Lime enhanced gasification of solid fuels: Examination of a process for simultaneous hydrogen production and CO 2 capture

Abstract

The lime enhanced gasification (LEGS) process uses CaO as a CO 2 carrier and consists of two coupled reactors: a gasifier in which CO 2 absorption by CaO produces a hydrogen-rich product gas, and a regenerator in which the sorbent is calcined producing a high purity CO 2 gas stream suitable for storage. The LEGS process operates at a pressure of 2.0 MPa and temperatures less than 800 °C and therefore requires a reactive fuel such as brown coal. The brown coal ash and sulfur are purged from the regenerator together with CaO which is replaced by fresh limestone in order to maintain a steady-state CaO carbonation activity ( a ave). Equilibrium calculations show the influence of process conditions and coal sulfur content on the gasifier carbon capture (>95% is possible). Material balance calculations of the core process show that the required solid purge of the sorbent cycle is mainly attributed to the necessary removal of ash and CaSO 4 if the solid purge is used as a pre-calcined feedstock for cement production. The decay in the CaO capture capacity over many calcination–carbonation cycles demands a high sorbent circulation ratio but does not dictate the purge fraction. A thermodynamic analysis of a LEGS-based combined power and cement production process, where the LEGS purge is directly used in the cement industry, results in an electric efficiency of 42% using a state of the art combined cycle.