1. Introduction
By covering the surface of farmland, plastic mulching has been widely applied, which could increase grain yield by improving the microenvironment to promote the utilization of water and fertilizer [
1,
2]. Mulch film produced with crop straw is a new pollution-free and completely biodegradable mulch, which is similar to traditional plastic mulch in terms of moisture retention, temperature regulation and increasing crop yield [
3]. Thus, straw mulch film may be an alternative to plastic film.
The raw material for straw mulch film was obtained by straw pulping. Referring to the traditional papermaking process, pulping methods include physical methods (mechanical grinding treatment [
4] and steam blasting [
5]), chemical methods (acid treatment [
6], alkali treatment [
7], ionic liquid treatment [
8], organic solvent treatment and oxidation treatment), biological methods [
9], coupling methods [
10,
11], etc. Based on the solid conveying theory [
12,
13,
14], Haitao Chen et al. developed a D200 straw fiber making machine, which is composed of a single-screw extrusion system, steam explosion structure and cooling system, to pulp the crop straw without pollution. The straw is crushed by the single screw and then puffed by the steam explosion structure to increase the fiber branches [
15,
16], which is an effective physical pulping preparation method.
Due to the tubular structure with soft and coarse morphology, straw has more specific physical characteristics such as easy entanglement and poor mobility [
17]. Thus, crop straw often accumulates at the feeding inlet of the single-screw fiber extractor to interrupt the pretreatment process [
18], which not only reduces the efficiency of fiber production but also wastes a large amount of raw material during the production process. A proper feeding method is required to keep the stability and efficient operation of the system. The most common feeding method in engineering is free feeding, which relies on frictional traction of the material [
19]. Liu Huanyu et al. [
20] conducted combination experiments to optimize the best operating combination parameters of a D200 straw fiber extractor, and the free feeding method of raw material was used in this experiment. During feeding process, the frictional force between the single screw and the straw was not sufficient to break the arch structure of the straw accumulating at the feeding inlet. The force-feeding method is another usually used in industries, which needs extra force to assist in feeding the raw materials into the devices. Referring to the principle of the anisotropic partial engagement double-screw extruder, a double-roll feeding mechanism was designed to serve the purpose of breaking the material arch structure and achieving positive conveying [
21]. By performing continuity analysis of the processes of mixing, melting and conveying the isotropic meshing double-screw extrusion expander, Ge xunyi et al. [
22] derived the optimal process parameters for the preparation of straw-containing aquafeed. Cao Xinlin et al. [
23] investigated the movement properties of mash at different speeds of anisotropic meshing double-screws with Pro-E and ANSYS. Yang Tao et al. [
24] designed a differential double screw kneader to study the feeding, mixing and kneading process of high viscosity materials under differential velocity field.
Based on the D200 single screw straw fiber extruder and rice straw as the research object, a force-feeding device was designed with an auxiliary roller to improve the feeding speed and reduce the loss. In addition, orthogonal rotation combination experiments involving four factors and five levels were performed to determine the best combination of process parameters to meet the high-efficiency and low-loss fiber preparation work effect.
2. Materials and Methods
2.1. D200 Single Screw Straw Fiber Extruder
As shown in
Figure 1, the D200 single screw straw fiber extruder, which was developed by the authors, was used in experiments. Four working areas were operated through with straw going from feed inlet to outlet, i.e., feeding section, compression section, shearing section and blasting section. After soaking in normal temperature water, the straw was fed into the device through the feeding inlet and then was compressed with a compression ratio of 3:1, which is convenient for crushing in the shearing section. At last, the straw was puffed in the blasting section, which would yield more straw branches. The double-roller feeding inlet is a starvation feeding device, which can adapt to different lengths and different feeding methods of straw feeding. For different feeding methods, not all affect the experimental results.
2.2. Force-Feeding Device
By supplying extra force on the straw, the force-feeding device, which is located in the feeding section of the D200 single screw straw fiber extrude, is used to cooperate with the single screw to solve the problems of entanglement and bridging for keeping stabilization and continuation of the feeding process. According to anisotropic double-screw structure, the force-feeding device is designed as shown in
Figure 2, which is composed of an auxiliary roll, gap adjustment device, transmission system and frame. With an auxiliary roll driven by a motor, the straw was brought to the gap between the auxiliary roll and the single strew with opposite rotation of the two rolls, which was promoted by the frictional forces between straw and the two rolls. With the help of the screw axial thrust, the compressive straw would move forward to the extrusion machine.
2.3. Mechanical Analysis and Kinematics Analysis of Straw
2.3.1. Kinematics Analysis
As shown in
Figure 3, assuming that the gap between the two rolls in the cross-section (perpendicular to the axial direction of the single screw) was always filled with straw, the straw filled in micro-elements with height dh, which would be compressed with density moving from p
1 to p
2, while ignoring the change in height. The geometric relations of structure parameters were also calculated.
By assuming no loss of straw during the compression process, the mass of straw in the two dh areas should be the same as the law of conservation of mass, as shown in Equation (1).
where
L is Y-directional length of the force-feeding device;
a and
b are the gap length between two rollers, respectively.
The deformation degree of the straw directly was influenced by the pressure force and frictional force from the screw spindle and the auxiliary roll, which were affected by the geometric characteristics of the auxiliary roll diameter and the gap between the roller and the spindle. In this study, the compression ratio
γ was used to characterize the degree of straw compression. The initial position of the straw micro-element in the spindle and auxiliary roll contact point was set as b
1(
x1,
y1), b
2(
x2,
y2), and its length was
Lb1b2. After Δ
t time, the straw was moved to the coordinates of a
1(
x1′,
y1′) and a
2(
x2′,
y2′), which could be calculated with Equations (2)–(7), and the compression ratio
γ was described by Equation (8).
where
is angle of spindle side rotation in Δ
t(°);
is angle of roll side rotation in Δ
t(°);
is spindle line speed;
is line speed difference;
is maximum turning angle of roller side;
is maximum turning angle of spindle side.
2.3.2. Mechanical Analysis
As shown in
Figure 4, the forces (pressure forces and frictional forces from two rollers) acting on the straw in the gap were described. To ensure success of passing through the gap, the resultant forces in the negative direction of the Z-axis should be greater than 0, which were described by the Equation (9) as follows:
where
F is active pressure on the straw;
FN is positive pressure of two round rollers;
Ff is frictional force;
α is side angle of spindle;
β is side angle of the roller.
From the above formula, it can be obtained that:
To ensure that the material can move downward in the gap, the friction force must be less than the maximum static sliding friction:
Substituting it into the Equation (10):
If the extra pressure force acting on straw was 0, the pressure forces and frictional forces from two rollers could be calculated with Equations (13)–(16):
where
is the coefficient of static friction between the material and the stainless steel;
is the coefficient of static friction between the material and the rubber.
2.4. Auxiliary Roller
The diameter of the auxiliary roll and the gap between the two rollers were important parameters affecting the feeding process. Based on the kinematic analysis mentioned above, the ratio of the density between the initial feeding state and the ultimate feeding state was chosen to calculate the compression ratio, i.e.,
The screw radius
r1 is 100 mm.
where
h is height before and after compression;
r1 is diameter of the screw spindle;
r2 is diameter of the auxiliary roll.
The compression ratio could be described as Equation (21).
The relationship of the compression ratio and the diameter of auxiliary roller can be obtained according to Equation (21), as shown in
Figure 5. The compression ratio measured in experiments of the feed inlet and outlet of the device was about 3. In this study, the compression ratio was set to 3–5, i.e.,
, and the gap between the two rolls was set as 6–12. The radius
r2 was calculated [51.5, 151.5].
2.5. Spindle Speed
The spindle speed of the D200 straw fiber making machine directly would affect the conveying performance and the shear rate of system [
24]. With an increase in the spindle screw speed, the straw near the spindle screw side in the force-feeding device was dragged into system more easily, which increased the feeding rate of the material until a certain value. However, the quality of the fiber would decrease with increasing spindle speed. Based on previous research, the spindle speed was selected as 85–105 rpm to keep the feeding rate of the force-feeding device and the fiber quality.
2.6. Line Speed Difference
In the feeding process, difference between the auxiliary rollers and the spindle causes the straw in the gap to be fed into the system by frictional forces, which increases the feeding rate and leads to straw losses. To keep a high feeding efficiency and low loss, a 3 × 3 orthogonal simulating test was designed with EDEM2018 software to optimize the best speed difference, as shown in
Figure 6. The physical parameters of straw were listed in
Table 1; the diameter of the screw spindle is 200 mm, and the gap is 11 mm.
A pellet sphere was used to establish the model of rice straw with a 70 mm length. The Hertz was applied to establish the bonding between the straw pellet spheres by considering the bending deformation of rice straw during the feeding process. The radius of pellet sphere was 2.5 mm, and the adhesion radius was 3 mm.
In this study, a straw length of 70 mm and saturated water content were selected, and spindle speed was set to 85 rpm, 100 rpm and 115 rpm, respectively. Then, the line speed differences of 2000 mm/s, 4000 mm/s and 6000 mm/s were selected for an orthogonal comparison test. The efficiency and loss rate of rice straw transported under different spindle speed and line speed differences were investigated. The simulation results are shown in
Table 2.
From the simulation result, it can be deduced that the accumulation phenomenon of straw in the gap could be eased effectively with increasing the line speed difference, but a high line speed difference will lead to more losses as short straw is pulled out. When the spindle speed was 85 rpm and when the line speed difference was 6000 mm/s, the maximum loss rate was 25.2%. For verifying the simulation result, the single factor test was performed, and the influence curve of line speed difference on feeding rate was shown in
Figure 7. From the experimental results, the performing parameters were obtained with a line speed difference of 4000 mm/s, which has the largest feeding rate and lowest loss rate. It was similar to the simulation results and provides a basis for the selection of the optimal process parameters.
The test results show that many small straws are wrapped and pulled by the winding characteristic of actual test materials, which increases the feeding rate and also causes the error of discrete element simulation. As shown in
Figure 8, the winding property of rice straw enhances the feeding effect and reduces the loss rate. At the line speed difference of 4000 mm/s, the feeding rate is the largest, and the loss rate is the lowest, which is similar to the simulation results and provides a suggestion for the optimal of process parameters.
2.7. Experimental Materials and Equipment
In this study, rice straw from Suixian No.9, Suilan County, Suihua City, was selected as the test material. Before the test, rice straw was soaked in normal temperature water for 8 h to reach saturated moisture content.
The D200 straw fiber making machine made by Northeast Agricultural University was used in the experiments. The test instrument adopts the ATV312HU75N4 inverter, Schneider Electric Co., LTD., frequency conversion range of 0~50 Hz; 6SE6440-2UD33-71B137KW inverter 0~60 Hz; ACS-30 electronic scale, Yongkang Jiangnan Weighing Instrument Factory, measuring range 30 kg, accuracy 10 g; Supo blast drying oven, Shaoxing Super Instrument Ltd., temperature control range 50 °C~300 °C; and a vernier caliper with an accuracy of 0.1 mm.
2.8. Experimental Design
A quadratic orthogonal rotating center combination method was designed with four factors and five levels. The diameter of the auxiliary roll, the line speed difference between the roller and the spindle, the spindle speed and the gap between the two rolls were selected as test factors. The diameter of the auxiliary roll X1 was 170–230 mm; the line speed difference X2 was 2000–4000 mm/s; the spindle speed X3 was 85–105 r/min; and the gap X4 was 8–14 mm. The test factor coding table was shown in
Table 3.
2.9. Detection Method
The average feeding rate of the force-feeding device was an important factor affecting the efficiency of straw fiber making. By selecting three points within a 9-min time frame, recording the feeding amount of straw in 3 min, and calculating the feeding rate recorded as
Y1
1,
Y1
2 and
Y1
3, in turn, each group of tests was repeated three times, and the final result was taken as the mean value of the three tests.
where
M was the amount of feeding;
t was the feeding time.
Fiber loss Y2 was used to evaluate the force-feeding effect of the force-feeding device of the D200 fiber mill by measuring the residual straw.
5. Conclusions
(1) The parameters of spindle speed, gap between the two rolls, diameter of the auxiliary roll and line speed difference have a highly significant effect on the feeding rate and loss of the D200 fiber making machine force-feeding device (p < 0.01). The influencing sequence of each parameter on the feeding rate of the device is spindle speed, line speed difference, gap and diameter of the auxiliary roll, while the order of influence of each parameter on the amount of loss is gap, line speed difference, spindle speed and roller diameter. This starvation feeding device provides a design reference for the further collection, processing and treatment of agricultural waste with saturated moisture content that is prone to entanglement.
(2) In this paper, optimization of the performance parameters of the D200 straw fiber making machine according to the requirements of a high feeding rate and low loss is demonstrated. With a diameter of the auxiliary roll of 230 mm, a line speed difference of 2810 mm/s, a gap between the two rolls of 14 mm and a spindle speed of 104.53 rpm, the average feed rate of the fiber making machine unit was 2.419 t/h, and the loss rate was 1.944 kg/h, which could achieve the purpose of continuous performance of the fiber making system with a high feeding rate and low loss. It can provide a process reference for the fibrous production, high value utilization and feed processing of agricultural crop straw.