AMAP Project 5

Sustainable aluminium recycling: Efficient melting

In the Advan­ced Metals And Pro­ces­ses Rese­arch Clus­ter (AMAP), various rese­arch units and com­pa­nies work tog­e­ther on pre-com­pe­ti­ti­ve pro­blems. The “Pro­ject 5 — Alu­mi­ni­um Recy­cling”, in which the IOB is invol­ved, deals with the model­ling of the hea­ting and mel­ting pro­cess of alu­mi­ni­um scrap (e.g. bevera­ge cans, so-cal­led UBCs — “used bevera­ge cans”, see Figu­re 1). Com­pared to the pro­duc­tion of pri­ma­ry alu­mi­ni­um, the recy­cling of used alu­mi­ni­um (such as bevera­ge cans) is more ener­gy-effi­ci­ent and at the same time leads to signi­fi­cant­ly lower CO2 production.

Figu­re 1: Packa­ge of lac­que­r­ed, used bevera­ge cans (UBCs)

The mel­ting down of alu­mi­ni­um scrap is an important pro­cess in the recy­cling chain and requi­res a detail­ed con­side­ra­ti­on of all the mecha­nisms invol­ved. A bet­ter under­stan­ding of the­se mecha­nisms, e.g. heat trans­fer or pyro­ly­sis, leads to a fur­ther opti­mi­sa­ti­on of the recy­cling con­cept. Ques­ti­ons con­cer­ning metal los­ses and orga­nic con­ta­mi­na­ti­on of alu­mi­ni­um scrap will be investigated.

The main task of the IOB within the AMAP P5 con­sor­ti­um is to gene­ra­te a CFD simu­la­ti­on of the dif­fe­rent inves­ti­ga­ted phe­no­me­na. The con­tri­bu­ti­ons inves­ti­ga­ted at the IOB are the model­ling of the com­bus­ti­on pro­cess, in par­ti­cu­lar the fla­me­l­ess com­bus­ti­on, the (radia­ti­on) heat trans­fer to the mate­ri­al to be mel­ted as well as the pyro­ly­sis of the adhe­ring orga­nic mate­ri­al (paints and coo­ling lubricants).

A cha­rac­te­ristic fea­ture of fla­me­l­ess com­bus­ti­on is the strong recir­cu­la­ti­on of the com­bus­ti­on pro­ducts, which results in a redu­ced com­bus­ti­on tem­pe­ra­tu­re and a spa­ti­al­ly exten­ded reac­tion zone. The absence of tem­pe­ra­tu­re peaks and thus of a fla­me front cau­ses a reac­tion zone that can­not be detec­ted by the human eye. A nume­ri­cal cal­cu­la­ti­on of this pro­cess with the alre­a­dy imple­men­ted models is not pos­si­ble and requi­res an exten­si­on of the che­mi­cal reac­tion mecha­nisms. Figu­re 2 shows the OH° con­cen­tra­ti­on on the midd­le pla­ne of a pilot fur­nace model. Com­pared to con­ven­tio­nal com­bus­ti­on, this is not con­cen­tra­ted at the bur­ner mouth, but pro­tru­des rela­tively far into the fur­nace. The tem­pe­ra­tu­re peaks, at approx. 900 °C, are also well below the adia­ba­tic com­bus­ti­on tem­pe­ra­tu­re and thus cor­re­spond to the fla­me­l­ess conditions.

Figu­re 2: OH con­cen­tra­ti­on in the pilot fur­nace model

The fur­nace walls hea­ted by the descri­bed com­bus­ti­on pro­vi­de part of the heat trans­fer to the mate­ri­al in the form of radi­ant ener­gy. In order to cal­cu­la­te the radi­ant heat trans­fer to a feed mate­ri­al model­led as a porous medi­um, the radia­ti­on models pro­vi­ded by com­mer­cial sol­vers must be fur­ther deve­lo­ped. This is also being work­ed on by the IOB.

Ano­ther part of the AMAP P5 is the inves­ti­ga­ti­on of the pyro­ly­sis gas emis­si­ons alre­a­dy men­tio­ned abo­ve from various orga­ni­cal­ly con­ta­mi­na­ted scrap metals during hea­ting. On the basis of expe­ri­men­tal inves­ti­ga­ti­ons the pyro­ly­sis gas emis­si­ons are cha­rac­te­ri­zed in order to be able to obser­ve the inter­ac­tions of such gases with the melt and to inte­gra­te the emis­si­on into CFD simu­la­ti­ons. For this pur­po­se, small quan­ti­ties of the mate­ri­al to be inves­ti­ga­ted are hea­ted on a labo­ra­to­ry sca­le with a defi­ned hea­ting rate from room tem­pe­ra­tu­re to just below the mel­ting point. At tem­pe­ra­tures from approx. 350 °C decom­po­si­ti­on reac­tions of the orga­nic com­pon­ents take place which lead to tar, oil and gas emis­si­ons. Resul­ting gases are ana­ly­zed with ana­ly­sis sys­tems (FTIR etc.). The know­ledge gai­ned allows con­clu­si­ons to be drawn about the decom­po­si­ti­on pro­ces­ses and reac­tion mecha­nisms as well as about the calo­ric con­tri­bu­ti­on of the gases to the fur­nace ener­gy balance.