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Na-containing minerals transformation behaviors during Na2CO3-catalyzed CO2 gasification of high-alumina coal Yangang Mei, Zhiqing Wang, Huibin Fang, Yongwei Wang, Jiejie Huang, and Yitian Fang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02491 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017
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Na-containing minerals transformation behaviors during Na2CO3-catalyzed CO2 gasification of high-alumina coal
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Yangang Meia,b, Zhiqing Wangb*, Huibin Fangb, Yongwei Wanga,b, Jiejie Huangb,
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Yitian Fangb
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a
University of Chinese Academy of Sciences, Beijing 100049, China
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b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
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Academy of Sciences, Taiyuan, Shanxi 030001, China KEYWORDS: Catalytic gasification; Minerals transformation; Al extraction;
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High-alumina coal
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ABSTRACT: Interactions between alkali metals in catalysts and silicon or aluminum
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minerals in coal are closely related to gasification reactivity, deactivation and
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recovery of alkali catalysts during coal catalytic gasification, and alkali-containing
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minerals and their transformation behaviors are key issues for understanding these
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interactions. In this paper, Na-containing minerals transformation behaviors and their
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influences on the catalytic performance during Na2CO3-catalyzed CO2 gasification of
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high-alumina coal were comprehensively investigated by TG, ICP, XRD and FTIR.
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Moreover, in order to have a better understanding of the minerals transformation
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during catalytic gasification, model compounds, i.e. kaolinite (Al2O3·2SiO2·2H2O)
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and boehmite (AlOOH), the main Al-containing minerals in high-alumina coal, were
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chosen as model compounds to investigate the minerals transformation behaviors. The
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results show that Na2CO3 firstly deactivates to generate inert sodium aluminum
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silicate (Na1.55Al1.55Si0.45O4) at 700 °C, this contributes to the deactivation of catalysts, 1 / 30
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and then various kinds of sodium aluminum silicates are formed with increasing
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temperature and Na2CO3 addition. Among them, sodium aluminum silicate
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((Na2O)0.33NaAlSiO4) has been testified as the most stable mineral during gasification.
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In addition, Na-containing minerals transformation and its resulting products are
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helpful to the recovery of Al from the ash of catalytic gasification, and 94% recovery
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rate can be obtained, which is considered to be a method to extract Al from
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gasification ash. Model kaolinite and boehmite can well explain the minerals
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transformation during Na2CO3-catalyzed CO2 gasification of high-alumina coal.
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INTRODUCTION
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Gasification is considered to be a key process in many clean coal technology. In
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order to obtain high carbon conversion and rapid reaction rate, gasification is usually
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operated at high temperature and high pressure,1-3 resulting in high capital investment
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but relatively poor operation stability. Catalytic gasification, which can be operated at
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moderate condition, has been attracting continuing interest and showing some
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prospects of commercial application, due to its lower reaction temperature,4,5
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remarkably enhanced gasification rate and higher methane content in syngas.5-8 Alkali
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metals,9 alkaline-earth metals,10,11 and transition metals12-14 are usually used as
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gasification catalysts, among which Na and K are considered to be the most excellent
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catalysts during coal gasification.5-10 Na and K can obviously enhance gasification
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rate, and thus the gasification temperature can be decreased dramatically. Furthermore,
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both catalytic effect and low temperature contribute much to the higher content of
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methane in syngas, which is very important for synthetic natural gas (SNG) industry,
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because the gap between methane supply and demand in some country has been
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expanding and dozens of coal to methane industry projects (SNG) have been running.
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Catalytic gasification has been attracting more and more attention.
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However, large-scale commercial application of catalytic gasification has not
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been successfully industrialized, which can be explained by four reasons: catalyst
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deactivation, low recovery rate of catalyst from gasification ash, slagging and
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corrosion problems derived from alkali addition.15-18 Catalyst deactivation and low
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recovery rate of catalyst result in huge alkali consumption and only parts of alkali act
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as catalysts; serious slagging can even stop the gasifier; corrosion decreases the life of 3 / 30
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steel equipment. The lower recovery rate of catalysts may attribute to the interactions
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between alkali metals in catalysts and minerals in coal, these interactions consume
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catalyst and most of the resulting products are water-undissolved, and thus only part
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of alkali can be reclaimed, resulting in poor competitiveness compared with
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traditional gasification technology. Therefore, understanding the migrating and
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transformation behaviors of alkali-containing matters during catalytic gasification is
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very important for industrial R&D of catalytic gasification. Basing on these, lots of
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researchers have focused on this issue, and many valuable results have been obtained.
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Ding6 investigated the co-gasification behaviors of coal and biomass char and found
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that potassium species in biomass exhibit better catalytic effect than that of coal char,
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and inactivation of K can well explain the different catalytic performance of K.
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Wang19 and Rozita20 found that the nepheline and sodium aluminum silicate are
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inactivation product of alkali catalysts and alkali metals can react with minerals in
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coal to form sodium aluminum silicate and nepheline. Kosminski21 and Li22,23
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investigated the migrating behaviors of inherent alkali during gasification. However,
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little work focuses on alumina-Na reactions and transformation behaviors of
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alkali-containing minerals during catalytic gasification.
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In this study, a high-alumina coal and sodium carbonate (Na2CO3) were chosen
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to amplify the alumina-Na reactions during catalytic gasification. The effect of
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Na2CO3 content on Na-containing minerals transformation and its catalytic
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performance were investigated. Moreover, in order to have a better understanding of
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minerals transformation, model compounds, i.e. kaolinite and boehmite, the 4 / 30
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dominating minerals in high-alumina coal, were chosen as model compounds to
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testify and investigate the minerals transformation behaviors. The objective is to
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provide technical support for industrialization of coal catalytic gasification.
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2. EXPERIMENTAL SECTION
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2.1. Samples and reagent.
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Sunjiahao bituminous coal, collected from east of Inner Mongolia and typically
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for its high alumina content in coal ash, was chosen. The raw SJH coal was ground,
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sieved to a particle less than 120 µm, dried in an oven at 105 °C for 12 h and then
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stored in a desiccator as coal sample (SJH). Sodium carbonate, washed kaolinite and
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boehmite were purchased from Tianjin Hengxing Chemical Co., Inc., China Kaolin
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Clay Inc. and Xuancheng Jingrui novel material Co., Inc., respectively. The proximate
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and ultimate analyses of SJH coal were listed in table 1, and the ash composition of
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SJH coal was listed in table 2. It can be seen that content of Al2O3 in the ash can reach
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46.34%. Table 1. Proximate and Ultimate Analyses of SJH Coal.
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proximate analysis (wt% ada) V FC A M SJH coal 29.6 51.2 16.9 2.3 Sample
93 94
a
ultimate analysis (wt% dafb) C H N S Oc 78.8 4.9 1.5 0.8 14.0
ad: air-dried basis. bdaf: dry ash-free basis. cBy difference Table 2. Ash Compositions (wt%) of SJH Coal. sample Al2O3 SiO2 Fe2O3 TiO2 CaO MgO Na2O SO3 K2O P2O5 SJH coal 46.34 36.29 6.38 3.57 2.61 2.18 0.76 0.45 0.42 0.03
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2.2. Preparation of Na2CO3 loaded coal samples. The Na2CO3 loaded coal samples were prepared by solution impregnation. The 5 / 30
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procedure was as follows: a certain amount of Na2CO3 was completely dissolved in
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50 mL deionized water to form Na2CO3 aqueous solution, and then 10 g SJH coal was
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added to the Na2CO3 aqueous solution and the mixed was kept at 80 °C under stirring
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until it changed into viscous slurry, and the slurry was dried in an oven at 105 °C for
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24 h. Finally, the dried samples were crushed and sieved to a particle less than 120 µm
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as Na2CO3 loaded coal samples. The amounts of Na2CO3 loaded on coal were 1-15
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wt%.
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2.3. CO2 gasification.
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A Setaram SETSYS TGA was employed to measure the isotherm reactivity of
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Na2CO3 loaded SJH coal at different temperature. In every runs, 6 mg sample was
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evenly spread on a platinum crucible and heated to the setting temperature
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(600-1000 °C) at a rate of 20 °C/min under a continuous N2 flow (99.9%, 100
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mL/min), kept for 30 min to stabilize the system, and then the N2 was switched to
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CO2 (99.8%, 100 mL/min) and gasification reaction began. The gasification continues
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until the total mass was stable.
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The carbon conversion (X) was calculated using the following equation:
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X=(m0-mt)/(m0-m∞)
(1)
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Where m0 represents the initial mass of char at the beginning of gasification, mt
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represents the mass of char during the gasification at a certain time, m∞ represents the
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final mass of gasification.
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The reactivity index (R0.5) was adopted to compare the gasification reactivity of
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various Na2CO3 additions at different temperature. The reactivity index was defined 6 / 30
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as follows: R0.5=0.5/τ0.5
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τ0.5 was the time when the X was 0.5.
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2.4. Preparation of coal gasification ash.
(2)
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Elema tubular furnace, with 40 mm internal diameter, 600 mm length was
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employed to prepare coal gasification ash. The tubular furnace was heated to the
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desired temperature at a rate of 10 °C/min. Then 2 g sample, evenly spread in a nickel
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boat, was quickly pushed to the flat-temperature zone with continuous CO2 flow
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(99.8%, 100 mL/min). When the experiment is over, the nickel boat was pulled out to
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the low temperature area for 50 min with the protection of N2 flow (99.9%, 600
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mL/min). The prepared ashes were used to investigate the minerals transformation
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behaviors and Na distribution during coal catalytic gasification, and the sample of
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15%Na-900 represents the ash prepared by CO2 gasification of SJH coal at 900 °C
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and with 15 wt% Na2CO3 addition, and the rest can be deduced by 15%Na-900.
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2.5. Water and acid leaching process.
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In order to investigate Na-containing minerals distribution during catalytic
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gasification and its reactions with Al-containing minerals, Na-containing minerals
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were divided into four parts: water dissolved, acid dissolved, remaining in acid
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leaching residue and volatile during gasification, and every part converted to content
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of pure Na and were recognized as Na distribution. Among these, remaining in acid
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leaching residue was the Na which can’t be leached during the process of acid
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leaching; Volatile was the Na which volatilizes as vapor during catalytic gasification,
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and it was calculated as follows:
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NaV=NaT-NaA-NaW-NaR
(3)
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Where NaV was the content of volatile Na, and NaA, NaW, NaR and NaT were the
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content of Na dissolved in acid, water dissolved, remaining in acid leaching residue
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and total Na, respectively. The leaching experiments were conducted using a
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thermostat water bath with magnetic stirrer. In every run, 10 mL 6 mol/L H2SO4 and a
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magnetic paddle were added to a PTFE cuvette, and the cuvette was fixed on the
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thermostat water bath. Once the desired temperature was obtained, 0.5 g sample was
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added into the solution. After two hours’ leaching, the hot solution was filtered
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through Buchner funnel, and the filter cake was washed by hot deionized water. The
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leaching solution and washing liquid were collected to a 100 mL volumetric flask and
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precisely set to 100 mL. The filter cake was dried at 105 °C in an oven, and
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R-15%Na-900 represented the leaching residue of 15%Na-900, and water leaching
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experiment was the same as above except for changing the leaching medium
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deionized water to H2SO4. ICP was employed to measure the Na, Al content in the
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leaching liquid. The alumina leaching efficiency was calculated as follows:
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X=ms/ml
(4)
Where ml was Al content in the leaching liquid, ms was Al mass in coal catalytic
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gasification ash.
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2.6 Characterization means
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The proximate and ultimate analyses were conducted following the Chinese
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National Standards (GB/T212-2008 and GB476-91, respectively). The Na, Al content
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in the liquid were tested by ICP (Thermo ICAP 6300, Thermo Fisher Scientific, USA).
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The phase analyses of coal gasification ash were performed by an X-ray diffraction
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analyzer (D8Advance, Bruker, Germany) using Cu Kα radiation (λ = 1.54056 Å). The
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accelerate voltage of 30 kV, a tube current of 15 mA, and a step size of 4 deg/min
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between 2θ = 10° and 80° were used. Infrared spectra were measured by a Thermo
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Scientific IS50FT-IR spectrometer, where KBr pellet method was used and the
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spectra in the region of 4000-400 cm-1 were obtained after 64 scans at 4 cm-1
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resolution.
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3. RESULTS AND DISCUSSION
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3.1. XRD spectrums of SJH coal and Na2CO3 loaded coal.
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The XRD results of SJH coal and Na2CO3 loaded coals are shown in Figure 1. It
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shows that kaolinite and boehmite are the main alumina-containing matters in coal,
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which is the reason why the alumina content in ash is relatively higher compared with
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other coals. Guo24 found the same minerals in coal gangue. In addition, 15 wt%
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Na2CO3 loaded SJH coal shows weak Na2CO3 diffraction peaks, which indicates that
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Na2CO3 exists in the form of highly dispersed Na2CO3, and only a little in the form of
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crystalline and bulk Na2CO3. It is recognized that highly dispersed Na2CO3 has better
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catalytic performance than that of crystalline and bulk Na2CO3, and thus the
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impregnating method used in this study is appropriate.
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Figure 1. The XRD spectra of raw coal and 15 wt% Na2CO3 loaded SJH coal. 1. Kaolinite 2. Boehmite 3. Na2CO3
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3.2. Catalytic performance of Na2CO3 and Na distribution.
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3.2.1. Catalytic gasification with 1-15 wt% Na2CO3 addition and Al leaching of the
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gasification residue.
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The concentration of Na2CO3 plays an important role in catalytic gasification.
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The isothermal gasification was conducted at 900 °C, and the curves of carbon
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conversion X versus time are shown in Figure 2. It’s generally accepted19,25 that the
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gasification activity firstly increases with increasing Na2CO3 addition and levels off at
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a certain concentration. It’s clearly shown that increase in Na2CO3 concentration can
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increase the gasification rate. 120 min is needed to complete the total gasification
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reaction in uncatalyzed CO2 gasification at 900 °C, while 80 min is needed for that of
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5 wt% Na2CO3 loaded sample. It is interesting that gasification reactivity rapidly
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increases when the amount of Na2CO3 increases from 5 wt% to 7 wt%. Whereas, the
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subsequent increase in Na2CO3 contributes little to the gasification reactivity, it seems
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that the content of catalyst has reached its saturation level. Popa3 and Wang8 also 10 / 30
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found that the amount of alkali catalyst has an optimal value during catalytic
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gasification. However, the saturation amount and rapidly increased gasification
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reactivity in this study is different from the above findings. Al-containing minerals in
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coal may answer this difference, and the reasons can be explained as follows.
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Figure 2. Effect of Na2CO3 loaded content on coal gasification at 900 °C.
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Figure 3. Al extraction yield and R0.5 change with various Na2CO3 additions at 900 °C.
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The obvious increase in gasification reactivity (fast growth in catalytic
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performance) from 5 wt% to 7 wt% Na2CO3 addition may be related to Na
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inactivation during catalytic gasification. With low loading amount, most of Na2CO3
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reacts with minerals in coal to form inactivated sodium aluminum silicate, which 11 / 30
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contributes little to the gasification reactivity, and thus the catalytic performance is
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poor. But the amount of minerals in coal is a constant, when Na2CO3 increases to a
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certain amount, Na2CO3 has been consumed off by reacting with minerals in coal. The
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furtherly added Na2CO3 can be saved and acts as catalyst, and thus gasification
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reactivity increases obviously. In order to certify these hypotheses, some other
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experiments were conducted.
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First of all, it’s a fact26 that Al-containing minerals in uncatalytic gasification ash
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(mainly mullite and alumina) are hard to be dissolved by acid, while most of the
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inactivated sodium aluminum silicate are easy to be dissolved by acid. So the Al
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extraction by acid can well represent the inactivated Na during catalytic gasification.
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In order to have a better understanding of the inactivated Na, the Al extraction yields
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versus different Na2CO3 additions and R0.5 versus various Na2CO3 additions are
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shown in Figure 3. Al extraction yield for uncatalyzed CO2 gasification ash can only
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reach 40%, but the yield for the gasification ash of 7 wt% Na2CO3 additions can reach
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83%, indicating most of Al-containing minerals have reacted with Na2CO3 to form
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acid dissolved matters. Obviously, Al extraction yield increases rapidly with the
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addition of Na2CO3, while obvious increase in R0.5 has not been observed when the
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amount of Na2CO3 increases from 1 wt% to 5 wt%, however, with successive addition
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of Na2CO3, the Al extraction increases slightly but R0.5 increases obviously. It seems
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that a small amount of Na2CO3 tends to inactivate to form sodium aluminum silicate
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rather than accelerate gasification, while the furtherly added Na2CO3 only influence
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gasification rather than Al extraction. When the amount of Na2CO3 is small and 12 / 30
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insufficient, most of Na2CO3 firstly reacts with the Al-containing minerals in coal and
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converts to other Na-containing matters with no catalytic activity, resulting in
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catalysts inactivation. This is consistent well with the slight increase in R0.5 when the
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amount of Na2CO3 increases from 1 wt% to 5 wt% (Figure 3). On the other hand,
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Al-containing minerals in raw coal usually can’t be extracted, and thus the increase in
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Al extraction yield is partly brought out by the reactions of Na2CO3 and Al-containing
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minerals in the coal, and thus the increase in Al extraction partly reflects the
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inactivated Na2CO3. The similar Al extraction behaviors were obtained by Guo27, who
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investigated the Al leaching behaviors using coal gangue as Al source and Na2CO3 as
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activator. Tang28 and Jiang29 also found that alkali metals can be inactivated during
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catalytic gasification. When the amount of Na2CO3 reaches 5 wt%, most of the
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minerals in coal have completed the inactive reaction and convert into acid-dissolved
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matters and there is no enough minerals to reaction with Na2CO3, so furtherly
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increased Na2CO3 addition from 7 wt% to 15 wt%, the Al extraction yield only
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increases by 11 percent (from 83% to 94%), which indicates that most of
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Al-containing matters have reacted with Na2CO3 to form acid-soluble substance. Thus
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the subsequently added Na2CO3 may exit in the forms of “active catalysts” and only
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then the high gasification and better catalytic performance can be obtained. In
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addition, the high Al extraction indicates that it can be considered to be a way to
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extract Al from coal ash.
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3.2.2. Catalytic gasification at different temperatures and Al leaching behaviors.
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Temperature is one of the key factors during catalytic gasification. Figure 4
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shows the effect of temperature on gasification reactivity with 15 wt% Na2CO3
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addition. As shown in Figure 4, catalytic gasification is sensitive to temperature. The
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carbon conversion X only reaches 0.5 in 120 min when it is gasified at 600 °C.
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However, when the gasification temperature increases to 800 °C, only 25 min is
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needed to finish the total conversion. In addition, Al-containing matters can react with
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Na2CO3 at a certain temperature. Figure 5 shows the results of Al extraction yield of
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gasification ash and R0.5 change at various temperatures with 15 wt% Na2CO3
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addition. Matjie30 obtained 18% Al extraction from coal fly ash using H2SO4 as
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leaching medium, indicating most of Al-containing minerals in coal ash can’t be
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extracted by acid, while catalytic gasification ash can achieve higher Al extraction
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yield. In addition, the increase in temperature (before 900 °C) can increase Al
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extraction yield, indicating inactivated Na increases at higher temperature, which is
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well accordance with the Na distribution and mineral composition results, which are
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listed in following context.
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Figure 4. Effect of temperature on gasification of coal with 15 wt% Na2CO3 addition.
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Figure 5. Al extraction yield and R0.5 change at various temperatures with 15 wt% Na2CO3 addition. 3.2.3 Na distribution during catalytic gasification.
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Na2CO3 acts as catalyst during catalytic gasification and it’s water dissolved;
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inactivated Na is water undissolved and it’s closely related to the Al leaching
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behaviors; and volatile Na is responsible for the low recovery of alkali and the
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corrosion problem of equipment, so Na distribution plays an important role in
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catalytic gasification. Figure 6 shows the Na distribution at 900 °C with various
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Na2CO3 additions. The water dissolved Na mainly results from the remaining Na2CO3,
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which doesn’t react with minerals in coal, and increase in Na2CO3 addition can
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increase the yield of water dissolved Na. For example, Figure 6 shows that the water
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dissolved Na increases from 6.6 to 11.8% when the amount of Na2CO3 increases from
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5 wt% to 10 wt%. Because most of water dissolved Na has catalytic performance,
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water dissolved Na is closely related to reactivity index of R0.5, increase in water
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dissolved Na results in that the R0.5 rapidly increases from 1.27 to 9.49 h-1 when the
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Na2CO3 addition increases from 5 wt% to 10 wt%. Na in catalytic coal gasification
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ash mainly exists is the forms of nepheline and sodium aluminum silicate, which can 15 / 30
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be dissolved by H2SO4, as a result, high acid dissolved Na is obtained. Figure 7 shows
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Na distribution in the ash produced from gasification ash at different temperatures
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with 15 wt% Na2CO3 addition. Some Na can volatilize during catalytic gasification,
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and the melting point of Na2CO3 is 851 °C, thus volatile Na increases with increasing
294
Na2CO3 addition and part of Na2CO3 volatilizes to gas when temperature is higher
295
than 851 °C. In addition, 52.4% Na is leached by water when gasification temperature
296
is 600 °C, showing that lots of loaded Na2CO3 hasn’t reacted with minerals at this
297
temperature. In order to investigate whether the water-dissolved Na is in the form of
298
Na2CO3, catalytic gasification residue was analyzed by FTIR (Figure 10(a)) and the
299
results show that the carbonate bond (880 and 1450 cm-1) appears, indicating some
300
Na2CO3 is reserved at 600 °C, which coincides well with the high water dissolved Na,
301
but with increasing temperature, more Na2CO3 can react with minerals in coal to form
302
water undissolved nepheline and sodium aluminum silicate, resulting in the decrease
303
in the amount of water dissolved Na. The result is also correspond with the low
304
content of volatile Na at 1000 °C (the volatile Na is only 12% at 1000 °C), and
305
1000 °C is much higher the melt point of Na2CO3, for the nepheline and sodium
306
aluminum silicate are hard to volatilize even at 1000 °C. What follow is some
307
evidences for these.
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Figure 6. Na distribution with various Na2CO3 additions.
309
310
Figure 7. Na distribution at different temperatures.
311
312
3.3. Minerals transformation during gasification.
313
3.3.1 Minerals transformation during catalytic gasification with 1-15 wt% Na2CO3
314
addition.
315
XRD was employed to investigate minerals transformation behaviors during
316
catalytic gasification. Figure 8 shows the XRD spectra of gasification ash with
317
various Na2CO3 additions at 900 °C. The 1%Na-900 XRD spectra show that the main
318
mineral in coal ash is mullite, indicating that most of kaolinite and boehmite have
319
converted to mullite with low Na2CO3 addition. When the amount of Na2CO3
320
increases from 1 wt% to 5 wt%, the mullite disappears and nepheline 17 / 30
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Page 18 of 30
321
(Na6.65Al6.24Si9.76O32) gradually appears. Corresponding to these transformation, the
322
Al extraction yield increases from 42% to 74% (Figure 3). With 7 wt% Na2CO3 is
323
added, part of nepheline (Na6.65Al6.24Si9.76O32) converts to sodium aluminum silicate
324
(NaAlSiO4). When Na2CO3 addition increases to 10%, the sodium aluminum silicate
325
(NaAlSiO4) converts to sodium aluminum silicate ((Na2O)0.33NaAlSiO4). With further
326
increase in Na2CO3 addition, the minerals keep unchanged. The sodium aluminum
327
silicate ((Na2O)0.33NaAlSiO4) is acid-dissolved, which also coincides well with high
328
Al extraction yield in Figure 3. It appears that nepheline and sodium aluminum
329
silicate
330
(Na6.65Al6.24Si9.76O32) and sodium aluminum silicate (NaAlSiO4) convert to sodium
331
aluminum silicate ((Na2O)0.33NaAlSiO4) with increasing Na2CO3 addition. Sodium
332
aluminum silicate ((Na2O)0.33NaAlSiO4) is the stable Na-containing minerals with
333
increasing Na2CO3 addition.
aren’t
stable
with
increase
in
Na2CO3
addition,
for
nepheline
334 335
Figure 8. The XRD spectra of coal gasification ash with various Na2CO3 additions at
336
900 °C.
337
1. Mullite 2. Nepheline (Na6.65Al6.24Si9.76O32) 3. Sodium aluminum silicate
338
(NaAlSiO4) 4. Sodium aluminum silicate ((Na2O)0.33NaAlSiO4) 18 / 30
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3.3.2. Minerals transformation during catalytic gasification at various temperatures.
340
Figure 9 shows the XRD spectra of ash at various temperatures and with 15 wt%
341
Na2CO3 addition. It shows that the minerals in coal react with Na2CO3 to form
342
Al-containing minerals with increasing temperature during gasification process,
343
which is well consistent with the increase in Al extraction (Figure 4). At 600 °C, the
344
reaction rate is slow and some newly generated trikalsilite appears in the ash, but the
345
main form of Na is still Na2CO3, which has been testified by the high water dissolved
346
Na (Figure 7) and the strong IR absorption peaks of carbonate bond (Figure 10(a)).
347
The presence of Na2CO3 indicates that Al-containing minerals in coal can’t
348
completely react with Na2CO3 at 600 °C. When gasifying at 700 °C, the dominant
349
mineral in catalytic coal ash is sodium aluminum silicates (Na1.55Al1.55Si0.45O4), which
350
indicates huge amount of Na inactivated. The sodium aluminum silicate
351
(Na1.55Al1.55Si0.45O4) isn’t a stable sodium aluminum silicate. When the temperature
352
continually increases, the sodium aluminum silicate (Na1.55Al1.55Si0.45O4) converts to
353
nepheline (NaAlSiO4) at 800 °C. Nepheline (NaAlSiO4) is also not the stable
354
substance in the process of catalytic gasification. With further increase in temperature,
355
the nepheline (NaAlSiO4) converts to sodium aluminum silicate ((Na2O)0.33NaAlSiO4)
356
at 900 °C, and keeps unchanged at 1000 °C, indicating sodium aluminum silicate
357
((Na2O)0.33NaAlSiO4) is a stable Na-containing mineral with limited condition. Figure
358
5 shows that Al extraction yield can reaches 94% when sodium aluminum silicate
359
((Na2O)0.33NaAlSiO4) is the main exited form in the ash, which has higher extraction
360
yield than others, indicating that sodium aluminum silicate ((Na2O)0.33NaAlSiO4) can 19 / 30
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361
be easily dissolved by acid. Tang28 and Jiang29 found the generation of nepheline and
362
sodium aluminum silicate during catalytic gasification with alkali addition, and that is
363
well consistent with the presence of Na-containing minerals in this study.
364 365 366 367 368
Figure 9. The XRD spectra of coal gasification ash at different temperatures. 1. Na2CO3 2. Trikalsilite 3. Sodium aluminum silicate (Na1.55Al1.55Si0.45O4) 4. Nepheline(NaAlSiO4) 5. Sodium aluminum silicate ((Na2O)0.33NaAlSiO4) 3.3.3. FTIR spectral of catalytic gasification ash.
369
Some information about amorphous minerals and poorly crystallized matters
370
can’t be obtained by XRD, and there FTIR can act as a good supplement. FTIR
371
spectrums of ash with various Na2CO3 additions at different temperature are shown in
372
Figure 10. The presence of carbonate bands in Fig 10(a), at about 880 and 1450 cm-1,
373
indicates presence of Na2CO3 during catalytic gasification, and with increasing
374
temperature, bands at about 880 and 1450 cm-1 become weaker and they finally
375
disappear at 1000 °C, which shows that all the Na2CO3 have reacted with minerals in
376
coal to generate sodium aluminum silicate (Figure 9). In addition, as shown in Figure
377
10(b), with increasing Na2CO3 addition, the carbonate band appears (1450 cm-1). In
378
addition, the stretching vibration of Si-O-Al bond31 at 686, 585 cm-1 gradually became 20 / 30
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weaker with increasing temperature and Na2CO3 addition. The main minerals in
380
uncatalytic gasification ash are metakaolinite and mullite, which coincides well with
381
the presence of Si-O-Al bond. The bands at 1100-1200 cm-1 is assigned to vibrational
382
vibration of Si-O-Si and Si-O-Al,32 these bands have shifted to low wave numbers
383
with increasing temperature and Na2CO3 addition, indicating that the degree of
384
polymeric Al-Si matters is decreased, which corresponds to the increase in the Al
385
extraction yield with increasing temperature and Na2CO3 addition in Figure 3 and
386
Figure 5.
387 388
(a)
389 390
(b) 21 / 30
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391
Figure 10. FTIRs of different gasification ashes: (a) at various temperatures (b) with
392
various Na2CO3 additions.
393
3.4. Study of model compounds boehmite and kaolinite.
394
Due to the complex in coal minerals, pure boehmite and kaolinite, which are the
395
dominant minerals in SJH coal, are chosen as model compounds to investigate the
396
minerals transformation behaviors during catalytic gasification. All Al-containing and
397
Si-containing matters in the raw coal are assumed as boehmite and kaolinite. The
398
proportion of boehmite and kaolinite are calculated on the base of the ash composition
399
of SJH coal. The XRD spectra of pure model boehmite and kaolinite are shown in
400
Figure 11. The boehmite and kaolinite with Na2CO3 addition at Na/(Al+Si) = 1 (mole
401
ratio) at 900 °C is designated as Kao+Bo-1-900.
402 403 404 405
Figure 11. The XRD spectra of boehmite and kaolinite. 1. Boehmite 2. Kaolinite 3.4.1. Minerals transformation of model compounds with various Na2CO3 additions.
406
Figure 12 shows XRD spectra of boehmite and kaolinite with various Na2CO3
407
additions at 900 °C. It could be seen that main mineral is quartz when no Na2CO3 was 22 / 30
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408
added, which is the decomposition product of kaolinite (Al2O3·2SiO2·2H2O). The
409
kaolinite decompose to mullite (3Al2O3·2SiO2) and quartz (SiO2), and the boehmite
410
decomposes to form alumina, while mullite and alumina, the main Al-containing
411
matters in coal ash, can’t be dissolved by H2SO4 at low temperature, which results in
412
the low Al extraction yield in Figure 3. The diffraction peak intensity of mullite and
413
alumina is relatively low and don’t show in the XRD of the gasification ash. When the
414
Na2CO3 was added with Na/(Al+Si) = 0.1, products are sodium aluminum silicate
415
(Na1.15Al1.15Si0.85O4) and quartz, but the quartz doesn’t appear in the gasification ash
416
and the sodium aluminum silicate (Na1.15Al1.15Si0.85O4) in model compounds differs
417
from the nepheline (Na6.65Al6.24Si9.76O32) in catalytic gasification ash. Only part of
418
Si-Al matters reacts with Na2CO3 to generate sodium aluminum silicate. With
419
increasing Na2CO3 addition, new Si-Al phases, i.e. sodium aluminum silicate
420
(NaAlSiO4) and nepheline (NaAlSiO4), appeared, and sodium aluminum silicate
421
(NaAlSiO4) is the main mineral in the gasification ash with 7% Na2CO3 addition.
422
When the Na/(Al+Si) increases to 1, main product is sodium aluminum silicate
423
((Na2O)0.33NaAlSiO4), which is the dominant mineral in catalytic gasification ash
424
(Figure 9) with 15% Na2CO3 addition. The products of model compound is same with
425
gasification ash with Na2CO3 addition, which indicates that the model compound
426
transformation behaviors are consistent with that of coal catalytic gasification, and the
427
model compounds can help to investigate the minerals transformation behaviors
428
during catalytic gasification.
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429 430 431
Figure 12. The XRD spectra of model compound with various Na2CO3 additions. 1. Quartz 2. Sodium aluminum silicate (Na1.15Al1.15Si0.85O4) 3. Sodium aluminum
432
silicate (NaAlSiO4) 4. Nepheline (NaAlSiO4) 5. Sodium aluminum silicate
433
((Na2O)0.33NaAlSiO4)
434
3.4.2. Minerals transformation of model compound at different temperatures.
435
Figure 13 shows the XRD spectra of model compound at different temperatures
436
and with Na/(Al+Si) = 1. When sintering the boehmite, kaolinite and Na2CO3 at 400
437
°C, the boehmite and kaolinite can’t react with Na2CO3, and the main product are
438
kaolinite and Na2CO3. As temperature increases to 600 °C, kaolinite decomposes to
439
metakaolinite and Na2CO3 keeps unchanged, indicating the Na2CO3 can’t react with
440
boehmite or kaolinite at 600 °C, while the gasification ash is made of trikalsilite and
441
Na2CO3. Other minerals and the carbon in coal may facilitate the Al-Na reactions. As
442
temperature furtherly increases to 800 °C, boehmite and kaolinite react with Na2CO3
443
to form sodium aluminum silicate (Na1.55Al1.55Si0.45O4) and nepheline (NaAlSiO4),
444
which are the main minerals in the ash of 15%Na-800 (Figure 8). The new produced
445
sodium aluminum silicate and nepheline are easy to be dissolved by acid at room
446
temperature. Guo26 investigated the Al leaching behavior of sintering coal ash with 24 / 30
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Na2CO3 and NaOH addition, and nepheline is their target product, thus the produce of
448
nepheline in this study favors Al leaching. The sodium aluminum silicate
449
(Na1.55Al1.55Si0.45O4) and nepheline (NaAlSiO4) converse to sodium aluminum silicate
450
((Na2O)0.33NaAlSiO4)
451
((Na2O)0.33NaAlSiO4) keeps unchanged at 1000 °C, so sodium aluminum silicate
452
((Na2O)0.33NaAlSiO4) is supposed as the stable matter with increasing temperature. It
453
is the main phase in the ash of 15%Na-900 (Figure 8), so kaolinite and boehmite
454
reaction with Na2CO3 at various temperatures can well reflect the minerals
455
transformation behaviors during catalytic gasification with Na2CO3 addition.
at
900
°C
and
the
sodium
aluminum
silicate
456 457
Figure 13. The XRD spectra of model compound with Na2CO3 addition at different
458
temperature.
459 460
461
1. Kaolinite 2. Na2CO3 3. Sodium aluminum silicate (Na1.55Al1.55Si0.45O4) 4. Nepheline (NaAlSiO4) 5. Sodium aluminum silicate ((Na2O)0.33NaAlSiO4) 4. CONCLUSIONS
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462
The
Na-containing
minerals
transformation
Page 26 of 30
behaviors
during
catalytic
463
gasification with Na2CO3 addition have been comprehensively investigated. Some
464
specific conclusions are as follows:
465
(1) The dominant minerals in coal are kaolinite and boehmite, which can react
466
with Na2CO3 to synthetize nepheline and sodium aluminum silicate, causing the
467
inactivation of the catalyst during catalytic gasification.
468
(2) Na2CO3 inactivates to form inert sodium aluminum silicate at 700 °C, and
469
several kinds of sodium aluminum silicate are formed at different temperatures with
470
various Na2CO3 additions, while sodium aluminum silicate ((Na2O)0.33NaAlSiO4) is
471
the most stable matter with increasing temperature and Na2CO3 addition.
472
(3) Reactions between minerals in coal and loaded Na2CO3 can produce some
473
acid dissolved products, resulting in 94% Al extraction yield by H2SO4 direct leaching
474
of catalytic gasification ash, which can be considered to be a way to extract Al from
475
coal ash.
476
(4) The reactions between Na2CO3 and model compounds, i.e. kaolinite and
477
boehmite, can well explain and reflect the minerals transformation behaviors during
478
catalytic gasification with Na2CO3 addition.
479
■AUTHOR INFORMATION *Corresponding author; Postal address: Institute of Coal Chemistry, Chinese Academy of Sciences, #27 Taoyuan South Road, Taiyuan, Shanxi 030001, China; Tel: +86 351 2021137-801; Fax: +86 351 2021137-802;
480
E-mail address:
[email protected].
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■ACKNOWLEDGEMENTS
482
The work is financially supported by the National Science Foundation of China
483
(21676289), the Strategic Priority Research Program of the Chinese Academy of
484
Sciences (XDA07050100), the Research Supported by the CAS/SAFEA International
485
Partnership Program for Creative Research Teams and Youth Innovation Promotion
486
Association (2014156).
487
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