Article pubs.acs.org/EF
Fuel Nitrogen Conversion in Chemical Looping with Oxygen Uncoupling of Coal with a CuO-Based Oxygen Carrier Tao Song,*,†,‡ Wanjun Guo,† and Laihong Shen† †
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing 210096, China Institute of Solids Process Engineering and Particle Technology, Hamburg University of Technology, Hamburg 21073, Germany
‡
ABSTRACT: The interest in chemical looping with oxygen uncoupling (CLOU) of coal as a method for CO2 enrichment has increased drastically during recent years. The objective of this work was to experimentally investigate fuel nitrogen conversion in the CLOU process of coal with a CuO-based oxygen carrier in a batch fluidized-bed unit. Three coals of different rank (anthracite, sub-bituminous, and bituminous) and their corresponding chars were used as fuels. Furthermore, experiments of coal combustion under air-fired conditions were performed to recognize the unique pathway of fuel nitrogen conversion in CLOU. Experimental results indicated that NO was formed in the largest amount of total NOx. For the Shenhua bituminous coal that was used, most of the NO, NO2, and N2O from fuel nitrogen was formed during the devolatilization stage of the coal. However, with Xuzhou sub-bituminous coal or Huaibei anthracite as fuels, a certain portion of NO was derived from char-N. Under the current experimental conditions (temperature of >900 °C), N2O formation was much less, because of its highly sensitive reactivity with temperature. Meanwhile, in the CLOU process, NO2 derived from fuel nitrogen accounted for a relatively large proportion, although it was lower than NO. The unique NO2 formation supported the occurrence of the heterogeneous reactions between gases and solid oxygen carrier particles in the reactor, where the oxygen carrier and coal were intensively mixed. Also, the gaseous oxygen from CuO particles participated in the reactions. Furthermore, with char as fuels, lower NOx formation was found, and the char nitrogen conversion to NOx was almost proportional to carbon conversion.
1. INTRODUCTION Chemical looping combustion (CLC) is one of important techniques used to combine fuel combustion and almost pure CO2 production in situ, allowing for CO2 sequestration.1 This occurs via indirect combustion whereby oxygen from air is transferred by solid oxygen carrier particles to fuel combustion. Various fuels (natural gas, syngas, coal, etc.) are burnt in two reactors designated as fuel and air reactors. In this way, the nitrogen from the air leaves the system from the air reactor, whereas the flue gas from the fuel reactor consists of only CO2 and water. After water condensation, almost pure CO2 can be obtained. Based on CLC, in 2005, Lyngfelt and Mattisson from Chalmers University of Technology first proposed another chemical looping conceptthe so-called “chemical looping oxygen uncoupling” (CLOU) process;2 they later presented some further publications on the subject in 2009.3,4 The CLOU process uses oxygen carrier particles based on some metal oxides that have the capability to release gaseous oxygen at high temperatures, and oxygen generated by the solid oxygen carrier particles reacts directly with fuels under oxyfiring conditions. After that, oxygen carriers are circulated to an air reactor to react with air for regeneration. The exit gases from fuel reactor mainly consist of CO2 and H2O. After a simple condensation, a high concentration of CO2 can be obtained. Therefore, the connection of the fuel reactor and the air reactor makes up the CLOU process for CO2 enrichment. The main reactions involved in the fuel reactor (R1 and R2) and the air reactor (R3) of the CLOU process are given as 2MexOy ↔ 2MexOy − 1 + O2 © 2015 American Chemical Society
⎛ m⎞ CnH 2m + ⎜n + ⎟O2 → nCO2 + mH 2O ⎝ 2⎠
(R2)
2MexOy − 1 + O2 ↔ 2MexOy
(R3)
As a key point in the application of CLOU, the oxygen carrier should have a good chemical and physical reactivity. Since from 2008, various types of oxygen carriers for CLOU have been proposed and tested, such as Cu-based, Mn-based oxygen carriers and so on, as summarized in two comprehensive reviews by Adánez et al.6 and Mattisson.7 Among all the feasible oxygen carrier materials for the CLOU process, the CuO-based oxygen carrier is attractive, with respect to the rate of oxygen release at relevant temperatures. Also, there are no thermodynamical restrictions, with respect to the complete conversion of fuel gases. Various types of CuO-based materials had been attempted to be used as oxygen carriers in the CLOU process, such as copper ores,8,9 synthesized materials with different supports of Al2O3, ZrO2, MgAl2O4, and so on.10−18 Coal is a less expensive and more abundant resource than other fossil fuels in the world, while, at the same time, being a reliable fuel for power production. As far as power production is concerned, the CLOU technology becomes competitive with coal as fuel. A schematic picture of the CLOU process of coal using a CuO-based oxygen carrier is shown in Figure 1. In this case, the char combustion takes place in the fuel reactor instead of a gasification which normally occurs in CLC. The oxyfiring Received: March 13, 2015 Revised: May 12, 2015 Published: June 4, 2015
(R1) 3820
DOI: 10.1021/acs.energyfuels.5b00533 Energy Fuels 2015, 29, 3820−3832
Article
Energy & Fuels
oxygen concentration and combustion temperature on fuel nitrogen conversion are strongly coupled. The CO2 stream from fuel reactor is better to be recycled into the fuel reactor again as a fluidizing agent. Then, because of the dilution effect of CO2, the oxygen concentration of the fuel reactor will become lower. That is, although coal is burned in the CLOU process by oxygen, the coal conversion rate is expected to be lower than that in an air-fired fluidized-bed combustion process. Because of different coal combustion behaviors in both cases, the fuel nitrogen conversion is very likely to be different. Furthermore, another oxidizing effect of nitrogen intermediates HCN and NH3 by the oxygen carrier particles through complex gas−solid reactions should be considered. That is, the heterogeneous reactions between gases and solid oxygen carrier particles may occur in the fuel reactor, where the oxygen carrier and coal are intensively mixed. The contact among the nitrogen intermediates and oxygen carrier particles are inevitable. In summary, three peculiar factors seem to have significant influence on the fuel nitrogen conversion in the CLOU process of coal. Those are a mild and continuous oxygen release from the oxygen carriers, a relatively low oxygen concentration coupled with combustion temperature for coal conversion and the presence of large amount of oxygen carriers in the fuel− oxygen-carrier system. The objective of this work was to present an investigation of the fate of fuel nitrogen during the CLOU process of coal. A series of experiments were conducted in a laboratory-scale fluidized-bed reactor with a CuO-based oxygen carrier. Three coals of different rank (anthracite, subbituminous, and bituminous) and their corresponding chars were used as fuels. Also, experiments of coal combustion in airfired condition were performed to recognize the unique pathway of fuel nitrogen conversion in CLOU.
Figure 1. Concept of the CLOU process of coal for CO2 capture.
condition in CLOU is accompanied by an increase in the char conversion rate of up to 1 or 2 orders of magnitude, compared to the char gasification.5 The fuel reactor inventory can be reduced significantly. The concept of using CuO to enhance the carbon gasification reaction was first demonstrated by Lewis et al. in 1951,19 when the importance of the O2 release from the decomposition of CuO in accelerating the rate of oxidation of solid fuels was recognized. The material and energy balances and process modeling relationships were discussed by Eyring et al.,20 with respect to the relevant engineering considerations. The first attempt of operating CLOU in a continuously operating 1.5 kW unit was carried out by Abad et al.,21 using a bituminous coal as solid fuel. Results found that carbon capture efficiency values close to 100% were obtained at a fuel reactor temperature of 960 °C. Nitrogen oxides are one of the mainly atmospheric pollutants for coal-fired power plant. However, according to the knowledge of authors, there are few investigations carried out to study the release of NOx during the CLOU of coal. The fate of fuel nitrogen during coal combustion in the CLOU process is unknown. Fuel nitrogen as an important part of solid fuels contributes significantly to the NOx formation in air-fired combustion and even more in the CLOU process when thermal-NOx and promote-NOx in CLOU are eradicated, because of its low operation temperature (
