We report on boundary layer flashback of a turbulent premixed, pure hydrogen flame using well-resolved LES. This numerical work is based on flashback experiments of the TU Delft (TUD) jet flame at a jet Reynolds number of Re=11000. Flashback is a highly sensitive process, which is why (i) the turbulent inflow conditions, (ii) chemistry modeling and (iii) the wall temperatures of the mixing tube are crucial parameters to predict accurately this transient process. The presence of thermo-diffusive flame instabilities is the main contributor for flashback in this setup. We identify quasi-coherent turbulent structures in the mixing tube, namely an ejection event, which transports slow, preheated and hydrogen-enriched fluid away from the wall and triggers the flashback event. As a result, the flame forms a convex cusp upstream of the tube exit pointing towards the unburnt gas mixture. During the transition from unconfined (no walls around the flame) to confined (flame surrounded by walls) boundary-layer flashback, this cusp further bends and propagates towards the jet exit center, while, at the same time, its curvature and the reaction rate of hydrogen significantly increase by a factor of two. We repeated the flashback simulations several times and also for various flow conditions: all cases feature the same FB characteristics and, hence, confirms the generality of the conclusions. Moreover, the numerical flashback mechanism confirms the process hypothesized by the experiments. Based on the identified governing key parameters that affect flame flashback, we performed parametric variations of the Lewis number and wall temperature. By varying the Lewis number, we can clearly state that the flashback is driven by thermo-diffusive instabilities, while a hotter wall significantly deteriorates the flashback behavior of this setup. Novelty and significance statement Hydrogen combustion plays a crucial role in various energy applications due to no CO₂ emissions. However, lean premixed hydrogen/air combustion can lead to safety challenges, particularly in the form of flame flashback, potentially causing catastrophic failures in combustion chambers. Understanding and controlling flashback is essential to ensure the safe and efficient use of hydrogen for instance in gas turbines. With this study, we address a number of open questions: (i) root cause of boundary layer flashback in turbulent premixed lean 100% hydrogen jet flames. (ii) transition from unconfined to confined boundary layer flashback. (iii) investigate key parameters that govern flame flashback: Lewis number and wall temperature. This study demonstrates for the first time that flashback in turbulent premixed lean hydrogen combustion is driven by the characteristic behavior of thermo-diffusive instabilities.

Mainstream partial nitritation and Anammox (PN/A) has been observed and studied in the step-feed activated sludge process at the Changi water reclamation plant (WRP), which is the largest WRP (800,000 m³/d) in Singapore. This paper presents the study results for enhanced biological phosphorus removal (EBPR) co-existing with PN/A in the activated sludge process. Both the in-situ EBPR efficiency and ex-situ activities of phosphorus release and uptake were high. The phosphorus accumulating organisms were dominant, with little presence of glycogen accumulating organisms in the activated sludge. Chemical oxygen demand (COD) mass balance illustrated that the carbon usage for EBPR was the same as that for heterotrophic denitrification, owing to autotrophic PN/A conversions. This much lower carbon demand for nitrogen removal, compared to conventional biological nitrogen removal, made effective EBPR possible. This paper demonstrated for the first time the effective EBPR co-existence with PN/A in the mainstream in a large full-scale activated sludge process, and the feasibility to accommodate EBPR into the mainstream PN/A process. It also shows EBPR can work under warm climates.

A laboratory fed-batch reactor has been used to study under controlled conditions the performance of partial nitritation/anammox for the 200,000 m³/day step-feed activated sludge process at the Changi Water Reclamation Plant, Singapore. The similarity of the concentrations of NH₄, NO₂, NO₃, PO₄, suspended chemical oxygen demand (sCOD), pH, and alkalinity (ALK) between the on-site process and laboratory reactor illustrates that the laboratory fed-batch reactor can be used to simulate the site performance. The performance of the reactor fed by primary effluent illustrated the existence of anammox and heterotrophic denitrification and apparent excessive biological phosphorus removal as observed from the site. The performance of the reactor fed by final effluent proved the presence of anammox process on site. Both the laboratory reactor and on-site process showed that higher influent 5-day biochemical oxygen demand/total nitrogen (BOD₅/TN) (COD/TN) ratio increases the nitrogen removal efficiency of the process.