J. Cent. South Univ. (2017) 24: 496-505

DOI: 10.1007/s11771-017-3452-6

Experimental investigation of performance properties and agglomeration behavior of fly ash from gasification of corncobs

YAO Xi-wen(姚锡文), XU Kai-li(许开立), LI Yang(李洋)

College of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2017

Abstract: The gasification industries make use of biomass residue as feedstock to produce synthesis gas, but the gasification of this waste biomass generates tons of ash everyday. Performance properties and agglomeration behavior of corncob ash (CCA) collected from the gasification of corncobs in a pilot-scale gasification station were investigated by using some experimental methods. Based on the chemical composition results, the agglomeration tendency of CCA from combustion and gasification process was also analyzed. Chemical analysis shows that the fly ash is mainly composed of inorganic matters formed by K, Mg, Ca, Na, Fe, Al, S, etc. The agglomeration characteristics indicate that the slagging degree increases with the increase of ashing temperature, and the slagging tendency of these CCA samples from gasification or combustion is different with various slagging indices. All CCA samples from combustion or gasification can cause slagging/fouling problems in thermal conversion systems. The applications of CCA are closely related to its performances, and CCA has the potential to be used in various fields, for example, as a material for ceramic products and activated carbon, as an adsorbent, as a crude fertilizer, and as a structural material.

Key words: biomass gasification; corncob; fly ash; agglomeration behavior; characterization

1 Introduction

Nowadays, owing to the depletion of limited fossil fuel and growing environmental problems, there has been an urgent demand for green renewable energy [1, 2]. Biomass fuel usually contains less sulphur and nitrogen than coal, which is considered to be a green renewable energy to cope with global warming and the exhaustion of fossil fuel reserves [3]. Thus, bio-energy has the potential of being more important in the future. Generally speaking, there exist a lot of thermo-chemical conversion techniques employed for converting biomass into higher heating value fuels, such as combustion, gasification, liquefaction, and pyrolysis, as well as co-combustion, co-pyrolysis, and co-gasification of coal with biomass [4-6].

Corn is one of the most extensively planted crops, and corncob is an abundant agricultural byproduct available worldwide. Every year large amounts of corncob are produced by the threshing process of corn. So it is of great significance to convert waste corncobs to produce high quality products by thermo-chemical conversion technologies. Gasification is a process that converts carbonaceous materials, and it has received worldwide attention for the convention of biomass into synthesis gas in recent years [7, 8]. However, the gasification of corncob as fuel always generates large amounts of residual ashes, which have great passive effects on the thermo-chemical conversion of corncob. Furthermore, the corncob ash (CCA) generated from gasification often contains various forms of alkali and alkali earth metals together with a mass of silica [9]. Consequently, biomass ash with high volatility often easily results in serious fouling and slagging problems in thermal conversion processing systems [10].

Since the composition of biomass ash is quite complex, so the properties of various biomass ashes are quite different. At present, considerable studies on the characterization of biomass ash have been carried out [2, 4, 5, 7-13]. However, there still remains a lack of further detailed investigations on the performance properties of CCA from biomass gasification station. On account of this reason, the potential applications of CCA obtained from biomass gasification station are still unclear, and this ash is also uncertain in terms of agglomeration characteristics. Hence, the further study on the agglomeration and performance characteristics of biomass ash is very important, which can provide a baseline not only for solving the slagging and fouling problems caused by fly ash, but also for promoting the effectively utilization of biomass ash.

In this work, the properties and agglomeration behavior of CCA derived from a pilot-scale biomass gasification station were studied by using a lot of experimental techniques. Besides, the effects of ashing conditions on the agglomeration characteristics of CCA were investigated by conducting muffle furnace burning tests. Based on the chemical composition results, the agglomeration behavior and slagging tendency of ash were also explored.

2 Materials and experimental methods

2.1 Materials and description of gasification station

The ash samples used in this study were collected from the cyclone dust collector of a corncob sourced gasification station in the rural area of Shenyang, China. The gasification of corncobs was carried out in a downdraft fixed bed, which has an internal diameter of 1.03 m and a height (from the gasifier top to the grate) of 1.88 m. This gasification station has a maximum thermal input of 175 kW, which provides syngas for approximately 300 households. The whole gasification plant covers an area of 58 m². Figure 1 shows the gasifier followed by a gas cleaning and cooling section equipped with a cyclone dust collector, a spray-dryer system, and a tar scrubber.

These collected ash samples were first pulverized by a ball mill due to the severe non-homogenity, then placed into a pan and oven dried at (105±0.5) °C for 24 h. After the ash samples were dried and cooled, they were again grounded and homogenized, and finally sieved with a 100 meshes sieve (about 0.154 mm) prior to analyses. Figures 2(a)-(c) respectively exhibit the appearance of raw ash samples, ground and sieved ash powders, and typical agglomerate samples. Some of these agglomerate samples are larger than 30 mm in size (see Fig. 2(c)), indicating the presence of serious agglomeration phenomena in the ash.

A laser particle size analyzer (Winner 3001, Jinan Micro-nano Particle Instrument Co., China) was used to determine the size distribution of the CCA powders with a mean diameter and a medium diameter of 12.96 μm and 10.23 μm, respectively (Table 1).

As presented in Table 1, both the spherical degree

value (Q) and the length-width radio (L) are close to 0.9, indicating that the pulverizing and sieving process makes these inhomogeneous ash powders become homogenized. Moreover, it can be inferred from Table 1 that the accumulated percentage for the seized ash powders within diameter of 30 μm occupies a percentage of greater than 90%. In addition, the granularity analysis also shows that the S/V (ratio of total surface area to volume) value for the sieved powders is close to 1.5 m²/cm³, which is much larger than the ash powders formed through combustion [8, 14]. This result also indicates that the gasification ash has more small pores due to the release of volatiles.

Fig. 1 Pictures of gasification station

Fig. 2 Appearance of raw ash (a), ground and sieved ash powders (b), and some typical agglomerate samples (c)

Table 1 Granularity analysis results of ash powders

2.2 Experimental apparatus

Chemical analysis of CCA was determined by X-ray fluorescence (ZSX100e. Rigaku Co., Japan). Mineral phases were determined by X-ray diffraction using a Cu K_α radiation of 0.15406 nm (X’Pert PRO, PANalytical B.V., Netherlands). Major peaks were identified through comparison with standards of High Score Plus software package. Scanning electron microscopy (Ultra Plus, Carl Zeiss Co. Ltd., Germany) and energy dispersive X-ray (Genesis, Edax DX-4) were employed to obtain more information on the ash, such as ash morphology, agglomeration phenomena, and ash surface composition. Prior to analysis, the ash was gold-coated for making it electrically conductive. Thermal analysis was finished in nitrogen, dry air and 40% O₂+60% N₂, respectively, by using a thermal balance (NETZCH-STA449 F3, Germany). For each test, about 5 mg samples were heated at the rate of 20 °C/min till a temperature of 1200 °C.

Moreover, the effects of ashing conditions on the slagging characterization, apparent morphology and structure of CCA were explored through conducting muffle furnace burning tests which were carried out in a chamber muffle furnace (SX2-15-12, Shuangyu Instruments Co., China). In this work, repeated tests for all the above experiments were performed at least three times, and finally the average values were used to minimize the error.

3 Results and discussion

3.1 Chemical and phase analysis

The chemical composition of CCA determined by XRF is listed in Table 2. The ash is mainly composed of inorganic matters formed by K, Mg, Ca, Na, Fe, Al, S, etc. mostly in various forms of oxides, silicates or chlorides [14]. From these results, it is found that K and Si are the predominant elements in CCA, and this result is confirmed by XRD analysis, which reveals the complex diffraction patterns of ash due to different phases present and several possible peak overlapping.

Table 2 Composition of CCA from gasification (mass fraction, %)

As seen from Table 2, these CCA powders mainly comprises of a mixture of compounds along with potassium and silica, and the XRD results indicate the presence of potassium mostly in the forms of arcanite (K₂SO₄), kalicinite (KHCO₃), kalsilite (KAlSiO₄), leucite (KAlSi₂O₆), sylvite (KCl) and the presence of silica in the form of quartz (SiO₂) as the major component. In addition, calcium and sodium are mainly presented in the form of calcite (CaCO₃) and natrite (Na₂CO₃), respectively. Similar compounds of CCA have been reported in other literature [15].

3.2 SEM analysis

SEM analysis can provide further insight into the morphology and external surface structure of the ash. Figure 3 shows the SEM images of CCA powders.

As seen from Fig. 3, these images reveal particles that are irregular and show some agglomeration, and there exist agglomerated particles (Fig. 3(a)), quadrate particles (Fig. 3(b)) and near-spherical particles (Fig. 3(c)) in CCA. As it can be seen from Fig. 3(c), most of the near-spherical particles are less than 2 μm in size.

For obtaining more detailed imaging information on ash morphology and ash agglomeration, the images of individual particles in the CCA are shown in Fig. 4.

As seen from Fig. 4(a), a piece of unburnt flaky ash particle shows the original and fibrous texture of the corncob feedstock. Figure 4(b) shows the presence of quadrate crystalline phases. Fig. 4(c) exhibits the surface morphology of a typical agglomerate sample composed of small particles. At a closer examination of Fig. 4(c), many voids can be found throughout the agglomerate, which can result in the adsorption of small ash particles step by step. Figure 4(d) shows a dumbbell-shaped particle attached with numerous floccules, indicating the release of alkali metal compounds and the appearance of slight caking. Figure 4(e) shows the internal surface of agglomerate that was broken before observation. After an inspection of internal surface, a porous structure caused by release of volatiles from solid substrates can be identified. Figure 4(f) shows the external surface of a tightly bonding agglomerate. The diverse morphologies of different ash particles reflect the complex existential forms of various inorganic elements in biomass.

Fig. 3 SEM images of agglomerated particles (a), quadrate particles (b), and near-spherical particles bonding together (c)

3.3 Analysis of EDX composition data

For better understand the agglomeration behavior of CCA, the EDX analyses which were conducted on ash samples have been organized and compared with each other, on the basis of the appearance of various points in SEM images. The results of EDX analyses of points a-f in Fig. 4 are listed in Table 3. In this section, in order to compare the composition at various points intuitively, the content of different elements for points a-f was presented in a histogram (Fig. 5).

Seen from the results, the elemental composition for particles in different morphologies is of great difference. As determined by EDX, the predominant elements of the ash surface composition are in various compounds such as C, O, Si, Cl, K and Ca. Lesser amounts of elements Na, Mg, Al, P, S and Fe can also be observed from these results.

As for the unburnt flaky ash particle (Fig. 4(a)), the results of Point a exhibit a high content of carbon (78.13%), indicating that there may still exist some organic carbon on the surface of flaky particle due to the inadequate burning. The EDX results of Point b indicate that Si and O are the predominant elements, which reach a high content of 30.85% and 47.08% respectively, hence, these crystalline phases observed in Fig. 4(b) are identified as quartz.

According to the EDX results of Point c on the surface of the agglomerate sample, it is easily found that the content of alkali metal K is up to 22.31%, and the content of alkaline-earth metal Ca and Mg is as high as 12.50% and 3.31%, respectively. XIAO et al [4] suggested that the ash fusion points decrease with the increase of alkali metal content, which aggravates fouling and slagging problems due to high viscosity of biomass ash. The EDX results of Point d show that the carbon content on the surface of floccules is 16.15% while the content of Si and that of O is respectively 32.05% and 50.58%. This result illustrates that some amorphous SiO₂ exists on the surface of floccules.

The results of Point e are similar to those of Point f. An enrichment of potassium and chlorine can be observed not only on the interior of agglomerate sample (Fig. 4(e)) but also on the external surface of molten particles (Fig. 4(f)), which indicates that the emission of potassium is mostly in the form of KCl. The high content of chlorides on ash surface is mostly due to the favorable volatility of chlorides [16]. As a result, the chlorides can be gradually released from the inside of biomass in gasification process, which can then stay on the surface of ash particles.

3.4 Effects of ashing conditions on agglomeration

In order to clearly understand the effects of ashing conditions on the agglomeration behavior of CCA, the prepared corncob samples with the same mass were put into a crucible, then kept at 600 °C for 2 h and 4 h respectively and 815 °C for 2 h, 4 h and 6 h in muffle furnace. Figure 6 shows the apparent morphology of the ash obtained from different ashing conditions. The ash content of the raw samples are presented in Table 4.

Fig. 4 SEM images of a piece of unburnt flaky ash (a), details of crystalline phases (b), a typical agglomerate sample (c), a dumbbell-shaped particle attached with floccules (d), internal porous structure of a broken agglomerate (e), external surface of a bonding agglomerate (f)

Table 3 Results of EDX spot analyses at points a-f in Figs. 4(a)-(f) (mass fraction, %)

As seen from Fig. 6, the agglomeration degree of 815 °C ash is more serious than that of 600 °C ash, revealing that the agglomeration degree increases with the increase of ashing temperature. As presented inTable 4, under the same burning time, the ash content of 600 °C ash is higher than that of 815 °C ash, and this is probably due to the fact that more combustible component remains in 600 °C ash compared with 815 °C ash. Besides, the ash content decreases with the increase of ashing temperature when the burning time is equivalent, revealing that the higher the temperature is, the more sufficiently the biomass burns, and the more easily the inorganic compounds volatilize.

Fig. 5 Comparison of elemental composition:

3.5 Agglomeration characteristics analysis

The agglomeration characteristics of fly ash are closely related to the chemical reactions of mineral matters in the biomass fuel [4]. For investigating the agglomeration characteristics of the gasification ash more accurately, the chemical compositions of 600 and 815 °C CCA were taken as a reference in this section. Just as for determining the composition of gasification ash earlier (Section 3.1), the oxides in 600 °C, 2 h CCA (Fig. 6(b)) and 815 °C, 2 h CCA (Fig. 6(d)) were also analyzed by XRF. The composition results of 600 °C, 2 h CCA, 815 °C, 2 h CCA, together with the gasification CCA are all summarized in Table 5.

Fig. 6 Apparent morphologies of raw ash samples obtained from different ashing conditions:

Table 4 Ash content of raw samples under different conditions

Table 5 Compositions of different CCA samples analyzed by XRF analysis (mass fraction, %)

According to Ref. [17], these formulas for agglomeration indices are described as follows:

The alkali index (I) is defined as the percentage of alkali metal (K₂O+Na₂O) in ash.

The index R_b/a is defined as base (Fe₂O₃, CaO, MgO, K₂O, Na₂O) to acid (SiO₂, TiO₂, Al₂O₃) ratio.

R_b/a=(Fe₂O₃+CaO+MgO+K₂O+Na₂O)/(SiO₂+TiO₂+Al₂O₃)         (1)

The index G is defined as silica ratio:

G=(SiO₂×100)/(SiO₂+Fe₂O₃+CaO+MgO)          (2)

The index S/A is defined as silica-to-alumina ratio:

S/A=SiO₂/Al₂O₃                                               (3)

H_w is defined as slagging/fouling index:

H_w= R_b/a×Na₂O, as Fe₂O₃/(CaO+MgO) > 1         (4)

H_w=Na₂O, as Fe₂O₃/(CaO + MgO) < 1 andCaO+MgO>2         (5)

All the values of different agglomeration indices are listed in Table 6 referring to the above formulas.

As seen from Table 6, all these CCA samples contain large amounts of alkali metals, indicating that all the ash samples fall in the serious agglomeration range. The R_b/a values of the three CCA samples are all greater than 0.7 and their agglomeration tendency is inferred to be serious.

Table 6 Calculating values of agglomeration indices

Howerer, the values of G for all these ash samples are in the medium agglomeration range from 66.1 to 78.8, indicating their agglomeration tendency is medium. Moreover, the S/A values for all the three kinds of CCA samples in this study are much greater than 2.65, which reveals that the agglomeration tendency is serious. As regards the H_w, the values of gasification CCA and 815 °C, 2 h CCA are within the easy agglomeration range (between 3 and 6), while the H_w value of 600 °C, 2 h CCA is greater than 6 and the agglomeration tendency is serious.

Based on the above analyses, it can be found that the agglomeration tendency of the ash respectively from gasification and combustion process is slightly different with various agglomeration indices, and all these CCA samples can cause slagging and fouling problems in thermal conversion systems.

3.6 Thermal properties analysis

The mass loss and the degradation rate together with the differential thermal behavior as a function of temperature recorded under the inert or oxidative atmosphere are exhibited in Fig. 7. As seen from Fig. 7(a), the total mass loss increases with the increase of oxygen concentration. The mass loss of the ash in thermal analysis, in most cases, can be explained from the ash composition.

As seen from Fig. 7(a), the TG patterns can be divided into four stages, less than 250 °C for the degradation of unstable constituents, accompanied by a sharp mass loss rate peak in the DTG patterns in Fig. 7(b), 250-600 °C for the oxidation of organic matters, accompanied by a broad exothermic peak caused by the volatilization of unburnt carbon and residual organics (Fig. 7(c)). The oxidation of unburnt carbon is expected to occur below 600 °C while the decomposition of alkali-containing carbonates is expected at above 600 °C [12]. Most of the mass loss from 600 to 800 °C can be attributed to the removal and reaction of inorganic matters, such as the CO₂ release from the decomposition of calcium carbonate (referring to Eq. (6)), while the mass loss at above 800 °C may be caused by the thermal transformation of residual inorganics:

CaCO₃(s)→CaO(s)+CO₂(g)                     (6)

Taking into account of others’ studies [9, 18], the continuous mass loss above 1000 °C may occur due to CO₂ release when K₂CO₃ reacts with SiO₂ (referring to Eq. (7)). Besides, the mass loss caused by the gas-phase release of a minor amount of K₂O from the K₂CO₃ decomposition (referring to Eq. (8)) can also make a significant contribution to the total mass loss when heating up to 1000 °C [19]:

K₂CO₃(s)+xSiO₂(s)→CO₂(g)+K₂O·xSiO₂(s) (x=1, 2, 4)         (7)

K₂CO₃(s)→K₂O(s) + CO₂(g)                    (8)

As seen from the DTA patterns in Fig. 7(c), a small endothermic peak appears within 620 and 630 °C in case of nitrogen while a small exothermic peak appears in this temperature zone in case of dry air or 40%O₂ in N₂. According to ABRAHAM et al [12], the small endothermic change in the nitrogen case is caused by the melting of KCl while this exothermic change in the oxidative cases is due to the combustion and volatilization of unburnt carbon residues.

Fig. 7 Thermal analysis curves for fly ash:

TG and DTG patterns are often used to establish thermal degradation profiles and parameters can be derived from these profiles. For evaluating the thermal performance of CCA, a volatile release characteristic index is defined as [20]

                   (9)

where (dw/dt)_max is the maximum mass loss rate and (dw/dt)_mean is the mean mass loss rate, T_s is the starting temperature for volatile release and mass loss, T_max is the temperature of maximum mass loss rate, △T_1/2 reflects the temperature of full width at half maximum for the main peak of DTG curves, and D reflects the volatile release characteristic index.

Table 7 summarizes these pyrolysis characteristic parameters and finish temperature of the main mass loss (T_f) and the total mass loss (W_t) determined from TG and DTG results in Figs. 7(a) and (b).

Table 7 Characteristic parameters from thermal degradation curves of CCA in different atmospheres

As seen from Table 7, the total mass losses in nitrogen, dry air, and 40%O₂+60%N₂ are 17.13%, 19.86%, and 23.12%, respectively. With the increase of oxygen content, the starting degradation temperature (T_s) shifts to a lower temperature zone while the finish degradation temperature (T_f) increases with a peak temperature (T_max) respectively at 193.6, 153.5, and 133.4 °C in the cases of nitrogen, dry air, and 40%O₂+60%N₂. The reason for the observations is that the high oxygen content can greatly accelerate the pyrolysis process.

Moreover, these pyrolysis parameters including (dw/dt)_max, (dw/dt)_mean, △T_1/2, and volatile release characteristic index (D) all increase with the increase of oxygen content, further indicating that the presence of oxygen is propitious to the combustion performance of unburnt carbon in the fly ash.

3.7 Ash utilizations

According to the analyses above, it is found that the presence of oxides in CCA makes it possible for making ceramic products, such as structural ceramics and membrane filters. Moreover, as CCA is devoid of toxic metals, it can potentially be used as adsorbents for clarification in various industries. As water soluble salt concentrate is specifically rich in potassium, chloride is of great interest in terms of the element extraction, basically the same to literature [12].

Through comparing the chemical composition results of CCA from gasification with those of other biomass ash, such as wood ash from gasification [8] and rice husk ash from combustion [21], it can be found that the proportions of nutrient elements (commonly K, Ca, and P) in CCA are much higher, indicating that the CCA has a good potential to be used as a crude fertilizer. This result is also consistent with that of PAN and EBERHARDT [7], who reported that the nutrient elements in gasification ash were more bioavailable than those in combustion ash.

Moreover, from the point of ash utilization, there also exist some differences between CCA and other biomass ashes. For example, the specific surface area of gasification ash is relatively higher than other biomass ash from combustion [8, 14], and the unburnt carbon separated from CCA is more available for use as activated carbon, which is a advantage of using the waste ash from biomass gasification station. Also, the CCA is largely composed of silica. Due to the high silica content, CCA has a potential to become a SiO₂ source for silicon compounds preparation. In addition, the suitability for CCA as a structural material has been ascertained by previous studies [22, 23].

4 Conclusions

The focus of this work is to obtain information about the performance properties and agglomeration behavior of fly ash from the gasification of corncobs in a pilot-scale gasification station. Based on the results of chemical composition, the agglomeration tendency of the ash from combustion and gasification process was studied. SEM images reveal that the ash particles are irregular, as well as show some agglomeration. These agglomeration characteristics reveal that the ash slagging degree can be aggravated with the increase of ashing temperature. The slagging tendency is different with various slagging indices. All CCA samples can cause slagging/fouling problems in thermal conversion systems. In addition, the thermal analysis results reveal that the starting degradation temperature tends to shift to a lower temperature zone while the finish degradation temperature and volatile release characteristic index increase with the increase of oxygen content. The utilizations of biomass ash are closely related to the performance properties.

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(Edited by YANG Bing)

Cite this article as: YAO Xi-wen, XU Kai-li, LI Yang. Experimental investigation of performance properties and agglomeration behavior of fly ash from the gasification of corncobs [J]. Journal of Central South University, 2017, 24(3): 496-505. DOI: 10.1007/s11771-017-3452-6.

Foundation item: Project(2013020137) supported by the Natural Science Foundation of Liaoning Province, China; Project(2015-36) supported by Rural Energy Comprehensive Construction Foundation of the Ministry of Agriculture, China

Received date: 2015-10-14; Accepted date: 2016-10-13

Corresponding author: XU Kai-li, Professor, PhD; Tel: +86-21-83678405; E-mail: kaili_xu@aliyun.com