NOx emissions during coal or biomass combustion, or coal-biomass cofiring depend on details of fluid mechanics, reaction kinetics, heat release, and local stoichiometry. It is difficult to quantitatively estimate such emissions. We can generalize how NOx emissions from coal-biomass blends compare to the emissions from the individual fuels in the blend. To do so, we measure the NOx emissions from the two pure fuels. We then interpolate between these fuels to estimate the emissions from the blend. We compare this predicted or interpolated emission with an experimentally measured result at the same blend ratio. Such data can be plotted on a parity diagram (see below). To the extent that the data fall along the diagonal of the parity diagram, the fuels behave from the standpoint of NOx emissions as though they were combusted separately and the combustion effluents were mixed. If data consistently fall on one side or the other of the diagram, the fuels are interacting during NOx reactions.
Representative results are shown here and illustrate that the first-order effect of cofiring is that the fuels appear to not be interacting. A more detailed examination indicates a relatively minor but consistent interaction, with the data for switchgrass (diamond symbols) falling generally above the line and data for wood (circular symbols) falling generally below it. These trends are consistent with theoretical expectations.