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Release and transformation pathway of various K-species during thermal conversion of agricultural straw. Part I: devolatilization stage Yanqin Huang, Huacai Liu, Hongyou Yuan, Xiuzheng Zhuang, Song Yuan, Xiuli Yin, and Chuangzhi Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02191 • Publication Date (Web): 04 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018
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Energy & Fuels
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Release and transformation pathway of various K-species during thermal
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conversion of agricultural straw. Part I: devolatilization stage
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Yanqin Huanga,b, Huacai Liua,b, Hongyou Yuana,b, Xiuzheng Zhuanga,b,c, Song
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Yuana,b,c, Xiuli Yin∗a,b, Chuangzhi Wua,b
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a
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Conversion, Chinese Academy of Sciences, Guangzhou 510640, China
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b
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Development, Guangzhou 510640, China
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c
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CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy
Guangdong Provincial Key Laboratory of New and Renewable Energy Research and
University of Chinese Academy of Sciences, Beijing, 10049, China.
Abstract: The serious K-induced ash problems during biomass thermal conversion are
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closely related to release and transformation behavior of K. The aim of this work is to
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explore contribution of different occurrence forms of K on K-release and
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transformation pathway among various K-species during devolatilization of
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agricultural straw. Devolatilization experiments under different heat treatment
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conditions (i.e. temperature, heating rate and atmosphere) were carried out using a
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fixed-bed apparatus, and occurrence forms of K in virgin sample and residual chars
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were quantitatively determined by sequential chemical fraction analysis combined
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with ICP-OES (inductively coupled plasma optical emission spectrometry). The
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unstable water-soluble and ion-exchangeable K in wheat straw accounted for about
∗
Corresponding author, tel.: +86 02087057731; fax: +86 02087057737. E-mail address:
[email protected] Address: No. 2, Nengyuan Rd, Wushan Tianhe District, Guangzhou, 510640 China 1
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89.3% of total K, and were mainly responsible for the K-release. Part of the unstable
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K were directly decomposed into atomic K or sublimated, and a large fraction of them
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were first bound to char matrix to form char-K and then transformed into other forms
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or released depending on specific conditions. The release and transformation of K was
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promoted under intensive conditions (i.e., high temperature and heating rate) and was
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strongly dependent on atmospheres. Compared with N2 condition, CO2 atmosphere
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inhibited K-release and facilitated formation of HCl-soluble K, while O2 atmosphere
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promoted K-release and enhanced formation of insoluble K. Finally, the functional
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mechanism for K-release and transformation among various K-species during
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volatilization was proposed.
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Key words: chemical forms of K; release; transformation pathway; mechanism
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Introduction
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The depletion of fossil fuel and worsening environmental issues have encouraged
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the exploration of renewable and clean energy. Biomass has attracted worldwide
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attention and is considered to be a promising fuel due to its renewability and
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CO2-neutral emission. According to its source, biomass can be generally classified
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into four categories, i.e., woody, agricultural, waste, and excrement.1, 2 Among them,
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the agricultural is one of the major biomass energy resource, and it covers harvesting
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residues, grass, energy plants, and residues from food industry.1 These residues are
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commonly used to supply power or heat through thermochemical conversion
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technologies (i.e. gasification and combustion). Unfortunately, these agricultural fuel
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usually contains high content of alkali metal potassium (for example, 1.0-2.5 wt% in 2
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straw),3 which easily enters into gas phase due to its strong sublimation tendency and
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gives rise to serious ash-related problems, such as fouling, corrosion and slagging.
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These K-induced problems will result in unscheduled shutdown of devices and
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increase in operation cost. Hence, comprehensive understanding of release and
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transformation behavior of K during thermal conversion is of great importance to
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solve K-related problems. Plenty of efforts have been attempted to investigate release behavior of K during
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thermal utilization of biomass. It was revealed that partitioning of K in gaseous phase
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and residual char was closely related to temperature,4-9 atmosphere,10-12 heating rate9,
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at elevated temperature and under combustion conditions,10 and the inlet of steam
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might facilitate K volatilization via widening or creating pores during char-steam
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gasification.12 The influence of heating rates on the release of alkali metals was
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dependent on their occurrence forms and experimental conditions, and there was no
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an agreement on relationship between them.9, 13 The vapor K-species in the product
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gas were comprehensively investigated by an online Molecular Beam Mass
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Spectrometry, and it was found that the volatilization of K was mainly in the form of
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KCl, KOH or atomic K depending on specific conditions,15, 18 In addition to heat
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treatment conditions, the K-release was also affected by the relative Cl-concentration
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and other inorganic constituents. It was revealed that Cl facilitated K-release through
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sublimation of KCl while Si and Al suppressed it by forming stable K-Si-Al
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compounds.4, 14, 16 Based on above-mentioned findings, mechanisms for K-release was
and other inorganic elements in biomass.14-17 The release of K would be promoted
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gradually developed, and the K-release was usually ascribed to sublimation of
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inorganic K and decomposition of organic K.19-22
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Previous studies mainly focused on the total release of K during biomass thermal
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conversion, while few afford was paid on interactions between K and biomass the
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thereby transformation behavior among various K-species. Potassium mainly exhibits
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four occurrence forms in biomass: water soluble K, ion-exchangeable organic K,
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acid-soluble K, and insoluble K.3 The K-release is closely associated to chemical
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forms of K, and not all occurrence modes of K will volatilize into gaseous phase and
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lead to K-induced deposition. Part of K might be transformed into other forms during
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heat treatment of biomass. Chen et al.23 investigated transformation of K during straw
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heat treatment at low temperature of 200-350 °C, and detected a conversion from
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water soluble K to ion-exchanged K. In the research of Deng et al.,10, 11 it was found
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that the contents of water-soluble and ion-exchangeable K were decreased, while the
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fractions of HCl-soluble and insoluble K were increased. Wang et al.8 pointed out that
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a considerable amount of organically bound K was formed during pyrolysis of KCl
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dropped pine wood. Many works on transformation of alkali metals were focused on
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thermal conversion of coal, while the main alkali metal in coal is sodium, and the
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chemical structure of biomass greatly distinguishes from coal. In the research of Wang
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et al., 24 release behavior of Na during pyrolysis of high-Na coals were investigated. It
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was indicated that speciation transformation of Na were significantly affected by
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operation conditions, and the contribution of different species of Na on Na-release
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was different. Liu et al.25 also found speciation transformation of alkali metals during 4
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combustion of high-alkali coal. Although amounts of work on K-release have been done over the past few
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decades, there are still many issues on functional mechanism for release and
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transformation of K, and it is challenging to forecast K-release during biomass
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thermal conversion according to existing findings. Devolatilization is the first step
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during thermal conversion of biomass. The speciation transformation among various
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K-species during devolatilization will conversely affect release behavior of retaining
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K in char during secondary stage of thermal conversion. Hence, the first step of
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solving the K-induced deposition is to understand release and transformation behavior
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of various K-species during devolatilization. Unfortunately, up to date, the
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contribution of different K species on K-release, especially mechanism on speciation
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transformation among different occurrence forms of K is still unclear.
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This work is aimed to clarify the release behavior and transformation pathway of
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different chemical forms of K during devolatilization stage of agriculture straw.
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Devolatilization experiments were carried out on a fix-bed pyrolysis apparatus. The
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subsequent chemical fractionation analysis combined with an inductively coupled
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plasma optical emission spectrometry (ICP-OES) was employed to explore the
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transformation behavior of various K-species. The influence of temperature, heating
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rate, atmosphere (inert, reducing and oxidizing) on release and transformation
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behavior of different K-species were intensively discussed. Based on this, the
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pathway for K-release and speciation transformation during devolatilization was
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proposed. This study will be of great importance to better understanding the 5
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functional mechanism of various K-species on K-induced deposition during thermal
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conversion of agricultural biomass.
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2. Experimental section
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2.1 Material Wheat straw (WS) is a byproduct for wheat production and usually left as residue.
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Owning to its abundance worldwide and high-K content, wheat straw is selected as a
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typical representative of agricultural straw. In this work, the WS was obtained from a
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rural area in Henan province, China. Before experiment, the material was dried and
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grounded into powder with size of
