Energy efficiency in dual fluidised bed fast pyrolysis

Abstract

The Combustion Reduction Integrated Pyrolysis System (CRIPS) is a dual fluidised bed fast pyrolyser that was developed at the University of Pretoria for the conversion of biomass waste to biofuels. The dual fluidised bed design allows in situ catalytic upgrading of bio-oil, by providing the conditions required for the regeneration and decoking of catalysts. The first version of the CRIPS process (CRIPS 1) emphasised the need for an energy balance approach to model the pyrolysis process rather than a mass balance. CRIPS 1 experienced severe energy losses and as a result very poor performance was observed. The energy balance was set up in the enthalpy reference level since no shaft work was produced and the entire process was operated under constant atmospheric conditions. The enthalpy balance approach was set up to analyse the process performance and energy efficiencies of a CRIPS process and possibly the bio-oil energy content and yield that could be expected from such a process. The approach was used to derive the bio-oil properties and energy efficiencies for a number of scenarios based on the CRIPS process. The Higher Heating Value (HHV) of the bio-oil was derived using the total energy balance of the CRIPS process. The validity of the approach was confirmed by comparing the derived bio-oil HHV from CRIPS 1 of 14,2 MJ/kg with that of similar processes, in the range of 17-23 MJ/kg, as well as comparison to the operating data and process yields. The enthalpy balance approach was able to accurately model the operation of CRIPS 1 using energy and mass balances and therefore the approach was used in the design of CRIPS 2 to limit heat losses and improve the process efficiency by recovering heat from the exhaust of the combustor. The heat recovery resulted in significant improvements in the efficiency of CRIPS 2 (74%) compared to CRIPS 1 (33%). The final design of the CRIPS 2 process featured an annular design in which the combustion bed is located in a refractory cylinder, with the pyrolysis bed around the refractory. The design allowed for the addition of a heat exchanger inside the combustor which is responsible for the increase in efficiency.