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Wine industry waste thermal processing for derived fuel properties improvement
Renewable Energy 57 (2013) 645e652
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Wine industry waste thermal processing for derived fuel properties improvement Cosmin Marculescu*, Simona Ciuta Department of Power Engineering, University Politehnica of Bucharest, 313 Splaiul Independentei, 6, Bucharest 060042, Romania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2012 Accepted 25 February 2013 Available online 28 March 2013
The paper presents the results of pyrolysis process applied as pre-treatment sequence for grape marc within energy conversion chain. Experimental campaign for proximate, ultimate and calorimetric characterization of the product was conducted. The product equivalent chemical formula was established e C4H7O3. The atmospheric pressure pyrolysis with external heat source at temperatures between 350 C e600 C was studied with respect to process parameters influence on end-products formation, distribution and physicalechemical properties. Based on experiments the product activation energy of 111.5 kJ/mol was determined. The process parameters influence on end-products energy content, pyrolysis energy consumption and global energy efficiency was established. The research focused on maximum net energy content delivered in pyrolysis products as high quality fuel for energy conversion processes. The study revealed that pyrolysis gas alone can ensure the process self-sustenance for the applied temperature range. The optimum treatment temperature with respect to global energy balance was found at 550 C. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Pyrolysis Biomass Kinetics Activation energy Energy balance
1. Introduction Biomass is considered one of the most important renewable sources of energy of the future. In the last few years advanced technologies of biomass into fuel conversion or efficient combustion processes have evolved. Agricultural activities generate large amounts of biomass residues. While most crop residues are left in the field to reduce erosion and recycle nutrients back into the soil, some could be used to produce energy without harming the soil. One of the most representative crops which can be cultivated in extensive surfaces worldwide is represented by vine. Wine making industry generates huge amounts of wastes every year, for each kilogram of grape which is pressed into wine, more than 20% is residue [1]. Grapes are grown commercially in over 90 countries worldwide on almost 7.6 million hectares. In Europe, average yields are 1.4 and 1.7 tons/ha and in the USA yields are twice the world average [2]. The total world production of grapes in 2009 is estimated to be about 68.9 million tons, next only to citrus and bananas and is followed by apples [3]. The main wine making industry wastes are represented by grape pomace (marc) resulted directly from grape pressing. * Corresponding author. Tel.: þ40 745133713; fax: þ40 214029675. E-mail addresses: [email protected] (C. Marculescu), ciuta_simona@ yahoo.com (S. Ciuta). 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.02.028
The components of pomace in wine-making differ on whether white or red wine is being produced, but generally contains skins, pulp, seeds, and stems of the fruit. Grape pomace has traditionally been used to produce pomace brandy (such as grappa) and grape seed oil. Today, it is mostly used as fodder or fertilizer. Over the world grape marc is treated in different manners: recovery of grape seeds for food consumption, in industry for paints and varnishes, in perfumery, in the pharmaceutical industry or in the manufacture of soap, as feed or fertilizer and in countries like Canada, Italy or France for energy recovery. The least exploited possibility for grape marc treating is energy recovery. This approach into energy conversion is presented in this paper, mostly because pomace became almost completely disposed of in the field, and represents an expense and an environmental problem for distilleries. Worldwide, the most extensively used technology for biomass power production is direct combustion followed by a steam cycle [4]. Indeed, this is expected to continue being the dominant technology in the short to medium term. Another technology with great future potential is biomass gasification followed by a combined cycle. The main problem in energy valorization of the product is represented by the seasonality of grapes, which are mainly available for 2e3 months per year. A solution for continuous operated power plants could be combustion, preceded by product pre-treatment for the decrease of high moisture content and fast degradation properties. The stabilization of the product, throughout pyrolysis process, can solve the
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C. Marculescu, S. Ciuta / Renewable Energy 57 (2013) 645e652
Abbreviations cbp cch p ctp cvp Ea Qi Qp Qchar Qtar Qgas DQ DQR
biomass specific heat (kJ kg1 K1) char specific heat (kJ kg1 K1) tar specific heat (kJ kg1 K1) volatiles specific heat (kJ kg1 K1) activation energy (kJ kg1) raw product energy content based on its LHV (kJ) pyrolysis process specific heat demand (kJ) pyrolysis char energy content based on its LHV (kJ) pyrolysis tar energy content based on its LHV (kJ) pyrolysis gas energy content based on its LHV (kJ) losses in pyrolysis process (kJ) losses in the batch reactor (kJ)
temporary availability of the pomace and storage by transforming it into high carbon content derived fuels. Pyrolysis products uses are diverse. The bio-oil, composed of aliphatic and aromatic hydrocarbons, can be used in electricity generation as a substitute fuel. However, several properties of the bio-oils such as high water content or poor ignition ability have to be ameliorated or existing equipment modified to meet competitive standards [5]. Charcoal, a carbon-rich solid residue, can be upgraded to activated carbon for use in chemical, pharmaceutical and food industries, or in syngas production in the gasification process. The dry pyrolysis gas mixture contains the main components: CO2, CO, CH4, H2 and C2 hydrocarbons [6] and can be used for heat production and power generation, but is often used to sustain the pyrolysis process in a biomass waste pyrolysis plant or to dry the feedstock. A specific pyrolysis installation will aim at the optimal production (yield, composition and properties) of one or more of the products. The aim of this paper is to determine the optimum pyrolysis process parameters, to maximize the derived fuels energy potential in terms of global energy balance. The sample mass decreasing rate during the process run is determined to establish the product behavior under non-oxidant conditions at atmospheric pressure with respect to process kinetics. Therefore, the main purpose is to ensure that the pyrolysis process is self sufficient, from an energy standpoint. From a sustainability point of view, it is intended to avoid the use of fossil fuels. As from economical point of view decentralized pretreatment establishments, placed near the biomass source bring many advantages in storage, handling and lower transport costs. The pyrolysis gases can be used to provide the energy required for the endothermic pyrolysis process. 2. Materials and methods
carbon mass (kg) char mass (kg) initial biomass mass (kg) tar mass (kg) volatile mass (kg) water mass (kg) initial temperature (K) water vaporization temperature (K) devolatilization starting temperature (K) process temperature (K) tar latent heat of vaporization (kJ kg1) water latent heat of vaporization (kJ kg1) low heating value (kJ kg1) high heating value (kJ kg1)
mc mch m0 mt mv mw T0 Tw Tv Tp
lt lw
LHV HHV
the end of the tube in a container. After water separation, the remaining liquid phase and the solid residues (char) were weighted at the end of the process. The batch reactor’s other collection tube permits sampling of the gas fraction in real time through a sonde connected to a gas analyzer. Schematic diagram of the main elements of fixed bed furnace is shown in the figure below. Working temperature range is between 20 C and 1300 C. Pyrolysis device is equipped with a control pad that allows temperature programming, working time (residence time at process temperature) and heating rate. Samples to be subjected to thermalechemical treatment processes such as pyrolysis/combustion are introduced into the furnace in a tubular parallelepiped crucible of refractory steel W4541, size: 100 cm long, 4 cm wide and 3 cm in height. To ensure an inert atmosphere in the oven during the pyrolysis process, nitrogen was blasted, its flow being measured using a flow meter. 2.2. Products and sampling The material used in this study is represented by grape marc provided by a Romanian wine producer, directly from the industrial processing line, after grape pressing. The moisture content determined for the material as arrived was approximately 60%. Different analyzes were performed on the dried product: proximate analysis, elemental analysis, high and low heating value determination. For the determination of volatile content the temperature was set at 800 C and the crucible with material previously graded, was subjected to a pyrolysis process for 40 min (nitrogen used for the inert atmosphere). The samples obtained in the first stage were subjected to combustion process (950 C for 15 min e until the complete combustion of the sample) in order to establish the total
2.1. Reactor The experiments for pyrolysis treatment process were carried out on a Nabertherm tubular fix bed reactor model RO 60/750/13 (Fig. 1) modified according to experiment configuration requirements. It consists of a refractory steel tube, exterior electrically heated and with an interior diameter of 60 mm. The samples are introduced into the refractory steel reactor by a tubular parallelepiped crucible of refractory steel with an active heating area of 750 mm. The reactor is equipped with two injection inlets for different experimental conditions and a gaseliquid fraction separation unit for discharges resulted from treatments applied to solid masses according to Fig. 1 legend. The collecting tube for the liquid reaction products allows the entire fraction to condense, being collected at
5 4
3
6
2 7
1
9
Active area, isothermal
8
Fig. 1. Tubular batch reactor: 1.Nitrogen injection; 2.Air injection; 3.Refractory steel crucible; 4.Refractory steel tube; 5.Electrically heated chamber; 6. Exhaust gases; 7.Thermocouple; 8.Liquid reaction products (tar); 9.Sample.
C. Marculescu, S. Ciuta / Renewable Energy 57 (2013) 645e652
content of the combustible fraction, respectively the one of inert fraction (non-combustible). Results are presented in Tables 1 and 2. High heating value was determined using the calorimeter IKA C200, the sample being subjected to combustion, in an oxidative environment (pure oxygen at 30 bars). The chemical composition of the material was established using an elemental analyzer, with a sample weight which varied around 1 mg. The EA 3000 Series analyzer uses the principle of dynamic flash combustion followed by gas chromatography separation of the resultant gaseous species (N2, CO2, H2O, and SO2) and Thermal Conductivity Detection (TCD). It can be concluded that the characteristics of these wastes are similar to different types of wood biomass when we refer in particular to volatile content and elemental composition. Regarding the high heating value its value is much closer to that of various conifers. As any type of biomass, grape marc consists of the main biopolymers: cellulose, hemicelluloses and lignin which influence strongly the pyrolysis process kinetics. The relatively high sulfur content of the material can be explained by treatment methods applied to vine that help keeping the grape berries not to alter for a longer period before being harvested. 3. Results and discussions 3.1. Process run With respect to waste to energy conversion chain, the high amount of energy required by the devolatilization process for product stabilization leads to a series of experimental based criteria necessary in global energy balance determination. In industrial applications (continuous flow) for the energy consumption quantification the main process parameters are the residence time and temperature. Laboratory specific parameters such as: nitrogen flow (for inert environment conditions) and heating rate are not to be considered at industrial scale in energy sector. During the devolatilization process the pyrolysis gases create the non-oxidant conditions and the nitrogen gas injection is not required. Even if the product ignition may occur at process start e up, the absence of air inhibits this phenomenon at maximum 1 min depending on reactor type. In industrial applications the heating rate, an important process parameter that influences the properties and yields of pyrolysis products, is conditioned by process temperature and product feed e in temperature. A higher heating rate increases the yield of liquid products resulting maximum oil yields, mainly because of a better heat and mass transfer [7]. The process temperature and pressure are parameters that influence char’s physicalechemical properties. In energy sector usually the thermalechemical processes are conducted at atmospheric pressure (or slightly lower). Previous works in the field revealed that at temperatures above 650 C most of pyrolysis products components are represented by gas. The quality of the char is also influenced by process temperature with respect to specific surface that increases with direct influence on the next treatment stage process run especially if gasification is applied [8]. For in-depth process analysis and product behavior, the range of treatment temperature varied between 350 C and 600 C, being performed six experiments on dried grape marc. Above 600 C the non-oxidizing process does not present a solution because air
647
Table 2 Elemental composition of dried grape marc. C [%]
H [%]
O [%]
N [%]
S [%]
Ash [%]
43.21
5.94
45.50
0.65
1.24
3.46
gasification can be applied. If two stage treatment is used for waste to energy conversion, the chain conversion efficiency should be determined for each stage separately and optimized globally. In terms of process kinetics, for example, a minimum pyrolysis temperature will deliver a char with compact structure (non porous) that will require a higher gasification temperature or longer treatment period, with increased energy consumption, due to lower specific surface and consequently lower constant rate of reaction (according to Arrhenius equation) [9]. The pyrolysis process was conducted in the tubular batch reactor described in detail above, the sample being exposed to a specific temperature in the preheated furnace, hence subjected to a flash pyrolysis. The mass of grape marc sample was 50 g imposed by crucible size and product specific mass. The conditions encountered in an industrial process are different from experimental size units, because heat and mass transport mechanisms will strongly influence the composition and distribution of process end-products [10]. After the inert conditions were created inside the reactor using nitrogen and the sample was introduced, a continuous flow of nitrogen was blasted into the reactor at 100 cm3/min. The continuously fed-in nitrogen was used to ensure a laminar flow of pyrolysis gases toward the reactor outlet, to be analyzed. The heating rate varied between 40 and 50 /min depending on the reactor preheating temperature. For this range of heating rate the differences between process by-products are minimal. Moreover the conditions are similar to industrial ones. 3.2. Process kinetics 3.2.1. Sample mass conversion rate The parameter that strongly influences the pyrolysis products as well as the process energy balance is the product residence time in the active zone of an installation. To establish the minimum treatment period required for the volatiles liberation and char stabilization (depending on temperature level) the product was first subjected to the same pyrolysis process in an electric furnace connected to a high precision balance. The results offer sample mass variation in time, providing information on product status during the treatment process. The time variable data enables the precise control of each transformation phase of the product and the possibility to correlate the flue gas composition with treatment stage. Fig. 2 shows the sample mass variation for six treatment temperatures in low-mid range. The samples consist in raw product directly from industrial processing line (60% moisture). The four main stages which the product undergoes are clearly presented in Fig. 2: zone I e heating; zone II - moisture loss and primary volatile release; zone III e devolatilization; zone IV e char stabilization. As expected, the temperature has the main influence on weight loss, because along with its increasing, the product loses moisture and volatiles faster. As it is shown in the figure, the drying stage (II) finalizes after 10e12 min, while the devolatilization (III) lasts up to
Table 1 Humidity content and proximate analysis of grape marc. Material
Moisture [%]
Volatile matter [%]
Fixed carbon [%]
Inert 2content [%]
HHV measured [kJ/kg]
LHV measured [kJ/kg]
Grape marc
60
72
24.5
3.5
20,050
19,729
648
C. Marculescu, S. Ciuta / Renewable Energy 57 (2013) 645e652
350 °C
400 °C
450° C
500 °C
550 °C
600 °C
100 90 80
Mass loss [%].
70 60 50 40 30 20 10
I
III
II
IV
81 00
36 00
15 00
12 00
10 50
90 0
80 0
70 0
60 0
55 0
50 0
45 0
40 0
35 0
30 0
25 0
20 0
15 0
10 0
0
50
0 Time [sec] Fig. 2. Raw product mass variation as function of pyrolysis temperature.
20e25 min for the medium temperatures. The period for complete release of volatiles at low operating temperatures is about 35e 45 min, while for medium to high temperature the thermal scission of chemical bonds in the individual constituents of biomass takes up to10 min at 550 C e 600 C, leading to permanent gas species (CO2, CO, CH4) and condensable species formation. This involves a significantly decrease of residence time of the product at temperatures higher than 500 C. Consequently the additional energy demand expected for higher operating temperatures, is limited by a shorter processing time. The hemicelluloses, cellulose and lignin distribution within biomass product influence the product conversion stages. Hence hemicelluloses begins to thermally decompose at 220 C, and then main devolatilization takes place at 250 C e 350 C up to 500 C, cellulose degrades approximately between 300 C and 400 C, while lignin degradation takes place at 200 C e 700 C. Degradation of cellulose and hemicelluloses causes formation of volatiles, whereas degradation of lignin mainly causes formation of char [11]. At temperatures higher than 500 C, the curves from Fig. 2 follow a different allure than those from lower temperatures (350 C e 450 C). The amount of stabilized char remaining in the crucible increases slightly at elevated temperatures, this being explained through cracking of condensable components and additional formation of char (polymerization), beside noncondensable gases. Thus, during tar release inside a particle, the coking can also lead to higher yields of (secondary) char [12]. For the industrial application, depending on technology type (reactor) the water content may have a significant influence on minimum pyrolysis treatment period. For the laboratory conditions, the available excess of heat flow conducts to similar treatment periods. The sample mass loss variation curves have similar decreasing rates for both raw and dried product. 3.2.2. Activation energy The energy demand for the pyrolysis conversion process was calculated using the product activation energy (Ea). The product consists in a mixture of different grape marc constituents. Consequently the experimental determination of activation energy using
differential thermo-gravimetric analysis will not provide reliable data due to limited sample size (milligrams). Moreover the Thermal-Gravimetrical analyzers (TG-DTA) cannot provide experimental conditions similar to industrial ones. The heat and mass transfer processes refer to particle size samples. For Ea determination a first order kinetic model based on Arrhenius equation was used together with sample mass decreasing rate. During the devolatilization process the mass loss is given by a differential equation as function of non-liberated volatile fraction and sample mass variation gradient [13].
dm ¼ k ½mðts Þ mc dt
(1)
For product direct feed-in conditions, equivalent to 43 /min heating rate, the activation energy under pyrolysis environment (O2 less than 0.5% in the flue gas) at atmospheric pressure is 111.5 kJ/mol. This value is similar to other biomass products subjected to pyrolysis processes [14,15]. For industrial applications the expression of activation energy related to 1 kg of product is required. Using the elemental composition of the grape marc and based on existing empirical biomass formulas the product chemical expression was set to C4H7O3. For a molar mass of 103 g/mol the activation energy is 1083 kJ/kg. 3.3. Pyrolysis products characterization As revealed in Fig. 2, the residence time for a complete pyrolysis process is established at 1 h. The solid and liquid process endproducts are collected weighted and characterized using calorimetric analysis. The third product is represented by gas species. The quantity is established by difference and the composition is determined using Testo 350 XL gas analyzer with additional CO2 infrared system and hydrocarbons sensor. 3.3.1. By-products distribution The main reactions within non-oxidant thermal treatment, which occur at different speed rates, are: cracking, reforming, dehydration, condensation, polymerization, oxidation and
C. Marculescu, S. Ciuta / Renewable Energy 57 (2013) 645e652
gasification reactions (the last two reactions are initiated by oxygen present in the product or by water vapors) [16]. These reactions result from both primary decomposition of the solid fuel and secondary reactions of volatile condensable products into lowmolecular weight gases and char, as they are transported through the particle and the reaction environment [17]. The distinction between primary pyrolysis and secondary pyrolysis is not well defined as the secondary reactions of volatiles can occur both in the pores of the particles and/or in the bulk gas. Thus, the primary and secondary reactions can occur simultaneously in different parts of a particle. The char resulting from the primary pyrolysis stage can also be active during the secondary reactions, mainly by catalyzing the conversion of organic vapors into light gases (cracking reactions) and secondary char (polymerization reactions) [12]. The results on pyrolysis products distribution are presented in Fig. 3. The distribution is specific to each product and can be explained by reactions that take place depending mainly on temperature. Hence the tar yield is significantly increasing, achieving a maximum 500 C, after this temperature its yield starts to decrease. This may be due to secondary reactions in the gas phase, leading to over cracking of the products formed, and reducing the liquid yield at higher temperatures (at 500 C the tar fraction in pyrolysis products represent 34% compared to 21% at 600 C) [17]. Instead, lower temperatures prevent the full decomposition of the biomass. It is observed that once the temperature increases, the product releases volatiles faster, therefore a decrease in the mass of char left in the crucible. The char yield decreases from 48.2% to 30.6% as the pyrolysis temperature increased from 350 C to 600 C. The decrease in the char yield with temperature increasing could be due to greater primary decomposition of biomass at higher temperature or to secondary decomposition of the remaining char [18]. The char undergoes changes in color, form, size and bulk volumes as the temperature is increased. The physical appearance of char obtained at low temperatures (<500 C) is brown/gold color, fibrous, and a little change in the size of material. Chars obtained at higher temperatures manifested noticeable changes, being black in color, fragile, dispersed and low dense (due to porous structure and increased specific surface) [19]. The gas yield was found to be minimum at 500 C and maximum of 49.5% at 450 C. The secondary decomposition of the char at higher temperatures may also give some non-condensable gaseous products, which also contributes to the increase of the gas yield [19]. The conversion of primary tar in gaseous phase can be described as two parallel reactions to form light gases and refractory tar, which is harder to crack than the primary tar. The sequential
649
transformation of the primary tars as a function of temperature proceeds through a stage of light hydrocarbons and oxygenates to the ultimate formation of small quantities of polynuclear aromatics. In addition, the secondary tar-cracking process can lead to the formation of soot (secondary char), through a gas-phase nucleation mechanism that is also favored at high temperatures [12]. 3.3.2. Pyrolysis products specific energy For the process energy balance the pyrolysis products low heating value is required. For solid and liquid fractions the direct determination using calorimetric analysis was performed. The energy content of gaseous fraction was calculated based on pyrolysis gas components and their LHV from specific data bases. The influence of process temperature on end-products specific energy content was quantified. The results for the high calorific values of pyrolysis char, oil and gas are presented in Fig. 4. 3.3.2.1. Solid fraction. The volatile matter content of the char, which contributes to its calorific value, decreases continuously with pyrolysis temperature increase. The rapid drop in volatile content is due to decomposition of cellulose and lignin, releasing a variety of volatiles, both tar and non-tar forming compounds [20]. The increase of fixed carbon and ash ratio is due to removal of last volatile matter remained in char sample. A more stable char and ashforming inorganic matter are delivered [21]. The high heating value of char reaches the maximum of about 28 MJ/kg at 350 C and decreases irregularly down to 25e26 MJ/kg at 600 C. Char formation is limited by high heating rates and high temperature. The carbon content in the charcoal and its heating value are increasing at low temperature and low heating rate [6]. In this case insignificant variations of char HHV occur, compared to different biomass types and similar treatment conditions [22], mostly because of higher heating rates. The heating rate is conditioned by process temperature similar to industrial applications as detailed above (Section 3.1). It increases proportionally with temperature, disfavoring carbon fixation in char. 3.3.2.2. Liquid fraction. The liquid fraction consisting in heavy hydrocarbons and water was separated from the pyrolysis gas using a special designed condenser. The tar was analyzed using the Calorimeter C200, being previously dehydrated. Fig. 4 shows the constant increase of HHV of tar, achieving its maximum value of 30 MJ/kg at 550 C. With the rise of temperature, the primary tars, that are constituted from light hydrocarbons, transform into secondary and
Tar Tar
Char
Char
Gas
35000
Gas
30000
45 By-product HHV [kJ/kg]
By-product mass distribution [%].
50
40 35 30 25 20 15
20000 15000 10000 5000
10 5 0
25000
0
350
400
450
500
550
600
350
400
450 500 Temperature [°C]
550
600
Temperature [°C]
Fig. 3. Pyrolysis products distribution between 350 C and 600 C.
Fig. 4. High heating value of products resulted through pyrolysis at temperatures between 350 C and 600 C.
C. Marculescu, S. Ciuta / Renewable Energy 57 (2013) 645e652
tertiary tar compounds, more stable molecules, with higher energetic value, leading to an increase of liquid fraction energy potential. Above 550 C the decrease of tar HHV can be noticed due to cracking of liquid phase and migration of rich carbon compounds into gas phase. The maximum energy potential of tar is achieved at 550 C, while the maximum yield of tar was obtained at 500 C. Thus, the maximum quantity and energy potential of tar may not be obtained at the same temperature. 3.3.2.3. Gaseous fraction. The energetic potential of the noncondensable gas fraction was determined based on its composition, considering the entire H2O vapors recovered in the liquid fraction. The main pyrolysis gases are formed through thermally reactions such as cracking, depolymerization, decarboxylation or oxidation [23]. The influence of temperature on pyrolysis gas composition is presented in Fig. 5. Identified gases are CO2, CO, H2, CH4, C2H4 and C2H6. The major components are CO2, CO, H2 and CH4. The other hydrocarbons do not exceed 1.5% in volume; therefore their influence on pyrolysis gas calorific value is minimal. At low temperatures (350 C e 450 C) CO2 represents 77%e64% in volume. The increase of pyrolysis temperature reduces the concentration of CO2 to 53% at 600 C. The origin of the CO2 is given by the decomposition of cellulose and hemicelluloses, while at higher temperatures CO2 presence can be due to the lignin degradation [20]. On the other hand, the CO2 fraction in the product gases decreases with temperature because the CO2 is produced by carboxyl released at relatively low temperature and the secondary reactions of volatiles produce mostly CO, H2 and CH4 rather than CO2 [24]. The CO content reaches a maximum of 28% at 500 C slowly decreasing after this temperature. The CH4 reaches its maximum value of 10% at 600 C. The formation of CH4 and other light hydrocarbons found could be mainly related to the degradation of lignin, since their concentration increases at high processing temperatures. The formation of methane is due to the release of methoxy groups, involving CeC rupture, and it is controlled by hydrogen transfer reactions. After 600 C, a second formation of CH4 occurs due to the lignin decomposition [20]. This demonstrates that at a higher reaction temperature, a higher heating value gas could be obtained by biomass pyrolysis. Higher temperatures favor cracking of the hydrocarbons in the gaseous products and increase the hydrogen yield [25]. High temperatures (>500 C) are also advantageous for H2 production that is attributable to the rearranging and condensing of aromatic rings while releasing H2 during the secondary reaction [11]. At 350 C the hydrogen content is about 2% and increases continuously with temperature up to 12% at 600 C. Hydrogen may have been also formed by direct dehydrogenation of secondary char formed through the polymerization of tar [26]. 3.4. Process energy balance Sustainable heat and power generation from this particular type of biomass with respect to source discontinuous availability (3 months per year) requires a self-sustained pyrolysis unit located near distilleries or wine producers. This solution will solve the
DQ ¼
ZTw
vt mw $cw p$
dt T0 |fflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflffl} a
þ mw $lw þ |fflfflfflffl{zfflfflfflffl} b
ZTp
vt mt $ctp $
dt T0 |fflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflffl} c
þ mt $lt þ |fflffl{zfflffl} d
100 CO2
90 Gas component distribution [%]
650
CO
H2
CH4
80 70 60 50 40 30 20 10 0 350
400
450 500 Temperature [°C]
550
600
Fig. 5. Composition of non-condensable gases released during pyrolysis process of grape marc.
problems related to industrial waste peak quantities and mid to long-term storage caused by product biodegradable characteristics. The pyrolysis solid and liquid products present stable physicale chemical properties similar to fossil fuels. The energy balance for the pyrolysis process was calculated based on experimental results for 1 kg of grape marc. The input energy is given by waste product energy content Qi (through its LHV) and the heat required by the process (pyrolysis with external heat input) Qp. The output energy is given by: pyrolysis products energy content Qchar, Qtar, Qgas (through their LHV and quantity), and by-products sensitive heat and pyrolysis reactor heat losses DQ. The process energy balance is given by Equation (2).
Qi þ Qp ¼ Qchar þ Qtar þ Qgas þ DQ
(2)
The thermal energy consumption Qp is given by Equation (3) where the heat entering the system is defined by four separate terms that represent: sample heating to minimum volatile release temperature (a); volatiles heating to process temperature (b); char heating to process temperature (c); sample devolatilization energy (d) as function of product activation energy under process conditions for biomass cracking, volatile and char formation.
ZTv Qp ¼
vt m0 $cbp $
dt T0 |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} a
ZTp þ
vt mv $cvp $
dt Tv |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} b
ZTp
þ
vt þ Ea $m0 dt |fflfflffl{zfflfflffl} Tv d |fflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflffl} mch $cch p $ c
(3) The heat loss in the reactor is quantified by Equation (4). The heat loss occurs with by-products sensitive heat: condensed water cooling from condensing temperature to reference temperature (a); water vapors heat of vaporization (b); tar cooling from condensing temperature to reference one (c); tar heat of vaporization (d); char cooling from process temperature to reference one (e) and installation heat losses through insulation (f).
ZTp
vt þ D QR dt |ffl{zffl} T0 f |fflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflffl} mch $cch p $ e
(4)
C. Marculescu, S. Ciuta / Renewable Energy 57 (2013) 645e652 Table 3 Values of physical parameters used in energy balance. Property
Value
Reference
Specific heat (kJ kg1 K1)
cbp ¼ 2:5 cch p ¼ 1 ctp ¼ 1:4 cvp ¼ 2 m0 ¼ 1 mch ¼ 0.34 mt ¼ 0.33 mv ¼ 0.66 mw ¼ 0.07 T0 ¼ 293 Tp ¼ 773 Tv ¼ 543 Tw ¼ 378 lt ¼ 350 lw ¼ 2257
[27]
Mass measured (kg)
Temperature (K)
Latent heat of vaporization (kJ kg1)
Qtar
Qgas
e e [27] [27] [27]
Qp
14000 12000
Energy [kJ]
10000 8000 6000 4000 2000 0
350
400
450
500
550
products (after ensuring the process heat consumption) is found at 550 C while the minimum is found at 450 C. Consequently if pyrolysis process will be used as pre-treatment process within grape marc to energy conversion chain the optimum temperature with respect to energy consumption will be 550 C. 4. Conclusions
e
The values of the physical parameters used in the energy balance calculation are presented in Table 3. The energy balance results are presented in Fig. 6. The maximum energy of pyrolysis products is contained by char based on its high quantity. Char LHV is quasi constant throughout all treatment temperatures range. At low temperatures the char production is maximized and consequently the energy content delivered with it. The decrease of this energy with pyrolysis temperature is conditioned by char fraction decrease and not by its LHV variation. The energy contained by tar is maximum at 550 C. Different from char, the energy associated to this product varies also due to its LHV. The tar LHV reaches its maximum of 30,177 kJ/kg at 550 C even if its maximum quantity is obtained at 500 C. Despite pyrolysis gas LHV continuous increase from 6100 kJ/kg to 11,000 kJ/kg, the maximum energy content is reached at 600 C together with its maximum yield. It can be noticed that pyrolysis gas represents the second energy carrier. Within 500 C e 550 C the tar occupies the second position after char when referring to energy content, based on its quantity. The maximum pyrolysis products energy is delivered at 550 C, excepting their sensitive heat (the products temperature is assumed to be the reference temperature 20 C). The heat required by pyrolysis process increases, as expected, with treatment temperature. For all treatment temperatures the energy balance is positive, the process being self-sustained. Moreover the pyrolysis gas energy alone is sufficient to sustain the process energy consumption. The maximum net energy content delivered by pyrolysis
Qchar
651
600
Temperature [°C]
Fig. 6. Pyrolysis products energy content and process heat consumption.
The proximate, ultimate and calorimetric analysis (LHV ¼ 19,729 kJ/kg) of grape marc revealed characteristics similar to wood biomass, even higher energy potential, recommending this waste as renewable energy source. The product activation energy of 111.5 kJ/mol, respectively 1083 kJ/kg, was experimentally determined for industrial pyrolysis process conditions to provide a reliable value for real scale applications. The product equivalent chemical formula was established as C4H7O3 and it can be used for thermalechemical processes simulations. The pyrolysis solid, liquid and gas products were investigated along with their formation mechanisms. For all treatment temperatures the process energy balance is positive, the process being self-sustained by the pyrolysis gas energy alone. The maximum net energy content found in pyrolysis products is achieved at 550 C. Consequently the pyrolysis process can be used as pre-treatment stage for this product conversion to derived high quality fuels. Acknowledgment This work was supported by POSDRU/89/1.5/S/62557 and POSDRU/88/1.5/S/61178 projects. References [1] Ministry of Agricultural and Rural Development; 2009. http://www.madr.ro/ [accessed 10.04.09]. [2] National Vineyard Growers and Wine; 2009. http://www.pnvv.ro/ [accessed 01.08.09]. [3] Food and Agriculture Organization of the United Nations; 2011. http://www. faostat.fao.org/ [accessed 10.10.11]. [4] Bain RL, Overend RP, Craig KR. Biomass-fired power generation. Fuel Process Technol 1998;54:1e16. [5] Czernik S, Bridgwater AV. Overview of applications of biomass fast pyrolysis oil. Energy Fuel 2004;18:590e8. [6] Becidan M, Skreiberg Ø, Hustad JE. Products distribution and gas release in pyrolysis of thermally thick biomass residues samples. J Anal App Pyrol 2006;78:207e13. [7] Choi HS, Choi YS, Park HC. Fast pyrolysis characteristics of lignocellulosic biomass with varying reaction conditions. Renew Energy 2012;42: 131e5. [8] Sharma RK, Wooten JB, Baliga VL, Lin X, Chan WG, Hajaligol MR. Characterization of chars from pyrolysis of lignin. Fuel 2004;83: 1469e82. [9] Ratnadhariya JK, Channiwala SA. Three zone equilibrium and kinetic free modeling of biomass gasifier e a novel approach. Renew Energy 2009;34: 1050e8. [10] Schröder E. Experiments on the pyrolysis of large beech wood particles in fixed beds. J Anal App Pyrol 2004;71:669e94. [11] Williams PT, Besler S. The pyrolysis of rice husks in a thermogravimetric analyser and static batch reactor. Fuel 1993;72:151e9. [12] Neves D, Thunman H, Matos A, Tarelho L, Gómez-Barea A. Characterization and prediction of biomass pyrolysis products. Prog Energy Combust Sci 2011;37:611e30. [13] Marculescu C, Stan C. Poultry processing industry waste to energy conversion. Energy Procedia 2011;6:550e7. [14] Yao F, Wu Q, Lei Y, Guo W, Xu Y. Thermal decomposition kinetics of natural fibers: activation energy with dynamic thermogravimetric analysis. Polym Degrad Stab 2008;93:90e8. [15] Haykiri-Acma H, Yaman S, Kucukbayrak S. Effect of heating rate on the pyrolysis yields of rapeseed. Renew Energy 2006;31:803e10. [16] Mohan D, Pittman CU, Steele PH. Pyrolysis of wood/biomass for bio-oil: a critical review. Energy Fuel 2006;20:848e89. [17] Di Blasi C. Modeling chemical and physical processes of wood and biomass pyrolysis. Prog Energy Combust Sci 2008;34:47e90. [18] Horne PA, Williams PT. Influence of temperature on the products from the flash pyrolysis of biomass. Fuel 1996;75:1051e9.
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C. Marculescu, S. Ciuta / Renewable Energy 57 (2013) 645e652
[19] Demiral I, Ayan EA. Pyrolysis of grape bagasse: effect of pyrolysis conditions on the product yields and characterization of the liquid product. Bioresour Technol 2011;102:3946e51. [20] Uzun BB, Pütün AE, Pütün E. Composition of products obtained via fast pyrolysis of olive-oil residue: effect of pyrolysis temperature. J Anal App Pyrol 2007;79:147e53. [21] Katyal S, Thambimuthu K, Valix M. Carbonisation of bagasse in a fixed bed reactor: influence of process variables on char yield and characteristics. Renew Energy 2003;28:713e25. [22] Gañan J, González JF, González eGarcía CM, Cuerda-Correa EM, Macías-García A. Determination of the energy potential of gases produced in the pyrolysis processes of the vegetal carbon manufacture industry. Bioresour Technol 2006;97:711e20.
[23] González JF, Encinar JM, Canito JL, Sabio E, Chacón M. Pyrolysis of cherry stones: energy uses of the different fractions and kinetic study. J Anal App Pyrol 2003;67:165e90. [24] Luo Z, Wang S, Liao Y, Zhou J, Gu Y, Cen K. Research on biomass fast pyrolysis for liquid fuel. Biomass Bioenergy 2004;26:455e62. [25] Zanzi R, Sjöström K, Björnbom E. Rapid pyrolysis of agricultural residues at high temperature. Biomass Bioenergy 2002;23:357e66. [26] Sharma RK, Wooten JB, Baliga VL, Hajaligol MR. Characterization of chars from biomass-derived materials: pectin char. Fuel 2001;80: 1825e36. [27] The Engineering ToolBox; 2012. http://www.engineeringtoolbox.com/ [accessed 01.03.12].
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Wine industry waste thermal processing for derived fuel properties improvement Cosmin Marculescu*, Simona Ciuta Department of Power Engineering, University Politehnica of Bucharest, 313 Splaiul Independentei, 6, Bucharest 060042, Romania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2012 Accepted 25 February 2013 Available online 28 March 2013
The paper presents the results of pyrolysis process applied as pre-treatment sequence for grape marc within energy conversion chain. Experimental campaign for proximate, ultimate and calorimetric characterization of the product was conducted. The product equivalent chemical formula was established e C4H7O3. The atmospheric pressure pyrolysis with external heat source at temperatures between 350 C e600 C was studied with respect to process parameters influence on end-products formation, distribution and physicalechemical properties. Based on experiments the product activation energy of 111.5 kJ/mol was determined. The process parameters influence on end-products energy content, pyrolysis energy consumption and global energy efficiency was established. The research focused on maximum net energy content delivered in pyrolysis products as high quality fuel for energy conversion processes. The study revealed that pyrolysis gas alone can ensure the process self-sustenance for the applied temperature range. The optimum treatment temperature with respect to global energy balance was found at 550 C. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Pyrolysis Biomass Kinetics Activation energy Energy balance
1. Introduction Biomass is considered one of the most important renewable sources of energy of the future. In the last few years advanced technologies of biomass into fuel conversion or efficient combustion processes have evolved. Agricultural activities generate large amounts of biomass residues. While most crop residues are left in the field to reduce erosion and recycle nutrients back into the soil, some could be used to produce energy without harming the soil. One of the most representative crops which can be cultivated in extensive surfaces worldwide is represented by vine. Wine making industry generates huge amounts of wastes every year, for each kilogram of grape which is pressed into wine, more than 20% is residue [1]. Grapes are grown commercially in over 90 countries worldwide on almost 7.6 million hectares. In Europe, average yields are 1.4 and 1.7 tons/ha and in the USA yields are twice the world average [2]. The total world production of grapes in 2009 is estimated to be about 68.9 million tons, next only to citrus and bananas and is followed by apples [3]. The main wine making industry wastes are represented by grape pomace (marc) resulted directly from grape pressing. * Corresponding author. Tel.: þ40 745133713; fax: þ40 214029675. E-mail addresses: [email protected] (C. Marculescu), ciuta_simona@ yahoo.com (S. Ciuta). 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.02.028
The components of pomace in wine-making differ on whether white or red wine is being produced, but generally contains skins, pulp, seeds, and stems of the fruit. Grape pomace has traditionally been used to produce pomace brandy (such as grappa) and grape seed oil. Today, it is mostly used as fodder or fertilizer. Over the world grape marc is treated in different manners: recovery of grape seeds for food consumption, in industry for paints and varnishes, in perfumery, in the pharmaceutical industry or in the manufacture of soap, as feed or fertilizer and in countries like Canada, Italy or France for energy recovery. The least exploited possibility for grape marc treating is energy recovery. This approach into energy conversion is presented in this paper, mostly because pomace became almost completely disposed of in the field, and represents an expense and an environmental problem for distilleries. Worldwide, the most extensively used technology for biomass power production is direct combustion followed by a steam cycle [4]. Indeed, this is expected to continue being the dominant technology in the short to medium term. Another technology with great future potential is biomass gasification followed by a combined cycle. The main problem in energy valorization of the product is represented by the seasonality of grapes, which are mainly available for 2e3 months per year. A solution for continuous operated power plants could be combustion, preceded by product pre-treatment for the decrease of high moisture content and fast degradation properties. The stabilization of the product, throughout pyrolysis process, can solve the
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Abbreviations cbp cch p ctp cvp Ea Qi Qp Qchar Qtar Qgas DQ DQR
biomass specific heat (kJ kg1 K1) char specific heat (kJ kg1 K1) tar specific heat (kJ kg1 K1) volatiles specific heat (kJ kg1 K1) activation energy (kJ kg1) raw product energy content based on its LHV (kJ) pyrolysis process specific heat demand (kJ) pyrolysis char energy content based on its LHV (kJ) pyrolysis tar energy content based on its LHV (kJ) pyrolysis gas energy content based on its LHV (kJ) losses in pyrolysis process (kJ) losses in the batch reactor (kJ)
temporary availability of the pomace and storage by transforming it into high carbon content derived fuels. Pyrolysis products uses are diverse. The bio-oil, composed of aliphatic and aromatic hydrocarbons, can be used in electricity generation as a substitute fuel. However, several properties of the bio-oils such as high water content or poor ignition ability have to be ameliorated or existing equipment modified to meet competitive standards [5]. Charcoal, a carbon-rich solid residue, can be upgraded to activated carbon for use in chemical, pharmaceutical and food industries, or in syngas production in the gasification process. The dry pyrolysis gas mixture contains the main components: CO2, CO, CH4, H2 and C2 hydrocarbons [6] and can be used for heat production and power generation, but is often used to sustain the pyrolysis process in a biomass waste pyrolysis plant or to dry the feedstock. A specific pyrolysis installation will aim at the optimal production (yield, composition and properties) of one or more of the products. The aim of this paper is to determine the optimum pyrolysis process parameters, to maximize the derived fuels energy potential in terms of global energy balance. The sample mass decreasing rate during the process run is determined to establish the product behavior under non-oxidant conditions at atmospheric pressure with respect to process kinetics. Therefore, the main purpose is to ensure that the pyrolysis process is self sufficient, from an energy standpoint. From a sustainability point of view, it is intended to avoid the use of fossil fuels. As from economical point of view decentralized pretreatment establishments, placed near the biomass source bring many advantages in storage, handling and lower transport costs. The pyrolysis gases can be used to provide the energy required for the endothermic pyrolysis process. 2. Materials and methods
carbon mass (kg) char mass (kg) initial biomass mass (kg) tar mass (kg) volatile mass (kg) water mass (kg) initial temperature (K) water vaporization temperature (K) devolatilization starting temperature (K) process temperature (K) tar latent heat of vaporization (kJ kg1) water latent heat of vaporization (kJ kg1) low heating value (kJ kg1) high heating value (kJ kg1)
mc mch m0 mt mv mw T0 Tw Tv Tp
lt lw
LHV HHV
the end of the tube in a container. After water separation, the remaining liquid phase and the solid residues (char) were weighted at the end of the process. The batch reactor’s other collection tube permits sampling of the gas fraction in real time through a sonde connected to a gas analyzer. Schematic diagram of the main elements of fixed bed furnace is shown in the figure below. Working temperature range is between 20 C and 1300 C. Pyrolysis device is equipped with a control pad that allows temperature programming, working time (residence time at process temperature) and heating rate. Samples to be subjected to thermalechemical treatment processes such as pyrolysis/combustion are introduced into the furnace in a tubular parallelepiped crucible of refractory steel W4541, size: 100 cm long, 4 cm wide and 3 cm in height. To ensure an inert atmosphere in the oven during the pyrolysis process, nitrogen was blasted, its flow being measured using a flow meter. 2.2. Products and sampling The material used in this study is represented by grape marc provided by a Romanian wine producer, directly from the industrial processing line, after grape pressing. The moisture content determined for the material as arrived was approximately 60%. Different analyzes were performed on the dried product: proximate analysis, elemental analysis, high and low heating value determination. For the determination of volatile content the temperature was set at 800 C and the crucible with material previously graded, was subjected to a pyrolysis process for 40 min (nitrogen used for the inert atmosphere). The samples obtained in the first stage were subjected to combustion process (950 C for 15 min e until the complete combustion of the sample) in order to establish the total
2.1. Reactor The experiments for pyrolysis treatment process were carried out on a Nabertherm tubular fix bed reactor model RO 60/750/13 (Fig. 1) modified according to experiment configuration requirements. It consists of a refractory steel tube, exterior electrically heated and with an interior diameter of 60 mm. The samples are introduced into the refractory steel reactor by a tubular parallelepiped crucible of refractory steel with an active heating area of 750 mm. The reactor is equipped with two injection inlets for different experimental conditions and a gaseliquid fraction separation unit for discharges resulted from treatments applied to solid masses according to Fig. 1 legend. The collecting tube for the liquid reaction products allows the entire fraction to condense, being collected at
5 4
3
6
2 7
1
9
Active area, isothermal
8
Fig. 1. Tubular batch reactor: 1.Nitrogen injection; 2.Air injection; 3.Refractory steel crucible; 4.Refractory steel tube; 5.Electrically heated chamber; 6. Exhaust gases; 7.Thermocouple; 8.Liquid reaction products (tar); 9.Sample.
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content of the combustible fraction, respectively the one of inert fraction (non-combustible). Results are presented in Tables 1 and 2. High heating value was determined using the calorimeter IKA C200, the sample being subjected to combustion, in an oxidative environment (pure oxygen at 30 bars). The chemical composition of the material was established using an elemental analyzer, with a sample weight which varied around 1 mg. The EA 3000 Series analyzer uses the principle of dynamic flash combustion followed by gas chromatography separation of the resultant gaseous species (N2, CO2, H2O, and SO2) and Thermal Conductivity Detection (TCD). It can be concluded that the characteristics of these wastes are similar to different types of wood biomass when we refer in particular to volatile content and elemental composition. Regarding the high heating value its value is much closer to that of various conifers. As any type of biomass, grape marc consists of the main biopolymers: cellulose, hemicelluloses and lignin which influence strongly the pyrolysis process kinetics. The relatively high sulfur content of the material can be explained by treatment methods applied to vine that help keeping the grape berries not to alter for a longer period before being harvested. 3. Results and discussions 3.1. Process run With respect to waste to energy conversion chain, the high amount of energy required by the devolatilization process for product stabilization leads to a series of experimental based criteria necessary in global energy balance determination. In industrial applications (continuous flow) for the energy consumption quantification the main process parameters are the residence time and temperature. Laboratory specific parameters such as: nitrogen flow (for inert environment conditions) and heating rate are not to be considered at industrial scale in energy sector. During the devolatilization process the pyrolysis gases create the non-oxidant conditions and the nitrogen gas injection is not required. Even if the product ignition may occur at process start e up, the absence of air inhibits this phenomenon at maximum 1 min depending on reactor type. In industrial applications the heating rate, an important process parameter that influences the properties and yields of pyrolysis products, is conditioned by process temperature and product feed e in temperature. A higher heating rate increases the yield of liquid products resulting maximum oil yields, mainly because of a better heat and mass transfer [7]. The process temperature and pressure are parameters that influence char’s physicalechemical properties. In energy sector usually the thermalechemical processes are conducted at atmospheric pressure (or slightly lower). Previous works in the field revealed that at temperatures above 650 C most of pyrolysis products components are represented by gas. The quality of the char is also influenced by process temperature with respect to specific surface that increases with direct influence on the next treatment stage process run especially if gasification is applied [8]. For in-depth process analysis and product behavior, the range of treatment temperature varied between 350 C and 600 C, being performed six experiments on dried grape marc. Above 600 C the non-oxidizing process does not present a solution because air
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Table 2 Elemental composition of dried grape marc. C [%]
H [%]
O [%]
N [%]
S [%]
Ash [%]
43.21
5.94
45.50
0.65
1.24
3.46
gasification can be applied. If two stage treatment is used for waste to energy conversion, the chain conversion efficiency should be determined for each stage separately and optimized globally. In terms of process kinetics, for example, a minimum pyrolysis temperature will deliver a char with compact structure (non porous) that will require a higher gasification temperature or longer treatment period, with increased energy consumption, due to lower specific surface and consequently lower constant rate of reaction (according to Arrhenius equation) [9]. The pyrolysis process was conducted in the tubular batch reactor described in detail above, the sample being exposed to a specific temperature in the preheated furnace, hence subjected to a flash pyrolysis. The mass of grape marc sample was 50 g imposed by crucible size and product specific mass. The conditions encountered in an industrial process are different from experimental size units, because heat and mass transport mechanisms will strongly influence the composition and distribution of process end-products [10]. After the inert conditions were created inside the reactor using nitrogen and the sample was introduced, a continuous flow of nitrogen was blasted into the reactor at 100 cm3/min. The continuously fed-in nitrogen was used to ensure a laminar flow of pyrolysis gases toward the reactor outlet, to be analyzed. The heating rate varied between 40 and 50 /min depending on the reactor preheating temperature. For this range of heating rate the differences between process by-products are minimal. Moreover the conditions are similar to industrial ones. 3.2. Process kinetics 3.2.1. Sample mass conversion rate The parameter that strongly influences the pyrolysis products as well as the process energy balance is the product residence time in the active zone of an installation. To establish the minimum treatment period required for the volatiles liberation and char stabilization (depending on temperature level) the product was first subjected to the same pyrolysis process in an electric furnace connected to a high precision balance. The results offer sample mass variation in time, providing information on product status during the treatment process. The time variable data enables the precise control of each transformation phase of the product and the possibility to correlate the flue gas composition with treatment stage. Fig. 2 shows the sample mass variation for six treatment temperatures in low-mid range. The samples consist in raw product directly from industrial processing line (60% moisture). The four main stages which the product undergoes are clearly presented in Fig. 2: zone I e heating; zone II - moisture loss and primary volatile release; zone III e devolatilization; zone IV e char stabilization. As expected, the temperature has the main influence on weight loss, because along with its increasing, the product loses moisture and volatiles faster. As it is shown in the figure, the drying stage (II) finalizes after 10e12 min, while the devolatilization (III) lasts up to
Table 1 Humidity content and proximate analysis of grape marc. Material
Moisture [%]
Volatile matter [%]
Fixed carbon [%]
Inert 2content [%]
HHV measured [kJ/kg]
LHV measured [kJ/kg]
Grape marc
60
72
24.5
3.5
20,050
19,729
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350 °C
400 °C
450° C
500 °C
550 °C
600 °C
100 90 80
Mass loss [%].
70 60 50 40 30 20 10
I
III
II
IV
81 00
36 00
15 00
12 00
10 50
90 0
80 0
70 0
60 0
55 0
50 0
45 0
40 0
35 0
30 0
25 0
20 0
15 0
10 0
0
50
0 Time [sec] Fig. 2. Raw product mass variation as function of pyrolysis temperature.
20e25 min for the medium temperatures. The period for complete release of volatiles at low operating temperatures is about 35e 45 min, while for medium to high temperature the thermal scission of chemical bonds in the individual constituents of biomass takes up to10 min at 550 C e 600 C, leading to permanent gas species (CO2, CO, CH4) and condensable species formation. This involves a significantly decrease of residence time of the product at temperatures higher than 500 C. Consequently the additional energy demand expected for higher operating temperatures, is limited by a shorter processing time. The hemicelluloses, cellulose and lignin distribution within biomass product influence the product conversion stages. Hence hemicelluloses begins to thermally decompose at 220 C, and then main devolatilization takes place at 250 C e 350 C up to 500 C, cellulose degrades approximately between 300 C and 400 C, while lignin degradation takes place at 200 C e 700 C. Degradation of cellulose and hemicelluloses causes formation of volatiles, whereas degradation of lignin mainly causes formation of char [11]. At temperatures higher than 500 C, the curves from Fig. 2 follow a different allure than those from lower temperatures (350 C e 450 C). The amount of stabilized char remaining in the crucible increases slightly at elevated temperatures, this being explained through cracking of condensable components and additional formation of char (polymerization), beside noncondensable gases. Thus, during tar release inside a particle, the coking can also lead to higher yields of (secondary) char [12]. For the industrial application, depending on technology type (reactor) the water content may have a significant influence on minimum pyrolysis treatment period. For the laboratory conditions, the available excess of heat flow conducts to similar treatment periods. The sample mass loss variation curves have similar decreasing rates for both raw and dried product. 3.2.2. Activation energy The energy demand for the pyrolysis conversion process was calculated using the product activation energy (Ea). The product consists in a mixture of different grape marc constituents. Consequently the experimental determination of activation energy using
differential thermo-gravimetric analysis will not provide reliable data due to limited sample size (milligrams). Moreover the Thermal-Gravimetrical analyzers (TG-DTA) cannot provide experimental conditions similar to industrial ones. The heat and mass transfer processes refer to particle size samples. For Ea determination a first order kinetic model based on Arrhenius equation was used together with sample mass decreasing rate. During the devolatilization process the mass loss is given by a differential equation as function of non-liberated volatile fraction and sample mass variation gradient [13].
dm ¼ k ½mðts Þ mc dt
(1)
For product direct feed-in conditions, equivalent to 43 /min heating rate, the activation energy under pyrolysis environment (O2 less than 0.5% in the flue gas) at atmospheric pressure is 111.5 kJ/mol. This value is similar to other biomass products subjected to pyrolysis processes [14,15]. For industrial applications the expression of activation energy related to 1 kg of product is required. Using the elemental composition of the grape marc and based on existing empirical biomass formulas the product chemical expression was set to C4H7O3. For a molar mass of 103 g/mol the activation energy is 1083 kJ/kg. 3.3. Pyrolysis products characterization As revealed in Fig. 2, the residence time for a complete pyrolysis process is established at 1 h. The solid and liquid process endproducts are collected weighted and characterized using calorimetric analysis. The third product is represented by gas species. The quantity is established by difference and the composition is determined using Testo 350 XL gas analyzer with additional CO2 infrared system and hydrocarbons sensor. 3.3.1. By-products distribution The main reactions within non-oxidant thermal treatment, which occur at different speed rates, are: cracking, reforming, dehydration, condensation, polymerization, oxidation and
C. Marculescu, S. Ciuta / Renewable Energy 57 (2013) 645e652
gasification reactions (the last two reactions are initiated by oxygen present in the product or by water vapors) [16]. These reactions result from both primary decomposition of the solid fuel and secondary reactions of volatile condensable products into lowmolecular weight gases and char, as they are transported through the particle and the reaction environment [17]. The distinction between primary pyrolysis and secondary pyrolysis is not well defined as the secondary reactions of volatiles can occur both in the pores of the particles and/or in the bulk gas. Thus, the primary and secondary reactions can occur simultaneously in different parts of a particle. The char resulting from the primary pyrolysis stage can also be active during the secondary reactions, mainly by catalyzing the conversion of organic vapors into light gases (cracking reactions) and secondary char (polymerization reactions) [12]. The results on pyrolysis products distribution are presented in Fig. 3. The distribution is specific to each product and can be explained by reactions that take place depending mainly on temperature. Hence the tar yield is significantly increasing, achieving a maximum 500 C, after this temperature its yield starts to decrease. This may be due to secondary reactions in the gas phase, leading to over cracking of the products formed, and reducing the liquid yield at higher temperatures (at 500 C the tar fraction in pyrolysis products represent 34% compared to 21% at 600 C) [17]. Instead, lower temperatures prevent the full decomposition of the biomass. It is observed that once the temperature increases, the product releases volatiles faster, therefore a decrease in the mass of char left in the crucible. The char yield decreases from 48.2% to 30.6% as the pyrolysis temperature increased from 350 C to 600 C. The decrease in the char yield with temperature increasing could be due to greater primary decomposition of biomass at higher temperature or to secondary decomposition of the remaining char [18]. The char undergoes changes in color, form, size and bulk volumes as the temperature is increased. The physical appearance of char obtained at low temperatures (<500 C) is brown/gold color, fibrous, and a little change in the size of material. Chars obtained at higher temperatures manifested noticeable changes, being black in color, fragile, dispersed and low dense (due to porous structure and increased specific surface) [19]. The gas yield was found to be minimum at 500 C and maximum of 49.5% at 450 C. The secondary decomposition of the char at higher temperatures may also give some non-condensable gaseous products, which also contributes to the increase of the gas yield [19]. The conversion of primary tar in gaseous phase can be described as two parallel reactions to form light gases and refractory tar, which is harder to crack than the primary tar. The sequential
649
transformation of the primary tars as a function of temperature proceeds through a stage of light hydrocarbons and oxygenates to the ultimate formation of small quantities of polynuclear aromatics. In addition, the secondary tar-cracking process can lead to the formation of soot (secondary char), through a gas-phase nucleation mechanism that is also favored at high temperatures [12]. 3.3.2. Pyrolysis products specific energy For the process energy balance the pyrolysis products low heating value is required. For solid and liquid fractions the direct determination using calorimetric analysis was performed. The energy content of gaseous fraction was calculated based on pyrolysis gas components and their LHV from specific data bases. The influence of process temperature on end-products specific energy content was quantified. The results for the high calorific values of pyrolysis char, oil and gas are presented in Fig. 4. 3.3.2.1. Solid fraction. The volatile matter content of the char, which contributes to its calorific value, decreases continuously with pyrolysis temperature increase. The rapid drop in volatile content is due to decomposition of cellulose and lignin, releasing a variety of volatiles, both tar and non-tar forming compounds [20]. The increase of fixed carbon and ash ratio is due to removal of last volatile matter remained in char sample. A more stable char and ashforming inorganic matter are delivered [21]. The high heating value of char reaches the maximum of about 28 MJ/kg at 350 C and decreases irregularly down to 25e26 MJ/kg at 600 C. Char formation is limited by high heating rates and high temperature. The carbon content in the charcoal and its heating value are increasing at low temperature and low heating rate [6]. In this case insignificant variations of char HHV occur, compared to different biomass types and similar treatment conditions [22], mostly because of higher heating rates. The heating rate is conditioned by process temperature similar to industrial applications as detailed above (Section 3.1). It increases proportionally with temperature, disfavoring carbon fixation in char. 3.3.2.2. Liquid fraction. The liquid fraction consisting in heavy hydrocarbons and water was separated from the pyrolysis gas using a special designed condenser. The tar was analyzed using the Calorimeter C200, being previously dehydrated. Fig. 4 shows the constant increase of HHV of tar, achieving its maximum value of 30 MJ/kg at 550 C. With the rise of temperature, the primary tars, that are constituted from light hydrocarbons, transform into secondary and
Tar Tar
Char
Char
Gas
35000
Gas
30000
45 By-product HHV [kJ/kg]
By-product mass distribution [%].
50
40 35 30 25 20 15
20000 15000 10000 5000
10 5 0
25000
0
350
400
450
500
550
600
350
400
450 500 Temperature [°C]
550
600
Temperature [°C]
Fig. 3. Pyrolysis products distribution between 350 C and 600 C.
Fig. 4. High heating value of products resulted through pyrolysis at temperatures between 350 C and 600 C.
C. Marculescu, S. Ciuta / Renewable Energy 57 (2013) 645e652
tertiary tar compounds, more stable molecules, with higher energetic value, leading to an increase of liquid fraction energy potential. Above 550 C the decrease of tar HHV can be noticed due to cracking of liquid phase and migration of rich carbon compounds into gas phase. The maximum energy potential of tar is achieved at 550 C, while the maximum yield of tar was obtained at 500 C. Thus, the maximum quantity and energy potential of tar may not be obtained at the same temperature. 3.3.2.3. Gaseous fraction. The energetic potential of the noncondensable gas fraction was determined based on its composition, considering the entire H2O vapors recovered in the liquid fraction. The main pyrolysis gases are formed through thermally reactions such as cracking, depolymerization, decarboxylation or oxidation [23]. The influence of temperature on pyrolysis gas composition is presented in Fig. 5. Identified gases are CO2, CO, H2, CH4, C2H4 and C2H6. The major components are CO2, CO, H2 and CH4. The other hydrocarbons do not exceed 1.5% in volume; therefore their influence on pyrolysis gas calorific value is minimal. At low temperatures (350 C e 450 C) CO2 represents 77%e64% in volume. The increase of pyrolysis temperature reduces the concentration of CO2 to 53% at 600 C. The origin of the CO2 is given by the decomposition of cellulose and hemicelluloses, while at higher temperatures CO2 presence can be due to the lignin degradation [20]. On the other hand, the CO2 fraction in the product gases decreases with temperature because the CO2 is produced by carboxyl released at relatively low temperature and the secondary reactions of volatiles produce mostly CO, H2 and CH4 rather than CO2 [24]. The CO content reaches a maximum of 28% at 500 C slowly decreasing after this temperature. The CH4 reaches its maximum value of 10% at 600 C. The formation of CH4 and other light hydrocarbons found could be mainly related to the degradation of lignin, since their concentration increases at high processing temperatures. The formation of methane is due to the release of methoxy groups, involving CeC rupture, and it is controlled by hydrogen transfer reactions. After 600 C, a second formation of CH4 occurs due to the lignin decomposition [20]. This demonstrates that at a higher reaction temperature, a higher heating value gas could be obtained by biomass pyrolysis. Higher temperatures favor cracking of the hydrocarbons in the gaseous products and increase the hydrogen yield [25]. High temperatures (>500 C) are also advantageous for H2 production that is attributable to the rearranging and condensing of aromatic rings while releasing H2 during the secondary reaction [11]. At 350 C the hydrogen content is about 2% and increases continuously with temperature up to 12% at 600 C. Hydrogen may have been also formed by direct dehydrogenation of secondary char formed through the polymerization of tar [26]. 3.4. Process energy balance Sustainable heat and power generation from this particular type of biomass with respect to source discontinuous availability (3 months per year) requires a self-sustained pyrolysis unit located near distilleries or wine producers. This solution will solve the
DQ ¼
ZTw
vt mw $cw p$
dt T0 |fflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflffl} a
þ mw $lw þ |fflfflfflffl{zfflfflfflffl} b
ZTp
vt mt $ctp $
dt T0 |fflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflffl} c
þ mt $lt þ |fflffl{zfflffl} d
100 CO2
90 Gas component distribution [%]
650
CO
H2
CH4
80 70 60 50 40 30 20 10 0 350
400
450 500 Temperature [°C]
550
600
Fig. 5. Composition of non-condensable gases released during pyrolysis process of grape marc.
problems related to industrial waste peak quantities and mid to long-term storage caused by product biodegradable characteristics. The pyrolysis solid and liquid products present stable physicale chemical properties similar to fossil fuels. The energy balance for the pyrolysis process was calculated based on experimental results for 1 kg of grape marc. The input energy is given by waste product energy content Qi (through its LHV) and the heat required by the process (pyrolysis with external heat input) Qp. The output energy is given by: pyrolysis products energy content Qchar, Qtar, Qgas (through their LHV and quantity), and by-products sensitive heat and pyrolysis reactor heat losses DQ. The process energy balance is given by Equation (2).
Qi þ Qp ¼ Qchar þ Qtar þ Qgas þ DQ
(2)
The thermal energy consumption Qp is given by Equation (3) where the heat entering the system is defined by four separate terms that represent: sample heating to minimum volatile release temperature (a); volatiles heating to process temperature (b); char heating to process temperature (c); sample devolatilization energy (d) as function of product activation energy under process conditions for biomass cracking, volatile and char formation.
ZTv Qp ¼
vt m0 $cbp $
dt T0 |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} a
ZTp þ
vt mv $cvp $
dt Tv |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} b
ZTp
þ
vt þ Ea $m0 dt |fflfflffl{zfflfflffl} Tv d |fflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflffl} mch $cch p $ c
(3) The heat loss in the reactor is quantified by Equation (4). The heat loss occurs with by-products sensitive heat: condensed water cooling from condensing temperature to reference temperature (a); water vapors heat of vaporization (b); tar cooling from condensing temperature to reference one (c); tar heat of vaporization (d); char cooling from process temperature to reference one (e) and installation heat losses through insulation (f).
ZTp
vt þ D QR dt |ffl{zffl} T0 f |fflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflffl} mch $cch p $ e
(4)
C. Marculescu, S. Ciuta / Renewable Energy 57 (2013) 645e652 Table 3 Values of physical parameters used in energy balance. Property
Value
Reference
Specific heat (kJ kg1 K1)
cbp ¼ 2:5 cch p ¼ 1 ctp ¼ 1:4 cvp ¼ 2 m0 ¼ 1 mch ¼ 0.34 mt ¼ 0.33 mv ¼ 0.66 mw ¼ 0.07 T0 ¼ 293 Tp ¼ 773 Tv ¼ 543 Tw ¼ 378 lt ¼ 350 lw ¼ 2257
[27]
Mass measured (kg)
Temperature (K)
Latent heat of vaporization (kJ kg1)
Qtar
Qgas
e e [27] [27] [27]
Qp
14000 12000
Energy [kJ]
10000 8000 6000 4000 2000 0
350
400
450
500
550
products (after ensuring the process heat consumption) is found at 550 C while the minimum is found at 450 C. Consequently if pyrolysis process will be used as pre-treatment process within grape marc to energy conversion chain the optimum temperature with respect to energy consumption will be 550 C. 4. Conclusions
e
The values of the physical parameters used in the energy balance calculation are presented in Table 3. The energy balance results are presented in Fig. 6. The maximum energy of pyrolysis products is contained by char based on its high quantity. Char LHV is quasi constant throughout all treatment temperatures range. At low temperatures the char production is maximized and consequently the energy content delivered with it. The decrease of this energy with pyrolysis temperature is conditioned by char fraction decrease and not by its LHV variation. The energy contained by tar is maximum at 550 C. Different from char, the energy associated to this product varies also due to its LHV. The tar LHV reaches its maximum of 30,177 kJ/kg at 550 C even if its maximum quantity is obtained at 500 C. Despite pyrolysis gas LHV continuous increase from 6100 kJ/kg to 11,000 kJ/kg, the maximum energy content is reached at 600 C together with its maximum yield. It can be noticed that pyrolysis gas represents the second energy carrier. Within 500 C e 550 C the tar occupies the second position after char when referring to energy content, based on its quantity. The maximum pyrolysis products energy is delivered at 550 C, excepting their sensitive heat (the products temperature is assumed to be the reference temperature 20 C). The heat required by pyrolysis process increases, as expected, with treatment temperature. For all treatment temperatures the energy balance is positive, the process being self-sustained. Moreover the pyrolysis gas energy alone is sufficient to sustain the process energy consumption. The maximum net energy content delivered by pyrolysis
Qchar
651
600
Temperature [°C]
Fig. 6. Pyrolysis products energy content and process heat consumption.
The proximate, ultimate and calorimetric analysis (LHV ¼ 19,729 kJ/kg) of grape marc revealed characteristics similar to wood biomass, even higher energy potential, recommending this waste as renewable energy source. The product activation energy of 111.5 kJ/mol, respectively 1083 kJ/kg, was experimentally determined for industrial pyrolysis process conditions to provide a reliable value for real scale applications. The product equivalent chemical formula was established as C4H7O3 and it can be used for thermalechemical processes simulations. The pyrolysis solid, liquid and gas products were investigated along with their formation mechanisms. For all treatment temperatures the process energy balance is positive, the process being self-sustained by the pyrolysis gas energy alone. The maximum net energy content found in pyrolysis products is achieved at 550 C. Consequently the pyrolysis process can be used as pre-treatment stage for this product conversion to derived high quality fuels. Acknowledgment This work was supported by POSDRU/89/1.5/S/62557 and POSDRU/88/1.5/S/61178 projects. References [1] Ministry of Agricultural and Rural Development; 2009. http://www.madr.ro/ [accessed 10.04.09]. [2] National Vineyard Growers and Wine; 2009. http://www.pnvv.ro/ [accessed 01.08.09]. [3] Food and Agriculture Organization of the United Nations; 2011. http://www. faostat.fao.org/ [accessed 10.10.11]. [4] Bain RL, Overend RP, Craig KR. Biomass-fired power generation. Fuel Process Technol 1998;54:1e16. [5] Czernik S, Bridgwater AV. Overview of applications of biomass fast pyrolysis oil. Energy Fuel 2004;18:590e8. [6] Becidan M, Skreiberg Ø, Hustad JE. Products distribution and gas release in pyrolysis of thermally thick biomass residues samples. J Anal App Pyrol 2006;78:207e13. [7] Choi HS, Choi YS, Park HC. Fast pyrolysis characteristics of lignocellulosic biomass with varying reaction conditions. Renew Energy 2012;42: 131e5. [8] Sharma RK, Wooten JB, Baliga VL, Lin X, Chan WG, Hajaligol MR. Characterization of chars from pyrolysis of lignin. Fuel 2004;83: 1469e82. [9] Ratnadhariya JK, Channiwala SA. Three zone equilibrium and kinetic free modeling of biomass gasifier e a novel approach. Renew Energy 2009;34: 1050e8. [10] Schröder E. Experiments on the pyrolysis of large beech wood particles in fixed beds. J Anal App Pyrol 2004;71:669e94. [11] Williams PT, Besler S. The pyrolysis of rice husks in a thermogravimetric analyser and static batch reactor. Fuel 1993;72:151e9. [12] Neves D, Thunman H, Matos A, Tarelho L, Gómez-Barea A. Characterization and prediction of biomass pyrolysis products. Prog Energy Combust Sci 2011;37:611e30. [13] Marculescu C, Stan C. Poultry processing industry waste to energy conversion. Energy Procedia 2011;6:550e7. [14] Yao F, Wu Q, Lei Y, Guo W, Xu Y. Thermal decomposition kinetics of natural fibers: activation energy with dynamic thermogravimetric analysis. Polym Degrad Stab 2008;93:90e8. [15] Haykiri-Acma H, Yaman S, Kucukbayrak S. Effect of heating rate on the pyrolysis yields of rapeseed. Renew Energy 2006;31:803e10. [16] Mohan D, Pittman CU, Steele PH. Pyrolysis of wood/biomass for bio-oil: a critical review. Energy Fuel 2006;20:848e89. [17] Di Blasi C. Modeling chemical and physical processes of wood and biomass pyrolysis. Prog Energy Combust Sci 2008;34:47e90. [18] Horne PA, Williams PT. Influence of temperature on the products from the flash pyrolysis of biomass. Fuel 1996;75:1051e9.
652
C. Marculescu, S. Ciuta / Renewable Energy 57 (2013) 645e652
[19] Demiral I, Ayan EA. Pyrolysis of grape bagasse: effect of pyrolysis conditions on the product yields and characterization of the liquid product. Bioresour Technol 2011;102:3946e51. [20] Uzun BB, Pütün AE, Pütün E. Composition of products obtained via fast pyrolysis of olive-oil residue: effect of pyrolysis temperature. J Anal App Pyrol 2007;79:147e53. [21] Katyal S, Thambimuthu K, Valix M. Carbonisation of bagasse in a fixed bed reactor: influence of process variables on char yield and characteristics. Renew Energy 2003;28:713e25. [22] Gañan J, González JF, González eGarcía CM, Cuerda-Correa EM, Macías-García A. Determination of the energy potential of gases produced in the pyrolysis processes of the vegetal carbon manufacture industry. Bioresour Technol 2006;97:711e20.
[23] González JF, Encinar JM, Canito JL, Sabio E, Chacón M. Pyrolysis of cherry stones: energy uses of the different fractions and kinetic study. J Anal App Pyrol 2003;67:165e90. [24] Luo Z, Wang S, Liao Y, Zhou J, Gu Y, Cen K. Research on biomass fast pyrolysis for liquid fuel. Biomass Bioenergy 2004;26:455e62. [25] Zanzi R, Sjöström K, Björnbom E. Rapid pyrolysis of agricultural residues at high temperature. Biomass Bioenergy 2002;23:357e66. [26] Sharma RK, Wooten JB, Baliga VL, Hajaligol MR. Characterization of chars from biomass-derived materials: pectin char. Fuel 2001;80: 1825e36. [27] The Engineering ToolBox; 2012. http://www.engineeringtoolbox.com/ [accessed 01.03.12].