Study and evaluation of electrolysis as a treatment option for non sulfur black liquor

Abstract

Electrolysis of black liquor, an effluent from paper industry, was carried out to study and ascertain its suitability as a treatment option for non sulfur black liquor. From the initial experiments it was found that, though, during electrolysis of black liquor hydrogen is evolved at the cathode negligible gaseous products were obtained at the anode. Energy efficiency in terms of HHV of hydrogen was found in the range of 84% - 96% whereas under similar conditions alkaline water electrolysis could not give more than 66% efficiency. Hydrogen evolution in black liquor electrolysis was possible even at an inter electrode potential of 1.17 V but in alkaline water electrolysis there was no hydrogen production below an inter electrode potential of 1.31 V. Cathodic hydrogen evolution reaction was studied for black liquor electrolysis using standard electrochemical methods and was compared with alkaline water electrolysis as control. In black liquor electrolysis the hydrogen evolution was found to be kinetically facile with exchange current densities of 2.194 mA/cm2, 1.585 mA/cm2 and 9.856 mA/cm2 for wheat straw black liquor, bagasse black liquor, and eucalyptus black liquor, respectively, in comparison to 1.075 mA/cm2 for alkaline water. The cathodic Tafel slopes were –112.8 mV dec-1, –116.9 mV dec-1, 137.9 mV dec-1, and –135.3 mV dec-1 for alkaline water, wheat straw black liquor, bagasse black liquor, and eucalyptus black liquor, respectively. The activation overpotential for hydrogen evolution was significantly lower for black liquor electrolysis. In addition to this, alkali lignin, amounting to 28 – 46 mg/mg of hydrogen produced, was separated at anode during black liquor electrolysis, which, on account of its good calorific value, has the potential of significantly improving the overall energy efficiency of the process. Black liquor electrolysis showed additional anodic electroactivity in a potential window of –0.2V to 0.2 V, well shifted from the region of oxygen evolution reaction. This reaction followed a sluggish one electron oxidative charge transfer as revealed by cyclic and square wave voltammograms. The anodic charge transfer coefficient was slightly higher than 0.5. Black liquor electrolysis could produce hydrogen with a much lower inter-electrode potential with the exclusion of gaseous products being formed at the anode. This can have significant bearing on the energy efficiency of the process.