Accepted Manuscript Comparison of slow and fast pyrolysis for converting biomass into fuel Saleh Al Arni PII:
S0960-1481(17)30376-2
DOI:
10.1016/j.renene.2017.04.060
Reference:
RENE 8755
To appear in:
Renewable Energy
Received Date: 22 December 2016 Revised Date:
5 April 2017
Accepted Date: 27 April 2017
Please cite this article as: Al Arni S, Comparison of slow and fast pyrolysis for converting biomass into fuel, Renewable Energy (2017), doi: 10.1016/j.renene.2017.04.060. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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1 Comparison of slow and fast pyrolysis for converting biomass into fuel
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Saleh Al Arni
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Chemical Engineering Department
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College of Engineering, University of Hai'l,
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Hai'l 81451
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Kingdom of Saudi Arabia
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Email:
[email protected]
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Abstract
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In this work, the conversion of sugarcane bagasse into fuel was studied as a low cost source material. The conversion was carried out experimentally in a batch pyrolysis reactor. Two pyrolysis methods were compared; namely, fast pyrolysis and slow or conventional pyrolysis. This comparison was based on the thermal decomposition of biomass into fuel and on the product yields. Since the yields are affected by the type of pyrolysis and the operating temperature of the reactor, the comparisons have been conducted at three fixed temperature values of 753, 853 and 953 K. The results revealed that the conventional pyrolysis produce more syngas yield with the increases of temperature. In the case of fast pyrolysis, it was observed that losses and solid yield increase with temperature increase. Moreover, it was found that the highest losses in both cases are less than 15% and that it was higher in conventional pyrolysis. Gases released during the thermal decomposition of biomass were identified as H2, CO, CO2, CH4 and some light molecular weight of hydrocarbons, such as C2H4 and C2H6. The low temperature was favored for the production of methane other than hydrogen for both processes, while high temperature was favored for the production of hydrogen. The produced H2 can be used in typical fuel cells.
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Keywords: Slow pyrolysis, Fast pyrolysis, Biomass pyrolysis, Sugarcane bagasse and agricultural waste.
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1. Introduction
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Nowadays, sugarcane bagasse is attracting an increased attention as it is a highly available material in the sugar industries and it has low cost. Consequently, this fibrous residue is considered as a raw material for biofuel [1-3]. Nevertheless, the majority of it can be combusted onsite for supply of energy; specifically for steam generation [4]. The use of biomass as a renewable energy source is very interesting because it offers a significant improvement towards being friendly with the environment. In fact, the use of biomass converting technology provides a solution to the pollution problem and creates new filed of jobs in the domain of innovative developments in agricultural waste utilization. Biomass conversion provides additional advantages, such as reducing the volume of biomass and making it more compacted which eases 1
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its transportation and storage, and save it from degradation if stored for long time. There are numerous studies reported in the literature that applied different techniques and processes to convert biomass into useful products [5]. In a previous study, Arni and Conveti, 2012, mentioned several methods that can be used to convert bagasse into energy products; these used thermochemical processes that include combustion, pyrolysis and gasification [6]. The pyrolysis is a technology for thermal treatment of biomass to recover a new material and energy. It is based on a thermochemical conversion of biomass without oxygen into the three material states; solid (called char), condensable liquid (some time called bio-oil or tar) and non-condensable (gas) [7, 8]. There are several types of processes of biomass pyrolysis; these include conventional or slow, vacuum, fast and flash pyrolysis. Any of these processes depends on the final target of products and the operating conditions and parameters of the reactor [9].
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Fast pyrolysis offers particular promising advantages in the conversion of biomass [10]. Generally, fast pyrolysis is employed to maximize the liquid product yield, while slow pyrolysis is employed to maximize the solid product yield [9].
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In this study, the main objective is to compare between the experimental results of slow pyrolysis and fast pyrolysis of bagasse aimed to convert biomass into syngas fuel.
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55 2. Materials and methods
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2.1. Sample
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The sugarcane bagasse used in this work was obtained from Brazil and supplied by Usina Guaranı´, Olı´mpia-SP. The analyzed Brazilian bagasse sample did not have a homogeneous structure, but it was a mixture of raw and ground sugarcane bagasse (Fig. 1).
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Bagasse is a waste produced when sugar is extracted from sugarcane, and, typically, in a moisture and ash free condition. It generally contains 33–48% cellulose, 19–43% hemicelluloses and 6– 32% lignin [11]. The chemical composition, the proximate analysis, and high heating value (HHV) of the Brazilian sugarcane bagasse used in this study are shown in Table 1. The biomass moisture has an average constant of about 8.5%. This might suffer minor changes from day to day due to changes in the atmospheric conditions. The effect of humidity on the biomass-heating rate was extensively studied in a previous work [8].
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Table 1. Composition deriving from proximate and ultimate analysis and higher heating value of Brazilian Sugarcane Bagasse.
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Ultimate analysis (wt.% on dry basis) H N
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O difference 47.43
Proximate analysis (wt.% on dry at room temperature basis) Fixed Volatile Ash Moisture carbon matter 1.64 84.00 5.86 8.5
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Heating value HHV MJ/kg 18.17
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Figure 1. Brazilian sugarcane bagasse
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2.2. Experimental apparatus and method
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The experiments on the pyrolysis of biomass material were performed using a laboratory-scale apparatus, whose photograph and scheme are reported in Fig. 2. The reactor was a Hastelloy-type horizontal cylindrical device of steel (8 of Fig. 2B). The device is provided with a heating system that is consisted of a ceramic furnace (11), an electrical power source (10), with a heating rate of about 1 oC/s, and a basket containing the biomass sample (7). The temperature of the furnace appeared on the integrated display and it could be regulated to reach the set-point. The maximum allowable temperature value is 1400 oC. The effective temperature of the biomass was measured using a K-type thermocouple (6), inserted into the biomass and its value appeared on a display (9). The accessories connected to the reactor were a flowmeter (3), for the regulation of the carrier gas (Helium), two gas meters, Gallus 1000 type from Schlumberger, fitted on the head (mount) (4) of the reactor and the end (valley) of the line (17), respectively, to evaluate the pyrolysis products, and four glass condensers (13) to collect the condensable material. The condensers, two with a capacity equal to 250 ml and two equal to 125 ml, were immersed in a aqueous solution (12) of 20% ethylene glycol, with a constant temperature around 0 oC during the run. An adsorbent tower (drying tower) (14), and a filter (15), for cleaning up the produced gas, were fitted after the condensers. The drying tower and the filter were both filled with silica– alumina wool and active carbon. Rubber pipes with an adjustable diameter were used to connect the on-line elements. The gaseous components generated by the biomass pass from the pyrolysis reactor to the condensers, then through a Teflon tube to the drying tower and to the filter and finally to the analysis section, before reaching the atmosphere through a chimney.
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During the experiment start-up, an inert gas (He) was used to purge the pyrolysis reactor; this flow was maintained for about 30 minutes to remove the air from every part of the plant before the test run. The inert gas flow value was kept at about 1 l/min for the entire batch run. The sample used in each run was 15 g of bagasse, it kept in the basket in the cooling zone of the reactor before and after the run. The pyrolysis began when the sample introduced to the center of the furnace after the temperature of furnace reached to the desired pyrolysis temperature. The effective temperature of the sample was measured by the thermocouple in the centre of the basket which was connected to a display. Gas produced was monitored on-line (16) by gas chromatography and recorded on a computer through a data acquisition system. At the end of every experiment, the heating was turned off and the basket moved back to the cooling zone and, once room temperature was reached, the carbon residues (char) remaining in the basket, and the
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condensed liquid (tar) in the collected apparatus were weighed. Later, the solid and liquid fractions were analysed.
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Figure. 2. A. A photograph of the laboratory equipment used, B. Scheme of the laboratory plant
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for the biomass batch pyrolysis. 1: Helium source; 2: gas line; 3. area meter; 4 & 17: gas meter; 5:
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cooling zone; 6: thermocouple; 7: biomass basket; 8: reactor; 9: temperature display; 10: power
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line; 11: furnace; 12: water solution tank; 13: condensers; 14: drying tower; 15: filter; 16: on-line
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zone for gas sampling; 18: gas exit.
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3. Results and discussion
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The conditions and experimental procedures, such as the variation of temperature, residence time, moisture content and particle size of biomass, have their effects on the yield of products. The effect of moisture content of biomass related to the heating rate was studied in a previous work [8]. It was noted that the moisture content caused a decrease in the quantity of syngas produced, while the effect of particle size of biomass not studied because it does not make part of the scope of this work. In this study, the conditions and procedures for the conventional and fast pyrolysis are rest same. Only the residence time and heating rate have been changed. For each experimental run, we observed losses. The volatile products condensed inside of connected tubes explain the losses, but the losses for both processes were less than 15%, while it was higher in the conventional pyrolysis which could be attributed to the long residence time.
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In the following paragraphs, we discuss in details these effects.
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3.1. Effect of temperature on the bagasse pyrolysis
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a) Conventional pyrolysis
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The effective temperature on biomass was measured by thermocouple, type K, welded into the biomass. The following graph, Fig. 3, shows the performance of the temperature inside the reactor in case of conventional pyrolysis, that obtained at the temperature of 853 K and rate 45-50 o C/min with residence time of about one hour. In this experiment, the conventional pyrolysis also known as slow pyrolysis or carbonization, it has been done as slow heating with a rate of 4550°C/min and a long residence time of about 60 minutes. Several experiments for conventional pyrolysis were performed in a temperature range between 663 to 1253 K; for each set temperature, two tests have been done then their average value has been calculated. The longer residence time and slow heating rate cause reducing in the liquid yield and increasing the gas production; these results are conformed with Bridgwater research [12].
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Figure 3. Trend of temperature inside the reactor at 853 K
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Table 2 shows the results of the slow pyrolysis process of sugarcane bagasse. The fraction yields of experimental runs in function with variation of reactor temperature that obtained on dry basis. The results indicates that an increase of temperature will lead to an increase in the syngas fraction; the high syngas fraction yield was 41.33% obtained at 1143 K.
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Products (%) Char Tar Gas Losses
663 33.67 27.11 26.43 12.79
753 37.64 26.11 25.1 11.15
803 35.67 24.13 26.20 14.00
Slow Pyrolysis Temperature (K) 853 873 893 953 33.33 32.33 31.67 27.67 22.67 26.67 29.33 21.67 30.50 29.40 26.30 35.67 13.50 11.60 12.70 14.98
1143 27.33 20.33 41.33 11.00
1203 31.30 19.20 36.10 13.40
1253 30.67 19.33 37.50 12.50
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Table 2. Fraction yield of experimental runs that obtained from the slow pyrolysis of bagasse (wt.% on dry basis) in function of variation of temperature.
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In the case of fast pyrolysis, the temperature reached to the set point more quickly (Fig. 4) in a time of 5 to 7 minutes, with a rate of 120-127 oC/min, and a residence time of about twenty minutes. The short time of residence and the high rate of temperature are favored the production of fluid fraction (Tar), Table 4. The results indicated that increasing temperature will decrease the tar fraction; the high tar fraction yield was 50.89% that has been obtained at 753 K.
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Figure 4. Trend of temperature inside the reactor at 653 K
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Table 3. Experimental results of fraction yields varying with temperature that obtained from the fast pyrolysis of bagasse (wt.% on dry Basis) 6
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Char Tar Gas Losses
47.13 50.89 11.33 14.12 13.21 9.65
42.81 39.77 38.11 15.46 17.94 21.32 11.87 12.76 13.31
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Fast Pyrolysis Temperature (K) 653 753 853 953 1053 28.33 25.34 29.86 29.53 27.26
170 171 3.2 Yield production of syngas
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The optimal decomposition of bagasse depends on the target of the product desired; in the aim of the present study is the investigation of the gasses products. We have selected three set points of temperature (753, 853 and 953 K) for comparison of syngas produced from the decomposition of sugarcane bagasse. The non condensable gas products were CH4, CO, CO2, H2, and some hydrocarbons such as C2H2, C2H4, C2H6 and C3H8. It can be observed that an increase in temperature leads to an increase in the gas yield. The results indicate that the maximum syngas obtained from the sugarcane biomass was about 41% at 1143 K by conventional pyrolysis. The comparison of the two results of slow and fast pyrolysis are presented in Fig. 5. The comparison shows an increase in the fraction of syngas of bagasse (wt. % on dry basis) with the increase of temperature for both pyrolysis processes. On the other hand, there was an increase in this fraction from 9, 15 to 18% in the case of fast pyrolysis, with rise of temperature from 753, 853 to 953 K, respectively. On the other hand, the increase in the slow pyrolysis was from 25, 30 to 35% at the same rising of temperature. The effect of the long residence time makes advantage of the gaseous products that they are higher.
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Comparison between slow and fast pyrolysis: Fraction of syngas of bagasse (wt.% on dry Basis)
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Slow 753 K
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Fig. 5 Comparison between slow and fast pyrolysis for fraction of syngas.
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193 Based on the results obtained, the syngas fraction produced in fast pyrolysis is less than that produced in conventional pyrolysis at same temperature; this indicates that the conventional pyrolysis favors the decomposition of bagasse toward syngas fraction.
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Table 4 shows the comparison between slow and fast pyrolysis for syngas produced from sugarcane bagasse (Vol%). The results shows that the sugarcane bagasse had some production of hydrocarbons gaseous in both pyrolysis processes, while the majority of syngas produced monoxide and dioxide of carbon that are in good agreement with those obtained from other researcher [13, 14]. The fast pyrolysis had the majority of syngas produced carbon dioxide and the maximum value is about 58 Vol% at 853K, while conventional pyrolysis had the majority of syngas produced carbon monoxide and the maximum value about 60 Vol% at 753K.
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Table 4. Comparison between slow and fast pyrolysis for syngas produced from sugarcane bagasse (Vol%) 753 K
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CH4 H2 CO2 CO C2H4 + C2H6 C3H8
21.5 8.7 52.4 13.9 2.2 1.4
17.6 9.6 11.7 60.1 0.6 0.4
6.9 15.2 58.1 18.7 0.7 0.4
31.1 21.3 20.7 25.1 1.3 0.5
17.0 45.3 14.4 20.5 1.7 1.1
7.2 28.8 23.9 37.7 0.9 1.6
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Fig 6 is a chart illustrates the yield of Hydrogen and methane. In general, hydrogen yield increases with the rise of temperature for both processes; it is agreed with other researchers [15, 16]. The maximum value is obtained at 953K (about 45 Vol%) by fast pyrolysis, while methane yield produced depends on the temperature and the process. The low temperature favors the production of methane other than hydrogen for both processes, while high temperature favors the production of hydrogen.
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Syngas (Vol% )
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853 K CH4
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Fig. 6 Comparison between slow and fast pyrolysis for H2 and CH4.
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Comparison between slow and fast pyrolysis. Yield production of H2 and CH4 (Vol%)
217 218 4. Conclusions
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The batch experimental of sugarcane bagasse was carried out for both conventional and fast pyrolysis. The heating rate of conventional pyrolysis was about 45-50 oC/min while it was 120127 oC/min for fast pyrolysis, and the residence times were about one hour and twenty minutes, respectively. The comparison of syngas produced was carried out for three set points of temperature; 753, 853, and 953 K. The yields of gas produced by fast pyrolysis were lower than of these produced by conventional pyrolysis. The gases identified during the thermal decomposition of biomass were H2, CO, CO2, CH4 and some low molecular weight hydrocarbons such as C2H4 and C2H6. The quality of syngas produced depends on the process type and the operating parameters of the reactor. It is observed that an increase of temperature leads to, in the conventional pyrolysis, an increase in the syngas yield produced, while in the fast pyrolysis it has an increase in the solid yield produced, also an increase of losses. However, the losses for both processes are less than 15% and it is higher in the case of conventional pyrolysis. The reason of these losses is due to the volatile products that are condensed inside the connected tubes. Their presence in higher concentrations in the case of conventional pyrolysis could provide the explanation for the longer residence time. The hydrogen yield increases with the rise of temperature for both processes; the maximum value was obtained at 953K (about 45 Vol%) by fast pyrolysis, while methane yield produced depends on the temperature and the process type. The maximum value of methane was obtained at 853K (about 30 Vol%) by conventional pyrolysis. The low temperature favored the production of methane other than hydrogen for both processes, while high temperature favored the production of hydrogen. Experimental work on conventional and fast pyrolysis requires additional investigations related to the effect of particle size on the products and the quantity and quality of yield products. Furthermore, the research could be extended to include other biomass such as date palm fiber.
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