**4. Formation of chemical deposits**

Chemical changes in the composition of fuels upon storage are a significant source of contamination and can result in the formation of particulates, slimes and other types of deposits in the fuel. The deposits can clog filters at the fueling stations, or if they pass through the filters cause operational problems in the aircraft. The changes in composition can be the result of the variable composition of fuels, low temperature oxidation of fuel components or other possible reactions of various fuel components. The reaction products are frequently insoluble in the fuel, resulting in their separation as a solid of film. The different types of reactions will be discussed separately in the sections below.

#### **4.1. Low temperature oxidation of fuel components**

The oxidation chemistry of jet fuel is normally divided into two distinct regions, with different mechanisms. The auto oxidation mechanism becomes important at about 140°C and the pyrolytic mechanism becomes important at about 300°C. Neither of these regions would appear to be operating at the normal storage temperatures of fuels. Studies of these mechanisms however focused on the chemistry of the hydrocarbon components of the fuel. They indicated that aromatic and cyclic hydrocarbons were substantially more reactive than the paraffinic compounds in the fuels [9]. It is known that free radical autoxidation of hydrocarbons occurs readily at temperatures between 30 and 60°C. The process is thought to be a radical chain reaction initiated by peroxides which abstract a hydrogen atom forming a free radical [10]. It has been shown that a number of factors are involved, but it is often the termination reaction in the radical chain mechanism that leads to the formation of deposits. A startling observation was that the less stable fuels formed less deposit [11]. This observation is likely due to the less stable fuels reacting more quickly, but not through a mechanism that leads to the formation of solid deposits. A series of steps, after the formation of the radical; leads to the formation of oxygenated products, such as primary alcohols [12]. The alcohols further react to form more complex oxygenated products such as dihydrofuranones [13].

What is frequently not accounted for in studies of the autoxidation mechanism of fuels is the presence of trace polar components in the fuel and the presence of catalytic metal ions in the fuel. Polar components have been shown to be correlated to deposit formation, with phenols, indoles and carbazoles showing the largest effect [14]. Some common classes of polar compounds are shown in **Figure 3**. It is well known that nitrogen containing compounds are oxidized much more rapidly that typical hydrocarbons. The enhanced reactivity of the polar compounds then leads to a wide range of new reactions that eventually result in the formation of particulates and films.

**4. Formation of chemical deposits**

**Figure 2.** Reaction of a ketal and an orthoester with water.

198 Flight Physics - Models, Techniques and Technologies

be discussed separately in the sections below.

**4.1. Low temperature oxidation of fuel components**

Chemical changes in the composition of fuels upon storage are a significant source of contamination and can result in the formation of particulates, slimes and other types of deposits in the fuel. The deposits can clog filters at the fueling stations, or if they pass through the filters cause operational problems in the aircraft. The changes in composition can be the result of the variable composition of fuels, low temperature oxidation of fuel components or other possible reactions of various fuel components. The reaction products are frequently insoluble in the fuel, resulting in their separation as a solid of film. The different types of reactions will

The oxidation chemistry of jet fuel is normally divided into two distinct regions, with different mechanisms. The auto oxidation mechanism becomes important at about 140°C and the pyrolytic mechanism becomes important at about 300°C. Neither of these regions would appear to be operating at the normal storage temperatures of fuels. Studies of these mechanisms however focused on the chemistry of the hydrocarbon components of the fuel. They indicated that aromatic and cyclic hydrocarbons were substantially more reactive than the paraffinic compounds in the fuels [9]. It is known that free radical autoxidation of hydrocarbons occurs readily at temperatures between 30 and 60°C. The process is thought to be a radical chain reaction initiated by peroxides which abstract a hydrogen atom forming a free radical [10]. It has been shown that a number of factors are involved, but it is often the termination reaction The oxidative addition reactions of a number of nitrogen, oxygen and sulfur containing compounds have been studied at 130°C. Considering that these compounds react completely in a few hours at this temperature, the mechanism could be valid at lower temperatures over a longer period of time. The oxidative addition reaction of several nitrogen and oxygen containing polar compounds are shown in **Figure 4**. It has been shown that the rates of oxidative addition depend greatly on the hetero atom present with N > O >> S [15].

The observation that the oxidative addition reaction produces molecules of significantly higher molecular weight and that those molecules can react further suggests that the problems would eventually become insoluble in the jet fuels, resulting in the formation of solid particles or oligomeric films.

A fundamental understanding of the mechanism of deposit formation under both thermal autoxidation and typical storage conditions has been a long-term goal. One possible mechanism that leads to the formation of high molecular weight products involves the oxidation of hydrogen containing heteroatomic aromatic compounds to form electrophilic quinone-like species. The quinone-like species reacts with nucleophile present in the fuels to eventually form soluble macromolecular oxidatively reactive species (SMORS) [16]. The initial step of the SMORS mechanism is the reaction of phenol with a peroxy radical to abstract a hydrogen atom. A similar reaction would also be possible with anilines and thiophenols that may

**Figure 4.** Reactions of polar compounds in jet fuel. (a) Reaction of 2, 3-dehydroindole to form higher molecular weight products and (b) reaction of benzofuran to form higher molecular weight products.

also be present [17]. The importance of the phenol has been demonstrated by conversion of active hydrogen containing species to silyl ethers which resulted in a dramatic decrease in deposit formation [18]. The phenoxy radical rapidly reacts to form an electrophilic quinone which then reacts with a nucleophilic aromatic heterocyclic compounds, such as pyroles, indoles, and carbazoles (see **Figure 5**). Further reactions lead to the formation of SMORS which can then react further to form insoluble deposits [19]. The understanding of this mechanism has led to some possible stabilizers which may improve the stability of stored fuel [20].

#### **4.2. Reactions between fuels with differing composition**

The oxidative addition reactions of a number of nitrogen, oxygen and sulfur containing compounds have been studied at 130°C. Considering that these compounds react completely in a few hours at this temperature, the mechanism could be valid at lower temperatures over a longer period of time. The oxidative addition reaction of several nitrogen and oxygen containing polar compounds are shown in **Figure 4**. It has been shown that the rates of oxidative

The observation that the oxidative addition reaction produces molecules of significantly higher molecular weight and that those molecules can react further suggests that the problems would eventually become insoluble in the jet fuels, resulting in the formation of solid

A fundamental understanding of the mechanism of deposit formation under both thermal autoxidation and typical storage conditions has been a long-term goal. One possible mechanism that leads to the formation of high molecular weight products involves the oxidation of hydrogen containing heteroatomic aromatic compounds to form electrophilic quinone-like species. The quinone-like species reacts with nucleophile present in the fuels to eventually form soluble macromolecular oxidatively reactive species (SMORS) [16]. The initial step of the SMORS mechanism is the reaction of phenol with a peroxy radical to abstract a hydrogen atom. A similar reaction would also be possible with anilines and thiophenols that may

**Figure 4.** Reactions of polar compounds in jet fuel. (a) Reaction of 2, 3-dehydroindole to form higher molecular weight

products and (b) reaction of benzofuran to form higher molecular weight products.

addition depend greatly on the hetero atom present with N > O >> S [15].

particles or oligomeric films.

200 Flight Physics - Models, Techniques and Technologies

Jet fuels obtained from different sources, refined in different ways have been shown to have somewhat different compositions. The differences are quite apparent when the classes of polar compounds from several different jet fuel samples are compared as is shown in **Table 3**.

The heteroatom containing components of the fuels are in large part responsible for the low temperature reactivity of the fuel [21]. This can be particularly problematic when a fuel that is high in basic nitrogen containing groups is added to a storage tank containing a fuel rich in acids or phenols. The product of the acid-base reaction is reasonably expected to form an insoluble film in the tank.

**Figure 5.** Mechanism for the formation of SMORS, and further reaction to form insoluble species. The figure shows the reaction of indole, however other nucleophilic heterocyclic compounds also react.


**Table 3.** Comparison of the classes of polar compounds found in various jet fuels.
