Cenosphere formation from heavy fuel oil: a numerical analysis accounting for the balance between porous shells and internal pressure

TL;DR: In this article, the critical diameter of the cenosphere is modelled based on the balance between two pressures developed in an HFO droplet, i.e., the pressure (Prpf) developed at the interfac...

Abstract: Heavy fuel oil (HFO) as a fuel in industrial and power generation plants ensures the availability of energy at economy. Coke and cenosphere emissions from HFO combustion need to be controlled by particulate control equipment such as electrostatic precipitators, and collection effectiveness is impacted by the properties of these particulates. The cenosphere formation is a function of HFO composition, which varies depending on the source of the HFO. Numerical modelling of the cenosphere formation mechanism presented in this paper is an economical method of characterising cenosphere formation potential for HFO in comparison to experimental analysis of individual HFO samples, leading to better control and collection. In the present work, a novel numerical model is developed for understanding the global cenosphere formation mechanism. The critical diameter of the cenosphere is modelled based on the balance between two pressures developed in an HFO droplet. First is the pressure (Prpf) developed at the interfac...

Summary

1. Introduction

• Heavy fuel oil (HFO) produces copious heat energy and is particularly useful in industrial settings and electric power stations [1].
• When all the volatiles have evaporated, the flame around the droplet runs out of fuel and dies out.
• The production of cenospheres has been observed in HFO combustion and has generally been ascribed to the presence of asphaltenes in the fuel [5].
• Asphaltenes can be schematically described as aromatic units linked by alkyl chains.
• A few numerical studies on cenosphere formation in HFO have previously been conducted [1, 8, 9].

2. Asphaltene reaction

• Several studies on the cracking of asphaltenes have been reported in the literature.
• Evaporation, product formation from asphaltene, and coke accumulation on the surface of the droplet are simultaneous processes.
• At every time step, the radial temperature distribution and the radius of the droplet are calculated.
• The reaction rate constants ( L) in the each annular zone (eK) is calculated according to the zone’s average temperature (3b, )).
• The initial Page 58 of 90 URL: http://mc.manuscriptcentral.com/tctm E-mail: ctm@tandf.co.uk Combustion Theory and Modelling For Peer Review O nly stage of droplet evaporation and thin coke layer formation during regression period are shown in Fig. 2a and b.

3. Mathematical Modeling

• With their multicomponent composition, HFO droplets possess individual physical and chemical properties.
• The temperature distribution in the radial direction is calculated based on the conduction equation [16-18], Eq. 7.
• Evaporation, product formation from asphaltene, and coke accumulation on the surface of the droplet are simultaneous processes.
• The entire process of volatile matter evaporation and shell Page 7 of 90 URL: http://mc.manuscriptcentral.com/tctm E-mail: ctm@tandf.co.uk Combustion Theory and Modelling For Peer Review O nly formation is analyzed in three phases: regression, shell formation and hardening, and after shell hardening is completed.
• A droplet is heated by convection from the surrounding hot gas.

3.1 Phase one: Regression Phase

• In the regression phase, the evaporation of the droplet follows Spalding’s evaporation equations for a single fuel drop [9, 18, 19], equations are shown in Eq. 8 to 11.
• Layer formation on the surface of the droplet due to coke accumulation is not considered until the total accumulated coke reaches a layer thickness of 10 µm.
• At every time step, the radial temperature distribution and the radius of the droplet are calculated.
• The reaction rate constants ( 6) in the each annular zone (Z5) is Page 8 of 90 URL: http://mc.manuscriptcentral.com/tctm E-mail: ctm@tandf.co.uk Combustion Theory and Modelling For Peer Review O nly calculated according to the zone’s average temperature (WS, ").
• The asphaltene reaction and rate of formation of its products in the each layer are calculated.

3.2 Phase two: Shell formation and hardening

• After the regression phase, the considerable amount of coke accumulated on the surface of the droplet forms a flexible and porous thin layer of coke.
• The evaporated fuel gases pass through the porous coke layer and develop pressure ( ) at the interface of droplet liquid surface and inner surface of the coke layer.
• The outer coke shell may expand due to the force due to .
• The is calculated by using the momentum equation in spherical porous medium (Eq. 14) [21, 22].
• The stages of pressure balance and when the shell reaches a critical diameter are shown in Fig. 2c.

3.3 Phase three: Flow through rigid shell

• After hard shell formation, Lee and Law [9] assumed that because the total volume of the droplet is fixed by the rigid shell, the continuous depletion of the liquid due to gasification must create a continuously expanding, vapor-saturated space at the core of slurry inside the shell.
• Moszkowicz et al. [1] also made the same assumption.
• Due to the spin or rotational motion of the droplet, liquid spreads on the inner surface of the shell and keeps the inner surface wet.
• It is assumed that inner surface of the shell is saturated with liquid that is flowing through porous Page 61 of 90 URL: http://mc.manuscriptcentral.com/tctm E-mail: ctm@tandf.co.uk Combustion Theory and Modelling For Peer Review O nly shell.
• Continuous evaporation of fuel takes place through this mechanism.

4.1 Radial temperature distibution and reaction rate constants

• The variations in the reaction constants (k1 to k6) at different temperatures are calculated according to Phillips et al.’s [10] kinetic parameters and are shown in Fig. 3a.
• The direct Page 12 of 90 URL: http://mc.manuscriptcentral.com/tctm E-mail: ctm@tandf.co.uk Combustion Theory and Modelling For Peer Review O nly products from the asphltene reaction are coke, gas and HFO.
• The middle oil formation rate constant is smaller, although it forms from HFO and the HFO concentration is high in the droplet.
• The radial temperature distibution in the droplet is shown in Fig. 3b.
• The kinetic reactions for asphaltene conversion take place only in the outer layers.

4.2 Pressure balance due to coke accumulation and flow through porous shell

• After a certain Page 63 of 90 URL: http://mc.manuscriptcentral.com/tctm E-mail: ctm@tandf.co.uk Combustion Theory and Modelling For Peer Review.
• The rate of fuel evaporation and the velocity of evaporated fuel passing through the porous shell influence the variation of .
• The variation of pressures at the interface of the droplet and coke shell is shown in Fig.
• In such droplets, the evaporation rate is also high and the mass flux of the evaporated fuel is less.
• It is considered that, when ≥ the shell starts to harden and the mechanical strength of the shell dominates rather than van der Walls force or the surface tension.

4.3 Variation in shell diameters

• Variations in shell and droplet dimensions are shown in Fig. 5a.
• When the shell starts to harden ( ≥ ), the internal motion (contraction and expansion) in the shell layer ceases and the outer diameter ( ) of the shell becomes fixed.
• Lee and Law [9] considered that the droplet’s diameter is equal to the shell’s inner diameter.
• The rate of decrease in shell inner Page 65 of 90 URL: http://mc.manuscriptcentral.com/tctm E-mail: ctm@tandf.co.uk Combustion Theory and Modelling For Peer Review O nly diameter increases for larger droplets.

4.4 Droplet evaporation, gas and coke formation

• The variations in droplet evaporation, gas and coke formation from the asphaltene reaction are shown in Figs. 6 and 7.
• The surface area of the droplet during the regression period and the inner surface area of the shell (after rigid shell formation) are high for larger droplets.
• The rate of fuel evaporation is proportional to the surface area of the droplet.
• The quantity of asphaltene at high temperatures is higher for larger droplets; hence, the rate of gas and coke formation from asphaltene is also higher for bigger droplets.
• The accumulation of coke during droplet evaporation is plotted in Fig. 7b.

4.5 Velocity of evaporated fuel through shell

• The evaporated fuel passes through the porous shell.
• The variations in the velocity of evaporated fuel through the shell for different sized droplets are shown in Fig.
• At any instant in time, the inner diameter of the shell is higher for larger droplets and vice versa.
• The total variation between the present computational model and the experimental results of Urban et al. [12] is in the range of 3 to 7%.

5. Validation

• The present numerical model is validated with the experimental results of Urban and Dryer [4] and Urban et al. [12].
• In that study [12], a range of droplets (100 to 700 µm) of different fuels are tested and the variations in particle diameter and droplet size are studied.
• The total variation between the present computational model and the experimental results of Urban et al. [12] is in the range of 3 to 7%.

6. Conclusions

• In the present work, a numerical model is developed to understand the mechanism of cenosphere formation from the evaporation/combustion of a heavy fuel oil (HFO) droplet.
• The reaction constant (k6) for gas formation from asphaltene is smaller than other constants (k1 to k5).

Full text

Cenosphere formation from heavy fuel oil: a
numerical analysis accounting for the balance
between porous shells and internal pressure
Item Type Article
Authors Vanteru, Mahendra Reddy; Rahman, Mustafa M.; Gandi, Appala;
Elbaz, Ayman M.; Schrecengost, Robert A.; Roberts, William L.
Citation Cenosphere formation from heavy fuel oil: a numerical analysis
accounting for the balance between porous shells and internal
pressure 2016, 20 (1):154 Combustion Theory and Modelling
Eprint version Post-print
DOI 10.1080/13647830.2015.1118556
Publisher Informa UK Limited
Journal Combustion Theory and Modelling
Rights This is an Accepted Manuscript of an article published
by Taylor & Francis in Combustion Theory and
Modelling on 18 Jan 2016, available online: http://
wwww.tandfonline.com/10.1080/13647830.2015.1118556.
Download date 10/08/2022 03:29:49
Link to Item http://hdl.handle.net/10754/596924

For Peer Review Only
Cenosphere formation
from heavy fuel oil: A numerical
analysis accounting for the balance between porous shells
and internal pressure
Journal:
Combustion Theory and Modelling
Manuscript ID
TCTM-2015-08-87.R1
Manuscript Type:
Original Manuscript
Date Submitted by the Author:
31-Oct-2015
Complete List of Authors:
Vanteru, Mahendra; KAUST, CCRC
Rahman, Mustafa; KAUST,
Gandi, Appala; KAUST,
Elbaz, A; KAUST,
Schrecengost, Robert
Roberts, William; KAUST, CCRC
Keywords:
Heavy fuel oil, Cenosphere, Numerical modeling, Pressure balance ,
Asphaltene
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Combustion Theory and Modelling

For Peer Review Only
Cenosphere formation from heavy fuel oil: A numerical analysis
accounting for the balance between porous shells and internal
pressure
V. Mahendra Reddy
1
, Mustafa M. Rahman
2
, Appala N. Gandi
3
, A. M. Elbaz
1,4
,
Robert A. Schrecengost
5
, William L. Roberts
1*
1
Clean Combustion Research Center, King Abdullah University of Science and Technology,
Thuwal, Saudi Arabia
2
Mechanical Engineering Program, PSE Division, King Abdullah University of Science and
Technology, Thuwal, Saudi Arabia
3
Computational Physics & Materials Science, PSE Division, King Abdullah University of
Science and Technology, Thuwal, Saudi Arabia
4
Mechanical power department, Faculty of Engineering Materia, Helwan University, Cairo,
Egypt
5
Alstom Power Inc. Windsor CT, USA
Abstract:
Heavy fuel oil (HFO) as a fuel in industrial and power generation plants ensures the availability
of energy at economy. Coke and cenosphere emissions from HFO combustion need to be
controlled by particulate control equipment such as electrostatic precipitators, and collection
effectiveness is impacted by the properties of these particulates. The cenosphere formation is a
function of HFO composition, which varies depending on the source of the HFO. Numerical
modeling of the cenosphere formation mechanism presented in this paper is an economical
method of characterizing cenosphere formation potential for HFO in comparison to experimental
analysis of individual HFO samples, leading to better control and collection.
1*
william.roberts@kaust.edu.sa; Phone: +966 12 808-4909
Page 1 of 90
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Combustion Theory and Modelling

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In the present work, a novel numerical model is developed for understanding the global
cenosphere formation mechanism. The critical diameter of the cenosphere is modeled based on
the balance between two pressures developed in an HFO droplet. First is the pressure (

)
developed at the interface of liquid surface and inner surface of the accumulated coke due to the
flow restriction of volatile components from the interior of the droplet. Second is the pressure
due to the outer shell strength (

) gained from van der Walls energy of the coke layers and
surface energy. In this present study it is considered that when 

≥ 

the outer shell starts
to harden. The internal motion in the shell layer ceases and the outer diameter (

) of the
shell is then fixed.
The entire process of cenosphere formation in this study is analyzed in three phases: regression,
shell formation and hardening, and post shell hardening. Variations in pressures during shell
formation is analyzed. Shell (cenosphere) dimensions are evaluated at the completion of droplet
evaporation. The rate of fuel evaporation, rate of coke formation and coke accumulation are
analyzed. The model predicts shell outer diameters of 650, 860 and 1040 µm, and inner
diamerers are 360, 410 and 430 µm respectively, for 700, 900 and 1100 µm HFO droplets. The
present numerical model is validated with experimental results available from the literature.
Total variation between computational and experimental results is in the range of 3 to 7 %.
1. Introduction
Heavy fuel oil (HFO) produces copious heat energy and is particularly useful in industrial
settings and electric power stations [1]. Soot formation of carbonaceous particulates from HFO
combustion results in ultrafine particles (~0.1 µm) and can cause adverse health effects in
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humans [2, 3]. Apart from health concerns, production of larger carbonaceous particulates
known as cenospheres (50-1000 µm) also results in increased fouling and corrosion of heat
transfer surfaces, which increases equipment maintenance costs and downtime. Formation of
cenospheres have been noted primarily in HFO combustion and generally this has been ascribed
to the presence of asphaltenes in such fuels [5, 6]. In general, emissions from HFO combustion
systems are gaseous pollutants such as CO
2
, CO, SO
2
, NOx, etc., and carbonaceous particulates
such as submicron soot and much larger cenospheres. Costly counter measures are required in
the combustion process or for installation of air quality control systems to meet emissions
standards [4].
When a droplet of HFO enters a combustor, heat causes vaporization of the volatiles, which
eventually ignite and burn. A rise in temperature leads to cracking (pyrolysis) reactions and local
formation of solids that accumulate on the surface of the droplet. The accumulated particles form
a film that is permeable to volatiles, allowing continuous evaporation of the inner liquid. When
all the volatiles have evaporated, the flame around the droplet runs out of fuel and dies out. The
so-formed remaining hard structure is called a cenosphere [1]. The production of cenospheres
has been observed in HFO combustion and has generally been ascribed to the presence of
asphaltenes in the fuel [5]. Asphaltenes can be schematically described as aromatic units linked
by alkyl chains. They are correspond to the part of the fuel which is soluble in benzene and
insoluble in heptane [7].
Asphaltenes are the fraction of the oil that produces the highest amount
of coke with activation energies of 94 to 135 kJ/mol [6]. When the as-formed cenosphere is
heated and subsequently ignites in an oxygen-poor atmosphere, it burns via a heterogeneous
surface reaction. The reaction rate is limited by the oxygen supply. Sharp falls in temperature
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