Coal gasification characteristics in a thermobalance and an internally circulating fluidized bed

Abstract

The catalytic activities of five different binary mixtures of alkali($K_2CO_3$, $K_2SO_4$, $Na_2CO_3$) and iron salts [$FeSO_4$, $Fe(NO_3)_3$] on steam-char gasification at 700~800 ℃ have been measured in a thermobalance reactor. Under the same experimental conditions with 3 wt% catalyst loading, the catalytic activities of pure salts are found to be ranked as : $K_2CO_3 > Na_2CO_3$, $FeSO_4 > K_2SO_4 > Fe(NO_3)_3$. For potassium sulphate and sodium carbonate, the gasification rate could be enhanced by addition of iron salt and the activity rank of the mixtures with 6 wt% loading is found to be $K_2SO_4+FeSO_4>NaCO_3+FeSO_4>K_2SO_4+Fe(NO_3)_3$ $K_2CO_3+FeSO_4>K_2CO_3+Fe(NO_3)_3$. Especially large cooperative effect of alkali and iron salts on the gasification rate was observed and the two mixtures of [$K_2SO_4+FeSO_4;K_2SO_4+Fe(NO_3)_3$] exhibit better catalytic activity than $K_2CO_3$ at 800 ℃. A kinetic parameter is proposed as a simple measure of activity of various catalysts based on the grain model. Based on the kinetic parameters and melting behavior, $K_2SO_4$ +$FeSO_4$ in the temperature range 700~800 ℃ is found to be an effective catalyst for the steam-char gasification reaction. The data obtained from the XRD patterns of char samples indicate that iron salts accelerate the conversion of alkali salt to the more active species. With 6 wt% loading of the mixed catalyst, the optimum weight ratio of potassium to iron salt is found to be 4 for its best activity in steam-char gasification. A ternary catalyst system Na+Fe+Ca has been tested with three different Ca-components in the steam gasification of Jinchun char. The mixture of Na+Fe shows better catalytic effect than the mixture of K+Fe, and the addition of Ca-based sorbents results in further increase in the reaction rate. The effectiveness of the Ca-based sorbents in the Na+Fe+Ca ternary catalyst strongly depends on the calcination rate of the Ca-based sorbents. Hydrodynamic properties in a circulat...