Computational Fluid Dynamics (CFD) Investigation of the Oxy

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CFD INVESTIGATION OF THE OXY-COMBUSTION CHARACTERISTICS OF DIESEL OIL, KEROSENE AND HEAVY OIL LIQUID FUELS IN A MODEL FURNACE Pervez Ahmed, Mohamed A. Habib, Rached Ben-Mansour, and Ahmed F. Ghoniem Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02794 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on February 1, 2016

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CFD INVESTIGATION OF THE OXY-COMBUSTION CHARACTERISTICS OF

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DIESEL OIL, KEROSENE AND HEAVY OIL LIQUID FUELS IN A MODEL

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FURNACE

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Pervez Ahmed*a, M. A. Habib a, Rached Ben-Mansour a and A. F. Ghoniem b a

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Faculty of Engineering, KFUPM, Dhahran 31261, Saudi Arabia

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KACST TIC # 32-753, KACST and Mechanical Engineering Department

b

Department of Mechanical Engineering, Massachusetts Institute of Technology, USA

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Abstract

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This study investigated the air- and oxy-combustion characteristics of liquid fuels (diesel oil,

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kerosene and heavy oil) using a computational fluid dynamics approach. Various key aspects of

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the combustion characteristics of these liquid fuels in a down-fired laboratory furnace are

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presented. The flow characteristics, flame structure, fuel evaporation and the formation of CO in

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turbulent non-premixed flames with different O2/CO2 fractions are discussed in detail. The

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results of oxy-combustion are also compared with air combustion. Three cases of oxy-

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combustion (i.e., OF21, OF30 and OF35 with 21%, 30% and 35% oxygen content (by volume),

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respectively) are considered. Evaporation rates were reduced when N2 in the air was replaced by

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CO2 in oxy-combustion; however, similar evaporation rates are obtained when the volume of O2

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in oxy-combustion was increased to 30%. Combustion temperature decreased when N2 in the air

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was replaced by CO2 in the oxy-combustion environments at the same mole fraction. However,

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when the O2/CO2 mole fraction was increased, the temperatures were similar to that of the air-

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combustion environments. Moreover, due to better evaporation of the fuel and the higher 1

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temperatures attained in oxy-combustion, the flame length decreased. By contrast, oxy-

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combustion yields high CO concentrations compared with the air-combustion environments. The

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CO concentrations decreased when oxygen content in the oxy-combustion cases was increased.

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In addition, among the three fuels considered, heavy oil predicted the highest CO concentrations,

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while diesel and kerosene were in a comparable range. Furthermore, soot concentrations are

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found to be lower in oxy-combustion compared to air-combustion environments.

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Keywords: lab furnace, liquid fuels, oxy-combustion, CFD

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Corresponding Author/Pervez Ahmed, Tel: +966 13 860 7869; Email: [email protected]

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1. Introduction

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Studies have concluded that global greenhouse gas emissions need to be greatly reduced and that

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one possible method of achieving this outcome is through carbon capture and storage (CCS)

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technology. CCS is believed to have the potential to mitigate global greenhouse gas emissions.

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Among the present CCS technologies, oxy-combustion is the most promising for various power

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and steam production plants [1, 2]. There are many advantages of oxy-combustion technology

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compared with conventional combustion technology including delivering high combustion

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efficiency, reducing flue gas volume, lowering or eliminating NOx and improving plant

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efficiency. Oxy-combustion leads to simultaneous near zero CO2 emissions and negligible NOx

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because N2 in the air is replaced by CO2. In addition, oxy-combustion technology can be

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retrofitted to existing steam and power plants without performing any major modifications to the

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plants themselves. Oxy-combustion technology application utilizes all types of fuels such as

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solids, liquids and gaseous fuels. Both experimental and numerical studies have been conducted 2


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on the use of gaseous fuels for oxy-combustion [3-16]. A lot of studies were conducted for oxy-

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fuel combustion modelling for improved performance in solid fuel cases as well [2, 17-29].

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However, limited work has been done on oxy-combustion of liquid fuels and therefore it is

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important to understand thoroughly the underlying processes in oxy-combustion of heavier liquid

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fuels before the technology is implemented on a large scale.

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Saario et al [30] simulated the combustion of heavy oil fuel in air in a laboratory furnace using

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K-ε and Reynolds stress models, and they found discrepancies for the gas species concentration

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near the burner tip. Barreiros et al [31] developed a mathematical model to predict the near

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burner region measurements of NOx and particulate emissions, and they found that rapid

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vaporization of liquid fuel droplets and long residence times in the internal recirculation zone of

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the burner tend to minimize both NOx and particulate emissions. Morris et al [32] conducted

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experiments in a down-fired laboratory combustor to determine the effects of the oxy-

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combustion of coal on soot and particle emissions, and they found that oxy-combustion produces

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less soot compared with air combustion, even under the same adiabatic temperatures. Those

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authors also demonstrated that most of the ultrafine particle emissions consisted of soot, which

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has a considerable effect on the black carbon pool of the atmosphere. Many researchers [33-35]

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have reviewed aspects of atomization and vaporization of liquid fuel droplets for spray

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combustion with an emphasis on combustion engines and gas-turbine combustor applications.

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Spray combustion provides instant vaporization and mixing of liquid fuels with an oxidizer that

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significantly affects the combustion rate. Spraying liquid fuels during the combustion process

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greatly affects ignition, heat release rate, exhaust emissions and pollutant formations. Watanabe

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et al [36] numerically studied the spray combustion of liquid kerosene oil including soot and NO 3


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models. They found that the temperature radiation model without soot was higher than that of the

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experimental data. However, including soot in the model reduced the differences considerably,

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as soot has significant effects on radiative heat transfer. Moreover, the mole fraction of NO in the

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exhaust-including-soot model was in agreement with the experimental data.

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Many [36-39] modeling efforts for liquid atomization and spray combustion have been attempted

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during the past few years, and satisfactory levels of agreement between experimental data and

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numerical simulations in flame structure and soot predictions have been successfully obtained.

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Fang et al [40] investigated the combustion characteristics of liquid n-heptane in a burner placed

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inside a stainless steel combustion chamber and analyzed the wall temperature distribution and

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combustion products under different equivalence ratios, gas velocities and combustion chamber

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materials. They found that the wall temperature decreased with an increase in the equivalence

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ratio, while the wall temperature gradient increased gradually. Saez et al [41] studied the

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combustion characteristics of liquid butane and diesel oil using a diesel oil burner, and they

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observed elongated conical flames during the combustion of liquid butane with higher radiation

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zones in the center and flame front locations. They also showed that the temperatures and NOx

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obtained through liquid butane combustion were much lower than those of the diesel oil flames.

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Combustion of propane using high velocity oxygen-fuel thermal spraying was investigated

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numerically by Kamnis and Gu [42]. They found that the flow in the combustion chamber is

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greatly affected by the turbulent fluctuations and that the global reaction mechanism of propane

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reasonably predicted the flame temperature and other parameters. Hosseini et al [43] compared

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exhaust gas emissions during the combustion of bio-diesel and gas oil, and they found that bio-

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diesel emits less CO and CO2 than gas oil does; however, the risk of NOx emissions is increased. 4


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Moreover, the authors concluded that with the increase of the equivalence ratio, the CO and CO2

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emissions increased.

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In their work on large-eddy simulations of an evaporating two-phase flow in a burner, Sanjose et

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al [44] demonstrated that simplified injection methods are appropriate for simulations of real-

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time combustion geometries. The extent of spray formation of fuels such as gasoline, n-pentane,

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n-butanol, ethanol and iso-octane at different temperatures and pressures were investigated by

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Aleiferis et al [45] using phase Doppler and laser diffraction techniques. Jager and Kohne [46]

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showed that developing burner systems for light fuel oil can reduce NOx emissions compared

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with conventional burners. They also modeled the phenomenon using CFX software for

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optimization using the RNG k-epsilon turbulence model. Cerea et al [47] investigated the

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combustion of liquid bio-fuels using a dual nozzle laboratory scale burner. Their investigations

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showed that dual nozzle burners can lower NOx and soot concentrations, indicating the possible

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use of a wide range of low-BTU liquid fuels. Liquid fuels such as heavy oil, kerosene and diesel

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oil are extensively used in industrial applications including in liquid oil fired combustion

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chambers, gas turbines, internal combustion engines and industrial furnaces. Modeling liquid

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fuel atomization and simultaneous combustion involves phenomena such as turbulence, heat

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transfer, mass transfer, droplet dynamics and phase changes that are strongly coupled, resulting

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in an intricate process. Therefore, a detailed understanding and analysis of atomization and

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combustion are critical to improve current combustion device efficiencies to meet future

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restrictions on pollutant emissions.

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Despite several studies presenting both numerical and experimental work on liquid fuels in oxy-

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combustion atmospheres, most are limited to plug flow reactors [48, 49]. Studies including

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detailed modeling of oxy-combustion of liquid fuels are very limited. Currently, computational

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fluid dynamics (CFD) models are extensively used as they provide the most appropriate

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representation of various thermo-physical and thermo-chemical processes such as flow

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distribution, flame front propagation, turbulent mixing, droplet atomization, reaction zones, and

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species distribution. The present study investigated the combustion characteristics of different

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liquid fuels in a down-fired laboratory furnace using a non-premixed, turbulent combustion

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model. This study focused on the validation of the models used. Characteristics such as flow

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distribution, temperature and flame structures, atomization, and CO emissions for the different

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fuels under oxy-combustion environments are investigated.

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2 Modeling

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2.1 Furnace Description:

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The combustion characteristics of different liquid fuels were examined using a down-fired

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laboratory furnace. A brief description of the geometry of the furnace is presented in Figure 1a.

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The furnace is a cylindrical chamber and is down-fired to facilitate soot particle removal

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produced during combustion. Owing to the symmetry of the furnace under study, it is modeled as

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a 2D axis-symmetric to reduce computational time and effort. The height of the chamber is 2.4 m

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with an internal diameter of 0.6 m. The upper half of the furnace is covered with a refractory

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lining that is 0.15 m thick, around which there is a blanket of 0.05 m thick ceramic. The lower

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half of the furnace is exposed to the atmosphere. To better illustrate the oxidizer and the fuel 6


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supply mechanism, a detailed view of the furnace near the inlet section and the burner gun are

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presented in Figure 1b. Figure 1b also shows the arrangement at the top wall of the furnace. It

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consists of a burner gun and a secondary air supply arrangement. The secondary air is supplied,

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as shown in Figure 1b, through a conventional double concentric configuration and a burner gun

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is used to introduce the liquid fuel into the furnace together with air-assisted atomizer supported

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with a water-cooled pipe. Further details of the furnace can be found in Saario et al [30].

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2.3 Modeling:

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Conservation of mass, momentum and energy equations are solved for each phase including

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equations for turbulent kinetic energy, turbulent energy dissipation, and species transport. The

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steady state equations for conservation of mass, momentum, energy and species transport

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equations are listed below.

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∂ ρUj =0 ∂x j

(1)

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∂ ∂ ∂2 (ρUiU j ) =− p +µ 2 Uj ∂x j ∂x i ∂x j

(2)

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(ρCp )f Ui

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∂ ∂ ∂ (ρU jY i ) − ρDi ,m .Y i = 0 ∂x j ∂x i ∂x j

∂ ∂ ∂ T= kf T ∂x j ∂x j ∂x j

(3)

(4)

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Uj is the velocity vector, ρ is density of the fluid,

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Discrete Ordinates (DO) radiation model is used for the present simulations. DO radiation model

p is the pressure, µ is the dynamic viscosity.

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solves the radiative transfer equation (RTE) for a finite number of discrete solid angles. Each

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angle is associated with a vector direction

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radiative transfer equation (RTE) of DO model in the direction

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fixed in the global Cartesian system (x,y,z). The

4 σ r r r r r 2 σT ∇.( I ( r , s ) s + ( a + σ s ) I ( r , s )) = an + s 4π π

is 4π

r r'

r r'

∫ I (r , s ) j ( s .s )d Ω

(5)

0

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Turbulent combustion modeling involves a wide range of coupled phenomenon such as chemical

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kinetics, reaction mechanisms, two or three phase systems and radiative heat transfer, in addition

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to turbulence. The model is governed by Navier-Stokes equations, species and energy transport

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equations with incorporated Reynolds and Favre averaging [44].

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function (PDF) approach is recommended for better accuracy compared with the eddy

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dissipation model (EDM) [45].

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For injection of liquid fuels, a group-type atomizer model was used that accelerates the liquid

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fuel through the nozzle walls. The liquid comes out of the nozzle orifice as a thinning sheet that

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is unstable forming droplets. This type of atomizer is largely used in furnaces and gas turbines.

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The transition from liquid flow to injection spray can be divided broadly into three steps: film

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formation, sheet break-up and atomization. For a group-type atomizer injection model, the

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number of particle streams in the group and spray cone have to be set. For group injections, the

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properties ( φ ) such as position x-y, velocity, initial diameter, initial temperature and mass flow

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rate of the injected particles of the first, 1, and the last point, N, in the group are assigned within

The probability density

8


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a range of values from φ1 to φN . The ith injection in the group is assigned a value between the first

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and last values of φ , using the linear variation

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φi = φ1 +

φN − φ1 N −1

( i − 1)

(6)

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In equation (6) φ represents any property such as position, velocity, diameter, temperature and

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mass flow rate of the injected particle. The size distributions of the droplet particles were

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determined using the Rosin-Rammler diameter distribution method. This method is based on the

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assumption of an exponential relationship between droplet diameter d and the mass fraction of

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droplets greater than d given by: −

Yd = e − ( d / d )

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n

,

−

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where

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A non-premixed combustion model was used for the present study. To use a non-premixed

174

approach, the flow must be turbulent and the chemical kinetics must be rapid so that the flow is

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near chemical equilibrium. The non-premixed modeling approach involves the solution of

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transport equations for one or two conserved scalars (the mixture fractions). Equations for

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individual species are not solved. Instead, species concentrations are derived from the predicted

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mixture fraction fields. The thermo-chemistry calculations are preprocessed in pre-PDF and

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tabulated for look-up in FLUENT. Interaction of turbulence and chemistry is accounted for with

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a probability density function (PDF).The non-premixed modeling approach has been specifically

d

is the mean diameter of the droplet and n is the spread parameter.

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developed for the simulation of turbulent diffusion flames with fast chemistry. The non-premixed

182

model allows intermediate (radical) species prediction, dissociation and rigorous turbulence

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chemistry coupling. The method is computationally efficient, in that, it does not require the

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solution of a large number of species transport equations. The turbulence-chemistry interaction is

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calculated using the discrete phase combustion model that is based on the mixture fraction (f)

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and probability density function (PDF) approach. The mixture fraction f is defined as the mass

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fraction of the primary (fuel) stream:

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f =

sYfu − Yox + Yox,0

(7),

sYfu,1 + Yox,0

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Where Yfu and Yox are the mass fractions of fuel and oxidizer respectively while subscript0 and

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subscript 1are used to designate oxidant stream and fuel stream respectively. The probability

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density function, defined as the fraction of time spent by the fluid at state f, is given by equation

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(2) as follows:

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p ( f ) ∆ f = lim

T →∞

1 T

∑τ

(8),

i

i

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where T is the time scale and τi is the amount of time that f spends in the ∆f band. The Favre

195

(density weighted) average of any other scalar quantities, such as temperature and species, can be

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calculated by integrating the products of the scalar & PDF [46]. A PDF table for the desired

197

composition of fuel and oxidizer stream is generated by performing the chemistry calculations

198

and co-relating the gas variables with the mixture fraction.

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The Euler-Lagrange approach is utilized to solve the discrete phase model. The fluid phase is

200

treated as continuum, while the dispersed phase is solved by tracking the particles/droplets 10


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through the calculated flow field, where both the phases can exchange mass, momentum and

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energy. The trajectory of discrete particles is predicted by integrating the force balance on the

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particle in the Lagrangian reference frame.

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du p dt

= FD (u − u p ) +

gx (ρ p − ρ )

ρp

+ Fx

(9)

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In this equation, FD(u – up) is the drag force and Fx is the force arising due to the pressure

206

gradient along the fluid. The heat and mass transfer for the discrete phase are solved by

207

incorporating three laws. When the droplet temperature is less than the vaporization temperature,

208

the inert heating law is implied:

209

mpcp

210

where the heat transfer coefficient is calculated using the correlation of Ranz & Marshall [50].

211

h=

212

The droplet vaporization law is applied when the droplet temperature is above the vaporization

213

temperature, but below the boiling point.

214

Nv = κc ( Cv,D − Cv,∞ )

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The droplet boiling law is applied to predict the convective boiling of the droplet when the

216

temperature of the droplet has reached the boiling point.

217

−r

218

The effect of soot concentration on the radiation absorption coefficient is accounted for by

219

determining the absorption coefficient for soot. The overall absorption coefficient was defined as

dT p dt

= hA p (T∞ − T p )+ ε p A pσ (θ R4 − T p4 )

k∞  2 + 0.6 Re1/d 2 Pr1/ 3  dD 

(10),

(11).

(12).

dmD = hAD (T∞ − TD ) + ε D ADσ (TR4 − TD4 ) dt

(13).

11


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the sum of the absorption coefficients of the radiating gas and soot. The soot formation and

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consumption are governed by the following transport equation, which was solved for the soot

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mass fraction,

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→ µ ∂ (ρY s ) +∇.(ρν Y s ) = ∇.( t ∇Y s ) + Rs ,net ∂t σs

(14),

224

where Ys is the soot mass fraction, σ s is the turbulent Prandtl number for soot transport and Rs, net

225

is the net rate of soot generation [kg/m3s]. Rs, net is calculated by

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Rs ,net = Rs ,gen − Rs ,ox .

227

The rate of soot formation was based on a simple empirical rate [36],

228

Rs , gen = Cs Pf Φre −ERT

229

where Cs and r are constants, Pf is the fuel partial pressure, and Φ is the local fuel equivalence

230

ratio. The constants Cs and r were set to 1.5 (kg.m.s) and 3, respectively. The Discrete Ordinate

231

(DO) model is used to solve radiative heat transfer in the reactor, with 5 flow iterations for each

232

iteration of radiation. The absorption coefficient of the gas mixture is determined by a domain-

233

based weighted sum-of-gray-gas model.

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2.2 Inlet and Boundary Conditions:

235

The combustion modeling of liquid fuels in a down-fired laboratory furnace was performed

236

using the commercially available CFD package FLUENT. An oxidizer is introduced into the

237

furnace from the top using mass flow rate boundary conditions. The outlet of the furnace is given

238

the pressure outlet boundary condition. The top and side walls of the upper half section of the

239

furnace are covered with a 0.15 m thick solid refractory lining, which is covered with a 0.05 m

(15),

12


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thick solid ceramic fiber blanket. Conduction equations are solved in these solid zones. The top

241

layer of the ceramic fiber is held at a constant temperature of 300 K to simulate the effect of inlet

242

water through the water jacket above the ceramic fiber blanket. The lower half section of the

243

furnace is exposed to the atmosphere where surface convention equations are solved. Liquid fuel

244

is injected at the top of the furnace centered at the secondary air inlet using the discrete phase

245

model. The total flow rate of the fuel is injected equally at three different angles (4°, 8°, and 12°)

246

with the axis. Fuel is injected at a temperature of 373 K and the oxidizers (secondary and

247

primary airs) are introduced at 293 K. The total mass flow rate of the fuel for all the cases was

248

kept constant, at a rate of 0.33 kg/s at the inlet, throughout the study, with only the fraction of the

249

oxidizer changed. The equivalence ratio φ = 0.91 was used throughout this study. The volume

250

fractions of O2/CO2 were varied to obtain the desired percentages of O2 and CO2. Approximately

251

10% excess oxidizer was supplied for all cases. Four different cases of air and oxy-combustion

252

are considered in this study. They are designated as AF21, OF21, OF30, and OF35 for air fuel

253

with 21% oxygen and remaining nitrogen, oxy-fuel with 21% oxygen, oxy-fuel with 30%

254

oxygen, oxy-fuel with 35% oxygen and remaining CO2, respectively. The cases are summarized

255

in Table 1. The flow is turbulent; the chemical kinetics were considered to be infinitely fast, and

256

the flow considered was near equilibrium.

257

2.4 Numerical solution

258

A finite-volume based commercial CFD-code was used to solve the governing equations. The

259

time-averaged equations for the conservation of mass, momentum, fuel mixture fraction and its

260

variance, and enthalpy were solved together with transport equations for k and ε. In this study, a

261

standard k-epsilon model is used to solve the turbulence chemistry as it predicted better results 13


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with less computational effort. These partial differential equations are discretized and

263

approximated by algebraic equations for the finite number of volumes in the domain [46]. The

264

non-premixed PDF combustion model is used to solve the species transport with 25 numbers of

265

species. The species considered are Carbon (C), Atomic hydrogen (H), Atomic Nitrogen (N) ,

266

Atomic-Oxygen (O), Sulphur (S), Nitrogen (N2), Oxygen (O2), Heavy Oil (C19H30), Diesel(

267

C10H22), Kerosene (C12H23), Methane(CH4), Hydrogen (H2), acetylene(C2H2), benzene(C6H6),

268

Carbon monoxide (CO), Carbon dioxide (CO2), Water vapor (H2O), diacetylene (C4H2), Sulfur-

269

solid (S), Hydroxyl (OH), Carbonyl-sulfide (COS), Hydrogen sulfide (H2S), Carbon-

270

sulfide(CS), Butadienyl-radical

271

Propargyl-radical (H2CCCH). Radiation is solved, both in the gas and the solid phase, using the

272

DO radiation model with 0.8 internal emissivity conditions. A one step soot model was used in

273

this study and a discrete phase model was used to model liquid fuel vaporization. The fuel is

274

injected at different angles for the combustion to reach the furnace side walls in order to have

275

higher heat fluxes. A group-type injection was used with 10 particles streams in each injection.

276

Therefore, the total number of particle streams considered for the total fuel injection was 30. The

277

mean diameter of the particle size (MSD) used was 2.68e-05 m, and the spread parameter was

278

1.42. To verify that the results obtained were independent of the grid, a grid independency test

279

was conducted on three different grids with nodes 9526, 15889 and 22795. The grid with 9526

280

nodes was unable to capture the distribution accurately. Grids with 15889 and 22795 nodes show

281

results that are in close proximity. The difference between the results obtained using grid sizes

282

15889 and 22795 is less than 1%. Grids with more than 25000 nodes were also tested but

(C4H), Carbon-disulfide vapor (CS2), Ethylene (C2H4),

14


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283

resulted in no obvious advantage and increased the computational effort and time. Therefore, an

284

optimal grid of 22795 nodes has been used throughout the study.

285

5. Results and Discussion

286

The validation for the present combustion model was conducted against the experimental results

287

presented by Saario et al [30]. Figure 2 shows the molar distribution of O2 and CO2 at three

288

different radial positions (20 mm, 300 mm and 600 mm) from the entrance of the furnace. The

289

figure shows that the numerical results obtained using this model is in good agreement with the

290

experimental data. There are some discrepancies, but the trends are encouraging. The present

291

study investigated the combustion characteristics of three different liquid fuels (diesel, kerosene

292

and heavy oil) in air and O2/CO2 environments. The flame structure, flow and temperature

293

characteristics, fuel evaporation and the formation of CO in turbulent non-premixed flames with

294

different O2/CO2 fractions are discussed in detail. The investigation comprised of one air-fuel

295

case and three oxy-fuel cases with different oxygen volume fractions. The different oxygen

296

volume fractions in the oxy-fuel cases simulated different recycled flue gasses. The air case is

297

designated as AF21 and the oxy-combustion cases are represented as OF21, OF30 and OF35,

298

where the number denotes the oxygen concentration in the volumetric percentage. The

299

stoichiometric fuel-oxidizer (O2/N2 in air and O2/CO2 in oxy-fuel cases) ratio φ was 0.91.

300

Additional oxidizer (10%) was provided for all cases under study.

301

5.1 Flow characteristics

15


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302

The flow characteristics of air and oxy-combustion of liquid fuels were studied and the velocity

303

vectors for diesel oil are presented in Figure 3. Figure 3 shows the velocity vector contours for

304

both air and oxy-combustion of diesel oil. The vectors are presented on the same scale and the

305

color bar at the bottom indicates the velocity magnitude. It is important to note here that the mass

306

flow rates are kept constant for both cases shown in this figure. The results clearly indicate that

307

the velocities are reduced and flow is shifted when N2 in air combustion is replaced by CO2 in

308

oxy-combustion. This phenomenon may be attributed to the higher density of CO2 (1.7878

309

kg/m3) compared with N2 (1.138 kg/m3), resulting in lower volume flow rates. These results are

310

confirmed by the plot presented in Figure 4. Figure 4 compares the densities of air and oxy-

311

combustion of diesel oil along the axis of the furnace. This plot shows that replacing N2 by CO2

312

in oxy-combustion leads to an increase in the density of the gases. Consequently, the velocities

313

of the gases are reduced. As the secondary oxidizer flow was a swirl type, a reverse pressure

314

gradient is created between the primary (atomizing) oxidizer and secondary oxidizer inlets, near

315

the axis of the furnace close to the burner. Due to the reverse pressure gradient, the hot gases

316

recirculate and mix with the unburned fuel emerging from the burner in an inner recirculation

317

zone (IRZ). To attain high heat fluxes along the furnace walls, the secondary oxidizer is directed

318

into the furnace at an angle of 30° that causes the flow to deflect towards the furnace side wall

319

creating two recirculation zones; one is the primary recirculation zone downstream represented

320

by PRZ, and the other is the secondary recirculation zone represented by SRZ. The SRZ is

321

created on the side wall of the furnace, near the top, which is also known as the fuel-lean side,

322

and PRZ is created downstream along the axis of the furnace, known as the fuel-rich side. The

323

diffusion flame emerges from the IRZ and is sheared between the PRZ and SRZ. The flame 16


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Energy & Fuels

324

intensity and stability of the diffusion flame is increased towards the furnace wall between these

325

two PRZ and SRZs. The PRZ and SRZs force the hot gases to recirculate and mix with the

326

incoming fuel and secondary oxidizer, causing heat transfer to the incoming gases, thereby

327

stabilizing the flame. Downstream of the furnace, the velocities are reduced. The reactions in the

328

combustion process occur between the fuel-rich side and the oxidizer-rich stream.

329

5.2 Temperature characteristics

330

The temperature characteristics of three different fuels for air combustion and three cases of oxy-

331

combustion are presented in this section. The air and oxy-combustion characteristics are

332

compared, and the effects of variation of O2/CO2 in the oxy-combustion for the three different

333

cases are discussed. The temperature contours for three different fuels in air combustion (AF21)

334

are compared with their corresponding oxy-combustion (OF21) (Figure 5). Note that the

335

temperature contours for the same fuel are compared on the same scale and the color bar at the

336

bottom of each contour highlights the temperature variation. It was observed that, for every fuel,

337

combustion in air yields higher temperatures than in oxy-combustion for the same O2/diluent

338

fraction. The peak temperatures in the air-fuel combustion of diesel, kerosene and heavy oil were

339

1805 K, 1801 K and 1936 K, respectively, and were 1557 K, 1557 K and 1567 K for oxy-

340

combustion, respectively.

341

combustion due to the increase in the density of the gases. As explained earlier (Figure 4), oxy-

342

combustion indicates a higher density than in air combustion, due to the higher molecular weight

343

of CO2 (44) compared with N2 (28). Thus, for the same mass flow rate of the gases, the volume

344

flow rate is lower in oxy-combustion than in air combustion. These observations are verified by

A decrease in the volume of gases was observed in the oxy-

17


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Page 18 of 54

345

the flame sizes of air and oxy-combustion of liquid fuels presented in the temperature contours

346

of Figure 5. The flames are defined as the zone of maximum temperature. Oxy-combustion

347

exhibits weak flames that are pushed towards the furnace wall; therefore, a slower temperature

348

rise is observed in oxy-combustion than in air combustion. As well, the temperature contours in

349

the oxy-combustion cases exhibited lifted flames (i.e., weak non-premixed flames) near the

350

furnace tip. Moreover, a thicker flame in the air combustion case appears to begin near the

351

nozzle inlet, but this is delayed in oxy-combustion. The delay in combustion in the oxy-fuel

352

cases can be attributed to the higher specific heat capacity (ρCp) of CO2 (Figure 6). Additionally,

353

the increase of ρCp leads to more absorption of heat, resulting in a reduction of activation energy

354

and delayed ignition. These results are confirmed by the observations of Shaddix and Molina

355

[51], who found that flame temperatures were reduced significantly when N2 in air is replaced by

356

CO2 in oxy-combustion. Similar results were reported by Wall et al [52] in experimental and

357

theoretical investigations of oxy-fuel flames.

358

Further analysis of the lower temperatures in oxy-combustion reveals that slower fuel burning

359

rates may also contribute to the delay in the combustion process. This results in slower

360

temperature increase that drives the flame zone towards the furnace wall. These observations are

361

consistent with the trends in the temperature contours discussed earlier. Though the correlation

362

between the fuel fraction and the temperature delay was expected, it is not clear from these

363

results why the fuel burning rate is slower. To further illustrate the effect of CO2 in oxy-

364

combustion on the burning rates of the fuel, a comparison of turbulence intensities, for air and

365

oxy-combustion of diesel oil, along the axis as well as along the radial distance of the furnace are

366

calculated. Figure 7a presents axial distribution of turbulent intensities along the furnace. This 18


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367

figure indicates that the turbulence intensity in the oxy-combustion case (OF21) is lower than in

368

the air-combustion case (AF21) near the furnace inlet region; however, the difference is small

369

downstream moving along the furnace axis. Thus, the lower turbulence intensities in oxy-

370

combustion which are expected to lead to slower burning rates may lead to shifted flame towards

371

the furnace wall.

372

Figure 8 presents the temperature contours of the air combustion and oxy-combustion (for

373

increasing percentages of O2/CO2) cases for the three different types of liquid fuels. The contours

374

are presented on the same scale and the color bar at the bottom highlights the temperature

375

variation. The variations in the flame structure are apparent from these figures. As discussed

376

earlier, the combustion process is delayed when N2 in air (AF21) is replaced by CO2 in oxy-

377

combustion (OF21). These results are in agreement with the study of Kiga et al [53], which

378

investigated ignition characteristics of pulverized coal in a O2/CO2 atmosphere. They found that

379

the flame propagation speed in oxy-combustion is reduced when N2 is replaced by CO2, and this

380

was attributed to the higher specific heat capacity of CO2 compared to N2 [53, 54]. The overall

381

trends obtained in the present study are consistent with earlier studies on combustion in O2/CO2

382

atmospheres [53-55]. It is observed from these temperature contours (Figure 8) that in air

383

combustion (AF21) the flame starts near the tip of the nozzle. As explained earlier, this may be

384

attributed to the higher turbulent intensities in air combustion (AF21) than in oxy-combustion

385

(OF21). The lower turbulence intensity in oxy-combustion (OF21) may cause a delay in the

386

mixing of the fuel with the oxidizer, resulting in flame lift-off. However, the increase in O2

387

volume fraction (or decrease in CO2) results in better mixing and increases the combustion

388

efficiency. 19


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389

To attain similar characteristics of combustion in oxy-combustion, the volume fraction of O2 in

390

the O2/CO2 mixture must be higher, approximately 30% than that of air combustion. This

391

phenomenon explains why the OF30 case was considered. To further analyze the increase in the

392

volume of O2 in the O2/CO2 mixture, the case OF35 was also considered in this study. Figure 8

393

shows that the flame structure and temperature levels in air combustion (AF21) and oxy-

394

combustion (OF30) are nearly identical. This is due to the increased volume fractions of O2 in

395

OF30, which is similar to air combustion (AF21); therefore, the local temperatures are expected

396

to reach similar values. Further increase in the O2 volume fraction (or decreases in CO2) (i.e.,

397

OF35) results in a temperature rise of 100 K more than the OF30 case. This outcome is due to

398

improved mixing and burning rates. The turbulent intensities along the axis of the furnace with

399

the increasing O2 volume fraction for diesel oil combustion (Figure 7a) explain the better mixing

400

and the better burning rates near the furnace inlet region moving from OF21 to OF35, thereby

401

increasing the combustion efficiency. However, moving downstream, the turbulent intensities

402

are lower for the higher oxygen volume cases (OF30 and OF35), even though the difference is

403

small. As most of the combustion process occurs in a region closer to the furnace inlet, it would

404

be appropriate to examine the turbulent intensities near the furnace inlet. Therefore, the turbulent

405

intensities of diesel oil combustion at a radial distance of 320 mm from the furnace inlet with

406

increasing O2 volume fraction are presented in Figure 7b. The figure clearly indicates that with

407

an increase in the O2 volume fraction, higher turbulent intensities are attained, resulting in better

408

mixing and combustion efficiency.

409

To further analyze the impact of O2/CO2 atmospheres on temperatures and burning rates, the

410

specific thermal heat capacity (ρCp) of gases in air and oxy-combustion are presented (Figure 9). 20


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Energy & Fuels

411

Figure 9 shows the axial distribution of the specific thermal capacity (ρCp) of gases in air and

412

oxy-combustion cases for diesel oil combustion. The figure indicates that oxy-combustion

413

(OF21) results in a higher specific thermal capacity (ρCp) compared with air combustion. This

414

phenomenon explains the flame cooling effects and lower temperatures obtained in the oxy-

415

combustion OF21. Further increases in the O2 volume fraction decrease the thermal heat

416

capacity. These results indicate that oxy-combustion cannot be achieved by mere replacement of

417

N2 by CO2, but requires a reasonable amount of oxygen to attain characteristics similar to those

418

of air combustion. Simply replacing N2 with CO2 may result in flame instability and may

419

ultimately lead to flame extinction. Notably, the flames are shortened with a decrease in CO2

420

moving from OF21 to OF35. This outcome is due to the increased volume fraction of O2 in the

421

oxidizer mixture. Moreover, as the percentage of CO2 decreases (or increases in the O2 volume

422

fraction) in the O2/CO2 mixture, further increase in the maximum flame temperatures (shown in

423

Figure 10) are observed. It is also observed that among all the fuels considered for the study,

424

heavy oil combustion exhibits high adiabatic flame temperatures, while diesel and kerosene

425

combustion shows similar peak temperatures. It is not clear why the temperatures in the heavy

426

oil combustion are higher even though the net calorific value (CV) per kg of the fuel (refer Table

427

2) is less compared to that of kerosene and diesel. However, this may be attributed to the product

428

of CV and density of the fuels (i.e., net calorific value per unit volume (m3)), so that heavy oil

429

yields high calorific values per volume (960 * 42000 = 40320.000 MJ/m3) compared to kerosene

430

(780 * 43000 = 33540.000 MJ/m3) and diesel (730 * 43400 = 31682.000 MJ/m3).

431

5.3 Devolatilization/Evaporation

21


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432

Devolatilization/evaporation of liquid fuels is an important factor in combustion. The higher the

433

evaporation, the better the reaction rates. These liquids must be atomized and well-mixed with

434

the combustion oxidizer before burning. The atomization in the present furnace is aided by a

435

flow of high velocity oxidizer (air in air combustion and oxygen in oxy-combustion) and is

436

called air-assisted atomization. This type of atomization is typically employed for high viscosity

437

fuels in industrial applications. To illustrate the atomization of liquid fuels in air and oxy-

438

combustion environments, contours of particle diameters of diesel oil are presented in Figure 11.

439

Larger droplet diameters are observed in oxy-combustion (OF21) than in air combustion (AF21)

440

(Figure 11). Moreover, the droplet penetration length in oxy-combustion (OF21) is slightly more

441

than in air combustion (AF21). However, with the increase in the volume percentage of oxygen

442

(i.e., in OF30 and OF35), the droplet diameter and droplet penetration length decreases. These

443

results indicate that evaporation rates are affected when N2 in air is replaced by CO2 in oxy-

444

combustion. The delay in evaporation may be attributed to the lower burning rates and lower

445

temperatures obtained in oxy-combustion (discussed in earlier sections). With an increase in the

446

O2 volume fraction or a decrease in the CO2 percentage in the O2/CO2 mixture (i.e., in OF30 and

447

OF35), the evaporation rates increase. Higher burning rates and temperatures obtained in the

448

OF30 and OF35 cases also contribute to better evaporation of the liquid fuel.

449

The volume averaged devolatilization/evaporation rates of all the fuels under study are compared

450

under air and oxy combustion environments in Figure 12a. As discussed earlier, the oxy-

451

combustion environment decreases the evaporation rates of liquid fuels. Moreover, it was

452

observed that, among the fuels studied, the heavy oil fuel predicts the highest evaporation rates

453

under the same conditions. This phenomenon may be attributed to the lower latent heat of heavy 22


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Energy & Fuels

454

oil fuel compared to diesel and kerosene. To match similar volume fractions in oxy-combustion

455

to that of air, the evaporation rates of the OF30 case are compared with AF21 in Figure 12b. It is

456

apparent from this figure that oxy-combustion of liquid fuels with 30% O2 volume fraction yields

457

similar evaporation rates close to those of air combustion except in diesel oil. The flame lengths,

458

shown in Figure 8, are increased moving from diesel oxy-combustion to heavy oil fuel oxy-

459

combustion. These results can be confirmed from the droplet diameter contours presented in

460

Figure 13. The results from these contours illustrate that heavy oil fuel is characterized by higher

461

values of droplet sizes and droplet penetration lengths compared with kerosene and diesel. The

462

higher droplet penetration lengths, in the case of heavy oil fuel, can be attributed to a higher

463

viscosity (refer Table 2). In addition, the higher droplet surface tension (refer Table 2) of heavy

464

oil fuel leads to lower Weber numbers, leading to slower atomization. Similar results were

465

reported by Kyriakides et al [56] in their investigation of heavy oil fuel for marine diesel engine

466

applications. These predictions are also in agreement with the experimental results of Fink et al.

467

[57], who measured higher mean diameter values for heavy oil compared to pure diesel. The

468

increase in SMD values of heavy oil fuel was attributed to the increased surface tension and

469

viscosity, compared with diesel. The Weber number, which is the ratio of inertia force to surface

470

tension force, is less for heavy oil, due to a higher surface tension of heavy oil fuel droplets,

471

which explains the slower atomization in the heavy oil fuel combustion compared with kerosene

472

and diesel. This phenomenon may cause increased local soot formation rates. Moreover, the high

473

heat of evaporation adds to the slower evaporation of heavy oil.

474

The maximum evaporation rates of all the three fuels for increasing volume fractions of O2 for

475

oxy-combustion are presented in Figure 14. Among the three fuels, heavy oil possessed the 23


Energy & Fuels

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Page 24 of 54

476

highest evaporation rates. This outcome may be attributed to the lower latent heat of the heavy

477

oil fuel (presented in Table 2) compared with that of kerosene and diesel. As the latent heat of

478

diesel is higher than that of kerosene, it exhibits the lowest evaporation rates among the three

479

liquid fuels. It is also apparent from this figure that the evaporation rate of all fuels increased as

480

the O2 percentage increased from 21% to 30%. At a lower O2 percentage (i.e., 21% of O2 (by

481

volume)), the evaporation rate of diesel oil is similar to kerosene due to comparable latent heats.

482

The evaporation rates increased with the increase in the O2 volume fraction (or decrease in CO2)

483

for all the liquid fuels. The maximum temperatures obtained in heavy oil fuel combustion also

484

contributed to better evaporation rates. Moreover, it is observed that with an increase in oxygen

485

content to 35%, the evaporation rates increase, except in heavy oil where it remains almost

486

constant.

487

5.4 CO characteristics

488

The concentrations of CO formation during the air and different oxy-combustion cases in a

489

laboratory furnace, under a reasonably defined reacting atmosphere, are presented in this section.

490

CO is formed as intermediate species during the combustion process. It is believed to be formed

491

from the reaction shown in equation (16) [58, 59]. This reaction is significantly slower compared

492

with the CO formation from fuel, leading to an increased CO concentration in the flame.

493

Figure 15 shows the maximum CO concentration obtained during the combustion of the three

494

fuels. High adiabatic flame temperatures are favorable conditions for the production of CO. As a

495

result, higher CO concentrations are observed in heavy oil combustion (Figure 15a), followed by

496

kerosene and diesel oil. Stoichiometry also plays an important role in CO production under both 24


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Energy & Fuels

497

air and oxy-fuel conditions. As CO concentration is a function of temperature and stoichiometry,

498

the volume fraction of O2 in oxy-combustion must be higher to maintain similar adiabatic flame

499

temperatures in air combustion. Accordingly, the oxygen mass fraction in OF30 is similar to

500

AF21, and this explains the reason for comparing the results of CO characteristics for the air-

501

combustion case (corresponding to AF21) with the oxy-combustion case (corresponding to

502

OF30) presented in Figure 15b. The CO concentrations in the air-combustion case are lower

503

compared with oxy-combustion. These results agree with the results of M. Barbas [60], who

504

conducted experiments under air and oxy-fuel conditions. Combustion in the oxy-fuel

505

environment has a significant impact on CO concentrations, which can be attributed to the

506

participation of CO2 in the chemical reactions. It has been documented [58-60] that the following

507

reactions enhance CO formation:

508

CO2 + H CO+OH (16), and

509

OH+H2 H+ H2O (17)

510

These equations signify the participation of CO2 in the reactions to form CO. The forward

511

reaction represented by equation (16) dominates the CO production rate and due to high

512

availability of CO2 in oxy-fuel combustion, higher CO concentrations are observed. Moreover,

513

the reaction represented by equation (17) is enhanced by higher CO2 concentrations resulting in

514

low H2 and high H2O concentrations in the oxy-combustion of liquid fuels. These results can be

515

verified by the contours of molar concentrations of H2 and H2O for diesel oil combustion (Figure

516

16). The results are compared for air fuel AF21 and oxy-combustion OF30 to maintain similar

517

peak temperatures and stoichiometry. Chen and Ghoniem [58] showed that the backward 25


Energy & Fuels

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Page 26 of 54

518

reaction, represented by equation (17), dominates the production of H2 on the fuel-rich side of

519

the oxy-combustion flame, whereas H2 is oxidized on the fuel-lean side. Figure 17 illustrates

520

these results. Figure 17a compares the mole fraction of H2 for air fuel and oxy-combustion at a

521

radial distance of 320 mm from the inlet. It is clear from this figure that the mole fraction of H2

522

on the fuel-rich side is higher for oxy-combustion flames and lower on the fuel-lean side.

523

However, it is important to note that the overall molar concentration of H2 (Figure 16a) is lower

524

in oxy-combustion than in air combustion. This difference is due to the enhancement of the

525

forward reaction rate of equation (17). As discussed earlier, in oxy-combustion, owing to high

526

concentrations of CO2, the forward reaction rate represented by equation 10 is enhanced. This

527

phenomenon increases the production of CO and OH. The OH radical produced reacts with H2,

528

pushing the reaction (equation 17) forward to produce H2O (Figure 17b). The higher molar

529

concentrations of H2O in oxy-combustion OF30 (Figure 16b) confirms these results. By contrast,

530

lower H2 production leads to low molar concentrations of H2O in air combustion of liquid fuels.

531

The oxy-combustion of liquid fuels results in CO concentrations that are several times higher

532

than for combustion of these fuels in air. Among the fuels, heavy oil fuel oxy-combustion

533

resulted in the highest CO concentration, while kerosene and diesel oil were more or less similar.

534

However, all the CO concentrations in oxy-combustion were in a significant range and were

535

comparable with one another. The equilibrium results cannot identify the pathway for higher CO

536

concentration in heavy oil, but it may be attributed to the higher carbon content reducing CO2 to

537

CO through the reaction C + CO2 2CO. The maximum CO concentrations during the oxy-

538

combustion of all the three fuels for increasing volume percentage of O2 in oxidizer mixture are

539

presented in Figure 18. It is apparent from the figure that increasing the oxygen percentage in the 26


Page 27 of 54

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Energy & Fuels

540

O2/CO2 mixture decreases the CO concentration dramatically for all the three fuels. These CO

541

trends are in line with the results obtained in the work by M. Barbas [60]. The CO concentrations

542

are reduced by 8 times approximately when the oxygen content is increased from 21% to 30%.

543

The higher concentration of CO in the OF21 is due to the fuel-rich stoichiometry that is

544

favorable for CO production. As the oxygen content is increased, CO concentration decreases.

545

However, when the oxygen content is increased from 30% to 35%, a slight increment in the CO

546

concentrations is observed. This phenomenon can be attributed to the higher temperatures that

547

are obtained in oxy-combustion OF35.

548

The contours of molar concentrations of CO for air and oxy-combustion cases of the three

549

different fuels under study are presented in Figure 19. Note that the contours of air and oxy-

550

combustion cases for the same fuel are presented on the same scale to compare the differences

551

clearly. Similar effects, as presented in earlier figures of CO concentrations, are shown in these

552

contours of molar concentrations of CO. It can be observed from these CO molar concentration

553

contours that the CO emissions are predicted higher in oxy-combustion compared with air-

554

combustion atmospheres. These results are in agreement with the results obtained by Amato et al

555

[61]. Similar results were found during the experimental and numerical investigations of Glaborg

556

and Bentzen [48] and Heil et al [9]. When N2 in air (for AF21 case) is replaced with CO2 in oxy-

557

combustion (OF21), a dramatic increase in CO concentration is predicted. An intense red color,

558

corresponding to the highest values of CO concentrations, appears in the contours of the oxy-

559

combustion OF21 case for all fuels considered. The fuel-rich stoichiometry in oxy-combustion

560

(OF21) provides favorable conditions for CO formation. The participation of CO2 in the forward

561

reaction rate in oxy-combustion, as represented by equation (16), also contributes substantially to 27


Energy & Fuels

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Page 28 of 54

562

the higher CO concentrations on the fuel-rich side of the flames, showing that the chemical effect

563

of CO2 is more prominent under fuel-rich conditions. The higher CO emissions in the oxy-

564

combustion cases may also be due to the slow oxidation of the intermediate CO formed in the

565

reaction near the flame. Among the three fuels, heavy oil fuel had the highest CO concentrations,

566

followed by kerosene and diesel oils. It can be concluded from these results that an excess

567

amount of oxygen in the oxy-combustion of these liquid fuels will reduce CO emissions

568

significantly.

569

Soot concentrations of three fuels under study i.e. heavy oil, kerosene and diesel for both air and

570

oxy-combustion cases are presented in Figure . The results for the air-fuel AF21 and oxy-fuel

571

OF21 cases for all the fuels at 21% oxygen corresponding to air and oxy-combustion conditions,

572

show that oxy-combustion yields less soot compared to air-combustion for all three different

573

fuels. These results are consistent with the results reported by Morris et al [32]. In their work

574

they found that at low flue gas oxygen concentrations, oxy-combustion yields lower soot

575

concentrations than air combustion, however, no significant differences were observed between

576

the two oxy-combustion cases. The results obtained by samanta et al [62] also supports these

577

statements.

578

6. Conclusions

579

This 2D investigation of the effects of liquid fuels on oxy-combustion characteristics was

580

conducted in a down-fired laboratory furnace. The liquid fuels, heavy oil, kerosene and diesel

581

were used for this study and various key aspects of the combustion characteristics of these liquid

582

fuel oils are presented. The flow characteristics, flame structure, fuel evaporation and the 28


Page 29 of 54

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

583

formation of CO in turbulent non-premixed flames with different O2/CO2 fractions are discussed

584

in detail. The results of the oxy-combustion of the three different fuels are also compared with

585

their corresponding air-combustion cases. Evaporation rates were reduced when N2 in air was

586

replaced by CO2 in oxy-combustion; however, similar evaporation rates were obtained when the

587

volume percentage of O2 in oxy-combustion was increased to 30. Lower temperatures were

588

obtained in the oxy-combustion environments; however, with higher O2 contents the

589

temperatures reached close to those of the air-combustion environments. Moreover, due to better

590

evaporation of the fuel, with higher temperatures attained in oxy-combustion, the flame length

591

decreased. It was found that oxy-combustion of liquid fuels yields high CO concentrations

592

compared with air-combustion environments, and this is decreased when the oxygen content in

593

the oxy-combustion cases is increased. In addition, among the three fuels considered, heavy oil

594

predicted the highest CO concentrations, while diesel and kerosene were in a comparable range.

595

Furthermore soot concentrations are found to be lower in oxy-combustion compared to air-

596

combustion environments.

597

7. Acknowledgements

598

The authors wish to acknowledge the funding received from the King Abdul-Aziz City for

599

Science and Technology Unit at King Fahd University of Petroleum and Minerals (KFUPM)

600

(project no. 13-ENE1961-04). The support of KACST-TIC on CCS, under the project CCS_10,

601

is also appreciated.

602

8. References

29


Energy & Fuels

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761 762 763 764 765 766 767 768 769 770 33


Energy & Fuels

Figures

771

2.4

Ceramic Fibre

0.15 0.05

Refractory

0.3

Burner

gravity

Y

Axis of symmetry

X

(a)

150

50

772 773

Cooling water

50

300

Atomizing Fluid Fuel

60

28

125

Burner Gun

Axis

15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 54

774 775

(b)

g

Figure 776 1: (a) The cylindrical down-fired furnace (all dimensions are in meters) (b) Diagram of the burner 777

furnace and the burner gun (all dimensions are in mm)

778 34


Page 35 of 54

779 780 781 782 783 0.2 0.15

CO2 (dry volume %)

O2 (dry volume %)

X = 20mm 0.15

0.1

0.05

0

0

50

100

150

200

250

0.1

0.05

0

300

X = 20mm

0

50

Radial Distance (mm)

100

150

200

250

300


0.2 0.15

CO2 (dry volume %)

O2 (dry volume %)

X = 320mm 0.15

0.1

0.05

0

0

50

100

150

200

250

0.1

0.05

0

300

X = 320mm

0

50


100

150

200

250

300


0.2 0.15

0.15

0.1

0.05

0

784

CO2 (dry volume %)

X = 620mm O2 (dry volume %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0

50

100

150

200

250

300

X = 620mm

0.1

0.05

0

0

50


100

150

200

250

300


785

Figure 2: Comparison of experimental (dots) and numerical (lines) mole fractions of O2 and CO2

786

at three different radial positions in the furnace

35


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 54

SRZ PRZ

IRZ

(a) SRZ PRZ

IRZ

(b)

787 788

Figure 3: Contours of the velocity vectors for (a) air combustion, AF21 and (b) oxy-combustion,

789

OF21, for diesel oil

36


Page 37 of 54

2.5

AF21 OF21

2 3

Density (kg/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1.5

1

(a)

0.5

0

790 791 792

0.5

1

1.5

Axial Distance (m)

2

2.5

Figure 4: Comparison of the densities along the axial line of the furnace for air and oxycombustion of diesel oil

793 794 795 796 797

37


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 54

Air-Combustion

Oxy-Combustion

0.5

0.5

AF21

OF21

Diesel

Diesel

0.3

0.3

0.1

0.1 0 293

0.5 509

725

941

1 1157

1373

1.5 1589

0

1805

0.5

293

0.5 509

725

941

1 1157

1373

1589

1805

0.5

AF21

OF21

Kerosene

Kerosene

0.3

0.3

0.1

0.1 0 293

0.5 508

724

939

1 1155

1370

0

1.5 1586

1801

0.5

293

0.5 508

724

939

1 1155

1370

1.5 1586

1801

0.5

AF21

OF21

Heavy Oil

Heavy Oil

0.3

0.3

0.1

0.1 0

798

1.5

293

0.5 528

763

997

1 1232

1467

0

1.5 1702

1937

293

0.5 528

763

998

1 1232

1467

1.5 1702

1937

799

Figure 5: Contours of the temperatures (K) of three different liquid fuels in air (AF21) and oxy-

800

combustion (OF21)

38


3

x 10

AF21 OF21

2

1

0

801

3

Diesel

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Specific thernal capacity (J/m -K)

Page 39 of 54

0.5

1 1.5 2 Distance along axis (m)

2.5

802

Figure 6: Comparison of the ρCp for air and oxy-combustion of diesel along the axis of the

803

furnace

804 805 806 807 808 809

39


Energy & Fuels

Turbulence Intensity (%)

6

AF21 OF21 OF30 OF35

5 4 3 2

(a) 1 0

0

810 811 1.5 Turbulence Intensity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 54

0.5 1 1.5 2 Distance along the furnace axis (m)

AF21 OF21 OF30 OF35

1.3 1.0 0.8 0.5

(b)

0.3 0.0

812

2.5

0

0.1 0.2 Radial distance (m)

0.3

813

Figure 7: (a) Axial distribution of turbulent intensities (b) Radial distribution (at x=320mm)

814

turbulent intensities in air and oxy-fuel environments for diesel oil combustion

815

40


Page 41 of 54

1 2 3 0.5 4 5 0.3 6 7 8 0.1 9 10 11 12293 13 0.5 14 15 0.3 16 17 18 0.1 19 20 21 22293 0.5 23 24 25 0.3 26 27 28 0.1 29 30 31 32293 0.5 33 34 35 0.3 36 37 38 0.1 39 40 41 42293 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

AF-21

0

0.5 533

772

1012

1 1251

1491

0.5 533

772

1012

1730

1 1251

1491

0.5 533

772

1012

1 1251

1491

533 816 817 818 819 820

0.5 772

1012

a) Diesel

1491

0.1 0 293

1730

772

1012

1251

1491

1730

0

1970

293

0.1

0.1

293

0.5 533

772

1012

1 1251

1491

0.5

293

0.3

0.1

0.1

293

0.5 533

772

1012

1 1251

1491

0.5

0.1

0.1

293

0.5 533

772

1012

1 1251

1491

b) Kerosene

1970

772

1012

1251

1491

0.5

1.5 1730

533

772

1012

1 1251

1491

0 293

0.5

1970

1.5 1730

533

772

1012

1 1251

1491

c) Heavy fuel

Figure 8: Temperature contours for air and oxy-combustion of three different fuels

821 822 823 41


1970

1970

OF-35

1.5 1730

533

293

0.3

1730

1

0.5

0.3

0

1491

0.5

0

1970

OF-35

1251

1.5

OF-30

1.5 1730

1012

0.5

0.3

0

772

0

1970

OF-30

533

1

OF-21

1.5 1730

0.5

0.5

0.3

0

AF-21

1.5

0.3

1970

1970

533

1

OF-21

1.5 1730

0.5

0.5

1.5

1 1251

0.1

1970

OF-35

0

0.3

1.5 1730

0.5

0.3

1970

OF-30

0

AF-21

1.5

OF-21

0

0.5

1.5 1730

1970

x 10 2

AF21 OF21 OF30 OF35

1.5

1

0.5

0

824

Page 42 of 54

3

Diesel

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Specific thernal capacity (J/m -K)

Energy & Fuels

0.5 1 1.5 2 Distance along furnace axis (m)

2.5

825

Figure 9: Axial specific thermal capacities in air and oxy-fuel environments for diesel oil

826

combustion

827 828 829 830 831 832 833 834

42


Page 43 of 54

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

835 836

Figure 10: Maximum temperatures obtained for three different fuels with increasing percentage

837

of O2 (or decreasing percentage of CO2)

43


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 54

838 839

Figure 11: Contours of the particle diameters (m) for diesel oil fuel in air and oxy-combustion

840

44


Page 45 of 54

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(a)

841

(b)

842 843

Figure 12: (a) Comparison of the evaporation rates of three different fuels in air AF21 and oxy-

844

combustion OF21 environments (b) Comparison of the evaporation rates of three different fuels

845

in air AF21 and oxy-combustion OF30 environments

846

45


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 54

847 848

Figure 13: Comparison of the contours of particle diameters (m) of diesel, kerosene and heavy

849

oil fuel for oxy-combustion OF30

850 851

46


Page 47 of 54

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

852 853

Figure 14: Evaporation rates of three different fuels in oxy-combustion with increasing O2

854

percentages (or decreasing percentage of CO2)

855 856 857 858

47


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 54

(a)

859

(b)

860 861

Figure 15: (a) Comparison of the molar concentrations of CO for three different liquid fuels

862

under air (AF21) and oxy-combustion (OF30) environments (b) Comparison of the molar

863

concentrations of CO for three different liquid fuels under air (AF21) and oxy-combustion

864

(OF30) environments

865 48


Page 49 of 54

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

866 867

0.5

0.5

AF21

Mol Con of H 2 0.3

0.3

0.1

0.1 0

0.0E+00

8.1E-05

0.5 1.6E-04

2.4E-04

3.3E-04

1 4.1E-04

4.9E-04

AF21

Mol Con of H 2O

5.7E-04

0.5

0 0.0E+00

3.8E-04

0.5 7.5E-04

1.1E-03

1.5E-03

1 1.9E-03

2.3E-03

2.6E-03

0.5

OF30

Mol Con of H 2

OF30

Mol Con of H 2O

0.3

0.1

0.1 0

868

0.0E+00

8.1E-05

1 1.6E-04

2.4E-04

3.3E-04

4.1E-04

4.9E-04

5.7E-04

0

0.0E+00

3.8E-04

0.5

7.5E-04

1.1E-03

1.5E-03

1

1.9E-03

2.3E-03

2.6E-03

869

Figure 16: Comparison of the molar concentrations of (a) H2 and (b) H2O for air-combustion

870

(AF21) and oxy-combustion (OF30) of diesel oil fuel

871 872 873 874 875 876

49


Energy & Fuels

AF21 OF30

0.02

Diesel AF21 OF30

Fuel rich Zone

Mole Fraction of H 2O

FLAME

0.04

0.03

0.25

Diesel

Fuel rich Zone

Fuel Lean Zone

0.01

0.2

FLAME

0.05

Mole Fraction of H 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 54

0.15

Fuel Lean Zone

0.1 0

877

0

0.1

0.2

0.3

0

Radial distance (mm)

0.1

0.2

0.3

Radial distance (mm)

878

Figure 17: Comparison of the mole fractions of (a)H2 and (b)H2O for air-combustion (AF21) and

879

oxy-combustion (OF30) of diesel and kerosene oil fuel at a radial distance of 320 mm from the

880

inlet

881 882 883 884 885

50


Page 51 of 54

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

886 887

Figure 18: Molar concentrations of CO for three different fuels under oxy-combustion

888

environments with an increasing O2/CO2 fraction

51


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.5

Diesel

0.5

0.000 0.001 0.001 0.002 0.002 0.003 0.004 0.004

AF-21

0.3

0.000 0.000 0.001 0.001 0.001 0.001 0.002 0.002

0.25

0.5

0.75

1

Diesel OF-21

0.3

0.25

0.5

0.75

0.5

0.75

OF-30

0.3

0.25

0.5

0.75

0.000 0.001 0.001 0.002 0.002 0.003 0.004 0.004

OF-35

0.3

0.25

0.5

0.75

1

Kerosene OF-30

0.25

0.5

0.75

1

0.25

0.5

0.75

1

Heavy Oil 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008

OF-30

0.3

0.1

0.25

0.5

0.75

1

Kerosene

0 0.5

0.000 0.001 0.001 0.001 0.002 0.002 0.002 0.003

OF-35

0.25

0.5

0.75

1

Heavy Oil 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008

OF-35

0.3

0.1

0

1

OF-21

0 0.5

0.3

0.1

0.75

0.1

0 0.5

0.5

0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007

0.000 0.001 0.001 0.001 0.002 0.002 0.002 0.003

1

Diesel

0.25

Heavy Oil

0.3

0.1

0

0 0.5

0.3

0.1

AF-21

1

OF-21

0 0.5

0.000 0.001 0.001 0.002 0.002 0.003 0.004 0.004

889

0.25

Kerosene 0.000 0.001 0.002 0.002 0.003 0.003 0.004 0.005

1

Diesel

0.000 0.001 0.001 0.001 0.002 0.002 0.002 0.003 0.3

0.1 0

Heavy Oil

0.1

0.3

0.1

0.5

AF-21

0 0.5

0.000 0.001 0.001 0.002 0.002 0.003 0.004 0.004

0.5

0.5

0.1

0 0.5

Kerosene

0.3

0.1

Page 52 of 54

0.1

0

0.25

0.5

0.75

1

0

0.25

0.5

0.75

1

890

Figure 19: Contours of the molar concentrations of CO of three different fuels under air and oxy-

891

combustion environments

52


Page 53 of 54

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

892

893 894

Figure 20: Soot concentrations of liquid fuels under air (AF21) and oxy-combustion (OF21)

895

environments

896

897

898

899

900

901

902

903 53


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Tables

904 905

Page 54 of 54

Table 1: Cases considered for present investigation

Gas composition (Vol%) Case AF21 OF21 OF30 OF35

Media Air-fuel O2/CO2-fuel O2/CO2-fuel O2/CO2-fuel

O2

N2

CO2

21 21 30 35

79 -

79 70 65

906 907

Table 2: Fuels and their properties

Formula Density (kg/m3) Cp(j/kg-k) Thermal conductivity w/m-k Viscosity (kg/m-s) Latent heat (j/kg) Net Calorific value, CV (kJ/kg) Vaporization temperature (K) Boiling Point (K) Molecular weight (kg/kg-mol) Droplet surface tension (n/m)

Diesel

Kerosene

Heavy Oil

C10H22 730 2090 0.149 0.0024 277000 43400 341 447.1 142.2847 0.026

C12H23 780 2090 0.149 0.0024 226000 43000 341 477 167.31 0.026

C19H30 960 1880 0.12 0.048 124000 42000 400 589 258.19 0.031

908

54


Computational Fluid Dynamics (CFD) Investigation of the Oxy

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