Release of Alkalis and Chlorine from Combustion of Waste Pinewood

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Release of alkalis and chlorine from combustion of waste pinewood in a fixed bed Xiaoxiao Meng, Wei Zhou, Emad Rokni, Guoyou Chen, Rui Sun, and Yiannis Angelo Levendis Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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Energy & Fuels

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Release of alkalis and chlorine from combustion of waste pinewood in

2

a fixed bed

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Xiaoxiao Meng1, Wei Zhou1, *, Emad Rokni2, Guoyou Chen3, Rui Sun1, *, Yiannis A. Levendis2,*

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1School

5 6

2 3

of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China

Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA, USA

Agricultural Products Quality and Safety Research Institute, Heilongjiang Academy of Agricultural Sciences, Harbin

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150001, PR China

8

9

Abstract

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Combustion of biomass in a boiler releases alkali metals and chlorine which, together

11

with silicon and sulfur, are responsible for slagging, fouling, corrosion and particulate

12

emissions. This research investigated the effects of the primary (under-fire) air flow-

13

rate, ṁair, and its preheating temperature on the ignition and burning rates of

14

pinewood chips in a laboratory fixed bed furnace, and on the release of alkali metals

15

and alkali earth metals (potassium (K), sodium (Na), calcium (Ca), magnesium (Mg))

16

and chlorine. The air flow rate, ṁair, through the bed was varied in the range of 0.085-

17

0.237 kg m-2s-1, resulting in an overall excess primary air coefficient λ varying from

18

0.58 to 1.1. The air was also preheated in the range of 20-135 °C. Results showed that

19

increasing either the ṁair or the air preheat temperature increased the flame

20

propagation rate (ignition rate) and the mass burning rate of the fuel. Moreover, the

21

release of Cl was nearly-complete (>99%) in all examined cases, whereas the release

1

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of alkalis was only partial. Calcium was the most predominant alkali in the pinewood,

23

however, potassium was the predominant alkali in the released gases. The mass

24

fraction of Na in the pinewood was much lower than that of K, but it was released

25

more comprehensively. Increasing the air flow rate enhanced the release of K and Na

26

significantly, while it enhanced the release of Ca and Mg only slightly. Preheating the

27

primary air preferentially increased the migration of K to the gas phase, whereas Na,

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Ca and Mg were affected only mildly. The preheated air promoted the transfer of

29

chlorine to HCl. Overall, moderately high primary air flow rates generate globally fuel

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lean conditions and mildly preheated air can enhance the mass burning rate of

31

pinewood and its conversion to fully- and partially-oxidized gases. However, they

32

result in enhanced gasification of the alkalis in the biomass. In the case of pinewood

33

this may be a minor concern, as the absolute values of such emissions are low relative

34

to other biomass fuels.

35 36

Key Words: Alkali metals, chlorine, pinewood residue, combustion, preheated air.

37 38

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1. Introduction

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In recent years, concerns about the environmental consequences of the ever

41

increasing greenhouse gas (GHG) emissions heightened the global interest in the

42

development of renewable energy sources, such as biomass, solar, wind, tide, etc.

43

1,2,3,4.

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for electricity generation, with probably annual increases averaging 2.9% from 2012

45

to 2040 5. Renewable biomass is a promising fuel because of low net CO2 emissions

46

and, thus, it can be used to substitute fossil fuels for heat and power generation 4,6.

As a result, renewables are expected to be the fastest growing source of energy

47

Combustion is a widely used technology for energy harvesting from biomass

48

throughout humankind`s history 7. However, biomass generally contains a number of

49

alkali and alkaline earth metals (AAEMs), Cl, Fe and S, etc., which can cause several

50

issues, such as fouling, slagging and corrosion of heat transfer surfaces, as well as

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particulate emissions 8,9. These issues can have a detrimental effect on the operational

52

efficiency and on the lifetime of combustion equipment

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typically the most important fouling species, and the non-metallic element of Cl in

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those species (e.g. KCl) may accelerate corrosion of equipment during biomass

55

thermochemical conversion 11,12. Therefore, a better understanding of the release and

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transformation of AAEMs, particularly K, and Cl during biomass combustion can help

57

address this particular technical problem caused by the biomass utilization.

10.

Therein, potassium is

3


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In addition, it is known that K is the most abundant cation in biomass for plant

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nutrition and growth etc., and as an important anion, Cl is responsible for balance 13;

60

both of K and Cl ions are highly mobile. It has been reported that K transformations

61

may occur during both the devolatilization and the char burnout stages, especially at

62

a high temperature

63

metal K is related to inorganic salts or it is organically bound to the carbonaceous

64

matrix 16,17,18. During the devolatilization stage, inorganic and organic K can transfer

65

to each other, therein the release of K may be promoted as the organic matrix can

66

decompose even at a low temperature 14,19.

14,15,16.

In addition, it is typically either attested that the alkali

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Baxter et al. 8 burned woody and herbaceous materials in biomass-fired boiler,

68

and reported that chlorine facilitates the mobility of many inorganic compounds,

69

particularly potassium. Moreover, they concluded that the concentration of chlorine

70

in biomass often dictates the amount of alkali metal vaporized during combustion

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more strongly than the concentration of alkali in the biomass. Biomass fuels with high

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potassium, chlorine, silicon and sulfur contents, such as straws and other herbaceous

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plants, are particularly problematic as they cause furnace slagging and rapid fouling of

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heat-transfer surfaces 8. In addition, the chloride such as KCl and NaCl, are known to

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increase the corrosion rate of stainless-steel surfaces of superheater heat exchangers

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used inside boilers at temperatures lower than the melting temperature of the salt

77

and their mixtures 20. It was reposted that both KCl and NaCl are equally corrosive in 4


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the steam boilers, despite their different corrosive structures [20]. Therefore, during

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biomass combustion attention should be paid to the emissions of alkali metals and HCl,

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which cause deposition to boiler surfaces, corrosion, and emit dust and incomplete

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combustion species, including CO and particulate matter (PM) 21. All of these problems

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can disturb the combustion process, impair efficiency and cause undesirable

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shutdowns. Furthermore, if HCl is released to the atmosphere, it can cause acid rain

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and respiratory diseases 22,23,24,25. Also, HCl can be precursor to dioxin formation in the

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presence of unburned carbon, as reported by Björkman and Strömberg 13.

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Previous work by Johansen et al. 26 investigated the release of Cl, S, and K during

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combustion of wood chips and corn stover, and observed that chlorine was nearly

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completely released, whereas K was only partially released. Meng et al.

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various blends of pinewood (a low Cl, K, S biomass) and corn straw (a high Cl, K, S

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biomass) in a fixed bed and they also found that the release of chlorine was nearly

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complete. Moreover, they reported that the molar Cl/K ratio increased in the fuel

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blend as the amount of corn straw increased, resulting in enhanced alkali release to

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the gas phase, in agreement with the aforesaid conclusion of Baxter et al. [8]. Ren et

94

al. 4 addressed the emissions of hydrogen chloride from burning corn straw (a high Cl,

95

K, S biomass) in a fixed bed and found that chlorine was only partially released to the

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gas phase. Moreover, they found that increasing the flowrate of the primary air

97

through the bed (ṁair) led to a higher conversion ratio of chlorine to hydrogen chloride.

27

burned

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Finally, Meng et al.

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moderate Cl, K, S blend ) in a fixed bed and found that increasing ṁair increased the

100

amounts of both alkalis and chlorine released to the gas phase. This study focused on

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important combustion performance parameters of waste pinewood chips (a low Cl, K,

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S biomass) in a fixed bed, and assessed the evolution of alkalis and chlorine. The

103

effects of ṁair through the bed, and the effects of preheating the air on the combustion

104

performance parameters and the evolution of alkali and alkali earth metals (AAEMs)

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and chlorine (such as HCl) were investigated.

28

burned a 50-50% blend of pinewood and corn straw (i.e.,

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Woody biomass is most commonly used as fuel in grate combustors and, to lesser

107

extent, in fluidized bed furnaces. In the former combustors, moving grate systems are

108

the prevalent equipment in medium and small-sized district heating and industrial

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units 29,30,31. Fixed bed reactors with stationary grates can be used in the laboratory to

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simulate the moving grate systems

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vertical gradients of temperature and concentrations of species in the bed layer, the

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horizontal gradients are relatively small. Hence, heat and mass transfer in the bed can

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be only considered in the vertical direction 33. A fixed bed reactor is simpler to operate

114

and collect data with minimum running costs 34. Therefore, this experimental study

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herein was performed in a fixed bed reactor with a stationary grate.

31,32.

This is because compared to the prevalent

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117

2. Experimental apparatus and biomass fuels 6


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Energy & Fuels

2.1 Experimental apparatus

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Biomass was burned in a one-dimensional fixed bed inside a vertically-oriented

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cylindrical combustion chamber, depicted in Fig. 1. The chamber is 1.30 m high, with

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an inner diameter of 180 mm, and it is composed of three layers of materials in the

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wall. The inner layer is made of 50 mm thick alumina refractory pouring; the middle

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insulating layer is composed of a 150 mm thick refractory fiber cotton wall; the outer

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protective sleeve is made by Cr18Ni9Ti steel. More details about the experimental test

125

rig are given in Ref. 27.

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The fixed bed furnace setup consists of four systems: the furnace, the air supply

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system, the air preheating system and the measurement systems. During the

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experiments heat was supplied to a fixed bed of biomass by burning propane gas. A

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gas nozzle was placed at a furnace height of 750 mm away from the grate, and it was

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tilted to 45°. At the beginning of each experiment, the propane torch was ignited and

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then its flow rate was kept at a constant value that was sufficient to maintain the

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chamber temperature at around 900 °C. The pinewood was heated by radiation and

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convection from the high temperature propane flame. The bed temperatures were

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monitored by armored K-type thermocouples at different bed heights, at 11 different

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positions specified in Table 1. The measuring range of the armored K- type

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thermocouples was 0 to 1390 °C, with a measurement error of ±2.5 °C. A gas-sampling

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probe was inserted at 388 mm above the grate to monitor the gaseous emissions of 7


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combustion. In a number of experiments, the primary combustion air (under-fire air)

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was heated using an electric preheater incorporating a spiral-tube heat exchanger

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and, afterwards, it was supplied to the bottom wind box of the furnace and then to

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the fuel bed through the porous metal grate. Its flow was controlled by a flow meter

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and its temperature was monitored by the bottommost level 1 thermocouple (T1)

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shown in Fig. 1.

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Figure 1. Schematic of the fixed bed furnace for the combustion of the biomass fuels

145 146

Table 1. Thermocouple locations in the fixed bed.

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Number

Distance to the grate surface (mm)

Number

Distance to the grate surface (mm)

1

748

6

208

2

560

7

118

3

478

8

28

4

388

9

-90

5

298

148 149

2.2 Experimental Procedures and Fuel Properties 8


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Air-dried pinewood was selected for these experiments based on its relative high

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planting rate in northern China. Pinewood residues were collected from a timber mill

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of Heilongjiang Province, China. The Proximate and Ultimate analyses of air-dried

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pinewood (moisture content – 4.72%) are listed in Table 2. Table 2. Chemical compositions (wt%) of the selected pinewood fuel.

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Value

Method

Reference

GB/T 476-2001

Ultimate analysis/wt%（dry basis） Carbon dioxide

Carbon (%)

47.65

Hydrogen (%)

5.79

Water absorption

Oxygen (%)

38.44

Calculation

Nitrogen (%)

0.16

Pyrolysis and titration

Sulfur (%)

0.02

Chlorine (%)

0.017

Calcium (%)

0.087

Sodium (%)

0.008

Potassium (%)

0.055

Magnesium (%)

0.054

absorption

Pyrolysis and coulometry

GB/T 214-1996

GB/T 211-1996

Proximate analysis /wt%（dry basis） Ash (%)

0.58

Slow ashing

Moisture content (%)

4.72

Air drying

GB/T 212-2001

Fixed Carbon (%)

12.78

Calculation

GB/T 213-2003

Volatile matter (%)

81.80

Thermostatic firing

Low Heating value (MJ/kg)

17.93

calorimeter

GB/T 213-2008

155

In these experiments, the pinewood chips were in the form of parallelepipeds

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with approximate dimensions of 5x2x1 cm and were packed in beds weighing approx.

157

2.5 kg and having heights of 540 mm and diameters of 180 mm. To investigate the 9


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effect of primary air flow on the AAEMs and Cl released to the gas phase during

159

pinewood combustion, experiments were conducted with four different primary air

160

flow rates through the grate at the bottom of the reactor (under-fire air) without

161

preheating, i.e., at a temperature of 20°C. The detailed experimental conditions are

162

listed in Table 3. In addition, experiments were also conducted to preheat the primary

163

air to the temperatures of 85 and 135 °C. Based on the stoichiometric fuel/air ratio of

164

the pinewood fuel (approximated as CH1.46O0.61N0.029S0.0016 from its Ultimate analysis)

165

and on the actual fuel/air ratio from experimental data, overall (global) excess air

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coefficients were calculated as λ = (mair/mfuel)actual/(mair/mfuel)stoichiometric = (Vair A Δt

167

ρair/mfuel)actual/(mair/mfuel)stoichiometric, (Δt indicates the time of combustion and Vair is the

168

air flow velocity and A represents the cross sectional area of the bed) and are also

169

shown in Table 3. Table 3. Experimental operating conditions.

170

Mass flux of primary air

Excess primary air

ṁair (kg m-2s-1)

coefficient (λ)

100

0.085

0.56

160

0.135

0.87

230

0.194

0.92

280

0.237

1.1

Primary air flow rate (L/min)

171 172

The combustion effluent gases were first heated to 180 ℃

to avoid

173

condensation of H2O, which can absorb HCl, and then they passed through a fiber filter

174

to collect particles. Thereafter, gas analysis was performed by Fourier Transform Infra-

175

Red (FTIR) spectroscopy (using a GASMET DX4000 instrument), which quantified HCl 10


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and other species, such as CO2, CO and CH4, in the hot and humid sample gas. The

177

LabVIEW software recorded the analyzer signals through a data acquisition card (Data

178

Translation PCI-6221). All experiments were repeated in triplicates, the mean values

179

of which and one standard deviation for each case are presented in the ensuing

180

Experimental Results Section.

181

During the pinewood combustion, the bottom ash and the fly ash were also

182

collected and analyzed by the following procedure. Solid ash samples were digested

183

as a solution first, following the method in reference 4: (i) Solid particles were first

184

dried at 105 °C for 2 hours. (ii) Thereafter, the samples were put into an ultra-high

185

purity (metal impurity content of less than 1ppb) solution consisting of 6 ml HNO3 and

186

2ml H2O2 for 2 hours. (iii) Subsequently, a new solution (HNO3:H2O2:HF = 4:2:2) was

187

added into the samples and the samples were heated at a rate of 20 °C /min to 120°C,

188

and were kept at this temperature for 5 min. (iv) Afterwards, the samples were heated

189

at a heating rate of 16 °C /min to 200 °C, and they were kept at this final temperature

190

for 1 hour. At the end of this procedure, the digested solutions were used to monitor

191

(a) the mass fractions of AAEMs (Ca, K, Na and Mg) in the ash by inductively coupled

192

plasma atomic emission spectroscopy (ICP-AES), and (b) the mass fractions of Cl by ion

193

chromatography (IC). The corresponding fraction of AAEMs and Cl are listed in Table

194

4.

195 11


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Table 4. Mass fractions of K, Na, Ca, Mg, and Cl in the ash.

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Species

Preheated air temperature

ṁair (kg m-2s-1)

(°C)

0.085

0.135

0.194

0.237

20

85

135

Chlorine (%)

0.02

0.01

0.03

0.02

0.01

0.01

0.02

Calcium (%)

24.11

25.51

26.91

28.41

25.51

25.99

26.59

Sodium (%)

0.60

0.44

0.45

0.48

0.38

0.44

0.41

Potassium (%)

5.59

5.68

5.39

5.08

5.68

5.49

4.58

Magnesium (%)

6.12

7.06

7.01

7.09

7.06

6.76

6.82

197

198

3. Experimental Results and Discussion

199

3.1 Effects of primary air flowrate, ṁair, on AAEMs and Cl release

200

3.1.1 Combustion characteristics

201

The fuel bed temperature histories at the different heights of the bed, which

202

were monitored with the thermocouples T2-T8, are shown in Fig. 2 at different ṁair

203

settings. The ignition delay, temperature gradient upon ignition (°C/s), the average

204

flame propagation rate (ignition rate), and the average burning rate are listed in Table

205

5. The bed of pinewood chips was ignited at the top by a propane flame which heated

206

the bed by radiation and convection, see Fig. 1. The temperature gradient upon

207

ignition was defined as the time elapsed for the temperature in each bed layer to

208

increase from 300 to 800 °C 35. Ignition delays were in the range of 1699-2370 s and

209

attributed to the times needed for heating up the fuel to its devolatilization

210

temperature. Once the pinewood was ignited, at about 160 °C, the corresponding bed 12


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temperature rose rapidly as shown in Fig. 2. With the increase of ṁair in the range of

212

0.085 – 0.237 kg m-2s-1, the temperature gradient increased from 7 to 11 °C/s (by

213

~60%) because of the additional supply of oxygen (i.e., a higher λ).

214

Table 5. Combustion parameters at different primary air flow rates, ṁair.

ṁair (kg m-2s-1) Ignition delay time (s) Temperature gradient (°C/s) Flame propagation rate (ignition rate) (mm/s) Burning rate (kg m-2s-1)

0.085 1699 7

0.135 1705 8

0.194 1764 9

0.237 2370 11

0.33

0.38

0.45

0.47

0.025

0.027

0.034

0.035

215 216

After the flame front passed by the location of a given thermocouple, the bed

217

temperature decreased mildly and then it leveled off. As hot combustion gases moved

218

upwards, passing by the thermocouple, they experienced heat losses to the inner

219

furnace walls, hence, their temperature dropped. At the same time, as the furnace

220

walls got progressively hotter with time the bed temperature at the location of that

221

thermocouple arose again 28,36. As the flame approached the bottom of the bed, the

222

temperatures in layers (T7 and T8) were the hottest, exceeding by ~400 °C the

223

temperatures of the upper bed layers, see Fig. 2. This was attributed to diminished

224

heat transfer, as the chamber wall heated up, and to widespread combustion of

225

accumulated chars in addition to combustion of any remaining volatiles. At

226

increasingly higher ṁair the peak temperatures of all layers increased, and it even

227

exceeded the upper limit of the K-type thermocouples used in these experiments. As 13


Energy & Fuels

228

a result, combustion became more intense and the fuel burnout time in each layer

229

decreased. (b) ṁair = 0.135 kg m-2s-1

1400

1400

1200

1200

Temperature (°C)

Temperature (°C)

(a) ṁair = 0.085 kg m-2s-1

1000 800 600 400

1000 800 600 400

200

200

0

0 0

230

1000 2000 3000 4000 5000 6000

0

Time (s)

(c) ṁair = 0.194 kg m-2s-1

1200

1200

Temperature (°C)

1400

1000

Time (s)

T8 T7

1000

800 600 400 200

T6

800

T5

600

T4

400

T3 T2

200

0 0

231

1000 2000 3000 4000 5000 6000

(d) ṁair = 0.237 kg m-2s-1

1400

Temperature (°C)

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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0

1000 2000 3000 4000 5000 6000

0

Time (s)

1000 2000 3000 4000 5000 6000

Time (s)

232

Figure 2. Bed temperature histories at different heights from the grate. The temperature of the

233

primary air entering the bed was 20 °C. The vertical dashed lines denote that the flame

234

approached the bottom of the bed and the char combustion became dominant near the grate

235

surface.

Furthermore, with the increase of ṁair the flame propagation rate (mm/s), which

236 237

represents the flame travel velocity

30,

increased by ~40%. This parameter is also

238

termed ignition rate and represents the average propagation rate of an ignition wave

239

35.

Accordingly, the burning rate (kg m-2s-1), defined herein as the mass loss of the 14


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Energy & Fuels

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biomass over time per unit cross-sectional area of the cylindrical perpendicular to its

241

vertical axis 37, increased from 0.025 to 0.035 kg m-2s-1, as listed in Table 5. Burning

242

rates of pinewood are higher than those of corn straw examined earlier [33], at

243

comparable ṁair. The experimental burning rates are plotted against ṁair in Fig. 3.

244

Therein calculated stoichiometric burning rates are also included. They are higher than

245

stoichiometric burning rates at the first three experimental conditions, where overall

246

fuel-rich conditions prevailed; this difference may be attributed to the contributions

247

of gasification reactions 35. As the burning rates increased with ṁair, the amounts of

248

unburned carbon remaining in the ashes on the grate, at the bottom of the bed

249

decreased, from 12% to 4.5%, as shown in Table 6. This trend is in line with the

250

increase of carbon conversion to CO2 and CO, which increased with the ṁair. Methane,

251

was the most predominant detected unburned hydrocarbon and accounted for a

252

significant fraction of the pinewood carbon, particularly so at the lower ṁair; this is in

253

agreement with the aforementioned possibility of significant gasification reactions.

254

Methane emissions decreased with increasing ṁair; such a phenomenon was

255

attributed to the increased heat generation upon ignition and sufficient oxygen at

256

higher ṁair 38. Therefore, it was confirmed that combustion of the pinewood bed was

257

more effective at high air flowrates.

15


Energy & Fuels

Burning Rate 0.04 0.035

kg m-2s-1

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

Page 16 of 40

0.03 0.025 0.02 0.015 0.01 0.06

0.11

0.16

0.21

0.26

0.31

ṁair (kg m-2s-1) burning rate

258

stoichiometric combustion

259

Fig. 3 Actual and calculated stoichiometric burning rates of pinewood biomass as a function

260

of ṁair

261 262

Table 6. Conversion of carbon to carbon-containing gases and unburned carbon in the ash at

263

different values of ṁair. C (%)

C (%)

C (%)

C (%)

C (%)

released

released as

released as

released as

retained in

as CO2

CO

CH4

C2H6

the ash

0.085

42.7

9.3

12.6

0.12

12.0

23.27

0.135

44.1

12.9

10.9

0.08

7.3

24.7

0.194

50.6

16.3

10.0

0.07

5.5

17.6

0.237

57.2

20.0

6.5

0.09

4.5

11.8

ṁair (kg m-2s-1)

C (%) not detected

264 265

3.1.2 The effect of ṁair on AAEMs release

266

The release of AAEMs (K, Ca, Na, and Mg) and of chlorine to the gas phase during

267

pinewood combustion in a fixed bed, was calculated based on the difference of the

268

initial contents of AAEMs and Cl in the raw pinewood, shown in Table 2, and their

269

contents in the residual ashes after burnout

29

shown in Table 4. Therefore, to 16


Page 17 of 40 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

270

investigate the release of K and other alkali metals during the pinewood combustion,

271

the mass-based emissions of AAEMs (mg/gram fuel) are represented in Fig. 4a, the

272

molar emissions (mmol/gram fuel) are shown in Fig. 4b, and the fractions of biomass

273

AAEMs released to the gas phase are shown in Fig. 4c. Results of this calculations show

274

calcium and potassium were the most prominent emissions of AAEMs followed by

275

magnesium and sodium. The amounts of AAEMs released to the gas phase increased

276

with increasing ṁair, as the bed temperatures got hotter, see Fig. 2. Higher

277

temperatures in the bed may have reached or exceeded the boiling points of K, Na

278

and Mg and Ca (759°, 883°, and 1070°, respectively) but barely that of Ca (1464° C).

279

The fractions of alkali earth metals (Ca and Mg) that were released to the gas

280

phase were lower (10-19% and 24-34% respectively) than those of the alkali metals (K

281

and Na which were 36-48% and 60-68%, respectively), whereas the mass emissions

282

and molar emissions of Ca and Mg were higher, see Figs 4a and 4b. In addition, in the

283

range of lower ṁair : 0.085-0.135 kg m-2s-1, the variation of ṁair had little effect on Ca

284

release, which attests that Ca is stable in the solid particles. As reported by Aho et al.

285

39,

286

reaction with sulfur oxide and the formation of stable CaSO4, which can adhere to the

287

surface of the solid particles. Therefore, increasing peak bed temperatures at higher

288

ṁair, in the range of 0.294-0.237 kg m-2s-1, exceeded the boiling point of Ca and caused

289

an additional release (by 28%) to the gas phase. Furthermore, it has been reported

the main possible reaction pathway of Ca during devolatilization stage is the

17


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

Page 18 of 40

that during the char burning stage, escaping silicates to the outer structure of the

290

16,17

291

biomass encounter the retained AAEMs and,

292

Mg, they remain in the ashes. This supports the observed low release of alkali earth

293

metals to the gas phase.

upon preferential reaction with Ca and

294

With the increase of ṁair, the fractions of K and Na released to the gas phase

295

increased. Even if the fraction of sodium released was higher than the fraction of

296

potassium, the total amount of the latter was higher than that of the former by a

297

factor of five or more. This is because the pinewood contains nearly ten times more K

298

than Na, see Table 1. Since potassium is the most important fouling species in

299

combustors 40, the release and transformation of K during pinewood combustion is

300

important. The increasing bed temperatures, with increasing ṁair, facilitate the release

301

of K, both from inorganic salts and from organically bound compounds 817. To assess

302

the main form of K in the pinewood, a chemical fractionation analysis was performed

303

41.

304

filtered fluid was tested for its K content by inductively coupled plasma atomic

305

emission spectroscopy (ICP-AES). Based on the content of K element in the filtered

306

fluid, it was determined that approx. 88.4% potassium could be dissolved in water;

307

hence, it was concluded that most of K in pinewood was in the form of inorganic salt.

308

The results were consistent with those of Zhang et al. 42, who reported that the main

309

anions in pinewood water solution are Cl- and HCO3-. Accordingly, the relatively lower

Powdered pinewood was mixed thoroughly with deionized water, and then the

18


Page 19 of 40 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

310

melting point of potassium chloride (m.p.KCl ~770 °C) than that of potassium carbonate

311

(m.p.K2CO3 ~900 °C)

312

devolatilization stage. Therefore, the main release of K during the combustion of

313

volatiles of pinewood was probably caused by the evaporation of KCl, which is in

314

agreement with Zhang et al. 42. In fact, the bed temperature in the volatile burning

315

stage did not reach the boiling point of KCl(s) (1,420°C). However, during the biomass

316

devolatilization period ion-exchange reactions with function groups in the organic

317

matrix, such as R-COOH(s) + KCl (s) → R-COOK (s) + HCl(g) 44 may have taken place.

318

This is supported by the time-revolved evolution of HCl, shown in Fig. 5. Most of the

319

HCl appears to have been released during the volatile matter burning stage. During

320

the char burning stage, with the increasing ṁair the peak bed temperature increased

321

to values reaching and exceeding 1400 °C. At those temperatures K could be released

322

to the gas phase, mostly as KCl and partially as K2CO3, etc. This is consistent with the

323

results of Knudsen et al. 16.

43

suggests that KCl should be more volatile during the

19


Energy & Fuels

(a) Mass emissions

mg/g of fuel

0.4 0.3 0.2 0.1 0 0.085 kg m¯²s¯¹

0.135 kg m¯²s¯¹ K

324

Na

0.194 kg m¯²s¯¹ Ca

0.237 kg m¯²s¯¹

Mg

mmol/g of fuel

(b) Molar emissions 0.01 0.008 0.006 0.004 0.002 0 0.085 kg m¯²s¯¹

0.135 kg m¯²s¯¹ K

325

Na

0.194 kg m¯²s¯¹ Ca

0.237 kg m¯²s¯¹

Mg

(c) AAEMs released to the gas phase 80

mass %

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

Page 20 of 40

72

71

60

60 40 20

48

46

37

36

68

14

12

11

35

29

26

25

18

0 0.085 kg m¯²s¯¹

326

0.135 kg m¯²s¯¹ K

Na

0.194 kg m¯²s¯¹ Ca

0.237 kg m¯²s¯¹

Mg

327

Figure 4. Integrated AAEMs (K, Na, Ca, and Mg) (a) mass emissions (mg/gram pinewood), (b)

328

molar emissions (millimoles/gram pinewood), and (c) mass fractions of AAEMs released to the

329

gas phase during pinewood combustion in a fixed bed furnace. The temperature of the primary

330

air entering the bed was 20 °C.

331

20


Page 21 of 40 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

332

Energy & Fuels

3.1.3 The effect of ṁair on chlorine release

333

The effect of ṁair on the release of Cl was investigated in this work. As shown in

334

Fig. 6, the chlorine release from pinewood was nearly complete at all ṁair conditions

335

with the specific mass emissions of 0.3 mg/gram fuel. This result is in contrast with the

336

combustion of corn straw in this laboratory 4, which released only 25-40% of its

337

chlorine to the gas phase, albeit with much higher specific mass emissions in the order

338

of 1-2 mg/gram fuel, as pinewood has much lower chlorine and potassium contents

339

than corn straw 28. The fact that corn straw retains a higher fraction of its chlorine in

340

the ash than pinewood can be explained based on the fact that it contains more

341

organically-bound chlorine. Whereas such chlorine can be more readily released

342

during the volatile phase it can be re-captured by the alkalis in the char. Since corn-

343

straw has a high potassium content, far in excess of its chlorine content

344

effectively capture more chlorine than pinewood in the chars. These results are in

345

qualitative agreement with those of Johansen et al.

346

combustion of corn and wood blends in a grate reactor.

26,

45,

it can

who investigated the

21


Energy & Fuels

(b) ṁair = 0.135 kg m-2s-1

1.2

1.2

1

1

µg/gram fuel

µg/gram fuel

(a) ṁair = 0.085 kg m-2s-1

0.8 0.6 0.4 0.2

0.8 0.6 0.4 0.2 0

0 0

2000

0

4000

1000

1.2

1

1

µg/gram fuel

µg/gram fuel

1.2 0.8 0.6 0.4 0.2 0 1000

2000

3000

4000

5000

(d) ṁair = 0.237 kg m-2s-1

(c) ṁair = 0.194 kg m-2s-1

0

2000

Time (s)

Time (s)

347

3000

4000

0.8 0.6 0.4 0.2 0

5000

0

1000

Time (s)

2000

3000

4000

5000

Time (s)

348 349

Figure 5. HCl mass emissions (µg/unit gram pinewood) versus time at different air flow rates,

350

ṁair. The temperature of the primary air entering the bed was 20 °C.

351 (a) Mass emissions of chlorine

(b) Release of chlorine 125

Cl released (%)

0.4

(mg/gram fuel)

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

Page 22 of 40

0.3 0.2 0.1

75 50 25 0

0 0.085

352

100

total Cl

0.135

0.194

ṁair (kg m-2s-1) Cl in HCl

Other Cl

0.237

0.085 total Cl %

0.135

0.194

ṁair (kg m-2s-1) Cl% as HCl

0.237

Other Cl%

22


Page 23 of 40

(c) Molar emissions of Cl and K 0.01

mmol/g of fuel

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

K

unidentified Cl

0.005

0 0.085

353

0.135

0.194

ṁair (kg m-2s-1)

0.237

354

Figure 6. Integrated chlorine (a) mass emissions (mg/gram pinewood); (b) fuel-Cl (%) fraction

355

released to the gas phase, Cl conversion to HCl, and CL conversion to other gaseous species; (c)

356

molar emissions (millimoles/gram pinewood) of K and Cl to the gas phase during pinewood

357

combustion in a fixed bed furnace. The temperature of the primary air entering the bed was

358

20°C.

359

The fractions of Cl converted to HCl, shown in Fig. 6, increased with the increase

360

of ṁair, until 0.237 kg m-2s-1 was reached and, thereafter they decreased. Most of the

361

HCl appeared to have been released during the volatile matter combustion stage, see

362

Fig. 5. The organic Cl can easily convert to HCl at a relative low temperature (˂500°C),

363

and up to 50 wt% of the Cl was released as HCl. At the same time, the release of other

364

unidentified chlorine-containing species followed the opposite trend of HCl. Such

365

species are likely to be mostly KCl, as previously mentioned, and in smaller amounts

366

CH3Cl and other alkali salts and carbonates. The likelihood that K is mainly released as

367

KCl, is supported by the fact that the molar emissions of K were nearly equivalent to

368

those of Cl, as shown in Fig. 6c. In addition, HCl emitted at the volatile combustion

369

stage could be recaptured by K+ in the char, since HCl-containing combustion gases

370

moved upwards in the bed through the accumulating thick layer of chars, which

23


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

Page 24 of 40

371

moved downwards. The overall reaction can be represented as char-O-K + (s) + HCl (g)

372

→ char-OH (s) + KCl (s,l)

373

burning stage (near the bottom of the bed), see Fig. 2, evaporation of KCl on the char

374

occurred to the gas phase. Hence, higher K emissions occurred at higher ṁair, which

375

induced higher char temperatures, see Fig. 2. Moreover, the less stable K2CO3 could

376

have reacted with HCl, according to K2CO3+ HCl (g) → 2KCl (g) + H2O (g)+CO2 (g)

377

reaction, which

378

However, with the increased of ṁair, the increasing CO2 partial pressure (shown in

379

Table 5) could have shifted the equilibrium to the opposite direction 47.

46.

As the maximum temperatures occurred at the char

could have captured additional HCl and released more KCl

46.

It is possible that CH3Cl could also have formed during biomass pyrolysis and

380 381

combustion along with HCl

48,49.

CH3Cl and HCl are competing for Cl radicals during

382

biomass pyrolysis and combustion. It has been reported that Cl radical-generated

383

CH3Cl accounts for about 3% of the total chlorine-bearing gases during combustion

384

5051,

or even less at high temperatures 4.

385 386

3.2 The effect of preheated air temperature on AAEMs and Cl release

387

The second part of this work investigated the effects of preheating the primary

388

(over-fire) air temperature on the release of AAEMs and Cl during pinewood

389

combustion in the fixed bed. In these experiments, the air flowrate was kept constant

390

at 0.135 kg m-2s-1. This particular flowrate was chosen because up to this value the 24


Page 25 of 40 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

391

release of K increased only mildly with ṁair whereas beyond this value the release of

392

K increased more drastically, see Fig. 6c. Thus higher flowrates were not chosen, as

393

they might have obscured the investigation of the effects of the preheated air on the

394

release of AAEMs. As shown in Fig. 7, increasing the air temperature from 20 to 135

395

°C, lengthened the ignition delay significantly (by 30%), and caused a shorter burn-out

396

time (by 18%). The experimental results confirmed that the biomass burns faster with

397

preheated air, and the burning rate increased from 0.027 to 0.032 kg m-2s-1 (by 18%)

398

during pinewood combustion in a fixed bed, as shown in Table 7. This can be attributed

399

to the fact that increasing the primary air temperature enhances moisture removal

400

and, thus, the drying of the biomass

401

increased with the preheating of the air, from 0.38 mm/s at 20 °C to 0.48 mm/s at 135

402

°C (by 26 %). These results are in agreement with those of Zhao et al. 36, who reported

403

that the flame propagation rate is controlled by both the burning rate of the fuel and

404

by the heat transfer to and from the flame zone.

405

52.

Meanwhile, the flame propagation rate

Table 7. Combustion parameters at primary (under-fire) air preheat temperatures Preheated primary air (°C)

20

85

135

Ignition delay time (s)

1705

2180

2230

0.38

0.44

0.48

0.027

0.029

0.032

Flame propagation rate or ignition rate (mm/s) Burning rate (kg/m2s) 406

25


Energy & Fuels

(b) 135 °C

(a) 20 °C

1400

1400

Temperature (°C)

1200

Temeprature (°C)

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

1000 800 600 400 200 0

1200

T8

1000

T7 T6

800

T5

600

T4

400

T3

200

T2

0 0

407

Page 26 of 40

1000 2000 3000 4000 5000 6000

Time (s)

0

1000 2000 3000 4000 5000 6000

Time (s)

408

Figure 7. Bed temperature histories at different heights in the bed (typical air cases: (a) 20 °C

409

without preheating air; (b) preheating air to 135 °C). The ṁair was kept at 0.135 kg m-2s-1. The

410

vertical dashed lines denote that the flame approached the bottom of the bed and the char

411

combustion became dominant near the grate surface.

412 413

The combustion effectiveness of the fuel in the fixed bed was assessed based on

414

the conversion of carbon in the fuel to CO2 and/or the amount of unburned carbon in

415

the ashes, shown in Fig. 8. The CO2 emission increased from 44% to 52% when the air

416

preheat temperature increased from at 20 °C to 85 °C; at which condition the amounts

417

of CO and unburned methane were the lowest. When the air was preheated further

418

to 135 °C the CO2 emissions decreased and those of CO and methane increased. The

419

unburned carbon was lowest at the preheated air temperature of 85 °C. This

420

observation suggests some irregularity of combustion in the bed, such as spot ignition

421

away from the flame front. Therefore, better combustion effectiveness for the

422

pinewood appeared to occur at a preheated air temperature of 85 °C.

26


Page 27 of 40

Carbon released to the gas phase 60 50

mass %

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

20 °C

85 °C

135 °C

40 30 20 10 0 (η) C% to CO₂

C% to CO

C% to CH₄

423

unburned carbon

424

Figure 8. Conversion of C to CO2, CO and CH4 and unburned carbon during pinewood combustion

425

at different preheated air temperatures. The ṁair was kept at 0.135 kg m-2s-1.

426

The mass amounts of AAEMs released to the gas phase increased with increasing

427

the temperature of the preheated air, shown in Fig. 9. Specifically, the emissions of K

428

rose from 0.2 to 0.39 mg/gram fuel, amounting to 37-61% of the potassium mass in

429

the fuel as the temperature was increased from 20 to 135 °C. The corresponding

430

sodium mass released to the gas phase was even higher, ranging from 69 to 73%, while

431

those of calcium and magnesium were lower, ranging from 12 to 13% and from 26 to

432

30%, respectively. These results show that the temperature of the preheated air

433

mostly affected the potassium release. At low temperatures potassium exists in the

434

char in the form of K-salts, mostly exposed on the surface of the solid particle. As the

435

air temperature increased, higher flame temperatures were generated (see for

436

instance T2 in Fig. 7), which are likely to have enhanced the migration of K-salt to the

437

gas phase. Moreover, it has been reported that increasing temperature leads to fuel

27


Energy & Fuels

438

particles with increased porosity 16, which may have aided the migration of K-salt to

439

particle surface, and therefrom to the gas phase. (a) Mass emissions of AAEMs 0.5

K

Na

Ca

Mg

mg/g of fuel

0.4 0.3 0.2 0.1 0 20 °C

85 °C

135 °C

Primary air temperature

440

(b) Molar emissions of AAEMs 0.012

mmol/g of fuel

K

Na

Ca

Mg

0.008 0.004 0 20 °C

85 °C

Primary air temperature

441

135 °C

(c) AAEMs released to the gas phase 100

K

Na

Ca

20

73

69

61

51

60 40

Mg

71

80

mass %

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

Page 28 of 40

37 12

30

28

26 12

13

85 °C

135 °C

0 20 °C

442

Primary air temperture

443

Figure 9. Integrated AAEMs (K, Na, Ca, and Mg) at different primary air temperatures: (a) mass

444

emissions (mg/gram pinewood), (b) molar emissions (millimoles/gram pinewood), and (c) mass 28


Page 29 of 40 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

445

fractions of AAEMs released to the gas phase during pinewood combustion in a fixed bed

446

furnace. The ṁair was kept at 0.135 kg m-2s-1.

447

The amounts of fuel Cl released to the gas phase and the specific mass emissions

448

of HCl, as well as those for other non-measured species are shown in Fig. 10a, at the

449

different preheated air temperatures examined herein. The corresponding fractions

450

of fuel Cl released to the gas phase as well as the fractions of fuel chlorine which were

451

converted to HCl and to other unidentified species (such as KCl) are shown in Fig. 10b.

452

In all cases the total Cl mass emissions were nearly 0.3 mg/gram fuel, which

453

corresponded to the entire mass of chlorine in the biomass, i.e., the Cl was released

454

in its entirety (100%), whether the air was preheated or not. Meanwhile, by increasing

455

the inlet air temperature from 20 °C to 135 °C, the mass emission of HCl rose from

456

0.124 to 0.157 mg/gram fuel, with the corresponding fractions of Cl been converted

457

to HCl increasing from 41 to 52%. The increased emissions of HCl may be attributed to

458

the faster flame propagation rate and burning rate at higher air temperature. In

459

addition, the preheated air accelerated the reaction rate of pinewood combustion at

460

the volatile combustion stage 5354. This increase in HCl emissions may be attributed to

461

the fact that high air preheat temperatures dried the fresh fuel in the bed. Then, as

462

the generated steam moved upwards, it increased the temperature of the upper bed

463

layers (seen in Fig. 7) and it enhanced devolatilization which, in turn, promoted HCl

464

release. This is supported by the findings of Yang et al.

465

increased moisture can produce higher flame temperatures. In such case, the

55,

who reported that the

29


Energy & Fuels

466

chemical equilibrium of the reaction H2O ⇌ OH + H is shifted to the products;

467

therefore, the generated OH and H radicals can readily react with Cl radicals and

468

augment the conversion of Cl to HCl at increasing preheat temperature. (a) Mass emissions of Cl

mg/g of fuel

0.4

total Cl

Cl in HCl

Other Cl

0.3 0.2 0.1 0 20 °C

85 °C

135 °C

Primary air temperature (°C)

469

(b) Cl Released to the gas phase total Cl % 100

Cl% as HCl 100

Other Cl% 100

100 80

mass %

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

Page 30 of 40

60

59 41

50

50

52

48

40 20 0 20 °C

470

85 °C

135 °C

Primary air temperature (°C)

471

Figure 10. (a) Mass emissions of Cl (mg/gram pinewood) and (b) release of fuel Cl (%) to the gas

472

phase, conversion of fuel Cl to HCl, and to other chlorinated species during pinewood

473

combustion in a fixed bed furnace. The ṁair was kept at 0.135 kg m-2s-1.

474

Besides HCl and KCl, other chlorinated species possibly released to the gas phase

475

may include methyl chloride (CH3Cl). Johansen47 reported that CH3Cl released from

476

wood chips was less than 0.4% of the total quantity of Cl. Therefore, methyl chloride

30


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Energy & Fuels

477

is not significant for the Cl balance, and KCl may be the primary compound in the

478

“other” category in Figs. 5 and 10.

479

Conclusions

480

This investigation studied the effect of the specific primary (under-fire) air flow

481

rate, ṁair, in a fixed bed and its degree of preheating on pinewood combustion

482

performance parameters. The release of fuel-bound AAEMs and chlorine to the gas

483

phase, and the evolution of HCl were also investigated. Results from this laboratory-

484

scale investigation revealed that:

485

(a) Increasing the ṁair, from 0.085 to 0.237 kg m-2s-1 increased the flame temperature,

486

the flame propagation rate (ignition rate) and the burning rate of the fuel bed.

487

(b) Increasing the ṁair, increases the release of K to the gas phase significantly (from

488

35 to 48%), and the release of Na also significantly (from 60-72%). As the amount

489

of K in the fuel was much higher than that of Na, the emitted K was more

490

pronounced than the emitted Na by fivefold.

491

(c) A large fraction (88 %) of K in the pinewood fuel was found to be in the form of

492

inorganic salts, which during combustion can generate gaseous KCl, and in lesser

493

amounts K2CO3, particularly at high ṁair.

494

(d) The entirety of the Cl mass in the pinewood fuel was released to the gas phase at

495

all combustion conditions examined. As the ṁair increased, the amount of Cl 31


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Page 32 of 40

496

converted to HCl increased tom 38 to 49%, and then decreased to 27% at the

497

highest flowrate.

498

(e) Preheating the primary air temperature from 20 to 135 °C increased the flame

499

temperature, the flame propagation rate and the burning rate, which resulted in

500

increased volatilization of all of the alkalis, particularly K the mass fraction of which

501

increased from 35 to 61%.

502 503

(f) Preheating the primary air temperature from 20 to 135 °C promoted the HCl emission. The release of Cl as HCl increased from 41 to 52%.

504

Overall, a sufficiently high primary air flow rate resulting in an overall excess air

505

coefficient bigger than unity (λ>1) and a mild air preheat can be recommended to

506

enhance the mass burning rate of pinewood and its conversion to fully- and partially-

507

oxidized gases (CO2 and CO). However, such conditions result in enhanced gasification

508

of the alkalis in the biomass. In the case of pinewood, which contains low mass

509

fractions of Cl and AAEMs, the absolute values of such emissions are low relative to

510

other biomass types, such as straw.

511

Acknowledgement

512

This work has been supported by the Innovative Research Groups of the National

513

Natural Science Foundation of China (Grant No. 51476046), the US National Science

514

Foundation (Award Number 1810961), Harbin University and Northeastern University. 32


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Release of Alkalis and Chlorine from Combustion of Waste Pinewood

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