On the effect of pellet density on biomass pyrolysis in a pressurized fixed bed reactor

In this work, pellets of different densities but the same particle size were prepared from pine wood. The pellets as well as the raw biomass sample, all in form of cubic blocks whose side was 5 mm, were pyrolysed in a pressurised fixed bed reactor at different temperatures (350 and 500 °C) and pressures (1 and 8 bar) to investigate how variation in biomass density affect the product distribution, tar and gas compositions, and char characteristics. More char and less tar were obtained from pellets of higher density at the same pyrolysis conditions. Chars from the pyrolysis of pellets of higher density contain more oxygen and less amount of crystallite clusters, making them more reactive in combustion tests run by thermogravimetric analysis (TGA). Tars were obtained from higher-density pellets in lower yields and were lighter (smaller molecular weight distribution) than those from less dense pellets. At higher temperatures extensive biomass decomposition forms chars with more crystallite clusters and less oxygen, while a higher char yield with less oxygen was obtained at higher pressures, along with smaller amount of a lighter tar. Higher temperature and pressure tend to produce a less reactive char. Through this systematic work, the similarities as well as some key differences between the effect of higher pressure and greater particle density on pyrolysis behavior have been identified.