A Numerical Investigation of Oxygen-Enriched Biogas Counter-Flow Diffusion Flames

ABSTRACT

Biogas is an attractive biofuel which contributes to renewable energy recovery and pollutant emission reduction. Biogas technology has won universal attention and is continuously in development over the world. Combustion is currently one of the major applications of biogas. However, it encounters several problems related to flame stability and insufficient heating value. In the present paper, oxygen-enhanced combustion is proposed as a solution to overcome these difficulties. A numerical investigation of biogas laminar non-premixed counter-flow oxygen enhanced flames is achieved. The fuel is biogas produced in El-Karma wastewater treatment plant situated in Algeria. Flame structure and NO_X pollutant emissions are examined over a wide range of scalar dissipations (from equilibrium to extinction) and O₂ contents (from 21% to 100% by volume), including all oxygen enhanced combustion regimes. The laminar flamelet model is considered with the detailed GriMech 3.0 chemical mechanism. Results show that flame temperature increases with oxygen enrichment and stability limits are improved. Radiation effects dissipated beyond an intermediate value of scalar dissipation for all O₂ levels in the oxidizer. This value increases with O₂ enrichment. NO_x emissions are promoted for low and intermediate oxygen levels but drop for high oxygen-level enrichment. Practical systems using biogas as a fuel should operate at O₂ enrichment levels upper than 70% under scalar dissipations slightly greater than 10 s⁻¹ in order to minimize NO_X emissions with acceptable levels of CO and CO₂.

KEYWORDS:

• Biogas
• oxygen-enhanced combustion
• NO_x emissions
• laminar diffusion counter-flow flames

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author contributions

Khadidja Safer: Original idea, supervision, software, writing

Meriem Safer: Investigation, methodology, writing, editing

Tabet Fouzi: writing, review and editing

Ahmed Ouadha: Review and editing, methodology, visualization

Nomenclature

acr	=	The critical strain rate, s−1
ai	=	Planck mean absorption coefficient of ith species, msup > −1.Pasup > −1
as	=	The flamelet strain rate, s−1
Cp	=	Specific heat at constant pressure, J.kgsup > −1.Ksup > −1
cp,n	=	nth species specific heat at constant pressure, J.kgsup > −1.K−1
erfcsup > −1	=	Inverse complementary error function
Len	=	nth species Lewis number
P	=	Pressure, Pa
T	=	Temperature, K
Tb	=	The far-field temperature, K
Ul	=	The laminar flame speed, m.ssup > −1
Xi	=	ith species mole fraction
Yn	=	nth species mass fraction
Z	=	Mixture fraction

Greek symbols
α	=	The thermal diffusivity, m2.s`
χ	=	Scalar dissipation rate, s−1
χst	=	Stoichiometric scalar dissipation rate, s−1
λ	=	Thermal conductivity, W.m−1.K−1
ρ	=	Density, kg.m−3
σ	=	The Stefan – Boltzmann constant, 5.67 × 10−8 W.m−2.K−4
${\dot{\omega }}_{\mathit{n}}$	=	nth species reaction rate, kg.m−3s