• Energy Research

This study examines the impact of biomass drying on the overall efficiency of a Biomass-to-Liquids (BTL) process co-producing Fischer-Tropsch (FT) syncrude and heat. The proposed BTL process is based on steam-blown dual fluidised-bed gasification of biomass, simplified gas clean-up and FT synthesis. Biomass drying was found to have a significant effect on both the FT product yield and the overall thermal efficiency of the BTL plant. In the considered concept, FT off-gases are used as supplementary feedstock in the oxidiser, alongside with char, to satisfy the heat demand in gasification. Drying of the biomass feedstock leads to reduced need of water evaporation during gasification. This in turn reduces the demand for off-gases in the oxidiser, and allows to produce more syncrude (and less off-gases) in the FT unit. The simulations indicated that more extensive drying could result in a significant increase in FT syncrude yield: the thermal efficiency of FT syncryde production increased from 47.7% to 54.6% as the moisture mass fraction of the biomass feed was reduced from 30% to 8%. The highest overall thermal efficiencies of 85–88% were obtained with a two-stage drying concept that combines a belt dryer and an indirectly-heated steam dryer.

Large-scale systems suitable for the production of synthetic natural gas (SNG), methanol or gasoline (MTG) are examined using a self-consistent design, simulation and cost analysis framework. Three basic production routes are considered: (1) production from biomass via gasification; (2) from carbon dioxide and electricity via water electrolysis; (3) from biomass and electricity via hybrid process combining elements from routes (1) and (2). Process designs are developed based on technologies that are either commercially available or successfully demonstrated at precommercial scale. The prospective economics of future facilities coproducing fuels and district heat are evaluated from the perspective of a synthetic fuel producer. The levelised production costs range from 18-37 euros/GJ for natural gas, 21-40 euros/GJ for methanol and 23-48 euros/GJ for gasoline, depending on the production route. For a given end-product, the lowest costs are associated with thermochemical plant configurations, followed by hybrid and electrochemical plants.

A model for pressurised steam/O2-blown fluidised-bed gasification of biomass with catalytic reforming of hydrocarbons and tars was developed using Aspen Plus simulation software. Seven main blocks were used to model the fluidised-bed gasifier and two for the catalytic reformer. Modelling blocks were complemented with FORTRAN subroutines to simulate the observed non-equilibrium behaviour of the process. The model was fitted with experimental data derived from a 0.5 MW scale test rig operated with crushed wood pellets and forest residues and was shown to be capable of predicting product gas composition from gasification of clean wood. A parametric analysis indicated that a significant improvement in the syngas efficiency could be achieved by rising the filtration temperature and reformer conversions. Other improvement possibilities include fuel drying and lower reforming temperature.

A process model for pressurised fluidized-bed gasification of biomass was developed using Aspen Plus simulation software. Eight main blocks were used to model the fluidized-bed gasifier, complemented with FORTRAN subroutines nested in the programme to simulate hydrocarbon and NH(3) formation as well as carbon conversion. The model was validated with experimental data derived from a PDU-scale test rig operated with various types of biomass. The model was shown to be suitable for simulating the gasification of pine sawdust, pine and eucalyptus chips as well as forest residues, but not for pine bark or wheat straw.

We develop a framework for comparing carbon-neutral synthetic fuels (CNSFs) with battery electric vehicles (BEVs) as alternatives to reducing CO2 emissions from light-duty vehicles. CNSFs can be divided into fuels produced from biomass via gasification and electrofuels produced from CO2 and water using electricity. We develop CNSF cost estimates for first-of-a-kind plants operating at commercial scale. Although already competitive over short distances, we find that longer-range BEVs are likely to remain more expensive than CNSFs even if low (∼$125/kWh) battery costs are achieved, and all three options would require carbon prices in excess of $130/tCO2 or oil prices in excess of $100/bbl to become commercially viable relative to petroleum. The viability of electrofuels ultimately depends on access to low-cost, ultra-low-carbon power systems or sources of zero-carbon electricity with high annual availability. Priorities should include deploying a portfolio of CNSF technologies to help appraise decarbonization pathways, economies of scale, and learning by doing.

Dedicated biomass gasification technologies are presently being developed in many countries for the production of second-generation liquid biofuels. Both fluidised-bed gasification and special entrained flow systems are under intensive development. These technologies can also be used for hydrogen production, which may become an interesting alternative in replacing part of fossil fuel input in oil refineries and chemical industries. In addition, fuel cell technology is being developed for hydrogen-rich gases. New and revolutionary production methods, capable of replacing the classical process routes, can not however be foreseen to emerge in the medium-term. Also the new hydrogen separation technologies, presently under development, seem to have only limited potential to reduce the production cost of hydrogen compared to commercially available technology. However, with rising prices of fossil fuels and locally depleting natural gas reserves, gasification route is likely to gain more ground as a credible production technology for hydrogen. The global needs to cut down the CO2 emissions can also make gasification of biomass an interesting possibility. Several biomass gasification processes are presently at demonstration phase, mostly aimed for the production of liquid transportation fuels. If and when this technology will be commercialized, it could easily be adopted to the production of hydrogen.