**Liquid Fuel Ageing Processes in Long-term Storage Conditions**

Marlena Owczuk and Krzysztof Kołodziejczyk

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/59799

#### **1. Introduction**

As a result of the ever growing demand on liquid fuels and the ever more stringent qualitative criteria, to guarantee the high quality of the fuels during long-term storage is a priority. In consequence of various internal or external factors, processes occur in the fuels which lead to adverse physical or chemical changes and affect product's performance characteristics. The problem is particularly observable in first-generation bio-fuels: they have unstable composi‐ tion, which causes problems with the development of efficient methods to protect them from ageing processes.

Discussed in this chapter are the nature and identification of changes which occur in petrole‐ um-derived products during long-term storage. The physical and chemical properties and stability of the various hydrocarbons the petroleum products consist of are discussed, and factors which affect the rate of fuel degradation processes during storage are presented. Critical properties of petroleum products and biocomponents are selected, related to normative parameters, and measurement methods are proposed which enable the monitoring of changes in product quality.

#### **2. The quality of engine fuels**

Fuels are expected to be of high quality and to remain stable, even after prolonged storage. These aspects are governed by applicable laws, resulting from Directive 2003/17/EC of the European Parliament and of the Council of 3 March 2003 amending Directive 98/70/EC concerning the quality of gasoline and diesel fuel [1]. The EU member states have implemented fuel quality monitoring systems based on standard sampling and testing procedures. The

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principles of the organization and operation of a system of monitoring and control of the quality of fuels for use in vehicles for various applications have been set out. According to applicable laws, all fuels which are intended to be transported, stored or placed on the market must comply with qualitative requirements set out for the given type of fuel.

According to the quality standards that apply in Europe for gasoline (EN 228) and for diesel fuel (EN 590), the maximum permissible content of additives is 5% for bioethanol and 7% for FAME (*Fatty Acid Methyl Esters*).

#### **3. Properties of fuels**

Fuels are characterized by the following physical and chemical properties:


Moreover, if a fuel has a content of biocomponents, it is important to know how susceptible it is to oxidation. Although no universal method exists to analyze every type of fuel, the following values are determined as a rule:


Fuels have different compositions so the above parameters may or may not be a reliable indicator, therefore, it is necessary to continue research works on the subject.

Since it is necessary to maintain a high quality of products to be supplied to the end user, optimum storage conditions must be specified. Different raw materials and different processes of technology are used to produce biocomponents and this affects their physico-chemical properties, often to a quite large extent. In addition, since petroleum-based fuels are different in respect of structure and chemical composition, optimum storage conditions will suit the properties and requirements of every type of fuel/biofuel. The choice of qualitative criteria for fuels ought to stem from the necessity to determine those parameters only of which the values may change in the process of oxidation during long-term storage. The present authors believe that chemical stability, stability at low temperatures, and resistance to corrosion are the crucial properties. These criteria take into account factors which may change the quality of fuels, such as: structure, use at low temperatures, and storage conditions. The importance of such criteria may change, depending on the critical factor.

#### **3.1. Chemical stability**

principles of the organization and operation of a system of monitoring and control of the quality of fuels for use in vehicles for various applications have been set out. According to applicable laws, all fuels which are intended to be transported, stored or placed on the market

According to the quality standards that apply in Europe for gasoline (EN 228) and for diesel fuel (EN 590), the maximum permissible content of additives is 5% for bioethanol and 7% for

**•** evaporative power and ability to form a fuel blend, which characterize engine starting properties through the fuel's fractional composition, vapor pressure, and volatility index,

**•** octane number (RON – Research Octane Number, MON – Motor Octane Number for gasoline) and cetane number (CN for diesel fuel), which characterize an ability of combus‐

**•** corrosive action and sulfur content, which characterize the effect of the fuel on metal parts

**•** chemical stability (induction period, oxidation stability) which characterize the rate of oxidation and degradation of the fuel, formation of resinous compounds which affect the

**•** tendency to form deposits – residue after coking and incineration, oxide ash and sulfate ash; they are a measure of mineral and organic contamination – metal compounds which may

**•** content of water and mechanical impurities which affect quality (clarity, emulsion forming,

**•** density and viscosity, which characterize flow along the tubes, filters and openings of nozzles, degree of atomization, range of evaporation stream range, and lubrication of

**•** behavior at low temperatures (cloud point, cold filter plugging point, flow temperature),

**•** lubrication properties, which have an effect on the wear and tear of the fuel system/fuel

must comply with qualitative requirements set out for the given type of fuel.

Fuels are characterized by the following physical and chemical properties:

tion without knocking in normative conditions,

corrosion, microbial growth, filter plugging),

which characterize stability of fuels at low temperatures,

of tanks, transport conduits and engine components,

**•** resin content, which characterizes a tendency to form deposits,

precipitate from the fuel when used in high-temperature conditions,

**•** ignition and self-ignition temperature, which characterize ignition properties,

FAME (*Fatty Acid Methyl Esters*).

fuel's quality during storage,

injection system components,

pump and on emission of pollutants.

**3. Properties of fuels**

102 Storage Stability of Fuels

Chemical stability of a fuel is understood as resistance to oxidation processes and to all chemical reactions that may be initiated by external factors (especially by atmospheric oxygen). Adverse changes which may occur lead to autocatalytic oxidation, polymerization and condensation of compounds which contain unsaturated bonds (olefins) and compounds of sulfur, oxygen, and nitrogen. As a result of oxidation, high-molecular resins are formed along with gums, insoluble deposits, and acidic compounds which may attack metals. Thermal stability of a fuel is defined as resistance to complex degradation processes that occur at elevated temperatures.

A number of theories describe the mechanisms of oxidation of petroleum products. The first known theories concerned, first of all, the mechanism of action of oxygen. Lavoisier and Schönbein [2] claimed that only the active form of oxygen (identified with ozone) is able to cause oxidation of fuel. A similar theory was proposed by Claussius [2], who maintained that there exist oxygen atoms with negative and positive polarity, which combine with neutral molecules of oxygen, forming anti-ozone and ozone. Other researchers were of the opinion that oxygen contains a certain balanced quantity of negative and positive ions of oxygen which oxidize substances to various extents (Vant Hoff [2]). That theory was challenged by still others, who believed that there is no difference between oxygen atoms and that substances are oxidized when breaking the oxygen molecule (Hoppe, Seiler [2]). At present, those theories have purely historical value.

Generally, it is assumed that the process of self-oxidation of a fuel occurs as the result of a number of radical-chain reactions, which can be divided into initiation, propagation, and termination reactions. The initiation reaction, also called initiation of the oxidation reaction chain, begins as a molecule of oxygen from the air attacks the C-H bond of hydrocarbons, of which the mechanism is described in the reactions below (1, 2):

$$\rm RH + O\_2 \rightarrow HO\_2\bullet + R\bullet \tag{1}$$

$$\text{2RH} + \text{O}\_2 \rightarrow = 2\text{R} \cdot + \text{H}\_2\text{O}\_2\tag{2}$$

The initiation reaction produces alkyl radicals, which further react with oxygen to form hydroxyperoxides; they are aggressive oxidants.

The next step of the reactions, during which the chain growth and branching occur, includes propagation reactions which run according to the mechanism described below, in (3...7)

$$\text{R}\bullet \,+\,\text{O}\_2 \rightarrow \text{RO}\_2\bullet\tag{3}$$

$$\text{RO}\_2\text{\textbullet + RH} \rightarrow \text{ROOH} + \text{R}\bullet \tag{4}$$

$$\text{ROOH} \rightarrow \text{RO} \cdot + \text{ } \text{HO} \tag{5}$$

$$\text{RO} \bullet + \text{HR} \rightarrow \text{ROH} + \text{R} \bullet \tag{6}$$

$$\text{HO}\bullet + \text{HR} \rightarrow \text{H}\_2\text{O} + \text{R}\bullet \tag{7}$$

Decomposition of peroxides gives active alkoxyl and hydroxyl radicals which, in the next steps of the reaction, detach more hydrogen atoms from the hydrocarbons. These changes lead to the formation of aldehydes, ketones, acids or alcohols, having an adverse effect on the performance properties of petroleum-derived fuel [3].

The last step of self-oxidation of hydrocarbons is the reaction termination, or closing phase. Owing to recombination, free hydrocarbon radicals and peroxide radicals that have been formed are deactivated according to the reactions (8...10):

$$\mathbf{R}\mathbf{R}\mathbf{\bullet} \to \mathbf{R} - \mathbf{R} \tag{8}$$

$$\text{R} \bullet + \text{RCOO} \rightarrow \text{ROOR} \tag{9}$$

$$\text{2RCOO} \text{• } \rightarrow \text{ROOR} + \text{O}\_2 \tag{10}$$

In addition to peroxide theories, descriptions of hydroxylation theories are found in available literature as well.

According to Bach and Engler [2], oxygen only reacts in its excitation state. One bond between oxygen atoms becomes weaker, according to (11):

$$\bullet \text{ O=O} \rightarrow \bullet \text{O-O-} \tag{11}$$

Readily oxidized substances A are oxidized by this form of oxygen, forming peroxides, an initial product of self-oxidation of hydrocarbons (the peroxide theory). A molecule of atmos‐ pheric oxygen, when contacted with the fuel, will behave like an unsaturated compound and may react without being broken into atoms beforehand, according to the reaction (12,13).

$$\mathbf{A} + \mathbf{O}\_2 = \mathbf{A}\mathbf{O}\_2 \tag{12}$$

$$\mathbf{AO}\_2 + \mathbf{B} = \mathbf{AO} + \mathbf{BO} \tag{13}$$

where:

oxidize substances to various extents (Vant Hoff [2]). That theory was challenged by still others, who believed that there is no difference between oxygen atoms and that substances are oxidized when breaking the oxygen molecule (Hoppe, Seiler [2]). At present, those theories

Generally, it is assumed that the process of self-oxidation of a fuel occurs as the result of a number of radical-chain reactions, which can be divided into initiation, propagation, and termination reactions. The initiation reaction, also called initiation of the oxidation reaction chain, begins as a molecule of oxygen from the air attacks the C-H bond of hydrocarbons, of

The initiation reaction produces alkyl radicals, which further react with oxygen to form

The next step of the reactions, during which the chain growth and branching occur, includes propagation reactions which run according to the mechanism described below, in (3...7)

Decomposition of peroxides gives active alkoxyl and hydroxyl radicals which, in the next steps of the reaction, detach more hydrogen atoms from the hydrocarbons. These changes lead to the formation of aldehydes, ketones, acids or alcohols, having an adverse effect on the

The last step of self-oxidation of hydrocarbons is the reaction termination, or closing phase. Owing to recombination, free hydrocarbon radicals and peroxide radicals that have been

RH + O HO + R 2 2 ® g g (1)

<sup>2</sup> 2 2 2RH + O 2R + H O ®= g (2)

R O RO + ®2 2 g g (3)

RO RH ROOH + R <sup>2</sup> g g + ® (4)

ROOH RO HO ® + g g (5)

RO + HR ROH + R g g ® (6)

HO + HR H O + R ® <sup>2</sup> g g (7)

which the mechanism is described in the reactions below (1, 2):

hydroxyperoxides; they are aggressive oxidants.

performance properties of petroleum-derived fuel [3].

formed are deactivated according to the reactions (8...10):

have purely historical value.

104 Storage Stability of Fuels

A – readily oxidized substance,

B – not-readily oxidized substance,

AO2 – peroxide.

The resulting peroxides may give away part of their oxygen to other, less readily oxidized substances, which results in oxidation of the latter. The theory was confirmed by Czernożukow and Iwanow [4, 5]. Iwanow adapted the peroxide theory to the chain mechanism of oxidation of hydrocarbons. He maintained that the oxidation reaction is initiated by active hydrocarbon particles (oxygen attacks the C-H bond rather than the C-C bond which is weaker) and runs in accordance with the mechanism shown in Fig.1 below for the respective hydrocarbon groups.

A different theory, proposed by Siemionow [6], explains that oxygen attacks the C-H rather than the C-C bond, in the first place. Although C-C is weaker, it takes extra energy to break the C-C bond, because of the shielding effect of hydrogen atoms.

#### **Paraffin hydrocarbons**

**Naphthene hydrocarbons** 
