Influence of woody biomass (cedar chip) addition on the emissions of PM10 from pulverised coal combustion

Fuel 90 (2011) 77–86

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Influence of woody biomass (cedar chip) addition on the emissions of PM10 from pulverised coal combustion Lian Zhang a,⇑, Yoshihiko Ninomiya b, Qunying Wang b,c, Toru Yamashita d a

Department of Chemical Engineering, Monash University, GPO Box 36, Clayton, Victoria 3800, Australia Department of Applied Chemistry, Chubu University, 1200, Matsumoto-Cho, Kasugai, Aichi 487-8501, Japan c Advanced Fuel Utilisation Sector, Energy Engineering Research Laboratory, Central Research Institute of Electric Power Industries, 2-6-1 Nagasaka, Yokosuka-Shi, Kanagawa 240-0196, Japan d Coal and Environment Research Laboratory, Idemitsu Kosan Co., Ltd., 3-1 Nakasode, Sodegaura, Chiba 299-0267, Japan b

a r t i c l e

i n f o

Article history: Received 20 April 2010 Received in revised form 17 August 2010 Accepted 17 August 2010 Available online 16 September 2010 Keywords: Co-firing PM10 Synergetic interaction Homogeneous coagulation Surface reaction

a b s t r a c t Co-combustion of pulverised coal with a woody biomass, cedar chip was conducted in a lab-scale droptube furnace (DTF) to investigate the synergetic interaction between the inorganic elements of different fuels and the emissions of sub-micron particles (particles smaller than 1.0 lm in size, PM1) and supermicron particles (particles in the size range of 1.0–10 lm, PM1+) during co-firing. The mass fraction of cedar chip in fuel blend ranged from 10% to 50%. All the fuels were burnt in air at two furnace temperatures, 1200 and 1450 °C. The results indicate that, under an identical calorific input, combustion of cedar chip alone favored the emission of sub-micron PM1, which is dominated by volatile elements including K, Ca, Fe, Na and P. A large fraction of K and Na were most probably present as gaseous vapors in the furnace. The other metals mainly condensed into nano-scale nuclei which subsequently coagulated into a variety of sizes in flue gas. Coal combustion alone favored the release of super-micron particles rich in Al and Si. Emission of PM upon co-firing was a function of both cedar chip share and furnace temperature. At a small mass fraction for cedar chip in fuel blend, e.g. 10% tested here, interaction between the inorganic elements of single fuels was insignificant at either furnace temperature. Accordingly, the quantities of PM1 and PM1+ emitted from co-firing at 10% cedar chip were slightly higher than from the combustion of coal alone, due to the contribution of cedar chip. Significant interaction between the inorganic elements of single fuels was observed for co-firing of coal with >10% cedar chip at the furnace temperature of 1450 °C. As has been confirmed, adding 20–30% cedar chip to coal resulted in the shift of approximately 90% of PM1 and 50% PM1+ into coarse ash particles. For the cedar chip-derived alkali vapors and nano-scale/sub-micron particles, the rates of their shift into larger particles were influenced by two competing routes, homogeneous coagulation and surface reaction with coal-derived kaolin. In contrast, the shift of super-micron particles was primarily determined by their collision probability with the coal-derived mineral grains in bulk gas. A sticky surface for particles is also essential. The shift of individual metals into coarse ash differed distinctly from one another. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction Co-firing of coal with biomass is one of the effective means for the reduction of greenhouse gas emissions from coal-fired power plants. It is also one of the most efficient and economical ways for the utilisation of biomass [1–3]. As has been identified in both laboratories and plant fields [1], a partial substitution of coal with biomass has insignificant impact on or at worst slightly decreases the overall power generation efficiency. Co-firing also potentially mitigates the gaseous pollutants including SO2 and NOx, since biomass usually contains less sulfur and nitrogen than coal [1,4–6]. ⇑ Corresponding author. Tel.: +61 3 9905 2592; fax: +61 3 9905 5686. E-mail address: [email protected] (L. Zhang).

Nevertheless, queries regarding the emission of particulate matter smaller than 10 lm (PM10) from biomass has been raised [1,4,5,7,8]. PM10 emission is one of the major concerns related to the environmental impact of coal-fired power generation, which has been proven carcinogenic and harmful from the public health perspective [9]. The combustion-driven inorganic PM10 constitutes of two major fractions: sub-micron particulates smaller than 1.0 lm in size, PM1, and super-micron particles ranging from 1.0 to 10.0 lm in size, PM1+ [10–12]. The former fraction is principally formed by the vaporisation–condensation pathway [11,13–17]. Vaporisation of inorganic constituents occurs in the near-burner zone in a furnace, resulting in the formation of inorganic vapors which have the potential to condense and coagulate in the cooling sections

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L. Zhang et al. / Fuel 90 (2011) 77–86

downstream. In contrast, PM1+ is dominated by refractory elements which are generated by either direct liberation or fragmentation of inherent mineral grains [11,12]. Regarding the differences of coal and woody biomass in mineralogical property, the latter fuel contains a relatively large quantity of alkali and alkaline earth metals (AAEM) which are ionic salts or organically bound cations in the carbon matrix. These species readily convert into sub-micron alkali chlorides and sulfates during woody biomass combustion [8,18–22]. The coarse PM1+ fraction emitted from biomass combustion, as has been confirmed in the literature [10,23], is minor and rich in Ca-based compounds such as CaO. This is contrary to the coal combustion-derived PM dominated by coarse PM1+ [11,12], PM1 formed from coal merely accounts for approximately 1–2% on the mass basis of overall ash [11,24]. Co-firing of coal with biomass can be simply realised by directly feeding biomass into existing coal-fired boilers [25]. The PM10 emission from such a process has been reported changing unpredictably. Field measurement indicated an enhancement in the amount of PM10, the reasons of which however have not been clarified yet [26]. Conversely, the lab-scale co-firing of coal with sawdust indicated that, since a portion of the biomass-derived submicron particulates are captured by coarse coal ash, the PM10 emitted from co-firing shows a large mean size compared to that released from coal alone [27]. Thermodynamic equilibrium modelling also indicated that, the biomass-derived chlorides, e.g. NaCl(g) and KCl(g), have the potential to undergo sulfation and/or reaction with kaolin [28]. Correspondingly, the gaseous HCl is released meanwhile alkali elements are fixed into bottom ashes [29]. Nevertheless, as lots of the metals except Na and K have not been examined yet, it is still not fully understood whether PM10 emitted from co-firing and its properties are merely affected by the dilution effect of biomass, or by the synergetic interactions between the inorganic components of different fuels. Except the relatively abundant AAEM, the other metals in woody biomass also potentially affect the characteristics of co-firing ash such as leachability and toxicity [30,31]. In the present paper, a woody biomass was added to coal at a broad mass fraction variation from 10% to 50% to investigate its influence on PM10 emissions during pulverised coal combustion. Through size-segregation and extensive analysis of individual sizes/particles, the emission rates of PM10 and individual metals have been investigated intensively to clarify the interactions, if any, among the different metals and different particles during cofiring. Intensive investigation has been made on the shift of cedar chip-derived inorganic vapors and sub-micron particles during co-firing. The shift rate of PM1+ during co-firing was also addressed. This study aims to elucidate the optimal cedar chip share in fuel blend to minimise PM10 emissions during co-firing.

2. Experimental 2.1. Fuel properties A pre-dried bituminous coal and a cedar chip sample pulverised to 6125 lm and 6420 lm, respectively were tested. As shown in Table 1, the calorific value of cedar chip is approximately 60% of that of coal. It contains a large fraction of volatile matter and less fixed carbon. The total carbon is only 53.3 wt.%, which is far lower than that in coal. Moreover, only 0.9% ash, 0.01% S and 0.13% N are present in cedar chip, relative to the high contents of these impurities in coal. These properties for cedar chip are similar to that of the woody biomasses reported in the literature [2,18,32]. The bulk low-temperature-ash (LTA) compositions of these two fuels in Table 2 indicate that, coal ash is made up almost exclusively of SiO2 (40.8%) and Al2O3 (31.1%), with minor amounts of

Table 1 Properties of the raw coal and cedar chip used in this study.

Caloric value, kcal/kg Proximate analysis, wt.%, as-received Moisture Ash Volatile matter Fixed carbon Ultimate analysis, wt.%, daf C H N S O (by difference)

Coal

Cedar chip

6980.0

4340.0

3.0 9.3 33.2 54.5

12.4 0.9 71.7 15.0

82.0 5.3 1.4 0.4 10.9

53.3 6.2 0.1 0.01 40.4

Table 2 Elemental composition of the LTAs of two single fuels, wt.%.

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O P2O5 SO3 TiO2 V2O5 MnO

Coal

Cedar chip

40.8 31.1 4.8 7.4 2.1 1.0 1.1 0.6 3.6 1.5 2.1 <0.1

37.2 8.7 6.2 23.0 4.9 6.3 7.5 0.9 1.8 0.3 0.0 0.1

CaO, Fe2O3 and SO3. In contrast, the biomass LTA ash is rich in SiO2 (37.2%), CaO (23.0%), Na2O (6.3%), K2O (7.5%) and Fe2O3 (6.2%). 2.2. Combustion conditions, PM10 sampling and characterisation methods Combustion was carried out in a lab-scale DTF. Its configuration and combustion procedure have been described elsewhere [33]. The reaction temperatures of 1200 and 1450 °C, air and a nominal gas residence time of 3 s were employed as the combustion conditions. Six fuels were tested, including two pure fuels and four blends with the cedar chip mass fraction varying from 10% to 20%, 30% and 50%. The calorific input of all the fuels was kept constant through regulating their feeding rates. PM sampling procedure is same as that has been adopted during coal studies [12]. In brief, ash particles were sucked iso-kinetically by a nitrogen-quenched water-cooled sampling probe in the reactor. Subsequently, the particles larger than 10.0 lm, defined as coarse ash hereafter, were collected by a cyclone installed underneath the sampling probe. The fine particles, i.e. PM10, were further introduced into a 13-stage low-pressure-impactor (LPI) for sizesegregation. PM1 defined here is the sum of the particulates with a d50 (a 50% collection efficiency for the size on a stage of LPI) from 0.03 through to 0.76 lm. The remaining sizes, ranging from 1 through to 10 lm, are assigned as PM1+. The mass balances of the metals are rather satisfactory, particularly in the case when coal was used as a single fuel or mixed with cedar chip. For cedar chip alone, the mass balances for a variety of metals including Na (40% recovered), K (74% recovered), Si (68% recovered) and Mg (71% recovered) were not closed. This is reasonable as the overall ash content in cedar chip is extremely low (see Table 2). The elemental compositions of coarse ash and particles in each size bin of PM10 were determined by X-ray Fluorescence spectroscopy (XRF). Morphologies and compositions of individual particles

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L. Zhang et al. / Fuel 90 (2011) 77–86

of Al and Si to the metals including Na, K, Ca, Mg and Fe is suitable for slag/liquid formation in these cases.

were determined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) coupled with Energy-dispersive X-ray spectroscopy (EDS). Computer-controlled SEM (CCSEM) was used to measure the size-dependent properties of inherent metals in the single fuels and the coarse ash particles derived from combustion.

3. Results and discussion 3.1. Mineralogical properties of single fuels

2.3. Thermodynamic equilibrium modelling

The inorganic elements in cedar chip are formed by plant uptake from soil and rain-water during growth [35,36], which are thus distinctly different from coal mineral grains in terms of chemical/physical properties. Sequential leaching with the initial use of Milli-Q water followed by 1 M ammonium acetate was employed to indirectly determine the modes of occurrence of the metals of interest. Water is expected to extract the water-soluble compounds, whilst ammonium acetate washing is available to quantify the amounts of ion-exchangeable cations associated with carboxylic acids. As evidenced in Table 3, approximately half of Ca and Mg are ion-exchangeable cations in cedar char. Their remaining fractions are mostly water-insoluble, which might be minerals such as calcium oxalate and/or calcite embedded deeply in carbonaceous matrix [36]. Regarding alkali elements, K is mainly ion-exchangeable whereas Na is principally present as complex species that are dissoluble in these two weak leaching agents. Fe is also mostly insoluble in water or ammonium acetate, indicative of a complex structure such as carboxydrate, chelate and/or organic sulfates that are organically associated [36]. P is mainly water-soluble phosphates. All these are significantly different from the corresponding metals in coal. Few of the metals in coal were found soluble in the two leaching agents used here. Typical microstructures of the cross-sections of two single fuels in Fig. 1, determined by SEM in back-scattered electron (BSE) mode, further prove the differences in the mineralogical properties of these two fuels. For the SEM-detectable particle grains (right

Thermodynamic modelling of ash formation based on the minimisation of the total Gibbs free energy of a system is a useful way for the simulation of the transformation of inorganic elements [34]. A commercial software program, FactSage 6.1, was employed in this study. For all the calculations the input supplied to the model is the elemental analysis of a fuel (C%, N%, H%, S%) and ash composition. The initial state for the metals in cedar chip was chosen as gas phase, considering that the abundant ion-changeable cations and high-volatile ionic salts (e.g. chloride) can quickly vaporise at low temperatures to form reactive radicals or metallic vapors. In the case of cedar chip combustion alone, the product database chosen includes real gases, pure liquids and solids. On the other hand, the database of solid solution and slag-liquid was also chosen for the combustion of coal alone and fuel blends as the molar ratio

Table 3 Sequential leaching results for the major inorganic elements in cedar chip, wt.%.

Ca Fe K Mg Na P

Water-soluble

Ion-exchangeable Cations

Insoluble fraction

4.0 10.5 10.0 18.1 21.2 87.8

40.3 0.0 90.0 46.1 1.9 12.2

55.7 89.5 0.0 35.8 76.9 0.0

Fig. 1. Cross-sections of cedar chip (a) and coal (b) observed by SEM in back-scattered electron (BSE) mode.

Table 4 Mineralogical properties of the bituminous coal tested here, wt.% on the mass basis of overall minerals. Category

Si–O Fe–O Ca–O Ca–Mg–O Si–Al–O Fe–S Unknown Totals

Particle diameter (lm)

Totals

0.5–1.

1–2.2

2.2–4.6

4.6–10.

10–22

22–46

46–100

0.2 0.0 0.1 0.1 1.5 0.0 0.7 2.6

1.2 0.4 0.6 0.7 7.9 0.5 1.8 13.9

0.4 1.2 1.1 0.4 12.3 0.7 4.0 20.1

1.2 0.1 0.4 0.8 13.4 0.7 4.4 21.0

0.3 0.0 0.1 1.3 6.3 1.5 2.4 11.9

0.3 0.0 0.5 1.1 7.4 1.2 2.7 13.2

3.0 0.0 1.1 0.0 9.2 2.0 2.1 17.4

6.9 1.7 4.0 4.4 57.9 6.5 18.6 100.0

L. Zhang et al. / Fuel 90 (2011) 77–86

spots) in cedar chip, nearly all of them are present as sub-micron species in the porous carbonaceous matrix. EDX analysis confirmed the presence of Si–O, Si–Al–O, Fe–S–O associates and complex species with more than three elements in non-stoichiometric ratios. Clearly, in addition to the sequential leaching results shown above, the result here further strongly indicates that a portion of the acidinsoluble fraction of the inorganic elements listed in Table 3 are sub-micron particles embedded deeply in carbon matrix in cedar chip. Due to the protection effect of carbon moieties, these species remained intact during acid leaching. The inorganic elements in coal are exclusively mineral grains with a portion being firmly embedded in coal matrix, as demonstrated in Fig. 1b. Statistic analysis of >3000 particles by CCSEM in Table 4 identified a broad size distribution with two major peaks at 4.6 and 46.0 lm for coal minerals, relative to a single distribution peaking at 4.6 lm for the detectable minerals in cedar chip (data not shown). The associates of Si–Al–O (kaolinite), Si–O (quartz), Fe–S (pyrite) and non-stoichiometric complexes are predominant in coal. 3.2. Combustion of pure fuels Combustion of coal alone led to the emission of abundant particles larger than 1.0 lm, as has been described explicitly elsewhere [12]. For instance, at the furnace temperature of 1450 °C, PM1+ emitted from coal alone accounts for 40.0 mg/g-coal, relative to less than 1.0 mg/g-coal for PM1. Moreover, the coal-derived PM1+ is rich in Al2O3 and SiO2, exhibiting a similar composition to the original mineralogical properties. The presence of these two elements in PM1+ was mostly caused by a direct liberation and/or fragmentation of the original kaolin and quartz in coal. The ash released from combustion of pure cedar chip is completely smaller than 10 lm in size. Fig. 2 illustrates its particle size distributions at two furnace temperatures. Note that the unit of Yaxis refers to the quantity of a particulate size on the mass basis of cedar chip with a calorific value equivalent to that of pure coal. Such a definition was used throughout this paper for the comparison of the PM properties emitted from different fuels on the same basis, i.e. equivalent coal mass with identical calorific value. As can be seen, the PM emitted from combustion of cedar chip alone at 1200 °C distributes rather uniformly among different sizes. In contrast, two major peaks centering at 0.1 and 2.0 lm were observed at 1450 °C. The larger quantity of the latter peak implies the intense coagulation of primary nuclei at 1450 °C.

The size-dependent distribution of individual elements in PM10 was further calculated, according to

C i;j  M j W i;j ¼ P  100 Mj

ð1Þ

where Mj denotes the mass of the jth size, Ci,j, refers to the mass content of the ith element in the jth size range. Therefore, Wi,j denotes the percentage of the ith element in the jth size range on the mass basis of overall PM10. Fig. 3 indicates a noticeable variation of the elemental distribution with particulate size. Fe and Ca are most abundant. Fe has a relatively uniform distribution, in comparison to Ca which is mostly present in the sizes larger than 0.76 lm. Regarding the other metals, K is most abundant and mainly distributed in nano-scale sizes and possesses a similar tendency with that of S and Cl. Increasing furnace temperature to 1450 °C caused substantial changes to the distribution of the elements in the cedar chip-derived PM. Abundant Si and Al was observed in the super-micron particles including 1.1 and 2.5 lm, implying the intense fragmentation and/or vaporisation of these two refractory metals at higher furnace temperatures. Fe is enriched in the sub-micron particles, whereas Ca shows a rather broad distribution with relative abundance in the super-micron sizes. The size distributions of K and P are also broad. Less S and Cl were found in the PM emitted at 1450 °C than 1200 °C.

40 3.6 µm 2.5 µm 1.1 µm 0.76 µm 0.50 µm 0.30 µm 0.20 µm 0.12 µm 0.05 µm 0.03 µm

(a) % distributed in overall PM

80

32

24

16

8

0 Si

Al

Fe

Ca

Mg

K

Na

P

S

Cl

40 3.6 µm 2.5 µm 1.1 µm 0.76 µm 0.50 µm 0.30 µm 0.20 µm 0.12 µm 0.05 µm 0.03 µm

% distributed in overall PM

(b) 32

24

16

8

0 Si

Fig. 2. Particle size distribution of PM10 formed cedar chip combustion.

Al

Fe

Ca

Mg

K

Na

P

S

Cl

Fig. 3. Elemental compositions of individual sizes in the PM10 emitted from cedar chip combustion at 1200 °C (a) and 1450 °C (b).

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L. Zhang et al. / Fuel 90 (2011) 77–86

Fig. 4. Microstructures of the particles formed from cedar chip combustion at 1450 °C. Panels (a) and (b) are for the size of 0.5 lm. Panels (c) and (d) are for the size of 2.0 lm. Note oxygen was excluded in panels (b) and (d).

100

SEM and TEM observation of two typical particulates, 0.5 and 2.5 lm in Fig. 4, further evidences the difference in the formation routes of these two sizes. The prevalence of nanoparticles of 20– 50 nm was confirmed in the first size, as demonstrated in panel a where the photograph embedded on the right bottom was obtained from TEM observation. EDS analysis in panel b confirmed the presence of a number of elements including Ca, Fe, Mg, Si, P, Cl and S. Such a finding is broadly consistent with the results in Fig. 3b. On the other hand, the particles in PM1+ exhibit a primary size of approximately 1.0–2.0 lm in Fig. 4c. Elemental analysis in panel d for the selected spot in panel c indicates the abundance of Ca, followed by Si, Al, K and Cl in a descending order. Vaporisation of metals plays a crucial role on the formation of nanoparticles and even sub-micron particles in PM1. Compared to discrete mineral grains, the ion-exchangeable cations and water-soluble ionic salts vaporises readily. The partitioning of the resulting metallic vapors however varies greatly with the elemental type. As confirmed by the thermodynamic modelling results in Fig. 5, among the metals studied here only the alkali metals are able to remain in gas phase after vaporisation. The resulting alkali metal vapors are mainly present as chloride and/or hydroxide with

80 Gases (e.g. NaOH, NaCl)

Wt%

60

40

Na2SO4

20 NaOH

0 100

80 Gases (e.g. KOH, KCl)

Wt%

60

40 K2SO4 Table 5 Thermodynamic modelling prediction on the chemical forms of super-micron PM1+ emitted from cedar chip combustion, wt.%.

20 KOH

0 600

800

1000

1200

1400

1600

1800

o

Temperature, C Fig. 5. Thermodynamic modelling prediction of the partitioning of alkali metals during cedar chip combustion.

CaO Ca2SiO4 MgO Ca2Fe2O5 Slag-liquid

1200 °C

1450 °C

29.1 26.6 6.6 2.7 35.0

20.9 28.0 5.7 0.0 45.4

L. Zhang et al. / Fuel 90 (2011) 77–86

4.0 3.0 2.0 1.0

30 20 10 0

Cedar Chip

Mixture, 50/50

Mixture, 70/30

Mixture, 80/20

Mixture, 90/10

0.0

40

Cedar Chip

5.0

(b) 50

Mixture, 50/50

6.0

60

Mixture, 70/30

(a)

Measured Calculated

Mixture, 80/20

7.0

Coal

Amount, mg/g_equivalent coal

Concentrations of the two fractions of PM emitted from co-firing at 1200 °C were plotted as a function of cedar chip fraction in fuel blend in Fig. 6. Here again, the concentrations of these two PM fractions were expressed on the mass basis of fuel with a calorific value equivalent to that of pure coal. The gray bars, termed measured, are the experimental results obtained from all the cases,

Mixture, 90/10

3.3. PM emissions during co-firing of coal and cedar chip

while the dotted white bars are the prediction results calculated by a linear combination of the experimental results for two pure fuels. On the basis of equivalent calorific input, cedar chip alone released approximately 6.0 mg/g PM1 at 1200 °C, relative to 0.8 mg/g from coal combustion alone. The quantity of PM1 emitted from co-firing was also found increasing proportionally with the increase in cedar chip share. Considering the relatively large experimental error for PM1 sampling, it is safe to conclude that the PM1 quantity emitted from co-firing at 1200 °C is a linear combination of the results for single fuels. In other words, cedar chip is the major source for PM1 emission at this low furnace temperature. Regarding the formation of PM1+, coal alone released approximately 50 mg/g, relative to less than 10 mg/g for cedar chip alone. Co-firing however led to the remarkable reduction in the emission of PM1+. At the high cedar chip shares of 30% and 50%, the PM1+ quantities measured are far lower than the corresponding prediction results.Fig. 7 illustrates the concentrations of the two factions of PM10 as a function of cedar chip fraction during co-firing at 1450 °C. Compared to coal combustion alone, co-firing of coal with 10% cedar chip released a noticeably large quantity of PM1, indicative of the noticeable contribution of cedar chip to the emission of sub-micron particles at a small share of cedar chip in flue blend. This finding is consistent with the plant field observations [26]. However, with the cedar chip fraction increasing to 30%, an obvious reduction in the PM1 amount was observed. Comparison of the experimental and prediction results further indicates that approximately 90% of the cedar chip-derived PM1 shifted into

Coal

high partial pressures in the furnace, which apparently nucleated into nano-scale particles during flue gas cooling/quenching. This explains the enrichment of K in the sub-micron particles. Formation of liquidus sulfates is also noteworthy, which are able to condense/coagulate into large particles through collision with other particles. The abundant water-insoluble Fe-bearing mineral grains in cedar chip were preferentially released as solid particles in gas stream during combustion. Even for Ca and Mg, in addition to their original mineral grains, the ion-exchangeable fraction of these two metals also preferentially coagulated into particles larger than 1.0 lm during combustion, as visualised in Fig. 4c. The thermodynamic equilibrium modelling results in Table 5 further indicate that, the solid oxides and solid solution (slag) are the most probable forms for the refractory metals containing Ca, Mg, Si and Fe in the super-micron sizes formed at either furnace temperature. Increasing temperature from 1200 to 1450 °C favored the melting of solid species in PM1+.

Amount, mg/g_equivalent coal

82

Cedar Chip

Mixture, 50/50

Mixture, 70/30

Mixture, 80/20

Mixture, 90/10

0.0

10 0

Fig. 7. Emissions of PM1 (a) and PM1+ (b) during co-firing at 1450 °C.

Cedar Chip

1.0

20

Mixture, 50/50

2.0

30

Mixture, 70/30

3.0

40

Mixture, 80/20

4.0

(b) 50

Mixture, 90/10

5.0

60

Coal

(a)

Measured Calculated

6.0

Amount, mg/g_equivalent coal

7.0

Coal

Amount, mg/g_equivalent coal

Fig. 6. PM1 (a) and PM1+ (b) emissions during co-firing of coal and cedar chip at 1200 °C.

L. Zhang et al. / Fuel 90 (2011) 77–86

coarse particles at the cedar chip shares larger than 10%. Similar phenomenon was observed for PM1+ emissions. Elemental analysis of individual particles further indicates the importance of fuel mixing on PM properties. Typical results for co-firing of coal with 50% cedar chip are shown in Fig. 8. Based on the comparison of Figs. 8a and 3a, one can clearly conclude that, the abundant Si and Al in the PM emitted from co-firing at 1200 °C were contributed from coal, as these two elements are predominant in the coal-derived PM. Except S, the other elements show a rather similar distribution with that in the PM emitted from cedar chip alone, evidencing the significant contribution of cedar chip to these metals in PM emission at 1200 °C. Comparison of the elemental composition of individual sizes in Figs. 8b and 3b also indicates the significant variation of PM composition with fuel type (i.e. single or blended fuel) at 1450 °C. Si, Al, 40 3.6 µm 2.5 µm 1.1 µm 0.76 µm 0.50 µm 0.30 µm 0.20 µm 0.12 µm 0.05 µm 0.03 µm

% distributed in overall PM

(a) 32

24

16

8

0 Si

Al

Fe

Ca

Mg

K

Na

P

S

Cl

40 3.6 µm 2.5 µm 1.1 µm 0.76 µm 0.50 µm 0.30 µm 0.20 µm 0.12 µm 0.05 µm 0.03 µm

% distributed in overall PM

(b) 32

24

16

8

0 Si

Al

Fe

Ca

Mg

K

Na

P

S

Cl

Fig. 8. Distribution of individual elements in the PM collected from co-firing of coal mixed with 50% cedar chip. Panels (a) and (b) are for 1200 and 1450 °C, respectively.

83

Fe and Ca are still dominant in the PM emitted from co-firing. Their contents in the sub-micron sizes are however greatly reduced. Moreover, less K and P were found in the PM emitted from co-firing, suggestive of a substantial shift of these two elements into coarse ash particles. The similar conclusion can be drawn for the four major metals from Si through to Ca, as their overall contents in the co-firing-derived PM was greatly lowered when compared with the results in Fig. 3b for the PM emitted from cedar chip alone. Microstructures of typical coarse ash particles generated from combustion of different fuels in Fig. 9 proven the significant interaction between inorganic constitutes during co-firing. The particles released from coal alone are exclusively composed of porous kaolin and/or its derivatives such as meta-kaolin and/or mullite (panel a), which are however slightly coated with ultra-fine nano-particle deposits when 10% cedar chip was added to coal (panel b). With the cedar chip mass fraction increasing to 50%, more nanoparticles and even those in micrometer size deposited on the coal-derived coarse particles, as demonstrated in panel c. The coarse particles derived from co-firing also greatly agglomerated with one another. 3.4. Transformation of individual elements during co-firing As discussed above, at the furnace temperature of 1200 °C, the PM1 emitted from co-firing was mainly contributed from cedar chip. Apparently, the routes for the growth of the cedar chip-derived inorganic vapors and primary nuclei underwent little interaction with the presence of coal mineral grains in the furnace, probably due to their low contents in bulk gas. In other words, irrespective of coal addition, the metallic vapors and ultra-fine particulates derived from cedar chip preferentially undertook self coagulation in free molecular range in the gas phase. The presence of gaseous alkali elements released from cedar chip is also beneficial for the capture of S in the bulk gas, as demonstrated in Fig. 10. The contents of S in either fraction of PM increased rather linearly with the increase of cedar chip share in fuel blend. A capture ratio as high as 25% was even achieved during co-firing of coal added with 50% cedar chip, which should be attributed to the formation of K sulfate, as indicated by a similar tendency for the variation of the contents of S and K with particle size in Fig. 8a. The alkali sulfate liquidus could readily condense and coagulate into super-micron particles in the high supersaturation regions in the temperature range of 723–1223 °C [36]. Formation of sulfate is thermodynamically unfavorable at 1450 °C. Accordingly, the vaporised metals and/or their primary nuclei underwent either heterogeneous condensation or chemical reaction/adsorption with coal-derived refractory particle grains. Chemical reactions between kaolin and most of the metals discussed here are thermodynamically favorable at elevated furnace temperatures. Fig. 11 illustrates the extents for a variety of metals from Na through to Fe shifting into the particulates larger than 1.0 lm during co-firing, which were calculated by comparing the measured quantity of a certain metal in the co-firing derived PM1

Fig. 9. Typical microstructures of coarse ash particles (>10 lm) formed during combustion of coal alone (a), co-firing of coal with 10% cedar chip (b) and 50% cedar chip (c) at 1450 °C.

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15

100 PM1+

12

80

Wt% of liquid-solution

Wt% transferred into PM

PM1

9

6

3

50

(3) (2) (1)

20

1200

1400

1600

1800

2000

Reaction temperature, C

ix tu

re

,5

0/ ,7 re

Fig. 12. Predicted liquid amounts in co-firing cases (the figures in parentheses denote the co-firing cases having various cedar chip shares of 10%, 20%, 30% and 50% in increasing order).

M

ix tu M

40

o

0/

30

20 ix tu

M

ix tu

re

re

,9

,8

0/

0/

10

l C oa

(4)

0 1000

0

M

60

Fig. 10. Transformation of S into PM during co-firing at 1200 °C.

100

Mg

K

Shift Percentage, %

Na

Fe P

Ca

80

60

40

20

0 0

100

200

300

400

500

600

700

Amounts Derived from Cedar Chip, µg/g-coal Fig. 11. Capture efficiencies of cedar chip-derived gaseous and sub-micron particulates during co-firing at 1450 °C.

and its predicted value based on a linear combination of the results for two single fuels. Irrespective of the elemental type, the shift percentages of these elements increased rapidly at low contents, which eventually leveled off at approximately 90% during the combustion of coal added with 30–50% cedar chip. More interestingly, it is obvious that Na shift exhibited the highest rate, which is followed by the shift of Mg, K, P, Ca and Fe in a descending order, although the amounts of these metals show a reverse trend. The modes of occurrence of these metals in the furnace can partly explain this phenomenon. As has been discussed before, except Na, K and P, the other elements discussed here preferentially converted into particles through oxidation (see Table 5). The resulting particles thus possess low diffusivity in continuum range relative to that of gaseous alkali metals in free molecular range. Accordingly, their collision with coal-derived mineral grains is less frequent. In contrast, the locally homogeneous coagulation could be enhanced and become more competitive. The larger the resulting particle cluster is, the lower the chance for these metals to travel to bulk gas and collide/react with other mineral grains, particularly with these derived from coal matrix. This explained both the flattening

of the shift ratio of a certain element at high contents and the lowest shift rate for Fe-bearing species with the largest content emitted during co-firing. Formation of molten species through the chemical reaction of cedar chip metals and coal-derived kaolin grain on its surface to cause the deactivation of kaolin is another plausible explanation [36]. As predicted by thermodynamic modelling (Fig. 12), the addition of 10% cedar chip to coal has the potential to yield up to 10% molten species at 1450 °C, the influence of which on kaolin structure/deactivation is thus insignificant. Indeed, a small amount of low-melting eutectics can even break down the closely packed crystal structure of kaolin to improve its capture efficiency [37]. This clearly applies to the metals with low contents such as Na, Mg and K. However, with the increase in the contents of cedar chip-derived metals, the eutectic melt amount can reach a level as high as approximately 60% at a cedar/coal mass ratio of 50/50, which is obviously excessive and hence potentially closes the pores of kaolin and triggered a severe agglomeration of sticky particles, thus deactivating kaolinite before its complete utilisation has been achieved. The super-micron particulates derived from cedar chip (i.e. PM1+) also shifted into coarse ash species. Their shift, however, should be primarily induced by the particle–particle collision. The probability for this type of collision is obviously low, as discussed above. Moreover, unless the surfaces are sufficiently sticky, the solid particles are not able to bind together. Due to these considerations, it is inferable that the shift ratios for metals in PM1+ could be lower than those in PM1. This is proven in Fig. 13. Irrespective of the metallic type, the super-micron particles showed a maximum shift ratio of 70% when 50% cedar chip was added to coal, in comparison to approximately 90% for the metals in PM1. Little shift was observed in the case of adding 10% cedar chip to coal, indicating that the cedar chip-derived super-micron particles in this case is too dilute to undertake any collision to grow up. With the increase of the content of a metal emitted as PM1+ from cedar chip, a linear increase of its shift ratio was observed, supporting the above argument that the shift of PM1+ is principally determined by the particle–particle collision probability in gas stream. Difference in the shift ratios of the PM1+-bound individual metals is also remarkable. Irrespective of furnace temperature, Na shift still dominated over the shift of other metals, followed by Mg, P, Fe and Ca in a descending order, although their concentrations show a reverse order. These observations are consistent with that ob-

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100

100

(a)

o

PM1, 1450 C o

PM1+, 1200 C

80

Mg

Reduction Efficiency, %

Shift Percentage, %

80 Fe Na

Ca

60 P

40

20

o

PM1+, 1450 C 60

40

20

0 0

500

1000

1500

2000

2500

Amounts Derived from Cedar Chip, µg/g-coal

0 10

20

30

40

50

Cedar Chip Mass Fraction, wt% 100

(b)

Fig. 14. Overall reduction efficiency of PM as a function of cedar chip share during co-firing.

Shift Percentage, %

80 Mg P

Fe Ca

60 Na

40

20

0 0

500

1000

1500

2000

2500

Amounts derived from Cedar Chip, µg/g-coal Fig. 13. Capture efficiencies of cedar chip-derived super-micron particle-bound metals during co-firing at 1200 °C (a) and 1450 °C (b).

served for the sub-micron particles in Fig. 11. The low shift of Ca relative to Mg is broadly consistent with the observations during combustion of coal added with Ca-/Mg-based sorbents [30,38,39]. The most plausible explanation is the modes of occurrence of the different metals in the PM1+ released from cedar chip. The alkali metals are still probably inorganic vapors, which thus readily diffuse through gas stream to reach coal-derived mineral grain surface. For the other metals, particularly Ca, they are mostly present as solid oxide (see Table 4). Apparently, even though these particle-bound metals could collide with coal-derived kaolin, they could quickly bounce away in the case that the particle surfaces are not sufficiently molten. Increasing furnace temperature showed very limited impact on the melting of the cedar chip-derived super-micron particles (see Table 4), thereby only affecting slightly on the shift ratios of the metals in the super-micron particles, as shown in Fig. 13b. Clearly, compared to the sub-micron particles, the super-micron particles are difficult to capture by coal-derived kaolin during co-firing. The results obtained for the competitive shift of the cedar chipderived multiple metals to coarse ash particles are useful for the management of the pollutant emissions during co-firing of coal with woody biomass. In summary, the overall PM1 and PM1+ emission reduction efficiencies were plotted versus cedar chip share in fuel blends in Fig. 14. Here again, the reduction efficiency of each

fraction of PM was calculated by comparing its measured quantity with the corresponding value predicted from linear combination of the results for single fuels shown in Figs. 6 and 7. It is clear that, a cedar chip fraction of around 20–30% and a furnace temperature higher than 1200 °C are essential for the minimisation of the PM emission during co-firing. Due to the control of both diffusion and kinetic, a small share of cedar chip such as 10% is insufficient to interact with coal combustion-driven particles. As a result, the PM emission at low cedar shares in fuel blend is increased, rather than being decreased. More ultra-fine particles will be released from the coagulation of the metallic vapors derived from the organically bound cations and ionic salts in cedar chip. Reduction in PM1 during co-firing at 1200 °C or less is not practical. In contrast, under the co-firing conditions of the furnace temperature of 1450 °C and the cedar chip mass P30%, a maximum reduction efficiency of approximately 90% can be achieved for the PM1 emission. PM1+ emission reduction is difficult, which reaches up to 40% under the afore-mentioned optimal conditions for PM1 emission control. Increasing the collision frequency and melting propensity of particles are two vital factors determining the PM1+ and overall PM emission control during co-firing. 4. Conclusions Co-firing of a bituminous coal with woody cedar chip was conducted at two furnace temperatures (1200 and 1450 °C). Apart from the bulk properties of PM and its two fractions, sub-micron PM1 and super-micron PM1+, the properties of individual particles/metals have been examined intensively. In consistence with the literature, increase in PM emission was observed at a low addition fraction of cedar chip to coal, 10% tested in this study. Adding more cedar chip in fuel blend is beneficial for the reduction of PM emission, particularly at 1450 °C, due to the synergetic interactions between the inorganic elements of single fuels. The major conclusions are drawn as follows. 1. Irrespective of furnace temperature, the ash particles derived from the combustion of cedar chip alone are exclusively smaller than 10 lm partitioning between the sub-micron particles and super-micron particles with nearly comparable amounts. Three metals including K, Fe and Ca are dominant. K could be initially

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present as gaseous chloride/hydroxide in the furnace. The other two metals are mostly present as solid oxides preferentially coagulating into super-micron sizes. 2. The shift ratios of cedar chip-derived PM1 and individual metals are dependent on furnace temperature and the elemental type. The interaction of cedar chip-derived PM1 with coal mineral grains is favored at the furnace temperatures above 1200 °C, which is also affected by a variety of influential factors including the diffusion rate of metal nuclei, homogeneous coagulation extent of metallic nuclei, and the melting and agglomeration extent of refractory kaolin particles. 3. The shift ratio of the cedar chip-derived PM1+ into coarse ash particles larger than 10 lm is low, and is limited by the low frequency for the collision of solid particles and their melting extents. Increasing furnace temperature showed very limited influence in improving these two issues. 4. A cedar chip mass fraction of 20–30% is essential for an efficient control of the emissions of PM1 and PM1+ during co-firing of coal with woody biomass under the typical pulverised coalfired conditions.

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