Slagging and Fouling Characteristics of Zhundong High-Sodium Low

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Slagging and fouling characteristics of Zhundong highsodium low-rank coal during circulating fluidized bed utilization Xiaobin Qi, Guoliang Song, Weijian Song, Shaobo Yang, Zhao Yang, and Qinggang Lyu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02053 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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Energy & Fuels

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Slagging and fouling characteristics of Zhundong high-sodium low-rank coal during circulating

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fluidized bed utilization

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Xiaobin Qi1, 2, Guoliang Song1, 2*, Weijian Song1, 2, Shaobo Yang1, 2, Zhao Yang1, 2, Qinggang Lyu1, 2

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1

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China

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2

University of Chinese Academy of Sciences, Beijing 100049, China

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*Corresponding author. Guoliang Song, Tel.: +86-010-82543129;

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E-mail address: [email protected]. (G. Song)

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Abstract

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Zhundong coal (ZDc) with a large reserve is faced with severe slagging and fouling during

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combustion in pulverized coal furnaces because of its high-Na content. Due to the low-temperature

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reaction characteristics of circulating fluidized bed (CFB), the ash-related problems might be

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alleviated when ZDc is used as fuel in CFB. In this study, the slagging and fouling characteristics of

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three types of ZDc were tested in a 0.4 t/d CFB test system. The influences of three aspects

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including reaction temperature, reaction atmosphere and coal property were studied in order to

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realize the utilization of ZDc in CFB. Experimental results indicate that slagging and fouling still

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occurred in CFB. During gasification, ZDc performed slagging behaviors in the riser, which was

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closely related to the used bed material of quartz sand. The slagging degree was increased with the

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rise of reaction temperature. In different reaction atmospheres, the migration and transformation

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behaviors of Na differed, which further contributed to the slagging in the riser during gasification

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and fouling on tail heating surfaces during combustion, respectively. Besides, the Na content and

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occurrence mode in ZDc greatly impacted slagging. For the used three kinds of ZDc, their slagging

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tendencies obeyed the order of ZDc-3 > ZDc-1 > ZDc-2.

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Keywords: Zhundong high-sodium low-rank coal, circulating fluidized bed, slagging, fouling,

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combustion and gasification

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1 Introduction

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Worldwide, coal is still the primary energy at present, especially in China. Compared to other fossil

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fuels such as petroleum, little or no processing needs to be carried out during coal utilization,1

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which significantly reduces the cost. With the continuous consumption of coal resources in recent 1

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years, more attention has been paid to the low-rank coal, accounting for about 50% of global coal

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reserves.2 However, the utilization of low-rank coal is limited due to its poor transportation and

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storage characteristics, spontaneous combustion3 and usually higher carbon dioxide emissions.4 In

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order to realize its utilization, studies on combustion,5,6 gasification,7 pyrolysis8,9 and

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liquefaction10,11 were attempted. Besides, some types of low-rank coal around the world such as the

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PRB sub-bituminous coal/Beulah lignite from US and the Victorian brown coal from Australia

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contain high Alkali and Alkaline Earth Metal (AAEM) species, especially Na species, which performed

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severe slagging and fouling during their utilization.12,13 Zhundong low-rank coal explored in Xinjiang,

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China earlier this decade was approximately estimated with 390 Gt reserve, and also faced the

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same ash-related problems because of its high Na content,14-16 which has been the biggest obstacle

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to its use.

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To date, lots of studies on this type of coal have been carried out so as to further investigate the ash

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deposition mechanism. Researches on Zhundong low-rank coal combustion in high-temperature

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reactors including pulverized coal furnace,17 drop tube furnace14 and tube furnace18 show

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temperature was the most important factor affecting these ash-related issues. Due to the low

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melting points of AAEM-based species, usually close to or even lower than the reaction

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temperature of the above high-temperature reactors, slagging often occurred on high-temperature

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radiant heating surfaces during Zhundong low-rank coal combustion.19,20 On the other hand, the

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release of AAEM-based species was enhanced by high reaction temperatures. At 1000 oC, over 50%

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Na was released into gas.21 The released Na species would deposit on fine ash particle surfaces or

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metal heating surfaces via condensation, thermophoresis and chemical reaction,22,23 and play a

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strong adhesion agent during ash deposition.14 Unfortunately, conventional soot blowers have little

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effect on the sediments. Besides temperature, pretreatment

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play an important role in the migration and transformation of Na species.

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Summarizing the above introduction, circulating fluidized bed (CFB) with low reaction temperatures

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(usually 850~950 oC), might be potential to realized the utilization of Zhundong high-Na low-rank

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coal through remitting slagging and fouling. Additionally, compared with the fixed bed and

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entrained flow bed, CFB has long been recognized as a promising technology because of its better

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2


and reaction atmosphere21, 25 also

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adaptability to fuel such as coal,26 biomass27 and other municipal solid waste,28 and effective control

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in pollutant emissions.29 Furthermore, previous experiences on utilization of Victorian high-Na

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low-rank coal in CFB further verified the feasibility of the utilization of high-Na low-rank coal in

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CFB.30-32 Therefore, studies on Zhundong coal in CFB are very valuable and promising. In our

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previous works,33-35 we have attempted it and preliminarily obtained some useful conclusions, while

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the previous studies placed more focus on the migration and transformation behaviors of sodium in

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Zhundong coal and ash agglomeration, and more studies on slagging and fouling are still needed in

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order to realize the ultimate use of this special coal in CFB. Furthermore, the coal property should

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also become the object of study before the large-scale application of Zhundong coal because the Na

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properties of Zhundong coal mined from different regions vary widely.

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In this paper, three types of Zhundong high-Na low-rank coal were used as fuel in a 0.4 t/d CFB test

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system to investigate their slagging and fouling characteristics. Ash samples were collected and

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characterized by a series of analytic techniques to study the migration and transformation behaviors

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of AAEM species, and slagging/fouling mechanisms. Influences of reaction temperature,

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atmosphere and coal properties on ash-related issues were respectively discussed. The aim of this

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study is to obtain more useful data for the industrial application of Zhundong coal in CFB.

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2 Experimental section

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2.1 Experimental material

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For Zhundong coalfield, its large area and wide east-west span result in the big property difference

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among coals from different mines.36 In these experiments, three types of high-Na low-rank

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Zhundong coal (ZDc) mined from different mines were used as fuel and referred to as ZDc-1, ZDc-2

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and ZDc-3, respectively. The properties of the used coal are given in Table 1. For the three types of

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ZDc, the Na2O contents in coal ash are very high, separately accounting for 3.92%, 4.38% and 7.28%,

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which are above that of other power coals in China. However, the big differences among them are

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reflected by their ash contents, Cl contents and certain ash composition contents such as SiO2, Al2O3

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and SO3.

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Quartz sand, which is mainly composed of SiO2 (about 95%), was selected as the experimental bed

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material. Before experiments, the used coals were crushed and sieved in a size range of 0.1-1.0 mm, 3


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and the used bed material was crushed and sieved in a size range of 0.18-0.71 mm.

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2.2 Test system

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Experiments were conducted in a 0.4 t/d CFB test system, as shown in Figure 1. The test system

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primarily consists of a riser, coal feeder, cyclone and loop seal. The riser is of 150 mm inner

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diameter and 4400 mm height. For the feeder, its maximum coal feed rate is up to 20 kg/h. The

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cyclone and loop seal could work together to continuously separate and re-circulate materials in the

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furnace in order to realize the steady run.

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An electric furnace was wrapped outside the riser to provide external heating during experiments.

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Moreover, the test main tubes were coated by the high-temperature resistant insulating cotton to

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minimize the heat loss. The K-type thermocouples and B0300 pressure sensors were set along the

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gas flow direction to real-time monitor the temperature and pressure. Ash samples can be collected

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from eight sampling positions (P1~P8). The experimental air including the primary air, spreading

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coal air and re-circulating air was controlled by mass flow meters. All data were recorded and

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displayed by a programmable logic controller (PLC) data acquisition system.

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2.3 Conditions and sampling system

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For each test, the highest temperature along the riser was considered as the operating temperature.

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The ZDc-1 and ZDc-2 were used to investigate the influence of operating temperature and reaction

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atmosphere in slagging and fouling, respectively. In addition, the influence of coal property was

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obtained through the comparison among ZDc-1, ZDc-2 and ZDc-3. Each experiment stably

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maintained for 4 h, and the main parameters under different test conditions were illustrated in

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Table 2.

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Bottom ash (P1) and fly ash (P4) were collected and analyzed. The two kinds of ash samples were

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sampled through the sampling cans which were independently controlled by a valve. The stainless

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steel ash deposited probes with outer diameter 20 mm and length 400 mm were separately set in

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P5 and P8. It is reported that the increasing wall temperature of the heating surfaces could enhance

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the slagging and fouling.37-39 Thus, no cooling media was used for the probes in order to obtain the

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maximum slagging and fouling effect within such short time. The deposited ash on the probes was

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focused to study the slagging and fouling characteristics of ZDc. 4


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Energy & Fuels

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2.4 Analysis

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In order to investigate the property difference of sodium in ZDc exploited from different mines, we

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paid much attention to sodium occurrence mode and its corresponding content through chemical

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extraction, which was widely used and introduced by other studies.21,34 For each coal sample, 1.0 g

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coal was put into a beaker and immerged in 50 ml ultrapure water at 60 oC for 24 hours. The solid

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residues were obtained by filtering the mixture. The solid residues were successively treated with

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the same operations above using 1.0 mol/l ammonium acetate solution and 1.0 mol/l hydrochloric

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acid solution. The final residues were dissolved with the digestion solution (HNO3: HF = 1: 3). The

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four kinds of filtrate were stored for further test. Besides, same operations were performed with

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coal ash.

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Moreover, a series of analytic techniques including inductively coupled plasma-atomic emission

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spectrometry (ICP-AES), X-ray diffractometry (XRD), X-ray fluorescence (XRF) and scanning electron

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microscopy (SEM) were used to characterize ash samples. These ash samples were treated with

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ashing before XRD and XRF analysis to eliminate the interference of carbon to analytic results. Due

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to the strong volatility of AAEM compounds in ZDc, the ashing process was conducted at 575 oC,

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which referred to the ASTM standard E1755-01 and could avoid the release of these AAEM

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species.40

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3 Results and discussion

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3.1 Influence of operating temperature

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The temperature variations along the flue gas flow direction at different operating temperatures for

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ZDc-1 are exhibited in Figure 2. For three tests, the gas temperatures near the ash deposited probe

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in P5 were separately 861, 931 and 945 oC, and the ones for the probe in P8 were 753, 814 and 826

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o

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During gasification of ZDc-1, slagging occurred on the ash deposited probe in P5 (see Figure 3),

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while only a layer of floating ash appeared on the probe in P8, indicating an insignificant ash

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deposition. Through collecting and weighing the slagging samples, it is found that the rise of the

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operating temperature enhanced ash deposition. The deposited amount was only 0.18 g at 900 oC,

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whereas it was up to 5.6 g for the test of 1000 oC. The molten phenomenon could be observed on

C, respectively.

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the surface of slags through SEM pictures, just as illustrated in Figure 4. The increasing temperature

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apparently resulted in more and bigger molten spherules, which reveals the promoting effect of

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temperature on slagging. In addition, these deposited slags were mainly concentrated on the

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leeward surface of the probe rather than the windward, indicating an inhibited slagging by the

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flushing action of the gas flow with high velocity.41 Therefore, the airflow dead zone should be

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avoided when designing the structure and layout of heating surfaces.

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Bottom ash and fly ash play an important role in slagging and fouling in the riser and the tails,

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respectively, thus their physical and chemical properties will offer lots of useful information for

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understanding the ash deposited mechanism. Figures 5 and 6 give their chemical compositions by

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ICP-AES and XRF, and crystal phases by XRD. The results suggest bottom ash was primarily rich in Si,

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Ca and Na, which might exist in forms of SiO2, Ca2Al2SiO7, CaSiO3 and NaAlSi2O7, while Ca and S were

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the top two mineral elements for fly ash, and believed in the form of CaSO4. For ZDc, Ca2Al2SiO7,

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CaSiO3 and NaAlSi2O7 were considered as the common phases in slags at high temperatures.42,43

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As illustrated in Figure 5, with the rise of temperature, Na content in bottom ash first decreased and

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then increased whereas the opposite was true for fly ash. It is strange that this variations of Na

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contents were not consistent with that of ash deposited amount on probe with temperature shown

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in Figure 3. Additionally, the results also failed to agree with those without mineral additives that

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the Na release (or retention) was monotone increased (or decreased) with temperature.21 Due to

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the strong volatility of Na species for low rank coals, their release would be enhanced as

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temperature increased.44 Meanwhile, when quartz sand was used as the bed material, at higher

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temperature, quartz sand would reacted with Na species more easily to form low-temperature

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eutectics.34 Hence, it is inferred that the participation of quartz sand caused the presence of

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competitive relation between the release of Na and the residing reactions of Na in coal when

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increasing temperature. For example, when temperature rose from 900 oC to 950 oC, more Na

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species were released into gas phase, resulting in lower Na retention in bottom ash and more Na

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condensation in tail-flue fly ash. As temperature further rose to 1000 oC, the enhanced reactions

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between quartz sand and Na species caused more Na species resided in bottom ash rather than fly

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ash. Furthermore, this view can be verified by XRD patterns of bottom ash. The decreasing peak 6


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intensity of SiO2 with the rise of temperature indicates quartz sand was gradually consumed. In

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particularly, the diffracted intensity of SiO2 was significantly reduced as temperature rose from 950

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o

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stronger than their release effect at 1000 oC. Besides, as temperature rose, Ca, Fe and S species

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exhibited the similar behaviors to Na species. This indicates vigorous chemical reactions occurred

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among these species at 1000 oC and resided them in bottom ash.

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In the test of 1000 oC, coking slags, mainly composed of agglomerated particles, were found in the

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riser. XRF analysis in Figure 7 shows the coking slags were rich in Ca, Si, S and Na, and these mineral

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elements were identified in forms of SiO2, Ca2Al2SiO7, CaSiO3, CaSO4 and NaAlSi2O6 through XRD

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analysis. As reported, Na species could cause a sticky coating layer on the bed particle surface,

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which would cause defluidization without effective measures.32,45 Therefore, defluidization is

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unavoidable for a longer-time run unless other bed materials replace the SiO2-rich quartz sand and

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the operating temperature is controlled below 950 oC.

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3.2 Influence of reaction atmosphere

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Temperature variations along the flue gas flow direction during ZDc-2 gasification (ER=0.48) and

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combustion (ER=1.13) at 950 oC in CFB are shown in Figure 8. At ER=0.48, temperature gradually

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reduced along the riser, whereas the temperature increased along the riser for the case of ER=1.13.

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Since most of ash particles were separated from the flue gas by the cyclone, the gas temperature

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was considerably reduced after the flue gas passed through the cyclone. However, the temperature

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decreased more rapidly at ER=1.13 than at ER=0.48, which might be attributed to fewer ash

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particles contributing to the heat load in the flue gas.

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Photos of the ash deposited probes for ZDc-2 at above two atmospheres are shown in Figure 9.

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Although Na2O content in ZDc-2 ash is higher than that in ZDc-1 coal, no deposited ash appeared on

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the probe in P5 for ZDc-2. To a certain extent this phenomenon suggests coal property has a great

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impact on slagging. Besides, the deposited ash on the tail probe in P8 was obvious. Compared to

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the case of gasification, the deposited ash that formed during ZDc-2 combustion clumped,

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agglomerated, and further connected tightly with metal surfaces. This implies the reaction

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atmosphere has a strong influence on fouling on tail heating surfaces.

C to 1000 oC. Thus, it is deduced that the retention effect of Na species in bottom ash was far

7


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It is believed through the above results that ZDc-2 has a weak slagging tendency in furnace, but the

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further analysis of bottom ash exhibits its different slagging tendencies between gasification and

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combustion. As shown in Figure 10, SiO2, NaAlSiO4 and Ca2Al2SiO7 were major phases in bottom ash.

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In the gasified bottom ash, Ca3Fe2(SiO4)3 was also detected. These crystalline phases were might

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formed by the following reactions (R1-R5) among Si-, Al-, Ca-, Fe- and Na-containing species in

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ZDc-2:46

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2CaO+Al2O3+SiO2→Ca2Al2SiO7

R1

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NaCl+3SiO2+H2O→Na2O·3SiO2+2HCl

R2

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Na2SO4+3SiO2→Na2O·3SiO2+SO2+0.5O2

R3

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Na2O·3SiO2+Al2O3→2NaAlSiO4+SiO2

R4

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3CaO+Fe2O3+3SiO2→Ca3Fe2(SiO4)3

R5

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In Figure 10, the stronger diffracted intensity of SiO2 in bottom ash at ER=1.13 signifies SiO2 was

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stable, and consumed little during ZDc-2 combustion. By comparison, at ER=0.48, the high

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consumption of SiO2 resulted in more Si bearing crystalline phases formed in the gasified bottom

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ash. Na contents in bottom ash and fly ash for ZDc-2 at ER=0.48 and 1.13 are shown in Figure 11. It

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is observed that the Na content in the gasified bottom ash was higher than that in the combusted

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bottom ash, while it was the opposite for fly ash. This might be ascribed to the influence of ER on

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the release of Na. Actually, the increasing release of Na with the carbon conversion has been

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reported.45 At ER=1.13, coal particles was easily divided into fine char fragments or ash particles

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because of the violent reaction, resulting in more released Na species. The released Na species

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subsequently deposited on the surface of fine ash particles, and enhanced fouling on tail heating

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surfaces. As the resided Na in furnace decreased, the reactions between Na species and SiO2 were

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inhibited. Then, the formed Na-containing eutectics reduced, causing a weaker slagging tendency.

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The XRD and XRF analysis results of the deposited ash in P8 for ZDc-2 at ER=0.48 and 1.13 are

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demonstrated in Figure 12. Through the XRD analysis, it is clear that the main crystalline phases

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were similar for above two types of deposited ash. Significantly, the diffracted intensity of NaCl in

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deposited ash was highest. Since NaCl is a common aggressive medium, the metal heating surfaces

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might face the corrosion risk via the following reaction (R6): 8


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Fe2O3+2NaCl+0.5O2→2NaFeO2+Cl2

R6

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It is also observed from Figure 12 that the diffracted intensity of NaCl relative to other minerals

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during gasification was higher than that during combustion. This signifies a stronger interaction

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among minerals occurred during combustion, causing the formation of new mineral phases and

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consumption of those high-content compounds. The chemical compositions of the deposited ash

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show Ca was the highest element, of which promotion in fouling has been verified.15 The S contents

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in the deposited ash were very low because of the low SO3 content (only 1.82%) in ZDc-2 ash.

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Therefore, the influence of S on fouling could be ignored in this study, whereas it could not be

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ignored for other ZDc.47 Generally, sulfation would occur between SO2 and Na species, forming

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Na2SO4 and then enhancing fouling.

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Amounts of residual carbon in deposited ash were relatively stable under the gasification condition,

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which was proved in Figure 12(a). The residual carbon played a role of inert diluent in deposited ash,

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and reduced the possibility of the tight association or chemical reactions among active constituents

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inside as well as the stickiness of deposits. Additionally, in comparison to the case of gasification,

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the particle size of fly ash formed during combustion was finer (see Figure 13). The finer particle

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more easily transferred towards the low-temperature heating surfaces due to the thermophoresis

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force, and sintered together forming sticky aggregates.37 Therefore, the fouling tendency was

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relieved at ER=0.48, while enhanced at ER=1.13. As a summary, regardless of gasification or

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combustion, the ash-related issue is an unavoidable problem of ZDc, but the problems (such as

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slagging in furnace for gasification and fouling in tails for combustion) can be separately targeted to

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solve.

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3.3 Influence of coal property

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For ZDc-3, its ash deposition situation during gasification at 950 oC was similar to ZDc-1: Obvious

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slagging occurred on the probe in P5 (Figure 14) and only a layer of floating ash appeared on the

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probe in P8. However, the deposited amount of slags for ZDc-3 was higher, about 2.4 g. It can be

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inferred, combining the previous results of sections 3.1 and 3.2, that the three types of ZDc behaved

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the following slagging tendency: ZDc-3 > ZDc-1 > ZDc-2.

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In view of the big differences in coal property among the three experimental coals, their diverse 9


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slagging tendencies were mainly ascribed to the diversity of coal ash composition, particularly those

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involving Na. For the three coals, their Na2O contents in coal ash via the ashing pretreatment at 815

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o

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(3.92%). However, the Na contents in raw coal through chemical extraction followed the order of

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ZDc-2 (9.87 mg/g) > ZDc-1 (4.45 mg/g) > ZDc-3 (4.18 mg/g), which is illustrated in Figure 15(a). The

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Na contents by the methods of GB/T1574-2007 and chemical extraction failed to match their

259

slagging tendencies. This indicates the prediction of slagging behavior by the Na content measured

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through the above two methods is not accurate. By comparison, the Na contents in coal ash

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undergoing the pretreatments of 575 oC ashing and chemical extraction in sequence were

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consistent with their slagging tendencies, just as shown in Figure 15(b), suggesting this method can

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be used to preliminarily estimate ZDc slagging tendency during gasification.48

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Besides Na content, the Na occurrence mode also played an important role in slagging and fouling.49

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For the three used coals, the water-soluble Na was dominant, occupying 81.1% (ZDc-1), 88.4%

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(ZDc-2) and 56.9% (ZDc-3), respectively. It is believed that the Na in water-soluble form is active and

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unstable at high temperature. Furthermore, the NH4Ac-soluble Na accounted for as high as 36.2% of

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the total Na in ZDc-3, whereas only 12.4% for ZDc-1 and 7.9% for ZDc-2. It is reported that NH4Ac

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solution could extract organically-bounded Na in form of carboxylate,50 which would generate Na2O

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at high temperatures.51 The main inorganic Na-based phases in three types of ZDc are given in Table

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3. It is known that the three coals all contained lots of Na silicates (Na2SiO3 and Na2Si3O7) and

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aluminosilicates (nepheline and maralite). However, the volatile hatile (NaCl) was dominant in ZDc-2.

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Na aluminosilicates are usually stable at the reaction temperature suitable for CFB (850~950 oC).

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Although Na2SiO3 and Na2Si3O7 are water-soluble under certain conditions,52 they are non-volatile

275

at high temperatures. This means the water-soluble Na greatly differed among the three coals,

276

which might cause their diverse slagging tendencies.

277

For the high-NH4Ac-soluble-Na ZDc-3, the generated Na2O would interact with other metal oxides

278

forming low-temperature eutectics such as SiO2-CaO-Na2O.33 Na2O also reacted with SiO2-rich bed

279

materials resulting in agglomeration even defluidization via R7 and R8.45

280

C according to GB/T1574-2007 (Table 1) obeyed the order of ZDc-3 (7.28%) > ZDc-2 (4.38%) > ZDc-1

SiO2 + Na2O → Na2SiO3 10


R7

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281

Na2SiO3 + nSiO2 → Na2O·nSiO2

R8

282

The formed and original Na2SiO3 performed strong viscosity and acted as the fluxing agent during

283

slagging. Under SiO2-rich and Na2O-rich conditions, the unsaturated Na2SiO3 could further react

284

with them forming Na2O·nSiO2 (n >1 for the SiO2-rich condition while n ZDc-1 > ZDc-2. 11


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

309

Acknowledge

310

This work was financially supported by the Strategic Priority Research Program of the Chinese

311

Academy of Sciences (No. XDA07030100) and the International Science & Technology Cooperation

312

Program of China (No. 2014DFG61680).

313

References

314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348

[1] Yang, Q.C.; Qian, Y.; Zhou, H.R.; Yang, S.Y. Energy Conv. Manag. 2016, 126, 898-908. [2] Wang, N.; Yu, J.L.; Tahmasebi, A.; Han, Y.N.; Lucas, J.; Wall, T.; Jiang, Y. Energy Fuels 2014, 28 (1), 254-263. [3] Yu, J.L.; Tahmasebi, A.; Han, Y.N.; Yin, F.K.; Li, X.C. Fuel Process. Technol. 2013, 106, 9-20. [4] Tahmasebi, A.; Yu, J.L.; Han, Y.N.; Yin, F.K.; Bhattacharya, S.; Stokie, D. Energy Fuels 2012, 26 (6), 3651-3660. [5] Roy, B.; Bhattacharya, S. Fuel Process. Technol. 2014, 117, 23-29. [6] Binner, E.; Zhang, L.A.; Li, C.Z.; Bhattacharya, S. Proc. Combust. Inst. 2011, 33, 1739-1746. [7] Li, C.Z. Fuel 2007, 86 (12-13), 1664-1683. [8] Jamil, K.; Hayashi, J.I.; Li, C.Z. Fuel 2004, 83 (7-8), 833-843. [9] Wu, H.W.; Hayashi, J.; Chiba, T.; Takarada, T.; Li, C.Z. Fuel 2004, 83 (1), 23-30. [10] Hao, P.; Bai, Z.Q.; Zhao, Z.T.; Yan, J.C.; Li, X.; Guo, Z.X.; Xu, J.L.; Bai, J.; Li, W. Fuel Process. Technol. 2017, 159, 153-159. [11] Huang, L.L.; Schobert, H.H. Coal Sci. Technol. 1995, 24, 1243-1246. [12] Miller, S.F.; Schobert, H.H. Energy Fuels 1994, 8 (6), 1208-1216. [13] van Eyk, P.J.; Ashman, P.J.; Alwahabi, Z.T.; Nathan, G.J. Combust. Flame 2011, 158 (6), 1181-1192. [14] Li, G.; Li, S.; Huang, Q.; Yao, Q. Fuel 2015, 143, 430-437. [15]Wang, X.B.; Xu, Z.X.; Wei, B.; Zhang, L.; Tan, H.Z.; Yang, T.; Mikulcic, H.; Duic, N. Appl. Therm. Eng. 2015, 80, 150-159. [16] Wu, X.J.; Zhang, X.; Yan, K.; Chen, N.; Zhang, J.W.; Xu, X.Y.; Dai, B.Q.; Zhang,J.; Zhang, L. Fuel 2016, 181, 1191-1202. [17] Wei, B.; Tan, H.; Wang, Y.; Wang, X.; Yang, T.; Ruan, R. Appl. Therm. Eng. 2017, 119, 449-458. [18] Zhou, B.; Zhou, H.; Wang, J.Y.; Cen, K.F. Fuel 2015, 150, 526-537. [19] Dai, B.-Q.; Low, F.; De Girolamo, A.; Wu, X.; Zhang, L. Energy Fuels 2013, 27 (10), 6198-6211. [20] Dai, B.Q.; Wu, X.J.; De Girolamo, A.; Zhang, L. Fuel 2015, 139, 720-732. [21] Li, G.Y.; Wang, C.A.; Yan, Y.; Jin, X.; Liu, Y.H.; Che, D.F. J. Energy Inst. 2016, 89 (1), 48-56. [22] Baxter, L.L.; Desollar, R.W. Fuel 1993, 72 (10), 1411-1418. [23] Laursen, K.; Frandsen, F.; Larsen, O.H. Energy Fuels 1998, 12 (2), 429-442. [24] Wang, C.a.; Liu, Y.; Jin, X.; Che, D. J. Energy Inst. 2015. [25] Wang, C.A.; Jin, X.; Wang, Y.K.; Yan, Y.; Cui, J.; Liu, Y.H.; Che, D.F. Energy Fuels 2015, 29 (1), 78-85. [26] Lee, J.M.; Kim, D.W.; Kim, J.S. Energy 2011, 36 (9), 5703-5709. [27] Alonso, M.; Diego, M.E.; Perez, C.; Chamberlain, J.R.; Abanades, J.C. Int. J. Greenh. Gas Control. 2014, 29, 142-152. 12


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[28] Li, S.Y.; Li, Y.Y.; Lu, Q.G.; Zhu, J.G.; Yao, Y.Y.; Bao, S.L. Waste Manage. 2014, 34 (12), 2561-2566. [29] Duan, L.B.; Duan, Y.Q.; Zhao, C.S.; Anthony,E. J. Fuel 2015, 150, 8-13. [30] Vuthaluru, H.B.; Zhang, D.K. Fuel Process. Technol. 1999, 60 (2), 145-156. [31] Vuthaluru, H.B.; Linjewile, T.M.; Zhang, D.K.; Manzoori, A.R. Fuel 1999, 78 (4), 419-425. [32] van Eyk, P.J.; Kosminski, A.; Ashman, P.J. Energy Fuels 2012, 26 (1), 118-129. [33] Qi, X.; Song, G.; Song, W.; Lu, Q. J. Fuel Chem. Technol. 2015, 43 (8), 906-913. (in Chinese) [34] Song, G.L.; Qi, X.B.; Song, W.J.; Lu, Q.G. Energy Fuels 2016, 30 (5), 3967-3974. [35] Song, G.; Qi, X.; Song, W.; Yang, S.; Lu, Q.; Nowak, W. Fuel 2016, 186, 140-149. [36] Zhou, J.B.; Zhuang, X.G.; Alastuey, A.; Querol, X.; Li, J.H. Int. J. Coal Geol. 2010, 82 (1-2), 51-67. [37] Li, J.B.; Zhu, M.M.; Zhang, Z.Z.; Zhang, K.; Shen, G.Q.; Zhang, D.K. Fuel Process. Technol. 2016, 144, 155-163. [38] Zhou, H.; Zhou, B.; Zhang, H.; Li, L. Energy Fuels 2014, 28 (12), 7701-7711. [39] Namkung, H.; Xu, L.H.; Shin, W.C.; Kang, T.J.; Kim, H.T. Fuel 2014, 117, 1274-1280. [40] Zhang, X.; Zhang, H.; Na, Y. Procedia Eng. 2015, 102, 305-314. [41] McCullough, D.P.; van Eyk, P.J.; Ashman, P.J.; Mullinger, P.J. Energy Fuels 2011, 25 (7), 2772-2781. [42] Wei, B.; Wang, X.; Tan, H.; Zhang, L.; Wang, Y.; Wang, Z. Fuel 2016, 181, 1224-1229. [43] Ma, Y.; Huang, Z.; Tang, H.; Wang, Z.; Zhou, J.; Cen, K. J. Fuel Chem. Technol. 2014, 42 (1), 20-25. (in Chinese) [44] Quyn, D.M.; Wu, H.W.; Li, C.Z. Fuel 2002, 81 (2), 143-149. [45] Kosminski, A.; Ross, D.P.; Agnew, J.B. Fuel Process. Technol. 2006, 87 (11), 953-962. [46] Kosminski, A.; Ross, D.P.; Agnew, J.B. Fuel Process. Technol. 2006, 87 (12), 1051-1062. [47] Song, G.; Song, W.; Qi, X.; Lu, Q. Energy Fuels 2016, 30 (4), 3473-3478. [48] Song, W.; Song, G.; Qi, X.; Lu, Q. J. Fuel Chem. Technol. 2016, 44 (2), 162-167. (in Chinese) [49] Zhang, J.; Han, C.L.; Yan, Z.; Liu, K.L.; Xu, Y.Q.; Sheng, C.D.; Pan, W.P. Energy Fuels 2001, 15 (4), 786-793. [50] Benson, S.A.; Holm, P.L. Ind. Eng. Chem. Prod. Res. Dev. 1985, 21 (1), 145-149. [51] Qi, X.; Song, G.; Song, W.; Lu, Q. J. Energy Inst. 2017, 90 (6), 914-922. [52] Kosminski, A.; Ross, D.P.; Agnew, J.B. Fuel Process. Technol. 2006, 87 (12), 1037-1049.

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Tables Table 1. Coal property of three types ZDc Coal ZDc-1 ZDc-2 Proximate analysis (%, air dry basis) Fixed carbon 45.27 43.86 Volatile 34.06 30.46 Ash 5.03 14.66 Water 15.64 11.02 LHV (MJ/kg) 17.63 17.93 Ultimate analysis (%, dry basis) C 64.50 57.92 H 2.02 2.65 O 26.23 22.17 N 0.82 0.65 S 0.47 0.12 Cl 0.123 1.279 Ash composition (%) SiO2 17.24 41.98 Al2O3 11.90 17.59 Fe2O3 5.76 6.76 CaO 28.74 19.39 MgO 5.34 2.49 TiO2 0.60 1.08 SO3 19.58 1.82 P2O5 0.05 0.18 K 2O 0.38 0.66 Na2O 3.92 4.38

ZDc-3 55.48 27.02 3.16 14.34 23.7 75.34 3.53 16.31 0.61 0.53 0.065 3.73 6.16 5.37 33.45 5.42 0.41 29.34 Not detected 0.45 7.28

382 383 384

Table 2. Experimental conditions Test Coal Operating temperature (oC) ER* I 900 0.46 ZDc-1 II 950 0.47 III 1000 0.43 IV 950 0.48 ZDc-2 V 950 1.13 VI ZDc-3 950 0.45 *Equivalence ratio (ER): the ratio between the actual air flow and fuel just complete combustion. 14


Fluidized velocity (m/s) 2.74 2.86 2.72 2.80 3.29 2.72 the theoretical air flow for

Page 15 of 21 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

385 386

Table 3. Main Na-based phases in the three experimental ZDc Coal

Na-based phase

ZDc-1

Na2SiO3, nepheline [KNa3(AISiO4)4], maralite

ZDc-2

Hatile (NaCl), Na2SiO3, nepheline [KNa3(AISiO4)4], Na2SO4, amphibole

ZDc-3

Na2SiO3, nepheline [KNa3(AISiO4)4], Na2Si3O7, maralite, dachiardite

387 388

15


Energy & Fuels

389 390

Figures T7

P6

T9

P7

P5 T6 P4 T10

P8

T5

3

1

T4

2

4

T8

T3

P3

T2 T1

P2

P1

391 392 393

1-riser, 2-coal hopper, 3-cyclone, 4-loop seal T1~T10: thermocouple positions, P1~P8: ash sampling positions

394

Figure 1. Schematic diagram of 0.4 t/d CFB test system

395 o

900 C

1000

o

950 C o

950

1000 C

o

Temperature ( 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 16 of 21

900 850 800 750 T1

396 397

T2

T3

T4

T5

T6

T7

T9

T10

Thermocouple position

Figure 2. Temperature variations along the flue gas flow direction for ZDc-1.

398 16


Page 17 of 21

399 400

Figure 3. Deposited amount and macro morphology of slags for ZDc-1.

401 402

Figure 4. SEM pictures of slags for ZDc-1. 40

Content (%)

30

Bottom Ash o 900 C

20 15

o

950 C

10

o

1000 C

5 Si

20

10

900

403

25

Bottom Ash Fly Ash

Content (%)

Na Content (mg/g)

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

950

o

Na

Al

Fe

Mg

K

o

950 C o

1000 C Ca

Na

Al

Fe

Mg

Element

Temperature ( C)

404 405

S

o 900 C Fly Ash

Si

1000

Ca

30 25 20 15 10 5

Figure 5. Chemical compositions of bottom ash and fly ash for ZDc-1. 17


S

K

Energy & Fuels

2000 o

Bottom Ash-1000 C

o

6

2000

1

1500

Fly Ash-1000 C

4

500

2 1 5 3 3 3

1000 1 2

1 3 18

1

1

o

Bottom Ash-950 C 1

10000 5000

1

1 1 1

1 23 1 1 1

1

0

1

1

1

15000

o

Bottom Ash-900 C

1

0 10

1 1 1 1

1

1

6

o

6

2000

Fly Ash-950 C

1000

7 3

6 3 66 6 76 6 6 6

7

6

0 2000 o

Fly Ash-900 C

6

1000

6

500

3 7

7 3

20

30

1

2

36 6 6 7 6 6 6 6

0

1500

10000 5000

6

3 7

0

15000

7

1

Diffracted intensity (counts)

1000


1

6 667 6 6 6 6

6

0 20

30

40

50

ο

60

70

80

10

90

40

o

50

60

70

80

2θ ( )

2θ ( )

406 407

1-SiO2; 2-CaSiO3; 3-Ca2Al2SiO7; 4-NaAlSi2O6; 5-KAlSi3O8; 6-CaSO4; 7-CaCO3; 8-CaO

408

Figure 6. XRD patterns of bottom ash and fly ash for ZDc-1 at 900, 950 and 1000 oC.

Diffracted intensity (counts) Content (%)

409

410 411

15 10 5 0

Si

Ca

Na

Al

Fe

Mg

S

K

Element 1 2000 1500

1

1000

1

7-NaAlSi2O6

37 1 53 4 32 2 6

500 0 10

20

1-SiO2, 2-Ca2Al2SiO7, 3-CaSiO3, 4-CaSO4, 5-KAlSi3O8, 6-CaO,

30

2 40

1

2 50 ο

60

70

1 80

90

2θ ( )

Figure 7. Macro morphology and properties of coking slags for ZDc-1 at 1000 oC. ER=1.13 ER=0.48

1000 950 o

Temperature ( 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 18 of 21

900 850 800 750 T1

412 413

T2

T3

T4

T5

T6

T7

T9

T10

Position

Figure 8. Temperature variations along the flue gas flow direction for ZDc-2 at ER=0.48 and 1.13. 18


Page 19 of 21

414 415 416

(a): the probe in P5 at ER=0.48, (b): the probe in P5 at ER=1.13, (c): the probe in P8 at ER=0.48, (d): the probe in P8

417

Figure 9. Photos of ash deposited probes for ZDc-2 at ER=0.48 and 1.13.

at ER=1.13

50000

1

ER=1.13

40000


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

30000 20000 10000

1 2

0

23

1 1 1 1

1

1

1

1

1

1

ER=0.48

6000 4000 2000 0 10

12 3 2 23 4 1 1 1 1

20

30

40

1 13 1

50

1

1

60

70

80

90

2θ(°)

418 419

1-SiO2, 2-NaAlSiO4, 3-Ca2Al2SiO7, 4-Ca3Fe2(SiO4)3

420

Figure 10. XRD patterns of bottom ash for ZDc-2 at ER=0.48 and 1.13.

19


Energy & Fuels

12

1.13 0.48

Na content (mg/g)

10 8 6 4 2 0

Bottom Ash

Fly Ash

Sample

421 422 Diffracted intensity (counts)

6000

1

5000

1-NaCl, 2-Ca2Al2SiO7,

4000

3-MgO, 4-Ca12Al14O13, 6

5-NaAlSiO4, 6-SiO2,

1

3000 2000

4 6 7 2

1000 0 10

20

7-CaSO4

4 5

30


Figure 11. Na contents in bottom ash and fly ash for ZDc-2 at ER=0.48 and 1.13.

4

43 40

1 22 4

6

50

60

o

1

1

70

80

90

1

1-NaCl, 2-Ca2Al2SiO7, 3-MgO,

2500

2 2000

4-Ca12Al14O13, 5-NaAlSiO4,

4

6-SiO2, 7-CaSO4

4

1500

65 5 51 7 2

1000

4

1 2 3

10

20

30

40

50

o

60

70

1 80

25

Content (%)

20

13.6%

15

8.8%

10

20 15 10

7.2%

5

0 Si

Ca

Na

Mg

Al

Fe

K

Cl

0

S

Si

Ca

Na

Mg

Al

Fe

Element

Element

(a) ER=0.48

(b) ER=1.13

K

Cl

Figure 12. Properties of the deposited ash in P8 for ZDc-2 at ER=0.48 and 1.13. 100

Cumulative percent (%)

90 80

ER=1.13 ER=0.48

70 60 50 40 30 20 10 0

425 426

90

30

Ashing No Ashing

5

424

1

2θ ( )

25

423

2 14

500

2θ ( ) 30

Content (%)

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 21

10

100

Partice size (µm)

1000

Figure 13. Particle size distribution of fly ash in P4 for ZDc-2 at ER=0.48 and 1.13.

427

428 20


S

Page 21 of 21

429

Figure 14 Picture of the ash deposited probe in P5 for ZDc-3 (test VI). Insoluble HCl Soluble NH4Ac Soluble

10 (a) 8

431

Water Soluble

4 2

ZD-1

Water Soluble

8 6 4

ZD-2

0

ZD-3

ZD-1

ZD-2

ZD-3

Coal ash

Coal

Figure 15. contents of Na with different occurrences in ZDc and ZDc ash.

Na content (mg/g)

Bottom Ash Fly Ash

40 30 20 10 0

433

10

2

50

432

Insoluble HCl Soluble NH4Ac Soluble

(b)

12

6

0

430

14

Na content (%)

Na content (mg/g)

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

ZD-1

ZD-2

ZD-3

Coal

Figure 16. Na contents in bottom ash and fly ash for the three experimental ZDc (tests II, IV and VI)

434 435

21


Slagging and Fouling Characteristics of Zhundong High-Sodium Low

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