Construction and Commissioning of a Novel Dual Fluidised Bed Reactor for Biomass Fast Pyrolysis

Abstract

The CRIPS 2 dual fluidised bed pyrolysis unit at the University of Pretoria was successfully constructed and subsequently commissioned through eight experimental runs. The scope of the investigation involved determining the mass and energy balances around the unit, the thermal efficiency during operation, and the pyrolysis capabilities.

The lignocellulosic biomass chosen for the experimental runs was Eucalyptus grandis wood chips with an average diameter of 448 to 489 μm. The products generated from the pyrolysis of lignocellulosic biomass are bio-oil, biochar, and non-condensable gases (NCG). The optimal experimental run involved the highest feed rate of biomass (5 kg/h) with a reduced NCG recycle flow-rate (140 l/ min) to produce a bio-oil with an HHV of 7.29 MJ/kg. The lower HHV and higher water content in the bio-oil was caused by oxygen transfer between the combustion bed and the pyrolysis unit. Nevertheless, the unit is very capable of processing higher biomass throughputs than the previous generation CRIPS unit. Residues found in the condensation unit and the bio-oil collection container were found to have an HHV of 22.11 MJ/kg, therefore confirming the pyrolysis potential of the unit. The biochar produced from the optimal run conditions had an HHV of 26.54 MJ/kg which can find a use in various heating applications. The BET surface area results for the collected biochar were very low. However, the wood feedstock was oversized to allow it to remain in the unit to produce additional combustion energy. As a result, the surface areas are not representative of the total biochar produced.

Mass balances were performed with an error of under one percent and confirmed that pyrolysis was taking place. However, energy balances indicated that 35 % of the heat was unaccounted for. This discrepancy was discovered due to thermal soaking of the unit, whereby the large thermal mass was still consuming the heat during operations and was not at a thermal-steady state. Nevertheless, analysis of the thermal conservation abilities could still be calculated, indicating that the operation of the unit was very close to its design values. The insulation and integrated combustion unit reduced the radial heat losses by 82 % during the experimental runs and by 40 % (theoretical) at full design capacity compared to the CRIPS 1 unit. The integrated APH (air preheater) also reduced the LPG flow rate requirements by 49 % and operated very close to the design values.

The recommendations for future work include various operational changes and unit modifications:

• Reduce the oxygen transfer through operation and unit modifications. • Change the biomass screw conveyor delivery arrangement. • Add cyclones in series for better biochar recovery. • Increase the cyclone capacity. • Add pilot burner and flame detection unit. • Add a separate distribution manifold for recycled NCG. • Provide additional insulation for the top of the unit.