In situ gasification Chemical-Looping Combustion of coal using limestone as oxygen carrier precursor and sulphur sorbent

Abstract

In-situ Gasification Chemical-Looping Combustion (iG-CLC) burning coal is achieving a great interest due to the possibility of using low cost oxygen carriers such as CaSO4. The Limestone Chemical Looping Combustion process (LCL-CTM) registered by Alstom Power Inc. proposes the use of a continuous CaCO3 feeding together with coal to produce CaSO4 as an oxygen carrier via sulphur retention. The operation is similar to what happens in a circulating fluidized bed boiler burning coal. In the present research work, the study of thermodynamic equilibrium limitations together with mass and enthalpy balances have been carried out for a CLC system in order to investigate whether the LCL-CTM process is a promising and energy efficient option to carry out the coal combustion with CO2 capture and in-situ desulphurisation. So, no limitations were found to transfer the required oxygen from air to fuel using sulphated limestone as oxygen carrier for whatever coal used. However, coals with sulphur content above 1 wt.% are advisable to perform this process. In addition, no drawback referred to thermal integration in the system has been detected. Thus, operation at 950 ºC in the fuel reactor to avoid SO2 release via side reactions, and 1000 ºC in the air reactor is feasible. Likewise, experimental tests have been performed in a thermogravimetric analyser to analyse the capability of a limestone to be sulphated and to transfer oxygen. A value of the oxygen transport capacity of about 16.7 wt.% was obtained. This value is four times higher than that of others typical inexpensive oxygen carriers published in literature.