(622c) Techno-Economic Feasibility of Thermocatalytic Conversion of CO2 into Renewable Natural Gas (RNG)

Introduction: The renewable natural
gas (RNG) generation process discussed in this talk relies on the
thermocatalytic Sabatier reaction:

CO₂ + 4H₂ = CH₄
+ 2H₂O + heat (165 kJ/mol)

The main idea is to convert the CO₂
contained in waste streams (e.g. landfill gas, biogas) into RNG on site using
electrolytic hydrogen (H₂) generated using renewable (wind, solar, hydro),
low carbon footprint (nuclear), or surplus electricity.

Problem Statement: Although
CO₂ separation technologies are already commercially available (e.g.
pressure swing adsorption (PSA) and membrane separation), the captured CO₂
has to be compressed on site, transported, and eventually stored or utilized
somehow. Storing the captured CO₂ in geological formations could be
prohibitively expensive. Some of that captured CO₂ can be utilized
in the chemical and food industry. However, these markets are limited in their
capacity to absorb megatons of separated CO₂. In addition, the
capital and operating costs associated with the CO₂ compression and
transportation may not be recoverable.

Innovation: The approach discussed
in this talk is an innovative and sustainable alternative that goes beyond
conventional CO₂ capture and storage (CCS), relying on CO₂
capture and utilization (CCU) instead. The hydrogen required for the reaction
is provided by water electrolysis using renewable or low carbon footprint
electricity. The water and heat generated in the process are recuperated to
improve the process efficiency. Oxygen (O₂), which is a byproduct of
water electrolysis can be utilized as well. With a proper postreatment, such
process would produce a pipeline quality RNG, which can be directly injected
into the existing natural gas infrastructure, replacing the fossil natural gas.

Results & Discussion: Technological
issues related to catalysis, reactor design and system integration are briefly outlined
first. The focus is on the technological and economic feasibility of this
conversion pathway. A case study is included, showing the projected production
cost and payout period. Design of the entire RNG production system, including upstream
and downstream treatment, is discussed in detail. High-level techno economic
assessments (TEA) were undertaken to evaluate the preliminary equipment,
utility and financial requirements for the project. Process design utilized
design software ASPEN HYSYS. Detailed TEA for the system has been conducted to
analyze the project costs, profit margins and overall economics. Sensitivity
analysis has been performed to test the effects of various parameters such as
the electricity price. Various configurations of plant design have been
considered and assessed to allow optimization.

Conclusions: The project economics is
highly depended on both the RNG selling price and the price of electricity. The
production cost can be as low as 10$/GJ and 15$/GJ at 4 and 8 cents per kWh
electricity prices, respectively. The majority of capital and operating cost
contributions are from the electrolyzer system. The maximum payout period for
the system could be as low as 6 years, showing project feasibility for moderately
low electricity prices.