Molten carboate fuel cell fed with biogas

Renewable and sustainable energy sources have to be considered to overcome problems such as energy security and climate change associated with fossil fuel. Drivers for bioenergy technologies development are environmental and socio-economic politics. First biomass has a carbon neutral life cycle and promotes climate change benefits that can be realized through reduction of GHG emissions. A sustainable energy crops production, which does not compete with the use of land and water for food production, can have a positive impact toward food security because promote rural development providing energy access to remote communities and creating employment. Finally bioenergy guarantee energy security because it promotes the energy supplies diversification and the reduction of dependency on a few exporters of oil and natural gas. Fuel cells play a key role to overcome the low energy content of the biomass, because they are highly efficient and environmentally friendly. High temperature fuel cells, such as molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC), are particularly suitable for bioenergy production because they can be fed with biogas directly: methane can be transformed to hydrogen by internal reforming , carbon dioxide is a safe diluent and carbon monoxide acts as both a direct fuel, because is an electroactive specie, and a hydrogen supplier by water gas shift reaction. However biogas contains other chemicals, biomass processing byproducts, affecting cell performances. Among these, hydrogen sulfide is the most important because is able to poison the cell at low ppm levels. This work includes studies of biogas composition on MCFC performance, studies of sulfur poisoning mechanisms, identification of selection criterions for sulfur-tolerant materials, and preliminary characterization of alternative anode materials with high corrosion resistance and with high recovering capability. Hydrogen sulfide affects cell performance because it reacts with both the anode and the electrolyte. Electrolyte poisoning occurs is due to the replacement of carbonate ions with sulfide and sulfate ions. Anode degradation is due to bulk chemical reactions, chemisorption, physical adsorption and electrochemical reactions. Multivariate analysis was performed to study poisoning mechanisms and develop a model with predictive capability. Results show that sulphur poisoning occurs via electrochemical mechanisms mainly. At low hydrogen sulfide levels just two poisoning reaction types occur: physical and chemical absorptions on nickel surface; replacement of carbonate ions with sulfide and sulfate ions. In fact, formations of bulk nickel sulfides are thermodynamically forbidden. However when it is applied a current at the cell, the anodic potential increases up to right values for nickel sulfide formations via electrochemical mechanisms. It means that under current load MCFC is more sensible to hydrogen sulfide attack. Poisoning levels depend on applied current, hydrogen sulfide concentrations and hydrogen concentrations.Irreversible poisoning effects are due to stable nickel sulfide formations, Ni3S2. Selection criteria for alternative anode materials with a high resistance to hydrogen sulfide corrosion have to include evaluations of both thermochemical parameters such as Gibbs free energy of sulfuration reactions and electrochemical parameters such as electrode potential for sulfide depositions. Proposed anode materials include to two different categories: anodes made of an electrocatalyst and a hydrogen sulfide trap such as NiCr covered with either CeO2 or CeO2-ZrO2; anode made of a hydrogen sulfide resistance electrocatalyst such as NiAl.