Introduction of advanced technology (solid oxide fuel cell) in the sugar cane industry : technical and sustainability analysis

In the present study, the feasibilities and potential of SOFC integration into a sugar-ethanol factory as a sustainable energy scenario future for Cuba have been evaluated by means of technical and sustainability criteria. Firstly, Chapter 1 reviews the state-of-the-art of the Cuban energy context, with emphasis on sugarcane as the most valuable renewable energy resource. The advantages of ethanol conversion to hydrogen by means of steam reforming, and the use of hydrogen in fuel cell (SOFC) to produce electricity is also discussed. It is concluded that the integration of SOFC technology into a sugar industry could increase the net electricity production, the efficiency and sustainability of the existing sugar factories, but also reducing the fossil fuel consumption as well as the pollutants released to the environment. Finally, an overview of actual available environmental sustainability assessment methods such as exergy analysis, Life Cycle Assessment and environmental external costs and its potential are giving in this chapter. In Chapter 2, a solid oxide fuel cell (SOFC) system using bioethanol as feedstock is evaluated considering the first and second law of thermodynamics. The power plant is integrated by the following stages: a vaporizer, bioethanol steam reformer, SOFC, post-combustor and a heat exchanger network. A novel thermodynamic model for the evaluation of the SOFC is provided and a kinetic model is used to assess the conversion of methane within the cell anode which is fed with the reformate gas produced in a bioethanol steam reforming unit. A detailed model of all components of the plant is introduced, with special attention to the kinetic pattern of the bioethanol steam reforming. On the base of this model, the effect of the fuel cell and the reforming operational parameters (reactor temperature (823<TESR<973K), reactants ratio (5<RAE<6.5) and SOFC fuel utilization coefficient (0.7<Uf<0.9)) on the process is studied, in terms of the performance (efficiency and exergy destruction) of the overall system. Also the distribution of irreversibilities generated in each device and the whole process is reported with exergy and energy efficiencies. The result shows an exergy efficiency of more than 30% for the SOFC power plant. Moreover, the system operates autonomously; no extra fuels (ethanol, fossil fuel, etc) are needed to supply energy for the global process. In a third part, Chapter 3, the SOFC technology evaluated previously is integrated into a sugar-bioethanol factory, which is evaluated by a Life Cycle Assessment approach (LCA) and exergy analysis to assess the environmental impacts and the exergy efficiency of the system. The sugarcane is the primary feedstock and sugar, bioethanol and electricity are the main products of the system, where the functional unit is defined as 9.86 t/h of sugar, 2.195 t/h of hydrated bioethanol (96 %w/w) and 847 kWh of electricity based upon an installed technology. Two technological schemes are considered for sugar, bioethanol and heat and power production from sugar cane. The first is the traditional integration of sugar and bioethanol production processes including cogeneration with bagasse (existing scenario). The second scenario includes the Solid Oxide Fuel Cell (SOFC) technology (future scenario) using an intermediate stream from the bioethanol production process, which would substitute the diesel combustion engines in a future energy scenario. Changes in the reformer temperature and the water-bioethanol fed molar ratio affect the efficiency of the reformer and SOFC power plant and hence its environmental performance as well, so that the effect of these operational variables on the efficiency and environmental profile of the products (sugar, ethanol and electricity) are determined. The environmental impact (greenhouse gases and air pollution), exergy efficiency, irreversibility and a renewability parameter are considered as indicators for the comparative assessment with conventional sugar, bioethanol and electricity production technologies. The integration SOFC scenario shows significant advantages from an environmental point of view, obtaining a reduction of 52 % greenhouse emissions and 60% fossil fuel consumption. At the same time, the overall process efficiency (38%) and renewability index (0.93) are higher than for the traditional sugar-ethanol process. As was discussed in chapter 3, the exhaust gases from bagasse and fossil fuel combustion are the main sources of atmospheric pollution for the traditional and future scenario. The necessity of finding out a balance between the costs of achieving a lower level of environmental and health impact and the benefits of providing electricity at a reasonable cost have lead to the process of estimating the external costs derived from these impacts, which are not included in the electricity prices. In Chapter 4, the external costs of the electricity generation integrating SOFC technology to sugar-bioethanol industry (future scenario) are compared with the traditional sugar-bioethanol factory (existing scenario). The Uniform World Model (UWM) has been used to estimate the health impacts. The impact of mortality is quantified in terms of the reduction in life expectancy, expressed as cumulative Years of Life Lost (YOLL) for the population at risk. Chronic bronchitis, emergency room visits, asthma attacks and medication use in adult and children, and emergency hospital admissions for respiratory causes are included under the morbidity effect by air pollution, which are considered in the present dissertation. This study considers primary (PM10, SO2 and NOx) and secondary pollutants (sulfate and nitrate aerosols). Monetary values of damage costs related to human health per kWh of generated electricity are determined, as well as its influence on the electricity production cost. The performance of the cumulative cash position profile (106 US$) and the payback period (years) are analyzed varying the cost per kW installed fuel cells ($400 < FC< $1500/kW forecasts published elsewhere) and the market price of electricity (EP) from 0.10 to 0.60 US$ cent/kW. The results of the external costs show that the use of a SOFC technology involves a reduction of health impacts by 25.76 YOLL yr-1 (12 %) and external costs by 52175 US$ yr-1 (12 %). The internalization of this external cost would increase the production cost and the electricity price by more than 30 and 10 %, respectively. The implementation of SOFC technology into sugar-ethanol factories has a net cost-benefit of 2.6*105 US$/year. Chapter 5 includes the general discussion, conclusions and perspectives. Based on a SWOT analysis the strengths, weaknesses, opportunities and threats of implementation of SOFC power plant into sugar-ethanol factories are discussed