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Releases of K and Cl and Emissions of NOx and SO2 during Reed Black Liquor Combustion in Fluidized Bed Xiaoyu Ji, Haipei Bie, Yaning Zhang, Pei Chen, Wenxuan Fang, and Rushan Bie Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02701 • Publication Date (Web): 02 Jan 2017 Downloaded from http://pubs.acs.org on January 8, 2017
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Energy & Fuels
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Release of K and Cl and Emissions of NOx and SO2 during
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Reed Black Liquor Combustion in Fluidized Bed
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Xiaoyu Ji1, Haipei Bie2, Yaning Zhang1, Pei Chen1, Wenxuan Fang1 and Rushan
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Bie1,*
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1. School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, P.R.
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China
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2. Faculty of Engineering, The University of Nottingham, Nottingham NG7 2RD, United
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Kingdom
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ABSTRACT: To predict and relieve ash-related problems and harmful gas emissions
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during fluidized bed combustion, release of K and Cl and emissions of NOx and SO2
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during reed black liquor (BL) combustion in fluidized bed were investigated in this
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study. The effects of bed material (calcium-based zeolite and limestone) and bed
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temperatures (700-900°C) were also studied. The results showed that higher
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temperature favored the K release for both calcium-based zeolite and limestone, and
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calcium-based zeolite caused lower K release due to the formation of K3Al(SO4)3,
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KAl(SO4)2 and K2O-Al2O3-SiO2 eutectic compounds. Cl was mainly released in the
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form of HCl above 700°C. Moreover, high temperature facilitated Cl release for
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calcium-based zeolite whereas had complicated effect on Cl release for limestone.
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Although higher temperature increased the NOx emission for both calcium-based
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zeolite and limestone, limestone resulted in higher NOx emission due to the
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conversion of fuel-N to NOx under the CaO catalysis. Temperature had slight effect on
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SO2 emission for calcium-based zeolite whereas decreased the SO2 emission for
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limestone. Reed BL combustion generated lower SO2 emissions with the help of
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Na2CO3 and K2CO3, and limestone resulted in much lower SO2 emission with the help
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of CaCO3 and CaO.
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Keywords: reed black liquor; limestone; calcium-based zeolite; potassium release;
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chlorine release; harmful gas emission
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Corresponding author:
[email protected] (Rushan Bie) 1
ACS Paragon Plus Environment
Energy & Fuels
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1. Introduction
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Black liquor (BL) is a kind of biomass waste liquid from chemical pulp process
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of papermaking,1 containing organic materials (lignin and polysaccharides) from
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straw or wood and inorganic cooking chemicals (mainly soluble salt ions).2 The main
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products of BL combustion are alkali salts3, such as Na2CO3, Na2SO4, K2CO3 and
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K2SO4, etc., and about 240 million tons of black liquor is produced worldwide
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annually.1 BL is therefore an important source for energy and chemicals. Traditionally,
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the Tomlinson recovery boiler is used to recover energy and chemicals from BL.
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However, this technology is only usable for wood BL but not for straw BL due to the
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facts that the straw BL has higher silica content, higher chlorine content, higher
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viscosity and lower heat value compared with wood BL. Nowadays, fluidized bed
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combustion has been recognized as a more economic and effective method for BL
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treatment due to the advantages of better fuel flexibility, higher heat transfer rate,
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lower investment and lower emissions, etc. However, as regard to this technology, the
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problem of bed material agglomeration may appear during the BL combustion in
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fluidized bed.4, 5 In some related studies, Cl and K were reported as the major cause of
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agglomeration 5-8 through the mechanisms that these two elements would be involved
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in reactions to form low-melting compounds, which behave like viscous liquid at high
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temperatures and then promote the agglomeration.9 In this context, the defluidization
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occurred at 700°C during the BL combustion in fluidized bed and the defluidization
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time was even shorter than 15 min when the temperature was above 800°C.5, 10
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In our previous studies, calcium-based zeolite (CaO·Al2O3·nSiO2) and limestone
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were successfully used as bed material to inhibit agglomeration during the
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combustion of reed BL in fluidized bed, and no defluidization was observed within 4h
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when the bed temperatures were in the range of 850-900°C.11,
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low-melting compounds containing K and Cl (such as KCl) were observed in the
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agglomerates, which indicated that they still played a role in the agglomeration. In
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addition, Cl and K are released to the gas phase during thermal conversion of biomass,
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and they are directly responsible for the corrosion and fouling of heat transfer
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surfaces.13-16 From the information above, it can be concluded that the severities of 2
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However, the
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Energy & Fuels
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corrosion, fouling and bed agglomeration depend significantly on the fractions of Cl
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and K in both gas phase and remained salts.
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The release of K and Cl during combustion and pyrolysis of different biomass
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fuels have been the focus of numerous previous studies. These investigations revealed
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that the release of K and Cl were affected by reactor type,17 reaction temperature, gas
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atmosphere,18, 19 element form, compound content,20, 21 residence time
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particle size.23 Further studies showed that the release of K mainly occurred above
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700°C through the evaporation of KCl and decomposition of K2CO3.18, 24-26 Moreover,
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the release of K would be facilitated if a high amount of Cl is present in the fuel. At
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temperatures below 700°C, only a limited release of K (
