Catalytic effects of shale ash with different particle sizes on characteristics of gas evolution from retorting oil shale

Abstract

Huadian oil shale samples were pyrolyzed in a small retort at 520 °C in the absence and presence of shale ashes with different particle sizes of 0–0.20 mm, 0–0.60 mm and 0–1.25 mm under argon atmosphere to evaluate the catalytic effects of shale ash on the products yield and non-condensable gases evolution characteristics. The shale ash samples were recorded as 0.20 mm, 0.60 mm and 1.25 mm shale ashes. The results indicate that the shale ash samples with good adsorption capacity and certain catalytic performance significantly affect the reactivity of oil shale pyrolysis. There are a slight decrease in the shale char yield and an increase in the total yield of volatile products with the particle size of shale ash decreasing from 1.25 to 0.20 mm. The shale oil yield increases after adding 0.20 mm and 0.60 mm shale ashes, especially 0.20 mm, but decreases after adding 1.25 mm shale ash compared with that obtained without adding shale ash, while the non-condensable gases yield decreases after adding shale ashes with three particle sizes, especially 0.60 mm. In addition, the gas products contain higher-volume content of CO2, CH4 and H2, and lower-volume content of CO and C2–C4 hydrocarbons. Adding the shale ash makes the peak concentrations of all gases shift to the lower temperatures and increases the maximum peak concentrations of C4H8 and H2, especially H2, but decreases those of other gases. The non-condensable gases mainly contain CO2 and CH4, and smaller amounts of other gases in terms of their mass distribution. The addition of shale ashes reduces the yields of several gases except CO2, H2, C2H6 and C4H8 compared with those obtained without adding shale ash. Adding three shale ash samples increase the H2 content, but cause different effects on the contents of CO2, C2H6 and C4H8. Among three shale ash samples, adding 1.25 mm shale ash produces the largest amount of CO2 and CO, as well as the lowest total content of C2–C4 alkenes, while adding 0.60 mm shale ash produces the largest amount of H2 and largest total content of C1–C4 alkanes. Adding the shale ash decreases the ethene/ethane ratio and increases the other alkene/alkane ratios compared with those obtained without adding shale ash. Adding 0.2 mm shale ash among three shale ash samples produces the lowest ethene/ethane and propene/propane ratios, being possibly related to the most significant coking reactions. Adding shale ashes with smaller particle sizes are more conducive to obtain non-condensable gases of higher heating value. The addition of 0.20 mm shale ash is the optimal for improving the shale oil yield and the conversion rate of kerogen to volatile products.