Oxygen Enriched Air Effects on Combustion, Emission, and Distributed Reaction

Abstract

A novel combustion technology which combines colorless distributed combustion (CDC) and oxygen enriched combustion (OEC) air is examined to achieve optimum benefits of both technologies and to foster novel technologies for cleaner environment. The influence of oxygen enriched air–methane flames under nonpremixed and premixed fuel-lean combustion conditions is examined with focus on emission of NO and CO, combustor exit temperature (Texit), and distribution of reaction zone in the combustor using OH* chemiluminescence intensity distribution. A cylindrical combustor was used at combustion intensity of 36 MW/m3·atm and heat load of 6.25 kW. Results are also reported with normal air (21% oxygen). Oxygen enrichment provided stable combustion operation at lower equivalence ratios than normal air and also reduced CO emission. Increase in oxygen concentration from 21% to 25% or 30% increased the NO and decreased CO emissions at all the equivalence ratios examined. Using 30% O2 enriched air in premixed case showed NO emissions of 11.4 ppm and 4.6 ppm at equivalence ratios of 0.5 and 0.4, respectively. Oxygen enrichment also reduced CO emission to 38 ppm at equivalence ratio of 0.5. Operating the combustor with normal air at these equivalence ratios resulted in unstable combustion. OH* chemiluminescence revealed increased intensity with the reaction zone to shift upstream at increased oxygen concentration. The exhaust temperature of the combustor increased with oxygen enrichment leading to lower CO concentration and increased combustion efficiency. The oxidizer injected at higher velocities moved the reaction zone to upstream location with simultaneous reduction of both NO and CO, specifically under nonpremixed combustion.

Nonpremixed Air/Oxygen Jet Burner to Improve Moderate or Intense Low-Oxygen Dilution Combustion Characteristics in Oxygen-Enriched Conditions

“…In common, increasing the oxidizer velocity is considered to overcome the high NO emission problem, 31 which requires high-performance blowers with more energy consumption. Nevertheless, there is a conflict of interest between NO emission reduction and energy saving.…”

“…The low NO x emission at an oxygen content of up to 35% is attributed to the notably low peak temperature due to the low wall temperature of the combustor (∼1153 K). In a cylindrical combustor, Said and Gupta examined the NO emission from methane MILD combustion using oxygen-enriched air at different oxygen contents and inlet velocities by an experimental investigation. They found that for the nonpremixed cases, the NO emission significantly increases from 13.2 to 160.3 ppm as the oxygen concentration is enriched from 21 to 30%, while it can be reduced by raising the oxidant velocity.…”

“…It is worth stressing that MILD combustion emits high NO x emissions when using high oxygen enrichment in air, suggesting that the low NO x emission characteristic of MILD combustion would not be maintained under such conditions when the original burner is still utilized. Although the NO x emission can be reduced by raising the oxidizer velocity, − it needs to be largely elevated to achieve a comparable NO x emission level, which requires high-performance blowers with more energy consumption. In other words, there is a conflict of interest between NO x emission reduction and energy saving.…”

Numerical Investigation of the Plasma-Assisted MILD Combustion of a CH4/H2 Fuel Blend Under Various Working Conditions

“…Achieving clean and efficient combustion remains as an important industrial priority and therefore continues to attract significant attention [1,2]. Moderate or intense low-oxygen dilution (MILD) [3][4][5] combustion is a well-demonstrated concept that provides preheating and dilution simultaneously and thus reduces the emissions significantly.…”

Fuel injection effects on distribution reaction in a high intensity combustor

“…Enhanced mixing between the hot gases and air dilutes the oxygen concentration to result in lower reaction rate. Distributed reaction avoids thin reaction zone and results in a uniform temperature distribution which mitigates thermal NO X formed from the Zeldovich mechanism (thermal NO X mechanism) [7][8][9][10][11][12][13].…”