CO2 as a substitute of steam or inert transport gas in a fluidised bed for biomass gasification

• Substitution of steam and inert transport gas by CO2 in a fluidised bed was investigated.
• Woody biomass gasification tests were performed at 850 °C.
• Substitution of steam by CO2 does not influence the process efficiency.
• It can be used to adjust the H2/CO ratio to the synthesis requirement for DME.
• If external H2 is used to adjust H2/CO, the impact on CO2 net emission is positive.

The objective of the present study is to investigate the influence of CO2 totally or partly replacing steam in a fluidised bed in conditions close to those of a gasifier of a dual fluidised bed. The influence of CO2 substituting inert gas in the biomass feeder is also investigated. All experiments are performed at 850 °C and 1.5 bara in a bubbling fluidized bed fed with 3 kg/h woody biomass. For constant biomass feeding rate and H2O + CO2 volume flowrate, the substitution of H2O by CO2 – from 0% to 100% – leads to modification of the gas composition mainly in relation with the water–gas shift reaction. The hydrocarbon and tar yields are not significantly influenced by this substitution. The net conversion of carbon into gas and the cold gas efficiency are neither influenced by the replacement of H2O by CO2. The H2/CO volume ratio decreases from 1.8 with 100% H2O to 0.5 with 100% CO2. The substitution of inert gas by CO2 in the feeding line induces an increase of total oxidant input in the gasifier. As (CO2 + H2O) input increases, carbon conversion into gas as well as cold gas efficiency both increase. These results show that substitution of H2O by CO2 does not have any interest in a standard biomass to fuel process, except from an adjustment of H2/CO ratio for a synthesis in which a low ratio is required, such as the direct DME synthesis. However, with an H2/CO ratio adjustment by addition of external H2, the substitution of H2O by CO2 has a positive influence on the biofuel yield and on the CO2 net emission for the global process.