(259b) Techno-Economic Screening Analysis of Terrestrial Enhanced Weathering of Igneous Rocks and Industrial Waste Materials

The Biden administration set the goal of achieving net-zero emissions by 2050. Meeting this goal requires investment in a portfolio of technologies including carbon dioxide removal (CDR). CDR technologies remove CO₂ from the atmosphere and can offer a means to decarbonize hard-to-abate CO₂ emissions (such as emissions from air travel) and meet the net-zero goal. Moreover, limiting global temperature increase to 1.5 °C in accordance with the Paris Agreement will require CDR to address legacy CO₂ emissions. One emerging CDR technology of interest is terrestrial enhanced weathering. Terrestrial enhanced weathering leverages the geologic process of weathering, which naturally removes CO₂ from the atmosphere. Weathering is the natural breakdown of alkaline rocks in the presence of rainwater, temperature changes, and/or living organisms. CO₂ in the air reacts with water to produce carbonic acid, which then reacts with alkaline rocks—containing hydroxide, silicate, and/or carbonates—to produce aqueous bicarbonate ions. The bicarbonate ions are eventually transported through soil, groundwater, and river water to the ocean where the CO₂ can remain in solution for more than 100,000 years. Sample reactions are presented in Figure 1.

While geologic weathering naturally removes CO₂ from the atmosphere, it is a slow process. Terrestrial enhanced weathering accelerates the geologic process of weathering by (1) crushing and grinding alkaline rocks and materials to increase their exposed surface area, and (2) distributing this material in favorable regions. Favorable regions include tropical areas or agricultural fields where the materials are exposed to water, microorganisms, and favorable pH and temperatures. The weathered material passively captures CO₂ over time and stores the reacted carbon in the form of a bicarbonate. Suitable materials for enhanced weathering include naturally occurring alkaline rocks or alkaline industrial waste. Examples of naturally occurring rocks include mined igneous rocks such as basalt (mafic rock) and dunite (ultramafic rock). Alternatively, alkaline materials other than rock can react with CO₂ in the atmosphere and, thus, be “weathered,” such as industrial waste materials like biomass ash or cement kiln dust.

Terrestrial enhanced weathering has emerged as a technology of interest due to its simplicity, low energy requirement, and potentially low capital investment requirement. Due to the novelty of this technology, there is limited literature exploring the techno-economics of the process. This presentation reports on a screening techno-economic analysis that examines the impact of key factors that influence terrestrial enhanced weathering performance and cost.

Two cases are considered for analysis based on the material used:

(1) Naturally occurring mined igneous rock

(2) Industrial waste materials (like biomass ash or cement kiln dust)

The analysis considers locating the terrestrial enhanced weathering system in the Midwestern United States, because of proximity to alkaline materials, availability of farmland, and appropriate ambient conditions for weathering. For the igneous rock case, the size of the system is based on the average amount of igneous rock available from a single mine—250,000 tonnes/year. For the industrial waste case, the size of the system is based on the amount of suitable waste material produced by an industrial hub—150,000 tonnes/year.

The analysis scope for the igneous rock case includes material purchase, comminution, transport by truck, distribution of the material on farmland, and measurement, verification, and reporting (MVR) of the CDR. The comminution occurs at a remote location and is processed to a particle size of 20 μm. For the industrial waste case, the scope of analysis includes material purchase, transport by truck, distribution of the material on farmland, and MVR. Comminution is not required, since biomass ash/cement kiln dust is already available in small particle sizes. For each case, a performance model was developed to estimate the mass and energy balances of the process, and a cost model was developed to calculate the capital costs, operational and maintenance (O&M) costs, and the levelized cost of CO₂ capture (LCOC).

The cost of material transportation is estimated as a function of the amount of material and the transport distance. It is assumed that existing farming infrastructure can be leveraged to apply the material to the land; thus, application equipment is not included in the capital costs. MVR costs are estimated by assuming that soil samples are periodically evaluated in order to confirm CDR is taking place. The reacted material is reapplied each year, and a 30-year project lifetime is assumed. Capital costs include the equipment for comminution, material handling, and material storage. O&M costs include the material costs, transport costs, energy requirements, labor costs, and MVR. Capital and O&M costs are determined using in-house cost estimating references informed by engineering procurement and construction firm (EPC) input, engineering judgment, and literature-reported values.

Results from the techno-economic models show that the igneous rock-based process is able to capture about 140,000 tonnes of CO₂ each year while the industrial waste-based process can capture about 90,000 tonnes of CO₂ each year. The corresponding LCOC for the igneous rock case is about $136/tonne of CO₂ captured. The industrial waste case results in an LCOC of about $119/tonne of CO₂ captured. The resulting breakdown of the LCOC can be found in Figure 2.

Since many factors can greatly impact the performance and the cost of the terrestrial enhanced weathering, multiple sensitivity analyses were performed. Factors considered included the weathering rate and weathering potential, amount of material applied, material cost, site application, energy cost, MVR cost, and transportation distance. For the igneous rock case, a sensitivity on the rock size was also performed. Material size influences the material-specific surface area and comminution energy requirement based on the material size. For the industrial waste case, the specific surface area is varied.

The weathering rate and weathering potential were found to have a large impact on the LCOC for both cases (see Figures 3 and 4). This highlights the importance of material selection since the weathering rate and potential vary across materials of different compositions. Additionally, the weathering rate and potential are significantly impacted by ambient conditions; this highlights the importance of application location selection.

A preliminary life cycle analysis indicates the net CDR rate is >50% of the CO₂ capture rate (conservatively assumed). Therefore, the net cost of CDR is no more than double the reported LCOC. This screening analysis indicates that implementing terrestrial enhanced weathering by utilizing ultramafic rock and/or cement kiln dust in suitable locations can be relatively low cost.

This transparent screening analysis based on systematic techno-economic analysis provides preliminary insight into factors that impact the performance and cost of terrestrial enhanced weathering. The results indicate that terrestrial enhanced weathering has the potential to offer a feasible approach to CDR. Future work will leverage these results to design relevant case studies for a more detailed techno-economic analysis of terrestrial enhanced weathering.

Disclaimer

This project was funded by the United States Department of Energy, National Energy Technology Laboratory an agency of the United States Government, through a support contract. Neither the United States Government nor any agency thereof, nor any of its employees, nor the support contractor, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.