Aggravated fine particulate matter emissions from heating-upgraded biomass and biochar combustion: The effect of pretreatment temperature

Fuel Processing Technology 171 (2018) 1–9

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Research article

Aggravated fine particulate matter emissions from heating-upgraded biomass and biochar combustion: The effect of pretreatment temperature ⁎

Zhongfa Hua, Xuebin Wanga, , Adewale Adeosunb, Renhui Ruana, Houzhang Tana, a b

MARK

⁎

MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China Department of Energy, Environmental & Chemical Engineering, Consortium for Clean Coal Utilization, Washington University in St. Louis, St. Louis, MO 63130, USA

A R T I C L E I N F O

A B S T R A C T

Keywords: Biomass combustion Pretreatment temperature Particulate matter Chlorine Potassium

Heat pretreatment is a promising method for biomass upgrading. However, PM formation from the combustion of such pretreated biomass has not been fully evaluated. In this work, the effect of pretreatment temperature on PM emission of the upgraded biomass and biochar combustion was studied in an entrained flow reactor. The physical and chemical properties of upgraded biomass, biochar and PMs at varied pretreatment temperatures were obtained to illustrate the PM formation mechanism. Results show that pretreatment temperature significantly affects the concentration and particle size distribution of PM emissions, through changing the char yield and K/Cl contents in char. With increase in pretreatment temperature, the PM1.0 emission of upgraded biomass and biochar combustion first increases, reaches maximum at 500 °C, and then decreases. A linear relationship between the PM1.0 emission and Cl content in upgraded biomass and biochar was found. This result indicates that the combustion of upgraded biomass and biochar produced at moderate temperatures of 250–500 °C result in aggravated fouling and PM emissions.

1. Introduction Utilization of more sustainable and environmentally-friendly sources of energy can help to abate the effect of greenhouse gas emissions. Biomass is an alternative source that is carbon-neutral with a potential to reduce CO2 and SOx/NOx emissions [1,2]. As a big agricultural country, China has an abundant biomass energy resource, and its direct combustion is of great potential to reduce fossil fuel utilization and ensure energy-supply security. However, utilization of raw biomass for commercial power generation still faces several drawbacks to its implementation [3,4]. These drawbacks include low bulk density, low energy density, high moisture content, high logistic cost and poor grindability [5,6]. Therefore, for industrial use, many technologies have been proposed to upgrade biomass materials, overcome these drawbacks and improve the efficiency of biomass utilization. Heat pretreatments, including torrefaction and pyrolysis, are simple, promising and effective methods for biomass upgrading [7,8]. Torrefaction is a mild thermolysis process at relative low temperatures of 200–300 °C, mainly to improve the thermo-chemical properties of biomass [9,10]. In contrast, pyrolysis results in the deep decomposition of biomass materials at relatively higher temperatures, mainly converted into bio-fuels with high energy density, such as liquid (biooil), gas and solid sample (biochar) [11]. The solid products from both

⁎

torrefaction and pyrolysis share significant improvement on fuel properties, with high energy density and good grindability, and are suitable for the utilization as alternative and supplementary fuels for power generation [12,13]. Due to the high chlorine and alkali metal contents in biomass, direct combustion or co-firing with coal contributes significantly to PM1.0 (particulate matter with aerodynamic diameter < 1.0 μm) emission [14,15], inducing ash deposition, fouling, and slagging [16,17]. The heat pretreatment of biomass results in complex transformation to biochar and the release of a considerable amount of alkali and chlorine into the flue gas [18,19]. However, the residual biochar pretreated at moderate temperatures still contains abundant species of alkali and chlorine, which may result in significant PM1.0 emission. Extensive studies on the characteristics of PM from raw biomass combustion [20,21] or its co-firing with coal [22,23] have been reported. Few studies were conducted on the PM emission from biochar combustion. Yani et al. [19] adopted a drop-tube furnace to investigate the behavior of PM10 emission from the combustion of biochar obtained through torrefaction at 220–280 °C. They observed bimodal mass-based particle size distribution (PSD) from biochar combustion, which is similar to that from raw biomass combustion. Nonetheless, when the biomass pretreatment temperature was further increased to 400–550 °C, a unimodal PSD was observed for the PM10 emission, and the PM1.0

Corresponding authors. E-mail addresses: [email protected] (X. Wang), [email protected] (H. Tan).

https://doi.org/10.1016/j.fuproc.2017.11.002 Received 10 August 2017; Received in revised form 3 November 2017; Accepted 7 November 2017 0378-3820/ © 2017 Elsevier B.V. All rights reserved.

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(particulate matter with diameter < 1.0 μm) emission is negligible [24]. The biomass feedstocks used in these two works on biochar combustion contain low chlorine contents (< 0.2%). However, straw, which is the most part of biomass energy resource with a share of 72% in China [25], has a relatively high chlorine. The chlorine content in straw [26–29] can be up to three times or higher than that in biomass used in previous studies [19,24]. The chlorine species in biomass have been widely reported to be the main cause for the formation of fine particles during biomass combustion [14,30]. These significant differences in the chlorine content of biomass may result in different characteristics of PM1.0 emission during biochar combustion. In addition, the effect of pretreatment temperature within a wide range on the PM emission from upgraded biomass and biochar combustion has not been fully explored. Therefore, a deep and systematic understanding on the fine particle emission from upgraded biomass and biochar combustion, is important for the high-efficient and environment-friendly utilization of biomass in China. In this study, the behaviors of PM emission from the combustion of upgraded biomass and biochar prepared in a wide temperature range of 250–1000 °C, were carried out in a laboratory-scale entrained flow reactor at 1200 °C. The biomass pretreated below 300 °C is called “upgraded biomass”, while the biomass pretreated above 350 °C is called “biochar”. The fine particles were collected by a Dekali low pressure impactor (DLPI). Morphologies and chemical composition of the fine particles were analyzed by a scanning electron microscopy equipped with energy dispersive X-ray spectrometry (SEM-EDS). Based on the experimental results, the PM formation mechanism during upgraded biomass and biochar combustion is illustrated.

Table 2 The composition of wheat straw ash. Component

MgO

Al2O3

SiO2

P2O5

SO3

Cl

K2 O

CaO

Straw (wt%)

2.42

2.83

42.77

3.55

4.52

6.15

27.68

10.08

ground and sieved into diameter below 100 μm. X-Ray fluorescence (XRF, S4-Pioneer, Bruker Co., Germany) was employed to directly determine the potassium and chlorine contents, and X-ray diffraction (XRD, X'pert MPD Pro, PANalytical, Netherlands) was used to analyze the mineral phases in the upgraded biomass and biochar at different pretreatment temperatures. The solid products were dried in oven for 6 h before XRF characterization, therefore, the presented elemental content in this paper is in dry basis. 2.2.2. Entrained flow reactor system The behavior of PM emission from upgraded biomass and biochar combustion was studied in an entrained flow reactor system as shown in Fig. 2. The corundum tube reactor is heated by silicon carbide of three zones. The temperature was measured by a B-type thermocouple ( ± 1 °C), generating an isothermal region of ~ 500 mm. The reactor is 1200 mm in height with an internal diameter of 50 mm. The fuel samples are fed by a micro-scale fluidized feeder, where the particles were entrained by the nitrogen of 1 L/min. Before each test, the feeding rate of 90–150 mg/min was determined first, and then the fuel particles were fed into the reactor via a water-cooled feeding probe. The experiments were operated at 1200 °C and at a total gas flow of 4 L/min with a N2:O2 ratio of 1:4. The residence time of fuel particles in the isothermal zone was ~3.5 s to ensure the complete combustion and avoid the carbonaceous particles. The fine particles in the flue gas were sampled by a water-cooled probe, where another N2 flow was introduced to quench and dilute the flue gas. The diluted particle-containing flue gas first passed the PM10 cyclone to separate the coarse particles with diameters > 10 μm, and then was introduced to DLPI (Dekati low pressure impactor, Finland) for size classification and collection. The sampling devices and related pipelines were heated up to 150 ± 5 °C, in order to avoid and prevent acid gas condensation. The mass of deposits on the substrates was analyzed by a high precision electronic micro-balance ( ± 0.001 mg, Sartorius M2P, Germany), to obtain the mass PSD of PM10. In each case, the sampling was repeated three times to ensure reproducibility. The particles collected on the substrates were analyzed by SEM-EDS (JEM7800F, JEOL, Japan) for morphologies and elemental compositions.

2. Materials and methods 2.1. Fuel properties The biomass used in the present study is the typical wheat straw from Baoji city of Shaanxi province. The proximate and ultimate analyses, and ash compositions are presented in Tables 1 and 2, respectively. As seen from Table 2, the chlorine and potassium contents are high, 6.15% and 27.68%, respectively. The straw samples were ground, sieved into diameter < 100 μm, and then put into sealing bag for use. The fuel samples were dried in an oven at 105 °C for 2 h before the preparation of upgraded biomass and biochar and combustion tests in entrained flow reactor. 2.2. Experimental setup

3. Results and discussion

2.2.1. Fixed-bed pyrolysis reactor The preparation of upgraded biomass and biochar was performed in a fixed-bed pyrolysis system as shown in Fig. 1. The quartz tube reactor is 40 mm in internal diameter and 2000 mm in length. The temperature was measured by a K-type thermocouple ( ± 1 °C). A corundum crucible with 1.0 g straw was placed into the reactor and exposed to nitrogen with gas flow of 1 L/min for 30 min. Then, the samples were heated to and maintained at the desired temperature for 120 min to ensure complete pyrolysis. Thereafter, the residual char was cooled to ambient temperature and weighed by an electronic balance ( ± 0.0001 g, AEL-200, Shimadzu, Japan) to obtain the char yield. Each test was repeated 5 times. The upgraded biomass and biochar was

3.1. The yield of char and the evolution of K/Cl during straw pyrolysis The yield of char as a function of temperature during the process of heat pretreatment is presented in Fig. 3(a). With the increasing in the pretreatment temperature from 250 °C to 350 °C, the char yield decreases from 55.5% at to 42.7%, and further decreases to 32% at 1000 °C. This trend of the temperature-dependent char yield is similar to previous research [7], while these yields are somewhat different. It might be due to the difference of fuel properties. Biomass mainly consists of cellulose, hemicellulose and lignin. The thermal decomposition of cellulose starts at < 150 °C, generating the more stable anhydrocellulose [31]. This process is the dominant below 300 °C, thereby resulting in a higher char yield as shown in Fig. 3(a). With increase in temperature, the cellulose depolymerizes and produces volatiles [32], leading to a decrease of char yield at 350 °C. Lower char yield with increase in temperature may be caused by the enhanced volatile-release through promotion of primary decomposition or secondary decomposition at higher temperatures [33,34]. It is well known that during biomass combustion, K and Cl in

Table 1 Properties of wheat straw used in this study. Proximate analysis (wt%, ad)

Ultimate analysis (wt%, ad)

Mad

Aad

Vad

FCad

Cad

Had

Oad

Nad

Sad

Clad

3.25

14.24

70.02

17.31

42.5

5.98

49.11

1.58

0.60

1.03

2

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Corundum Boat

Fig. 1. Schematic of fixed-bed reactor system.

Mass Flowmeter

Valve

To air

Filter

N2 Condensation Unit Fig. 2. Schematic of entrained flow reactor system.

Water in

Flowmeter

Water out

O2

N2

DLPI

Water out Water in

Micro-fuel feeder

Cyclone

Quench N2 in

Pump

biomass are released into the flue gas as gaseous species, the condensation of which dominates the formation of PM1.0 and induces the corrosion and fouling on heating exchanger surfaces [15,24]. By comparison, the evolutions of K and Cl during biomass pyrolysis have not

been well demonstrated. In this work, the contents of K and Cl in the upgraded biomass and biochar at different pretreatment temperature were determined by XRF, and the evolution curves are also presented in Fig. 3(a) within the temperature range of 250–1000 °C. It can be seen 3

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80

Char yield (%)

K

10

Char yield

40

5

Cl or K content (%)

60

Cl

20

Release ratio of K or Cl (%)

100

60

40

K release 20

0

0

0 200

400

600

800

Cl release

80

1000

200

Pretreatment temperature (°C)

(a)

400

600

800

Pretreatment temperature (°C)

1000

(b)

Fig. 3. Effect of pretreatment temperature on (a) char yield and K/Cl contents in upgraded biomass and biochar, and (b) the K/Cl release ratio.

HCl [36,37]. From the XRD patterns at higher temperatures (≥ 815 °C) in Fig. 4, there is no observable KCl in the char. As to the release of K in Fig. 3(b), an approximately linear correlation exists for the K release with pretreatment temperature below 815 °C, while the K release remains unchanged with the pretreatment temperature above 815 °C. It has been reported that part of the alkali metal in biomass is bound with char matrix as alkali carboxylates [38], which become thermally unstable and decompose with the increase in temperature. However, at a higher temperature around 1000 °C, the silico-aluminate oxides in biomass become active to capture the potassium and form stable potassium aluminosilicates, e.g. K4Al2Si2O9 and KAlSi2O6 as observed from the XRD patterns in Fig. 4.

that as the pretreatment temperature increases, the absolute content of Cl increases first, reaches the maximum at 500 °C, and then decreases. At 1000 °C, no Cl is found in the residual char. The evolution of K follows the similar tread, while the maximum content in char is observed at 350 °C. Previous studies indicated that a negligible fraction of K and Cl can be released below 250 °C [18,19]. Therefore, we use the absolute K and Cl quantities in the char produced at 250 °C as basis for determining the release ratio of K and Cl as pretreatment temperature changes, and the result is shown in Fig. 3(b). In Fig. 3(b), the release of Cl shows two-step: a slow release below 500 °C and a rapid release thereafter. At lower temperatures, only a small portion of Cl is released as HCl during devolatilization, due to the destruction of biomass matrix [35]. The retained Cl in char matrix prepared below 500 °C mainly existed as KCl as shown in the XRD patterns in Fig. 4. However, with increase in the pretreatment temperature, a fraction of the KCl would be vaporized via reaction R1 and a certain fraction will also react with silico-aluminate oxides via reactions R2-R4, forming aluminosilicates with high meting point and releasing

KCl(s, l) → KCl(g)

(R1)

4KCl + 2SiO2 + Al2O3 + 2H2 O → K 4 Al2Si 4 O9 + 4HCl

(R2)

2KCl + 4SiO2 + Al2 O3 + H2 O → 2KAlSi2 O6 + 2HCl

(R3)

2KCl + 2SiO2 + Al2 O3 + H2 O → 2KAlSiO4 + 2HCl

(R4)

Fig. 4. XRD patterns of upgraded biomass and biochar at different pretreatment temperatures.

70000 1—KCl 2—SiO2 3—KAlSi2O6 4—K4Al2Si2O9 5—KAlSiO4 6—K2O 2

60000

2

4

5

Pretreatment temperature 2

1000 C

50000 2

Intensity (1)

3

2 4

2

40000

815 C 2 1

2

30000

6

2

1

500 C

20000

2

10000

2

2

1

6

2

1

350 C 2

1

6

2

1

250 C

0 10

20

30

40

50

2 4

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dM/dDp (mg/g_fuel)

20

10 Pretreatment temperature 105 °C (raw biomass) 250 °C 350 °C 500 °C 815 °C 1000 °C

8

PM yields (mg/g_fuel)

25

15

10

PM1.0-10 yield

4

2

5

0 0.01

PM1.0 yield

6

0

0.1

Dp ( µm)

1

0

10

200

400

600

800

1000

Pretreatment temperature (°C)

(a)

(b) Fig. 5. (a) Mass-based PSDs of PM10 and (b) PM yields.

3.2. PSDs and yields of fine particles

below 500 °C, the Cl content in upgraded biomass and biochar gradually increases, which follows the same tendency of PM1.0 yield in Fig. 5(b). At higher temperatures, the significant decrease of Cl content at 815 °C and none Cl at 1000 °C result in the significant reduction on PM1.0 and almost none PM1.0 emission, respectively. The enhanced release of chlorine and potassium from upgraded biomass and biochar is indicated by the higher concentration of chloride in gas phase, resulting in intensified homogenous nucleation and heterogeneous condensation [40]. As to the PM1.0–10 of coarse-mode, the charring process does not change the location of the PSD peak center at ~4.0 μm. The PM1.0–10 yield from the combustion of biochar prepared at 350–815 °C is about 1.5–2 times that of the raw straw. However, the PM1.0–10 yields from the upgraded biomass at 250 °C and biochar at 1000 °C are almost the same to that produced from the raw biomass. These results of PM yields from the combustion of upgraded biomass and biochar at varied pretreatment temperatures strongly suggest that a moderate temperature range of 350–815 °C aggravates the emissions of fine particles.

Fig. 5 presents the mass-based PSDs and yields of fine particles generated from the combustion of biomass and its derived upgraded biomass and biochar. As shown in Fig. 5(a), the PSDs of PM10 from the combustion of both raw biomass and biochar generally show a bimodal distribution, with a fine mode < 1.0 μm and a coarse mode > 1.0 μm. The only exception is the case of the combustion of the biochar prepared at 1000 °C, in which the fine mode of PM emissions disappears and a unimodal distribution is observed. By comparing the PSDs of fine particles from raw biomass and upgraded biomass and biochar combustion in Fig. 5, it can be found that the yields of PM1.0 (fine mode particles) from the combustion of upgraded biomass and biochar produced at 250–500 °C are higher than that from raw biomass combustion. The PM1.0 yield from upgraded biomass and biochar combustion increases with the increasing in pretreatment temperature and reaches the maximum at 500 °C. When this temperature increases from 250 °C to 500 °C, the fine-mode peak center moves from 138 nm to 256 nm. However, with further increase to 815 °C, the PM1.0 yield is significantly reduced and is lower than that from raw biomass combustion, and the fine-mode peak center decreases to 65 nm. The formation of PM1.0 from biomass combustion starts with the vaporization of the volatile elements (mainly K, Cl and S), followed by the homogeneous nucleation and/or heterogeneous condensation of these vapors [14,15,30]. The presence of Cl in raw biomass and its derived upgraded biomass and biochar favors the formation of stable gaseous KCl during combustion [19,39]. The stable gaseous KCl further undergoes the homogeneous and heterogeneous nucleation, dominating the formation of PM1.0 [14,19]. As calculated by the content of Cl and K in biomass shown in Tables 1 and 2, the absolute mass of K in biomass is much higher than that of Cl, which indicates the concentration of KCl vapor and PM1.0 in flue gas might be mainly dominated by the Cl content in biomass. In high-temperature environment, the released K from biomass can either be captured by silico-aluminate oxides to form potassium-aluminosilicates in solid phase or combine with Cl to form KCl vapor in gas phase. For these two conversion paths, the K in flue gas prefers the second one to produce stable KCl in gas phase [39]. Therefore, the presence of Cl facilitates the formation of KCl molecules in flue gas, which further undergo homogeneous/heterogeneous nucleation, thus dominating the PM1.0 formation [19]. A proportional relationship between the PM1.0 yield from the combustion of different biomass in practical furnaces and the chlorine content in biomass was concluded by Christensen et al. [14]. In our study, as shown in Fig. 3(a),

3.3. Elemental compositions and morphologies of PM10 Elemental compositions of PM10 collected in each stage were analyzed by EDS, and the elemental mass size distribution of K, Cl, Ca, Mg and Si in PM10 from the combustion of raw biomass and its derived upgraded biomass and biochar are presented in Fig. 6. As shown in Fig. 6, K, and Cl are enriched and dominantly in PM1.0, which is consistent with the previous results [19,41]. This further demonstrates the dominant presence of K and Cl in PM1.0, which is mainly formed through the homogeneous nucleation and heterogeneous condensation of potassium-containing and chlorine-containing vapors [14,30]. However, Mg, Ca and Si are enriched in PM1.0–10, with tiny amount in PM1.0. It is probably attributed to the different formation mechanisms between PM1.0 and PM1.0–10, where PM1.0–10 are formed through ash/ char fragmentation and coalescence of inorganic refractory elements. Considering the dominance of K and Cl in the PM1.0 as shown in Fig. 6, the mole ratio of K/Cl along with particle size was calculated and shown in Fig. 7. The stoichiometric correlation between K and Cl indicates that those fine particles from biomass, and its derived upgraded biomass and biochar combustion are composed of only KCl. These results agree well with the previous results of PM emissions from biomass and biochar combustion [15,24]. The morphologies of fine- and coarse-mode particles generated from the combustion of upgraded biomass at a pretreatment-temperature of 250 °C by SEM are compared in Fig. 8. In fine-mode particles, it is 5

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8

K

K

K

K

K

Cl

Cl

Cl

Cl

Cl

Mg

Mg

Mg

Mg

Mg

Ca

Ca

Ca

Ca

Ca

Si

Si

Si

6 4 2 0 8

dm/dlog(Dp) (mg/g_fuel)

6 4 2 0 0.15 0.10 0.05 0.00 1.0

1.0

0.5

0.5

0.0 1.5

0.0

Si

Si

1.0 0.5 0.0 0.01

0.1

1

10 0.01

(a)

0.1

1

10 0.01

0.1

1

Dp (µm) (c)

(b)

10 0.01

0.1

1

(d)

10 0.01

0.1

1

10

(e)

Fig. 6. Elemental mass size distribution of K, Cl, Mg, Ca and Si in PM10 from the combustion of (a) raw biomass, (b) upgraded biomass produced at 250 °C, and biochar produced at (c) 350 °C, (d) 500 °C and (e) 815 °C.

Mole ratio of (Na+K)/Cl

2.0

1.5

3.4. Discussion on the K-Cl evolution during upgraded biomass and biochar combustion

Pretreatment temperature 105 C (raw biomass) 250 C 350 C 500 C 815 C

The relationship between the PM1.0 yield and the K-Cl content in upgraded biomass and biochar at different pretreatment temperatures is presented in Fig. 9. The result shows a linear relationship between the PM1.0 yield and the Cl content in upgraded biomass and biochar, whereas, no relationship is observed between the PM1.0 yield and the K content in upgraded biomass and biochar. This indicates that the formation of PM1.0 from upgraded biomass and biochar combustion is dominated by the Cl content in fuels but not K content. Yang et al. [20] compared the PM emissions from agricultural biomass with high Cl content and woody biomass with ignorable Cl content. It was found that the PM1.0 emission from woody biomass with ignorable Cl content is far less than that from agricultural biomass with high Cl content. The results of PM1.0 emissions from agricultural biomass and woody biomass strongly support the hypothesis that the Cl content of fuel determines the PM1.0 emission from biomass combustion as observed in our study. On the basis of our results, a schematic of K and Cl transformation from the upgraded biomass and biochar combustion is proposed in Fig. 10. Before the discussion, we summarized the reported data of the absolute K and Cl contents in biomass [14,42–44] and compared them in Fig. 11, which shows that the K content in biomass is higher than the Cl content. During the pretreatments, the chlorine in biomass is easier to release as devolatilization, resulting in a much higher potassium content than chlorine in upgraded biomass and biochar. After that, during combustion at high temperatures (e.g., 1200 °C, in this study), the organic and inorganic potassium and chlorine would be easy to release to the gas phase. In the gas phase, apart from potassium chlorides, the excess potassium exists mainly as potassium hydroxides

1.0

0.5

0.0 0.01

0.1

1

Dp ( m) Fig. 7. Mole ratios of K/Cl in PM1.0 along with the particle size.

dominated by the accumulation of regular-shape cube crystals, which is the representative structures of KCl. In coarse-mode particles, it is dominated by the smooth larger spheres of molten materials, and on the surfaces, a certain amount of unregular-shape substances can be observed, which is because of the adhesion of these accumulated finemode particles or heterogeneous condensation of gaseous KCl vapors. This is further confirmed by a certain amount of K and Cl in PM1.0–10 as shown in Fig. 6.

6

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KCl

KCl

(a)

(b)

Fig. 8. SEM morphologies of PM from the combustion of biomass pretreated at 250 °C: (a) fine-mode particles; (b) coarse-mode particle.

and affects the safety and efficiency of furnaces burning biomass fuels. KCl in the deposited particles on heating surfaces can directly react with iron, chromium, nickel and their oxides, and destroy the protective oxide film, thus accelerating the corrosion [47,48]. In addition, in the presence of both KCl and water vapor, the reduction of oxygen on the scale surface results in the formation of potassium hydroxide, releasing the chloride ions. The released chloride ions diffuse inwards, react with iron ions, and form iron chloride [49]. Note that this study shows the aggravated KCl-rich PM1.0 emissions for typically used temperature range of 250–500 °C for biomass upgraded. This indicates that it is necessary to take measures to address the aggravated fouling and emission problems associated with burning these upgraded biomass or solid residuals produced at moderate temperatures. Water-leaching treatment, co-firing with coal, or kaolin addition in upgraded biomass and biochar can be adopted to address these negative effects from the enrichment of K-Cl. Our previous studies have presented the high removal ratios of > 70% and 100% for potassium and chlorine, respectively [50]. On the other hand, the co-firing with coal and kaolin additives can enhance the conversion of alkali chlorides from gas phase into the solid phase via the reaction R2-R4 as described above [51,52].

and potassium atoms, which are very reactive and unstable. These two reactive and unstable species can be captured by the silico-aluminate oxides to produce potassium-aluminosilicates as the stable substances in solid phase, thereby to transfer a certain amount of K in coarse-mode particles (PM1.0–10). At the same time, a certain fraction of these two species can also be converted into KCl via the reactions with gaseous HCl in the gas phase [45]. The gaseous KCl, because of its better chemical stability, once formed, most KCl will be as the precursor of finemode particles. The KCl vapor then experiences a series of homogeneous nucleation and heterogeneous condensation on the surfaces of the existing fine-mode particles. This route is responsible for the PM1.0 formation during upgraded biomass and biochar combustion, and can explain why the higher Cl content leads to the higher PM1.0 emissions as shown in Fig. 5. The enhancement of K-Cl in and the high PM emissions from upgraded biomass and biochar give an alarm that heating pretreatment of biomass can result in the aggravated environment and safety problems during the upgraded-fuel combustion in industrial furnaces. It was widely reported that these KCl-rich particles are more inclined to adhere on the heating surfaces, aggravating the ash deposition and hot corrosion [16,46], which induces the deterioration of heat exchangers

10

PM1.0 yield (mg/g_fuel)

Solid: K Open: Cl Linear Fitting of Cl

Fig. 9. Relationship between the K/Cl content in upgraded biomass and biochar and the PM1.0 yield.

Pre-treatment temperature 815 C

815 C 350 C

8 350 C 250 C 250 C

6

4 500 C

500 C

2 1000 C

1000 C

0 0

1

2

3

9

10

11

K or Cl content in upgraded biomass and biochar (%) 7

12

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KCl(g)

Fig. 10. Schematics of K/Cl transformation during upgraded biomass and biochar combustion.

Nucleation/ Condensation KCl(s)

HCl(g) KOH(g) Evaporation/ Decomposition Organic-K/Cl

Reaction with HCl

K(g)

KCl aerosol (PM1.0)

Decomposition/ Oxidation

Reaction with Silicoaluminate Oxides

Char-K Pyrolysis Upgraded biomass and U biochar Particle Char Particle Inorganic-K/Cl (KCl(s))

K-aluminosilicate (PM1.0+)

3.0

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K content in fuel (%)

2.5

2.0

1.5

1.0

Christensen et al.[14] Steenari et al.[40] Gilbe et al.[41] Kundsen et al.[42]

0.5

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Cl content in fuel (%) Fig. 11. Summary of the K/Cl contents in biomass fuels.

4. Conclusions (1) Pretreatment temperature significantly affects the yield and PSD of PM emissions, through changing the char yield and K/Cl contents in char. With increase in pretreatment temperature, the PM1.0 emission of upgraded biomass and biochar combustion first increases because of the enhancement of K/Cl in upgraded biomass and biochar, reaches the maximum at 500 °C, and then decreases. A linear relationship between the PM1.0 emission and Cl content in upgraded biomass and biochar was found. (2) The combustion of biomass pretreated at 250–500 °C presents 1.5–2 times PM1.0 emissions than raw biomass, which alerts that the biomass upgrading at moderate temperatures result in aggravated fouling and fine particle emissions during upgraded biomass and biochar combustion.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51676157, 5161101654 and 91544108), the National Key Research and Development Program of China (No. 2016YFC0801904), and the Fundamental Research Funds for the Central Universities. 8

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