Numerical Investigation of the Effect of CO₂/H₂O Composition in Oxidizer on Flow Field and Combustion Behavior of Oxy-pulverized Coal Combustion

ABSTRACT

Oxy-coal combustion is the most promising technology for the reduction of greenhouse gases from pulverized coal-fired power plants. Oxy-coal combustion employs recirculated flue gas mainly consisting of CO₂ and water vapor as a diluent. Thus, the pulverized coal particles are surrounded and burned under steam rich atmosphere. The addition of H₂O would have a significant impact on the combustion characteristics of oxy-coal combustion. The chemical and physical properties of steam are different than the CO₂, replacement of CO₂ with steam will alter heat capacity, gasification, and radiation properties considerably. The present article numerically compares ideal dry recycle oxy-coal combustion (0% H₂O) with wet recycle oxy-coal (10-50% H₂O) and oxy-steam combustion (H₂O replaces whole CO₂ from oxidant) in terms of flow field, temperature distribution, oxidizer distribution, radiative heat transfer, char consumption, and species concentration. Higher flame temperatures under enriched steam oxy-coal combustion cases were found due to lower volume heat capacity of H₂O than CO₂. Steam enrichment also enhanced char gasification reaction, which has affected temperature distribution and incident radiation profile inside the combustion chamber. Peak temperature obtained under oxy-steam case is around 10% higher than ideal dry recycle case (0% H₂O) and 2-5% higher than the wet oxy-coal combustion cases having 50-10% H₂O in the oxidizer.

KEYWORDS:

• Oxy-coal combustion
• oxy-steam combustion
• Steam
• gasification
• char mass fraction

Nomenclature

Apre-exponential factor (s⁻¹)

Eactivation energy (J.kg⁻¹)

${\bar{U}}_i$mean velocity in tensorial notation (m.s⁻¹)

${\bar{U}}_{\theta }$mean tangential velocity (m.s⁻¹)

${\bar{U}}_z$mean axial velocity (m.s⁻¹)

$V_i$velocity of particle (m.s⁻¹)

$Y_i$mass fraction of species i

Eactivation energy (kJ.kg⁻¹)

q_riradiative heat flux (W.m⁻²)

$d_i$particle diameter of i_th class of particle (m)

$\stackrel{.}{S}$source term gas phase due to particles (kg.m⁻³s⁻¹)

$\underset{M_i}{\overset{.}{S}}$momentum source term in gas phase due to particles (kg.m⁻²s⁻²)

$\underset{E}{\overset{.}{S}}$energy source term in gas phase due to particles (W.m⁻³)

Ddiffusion coefficient (m².s⁻¹)

$G^{\prime }\left(d_i\right)$mass fraction of spray having diameter above $d_i$

bsize parameter

nspread parameter

$d_{max,i}$maximum diameter of i_th class of particle (m)

$d_{min,i}$minimum diameter of i_th class of particle (m)

$\mathrm{\Delta }{H}_{devo}$enthalpy of devolatilization (J.kg⁻¹)

$\mathrm{\Delta }{H}_{devo}$enthalpy of char reaction (J.kg⁻¹)

c_pspecific heat at constant pressure (J.kg⁻¹. K⁻¹)

A_pparticle surface area (m²)

m_pparticle mass (kg)

m_p,0initial particle mass (kg)

f_v,0initial mass fraction of volatiles in coal

f_w,0initial mass fraction of moisture in coal

kkinetic rate (s⁻¹)

ReReynolds number

NuNusselt number

PrPrandtl number

ppressure (Pa)

$\overrightarrow{r}$position vector

$\overrightarrow{s}$direction vector

${\vec{s}}^{\prime }$scattering direction vector

${\sigma }_s$scattering coefficient

$I_{\lambda }\left(\vec{r},\vec{s}\right)$ spectral radiation intensity

nrefractive index

$\mathrm{\varnothing }$phase function

${\mathrm{\Omega }}^{\prime }$solid angle

k_ipressure absorption coefficient of absorbing gas i

p_ipartial pressure (Pa)

$a_{\epsilon ,i}$weighting factor of emissivity for gray gas i

$G_k$generation of turbulent kinetic energy due to mean velocity gradient

$G_b$generation of turbulent kinetic energy due to buoyancy

$Y_M$contribution of fluctuating dilatation in compressible turbulence to the overall dissipation rate

${\mu }_t$turbulent viscosity

$a\ast$damping coefficient for turbulent viscosity

$q_{ri}$heat flux due to radiative heat exchange between gas and particle phase

$Y_i$mass fraction of species i

$R_i$net rate of production of species i by chemical reaction

$\stackrel{.}{S_i}$source term

${\upsilon }_{i,r}^{\prime }$stoichiometric constant of reactant i

${\upsilon }_{j,r}^{{ }^{\prime \prime }}$stoichiometric constant of product j

M_wmolecular weight

Aempirical model constant (4.0)

Bempirical model constant (0.5)

Greek symbols

${\alpha }_t$turbulent thermal diffusivity (m².s⁻¹)

${\mu }_t$eddy viscosity (kg.s.m⁻¹)

${\theta }_R$radiation temperature (K)

$\rho$density (kg.m⁻³)

$\sigma$Stefan-Boltzmann constant (W.m⁻². K⁻⁴)

$k$absorption coefficient (m⁻¹)

$\mathrm{\forall }$volume of computational cell (m³)

Subscripts

iinitial state

jtensor notation’s index

ggas phase

pparticle phase

Supplementary material

Supplemental data for this article can be accessed on the publisher’s website