Article pubs.acs.org/EF
Influences of Potassium Hydroxyde on Rate and Thermicity of Wood Pyrolysis Reactions C. Di Blasi,*,† C. Branca,‡ and A. Galgano‡ †
Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli “Federico II”, P.le V. Tecchio, 80125 Napoli, Italy ‡ Istituto di Ricerche sulla Combustione, C.N.R., P.le V. Tecchio, 80125 Napoli, Italy S Supporting Information *
ABSTRACT: Pyrolysis of KOH impregnated wood is investigated by means of thermogravimetric analysis and a packed-bed reactor. Apart from a confirmation in the trend of product yields and composition versus the additive content, profound modifications in the devolatilization curves and the bed thermal field are for the first time observed. For KOH contents up to about 1 wt %, the effects are essentially of catalytic type. The usual shape of the thermogravimetric curves is preserved though with a progressive displacement at significantly lower temperatures. For higher KOH contents, also associated with wood structure deterioration during aqueous impregnation, even the qualitative features are altered. The bed heating dynamics are strongly modified by the enhanced reaction exothermicity which, for high KOH contents, tend to turn the role of the external heat source simply into a starter for a self-sustaining process. The magnitude of the exothermic effect display, also including pyrolytic runaway, depends on the additive percentage and the heating conditions.
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INTRODUCTION Alkali compounds, indigenous or deliberately added to lignocellulosic fuels, highly affect the primary and secondary pyrolysis reactions.1−17 The modifications introduced in the pyrolysis reaction paths and the product distribution depend on the chemical state of the compound and the nature of the metal5 and are the result of both irreversible modifications in the wood structure during impregnation, whose entity depends on the basicity of the aqueous solution, and the intrinsic catalytic action of the metallic ions.9 The practice of adding large amounts of alkaline compounds to wood has long been known to increase the fire resistance18 which, in pyrolysis, is manifested by a remarkable increase in the production of char, water, and carbon dioxide, at the expense of condensable organic products.5,6,9 Instead, though still associated with a global decrease in the production of organics, the addition of small quantities of alkali compounds is apt to increase the yields of some specialty chemicals such as cyclopentenones and phenols.5,6,9 Thus, also for this process, similar to acidic pyrolysis, profitable applications can be found in integrated biorefineries. However, the changes induced by alkali metal impregnation on the energetics of the pyrolysis process have been scarcely investigated. In particular, it should be stressed that, also for the studies carried out by this research group and focused on the yields/composition of the products,5,6,9 the modifications in the reaction heats, consequent to those of the conversion pathways, have not been given consideration. This aspect is especially important when the production of chemicals is of interest as their yields are highly affected by the actual reaction temperature. On the other hand, it is proved that the exothermicity of biomass pyrolysis reactions, in the absence of additives, may have dramatic effects on the thermal field and the pyrolytic conversion.19 © XXXX American Chemical Society
Previous studies have shown that external heating applied along the later surface of the cylindrical packed bed of biomass particles exalts the display of reaction heat effects which, inducing large modifications in the actual bed temperature, in their turn, affect the activity of reactions and the associated products.20−23 The dynamics of the thermal field depend on both the external heating conditions and the feedstock properties. From the qualitative point of view, temperature measurements (bed cross section at the median height) show the existence of two main zones: an external annulus, with high spatial gradients, and a cylindrical core with much smaller spatial variations. Owing to the symmetry condition, the latter zone permits a clear display of reaction heat effects that always appear as temperature overshoots with respect to the final values achieved by the inert char bed. Moreover, the center bed heating rate exhibits a peculiar behavior consisting of two peaks separated by a valley testifying exothermic reactions or endothermic/thermally neutral reactions, respectively. These, on the basis of the corresponding temperatures, are associated with the decomposition of the three main chemical components of lignocellulosic materials or, more precisely, pseudocomponents, as the temperature range of the decomposition reactions are derived from thermogravimetric analysis.24,25 In fact, the kinetic analysis of the thermogravimetric curves does not allow the contribution of each component to be exactly defined owing to the more or less wide overlap with the others. However, the main contributor for the low, intermediate, and high temperature ranges can always be identified and is indicated as a pseudocomponent. Though the energetics of component decomposition are highly dependent Received: February 22, 2017 Revised: April 27, 2017 Published: May 3, 2017 A
DOI: 10.1021/acs.energyfuels.7b00536 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels on their specific nature and the conversion conditions, it can be stated that the first exothermic contribution results from hemicellulose degradation, the endothermic/thermally neutral valley is characterized by the simultaneous degradation of both cellulose and lignin and the second exothermic peak by the residue lignin degradation. It is important pointing out that, given the heat and mass transfer controlling role, the reactions and related heat display refer to both primary degradation of the substrate and secondary degradation of the generated tarry vapors. Moreover, for extremely mild heating conditions and feedstocks owning scarcely porous structure and/or a low initial degradation temperature, the phenomenon of pyrolytic runaway may occurs,19,21 that is, decomposition occurs almost instantaneously across the entire bed volume with temperature overshoots that may exceed 200 K. Very high temperature overshoots and pyrolytic runaway are also observed for wood preimpregnated with acidic compounds,26,27 testifying that reaction paths and reaction heats are simultaneously deeply modified. Moreover thick wood particles, preloaded with alkali compounds, also show enhanced exothermic effects during pyrolytic conversion.11 In this study, an analysis is carried out of the thermal field (and the yields of the lumped classes of products) of a packedbed reactor during the pyrolysis of beech wood particles impregnated with variable percentages of potassium hydroxide (KOH). In this case, conversion results from a strong interaction between (primary and secondary) pyrolysis reactions and transport phenomena (i.e., heat and mass transfer). Thermogravimetric curves of the various samples are also measured, aimed at evaluating the modifications in the devolatilization rates and the actual reaction temperatures for conditions of primary pyrolysis reactions control and absence of significant temperature deviations with respect to the programmed values. It can be understood that the results of the thermogravimetric analysis, apart from providing additional knowledge about the process fundamentals, are of paramount importance for understanding the packed-bed conversion which mimics the conditions of practical applications.
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Figure 1. KOH content in wood (wt %) versus the KOH content in water (g/L) as measured (symbols) and predicted by fit lines for low (purple) and high (yellow) values. applied. Milling and grinding of the samples result into particle sizes below 80 μm for a fraction of about 50 wt % and in the range 80−280 μm for the remaining amount. The bench-scale pyrolysis experiments are conducted for a 15 g sample mass by means of a cylindrical packed bed (4 cm diameter and length) instantaneously and uniformly heated along the lateral surface, as already described in detail elsewhere20−23 and whose main features are summarized, also with the aid of a schematic, in the Supporting Information. The thermal field (five thermocouples positioned at the median section starting from r = 0 (center) to 1.9 cm (subsurface)) and the yields of the lumped product classes and the gas composition are measured. A significant number of experiments, aimed at investigating the effects of the KOH content in wood, is made for a radiant heat flux density of 30 kW/m2, corresponding to a heating temperature around 690 K (the heating temperature is that reached by the char bed, at the conclusion of the conversion process, at a distance of 5 mm from the heat exposed surface, which depends on the properties of the origin feedstock) and corresponding to the maximum temperature overshoot for unimpregnated wood. However, the effects of a lower heat flux density (22 kW/m2, corresponding to heating temperature of 570 K) are also studies for a KOH content in wood of 1.9 wt %. In all cases, the typical mass closures for the packed-bed experiments are around 95−96%. During the pyrolysis of KOH impregnated wood, the catalyst undergoes physical and chemical processes as already described in detail,6 where the related references can be found, so only the chief possible events are very briefly summarized. KOH presents a fusion temperature of 633−653 K with a fusion heat of 149.3 kJ/kg. For the temperature of interest in pyrolysis, it can form potassium carbonate and/or undergo dehydration again with the subsequent formation of potassium carbonate. This compound, for temperatures above 1073 K or lower in the presence of carbon, may decompose. It is also possible that metallic potassium can be formed due to reduction of K2O with hydrogen or carbon at temperatures above 973 K. It is difficult to actually establish whether these reactions or some of them actually occur for the pyrolysis conditions of this study. However, given the relatively small amounts of additive examined, it can be assumed that the effects of KOH modification are of minor importance compared with its intrinsic action on wood pyrolysis.
EXPERIMENTAL DETAILS
The feedstock, beech wood, is the same as in previous works using acidic catalysts,26,27 though the particle sizes are smaller (3mm). The impregnation procedure is also the same: the sample is soaked in KOH aqueous solutions (a mass ratio of wood to water of 1/5) for 3 h with stirring. Then it is dried with a forced air flow at 343 K for 2 h, followed by oven drying for 17 h at 373 K. As expected, the content of the additive in wood increases with its concentration in the aqueous solution. For low concentrations (up to about 1 wt % on wood dry basis), the measured (weight difference between the dry samples after and before impregnation) and estimated (estimation based on the assumption that, following impregnation, the sample weight doubles as for raw wood) contents are approximately coincident. Instead, as already reported for fir wood,6 the impregnation becomes more effective for high KOH concentrations in water, most likely owing to increased swelling (and wetting) of the particles consequent to the enhanced basicity of the solution or, in any case, to modifications in the alkaline gradient (Figure 1). In particular, for KOH concentration in water of 20 g/L, the amount of impregnated additive in wood corresponds to 2.2 wt %, which is the maximum examined in this study (range of 0 (untreated material) to 2.2%). Experiments are carried out by means of a thermogravimetric analyzer and a packed-bed reactor. Thermogravimetric curves are measured using the commercial system Mettler TGA/1, with a pulverized sample mass of 5 mg, preliminarily subjected to KOH impregnation, heated under a nitrogen flow of 50 mL/min. A heating rate of 5 K/min up to 773 K is always
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RESULTS AND DISCUSSION The thermogravimetric curves are first examined to evaluate the influence of the KOH content on the characteristic temperatures, devolatilization rates, and mass fractions of wood. Then the thermal profiles and the yields/composition of products of the packed bed are commented on. Thermogravimetric Analysis. The measured integral and differential thermogravimetric curves of the KOH loaded beech wood samples are reported versus temperature in Figure 2. On the whole, the devolatilization process is displaced at lower temperatures, as already noticed in the presence of alkaline B
DOI: 10.1021/acs.energyfuels.7b00536 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 3. Characteristic temperatures of the thermogravimetric curves: initial degradation temperature, Tinitial, temperatures Tonset, Tshoulder, Tpeak, and Toffset at the beginning of the shoulder, shoulder, peak, and tail of the mass release rate curve and temperature range corresponding to full width at half-maximum rate, fwhm, versus the KOH content in wood.
Figure 2. Thermogravimetric curves for the mass fraction, Y, and mass loss rate, −dY/dt, versus temperature of beech wood loaded with different KOH contents (heating rate 5 K/min up to 773 K).
compounds by other authors.2,3,11,15,16 For low KOH contents (approximately below 0.5 wt %), the rate curves show the usual features of beech wood devolatilization, that is, a shoulder, a peak, and a tailing zone, usually associated with the decomposition of the pseudocomponents hemicellulose, cellulose, and lignin. For intermediate contents (approximately below 1 wt %), the shoulder zone of the rate curves becomes barely visible, and the wide reaction zone, after the peak, is characterized by high values (the shape of the rate curve is, for a large part, concave). For high contents (above 1 wt %), large quantities of volatiles are released before the peak rate is attained so that the high temperature zone is characterized by low rates (the shape of the rate curve is, for a large part, convex). Therefore, KOH introduces not only quantitative but also qualitative modifications, for sufficiently high values, in the devolatilization rate of beech wood. In general, the parameters employed to characterize the thermogravimetric behavior of lignocellulosic fuels include28 the initial degradation temperature corresponding to a mass fraction of 0.98, Tinitial, the temperature Tpeak of the maximum devolatilization rate (peak rate of pseudocellulose decomposition), with the corresponding −(dY/dt)peak and Ypeak, Tonset, and Tshoulder (beginning and peak rate of pseudohemicellulose decomposition), with the corresponding −(dY/dt)shoulder and Yshoulder, and Toffset, which demarcates the beginning of the final, tailing region of the rate curve (pseudolignin decomposition). Other parameters are the char yield, Y773 (the solid mass fraction detected at 773 K), and the full width of the rate curve at half-maximum, fwhm, expressed in K. However, following the changes in the shape of the thermogravimetric curves with the KOH content, all these parameters can be entirely evaluated only for KOH contents up to about 1 wt % being understood that some of them, that is, Tinitial, Tpeak, fwhm, Ypeak, Y773, and dY/dtpeak can always be easily evaluated. Their dependence on the catalysts content can be examined by means of Figures 3 and 4. The parameter fwhm, providing an indication about the temperature range where a large part of the conversion takes place, is useful to summarize the changes in the shape of the rate curve. For KOH contents between 0 and 0.5 wt %, it rapidly increases (from about 45 to 65 K). Hence, the most evident effect of KOH impregnation is an enlargement of the reaction zone, essentially related to the peak and tail zones (Tpeak decreases by about 25 K and Toffset by about 15K) while
Figure 4. Characteristic mass fractions, Yshoulder and Ypeak, and rates of mass loss, −dY/dt shoulder and −dY/dt peak, of the thermogravimetric curves in correspondence of the shoulder and peak, and mass fraction for temperature of 773 K, Y773, versus the KOH content in wood.
the displacement in the initial decomposition zone is small (both Tinitial and Tshoulder decrease by about 10 K). As a consequence, the peak rate decreases and it is attained at successively higher mass fractions or, in other words, the highrate zone tends to be displaced toward lower temperatures. The mass fraction and the devolatilization rate of the shoulder zone slightly increase. Moreover the char yields continuously increase (from about 20 to 25 wt %), also taking into account that the intrinsic contribution of the catalyst is very small. For intermediate catalyst contents (approximately 0.5−1 wt %) the characteristic parameters of the thermogravimetric curves show smaller variations. The fwhm temperature range approximately retains its maximum (around 65K) with an exalted overlap between component decomposition though the parameters characterizing the hardly visible shoulder zone can still be mathematically defined. The displacement of the conversion process at successively lower temperature persists with figures that are comparable for both the initial and the peak zones (Tinitial and Tpeak decrease by about 15 K) and almost negligible for the shoulder and tail zones (temperature decrease around 3 K). The decrease in the peak rate and the increase in the corresponding mass fraction is confirmed together with a further increase in the rate and mass fractions of the shoulder C
DOI: 10.1021/acs.energyfuels.7b00536 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 5. Temperature versus time profiles at several radial positions of the packed bed for an external heat flux density of 30 kW/m2 and KOH contents in wood of 0 (A), 0.5 (B), 0.8 (C), and 1.9 (D) wt %.
0.5 wt % and then causes a tendency toward asymptotic values. In summary, for percentages in the range 0−1 wt %, the process starts at lower temperatures (by about 25 K) with the most remarkable effect on pseudocellulose decomposition (Tpeak decreases by about 40 K), and smaller effects on the decomposition of pseudolignin (Toffset reduced by 18 K) and pseudohemicellulose (Tonset approximately constant and Tshoulder decreased by 10 K). Moreover the reaction zone is enlarged (by about 20K) with a reduction of about 25% on the peak rate and an increase in the char yield by about 35%. As already stated, these effects can be mainly attributed to the alkaline catalysis of the wood pyrolysis reactions. The further increase in the KOH content, for percentages well above 1 wt %, also enhances the degradation of the wood structure during the impregnation stage so that pyrolysis occurs of a substrate that is progressively altered. The displacement of the process at lower temperature continues, but the most remarkable effect is on the decomposition of pseudolignin with respect to the other components (Toffset decreases by about 25 K versus 8 or 16 K for Tinitial or Tpeak). The rates present more complex dynamics but, on the whole, the variations are small. The evident acceleration in the devolatilization process and the temporary reduction in the growth rate of the char yield can be reasonably attributed to depolymerization/chemical bond rupture during the impregnation stage. Packed-Bed Heating Dynamics (High External Heat Flux Density). An important part of packed-bed pyrolysis results is represented by the thermal field. A summary of the qualitative changes induced by KOH on the bed thermal field for Q0 = 30 kW/m2 and KOH contents of 0, 0.5, 0.8, and 1.9 wt % can be made by means of Figure 5A−D, reporting the temporal temperature profiles measured at various radial positions, Figure 6, reporting the radial profiles at t = tmax, that is, the time when the maximum temperature at the bed center is achieved, and Figure 7, reporting the time derivative of the center bed temperature (heating rate). It can be seen that the qualitative features of the bed core heating are not modified by the KOH addition. In fact, as for unimpregnated wood, this
zone but, as already noticed, variations are significantly milder. Moreover the increase in the char yield is also rather small (from about 25 to 27 wt %). On the whole, it can be stated that the most evident effect of intermediate catalyst contents is to push the pseudocellulose decomposition zone toward that of the pseudohemicellulose, so that a distinction between the two processes in no longer possible. For high KOH contents in wood (1−2.2 wt %), the characteristic temperatures Tinitial, Tpeak, and Toffset confirm the decreasing trends whereas the other parameters exhibit a more complicated dependence. The temperatures Tinitial and Tpeak continue to decrease (by about 10 K), but the most remarkable effect of the additive is on the temperature Toffset, which decreases by about 25 K, testifying the displacement of the entire reaction process at lower temperatures. The fwhm initially decreases with a reduction from about 65 to 55 K (KOH percentage from about 1 to 1.6 wt %) to finally attain an approximately constant value. These features are associated with a slow increase in the char yield (from about 27 to 30 wt %). Following the decrease in the fwhm and the barely increasing solid yield, the peak rate initially tends to remain constant or to weakly increase (the corresponding mass fraction remains approximately constant). Then, after a local maximum for KOH percentages around 1.3 wt %, it resumes the decreasing trend (the same value is observed for KOH percentages around 1 and 2.2 wt %, which demark the extension of the zone under study). KOH exerts a 2-fold action on the decomposition of wood whose intensity depends on the quantity actually loaded, consisting of the catalytic action of the K ion on the reaction paths and the progressive degradation of the wood matrix during the impregnation stage. The latter becomes especially significant for KOH percentages above 1 wt %, as indirectly indicated by the impregnation curve (Figure 1). In fact the results of the thermogravimetric analysis show a low- and a high-percentage action of KOH, using as a demarcation a value around 1 wt %. The low-concentration action (mainly K catalysis) reaches its maximum effect for KOH contents around D
DOI: 10.1021/acs.energyfuels.7b00536 Energy Fuels XXXX, XXX, XXX−XXX
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manifest a much rapid increase, followed by a plateau or a local minimum, corresponding to the occurrence and the conclusion of the decomposition reactions, respectively. Finally, the new rise is most likely consequent to heat transfer from the hot degrading core of the bed toward the surface. The radial temperature profiles at t = tmax can be especially useful to understand the changes induced by the additive for both the external and the more internal bed portions. As the KOH percentage in wood is increased, initially (i.e., KOH contents of 0.5 wt %) the thickness of the reaction zone enlarges. Temperatures significantly higher than those observed for the unimpregnated sample are reached across the entire bed, despite the shorter observation time (tmax value of 454 versus 627 s), indicating that the actual thermal conditions are mainly determined by reaction-induced self-heating with respect to the external heat flux. Then, for higher KOH contents, the spatial temperature profiles at tmax show continuously decreasing values mainly at the surface. Indeed, given the progressive reduction in the reaction temperature and observation time (tmax continuously decreases), the contribution from the external heating, for sample heating, with respect to selfheating is increasingly reduced. In this way, the advancement in the reaction zone, across the unconverted portion of the bed, is for a large part due to the associated rate of heat release that permits self-support of the process. In other words, the faster propagation rate of the external reaction zone toward the bed core does not allow the attainment of tmax values sufficiently long for the surface temperature to increase at the levels previously observed. In reality, for the highest KOH contents examined, it can be thought that the external heat flux practically assumes the role of a starter for the pyrolytic conversion and it is the reaction itself which then causes the advancement in the reaction zone and determines the conversion times. On the other hand, as the surface is the site where the reaction begins and given the low activation temperatures (or short times), the heat generated is promptly transferred inward toward the still cold internal portion of the bed, so that, on the whole, the temperature of the external bed annulus decreases. Hence, the changes in the mechanisms responsible for bed heating prior to the beginning of the reactions cause the profound alteration in the shape of the radial profiles of temperature. At low KOH percentages, the heating associated with the reaction exothermicity and the external heating takes place at approximately the same rate, so that the maximum temperatures tend to increase owing to the increased exothermicity of catalyzed pyrolysis. Then, for sufficiently high KOH percentages, the decomposition rate and the related heat release rate become much faster than the external heating rate or, in other words, when adequate activation temperatures are reached, the decomposition process first causes heating of the superficial layer of the bed (also with modifications in the thermal capacity and other thermal properties) that is subsequently heated by the external radiant heat source. In this way, the heat locally released by the reaction is promptly transferred toward the internal colder zone not promoting the local attainment of high temperatures. It is evident that, following the progressive reduction in the reaction temperature for the KOH impregnated wood (and the conversion time), the role of the external heating, for the preheating stage, becomes progressively less important compared with the chemically generated heat.
Figure 6. Spatial profiles of the packed-bed temperatures for the times, tmax, corresponding to the attainment of the maximum temperature, Tmax, and KOH contents in wood of 0, 0.5, 0.8, and 1.9 wt % (external heat flux density of 30 kW/m2).
Figure 7. Time derivative of the center bed temperature, dTc/dt, versus time (external heat flux density equal to 30 kW/m2) for KOH contents in wood of 0, 0.5, 0.8, and 1.9 wt %.
is the site where the maximum temperature, Tmax, is attained with values that are higher than the final one, determined by the external heat source and taken as a reference to evaluate the maximum overshoot, ΔT. The center bed heating rate, given sufficiently high temperatures for the reactions to become active, shows the usual two peaks separated by a valley associated, in the order, with the characteristic temperatures, T Hm , T Lm , and T Cm , essentially corresponding to the decomposition of the pseudocomponents hemicellulose, lignin, and cellulose and related generated vapors. However, quantitative differences are huge. In the first place, the central bed core, always characterized by radial gradients smaller than those of the more external annulus, exhibits higher temperature overshoots, testifying a highly increased exothermicity of the KOH catalyzed reactions. Moreover, the conversion process requires shorter times (the time tmax can be considered a rough approximation of the conversion time) and the bed center heating rate becomes faster (also the first peak and the valley become less visible and the magnitude of the second peak largely surpasses that of the first). However, the latter variable confirms that, after KOH impregnation, the dynamics of wood conversion are still affected by the different decomposition rates of the main chemical components. This finding is important also in relation to the formulation of kinetic mechanisms and the associated estimates of the reaction heats, as recently demonstrated for untreated wood.29 The heating dynamics of the more external bed annulus are also significantly influenced by the KOH percentage in wood. For the most external radial positions (r = 1.9 and 1.5 cm), for values above 500−550 K, the temporal profiles of temperature E
DOI: 10.1021/acs.energyfuels.7b00536 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Packed-Bed Thermal Parameters. The temporal profiles of temperature of the packed-bed can be used to understand how the maximum temperatures, Tmax, and the corresponding overshoot, ΔT, and time, tmax, depend on the KOH percentage in wood, given an assigned external heat flux density. Other parameters that can also be useful to characterize the process, especially in relation to the total amount of heat generated from the decomposition process, include the maximum surface temperature (r = 1.9 cm), Tsmax, the average conversion temperature (evaluated over time interval 0−tmax and the space interval 0−20 mm, assuming that the thermal properties are constant and that the temperature profiles of the median section are the same as along the entire length of the bed), Taver, and the characteristic temperatures associated with the two peaks and the valley in the center bed heating rate, THm, TLm, and TCm. These parameters are reported versus the KOH percentage in wood in Figures 8−10. The largest variation on
Figure 10. Characteristic temperatures, THm, TCm, TLm, (associated with the maximum in the decomposition rate of pseudocomponents hemicellulose, cellulose, and lignin and defined in correspondence to the first local maximum, the valley, and the absolute maximum of the bed center heating rate, see Figure 7) versus the KOH content in wood (external heat flux density of 30 kW/m2).
are observed. The same trend is also shown by the maximum temperature at the surface (and at the other radial positions), but the decrease is more marked. The maximum temperature overshoot, ΔT, evaluated at the bed center and assuming a steady average temperature of 670 K, shows the same qualitative trend as Tmax, with values roughly increasing from 60 to 100 K and then decreasing to about 85 K. These trends are well explained by the changes discussed above in the shape and values of the temperature profiles caused by the modified ratio between the heat amount provided to the bed by the external source and the occurrence of the decomposition reactions. The simultaneous analysis of the average conversion temperature, Taver, and the conversion time (tmax) provides further evidence about the modification in the heat transfer process. The strongest variation for tmax takes place for KOH contents up to about 0.2 wt % (625−500 s), followed by milder changes for higher values (500−370 s). For low KOH contents, Taver also decreases but with a slower rate with respect to tmax. Therefore, the reduction in the amount of heat provided by the external heat source, consequent to a shorter tmax, is partly compensated by the increase in the amount of heat released by the reaction. For intermediate KOH contents, where the reduction in tmax is less marked, the increase in the amount of heat released from the reaction is sufficient to exactly compensate the diminished amount provided from the external radiation and Taver remains approximately constant. For very high KOH contents, it can be postulated that initially there is a prevalence of the increase in the amount of heat released from the reaction with respect to the decrease in the externally provided amount, leading to an increase in Taver. Then both tmax and Taver tend to weakly decrease, owing to the still decreasing reaction temperatures, as indicated by the thermogravimetric analysis results and the progressively lower amount of external heat with respect to the quantity of heat released by the reaction, which most likely approaches a constant value. All the characteristic temperatures of pseudocomponent decomposition show the same qualitative dependence on the KOH percentages, apart from the obvious quantitative differences originated from the intrinsic thermal stability of pseudocomponents.24,25 For low KOH contents, an increase is observed attributable to the increased thermal severity of the
Figure 8. Maximum temperature, Tmax, (attained at the bed center), maximum subsurface temperature, Tsmax, (measured at r = 19 mm), and maximum temperature overshoot, ΔT, (difference between the maximum temperature and the corresponding final value of the inert char bed) versus the KOH content in wood (external heat flux density of 30 kW/m2).
Figure 9. Time tmax (time corresponding to the attainment of the maximum temperature of the bed) and average bed temperature, Taver, (evaluated at tmax) versus the KOH content in wood (external heat flux density of 30 kW/m2).
Tmax takes place for very low KOH contents with a rapid increase from about 725 to 770 K (KOH percentage from 0 to about 0.2 wt %) followed by a roughly constant value (up KOH percentages around 0.7 wt %). Then slightly decreasing values F
DOI: 10.1021/acs.energyfuels.7b00536 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels process induced by the higher exothermicity of the reactions. Indeed, for untreated wood, the characteristic temperatures tend to increase with the heating temperature.20,21 Then after a wide zone of approximately constant values, for very high KOH percentages, the reduction in the actual degradation temperatures, registered by the thermogravimetric curves, and the resizing of the external heat flux role in the conversion process prevail over the increased exothermicity. Packed-Bed Heating Dynamics (Low External Heat Flux Density). It should be noticed that the qualitative features of the temperature profiles discussed above are associated with an external heat flux density that, for untreated wood, approximately corresponds to the attainment of the maximum temperature overshoot20,21,30 but, given the progressive reduction in the decomposition temperatures caused by KOH loading, it becomes very severe for the impregnated samples, somewhat obscuring the thermal effects of the decomposition reaction. Indeed, the packed bed pyrolysis of wood particles, loaded with remarkable contents of KOH, and exposed to lower external heat flux densities reproduce the qualitative features of pyrolytic runway observed for scarcely porous lignocellulosic wastes.21,22 These features can be clearly observed from the temperature profiles reported in Figure 11
Figure 12. Yields (as wt %, dry basis) of the lumped classes of products (char, organics, water, and gas) from the packed pyrolysis versus the KOH content in wood (external heat flux density 30 kW/ m2).
is increased, the yields of water and total gas also increase. The increase in the total gas yield should be attributed mainly to CO2 production (the increase in CO is much less marked and the CH4 yield remains approximately constant or slightly decreases for very high KOH percentages), as shown in Figure 13. This result is in agreement with previous findings of this
Figure 11. Temperature versus time profiles at several radial positions of the packed bed for external heat flux densities of 22 and 30 kW/m2 and wood with a KOH content of 1.9 wt %.
Figure 13. Yields (as wt %, dry basis) of the gaseous species from the packed pyrolysis versus the KOH content in wood (external heat flux density 30 kW/m2).
for an external heat flux density of 22 Kw/m2 and a KOH concentration of 1.9 wt % (the profiles measured for an external heat flux density of 30 kW/m2 are also reported for comparison). While unimpregnated wood at this low heating temperature scarcely reacts with hardly visible exothermic display,20,21,30 the treated sample, after an induction period, exhibits a very rapid rise of the temperature, well above the external heating value, across the entire bed. As already observed for the events of pyrolytic runaway, the maximum temperatures are still attained at the inner core of the bed but the differences with the more superficial layer are reduced with maximum temperature overshoots exceeding 200 K. Though not investigated, it can be reasonably guessed that the qualitative trend of this parameter (and the intensity/violence of the pyrolytic runaway) on dependence of the KOH content in wood is the same as shown in Figure 8 for the more severe external heat flux. Product Yields. The yields of the lumped classes of products versus the KOH content in wood are reported in Figure 12 (Q0 = 30 kW/m2). As the KOH percentage in wood
group5,6,9 and confirm the flame retardancy properties of KOH consisting in a remarkable reduction in the production of flammable volatile products. Indeed, again in accordance with previous results,5,6,9 the yields of the total condensable organic products also decrease. The composition of the organic fraction has not been determined but, based on the previous detailed characterization,5,6,9 a narrow range of low KOH contents (0.2−05 wt %) permits the maximization of specialty chemicals. On the basis of the results of thermogravimetric analysis, it can be stated that it mainly corresponds to the catalytic action of the additive and the attainment of maximum rates of heat release from the decomposition reactions. In fact, the increased formation of char and its more cross-linked structure, in the presence of alkaline compounds, are indicated as the main cause of the increased exothermicity of the process,16 though the contribution of secondary reaction activity is certainly very important when transport phenomena are controlling.11 Finally, it is worth mentioning that, as demonstrated in a previous G
DOI: 10.1021/acs.energyfuels.7b00536 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels work,23 for the reactor configuration used in this study, the product distribution is determined not only by the modifications in the reaction paths, caused by KOH addition, but also by the associated changes in the amount of heat released/required by the process that consequently introduces modifications with respect to the programmed external heating temperature. A significant number of investigations has been carried out about the catalytic pyrolysis of lignocellulosic biomass focused on the induced modification in the product distribution, especially the maximization of valuable chemicals. For instance, it has been demonstrated31 that acidic pyrolysis could be a practical alternative to acid hydrolysis for the production of furfural, also with a reduced environmental impact. Comprehensive evaluations, including the economic aspects related to the impregnation pretreatment, are not available for the alkaline pyrolysis also because precise prerequirements on the yields and nature of the specialty products should be first assigned. This study also puts into evidence that, as a consequence of the strong enhancement in the process exothermicity, the control of the thermal conditions during conversion could introduce additional difficulties and require more effective (and costly) control systems.
not only to confirm trends already reported for the product distribution but to discover that KOH impregnation, as a consequence of the modifications introduced in the reaction paths, also causes a remarkable increase in the associated global rate of heat release.The enhancement in the reaction exothermicity is, in the first place, displayed by higher temperature overshoots and faster conversion rates. In reality, the magnitude of the temperature overshoots as well as the bed temperature dynamics are highly dependent on the KOH content in wood and the external heating temperature (or intensity of the radiant heat flux). For relatively high external heating temperatures (around 690 K), reaction-induced overheating becomes progressively more important than external heating in relation to sample preheating to activate thermal decomposition so that the process tend to self-sustain. For low external heating temperature (around 570 K) and sufficiently high KOH percentages, pyrolytic runaway occurs. The heating dynamics and the products of packed bed pyrolysis as well as the operating conditions (particle sizes and heat flux density) reproduce those of practical applications typical of fixed-bed reactors though the magnitude of reaction heat effects is expected to be also affected, among others, by the process scale. Therefore, the results of this study are useful not only for improving the knowledge of process fundamentals. The comprehension of the interaction between chemical reactions, in particular secondary reaction activity, and transport phenomena can be facilitated with the aid of comprehensive transport models. However, the action exerted by KOH, consisting in the alteration of the wood structure already during the impregnation stage and the alkaline catalysis of the decomposition reactions, still requires further investigation in relation to the determination of the kinetic mechanisms and reaction heats. Other aspects that deserve consideration concern the effects of the chemical state of the alkaline compound and nature of the alkali metal, in particular the role of the sole alkaline pyrolysis by means of sample impregnation with pH-neutral alkaline compounds.
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CONCLUSIONS The addition of KOH, by means of aqueous impregnation, modifies the reaction paths and the product distribution from beech wood pyrolysis. For reduced effects of secondary decomposition and heat/mass transfer intrusions, such as in thermogravimetric analysis, the chief action of KOH can be summarized in a progressive reduction in the reaction temperature and the peak of the devolatilization rate, also corresponding to an enlargement on the temperature range where decomposition takes place and a reduction in the amount of volatile products. In figures, for KOH contents in wood up to 2.2 wt %, the temperature corresponding to the peak of the devolatilization rate decreases by about 50 K and the maximum enlargement in the reaction temperature range is around 20 K with a global reduction in the peak devolatilization rate of about 25%. Moreover the maximum effects of KOH on the char yields are observed for low percentages (below 1 wt %). The shape of the thermogravimetric curves also suggests two different actions of low and high KOH percentages. The lowconcentration action reaches its maximum effect for contents around 0.5 wt % and the further increase of the loaded quantity up to about 1 wt % does not lead to any new significant change. Then, at higher KOH contents, the high-concentration action dominates with its maximum effect approximately between 1 and 1.4 wt %, again followed by a tendency toward constant values. The thermogravimetric measurements do not indicate the origin or cause of the observed KOH action. However, a change in the slope of the impregnation curve supports the speculation that for a KOH percentage above 1 wt % the effects of the wood matrix deterioration sum up with alkaline catalysis. Contrary to the case of thermogravimetric analysis which essentially allows the modifications in the primary reaction paths to be investigated for assigned thermal conditions, the packed-bed pyrolysis results from the activity of both primary and secondary reactions, the interaction between chemical processes and transport phenomena, and the reaction-induced overheating, affected by the KOH impregnation of the wood samples. In fact, the packed-bed experiments have been useful
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b00536.
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Details of the experimental setup and methods (PDF)
AUTHOR INFORMATION
Corresponding Author
*Phone: 39-081-7682232. Fax: 39-081-2391800. E-mail:
[email protected]. ORCID
C. Di Blasi: 0000-0001-5499-6251 Notes
The authors declare no competing financial interest.
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NOMENCLATURE
Thermogravimetric Analysis
−dY/dtpeak = peak rate −dY/dtshoulder = shoulder rate fwhm = temperature range defined as the full width of the rate curve at half-maximum H
DOI: 10.1021/acs.energyfuels.7b00536 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
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Tinitial = initial degradation temperature (in correspondence of a mass fraction of 0.98) Toffset = temperature at the tail of the rate curve Tonset = temperature at the beginning of the shoulder of the rate curve Tpeak = temperature at the peak of the rate curve Tshoulder = temperature at the shoulder of the rate curve Ypeak = mass fraction in correspondence of the peak rate Yshoulder = mass fraction in correspondence of the shoulder rate Y773 = mass fraction for a temperature of 773 K Packed-Bed Pyrolysis
Q0 = radiant flux density Taver = average bed temperature (evaluated over the interval 0−tmax) Tc = center bed temperature TCm = center bed temperature in correspondence of the valley in the heating rate (pseudocellulose decomposition) THm = center bed temperature in correspondence of the first peak in heating rate (pseudohemicellulose decomposition) TLm = center bed temperature in correspondence of the second peak in the heating rate (pseudolignin decomposition) Tmax = maximum temperature at the bed center tmax = time corresponding to Tmax Tsmax = maximum subsurface (r = 19 mm) temperature ΔT = maximum temperature overshoot at the bed center
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DOI: 10.1021/acs.energyfuels.7b00536 Energy Fuels XXXX, XXX, XXX−XXX
