Potassium-Promoted Î³-Alumina Adsorbent from K2CO3 Coagulated

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Potassium Promoted #-alumina Adsorbent from K2CO3 Coagulated Alumina Sol for Warm Gas Carbon Dioxide Separation Shuang Li, Yixiang Shi, and Ningsheng Cai ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 08 Dec 2014 Downloaded from http://pubs.acs.org on December 9, 2014

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ACS Sustainable Chemistry & Engineering

Potassium Promoted γ-alumina Adsorbent from K2CO3 Coagulated Alumina Sol for Warm Gas Carbon Dioxide Separation Shuang Li, Yixiang Shi*, Ningsheng Cai

Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China.

*

Email Address: [email protected]

KEYWORDS. Potassium-promoted γ-alumina; Alumina sol; Warm gas clean-up; Pre-combustion CO2 capture; Carbon dioxide adsorbent;

ABSTRACT. This paper provided a practical synthesis method of K-promoted γ-alumina for CO2 pre-combustion capture integrating adsorbent pelletting and catalytic active component promotion into one single process. The precursor of absorbent, K-promoted pseudo boehmite (PB), was synthesized from alumina sol and K2CO3 powder by mixing, aggregating, drying and aging. K-promoted γ-alumina was obtained after calcination at 400oC from K-promoted PB. Characteristic crystallines of several stages of products in the synthesis process had been detected by X-ray diffraction. The optimal Al:K ratio in view of CO2 capacity and cyclic performance of K-promoted γ-alumina were obtained by a thermal gravimetric analyzer. The optimal ratio of K-promoted γ-alumina showed a capacity of 0.67 mmol/g (dry base) for the first cycle and 0.34 mmol/g (wet base) after 30 pressure swing adsorption (PSA) cycles at 300oC. K-promoted γ-alumina adsorbent could be favorable for low energy

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penalty pre-combustion CO2 capture application. It showed a variety of advantages such as low adsorption heat, good mechanical strength, low cost in synthesis and fair stable capacity.

Introduction

Fossil fuel-based power generation brings about large amount of anthropological CO2 emission, which gives rise to global warming nowadays. However, CO2 capture from power generation process is energy consuming, consequently thermal efficiency of powder plants will fall by 8-10% inevitably.1 Pre-combustion CO2 capture in a warm gas clean-up process enjoys following two advantages. First, volumetric concentration of CO2 in shift gas is 15%-40%, relatively higher compared to its post-combustion counterpart. Secondly, if solid adsorbents were used in the elevated temperature range (250oC -400oC) of syngas or shift gas, another 0.2-0.7% of HHV based thermal efficiency penalty caused by carbon capture will be saved compared to cold clean-up such as Selexol. Moreover, it is possible to cut down on additional regeneration heat if pressure swing adsorption (PSA) was adapted for cyclic CO2/H2 separation process.2 From this concept, a combination of water-gas-shift and CO2 pre-combustion capture, called sorption enhanced water gas shift (SEWGS), have been comprehensively studied by various scientists. 3-7

Selection of proper adsorbents/sorbents is priority in turning SEWGS into practice. Appropriate adsorbents/sorbents should meet the following requirements, 1) relative fast adsorption and desorption kinetics, in order to match fast PSA processes; 2) stable


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cyclic CO2 capacity at the temperature range of 250-400oC; 3) low adsorption heat/regeneration heat to meet the demand of PSA. Therefore, magnesium and aluminum oxides based chemical adsorbents, including hydrotalcite-like compounds (HTls), are proved to be promising options by previous researchers.8-11 The chemical adsorption of CO2 takes place on the surface by forming partially reversible chemical bond. Unlike bulk chemical sorbents such as CaO and Li2ZrO3(adsorption heat is 178 kJ/mol and 160 kJ/mol respectively), the adsorbents which are weakly boned with CO2 will not transform into new products when adsorption takes place, thus the adsorption heat is rather small. As it is shown in literature, the adsorption heat of CO2 on MgO and calcined hydrotalcite is 30-39 kJ/mol12 and 10-17 kJ/mol8 respectively. Alkaline potassium salt is often promoted or doped to enhance the CO2 capacity and adsorption kinetics of pristine magnesium oxide,13 aluminum oxide14,15 and HTlc derived materials.16-18 Promoted surface basic sites elevate the adsorption heat to 37.5 kJ/mol for Na2O promoted alumina,19 and 42 kJ/mol for K2CO3 promoted hydrotalcite.20 In above cases, The fact that the adsorption only takes place at adsorbent’s surface ensures continual CO2 separation by MgO, Al2O3 and HTlcs in a PSA process.

For above applicable surface adsorbents, good reversibility and mechanical stability are essential to be discussed. Magnesium oxide may show higher capacity than its alumina counterpart; however it suffers from poor stability in both capacity and mechanical strength. The adsorption temperature of CO2 on MgO is usually between 25-100oC, while desorption takes place at 400-800oC.13,21 Hydrotalcite (most often



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discussed HTls) transforms to amorphous adsorbent after calcination at 400oC.22 It can be viewed as a mixture of magnesium oxide and aluminum oxide at atomic scale, thus it combines the advantage of chemically active MgO and inert Al2O3. Though irreversible Mg ions are well dispersed at atomic scale in the calcined hydrotalcite molecule, MgCO3 is likely to form in pellets after 1200 cycles, giving rise to cyclic capacity degradation and mechanical failure.23 Additionally, steam concentration and intense pressure change in real SEWGS and PSA processes should be considered. Therefore, chemically inert and stable Al-O bond is more favorable for PSA than magnesium oxide based adsorbents at the temperature range of 250-400oC, especially taking adsorbent costs, cyclic reversibility and mechanical strength into consideration.

It is reported that surface promotion by basic potassium salts, and analogous alkaline earth or alkaline metal earth salts, enables robust but poor CO2 capacity alumina to become

competitive

adsorbent

in

removal

of

CO2

from

WGS

processes.14,15,24-27Traditionally, in order to obtain adsorbents in pellet form, shaped alumina pellets are directly doped or impregnated by potassium cation based solution. Iordan24 studied interfacial chemistry of K+ on alumina molecular in the impregnation process, K-O-Al bond was existed on the liquid-solid surface in K2CO3 solution, and calcination enabled a uniform surface dispersion of K promotion. Walspurger synthesized15 K-promoted alumina from cheap alumina pellets. K-dawsonite was formed after the reaction with CO2 and steam at 300oC, and regenerated at 500oC by a temperature swing adsorption process. Wang25 focused on the effects of various potassium salts on the surface basicity of alumina. Previous researches have


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contributed to a better understanding of fundamental mechanism and reaction process of CO2 adsorption on K-promoted alumina. It is high time that focus should be paid on K-Alumina’s cyclic performance in real WGS operation condition to look for possible application for industrial use.

On one hand, Alumina sol is one of the mostly used starting materials in adhesive and additive synthesis. On the other, simple electrolytes are often employed as coagulant to condense colloidal alumina or silica in industrial production of functional catalytic supports. A combination of both may become a potential alternative for traditional K+ solution impregnation method when taking integration of pelleting and promotion in one process into consideration. Mechanically stable functional alumina skeleton will be formed in-situ to adhere and support the porous material after drying and calcination.

This research synthesized K2CO3 promoted γ-alumina by coagulating colloidal alumina sol with K2CO3 electrolyte solution. The optimal ratio and various alkaline metal based electrolytes have been studied. Crystalline phases of products after each synthesis step, including coagulation, drying and calcinations, is detected by X-ray Diffraction (XRD). Cyclic performance of K-promoted alumina with optimal K:Al ratio was tested isothermally at 300oC, 1 atm, both in dry and wet conditions, by a thermal gravimetric analyzer (TGA) setup.

Synthesis of K-promoted γ-alumina



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Figure 1. An illustration of alumina sol micelle

Anhydrous potassium carbonate (A.R., Beijing Chemicals) and commercial alumina sol (A-4-20, Snow chemical, China) are starting materials for systhesizing K-promoted alumina. The main component of alumina sol is positively charged [AlOOH]n crystal and its detailed properties are available in Table 1.

Table 1.

AlOOH

pH

(wt.%)

20±2%

Properties of A-4-20 alumina sol

Average particle

Appearance

Crystal type

Impurities

Slight white

Boehmite

NO3-

Potassium-Promoted Î³-Alumina Adsorbent from K2CO3 Coagulated

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