Numerical investigation of spray combustion towards HITAC conditions

The features of High Temperature Air Combustion (HiTAC), i.e. high-efficiency combustion processes creating a uniform temperature distribution with low NOX and CO emissions, lend itself ideally for the combustion of all sorts of "difficult” fuels, ranging from low-calorific gases such as waste-gases, to heavy fuel-oils. However, to date most of the applications of HiTAC are for gaseous fuels and solid fuels, while little has been investigated on liquid fuel spray combustion in such combustion regimes. The objective of the research presented in this thesis is to identify and specify the important parameters for achieving good model performance and to understand how HiTAC conditions can be achieved for spray combustion. For this purpose numerical investigations have been performed on the NIST (National Institute of Standards and Technology) methanol spray flame under a conventional condition, the DSHC (Delft Spray-in-Hot-Coflow) ethanol spray flames in both cold and hot co-flow conditions, and the heavy fuel oil spray combustion in a 9 MW boiler with flue gas recirculation using Stork Double Register Burner (DRB). The NIST methanol spray flame was numerically studied using an Eulerian- Lagrangian RANS model. With steady laminar flamelet model good agreements between experimental data and numerical results were observed. Then we extended the validated models and methods to the the simulation of DSHC flames, and also extended the limited co-flow conditions of experiment to a series of combinations of temperatures and O2 concentrations for comparative study. The results showed that the low O2 concentration plays a key role in the reduction of peak temperature and thus thermal NOX emissions. The flame profiles and SMD at various elevations showed good agreements with experimental data under the similar conditions. The analysis of spray trajectories has been discussed in order to perform proper model validation. Heavy fuel-oil combustion in a 9MW boiler was numerically investigated with the Euler-Lagrange method as well. For Combustion, Eddy Dissipation model with a two-step global reaction mechanism was used instead. The results showed that a more uniform temperature distribution in the boiler can be achieved by diluting the primary and secondary air flow with flue gas recirculation. In this way the thermal NOX can be effectively reduced, while the remained fuel NOX formation is mainly dependent on the local combustion characteristics and the initial concentration of nitrogen-bound compounds. Besides, soot formation should be included in the simulation since it shows considerable influence on peak temperature and NOX formation. It is also concluded that the realization of HiTAC-like conditions in heavy fuel-oil combustion depends on the possibility to guarantee a sufficiently high level of flue gas recirculation flow into the evaporating spray jet.