Industrial and domestic wastewater produce large quantities of residual sludge.
Nowadays, a wide reduction of sludge volumes is necessary.
Sludge drying is a very effective volume reduction method, and also an important intermediate treatment process as compared with other sludge building materials, etc.
Thermal drying of sewage sludge is widely recognized as an efficient means to transform wet sludge into a solid form that allows easy handling, storage, and recycling [1, 2].
There are three categories of dryers for thermal drying of sludge, i.e. indirect dryers [3], direct dryers, and combined dryers [4, 5].
The indirect dryers have the advantages of producing minimal amounts of vapors, no pollution of heat carrier and lower energy consumption than direct ones.
Reports on drying kinetics of sludge in indirect dryers can be found abundantly [6-11]. Saturated steam and thermal oil are typical heat carriers for indirect dryers.
Flue gas is only occasionally used in indirect dryers[8].
Saturated steam, thermal oil and flue gas are markedly different in physical properties and heat transfer (HT) characteristics.
In this study, the HT characteristics of saturated steam, thermal oil and flue gas in a Nara-type paddle dryer were investigated based on theoretical simulation.
Drying kinetics of a municipal sewage sludge in the paddle dryer with different heat carriers were also presented.