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Influence of feed characteristics on the microwave-assisted pyrolysis used to produce syngas from biomass wastes
Journal of Analytical and Applied Pyrolysis 91 (2011) 316–322
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Influence of feed characteristics on the microwave-assisted pyrolysis used to produce syngas from biomass wastes Y. Fernández, J.A. Menéndez ∗ Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain
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Article history: Received 9 December 2010 Accepted 11 March 2011 Available online 24 March 2011 Keywords: Microwave-assisted pyrolysis Pseudo-catalytic effect Syngas Biomass Waste
a b s t r a c t A series of biomass wastes (sewage sludges, coffee hulls and glycerol) were subjected to pyrolysis experiments under conventional and microwave heating. The influence of the initial characteristics of the raw materials upon syngas production was studied. Glycerol yielded the highest concentration of syngas, but the lowest H2 /CO ratio, whereas sewage sludges produced the lowest syngas production with the highest H2 /CO molar ratio. Coffee hull displayed intermediate values for both parameters. Microwave heating produced greater gas yields with elevated syngas content than conventional pyrolysis. Moreover, microwave pyrolysis always achieved the desired effect with temperature increase upon the pyrolysis products, whatever biomass material was employed. This could be due to the hot spot phenomenon, which only occurs under microwave heating. In addition, a comparison of the energy consumption of the traditional and microwave-assisted pyrolysis is also presented. Results point at microwave system as less time and energy consuming in comparison to conventional system. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Given the serious shortcomings in the international energy scene, regarding the uncertainty and environmental impact of the current energy supplies, it is imperative to look for nonconventional sources of energy. Biomass wastes are considered as possible alternatives, not only because such wastes are growing and posing environmental problems, but also because they are renewable sources that could contribute to the fight against environmental degradation. Various biochemical and thermochemical processes have been developed for the recovery of resources from biomass wastes [1]. Among them, pyrolysis is one of the most appropriate conversion routes since it allows a higher energy recovery from waste and produces fewer pollutants than other options, e.g. incineration. Moreover, this process generates a wide spectrum of products (solid [2], liquid [3] and gas [4–6]) with numerous applications, thus making pyrolysis treatment self-sufficient in terms of energy usage and significantly reducing operating costs. The production of synthesis gas (i.e. H2 + CO) is one of the outcomes of the pyrolysis process. The H2 /CO molar ratio is important since it is this that determines its possible application. For example, synthesis gas that has a high H2 /CO molar ratio is employed to produce hydrogen for ammonia synthesis. Moreover, this ratio can
be further increased during the water–gas shift reaction to remove CO. In spite of the fact that the present methods for syngas production from biomass are mainly based on gasification processes, novel operating conditions in the pyrolysis process may contribute to maximizing the yield, and therefore, to a reduction in costs since no steam supply is necessary. In short, the application of microwave heating to the pyrolysis process is responsible for a new temperature distribution, higher heating rates and for the appearance of unexpected physical behaviours such as the “hot spots” phenomenon, factors which increase the gas yield and ensure a higher syngas content. These results have already been reported for different biomass materials, such as sewage sludges [4], coffee hull [5], glycerol [6] and rice straw [7]. Hitherto research activity on pyrolysis has mainly dealt with the influence of the operating conditions on the final products and only few studies have centered on the feed characteristics. This paper evaluates the initial properties of different biomass materials (i.e. sewage sludges from a waste water treatment plant, an agricultural residue (coffee hulls), and a by-product of biodiesel production (glycerol)) in order to assess their suitability for producing synthesis gas using conventional and microwave pyrolysis systems. In this paper, a comparative study of the final pyrolysis yields and detailed description of the gas fractions, as well as time and energy consumptions under both heating systems, are presented. 2. Experimental
∗ Corresponding author at: Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain. E-mail address: [email protected] (J.A. Menéndez). 0165-2370/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2011.03.010
Two sewage sludges (SS) from waste water treatment plants subjected to aerobic (SS-V) and anaerobic treatments (SS-L), an
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Fig. 2. Comparative diagram of the yields obtained during the pyrolysis of glycerol, coffee hulls and sewage sludges under EF and MW heating.
Fig. 1. Ultimate (up) and elemental analysis (down) of the raw materials studied during the pyrolysis experiments. The ash content for GL corresponds to the activated carbon used as catalyst during the pyrolysis experiments [6].
agricultural residue from the coffee industry (i.e. coffee hull (CH)) and a subproduct of biodiesel production (i.e. glycerol (GL)) were subjected to pyrolysis experiments under conventional (EF) and microwave heating (MW). The SS pyrolysis experiments were carried out at 1000 ◦ C and with different moisture contents: dry SS (L0 and V0) and wet SS (V71 and L81). The influence of temperature was evaluated in the pyrolysis of CH (500, 800 and 1000 ◦ C) and GL (400, 500, 600, 700, 800 and 900 ◦ C). The experimental conditions employed in SS [4], CH [5] and GL [6] pyrolysis, as well as an analysis of the raw materials and pyrolysis products have been reported in previous articles. Nevertheless, a comparative illustration of the ultimate and proximate analysis of the raw materials is presented in Fig. 1.
during the condensation reactions [9,10]. However, its ash content (5.6%) is much lower than that of SS, which might explain the lower solid yield for CH. In order to explain these results, the yields were expressed on an ash free basis (see Fig. 3). The solid residue for CH still remains below that of SS, which suggests that the carbonaceous residues from SS are less reactive and have more stable chemical structures. Hence, higher activation energies are necessary to convert them into a final gas yield [11,12]. From this it can be inferred that the self-gasification of the carbonaceous residue with the volatiles occurs to a larger extent with the char of CH. Furthermore, the K and Ca catalysts that favour the gasification reaction [13], are present in a higher proportion in the ash composition of CH (40.3
3. Results and discussion 3.1. Yields The initial characteristics of the raw materials determine, to a large extent, the final pyrolysis yields (see Fig. 2). GL produces the highest gas fraction and the lowest carbonaceous residue as a result of its elevated volatile matter content (100%). On the other hand, both SS generate the lowest/highest gas/solid yields, respectively, as a consequence of their lower volatile matter (54.7% for SS-L and 62.3% for SS-V), and larger ash content (38.1% for SS-L and 31.2% for SS-V) which remains in the final solid. Moreover, the presence of oxygen in the raw material, to a lesser degree in SS, has been reported to be directly related to oxygenated functional groups, and therefore, to the degree of devolatilization achieved during the pyrolysis process [8]. CH might be expected to produce the highest solid yield due to its high content in fixed carbon (17.4%), and the presence of P2 O5 (5.2%) in its inorganic composition [5], which has a catalytic effect
Fig. 3. Comparative diagram of the yields (ash free basis) obtained during the pyrolysis of glycerol, coffee hulls and sewage sludges under EF and MW heating.
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and 29.6%, respectively) [5] than in that of SS-L (0.97 and 5.29%, respectively) and SS-V (0.76 and 5.97%, respectively) [6]. In addition, the composition of the sewage sludges from the waste water treatment plants is much more heterogeneous than that of the agricultural residue. Conesa et al. [14] have considered several possible kinetic models for the pyrolysis of two sewage sludges, one of which is anaerobically digested whereas the other is not digested. The best model assumes the presence of three different organic fractions in the sludges: biodegradable organic matter, dead bacterias and non-biodegradable compounds that give rise to volatiles and carbonaceous residue. Each of these fractions is associated with different thermal behaviours, the non-biodegradable matter being responsible for the highest solid residue. When both SS are compared, a greater solid yield is observed in SS-L, which is in accordance with the results of Conesa et al. [15]. They consider a higher amount of non-biodegradable matter to be present in anaerobically digested SS. Font et al. [16] consider the biodegradable matter to be related to lignocellulosic material, and the non-biodegradable matter to polymers. In contrast, the composition of the agricultural residue is only linked to that of lignocellulosic materials, the composition being cellulose (40–50%), hemicellulose (20–30%), lignin (20–25%) and ash (1–5%) [17]. Hence, CH generates less carbonaceous residue than SS, since only biodegradable matter is found in its composition. However, CH pyrolysis produces solid yields that are slightly higher than those found in other lignocellulosic materials [18]. Nabais et al. [19], using CH for the production of activated carbon, ascertain CH to be composed of around 54% of holocellulose (i.e. hemicellulose + cellulose) and 46% of lignin. A higher lignin content is deduced compared to other agricultural residues, this component being strongly involved in the increase in the solid fraction [18]. As for the liquid fraction, GL has the greatest yield, followed by CH and SS. Since GL is liquid, its solid fraction is almost nonexistent. Glycerol conversion during the pyrolysis process also has to be considered, since unconverted GL could be present in this fraction. Apart from the feed characteristics, the operating conditions also have an influence on the final pyrolysis yields. An increase in temperature and the use of microwaves during the pyrolysis process give rise to higher gas productions and lower oil yields. However, the trend is not clearly defined for the solid fraction, since this fraction is determined by the nature of the raw material. In the case of solid materials, such as SS and CH, lower carbonaceous residues are obtained under microwave-assisted pyrolysis and with the increase in temperature. Both of these solids favour higher degrees of devolatilization and heterogeneous reactions between the volatiles and the carbonaceous residue. In contrast, glycerol is liquid, and therefore, the solid fraction must come from the carbon deposits produced during hydrocarbon decomposition. High temperatures and microwave heating favour the aforementioned reactions, and so higher solid yields are obtained. From these data, it can be concluded that microwave heating boosts the effect attained with the temperature increase, i.e. less solid residue when solid raw material is employed and more char when material in liquid state is used. The presence of hot spots or “microplasmas” during microwave heating seems to be responsible for this, since the temperatures of these spots are much higher than the temperature of the bulk of the material.
absolute error has been estimated for each gas volume, being lower than 5% in all values. 3.2.1. Syngas volume Fig. 4 shows the volume of syngas for all the materials subjected to pyrolysis under conventional and microwave heating, with different moisture contents (SS) and at different temperatures (CH and GL). This figure is a bubble graph, which presents a global view of the gas yield (expressed as wt.%) and syngas content (expressed as vol.%). The size of the bubble represents the volume of syngas (l g−1 ). The ideal conditions appear in quadrant I, where the highest values for gas yield and syngas content are reached. These values were obtained from the experiments carried out on GL, and for the MW pyrolysis of CH. The highest syngas productions correspond to values of 0.93 l g−1 and 0.62 l g−1 for GL and CH, respectively, and were obtained under MW and at high temperatures. At low temperatures, the differences between GL and CH are less marked but the amount of syngas produced by CH (0.41 l g−1 ) is even greater than that obtained from GL (0.34 l g−1 ) at 500 ◦ C using MW. The pyrolyses of CH under EF are reflected in quadrant II. In quadrant IV, the gas yield is lower than 50%, although the syngas content is higher than this. All the SS pyrolyses are found in this quadrant, regardless of the moisture content (wet or dry SS). The largest syngas volume was achieved under MW pyrolysis using dry SS, i.e. 0.56 l g−1 for SS-L and 0.53 l g−1 for SS-V. Nevertheless, the results obtained with wet SS do not significantly differ from those of dry SS, 0.53 and 0.50 l g−1 , respectively. An unexpected result was obtained under the EF pyrolysis experiments: the wet SS yielded higher syngas volumes (0.40 and 0.29 l g−1 , respectively). Previous works [4] had demonstrated that homogeneous reactions, such as those involving steam and released volatiles, are favoured under EF. In spite of their lower gas yields, both SS yielded higher syngas contents than the other materials, the highest syngas content corresponding to the anaerobically digested SS (SS-L). The type of heating is also an important factor for syngas production. For all the materials and temperatures studied, the volume of synthesis gas is higher under MW than EF, the differences between these two heating methods being especially pronounced at low temperatures. It is also observed that MW promotes the effect as the temperature increases, which could therefore be interpreted as a catalytic effect. Zhao et al. [20] studied the catalytic activity of Ni during the pyrolysis of cellulose. They found that the use of supported Ni catalysts affects the yield and composition of the gaseous fraction, and more specifically the H2 content in the same way as MW. Three main steps serve to explain the mechanism followed by the Ni catalysts: (i) catalysis of C–C bond breakage, favouring the cracking reactions of volatiles; (ii) catalysis of C–H bond cleavage, favouring light hydrocarbon reforming; and (iii) catalysis of the water–gas shift reaction resulting in the reduction of CO. With the exception of last step, the use of microwaves during pyrolysis processes achieves similar results. Hence, it might be possible to talk of a pseudo-catalytic effect in microwave heating. The characteristics of MW heating (i.e. volumetric heating [21]), and the associated “hot spot” or “microplasma” phenomenon may be responsible for the mechanism that gives rise to a higher syngas production, since heterogeneous and catalytic heterogeneous reactions take place, to a greater extent, with this heating method.
3.2. Gas composition The gas composition during the pyrolysis experiments of SS [4], CH [5] and GL [6] is mainly H2 + CO, and smaller amounts of CO2 and light hydrocarbons (CH4 , C2 H4 and C2 H6 ). From their concentrations in the final gas (vol.%), together with the gas yield (wt.%), the volume of the final gases has been calculated and expressed in l g−1 under standard conditions of pressure and temperature. The
3.2.2. H2 /CO molar ratio More detailed information about the synthesis gas produced from pyrolysis experiments under different conditions appears in Fig. 5. The H2 volume (l g−1 ) is represented against CO volume (l g−1 ), whilst the size of the bubble stands for the H2 /CO molar ratio. H2 and CO production occurs within the intervals of 0–0.5 and 0–0.6 l g−1 , respectively.
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Fig. 4. Comparative diagram between the gas yield (wt.%) and syngas production (vol.%) during the pyrolysis of glycerol, coffee hulls and sewage sludges under EF and MW heating. The size of the bubble represents the amount of syngas production (l g−1 ).
Quadrant II shows a higher H2 production. Accordingly, large H2 /CO molar ratios are found in the final synthesis gas. Most of the MW pyrolysis experiments carried out with SS and CH are found in this quadrant, the proportions of H2 /CO being around 1.1 for both of the SS and 1.4 for CH. In the case of SS, moisture content increases this proportion because H2 is donated by the water through the steam gasification of the carbonaceous residue and the steam reforming of hydrocarbons. Both SS and CH present H2 /CO molar ratios close to 1 for pyrolysis processes under EF (found in quadrant III), although their syngas volumes are much smaller. A completely different picture is presented by the GL pyrolysis experiments in quadrants I and IV, where CO production exceeds that of H2 . This implies H2 /CO molar ratios of less than one unit despite the fact that the highest syngas productions are obtained with glycerol. Dauenhauer et al. [22] have proposed an adsorption and decomposition mechanism of glycerol over metallic catalysts to explain the higher CO production during the pyrolysis of this material. Glycerol is an oxygenated compound composed of hydroxyl groups, which are adsorbed over active centers of the catalyst surface through one or more oxygen atoms. The decomposition of glycerol takes place through the breaking of bonds in O–H, C–H, and possibly C–C, giving rise to the adsorption of H, C and O, which is responsible for syngas production. Additionally, Zum Mallen and Schmidt [23] observed no breaking of the C–O bond when CO was adsorbed over an active center. It is well-known that MW pyrolysis experiments with GL give rise to higher H2 /CO ratios than under EF. The main differences between the two heating systems practically disappear at high temperatures, as happens at 900 ◦ C, when the pyrolysis of GL produces
a synthesis gas with a H2 /CO ratio equal to 0.8 under both MW and EF heating. In the production of the synthesis gas, not only its volume important, but also its H2 /CO molar ratio, since this determines the possible applications for which it can be used. GL is the material with the highest syngas volumes (0.9 l g−1 ) even though it has the lowest H2 /CO ratios. Both of the SS, unlike the GL, produce less syngas with H2 /CO ratios above one unit. In the case of CH, intermediate values are obtained for both the syngas volume (0.62 l g−1 ) and the H2 /CO molar ratio (1.4), the latter occurring even at low temperatures (500 ◦ C). 3.2.3. Accompanying gases In addition to the synthesis gas, the gas composition during the pyrolysis experiments includes other gases that provide useful information about the process and the materials involved. This is the case of CO2 , whose values are below 0.1 l g−1 for SS and GL, but around 0.2 l g−1 for CH (see Fig. 6). Other authors [24–27] have also detected CO2 production during the pyrolysis of biomass. The mechanism that controls the pyrolysis of lignocellulosic materials and gas evolution takes place in two stages during the pyrolysis process. The first is located at the beginning of the experiments, where higher CO2 values are observed as a result of the primary reactions (primary pyrolysis). During the course of the pyrolysis process a second region appears, characterized by an increase in the CO from the primary decomposition of volatiles during the secondary reactions (secondary pyrolysis). The temperature and residence time of the volatiles in the reactor determine the extent of the secondary reactions, and therefore, the CO2 and CO yields in the final gas. This explains why the CO2 content is higher at low temperatures, and
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Fig. 5. Comparative diagram between H2 and CO production (l g−1 ) during the pyrolysis of sewage sludges, coffee hulls and glycerol under EF and MW heating. The size of the bubble represents the H2 /CO molar ratio.
why, as the temperature increases, CO2 falls and CO production rises. Compared to the initial characteristics of the raw materials, higher CO2 productions are observed for CH. The amount of cellulose present in the raw material is an important factor in determining the amount of carbon oxides produced, since cellulose itself is a highly oxygenated polymer [25–27], and the higher amount of CO2
in the CH pyrolysis experiments occurs because this material has a higher cellulosic content, as reflected by its oxygen value in Fig. 1. GL can be ignored since no cellulose appears in its composition (pure glycerol). On the other hand, lower CO2 and higher CO concentrations are observed under MW pyrolysis. Considering that the residence times of the volatiles in both heating systems are similar, i.e. the same
Fig. 6. CO2 production (l g−1 ) during the pyrolysis of sewage sludges, coffee hulls and glycerol under EF and MW heating.
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Fig. 7. Light hydrocarbon (CH4 + C2 H4 + C2 H6 ) production (l g−1 ) during the pyrolysis of sewage sludges, coffee hulls and glycerol under EF and MW heating.
3.3. Time and energy consumption A brief comparative study between conventional and microwave heating systems was carried out in terms of time and energy consumption. Same volumes (0.05 l) of the different samples studied (sewage sludge L, sewage sludge V, coffee hull, and the activated carbon used as catalyst during glycerol pyrolysis (BC)) have been heated at different temperatures in both heating systems. Conventional and multimode microwave systems have been previously described in [4–6] and in page 730 of Ref. [28], respectively. Time saving of MW with respect to EF has been calculated considering the heating time that each pyrolysate needs to reach a final temperature under both heating systems. In all cases, MW achieves lower heating times than EF, and therefore, MW attain higher time saving, even up to 60%. Interestingly, time saving rises as the temperature increases, e.g. time saving for the coffee hull corresponds to values of 40 and 60% at temperatures of 500 and 1000 ◦ C, respectively. In the case of the activated carbon used as catalyst in glycerol pyrolysis, time saving ranges from 50% to 60% when the temperature rise from 400 to 900 ◦ C, respectively. Finally, time saving for the heating of sewage sludges is found to be around 60% for both L and V at 1000 ◦ C. Related to energy consumption, wattmeters were employed in EF and MW to register the power used to maintain the sample (carbonaceous residues of sewage sludge L and coffee hull, and the activated carbon used as catalyst in glycerol pyrolysis) at a specific and stable temperature. The volume of heated sample was optimized for each system (0.05 l) and similar times (30 min) were
necessary for completing the pyrolysis processes in both devices, once the temperature became constant. For all materials and studied temperatures (700–900 ◦ C), the specific power (expressed as W g−1 ) under MW resulted to be lower than that employed under EF, increasing with the rise in temperature. Thus, it was found (see Fig. 8) that the specific power required to keep the pyrolysis temperature in the MW system was below 8 W g−1 for all materials and over 10 W g−1 when EF was used. Note that different slopes appear for both MW and EF, which suggests that the temperature increase under EF requires higher energy consumption. Under MW, the slope of the line is smoother than that of EF, indicating a higher energy saving at high temperatures. On the other hand, not all materials are heated in the same way under both heating systems. In the case of EF, the density of the material is an important factor to take into account. For instance, the char of sewage sludge, less dense (0.25 g cm−3 ) than coffee hull (0.39 g cm−3 ) and the activated carbon (0.43 g cm−3 ), needs more energy to reach a final temperature. Therefore, higher energy efficiencies are achieved with MW when the material is suitable for heating. However, it is important to note that the above values are only valid for the laboratory equipments and operating conditions employed in this study, i.e. they cannot be generalised, since the 40 SS-EF
Specific power (W g-1)
carrier gas flow was used (60 ml min−1 ); the concentrations of CO2 and CO must be due to the fact that MW favours gas–solid heterogeneous reactions, such as CO2 gasification of the carbonaceous residue yielding CO. The presence of light hydrocarbons (LH), such as CH4 , C2 H4 and C2 H6 was also detected in the final gas composition during the pyrolysis experiments. Fig. 7 represents LH production (CH4 + C2 H4 + C2 H6 ) at different temperatures under MW and EF. The higher yields correspond to GL pyrolysis, around 0.2 l g−1 . For the other materials (SS and CH), LH concentrations are even lower than 0.1 l g−1 , as in the case of SS.
30 BC-EF
20
CH-EF
10
BC-MW
SS-MW
CH-MW
0 600
700
800
Temperature (ºC)
900
1000
Fig. 8. Specific power (W g−1 ) used in both MW (smooth lines) and EF (striped lines) heating systems to maintain the char of sewage sludge L (SS), the char of coffee hulls (CH) and the activated carbon used as catalyst in glycerol pyrolysis (BC) to different final temperatures. The lines represent the trend lines for the power values obtained.
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use of different experimental or industrial devices could invert the final result. Considering the aforementioned data, it was possible to calculate energy consumptions (expressed in kW h) and energy saving (%) of MW with respect to EF. From specific powers used to maintain the pyrolysis temperature of the sample, and taking into account the final time, energy consumptions were found to be lower than 0.004 kW h and higher than 0.005 kW h under MW and EF, respectively. Alibas [29] observed energy consumption of 0.07 kW h during the microwave drying period of nettle leaves, since more heat is needed to evaporate the moisture content. Regarding energy saving, values lying from ca. 50 to 77% were obtained in our pyrolysis experiments under MW, which is in agreement with other authors [30] that, using different equipments in a drying process, found energy saving of 70%. 4. Conclusion This paper has focused on the influence of feed characteristics on the final yields and gas compositions during the pyrolysis process under conventional and microwave heating. More specifically, the physical state and chemical properties (ultimate and proximate analysis) of several biomass wastes have been related to syngas production. Glycerol provided the highest concentration of syngas in the gas fraction, although it had the lowest H2 /CO ratio. Sewage sludges showed the opposite tendency, i.e. the lowest syngas production with the highest H2 /CO molar ratio, coffee hull displaying intermediate values for both parameters. The use of microwaves always produces a greater gas yield with a higher level of syngas than conventional pyrolysis. Moreover, microwave pyrolysis always achieves the desired effect with temperature increase whichever the biomass material is employed. This may be due to the hot spot phenomenon, which only occurs under microwave heating. Acknowledgements Y. Fernández is grateful to CSIC of Spain and the European Social Fund (ESF) for financial support under the Ph.D. Grant I3P-BDP2006. References [1] A.P.C. Faaij, Bio-energy in Europe: changing technology choices, Energy Policy 34 (2006) 322–342. [2] J.F. González, S. Román, J.M. Encinar, G. Martínez, Pyrolysis of various biomass residues and char utilization for the production of activated carbons, J. Anal. Appl. Pyrol. 85 (2009) 134–141. [3] A. Domínguez, J.A. Menéndez, M. Inguanzo, J.J. Pis, Production of biofuels by high temperature pyrolysis of sewage sludge using conventional and microwave heating, Bioresour. Technol. 97 (2006) 1185– 1193. [4] A. Domínguez, Y. Fernández, B. Fidalgo, J.J. Pis, J.A. Menéndez, Bio-syngas production with low concentration of CO2 and CH4 from microwave-induced pyrolysis of wet and dried sewage sludge, Chemosphere 70 (2008) 397– 403.
[5] A. Domínguez, J.A. Menéndez, Y. Fernández, J.J. Pis, J.M. Valente Nabais, P.J.M. Carrott, M.M.L. Ribeiro Carrott, Conventional and microwave induced pyrolysis of coffee hulls for the production of a hydrogen rich fuel gas, J. Anal. App. Pyrol. 79 (2007) 128–135. [6] Y. Fernández, A. Arenillas, M.A. Díez, J.J. Pis, J.A. Menéndez, Pyrolysis of glycerol over activated carbons for syngas production, J. Anal. App. Pyrol. 84 (2009) 145–150. [7] Y.F. Huang, W.H. Kuan, S.L. Lo, C.F. Lin, Total recovery of resources and energy from rice straw using microwave-induced pyrolysis, Bioresour. Technol. 99 (2008) 8252–8258. [8] D. Savova, E. Apak, E. Ekinci, F. Yardim, N. Petrov, T. Budinova, M. Razvigorova, V. Minkova, Biomass conversion to carbon adsorbents and gas, Biomass Bioenergy 21 (2001) 133–142. [9] G. Dobele, D. Meier, O. Faix, S. Radtke, G. Rossinskaja, G. Telysheva, Volatile products of catalytic flash pyrolysis of celluloses, J. Anal. App. Pyrol. 58–59 (2001) 453–463. [10] D.J. Nowakowski, C.R. Woodbridge, J.M. Jones, Phosphorus catalysis in the pyrolysis behaviour of biomass, J. Anal. App. Pyrol. 83 (2008) 197–204. [11] A. Demirbas, Yields of hydrogen-rich gaseous products via pyrolysis from selected biomass samples, Fuel 80 (2001) 1885–1891. [12] M. van der Velden, J. Baeyens, A. Brems, B. Janssens, R. Dewil, Fundamentals, kinetics and endothermicity of the biomass pyrolysis reaction, Renewable Energy 35 (2010) 232–242. [13] J.A. Menéndez, A. Domínguez, Y. Fernández, J.J. Pis, Evidence of self-gasification during the microwave-induced pyrolysis of coffee hulls, Energy Fuel 21 (2007) 373–378. [14] J.A. Conesa, A. Marcilla, D. Prats, M. Rodríguez-Pastor, Kinetic study of the pyrolysis of sewage sludge, Waste Manage. Res. 15 (1997) 293–305. [15] J.A. Conesa, A. Marcilla, R. Moral, J. Moreno-Caselles, A. Pérez-Espinosa, Evolution of gases in the primary pyrolysis of different sewage sludges, Thermochim. Acta 313 (1998) 63–73. [16] R. Font, A. Fullana, J.A. Conesa, Kinetic models for the pyrolysis and combustion of two types of sewage sludge, J. Anal. App. Pyrol. 74 (2005) 429–438. [17] J.M. Encinar, F.J. Beltrán, A. Ramiro, J.F. González, Pyrolysis/gasification of agricultural residues by carbon dioxide in the presence of different additives: influence of variables, Fuel Process. Technol. 55 (1998) 219–233. [18] B. Cagnon, X. Py, A. Guillot, F. Stoeckli, G. Chambat, Contributions of hemicellulose, cellulose and lignin to the mass and the porous properties of chars and steam activated carbons from various lignocellulosic precursors, Bioresour. Technol. 100 (2009) 292–298. [19] J.M.V. Nabais, P.J.M. Carrott, M.M.L. Ribeiro Carrott, Patente Portuguesa No. 103520 (11/07/2006). [20] M. Zhao, N.H. Florin, A.T. Harris, The influence of supported Ni catalysts on the product gas distribution and H2 yield during cellulose pyrolysis, Appl. Catal. B Environ. 92 (2009) 185–193. [21] E.T. Thostenson, T.W. Chou, Microwave processing: fundamentals and applications, Compos. Part A Appl. Sci. 30 (1999) 1055–1071. [22] P.J. Dauenhauer, J.R. Salge, L.D. Schmidt, Renewable hydrogen by autothermal steam reforming of volatile carbohydrates, J. Catal. 244 (2006) 238–247. [23] M.P. Zum Mallen, L.D. Schmidt, Oxidation of methanol over polycrystalline Rh and Pt: rates, OH desorption, and model, J. Catal. 161 (1996) 230–246. [24] A. Fontana, P.H. Laurent, C. Kestemont, C. Braekman-Danheux, The behaviour of chlorine with cellulose and lignin, Erdöl Erdgas Kohle 116 (2000) 89–92. [25] I. Ahmed, A.K. Gupta, Syngas yoield during pryolysis and steam gasification of paper, Appl. Energy 86 (2009) 1813–1821. [26] C. Couher, J.M. Commandre, S. Salvador, Failure of the component additivity rule to predict gas yields of biomass in flash pyrolysis at 950 ◦ C, Biomass Bioenergy 33 (2009) 316–326. [27] C. Couhert, J.M. Commandre, S. Salvador, Is it possible to predict gas yields of any biomass after rapid pyrolysis at high temperature from its composition in cellulose, hemicellulose and lignin? Fuel 88 (2009) 408–417. [28] Y. Fernández, A. Arenillas, J.A. Menéndez, Microwave heating applied to pyrolysis, in: S. Grundas (Ed.), Advances in induction and microwave heating of mineral and organic materials, Intech, Croatia, 2011, pp. 723–752. [29] I. Alibas, Energy consumption and colour characteristics of nettle leaves during microwave, vacuum and convective drying, Biosyst. Eng. 96 (2007) 495–502. [30] G.P. Sharma, S. Prasad, Specific energy consumption in microwave drying of garlic cloves, Energy 31 (2006) 1921–1926.
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Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Influence of feed characteristics on the microwave-assisted pyrolysis used to produce syngas from biomass wastes Y. Fernández, J.A. Menéndez ∗ Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain
a r t i c l e
i n f o
Article history: Received 9 December 2010 Accepted 11 March 2011 Available online 24 March 2011 Keywords: Microwave-assisted pyrolysis Pseudo-catalytic effect Syngas Biomass Waste
a b s t r a c t A series of biomass wastes (sewage sludges, coffee hulls and glycerol) were subjected to pyrolysis experiments under conventional and microwave heating. The influence of the initial characteristics of the raw materials upon syngas production was studied. Glycerol yielded the highest concentration of syngas, but the lowest H2 /CO ratio, whereas sewage sludges produced the lowest syngas production with the highest H2 /CO molar ratio. Coffee hull displayed intermediate values for both parameters. Microwave heating produced greater gas yields with elevated syngas content than conventional pyrolysis. Moreover, microwave pyrolysis always achieved the desired effect with temperature increase upon the pyrolysis products, whatever biomass material was employed. This could be due to the hot spot phenomenon, which only occurs under microwave heating. In addition, a comparison of the energy consumption of the traditional and microwave-assisted pyrolysis is also presented. Results point at microwave system as less time and energy consuming in comparison to conventional system. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Given the serious shortcomings in the international energy scene, regarding the uncertainty and environmental impact of the current energy supplies, it is imperative to look for nonconventional sources of energy. Biomass wastes are considered as possible alternatives, not only because such wastes are growing and posing environmental problems, but also because they are renewable sources that could contribute to the fight against environmental degradation. Various biochemical and thermochemical processes have been developed for the recovery of resources from biomass wastes [1]. Among them, pyrolysis is one of the most appropriate conversion routes since it allows a higher energy recovery from waste and produces fewer pollutants than other options, e.g. incineration. Moreover, this process generates a wide spectrum of products (solid [2], liquid [3] and gas [4–6]) with numerous applications, thus making pyrolysis treatment self-sufficient in terms of energy usage and significantly reducing operating costs. The production of synthesis gas (i.e. H2 + CO) is one of the outcomes of the pyrolysis process. The H2 /CO molar ratio is important since it is this that determines its possible application. For example, synthesis gas that has a high H2 /CO molar ratio is employed to produce hydrogen for ammonia synthesis. Moreover, this ratio can
be further increased during the water–gas shift reaction to remove CO. In spite of the fact that the present methods for syngas production from biomass are mainly based on gasification processes, novel operating conditions in the pyrolysis process may contribute to maximizing the yield, and therefore, to a reduction in costs since no steam supply is necessary. In short, the application of microwave heating to the pyrolysis process is responsible for a new temperature distribution, higher heating rates and for the appearance of unexpected physical behaviours such as the “hot spots” phenomenon, factors which increase the gas yield and ensure a higher syngas content. These results have already been reported for different biomass materials, such as sewage sludges [4], coffee hull [5], glycerol [6] and rice straw [7]. Hitherto research activity on pyrolysis has mainly dealt with the influence of the operating conditions on the final products and only few studies have centered on the feed characteristics. This paper evaluates the initial properties of different biomass materials (i.e. sewage sludges from a waste water treatment plant, an agricultural residue (coffee hulls), and a by-product of biodiesel production (glycerol)) in order to assess their suitability for producing synthesis gas using conventional and microwave pyrolysis systems. In this paper, a comparative study of the final pyrolysis yields and detailed description of the gas fractions, as well as time and energy consumptions under both heating systems, are presented. 2. Experimental
∗ Corresponding author at: Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain. E-mail address: [email protected] (J.A. Menéndez). 0165-2370/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2011.03.010
Two sewage sludges (SS) from waste water treatment plants subjected to aerobic (SS-V) and anaerobic treatments (SS-L), an
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Fig. 2. Comparative diagram of the yields obtained during the pyrolysis of glycerol, coffee hulls and sewage sludges under EF and MW heating.
Fig. 1. Ultimate (up) and elemental analysis (down) of the raw materials studied during the pyrolysis experiments. The ash content for GL corresponds to the activated carbon used as catalyst during the pyrolysis experiments [6].
agricultural residue from the coffee industry (i.e. coffee hull (CH)) and a subproduct of biodiesel production (i.e. glycerol (GL)) were subjected to pyrolysis experiments under conventional (EF) and microwave heating (MW). The SS pyrolysis experiments were carried out at 1000 ◦ C and with different moisture contents: dry SS (L0 and V0) and wet SS (V71 and L81). The influence of temperature was evaluated in the pyrolysis of CH (500, 800 and 1000 ◦ C) and GL (400, 500, 600, 700, 800 and 900 ◦ C). The experimental conditions employed in SS [4], CH [5] and GL [6] pyrolysis, as well as an analysis of the raw materials and pyrolysis products have been reported in previous articles. Nevertheless, a comparative illustration of the ultimate and proximate analysis of the raw materials is presented in Fig. 1.
during the condensation reactions [9,10]. However, its ash content (5.6%) is much lower than that of SS, which might explain the lower solid yield for CH. In order to explain these results, the yields were expressed on an ash free basis (see Fig. 3). The solid residue for CH still remains below that of SS, which suggests that the carbonaceous residues from SS are less reactive and have more stable chemical structures. Hence, higher activation energies are necessary to convert them into a final gas yield [11,12]. From this it can be inferred that the self-gasification of the carbonaceous residue with the volatiles occurs to a larger extent with the char of CH. Furthermore, the K and Ca catalysts that favour the gasification reaction [13], are present in a higher proportion in the ash composition of CH (40.3
3. Results and discussion 3.1. Yields The initial characteristics of the raw materials determine, to a large extent, the final pyrolysis yields (see Fig. 2). GL produces the highest gas fraction and the lowest carbonaceous residue as a result of its elevated volatile matter content (100%). On the other hand, both SS generate the lowest/highest gas/solid yields, respectively, as a consequence of their lower volatile matter (54.7% for SS-L and 62.3% for SS-V), and larger ash content (38.1% for SS-L and 31.2% for SS-V) which remains in the final solid. Moreover, the presence of oxygen in the raw material, to a lesser degree in SS, has been reported to be directly related to oxygenated functional groups, and therefore, to the degree of devolatilization achieved during the pyrolysis process [8]. CH might be expected to produce the highest solid yield due to its high content in fixed carbon (17.4%), and the presence of P2 O5 (5.2%) in its inorganic composition [5], which has a catalytic effect
Fig. 3. Comparative diagram of the yields (ash free basis) obtained during the pyrolysis of glycerol, coffee hulls and sewage sludges under EF and MW heating.
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and 29.6%, respectively) [5] than in that of SS-L (0.97 and 5.29%, respectively) and SS-V (0.76 and 5.97%, respectively) [6]. In addition, the composition of the sewage sludges from the waste water treatment plants is much more heterogeneous than that of the agricultural residue. Conesa et al. [14] have considered several possible kinetic models for the pyrolysis of two sewage sludges, one of which is anaerobically digested whereas the other is not digested. The best model assumes the presence of three different organic fractions in the sludges: biodegradable organic matter, dead bacterias and non-biodegradable compounds that give rise to volatiles and carbonaceous residue. Each of these fractions is associated with different thermal behaviours, the non-biodegradable matter being responsible for the highest solid residue. When both SS are compared, a greater solid yield is observed in SS-L, which is in accordance with the results of Conesa et al. [15]. They consider a higher amount of non-biodegradable matter to be present in anaerobically digested SS. Font et al. [16] consider the biodegradable matter to be related to lignocellulosic material, and the non-biodegradable matter to polymers. In contrast, the composition of the agricultural residue is only linked to that of lignocellulosic materials, the composition being cellulose (40–50%), hemicellulose (20–30%), lignin (20–25%) and ash (1–5%) [17]. Hence, CH generates less carbonaceous residue than SS, since only biodegradable matter is found in its composition. However, CH pyrolysis produces solid yields that are slightly higher than those found in other lignocellulosic materials [18]. Nabais et al. [19], using CH for the production of activated carbon, ascertain CH to be composed of around 54% of holocellulose (i.e. hemicellulose + cellulose) and 46% of lignin. A higher lignin content is deduced compared to other agricultural residues, this component being strongly involved in the increase in the solid fraction [18]. As for the liquid fraction, GL has the greatest yield, followed by CH and SS. Since GL is liquid, its solid fraction is almost nonexistent. Glycerol conversion during the pyrolysis process also has to be considered, since unconverted GL could be present in this fraction. Apart from the feed characteristics, the operating conditions also have an influence on the final pyrolysis yields. An increase in temperature and the use of microwaves during the pyrolysis process give rise to higher gas productions and lower oil yields. However, the trend is not clearly defined for the solid fraction, since this fraction is determined by the nature of the raw material. In the case of solid materials, such as SS and CH, lower carbonaceous residues are obtained under microwave-assisted pyrolysis and with the increase in temperature. Both of these solids favour higher degrees of devolatilization and heterogeneous reactions between the volatiles and the carbonaceous residue. In contrast, glycerol is liquid, and therefore, the solid fraction must come from the carbon deposits produced during hydrocarbon decomposition. High temperatures and microwave heating favour the aforementioned reactions, and so higher solid yields are obtained. From these data, it can be concluded that microwave heating boosts the effect attained with the temperature increase, i.e. less solid residue when solid raw material is employed and more char when material in liquid state is used. The presence of hot spots or “microplasmas” during microwave heating seems to be responsible for this, since the temperatures of these spots are much higher than the temperature of the bulk of the material.
absolute error has been estimated for each gas volume, being lower than 5% in all values. 3.2.1. Syngas volume Fig. 4 shows the volume of syngas for all the materials subjected to pyrolysis under conventional and microwave heating, with different moisture contents (SS) and at different temperatures (CH and GL). This figure is a bubble graph, which presents a global view of the gas yield (expressed as wt.%) and syngas content (expressed as vol.%). The size of the bubble represents the volume of syngas (l g−1 ). The ideal conditions appear in quadrant I, where the highest values for gas yield and syngas content are reached. These values were obtained from the experiments carried out on GL, and for the MW pyrolysis of CH. The highest syngas productions correspond to values of 0.93 l g−1 and 0.62 l g−1 for GL and CH, respectively, and were obtained under MW and at high temperatures. At low temperatures, the differences between GL and CH are less marked but the amount of syngas produced by CH (0.41 l g−1 ) is even greater than that obtained from GL (0.34 l g−1 ) at 500 ◦ C using MW. The pyrolyses of CH under EF are reflected in quadrant II. In quadrant IV, the gas yield is lower than 50%, although the syngas content is higher than this. All the SS pyrolyses are found in this quadrant, regardless of the moisture content (wet or dry SS). The largest syngas volume was achieved under MW pyrolysis using dry SS, i.e. 0.56 l g−1 for SS-L and 0.53 l g−1 for SS-V. Nevertheless, the results obtained with wet SS do not significantly differ from those of dry SS, 0.53 and 0.50 l g−1 , respectively. An unexpected result was obtained under the EF pyrolysis experiments: the wet SS yielded higher syngas volumes (0.40 and 0.29 l g−1 , respectively). Previous works [4] had demonstrated that homogeneous reactions, such as those involving steam and released volatiles, are favoured under EF. In spite of their lower gas yields, both SS yielded higher syngas contents than the other materials, the highest syngas content corresponding to the anaerobically digested SS (SS-L). The type of heating is also an important factor for syngas production. For all the materials and temperatures studied, the volume of synthesis gas is higher under MW than EF, the differences between these two heating methods being especially pronounced at low temperatures. It is also observed that MW promotes the effect as the temperature increases, which could therefore be interpreted as a catalytic effect. Zhao et al. [20] studied the catalytic activity of Ni during the pyrolysis of cellulose. They found that the use of supported Ni catalysts affects the yield and composition of the gaseous fraction, and more specifically the H2 content in the same way as MW. Three main steps serve to explain the mechanism followed by the Ni catalysts: (i) catalysis of C–C bond breakage, favouring the cracking reactions of volatiles; (ii) catalysis of C–H bond cleavage, favouring light hydrocarbon reforming; and (iii) catalysis of the water–gas shift reaction resulting in the reduction of CO. With the exception of last step, the use of microwaves during pyrolysis processes achieves similar results. Hence, it might be possible to talk of a pseudo-catalytic effect in microwave heating. The characteristics of MW heating (i.e. volumetric heating [21]), and the associated “hot spot” or “microplasma” phenomenon may be responsible for the mechanism that gives rise to a higher syngas production, since heterogeneous and catalytic heterogeneous reactions take place, to a greater extent, with this heating method.
3.2. Gas composition The gas composition during the pyrolysis experiments of SS [4], CH [5] and GL [6] is mainly H2 + CO, and smaller amounts of CO2 and light hydrocarbons (CH4 , C2 H4 and C2 H6 ). From their concentrations in the final gas (vol.%), together with the gas yield (wt.%), the volume of the final gases has been calculated and expressed in l g−1 under standard conditions of pressure and temperature. The
3.2.2. H2 /CO molar ratio More detailed information about the synthesis gas produced from pyrolysis experiments under different conditions appears in Fig. 5. The H2 volume (l g−1 ) is represented against CO volume (l g−1 ), whilst the size of the bubble stands for the H2 /CO molar ratio. H2 and CO production occurs within the intervals of 0–0.5 and 0–0.6 l g−1 , respectively.
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Fig. 4. Comparative diagram between the gas yield (wt.%) and syngas production (vol.%) during the pyrolysis of glycerol, coffee hulls and sewage sludges under EF and MW heating. The size of the bubble represents the amount of syngas production (l g−1 ).
Quadrant II shows a higher H2 production. Accordingly, large H2 /CO molar ratios are found in the final synthesis gas. Most of the MW pyrolysis experiments carried out with SS and CH are found in this quadrant, the proportions of H2 /CO being around 1.1 for both of the SS and 1.4 for CH. In the case of SS, moisture content increases this proportion because H2 is donated by the water through the steam gasification of the carbonaceous residue and the steam reforming of hydrocarbons. Both SS and CH present H2 /CO molar ratios close to 1 for pyrolysis processes under EF (found in quadrant III), although their syngas volumes are much smaller. A completely different picture is presented by the GL pyrolysis experiments in quadrants I and IV, where CO production exceeds that of H2 . This implies H2 /CO molar ratios of less than one unit despite the fact that the highest syngas productions are obtained with glycerol. Dauenhauer et al. [22] have proposed an adsorption and decomposition mechanism of glycerol over metallic catalysts to explain the higher CO production during the pyrolysis of this material. Glycerol is an oxygenated compound composed of hydroxyl groups, which are adsorbed over active centers of the catalyst surface through one or more oxygen atoms. The decomposition of glycerol takes place through the breaking of bonds in O–H, C–H, and possibly C–C, giving rise to the adsorption of H, C and O, which is responsible for syngas production. Additionally, Zum Mallen and Schmidt [23] observed no breaking of the C–O bond when CO was adsorbed over an active center. It is well-known that MW pyrolysis experiments with GL give rise to higher H2 /CO ratios than under EF. The main differences between the two heating systems practically disappear at high temperatures, as happens at 900 ◦ C, when the pyrolysis of GL produces
a synthesis gas with a H2 /CO ratio equal to 0.8 under both MW and EF heating. In the production of the synthesis gas, not only its volume important, but also its H2 /CO molar ratio, since this determines the possible applications for which it can be used. GL is the material with the highest syngas volumes (0.9 l g−1 ) even though it has the lowest H2 /CO ratios. Both of the SS, unlike the GL, produce less syngas with H2 /CO ratios above one unit. In the case of CH, intermediate values are obtained for both the syngas volume (0.62 l g−1 ) and the H2 /CO molar ratio (1.4), the latter occurring even at low temperatures (500 ◦ C). 3.2.3. Accompanying gases In addition to the synthesis gas, the gas composition during the pyrolysis experiments includes other gases that provide useful information about the process and the materials involved. This is the case of CO2 , whose values are below 0.1 l g−1 for SS and GL, but around 0.2 l g−1 for CH (see Fig. 6). Other authors [24–27] have also detected CO2 production during the pyrolysis of biomass. The mechanism that controls the pyrolysis of lignocellulosic materials and gas evolution takes place in two stages during the pyrolysis process. The first is located at the beginning of the experiments, where higher CO2 values are observed as a result of the primary reactions (primary pyrolysis). During the course of the pyrolysis process a second region appears, characterized by an increase in the CO from the primary decomposition of volatiles during the secondary reactions (secondary pyrolysis). The temperature and residence time of the volatiles in the reactor determine the extent of the secondary reactions, and therefore, the CO2 and CO yields in the final gas. This explains why the CO2 content is higher at low temperatures, and
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Fig. 5. Comparative diagram between H2 and CO production (l g−1 ) during the pyrolysis of sewage sludges, coffee hulls and glycerol under EF and MW heating. The size of the bubble represents the H2 /CO molar ratio.
why, as the temperature increases, CO2 falls and CO production rises. Compared to the initial characteristics of the raw materials, higher CO2 productions are observed for CH. The amount of cellulose present in the raw material is an important factor in determining the amount of carbon oxides produced, since cellulose itself is a highly oxygenated polymer [25–27], and the higher amount of CO2
in the CH pyrolysis experiments occurs because this material has a higher cellulosic content, as reflected by its oxygen value in Fig. 1. GL can be ignored since no cellulose appears in its composition (pure glycerol). On the other hand, lower CO2 and higher CO concentrations are observed under MW pyrolysis. Considering that the residence times of the volatiles in both heating systems are similar, i.e. the same
Fig. 6. CO2 production (l g−1 ) during the pyrolysis of sewage sludges, coffee hulls and glycerol under EF and MW heating.
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Fig. 7. Light hydrocarbon (CH4 + C2 H4 + C2 H6 ) production (l g−1 ) during the pyrolysis of sewage sludges, coffee hulls and glycerol under EF and MW heating.
3.3. Time and energy consumption A brief comparative study between conventional and microwave heating systems was carried out in terms of time and energy consumption. Same volumes (0.05 l) of the different samples studied (sewage sludge L, sewage sludge V, coffee hull, and the activated carbon used as catalyst during glycerol pyrolysis (BC)) have been heated at different temperatures in both heating systems. Conventional and multimode microwave systems have been previously described in [4–6] and in page 730 of Ref. [28], respectively. Time saving of MW with respect to EF has been calculated considering the heating time that each pyrolysate needs to reach a final temperature under both heating systems. In all cases, MW achieves lower heating times than EF, and therefore, MW attain higher time saving, even up to 60%. Interestingly, time saving rises as the temperature increases, e.g. time saving for the coffee hull corresponds to values of 40 and 60% at temperatures of 500 and 1000 ◦ C, respectively. In the case of the activated carbon used as catalyst in glycerol pyrolysis, time saving ranges from 50% to 60% when the temperature rise from 400 to 900 ◦ C, respectively. Finally, time saving for the heating of sewage sludges is found to be around 60% for both L and V at 1000 ◦ C. Related to energy consumption, wattmeters were employed in EF and MW to register the power used to maintain the sample (carbonaceous residues of sewage sludge L and coffee hull, and the activated carbon used as catalyst in glycerol pyrolysis) at a specific and stable temperature. The volume of heated sample was optimized for each system (0.05 l) and similar times (30 min) were
necessary for completing the pyrolysis processes in both devices, once the temperature became constant. For all materials and studied temperatures (700–900 ◦ C), the specific power (expressed as W g−1 ) under MW resulted to be lower than that employed under EF, increasing with the rise in temperature. Thus, it was found (see Fig. 8) that the specific power required to keep the pyrolysis temperature in the MW system was below 8 W g−1 for all materials and over 10 W g−1 when EF was used. Note that different slopes appear for both MW and EF, which suggests that the temperature increase under EF requires higher energy consumption. Under MW, the slope of the line is smoother than that of EF, indicating a higher energy saving at high temperatures. On the other hand, not all materials are heated in the same way under both heating systems. In the case of EF, the density of the material is an important factor to take into account. For instance, the char of sewage sludge, less dense (0.25 g cm−3 ) than coffee hull (0.39 g cm−3 ) and the activated carbon (0.43 g cm−3 ), needs more energy to reach a final temperature. Therefore, higher energy efficiencies are achieved with MW when the material is suitable for heating. However, it is important to note that the above values are only valid for the laboratory equipments and operating conditions employed in this study, i.e. they cannot be generalised, since the 40 SS-EF
Specific power (W g-1)
carrier gas flow was used (60 ml min−1 ); the concentrations of CO2 and CO must be due to the fact that MW favours gas–solid heterogeneous reactions, such as CO2 gasification of the carbonaceous residue yielding CO. The presence of light hydrocarbons (LH), such as CH4 , C2 H4 and C2 H6 was also detected in the final gas composition during the pyrolysis experiments. Fig. 7 represents LH production (CH4 + C2 H4 + C2 H6 ) at different temperatures under MW and EF. The higher yields correspond to GL pyrolysis, around 0.2 l g−1 . For the other materials (SS and CH), LH concentrations are even lower than 0.1 l g−1 , as in the case of SS.
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900
1000
Fig. 8. Specific power (W g−1 ) used in both MW (smooth lines) and EF (striped lines) heating systems to maintain the char of sewage sludge L (SS), the char of coffee hulls (CH) and the activated carbon used as catalyst in glycerol pyrolysis (BC) to different final temperatures. The lines represent the trend lines for the power values obtained.
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use of different experimental or industrial devices could invert the final result. Considering the aforementioned data, it was possible to calculate energy consumptions (expressed in kW h) and energy saving (%) of MW with respect to EF. From specific powers used to maintain the pyrolysis temperature of the sample, and taking into account the final time, energy consumptions were found to be lower than 0.004 kW h and higher than 0.005 kW h under MW and EF, respectively. Alibas [29] observed energy consumption of 0.07 kW h during the microwave drying period of nettle leaves, since more heat is needed to evaporate the moisture content. Regarding energy saving, values lying from ca. 50 to 77% were obtained in our pyrolysis experiments under MW, which is in agreement with other authors [30] that, using different equipments in a drying process, found energy saving of 70%. 4. Conclusion This paper has focused on the influence of feed characteristics on the final yields and gas compositions during the pyrolysis process under conventional and microwave heating. More specifically, the physical state and chemical properties (ultimate and proximate analysis) of several biomass wastes have been related to syngas production. Glycerol provided the highest concentration of syngas in the gas fraction, although it had the lowest H2 /CO ratio. Sewage sludges showed the opposite tendency, i.e. the lowest syngas production with the highest H2 /CO molar ratio, coffee hull displaying intermediate values for both parameters. The use of microwaves always produces a greater gas yield with a higher level of syngas than conventional pyrolysis. Moreover, microwave pyrolysis always achieves the desired effect with temperature increase whichever the biomass material is employed. This may be due to the hot spot phenomenon, which only occurs under microwave heating. Acknowledgements Y. Fernández is grateful to CSIC of Spain and the European Social Fund (ESF) for financial support under the Ph.D. Grant I3P-BDP2006. References [1] A.P.C. Faaij, Bio-energy in Europe: changing technology choices, Energy Policy 34 (2006) 322–342. [2] J.F. González, S. Román, J.M. Encinar, G. Martínez, Pyrolysis of various biomass residues and char utilization for the production of activated carbons, J. Anal. Appl. Pyrol. 85 (2009) 134–141. [3] A. Domínguez, J.A. Menéndez, M. Inguanzo, J.J. Pis, Production of biofuels by high temperature pyrolysis of sewage sludge using conventional and microwave heating, Bioresour. Technol. 97 (2006) 1185– 1193. [4] A. Domínguez, Y. Fernández, B. Fidalgo, J.J. Pis, J.A. Menéndez, Bio-syngas production with low concentration of CO2 and CH4 from microwave-induced pyrolysis of wet and dried sewage sludge, Chemosphere 70 (2008) 397– 403.
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