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 modeled 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 set of technologies for chemically separating CO₂ from the atmosphere so it can be sequestered. The amount of CO₂ removed by DACCS 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 costs and potential incentives, which can come from carbon price or direct subsidies. DACCS capacity model has orders, completions and retirement, subject to delays and limits on construction capability. The cost of DACCS changes over time due to two competing dynamic forces. Learning, from accumulated experience, tends to lower costs. The sum of the pending and installed capacity, representing using up the sites with best access to CO₂ transport and storage, tends to raise costs. 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, including a carbon price and subsidies, 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 and application, i.e., nonelectric industry and electricity generation for all end use 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 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.

For each fuel and application, CCS capacity adjusts over time to the desired amount, which is determined as the fraction of total energy capacity indicated by an s-shaped function of the ratio of marginal incentives to costs. As the incentives increase or costs decrease, more CCS projects are initiated up to a maximum where all energy capacity has CCS. Incentives for CCS can come from a carbon price, subsidies, or from a clean electricity standard. 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 includes capture and transport equipment costs, the cost of storage, and the cost of energy, which is assumed to equal the market price of electricity from the Market Clearing sector. Equipment costs, and the energy needed to operate CCS, tend to decline following endogenous learning curves. The sum of the pending and installed capacity, representing using up the sites with best access to CO₂ transport and storage, tends to raise costs. Storage costs increase with cumulative use of storage. The balance of these dynamic changes can raise or lower total CCS costs over time, which will alter the amount of CCS for each application. Unit costs of existing CCS capacity are determined by sum of the embodied capital costs of CCS and the product of market price of electricity and the embodied energy intensity of using CCS, minus the variable incentives from the carbon tax avoided by what is captured and unit subsidies. The net of variable costs and incentives affect the market clearing utilization of each fuel that might be equipped with CCS compared to other energy sources. The ratio of incentives to variable costs determines the CCS utilization; if a plant has CCS equipment but the CCS incentives are no longer greater than its variable costs, the plant can still operate but without using its CCS. The net of costs and incentives also average into the unit costs of the total energy capacity for each fuel and application for decisions of investments in new energy (weighted by indicated CCS capacity), and in the effects of market prices and profitability effects (weighted by existing capacity).

Model Equations🔗