A Comparative Analysis of Heat Recovery and Product Stabilization in Fluid-Bed and Ablative Pyrolysis Systems

Fast pyrolysis is a key process for converting renewable biomass residues into char, bio-oil and off gases. Rapid biomass heating and rapid vapour quenching have been the essence of fast pyrolysis in order to maximize the... [ view full abstract ]

Fast pyrolysis is a key process for converting renewable biomass residues into char, bio-oil and off gases. Rapid biomass heating and rapid vapour quenching have been the essence of fast pyrolysis in order to maximize the yield of bio-oil, but often sacrificing the quality of the bio-oil. Nearly all existing commercial pyrolysis technologies employ single-step rapid condensation of vapours from 500 ^oC to 50 ^oC using sprays of cold bio-oil or liquid hydrocarbon as a quench fluid. While single step rapid quench helps to maximize the yield of liquid product, the raw bio-oil product is a non-homogenous mixture of hundreds of oxygenated organic compounds including organic acids, and it has a significant fraction of water. One-step quench also results in high quality heat being lost to the surroundings (primarily cold water utility or ambient air) with no possibility to utilize this energy for the high-temperature heating requirements in the upstream process.

A novel 3-stage fractional condensation approach has been investigated.  The intent is to produce targeted stable products for value added applications as well to enhance the overall efficiency of pyrolysis processes. The first phase of this research involved modelling and simulation of staged condensation of pyrolysis vapours using Pro/2 process software.  A comprehensive pyrolysis model with 13 representative compounds was developed and validated with NREL’s 2013 model built upon Aspen Plus. The Pro/2 model is able to simulate complex condensation of lignin and sugar fractions at high temperatures.

Multiple cases involving staged condensation in ablative and fluid bed pyrolysis systems were investigated. The two systems represented a low and high ratio of carrier gas to biomas, respectively. In each case, there was a trade-off between high-quality heat recovery and early separation of lignin and sugars from organic acids. Results demonstrated that judicious selection of condenser temperatures offers opportunity for early isolation of sugars and lignins from acids, thereby improving product stability. Given the high ratio of fluidization gas to biomass in fluid bed pyrolysis, dew point depression adds additional complexity and limits heat recovery.