Ind. Eng. Chem. Res. 2002, 41, 4965-4975
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Experimental Study of Cellulose Fast Pyrolysis in a Flow Reactor V. Visentin, F. Piva, and P. Canu* Dipartimento di Principi e Impianti di Ingegneria Chimica “I. Sorgato”, DIPIC, Universita` di Padova, Via Marzolo 9, 35131 Padova, Italy
The main purpose of this work is the development of a reproducible experimental technique for supplying new data from cellulose pyrolysis at high heating rates and low residence times. The actual behavior of solid materials under such conditions (fast pyrolysis) is quite difficult to investigate experimentally. Data were collected in an entrained-flow reactor, accurately optimized to operate at low solids/gas ratios. Cellulose powder, mostly Avicell PH102, was conveyed by an inert carrier gas into a catalytically inert quartz tube with different heating sections. Weight loss and composition measurements were collected at temperatures between 400 and 650 °C, residence time between 50 and 140 ms, and nominal heating rates ranging from 6000 to 11 000 K/s. The disturbance effect of aggregative flow was demonstrated and quantified; all of the meaningful data were collected under conditions approaching the single-particle (homogeneous flow), while trying to keep the increasing experimental error bounded. The role of water loss during the initial heating of the particles is specifically addressed, together with its contribution to aggregation and further reactions. It is demonstrated that dilute particle suspensions lose water very rapidly. The particle size effect has been quantified as well, confirming that 90-µm particles are small enough to prevent any intraparticle processes. Introduction It has now been confirmed that biomass pyrolysis is a viable route for producing gases and liquids that can replace some petroleum-derived products. The possibility of obtaining useful chemicals has also been demonstrated.1 The interest in these technologies is based on the widespread availability of a renewable source of energy that does not contribute to increasing atmospheric CO2 levels. Although pyrolysis is meant to be applied to raw biomass, laboratory experiments must concentrate on a more chemically defined material, to limit the variability of the results. It is commonly agreed that cellulose can be a model species for biomass because it is the most frequent and sometimes the most abundant biomass component. Such an assumption is not fully satisfactory, both because cellulose is only partially representative of biomass and because cellulose itself is not a precisely defined chemical species, given that its properties depend on the polymerization degree and the production process. In any case, because of the complexity of biomass pyrolysis, data obtained on cellulose represent a good starting point. Several fast pyrolysis techniques have been suggested,2 some of which have been developed to a commercial scale. Reference 3 provides a thorough review of the pyrolysis processes involving rapid heating and low residence times, now recognized as fast pyrolysis, and the status of their applicability. Many researchers agree that variations in the contact time and the temperature can lead to completely different products. This fact clearly indicates the existence of a nonelementary mechanism for the overall process, including massand energy-transport steps. To understand how selectivity is influenced by process conditions, some laboratory experiments in which these variables are carefully controlled and hopefully single processes are isolated * Corresponding author. Tel.: (+39) 049-8275463. Fax: 8275461. E-mail:
[email protected].
are required. At the least, physical and chemical processes must be distinguished to be quantified independently. Note that the combination of temperature level and contact time is frequently addressed in the literature in terms of the heating rate (HR), somewhat drawing the attention away from the prime variables. Among the many laboratory studies, TGA is the most common technique used. Several authors have pointed out the limitations and sensitivity of TGA results at high heating rates, which can be explained by nonuniform heating of the samples4 or mass-transfer limitations.5 In any case, values of the heating rate in TGA experiments are always too far from those typical of fast pyrolysis, despite the development of modified devices (HR > 2 K/s, ref 6) and more efficient standard instruments (HR e 1.33 K/s, ref 7). On the other hand, a relatively slow technique such as TGA allows real-time analysis of the produced gas through coupling with FTIR spectroscopy or MS.8 Transfer of these results to conditions typical of fast pyrolysis is questionable, however. Interesting speculations about the thermodynamics of the pyrolysis process have also been published by Suuberg and co-workers,5,9 highlighting the role of phase changes together with decomposition reactions. Unfortunately, their investigation is limited to relatively slow heating rates, but the importance of liquid intermediate products has been confirmed by others.10 Pyroprobe (e.g., ref 11) has been suggested as a more appropriate technique, given that a higher rate of heating can be achieved and the use of flowing gas can help to minimize secondary reactions. Again, rather large samples must be used, leading to concerns about mass- and heat-transport limitation. In an attempt to closely match full-scale conditions, laboratory bubbling fluidized beds have been used as well. Although high temperatures can be reached in fractions of a second,12 approaching an estimated heating rate of 105 K/s, the residence time of the solid is difficult to control, and significant backmixing occurs. Short volatile residence
10.1021/ie011034y CCC: $22.00 © 2002 American Chemical Society Published on Web 08/27/2002
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times call for shallow beds and high gas flow rates. For these reasons, the fluidized bed is not well-suited to scientific investigations of the mechanism of pyrolysis, but it is a valid option at the process scale.13 The Curie-point pyrolyzer is an interesting technique for investigating the devolatilization behavior of solids following rapid heating.14,15 Theoretically, HRs of 104 K/s can be achieved, but the experiment is intrinsically dynamic, i.e., short contact times require an extremely fast analytical technique. Our experience16,17 has demonstrated that the residence time distribution of the products in the sampling line is sufficient to mislead the investigation of the kinetics. Pulsed radiant heating has been used,10 allowing the cellulose samples to be treated for short times and the intermediate products to be observed. Unfortunately, cellulose absorbs a small fraction of the incoming radiation, and the temperature of the sample is rather uncertain. Radiant heating has been used also by Lanzetta et al.18 in an interesting contribution where the existence of an upper bound to the applicable heating rate is suggested. Heating grids have often been used to study the pyrolysis of solid samples.14,19-21 Disadvantages are the possibility that stainless steel screens can catalyze the reaction and the inconvenient measurement of temperature and residence time, the two critical variables in determining the thermal decomposition behavior. Extremely low exposure times and high temperatures have been obtained by means of a shock tube,22 which gave very detailed gas composition measurements and kinetics, but the values of temperature and residence time obtained were quite far from those that can be achieved industrially. Reactors in which the solid flows cocurrently with the gas, usually downward, have been used occasionally23 and more frequently with coal24,25 or other materials.27 Apparently, such reactors have not received enough consideration as a tool for kinetics measurements because of a few unusual features, including the establishment of a temperature profile along the reactor and the fact that time does not explicitly appear in the experiment. In addition, some difficulties have been assumed to compromise the usefulness of the technique, i.e., the actual sample temperature is impossible to measure, and several experiments provide the same information as a single thermogravimetric analysis, resulting in tedious operation. In our view, the features of the entrained-flow reactor (EFR) are unique with respect to the investigation of fast processes with interactions between chemical and physical processes, and its difficulties can be overcome by means of suitable modeling of the results.28 It is worth emphasizing that a flow reactor can, in principle, overcome the limitations of the dynamics of both the heating system (heating coils, heating grids, Curie-point supports, radiant lamps) and the measuring technique (temperature and composition). These features appear to have been clearly exploited in the two recent papers,29,30 where a significant effort was devoted to the in-depth characterization of the reactor and the reacting cellulose so that a kinetically controlled pyrolysis could be obtained. The conclusion of these studies was that laminar EFR is a useful tool for generating unique and significant data on the high-temperature behavior of reacting solids. These authors extensively used CFD modeling to design and characterize their EFR and compared their findings to literature kinetics results. In contrast, we report
modeling characterization and kinetics speculation, independently of literature values, in a companion paper.28 The use of so many diverse techniques demonstrates how difficult the investigation of the pyrolysis of solids at high temperatures and short residence times is. Experimental Section Materials. In this study, we used only microcrystalline cellulose. All of the different celluloses were obtained from FMC Corp. and used as received, except for a few tests in which we dried the cellulose in an oven at 110 °C before use. We tested three types of cellulose from the same supplier, all of which are called Avicell, with different identification codes: PH102, PH112 (low moisture), and PH200 (larger size). According to the certificate of analysis accompanying the samples of Avicell PH102, the one we mostly used, this material has a bulk density of 312 kg/m3; an average degree of polymerization of 227 units; a moisture content between 3 and 5 wt % (4.05 in our sample at the time of the analysis); and air jet particle distribution of 0.67 wt % above 60 mesh, 60.69 wt % above 200 mesh, and nothing below 38 µm. An average diameter of 90 µm is assumed. The PH112 and PH200 types have properties comparable to those of PH102 except for the moisture content (about 1wt % for PH112) and size (average diameter of 180 µm for PH200). Apparatus. The entire experimental setup is shown in Figure 1. It consists of a cellulose feeder, a heated vertical quartz tube, a quench section, two cold traps, and the gas sampling line. Cellulose powder is fed into the reactor by a suitable feeder, where a suspension of particles in a carrier gas (N2) is created. An additional dilution with some secondary nitrogen takes place before the stream enters the reactor so that the concentration of solids and the overall gas flow rate through the reactor can be controlled independently. The reactor is a quartz tube inside three independent heating sections. Part of the quartz tube exceeds the length of the heating sections, and it is used for rapid quenching and for connections. An initial cooling is obtained by water evaporation. Latent heat can provide the large heat sink required for fast quenching. Evaporation is achieved by keeping wet a thin porous coating along a free portion of the quartz tube (approximately 0.15 m) after the last heating element but before the first cold trap. At the outlet, there are two cold traps, in accordance with standard procedures. The first trap uses flowing water or an ice bath, and the second trap is cooled by liquid nitrogen. A gas chromatograph provides the analysis of uncondensable gases. The efficiency of the quenching system and solids and liquids collection is demonstrated by the absence of tars in the gas analysis results and vent lines. Note that the cooling and separation section is not designed specifically for collecting pure fractions of products (tars, char, unreacted cellulose), but for separating any condensable or solid species and collecting them as much as possible. Feeder. Accurately and steadily feeding a very low amount of powder into a flowing gas is a rather difficult task. After some attempts with different arrangements, the feeder suggested by Scott and Piskorz31 was built, with a few modifications to fit the characteristics of our powder and the purpose of this study. The feeder must provide a low solids flow rate to approximate the
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Figure 1. Experimental apparatus.
conditions of homogeneous flow,32 where particles travel independently in the reactor, and to the formation of agglomerates, which leads to uneven temperatures among the different particles. At the same time, the feeder must give a constant solids flow rate for good reproducibility of the experiments as well as for the assurance of stationary conditions for the temperature and concentration profiles. The actual feeder is a PVC cylinder (80 mm i.d.) partially filled with powder that has a screw cover and a vertical shaft bearing two sets of paddles. A horizontal entrainment tube (4 mm i.d.) intersects the cylindrical body of the feeder at a half-radial position. A small hole, either 0.75 or 0.60 mm in diameter, depending on the application and the desired feed rate, was drilled into the entrainment tube inside the feeder. The hole diameter is approximately 4 or 5 times the largest particle in the solid. Gas is fed to the entrainment tube at a sufficient rate to convey the solids entering through the small hole without deposition to the secondary gas inlet after the feeder (shown in Figure 1), where the velocity increases and, at the same time, a higher dilution is obtained. The line carrying the gas to the feeder is split between the entrainment tube and the freeboard at the top of the cylinder. Between this split and the feeder, a throttling valve is used to produce a pressure head above the bed that forces the solids to enter the entrainment tube orifice together with some nitrogen. A water manometer is connected across the two inlets
to measure this gap. The operating principle is similar to that of a microfluidized bed, although here the cellulose fluidization is mechanically obtained. Because of the small diameter of the entrainment tube, the gassolids suspension flow is laminar (Re ) 100-1500, depending on the gas temperature). The primary nitrogen flow rate is controlled by a mass flow controller (range of 0.1-2.0 L/min), while the secondary gas flow rate is measured by a rotameter (range of 0.5-5 L/min). In principle, a number of variables could affect the solids flow rate Qs, including the stirring speed, the pressure drop between the free surface of the cellulose bed and the orifice of the entrainment tube, the carrier gas flow rate, the hole position, and the hole diameter. We carried out a number of experiments to assess the effect of varying these parameters and eventually obtain a correlation for predicting the conditions required for a given flow rate. A typical test is performed at ambient conditions with cellulose Avicell PH102, with the weight of the trap at the reactor outlet being measured continuously. Figure 2 shows an example of the results obtained for two diameters of the hole in the tube. We can see that the slope (i.e., the solids flow rate, Qs) remains approximately constant for rather limited periods of time, which are, however, long enough for significant pyrolysis experiments. The values of the solids flow rate that can be evaluated from the weight measurements are indeed quite uniform, and its dependence on the operating parameters can be assessed. The solids flow rate can be selected through the carrier gas
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Figure 3. Longitudinal section of the reactor with actual dimensions (in millimeters).
Figure 2. Test of the cellulose feeder. Stability of the measurements and effect of the diameter of the hole in the entrainment tube. Q0 ) 2 L/min, ω ) 200 rpm, ∆P ) 0.5 cm H2O.
flow rate. A linear correlation is observed beyond a minimum value of the carrier gas flow rate required to trigger entrainment. The result is consistent with the equation suggested in ref 31
Qs ) a + b∆P + cω
(1)
where a, b, and c are constants. The effect of the gas flow rate, V0, is indeed the same as the pressure loss across the feeder
Qs ) a′ + b′V0 + c′ω
(2)
In other words, at constant stirring speed ω, the amount of solid is directly proportional to ∆P or, equivalently, to the gas flow rate, a variable that is easier to control in our apparatus. Indeed, the pressure loss of the gas flowing through the hot reactor is much higher than the pressure drop in the feeder, so that the latter becomes difficult to measure quantitatively. The stirring speed can affect the solids flow rate, but its influence is not simply additive with respect to that of the gas flow rate (or pressure loss). An equation of the form
Qs ) a′′ + b′′V0 + c′′ω + dωV0
(3)
turns out to be more appropriate, as the slope with respect to V0 depends on ω as well. The actual solids flow rate in a standard pyrolysis experiment is set according to the equation above and globally verified at the end of the run by comparing the total amount of solids fed to the duration of the experiment. Such an average solids flow rate can be assumed to be a good approximation of the actual instantaneous solids flow rate in light of the good stability of the feeder, as discussed above. All of the weight measurements of the feeder and the traps were obtained with an analytical balance accurate to within 0.01 g. The stability of the feeder cannot be assessed at high temperature, because weight loss due to devolatilization takes place. By means of FCC powders of the same size (but different density), the above behavior has been confirmed up to 550 °C. Reactor. The reactor is a 5-mm-i.d., 7-mm-o.d. quartz tube inside three independent, 0.4-m-long heating sections, each equipped with PID controllers and K-type thermocouples. The heated length of the reactor can be changed by turning on or off some of the sections, leading to different residence times. Note that 1.2 m is the maximum heated length, although the quartz tube is 1.5 m long. The remainder is used for quenching and
Figure 4. Temperature profiles of the gas along the reactor for several nominal temperatures.
connections. The length/diameter ratio ranges between 0.0042 and 0.0125. A longitudinal section of the reactor with actual dimensions is shown in Figure 3. Two coaxial alumina tubes surround the quartz reactor, and in the interstice between them is a Ni/Cr 80/20 spiral wire (0.9541 Ω/m) that delivers about 500 W at 48 V. The internal refractory tube (φi/o ) 8/12 mm) is used as a thermal capacity to make the heat flow uniform. It also prevents uneven radiant heat flow from the heating wires. The external refractory tube, with some ceramic fibers around it, is used to limit thermal leaks. Note that a thermocouple is fit to the external wall of the quartz reactor in the middle of each heating element. As a consequence, the temperature reading turns out to be higher than the actual temperature inside the reactor. A suitably increased set point must be selected for the temperature controllers to achieve the desired temperature in the reactor. The actual gas temperature inside the quartz tube must be independently measured through a shielded thermocouple inserted into the reactor during calibration runs with no solids flow. Such empirical temperature set points vary with the operating conditions, namely, the gas flow rate and temperature, of the experiments. Typical temperature profiles are shown in Figure 4 for three heating elements and six different nominal temperatures. The first heating element provides an almost linear increase in temperature up to the desired set temperature, which is kept constant in the following two sections. Note that some boundary effects are present also at the exit. It turns out that the reactor is not isothermal. Although the results are reported for the nominal temperature, i.e., the value of the constant portion of the profile or the outlet value in the case of a single heating element, it must be kept in mind that the temperature history experienced by the particles varies. Such an observation is extremely important in the modeling of the data from these experiments.24,28 However, the data in Figure 4 are reported as a function of space instead of time to allow for a simpler comparison between the profiles at different temperatures. The
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effect of cellulose pyrolysis on the gas temperature is considered to be negligible because of the very low solids/gas ratio. A quantitative discussion of this point can be found in ref 28. An order-of-magnitude estimate assuming that the reaction is endothermic by 129 kJ/ kgvolatiles5 results in a heat sink from the reaction of less than 2 W, which is quite small. Gas Sampling and Analysis. Gas analysis was obtained by gas chromatography. Care must be exercised that only uncondensable gases are sampled and that they not be allowed to react further. Moreover, insertion of a probe in the reactor tail severely disturbs the gas flow into the reactor, leading to increased char formation. The best arrangement identified is shown in Figure 1. Gases are withdrawn between the two traps by means of a vacuum pump. The sampling flow rate is adjusted through a valve according to the actual velocity of the gas in the reactor, to prevent flow modification in the reaction tube. For this purpose, it is crucial that the gas leaving the reactor can also escape independently of the vacuum pump. An additional expansion is allowed before the instrument is reached to prevent the entrainment of oil droplets. Analyses were performed on a subset of experiments at 500 and 650 °C, with one, two, and three heating elements. We measured the quantities of CO, CO2, and water in the pyrolysis gas with a GC HP6890 instrument using He as the carrier gas, a TCD detector, and two different steel columns: a Porapack Q column for CO2 and water and a molecular sieve 5A column for CO. Both columns have a 1/8-in. diameter and are 6 ft long. Samples are injected through a six-port sampling valve with a 4.9-mL loop. Three analyses were performed during each run, and the results were averaged. Note that, because of the extreme dilution of the solids in the inert carrier gas, gaseous products of pyrolysis are present at very low concentrations, making the analysis rather difficult. Experimental Procedure When the number of heating elements to be used has been selected, the temperature profile must be allowed to stabilize under flowing nitrogen, which takes about 20 min. Then, the feeder is loaded with cellulose, and cleaned and dried traps are positioned at the outlet of the quartz tube. The traps and the feeder are weighed in advance. Once all of the connections are established, the primary nitrogen line is opened, followed by the secondary line to prevent back-flow of the cellulose. The run starts when the stirrer of the feeder is turned on. A typical run lasts about 15 min as a compromise among (1) the stability of the feeder, (2) the need to reestablish a constant temperature profile in the reactor because of transient effects of the start-up operations and because of some oxygen coming from the feeder, and (3) the need to accumulate a significant amount of solid in the traps to minimize the weighing error. In the first trap, we collect the unreacted cellulose plus char and traces of heavy tars, whereas in the second, we collect the tars. The collected data are weight loss measurements and some gas analysis results. We measure the difference between the weight of the feeder, the traps, and the final filter at the beginning of one run and at its end. Accordingly, the weight loss reported in the following results is always
WL (%) ) (∆Wfeeder - ∆Wtraps - ∆Wfilter)/∆Wfeeder
Figure 5. Influence of the gas/solids ratio on the weight loss at two values of reactor length (i.e., residence time) and T ) 450 °C (top) or T ) 500 °C (bottom). Avicel PH102.
Experimental Results and Discussion Influence of the Solids/Gas Ratio. An extensive part of the investigation was devoted to isolation of the chemical process in the whole transformation. The first concern is a proper understanding of the heat- and mass-transfer influences. One can try to eliminate their effect or alternatively to reduce them to a form that is easily quantifiable. For this purpose, the ideal configuration is that of a single particle travelling through the reactor. However, fluidization of powders at high gas-to-solids ratios is known to promote the formation of aggregates that are elongated in the flow direction.25 In this case, the particles in the clusters are unevenly heated, resulting in a lower conversion. Because the size and the appearance of such aggregates are difficult to predict in view of a quantitative description of the limitation induced, they must be avoided. Accordingly, several measurements were taken at different solids feed rates, Qs, for a constant gas flow rate. Qs was varied between 0.05 g/min and 1.3 g/min always with 5 L/min of nitrogen as the carrier gas. Simultaneously, two values of temperature (450 and 500 °C) and two reactor lengths were studied, resulting in four values of residence time. The results are summarized in Figure 5, where the solids flow rate has been changed while the gas flow rate was kept constant, resulting in a variation of the gas/solids ratio in the reactor. The general behavior confirms that a larger concentration of solids in the reactor (smaller 1/Qs) limits the conversion. On the other side, the conversion approaches an asymptotic value, clearly calling for low solids feed rates to obtain meaningful data. The effect is always observed, independently of the residence time and temperature values. Obviously, higher residence times and temperatures both result in higher conversions. It can be observed from Figure 5 that the experimental error increases when less solid is fed.
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Figure 6. Reproducibility: weight loss measurements, averages and (σ intervals (shaded) of values at Qs < 0.15 g/min (1/Qs > 6.7). T ) 450 °C (top) and T ) 600 °C (bottom) for three reactor lengths at low gas/solids ratio after apparatus optimization. Avicel PH102.
Indeed, small fractions of cellulose particles tend to stick to the walls and deposit at junctions and bends. Amounts of cellulose as small as 0.05 g that are lost in the reactor during a standard experiment (15 min) can affect the weight loss results up to 8% at the lowest solids feed rate As a compromise, all of the measurements were made at a solids feed rate approximately equal to or less than 0.15 g/min after some special modifications of the experimental apparatus were made to minimize the loss of solids along the reactor and piping. Adjustments of the apparatus were mandatory to operate at such low solids flow rates. A precise setting of the rate of solids delivery by the feeder was not possible, particularly at such low values. The actual solids feed rate was calculated at the end of the experiment, as discussed above. A solids flow rate of 0.15 g/min implies a solids volumetric fraction ranging from 3.85% at the inlet (cold N2) to 1.35% at the point where 600 °C is reached, not considering the solids consumption due to the pyrolysis. Errors and Reproducibility. Each experimental configuration was tested between four and six times, allowing for an estimation of the confidence of each point. Because the overall number of data points is quite large, only some data are reported in Figure 6, representative of low- and high-temperature behavior. The same issue of reproducibility of the measurements at T ) 450 and 600 °C can be discussed. At the lower temperatures, the variability of the results is higher because of the lower conversion, which is more affected by uncontrolled losses of material in the apparatus, even in extremely small amounts. In trying to extract a representative value of weight loss for each set of conditions, no doubts arise at T ) 600 °C, whereas at low temperature, the instabilities in the results are
Figure 7. Weight loss measurements at T ) 400 and 450 °C (top) and at T ) 500, 550, and 600 °C (bottom) for three reactor lengths. Averages of the data at Qs < 0.15 g/min and (σ intervals. Avicel PH102.
relatively larger because of the lower values of WL. The representative WL values are identified by lines in Figure 6 that are based on the averages of the values at the lower solids flow rates, after stabilization is observed. Data at high temperature are suitable for identification of the experimental error: values of σ/mean range between 19, 10.7, and 3% for one, two, and three heating elements, respectively. Again, lower values of weight loss (i.e., fewer heating elements) are more affected by the experimental errors, as expected, indicating the difficulty inherent in obtaining reproducible data at high heating rates and short residence times. Careful design and assembly of the apparatus eventually led to minimization of the errors, yielding the reliable results discussed in the following sections. The most important details are related to the flow path of the solid suspension, where discontinuities (bends and junctions) must be avoided, limited, or assembled so that minimum opportunity for the solid powder to deposit is provided. Because the amount of solid processed is so small, even minute deposits along the transfer lines can seriously affect the precision of the results. Weight Loss of Avicell PH102. Most of the weight loss data were recorded with Avicell PH102. Measurements were made at five temperature levels (400, 450, 500, 550, and 600 °C) and three reactor lengths (one, two, and three heating elements). We considered both gas and light tars as volatile matter. All of the data are summarized in Figure 7 in terms of weight loss vs reactor length. The averages are reported, together with the (σ interval. Data at low temperature (
