The calcium looping cycle for CO₂ capture from power generation, cement manufacture and hydrogen production

Abstract

Calcium looping is a CO₂ capture scheme using solid CaO-based sorbents to remove CO₂ from flue gases, e.g., from a power plant, producing a concentrated stream of CO₂ (∼95%) suitable for storage. The scheme exploits the reversible gas–solid reaction between CO₂ and CaO(s) to form CaCO₃(s). Calcium looping has a number of advantages compared to closer-to-market capture schemes, including: the use of circulating fluidised bed reactors—a mature technology at large scale; sorbent derived from cheap, abundant and environmentally benign limestone and dolomite precursors; and the relatively small efficiency penalty that it imposes on the power/industrial process (i.e., estimated at 6–8 percentage points, compared to 9.5–12.5 from amine-based post-combustion capture). A further advantage is the synergy with cement manufacture, which potentially allows for decarbonisation of both cement manufacture and power production. In addition, a number of advanced applications offer the potential for significant cost reductions in the production of hydrogen from fossil fuels coupled with CO₂ capture. The range of applications of calcium looping are discussed here, including the progress made towards demonstrating this technology as a viable post-combustion capture technology using small-pilot scale rigs, and the early progress towards a 2 MW scale demonstrator.

Introduction

Carbon Capture and Storage (CCS) is a range of technologies being developed to help mitigate climate change by isolating from the atmosphere a significant fraction of the CO₂ produced during fuel combustion (e.g., coal, gas and biomass). In the case of coal-fired power stations, the technology could prevent up to ∼90% of the CO₂ from being emitted to the atmosphere; greater net emission reduction may be possible if coal is co-fired with biomass by accounting for the CO₂ removed from the atmosphere during the biomass growth. CCS involves capture, purification and compression of the CO₂ ready for transportation to a permanent storage location. This review focuses on CO₂ capture, which is the most costly stage in the CCS process; for information on the topics of transport and storage please refer to Steeneveldt et al. (2006). The CO₂ capture technology closest to market is post-combustion ‘scrubbing’ using amine-based solvents, which has been used for industrial-scale separation of CO₂ for several decades. However, there are issues associated with amine-scrubbing for CO₂ capture from a combustion flue-gas, including the high cost of manufacturing the solvent (e.g., MEA at ∼USD 1250/tonne) (Rao and Rubin, 2002), the high efficiency penalty (∼9.5–12.5 percentage points) (Xu et al., 2010), degradation of the solvent due to reactions with O₂ and SO₂ (frequently present in industrial flue gases), and the handling and disposal of large quantities of degradation products/waste solvent (which raises both environmental and health and safety concerns).

Research into a range of alternative CO₂ capture technologies, which aim to address such issues, are being pursued all over the world and one promising technology involves the use of a solid CO₂ sorbent using calcium-based materials, known as ‘The Calcium Looping Cycle’ or ‘Ca-looping’. This process offers a number of advantages, including; the use of well established fluidised bed technology; it is a high-temperature process and so high-grade excess heat can be recovered to provide additional energy to drive a steam cycle, thus reducing the overall efficiency penalty of the process; the materials used to perform the CO₂ capture are widely available and environmentally benign (derived from limestones/dolomites); the affinity of the materials to SO₂ provides simultaneous partial desulphurisation of the industrial flue gas; and, the waste material from the process has potential uses elsewhere in industry, most notably the cement industry. These advantages are discussed in detail herein.