1. Introduction

The typical fuel gases used in integrated iron and steelworks are listed in CF62 and introduced in more detail in CF100. This group of industrial fuel gases includes the byproduct of [GLOSS]Basic Oxygen Steelmaking[/GLOSS] ([GLOSS]BOS[/GLOSS]), referred to here as [GLOSS]Basic Oxygen Furnace Gas[/GLOSS] or [GLOSS]BOFG[/GLOSS], but also known as [GLOSS]Basic Oxygen Steelmaking Gas[/GLOSS].

BOFG is produced as a by-product of the process of decarburising liquid iron ([GLOSS]hot metal[/GLOSS]) in a [GLOSS]Basic Oxygen Furnace[/GLOSS], [GLOSS]BOF[/GLOSS]. Decarburised liquid iron is called liquid steel.

BOFG is a low to medium [GLOSS]calorific value[/GLOSS] fuel gas, which can be used as gaseous fuel in various applications inside and outside the steel plant.

The production of BOFG and its applications are described in associated Combustion File numbers 219, and 247 respectively.  The present CF concentrates on the fuel, combustion and [GLOSS]flue gas[/GLOSS] properties of Basic Oxygen Furnace Gas.

2. Fuel properties of Basic Oxygen Furnace Gas

The fuel properties of Basic Oxygen Furnace Gas are mainly dependent on the point in time during the blowing cycle and the gas cleaning employed.

Normally, the Basic Oxygen Steelmaking Gas, which is generated in a part of the blowing cycle, is stored to be used as fuel. In Table 1 a typical fuel gas composition is shown.

Table 1. Typical composition of Basic Oxygen Furnace Gas

As shown in Table 1, the main combustible specie in BOFG is carbon monoxide. However, the concentration if this component is much higher than in Blast Furnace Gas (typically 22%). From the composition of the BOFG, other properties, such as the [GLOSS]net calorific value[/GLOSS] and [GLOSS]net Wobbe Index/Number[/GLOSS] can be calculated.

The net calorific value of dry BOFG is about 8.7MJ/m_o³. The net Wobbe index of Basic Oxygen Steelmaking Gas is about 8.5 MJ/m_o³. The net calorific value of BOFG is almost equal to the Wobbe index since the relative density of BOFG equals one.

However, since BOFG is normally cleaned by wet scrubbing, the gas is always saturated with water vapour when it is used. The amount of water vapour in the gas is dependent on the temperature of the gas when it is used.

When BOFG is used at a temperature of 40°C, the water vapour concentration is 7.3% (as indicated in Table 1). When the temperature of the gas is 50°C, the water vapour concentration is about 12%. In that case the net calorific value and the net Wobbe index reduce to about 7.6 MJ/m_o³.

3. Combustion and flue gas properties of Basic Oxygen Furnace Gas

Combustion properties that are important when BOFG is fired in industrial furnaces and boilers are:

o         The [GLOSS]stoichiometric air requirement[/GLOSS],

o         The [GLOSS]adiabatic flame temperature[/GLOSS],

o         The [GLOSS]flammability limits[/GLOSS],

o         The [GLOSS]auto-ignition temperature[/GLOSS] and;

o         The flue gas composition.

Stoichiometric Air Requirement

To fire BOFG under stoichiometric conditions, the air requirement is about 1.5m_o³/m_o³ fuel. This is a relatively small air requirement compared to that needed for natural gas (8.5m_o³/m_o³ to 10.0m_o³/m_o³⁾.

Adiabatic Flame Temperature

Since the main combustible specie in BOFG is carbon monoxide, this results in a relatively high adiabatic flame temperature of about 2000°C. This is almost equal to the adiabatic flame temperature of a natural gas flame at stoichiometric conditions.

Flammability Limits

A dry BOFG – air mixture at ambient temperature and pressure is explosive if the concentration of BOFG in the mixture is 18.8 – 71.4% v/v [4].

Auto-Ignition Temperature

The auto-ignition temperature of a gas mixture is determined by the lowest auto-ignition temperature of one of the species in the gas mixture. In general, this is the hydrocarbon component with the largest molecule or hydrogen. In a BOFG – air mixture, the auto-ignition temperature of hydrogen determines the auto-ignition temperature. This is 400°C, which is relatively low compared to the auto-ignition temperature of natural gas: 617°C [5].

Flue Gas Composition

In Table 1 the flue gas composition of a stoichiometric Basic Oxygen Steelmaking Gas flame, assuming complete combustion, is presented.

Table 2 Flue gas properties of stoichiometric Basic Oxygen Furnace Gas flames, assuming complete combustion

4. An overview of the combustion engineering related properties of BOFG

An overview of the gas properties, combustion and flue gas properties of a typical Basic Oxygen Furnace Gas is shown in Table 3.

Table 2 Typical example of combustion related properties of Basic Oxygen Furnace Gas at stoichiometric combustion

Sources

[1] American Iron and Steel Institute, www.steel.org.

[2] Stubbles, J., The basic oxygen steelmaking BOS process, www.steel.org.

[3] AISE Steel Foundation, 1998, Making, Shaping, and Treating of Steel, 11th Edition, Steelmaking And Refining Volume. Pittsburgh PA

[4] Zabetakis, M.G., Flammability characteristics of combustible gases and vapors, Bulletin 627, Bureau of Mines, 1965.

[5] N.V. Nederlandse Gasunie, Physical properties of natural gases, Groningen, Netherlands, 1980.

[6] Informatie Chemische Stoffen, werkinstructiekaart oxygas, Corus, 2002.