The biomass to liquid (BTL) process is the process in which biomass is converted to biofuels through gasification and Fischer Tropsch synthesis. Lignocellulose, such as straw, wood, and agricultural wastes, are the basis for the production. The products from the process are liquid biofuels with practically no sulfur and, in the case of biodiesel, with better ignition properties than diesel from fossil oil. Because the biomass, on a dry basis, has a lower heating value of ca 19 MJ/kg and the Fischer-Tropsch biofuel contains ca 43 MJ/kg, a significant amount of carbon must be exhausted as CO2 unless extra energy is added to the system. Currently, the carbon efficiency of the conversion of biomass to biofuel is approximately 30 %. To increase carbon efficiency, energy must be added to the process resulting in increased fuel production with the same amount of biomass, and as a result, less CO2 is expelled from the process. The idea of this project is to add energy in the form of hydrogen produced by electrolysis of water or steam. By utilizing thermal heat from the BTL process to produce steam, less electrical work is required to split water. In this process heat at both high and low temperatures can be utilized. Hydrogen can be produced efficiently by electrolysis of steam at high temperature with the use of solid oxide electrolysis cells. Reversed electrodialysis can utilize low-temperature energy to produce hydrogen. By splitting water in the electrolysis we are also producing sufficient oxygen for the gasification of biomass. This process concept has the potential of obtaining a carbon efficiency of more than 90 %. The process will be more environmentally sustainable as less biomass will be required for the same amount of fuel and less CO2 will be expelled from the process. An economic analysis shows that the process will be more profitable than a conventional process, but still, the Levelized cost is high.
Betydelig større karbon effektivitet kan oppnås med bruk av hydrogen tilsatt et BtL anlegg. Beregninger viser at den kan økes fra 30 til 90%. Det betyr at vi trenger bare en tredel mengde biomasse for å produsere samme mengde drivstoff når hydrogen fra fornybar kraft tilsettes. Investeringene blir betydelig større med hydrogenproduksjon, men lønnsomheten er blitt bedre fordi vi kan produsere så mye mer drivstoff. Med varmeintegrasjon av høytemperatur dampelektrolyse (SOEC) kan vi redusere kraftbehovet per produsert enhet hydrogen sammenliknet med tradisjonell alkalisk vannelektrolyse. Investeringskostnadene av SOEC er fremdeles høy og det er driftstekniske utfordringer med teknologien. Ved å brenne gassen fra FT syntesen på anoden gir positive effekter ved at termisk energi blir tilført hoved-reaksjonen, fortynning av oksygen, samt at det reversible potensialet blir redusert. Brenngassen kan ha to kjemiske effekter.
At present, entrained flow gasification followed by catalytic Fischer-Tropsch synthesis has been considered as one of the most attractive technological routes for the production of marketable liquid biofuels for the heavy-road road and aviation transport sector. However, the overall carbon conversion efficiency from biomass to biofuels achieved in this route is still low, about 30-40%. The REN-BTL project aims to improve significantly the carbon-efficiency, and thus the economics and the environmental performance, of liquid biofuels production by integrating the production of H2 (+ O2) using the available heat from the biomass-to-biofuels conversion process as well as renewable electricity. Two different technologies for production of hydrogen (+O2) are considered: 1) Solid oxide electrolytic cells (SOEC) using high-temperature steam produced from the gas cooling after gasification and input electricity. and 2) Reversed Electro-dialysis (RED) using low-temperature waste heat from Fischer Tropsch. The hydrogen produced can then be used in the biofuels production route for three different purposes: 1) to increase the H2 to CO ratio convert CO2 to CO (via reversed water gas shifting) after gasification, 2) to improve the Fischer-Tropsch synthesis, and 3) to upgrade the Fischer-Tropsch product to marketable diesel-type and aviation fuels. In addition, the oxygen (O2) produced can be use directly in the gasification replacing the Air Separation Unit. To the acknowledge of the participants in this project, the use of the high-temperature and low-temperature waste heat from the biofuels production route for producing H2 and O2 represent a research novelty. The project will contribute to pave the way to achieve sustainable and low-carbon energy systems through more efficient utilization of biomass resources for production of liquid biofuels and better integration between renewable energy systems.