In order to supply the syngas (CO + H
2) to the liquid fuel synthesis reaction in the biomass-to-liquid (BTL) system, woody biomass gasification was carried out using a mixture of CO
2 and O
2 as a main gasifying agent. Gasification of biomass using a CO
2/O
2 mixture (79/21(v/v)) exhibited a maximum value of the conversion to gas (88 C-mol%), compared to those (78 C-mol%, 80 C-mol%) using pure CO
2 and air, respectively The effects of [H
2O]/[C] ratio and O
2 content on the gasification behavior with CO
2/O
2 were investigated. The H
2/CO ratio in the product gas increased from 0.3 to 1.0 due to the promotion of water-gas shift reaction when the [H
2O]/[C] ratio was increased in the range of 0 - 5. The conversions to gas and tar increased from 88 to 94 C-mol% and from 0.1 to 0.6 C-mol%, respectively; on the other hand, conversion to char decreased from 8.7 to 0.7 C-mol%. The CO
2 and H
2 contents increased from 53 to approximately 58 mol% and from 11 to 19 mol%, respectively; however, the CO content decreased from 32.3 to 18.1 mol%. The conversion to gas increased with the O
2 content (0 - 50 vol%) in the gasifying agent, while the conversions to tar and char decreased from 0.8 to 0.0 C-mol% and from 16 to 4.1 C-mol%, respectively. The CO and H
2 contents increased from 36 to 42 mol% and from 9.5 to 20 mol%, respectively, while the CO
2 content decreased from 49 to 32 mol%. In all the cases, H
2S and COS were detected in the product gas. The conversions to gas on a sulfur basis were approximately 60 S-mol% at all [H
2O]/[C] ratios. However, with an increase in the [H
2O]/[C] ratio, the conversion to H
2S increased from 38 to 46 S-mol%, while that to COS decreased from 25 to 16 S-mol%; this was attributed to the occurrence of the following reaction: COS + H
2O → H
2S + CO
2. With an increase in the O
2 content, the conversion to H
2S increased from 38 to 44 S-mol%, while that to COS decreased from 25 to 18 S-mol%. The addition of O
2 to the gasification with CO
2 promoted the gasification of tar and char to produce H
2S than COS regardless of the oxidation atmosphere. From the results obtained, the BTL process including the woody biomass gasification with a CO
2/O
2 mixture, dry gas cleaning, water-gas shift reaction, removal of CO
2 and liquid fuel synthesis reaction was proposed.
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