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

Oxygen carriers in CLC process. State of art. General oxygen carriers characteristics. Dry impregnation method. Fluidized Beds. Advantages and disadvantages of the Fluidized-Bed Reactor. Gamma alumina. Preparing of solution. Impregnation calculations.
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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...