Technological advances of industrial biomass pyrolysis

Pyrolysis is the thermal process of decomposition of organic material under elevated temperatures and atmospheres of little or no oxygen, producing biogas, bio-oil and char. The growing interest in industrial biomass pyrolysis is the potential for upgrading of the pyrolysis bio-oils to liquid transport fuels, while the solid char, which is rich in carbon, can be used as a renewable solid fuel alternative, specialty carbon product, such as metallurgical carbon or can be blended into soils thereby sequestering carbon and improving soil quality. Industrial biomass pyrolysis processes can operate under fast or slow pyrolysis conditions, depending on the residence time and heating rate of the material in the reactor. Fast pyrolysis is characterised by rapid heating rates that are usually within the range of 1,000 to 10,000 °C/s. On a dry feed basis the yields of liquid oil from fast pyrolysis are between 50 and 80 wt% with gas and char products accounting for the remainder in approximately even proportions. Slow pyrolysis applies gradual heating rates that can be as low as several degrees per minute, where the products may range from being equally distributed amongst the solid, liquid and gas phases to producing solid and gas only. Fast pyrolysis reactor types include bubbling fluidized beds, circulating fluidized beds, rotating cone transported beds and ablative pyrolysers. Slow pyrolysis reactor designs include vacuum pyrolysers, bubbling fluidized beds, augers and heated kilns of various types. At present, most of the reactor designs have been scaled to either pilot or demonstration plant levels. The factors hindering further commercialization of biomass pyrolysis technologies are associated with the inherent limitations of the various reactors. These include raw material feed size and moisture requirements, energy efficiency, scalability restrictions, plant capital intensity and product quality. This chapter reviews and compares the strengths and weaknesses of each reactor design and elucidates the likely pathway for further development of low capital cost, scalable and energy efficient pyrolysis technology.