El panorama energético internacional, basado en el uso de combustibles fósiles, está caracterizado por el crecimiento continuo de la demanda energética que origina escases de recursos y deterioro del medio ambiente. Por tal motivo, el uso de tecnologías renovables se ha incrementado a nivel mundial para asegurar la sostenibilidad del futuro. Una de las tecnologías renovables más utilizadas en la actualidad se basa en la digestión anaerobia de residuales biodegradables para producir biogás, un gas combustibles muy versátil que puede reemplazar combustibles convencionales en plantas generadoras de electricidad y calor. El valor energético del biogás y su potencial para mitigar las emisiones de de carbono de conjunto con la producción de compost y la remoción de la carga contaminante son los principales beneficios que puede reportar la digestión anaerobia.
Las vinazas obtenidas en el proceso de destilación del etanol en la industria azucarera (vinazas de molasas de caña) son sustratos de altos contenidos de materia orgánica y azufre que pueden ser utilizados para producir biogás. Por tanto la digestión anaerobia de las vinazas promueve la actividad de las bacterias sulfato-reductoras (SRB), produciendo sulfuro de hidrógeno que se distribuye entre las fases líquida y gaseosa del este proceso. La producción y características de la vinaza son variables y dependientes de la materia prima y el tipo de proceso empleados en producción del etanol. Las fluctuaciones en la concentración de las vinazas causan variaciones en el contenido de materia orgánica (COD) y azufre (SO42-) que a su vez pueden causar respuestas dinámicas durante su tratamiento anaerobio, influenciando la calidad del biogás y el funcionamiento del proceso. Por tal motivo, resulta necesario estudiar el proceso de reducción de sulfatos en la digestión anaerobia de las vinazas de molasas de caña con el fin de potenciar el uso energético del biogás y el buen funcionamiento del proceso mediante la predicción de las concentraciones de sulfuro de hidrógeno en las fases líquida y gaseosa respectivamente.
In the present research the sulfate reduction process in the anaerobic digestion of a very high strength and sulfate rich vinasse (characterization and modeling) and the impacts of anaerobic digestion power plants as alterative for lagooning Cuban vinasse have been studied. Firstly, Chapter 1 reviews the state-of-the-art of the sulfate reduction process in the anaerobic digestion of vinasse, with emphasis on the modeling of sulfate reduction including the process and reaction involved, the kinetics (growth, inhibition and endogenous processes), the acid–base equilibrium and the gas–liquid transfer equations. In addition, the principal approaches to model sulfate reduction were discussed. It is concluded that vinasse is a typical sulfate-rich liquid substrate for anaerobic digestion but the sulfate reduction processes in the anaerobic digestion of vinasses with similar simultaneous high levels of COD and SO42- should be studied. In addition, it is shown that the model equations are available in literature but no results have been shown (as an extension of ADM1) to predict the concentrations of total aqueous sulfide (Sh2s), free sulfides (Sh2s,free) and gas phase sulfides (Sgas,h2s). Therefore, kinetic coefficients to model sulfate reduction in the anaerobic digestion of vinasse have not been reported in literature and this fact is limiting the prediction of the sulfur compound in the gas and liquid phases to assist the energetic use of the biogas and the process performance. Finally, this chapter shows an overview of the currently available environmental sustainability concepts of life cycle assessment and exergy analysis. In Chapter 2, the sulfate reduction process in the anaerobic digestion of a very high strength and sulfate rich vinasse is characterized, by giving COD and SO42- pulses at different SO42-/COD ratios to obtain dynamical responses. A set of dynamic data reliable for calibrating mathematical models, when sulfate reduction in the anaerobic digestion of a very high strength and sulfate rich vinasse is of primary interest, is given in this Chapter 2. The results shows deterioration of the biogas quality at a SO42-/COD ratio of 0.05 (when inf_COD and inf_SO42- increased), strong contribution of pSRB to the degradation of propionate at SO42-/COD ratios ≤ 0.10 (in contrast to literature results), inhibition by H2Saq, [H2S]free and propionic acid to SRB, methanogens or both at a SO42-/COD ratio of 0.10 (deterioration of the anaerobic digestion process) and severe inhibition for methanogens and SRB at a SO42-/COD ratio of 0.15 and 0.20 (leading to reactor failure). The results of the mass balance calculations (COD and sulfur) are also shown in this chapter.
In Chapter 3, the modeling of the anaerobic digestion of cane-molasses vinasse, extending the Anaerobic Digestion Model No. 1 with sulfate reduction for very high strength and sulfate rich wastewaters is presented. Results of the sensitivity analysis based on the local relative sensitivity methods are shown. The model predictions were mostly classified as high
(±10%) or medium (10% - 30%) accuracy quantitative predictions during model calibration and validation, based on a mean absolute relative error for the process variables: sulfates, total aqueous sulfide, free sulfides, methane, carbon dioxide and sulfide in the gas phase, gas flow, propionic and acetic acids, chemical oxygen demand (COD), and pH. As a result, the model is considered as valid to assist the sulfate reduction process in the anaerobic digestion of cane-molasses vinasse when sulfate and organic loading rates range from 0.36 to 1.57 kg SO42- m-3 d-1 and from 7.66 to 12 kg COD m-3 d-1, respectively.
In Chapter 4, the impacts of eighteen alternatives of anaerobic digestion power plants for lagooning Cuban vinasse was assessed by using Life Cycle Assessment and exergy analysis. The environmental profiles of anaerobic digestion power plants with respect to the lagooning of Cuban vinasse for the endpoint impact categories “ecosystem quality”, “human health” and “natural resources” are shown. The exergy efficiency was used to assess potential process improvement and irreversibilities in the subprocesses that form the anaerobic digestion power plants. The alternatives with the highest benefits for the Life Cycle Assessment and the exergy analysis are discussed. In general, the treatment of 1072 ton of vinasse (exergy content of 740.6 GJex) in anaerobic digestion power plants can produce 143 GJex as electricity and heat, 179 GJex as sludge (65% dry matter, w/w), 22.4 GJex as ferti-irrigation water and 0.38 GJex as sulfur in the filter cake. This way, 44% of the exergy contained in vinasse is converted to electricity, heat and sludge, becoming vinasse a potential renewable resource that can replace 402590 GJex (electricity, heat and sludge) per year in Cuba and reduce the negative environmental impacts for the studied categories. Chapter 5 includes the general conclusions and perspectives of this research to support decision-makers in the further implementation of anaerobic digestion power plants for the treatment of vinasses in Cuba
