In this study, in terms of the mechanisms of nitric oxide (NO) emissions, research was carried out to consider the impact of physical and chemical properties of wood and herbaceous biomass processed into pellets and briquettes in the course of the combustion process (in individual phases) in a low-power heating device. Combustion tests in the grate heating device showed statistically significant differences in the combustion process and thus carbon monoxide (CO), nitric oxide (NO), and sulfur dioxide (SO2) emissions in the fuel form and the combustion phase. In terms of assessing the ecological and energy parameters of the combustion process, the nitrogen content in biomass was not the most important factor indicating the formation of NO emissions. Usually, the strongest correlations were observed with the formation of NO emissions in the first phase of combustion, which was related to the emissions of CO and SO2. In the second and third flame phases, a significant reduction in NO emissions was observed, which was poorly positively dependent on the nitrogen contained in the fuel. In addition, it has been shown that the fuel geometric features greatly influence NO content in the exhaust gases in the first combustion phase. It is also indicated that further research is required, considering the possibility of reducing volatile flue gas fractions, which will lead to the development of low-emission and highly efficient biofuel combustion technologies in low-power heating devices.