In desalination processes, it is important to ensure that the pure water, which is the main product, is free from any acidic components. After the process of direct-contact evaporation of seawater, the formation of acids such as carbonic and nitric acid must be considered for the health of consumers. This paper investigates the possible formation of nitric acid in a direct-contact heat exchanger (DCHE), based on the chemical equilibrium composition of the combustion products when the fuel is propane or hydrogen. Association and dissociation reaction mechanisms of the main products are written for both fuels. A detailed comparison of the combustion products is given for these fuels. The results obtained from the analysis indicate that there is low risk of nitric acid formation as a product in the evaporator products when propane or hydrogen is used as fuel.