Techno-economical method for the removal of alkali metals from

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Techno-economical method for the removal of alkali metals from agricultural residue and herbaceous biomass and its effect on slagging and fouling behavior Young-Joo Lee, Jong-Won Choi, Ju-Hyoung Park, Hueon Namkung, Gyu-Seob Song, SeJoon Park, Dong-Wook Lee, Joeng-Geun Kim, Chung-Hwan Jeon, and Young-Chan Choi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02588 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Sustainable Chemistry & Engineering

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Techno-economical method for the removal of alkali metals from

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agricultural residue and herbaceous biomass and its effect on

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slagging and fouling behavior

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Young-Joo Lee 1†, ǁ, Jong-Won Choi 1†, Ju-Hyoung Park †, Hueon Namkung †,

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Gyu-Seob Song †, Se-Joon Park †, Dong-Wook Lee †, Joeng-Geun Kim †,

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Chung-Hwan Jeon *ǁ, and Young-Chan Choi *†

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†

Korea Institute of Energy Research (KIER), 152, Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic

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of Korea ǁ

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Pusan Clean Coal Center, School of Mechanical Engineering, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea

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Corresponding Author

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* E-mail: [email protected]. Tel. : +82-51-510-3051 (C.-H.J.).

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* E-mail: [email protected]. Tel. : +82-42-860-3784 (Y.-C.C.).

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These authors equally contributed to this work.

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

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ABSTRACT

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Nowadays, it is widely recognized that biomass combustion processes can contribute to the

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mitigation of greenhouse gas emissions, and thus becomes a viable option as an alternative

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energy source for the power industry. Among various biomasses, the herbaceous biomass is

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regarded as abundant and relatively inexpensive fuel. However, it contains high ashes

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(especially high levels of alkali metals), causing to operation troubles such as slagging and

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fouling inside a heat exchanger or the efficiency deterioration. Accordingly, we herein

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propose an economical method to remove the inherent ashes in the biomass using 16.6 M

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acetic acid pre-treatment at 60oC for 10 min. Seven different biomasses were investigated to

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validate the effects of method. The Kenaf shows the total mineral rejection of 93.48%. In

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particular, the potassium and sodium elements in the Kenaf, which are major factors

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influencing on fouling and slagging in a boiler, were removed up to 99.46 and 100%,

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respectively. Furthermore, the proposed wet treatment was more effective for biomass with

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higher surface areas.

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Keywords: Ashless biomass, ash extraction, wet treatment, slagging, fouling

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■ INTRODUCTION

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Recently, global warming issues associated with the indiscriminate use of fossil fuels have

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motivated many researchers to focus on the reduction of carbon dioxide (CO2) emissions

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from the combustion process. The use of biomass for heat and electricity generation via

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thermo-chemical processes such as combustion, pyrolysis, and gasification, as well as for the

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production of various biofuels, is one means of reducing CO2 emissions.1-4 However, several

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bottlenecks exist that affect the commercialization of biomass. Among these, the high content

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of alkali and alkaline earth metals (AAEM) in the biomass lowers its melting temperature,

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resulting in ash related problems such as slagging and fouling during the combustion, which

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interrupts stable power plant operation.5-9 Therefore, to inhibit the severe ash deposition

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phenomenon during biomass combustion, significant research effort has been expended to

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investigate and develop operational technologies such as the blending of coal and biomass, 10

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controlling the operation conditions,11 and the addition of additives12-13.

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Priyanto et al.10 have investigated co-firing 4 types of woody biomasses with coal. They

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concluded that the use of 30% of a biomass containing the ash higher than 1.0wt% for co-

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firing produces a large number of molten ash particles with a higher calcium mineral content,

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which is still impeding the commercial use of biomass for the power generation.

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Szemmelveisz et al.11 reported that the softening properties of the alkaline minerals such as

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K, Na and Cl inside three herbaceous biomasses may critically influence on operational

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problems (slagging and fouling) during a co-combustion with coal. They suggested that co-

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firing of coal with herbaceous biomass requires lowering the operation temperature of the

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boiler to avoid the deposits and slagging. However, the lower thermal efficiency of boiler is

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accompanied by lowering the combustion temperature.

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Kassman et al.12 reported combustion of a biomass-only resulted in enhanced content of



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chlorine deposit outside a super-heater surface, and thus suggested the injection of

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ammonium sulfate to significantly lower the level of gas phase KCl. Accordingly, they found

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almost no chlorine in the deposits. However, the effect of ammonium sulfate on other

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environmental problems such as gaseous pollutant and PM emission has not been clear yet.

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Wang et al.13 reported that severe melting of the ash was due to formation and fusion of

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low temperature melting potassium silicates, thereby investigating the effects of two mineral

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additives: zeolite 24A and kaolin to capture KCl. They concluded that addition of kaolin and

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zeolite 24A significantly reduced the sintering tendency of the ash inside barely straw up to

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50%.

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However, the problem of fouling and slagging still remains a key challenge. Furthermore,

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such modifications or optimizations typically require extensive changes of the external

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environment, which can be very costly to implement. Recent research efforts have been

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shifted towards biomass pre-treatment prior to the combustion. For example, the removal of

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the alkali metals (K and Na), and Cl with pure water,14-16 acidic solutions,17-19 and alkaline

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solutions20 have been investigated. The hydrothermal pretreatment (HTP) process with pure

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water have been revealed to consume long residence time, high cost and high temperature

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even with low yield.14-16 The previous researchers have employed various strong acids such

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as HCl, H2SO4 and HNO3 or strong base (NaOH), all of which can easily break the cellulose

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and hemicellulose inside a biomass into glucose and xylose, respectively.17-20 Unlike the

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cellulose and hemicellulose, glucose and xylose are soluble, leading to low solid yield during

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the treatment process. Thereby, those reasons derived us to select the use of acetic acid as a

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weak acid with a low solid loss.

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It is well known that the cellulose, hemicellulose, and lignin constituents of biomass are

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responsible for the combustion sources. Hence, a minimization of the loss of these three

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constituents from biomass during the pre-treatment and optimization processes are necessary.


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We herein propose a commercially accessible and acceptable pre-treatment method for the

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production of ashless biomass, which entails an acidic treatment of the biomass at a

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reasonable temperature (60°C) for a substantially short time (less than 10 min) compared to

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analogous methods already presented in the literature. The changes in the ash and carbon

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contents of the biomass were examined with respect to the reaction temperature, residence

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time, and pH, and were compared to other studies. In addition, we have experimentally

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investigated the enhancement of the ash fusion temperature after the pre-treatment stage so as

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to generate ashless biomass applying for commercial power plants.

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■ MATERIALS AND METHODS

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Preparation of raw materials. Seven biomass samples were selected for this work:

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Miscanthus, Kenaf, Corn stalk (grown in Korea), Napiergrass, Cashew Nut Shell (CNS;

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imported from Vietnam), Empty Fruit Bunch (EFB), and Palm Kernel Shell (PKS; imported

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from Malaysia). All the samples were cultivated in 2017. Before leaching the alkali metals by

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acid solution pre-treatment, all the raw biomass were dried at 105°C for 6 h in an oven to

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reduce the water content to < 1wt%, and thereby easily chopped by the shredder (D3V-10,

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Japan). With the exception of the PKS and CNS due to proper initial size (average 30-40 mm),

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all the samples were crushed into small fragments and separated through a sieve, with the size

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ranging from 10 to 20 mm.

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Preparation of the ashless biomass. In this study, we used acetic acid (CH3COOH) to

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remove the ash from the biomass. The mineral extraction was carried out in a 500 mL

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autoclave reactor with a heating rate of 2°C/min up to the desired temperature. The samples

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were stirred with a magnetic stirrer at 100 rpm. After a certain reaction time, the solid residue

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was separated using a filter paper, with the pore size of 1 µm, washed continuously with

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ultrapure water until it reached a neutral pH, and finally oven-dried at 105°C for 6 h. The

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filtrate obtained from the filtration stage can be also treated to produce high-valuable mineral

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products through ion fractionation, and the resulting fraction with the continuously depleting

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mineral content is re-used in the ashless biomass process to extract the ash minerals. The

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detailed ashless biomass process is shown in Figure 1.

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In general, the mineral ions contained in the biomass can be extracted as shown by the following reactions: (Na + CH3COOH + H2O → CH3COONa + H3O)


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(Mg + CH3COOH + H2O → [CH3COO]2Mg + H3O)

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(K + CH3COOH + H2O → CH3COOK + H3O)

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(Ca + CH3COOH + H2O → [CH3COO]2Ca + H3O)

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(Mn + CH3COOH + H2O → [CH3COO]2Mn + H3O)

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(Cl2 + CH3COOH → CH2ClCOOH + HCl)

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There is an important concern to achieve a successful ash rejection. The ash rejection

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efficiency was calculated from the difference in the ash weight between the initial and the

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treated biomass (dry basis) based on the proximate analysis. The ash rejection can be defined

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by Eq. (1).

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Ash rejection (%) = =

Ash content of treated one without water Ash content of raw biomass without water M ash , treated , dry basis M ash , raw , dry basis

(1)

from Proximate analysis

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Where, M denotes the weight.

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The biomass sample needs to be fully immersed in the catalytic solution (i.e. acetic acid) to

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increase the mobility of the minerals via solvation as well as the reacting surface area. The

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biomass solid/liquid ratio is dependent on the type of biomass used, because the amount of

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liquid absorption of each biomass differs. For example, the highly porous Miscanthus sample

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tends to absorb more water than other samples. Therefore, a larger amount of acetic acid

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solution is necessary to fully immerse the Miscanthus in the reaction chamber (Figure 2). To

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evaluate the pre-treatment performance, we investigated the solid yield of the ashless biomass

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and the amount of ash rejection in this study. The solid yield can be expressed in Eq. (2),



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which was calculated on the basis of the amount of the organic solids between the original

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and the treated biomass sample.

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Solid yield (%) = =

Carbon and VM content of treated one without water and ash Carbon and VM content of raw biomass without water and ash ( M F .C . + MVM ) treated , dry basis ( M F .C . + MVM ) raw , dry basis

(2)

from Proximate analysis

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Where, F.C. and VM denote the fixed carbon and volatile matter contents, respectively.

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The mass variation was measured by a precise balance with the accuracy of 0.001 g (ME403,

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Mettler-Toledo).

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Experimental parameters. In the first, we observed the ash rejection efficiency with

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different concentrations of the acetic acid solution at 40°C for 10 min. The optimization of

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the solution concentration is a crucial concern between the solid yield of the ashless biomass

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and ash rejection, because the highly concentrated solution dissolves more of the organic

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solids. Therefore, it results in a lower organic solid yield even though it can guarantee a high

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amount of ash rejection. Therefore, we carefully decided the solution concentration to

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minimize the loss of the organic solid yield as well as to maximize the ash rejection. The

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concentration of the acetic acid solution was 16.61 M, and then, at a given solution

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concentration, we observed the amount of ash rejection and organic solid yield at the different

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temperatures and residence times. The contents of pH, temperature and residence time are

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detailed in influence of experimental parameters of the Results and discussion.

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Analysis of the ashless biomass. The treated biomass samples were crushed and separated

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into ~75 µm sizes for further analysis such as the proximate analysis (TGA-701, LECO Co.),

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elemental analysis (TruSpec elemental analyzer and SC-432DR sulfur analyzer, LECO Co.),

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and calorific value analysis (AC600 Semi-auto calorimeter, LECO Co.). In addition, we have

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compared the surface elements between the raw and treated samples to directly identify the


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effectiveness of the proposed treatment method using scanning electron microscope and

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energy dispersive x-ray spectroscopy (SEM-EDX, S-4700, HITACHI). Moreover, the

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prepared biomass samples were ashed in an electric muffle furnace at 600°C for 6 h (ASTM

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Standard Method Number E1755-01) to identify the inorganic chemical content using X-ray

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fluorescence (XRF, Primus II, RIGAKU Co.). The ash fusion temperature was obtained from

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the melting test according to ASTM D1857 (5E-AF4000).

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■ RESULTS AND DISCUSSION

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Influence of experimental parameters. Figure 3 shows the variations in the biomass

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constituents, ash rejection, and solid yield of Miscanthus at different pH (Figure 3-a),

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temperature (Figure 3-b), and residence time (Figure 3-c) in the acetic acid solution,

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respectively. For the acetic acid treatment, the initial ash content decreased with decreasing

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pH at a fixed temperature and time, and the relative composition of the fixed carbon and

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volatile matter increased. In more detail, we obtained the maximum ash rejection at pH value

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of 1.76. The ash rejection increased from 79.25 to 84.92% when pH decreased from 2.24 to

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1.76. However, it decreased down to 75.7% at pH value of 1.28. In contrast, the solid yield

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continuously decreased from 99.23 to 96.62% when varying the pH from 2.24 to 1.28. This

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was due to the increased dissolution of hemicellulose in the stronger (more concentrated) acid

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solution. At temperatures above 170°C in the presence of acids, it is known that the xylose

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(C5) component in biomass is converted into organic acids (e.g. glycolic acid, furfural, and

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acetic acid) with an excessive reaction time.21 Considering the aforementioned reason, our

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proposed pre-treatment method was performed below 100°C in order to minimize the organic

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solid loss. In particular, at pH 1.76, we obtained an ash rejection of 84.92% and a solid yield

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of 98.91%. In addition, above this pH, the efficiency slope became steady, because the acetate



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ions in solution were sufficient to react with potassium and, hence, the excessive ions did not

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contribute towards enhancing the leaching reaction. As a result, we set the initial solution

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concentration as pH 1.76 for testing the influence of temperature and time.

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According to Figure 3-b, the experimental results showed that the ash rejection increased

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from 77.38 to 85.41% with increasing the temperature from 20oC to 60oC, however at higher

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temperature than 60oC the ash rejection decreased down to 70.31%. The solid yield almost

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linearly decreased from 99.91 to 95.46% when rising the temperature from 20oC to 100oC.

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As seen in Figure 3-b, the left-side bar chart shows that volatile matter component variation

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behaves similarly to the ash rejection. The volatile matter reduction at higher than the

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temperature 60oC seems dominant, attributing to the relative increase of ash portion from

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Proximate analysis. According to Eq. (1), the smaller remaining VM after a treatment can

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increase the numerator value, and thereby showing the lower ash rejection. Thus, we decided

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this liquid concentration (pH 1.76) and temperature (60°C) for further experiments to identify

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the variation in ash rejection and solid yield by varying the residence time from 0 to 90 min

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and the results are reported in Figure 3-c. Differing from pH and temperature tests results, we

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observed the maximum ash rejection for the shortest treatment time, of which value was

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85.41% for 10 min. Further study on the shorter residence time than 10 min will give us

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information of more optimal residence time, which is on-going research in our laboratory. In

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more detail, the ash rejection decreased from 85.41 to 80.52% for the treatment time from 10

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to 90 min. The solid yield decreased from 98.75 to 94.89% for the same residence time

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conditions.

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The efficiency slope decreased after 10 min, which was attributed to the solution being more

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diluted with time owing to the water generated as a by-product of the reaction. As shown in

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Figure 4, the rejection efficiency of potassium was evaluated depending on the particle size of

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the biomass at fixed reaction conditions (pH 1.76, 60°C and 10 min), which were already


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shown to be the optimal extraction conditions. As the particle size of Miscanthus decreases,

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the available particle surface area that can be reacted and the rejection efficiency of

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potassium both increase. Due to the low density characteristics of the biomass, the pelletizing

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process is indispensable at the end of the ashless biomass process. For the biomass pelletizing,

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the optimal size of the powdery biomass is about 10 mm, because the rejection efficiency of

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potassium is more than 99wt% at that size. The more pulverization required, the more

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uneconomical is the process due to an increase in the processing cost.

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Basic characteristics of ashless biomass. In general, biomass contains both external ash

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(i.e. soil) and inherent ash. While the external ash in the biomass surface is easily rinsed out

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by water, the inherent ash is hardly extracted due to the strong chemical interactions with the

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carbonic moieties of the cellulose, hemicellulose and lignin constituents.22,23 This is the main

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reason for using the catalytic solution instead of just water to efficiently reduce the ash

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contents in biomass. In this study, the acetic acid with a pH adjustment was used to remove

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the internal ash. The acetic acid was used to mainly remove alkali-based ash such as the

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potassium and sodium.

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Table 1 summarizes the results of the fuel characterization. As shown, we observed a

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significant ash rejection between the initial and the treated sample, while the difference in the

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carbon content was relatively small. In addition, the calorific values of most samples

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increased up to 100-600 kcal/kg, which was attributed to increase in portion of the fixed

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carbon and volatile matter due to the higher rejected ash content per fuel. Miscanthus and

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Corn stalk showed a relatively higher difference in their calorific values after the treatment,

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implying that the proposed treatment method is more effective for the naturally low calorific

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value biomasses in terms of the fuel upgrade. In addition, when the five organic components

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such as carbon (C), hydrogen (H), nitrogen (N), oxygen (O) and sulfur (S) were investigated

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by ultimate analysis, the precursors of the NOx and SOx also decreased by up to 80%, while



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the C, H and O contents remained constant. In other words, our proposed treatment method

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can be evaluated to improve the fuel quality in terms of environmental benefits. Since the

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pre-treatment temperature (60°C) of the ashless process for extracting the ash is very low, the

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biomass studied in this experiment has a very high solid yield, above 97%. The overall mass

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balance for the production process of the ashless biomass using Miscanthus is illustrated in

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Figure 5. The constituents of the biomasses were analyzed according to the NREL Laboratory

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Analytical Procedures (NREL / TP-510-42618, structural carbohydrates and lignin, NREL /

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TP-510-42623, sugars and by-products in the liquid fraction). In case of Miscanthus, the solid

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to aqueous acetic acid solution ratio is 1:8, meaning that 1 kg of many pieces of the

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Miscanthus and 8 kg of solution are mixed for the mineral extraction. As a next step, the

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separation with a filter produces 0.876 kg of ashless biomass fuel and 8.124 kg of solution

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waste, respectively. And then, after extracting 0.1932 kg of the minerals, the remaining

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7.9308 kg acid liquid is further re-used in the extraction process. Therefore, in this study, the

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extraction process was designed for minimizing the wastewater by reusing the acidic solution

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while increasing the yield of the ashless biomass to be used as fuel.

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According to the previous literatures, an ash rejection of 10-50%, with water as a pre-

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treatment solution,15,24,25 and an ash rejection of 20-70% with either an acidic or alkaline

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catalyst solution17,18,20 have been reported. Furthermore, the ratio of the solid (biomass) to

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liquid (solution) and residence time has been reported as 1:20~50 and 4-24 h, respectively.

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However, in this study, the solid-liquid ratio was reduced to 1: 8 or less, and the residence

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time was set to 10 minutes, which was different from the previous study. In addition, we

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prepared the high concentration solution with 16.6 M, which resulted in high ash rejection

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while the previous researchers employed the molar concentration of acetic acid as below 3 M.

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Considering the relatively high ash rejection, solid yield, low solid-liquid ratio, and short

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residence time, we believe that the proposed method is fairly competitive for practical


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applications.

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Comparison of mineral composition. Ash emission is directly related to the initial amount

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of minerals present inside the biomass prior to combustion. As summarized in Table 2, the

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ash content of the raw Miscanthus is 30 times higher than that of ashless-treated biomass

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after combustion. Although the initial compositions of the minerals are dependent on the

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different types of biomass, we found that all the biomass mainly contained a large portion of

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potassium, calcium and silicon. Therefore, the development of the ash rejection method for

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ashless biomass production is important. For instance, the acetic acid solution may be more

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effective on biomass stalk, rather than the shell of the biomass such as PKS and CNS,

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because the higher surface area of the stalk can accelerate the reaction between the solid and

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the liquid agent, which has been confirmed by our comparisons of the ash rejection values

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(See Table 1). Particularly, almost all the groups of biomass used in this experiment showed a

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high rejection value of up to 98-99% for potassium oxide, 70-93% for calcium oxide, and 60-

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95% for silica, which was significantly higher compared to the previous works in the

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literature24-27. Karnowo et al.26 have suggested the potassium extraction method, from which

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the potassium present inside of the rice husk was removed up to 94.0% via the use of bio-oil

289

containing phenolic and dehydrated sugars with the ratio of oil to rice husk of 20 for 24 h.

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Liaw et al.27 reported that the hydrothermal process with the operating conditions of 10Mpa

291

and 150°C removed more than 90% of the calcium contained inside the wood chips. On the

292

other hand, our proposed extraction process uses relatively lower temperature, lower pressure

293

and a shorter residence time than those found in the previous works24-27. We believe that our

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process can maximize the solid yield of the hemicellulose and cellulose.

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As depicted in Figure 6 and 7, the raw and ashless Miscanthus were produced as a char at

296

700-900°C, and the characteristics of the char surface were examined at various

297

magnifications. The difference between two samples was apparent above 1,000 magnification



298

condition. The observed raw Miscanthus char surface was covered with many mineral grains

299

whereas the cylindrical shape was not clearly visible. However, the char surface of the treated

300

Miscanthus was clearer than the raw sample, allowing for a smoother appearance. During the

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surface elemental analysis via the EDX, a few peaks of inorganic matter such as K, Fe, Si, Al,

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and Ca were observed for the raw biomass. All of these materials result in fouling, slagging,

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and clinker during biomass combustion. C and O were the main elements that remained in the

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ashless biomass (Figure 8).

305

Influence of the ash melting behavior by the mineral extraction. The melting

306

characteristics after ash extraction in biomass are shown using a ternary phase diagram.

307

Figure 9 shows the seven biomasses plotted in the diagram with the three points of SiO2 +

308

Al2O3 + Fe2O3 + Na2O + TiO2, K2O + P2O5 + SO3 + Cl2O and CaO + MgO + MnO. According

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to the composition of the mineral oxides, the upper part is composed of the silicon type “S

310

type”, the lower left is the calcium type “C type”, the right side is the potassium type “K

311

type”, and the middle type is “CK type”, which is an intermediate between calcium and

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potassium.28 The melting temperature of the ash is the highest (1,300°C) near silicon and

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calcium, while the lowest temperature is 1,100°C for the K type, and the remaining area is

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1,100-1,300°C. In addition, the upper part of the plot shows the characteristics of high acidity

315

due to the increase in SiO2, Al2O3, and TiO2 belonging to the acid series. To avoid fouling and

316

slagging problems, the biomass should be desirably placed as far as possible from the right

317

corner, in other words, low potassium content. For instance, raw Kenaf and Napiergrass are

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initially located in the right corner (K type) due to the high K2O contents. After the treatment,

319

they migrate to the upper S type and C type, which means the alleviation of the low melting

320

risk during combustion. Likewise, the other biomass are also shifted toward the top or left

321

corner of the triangle by means of this proposed process.

322

Furthermore, the ash fusion temperature and deposition tendency with the base to acid (B/A)


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323

ratio, comparison of silica and alkali, slagging, fouling, and the chlorine indices are evaluated

324

and listed in Table 3. The ash fusion temperature of the biomass is very important, because it

325

can be used to determine the melting behavior of the ash. Most biomass causes slagging and

326

fouling to occur in the boiler, since the initial deformation temperature (IDT) is lower than

327

1,150°C. However, when the alkali metals are extracted from the biomass, the IDT of the

328

ashless biomass is increased to a value higher than 30% compared to that of the raw biomass.

329

When all the biomass are treated under the ashless biomass process, the IDT is higher than

330

1,200°C. Among them, in particular, the IDT of the Kenaf is higher than 1,550°C after the

331

pre-treatment. There is a significant level of ash composition in the biomass;29-34 (a) B/A ratio:

332

>1.0, (b) silica percentage: 4.0, (f) chlorine content: >0.5 (Table 4). The raw biomass possesses a severe

334

slagging/fouling potential as determined based on the B/A ratio and the total alkali indicators,

335

thus presenting a dangerous impact in the boilers. However, the ashless biomass showed a

336

near zero value. Similar to the data presented in Table 2, it can be seen that the chlorine,

337

which causes high temperature corrosion, can be removed by the ashless biomass process by

338

more than 90%. Thus, one can reduce the generation of chlorine compounds such as KCl and

339

HCl, which may corrode the inner part of the boiler or the super-heater and the economizer

340

tube. Therefore, the proposed ash extraction process in this study can contribute to the clean

341

biomass combustion technology in the near future.

342 343

■ CONCLUSIONS

344

In this study, we proposed a acetic acid based pretreatment method to extract the alkali

345

minerals inside herbaceous biomass and investigated the effect of treatment parameters such

346

as concentration, temperature and residence time. The optimal pretreatment conditions for the



347

maximum ash rejection were found to be 1.76 in pH, 60°C in temperature and 10 min in

348

residence time, at which the hemicellulose, cellulose, and lignin losses were minimized. The

349

further investigation of shorter treatment time than 10 min will be the next step in our group.

350

Among seven biomass samples, Kenaf showed the maximum ash rejection of 93.48%. In

351

particular, the potassium and sodium elements in Kenaf, which are the major chemical factors

352

influencing the fouling and slagging in a boiler, were removed up to 99.46 and 100%,

353

respectively. Comparing the wet treatment effect between stalk-based and shell based

354

biomasses, the proposed treatment seems more effective for the stalk-based biomass such as

355

Miscanthus, Kenaf, Corn stalk, Napiergrass, and EFB than the shell-based biomass such as

356

CNS and PKS due to their higher surface areas, of which result was validated by BET

357

measurement. From the experimental results, we opine that the proposed pretreatment

358

method may contribute to the significant reduction of chloride-induced corrosion and the

359

slagging/fouling by ash deposition in a boiler during combustion. Finally, we believe that the

360

proposed method will pave a way to the fuel switching from coals to agricultural residues and

361

herbaceous biomass, which have a shorter harvest interval than the lignocellulosic biomass.

362 363 364 365


Page 16 of 36

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366

■ AUTHOR INFORMATION

367

Corresponding Authors

368

*C.-H. JEON. E-mail: [email protected]. Tel: +82-51-510 3051

369

*Y.-C. CHOI. E-mail: [email protected]. Tel: +82-42-860 3784

370

Notes

371

The authors declare no competing financial interest.

372 373

■ ACKNOWLEDGMENTS

374

This research was supported by the Clean Power Core Technology Program of the Korea

375

Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource

376

from the Ministry of Trade, Industry & Energy, and Republic of Korea (No.

377

20151120100180).

378



379

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Table 1 Fuel characteristics of raw and ashless biomass

Contents

Proximate analysis (dry basis, wt %)

Ultimate analysis (daf, wt %)

Solid yield (daf*, wt %)

V. M.

Ash

F.C.

C

H

N

O

S

O/C

H/C

-

71.26

13.25

15.49

49.30

6.67

1.28

42.72

0.03

0.65

98.75

81.14

1.93

16.93

52.30

5.59

0.36

41.72

0.03

-

75.45

5.13

19.42

49.97

5.84

1.77

42.33

97.98

84.54

0.33

15.13

51.67

6.01

0.07

-

71.57

11.70

16.73

50.77

6.24

97.84

81.67

1.98

16.35

51.43

-

74.05

6.10

19.85

98.52

80.87

2.16

-

79.22

95.25

Raw Miscanthus Ashless miscanthus Raw Kenaf Ashless kenaf Raw Corn stalk Ashless Corn stalk Raw Napierglass Ashless Napierglass Raw EFB Ashless EFB Raw PKS Ashless PKS Raw CNS Ashless CNS

Atomic ratio

HHV [kcal/kg]

LHV [kcal/kg]

1.62

4,020

3,650

0.59

1.28

4,340

4,040

0.08

0.85

0.12

4,390

4,050

42.24

0.02

0.82

0.12

4,290

3,940

1.02

41.82

0.15

0.82

0.12

3,980

3,650

5.91

0.05

42.57

0.03

0.83

0.11

4,590

4,260

48.83

6.41

1.24

43.44

0.08

0.89

0.13

4,270

3,890

16.97

53.56

5.97

1.56

38.85

0.07

0.73

0.11

4,490

4,120

5.01

15.78

55.59

6.30

1.82

36.23

0.07

0.65

0.11

4,870

4,240

81.15

1.40

17.46

53.19

6.30

1.68

38.75

0.08

0.73

0.12

4,630

4,290

-

72.73

3.64

23.63

57.04

5.67

1.51

35.75

0.03

0.63

0.10

4,860

4,510

98.46

75.92

1.78

22.30

56.52

6.00

0.07

37.38

0.03

0.66

0.11

4,880

4,540

-

80.27

1.47

18.26

58.88

6.64

1.49

33.00

0.00

0.56

0.11

5,430

5,010

98.58

78.56

0.62

20.82

54.28

6.01

0.11

39.56

0.04

0.73

0.11

4,970

4,640

*daf : dry and ash free


ACS Sustainable Chemistry & Engineering 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

Page 24 of 36

Table 2 Mineral oxide contents of raw and ashless biomass Mineral oxide (mg/kg) Contents Na2O

MgO

Al2O3

SiO2

P2O5

K2O

CaO

TiO2

MnO

Fe2O3

Raw Miscanthus

398

2,346

7,793

98,187

2,479

14,699

3,685

570

398

2,001

Ashless Miscanthus

21

46

1,004

16,988

166

188

248

104

12

559

2,368

4,142

630

1,579

8,109

28,627

5,372

82

92

267

Ashless Kenaf

0

51

99

499

632

155

1,495

31

11

369

Raw Corn stalk

105

7,485

3,158

50,841

20,818

24,760

7,883

363

129

1,427

0

39

250

16,117

1,212

219

1,269

61

13

575

122

878

1,098

9,103

4,841

38,242

1,768

177

79

4,658

Ashless Napierglass

0

192

2,162

11,490

2,285

214

2,631

296

22

2,329

Raw EFB

60

2,263

1,842

18,467

2,353

21,145

2,723

235

65

906

Ashless EFB

0

0

0

1,844

0

380

380

243

151

10,964

Raw PKS

73

1,862

2,121

16,669

1,137

2,357

10,428

168

44

1,581

Ashless PKS

2

81

5421

6,436

387

795

2,421

280

15

1,967

141

1,068

181

347

1,545

9,913

1,002

100

118

300

0

404

267

542

518

989

1,843

180

188

1,263

Raw Kenaf

Ashless Corn stalk Raw Napierglass

Raw CNS Ashless CNS


Page 25 of 36 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


Table 3 Ash fusion temperature and deposition tendency of raw and ashless biomass Ash fusion temperature (oC)

Slagging/Fouling indices

Contents IDT

ST

HT

FT

B/A ratio

Silica Percentage

Slagging index

Fouling index

Total Alkali

Chlorine contain

Raw Miscanthus

1,174

1,261

1,297

1,329

0.22

0.92

1,198

0.07

11.39

1.22

Ashless Miscanthus

1,363

1,411

1,444

1,464

0.06

0.95

1,379

0.01

1.08

0.01

Raw Kenaf

1,178

1,220

1,263

1,291

17.80

0.14

1,195

82.22

60.47

0.55

Ashless Kenaf

>1,550

>1,550

>1,550

>1,550

3.29

0.21

1,550

0.00

4.63

0.02

Raw Corn stalk

1,085

1,139

1,161

1,178

0.77

0.75

1,100

0.07

21.26

0.65

Ashless Corn stalk

1,344

>1,500

>1,500

>1,500

0.13

0.90

1,375

0.00

1.11

0.01

Raw Napierglass

1,040

1,128

1,189

1,213

4.40

0.55

1,069

0.88

62.92

0.22

Ashless Napierglass

1,258

1,282

1,290

1,301

0.38

0.69

1,264

0.00

0.99

0.01

918

1,124

1,151

1,185

1.32

0.76

964

0.16

42.36

0.31

Ashless EFB

1,393

1,483

1,492

1,498

5.62

0.14

1,412

0.00

2.72

0.01

Raw PKS

1,205

1,220

1,224

1,229

0.86

0.55

1,208

0.17

6.67

0.01

Ashless PKS

1,318

1,452

>1,500

>1,500

0.43

0.59

1,354

0.01

4.47

0.01

Raw CNS

1,084

1,399

1,461

>1,500

19.78

0.13

1,159

18.98

68.33

0.10

Ashless CNS

1,210

1,258

1,261

1,266

4.55

0.13

1,220

0.00

15.96

0.01

Raw EFB


ACS Sustainable Chemistry & Engineering 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

Page 26 of 36

Table 4 Summary of deposition tendency Indices

Low

Medium

High

Severe

(a) Base/acid ratio : (Fe2O3 + CaO + MgO + Na2O + K2O)/(SiO2 + Al2O3 + TiO2)

< 0.5

0.5-0.7

0.7-1.0

> 1.0

(b) Silica percentage : (SiO2)/( SiO2 + Fe2O3 + CaO + MgO) × 100

> 50

50-30

30-5

1,340

1,340-1,250

1,250-1,150

< 1,150

(d) Fouling index : (Fe2O3 + CaO + MgO + Na2O + K2O)/(SiO2 + Al2O3 + TiO2) × Na2O

< 0.2

0.2-0.5

0.5-1.0

> 1.0

(e) Total alkali : Na2O + K2O

< 2.0

2.0-3.0

3.0-4.0

> 4.0

(f) Chlorine contain

< 0.2

0.2-0.3

0.3-0.5

> 0.5

(c) Slagging index : (Rs) = 4 IT + HT/5


Page 27 of 36 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


Figure 1 Flow chart of the proposed ashless biomass process

27



Figure 2 S/L ratio during ash rejection experiments

28


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Figure 3 Comparison of constituents, ash rejection and solid yield of Miscanthus at different pH ((a), fixed in 40oC and 10 min), temperature ((b), fixed in 16.61M CH3COOH and 10 min) and residence time ((c), fixed in 16.61M CH3COOH and 60oC)

29



Figure 4 The rejection efficiency of potassium from Miscanthus depending on particle size

30


Page 30 of 36

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Figure 5 The overall mass balance for the production of ashless biomass (Miscanthus)

31



Figure 6 SEM on the surface of raw Miscanthus char

32


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Figure 7 SEM on the surface of ashless Miscanthus char

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Figure 8 EDX on the surface of raw and ashless Miscanthus char at 900oC

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Figure 9 Ternary phase diagram using new chemical classification system

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Synopsis: We suggest the biomass pretreatment method, including the successive acidic solutions based treatment at below 60oC for 10 min.

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Techno-economical method for the removal of alkali metals from

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