5.2 Fuel reactor modelling in CLOU
5.2.10 The effect of carbon separation system
As shown by the simulations, the oxygen carrier stream exiting the fuel reactor will con-tain a small amount of unreacted char. This char will burn in the air reactor, but the resulting CO2 cannot be captured. To improve the char conversion in the fuel reactor, a carbon stripper can be implemented between the reactors. The carbon stripper allows the separation of char from the oxygen carrier based on selective entrainment of particles in a fluidized bed (Gayan et al., 2013; Kramp et al., 2012a). The device has to be supplied with sifting gas, which could be steam or recirculation gas from the fuel reactor. Either way, char gasification and combustion of the gasification products would occur, and thus enhance the char conversion. Also, direct combustion of the char is possible as the oxy-gen carrier will release some gaseous oxyoxy-gen. However, since the gasification process is rather slow and the residence time of the solids in the carbon stripper is assumed low, the described reaction scheme is not taken into consideration in the current modelling ap-proch.
The performance of the carbon stripper is indicated by carbon separation efficiency, ηcs, which determines the fraction of the char separated and returned back to the fuel reactor.
Atηcs = 1, no char escapes to the air reactor. The main influence of the carbon stripper is to extend the residence time of char particles in the fuel reactor (Abad et al., 2013).
As illustrated in Figure 5.15, both the char conversion and the CO2 capture efficiency increases with the increasingηcs. In the reference case conditions, it is possible to reach aCO2capture efficiency as high as 89.2% without using the carbon stripper. Therefore, the influence of the carbon separation remained rather moderate. A separation efficiency of approximately 60% is required to capture 95% of the carbon introduced in the system.
The analysis was additionally conducted for a low-reactive coal having an apparent char combustion rate of one third of that of the reference coal. According to Figure 5.15, a CO2 capture efficiency of only 61.9% is obtained if the carbon stripper is not utilized.
In this case, the relevance of the carbon stripper became more evident as the char con-centration at the reactor exit increased compared to the reference case. Furthermore, a greater concern should be given to the performance of the carbon stripper, as it is neces-sary to have a separation efficiency of almost 90% to obtain a 95% of carbon capture. The experimental work of Adanez-Rubio et al. (2013) also underlined the need of a carbon separation system when low reactive coals are used.
84
5 Analysis and modelling of chemical looping with oxygen uncoupling (CLOU) process for solid fuels
0 0.2 0.4 0.6 0.8 1
0.4 0.5 0.6 0.7 0.8 0.9 1
Carbon separation efficiency (−) Char conversion (−), CO 2 capture eff. (−)
Xchar
ηcc Ref. case coal
Low reactive coal
Figure 5.15: The effect of carbon separation system on char conversion andCO2capture efficiency. The char combustion rate of the low reactive coal was one third of that of the reference coal (TFR= 960◦C,mOC = 400kg/MWf,ug= 5.0m/s).
5.2.11 Discussion
A one-dimensional fluidized bed model to describe the behavior of the fuel reactor in-volved in chemical looping with oxygen uncoupling (CLOU) process has been devel-oped. The model considers the physical phenomena relevant to CLOU, such as release of oxygen by oxygen carrier decomposition, combustion of coal with molecular oxygen, fluidized bed hydrodynamics, and transfer of heat within the reactor.
The performance of a CLOU fuel reactor fed by bituminous coal and CuO-based oxy-gen carrier was evaluated by means of model simulation. A reference case was defined and simulated, after which the effect of relevant design and operational parameters on the results was assessed by parameter variations. For the reference case conditions with a temperature of 960◦C and solids inventory of 400 kg/MWf in the reactor, aCO2capture efficiency of approximately 90% was predicted without the utilization of the carbon strip-per. The oxygen release rate was high enough to supply an excess of gaseous oxygen for combustion, thus the performance was limited by the coal reactivity. Solids inventories below 100 kg/MWthresulted in a lack of oxygen, and thus, a sharp reduction in the char conversion andCO2capture was observed.
The temperature showed an important effect on the conversion of coal in the fuel reac-tor, and theCO2capture efficiency increased from 81.9% at 935◦C to 93.7% at 985◦C.
Therefore, a high temperature in the fuel reactor is desired. The model includes a de-tailed formulation of the energy conservation and thereby allows the investigation of heat
5.2 Fuel reactor modelling in CLOU 85
transfer within the reactor. Hence, the arrangement of heat transfer surfaces and gas and solids inlets could be chosen to minimize the intrareactor temperature gradients affecting negatively the chemical equilibrium.
For a fixed solids inventory, an increase in the solids circulation rate gave a decrease in the CO2 capture efficiency. This was due to the decreased residence time of solids in the reactor. It was also shown that the coal rank has an important effect on the overall performance, as low reactive coals required higher residence times to attain feasible char conversion. In such cases, it is necessary to utilize a carbon separation system with a sep-aration efficiceny of>90% to reachCO2capture efficiencies>95%. When using highly reactive coals, the carbon separation unit becomes unnecessary.
Overall, the model predictions appeared to be in agreement with the results presented in the literature. As the model structure and submodel forms turned out to be appropriate to describe the studied process, the model will be further developed for investigations of the whole CLOU process loop comprising two interconnected fluidized bed reactors.
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5 Analysis and modelling of chemical looping with oxygen uncoupling (CLOU) process for solid fuels
87
6 Conclusions
In this thesis, chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU) processes were investigated by means of modelling. In the previous sections, the results and discussions were presented, whereas this section contains the concluding remarks.
As the main objective of the thesis, a one-dimensional, dynamic fluidized bed model frame for simulating the above-mentioned processes was developed. The model com-prises fundamental continuum equations combined with semi-empirical correlations. Such a modelling approach was chosen for low computational cost while maintaining a suffi-cient accuracy in results. The best available knowledge from the literature on fluidized bed reactors and the chemical looping technology was utilized for describing the physical phenomena relevant to the processes studied.
The model frame was succesfully validated based on the operation of a 150 kWthCLC pilot unit fed by methane and Ni-based oxygen carrier. The model was capable of simu-lating the behavior of the pilot unit and reproducing the results obtained experimentally.
Then, a conceptual design for a CLC reactor system at a pre-commercial scale of 100 MWth was created, after which the validated 1D model was used to predict the perfor-mance of the system. The perforperfor-mance was greatly influenced by the hydrodynamics.
Thus, to achieve optimal operation conditions, effective solids transport and control sys-tems are needed. The air reactor would be very similar to a conventional CFB boiler, and its design can be handled without major difficulties. From the reaction engineering point of view, the main focus should be given to the fuel reactor.
The integration of CLC and steam turbine cycle was studied resulting in a power plant configuration which included only conventional power plant components, and the 100 MWthCLC unit was considered suitable for a retrofit arrangement where it replaced the natural gas steam generator. A process flow sheet of the whole plant was set up and simulated, and taking into account the efficiency drop of about 2%-points by the CO2 compression, a net plant efficiency similar to that currently achievable in a modern steam power plant was obtained. The degree of fuel conversion was found to have a significant effect on the net power plant efficiency; thus the reactor system should be designed care-fully for maximal fuel conversion.
The operation of a hypothetical CLOU system fed by bituminous coal and CuO-based oxygen carrier was characterized via mass, energy, and exergy balance analysis. Various process parameters were evaluated, and as a result, possible operational regions and ma-terial requirements for the given oxygen carrier were determined. In addition, a thermal assessment with respect to reactor temperatures and extraction of heat was carried out for envisioning different heat balance scenarios. With an exergy analysis, the effect of fun-damental operating parameters on the second-law efficiency of the system was evaluated and the magnitudes of various inefficiencies and irreversibilities were estimated.
88 6 Conclusions
An essential part of the viability of a CLOU system is based on the behavior of the fuel reactor, which determines the conversion of the solid fuel. Therefore, the 1D fluidized bed model was modified suitable for CLOU, and the operation of a CLOU fuel reactor scaled up to 500 MWthwas evaluated. A reference case was defined and simulated, after which the effect of relevant design and operational parameters on the results was assessed by parameter variations.
Overall, the 1D model predictions appeared to be in agreement with the results presented in the literature, and the model structure and submodel forms turned out to be appro-priate to describe the studied processes. Obviously, further improvement and validation of the submodels is a continuous task, and applicable empirical data is always needed.
So far, only stationary cases have been simulated, but since the model is based on time-dependent balance equations, dynamic simulations can be conducted for further analysis of the processes.
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