Article pubs.acs.org/EF
Physicochemical Composition and Energy Property Changes of Wheat Straw Cultivars with Advancing Growth Days at Maturity Wenjuan Niu, Xian Liu, Guangqun Huang, Longjian Chen, and Lujia Han* Biomass and Bioresource Utilization Laboratory, College of Engineering, China Agricultural University, Beijing 100083, People’s Republic of China S Supporting Information *
ABSTRACT: Physicochemical properties of wheat straw play crucial roles in bioenergy and chemical conversion processes. Four wheat straw cultivars at different maturity levels were collected in Beijing, China, and the growth days (GDs) ranged from 236 to 263 days. The physicochemical compositions and energy properties were analyzed. Cellulose (Cel), hemicelluloses (Hem), monosaccharidic composition of hemicelluloses (xylan, arabinan, hemicellulosic glucan, galactan, and mannan), and dry matter (DM) increased from 236 to 263 days, which can be explained by the increasing lignification of the cell walls in wheat straw with maturity. Water-soluble carbohydrates (WSC), crude protein (CP), nitrogen (N), phosphorus (P), and copper (Cu) decreased initially and then leveled off, with leaves decreasing. Lignin (Lig), ash, sulfur (S), potassium (K), and sodium (Na) increased first and then decreased slightly to 263 days. Carbon (C), hydrogen (H), oxygen (O), volatile matter (VM), fixed carbon (FC), and higher heating value (HHV) had litter variation with advancing maturity. A number of significant correlations were found among different physicochemical compositions. Regression equations for Cel and DM based on GDs showed excellent performance for prediction, while models for WSC and CP showed good prediction.
1. INTRODUCTION Wheat straw is an important potential bioenergy and feed resource. In 2011, a total of 842 million metric tons of crop straw and 127 million metric tons of wheat straw were obtained, as reported by National Bureau of Statistics of China. Wheat straw is produced in great numbers every year, and most of this abundant agricultural byproduct is used for bioethanol production,1 animal feed,2 burning,3 papermaking and pulping.4 The use value of wheat straw varies depending upon the growth process and maturity. Variable chemical composition could have a great effect on both the feed nutrition value and energy conversion efficiency.5,6 Ultimate composition was usually applied to estimate the energy value.7 The proximate composition and calorific value were often used as indicators in thermal chemical engineering.8 The mineral element was often related to combustion behavior; especially, alkali metals potassium (K) and sodium (Na) in the gas phase may lead to serious corrosion of the gas turbine.7 Learning more about the knowledge of physicochemical compositions and energy properties allows for better use of wheat straw at maturity. Studies on the changes of physicochemical compositions and energy properties have been reported in corn stover,9 wheat straw,10 cotton stalks,11 etc.,12 but the span of most studies was just on several stages, with no specific continuous growth days (GDs) being reported. The most frequently studied physicochemical compositions were cellulose (Cel), hemicelluloses (Hem), lignin (Lig), water-soluble carbohydrates (WSC), crude protein (CP), higher heating value (HHV), and dry matter (DM).9,12,13 Carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), volatile matter (VM), fixed carbon (FC), ash, phosphorus (P), potassium (K), sodium (Na), and copper (Cu) were studied less extensively with advancing GDs at maturity. © 2013 American Chemical Society
Traditional laboratory determination methods of physicochemical compositions are labor-intensive, time-consuming, and high-cost. Current prediction methods generally use easily available indicators to estimate physicochemical composition contents and energy values according to their high correlations, which have been used in biomass, food, and other fields.12,14,15 Regression equations have been reported to calculate the DM of corn stover from plant dimensions.16 WSC were estimated from DM for ranking genotypic differences.17 DM and CP of corn forage have been predicted from daily growing degree units (GDUs).11 The HHV of biomass has been correlated to ultimate composition and ash accurately.18 The aim of this study is to investigate the changes of physicochemical compositions and energy properties of wheat straw at different GDs and use the correlations for fast estimation.
2. EXPERIMENTAL SECTION 2.1. Wheat Straw Collection and Preparation. A total of 40 wheat straw samples at maturity, including four different cultivars, were collected from the Chinese Academy of Agricultural Sciences in Beijing, China. “Zhongzuo 9504”, “Xinmai 18”, “Aikang 58”, and “Xiaoyan 6” of wheat plants were named “cultivar 1”, “cultivar 2”, “cultivar 3”, and “cultivar 4” in this study, respectively. Their grown conditions were the same, including the date of sowing (October 1, 2011), fertilizer input (126 kg/ha N, 90 kg/ha P2O5, and 90 kg/ha K2O were uniformly applied before sowing; immediately, 84 kg/ha N was broadcast prior to booting in spring), agrochemical input, etc. There were no field activities from the first day of collection, and the collected whole crop wheat had the ears of grain removed. Samples Received: July 8, 2013 Revised: August 24, 2013 Published: September 10, 2013 5940
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Figure 1. Physicochemical compositions and energy content of the four wheat straw cultivars at maturity. Error bars correspond to the standard deviation. of wheat straw sample in an IKA C200 oxygen bomb calorimeter (IKA Analysetechnik, Heitersheim, Germany) according to ASTM E71187.23 About 0.50 g of crop straw sample was digested with 7 mL of concentrated nitric acid in a closed reactor in a microwave oven (Milestone Italy, equipped with a HPR 1000/6S rotor), and an AAS Vario 6 atomic absorption spectrometer (Jenoptik Company) was used for K, Na, P, and Cu analyses. All of the chemical analyses were performed in duplicate aliquots, and the recorded result was the average. 2.3. Data Analysis. Origin 8.0 and SPSS 17.0 were used for chemical composition, ultimate composition, proximate composition, calorific value, and mineral element statistics of wheat straw. One-way analysis of variation (ANOVA), Pearson coefficient of correlation, and curve estimation were applied to analyze the relationship among physicochemical compositions and energy properties. In regression analysis, the following statistical parameters were calculated to evaluate the performance of regression equations: Pearson coefficient of correlation (r), regression coefficient of correlation (R2), ratio of standard error of performance to standard deviation (RPD), and rootmean-square error of prediction (RMSEP). R2 > 0.81 made good prediction, and 0.66 < R2 < 0.81 was assessed approximate quantitative prediction. RPD > 3.0 indicated excellent prediction; 2.5 < RPD < 3.0 can be practically used for prediction; 2.0 < RPD < 2.5 indicated approximate quantitative prediction; and RPD < 2.0 was considered insufficient for application24
were collected at 3 days from May 23, 2012, and the corresponding GDs were 236, 239, 242, 245, 248, 251, 254, 257, 260, and 263, substantially across the milky stage, dough stage, and full ripe stage. Wheat straw samples were dried at 45 °C in an oven for 48 h, fed to a ZM100 mill (Rersch GmbH and Company, Germany), and milled to pass through a 20-mesh sieve for physicochemical composition and energy property analyses. 2.2. Physicochemical Composition and Energy Property Analyses. GDs were counted from the sowing day to the collected day in this study. DM of freshly collected samples was analyzed at 45 °C in a drying oven according to ASTM E1757-01. Other physicochemical composition and energy property analyses were made on 45 °C oven-dried wheat straw samples. Cel, Hem, and Lig was measured according to the National Renewable Energy Laboratory (NREL) method:19 A total of 300 mg of wheat straw sample was hydrolyzed with 3 mL of 72% H2SO4 at 30 °C for 2 h in a highpressure tube, followed by adding 84 mL of deionized water to 4% H2SO4, then placed in an 121 °C autoclave for 1 h, and cooled to room temperature. A total of 20 mL of supernatant was neutralized to pH 5−6 by adding 0.82−0.85 g of CaCO3 and filtered. The monosaccharidic composition of hemicelluloses in the neutralized liquid by filtrate was determined by high-performance liquid chromatography (HPLC) using a Hitachi refractive index (RI) detector L-2490 and Biorad Aminex HPX-87P to obtain Cel and Hem, while Lig was determined by the remaining residue burned in a 575 ± 25 °C muffle furnace for 3 h. WSC were extracted by weighing 300 mg of wheat straw sample in 98 ± 2 °C and 10 mL of deionized water for 1 h and determined by an anthrone sulfuric acid method using an UV-2550 spectrophotometer. CP was analyzed according to the AOAC official method:20 Weigh 1 g of wheat straw sample with 2 mL of catalyst Cu3.5 (7 g of K2S04 and 0.8 g of CuS04·5H2O) and 12 mL of H2SO4 in a tube, place it on a digestion oven from room temperature to 200 °C, hold at 200 °C for 30 min, then ramp to 420 °C, and hold at 420 °C for 2 h. CP was measured by a Kjeltec 2300 (FOSS Tecator AB, Sweden). C, H, O, N, and S were determined by putting 40 mg of wheat straw sample in an Elementar Vario EL II (Vario Macro, Germany). Ash was determined at 575 ± 25 °C for 3 h of burning in a muffle furnace.21 VM was analyzed at 900 °C for 7 min of burning in a muffle furnace, and FC was determined by 100% minus both ash and VM, following the method by ASTM.22 The HHV was measured by burning 500 mg
n
RMSEP = RPD =
∑i = 1 (yi − yi p )2
SD RMSEP
n
(1) (2)
where yi is the actual value for the ith wheat straw sample, yip is the predicted value for the ith wheat straw sample, SD is the standard deviation of all of the wheat straw samples, and n is the number of wheat straw samples.
3. RESULTS AND DISCUSSION 3.1. Physicochemical Compositions and Energy Properties. Figure 1 and Table A1 of the Supporting 5941
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Figure 2. Chemical composition of wheat straw cultivars with advancing GDs at maturity. Error bars correspond to the standard deviation.
Figure 3. Monosaccharidic composition of Hem of wheat straw cultivars with advancing GDs at maturity. Error bars correspond to the standard deviation.
“cultivar 4” samples were not significant (see Table A1 of the Supporting Information), indicating that cultivars had little effect.27 Figure 3 shows the monosaccharidic composition of Hem of wheat straw cultivars with advancing GDs at maturity (details can be found in Table A3 of the Supporting Information). Xylan is the major component of Hem, while arabinan, hemicellulosic glucan, galactan, and mannan are also the Hem components. An increase of the total Hem content is mainly due to an increase of xylan, arabinan, hemicellulosic glucan, galactan, and mannan, which can be explained by the increasing degree of lignification of the cell walls with maturity.12 Cel, Hem, and Lig are structural compounds, while WSC and CP are non-structural compounds of wheat straw. The leaf part decreases while the stem part increases with maturity. Leaves have less lignified cell walls, and stems are rich in secondary cell walls, which cause that structural compounds to increase and
Information list the descriptive statistics for chemical composition, ultimate composition, proximate composition, calorific value, and mineral element of the four wheat straw cultivars. Figures 2−7 and Tables A2−A7 of the Supporting Information show the chemical composition, ultimate composition, proximate composition, calorific value, and mineral elements of four wheat straw cultivars at different maturities. 3.1.1. Chemical Composition. Cel and Hem marginally increased with GDs (p < 0.01), while Lig stayed relatively unchanged with advancing GDs in Figure 2 (details in Table A2 of the Supporting Information). WSC decreased continuously as GDs increased from 236 to 251 days (p < 0.01) and remained relatively constant afterward. CP decreased initially from 236 to 251 days (p < 0.01) and stayed stable thereafter (Figure 2), which is consistent with the previous reports of wheat and corn forage.11,25,26 The differences of Cel, Hem, Lig, WSC, and CP among “cultivar 1”, “cultivar 2”, “cultivar 3”, and 5942
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Figure 4. Ultimate composition of wheat straw cultivars with advancing GDs at maturity. Error bars correspond to the standard deviation.
Figure 5. Proximate composition of wheat straw cultivars with advancing GDs at maturity. Error bars correspond to the standard deviation.
wheat straw produced by photosynthesis is little at late maturity. C, H, and O are used as major variables in predicting and calculating the HHV.30 N and S are obtained via different processes from soil. The reduction of N indicates the transfer of N from the stem to grain.31 N forms relatively stable and complex compounds in wheat straw. N and S can form higher energy chemical bonds, such as C−N, C−S, H−N, and H−S, than those of CO, C−O, and O−H,32 and the two elements may result in environmental consequences, such as acid rain and smog.33 3.1.3. Proximate Composition. VM, FC, and ash had small variation (Figure 5 and Table A5 of the Supporting Information). The differences of ash of “cultivar 1”, “cultivar 2”, “cultivar 3”, and “cultivar 4” were significant (see Table A1 of the Supporting Information). VM is the sum of liquid and gas decomposed by organic matter of wheat straw at 900 °C without air, while there is little accumulation of organic matter at maturity. FC is a non-volatile substance and present in the form of elemental carbon in wheat straw. Ash is one of the most studied physicochemical compositions of wheat straw and originates simultaneously from natural and inorganic, organic, and fluid matter during combustion. Ash is the complement of the organic matter content. Therefore, the ash content is
non-structural compounds to decrease with advancing maturity.12 Cel, Hem, and Lig are important indices for thermal decomposition and hydrolysis applications. Lig is a substance that is difficult for ruminants to use and has an adverse influence on the quality of rough forage.19 WSC and CP are regarded as important nutrient indices in forage use.11,1213 WSC and CP are very important indicators of the nutrition value of feed. Feed with a low nutritional value is not good for animals, which indicates that wheat straw with low WSC and CP contents at extended GDs beyond 251 GDs is not suitable as feed. 3.1.2. Ultimate Composition. The levels of C, H, and O in wheat straw remained pretty unchanged with advancing maturity days. The N level in wheat straw generally decreased as GDs increased from 236 to 257 days (p < 0.01). The S initially increased, reaching the highest level at 245 days, but decreased afterward to a level slightly lower than the initial level (Figure 4 and Table A4 of the Supporting Information). These changes of wheat straw were consistent with crop barley and corn stover.28,29 C, H, O, N, and S can form complex organic compounds, which precipitate the formation of volatiles in thermal decomposition. The contents of C, H, and O are direct results of photosynthesis of wheat straw, and organic matter of 5943
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Figure 6. Calorific value of wheat straw cultivars with advancing GDs at maturity. Error bars correspond to the standard deviation.
Figure 7. Mineral element of wheat straw cultivars with advancing GDs at maturity. Error bars correspond to the standard deviation.
3.1.5. Mineral Element. K and Na initially increased, reaching their highest level at 251 days, but decreased slightly afterward to a low level. P and Cu decreased initially from 236 to 251 days (p < 0.01) and stayed stable thereafter in Figure 7 (details in Table A7 of the Supporting Information). The mineral element is primarily present in the soil and absorbed into the plants with water by the roots. K and Na are present in ionic form and can be transferred. The ash-melting behavior, slag, and hard deposit were greatly influenced by the alkaline metal K and Na contents. Increased K and Na contents can not only lead to a decreased ash-melting point but also increase the amount of aerosols formed during combustion and, thus, fouling.36 P is the presence of unstable compounds and can be transferred. P is an essential mineral element for crop growth, and knowledge of the dynamic changes of P in wheat straw could help make reasonable fertilization on crops. Cu is a trace element, and too much intake in a biological body will lead to
strongly negatively correlated to the HHV, and its content can also affect the conversion process of wheat straw.8 3.1.4. Calorific Value. Figure 6 showed that the HHV had small variation. The HHV is the heat released by complete combustion of organic matter in wheat straw and is used to characterize the quality of the fuel. DM of wheat straw remained relatively low from 236 to 248 days. There is a sharp increase in DM from 248 to 254 days (p < 0.01), after which DM changed little (Figure 6 and Table A6 of the Supporting Information). The data suggest that, until 254 GDs of wheat straw, DM is still accumulating. There are two sources of DM: namely, plant photosynthesis and the uptake of mineral elements. Nutrients produced by photosynthesis are mostly for the growth of wheat grain, and only a small part of DM is supplied for wheat straw.34 The dry weight accumulation of wheat grain follows a slow−fast−slow pattern, and the water loss of wheat grain was fast at maturity.35 5944
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Table 1. Correlation Analysis Results among Physicochemical Compositions and Energy Properties (n = 40) GDs Cel Hem Lig WSC CP C H O N S VM FC ash HHV DM K Na P Cu S VM FC ash HHV DM K Na P Cu a
GDs
Cel
Hem
1 0.90a 0.65a 0.00 −0.85a −0.82a −0.14 −0.76a 0.13 −0.82a −0.38b −0.01 −0.08 0.12 0.16 0.95a 0.04 0.49a −0.80a −0.80a S
1 0.61a 0.13 −0.94a −0.83a −0.08 −0.58a 0.14 −0.84a −0.29 0.04 −0.09 0.07 0.20 0.83a 0.15 0.56a −0.80a −0.71a VM
1 0.19 −0.75a −0.75a −0.16 −0.36b 0.20 −0.76a −0.01 0.14 −0.25 0.16 0.38b 0.55a −0.04 0.59a −0.72a −0.62a FC
1 0.19 −0.08 −0.15 −0.21 −0.48a −0.15 −0.23 0.24 0.25
1 −0.74a −0.30 0.35b −0.03 −0.52a −0.06 0.12 0.04
Lig
1 −0.42a −0.04 0.02 0.25 −0.03 0.11 0.12
WSC
1 −0.24 −0.28 −0.14 0.12 0.09 −0.33b 0.07 −0.21 0.05 0.23 0.14 −0.06 0.34b 0.02 −0.29 −0.16 ash
CP
1 0.90a 0.21 0.50a −0.15 0.90a 0.23 0.06 0.15 −0.30 −0.19 −0.75a −0.25 −0.63a 0.90a 0.70a HHV
1 −0.59a 0.00 0.34b 0.12 0.32b −0.22
1 0.22 −0.01 0.32 −0.11 −0.15
1 0.34b 0.52a −0.17 0.99a 0.26 0.13 0.11 −0.33b −0.15 −0.68a −0.28 −0.55a 0.98a 0.81a DM
C
H
1 0.19 0.02 0.34b 0.33b 0.18 0.31 −0.69a 0.33b −0.12 −0.31 −0.19 0.33b 0.22 K
1 0.04 0.50a −0.66a −0.71a
1 0.04 0.51a 0.67a 0.06 −0.02 −0.05 −0.13 −0.83a 0.07 −0.32b 0.49a 0.57a Na
1 0.09 −0.29 −0.07
O
N
1 −0.20 0.11 0.07 −0.04 −0.05 0.27 0.13 0.08 0.08 −0.14 −0.05 P
1 −0.55a −0.33
1 0.23 0.11 0.13 −0.32b −0.15 −0.67a −0.28 −0.51a 0.97a 0.81a Cu
1 0.79a
1
Correlation is significant at the 0.01 level (two tailed). bCorrelation is significant at the 0.05 level (two tailed).
Table 2. Regressions of Physicochemical Compositions with GDs of Wheat Straw at Maturity (n = 40) parameter chemical composition
ultimate composition calorific value mineral element
regression Cel (%) WSC (%) CP (%) N (%) DM (%) P (g/kg)
Cel = −708.812 + 5.475GDs − 0.010GDs WSC = 1837.001 − 14.333GDs + 0.028GDs2 CP = 779.341 − 6.101GDs + 0.012GDs2 N = 150.155 − 1.098GDs + 0.002GDs2 DM = 2633.292 − 23.551GDs + 0.053GDs2 P = 127.916 − 0.997GDs + 1.95 × 10−3GDs2 2
poisoning. Cu is also a kind of heavy metal and can be treated as a catalyst in the combustion system. Ash quality and particulate emissions can also be impacted by heavy metals. Therefore, a heavy metals, such as the Cu content in crop straw fuels, had better be kept in suitable value ranges, especially concerning their use in small combustion equipment systems.36 3.2. Estimation of Physicochemical Compositions and Energy Properties. 3.2.1. Correlations of Physicochemical Compositions and Energy Properties. Correlation analysis results of physicochemical compositions and energy properties of wheat straw samples were shown in Table 1. The study on the correlations among chemical composition, ultimate composition, proximate composition, calorific value, and mineral element is important for understanding the life form of wheat straw at maturity. GDs can be obtained directly with the sowing time. Cel, WSC, CP, N, DM, P, and Cu of wheat
R2
RPD
RMSEP
F(P)
0.85 0.88 0.87 0.87 0.91 0.87
3.08 2.51 2.94 0.72 3.35 1.69
1.70 1.85 0.81 0.61 8.05 0.16
