Overall Heat Transfer Coefficients in Organic Substrates Composting

Abstract

The overall heat-transfer coefficients for four self-heating composting experiments were determined and mathematically modeled based on an energy balance approach. Heat dissipation to the surroundings through combined conductive/convective/radiative heat transfer mechanisms has been considered. Energy balance parameters were mathematically modeled with time for the data collected for the composting of mixed yard/green vegetables wastes, broiler litter and bagasse in PVC drum composters. The rates of reaction for the mixes varied between 7.5 and 8.75 K/day for the first 4 days, after which the rate of temperature decrease varied from 1.32 to 3.01 K/day. The rate of temperature change for all mixes best fitted a first order intermediate with equilibrium variation in time with R2 values varying from 0.9809 to 0.9999 for day 3 to day 33. Variations of U-values for the composting experiments in this study were two-tiered. Initial U-values (before day 3) were much larger than the values after day 3. Maximum U-values varied between 35.5 and 263.9W/m2•K. U-values dropped to 88.7-95.0% the initial values just after the active phase, thereafter remaining quasi steady in the range of 2.44-8.15 W/m2•K. Mathematical correlations for U-values from day 1 to 30 fitted best with balanced order polynomials degree four for R2 values of 0.9953 and 0.9999 at a 95% confidence interval. ANOVA tests showed that the values varied significantly (p < 0.05).

Keywords: Composting, energy balance parameters, heat transfer, mathematical correlations, overall heat-transfer coefficients