Chemical-looping combustion in a reverse-flow fixed bed reactor
Introduction
CLC (Chemical-looping combustion) is a promising technology for power generation with cost effective CO2 capture. CLC is a two-step combustion process that produces a high purity stream of CO2, ready for compression and sequestration. A solid oxygen carrier, in the form of metal oxide particles, transports the oxygen from air to combust a gaseous fuel. Since the fuel is never mixed with air, the combustion products (H2O/CO2) are not diluted in N2 and CO2 can be captured after condensing the water vapor. CLC is a pre-commercialization technology, but simulation studies on its integration with power plants indicate that it has the potential to be more efficient than all other CO2 capture technologies [1], [2], [3], [4], [5], [6], [7]. The overall efficiency of the corresponding power plant would be higher if CLC was to be integrated with CC (combined cycle) power plants that take advantage of the high efficiencies of the Brayton cycle. Therefore, several process configurations have been proposed that integrate CLC reactors with CC power plants utilizing natural gas and synthesis gas fuels. The thermal efficiency of natural gas-fired CC with CLC is 52–53% [8], [9], [10], which is about 3–5% higher than that of post-, oxy-fuel, or pre-combustion CO2 capture methods [11]. For coal feedstocks, the thermal efficiency of CLC combined with an IGCC (integrated gasification combined cycle) was estimated to be close to that of conventional IGCC without CO2 capture [12] and about 2% higher than IGCC with pre-combustion CO2 capture [13], [14].
CLC reactors must operate at high pressures (>20 bar) and high temperatures (>1200 °C) [15], [16] for their seamless integration with downstream gas turbines. A major challenge in the development of CLC reactors, in the case of the more conventional fluidized bed configurations, is the establishment and maintenance of stable fluidization at high-pressure and at large-scale. Therefore, the design of the main candidate reactor configuration for commercial CLC realization, consisting of two interconnected fluidized beds between which the oxygen carrier is circulating [17], [18], [19], [20], [21], [22], [23], [24], [25], suffers from immaturity of its fluidization technology. The main advantages of fluidized bed implementations of CLC are the high gas/solid heat and mass transfer rates, stable operating temperature, and continuous operation. Most of the operational experience of fluidized bed CLC units was gained at atmospheric pressure and research is on-going to design and operate reactors that can accomplish stable circulation and fluidization of the oxygen carrier at the scale conceptualized for power generation [26], [27], [28].
An alternative to interconnected fluidized bed CLC configurations is the much simpler fixed bed design [29], [30], [31], [32]. In a fixed bed reactor configuration for CLC, the oxygen carrier is statically contained and alternatively exposed to oxidizing and reducing gases. Since solid circulation (and thus gas–solid separation) is not required, high-pressure operation can be achieved without much difficulty. The fixed bed reactor is more compact than the fluidized bed, which offers potential for better utilization of the oxygen carrier, lower capital cost, and smaller process footprint. The main disadvantages of the fixed bed reactor are its dynamic operation and the necessity of high temperature valves. The main challenge of interest to this work is the batch operating principle of a fixed bed CLC reactor configuration, in conjunction with the requirement for stable, smooth steady state operation of power plants. This is a traditional chemical engineering challenge, but relatively new to the power generation sector.
In previous work, we used dynamic models to study and compare the performance of fixed bed and fluidized bed reactors for gaseous CLC [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]. We showed that the fixed bed reactor can operate at near 100% CH4 conversion, when the reduction cycle is stopped early enough to ensure high CO2 capture efficiency [37]. In other relevant work, Hamers et al. [32] and Spallina et al. [43] showed that a proper heat management strategy is essential to the integration of fixed bed CLC into a power plant. Spallina et al. [13] estimated the thermal efficiency of a CLC-based IGCC plant using fixed bed reactors to be 40–41%, which was similar to an IGCC-CLC plant with fluidized bed reactors [44]. Heat management in the form of CLC operating strategy was formulated as an optimization problem by Han and Bollas [45], with the objective of maximizing energy efficiency as a function of the CLC cycling procedure. Here, we expand upon previous work by allowing the operating strategy of CLC to change the direction of the gas flow during each cycle.
Section snippets
Basic operating principle of fixed bed CLC
The fixed bed CLC reactor is a batch process, operated in successive cycles of reduction, oxidation, and heat removal. During reduction, a gaseous fuel (such as natural gas or syngas) is fed to a fully oxidized bed, where the oxygen carrier is reduced and the fuel is converted mainly to CO2 and H2O. The reduction of the oxygen carrier is usually a combination of endothermic and exothermic reactions, depending on the fuel and metal oxide. The reduction phase is stopped when either the fuel
Novelty and scope of this work
To summarize the scope of this work, we are interested in fixed bed reactors, because of their capability to operate at high pressures, with gaseous fuels and we wish to explore the optimality of reactor design and operation strategy for the maximization of energy efficiency and CO2 capture. The basic process elements that control these efficiencies are the uniformity of the bed in terms of temperature and oxygen carrier conversion profiles. In an effort to impose uniformity to the
Dusty gas fixed bed reactor model
In this work, we are interested in the intensified integration of CLC in power plants. We thus diverge from laboratory practices and use pure CH4 or synthesis gas as the feedstock. This is accomplished by extension of our previously developed heterogeneous models [34], [35], to the so-called dusty-gas modeling framework, which can more accurately capture the heat and mass transfer phenomena of concentrated gas–solid flows. The dynamic mass balance for the gas phase inside a spherical particle
Base case for comparative analysis of the reverse-flow reactor
In this Section, the reverse-flow reactor performance is compared against an equivalent one-directional flow fixed bed reactor. This comparison assumes a clean methane and syngas fuel source, from natural gas or coal gasification respectively. Synthetic oxygen carriers of supported CuO and NiO were used to improve reactivity and selectivity of the reactor and reduce its footprint. Oxygen carrier reduction and oxidation kinetics were adapted from Nordness et al. [41], who experimentally studied
Results and discussion
In Section 5, we proposed reverse-flow fixed bed reactors as a process intensification option for CLC. Here we evaluate the advantages of reversing the feed flow operation during the reduction step, which was shown as the most impactful on the overall CLC performance. The benefits of reversing the feed flow in the reduction step are illustrated for the entirety of the CLC sequence (oxidation, heat removal and reduction), at cyclic steady-state conditions. Cyclic steady-state refers to the
Conclusions
The use of a reverse-flow reactor was explored as a process intensification option to improve the energy efficiency of chemical-looping combustion systems integrated into a combined cycle power plant. Periodic flow reversal of the fuel was shown to enhance the contact between the fuel and oxygen carrier, improve reactor bed temperature and conversion uniformity, and suppress carbon deposition. We addressed the operational challenges of the batch fixed bed CLC reactor using a model-based
Acknowledgment
This material is based upon work supported by the National Science Foundation under Grant No. 1054718.
References (56)
- et al.
Evaluation of a chemical-looping-combustion power-generation system by graphic exergy analysis
Energy
(1987) - et al.
Effect of oxygen carriers on performance of power plants with chemical looping combustion
Proced Eng
(2013) - et al.
Comparison of nickel- and iron-based oxygen carriers in chemical looping combustion for CO2 capture in power generation
Fuel
(2005) - et al.
Multi-stage chemical looping combustion (CLC) for combined cycles with CO2 capture
Int J Greenh Gas Control
(2007) - et al.
A quantitative comparison of gas turbine cycles with CO2 capture
Energy
(2007) - et al.
Integration of coal gasification and packed bed CLC for high efficiency and near-zero emission power generation
Int J Greenh Gas Control
(2014) - et al.
Comparison of carbon capture IGCC with pre-combustion decarbonisation and with chemical-looping combustion
Energy
(2011) - et al.
Syngas combustion in a chemical-looping combustion system using an impregnated Ni-based oxygen carrier
Fuel
(2009) - et al.
Progress in chemical-looping combustion and reforming technologies
Prog Energy Combust Sci
(2012) - et al.
A fluidized-bed combustion process with inherent CO2 separation; application of chemical-looping combustion
Chem Eng Sci
(2001)
300W laboratory reactor system for chemical-looping combustion with particle circulation
Fuel
Combustion of syngas and natural gas in a 300 W chemical-looping combustor
Chem Eng Res Des
Chemical-looping combustion in a 300 W continuously operating reactor system using a manganese-based oxygen carrier
Fuel
160h of chemical-looping combustion in a 10kW reactor system with a NiO-based oxygen carrier
Int J Greenh Gas Control
Pressurized chemical-looping combustion of coal with an iron ore-based oxygen carrier
Combust Flame
Pressurized chemical-looping combustion of coal using an iron ore as oxygen carrier in a pilot-scale unit
Int J Greenh Gas Control
A theoretical investigation of CLC in packed beds. Part 1: particle model
Chem Eng J
Conceptual design of a Ni-based chemical looping combustion process using fixed-beds
Appl Energy
Modeling of Cu oxidation in an adiabatic fixed-bed reactor with N2 recycling
Appl Energy
A novel reactor configuration for packed bed chemical-looping combustion of syngas
Int J Greenh Gas Control
Model-based analysis of bench-scale fixed-bed units for chemical-looping combustion
Chem Eng J
Heterogeneous modeling of chemical-looping combustion. Part 1: reactor model
Chem Eng Sci
Heterogeneous modeling of chemical-looping combustion. Part 2: particle model
Chem Eng Sci
Model-assisted analysis of fluidized bed chemical-looping reactors
Chem Eng Sci
Kinetics of NiO reduction by H2 and Ni oxidation at conditions relevant to chemical-looping combustion and reforming
Int J Hydrogen Energy
Continuous regime of chemical looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) reactivity of CuO oxygen carriers
Appl Catal B Environ
Investigation of heat management for CLC of syngas in packed bed reactors
Chem Eng J
Comparison on process efficiency for CLC of syngas operated in packed bed and fluidized bed reactors
Int J Greenh Gas Control
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2019, EnergyCitation Excerpt :Thermal cracking of the liquid hydrocarbons was carried out in a MAT fixed-bed reactor illustrated in Fig. 1 [33]. Although a fluidised bed reactor is the most common reactor type used for CLC tests, such experiments have also been conducted in fixed-bed reactors [3,5,37,38]. The experimental unit comprised a Pyrex glass cylindrical reactor with an internal diameter of 15 mm and a length of 270 mm, a temperature controlled tubular furnace, a syringe pump, a liquid product receiver and a gas bag.