Fast pyrolysis of agricultural wastes: Characterization of pyrolysis products

Fuel Processing Technology 88 (2007) 942 – 947 www.elsevier.com/locate/fuproc

Fast pyrolysis of agricultural wastes: Characterization of pyrolysis products Jale Yanik ⁎, Christoph Kornmayer, Mehmet Saglam, Mithat Yüksel Institute for Technical Chemistry, Division of Chemical–Physical Processing Forschungszentrum Karlsruhe, Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen, Germany Received 7 March 2007; received in revised form 3 May 2007; accepted 7 May 2007

Abstract This study deals with pyrolysis of three agricultural wastes (corncob, straw and oreganum stalks) at 500 °C in a fluidized bed reactor. The yields of char, liquid and gas were quantified. Pyrolysis liquids produced were in two separate phases; aqueous phase and oil. Oil yields varied between 35 and 41%, depending on biomass type, whereas the yields of aqueous phases were almost similar, around 6%, for all feedstock. For characterization, oils were fractionated by water extraction into two fractions; water solubles and water unsolubles. Both aqueous phase and watersoluble fraction were analyzed by gas chromatography–mass spectrometry and high-performance liquid chromatography. In addition, water content and elemental analysis of the oils were determined. Chemical compositions of gas and char products relevant to fuel applications were determined. © 2007 Elsevier B.V. All rights reserved. Keywords: Biomass; Pyrolysis; Liquefaction

1. Introduction Because of the global climate changes, environmental pollution and reduction of availability of fossil energy resources, renewable energy is of growing importance. Today, biomass is considered as a renewable resource with high potential for energy production. Biomass can be converted to various forms of energy through numerous thermochemical conversion processes, depending upon the type of energy desired. Among the thermochemical processes, pyrolysis is a promising tool for providing bio-oil that can be used as fuel or chemical feedstock. The pyrolysis of biomass is a very old energy technology that is becoming interesting again among various systems for the energetic utilisation of biomass. Depending on the operating conditions, the pyrolysis process can be divided into two classes: conventional pyrolysis and fast or flash pyrolysis. Conventional pyrolysis is a known technology for producing charcoal (mainly) and chemicals such as methanol and acetic acid for hundred years. The main ⁎ Corresponding author. Permanent address: Department of Chemistry, Faculty of Science, Ege University, 35100 Izmir, Turkey. E-mail address: [email protected] (J. Yanik). 0378-3820/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2007.05.002

goal of fast pyrolysis is to convert the biomass into a liquid. In practice, about 40–75% of the biomass (on dry basis) is converted into pyrolytic oil. About 10–20% of the biomass is converted into char. Fast pyrolysis offers a flexible and attractive way of converting biomass into a liquid, which can be easily stored, transported and handled for the production of heat, power and chemicals. Principles and practice of biomass fast pyrolysis and applications for the liquid product were reviewed by Bridgwater [1]. Bio-oil has a number of undesirable characteristics as fuel, such as thermally unstable components leading to gum formation, low heating value due to the water content and highly oxygenated compounds, a corrosive organic acid component, and phase instability with a tendency towards phase separation [1,2]. The aim of the present study is to determine the pyrolysis behaviour and yields of products from fast pyrolysis of three agricultural wastes (corncob, wheat straw and oreganum stalk). Corncob and straw are the widely planted in the world. Oregano is an aromatic and medical plant and grows wild in the Mediterranean. Turkey is one of the world's largest suppliers of oregano. Although, activated carbon manufacture from corncob was studied by many researchers, the gas and liquid products from pyrolysis of corncob were rarely investigated in the

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literature. As far as we know, pyrolysis of oreganum stalks is the first time to be investigated. 2. Materials and methods 2.1. Materials In this study, three agricultural wastes (straw, corncob and oreganum stalk) were used. The biomasses were crushed to a particle size less than 1 mm. The composition of the biomasses are given in Table 1.

Fig. 1. Fractionation of oil.

2.2. Experimental setup

2.3. Characterization of oils

The pyrolysis experiments were conducted in a bench scale continuous fluidized bed reactor (ID: 40 mm, length 300 mm). Details about the reactor were presented in Ref. [3]. The material bed was 450 μm silica sand with a static bed depth of 37 mm. The feeder system consisted of a hopper and a screw feeder. Nitrogen was divided into two streams, one to the reactor as a fluidizing gas and the other (b10%) to the feeder to entrain the biomass. Gas velocities were determined by preliminary experiments and were between 0.25 and 0.3 m/s at operating temperatures. The residence time of gas in reactor had values around 1–2 s. In each experiment, after the system reached steady state at 500 °C, the screw conveyor was switched on and biomass of 100 g was uniformly fed to the reactor. The volatile products and coke left the reactor to a cyclone where most of the coke was separated and collected in a charpot under the cyclone. Fluidized bed reactor, cyclone and charpot were all contained within an electrically heated furnace. From the cyclone, the gases were passed through a cool water heat exchanger at 14 °C, where some liquid products along with some cokes were condensed. The aqueous phase and oily phase were condensed using an icebath. Two electro filters were used to make the droplets of heavier components coalesce from the stream. The rest of the flow from the electro filters was passed through a filter packed with cotton wool, which effectively trapped any remaining aerosols. Following the filter, the exhaust gas was vented after gas samples were taken using a bypass. The experiments lasted from 45 to 55 min. All system components were weighed separately before and after each run to determine the total weight of coke and pyrolytic oil. The amount of gas was determined by difference. Aqueous phase and oily compounds were collected in traps and weighed. Heat exchanger, electro filters and cotton wool were first washed out with CH2Cl2 and then with methanol. CH2Cl2 and methanol solutions were separately collected. The CH2Cl2 was evaporated under vacuum at 20 °C and CH2Cl2 solubles were added into the oily compounds received from the traps. The methanol solution was filtered, evaporated under vacuum at 50 °C and methanol solubles were weighed as “high molecular weight lignin” (HMWL). The weights of coke collected in the charpot and methanol unsolubles are given as “total coke amount”. The “oil” yield from biomass pyrolysis was defined as the sum of the oil phase received in traps and recovered by CH2CL2 and CH3OH washing.

For characterization, oil was extracted with water (1:10 w/w). Water insolubles were dried under vacuum at 60 °C, residue was weighed as water insoluble. The amount of water solubles was determined by difference. Fractionation of pyrolytic oil can be represented as in Fig. 1.

Table 1 Properties of biomass Corncob

Oreganum stalk

Proximate analysis (wt.%) Moisture 6.3 9.0 Ash 2.1 4.0 Ultimate analysis (dry, wt %) C 42.90 42.50 H 6.40 6.00 N 0.60 0.70 S 0.29 0.29 Cl 0.59 1.15 Composition of lignocellulosic material (wt.%) Cellulose 31.7 33.8 Lignin 31.7 10.9 Hemicellulose 3.4 9.3

Straw 7.2 4.1 43.89 6.54 0.42 0.51 1.88 34.5 12.2 14.2

2.4. Analysis Gas analysis was carried out by GC with two columns as well as thermal conductivity and flame ionization detectors were connected in series. The chemical analysis of water-soluble fraction of oils and water phase collected in trap was performed by GC/MS series 6890. Gas chromatography was performed using a 60 m × 0.25 mm capillary column (chrompack DB 1701) with 0.25 μm film. The injector temperature and detector temperature were kept at 250 °C and 280 °C, respectively. The oven program was 4 min isothermal at 45 °C, then 3 °C/min to 280 °C and finally 15 min at 280 °C. The compounds were quantified using the internal standard method with fluoranthene as internal standard. For all the detected constituents, relative standard deviation (%RSD) was varied from 1.8 to 5.6%. The amounts of some volatile compounds in water-soluble fraction and water phase were measured by an ion chromatograph equipped with a highperformance liquid chromatograph pump and an L-4250 UV–vis detector. Relative standard deviation values were found to be less than 0.2%. The amounts of total phenol are determined colorimetrically (reaction of phenols with 4-nitroaniline to a yellow complex) with the photometer Cadras 200 by Lange-Hach. The RSD values of the method was reported to be less than 5%. Elemental analysis of chars and oil were performed at the Engler-Bunte-Institut, Karlsruhe. The water content of oil was analyzed by Karl-Fischer titration according to ASTM D 1774 (0.02–0.86% RSD).

3. Results and discussion 3.1. Pyrolysis yields The biomass pyrolysis behaviour is influenced crosswise by many parameters, such as temperature, particle size, heating rate, feed rate and biomass species. Temperature plays a major role in biomass pyrolysis. In this study, the pyrolysis of biomass in the fluidized bed reactor was carried out at 500 °C. The pyrolysis temperature was chosen on a literature basis. Many researchers studied biomass pyrolysis in both fluidized bed reactor and fixed-bed reactor. Mostly, they focused on the temperature effect. Their studies showed that a maximum for bio-oil yield was found in a temperature range from 450– 550 °C [4–11]. At higher temperatures, secondary reactions of volatiles led to a significant lower bio-oil yield. Table 2 shows the char, liquid (aqueous fraction + oil) and gas yields resulting from the fast pyrolysis of biomass. As seen from Table 2, pyrolysis yields changed not significantly with the biomass species, especially in the case of oreganum stalk and

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Table 2 Product distributions from pyrolysis of agricultural wastes, wt.% Feed

Oreganum stalk

Corncob

Straw

Gasa Aqueous phase Oil Char

32 6 ± 0.3 39 ± 3.1 23 ± 1.9

30 6 ± 1.3 41 ± 0.9 23 ± 1.5

39 6 ± 0.5 35 ± 1.3 20 ± 0.4

a

Calculated from mass balance.

corncob. The highest amount of gas was obtained from straw. Although, the general idea is that char formation increases with the lignin content [12], the amount of char from straw was not higher than that of other biomasses. The reason may be linked to high heating rates [13]. The oil yields seem to be reasonable, considering other fast pyrolysis studies. It was reported that the yields of bio-oil were ranging between 13 and 17 wt.% for fast pyrolysis of rapeseed [6] and lower than 14 wt.% for rice straw, sugarcane bagasse and coconut shell [4]. On the other hand, in another study, fast pyrolysis of rice straw produced the oil (containing 43 wt.% water) with a yield of around 57% [14]. Nokkosmaki et al. obtained bio-oil (containing 20 wt.% water) with a yield of 66 wt.% from pyrolysis of pine sawdust [15]. Biomass pyrolysis liquids are either a homogeneous onephase oil containing large amounts of water or a heterogeneous fluid separated into an aqueous and an oily phase. In this study, the aqueous phase could be received separately from the oil. It is noted that, the amount of aqueous fraction was smaller than the amount of moisture in the biomasses, except corncob. This is reasonable, because the some part of water both originated from a dehydration reaction of organic compounds and free water in the biomass can be dispersed into oil. This can be clearly seen from Table 5. 3.2. Characterization of pyrolytic oil The chemical composition of liquids (pyrolytic oil) derived from lignocellulosic materials is complicated and differs considerably from that of petroleum based fuels. Because of the large number of oxygen containing reactive functional groups, pyrolytic oil does not exhibit thermal stability and cannot be effectively fractionated by conventional techniques such as distillation. It is known that biomass-based pyrolysis oils consist of mainly water, carboxylic acids, carbohydrates and lignin-derived substances [16]. In the fast pyrolysis processes, short vapor residence times lead to incomplete depolymerisation of the lignin resulting in a less homogenous liquid product. Secondary reactions are minimized [1]. Due to this reason, the characterization procedure in this study was carried out as based on the fractionation of pyrolytic oil into two fraction, namely water solubles and water insolubles which developed by Sipila et al. [16]. Sipila et al. demonstrated that the water-soluble fraction generally contained water (product water + feed water), volatile acids, alcohols and sugars. In the present study, the yellowcolored water-soluble fractions comprised a considerable part of

the pyrolytic oils. The corncob gave the highest water soluble (68%), probably due to the high content of carbohydrates (cellulose/hemicelluloses), whereas the amount of water solubles of pyrolytic oils from straw and oreganum stalk was around 53%. The water-insoluble fraction of pyrolytic oils mainly consists of lignin-derived materials [15,16] and is called pyrolytic lignin [17]. It was found that the yields of pyrolytic lignin were about 22–28% of the oil from wood pyrolysis depending on the nature of the biomass [18]. In this study, the water-insoluble fractions obtained were heavy oil-like, sticky materials. As seen from Fig. 2, these fractions consist of extractives, low molecular weight lignin and high molecular weight lignin compounds. Extractives contained hexane-soluble compounds, consisting mainly of hydrocarbons. In some of the studies relating to pyrolysis of lignocellulosic materials in a fixed-bed reactor, hexane solubles in oil were separated into three fractions: aliphatic, aromatic and polar fractions by column chromatography [19–21]. The results showed that the polar fraction was dominant. Because of their low amounts, a more detailed investigation of extractives has not been conducted in this work. In this study, water-soluble fractions of oil and aqueous phase were analyzed by means of several systems. The GC/MS characterization showed about 8–20% of the total organics in water solubles and aqueous phases. Although, 49 species of the organic products in all water-soluble fractions have been identified, the number of species in aqueous phase varied with the biomass type, changing between 20 and 50. Identified compounds were listed as groups and the concentrations of each group, as wt.% are given in Table 3. Aqueous phase and watersoluble fraction were donated as AP and WS. The presented compounds in Table 3 are, as expected, degradation products of lignin and polysaccharides. As seen from Table 3, acetic acid and nonaromatic ketones were found to be the largest amount. Nonaromatic ketones consisted principally of hydroxypropanone, butanone derivates, saturated and unsaturated five membered cyclic ketones. In the case of corncob and straw, the water-soluble fraction contained

Fig. 2. Composition of oil obtained from pyrolysis of agricultural wastes.

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Table 3 The compounds detected by GC/MS and their concentration, wt.% Corncob

Acids Acetic acid Propanoic acid Nonaromatic aldehydes Hydroxyacetaldehyde Propanal, 3-hydroxy Nonaromatic ketones Furans Furaldehyde, 2Furfuryl alcohol, 2Furan, 2-acetyl Furaldehyde, 5-methylButyrolactone, γ(5H)Furan-2-one (5H)Furan-2-one, 5-methylPyrans Sugars α-d-Glucopyranose, 1,4:3,6-dianhydro Arabinofuranose, 1,5-anhydro Catechols Hydroquinone; benzene, 1,4-dihydroxyBenzene, dihydroxy-methylLignin derived phenols Phenol Cresol, oCresol, pCresol, mGuaiacols/methoxy phenols Guaiacol, phenol, 2-methoxyGuaiacol, 4-methylGuaiacol, 4-ethylGuaiacol, 4-vinylVanilin Guaiacyl acetone Syringols/dimethoy phenols Syringol; phenol,2–6-dimethoxySyringol, 4-methylSyringol, 4-ethylSyringol, 4-allylSyringyl acetone

Oreganum

Straw

AP

WS

AP

WS

AP

WS

2.931 0.202

5.099 0.444

4.065 0.314

2.560 0.324

2.236 0.114

2.599 0.296

1.942 0.419 2.460 0.132

2.233 0.5001 6.896 1.445

0.821 0.287 5.535 0.665

0.202 7.367 1.303

4.627 0.219 1.892 0.027

4.408 0.588 5.119 1.254

nd 0.297

0.039 2.276

nd 0.682

nd 0.471

0.004 0.147

0.053 1.503

nd

0.212

0.211

0.279

nd

0.123

0.017

0.439

0.096

0.295

nd

0.186

0.009

0.304

0.206

0.560

nd

0.340

nd

0.163

0.236

0.542

nd

0.401

more amounts of acids, sugars, catechols and phenols than the water phase. However, for oreganum stalk, the amount of acetic acid in the water phase was more than that of water-soluble fraction. Some of the low molecular-mass compounds such as acetone, formaldehyde, formic acid and methanol, could not be detected because the MS detector gives no response for species eluated after 5 min. Therefore, the concentrations of these compounds were detected by HPLC-UV–vis detector. In addition, the amount of total phenol was determined colorimetrically. Results are given in Table 4. From Tables 3 and 4, it is concluded that, water phase from pyrolysis contained mostly low molecular weight degradation products. Overall, by considering the yields of aqueous phase and oil from pyrolysis, we can mention that most of the oxygenated organic compounds formed during pyrolysis were accumulated in oil.

3.3. Fuel characteristics of pyrolysis products As known, pyrolysis of carboneous materials produces useful products; gas, oil and solid char which may be used as fuel or a feedstock for petrochemicals and other applications. Generally, the pyrolytic gas can be used as a make up heat

Table 4 The concentration of some compounds detected by HPLC and photometer Corncob

Acetone, v/v % Formic acid, wt.% Formaldehyde Methanol, v/v% Total phenols, wt.%

Oreganum

Straw

AP

WS

AP

WS

AP

WS

7.8 0.34 3.15 2.04 0.18

5.0 1.22 1.22 1.70 0.66

2.4 0.03 n.d. 3.05 nil

1.0 0.15 n.d. 1.30 12.54

3.3 0.46 2.52 2.49 0.08

14.70 1.78 6.21 1.29 13.5

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source for the process in the case of pyrolysis of carbon rich materials, such as coal, waste plastics, tire, etc. Pyrolysis of lignocellulosic materials produces a gas rich in carbon oxides due to the high oxygen content of feed material. In this study, carbon oxides (mainly CO2) were dominant about 84–90 v/v% in the pyrolysis gas products. The methane amount were about 6–8 v/v% where as C2–C4 gases were minor. Hydrogen could not be detected due to the strong dilution with fluidizing gas (N2). The composition of gas products did not vary a lot with the kind of biomass. These results were in well agreement with the previous works on biomass fast pyrolysis [22–24]. For example, in the pyrolysis of rice straw in a fluidized bed, the hydrogen content was b1 wt.% and CH4 content was b 10 wt.% [22]. CO2 is a product of the primary pyrolysis of cellulose and hemicelluloses [25] where CH4 and CO are mainly formed from secondary cracking of volatiles, followed by a reduction of CO2 (C + CO2 = 2⁎CO) [26]. It was observed that high temperature (N 500 °C) led to a high proportion of CO and CH4 while a lower temperature led to a high proportion of CO2 [24]. Due to the high proportion of carbon oxides the obtained pyrolysis gases in this study had a low calorific value. Anyway, they can provide some part of the energy requirements of the pyrolysis plant. The characteristic of chars obtained from pyrolysis of biomass is depended on the pyrolysis conditions such as temperature and heating rate as well as the composition of the biomass. Both the hydrogen and oxygen contents of char decrease as the temperature is increased (P10, P10–15) accompanying with the loss of the hydroxyl and carbonyl groups [27]. The heating value of the char is also increased with temperature [28]. The high oxygen content in the char is due to the high heating rate and the moderate pyrolysis temperature. The ash content, calorific value and elemental composition of chars are given in Table 5. The chars from pyrolysis of corncob, oreganum stalk and straw were donated as CC, OC and SC, respectively. The calorific value of chars was calculated from the following equation [29]. H0 ¼ 0:3491C þ 1:1783H þ 0:10005S  0:1034O  0:0151N  0:0211AS where, H0 is the gross calorific value, MJ kg− 1 and C, H, O, N, S and AS are the weight percentage of the carbon, hydrogen, oxygen, nitrogen, sulphur and ash content of the char. The difference in elemental composition of chars from three biomasses is linked to the differences in nature and structure of

Table 5 Properties of chars from pyrolysis of biomasses

Ash, wt.% GCV, kcal kg− 1 Proximate analysis, wt.% C H N S

CC

OC

SC

10.06 25

14.91 24

38.37 19

74.25 2.91 0.78 0.74

64.81 5.00 2.04 0.55

62.77 2.81 0.75 0.62

Table 6 Properties of oil from pyrolysis of biomasses

Water content, wt.% C H N S Oa

CO

OO

SO

14.32 45.01 8.48 1.10 0.15 45.26

6.18 53.70 5.78 2.64 0.44 37.57

4.68 48.34 6.16 1.25 0.27 43.99

a

From difference.

the biomass. Due to the higher content of oxygen, the char obtained from straw had a lower calorific value than those from oreganum stalk and corncob. All chars obtained are appropriate for household briquette production due to the low sulphur content. They can be also burned in a steam boiler with an appropriate emission control. Also, chars are one of the raw materials for activated carbon production. Their hydrophilic surface due to the presence of more-oxygen containing groups is suitable for removing of ions and other pollutants from water [30]. Pyrolysis oils were reddish brown in color with irritable odor, as all pyrolysis liquids obtained from biomasses [4,15]. The moisture content and elemental composition of the oils are given in Table 6. The oils obtained from corncob, oreganum stalk and straw were donated as CO, OO and SO, respectively. It is clear that the oils should be upgraded to receive an improved oil composition for the direct utilisation as a fuel. The studies related to upgrading of bio-oil has been reviewed by Huber et al. [2]. 4. Conclusion In this study, fast pyrolysis of three biomass species (corncob, oreganum stalks and straw) was carried out at 500 °C. The gas products obtained with a yield around 30–40% were composed of mostly carbon oxides. Although, the higher heating value of the gas in this study is quite low, pyrolysis gas, in general, can be burned to provide process heat. The aqueous phase and oil phase were separately obtained as pyrolysis liquids. The oil yield from biomass species ranged from 35 to 41%. The lowest oil yield was obtained from straw. The oils were fractionated as water-soluble and water-unsoluble fractions. The oil containing 68% of water-soluble fraction was obtained from corncob, whereas the fraction of water soluble was around 52% in the case of both straw and oreganum stalk. GC–MS and HPLC analyses showed that carboxylic acids, mainly acetic acid, nonaromatic ketones, mainly acetone, methanol and phenols are the main organic compounds of the water-soluble fraction. These compounds were also identified in the aqueous phase. Based on the above results, oil can constitute one important source of speciality chemicals. Although, pyrolysis yields did not significantly change with the type of biomass, the composition of oil varied depending on the feedstock. Overall, oil was found to be highly oxygenated. Therefore, it was concluded that the oil obtained from biomasses needs to be upgraded for use as a fuel. Since the

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