Comparison of gasification performances between raw and torrefied biomasses in an air-blown fluidized-bed gasifier

Abstract

A CFD-DEM model already developed by the authors has been extended to directly compare the gasification performances between raw and torrefied grassy biomasses in an air-blown fluidized-bed gasifier. The bed material is non-calcined olivine which acts as the solid heat carrier. Furthermore, effects of four key operating parameters (i.e., gasification temperature Tr, excess air ratio λ, steam addition, and biomass feed location) are also systematically examined. The results are analyzed both qualitatively and quantitatively by various indices: the fluidization behavior, bed pressure drop, product gas concentration profiles, total gas yield, and carbon conversion (CC). For both raw and torrefied biomasses, increasing Tr can enhance both the total gas yield and CC; rising λ decreases the H2 yield but increases the CO2 yield and CC; the steam addition has a positive influence on the total gas yield and CC and it can also be used to adjust the H2/CO ratio in the product gas; both the total gas yield and CC decrease with raising the height of the biomass feed location. For all cases, the torrefied biomass obtains a lower H2 yield and CC but a higher CO yield than its raw counterpart under the same operating conditions, suggesting that torrefied biomass requires a longer conversion process compared with raw fuel. Moreover, the gasification behavior of torrefied biomass is more dependent on the fuel feed location than raw fuel and such knowledge is important for the optimal design of fluidized-bed gasifier for torrefied feedstock.