Updated: July 2002

1. Outline of system

At present, sewage sludge is mainly treated by incineration. Out of the running costs required for incineration systems, it is possible to decrease fuel cost remarkably by adopting"indirect heating type drying process". Aiming at further energy saving then at energy creation, we have completed the development of Mitsubishi super-highly efficient drying systems.
As shown in Figure 1, this is an indirect heat pump system consisting of circulating steam and exhaust steam. (Heat pump: absorbing heat from low-temperature heat sources and provides heat for high-temperature heat sources) It is possible to cover the heat required for sludge drying by taking the waste heat contained in exhaust steam into circulating steam using exhaust condensers and further by supplementing the quantity of heat lost (by radiation, etc.) using compressors. Therefore, the energy necessary for drying sludge is the power to drive the compressors.

2. Characteristics

This system intends to use effectively the "waste heat from indirect heating type dryers" so far thrown away, enables further energy saving. This system features the following merits:
(1) High energy efficiency:
High energy efficiency is attainable by adopting a mechanical compression type indirect heat pump system and recovering heat from low-temperature heat sources.
(2) Effective use of energy (effective use of waste heat produced by incineration):
When this system is adopted for the steam used so far as the heat source for indirect heating type dryers, effective use of energy is possible for power generation, usage of remaining heat and other applications.
(3) Stabilized operation:
Stabilized heat recovery is attained from low-temperature (100ºC) and dirty dry exhaust steam by using indirect type heat pump systems (separating the circulating steam line and the exhaust steam line).
(4) Compact system:
It is compact the system by concentrating necessary devices around the dryer.

Figure 1 Flow diagram of a super-highly�@efficient drying system

3. Performance

The energy required for this system is limited to that necessary for driving the compressor, only (the remainder is recovered from the heat contained in dry exhaust steam). Thus, it is possible to reduce the energy required for this system down to about 20% compared to that necessary for drying wastes with high water content, for example, sewage sludge.

Figure 3 Energy required for drying

4.Applicability and objectives

(1) Applicability: waste dryers (sludge dryers)
(2) Objectives: wastes with high water content, for example, sewage sludge

Figure 2 Outside appearance of a demonstration system (capacity: 3.6 tons per day)