Coke Making Fundamentals

By David Riley

July 2013

2

WHAT IS COKING COAL

A coal that, during slow heating, softens to form a liquid or plastic phase, agglomerates, swells then resolidifies with shrinkage to form a solid grey mass known as coke.

3

HISTORICAL VALUE OF COAL

4

HOW IS COKE FORMED Coking coal is heated in a refractory lined oven to approx

1100°C for 18 hours in the absence of oxygen to form a grey carbon mass.

The coke is then pushed from the oven while at temperature and quenched with water.

5

WHAT HAPPENS DURING COKING

Approximate Temperature Coking Coal Non-coking & Thermal Coal

0-200oC Slow decomposition Loss of water and carbon dioxide

Slow decomposition Loss of water & carbon dioxide.

200-400oC Rapid decomposition, fragmentation of coal molecules, loss of tars

Rapid decomposition, fragmentation of coal molecules, loss of tars.

400-500oC Formation of liquid (plastic phase), swelling, fusion of particles and agglomeration

Does not form a liquid / plastic phase.

450-550oC Resolidification to form a semi-coke, first stage of shrinkage

No resolidification.

550-1000oC Loss of hydrogen and oxygen, second stage of shrinkage

End State Coke Lumps Char – fine powder.

6

STAGES IN COKING CYCLE

7

SGS MOVABLE WALL OVEN

8

WALL PRESSURE DURING COKING

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 2 4 6 8 10 12 14 16 18 20Time (Hrs)

Ove

n W

all P

ress

ure

(psi)

Expanding plastic layers generates force on oven wall

Expanding plastic layers generates force on oven wall

Plastic layers meet. No gas escape through coal creates a pressure spike

Plastic layers meet. No gas escape through coal creates a pressure spike

Balance of semi-coke shrinkage/gas escape and pressure generation in plastic layer

Balance of semi-coke shrinkage/gas escape and pressure generation in plastic layer

Pressure generated during coking is transferred through the coke and into the wall of the oven. These forces can damage the oven.

9

PRIMARY USE FOR COKE – STEEL PRODUCTION

Coke Oven Sinter Plant

Slag

Molten pig iron

Oxygen Converter Sintered

ore Coke

Blast Furnace

Molten Steel

10

ROLE OF COKE IN STEEL PRODUCTION

Thermal • 60-80% of the blast furnace’s thermal requirements are supplied by the burning of coke

Chemical • Provides the majority of the CO gas which is the

principal reducing reagent in the furnace. • Provides carbon for dissolution in the metal

Structural • Supports burden materials at high temp while its own

mass is diminishing • Provides permeability for gas flow & molten liquids

through the furnace

11

HOW IS COKE QUALITY DETERMINED

Pilot Scale Testing • Coal is crushed to approx 90% -3.0mm and coked in either a large or small scale

coke oven.

• Large scale – 300kg of 5% moisture coal is coked for 18 – 20 hours. Coking is deemed complete after centre of charge has reached 1000°C plus 1 hour soak time.

• Coke sample is pushed from the oven and water quenched.

• Sample is dropped from a standard height to stabilise material and simulate travel from coke oven to blast furnace. Historically these heights can range from 3.6m to 12m depending on customer requirements.

• Sample is dried to <1% H20 and then sized analysis performed to determine coke mean size.

• Representative tests are then made up to determine cold & hot strength (CSR / CRI).

• Small scale – 7kg of 5% moisture coal is coked for 3 – 4 hours. The same definition of end coking is applied. Enough sample generated to determine % yield and hot strength (CSR / CRI).

12

COKE QUALITY – COLD STRENGTH

Test Standard Sample Size (kg)

Revs Screen Size (mm)

ASTM USA 10 1400 25 & 6.35

Micum / Irsid

Euro 25 100-400 40, 20 & 10

JIS Japan 10 30 -120 50 & 15

Coke must maintain its integrity with minimum breakage from the coke oven to the blast furnace. Excessive breakage will result in fines, reducing permeability and blast furnace productivity.

Unfortunately at every point of loading and unloading the coke will suffer abrasion and breakage.

Many international standards have been developed to determine a coke’s resistance to impact and abrasion (below)

13

COKE QUALITY – HOT COKE STABILITY

To measure the performance of coke in the blast furnace a method was developed by Nippon Steel that is accepted as the industry standard – CSR (Coke Strength after Reaction) & CRI (Coke Reactivity Index).

Strength is important due to degradation of the coke during its journey through the blast furnace. The coke weakens as carbon is liberated for dissolution in the metal to form pig iron (approx 4% carbon steel).

Reaction taking place in CSR / CRI test below:

CO2 + C (coke) = 2CO

14

CSR TEST – BLAST FURNACE SIMULATION

200g of -21mm +19 mm coke is heated to 1100°C under nitrogen.

After the coke reaches 1100°C the nitrogen gas is replaced with carbon dioxide for a period of 2 hours.

The sample is then cooled to room temperature under nitrogen and the % weight loss measured (CRI Index).

The material is then tumbled for 600 revs to determine the coke strength by screening tumbled material (CSR Index = % +10mm).

The test is performed in duplicate and repeatability must be within 5 CSR / CRI points.

15

DOLLAR VALUE OF CSR INDEX

16

SGS’ BRISBANE COKE TESTING FACILITY IN PINKENBA

Coke Testing Facility Includes

255kg movable wall coke oven

7 kg small scale oven

ASTM, Tumble

Micum / Irsid Tumble

JIS Tumble

CSR / CRI Furnaces

SGS Pinkenba employs 57 people across 8 divisions.

17

SGS – WEST VIRGINIA USA

Largest SGS coal lab in the U.S (approx 45 employees)

300 kg movable wall oven

30 lb small scale oven

Full coke testing facilities

Full coal testing facilities

18

ACKNOWLEDGEMENTS

AS Standards 2267-1997, 1038.13-1990, 4264.2-1996

5th ICCP Course Organic Petrology 2012 – CSIRO Seminar by Prof Claus Diessel & Dr Walter Pickel

Introduction to Iron & Steel Making Swinburne University of Technology 2013 – CSIRO Seminar by Prof Geoff Brooks

McMahon Coal Quality Resources - Coal Quality Command Module Nov 2009.

Coal & Coke Course BHP Billiton 2005

Reifenstein 2003 ACARP Report - The Coke Reactivity Test Critical Parameters

Google Maps (www.maps.googe.com.au)