NO x Reduction by Air-Side Versus Fuel-Side Dilution in Hydrogen Diffusion Flame Combustors

Abstract

Lean-direct-injection (LDI) combustion is being considered at the National Energy Technology Laboratory as a means to attain low NOx emissions in a high-hydrogen gas turbine combustor. Integrated gasification combined cycle (IGCC) plant designs can create a high-hydrogen fuel using a water-gas shift reactor and subsequent CO2 separation. The IGCC’s air separation unit produces a volume of N2 roughly equivalent to the volume of H2 in the gasifier product stream, which can be used to help reduce peak flame temperatures and NOx in the diffusion flame combustor. Placement of this diluent in either the air or fuel streams is a matter of practical importance, and it has not been studied to date for LDI combustion. The current work discusses how diluent placement affects diffusion flame temperatures, residence times, and stability limits, and their resulting effects on NOx emissions. From a peak flame temperature perspective, greater NOx reduction should be attainable with fuel dilution rather than air or independent dilution in any diffusion flame combustor with excess combustion air, due to the complete utilization of the diluent as a heat sink at the flame front, although the importance of this mechanism is shown to diminish as flow conditions approach stoichiometric proportions. For simple LDI combustor designs, residence time scaling relationships yield a lower NOx production potential for fuel-side dilution due to its smaller flame size, whereas air dilution yields a larger air entrainment requirement and a subsequently larger flame, with longer residence times and higher thermal NOx generation. For more complex staged-air LDI combustor designs, the dilution of the primary combustion air at fuel-rich conditions can result in the full utilization of the diluent for reducing the peak flame temperature, while also controlling flame volume and residence time for NOx reduction purposes. However, differential diffusion of hydrogen out of a diluted hydrogen/nitrogen fuel jet can create regions of higher hydrogen content in the immediate vicinity of the fuel injection point than can be attained with the dilution of the air stream, leading to increased flame stability. By this mechanism, fuel-side dilution extends the operating envelope to areas with higher velocities in the experimental configurations tested, where faster mixing rates further reduce flame residence times and NOx emissions. Strategies for accurate computational modeling of LDI combustors’ stability characteristics are also discussed.

Experimental and Kinetic Evaluation of Pressurized Lean Premixed Hydrogen-Air Flame Stability With Carbon Dioxide and Steam Dilution

“…Fuel and/or oxidizer dilution can also be employed to control the reactivity and emissions formation of H 2 flames, as shown by Weiland and Strakey [12], who utilized excess N 2 for NO x control of a H 2 -air diffusion flame, and Tanneberger et al [13] who utilized steam dilution for stoichiometric operation of non-premixed H 2 -O 2 flames. However, both approaches imply air separation as part of the process, incurring an overall system efficiency penalty.…”

The Effect of Swirl Number and Oxidizer Composition on Combustion Characteristics of Non-Premixed Methane Flames

“…Gas turbine engines for power generation have used combustors operated with diffusion flames thanks to their reasonable performance and higher stability characteristics. , However, these kinds of combustors are no longer preferred to be used due to their high combustion pollutant emissions with unacceptable higher concentrations of NOx. − Power generation industries rely mainly on combined cycle steam power plants and gas turbine engines for energy production from gaseous fuels. Thanks to the new heat recovery technologies which utilize the waste heat to convert it into mechanical energy.…”