NOX as a Function of Fuel Type: C1-to-C16 Hydrocarbons and Methanol

Abstract

The response of NOX to fuel type is determined for lean-prevaporized-premixed combustion in on atmospheric pressure jet-stirred reactor (JSR). Pure, straight chain, alkane fuels from C1 (methane) to C16 (hexadecane) and methanol are tested. Testing is conducted under three conditions, each of which is obtained using a particular inlet-jet nozzle for the JSR. For a selected condition, the fuels are burned under nearly identical macroscopic thermal and fluid mechanical fields in the JSR. A constant mixture inlet total temperature is used that provides for complete vaporization of all of the fuels tested without causing pre-flame reactions in the premixer. Two of the nozzles used have single, centered jets and provide a nominal combustion temperature of 1790 K. The third nozzle has eight diverging jets. In this case, the fuel-air equivalence ratio is increased, providing a nominal combustion temperature of 1850 K. Lowest levels of NOX are measured when methanol is burned. Upon switching to methane, the NOX concentration increases by 62±10%. For the alkane fuels, methane combustion yields the least amount of NOX. Upon switching from methane to ethane, the NOX increases by 22±2%. Further increases in the size of the alkane fuel molecule (from C2 to C16) show small increases (8±5%) in the NOX concentration. Although the JSR-condition selected affects the absolute NOX concentration, the trends in NOX concentration with respect to fuel-type are the same for the three conditions used. The JSR is modeled as a single perfectly stirred reactor (PSR) operating at the experimental residence time and combustion temperature. This modeling shows that the increase in the NOX measured for the alkane fuels may be explained in terms of the increase in O-atom concentration with increasing C/H ratio. In order to explore the effect of Fenimore prompt NOX, a two-PSRs-in-series model is used. The first PSR is assigned a residence time equal to 5% the total residence time of the reactor. The second PSR accounts for the remaining 95% of the reactor residence time. Because of its short lifetime, the CH radical is effectively restricted to the first PSR, and prompt NOX mainly forms in this zone. Mechanisms directly dependent on the O-atom form NOX throughout the reactor. The modeling suggests that prompt NOX is responsible for the greater concentration of NOX found in the methane combustion compared to the methanol combustion.