Proof-of-Concept of a Novel Solid-Solid Heat Exchanger Based on a Double L-Valve Concept

Indirect contact type heat exchangers (HEs) are broadly used in fluidized bed systems, usually in the form of heat transfer pipes containing a liquid or a gas medium. An alternative method of transferring heat to a gas-solid system is by beneficiating from the flow of another granular solid stream without necessarily requiring mixing of them. This can be mainly achieved though their continuous flow in concentric heat pipes. This paper deals with the proof-of-concept of a novel parallel-flow solid-solid HE, consisting of two L-Valves with concentric vertical tubes, named as Double L-Valve. This type of HE can be used in dual fluidized bed systems, as for instance in calcium looping process (oxy-fired or indirectly heated -IHCaL) for heating up the solid stream coming from the carbonator reactor by absorbing heat from the hotter stream coming from the calciner. This type of HE has been constructed on a small scale with acrylic transparent tubes and hydrodynamic tests have been conducted under cold flow conditions, in order to study the hydrodynamic response of the system at different designs and airflow rates. Apart from the cold flow pilot model, a theoretical assessment of the HE, operating under real conditions (hot flow conditions) at an industrial scale has been carried out by means of proper heat transfer models. Several parametric studies have been carried out to optimize its design, whereas its parametric costing has been undertaken. Finally, the structural integrity of the double L-Valve under high temperature conditions has been thoroughly investigated by Finite Element Analysis simulations and analytical models. All in all, the proposed HE concept offers several advantages to be integrated in a IHCaL system including design flexibility, simple construction, rather sufficient heat transfer efficiency, durability and rather low cost.