Surface morphology of blended molding pellets made of desert caragana and maize stover

Abstract

The surface morphology of biomass pellets can provide data for the contact mechanics fractal model between the molding pellet and hole in biomass molding machines and for predicting wear of the molding hole. In this study, desert caragana (Caragana korshinskii) was mixed with residue from maize production, crushed, and compressed into pellets, which were used to collect data on their circumferential surface roughness profile, density, diameter, and hardness. The results showed that frictional wear occurs during contact between the forming hole and the molding particles, increasing the diameter d₀ of the forming hole and the diameter d of the molding particles. The density ρ and hardness HD of the molded pellets decreased as their diameter d increased, and the ρ and HD of caragana were higher in autumn than in summer. The values of the roughness parameters R_a, R_c and R_z of the molded particles increased with their diameter d. The maximum material rate M_r2 value of the roughness’s central profile remained at 86% with the increase of diameter d. Molded particles had surface roughness kurtosis R_ku>3 and roughness skew R_sk<0 with the increase of diameter d. The surface of molded particles was spiky and negatively skewed, and with a low number of spikes.

Full Article

Surface Morphology of Blended Molding Pellets Made of Desert Caragana and Maize Stover

Xuehong De,^a Bowen Zhang,^a Wenbin Guo,^a,* Jiawei Yang,^a Jianchao Zhang,^a and Xiaomin Zhai ^b

The surface morphology of biomass pellets can provide data for the contact mechanics fractal model between the molding pellet and hole in biomass molding machines and for predicting wear of the molding hole. In this study, desert caragana (Caragana korshinskii) was mixed with residue from maize production, crushed, and compressed into pellets, which were used to collect data on their circumferential surface roughness profile, density, diameter, and hardness. The results showed that frictional wear occurs during contact between the forming hole and the molding particles, increasing the diameter d₀ of the forming hole and the diameter d of the molding particles. The density ρ and hardness HD of the molded pellets decreased as their diameter d increased, and the ρ and HD of caragana were higher in autumn than in summer. The values of the roughness parameters R_a, R_c and R_z of the molded particles increased with their diameter d. The maximum material rate M_r2 value of the roughness’s central profile remained at 86% with the increase of diameter d. Molded particles had surface roughness kurtosis R_ku>3 and roughness skew R_sk<0 with the increase of diameter d. The surface of molded particles was spiky and negatively skewed, and with a low number of spikes.

DOI: 10.15376/biores.18.2.3019-3032

Keywords: Desert caragana; Molding pellets; Roughness parameters; Density; Morphological analysis

Contact information: a: College of Mechanical and Electrical Engineering, Inner Mongolia Agricultural University, Hohhot 010020, PR China; b: Sixth Research Institute, China Aerospace Science & Industry Corp, Hohhot 010076, PR China; *Corresponding author:wenbingwb2000@sina.com

INTRODUCTION

The particles molded by mixing desert caragana (Caragana korshinskii) and maize stover were made from caragana with a certain particle size and moisture content and different proportions of stover, which were compressed by a biomass ring molding machine under external force to make pellets with a certain diameter size and density (Jiang et al. 2020; Zhang et al. 2020a). In the process of being compressed, contact friction is generated between the forming particles and the ring die forming holes in contact with each other, and when the frictional heat reaches the lignin softening temperature of about 100 °C, the raw material is extruded into pellets (De et al. 2014). In this process, the forming particles cause some wear on the contact surface inside the forming die holes. When the amount of wear on the contact surfaces in the mold hole reaches a certain value, the ratio of the length to the diameter of the mold hole (Length to diameter ratio) h/d₀ decreases, affecting the quality of the molded particles. When h/d₀ drops to a certain limit, the forming die hole cannot compress out qualified forming particles and the forming machine is scrapped (Ning et al. 2016; Jiao 2020; Wang et al. 2021; Yu 2021). Research on the microstructure of the circumferential surface of molding particles is the basis for the establishment of a contact fractal model between the molding holes and the circumferential surface of forming particles, the analysis of the contact friction wear mechanism, and the prediction of wear (Duan and Chen 2017; Nieslony et al. 2018; Liu et al. 2020b; Qiu et al. 2020; Mao 2021; Song et al. 2021; Zhang et al. 2021).

It is possible to analyze the surface profile morphology of the particles based on the following: the shape sharpness R_ku (roughness kurtosis) of the roughness profile height amplitude curve relative to the average line; and the asymmetric skew R_sk (roughness skew) of the profile height amplitude curve relative to the average line. R_ku>3 indicates it is a spiky surface with a high kurtosis value; R_ku=3 indicates it is an excellent arbitrary surface kurtosis value; R_sk<0 (high grinding surface) indicates it is an asymmetrical negative skewed surface with fewer profile spikes and a strong surface bearing capacity; R_sk=0 (geodesic surface); and R_s>0 (rotary surface) indicates it is an asymmetrical positive skewed surface with more profile spikes and a weaker surface bearing capacity, requiring grinding. This system can be used as an important basis for modeling the mechanics of the contact between the mold hole and the molded particles (An et al. 2020; Huang et al. 2020; Xiao et al. 2020; Shi et al. 2020; Zhang et al. 2020b).

For this study, molding particles were chosen for mixing caragana and stover in different proportions, which are compressed by a ring molding machine with a mold hole diameter of d₀ of 8 mm. Data were collected for their circumferential surface roughness profile, density, diameter, and hardness. Correlations among these quantities were analyzed. By analyzing the surface morphological characteristics, the pattern between the surface roughness parameters of the biomass molding pellets and the forming die holes of the key components of the ring die was explored. Such work provided base data for modeling the fractal contact between the forming pellet and the forming die hole, as well as for accurately analyzing the mechanical contact between the surface of the biomass molding pellets and the forming die (Falconer 1989; Huang 2020; Liu et al. 2020a; Schweinhart 2020; De et al. 2021).

Theoretical Analysis

The rough surface profile of the biomass forming pellet circumference is the basis for an accurate analysis of the interaction between the surface of the biomass forming pellet and the forming die. It is also the key to analyze the frictional wear mechanisms to which the forming die is subjected and to predict the wear. Fractal theory can be used to gain insight into the fine structure hidden in the chaotic natural phenomena within things. It is well suited to describing phenomena characterized by scalar rates, and the rough surface profile of the circumference of caragana strip-formed particles has statistical self-similarity and scale-invariance. The use of fractal theory to study the real contact between metal contact surfaces has been carried out and can also be used to explore the real contact between biomass forming particles and the inner wall of the forming hole (Falconer 1989; Huang et al. 2020; Liu et al. 2020).

Frictional Surface Contact Condition

It was assumed that the forming channel and the biomass pellet are element O1 and O2 of the friction pair, respectively, and the corresponding contour lines of the contacting surface S1 and S2 before contact deformation happens are z₁(x) and z₂(x), respectively. At the smallest scale, there is a point B₁ in contact with a certain micro-convex body pair i at a certain moment, and after a certain time and relative sliding distance S, the dashed line indicates the profile when undeformed. Considering the elastic-plastic deformation, let A_i be the contact surface and let δ₁⁽ⁱ⁾ and δ₂⁽ⁱ⁾ be its deformations, respectively, projected in the horizontal x and vertical z directions δ_x1⁽ⁱ⁾, δ_z1⁽ⁱ⁾ and δ_x2⁽ⁱ⁾, δ_z2⁽ⁱ⁾, when due to deformation and relative sliding along the contact surface (Ge et al. 2005), the amount of change in the vertical distance between the two contact surfaces ∆h=Stanβ-(δ_z1⁽ⁱ⁾+δ_z2⁽ⁱ⁾)=Stanβ-δ_zi (β is the inclination angle of the micro-contact surface, tanβ=∂z1/∂x1, z₁ and x₁ are its projections in the z,x direction). For a certain relative sliding velocity V_x along the x-direction of the surface contact friction sub-element, its relative sliding velocity V_z in the z-direction depends on the surface morphological characteristics and mechanical properties, set V_x = ∂S/∂t, V_z = ∂h/∂t, the two sides of ∆h to time t derivative has V_z = ∂h/∂t = V_x – ∂z1/∂x1 – ∂δ_z⁽ⁱ⁾/∂t.

Fig. 1. Analysis of the contact state and forces of a pair of micro asperity after a relative sliding distance

Microscopic Contact Mechanics Analysis

In the contact region (Fig. 1), the microconvex body has a mechanical deformation resistance q and a molecular adhesion resistance p, and the total contact pressure p_z can be decomposed as p⁽ⁱ⁾_z = p⁽ⁱ⁾ + q⁽ⁱ⁾. If the two contact surfaces move only in the xz plane, then they can be decomposed in the x and z directions as p₁⁽ⁱ⁾ = p_x1⁽ⁱ⁾x + p_z1⁽ⁱ⁾z, p₂⁽ⁱ⁾ = –p_x2⁽ⁱ⁾x – p_z2⁽ⁱ⁾z and q₁⁽ⁱ⁾ = q_x1⁽ⁱ⁾x – q_z1⁽ⁱ⁾z, q₂⁽ⁱ⁾ = -q_x2⁽ⁱ⁾x + q_z2⁽ⁱ⁾z, where, p₁⁽ⁱ⁾ and p₂⁽ⁱ⁾, q₁⁽ⁱ⁾ and q₂⁽ⁱ⁾ are projected with equal magnitude and opposite direction in the corresponding directions. The relationship between the slope at each contact point and the contact pressure at that point is as follows: ∂z1/∂x1=-∂z2/∂x2=q_x1⁽ⁱ⁾/q_z1⁽ⁱ⁾=q_x2⁽ⁱ⁾/q_z2⁽ⁱ⁾=p_z1⁽ⁱ⁾/p_x1⁽ⁱ⁾=p_z2⁽ⁱ⁾/p_x2⁽ⁱ⁾, combined with V_z over its spatial micro-contact area A_i (i = 1, 2, …, n) to sum up separately as follows:

(1)

(2)

Integrating the sum over A_i for both sides of V_z = ∂h/∂t gives the true contact area (A_r) of the two contact surfaces as,

(3)

where A₀ is a constant, A_r is the true contact area, and E₀ is the rate of change in time of the deformation in the z-direction, respectively:

(4)

Suppose that the z direction component and the x direction component of molecular interaction force on the whole contacting surface are R_z1 and R_z2, respectively. The z direction component and the x direction component of mechanical deformation resistance on the whole contacting surface are T_z1 and T_z2, respectively. Here set the left side of Eqs. 1 and 2 as W(1) and W(2) respectively and descend into Eq. 3 to get V_z[R_x1/W(1)-A_r/E₀]+V_x[A₀/E₀–R_z1/W(1)]=0, V_z[T_z1/W(2)-A_r/E₀]+V_x[A₀/E₀–T_x1/W(2)]=0. This yields R_z1=(A₀/A_r)R_x1=cR_x1,and T_x1=(A₀/A_r)T_x1=cT_x1 (let c=A₀/A_r). Let the shear strength of the mould hole material be τ and A_rp be the plastic contact area, then we have A_rp=R_x1/τ. If the normal load between the inner surface of the mould hole and the contact surface of the moulded particles is F_N, then we know that F_N=R_z1+T_z1, so the friction force F=R_x1+T_x1=cF_N+(1-c²)τA_rp is generated between the inner surface of the mould hole and the moulded particles.

The rough surface self-affine fractal features, therefore based on the rough surface fractal characterization and Hertz theory (Liu et al. 2020), then get the equation as follows,

(5)

where D is the fractal dimension of the surface profile; Ψ is the coefficient; a_c is the critical area at the micro-contact point when plastic change occurs (m²); G is the fractal dimensional characteristic coefficient (m); σ_y is the yield strength of the material (Pa); and E is the composite modulus of elasticity of the friction substrate (Pa).

EXPERIMENTAL

Materials

Two biomass materials Caragana korshinskii twig (known as desert caragana) and maize stover (without the cobs) were selected in summer and autumn in the sandy areas of western Inner Mongolia) to prepare the caragana material molding pellets (Fig. 1). The granularity of the raw material pellets ranged from 1 to 4 mm and the average value of moisture content was 13%.

Instruments

The main test equipment included a JB-8C type precision roughness meter (YDYQ Precision Instrument Co., Ltd., Guangzhou, China); DHS-10A Rapid Moisture Tester (Lichen-BX Instrument Technology Co., Ltd., Shanghai, China); standard test sieve with 3-mm sieve pore as per GB/T 6003.1-2012 “Test Sieve Technical Requirements and Inspection” (Xinxiang Xiyang Yang Screening Machinery Manufacturing Co., Ltd., Xinxiang, China); JAEIHAENE type electronic scale (accuracy 0.01 g, Ruian Deli Business Electronics Co., Ltd., Ruian, China); Model LX-D Shore hardness tester (Leqing Eidelberg Instruments Co., Ltd., Leqing, China); 100 mL measuring cylinder (accuracy 1 mL, Tianchang Tianhu Analytical Instruments Co., Ltd., Tianchang, China); and vernier calipers (Wuxi Xigong Gauge Co., Ltd., Wuxi, China).

Parameters Determination

Six types of molding pellets were compressed using a ring molding machine with a molding die-hole diameter d₀ of 8 mm. These 6 kinds of pellets were compressed from a mixture of 3 ratios of maize stover and caragana in summer and autumn. A total of 480 of them were taken for the study, and their diameter d, density ρ, hardness, and water content were measured. A roughness meter was used to measure the roughness morphology of its circumference surface and extract data. Then the parameters were compared for analysis of factors affecting the surface roughness morphology of the molding particles.

RESULTS AND DISCUSSION

Surface Parameters of Caragana and Stover Mixed Molding Pellets

Eighty samples of each of the six types of caragana branches mixed with stover in different proportions with a diameter of Φ8 mm were randomly selected. The actual diameter d, mass m, and hardness of the samples were measured. The average density value ρ of the sample was calculated from the volume and the mass m, and the volume could be obtained from the diameter d and the length. The surface roughness of the circumference of the molding particles was measured using a roughness meter, and its data were extracted. Then, according to the actual diameter d value, the 80 samples were divided into 8 groups and the average value of each parameter was taken. The results are shown in Tables 1 and 2. R_a indicates the arithmetic mean deviation of the assessed profile, R_c indicates the profile support length rate, R_z indicates the ten-point average height of microscopic unevenness, R_sm indicates the average width of the profile, and R_pc denotes the number of wave crests, M_r2 indicates the maximum material rate of the roughness center profile, R_ku indicates roughness kurtosis, and R_sk indicates roughness skew.

Fig. 2. Molding particle test rig as well as density, hardness, and circumferential rough surface topography measurement system

Table 1. Mean Values for Each Parameter of the Circumferential Surface of Caragana and Stover Mixed Molding Pellet in Summer

Table 2. Mean Values for Each Parameter of the Circumferential Surface of Caragana and Stover Mixed Molding Pellet in Autumn

Relationship between the Diameter of the Molding Particles and their Density, Hardness, and Rough Surface Parameters

Figure 3 shows the relationship between pellet diameter d and density ρ for caragana mixed with maize stover in summer and autumn. Figure 4 shows the relationship between diameter d and hardness (HD) for caragana mixed with maize stover in summer and autumn. The density ρ and hardness HD of the six molded pellets tend to decrease as diameter d of the molded pellets increases. These changes were attributed mainly to an increase in diameter d₀ caused by the frictional wear of the material during the extrusion process of the mould holes in the molding machine. The length to diameter ratio h/d₀ of the molded holes decreased, which in turn made the density ρ and hardness HD of the molded pellets decrease. It can also be seen from the graph that the density ρ and hardness HD of the caragana mixed with stover molding pellets was somewhat greater in autumn than in summer overall, mainly because the fact that autumn lemons have thicker rhizome and higher biophysical properties (modulus of elasticity, shear modulus) than summer ones, which could also indicate that the summer caragana is suitable for feed and the autumn caragana could be considered for development as a biomass fuel.

Fig. 3. Relation between diameter d and density ρ