Turbulent premixed combustion: Flamelet structure and its effect on turbulent burning velocities

Recent advances in heat-recirculating combustion in industrial furnaces, particularly of the alternating flow type, are reviewed. A large amount of waste heat can be recovered by this type of system. Highly preheated combustion air, typically above 1300 K, is easily obtained due to advanced design and metarials employed. Although preheated air combustion generally produces high nitric oxide emissions, it has been used to generate high-temperature flames for some special applications. The energy saving achieved simultaneously by heat recirculation has become more attractive, from an ecological point of view. However, to enjoy the energy saving brought by a high rate of heat recirculation by applying highly preheated air combustion to generic industrial furnances, a reduction of nitric oxide emission is required. The possibility of low nitric oxide emission from highly preheated air combustion is intensively discussed. Dilution of the air with burned gases and combustion occurring in air with low oxygen concentration are shown to be indispensable factors in realizing low nitric oxide emissions. This has led to advanced furnance technology.

The science and technology of combustion in highly preheated air

Moderate and intense low oxygen dilution combustion is a newly implemented and developed concept to achieve high thermal efficiency and fuel savings while maintaining emission of pollutants at very low levels. It utilizes the concept of heat and exhaust gas recirculation to achieve combustion at a reduced temperature, a flat thermal field, and low turbulence fluctuations. An experimental burner is used in this study to simulate the heat and exhaust gas recirculation applied to a simple jet in a hot coflow. Temporally and spatially resolved measurements of reactive scalars are conducted on three different turbulent nonpremixed flames of a H2/CH4 fuel mixture at a fixed-jet Reynolds number, and different oxygen levels in the hot oxidant stream. The data were collected using the single-point Raman-Rayleigh-laser-induced fluorescence technique. The results show substantial variation in the flame structure and appearance with the decrease of the oxygen level. By reducing the oxygen level in the hot coflow, the flame becomes less luminous, the temperature increase in the reaction zone can get as low as 100 K, and the levels of CO and OH are substantially lowered. The levels of NO also decrease with decreasing the oxygen levels and at 3% by mass, it is less that 5 ppm. For this case, a widely distributed NO profile is found which is not consistent with profiles for other oxygen levels.

Structure of turbulent non-premixed jet flames in a diluted hot coflow

Analysis of non-adiabatic heat-recirculating combustors

The fundamental combustion and emissions properties of advanced biofuels are reviewed, and their impact on engine performance is discussed, in order to guide the selection of optimal conversion routes for obtaining desired fuel combustion properties. Advanced biofuels from second- and third-generation feedstocks can result in significantly reduced life-cycle greenhouse-gas emissions, compared to traditional fossil fuels or first-generation biofuels from food-based feedstocks. These advanced biofuels include alcohols, biodiesel, or synthetic hydrocarbons obtained either from hydrotreatment of oxygenated biofuels or from Fischer–Tropsch synthesis. The engine performance and exhaust pollutant emissions of advanced biofuels are linked to their fundamental combustion properties, which can be modeled using combustion chemical-kinetic mechanisms and surrogate fuel blends. In general, first-generation or advanced biofuels perform well in existing combustion engines, either as blend additives with petro-fuels or as pure “drop-in” replacements. Generally, oxygenated biofuels produce lower intrinsic nitric-oxide and soot emissions than hydrocarbon fuels in fundamental experiments, but engine-test results can be complicated by multiple factors. In order to reduce engine emissions and improve fuel efficiency, several novel technologies, including engines and fuel cells, are being developed. The future fuel requirements for a selection of such novel power-generation technologies, along with their potential performance improvements over existing technologies, are discussed. The trend in the biofuels and transportation industries appears to be moving towards drop-in fuels that require little changes in vehicle or fueling infrastructure, but this comes at a cost of reduced life-cycle efficiencies for the overall alternative-fuel production and utilization system. In the future, fuel-flexible, high-efficiency, and ultra-low-emissions heat-engine and fuel-cell technologies promise to enable consumers to switch to the lowest-cost and cleanest fuel available in their market at any given time. This would also enable society as a whole to maximize its global level of transportation activity, while maintaining urban air quality, within an energy- and carbon-constrained world.

J. A. Wunning

Papers

Flameless oxidation to reduce thermal no-formation