Introduction
With the aim of recovering waste for the production of alternative fuels, Centro Nacional del Hidrógeno (CNH2), Fundación CIDAUT and the University of Valladolid are participating in a project focused on the development of lignocellulosic biomass oxy-gasification technology.
The process consists of a first pyrolysis stage, heating the biomass by burning part of the recirculated synthesis gas, and a second stage carried out in an EFG (Entrained Flow Gasification) reactor, in which the high operating temperatures achieved by using oxygen as a gasifying agent facilitate the reaction of solids and the decomposition of tars and heavy hydrocarbons, as well as the production of a synthesis gas with a higher calorific value relative to air-based gasification technologies.
This synthesis gas is converted into synthetic fuels, such as petrol or SAF, using a Fischer–Tropsch process. Additional inputs to both reactors are oxygen and hydrogen, respectively. Both gases are obtained from an alkaline electrolyser, achieving total integration between the processes.
This work focuses on pyrolysis and oxy-gasification; therefore, no modelling results for electrolysis or the Fischer–Tropsch process are presented.
Methods
In order to optimise the process and design and size the equipment, a pyrolysis model was developed in 0D and 1D, and a model of the complete process in 1D and 3D, which includes both pyrolysis and gasification of pine wood chips. This approach allows an initial prediction of the behaviour of the system with the use of other types of biomass.
The pyrolysis model was constructed based on the kinetic model proposed by E. Ranzi [1], which details all the reactions involved in the process. It was developed in both Visual Basic for Applications and Python. In both cases, the results were validated using information available in the literature and through thermogravimetric tests.
The 1D model of the complete process, programmed in C++, enabled rapid parametric studies of the EFG to be carried out to determine its optimal dimensions (diameter and length) in accordance with efficiency requirements, mechanical design and maximum permissible dimensions. It also provides the necessary boundary conditions (mass flow, composition and temperature of the gases at the EFG inlet) to be entered into the 3D model.
The 3D model, created in ANSYS Fluent, which is more computationally expensive, provides detailed information on the behaviour of the fluid inside the reactor. A reactor mesh was generated, with the dimensions considered optimal for the EFG through parametric study, and the boundary conditions obtained at the EFG inlet of the 1D model are introduced so that the two models are similar.
The main methodological contribution of this work lies in the development of a coupled multiscale modelling framework that integrates detailed kinetic pyrolysis modelling with rapid 1D reactor optimisation and CFD-consistent 3D validation for entrained-flow gasifier design within an integrated process for the production of synthetic fuels from biomass.
Results
Simulations were carried out using the pyrolysis model, which enabled the endothermic nature of the process to be analysed through the evolution of temperature in an adiabatic system. In addition, an estimated 50% tar and 12% char production was calculated, as well as the calorific power required for the pyrolyser.
The results obtained from the simulations of the complete 1D and 3D models carried out with the optimal design geometry and a feed rate of 50 kg/h of pine wood chips have been compared, obtaining a flow rate of 81.11 kg/h of synthesis gas with a composition (dry basis) of 59.14% CO, 27.92% H2, 10.52% CO2 and 2.41% CH4, corresponding to a lower heating value (LHV) of approximately 11.3 MJ/Nm3.
Conclusion
The proposed framework supports the design and pilot-scale integration of an oxy-gasification unit coupled with downstream Fischer–Tropsch synthesis and on-site electrolysis.
Regarding the estimation of the heat output required in the pyrolyser, the results obtained are very close to those published by other researchers in the same field [2]. The results obtained from the 1D and 3D models allow the EFG and pyrolyser to be dimensioned, the recirculated gas and oxygen flow rates to be added at the different stages to be defined, and the size of the carbon particles entering the reactor to be determined, thereby achieving an acceptable degree of solid-to-gas conversion.
References
[1] Ranzi, E. et al.: “Chemical kinetics of biomass pyrolysis,” Energy and Fuels, vol. 22, no. 6, pp. 4292–4300, Nov. 2008,
[2] Daugaard, D. E. and Brown, R. C.: “Enthalpy for pyrolysis for several types of biomass,” Energy and Fuels, vol. 17, no. 4, pp. 934–939, Jul. 2003,
