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Microwave Pyrolysis of Wood Pellets J. P. Robinson,* S. W. Kingman, R. Barranco, C. E. Snape, and H. Al-Sayegh Department of Chemical & EnVironmental Engineering, UniVersity of Nottingham, Nottingham NG7 2RD, U.K.
The pyrolysis of wood pellets was investigated using a single-mode microwave cavity, and the dielectric properties of the wood were measured at temperatures up to 750 °C. Below 600 °C, the only microwaveabsorbing phase within wood is water. This study has shown categorically that microwave pyrolysis can be achieved without the use of carbon-rich dopants and that the heating of water alone can be used to induce pyrolysis of wood. A number of potential mechanisms are discussed that relate to the power density within the heated material. The yield of bio-oil and biogas is a function of the heating rate and power density, and for the samples used in this study, a threshold power density of 5.0 × 108 W/m3 was found, below which microwave pyrolysis did not occur. The results and hypotheses presented in this article represent the first steps in understanding the fundamental mechanisms of microwave pyrolysis. Introduction The extraction of oils and gases from renewable feedstocks such as wood chippings and other biomass sources is of significant interest in the move toward carbon-free energy production. Bio-oils are typically produced by pyrolysis processes, where the biomass is heated in the absence of oxygen, which leads to thermal decomposition of the solid and the evolution of volatile and semivolatile compounds. Pyrolysis occurs across a range of temperatures from 200 to 600 °C, depending on the exact composition of the biomass. In general, pyrolysis can be split into two distinct classifications: (i) fast pyrolysis, where high heating rates and rapid quenching are used to produce high yields of bio-oil, and (ii) slow pyrolysis, where longer residence times lead to further thermal decomposition of the initial liquid pyrolysis products to give a high biogas yield. A comprehensive review of biomass pyrolysis processes is available in Mohan et al.1 and will not be duplicated here. A number of studies have investigated the feasibility of using microwave heating instead of conventional heating for pyrolysis processes, with specific examples in the areas of municipal waste,2 sewage sludge,3 plastic and rubber wastes,4,5 and wood.6 There are many perceived advantages of microwave heating for pyrolysis processes, including energy efficiency, rapid and controlled heating, and the ability to operate from an electrical source. Underpinning these proposed advantages are two main fundamental differences between microwave heating and conventional heating: (1) volumetric heating, in which each individual molecule is heated directly and instantaneously through interactions with the electric field, and (2) selective heating, in which different substances in a heterogeneous material can be heated to different extents depending on their dielectric properties. In microwave heating, the power absorbed per unit volume, or power density (Pd), is given for a uniform electric field by the equation7 Pd ) 2πfε0ε′′|E| 2
(1)
where f is the microwave frequency, ε0 is the permittivity of free space (8.85 × 10-12 F/m), ε′′ is the dielectric loss factor, and E is the magnitude of the electric field. The loss factor represents the ability of a material to convert electromagnetic * To whom correspondence should be addressed. E-mail:
[email protected].
energy into heat. Materials with a relatively high value of ε′′ are microwave-absorbent, whereas materials whose ε′′ is close to zero are microwave-transparent. Materials that contain a mixture of absorbent and transparent phases will therefore undergo selective heating in a microwave environment. Previous Microwave Pyrolysis Work. Biomass contains water, so it will absorb microwaves at room temperature. Once the water is lost, however, the biomass becomes more transparent to microwaves and, therefore, more difficult to heat. Many previous studies have identified that the temperatures required for pyrolysis of their materials could not be achieved using their microwave equipment.3-5 To overcome this problem, activated carbon or char was added as a microwave-absorbing dopant. Once pyrolysis starts to occur however, the removal of volatiles produces char, which will act as a microwave absorber, so that the pyrolysis process can then be sustained. Comparatively, very little research has been carried out on the microwave pyrolysis of biomass/wood. Most research in the field uses off-the-shelf multimode cavities and doping agents to improve the microwave absorption.8,9 However, Miura et al.6 used relatively large samples of wood in a multimode cavity without a doping agent to facilitate the heating. They investigated the effect of sample size and obtained a relatively high bio-oil yield of 32.5% with char yields consistently of 20%. Interestingly, they concluded from their experiments that larger wood blocks can be pyrolyzed with less electric power consumption per unit weight than smaller wood blocks. Previous studies have demonstrated that microwaves can be used as a heating source for pyrolysis processes, but there is little understanding of the fundamental mechanisms and little indication of why microwave heating offers advantages over conventional heating. This aim of this study was to quantify the microwave-absorbing properties of biomass during the pyrolysis process and to investigate microwave pyrolysis of biomass using water alone as the microwave receptor. Experimental Details The moisture content of the sample was determined using the primary oven-dry method, as defined by standard test method ASTM D4442, which involves weighing the sample before and after drying at 105 °C. Proximate analysis of wood gives the weight fraction of moisture, volatiles, fixed carbon, and ash. A PerkinElmer Pyris
10.1021/ie901336k 2010 American Chemical Society Published on Web 12/03/2009
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Ind. Eng. Chem. Res., Vol. 49, No. 2, 2010 Table 2. Physical Properties of Wood Pellets Used in This Studya
Table 1. Ranges of Variables Studied variable
range studied
applied microwave power treatment time water content total energy input initial power density
0.5-3.0 kW 50-300 s 6.3-21.9% 7000-26500 kJ/kg (7.7 × 108)-(4.8 × 109) W/m3
1 thermogravimetric analyzer (TGA) was used for this purpose by closely simulating the general ASTM standard test method E870-82. In these experiments, 10-20 mg of finely ground wood was loaded into the crucible of the TGA analyzer and heated to 105 °C, at 50 °C min-1, in an inert atmosphere, using nitrogen at a flow rate of 30 mL min-1. The sample was held at 105 °C for 5 min to allow the water to evaporate. The volatile matter was determined by heating the sample to 950 °C for 20 min at the same heating rate. The temperature was then decreased to 600 °C and held for 10 min for the fixed carbon to be determined, under an air atmosphere. The residue left in the crucible corresponded to the ash content of the sample. Four replicates of the sample were analyzed in order to provide an estimate of the repeatability of the analysis. The calorific value was determined with an IKA calorimeter system, in which about 1 g of wood sample, in duplicate, was completely combusted under a pressurized oxygen atmosphere of 30 bar. The rise in temperature of the bomb allowed for the calculation of the calorific value. Elemental analysis (CHN) of the wood sample was determined using a Thermo Flash EA 1112 analyzer couple with an MAS 200R autosampler and Eager software. The dielectric constant (ε′) and dielectric loss factor (ε′′) were measured at 2.45 GHz using a cavity perturbation apparatus capable of measurements at temperatures up to 800 °C. A vector network analyzer was used to measure the frequency shift when a known volume of sample was inserted into a resonant cavity at a controlled temperature. The Q factor of the cavity was calculated and used, in turn, to calculate ε′ and ε′′. A detailed description of this equipment is given elsewhere.10 Single-Mode Microwave Tests. Microwave pyrolysis of the wood samples was carried out using a 3 kW single-mode system over a range of powers, treatment times, and moisture contents (see Table 1). The system operated at a frequency of 2.45 GHz and included a 3 kW microwave generator, a cylindrical singlemode TE10n cavity, a WR430 waveguide, and an automatic EH tuner to improve the impedance matching. Quartz reactors of 35-mm diameter were used to accommodate the wood sample within the cavity, and quartz wool was used to support the sample. A schematic diagram of the installation and reactor used is shown in Figure 1. The wood sample was weighed and placed in the reactor within the single-mode cavity. Nitrogen, at a flow
Figure 1. Single-mode microwave pyrolysis system.
moisture content volatile matter fixed carbon ash C H N calorific value (MJ/kg)
6.3 ( 0.2 85.4 ( 0.3 14.2 ( 0.2 0.4 ( 0.3 45.30 6.1 0.21 17.9 ( 0.6
a All quantities are expressed as percentages of the original mass (dry basis), except for calorific value.
rate of 3 L min-1, was used as a sweep gas to aid the removal of volatiles and to maintain an oxygen-free atmosphere within the system. The volatiles were passed through a series columns packed with glass raschig rings and filled with solvents in order of increasing polarity, namely, hexane, dichloromethane (DCM), and a 1:1 mix of DCM and methanol. The liquid/solvent mix obtained from the columns was subjected to rotavaporation at 40 °C to determine the total liquid yield. The solid residue left was weighed, and the gas yield was estimated by difference following a mass balance. The incondensable gas leaving the columns was passed through a scrubbing system before being vented to the atmosphere. Any reflected microwave power was absorbed by a water load in the circulator, which protected the magnetron from large amounts of reflected power. Pellets made from European Pine were obtained in two size classes, 8-mm pellets containing 6.3% water. The liquid and gas yields were measured and are plotted in Figures 4 and 5, respectively, against the specific energy input, which was calculated from the applied power, treatment time, and total sample mass. Figures 4 and 5 demonstrate that pyrolysis can occur without the use of microwave-absorbing additives, as oil yields of 10-35% and gas yields of 20-30% were obtained using only the inherent moisture as the microwave-receptive phase. From Figure 4, it is apparent that increasing the amount of energy
Figure 4. Effect of energy input on liquid yield at different applied power levels. The repeatability of the liquid yields is (1.5%.
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Figure 5. Effect of energy input on gas yield at different applied power levels. The repeatability of the gas yields is (4%.
supplied to the sample leads to an increase in liquid yield. A similar trend could be interpreted from Figure 5, although the gas yield appears to be influenced to a much lesser extent than the liquid yield and might even be independent of it within the scatter of the data. Increasing the amount of energy supplied to the wood sample leads to higher temperatures, so it would be expected that more thermal decomposition would occur and, therefore, higher product yields would be obtained. In a number of cases, the data in Figures 3 and 4 allow a comparison of different power levels at the same energy input. For example, in Figure 3, at ∼15000 and ∼26500 kJ/kg, it is possible to identify a trend of higher liquid yields at higher power. It appears that applying a high power for a short time leads to a higher yield than a lower power for a longer time, at equivalent energy inputs. In practical terms, this results in a difference in the heating rate, with high heating rates giving improved yields over low heating rates. It is thought that this is a result of using water alone as the microwave receptor and can potentially be explained by the rate of water loss from the wood: (1) One contribution is from vaporization of free water. Above a certain critical moisture content, the water in the wood will be present as free water, and it will evaporate from the wood at the same rate as if it were a droplet of water. For free water, the presence of the wood has no impact on the rate of vaporization.12 (2) A second contribution is from vaporization of capillary water. Below the critical moisture content, the structure of the wood will affect the transfer of the water because, for vaporization to occur, the water must diffuse to the surface of the particle. The diffusion will occur at a finite rate, but the mass transfer will be enhanced by the buildup of vapor pressure from heating the capillary water. If the heating occurs slowly, then the energy absorbed by the water will be dissipated into the surrounding wood by conventional heat transfer. At the same time, the water will move to the surface of the particle, where it vaporizes. If the heating rate were much higher, then the capillary water could be superheated above its normal boiling point before it diffused out of the capillary structure. It is also possible that the conversion of some capillary water to steam will result in very high localized pressures within the wood structure, meaning that significant superheating of the remaining water is possible. However, the maximum temperature achievable by this mechanism is determined by the critical point of water, 374 °C and 221 bar.13 (3) A third contribution is from bound water. Water that is bound within the cellulose structure of the wood does not exert its full vapor pressure.11 Significantly higher temperatures are
required for bound water to vaporize (demonstrated in Figure 2) because of its lower vapor pressure. When steam is created, it is possible that high pressures occur in localized regions within the wood, leading to much higher temperatures required for bound water to vaporize. Under microwave heating, the capillary water could be restricted to 374 °C, but the bound water will not because of its lower vapor pressure. It is thought, therefore, that the onset of pyrolysis at around 500 °C could be caused by superheating of the bound water within the wood. Once pyrolysis begins, the produced char acts as a microwave absorber, allowing further pyrolysis of the wood. In Figure 3, it is shown that adding more water results in lower liquid and gas yields. This is because the added water occurs as either free water or capillary water, not as bound water. The added water will not be able to attain temperatures in excess of 374 °C therefore it cannot contribute to the pyrolysis process, but it does absorb the applied microwave power and results in a lower-efficiency process. For pyrolysis of the wood to occur using water alone, it is imperative that steam be produced quickly, leading to very high localized temperatures. The heating rate is proportional to the power density,14 according to Pd ∆T ) ∆θ FCp
where Pd ) 2πfε0ε′′|E| 2
At low power density, the heating rate is low, meaning that any produced steam or evaporated water can escape from the wood structure without a significant pressure buildup. This potentially explains why pyrolysis of wood is not reported in studies using domestic microwave ovens, as the power density is not high enough in this equipment. The single-mode microwave apparatus used in this study gives much higher power densities, leading to higher heating rates, causing a pressure buildup, and therefore allowing temperatures in excess of 500 °C to be achieved from superheating the bound water. This hypothesis can potentially explain the observations in Figure 4, where higher powers lead to higher yields at the same energy input. In this case, higher applied power leads to a higher power density and therefore a higher heating rate. The power density will depend on the electric field strength within the wood, which, in turn, depends on the geometry of the microwave cavity and the applied microwave power. There will be an electric field distribution within the wood and therefore, a range of power densities will exist. The power density distribution will also change over time as (a) water is lost and (b) graphitization occurs. An average power density can be estimated based on the initial water content and the amount of power absorbed. The effects of power density on liquid and gas yields are shown in Figures 6 and 7 for a range of different energy inputs. Effect of Power Density on Liquid and Gas Yields. Figures 6 and 7 show that increasing the power density leads to a corresponding increase in the respective liquid and gas yields during the pyrolysis process. The data are grouped according to the total energy input, and it is apparent that these data do not necessarily follow the same trend as for power density. For example, in Figure 7, the experiments performed at 20.0 MJ/ kg give a lower gas yield than those performed at 13.0 and 16.5 MJ/kg. At a constant power density of 2.3 × 109 W/m3, there is a trend of increased liquid and gas yields at higher energy inputs, which correspond to longer treatment times. Over the range of experimental parameters studied it is apparent from Figures 6 and 7 that power density has a much greater effect on the pyrolysis process than the total energy input. From the
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The results and hypotheses presented in this article represent the first steps in understanding the fundamental mechanisms of microwave pyrolysis, and future work will focus on a systematic evaluation of why pyrolysis can be achieved using water as the only microwave-absorbing phase. Conclusions
Figure 6. Effect of power density on liquid yield for four energy inputs.
At temperatures up to 600 °C, the only microwave-absorbing phase within wood is water. This study has shown categorically that microwave pyrolysis can be achieved without the use of carbon-rich dopants, and that the heating of water alone can be used to induce pyrolysis of wood. The yield of bio-oil and biogas is a function of the heating rate and power density, and a threshold power density of 5.0 × 108 W/m3 exists, below which microwave pyrolysis will not occur. Most domestic microwave ovens cannot achieve this threshold, and hence, microwave cavities which operate at higher powers or more concentrated electric field strengths are required to induce pyrolysis of wood. Literature Cited
Figure 7. Effect of power density on gas yield for four energy inputs.
hypothesis presented earlier, high power density leads to a high heating rate, a large pressure buildup and significant superheating of the bound water, leading to enhanced pyrolysis and greater liquid and gas yields. Low power densities give lower heating rates, less pressure buildup, lower temperatures, and thus lower liquid and gas yields. From the data shown in Figures 6 and 7, it is possible to extrapolate the minimum power density required for pyrolysis to occur, that is, the power density at which the liquid and gas yields are both zero. Using this method, the estimated minimum power density for pyrolysis is on the order of 5.0 × 108 W/m3. For comparison, the average power density achieved in a 800 W domestic microwave oven used to heat the same wood pellets would be 4.0 × 107 W/m3, over an order of magnitude lower than the minimum power density identified in Figures 6 and 7. This explains why, in the majority of previous work, microwavesusceptible dopants are needed when using domestic microwave ovens. Unfortunately, the operating range of the equipment used in this work prevented further investigation of power densities close to the identified threshold. However, this issue will be studied in more detail in a future work program.
(1) Mohan, D.; Pittman, C. U.; Steele, P. H. Pyrolysis of wood/biomass for bio-oil: A critical review. Energy Fuels 2006, 20, 848–889. (2) Robinson, J. P.; Kingman, S. W.; Snape, C. E.; Shang, H. Pyrolysis of biodegradable wastes using microwaves. Waste Resource Manage. 2007, 160 (3), 97–103. (3) Menendez, J. A.; Inguanzo, M.; Pis, J. J. Microwave-induced Pyrolysis of Sewage Sludge. Waste Res. 2000, 36, 3261. (4) Ludlow-Palafox, C.; Chase, H. A. Microwave-Induced Pyrolysis of Plastic Wastes. Ind. Eng. Chem. Res. 2001, 40, 4749. (5) Adhikari, B.; De, D.; Maiti, S. Reclamation and Recycling of Waste Rubber. Prog. Polym. Sci. 2000, 25, 909. (6) Miura, M.; Kaga, H.; Sakurai, A.; Kakuchi, T.; Takahashi, K. Rapid Pyrolysis of Wood Block by Microwave Heating. J. Anal. Appl. Pyrol. 2004, 3, 187. (7) Clark, D. E.; Folz, D. C.; West, J. K. Processing materials with microwave energy. Mater. Sci. Eng. A 2000, 287, 153. (8) Krieger-Brockett, B. Microwave pyrolysis of biomass. Res. Chem. Intermed. 1994, 20, 39. (9) Xiaoya, G.; Yong, Z.; Zhou, B. Influence of Absorption Medium on Microwave Pyrolysis of Fir Sawdust In 2nd International Conference on Bioinformatics and Biomedical Engineering; IEEE Press: Shanghai, 2008; pp 798-800 (doi: 10.1109/ICBBE.2008.195). (10) Lester, E.; Kingman, S. W.; Dodds, C.; Patrick, J. The potential for rapid coke making using microwave energy. Fuel 2006, 85, 2057. (11) Meredith, R. J. Engineers Handbook of Industrial MicrowaVe Heating; IEEE: London, 1998. (12) Kowalski, S. J. Thermomechanics of Drying Processes; Springer: Berlin, 2003. (13) Perry, R. H.; Green, D. W. Perry’s Chemical Engineers Handbook; McGraw-Hill: London, 1998. (14) Shang, H.; Snape, C. E.; Kingman, S. W.; Robinson, J. P. Microwave treatment of oil-contaminated North Sea drill cuttings in a high power multimode cavity. Sep. Purif. Technol. 2005, 44 (17), 6837–6844.
ReceiVed for reView August 26, 2009 ReVised manuscript receiVed November 11, 2009 Accepted November 16, 2009 IE901336K
