Improving design and operating parameters of the recuperator for waste heat recovery from rotary kilns

Depending on their applications, heat losses from the shells of rotary kilns account for 3 to 25% of the total heat input. Over the hottest zone of the kiln shell, an annular duct with a variable diameter is formed. Two air streams entering the annulus at both ends flow to a common extraction point to receive the thermal power equal to the ambient heat loss of the bare kiln. The design does not require airtightness, utilizes the entire heat loss, and by the variation of the airflow can be used over the kilns with variable operating parameters (±20% heat loss), which show similar surface temperature patterns. The main disadvantage of the design is the approaching of the surfaces of the kiln and the recuperator at the outlet of preheated air. This means that for a given heat loss and a surface temperature pattern, the rotational eccentricity of the kiln shell is the most influencing parameter that defines the air preheating temperature and the efficiency of the recuperator. To solve the problem, 4 redesigns with: (i) double annuluses, (ii) the usage of radiation fins, (iii) air addition, and (iv) a combination of two basic designs are analyzed by the use of analytical and CFD models. From the listed redesigns: (i) could be used only to prevent overheating, (ii) has a modest positive effect, (iii) should be applied in combination with (iv).