Mechanical-press torrefaction of Napier grass for bio-coke production: effects of temperature and pressure on densification, fuel properties, and CO2 gasification behavior

Abstract

This study investigated a mechanical-press torrefaction-assisted carbonization route for producing Napier grass-derived bio-coke. Napier grass was first torrefied under mechanical pressure at 280–340 °C and 20–70 MPa, followed by carbonization at 1000 °C. The effects of torrefaction temperature and pressure on the properties of the torrefied products and resulting bio-cokes were evaluated. Mechanical-press torrefaction densified the biomass and enabled the formation of consolidated bio-cokes after carbonization. Although the carbonization yield increased on a torrefied-precursor basis, severe pretreatment reduced the overall bio-coke recovery from the original biomass. On a whole-sample dry basis, the carbon contents and HHVs of the pretreated bio-cokes ranged from 69.20 to 72.65 wt% and from 25.97 to 27.30 MJ kg −1 , respectively, compared with 70.06 wt% and 26.48 MJ kg −1 for BC-Raw NP. BC-TNP-340-45 exhibited the highest carbon content, HHV, and apparent density of 1.03 g cm −3 , whereas the highest compressive strength of 41.7 MPa was obtained for BC-TNP-320-70. Raman analysis showed a slight decrease in the I D /I G ratio from 1.158 for BC-Raw NP to 1.037 for BC-TNP-340-45, indicating a modest change in carbon structure. CO 2 -TGA-DTG-DSC analysis showed that pretreatment shifted the principal CO 2 gasification region toward higher temperatures, with T max increasing from 881.34 to 970.26 °C. A preliminary Fe 2 O 3 reduction test provided qualitative evidence of metallic Fe formation. Overall, mechanical-press torrefaction effectively modified and densified Napier grass before carbonization, producing bio-cokes with improved mechanical integrity and higher characteristic CO 2 gasification temperatures, while also upgrading fuel characteristics under selected pretreatment conditions.