It has been discussed that increasing the amount of water vapor for various combustion processes can improve performance in the form of operational performance and pollution reduction. This investigation explores the use of water spray injection in a waste-heat-recovery boiler (WHRB) to improve the thermal efficiency and limit Nitrogen Oxide (NOx) pollution. A WHRB operates by warming the intake air via a heat exchanger (HE-X) that extracts thermal energy held by flu gases exiting the system. This preheated air is then flows to the combustion chamber where it is mixed with the fuel and used to heat water at the main HE-X. After combustion, the flue gases pass through the recovery HE-X to transfer waste heat to the intake air, repeating the process. When this type of heat recovery method is used without water injection only the small amount of water vapor already present in the ambient air plus the water vapor generated from the combustion process is available to condense at the recovery HE-X. When combined with a water spray system incorporated into the air intake, the boiler system can take advantage of a phase change in both the exhaust gas and the intake air. To recover greater amounts of thermal energy from the recovery HE-X, water is sprayed into the air intake where the ambient air meets the recovery HE-X. The difficulty is the water injection mass rate requires precise control while also distributing the finest spray possible. This homogenized spray, both in the volumetric as well as the temporal domain, improves the vaporization rate of the injected water. This requires a spray with the greatest distribution of droplets as small as possible. To achieve this, increasing the in-nozzle turbulence of the injector is required. For the first time, this study introduces an injection strategy utilizing a high frequency that can achieve the necessary in-nozzle turbulent forces. In order to first evaluate the water spray method, the development of the water spray is visualized in the axial and radial planes by employing the Mie-scattering technique. Average droplet diameter is measured with the Laser Diffraction Method (LDM). In addition to evaluating changes to the spray characteristics, practical testing with an experimental boiler is performed to verify the effects the high-frequency injection strategy has on the boiler's performance. LDM revealed the mean droplet size decreased at higher injection frequencies. Concurrently, Mie-scattering showed the uniformity of the spray distribution increased in both the spatial and temporal dimensions at the higher injection frequencies. The boiler intake air's evaporation rate improved as a result of the increasingly homogenous spray distribution. This improved evaporation rate allowed greater amounts of water to enter the boiler's combustion process, saturating the flu gases. This saturated exhaust gas flows to the recovery HE-X where the water vapor's heat of condensation is transferred to the intake air. This increase in water content in the combustion process and in the flu gases decreased NOx emissions and allowed greater amounts of thermal energy to be captured and recycled, improving thermal efficiency. Because not all the water sprayed into the boiler system evaporated at the highest injection frequency, the system performance can still be improved by narrowing down the exact periodicity the boiler requires of the water injection system.