EVALUACIÓN DE LOS IMPACTOS AMBIENTALES IDENTIFICADOS

Combustion is the oldest technology for energy production. In fact today, most of the world’s energy requirements are directly or indirectly satisfied through combustion. That is why it is worthwhile to study this process in detail and develop a mathematical model for it. Fuel combustion is a combined process of many sub-processes. According to Ikegami et al.[23] the combustion process of heavy fuel oil is divided into two main phases;

1. Liquid droplet phase 2. Solid coke phase

The liquid droplet phase is complicated and it includes many heat and mass transfer processes and some chemical reactions. Solid coke phase is the heterogeneous oxidation of polymer residue. Liquid phase combustion can be further subdivided into four successive stages;

1) Pre-ignition heating 2) Evaporation

3) Thermal decomposition 4) Polymerisation

Williams [11] also divided the entire combustion course as summation of five different phases;

I. Heating up and vaporization of low boiling point fraction.

II. Self ignition with little thermal decomposition and continued vaporization of the droplet’s light components.

III. Disruptive boiling that can occur due to boiling of the low boiling point component within the droplet and swelling together with thermal decomposition.

IV. Formation of carbonaceous residue.

V. Heterogeneous combustion of carbonaceous residue.

Similarly, Chen & EI-Wakil [31] summarised the combustion history of burning the droplet in a flowchart which is shown in Figure 2-5.

Figure 2-5: The combustion history of a burning droplet (from Chen & EI-Wakil [31]).

• Viscous shell formation due to evaporation of lighter components from the surface and high viscous residue,

• Liquid pyrolysis (includes thermal decomposition and polymerisation) due to convective and radiative heating,

• And also the disruptive boiling.

2.3.1 Overview of Combustion Studies

In the late 1950’s, pioneering researchers of combustion modelling behaviour, Hottel et al.[32] and Godsave [33], made efforts to understand the combustion behaviour of fuel droplets. Hottel et al.[32] proposed three stages of the burning process of droplets of heavy oil. First, the pre heat stage, second evaporation and third the combustion of volatile matter. In 1960, Wood et al.[34] studied the heterogeneous combustion of multicomponent fuels. Their result showed that during combustion, the composition of multicomponent fuel changes by a simple process of batch distillation. In 1967, Michael & EI-Wakil [35] divided the burning process into liquid and residue phases. They further divided the liquid phase into an evaporation and thermal decomposition phase. Chen & EI-Wakil [31] and Shyu et al. [36] proposed a mathematical model for the evaporation and combustion of heavy fuel oil droplets, they treated fuel as a single component liquid with variable boiling points. Lightman & Street [37], and Marrone et al.[38] studied detailed combustion behaviour of various fuel components and their tendency to form coke residue. Urban and Dryer [39] had concerns about the coke oxidation process and they studied the structure of cenospheres. It has been recognised that the residual portion of the oil is the major source of the coke formation. Generally, the residual portion of heavy fuel oil is characterised by a high content of large, stable hydrocarbon molecules, including aromatics and asphaltenes. The contribution of asphaltenes to the formation of coke is still a controversial issue.

In 1993, Baert [14] divided the liquid phase (droplet) into four components and using the block type distribution to represent each of them, developed a simple evaporation and pyrolysis mathematical model of heavy fuel oil which predicts coke formation and evaluation of gases from the fuel. Baert [14] demonstrated that asphaltenes is the main source of coke formation. The present work has drawn having on Baert’s pyrolysis

chemical kinetics, but has taken the more rigorous approach to evaporation modelling as described in later chapters. In that simple evaporation model, Baert assumed that fuel components remain in the liquid phase until the droplet temperature reaches 90% of its boiling point (BP). This assumption is quite adequate for the representation of highly volatile component’s evaporation, if boiling points of the components are known. Baert [14] used a block type molecular weight distribution instead of an actual gamma type distribution of the molecular weight. A block type distribution requires four parameters to represent the fuel compositions in evaporation and pyrolysis modelling.

Takasaki et al.[1] studied the combustion characteristics of marine fuel oil. They used two different fuels BFO-S and BFO-A. Results of composition analysis of both fuels showed that BFO-A (poor fuel) contains 24% saturated hydrocarbon, which is lower than good fuel BFO-S (31%). The percentage of aromatic hydrocarbon is very high (67%) in BFO-A compared to BFO-S (47%). Both these fuels contain residue of more than 50% by weight and its properties have not been verified [1]. As mentioned in other literature, there is no uniformity in the composition of HFO since it depends on the crude oil source and also on the type of vacuum residue and cutter stock used [40]. Chromatographic analysis of cutter stock shows that in trouble free BFOs sharp peaks of n-paraffins are observed while in trouble making BFOs naphthalene peaks are found [1].

Recently, Goldsworthy [2] proposed a basic combustion and ignition model of heavy fuel oil. According to Goldsworthy [2], the cutter stock’s properties determine the ignition quality of the heavy fuel oil because it is the first component to evaporate. The combustion process of heavy fuel oil is very complex due to large range of the molecular weights and molecule types. Higher aromatic content can lead to late ignition and poor combustion of fuel [2]. The purpose of the addition of cutter stock in HFO is not only to reduce the viscosity for convenience in handling, but it also helps the residue to complete the combustion. The role of cutter stock is to evaporate and ignite first, and then it forms the flame surrounding the unevaporated residual portion. Though the residual portion is heavy, it can burn perfectly. Hence, the role of cutter stock is more important when the residue is heavy [1]. Formation of the carbonaceous residue (cenospheres) is a function of the aromatic content of the heavy fuel oil. It is a characteristic of the high asphaltene content of the fuel [41]. To some extent, Organo-sulfur and nitrogen compound are also

findings of Bomo et al.[42] that the formation of cenospheres are not correlated to the asphaltenes content but to the chemical structure of molecular units of asphaltenes. Similarly, Whitehead et al.[43] also ruled out any relationship between the fuel properties and corresponding particulate emission.