Self-heating characteristics of an 80-m³ wheat straw pile considering the internal convection effect

The self-heating of biomass piles poses a significant risk of spontaneous ignition during storage. Studies on medium- to large-scale stockpiles remain relatively scarce and lack a systematic analysis of the evolutionary characteristics of various heat sources. This research conducted a 74-day experiment with the objective of accurately measuring the distributions of temperature, humidity, and oxygen concentration inside the biomass pile. The study provided comprehensive and reliable data for the purpose of verifying the accuracy of subsequent numerical simulations. In addition, a Matlab-based numerical calculation platform was developed, which coupled the biomass self-heating sub-models previously developed by the author’s team and incorporated the internal convection effect by directly solving the gas velocity distribution within the piles, which is described by the modified Ergun model. This quantitative approach revealed the evolution rules of various heat sources and the influence mechanism of internal convection on the self-heating characteristics of the biomass pile. The experimental results indicate that the maximum temperature within the pile was 79.4 °C and that the minimum oxygen concentration was less than 11% during the first three days. After two months, the internal temperature remained at approximately 60 °C, with an internal oxygen concentration of approximately 16%. High temperatures were predominantly concentrated in the upper-central region of the pile due to internal convection. The simulation results show deviations of less than 5 °C for the temperature and less than 2% for the oxygen concentration. The heat generation from anaerobic metabolism in the central region of the pile was approximately 25 W/m³, whereas that from aerobic metabolism in the periphery reached approximately 90 W/m³. The maximum temperature did not increase further, as heat released from chemical oxidation processes was suppressed by relatively low internal temperatures and moisture evaporation absorbed substantial heat. Neglecting the internal convection effect resulted in a calculated maximum temperature 5 °C higher than the experimental data and an unrealistic estimation of the oxygen concentration at 0%, which deviated significantly from the actual measurements. Consequently, incorporating convection is imperative for enhancing the prediction accuracy of self-heating in biomass.