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 / Dissertation (MEng)--University of Pretoria, 2013. / Chemical Engineering / unrestricted

This thesis describes the development and experimental testing of a 100 kW dual fluidized bed biomass gasifier (also called a Fast Internally Circulating Fluidized Bed (FICFB) biomass gasifier). This steam-blown gasifier is being studied for its suitability within combined heat and power plant systems for the New Zealand forest products industry. This advanced design of gasifier has the ability to generate producer gas with a lower heating value (LHV) of 11.5-13.4 MJ/Nm3, which is two to three times higher than yielded by conventional gasification systems. This is accomplished because the gasification and combustion processes occur in two physically separated reactors. Several modifications to the gasifier were required after it was first constructed in order to achieve stable and reliable operation. Producer gas yields were measured through the use of helium as a tracer gas. A new simultaneous producer gas and tar sampling system was developed, allowing accurate samples to be obtained in a matter of minutes. Experimental testing included a cold testing exercise which provided valuable information on the circulation behaviour of the bed material and char within the gasifier. This helped in achieving stable and reliable operation of the plant. Producer gas yields of 14.6 Nm3/h were recorded with a fuel (radiate pine wood pellets) feed rate of 18.9 kgdry/h. The cold gas efficiency ranged from 16-40 % with limited heat recovery in place, but depended noticeably on the plant operating conditions especially gasification temperature. The amount of polycyclic aromatic hydrocarbon (PAH) tars measured in the producer gas ranged between 0.9-4.7 g/Nm3 with naphthalene and acenapthylene being the most abundant compounds. The moisture content of the producer gas was determined to be 0.9-1.2 g/gdry gas. It was found that a steam to biomass ratio of 0.45-0.7 kg/kgdry was most favourable for generating a 12-13.4 MJ/Nm3 producer gas while limiting the amount of steam generation. Gasification temperatures above 750 °C encouraged higher producer gas yields and higher cold gas efficiencies. The catalytic bed material olivine (forsterite olivine) was found to increase the producer gas yield by approximately 20 % compared to the non-catalytic bed material greywacke. The use of olivine meant higher cold gas efficiencies were achieved for a given wood feed rate.

Production of high-quality syngas from biomass gasification in a dual fluidised bed gasifier (DFBG) has made a significant progress in R&amp;D and Technology demonstration. An S&amp;M scale bio-automotive fuel plant close to the feedstock resources is preferable as biomass feedstock is widely sparse and has relatively low density, low heating value and high moisture content. This requires simple, reliable and cost-effective production of clean and good syngas. Indirect DFBGs, with steam as the gasification agent, produce a syngas of high content H2 and CO with 12-20 MJ/mn3 heating value. The Mid Sweden University (MIUN) gasifier, built for research on synthetic fuel production, is a dual fluidised bed gasifier. Reforming of tars and CH4 (except for methanation application) in the syngas is a major challenge for commercialization of biomass fluidised-bed gasification technology towards automotive fuel production. A good syngas from DFBGs can be obtained by optimised design and operation of the gasifier, by the use of active catalytic bed material and internal reforming. This thesis presents a series of experimental tests with different operation parameters, reforming of tar and CH4 with catalytic bed material and reforming of tar and CH4 with catalytic internal reformer.   The first test was carried out to evaluate the optimal operation and performance of the MIUN gasifier. The test provides basic information for temperature control in the combustor and the gasifier by the bed material circulation rate.    After proven operation and performance of the MIUN gasifier, an experimental study on in-bed material catalytic reforming of tar/CH4 is performed to evaluate the catalytic effects of the olivine and Fe-impregnated olivine (10%wtFe/olivine Catalyst) bed materials, with reference to non-catalytic silica sand operated in the mode of dual fluidised beds (DFB). A comparative experimental test is then carried out with the same operation condition and bed-materials but when the gasifier was operated in the mode of single bubbling fluidised bed (BFB). The behaviour of catalytic and non-catalytic bed materials differs when they are used in the DFB and the BFB. Fe/olivine and olivine in the BFB mode give lower tar and CH4 content together with higher H2+CO concentration, and higher H2/CO ratio, compared to DFB mode. It is hard to show a clear advantage of Fe/olivine over olivine regarding tar/CH4 catalytic reforming.    In order to significantly reduce the tar/CH4 contents, an internal reformer, referred to as the FreeRef reformer, is developed for in-situ catalytic reforming of tar and CH4 using Ni-catalyst in an environment of good gas-solids contact at high temperature.  A study on the internal reformer filled with and without Ni-catalytic pellets was carried out by evaluation of the syngas composition and tar/CH4 content. It can be concluded that the reformer with Ni-catalytic pellets clearly gives a higher H2 content together with lower CH4 and tar contents in the syngas than the reformer without Ni-catalytic pellets. The gravimetric tar content decreases from 25 g/m3 down to 5 g/m3 and the CH4 content from 11% down below 6% in the syngas.   The MIUN gasifier has a unique design suitable for in-bed tar/CH4 catalytic reforming and continuously internal regeneration of the reactive bed material. The novel design in the MIUN gasifier increases the gasification efficiency, suppresses the tar generation and upgrades the syngas composition. / Gasification-based Biorefinery for Mechanical Pulp Mills