Fuel gases used in steel industry


Fuel gases used in steel industry

Fuel gas is a fuel which under ordinary conditions is in the form of gas. Fuel gases are used in steel plants for different applications which include (i) a source of heat (ii) as a reductant and (iii) cutting and welding application. Fuel gases usually used in steel industry are natural gas (NG), liquefied petroleum gas (LPG), acetylene, by product gases (blast furnace gas, coke oven gas and converter gas).

Natural gas

Natural gas is a gaseous fossil fuel which is extracted from deposits in the earth. It is a mixture of hydro carbons consisting primarily of methane (generally greater than 80 %) but includes varying amounts of other higher alkanes such as ethane, propane and butane etc. It may even contain some small percentage of nitrogen, carbon dioxide and hydrogen sulphide. It is an odorless, colourless, tasteless and non toxic gas. Natural gas is lighter than air and it burns with a clean blue flame when mixed with the requisite amount of air and ignited. It is considered one of the cleanest burning fuels. On burning it produces primarily heat, carbon dioxide and water.

Quantities of natural gas are measured in normal cubic meters (corresponding to 0 deg C and I Kg/Sq cm pressure) or standard cubic feet (corresponding to 16 deg C and 14.73 psia pressures). The higher heat value of one cubic meter of natural gas varies from around 9500 Kcal to 10,000 Kcal. Its density is around 0.85 Kg/Cum.

The main usage of natural gas in the steel industry is in iron making. For production of direct reduced iron it is reformed to produce reducing gases which are then used for the reduction of iron ore. The main reforming reactions are as follows.

2CH4 + O2 = 2CO + 4 H2

CH4 + H2O = CO + 3 H2

CO2 + H2 = CO + H2O

In case of iron making in blast furnace, natural gas is being injected in some of the blast furnaces at the tuyeres to reduce the consumption of BF coke and to increase blast furnace productivity. In blast furnace, natural gas plays a double role of a reductant and a fuel.

Liquefied petroleum gas

Liquefied petroleum gas (LPG) is extracted from crude oil. The main components of LPG are hydro carbons containing 3 or 4 carbon atoms. The normal components of LPG are propane (C3H8) and butane (C4H10). Small percentage of other hydro carbons may also be present in LPG.

LPG is a gas at atmospheric pressure and ambient temperature, but it can be liquefied when moderate pressure is applied or when the temperature is sufficiently reduced. It can be easily condensed, packaged, stored and utilized, which makes it an ideal energy source for a wide range of applications. Normally LPG is stored in liquid form under pressure in a steel container, cylinder or tank. It is a colourless, odorless and non toxic gas. LPG is highly inflammable. LPG vapour is heavier than air, so any leakage will sink to the ground and accumulate in low lying areas and may be difficult to disperse. LPG expands rapidly when its temperature rises.

Calorific value of LPG is around 11,000 Kcal/Kg or around 22500 Kcal/Cum. Its liquid density is around 0.5 to 0.58 Kg/Cum. As its boiling point is below room temperature, LPG evaporates quickly at normal temperature and pressure. The ratio between the volumes of the vaporized gas and liquefied gas varies depending on composition, pressure and temperature but typically it is around 250:1.

In steel plants, LPG is stored in pressure vessels. These containers are either spherical or cylindrical and horizontal (Fig 1). LPG containers have pressure relief valves, such that when subjected to exterior heating sources, they will vent LPGs to the atmosphere.

LPG gas storage

Fig 1 LPG storage tanks

In steel plant LPG used as fuel as such or by mixing it with low calorific value blast furnace gas, in gas cutting of steel and other metals and in gas cutting torches of continuous casting machines.

Acetylene

Acetylene is the chemical compound with the formula C2H2. It is an unsaturated hydrocarbon and the simplest alkyne. An acetylene molecule is composed of two carbon atoms and two hydrogen atoms. The two carbon atoms are held together by what is known as a triple carbon bond. This bond is useful in that it stores substantial energy that can be released as heat during combustion. However, the triple carbon bond is unstable, making acetylene gas very sensitive to conditions such as excess pressure, excess temperature, static electricity, or mechanical shock.

It is a colourless and odorless gas. At atmospheric pressure, acetylene cannot exist as a liquid and does not have a melting point. The adiabatic flame temperature (AFT) in air at atmospheric pressure is 2534 deg C. Its specific gravity is 0.91 at 21 deg C.

It is produced by the hydrolysis of calcium carbide by the following chemical reaction.

CaC2 + 2H2O = Ca(OH)2 + C2H2

These days acetylene is mainly manufactured by the partial combustion of methane or appears as a side product in the ethylene stream from cracking of hydrocarbons.

Acetylene must be stored under special conditions because of its unstable nature. This is accomplished by dissolving the acetylene in liquid acetone. The liquid acetone is then stored in the acetylene cylinder, which in turn, is filled with a porous (sponge like) cementitious material.

Acetylene is used in steel plants for oxyacetylene gas cutting and welding and in flame cutting machines of continuous casting machines. It is sometimes used for carburization of steel.

Coke oven gas

Coke oven (CO) gas is produced during carbonization of coking coals in by product coke oven batteries. The raw coke oven gas which comes out of the battery is cleaned in the byproduct plant to produce clean coke oven gas. The main components of coke oven gas are hydrogen (H2), methane (CH4), carbon mono oxide (CO) and hydro-carbons (CmHn). It also contains inert gases like nitrogen (N2) and carbon dioxide (CO2). A small percentage of oxygen is also present in the gas.

The density of CO gas at standard temperature and pressure is 0.545 Kg/Cum. CO gas has a heating value between 4200 to 4800 Kcal/N Cum. It has a theoretical flame temperature of 1982 deg C. It has a rate of flame propagation which allows its actual flame temperature to be close to its theoretical flame temperature.

Coke oven gas is normally used in coke oven battery heating, heating in other furnaces and for power generation. Coke oven gas can be used as such or can be mixed with BF gas and/or converter gas before being used as fuel in the furnaces. CO gas injection at tuyere level has been successfully tried in the blast furnace in some steel plants.

Blast furnace gas

Blast furnace (BF) gas is a gaseous by product which is generated while producing hot metal (liquid iron) in a blast furnace. The total amount of carbon mono oxide (CO) and carbon dioxide (CO2) gases by volume in the BF gas at the top of the blast furnace is around 40 % of the total gas volume. The CO/CO2 ratio can vary in a blast furnace from 1.25:1 to 2.5:1. Higher percentage of CO in the gas makes the BF gas hazardous. The hydrogen content of the gas can vary from 2 % to 5 % depending upon the type and amount of fuel injected in the tuyeres of the blast furnace. The balance component of the BF gas is nitrogen. Methane (CH4) can also be present in the BF gas up to 0.2 %. BF gas can contain these cyano compounds in the range of 200 mg to 2000 mg/Cu m. BF gas has the following characteristics:

  • Very low calorific value (CV) in the range of 700 to 850 Kcal/Cu m.
  • It has a high density. It is around 1.250 Kg/Cu m at the standard temperature and pressure (STP) which is 0 deg C and 1 Kg/Sq cm pressure. This density is highest amongst all the gaseous fuel. Since the density is higher than the density of air it settles in the bottom in case of a leakage.
  • It has low theoretical flame temperature which is around 1455 deg C.
  • It has low rate of flame propagation. It is lower than any other common gaseous fuel.
  • BF gas burns with a non luminous flame.
  • Auto ignition point of BF gas is around 630 deg C.
  • BF gas has lower explosive limit (LEL) of 27 % and upper explosive limit (UEL) of 75 % in an air gas mixture at normal temperature and pressure.

In steel plants, BF gas is normally being used mixed with either coke oven gas or converter gas or both. The mixed gas is used as a fuel in various furnaces of the steel plant. BF gas without mixing and without preheat can be used in BF stoves, soaking pits, normalizing and annealing furnaces, foundry core ovens, gas engines for blowing, boilers for power generation, gas turbines for power generation.

Converter gas

During the process of steel making in basic oxygen furnace (BOF), converter gas is generated at a rate around 75 to 95 Cum /ton of crude steel. This gas has got a calorific value of 1600 to 1800 Kcal/N Cum of the gas. The composition of converter gas is Carbon mono oxide in the range of 58 % to 65 %, Carbon dioxide in the range of 15 % to 20 %, nitrogen in the range of 15 % t0 20 % , hydrogen around 1 % and oxygen around 0.2 %. Density of converter gas is 0.865 Kg/Cum.

LD gas is normally mixed with the blast furnace gas in various proportion and the mixed gas is used for heating purpose in various furnaces.