approximately 17–70%, depending on the fuel used, and can then be captured,
stored, or used (Buhre et al. 2005).
By determining the physical and chemical processes that the fuel experiences
during oxycombustion, characteristics such as heat and mass transfer, temperature,
stability and flame velocity, ignition, and pollutant formation are affected globally
(Chen et al. 2012a, b). The main impacts are related to the differences in properties
of CO 2 , the diluent gas in oxycombustion, and N 2 , the diluent in the combustion
with air (Yin and Yan 2016). Table 1 shows the different physical properties and
chemical effects of main gases resulting from oxycombustion (CO 2 and H 2 O) and
conventional combustion with air (N 2 and O 2 ), which induces substantial changes
in combustion processes.
The total heat and mass transfer in a furnace include radiative and convective heat
transfer and depend especially on the flame temperature and gas properties.
Radiation is the principal mode of heat transfer in combustion processes, playing a
dominant role in the furnace. The entire flame is considered to be a constant source
of radiation, and its radiative energy release rate is improved when the emissivity (e)
is higher, that is, when the capacity of a substance to emit heat is greater. Thus,
unlike diatomic molecules, such as N 2 , triatomic molecules such as CO 2 and H 2 O are
radiating species and have higher partial pressures, and consequently, the absorptivity and emissivity of the flue gas substantially increase (Chen et al. 2012b).
As for convection, there is a greater contribution to heat exchange, which is
influenced by flow velocity of gases, density, viscosity, thermal conductivity, and
specific heat capacity, which are also functions of flame temperature. The rate of
convective heat transfer coefficient in both oxycombustion and combustion with air
can be expressed in terms of dimensionless numbers, such as the Reynolds and the
Prandtl numbers, and by fluid thermal conductivity. Thus, the thermal conductivity
of CO 2 is slightly higher than that of N 2 , not significantly altering the heat transfer.
However, the lower kinematic viscosity of CO 2 and its higher density, due to the
higher molecular weight (44.09) when compared to N 2 (28.01), results in a larger
Reynolds number and, therefore, a higher convective heat transfer coefficient (Yin
and Yan 2016). In terms of specific heat capacity, it is observed that at 1000 °C, the
N 2 presents a Cq of 34.18 kJ/k mol, whereas CO 2 has Cq 57.83 kJ/k mol, further
highlighting the high heat transfer of these gases in oxycombustion conditions
(Cengel 2003).
In relation to flame temperature, it is necessary to recirculate between 60 and
80% of the oxycombustion of gases into the furnace, aiming to moderate excess
temperature due to the increase of oxygen concentration injected, and also to
achieve a similar profile of heat transfer in relation to combustion with air. High O 2
concentrations increase the adiabatic flame temperature, which is the largest
attained temperature in the combustion products without heat exchanging inside or
outside the system, and this occurs due to lack of N 2 dilution. In this case, to
moderate excess temperature, the proportion of recycled flue gas and the O 2 concentration to be injected must be adjusted in order to achieve the same flame
temperature as in the combustion with air. On the other hand, if the recycled flue
gas amount is higher, it will result in a lower average O 2 concentration for furnace
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stored, or used (Buhre et al. 2005).
By determining the physical and chemical processes that the fuel experiences
during oxycombustion, characteristics such as heat and mass transfer, temperature,
stability and flame velocity, ignition, and pollutant formation are affected globally
(Chen et al. 2012a, b). The main impacts are related to the differences in properties
of CO 2 , the diluent gas in oxycombustion, and N 2 , the diluent in the combustion
with air (Yin and Yan 2016). Table 1 shows the different physical properties and
chemical effects of main gases resulting from oxycombustion (CO 2 and H 2 O) and
conventional combustion with air (N 2 and O 2 ), which induces substantial changes
in combustion processes.
The total heat and mass transfer in a furnace include radiative and convective heat
transfer and depend especially on the flame temperature and gas properties.
Radiation is the principal mode of heat transfer in combustion processes, playing a
dominant role in the furnace. The entire flame is considered to be a constant source
of radiation, and its radiative energy release rate is improved when the emissivity (e)
is higher, that is, when the capacity of a substance to emit heat is greater. Thus,
unlike diatomic molecules, such as N 2 , triatomic molecules such as CO 2 and H 2 O are
radiating species and have higher partial pressures, and consequently, the absorptivity and emissivity of the flue gas substantially increase (Chen et al. 2012b).
As for convection, there is a greater contribution to heat exchange, which is
influenced by flow velocity of gases, density, viscosity, thermal conductivity, and
specific heat capacity, which are also functions of flame temperature. The rate of
convective heat transfer coefficient in both oxycombustion and combustion with air
can be expressed in terms of dimensionless numbers, such as the Reynolds and the
Prandtl numbers, and by fluid thermal conductivity. Thus, the thermal conductivity
of CO 2 is slightly higher than that of N 2 , not significantly altering the heat transfer.
However, the lower kinematic viscosity of CO 2 and its higher density, due to the
higher molecular weight (44.09) when compared to N 2 (28.01), results in a larger
Reynolds number and, therefore, a higher convective heat transfer coefficient (Yin
and Yan 2016). In terms of specific heat capacity, it is observed that at 1000 °C, the
N 2 presents a Cq of 34.18 kJ/k mol, whereas CO 2 has Cq 57.83 kJ/k mol, further
highlighting the high heat transfer of these gases in oxycombustion conditions
(Cengel 2003).
In relation to flame temperature, it is necessary to recirculate between 60 and
80% of the oxycombustion of gases into the furnace, aiming to moderate excess
temperature due to the increase of oxygen concentration injected, and also to
achieve a similar profile of heat transfer in relation to combustion with air. High O 2
concentrations increase the adiabatic flame temperature, which is the largest
attained temperature in the combustion products without heat exchanging inside or
outside the system, and this occurs due to lack of N 2 dilution. In this case, to
moderate excess temperature, the proportion of recycled flue gas and the O 2 concentration to be injected must be adjusted in order to achieve the same flame
temperature as in the combustion with air. On the other hand, if the recycled flue
gas amount is higher, it will result in a lower average O 2 concentration for furnace
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