JP3889827B2

JP3889827B2 - Reducing gas heating method and apparatus

Info

Publication number: JP3889827B2
Application number: JP00516096A
Authority: JP
Inventors: 孝明毛利; 良亮清水; 泰正森田
Original assignee: Chiyoda Corp
Current assignee: Chiyoda Corp
Priority date: 1996-01-16
Filing date: 1996-01-16
Publication date: 2007-03-07
Anticipated expiration: 2016-01-16
Also published as: JPH09194918A

Description

【０００１】
【発明の属する技術分野】
本発明は、主として直接還元製鉄プロセスに用いられる一酸化炭素含有還元性ガスの加熱方法に関する。
【０００２】
【従来の技術】
直接還元製鉄プロセスでは、８００〜１０００℃に加熱した還元性ガスを用いて鉄鉱石（酸化鉄）を還元することにより、金属鉄を製造する。ここで用いられる還元性ガスは一般に水素、一酸化炭素、二酸化炭素及び水蒸気を含み、これに加えてメタン、窒素などを含む場合も多い。このような還元性ガスを８００〜１０００℃に加熱するには炉内にガス加熱管を配した加熱炉を用いる。加熱炉は一般にバーナを備えた燃焼室（輻射伝熱部）と燃焼ガスを通す対流伝熱部とからなり、還元性ガスはまず対流伝熱部に配されたガス加熱管内を通過して約４００〜４５０℃に予熱され、次いで燃焼室に配されたガス加熱管内を通過して所望の温度にまで加熱される。
【０００３】
燃焼室に配備されたガス加熱管は、外径１５０〜１９０ｍｍ程度の管が図１に示すように折り畳まれた形状をしているため伝熱コイルと呼ばれており、一般に複数のこのような伝熱コイルが並列に接続されている。伝熱コイルは伝熱コイルの両側の壁面に設けられたラジアントウォールバーナーの火炎からの輻射熱を受けて加熱され、これにより伝熱コイル内を通過する還元性ガスが所定の温度まで加熱される。このときのガスの昇温速度は２００〜３００℃／秒程度である。
【０００４】
【発明が解決しようとする課題】
ところが、継続的に加熱炉を使用していると上記ガス加熱管の一部が消耗減肉するという現象（メタルダスティング）が生ずることがわかり、そのたびに消耗減肉した部分のガス加熱管を交換しなければならないという不都合があった。この現象は特に燃焼室に配されたガス加熱管の一部（前半部、図１参照）に多く見られる。これはガス加熱管内面の浸炭脆化が原因であると思われる。すなわち、一酸化炭素を含む還元性ガスを加熱すると次のような不均化反応（ブドアール反応）によりカーボンが析出すると考えられる。
２ＣＯ ⇔ ＣＯ₂ ＋Ｃ↓
こうして析出したカーボンはガス加熱管内面に付着し、高温のガス加熱管の材料（主要成分）である鉄等と反応して浸炭現象を生じ、ガス加熱管内面を脆化消耗させると考えられるのである。
【０００５】
このような浸炭脆化を防ぐ方策として、ガス加熱管（特にその内面）の材質を改良してカーボンが析出しても浸炭脆化が起こらないようにすることも考えられる。しかしながら、加熱炉の使用条件やコストを考慮すると、必ずしもすべての場合にそのような材質改良のみで対処することができるとはいえず、あるいは不可能ではないにしても現実的な解決策とはいえない場合がしばしばある。
【０００６】
本発明は以上のような要請を念頭においてなされたものであり、材質改良とは別の観点からガス加熱管の浸炭脆化による消耗減肉を防止する方策として、一酸化炭素を含む還元性ガスを加熱する加熱炉のガス加熱管内面へのカーボンの析出自体を防止しようとするものである。
【０００７】
【課題を解決するための手段】
本発明は、一酸化炭素含有還元性ガスを伝熱面を介して加熱する方法を提供する。その方法は、該ガスからカーボンが析出しやすい危険温度帯域を定め、該ガスの該危険温度帯域における昇温速度が１０００℃／秒以上となるように急速加熱するものである。
【０００８】
或いはその方法は、該ガスからカーボンが析出しやすい危険温度帯域を定め、該ガスの該危険温度帯域における滞留時間が０．３５以下となるように急速加熱するものである。
【０００９】
本発明はまた、上記方法を実施するために特に適した装置をも提供するものである。そのような装置は、燃焼室内にバーナーとガス加熱管が配備された加熱部を有し、該ガス加熱管を該バーナーの火炎で外側から加熱する一方、一酸化炭素含有還元性ガスを該ガス加熱管内に流通させることにより、該一酸化炭素含有還元性ガスを昇温するガス加熱炉であって、該ガス加熱管が外径３０〜８０ｍｍの直管からなるマルチパス構造をとる。
【００１０】
【作用】
本発明において加熱の対象となる一酸化炭素含有還元性ガスとしては、まず第一に直接還元製鉄用の還元炉テールガスが上げられる。このガスの典型的な組成の一例を示せば、
【表１】
還元炉テールガス（脱ＣＯ ₂ 後）
Ｈ₂ ７０．７％（体積％、以下同じ）
ＣＯ１６．７％
ＣＯ₂ １．０％
ＣＨ₄ ３．５％
Ｎ₂ ６．８％
Ｈ₂Ｏ１．３％（飽和量）
となる。
【００１１】
もちろん本発明において加熱の対象となるガスは上記還元炉テールガスに限られるものではなく、典型的な組成を下記に示すような改質器からのリフォームドガス、コークス炉ガス、高炉ガス、転炉ガスなど、各種の一酸化炭素含有還元性ガスが対象となる。
【表２】
改質器からのリフォームドガスの組成の一例
Ｈ₂ ５８．２％（体積％、以下同じ）
ＣＯ１２．９％
ＣＯ₂ ５．７％
ＣＨ₄ ０．３％
Ｈ₂Ｏ２２．９％
【表３】
コークス炉ガスの組成の一例
Ｈ₂ ５０〜５７％（体積％、以下同じ）
ＣＯ５〜７％
ＣＯ₂ ２〜５％
ＣＨ₄ ２２〜３０％
Ｃ₂ ⁺ ２〜５％
Ｎ₂ ２〜６％
Ｈ₂Ｏ飽和量
【表４】
高炉ガスの組成の一例
Ｈ₂ ３〜４％（体積％、以下同じ）
ＣＯ２１〜２４％
ＣＯ₂ ２０〜２３％
Ｎ₂ ５２〜５６％
Ｈ₂Ｏ飽和量
【表５】
転炉ガスの組成の一例
Ｈ₂ １．５〜３．３％（体積％、以下同じ）
ＣＯ６５〜６７％
ＣＯ₂ １２〜１４％
ＣＨ₄ ０．２〜０．４％
Ｎ₂ １６〜１９％
Ｈ₂Ｏ飽和量
なお、一酸化炭素含有率が４％未満のガスはカーボン析出の傾向が極めて小さいので、本発明の手段によらなくてもガス加熱管の消耗減肉が実質的に生ずることはない。
【００１２】
一酸化炭素を含む還元性ガスからカーボンを析出させる前記ブドアール反応は発熱反応であり、化学平衡論的には低温ほど反応が右に進むためカーボンが析出しやすいといえる。すなわち、ブドアール反応の化学平衡定数
Ｋp（eq）＝Ｐ_CO2 ^*／（Ｐ_CO ^*）²
（Ｐ_CO2 ^* ………平衡二酸化炭素分圧、単位kg/cm²）
（Ｐ_CO ^* ………平衡一酸化炭素分圧、単位kg/cm²）
は熱力学的には、所定の圧力条件下に温度に対して単調減少する関数として表される。例えば圧力３．５Kg/cm²Ｇの下で常温より９００℃まで加熱する場合を想定し、熱力学的に計算したＫp（eq）の値を温度に対してプロットすれば図２に示すような曲線となる。また別に、前記に示した還元炉テールガスの組成から全圧３．５kg/cm²Ｇの下での実際の一酸化炭素分圧Ｐ_CO及び二酸化炭素分圧Ｐ_CO2 を求め、
Ｐ_CO2／（Ｐ_CO）²＝Ｋp（obs）
［（obs）は観測値（observed value）であることを示す。］
とおいたものを同じく図２に示す。Ｋp（eq）＞Ｋp（obs）である温度領域がカーボンが析出する可能性のある領域であり、Ｋp（eq）／Ｋp（obs）の比が大きいほどカーボンが析出しやすい。なお、Ｋp（obs）＞２．０（kg/cm²）^-1となるようなガスはカーボン析出の傾向が極めて少ないので、本発明の手段によらなくてもガス加熱管の消耗減肉が実質的に生ずることはない
【００１３】
一方、低温では前記ブドアール反応の反応速度は大きくないから、カーボンが析出する可能性がある領域においては高温の方が析出しやすいともいえる。従って、カーボンが析出しやすい危険温度帯域とは、熱力学的にカーボン析出のポテンシャルが高く、かつ反応速度も比較的大きな温度範囲と定義される。一般に、Ｋp（eq）＞２．５Ｋp（obs）であるとき熱力学的にカーボン析出のポテンシャルが高いといえ、また４５０℃以下では反応速度が小さいためカーボン析出の傾向は小さい。例えば図２に示す例では、カーボンが析出しやすい危険温度帯域は一般に５００℃〜７７５℃と定められる。
【００１４】
上記のように、カーボンが析出しやすい危険温度帯域は、加熱対象となるガスの組成（特に一酸化炭素濃度と二酸化炭素濃度）及び操作圧力によって異なる。対象となる系について定められた危険温度帯域を、当該ガスは１０００℃／秒以上、好ましくは１５００〜３５００℃／秒の昇温速度で加熱される必要がある。これは前記した従来の昇温速度（高々３００℃／秒）に比べてかなり大きな昇温速度である。なお、危険温度帯域以外の温度域では、従来の昇温速度で加熱して差し支えない。
【００１５】
一般に危険温度帯域は広くても３５０℃（４５０℃から８００℃まで昇温する場合）であるから、１０００℃／秒の昇温速度では０．３５秒以下、１５００℃／秒及び３５００℃／秒の昇温速度ではそれぞれ０．２３秒以下及び０．１秒以下でガスは当該危険温度帯域を通過することになる。かくして、ガスの危険温度帯域での滞留時間は極めて短くなり、ブドアール反応によるカーボンの析出は有効に抑制される。なお、上記のように、４５０℃から８００℃という温度範囲は通常の条件下における危険温度帯域をほぼカバーする範囲であるから、４５０℃から８００℃までを１０００℃／秒以上の速度で昇温させるように設計すれば、一般に本発明の昇温条件は自動的に（予め危険温度帯域を定義しなくても）満たされることになる。
【００１６】
１０００℃／秒以上という大きな昇温速度を得るためには、従来より伝熱係数を大きくする必要がある。この場合の伝熱係数は火炎からガス流への伝熱についてのものということになるが、これは火炎から管表面への輻射伝熱、管外表面から管内表面への伝熱、及び管内表面からガス流への対流伝熱が総合的に寄与する総括伝熱係数である。ここで、管内表面からガス流への対流伝熱を促進するためには、管内表面近傍に形成される境膜（層流）部分の厚みを減らすことが有効である。境膜の厚みはガス流速を大きくすれば減少するが、従来は外径１５０〜１９０ｍｍ程度の比較的太い管を用いていたため、管内ガス線速度を上げようとすると（加熱管の本数が減少することにより）ガス流量に対して伝熱面積が小さくなり、それを避けようとすれば（加熱管の長さが長くなることにより）ガスの対流時間が長くなるという欠点があった。これに対して、本発明においては、当該危険温度帯域に相当する部分のガス加熱管として従来のものに比べて細い管（外径３０〜８０ｍｍ）を用いるので上記欠点が解消する。当該危険温度帯域における平均ガス線流速は約４０ｍ／秒以上とすることが好ましい。
【００１７】
また、従来は図１に示すような折り畳み形状の加熱管を用いているため、Ｕターン部におけるガスの滞留やカーボンの局所的沈積、あるいはＵターン部におけるエロージョンに起因すると思われる消耗減肉が見られる。このような問題を解決するにはＵターン部を設けないことが好ましい。従って、本発明においては、当該危険温度帯域部分を直管で構成した直管マルチパス方式を採用する。これは不必要な圧損の増大を避けるため、ガス流速の増大に対しても好結果をもたらすと考えられる。
【００１８】
通常、カーボンが析出しやすい危険温度帯域は燃焼室に設けられたガス加熱管の前半部（一般に約４５０℃から約８００℃まで昇温する部分）にある。すなわち、必ずしも燃焼室に設けられた全ガス加熱管を上記のように比較的細い直管で構成し、かつ全断面積を絞ってガス線流速を上げる必要はない。従って、燃焼室内のガス加熱管を前半部と後半部とに分割し、前半部の加熱管（当該危険温度帯域を含む部分）のみについて上記構成をとれば、本発明の目的は達成される。
【００１９】
【発明の実施の形態】
実施例１
外径６０．５ｍｍ、肉厚８．５ｍｍ、長さ１３ｍの直管５本を並列に接続したガス加熱管を有する実験用加熱炉を製作し、次の組成を有する一酸化炭素含有還元性ガスの加熱を行った。
【表６】
ガス組成
Ｈ₂ ７０．７％（体積％、以下同じ）
ＣＯ１６．７％
ＣＯ₂ １．０％
ＣＨ₄ ３．５％
Ｎ₂ ６．８％
Ｈ₂Ｏ１．３％
【００２０】
上記ガス加熱管入口部におけるガス温度は４５０℃、出口部におけるガス温度は７７５℃とした。ガス圧力は３．５kg/cm²Ｇとし、ガス流量及び加熱量を調整して、次の２つの条件で運転を行った。
【表７】

【００２１】
上記２条件でそれぞれ６０００時間運転し、その時点で加熱管の肉厚を測定したが、減肉はほとんど観察されなかった。
【００２２】
実施例２
図３及び図４は本発明の実施に好適な装置（ガスヒーター）の例を示すものである。図３は同装置の正面図、図４は同装置の側面図である。同装置は大きく対流伝熱部Ａと輻射伝熱部Ｂからなり、輻射伝熱部Ｂは一次加熱部Ｂ１及び二次加熱部Ｂ２からなる。対流伝熱部Ａは輻射伝熱部Ｂからの燃焼ガスの排気路に当たり、高温の燃焼ガスの熱を利用して一酸化炭素含有還元性ガスの予熱を行う部分である。また輻射伝熱部Ｂ（一次加熱部Ｂ１及び二次加熱部Ｂ２）では、バーナーの火炎からの輻射熱により対流伝熱部Ａで予熱された一酸化炭素含有還元性ガスが所定の温度までさらに加熱される。対流伝熱部Ａ、一次加熱部Ｂ１及び二次加熱部Ｂ２内には、それぞれ図に示すように対流伝熱部加熱管１、輻射伝熱部一次加熱管２及び輻射伝熱部二次加熱管３が配備されており、それらの間はクロスオーバー管４及び５で連絡されている。一酸化炭素含有還元性ガスは対流伝熱部加熱管１を通過することにより常温から約４５０℃に加熱され、クロスオーバー管４を経て入口ヘッダー６から輻射伝熱部一次加熱管２を通過することにより約７７５℃に加熱され、さらにディストリビューター７からクロスオーバー管５を経て輻射伝熱部二次加熱管３を通過することにより約９２０℃に加熱され、トランスファーライン８により目的のプロセスへ移送される。一次加熱部Ｂ１の炉壁には１段目バーナー９及び２段目バーナー１０が配備されており、これらは通常の長炎型バーナーであって火炎の輻射熱により輻射伝熱部一次加熱管２を加熱する。二次加熱部Ｂ２の炉壁には長炎型の１段目バーナー１１及びラジアントウォールバーナー１２が配備されており、これらにより輻射伝熱部二次加熱管３を加熱する。二次加熱部の２段目バーナーとしてこのようにラジアントウォールバーナーを用いるのは、長炎型バーナーと違って加熱管に直接炎が当たらないため、加熱管をオーバーヒートする心配がないからである。輻射伝熱部一次加熱管は入口ヘッダー６からディストリビューター７までの間を多数本が平衡に並んで走っており、それぞれ一次加熱部の直下で垂直に立ち上がってそのまま一次加熱部内をまっすぐに上昇する。輻射伝熱部二次加熱管はディストリビューター７から水平に（かつ横方向に多少広がりながら）複数本突き出ているクロスオーバー管５に直接つながっており、従来型のコイル状の形態をなしている。なお、トランスファーライン８には高温に加熱されたガスが通るため、その内部を耐火断熱キャスタブル１３で断熱する。
【００２３】
比較例
外径１７０ｍｍ、肉厚１０ｍｍ、長さ８ｍの直管６本をＵ字管でつないでなる伝熱コイルをガス加熱管とする、図１に示す従来型加熱管と同型のガス加熱管を有する実験用加熱炉を製作し、実施例１と同じ組成の一酸化炭素含有還元性ガスを加熱した。
【００２４】
上記ガス加熱管入口部におけるガス温度は４５０℃、出口部におけるガス温度は９２０℃とした。ガス圧力は３．５kg/cm²Ｇとし、ガス流量及び加熱量を調整して、次の条件で運転を行った。
【表８】
運転条件
昇温速度３００℃／秒
ガス滞留時間１．５７秒
ガス平均流速３１ｍ／秒
【００２５】
上記条件で６０００時間運転し、その時点で加熱管の肉厚を測定した結果、入口部に近い側の直管２本分にわたって最大５ｍｍ程度の減肉が観察された。
【図面の簡単な説明】
【図１】従来のガス加熱管とその消耗減肉箇所を示す。
【図２】ブドアール反応の平衡定数の温度依存性及びカーボン析出危険帯域を示す。
【図３】本発明の実施に好適な装置の例を示す正面図である。
【図４】図３の装置のＸ−Ｘ矢視及びＹ−Ｙ矢視を示す側面図である。
【符号の説明】
Ａ対流伝熱部
Ｂ輻射伝熱部
Ｂ１一次加熱部
Ｂ２二次加熱部
１対流伝熱部加熱管
２輻射伝熱部一次加熱管
３輻射伝熱部二次加熱管
４クロスオーバー管（対流伝熱部から輻射伝熱部へ）
５クロスオーバー管（一次加熱部から二次加熱部へ）
６入口ヘッダー
７ディストリビューター
８トランスファーライン
９１段目バーナー
１０２段目バーナー
１１１段目バーナー（二次加熱部）
１２ラジアントウォールバーナー
１３耐火断熱キャスタブル
１４対流伝熱部床面[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a heating method of a carbon monoxide-containing reducing gas mainly used in a direct reduction iron making process.
[0002]
[Prior art]
In the direct reduction iron making process, metallic iron is produced by reducing iron ore (iron oxide) using a reducing gas heated to 800 to 1000 ° C. The reducing gas used here generally contains hydrogen, carbon monoxide, carbon dioxide and water vapor, and in addition to this, often contains methane, nitrogen and the like. In order to heat such a reducing gas to 800 to 1000 ° C., a heating furnace provided with a gas heating tube is used. A heating furnace generally consists of a combustion chamber (radiant heat transfer section) equipped with a burner and a convection heat transfer section through which combustion gas passes, and the reducing gas first passes through the gas heating pipe arranged in the convection heat transfer section. It is preheated to 400 to 450 ° C. and then heated to a desired temperature through a gas heating pipe disposed in the combustion chamber.
[0003]
The gas heating pipe provided in the combustion chamber is called a heat transfer coil because a pipe having an outer diameter of about 150 to 190 mm is folded as shown in FIG. Heat transfer coils are connected in parallel. The heat transfer coil is heated by receiving radiant heat from a flame of a radiant wall burner provided on both wall surfaces of the heat transfer coil, whereby the reducing gas passing through the heat transfer coil is heated to a predetermined temperature. At this time, the temperature rising rate of the gas is about 200 to 300 ° C./second.
[0004]
[Problems to be solved by the invention]
However, it can be seen that when the furnace is continuously used, a phenomenon (metal dusting) occurs in which a part of the gas heating pipe is consumed and thinned, and the gas heating pipe of the portion where the consumption is reduced each time. Had the inconvenience of having to be replaced. This phenomenon is often observed particularly in a part of the gas heating pipe (first half, see FIG. 1) arranged in the combustion chamber. This seems to be caused by carburization embrittlement on the inner surface of the gas heating tube. That is, it is considered that when a reducing gas containing carbon monoxide is heated, carbon is precipitated by the following disproportionation reaction (Budard reaction).
2CO ⇔ CO ₂ + C ↓
The carbon deposited in this way adheres to the inner surface of the gas heating tube and reacts with iron, which is a material (main component) of the high-temperature gas heating tube, to cause carburization and to cause the inner surface of the gas heating tube to become brittle and consumed. is there.
[0005]
As a measure for preventing such carburization embrittlement, it is conceivable to improve the material of the gas heating pipe (particularly the inner surface thereof) so that carburization embrittlement does not occur even if carbon is deposited. However, considering the use conditions and costs of the heating furnace, in all cases, it cannot be said that only such material improvement can be dealt with, or a practical solution if not impossible. Often not.
[0006]
The present invention has been made with the above demands in mind, and a reducing gas containing carbon monoxide as a measure for preventing consumption thinning due to carburization embrittlement of the gas heating pipe from a viewpoint different from material improvement. It is intended to prevent the deposition of carbon on the inner surface of the gas heating tube of the heating furnace that heats the steel.
[0007]
[Means for Solving the Problems]
The present invention provides a method of heating a reducing gas containing carbon monoxide via a heat transfer surface. In this method, a dangerous temperature zone in which carbon is likely to precipitate from the gas is determined, and rapid heating is performed so that the temperature rising rate of the gas in the dangerous temperature zone is 1000 ° C./second or more.
[0008]
Alternatively, in the method, a dangerous temperature zone in which carbon is likely to precipitate from the gas is determined, and the gas is rapidly heated so that the residence time of the gas in the dangerous temperature zone is 0.35 or less.
[0009]
The present invention also provides an apparatus particularly suitable for carrying out the above method. Such an apparatus has a heating section in which a burner and a gas heating pipe are arranged in a combustion chamber, and the gas heating pipe is heated from the outside by a flame of the burner, while carbon monoxide-containing reducing gas is supplied to the gas. A gas heating furnace that raises the temperature of the carbon monoxide-containing reducing gas by circulating in the heating tube, and the gas heating tube has a multi-pass structure composed of a straight tube having an outer diameter of 30 to 80 mm.
[0010]
[Action]
As the carbon monoxide-containing reducing gas to be heated in the present invention, firstly, a reducing furnace tail gas for direct reduction iron making is raised. An example of the typical composition of this gas is
[Table 1]
Reduction furnace tail gas ( after de-CO ₂ )
H ₂ 70.7% (volume%, the same shall apply hereinafter)
CO 16.7%
CO ₂ 1.0%
CH ₄ 3.5%
N ₂ 6.8%
H ₂ O 1.3% (saturation)
It becomes.
[0011]
Of course, the gas to be heated in the present invention is not limited to the reducing furnace tail gas, but a reformed gas from a reformer, a coke oven gas, a blast furnace gas, and a converter having typical compositions shown below. Various carbon monoxide-containing reducing gases such as gases are targeted.
[Table 2]
Example of reformed gas composition from reformer 58.2% H ₂ (volume%, the same shall apply hereinafter)
CO 12.9%
CO ₂ 5.7%
CH ₄ 0.3%
H ₂ O 22.9%
[Table 3]
Example of composition of coke oven gas H ₂ 50-57% (volume%, the same shall apply hereinafter)
CO 5-7%

CO

₂ 2~5%
CH ₄ 22~30%
C ₂ ⁺ 2-5%
N ₂ 2-6%
H ₂ O saturation amount [Table 4]
Example of blast furnace gas composition H ₂ 3-4% (volume%, the same shall apply hereinafter)
CO 21-24%
CO ₂ 20~23%
N ₂ 52~56%
H ₂ O saturation amount [Table 5]
Example of converter gas composition H ₂ 1.5-3.3% (volume%, the same shall apply hereinafter)
CO 65-67%

CO

₂ 12~14%
CH ₄ 0.2~0.4%
N ₂ 16-19%
H ₂ O saturation amount A gas having a carbon monoxide content of less than 4% has a very small tendency to precipitate carbon, and therefore, the gas heater tube is substantially reduced in thickness even without using the means of the present invention. There is no.
[0012]
The Boudal reaction, in which carbon is precipitated from a reducing gas containing carbon monoxide, is an exothermic reaction, and it can be said that carbon tends to precipitate because the reaction proceeds to the right at lower temperatures in terms of chemical equilibrium. That is, the chemical equilibrium constant Kp (eq) of the Butard reaction = P _CO2 ^* / (P _CO ^* ) ²
(P _CO2 ^* ……… equilibrium carbon dioxide partial pressure, unit kg / cm ² )
(P _CO ^* ……… Equilibrium carbon monoxide partial pressure, unit kg / cm ² )
Is expressed thermodynamically as a function that decreases monotonically with temperature under a given pressure condition. For example, assuming a case where heating is performed from room temperature to 900 ° C. under a pressure of 3.5 kg / cm ² G, if the value of Kp (eq) calculated thermodynamically is plotted against the temperature, as shown in FIG. It becomes a curve. Separately, the actual carbon monoxide partial pressure P _CO and carbon dioxide partial pressure P _CO2 under a total pressure of 3.5 kg / cm ² G are obtained from the composition of the reducing furnace tail gas shown above,
P _CO2 / (P _CO ) ² = Kp (obs)
[(Obs) indicates an observed value. ]
2 is also shown in FIG. The temperature region where Kp (eq)> Kp (obs) is a region where carbon may be deposited, and the larger the ratio of Kp (eq) / Kp (obs), the easier the carbon is deposited. Note that a gas that satisfies Kp (obs)> 2.0 (kg / cm ² ) ⁻¹ has a very small tendency of carbon deposition. It does not occur substantially. [0013]
On the other hand, since the reaction rate of the Butard reaction is not large at low temperatures, it can be said that high temperatures are more likely to precipitate in a region where carbon may be precipitated. Therefore, the dangerous temperature zone in which carbon is likely to precipitate is defined as a temperature range in which the potential for carbon deposition is thermodynamically high and the reaction rate is relatively large. Generally, when Kp (eq)> 2.5 Kp (obs), it can be said that the potential for carbon deposition is high thermodynamically, and at 450 ° C. or less, the reaction rate is small, so the tendency for carbon deposition is small. For example, in the example shown in FIG. 2, the dangerous temperature range in which carbon is likely to precipitate is generally defined as 500 ° C. to 775 ° C.
[0014]
As described above, the dangerous temperature zone where carbon is likely to precipitate varies depending on the composition of the gas to be heated (particularly the carbon monoxide concentration and the carbon dioxide concentration) and the operating pressure. The gas needs to be heated at a rate of temperature increase of 1000 ° C./second or more, preferably 1500 to 3500 ° C./second, in the dangerous temperature zone defined for the target system. This is a considerably large heating rate compared to the conventional heating rate (300 ° C./second at most). In the temperature range other than the dangerous temperature range, heating may be performed at a conventional temperature increase rate.
[0015]
In general, the dangerous temperature range is 350 ° C at maximum (when the temperature is increased from 450 ° C to 800 ° C), so at a temperature increase rate of 1000 ° C / second, 0.35 seconds or less, 1500 ° C / second, and 3500 ° C / second. The gas passes through the dangerous temperature zone at 0.23 seconds or less and 0.1 seconds or less, respectively. Thus, the residence time of the gas in the dangerous temperature zone is extremely short, and the carbon deposition due to the Butard reaction is effectively suppressed. As described above, since the temperature range of 450 ° C. to 800 ° C. is a range that almost covers the dangerous temperature band under normal conditions, the temperature is increased from 450 ° C. to 800 ° C. at a rate of 1000 ° C./second or more. In general, the temperature raising condition of the present invention is automatically satisfied (even if the dangerous temperature band is not defined in advance).
[0016]
In order to obtain a large heating rate of 1000 ° C./second or more, it is necessary to increase the heat transfer coefficient. The heat transfer coefficient in this case is for heat transfer from the flame to the gas flow, which is the radiant heat transfer from the flame to the tube surface, the heat transfer from the tube outer surface to the tube inner surface, and the tube inner surface. This is the overall heat transfer coefficient to which the convective heat transfer from the gas flow to the gas flow contributes comprehensively. Here, in order to promote convective heat transfer from the tube inner surface to the gas flow, it is effective to reduce the thickness of the boundary film (laminar flow) portion formed in the vicinity of the tube inner surface. The thickness of the boundary film decreases as the gas flow rate is increased. Conventionally, a relatively thick tube having an outer diameter of about 150 to 190 mm has been used. Therefore, an attempt to increase the gas linear velocity in the tube (the number of heating tubes decreases). Therefore, the heat transfer area becomes smaller with respect to the gas flow rate, and if it is attempted to avoid it, there is a disadvantage that the convection time of the gas becomes longer (by increasing the length of the heating tube). On the other hand, in the present invention, since the tube (outer diameter 30 to 80 mm) thinner than the conventional one is used as the gas heating tube corresponding to the dangerous temperature zone, the above disadvantage is solved. The average gas flow velocity in the dangerous temperature zone is preferably about 40 m / second or more.
[0017]
In addition, since a conventional heating tube having a folded shape as shown in FIG. 1 has been used, there is a consumption thinning that may be caused by stagnation of gas in the U-turn portion, local deposition of carbon, or erosion in the U-turn portion. It can be seen. In order to solve such a problem, it is preferable not to provide a U-turn portion. Therefore, in the present invention, a straight pipe multipath method is adopted in which the dangerous temperature zone portion is constituted by a straight pipe. This is thought to have a positive effect on increasing gas flow rates, as it avoids unnecessary increases in pressure loss.
[0018]
Usually, the dangerous temperature zone where carbon is likely to deposit is in the first half of a gas heating tube provided in the combustion chamber (generally, the temperature is raised from about 450 ° C. to about 800 ° C.). That is, it is not always necessary to configure the entire gas heating pipe provided in the combustion chamber as a relatively thin straight pipe as described above, and to reduce the total cross-sectional area to increase the gas flow velocity. Therefore, the object of the present invention can be achieved by dividing the gas heating pipe in the combustion chamber into the first half and the second half, and taking the above configuration for only the first half heating pipe (the part including the dangerous temperature zone).
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Example 1
A reductive gas containing carbon monoxide having the following composition was manufactured by producing a test heating furnace having a gas heating pipe in which five straight pipes having an outer diameter of 60.5 mm, a wall thickness of 8.5 mm, and a length of 13 m were connected in parallel. Was heated.
[Table 6]
Gas composition H ₂ 70.7% (volume%, the same shall apply hereinafter)
CO 16.7%
CO ₂ 1.0%
CH ₄ 3.5%
N ₂ 6.8%
H ₂ O 1.3%
[0020]
The gas temperature at the inlet of the gas heating tube was 450 ° C., and the gas temperature at the outlet was 775 ° C. The gas pressure was 3.5 kg / cm ² G, the gas flow rate and the heating amount were adjusted, and the operation was performed under the following two conditions.
[Table 7]

[0021]
Each of the above two conditions was operated for 6000 hours, and the thickness of the heating tube was measured at that time, but almost no thinning was observed.
[0022]
Example 2
3 and 4 show an example of an apparatus (gas heater) suitable for carrying out the present invention. 3 is a front view of the apparatus, and FIG. 4 is a side view of the apparatus. The apparatus is largely composed of a convection heat transfer section A and a radiant heat transfer section B, and the radiant heat transfer section B is composed of a primary heating section B1 and a secondary heating section B2. The convection heat transfer part A hits the exhaust path of the combustion gas from the radiant heat transfer part B, and is a part that preheats the reducing gas containing carbon monoxide using the heat of the high temperature combustion gas. In the radiant heat transfer section B (primary heating section B1 and secondary heating section B2), the carbon monoxide-containing reducing gas preheated in the convection heat transfer section A by the radiant heat from the burner flame is further heated to a predetermined temperature. Is done. In the convection heat transfer part A, primary heating part B1 and secondary heating part B2, as shown in the figure, the convection heat transfer part heating tube 1, the radiant heat transfer part primary heating pipe 2 and the radiant heat transfer part secondary heating are respectively shown. Tubes 3 are provided and communicated by

crossover tubes

4 and 5 between them. The reducing gas containing carbon monoxide is heated from room temperature to about 450 ° C. by passing through the convection heat transfer section heating tube 1, and passes through the radiant heat transfer section primary heating pipe 2 from the inlet header 6 through the crossover pipe 4. Then, it is heated to about 775 ° C., further passes through the crossover pipe 5 from the distributor 7, passes through the secondary heating pipe 3 of the radiant heat transfer section, is heated to about 920 ° C., and is transferred to the target process by the transfer line 8. Is done. A first stage burner 9 and a second stage burner 10 are arranged on the furnace wall of the primary heating section B1, and these are ordinary long flame type burners, and the radiant heat transfer section primary heating tube 2 is connected by the radiant heat of the flame. Heat. A long flame type first stage burner 11 and a radiant wall burner 12 are disposed on the furnace wall of the secondary heating section B2, and the radiant heat transfer section secondary heating tube 3 is heated by these. The reason why the radiant wall burner is used as the second stage burner of the secondary heating unit is that, unlike the long flame type burner, the flame does not directly hit the heating tube, so there is no fear of overheating the heating tube. A large number of primary heating tubes of the radiant heat transfer section run in parallel from the inlet header 6 to the distributor 7, and each of them rises vertically immediately below the primary heating section and rises straight in the primary heating section. . The secondary heating pipe of the radiant heat transfer section is directly connected to the crossover pipe 5 projecting horizontally from the distributor 7 (and extending slightly in the lateral direction), and has a conventional coil shape. . In addition, since the gas heated at high temperature passes through the transfer line 8, the inside thereof is insulated by the refractory heat insulating castable 13.
[0023]
Comparative example The same type as that of the conventional heating tube shown in Fig. 1, wherein a heat transfer coil formed by connecting six straight tubes having an outer diameter of 170 mm, a wall thickness of 10 mm, and a length of 8 m with a U-shaped tube is used as a gas heating tube. An experimental heating furnace having a gas heating tube was manufactured, and a carbon monoxide-containing reducing gas having the same composition as in Example 1 was heated.
[0024]
The gas temperature at the inlet of the gas heating tube was 450 ° C., and the gas temperature at the outlet was 920 ° C. The gas pressure was 3.5 kg / cm ² G, the gas flow rate and the heating amount were adjusted, and the operation was performed under the following conditions.
[Table 8]
Operating conditions Temperature rising rate 300 ° C / second Gas residence time 1.57 seconds Gas average flow velocity 31m / second
As a result of measuring the thickness of the heating tube at that time after operating for 6000 hours under the above conditions, a thickness reduction of about 5 mm at maximum was observed over two straight tubes on the side close to the inlet.
[Brief description of the drawings]
FIG. 1 shows a conventional gas heating pipe and its depleted thickness portion.
FIG. 2 shows the temperature dependence of the equilibrium constant of the Butard reaction and the danger zone for carbon deposition.
FIG. 3 is a front view showing an example of an apparatus suitable for carrying out the present invention.
4 is a side view showing the XX arrow view and the YY arrow view of the apparatus shown in FIG. 3; FIG.
[Explanation of symbols]
A convection heat transfer part B radiant heat transfer part B1 primary heating part B2 secondary heating part 1 convection heat transfer part heating pipe 2 radiant heat transfer part primary heating pipe 3 radiant heat transfer part secondary heating pipe 4 crossover pipe (convection transfer (From heat to radiant heat transfer)
5 Crossover tube (from primary heating section to secondary heating section)
6 Inlet header 7 Distributor 8 Transfer line 9 First stage burner 10 Second stage burner 11 First stage burner (secondary heating section)
12 Radiant Wall Burner 13 Fireproof and Insulated Castable 14 Convection Heat Transfer Floor

Claims

In a method of heating a carbon monoxide-containing reducing gas through a heat transfer surface, a primary heating step of rapidly heating the gas from a temperature of 450 ° C. or lower to a temperature of 800 ° C. or higher at a rate of temperature increase of 1000 ° C./sec or higher; , Including a secondary heating step for further heating the gas heated in the primary heating step, wherein the primary heating step is provided with a gas heating pipe having a multi-pass structure consisting of a straight pipe having an outer diameter of 30 to 80 mm in the combustion chamber The gas heating tube is heated from the outside while the gas is circulated in the gas heating tube .

The method according to claim 1, wherein a temperature increase rate in the primary heating step is 1500 to 3500 ° C./second.

The method according to claim 1 or 2, wherein a residence time in the primary heating step is 0.35 seconds or less.

The method according to claim 3, wherein a residence time in the primary heating step is 0.1 to 0.23 seconds.

The method according to any one of claims 1-4 wherein the gas is a reducing furnace tail gas for reduced iron directly.

The method according to any one of claims 1 to 4 , wherein the gas has a carbon monoxide content of 4% by volume or more (on a dry basis).

The method according to any one of claims 1 to 4 , wherein the gas has a Kp (obs) value of 2.0 or less as appropriate.
Kp (obs) = P _CO2 / (P _CO ) ²
P _CO : Carbon monoxide partial pressure (unit: kg / cm ² )
P _CO2 : Carbon dioxide partial pressure (unit: kg / cm ² )

The method according to any one of claims 1 to 7 , wherein an average value of velocity components parallel to the heat transfer surface of the gas in the primary heating step is 40 m / sec or more.

A gas heating furnace for heating a reducing gas containing carbon monoxide through a heat transfer surface, and a gas heating tube having a multipath structure consisting of a burner and a straight pipe having an outer diameter of 30 to 80 mm in a combustion chamber. By deploying and heating the gas heating tube from the outside with a flame of the burner, the gas flowing in the gas heating tube is rapidly increased from a temperature of 450 ° C. or lower to a temperature of 800 ° C. or higher at a heating rate of 1000 ° C./second or higher. A primary heating unit configured to heat, a convection heat transfer unit for preheating the gas supplied to the primary heating unit, and a secondary heating unit for further raising the temperature of the gas heated by the primary heating unit gas furnace, characterized in that comprising a.

The gas heating furnace according to claim 9, wherein the secondary heating unit has a radiant wall burner.