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 [GLOSS]Coke Oven Gas[/GLOSS] or [GLOSS]COG[/GLOSS].

COG is produced as a by-product from [GLOSS]coke[/GLOSS] production.  COG is a valuable since it can be used as a relatively rich gaseous fuel in various applications inside or outside the steel plant. One of the biggest consumers is the coke plant itself, where up to 50% is used as the coke plant fuel.

The production of COG, its combustion and flue gas properties, and its the applications are described in associated Combustion File numbers 217, 240 and 241 respectively.

The present combustion file concentrates on the fuel properties of COG.  The fuel properties are dependent on the [GLOSS]coal[/GLOSS]s used for coke production, the coke oven operating conditions and the degree of chemical recovery and gas cleaning employed.

The primary aim of a coke plant manager is to make a good and constant quality coke, at a sufficient rate, with minimum environmental impact, and at low cost.

The properties of the Coke Oven Gas by-product also depend on these factors.

2. What influences the properties of coke?

Properties of coke can be classified into three main categories:

q       Physical properties, e.g. size distribution, ASTM tumbler strength (stability and hardness) and porosity.

q       Thermo-Chemical properties, e.g. coke reactivity and coke strength after reaction with CO₂.

q       Chemical properties e.g. fixed carbon content, sulphur content, alkali content, ash content, phosphorous content, volatile matter and moisture content.

Coking coals are defined as those coals that on carbonization pass through softening, swelling and re-solidification to coke.  An important consideration in selecting a coking coal is that it should not exert a high coke oven wall pressure and should contract sufficiently to allow the coke to be pushed from the oven.

The coal may be either from a single mine, or a blend of [GLOSS]bituminous coal[/GLOSS]s from different mines.  Some steel plants are located near a mine, hence always using the coal from this mine. Other steel plants use coal blends. With coal blends a steel plant is less dependent on the conditions of a single mine. In this case the quality of the coking coal mix can be kept constant.

The operating conditions that can influence the coke production and coke quality are the coking temperature, the coking rate, soaking time, quenching practice and coke handling.

3. What influences the fuel properties of COG?

To recall, the fuel properties of Coke Oven Gas are dependent on:

q       The quality of the coals used for coke production,

q       The coke oven operating conditions such as:

o         Coking temperature

o         Coking rate

q       The degree of chemical recovery and gas cleaning employed.

Due to these factors, fuel gas properties of Coke Oven Gas are different from site to site and may also vary in time.

In Table 1 the average fuel gas composition of four different coke plants are shown.

4. The potential range of COG fuel properties

From Table 1, it may be seen that the main combustible species in Coke Oven Gas are hydrogen and methane.

As these are highly combustible fuel components, Coke Oven Gas is a high quality fuel gas.

From the composition of the COG, the ranges of other properties, such as the [GLOSS]Calorific value[/GLOSS] and [GLOSS]Wobbe Index/Number[/GLOSS] can be calculated as listed in Table 2.

It should be noted that, although the Net Calorific Value of Coke Oven Gas is much lower than that of [GLOSS]natural gas[/GLOSS], the Net Wobbe index of Coke Oven Gas approaches that of Natural Gas – see Combustion Files 216 and 230.

This is due to the high concentration of hydrogen in Coke Oven Gas, which results in a relatively low relative density (approximately 0.3) and a high net Wobbe index.

Table 1 Average Coke Oven Gas composition of different coke plants

Table 2 Calorific Value and Wobbe Number Ranges for Coke Oven Gas

Sources

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

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

[3] Militzer, M.R., UEC Coal & Coke Laboratory, Coke Production for Blast Furnace Ironmaking.

[4] Valia, H.S. Coke production for blast furnace ironmaking, Ispat Inland Inc, American Iron and Steel Institute.

[5] Platts, M. The coke oven by-product plant, ThyssenKrupp EnCoke, American Iron and Steel Institute.