Article pubs.acs.org/EF
Sintering Characteristics and Mineral Transformation Behaviors of Corn Cob Ashes Liang Wang,* Johan E. Hustad, and Morten Grønli Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway ABSTRACT: The aim of this work was to investigate the sintering characteristics and mineral transformation behaviors of three corn cob ashes using a combination of inductively coupled plasma−atomic emission spectroscopy (ICP−AES), ash fusion analysis, X-ray diffraction (XRD), and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM− EDX). The sintering degrees of the corn cob ashes at elevated temperatures were graded by performing laboratory-scale sintering tests. The WCob ash has a significantly low melting temperature of 834 °C and showed severe sintering behaviors during testing. The SEM−EDX and XRD analyses revealed that the fused WCob ash consisted of a mixture of potassium-rich silicate− phosphate melts. The WCob is dominated by potassium, silicon, and phosphorus, along with small amounts of alkali earth metals. Therefore, the formation and melting of potassium-rich silicates and phosphates are favored, causing severe sintering of the WCob ash at elevated temperatures. In contrast, a relatively higher melting temperature of 998 °C and a moderate sintering degree were observed from the PCob ash. High contents of chlorine, calcium, and magnesium in the PCob may promote potassium release from ash residues, instead of being incorporated into the silicate and phosphate structures. This process could inhibit the formation of low-temperature-melting silicates and phosphates and reduce ash sintering consequently. The abundance of calcium and magnesium in the PCob also led to the formation of high-temperature-melting silicates and phosphates, restraining ash melt formation and the extent of ash sintering. Results from the experimental work and analyses indicate that the combustion of corn cob may be challenging because of ash sintering. The transformation and melting behaviors of corn cob ashes are highly dependent upon the fuel compositions.
1. INTRODUCTION Using biomass for sustainable energy production is continuously gaining interest because of concerns of greenhouse gas emissions and worldwide energy demands.1−3 Thus, new biomass materials and residues are now being explored and introduced into the market. Examples of such biomass materials include agricultural residues, forest residues, and food processing wastes.4−11 Agricultural residues, a widely available energy source, are especially interesting for heat and electricity production by means of combustion.3 However, the combustion of the agricultural residues can be heavily hampered by problems, such as ash sintering and slagging. These problems are closely related to the presence and transformation of ash-forming elements in the fuel. In comparison to woody biomasses, agricultural residues typically contain higher amounts of ashforming elements.6,8,12 Most significant ash-forming elements in agricultural residues are nutrients required for plant growth, including potassium (K), phosphorus (P), chlorine (Cl), sulfur (S), silicon (Si), calcium (Ca), and magnesium (Mg). K and P are two essential nutrients that promote plant root growth, increase grain yields, and enhance the strength of fiber structures.13 Moreover, potassium and phosphorus fertilizers are currently being used to improve the soil quality and grain production, enhancing the K and P concentrations in agricultural residues.3,13 Cl is an abundant micronutrient that plays a number of biochemical roles during plant growth.13 Si is a key element for the formation of the silicate skeleton in plant cells, which provides structural strength and protection against fungal pathogens.13 Sand/soil contamination during plant © 2012 American Chemical Society
harvesting and handling can also increase the Si content in agricultural residues.13,14 The alkali earth metals Ca and Mg are important for the formation and stability of cell walls in the living plants.13 Ash-forming elements undergo complex transformation reactions during the combustion of agricultural residues. The reactions involving potassium are often directly responsible for ash-sintering problems occurring in combustion appliances. During the devolatilization and char burnout stages, large amounts of K, S, and Cl are released directly from the fuel particles and form potassium salts (i.e., KCl, K2SO4, and K2CO3) in either gaseous or condensed phases. The potassium salts have melting temperatures as low as 770 °C. Binary systems containing KCl and K2SO4 may melt at an even lower temperature around 550 °C.15 Some potassium salts or mixtures of these salts are present as molten phases at normal biomass combustion temperatures (700−1000 °C), leading to ash sintering and deposition on heat-transfer surfaces.4,5 On the other hand, a considerable fraction of K remains in the char/ash residues and readily reacts with silicon that is either organically dispersed in the fuel particles or from sand contamination. The gradual incorporation of K into silicates begins at approximately about 600 °C and is far progressed up to 900 °C.13 This process leads to the formation of low-temperature-melting potassium silicates that readily melt and initiate ash sintering.4 Received: February 6, 2012 Revised: August 28, 2012 Published: August 28, 2012 5905
dx.doi.org/10.1021/ef300215x | Energy Fuels 2012, 26, 5905−5916
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purposes. Therefore, it is imperative to obtain a better understanding of the ash transformation and sintering behaviors before new fuels from the agricultural sector are used in industrial applications. Corn is one of the most important crops, with a worldwide yearly production of more than 800 million tons.30 The production of corn will continuously increase to secure the food supply for the growing world population.31 The corn cob is the kernel part left after separating the grains from the corn. Approximately 18 kg of corn cobs is generated for every 100 kg of corn grains produced.30 As a major byproduct from the corn harvesting, the worldwide annual corn cob production is very huge. Currently, corn cobs are mainly used as raw materials for industry processing applications, animal feeding, and charcoal production.30−32 However, significant amounts of corn cobs are still unused and considered worthless waste material.33 There is great potential for producing heat and electricity from corn cobs by means of combustion. However, literature on the transformation and sintering behaviors of the corn cob ash is scarce. Arvelaskis et al. reported that the studied corn cob ash is rich in alkali metals, chlorine, and sulfur.34 This corn cob ash had a low sintering temperature of 750 °C and was completely melted at 900 °C. Even leached with tap water, considerable ash sintering was still observed during combustion. In another study, the bed agglomeration characteristics of a kind of Thai corn cob were experimentally studied in a laboratory-scale fluidized-bed combustor using quartz sand as the bed material. It was found that bed agglomeration and consequent defluidization took place rapidly during combustion of the corn cob at 800 °C.35 Large amounts of potassium silicate melts formed during the corn cob combustion, resulting in considerable agglomeration of bed materials.35 Obviously, these available results are deficient for a comprehensive understanding of the transformation and sintering mechanisms of corn cob ashes. The aim of the present work was to investigate the chemical compositions, sintering properties, and mineral transformation behaviors of three corn cob ashes at elevated temperatures. The results will provide valuable information for the efficient combustion of corn cobs for heat and power production.
However, the incorporation of potassium into the silicate structures can be restrained by the presence of Cl in the fuel. In the temperature interval of 700−830 °C, chlorine plays a dominant role in facilitating the release of potassium out of the fuel and preventing the incorporation of K into silicate structures.13,15,16 Knudsen et al.13 reported that the potassium released at 830 °C was significantly increased from 16 to 85% when excess Cl was added to the raw wheat straw sample. Further equilibrium calculations indicated that all K in the wheat straw can be driven out and the formation of potassium silicates can be completely depressed, when Cl was present in great molar excess to K.17 These findings have been confirmed by other experimental studies and are supported by thermodynamic calculations.15,16,18−20 Furthermore, some agricultural residues are rich in both phosphorus and potassium. Several authors have stated that phosphorus has a high affinity toward potassium, binding it in potassium phosphates in ash.12,21,22 For this reason, phosphorus may reduce the amount of potassium available to form low-temperature-melting silicates. However, some of the potassium-rich phosphates have low melting points, which are present as molten phases and enhance the sintering of residual ash at combustion temperatures.12 Agricultural residues also contain moderate amounts of the alkali earth metals Ca and Mg. During combustion, the alkali earth metals in oxides may dissolve in the molten potassium silicates and, to some extent, increase the volatility of K as a result of the competition for network positions.4,23 Equilibrium calculation results showed that, during combustion of a Si-rich wheat straw, more K was present in the salt form rather than potassium silicates when extra calcium was introduced into the fuel.17 As the reaction temperature increased above 1100 °C, the total amount of formed potassium salts in the gas phase increased radically.17 Additionally, reactions of Ca and/or Mg oxides with potassium silicates lead to the formation of high-temperature-melting Ca/ Mg−K−silicates.4 Therefore, an increase of Ca content in the raw fuel generally limits the formation of potassium-rich silicate melts and mitigates the ash sintering during combustion of agricultural residues.6,10,24,25 In addition, the presence of inherent and/or extraneous Ca and Mg in the fuel also promotes the formation of high melting point phosphates with high Ca(Mg)/K ratios.12,21,22 The incorporation of perceptible amounts of CaO can increase the melting temperature of potassium-rich phosphates from as low as 700 to over 1000 °C.26 Furthermore, enhancing the Ca content could also enhance volatilization of K from phosphorus-rich biomass fuels.9,26,27 Altogether, the chemical compositions of biomass fuels, especially the relative concentrations of key ash-forming elements, have a significant impact on ash transformation sequences and sintering behaviors. The tendency for ash sintering is generally high during the combustion of agricultural residues. For fixed-bed combustion systems, the ash sintering proceeds with slag formation as a result of progressive bridging, coalescence, and accumulation of the sintered ash residues on grates. The slag with large sizes cannot be transported out from the grate/furnace, which then interferes with the combustion processes and reduces the performance of combustion appliances.10−12 In fluidized-bed boilers, ashes derived from agricultural residues melt into highly viscous liquids, resulting in the agglomeration and defluidization of bed materials.28,29 Ash-sintering problems reduce the energy conversion efficiency of combustion systems and limit further use of agricultural residues for energy conversion
2. EXPERIMENTAL SECTION 2.1. Fuel Characterization. Three types of corn cob were obtained from Surcin, Belgrade’s municipality in Serbia (SCob, ZP Maize Hybrid-ZP 505), Pioneer Hi-Bred International, Oahu, Hawaii (PCob), and the Waimanalo farm at the University of Hawaii, Oahu, Hawaii (WCob). The corn cob samples were first crushed and ground in a cutting mill mounted with a 1 mm sieve. The milled corn cobs were dried in an oven at 105 °C for 48 h to obtain a stable weight. After drying, the volatile matter and ash content of each sample were determined by following American Society for Testing and Materials (ASTM) standards E871 and D1102, respectively. The fixed carbon content of each sample was calculated by difference of 100% and the sum of the volatile matter and ash content. Ultimate analyses of the samples were conducted with an elemental analyzer (Vario MACRO Elementar) according to the standards ASTM E777 (carbon and hydrogen) and ASTM E778 (nitrogen). The oxygen content was determined by difference of 100% and the sum of ash, C, H, N, and S contents. The properties of the three corn cobs are given in Table 1. The major elements in the three corn cobs were analyzed by inductively coupled plasma−atomic emission spectroscopy (ICP− AES). Prior to ICP analysis, the corn cobs were prepared by multi-step pressurized microwave digestion in a mixture of HNO3/HF/H3BO3. The chlorine content in the fuel was detected by an ion chromatograph (Dionex ICS-90; standard, CEN/TS 15408:2007). 5906
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hard sintered ash residues with partial melting, (4) very hard sintered ash residues with slag formation, and (5) completely melted ash residues.6,25,36 A similar grading scale was applied in other studies and proved to be reliable for obtaining valuable information with rather quick tests.6,25,36 2.3. X-ray Diffraction (XRD) and Scanning Electron Microscopy Coupled with Energy-Dispersive X-ray Spectroscopy (SEM−EDX) Analyses. The mineral compositions of the standard ash samples and sintered residues were identified with an XRD analyzer (Bruker D8 Focus) equipped with Cu Kα radiation and a LynxEye detector. Each ash sample was ground into powder with a particle size smaller than 15 μm and spread on a silicon sample holder for X-ray scanning. The major crystalline phases in the sample were identified using the TOPAS evaluation program plus the ICDD-PDF2 database. One should note that the cooling history may influence the amount of amorphous materials contained in one ash sample.37 However, the XRD analysis results are still comparable, because the same cooling procedure was performed for all of the ash residues after the heating treatment. The morphology and microchemistry of ash samples sintered at increasing temperatures were examined by SEM−EDX. Representative ash residues with loose or partially sintered structures were spread on carbon tapes mounted on top of sample tabs. The severely/completely melted ash residues were embedded into resin, which were then cut and ground to produce smooth cross-sections. SEM was operated to take backscattered electron (BSE) images, providing information about the distribution of different elements in a scanned area. Furthermore, the microchemistry and associations of the main elements in a sample were examined with EDX by means of spot and element mapping methods.
Table 1. Fuel Characteristics and Ash Chemical Compositions sample
PCob
WCob
Proximate Analysis (wt %, dba) volatile matter 79.65 80.32 fixed carbon 17.75 17.64 ash content 2.61 2.04 heating value (MJ/kg) 18.94 18.22 Ultimate Analysis (wt %, db) C 47.26 48.07 H 6.39 6.35 N 0.54 0.47 Ob 43.11 43.31 S 0.10 0.10 Cl 0.39 0.17 Ash-Forming Elements (wt %, db) K 0.575 0.977 Si 0.253 0.472 P 0.085 0.223 Ca 0.43 0.26 Mg 0.065 0.048 Na 0.002 0.001 Al 0.006
