Results and discussion

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experimental runs (min 3 runs-max 6 runs) were performed to ensure the reproducibility of the results.

7.2.2. Data analysis

The inventory trapped inside the riser and the return leg are weighed separately and compared with the measured pressure drop. The static pressure drop ∆𝑝_{𝑠𝑡𝑎𝑡𝑖𝑐} for a given experiment is calculated from Eq. (16) as follows, by neglecting the static pressure drop of the gas in the riser.

∆𝑝_{𝑠𝑡𝑎𝑡𝑖𝑐} = ∫(1 − 𝜀)𝜌_𝑠𝑔𝑑ℎ =𝑀_{𝑟𝑖𝑠𝑒𝑟}𝑔 𝐴_{𝑟𝑖𝑠𝑒𝑟}

(51) Where, 𝑀_{𝑟𝑖𝑠𝑒𝑟} is the trapped inventory in the riser and 𝐴_{𝑟𝑖𝑠𝑒𝑟} is the cross sectional area of the riser. In this study a separate friction and acceleration pressure drop could not be calculated, therefore for simplification reasons both pressure drops are summed up as (∆𝑝_{𝑓𝑟+𝑎𝑐𝑐}). Eq. (17), Eq. (18) and Eq. (19) could be simplified into Eq. (52) as

where

∆𝑝_𝑓𝑟 = ∆𝑝_{𝑓𝑟 𝑔}+ ∆𝑝_{𝑓𝑟 𝑠} (53)

and ∆𝑝_{𝑟𝑖𝑠𝑒𝑟} is the time averaged pressure drop measured during the experimental steady state. The fraction of friction and acceleration pressure to the total riser pressure drop measured is represented by 𝜓, and defined by Eq. (54)

𝜓 = ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐}

∆𝑝_{𝑟𝑖𝑠𝑒𝑟} (54)

The inventory trapped in the return leg is weighed and the particle height in the standpipe is also measured (only in cold model) during the experiment and after valve closure only for cold model experiments. The inventory trapped in the return leg and the one in the standpipe is later used in the analysis of inventory distribution of CFB system. In bench scale test plant these measurements were not possible.

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velocity on the friction and acceleration losses in the cold model set up. In this set of experiments the ∆𝑝_{𝑟𝑖𝑠𝑒𝑟}is kept constant around 80 mbar. The inventory trapped in the riser is converted to ∆𝑝_{𝑠𝑡𝑎𝑡𝑖𝑐} using Eq. (51) and ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} is calculated using Eq. (52).

The values of ∆𝑝_{𝑠𝑡𝑎𝑡𝑖𝑐} and ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} shown in Figure 58 are the average values of multiple runs. In Figure 58 it is clear that an increase in riser velocity increases

∆𝑝_{𝑓𝑟+𝑎𝑐𝑐}. However, it is interesting to note that at low velocities the ∆𝑝_{𝑠𝑡𝑎𝑡𝑖𝑐} measured is more than the ∆𝑝_{𝑟𝑖𝑠𝑒𝑟}. Therefore ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} values are calculated as negative values.

Above 3.3 m/s the ∆𝑝_{𝑠𝑡𝑎𝑡𝑖𝑐} is found to be lower than the ∆𝑝_{𝑟𝑖𝑠𝑒𝑟} and continues to decrease with increase in velocity, thus ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} are recorded as positive and increase with increase in velocity. From Eq. (16) to (19) it is observed that the friction and acceleration pressure drop is highly dependent on the velocity of gas and particles in the riser. Therefore, the increment in ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} can be explained. The % friction and acceleration pressure drop as per Eq. (54) for the above results is calculated between -55 to +70 % of the riser pressure drop.

The ‘’negative’’ values of ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} should not be mistaken as the negative friction.

The fluidized bed is a combination of annular and core region. The total pressure drop is defined as a combination of static pressure drop caused by the core and the annular region particles. The particles at the wall (annular region) experience a very low gas velocity zone, below 𝑢_𝑚𝑓 thus the already suspended particles flow downwards or stop fluidizing. Below 𝑢_𝑚𝑓, the pressure drop caused by the particles is lower than the weight of the particles [9]. At lower velocities, the riser inventory was found more than the one calculated through the observed pressure drop. This phenomenon indicates that the population of such down flowing particles is significant in such small cross section, as in our experiments. The volume fraction of the annular region could be higher than the one of a riser with bigger cross section. A similar phenomena has been

Figure 58 – Variation of static head and the friction - acceleration pressure drop in a CFB riser with the riser velocity

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previously reported by Sankar and Smith [190] and Rautiainen et al. [115,117].

According to Rautianen et al. [115] the solid friction factor (𝑓_𝑠) mentioned in Eq. (19) varies with the solid velocity and at low velocity the values are ‘’negative’’. Rautianen et al. [115] also developed a correlation for friction factor which takes this observation into consideration.

From Figure 58 it is clear that the negative values of ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} occur between 𝑢_𝑐 and 𝑢_𝑘while above 𝑢_𝑘 most of the ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} values were reported to be positive. These values of 𝑢_𝑐 and 𝑢_𝑘 are calculated using the formulas proposed by Kunii and Levenspiel [79]. These velocities mark the onset and end of transition to the turbulent regime or as per [191] it is the turbulent regime. Since no similar observation could be found in literature, co-relating the turbulent regime and the observed phenomenon is not appropriate. Further investigation is suggested in this issue.

To conclude, friction and acceleration pressure drop show considerable variation.

Neglecting this influence could bring significant error in calculating riser inventory in small scale cross section reactor. Without friction and acceleration taken into consideration, at low riser velocities below 𝑢_𝑘within transition to turbulent phase the riser inventory could be underestimated and above 𝑢_𝑘overestimated.

7.3.2. Effect of total riser pressure drop (cold model)

In previous section the effect of riser velocity on the friction and acceleration losses is shown at constant total pressure drop in the riser. However, it is interesting to see the effect of total pressure drop on the friction and acceleration pressure drop.

Figure 59a shows ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} subject to variation in ∆𝑝_{𝑟𝑖𝑠𝑒𝑟} and riser velocity 𝑢₀. The

∆𝑝_{𝑟𝑖𝑠𝑒𝑟} selected were ca. 40, 60 and 80 mbar at the velocities of 3.15, 3.5 and 3.9 m/s.

The velocities were selected such for providing negative, near zero and positive

∆𝑝_{𝑓𝑟+𝑎𝑐𝑐}.

As it can be seen, that the ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} measured for a velocity of 3.15 m/s is negative for all three ∆𝑝_{𝑟𝑖𝑠𝑒𝑟} as observed earlier in Figure 58. For 60 mbar and 80 mbar the values were close in the range of -35 mbar and -40 mbar, while for 40 mbar the value was - 20 mbar. At a riser velocity of 3.5 m/s the ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} is also measured as close to zero for the ∆𝑝_{𝑟𝑖𝑠𝑒𝑟} of 60 mbar and 80 mbar. However, for 40 mbar ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} was measured as 16.5 mbar. For a riser velocity of 3.9 m/s the ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} measured is in the range of 16-18 mbar for all three ∆𝑝_{𝑟𝑖𝑠𝑒𝑟}, which is very close. In other words, at this velocity the effect of increased ∆𝑝_{𝑟𝑖𝑠𝑒𝑟}through addition of total solid inventory had less effect on ∆𝑃_{𝑓𝑟+𝑎𝑐𝑐}. Thus comparing Figure 58 and Figure 59a it can be deduced that the riser velocity is the major factor which determines the magnitude of friction and acceleration pressure drop in the riser. The discrepancy of the results of 40 mbar at the velocity of 2.1 and 3.5 m/s could not be yet explained. Figure 59b shows the results of Figure 59a, as 𝜓 defined by Eq. (54). Since 𝜓 represents ∆𝑃_{𝑓𝑟+𝑎𝑐𝑐} as a fraction of

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the total pressure drop, and may be useful to compare the other results in literature which are often gives information of frictional pressure drop as a percentage of the total pressure drop [118]. For a riser velocity of 3.9 m/s, it can be seen that increasing

∆𝑝_{𝑟𝑖𝑠𝑒𝑟} the 𝜓 decreases. This is explained since the ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} nearly remained constant and ∆𝑝_{𝑟𝑖𝑠𝑒𝑟} increased. Issangya [121] also observed that for high density CFB risers the contribution of pressure drop due to friction is less compared to low density risers.

7.3.3. Experiments in the bench scale test plant- influence of riser velocity on friction and acceleration pressure drop

The results presented in the previous sections show an interesting phenomenon.

However, it is hard to conclude that such phenomenon could occur in the large scale CFB risers. Therefore, similar experiments were performed in the bench scale test plant facility shown in Figure 57b. The facility and the experimental conditions are described in Table 18. These conditions are exact conditions for calcium looping process, except the fluidizing gas was air instead of flue gas in order to avoid carbonation reaction. The riser superficial velocity was the main parameter investigated. Figure 60 shows the static pressure drop (static head) and the friction and acceleration pressure drop. It can be observed that even during the test plant

Figure 59: Influence of riser inventory or total riser pressure drop on the friction and acceleration pressure drop

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experiments the static head was recorded higher than the actual pressure drop. Thus as per Eq. (52) negative values of friction and acceleration pressure drop are obtained.

For velocities between 3.5 to 6.1 m/s the ∆𝑝_{𝑓𝑟+𝑎𝑐𝑐} was calculated to be -20 to +5.3 mbar. The results of Figure 60 are analogues to the results as per Figure 58, if we consider the scaling ratio of velocity (Table 18) except the results at velocity higher than 6.1. The term “analogues” is used here, since the cold model and bench scale test plant showed similar results.

The experiments could not be performed at higher velocity due to facility related limitations. Although, the phenomenon has been repeated in the scaled up facility, it is still audacious to conclude that such behaviour will be observed in larger scale risers also. Contrary, employing similar experimental method or quick closing valve is not suitable for large scale risers. The use of optical fibre probes will be a suitable method.

The space time (τ́_𝐶𝑎) values as per (Eq. (31) to Eq. (34)) are shown in Figure 61 for the results obtained from test plant. The calculations are done assuming a CO2 inlet concentration of 15 vol% and bed material as pure CaO at 630°C. The actual space time is calculated using the solid inventory of riser captured using quick closing valve method (𝑀́_{𝑟𝑖𝑠𝑒𝑟}) of Figure 60 and estimated space time is calculated using the riser total of Figure 60. As seen in Figure 61, for all cases except at 6.1 m/s the actual space time is higher than the estimated space time. Nevertheless, the suitable velocity range for calcium looping process is also between 4-6 m/s. Therefore, the scientists who rely on the pressure drop values for space time calculations (Eq.(31)) can consider pressure drop values as a conservative approach. Thus, space time values predicted from pressure drop readings will be lower than the real space time values.

Figure 60 – Influence of riser velocity on the friction and acceleration pressure drop in hot condition in bench scale test plant

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