A flame stability diagram for piloted non-premixed oxycombustion of low calorific residual gases

摘要

Residual gases having a low calorific value are by-products of many industrial combustion systems. Subsequently burning those exhaust mixture, as blast furnace gases (BFG), is a direct way to increase the overall efficiency of processes. Flame stability is a crucial issue when burning such low calorific fuels and the choice of a piloted burner with pure oxygen as oxidizer appears as an interesting option. To progress in this direction, a new concept of piloted burner is discussed in which oxycombustion of a CH_4/BFG mixture is studied over a large range of global Net Calorific Value. From the results, the parameters controlling the major flame properties are determined and an attempt is made to draw a generic stability map to delineate between various combustion regimes. The burner consists of four coaxial jets. Starting from the axis of symmetry, it is composed of an oxygen (O_(2i) center jet, surrounded by a synthetic BFG flow, followed by a CH_4 jet, and oxygen (O_(2e)) at the periphery. The objective of such geometry is to stabilize low calorific BFG combustion with an external CH_4-O_(2e) pilot flame. The flame structure is studied according to the proportion of residual BFG gases injected. CH~* chemiluminescence imaging is performed. Three types of flames are observed and organized in stability diagrams for such piloted non-premixed burner. In Type 1, two concentric flames are attached at the burner nozzle. In Type 2, the internal BFG flame is lifted. In Type 3, a single annulus flame burning a fuel mixture of CH_4 and BFG with the external oxygen is observed. Characteristic numbers, as ratio of jets velocities and Damkoehler numbers are found to control transition between the various types of flames. They are used to build the stability diagrams.