Synthesis and thermo-economic design optimization of wood-gasifier-SOFC systems for small scale applications

The conceptual design of a biomass integrated gasification fuel cell system for small scale applications (40 kg h−1 woody biomass input with 50% mass fraction water content) is discussed in this work. Two different biomass gasifiers (circulating fluidized bed and downdraft), two different reformers, a solid oxide fuel cell, a gas turbine and a heat recovery steam cycle were investigated. A two-step optimization procedure was used to perform thermo-economic design optimizations of nine system configurations generated by combining different technologies. At the master level an evolutionary algorithm is used to optimize the system intensive parameters following the minimization of the system costs and the maximization of the net power production simultaneously (two-objective). At the inner level, system mass flow rates are optimized by linear programming subject to the thermal balance and to the heat transfer feasibility constraints which were formulated by means of Pinch Analysis techniques. The degree of system internal heat recovery was studied by including the minimum temperature difference between hot and cold streams as a decision variable at the master optimization level. Optimization results are shown by means of an optimal Pareto front for each configuration. The degree of system internal heat recovery of some specific solutions is discussed by means of Pinch Analysis composite curves. The study shows that very high system efficiencies can be obtained but only at the expense of really high system costs mainly because of the high costs of the fuel cell and of the gasifier especially at the small scale level considered here. Minimum specific plant costs of the most cost-effective configuration, based on a hybrid cycle, greater than 7000 $ kW−1 (2010 dollars) are found. The indirect circulating fluidized bed gasifier appears the most promising choice both in terms of cost and of system performance since it allows for better thermal integration at high temperatures and greater hydrogen yields. Auto-thermal reforming is a cheaper solution compared to steam reforming but does not benefit of the system internal heat recovery thus leading to comparably lower system efficiency. Steam reforming is particularly convenient when the system is pressurized and extra power can be recovered by gas expansion since a great amount of steam can be injected prior the reformer and vaporized by recovering the heat from exhaust gases