Ash Characterization and Transformation Behavior ... - ACS Publications

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Ash Characterization and Transformation Behavior of the Fixed-Bed Combustion of Novel Crops: Poplar, Brassica, and Cassava Fuels Maryori Díaz-Ramírez,*,†,‡,§ Christoffer Boman,§ Fernando Sebastián,‡ Javier Royo,† Shaojun Xiong,∥ and Dan Boström§ †

Department of Mechanical Engineering, University of Zaragoza, María de Luna, 3, E-50018 Zaragoza, Spain Centre of Research for Energy Resources and Consumption (CIRCE) Foundation, E-50018 Zaragoza, Spain § Energy Technology and Thermal Process Chemistry, Umeå University, SE-901 87 Umeå, Sweden ∥ Biomass Technology and Chemistry, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden ‡

ABSTRACT: New biofuel raw materials for energy pellet production are now being studied as potential energy sources for the heating market. Because of the complexity of the chemical and physical properties of novel fuels, such as some agricultural residues and energy crops, the study of their ash-related aspects is crucial for the sustainable development of this potential energy sector. Ash fractions formed during fixed-bed combustion of different pelletized novel crops; i.e., two Mediterranean crops (one herbaceous, brassica, and one woody species, poplar) and three Chinese cassava stems (cassava species from three different Chinese regions), and three Chinese cassava stems (cassava species from three different Chinese regions), were characterized, and their formation paths assessed in this study. Special emphasis was placed on elucidating the role of major ash-forming elements in the fractionation and transformation behavior, leading to the formation of bottom ash, deposits, and particulate emissions (fine and coarse ash particle fractions) on the basis of experimental data. In the Mediterranean fuels, the predominant ash fraction obtained was bottom ash, mainly characterized by silicates. Phosphates were found to be the main crystalline phases in the Chinese fuels. The slagging tendency was low for all of the fuels, although more significant for the cassava species under the studied conditions. Further, combustion of the studied Chinese energy crops resulted in a considerably finer particle fraction compared to the Mediterranean fuels. Deposits and particulate matter were dominated by K-sulfates as well as K-chloride in all fuels (except poplar), with the occurrence of K-phosphates for cassava pellets. Overall, this study showed fundamental differences in ash transformation behavior during combustion of P-rich fuels (i.e., cassava mixtures) compared to Si-rich fuels (i.e., poplar and brassica mixtures). Of major importance is the experimental verification of the higher thermodynamic stability of phosphates in relation to silicates. Furthermore, in P-rich fuels at high (K + Na)/(Ca + Mg) ratios, a significant degree of alkali metal volatilization occurs, which forms larger amounts of particulate matter, whereas this ratio has no/low effect in Si-rich fuels at high alkali metal ratios. tion,4,5 the use of novel crops, such as cassava-based (i.e., Manihot esculenta) fuels, for thermal energy application has recently been explored. Energy crops cultivated today can generally be described as “problematic” fuels largely because of their high ash content and their ash-forming matter, unlike the biofuels used traditionally, such as stemwood-based assortments.6−9 Consequently, a higher tendency to cause fouling, slagging, and/or corrosion problems is expected from their thermochemical conversion. Therefore, gaining knowledge related to the understanding of the environmental and thermal behavior of these fuels is relevant for the sustainable development of the global energy crop market. Because literature on energy crops for fuel purposes is scarce, some experimental ventures with these and similar kinds of fuels, including agricultural residues, have recently focused on achieving a better understanding of their combustion behavior that is often based on the chemical aspects of ash.10−22 Previous

1. INTRODUCTION Final selection of the biomass feedstock for a local heating market is largely determined by resource availability and global economic considerations. Whereas in northern European countries stemwood-based assortments have generally been used as the main biofuel raw materials for pellet production, energy crops are now being studied as potential future biofuels in other regions.1 In Mediterranean areas, such as Spain, all of the bioenergy chain steps, from the energy crop production to its transformation in the energy conversion system, have been evaluated. This entails the assessment of the properties of the energy crop pellets, e.g., their physical and chemical aspects, as well as the operating and design conditions of thermal conversion technologies adapted to combustion characteristics of novel fuels. The development of a highly reliable facility that thermochemically converts pellets from Mediterranean energy crops in an efficient, automatic, and cost-effective way means that ash-related problems also have to be addressed and measures for their prevention have to be developed. Residues from agricultural production and also energy crops are currently being studied as potential fuels in China.2,3 Besides their promising application for bioethanol produc© 2012 American Chemical Society

Received: November 25, 2011 Revised: April 12, 2012 Published: April 13, 2012 3218

dx.doi.org/10.1021/ef2018622 | Energy Fuels 2012, 26, 3218−3229

3219

(wt %, db)

Cl db) db) db) db) db) db) db) db) db) db)

db) db) db) db)

ICP−AES/SS 028113-1 ICP−AES/SS 028113-1 ICP−AES/SS 028113-1 ICP−AES/SS 028113-1 ICP−AES/SS 028113-1 ICP−AES/SS 028113-1 ICP−AES/SS 028113-1 ICP−AES/SS 028113-1 ICP−AES/SS 028113-1 ICP−SFMS/SS 028113-1

SS 187154:1;SS 187185

+

< 0.01

6.8 Ultimate Analysis 51.3 6.3 < 0.1 42.0

19.2 Physical Dimensions 8 1.08 ± 0.41 Proximate Analysis 0.3

stemwood

+

0.24+

50.1* 6.0* 0.4* 40.4

2.8 15.5 81.7* 8.4

6 1.22 ± 0.41

17.5*

poplar

< 0.01 0.02 Percentage of Dry Substance in Ashes 6.5 16.5 1.63 0.76 31.67 32.68 1.17 0.62 14.33 9.79 7.67 5.11 3.33 0.04 0.4 0.92 3.03 3.24 0.004 0.081

SS 187186:1;(ICP−AES/SS 028113-1)+

LECO method 1; EN 15104* LECO method 1; EN 15104* LECO method 1; EN 15104*

SS-ISO 562:1; EN 15148* SS 187170:3

SS 187171:1

SS-ISO 1928:1; EN 14918*

testing method and standards

14.7 2.31 23.41 1.07 23.53 2.02 0.03 0.62 5.95 0.021

0.12

+

0.47+

47.1* 6.6* 1.8* 35.6

8.5 17.2 74.3* 11.9

6 1.28 ± 0.41

3.7 18.6*

brassica

0.46 0.15 18.99 0.17 32.12 8.14 0.22 0.13 9.62 0.092

0.09

0.17

45.9 6.1 1.4 41.8

4.5 16.8 78.7 9.4

8 1.73 ± 0.74

2.2 18.5

CassG

0.33 0.09 17.68 0.12 31.79 8.93 0.46 0.18 12.66 0.088

0.11

0.16

46.1 6.1 1.0 42.9

3.6 17.5 78.9 9.6

8 1.39 ± 0.41

1.8 18.4

CassL

0.34 0.10 10.63 0.14 43.36 2.00 0.52 0.25 7.59 0.074

0.29

0.13

46.3 6.2 1.0 42.6

3.5 18.7 77.8 8.5

8 1.81 ± 0.48

1.7 18.7

CassW

Values are given in weight percent, dry basis (wt %, db), except for moisture and heating value, which are given in weight percent, wet basis (wt %, wb) and megajoules per kilogram, dry basis (MJ/kg, db) respectively. Items marked with an asterisk mean values were determined by a different accredited laboratory. Items marked with a plus mean values were determined with a different method by the same accredited laboratory. bBr50%−W50%, CassG50%−W50%, CassL50%−W50%, and CassW50%−W50%. cDetermined by balance according to calculated values. dHigher heating value (HHV) of dry fuel.

a

(wt %, db)

S

(wt %, (wt %, (wt %, (wt %, (wt %, (wt %, (wt %, (wt %, (wt %, (wt %,

(wt %, (wt %, (wt %, (wt %,

C H N Oc

SiO2 Al2O3 CaO Fe2O3 K2O MgO MnO Na2O P2O5 ZnO

(wt %, (wt %, (wt %, (wt %,

ash content in pellets fixed carbonc volatile matter moisture

db) db) db) wb)

(mm) (mm ± SD)

(wt %, db) (MJ/kg, db)

diameter average length

ash content in mixtures HHVd

b,c

parameter

Table 1. Fuel Characteristicsa

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Figure 1. Fuel inorganic constituents. Amount of element is calculated based on the HHV of dry solid fuel at constant pressure.

Figure 2. Relative amount of the Mediterranean and cassava major ash-forming elements. Contents of the x, y, and z axis are given in mole fractions and normalized to 100 %.

research23−26 on ash-related problems during combustion of woody biofuels has shown that relative proportions among potassium (K), silicon (Si), calcium (Ca), and magnesium (Mg) strongly influence the slagging tendency in the stemwood-based assortments typically used. In general, aluminum (Al) and phosphorus (P) can also occur in the slag formed during biofuel combustion but to a lesser content. Nevertheless, the P content tends to increase when P-rich fuels, such as rapeseed meal, reed canary grass, or cereal grains (e.g., oat, barley, rye, or wheat), are combusted.10,11,25 The high concentration of K in combination with the contents of chlorine (Cl) and sulfur (S) is significantly associated with the formation and properties of fine aerosol particles.13,14,27−32 Furthermore, deposits of these elements on heat-exchanging surfaces may lead to corrosion phenomena.33−35 The relative concentration of these elements in energy crops identifies these

novel sources as potentially problematic fuels during their thermochemical conversion in biomass boilers. Some ash indices, such as molar ratios based on the relative amount of the main ash components (e.g., K, Si, Ca, Mg, P, S, and Cl), are generally used to evaluate the quality of biofuels and to predict and/or compare the combustion behavior of biomass fuels.8,36−38 Nevertheless, their validation could result in a very complex task. Factors related to the thermodynamic equilibrium, order of reactivity and availability of ash-forming elements, thermal conversion system design, and its operating conditions might influence ash transformation and fractionation behavior in the biomass combustion unit. Much is still unknown about detailed ash formation, and consequently, a more general understanding of the mechanisms behind the observed behavior is needed. It is not only required to evaluate the suitability of energy crops for the available thermal conversion facilities but also for suggesting which improve3220


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Table 2. Applied Molar Ratio Indices molar ratio of the fuels

ratio label

poplar

brassica

CassG

CassL

CassW

Si/P (K + Na + Ca + Mg)/(Si + P) (K + Na)/(Ca + Mg)

I II III

6.04 2.95 0.33

2.92 3.00 1.11

0.06 8.56 1.27

0.03 6.62 1.27

0.05 10.36 3.88

Figure 3. Schematic figure of the experimental setup (equipment not true to scale). fuels were three different pelletized cassava stems, namely, cassava G (CassG), cassava L (CassL), and cassava W (CassW), from three different regions in Guangxi, China. Characteristics of the tested pellets with regard to fuel properties are given in Table 1, and a comparison of the content of ash-forming elements is shown in Figure 1. As seen from Table 1, these fuels are characterized by high ash content compared to standard softwood pellets, which have been included as reference fuels. Their combustible fractions are quite similar. With regard to the fuel inorganic composition, it is shown in Figure 1 that K, Ca, Si, S, P, Mg, and Cl are the main inorganic constituents in the Mediterranean and Chinese fuels. 2.2. Fuel Categorization. To identify the main differences among fuels from a more general ash composition perspective, molar relative proportions of the six major ash-forming elements, K, Na, Ca, Mg, Si, and P, are represented in four ternary diagrams in Figure 2. The selection criterion for these six elements is mainly based on findings from previous research experience,39 which identified them as major players in the primary ash transformation routes. Cl and S have not been considered here because they largely contribute to secondary transformations. Considering the relative amounts of the major ash-forming elements, Si and P (see Figures 1 and 2), two distinct categories can be distinguished: high Si/P ratio (poplar and brassica) and low Si/P ratio (cassava fuels). In the first category, brassica has a somewhat lower Si/P ratio and a comparable range of alkali metal and alkaline earth elements with the ones in poplar. Furthermore, a comparison of the expected and experimentally determined primary ash transformation behavior for the Mediterranean crops and inorganic major constituents of cassava was performed using three molar ratios, Si/P (I), (K + Na + Ca + Mg)/(Si + P) (II),

ments to the traditional technologies implemented for softwood pellets are demanded. Considering these two important issues, it will be possible to guarantee the use of these biofuels achieving high efficiency, low cost, and low environmental impact. For these reasons, an initial objective of this work was to elucidate the ash characteristics and fractionation behavior associated with the chemical composition of two Mediterranean energy crop alternatives: a herbaceous one, i.e., Brassica carinata, and a woody one, i.e., Populus sp., when combusted in fixed-bed residential appliances. These two Mediterranean fuels are relatively rich in Si compared to other agriculture-related opportunity fuels of increasing research interest that are more P-rich. To facilitate a more general evaluation in the context of the combustion properties of energy crops and agricultural residues, an additional objective was to investigate the ash-related aspects during combustion of three pelletized cassava stems, which are P-rich fuels. Finally, the experimental results on ash transformation routes during fixed-bed combustion of these novel fuels were discussed considering availability aspects of the ashforming elements during the fuel conversion process.

2. MATERIALS AND METHODS 2.1. General Fuel Characteristics. Five biomass fuels were tested in this study. Two of them were pellets from two Mediterranean energy crops that are currently being assessed in Spain, Brassica carinata (brassica, Br) and Populus sp. (poplar, Pop). The other tested 3221


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and (K + Na)/(Ca + Mg) (III), which are shown in Table 2. They were selected as the most significant ratios on the basis of results from the comparison of the major ash-forming elements as presented in Figure 2 and knowledge acquired from previous research work.39 The main purpose of the introduction of these molar ratios was to facilitate comprehension of the primary ash transformation routes from a simplified perspective. Consequently, Cl and S have been excluded from the molar ratio comparison. 2.3. Experimental Setup. The combustion experiments were performed in a reference boiler, which is also used in the Swedish national certification system (P-marking) for residential pellet burners. A schematic view of the experimental setup is shown in Figure 3. Besides the integrated heat exchanger (see A in Figure 3), the boiler walls are also water-jacketed (see B in Figure 3). The type of burner used in the experimental campaign was chosen from four different small-scale pellet and grain burners installed in the reference boiler. Preliminary experimental tests showed that a top-fed

residual ash in brassica and cassava fuels, leading to the total saturation of the burner section. Consequently, temperatures in the burner area decreased significantly, leading to unscheduled shutdowns even in really short test periods. To enable longer sampling periods, the brassica and cassava fuels were mixed with standard softwood pellets (W) with low ash content (0.35 wt %, db) in the proportion of 50 wt %, wb of brassica or cassava pellets and 50 wt %, wb of softwood pellets. The ash content in the mixtures (i.e., Br50%−W50%, CassG50%− W50%, CassL50%−W50%, and CassW50%−W50%) is shown in Table 1. Consequently, stemwood pellets were used as a “dilution factor” of the total amount of ash in the burner. Combustion temperatures were continuously measured at three different positions in the vicinity of the burner grate (see T1, T2, and T3 in Figure 3), at two different positions on the rear boiler wall (see T4 and T5 in Figure 3), and at one position immediately underneath an air-cooled probe (see T6 in Figure 3), with N-type thermocouples. A summary of the boiler thermal profile during the combustion tests is presented in Table 3. The concentrations of the gaseous flue gas components O2 and CO were continuously measured with electrochemical sensors (flue gas analyzer, Testo 350XL) in all of the tests. Generally, combustion conditions were controlled and relatively stable with an average O2 concentration in the range of 9−11 vol % dry gas (dg). CO emissions were under the European EN 303-5 requirements set at 3000 mg/Nm3 at 10 vol % O2 dg for boilers of nominal output ≤ 50 kWth class 340 (the strictest class in this standard) for all test, all tests, except for cassava L and cassava W, which had somewhat higher CO emissions. Despite that, CO emission levels were acceptable for high combustion quality (loss of ignition matter was less than 0.02 kg/ kg solid residue, db) and to avoid interference in the ash fractionation behavior. 2.5. Sampling of Sintering Ash, Slag, and Deposits. After each experiment, the combustion equipment was inspected for slag and/or sintering formation in the residual ash (bottom ash). The total amount of slag and/or sintering ash was quantified and further chemically characterized by qualitative and semi-quantitative methods, as described in section 2.7. The deposits on a specific area of 12 cm × 12 cm (cleaned and polished before each test) of the rear boiler wall (see B in Figure 3), namely, BW fraction, were also collected and chemically characterized. An air-cooled controlled temperature deposition probe with an exchangeable stainless-steel sampling ring (see C in Figure 3) was used to estimate the ash material potentially deposited on the heat-exchanging tubes, namely, DP fraction. The sampling ring was cooled to between 140 and 190 °C and placed just in front of the heat-exchanging tubes for an accumulation time of approximately 40 min. The deposits collected by the sampling ring were further chemically characterized. 2.6. Particle Sampling. Particle sampling was carried out after stable conditions regarding temperatures and gaseous emissions were reached. To determine both the particle mass concentration in the flue gases and size distribution, a 13-step low-pressure impactor (LPI, Dekati, Tampere, Finland), which classifies particle sizes in the range of 0.03−10 μm according to the aerodynamic diameter, was used. Isokinetic sampling was carried out in the flue gas channel for the

Figure 4. Residential pellet top-fed burner (20 kWth) implemented in the experimental setup. burner (see Figure 4) was the most suitable for the pelletized energy crops used in this study. This burner, constructed according to the main principles required for pellet combustion, is like a small-grate burner. It is designed as a cup, which has one plate resembling a grate, where primary air is underfed by several nozzles. This plate also acts as an ash removal system (see 5 in Figure 4), which scrapes away ash and other combustion residues and moves them into the boiler combustion chamber. The movement frecuency of this plate is automatically controlled by setting the burner operation conditions. Secondary air is distributed inside the burner cup by nozzles placed on its sides. Although nozzles arrangement for primary and secondary air is different, their supply is not separately controlled. 2.4. Combustion Tests, Procedure, and Conditions. Preliminary experiments were carried out to identify the optimal operation conditions of the system for each kind of fuel. In these tests, the burner showed some limitations in terms of handling the high amount of

Table 3. Boiler Thermal Profile and Deposition Tendency during Combustion Conditionsa parameter temperature in the vicinity of the burner grate (°C) (range of maximum values) temperature on the rear boiler wall (°C) temperature in flue gases around the sampling ring (°C) exhaust gas temperature (°C) deposition rates on the sampling ringb (mg cm−2 h−1) deposition rates on the rear boiler wallc (mg cm−2 h−1)

Pop

Br50%−W50%

CassG50%−W50%

CassL50%−W50%

CassW50%−W50%

820 ± 62 (920−1020) 422 ± 22 372 ± 29 169 ± 5 0.20 2.03

851 ± 94 (920−1000) 322 ± 65 291 ± 29 115 ± 9 0.33 1.23

751 ± 109 (880−1000) 335 ± 18 285 ± 6 130 ± 4 0.36 1.39

762 ± 180 (960−1057) 361 ± 23 288 ± 17 138 ± 2 0.26 1.24

673 ± 184 (960−1053) 325 ± 20 312 ± 10 137 ± 3 0.31 1.31

a

Average values are given with standard deviations. bThe deposition rate on the sampling ring is defined as the mass deposited on the surface area of the cylinder side and collected during the deposition probe sampling time. It is expressed as the mass deposited per unit of time and area. cThe deposition rate on the rear boiler wall is defined as the mass deposited on the surface area of the rear boiler wall polished area during the duration of the test. It is expressed as the mass deposited per unit of time and area. 3222


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impactor. It was heated to the same temperature as the flue gases (i.e., around 130 °C). Aluminum foils (nongreased) were used as substrates in the impactor. The particle mass concentration in the flue gases was also quantified by a standard total dust filter, and sampling was performed in similar collection conditions to those of the impactor to verify the impactor measurements. 2.7. Chemical Characterization. The chemical composition of the bottom ash fraction, deposits, and particulate matter was semiquantitatively analyzed by scanning electron microscopy (SEM, Philips XL30) equipped with energy-dispersive spectroscopy (EDS) and qualitatively assessed with powder X-ray diffraction (P-XRD, Bruker D8 Advance Instrument). Diffraction data for the XRD analysis were obtained from TOPAS R 2.141 and the Inorganic Crystal Structure Database (ICSD).42 The so-called Rietveld technique was used to evaluate the P-XRD results to obtain semi-quantitative information about the relative content of the crystalline phase. The P-XRD measurements were performed on ground samples, which were subsequently analyzed by SEM−EDS spot and area methods. Three areas measuring 1 mm × 1 mm on each sample were examined to obtain good reproducibility. Thus, P-XRD and SEM−EDS analyses were carried out on identical samples, allowing for a direct comparison of the two complementary methods. 2.8. Assessment of the Sintering Degree. The sintering degree for the collected bottom ash samples was assessed by both visual inspection and using a simple strength test. Samples were classified in five ash fractions based on a revised classification of the sintering degree defined in previous works:11,23

Figure 5. Ash fractions from the mass balance (unburnt matter in the residual ash is not considered). In all cases categories 1 and 2a, (i.e., S1 and S2a) ash fractions are grouped together as the S1−S2a fraction because they could not be separated by sieving. SM is defined as the fine particle mode, and ID corresponds to the remaining ash fraction.

samples clearly had differences in terms of their ash fractionation behavior. For Mediterranean fuels, bottom ash was found to be the main ash fraction, whereas for Chinese fuels, a significantly higher fraction formed fine sub-micrometer particles (see SM fraction in Figure 5). As shown in Figure 5, sintering tendency differed among Mediterranean and Chinese fuels. S1 and S2a fractions were mainly identified in both poplar and brassica, but there were also very small amounts (traces) of the S2b fraction in the latter. Further, a somewhat higher fraction of sintered matter category S2b was obtained from cassava tests in the following order: CassW50%−W50% > CassL50%−W50% > CassG50%−W50% (see S2b fraction in Figure 5). 3.2. Bottom Ash, Deposits, and Particulate Matter Chemical Characterization. The results from SEM−EDS and P-XRD analyses of collected bottom ash fractions (i.e., S1, S2a, and S2b), deposits on the rear boiler wall (BW fraction) and sampling ring (DP fraction), and fine particulate matter (SM fraction) are given in Figure 6 and Tables 4 and 5. As seen, bottom ash fractions were dominated by somewhat refractory phosphates and silicates depending upon the relative content of Si and P in each fuel (see Table 4). The phase composition of deposits on the rear boiler wall (see Table 5) resembles the corresponding one for bottom ash fractions with the addition of a certain amount of K-chloride in cassava mixtures, which was less significant but not negligible in brassica. DP samples (see Table 5) were dominated by K-sulfates as well as K-chloride in all fuels (except poplar), with the occurrence of K-phosphates for cassava. Chemical composition of fine particulate matter (see SM fraction in Table 5) was similar to results from the DP fraction but mainly dominated by K-sulfates. 3.3. Particulate Emissions and Size Distributions. The particle mass concentrations and size distributions are given in Figure 7. Even though particulate emissions in all cases were dominated by fine sub-micrometer particles (SM fraction), considerable differences between the tested Mediterranean and Chinese fuels were apparent with regard to particle mass concentration. All values were higher than the European EN 303-5 requirements set at 150 mg/Nm3 at 10 vol % O2 dg for boilers of nominal output < 50 kWth class 3.40

• Category 1: non-sintered ash residue, i.e., nonfused ash (clear grain structure).

• Category 2a: partly sintered ash, i.e., particles containing clearly fused ash that break at a light touch (distinguishable grain structure). • Category 2b: partly sintered ash, i.e., particles containing clearly fused ash that hold together at a light touch but are easily broken apart by hand (distinguishable grain structure). • Category 3: totally sintered ash, i.e., deposited ash fused to smaller blocks that are still breakable by hand (slightly distinguishable grain structure). • Category 4: totally sintered ash, i.e., deposited ash totally fused to larger blocks that are not possible to break by hand (no distinguishable grain structure). 2.9. Ash Mass Balances. To estimate quantitatively the ash fractionation corresponding to each fuel, an ash mass balance was carried out for each combustion test. The total input of ash in the tested poplar or in the 50 % stemwood pellet mixtures (i.e., brassica and cassava experiments) were considered as input stream to calculate the mass balance. Measured outputs included in the ash mass balance were the bottom ash fraction (i.e., slagging and/or sintering formation in both burner ash and ash scraped by the pusher into the boiler combustion chamber according to their sintering degree as S1, S2a, S2b, S3, and S4 fractions) and the particle content in the flue gas determined by particle sampling, namely, SM fraction. Deposits collected from the specific area on the rear boiler wall (BW fraction) as well as particles deposited on the heat-exchanging tubes estimated by the deposition probe (DP fraction) and on the stainless-steel plate (see D in Figure 3) were considered and counted as unquantified matter, namely ID, the remaining ash fraction. Although ashes deposited in these boiler sections were gathered after each test, the boiler was not completely and carefully cleaned as performed for the rear boiler wall deposit measurements. For that reason, deposits (ID fraction) were calculated by the difference between the total amount of ingoing ash and the measured outputs previously mentioned to determine the closure of the ash mass balance.

3. RESULTS 3.1. Ash Fractionation. Measured and estimated ash fractions are presented in Figure 5. As seen, the tested fuel 3223


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Figure 6. Average elemental content of the collected ash fractions obtained from several area analyses using SEM−EDS. Results from SEM−EDS were normalized to 100 % for the major ash-forming elements.

3.4. Deposition Rates. The deposition rates estimated by the deposition probe and deposit measurements on a specific area of the rear boiler wall are presented in Table 3. Poplar showed the lowest built up rates, whereas minor differences were observed among the other fuels.

certain compositional ranges, and frequently, one has to consider other ratios and concentrations as well. The use of indices, however, can be suggested, owing to the simplification that they provide once the underlying ash transformation reactions are understood and potential interfering reactions and conditions are considered. Taking this into account, a further discussion is based on the results of the chemical characterization of the ash fractions and also the relative compositional differences in the fuels by introducing the molar ratios Si/P (I), (K + Na + Ca + Mg)/(Si + P) (II), and (K + Na)/(Ca + Mg) (III) (see Table 2) and the four ternary diagrams shown in Figure 2. It must be stated that the implemented ratios are not presented as indices to be used solely to predict the ash chemical behavior of biomass fuels but as a simplified support to discuss and structure the most important primary ash transformation routes. 4.2. Preliminary Remarks about Ash Transformations and Fractionation. As mentioned in section 2.2, the assessed fuels can be grouped into two different categories, one

4. DISCUSSION 4.1. General Comments. In the literature, a rather large number of attempts to rationalize features of biomass ash transformation reactions and related ash problems in terms of various “indices” can be found.8,36−38,43−45 These indices are largely expressed as concentration ratios between some of the major ash-forming elements. In many cases, the bases of the correlations are empirical observations, with rather vague theoretical considerations concerning the underlying ash transformation chemistry. In the experience of the authors, many of these indices are useful but have to be used with care. They often cannot be generally used but are restricted to 3224


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Table 4. Major and Minor Crystalline Phases Identified by P-XRD in Bottom Ash Fractionsa Br50%−W50%

Pop

CassG50%−W50%

formula

S1−S2a

S1−S2a

SiO2 CaO MgO CaCO3 K2Ca(CO3)2 K2SO4 K3Na(SO4)2 CaKPO4 CaK2P2O7 Ca5(PO4)3(OH) Ca2SiO4 Ca3Mg(SiO4)2 sum

7 16 5

2 11 7

14

12

23

7 9

14

52

63

100

100

40 17 99

S2b

CassL50%−W50%

CassW50%−W50%

S1−S2a

S2b

S1−S2a

S2b

S1−S2a

S2b

22 21 2 6 7 3 39

15 20

13 20

8 17

13 10

19 7 3 27

6 11

15 14

44

38

11 7 5 48

12 10 4 11 7 5 44

9

6

7

5

7

100

100

99

99

100

100

a

Values in the table give the contents of crystalline phases (wt %) in the different samples as the result of semi-quantitative refinement of the XRD data with the Rietveld technique.

Table 5. Major and Minor Crystalline Phases Identified by P-XRD in Deposits and Particulate Mattera Br50%−W50%

Pop formula

BW

DP

SiO2 CaO MgO CaCO3 K2Ca(CO3)2 K2SO4 K3Na(SO4)2 KCl KPO3/KH2PO4 CaKPO4 Ca5(PO4)3(OH) Ca2SiO4 Ca3Mg(SiO4)2 sum

1 40 11

15 4

15

57 24

SM

BW

DP

CassG50%−W50% SM

BW

28 4

90 10

CassL50%−W50% SM

20 13

23

78

77

5

22

23

5 22 3 9 5 15 9

16 21 11 99

DP

5 7 24 19 7 9 28

BW

DP

11 11

3

6 47 20 14 12

3 33

CassW50%−W50%

SM

2

19

52 10 30 5

29 6 42

99

100

DP

SM

9 7

1

15 8 15 3

BW

29

12 30

34

40

7

17

84

99

33 15 53

23 100

100

99

100

100

101

99

99

99

100

a

Values in the table give the contents of crystalline phases (wt %) in the different samples as the result of semi-quantitative refinement of the XRD data with the Rietveld technique.

involving a high Si/P ratio (poplar and brassica crops) and one with a low Si/P ratio (the three cassava stems). As mentioned in section 2.2, however, brassica has a somewhat lower Si/P ratio in the former category. Thus, in the first category, the

obtained ash transformation reactions are mainly dominated by silicate formation, and in the second category, the obtained ash transformation reactions are mainly dominated by phosphate formation. There is apparently a major difference between early ash transformation forms of Si and P during the thermal conversion of biofuel. Si is believed to form very small silica (SiO2) particles quite rapidly, whereas P is expected to be liberated as gaseous P2O5. The “early” forms of both of these ash-forming components will readily react with any available basic components, primarily alkali metals. From both a thermodynamic and an availability perspective, however, phosphate formation will be favored.11 Strictly employing the “thermodynamic order of stability”,46 the formation of potassium silicates will take place once and only if there is no P left. Both categories of phases are associated with low-temperature eutectics and peritectics around 750 °C in the system K2O−CaO−SiO2 (near 30 wt % K2O, close to binary region K 2 O−SiO 2 ) 4 7 and around 590 °C in the system CaO−K2O−P2O5 (between 40 and 60 wt % K2O, close to binary region K2O−P2O5),48−52 respectively. This may thus cause ash-related problems, such as slagging and bed

Figure 7. Particle mass concentrations (TM) and size distributions. 3225


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Figure 8. SEM micrographs and EDX spot and area analyses (values in atomic percent and on a C- and O-free basis) of the ash residue samples: (a) poplar, large fused calcium−magnesium−potassium silicate particle from the sintering category S1−S2a fraction (Si−K−Ca); (b and c) brassica, particle from the sintering category S2b sample (Si−K−Ca−P); and (d) cassava G, fused K/Ca phosphate particle from the sintering category S2b sample.

particulate emissions and deposits. At ratios of (K + Na + Ca + Mg)/(Si + P) roughly above 3, there will be a surplus of the basic oxides (i.e., the silicates and phosphates are “saturated” with respect to the basic components).47−52 The fate of the volatilized gaseous alkali metal part of that surplus is related to the concentrations of SO2/SO3 and HCl in the flue gases38,39,43 because it will form fine particles and/or deposits as sulfates and chlorides in colder parts of the flue gas path throughout the conversion system via condensation/nucleation processes. The surplus of alkaline earth oxides will stay in the bottom ash. Dependent upon the proportions of ash-forming elements and the overall process temperature, the eventual presence of sulfates and carbonates in the residual ash will occur.39 The physical and chemical characteristics of these salts are very different from those of the phosphate−silicate slag described above. The formation mechanisms are also different. They are mainly related to secondary ash transformation reactions, which are beyond the scope of this study. Usually, these compounds are absent, owing to excessively high bed and boiler temperatures, but they have been observed in thermal conversion systems running at lower bed temperatures. The occurrence of carbonates and sulfates in the residual bottom ash can also be affected by the association forms of the elements and interactions among other compounds in the system. For instance, the thermal stability of alkali metal sulfate is affected by the presence of the silicate matrix at higher temperatures than 850 °C.38,55,56 4.3. Poplar. The ash-forming matter in poplar is characterized by the highest Si/P (I) ratio (see Table 2), indicating that the residual ash will be mainly dominated by silicates. The relatively high (K + Na + Ca + Mg)/(Si + P) (II) ratio together with the relatively low (K + Na)/(Ca + Mg) (III) ratio (see Figure 2) suggests that bottom ash consists of silicates rich in Ca and Mg and a certain “free” surplus of these alkaline earth oxides. These ratios also imply that, in

agglomeration. The behavior of the pure silicate and phosphate systems at very low ratios is different, however. In the melted form, the silicate and phosphate systems may interact. In the light of empirical evidence in terms of both morphological and compositional observations of molten ash in mixed silicate and phosphate systems,11,20 a complex melt involving both silicates and phosphates along with alkali metal and alkaline earth components is assumed to be formed. The physical and chemical properties of such melts are to a large extent unknown, implying, for instance, that no information is available about the liquidus or solidus properties of these multicomponent systems. It appears (because of thermodynamic reasons) that the precipitation of ternary phosphate phases is favored from a complex phosphate−silicate melt of a given composition upon temperature decreases, leaving a residual melt enriched in silicate. Consequently, at certain high P concentrations, the silicate formation will be partially or nearly completely canceled. This condition is occasionally displayed as silica appears in the form of cristobalite, i.e., unreacted and “out-conquered” by the phosphate.11,20 Despite the unknown properties of these complex systems, the inclusion of alkaline earth oxides may raise melting temperatures during the formation of solid ternary phosphates and silicates, which largely contributes to overcoming such problems as slagging and bed agglomeration. Generally, from ternary and binary phase diagrams for both the silicate and phosphate systems, there is a trend of increasing liquidus temperatures with decreasing (K + Na)/(Ca + Mg) ratios.11,20,47−54 If a surplus of the basic components (e.g., K2O and CaO) in relation to the acidic components (SiO2 and P2O5) exists, the behavior of alkali metal and alkaline earth oxides will differ, respectively.20,21,53,54 The latter will largely remain in the residual ash, whereas the former will be volatilized to a high degree and play a crucial role in the formation of fine 3226


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of Figure 8) showed that Ca, Si, K, and P compounds were available for trace amounts of formation of a multiphase melt. A relatively low proportion of sulfates was identified in bottom ashes by XRD analysis (see Table 4). As mentioned in the poplar case, the low thermal profile in the vicinity of the burner and in the boiler (see Table 3) certainly influenced their occurrence in the residual ash. In comparison to poplar, the total amount of the remaining ash fraction (ID) obtained in the brassica test was lower, possibly the result of the slightly increased “stickiness” of this ash, as previously suggested. The composition of the BW and DP ash compared to the woody crop also included a phosphate (CaKPO4) in the BW fraction and KCl in the DP fraction (see Table 5). As expected from the molar ratio comparisons, the amount of fine particles (SM) was larger than for poplar (see Figures 5 and 7), which is related to both the higher ash content and the lower Si/P (I) and higher (K + Na)/(Ca + Mg) (III) ratios. 4.5. Cassava. As shown in Table 2 and Figure 2, cassava fuels are characterized by an extremely low I ratio, a very high II ratio, and a reasonably high III ratio. These conditions mean that cassava ash transformation reactions and fractionation are quite different compared to the ones described for the two studied Mediterranean fuels. First, a domination of the residual ash by phosphates is expected. Second, a relatively large surplus of alkaline earth oxides is assumed together with extensive fine particle formation. As shown in Figure 5, the sintering degree for the three Chinese fuels was higher (i.e., significant amount of the S2b fraction) than for the Mediterranean fuels but still relatively low. The elemental composition of the formed ash was generally dominated by a very high amount of Ca, Mg, and K, together with considerable amounts of P (see panels c−e of Figure 6). Similar to brassica, a high concentration of K was found in all bottom ash fractions, as well as a significant Ca content. The formed ash was more P-rich than both brassica and poplar, however. According to P-XRD results (see Table 4), the residual bottom ash from cassava combustion tests was mainly characterized by both the ternary phosphates (CaKPO4) and the double carbonate [K2Ca(CO3)2]. The occurrence of the latter phase is a consequence of the high II and III ratios. Furthermore, temperature conditions in the system (see Table 3) certainly influenced their formation as well as the presence of the alkali metal sulfates [K2SO4 and K3Na(SO4)2] in bottom ash fractions. As expected from the assessment of the three molar ratios compared in this work, significant amounts of alkaline earth oxides (CaO and MgO) are also contained in these ash fractions. SEM micrographs of the formed trace S2b fraction samples (see Figure 8d) identified the existence of small amounts of a glassy phase mainly dominated by K, Ca, and P, which is the reason for the sticky behavior leading to the formation of such material. The proportion of particulate matter (fine and ash particle fractions) for the cassava fuels was in all cases more than 50% (see Figure 5), and the major part consisted of fine particles (SM fraction). As shown in Table 5, the BW and DP samples resembled the residual ash with one important exception; i.e., there were significant amounts of potassium phosphates (KPO3 or KH2PO4). The presence of K-phosphates is presumably a consequence of the high K and P concentrations in cassava (see Figure 1), together with the relatively high vapor pressure of these species or their constituents (i.e., KOH and P2O5), as well as an assumed insufficient time for reaction with CaO. Only a fraction, however, of the K and P is volatile and “released” from the fuel

comparison to the other tested fuels, combustion of poplar should result in the least amount of fine particulate matter. The II and III ratios for poplar are the most advantageous among the tested fuels from a slagging perspective. This was observed in the assessment of the sintering degree when bottom ashes were mainly characterized by a significant proportion of the S1 ash fraction and very small amounts of the S2a ash fraction. Furthermore, some ash particles, clearly melted, were identified by SEM micrographs of these residues, as presented in Figure 8a. Although their occurrence was as a minor fraction, this melted ash could act as a sticky matter and, therefore, contribute to the mild sintering tendency determined. Area analysis on these particles showed the formation of solidified fused matter, which mainly consisted of K, Si, and Ca (calcium−potassium silicate). The phase composition of the bottom ash fractions shown in Table 4 was dominated by refractory phases, such as the basic silicates [Ca2SiO4 and Ca3Mg(SiO4)2] and the alkaline earth oxides (CaO and MgO), in agreement with the high proportion of corresponding nonvolatile ash elements (Si, Ca, and Mg) in the fuel. Sulfates were also identified in bottom ash but in a less significant amount. The presence of alkali sulfates is certainly related to the fairly low temperatures in the vicinity of the burner (< 900 °C) and in the boiler (see Table 3). The amount of particulate matter is relatively large if the ID fraction is included along with the SM fraction (see Figure 5). An apprehension of the composition of the former can be obtained from the phase analysis of the BW and DP ash fractions (see Table 5), where the BW ash mainly reflects the residual bottom ash fractions and the composition of the DP ash has shifted toward the composition of the SM fraction (see Figure 6a and Table 4). Consequently, the amount of fine sub-micrometer particles (SM) was the lowest of all tested fuels, as shown in Figure 7, entirely in accordance with expectations from the three molar ratio comparisons. 4.4. Brassica. The Si/P (I) ratio (see Table 2) is about half that of poplar, indicating that a higher amount of phosphates in the residual ash can be expected. The II ratio is about the same as for poplar (see Table 2 and Figure 2d), but the III ratio is higher (see Table 2 and panels a−c of Figure 2). On the basis of these ratios, the difference in sintering and slagging behavior between poplar and brassica is not obvious or easily anticipated. An increased amount of volatilized deposit and fine particle formed from alkali metals is expected, however. As shown in Figure 5, the fraction of residual ash (mainly S1 and S2a and trace amounts of S2b fraction) as well as fine particles (SM) was somewhat higher for brassica than for poplar. Differences among the higher fraction of residual bottom ash and fine particles in fuels might be explained by the ash chemical composition in brassica, if the remaining fraction (ID) is assumed to be “entrained fine and coarse ash particle fractions” deposited in the boiler. On the basis of results from SEM−EDS area analysis in the samples, the concentration of Ca in brassica bottom ash was lower compared to poplar, whereas the concentration of K and P was higher (see panels b and a of Figure 6). These conditions result in a residual ash that has a slightly higher sintering tendency for brassica, leading to less entrainment of coarse ash particles from the burner/boiler. The phase composition of the residual ash (see Table 4) resembles the corresponding one for poplar with the addition of two ternary phosphates (CaKPO4 and CaK2P2O7). Furthermore, results from SEM−EDS spots and area analysis of selected particles of the minor S2b fraction samples (see panels b and c 3227


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bed/bottom ash during combustion, whereas some are captured in more refractory compounds (K/Ca-phosphates) in bottom ash. Once such ternary phosphates have been formed, the vapor pressure of potassium phosphate is considerably lowered. As shown in Figure 7, cassava fuels exhibited extremely high concentrations of fine particles formed during combustion and mainly dominated by potassium chloride and sulfates with minor amounts of potassium phosphate (see panels c−e of Figure 6 and Table 5).

sintering tendency, however, all tested fuels can be classified as non-severe slagging fuels under the studied conditions.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +34-976-76-25-82. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was financially supported by the collaboration between the Swedish Institute Guest Scholarship Program for Research in Sweden 2008/09, which awarded Maryori Dı ́az-Ramı ́rez, and the Energy Technology and Thermal Process Chemistry (ETPC), Umeå University, which was her host university in Sweden, and the Spanish Education and Science Ministry, through Project “Bio3”, ENE200803194/ALT, and Project PSE “On Cultivos”, PS-1200002005-6. Thanks are also owed to the Swedish Energy Agency (30646-1, WP7). The authors also thank Håkan Ö rberg, Swedish University of Agricultural Sciences, Wanbin Zhu, and China Agricultural University. Finally, the corresponding author gives particular thanks to ETPC technicians and Ph.D. students for their support.

5. CONCLUSION This paper has discussed the ash transformation and fractionation behavior of five pelletized energy crops: two Mediterranean Si-rich fuels, i.e., brassica (a herbaceous energy crop) and poplar (a woody energy crop), and three sorts of P-rich Chinese fuels (i.e., three types of cassava stem). In this work, ash transformation has been discussed in relation to fuel compositions according to three molar ratios, i.e., Si/P (I), (K + Na + Ca + Mg)/(Si + P) (II), and (K + Na)/(Ca + Mg) (III), because these are considered to govern the primary ash transformation routes. Results from the experimental work showed that the amount of major ash-forming elements, together with the relative thermodynamic stability of formed ash compounds, has substantial influence on ash transformation and fractionation. Significant and important differences in the relative composition of the most important ash-forming elements among the fuels (Si-rich compared to P-rich) resulted in very different ash chemical behavior. In both Mediterranean fuels, slightly sintered bottom ash was the main ash fraction dominated by Ca/Mg-silicates and Ca/Mg-oxides. Minor amounts of K sulfates were identified in this fraction, as well. The occurrence of crystalline Ca/K phosphates in a lowmelting Ca/K/Mg silicate was also found in brassica. This behavior confirms the higher thermodynamic stability of phosphate systems in relation to silicates at a lower Si/P ratio in fuel in accordance with earlier experiments. In comparison to the Si-rich Mediterranean crops, a lower fraction of the total ash formed residual ash during P-rich cassava fuel combustion but with a more significant sintering degree. It was predominantly characterized by K/Ca phosphates, and similar ash-forming element fractionation was observed among sintering fractions for the other cassava stems. In comparison to the relative fractionation, the determined mass concentrations in fuels were a result of both the total ash content and relative compositions of ash-forming elements. Furthermore, combustion of the P-rich cassava fuels resulted in extremely high concentrations of fine particulate emissions (> 800 mg/Nm3 at 10 % vol O2 dg, in a 50 % mixture with stemwood), composed mainly of K-chloride, -sulfates, and -phosphates, which indicates that alkali metal volatilization is favored in the P-rich fuels at high alkali metal ratios. For Mediterranean crops, the particle emissions were around 500 and 200 mg/Nm3 at 10 vol % O2 dg for brassica in a 50 % mixture with stemwood and poplar, respectively, which is also high compared to corresponding values for typical stemwoodbased fuels and considering the “dilution factor” achieved by adding standard softwood pellets. The fine particles from these two fuels were dominated by K-sulfates, with minor amounts of K-chloride for brassica. Finally, results from this study suggest that these energy crops are less useful in small- and mediumscale combustion applications, because of the high formation of fine particle and deposit-forming ash matter. With regard to

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