(267d) Utilization of Reburning Reactions for NOx Control in Oxy-Fuel Combustion

Oxy-fuel combustion presents several new options for NOx control, but for optimization purposes most of these options require further exploration. The present work focuses on reburning of NOx in oxy-fuel combustion. It is concluded in several studies on oxy-fuel combustion that a large amount of the NOx in the recycled flue gas is reduced by reburning reactions in the flame. Typically, this reduction lowers the emission of NOx with around 70% per unit of fuel supplied compared to the emission during air-firing under otherwise similar condition. Although the reduction of NOx by reburning reactions has been proven in several studies, the utilization and optimization of this technique is poorly investigated within the framework of oxy-fuel combustion. In the present work combustion parameters, crucial for the reburning process, are investigated, with the aim of creating a process that fully utilizes reburning of recycled NOx as a primary measure for NOx control in oxy-fuel combustion. The combustion chemistry and reduction performance of reburning in oxy-fuel combustion is compared against the established reburning process during air-firing; important differences are highlighted and discussed.

The objective of reburning is to create hydrocarbon radicals (CHi) which can break up the NO formed and then convert the formed nitrogen-containing volatiles (mostly HCN and NH3) into N2. A reburning process consists of a reburn-zone and a burnout-zone. NO is supplied to the reburn-zone, which should be oxygen lean to promote the formation of radicals. In the burnout-zone, O2 is added in sufficient amount to complete the combustion. In the air-fired application of reburning, a primary combustion zone where most of the fuel is converted is the source of NO and a secondary fuel stream is supplied to create the reburn-zone. In oxy-fuel combustion NO is instead supplied by the recycle stream and the primary combustion acts as a reburn-zone.

The reburning chemistry is investigated with a detailed chemical kinetic mechanism, which consists of a reaction system for light hydrocarbons and the nitrogen chemistry active in reburning of NO. To achieve a similar basis for a comparison between air and oxy-fuel combustion, the replacement of N2 with CO2 is the sole change, and the oxygen concentration in the oxidizer is set to 21 vol-% in both cases. In practice, the oxygen concentration in oxy-fuel combustion will probably be higher and due to the higher heat capacity of CO2 a similar temperature profile to air combustion is achieved with around 30 vol-% of O2. The simulations carried out in this work are isothermal and to eliminate the influence of temperature, the same temperature is applied in the air and the oxy-fuel case. Furthermore, the reburning process is modeled as performed during oxy-fuel combustion, i.e. there is an initial concentration of NO present in the oxidant stream also during air-firing.

The oxidation of CO by OH radicals, according to Reaction 1, is central for the combustion process.

CO+OHCO2+H (1)

It has been concluded in previous studies that in some parts of the flame the elevated concentration of CO2 during oxy-fuel combustion could drive Reaction 1 in the reverse direction, which would cause higher concentrations of CO and at the same time decrease the concentration of H-atoms and other chain-carrying radicals. The H-atom is crucial for the combustion process and the modeling results show that the oxidation of the fuel is faster during air combustion than during oxy-fuel combustion. As a consequence, the early stage of the combustion processes, where the reburning reactions are active, is shorter during air-firing. During the second stage when CO starts to oxidize to CO2 (forward Reaction 1), the higher CO concentration of oxy-fuel combustion reduces the relative rate of radical consumption and, eventually, the concentration of radicals during oxy-fuel combustion exceeds that of air combustion. In the burnout zone, where more oxygen is added, the same pattern as in the reburn zone can be detected: Initially the concentration of radicals is higher during air-firing but the decrease is slower during oxy-fuel combustion. The nitrogen chemistry is highly dependent on the radical reactions and in the present work these observations are used to explain and optimize the performance of reburning during oxy-fuel combustion.