Article pubs.acs.org/EF
Oxidation of Ammonia by Ilmenite under Conditions Relevant to Chemical-Looping Combustion Mao Cheng,† Fredrik Normann,‡ Dongmei Zhao,*,§ Zhenshan Li,*,† Ningsheng Cai,† and Henrik Leion§ †
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China ‡ Department of Energy and Environment, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden § Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden ABSTRACT: The oxidation of ammonia to NOx and N2 was investigated under conditions that pertain to the fuel reactor during the chemical-lopping combustion of coal with ilmenite. The catalytic decomposition of NH3, the oxidation of NH3 over ilmenite, and the reduction of NO by reduced ilmenite and NH3 were studied experimentally. The catalytic decomposition of NH3, NO reduction by NH3, and reduced ilmenite were found to be important for N2 formation. NH3 oxidation over ilmenite was the only way in which NO could be formed in this system, with around 18% of the NH3 being converted to NO at 850−950 °C. The oxidation of NH3 was only slightly influenced by the reactor temperature but was strongly influenced by the concentrations of NH3 and syngas. NO formation was promoted by high concentrations of NH3 and decreased by high concentrations of syngas. The selectivity of the NH3 toward NO formation was favored at low concentrations of NH3 and syngas. The conversion of NH3 was complete in most cases, although 15−25% of the NH3 was not converted when the inlet syngas concentration increased to levels higher than 10−30%.
1. INTRODUCTION Chemical-looping combustion (CLC) is a technique that enables the capture of carbon dioxide (CO2) without gas separation, thereby minimizing the energy penalty. CLC uses a metal oxide as an oxygen carrier (OC) to transfer oxygen between two interconnected fluidized bed reactorsan air reactor and a fuel reactor.1 The metal oxide is reduced as it oxidizes the fuel in the fuel reactor. The reduced metal oxide is then transferred to the air reactor where it is oxidized. The overall reaction between fuel and oxygen, as well as the heat release in the CLC system is equivalent to a conventional combustion process with air. However, the fuel is never mixed with air and the gas from the fuel reactor consists mainly of CO2 and H2O, albeit without N2 dilution. Figure 1 illustrates the operating principle of a CLC system and the process of coal conversion in the fuel reactor. Coal is an important fuel, in that it accounts for around 30% of the global primary energy supply.2 Much research has been performed on CLC with coal as the fuel.3 Ensuring complete and rapid conversion of coal in the fuel reactor is a key priority for the CLC system. The most commonly used method to convert coal in CLC is through so-called “in situ gasification chemical looping combustion” (iG-CLC; Figure 1), whereby the processes of pyrolysis, gasification, and oxidation of fuel gases all take place in the fuel reactor.4 Most countries have strict regulations regarding emissions from coal-fired power plants. In China, the latest policies for newly built Circulating Fluidized Bed (CFB) boilers require that the concentration of NOx and SOx are
