Article pubs.acs.org/EF
Experimental Study on the Combustion Characteristics and Alkali Transformation Behavior of Straw Yanfen Liao,* Guang Yang, and Xiaoqian Ma School of Electric Power, South China University of Technology, Guangzhou 510640, People’s Republic of China ABSTRACT: Potassium, a key nutrient involved in biomass growth, contributes to problematic ash fouling and slagging during combustion. This study was conducted to investigate the combustion behavior of typical rice straw grown in south China, as well as potassium transformation and evolution behaviors. The results showed that the volatile components of the straw have good devolatilization and combustion characteristics, while the residual char is relatively slow to burn out. Under low-temperature conditions, the combustion of char beginning after the devolatilization is almost complete. Chlorine has a higher volatility than alkali metals under combustion conditions. In addition, the migration of potassium has the same tendency as the release of volatiles, indicating that some potassium might evolve together with volatiles at the initial stage of combustion, i.e., in the form of organic potassium. Above 500 °C, more inorganic potassium is released to the gas phase, primarily as KCl. These results imply that low-temperature combustion is effective for the retention of potassium within straw ash. A high silica content in straw material is also helpful to constrain inorganic potassium within the biomass residual but might induce agglomeration of the straw ash.
1. INTRODUCTION With the concerns over energy shortage and CO2 emissions, biomass is now being considered as an inexhaustible and clean energy resource worldwide. As a large agricultural country, China is producing high amounts of agricultural residue annually. Use of this biomass resource would help alleviate the current energy shortage and environmental pollution. The New Energy and Renewable Energy Development Program of China declared that the installed capacity of the biomass power plant will reach 30 GW in 2020, and a series of subsidy policies have been implemented by the government to support the development of biomass energy technology. Biomass fuels contain inherent inorganic elements, including alkali metals and chlorine, as well as minor quantities of Si, Al, Ca, Fe, Mg, P, and S.1,2 Potassium, which is the most important alkali metal, is involved in reactions leading to ash fouling and slagging in combustion systems.3−11 Under high-temperature combustion conditions, alkali-metal-related compounds might be released to the gas phase and subsequently deposited and condensed on the rear flue surface as coarse fly ash, causing fouling and high-temperature corrosion.3−8 In the fluidized-bed combustion process, some reactions occur between alkali metals and bed solids, producing low-melting-point eutectic compounds that may lead to particle agglomeration.5,9−11 Most studies that have been conducted to date have investigated the behavior of alkali transformation and deposition.12−14 A chemical fractionation study of biomass alkali metal found that more than 90% of alkali in straw is water-soluble and that this part of alkali is crucial to the alkali pyrolysis release.15 Knudsen and Jensen proposed a law of transformation behaviors of potassium and its related elements during the straw combustion.16 Additionally, a detailed reaction mechanism for the sulfation of alkali metals was suggested by Peter Glarborg, and ionic behavior was observed for some association reactions involved.17 Dependent upon the combustion or gasification conditions, some potassium ions in biomass © 2012 American Chemical Society
have evolved and precipitated as salts (e.g., chloride, hydroxide, carbonate, oxalate, and sulfate), while some remained in the residue as K2O·nSiO2(s) and KAlSiO4(s).18 The temperature is another crucial factor that determines the formation of these compounds, and high-temperature evolution (>600 °C) is thought to lead to the greatest amount of ash fouling and slagging.16,4 Many of the aforementioned studies were conducted in batch (fixed-bed) experiments, often with high temperature. There is currently great interest in the staged combustion of biomass. Under these conditions, the biomass particles are first pyrolyzed or partially burned under low temperature, after which the volatile portions emitted are burned under high temperature. Consequently, this study was conducted to investigate the combustion features of biomass at 350−800 °C. This work also investigated the influence of the combustion temperature and time on devolatilization, char combustion, potassium transformation, and evolution behaviors and attempted to establish whether potassium could be constrained within the biomass residual under low-temperature combustion conditions.
2. EXPERIMENTAL SECTION The rice straw used in this study was paddy rice, grown in Mei City, Guangdong Province, and was a typical crop grown in south China. After the samples were cut into 1−3 cm pieces, they were dried in an oven at 90 °C for 5 h, ground, and sieved to less than 250 μm. The samples were subsequently analyzed to determine the main parameters that influenced thermal conversion. Proximate determinations were made according to the Standard Practice for the Proximate Analysis of Biomass (ASTM E0870-82R98E01). In addition, an ash test was conducted according to the Standard Test Method for Ash in Biomass (ASTM E1755-01). A GmbH VarioEL Received: October 17, 2011 Revised: January 2, 2012 Published: January 3, 2012 910
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Table 1. Ultimate and Proximate Analyses of Straw ultimate analysis (wt %, daf)
proximate analysis (wt %, ad)
C
H
O
N
S
moisture
volatile matter
fixed carbon
ash
49.73
8.36
40.17
1.39
0.35
5.84
59.7
18.22
16.24
equipment model CHNS analyzer was used to determine the carbon, hydrogen, nitrogen, and sulfur contents. The results of the proximate and ultimate analyses are shown in Table 1. The straw was found to have a high oxygen content and volatility, indicating good ignition characteristics. In addition, the straw sample had a high ash fraction and a high alkali content (as shown in Table 2).
On the basis of the main component of straw ash and the comelting during the straw combustion process, two main databases are selected in the simulation, which come along with the code. The database SLAGD contained some oxide, which exited in straw ash (MgO, Na2O, SiO2, CaO, Al2O3, and K2O), and some other inorganic salts, such as K2SO4, KCl, Na2SO4, NaCl, CaSO4, MgSO4, CaCl2, MgCl2, etc. The molten database SLATA contained the inorganic salts molten database, which is used to analysis the low-temperature comelting of inorganic salts and the aggregation and slagging caused by K2O and other oxide melting.
Table 2. Composition Analyses of Straw Ash (wt %) K2O
Na2O
Al2O3
Fe2O3
CaO
MgO
ZnO
SiO2
13.27
1.49
1.65
0.72
4.23
1.86
6.92
58.90
3. RESULTS AND DISCUSSION 3.1. Combustion Behavior of Straw. The combustion experiments were carried out in the fixed-bed reactor, as shown in Figure 1. The combustion temperature (referred to as the reactor temperature) was set at 350, 400, 500, 600, 700, and 800 °C, and the residual time in the reactor hot zone was controlled at 10−1800 s. The weight loss ratio α (wt %) was measured to illustrate the combustion characteristics of rice straw, according to eq 1
The experiments were carried out in a fixed-bed reactor, as shown in Figure 1. The quartz glass reactor had dimensions of 50 × 5 × 1200 mm (outside diameter × thickness × height) and was surrounded by silicon carbide pipes to supply heat by radiation. The sample was contained in a ceramic container with a mass density of less than 0.1 g/cm2 to ensure even heating and low heat-transfer resistance. In addition, 0.5 g of sample was used in each test to ensure reproducibility. The reactor temperature was controlled by the proportional− integral−derivative (PID) controller. The combustion experiment was carried out as follows: (1) The reaction zone was heated to a preselected temperature. (2) The ceramic container loaded with the sample was inserted into the pre-cooling chamber of the reactor. (3) The inlet port of the reactor was sealed, and the atmosphere was controlled by the flowmeters. (4) After a few minutes of purging, the sample was rapidly inserted into the hot zone using a handspike. (5) At the given combustion time, the atmosphere was regulated into a nitrogen condition and the sample was quickly withdrawn from the hot zone and set into a cooling tank with a nitrogen atmosphere to quench the combustion process. All experiments were repeated 3 times to ensure reproducibility. After quenching, all ash residues were analyzed to determine the devolatilization behavior and combustion regulation of char by proximate analysis. Inductively coupled plasma−atomic emission spectrometry (ICP−AES, Thermo Jarrell Ash, IRIS1000) was adopted to monitor the content of potassium in ash. X-ray diffraction (XRD) patterns of alkali metal compounds present in ash were measured on a D/max-IIIA device (Rigaku Denki Co., Ltd.) with a scan angle of 2θ at 5−65°. Morphology and elements present in the ash were investigated using Hitachi S-3700N scanning electron microscopy (SEM), combined with Bruker EDX microanalysis (voltage, 20 keV; angle, 35°). The equilibrium analysis software FactSage was used to determine thermodynamic stable chemical and physical forms in the chemical system. When the parameters, such as the elementary composition of the fuel and air, temperature, and pressure, are inputted, FactSage will search the species including these elements from the database.18
α=
(m 0 − m t ) × 100% m0
(1)
where m0 is the initial weight of the sample and mt is the weight of the residue after burning for time t. The combustion process was completed within 720 s at all temperatures investigated; therefore, the weight loss curves of straw combustion within 720 s are shown in Figure 2. Rapid weigh loss occurred when α (wt %) was less than 60%, after which the combustion speed decreased. The temperature plays a key role in the combustion process. At low temperatures (350 °C), the weight loss ratio is only 8.3% after burning for 60 s. When the temperature increases to 500 °C, the combustion process is improved and the weight loss ratio approaches 52% within 60 s. This acceleration is more apparent for temperatures over 600 °C, at which the combustion process is so fast that the three weight loss curves were almost identical at the initial stage. The ratio of residual char or volatile matter to that contained in the initial sample was used to illustrate the combustion characteristics, χ (wt %), as follows:
χ=
X i mt × 100% Xi0m0
(2)
Figure 1. Schematic diagram of the fixed-bed reactor. 911
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occurred throughout the combustion process when the reactor temperature was greater than 600 °C. These findings indicate that the early weight loss of straw is primarily caused by devolatilization throughout the combustion process, while the late weight loss is due to the combustion of char, which takes longer to burn out (as shown in Figure 2). When the combustion feature was observed from the outlet port of the quartz glass reactor, white smoke was observed at 75 s under low-temperature combustion (350 °C). These findings indicate that a large amount of volatiles are beginning to be released. This devolatilization process lasted for another 55 s until a bright flame was observed. The volatile combustion was identified by the flame above the straw sample and the unchanged mass of char at this time (as shown in Figure 4). When the volatile combustion flame disappeared, the remaining char particle began to combust and glowed in the reactor. As the temperature increased, the starting time of both devolatilization and the char combustion moved ahead, while the devolatilization duration time was shorten. Observation of the residual ash of different reactor temperatures revealed that ash that formed at less than 400 °C is loose powder, while slight slagging occurs between the residue and the bottom of the ceramic boat as the temperature increases. Above 700 °C, the slagging becomes serious and cannot be scratched out. The was likely a result of the reaction between the silica of the ceramic container surface and KCl in the biomass, which is described as follows:19
Figure 2. Weight loss curves of straw combustion.
where Xi is the mass fraction of char or volatile matter, mt is the residual mass of the sample, Xi0 is the initial mass fraction of char or volatile matter, from the proximate analysis shown in Table 1, and m0 is the initial mass of the sample. As shown in Figures 3 and 4, the char combustion is much slower than the volatile matter emissions and lower temperatures
2KCl + nSiO2 + H2O(g) → K2O·nSiO2 + 2HCl(g)
(3)
Therefore, to induce as much potassium to remain in the straw ash as possible, the temperature of combustion or pyrolysis should be below 600 °C. 3.2. Emission of Potassium. Alkali metallic species are important plant nutrients that are absorbed into plant materials through the root system and transported to all areas of the growing plant. Alkali metallic species are highly mobile and tend to volatilize during biomass thermochemical conversion, which results in deposition and corrosion of the heating surface. Potassium is the most important alkali metal because of its being present in high concentrations in many biomass fuels. Accordingly, understanding the emission behavior of potassium during combustion is important to the development of advanced biomass combustion technologies. ICP−AES (Thermo Jarrell Ash, IRIS1000) was employed to measure the content of potassium in residual matter after the samples were burned for a given time. The retention ratio β (wt %) was used to illustrate the emission of potassium, which is defined as the ratio of potassium retained in ash to the total content in the initial sample m β = Kt × 100% mK0 (4)
Figure 3. Devolatilization characteristics of straw.
Figure 4. Combustion of char of straw.
where mK0 is the potassium content of the initial sample and mKt is the potassium content of residue after burning for time t. The relationship between the retention ratio and reactor temperature or combustion time is shown in Figure 5. The rapid emission of K primarily occurs in the first 180 s, after which the emission speed decreases with the retention rate, gradually reaching an asymptote. In comparison of Figure 3 to Figure 4, a similar tendency was observed between the K emission behavior and devolatilization feature, implying that
were associated with greater backward delays for char combustion. These two distinct events were easily observed under low-temperature conditions (350 °C), when the release of volatile compounds was almost completed at 240 s, with the char was just beginning to burn. As the temperature increased, some char combustion occurred simultaneously with the release of volatiles, and this 912
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Overall, these results indicate that, at a low combustion temperature or during the initial burning stage, the evolution of potassium is associated with the decomposition of the organic structure containing organic potassium components, while the emission at high temperature corresponds to the evaporation of inorganic potassium salt. 3.3. SEM/EDX and XRD Analysis of the Combustion Residue. Scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis was applied to investigate the morphology of the residual ash and the concentration of metal species in ash. SEM photomicrographs of the residues burned at 400, 600, and 800 °C for 720 s are shown in Figure 7. The fiber structure of straw ash burned at 400 °C was incompletely destroyed; however, as the temperature increased, a rod-shaped structure was observed and the fiber structure began to collapse. Many angular particles and discrete grains of minerals appear at 800 °C, along with a certain amount of melted slagging on the surface (as shown in Figure 7c). EDX analyses revealed that the angular particles mainly consisted of Si, O, K, and Cl and a small amount of C, Na, Ca, Mg, Zn, Fe, Al, and Mn. To decrease the measurement error because of the unevenness of the sample, EDX analysis of the same sample was repeated for different sites. The average content of each element in ash is shown in Table 3. As shown in Table 3, some carbon is found in the lowtemperature ash, which is considered to be the support matter that maintains the fiber structure of the residue (as shown in Figure 7a). The concentrations of Ca, Zn, Fe, Al, and Mn have only shown small fluctuations with an increasing temperature. It was reported that these elements always exist as oxides or multiple oxides in ash with good stability. Sulfur was not confirmed by EDX analysis because of the superposition of several spectrum peaks. A stoichiometric excess of potassium and sodium relative to chlorine was found. The molar ratios of (K + Na)/Cl at 400, 600, and 800 °C were 3.7, 5.3, and 8.4, respectively, whereas the initial ratio in raw straw was 2.02. These comparisons reveal that chlorine in straw has higher volatility than alkali metals under the combustion conditions. It has been proposed22 that KCl in biomass reacts with functional groups in the char (e.g., carboxylic or phenolic groups) in a charge-transfer reaction, in which HCl is volatilized and K+ is ion-exchanged with functional groups. Over half of the mass of the ash was silicon and oxygen, indicating that SiO2 (a typical component of quartz) was predominant in ash. Indeed, this compound was likely the angular particles shown in Figure 7c. A relatively higher concentration of oxygen than silicon (Si/O = 1:5) was observed, which confirmed that other oxides, such as K2O, Na2O, and CaO, were present in the ash. To determine which of these compounds were associated with potassium, XRD analysis (diffraction angle 2θ, 5−75°) was performed separately on the combustion residues. As shown in Figure 8, the XRD spectra of samples formed under all three temperatures have the same patterns, and the main crystalline phases are quartz (2θ = 20.8°, 21.8°, and 35.96°), KCl (2θ = 28.34°, 40.52°, and 50.22°), K2SO4 (2θ = 29.76° and 30.9°), Na2SO4 (2θ = 58.68°), and NaAlSi3O8 (2θ = 27.88°). As the temperature increased, all peak intensities associated with KCl and K2SO4 decreased, especially for the peaks of KCl. It should be noted that KCl plays a dominant role in potassium evolution, while the contribution of K2SO4 is limited.
Figure 5. Emission behavior of potassium.
some potassium might evolve together with volatile matter during the initial stage of combustion, i.e., as the form of organic potassium. It has also been reported that potassium may be present in an aqueous salt form (e.g., chloride, hydroxide, carbonate, and sulfate)20 or distributed via ion exchange with hydroxyl groups in organic biomass components, such as proteins and other cellwall material.16,20,21 When the sample is inserted into the hot reactor zone, these salt solutions within the sample are dried rapidly, leaving salts distributed throughout the straw particle. During this stage, the temperature of the particle is still too low for potassium salts to have a sufficient vapor pressure to evaporate. Therefore, it is assumed that, in the initial stage, the potassium is released through decomposition of the organic structure. The inorganic potassium then evaporates and is released with a rising temperature because of the increased vapor pressure. At high combustion temperatures (>500 °C), the K retention ratio further decreases continuously when the release of volatiles is almost complete, implying the release of inorganic potassium. The relationship between K retention ratios and the combustion temperature is plotted in Figure 6. The results
Figure 6. Emission of potassium versus temperature (720 s).
showed that the emission of K occurs over two regimes. In the low-temperature region (
