Experimental and numerical analyses of flameless combustion using biogas from palm oil mill effluent

Abstract

Flameless combustion as a clean combustion technology has been recently developed due to simultaneous low emission formation as well as efficient combustion process. Biogas has also recently been identified as a potential alternative fuel for flameless combustion. Biogas has attracted attentions because its generation is not limited to the specific geography. Since Malaysia is currently one of the world's largest producer of palm oil, biogas released from palm oil mill effluent (POME) has great capability to be applied as a source of energy in the country. Since the calorific value of POME biogas is relatively low (around 22 MJ/m3), producing a stable flame o f POME biogas premixed combustion is quite difficult. Indeed, high temperature of flame front and high rates of thermal NOx formation, complicated setting and low efficiency of the conventional biogas combustion are crucial problems in applying biogas in premixed combustion system. Since upgrading o f POME biogas is a complicated and expensive process, direct injection o f POME biogas in flameless combustion system is a candidate for efficient POME biogas energy extraction. The objectives of this study are to investigate performance o f a laboratory-scale flameless combustion furnace fuelled by POME biogas in terms of flameless stability, temperature distribution and pollutant formation. The effects of burner configuration on the performance of POME biogas flameless combustion are evaluated. Moreover, various aspects of biogas flameless mode in terms of burned gas recirculation inside the chamber and relationship between mixing and chemical reactions, effects of various preheated diluted oxidizer on the flameless combustion system are investigated numerically. The results confirm that flameless combustion of POME biogas is feasible in the lean, stoichiometry and rich fuel circumstances and the axial temperature of the chamber is higher in stoichiometric condition. Extremely low O2 and CH4 concentration are recorded in highly diluted oxidizer in ultra-lean flameless combustion. Due to the low calorific value of POME biogas and the distance between fiiel/oxidizer jets, Damkohler number is found higher than unity and consequently eddy dissipation method (EDM) is proposed for turbulence chemistry interaction of POME biogas flameless combustion. The numerical results are in good agreement with experimental results. The stability of POME biogas flameless combustion is discussed based on the internally burned gas recirculation. It is found that POME biogas flameless combustion is sustained when recirculation ratio (Kv) is greater than 2.6. Flameless combustion of POME biogas is found to be limited to Kv o f less than 4.6 in coaxial burner configuration. In tangential burner configuration, POME biogas flameless combustion is sustained in higher recirculation ratios (Kv =6.3). The efficiency o f POME biogas flameless combustion is 62% and 66% in coaxial and tangential burner configurations respectively. Temperature uniformity is calculated 0.92 and 0.96 in coaxial and tangential burner configurations respectively. When equivalence ratio increases from 0.6 to 1.2, NOx emission decreases from 2.4 ppm to less than 1 ppm in coaxial burner and from 3.1 ppm to 1.1 ppm in tangential burner.