Abstract
This paper describes the experiences with the combustion of gaseous fuel that contains as one of the components the nitrogen of low concentration. Such gaseous fuel with the bound nitrogen is mostly used in the primary steam reformer furnaces.
The main goal of the research was the investigation of the influence of nitrogen content in the gaseous fuel on the formation of NOx during the combustion process. Two types of low-NOx burners were tested. The first type of burner was the air-staged burner with the heat output of 0.5 MW and combusted the premixed mixture of fuel and combustion air. The second type of burner was the non-premixed fuel-staged burner with the heat output of 0.9 MW. The main difference between the used burners consists in different way of mixing of fuel with the oxidant (combustion air). As for the premixed burner, the fuel-oxidant mixture enters the combustion chamber premixed with the sub-stoichiometric content of oxygen; the left oxidant comes by the secondary air. As for the non-premixed fuel-staged burner, the all oxidant is mixed with the primary fuel at the outlet of the burner (the air excess is high) and the secondary fuel is combusted downstream in the second stage. The concentration of ammonia NH3 in the fuel was ranging between 0 - 1 % by volume. The oxygen concentration in the flue gas was maintained in the neighborhood of 3 % on dry basis in all tests. Natural gas was used as the reference “clean” fuel that does not contain the fuel-bound nitrogen.
The results showed that the presence of fuel-bound nitrogen, in this case in the form of ammonia, significantly affects the formation of nitrogen oxides. As for the combustion of pure natural gas, the measured concentration of nitrogen oxides in the flue gas at the outlet of the radiation chamber was 150mg/m3, respectively 100 mg/m3, when the premixed air-staged burner, respectively non-premixed fuel-staged burner, was used. However, when the fuel containing the ammonia was combusted, the concentration of nitrogen oxides increased several times.
Furthermore it was revealed that during the combustion of gaseous fuel containing the ammonia, ca. 25 % of chemically bound nitrogen in ammonia, respectively ca. 15 %, passed to the form of fuel NO when the premixed air-staged burner, respectively non-remixed fuel-staged burner, was used. Thus it can be concluded that the conversion of chemically fuel-bound nitrogen to the form of nitrogen oxides is also dependent on the combustion regime and type of burner.
