Carbon Mineralization: From Natural Analogues to Engineered Systems

Carbon sequestration research and technology is motivated by concerns that increasing atmospheric CO concentrations are causing changes to Earth's climate and ecosystems that have the potential to cause serious, negative impacts on human welfare (IPCC 2005, 2007). As a global society, we will need to greatly improve energy efficiency and conservation, and develop alternative and renewable energy sources, while implementing carbon sequestration strategies to stabilize the concentration of atmospheric CO . The carbon mineralization strategies reviewed in this chapter complement CO storage in subsurface pore space. This promising approach for sequestering CO is grounded in the fundamental processes that govern natural mineral dissolution and carbonate precipitation. Natural analogue sites allow for the study of the geochemical and biological transformation of CO at the field-scale; drawing our attention to potential reaction pathways that can be exploited and utilized, but also to the limitations that must be overcome in geoengineered and industrial systems designed to accelerate carbonation. Further study of natural analogues may yield a better understanding of the reaction pathways required for efficient carbonation, the long-term stability of carbonate minerals at Earth's surface, and the monitoring required for long-term storage. Enhanced weathering of natural minerals or alkaline wastes under near-surface conditions offers a low-energy means of sequestering CO. Although this method offers the ability to aid in remediating the atmosphere, its effectiveness remains untested at large-scales. Accelerated carbonation of alkaline wastes may offer a means of reducing net greenhouse gas emission at the industrial level, while providing a testing ground for more widespread implementation. Biologically mediated carbonate precipitation is an alternate, low-energy means of sequestering CO that could be incorporated into efforts to produce biofuels. In situ carbon mineralization of peridotite offers substantial capacity and relatively fast carbonation rates. Industrial reactors for ex situ carbonation are technologically feasible, yet the estimated costs exceed current carbon prices. Further research and development of process routes is therefore required. Industries that produce alkaline wastes may adopt these technologies as a means of reducing their carbon footprints, while helping to further develop these technological solutions. The largest scale geologic carbon capture and storage operations currently inject ~1-3 Mt CO/yr into subsurface pore space (Michael et al. 2009; Whittaker et al. 2011). Use of industrial wastes for carbonation may rival these rates. In the future, these two strategies may be roughly equivalent in rate and capacity: global implementation of accelerated waste carbonation could exceed the sequestration capacity of 700 CO injection sites. Use of a variety of industrial wastes in parallel could provide ~45% of a "stabilization wedge," and deliver significant offsets at the industry-specific level (Figs. 10 and 11). Implementation of accelerated waste carbonation technologies may allow establishment of viable ex situ technologies that could then be applied to larger scale carbonation of abundant, rock forming minerals, both ex situ and in situ. Although mafic and ultramafic deposits are present in sufficient quantity to completely offset anthropogenic CO emissions for more than 1000 years, large-scale deployment of ex situ carbonation would require new mining activities at a scale comparable to total existing global mining operations (Power et al. 2013b). In principle, enhanced weathering and/or in situ carbonation of natural deposits could comprise an entire "stabilization wedge," but these techniques are very much at the basic research stage. The capacity and rates of carbon mineralization are sufficient to offset significant portions of global greenhouse gas emissions. To realize this potential requires an interdisciplinary effort from fields ranging from the physical sciences to engineering to social sciences. Many of the strategies discussed in this chapter are technologically feasible at a level required for large-scale experimentation and even implementation at the industrial scale. In practice, a combination of ex situ carbonation of industrial waste and natural minerals, in situ carbonation of rock formations, and ongoing CO storage in subsurface pore space, could achieve a "stabilization wedge" (Fig. 11). However, financial incentives, either via a cap-and-trade mechanism or a carbon tax, are required to stimulate further innovation and research of CO sequestration technologies that will lead to significant CO sequestration via carbon mineralization or any other method proposed to date. Investigation of all of these techniques should proceed in parallel, followed by gradual adoption of a range of successful methods, using a variety of optimal strategies that depend on specific local conditions and opportunities.