Effect of CO₂/N₂ Dilution on Characteristics of Liquid Fuel Combustion in Flameless Combustion Mode

Abstract

This paper presents the effect of CO 2 and N 2 dilution in the oxidizer stream on combustion and emission characteristics in a flameless combustor operating with kerosene and biodiesel as a fuel. The thermal input is varied from 30 kW (7.5 MW/m 3 ) -53 kW (13.3 MW/m 3 ) and inlet air temperature are varied from 300 K to 800 K. CO 2 /N 2 are added to dilute the oxidizer stream in a proportion of 15% of total air volume to reduce the O 2 concentration in the air stream. Reacting and nonreacting flow simulations show that the flow field improves significantly as both the flow velocity and temperature distributions become uniform. Distribution of the reactant dilution ratio becomes more uniform and a maximum value of R dil = 4.89 is achieved for 30 kW conditions. The reduced O 2 concentration in the oxidizer results in enhanced dilution of fresh air-fuel mixture thereby enables uniform temperature and flow field. For the air preheating case, CO 2 dilution does not affect the velocity and thermal fields significantly. The measured CO emissions are marginally increased due to CO 2 dilution; however, NO x emissions are significantly reduced. N 2 dilution results in increased levels of NO x , contrary to the perception of the inert role of N 2 addition even though the combustor operating temperatures are lower than 1800 K. The measurements of acoustic pressure emissions show a sharp decrease during flameless combustion mode.

Moderate and Intense Low-Oxygen Dilution (MILD) Combustion of Liquid Fuels: A Review

“…On the basis of the studies ,,− mentioned above, it can be concluded that the operating parameters, e.g., the air preheating temperature, the fuel type, the fuel–air premixing pattern, and the fuel–air equivalence ratio, all substantially influence the in-furnace liquid fuel MILD combustion characteristics. Several significant and relevant points can be drawn as follows: (1) The high temperature air preheating is not necessary for MILD combustion.…”

“…Apart from the fundamental research on the open JHC flames, the MILD combustion of firing liquid fuels has also been experimentally investigated in the closed furnaces for its establishment, emissions, and combustion characteristics. ,,− Table summaries the experimental conditions performed by several research groups ,,− during the years from 2004 to 2020. The previous studies mainly burned diesel, kerosene, and gasoline.…”

“…It is obvious that the stabilization and efficiency of the liquid fuel combustion depend greatly upon those processes. For the experiments summarized in Table , the liquid fuels were atomized by the commercial cone pressurized atomizer ,,− ,,,,, or vaporized ,, by preheating. The Sauter mean diameter ( d SMD ) of the fuel droplet is a key parameter to the evaporation.…”

“…The previous studies ,,− revealed that the stable MILD combustion of burning liquid fuels can be successfully established in different enclosed combustors, where very low emissions, high thermal intensity (even >100 MW/m 3 ), and high combustion efficiency can be realized simultaneously. Therefore, the MILD combustion has excellent potential to apply to practical devices, such as diesel engines, oil-fired industrial boilers, and gas turbine engines.…”

“…Most of the previous studies − on MILD combustion of liquid fuels were carried out with lab-scale experiments. Both pilot-scale and industrial-scale experiments and numerical simulations are lacking.…”