The FlexHeat2Anneal pro­ject is being car­ri­ed out by the Insti­tu­te for Indus­tri­al Fur­naces and Heat Engi­nee­ring (IOB), tog­e­ther with WS Wär­me­pro­zess­tech­nik GmbH and thys­sen­krupp Ras­sel­stein GmbH, to inves­ti­ga­te the fle­xi­ble use of hydro­gen as a fuel in con­ti­nuous anne­al­ing lines and hot-dip coa­ting plants.

The back­ground of the pro­ject is the Euro­pean goal to beco­me cli­ma­te-neu­tral by 2050, which has resul­ted in the need for the steel indus­try to find alter­na­ti­ve ener­gy sources to mas­si­ve­ly redu­ce the CO₂-emis­si­ons from heat tre­at­ment plants. Hydro­gen pro­du­ced from rene­wa­ble ener­gies, so-cal­led green hydro­gen, repres­ents a pro­mi­sing alter­na­ti­ve to the pre­vious­ly used fos­sil natu­ral gas, as it burns local­ly wit­hout CO₂-emis­si­ons. Thys­sen­krupp Steel Euro­pe GmbH and thys­sen­krupp Ras­sel­stein GmbH alo­ne ope­ra­te ten fire coa­ting plants and four con­ti­nuous anne­al­ing lines with an annu­al pro­duc­tion capa­ci­ty of 5.5 mil­li­on ton­nes of steel. The com­ple­te con­ver­si­on of the­se plants to hydro­gen would result in a reduc­tion of 420,000 ton­nes of CO₂ per year.

The pro­duc­tion of steel strip requi­res an addi­tio­nal anne­al­ing step after cold rol­ling. This pro­cess is used to adjust the mecha­ni­cal pro­per­ties and res­to­re the for­ma­bi­li­ty of the strip. The strip is recrystal­li­zed at tem­pe­ra­tures bet­ween 670 °C and 900 °C and the heat tre­at­ment can be car­ri­ed out in both bell anne­al­ing fur­naces and con­ti­nuous anne­al­ing lines. In the lat­ter, the strip pas­ses through the fur­nace under a defi­ned pro­tec­ti­ve gas atmo­sphe­re (usual­ly a mix­tu­re of 95% N₂ and 5% H₂). This pro­tec­ti­ve gas atmo­sphe­re pre­vents oxi­da­ti­on of the strip sur­face during heat tre­at­ment. The hea­ting of con­ti­nuous anne­al­ing fur­naces is usual­ly done with self-recup­er­a­ti­ve bur­ners that use pre­hea­ted air to com­bust fuel gases. Howe­ver, due to the defi­ned atmo­sphe­re in the fur­nace, the sepa­ra­ti­on bet­ween com­bus­ti­on gases and pro­tec­ti­ve gas atmo­sphe­re must be ensu­red using radi­ant hea­ting tubes. To ensu­re even hea­ting of the strip in ver­ti­cal plants, seve­ral radi­ant tubes, each with a bur­ner, are instal­led. Howe­ver, bur­ner and radi­ant tube sys­tems have so far been opti­mi­zed pri­ma­ri­ly for appli­ca­ti­ons with natu­ral gas or blast fur­nace gases.

Fig.1: Sche­ma­tic view of a con­ti­nuous anne­al­ing line for tin­p­la­te [Source: thys­sen­krupp Ras­sel­stein GmbH]

When inte­gra­ting hydro­gen into exis­ting sys­tems, it is important to keep fle­xi­bi­li­ty in mind and give the ope­ra­tor the abili­ty to respond to pri­ce or avai­la­bi­li­ty fluc­tua­tions in the coming years. The­r­e­fo­re, the deve­lo­p­ment of a fuel-fle­xi­ble bur­ner-radi­ant tube sys­tem is aimed at. The addi­ti­on of hydro­gen to fos­sil natu­ral gas should be pos­si­ble from 0 to 100 per­cent by volu­me wit­hout requi­ring manu­al adjus­t­ments to the sys­tem. Such a sys­tem offers a prac­ti­cal solu­ti­on for the short- and long-term ope­ra­ti­on of low-emis­si­on plants.

To enable a switch to hydro­gen, exis­ting sys­tems (PP and W hea­ting tubes) will first be exami­ned for the use of hydro­gen and natu­ral gas-hydro­gen blends within the frame­work of FlexHeat2Anneal. Sub­se­quent­ly, a fuel-fle­xi­ble and ener­gy-effi­ci­ent bur­ner-radi­ant tube sys­tem will be deve­lo­ped tog­e­ther with WS Wär­me­pro­zess­tech­nik GmbH. The new radi­ant tube sys­tems will then be used and tes­ted on an indus­tri­al con­ti­nuous anne­al­ing line of thys­sen­krupp Ras­sel­stein GmbH for steel strip using two demonstrators.

Fig.2: Modu­lar test rig for the inves­ti­ga­ti­on of dif­fe­rent radi­ant tube geo­me­tries (left: PP-type radi­ant tube, right: W‑type radi­ant tube).

The chal­lenges in opti­mi­zing the sys­tem for natu­ral gas-hydro­gen mix­tures lie in the dif­fe­rent com­bus­ti­on pro­per­ties of natu­ral gas, hydro­gen, and natu­ral gas-hydro­gen mix­tures. The vary­ing fuel com­po­si­ti­on makes it dif­fi­cult to ensu­re high ener­gy effi­ci­en­cy and con­stant power out­put, as well as to gua­ran­tee low NO_x emis­si­ons. Accor­din­gly, approa­ches to redu­ce NO_x emis­si­ons, such as fuel sta­ging or the FLOX® tech­no­lo­gy, are being tes­ted and opti­mi­zed for hydro­gen and natu­ral gas-hydro­gen mixtures.

In addi­ti­on to opti­mi­sing and repla­cing the sys­tems, the gas sup­p­ly at the plant must also be adapt­ed. Fur­ther­mo­re, the radi­ant tubes and bur­ners must be equip­ped with mea­su­re­ment equip­ment to eva­lua­te fur­nace ope­ra­ti­on, ener­gy effi­ci­en­cy and pol­lutant emis­si­ons. Based on the results, the effects of swit­ching from fos­sil natu­ral gas to hydro­gen for con­ti­nuous anne­al­ing lines and hot-dip gal­va­ni­s­ing lines will be inves­ti­ga­ted and the true CO₂ reduc­tion poten­ti­al will be determined.