Characterization of Residual Carbon in Fly Ashes from Power Plants

Jan 8, 2013 - ABSTRACT: Fly ash samples were collected from grate fired power plant units burning mixed fuels of agricultural residues and woody bioma...
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Characterization of Residual Carbon in Fly Ashes from Power Plants Firing Biomass Mingyan Zhao,† Zongna Han,† Changdong Sheng,*,† and Hongwei Wu‡ †

School of Energy and Environment, Southeast University, Si Pai Lou 2, Nanjing 210096, P.R. China Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia

‡

ABSTRACT: Fly ash samples were collected from grate fired power plant units burning mixed fuels of agricultural residues and woody biomass. The fly ashes were characterized in terms of the residual carbon mainly via nonisothermal thermogravimetric analysis (TGA) technique and loss-on-ignition (LOI) approach. TGA by heating of the fly ashes at 7 °C/min to 900 °C in air atmosphere showed three distinct peaks of weight loss in the temperature ranges of 350−530 °C, 530−660 °C, and 660−900 °C, which were confirmed to result mainly from the oxidation of residual carbon, the decomposition of calcium carbonate, and the release of alkali chlorides related species, respectively. It was shown that nonisothermal air TGA can distinguish the contribution of residual carbon oxidation to weight loss from those of the reactions of inorganic matter and, therefore, can be used as a simple technique for estimating the residual carbon content in biomass combustion fly ashes for engineering applications. The residual carbon content determined by TGA was compared with that from LOI measurements at 450 and 550 °C. The size distribution of the residual carbon in the fly ashes and the combustion reactivity of the residual carbon in large size fractions (>200 μm) of these fly ashes were also investigated, aiming at exploring the recovery and application of the residual carbon. quality and affecting ash utilization.6 A high content of unburned carbon hinders the process of ash granulation and chemical hardening,11,12 leading to potentially undesired influences on ash handling and spreading operations when using wood ash as fertilizer. Ashes with the residual carbon content >5% may not be suitable for agricultural and forestry uses because of the potential high contents of harmful organic species such as polyaromatic hydrocarbons.13 Chusilp et al.14 reported that, when used as a partial replacement for Portland cement in mortar, bagasse ash with a higher loss-on-ignition (LOI) shows a slower development of compressive strength of the mortar and a weaker resistance against sulfate attack. Chandara et al.15 found palm oil fuel ash with a higher unburned carbon content requires more superplasticizer to be added to the paste or concrete for improving the fluidity of cement pastes. Rajamma et al.16 also mentioned the necessity of controlling the carbon content to maintain the highlighted benefits of employing biomass fly ashes in cement formulation. There are several methods for evaluating unburned carbon in combustion residues. One is the determination of LOI, which is the most commonly used for assessing combustion efficiency of boilers and evaluating commercial value of ashes in engineering practices. The main advantage of this approach is its simplicity, although the LOI value may not be always a true measure of unburned carbon in combustion residues such as coal fly ashes.17−19 However, for the ashes from biomass combustion, there is no standard for the temperature used for determining LOI, most commonly 550 °C7,8,19,20 or other temperatures such as 450 °C, 600 °C, 750 °C, and even higher.19−22 The other method is thermogravimetric analysis (TGA), which was

1. INTRODUCTION Electricity generation from directly firing biomass is playing an important role in the penetration of biomass-based renewable energy in Chinese energy market, resulting in a total installation of ∼2 GW biomass-fired power capacity during 2006−2010.1 While making a great contribution to converting biomass into green electricity, these plants produce a substantial amount of combustion residues, i.e., ash products. For example, in Jiangsu Province alone, the production of ashes, including bottom and fly ash, from biomass-fired power plants increased from 42,000 t in 2007 to 180,000 t in 2010.2 Therefore, utilization and management of biomass combustion ash is becoming one of the important issues in sustainable utilization of biomass for electricity generation. In China, biomass fuels used in power plants are mainly agricultural residues, particularly crop straws including rice straw, wheat straw, and corn stover. To ensure fuel supply and to avoid boiler operation problems related to firing sole crop straws, burning the straws mixed with other agricultural residues such as rice husk, cotton stalk, and even wood barks is a common practice. Biomass typically is of low energy density and poor fuel quality (e.g., high moisture content and poor grindability3). Therefore, fly ashes produced from grate fired power plants are commonly high in unburned carbon content,2 leading to the loss of combustion efficiency and the adverse impact on subsequent ash utilization. Biomass ash can find a variety of applications including using directly as fertilizer or raw material in fertilizer production, building material or component in the manufacture of building material, etc.4−6 With separation, residual carbon from the ash may also be a valuable material and has the potential to be used as fuel4 or as raw material for activated carbon production.7−10 In practical applications, the unburned carbon content is considered as an important parameter in determining ash © 2013 American Chemical Society

Received: October 22, 2012 Revised: January 3, 2013 Published: January 8, 2013 898

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Therefore, fly ash samples, named as A-FA and B-FA respectively, were collected under the filter hoppers. Unit C was installed with cyclones and baghouse filters for removing coarse and fine fly ash, respectively. Although the cyclone and filter ashes were mixed during operation and conveyed to an ash silo and then transported as a single ash product, separate sampling was carried out to collect three fly ash samples, i.e., cyclone ash (C-CFA), filter ash (C-FFA), and mixed ash (C-FA), respectively. The as-received ash samples were subjected to determination of the elemental inorganic composition based on a combination of X-ray fluorescence (XRF) for Si and Al and inductively coupled plasma− optical emission spectrometry (ICP−OES) for the rest of inorganic elements. Briefly, Si and Al were quantified using an EAGLE III (EDAX Inc.) XRF spectrometer. The method based on ICP−OES followed European standard EN 15290:2011,25 which first digested a sample using H2O2/HNO3/HF into a solution that was then analyzed using a Perkin-Elmer OPTIMA 2000DV ICP−OES. 2.2. Ash Sieving and Particle Size Distribution. The asreceived fly ash samples were all in the form of fine powder and with wide size distributions. In order to investigate the size distributions of the residual carbon in the samples, the as-received ashes were sieved into four size fractions by using a mechanical shaker with mesh screens of 50, 90, and 200 μm. By sieving, the mass-size distributions of the fly ashes were obtained. With the unburned carbon content of each size fraction determined, the carbon-mass distribution among various size fractions was also determined. The contents of the unburned carbon in the as-received ashes and their size fractions were measured with the TGA approach and the LOI method. 2.3. TGA of the Fly Ashes. The as-received ashes and their size fractions were subjected to thermal analysis in air atmosphere in order to develop the TGA technique for estimating the content as well as the combustion reactivity of residual carbon in biomass combustion fly ashes. Nonisothermal TGA was conducted by using a Setaram SETSYS thermogravimetric analyzer. Prior to TGA, the as-received samples and the coarse size fractions were ground to be less than 105 μm. Generally, ∼4 mg of an ash sample was loaded in a TGA sample crucible, heated from room temperature to 105 °C, held for 30 min for removing moisture, and followed by a further heating to 900 °C at 7 °C/min using air (synthesized with ultra high purity O2 and N2) as purge gas. To distinguish the contribution of unburned carbon oxidation from those of the reactions of inorganic species, e.g., carbonate decomposition, to the weight loss in thermal analysis, another set of TGA experiments was carried out under inert atmosphere (ultra high purity N2). Selected samples were also washed with deionized water and 1 M HCl solution at a liquid/solid ratio of 30:1 (mL/g) to remove watersoluble and acid soluble inorganic species, respectively. The resulted samples were then subjected to TGA in air atmosphere. To further understand the behavior of different inorganic species contributing to weight loss in air TGA measurement, the leachates were analyzed to determine the removals of major elements (Cl was measured with Ion Chromatograph and the others with ICP−OES). Additionally, X-ray diffraction (XRD) analysis was also conducted using a Bruker D8 ADVANCE powder diffractometer for the as-received, water- and HCl-washed ash samples. Based on the measured weight of a sample as a function of time in TGA, the rate of weight loss (i.e., derivative thermogravimetry, DTG) was used to represent the behavior of an ash during the thermal analysis

developed and employed for measuring the contents of unburned carbon in coal power plant fly ashes.17,18 TGA was also often used for characterizing residual carbons from the combustion of biomass and biomass originating materials.7,9,10,19,23,24 Nevertheless, there is still considerable scope in developing and applying this technique for evaluating the residual carbon in biomass ashes. In the open literature, there are only limited and scattered studies7,9,10,19 on the characterization of residual carbon in the ashes from biomass combustion plants. Batra et al. 7 characterized the unburned carbon in bagasse fly ash by using thermal analysis, electron microscopy, and adsorption and suggested that the unburned carbon obtained by floating and sieving could be used as a low cost precursor for the production of activated carbon or as fuels. Girón et al.9,10 characterized the texture and thermal properties of the unburned carbons in the fly ashes from a grate fired furnace and a fluidized bed combustor firing eucalyptus wood bark to consider their utilization as precursor material for activated carbon production. Bjurströ m et al. 19 conducted a systematic investigation on the content and nature of unburned carbon in the ashes of biofuels (mainly woody biofuels) from 18 Swedish combustion plants using various analytical methods. Nevertheless, it is expected that the properties of the ashes as well as the residual carbon depend on the types and quality of the fuels burned and the combustion conditions. The complex chemistry of inorganic matter and carbon material in biomass ashes implies the difficulty and complexity in determining the content and properties of unburned carbon. As a result, a simple analytical procedure for assessing the quality of or even the amount of unburned carbon in biomass ash has not been established yet,19 although it would be practically very useful. Additionally, it is also desirable to establish the linkage between the properties of unburned carbon and the effect on ash utilization. Therefore, the objective of the present work is to characterize the residual carbon in fly ashes collected from grate fired power plants burning mixed fuels of agricultural residues and woody biomass. Thermal analysis of fly ashes was conducted to characterize the residual carbon via the nonisothermal TGA technique as well as the LOI approach to distinguish residual carbon oxidation from inorganic matter behavior. The residual carbon content determined by TGA was compared with that from LOI measurements. Additionally, the size distribution of the residual carbon in fly ashes and the oxidation reactivity of the residual carbon were also investigated, aiming at exploring potential applications of the residual carbon. The implications of the study to determination of the unburned carbon content and recovery and use of the residual carbon from biomass combustion fly ash are also discussed.

2. EXPERIMENTAL SECTION 2.1. Fly Ash Samples. Fly ash samples were collected from three power plant units. The units are all equipped with a 15 MW watercooled vibrating grate furnace, designed to directly burn crop straws but actually be fired with mixed biomass fuels. When doing the sampling, two units (Unit A and B) were burning the mixtures of 35− 45% of agricultural residues (including rice straw, wheat straw, corn stover, and rice husk) and the rest of wood bark. The third unit (Unit C) was being fired with mixed fuels consisting of ca. 60% of agricultural residues, mainly rice straw, rice husk, and a small fraction of mushroom planting wastes (originally sawdust and straws), and ca. 40% of woody wastes (mainly wood bark). Unit A and B were installed with baghouse filters to remove fly ash in flue gas leaving the boilers.

DTG = −

1 dm × 100%, m0 dt

%/min

(1)

where m and m0 are the sample weight at time t and its initial on a dry mass basis, respectively. Based on the comparison of ash thermal behavior in air and N2 atmosphere, the weight loss due to the combustion of residual carbon, i.e., the content of unburned carbon, was determined from the air TGA. Subsequently, the approach developed by Russell et al.26 was employed to derive the rate constant 899

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and kinetic parameters of the combustion of the residual carbon. The combustion rate constant k is expressed as k=−

1 dmc = k 0 exp( − E/RT ), mc dt

min−1

(2)

where mc is the weight of the residual carbon at time t on a dry ash free basis, k0 and E are the pre-exponential factor and activation energy of carbon oxidation, respectively, T is the temperature, and R is the universal gas constant. The values of k0 and E were determined by fitting the Arrhenius plot with the combustion rate constant for each sample. 2.4. LOI Determination. LOIs of the as-received ashes and their size fractions were determined following European standard EN 13039:201127 − a standard designated originally for determination of total organic matter and ash contents of soil improvers and growing media. Briefly, ∼5 g sample was first dried and then slowly heated to 450 °C and held in air for six hours by using a muffle furnace. The weight loss was taken to be the LOI and expressed as the mass percentage of the dried sample. For comparison, LOI at 550 °C was also measured by heating the dried sample to 550 °C and then held for two hours in the muffle furnace. The LOIs at 450 and 550 °C were compared with the residual carbon content determined by nonisothermal air TGA.

Figure 1. DTG curves of as-received fly ashes heated in air.

3. RESULTS AND DISCUSSION 3.1. Thermal Behavior of Fly Ashes. Table 1 presents the composition of the five as-received fly ashes. The values of LOI at 450 °C indicate considerably high residual carbon contents in the ashes, reflecting the poor combustion performance in the grate furnaces. Figure 1 shows DTG curves obtained by heating the five asreceived fly ash samples in TGA reactor at 7 °C/min under air atmosphere. All the samples have three distinct stages of weight loss in the temperature ranges of 350−530 °C, 530−660 °C, and 660−900 °C, respectively, although the peak temperatures and weight loss rates of each stage vary with the samples. As shown in Figure 2, using the C-FFA sample as an example, the weight losses of the size fractions of the same sample also take place at the same three stages. Similar trends were also observed for other ash samples (the data are not shown here). Additionally, Figure 2 indicates that, as the ash particle size increases, the peak rate of weight loss at the first stage (350−

Figure 2. DTG curves of C-FFA fly ash and its size fractions heated in air. The sizes of ash fractions are showed in micrometers in the legend.

530 °C) increases, with the peak temperature remaining nearly unchanged, while the peak rates of the second and third stage decrease and occur generally at lower temperatures. To identify the causes leading to the presence of such three stages in weight loss, selected ash samples were also subjected to nonisothermal TGA under N2 atmosphere. Figure 3 compares the DTG curves obtained from air and N2 analysis for A-FA and C-FFA samples. It is clear that, in the absence of oxygen (under N2 atmosphere), nearly no weight loss takes place over the temperature range of 350−530 °C, while the weight losses at the temperatures of 530−660 °C and 660−900 °C are still obvious. Therefore, it is plausible to conclude that the weight loss occurring at the temperature of 350−530 °C in air is mainly due to the oxidation of residual carbon in the fly ash samples. The weight losses at the two higher temperature stages are more likely to result from the thermal behavior of inorganic matter of the fly ashes. Thermal behavior of water- and HCl-washed fly ashes in air TGA is compared with that of the as-received samples, as shown in Figure 4 for A-FA and C-FFA samples. It can be seen that, compared to the DTG curves of the as-received ashes, the first weight loss peak was not significantly affected by waterand HCl-washing, which further confirms that it is attributed to the burning of the residual carbon. On the other hand, water washing eliminated the third weight loss peak, while HCl washing eliminated the second and third one. The results suggest that the weight loss of the fly ash at the highest temperatures is due to the behavior of water-soluble inorganic species, while that at the mid temperatures is related to the behavior of water insoluble but diluted HCl soluble species.

Table 1. Composition of As-Received Fly Ashes on a Dry Mass Basis ash content, %a LOI, %a elements Si, %b Al, %b Fe, mg/kgc Ca, mg/kgc Mg, mg/kgc Na, mg/kgc K, mg/kgc Ti, mg/kgc P, mg/kgc Cl, mg/kgc S, mg/kgc

A-FA

B-FA

C-CFA

C-FFA

C-FA

92.21 7.79

80.34 19.66

88.61 11.39

81.61 18.39

83.21 16.79

18.35 4.58 15349 96092 14729 9317 43299 2316 3700 24806 17154

19.80 2.65 10152 58641 10626 6756 42930 1313 3538 22022 10210

26.73 2.67 9068 40976 8975 5043 30822 1286 3593 6212 3069

17.10 1.03 4704 66754 12036 3741 69208 490 5728 53087 20708

24.15 1.70 6235 44058 9240 4722 35234 752 4172 17077 7374

a Ash content and LOI were determined at 450 °C following EN 13039:2011. bContents were measured by using XRF. cContents were determined following EN 15290:2011.

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Figure 3. Comparison of DTG curves of fly ashes heated in air and N2 for (a) A-FA and (b) C-FFA.

Table 2. Removal of Major Elements in the As-Received Fly Ashes by Water- and HCl-Washing, Expressed as % of the Total Elemental Amount Extracted on a Dry Matter Basis A-FA Cl S K Na Ca Mg Fe a

C-FFA

water washed

HCl washed

water washed

HCl washed

94.14 37.71 41.33 11.89 9.35 200 μm) contain a substantial amount of residual carbon, these samples may find relevant applications in practice such as

reuse as fuel or as feedstock for activation carbon production. Therefore, based on the air TGA data at the stage of 350−530 °C, the combustion reactivity of the residual carbon in the fly ashes was further determined for these large size fractions (>200 μm), along with their water- and HCl-washed samples. Figure 8 presents Arrhenius plots for the oxidation of the residual carbon in the large size fractions (>200 μm) of the set of fly ash samples. The plots clearly indicate that the residual carbons in these large size fractions (>200 μm) of the fly ashes have similar combustion reactivity. Both water- and HClwashing lead to considerable decreases in the reactivity. This is expected as such washings removed a substantial amount of inorganic species (Table 2), which are known to be good catalysts for carbon combustion.45−50 Consequently, the oxidation of the residual carbon occurs at higher temperatures, and the produced CO2 from the oxidation depresses the decomposition of calcium carbonate, resulting in the 904

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Therefore, the influence of inorganic matter on reactivity measurement is expected to be small. Table 3 indicates that activation energy values of the residual carbon oxidation for all the large size fractions of these biomass fly ashes are between 128.4 and 174.2 kJ/mol, which is within the range of the data reported in the literature for biomass char oxidation.50 Although E values of the samples are quite different, it can be seen in Table 3 that the sample with a lower E generally has a lower pre-exponent factor k0 because of the kinetic compensation effect.51 This is reflected by the similar reactivity of the residual carbon for these large size fractions of fly ash samples, as shown in Figure 8. Figure 8 also compares the reactivity of recoverable residual carbon in the large size fractions (>200 μm) with that of laboratory chars of wheat straw (WS), rice husk (RH), and fir wood (FW), the components of fuel mixtures fired in the power plant units from which the fly ashes were sampled. The chars were generated by pyrolyzing the biomasses at 900 °C in N2 in a tube furnace, with the procedure described elsewhere.49 It can be seen that the residual carbons of all the fly ashes have the reactivities much lower than WS char, comparable or even higher than RH char, and much higher than FW char. Even after water- and HCl-washing, the residual carbons in the resulted C-FFA samples are still much more reactive to oxygen than FW char. It is worth noting that the residual carbons had experienced the temperatures (up to 1100−1200 °C on the grates and 800−1000 °C in the furnaces) higher than the laboratory chars, implying undergoing more severe thermal deactivation.45 Nevertheless, the residual carbons still have good reactivity, comparable to those of the 900 °C laboratory chars prepared from the component biomasses, suggesting that the residue carbon to be suitable applications as fuel or feedstock for producing activation carbon.

Figure 8. Arrhenius plots of oxidation reactivity of the recoverable residual carbon in large size fractions (>200 μm) of the fly ash samples, in comparison to those of biomass chars generated by pyrolyzing wheat straw, rice husk, and fir wood at 900 °C under N2 atmosphere.

decomposition peaks of water-washed fly ashes also shifting to higher temperatures (Figure 4). Figure 8 also shows that the Arrhenius plots are very close to straight lines over a wide range of carbon conversion levels (10−80%). Therefore, the approach of Russell et al.26 developed for nonisothermal air TGA (eq 2) was followed to derive kinetic parameters including activation energy E and preexponent factor k0 for the oxidation of the residual carbon. The obtained parameters for the unburned carbons in the >200 μm size fractions of the fly ashes and some washed samples are summarized in Table 3. It is noted that Russell et al. method Table 3. Combustion Kinetic Parameters of Residual Carbons in Large Size Fractions (>200 μm) of Fly Ashes and Their Water-Washed (WW) and Acid-Washed (AW) Samples sample

E, kJ/mol

A-FA B-FA C-FFA C-CFA C-FA WW C-FFA AW C-FFA WW A-FA AW A-FA

128.4 133.0 174.2 139.4 167.5 148.2 139.5 106.8 107.5

4. FURTHER DISCUSSION AND IMPLICATIONS The data presented in this paper clearly demonstrate the complex thermal behavior of biomass fly ashes. In addition to the burnout of residual carbon, other thermal events also take place, including mainly the decomposition of carbonates and the release of alkali related species. It is also shown that such complex ash thermal behavior complicates using the LOI approach to estimate the residual carbon content as well as selecting proper temperature for LOI determination. Besides the oxidation of unburned carbon (char), the weight loss at 350−530 °C in air TGA might also result from the hydration or decomposition of mineral species such as MgCO39 as well as the evaporation of heavy metals (if any).44 Nevertheless, no obvious weight loss observed in this temperature range in N2 TGA (Figure 3) implies that the contribution from the reactions of mineral species to the weight loss in air in this temperature range is negligible for the ashes studied. It is clear that nonisothermal air TGA is capable of distinguishing the contribution of residual carbon oxidation to weight loss from those of inorganic matter reactions, particularly the decomposition of calcium carbonate and the release of alkali chlorides related species. Therefore, nonisothermal air TGA can be used as a simple technique for estimating the residual carbon content in biomass ashes for engineering applications. The TGA measurements also suggest that the decomposition of calcium carbonate in biomass ashes can start at the temperatures as low as ∼530 °C. Considering that the carbonate is a common species occurring in both woody and

k0, 1/min 5.93 4.57 1.34 4.52 2.94 4.09 4.59 5.53 4.27

× × × × × × × × ×

108 108 1012 109 1011 109 108 106 106

was originally developed to evaluate the reactivity of coal chars with high carbon contents. Nevertheless, similar methods were applied for unburned carbons of coal fly ashes and high ash coal chars,52,53 although suggestion on using carbon concentrated samples was made to avoid the interference of the large quantity of inorganic matter with accurate measurement.52 In this study, the unburned carbon contents are very high for the large size fractions (>200 μm) of biomass fly ashes (see Figure 7) and their washed samples. Furthermore, the comparison between air and N2 TGA (Figure 3) clearly shows a negligible effect of the possible reactions of inorganic compounds on the weight loss rate of the carbon oxidation. Additionally, the determination of the reactivity in the carbon conversion range of 10−80% can avoid the possible interference of inorganic matter with the resolution in the weight measurements at the stages of very lower and very higher carbon conversion. 905

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a simple method to estimate the residual carbon content in biomass fly ashes if the ashes are burned for an efficiently long time at the temperature below the decomposition temperature of calcium carbonate. It was also found that sieving is not an effective approach for pretreating biomass fly ashes to lower the residual carbon content for forestry and agricultural utilization of biomass combustion fly ashes, but it may be an easy technique for recovering a quite large part of residual carbon containing in large fly ash particles. The recoverable residual carbon from the biomass fly ashes is very reactive to oxidation, implying its good quality for reuse as fuel or as feedstock for activation carbon production.

herbaceous biomass ashes, it is clear that LOI measured at the temperatures higher than 530 °C may lead to an overestimation if LOI is used for estimating the residual carbon content in biomass ashes. Figure 6 shows that LOIs at 450 °C are well consistent with TGA measured unburned carbon contents, while LOIs at 550 °C are systematically higher than TGA measured values for the ashes with the LOI values of 10−30%. The reason is that a considerable amount of carbonates may have decomposed when the ash samples held at 550 °C for a long period (in the scale of hours). In other words, LOI at the temperature of