Characteristics of Particulate Matter Emitted from Agricultural Biomass

May 30, 2017 - In this work, the emission of particulate matter (PM) from combustion of agricultural biomass was investigated in comparison to woody b...
0 downloads 0 Views 1MB Size
Subscriber access provided by Binghamton University | Libraries

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

Characteristics of particulate matter emitted from

2

agricultural biomass combustion

3

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

4

1

5

University of Science and Technology, Wuhan 430074, PR China

6

2

7

Henan 450002, PR China

8

*

9

[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,

10

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11

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

12

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

13

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

14

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

15

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

16

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

17

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

18

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

19

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

20

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

21

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

22

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

23

resource.

24

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

ACS Paragon Plus Environment

Page 2 of 28

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

25

1. Introduction

26

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

27

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

28

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

29

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

30

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

31

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

32

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

33

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

34

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

35

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

36

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

37

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

38

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

39

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

40

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

41

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

42

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

43

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

44

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

45

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

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

46

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

47

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

48

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

49

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

50

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

51

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

52

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

53

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

54

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

55

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

56

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

57

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

58

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

59

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

60

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

61

grains.

62

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

63

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

64

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

65

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

66

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

67

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

ACS Paragon Plus Environment

Page 4 of 28

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

68

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

69

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

70

characteristics in the combustion.

71

2. Experimental

72

2.1 Fuel properties

73

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

74

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

75

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

76

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

77

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

78

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

79

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

80

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

81

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

82

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

83

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

84

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

85

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

86

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

87

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

88

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

89

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

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

90

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

91

2.2 Combustion experiment

92

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

93

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

94

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

95

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

96

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

97

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

98

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

99

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

100

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

101

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

102

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

103

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

104

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

105

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

106

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

107

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

108

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

109

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,

110

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

111

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

ACS Paragon Plus Environment

Page 6 of 28

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

112

condensation and gravitational settling deposition during the sampling process.

113

2.3 PM sample

114

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

115

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

116

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

117

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

118

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

119

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

120

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

121

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

122

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

123

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

124

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

125

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

126

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

127

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

128

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

129

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

130

3. Result and discussion

131

3.1 Emission of particulate matter from combustion of different biomass fuels

132

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

133

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

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

134

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

135

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

136

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

137

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

138

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

139

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

140

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

141

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

142

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

143

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

144

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

145

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

146

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

147

pathways for agricultural biomass and woody biomass.

148

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

149

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

150

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

151

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

152

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

153

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

154

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

155

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

ACS Paragon Plus Environment

Page 8 of 28

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

156

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

157

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

158

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

159

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

160

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

161

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

162

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

163

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

164

mode peak for these two biomass.

165

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

166

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

167

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

168

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

169

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

170

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

171

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

172

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

173

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

174

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

175

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

176

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

177

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

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

178

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

179

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

180

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

181

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

182

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

183

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

184

the PM particles.

185

3.2 Effect of temperature on the PM emission

186

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

187

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

188

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

189

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

190

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

191

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

192

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

193

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

194

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

195

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

196

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

197

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

198

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

199

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

ACS Paragon Plus Environment

Page 10 of 28

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

200

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

201

accordingly.

202

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

203

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

204

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

205

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

206

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

207

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

208

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

209

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

210

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

211

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

212

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

213

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

214

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

215

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

216

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

217

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

218

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

219

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

220

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

221

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

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

222

3.3 SEM-EDS analysis of the PM

223

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

224

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

225

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

226

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

227

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

228

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

229

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

230

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

231

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

232

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

233

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

234

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

235

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

236

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

237

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

238

to cotton stalk is proposed.

239

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

240

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

241

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

242

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

243

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

ACS Paragon Plus Environment

Page 12 of 28

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

244

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

245

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

246

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

247

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

249

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

250

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

251

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

252

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

253

through complex physical and chemical effects[9].

254

4. Conclusion

255

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

256

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

257

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

258

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

259

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

260

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

261

compounds.

262

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

263

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

264

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

265

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

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

266

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

267

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

268

important role during combustion of cornstalk and rice husk.

269 270

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

271

ACS Paragon Plus Environment

Page 14 of 28

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

272

Acknowledgement

273

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

274

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

275

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

276

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

277

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

278

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

279

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

280

opinions and gave a lot of practical guidance.

281

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

282

Reference

283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324

[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 (