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Evaluating the effect of inert supports and alkali sodium on the performance of red mud oxygen carrier in chemical looping combustion Jinhua Bao, Liangyong Chen, Fang Liu, Zhen Fan, Heather Nikolic, and Kunlei Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01835 • Publication Date (Web): 07 Jul 2016 Downloaded from http://pubs.acs.org on July 9, 2016
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Evaluating the effect of inert supports and alkali sodium on the performance of red mud oxygen carrier in chemical looping combustion Jinhua Bao a, Liangyong Chen a, Fang Liu a, Zhen Fan a, Heather S Nikolic a, Kunlei Liu a, b, * a
Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511, USA b
Department of Mechanical Engineering, University of Kentucky, Lexington, KY 40506, USA
*Corresponding author. Tel +1 859 257 0293; Fax +1 859 257 0302; E-mail address:
[email protected] Abstract: Chemical looping combustion (CLC) is an advanced technology with inherent CO2 capture in which a solid oxygen carrier circulates between an air reactor and a fuel reactor. For coal-fueled CLC, the existence of solid impurities requires the oxygen carrier not only to have good reactivity, but also to be contaminant-resistant, lowcost, and readily available. Therefore, the development of cost-effective and well-performing oxygen carriers is very meaningful for the coal-fueled CLC process. Natural red mud, a by-product from the aluminium industry, was found to function well as an oxygen carrier and has also been found to have in-situ coal catalytic gasification behaviour. A thorough study on the long-term cyclic performance of red mud with coal char in a fluidized reactor was conducted in this work. For the purpose of comprehensively understanding the functions of inert supports as well as the sodium content in red mud, the effect of various inert oxides (Al2O3, SiO2, TiO2, and CaO) and the addition of sodium was evaluated. It has been proven that inert supports, Al2O3, SiO2, and TiO2, have a positive effect on both the reduction and oxidation reactivity of iron-based oxygen carriers by developing a porous structure in the particle. Al2O3 and SiO2 show the ability to stabilize the reactivity of iron oxide with a gaseous reductant (CO), even under fluidized conditions. Both Al2O3 and TiO2 can assist in maintaining the mechanical strength of the oxygen carrier after many cycles in a fluidized-bed reactor. The addition of sodium (Na) to red mud does not exhibit much effect on the reactivity of OC with CO as the fuel. However, it can significantly enhance the char gasification rate due to its catalytic function. Additionally, interaction between the active iron oxide and inert 1
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supports or sodium in the form of red mud at high temperatures leads to the formation of spinel phases. The growth of spinel phases results in the reduction of the oxygen carrying capacity. However, it helps fix sodium as a relatively stable chemical compound (NaAlSiO4 or NaFe0.25Al0.75O2). Both inert supports and sodium in natural red mud play critical roles in the performance of red mud as an oxygen carrier from either physical or chemical aspects. Keywords: Chemical looping, red mud oxygen carrier, alkali metal, reactivity, gasification 1.
Introduction Facing the pressure from climate change, reducing the emission of CO2 from coal-fired power plants has
become very urgently needed. Conventional power plant coal combustion requires direct contact between air and coal. The presence of nitrogen (N2) in air strongly dilutes the concentration of CO2 in the flue gas. Typically, the CO2 concentration in the flue gas from traditional utility coal combustion is 12-15 % by volume. 1 Such a low CO2 concentration makes the separation and capture of CO2 energy intensive. Chemical-looping combustion (CLC) inherently isolates the CO2 stream, avoiding the requirement for costly separation processes and significant efficiency penalties 2,3, making it a very attractive option. The CLC process consists of two interconnected reactors, an air reactor and a fuel reactor. An oxygen carrier, usually a transition metal oxide, circulates between the reactors. In the fuel reactor, the oxygen carrier is reduced by the fuel and in the air reactor, the oxygen-depleted carrier is oxidized back to its original state by oxygen from the air. By employing the intermediate oxygen carrier, the oxygen needed for combustion can be transported from the air to the fuel without direct contact. Therefore, the exhausted gas from the fuel reactor contains CO2 and H2O ideally, and cannot be diluted by N2 from the air. By condensing the steam, high-purity CO2 can be obtained without significant energy penalty for separation. The CLC process based on two interconnected fluidized bed reactors was proposed by Lyngfelt et al 4 in 2001, and later successfully demonstrated at Chalmers University of Technology for both gaseous fuels and solid coal at the scale of 10 kWth. 5,6 A larger scale CLC unit of 120 kWth for gaseous fuel was developed in Vienne in 2010. 7 2
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CLC with gaseous fuels (natural gas or syngas) has been widely proven to be feasible with synthetic carriers in the last few years. 8,9,10,11 If coal continues to be a primary energy source in the medium-term, adapting the CLC process for solid coal is highly advantageous in a future business environment with restriction of CO2 emissions. 12 One option for CLC with solid coal is to directly introduce coal into the fuel reactor and physically mix it with oxygen carriers for combustion via solid-solid 13 or gas-solid reactions. However, the direct solid-solid reaction between coal and oxygen carriers is limited by small contact areas and slow solid-state diffusion, and thereby is kinetically much slower than the indirect gas-solid reaction. 14 Instead, coal particles need to first undergo pyrolysis, yielding volatiles and char; then, the resulting char is gasified in situ by a gasification agent (H2O and/or CO2) producing synthesis gas (CO+H2), which subsequently reacts with the oxygen carrier to form CO2 and steam and further enhances the in-situ gasification. Generally, char gasification is the rate limiting step. 15 Application of the CLC technology for solid coal has already been demonstrated at the pilot scale, such as the 100 kWth plant in Chalmers 16, the 100 kWth pressurized CLC in Southeast 17, the 1 MWth plant in Damstandt 18, and the 3 MWth Plant in Alstom. 19 The oxygen carrier is a key factor for the CLC technology development. The reliable oxygen carriers must have high reactivity during reduction and oxidation, long-term stability, good oxygen cyclic capacity, robust mechanical durability, agglomeration absence, low cost, and be environmentally friendly. Up to present, there has been a lot of research focusing on developing different types of oxygen carriers, including synthetic oxygen carriers, natural minerals, industry residuals, etc. 12 Synthetic oxygen carriers are generally metal oxides based on Fe, Cu, Ni, Mn, and Co, which are supported by inert oxides, such as Al2O3, SiO2, MgO, TiO2, ZrO2, cement 20, sepiolite, etc. Due to the contamination of coal ash and the loss of oxygen carriers together with ash removal from the reactor, coal-fueled CLC requires frequent make-up of fresh oxygen carriers. Therefore, the low-cost and abundant or easily available oxygen carriers are more attractive and preferred for coal-fueled CLC. From this point, many researchers 3
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start to focus on seeking natural ores and industry residuals as oxygen carriers, such as Fe- 21,22,23,24, Mn- 25,26,27, Cu-based 28 ores, and by-products from iron and aluminium purification processes. 29,30 Ilmenite, a natural mineral composed of FeTiO3, has been extensively tested on different scale reactors and has been proven as a promising oxygen carrier for coal-fueled CLC.
7,18,31 ,32 ,33 , 34 , 35 ,36 ,37
However, one severe problem for ilmenite is its low
reactivity during reduction. The low reactivity results in a long reaction time and thus a large inventory is needed to achieve the equivalent fuel-conversion efficiency, which will enlarge the reactor size and increase the capital cost. It has been verified that impregnating an alkali metal on ilmenite can promote the reactivity with reductant gas (CO) due to the self-diffusion character of alkali metal. 38,39 However, volatilization of the alkali metals under CLC reaction conditions was notable; the fraction of potassium (K) in ilmenite after 40 cycles in a fluidized-bed reactor decreased to only 1/5 of that in the initial sample. 39 In fact, the needed amount of alkali metal for promoting the reactivity of ilmenite is much less than the impregnation percentage. The impregnation of alkali metal is acceptable for fundamental research. However, the impregnation process may not be appropriate for industrial operation of CLC unit in the future because of the instability of alkali metals, which is likely to cause downstream problems like ash fouling and corrosion. Therefore, searching for a low-cost oxygen carrier which naturally contains an alkali metal in the stable form becomes very meaningful. Red mud (RM), an iron-based solid by-product from the alumina industry Bayer process, is mainly comprised of Fe2O3, Al2O3, SiO2, and TiO2 with a pH ranging from 10-13. 40 Productive utilization of RM is very meaningful since RM disposal is one of the most significant problems in the alumina industry. It has been demonstrated that RM can function well as an oxygen carrier in principle by UKy-CAER. 41 , 42 The reduction and regeneration behaviour of the RM material has been investigated under a simulated CLC atmosphere using a thermogravimetric analyser. 41 Considering that the practical CLC process is generally based on fluidized-bed technology, a detailed study of RM under fluidizing conditions over a long-term cycle is necessary. Meanwhile, examining the function of 4
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each content of RM is valuable for a comprehensive understanding of the RM oxygen carrier, which could be instructive for future oxygen carrier development. Accordingly, the objective of this work is to evaluate the performance of RM during long-term char-fueled CLC experiment under fluidizing conditions. The effect of inert oxides (Al2O3, SiO2, TiO2, and CaO) and sodium, dominant components of RM, on the performance of RM as an oxygen carrier will be determined as well. 2.
Experimental
2.1 Oxygen carriers Different types of oxygen carriers were prepared in this study, including red mud, ilmenite, pure iron oxide, iron oxide containing inert compounds (Al2O3, SiO2, TiO2, or CaO), and synthetic RM doped with/without Na+. Pure iron oxide and ilmenite were used as references. All the carrier particles were sieved to 125-355 µm. i)
Natural RM and ilmenite were supplied by Alcoa World Alumina LLC and QIT Iron and Titanium Inc., respectively. The received RM and ilmenite particles were directly heat treated in air for 6 h at 1150 °C and 950 °C, respectively, in order to remove moisture, gain mechanical strength, and reach fully oxidized state.
ii)
Pure iron oxide particles were prepared from Fe2O3 powder (Alfa Aesar, purity > 99.5 wt. %, diameter < 5 µm). De-ionised water (purified by US filter plus 150) was sprayed on the powder, with mechanical mixing, to form agglomerates, which were subsequently sieved to 125-355 µm.14 These granules were calcined in air at 1300 °C for 6 h, then cooled and re-sieved.
iii)
Iron oxides supported on different types of inert compound were prepared by mechanically mixing Fe2O3 powder and inert oxide powder (γ-Al2O3, SiO2, TiO2, or CaO,
