Soot control by fuel additives

Soot is the particulate matter resulting from incomplete combustion of hydrocarbon fuels. When present in sufficient particle size and quantity, soot in exhaust gases constitutes a black smoke. Soot formation is undesirable when it leads to environmental pollution or damages the performance of combustion equipment. Although soot is not the most abundant pollutant, it may be one of the most hazardous since soot particles are of the proper size (50-2000 A) to be ingested deep into the lungs. ~ The observation that soot can cause cancer of the skin in man was first made by Pott in 1775. 2 The polynuclear aromatic hydrocarbons (PAH) absorbed on soot are believed to be responsible for this effect, 3'4 as many PAH are known to be carcinogenic. 5 The prevention of soot emission is an important constraint in the design and operation of combustion systems. An important example that illustrates the type of considerations involved in the control of soot is combustion in gas turbines employed in aircraft engines. Turbine combustor performance is usually limited by the temperatures which the construction materials can withstand. 6 Increased emmissivity and higher radiative heat transfer caused by soot formation in the combustor can cause overheating and damage. Excessive quantities of soot particles can erode turbine blades and cause carbonaceous deposits leading to fuel-spray distortion. These problems would become even more serious with any trend toward more aromatic fuels. 7 Militarily, a fuel-rich primary combustion zone is desirable as it improves high altitude relight capability, s However, a sooty exhaust trailing jet aircraft is equally undesirable since it enhances detection by an adversary, s The U.S. Navy has also reported problems with carbon deposits left on aircraft carrier decks, 8 and in 1965 launched a program aimed at ridding their planes of smoke. 8'9 Soot formation in aircraft turbines first received attention by the U.S. Air Force during the Korean conflict. However, inquiries to pilots produced no complaints, and the problem was temporarily set aside, it The first commercial jet aircraft introduced in 1959 were marginal on power, and therefore used water injection during takeoff. This generated much more soot than dry operation, probably by prematurely quenching combustion reactions.t ~ Complaints about jet smoke from residents in areas around the Los Angeles International Airport provoked a study, 12 which showed that although the overall contribution to air pollution by aircraft in metropolitan areas was only 1 or 2% of the total, it was responsible for around 10% of the total in the immediate area of the airport. 12.13 Nevertheless, industry was slow to respond, probably since even a very smoky exhaust resulted in only a minor loss (less than 1⁄2%) in combustion efficiency, s'~° and because the health hazards of soot and PAH were not as well documented as they are today. 3'5 Finally, in 1968 Pratt and Whitney released information on a new combustion chamber for the JT8D engine which had been successful in reducing smoke emissions. 13 However, problems with soot formation still exist, especially with the new and more powerful turbines employing higher combustor pressures and heat release rates.11 Many investigations of jet engine emissions have reached the conclusion that combustor design modifications are the best method of assuring clean exhaust. 6"8'14-16 Design modifications are undoubtedly the most economical lont-term approach, but they may not be possible in all cases. Considerations such as poser and reliability at all operating altitudes sometimes make other solutions more desirable. The use of fuel additives is one such solution which has gained much attention. Additionally, the chemical properties of fuels are known to have a substantial effect on sooting tendency. In well-mixed systems burning with excess air, acetylene has the least tendency to form soot, followed by (in order of increasing tendency to form soot) alkenes, alkanes, and aromatics. 7 Thus, as petroleumbased fuels dwindle in supply and are supplemented by the more aromatic coal-derived liquids, more problems with soot formation can be expected. The staged combustion systems now being considered for NOx reduction also increase the tendency for sooting problems due to the lower-temperature, fuel-rich primary zone. Thus, even the best possible design modifications may still need help from fuel additives in some cases to keep soot formation within acceptable levels.