Characterization of Products From High-Pressure Biomass Pyrolysis in an Entrained-Flow Reactor

A major advantage of biomass gasification is that any lignocellulosic material can be converted to syngas and subsequently to biofuels. There are techno-economic advantages of using high pressure in gasifying biomass. In a gasifier, pyrolysis and char gasification steps are known to proceed in series. Thus it becomes important to study the characteristics of pyrolysis products, since these have an effect on the gasification kinetics.  

Pyrolysis experiments on pine and switchgrass (sieved to 180-250 µm) were performed in entrained flow reactor at high temperatures (600-1000 °C) and high pressures (1-20 bar). Heating rates as high as 10⁴ K/s were achieved with solids residence time from 3-10 s. Char, gases and condensable tars were characterized using SEM, N₂ physisorption, GC and MS respectively.

SEM was used to study the influence of pyrolysis temperature and pressure on char morphology. The results show that with increase in pyrolysis temperature and pressure the particles melted and gained more spherical features. At ambient pressures, a significant fraction of particles burst at higher temperatures. At high pressures, pyrolysis gases tend to be trapped in char particles forming distinct gas filled pockets. Char structure and surface area, is strongly impacted by the increased temperature and pressure during pyrolysis, and is likely to affect char gasification activity.

The yields of light gases such as CO, CO₂, H₂, CH₄, C₂H₄ and C₂H₂  were quantified using GC analysis. The condensable organics from pyrolysis were extracted and analyzed using MS. A variety of tar species from phenolics to polynuclear aromatic hydrocarbons (PAHs) were identified and their relative abundance rated as a function of temperature and pressure.

The present study will provide the basis for improved mathematical model for char gasification based on meaningful descriptors.