Pyrolysis of Olive Stone for Energy Purposes

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 82 (2015) 374 – 380

ATI 2015 - 70th Conference of the ATI Engineering Association

Pyrolysis of olive stone for energy purposes P. Bartoccia*, M. D’Amicob, N. Moriconib, G. Bidinia, F. Fantozzia a

Department of Engineering, University of Perugia, Via G. Duranti, 06125 Perugia, Italy b Biomass Research Centre, University of Perugia

Abstract Pyrolysis of biomass is a promising technology for the production of distributed and renewable energy on small and micro-scale since it produces a gas with relatively high calorific value, which can be burned in an internal combustion engine or in a microturbine; pyrolysis also generates byproducts (char and tar) which can be used to provide energy to the process or for cogeneration purposes. This research is aimed at the exploitation of waste from agricultural production processes, in particular olive mill wastes whose management has critical environmental and disposal costs; the yields of pyrogas, tar and char obtained from the pyrolysis of olive stone in a batch reactor was measured. Pyrogas produced is sampled through a line for the sampling of condensable substances in accordance with existing regulations, CEN/TS 15439, and once purified from water vapor and tars is analyzed with micro-GC. The data collected is used to perform mass and energy balances and to determine the content of tars and the Low Heating Value (LHV) of the gas produced.

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-reviewofunder responsibility of ATI Peer-review under responsibility the Scientific Committee of ATI 2015

Keywords: Pyrolysis, Olive, Batch, Pyrogas, Wastes

1. Introduction The management of by-products of the vegetal oil industry is a typical problem of Mediterranean countries where annually more than 30 million m3 of waste from mills are produced both as liquid (waste waters) and solid (dried pomace and olive stone). Olive oil extraction is currently carried out either in two or three phases. With the three phases method a large part of polyphenols is washed out because of the higher quantity of added water, therefore, a large quantity of wastewater that needs to be processed is * Corresponding author. Tel.: +39 0755853773. E-mail address: [email protected] (P. Bartocci).

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Scientific Committee of ATI 2015 doi:10.1016/j.egypro.2015.11.808

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produced. With a two phases process the olive paste is separated into oil and wet pomace and the water is expelled with the pomace. With both processes olive stone is present in the pomace which shows different humidity content. Olive stone can be recovered mechanically from pomace through centrifugation. Olive pomace and waste waters are two waste products from the olive oil industry and considering their world production [1], the potentiality of their reuse for energy purposes is evident. Wastewater produced by olive mills are characterized by high pollution load due to the presence of organic complexes not easily biodegradable without the adoption of appropriate technologies [2]. The energy recovery of residual biomass from the oil industry [3-5] can be carried out by anaerobic digestion for biogas production [6-9], and several studies are focused on the preparation of activated carbons from olive waste products [10,11]. While wastewater and pomace may be used in anaerobic digestion plants, residual olive stone is an interesting fuel for households stoves or can be used for CHP applications on small scale through thermochemical processes, such as pyrolysis or gasification. Both processes produce a gas characterized by a relatively high calorific value, usable in internal combustion engines or micro-turbines, and byproducts, such as char and tar, that can be used to support the process or for co-generation purposes [1216]. This research activity is focused on testing the potentiality of energy valorization of olive stone through pyrolysis; tests were carried out in a batch reactor, 5 liter capacity. In this work the composition of the produced gas and mass and energy balances are measured. 2. Materials and methods 2.1. Feedstock In order to perform mass and energy balances and characterize energetically, physically and chemically the olive stone and the products of pyrolysis, TGA and CHN analysis were carried out. The results are shown in Table 1. The olive stone is characterized by high content of volatiles, carbon and oxygen. The sample of olive stone used in the experimental test is shown in Table 1. The olive stone comes from both two and three phases method. Table 1. TGA, CHN e high calorific value of olive stone and wet pomace TGA

Olive stone

Wet pomace

Moisture [% wb] Ash [% db] Volatiles [% db] Fixed Carbon [% db] CHN Carbon [% wb] Hydrogen [% wb] Nitrogen [% wb] Oxygen [% wb] HHVwb [MJ/kg]

4.53 0.49 87.06 12.45

49.02 0.84 42.35 7.79

50.00 6.17 0.42 43.41 19.21

55.54 7.98 1.98 34.5 5.7

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The essential difference between the two methods is the type of decanter that is used. The study of Roig et al. (2006) [17] shows that from a three phase extraction system about 1-1.6 OMW m3/t of olives can be obtained (where OMW indicates Olive Mill Wastes), assuming that the stone content in the olive is about 18%w, through separation we achieve about 180 kg of olive stone per ton of worked olives. The two phase system instead produces about 800 kg of olive cake and 0.2 m3 of waste water per ton of worked olive. 2.2. Batch pyrolysis plant In the Literature there are several laboratory reactors of batch pyrolysis. The main types are represented by steel reactors heated with electric heaters, electric muffle ovens, in which the sample is inserted through ceramic crucibles, or thermogravimetric microbalances [18-21]. The pyrolysis batch reactor at the University of Perugia is a cylindrical reactor of 15 cm internal diameter and height equal to 30 cm. The layout of the laboratory plant is shown in [22,23] and in figure 1. Main operating parameters of the reactor are shown in table 2. A volumetric flow meter measures the amount of pyrogas produced during the experimental test and sampled by CEN / TS 15439 [24], according to the current regulations. At the end of the test the isopropanol content in the impinger bottles was evaporated to determine gravimetric tar by rotary evaporator Strike 300. The remaining char inside the reactor was weighed and subjected to TGA, CHN and calorimetric analysis and mass and energy balance. Gravimetric tar was performed according to the current regulations CEN/TS 15439. The pyrogas from olive stone pyrolysis was collected in Tedlar bags and subsequently analyzed by Micro-GC Agilent 490-PRO to determine the energy content. The operating temperature, inside the reactor, was set to 600 °C.

Fig. 1. Batch plant layout Table 2: Main operating parameters of the reactor Parameter

Quantity

Pyrogas avegare flow Biomass flow Working pressure Temperature inside the reactor Electrical heaters power Reactor volume

1 l/minute 200 gr/hr atmospheric 600°C 10 kWe 5l

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3. Results of experimentation After heating the reactor for about 1 hour and a half, the temperature inside the reactor remained constant and equal to about 600°C. Then biomass was fed to the reactor, 100 gr of biomass were used and the test lasted 30 minutes. Operating temperature was chosen based on previous TGA tests on different biomass materials [23, 25, 26], reaching 80% of volatiles extraction at 600°C. A higher temperature would produce a higher volatile extraction but would require too much energy to promote the process. The maximum pressure achieved was 0.2 bar. The percentage yields in mass of char, tar and syngas obtained at the end of the experimental test are shown in Figure 2 and are respectively 17%, 25% and 44%. Tar that was deposited along the exiting pipes was evaluated weighing the pipes before and after the test. In Figure 2 water is not considered. The high yield of gas compared to other studies in the Literature [27] can be attributed to different conditions adopted for the tests; while having the same reference value for the temperature the experimental test of this work was characterized by a higher heating rate due to the introduction of the olive stone sample finally grinded in a batch reactor at a temperature of 600 °C.

Fig. 2. Mass yields and energy yields from olive stone pyrolysis Table 3. Gas composition and LHV. Chemical specie (% vol)

LHV [MJ/kg]

H2 29,47

O2 0.80

N2 3.65

±1.85 SD

±0.35 SD

±0.77 SD

CO2 14.10

CO 31.48

CH4 20.50

±0.59 SD

±1.16 SD

±0.29 SD

16.03 ±0.6

Fig. 3. Drechsel bottles, flammability test and sample of char

Table 3 shows the composition of the produced gas and its relative energy content as average of 4 samples carried out. The high yield of methane, carbon monoxide and hydrogen give the gas an energy content of particular interest. Table 4 shows the TGA, CHN and calorimetric analysis of the produced char. The presence of a residual percentage of volatiles in the char, equal to approximately 12%, shows

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that the pyrolysis reactions at the set temperature of 600 °C was not complete; the energy content of the produced gas is therefore potentially improvable. Table 5 shows the mass balance of the experimental test. Figure 3 shows a detail of the tar deposited inside the Drechsel bottles, the flammability test and the char produced. Table 4. TGA, CHN e high calorific value of produced char TGA Moisture [% wb] Ash [% db] Volatiles [% db] Fixed Carbon [% db] CHN Carbon [% wb] Hydrogen [% wb] Nitrogen [% wb] Oxygen [% wb] HHVwb [MJ/kg]

2.25 11.80 11.45 76.75 83.39 1.51 0.54 14.56 29.85

Table 5. Mass balance. Product Char Tar Pyrogas Water Gravimetric tar

Weight (g) 17.00 24.83 44.17 14.00 7.00

4. Conclusions The batch pyrolysis reactor designed and built at the University of Perugia was employed for an experimental test on olive stone as a residual of the oil production process. The yields of pyrolysis products and their relative energy content were measured. The high content of volatiles allowed to obtain a gas yield of more than 40% by weight and the significant presence of methane, carbon monoxide and hydrogen gave to the produced pyrogas an interesting low calorific value equal to 16 MJ/kg. The high yield of hydrogen (29.47%) suggests a possible coupling to a fuel cell. Future developments will consider further tests of pyrolysis at different reference temperature to verify the possibility of use of the produced gas in an internal combustion engine. 5. Copyright Authors keep full copyright over papers published in Energy Procedia Acknowledgements The authors gratefully acknowledge Mr Giacomo Iraci for providing the olive stone and the EU contribution through Umbria Region, in the framework of the Rural Development Plan of Umbria 2007/2013, Ass 1, Measure 1.2.4, to the project BIOL for the "Enhancement of wastes from mills and livestock manure through the development of an innovative lay-out for their use in the production of agro energy" .

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Biography Dr. Bartocci is a researcher of the University of Perugia, Engineering Department. He is a PhD in Energetics. He is involved in biomass cultivation and biomass resources assessment and bioenergy production through pyrolysis.