Techno-economic analysis of a Fischer-Tropsch fuels and waxes production plant with digestate gasification

A simulation on Aspen Plus for the synthesis of synthetic fuels and waxes is carried out. A 3MWth dual fluidized bed gasifier (DFBG) is employed to produce a syngas and it is fed with digestate or pine wood: the first biomass is the real investigation of the work while the latter is used as a reference. The mass flow of steam used in the gasification process is varied to keep the H2/CO ratio around 2. The syngas is cleaned removing sulfur compounds, ammonia and chlorine by wet scrubbers, the CO2 is removed by a pressure swing adsorption (PSA). The Fischer-Tropsch synthesis process (FTS) is simulated adopting the Anderson-Schulz-Flory model (ASF) with a chain-growth probability as a function of the temperature and H2/CO ratio. The FT products are separated into off-gas and LPG, naphtha, middle distillate and waxes. Three different scenarios are investigated: in two of these, the off-gas and LPG are burnt in a gas turbine (GT) to produce electric energy; in the third, they are recirculated back to the gasifier in order to increase the production. 105-168 kg of syncrude are produced every ton of dry digestate while 144-225 kg using pine wood. The maximum efficiencies found are 54.5% and 51.3% respectively. The minimum levelized cost of syncrude (LCOS) and waxes (LCOW) are 1.71€/kg and 3.38€/kg respectively in case of using digestate and 1.96€/kg and 4.22€/kg using pine wood.