Syngas production via Biomass Chemical Looping Gasification (BCLG) in a 50 kWth unit using ilmenite as oxygen carrier

The use of liquid biofuels is considered one of the most relevant ways to achieve the reduction of greenhouse gas (GHG) emissions in the sector of transport, and more specifically in aviation. A possible route is the use of biogenic waste from the agriculture and forestry sector to obtain synthesis gas via gasification and then the Fischer-Tropsch synthesis to obtain liquid biofuels. Among the different technologies available for the gasification process, Biomass Chemical Looping Gasification (BCLG) represents an innovative process that allows the generation of non-nitrogen diluted synthesis gas under autothermic conditions and with low tars content. By using carbon-neutral biomass as fuel, even net negative emissions could be achieved if the BCLG process is combined with CO2 storage. This work presents an experimental campaign of 40 hours of continuous operation in a 50 kWth BCLG unit developed inside the EU H2020 CLARA Project. Wheat straw pellets with additives were used as fuel feedstock and ilmenite as solid oxygen carrier. This work aimed to investigate the effect of different operational variables such as the fuel reactor temperature, the mean residence time of solids in the fuel reactor, the oxygen-to-biomass ratio (controlled by the oxygen fed into the air reactor) and the gas velocity in the carbon stripper, on the process performance and the syngas yield. Unlike previous studies, effective oxygen-to-biomass ratios that consider the right amount of oxygen that reacts with the oxygen carrier both in the fuel and air reactors (λeff,FR and λeff,AR, respectively) were defined The main factor affecting the syngas yield and the cold gas efficiency was λeff,FR, since it represents the amount of lattice oxygen that actually reacts in the fuel reactor during syngas combustion. However, it showed a low impact on the production of light hydrocarbons (CH4 and C2-C3). The temperature and mean residence time in the fuel reactor showed a considerable effect on the char conversion in the fuel reactor, since an increase in those factors caused an increase in the reaction rates or in the contact time between the oxygen carrier and biomass in the fuel reactor, respectively. Finally, the carbon stripper did not show the ability to separate the solid oxygen carrier from the unconverted char entrained from the fuel reactor, as the char still retained its initial pellet form. Therefore, an increase in gas velocity of carbon stripper promoted the char by-pass to the air reactor, increasing both the biomass conversion in the unit and the fraction of carbon emitted in the air reactor.