Eskom Holdings SOC Ltd is currently in the process of retrofitting 4 multi-unit power stations (8x200MWe units, 18 x 600MWe units) with Low NOx Burner technology in order to meet the requirements set out in the South African National Environmental Management, Air Quality Act of 2004. This paper outlines and shares the learning experiences of the complex engineering challenges faced during the design of the Matla Power Station combustion system, for the reduced emissions of Oxides of Nitrogen (NOx) to the atmosphere. The paper covers the role of modelling by Computational Fluid Dynamics (CFD) when designing modern combustion systems, while highlighting the importance of verification and validation of such models. The paper further details the inherent challenges faced when retrofitting older technology boilers with state of the art, modern firing systems. In solving these challenges, a multi-disciplinary international collaboration had revisited the fundamentals of NOx formation and emissions reduction.