On the nitrogen chemistry in jet and swirled pilot-scale PF flames

Other conference contribution, 2019

This paper discusses the differences that may occur in terms of NOx formation while using different suspension-fired technical and pilot-scale units. The aim with the work is to examine the NOx formation and to show how the fuel volatile-composition can be a released and exposed, and, in turn, influence the NOx formation. The flames studied are both jet-type, fully developed turbulent and low-Reynolds number type flames. The units used are located at the University of Utah: the L1500 unit (1.5 MW unit) and the OFC unit (100 kW). The same fuel is used in all tests: Sufco coal.

By using these two different units together with three different flame types, we can discuss the influence of both scale and flame type on the formation of NOx. This problem is well-known since previous pilot-scale combustion research (for example the work conducted by the IFRF in the late 1970s). However, the issue is certainly not always recognized in more recent studies. Furthermore, in targeting NOx emission issues in both power plants and industrial units, these scaling issues become critical in todays’ NOx emission research. Therefore, this paper aims to bring this issue some attention.

This work demonstrates that NOx formation is closely related to both scale and flame-type. The reason is that the NOx formation is mostly dependent on the time-temperature development combined with oxygen-exposure throughout the suspension. This will be clearly demonstrated in this paper by the experimental results, and then further analyzed by kinetic modelling. In future NOx formation work, these results should be used to create a model fully capable of recreating the differences in scale and flame type in suspension-fired units. Although NOx formation remains a classical combustion problem it is still difficult to tackle in modern large-scale solid-fuel units that cannot use secondary emission control (for process-related or economic reasons). This should prompt new research, related to both fuel-switch in existing units, load-variation in coal PF-units, rotary-kilns, and other coal-based applications; all needing to reduce and control their NOx emissions.