About the Carbon Dioxide Utilization Data and Analysis

Modeled reductive CO₂ utilization processing steps include the core CO₂ conversion step, recycle of unconverted feedstocks, and product purification stages. For each pathway, the materials and energy required to convert a fixed CO₂ stream are quantified and used to estimate capital and operating expenses with raw material unit prices from open literature, the U.S. Energy Information Administration, and other commercial databases. The modeled system boundary does not include activities and processes common to all pathways such as electricity generation or the production and capture of waste CO₂, which are instead captured as constant operating expenses.

The scale basis for the modeled conversion pathways is a 200-million-gallon-per-year (MGY) bioethanol plant as it represents an appealing entry point with high purity and low-cost CO₂. The mass flowrate of emitted CO₂ (M_CO2 ), kg per hour) during ethanol fermentation is defined by ethanol production (E_g,f , 200MGY), CO₂ emissions factor (θ_E.f , 6.6lb CO₂ per gal ethanol) and operating hours per year (O_t , 7,884 hours per year) resulting in a value of 75,931 kg CO₂/hr:

The CO₂ mass flowrate from a 200 MGY bioethanol plant is also equivalent to the amount of CO₂ emitted from a 100-megawatt (MW) power plant with an emission rate of 820 kg CO₂/MWh and ~90% CO₂ capture efficiency, which is 1 order of magnitude smaller than a typical 1,000-MW coal-fired power plant

A direct-air capture plant with airflow of ~100 million cubic meters (MCM) per hour at CO₂ concentration of 415ppm, which is about half of the largest 1 megatonne (Mt) CO₂/year direct air capture plant in development in the United States.

Variable operating costs are based on material and energy balance calculations under current, future, and theoretical scenarios. We assume a CO₂ price of $40, $20, and $0/tonne for current, future, and theoretical scenarios, respectively.

We selected $40/tonne CO₂ as a middle ground of reported cost of CO₂ avoided among different carbon capture and storage (CCS) technologies; $20/tonne CO₂ reflects high-purity CO₂ with minimal clean-up; and $0/tonne CO₂ is modeled to reflect a best-case scenario in the absence of policy credits. Electricity is priced at $0.068, $0.030, and $0.020/kWh for the current, future, and theoretical scenarios.

For the current scenario, $0.068/kWh is based on the 2017 average industrial electric rate in the United States, where $0.030 and $0.020/kWh are selected as reasonable long-term estimates for average deliverable electric rates. All hydrogen (H₂) needed is assumed to be produced via water electrolysis.

NREL’s H2A: Hydrogen Production Cash Flow Analysis Model (version 3.2018) for polymer electrolyte membrane electrolysis was used to calculate the electrolytic H₂ price. Applying the aforementioned electricity costs under current, future, and theoretical scenarios in the U.S. Department of Energy (DOE) H2A model, estimated production costs for electrolytic H₂ are $3.91, $1.80, and $1.20/kg for the three respective scenarios.

All costs are inflation-adjusted to 2016 U.S. dollars (2016$) using the Plant Cost Index from Chemical Engineering Magazine, the Industrial Inorganic Chemical Index from SRI Consulting, and the labor indices provided by the U.S. Department of Labor Bureau of Labor Statistics.

For all five conversion pathways, annual maintenance costs are estimated at 2.5% of the total installed equipment costs, and annual fixed operating costs (i.e., salaries, property insurance, and tax) are modeled at 3% of the total installed cost (TIC). Low-temperature CO₂ electrolyzers are assumed to be replaced on 7/10 year intervals at 15%/12% of total installed electrolyzer costs for current and future scenarios respectively based on previous water electrolyzer assumptions in the H2A: Hydrogen Analysis Production Model. High temperature solid oxide CO₂ electrolyzers are assumed to be replaced on 4/7 year intervals at 7.5%/4% of total installed electrolyzer costs. In the theoretical scenario, it was assumed that no electrolyzer replacement was required in either case.

Electrolyzer capital costs were based on the established polymer electrolyte membrane (PEM) water electrolyzer systems published by the U.S. Department of Energy H2A program. However, whereas PEM water electrolyzers typically use platinum on the cathode, CO₂ electrolyzers more often utilize cheaper metals such as copper, tin, and silver depending on the desired product. Installed electrolyzer capital costs were adjusted accordingly to account for these savings yielding installed PEM electrolyzer costs on a per square-meter (m²) basis of $18,000, $13,000, and $6,000/m² for current, future, and theoretical scenarios. In the microbial electrosynthesis pathway, alkaline water electrolyzers are selected as the most similar configuration based on reported lower current density and aqueous environments due to constraints dictated by microorganisms. Using comparable H2A models, the installed alkaline electrolyzer costs on a per m² basis are estimated at $1,400, $1,100, and $800/m² for current, future and theoretical scenarios. CO₂ electrolyzers in high-temperature electrolysis pathway are modeled based on the solid oxide electrochemical cell (SOEC). The calculated installed SOEC costs on a per m² basis are $8,000, $6,000, and $4,000/m² for the three scenarios.

The total electrode area needed for each pathway-product electrolysis combination is then defined by the total current needed to reduce the incoming CO₂ and current density (CD) under the three respective scenarios, where I is the current, z is the number of required electrons to produce one mole of product, n is the number of moles of the given product, FE is faradaic efficiency, F is Faraday's constant, t is the operating time, and Q is the total charge in Coulombs per time:

The reactor costs in biochemical (BC) and thermochemical (TC) pathways and product separation equipment costs are primarily based on industry quotes found in published NREL design reports. The key assumptions for bio- and thermo-reactor cost estimations are summarized, along with the general equipment cost assumptions for product separation processes under Capital Costs below.

With capital and operating cost data, a discounted cash flow rate-of-return analysis is generated using published engineering methods. Major financial assumptions include 40% equity financing and 3 years of construction plus 6 months for start-up.

The plant’s life is assumed to be 20 years. The working capital is assumed 5% of the fixed-cost investment, and income tax is 21%. For each pathway-product combination, the calculated minimum selling price (MSP) is the minimum price at which the product must sell to generate a net present value of zero for a 10% internal rate of return. It should be emphasized that although MSP is calculated as a single point value, uncertainty exists around the conceptual cost estimates. Based on availability of data, the technology readiness level of these technologies, and our analysis approach, our analysis and data reported herein are analogous to a “Class 4” study of feasibility as defined by the Association for the Advancement of Cost Engineering International Recommended Practice No. 18R-97.  As such, the implied accuracy of the estimates is likely similar to the expressed class 4 range of -30% to +50%.

From the U.S. Department of Energy H2A analysis for central electrolysis, the reported capital costs for polymer electrolyte membrane (PEM), solid oxide electrochemical cell (SOEC), and alkaline water electrolyzers are used to estimate the CO₂ electrolyzer capital costs for low-temperature electrolysis, high-temperature electrolysis, and microbial electrosynthesis pathways, respectively. The equation below is used to calculate CO₂ electrolyzer capital costs on the basis of $/m².

CO₂ electrolyzer installed capital cost ($/m²) = uninstalled capital cost ($/kW) × Cell voltage (V) × Current Density (mA/cm²) × (1,000mA/A) × (1kW/1,000W) × (10,000cm²/m²) × installation factor

Low-Temperature and High-Temperature Electrolysis, and Microbial Electrosynthesis CO₂ Electrolyzer Cost Assumptions

Separation Process Cost Assumptions

Listed below are the major process assumptions for the five conversion pathways and their products under current, future, and theoretical scenarios.