Calculating Flammability Limits of Combustible Gas Mixtures containing Water Vapour
A suitable method that meets the requirements of AS2381 has been developed for determining the flammability limits of gas mixtures containing water vapour. AS2381 provides a method (Appendix J) for determining the LEL (lower explosive limit) and UEL (upper explosive limit) for gas mixtures containing combustible and non-combustible (CO2 and N2) components. This method is used typically in the natural gas industries to determine the flammable limits of gas mixtures for the design of electrical systems.
However, this method does not include the inerting affects of water. This is of particular importance in the minerals processing industries where the unit operations of drying and reduction are combined in a single process step. These processes use sub-stoichiometric combustion to provide both an atmosphere for the reduction of metal oxides and heat for drying. Sub-stoichiometric combustion is used to create reducing agents (CO and H2) used in the reduction of metal oxides to metal. These reductants are flammable and residual concentrations of these components remain in the units offgas system thus creating the potential for ignition.
This paper presents the results of a literature search instigated to determine a suitable method to account for the effects of water vapour on flammability limits of gas mixtures. An update to the method of AS2381 is provided that accounts for water. The paper then investigates the impact of water vapour on the flammability of gas mixtures created by the sub-stoichiometric combustion of hydrocarbon fuels. Recommendations on suitable methods to ensure safe operation of these processes are described.