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Sustainable bio-energy potential of perennial energy grass from reclaimed coalmine spoil (marginal sites) of India
Renewable Energy 123 (2018) 475e485
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Sustainable bio-energy potential of perennial energy grass from reclaimed coalmine spoil (marginal sites) of India Sanjoy Kumar*, Prosenjit Ghosh Centre for Earth Science, Indian Institute of Science, Bangalore, Karanataka 560012, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2017 Received in revised form 12 December 2017 Accepted 9 February 2018 Available online 20 February 2018
Usage of marginal lands to grow perennial grass for biomass feedstocks is a promising option to meet the bioenergy demand in India. In this context, the present work investigated the potential utility of two perennial grass species Cenchrus ciliaris (L.) and Pennisetum pedicellatum (Tan.) to be a new promising energy source for bioenergy. This study entails a detailed characterization of biomass feedstocks using proximate and ultimate analysis, and lignocellulosic fractions and thermogravimetric behaviour using TG-FTIR and Py-GC/MS spectrophotometry to evaluate their potential as an alternate green fuel to fossil fuels by measuring their thermochemical conversion functioning. Property analysis of perennial grass species showed a significant difference in moisture content (7.2e8.5%), volatile matters (80.5e82.4%), fixed carbon (11.3e18.9%), HHV (15e17.8 MJ/kg) and LHV (14.3e16.5 MJ/kg), which is very promising for bioenergy generation. Lignocellulosic fractions of biomass feedstocks are comparable to the previous studied biomass species including switchgrass and elephant grass. The individual decomposition experiments indicated that biomass feedstocks possess higher thermochemical reactivity and shorter devolatilization time. According to Py-GC/MS study, carbonyl compounds including aldehyde and ketones are the major volatile products, in addition to furans, benzenes, phenols, acids, and others. The TGFTIR results showed that main gaseous products evolved during devolatilization are CO, CO2, CH4, and H2O. All of the results and findings would help in characterizing the biomass as potential bioenergy feedstocks compatible with other biomass currently in use as supplementary fuel for power generation. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Bioenergy feedstock Lignocellulosic biomass Thermogravimetric Py-GC/MS-FTIR
1. Introduction Globally, the energy sector faces a major challenge of providing energy at an affordable cost without adversely affecting the environment [1]. Almost one-quarter of world human populations are living under deficient basic energy demands that are not being met. Efforts to increase renewable energy resources in developing countries where per capita energy availability is low are needed [2]. Therefore, in recent years production of alternate energy, fuels and a variety of chemicals from lignocellulosic biomass has attracted scientific attention worldwide [3]. In this context, renewable bioenergy feedstocks act as a promising alternative resource to provide about 14% of the world's energy demand [4]. Moreover, lignocellulosic biomass mainly includes agricultural residue, wood waste, energy crops, noted energy grass species (switchgrass,
* Corresponding author. E-mail address: [email protected] (S. Kumar). https://doi.org/10.1016/j.renene.2018.02.054 0960-1481/© 2018 Elsevier Ltd. All rights reserved.
miscanthus, napier grass), and aquatic plants. Therefore, they are considered as a potential resource for meeting bioenergy demands [5]. Bioenergy can be readily converted to produce heat and electricity, and production of biofuels. As a sustainable energy sources, bioenergy feedstocks are widely available and receiving increased attention as a renewable substitute for fossil fuels. Conversely, if produced sustainably and used efficiently, it will reduce oil demand, addressed major environmental problems, and it can induce economic growth in a developing country. Other major prospective benefits of bioenergy cultivation include restoration of soil productivity of degraded land, improving access to quality of energy services, and reduction of major greenhouse gas emission [6]. In India, a number of different species variety of perennial grasses are available throughout the year which often grows in wild conditions and characterized as low input-high yielding biomass. Therefore, these energies grasses can be act as a potential resource for bioenergy feedstock without adversely affecting the soil C stocks [6]. Accordingly, the different varieties of perennial grasses (switchgrass, bermudagrass, elephantgrass, timothygrass) and
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lignocellulosic biomass (wheat straw and pinewood) would be used for the thermochemical conversion (pyrolysis) process [7]. However, the availability of land to grow energy crops is the primary concern for the development of such bioenergy resources. Recently, several studies have found that the marginal lands should be used for sustainable production of biomass feedstocks (bioenergy crops) based on the profitability of current land use, soil health, and environmental degradation indicators [8]. Cai et al. [9] estimated that around 1107e1411 Mha of degraded land available globally for energy crop production constitute up to 26e55% of the world energy from biomass [9,10], and the energy crop yields on such soils were estimated to vary from 1 to 10 t ha1 yr1. Likewise, open cast mining activities degrade 2e10 times more land as compared to underground mining [11], which leaves a landscape with no vegetation with adverse effects on the soil erosion, water, and air pollution. Therefore, utilization of such degraded lands for cultivating bioenergy feedstocks is an efficient way to conserve natural resources, such as lowered soil erosion and run-off, enhanced soil C-sequestration, and yielding biomass for thermochemical conversion while contributing to domestic energy demand [12]. This may be main reason why there is increasing trend in this research area. Recently pyrolysis of biomass has received strong interest due to limited availability of fossil resources. In addition, pyrolysis is a simple thermochemical process to transform lignocellulosic into low-molecular-weight compounds. Provided, the yield and composition of pyrolysis products strongly depend on the sources and quality of the biomass feedstocks as well as on the process parameters [13]. However, several new analytical techniques were developed to measure the degradation process as well as the composition of the products. In thermochemical conversions, the forestry and wood species were mainly used as reference feedstocks, while herbaceous perennial grass biomass has received little attentions. The woody biomasses are characterized with high C/N ratio, low ash content, and their ash contains low mineral constituent avoiding slagging, fouling, and less corrosion to the instruments [14]. Further, they possess a higher heating values and biomass density on one hand, and increasing energy conversion efficiency on the other hand. However, woody species showed relatively less importance due to limited land availability than many herbaceous perennial grasses, and their annual biomass yield is generally lower due to slow growth rate in the postestablishment years [15]. In addition, the harvest is also quite major problem because its required more energy for biomass comminution due to greater biomass recalcitrant [15]. Therefore, further research on low cost production biomass feedstocks required for energy production. Accordingly, we have selected two perennial grasses species namely Cenchrus ciliaris (L.) and Pennisetum pedicellatum (Tan.) mainly used for reclamation in coalmine degraded landscapes in Jharia Coalfield, India, with the aim to estimated their bioenergy potential. Cenchrus ciliaris (L.) and Pennisetum pedicellatum (Tan.) species are two perennial grass species which is widely adapted to degraded soils with poor in water content and nutrients availability, and can grow upto a height of 0.5e1.2 m. Both the genus contains several species, most of them is considered as commercial biological resources due to low cost production, adaptation in poor soils, and grows on limited water requirement. Thus, due to its abundance and low cost to grow on degraded landscape, the perennial energy grasses was selected for thermochemical conversions for the first time from mine degraded lands in India. The proximate and ultimate and thermochemical conversion behaviors (TG-FTIR-GC/MS) shown that both the perennial grass species has pyrolysis and energy properties compare to other traditional energy grasses including miscanthus and switch grass.
2. Materials and methods 2.1. Study area and sample collections The study was carried out in an ecologically restored coalmine overburden dumps located in the Damoda colliery, Barora area, BCCL (Bharat Coking Coal Limited), Jharia Coalfields, Jharkhand, India. The Jharia coalfields fall between latitudes 23 390 e23 480 N and longitudes 86 11’ e 86 270 E covering an area of 450 km2 (Suppl. Fig. 1). The total study area was approximately 4 ha (40, 000 m2) and the age of the entire restored site is 4 years. The climate of the study area is dry tropical, having hot and dry summer season (March to June), followed by wet season (July to September) and winter season (November to February). However, the area receives an annual rainfall of about 1140e1700 mm with an average annual relative humidity of about 68%. Mine spoils were afforested with grass-legume mixtures before the onset of monsoon (MayeJune 2014) through social forestry program, BCCL, India. Upon maturation, the biomass samples of two most dominant grass species Pennisetum pedicellatum (Tan.) and Cenchrus ciliaris (L.) collected during winter seasons i.e. January 2015. Although, the grass species were selected on the basis of their capacity to produce biomass annually. The dry matter yield of Pennisetum pedicellatum is 7.5e8 t ha1 yr1 and for Cenchrus ciliaris is 4e8 t ha1 yr1, respectively. The biomass samples were air dried, grinded and sieved using a Wiley mill to produce particle size <2.5 mm, according to ASTM E828-81 protocol. 2.2. Methods To correlate the respective composition with the thermal behavior of the biomass feedstocks, chemical and physical analysis was performed. 2.2.1. Proximate and ultimate analyses The proximate analysis was performed for the determination of moisture, ash and fixed carbon content and volatile matter following the ASTM standardized procedures. The measurement was done for moisture content using ASTM E871-82, volatile matter following ASTM E872-82, ash content according to ASTM E1755-01. This process allows removal of volatile and fixed carbon content. Fixed carbon content was measured as according to [16]. % FC ¼ 100 (% Ashe% VM)
(1)
The ultimate analysis was performed to determine the basic elemental composition of biomass. The carbon (C), nitrogen (N), hydrogen (H), and sulfur (S) content of the samples were measured in the Perkin-Elmer CHN 2400 elemental analyzers at Organic Chemistry Laboratory, Indian Institute of Science (IISc) Bangalore, India. The samples were burnt in a pure oxygen atmosphere and the combustion gases were automatically measured. The oxygen content was determined by subtracting the sum of the other element contents from 100%. 2.2.2. Higher heating value The higher heating value (HHV) of the biomass feedstocks were estimated in a static and adiabatic bomb calorimeter, Parr 1241, according to the ASTM D2015 for determination of the gross calorific value [17]. Biomass feedstocks were grounded to obtain a homogeneous fine powdered sample. These powder samples were well mixed and dried at 105 C for 6 h and stored in a desiccator for a period of 12 h. Finally, the powder samples were pressed in the form of pellets of 1 cm in diameter with a mass content of approximately 0.5 g. The combustion of samples was done in the
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477
presence of O2 at a pressure of 20e30 atm. Benzoic acid was used as a standard for determining the heat capacity of the calorimeter. Lower heat value (LHV) was calculated using the relationship between HHV and LHV:
obtained DSC curves give the heat flux (Wg1) in function of the temperature or as a function of the time (constant DT/ Dt ¼ 10 K min1).
LHV ¼ HHV - hg (9H þ M)/100
2.2.5. TGA-FTIR analysis Thermogravimetric analysis was conducted using a hyphenated PerkinElmer TL9000 transfer line (TGA-FTIR) facilities available at Interdisciplinary Centre for Energy Research (ICER), IISc Bangalore, India. The experiments were carried out with powder samples (~10 mg) loaded into a ceramic container (Al2O3) for each run to avoid the possible temperature gradient and to ensure kinetic control of the process. All the experiments were conducted in the temperature range of 25e900 C at four different heating rates 5, 10, 15, and 20 C min1 in a nitrogen atmosphere (50 ml min1). The transfer line and gas cell were heated at 200 C to minimize secondary reaction. The FTIR spectra were collected at a resolution of 1 cm1 and with a wavelength range from 500 to 4000 cm1. Temperature calibration was performed using the Curie point of four magnetic standardsdalumel (TCurie of 154.2 C), nickel (TCurie of 355.3 C), perkalloy (TCurie of 596.0 C) and iron (TCurie of
(2)
Where H and M are the hydrogen and moisture percentage of the tested fuel. Here, hg is the latent heat of steam in the same unit as HHV and LHV. 2.2.3. Lignocellulosic fractions Lignocellulosic fractions (cellulose, hemicellulose, and lignin) of the biomass samples were determined using the method described in Ref. [18]. The hemicellulose content was determined from the difference between the acid detergent fiber and neutral detergent fiber. The lignin was determined by the mass difference between the sample digested with the detergent acid and the oxidation carried out with the buffered solution of the acetic acid and potassium permanganate. The cellulose content was determined by the difference in mass between the dry residue generated in the lignin analysis and the same residue calcined in a muffle furnace at 500 C for 2 h.
Table 3 Lignocellulosic fractions data for perennial energy grass.
2.2.4. Differential scanning calorimeter (DSC) The combustion experiments were performed using differential scanning calorimeter (DSC) analysis at the temperature range of 20 Ce600 C at three different heating rates as 5, 10, and 15 C min1 facility available at Material Engineering, IISc Bangalore. The biomass samples used in this work were prepared according to the ASTM Standards (D 2013e72). Weight loss, heating rate and the amount of heat phenomenon were continuously recorded for all the runs. The experiments were carried out with the powdered sample (~10 mg) whereas the airflow rate (50 ml min1) was kept constant during the experiments. However, the DSC instrument was calibrated regularly with indium as reference material. The
Biomass materials
Cellulose Hemicellulose Lignin
Lignocellulosic fractions (%) C. ciliaris [present study] 29 P. pedicellatum [present study] 31 Composite biomass 35 (C. ciliaris þ P. pedicellatum) [present study] Rice husk [24] 35 Elephant grass [25] 30 Soft wood [19] 35e40 Switch grass [26] Sugarcane bagasse [27]
25 40
33 29 31
14 16 18
33 30 25e30
23 10 27 e30 24 18 e23
38 23e32
Table 1 Proximate analysis, high heating value and low heating value data for perennial energy grass. Biomass material
Moisture content (%)
Volatile matter (%)
Ash (%)
Fixed carbon (%)
HHV (MJ/kg)
LHV (MJ/kg)
C. ciliaris [present study] P. pedicellatum [present study] Composite biomass (C. ciliaris þ P. pedicellatum) [present study] Rice husk [24] Elephant grass [25] Wood [19] Vegetal coal [17] Switch grass [26] Sugarcane bagasse [27]
8.5 7.2 7.8 8.8 14.1 20.1 5.3 11.3 11.0
81.2 80.5 82.4 59.2 74.0 82.0 26.2 83.2 85.3
8.9 6.8 7.1 26.1 11.2 1.1 5.9 4.4 6.7
12.2 11.3 18.9 14.6 15.1 17.3 68.1 12.3 8.0
16.1 15.6 17.8 13.2 16.8 18.6 29.7 19.7 19.4
14.8 14.3 16.5 12.1 15.4 16.9 29.0 18.2 17.9
Table 2 Ultimate analysis, high heating value and low heating value data for perennial energy grass. Biomass material
Carbon (wt%)
Hydrogen (wt%)
Nitrogen (wt%)
Sulphur (wt%)
Oxygena (wt%)
H/Cb
O/Cb
C. ciliaris [present study] P. pedicellatum [present study] Composite biomass (C. ciliaris þ P. pedicellatum) [present study] Rice husk [24] Elephant grass [25] Wood [19] Vegetal coal [17] Switch grass [26] Sugarcane bagasse [27]
39.8 40.9 43.0 35.6 41.6 51.6 79.3 38.0 57.2
5.7 5.8 5.7 4.5 5.8 6.3 2.7 6.2 6.1
1.7 1.0 1.8 0.2 1.8 e 0.7 0.6 0.4
0.1 0.1 0.1 0.02 e 0.1 0.3 0.3 e
52.7 53.0 49.4 59.7 51.1 41.5 17.0 50.6 36.4
0.14 0.14 0.13 0.13 0.14 0.12 0.03 0.16 0.11
1.32 1.29 1.15 1.67 1.23 0.80 0.21 1.33 0.64
a b
Calculated difference. Molar ratio.
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780.0 C). Calibration for mass was carried out using a 100 mg calibration masses provided by the equipment manufacturer. 2.2.6. Pyrolysis-GC/MS analysis Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) system was used to separate and identify the evolved volatile substances. For this purpose, a hyphenated Perkin-Elmer TGA spectrum was attached to a GC/MS (Clarus 680 GC/Clarus sq 8T MS) system and used for characterization process at a pyrolysis temperature from 30 to 800 C. Then, the chromatographic separation of evolved gases were carried out using an Agilent HP-5 capillary column (30 m 250 mm film thickness) with the capillary column maintained at 280 C for about 15 min. In this experiments, Helium was used as a carrier gas at a constant flow rate of 1.0 ml min1. The GC inlet was fixed at 280 C and 400 C with a split ratio of 20:1. Mass spectra were recorded under electron ionization (70ev) with the m/z range of 50e550 au. Peak identification was carried out with NIST mass spectral library and literature. 3. Results and discussion 3.1. Feedstock characterization The results of a proximate and ultimate analysis and lignocellulosic fractions were obtained for biomass species along with other biomass materials previously studied (taken from the literature) using pyrolysis method are shown in Tables 1e3. The moisture content of biomass samples varies between 7.2 and 8.5% and is the most important characteristics that influences mechanical, physical and thermal properties for thermochemical conversion technology. However, the moisture contents found in studied biomass samples were found comparatively lesser than other previously reported biomass samples (sugar bagasse, switch grass, wood, and elephant grass) (Table 1) with values ranges from 11 to 14%. The major factors leading to relatively lesser percentage of moisture contents in the studied biomass samples is due to seasonal effects (winter) having relative humidity close to 60% during harvesting. Therefore, the results showed that biomass feedstock's are found more suitable for thermal conversion, while those having high moisture content are more suited for a biochemical process using fermentation or hydrothermal process [19]. Further, the volatile matters and inorganic components (ash content) of biomass affect both the handling and processing costs of overall biomass energy conversion. Therefore, the comparison
Fig. 1. A van Krevelen diagram of H/C against O/C molar ratio for bituminous coal, perennial energy grass, rice and present study (biomass species) obtained from reclaimed coal mine spoils.
Fig. 2. Differential Scanning Calorimeter (DSC) curve of (a) Cenchrus ciliaris (L.); (b) Pennisetum pedicellatum (L.); and (c) composite biomass (Cenchrus ciliaris þ Pennisetum pedicellatum) samples at three different heating rates (5 C min1, 10 C min1, and 15 C min1).
S. Kumar, P. Ghosh / Renewable Energy 123 (2018) 475e485
between the different biomass shows higher amounts of volatile matter ranging from 80.5 to 82.4% for all species and found similar to other biomass materials (Table 1). This observation may be due to a higher cellulose and hemicellulose percentage [20]. Accordingly, biomass with a high content of volatile matter is more sensitive to thermal degradation and this strongly influences its combustion behavior, and thermal decomposition. It also provides a potential feedstock for energy production using pyrolysis [20]. In addition, the ash contents of studied biomass feedstock's ranged from 6.8 to 8.9% which is found higher in comparison to switchgrass (5.7%), wheat straw (4.9%) and red canary grass (4.6%) [21]. However, it has been already reported that fuels with over 20% higher ash contents are not a suitable for heat generation during pyrolysis. This leads to the operational constraints, such as slagging, fouling, sintering and corrosion are closely related to the chemical compositions of the ash [22]. Moreover, the higher values of fixed carbon percentage (FC) reported in the studied biomass feedstock's (11.3e18.9%) in comparison to other biomass species is due to high content of lignin (Table 1) [19]. Generally, the biomass fuels having higher volatile matters and low fixed carbon lead to rapid gas evolution during pyrolysis needs a dedicated combustion design. However, biomass fuels having low volatiles take a long time to burn out unless they are pulverized to a very small size [20]. Therefore, the studied biomass samples have advantages compared to other fuels owing to low moisture and ash content, and high volatile matter making them the ideal biomass feedstock for thermochemical conversions. Additionally, the higher heating value (HHV) is one of the most important parameter among physical properties related to assess the calorific values of biofuels from biomass feedstocks (Table 1). The HHV of studied biomass samples were varied from 15.6 to 17.8 MJ/kg on a dry basis, and the low heating value (LHV) ranged from 14.3 to 16.5 MJ/kg, respectively (Table 1) which was found relatively lower than that of vegetal coal (29.7 MJ/kg), and found within the range of previously studied energy crops including switch grass (19.7 MJ/kg) and sugarcane bagasse (19.4 MJ/kg) [20,29,30]. This property likely due to cell wall structure, namely higher lignin content, volatile and carbon content in bioenergy feedstocks. The lignin structure mainly contains CeC bonds having higher calorific value potential than CeH and CeO bonds [19]. As a consequences the higher the lignin, the higher the HHV and LHV. Thus, the high HHV and LHV make the biomass feedstocks attractive bioenergy resources to exploit as an alternative fuel [23]. Carbon (C), hydrogen (H) and oxygen (O) are the main components of biomass feedstocks in a bioenergy fuel. C and H get oxidized during combustion by exothermic reactions (produced CO, CO2, and H2O) and therefore control the higher heating value of the fuel. The results of ultimate analysis of biomass samples showed
479
that C, H, and O content varies from 39.8 to 43.0%, 5.7e5.8%, and 49.4e53%, respectively (Table 2). These values were found similar to other biomass samples [21]. However, the percentage of carbon content (C. ciliaris-39.8%, P. pedicellatum-40.9%, and composite biomass e 43.0%) in studied biomass samples were found to be lower than wood (51.6%), sugarcane bagasse (57.2%), and bituminous coal (73.1%), and found higher than rice straw (41.4%) [19]. It has been reported that a higher proportion of O and H relative to C reduces the energy value of biomass feedstocks as compared to fossil fuel due to the lower energy content of CeO and CeH bonds than in CeC bonds [19]. Moreover, the low content of carbon (C), fixed carbon (FC), and a higher proportion of oxygen (O) and hydrogen (H) in biomass samples compared to vegetal coal have significant benefit for reducing CO2 emissions during biomass conversion. Similarly, the low content of nitrogen (N) (<2.0%) and sulfur (S) (<0.3%) indicate that biomass species contribute less emission to the environment in comparison to fossil fuels (Table 2). This is due to a reduction in quantities of sulfur oxides (SOx), nitrogen oxides (NOx), toxic and corrosives gases generated during the thermochemical conversion [24]. Finally, the high value of hydrogen (H) in biomass is an advantage due to increased role of combustible H2, H2S, carbohydrate, hydrocarbons, and H-containing functional groups, greater volatility and high reactive nature of this fuel [25]. A very useful mean of comparing biomass feedstocks and fossil fuels is in terms of their O/C and H/C ratios, known as a Van Krevlen diagram (Fig. 1). Although, the increased H/C and O/C ratios in biomass imply decreasing aromaticity and increasing the role of the oxygen-containing hydroxyl, carboxyl, ether, and ketone functional groups [25]. Therefore, lower the respective ratios, greater the energy content of the biomass materials due to increased carboncarbon bonding. In this present study, the H/C and O/C molar ratios were calculated from the elemental composition varied from 0.13 to 0.14 (H/C) and 1.15 to 1.32 (O/C), respectively (Table 2). However, the molar (O/C) composition of biomass samples was found relatively higher than the coal and other biomass materials indicate minimum energy density and HHV. According to chemical compositions of the biomass feedstocks (Table 3), the fractions of cellulose, hemicellulose, and lignin in the feedstock are varied but found relatively similar to other biomass materials (Table 3). 3.2. DSC-TGA/DTG analysis The two different exothermic reaction regions were observed in differential scanning calorimeter (DSC) experiments (Fig. 2). First region or shoulder is due to decomposition of light volatile matters, which again provide reactivity to biomass samples. However, the second shoulder or region is due to decomposition of fixed carbon.
Table 4 Reaction regions of perennial energy grass biomass samples at different heating rates. Biomass samples DSC Cenchrus ciliaris Pennisetum pedicellatum Composite (C. ciliaris þ P. pedicellatum) TGA-DTG Cenchrus ciliaris Pennisetum pedicellatum Composite (C. ciliaris þ P. pedicellatum)
5 C/min
10 C/min
15 C/min
20 C/min
light volatiles heavy volatiles light volatiles heavy volatiles light volatiles heavy volatiles
220e350 350e490 210e350 350e480 250e340 340e510
240e375 375e520 240e360 360e530 220e370 370e550
250e390 390e560 250e380 380e590 250e405 405e580
e e e e e e
of of of of of of
180e370 370e450 150e380 380e650 210e380 380e480
190e380 380e460 150e370 370e620 205e390 390e460
190e400 400e580 160e400 400e650 110e300 300e450
170e400 400e560 200e400 400e700 210e400 400e460
Reaction types Combustion Combustion Combustion Combustion Combustion Combustion
of of of of of of
Decomposition Decomposition Decomposition Decomposition Decomposition Decomposition
light volatiles heavy volatiles light volatiles heavy volatiles light volatiles heavy volatiles
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In addition, we had also observed that with an increase in heating rates, the starting and end point of temperature increases due to thermal lag (Table 4) [26]. Generally, as the heating rate was increased, the pyrolysis temperature and all characteristic temperatures shifted to higher values (Table 4). However, this observation may be due to the decrease in maximum weight loss rate as the heating rate increased. Consequently, these changes could be mainly attributed due to change in the decomposition kinetics and the fact that an increase in the heating rates provided higher thermal energy, ensuring a better heat transfer between the surrounding environment and the sample inside [27]. Finally, the higher heating rate also lowers the yield of residue at the end of the reaction. This is due to the lower heating rates resulted in longer residence times inside the reactor favoring secondary reactions such as cracking, re-polymerization and recondensation, thus leading to char formation [28]. The observed thermal behavior of (TG-DTG) biomass feedstocks during pyrolysis is shown in Fig. 3. Based on TG-DTG curves, the weight mass loss range can be divided into three zones. First zone start at 30 C and finishes at the 130e210 C with intervals, which is according to each heating rate. This zone represents a mass loss percent by about 5% which is due to the release of moisture content present in biomass materials and external moisture bounded by the surface tension. In addition to moisture content, trace losses of CO2, formic acid, and acetic acid also released during decomposition of the biomass particles. The second zone is deemed to start at 130e210 C and ends at around 550 C with a significant mass loss of 60%, where the main devolatilization occurs with the maximum rate. However, the corresponding area on the DTG curve is represented by an overlapping peak. In addition, the minor and major reactions observed in active pyrolysis of this zone can mainly be attributed to the degradation of volatile matters including the hemicellulose, cellulose, and part of lignin fraction in the biomass samples [29,30]. In general, there was a slight increase in mass loss rate with increase in different heating rate within the temperature range of 130e550 C due to thermal lag at a higher heating rate during pyrolysis [26]. Miranda et al. [29] and Hu et al. [30] also reported similar trends in mass loss rates with heating rates for textile wastes and cellulose. Finally, the third zone can be related to the passive pyrolysis zone (550 Ce900 C), where lignin decomposition was prominent and reflected as a weight loss of ~7.1e10.8%. Nevertheless, the thermal degradation of all biomass samples were almost completed at the end of third reaction zone as indicated by the linear segment of the TG-DTG curve (Fig. 3), and the reactivity of decomposition regions was proportional to a height of DTG peak. Therefore, it's should be noted that the peak height is directly proportional to the reactivity, and the temperature corresponding to peak height is inversely proportional to the reactivity [31]. This experimental evidences suggest that the principal reactions of biomass feedstocks during pyrolysis include depolymerization, decarboxylation and cracking, which mostly held in the temperature range of 130 Ce550 C. It is worth noted that between temperature range of 130 C and 550 C, the cellulose and lignin degrade under highly exothermic conditions, producing high quantities of CO2, CO, CH4, H2, ethanol, acetic acid, formic acid, and formaldehyde and tar (volatile and pyrolignous) along with low quantities of ethane and ethylene. However, as the pyrolysis temperature increases above 550 C, reactions between the formed gases and the solid occur, generating highly combustible products [32]. The heating rate also affects the thermal decomposition characteristics [29]. The increase in heating rate results in shifting of the peak temperature to higher values (from 310 C at a heating rate of 5 C min1 to 340 C at 20 C min1) (Table 4). This is due to the combined effects of heat transfer at different heating rates and the
kinetics of the decomposition, resulting in delayed decomposition [29,33]. Provided, the maximum mass loss rates were also shifted to higher temperatures with increasing heating rate as a consequence of the increasing effect of the inertia of devolatilization process as
Fig. 3. TGA-DTG curve of (a) Cenchrus ciliaris (L.); (b) Pennisetum pedicellatum (L.); and (c) composite biomass (Cenchrus ciliaris þ Pennisetum pedicellatum) samples at four different heating rates (5 C min1, 10 C min1, 15 C min1 and 20 C min1).
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Fig. 4. Evolution of FTIR spectrum recorded during TG analysis of biomass samples at the heating rate of 10 C min1: (a) C. ciliaris, (b) P. pedicellatum, and (c) composite biomass sample (C. ciliaris þ P. pedicellatum).
the characteristic time of the process is decreased. However, the data also showed that higher heating rate led to the higher yields of volatile materials and lower yield of chars [34]. On the other hand, the values of mass loss rates are within the range that has been reported in others cellulosic biomass samples (cotton residue, olive kernel, forest residue [26], cotton straw and rice husk [21]; and coconut and cashew nut shells [34]). 3.3. 3 Pyrolysis-FTIR spectra analysis Another observation during the qualitative analysis of complex organic and inorganic compounds during pyrolysis was distinguished using TGA-FTIR analysis. The FTIR spectra of the evolved gas products at different pyrolysis temperatures for biomass
samples at the heating rate of 10 C min1 is shown in Fig. 4 and Suppl. Table 1. The pyrolyzed molecular gaseous products released were mostly H2O, CH4, CO, and CO2 at a temperature of 300 C. These compounds were identified by their characteristic absorbance band of 4000e3000 cm1 corresponded to the stretching of OeH bonds related to an H2O release. It is also well known that the band spectrum at 3100-2800 cm1 indicated the existence of CH4, and CO at the band of 2250e2000 cm1, CO2 at the band of 2400e2250 cm1 and 780-600 cm1 were also detected [31]. It is worth noting that at higher pyrolytic temperature about 400 C, gaseous products continued to increase and some more related volatiles were also produced. Moreover, the releasing rate of evolving gas reached its maximum at 400 C, and then with pyrolysis progression, the content of gaseous products decreased
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Table 5 Pyrolysis products of bioenergy grass species by PyeGC/MS at a temperature profile of 300 C and 400 C. (a) C. ciliaris at 300 C S. No
R. T. (min)
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.02 1.14 1.60 2.29 5.19 7.51 9.93 9.99 11.65 12.04 23.51 24.00 24.14 24.30 25.26 25.95 26.53 26.81 27.29 30.08
1.41 1.59 3.12 1.73 2.13 1.38 2.14 1.55 1.82 1.06 1.41 1.72 3.49 1.23 1.07 2.01 1.35 1.90 0.99 0.97
Furan Ethanol Acetic acid, anhydride with formic Acetaldehyde Acetone Toluene Dimethyl oxalate Methyl pyruvate 2,3-dimethyl-2-cyclopenten-1-one Styrene Creosol Phenanthrene Nonanoic acid, 6-phenyl-, methyl ester 3,5-dimethoxy-4-methylbenzoic acid Heptanoic acid 4-Hydroxy-3,5-dimethoxy-benzaldehyde 7-Methoxybenzofuran 2-Methoxy-4-ethyl-phenol 2-Methoxy-4-vinyl-phenol 2,6-Dimethoxy-pheno
68 44 88 44 58 92 118 102 110 104 138 180 248 136 130 182 148 152 150 154
C6H12O2 C2H4O C3H4O3 C2H4O C3H6O C7H8 C4H6O4 C4H6O3 C7H10O C8H8 C8H10O2 C14H1 C16H30O2 C8H8O2 C7H14O2 C9H10O4 C9H8O2 C9H12O2 C9H10O2 C8H10O3
Furan Alcohol Acid Aldehyde Ketone Benzene Acid Methyl ester Phenol Benzene Phenol Polycyclic aromatic hydrocarbon Esters Esters Acid Monocarboxylic acid Carboxylic acid Phenol Phenol Alcohol
C. ciliaris at 400 C S. No
RT
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.60 3.67 3.71 4.50 4.89 5.04 6.96 8.03 8.67 9.88 10.33 10.52 10.89 11.18 11.32 11.88 12.02 23.68 23.77 25.35
1.25 3.21 4.03 4.76 2.18 1.03 9.02 0.78 1.09 11.6 1.3 4.06 0.88 1.78 1.19 2.05 0.97 0.68 3.44 3.67
Acetic acid, anhydride with formic Acetone Benzene Pyrrolidine 2,3-Butanedione Cyclohexanone 6-Hydroxyhexan-2-one 2-Butenal 3,3-difluoroprop-2-enyl (trimethyl) silane o-Cresol Methyl pyruvate Benzene, ethoxy Acetone, acetylhydrazone 2,3-dimethyl cyclohexanol 3-methyl-1,2-cyclopentanedione 2-methyl-2-pentenal Methylidenecyclooctane 2-Methoxy-4-methyl-phenol Phenanthrene 2-Propenoic acid
88 58 78 71 86 98 116 70 150 108 102 384 114 128 112 98 124 138 180 150
C3H4O3 C3H6O C6H6 C4H9N C4H6O2 C6H8O2 C6H12O2 C4H6O C6H12F2Si C7H8O C4H6O3 C15H21BrCl2O2 C5H10N2O C8H16O C6H8O2 C6H10O C9H16 C8H10O2 C14H10 C3H3BrO2
Acid Acetone Benzene Amine Ketone Ketone Ester Aldehyde Others Phenol Others Benzene Ketone Aldehyde Ketone Ketone Others Phenol Polycyclic aromatic hydrocarbon Others
(b) P. pedicellatum at 300 C S. No
R.T. (min)
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
0.86 1.59 5.12 5.27 6.22 8.45 8.68 8.81 9.32 9.92 10.35 10.55 10.70 11.08 11.22 11.31 11.91 17.62 26.69 26.78
1.07 1.57 1.21 0.88 1.01 1.18 0.99 1.01 2.07 8.78 1.23 1.49 1.24 1.25 1.16 1.29 1.06 1.07 1.21 1.53
2,4-hexadienoic acid Butyl acetate Pyridine, 3-methyl Furfural 2-Cyclopentene-1,4-dione 1-Hexyne, 6-(1-ethoxyethoxy)Alpha-d-xylopyranose 1,2-Cyclopentanedione, 3-methyl 2-hydroxy-2-cyclopenten-1-one o-Cresol 4-amino-2(1H)-pyrodinone 2-Pyrroline, 1,2-dimethylCyclooctane-1,2-diol Pyridine, 4-methyl 2,3-dimethyl cyclohexanol g-Decalactone Acetic acid, butyl ester Undecylenic acid 4-Hydroxy-4-methylpentanoic acid 4-Methylguaiacol
112 116 93 96 98 170 150 112 98 108 110 97 144 93 128 170 116 184 132 138
C6H8O2 C6H12O2 C6H7N C5H4O2 C5H6O2 C10H18O2 C5H10O5 C6H8O2 C5H6O2 C7H8O C5H6ON2 C6H11N C8H16O2 CH3C5H4N C8H16O C10H18O2 C6H12O2 C11H20O2 C6H12O3 C8H10O2
Ether Acetate ester Aromatic Amine Furan Furan Others Carbohydrate Ether Furan Toluene Others Others Heptane Others Aldehyde Benzene Ester Benzene Aldehyde Aromatic ether
S. Kumar, P. Ghosh / Renewable Energy 123 (2018) 475e485
483
Table 5 (continued ) (b) P. pedicellatum at 300 C S. No
R.T. (min)
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
P. pedicellatum at 400 C S. No
R.T. (min)
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.60 7.40 8.04 8.63 9.32 9.95 10.39 11.34 11.70 12.26 12.93 26.92 27.62 27.93 29.04 29.18 29.90 30.09 30.29 31.11
1.41 1.29 2.56 3.66 2.42 11.33 7.16 2.94 5.35 2.6 2.42 0.68 0.64 0.98 0.71 0.7 0.64 0.81 0.92 0.63
Tetrahydropyrrole-3-amino-2,5-dione 1-Hydroxy-2-propanone 1,3-Dioxolane, 2-(1-methylpropyl)2-Butenal 2-Propanone, 1-hydroxyMethyl isobutyl ketone 4-amino-2(1H)-pyrodinone 3-methyl-1,2-cyclopentanedione 2-methyl-2-pentenal 9-oxabicyclo[3.3.1]non-2-yl-acetate Phenol, 2,6-dimethoxy 4-Methylguaiacol Toluene, 3,4-dimethoxyEthyl isovalerate Retene 3-Methylcatechol 1,2,3,4-tetrahydrotriphenylene Benzaldehyde, 3-methyl4-Methylcatechol 1,2,3,4,5-Cyclopentanepentol
71 74 130 70 74 100 110 112 98 184 154 138 152 130 234 124 232 120 124 150
C11H20O2 C3H6O2 C7H14O2 C4H6O C3H6O2 C6H12O C5H6ON2 C6H8O2 C6H10O C10H16O3 C8H10O3 C8H10O2 C9H12O2 C7H14O2 C18H18 C7H8O2 C18H16 C8H8O C7H8O2 C5H10O5
Ester Carboxylate ester Ester Aldehyde Aldehyde Alkyl aldehyde Aromatics Furan Cyclic ketone Benzene Phenol Phenol Phenol Ester Benzene Phenol Benzene Acetone Phenol Others
(c) Composite bioenergy samples 300 C S. No
RT
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.61 1.72 1.89 6.23 7.23 7.93 8.01 8.15 8.24 8.45 8.72 8.84 9.41 9.46 9.59 10.03 10.56 11.39 25.77 31.03
2.21 0.69 1.58 0.6 1.66 0.63 0.76 0.84 0.6 1.01 2.08 0.74 1.78 0.69 0.76 7.98 3.19 0.73 0.63 0.87
Tetrahydropyrrole-3-amino-2,5-dione Methanol cis-1-Cyclopentene-3,4-diol Pyruvic acid methyl ester Furfural 2-Furanmethanol 1-Butanol 6-Hydroxy-2-hexanone 6-Methyl-3-oxatricyclo[3.2.1.02,4]octane-6-carboxamide 4,4-Dimethylcyclopentene 1,3-Dioxan-5-ol 3-Methyl-1,2-cyclopentanedione 2-Propanone, 1-hydroxyDisulfide, dimethy 1,2-diazaspiro(2.5)octane Methyl pyruvate Benzene, ethoxy Dodecane 2,4-Dimethylphenol Benzaldehyde, 3-methyl-
71 32 100 102 96 98 74 116 167 96 104 112 74 94 112 102 122 170 122 120
C11H20O2 CH4O C5H8O2 C4H6O3 C5H4O2 C5H6O2 C4H10O C6H12O2 C9H13NO2 C7H12 C4H8O3 C6H8O2 C3H6O2 C2H6S2 C6H12N2 C4H6O3 C8H10O C12H26 C8H10O C8H8O
Ester Alcohol Acetylacetonate Methyl ester Furan Furan Alcohol Ester Ester Cyclohexane Hydroxy fatty acid Furan Ester Others Others Carbonate ester Phenol Alkane Benzene Benzene
Composite bioenergy samples 400 C S. No
R.T. (min)
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.60 2.68 2.88 7.27 7.34 7.44 7.76 7.82 8.05 8.23 9.62 10.08 10.18 10.35 10.49 11.00 19.46 19.62 20.21 20.91
2.56 1.93 1.20 1.23 1.37 1.78 0.76 1.18 0.66 1.80 1.10 2.57 1.44 1.02 3.17 0.66 0.88 2.83 1.33 0.68
2-Pyrroline, 1,2-dimethylMethyl acetate 1,3,5-Cycloheptatriene 4,4-Dimethyl-2-oxazoline 1-Hydroxy-2-propanone 6-Hydroxy-2-hexanone Toluene 1-Butanol 2-Butenal Phenol Phenol, 2-methylPhenol, 4-methyl2-Heptanol Methyl pyruvate 2,3,5-Trimethylfuran 2,3-dimethyl cyclohexanol 2-Methyl-phenol 1,4-Dimethoxy-2-methylcyclohexane 2,5-Cyclooctadien-1-ol 1,2-dimethoxy-4-n-propyl benzene
97 74 92 99 74 116 92 74 70 94 108 108 116 102 110 128 108 158 124 164
C6H11N C3H6O2 C7H8 C5H9NO C3H6O2 C6H12O2 C7H8 C4H10O C4H6O C6H5OH C7H8O C7H8O C7H16O C4H6O3 C7H10O C8H16O C7H8O C9H18O2 C8H12O C11H16O
Others Ester Benzene Others Propanals Acid Benzene Alcohol Ether Phenol Furan Furan Alcohol Ester Furan Aldehyde Furan Ester Furan Phenol
484
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(Fig. 4). In addition, the IR-spectra also proves the presence of volatile organic compounds with the well-distinguished absorption bands of carbonyl compounds (at 1733 cm1) including aldehyde, ketones, and acids. On the other hand, absorption band at 1560 cm1, 1480 cm1, and 1382 cm1 might be associated with CeOeC stretching vibrations in furans. Meanwhile the absorption peak at 1184 cm1 might result from stretching of the hydroxyl bond of phenol or alcohol structure. In addition, the absorption band between 952 and 676 cm1 indicates the mono, polycyclic, and substituted aromatic groups [35]. Moreover, the simultaneous evolution of different volatile compounds having similar chemical structure was not identified by FTIR spectra. Therefore, the detailed information about organic compounds of evolved gaseous composition formed during pyrolysis were thus obtained using TGA-GC/MS analysis.
biomasses. The gaseous products analysis using online TGA-FTIR hyphenated system, mainly at 300e400 C, which was consistent with the observation obtained from TGA-DTG analysis. The Py-GC/ MS analysis showed that the pyrolyzed products mainly consist of hydrocarbons such as carbonyl compounds of aldehydes and ketones followed by phenols, furans, and the other most prominent volatile organic decomposition products. Biomass feedstocks showed similar thermal events with distinct proportion, with exception of mass loss at different heating rates. The results obtained in the thermochemical characterization of the feedstocks were similar to the biomass currently used to generate energy and to obtain value-added products through pyrolysis, indicating that biomass species could be used as bioenergy fuel in a cost-effective and energy-efficient manner in future. Acknowledgment
3.4. 4 Pyrolysis- GC/MS analysis To investigate the formation of main pyrolyzed product components in biomass samples Py-GC/MS analysis was used during the thermogravimetric process (Suppl. Fig. 2). The GC/MS data were used to identify the mixture of volatile and non-volatile compounds in pyrolyzed gaseous products. However, the main chemical constituents were identified using the PerkinElmer NIST library and published data and, consequently, the highest likelihood of compounds were presented in Table 5. A total of 40 compounds were identified, and peaks of total ion count were characterized in Suppl. Fig. 2. The main compounds with the corresponding % area identified as carbonyl compounds [aldehydes (2, 3-dimethyl cyclohexanol, 0.66%; 4-Hydroxy-4-methylpentanoic acid, 1.21%; 2-Butenal, 3.66%), and ketones (acetone, 2.13%; 2, 3-Butanedione, 2.18%)], furans (2,3,5-Trimethylfuran, 3.17%; 3-methyl-1,2cyclopentanedione, 2.94%; 2-cyclopentene-1,4-dione, 1.01%; 2hydroxy-2-cyclopenten-1-one, 2.07%; furfural, 1.66%; 2Furanmethanol, 0.63%; phenol, 2-methyl-, 1.10%), and phenols (benzene, ethoxy, 3.19%; phenol, 2,6-dimethoxy, 2.42%; 2methoxy-4-vinyl-phenol, 0.99%; o-Cresol, 11.6%; 3methylcatechol, 0.70%; 4-Methylguaiacol and 1,2-dimethoxy-4-npropyl benzene, 0.68%) were most prominent products. The pyrolyzed products also contain many other aromatic derivatives (1ethyl-3-methyl-benzene and 1, 2, 4-trimethoxybenzene) and other trace products (methyl pyruvate and pyran derivatives). Toluene, a major variant of benzene type product was mainly generated from the decomposition of the intermediate of anisole. A larger amount of simple phenol and its derivatives especially cresol, which was product of OCH3 rearrangement were generated from pyrolysis of biomass samples. 4. Conclusions The present studies suggest selecting the new appropriate perennial grass species for bioenergy conversion with low cost input and high yielding phenomenon, as well as the thermochemical conversions to maximize efficiency of herbaceous materials. According to the property of biomass feedstocks, the lowest moisture and ash content and high volatile matter suggest biomass feedstocks (perennial grasses) as a potential candidate for the thermochemical conversion process and for energy generation due to higher H/C and/or O/C ratios in all the tested biomass species. The calorific value (15e17.8 MJ/kg) and proximate analysis confirm the biomass feedstocks are suitable for exploitation as bio crude/ biochemical fuel. The nitrogen and sulfur contents were relatively low, contributing to a reduction in pollutant gaseous emissions during decomposition. The hemicellulosic, cellulose and lignin contents of the tested biomass feedstocks were similar to the other
The financial support to the first author by the Science and Engineering Research Board, Ministry of Science and Technology, Government of India (Fast Track Scheme for Young Scientist, SERBDST, SR/FTP/ES-3/2014) is greatly acknowledged. The authors would like to thanks to the Chairman of Centre for Earth Science & Director of the Indian Institute of science, Bangalore, for supporting this work. The authors would like to thanks the authority of Environmental Division, Bharat Coking Coal Limited (BCCL), Dhanbad, India for granting permission to study and take samples from their reclaimed coalmine spoils dumps. The authors would like to thanks the Department of Material Engineering, Interdisciplinary Centre for Energy Research (ICER), and Department of Organic Chemistry, IISc Bangalore for providing the research facilities. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.renene.2018.02.054. References [1] S.G. Sahu, N. Chakraborty, P. Sarkar, Coalebiomass co-combustion: an overview, Renew. Sustain. Energy Rev. 39 (2014) 575e586. [2] Mendu, et al., Global bioenergy potential from high-lignin agricultural residue, Proc. Natl. Acad. Sci. U.S.A. 109 (10) (2012) 4014e4019, https://doi.org/ 10.1073/pnas.1112757109. [3] H.J. Huang, S. Ramaswamy, W. Al-Dajani, U. Tschirner, R.A. Cairncross, Effect of biomass species and plant size on cellulosic ethanol: a comparative process and economic analysis, Biomass Bioenergy 33 (2009) 234e246. pez-Gonza lez, M. Fernandez-Lopez, J.L. Valverde, L. Sanchez-Silva, [4] D. Lo Thermogravimetric-mass spectrometric analysis on combustion of lignocellulosic biomass, Bioresour. Technol. 143 (2013) 562e574. [5] S.S. Toor, H. Reddy, S. Deng, J. Hoffmann, D. Spangsmark, L.B. Madsen, et al., Hydrothermal liquefaction of Spirulina and Nannochloropsis salina under subcritical and supercritical water conditions, Bioresour. Technol. 131 (2013) 413e419, https://doi.org/10.1016/j.biortech.2012.12.144. [6] Sarkhot, et al., Total and available soil carbon fractions under the perennial grass Cynodon dactylon (L.) Pers and the bioenergy crop Arundo donax L, Biomass Bioenergy 41 (2012) 122e130. [7] S. Nanda, J. Mohammad, S.N. Reddy, J.A. Kozinski, A.K. Dalai, Pathways of lignocellulosic biomass conversion to renewable fuels, Biomass Conv. Biorefinery 4 (2014) 157e191. https://doi.org/10.1007/s13399-013-0097-z. [8] Jones, et al., Morphological and physiological traits for higher biomass production in perennial rhizomatous grasses grown on marginal land, GCB Bioenergy 7 (2015) 375e385, https://doi.org/10.1111/gcbb.12203. [9] Cai, et al., Land availability for biofuel production, Environ. Sci. Technol. 45 (1) (2011) 334e339, https://doi.org/10.1021/es103338e. [10] Vassilev, et al., Advantages and disadvantages of composition and properties of biomass in comparison with coal: an overview, Fuel 158 (2015) 330e350. https://doi.org/10.1016/j.fuel.2015.05.050. [11] J.E. Campbell, D.B. Lobell, R.C. Genova, C.B. Field, The global potential of bioenergy on abandoned agriculture lands, Environ. Sci. Technol. 42 (15) (2008) 5791e5794. [12] M.S. Li, Ecological restoration of mineland with particular reference to the metalliferous mine wasteland in China: a review of research and practice, Sci. Total Environ. 357 (2006) 38e53, https://doi.org/10.1016/
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Sustainable bio-energy potential of perennial energy grass from reclaimed coalmine spoil (marginal sites) of India Sanjoy Kumar*, Prosenjit Ghosh Centre for Earth Science, Indian Institute of Science, Bangalore, Karanataka 560012, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2017 Received in revised form 12 December 2017 Accepted 9 February 2018 Available online 20 February 2018
Usage of marginal lands to grow perennial grass for biomass feedstocks is a promising option to meet the bioenergy demand in India. In this context, the present work investigated the potential utility of two perennial grass species Cenchrus ciliaris (L.) and Pennisetum pedicellatum (Tan.) to be a new promising energy source for bioenergy. This study entails a detailed characterization of biomass feedstocks using proximate and ultimate analysis, and lignocellulosic fractions and thermogravimetric behaviour using TG-FTIR and Py-GC/MS spectrophotometry to evaluate their potential as an alternate green fuel to fossil fuels by measuring their thermochemical conversion functioning. Property analysis of perennial grass species showed a significant difference in moisture content (7.2e8.5%), volatile matters (80.5e82.4%), fixed carbon (11.3e18.9%), HHV (15e17.8 MJ/kg) and LHV (14.3e16.5 MJ/kg), which is very promising for bioenergy generation. Lignocellulosic fractions of biomass feedstocks are comparable to the previous studied biomass species including switchgrass and elephant grass. The individual decomposition experiments indicated that biomass feedstocks possess higher thermochemical reactivity and shorter devolatilization time. According to Py-GC/MS study, carbonyl compounds including aldehyde and ketones are the major volatile products, in addition to furans, benzenes, phenols, acids, and others. The TGFTIR results showed that main gaseous products evolved during devolatilization are CO, CO2, CH4, and H2O. All of the results and findings would help in characterizing the biomass as potential bioenergy feedstocks compatible with other biomass currently in use as supplementary fuel for power generation. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Bioenergy feedstock Lignocellulosic biomass Thermogravimetric Py-GC/MS-FTIR
1. Introduction Globally, the energy sector faces a major challenge of providing energy at an affordable cost without adversely affecting the environment [1]. Almost one-quarter of world human populations are living under deficient basic energy demands that are not being met. Efforts to increase renewable energy resources in developing countries where per capita energy availability is low are needed [2]. Therefore, in recent years production of alternate energy, fuels and a variety of chemicals from lignocellulosic biomass has attracted scientific attention worldwide [3]. In this context, renewable bioenergy feedstocks act as a promising alternative resource to provide about 14% of the world's energy demand [4]. Moreover, lignocellulosic biomass mainly includes agricultural residue, wood waste, energy crops, noted energy grass species (switchgrass,
* Corresponding author. E-mail address: [email protected] (S. Kumar). https://doi.org/10.1016/j.renene.2018.02.054 0960-1481/© 2018 Elsevier Ltd. All rights reserved.
miscanthus, napier grass), and aquatic plants. Therefore, they are considered as a potential resource for meeting bioenergy demands [5]. Bioenergy can be readily converted to produce heat and electricity, and production of biofuels. As a sustainable energy sources, bioenergy feedstocks are widely available and receiving increased attention as a renewable substitute for fossil fuels. Conversely, if produced sustainably and used efficiently, it will reduce oil demand, addressed major environmental problems, and it can induce economic growth in a developing country. Other major prospective benefits of bioenergy cultivation include restoration of soil productivity of degraded land, improving access to quality of energy services, and reduction of major greenhouse gas emission [6]. In India, a number of different species variety of perennial grasses are available throughout the year which often grows in wild conditions and characterized as low input-high yielding biomass. Therefore, these energies grasses can be act as a potential resource for bioenergy feedstock without adversely affecting the soil C stocks [6]. Accordingly, the different varieties of perennial grasses (switchgrass, bermudagrass, elephantgrass, timothygrass) and
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lignocellulosic biomass (wheat straw and pinewood) would be used for the thermochemical conversion (pyrolysis) process [7]. However, the availability of land to grow energy crops is the primary concern for the development of such bioenergy resources. Recently, several studies have found that the marginal lands should be used for sustainable production of biomass feedstocks (bioenergy crops) based on the profitability of current land use, soil health, and environmental degradation indicators [8]. Cai et al. [9] estimated that around 1107e1411 Mha of degraded land available globally for energy crop production constitute up to 26e55% of the world energy from biomass [9,10], and the energy crop yields on such soils were estimated to vary from 1 to 10 t ha1 yr1. Likewise, open cast mining activities degrade 2e10 times more land as compared to underground mining [11], which leaves a landscape with no vegetation with adverse effects on the soil erosion, water, and air pollution. Therefore, utilization of such degraded lands for cultivating bioenergy feedstocks is an efficient way to conserve natural resources, such as lowered soil erosion and run-off, enhanced soil C-sequestration, and yielding biomass for thermochemical conversion while contributing to domestic energy demand [12]. This may be main reason why there is increasing trend in this research area. Recently pyrolysis of biomass has received strong interest due to limited availability of fossil resources. In addition, pyrolysis is a simple thermochemical process to transform lignocellulosic into low-molecular-weight compounds. Provided, the yield and composition of pyrolysis products strongly depend on the sources and quality of the biomass feedstocks as well as on the process parameters [13]. However, several new analytical techniques were developed to measure the degradation process as well as the composition of the products. In thermochemical conversions, the forestry and wood species were mainly used as reference feedstocks, while herbaceous perennial grass biomass has received little attentions. The woody biomasses are characterized with high C/N ratio, low ash content, and their ash contains low mineral constituent avoiding slagging, fouling, and less corrosion to the instruments [14]. Further, they possess a higher heating values and biomass density on one hand, and increasing energy conversion efficiency on the other hand. However, woody species showed relatively less importance due to limited land availability than many herbaceous perennial grasses, and their annual biomass yield is generally lower due to slow growth rate in the postestablishment years [15]. In addition, the harvest is also quite major problem because its required more energy for biomass comminution due to greater biomass recalcitrant [15]. Therefore, further research on low cost production biomass feedstocks required for energy production. Accordingly, we have selected two perennial grasses species namely Cenchrus ciliaris (L.) and Pennisetum pedicellatum (Tan.) mainly used for reclamation in coalmine degraded landscapes in Jharia Coalfield, India, with the aim to estimated their bioenergy potential. Cenchrus ciliaris (L.) and Pennisetum pedicellatum (Tan.) species are two perennial grass species which is widely adapted to degraded soils with poor in water content and nutrients availability, and can grow upto a height of 0.5e1.2 m. Both the genus contains several species, most of them is considered as commercial biological resources due to low cost production, adaptation in poor soils, and grows on limited water requirement. Thus, due to its abundance and low cost to grow on degraded landscape, the perennial energy grasses was selected for thermochemical conversions for the first time from mine degraded lands in India. The proximate and ultimate and thermochemical conversion behaviors (TG-FTIR-GC/MS) shown that both the perennial grass species has pyrolysis and energy properties compare to other traditional energy grasses including miscanthus and switch grass.
2. Materials and methods 2.1. Study area and sample collections The study was carried out in an ecologically restored coalmine overburden dumps located in the Damoda colliery, Barora area, BCCL (Bharat Coking Coal Limited), Jharia Coalfields, Jharkhand, India. The Jharia coalfields fall between latitudes 23 390 e23 480 N and longitudes 86 11’ e 86 270 E covering an area of 450 km2 (Suppl. Fig. 1). The total study area was approximately 4 ha (40, 000 m2) and the age of the entire restored site is 4 years. The climate of the study area is dry tropical, having hot and dry summer season (March to June), followed by wet season (July to September) and winter season (November to February). However, the area receives an annual rainfall of about 1140e1700 mm with an average annual relative humidity of about 68%. Mine spoils were afforested with grass-legume mixtures before the onset of monsoon (MayeJune 2014) through social forestry program, BCCL, India. Upon maturation, the biomass samples of two most dominant grass species Pennisetum pedicellatum (Tan.) and Cenchrus ciliaris (L.) collected during winter seasons i.e. January 2015. Although, the grass species were selected on the basis of their capacity to produce biomass annually. The dry matter yield of Pennisetum pedicellatum is 7.5e8 t ha1 yr1 and for Cenchrus ciliaris is 4e8 t ha1 yr1, respectively. The biomass samples were air dried, grinded and sieved using a Wiley mill to produce particle size <2.5 mm, according to ASTM E828-81 protocol. 2.2. Methods To correlate the respective composition with the thermal behavior of the biomass feedstocks, chemical and physical analysis was performed. 2.2.1. Proximate and ultimate analyses The proximate analysis was performed for the determination of moisture, ash and fixed carbon content and volatile matter following the ASTM standardized procedures. The measurement was done for moisture content using ASTM E871-82, volatile matter following ASTM E872-82, ash content according to ASTM E1755-01. This process allows removal of volatile and fixed carbon content. Fixed carbon content was measured as according to [16]. % FC ¼ 100 (% Ashe% VM)
(1)
The ultimate analysis was performed to determine the basic elemental composition of biomass. The carbon (C), nitrogen (N), hydrogen (H), and sulfur (S) content of the samples were measured in the Perkin-Elmer CHN 2400 elemental analyzers at Organic Chemistry Laboratory, Indian Institute of Science (IISc) Bangalore, India. The samples were burnt in a pure oxygen atmosphere and the combustion gases were automatically measured. The oxygen content was determined by subtracting the sum of the other element contents from 100%. 2.2.2. Higher heating value The higher heating value (HHV) of the biomass feedstocks were estimated in a static and adiabatic bomb calorimeter, Parr 1241, according to the ASTM D2015 for determination of the gross calorific value [17]. Biomass feedstocks were grounded to obtain a homogeneous fine powdered sample. These powder samples were well mixed and dried at 105 C for 6 h and stored in a desiccator for a period of 12 h. Finally, the powder samples were pressed in the form of pellets of 1 cm in diameter with a mass content of approximately 0.5 g. The combustion of samples was done in the
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presence of O2 at a pressure of 20e30 atm. Benzoic acid was used as a standard for determining the heat capacity of the calorimeter. Lower heat value (LHV) was calculated using the relationship between HHV and LHV:
obtained DSC curves give the heat flux (Wg1) in function of the temperature or as a function of the time (constant DT/ Dt ¼ 10 K min1).
LHV ¼ HHV - hg (9H þ M)/100
2.2.5. TGA-FTIR analysis Thermogravimetric analysis was conducted using a hyphenated PerkinElmer TL9000 transfer line (TGA-FTIR) facilities available at Interdisciplinary Centre for Energy Research (ICER), IISc Bangalore, India. The experiments were carried out with powder samples (~10 mg) loaded into a ceramic container (Al2O3) for each run to avoid the possible temperature gradient and to ensure kinetic control of the process. All the experiments were conducted in the temperature range of 25e900 C at four different heating rates 5, 10, 15, and 20 C min1 in a nitrogen atmosphere (50 ml min1). The transfer line and gas cell were heated at 200 C to minimize secondary reaction. The FTIR spectra were collected at a resolution of 1 cm1 and with a wavelength range from 500 to 4000 cm1. Temperature calibration was performed using the Curie point of four magnetic standardsdalumel (TCurie of 154.2 C), nickel (TCurie of 355.3 C), perkalloy (TCurie of 596.0 C) and iron (TCurie of
(2)
Where H and M are the hydrogen and moisture percentage of the tested fuel. Here, hg is the latent heat of steam in the same unit as HHV and LHV. 2.2.3. Lignocellulosic fractions Lignocellulosic fractions (cellulose, hemicellulose, and lignin) of the biomass samples were determined using the method described in Ref. [18]. The hemicellulose content was determined from the difference between the acid detergent fiber and neutral detergent fiber. The lignin was determined by the mass difference between the sample digested with the detergent acid and the oxidation carried out with the buffered solution of the acetic acid and potassium permanganate. The cellulose content was determined by the difference in mass between the dry residue generated in the lignin analysis and the same residue calcined in a muffle furnace at 500 C for 2 h.
Table 3 Lignocellulosic fractions data for perennial energy grass.
2.2.4. Differential scanning calorimeter (DSC) The combustion experiments were performed using differential scanning calorimeter (DSC) analysis at the temperature range of 20 Ce600 C at three different heating rates as 5, 10, and 15 C min1 facility available at Material Engineering, IISc Bangalore. The biomass samples used in this work were prepared according to the ASTM Standards (D 2013e72). Weight loss, heating rate and the amount of heat phenomenon were continuously recorded for all the runs. The experiments were carried out with the powdered sample (~10 mg) whereas the airflow rate (50 ml min1) was kept constant during the experiments. However, the DSC instrument was calibrated regularly with indium as reference material. The
Biomass materials
Cellulose Hemicellulose Lignin
Lignocellulosic fractions (%) C. ciliaris [present study] 29 P. pedicellatum [present study] 31 Composite biomass 35 (C. ciliaris þ P. pedicellatum) [present study] Rice husk [24] 35 Elephant grass [25] 30 Soft wood [19] 35e40 Switch grass [26] Sugarcane bagasse [27]
25 40
33 29 31
14 16 18
33 30 25e30
23 10 27 e30 24 18 e23
38 23e32
Table 1 Proximate analysis, high heating value and low heating value data for perennial energy grass. Biomass material
Moisture content (%)
Volatile matter (%)
Ash (%)
Fixed carbon (%)
HHV (MJ/kg)
LHV (MJ/kg)
C. ciliaris [present study] P. pedicellatum [present study] Composite biomass (C. ciliaris þ P. pedicellatum) [present study] Rice husk [24] Elephant grass [25] Wood [19] Vegetal coal [17] Switch grass [26] Sugarcane bagasse [27]
8.5 7.2 7.8 8.8 14.1 20.1 5.3 11.3 11.0
81.2 80.5 82.4 59.2 74.0 82.0 26.2 83.2 85.3
8.9 6.8 7.1 26.1 11.2 1.1 5.9 4.4 6.7
12.2 11.3 18.9 14.6 15.1 17.3 68.1 12.3 8.0
16.1 15.6 17.8 13.2 16.8 18.6 29.7 19.7 19.4
14.8 14.3 16.5 12.1 15.4 16.9 29.0 18.2 17.9
Table 2 Ultimate analysis, high heating value and low heating value data for perennial energy grass. Biomass material
Carbon (wt%)
Hydrogen (wt%)
Nitrogen (wt%)
Sulphur (wt%)
Oxygena (wt%)
H/Cb
O/Cb
C. ciliaris [present study] P. pedicellatum [present study] Composite biomass (C. ciliaris þ P. pedicellatum) [present study] Rice husk [24] Elephant grass [25] Wood [19] Vegetal coal [17] Switch grass [26] Sugarcane bagasse [27]
39.8 40.9 43.0 35.6 41.6 51.6 79.3 38.0 57.2
5.7 5.8 5.7 4.5 5.8 6.3 2.7 6.2 6.1
1.7 1.0 1.8 0.2 1.8 e 0.7 0.6 0.4
0.1 0.1 0.1 0.02 e 0.1 0.3 0.3 e
52.7 53.0 49.4 59.7 51.1 41.5 17.0 50.6 36.4
0.14 0.14 0.13 0.13 0.14 0.12 0.03 0.16 0.11
1.32 1.29 1.15 1.67 1.23 0.80 0.21 1.33 0.64
a b
Calculated difference. Molar ratio.
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780.0 C). Calibration for mass was carried out using a 100 mg calibration masses provided by the equipment manufacturer. 2.2.6. Pyrolysis-GC/MS analysis Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) system was used to separate and identify the evolved volatile substances. For this purpose, a hyphenated Perkin-Elmer TGA spectrum was attached to a GC/MS (Clarus 680 GC/Clarus sq 8T MS) system and used for characterization process at a pyrolysis temperature from 30 to 800 C. Then, the chromatographic separation of evolved gases were carried out using an Agilent HP-5 capillary column (30 m 250 mm film thickness) with the capillary column maintained at 280 C for about 15 min. In this experiments, Helium was used as a carrier gas at a constant flow rate of 1.0 ml min1. The GC inlet was fixed at 280 C and 400 C with a split ratio of 20:1. Mass spectra were recorded under electron ionization (70ev) with the m/z range of 50e550 au. Peak identification was carried out with NIST mass spectral library and literature. 3. Results and discussion 3.1. Feedstock characterization The results of a proximate and ultimate analysis and lignocellulosic fractions were obtained for biomass species along with other biomass materials previously studied (taken from the literature) using pyrolysis method are shown in Tables 1e3. The moisture content of biomass samples varies between 7.2 and 8.5% and is the most important characteristics that influences mechanical, physical and thermal properties for thermochemical conversion technology. However, the moisture contents found in studied biomass samples were found comparatively lesser than other previously reported biomass samples (sugar bagasse, switch grass, wood, and elephant grass) (Table 1) with values ranges from 11 to 14%. The major factors leading to relatively lesser percentage of moisture contents in the studied biomass samples is due to seasonal effects (winter) having relative humidity close to 60% during harvesting. Therefore, the results showed that biomass feedstock's are found more suitable for thermal conversion, while those having high moisture content are more suited for a biochemical process using fermentation or hydrothermal process [19]. Further, the volatile matters and inorganic components (ash content) of biomass affect both the handling and processing costs of overall biomass energy conversion. Therefore, the comparison
Fig. 1. A van Krevelen diagram of H/C against O/C molar ratio for bituminous coal, perennial energy grass, rice and present study (biomass species) obtained from reclaimed coal mine spoils.
Fig. 2. Differential Scanning Calorimeter (DSC) curve of (a) Cenchrus ciliaris (L.); (b) Pennisetum pedicellatum (L.); and (c) composite biomass (Cenchrus ciliaris þ Pennisetum pedicellatum) samples at three different heating rates (5 C min1, 10 C min1, and 15 C min1).
S. Kumar, P. Ghosh / Renewable Energy 123 (2018) 475e485
between the different biomass shows higher amounts of volatile matter ranging from 80.5 to 82.4% for all species and found similar to other biomass materials (Table 1). This observation may be due to a higher cellulose and hemicellulose percentage [20]. Accordingly, biomass with a high content of volatile matter is more sensitive to thermal degradation and this strongly influences its combustion behavior, and thermal decomposition. It also provides a potential feedstock for energy production using pyrolysis [20]. In addition, the ash contents of studied biomass feedstock's ranged from 6.8 to 8.9% which is found higher in comparison to switchgrass (5.7%), wheat straw (4.9%) and red canary grass (4.6%) [21]. However, it has been already reported that fuels with over 20% higher ash contents are not a suitable for heat generation during pyrolysis. This leads to the operational constraints, such as slagging, fouling, sintering and corrosion are closely related to the chemical compositions of the ash [22]. Moreover, the higher values of fixed carbon percentage (FC) reported in the studied biomass feedstock's (11.3e18.9%) in comparison to other biomass species is due to high content of lignin (Table 1) [19]. Generally, the biomass fuels having higher volatile matters and low fixed carbon lead to rapid gas evolution during pyrolysis needs a dedicated combustion design. However, biomass fuels having low volatiles take a long time to burn out unless they are pulverized to a very small size [20]. Therefore, the studied biomass samples have advantages compared to other fuels owing to low moisture and ash content, and high volatile matter making them the ideal biomass feedstock for thermochemical conversions. Additionally, the higher heating value (HHV) is one of the most important parameter among physical properties related to assess the calorific values of biofuels from biomass feedstocks (Table 1). The HHV of studied biomass samples were varied from 15.6 to 17.8 MJ/kg on a dry basis, and the low heating value (LHV) ranged from 14.3 to 16.5 MJ/kg, respectively (Table 1) which was found relatively lower than that of vegetal coal (29.7 MJ/kg), and found within the range of previously studied energy crops including switch grass (19.7 MJ/kg) and sugarcane bagasse (19.4 MJ/kg) [20,29,30]. This property likely due to cell wall structure, namely higher lignin content, volatile and carbon content in bioenergy feedstocks. The lignin structure mainly contains CeC bonds having higher calorific value potential than CeH and CeO bonds [19]. As a consequences the higher the lignin, the higher the HHV and LHV. Thus, the high HHV and LHV make the biomass feedstocks attractive bioenergy resources to exploit as an alternative fuel [23]. Carbon (C), hydrogen (H) and oxygen (O) are the main components of biomass feedstocks in a bioenergy fuel. C and H get oxidized during combustion by exothermic reactions (produced CO, CO2, and H2O) and therefore control the higher heating value of the fuel. The results of ultimate analysis of biomass samples showed
479
that C, H, and O content varies from 39.8 to 43.0%, 5.7e5.8%, and 49.4e53%, respectively (Table 2). These values were found similar to other biomass samples [21]. However, the percentage of carbon content (C. ciliaris-39.8%, P. pedicellatum-40.9%, and composite biomass e 43.0%) in studied biomass samples were found to be lower than wood (51.6%), sugarcane bagasse (57.2%), and bituminous coal (73.1%), and found higher than rice straw (41.4%) [19]. It has been reported that a higher proportion of O and H relative to C reduces the energy value of biomass feedstocks as compared to fossil fuel due to the lower energy content of CeO and CeH bonds than in CeC bonds [19]. Moreover, the low content of carbon (C), fixed carbon (FC), and a higher proportion of oxygen (O) and hydrogen (H) in biomass samples compared to vegetal coal have significant benefit for reducing CO2 emissions during biomass conversion. Similarly, the low content of nitrogen (N) (<2.0%) and sulfur (S) (<0.3%) indicate that biomass species contribute less emission to the environment in comparison to fossil fuels (Table 2). This is due to a reduction in quantities of sulfur oxides (SOx), nitrogen oxides (NOx), toxic and corrosives gases generated during the thermochemical conversion [24]. Finally, the high value of hydrogen (H) in biomass is an advantage due to increased role of combustible H2, H2S, carbohydrate, hydrocarbons, and H-containing functional groups, greater volatility and high reactive nature of this fuel [25]. A very useful mean of comparing biomass feedstocks and fossil fuels is in terms of their O/C and H/C ratios, known as a Van Krevlen diagram (Fig. 1). Although, the increased H/C and O/C ratios in biomass imply decreasing aromaticity and increasing the role of the oxygen-containing hydroxyl, carboxyl, ether, and ketone functional groups [25]. Therefore, lower the respective ratios, greater the energy content of the biomass materials due to increased carboncarbon bonding. In this present study, the H/C and O/C molar ratios were calculated from the elemental composition varied from 0.13 to 0.14 (H/C) and 1.15 to 1.32 (O/C), respectively (Table 2). However, the molar (O/C) composition of biomass samples was found relatively higher than the coal and other biomass materials indicate minimum energy density and HHV. According to chemical compositions of the biomass feedstocks (Table 3), the fractions of cellulose, hemicellulose, and lignin in the feedstock are varied but found relatively similar to other biomass materials (Table 3). 3.2. DSC-TGA/DTG analysis The two different exothermic reaction regions were observed in differential scanning calorimeter (DSC) experiments (Fig. 2). First region or shoulder is due to decomposition of light volatile matters, which again provide reactivity to biomass samples. However, the second shoulder or region is due to decomposition of fixed carbon.
Table 4 Reaction regions of perennial energy grass biomass samples at different heating rates. Biomass samples DSC Cenchrus ciliaris Pennisetum pedicellatum Composite (C. ciliaris þ P. pedicellatum) TGA-DTG Cenchrus ciliaris Pennisetum pedicellatum Composite (C. ciliaris þ P. pedicellatum)
5 C/min
10 C/min
15 C/min
20 C/min
light volatiles heavy volatiles light volatiles heavy volatiles light volatiles heavy volatiles
220e350 350e490 210e350 350e480 250e340 340e510
240e375 375e520 240e360 360e530 220e370 370e550
250e390 390e560 250e380 380e590 250e405 405e580
e e e e e e
of of of of of of
180e370 370e450 150e380 380e650 210e380 380e480
190e380 380e460 150e370 370e620 205e390 390e460
190e400 400e580 160e400 400e650 110e300 300e450
170e400 400e560 200e400 400e700 210e400 400e460
Reaction types Combustion Combustion Combustion Combustion Combustion Combustion
of of of of of of
Decomposition Decomposition Decomposition Decomposition Decomposition Decomposition
light volatiles heavy volatiles light volatiles heavy volatiles light volatiles heavy volatiles
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In addition, we had also observed that with an increase in heating rates, the starting and end point of temperature increases due to thermal lag (Table 4) [26]. Generally, as the heating rate was increased, the pyrolysis temperature and all characteristic temperatures shifted to higher values (Table 4). However, this observation may be due to the decrease in maximum weight loss rate as the heating rate increased. Consequently, these changes could be mainly attributed due to change in the decomposition kinetics and the fact that an increase in the heating rates provided higher thermal energy, ensuring a better heat transfer between the surrounding environment and the sample inside [27]. Finally, the higher heating rate also lowers the yield of residue at the end of the reaction. This is due to the lower heating rates resulted in longer residence times inside the reactor favoring secondary reactions such as cracking, re-polymerization and recondensation, thus leading to char formation [28]. The observed thermal behavior of (TG-DTG) biomass feedstocks during pyrolysis is shown in Fig. 3. Based on TG-DTG curves, the weight mass loss range can be divided into three zones. First zone start at 30 C and finishes at the 130e210 C with intervals, which is according to each heating rate. This zone represents a mass loss percent by about 5% which is due to the release of moisture content present in biomass materials and external moisture bounded by the surface tension. In addition to moisture content, trace losses of CO2, formic acid, and acetic acid also released during decomposition of the biomass particles. The second zone is deemed to start at 130e210 C and ends at around 550 C with a significant mass loss of 60%, where the main devolatilization occurs with the maximum rate. However, the corresponding area on the DTG curve is represented by an overlapping peak. In addition, the minor and major reactions observed in active pyrolysis of this zone can mainly be attributed to the degradation of volatile matters including the hemicellulose, cellulose, and part of lignin fraction in the biomass samples [29,30]. In general, there was a slight increase in mass loss rate with increase in different heating rate within the temperature range of 130e550 C due to thermal lag at a higher heating rate during pyrolysis [26]. Miranda et al. [29] and Hu et al. [30] also reported similar trends in mass loss rates with heating rates for textile wastes and cellulose. Finally, the third zone can be related to the passive pyrolysis zone (550 Ce900 C), where lignin decomposition was prominent and reflected as a weight loss of ~7.1e10.8%. Nevertheless, the thermal degradation of all biomass samples were almost completed at the end of third reaction zone as indicated by the linear segment of the TG-DTG curve (Fig. 3), and the reactivity of decomposition regions was proportional to a height of DTG peak. Therefore, it's should be noted that the peak height is directly proportional to the reactivity, and the temperature corresponding to peak height is inversely proportional to the reactivity [31]. This experimental evidences suggest that the principal reactions of biomass feedstocks during pyrolysis include depolymerization, decarboxylation and cracking, which mostly held in the temperature range of 130 Ce550 C. It is worth noted that between temperature range of 130 C and 550 C, the cellulose and lignin degrade under highly exothermic conditions, producing high quantities of CO2, CO, CH4, H2, ethanol, acetic acid, formic acid, and formaldehyde and tar (volatile and pyrolignous) along with low quantities of ethane and ethylene. However, as the pyrolysis temperature increases above 550 C, reactions between the formed gases and the solid occur, generating highly combustible products [32]. The heating rate also affects the thermal decomposition characteristics [29]. The increase in heating rate results in shifting of the peak temperature to higher values (from 310 C at a heating rate of 5 C min1 to 340 C at 20 C min1) (Table 4). This is due to the combined effects of heat transfer at different heating rates and the
kinetics of the decomposition, resulting in delayed decomposition [29,33]. Provided, the maximum mass loss rates were also shifted to higher temperatures with increasing heating rate as a consequence of the increasing effect of the inertia of devolatilization process as
Fig. 3. TGA-DTG curve of (a) Cenchrus ciliaris (L.); (b) Pennisetum pedicellatum (L.); and (c) composite biomass (Cenchrus ciliaris þ Pennisetum pedicellatum) samples at four different heating rates (5 C min1, 10 C min1, 15 C min1 and 20 C min1).
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Fig. 4. Evolution of FTIR spectrum recorded during TG analysis of biomass samples at the heating rate of 10 C min1: (a) C. ciliaris, (b) P. pedicellatum, and (c) composite biomass sample (C. ciliaris þ P. pedicellatum).
the characteristic time of the process is decreased. However, the data also showed that higher heating rate led to the higher yields of volatile materials and lower yield of chars [34]. On the other hand, the values of mass loss rates are within the range that has been reported in others cellulosic biomass samples (cotton residue, olive kernel, forest residue [26], cotton straw and rice husk [21]; and coconut and cashew nut shells [34]). 3.3. 3 Pyrolysis-FTIR spectra analysis Another observation during the qualitative analysis of complex organic and inorganic compounds during pyrolysis was distinguished using TGA-FTIR analysis. The FTIR spectra of the evolved gas products at different pyrolysis temperatures for biomass
samples at the heating rate of 10 C min1 is shown in Fig. 4 and Suppl. Table 1. The pyrolyzed molecular gaseous products released were mostly H2O, CH4, CO, and CO2 at a temperature of 300 C. These compounds were identified by their characteristic absorbance band of 4000e3000 cm1 corresponded to the stretching of OeH bonds related to an H2O release. It is also well known that the band spectrum at 3100-2800 cm1 indicated the existence of CH4, and CO at the band of 2250e2000 cm1, CO2 at the band of 2400e2250 cm1 and 780-600 cm1 were also detected [31]. It is worth noting that at higher pyrolytic temperature about 400 C, gaseous products continued to increase and some more related volatiles were also produced. Moreover, the releasing rate of evolving gas reached its maximum at 400 C, and then with pyrolysis progression, the content of gaseous products decreased
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Table 5 Pyrolysis products of bioenergy grass species by PyeGC/MS at a temperature profile of 300 C and 400 C. (a) C. ciliaris at 300 C S. No
R. T. (min)
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.02 1.14 1.60 2.29 5.19 7.51 9.93 9.99 11.65 12.04 23.51 24.00 24.14 24.30 25.26 25.95 26.53 26.81 27.29 30.08
1.41 1.59 3.12 1.73 2.13 1.38 2.14 1.55 1.82 1.06 1.41 1.72 3.49 1.23 1.07 2.01 1.35 1.90 0.99 0.97
Furan Ethanol Acetic acid, anhydride with formic Acetaldehyde Acetone Toluene Dimethyl oxalate Methyl pyruvate 2,3-dimethyl-2-cyclopenten-1-one Styrene Creosol Phenanthrene Nonanoic acid, 6-phenyl-, methyl ester 3,5-dimethoxy-4-methylbenzoic acid Heptanoic acid 4-Hydroxy-3,5-dimethoxy-benzaldehyde 7-Methoxybenzofuran 2-Methoxy-4-ethyl-phenol 2-Methoxy-4-vinyl-phenol 2,6-Dimethoxy-pheno
68 44 88 44 58 92 118 102 110 104 138 180 248 136 130 182 148 152 150 154
C6H12O2 C2H4O C3H4O3 C2H4O C3H6O C7H8 C4H6O4 C4H6O3 C7H10O C8H8 C8H10O2 C14H1 C16H30O2 C8H8O2 C7H14O2 C9H10O4 C9H8O2 C9H12O2 C9H10O2 C8H10O3
Furan Alcohol Acid Aldehyde Ketone Benzene Acid Methyl ester Phenol Benzene Phenol Polycyclic aromatic hydrocarbon Esters Esters Acid Monocarboxylic acid Carboxylic acid Phenol Phenol Alcohol
C. ciliaris at 400 C S. No
RT
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.60 3.67 3.71 4.50 4.89 5.04 6.96 8.03 8.67 9.88 10.33 10.52 10.89 11.18 11.32 11.88 12.02 23.68 23.77 25.35
1.25 3.21 4.03 4.76 2.18 1.03 9.02 0.78 1.09 11.6 1.3 4.06 0.88 1.78 1.19 2.05 0.97 0.68 3.44 3.67
Acetic acid, anhydride with formic Acetone Benzene Pyrrolidine 2,3-Butanedione Cyclohexanone 6-Hydroxyhexan-2-one 2-Butenal 3,3-difluoroprop-2-enyl (trimethyl) silane o-Cresol Methyl pyruvate Benzene, ethoxy Acetone, acetylhydrazone 2,3-dimethyl cyclohexanol 3-methyl-1,2-cyclopentanedione 2-methyl-2-pentenal Methylidenecyclooctane 2-Methoxy-4-methyl-phenol Phenanthrene 2-Propenoic acid
88 58 78 71 86 98 116 70 150 108 102 384 114 128 112 98 124 138 180 150
C3H4O3 C3H6O C6H6 C4H9N C4H6O2 C6H8O2 C6H12O2 C4H6O C6H12F2Si C7H8O C4H6O3 C15H21BrCl2O2 C5H10N2O C8H16O C6H8O2 C6H10O C9H16 C8H10O2 C14H10 C3H3BrO2
Acid Acetone Benzene Amine Ketone Ketone Ester Aldehyde Others Phenol Others Benzene Ketone Aldehyde Ketone Ketone Others Phenol Polycyclic aromatic hydrocarbon Others
(b) P. pedicellatum at 300 C S. No
R.T. (min)
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
0.86 1.59 5.12 5.27 6.22 8.45 8.68 8.81 9.32 9.92 10.35 10.55 10.70 11.08 11.22 11.31 11.91 17.62 26.69 26.78
1.07 1.57 1.21 0.88 1.01 1.18 0.99 1.01 2.07 8.78 1.23 1.49 1.24 1.25 1.16 1.29 1.06 1.07 1.21 1.53
2,4-hexadienoic acid Butyl acetate Pyridine, 3-methyl Furfural 2-Cyclopentene-1,4-dione 1-Hexyne, 6-(1-ethoxyethoxy)Alpha-d-xylopyranose 1,2-Cyclopentanedione, 3-methyl 2-hydroxy-2-cyclopenten-1-one o-Cresol 4-amino-2(1H)-pyrodinone 2-Pyrroline, 1,2-dimethylCyclooctane-1,2-diol Pyridine, 4-methyl 2,3-dimethyl cyclohexanol g-Decalactone Acetic acid, butyl ester Undecylenic acid 4-Hydroxy-4-methylpentanoic acid 4-Methylguaiacol
112 116 93 96 98 170 150 112 98 108 110 97 144 93 128 170 116 184 132 138
C6H8O2 C6H12O2 C6H7N C5H4O2 C5H6O2 C10H18O2 C5H10O5 C6H8O2 C5H6O2 C7H8O C5H6ON2 C6H11N C8H16O2 CH3C5H4N C8H16O C10H18O2 C6H12O2 C11H20O2 C6H12O3 C8H10O2
Ether Acetate ester Aromatic Amine Furan Furan Others Carbohydrate Ether Furan Toluene Others Others Heptane Others Aldehyde Benzene Ester Benzene Aldehyde Aromatic ether
S. Kumar, P. Ghosh / Renewable Energy 123 (2018) 475e485
483
Table 5 (continued ) (b) P. pedicellatum at 300 C S. No
R.T. (min)
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
P. pedicellatum at 400 C S. No
R.T. (min)
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.60 7.40 8.04 8.63 9.32 9.95 10.39 11.34 11.70 12.26 12.93 26.92 27.62 27.93 29.04 29.18 29.90 30.09 30.29 31.11
1.41 1.29 2.56 3.66 2.42 11.33 7.16 2.94 5.35 2.6 2.42 0.68 0.64 0.98 0.71 0.7 0.64 0.81 0.92 0.63
Tetrahydropyrrole-3-amino-2,5-dione 1-Hydroxy-2-propanone 1,3-Dioxolane, 2-(1-methylpropyl)2-Butenal 2-Propanone, 1-hydroxyMethyl isobutyl ketone 4-amino-2(1H)-pyrodinone 3-methyl-1,2-cyclopentanedione 2-methyl-2-pentenal 9-oxabicyclo[3.3.1]non-2-yl-acetate Phenol, 2,6-dimethoxy 4-Methylguaiacol Toluene, 3,4-dimethoxyEthyl isovalerate Retene 3-Methylcatechol 1,2,3,4-tetrahydrotriphenylene Benzaldehyde, 3-methyl4-Methylcatechol 1,2,3,4,5-Cyclopentanepentol
71 74 130 70 74 100 110 112 98 184 154 138 152 130 234 124 232 120 124 150
C11H20O2 C3H6O2 C7H14O2 C4H6O C3H6O2 C6H12O C5H6ON2 C6H8O2 C6H10O C10H16O3 C8H10O3 C8H10O2 C9H12O2 C7H14O2 C18H18 C7H8O2 C18H16 C8H8O C7H8O2 C5H10O5
Ester Carboxylate ester Ester Aldehyde Aldehyde Alkyl aldehyde Aromatics Furan Cyclic ketone Benzene Phenol Phenol Phenol Ester Benzene Phenol Benzene Acetone Phenol Others
(c) Composite bioenergy samples 300 C S. No
RT
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.61 1.72 1.89 6.23 7.23 7.93 8.01 8.15 8.24 8.45 8.72 8.84 9.41 9.46 9.59 10.03 10.56 11.39 25.77 31.03
2.21 0.69 1.58 0.6 1.66 0.63 0.76 0.84 0.6 1.01 2.08 0.74 1.78 0.69 0.76 7.98 3.19 0.73 0.63 0.87
Tetrahydropyrrole-3-amino-2,5-dione Methanol cis-1-Cyclopentene-3,4-diol Pyruvic acid methyl ester Furfural 2-Furanmethanol 1-Butanol 6-Hydroxy-2-hexanone 6-Methyl-3-oxatricyclo[3.2.1.02,4]octane-6-carboxamide 4,4-Dimethylcyclopentene 1,3-Dioxan-5-ol 3-Methyl-1,2-cyclopentanedione 2-Propanone, 1-hydroxyDisulfide, dimethy 1,2-diazaspiro(2.5)octane Methyl pyruvate Benzene, ethoxy Dodecane 2,4-Dimethylphenol Benzaldehyde, 3-methyl-
71 32 100 102 96 98 74 116 167 96 104 112 74 94 112 102 122 170 122 120
C11H20O2 CH4O C5H8O2 C4H6O3 C5H4O2 C5H6O2 C4H10O C6H12O2 C9H13NO2 C7H12 C4H8O3 C6H8O2 C3H6O2 C2H6S2 C6H12N2 C4H6O3 C8H10O C12H26 C8H10O C8H8O
Ester Alcohol Acetylacetonate Methyl ester Furan Furan Alcohol Ester Ester Cyclohexane Hydroxy fatty acid Furan Ester Others Others Carbonate ester Phenol Alkane Benzene Benzene
Composite bioenergy samples 400 C S. No
R.T. (min)
Area %
Name of compound
Mol. Wt.
Chemical formula
Type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.60 2.68 2.88 7.27 7.34 7.44 7.76 7.82 8.05 8.23 9.62 10.08 10.18 10.35 10.49 11.00 19.46 19.62 20.21 20.91
2.56 1.93 1.20 1.23 1.37 1.78 0.76 1.18 0.66 1.80 1.10 2.57 1.44 1.02 3.17 0.66 0.88 2.83 1.33 0.68
2-Pyrroline, 1,2-dimethylMethyl acetate 1,3,5-Cycloheptatriene 4,4-Dimethyl-2-oxazoline 1-Hydroxy-2-propanone 6-Hydroxy-2-hexanone Toluene 1-Butanol 2-Butenal Phenol Phenol, 2-methylPhenol, 4-methyl2-Heptanol Methyl pyruvate 2,3,5-Trimethylfuran 2,3-dimethyl cyclohexanol 2-Methyl-phenol 1,4-Dimethoxy-2-methylcyclohexane 2,5-Cyclooctadien-1-ol 1,2-dimethoxy-4-n-propyl benzene
97 74 92 99 74 116 92 74 70 94 108 108 116 102 110 128 108 158 124 164
C6H11N C3H6O2 C7H8 C5H9NO C3H6O2 C6H12O2 C7H8 C4H10O C4H6O C6H5OH C7H8O C7H8O C7H16O C4H6O3 C7H10O C8H16O C7H8O C9H18O2 C8H12O C11H16O
Others Ester Benzene Others Propanals Acid Benzene Alcohol Ether Phenol Furan Furan Alcohol Ester Furan Aldehyde Furan Ester Furan Phenol
484
S. Kumar, P. Ghosh / Renewable Energy 123 (2018) 475e485
(Fig. 4). In addition, the IR-spectra also proves the presence of volatile organic compounds with the well-distinguished absorption bands of carbonyl compounds (at 1733 cm1) including aldehyde, ketones, and acids. On the other hand, absorption band at 1560 cm1, 1480 cm1, and 1382 cm1 might be associated with CeOeC stretching vibrations in furans. Meanwhile the absorption peak at 1184 cm1 might result from stretching of the hydroxyl bond of phenol or alcohol structure. In addition, the absorption band between 952 and 676 cm1 indicates the mono, polycyclic, and substituted aromatic groups [35]. Moreover, the simultaneous evolution of different volatile compounds having similar chemical structure was not identified by FTIR spectra. Therefore, the detailed information about organic compounds of evolved gaseous composition formed during pyrolysis were thus obtained using TGA-GC/MS analysis.
biomasses. The gaseous products analysis using online TGA-FTIR hyphenated system, mainly at 300e400 C, which was consistent with the observation obtained from TGA-DTG analysis. The Py-GC/ MS analysis showed that the pyrolyzed products mainly consist of hydrocarbons such as carbonyl compounds of aldehydes and ketones followed by phenols, furans, and the other most prominent volatile organic decomposition products. Biomass feedstocks showed similar thermal events with distinct proportion, with exception of mass loss at different heating rates. The results obtained in the thermochemical characterization of the feedstocks were similar to the biomass currently used to generate energy and to obtain value-added products through pyrolysis, indicating that biomass species could be used as bioenergy fuel in a cost-effective and energy-efficient manner in future. Acknowledgment
3.4. 4 Pyrolysis- GC/MS analysis To investigate the formation of main pyrolyzed product components in biomass samples Py-GC/MS analysis was used during the thermogravimetric process (Suppl. Fig. 2). The GC/MS data were used to identify the mixture of volatile and non-volatile compounds in pyrolyzed gaseous products. However, the main chemical constituents were identified using the PerkinElmer NIST library and published data and, consequently, the highest likelihood of compounds were presented in Table 5. A total of 40 compounds were identified, and peaks of total ion count were characterized in Suppl. Fig. 2. The main compounds with the corresponding % area identified as carbonyl compounds [aldehydes (2, 3-dimethyl cyclohexanol, 0.66%; 4-Hydroxy-4-methylpentanoic acid, 1.21%; 2-Butenal, 3.66%), and ketones (acetone, 2.13%; 2, 3-Butanedione, 2.18%)], furans (2,3,5-Trimethylfuran, 3.17%; 3-methyl-1,2cyclopentanedione, 2.94%; 2-cyclopentene-1,4-dione, 1.01%; 2hydroxy-2-cyclopenten-1-one, 2.07%; furfural, 1.66%; 2Furanmethanol, 0.63%; phenol, 2-methyl-, 1.10%), and phenols (benzene, ethoxy, 3.19%; phenol, 2,6-dimethoxy, 2.42%; 2methoxy-4-vinyl-phenol, 0.99%; o-Cresol, 11.6%; 3methylcatechol, 0.70%; 4-Methylguaiacol and 1,2-dimethoxy-4-npropyl benzene, 0.68%) were most prominent products. The pyrolyzed products also contain many other aromatic derivatives (1ethyl-3-methyl-benzene and 1, 2, 4-trimethoxybenzene) and other trace products (methyl pyruvate and pyran derivatives). Toluene, a major variant of benzene type product was mainly generated from the decomposition of the intermediate of anisole. A larger amount of simple phenol and its derivatives especially cresol, which was product of OCH3 rearrangement were generated from pyrolysis of biomass samples. 4. Conclusions The present studies suggest selecting the new appropriate perennial grass species for bioenergy conversion with low cost input and high yielding phenomenon, as well as the thermochemical conversions to maximize efficiency of herbaceous materials. According to the property of biomass feedstocks, the lowest moisture and ash content and high volatile matter suggest biomass feedstocks (perennial grasses) as a potential candidate for the thermochemical conversion process and for energy generation due to higher H/C and/or O/C ratios in all the tested biomass species. The calorific value (15e17.8 MJ/kg) and proximate analysis confirm the biomass feedstocks are suitable for exploitation as bio crude/ biochemical fuel. The nitrogen and sulfur contents were relatively low, contributing to a reduction in pollutant gaseous emissions during decomposition. The hemicellulosic, cellulose and lignin contents of the tested biomass feedstocks were similar to the other
The financial support to the first author by the Science and Engineering Research Board, Ministry of Science and Technology, Government of India (Fast Track Scheme for Young Scientist, SERBDST, SR/FTP/ES-3/2014) is greatly acknowledged. The authors would like to thanks to the Chairman of Centre for Earth Science & Director of the Indian Institute of science, Bangalore, for supporting this work. The authors would like to thanks the authority of Environmental Division, Bharat Coking Coal Limited (BCCL), Dhanbad, India for granting permission to study and take samples from their reclaimed coalmine spoils dumps. The authors would like to thanks the Department of Material Engineering, Interdisciplinary Centre for Energy Research (ICER), and Department of Organic Chemistry, IISc Bangalore for providing the research facilities. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.renene.2018.02.054. References [1] S.G. Sahu, N. Chakraborty, P. Sarkar, Coalebiomass co-combustion: an overview, Renew. Sustain. Energy Rev. 39 (2014) 575e586. [2] Mendu, et al., Global bioenergy potential from high-lignin agricultural residue, Proc. Natl. Acad. Sci. U.S.A. 109 (10) (2012) 4014e4019, https://doi.org/ 10.1073/pnas.1112757109. [3] H.J. Huang, S. Ramaswamy, W. Al-Dajani, U. Tschirner, R.A. Cairncross, Effect of biomass species and plant size on cellulosic ethanol: a comparative process and economic analysis, Biomass Bioenergy 33 (2009) 234e246. pez-Gonza lez, M. Fernandez-Lopez, J.L. Valverde, L. Sanchez-Silva, [4] D. Lo Thermogravimetric-mass spectrometric analysis on combustion of lignocellulosic biomass, Bioresour. Technol. 143 (2013) 562e574. [5] S.S. Toor, H. Reddy, S. Deng, J. Hoffmann, D. Spangsmark, L.B. Madsen, et al., Hydrothermal liquefaction of Spirulina and Nannochloropsis salina under subcritical and supercritical water conditions, Bioresour. Technol. 131 (2013) 413e419, https://doi.org/10.1016/j.biortech.2012.12.144. [6] Sarkhot, et al., Total and available soil carbon fractions under the perennial grass Cynodon dactylon (L.) Pers and the bioenergy crop Arundo donax L, Biomass Bioenergy 41 (2012) 122e130. [7] S. Nanda, J. Mohammad, S.N. Reddy, J.A. Kozinski, A.K. Dalai, Pathways of lignocellulosic biomass conversion to renewable fuels, Biomass Conv. Biorefinery 4 (2014) 157e191. https://doi.org/10.1007/s13399-013-0097-z. [8] Jones, et al., Morphological and physiological traits for higher biomass production in perennial rhizomatous grasses grown on marginal land, GCB Bioenergy 7 (2015) 375e385, https://doi.org/10.1111/gcbb.12203. [9] Cai, et al., Land availability for biofuel production, Environ. Sci. Technol. 45 (1) (2011) 334e339, https://doi.org/10.1021/es103338e. [10] Vassilev, et al., Advantages and disadvantages of composition and properties of biomass in comparison with coal: an overview, Fuel 158 (2015) 330e350. https://doi.org/10.1016/j.fuel.2015.05.050. [11] J.E. Campbell, D.B. Lobell, R.C. Genova, C.B. Field, The global potential of bioenergy on abandoned agriculture lands, Environ. Sci. Technol. 42 (15) (2008) 5791e5794. [12] M.S. Li, Ecological restoration of mineland with particular reference to the metalliferous mine wasteland in China: a review of research and practice, Sci. Total Environ. 357 (2006) 38e53, https://doi.org/10.1016/
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