Combustors smoke

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

Unlike the aircraft turbiae, the ground-based gas turbiae operates continuously at the same power setting, usually 60—80% of maximum power output except when starting. Air and fuel flow patterns are constant and the combustor can be tuned to minimise smoke formation and high metal temperatures. 

With torroidal air flow, combustors will operate without visible smoke when properly developed for a primary-zone equivalence ratio below 1.5. Visible smoke is an air-pollution problem. 

Balir, D.W., Smith, J.R., and Kenwortliy, N.J., Development of Low Smoke Emission Combustors for Large Aircraft Turbine Engines, AIAA Paper Number 69-493. 

The air flow was visualized by injecting smoke into the combustor settling chamber. Without air forcing, the naturally existing axial and helical vortices are weak and disorganized. Images of these natural vortical structures could not be clearly captured during this experiment. 

Other issues of importance to combustor performance include soot production, pressure loss, and mechanical lifetime of the material. Too much soot in the exhaust could indicate poor combustion efficiency and unwanted particulate (smoke) emissions. For the baseline case without any inserts in the combustor, a slightly sooting flame was found. When one or two porous layers were inserted into the flame, no soot residue was found in the porous foams. It was thought... 

COMBUSTOR INLET-AIR PRESSURE. Increased pressure accelerates smoke formation in both laboratory flames and combustors. Coke deposits are, in general, affected similarly. A leveling-off in deposit rate has been found once the pressure is increased to 2 to 3 atmospheres. This is attributed to increased rate of erosion with increased air density. Coke deposition would be expected to increase with pressure because smoke forms more readily at the higher pressures and because the evaporation of fuels is retarded. 

COMBUSTOR INLET-AIR TEMPERATURE. Inlet-air temperature has little or no effect on smoke formation. The influence of inlet-air temperature on coke deposition is a complex process depending on design, fuel used, and operating conditions. Different investigators have reported decreases, increases, maxima, and minima. The different basic processes resulting on coke deposition are important at different temperature levels. If the inlet-air temperature is above the temperature at which coke will bum, coke will not deposit on the hot metal surfaces. 

COMBUSTOR INLET-AIR VELOCITY. Inlet-air velocity effects on both smoke and coke deposition arc similar to inlet temperature effects—i.e., depending on operating and design variables, smoke and deposits may increase or decrease with increase in velocity. Velocity can increase coke deposition, for a given combustor design, by increasing recirculation of air currents that would cause more fuel impingement on walls... 

COMBUSTOR OVER-ALL FUEL-AIR RATIO. In general, coke and smoke both increase with increasing fuel-air ratio, although some investigations have shown that smoke can attain a peak point beyond which it decreases. However, the location of this peak value was variable and dependent on other factors. These fuel-air ratio effects can be attributed to more fuel wash on surfaces, richer local fuel-air ratios, and increased thermal cracking of the fuel. Increased burning and erosion might lower coke and smoke formation, however. 

Coke and smoke formation was found to increase up to an equilibrium level with increasing pressure, fuel-air ratio, and time of test. Variation in velocity and temperature produced conflicting results, with smoke and deposits increasing or decreasing, depending on operating conditions and design of the combustor. 

Synthetic liquid fuels derived from coal and shale will differ in some characteristics from conventional fuels derived from petroleum. For example, liquid synfuels are expected to contain significantly higher levels of aromatic hydrocarbons, especially for coal-derived fuels, and higher levels of bound nitrogen. These differences can affect the combustion system accepting such fuels in important ways. In continuous combustors, i.e. gas turbines, the increased aromatics content of coal-derived fuels is expected to promote the formation of soot which, in turn, will increase radiation to the combustor liner, raise liner temperature, and possibly result in shortened service life. Deposit formation and the emission of smoke are other potential effects which are cause for concern. Higher nitrogen levels in synfuels are expected to show up as increased emissions of N0X (NO+NO2) An earlier paper presented results of an experimental study on the effect of aromatics and combustor... 

Smoke emissions did depend on fuel properties and ranged between a SAE Smoke Number of 20 to 45 at baseload operation. Indication of increased smoke and liner heating with reduced fuel hydrogen content was observed, although the indicated trends were not as consistent as those for lean combustors. 

Smoke Dependence on Primary Combustor Equivalence Ratio for Shale Oil Residual... 

The staged combustor data have been normalized at a primary equivalence ratio of 1.55 for alltest fuels (Figure 16). The fractional increase in heat flux is generally consistent with the lean combustor temperature parameter data presented by Westinghouse ( 3). As with the correlation of the rich-lean smoke data, the heat flux parameter does not display a unique correlation to fuel hydrogen content. 

Measurements from synthetic fuel spray flames and laboratory droplet reactors indicated the extent to which fuel properties and combustion conditions influenced particulate yields. A series of seven fuels were tested in a 21 kW spray combustor for total particulate by gravimetric sampling and soot by Bacharach smoke number. Variations in total particulate were dominated by the tendency of the fuel to form ceno-spheres while smoke number correlated with the C H ratio of the fuel. The laboratory droplet studies were performed in a gas flame supported reaction environment. These results confirmed the correlation between soot yield and C H ratio. In addition, two distinct forms of disruptive droplet combustion were observed. 

The feasibility of the dispersion fuels concept for applica--tion to gas turbine power plants is evaluated from disper-sion fuels formulation studies, from the results of single droplet tests directed toward demonstration of the droplet shattering process, and from the results of initial burner tests of dispersion fuels. Results demonstrate the existence of the microexplosion phenomenon in single-droplet combustion experiments. Gas turbine combustor tests indicate that fuel emulsification may alter favorably the efficiency of a practical gas turbine combustor without adversely affecting the turbine inlet temperature profile or CO, and smoke emissions. 

Figure 11, Combustion efficiency and smoke emissions—pressure-atomizing nozzle, FT12/FT4 combustor, T ir = 530 K Redwood 650 oil P = iO atm Tf i = 50 K,...

Smoke emissions, also shown in Figure 11, followed a trend opposite to the combustion eflBciency, first decreasing to a minimum value and thereafter increasing. A net reduction in SAE smoke number (2) of 16% was measured for approximately 4% water addition. Thus, the data indicate that fuel emulsification can alter favorably the eflBciency of a practical gas turbine combustor without aflFecting adversely the turbine inlet temperature profile or the smoke emissions. It is speculated that part or all of this improvement may be attributed to improved atomization. 

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