Article pubs.acs.org/EF
Ash Composition in Cassava Stems Originating from Different Locations, Varieties, and Harvest Times Maogui Wei,†,‡,§ Wanbin Zhu,*,†,‡,§ Guanghui Xie,†,∥ Torbjörn A. Lestander,‡ Jishi Wang,† and Shaojun Xiong‡ †
College of Agronomy and Biotechnology, China Agricultural University, Beijing 100193, People’s Republic of China Department of Forest Biomaterials and Technology, Swedish University of Agricultural Sciences, SE 901 83 Umeå, Sweden § Center of Biomass Engineering, China Agricultural University, Beijing 100094, People’s Republic of China ∥ National Energy R&D Center for Non-food Biomass, China Agricultural University, Beijing 100193, People’s Republic of China ‡
ABSTRACT: The influence of growth location, variety, and harvest time on ash composition and calorific value of cassava stems was evaluated using 180 samples from a full factorial-designed experiment (three locations × three varieties × five harvest times) in Guangxi, China. The calorific value of cassava stems showed only small variations (1500 °C), which was consistent with our published results18,19 and suggested limited sintering during combustion, if any. K and Cl contents varied the most and did so significantly between locations (growth environments), which was attributed to differences in soil composition. On the basis of theoretical and empirical indices of molar ratios, cassava stems across all treatments generally exhibited a risk of particle emissions when combusted but stems from one location (Heng) could have much better combustion behavior than those from others in terms of slagging and corrosive Cl-rich deposit tendencies indicated by the indices K/(Ca + Mg) and combinations of S/Cl and Cl/(K + Na), respectively. Stems from Wuming showed a higher risk for the induction of particle emission, according to the index (K + Na + Ca + Mg)/(P + Si), while biomass from Longan and the variety Xinxuan048 tended to show risk of forming corrosive Cl-rich deposits.
1. INTRODUCTION
and prediction of biomass properties but also for the development of tailored biofuel feedstocks. Cassava (Manihot esculenta Crantz.) stems, the residues after cropping starchy roots, have great potential to be a feedstock for biofuel production and such for the development of a sustainable society.18,19 The cassava stems produced in the world amount to about 32 Tg of dry mass based on a stem/root mass ratio of 50% and the production of cassava roots in 2011.19,20 However, about 80% of these cassava stems are abandoned or burned in the wild. The remaining stems are used for propagation and substrates for growing mushrooms or are recycled to maintain soil fertility.21 The use of cassava stems for industrial biofuel production have been overlooked until recently.13,18,19,22−24 Very few studies, if any, have focused on the ash composition of cassava stems because it is a “new” solid biofuel. In previous studies,18,19 we suggested that cassava stems could be a promising feedstock for the production of electricity, heat, and pellets based on the abundance of resource availability and a few fuel characteristics, such as heating value (17.2−18.7 MJ kg−1), ash content (4.3%), and ash fusion temperature (1340−1550 °C). In a collaborated report,13 however, combustion of cassava stem pellets of the same origin as Tao et al.18 in a 20 kW burner generated considerable deposition and particle emissions and the fuel from Wuming
As a renewable energy source, existing crop residues are expected to contribute significantly to fuel security for social development without expanding land use and otherwise endangering food security, biodiversity, habitats, or soil conservation.1−4 Biomass residues can also be potential feedstock for different bio-based products.5 Therefore, it is important to study the possibilities of using non-food biomass for bioenergy and biorefinery.6,7 Knowledge of biomass feedstock properties, such as physical characteristics and chemical compositions, is a prerequisite for designing effective industrial processes because biomass properties directly impact performance in different energy conversion processes, e.g., the effect of ash and its composition on combustion, gasification, pyrolysis, fermentation, and anaerobic digestion.8−11 Biomass ash composition can have a more profound and crucial role than energy properties in affecting energy conversion processes, such as combustion.3 This is especially true for crop residues that are often showing complex behaviors in combustion.12,13 Ash-forming elements are embedded in biomass, such as potassium (K), calcium (Ca), magnesium (Mg), silicon (Si), sodium (Na), phosphorus (P), sulfur (S), and chlorine (Cl), among others.12 They may vary with genotype,13,14 environment,15,16 harvest time,11,15 and management.17 Few studies have explored variation in these biomass properties regarding the influence of biotic and abiotic factors. This information is of importance not only in the management © XXXX American Chemical Society
Received: February 25, 2014 Revised: July 28, 2014
A
dx.doi.org/10.1021/ef5009693 | Energy Fuels XXXX, XXX, XXX−XXX
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Table 1. Physical and Chemical Propertiesa of Topsoil (0−20 cm)b Heng pH organic carbon (g kg−1) nitrogen (N) (%) sulfur (S-al) (mg kg−1) chlorine (Cl) (mg kg−1) phosphorus (P) (%) potassium (K) (%) calcium (Ca) (%) magnesium (Mg) (%) silicon (Si-al) (mg kg−1) a
Longan
Wuming
mean
range
mean
range
mean
range
5.6 8.6 0.07 11.5 41.2 0.08 0.14 0.07 0.12 128.5
5.4−6.0 7.1−9.8 0.02−0.11 9.4−17.9 18.8−67.4 0.06−0. 11 0.11−0.16 0.05−0.09 0.09−0.15 108.0−147.0
3.9 18.9 0.23 21.9 21.2 0.06 0.17 0.03 0.20 96.9
3.6−4.1 18.0−20.6 0.21−0.25 15.8−31.4 14.2−39.0 0.05−0.07 0.12−0.21 0.02−0.05 0.17−0.22 86.1−125.0
5.5 26.9 0.28 29.9 30.2 0.14 0.49 0.15 0.24 234.8
5.0−5.8 23.9−29.8 0.24−0.35 24.5−44.7 14.2−42.6 0.12−0.17 0.39−0.71 0.08−0.23 0.14−0.35 200.0−258.0
S-al and Si-al = available S and Si, respectively. bSamples were taken from study sites by the end of the season. 2.2. Sampling of Stems and Soil. A previous study19 has shown that the chemical composition in the 40−60 cm section of the aboveground stem is approximately equal to the average of the whole stem. Hence, the 40−60 cm aboveground sections of each plant were sampled. Cassava stem samples were harvested at the designed sampling times and sent immediately after the harvest to Guangxi University, where they were cleaned, chopped, and pre-dried at 75 °C to constant weight, to facilitate storage and transportation because of a high moisture content (about 75%, wet weight based) at harvest. All samples were then shipped to a laboratory at China Agricultural University (CAU) for further analysis. To examine the soil environment, 10 subsamples of topsoil in each plot were randomly taken at a near range (10−50 cm) of the plant, on 290 DAP, and then mixed to form one sample. In total, 36 samples (12 per location) were taken. All soil samples were dried at room temperature and sieved through a 100 mesh screen for further analysis. 2.3. Fuel and Soil Analyses. Fuel analysis included calorific value, ash content, and contents of the major ash-forming elements S, Cl, P, K, Ca, and Mg of plant samples. Samples were milled to less than 0.5 mm in size prior to analysis. Before fuel analyses, the biomass (milled) moisture content was redetermined at 105 °C. The gross calorific value (GCV) was measured using an autocalculating bomb calorimeter (ZDHW-YT8000, Yingtai Electronic Appliance Corporation, China). The ash content was analyzed at 550 °C according to ASTM standard D1102-84. Microwave-assisted HNO3 was used to extract elements P, K, Ca, and Mg25 that were then determined by Mo−Sb colorimetry,26 flame photometry,27 and atomic absorption spectrometry,28 respectively. Total Cl was determined using the AgNO3 titration method. Sulfur was extracted by HNO3−HClO4 digestion and measured using inductively coupled plasma (ICP) spectrometry (Varian 715-ES, Varian, Palo Alto, CA). Sulfur and Cl analyses were performed at the Institute of Plant Nutrition and Resources, Beijing, China. Other analyses were conducted in a laboratory at CAU, where the soil samples were also analyzed. Soil pH was measured on a 1:2.5 (v/v) soil/double-deionized water mixture. Organic carbon was analyzed by the Walkley−Black method. Total N in soil was determined by the Kjeldahl method, while total P, K, Ca, and Mg were determined using the same methods as for biomass. The determinations of water-soluble chloride ion, S available, and Si available in the soil were made using the ion-chromatography method, ICP spectrometer, and colorimetry technique by the silico− molybdenum blue complex, respectively. The results of the soil analysis can be found in Table 1. 2.4. Prediction of Ash Transformation. One of the main objectives of this study focused on biomass ash transformation and related problems, such as slagging, deposition, particle emission, and corrosion. To evaluate fuel quality and predict/compare combustion behavior of cassava stems, four molar ratios based on relative amounts of the major ash-forming elements were adopted: K/(Ca + Mg), Cl/ (K + Na), S/Cl, and (K + Na + Ca + Mg)/(Si + P). The ratio K/(Ca + Mg) has been shown to be positively correlated to slag formation during combustion of corn stover and wheat16,29 but has also been
showed even corrosion tendencies. With few observations thus far, it is difficult to conclude whether the results from the above-mentioned studies are common phenomena for cassava stems from different origins. Still, there are many questions to answer: (1) How large is the variability in cassava stem ash composition? (2) What is the influence of management practice and/or origin-based parameters on ash compositions, e.g., the influence of location, variety, and harvest time? (3) What consequences in ash transformation may result from the variability? Answering questions like these is important in the continuing learning process aimed at sustainable use of cassava stems. The main objectives of this study were to examine and document variation in cassava stem ash composition and to evaluate different impacts of parameter location, variety, and harvest time on ash composition and ash behavior in combustion, e.g., slagging, deposition, and particle emission, as well as corrosion. The chosen parameters are of major interest because these parameters are known to influence embedded ash elements in biomass but also because of their ease of control in fuel production management.
2. MATERIALS AND METHODS 2.1. Plant Materials and Their Origin. A total of 180 samples were collected in 2011 from a full factorial experiment composed of three locations, three varieties, and five harvest occasions. There were 45 treatments with four replicates each, resulting in a total of 180 plots. The three locations were Heng (22° 48′ 25″ N, 109° 05′ 47″ E), Longan (23° 03′ 40″ N, 107° 52′ 27″ E), and Wuming (23° 08′ 56″ N, 108° 09′ 17″ E), situated in Guangxi, China. Varieties South China 205 (SC205), South China 5 (SC5), and Xinxuan 048 (XX048) were examined. The study areas are within the three largest cassava production counties in Guangxi, and the varieties are the most common ones in the region. The five harvest times were 230, 245, 260, 275, and 290 days after planting (DAP), covering from the shortest to the longest possible harvest periods in practice in the region. It is understandable that plant chemical composition may be changing during the growing and aging and that harvest time can influence biofuel quality.11,15 Plots were arranged in the field by a randomized complete block design (eight plants per plot plus guarding rows) and were managed identically, for example, plant density at 12 500 plant ha−1 fertilization of N/P/K at 60:6.5:41.5 kg ha−1 on soil. The fertilizers were divided in three doses and applied manually on 0 (basal fertilizer in soil), 70 (after manuring with soil covered), and 150 (top application on soil) DAP, after consulting with local protocol and ref 21. Care and efforts were taken to avoid any possible fertilizer loss and/or other practice that may cause management difference between the sites, when designing, setting, and conducting the experiment. Weeds were controlled manually, and plots were rain-fed only. B
dx.doi.org/10.1021/ef5009693 | Energy Fuels XXXX, XXX, XXX−XXX
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Figure 1. Gross calorific value (GCV), ash-free calorific value (GCVaf), and ash content (%) of cassava stems from 45 treatments. Each bar refers to the mean ± standard error based on dry mass and calculated from four replicates. introduced as an index of particle emission.13 According to Stromberg,30 the ratio Cl/(K + Na) may indicate the proportion of alkali that could easily be vaporized as volatile chloride. When this ratio was >0.3, there was a risk of the formation of corrosive chlorinerich deposits.30 It is also suggested that the ratio of S/Cl can be an indication of whether there is sufficient surplus S to reduce the risk of corrosion in conjunction with alkali chlorides. When S/Cl is 2−4, the risk of Cl-induced corrosion is reduced.30 The ratio (K + Na + Ca + Mg)/(Si + P) was also introduced in a previous study13 to indicate the probability that cassava stems produce fine particulate emissions when combusted. At (K + Na + Ca + Mg)/(Si + P) ratios approximately >3, a surplus of basic oxides (i.e., silicates and phosphates are “saturated” with respect to basic components) will be present. Thus, some alkali metals may be volatilized to a high degree and play a crucial role in the formation of fine particle emission and deposits. Previous studies indicated that Si and Na contents in cassava stems were very low (2 times higher in Wuming samples than in Longan (Figure 1c). Almost all studied ash-forming elements (Figure 2) varied considerably across treatments, and the largest variations were observed for Cl (0.7−3.8 g kg−1) and K (2.6−27.8 g kg−1), which again were largely attributed to differences between locations rather than between varieties and between harvest times. The six examined ash-forming elements (S, Cl, P, K, Ca, and Mg), which are involved in ash transformation during combustion,31−34 made up the majority of the biomass ash: 66.2% for Longan, 65.7% for Wuming, and 62.6% for Heng. Potassium and Ca contents in cassava stems were considerably greater than other elements and constituted 37−51% of the ash, depending upon treatments. It should be noted that the K content was highest in Wuming but lowest in Longan stems (Figure 2c), which is consistent with the pattern of ash (Figure 1c). Remarkably, nearly 40% of cassava stem ash was K in the samples from Wuming on average (Table 2). Table 2 summarizes the overall mean values of the fuel variables for each parameter, location, variety, or harvest time, C
dx.doi.org/10.1021/ef5009693 | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Figure 2. Ash-forming elements S, Cl, K, Ca, Mg, and P contents in cassava stems of 45 treatments. Each bar refers to the mean ± standard error based on dry mass and calculated from four replicates.
and the two-way interaction location × variety and shows corresponding data for the cassava pellet fuels used in a previous combustion experiment.13 It is worth noting that the fuel characteristics of previously used cassava stem pellets (diameter of 8 mm and bulk density of about 680 kg m−3 13) were generally within the range of variations that resulted from the current experiment. 3.2. Effects of the Location, Variety, and Harvest Time. Statistical analysis indicates that all nine response variables were significantly affected by the location and variety (phosphorus at p < 0.05 and all others at p < 0.001) (Table 3); six of nine responses (not Cl, Ca, and Mg) were also significantly affected by the harvest time (p < 0.05). Location produced the largest contribution to total mean square of the variance for most response variables, except for S and Ca, whose variations were mostly affected by varieties (Table 3). Harvest time had almost no effect on ash composition; its contribution to total mean square of variance was less than 10% for all fuel variables (Table 3 and Figure 2). Location × variety (L × V) was shown to be the most important interaction, followed by location × harvest time (L × T); the former significantly affected all nine variables (p < 0.001; Table 3), and the latter was significant for six of them. The effect of V × T was significant on GCVaf only, and the
three-way interaction (L × V × T) did not have any significant effects at all. Eight of nine response variables were largely affected by the main effects of the individual factors; phosphorus was the only response variable that was mostly influenced by interactions, as indicated by the relative contribution to the total sum of mean squares (Table 3). The fact that location had the most important influence on cassava stem ash composition can largely be attributed to soil conditions, where the studied cassava was growing. The three locations were in very close geographic range (
