Influence of Torrefaction on Biomass Gasification Performance in a High-Temperature Entrained-Flow Reactor

摘要

In the present work, the gasification performances of two pairs of raw and torrefied biomasses, including raw forest residue and torrefied forest residue, and raw spruce and torrefied spruce in a high-temperature entrained-flow reactor, are numerically examined and compared to each other using a Eulerian-Lagrangian CFD model developed based on OpenFOAM. Moreover, the sensitivities of three important operating parameters (excess air ratio, steam/carbon molar ratio, and biomass particle diameter), which vary in the range of practical significance, are also tested. The calculated results are analyzed both qualitatively and quantitatively by five indicators: isothermal profiles, char consumption rate, gas compositions, species yield, and carbon conversion along the reactor length. The obtained results show that torrefied biomass can increase the maximum temperature in the reactor as compared to its raw parent fuel. During gasification, CO, H-2, and CO2 are the major species in the product gas and CH4 accounts for only a very small fraction of the syngas at such a high operating temperature of 1400 degrees C. As expected, the higher the excess air ratio, the lower the H-2 yield and higher CO2 production and carbon conversion; a rise in the steam/carbon molar ratio promotes the H-2 yield but reduces the CO production; and both the H-2 and CO yields and carbon conversion decrease with increasing particle diameter. In addition, three reaction zones can be recognized from the carbon conversion along the reactor length, and such information is useful for optimal reactor design. In all cases, torrefaction consistently reduces both the H-2 production and carbon conversion as compared to its raw biomass under the same operating conditions, which suggests that torrefied biomass requires a bigger length entrained-flow gasifier or longer particle residence time to achieve the same level of conversion as that for its raw biomass.