Process Simulation Development of Fast Pyrolysis of Wood Using Aspen Plus

A steady-state Aspen Plus simulation model has been developed that provides estimated mass and energy balances for an industrial fluidizing-bed fast pyrolysis process to produce bio-oil. The tool can be used to assess plant performance under varying process conditions using different feedstocks. A 30 MW lower heating value (LHV) bio-oil plant was modeled utilizing two different feedstock types (pine and forest residues). The fast pyrolysis product yields are functions of feedstock ash content and were calculated on the basis of data generated by a 0.5 t/d fast pyrolysis test unit. The UNIQUAC activity coefficient method was used for the calculation of the liquid phase, and the ideal-gas fugacity coefficient method was used for the vapor-phase calculations. Modeling of the condensation of fast pyrolysis vapors was also verified against experimental data gained from the 0.5 t/d test unit. Production costs were estimated for the two concepts. The results show that the pine-based fast pyrolysis process has better process efficiency and lower production costs compared with the forest-residue-based process. The total estimated capital investment costs including plant fixed capital investment (FCI), startup, working capital, and interest over construction period were estimated to be 24 and 28 M epsilon for the pine- and forest-residue-based processes, respectively. Sensitivity analyses showed that the bio-oil quality and bio-oil production efficiency can be improved by drying the recycle gas. Varying the production cost parameters within an industrially relevant range resulted in a production cost of bio-oil between 50 and 70 epsilon/MWh. However, unless the wood price is lower than current market price (20 epsilon/MWh assumed here) or excess heat may be valued higher than the fuel price, production is not currently competitive compared with fossil alternatives.