Influence of carbonization pretreatment on the gasification reaction characteristics of empty fruit bunch pellets

Abstract

Low-temperature pretreatment methods such as hydrothermal carbonization and dry terrefaction can effectively improve the fuel characteristics of biomass, achieving an efficient biomass utilization. Empty fruit bunch (EFB) was selected as the experimental raw material, and was pretreated by hydrothermal carbonization and dry terrefaction in the temperature range of 180−220 ℃, to study the effect of carbonization pretreatment on the physicochemical properties of biomass. The results showed that both hydrothermal carbonization and dry terrefaction could increase the carbon content and heating value of the biomass, reduce the H/C and O/C, and the quality enhancement effect of hydrothermal carbonization was more obvious at the same reaction temperature. A study on the CO2 gasification reaction characteristics of carbonization product pellets was carried out at 800−950 ℃ using large pellet thermogravimetry, and the experimental results showed that the carbon conversion rate of EFB feedstock pellets was only 30% at 800 ℃ for 60 min, but could be up to 100% at 850−950 ℃, and the peak gasification reaction rate of the pellets increased from 0.007%/min at 800 ℃ to 0.042%/min at 950 ℃. When the gasification temperature was 950 ℃, the gasification reactivity of hydrothermal carbon particles, R0.5 (reactivity corresponding to 50% carbon conversion), decreased from 0.034 min−1 for raw material particles to 0.023 min−1 for HT-180, and decreased to 0.011 min−1 when the hydrothermal temperature was increased from 180 ℃ to 220 ℃; the R0.5 of 200 ℃ terrefaction particles was the highest at 0.039 min−1. The analysis of the surface morphology and specific surface area of the granular gasification char at different gasification stages at 950 ℃ proved that the pellets formed by the melting of ash during the gasification process of EFB feedstock and terrefaction pellets adhered to their surfaces, and the pellet pores were clogged in the process of increasing the carbon conversion from 50% to 80%, and the specific surface areas were reduced by 502.40 m2/g and 452.44 m2/g respectively. During the process of increasing carbon conversion from 80% to 98%, the ash pellets gradually fused and coated the surface of the gasification char, resulting in the specific surface area of the pellets to reduce to about 1 m2/g, and the R0.98 (reactivity corresponding to 98% carbon conversion) to reduce to 0.017 min−1 and 0.018 min−1, respectively. The specific surface area of hydrothermal carbon pellets increased by 126.98 m2/g during the increase of carbon conversion from 80% to 98%, which was attributed to the fact that the removal of AAEMs by hydrothermal carbonization alleviated the slagging phenomenon during the gasification of pellets, resulting in no significant change in the reactivity of the pellets for gasification.

Keywords

• biomass
• hydrothermal carbonization
• dry terrefaction
• pellets gasification
• ash melting