Modeling and Experimental Studies of the Effects of Volume Shrinkage on the Pyrolysis of Waste Wood Sphere

Abstract

When the shape of single wood spheres was captured and processed under different pyrolysis environments instantaneously, the evolution of sample geometry was obtained and a shrinkage model was proposed in this paper. The proposed shrinkage model was coupled with a one-dimensional unsteady wood pyrolysis model to predict the temperature profiles and mass variation as well as product distribution within wood spheres. A one-step drying mechanism and three parallel primary decomposition reactions as well as three secondary cracking reactions were used to describe the entire pyrolysis process. Experiments were carried out to assess the efficiency of the model prediction, and the effects of volume shrinkage on the temperature, weight loss, and product distribution were analyzed and discussed. Experimental results show that the shrinkage rate is proportional to the furnace temperature, and the average shrinkage rate increases from 0.39 to 0.53 mm/min for 20 mm wood spheres and from 0.26 to 0.55 mm/min for 30 mm wood spheres when the furnace temperature increases from 673 to 973 K. However, the final shrinkage ratio is inversely proportional to the furnace temperature and decreases with the increased sphere diameter. The peaks of simulated temperature profiles are in good agreement with experimental results when shrinkage is considered. Simulated mass loss profiles with shrinkage agree well with experimental data. On the contrary, if a constant particle size is used, the deviation between simulated and measured residual fractions is about 24% for the wood spheres studied in this paper.

Experimental and modelling study on the influence of wood type, density, water content, and temperature on wood devolatilization

“…Moreover, the thermal conductivity along and across the fiber direction is not easily measured from experiments, while there are available and validated expression of effective dry wood conductivity. Furthermore, previous studies [10][11][12][13][14][15][16][17][18] have shown that appropriate devolatilization modelling can be obtained without consider the anisotropy of the wood. Thus, the anisotropy of wood is neglected in the present model.…”

“…The convective heat transfer coefficient (hc) of a spherical particle is determined by Ranz and Marshall equation [32]. The shrinkage factor (, volume based) which is dependent of the experimental conditions and wood type as reported in previous studies [10,16,33], is assumed to be 0.2 in this work. All of the thermos-physical properties described above are summarized in Table 4.…”

“…Detailed one-dimensional (1D) models have been developed to describe the devolatilization of single biomass particles [10][11][12][13][14][15][16][17][18]. The existing models have been used to study the effects of particle shape [10], moisture content [10,14], particle size [18], heat flux [17,18] on the biomass devolatilization and validated mainly by experiments under relatively low-temperature conditions (Tg <1200℃).…”

Investigation of regularities of pelletized biomass thermal deformations during pyrolysis

“…Volumetric shrinking upon pyrolysis was studied for different types of wood and temperatures and it was found that a birch particle shrinks 80% of its original size at 700 °C [3], a cubic wood particle -from 45-70% , respectively, at 350-900 °C [4]. Radial shrinkage of a spherical wood particle was examined by Huang et al, [5] pyrolyzing a sample from 400-700 °C and the results showed decrease of a wood sphere from 84.6-74% of the original size. Investigations of wood particle shrinkage in three directions were expanded by performing experiments upon pyrolysis, combustion, and gasification processes: Japanese Cyprus wood cylinders shrink about 70% and 80% in longitudinal direction and about 60% and 20% in axial direction, respectively, at 1 C/s and 30 C/s heating rate during pyrolysis [6], while at carbonization process [2] cubic tulip poplar particles shrinks from 10-21% in axial direction, from 20-32% in radial direction, and from 33-41% in tangential direction at 400-1600 °C temperature.…”

A review on the modeling and validation of biomass pyrolysis with a focus on product yield and composition

“…As a notable study in the Lagrangian context, Di Blasi et al (1993 and proposed a three-parameter shrinkage model in which the particle shrinkage was considered as a variable for a single biomass particle at a limited temperature condition. It should be noted that biomass motion is influenced by the drag force, gas flow, biomass residence time, as well as particle size and density that are changeable during the pyrolysis (Huang et al, 2014). These changes could also affect the secondary reaction rate and the results obtained (Wang et al, 2014).…”