In the last several decades

circulating fluidized bed reactors have been studied in many aspects including hydrodynamics

heat and mass transfer and gas-solid two phase contacting. However

despite the abundance of review papers on hydrodynamics

there is no summary paper on gas-solid contact efficiency to date

especially on high density circulating fluidized beds (CFBs). This paper gives an introduction to

and a review of the measurement of contact efficiency in circulating fluidized bed riser. Firstly

the popular testing method of contact efficiency including the method of heating transfer experiment and hot model reaction are discussed

then previous published papers are reviewed based on the discussed methods. Some key results of the experimental work are described and discussed. Gas-solid contact efficiency is affected by the operating conditions as well as the particle size distribution. The result of the contact efficiency shows that the CFB riser is far away froman ideal plug flowreactor due to the characteristics of hydrodynamics in the riser. Lacunae in the available literature have been delineated and recommendations have been made for further work.