3.2.1 Cold flow bubbling fluidized bed
The cold flow bubbling fluidized bed (CBFB) model was used to study the fluidization characteristics in the fluidized bed under different flow conditions. This setup was easy to control, and due to the cold operating environment, it was possible to add agglomerates and thus register how the agglomerated particles affected the important operating bed parameters, i.e. the minimum fluidization velocity (umf) and the bed pressure drop (Δp/L).
Figure 3-2 shows the CBFB system. The column is constructed with a transparent material and has a diameter of 8.4 cm and a height of 140 cm. The gasifying medium is compressed air at ambient temperature. The air flows into the column through a porous plate, which ensures even air distribution throughout the bed. Sierra mass-flow controllers are used to accurately adjust the airflow. Nine pressure transducers along the height of the column are constantly monitoring the pressure drop across the bed.
The first pressure transducer is located 3.5 cm below the gas distributor and the second transducer is located 6.5 cm above the gas distributor. The distance between the each of the pressure transducers is 10 cm. The pressure transducers are connected to the LabVIEW software for data acquisition. The top of the column is open to the atmosphere.
The minimum fluidization velocity for the bed material in each experiment was determined based on the measured bed pressure drop at the selected superficial air velocities. The results were used for validation of CPFD models, which can simulate the flow behaviour in any bubbling fluidized bed.
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Figure 3-2. Cold flow model of a bubbling fluidized bed gasifier (CBFB).
3.2.2 Laboratory scale 20 kW bubbling fluidized bed gasifier
The bubbling fluidized bed gasifier (BFBG) model was used to study the fluidization characteristics under different hot flow conditions. By using the hot flow setup, it was possible to provoke the formation of agglomerates and with this examine the agglomeration tendency for different types of biomass. The flow behaviour and the agglomeration tendency were studied for three separate works: (I) Study of the bed conditions in agglomerated fluidized bed processes (II) study and comparison of agglomeration tendency for different types of biomass, and (III) study of agglomeration tendency for fluidized bed processes with different particle size of the bed material.
Figure 3-3 shows the 20 kW BFBG system. The gasification reactor is a cylindrical column built in stainless steel, insulated with a refractory material on the inside and a 200 mm thick fiberglass layer on the outside to minimize the heat losses. The inner diameter of the reactor is 10 cm and the height is 1.3 m. The gasifying medium is preheated air that flows into the gasifier through two 10 mm steel pipes placed 27.5 mm from the bottom of the reactor. The air mass flow rate is controlled with a Brook air flowmeter. A screw conveyor installed 21.2 cm above the air inlet, ensures a steady supply of biomass to the process. The BFBG is typically operated with temperatures ranging between 700C and 900C. Three electrical heating elements are coiled around the wall of the reactor and
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are used for external heating of the gasification process. The gasifier is heated to 400ᵒC by the external heating source. Additional heating is obtained from the heat released from the combustion of the biomass. Five thermocouples and five pressure transducers placed along the height of the column are continuously monitoring the operating bed conditions. The distance between the temperature and pressure sensors are 10 cm, whereas the first sensor is at the same level as the air supply. Each pressure transducer measures the gauge pressure, i.e. the air pressure in excess of the atmospheric pressure, in the given position. The temperature and pressure sensors are connected to the LabWiew software for data acquisition. The producer gas leaves the reactor from the top.
Figure 3-3. 20 kW bubbling fluidized bed gasification (BFBG).
The remaining bed particles, ash and any agglomerates were removed from the gasifier after each of the test runs. According to the different types of biomass and the relevant bed conditions, the morphologies and structures of the agglomerates were compared and examined. The results were combined with CPFD modelling in order to simulate the effect of bed agglomeration on bed de-fluidization, as well as to predict the critical amount of agglomerates in the bed.
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3.2.3 Micro-scale fluidized bed reactor
The micro-scale fluidized bed (MBFB) model was designed to determine the onset of de-fluidization and the agglomeration tendency for different biomass ashes. The MBFB maintains stable operating conditions and provides relatively fast and flexible test runs for controlled bed agglomeration processes.
Figure 3-4 shows the MBFB system. The fluidized bed is a cylindrical column built in transparent quartz glass with an inner diameter of 43.6 mm and height 150 mm. The gasifying medium is air that flows in 7 mm thick pipe and enters the bed from the bottom of the column. The top of the bed is open to the atmosphere. The air flowrate is controlled with a Sierra mass-flow controller and flows into the bed through a 5 mm thick sintered disc distributor. The MBFB is placed in a Nabertherm muffle furnace to ensure stable and controllable temperature conditions in the bed. The muffle furnace is equipped with a quartz glass observation window that allows the user to see inside the chamber without disturbing the ongoing process. The experiments were carried out at temperatures of 700°C, 800°C, 850°C, 900°C, 950°C and 1000°C. The bed conditions were continuously observed throughout the experiments. The results were based on visual observations of changes in the fluidized conditions in the bed under different operating temperatures.
Figure 3-4. Micro-scaled fluidized bed reactor (MBFB).
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The amount of accumulated ash in the bed was measured at the time of de-fluidization.
The observations gave a multiple variable data set that formed the basis for a mathematical model for accurate predictions of de-fluidization and bed agglomeration for different types of biomass. Any agglomerates formed during the experiments were collected and examined with respect to their structural composition.