OCAC for Fuel Conversion Without CO₂ Capture

Abstract

As a new concept, oxygen carrier aided combustion (OCAC) technology proposed in 2013 by Chalmers University of Technology’s group, can alleviate the problem of uneven distribution of oxygen in the reactors. In the past 10 years,various research institutions, including Chalmers University of Technology, University of Cambridge, Tsinghua University, Friedrich-Alexander University and University of Nottingham, have conducted a series of studies on OCAC technology. It is worth mentioning that Chalmers University of Technology has complied with most of these studies from laboratory to industry scales. In particular, they carried out a serious of semi-industrial scale experiments in the 12 MW_thCFB boiler, which is well-known research boiler. OCAC technology is comprehensively introduced from six aspects: combustion characteristics, NO_x/SO_x emission, ash-related issues, aging of oxygen carrier, oxygen carrier recovery and physicochemical characteristics of oxygen carrier. In this chapter, allsummarized studies were performed under traditional air-combustion conditions without much consideration of CO₂ capture.

Since 2013, various research institutions, including Chalmers University of Technology, University of Cambridge, Tsinghua University, Friedrich-Alexander-University and University of Nottingham, have conducted a series of studies on OCAC technology. It is worth mentioning that Chalmers University of Technology has complied with most of these studies from laboratory to industry scales. In particular, they carried out a serious of semi-industrial scale experiments in the 12 MW_th CFB boiler (as shown in Fig. 3.1), which is well-known research boiler. In this section, all summarized studies were performed under traditional air-combustion conditions without much consideration of CO₂ capture. The relevant experimental studies with detailed information are summarized in Table 3.1.

Fig. 3.1

Chalmers boiler/gasifier reactor system (reprinted from [1] with permission of Elsevier)

Table 3.1 Summary of relevant experimental studies on the use of OC

3.1 Operation Experience

3.1.1 Combustion Performance

In 2014, Chadeesingh and Hayhurst [2] investigated the methane combustion in a bubbling fluidized bed with three kinds of bed material: silica sand (1.9 kg), silica sand (1.9 kg) + Fe₂O₃ (2.5 g) and silica sand (1.9 kg) + Fe₂O₃ (25 g). Although the term of OCAC has not been used in this work, it was actually an early trial of using OCs as bed material to facilitate fuel conversion. When there was only silica sand in the bed, sharp and loud ‘‘popping’’ noises were clearly heard, which seemed to be caused by the deflagration of hot CH₄ bubbles above the bed [19, 20]. The sound was still present when 2.5 g Fe₂O₃ was added to the bed material, while it disappeared when the share of Fe₂O₃ increased to 25.0 g. When only silica sand was used as the bed material, the combustion of hydrocarbon gases (such as CH₄ and C₃H₈) is inhibited. This is because silica sand provides a large surface area, on which free radicals readily recombine [21, 22]. As a result, the methane hardly burned in the dense bed. Instead, it burned in the splash zone. This assumption can be proven by the temperature distribution in the reactor (as shown in Fig. 3.2a). When Fe₂O₃ particles were present in the dense bed, a portion of CH₄ can be oxidized on the surface of the Fe₂O₃ particles, resulting in gaseous products of CO, CO₂ and trace C₂H₄ detected in the bed, as shown in Fig. 3.2c–e. Therefore, the oxidation of methane is promoted when Fe₂O₃ is added to the bed, even if the Fe₂O₃/silica sand ratio was small (<0.1 wt%). This important study demonstrated that the chemical-looping combustion of CH₄ can be achieved with either Fe₂O₃ or metallic Fe even in a single fluidized-bed reactor.

Fig. 3.2

(adapted from [2])

Plots of relevant parameter against height above the distributor

Schneider et al. [6] investigated the combustion of methane in a bubbling fluidized bed (the schematic illustration is shown in Fig. 3.3) with silica sand and ilmenite as bed materials. As shown in Fig. 3.4, the in-bed CO₂ yield can be significant increased by replacing the sand with ilmenite at different excess-air ratios. With silica sand as a bed material, the combustion of fuel in the bed was very weak, and occurred mostly above the bed surface. When the silica sand was replaced with ilmenite, the proportion of fuel conversion in the bed increased, and the combustion proportion above the surface of the bed decreased correspondingly, and the deflagration disappeared. With OC as a bed material, the proportion of fuel conversion in dense zone increased with the increase of bed height. In addition, the results also indicate that OCAC has more advantages in the application at low excess-air ratio.

Fig. 3.3

The schematic illustration of the bubbling fluidized bed reactor (reprinted from [6] with permission of American Chemical Society)

Fig. 3.4

CO₂ yield depending on excess-air ratio and height for silica sand and ilmenite. u/u_mf = 2.5, and bed temperature = 800 °C, the CO₂ in the bed is oxidized by CO (reprinted from [6] with permission of American Chemical Society)

In 2014, Källén et al. [3] used two kinds of manganese ore as bed material to investigate the performance of OCAC in a 300 W CFB combustor. The measured concentrations of CO and O₂ in the gas exhaust are shown in Fig. 3.5. When the bed material was solely silica sand, the CO concentration remained at a few hundred ppm until the air-to-fuel ratio decreased below 1.05. As the air-to-fuel ratio approached 1, the concentration of CO reached several thousand ppm at the two investigated temperatures. The same tests were repeated while shifting the bed material to a mixture of manganese ore and silica sand at 50/50 mass ratio. The CO concentration was significantly reduced in both cases when these two manganese ores were used as bed materials, especially at the lower air-to-fuel ratios. The study found that both manganese ores can release gaseous oxygen in the inert atmosphere, because the oxygen concentration of the exhaust gas in the cases of Mn ore were higher than that in sand case. This study demonstrated the great advantages of OCAC, yielding higher fuel conversion at low air-to-fuel ratios, resulting in higher combustion efficiency and lower energy consumption due to lower air supply.

Fig. 3.5

Measured outlet concentrations of CO and O₂ as a function of air-to-fuel ratio during operation with 100 wt% sand, 50 wt% Mn ore in the sand, SIB and UMK are two kinds of Mn ores from Sibelco and UMK companies (Adapted from [3])

The improved combustion performance of the OCAC process was also validated in a small bubbling fluidized bed with batch feeding of biomass char. Pour et al. [4] employed three kinds of ilmenite ore as bed material for the tests. Compared with the case of 100% sand as bed material, less oxygen was found in the outlet of the reactor for all ilmenite ore cases, and the fuel conversion increased, which means that the oxygen supply was used more efficiently. The OCAC processes using ilmenite ores as OCs displayed enhanced combustion characteristics regardless of the ore type. Wang et al. [5] investigated the effects of diverse OCs, including ilmenite, manganese ore and two industrial iron oxide scales (AQS and LDst), during the OCAC of biomass char at different air-to-fuel ratios. The combustion evidenced by the CO₂ yield (as shown in Fig. 3.6) and unburned volatiles showed that the various OCs perform differently. The promotion effect of OCs follows the order: Mn ore > LDst > AQS > ilmenite > sand.

Fig. 3.6

Average CO₂ yields of solid fuel combustion at different λ values (reprinted from [5] with permission of American Chemical Society)

Garcia et al. [7] used silica sand and ilmenite as bed material and a mixture of bituminous coal and wheat straw as fuel to investigate the combustion of OCAC in a 30 kW_th BFB combustor (the schematic diagram and photo of the pilot-scale BFB combustion system are shown in Fig. 3.7). The results verified that OCAC technology leads to lower CO emissions and smaller efficiency loss. Compared with 100% quartz sand, the combustion loss in 100% ilmenite case caused by CO emission and unburned carbon decreased from 0.2% and 2.59% to 0.17% and 1.76%, respectively. In addition, the results showed that the OCAC operation could slow down bed agglomeration. Compared with the operation of silica sand as a bed material, much less and smaller agglomerates were found in the case of ilmenite, and the agglomerates formed in the OCAC operation appeared to be weak and easily broken.

Fig. 3.7

a Schematic diagram and b photo of the pilot-scale BFB combustion system (reprinted from [7] with permission of Elsevier)

In 2013, Thunman et al. [1] carried out an OCAC combustion test by using ilmenite ore as OC, which was the first attempt at a semi-industrial scale CFB boiler (12 MW_th). The gas composition was measured at different heights of the furnace as well as at different intervals along the convection path. The results verified the expectation that CO can be reduced if the inert bed material is replaced by ilmenite ore. Figure 3.8 shows the change of CO concentration in the convection path after OCs were used to replace different proportions of quartz sand in the bed. The CO concentration in the exhaust gas was lower when more silica-sand was replaced by OCs. Compared to the 100% silica-sand case, the CO concentration in the exhaust gas was reduced by 80% when the OC content in the bed material was 40 wt%. By measuring CO and hydrocarbons at different positions of the boiler, it was found that the addition of OC can not only promote the oxidation of fuel, but it also improves the mixing of fuel and oxygen in a cross-section of the furnace. In addition, more fuel was oxidized within the furnace and less combustible gas was released to the cyclone, which is another benefit of OCAC: it avoids serious post-combustion.

Fig. 3.8

The CO concentration in the convection path (mg/nm³, at 6% O₂) versus air-to-fuel ratio, the curves show different fractions of OC (adapted from [1])

Rydén et al. [9, 11] and Hildor et al. [15] did similar experimental studies on the same CFB boiler, while calcined manganese ore and steel slag were used as OCs instead of ilmenite ore. However, the conclusions of the investigators seem different. Figure 3.9 shows the CO emission when different OC ratios were added to the bed material. The results indicate that the manganese ore and steel slag show excellent performance when used as OC, which facilitated the fuel conversion inside the boiler. On the other hand, the CO emission did not decrease monotonously with the increase of the OC content in the bed material. This is because both OCs have a poor ability to absorb alkali metals, and there is a high concentration of alkali metals (such as K) entering the gas phase, which according to the authors might inhibit the oxidation of CO [23,24,25]. To confirm this hypothesis, the authors measured the change of CO emission by adding sulfate to the furnace to reduce the alkali metals. When all bed materials were replaced by OCs, the addition of sulfate or ammonium sulfate reduced the CO emission. At the same time, the concentrations of K and S in the fly ash increased significantly. This indicates that the post-added sulfate reacts with the K salts to form a stable solid K₂SO₄ [23], which may reduce the inhibition of CO oxidation by alkali compounds in the flue gas [16]. In fact, sulfate fed to biomass boilers is occasionally applied to reduce CO emission [24, 25], but the authors pointed out that this explanation was not proven yet.

Fig. 3.9

CO Concentrations in the convection path (mg/nm³, at 6% O₂) as a function of air-to-fuel ratio (adapted from [9, 11])

Lind et al. [17] conducted an OCAC test in a 75 MW_th CFB boiler for burning municipal solid waste (MSW) using ilmenite ore as OCs, the study showed that the CFB boiler could be operated well, and the logistics were manageable. With over 12,000 h of operation, it has been demonstrated that the OCAC technology not only drastically reduced the CO concentration in the flue gas, but it can also improve the combustion of volatiles at the top of the combustion chamber. Particularly, compared with the 100% sand case, the average CO concentration was reduced by more than 45% (Fig. 3.10).

Fig. 3.10

Schematic representation of the 75 MW_th CFB-boiler [17]

Moldenhauer et al. [18] reported an OCAC operation in a commercial CFB boiler using rock ilmenite OC as bed material. CFB-boiler is located in the south of Sweden and consists of a biomass fired CHP cycle (112 bar, 540 °C) with a nominal thermal capacity of 115 MW_th. The amount of bed material in the system is about 60 tons under normal operation. The cross-section of the furnace is 2.2 m × 8.8 m at the height of the fluidization nozzles and expands to 5.5 m × 8.8 mm in the upper part of the furnace. The height from of the furnace is 28.4 m. Furnace and separators are equipped with ammonia injection (SNCR) systems to reduce the emissions of NO_x. A schematic of the system is illustrated in Fig. 3.11.

Fig. 3.11

Schematic representation of the 115 MW_th CFB-boiler [18]

As shown in Figs. 3.12 and 3.13, compared to the conventional operation with silica sand, the OCAC process made it possible to reduce the excess air by up to 30%, evidenced by the decrease of O₂ concentration at the outlet from 2.5 to 1.8%, while the boiler load increased from 115 to 123 MW_th. The total airflow in the OCAC operation at 123 MW_th was approximately the same as that in the conventional operation at 113 MW_th by silica sand. Consequently, the boiler can be operated in overload mode without increasing the gas velocity in the boiler and convection path. At the same load, OCAC operation significantly reduced the airflow volume, which improves the economics of operation. Keeping the proper gas velocity was also beneficial for the anti-erosion of the heat-transfer surfaces in the furnace.

Fig. 3.12

O₂ concentration during ilmenite operation in the horizontal pass for moist gas and O₂ fractions in the stack for dry and wet gas as a function of the net boiler load (dashed lines symbolizes set points for normal silica-sand operation, i.e. average O₂ and average net boiler load) [18]

Fig. 3.13

Total gas flow as primary air, secondary air, and recirculated flue gas as a function of the net boiler load during operation with ilmenite and silica-sand [18]

Combined with the studies of the OCAC technology from lab-scale, semi-industrial, and industrial scale, it is found that the employment of OCs as bed material significantly promotes the combustion and reduces the CO emission compared to traditional FBC technology, especially at low air-to-fuel ratios. This provides the possibility to operate the boiler with low excess air, reducing the energy consumption. However, it should be pointed out that the type of OC, the mass fraction of OC in the bed material, the absorption properties of OC over alkali, and the fuel characteristics need to be comprehensively considered, since they can affect the CO emission.

3.1.2 Oxygen Buffering Ability

In OCAC technology, the OCs transfer oxygen from oxidizing to reducing regions, thereby improving the uniformity of the oxygen distribution inside the boiler. It acquires oxygen in the oxidizing regions, while donating its stored oxygen in the reducing regions. Therefore, the oxygen buffering ability of OC is an important index for evaluating the OC performance in OCAC technology. An OC with desired oxygen buffering ability not only can improve the combustion efficiency of the boiler but also the stability and safety of boiler operation. For example, during the operation of a commercial CFB boiler, fluctuations in supply air and fuel-feeding inevitably occur. Under such circumstances, using OCs can provide a large amount of oxygen in a short time and so maintaining smooth boiler operation and providing valuable recovery time for the operator.

Lind et al. [10] investigated the oxygen buffering ability of ilmenite ore in the CUT 12 MW_th CFB boiler. During the test, the boiler load was first stable at 6 MW_th, then a step response of the process was studied by introducing a fuel pulse corresponding to an increase in fuel load to 8 MW_th in 5 s, as shown in Fig. 3.14. The response of unburnt fuel and oxygen in dry flue gases are shown in Fig. 3.15 and Fig. 3.16. When the fuel pulse was introduced, the oxygen concentration was reduced regardless of using OC or silica sand as bed materials. The CO concentration shows a significant peak during the fuel injection when using silica sand, while it was almost constant when using OC. The same pulse of fuel injection was repeated three times, all showing consistent results. The constant CO concentration during the fuel injection was attributed to the oxygen buffering ability of ilmenite ore, which has been proven to be significant. Figure 3.16a also confirmed that when the available oxygen in the furnace was not globally sufficient to fully convert the fuel pulse, the oxygen in the OCs can make up for the lack of oxygen in space and time in the furnace. In addition, the OC can be re-oxidized to the initial stage and capable of oxygen buffering when the pulse of volatiles disappeared and gaseous oxygen becomes available.

Fig. 3.14

The load of fuel feed (dashed line) and volatile release (solid line) as function of time (reprinted from [10] with permission of Elsevier)

Fig. 3.15

Response of unburnt fuel (CO (ppmv), solid line) and oxygen (%, dashed line) in dry flue gases. The O₂ concentration in the flue gas was 3.5 mol % on dry basis (reprinted from [10] with permission of Elsevier)

Fig. 3.16

Evaluated dynamic response of oxygen (dashed line) and unburnt components (solid line) in flue gases during the fuel pulse, a silica sand, b MeO (reprinted from [10] with permission of Elsevier)

3.2 Emissions

3.2.1 NO_x Emission

The characteristics for the formation and emission of NO_x during typical OCAC processes have been studied in fluidized bed units on the scale of bubbling fluidized bed, 12 MW_th CFB, and 115 MW_th CFB. However, the results related to the NO_x emission during OCAC investigated at different scales seem inconsistent.

Wang et al. [5] investigated the effect of different OCs as bed materials on the NO emission by varying air-to-fuel ratio for burning wood char in a bubbling bed. Under the same oxidation conditions (the same CO concentration), a lower NO emission was observed during operation with OCs compared to operation with sand (as shown in Fig. 3.17). At the same time, lower release of unburned volatiles and ammonia was observed in the outlet of the reactor in the OCAC case (as shown in Fig. 3.18), which means that more ammonia and NO were consumed. The mechanism of this reduction is not clear, and a possible inference is that the metal oxides with variable metal valence states contained in the OCs might promote the reduction of NO via catalytic reactions, which is widely accepted in selective catalytic reduction (SCR) technology [5]. Taking iron oxide as an example, the reaction could be as follow:

$$ {\text{3CO}} + {\text{Fe}}_{2} {\text{O}}_{3} \to {\text{3CO}}_{2} + {\text{2Fe}} $$

(3.1)

$$ {\text{4Fe}} + {\text{4NO}} \to {\text{3N}}_{2} + {\text{Fe}}_{2} {\text{O}}_{3} $$

(3.2)

Fig. 3.17

NO emission as a function of the CO concentration for different bed materials (reprinted from [5] with permission of American Chemical Society)

Fig. 3.18

Effects of oxygen carriers on rates of release of volatiles and ammonia (reprinted from [5] with permission of American Chemical Society)

Therefore, when there was CO in the reactor, even if the temperature was as high as the bed temperature, the metal oxide can still catalyze the reaction between NO and CO [26, 27]. This was also the reason why iron oxides have been widely used as additives in reburning technology for denitration [28].

Based on an investigation in the CUT 12 MW_th CFB boiler (fuel: wood chips), Thunman et al. [1] also found that the OCAC process with ilmenite as OC could significantly reduce the NO emission, as shown in Fig. 3.19. However, it was not clear whether the formation of NO was reduced, or if the reduction of NO was enhanced. Interestingly, a lower NO concentration was achieved in the case of 17 wt% ilmenite ore than with 20 wt%. It should be noted that the residence time of the ilmenite in the reactor at 17 wt% case was longer than that in the 20 wt% case. According to the relevant knowledge on the ilmenite ore in CLC, the continuous redox cycles of ilmenite at high temperature will cause the migration of iron to the particle surface and increase the active sites [29,30,31]. This suggests that the presence of ilmenite ore improves the catalytic reduction of NO [1]. However, an opposite result was obtained on the same boiler by Rydén et al. [9, 11] when manganese ore and steel slag were used as OCs (fuel: wood chips). Figure 3.20 shows that the NO emission increases with the increase of OC ratio in the bed inventory, while adding S or using regenerated bed materials reduces the NO emission. This can be explained by the consumption of reducing gas (such as H₂ and CO) by OCs, that results in more NO formation and emission. In addition, the presence of OC may catalyze the oxidation reaction to produce NO. The effects of S addition and regenerated bed material on the NO emission, however, have not been explained [9, 11]. All the results indicate that higher air–fuel ratio leads to higher NO emission, which is not beyond the expectation.

Fig. 3.19

Measured concentrations of NO (mg/nm³, at 6% O₂) in the convection path at various OC concentrations in the bed (adapted from [1])

Fig. 3.20

Measured concentrations of NO (mg/nm³, at 6% O₂) in the convection path at various Mn concentrations in the bed (adapted from [9])

From an OCAC operation on a 115 MW_th commercial CFB boiler with ilmenite ore as bed material (fuel: waste wood), Moldenhauer et al. [18] found that the NO_x emissions and ammonia consumption were all slightly higher compared to the traditional operation with silica sand as bed materials. However, the NO concentration of these gases decreased with the increase of furnace temperature, which was unexpected. This may be because the reduction of excess air in the OCAC case led to a more reducing local atmosphere, thereby reducing NO emissions (Fig. 3.21).

Fig. 3.21

NO_x concentration measured as dry-gas at 6% O₂ as a function of the temperature, the red and green circles represent silica sand case and OCAC case, respectively [18]

In conclusion, the micromechanism of NO formation and emission during the OCAC process is not that clear, which makes it worthy of further investigation. The application of OCs in CFB boiler may impose either positive or negative effects on the NO emission depending on operation parameters such as fuel type, OC type, temperature, excess air, and the ratio of primary and secondary air, etc. The possible influence of OCAC on the NO emission in fluidized bed combustion can be divided into direct influence and indirect influence. Direct effects may include: (1) Active components such as Fe₂O₃, CuO and MnO₂, etc., contained in the OC may be directly or catalytically oxidized through the nitrogen-containing gas components, such as NH₃ and HCN, to generate more NO_x; (2) The reduced OC may directly or catalytically reduce nitrogen-containing gas components (HCN, NH₃ and NO_x, etc.), resulting in a lower NO_x emission. Indirect effects may include: (1) the application of OCAC can reduce the hot spots and improve the uniformity of temperature distribution in a furnace, which can reduce the NO formation; (2) the presence of OC makes the reduction zones smaller in the furnace, which will weaken the reduction of NO_x; (3) Under OCAC operation, the physical property parameters (such as particle density, size and specific heat capacity) of the bed material are different from those found during operation with inert bed material. This will affect the heating of the fuel and may change the type and concentration of the nitrogen-containing gas components in the pyrolysis products.

3.2.2 SO_x Emission and Desulfurization

At present, the effect of OCAC technology on the SO_x emission characteristics under air combustion has not been deeply studied. This may be since most experimental studies have applied pure biomass or MSW as fuel, and the sulfur content in such fuels is very low. However, the SO_x emission should be considered when burning fuels with high sulfur content, such as coal and petroleum coke.

When Vigoureux et al. [16] performed OCAC combustion with ilmenite in the CUT 12 MW_th CFB boiler, they added sulfur into the furnace during combustion and found that both the outer ash layer and the core contained sulfur. With an increase of the test time, the potassium and sulfur contents in the ilmenite particles were gradually increased, which indicates that the application of OCAC technology may help to reduce SO_x emissions. During normal CFB operation, limestone is injected into the furnace as a desulfurization sorbent. It decomposes into CaO and then captures SO₂ through the sulfation reaction [32]:

$$ {\text{CaCO}}_3 = {\text{CaO}} + {\text{CO}}_2 $$

(3.3)

$$ {\text{CaO}} + {\text{SO}}_2 + {1}/{\text{2O}}_2 = {\text{CaSO}}_4 $$

(3.4)

On the one hand, OCAC technology can make the temperature and oxygen distribution in the furnace more uniform, which promotes the desulfurization reaction. On the other hand, although it is difficult to directly oxidize SO₂ to SO₃, it can be achieved through a catalytic reaction [33]. Existing studies [34, 35] have shown that metal oxides can be used as catalysts to oxidize SO₂. One of the most widely known oxidation catalysts is Fe₂O₃ [35]. Jørgensen et al. [34] experimentally studied the oxidation reaction of SO₂ to SO₃ in the temperature range of 873–1323 K under semi-dry and moist conditions, and evaluated the catalytic effects of quartz, alumina, and iron oxide on this reaction. They found that the homogeneous oxidation reaction between O₂ and SO₂ was slow. However, alumina and iron oxide could promote the SO₂ oxidation, and the iron oxide is a strong oxidation catalyst for the reaction between O₂ and SO₂.

When the temperature is below 400 °C [36], SO₃ is highly reactive with water vapor to form H₂SO₄ and can be completely transformed at around 200 °C [35]. The generated H₂SO₄ is highly corrosive to the heated surface. Higher SO₃ concentration in the flue gas increases the acid dew point. If the desulfurization efficiency in the furnace is low, corrosion of the low-temperature heating surfaces, such as air preheaters may be enhanced. At present, there is no research on the characteristics of SO₃ emission under OCAC operation, but further research is needed. For high-sulfur fuels, the impact of OCAC on SO₂ and SO₃ emissions should be considered, because it is critical for the safe operation of downstream heat exchangers. The desired research includes: the potential impact of OC addition on the formation and emission of SO_x, migration paths and desulfurization reactions. In addition, high concentrations of SO_x may reversely affect the physicochemical characteristics of OCs, and this needs to be considered.

3.3 Ash-Related Issues

Solid fuels (such as coal and biomass) are usually containing a heterogeneous mixture of organic and inorganic matter [37, 38]. During the thermal conversion of fuel in a CFB system, the inorganic matter is mainly transferred from the fuel to the solid residues, mainly ash, and may cause problems, such as agglomeration, ash deposit, fouling and corrosion of heat-transfer equipment [39, 40].

3.3.1 Bed Agglomeration

Bed agglomeration is caused by sintering of bed material may lead to defluidization. This can collapse the entire bed into a solid mass [41]. Consequently, bed agglomeration can cause unplanned shut down of a boiler. Inert bed material, such as silica sand or ashes, is most used for CFB boilers, and their agglomeration mechanism has been extensively studied [40]. The agglomeration mechanism is generally dominated by two regimes: melt-induced agglomeration (Fig. 3.22) and coat-induced agglomeration (Fig. 3.23) [42]. Melt-induced agglomeration is caused by the collision of large molten ash particle and bed material, where the molten ash acts as a “viscous glue” [43] bonding the latter into tough agglomerates. Scala and Chirone [44] pointed out that a char particle in the bed can create a local hot spot, which can enhance the adhesive potential of this “viscous glue” and intensify agglomeration. The coat-induced agglomeration is initiated by ash deposition on the surface of bed particles via the attachment of small ash particles, the condensation of vaporized alkali species, and/or the chemical reactions at the surface of bed particles. Then, the generated alkali-silicate (usually K-silicate) layer grows inwards after reaction with silicate species in the bed material, which causes the bed particles to possess two- or three-layer coatings [45, 46]. Lastly, the bed particles would experience a sintering and homogenizing process, resulting in partial melting, agglomeration, and defluidization [41].

Fig. 3.22

The schematic of coating-induced agglomeration mechanism (reprinted from [42] with permission of Elsevier)

Fig. 3.23

The schematic of melt-induced agglomeration mechanism (reprinted from [42] with permission of Elsevier)

In order to avoid bed agglomeration, the bed material needs to be repeatedly regenerated according to the fuel and bed material characteristics. Within the existing experimental studies of OCAC technology in 12 MW_th, 75 MW_th and 115 MW_th boilers, bed agglomeration was not reported. The used OCs were ilmenite ore, manganese ore, and LD-slag, which possessed high melting points, and the operation time was relatively short in these studies. However, when OCs with lower melting points such as CuO was considered to be applied in OCAC, agglomeration must be carefully looked after. One solution is to frequently replace the bed material, which means the loss of OCs. Another solution is to use some supports to fabricate the OC particles and elevate their melting point. It should be noted that both ways can be costly [47].

3.3.2 Ash-Layer Formation of OC

Elements found in ash-layers mainly originate from the bed material and the fuel [48, 49]. As shown in Fig. 3.24, there are three growth modes that have been proposed from previous studies for the formation of ash-layer [41, 50]:

Fig. 3.24

Illustration of the different mechanisms for ash layer formation on bed particles

• Mode (1): An ash-layer is attached to the inert bed particle and grows outward, while the elements contained in the ash-layer originate from the fuel;

• Mode (2): The initially formed ash-layer may react with the bed particles resulting in further inward growth;

• Mode (3): The ash layer grows both inward and outward.

Several research groups have investigated layer formation on bed materials during solid fuel combustion and found that the formation of an ash-layer usually follows the growth Mode (3) [45, 49, 51, 52], and the outer section of the ash-layer is more heterogeneous in elemental composition than the inner section. The heterogeneous outer layer grows outward on the surface of bed particles with a similar composition to fly ash. The less heterogeneous inner layer is often dominated by Ca and/or K, which can diffuse into the bed material and reciprocally react together [48, 49, 51, 53]. Most research on ash-bed material interaction focused on Si-based bed particles, while few studies have been centered on the interaction between OC and coal ash [54,55,56].

3.3.2.1 Ilmenite Ore as Bed Material

Corcoran et al. [8] have investigated the physical and chemical changes of ilmenite when it is used as OC during a typical OCAC process in the CUT 12 MW_th CFB boiler. As shown in Fig. 3.25, the particle of ilmenite ore underwent an element segregation with an iron moving to the surfaces and titanium enriched in the core. Such a phenomenon is quite like the observation in a CLC process when the ilmenite is subjected to redox cycles. Figure 3.26 showed the EDX line profiles of the outer parts of an ilmenite particles cross-section from sample II (24 h), a calcium-enriched double lamellar structure with iron layer surrounded by calcium was observed on the ilmenite particles. Interestingly, the potentially problematic element compound (potassium) was found to diffuse into the ilmenite particles to form a homogeneous compound, and the concentration of K in the particle increases sharply with time on stream. The XRD analysis shows that potassium titanium oxide (KTi₈O₁₆) was formed in the particle’s core, which suggested the reaction between K and TiO₂. In addition, a 72 h’ leaching experiment was carried out with Sample II in Fig. 3.16 by deionized water to understand the leaching properties of potassium titanium oxide, the results as shown in Fig. 3.27. Figure 3.27 showed that the highest amount of K and Ca in the leachate were a very limited degree, namely, to less than 32 ppm (<1%) and 7 ppm (<0.2%), respectively. This indicates that the contents of water-leachable Ca and K were very limited. The water-leachable K might be derived from the unreacted K, and it became water-unleachable once forming KTi₈O₁₆.

Fig. 3.25

EDX maps of the distribution of iron, titanium, calcium, and potassium in the cross-section of an ilmenite particle from sample in 24 h, (Reprinted from [8] with permission of American Chemical Society)

Fig. 3.26

EDX line profiles of the outer parts of an ilmenite particles cross-section from sample II (24 h) (reprinted from [8] with permission of American Chemical Society)

Fig. 3.27

Elemental composition in leachates from fresh ilmenite as well as samples I (1 h), II (24 h), III (48 h), and IV (72 h) exposed in the CFB boiler (reprinted from [8] with permission of American Chemical Society)

Corcoran et al. [13, 41] studied the mechanisms of element migration and ash-layer growth on the ilmenite OC during the OCAC of biomass in the CUT 12 MW_th CFB combustor. The fresh ilmenite included over 50 wt% of fine particles (<125 μm), less than 1 wt% of large-sized particles (250–355 μm). During the 364 h test, the irregular bed material was sampled. The resulting cumulative size distribution curve of the bed material is shown in Fig. 3.28. With the increase of time, the size of the particles increased significantly. For the fresh OCs, most of the particles are found to be in the size range of 90 − 180 μm, which increased to 125 − 250 μm after exposure. When the OC bed materials were used for 147 h, over 95 wt% of the particles were larger than 125 μm, and over 10 wt% of particles were above 250 μm. It should be emphasized that particles with diameter larger than 355 μm were found in all samples, regardless of exposure time. Only the sample exposed for more than 364 h shows a deviation from this trend. This deviation could have two reasons: (1) new material was added to the boiler to maintain the pressure drop over the bed; (2) a structural damage can occur after long exposure in the reactor [12]. In addition, agglomeration was not found in any bed sample.

Fig. 3.28

The size distribution curve of bed particles at different test times (reprinted from [13] with permission of American Chemical Society)

Figure 3.29 shows images of the calcium and potassium elemental cross-sections of OC particles after different times of operation. Both Ca and K migrate to the interior of bed particles, and the K migrates much faster than Ca. Concurrently, Ca and other ash components (such as Si and P) accumulate on the surface of the bed particles forming an outwardly growing ash layer. No K was found in this layer (as shown in Fig. 3.30). In addition, the authors found that the particles became more porous as the time-on-stream increased, which might be due to the replacement of Fe within the Titanate complex by guest elements (such as K and Ca) from the ash-layer. Consequently, the increase of particle size for OC bed particles probably contributed to the growth of the ash-layer.

Fig. 3.29

The distribution of Ca and K on the ilmenite ore particles [41]

Fig. 3.30

Migration of Ca and K into the particle, the content is the weight percentages [41]

The reactivity of OC samples can be tested using a laboratory scale bubbling fluidized bed. In the tests, syngas was used as fuel and the oxygen transfer capacity of the OC was established by the oxidation rate of CO (also called CO₂ yield). The CO₂ yield of OCs with different exposure time from a 12 MW_th CFB-boiler are summarized in Fig. 3.31. The relatively inert quartz sand was used as a reference. The fresh ilmenite ore shows a low initial CO₂ yield, while that of used OC samples increases with exposure time, except for the OC after 322 h exposure, where the CO₂ yield decreased slightly. This was because as the reaction time increases, ilmenite ore undergoes more redox cycles, more iron atoms migrate to the outer surface of the ilmenite particles to form a Fe-rich shell [57, 58]. At the same time, the pore structure of the oxygen carrier particles would develop [58], resulting in a high reactivity of OCs. These phenomena were also found in CLC technology. Furthermore, the decrease in reactivity of the sample with a longer test time (322 h) indicates that the ilmenite has a limited lifetime as an oxygen carrier, which is due to the reduction in the number of reactive sites. There are two possible reasons: (1) the ash layer covers the surface of the bed particles, and (2) the abrasion of the surface.

Fig. 3.31

CO₂ yield of bed samples with different exposure time from bench scale experiments with syngas as fuel. Solid lines denote materials obtained from the 12MW_th CFB-boiler, while dashed lines represent fresh materials (adapted from [59])

Vigoureux et al. [16] investigated the accumulation of alkali metals and S on OC during the OCAC of wood with the addition of elementary S in the 12 MW_th CFB boiler (as shown in Fig. 3.32). It was found that ilmenite as bed material can absorb not only Ca and K but also S. Besides, the absorption of S by aged ilmenite with an ash-layer is higher than that of a fresh one. This was because the aged ilmenite has higher contents of Ca and K, especially in the ash-layer, which could react with S [38]. Furthermore, the aged ilmenite exhibits a more developed pore structure with higher surface area, which facilitates the accumulation of S on and within the particles [58]. Therefore, when fresh ilmenite was used, S accumulated mainly in the ash-layer, while when aged ilmenite was used, S was found in the particle core. In addition, the quantitative analysis suggested that K₂SO₄ may be formed.

Fig. 3.32

The element distribution of Ti, Fe, C, K and S on the cross-section of OC of ilmenite ore after the OCAC process. The “Un” means fresh ilmenite particle, I and II means using the fresh and used ilmenite particle as bed materials, respectively. The asterisk means undetectable (reprinted from [16] with permission of American Chemical Society)

3.3.2.2 Manganese Ore as Bed Material

Hanning et al. [14] investigated the interaction between biomass and manganese ore as a bed material in the same 12 MW_th CFB boiler. No bed material was regenerated during the one-week experiment period. The concentration of different components in bed samples as a function of operational time are shown in Fig. 3.33, the results indicated that Si, Ca, K, and S elements accumulate on the bed material. The SEM–EDS results of the samples taken after 172 h of operation are shown in Fig. 3.34. The Si, Ca and K could be found on the OC particles, both inside and outside, while S was mainly in the outer ash-layer. The formation of an ash-layer on the particle surface leads to a significant increase of particle size, which continues with a prolonged OCAC operation. The fresh OC particle-size range was 100–400 μm, with an average size of 200 μm. After 172 h of exposure, a large number of bed particles in the sample were larger than 500 μm. Despite the increase of the size of the OC particles, the bed material did not show any severe agglomeration during the one-week test. Interestingly, some hollow particles could be found after a 172-h operation. The elemental distribution of a hollow particle is shown in Fig. 3.35, showing that Ca and Si are enriched in shell of the hollow particle, while Si, K, Mn, Fe and S exist inside the particle. The study concluded that the formation of hollow particles is mainly due to the collision between ash and char, resulting in a char particle surrounded by an ash-layer. With the burn-out of char core, a hollow particle with a shell of ash-layer forms [43].

Fig. 3.33

Concentration of different components in the fresh material and used bed samples as a function of operational time (reprinted from [14] with permission of Elsevier)

Fig. 3.34

Elemental distribution of manganese, aluminium, potassium, calcium, silicon and sulphur in the sample taken after 172 h of operation (reprinted from [14] with permission of Elsevier)

Fig. 3.35

Elemental distribution of Mn, Fe, K, Ca, Si and S in the hollow particle from the sample taken after 172 h (reprinted from [14] with permission of Elsevier)

As shown by Fig. 3.36, the reactivity of the bed particles towards syngas decreases as the OCAC operation time increases. There are two possible reasons: (1) during the operation of the boiler, the structure of the bed particles may change, affecting their reactivity, such as the deactivation caused by accumulation of ash elements on the surface; (2) the ash elements, migrating into the particles, may react with the active components to form inactive substances.

Fig. 3.36

CO₂ yield of bed samples with different exposure time from the boiler. The conversion is tested with syngas (adapted from [59])

With silica sand as bed material, it is usually necessary to regenerate the bed material regularly to avoid bed agglomeration, as mentioned above. However, a bed of manganese ore did not show any tendency of agglomeration, even if the boiler was operated at 870 °C for a whole week. This is because the manganese ore is not prone to ash melting, which is positive for the continued safe operation of the boiler. From another point of view, the concentration of alkali vapor in the flue gas may increase if less alkali is absorbed by the manganese ore, which may affect the combustion, fouling and corrosion inside the boiler. The fouling and corrosion behavior of the boiler with manganese ore as OC should be further investigated in detail.

3.4 The Aging of Oxygen Carrier

In the general operation of a biomass-fired CFB boiler, the used bed material is partially and continuously replaced by fresh one to avoid agglomeration issues. As the residence time of the bed material increases, the physicochemical properties of the bed material change to a certain extent. This process is called bed-material aging. Several phenomena cause aging in the thermal conversion of biomass, including the formation of an ash layer and of pores, cracks, and cavities [60,61,62].

Corcoran et al. [12, 13] evaluated the structural development and mechanical change of ilmenite OCs (rock- and sand-ilmenite) during long-term OCAC operation. They found that the cavities within particles and the cracks and clusters on the particle’s surface developed with the exposure time (the SEM micrographs of ilmenite particles extracted from different exposure time are shown in Fig. 3.37 and Fig. 3.38). The study evaluated samples of fresh ilmenite and of the same OC collected after a certain period of operation in the CUT 12 MW_th CFB boiler. During the operation, the sand-ilmenite generated several cavities. The cavity size increased, and more cavities emerged with longer operation time. The cavities were held together by an ash-layer before the particles were shattered into numerous pieces. The rock ilmenite particles first developed cracks, then the cracks extended inward, leading eventually to the splitting of the particle. The mechanical strength of both materials was weakened due to the development of cavities and cracks. The possible ways for particle degradation of sand and rock ilmenite particles are displayed in Fig. 3.39.

Fig. 3.37

SEM images of ilmenite particles (0, 5, 27, 51, 147 and 364 h). The top row shows overviews of particles, and the bottom row shows the cross-sections of particles (reprinted from [13] with permission of American Chemical Society)

Fig. 3.38

SEM micrographs of cross-section of ilmenite particles extracted after 2 and 15 days of exposure where a and b are sand ilmenite and c and d are rock ilmenite (reprinted from [12] with permission of Elsevier)

Fig. 3.39

The schematic of possible ways for particle degradation of sand and rock ilmenite particles [41]

Combined with the attrition test, it was found that the mechanical resistance of both fresh ilmenite OC is similar, but that of sand is slightly higher. After the OCAC operation, the sand ilmenite particles became more prone to attrition, while its mechanical resistance did not change dramatically over time. The rock ilmenite was initially sensitive to mechanical stress, but it became more resistant over time [12].

Appropriate determination of bed regeneration frequency is the key to the economy of operation of OCAC. Higher regeneration frequency of bed material reduces the risk of agglomeration and ensures the mechanical strength and high reactivity of OCs but increases the cost of OC procurement. However, if the regeneration frequency of bed materials is too low, it will not only increase the risk of agglomeration, but also reduce the reactivity of OCs due to ash-related problems (especially using high-ash fuel). Moreover, mechanical crushing and elutriation may lead to the deficit of OC particles in the reactor, resulting in excessive renewal. The optimal regeneration frequency is determined by the fuel properties (volatile content, ash content and elemental composition) and the OC physicochemical characteristics (composition, reactivity, abrasion resistance and crushing characteristic), which should be considered comprehensively in OCAC applications.

3.5 Bed Material Recovery

To reduce the cost of OC, it is very important to recover and reuse OC from slag discharge. Moldenhauer et al. [18], Gyllén et al. [59] and Lind et al. [63] presented an idea of magnetic separation for ilmenite OCs in operating large-scale CFB boilers. The proposed magnetic separator consists of a belt stretched across two horizontal cylinders. It can be seen from Fig. 3.40 that the left cylinder comprises an electric drum, and the right cylinder includes four sections of powerful neodymium magnets. The pre-treated bed materials is fed evenly to the belt by a distributor. Due to inertial force, the trajectory non-magnetic material shows a parabola and falls in front of the separator, while the magnetic material will fall from the belt behind the magnetic drum due to the attraction of the magnetic drum. The magnetic separator has been used in both the 75 MW_th and the 115 MW_th CFB boilers. It should be noted that the bed material should be sieved to remove impurities, including nails and stones, before added to separator.

Fig. 3.40

The schematic diagram of the roll belt magnetic separator (reprinted from [59] with permission of Elsevier)

Moldenhauer et al. [18] investigated the effect of the ratio of ilmenite ore in the bed material by measuring its magnetic content and the feed rate of fresh ilmenite to the 115 MW_th CFB boiler. In the first day, the feeding rate of fresh ilmenite was set to 15 t/day, then the feed rate was increased to 25 t/day in the second day. On the third day, the feed rate was set to 15 t/day, where it was kept for about two weeks followed by three days at 8 t/day, two days at 4 t/day and finally a little more than four days at 2 t/day. During the transition back to silica-sand operation, the feed rate was set to the nominal value for silica-sand operation, i.e. 8 t/day.

The magnetic fraction in the bed ash and feeding rate of fresh ilmenite as shown in Fig. 3.41. It should be noted that the bed particles with a size over 710 mm were discarded, which accounted for almost 6 wt% of the total bed inventory. The results showed that the magnetic content increased rapidly during the first 130 h, since a significant portion of the silica sand was gradually replaced by ilmenite ore. The magnetic susceptibility of the bed particles increases along with residence time. Interestingly, when the regeneration rate of bed material fell from 15 to 8 t/day, the magnetic fraction in the bed continued to increase. Even when the regeneration rate dropped from 8 to 2 t/day (approximately 0.72 kg/MWh), the magnetic fraction still rose. This indicates that the longer the residence time of ilmenite is in the furnace, the more magnetic it becomes. By testing the reactivity of the magnetic and non-magnetic fractions in a lab-scale bubbling bed with syngas as fuel, the results are plotted together with results from the Chalmers 12 MW_th boiler for comparison, as shown in Fig. 3.42. The reactivity of the sample accepted by the separator was close to that of 303 h sample from 12 MW_th CFB boiler, while the sample rejected by the separator has quite low reactivity. These results indicated that magnetic separation was a feasible way to recover the ilmenite ore from the boiler ash [59].

Fig. 3.41

Measured magnetic fraction in the bed ash and feeding rate of fresh ilmenite to the furnace (reprinted from [14] with permission of Elsevier)

Fig. 3.42

Conversion rate of syngas for different samples (adapted from [59])

In addition, it is essential to note that the above OC recovery case was for biomass combustion, whose ash content is small. Therefore, bed material must always be added to the boiler, and the replaced bed material (OC) is easy to add and discharge which facilitates OC recovery. However, for boilers burning high ash fuels (such as coal), the bed inventory in the boiler is always more than sufficient, and bottom ash needs to be discharged continuously. Therefore, increasing the residence time of OC in CFB boiler and low-cost recovery of OC from the discharge of the bottom ash is the basis for OCAC application in the case of high-ash fuel. On the one hand, the residence time of OC in the boiler can be increased and the amount of OC in slag can be reduced by reducing the size of OC; on the other hand, combined with the differences of magnetism, density and particle size between OC and ash, magnetic separation, pneumatic concentration, and sieving can be used to recover the OC from the ash discharge.

In addition, a separation procedure can combine the changes in the physicochemical characteristics of OC after being used in order to reduce the production costs of downstream companies providing them with better raw material. For example, after ilmenite ore has undergone multiple redox cycles, an iron-rich layer will be formed on the surface of the particles [29, 60]. The ilmenite ore particles are abrased for a long time before being discharged [41], which will move the Fe content to the ash and correspondingly increase the Ti concentration in the spent particles. High-concentration Ti-containing ore particles may benefit downstream industrial applications that facilitate the production of Ti-related products. Different schemes for the utilization of OC require comprehensive consideration of the complex factors in the furnace environment, such as fuel characteristics, physicochemical properties of OCs, impurities of OCs, potential downstream industries and their own processing. Therefore, the selection and low-cost utilization of OCs under different OCAC application scenarios require specific case analysis with contributions from diverse areas.

3.6 Characteristic Properties of OC

The selection of an OC is mainly determined by its reactivity, its price and whether it is suitable for long-term operation. General, important criteria for a good OC are as followed: (1) low production cost; (2) high reactivity with fuel and oxygen; (3) low fragmentation and attrition, as well as low tendency for agglomeration [64]. In the existing studies of OCAC, the selected OC is frequently ilmenite ore, manganese ore, and steel slag, of which ilmenite ore was used in most cases. Ilmenite ore is widely used by researchers due to its low price, acceptable reactivity, and good resistance to sintering and wear in fluidized beds [13, 29, 41, 59]. The disadvantages of manganese ore and steel slag are that they are easy to wear and have low reactivity.

In addition, the research on OC characteristics has accumulated a good knowledge in the CLC technology [65,66,67,68], which can provide a strong guarantee for the application of OCAC, including various natural ores and synthetic oxygen carriers. The choice of bed material needs to consider the reactivity of the OC and the mechanical properties (wear resistance), economy, and whether it will have other effects on the safe and efficient operation of the system. For the application of natural ore/synthetic materials with moderate reactivity but low cost, the product gas can also be regulated by controlling the addition amount of OC and adjusting the operation mode (such as adjusting the fluidization state). Overall, the bed material should be selected based on a comprehensive consideration of various characteristics of the material and the operating characteristics of the boiler, rather than a single characteristic such as reactivity, oxygen-carrying capacity and cost.