NO Emission Behavior in Oxy-fuel Combustion Recirculated with Carbon Dioxide

Abstract

A numerical study is conducted to grasp the flame structure and NO emissions for a wide range of oxy-fuel combustion (covering from air-blown combustion to pure oxygen combustion) and various mole fractions of recirculated CO 2 in a CH 4 -O 2 /N 2 /CO 2 counterflow diffusion flame. Special concern is given to the difference of the flame structure and NO emissions between air-blown combustion and oxy-fuel combustion w/o recirculated CO 2 and is also focused on chemical effects of recirculated CO 2 . Air-blown combustion and oxy-fuel combustion without recirculated CO 2 are shown to be considerably different in the flame structure and NO emissions. Modified fuel oxidation reaction pathways in oxy-fuel combustion are provided in detail compared to those in air-blown combustion without recirculated CO 2 . The formation and destruction of NO through Fenimore and thermal mechanisms are also compared for air-blown combustion and oxy-fuel combustion without recirculated CO 2 , and the role of the recirculated CO 2 and its chemical effects are discussed. Importantly contributing reaction steps to the formation and destruction of NO are also estimated in oxy-fuel combustion in comparison to air-blown combustion.

Experimental and Kinetic Studies of Chemical Role of CO₂ in Hydrocarbon Oxidation during Fuel-Rich O₂/CO₂ Combustion

“…typically coal, a large part of the fuel conversion occurs in the gas phase, and hydrocarbon oxidation plays an important role in combustion process (1)(2)(3)(4) . CH 4 is not only a natural gas component but also one of the major volatile matter released from coal.…”

“…CH 4 is not only a natural gas component but also one of the major volatile matter released from coal. Therefore, CH 4 is often the focus of O 2 /CO 2 combustion studies.…”

“…CH 4 is not only a natural gas component but also one of the major volatile matter released from coal. Therefore, CH 4 is often the focus of O 2 /CO 2 combustion studies. Many studies have reported that CO 2 is not inert but participates in chemical reactions in its gas phase primarily through the reaction CO 2 + H = CO + OH (R1) (5)(6)(7)(8)(9)(10)(11)(12)(13) .…”

“…The effect of R1 on the combustion process has been extensively studied (5)(6)(7)(8)(9)(10)(11)(12)(13) , and some studies have shown that CO 2 inhibits the oxidation of CH 4 because of the competition between reactions R1 and H + O 2 = O + OH (R3), the main chain branching reaction in the combustion process in a dilute flow reactor (7,10) . In contrast, our previous paper shows the role of CO 2 was to advance CH 4 oxidation during fuel-rich O 2 /CO 2 combustion where the concentrations of reactants were high because R3 became insignificant under fuel-rich condition, and R1 produced an abundance of OH radicals that were more active than the H radicals in hydrocarbon reactions in the specific temperature range (12,13) .…”

“…Measurement of hydrocarbons is performed in a flat CH 4 flame under air and O 2 /CO 2 environments. CHEMKIN-PRO is used to investigate sensitivity coefficients for CH 4 and hydrocarbon reaction pathways. Figure 1 shows the schematic of the experimental setup.…”