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Introduction to frictional pyrolysis (FP) – An alternative method for converting biomass to solid carbonaceous products
Fuel 175 (2016) 49–56
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Introduction to frictional pyrolysis (FP) – An alternative method for converting biomass to solid carbonaceous products S. Vakalis a,⇑, R. Heimann b, A. Talley b, N. Heimann b, M. Baratieri a a b
Free University of Bozen - Bolzano, Faculty of Science and Technology, Piazza Università 5, 39100 Bolzano, Italy Enginuity Worldwide LLC, 651 Commerce Road, Mexico, MO 65265, United States
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Introduction of a novel method of
pyrolysis via friction and pressure. The process time is less than 200 s. The final solid yield is higher than
80% and has 96% of the initial energy content. The net energy balance is 92.2% on HHV basis. The product has increased fixed carbon content in comparison to torrefied biomass.
a r t i c l e
i n f o
Article history: Received 24 September 2015 Received in revised form 9 December 2015 Accepted 10 February 2016 Available online 15 February 2016 Keywords: Biocoal Charcoal Carbonization Steam explosion Corn stover STA
a b s t r a c t In the biomass sector, technologies like carbonization and torrefaction have been utilized for the production of solid carbonaceous biofuels and materials. In the framework of this manuscript a novel method for production of solid carbonaceous materials is introduced and is defined from now on as frictional pyrolysis. It uses only the application of pressure and friction whereas no external heat transfer is needed for the propagation of the process. This novel method is compared with torrefaction in order to assess its potential to process corn stover which has strongly bounder water and high content of deciduous xylan. Mass balances have been implemented for both technologies. Characterization of the products has been done by means of Simultaneous Thermal Analysis and Elemental analysis. Frictionally pyrolyzed corn stover has higher recovered mass yield, higher recovered energy yield and fixed carbon content than torrefied corn stover. Although external energy is provided by means of an internal combustion engine the net energy content of the final solid yield contained 92.2% of the input energy. The differential scanning calorimetry analysis showed that under the same heating rate regime and in oxygen-rich environment, the frictionally pyrolyzed corn stover had more exothermic decomposition than the torrefied material. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction ⇑ Corresponding author. Tel.: +39 0471 017635; fax: +39 0471 017009. E-mail addresses: [email protected] (S. Vakalis), bob@enginuityww. com (R. Heimann), [email protected] (A. Talley), [email protected] (N. Heimann), [email protected] (M. Baratieri). http://dx.doi.org/10.1016/j.fuel.2016.02.045 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
Utilization of solid biofuels has been a crucial fraction in the energy production mix. With a share of 30%, coal is the second largest source of primary energy and is responsible for over 40% of worldwide electricity production. In the year 2013 coal and was
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the fastest-growing fossil fuel by adding more primary energy into the energy mix than any other fuel. It is expected that the global demand for coal by the year 2019 will reach the level of 9 billion tons [1]. Coal has been the dominant fuel for the power sectors of several developed countries like Australia, Germany and United Kingdom [2]. According to Mohr et al., the intensive use of coal in countries like USA and Chine will eventually lead to ‘peak-coal’ production before 2025 [3]. Therefore utilization of alternative renewable fuels needs to increase in the scheme of a more sustainable power production sector. This shift is also supported by the European Council which, in October 2014, agreed on a new framework for reducing EU’s domestic greenhouse gas emissions by at least 40% by 2030 relative to 1990 levels. Especially in the European Union Emission Trading Scheme, or EU ETS, a reduction of at least 43% (base year 2005) is expected [4]. ETS started in 2005 and from the beginning approximately 1.500 power generation plants participated in this Scheme. In total, 86% of the total capacity was from thermal power plants that use coal and natural gas. The scope of ETS is to set an annual carbon dioxide emission cap which is a limit that the power generation facilities (among other industries) should not exceed [5]. The aim to reduce CO2 emissions from the coal sector has been partially pursued by of co-combusting coal with biomass which is a renewable and carbon neutral resource [6]. This behavior is also enhanced by the significant potential of agricultural residues. In the framework of this study corn stover will be examined as a feedstock and thus analyzed through the experiments that will be presented. The potential and the utilization of agricultural residues for energy production has been thoroughly examined by several researchers and the results have been very promising [7–10]. Nonetheless, it should be pointed out that biomass may have significant drawbacks when co-combusted with coal due to the different combustion characteristics, i.e. ignition behavior and flame stability [6]. In addition Vassilev et al. have noted as well the issues of high moisture and low energy density [11]. Thus a tendency to create bio-coal, charcoal and other carbon-rich products from biomass resources has been observed. These products have several differences with each other but they also share some similarities. After treatment, the materials become more energy dense, a factor that enhances transportation and storage. Additional aspects that favor the storing are that resulting materials become more water resistant and that the decomposition/rotting process of biomass is reduced significantly. The two most applied methods for thermal conversion of biomass into carbon-rich products are mentioned below. The method of producing charcoal/biochar by means of partial oxidation in an air restricted environment has been a wellknown practice. Early applications included charcoal production in pit kilns within forest areas. Nonetheless this traditional production method has gradually been replaced by more permanent structures, i.e. brick and metal kilns [12]. At present the production of charcoal in metal rotary kilns is by far the most representative for the most areas worldwide, and is generally defined as carbonization. The process can mainly be divided in three fundamental stages, although further and more analytical clustering of the stages has also been identified in the literature [12,13]. These three main stages are drying, decomposition/pyrolysis and a final heating stage which enhances the removal of tar compounds and the increase of the fixed carbon content. The final charcoal product after the final heating stage is known as soft-burned and may contain tar compounds up to 30% of its weight [14]. Wu et al. (2014) recovered wood vinegar yields of 25% by carbonizing fir sawdust and cotton stalk at 350 °C. This result shows that several liquid products are also an important part of the final product mix. In this case, the produced wood vinegar contained primarily acids, phenols and ketones [15]. Nonetheless, the solid fraction of the process
is of high interest in most cases, due to the applications in the energy production sector but also due to the utilization as soil amendment [16]. A notable feature of the process is the variety of types of biomass that have been used as feedstock for biochar production. Characteristic examples are the carbonization of secondary forest wood [17], poultry litter [18], paper mill sludge [19] and oil palm biomass [20]. Charcoal products have increased heating value in comparison to the raw feedstock, an aspect that is advantageous in energy production. On the other hand, the application of biochar as soil amendment has both economic and environmental benefits due to carbon sequestration and avoided emissions due to the utilization of biochar instead of lime [21]. The upper temperature range of carbonization process is between 500 °C and 600 °C. The temperature of operation during the process directly influences the quality of the final product [12]. Throughout the years, several optimization strategies have been applied in order to improve the process including new kiln designs and utilization of the produced gaseous fraction for pre-heating [13]. However this process still remains rather inefficient with an average energy loss of 60%. In addition the production costs are high, the process is not continuous and the amount of produced waste is significant [14]. The second representative process for converting biomass into an energy dense carbon-rich material is torrefaction. It can be defined as mild and slow pyrolysis, where the objective is the production of roasted/mildly pyrolyzed solid yield regularly referred as char or biochar. The usual temperature of operation is between 200 and 300 °C and the process takes place in principle under oxygen – free environment. The heating source is provided externally contrary to the carbonization that was mentioned above [23]. Comparing the attributes of the products from the two processes mentioned above has been the aim of several studies [22–24]. Park et al. concluded that torrefaction tends to produce products with higher heating value than carbonization. On the other hand, products from carbonization have combustion and decomposition properties that are closer to coal in comparison to the properties of torrefied biomass [25–28]. Torrefaction of biomass may produce a wide range of produced solid yield which varies from 24% to 95% mainly according to the characteristics of the feedstock and the operating conditions, i.e. temperature and retention time of the process. The regulation of these operating conditions has the scope of optimal solid yield and can be altered according to the type of the biomass feedstock [26]. In the aspect of product characteristics, torrefaction of lignocellulosic (woody) biomass produces condensed aromatic structures [29]. Moreover the presence of acetoxy- and methoxy-groups in xylose results to the formation of volatiles like acetic acid and methanol [30]. Torrefied biomass has interesting characteristics like hydrophobic behavior, higher grindability and increased recovered energy yield. Reppellin et al. showed that the energy demand for grinding of torrefied biomass can be up to 90% decreased for specific types of biomass [31]. The authors also indicated that there is a correlation between the temperature of torrefaction and the decease of energy demand for grinding the torrefied product. This correlation has also been studied by Ohliger et al. and the results were of similar nature [32]. Pellets from torrefied biomass in principle absorb less moisture and have higher heating value but also with reduced hardness than the raw biomass pellets [33,34]. Finally, there is a wide range of alternative biomass types which have been studied for their torrefaction behavior in addition to the conventional lignocellulosic biomass. Toscano et al. studied the torrefaction of residues from the tomato industry. According to the authors the effects of torrefaction become evident from relatively low temperatures, i.e. 214 °C. In this case torrefaction was utilized not only for increasing the heating value of the feedstock but also to stabilize it [35]. Torrefaction of food waste showed that the increasing temperature of
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treatment results to decreasing O/C ratios due to the volatilization of oxygen-rich compounds which occurs in lower temperatures than the volatilization of hydrocarbon gases [36]. Similar results concerning the reduction of oxygen content were observed also in torrefaction of olive mill waste [37]. On the other hand, torrefaction has several difficulties in the treatment of agricultural residues with strongly bounded saturated water like corn stover, which is our examined fuel [38,39]. The feedstock in these cases does not start to convert for temperatures that the reaction is observable for other fuels like larch. Also for torrefaction of biomass with high content of deciduous xylan like straw and corn stover, a higher mass loss is observed with increasing temperature [29]. Hence, torrefaction of corn stover faces several difficulties due to the low conversion rate in lower temperatures of treatment and high mass loss (and energy loss) for higher temperatures of treatment. Finally, a major drawback for torrefaction reactors is that pretreatment is necessary in order to ensure uniformity but also to control the level of humidity and the particle size [40]. In each case the pretreatment depends also from the type of the reactor and exact conditions of the process. In addition, most of the torrefaction processes take place in screw reactors, drum reactors or belt conveyors which have limited scalability. Also not all the reactors can operate on a continuous-mode [41]. The existing state-of-the-art technologies for converting biomass into carbonaceous products have numerous applications and some of them are very promising for the future. Nonetheless at the moment there is not a reliable method that can operate continuously and convert moisture resistant, fibrous and nonpretreated biomass feedstock to a carbon-rich product with high heating value. Thus, the scope of this manuscript is the introduction of an alternative method for converting biomass into a carbon-rich product that will be from now on defined as frictional pyrolysis (FP). The method has been invented and patented by Heimann [42]. The method is comprised of a rotary friction auger that dries and subsequently pyrolyzes the input feedstock only by means of friction and pressure. It has the advantage of operating on a continuous mode and can successfully treat materials with strongly bounded saturated water. In principle, the method can be divided in three fundamental steps: pressurized drying, known also as steam explosion, subsequent pyrolysis of the feedstock only by the application of increased pressure and friction and the final stage of cooling and tar condensation. Additional novel patented concepts have been integrated in order to enhance different parts of the process [43–46]. A detailed description of frictional pyrolysis can be found in chapter 3 of this manuscript. In the framework of this paper the operating principles behind frictional pyrolysis are described. Moreover, the performance of frictional pyrolysis is assessed in comparison to torrefaction of corn stover. The raw feedstock along with the products from the two processes is thermochemically analyzed. Mass balances and recovered energy yield calculations have been performed.
taken into consideration that in industrial applications of torrefaction, the volatile compounds can be utilized in order to run the process. Thus, this study acknowledges fully the potential of torrefaction as an industrial process. Initially, frictional pyrolysis was performed with a rotary friction unit with a nominal production of 1 ton of input per hour. The final mass yield was close to 90% and consists of the solid yield and tar compounds that are trapped in the pores of the char. Recapturing of the tar back to the solid fraction is a characteristic of this technology which is described in detail in the following chapter. Subsequently the same corn stover feedstock has been torrefied at a bench scale reactor. The bench scale reactor is a quartz reactor a tubular with a 20 mm diameter and 45 mm length that is surrounded by an electric furnace. A picture of the apparatus is shown on Fig. 1. The nitrogen flow was controlled by means of a digital flux-meter and the temperature was measured with a mobile data logger and K-type thermocouples. The applied temperatures were 265 °C, 285 °C and 295 °C. In lower temperatures the observable carbonization was very low. The torrefied product at 265 °C was selected for the analysis because it had the highest yield, i.e. 70%, but still was lower than the yield from frictional pyrolysis. For the higher temperatures the yield decreased significantly until the level of 52% for treatment at 295 °C. Again it has to be pointed out that the experiment was performed on a lab scale. Nonetheless, the recovered yields are within the range of values that can be found in relevant endeavors. Torrefied material was used as a reference for comparison on a qualitative basis with the materials produced from frictional pyrolysis. A full scale comparison between the two technologies is beyond the scope of this manuscript. A Simultaneous Thermal Analyzer (STA 449F3, Netzsch) has been utilized for the implementation of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). By means of STA, several physicochemical characteristics of the input and produced solid materials have been assessed, i.e. the decomposition rates and the endothermicity/exothermicity under pyrolysis and combustion conditions. In both cases the heating rate has been set to 50 °C/min and the temperature range was from 40 °C to 1000 °C. The utilized atmosphere was nitrogen (N2) for pyrolysis and air for combustion experiments. The analysis of the feedstock and the products from frictional pyrolysis and torrefaction, has been performed at the laboratories of RE-CORD consortium (Florence, Italy) by means of the standard UNI EN 15104 for elemental analysis, the standard UNI EN 14774 for moisture, the standard
2. Materials and methods As mentioned above, two different methods have been applied for the production of carbon-rich products from corn stover. The same feedstock has been treated both with frictional pyrolysis and torrefaction. It has to be mentioned that the torrefaction experiments have been implemented on a pilot/lab scale. Thus energy balances have been implemented only for the case of frictional pyrolysis. An internal combustion engine is utilized to run the frictional pyrolysis process. The engine is used for the production of mechanical energy for the rotation of the auger. The engine does not provide any heat to system. On the other hand it has to be
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Fig. 1. Apparatus utilized for torrefaction experiments.
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UNI EN 14775 for ash and the standard UNI EN 14918 for the determination of higher heating value. By utilizing the measurements from the processes and the results from the analysis mass balances and recovered energy yield calculations have been performed. The mass balances have been implemented by means of STAN 2.5. For the case of frictional pyrolysis also an energy balance is performed. For the purpose of this analysis the rotary unit has been operated both with and without the reflux condenser in order to calculate the energy (by means of balancing methods) of the hydrocarbons that are volatilized and recaptured. The heating values have been measured along the process with an IKA C200 bomb calorimeter which is the same brand and type (but different actual instrument) that was used for the determination of the heating value of the other materials throughout the manuscript. Except from the elemental composition analysis, further analysis was performed in order to retrieve more information about the nature of the materials. The different functional groups of the chemical compounds that compose the feedstock and the products have been identified by means of Attenuated Total Reflectance (ATR). For this analysis a Brucker Tensor II was used. 3. Theory The method of frictional pyrolysis primarily consists of a friction auger which is defined by Heimann [29] as ‘‘Rotary Compression Unit”. The reason behind this terminology is that the friction auger can be adjusted in order to compress the feedstock under different levels of pressure and thus regulate the level of transformation that the material undergoes. Hence, it is possible to regulate the operating condition in the chamber and thus control the characteristics of the product. Characteristically, by operating the system in mild conditions, the feedstock can be dried and densified. Nonetheless, one of the main scopes of this manuscript is to describe the conversion of biomass into a carbonaceous solid product and the analysis of the operating conditions that drive the conversion process. The successful conversion of the materials requires not a single auger but a series of augers which represent the main subprocesses. Frictional pyrolysis can mainly be divided into three main sub-processes: (i) steam explosion with or without the enhancement of acid, (ii) pyrolysis and (iii) tar condensation and cooling. The process scheme is shown on Fig. 2 where the different parts are clearly separated. The unit that was utilized for the implementation of the experiments – that will be presented in the framework of this paper – is driven by a 74.57 kW electric motor. As already mentioned above, the engine does not provide
any heat to the system but only rotates the set of augers in order to produce friction, a technological possibility that is novel. However similar endeavors can be found in the literature, e.g. the concept of destructive distillation that was introduced by Loomans et al. [46]. The first step of the process is to remove via steam explosion the water from the rest of the feedstock by means of the heat generated by friction. Conventional steam explosion is the process when water violently flashes into steam due to rapid heating [47]. Steam explosion is triggered by a pressure pulse which results to a particle velocity in the coolant liquid [48]. Alvira et al. mention that acid presoaking is a rather common technique in order to enhance steam explosion [49]. In the aspect of acid presoaking, several other endeavors have been implemented where lignocellulosic biomass has been premixed with acids (and in most cases also starch) before the initiation of the process [50–52]. Nonetheless, the pretreated feedstock is utilized mainly for non-thermal processes like anaerobic digestion and ethanol production. The same concept, i.e. of acid impregnation, is applied in the method of frictional pyrolysis, in order to enhance the hydrolysis of the feedstock. Before the initiation of the friction the material is mixed with hydroxide which has mass yield less than 0.5% for the case of corn stover and may reach up to 1% for other biomass fuels. The process has been patented and analytically described by Heimann [43,44]. Steam explosion takes place after the drop in the pressure in the friction auger and in this area due to the rapid evaporation of water the structure of the biomass is ruptured [53]. The augers are air-tightly connected with the surrounding frame of the Rotary Friction Unit. There are parts along the surrounding frame that have additional depth/volume where the release of pressure takes place and this results to steam explosion but also to the release of the tar compounds in the second stage.
4. Results and discussion Table 1 shows the results from the elemental analysis for corn stover, torrefied corn stover and frictionally carbonized corn stover on dry basis. It has to be stated that oxygen is calculated with balancing methods. The increase in carbon content for both treatments is significant and comparable with 53.8% for torrefaction and 53.2% for frictional carbonization from the initial 42.5% for raw corn stover. Furthermore, it is interesting to denote that for frictional pyrolyzed material the hydrogen content is lower, i.e. 4.6%, in comparison to 5.1% for torrefaction, nonetheless the difference is only 0.5%. Finally the ash content for torrefaction is higher than the one for frictional carbonization, which is an indication for a higher solid yield recovered by the latter process. Fig. 3 presents the mass balances for the two processes and the recovered energy yield for each process. The solid mass that was recovered from torrefaction was 70.1% and the recovered energy yield was 91.32%, values that are typical for torrefaction at this temperature, i.e. 250–260 °C, and a retention time of 30 min and consistent with the values that can be found in the literature [26,27]. On the other hand, the solid mass that was recovered from
Table 1 Elemental analysis of raw and treated corn stover.
Fig. 2. Frictional pyrolysis process scheme – series of augers.
C H N O Ash
Corn Stover % db
Torrefaction (265 °C)
Frictional pyr.
42.5 5.5 1.1 36.08 14.81
53.8 5.1 1.2 11.27 28.63
53.2 4.6 1.4 18.27 22.53
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Fig. 3. Mass balances and recovered energy yield for frictional pyrolysis and torrefaction.
Table 2 Analysis and balances of raw and treated materials.
Ash (% wb) H2O (% wb) Solid yield (mass) HHV (MJ/kg) LHV (MJ/kg) Fixed carbon Recovered energy yield
Corn stover
Torrefaction (265 °C)
Frictional pyr.
13.41 9.5 100% 18.235 16.746 6.09% 1
26.57 7.2 70.1% 22.868 21.529 22.4% 91.32%
20.73 8 80.4% 21.249 20.002 28.31% 96.05%
frictional pyrolysis was 80.4% and the recovered energy yield was 96.05%. The full range of numbers that were used for this analysis, like heating values and recovered masses, but also values from other analysis like fixed carbon content can be found on Table 2. In detail the lower heating values increased 12.85% for torrefied corn stover and 11.94% for frictionally carbonized corn stover in comparison to the initial 16.746 MJ/kg of the raw corn stover. It is interesting that although the torrefied material has higher ash content than the frictionally pyrolyzed product, it also has higher carbon content and lower moisture content and oxygen content. These factors result to torrefied biomass having a higher LHV. The process of frictional pyrolysis has been performed on a pilot scale unit which operates with external energy provided by an engine. Fig. 4 shows the energy balance and the energy flows of the system for an hour of operation. The energy balance is presented on a HHV basis and the final yield contains 92.2% of the input energy. The inputs of the balance are the energy content of the feedstock and the energy provided by the engine. The energy that is lost consists primarily of two
streams. The water is removed from the system in the form of steam and the other stream represents the losses. The energy content of the steam is the enthalpy of the steam at the temperature of the exit, i.e. 120 °C. The amount of steam has been calculated by measuring the moisture of the feedstock before and after the removal and is approximately 85 kg (for 1 ton of input). As mentioned in the ‘Materials and Methods’ section the Rotary Compression Unit has been operated both with and without the reflux condenser in order to measure the heating value of the solid fraction on a tar-free basis. The mass yield of this solid fraction was approximately 72.56% and higher heating value was measured to be 21.29 MJ/kg. Fig. 5 shows the TG pyrolysis curves for raw corn stover, torrefied stover and frictionally carbonized stover. The decomposition curve for the torrefied stover is relatively typical for torrefied material, which mainly consists by cellulosic compounds due to the volatilization of hemicellulose and lignin compounds during torrefaction [54,55]. An interesting aspect is that the decomposition of the frictionally pyrolyzed material starts in lower temperatures, i.e. from 180 °C, which indicates the presence of hemicellulose and lignin compounds. Hence, this decomposition curve shows that the process of frictional carbonization follows a different transformation mechanism than the conventional combination of devolatilization and surface roasting. Developing a mechanism for this process goes beyond the scope of this paper and will be addressed by the authors in future publications. Chaula et al. stated that primarily hemicellulose is removed due to steam explosion [56]. Hence a possible driving factor could be identified as the exothermicity of pyrolysis which in principle is enhanced by the higher presence of lignin as state by Roberts et al. [57] in the addition to the transformation of acetyl group compounds to acids due to the autocat-
Fig. 4. Energy balance for frictional pyrolysis.
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Fig. 5. TG pyrolysis curves for raw corn stover, torrefied stover and frict. carbonized stover.
Fig. 6. TG combustion curves for raw corn stover, torrefied stover and frict. pyrolyzed stover.
alytic effect of steam which drives the rupturing of the biomass and the roasting [50]. Nonetheless, further studies and analysis are necessary for the comprehension and the modelling of frictional pyrolysis. Furthermore on Fig. 5 the final solid yield, which is constituted from the fixed carbon and the ash, is increased for both torrefaction and frictional carbonization. The levels are relatively similar with fractional pyrolysis having a small edge with 49.1% contrary to 48.3% for torrefaction. This increased solid yield for both treatment
methods implied significantly increased fixed carbon content for both cases in comparison to raw corn stover. Biswas has observed that steam pretreatment increases the fixed carbon content [58] and Pourmakhdomi has correlated steam explosion with increased fixed carbon as well [59]. Therefore the combination of steam explosion and carbonization/pyrolysis may have several other interesting applications. Surface combustion and decomposition combustion are the main types of combustion in industrial processes. It propagates
Fig. 7. DSC curves for combustion for raw corn stover, torrefied stover and frict. pyrolyzed stover.
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Fig. 8. Absorbance spectrums of frictionally pyrolyzed corn stover (a) and torrefied corn stover (b).
at temperatures higher or equal to the ignition point and requires fuels of high carbon content (fixed carbon). Thus, determining the fixed carbon content is a good indicator of the fuel quality. Fig. 6 shows the combustion TG curves; frictionally carbonized corn stover has lower ash content than the torrefied one. These ash content results have also been verified by means of applying the standard UNI EN 14,775, which provided identical results. The fixed carbon content for frictional pyrolysis is 28.3% in comparison to 22.4% for torrefaction. Fig. 7 shows the enthalpy changes from differential scanning calorimetry. Although torrefied corn stover is more exothermic than raw corn stover, frictionally torrefied corn stover has almost double levels of exothermicity than torrefied stover with 15 lV/mg vs 8 lV/mg. Finally, Fig. 8 shows the absorbance spectrum results from the ATR analysis. For the frictionally pyrolyzed stover (Fig. 8a) there are main peaks for the spectrum 900 cm 1 and the spectrum range from 550 to 600 cm 1. These peaks can be explained from the presence of carboxylic acids, alkyl halides – with a C–Br stretch and a N–H wag – but also the presence of aromatic hydrocarbons with an O–H bend. Finally, the peak at the range from 2200 to 2400 cm 1 indicates the presence of nitriles with a C (triple bond)N stretch which could be also justified by the high presence of nitrogen in the frictionally carbonized product. Contrary to the frictionally pyrolyzed stover, the torrified stover (Fig. 8b) totally lacks the peak at the range from 2200 to 2400 cm 1 which as mentioned before would indicate the presence of nitriles. On the other hand a peak at the wavelength of 1100–2400 cm 1 indicates the presence of aliphatic amines with C–N stretch. 5. Conclusions In conclusion, a novel method for producing charcoal or biocoal from lignocellulosic biomass is introduced in the framework of this paper and is defined from now on as frictional carbonization. The process takes place in a Rotary Compression Unit and consists of a set of friction augers, i.e. a rotary dryer, a reflux condenser and
an after-cooler. Primarily, it can be divided into three stages: steam explosion with and without acid enhancement, carbonization, and tar cooling and condensation. In order to provide a comparison basis with an advanced but also established technology, corn stover treated with frictional carbonization has been compared to corn stover treated with torrefaction. This novel process recovers more than 80% of the initial mass yield and 96% of the energy compared to torrefaction that has 70% recovered yield and 91% of the energy. In addition it has higher fixed carbon content with 28.3% versus 22.4% for torrefaction, although more volatiles have been removed during torrefaction. Frictionally carbonized corn stover shows higher exothermic behavior during combustion 15 lV/mg at 580 °C vs 8 lV/mg at 580 °C for torrefied material. Finally, thermogravimetric decomposition curves show that frictionally carbonized corn stover has also high content of hemicellulose and lignin contrary to torrefied corn stover which primarily consists of cellulosic compounds. References [1] International Energy Agency. Medium-term coal report 2014: market analysis and forecasts to 2019; 2014. ISBN 978-92-64-22188-8. [2] World Bank. Electricity production from coal sources (% of total). Retrieved online on 18th June 2015.. [3] Mohr SH, Wang J, Ellem G, Ward J, Giurco D. Projection of world fossil fuels by country. Fuel 2015;141:120–35. http://dx.doi.org/10.1016/j.fuel.2014.10.030. ISSN 0016-2361. [4] Bonn M, Heitmann N, Reichert G, Voßwinkel JS. EU climate and energy policy 2030 comments on an evolving framework. CEP; 2015. [5] Berghmans N, Alberola E. The power sector in phase 2 of the EU ETS: fewer CO2 emissions but just as much coal. [6] Sahu SG, Chakraborty N, Sarkar P. Coal–biomass co-combustion: an overview. Renew Sustain Energy Rev 2014;39:575–86. http://dx.doi.org/10.1016/j. rser.2014.07.106. ISSN 1364-0321. [7] Matsumura Y, Minowa T, Yamamoto H. Amount, availability, and potential use of rice straw (agricultural residue) biomass as an energy resource in Japan. Biomass Bioenergy 2005;29(5):347–54. http://dx.doi.org/10.1016/j. biombioe.2004.06.015. ISSN 0961-9534. [8] Ergüdenler A, Isßiǧigür A. Agricultural residues as a potential resource for environmentally sustainable electric power generation in Turkey. Renewable Energy 1994;5(5–8):786–90. http://dx.doi.org/10.1016/0960-1481(94)900884. ISSN 0960-1481.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Introduction to frictional pyrolysis (FP) – An alternative method for converting biomass to solid carbonaceous products S. Vakalis a,⇑, R. Heimann b, A. Talley b, N. Heimann b, M. Baratieri a a b
Free University of Bozen - Bolzano, Faculty of Science and Technology, Piazza Università 5, 39100 Bolzano, Italy Enginuity Worldwide LLC, 651 Commerce Road, Mexico, MO 65265, United States
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Introduction of a novel method of
pyrolysis via friction and pressure. The process time is less than 200 s. The final solid yield is higher than
80% and has 96% of the initial energy content. The net energy balance is 92.2% on HHV basis. The product has increased fixed carbon content in comparison to torrefied biomass.
a r t i c l e
i n f o
Article history: Received 24 September 2015 Received in revised form 9 December 2015 Accepted 10 February 2016 Available online 15 February 2016 Keywords: Biocoal Charcoal Carbonization Steam explosion Corn stover STA
a b s t r a c t In the biomass sector, technologies like carbonization and torrefaction have been utilized for the production of solid carbonaceous biofuels and materials. In the framework of this manuscript a novel method for production of solid carbonaceous materials is introduced and is defined from now on as frictional pyrolysis. It uses only the application of pressure and friction whereas no external heat transfer is needed for the propagation of the process. This novel method is compared with torrefaction in order to assess its potential to process corn stover which has strongly bounder water and high content of deciduous xylan. Mass balances have been implemented for both technologies. Characterization of the products has been done by means of Simultaneous Thermal Analysis and Elemental analysis. Frictionally pyrolyzed corn stover has higher recovered mass yield, higher recovered energy yield and fixed carbon content than torrefied corn stover. Although external energy is provided by means of an internal combustion engine the net energy content of the final solid yield contained 92.2% of the input energy. The differential scanning calorimetry analysis showed that under the same heating rate regime and in oxygen-rich environment, the frictionally pyrolyzed corn stover had more exothermic decomposition than the torrefied material. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction ⇑ Corresponding author. Tel.: +39 0471 017635; fax: +39 0471 017009. E-mail addresses: [email protected] (S. Vakalis), bob@enginuityww. com (R. Heimann), [email protected] (A. Talley), [email protected] (N. Heimann), [email protected] (M. Baratieri). http://dx.doi.org/10.1016/j.fuel.2016.02.045 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
Utilization of solid biofuels has been a crucial fraction in the energy production mix. With a share of 30%, coal is the second largest source of primary energy and is responsible for over 40% of worldwide electricity production. In the year 2013 coal and was
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the fastest-growing fossil fuel by adding more primary energy into the energy mix than any other fuel. It is expected that the global demand for coal by the year 2019 will reach the level of 9 billion tons [1]. Coal has been the dominant fuel for the power sectors of several developed countries like Australia, Germany and United Kingdom [2]. According to Mohr et al., the intensive use of coal in countries like USA and Chine will eventually lead to ‘peak-coal’ production before 2025 [3]. Therefore utilization of alternative renewable fuels needs to increase in the scheme of a more sustainable power production sector. This shift is also supported by the European Council which, in October 2014, agreed on a new framework for reducing EU’s domestic greenhouse gas emissions by at least 40% by 2030 relative to 1990 levels. Especially in the European Union Emission Trading Scheme, or EU ETS, a reduction of at least 43% (base year 2005) is expected [4]. ETS started in 2005 and from the beginning approximately 1.500 power generation plants participated in this Scheme. In total, 86% of the total capacity was from thermal power plants that use coal and natural gas. The scope of ETS is to set an annual carbon dioxide emission cap which is a limit that the power generation facilities (among other industries) should not exceed [5]. The aim to reduce CO2 emissions from the coal sector has been partially pursued by of co-combusting coal with biomass which is a renewable and carbon neutral resource [6]. This behavior is also enhanced by the significant potential of agricultural residues. In the framework of this study corn stover will be examined as a feedstock and thus analyzed through the experiments that will be presented. The potential and the utilization of agricultural residues for energy production has been thoroughly examined by several researchers and the results have been very promising [7–10]. Nonetheless, it should be pointed out that biomass may have significant drawbacks when co-combusted with coal due to the different combustion characteristics, i.e. ignition behavior and flame stability [6]. In addition Vassilev et al. have noted as well the issues of high moisture and low energy density [11]. Thus a tendency to create bio-coal, charcoal and other carbon-rich products from biomass resources has been observed. These products have several differences with each other but they also share some similarities. After treatment, the materials become more energy dense, a factor that enhances transportation and storage. Additional aspects that favor the storing are that resulting materials become more water resistant and that the decomposition/rotting process of biomass is reduced significantly. The two most applied methods for thermal conversion of biomass into carbon-rich products are mentioned below. The method of producing charcoal/biochar by means of partial oxidation in an air restricted environment has been a wellknown practice. Early applications included charcoal production in pit kilns within forest areas. Nonetheless this traditional production method has gradually been replaced by more permanent structures, i.e. brick and metal kilns [12]. At present the production of charcoal in metal rotary kilns is by far the most representative for the most areas worldwide, and is generally defined as carbonization. The process can mainly be divided in three fundamental stages, although further and more analytical clustering of the stages has also been identified in the literature [12,13]. These three main stages are drying, decomposition/pyrolysis and a final heating stage which enhances the removal of tar compounds and the increase of the fixed carbon content. The final charcoal product after the final heating stage is known as soft-burned and may contain tar compounds up to 30% of its weight [14]. Wu et al. (2014) recovered wood vinegar yields of 25% by carbonizing fir sawdust and cotton stalk at 350 °C. This result shows that several liquid products are also an important part of the final product mix. In this case, the produced wood vinegar contained primarily acids, phenols and ketones [15]. Nonetheless, the solid fraction of the process
is of high interest in most cases, due to the applications in the energy production sector but also due to the utilization as soil amendment [16]. A notable feature of the process is the variety of types of biomass that have been used as feedstock for biochar production. Characteristic examples are the carbonization of secondary forest wood [17], poultry litter [18], paper mill sludge [19] and oil palm biomass [20]. Charcoal products have increased heating value in comparison to the raw feedstock, an aspect that is advantageous in energy production. On the other hand, the application of biochar as soil amendment has both economic and environmental benefits due to carbon sequestration and avoided emissions due to the utilization of biochar instead of lime [21]. The upper temperature range of carbonization process is between 500 °C and 600 °C. The temperature of operation during the process directly influences the quality of the final product [12]. Throughout the years, several optimization strategies have been applied in order to improve the process including new kiln designs and utilization of the produced gaseous fraction for pre-heating [13]. However this process still remains rather inefficient with an average energy loss of 60%. In addition the production costs are high, the process is not continuous and the amount of produced waste is significant [14]. The second representative process for converting biomass into an energy dense carbon-rich material is torrefaction. It can be defined as mild and slow pyrolysis, where the objective is the production of roasted/mildly pyrolyzed solid yield regularly referred as char or biochar. The usual temperature of operation is between 200 and 300 °C and the process takes place in principle under oxygen – free environment. The heating source is provided externally contrary to the carbonization that was mentioned above [23]. Comparing the attributes of the products from the two processes mentioned above has been the aim of several studies [22–24]. Park et al. concluded that torrefaction tends to produce products with higher heating value than carbonization. On the other hand, products from carbonization have combustion and decomposition properties that are closer to coal in comparison to the properties of torrefied biomass [25–28]. Torrefaction of biomass may produce a wide range of produced solid yield which varies from 24% to 95% mainly according to the characteristics of the feedstock and the operating conditions, i.e. temperature and retention time of the process. The regulation of these operating conditions has the scope of optimal solid yield and can be altered according to the type of the biomass feedstock [26]. In the aspect of product characteristics, torrefaction of lignocellulosic (woody) biomass produces condensed aromatic structures [29]. Moreover the presence of acetoxy- and methoxy-groups in xylose results to the formation of volatiles like acetic acid and methanol [30]. Torrefied biomass has interesting characteristics like hydrophobic behavior, higher grindability and increased recovered energy yield. Reppellin et al. showed that the energy demand for grinding of torrefied biomass can be up to 90% decreased for specific types of biomass [31]. The authors also indicated that there is a correlation between the temperature of torrefaction and the decease of energy demand for grinding the torrefied product. This correlation has also been studied by Ohliger et al. and the results were of similar nature [32]. Pellets from torrefied biomass in principle absorb less moisture and have higher heating value but also with reduced hardness than the raw biomass pellets [33,34]. Finally, there is a wide range of alternative biomass types which have been studied for their torrefaction behavior in addition to the conventional lignocellulosic biomass. Toscano et al. studied the torrefaction of residues from the tomato industry. According to the authors the effects of torrefaction become evident from relatively low temperatures, i.e. 214 °C. In this case torrefaction was utilized not only for increasing the heating value of the feedstock but also to stabilize it [35]. Torrefaction of food waste showed that the increasing temperature of
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treatment results to decreasing O/C ratios due to the volatilization of oxygen-rich compounds which occurs in lower temperatures than the volatilization of hydrocarbon gases [36]. Similar results concerning the reduction of oxygen content were observed also in torrefaction of olive mill waste [37]. On the other hand, torrefaction has several difficulties in the treatment of agricultural residues with strongly bounded saturated water like corn stover, which is our examined fuel [38,39]. The feedstock in these cases does not start to convert for temperatures that the reaction is observable for other fuels like larch. Also for torrefaction of biomass with high content of deciduous xylan like straw and corn stover, a higher mass loss is observed with increasing temperature [29]. Hence, torrefaction of corn stover faces several difficulties due to the low conversion rate in lower temperatures of treatment and high mass loss (and energy loss) for higher temperatures of treatment. Finally, a major drawback for torrefaction reactors is that pretreatment is necessary in order to ensure uniformity but also to control the level of humidity and the particle size [40]. In each case the pretreatment depends also from the type of the reactor and exact conditions of the process. In addition, most of the torrefaction processes take place in screw reactors, drum reactors or belt conveyors which have limited scalability. Also not all the reactors can operate on a continuous-mode [41]. The existing state-of-the-art technologies for converting biomass into carbonaceous products have numerous applications and some of them are very promising for the future. Nonetheless at the moment there is not a reliable method that can operate continuously and convert moisture resistant, fibrous and nonpretreated biomass feedstock to a carbon-rich product with high heating value. Thus, the scope of this manuscript is the introduction of an alternative method for converting biomass into a carbon-rich product that will be from now on defined as frictional pyrolysis (FP). The method has been invented and patented by Heimann [42]. The method is comprised of a rotary friction auger that dries and subsequently pyrolyzes the input feedstock only by means of friction and pressure. It has the advantage of operating on a continuous mode and can successfully treat materials with strongly bounded saturated water. In principle, the method can be divided in three fundamental steps: pressurized drying, known also as steam explosion, subsequent pyrolysis of the feedstock only by the application of increased pressure and friction and the final stage of cooling and tar condensation. Additional novel patented concepts have been integrated in order to enhance different parts of the process [43–46]. A detailed description of frictional pyrolysis can be found in chapter 3 of this manuscript. In the framework of this paper the operating principles behind frictional pyrolysis are described. Moreover, the performance of frictional pyrolysis is assessed in comparison to torrefaction of corn stover. The raw feedstock along with the products from the two processes is thermochemically analyzed. Mass balances and recovered energy yield calculations have been performed.
taken into consideration that in industrial applications of torrefaction, the volatile compounds can be utilized in order to run the process. Thus, this study acknowledges fully the potential of torrefaction as an industrial process. Initially, frictional pyrolysis was performed with a rotary friction unit with a nominal production of 1 ton of input per hour. The final mass yield was close to 90% and consists of the solid yield and tar compounds that are trapped in the pores of the char. Recapturing of the tar back to the solid fraction is a characteristic of this technology which is described in detail in the following chapter. Subsequently the same corn stover feedstock has been torrefied at a bench scale reactor. The bench scale reactor is a quartz reactor a tubular with a 20 mm diameter and 45 mm length that is surrounded by an electric furnace. A picture of the apparatus is shown on Fig. 1. The nitrogen flow was controlled by means of a digital flux-meter and the temperature was measured with a mobile data logger and K-type thermocouples. The applied temperatures were 265 °C, 285 °C and 295 °C. In lower temperatures the observable carbonization was very low. The torrefied product at 265 °C was selected for the analysis because it had the highest yield, i.e. 70%, but still was lower than the yield from frictional pyrolysis. For the higher temperatures the yield decreased significantly until the level of 52% for treatment at 295 °C. Again it has to be pointed out that the experiment was performed on a lab scale. Nonetheless, the recovered yields are within the range of values that can be found in relevant endeavors. Torrefied material was used as a reference for comparison on a qualitative basis with the materials produced from frictional pyrolysis. A full scale comparison between the two technologies is beyond the scope of this manuscript. A Simultaneous Thermal Analyzer (STA 449F3, Netzsch) has been utilized for the implementation of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). By means of STA, several physicochemical characteristics of the input and produced solid materials have been assessed, i.e. the decomposition rates and the endothermicity/exothermicity under pyrolysis and combustion conditions. In both cases the heating rate has been set to 50 °C/min and the temperature range was from 40 °C to 1000 °C. The utilized atmosphere was nitrogen (N2) for pyrolysis and air for combustion experiments. The analysis of the feedstock and the products from frictional pyrolysis and torrefaction, has been performed at the laboratories of RE-CORD consortium (Florence, Italy) by means of the standard UNI EN 15104 for elemental analysis, the standard UNI EN 14774 for moisture, the standard
2. Materials and methods As mentioned above, two different methods have been applied for the production of carbon-rich products from corn stover. The same feedstock has been treated both with frictional pyrolysis and torrefaction. It has to be mentioned that the torrefaction experiments have been implemented on a pilot/lab scale. Thus energy balances have been implemented only for the case of frictional pyrolysis. An internal combustion engine is utilized to run the frictional pyrolysis process. The engine is used for the production of mechanical energy for the rotation of the auger. The engine does not provide any heat to system. On the other hand it has to be
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Fig. 1. Apparatus utilized for torrefaction experiments.
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UNI EN 14775 for ash and the standard UNI EN 14918 for the determination of higher heating value. By utilizing the measurements from the processes and the results from the analysis mass balances and recovered energy yield calculations have been performed. The mass balances have been implemented by means of STAN 2.5. For the case of frictional pyrolysis also an energy balance is performed. For the purpose of this analysis the rotary unit has been operated both with and without the reflux condenser in order to calculate the energy (by means of balancing methods) of the hydrocarbons that are volatilized and recaptured. The heating values have been measured along the process with an IKA C200 bomb calorimeter which is the same brand and type (but different actual instrument) that was used for the determination of the heating value of the other materials throughout the manuscript. Except from the elemental composition analysis, further analysis was performed in order to retrieve more information about the nature of the materials. The different functional groups of the chemical compounds that compose the feedstock and the products have been identified by means of Attenuated Total Reflectance (ATR). For this analysis a Brucker Tensor II was used. 3. Theory The method of frictional pyrolysis primarily consists of a friction auger which is defined by Heimann [29] as ‘‘Rotary Compression Unit”. The reason behind this terminology is that the friction auger can be adjusted in order to compress the feedstock under different levels of pressure and thus regulate the level of transformation that the material undergoes. Hence, it is possible to regulate the operating condition in the chamber and thus control the characteristics of the product. Characteristically, by operating the system in mild conditions, the feedstock can be dried and densified. Nonetheless, one of the main scopes of this manuscript is to describe the conversion of biomass into a carbonaceous solid product and the analysis of the operating conditions that drive the conversion process. The successful conversion of the materials requires not a single auger but a series of augers which represent the main subprocesses. Frictional pyrolysis can mainly be divided into three main sub-processes: (i) steam explosion with or without the enhancement of acid, (ii) pyrolysis and (iii) tar condensation and cooling. The process scheme is shown on Fig. 2 where the different parts are clearly separated. The unit that was utilized for the implementation of the experiments – that will be presented in the framework of this paper – is driven by a 74.57 kW electric motor. As already mentioned above, the engine does not provide
any heat to the system but only rotates the set of augers in order to produce friction, a technological possibility that is novel. However similar endeavors can be found in the literature, e.g. the concept of destructive distillation that was introduced by Loomans et al. [46]. The first step of the process is to remove via steam explosion the water from the rest of the feedstock by means of the heat generated by friction. Conventional steam explosion is the process when water violently flashes into steam due to rapid heating [47]. Steam explosion is triggered by a pressure pulse which results to a particle velocity in the coolant liquid [48]. Alvira et al. mention that acid presoaking is a rather common technique in order to enhance steam explosion [49]. In the aspect of acid presoaking, several other endeavors have been implemented where lignocellulosic biomass has been premixed with acids (and in most cases also starch) before the initiation of the process [50–52]. Nonetheless, the pretreated feedstock is utilized mainly for non-thermal processes like anaerobic digestion and ethanol production. The same concept, i.e. of acid impregnation, is applied in the method of frictional pyrolysis, in order to enhance the hydrolysis of the feedstock. Before the initiation of the friction the material is mixed with hydroxide which has mass yield less than 0.5% for the case of corn stover and may reach up to 1% for other biomass fuels. The process has been patented and analytically described by Heimann [43,44]. Steam explosion takes place after the drop in the pressure in the friction auger and in this area due to the rapid evaporation of water the structure of the biomass is ruptured [53]. The augers are air-tightly connected with the surrounding frame of the Rotary Friction Unit. There are parts along the surrounding frame that have additional depth/volume where the release of pressure takes place and this results to steam explosion but also to the release of the tar compounds in the second stage.
4. Results and discussion Table 1 shows the results from the elemental analysis for corn stover, torrefied corn stover and frictionally carbonized corn stover on dry basis. It has to be stated that oxygen is calculated with balancing methods. The increase in carbon content for both treatments is significant and comparable with 53.8% for torrefaction and 53.2% for frictional carbonization from the initial 42.5% for raw corn stover. Furthermore, it is interesting to denote that for frictional pyrolyzed material the hydrogen content is lower, i.e. 4.6%, in comparison to 5.1% for torrefaction, nonetheless the difference is only 0.5%. Finally the ash content for torrefaction is higher than the one for frictional carbonization, which is an indication for a higher solid yield recovered by the latter process. Fig. 3 presents the mass balances for the two processes and the recovered energy yield for each process. The solid mass that was recovered from torrefaction was 70.1% and the recovered energy yield was 91.32%, values that are typical for torrefaction at this temperature, i.e. 250–260 °C, and a retention time of 30 min and consistent with the values that can be found in the literature [26,27]. On the other hand, the solid mass that was recovered from
Table 1 Elemental analysis of raw and treated corn stover.
Fig. 2. Frictional pyrolysis process scheme – series of augers.
C H N O Ash
Corn Stover % db
Torrefaction (265 °C)
Frictional pyr.
42.5 5.5 1.1 36.08 14.81
53.8 5.1 1.2 11.27 28.63
53.2 4.6 1.4 18.27 22.53
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Fig. 3. Mass balances and recovered energy yield for frictional pyrolysis and torrefaction.
Table 2 Analysis and balances of raw and treated materials.
Ash (% wb) H2O (% wb) Solid yield (mass) HHV (MJ/kg) LHV (MJ/kg) Fixed carbon Recovered energy yield
Corn stover
Torrefaction (265 °C)
Frictional pyr.
13.41 9.5 100% 18.235 16.746 6.09% 1
26.57 7.2 70.1% 22.868 21.529 22.4% 91.32%
20.73 8 80.4% 21.249 20.002 28.31% 96.05%
frictional pyrolysis was 80.4% and the recovered energy yield was 96.05%. The full range of numbers that were used for this analysis, like heating values and recovered masses, but also values from other analysis like fixed carbon content can be found on Table 2. In detail the lower heating values increased 12.85% for torrefied corn stover and 11.94% for frictionally carbonized corn stover in comparison to the initial 16.746 MJ/kg of the raw corn stover. It is interesting that although the torrefied material has higher ash content than the frictionally pyrolyzed product, it also has higher carbon content and lower moisture content and oxygen content. These factors result to torrefied biomass having a higher LHV. The process of frictional pyrolysis has been performed on a pilot scale unit which operates with external energy provided by an engine. Fig. 4 shows the energy balance and the energy flows of the system for an hour of operation. The energy balance is presented on a HHV basis and the final yield contains 92.2% of the input energy. The inputs of the balance are the energy content of the feedstock and the energy provided by the engine. The energy that is lost consists primarily of two
streams. The water is removed from the system in the form of steam and the other stream represents the losses. The energy content of the steam is the enthalpy of the steam at the temperature of the exit, i.e. 120 °C. The amount of steam has been calculated by measuring the moisture of the feedstock before and after the removal and is approximately 85 kg (for 1 ton of input). As mentioned in the ‘Materials and Methods’ section the Rotary Compression Unit has been operated both with and without the reflux condenser in order to measure the heating value of the solid fraction on a tar-free basis. The mass yield of this solid fraction was approximately 72.56% and higher heating value was measured to be 21.29 MJ/kg. Fig. 5 shows the TG pyrolysis curves for raw corn stover, torrefied stover and frictionally carbonized stover. The decomposition curve for the torrefied stover is relatively typical for torrefied material, which mainly consists by cellulosic compounds due to the volatilization of hemicellulose and lignin compounds during torrefaction [54,55]. An interesting aspect is that the decomposition of the frictionally pyrolyzed material starts in lower temperatures, i.e. from 180 °C, which indicates the presence of hemicellulose and lignin compounds. Hence, this decomposition curve shows that the process of frictional carbonization follows a different transformation mechanism than the conventional combination of devolatilization and surface roasting. Developing a mechanism for this process goes beyond the scope of this paper and will be addressed by the authors in future publications. Chaula et al. stated that primarily hemicellulose is removed due to steam explosion [56]. Hence a possible driving factor could be identified as the exothermicity of pyrolysis which in principle is enhanced by the higher presence of lignin as state by Roberts et al. [57] in the addition to the transformation of acetyl group compounds to acids due to the autocat-
Fig. 4. Energy balance for frictional pyrolysis.
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Fig. 5. TG pyrolysis curves for raw corn stover, torrefied stover and frict. carbonized stover.
Fig. 6. TG combustion curves for raw corn stover, torrefied stover and frict. pyrolyzed stover.
alytic effect of steam which drives the rupturing of the biomass and the roasting [50]. Nonetheless, further studies and analysis are necessary for the comprehension and the modelling of frictional pyrolysis. Furthermore on Fig. 5 the final solid yield, which is constituted from the fixed carbon and the ash, is increased for both torrefaction and frictional carbonization. The levels are relatively similar with fractional pyrolysis having a small edge with 49.1% contrary to 48.3% for torrefaction. This increased solid yield for both treatment
methods implied significantly increased fixed carbon content for both cases in comparison to raw corn stover. Biswas has observed that steam pretreatment increases the fixed carbon content [58] and Pourmakhdomi has correlated steam explosion with increased fixed carbon as well [59]. Therefore the combination of steam explosion and carbonization/pyrolysis may have several other interesting applications. Surface combustion and decomposition combustion are the main types of combustion in industrial processes. It propagates
Fig. 7. DSC curves for combustion for raw corn stover, torrefied stover and frict. pyrolyzed stover.
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Fig. 8. Absorbance spectrums of frictionally pyrolyzed corn stover (a) and torrefied corn stover (b).
at temperatures higher or equal to the ignition point and requires fuels of high carbon content (fixed carbon). Thus, determining the fixed carbon content is a good indicator of the fuel quality. Fig. 6 shows the combustion TG curves; frictionally carbonized corn stover has lower ash content than the torrefied one. These ash content results have also been verified by means of applying the standard UNI EN 14,775, which provided identical results. The fixed carbon content for frictional pyrolysis is 28.3% in comparison to 22.4% for torrefaction. Fig. 7 shows the enthalpy changes from differential scanning calorimetry. Although torrefied corn stover is more exothermic than raw corn stover, frictionally torrefied corn stover has almost double levels of exothermicity than torrefied stover with 15 lV/mg vs 8 lV/mg. Finally, Fig. 8 shows the absorbance spectrum results from the ATR analysis. For the frictionally pyrolyzed stover (Fig. 8a) there are main peaks for the spectrum 900 cm 1 and the spectrum range from 550 to 600 cm 1. These peaks can be explained from the presence of carboxylic acids, alkyl halides – with a C–Br stretch and a N–H wag – but also the presence of aromatic hydrocarbons with an O–H bend. Finally, the peak at the range from 2200 to 2400 cm 1 indicates the presence of nitriles with a C (triple bond)N stretch which could be also justified by the high presence of nitrogen in the frictionally carbonized product. Contrary to the frictionally pyrolyzed stover, the torrified stover (Fig. 8b) totally lacks the peak at the range from 2200 to 2400 cm 1 which as mentioned before would indicate the presence of nitriles. On the other hand a peak at the wavelength of 1100–2400 cm 1 indicates the presence of aliphatic amines with C–N stretch. 5. Conclusions In conclusion, a novel method for producing charcoal or biocoal from lignocellulosic biomass is introduced in the framework of this paper and is defined from now on as frictional carbonization. The process takes place in a Rotary Compression Unit and consists of a set of friction augers, i.e. a rotary dryer, a reflux condenser and
an after-cooler. Primarily, it can be divided into three stages: steam explosion with and without acid enhancement, carbonization, and tar cooling and condensation. In order to provide a comparison basis with an advanced but also established technology, corn stover treated with frictional carbonization has been compared to corn stover treated with torrefaction. This novel process recovers more than 80% of the initial mass yield and 96% of the energy compared to torrefaction that has 70% recovered yield and 91% of the energy. In addition it has higher fixed carbon content with 28.3% versus 22.4% for torrefaction, although more volatiles have been removed during torrefaction. Frictionally carbonized corn stover shows higher exothermic behavior during combustion 15 lV/mg at 580 °C vs 8 lV/mg at 580 °C for torrefied material. Finally, thermogravimetric decomposition curves show that frictionally carbonized corn stover has also high content of hemicellulose and lignin contrary to torrefied corn stover which primarily consists of cellulosic compounds. References [1] International Energy Agency. Medium-term coal report 2014: market analysis and forecasts to 2019; 2014. ISBN 978-92-64-22188-8. [2] World Bank. Electricity production from coal sources (% of total). Retrieved online on 18th June 2015.
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