Energy Balance of a Biodrying Process for Organic Wastes of High Moisture Content: A Review

Abstract

Biodrying is a technology that makes use of bioenergy from organic waste with high water content to remove moisture and improve the utilization value and treatability of the waste. The essential feature of biodrying is the utilization of thermal energy, generated by aeration degradation of organic matter in waste, to evaporate water, thus achieving self-drying. The effectiveness of biodrying depends on the energy balance of the system, in which simultaneous heat and mass transfer are driven by biogenerated heat. We have focused on improving drying efficiency of these wastes and in this article we review the energy balance characteristics of the biodrying process. This encompasses the framework of biodrying energy balance analysis methodology, modeling expressions for energy release and utilization, and identification of the main model parameters and the main factors that affect the energy balance of the process. Finally, this review investigates possible ways to improve the efficiency of energy utilization. The heat source of the biodrying process can be calculated by combining a first-order biodegradation kinetics model of organic components with combustion heat (H _c ) of biodegradable volatile solids (BVS). Heat consumption is mainly described by the enthalpy difference of the ventilation air flow through the system. The biodegradation rate and ventilation mode—which includes the ventilation rate, ventilation time, and ventilation distribution—all influence the energy balance of the biodrying system. There are some possible alternatives that can be considered in order to optimize the energy balance and improve the drying efficiency, such asoptimizing organic biodegradation, improving material physical structures, utilizing sensible heat in exhaust gas in biodrying, and utilization of external low-grade heat sources, such as solar and industrial waste heat, to assist drying.

Keywords:

• Biodegradation rate
• Biodrying
• Energy balance
• Sludge
• Ventilation

ACKNOWLEDGMENTS

This work was financially supported by the National Program on Key Basic Research Project (973 Program, No. 2011CB201500), National Key Technology R&D Program of China (No. 2010BAK69B16-1-5), and Program of Shanghai Subject Chief Scientist (No. 10XD1404200).

Notes

^a R² = correlation coefficient; RMS = residual mean square.

^b Data obtained from constant temperature and pressure respirometry, with substrates present in solution. The data were tabulated as fast coefficient (k ₁)/slow coefficient (k ₂).

^c At 25°C.

^d At 50°C.

^a Mixture: The sludge was mixed with straw and sawdust with the proportion of 15:1:2.

^a Calculated based on VS loss.

^a Percentage of heat loss in total consumption.