Riser pressure profiles, inventories and entrainment rates

4.4.2.1. Carbonator

A typical riser axial pressure profile and corresponding solid fraction profile of the carbonator is shown in Figure 23. The pressure is shown relative to the pressure at the exit of the riser. The pressure profile shown in Figure 23 is a representative one and subjected to change with change in operational parameters such as riser velocity, total solid inventory etc. The difference of pressure values between two measurement points is the pressure drop, the pressure value at bottom of the riser (height = 0 m) is the value of total pressure drop in the riser (∆𝑝_{𝑟𝑖𝑠𝑒𝑟 𝐶𝑎}). Using pressure drop values between two points and using Eq. (13), the solid fraction (𝜀_𝑠 or 1-) is calculated. The solid fraction between two pressure points is assumed constant, therefore the solid fraction value is indicated at the middle of two pressure points. As seen in Figure 23 the carbonator is clearly divided in two zones, a dashed line marks the level at which the riser diameter changes from wide bottom (𝐷_{𝐶𝑎 𝑏𝑜𝑡}=140 mm) to 92 mm. The wide bottom zone shows explicitly high pressure drop and high solid fraction. In Figure 23 the solid fraction in bottom zone is high as 0.25, which is considerably higher than the solid fraction in a normal CFB and resembles the solid fraction in a turbulent bed. Wide bottom geometry creates a low velocity zone of turbulent regime in the bottom region, therefore turbulent fluidized bed conditions are created in this zone. Above 0.4 m level where diameter is 92 mm, the gas velocity is higher; the solid fraction shows drastic reduction to a very low value of 0.01. The visual observation confirms a core-annulus structure in this zone. The exit region shows slightly increased solid fraction, showing signs of exit effect [108]. The abrupt T-shaped exit creates slight solid back mixing in the riser exit region and increases the solid fraction. The solid fraction values recorded in different sections of the carbonator are compared with the solid fractions of a normal

Figure 23 - Typical carbonator pressure profile and solid fraction (𝟏 − 𝜺) profile

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CFB combustor (CFBC) and CFB carbonator used by Charitos et al. [71] (see Table 13). The carbonator in this study clearly displays much denser bottom zone compared to other carbonator and CFBC. Contrarily the upper lean region and exit region solid fractions are recorded lower than by [71] but are higher or equivalent to CFBC.

Therefore for a carbonator the solid fraction profile is satisfactory. In other work [34] it is shown that the significant portion of CO2 capture takes place in bottom dense zone of a CFB. Several other works have also implied that the gas solid contacting in a turbulent bed is very good [181]. Therefore such design with wide bottom riser may be suitable for carbonator.

Carbonator riser inventory (𝑀_𝐶𝑎) calculated for pressure profile shown in Figure 23 using Eq. (13) equals 11.8 kg. This includes both bottom dense and top lean region.

Using the scaling ratio from Table 12, the carbonator inventory in the test plant (𝑀́_𝐶𝑎) will be 64 kg, much higher than the required as shown in Table 12. The cold model shows that the required solid inventory in the carbonator is met. In case without wide bottom diameter, for the same cold model carbonator total pressure drop, 𝑀_𝐶𝑎 would have been 5.6 kg; (𝑀́_𝐶𝑎=32.4 kg). Therefore a wide bottom design may prove to be an advantage.

Table 13 - Solid fraction (𝜺𝒔) values of the hydrodynamic regions of the CFB carbonator and the CFB combustor

Dense bottom zone

Lean core annulus region

Exit region

Carbonator in this study 0.15-0.32 0.005-0.02 0.01-0.02 Carbonator Charitos et al.[71] 0.1-0.22 0.01-0.025 0.01-0.07

CFB combustor [71] <0.2 < 0.01

Figure 24 - Carbonator entrainment flux (𝑮𝒔 𝑪𝒂) variation with carbonator velocity 𝒖𝟎 𝑪𝒂. (Particles iron oxide, 𝒅𝒑50=166 µm)

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The carbonator solid entrainment flux (𝐺_{𝑠 𝐶𝑎}) against carbonator velocity (𝑢_{0 𝐶𝑎}) in a single loop operation is shown in Figure 24. 𝑢_{0 𝐶𝑎} is corresponding to the velocity in the top region diameter (92 mm) of the riser. As seen entrainment flux 𝐺_{𝑠 𝐶𝑎} clearly increases with increase in 𝑢_{0 𝐶𝑎} with an exponential growth function. Such trends are common in a loop seal operated CFB riser [71,104,140]. The test plant entrainment rate 𝐺́_𝐶𝑎 in kg/h is shown as secondary axis; by using the scaling ratio in Table 12. The entrainment rates corresponding to sorbent looping ratio (𝑅́_𝐿) of 14 is equal to 1200 kg/h, which is achieved at test plant carbonator velocity (𝑢́_{0 𝐶𝑎}) of 5.3 m/s. Thus, the entrainment rates from carbonator are also found to be within the required range.

4.4.2.2. Regenerator

The air or oxidant staging is a vital factor which influences the pressure and solid fraction profile of the regenerator. In air staging operation the total volumetric flow rate (𝑉_𝑅𝑒̇ ) to the regenerator is divided in primary air (PA), secondary air (SA) and tertiary air (TA). The air staging ratio (ASR) here is denoted as the fraction of 𝑉_𝑅𝑒̇ given in PA, SA and TA nozzles respectively. e.g. ASR 70:24:6 in Figure 25 represents 70% of 𝑉_𝑅𝑒̇ in the PA nozzle, 24% in the SA nozzle and remaining 6% in the TA nozzle respectively.

In without air staging operation, 𝑉_𝑅𝑒̇ is given only through the PA nozzle. The regenerator velocity 𝑢_{0 𝑅𝑒} corresponds to 𝑉_𝑅𝑒̇ . Figure 25 shows the pressure profile and solid fraction profile of the regenerator with and without air staging operation. The solid fraction is measured using the pressure drop between the two pressure points as explained earlier (Page 59). In an air staged operation, at the bottom of regenerator (part of regenerator below SA nozzle) significant pressure drop and high solid fraction (0.1 to 0.27) is observed. The part above the SA nozzle can be called a lean zone, since a low pressure drop and a low solid fraction (average 0.011) is observed. The

Figure 25 - Typical regenerator pressure and solid fraction (s) profile, with and without air staging (Conditions: u0 Re = 3.7 m/s, ASR 70:24:6% in air staging experiment).

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dense zone at the bottom occurs due to the air staging, the gas velocity in the bottom zone is low therefore the turbulent conditions are created. Additionally as per Ersoy et al. [168] the insertion of secondary air causes further densification of the bottom zone.

Without air staging, for the same 𝑢_{0 𝑅𝑒} and slightly lower total pressure drop value, the pressure profile is different. The pressure profile resembles an S- shape curve. The bottom zone is comparatively leaner (0.05-0.17), but the pressure drop and solid fraction in the upper lean region is higher compared to air staged operation.

The regenerator entrainment flux (𝐺_{𝑠 𝑅𝑒}) against the regenerator superficial velocity (𝑢_{0 𝑅𝑒}) in a single loop operation is shown in Figure 26. The 𝑢_{0 𝑅𝑒} corresponds to 𝑉_𝑅𝑒̇ . As seen in Figure 26, 𝐺_{𝑠 𝑅𝑒} is maximum without air staging. Entrainment flux clearly decreases with the application of air staging in comparison to without air staging. The increase in PA fraction increases the entrainment flux. This observation is consistent with the literature [140,154]. The test plant entrainment rate 𝐺́_𝑅𝑒 in kg/h is shown as secondary axis; by using the scaling ratio in Table 12. The entrainment rates corresponding to sorbent looping ratio (𝑅́_𝐿) of 14 is equal to 1200 kg/h, would be achieved at test plant regenerator velocity 𝑢́_{0 𝑅𝑒} = 6.7 m/s, and at cold model 𝑢_{0 𝑅𝑒}=4.3 m/s without oxidant staging. This operating velocity is higher than the normal range of operating velocity and furthermore the air staging effect will reduce entrainment rates even more. In short the regenerator cannot generate enough entrainment flux to fulfill the required solid looping rates of up to 1200 kg/h (𝑅_𝐿 = 14), but can fulfill partially up to 800 kg/h. This observation is the main reason why the solid looping rates reported in section 4.4.1 are limited to a value, and a simple conclusion can be made that the regenerator requires modifications in the design.

Figure 26 - Regenerator entrainment flux variation with and without air staging and riser velocity. Particles iron oxide, dp50 = 166 µm

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4.4.2.3. Fluctuations analysis

Understanding the pressure fluctuations in a CFB operation is important as it helps identifying regimes. Pressure fluctuations in a CFB are represented in various ways, in this work the pressure fluctuations are quantified by the ratio of standard deviation of riser total pressure drop (𝜎(∆𝑝_{𝑟𝑖𝑠𝑒𝑟 𝑖})) and riser total pressure drop ∆𝑝_{𝑟𝑖𝑠𝑒𝑟 𝑖}.

Figure 27 shows the variation of ^𝜎(∆𝑝_∆𝑝^{𝑟𝑖𝑠𝑒𝑟 𝐶𝑎)}

𝑟𝑖𝑠𝑒𝑟 𝐶𝑎 in the carbonator against the carbonator velocity (𝑢_{0 𝐶𝑎}). As seen, ^𝜎(∆𝑝_∆𝑝^{𝑟𝑖𝑠𝑒𝑟 𝐶𝑎)}

𝑟𝑖𝑠𝑒𝑟 𝐶𝑎 reduce substantially with increase in 𝑢_{0 𝐶𝑎} and becomes stable around 5% at 3.7 m/s.

The riser pressure fluctuations are widely used in qualitative identification of fluidization regimes. As discussed in section 2.2.1 and Figure 7, the pressure fluctuations in a riser follow a typical variation with change in riser velocity. It reaches a maximum at a velocity namely 𝑢_𝑐 and then steadily reduces and stabilizes at around certain value at velocity namely 𝑢_𝑘. As discussed earlier, it is generally accepted that the turbulent regime begins at 𝑢_𝑐 and ends at 𝑢_𝑘. Above 𝑢_𝑘 the regime is mainly fast fluidized regime. Thus up to 3.7 m/s the fluidization regime is mainly turbulent regime and above 3.7 m/s is fast fluidization regime. The experiments are not carried out to reveal the exact value of 𝑢_𝑐.

In the regenerator, the variation of pressure fluctuations with riser velocity shows a similar trend as in carbonator only for the without air staging case. The pressure fluctuation behaviour changes significantly with the application of the air staging.

Figure 28 shows the variation of the pressure fluctuation with various staging ratios, for a constant regenerator velocity (𝑢_{0 𝑅𝑒}). As seen in Figure 28, with a decrease in PA fraction, the pressure fluctuations increase rapidly; for PA flow of 50% of total VFR,

Figure 27- Carbonator's pressure fluctuations at different carbonator superficial velocities

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∆𝑝_{𝑟𝑖𝑠𝑒𝑟 𝑅𝑒} is observed as high as 35%, and for PA flow of 80% of total 𝑉_𝑅𝑒̇ for the same 𝑢_{0 𝑅𝑒}, the ^𝜎(∆𝑝_∆𝑝^{𝑟𝑖𝑠𝑒𝑟 𝑅𝑒)}

𝑟𝑖𝑠𝑒𝑟 𝑅𝑒 is observed as low as 5%. The increase of pressure fluctuations is a problem for the stability of the loop seal since it was observed that at high fluctuations gas escapes frequently through the cone valve. From these observations it is advised to operate the regenerator with PA with at least 70% 𝑉_𝑅𝑒̇ .

4.4.3. Solid flow diversion through cone valve and cone valve characterization