Avaliação do aproveitamento da vinhaça na produção de H2 via processo de eletrólise

Abstract

Vinasse is an agro-industrial effluent composed of approximately 90% water, with a solid fraction rich in mineral and organic substances of high fertilizing value, but with a polluting power about one hundred times greater than domestic sewage due to its low pH, high biochemical oxygen demand (BOD) levels, and corrosiveness. Its composition varies mainly depending on the composition of the mash, which may result from the distillation of the fermentation liquor from sugarcane ethanol production, molasses mash, or a combination of both. Typically, for every liter of ethanol produced, between 10 and 15 liters of this effluent are generated, which is primarily destined for fertigation. A very common process for hydrogen production is electrolysis, which, through an electric current, separates the water molecule (H2O) into hydrogen (H2) and oxygen (O2). The objective of this work is to evaluate the application of electrolysis to untreated sugarcane vinasse, focusing on hydrogen production, considering the use of different types of electrodes (aluminum, copper, stainless steel 304) and process conditions. An initial experimental design with 15 trials was conducted to evaluate the effects of the following variables: vinasse dilution (in distilled water), type of electrode (material), and scan rate (of the electric current) on the electrolysis process of vinasse for H2 generation. Next, using the best result from the previous trials, and following a mixture design with 10 experiments, the effects of adding supporting electrolytes — formic acid and NaOH — to the vinasse were also evaluated. For electrochemical analysis, cyclic voltammetry was used to obtain information about the redox behavior (reduction and oxidation) of the solution, electron transfer mechanisms, and electrochemical kinetics associated with adsorption processes. Chronoamperometry was also employed to measure the current generated over a fixed period (600 s), and with the aid of Faraday’s laws, to calculate the amount of hydrogen gas released between the electrode’s contact surface (area in cm²) and the analyzed solution. The best result was obtained for a solution containing 50% vinasse (diluted to 15%), 25% formic acid, and 25% sodium hydroxide, totaling 100 mL of solution, using a stainless steel electrode with a contact area of 0.15 cm² and a scan rate of 10 mV, resulting in 19,6.10-2 mol cm-2 de H2 generated for a current density of 75.6 mA. Samples of raw vinasse, the solution with electrolytes obtained at the best point of the mixture design at the initial time (before electrolysis), and the same sample after electrolysis were analyzed for physicochemical parameters and multi-elemental composition by total X-ray fluorescence. This allowed the verification of the potential and sustainable use of vinasse for hydrogen production under controlled process conditions, resulting in environmental, social, and financial benefits in line with the modern concept of a circular economy. The production cost of hydrogen via vinasse electrolysis was evaluated considering the average national electricity price of R$ 0.74/kWh (excluding taxes). The value of the electricity consumed in the system was obtained as a function of the theoretical efficiency of the electrolyzer cell (Faradaic efficiency). The estimated cost was 55,40 (R$ cm-2 mol-1 H2).