Design, modelling and construction of a scalable dual fluidised bed reactor for the pyrolysis of biomass

Abstract

The pyrolysis of biomass is a thermochemical process in which woody biomass is converted to several high-value products such as bio-oil, bio-char and syngas. The forestry sector has shown particular interest in this process as a large quantity of biomass is produced as an underutilised by-product in this sector annually. Dual fluidised beds (DFBs) have been identified as a feasible reactor system for this process. However, little attention has been given to the optimisation or to the design of a scalable DFB for the pyrolysis of biomass process. Therefore, the objective of the current project was the design, modelling and construction of a scalable dual fluidised bed system for the pyrolysis of biomass. In order to achieve this objective, several tasks were performed, which included the following: <ul> <li> A literature study was done in order to obtain a theoretical foundation for the current project.</li> <li> A novel dual fluidised bed reactor system was designed, which included the block flow diagram and the process and instrumentation diagram for the system.</li> <li> A cold unit of the system was built in order to test the performance of the system.</li> <li> A comprehensive model for the system was developed, which included mass and energy balance considerations, hydrodynamics and reaction kinetics.</li> <li> A complete pilot-scale system of the proposed design was built and tested at the University of Pretoria.</li></ul> Solids are heated by means of combustion reactions in one of the fluidised beds in the proposed dual fluidised bed design. An overflow standpipe is then used to transport the solids to a second fluidised bed in order to provide the energy required for the endothermic pyrolysis reactions. The cooler solids are then fed back to the combustion fluidised bed by means of a screw-conveyor, creating a circulating system. A two-stage model was used to model the pyrolysis reactions. In this model, the wood is converted to bio-char, syngas and tar compounds. The tar compounds are the desired product as they can be condensed to form liquid bio-oil. However, these compounds undergo a second reaction in the gas phase in which they are converted to bio-char and syngas. It is therefore necessary to quench these gases rapidly in order to maximise the yield of bio-oil obtained from the system. Bio-oil is a source of many high-value chemicals and can also be upgraded to produce liquid bio-fuels. A portion of the syngas is recycled back to the pyrolysis fluidised bed in order to fluidise the bed. In this way, oxygen is prevented from entering the pyrolysis fluidised bed, which would cause the biomass in the bed to undergo combustion rather than pyrolysis. The operating temperatures of the combustion and pyrolysis fluidised beds were optimised at 900°C and 500°C respectively. A cold unit of the system was built at the Agricultural Research Service in Wyndmoor, Pennsylvania, USA. From the experiments performed on this unit it was found that the solid transport mechanism designed during the project is suitable for the pyrolysis of biomass process. In addition, the solids circulation rate between the two beds was easy to control, which is necessary in order to maximise the yield of bio-oil obtained from the system. A pilot-scale unit of the dual fluidised bed design was built in order to finalise the design and ensure that it could be scaled up. This system included all the downstream units, which had to be designed for the dual fluidised bed system. Several cold-run experiments were also performed on the pilot-scale system in order to ensure that it would perform as required during operation. It was found that the combustion fluidised bed could be fluidised as required and that the circulation of solids between the combustion and pyrolysis fluidised beds functioned well and could be easily controlled. Therefore, it was concluded that the proposed dual fluidised bed system is suitable for the pyrolysis of biomass process and is a feasible reactor system for the large-scale pyrolysis of biomass. The large-scale operation of the proposed dual fluidised bed system offers several advantages, particularly within the forestry sector. These advantages have important implications, as follows: <ul> <li> The current research offers the opportunity for the forestry sector to shift its focus from the production of traditional wood products, such as pulp and paper, to products such as specialised chemicals.</li> <li> The bio-oil produced in the dual fluidised bed system can be upgraded to renewable liquid fuels, which may help reduce the dependence of the infrastructure on fossil fuels.</li> <li> The dual fluidised bed system provides an opportunity for capturing and removing CO2 from the atmosphere in the form of bio-char. It is therefore considered to be a carbon-negative process, and may help reduce the concentration of greenhouse gases.</li> <li> The bio-char produced in the dual fluidised bed system can be used to feed nutrients back to plantation floors in the forestry sector, thereby aiding the growth of further plantations.</li></ul> Copyright