1. Introduction

[GLOSS]Natural gas[/GLOSS] – [GLOSS]NG[/GLOSS] – frequently forms a major part of the fuel gas consumed in a modern integrated iron and steelworks. However the major feedstock is [GLOSS]coal[/GLOSS]. Coal is introduced in Combustion File (CF) 62 and described in more detail in CF177.

Coal is used to produce metallurgical coke, in [GLOSS]Coke Ovens[/GLOSS], giving as a by-product fuel gas, [GLOSS]Coke Oven Gas[/GLOSS] – [GLOSS]COG[/GLOSS].

The coke is used as a reducing agent and energy supply in [GLOSS]Blast Furnaces[/GLOSS] to reduce iron ore to “[GLOSS]hot metal[/GLOSS]”, giving as a by-product fuel gas, [GLOSS]Blast Furnace Gas[/GLOSS] – [GLOSS]BFG[/GLOSS].

The liquid iron is refined in a [GLOSS]Basic Oxygen Furnace[/GLOSS]. A major process is decarburisation, effected using relatively pure oxygen, a process which gives rise a by-product fuel gas, [GLOSS]Basic Oxygen Furnace Gas[/GLOSS] – [GLOSS]BOFG[/GLOSS].

These four fuel gases together form the typical fuel gases used in a integrated iron and steel works. The characteristics of these fuel gases are briefly introduced in this combustion files.

For the purposes of this introduction, the fuel gases supplied by the integrated iron and steelworks of CORUS, at IJmuiden in the Netherlands, to the IFRF Research Station, are used as a “baseline”.  Variations from these baselines are presented in a series of more detailed combustion files, noted in each of the sections below.

2. Natural Gas

Natural Gas – NG, introduced in CF62, is a high calorific value, gaseous fossil fuel, composed mainly of methane – CH₄– often found in association with crude petroleum deposits. The use of natural gas as an energy supply for integrated iron and steelworks depends on local availability. Further details are supplied in CF216.

Natural Gas can be used to fire a wide range of steel process heating furnaces

The “baseline” gas for this series has the properties given in Table 1.

3. Coke Oven Gas

Coke oven Gas COG, is produced as a by-product from coke production typically in horizontal retorts exemplified in Figure 1.

The raw coal is heated (without adding oxygen) in order to drive off the volatile components of the coal to produce a reactive coke with sufficient mechanical strength for use in Blast furnaces.

The gaseous by-product is a high quality fuel gas known as Coke Oven Gas. Apart form the fact that it contains significant amounts of carbon monoxide, it is the combustion engineers dream – a relatively rich gas with a calorific value about half that of Natural Gas with wide flammability limits.

The typical COG cited here has the properties given in Table 1.

The coals used, the coke oven operating parameters, along

with the degree of chemical recovery and gas cleaning employed locally affects the composition. These aspects are considered in CF217.

Figure 1: Coke Ovens

4. Blast Furnace Gas

The basic reducing agent in iron making is carbon (C), which stems from the coke and coal or heavy oil fed to the Blast Furnace along with the iron ore, limestone and other raw materials.

The top gas from the system is Blast Furnace Gas – BFG a very lean Fuel Gas with a Calorific Value around 1/10 that of Natural Gas

The typical BFG cited here has the properties given in Table 1.

The quality of the BFG is affected by the blast furnace operating parameters, such as the injection of powder coal or heavy oil. These aspects are considered in CF218.

Figure 2: Blast Furnace System

5. Basic Oxygen Furnace Gas

Basic oxygen “steel making” gas – BOFG is a by-product of the process of decarburising liquid iron typically of the type illustrated in figure 3. This is a process where a batch of liquid iron (“hot metal”) is added to the vessel along with scrap and steel slagging materials.

The excess carbon in the hot metal is oxidized using industrial grade oxygen. The oxygen is introduced via a water-cooled copper lance through which oxygen is blown into the liquid iron bath.

The combustion of the carbon results in very high local temperatures, which result in relatively high carbon monoxide concentrations in the “off-gas”, collected above the vessel. This results in a valuable by-product fuel gas after cleaning – Basic Oxygen Furnace Gas – BOFG – which as a CV about 1/4 that of Natural Gas.

As mentioned, the furnace is operated as a batch production process, which results in a variation of the composition with time. The typical average BOFG cited here has the properties given in Table 1. Further details are presented in CF219.

6. Summary of Fuel Gas Properties

Table 1: Fuel Gas Properties

Sources

[1] CORUS IJmuiden – Gas analysis data.

[2] American Iron and Steel Institute – Graphics – http://www.steel.org/learning/flowline/index.htm