Variation of Char Reactivity during Catalytic Gasification with Steam

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Variation of char reactivity during catalytic gasification with steam: comparison among catalytic gasification by ion-exchangeable Na, Ca and Na/Ca mixture Changshuai Du, Li Liu, and Penghua Qiu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02702 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 3, 2017

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Energy & Fuels

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Variation of char reactivity during catalytic gasification with steam: comparison among catalytic gasification by ion-exchangeable Na, Ca and Na/Ca mixture

5

Changshuai Du†, Li Liu†, Penghua Qiu*, †

6

†

7

150001, China.

8

* Corresponding author. E-mail: [email protected] (Penghua Qiu)

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Abstract: Reactivity profile of char during catalytic gasification is crucial for

10

designing and optimizing the gasification process, and it is greatly affected by the

11

types and changes in activity of the catalyst during gasification. The catalytic

12

gasification of Na-Char, Ca-Char and Na/Ca-Char mixtures with different

13

concentrations of steam was conducted within the temperature range of 700-900°C

14

using a micro fluidized bed reaction analyzer. The results indicate that the reactivity

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of Na-Char was always higher than that of Ca-Char during the initial gasification

16

stage (0~Xi) and then lower than that of Ca-Char in the later stage. The observations

17

were mainly attributed to the different deactivation paths for the Na and Ca catalysts.

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The drastic loss of Na during gasification is well corresponding to the sharp decrease

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of Na-Char reactivity within the initial carbon conversion, and the gradual changes of

1 2 3

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin,

1

ACS Paragon Plus Environment

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Ca dispersion contribute to the deactivation of the Ca catalyst. It is also demonstrated

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that the catalysis of Ca species exhibits better persistence than that of Na species at

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relatively high gasification temperatures (800-900°C). With an increasing steam

23

concentration, the catalytic activity of Ca is substantially promoted, resulting in a

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higher reactivity of Ca-Char than that of Na-Char. In addition, a cooperative effect

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between Na and Ca on the gasification reactivity is revealed, and the Na-Char and

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Ca-Char in the 30%:70% mixed ratio is an optimum for promoting char reactivity

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under the current conditions. The good performance of the Ca catalyst while

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interacting with the mineral matter enhances both the Na/Ca-Char reactivity and the

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resistance of Na to deactivation. The Na additive not only increases the gasification

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reactivity but also ensures the persistence of the Ca catalyst to deactivation, due to the

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formation of low-temperature melting eutectic catalyst. This work provides new

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insights into the high-efficiency gasification of low-rank coal with in-situ

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ion-exchangeable Na or Ca catalysts.

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Keywords: Gasification reactivity; Catalysis; Ion-exchangeable Na and Ca,

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Zhundong coal

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1. INTRODUCTION

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The catalytic steam gasification of coal has great potential for providing energy

38

sources and chemical feedstocks with high thermal efficiency. The feasible catalysts

39

for carbon gasification vary widely depending on their speciation and mode of 2


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occurrence. Examples of such catalysts include oxides, hydroxides, the salts or

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organic bonds of alkali and alkaline earth metallic (AAEM) species, and nickel- and

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iron-based materials.1-3 The AAEM species are considered superior catalysts for

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combustion or gasification, irrespective of their inherent presence within solid fuels or

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additives.4-8 Among these catalysts, the ion-exchangeable species are most promising.

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Therein, ion-exchangeable Na and Ca are generally considered excellent in-situ

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catalysts for low-rank coal because low-rank coal has large amounts of functional

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surface groups that act as cation-exchange sites.9-12 Taking full advantage of in-situ

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ion-exchangeable Na or Ca will play a significant role in the gasification of low-rank

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coal. In addition, using such catalysts, the side effects of AAEM species, e.g., fouling

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and slagging ash-related problems, device corrosion and bed agglomeration, can be

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retarded without requiring a large amount of catalyst additives. 13-15 Therefore, much

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attention should be given to developing the catalytic gasification of low-rank coal

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using ion-exchangeable Na or Ca.

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The reactivity profile of char during catalytic gasification is crucial for designing

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and optimizing the gasification process. However, due to the complexity and variety

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of the reaction conditions, different char reactivity profiles have been presented in the

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literatures.16-21 In particular, the types and changes in the activity of the catalyst

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greatly affect the reactivity profiles of char during gasification. For the catalytic

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gasification by Na and Ca catalysts, Ye et al.22 reported that the reactivity of char 3


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catalyzed by Na was higher than that of Ca in the steam gasification. Hippo et al.9

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found that reactivity of char gasification with steam catalyzed by Na approximated to

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that using Ca. However, Chen et al.23 reported the catalytic activities of AAEM

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catalysts for graphite gasification with carbon dioxide or steam followed the order Ca >

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Na. Such different orders of the reactivity of char catalyzed by Na and Ca mainly

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attribute to the disregard of the changes in the catalytic activity of Na and Ca during

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the overall gasification. The catalyst behaviors including the loss by evaporation, the

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formation of inactive matter, the poisoning of catalyst and the interaction between

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catalyst and mineral could almost change the catalytic activity of Na and Ca, and then

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contribute to the variations in the profiles of gasification reactivity. If the reactivity

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profiles of Na and Ca catalytic gasification are determined, it can establish the

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comprehensive and accurate comparisons of catalytic gasification reactivity by Na

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and Ca. Hag et al.24 observed that the reactivity increased with the fuel conversion in

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the case of steam gasification catalyzed by Na, while the reactivity showed an inverse

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trend for the catalytic gasification by Ca. Likewise, Moreover, different patterns of

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char reactivity profile have been summarized and used to interpret the vulnerable

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catalytic behaviors of various catalysts.25-27 Many studies regarding gasification

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reactivity have been conducted using catalytic additives, e.g. the AAEM species of

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carbonates, halides and other compounds, but few studies have been conducted on the

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gasification reactivity profile using ion-exchangeable Na and Ca catalysts. Therefore,

4


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comprehensive knowledge regarding the order of reactivity and reactivity profiles of

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char catalyzed by Na and Ca during steam gasification should be gained for the

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application of ion-exchanged Na and Ca.

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As reported extensively in previous publications, most studies have emphasized on

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the application of individual catalysts. Actually, two or more types of catalysts are

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always present in solid fuels during gasification. Wang et al.28-30 found that

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K2CO3/Ca(OH)2 exhibited a synergic effect on the char reactivity of gasification, as

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the calcium additive could inhibit the deactivation of potassium. Carrazza et al.31,32

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reported that the Ni/K mixture showed superior catalytic activity than that of Ni or K

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alone in the steam gasification because of the resistance to poisoning of Ni/K catalysts.

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Likewise, Pereira et al.33 observed that K-Ca-Ox catalyst exhibited resistance to sulfur

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poisoning. Additionally, some studies have indicated the molten nature of binary and

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ternary eutectic catalysts could enhance the catalyst/carbon contact, promoting the

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gasification reactivity.34-36 To date, only a few research groups have conducted the

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catalytic gasification by some certain catalyst mixtures. Little information regarding

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the catalytic effect of Na and Ca catalyst mixtures derived from ion-exchangeable Na

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and Ca on gasification reactivity has been given.

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As stated above, information on the catalytic gasification of coal concerning

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ion-exchangeable Na and Ca is lacking, particularly in terms of the char reactivity

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profile. Therefore, in this paper, a comparative study on the catalytic effects of 5


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ion-exchangeable Na and Ca is conducted in the temperature range of 700-900°C

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using different steam concentrations. Additionally, the effects of binary Na and Ca

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catalysts are compared to using Na and Ca individually during steam gasification.

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This paper aims to provide a comprehensive analysis of the catalysis behavior of

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ion-exchangeable Na and Ca during steam gasification and to contribute toward the

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design

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ion-exchangeable catalysts that are inherently present in low-rank coal.

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2. EXPERIMENTAL SECTION

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2.1 Sample Preparation

and

development

of

new

catalytic

gasification

technology

using

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Zhundong coal, which are abundant in China, were used in this paper. The coal

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samples were ball milled and sieved to a particle diameter range of 106-150 µm. The

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proximate and ultimate analyses of the coal samples are provided in Table 1.

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Table 1. Proximate and ultimate analyses of Zhundong coal Proximate analysis (wt%, ad) Moisture content 9.63 Volatile matter 40.30 Fixed carbon 44.57 Ash content 5.50 a by difference

Ultimate analysis (wt%, ad) Carbon 61.40 Hydrogen 4.41 Oxygen 17.69a Nitrogen 0.89 Sulfur 0.48

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The raw coal samples were first washed using a 0.1 M HCl aqueous solution (1g:

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50ml) and stirred for 24 h under a N2 atmosphere. The coal slurry was then washed

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and filtered with deionized water repeatedly until achieving a neutral pH and Cl-free

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solution. The wet samples were dried in an oven at 35°C for 48 h and then stored in a 6


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Teflon container. The dried coal samples were referred to as H-form coal. Sodium

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acetate and calcium acetate were employed for loading the ion-exchangeable Na and

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Ca coal samples, respectively (referred to as Na-form and Ca-form coal). The

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Na-form and Ca-form coal samples were prepared by mixing the H-form samples

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with 1.5 M sodium and calcium acetate aqueous solutions (1 g:30 ml), respectively.

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The pH of the coal slurry solution was key to the progress of the ion exchange

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solution,37,38 and the pH value was maintained at approximately 8.3 by adding either

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sodium or calcium hydroxide.39,40

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The char samples, namely, Na-Char, Ca-Char and H-Char, were prepared from the

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Na-form,

Ca-form

and

H-form

coal

samples,

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fluidized-bed/fixed-bed quartz reactor. This experimental system is mainly composed

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of the body of the quartz reactor, feeding system, gas distribution system, heating

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system and temperature control system. The fluidized-bed/fixed-bed quartz reactor is

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described in detail in a previous study.41-43 High-purity argon was used as the

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fluidizing and reacting agent. A thermocouple was inserted into the reaction zone for

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monitoring the temperature. After the reaction temperature reached 900°C, a solenoid

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valve was switched on for 20 ms to supply a fast gas flow (Ar) into the feeding tube,

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and the preplaced coal samples were immediately carried into the fluidized zone. The

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coal samples were pyrolyzed at a fast heating rate (>103 K/s) and maintained for 2

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min at 900°C. All char samples were prepared under the same conditions and then 7


respectively,

using

a

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used for steam gasification. The compositions of the AAEM species in the samples

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are shown in Table 2.

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Table 2. Mass concentrations of AAEM species in the samples AAEMs

140 141

Raw coal Na 0.234 K 0.022 Mg 0.179 Ca 1.003 a n/d: not detected

Samples (wt%, dried basis) H-form Na-form Ca-form Na-Char 0.008 1.885 0.015 2.017 a 0.014 0.022 n/d 0.004 0.023 0.020 0.019 0.027 0.065 0.053 2.562 0.069

Ca-Char 0.019 0.006 0.055 4.00

2.2 Steam gasification experiment

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The steam gasification of the char samples was conducted at atmospheric pressure

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using a micro fluidized bed reaction analyzer (MFBRA). The schematic diagram of

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this experimental system is shown in Figure 1. This system mainly consists of a

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fluidized-bed quartz reactor, a sample feeding system, a reacting agent supply system,

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and an online gas analysis system. The quartz reactor has a 20 mm internal diameter

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and is 60 mm long. Two porous sintered plates are installed in the reactor: the bottom

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one is used to load the fluidized medium and to distribute the fluidizing agent, and the

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top one is used to prevent the solid particles from escaping. The quartz sand (weight

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4.2 g, diameter range of 90-150 µm) was used as fluidized material while the height

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of static bed was 6 mm. The reacting agent supply system converts two different

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reacting agents using a four-way ball valve. A steady H2O/Ar gas flow is produced by

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intensively mixing of steam vapor and argon gas. A precalibrated syringe pump is

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used to feed hyper-pure water into the steam generator at a given rate. Meanwhile, 8


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high-purity argon gas is flowed into the steam generator through a T-joint and carries

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the steam into the buffer vessel. Mixture of H2O/Ar is fed into the reactor

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continuously, where the steam concentration is defined as the concentration of steam

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contained in the mixture. For all the experimental cases, the steam concentration is

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carefully controlled between 10% and 40%. While for cases under different

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temperatures, the same apparent flow rate (1.8 L/min) is set by adjusting the water

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pumping rate and Ar gas flow rate. Another O2/Ar gas flow stream is a mixture of

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oxygen and argon gas flow that is quantified using calibrated mass flow meters, and it

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is used to burn off the residual carbon after the gasification. Dried gas,180ºC

T-joint

On-line gas analysis system

MS Process mass spectrometer

Micro fluidizedbed quartz reactor

Cooling water

Exhaust

Condenser

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Energy & Fuels

Temperature controller

Ice-water 900º C

Sample feeding system

Samples

Char sample: 10 ± 0.1 mg Gas flow: 3.6Nml

Feed tube

Carrying gas Pulse valve

Infrared lamp (Heater)

One-way valve

Ar

Cooling water

Steam generator Syringe pump

Model Vol

Four-way ball valve

Buffer vessel

Mixing tank

O2 Ar

Heat tracing ribbon

164 165

Ar

Steam Syringe pump

Reacting agent supply system

Exhaust

Figure 1. Schematic diagram of a micro fluidized-bed reaction analyzer system.

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The char samples (10 ± 0.1 mg) were placed at the entrance of the feed tube, which

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was inserted into the reaction zone. A fast gas flow stream (3.6 Nml, argon) was

168

instantaneously passed through the feed tube, using a solenoid value, to carry the 9


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samples into the fluidized bed. When the char contacted the steam, the gasification

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reaction occurred immediately and produced gaseous products (mainly H2, CO, CO2,

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CH4). The released wet gas was dehumidified using an ice-water bath, resulting in

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traces of H2O ( RNa

0.2

0.4

0.6

0.8

0.015

Xi RCa< RNa

RCa> RNa

0.010

Reaction temperature: 850C 0.005

0.000 0.0

1.0

Ca-Char Na-Char H-Char

0.2

Carbon conversion X

0.4

0.6

0.8

1.0

Carbon conversion X 0.008

0.020

(c)

(d)


Reaction temperature: 800C

0.006





0.004

0.015

Reactivity dx/dt (s-1)


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Energy & Fuels

Xi 0.010

RCa< RNa

RCa> RNa

0.005

0.002 0.000 0.006

0.004

0.002

294

0.000 0.0

0.2

0.4

0.6

0.8

Carbon conversion X

1.0

0.000 0.0

0.2

0.4

Carbon conversion X

0.6

0.8

295

Figure 4. Reactivity profiles of Na-, Ca- and H-Char at a function of char conversion

296

at different reaction temperature.

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As shown in Figure 4, the reactivity of catalytic gasification (Na- and Ca-Char)

298

was largely higher than that of non-catalytic gasification (H-Char). Without the

299

catalysis in the H-Char gasification, the changes of char structure should play a

300

dominant role on the reactivity. The release of active structure (mainly small aromatic

301

ring) gradually occurred with the progress of carbon conversion.51,52 However, the

302

resulted decrease of H-Char reactivity can be completely ignored, comparing with the

303

sharp decrease of Na- and Ca-Char reactivity with the increasing carbon conversion. 17


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It is illustrated that the char structure has little effect on the reactivity of Na-Char and

305

Ca-Char gasification. Contrarily, the reactivity profiles of Na-Char and Ca-Char

306

gasification were mainly associated with the evolutions of Na and Ca catalysts during

307

gasification. Table 3 shows the reactivities of the gasified chars (X=20%) for the

308

gasification at different temperatures. It can be seen that the increases of Na-Char and

309

Ca-Char reactivities with the increasing temperature were largely higher than that of

310

H-Char by 1~2 orders of magnitude. Thus, it is considered that Na and Ca catalysts

311

were greatly influenced by gasification temperature compared with H-Char, resulting

312

in large changes of reactivity profile.

313

Table 3. Comparison of the reactivity increases of Na-, Ca- and H-Chars with

314

increasing temperature Samples

Reactivities of chars for the gasification at 700~900°C 700°C

750°C

800°C

850°C

900°C

H-Char

1.37E-04

1.50E-04

1.69E-04

3.44E-04

1.03E-03

Na-Char

6.40E-04

1.26E-03

3.71E-03

4.80E-03

6.00E-03

Ca-Char

2.31E-04

8.00E-04

1.86E-03

2.68E-03

5.40E-03

315

The reactivity of Na-Char was higher than that of the Ca-Char at different initial

316

conversion ranges, depending on the gasification temperature. When the carbon

317

conversion reached a characteristic value (Xi), the reactivity of Ca-Char became

318

higher than that of Na-Char in a few cases. The conversion values (Xi) were

319

approximately 0.2, 0.32 and 0.43 for the gasification at 900°C, 850°C and 800°C,

320

respectively. Moreover, the reactivity of Na-Char was always higher than that of

18


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Ca-Char at low conversion levels or approximate to that of Ca-Char at high

322

conversion levels, and a characteristic value (Xi) for gasification was not observed at

323

the relatively low temperatures of 750°C and 700°C (Figure 4d). The shift in the

324

conversion value (Xi) indicated that Na and Ca exhibited different behaviors with the

325

increasing temperature during steam gasification. For the gasification at 900°C, a

326

sharp decrease in the reactivities of Na- and Ca-Char occurred before the conversion

327

value (Xi) was reached; the reactivity of Ca-Char then began to decrease slowly and

328

maintain a relatively high reactivity. In contrast, the reactivity of Na-Char continued

329

to decrease sharply and began to exhibit the similar reactivity as H-Char at a

330

conversion of approximately 0.35. This similar tendency also occurred during

331

gasification at 850°C and 800°C, but the difference in the reactivity between Na-Char

332

and Ca-Char at high conversion levels (>Xi) was smaller than that during gasification

333

at 900°C. Additionally, the decrease in the reactivity of Na-Char occurred more

334

slowly with the decreasing temperature. Hence, the catalytic activity of Na resisted

335

degradation with the decreasing temperature. In contrast to sodium, a higher Ca-Char

336

reactivity occurred at high conversion levels (>Xi), especially for the cases of

337

gasification at 900°C and 850°C, illustrating that the catalytic activity of Ca remained

338

more persistent during carbon conversion but lost its capacity with the decreasing

339

temperature.

19


Energy & Fuels

0.045

Na Ca

Catalyst/Carbon (mol/mol-C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 41

0.036

0.027

0.018

0.009

0.000 X=0

340

X=0.1

X=0.3

X=0.5

X=0.7

Carbon conversion X

341

Figure 5. Mole ratios of catalyst and carbon in the Na-Char and Ca-Char at different

342

carbon conversion levels.

343

Previous studies have revealed that the dispersion of the catalyst is significant for

344

the catalytic activity,53,54 and the loss and mobility of the catalyst are also important

345

properties that affect the catalytic activity.55,56 In order to elucidate the different roles

346

of Na and Ca during gasification, the quantitative analyses of Na and Ca catalysts and

347

element compositions of char samples derived from different carbon conversion levels

348

for the gasification with 15% steam at 900°C are determined by ICP-OES and

349

SED/EDS, respectively, were shown in Figure 5 and Table 4. The total content of

350

catalyst in the char prior to gasification consists of the released catalyst and the

351

residual catalyst in the char along with the gasification. The released catalyst includes

352

three portions: 1) captured by bed materials; 2) deposited on the inner surface of

353

quartz reactor; 3) escaped from the reactor. The changes of catalyst content in the char 20


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samples are easier to certify the releasing extent of catalyst during gasification,

355

regardless of the releasing types of catalyst. It can be seen that the release of Na was

356

significant during gasification, but almost no vaporization loss of Ca was observed.

357

Combined with the SEM/EDX mapping analysis, the data (calculated by eq 8)

358

indicated that just trace of Na species present in the gasified char of 50% carbon

359

conversion above, but the atomic concentration of Ca was still high. Exactly, the loss

360

of Na catalyst during gasification is well corresponding to the decrease of char

361

reactivity with carbon conversion (Figure 4a). However, the increasing content of Ca

362

catalyst in the char was not always contributed to the promotion of char reactivity.

363

Some studies have reported that an increasing concentration of Ca species beyond a

364

certain level will not increase the char reactivity, but instead promote the sintering of

365

calcium inducing a remarkable poor dispersion of catalyst.49,57 The dispersion of Ca

366

was regarding as an important fundamental property influencing the catalytic activity.

367

Therefore, the standard deviations (calculated by eq 9) for different char samples was

368

given in Figure 6, illustrating the dispersion of catalyst in the char samples. From the

369

values of the standard deviations for the concentrations of Na and Ca, it allows the

370

interpretation of the poor dispersion of Ca in the Ca-Char. Moreover, the dispersion of

371

Ca will become worse with increasing carbon conversion, and the drastic progress

372

occurs for the 30% carbon conversion char, corresponding to the sharp decrease of

373

Char reactivity (Figure 4a). In contrast, the catalyst Na always disperses well in the

21


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Na-Char during gasification. Therefore, it can be concluded that the loss of the Na

375

catalyst and the poor dispersion of the Ca catalyst were the dominant factors on the

376

deactivation of Na and Ca during gasification, respectively. However, the deactivation

377

of Na caused by the reaction of the alkali salts with mineral matter during gasification

378

has also been reported in the literature.5 The occurrence mode of the Na catalyst may

379

have been associated with the different paths of Na deactivation during gasification.

380

The ion-exchanged Na may bond to the organic matter in the char after pyrolysis, and

381

the cleavage of the Na bonds occurs during the gasification reaction on the catalytic

382

sites, resulting in the release of Na. In contrast, it is hard for alkali salts to form

383

alkali-carbon complexes, due to the affinitive bonds of the anion.58 In addition, the

384

low content of ash (5.50%, in weight, ad) in Zhundong coal may be one reason for the

385

deactivation of Na by the reaction with the mineral matter. From the element

386

compositions of the char samples in Table 3, the concentrations of trace Al or Si on

387

the char surface were not proportional to the content of Na. This result is in agreement

388

with the results in the study by Matsuka et. al.56 It was also not observed that the Na

389

and K in two sub-bituminous coals (ash content: 2.8% and 6.1%, db) interacted with

390

inherent mineral matters during gasification. Thus, the deactivation mechanism of Na

391

during gasification is likely dependent on the occurrence mode of Na and the solid

392

fuels support. In the case of the Ca catalyst, the dispersion of Ca in gasified char has

393

also

been

characterized

using

other

methods,

22


i.e.,

XRD

and

CO2

Page 23 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

394

chemisorption.53,54,57,59 XRD limited in its ability to measure small calcium particles

395

(size < 5 nm) which are mainly concern with the catalytic activity. The CO2

396

chemisorption method was used to determine the dispersion of the Ca catalysts,

397

assuming that the CO2 molecule chemisorbs on the CaO site. Taking into account the

398

formation of the complex calcium compounds during gasification, the chemisorption

399

method was not able to detect all calcium elements on the char surface. The elemental

400

compositions and dispersions characterized by SEM/EDS in this study utilized the

401

advantages of X-rays to detect elements, and the statistics of the examined data

402

accurately reflected the representative values of the elements.

403

Table 4. Element compositions of char samples characterized by SEM/EDS Mapping Ca-Char, X=0.0 Ca-Char ,X=0.1 Ca-Char, X=0.3 Ca-Char, X=0.5 Ca-Char, X=0.7 Na-Char, X=0.0 Na-Char, X=0.1 Na-Char, X=0.3 Na-Char, X=0.5 Na-Char, X=0.7

C 84.51 82.8 77.16 75.11 73.05 84.48 82.86 82.26 94.34 95.25

Elemental relative concentrations (C0, At%) O Si Al F S Mg Na 12.96 0.00 0.00 0.00 0.22 0.00 0.00 13.92 0.00 0.08 0.00 0.47 0.00 0.00 19.15 0.10 0.00 0.00 0.26 0.00 0.00 19.37 0.13 0.00 0.84 1.12 0.00 0.00 21.71 0.00 0.12 0.96 0.18 0.17 0.00 13.35 0.00 0.00 0.00 0.06 0.00 1.94 14.79 0.00 0.00 0.00 0.26 0.00 2.03 13.92 0.00 0.00 1.44 0.35 0.00 1.89 4.89 0.00 0.04 0.00 0.036 0.06 0.22 4.02 0.00 0.00 0.00 0.56 0.00 0.12

23


Ca 2.31 2.73 3.33 3.44 3.81 0.06 0.00 0.07 0.09 0.05

Energy & Fuels

Standard deviations of Na dispersion

0.25

Na-Char 0.20

0.15

0.10

0.05

0.00 X=0.0

X=0.1

X=0.3

X=0.5

X=0.7

Carbon conversion X

404 1.8

Standard deviations of Ca dispersion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 41

Ca-Char 1.5

1.2

0.9

0.6

0.3 X=0.0

405 406

X=0.1

X=0.3

X=0.5

X=0.7

Carbon conversion, X Figure 6. Catalyst concentrations and the deviations of Na- and Ca-Char samples.

407

From the discussions stated above, it was concluded that the catalysis of the Ca

408

species was more persistent than that of the Na species at relatively high gasification

409

temperatures (800-900°C), and the reactivity of the char catalyzed by Na was higher

410

than that of the char catalyzed by Ca within a wider conversion range with the 24


Page 25 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

411

decreasing temperature. The loss of Na during Na-Char gasification is well

412

corresponding to the sharp decrease of reactivity with increasing carbon conversion.

413

Although almost all Ca catalyst present in the gasified char, the poor dispersion of Ca

414

contributes to the deactivation of Ca inducing the decreasing reactivity of Ca-Char.

415

3.2 Effects of Steam concentrations on the Na-Char and Ca-Char gasification

416

reactivity profiles

417

Figure 7 shows the reactivity profile as a function of the char conversion for Na-,

418

Ca- and H-Char during gasification with different concentrations of steam at 900°C.

419

The case of using a 15% steam concentration can be seen in Figure 4a. Clearly, the

420

reactivity profile of the char catalyzed by Na and Ca changed greatly with the

421

increasing steam concentration. Table 5 shows the reactivities of the gasified chars

422

(X=50%) for the gasification with different steam concentrations. It can be found that

423

the increases of Na-Char and Ca-Char reactivities with the increasing steam

424

concentration were largely higher than that of H-Char, especially for Ca-Char. Thus,

425

it is considered that the evolutions of Na and Ca catalysts by different steam

426

concentrations would account for the changes in the reactivity profiles of Na- and

427

Ca-Char. Specifically, the characteristic value (Xi) of the carbon conversion described

428

above still existed at relatively low steam concentrations (10% and 15%), and the

429

reactivity of Na-Char was higher than that of Ca-Char in the range of 0-Xi. When the

430

steam concentration exceeded 20%, the reactivity of Ca-Char became much higher 25


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 41

431

than that of Na-Char. The differences between the Na- and Ca-Char reactivities were

432

quantified and compared at the 50% conversion level (RX=50%), and the differences

433

increased steadily with the increasing steam concentration. These results suggest that

434

the high steam concentrations enhanced the catalytic effects of the Ca species during

435

char gasification.

436

Table 5. Comparison of the reactivity increases of chars with the increasing

437

steam concentration Samples

Reactivities of chars for the gasification with 10~40% steam 10%

15%

20%

30%

40%

H-Char

5.35E-04

6.80E-04

7.24E-04

8.47E-04

9.23E-04

Na-Char

6.40E-04

7.44E-04

8.69E-04

1.07E-03

1.31E-03

Ca-Char

2.22E-03

2.90E-03

3.32E-03

4.27E-03

4.85E-03

438

During steam gasification, the catalytic and non-catalytic reactions occur in

439

parallel. According to the low reactivity of the H-Char during the non-catalytic

440

gasification, the overall gasification process was dominated by the catalytic

441

gasification of the Na- and Ca-Char. The catalytic reactions occurred at the interface

442

of coexisting catalyst, H2O and carbon. The number of catalytic sites in the char was

443

crucial for the catalytic gasification. When the gasification occurred at low steam

444

concentrations, the Na catalyst could provide more catalytic sites than the Ca catalyst

445

to promote contact between the catalyst, H2O and carbon, possibly due to the well

446

dispersed and mobile of Na species. In comparison with the behavior of Na, the poor

447

dispersion and mobility of Ca catalyst are regarded as the main factors limiting its

26


Page 27 of 41

448

catalytic activity.54,55,60 Thus, it is considered that improving the dispersion and

449

mobility of Ca catalyst in char may greatly enhance the char reactivity. From our

450

experimental results, the enhancement of char reactivity with increasing steam

451

concentration was more pronounced in Ca-Char gasification. From this point of view,

452

it is inferred that the dispersion and mobility of Ca catalyst were likely improved by

453

the high steam concentration, then promoting the char reactivity 0.025

0.020

(a)

(b)



0.020



0.015

0.010

Steam concentration: 10 vol%

0.015

0.010


0.005 0.005

0.000 0.0

454

0.2

0.4

0.6

Carbon conversion X

0.8

0.000 0.0

1.0

0.4

0.6

0.8

1.0

0.025

(c)

(d)


0.015

0.010


0.015

0.010


0.005

0.005

0.000 0.0


0.020


0.020

455

0.2

Carbon conversion X

0.025


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.2

0.4

0.6

Carbon conversion X

0.8

1.0

0.000 0.0

0.2

0.4

0.6

0.8

1.0

Carbon conversion X

456

Figure 7. Reactivity profiles of Na-, Ca- and H-Char at a function of conversion for

457

the steam gasification with different steam concentrations.

458

3.3 Effects of the Na and Ca catalyst mixtures on the gasification reactivity

459

The Na-Char and Ca-Char samples were mixed in different mass ratios to achieve

460

the different mixtures of Na and Ca catalysts in the samples. In the absence of 27


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

461

interactions between the Na and Ca catalysts and with the Na-Char and Ca-Char

462

gasified with steam individually, the reactivity of the mixture samples could be

463

calculated using the individual reaction laws of Na-Char and Ca-Char (eq 6 and 7). If

464

the experimental reactivity of the Na/Ca-Char mixture was higher than that of the

465

calculated value (Rexp > Rcal), a cooperative effect between the Na and Ca was

466

indicated. The comparison between the experimental and calculated reactivity profiles

467

from the Na/Ca-Char gasification with 15% steam at 900°C is shown in Figure 8. The

468

experimental reactivity (Rexp) was higher than the calculated reactivity (Rmix),

469

regardless of the mass ratios of Na-Char and Ca-Char. According to the definition of

470

the comparison between the experimental and calculated reactivity (Rexp > Rcal), a

471

cooperative effect between Na and Ca on the reactivity was evidenced for the

472

gasification at 900°C. In addition, the reactivity of the 70%Na/30%Ca-Char mixture

473

was much higher than that of the pure Na-Char, especially within the low carbon

474

conversion range (Figure 8a). As stated in section 3.1, the reactivity of Na-Char was

475

higher than that of Ca-Char within the conversion range of 0-Xi. Thus, the higher

476

reactivity of the 70%Na/30%Ca-Char gasification was not attributed to the increase in

477

the catalyst content but rather to the cooperative interactions between Na and Ca.

28


Page 28 of 41

Page 29 of 41

0.025

(a)

R-exp R-cal R-Na-Char

-1

Reactivity dx/dt (s )

0.020

Reaction temperature:900C Reacting agents:15%steam+Ar Samples:70%Na-Char+30%Ca-Char

0.015

0.010

0.005

0.000 0.0

478

0.2

0.4

0.6

Carbon conversion X

0.8

1.0

0.025

(b)

R-exp R-cal


0.020


0.015

0.010

0.005

0.000 0.0

479

0.2

0.4

0.6

0.8

1.0

Carbon conversion X 0.025

(c)

R-exp R-cal R-Ca-Char

0.020


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels


0.015

0.010

0.005

480

0.000 0.0

0.2

0.4

0.6

0.8

1.0

Carbon conversion X

481

Figure 8. Comparison of experimental and calculated reactivity profile of

482

Na/Ca-Char gasification with 15% steam at 900°C.

483

The reactivity of the pure Na-Char decreased sharply and was similar to the 29


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

484

magnitude of the H-Char reactivity at the conversion of approximately 0.35. As

485

shown in Figure 8a, the addition of 30% Ca-Char clearly slows the decreasing speed

486

of gasification reactivity, and the reactivity reached the level of the H-Char reactivity

487

at the conversion of approximately 0.8. This result indicates that the Na/Ca mixture

488

had a better resistance to deactivation. A few studies have mentioned that catalyst

489

deactivation is mainly due to reactions with the mineral matter in the substance,

490

forming inactive silicates or aluminosilicates.3,61,62 To clarify the interactions between

491

the Na and Ca species and the mineral matter, the Na-Char/H-Char and

492

Ca-Char/H-Char mixtures were gasified with 15% steam at 900°C. The addition of

493

H-Char increased the ash content of the reacting samples. The comparison between

494

the experimental and calculated reactivity values of the Na/H-Char and Ca/H-Char are

495

shown in Figure 9. The experimental reactivity of the Ca/H-Char gasification was

496

higher than the calculated reactivity within the overall conversion range. The added

497

Ca catalyst increased the gasification of H-Char and enhanced the gasification

498

reactivity. According to the description by Wang et al.,28 the calcium additive could

499

suppress the interactions between the K2CO3 and mineral matter and thus increase the

500

gasification reactivity. The results from that study indicate that the increased ash

501

content from the H-Char additive intensified the interactions between the calcium

502

catalyst and the mineral matter, but the calcium species still exhibited superior

503

catalytic activity (Rexp > Rcal). In contrast, the higher experimental reactivity of 30


Page 30 of 41

Page 31 of 41

504

Na/H-Char than the calculated values occurred only in the initial conversion range

505

(approximately 0-0.18), supposedly due to the mobility of the alkali catalyst.19 Past

506

this range, the experimental reactivity of Na/H-Char remained lower than the

507

calculated values, illustrating that the added H-Char promoted the interactions

508

between the Na species and minerals and induced the deactivation of the Na catalyst.

509

From the above description, it can be concluded that the enhanced performance of the

510

calcium catalyst while interacting with mineral matter in the char substantially

511

contributed to the cooperative effect between Na and Ca species on the gasification

512

reactivity. 0.015

(a)

R-cal R-exp

-1

Reactivity dx/dt (s )

0.012

0.009

Reaction temperature:900C Reacting agents:15%steam+Ar Samples:30%Ca-Char+70%H-Char

0.006

0.003

0.000 0.0

513

0.2

0.4

0.6

0.8

1.0

Carbon conversion X

0.015

(b)

R-cal R-exp

0.012


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.009

Reaction temperature:900C Reacting agents:15%steam+Ar Samples:30%Na-Char+70%H-Char

0.006

0.003

0.000

514

0.0

0.2

0.4

0.6

Carbon conversion X

31


0.8

1.0

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

515

Figure 9. Comparison of experimental and calculated reactivity profile of Ca/H-Char

516

and Na/H-Char gasification with 15% steam at 900°C.

517

As shown in Figure 8c, the experimental reactivity of 30%Na/70%Ca-Char was

518

higher than the calculated values within the overall conversion range and obviously

519

higher than the reactivity of the pure Ca-Char in the conversion range of 0-0.5. The

520

catalytic activity of Na-Char was higher than that of Ca-Char in the conversion range

521

of 0-0.2 (Figure 4a). Moreover, the Na additive enhanced the Ca-Char reactivity.

522

Thus, the characteristic value of Xi increased to 0.5 during the Na/Ca-Char mixture

523

gasification. Interestingly, the experimental reactivity of the 30%Na/70%Ca-Char

524

mixture gasification was nearly equal to that of the pure Ca-Char gasification in the

525

conversion range of Xi above, despite the decrease in the Ca content in the sample.

526

This result suggests that the small quantity of Na additive, compared to the catalytic

527

effect of pure Ca-Char, not only greatly enhanced the gasification reactivity within a

528

wider conversion range but also ensured the equivalent and persistent catalytic

529

activity of the Ca species. As descripted in section 3.1, the poor dispersion of Ca

530

during carbon conversion was the primary cause of catalyst deactivation. However,

531

with the addition of the Na catalyst, the interaction between Na and Ca may have led

532

to the formation of low-temperature eutectic catalyst. Na has a remarkable effect on

533

reducing ash fusion temperature, which enable it easily to form a eutectic compound

534

with other components in ash. 63 Moreover, Pereira et al. 33 reported that the Na-Ca-Ox 32


Page 32 of 41

Page 33 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

535

catalyst had a higher activity than K-Ca-Ox during gasification, though K always

536

appeared the superior catalytic activity. It is because that well wetting of carbon by

537

mixed catalyst (Na/Ca) in the molten state partially contributed to the enhancement of

538

catalytic activity. Eutectic catalysts are important for wetting carbon surfaces and can

539

enhance the catalyst/carbon contact.28,29,33 Figure 10 shows the surface morphology

540

and BSE-mapping elemental analysis of the 30%Na70%Ca-Char at the carbon

541

conversion levels of 0.1 and 0.5. The coexistence of Na and Ca always occurred in the

542

gasified char at different carbon conversion levels. The formation of a

543

low-temperature eutectic catalyst may have been another critical factor for the

544

cooperative effect between Na and Ca. The better dispersion of the Ca catalyst on the

545

char surface may have been achieved in the presence of the Na additive.

546 547

Figure 10. surface morphology and BSE-mapping elemental analysis of

548

30%Na/70%Ca-Char at the carbon conversion levels of 0.1 and 0.5. 33


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

549

In addition, we investigated the optimal mixed proportion of Na- and Ca-Char to

550

enhance the char reactivity under the current conditions. As shown in Table 6, the

551

reactivity ratios of several chars and the H-Char (non-catalytic gasification) at

552

different carbon conversion levels were calculated. The reactivity of the Na/Ca-Char

553

mixture at the initial gasification stage increased with the addition of more Na-Char.

554

In contrast, the reactivity during the later gasification stage increased with the

555

addition of more Ca-Char. These results correspond well to the Na and Ca catalyst

556

results described in section 3.1. It is known that the reactivity will became lower at

557

the later stage of gasification. The reaction time typically has to be prolonged to

558

completely convert carbon. The limitation of gasification is the relatively low

559

reactivity of char after the initial gasification stage. From this view of point, the better

560

persistence of the Ca catalyst to deactivation potentially contributes to the complete

561

gasification of char. However, considering the relatively low reactivity of the Ca-Char

562

during the initial stage, adding a small quantity of Na-Char (30%) into the Ca-Char

563

samples clearly enhanced the reactivity during the initial gasification stage. Moreover,

564

the fast gasification rate during the initial stage will not cause large differences on the

565

reaction time for these three char samples. Additionally, the gasification reactivity of

566

30%Na/70%Ca-Char sample after 30% conversion was higher than that of the

567

70%Na/30%Ca-Char and 50%Na/50%Ca-Char samples. Therefore, the cooperative

568

effect between the Na and Ca catalysts in the 30%:70% mixed ratio is potential to

34


Page 34 of 41

Page 35 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

569

enable efficient gasification. Detailed information regarding the gasified char from

570

30%Na/70%Ca-Char gasification will be obtained in future study, to elucidate the

571

cooperative mechanism between the Na and Ca catalysts during gasification.

572 573

Table 6. Comparison of char reactivity in steam gasification for single and mixed catalysts Reactivity ratio of catalytic gasification and non-catalytic gasification (H-Char) Samples X=0.05

X=0.1

X=0.2

X=0.3

X=0.4

X=0.5

X=0.6

X=0.7

X=0.8

H-Char

1(1.03E-03)a

1(1.02E-03)

1(1.0E-03)

1(9.31E-04)

1(8.12E-04)

1(6.82E-04)

1(5.34E-04)

1(4.04E-04)

1(2.36E-04)

Na-Char

13.43

10.53

5.58

2.31

1.32

1.24

1.28

1.30

1.34

Ca-Char

13.12

8.96

5.16

4.76

4.42

4.26

4.12

4.04

4.21

70%Na/30%Ca-Char

18.09

14.51

8.82

4.96

2.98

2.69

2.25

1.83

1.81

50%Na/50%Ca-Char

17.42

12.66

6.27

4.61

3.51

3.07

2.70

2.27

2.11

30%Na/70%Ca-Char

16.10

12.47

6.89

5.49

4.53

4.21

4.14

3.99

4.08

574 575

a

: Figures in parentheses are actual values of H-Char reactivity

4. CONCLUSIONS

576

A systematic study on the char reactivity of catalytic steam gasification using

577

ion-exchangeable Na and Ca and Na/Ca mixtures was conducted in this study. The

578

main conclusions are as follows:

579

1.

The reactivity of Na-Char was always higher than that of Ca-Char during the

580

initial gasification stage (0~Xi) and then less than that of Ca-Char, owing to the

581

different deactivation paths for the Na and Ca catalysts. The drastic loss of Na during

582

gasification was well corresponding to the sharp decrease of Na-Char reactivity

583

within the initial carbon conversion, and the gradual changes of Ca dispersion

584

contributes to the deactivation of the Ca catalyst. It was also demonstrated that the 35


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

585

catalysis of Ca species exhibits better persistence than that of Na species at relatively

586

high gasification temperatures (800-900°C).

587

2.

The steam concentration played an important role on the catalyst activity of

588

Na and Ca, particularly for the Ca catalyst. With the increasing steam concentration,

589

the Ca/H2O/carbon contact clearly increased, resulting in the high reactivity.

590

3.

A cooperative effect between Na and Ca on the gasification reactivity is

591

revealed, and the Na-Char and Ca-Char in the 30%: 70% mixed ratio is an optimum

592

for promoting char reactivity under the current conditions. The good performance of

593

the Ca catalyst while interacting with the mineral matter enhances both the

594

Na/Ca-Char reactivity and the resistance of Na to deactivation. The Na additive not

595

only increases the gasification reactivity but also ensures the persistence of the Ca

596

catalyst to deactivation, due to the formation of low-temperature melting eutectic

597

catalyst.

598

AUTHOR INFORMATION

599

Corresponding Author*. Telephone: 0086-451-86413231 ext 804. E-mail:

600

[email protected]

601

ACKNOWLEDGMENTS

602

This work was financially supported by the National Natural Science Foundation of

603

China (Grant No. 51376053).

36


Page 36 of 41

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Energy & Fuels

604

REFERENCES

605

(1) Mims, C.A. Fundamentals and Mechanism. Fundamental Issues in Control of

606

Carbon Gasification Reactivity. 1991, 192: 383-407.

607

(2) Wood, B. J.; Sancier, K. M. Catalysis Reviews 1984, 26, 233-279.

608

(3) Nzihou, A.; Stanmore, B. Sharrock, P. Energy 2013, 58, 305-317.

609

(4) McKee, D. W. Carbon 1979, 17, 419-425.

610

(5) McKee, D. W.; Spiro, C. L.; Kosky, P. G.; Lamby, E. J. Fuel 1983, 62, 217-220.

611

(6) Wen, W.-Y. Catalysis Reviews 1980, 22, 1-28.

612

(7) Li, C.-Z. Fuel 2007, 86, 1664-1683.

613

(8) Chen, S. G.; Yang, R. T. Energy Fuels 1997, 11, 421-427.

614

(9) Hippo, E. J.; Jenkins, R. G.; Walker Jr, P. L. Fuel 1979, 58, 338-344.

615

(10) Hüttinger, K. J.; Minges, R. Fuel 1985, 64, 486-490.

616

(11) Takarada, T.; Tamai, Y.; Tomita, A. Fuel 1985, 64, 1438-1442.

617

(12) And, Y. O.; Asami, K. Energy Fuels 1996, 10, 431-435.

618

(13) Niu, Y.; Tan, H.; Hui, S. e. Progress in Energy and Combustion Science 2016, 52,

619

1-61.

620

(14) Narayan, V.; Jensen, P. A.; Henriksen, U. B.; Glarborg, P.; Lin, W.; Nielsen, R. G.

621

Biomass and Bioenergy 2016, 91, 160-174.

622

(15) Nutalapati, D.; Gupta, R.; Moghtaderi, B.; Wall, T. F. Fuel Processing Technology 37


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623

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