title: Principles of combustion engineering for boilers authors: C. J. Lawn year: 1987 –-
Chapter 2 - The combustion of HFO
Main constituents: paraffins, naphthenes ($C_5-C_9$), and aromatics. Specially useful due to its high specific energy content and high density (easy to transport), easily atomized, and almost completely combustible.
Correlation index $CI$ against relative density $S$ and reciprocal of average boiling point is a good indicator of oil major components:
\[CI = 473.9S - 456.8 + \frac{48640}{T_B} \quad\begin{cases} CI > 50 & \text{Predominantly aromatic components}\\[6pt] 15 < CI < 50 & \text{Mainly naphthenes or a mixture of all types}\\[6pt] 15 > CI & \text{Predominantly paraffinic components} \end{cases}\]
Typical elemental composition (given in mass percentages):
| Element | wt% |
|---|---|
| Carbon | 83-87 |
| Hydrogen | 10-14 |
| Suphur | 0.05-6 |
| Nitrogen | 0.1-2 |
| Oxygen | 0.05-1.5 |
Viscosity is highly dependent on temperature and used to control the characteristics of atomization in burners; for class G (main industrial application) of BS 2869 the typical kinematic viscosity at 80 °C is limited to 85 cSt; its maximum water content is 1% and ash production 0f 0.25% (see Table II for details and other classes). Viscosity is known to deviate from Newtonian at near pour point and the boiling point of its more volatile components.
When decreasing the hydrogen to carbon ratio the specific energy is also decreased, and this follows an increase of specific density of fuels. According to BS 2869, the net calorific value can be estimated from the density of the oil $\rho_{l}\:[kg\cdotp{}l^ {-1}]$ at 15 °C:
\[\Delta{}H_{net}=\left(46.423-8.792\rho_{l}^{2}+3.170\rho_{l}\right)\left(1-x-y-s\right)+9.420s-2.449x\]
where $x$ is the mass fraction of water, $y$ the mass fraction of ash, and $s$ the mass fraction of Sulphur; the resulting value is provided in $MJ\cdotp{}kg^{-1}$. Deviations are reported to be less than 1% when compared to bomb calorimetry data. Regarding specific heat a quite old (Cragoe, 1929) relationship based on relative density $S$ is given in $J\cdotp{}kg^{-1}$ as:
\[c_{p} = \frac{1683 + 3.39T}{S}\]
Typical value of surface tension are generally around $24-38\times{}10^{-3}\:N\cdotp{}m^{-1}$ at 20 °C and present a typical slope of $-0.07\times{}10^{-3}\:N\cdotp{}m^{-1}K^{-1}$ from room temperature up to 120 °C. Notice that this may be highly affected by actual fuel composition, so estimations must be used with care.
Combustion characteristics in general: maximum droplet heating rate around $10^5\:{}K\cdotp{}s^{-1}$ and flame temperatures of $2120\:{}K$, with a fuel residence time $<2\:{}s$. Droplets are generally injected from fractions of micron to $500\:{}\mu{}m$. The mechanism of combustion generally happens as follows:
- Volatiles phase: occurs between 250-350 °C and is characterized by the formation of a volatiles shell around the droplets, giving an aspect of granularity. It evolves till the unset of boiling, which occurs quite sharply somewhere in range 300-400 °C. Boiling leads to distortion and swealing, favoring breakup; several ejections might occur at this stage. The competing mechanisms during this phase are distillation and pyrolysis; the fuel composition and heating rate will determine the prevailing one.
See figures 8-9 of reference for more details regarding droplet history.