Development of a System for Electric Energy Production Based on Fuel Cells Fueled by Biogas Produced with Anaerobic Digestion of Biomass

Abstract

research activity regards the field of innovative technologies for energy production with high efficiency and low environmental impact from renewable sources. The steady increase in energy demand associated with the reduced availability of fossil fuels and the urgent need to reduce greenhouse gas emissions, are promoted the use of high-efficiency and alternative fuel technologies. use of biogas is an effective solution to the problems of energy supply with further benefit of recovery and reutilization of the organic waste to energy scope. biogas, a mixture consisting mainly of methane, carbon dioxide and hydrogen can be produced through thermochemical processes (i.e. pyrolysis and gasification) or biological (i.e. biophotolysis and biofermentation). last one have the advantage of a significant energy savings and reduced pollutant emissions. The technology of fuel cells (FCs) is today the most valid alternative to traditional thermal engines for different aspects: the higher efficiency (up to 80-85% in cogeneration systems), the negligible environmental impact, flexibility and modularity that allow to obtain power range from a few W up to MW. Thanks to these advantages the interest of the international scientific community to the FC continues to grow and the technology is beginning to have sufficient maturity to be developed on large scale. The objective of this PhD thesis is the development of an integrated system for production of electrical energy based on FCs, in particular on high-temperature fuel cells (Solid Oxide Fuel Cells) fueled with biogas and biohydrogen produced by anaerobic digestion of biomass. The development of integrated systems of this type needs further upgrades to became competitive for industrial applications. The biomethane production from biomass waste, despite being a well-established process, still needed improvement in relation to the nature of the starting biomass and operating conditions of the digester. production of biohydrogen for energy applications is still a challenge, and further studies are needed to get hydrogen rich streams and to optimize the digestion process towards H2 production rather than CH4. The engineering of the integrated system digester/FC deserves particular attention in order to make compatible the characteristics of biogas with the specifics of the FC. This refers in particular to the presence of impurities such as H2S and HCl that are harmful for FCs and require a specific design for clean-up systems and the choice of suitable adsorbent materials. The PhD thesis is divided in four chapters. The first chapter, “Introduction”, reports the state of the climate change, analyzing their causes and foreseeable future scenarios. It was decided to report the regulations introduced by European and international community that promoted the development of new technologies in the energy field. FCs are then treated as an emerging high-efficiency technology for the production of energy describing the general principles, the main types and applications already present on an industrial scale or foreseeable in a short-medium term. Relevant importance is dedicated to the anaerobic digestion process describing the general characteristics such as the reaction network, the steps involved, the operating conditions (T, pH, retention time) and the nature of the microbial consortia. The second chapter Biohydrogen Production concerned the study and optimization of fermentation processes (Dark Fermentation) aimed to the production of biohydrogen (and biomethane) evaluating the potential for energy exploitation of different biomass. Different types of biomass have been studied: two lignocellulosic, Arundo donax (not treated and pretreated by steam-explosion process) and a litter from animal house for mice; the organic fraction of municipal solid waste and manure from a cattle farm. Sewage sludge, used as inoculum, has been collected from the wastewater treatment plant in Nola (Na). All tests have been conducted under mesophilic conditions at 38 ° C. It has been developed an innovative experimental procedure aimed to the selection of the bacteria hydrogen producers through the use of a salts and nutrients medium specific for these bacteria. Compared with literature data the procedure operates under the same conditions of anaerobic digestion and does not require further additional costs. It was decided to make attractive the development of the process from an industrial point of view. The results obtained are very interesting due to high biogas production and H2 concentration (up to 70% vol). This data confirms the efficiency of experimental procedure able to enhance the growth of H2-producer bacteria to the detriment of methanogenic ones. The third chapter The Use of Biogas in Fuel Cells Technology: Adsorption Processes and Materials Adsorbent for Removal of Noxious Compounds deals with the purification system of the biogas stream from impurities, which can cause irreversible poisoning phenomena for FCs (particularly H2S and HCl). In order to be fed to the FCS, the biogas must have concentration of H2S and HCl lower than 1 ppmv. removal technique used is based on adsorption on microporous materials such as activated carbons and 13X zeolites. Such materials have been properly functionalized in order to increase the adsorbent capacity and selectivity toward H2S and HCl. experimental activity was conducted on two laboratory plants, that appear innovative compared to literature, which allow the continuous monitoring of the H2S and HCl concentrations with high accuracy of analysis. From adsorption data breakthrough curves of different materials have been obtained making able to compare the performance of the sorbents and to identify the optimal ones. results obtained are encouraging because have been obtained high purity levels with H2S and HCl concentrations lower than 1 ppm and longer saturation time compared to literature data. The fourth and final chapter Integrated Anaerobic Digester System/Fuel Cell reports the feasibility study and the development of the integrated plant anaerobic digester/FCs. Part of the research activity has been developed in Switzerland at the Paul Scherrer Institute (Villigen). In the contest of the project Biosweet (Biomass Energy Future for Swiss) has developed the line of research Manure to Electricity which includes the development of small plants (2-5 up to 100 kWe) using SOFCs fueled with biogas produced by anaerobic digestion of manure. project coordinator is Dr. Serge Biollaz. In addition to the participation of Prof. Turco’s research group, the project has been developed in collaboration with various research centres (WSL, the Swiss Federal Research Institute) and Swiss Universities (EPFL, University of Applied Sciences in Zurich). The feasibility study of the integrated system is carried out through the designing of the different sections of the plant: anaerobic digestion (digester, storage tanks), clean-up (upgrading and adsorbent beds), SOFC stack. In particular, a market survey aimed to identify and select the SOFC yet available in the market analyzing several characteristics such as power density, lifetime and resistance to harmful compounds. In addition, an economic analysis with an initial assessment of the costs has been developed. The integrated plant Digester/SOFCs for the production of small/medium plant size for electrical energy production has characteristics of novelty and may be competitive with other higher power plant engineering realities having lower flexibility for the reuse of the agro-industry waste materials.