Abstract: The accumulation of ash and slag in a radiant syngas cooler (RSC) of entrained flow gasifier can reduce the heat transfer efficiency and the production capacity.The mechanical properties of slag,which determine the adhesion strength and the removal difficulty,are the key for the understanding of the slag deposit phenomenon and the effective ash removal.However,the researches on the mechanical properties of slag in the RSC are currently limited.The slag from different positions in the RSC (including the slag deposited on the wall tubes and the slag on the RSC bottom) and the sintered slag were investigated experimentally,including the chemical composition,crystal mineral composition,ash fusion temperature,microstructure,and mechanical properties.The results showed that the samples of slag deposited on the wall tubes had higher fusion temperature,forming crystal minerals with more Ca and Al than other samples.The slag deposited on the RSC bottom and the sintered slag contained more calcite,and the sintered slag was rich in K and Na and had more amorphous composition.The microstructure of the slag on the wall tubes was relatively regular,mainly composed of ash particles and small cavities,while there were many bubbles of different sizes in the sintered slag.Higher sintering temperature bonded the slag particle closer and produced larger bubbles,but smoothed the micro surfaces.The mechanical properties of slag were related to its positions in the RSC.The elastic modulus (E) and the compressive strength (σ) of the slag on the wall tubes were between 550 to 750 MPa and 20-22 MPa respectively,which were much larger than the elastic modulus (7-54 MPa) and the compressive strength (1.2-3 MPa) of the slag on the RSC bottom and the sintered slag.The values of E and σ were close among the slags on the RSC bottom and the sintered slag,but there were still large coefficients of variation (0.35-0.50).This indicates that the mechanical parameters of the slag from the RSC vary widely and are strongly related to the local slag forming temperature,the ash particle composition,the fusing characteristics and the deposit porosity.