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Fuel Gases used in Steel Industry

• satyendra
• July 20, 2013
• 0 Comments
• acetylene, BF gas, By-product gas, CO gas, Converter gas, CV, DRI, Fuel gas, Liquefied petroleum gas, natural gas,

Fuel Gases used in Steel Industry

Fuel gas is a fuel which is in the form of gas under ordinary conditions. Several of the gases contain hydrocarbons (such as methane or propane), hydrogen (H2), carbon monoxide (CO), or mixtures of these gases. Such gases are source of heat energy and can be readily transmitted and distributed through pipe lines from the point of origin directly to the place of consumption. Fuel gases are different from liquid fuels and from solid fuels, though some fuel gases can be liquefied for ease of storage or transport.

Fuel gases are used in steel plants for different applications which include (i) a source of heat (ii) as a reductant, (iii) power generation, and (iv) cutting and welding application. Fuel gases normally used in steel plants include (i) natural gas (NG), (ii) liquefied petroleum gas (LPG), (iii) by-product gases such as blast furnace (BF) gas, coke oven gas (COG), and converter gas, and (iv) acetylene. Fig 1 shows types of fuel gases and their applications in steel plants.

Fig 1 Types of fuel gases and their applications in steel plants

Natural gas

NG is an environmentally friendly non-renewable gaseous fossil fuel which is extracted from deposits in the earth. It is a clean fuel with a high efficiency. It is transported to long distances (upto 5,000 kms) through a pipeline network. It is normally supplied to the consumers as (i) piped natural gas (PNG), (ii) compressed natural gas (CNG), and (iii) liquefied natural gas (LNG).

NG which is supplied to the consumer through pipe is PNG. The pipeline pressure at the consumer end is normally less than 16 atmospheres. CNG is a form of natural gas which undergoes compression (200 atmospheres to 250 atmospheres) into containers. LNG is made by cooling natural gas to a temperature of minus 162 deg C. At this temperature, NG becomes a liquid and its volume is reduced by 600 times.

NG is a mixture of hydro-carbons consisting primarily of methane (CH4), normally in a percentage of over 85 % by volume. Other hydro-carbons in NG include varying amounts of various higher alkanes such as ethane, propane, and butane etc. It also contains water vapour (H2O) at varying degrees of saturation, or condensed water. It can also contain some small percentage of nitrogen (N2), carbon dioxide (CO2) and hydrogen sulphide (H2S) and helium (He) etc.

NG is an odourless, colourless, tasteless and non toxic gas. It is lighter than air and 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, CO2, and water.

Quantities of NG are measured in normal cubic meters (corresponding to 0 deg C and 1 atmosphere pressure) or standard cubic feet (corresponding to 16 deg C and 14.73 pounds per square inch absolute pressure). The higher heat value of one cubic meter (cum) of natural gas varies from around 9,500 kcal to 10,000 kcal. Its density is around 0.85 kg/cum.

The main usage of NG in the steel industry is in iron making where it is used as a reductant. For using NG for the production of the direct reduced iron (DRI) in gas based DRI production processes, NG is needed to be reformed into a usable reducing gas which has a high in H2 and CO content. More than 90 % of the global DRI plants use NG. For the production of DRI, it is reformed to produce reducing gases which are then used for the reduction of iron ore. The main reforming reactions are (i) 2CH4 + O2 = 2CO + 4 H2, (ii) CH4 + H2O = CO + 3 H2, and (iii) CO2 + H2 = CO + H2O.

NG is injected in the tuyeres of the BF as an auxiliary fuel. It is injected along with the O2 enrichment of the hot blast air. The purpose of injection of NG as an auxiliary fuel is to have reduction in the specific consumption of coke. The replacement ratios of coke being achieved with NG gas injection in the BF are in the range of 1.3 to 1.4. The injected NG in the BF provides reducing gases consisting of H2 and CO to the furnace which moves up the furnace shaft and takes part in the reduction reactions of the iron oxides

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 can 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.

LPG is a colourless, odourless and non toxic gas. It is highly inflammable. Vapour of LPG is heavier than air, so any leakage sinks to the ground and accumulates in low lying areas and can be difficult to disperse. LPG expands rapidly when its temperature rises.

LPG is a colourless liquid which readily evaporates into a colourless and odourless gas. Normally foul smelling ethyl mercaptan is added as an odourizer to LPG so that leaks can be easily detected. During leakage, the vapourization of liquid cools the atmosphere and condenses the water vapour contained in it to form a whitish fog which can make it possible to see the escape of LPG.

In the case of mixing with air, the gas can burn or explode when it meets a source of ignition. It is heavier than air, so it tends to sink towards the ground. LPG can flow for long distances along the ground, and can collect in drains, gullies and cellars.

Specific gravity of LPG in liquid form at 15 deg C ranges from is 0.51 to 0.58 (water = 1). Specific gravity of LPG in gaseous form is 1.52 to 2.01 (air = 1). Boiling point of LPG is in the range of – 42 deg C to 0 deg C. The physical state of LPG is gas at 15 deg C and 1 kg/sq cm pressure.

Calorific value (CV) of LPG is around 11,000 kcal/kg or around 22,500 kcal/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. Solubility of LPG in water at 20 deg C is less than 200 ppm (parts per million). LPG is soluble in organic solvents like alcohol.

Vapour pressure of LPG at 40 deg C is 5.3 kg/sq cm to 15.6 kg/sq cm. The pressure inside a LPG storage vessel is equal to the vapour pressure corresponding to the temperature of LPG in the storage vessel. The vapour pressure is dependent on temperature as well as on the ratio of mixture of hydrocarbons. At liquid full condition any further expansion of the liquid, the storage vessel pressure rises by around 14 to 15 kg/sq cm for each deg C. This clearly explains the hazardous situation which can arise due to the overfilling of storage vessels.

LPG has a flash point of – 104.4 deg C.  Auto ignition temperature for propane is 46.1 deg C and that for butane is 405 deg C. Hence LPG does not ignite on its own at normal temperature. LPG is highly flammable with a lower explosive limit (LEL) of 1.9 % and upper explosive limit (UEL) of 9.5 %. This explosive range is considerably narrower than other common gaseous fuels. This gives an indication of the presence of hazard from LPG vapour accumulated in low lying area in the eventuality of the leakage or spillage. It has explosion sensitivity to static electricity.

In steel plants, LPG is used as fuel as such or by mixing it with BF gas having lower CV values, in gas cutting of steel and other metals, and in gas cutting torches of continuous casting machines. It is stored in pressure vessels. These containers are either spherical or cylindrical and horizontal (Fig 2). LPG containers have pressure relief valves, such that when subjected to exterior heating sources, they vent LPGs to the atmosphere.

Fig 2 Storage tanks for liquefied petroleum gas

By-product gases

During the production of iron and steel in an integrated steel plant, three by-product gases are produced which are categorized as fuel gases since they have got considerable calorific value. These gases are (i) blast furnace (BF) gas, (ii) coke oven (CO) gas, and (iii) converter gas.

Blast furnace gas – BF gas is a gaseous by product which is generated while producing hot metal (liquid iron) in a BF.  It has a high density, which is around 1.25 kg/cum at 0 deg C of temperature and 1 atmosphere of pressure. This density is highest amongst all the gaseous fuel. Since the density is higher than the density of air, it settles at the bottom in case of a leakage.

The four main components of the BF gas are N2, CO, CO2, and H2. The percent of these components in the BF gas normally ranges by volume as N2 – 40 % to 60 %, CO – 20 % to 28 %, CO2 – 17 % to 25 %, and H2 – 1 % to 7 %. CH4 can also be present in the BF gas upto 0.2 %. BF gas can also contain some hydro-cyanide (HCN) and cyanogen gas (CN2) which are formed due to the reaction of N2 in the hot air blast and carbon of the coke. The reaction is catalyzed by the alkali oxides. These gases are highly poisonous. BF gas can contain these cyano compounds in the range of 200 mg/cum to 2,000 mg/cu m. The H2 content of the gas varies because of the type and amount of fuel injected in the tuyeres of the BF.

The total amount of CO and CO2 by volume in the BF gas at the top of the BF is around 40 % to 45 % of the total gas volume. The CO/CO2 ratio can vary from 1.25:1 to 2.5:1. High percentage of CO in the gas makes the BF gas hazardous. The CV of the BF gas varies but is normally low which ranges between 650 kcal/N cum to 900 kcal/N cum and depends on the CO content of the gas. Hence, BF gas is frequently enriched by COG or NG prior to its use.

The main characteristics of BF gas are (i) it is almost a colourless gas (mild whitish) and normally odourless, however it can have sometimes a slight sulphur odour, (ii) low CV, (iii) low theoretical flame temperature which is around 1455 deg C, (iv) low rate of flame propagation which is normally lower than any other common gaseous fuel, (v) it burns with a non luminous flame, (vi) it has auto ignition point of around 630 deg C, (vii) it has LEL of 27 % and UEL of 75 % in an air gas mixture at normal temperature and pressure, (viii) it is flammable and can form explosive mixtures with air, which can be easily ignited by a static charge, and (ix) it has a stable chemical stability under normal storage and handling conditions.

In steel plants, BF gas is being normally used mixed with either CO 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, sintering plant, soaking pits, normalizing and annealing furnaces, foundry core ovens, gas engines for blowing, boilers for power generation, gas turbines for power generation.

Coke oven gas – COG is produced during carbonization of coking coals in by-product coke oven batteries. The raw COG, which comes out of the battery, is cleaned in the by-product plant where tar, ammonia, and benzol etc. to produce clean COG. The main components of clean COG are H2 (42 % to 65 %), CH4 (17 % to 34 %), CO (4.6 % to 7.5 %) and hydro-carbons (CmHn). It also contains inert gases like N2 (1.2 % to 18 %) and CO2 (0.2 % to 3.2 %). A small percentage of oxygen (O2) is also present in the gas.

Clean COG is a colourless gas with an odour characteristic of hydrogen sulphide and hydrocarbons. It has a LEL of 4.4 % and UEL of 34 %. Its vapour density is 0.36 (air=1).  The density of COG at standard temperature and pressure is in the range of 0.45 kg/cum to 0.50 kg/cum. The CV of the COG is in the range of 4,000 kcal/N cum to 4,600 kcal/N cum.

COG has a theoretical flame temperature of 1,982 deg C. It has a rate of flame propagation which allows its actual flame temperature to be close to its theoretical flame temperature. When exposed at high concentration, COG acts as a simple asphyxiant. It displaces O2 and cause rapid suffocation by showing symptoms of O2 deprivation.

Clean COG is normally used in coke oven battery heating, heating in other furnaces and for power generation. It can be used as such or can be mixed with BF gas and / or converter gas before being used as fuel in the furnaces. COG injection at tuyere level has been successfully tried in the BF in some steel plants. COG is also being used for the production of DRI where it replaces NG.

Converter gas – During the process of steelmaking in the basic oxygen furnace (BOF), significant amount of gases, rich in CO content, are generated during the blow time at a temperature of around 950 deg C. This gas mixture is termed as converter gas or BOF gas. Converter gas is also known as LD gas.

The main constituents of converter gas are CO, CO2, O2, and N2.  Composition wise it is similar to BF gas but with lesser percentage of N2 in it. The composition of converter gas is CO in the range of 58 % to 70 %, CO2 in the range of 15 % to 20 %, N2 in the range of 12 % to 20 %, H2 in the range of around 0.9 % to 1 %, and O2 in the range of around 0.1 % to 0.3 %. Density of converter gas is 0.865 kg/cum.

During the process of steelmaking in the BOF, converter gas is generated at a rate around 75 cum to 95 cum per ton of crude steel. The CV of the converter gas varies in the range of 1,600 kcal/N cum to 2,400 kcal/N cum. It is the function of the air ratio. Lower is the air ratio higher is the CV, since N2 percentage of the gas reduces. Lower air ratio also means lower specific yield of the gas.

Converter gas is highly poisonous and explosive and requires high degree of disciplined operation at the time of recovery .The gas is invisible and colourless. It cannot be detected by odour. It can readily form explosive mixtures with air, which are easily ignited by a static charge. It is a chemical asphyxiant. It displaces O2 and cause rapid suffocation by showing symptoms of O2 deprivation.

Converter gas is normally mixed with the BF gas in various proportions and the mixed gas is used for heating purpose in various furnaces.

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 H2 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 which 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.

Acetylene is a flammable, colourless, and odourless gas. Its molar mass is 26.04 g/mol. It is unstable in pure form and thus is normally handled as a solution. Pure acetylene is odourless, but commercial grades normally have marked garlic like odour due to the impurities. At atmospheric pressure, acetylene cannot exist as a liquid and does not have a melting point. The gross and net heating values of acetylene gas is 11,932 kcal/kg (13,980 kcal/N cum) and 11,514 kcal/kg (13,490 kcal/N cum) respectively.

Acetylene has a density of 1.11 kg/cum at 1 atmosphere pressure and 15 deg C. The specific gravity is 0.91 (air=1).  It is lighter than air so does not accumulate at low levels, where it can cause a potential hazard. The boiling point of gas is minus 84.7 deg C and melting point is minus 80.75 deg C. The adiabatic flame temperature (AFT) in air at atmospheric pressure is 2,534 deg C. The auto ignition temperature of acetylene gas is 305 deg C.  Its lower and upper explosive limits in air under STP conditions (0 deg C and 1.02 kg/sq cm) are 2.4 volume % to 83 volume %. The upper limit can reach 100 %. Its solubility in water is 1.2 g/litre.

Acetylene is produced by the hydrolysis of calcium carbide as per the chemical reaction CaC2 + 2H2O = Ca(OH)2 + C2H2. These days acetylene is mainly produced by the partial combustion of CH4 or appears as a side product in the ethylene stream during the cracking of hydrocarbons.

Acetylene is required to 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 oxy-acetylene gas cutting and welding and also in flame cutting machines of continuous casting machines. It is sometimes used for carburization of steel, flame heating, flame gouging, flame hardening, flame cleaning, flame straightening, thermal spraying, spot-heating, brazing, texturing and profile-cutting, and carbon coating.