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THESIS FOR THE DEGREE OF BACHELOR OF CHEMICAL ENGINEERING

Preparation of Cu-based oxygen carriers for Chemical-looping combustion

By Rosen Angelov

Professor: Prof. Dr.-Ing. Stefan Heinrich

Supervisors: Dipl.-Ing. Marvin Kramp

Dipl.-Ing. Andreas Thon

Hamburg 2012

Contents

Introduction

1. Theoretical part

1.1 Capturing of CO₂

1.2 Chemical Looping Combustion (CLC)

1.3 Oxygen carriers in CLC process

1.3.1 State of art

1.3.2 General oxygen carriers characteristics

1.3.3 Dry impregnation method

1.4 Fluidization

1.5 Fluidized Beds

1.5.1 Advantages and disadvantages of the Fluidized-Bed Reactor

1.6 Objectives

2. Experimental part

2.1 Materials

2.1.1 Gamma alumina

2.1.2 Copper(II) nitrate trihydrate

2.2 Preparing of solution

2.3 Impregnation calculations

2.4 Description of Fluidized bed reactor

2.5 Description of process of impregnation

2.6 Calcination

3. Results

3.1 SEM analyze of materials

3.1.1 SEM pictures at 250x zoom

3.1.2 SEM pictures at 1500x zoom

3.1.3 Discussion of the SEM pictures

3.2 Fluidization of the materials. Calculating the minimum fluidization velocity

3.2.1 Puralox (?-Al₂O₃)

3.2.2 Impregnated/dried Al₂O₃/Cu(NO₃)₂

3.2.3 Al₂O₃/CuO used in CLC facility

3.2.4 Discussion on fluidized tests

Conclusions

Acknowledgement

References

Introduction

These days are generally accepted that the greenhouse gas in developed industrial countries must be reduced as much as possible. Carbon dioxide is one of the most important greenhouse gases contributing to global warming. The CO₂ capture and storage (CCS) is a process involving the separation of CO₂ and the storage over the long term. There are different CCS technologies available or under development, but most of them are consuming a lot of energy and cost a lot of money.

The original idea of Chemical-looping combustion with using “solid oxidizing agents”, or as they are called now oxygen carriers, was born back in 1954 by Warren Lewis and Edwin Gilliland. Their original idea was producing pure CO₂, which was free of inert gasses like nitrogen [1]. CLC as a term was used for first time in 1987 by Ishida et al. [2]. The next few years there are several publications in literature about CLC (Ishida & Jin, 1994; Anheden, Nasholm, & Svedberg, 1995) [3]. The CLC as a combustion method develops rapidly in the last few years because of its advantages of CO2 capturing and low (minimum) energy cost used for separation [4].

1. Theoretical part

1.1 Capturing of CO₂

It has been known since 1896 that carbon dioxide is a greenhouse gas which is released from fossil fuel combustion and may affect the climate of the earth. In the last decade the concerns of growing emissions of the greenhouse gas increased significantly. In the developing countries, the economic growth results in a rapid increase in the demand for energy supplied by fossil fuels, while the developed countries have not yet found means for sufficient decrease their own use of these fuels. Furthermore, various options need to be investigated in the future) [2].

CO₂ removal from gases is a process of acid gas removal generally referred to as gas treating. The CO₂ separation requirements depend from different circumstances on the process used and the plant feedstock's, which are mostly light or heavy hydrocarbons or coal. The process requires a lot of energy and money. One of the strategies to decrease the amount of CO₂ emissions is to separate the CO₂ from the fuel gas and to store it. The scheme of CCS technology is shown in figure 1.

Figure 1. CCS Technology [5]

In the Post combustion fuel (gas/coal/biomass) and air are mixed in power plant where inert N₂ and unreacted O₂ are separated and almost pure CO₂ is captured.

In Pre combustion the solid fuel (coal or biomass) is gasificated with air flow and steam and then it went through process of reforming where the separation of CO₂ happened. Reforming is followed by process of hydrogenation and then the fuel enters the reactor. Exit gasses are N₂ and O₂.

During the oxyfuel process air is separated in advance of inert nitrogen and oxygen. Oxygen and fuel are mixed in reactor where the exit gas is CO₂.

There are several possibilities for such sequestration has been proposed [2]:

Ш storage in used oil and gas fields;

Ш storage in deep coal beds;

Ш storage in aquifers;

Ш deep sea storage;

Ш deep sea bottom storage.

There is other useful option which includes some energy efficiency improvements, the switch of less carbon-intensive fuels, renewable energy sources like sunlight and wind power, nuclear power and others that must be considered [6].

The global growth and distribution of CO₂ emissions is given in figure 2 bellow.



Figure 2. Global growth and distribution of CO₂ emissions 2000-2005 according to World Resources Institute [7]

From this figure can be concluded that the most of released emission are coming from electricity and heating, followed by transport and industry. There is a trend of decreasing the amount of released CO_2.

1.2 Chemical-looping Combustion

Chemical-looping combustion is a novel technology for carbon containing fuels preventing the CO₂ emissions released at atmosphere by inherent separation of the greenhouse gas carbon oxide. In the chemical-looping combustion (CLC) process, fuel gas (natural gas, syngas,) is burned in two interconnected reactors. In the first one, an oxygen carrier (metal oxide) that is used as oxygen source is reduced by the feeding gas to a lower oxidation state, where CO₂ and steam are reaction products. In the second reactor, the reduced solid is regenerated with air to the fresh oxide, and the process can be repeated for over 100 successive cycles. The carbon dioxide can be easily isolated from the outlet gas coming from the fuel reactor by steam condensation [8].

The CLC system is made of two interconnected reactors - air and fuel reactor, as shown in figure 3:

Fig. 3. Chemical-looping combustion. MeO/Me denote recirculated oxygen carrier solid material.

In the fuel reactor, the fuel gas is oxidized to CO₂ and H₂O by a metal oxide through the chemical reaction:

(2n + m)MeO + CnH₂m > (2n + m)Me + mH₂O + nCO₂ (1)

The exit gas stream from the fuel reactor contains CO₂ and H₂O, and almost pure CO₂ is captured water is condense. The reduced metal oxide, Me, is transferred into the air reactor where the metal is oxidized according to equation (2):

Me + ?O₂ > MeO (2)

The flue gas leaving the air reactor contains N₂ and unreacted O₂. The exit gas from the fuel reactor contains CO₂ and H₂O, which are kept apart from the rest of the flue gas. After water condensation, almost pure CO₂ can be obtained with no energy lost for component separation. Depending upon the metal oxide used, reaction (1) is often endothermic, while reaction (2) exothermic. The total amount of heat evolved from reactions (1) and (2) is the same as for normal combustion, where the oxygen is in direct contact with the fuel [2].

The reactors in Fig. 2 could be designed in different ways, but two interconnected fluidized beds have an advantage over other alternative designs, because the process requires a good contact between gas and solids [1]. The system proposed is a circulating system composed of two connected fluidized beds, a high-velocity riser and a low-velocity bubbling fluidized bed (figure 4).

In the Chemical-looping combustion the cornerstone are metal oxides which release oxygen - oxygen carriers. They circulate between fuel and air reactor and fuel is never in direct contact with air [9]. Oxygen carriers are first placed in air reactor. In air reactor the fuel gas is oxidized by the metal oxides. The exhaust gases here are inert nitrogen mostly and some unreacted oxygen. The driving force here is gas flow, which makes particles in fluidized state. With increasing the gas flow rate, velocity is increasing too.. Oxidized particles from air reactor through a cyclone device are transferred in the fuel reactor. Here in the fuel reactor, the fuel gas is reduced to CO₂ and H₂O. After water condensation almost pure CO₂ can be derived with minimum energy lost. By-product here is ashes, which are result from the combustion. In addition the fuel reactor can be considered as a bubbling fluidized bed and particles from the fuel reactor are transferred to the air reactor back by the force of gravity.



Figure 4. Layout of chemical-looping combustion process, with two interconnected fluidized beds [3].

1- Air reactor

2- Cyclone

3- Fuel reactor

1.3 Oxygen carriers in CLC process

1.3.1 State of art

This work was based on previous investigations. The dry impregnation method technic was based on the investigation of Luis F. Diego et. al [12]. He used CuO/Al₂O₃ oxygen carriers with content of CuO between 10 and 26 wt %, prepared by wet and dry impregnation methods. These oxygen carriers were analyzed in a fluidized bed facility during 100 reduction-oxidation cycles. He used CH₄/N₂ as fuel to determine. It was found that CuO/Al₂O₃ oxygen carriers with a CuO content lower than 10 wt % never agglomerated and those with a CuO content greater than 20 wt % always agglomerate. It was concluded that this phenomenon was indipendent of the preparation method and the calcination temperature used in their muffle oven. On the other hand, the behaviour CuO/Al₂O₃ oxygen carriers with intermidiate CuO content (15-17 wt %) depended on the calcination temperature used [12].

The oxygen carrier transfers oxygen from air to the fuel, avoiding the direct contact between them. The metal oxide, used as an oxygen carrier in chemical-looping combustion, must have sufficient mechanical strength in multiple successive cycle reactions, good rates of reduction and oxidation, must not agglomerate and must have pore structure. It is also an advantage if the metal oxide is cheap and environmentally safe. There are certain metals and their corresponding oxides which are proposed in literature such as: Fe, Ni, Co, Cu, Mn, and Cd. These metals and their oxides are combined with an inert that acts as a porous support enhancing the reactivity properties and stability of the active phase. Moreover, this inactive binder increases the mechanical strength and the attrition resistance of the oxygen carrier. In the previous works, oxygen carriers based on Ni or Cu show the highest reactivity with different support materials [10].

In the literature of chemical-looping combustion, the conversion rate X is often used to determine the degree of conversion of the oxygen carriers. The value is defined as the fraction of the difference between the mass of the oxygen carrier (m) and the mass of the oxygen carrier in its reduced state (m_red), and the difference between the mass of the oxygen carrier in its oxidized state (m_ox) and in its most reduced state:

 (3)

The degree of mass-based conversion ? is sometimes also used as a measure of the oxygen carrier conversion, as it is convenient for comparisons of different materials of different oxygen carrying capacity.

(4)

X can be converted to ? with the following equation:

(5)

where R₀ is the oxygen transfer capacity, which from the other side is the fraction of available oxygen in the oxygen carrier:

(6)

1.3.2 General oxygen carrier characteristics

There are several very important characteristics that oxygen carriers must possess:

v Good mechanical strength.

One of the most important qualities that the oxygen carriers must have is good mechanical strength. This gives them sufficient durability in cycle reactions and lowers the attrition rate of the particles during the CLC process.

v Good gas conversion.

For the efficiency of the process sufficient gas conversion must be achieved in both fuel and air reactors. This characteristic depends on several circumstances - type of used oxygen carriers, support material, reactor type.

v Sufficient oxygen capacity

The maximum oxygen capacity is determined mainly by the type of oxygen carriers which were used. For example in previous work where this property was investigated, it was conclude that the oxygen capacity of the Cu- and Mn-based oxygen carriers have the higher rates [11].

v High rates of reaction

A proper selection of primary metal oxides, supports, particle synthesis techniques and reaction conditions can enhance the reaction rate. Higher rates of reaction even allow a small reactor can be used to achieve the same results like bigger [12].

v Low cost price

The raw material cost and the cost of synthesizing the particles are of important economic consideration. This must be taken on mind in case of large rates of production.

1.3.3 Dry impregnation method

The dry impregnation method is often used for preparing oxygen carriers for Chemical-looping combustion process. In this work Cu-based oxygen carriers were prepared by this method and results that appeared was concidered as satisfied. The first step is selection of the support material. In that work material used as support was Puralox (?-Al₂O₃). The support is used in its powder form. Like it was mancioned in previous chapter, support material improves the particles strenght and reactivity abilities. This process were performed in fluidized bed reactor with Wurster-draft tube. After being doped on the support, the oxygen carriers must be calcined. Particles are first calcined at lower temperature for at least two hours. The calcination results in reduction of the metal nitrates to metal oxides. Finally, high-temperature calcination is carried out to obtain the desired physical stability..

The method is suitable for the synthesis of all types of oxygen carriers. However the method predominantly is used for a copper-based looping medium to reduce the effect of copper agglomeration. It is believed that the reduced agglomeration effect results from limited metal loading in the pores of the support [12].

1.4 Fluidization

Fluidization is process in which solid particles are transformed into a fluidlike state through suspension in a gas or liquid [13]. The ?uidization principle was ?rst used on an industrial scale in 1922 for the gasi?cation of ?ne-grained coal [14]. Since then, ?uidized beds have been applied in many industrially important processes. Fluidized beds can be used for a large scale of processes such as cooling-heating, drying, sublimation-desublimation, adsorption-desorption, coating, and granulation, to many heterogeneous catalytic gas-phase reactions as well as noncatalytic reactions [15].

In ?uidization a packed bed of solid particles is brought to a “?uidized” state by an upward stream of gas or liquid as soon as the gas velocity rate of the ?uid exceeds a certain limiting value Umf (where mf denotes minimum ?uidization). In the ?uidized bed the pressure drop ?p_fb of the ?uid on passing through the ?uidized bed is equal to the weight of the solids minus the buoyancy, divided by the cross-sectional area At of the ?uidized-bed vessel (7):

(7)

In Equation (7), the porosity ? of the ?uidized bed is the void volume of the ?uidized bed (volume in interstices between grains, not including any pore volume in the interior of the particles) divided by the total bed volume; s is the solids apparent density; and H is the height of the ?uidized bed. In many respects, the ?uidized bed behaves like a liquid.

Figure 5. Pressure drop in ?ow through packed and ?uidized bed

1.5 Fluidized Beds

oxygen impregnation alumina solution

The driving force of the fluidized bed reactors is the upward flow of gas or liquids. There are different types of fluidized beds reactors, depending of state that particles inside are. If the flow rate is very low and the particles are still, then this bed is called Fixed (figure 6 a). When the flow rate rise the bed come in the condition where the particles start rising a bit and this bed is called Minimum fluidization bed (figure 6 b). If the flow rate continues to rise, the process of forming bubbles has begun and this bubbles move through the particles unitill they reach the surface of the bulk. This is called Bubbling fluidized bed (figure 6 c). If the flow rate is to high, the particles are carried out of the bed and there is no bulk formation. In this case the fluidized bed is called Lean-phased (figure 6 e ).



Figure 6. Forms of fluidized beds [15].

1.5.1 Advantages and disadvantages of the Fluidized-Bed Reactor

There are several advantages of fluidized bed reactors that were concidered during the experiments:

v The fluidized bed is easy to work with

v The bad can be applied for particles with different size

v Can be used for both small and large quantities

v There are no hot spots during the process where the temperature is higher

v Some of the fluidized beds got filters, that can be autocleaned

There are some disadvantages of fluidized beds:

v It can not be work on high temperatures

v If the particles inside are with low size, they can adhere to the fluidized bed shapes

v Some beds require specific software

1.6 Objectives

The main goal of this work was to synthesize large quantities of Cu-based oxygen carriers with gamma alumina as support (?-AL₂O₃), which are used in Chemical-looping combustion (CLC) technology. The produced oxygen carriers should have good qualities. The work in this thesis was mainly experimental. For the most of the experiments was used fluidized bed reactor with Wurster-draft tube. According to calculations before experiments, the produced Cu-based oxygen carriers should have 13% content of CuO.

The other part of that work was to analyze the produced material using Scanning Electron Microscope (SEM) and fluidization test performed in fluidized bed reactor.

2. Experimental

2.1 Materials

For preparing of Cu-based oxygen carriers in this work were used as support commercial ?-alumina (Puralox NWa-155, Sasol, Germany GmbH) particles of 0.1-0.5 mm with a density of 0.76 g/cm3 and a porosity of 55.4 % were used as support and dry impergnation method should be applied. The properties of the materials used are given below.

2.1.1 Puralox (?- Al₂O₃)

Gamma-Al₂O₃ is white fine white powder (see figure 7) used as support for Cu-based oxygen carriers. It enhances the mechanical strength of the oxygen carriers and also lowers the attrition rate. Gamma alumina has large specific surface (164 m²/g) which made this support suitable for the purpose of this work. Other advantage of this material is its relatively large porosity (54.5%). The higher porosity allows more copper nitrate to be impregnated in the alumina particles. It is not enviormentaly danger.

Figure 7. Gamma alumina powder

Some of the material characteristics are given in Table 1 and Table 2.

Table 1. Product information [16]

Synonyms	Gamma alumina, Puralox NWa-155
Chemical formula	?- Al2O3
Molar mass	101.96 g/mol

Table 2. Chemical and physical data [16]

Test	Units	Value
Specific surface	m2/g	164.0
Al2O3 - content	%	98.4
SiO2 - content	ppm	51.0
Fe2O3 - content	ppm	54.0
Na2O - content	ppm	4.0
Bulk density	g/cm3	0.76
Porosity	%	54.5
Particle distribution <100 Mikron	%	1.0
Particle distribution <500 Mikron	%	99.9
Solubility	g/l	2670

Particles distribution of the Puralox NWa-155 is shown in figure 8.

Figure 8. Particles distribution of Puralox NWa-155

2.1.2 Copper(II) nitrate trihydrate

The copper(II) nitrate trihydrate is small blue crystals which are highly hygroscopic (figure 9). Left in contact with air they agglomerate fast and become hard. The copper nitrate trihydrate has good solubility in water. The measured solubility was 600 g/l and the solubility given from the distributor is over four times higher (2670 g/l). The copper nitrate has pH value 3-4, which makes it with acid characteristics.

Figure 9. Copper(II) nitrate trihyrdate crystals

Some of the characteristics of copper nitrate trihydrate are given in Tables 3 and 4.

Synonyms	Copper dinitrate trihydrate
Chemical formula	Cu(NO3)2*3H2O
Molar mass	241.6 g/mol

Table 3. Product information [17]

Properties	Units	Value
Melting point	°C	114
Density	g/cm3(20°C)	2.05
Bulk density	kg/m3	1050
pH value	lgH+	3-4
Solubility	g/l (20?C)	2670

2.2 Preparing of solution

The first part of experimental part was to prepare the oxygen carriers. The first step was preparing solution of Cu(NO₃)₂*3H₂O for impregnation. Before making the solution, we need to investigate the solubility of Cu(NO₃)₂*3H₂O in water. To investigate the solubility sample of 100g of the material was taken and placed in the flask of glass, which weight is measured in advance. The flask was placed on magnetic stirrer and small amounts of water were added during the mixing process while the copper nitrate crystals are fully dissolved. After it's fully dissolved the flask is put on the balance. From the difference in weights we can calculate the amount of water we used. The results from this experiment are given in the table below:

Table 5. Calculation of solubility

flask+fish, g	material, g	Material+water+flask,g	water only,g (18?C)
182,4	100	343	60.6

The results are showing that 100g of Cu(NO₃)₂*3H₂O can be dissolved in 60.6g of water. Since solubility is known, the next step is to prepare a solution in larger quantities. For this purpose is used larger container and stirrer like ones shown in figure 10:



Figure 10. Experimental setup for copper nitrate solution

When the exact amount of solution need and proportions material/water are calculated, first the water was placed in container, and then while stirrer is working on small portions was added the copper nitrate. After about hour of mixing the copper nitrate was dissolved and the solution was ready. The used copper nitrate trihydrate was not with sufficient quality and after dissolving of the solution in the bottom of the container there was white sludge left. Then optimization of method was tried like using the gravitational sedimentation and pumping the upper layer with limpid solution in a 10 litter tube. The final solution can be seen in figure bellow:



Figure 11. Solution of Cu(NO₃)₂*3H₂O

2.3 Impregnation calculations

For calculating how much solution was needed for exact amount of alumina, the following equations were used:

1) Finding the mass of CuO needed. We know in advance that the copper oxide in the material should be 13% of total weight.

m_CuO = m_Al2O3*0.13, g

2) Calculating the mole fraction of CuO:

n_CuO = m_CuO/M_CuO, mol

where M_CuO is molar mass of copper oxide and it is:

M_CuO = M_Cu + M_O, g/mol

3) Calculating the mass of the solid crystals Cu(NO₃)₂*3H₂O:

m[Cu(NO₃)₂*3H₂O] = M[Cu(NO₃)₂*3H₂O]*n[Cu(NO₃)₂*3H₂O], g

From the mole equation the followed conclusion was made:

n[Cu(NO₃)₂*3H₂O]:n[CuO] = 1:1

4) Calculating the amount of water needed to dissolve the solid material:

m_H2O = (m[Cu(NO₃)₂*3H₂O]*60.6)/100, g

These values come from the conclusion, that 100g of the copper nitrate trihydrate are dissolved in 60g of water.

5) Calculating the final amount of solution

m_solution = m[Cu(NO₃)₂*3H₂O] + m_H2O, g

2.4 Description of the fluidized bed reactor

Figure 11. Technical scheme of Fluidized bed

In figure 11 the inlet air flow is marked with arrow. The air flow is transported via pipes and first passes through the temperature sensor where the temperature is measured and also for the moisture . There is a heater before the air flow reaches the fluidized bed from below and brought particles inside to fluidized state. There is a nozzel in the bottom side of the draft tube which sprays the solution over the fluidized particles. Inside the fluidized bed there is another termo sensor, which measure the temperature of the particles. There are filters at the top of the fluidized bed which are autocleaned over period of time. Compressed air is used to create a vacuum in the pipe where the solution is sucked, but also is used in the cleaning of filters. The exhoust gas is brought of the room via hose-pipe where are also placed sensors for moisture and temperature.

2.5 Description of impregnation process

The next step after the solution was ready was dry impregnation (discussed in 3.3.3) performed in fluidized bed reactor shown in figure 12. The alumina powder is placed in the Wurster-Coating cylinder shown in figure 12, which has a perforated bottom with applied filters. These filters retain large and fine particles, and also allow air flow through them. The alumina powder during the process must not outflow through them. The average amount of alumina used for one cycle of process was 4.1 kg.

Figure 12. Wurster-Coating cylinder

After measuring the exact amount of alumina was putted in the cylindrical part, the fluidized bed was hermetically sealed and the process was ready to begin. First was needed to set the air flow rate, which determines how high the bulk would rise. With the higher flow rate set, the particle will reach higher.

The fluidized bed got heater that was set to 40°C over the process of spraying. Peristaltic pump (figure 13) was connected to the bed via hose and automatically controlled by the system software according needed rate.

Figure 13. Peristaltic pump

The rate of the pump was set to 45g/min. After solution was pumped, it was sprayed over the cylindrical part from the nozzle in the middle. The process of spraying can be seen in figure 14:

Figure 14. Spraying process in Wurster-Coater fluidized bed reactor

During the process of spraying the moisture of the material was changing with the amount of solution was sprayed because of the adsorption of the material.

The next step after the whole amount of solution was sprayed was to dry the particles. For the drying process the temperature of the heater was set to 80°C. The material was dry when the moisture remained.

2.6 Calcination

Calcination involves heating the particle to temperatures close to or higher than the sintering temperature for an extended period of time. The calcination procedure was carried out under an air atmosphere. Therefore, metal salts in the particle such as metal nitrates will be converted to metal oxides according to equation (0). For this purpose the oxygen carriers were put for at least two hours in muffle oven at 550 °C. The resulting, poisonous nitrogen dioxide must be removed. After over two hours at this temperature, the sample of oxygen carriers looks like these in figure 15.

Figure 15. Oxygen carriers calcined at 550°C Figure 16. Oxygen carriers calcined at 850°C

After calcination at 550°C a typical blackening for copper was spotted. Some of the particles went from light blue color to green blue and this was indication that hydroxide still content in the oxygen carriers.

The next step was calcination at 950°C for at least two hours again. The color of the oxygen carriers can be seen in figure 16. After second calcination they were brighter with a brown shade

3. Results

3.1 SEM images of materials

For analyzing the surface of the materials, JEOL 840a scanning electron microscope (figure 16) was used to make the SEM pictures.

Figure 16. JEOL 840a SEM

Before working on SEM, the probes were processed with Au(gold) film for 40 seconds at 40kV. That step was made for better SEM pictures. Gold particles cover the probes and as is known the gold is the best metal conductor. Covering of the particles with this film improves significantly the quality of the taken pictures.

For better analysis of the probes, four materials will be observed together at different zoom and compared.

3.1.1 SEM pictures at 250x zoom

A - Puralox NWa-155 (?- Al2O3)

B - Impregnated/dried Al2O3/Cu(NO3)2

C - Al2O3/CuO used in CLC facility

D - Al2O3/CuO calcined at 850?C

3.1.2 SEM pictures at 1500x zoom

A - Puralox NWa-155 (?- Al2O3)

B - Impregnated/dried Al2O3/Cu(NO3)2

C - Al2O3/CuO used in CLC facility

D - Al2O3/CuO calcined at 850?C

3.1.3 Discussion of the SEM pictures

Pictures showed that alumina particles, which were not involved in any process, have round shape (A). Their surface is smooth and there are no sharp edges. After the impregnation of the material there are crystals of CuO on the surface of alumina particles (B). These crystals are distributed evenly over the surface. That can be sign for successive impregnation of the fresh material, even the desired result is impregnation in the inner part of alumina.

In the pictures of Al₂O₃/CuO used in CLC facility can be observed that the particles are deformed from the attrition, which fluidization process caused (C). There are sharp edges and the surface is almost flat. Particles calcined at 850?C shown indications for CuO crystals on their surface, but these crystals are significantly less than fresh impregnated one.

3.2 Fluidization of the materials. Calculating the minimum fluidization velocity u_mf

The fluidization tests were performed in small fluidized bed shown in figure 17 below:

Figure 17. Fluidized bed for testing u_mf

Measurements were made during different pressure by two different controllers.

3.2.1 Puralox (?-Al₂O₃)

The calculated minimum fluidization velocity u_mf for ?-Al₂O₃ is:

Material	Umf, cm3/s	?bulk, kg/m3
Puralox Nwa-155	1,60	760

3.2.2 Impregnated/dried Al2O3/Cu(NO3)2

The calculated minimum fluidization velocity u_mf for Impregnated/dried Al₂O₃/Cu(NO₃)₂ is:

Material	Umf, cm3/s	?bulk, kg/m3
Al2O3/Cu(NO3)2 impregnated	2,32	1064,9

3.2.3 Al₂O₃/CuO used in CLC facility

The calculated minimum fluidization velocity u_mf for Al₂O₃/CuO used in CLC facility is:

Material	Umf, cm3/s	?bulk, kg/m3
Al2O3/CuO used in CLC facility	2,42	939,5

3.2.4 Discussion of fluidized tests

The minimum fluidization velocity raised gradually. The results were compared with previous works [18], which shown normal fluidization behavior of the particles.

Conclusions

Based on previous investigations a large scale method for Cu-based oxygen carriers preparation was established by dry impregnation method. During the preparation of spraying solution it was observed the formation of a white precipitate, which is a sign that the material used is not pure.

It was conclude that the fluidized bed reactor with Wurster-draft tube show good results as example a large amount of material (4,2 - 4,4 kg) can be prepared on one cycle. Also using of the Wursted-Coater has advantages because of the position of spraying nozzle, which is in the bottom side and it is in the bed of particles. This allows good contact, sufficient efficiency, between particles and sprayed solution compared with previous works where the nozzle was above the particles and the nozzle often blocked. It is also was concluded that the particles did not agglomerate during the process.

An amount of 180 kilograms Cu-based oxygen carriers with Puralox as support with 13% content of CuO was prepared. After calcination at 550°C a typical blackening for copper was spotted. Some of the particles went from light blue color to green blue and this was indication that hydroxide still content in the oxygen carriers. Sample of the material after calcination lose 18,9 % of its weight because of evaporation of the water molecules. After the calcination at 850°C material went light brown from almost black.

Samples of the Prualox, impregnated Al₂O₃/Cu(NO₃)₂, Al₂O₃/CuO used in CLC facility and Al₂O₃/CuO calcined at 850°C were analyzed using Scanning Electron Microscope (SEM) pictures. In these pictures can be seen that the surface of the impregnated Al₂O₃/Cu(NO₃)₂ content equally distributed crystals of CuO, which is sign for successive impregnation. After calcination in CLC facility the surface of particles was flat most is inside.

The same samples beside Al₂O₃/CuO calcined at 850°C were put to fluidization test performed in fluidized bed reactor. The minimum fluidization velocity were calculated and compared with results from previous works, which shown normal fluidization behavior of the particles.

Acknowledgements

First of all I want to thank Prof. Dr.-Ing. Stefan Heinrich for the given opportunity to do my thesis in Hamburg University of Technology (TUHH). Also I would not succeed without the support and help of my supervisors Dipl.-Ing. Marvin Kramp and Dipl.-Ing. Andreas Thon. I want to thank the entire department “Solid Processes Engineering and Particle Technology” which helped me in my work: Dr. Ernst-Ulrich Hartge, Dipl.-Ing. Miika Franck, Heiko Rohde and all the rest. Last but not least I want to thank the Bulgarian practicant Vassil Traychev for helping me with the experimental work.

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Thesis “ Capture of CO from Solid Fuels using Chemical-Looping Combustion and Chemical-Looping with Oxygen Uncoupling”

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