Division BAT 4: In order to reduce NOX emissions to air from process furnaces/heaters, BAT is to use one or a combination of the techniques given below.
United Kingdom
| Technique | Description | Applicability | |
|---|---|---|---|
| a. | Choice of fuel | See Section 12.3. This includes switching from liquid to gaseous fuels, taking into account the overall hydrocarbon balance | The switch from liquid to gaseous fuels may be restricted by the design of the burners in the case of existing plants |
| b. | Staged combustion | Staged combustion burners achieve lower NOX emissions by staging the injection of either air or fuel in the near burner region. The division of fuel or air reduces the oxygen concentration in the primary burner combustion zone, thereby lowering the peak flame temperature and reducing thermal NOX formation | Applicability may be restricted by space availability when upgrading small process furnaces, thus limiting the retrofit of fuel/air staging without reducing capacity For existing EDC crackers, the applicability may be restricted by the design of the process furnace |
| c. | Flue-gas recirculation (external) | Recirculation of part of the flue-gas to the combustion chamber to replace part of the fresh combustion air, with the effect of reducing the oxygen content and therefore cooling the temperature of the flame | For existing process furnaces/heaters, the applicability may be restricted by their design. Not applicable to existing EDC crackers |
| d. | Flue-gas recirculation (internal) | Recirculation of part of the flue-gas within the combustion chamber to replace part of the fresh combustion air, with the effect of reducing the oxygen content and therefore reducing the temperature of the flame | For existing process furnaces/heaters, the applicability may be restricted by their design |
| e. | Low-NOX burner (LNB) or ultra-low-NOX burner (ULNB) | See Section 12.3 | For existing process furnaces/heaters, the applicability may be restricted by their design |
| f. | Use of inert diluents | ‘Inert’ diluents, e.g. steam, water, nitrogen, are used (either by being premixed with the fuel prior to its combustion or directly injected into the combustion chamber) to reduce the temperature of the flame. Steam injection may increase CO emissions | Generally applicable |
| g. | Selective catalytic reduction (SCR) | See Section 12.1 | Applicability to existing process furnaces/heaters may be restricted by space availability |
| h. | Selective non-catalytic reduction (SNCR) | See Section 12.1 | Applicability to existing process furnaces/heaters may be restricted by the temperature window (900–1 050 °C) and the residence time needed for the reaction. Not applicable to EDC crackers |
BAT-associated emission levels (BAT-AELs): See Table 2.1 and Table 10.1.