The development of simulation tools is essential for the design, optimization, and scale-up of the chemical looping technology. Therefore, two chemical looping processes, namely chemical looping combustion (CLC) and chemical looping with oxygen uncou-pling (CLOU), are investigated by means of modelling. The main objectives of the thesis are described as follows:
• To apply a computational modelling approach to gaseous fuel CLC for predicting the operation of the process at different scales. A one-dimensional model frame comprises two interacting fluidized bed reactors. Reactor models are based on the conservation of mass and energy, expressed as balance equations for solids and gases. Several submodels describe the two-phase flow phenomena, chemical re-actions, and transfer of heat in the system. Fundamental continuum equations are combined with semi-empirical correlations for low computational cost while main-taining a sufficient accuracy in results. The model must be capable of describing the physical phenomena relevant to the process studied, and for validation purposes, the results should be compared to experimental data. (Publication I)
• To create a conceptual design for a large-scale CLC reactor system. The 1D flu-idized bed model is used to predict the performance of the system. (Publication II)
• To investigate the integration of CLC and a power cycle for energy production. A flow sheet model of CLC-combined steam power plant is developed, and the viabil-ity of the suggested plant layout is evaluated. In order to investigate different plant configurations, the model must be flexible and easily modifiable. (Publication II)
• To determine possible operational regions of a solid fuel CLOU process. The ba-sic relations between important process parameters are quantified via mass, energy, and exergy balance analysis. (Publication III)
• To apply the 1D fluidized bed model for simulations of a CLOU fuel reactor fed by coal. The effect of various process parameters on the results is assessed by param-eter variations. (Publication IV and Publication V)
1.2 Motivation and objectives of the study 17
This thesis is divided into 6 chapters. Chapter 1 introduces the research problem and objectives of the thesis. Chapter 2 describes the fundamentals of the processes studied.
In chapter 3, the one-dimensional dual fluidized bed model frame for CLC is introduced and verified by comparing the results to experimental data (see Publication I). In chapter 4, a scale-up procedure for CLC is presented, and the fluidized bed model is used to de-scribe the large-scale operation of the process. In addition, the designed reactor system is integrated with a steam turbine cycle and the viability of the plant is evaluated by flow sheet simulations (see Publication II). Chapter 5 is devoted to CLOU, and at first, the ba-sic relations between the relevant process parameters are quantified via mass, energy, and exergy balance analysis (see Publication III). Then, the fluidized bed model is adopted for CLOU and used for case simulations (see Publication IV and Publication V). Finally, chapter 6 concludes the work and gives recommendations for possible research work in the future. This thesis contains only the main findings of the research conducted; the de-tailed findings can be found in Publication I–Publication V. The thesis is concluded with an appendix containing the publications.
18 1 Introduction
19
2 Chemical looping technology
The term “chemical looping” has been given for cycling processes that use a solid material as oxygen carrier containing the oxygen required for the conversion of the fuel. After being reduced, the oxygen carrier must be reoxidized before the starting of a new cycle.
Chemical looping processes can be utilized to produce energy and/or hydrogen, both with CO2 capture. Different chemical looping concepts proposed in the literature have been summarized by Adanez et al. (2012).
2.1 Chemical looping combustion (CLC)
Chemical looping combustion (CLC) has been introduced as a promising combustion process with an inherent separation of the greenhouse gas CO2, initially by Lewis and Gilliland (1954) and later, for example by Richter and Knoche (1983), Ishida et al. (1987), and Ishida and Jin (1994). During the recent years, industry and academic institutions have noticed CLC’s potential for delivering the most efficient and economic technology in the case ofCO2capture (Lee et al., 2005). A great number of scientific papers con-sidering different areas of CLC research have been listed and reviewed recently (Adanez et al., 2012).
In traditional combustion, the fuel is in direct contact with air. Most of the technologies using this combustion method require a large amount of energy to separate and collect CO2 from the exhaust gas, because theCO2is diluted byN2 from the combustion air.
The conventional gas-phase combustion reaction, when using air as the oxygen source, is exothermic and can be written as
CxHy+
x +y 4
O2+ 3.76
x + y 4
N2→xCO2+y
2H2O + 3.76
x +y 4
N2 (2.1) In a CLC system, the process shown in Equation 2.1 is split into two interconnected fluidized bed reactors: an oxidizer (air reactor, AR) and a reducer (fuel reactor, FR) where two consecutive gas-solid reactions occur forming a chemical loop (see Fig. 2.1). A solid oxygen carrier (metal oxide) is used to transfer the oxygen from the air to the fuel. The oxygen carrier loops between the AR, where it is oxidized by the air (Eq. 2.2), and the FR, where it is reduced by the fuel (Eq. 2.3):
Me +1
2O2→MeO (2.2)
(2x + y)MeO + CxH2y→(2x + y)Me + yH2O + xCO2 (2.3) Depending upon the metal oxide used, the reduction reaction is often endothermic (∆Hred>0) while the oxidation reaction is highly exothermic (∆Hoxd <0). The total amount of heat released,∆Hc, is the same as for normal combustion. In CLC, the combustion air is not mixed with the fuel, and theCO2does not become diluted by the nitrogen of the flue gas,
20 2 Chemical looping technology
like in the conventional combustion process. The outgoing gas from the oxidation step (AR) will containN2and unreactedO2while the gas from the reduction step (FR) will be a mixture ofCO2and water vapor. The water vapor can be condensed, and close to pure CO2 is then obtained. Some energy is needed to compress theCO2 into a liquid form, suitable for transportation and storing (Lyngfelt et al., 2001).
MeO (+ Me)
Me (+ MeO) Air
reactor
Fuel reactor
Air Fuel
CO2, H2O O2, N2
Figure 2.1: CLC process loop between two interconnected fluidized bed reactors.
In CLC, gaseous fuels are preferred due to the favourable nature of heterogeneous gas-solid reactions. In the case of gas-solid fuel, like coal and biomass, problems arise as ho-mogeneous solid-solid reactions are not likely to occur at any reasonable rate and an intermediate fuel gasification step would be needed. The gasification can be proceeded in-situorex-situ; nevertheless, it will be the time limiting step in the process (Leion et al., 2008). Regarding the intensive use of coal for energy generation, there is an increasing interest in the use of CLC for solid fuels. Thus, in the last years, important work has been dedicated to adapting the process to solid fuels (Lyngfelt, 2013). Overall for CLC, more than 700 different oxygen carriers, mainly based on nickel, copper, and iron impregnated with a suitable inert binder, have been manufactured and characterized (Lyngfelt, 2011).