Experimental procedure

In the scope of the present work, using a single standard experimental procedure for the entire scope of the thesis is not possible. The experimental procedures used in this work are broadly classified into three categories, mainly

Figure 18 – Purging arrangement for preventing blocking of pressure ports

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1. Single loop CFB experiments

2. Dual loop or DFB experiments: these are the ones mainly used for CaL mode and SER mode experiments.

3. Special experiments: these experimental procedures are explained further in the manuscript. These special experiments are mixing and segregation experiments in BFB (see Chapter 5) and estimation of friction and acceleration magnitude in a CFB riser (see Chapter 7).

Single loop CFB experiments are performed on a single CFB. These experiments are mainly performed to evaluate the performance of a CFB, without the influence of the other fluidized bed of the DFB. The experimental procedure described here is used for majority of the experiments. At the beginning of an experiment, the solid particles from entire system are emptied and no gas flows are used. The selected solid inventory is weighed and termed as total solid inventory (𝑀_𝑇𝑜). This total solid inventory, 𝑀_𝑇𝑜 is poured into the standpipes of a CFB. Once the solid inventory is set, the air flow is introduced into the CFB as primary air at a given riser superficial velocity. Next first step is to initiate solid circulation in a CFB. For this the loop seal of the CFB in operation is steadily supplied with the air, the solid flow initiates from the loop seals and over the weir solid particles drop into the riser, the riser with enough transport velocity carries the particles out of the riser and these particles are separated from the gas flow into the cyclones and particles fall back into the standpipe. Thus solid circulation is initiated.

The loop seal aeration is adjusted for a trouble free operation. Since the solid flow from the loop seal- standpipe is initiated the pressure in the riser is increased and visually the riser is now full of solid particles. Over the data acquisition system the pressure in the riser can be monitored. In few minutes of operation, the pressure in the riser, cyclone and standpipe gets stabilized. The period after stabilization of the pressure profile, can be considered as steady state. During steady state, the average pressure drop of the riser is constant over a period of time, the pressure fluctuations are also uniform and the height of particles in the standpipe is very stable. The steady state is determined when the average total riser pressure drop remains constant with uniform fluctuations for a period of more than 10 minutes. The time remains is recorded for a steady state.

The riser entrainment rates are measured mainly using the discontinuous method. The loop seal aeration is stopped for certain span of time, the stoppage in the loop seal aeration stops material flow from loop seal to riser, therefore solid flow coming from the riser is accumulated in the standpipe (∆𝐻_𝑠𝑡𝑝).The time required to accumulate certain height (∆𝑡) in the standpipe is noted and then based on the bulk density (𝜌_{𝑠 𝑏𝑢𝑙𝑘}) of the particles the entrainment flux rates are calculated using the formula below in Eq. (30).

𝐺_𝑠 =

𝐴_𝑠𝑡𝑝. ∆𝐻_𝑠𝑡𝑝. 𝜌_{𝑠 𝑏𝑢𝑙𝑘} 𝐴_{𝑟𝑖𝑠𝑒𝑟}∆𝑡

(30)

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The loop seal aeration is resumed once the height and time is noted. However, with this method one should be careful, that the time span of accumulation should not extend to an extent that it affects the riser hydrodynamics, causing the errors in the measurements. Therefore, the time span should be as small as possible. In our experience 3-10 s is an optimum time span for measurements. Readings less than 1 second are not considered because of significant errors involved. The measurements are generally taken at the end of steady state. Minimum three such readings are taken for every steady state with a gap of at least 3 minutes to allow the system to regain the original hydrodynamic state, i.e. original mean pressure drop and fluctuations.

Sometimes it used to happen that after taking the entrainment rate reading the system may not regain original pressure drop after 3 minutes. In such cases more waiting time is considered till the original hydrodynamics is reached or the entire experiment is discarded. It is observed that loop seal aeration patterns get disturbed in such discontinuous methods and causes irregularities in achieving the original hydrodynamics in the riser.

The dual loop or DFB experiments are similar to single loop experiments, except the use of two fluidized beds coupled with each other. In CaL mode two CFB’s R2 and R3 are fed with total solid inventory (𝑀_𝑇𝑜), initially two CFBs are operated as single loop CFB system without coupling with cone valve. Once the CFB’s appear steady on the computer screen, cone valves from both CFB’s are opened. Once the cone valves are opened, the solid inventory is redistributed again within R2 and R3, and within few minutes, the DFB operation is stabilized. The stable steady DFB operation is identified just as single loop CFB system, i.e. steady pressure drops, uniform fluctuations and steady particle height in both CFB’s. If the steady situation persists for more than 10 minutes, the timings are noted for steady state pressure measurement data. Once these 10 minutes are over, the riser entrainment fluxes are measured as explained earlier in single loop CFB experiments (one CFB at a time) and cone valve flow rates are measured after that. To measure the cone valve flow rate, the solid flow from the cone valve is diverted into the sampling port shown in Figure 19 for certain time and the diverted solid inventory is removed and weighed. After weighing the removed inventory is given back to the respective risers.

SER mode is operated with BFB R1 and CFB R3. Initially, the total solid inventory is weighed and distributed into the R3 standpipe, L-valve and R1 BFB. To operate at first, the R3 is operated in a single loop CFB operation, and R1 is only fluidized through spargers. The aeration in the L-valve and gasifier loop seal of R3 is not initiated. Once the R3 single loop CFB operation becomes steady, the aeration in the gasifier loop seal of R1 is initiated and immediately the L-valve aeration is also applied. This step initiates the solid flow circulation between R1 and R3 making the system coupled. The redistribution of solid inventory takes some time the pressure profile of the entire DFB system is stabilized. In SER mode the steady state is considered when the pressure profile of R3 is uniform and the particle bed height in R1 and standpipe of R3 remains

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constant. If steady situation persists for more than 10 minutes, the timings are noted for steady state pressure measurement data. Once these 10 minutes are over, the riser entrainment fluxes are measured as explained earlier in single loop CFB experiments and L-valve flow rates are measured after that. To measure the L-valve flow rate, a measuring cup is installed in the R1, near the exit of L-valve as shown in Figure 19b.

The flow from the L-valve is directly trapped into the measuring cup and time required to fill the cup is recorded. The inventory trapped in the cup is immediately released into the R1 bed.