1. Background

[GLOSS]Blast Furnace Gas[/GLOSS] ([GLOSS]BFG[/GLOSS]) is a by-product of the iron making process (See linked Combustion File 100). This low calorific value gas is available in large quantities within integrated iron and steel works. Details of its production and properties may be found in linked combustion files (CFs 218, 242, 243).

Although BFG is often used within steam boilers for power generation, its use as a substitute for conventional fuels (such as [GLOSS]natural gas[/GLOSS]and[GLOSS]oil[/GLOSS]) in high temperature processes within the steel industry has been limited by its low [GLOSS]calorific value[/GLOSS]. The calorific value of BFG is about ten times lower than that of natural gas or fuel oils. Direct use of such a low [GLOSS]CV[/GLOSS]fuel leads to reduction in heat transfer within high temperature furnaces, and in many cases, the required process temperature may not be achieved.

In this Combustion File, the options for enhancing the heat transfer capabilities of BFG in high temperature furnaces are considered.

2. What are the main options to enhance the heat transfer from BFG?

The following options will increase the combustion temperature of BFG flames and enhance heat transfer in high temperature furnaces.

2.1 Oxygen enrichment of the [GLOSS]combustion air[/GLOSS]

By reducing the amount of nitrogen required to achieve a stoichiometric mixture of BFG and [GLOSS]comburent[/GLOSS], [GLOSS]oxygen enrichment[/GLOSS] reduces the thermal ballast in the flame and in the flue gases, leading in turn to increased combustion temperatures and reduced [GLOSS]sensible heat[/GLOSS] losses in the flue gases. These all enhance the performance of a high temperature furnace. Oxygen enrichment of up to 100% oxygen (pure oxygen) may be envisaged, the actual level chosen depending on the required furnace performance and on the economics of oxygen supply.

2.2 Addition of a [GLOSS]Rich Gas[/GLOSS] to the BFG

Addition of a richer gas to the BFG reduces the quantity of inert gases in a [GLOSS]stoichiometric[/GLOSS] mixture of the gaseous fuel and the comburent. The thermal effect of this is similar to that obtained by adding oxygen to the combustion air as described above. The rich gases that may be considered for this duty include natural gas (CF 216), [GLOSS]LPG[/GLOSS] ([GLOSS]Liquified Petroleum Gas[/GLOSS] – typically a mixture of propane and butane) and [GLOSS]Coke Oven Gas[/GLOSS] (CF 239). The latter is available as a by-product of the coke making process within integrated steel works. Its technical disadvantage compared to the two others is its relatively low CV (about half that of natural gas), which limits its effectiveness as a BFG enrichment agent.

2.3 Preheating the combustion air and/or BFG

Combustion air preheat, particularly when the preheat energy is taken from the sensible heat in the flue gases, by recuperation or regeneration, is an effective way of increasing the combustion temperature of a BFG flame while at the same time, recycling heat from the flue gases to the burner to enhance furnace efficiency (CF 172).

In the case of BFG, because of the quantity of inert gases in the BFG passing through to the flue gases, the capacity of combustion air is often not sufficient to absorb all the heat available in the flue gases. A more effective heat recovery maybe obtained by preheating both the combustion air and the fuel gas.

3. How do I assess the effect of these approaches on a high temperature process

Studies undertaken by the IFRF [1] have shown that one of the key parameters that may be used to assess the effect of these options on the efficiency of high temperature processes is the [GLOSS]adiabatic flame temperature[/GLOSS]. Guidance on the relationship between this parameter and the efficiency of a furnace fired by BFG is given in CF 250. Approaches to estimate the adiabatic flame temperature are outlined in CF 251.

Sources

[1] Michel J-B and Payne R. The use of blast furnace gas as a fuel in high temperature furnaces of the steel industry. Final report of the IFRF G1 trials, IFRF Doc No F01/a/100, March 1979.

Acknowledgements

The author acknowledges the support and encouragement of Jean-Bernard Michel and Roy Payne during the adaptation of their original report to create this Combustion File.