Acta Polytechnica


doi:10.14311/AP.2016.56.0312
Acta Polytechnica 56(4):312–318, 2016 © Czech Technical University in Prague, 2016

available online at http://ojs.cvut.cz/ojs/index.php/ap

SPECIFIC FEATURES OF THE OXYFUEL COMBUSTION
CONDITIONS IN A BUBBLING FLUIDIZED BED

Pavel Skopec∗, Jan Hrdlička

CTU in Prague, Faculty of Mechanical Engineering, Department of Energy Engineering, Technicka 4, 166 07
Prague, Czech Republic

∗ corresponding author: p.skopec@fs.cvut.cz

Abstract. Oxyfuel combustion is a promising approach for capturing CO2 from power plants. This
technology produces a flue gas with a high concentration of CO2. Our paper presents a verification
of the oxyfuel combustion conditions in a bubbling fluidized bed combustor. It presents a theoretical
analysis of oxyfuel combustion and makes a comparison with combustion using air. It is important to
establish a proper methodology for stoichiometric calculations and for computing the basic characteristic
fluidization properties. The methodology presented here has been developed for general purposes, and
can be applied to calculations for combustion with air and with oxygen-enriched air, and also for full
oxyfuel conditions. With this methodology, we can include any water vapour condensation during
recirculation of the flue gas when dry flue gas recirculation is used. The paper contains calculations for
a lignite coal, which is taken as a reference fuel for future research and for the experiments.

Keywords: oxyfuel combustion; bubbling fluidized bed; flue gas recirculation.

1. Introduction
Energy production from the combustion of fossil fuels
is accompanied by the formation of harmful emissions.
For several decades, there have been discussions about
climate change caused by the production of green-
house gases, and efforts are being made to reduce
their production as much as possible. CCS technolo-
gies (Carbon Capture and Storage, or Carbon Capture
and Sequestration) are one of the main approaches to
the reduction of carbon dioxide emissions from sta-
tionary energy sources. Several technological options
are under investigation, including oxyfuel combustion.
Oxyfuel combustion uses oxygen as an oxidant with a
high degree of flue gas recirculation. This increases
the CO2 concentration in the flue gas by eliminating
molecular nitrogen from the combustion air. It is
then easier to remove the remaining minor compo-
nents from the CO2-rich flue gas. The maximum CO2
concentration in the flue gas during air combustion
is not higher than 20%, but oxyfuel combustion can
increase this concentration up to 90% in the dry state.
Another benefit of oxyfuel combustion is that it re-
duces the emissions of other pollutants and increases
the combustion efficiency [1].

The basic design of oxyfuel combustion technology
has been known since 1982. The technology that
was developed at that time provided a rich CO2 gas
for enhanced oil recovery. During the 1990s, oxy-
fuel combustion was initiated as a CCS technology.
Current research focuses on pulverized coal combus-
tion, and there is also interest in circulated fluidized
bed combustion. Several laboratory and pilot exper-
imental facilities focusing on a circulating fluidized

bed and pulverized coal combustion have been set up
worldwide [1, 2]. The Schwarze Pumpe pilot power
plant in Germany [3] is a major project. It has power
output of about 30MWth(though it has not been in
operation recently). The Callide Power Station in Aus-
tralia [4] is the facility with the highest power output
until now, with an electric power output of 30MW.
Major facilities, with 30MWthpulverized coal and
30MWthcirculating fluidized bed boilers, are under
investigation by CUIDEN in Spain [5]. A 3MWthfull
chain system has been constructed in China, and a
35MWthunit is ready for commissioning. In addition,
some commercial-scale oxyfuel power plants are cur-
rently under preparation: a 200MWefacility, by the
Shenhua Group in China, and a 426MWefacility by
Whiterose in the UK [6]. Other important projects
are flexiburn facilities, which are able to operate under
oxy-combustion and air combustion mode. They have
been proposed by Foster Wheeler and by researchers
from the Czestochowa University of Technology [7].
A few investigations have been made in the field of
bubbling fluidized bed combustion (BFBC), which is
more suitable for lower power capacities. Only a few
published results are available on BFBC.

Our paper reports on a theoretical analysis of oxy-
fuel combustion, and contains a comparison with com-
bustion using air. A suitable methodology is proposed
for stoichiometric calculations and for computations
of basic characteristic fluidization properties. The
methodology presented here has been developed for
general purposes, and can be applied to calculations
for combustion using air, using oxygen-enriched air,
and also for full oxyfuel conditions.

312

http://dx.doi.org/10.14311/AP.2016.56.0312
http://ojs.cvut.cz/ojs/index.php/ap


vol. 56 no. 4/2016 Specific Features of Oxyfuel Combustion Conditions

ωO2 ωCO2 ωN2 ωAr ωH2O

Air 0.2062 0.0004 0.7685 0.0092 0.0157
Oxygen 1 1 0 0 0 0
Oxygen 2 0.95 0 0.05 0 0
Air+oxy 30 0.3 0.0003 0.6777 0.0081 0.0139
Air+oxy 50 0.5 0.0002 0.4841 0.0058 0.0099

Table 1. Volume concentration of the substances in different types of oxidant.

Figure 1. A comparison of the volume flow of all
streams of gases for combustion with air (A) and with
oxygen (B).

2. Characteristics of oxyfuel
combustion

Conventional combustion technology uses air as an
oxidant for combustion. Air contains 79% nitrogen,
which is therefore the main component of the flue gas.
This means that there is a low CO2 low concentration,
and the available separation methods (e.g. the amine
method) are relatively complicated and expensive.
Oxyfuel combustion uses a mix of oxygen (usually
with economical purity of 95%) and recirculated flue
gas. The flue gases are formed mainly by a mixture
of water vapour and CO2. The water vapour is easily
removed by condensation, and only minor components
need to be removed from the CO2.
Oxyfuel combustion and air combustion use dif-

ferent volumes of all streams of gases (Fig. 1). All
streams are displayed in the real scale of real flows
(normal cubic meters).

The flue gas recirculation stream for oxyfuel com-
bustion is shown in Fig. 1. Part B is set up to ensure
the same volume of fluidization medium as there would
be for combustion using air.

There are two reasons for recirculating the flue gas:
• It replaces the volume of nitrogen particularly by
CO2, and thus provides a sufficient amount of heat
carrier.

• In the case of fluidized bed boilers, it provides a
sufficient volume of fluidization medium.

Flue gases from oxyfuel combustion consist mainly of
CO2 and water vapour (Fig. 2). This has different
physical characteristics from those of the flue gases

from air combustion, which consist mainly of N2. The
main differences during oxyfuel combustion are:
• In order to reach the same adiabatic flame temper-
ature, it is necessary to reduce the oxygen concen-
tration in the fluidization medium to below 30%.
More than 60% of the total flue gas from the boiler
needs to be recirculated.

• The high concentration of CO2 and water vapour
causes higher emissivity of the gas, which leads to
greater radiation heat exchange.

• The density of the flue gas is higher due to the higher
molar mass of CO2 (44 g/mol) in comparison with
N2 (28 g/mol).

• The molar CO2 heat capacity is 1.25 times higher
than that of N2 because of its larger molecular
weight [8]. The flue gas heat capacity from oxy-
combustion is about one quarter higher than the flue
gas heat capacity from air combustion, taking into
account wet flue gas recirculation and an O2/CO2
ratio of 30/70.

• The excess of air during air combustion is usually
around 20% to 40%. To reach the same concentra-
tion of oxygen in flue gas during oxyfuel combustion,
the excess of oxygen is in just units of percent.

3. Methodology for
stoichiometric calculations

Stoichiometric calculations are volumetric calculations.
They are used to determine the amount of oxidant re-
quired to burn a unit amount of fuel, and to determine
the volume of the flue gases that arise. Stoichiometric
calculations are based on chemical reaction equations
and the balance of the amount of substance. In gen-
eral, we consider two basic models here - a complete
combustion model, and an incomplete combustion
model. The complete combustion model considers
complete combustion of all combustible substances.
The incomplete combustion model can calculate with
only partial fuel burn out and with unburned car-
bon formation. Although the incomplete combustion
model is more appropriate, the complete combustion
is used in most model technical applications uses, and
it is also used here [6].
Table 2 contains all equations that are needed for

computing all volume streams in the boiler. The vol-
umes are stated in standard cubic meters per kilogram
of fuel.

313



Pavel Skopec, Jan Hrdlička Acta Polytechnica

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Table 2. List of equations for calculating the volume flows in the boiler.

314



vol. 56 no. 4/2016 Specific Features of Oxyfuel Combustion Conditions

Cr[ %] Hr[ %] Nr[ %] Sr∗ [ %] Ar[ %] Wr[ %] Qr
i [MJ/kg]

39.05 3.5 0.51 0.65 15.23 28.5 16.9
∗ combustible sulphur

Table 3. Proximate and ultimate analysis of the coal.

Excess of air Recirculation coefficient r Temperature of the recirculated flue gas
1.4 0.3 120 °C

Table 4. Combustion properties of the reference state.

Mean diameter [mm] Bulk density [kg/m3] Density [kg/m3] Sphericity [–]
0.5 790 2195 0.75

Table 5. Characteristics of the fluidized bed particles (coal ash).

3.1. Volume of oxidant
The key step in stoichiometric calculations is to make
a proper calculation of the concentrations of the sub-
stances in the oxidant. In this sense, the term oxidant
refers to the air, the oxygen or the mixture of air and
oxygen in the case of enriched oxygen combustion, as
opposed to the volume of fluidization medium, which
expresses the mixture of oxidant and recirculated flue
gas (see Fig. 1). Table 1 shows the concentrations
of the substances for various oxidants. Oxygen 1 de-
notes 100% pure oxygen, and is the value used for
the calculations. In practice, it is assumed that some
economical optimum of oxygen purity will be used. A
value of 95% is usually quoted [1]. This is denoted as
oxidant Oxygen 2. Table 1 also shows the concentra-
tions for two types of oxygen-enriched air: Air+oxy 30
and Air+oxy 50, where the numbers correspond to the
oxygen concentrations. The concentration of water
vapour in air corresponds to 70% relative moisture at
20 °C.

First, the minimum volume of oxygen needed for
complete combustion is calculated (Tab. 2). The ex-
cess of oxidant is defined as the ratio of the real volume
of oxidant used for combustion and the minimum vol-
ume of oxidant:

α = VO,W

VO,W,min
= VD,W

VD,W,min
. (1)

To correctly determine the flow properties of the flu-
idization medium, it is necessary to know the compo-
sition and the concentration of the individual compo-
nents. The fluidization medium comprises the volume
of recirculated flue gas and the volume of oxidant (see
Tab. 2).

3.2. Volume of the fluidization medium
The volume of the fluidization medium is defined as
the mixture of oxidant and recirculated flue gas. This
stream depends mainly on the recirculation coefficient,
defined by (2). The volume concentration of oxygen
must usually be higher than 21% for oxy-combustion

(typically about 30%), in order to reach the same
conditions as during air combustion. The calculated
concentrations of the fluidized medium components
are stated in Tab. 6 and Fig. 3.

3.3. Volume of flue gas and total
volume of flue gas

The volume of flue gas is given by the sum of the
components that are formed during combustion (CO2,
SO2, N2 from the fuel, H2O) and the components that
are contained in the oxidant but do not participate in
the combustion process (excess of O2, N2, Ar, H2O
from the air humidity).

The volume of water vapour in the flue gas is influ-
enced by the potential condensation of water vapour
in the recirculated flue gas, which will be explained
below. This reduction is expressed by coefficient C
(Tab. 2). If there is no condensation of water vapour,
coefficient C equals 0.
Total volume of flue gas is the real volume that is

released from the boiler (see Fig. 1). It is the sum of
the recirculated flue gas and the flue gas that arises
from the oxidant. This stream passes through the
boiler, and is the main heat carrier.

3.4. Volume of recirculated flue gas
Flue gas recirculation (FGR) is a very important as-
pect of combustion in fluidized bed boilers. Air com-
bustion uses FGR to control the temperature of the
fluidized bed and to ensure proper fluidization. In the
case of oxyfuel combustion, it is the key approach for
reducing the temperature in the combustion chamber
and for keeping the necessary amount of fluidization
medium flow. FGR is extracted after all heat ex-
changing parts of the boiler. The amount of FGR is
expressed using a proportional recirculation coefficient
r [–], which is defined in this work as the ratio of the
volume of the recirculated flue gas to the volume of
the residual flue gas leaving the combustor:

r = VREC,W

VFG,W
. (2)

315



Pavel Skopec, Jan Hrdlička Acta Polytechnica

Air OXY1 OXY2

Total volume of oxidant VO,D (m3
n/kgfuel) 5.61 0.89 0.89

Volume of flue gas VFG,W (m3
n/kgfuel) 6.33 1.52 1.52

Oxidant excess α (–) 1.4 1.06 1.06
Recirculation coefficient r (–) 0.3 3.24 4.41
Total volume of flue gas VTFG,W (m3

n/kgfuel) 8.23 6.45 8.23
Total volume of fluidization medium VFM,W (m3

n/kgfuel) 7.6 5.81 7.6
O2/CO2 ratio (dry) in fluidization medium (–) – 30/70 25/75
SO2 concentration in dry flue gas (ppmv) 800 5800 5800
Adiabatic temperature of the flame TAD (°C) 1316 1316 1082
Minimum fluidization velocity u∗

fm (m/s) 0.33 0.32 0.32
Minimum terminal velocity u∗

t (m/s) 2.91 2.84 2.84
* The minimum fluidization velocity and the minimum terminal velocity were calculated from the
composition of the fluidization medium at the temperatures of the fluidized bed (850 °C).

Table 6. Results of the calculations.

When the recirculated flue gas is cooled down be-
low of the dew point temperature (e.g. because of
the low operation temperature of the FGR fan), the
water vapour begins to condense. The computation
methodology given in Tab. 2 includes the reduction
in water vapour due to condensation.
If we introduce the simplifying assumption of an

ideal gas and the assumption that the absolute gas
pressure is equal to 0.1 MPa, we can say that the
water vapour concentration is equal to the partial
pressure of the water vapour. The maximum water
vapour concentration will therefore be equal to the
saturation pressure for a given temperature.
If the gas temperature in the system does not fall

below the dew point temperature (for the oxyfuel
mode below approx. 80 °C), there is no condensation
and the volume of the water vapour in the recirculated
flue gas is calculated using the same relation as in the
calculation for other compounds:

VREC,H2O = rVFG,H2O. (3)

The total volume of wet recirculated flue gas is then:

VREC,W = VREC,D + VREC,H2O. (4)

4. Results and discussion
The oxyfuel combustion in a bubbling fluidized bed
(BFB) was verified using theoretical complete com-
bustion methodology introduced above. Verifying
the hydrodynamic characteristics of the BFB involves
computing the minimum fluidization velocity, using
the Ergun equation and the terminal velocity of the
particles [7]. The fuel used for the computations is
lignite coal. A proximate and ultimate analysis of the
fuel is presented in Tab. 3.

Computations were first performed for the reference
state of combustion using air. The combustion prop-
erties of the reference state are summarized in Tab. 4.
Table 5 summarizes the characteristics of the fluidized
bed particles (coal ash).

It has already been verified that these conditions
account for the real operation conditions for two ex-
perimental BFB boilers (one 0.5MW [11] and one
25 kW [12]).

Calculations were then made for oxyfuel combustion.
The main aim was to find an oxyfuel mode for which
the temperature and flow ratios would be similar to
the values for air combustion, meaning that it is also
applicable for BFB technology. Because it is difficult
to determine theoretically the exact temperature in
the fluidized bed, the adiabatic combustion tempera-
ture was chosen as one of the correlation parameters.
It was derived from the total heat released in the
furnace. This alternative will be denoted as OXY1.
The second approach uses a comparison based on the
same flow of the fluidizing medium, which would en-
sure sufficient fluidization of the fluidized bed (which
will be denoted as OXY2). All the alternatives are
summarized in Tab. 6.
Figures 2 and 3 show the concentrations of the

main components in the flue gas and in the fluidiza-
tion medium, respectively. The concentrations are
calculated in wet flue gas.
On the basis of Tab. 4, Fig. 2 and Fig. 3, the fol-

lowing statements can be made:
(1.) The volume of oxidant is reduced by more than
84% for oxyfuel combustion. It is therefore im-
possible to ensure fluidization when only oxygen is
used.

(2.) Oxyfuel combustion produces about a 76% lower
amount of flue gas. Flue gas absorbs most of the
heat that is released during combustion. This low
volume of flue gas would absorb only a minor frac-
tion of the heat that is released, and the adiabatic
flame temperature without the use of recirculation
would be as high as 3700 °C. It is therefore impos-
sible to operate the oxyfuel BFBC (or any oxyfuel
boiler) without using a high degree of flue gas recir-
culation. Flue gas recirculation is also required in
order to keep a sufficient volume of the fluidization
medium.

316



vol. 56 no. 4/2016 Specific Features of Oxyfuel Combustion Conditions

Figure 2. Concentration of the main substances in
the flue gas.

(3.) Although the fluidization medium has a different
share of components, it has only a marginal effect
on the computation of the minimum fluidization ve-
locity. The effect on the terminal velocity is slightly
greater, but is still only in units of percent.

(4.) It is impossible to achieve equal oxy-combustion
and air-combustion regimes in terms of simultane-
ously having the same thermodynamic and hydro-
dynamic parameters. Two boundary limit cases
were calculated. OXY1 mode has the same adia-
batic temperature of the flame, but the volume of
the fluidization medium is about 24% lower than
the volume of the fluidization medium for air. To
ensure a well-developed BFB, it is recommended
to use at least twice the minimum fluidization ve-
locity. If a larger amount of recirculated flue gas is
used, the same volume of the fluidization medium
can be ensured. However, this lowers the combus-
tion temperature. This could lead, e.g., to lower
desulphurization or to increased carbon monoxide
emissions.

5. Conclusions
A universal methodology for stoichiometric calcula-
tions has been developed for the theoretical verifica-
tion of oxyfuel combustion in BFB. The methodology
is summarized in Tab. 2. The input parameters are
the concentrations of the substances in the oxidant
medium. This methodology allows us to calculate the
flow of all streams in the boiler for combustion with
air, with oxygen-enriched air, and with pure oxyfuel.
According to the calculations for the two different oxy-
fuel modes and for the air combustion mode presented
here, the following conclusions can be drawn:
(1.) There is an 84% reduction of in the oxidant vol-
ume in comparison with air combustion.

(2.) The volume of flue gas is more than 76% lower
than for air combustion.

Figure 3. Concentration of the main substances in
the fluidization medium.

(3.) A high degree of flue gas recirculation is therefore
required.

(4.) Different compositions of the oxyfuel fluidization
medium and air have only a marginal effect on the
minimum fluidization and terminal velocities.

(5.) It is impossible simultaneously to reach appro-
priate temperature volume flow ratios. The most
typical mode in practical applications is a mode
between OXY1 and OXY2 cases. It is always im-
portant to ensure proper fluidization for BFBC. A
lower combustion temperature can be compensated
by a higher thermal load.

List of symbols
C Coefficient for reducing water vapour when there is

condensation [m3
n/kgfuel]

r Recirculation coefficient [–]
VFG Volume flow of flue gas [m3

n/kgfuel]
VFM Volume flow of fluidization medium [m3

n/kgfuel]
VO Volume flow of oxidant [m3

n/kgfuel]
VREC Volume flow of recirculated flue gas [m3

n/kgfuel]
VTFG Volume flow of total flue gas [m3

n/kgfuel]
α Excess of oxidant [–]
ωi Concentration of substance i [m3/m3]
ω′′H2O Saturated concentration of the water vapour

[m3/m3]

Subscripts
D Dry state
min Minimal amount
W Wet state

Acknowledgements
This work was supported by the Grant Agency of the Czech
Technical University in Prague, in the framework of grant
No. SGS13/181/OHK2/3T/12, and by project TA3020312
Research of Oxyfuel Combustion in a Bubbling Fluidized
Bed for CCS Technologies. We gratefully acknowledge
support from these grants.

317



Pavel Skopec, Jan Hrdlička Acta Polytechnica

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http://www.vattenfall.com/en/ccs/schwarze-pumpe.htm
http://www.vattenfall.com/en/ccs/schwarze-pumpe.htm
http://www.globalccsinstitute.com/insights/authors/dennisvanpuyvelde/2013/02/13/callide-oxyfuel-project
http://www.globalccsinstitute.com/insights/authors/dennisvanpuyvelde/2013/02/13/callide-oxyfuel-project
http://www.globalccsinstitute.com/insights/authors/dennisvanpuyvelde/2013/02/13/callide-oxyfuel-project

	Acta Polytechnica 56(4):312–318, 2016
	1 Introduction
	2 Characteristics of oxyfuel combustion
	3 Methodology for stoichiometric calculations
	3.1 Volume of oxidant
	3.2 Volume of the fluidization medium
	3.3 Volume of flue gas and total volume of flue gas
	3.4 Volume of recirculated flue gas

	4 Results and discussion
	5 Conclusions
	List of symbols
	Acknowledgements
	References