Flow property of coal ash and slag is an important parameter for slag tapping of entrained flow gasifier. The viscosity of slag with high contents of calcium and iron exhibits the behavior of a crystalline slag

of which viscosity sharply increases when temperature is lowered than temperature of critical viscosity (

T

CV

). The fluctuation in temperature near the

T

CV

can cause an accumulation of slag inside the gasifier. In order to prevent slag blockage

it is necessary to adjust the ash composition by additive to modify the flow property of coal rich in calcium and iron. Main components of coal gangue are Al

2

O

3

and SiO

2

which is a potential additive to modify the ash flow properties of these coals. In this work

we investigated the ash flow properties of a typical coal rich in calcium and iron by adding coal gangue with different SiO

2

/Al

2

O

3

ratio. The results showed that the ash fusion temperatures (AFTs) firstly decreased

and then increased with increasing amount of coal gangue addition. Chemical composition of coal ash rich in calcium and iron moved from gehlenite primary phase to anorthite

quartz and

corundum primary phases. The slags with coal gangue addition behaved as a glassy slag

of which the viscosity gradually increased as temperature decreased. Besides

a high SiO

2

/Al

2

O

3

ratio of coal gangue was beneficial to modify the slag viscosity behavior. Addition of coal gangue with a high SiO

2

/Al

2

O

3

ratio impeded formation of crystalline phases during cooling. This work demonstrated that coal gangue addition was an effective way to improve the ash flow properties of the coal rich in calcium and iron for the entrained flow gasifier.