Characteristics of particulate matter emitted from agricultural biomass

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Characteristics of particulate matter emitted from agricultural biomass combustion Wei Yang, Youjian Zhu, Wei Cheng, Huiying Sang, Haiping Yang, and Hanping Chen Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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Characteristics of particulate matter emitted from

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agricultural biomass combustion

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Wei Yang1, Youjian Zhu2, Wei Cheng1, Huiying Sang1, Haiping Yang*1, Hanping Chen1

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University of Science and Technology, Wuhan 430074, PR China

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Henan 450002, PR China

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*

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[email protected]

State key Laboratory of Coal Combustion, Department of Energy and Power, Huazhong

School of Energy and Power Engineering, Zhengzhou University of Light Industry, Zhengzhou,

Corresponding author: Haiping Yang, +86-27-87559358, +86-27-87545526,

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ACS Paragon Plus Environment

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Abstract：：In this work, the emission of particulate matter from combustion of agricultural

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biomass was investigated in comparison with woody biomass. The mechanism of particulate

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matter emission was studied by means of mass-based particle size distribution, inorganics

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elemental component analysis and morphology at variant combustion temperatures, and different

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biomass feedstocks. The mass-based particle size distributions (PSDs) of PM10 of cotton stalk, rice

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husk and camphorwood exhibits a bimodal distribution, while cornstalk with a unimodal

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distribution. The emission of PM10 of agricultural biomass is much higher than that of woody

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biomass, and it is mainly composed of PM1 in which Na, K are enriched as alkali metal chloride

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and sulfide. On the other hand, Mg and Ca are enriched as the main inorganic compounds in

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PM1-10 for woody biomass. Higher combustion temperature is favorable for the formation of fine

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PM particles against a reduction of PM10. PM1 and PM1-10 formation mechanisms are different for

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different biomass feedstocks, and their formation pathways are hereby proposed for each biomass

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resource.

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Keywords：：agricultural biomass, particulate matter, combustion, AAEM


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1. Introduction

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In the biomass combustion process, a part of volatile inorganic species, such as alkali metal

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containing compound (KOH, KCl), is released to the gas phase and then form fine particulate

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matters (PM) via complex chemical and physical reactions[1]. PMs could cause serious

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environmental pollution and pose a great threat on human health[2], which is one of key issues

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limiting effective utilization of rich agricultural biomass residues in many countries, particularly

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in China for the deteriorating atmosphere environment on the background of fast economic growth.

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Thus, more and more attention is paid to the PM research recently.

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Previous research[3-6] on PM emission mainly focuses on coal combustion. Minerals as clay

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and pyrite are abundant in coal ash[7, 8] and are transformed into ash particles mainly in the range

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of 1-10 µm via fragmentation and coalescence[3] in coal combustion. The trace elements (Na, Zn,

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etc) in coal also evaporate and subsequently condense to form PM1 (particulate matter diameter

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less than 1 micron) particles[9] in a less content during the combustion process. The biomass fuel,

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on the other hand, differ significantly from coal in properties and result in different PM emission

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characteristics in the combustion. Comparing to coal, more PM is generated in wood

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combustion[10, 11], and also, the physical characteristics and elemental composition of the PM are

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changed significantly[9, 12, 13]. Sippula et al.[14] found that the inorganic components in PM1 during

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Finnish wood combustion were K2SO4, KCl, K2CO3, KOH, and organic species. The PM

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formation pathway was suggested from thermodynamic calculation as follows: K2SO4 in the vapor

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starts to form small particles by homogeneous/heterogeneous condensation at 950-1050 oC,

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followed by the condensation of K2CO3, KOH and KCl as the temperature decreases.


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Agricultural biomass residues are abundant in China and are considered as the potential fuel

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to replace fossil fuel to a certain extent. Different from woody biomass, agricultural biomass has

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higher ash content (especially higher alkali metal content) and leads to different PM emission

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behavior[15]. More PM is generated in the combustion of agricultural biomass than woody

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biomass[16]. Carroll and Finnan[17] found that the total PM emission from wood combustion was in

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22-51 mg/Nm3, while this value increased to 100-399 mg/Nm3 for straw biomass. Garcia-Maraver

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et al.[18] found that the total PM emissions were in the range of 50-100 mg/Nm3 in the case of the

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pine pellets, while the values increased 100-600 mg/Nm3 for the olive biomass pellets. Results

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also indicated that the produced PM was dominated by PM2.5 (particulate matter diameter less than

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2.5 microns) for olive biomass. The ultra-fine particles were formed by homogeneous nucleation

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of alkali vapors, they also pointed out that heterogeneous condensation and particle growth played

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an important roles in the PM1 formation considering the higher particle concentration compared to

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pine wood[18]. On the other hand, both of fragmentation of minerals (rich in Mg, Ca, P, Fe and Si)

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and condensation of alkali vapor and sulfates contributed to the coarse particle formation[18].

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Additionally, Bäfver et al.[19] detected certain amount of P in the PM during combustion of oat

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grains.

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These studies, however, are usually conducted in the pellet burner under constant operation

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conditions[17, 20-22]. Researches about the effects of operation parameters on the PM emissions are

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limited. It is imperative to illuminate the PM formation mechanism of different agricultural

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biomass fuels to facilitate a clean and efficient utilization. In this work, the main purpose is to

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further study the formation mechanism of PM from agricultural biomass combustion. Three

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typical Chinese agricultural biomass residues were selected to investigate the effects of the


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feedstock properties on the PM emission characteristics in the combustion. Cotton stalk was

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selected as the representative fuel to investigate the effects of temperature on the PM emission

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characteristics in the combustion.

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2. Experimental

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2.1 Fuel properties

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The biomass fuels used in the experiment include three agricultural biomass residues, rice

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husk, cornstalk and cotton stalk, and one woody biomass, camphorwood. The agricultural biomass

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residues are collected in the rural area of Hubei Province and are selected due to the fact that these

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fuels are abundant in China and are considered as potential biomass fuels for heat and electricity

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production[23-26]. Camphorwood, which is relatively common in the local and is a potential

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renewable energy for heat production, was selected for comparison. The samples were crushed

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and sieved to a particle size of 125-177 µm. The particle size was chosen to ensure complete

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combustion and avoid operational difficulties based on previous results[4, 27, 28]. The proximate and

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ultimate analysis were made by means of the SDTGA-2000 industrial analyzer (Las Navas, Spain)

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and EL-2 type elemental analyzer (Vario, Germany), respectively. The heating value of the

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samples was analyzed using the automatic calorimeter (model: 6300, America). The low heating

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value, proximate and ultimate analysis of the fuels are presented in Table 1. The sample was ashed

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at 823 K and then analyzed using an X-ray fluorescence spectrometer (Jasco FP-6500, Japan) to

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get the chemical composition. The result is presented in Table 2. It can be seen from Table 1 that

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the ash content of camphorwood is the lowest, and the volatile matter is the highest. The contents

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of N and S in three agricultural biomass residues are higher than those in camphorwood. The K

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content in cornstalk is the highest as shown in Table 2, while the Mg/Ca content of cotton stalk


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and camphorwood is relatively high. Rice husk ash is mainly composed of silica.

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2.2 Combustion experiment

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The combustion tests were carried out in a drop tube furnace (DTF) as shown in Fig 1. The

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system is mainly composed of a Sankyo Piotech Micro Feeder (MFEV-10), an electrical heated

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furnace, a tube reactor, a gas supply section, a particulate matter collection section and related

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pipeline. The reactor is made of corundum tube with a height of 2000 mm and an inner diameter

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of 52 mm. The fuel was fed into the reactor at 0.15 g/min together with a primary air (0.5 L/min).

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To ensure complete combustion, a secondary air was fed at 1.5 L/min into the external reactor

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chamber, in which it is heated up and then entered the internal reaction chamber. The residence

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time of the particles in the furnace is about 3.6 seconds.

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The particle size mass distribution was measured by a Dekati low pressure impactor (DLPI)

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(Dekati, Finland). The morphology and chemical composition of the PM was also analyzed to

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provide information on the formation mechanism of PM. The combustion temperature was

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changed from 1073 to 1473 K for study on temperature effect. The burnout rates of the fuels were

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larger than 95% for all the tests. After combustion, the gas sample was firstly diluted by nitrogen

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at 8L/min to prevent secondary condensation reactions and ensure a sufficient sample flow rate for

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the instrument. Then the gas sample went through a cyclone and DLPI to collect fly ash (particles

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larger than 10 µm) and PM10 (particulate matter diameter less than 10 microns) samples,

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respectively. PM10 sample is divided into 13 stages with the corresponding 50% aerodynamic

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cutoff diameters are 0.028, 0.057, 0.094, 0.15, 0.26, 0.38, 0.61, 0.94, 1.58, 2.36, 3.95, 6.6, 9.8 µm,

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respectively[27]. The detailed operating procedures of DLPI have been previously described[4, 29, 30].

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All the sampling devices and related pipelines were kept at 393K to avoid possible acid gas


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condensation and gravitational settling deposition during the sampling process.

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2.3 PM sample

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In this work, each combustion experiment was conducted for three times with aluminum

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membrane to validate the repeatability and for another three times with polycarbonate membrane

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to get samples for the following analysis. The standard deviations indicated that the

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reproducibility of measurement is in acceptable range. The PM collected from the aluminum and

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polycarbonate membranes were used for gravimetric and elemental analysis, respectively.

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For the gravimetric analysis, the collected aluminum substrates were weighed by a

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micro-electrical balance (0.001 mg, Sartorius M2P, Germany) to obtain the mass size distribution

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of the PM. Polycarbonate substrates were used for elemental and morphological analysis. The

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collected sample was digested in a microwave oven with a mixture of HNO3(70% v/v)/H2O2(20%

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v/v)/HF(10% v/v), and then the alkali metal and alkaline earth metal (AAEM) species content was

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analyzed by an inductively coupled plasma mass spectrometry (ELAN DRC-e, America.

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Detection limit: 0.1-10 ppm). Cl was analyzed by ion chromatograph (881 Compact IC pro,

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Switzerland. Detection limit: 0.1-10 ppm) following rinsed in deionized water for 24 hours. For

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the surface morphological and elemental analysis of the PM, samples were mounted on carbon

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tape and then analyzed by an environmental scanning electron microscope with energy dispersive

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X-ray analysis in secondary electron mode (ESEM-EDS, Quanta 200, Netherlands).

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3. Result and discussion

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3.1 Emission of particulate matter from combustion of different biomass fuels

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The particle size distributions (PSDs) of PM10 from the combustion of four fuels at 1273K

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are shown in Fig 2. It is clear that the mass-based particle size distributions (PSDs) of PM10 from


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the combustion of cornstalk presents a unimodal distribution with the peak at around 0.6 µm.

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While for the other three fuels a bimodal distribution is presented with the coarse mode at around

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4 µm and the fine mode at 0.6, 0.3 and 0.2 µm respectively for cotton stalk, rice husk and

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camphorwood. This is consistent with the previous study[31, 32]. The yields of PM0.1, PM1, PM2.5,

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PM10, PM1-2.5, PM2.5-10, PM1-10 from the combustion experiments are presented in Table 3. As can

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be seen, the PM10 emissions of cotton stalk and cornstalk are 45.58 and 88.35 mg/Nm3, which are

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greatly higher than camphorwood (6.10 mg/Nm3). However, the PM10 emission of rice husk is

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quite low (12.37 mg/Nm3) in spite of its high ash content (16.20 wt%). The rice husk ash is

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dominated by SiO2 (96.36 wt%) which has a high melting point and would not evaporate to the

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gas phase under the current combustion temperature[15]. On the other hand, SiO2 could also inhibit

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the evaporation of alkali metal via the formation of silicates[31-33]. This caused the low PM

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emission from rice husk. It can be observed that the particles from agricultural biomass are mainly

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composed of PM1, which is different from camphorwood, which suggests different PM formation

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pathways for agricultural biomass and woody biomass.

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In order to explore the formation pathway of the PM during combustion, the key inorganic

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elements (Na, K, Ca, Mg, Cl) in each particle size range was analyzed and the results are

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presented in Fig 3. As can been seen, Na, K and Cl are enriched in PM1 and depleted in PM1-10 for

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all the agricultural biomass, whereas Ca and Mg are enriched in PM1-10. During the combustion

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process, the volatile alkali chlorides and alkali hydroxides vapors are initially released from the

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fuel[1]. Then ultra-fine-particles are formed when the inorganic vapors reach the condensation

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temperature through homogeneous nucleation[9]. Meanwhile, the inorganic vapors could also

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condense on the newly-formed/existing particles through heterogeneous condensation and


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increase the particle size at the same time[9, 34]. Additionally, agglomeration and coalescence also

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contributes to PM1 formation[9, 35]. On the other hand, it can be observed in Fig 2 and Fig 3 that the

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fine mode peak for cotton stalk and cornstalk situate at 0.6 µm which is higher than that of rice

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husk (0.3 µm) and camphorwood (0.2 µm). This can be explained by the enhanced particle

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agglomeration and coalescence[18] for these two biomass. As indicated earlier, the total PM

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emission of cotton stalk and cornstalk are significantly higher than camphorwood and rice husk.

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This makes heterogeneous condensation on the pre-existing particles, agglomeration and

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coalescence are prevailing during the particle formation process[18], which explains the higher fine

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mode peak for these two biomass.

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The coarse mode peak situates at approximately 4 µm for all biomass fuels if appears as seen

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in Fig 2. However, different formation pathways are suggested based on the elemental

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composition of the PM. Mg and Ca show a unimodal distribution in PM10 for cotton stalk and

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camphorwood and its concentration is under detection limit in PM1. The prevailing existence of

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Mg and Ca in PM1-10 was reported in previous study on combustion of torrefied and spent mallee

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leaf[31, 32]. It is probably attributed to two different formation mechanisms. The first one is the

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direct release of organic bound Ca and Mg during the devolatilization and char combustion

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process[36]. The released Ca and Mg will be oxidized to CaO and MgO[37], and subsequently form

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large Ca/Mg-rich particles through catalyzed sintering[31] and heterogeneous condensation of

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volatile inorganic species (e.g. KCl, K2SO4)[34]. The existence of certain amount K, Na and Cl in

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PM1-10, as seen from Fig 3, confirms the heterogeneous condensation on the existing particles. On

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the other hand, the AAEM compounds could also react with minerals to form silicates and

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phosphates and generate PM1-10 via coalescence and fragmentation[13, 31].


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However, these mechanisms can not be applied to rice husk and cornstalk due to the

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undetectable Mg and Ca contents in the particulate matters. The rice husk ash is dominated by Si

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so that PM1-10 is expectedly composed of Si compounds as SiO2 and silicates. For cornstalk, alkali

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silicates are probably responsible for the PM1-10 formation[38] in light of the high alkali metal and

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silicon content in the ash. Unfortunately, Si is not measured in this work. The PM1-10 formation

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pathway for these two fuels will be discussed later in Section 3.3 with the help of EDX analysis of

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the PM particles.

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3.2 Effect of temperature on the PM emission

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Cotton stalk was chosen to study the effect of temperature on PM emissions considering: 1)

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the dominated roles of AAEM species; and 2) the comparatively alkali and alkali metal content in

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the ash. The PSDs of PM10 at different temperature is shown in Fig 4. Although bimodal

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distributions are presented at all the combustion temperatures, clear differences in the PM1 and

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total PM emission are observed. The total PM10 emission at 1073 K is 90.59 mg/Nm3 and

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decreases to 16.30 mg/Nm3 at 1473 K as seen in Table 3. PM1 emission also decreases. This

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conflicts with the previous studies[39-41] that the total PM1 emission increased along with the

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notably increase of PM0.1 as the temperature increased from 1473 to 1723 K during the

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combustion of Chinese bituminous coal. Generally the enhanced PM1 emission at higher

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temperature results from the higher vaporization of volatile inorganic elements such as alkali

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vapors and chlorides[39-41]. The temperature effect on PM1-10 is much more complicated. Firstly

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higher temperature aggravates the char fragments and leads to the formation of smaller char

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particles[27]. This reduces the possibility of ash particles contact/interaction and thus weakens the

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melting, agglomeration and coalescence effects[27]. The minerals matter (especially these have a


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particle size less than 10 µm) could also release after the burn of char fragment and form PM1-10

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accordingly.

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In our experiment, the PM0.1 tends to increase slightly as the temperature increases. This

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implies the intensified vaporization of alkali vapors and chlorides under high temperature. On the

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other hand, PM10 emission decreases significantly as aforementioned. There are two possible

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reasons for the decrease of PM10. First, the high PM emission at 1073 and 1173 K could be due to

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the incomplete combustion. Thermogravimetric analysis was made to check the combustible

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fraction in the PM samples. Weight losses were found for the PM samples collected at 1073 and

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1173 K. This suggests an incomplete combustion and certain amount of carbon-containing

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compounds such as soot exists in the PM sample. The content of key inorganic elements (Na, K,

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Ca, Mg, Cl) in each range of particle size are presented in Fig 5. It can be observed that the Na and

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K contents in PM decrease notably with a slightly increase of Ca and Mg as the combustion

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temperature increases. Therefore, it is obvious that other reasons are also responsible for the PM

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losses under high temperature. Previous study[42] indicated that alkali hydroxides and chloride

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could react with the corundum tube (consist of high-purity alumina) by reacting with Al2O3 and

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form instable NaAlO2. Alkali chloride could also deposit on the reactor wall through van der

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Waals forces[42]. Therefore, it is speculated that certain amount of alkali compounds react with the

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corundum tube or deposit on the surface of tube under high combustion temperature and

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eventually reduce the K and Na content in PM10 emission. The slightly increase of Ca and Mg

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content in PM1-10 under high combustion temperature could be attributed to the intensified

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particles collision and fragments, which prompted Mg and Ca element to be broken into tiny

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particles and react with other elements to form particle matters[5, 6].


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3.3 SEM-EDS analysis of the PM

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The typical SEM images of PM samples are shown in Fig 6, and the corresponding EDS results

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are given in Table 4. Fig 6 a-c shows typical SEM images of PM samples from the combustion of

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cotton stalk. PM particles shown in Fig 6 a and b mainly composed of submicrometer sized

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agglomerates. The EDX analysis indicates that K, Cl, and S are the dominating elements. This

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further confirms that the fine mode of PM1 is formed by homogeneous nucleation, heterogeneous

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condensation of alkali chlorides and sulfates and agglomeration. For Fig 6 c, few fine spherical

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particles are also found except for the irregularly shaped large agglomerate particles. From Table 4,

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it can be seen that the Ca, Mg, Si and P are also present in addition to K, Cl and S. These elements

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generally are less volatile and tend to retained in the residual ash. This further confirms the two

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formation pathways for PM1-10 as proposed in section 3.1: 1）fragments and direct transformation

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of the non-volatile species; and 2) condensation of the alkali compounds on the surface of the

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coarse particles. Elemental composition of typical PM particles from camphorwood combustion is

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presented in Fig 6 j-l. The morphological characteristic is similar to cotton stalk except for the

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diminished agglomeration extent due to the low concentration of PM. Meanwhile, The EDX

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analysis also indicates a similar elemental composition. Thus a similar PM formation mechanism

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to cotton stalk is proposed.

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Fig 6 d-f shows typical SEM images of PM samples from the combustion of cornstalk. It shows

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that PM1 particle shown in Fig 6 d and e mainly composed of particles agglomerates with K and

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Cl as the main elements (Table 3). Irregularly shaped agglomerates and fine spherical particles are

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abundant for Fig 6 f. The EDX analysis indicates that it was mainly composed by K, Cl, Si with a

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less content of P, Ca, Na and Mg. The spherical particles imply the formation of melted alkali


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silicates in PM2.5-10. This confirms our previous speculation and indicates the dominated roles of

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alkali silicates and phosphates in the PM2.5-10 formation.

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SEM images and EDX analysis of typical PM particles from rice husk combustion are presented

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in Fig 6 g-i and Table 4, respectively. It can be seen that PM particle shown in Fig 6 g and h are

248

mainly composed of K and Cl suggest as similar PM1 formation mechanism with cotton stalk and

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cornstalk. For the coarse mode particles shown in Fig 6 i, Si and Al are prevailing. Two pathways

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are responsible for this phenomena: 1) Si is oxidized into small particles in the form of silicon

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oxide and form particulate matter with the size of 1-10 µm[43]; and 2) the reaction between SiO2

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and alkali metal leads to the formation of the alkali metal silicate and later forms PM1-10 particle

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through complex physical and chemical effects[9].

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4. Conclusion

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(1) The mass-based particle size distribution of PM10 from the combustion of all biomass

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fuels exhibits a bimodal distribution, except for cornstalk which shows a unimodal distribution.

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The total PM emission of agricultural biomass is much higher than that of woody biomass. The

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PM emitted from agricultural biomass combustion are mostly small particles under 1µm in

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aerodynamic diameter, and mainly composed of Na, K as alkali metal chloride and sulfide.

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Whereas for woody biomass PM1-10 is dominant with Mg and Ca as the main inorganic

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compounds.

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(2) For PM1, vaporization-condensation of alkali compound is the main formation pathway

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for all biomass. However, heterogeneous condensation, agglomeration and coalescence contribute

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significantly in PM1 formation during the combustion of cotton stalk and cornstalk.

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(3) For PM1-10, the following two formation pathways are proposed: 1) direct transformation


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of Ca/Mg and Si-rich particles with subsequent heterogeneous condensation; 2) formation of

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silicates and phosphates. In addition, the formation of alkali silicates and silicon dioxide plays an

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important role during combustion of cornstalk and rice husk.

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(4) The total PM10 emission decreases and PM0.1 emission increases when the combustion temperature increases from 1073-1473 K.

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Acknowledgement

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The authors wish to express sincere thanks for the National Natural Science Foundation of China

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(51476067 and 51622604), the financial support from the National Basic Research Program of

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China (2013CB228102), and the Special Fund for Agro-scientific Research in the Public Interest

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(201303095). The authors are also grateful for the assistance on the experimental studies provided

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by the Analytical and Testing Center in Huazhong University of Science & Technology

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(http://atc.hust.edu.cn), Wuhan 430074, China. The authors also would like to express our heartfelt

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thanks for Professor Wennan Zhang from Mid Sweden University, who put forward many valuable

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opinions and gave a lot of practical guidance.

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Reference

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[1] Dan Bostrom, Nils Skoglund, Alejandro Grimm, et al., Ash Transformation Chemistry during Combustion of Biomass. Energy & Fuels 2012, 26: 85-93. [2] Joey Villeneuve, Joahnn H. Palacios, Philippe Savoie, et al., A critical review of emission standards and regulations regarding biomass combustion in small scale units (

Characteristics of particulate matter emitted from agricultural biomass

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