Characterization of Biomass Derived Chars Using Raman Spectroscopy

A major advantage of biomass gasification is that any lignocellulosic material can be converted to syngas and subsequently to bio-fuels. In a gasifier, pyrolysis and char gasification steps are known to occur in series. Thus it becomes important to study the characteristics of pyrolysis products such as chars, gases and tars, since these have an effect on the char gasification kinetics.  

Pyrolysis of pine, sugarcane bagasse, and switchgrass was performed in a pressurized entrained flow reactor (PEFR) at high temperatures (600-1000 °C) and high pressures (1-20 bar). Heating rates as high as 10⁴ K/s can be achieved with solids residence time from 3-30 s. In order to explain wide variations in their gasification reactivity, the chars were characterized using Raman spectroscopy. 

A change in pyrolysis temperature and pressure results in a change in carbon structures of the char, and therefore its gasification reactivity. The Raman spectroscopy results were fit to the ten band model (each corresponding to a specific char structure).  Of these structures, five were of real interest: graphitic carbon (G band), medium aromatics (D band) and small aromatics (v_r, v_l, and g_r bands). The Raman spectrum was analyzed from 800 to 1800 cm^-1 and the bands were fit to the spectrum, with constraints placed on the bands’ wavelength range.  Chi² was used to determine the wellness of fit.  The intensity of each peak, or area of the band, relates to the prevalence of that specific structure in the char.   How the ratio of band areas changed with pyrolysis conditions provides insight into the evolution of carbon structures.  These results provide a good basis for understanding the observed changes in char composition and relating these to gasification reactivity.