Crushing pretreatment parameter optimization of soybean straw used as raw material of mulch

Abstract: Recently, a large amount of soybean straw was treated as waste and burned, which caused serious pollution. At the same time, for increasing the grain yield, the plastic mulch was applied widely in China which caused serious white pollution. Soybean straw was an excellent raw material of non-wood fiber, and manufacturing biodegradable mulch of soybean straw fiber could not only reduce the pollution, but also raise the value of soybean straw. Before the manufacturing of biodegradable mulch, the raw soybean straw need crushing pretreatment. There have been many domestic and international researches about the equipment of straw crushing. However, these equipments are mainly used for the preparation of straw returning, feeding or the formation of biomass solid fuel, and the straw crushing length is usually too short. According to the requirement of the plant fiber preparing machine, straw crushing length of 70- 85 mm is suitable for manufacturing soybean straw fiber mulch. As there is no special crushing equipment used for preparation raw material of straw fiber mulch, the existing rubbing and cutting machine was improved to process soybean straw with crushing length of 70-85 mm. By removing the chopper, the shearing effect was reduced, and the crushing was mainly realized by hammering effects of chopper holder, hammering on the soybean straw. After improvement the usual operation parameters are not applicable; thus, to optimize soybean straw crushing pretreatment parameters, the machine spindle speed, feed quantity and straw moisture content were selected as experiment factors, and a quadratic orthogonal rotatable central composite design experiment with three factors and five levels was conducted. The results showed that: 1) For the straw crushing length of improved rubbing and cutting machine, from high to low ranking of contribution rate, the factors are the spindle speed, moisture content and feed rate. The spindle speed and moisture content show significant interaction effect on soybean straw crushing length. There are two reasons for the interaction effect. Firstly, the increase of soybean straw moisture content causes the increase of straw plasticity, which weakens the shearing effect caused by increasing spindle speed. Secondly, with the increase of straw moisture content, lubrication film forms between straw and chopper holder, which also weakens the shearing effect caused by increasing spindle speed. 2) The optimized pretreatment parameters are as follows: the feeding quantity is 168 kg/h, moisture content of soybean straw is in range of 36%-58%, and the machine spindle speed is in range of 1 100- 1 300 r/min. Under the optimized parameters, the processed length of soybean straw is in range of 70-85 mm, which meets length requirement of the plant fiber preparing machine for raw materials. To verify the optimization results, 10 random groups of soybean straw were crushed under the optimal combination of parameters, the verification experiment results showed that there was no significant difference between predicted values and observed values.