Iron and steel

Summary

The steel industry produces 3-8% of the world's CO2 emissions.

1.8 T of CO2 is emitted for 1T of steel. The steel industry is under pressure to reduce CO2 emissions.However the best plants are now operating close to their theoretical limit so there is no chance of improvement with the current blast furnace design. Breakthrough technologies are being developed.

The research work is in the following approaches:

  • Increasing the efficiency of a blast furnace
  • Using oxygen, so pure CO2 is produced, which can be stored.
  • Replacing C  with H2 to produce H2O instead of CO2.
  • Using CH4 (methane) so 2/3 of the reduction is carried out by H2.
  • Electrolysis

 ULCOS research

Click on images for source and more information

Production method CO2 Energy required
  KG/T of Fe MJ/KG = GJ/T
Blast furnace 1671 19.5
Hlsarna blast furnace 1340 15.5
DRI Hydrogen reduction 71 12
DRI Methane  650 12
Coal gasified 1145 12
Electrolysis - (target) 0 11

Novel Ironmaking Technology with Low Energy Requirement and CO2 Emission

How iron and steel are made

1) making iron -- remove oxygen and silica from iron ore

Blast furnace using coke to remove oxygen to make pig iron with 4-5% C.

or:

DRI - Direct reduction of iron by using CO and/or H2 to remove the oxygen.

2) steelmaking - purifying the pig, scrap, or direct iron.

Basic oxygen furnace - oxygen is injected into the molten pig iron to oxidise the C and remove other impurities. Other metals can be added to make alloys.

or

EAF - Electric Arc Furnace uses mainly electricity to melt the scrap, pig, or direct iron. It also uses oxygen to remove carbon and imputies, and fuel to help the heating. Carbon is added to reduce iron oxides produced by the addition of oxygen. Wikipedia

CO2 emitted by different processes
  Traditional KG/T Modern KG/T
Ironmaking Blast furnace 1550 DRI 175
Steelmaking Basic oxygen furnace 350 Electric arc furnace 300
  Total 1900   475

 

Blast furnace process

Normally iron is produced in a blast furnace. Iron ore lumps, coke, and limestone are mixed and ignited. The coke burns in the limited air producing CO carbon monoxide. The CO then removes oxygen from the iron oxide leaving molten iron and CO2. The coke has to have enough strength to support the weight of the iron ore and limestone allowing gases to pass through the porous mass. Thus the need for coking, or metallurgical coal, to produce the required coke. The limestone combines with the silica and alumina, etc to produce slag which floats on top of the molten iron. The limestone also gives off CO2.

Details of the chemistry in a blast furnace are at the end of this page.

Steel produced by:   

 blast furnace 92%   

Methane 5%   

coal 2%   

charcoal 1%   

Making the blast furnace more eficient -Top Gas Recycling

The reaction is never 100%, so some of the CO is carried away in the exhaust gas. The CO2 can be removed and the remaining CO sent back into the blast furnace. This is called Top Gas Recycling. This reduces the amount of coke needed in the furnace.

If pure oxygen is used instead of air, then without nitrogen present, the remainnig gas will be pure CO2 which would be captured and stored (CCS).

Source - ULCOS

Blast furnace design - HIsarna steel making process

The HIsarna process  is based around a new type of blast furnace called a Cyclone Converter Furnace.

The powdered iron ore is dropped into the cyclone and on the way down it reacts with oxygen and powdered coal to form a pool of molten iron. The CO2 and energy saving is 20%.

Because the HIsarna process operates with pure oxygen the off gases are practically nitrogen free making it easy to store the CO2. 

It does not need coking coal, and can use source of carbon such as biomass.

Source Wikipedia

Cyclone converter

Cyclone converter furnace

Direct reduction of iron - DRI

The Direct Reduction of Iron process (DRI) uses H2 and/or CO  to remove the oxygen from the iron oxide ore. The reducing gas can be reacted with the iron ore in a fixed bed with lumps, a fluidised bed with fine ore, or a rotating kiln.The process does not remove silica or alumina so the iron ore needs to be of high quality. (generally having 65 to 70 percent iron). 

The process works below the melting point of iron, which is 1538°C. The output is sponge iron which can rust quickly, or even spontaneously catch fire. For transport it is usually rolled into briquettes while still hot. If the hot sponge iron is sent straight to an electric arc furnace then this saves reheating.

In Western Australia two tonnes of fines are produced for every tonne of the more valuable lump form. So the fines are being upgraded by the DRI process.

The reducing gas can be made by burning any organic material in limited air. Methane, coal, or biomass. It will work with pure CO, or H2, or any mixture of both.

DRI offers a carbon footprint that is one-third of that for the coke oven/blast furnace route, but at less than half the capital cost.  Source

Production uses 14.3 MJ  per KG of iron.

http://www.chemlink.com.au/cipbhp.htm     http://www.chemlink.com.au/dri.htm

 

REDUCTION

Fe3O4 + CO  → 3FeO + CO2
Fe3O4 + H2   → 3FeO + H2O
FeO     + CO  → Fe     + CO2
FeO     + H2   → Fe     +H2O

REFORMING
CH4     + CO2  → 2CO  +2H2
CH4     + H2O  → CO    +3H2

 

Most DRI is produced in a MIDREX® Shaft Furnace can use CO, H2, or CH4 in any combination as the reductant. 

It is a bed where the hot gases flow up through a bed of particles reacting as they slowly fall downward under gravity.

It can produce cold briquettes for transport and use in blast furnaces, or hot briquettes that can go straight into an electric arc furnace for removal of impurities.

Hydrogen reduction of iron

Hydrogen can be used to produce iron with very little CO2 being emitted.

At present hydrogen is expensive

Reactions within the MIDREX® Shaft Furnace
Reaction Heat Description
3Fe2O3 + CO →  2Fe3O4 + CO2 Exothermic Reduction by CO
3Fe2O3 + H2 →  2Fe3O4 + H2O Exothermic Reduction by H2
Fe3O4 + CO →  3FeO + CO2 Endothermic Reduction by CO
Fe3O4 + H2 → 3FeO + H2O Endothermic Reduction by H2
FeO + CO → Fe + CO2 Exothermic Reduction by CO
FeO + H2 → Fe + H2O Endothermic Reduction by H2
3Fe + CH4 →  Fe3C + 2H2 Endothermic Carburizing Reaction
3Fe + 2CO →  Fe3C + CO2 Exothermic Carburizing Reaction
3Fe + CO + H2 → Fe3C + H2O Exothermic Carburizing Reaction

Reduction with methane

Methane CH4 can be used for DRI. It produces far less CO2 than coal because 2/3 of the oxygen removal is done by hydrogen, producing only water. So methane produces only one third the CO2 of coal.

This CO2 could be buried as it should have no nitrogen in it.

Fe3O4 + 4CH4          -> 3Fe + 4CO + 8H2

Fe3O4  + 4CO           -> 3Fe + 4CO2 

Fe3O4  + 4H2            -> 3Fe + 4H2O

Fe3O4  + CO + 3H2   -> 3Fe + CO2 + 3H2O

Use of tyres / tires for iron and steel making

170 KG/T of crumbed tyres with their steel, can be added to a blast furnace. Tyres can replace 25-40% of the coke. Patent

Tyres can be heated to produce syn. gas (H2 and CO) that can be used for DRI.

Electric arc furnace, EAF, in spite of its name, needs carbon as a reactant, as an alloying element and as fuel. Tyres can be substiuted for coke in an electric arc furnace at the rate of 8-12 KG / T of steel. 1.7 KG or tyre is equivalent of 1 KG of coke.

By using crumbed tyres in their EAF, OneSteel's has cut electricity consumption during steelmaking by 3 per cent, and its use of coking coal by 12 -16:

Components of tyres  
Elastomer (rubber) 47%
Carbon black 21.5%
Steel 16.5%
Textile 5.5%
Sulfur 1%

 

Electrolysis, or electrowinning of iron - 

It is possible to make iron by electrolysis similar to the process for making aluminium. There are two main processes being developed ULCOWIN and ULCOLYSIS.

In the ULCOLYSIS process, iron ore is dissolved in a molten silica and calcium oxide mixture at 1600°C. This unusual electrolyte medium is chosen in order to operate at a temperature high enough to be above the melting point of iron metal. The anode, made of a material inert towards the oxide mixture, is dipped in this solution. The electrical current is sent between this anode and a liquid iron pool connected to the circuit as the cathode. Oxygen(O2) evolves as a gas at the anode and iron is produced as a liquid metal at the cathode.

ULCOWIN is at the pilot plant stage.

The aim is 3 MWh/t = 10.8 MJ/KG

 

  ULCOWIN pilot palnt

Chemical reactions in a blast furnace

From - ChemED DL

The oxides present in most iron ores are Fe2O3 and Fe3O4. These oxides are reduced stepwise: first to FeO and then to Fe. Ore, coke, and limestone are charged to the furnace through an airlock-type pair of valves at the top. Near the bottom a blast of air, preheated to 900 to 1000 K, enters through blowpipes called tuyères. Oxygen from the air blast reacts with carbon in the coke to form carbon monoxide and carbon dioxide, releasing considerable heat. The blast carries these gases up through the ore, coke, and limestone, and they exit from the top of the furnace.

By the time the ore works its way to the lower part of the furnace, most of the Fe2O3 has already been reduced to FeO. In this region, temperatures reach 1600 to 2000 K, high enough to melt FeO and bring it into close contact with the coke. Most of the FeO is reduced by direct reaction with carbon, the latter being oxidized to carbon monoxide:

2C(s) + 2FeO(l) → 2Fe(l) + 2COΔGm° (2000 K) = –280 kJ mol–1      (1)

The molten iron produced by this reaction drips to the bottom of the furnace where it is collected and occasionally tapped off.

Higher in the furnace, temperatures fall below the melting points of the iron oxides. Because there is little contact betweensolid chunks of ore and of coke, direct reduction by solid carbon is rather slow. Gaseous carbon monoxide contacts all parts of the ore, however, and reacts much more rapidly:

CO(g) + Fe2O3(s) → CO2(g) + 2FeO(s)      (2)

CO(g) + FeO(s) → CO2(g) + Fe(s)      (3)

Thus much of the “carbon reduction” in ironmaking is actually carried out by carbon monoxide.

The gangue in iron ore consists mainly of silicates and silica, SiO2. These impurities are removed in slag. Limestone added with coke and ore is calcined (decomposed to the oxide) by the high temperatures of the blast furnace:

CaCO3(s\xrightarrow{\text{1100 K}} CaO(s) + CO2(g)

Lime (CaO) serves as a flux, reducing the melting points (mp) of silica (SiO2) and silicates:

\underset{\text{mp = 2853 K}}{\mathop{\text{CaO(}s\text{)}}}\,\text{ + }\underset{\text{mp = 1986 K}}{\mathop{\text{SiO}_{\text{2}}\text{(}s\text{)}}}\,\text{ }\to \text{ }\underset{\text{mp = 1813 K}}{\mathop{\text{CaSiO}_{\text{3}}\text{(}l\text{)}}}\,

The liquid silicates flow rapidly down through the hottest part of the furnace. This helps to prevent reduction of silica to silicon, hence yielding purer iron. The slag is less dense than molten iron and immiscible with it. Therefore the slag floats on the surface of the iron and can easily be tapped off.

Although most blast-furnace iron now goes directly to a steelmaking furnace in molten form, much of it used to be run into molds where it hardened into small ingots called pigs because of their shape. Consequently blast-furnace iron is still referred to as pig iron. A single large blast furnace may produce more than 106 kg iron per day. For each kilogram of iron, 2 kg iron ore, 1 kg coke, 0.3 kg limestone, 4 kg air, 63 kg water, and 19 MJ of fossil-fuel energy are required. The furnace produces 0.6 kg slag and 5.7 kg, flue gas per kg iron. Nearly 5 percent of the iron ore is lost in the form of small particles suspended in the flue gas unless, as in the furnace shown in Fig. 1, air-pollution controls are installed. The latter trap FeO particles for recycling to the furnace and also make the flue gas (which contains about 12% CO and 1% H2) suitable as a fuel for preheating air fed to the tuyères. Thus control of blast-furnace air pollution (a major contributor to the one-time “smoky city” reputations of Pittsburgh, Pennsylvania and Gary, Indiana) also conserves ore supplies and energy resources.

Schematic Diagram of a Blast Furnace for Smelting Iron

1. Hot blast from Cowper stoves
2. Melting zone
3. Reduction zone of ferrous oxide
4. Reduction zone of ferric oxide
5. Pre-heating zone
6. Feed of ore, limestone and coke
7. Exhaust gases
8. Column of ore, coke and limestone
9. Removal of slag
10. Tapping of molten pig iron
11. Collection of waste gases.