Tratamiento de residuos de biomasa mediante oxidación y gasificación en agua supercrítica

Abstract

The development of highly productive industrial technologies brings about an increment of industrial waste. The management of industrial wastes and wastewaters in an economically and environmentally acceptable manner, is one of the most critical issues that modern industry is facing at present. Therefore, the development of innovative technologies to maximize the recovery of useful materials and/or energy from general waste and wastewaters in a sustainable way has become imperative. Within those new technologies to be developed, hydrothermal processes such as Supercritical Water Oxidation (SCWO) and Supercritical Water Gasification (SCWG) have stirred up great interest because of their huge advantages when compared to conventional treatment processes. This is due to their high efficiency levels to eliminate highly loaded industrial organic waste and the possibility to put these into value. SCWG appears as an option to use waste as a resource and turn it into a fuel gas that is rich in hydrogen and light hydrocarbons. In order to reach a better understanding of the complex reactions that occur during SCWG processes, the present work studies how different heating rates affect the generation of intermediate compounds and final products in a SCWG process of a model compound (cellulose). For this purpose, liquid and gaseous effluents produced at different temperatures (200, 250, 300, 350, 471 ºC) were analyzed until the operating conditions (525 ºC and 250 bar) were reached. The final effluents were also analyzed. In addition, the SCWG of the diapers was studied in a discontinuous reactor at 525 ºC and 240 bar. Olive mill waste has been treated with subcritical and supercritical water to produce mainly gaseous fuel by gasification. To achieve this goal, different batch reactors at temperatures between 200 and 530 ºC and pressure levels between 150 and 250 bar were used. This work has also studied the effect of different types of homogeneous (KOH, NaOH, Na2CO3, K2CO3) and heterogeneous catalysts (TiO2, V2O5 and Au-Pd) in SCWG processes. How the use of different waste concentrations (23, 35 and 80 g O2/l of chemical oxygen demand (COD)), and the mixture of solid and liquid waste from the olive oil industry, affects the process of biomass conversion (elimination of COD) and gas yield (production of H2, CH4, CO2 and CO) was also studied. During the course of the Doctoral Thesis, a research stay was held at the State University of Maringá, Brazil. Thanks to such stay, several studies were carried out in a system that operates in a continuous regime, and at temperatures 175 ºC higher than those studied by our research group. Consequently, SCWG using both olive mill waste and paper mill sludge at temperatures and pressures between 500-700 ºC and 230 bar, respectively, were studied. With regards to paper mill sludge, the effect of adding NaOH as a catalyst to the SCWG process was also studied. On the other hand, SCWO is an innovative technology used to successfully eliminate different waste types. Nevertheless, its industrial implementation is scarce and only a short number of companies commercialize this technology. Further research works are required to solve the problems derived from the high pressure and temperature operating conditions. For this reason, in the present work, the combination of SCWO-SCWG processes in a demonstration plant has been studied as a method to improve both processes and to turn some problems (or disadvantages) into solutions (or advantages). For this pioneering study, two model compounds were used, ethanol as the feed for the SCWO process and isopropanol as the feed for the SCWG process, achieving temperatures around 550 ºC for the combined process. This method has been patented, and for this reason a detailed bibliographic study was carried out for all previous patents related to GASC technology, being published as a review paper.