This article investigated the effects of coal bed thickness

which could change heating rate and product distribution

on pyrolysis of coal in a rectangular radial flow fixed-bed reactor. Increasing coal bed thickness prolonged the time required to heat up coal at bed center to the preset temperature

increased the yield of pyrolysis products water and gas

decreased the yield of char and tar oil

but the content of light component in tar oil tended to increase

and the calorific values of char and gas slight decreased. For example

at furnace wall temperature of 900℃

when coal bed thickness increased from 45 mm to 105 mm

the tar oil yield lowered by 12.7% from 7.17%(mass

the same below) to 6.26% (dry coal basis)

char decreased from 80.0% to 77.0% and heat value from 24348.5 kJ·m-3 to 20649.2 kJ·m-3

and the light fraction in tar oil in tar raised by 8.5% from 67.0% to 72.7%

and water and gas yields increased from 6.96% and 5.91% to 8.85% and 7.90%

respectively. It was found that the heat value of char was varied with the bed position and more coal bed thickness

more difference: on the radial direction the heat value was higher for the char at region with higher temperature than for that with lower temperature

while on the axial direction the heat value for all char at different region was almost the same. The calorific value of the char in the bed increased gradually from the high-temperature zone to low-temperature zone

indicating the fact that the deeper pyrolysis of coal occurred in the high-temperature zone. It is well known that there is the flow of gas and liquid products by pyrolysis from high temperature zone to low temperature zone and heavier fraction in tar oil can be trapped by low-temperature coal layer if coal bed was thicker. The trapped heavy fraction can then be cracked to form light fraction once the temperature of this coal layer raised enough high as the pyrolysis process proceeded. This indicated that the total tar oil yield decreased due to formation of much light fraction for the case of thicker coal bed. As a consequence

controlling the flow of pyrolysis product from high-temperature zone to low-temperature zone is critical for manipulation of the secondary reactions of primary pyropysis products and thereby for increase of the final yield and quality of the expected pyrolysis products. The study clarified again that in the pyrolysis reactor the optimal matching between the flow of gaseous pyrolsyis product and the gradients of temperature and tar composition is critical for realization of the selective cracking of heavy species and for improvement of the yield and quality of pyrolysis tar product.