Process Integration of Gasification and Electrolysis for Biofuel Production

Abstract

The decarbonization of the transport sector is a major challenge for the transition towards a sustainable economy. Amongst the different measures to lower the impact of road transportation on the environment is the use of biofuels, especially for heavy duty vehicles. For biofuels to be affordable and well integrated, improvements in the production process in terms of energy performance and economic feasibility need to be made.

Consequently, this thesis aims at the techno-economic assessment of the integration of pyrolysis oil gasification with electrolysis and syngas catalytic upgrading. Oxygen, being a co-product of electrolysis, is used as an oxidizer for entrained flow gasification of biomass derived pyrolysis oil. Hydrogen from the electrolyzer is used to enhance the production of biofuel. The process was designed to have a high-temperature solid oxide electrolysis cell (SOEC) and the biofuel was chosen to be compressed natural gas (bio-CNG). The modeling of the process was performed on Aspen Plus software and calculations of parameters for the gasifier and electrolyzer were done using Fortran and Matlab respectively.
As aimed by the integration of electrolysis, the hydrogen produced is added to the syngas to obtain a feed ratio of 3, required for an optimum methane production. The increase in the feed ratio at the level of the gasifier is mainly limited by the oxygen equivalence ratio (OER), considering the extent of combustion reactions and the operating temperature. The amount of steam inputted was found to increase the molar ratio of H2 to CO without significant change in the feed ratio due to the increase in the concentration of CO2. However, steam was still necessary for adjusting the temperature of the gasifier to values that are in accordance with experimental ones in literature. The obtained final product stream of bio-CNG at 250 bar and 15℃ has a purity of 98.63 mol% and 99.15 wt%.

Moreover, by applying heat integration to the process, the resulting surplus heat was 10.71 MW, which was assumed to be used to generate steam that is inputted to the SOEC for producing hydrogen as an additional product. The process was found to have an overall efficiency of 73.8% (LHV basis) and 78.2% (HHV basis). Heat integration increased the process efficiency by 15% (LHV basis) and 23% (HHV basis), by removing to a great extent the need for external heat and by having hydrogen as an energy output added to CNG.
As a complement to the energy performance, the economic feasibility of the process was assessed. The total capital investment was found to be 889.5 Euros/kWinput, the net present value was 6.0342 million Euros and the rate of return on investment 30.02%.