Calcium catalytic behavior during in-situ gasification of different aged coal chars in CO2 and steam

Abstract

Exchangeable calcium (Ca) in coals has significant catalysis during gasification. In previous work, coals were usually pyrolyzed for different time periods before char gasification. The extent of char aging during pyrolysis may change the state of Ca, and affect its catalytic behavior during gasification. This speculation is investigated by characterizing different aged chars and their in-situ gasification in a thermogravimetric reactor. Calcium-exchanged coal samples were first isothermally pyrolyzed at different pyrolysis time and then in-situ gasified in CO2 or steam at different temperatures. The pyrolyzed and partially gasified chars were collected and analyzed. The results show that char aging significantly changes the state and dispersion of Ca. While the nascent chars (product of 1-min pyrolysis) show no visible Ca-containing particles on the surfaces, the aged chars (product of 20-min pyrolysis) show evident CaO particles ranging from a few to tens of nanometers. During gasification, the extent of char aging does not change the catalytic action of Ca in either CO2 (surface recession mode) or steam (channeling mode). Increasing temperature favors the gasification reactivity of both nascent and aged chars in CO2, and that of aged chars in steam. However, the gasification reactivity of the nascent chars in steam does not seem to be considerably affected by increasing temperature. The increase of catalytic activity may be mostly offset by the decrease of Ca dispersion.