Article pubs.acs.org/IECR
Performance of Hematite/Ca2Al2SiO7 Oxygen Carrier in Chemical Looping Combustion of Coal Tao Song, Laihong Shen,* Siwen Zhang, Dingqian Chen, and Jun Xiao
Ind. Eng. Chem. Res. 2013.52:7350-7361. Downloaded from pubs.acs.org by UNIV OF TEXAS SW MEDICAL CTR on 10/08/18. For personal use only.
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, 210096, China ABSTRACT: In this work, a novel Ca-enhanced hematite oxygen carrier was developed for chemical looping combustion of coal. Calcium aluminate cement was used to bind hematite particles to enhance its mechanical strength and improve its chemical properties. An anthracite coal was used as the feed stock. Coal gasification and combustion tests at 900 °C were carried out with the silica sands particles, calcium aluminate cement particles, pure hematite, and the combined Ca-enhanced hematite oxygen carrier particles in a fluidized-bed reactor at atmospheric pressure. It was found that, by the presence of the contents of CaO, Al2O3, as well as SiO2, a material of Ca2Al2SiO7 with high melting point was produced. This Ca2Al2SiO7 material was confirmed to improve the mechanical strength for the newly developed oxygen carriers. Also, during the initial reduction process, the coal gasification rate was improved by nearly twice with the addition of the calcium aluminate cement. The physical and chemical properties for the combined oxygen carrier were characterized by X-ray powder diffractometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy and measurements of BET surface area. Multiple cycles tests confirmed that a stable reactivity for the newly developed oxygen carrier. The presence of Ca2Al2SiO7 prevented the sintering process during multiple cycles.
1. INTRODUCTION At present, chemical looping combustion (CLC) is known to be an interesting and attractive combustion technology to produce a relatively high concentration of CO2 in the combustion process. The concept of CLC is based on the use of oxygen carrier materials from oxidation−reduction cycles that can either oxidize or reduce fuels. The 1st CLC design based on the circulating fluidized bed principle was proposed by Lyngfelt et al.1 This concept develops as a well-accepted approach to the conduct chemical looping process in two fluidized-bed reactors (Fuel reactor and Air reactor) connected by solid transportation lines. Between these two reactors, the oxygen carriers are transported and cyclically used to supply lattice oxygen. Thereby the direct contact between fuel and air is avoided. In this way, the exit gas from the fuel reactor is mainly the high concentration of CO2 and water. After water is condensed, nearly pure CO2 is readily obtained, and then compressed into a liquid for storage. Some overviews of development of CLC technologies have been given by Lyngfelt et al.,2,3 Hossain and de Lasa,4 Fan and Li,5 Moghtaderi,6 and Adánez et al.7 One of the alternatives of CLC with solid fuels concepts is the so-called in situ gasification chemical looping combustion (iG-CLC). For coal as an example, in this approach, the coal and oxygen carriers are mixed in the fuel reactor, where coal is gasified by means of fluidizing agents of steam or CO2 (R1− R4). The coal pyrolysis/gasification products subsequently react with the oxygen carrier particles to yield CO2 and water (R5). The reduced oxygen carriers are transferred to the air reactor, where they are regenerated to its original state. fast pyrolysis
coal ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ volatile matter + resultant char
(R1)
char(C) + H 2O → CO + H 2
(R2)
© 2013 American Chemical Society
char(C) + CO2 → 2CO
(R3)
CO + H 2O → CO2 + H 2
(R4)
n MexOy + H 2 , CO, volatile matter → n MexOy − 1 + CO2 + H 2O
(R5)
The oxygen carrier is a significant component in the CLC process. It needs a high chemical reactivity when it is cyclically transferred in the two reactors. The manufacturing cost and environmental compatibility of oxygen carrier are other important factors needed to be considered. A range of oxides might be used as oxygen carriers, such as Ni-based, Mn-based, and Cu-based oxygen carriers. As an attractive option for iGCLC applications, Fe-based oxygen carriers are attractive due to low cost and without toxicity, though they have weak redox characteristics, low methane conversion and low oxygen transport capacity.8 Different types of Fe-based oxygen carriers have been investigated and analyzed including synthetic materials,9−14 natural ores15−21 and industrial residues or byproducts.8,22−24 In the iG-CLC process, a main challenge is to increase the char gasification rate, since the char gasification is the limiting step in this process.13,16,25 Some investigations have been carried out to improve the char gasification rate in iG-CLC process. These works were performed by Teyssié et al.,26 Schwebel et al.,27 Cuadrat et al.,19 and Gu et al.29 During these investigations, some additives such as Ca, alkali carbonates, Received: Revised: Accepted: Published: 7350
April 10, 2013 May 12, 2013 May 14, 2013 May 14, 2013 dx.doi.org/10.1021/ie401142w | Ind. Eng. Chem. Res. 2013, 52, 7350−7361
Industrial & Engineering Chemistry Research
Article
based on the particle size of the raw hematite as follows:
