Effect of water injection on boiler performance

A 20kW commercial boiler has been modified to enable the injection of water into its combustion air, with the aim of reducing the emissions of carbon monoxide (CO) and oxides of nitrogen (NOx) and increasing heat transfer efficiency.

It was identified that water injection had been used for efficiency and emissions control in both gas turbine and internal combustion engines. NOx reductions were consistently achieved however CO reductions were application dependant. The lack of literature relating to water injection in boilers provided an opportunity for novel research.

An experimental setup was designed to investigate the effect of water injected into the combustion air on the heat-transfer efficiency of the boiler system, as well as its emissions of CO and NOx. The differences between liquid water and steam, injecting at points internal and external to the burner, and with or without the use of nozzles was also explored.

NOx and CO reductions of up to 40% and 93% were achieved with water injected inside the burner through a nozzle with no significant change in heat-transfer efficiency. The CO reduction effectiveness was found to be dependent on several factors. These included: the method and location of the injection, whether the fluid was vaporised, and the air-to-fuel equivalence ratio. The majority of experimental cases resulted in NOx reductions.