Reactor for chemical-looping combustion
Systems and methods are provided for enhancement of gaseous CLC in a fixed-bed process, marked by an increase in CO 2 capture efficiency and oxygen carrier utilization, while reducing disadvantages of a conventional fixed-bed operation. The disclosed systems/methods provide a CLC fixed-bed reactor design in which the direction of the fuel gas is intermittently reversed during a single fuel oxidation step. In this reverse-flow mode, oxygen carrier reduction reactions are displaced over the ends of the reactor, which increases contact between fuel and oxidized solids and alleviates and/or mitigates problems of carbon deposition encountered by most oxygen carriers.
1. A chemical-looping process, comprising:
(a) delivering fuel gas to a fixed-bed reactor in a first fuel gas flow direction relative to the fixed-bed reactor, the fixed bed reactor including a metal oxygen carrier that oxidizes the fuel gas;
(b) intermittently reversing flow direction of the fuel gas to a second fuel gas flow direction that is opposite to the first fuel gas flow direction based at least in part on real-time analysis of gas exiting the fixed-bed reactor, thereby reducing the metal oxygen carrier from both sides of the fixed-bed reactor so as to improve utilization of the metal oxygen carrier, mitigate cold zones within the fixed-bed reactor, and reduce carbon deposition;
(c) regenerating the metal oxygen carrier within the fixed-bed reactor in an oxidation cycle by delivering an oxygen source to the fixed-bed reactor in a first oxygen source flow direction relative to the fixed-bed reactor;
(d) intermittently reversing flow direction of the oxygen source to a second oxygen source flow direction that is opposite to the first oxygen source flow direction, thereby increasing heat extraction in the oxidation cycle;
wherein the intermittent reversal of flow direction of the fuel gas and the oxygen source is effectuated at an interval and with a frequency to achieve a predetermined metal oxygen carrier utilization of at least sixty percent (60%), a fuel conversion of at least ninety five percent (95%), and a reduction product capture of at least ninety percent (90%).
2. The chemical-looping process of claim 1 , further comprising separating a pure stream of carbon dioxide after condensing water vapor from the fuel oxidation step.
3. The chemical-looping process of claim 1 , wherein autothermal reforming is accomplished without a need for oxygen or carbon dioxide separation.
4. The chemical-looping process of claim 1 , wherein the flow direction of the fuel gas is reversed at controlled time intervals during a single fuel oxidation step.
5. The chemical-looping process of claim 4 , wherein the controlled time intervals are between about 0.5 seconds and 60 seconds.
6. The chemical-looping process of claim 4 , wherein the controlled time interval is about 2 seconds.
7. The chemical-looping process of claim 1 , wherein the fuel oxidation step is a chemical-looping combustion (CLC) process.
8. The chemical-looping process of claim 1 , wherein the metal oxygen carrier is selected from the group consisting of a nickel oxide, a copper oxide, a manganese oxide, a NiAl oxide, a Mg—Al oxides, an iron oxide, and mixtures and combinations thereof.
9. The chemical-looping process of claim 1 , wherein the metal oxygen carrier is nickel oxide.
10. The chemical-looping process of claim 1 , wherein the fixed-bed reactor includes one or more ports, valves and control systems to allow the direction of the fuel gas flow to be switched intermittently during the fuel oxidation step.
11. The chemical-looping process of claim 1 , wherein the fuel gas is selected from the group consisting of methane, natural gas, syngas and bio-syngas.
12. The chemical-looping process of claim 1 , wherein oxygen carrier reduction reactions associated with the fuel oxidation step are displaced over ends of the fixed-bed reactor, thereby increasing contact between fuel and oxidized solids.