Effects of steam disinfection pipe structure parameters on the heating efficiency of red loam steam disinfection

Soil continuous cropping has been an ever-increasingly serious threat to modern agriculture in recent years. The soil disinfection method can be expected to effectively alleviate the soil's continuous cropping. Furthermore, the soil steam disinfection can be sown in a short period of time after disinfection, due to the non-toxic, pollution-free, compared with the chemical methods. But the low disinfection and heating efficiency has been limited in the process of red loam steam disinfection in practice. In this study, the disinfection pipe structural parameters were optimized to clarify the influence on the soil disinfection heating efficiency. Firstly, image processing was used to construct a soil pore structure discrete model. Secondly, the simulation was implemented to analyze the steam flow field in the disinfection pipe and the single-factor soil steam disinfection. Finally, the experiment was performed on the optimized structural parameters of multi-factor disinfection pipes using the soil vapor disinfection test platform. The results show that: 1) The circumferential and axial outlets of the disinfection pipe posed a significant effect on the heating efficiency of steam disinfection (P<0.01). The optimal structural parameters of the disinfection pipe were achieved: 2 mm pore size of the outlet pore, 3 circumferential pores, 2 axial pores. The average disinfection duration of the test when the average soil temperature reaches 80 ℃ is 394 s, and the average error rate between the test results and the predicted results is 5.3%; 2) The soil heating rates of each treatment reached the peak when the steam disinfection was carried out for 200 s. The efficiency of disinfection and heating gradually decreased after 200 s of disinfection. Future practical operations need to be guided by the variation of soil heating rate. Specifically, the steam valve should be gradually closed, the steam flow should be suspended, and the excess water in the soil should be removed to provide the soil pore permeability when the heating rate decreased gradually. Therefore, an intelligent control system can be expected to be combined, when conducting steam disinfection operations on Yunnan red loam in the later stage. The steam flow rate should be gradually reduced when disinfecting for 200 s. The soil temperature was redistributed and gradually decreased, as time increased. Once the soil temperature was below 60 ℃, the steam flow rate should be increased again, and the flow rate should be reduced and increased in a cyclic manner to ensure that the soil temperature was always above 60 ℃, in order to achieve the goal of efficient disinfection. The findings can provide a theoretical basis for efficient disinfection operations. 3) The range of single-pipe high-temperature areas in each treatment was mainly concentrated in the horizontal direction 0-150 mm and the vertical direction 0-200 mm. This study can provide a theoretical basis for the structural design of the disinfection pipes in the steam sterilizers and the intelligent disinfection operation, and lay a theoretical research foundation for the design of multi-pipe spacing of end actuators