Carbon Dioxide Removal (CDR)🔗

The carbon dioxide removal (CDR) submodel governs the storage of carbon by biological, chemical, and industrial means. It includes both CDR proper, and Carbon Capture and Storage (CCS). CDR refers to methods that take CO₂ from the atmosphere and sequester it as carbon somewhere else. The CDR methods we include are afforestation, soil carbon management, biochar, enhanced mineralization, direct air carbon capture and storage (DACCS), and bioenergy with carbon capture and storage (BECCS). CCS refers to methods that capture carbon from a fuel before or after combustion, so that less CO₂ is released to the atmosphere. CCS is modeled for fossil fuels (coal and gas) and bioenergy. There is overlap between CCS, BECCS, and DACCS including common technologies, storage sites, economic drivers, and infrastructure.

CCS and CDR methods are model at various degrees of detail. The amount and timing of removals are either set by, calibrated to, or grounded in a synthesis of literature, most frequently the Royal Society Report.

The carbon flows calculated by CCS and CDR structures are passed to the Carbon Cycle model and flow into biomass, soil, or sequestration stocks as appropriate. Each storage stock is subject to a leak or loss rate, adjustable in assumptions. In addition to carbon flows, this sector calculates the expenditures, energy needs, material flows, and land needs to show the impacts of relying on these techniques.

CDR Methods🔗

Afforestation includes the land deliberately planted with trees as a means of carbon sequestration. Additional new forests might occur endogenously if farmland is abandoned, but that is not counted as "afforestation". Afforestation is specified by the user as a percent of the maximum area available for planting, adjustable as an assumption, and potentially limited by the area of Other Land available. Once the land is specified as afforested land, the growing forests sequester and store carbon according to the NPP and respiration drivers defined in Terrestrial Biosphere Carbon Cycle.

Agricultural soil carbon refers to techniques that increase the amount of carbon in farmland soil. It is specified as a percent of the peak rate of CO₂ removal, adjustable in the assumptions. The farmland carbon transfer parameters in the Terrestrial Biosphere Carbon Cycle submodel are then adjusted to achieve that rate, potentially limited by land availability. The assumption on soil carbon loss rate adjusts the parameters of the Terrestrial Biosphere Carbon Cycle as well.

Biochar refers to turning biomass into charcoal then burying the carbon in farmland as a soil amendment. It is specified as a percent of the peak rate of CO₂ removal, adjustable in the assumptions, subject to a loss rate.

Mineralization is a chemical process, also called enhanced weathering, where certain kinds of rock are spread onto farmland, where they absorb CO₂. This also has a beneficial effect on agriculture if the soil is too acidic. The user input sets the percent of suitable farmland (adjustable in the assumptions) and the rate of CO₂ absorption the amount of rock applied and the specific absorption potential. Gross absorption is adjusted by a loss rate (default zero) and the emissions from the energy used to mine, grind and transport the necessary rock.

Direct air carbon capture and storage (DACCS) (sometimes called DAC) is a function of the capture equipment and the capacity to transport the captured CO₂, which is shared with CCS. The desired DACCS capacity is a function of the target percent of maximum Gtons CO₂ per year, which results in funding for DACCS infrastructure. DACCS capacity model has orders, completions and retirement, subject to delays and limits on construction capability. The amount of CO₂ captured is the lower of DACCS capacity and available CO₂ transport capacity. The cost of DACCS declines over time with learning, but can increase if CO₂ transport and storage constraints are exceeded. The energy required to operate DACCS equipment increases the energy demand for electricity, potentially increasing emissions. The gross capture by DACCS is stored in geological formations; an estimate of CO₂ emitted by its energy demand is subtracted to plot net removals.

Bioenergy with CCS (BECCS) is modeled under the CCS section below. It responds to price signals, i.e. carbon price, rather than having a user input under the CDR section.

Carbon Capture and Storage (CCS)🔗

Both fossil and bioenergy CCS are modeled as stocks of transport capacity (shared with DACCS), and individual capture capacities for each fuel, for industry and electric sectors. Completion is subject to both development and construction delays, with a limit on overall growth rates as construction capability itself takes time to construct. The time delays on transport capacity are assumed to ste the limit on deploying capture capacity as well. The amount of CO₂ captured for each fuel and application is the least of: CO₂ created in combustion, capture capacity, and available transport capacity. If transport capacity is limiting, it is shared in proportion to capture capacity.

CCS capacity adjusts over time to the amount indicated by a simple economics model. The available revenue for CCS can come from a carbon price or the incentive of a clean electricity standard. The market is defined by price sensitivity and a reference demand - the demand for CCS when its cost equals available revenue. There is additional exogenous construction of CCS representing the historical and expected construction for R&D, demonstration projects and the like, calibrated to historical CCS data. The cost of CCS capture and transport equipment costs, which follow endogenous learning curves, and the cost of energy assumed to equal the market price of electricity from the Market Clearing sector. Costs also increase as constraints on storage and transport capacity are reached. Within the overall market size, the different fuels and applications of CCS compete on price and remaining potential.

Model Equations🔗