Chemical Looping

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Contents

Introduction

Chemical looping or chemical looping combustion (CLC) is a novel technology that could provide the means to convert fossil fuels to electricity and provide carbon capture without significant efficiency or cost penalties. Chemical looping combustion is very similar to oxy-fuel combustion where there is no direct contact between air and fuel.[1] Oxygen is extracted from air, then the oxygen is reacted with the hydrocarbon fuel producing an exhaust gas composed of carbon dioxide and water vapor.[2] The water vapor is condensed out of the gas resulting in near 100% carbon dioxide stream that could be sequestered in the ground.

Process Description

Basic chemical looping combustion schematic

In its most basic form, chemical looping consists of an air reactor and a fuel reactor. Usually these reactors consist of interconnected fluidized beds.[2] An oxygen carrier is circulated between the two reactors. This oxygen carrier usually consists of a metal that is easily oxidized such as Fe, Ni, or Cu. In order to describe the chemical looping process, the oxygen carrier in its none oxidized form will be known as Me and the oxidized form of the carrier will be MeO. We will first start in the air (or oxidizing) reactor were Me is entrained in a fluidized bed with air as the fluidizing agent. At an elevated temperature (around 700 to 900⁰C)[1] Me reacts with the oxygen in the air in an exothermic reaction producing MeO according to the reaction:

2Me + O_2 \Rightarrow 2MeO

MeO is then separated from the N2 and transported to the fuel (or reducer) reactor. MeO reacts with a hydrocarbon fuel in the fuel reactor, around 900⁰C, to produce CO2 and H2O while reducing MeO to Me according to the reactions:

2MeO + C \Rightarrow 2Me + CO_2
MeO + H_2 \Rightarrow MeO + H_2O

The Me is then transported back to the air reactor to repeat the process.

Benefits

"Chemical looping can use coal to produce both hydrogen from coal, or a stream of combustion products that primarily consist of CO2 and steam. A relatively pure stream of CO2 that is sequestration ready can be produced by simply condensing the steam. This avoids the energy penalty traditional fossil fuel fired systems must pay to produce a pure stream of CO2. Additionally, Chemical Looping for Combustion (CLC) minimizes production of NOX that is produced in almost all other combustion processes."[3]

Oxygen Carriers

Oxygen carriers could include metals and none metals to "carry" oxygen from the air reactor to the fuel reactor. Items that need to be taken into consideration include cost, melting point, environmental impact, reactivity, and the amount of oxygen carried per weight. Typically these metals are support on inert materials in order to increase the strength of the particles. Some possible carriers include:

Metals
Carrier Oxidizer (Air Reactor) Reducer (Fuel Reactor)
Copper (Cu) 2Cu + O_2 \Rightarrow 2CuO

CuO + H_2 \Rightarrow Cu + H_2O

2CuO + C \Rightarrow 2Cu + CO_2

Iron (Fe) 2Fe_3O_4 + {1 \over 2}O_2 \Rightarrow 3Fe_2O_3

3Fe_2O_3 + H_2 \Rightarrow 2Fe_3O_4 + H_2O

6Fe_2O_3 + C \Rightarrow 4Fe_3O_4 + CO_2

Multimedia

Patents

Patent No Author Title Year
2009/0265978 A1 (Pub. No.) Cichanowicz et al. Rotary Regenerative Chemical Looping Combustion 2009
US 2007/0049489 A1 (Pub. No.) Becue et al. Redox active mass for a chemical looping combustion process 2006
2005/0175533 A1 (Pub. No.) Thomas et al Combustion looping using composite oxygen carriers 2004
5447024 Ishida et al. Chemical-looping combustion power generation plant system 1995

References

  1. 1.0 1.1  "Power Magazine, "Chemical Looping and Coal""
  2. 2.0 2.1 2.2  "Mattisson, T. and Lyngfelt, A., "Applications of chemical-looping combustion with capture of CO2""
  3.  "NETL Fact Sheet, "Chemical Looping for Combustion and Hydrogen Production""

External Links