Thermo gravimetric and kinetic studies on dried solid waste of post-methanated distillery effluent under oxygen and nitrogen atmosphere

Bioresource Technology 174 (2014) 126–133

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Thermo gravimetric and kinetic studies on dried solid waste of postmethanated distillery effluent under oxygen and nitrogen atmosphere C. Naveen, M. Premalatha ⇑ Solar Energy Laboratory, Centre for Energy & Environmental Science and Technology (CEESAT), National Institute of Technology (NIT), Tiruchirappalli 620 015, Tamil Nadu, India

h i g h l i g h t s  Post-methanated distillery effluent (PME) solid waste was analysed using TGA.  Activation energy of combustion increased with the increase of mass conversion. 2

 Arrhenius kinetics fits the experimental data with R value greater than 0.80.  Order of reaction of thermal degradation is nearer to zero.  Solid waste almost completely degrades at isothermal condition in pyrolysis mode.

a r t i c l e

i n f o

Article history: Received 10 August 2014 Received in revised form 2 October 2014 Accepted 4 October 2014

Keywords: PME (Post-Methanation distillery Effluent) dried waste Pyrolysis Combustion Thermogravimetric analysis Kinetics

a b s t r a c t This work seeks for the possibility of using solid waste generated by drying the post-methanated distillery effluent, as fuel. TGA has been employed to analyse the kinetics of thermal degradation of the solid waste at different heating rates of 10, 20, 30, and 40 °C min1 in pyrolysis and combustion modes. In combustion mode, the activation energy changes from 253.58 to 87.91 kJ mol1, corresponding to the changes in heating rates of 10 °C min1 to 40 °C min1, whereas, there is no significant change of activation energy in pyrolysis mode. The Arrhenius equation based kinetic model with regression analysis using LINEST function is able to predict the kinetic variables of dried solid waste in both the modes. Solid waste almost completely degrades at the end of isothermal condition in pyrolysis mode. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Energy efficiency, environmental sustainability and water conservation are the focus areas required for sustainable growth of any Industrial sector. Sustainable growth of an industry could happen when the treatment scheme of industrial effluent leads to water recycling, energy recovery and by-product recovery from the waste while getting treated. Any Industry discharging large quantity of waste water with high organic loading rate should be analysed for economical methods of energy recovery from the waste water. Distilleries, is one such potential industry to be considered for energy recovery from effluent. Distilleries are one of the major industrial sector in Asia and South America are also one of the most intensive consumers of water as well as energy. In India,

⇑ Corresponding author. Tel.: +91 04312503132; fax: +91 04312500133. E-mail address: [email protected] (M. Premalatha). http://dx.doi.org/10.1016/j.biortech.2014.10.013 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

the distilleries are using sugarcane molasses for producing alcohol and its derivatives (Mohana et al., 2009). Molasses a gel like substance (a complex waste) is discharged from sugar industries, where sugar cane is processed through a number of chemical (purification) and thermal treatment (evaporation) steps to produce sugar. The unrecovered sugar molecules present in the molasses are in complicated form due to high temperature processes followed in sugar industry. Molasses having a 85–92 brix (specific gravity of 1.38–1.52) (Satyawali and Balakrishnan, 2008) is diluted to 20–25 brix (specific gravity of 1.08) in order to obtain a desired sugar concentration, suitable for fermentation. So large amount of water is used for diluting the molasses to adjust the sugar concentration fermentable by yeast (Mohana et al., 2009). The dilution of molasses ultimately leads to 8–15 litres of effluent generation for every one litre of alcohol produced (Saha et al., 2005). Currently in India 319 distilleries are producing 3.25 billion litres of alcohol and discharge approximately 40.4 billion litres of effluent per annum (Pant and Adholeya, 2007).

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The fermented wash containing 8–10% alcohol is sent to distillation column, where it gets concentrated to more than 95–99% alcohol. The bottom product of the distillation column is termed as spent wash (Mohana et al., 2009). The unutilised sugar available in the spent wash is in complicated form called melanoidin, being the major reason for its colour and odour (Sankaran et al., 2014). The formation of melanoidin is due to high temperature maintained in distillation column. Spent wash gets cooled by plate heat exchangers and is sent to anaerobic digester for biogas generation. Anaerobic treatment has several advantages such as high degree of solids breakdown, high organic loading rate and recovery of biogas, a renewable energy which will be useful for other distillery operations (Mallick et al., 2010). The effluent received after anaerobic treatment is called Post-Methanation distillery Effluent (PME). The PME still contains, total suspended solid of (22,000– 34,000 ppm), total dissolved solids of (35,000–45,000 ppm) and C.O.D of (25,000–40,000 ppm) (Sankaran et al., 2014), which indicates the possible high organic loading rate of the treated effluent. The industries are aiming at zero discharge, by recycling the water after required treatment process (Chauhan and Dikshit, 2012). Presently, distillery plants are using reverse osmosis (R.O) process or mechanical evaporator system after anaerobic digester for zero discharge. The problem with the reverse osmosis process is the high initial investment due to membrane cost, frequent membrane replacement and higher operational cost due to high energy requirement to overcome high pressure drop across the membrane (Sankaran et al., 2014). R.O reject will contains more complicated which requires further treatment process before discharging. In case of mechanical evaporator system, it recycles the condensed water, but it is also energy intensive scheme since solid content of the effluent has to be concentrated by evaporating from 5–7% to 60–70%. Theoretically to evaporate one litre of pure water 2441.8 kJ of energy is required at 25 °C. In practices, the energy required is still more, due to heat losses. But to evaporate the same one litre of distillery effluent, the energy required becomes still higher due to boiling point elevation due to presence of high salt content. If this energy could be met from solar energy, then this scheme of mechanical evaporator could be preferred over the reverse osmosis process. After anaerobic digester a series of parabolic trough collector could be used to provide the required heat to concentrate the PME up to 60–70%, and then it is discharged to solar open pond to dry further. Completely dried PME solid waste could be burn as a fuel in boiler for generating power. The ash collected after combustion is rich in inorganic salt content like potassium, magnesium, etc. which could be recovered by suitable chemical process. Potassium rich ash can be used as land fill (Satyawali and Balakrishnan, 2008). An industry producing 50,000 litres per day of alcohol will approximately discharge 7,00,000 litres of Post-Methanation distillery Effluent (PME) containing 65,000 ppm of solids per litre. Hence the potential of solid waste for such an industry is 45.5 tonnes per day. Since the major fraction of the dried solid waste is organic compound, it may be useful to seek whether this solid waste could be used as a fuel. This type of organic waste from industries is termed as second generation bio fuels (Huang et al., 2011). Dried powder of molasses spent wash can be used as a fuel by mixing with 20% of agricultural waste by burning in a boiler (Satyawali and Balakrishnan, 2008). A number of studies on thermal degradation kinetics of biomass have been reported, but very few studies on thermal degradation kinetics of industrial biomass wastes are available in literature (Li et al., 2013; El may et al., 2012). The most common thermo chemical processes are pyrolysis, gasification, and combustion. Pyrolysis is defined as the thermal degradation of carbonaceous materials in the absence of oxygen to produce volatile organic carbon, pyrolysed oil and solid residue

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rich in carbon. Gasification is defined as the thermal degradation of carbonaceous materials in the presence of oxygen less than the stoichiometric quantity and always precedes combustion process. The kinetics of these processes is of great importance in avoiding energy losses and reducing overall energy consumption. Thermal degradation analysis of a fuel is useful in designing an effective combustion system (Ledakowicz and Stolarek, 2002). Thermo gravimetric analysis (TG) is a simple, fast and high-precision method used for thermal degradation study under well defined conditions (Li et al., 2013; Sanchez et al., 2009). The present research work is probably the first attempt to focus on the thermal degradation kinetics of pyrolysis and combustion process at different condition for PME solid waste by using TGA technique. Hence thermo gravimetric analysis at different conditions was done to studies the thermal degradation behaviour and its kinetics of dried solid waste obtained from PME. Kinetic parameters are calculated by Arrhenius equation based model and compared with experimental results. 2. Methods 2.1. Sample preparation Fifteen litres of Post-Methanation distillery Effluent (PME) sample were collected from local distillery at different time periods over the day and mixed well. Collected samples were kept for sun drying for two days. Sun dried sample was grinded using mortar and pestle. The fine powder was allowed to pass through a 105 lm sieve to confirm the uniform particle size distribution suitable for TGA pan and kept in air tight bag. 2.2. Calorific value analysis The calorimeter system IKA/C 5000 control was used for the determination of the gross calorific value of the samples. The analysis was conducted on adiabatic mode. The calorimeter bomb, after the sample charge, was saturated with 30 bar of pure oxygen. The calorimeter was standardized by combustion of benzoic acid. Measurements were replicated thrice to avoid error due to sampling. Average results are reported with the standard deviation from the average value. The energy content of the sun dried distillery wastes was 2299.6 kcal/kg or 9.626 MJ/kg on dry basis, SD-0.6. This value is much lower than the calorific value of coal and higher than that of municipal waste done by Tan et al. (2014). 2.3. Proximate analysis The thermal degradation of the sample was performed using a Perkin Elmer TGA 4000 series. ASTM E1775 standard for biomass was used for proximate analysis to find ash content of the dried sample. According to this method, biomass gets completely converted into ash, by holding at 600 °C for 30 min in oxidising atmosphere. However it was noticed that, the residue obtained at 600 °C for PME solid waste under oxidation atmosphere was black in colour, indicating that complete degradation of the sample did not happen at 600 °C. So the sample requires a higher temperature for complete decompose. Hence ASTM procedure D 5142–09 for coal and coke was followed. According to this procedure the maximum temperature for complete degradation of the sample is 950 °C in oxygen atmosphere. Complete degradation was observed by holding the sample at 950 °C for 30 min in oxygen atmosphere. The percentage of inorganic salt concentration in PME waste ash was quantified indirectly by using inductively coupled plasma optical emission spectrometry (ICP-OES). For this analysis the PerkinElmer Optima 2000 DV was used. All measurements were

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replicated thrice to avoid error due to sampling. Average results are reported with the standard deviation from the average value. 2.4. Ultimate analysis The ultimate analysis to find elemental composition such as C, H, O, N and S contents of the samples was performed using a PerkinElmer 2400 Series II CHNS/O Elemental Analyzer. The Sample was weighed with a precision of 0.00001 mg (Perkin Elmer Auto balance AD 6000) in a tin foil cup. Sample range of 1–2 mg was taken for analysis. In presence of excess oxygen and combustion reagents, samples were combusted and reduced to elemental gases such as CO2, H2O and N2. All measurements were carried out thrice to avoid error due to sampling. Average results are reported with the standard deviation. 2.5. Thermal degradation analysis-TGA Influence of different heating rates on dried PME waste at pyrolysis and combustion modes were observed using thermo gravimetric analyzer (TGA) under inert (nitrogen) and oxidizing (oxygen) atmosphere respectively. Dried PME solid residue were subjected to thermal degradation studies using a Perkin Elmer TGA 4000 series. The thermo gravimetric analyzer has a sensitive microbalance of 0.1 lg resolution and accuracy of ±0.02%. Its furnace can attain a maximum temperature of about 1000 °C with a temperature precision of ±0.8 °C. The instrument was connected to a computer for data logging and Pyris software was used to analyse the data. In order to create a good contact between the crucible and the sample, approximately 20 ± 5 mg of representative samples were placed onto the alumina pan. Thermal conversion processes, pyrolysis (at nitrogen atmosphere) and combustion (at oxygen atmosphere) of the sample were analysed by conducting the experiments under non-isothermal condition for temperature range of 30 °C to 950 °C at four different heating rates of 10, 20, 30 and 40 °C per minute respectively. Each experimental run was followed with isothermal mode where, the sample was held at 950 °C for 30 min to confirm the further possibilities of thermal degradation. The purge gases used for this study were 99.99% ultra-high purity nitrogen and oxygen with a flow rate of 20 cm3/min. Each run was conducted twice to check the reproducibility of the results and the maximum variation in mass taken for analysis was within ±5%. Before performing the actual experiment a blank run was performed under the same conditions using empty alumina pan to eliminate the effect of system error on the experimental result. To ensure complete combustion turbulence, residence time, temperature range, could be increased. Turbulence cannot be increased for TGA experiments since the samples are analysed in a batch mode. Hence to study complete degradation residence time was increased to 30 min at 950 °C using TGA 4000. For higher temperature range Perkin Elmer TGA 7 having specification of higher temperature range (30 °C to 1300 °C) was compared. The TGA 7 analyzer has a sensitive microbalance of 0.1 lg resolution, accuracy is ±0.02%, and its furnace can attain a maximum temperature of about 1300 °C with a temperature precision of ±0.2 °C. In order to create a good contact between crucible and the sample, approximately 20 ± 5 mg of representative samples were weighed on the platinum pan. The instrument was connected to a computer for data logging and Pyris software was used to analyse the data. 2.6. Evaluation of kinetic parameters TG (thermogravimetric) and DTG (derivative thermogravimetric) curves of the dried PME waste were analysed for different heat-

ing rates at different atmosphere. The reaction kinetics were determined using the procedure proposed by Duvvuri et al. (1975) and applied by (Kumar et al., 2008; Mansaray and Ghaly, 1999). This is good enough for the purpose of observing the relative effect of heating rate and atmosphere on activation energy, order of reaction and frequency factor. Reaction constant k is affected by the temperature and this dependence may be represented by the Arrhenius decomposition equation:

k ¼ A  eEa=RT

ð1Þ

where Ea is activation energy in (kJ/mol) and A is the pre-exponential or frequency factor in (s1) are the Arrhenius parameters, R is the universal gas constant 8.3143 kJ/mol/K and T is the temperature in K. The Pre-exponential factor A is assumed to be independent of temperature. Global kinetics of the vitalization reaction can be written as (Jeguirim and Trouvé, 2009)

 n w  wf 1 dw ¼k w0  wf dt w0  wf

ð2Þ

where w is the weight of sample in (mg) at time t, wf is the weight of the residue in (mg), w0 is the initial weight of the sample in (mg), dw/dt the ratio of change in weight to change in time and n the order of reaction The combined form of the above two Eqs. (1) and (2) can be written in linearized form as:

ln



     w  wf 1 dw E ¼ lnðAÞ  þ n ln ; w0  wf dt RT w0  wf

ð3Þ

This Eq. (3) is resembled the simplified form of equation as

y ¼ B þ Cx þ Dz; where

 y ¼ ln

 1 dw ; w0  wf dt

B ¼ lnðAÞ;

C¼

x¼

  E  ; R

1 ; T

 z ¼ ln

w  wf w0  wf



D ¼ n:

TGA curves are analysed using PYRIS software data to find the temperature at which maximum weight loss occurred at different heating rates in both pyrolysis and combustion modes were found. Using this, the constants X, Y and Z were calculated. By regression analysis B, C and D were quantified and subsequently, A, E and n values were determined. Regression analysis was performed using LINEST function in Microsoft Excel.

3. Results and discussion 3.1. Proximate and ultimate analysis of the sample Proximate analysis results showed that the moisture, volatile matter, fixed carbon and ash contents of dried PME waste were 8.23%, 48.27%, 22.91% and 20.59% (Table 1), respectively. Ultimate analysis showed that PME wastes consisted of moderate amount of carbon content (33.84%). The ash content is 18.09% this was due to the presence of total inorganic salts as shown in Table 2. The repeatability of data was good with standard deviation of less than 5% was observed.

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C. Naveen, M. Premalatha / Bioresource Technology 174 (2014) 126–133 Table 1 Proximate and ultimate analyses of dried PME waste. Characteristics of dried distillery wastes

Content in (wt%)

SD

Proximate analysis

Moisture Volatile matter Fixed carbon Ash

8.23% 51.27% 21.91% 18.09%

0.72 3.80 2.54 3.29

Ultimate analysis

C H N S O

33.84% 4.67% 3.87% 1.84% 34.89%

3.09 0.53 0.62 0.20 3.45

Table 2 Percentage of inorganic salt concentration in ash of dried PME waste. S. No.

Constituents

Concentration range in ash (wt%)

SD

1. 2. 3. 4. 5. 6. 7. 8. 9.

SiO2 Fe2O3 Al2O3 CaO MgO SO3 Na2O K2O Remaining are Cl and other traceable

12.4 0.47 0.30 2.05 11.30 0.80 3.60 45.10 23.98

0.79 0.18 0.14 0.49 1.30 0.35 0.55 4.47 2.21

3.2. Effect of different heating rates on thermal degradation of dried PME waste at nitrogen atmosphere/pyrolysis mode TG and DTG curves of PME dried solid sample at four different heating rates of 10, 20, 30, 40 °C/min from 35 °C to 950 °C in nitrogen atmosphere are shown in Figs. 1 and 2. From the Figs. 1 and 2, we can observe that, three distinct stages of weight loss occurred at all the four different heating rates during thermal degradation. DTG curve of the sample (Fig. 2), was similar to the pyrolysis of fungal pretreated corn stover (Ma et al., 2013), and Spirulina wastes (Li et al., 2013) i.e.: The first stage is dehydration stage (35–150 °C) where moisture is completely removed from the sample, the second stage is primary or active pyrolysis stage (200– 450 °C) where maximum weight loss occurred in the process with a sharp weight loss peak, and the third stage is passive pyrolysis

stage (450–950 °C), where a small amount of complex compounds were further degraded because of higher temperature range. Thermal degradation of the sample started above 150 °C and was not complete even at 950 °C due to complex composition of the sample. From the DTG curve (Fig. 2), we can clearly observe the influence of different heating rates. The maximum rate of degradation at active pyrolysis stage are increased with increase in heating rate (1.26%/min, 2.68%/min, 4.10%/min, 5.44%/min 10, 20, 30 and 40 °C/min rate respectively) because at higher heating rates thermal energy obtained by the sample for a given time is more, which will facilitated better heat transfer between the surroundings and the samples (Li et al., 2013). In DTG curve (Fig. 2), it is clearly visible that there is a shift towards right in peak temperature of maximum mass loss with the increase in heating rate under nitrogen

Fig. 1. TG curves at different heating rates under nitrogen atmosphere.

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Fig. 2. DTG curves at different heating rates under nitrogen atmosphere.

atmosphere (291, 296, 303 and 309 °C at heating rates of 10, 20, 30 and 40 °C/min rate, respectively). Same trend of shift towards right in peak temperature was found in pyrolysis mode degradation of biomass (Ghaly and Mansaray, 1999; Ghaly and Ergudenler, 1991). The temperature range of peak weight loss along with corresponding percentage of weight loss for different heating rates is given in Table 4. It is observed from the Table 3 that increasing in heating rates didn’t have an appreciable influence on the final residual weight at pyrolysis mode at both non-isothermal and isothermal mode. When the holding time was increased from one minute to 30 min at 950 °C, at least 30% increase in weight loss was observed irrespective of any heating rates. Table 3 shows the difference in weight between non-isothermal mode and non-isothermal followed with isothermal (holding sample at 950 °C for 30 min). The increased weight loss at isothermal mode in N2 atmosphere indicates the removal of complex volatiles and the available O2 in the sample is oxidising the char. White final residue (ash) confirms the occurrence of oxidation in inert atmosphere. The results of increased residence time at 950 °C for 30 min and exposing the sample at higher temperature range (up to 1300 °C) are compared, Perkin Elmer TGA7 was used to heat the sample from 35 °C to 1300 °C under the same condition of 10 °C/min and nitrogen atmosphere. From the Fig. 3, it is understood that both conditions are having almost same pattern of degradation up to 950 °C and at the end both are having almost same amount of weight remaining (24% ± 4).

3.3. Effect of different heating rate on thermal degradation of dried PME waste at oxygen atmosphere/combustion mode TG and DTG curves of PME dried solid sample at four different heating rates of 10, 20, 30, 40 °C/min from 35 °C to 950 °C in oxygen atmosphere are shown in Figs. 4 and 5. From the Figs. 4 and 5, we can observe that, four distinct stages of weight loss occurred at all the four different heating rates during thermal degradation, as shown by DTG curve of the sample (Fig. 5), which was similar to the oxidation of biomass like Spirulina wastes (Li et al., 2013) i.e.: The first stage is dehydration stage (35–150 °C) where moisture is completely removed from the sample, the second stage is devolatilization stage (200–450 °C) where the second largest weight loss occurred in the process which is similar to almost active pyrolysis region of pyrolysis mode, and the third stage is active oxidation stage (450–700 °C), where maximum amount of weight loss occurred. Further their will be a passive oxidation stage where slow oxidation of complex compound occurred from (700– 950 °C) because of higher temperature range. From the DTG curve (Fig. 5), we can clearly observe the influence of different heating rates. The maximum rate of degradation at oxidation stage tends to increase at higher heating rate (10.221%/min, 10.638%/min, 11.194%/min, 14.039%/min 10, 20, 30 and 40 °C/min rate, respectively) because at higher heating rate, thermal energy obtained by the sample for a given time is more, which will facilitate better heat transfer between the surroundings and the samples (Li et al., 2013). In DTG curve (Fig. 5),

Table 3 Final residual weight of dried PME sample at different heating rates and atmosphere at the end of both Non-isothermal and isothermal mode. Degree

Residue weight (%) up to 950 °C (Non-isothermal) condition

Residue weight (%) after 30 min hold at 950 °C (Isothermal) condition

N2

O2

N2

O2

10 °C/min 20 °C/min 30 °C/min 40 °C/min

48.70 43.30 48.05 49.00

36.00 38.70 37.30 36.50

24.70 16.10 16.89 19.18

17.79 17.47 16.40 15.60

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Fig. 3. Comparison between the effect of holding the sample at 950 °C for 30 min and heating to a higher heating range (35 °C to 1300 °C) using TGA7 at same operating condition.

Fig. 4. TG curves at different heating rates under oxygen atmosphere.

it is clearly visible that there is a shift towards right in peak temperature of maximum mass loss with the increasing of heating rate under oxygen atmosphere (530, 539, 547 and 557 °C at heating rates of 10, 20, 30 and 40 °C/min rate, respectively). Same trend of shift towards right in peak temperature was found in combustion mode degradation of biomass (El may et al., 2012; Li et al., 2013). The temperature ranges where peak weight loss occurred at different heating rates and its percentage of weight loss shown in Table 4. It is observed from the Table 3 that increasing the heating rates did not have an appreciable influence on weight loss in combustion mode (oxygen atmosphere) at both non-isothermal and isothermal mode. Thermal degradation of the sample started at above 150 °C and was not complete even at 950 °C due to high complex composition of the sample which is similar to pyrolysis mode. Hence the sample needs to be heated to a higher temperature or to be held at 950 °C for a longer period in order to drive off all volatiles and carbon. When the sample was hold at 950 °C for 30 min there was a considerable amount of weight loss, indicating that the complex carbon structures got further combusted.

3.4. Comparing the effect of different heating rates and different atmospheres on thermal degradation of dried PME waste The first two stages of moisture removal zone and active pyrolysis zone in the inert and oxidation atmosphere resulted in the same percentage of weight loss. The final residual weight in combustion mode at non-isothermal condition was less when compared to the pyrolysis mode because peak weight loss occurred in oxidation stage of combustion mode, sample decomposed quickly at a lower temperature due to the presence of oxygen. Similar scenario was mentioned in degradation of Spirulina in oxidation atmosphere (Li et al., 2013). The differences in the rate of degradation at combustion mode were significantly higher in active oxidation zone for all heating rates when compare to pyrolysis mode. Similar scenario was mentioned in thermal analysis of waste biomass (Munir et al., 2009). The weight of final residue in combustion mode was 12% lesser than that was obtained in pyrolysis modes at non-isothermal conditions up to 950 °C which was due to the supply of oxygen. From the Table 3 we can observe that the weight of final residue after

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Fig. 5. DTG curves at different heating rates under oxygen atmosphere.

Table 4 Temperature range (°C) of maximum weight loss with corresponding weight loss (%) at various heating rates and atmospheres of dried PME sample. Atmosphere

Heating rate (°C/min)

Temperature range at max weight loss occurs (°C)

Percentage of weight loss (wt%)

Nitrogen

10 20 30 40

220–390 220–405 220–420 231–429

15.00 16.00 18.00 18.00

Oxygen

10

220–390 492–585 220–405 494–626 220–420 486–623 230–430 513–664

15.50 24.33 16.00 22.78 17.00 19.59 17.20 20.16

20 30 40

30 min hold at 950 °C in both pyrolysis and combustion mode resulted in almost same residual weight for the heating rate of 20, 30, 40 °C/min and the residual weight was 6% lesser at the heating rate of 10 °C/min. This is very low when compared to the amount of fixed carbon present in the dried sample. The final weight of residue obtained in the pyrolysis mode under isothermal conditions is the result of both the release of volatiles from the sample and a partial oxidation of the solid material due to oxygen released by the sample. Because of this in pyrolysis process we were getting very less char yield. This is also confirmed by the white colour of ash (final residue) obtained at the end of pyrolysis process. If isothermal condition or higher temperature furnace could be used then both pyrolysis and combustion are almost having same residual weight. If we use only non-isothermal condition then combustion mode is more efficient. 3.5. Kinetic analysis of pyrolysis and oxidation mode at different heating rate The kinetic parameters such as pre-exponential factor, activation energy, order of reaction were determined for maximum weight loss zones using Linear regression analysis using LINEST function in Microsoft Excel. Maximum weight loss regions are considered to determine the parameters of reaction kinetics. Only one stage of weight loss in pyrolysis mode and two stages of weight losses were considered in combustion mode for calculation. Kinetic parameters at different

heating rates and at different atmosphere are shown in Table 5. For maximum weight loss region of both pyrolysis and combustion mode at all heating rates, correlation coefficient (R2) values for the multiple-regression using LINEST function in Microsoft Excel were greater than 0.80. The similarity between kinetic method selected and experimental results are verified using P-value. Degrees of freedom is obtained from LINEST function and using Microsoft Excel by applying TDIST function P-Value was found. The significance of the model was confirmed if (P-value of <0.05). The inference based on the P-values obtained from the analysis indicates that the overall model is statistically significant. So the kinetic parameters obtained are more reliable to predict the peak weight loss at different conditions. This will be more useful to design a proper thermal conversion system. On pyrolysis mode, the heating rate did not have much appreciable effects on activation energy and frequency factor but in combustion mode when heating rates increased, then activation energy and frequency factor were reducing significantly. In pyrolysis mode the activation energy required was very low in different heating rates, in the range of 17–20 kJ mol1. In combustion mode, required activation energy at devolatilization zone is in the range of 23–40 kJ mol1. In combustion mode, required activation energy of active oxidation zone at different heating rates are much higher, in the range of 253.58–87.91 kJ mol1 at combustion zone. It is similar to the value obtained for organic fraction of municipal solid waste by (Sanchez et al., 2009) using Ozawa–Flynn–Wall method. Activation energy reduces with higher heating rates. In combustion mode,

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C. Naveen, M. Premalatha / Bioresource Technology 174 (2014) 126–133 Table 5 Kinetic parameters at various heating rates and at different atmosphere. Atmosphere

Heating rate (°C min1)

Temperature range at max weight loss occurs (°C)

Tpeak (°C)

N2

10 20 30 40

220–390 220–405 220–420 220–429

291.44 296.05 303.21 309.14

20.29 18.02 19.63 17.99

O2

10

200–342 492–563 200–355 494–578 236–373 486–612 236–377 486–627

288.02 527.83 289.76 538.76 294.38 547.44 299.51 556.86

23.17 253.58 25.13 185.72 23.69 121.29 40.41 87.91

20 30 40

when heating rate increases the required activation energy decreases shows that at higher heating rate the required energy to supplied by supporting fuel will get reduce to ignite the waste inside a boiler. The n value is almost nearer to zero in every condition, implying the thermal degradation follows the zero-order reaction. 4. Conclusion The kinetic characteristics of combustion have a considerable deviation from the inert atmosphere in non-isothermal condition. The activation energy of PME dried waste at various heating rates (10–40 °C/min) was almost the same in pyrolysis mode, ranging from 17 to 20 kJ mol1 and for combustion mode it varies from 253.58 to 87.91 kJ mol1. At isothermal mode residual weight was same irrespective of the atmosphere. The n value is almost nearer to zero at all heating rates in both modes. R2 values using LINEST function were greater than 0.80, the predicted kinetic variables using selected kinetic model is more reliable. References Chauhan, M.S., Dikshit, A.K., 2012. Indian Distillery Industry: Problems and Prospects of Decolourisation of Spent wash, in: International Conference on Future Environment and Energy. pp. 119–123 Duvvuri, M.S., Muhlenkamp, S.P., Iqbal, K.Z., Welker, J.R., 1975. The pyrolysis of natural fuels. J. Fire Flammability 6 (2), 468–477. El may, Y., Jeguirim, M., Dorge, S., Trouvé, G., Said, R., 2012. Study on the thermal behavior of different date palm residues: characterization and devolatilization kinetics under inert and oxidative atmospheres. Energy 44, 702–709. Ghaly, A.E., Ergudenler, A., 1991. Thermal degradation of cereal straws in air and nitrogen. J. Appl. Biochem. Biotechnol. 27, 111–126. Ghaly, A.E., Mansaray, K.G., 1999. Comparative study of the thermal degradation of rice husks in various atmospheres. Energy Resour. J. 21, 867–882.

E (kJ mol1)

A (min1)

n

R2

P value

4.426 4.878 9.064 9.074

0.154 0.148 0.182 0.164

0.921 0.929 0.933 0.883

0.0005 0.0056 0.0129 0.0436

9.874 2.293  1015 10.569 1.419  1011 37.665 10.824  106 40.416 8.193  104

0.085 0.163 0.076 0.166 0.167 0.223 0.160 0.140

0.974 0.935 0.968 0.905 0.910 0.870 0.936 0.829

0.0005 0.0001 0.0056 0.0032 0.0556 0.0096 0.0512 0.0197

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