied hydrogen) at the outlet side of the hydrogen gas. Eventually, 20-40% of the supplied hydrogen is only available as heat, and in the extreme case of excess heat, it is the same as throwing away fuel. In other words, when the fuel utilization rate is high, the amount of fuel discharged to the outside of the fuel cell is reduced, so that the energy conversion efficiency is increased. When the fuel cell is low, the output density is high because overvoltage of the fuel electrode is not reduced and the stray current does not occur. I will. For this reason, the fuel utilization of a general fuel cell is suppressed to 60 to 70%.
[0008]
It is conceivable to burn the hydrogen that is not used in the cell reaction and discharged, and use the heat of combustion as a heat source for preheating the supplied fuel or as a heat source for steam reforming of hydrocarbon fuels. Requires a lot of energy, and considering the overall energy efficiency, burning hydrogen that was not used for the battery reaction without using it for power generation would greatly increase the thermal efficiency of the entire fuel cell power generation system. This results in a decrease.
[0009]
Therefore, an object of the present invention is to provide a fuel cell power generation method and a fuel cell power generation system that can make the fuel utilization rate 100%.
[0010]
[Means for Solving the Problems]
In order to achieve this object, a fuel cell power generation method according to claim 1 generates a fuel gas containing hydrogen by reacting a hydrocarbon-based fuel and steam in a reforming reactor, and generates the fuel gas. Circulating, the fuel electrode of the first fuel cell in which the electrolyte is an oxide ion conductor, and the fuel electrode of the second fuel cell in which the electrolyte is a proton conductor, disposed so as to contact the circulating fuel gas, An air electrode of the first fuel cell and an air electrode of the second fuel cell are arranged so as to be in contact with an oxidizing gas flowing outside the reforming reactor, and the reforming is performed by power generation by the first fuel cell. While consuming the hydrogen generated in the reforming reactor and supplying steam into the reforming reactor, and consuming the hydrogen generated in the reforming reactor by the power generation by the second fuel cell and performing the reforming. Water outside the reactor It is to discharge care.
[0011]
Further, the fuel cell power generation system according to claim 2 includes a reforming reactor that mainly generates hydrogen by reacting a hydrocarbon-based fuel and steam, a first fuel cell, and a second fuel cell, The first fuel cell includes an air electrode that oxygenates an oxidizing gas present outside the reforming reactor, an oxide ion conductor as an electrolyte through which the oxygen ions move, and the oxide ion conductor. A fuel electrode that reacts the transferred oxygen ions with hydrogen generated in the reforming reactor to generate water, wherein the second fuel cell converts the hydrogen generated in the reforming reactor into hydrogen The fuel electrode to be ionized, the proton conductor as an electrolyte in which the hydrogen ions move, and the hydrogen ions having moved through the proton conductor reacting with the oxidizing gas present outside the reforming reactor to form water. Generate So that and a Kikyoku.
[0012]
Therefore, the hydrogen generated in the reforming reactor is used for power generation in the first fuel cell and the second fuel cell. By the power generation of the first fuel cell, hydrogen in the reforming reactor is consumed, and steam and heat are supplied into the reforming reactor. On the other hand, the amount of hydrogen generated by the steam reforming is larger than the amount of hydrogen consumed by the power generation of the first fuel cell, and excessive hydrogen is generated in the reforming reactor. And discharged in the form of steam outside the reforming reactor. Thereby, the mass balance of hydrogen in the reforming reactor can be controlled by the power generation amount of the first fuel cell and the power generation amount of the second fuel cell. Further, by circulating the hydrocarbon-based fuel until it participates in the above reaction, it is possible to use 100% of the supplied hydrocarbon-based fuel and the generated hydrogen for the power generation of the first fuel cell and the second fuel cell. it can.
[0013]
According to a third aspect of the present invention, in the fuel cell power generation system according to the second aspect, a carbon dioxide separating means for selectively moving carbon dioxide in the reforming reactor to outside the reforming reactor is further provided. I am preparing for it.
[0014]
Therefore, the material balance of the carbon component in the reforming reactor can also be controlled. Also, at the same time that the hydrogen in the reforming reactor is consumed by the first fuel cell and the second fuel cell, the carbon dioxide in the reforming reactor is released to the outside of the reforming reactor by the carbon dioxide separating means. Therefore, the hydrogen generation reaction in the reforming reactor can be promptly advanced.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the configuration of the present invention will be described in detail based on an embodiment shown in the drawings.
[0016]
1 to 10 show an embodiment of a power generation method and a fuel cell power generation system using a fuel cell according to the present invention. In this power generation method using a fuel cell, a hydrocarbon-based fuel and steam are reacted in a reforming reactor 2 to generate a fuel gas containing hydrogen, and the fuel gas is circulated. The fuel electrode 31 of the first fuel cell 3, which is the fuel cell 30, and the fuel electrode 41 of the second fuel cell 4, the electrolyte of which is the proton conductor 40, are arranged so as to come into contact with the circulating fuel gas. The air electrode 32 of the second fuel cell 4 and the air electrode 42 of the second fuel cell 4 are arranged so as to be in contact with the oxidizing gas flowing outside the reforming reactor 2. Consumes the hydrogen generated in the reforming reactor 2 and supplies steam into the reforming reactor 2, consumes the hydrogen generated in the reforming reactor 2 by the power generation by the second fuel cell 4, and Water vapor is discharged to the outside
[0017]
Further, the fuel cell power generation system 1 includes a reforming reactor 2 that mainly generates hydrogen by reacting a hydrocarbon-based fuel with water vapor, a first fuel cell 3, and a second fuel cell 4, and a first fuel cell The fuel cell 3 includes an air electrode 32 that oxygenates an oxidant gas existing outside the reforming reactor 2, an oxide ion conductor 30 as an electrolyte through which the oxygen ions move, and an oxide ion conductor 30. A fuel electrode 31 that reacts oxygen ions that have been transferred with the hydrogen with the hydrogen generated in the reforming reactor 2 to generate water. The second fuel cell 4 includes a hydrogen electrode generated in the reforming reactor 2. A fuel electrode 41 for hydrogen ionizing the proton, a proton conductor 40 as an electrolyte in which the hydrogen ions move, hydrogen ions moving in the proton conductor 40 and an oxidizing gas existing outside the reforming reactor 2. React to produce water So that and an air electrode 42. Further, for example, the fuel cell power generation system 1 of the present embodiment includes a carbon dioxide separation membrane 5 as carbon dioxide separation means for selectively moving carbon dioxide in the reforming reactor 2 to the outside of the reforming reactor 2. Like that.
[0018]
The first fuel cell 3 and the second fuel cell 4 of the present embodiment are solid oxide fuel cells (SOFC) that operate in a temperature range of, for example, about 500 to 1000 ° C. SOFC has advantages over a polymer electrolyte fuel cell (PEFC) and the like that it does not require an expensive catalyst using platinum and that the fuel gas does not need to be high-purity hydrogen gas. The first fuel cell 3 and the second fuel cell 4 may be, for example, either a flat plate type or a cylindrical type. For example, in the present embodiment, as shown in FIG. 1, FIG. 2 or FIG. 5, FIG. Type.
[0019]
The number or size of the first fuel cell 3, the second fuel cell 4, and the carbon dioxide separation membrane 5 depends on the size of the target power generation capacity, the type of hydrocarbon fuel used as a raw material, and the like. It is appropriately determined so that the material balance in the vessel 2 matches, and is not particularly limited. 1 and 2 show an example in which the first fuel cell 3 and the second fuel cell 4 are single cells. On the other hand, when a plurality of first fuel cells 3 or second fuel cells 4 are provided, adjacent single cells are electrically connected in series by an interconnector 7 as shown in FIGS. 5 and 6, for example.
[0020]
For example, in the present embodiment, the air electrodes 32 and 42 of the first fuel cell 3 and the second fuel cell 4 are brought into contact with the oxidizing gas (for example, air) supplied by the oxidizing gas supply means (for example, the air introducing unit) 8. Then, the fuel electrodes 31 and 41 of the first fuel cell 3 and the second fuel cell 4 are arranged outside the reforming reactor 2 so as to contact the fuel gas containing hydrogen in the reforming reactor 2. It is arranged inside the quality reactor 2. The oxide ion conductor 30 as an electrolyte is disposed between the air electrode 32 and the fuel electrode 31, and the proton conductor 40 as an electrolyte is disposed between the air electrode 42 and the fuel electrode 41, respectively. The fuel gas containing hydrogen is circulated in the reforming reactor 2. The hatched arrows in FIGS. 1, 2, 5 and 6 indicate the flow of the fuel gas (mixed gas containing hydrocarbon fuel, steam, hydrogen, carbon dioxide, etc.) in the reforming reactor 2. , The dotted arrow indicates the flow of hydrogen gas, the dashed arrow indicates the flow of water vapor, and the two-dot chain arrow indicates the flow of carbon dioxide. 2 and 6 indicate the flow of the oxidizing gas (for example, air).
[0021]
When the fuel electrodes 31 and 41 are installed inside the reforming reactor 2, the first fuel cell 3 and the second fuel cell 4 are integrated with the reforming reactor 2, and the fuel cell power generation system 1 can be made compact. . The gas circulation in the reforming reactor 2 may be realized, for example, by natural convection due to a difference in the density of the fluid (in this case, the gas is circulated in the reforming reactor 2 so as to determine the flow of the fluid in the reforming reactor 2). A partial partition or the like may be provided), or an impeller or the like may be provided in the reforming reactor 2 to realize the forced convection by the rotational motion of the impeller or the like.
[0022]
However, the configuration is not limited to the configuration in which the fuel electrodes 31 and 41 are necessarily installed inside the reforming reactor 2. For example, a circulation path (not shown) is provided in which the fuel gas in the reforming reactor 2 is once taken out of the reforming reactor 2 and then returned to the inside of the reforming reactor 2. The fuel electrodes 31 and 41 may be provided. In this case, for example, a fuel flow passage of a cell stack of a flat-plate SOFC may be formed in the middle of the circulation path.
[0023]
Oxide ion (oxygen ion O^2-The principle of the first fuel cell 3 using the conductor 30 as an electrolyte will be described with reference to FIG. In the first fuel cell 3, a fuel gas containing hydrogen is supplied to the fuel electrode 31 (reference numeral 11 in FIG. 3 indicates a hydrogen molecule), and an oxidant gas (for example, air) is supplied to the air electrode 32. (Indicated by reference numeral 12 in FIG. 3 indicates oxygen molecules and reference numeral 13 indicates nitrogen molecules), oxygen in the air changes to oxygen ions and moves to the fuel electrode 31 side through the oxide ion conductor 30. I do. On the fuel electrode 31 side, hydrogen in the fuel gas is converted into hydrogen ions (protons), and the hydrogen ions combine with oxygen ions that have moved through the electrolyte 30 to generate water on the fuel electrode 31 side ( Reference numeral 14 in FIG. 3 indicates a water molecule). In other words, in the first fuel cell 3, oxygen existing outside the reforming reactor 2 is moved into the reforming reactor 2 and reacted with hydrogen in the reforming reactor 2 to change it to water. At this time, a voltage is generated in the first fuel cell 3 by an electrochemical reaction between the air electrode 32 and the fuel electrode 31. When the air electrode 32 and the fuel electrode 31 are connected by a conductor for flowing electrons to form an external circuit, electrons flow from the fuel electrode 31 to the air electrode 32 through the external circuit. The stream does its work (i.e. generates electricity) due to its potential difference. Note that Joule heat is generated when electrons flow through the members of the first fuel cell 3. This Joule heat can be supplied to the reforming reactor 2 using, for example, water generated on the fuel electrode 31 side as a medium. That is, water and heat can be simultaneously supplied to the reforming reactor 2. The chemical reaction formula at the air electrode 32 is represented by Formula 1, the chemical reaction formula at the fuel electrode 31 is represented by Formula 2, and the overall reaction is represented by Formula 3.
[0024]
(Equation 1)

(Equation 2)

(Equation 3)

[0025]
Hereinafter, preferred examples of the material used for the first fuel cell 3 will be described. As a material used for the electrolyte 30 of the first fuel cell 3, for example, yttria-stabilized zirconia (Zr_0.9Y_0.1O_1.95), Gadria-doped ceria (Ce)_0.8Gd_0.2O_1.9), Lanthanum gallate (La_0.8Sr_0.2Ga_0.85Mg_0.15O_2.825) Can be used in a temperature range of about 500 to 1000 ° C. and exhibit good oxide ion conductivity.
[0026]
The material used for the air electrode 32 of the first fuel cell 3 is, for example, La_1-xAE_xCo_1-yFe_yO₃And La_1-xAE_xMn_1-yCo_yO₃Perovskite oxides such as AE (alkaline earth metal elements such as Ca and Sr) are preferred examples. In addition, La_1-xAE_xCo_1-yFe_yO₃In the case of, generally, the composition ranges of 0.1 ≦ x ≦ 0.4 and 0.1 ≦ y ≦ 0.4 are preferable because they show a favorable catalytic reaction for oxygen ionization of oxygen. Also, La_1-xAE_xMn_1-yCo_yO₃In this case, the composition ranges of 0.1 ≦ x ≦ 0.4 and 0 ≦ y ≦ 0.4 are generally preferred. In particular, the Mn-based perovskite oxide is (La_1-xAE_x)_1-aMn_1-yCo_yO₃Can be present as a chemically stable electrode material, and 0 ≦ a ≦ 0.1 is considered as a preferable composition range. The method for producing the air electrode 32 is not particularly limited, and may be produced by a known method such as a solid phase reaction method, a citric acid method, and a coprecipitation method. As a starting material, for example, lanthanum oxide (La₂O₃), Alkaline earth metal carbonate (CaCO₃, SrCO₃Manganese oxide (Mn)₂O₃), Cobalt oxide (Co₃O₄), Iron oxide (Fe₂O₃) Etc. can be used. La as an example_0.8Sr_0.2Mn_0.9Co_0.1O₃The case where the air electrode 32 is formed by a solid-state reaction method will be described. The molar ratio of lanthanum oxide, strontium carbonate, manganese oxide, and cobalt oxide as starting materials is 0.8: 0.2: 0.9: After weighing and mixing so as to be 0.1, calcining at 1200 ° C. for about 10 hours, pulverizing again, mixing and firing to obtain an oxide powder having a perovskite structure. The powder is coated on the electrolyte 30 using an organic solvent such as turpentine oil, dried, and sintered at about 1200 ° C. for about 4 hours to form the air electrode 32.
[0027]
As a material used for the fuel electrode 31 of the first fuel cell 3, a mixture of TE (TE = Ni, Fe, Co, Cu) metal and ceramics (TE-based ceria cermet, TE-based zirconia cermet, etc.) is preferable. As In addition, from the results of many experiments and the theory of percolation, cermets of TE metal and ceramics generally provide sufficient electrical conductivity when TE metal is contained in about 40% by volume, and 40% by volume of TE metal and 60% by volume. It is known that many cermets composed of% ceramics have particularly excellent electrode characteristics. The method for producing the fuel electrode 31 is not particularly limited, and may be produced by a known method such as a solid phase mixing method, a spray pyrolysis method, and a combustion synthesis method. As an example, a case in which the fuel electrode 31 made of nickel zirconia cermet is manufactured by a solid-phase mixing method will be described. Is coated on the electrolyte 30 using the organic solvent described above, dried, and sintered at about 1400 ° C. for about 5 hours to obtain the fuel electrode 31. When fuel (hydrogen) is supplied to the fuel electrode 31, nickel oxide is reduced to form a nickel zirconia cermet.
[0028]
The principle of the second fuel cell 4 using the proton conductor 40 as the electrolyte will be described with reference to FIG. In the second fuel cell 4, a fuel gas containing hydrogen is supplied to the fuel electrode 41 (reference numeral 11 in FIG. 4 indicates a hydrogen molecule), and an oxidant gas (for example, air) is supplied to the air electrode 42. (Reference numeral 12 in FIG. 4 indicates an oxygen molecule, and reference numeral 13 indicates a nitrogen molecule), so that hydrogen in the fuel gas changes to hydrogen ions (protons) and passes through the proton conductor 40 to the air electrode 42 side. Go to On the air electrode 42 side, oxygen in the air is changed to oxygen ions, and the oxygen ions combine with the protons that have moved through the electrolyte 40 to generate water on the air electrode 42 side. In other words, in the second fuel cell 4, hydrogen in the reforming reactor 2 is moved to the outside of the reforming reactor 2, and is reacted with oxygen outside the reforming reactor 2 to change to water. At this time, a voltage is generated in the second fuel cell 4 by an electrochemical reaction in the air electrode 42 and the fuel electrode 41. If the air electrode 42 and the fuel electrode 41 are connected by a conductor for flowing electrons to form an external circuit, electrons flow from the fuel electrode 41 to the air electrode 42 through the external circuit, and the load 15 is taken in the external circuit. The electron flow works (ie, generates power) due to the potential difference. Note that Joule heat is generated when electrons flow through the members of the second fuel cell 4. The Joule heat is used as a heat source for preheating the hydrocarbon fuel to be supplied or steam reforming the hydrocarbon fuel by using a heat exchanger or the like with water generated on the air electrode 42 side as a medium. be able to. Further, high-temperature steam generated on the air electrode 42 side can be reused as a heat source for cogeneration. Note that the chemical reaction formula at the fuel electrode 41 is represented by Formula 4, the chemical reaction formula at the air electrode 42 is represented by Formula 5, and the overall reaction is represented by Formula 6.
[0029]
(Equation 4)

(Equation 5)

(Equation 6)

[0030]
Hereinafter, preferred examples of the material used for the second fuel cell 4 will be described. As a material used for the electrolyte 40 of the second fuel cell 4, for example, a rare earth element-doped barium-based serate oxide (for example, BaCe_1-xNd_xO_3-z, BaCe_0.9Nd_0.1O_2.95), Strontium zirconate doped with rare earth elements (for example, SrZr_1-xY_xO_3-z, SrZr_0.9Y_0.1O_2.95), Strontium-based serate oxides doped with rare earth elements (for example, SrCe_1-xYb_xO_3-z, SrCe_0.95Yb_0.05O_2.975) Can be used in a temperature range of about 500 to 1000 ° C. and exhibit good proton conductivity, and are therefore preferred examples. Note that z represents the amount of oxygen vacancies generated according to the valence of the element to be replaced. The method for producing the electrolyte 40 is not particularly limited, and may be produced by a known method such as a solid phase reaction method, a citric acid method, and a coprecipitation method. As a starting material, for example, cerium oxide (CeO₂), Zirconium oxide (ZrO)₂), Alkaline earth metal carbonates (SrCO₃, BaCO₃Etc.), rare earth oxides (Nd₂O₃, Y₂O₃, Yb₂O₃Etc.) can be used. BaCe as an example_0.9Nd_0.1O₃The case where the electrolyte 40 is prepared by the solid-state reaction method will be described. The starting materials are weighed such that the molar ratio of barium carbonate, cerium oxide, and neodymium oxide is 1: 0.9: 0.1. After mixing and calcining at 1200 ° C. for about 10 hours, pulverizing again, mixing and press-forming, and baking at 1500 to 1600 ° C. for about 10 hours to obtain an electrolyte 40.
[0031]
The material used for the air electrode 42 of the second fuel cell 4 is La_1-xAE_xCo_1-yFe_yO₃And La_1-xAE_xMn_1-yCo_yO₃Perovskite oxides such as AE (alkaline earth metal elements such as Ca and Sr) are preferred examples. In addition, La_1-xAE_xCo_1-yFe_yO₃In the case of, generally, the composition ranges of 0.1 ≦ x ≦ 0.4 and 0.1 ≦ y ≦ 0.4 are preferable because they show a favorable catalytic reaction for oxygen ionization of oxygen. Also, La_1-xAE_xMn_1-yCo_yO₃In this case, the composition ranges of 0.1 ≦ x ≦ 0.4 and 0 ≦ y ≦ 0.4 are generally preferred. In particular, the Mn-based perovskite oxide is (La_1-xAE_x)_1-aMn_1-yCo_yO₃Can be present as a chemically stable electrode material, and 0 ≦ a ≦ 0.1 is considered as a preferable composition range. The method for producing the air electrode 42 is not particularly limited, and may be produced by a known method such as a solid phase reaction method, a citric acid method, and a coprecipitation method. As a starting material, for example, lanthanum oxide (La₂O₃), Alkaline earth metal carbonate (CaCO₃, SrCO₃Manganese oxide (Mn)₂O₃), Cobalt oxide (Co₃O₄), Iron oxide (Fe₂O₃) Etc. can be used. La as an example_0.8Sr_0.2Mn_0.9Co_0.1O₃The case where the air electrode 42 is formed by the solid-state reaction method will be described. The molar ratio of the starting materials lanthanum oxide, strontium carbonate, manganese oxide, and cobalt oxide is 0.8: 0.2: 0.9: After weighing and mixing so as to be 0.1, calcining at 1200 ° C. for about 10 hours, pulverizing again, mixing and firing to obtain an oxide powder having a perovskite structure. The powder is coated on the electrolyte 40 using an organic solvent such as turpentine oil, dried, and sintered at about 1200 ° C. for about 4 hours to form the air electrode 42.
[0032]
Materials used for the fuel electrode 41 of the second fuel cell 4 include TE (TE = Ni (nickel), Fe (iron), Co (cobalt), Cu (copper)) metal and ceramics (for example, proton conductor oxide) (Cermet) is a preferred example. In addition, from the results of many experiments and the theory of percolation, cermets of TE metal and ceramics generally provide sufficient electrical conductivity when TE metal is contained in about 40% by volume, and 40% by volume of TE metal and 60% by volume. It is known that many cermets composed of% ceramics have particularly excellent electrode characteristics. The method for producing the fuel electrode 41 is not particularly limited, and may be produced by a known method such as a solid phase mixing method, a spray pyrolysis method, and a combustion synthesis method. As an example, a case where the fuel electrode 41 made of nickel-strontium serrate cermet is produced by a solid-phase mixing method will be described._0.95Yb_0.05O_2.975Coarse and fine particles, and nickel oxide are mixed, then coated on the electrolyte 40 using an organic solvent such as turpentine oil, dried, and sintered at about 1400 ° C. for about 5 hours to obtain a fuel electrode. 41. When the fuel (hydrogen) is supplied, nickel oxide is reduced in the fuel electrode 41 to form a nickel-strontium serrate cermet.
[0033]
The reforming reactor 2 of the present embodiment includes a hydrocarbon-based fuel (for example, methane (CH₄) And water vapor (H₂O) to react mainly with hydrogen (H₂) And carbon dioxide (CO₂) To produce the reaction of the following equation (7).
[0034]
(Equation 7)

[0035]
The gas in the reforming reactor 2 is actually steam (H₂O), carbon monoxide (CO), carbon dioxide (CO₂), Methane (CH₄), Hydrogen (H₂), It is described as "mainly producing hydrogen". The reforming reactor 2 is provided with a reforming catalyst 9 for causing the reaction of Formula 7 to occur. The reforming catalyst 9 is, for example, a nickel-based catalyst such as metallic nickel, or a mixture of a nickel-based catalyst and an iron-based oxide such as iron oxide or iron-chromium composite oxide at the rear in the gas flow direction. A known catalyst used for causing the reaction of Formula 7 may be used.
[0036]
From the reaction kinetics, the equilibrium reaction constant (K) in Equation 7 can be expressed by the following equation.
[0037]
(Equation 8)

However,
K: equilibrium reaction constant
P_CO2： Partial pressure of carbon dioxide
P_H2： Partial pressure of hydrogen
P_CH4： Partial pressure of methane
P_H2O: Partial pressure of water vapor
[0038]
The equilibrium reaction constant (K) takes a constant value for each temperature. As is known by Le Chatelier-Brown's law, in the equilibrium reaction of Equation 7, as the pressure in the reforming reactor 2 increases, the reaction proceeds to the left in the equation, so that the generated hydrogen Is reduced.
[0039]
The reaction in the case where the steam reforming proceeds, that is, the reaction in the case where the equilibrium reaction of Equation 7 proceeds in the right direction, can be expressed by the following equation.
[0040]
(Equation 9)

[0041]
As is apparent from Equation 9, in the steam reforming reaction in the reforming reactor 2, 4 moles of hydrogen molecules are generated from 1 mole of methane molecules and 2 moles of water molecules. On the other hand, in the first fuel cell 3 using the oxide ion conductor 30 as an electrolyte, as shown in Expression 3, the principle is that one mole of water molecule is generated for one mole of hydrogen molecule. Therefore, when power is generated only by the first fuel cell 3 and steam reforming of Expression 7 is performed using steam generated by the power generation, the amount of hydrogen generated by the steam reforming depends on the power generation of the first fuel cell 3. There is a problem that the amount of hydrogen consumed is twice as large as the amount of hydrogen consumed, the material balance in the reforming reactor 2 does not match, and the pressure in the reforming reactor 2 continues to increase. If the gas in the reforming reactor 2 is extracted as exhaust gas in order to lower the pressure in the reforming reactor 2, the fuel utilization of the fuel cell power generation system 1 decreases, and the exhaust gas is burned. Even if the fuel generated by the combustion is used as a heat source for preheating the hydrocarbon-based fuel for supplying heat or steam reforming of the hydrocarbon-based fuel, the thermal efficiency of the fuel cell power generation system 1 is greatly reduced.
[0042]
On the other hand, in the fuel cell power generation system 1 of the present embodiment, the first fuel cell 3 and the second fuel cell 4 are used together. Therefore, hydrogen obtained by steam reforming in the reforming reactor 2 is used for power generation in the first fuel cell 3 and the second fuel cell 4. The first fuel cell 3 consumes hydrogen in the reforming reactor 2 to supply steam to the reforming reactor 2, while the second fuel cell 4 consumes hydrogen in the reforming reactor 2. In addition, steam is discharged outside the reforming reactor 2. That is, excessive hydrogen generated in the reforming reactor 2 due to the interaction between steam reforming in the reforming reactor 2 and power generation of the first fuel cell 3 is reformed by power generation of the second fuel cell 4. It is discharged in the form of steam outside the reactor 2. Therefore, the mass balance of hydrogen and water vapor in the reforming reactor 2 can be controlled by the amount of power generated by the first fuel cell 3 and the amount of power generated by the second fuel cell 4.
[0043]
Here, in the reforming reactor 2 of the present embodiment, hydrogen and carbon dioxide are generated by the reaction between the hydrocarbon fuel and the steam (see Equation 7). Continue to remain, it becomes difficult for the equilibrium reaction of Expression 7 to proceed rightward. Therefore, for example, in the fuel cell power generation system 1 of the present embodiment, the reforming reactor 2 is provided with a carbon dioxide separation membrane 5 as carbon dioxide separating means.
[0044]
The carbon dioxide separation membrane 5 serves as a partition (partition) between the reforming reactor 2 and the carbon dioxide separation chamber 6. For example, in the present embodiment, the material of the carbon dioxide separation membrane 5 is an alkali metal-based composite oxide A₂BO₃(A = alkali metal element such as Li, Na, K, etc., B = titanium group element such as Ti, Zr, etc. However, the element corresponding to A or B may be not only one kind but plural kinds of elements, For example, LiKZrO₃, Li₂Zr_0.5Ti_0.5O₃But it's fine. ) Or alkali metal orthosilicate A₄SiO₄(A = Alkali metal element such as Li, Na, K, etc. However, the element corresponding to A may be not only one kind but also plural kinds of elements, for example, Li₂K₂SiO₄But it's fine. ) Is used at least. Then, a pressure difference is provided so that the partial pressure of carbon dioxide in the carbon dioxide separation chamber 6 is lower than the partial pressure of carbon dioxide in the reforming reactor 2. For this reason, the partial pressure of carbon dioxide in the carbon dioxide separation chamber 6 is set to a negative pressure (for example, atmospheric pressure or less) using, for example, an intake device.
[0045]
The action of the carbon dioxide separation membrane 5 is represented by A₂BO₃Will be described as an example with reference to FIG. On the surface of the carbon dioxide separation membrane 5 on the reforming reactor 2 side, A₂BO₃Is carbon dioxide (CO₂A)₂CO₃And BO₂(That is, the reaction of Equation 10 proceeds).
[0046]
(Equation 10)

(Equation 11)

(Equation 12)

[0047]
A generated by the reaction₂CO₃Or CO₃ ^2-Is in a molten state and diffuses in the carbon dioxide separation membrane 5 toward the carbon dioxide separation chamber 6. Then, on the surface of the carbon dioxide separation membrane 5 on the carbon dioxide separation chamber 6 side, A₂CO₃Is dissociated and carbon dioxide (CO₂) Is released to the carbon dioxide separation chamber 6 (that is, the reaction of Formula 11 proceeds). Or CO₃ ^2-Is dissociated and carbon dioxide (CO₂) Is discharged into the carbon dioxide separation chamber 6. A generated by the dissociation₂O diffuses back into the reforming reactor 2 through the carbon dioxide separation membrane 5. The despread A₂O is BO₂Reacts with A₂BO₃(That is, the reaction of Formula 12 proceeds), and the carbon dioxide (CO 2)₂), And again in the molten state A₂CO₃Or CO₃ ^2-Then, it diffuses in the carbon dioxide separation membrane 5 toward the carbon dioxide separation chamber 6. Thus, a mass transfer cycle is realized, and absorption of carbon dioxide on the reforming reactor 2 side and emission of carbon dioxide on the carbon dioxide separation chamber 6 side are continuously performed.
[0048]
A₄SiO₄In the case of the above, carbon dioxide can be separated from the reforming reactor 2 to the carbon dioxide separation chamber 6 by the same principle. That is, on the surface of the carbon dioxide separation membrane 5 on the side of the reforming reactor 2, A₄SiO₄Is carbon dioxide (CO₂A)₂CO₃And A₂SiO₃(That is, the reaction of Expression 13 proceeds).
[0049]
(Equation 13)

[Equation 14]

[Equation 15]

[0050]
A generated by the reaction₂CO₃Or CO₃ ^2-Is in a molten state and diffuses in the carbon dioxide separation membrane 5 toward the carbon dioxide separation chamber 6. Then, on the surface of the carbon dioxide separation membrane 5 on the carbon dioxide separation chamber 6 side, A₂CO₃Is dissociated and carbon dioxide (CO₂) Is released to the carbon dioxide separation chamber 6 (that is, the reaction of Expression 14 proceeds). Or CO₃ ^2-Is dissociated and carbon dioxide (CO₂) Is discharged into the carbon dioxide separation chamber 6. A generated by the dissociation₂O diffuses back into the reforming reactor 2 through the carbon dioxide separation membrane 5. The despread A₂O is A₂SiO₃Reacts with A₄SiO₄(That is, the reaction of Formula 15 proceeds), and the carbon dioxide (CO 2)₂), And again in the molten state A₂CO₃Or CO₃ ^2-Then, it diffuses in the carbon dioxide separation membrane 5 toward the carbon dioxide separation chamber 6. Thus, a mass transfer cycle is realized, and absorption of carbon dioxide on the reforming reactor 2 side and emission of carbon dioxide on the carbon dioxide separation chamber 6 side are continuously performed.
[0051]
Li₂ZrO₃As an example, the operation of the carbon dioxide separation membrane 5 will be described in more detail. FIG.₂ZrO₃CO₂Shows absorption and emission characteristics. The vertical axis of FIG.₂Li in the atmosphere₂ZrO₃CO₂Weight change rate due to absorption (100 x (weight measured at each temperature on the horizontal axis in the figure-initial weight before measurement) / (initial weight before measurement) [%]), that is, Li₂ZrO₃CO₂Indicates the amount of absorption. The graph indicated by the symbol A in the figure shows the case where the carbon dioxide partial pressure is 0.7 MPa, the graph indicated by the symbol B in the diagram shows the case where the carbon dioxide partial pressure is 0.5 MPa, and the graph shown by the symbol C in the figure. The graph shows the case where the partial pressure of carbon dioxide is 0.3 MPa, and the graph shown by the symbol D in the figure shows the case where the partial pressure of carbon dioxide is 0.1 MPa. CO₂When the temperature becomes about 460 ° C. or more under an atmosphere, Li₂ZrO₃Is CO₂To absorb Li₂CO₃And ZrO₂(That is, the reaction of Expression 16 proceeds). At a higher temperature (eg, about 900 ° C.), Li₂CO₃Is CO₂To release Li₂ZrO₃(That is, the reaction of Expression 17 and Expression 18 proceeds).
[0052]
(Equation 16)

[Equation 17]

(Equation 18)

[0053]
As shown in FIG. 9, the higher the carbon dioxide partial pressure is,₂Absorption of CO and absorption of CO₂Temperature until it releases. Therefore, if a partial pressure difference of carbon dioxide is provided across the carbon dioxide separation membrane 5 in a certain temperature range (about 600 ° C. to 800 ° C., more preferably around 750 ° C. in the example of FIG. 9), The separation membrane 5 is provided with CO₂Absorbs CO on the low pressure side₂Will be released. The certain temperature range is the operating temperature of the carbon dioxide separation membrane 5.
[0054]
Therefore, on the high pressure side (the reforming reactor 2 side), the reaction of Formula 16 proceeds, and Li₂ZrO₃Is CO₂Reacts with Li in the molten state₂CO₃And solid ZrO₂Occurs. Li in molten state₂CO₃Or CO₃ ^2-Diffuses in the carbon dioxide separation membrane 5 to the low pressure side (the carbon dioxide separation chamber 6 side). On the low pressure side, the reaction of Formula 17 proceeds, and Li₂CO₃Or CO₃ ^2-Is CO₂To release Li₂It becomes O and conversely diffuses to the high pressure side. Li diffused to high pressure side₂O is ZrO₂Reacts with Li₂ZrO₃Return to CO₂Reacts with Li in the molten state₂CO₃Or CO₃ ^2-And diffuses in the carbon dioxide separation membrane 5 to the low pressure side. This allows CO₂Can be continuously separated from the reforming reactor 2 into the carbon dioxide separation chamber 6.
[0055]
In this embodiment, for example, the carbon dioxide separation membrane 5 in the form of a thin film is supported by the porous membrane 19. As a material of the porous film 19, for example, zirconia stabilized with a rare earth element such as alumina or yttria is preferable. For example, in the present embodiment, A₂BO₃And BO₂Particles (inert nanoparticles) 18 which hinder grain growth and have a particle size of the order of nanometers are added to the material of the carbon dioxide separation membrane 5. As the inert nanoparticles 18, for example, alumina is suitable. In addition, the code | symbol 17 in FIG.₂BO₃(Eg Li₂ZrO₃) Indicates particles, and reference numeral 16 denotes BO₂(For example, ZrO₂) Indicates particles, and reference numeral 20 indicates A in a molten state.₂CO₃(Eg Li₂CO₃).
[0056]
Where CO₂In order to perform continuous separation, smooth mass transfer in the carbon dioxide separation membrane 5 is required.₂CO generated on the low pressure side by the absorption reaction₂(Eg, Li)₂CO₃) Is preferably in a molten state. Therefore, by adjusting the melting point of the above substance, the operating temperature of the carbon dioxide separation membrane 5 (that is, CO₂While absorbing CO and on the low pressure side₂It is thought that it is possible to control the temperature at which For example, by doping (adding) an alkali metal-based carbonate to the film material, the melting point of the substance can be lowered.
[0057]
Li₂CO₃Has a melting point of 732 ° C., but Li₂CO₃And K₂CO₃Is known to have a melting point of 500 ° C. or lower (for example, see FIG. 10A). Furthermore, Li₂CO₃And K₂CO₃And Na₂CO₃Is known to have a melting point of 400 ° C. or lower (for example, see FIG. 10B). Thus, for example, K₂CO₃And Na₂CO₃One or both of the film materials Li₂ZrO₃, And by adjusting the amount thereof, the operating temperature of the carbon dioxide separation membrane 5 can be controlled within the range of about 400 to 700 ° C. For example, K₂CO₃And Na₂CO₃One or both of the film materials Li₂ZrO₃To Li_2-xNa_xZrO₃Or Li_2-xK_xZrO₃Or Li_2-xyNa_xK_yZrO₃And CO₂Li by absorption reaction₂CO₃Or Li_2-xNa_xCO₃Or Li_2-xK_xCO₃Or Li_2-xyNa_xK_yCO₃Are produced, and these products are brought into a molten state in a target temperature range. As a result, these substances diffuse well in the carbon dioxide separation membrane 5, and₂Separation is performed continuously.
[0058]
The operating temperature of the carbon dioxide separation membrane 5 is preferably adjusted to the operating temperatures of the reforming catalyst 9, the first fuel cell 3, and the second fuel cell 4, from the viewpoint of facilitating temperature adjustment in the fuel cell power generation system 1. Further, the temperature inside the reforming reactor 2 is preferably, for example, about 500 ° C. to 1000 ° C. This is because the operating temperature of the first fuel cell 3 and the second fuel cell 4 of the present embodiment is about 500 ° C. to 1000 ° C.₂ZrO₃Does not absorb carbon dioxide, it is considered that the operation of the carbon dioxide separation membrane 5 is considered to be difficult, and the like.
[0059]
Since the carbon dioxide separation membrane 5 functions at a high temperature such as the reaction field of Equation 7, a step of cooling the mixed gas in the reforming reactor 2 is unnecessary. Further, in the carbon dioxide separation membrane 5, since the absorption and release of carbon dioxide are performed continuously, the step of regenerating the absorbent or the like is unnecessary. For this reason, a cooling step or a regeneration step that consumes a lot of energy becomes unnecessary, and the energy efficiency can be greatly improved. In principle, since only carbon dioxide can pass through the carbon dioxide separation membrane 5, high-purity carbon dioxide can be recovered, and the recovery of carbon dioxide can contribute to prevention of global warming.
[0060]
The carbon dioxide separated from the reforming reactor 2 has high heat. Therefore, the thermal efficiency of the fuel cell power generation system 1 can be further improved by effectively utilizing the heat of the carbon dioxide. For example, in the present embodiment, the heat of the carbon dioxide is transmitted to the hydrocarbon-based fuel and water supply pipe 22 using the heat exchanger 21 and is used for preheating the hydrocarbon-based fuel and water. (See FIG. 7). Alternatively, the heat of carbon dioxide may be transferred to the reforming reactor 2 or a reactor for preliminary steam reforming using a heat exchanger or the like, and used for the endothermic reaction of steam reforming. Further, the high-temperature carbon dioxide separated from the reforming reactor 2 can be reused as a heat source for cogeneration.
[0061]
An example of the operation of the fuel cell power generation system 1 configured as described above will be described mainly with reference to FIG. The hydrocarbon fuel required for the reaction of Formula 7 is supplied from fuel supply means (for example, a tank filled with natural gas containing methane as a main component) 23. Water required for the initial stage of operation is supplied from a water supply unit (for example, a water supply tank) 24, and heat required for the initial stage of operation is supplied by a heat supply unit (for example, a heat exchanger or a heating device) 25. For example, the high-pressure natural gas supplied from the fuel supply unit 23 and the water supplied from the water supply unit 24 are mixed, and both are heated to a high temperature of, for example, 500 ° C. to 1000 ° C. by the heat supply unit 25. To the reactor 2.
[0062]
In the reforming reactor 2, the hydrocarbon-based fuel reacts with steam under the reforming catalyst 9 to produce mainly hydrogen and carbon dioxide. The generated hydrogen is used for power generation in the first fuel cell 3 and the second fuel cell 4. Hydrogen in the reforming reactor 2 is consumed by the power generation of the first fuel cell 3, and steam and heat are supplied into the reforming reactor 2. Water is required to advance the steam reforming represented by Expression 7, and the reforming reaction is an endothermic reaction and thus requires heat. Power generation. On the other hand, the amount of hydrogen generated by the steam reforming is larger than the amount of hydrogen consumed by the power generation of the first fuel cell 3, and excessive hydrogen is generated in the reforming reactor 2. 4 and is discharged in the form of steam to the outside of the reforming reactor 2. That is, the mass balance of hydrogen in the reforming reactor 2 is controlled by the power generation amount of the first fuel cell 3 and the power generation amount of the second fuel cell 4. Furthermore, in the fuel cell power generation system 1 of the present embodiment, the carbon dioxide generated in the reforming reactor 2 by the carbon dioxide separation membrane 5 is released to the outside of the reforming reactor 2. That is, the mass balance of the carbon component in the reforming reactor 2 is controlled by the carbon dioxide separation membrane 5. At the same time as the hydrogen inside the reforming reactor 2 is consumed by the first fuel cell 3 and the second fuel cell 4, the carbon dioxide inside the reforming reactor 2 is removed from the outside of the reforming reactor 2 by the carbon dioxide separation membrane 5. Therefore, the equilibrium reaction of the equation (7) in the reforming reactor 2 can be promptly advanced rightward. The hydrocarbon-based fuel circulates in the reforming reactor 2 until participating in the above reaction. The high-temperature steam released to the outside of the reforming reactor 2 by the power generation of the second fuel cell 4 is used as a heat source for preheating the supplied hydrocarbon-based fuel or steam reforming using a heat exchanger or the like. can do. The high-temperature carbon dioxide separated from the reforming reactor 2 can be used as a heat source for preheating the hydrocarbon-based fuel to be supplied by using the heat exchanger 21.
[0063]
According to this fuel cell power generation system 1, by combining the two types of fuel cells 3 and 4 and the carbon dioxide separation membrane 5, the material balance in and out of the fuel cell power generation system 1 can be matched (in other words, mass transfer). Is achieved), and the supplied hydrocarbon-based fuel and the generated hydrogen can be used 100% for the power generation of the first fuel cell 3 and the second fuel cell 4. Then, if the initially required water and heat are supplied to the reforming reactor 2 by the water supply means 24 and the heat supply means 25, the water is thereafter naturally supplied by the power generation of the first fuel cell 3, and Is naturally supplied by the power generation of the first fuel cell 3 and the second fuel cell 4 and the high-temperature carbon dioxide separated from the reforming reactor 2. Only the system fuel needs to be supplied. Therefore, a closed cycle for water and heat is realized, and the energy efficiency and energy saving of the fuel cell power generation system 1 can be improved. Further, by establishing a technology for realizing a fuel utilization rate of 100% according to the present invention, it is possible to solve the cost problem relating to the SOFC. In addition, in the present invention, the fuel utilization rate of 100% is achieved by matching the material balances in and out of the fuel cell power generation system 1, so that it is not necessary to set the hydrogen concentration on the fuel gas outlet side to zero. In addition, phenomena such as an increase in overvoltage and generation of stray current caused by extremely low hydrogen concentration at the fuel gas outlet side do not occur. According to the fuel cell power generation system 1, it is considered that the fuel cell body can achieve a thermal efficiency of 70% or more.
[0064]
The above embodiment is an example of a preferred embodiment of the present invention, but the present invention is not limited to this, and various modifications can be made without departing from the gist of the present invention. For example, when water or heat in the reforming reactor 2 is insufficient due to operating conditions or the like, the water or heat may be supplied appropriately by the water supply unit 24 or the heat supply unit 25 as a matter of course. is there.
[0065]
【The invention's effect】
As is clear from the above description, according to the power generation method by the fuel cell according to the first aspect and the fuel cell power generation system according to the second aspect, the material balance in the reforming reactor is matched, in other words, the mass transfer of the mass transfer is performed. The hydrocarbon-based fuel to be balanced and supplied and the generated hydrogen can be used 100% for the power generation of the first fuel cell and the second fuel cell. If the initially required water and heat are supplied to the reforming reactor, the required water and heat are naturally supplied by the power generation of the first fuel cell and the second fuel cell. The vessel only needs to be supplied with hydrocarbon fuel. Therefore, a closed cycle for water and heat is realized, and the energy efficiency and energy saving of the fuel cell power generation system can be improved. Further, by establishing a technology for realizing a fuel utilization rate of 100% according to the present invention, it is possible to solve the cost problem relating to the SOFC. Further, in the present invention, a fuel utilization rate of 100% is achieved by matching the material balances, and it is not necessary to set the hydrogen concentration on the fuel gas outlet side to 0. No harm occurs.
[0066]
Furthermore, according to the fuel cell power generation system of the third aspect, it is also possible to control the mass balance of the carbon component in the reforming reactor, and the hydrogen in the reforming reactor is charged with the first fuel cell and the second fuel. Simultaneously with the consumption of the battery, the carbon dioxide in the reforming reactor is released to the outside of the reforming reactor by the carbon dioxide separating means, so that the hydrogen generation reaction in the reforming reactor can proceed promptly. It is possible to efficiently perform fuel cell power generation.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing one embodiment of a fuel cell power generation system of the present invention.
FIG. 2 is a schematic configuration diagram showing a cross section taken along line II-II of FIG.
FIG. 3 is a conceptual diagram showing the principle of a first fuel cell in which an electrolyte is an oxide ion conductor.
FIG. 4 is a conceptual diagram showing the principle of a second fuel cell in which the electrolyte is a proton conductor.
FIG. 5 is a schematic configuration diagram showing another configuration example of the fuel cell power generation system of the present invention (when a plurality of single cells of a fuel cell are stacked).
FIG. 6 is a schematic configuration diagram showing a cross section taken along line VI-VI of FIG. 5;
FIG. 7 is a schematic configuration diagram showing one embodiment of a fuel cell power generation system of the present invention.
FIG. 8 is a conceptual diagram showing the principle of the carbon dioxide separation means of the present invention.
FIG. 9: Li₂ZrO₃CO₂5 is a graph showing absorption and emission characteristics, in which the horizontal axis represents temperature [° C.] and the vertical axis represents CO.₂Li by absorption₂ZrO₃(100 × (weight measured at each temperature on the horizontal axis−initial weight before measurement) / (initial weight before measurement) [%]).
FIG. 10 (A) shows Li₂CO₃And K₂CO₃(B) shows the phase diagram of Li₂CO₃And K₂CO₃And Na₂CO₃FIG.
[Explanation of symbols]
1 fuel cell power generation system
2 Reforming reactor
3 First fuel cell
4 Second fuel cell
5 Carbon dioxide separation membrane (carbon dioxide separation means)
30 Oxide ion conductor
31 Fuel electrode of the first fuel cell
32 Air electrode of the first fuel cell
40 Proton conductor
41 Fuel electrode of the second fuel cell
42 Air electrode of the second fuel cell

Claims (3)

A fuel gas containing hydrogen is generated by reacting a hydrocarbon-based fuel and steam in a reforming reactor, and the fuel gas is circulated, and the fuel electrode of the first fuel cell in which the electrolyte is an oxide ion conductor And a fuel electrode of a second fuel cell whose electrolyte is a proton conductor is disposed so as to be in contact with the circulating fuel gas, and an air electrode of the first fuel cell and an air electrode of the second fuel cell are disposed. Is disposed so as to be in contact with the oxidizing gas flowing outside the reforming reactor, and consumes the hydrogen generated in the reforming reactor by the power generation by the first fuel cell and enters the reforming reactor. A fuel cell for supplying steam and consuming hydrogen generated in the reforming reactor by power generation by the second fuel cell and discharging steam to the outside of the reforming reactor; Power generation method. A reforming reactor that mainly generates hydrogen by reacting a hydrocarbon-based fuel with steam; a first fuel cell; and a second fuel cell, wherein the first fuel cell is provided outside the reforming reactor. An air electrode for oxygen ionizing the oxidizing gas present in the air, an oxide ion conductor as an electrolyte to which the oxygen ions move, and oxygen ions having moved through the oxide ion conductor and formed in the reforming reactor. A fuel electrode for reacting the generated hydrogen with water to produce water, wherein the second fuel cell comprises a fuel electrode for hydrogen ionizing the hydrogen generated in the reforming reactor, and an electrolyte for moving the hydrogen ions. As a proton conductor, and an air electrode that generates water by reacting hydrogen ions having moved through the proton conductor with an oxidizing gas present outside the reforming reactor. Fuel cell power generation Stem. 3. The fuel cell power generation system according to claim 2, further comprising a carbon dioxide separating means for selectively moving carbon dioxide in the reforming reactor to outside the reforming reactor.

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