Experimental and theoretical investigation of wood pellet shrinkage during pyrolysis (original) (raw)
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Shrinkage models have been developed and included in a model for the pyrolysis of large wood particles. Shrinkage is modelled in three different ways: uniform shrinkage, shrinking shell and shrinking cylinders. These models and a reference model without shrinkage are compared with experimental data for mass loss versus time during pyrolysis of birch cylinders at different temperatures. In the experiments a wood particle was introduced into a pyrolysis furnace held at constant temperature. The particle mass and volume were recorded using a balance and a video camera. Uniform shrinkage slows down the pyrolysis whereas shrinking shell and cylinder models enhance the pyrolysis rate. The effect was sufficiently small to be neglected given the uncertainty about some wood physical properties. q
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A transient unimodel for reacting pellets is considered with various modes of heat and mass transfer and structural changes for wood pyrolysis. Pellet breakup was found to be possible from strength calculations. This leads to an increase in the number of pellets and a decrease in the resistance to heat and mass transfer. The pressure and temperature buildup within 2.7 mm thick pellets was measured for wood pyrolysis/combustion experimentally. The bimodal wood pyrolysis was analyzed, and the rate constant and activation energy were found. Pellet breakup may also be used as a transient catalytic process where the catalysts become smaller as they break up in time.
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Pyrolysis of centimeter-scale wood particles of various sizes and shapes needs to be understood to determine their burning rate and life. Such particles may be thought of as firebrands, which are a major reason for spotting ignition in wildland and wildland-urban interface fires. The burning lifetime of firebrands controls the maximum distance they can travel to cause spotting. To understand and model this, experiments are done in a vertical tube furnace with wood particles of different sizes and shapes. For computations, two classes of shapes, prolate and oblate ellipsoids, were chosen to represent the arbitrary geometry of such particles. Prolate ellipsoids include shapes ranging from thin needles to spheres, whereas, oblate ellipsoids include shapes ranging from thin disks to spheres. The choice of these smooth shapes, while facilitating expedient computations also enables the coverage of wide ranges of particle shapes and surface area to volume ratios (SVR). Model simulations show satisfactory agreement with relevant literature and experimental data. Particle aspect ratio (ϵ, the ratio of minor and major axes), SVR, and equivalent radius (R e) are used to define the particle geometry. Mass loss and center temperature profiles are presented and discussed. It is shown that with the decreasing of aspect ratio, wood particle decomposes faster and the final char fraction becomes smaller. A power-law based correlation between conversion time (t con) and SVR is derived and verified against experiments. Further, it is shown that an increase in the SVR enhances the production of tar and decreases the yield of char while leaving the yield of gas mostly unaffected.
MODELING AND SIMULATION OF PYROLYSIS PROCESS FOR A BEECH WOOD MATERIAL
Modeling and simulation of beech wood was carried out using Aspen Plus simulation commercial package. The model was created based on pyrolysis product yield, proximate and ultimate analysis of the wood species. In the model development, RYield was used to represent pyrolysis reactor as a non-stoichiometric type that decomposes the wood into categories of conventional compounds. The model was simulated to give the components compositions in both gaseous and liquid products. The simulation was first conducted at a temperature of 450 o C, for range of feed particle sizes from 1.6-2.0 mm, using atmospheric pressure. Five different runs were carried out by varying their temperatures and particle size. The investigation revealed the effect of pyrolysis temperature and wood particle size on compositions of liquid and gaseous products. The results showed that production of methanol increases with temperature but decreases at temperatures beyond 550 o C. Carbon dioxide yield decreases with increase in temperature while that of carbon monoxide and methane get higher as temperature increases.
Numerical Analysis on Wood Pyrolysis in Pre-Vacuum Chamber
In the previous experimental work, a new technology system for wood pyrolysis was developed to aim at mitigating climate change, global warming, and energy crisis as well as enhancing low electrification in rural areas in developing countries. The new technology system equipped with a pre-vacuum chamber requires low cost and less maintenance. However, large wood pyrolysis in the pre-vacuum chamber is rather complicated. To obtain a good understanding of the previous experimental results, a numerical analysis taking account of heat-mass transfer and chemical reaction is carried out. Two-step general reaction model is proposed for the numerical analysis. The first stage is volatile and char formation from the wood pieces and the second state is decomposition of the volatile to five species including vapor of tar. In this analysis, chemical formulae of the volatile and the tar are successfully identified hypothetically. The results obtained by this numerical analysis can explain the experimental results reasonably and provide useful information about time evolution of volatile formation, temperature change in pre-vacuum chamber with time, and species mole concentration decomposed from the volatile.