Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
pubs.acs.org/EF
Novel Binders-Promoted Extrusion-Spheronized CaO-Based Pellets for High-Temperature CO2 Capture Shuai Pi, Zonghao Zhang, Donglin He, Changlei Qin,* and Jingyu Ran*
Energy Fuels Downloaded from pubs.acs.org by UNIV OF TEXAS AT DALLAS on 03/15/19. For personal use only.
Key Laboratory of Low-Grade Energy Utilization Technologies and Systems of Ministry of Education, School of Energy and Power Engineering, Chongqing University, Chongqing 400044, China ABSTRACT: Calcium looping (CaL) is a well-acknowledged approach in CO2 removal via the circulation of CaO-based sorbents between a carbonator and a calciner. During circulation, there is usually a quick decrease in CO2 sorption capacity and sorbent pellets experience severe attrition in fluidized-bed reactors, which are the two major challenges in CaL application. To develop sorbent pellets with high and stable sorption capacity and sufficient mechanical strength simultaneously, in this work, three kinds of novel phosphates were screened and utilized as binders in producing sorbent pellets via extrusion and spheronization, and their mechanical and chemical properties were tested and analyzed systematically. It was found that Mg3(PO4)2·4H2O was a good binder candidate in preparing CaO-based sorbent pellets with promoted mechanical and chemical performance. Under the optimum conditions of 10 wt % loading and 900 °C precalcination temperature, pellets of Ca8Mg1 (80 wt % Ca(OH)2 + 10 wt % Mg3(PO4)2·4H2O) demonstrated a compressive stress of 12.14 MPa and a CO2 uptake of 0.23 g-CO2/g-sorbent after 25 cycles, which were almost three times larger and 64% improved, respectively, compared to the typical Ca10 (pure Ca(OH)2). Additionally, the main reasons for performance enhancement were confirmed to be the production of Ca5(PO4)3(OH) with good hardness and the “spacer” effect of MgxCa1−xCO3.
1. INTRODUCTION CO2 is acknowledged to be the major greenhouse gas that contributes to severe environmental problems, including glaciers melting, sea-level rising, and so on.1,2 Combustion of fossil fuel is the primary source of anthropogenic CO2 emission, especially power generation that consumes a huge amount of coal, giving rise to a large proportion of CO2 emission.3,4 One effective solution for this concern is CO2 capture and storage technology, where the process of CO2 capture accounts for about 75% of the total cost.5,6 Up to now, researchers have proposed various technologies to separate CO2, and the only one that has been commercialized is solvent absorption with monoethanolamine (MEA). However, its application is limited by too much energy penalty.7,8 Hightemperature sorbents such as Li4SiO4 have also been studied but its maximum CO2 sorption capacity is only 0.367 g-CO2/ g-sorbent, and it costs too much to obtain these sorbents.9,10 Recently, a method named calcium looping (CaL) has been proposed and identified as one of the potential technologies for efficient and economical CO2 capture.11 CaL is based on a reversible chemical reaction: CaO + CO2 ⇔ CaCO3. First, CO2 in the exhaust is captured by CaO in a carbonator (at around 650−700 °C) to form CaCO3. Then, CaCO3 is transferred to a calciner for complete decomposition (at 900−950 °C) with CaO produced, which will be sent back to the carbonator to close the looping.12−16 There are many advantages with CaL, such as high CO2 theoretical uptake capacity (0.786 g-CO2/g-CaO) and cheap and abundant precursors of the sorbents.11,17,18 However, this method also faces some challenges. On one hand, CO2 sorption capacity of CaO-based sorbents reduces sharply with increasing the carbonation/calcination cycles because of sintering of CaCO3, and it reserves only 8−10% of its maximum theoretical capacity after long-term operation.19 On the other hand, the © XXXX American Chemical Society
sorbents are likely to be attrited and broken while circulating between the carbonator and calciner.20,21 In addition, thermal stress caused by temperature difference between the two reactors is detrimental to mechanical strength of the sorbent. Therefore, it is necessary to improve both the chemical adsorption stability and mechanical strength of CaO-based sorbents to suit long-term sorption/desorption application.22,23 In previous works, many researchers suggested that the addition of solid binders was beneficial for solving the loss-incapacity and attrition problems as aforementioned. These binders should have high temperature resistance, so that they could act as solid skeletons to isolate CaO or CaCO3 to prevent sintering. At the same time, they were able to make the sorbent assemble closely to improve its mechanical strength. Manovic and Anthony24 tested calcium aluminate cement and calcium bentonites as binders and reported that the former was a promising binder for pelletization, whereas the latter was limited by the generation of Ca2(SiO4) and Ca5(SiO4)2CO3. Ridha et al.25 prepared pellets with natural kaolin and Al(OH)3 obtained from acid leaching of kaolin and found that acetified sorbents with Al(OH)3 as binders had higher CO2 sorption capacity owing to the creation of a porous structure. Qin et al.17 proposed the large-scale manufacture of CaO-based particles by extrusion using calcium aluminate cement (≥39.8 wt % Al2O3) as the binder. With increasing the content of calcium aluminate cement, mechanical strength of the sorbent particles was observed to become much better, but there was still a slight and continuous decrease in CO2 sorption capacity. Chen et al.26 prepared sorbent particles with starch (pore Received: December 4, 2018 Revised: February 15, 2019 Published: February 20, 2019 A
DOI: 10.1021/acs.energyfuels.8b04189 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
temperature, the gas is switched to 85 mL/min of N2 and 15 mL/min of CO2 for CO2 adsorption with a duration of 20 min. Then, the temperature is increased to 900 °C at a rate of 40 °C/min again to regenerate the sorbents for 10 min under pure nitrogen. The above sorption/desorption process is repeated 25 times to investigate the cyclic performance of sorbents. CO2 sorption capacity (Cn, g-CO2/gsorbent) and CaO conversion (Xn, %) of sorbents are calculated according to the weight change of the pellets during test, and the detailed calculations of Cn and Xn are as follows m − m0 Cn = m0
forming agents) and aluminate cement and reported an improvement in both the mechanical strength and CO2 sorption capacity. Sun et al.27 identified pumice as an ideal binder for pellets preparation via hydration-mixing, but the improvement in CaO conversion was not very satisfactory. In summary, until now, the most commonly used binders are calcium aluminate cement and natural minerals containing much Al and Si, such as kaolin and pumice. Although these materials are helpful in pellets manufacture and beneficial for increasing attrition resistance of sorbents, there is relatively limited effectiveness in improving the sorbents’ sorption capacity. Besides, researchers reported that some kinds of metal oxides with high melting points such as MgO,28 ZrO2,29 CeO2,30 and Yb2O331 can act as solid “spacers” in CaO-based sorbents to slow down the sintering of CaCO3, thereby improving CO2 sorption stability of the sorbents. Therefore, it is significant to find novel additives that can both advance mechanical strength and CO2 uptake capacity of CaO-based sorbents effectively. In this paper, three kinds of high temperature-resistant phosphates (AlPO4, Mg3(PO4)2·4H2O, and Ca3(PO4)2) are studied as sorbent binders for the first time to our knowledge. Calcium hydroxide and polyethylene (PE) are chosen as the calcium precursor and pore former, and sorbent pellets are prepared via extrusion and spheronization with the selected novel binders. Then, mechanical and chemical properties of these sorbent pellets are investigated and analyzed, respectively. This work is aiming to identify an efficient binder that is suitable for pellet preparation and further determine its optimal synthesis technology. The influence of precalcination temperature in pellet manufacture is investigated as well.
Xn =
m − m0 MCaO · m0φ MCO2
where m is the mass of sorbent after 20 min carbonation stage, m0 is the mass of sorbent after first calcination, ϕ is the mass fraction of CaO in the fresh sorbent, and MCaO and MCO2 are mole mass of CaO and CO2. 2.3. Mechanical Properties Test. Mechanical properties of sorbent pellets are evaluated based on two aspects: compressive stress and particle-size distribution after impact crushing. Schematic diagram of the devices is shown in Figure 1.
2. EXPERIMENTAL SECTION 2.1. Materials and Pellets Preparation. In this work, three phosphates: AlPO4 (AR), Mg3(PO4)2·4H2O (≥98%), and Ca3(PO4)2 (AR) are used as sorbent binders, and their sizes are all in the range 48−75 μm. Calcium hydroxide (≥95%) and PE are chosen as the calcium precursor and pore former, particle sizes of which are 30−50 and 15 μm, respectively. CaO-based sorbent pellets are prepared by the following steps: first, 40 g raw materials consisting of calcium hydroxide, PE, and phosphate binders are weighed, in which the content of PE is fixed as 10 wt %, and the left are binders and Ca(OH)2 with a mass ratio of 1:8, 2:7, 3:6, and 4:5. These materials are dry-blended for 10 min and wet-mixed for 5 min with about 15 g deionized water. Then, cylindrical sorbents with the diameter of 1 mm are produced in an extruder (E-26). After that, the cylinders are dried in air for 5 min before they are loaded in a spheronizator (R-120) to prepare sorbent pellets via spheronization. The extruder and spheronizator rotate at a speed of 3000 rpm, and the two processes both last for 20 min. Finally, two kinds of pellets whose particle sizes are around 500 and 750 μm are selected (named as fresh pellets) and a part of them are calcined at 900 °C for 3 h in a muffle furnace (called precalcination). The sorbent samples manufactured are named as the binder symbol followed by the ratio of raw materials. For example, the samples with 60 wt % calcium hydroxide, 30 wt % binders, and 10 wt % PE are named Ca6Al3, Ca6Mg3, and Ca6Ca3, respectively. Additionally, the sorbent pellets prepared from fresh calcium hydroxide are named Ca10, and the sample with 90 wt % calcium hydroxide and 10 wt % PE is represented as Ca9. 2.2. CO2 Sorption/Desorption Test. CO2 sorption/desorption of sorbent pellets is tested in a thermogravimetric analyzer (NETZSCH TG 209 F3, Germany). About 15−20 mg fresh pellets with the particle size of 500 μm are placed in the thermogravimetric analysis, and it is heated to 900 °C at a rate of 40 °C/min under a nitrogen flow of 100 mL/min. After calcination for 10 min, the temperature is decreased at a rate of 30 °C/min to 650 °C, and at this
Figure 1. Schematic diagram of experimental devices. 2.3.1. Compressive Stress Test. Compressive stress of spherical pellets is measured using a precision digital force gauge (SHIMPO FGP-10, Japan), and its accuracy is 0.2% R.C. (R.C. = 100 N). A particle is transported on the testing platform, and the force of the probe is slowly increased until the particle is broken. And the maximum force is considered as the pressure that the particle can withstand. Each sample with or without precalcination is measured 20 times, and the average crushing force is calculated and divided by the maximum cross-sectional areas of these particles. In this case, the maximum compressive strength of pellets with different sizes can be compared conveniently. 2.3.2. Impact Crushing Test. Impact crushing experiment of pellets was performed in an apparatus built by ourselves, which is similar to the facilities described in the literature.32−34 It consists of two valves, a hopper, an eductor, a stainless target, and a collection chamber connected to a cylinder feeding with dry air, whose flow is accurately controlled by a mass flow controller. Sorbent pellets (about 500 mg) B
DOI: 10.1021/acs.energyfuels.8b04189 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels are accelerated by air flow at a speed of 18 m/s and impact the stainless target at the bottom of the system vertically via the eductor. Two valves are used to prevent the gas from escaping and to control the particles to fall into the eductor slowly. When pellets hit the target inclined by 60° with respect to the vertical direction, they will break into particles with smaller sizes. By sieving and weighing the mass of pellets with different sizes, we can evaluate the attrition resistance of sorbent pellets. 2.4. Characterization of Pellets. Phase compositions of sorbents are tested by an X-ray diffractometer (Bruker D8 ADVANCE, Germany). Surface morphology and element distributions of sorbents are investigated using field emission scanning electron microscopy (Hitachi SU8020, Japan) equipped with an energy dispersive spectrometer (HORIBA EX-350). The specific Brunauer−Emmett− Teller (BET) surface area and the average pore width of the sorbent pellets are determined via N2 sorption/desorption analysis obtained at approximately −196 °C using a Micromeritics TriStar II 3020 instrument after out-gassing under vacuum for 18 h at 200 °C.
3. RESULTS AND DISCUSSION 3.1. Roles of Various Novel Binders. Three kinds of phosphates (Al, Mg, Ca) were chosen as binders combined with calcium hydroxide and PE to prepare sorbent pellets via sequential extrusion and spheronization. To understand the potential roles of additives in sorbents, pyrolysis of the PE and binders was first conducted and their mass changes with temperature are depicted in Figure 2. It can be seen that PE Figure 3. Phase compositions of sorbent pellets with 30 wt % binders after precalcination at 900 °C.
occurrence of binder pyrolysis and consumption of partial active CaO during calcination. 3.1.1. Mechanical Behavior of Pellets with Different Binders. Two kinds of pellets with diameters of 750 and 500 μm were selected for the compression test, and the results are shown in Figure 4. It is obvious that pellets with 500 μm diameter exhibit much higher compressive stress than those with diameters of 750 μm, which is consistent with the work of Chen et al.26 With the addition of PE as the pore former in pellet manufacturing, the compression resistance of pellets decreases regardless of whether they are calcined or not. Before precalcination, some extent of but not too much improvement in compressive stress of sorbent pellets with 30 wt % binders is seen compared to Ca9. In contrast, after the calcination process, compressive stress of sorbents with 30 wt % AlPO4 and Ca3(PO4)2 is about two times higher than the fresh calcium hydroxide (3.97 MPa), and Ca6Mg3 also reaches a value of 6.34 MPa. As the compressive stress after precalcination is more significant and meaningful in CaL application, thus, these three phosphates are seen as good binder candidates for further investigation. Figure 5 shows particle-size distribution of sorbent pellets after the impact crushing test. About 500 mg of the sorbent pellets with diameters ranging in 500−750 μm are selected for the experiment, and after the test, the residual particles are collected, sieved into six size categories: 500−750, 375−500, 250−375, 187.5−250, 100−187.5, and
