A techno-economic analysis on water resource recovery facilities employing carbon capture strategies in biogas upgrading practices

OBJECTIVE: The increasing emphasis on circular economies has led wastewater treatment in recent years to become a renewable source for energy, heat, water, fertilizers, carbon products, and soil amendments. Despite these advancements, one significant and potential resource that remains unrealized is biogenic CO2, which can be used for geological sequestration or other manufacturing. Given the arising climate change policies and markets for biogenic, or green CO2, there will be greater financial incentives in the future for WRRFs to implement bioenergy programs with carbon capture, utilization, and sequestration (BECCUS) strategies. This paper will explore a market assessment for biogenic CO2 end-uses, funding mechanisms and economic incentives, regulations and policies, technological solutions, and a techno-economic analysis (TEA) of a case study for carbon capture at a WRRF. CURRENT CO2 MARKET In 2022, commodity CO2 demand in the United States was estimated at 10.3 million MT per year with over 64% of that sourced from fossil fuel derivatives (Figure 1). About 25 percent of CO2 are mined, with the remaining demand supplied as a byproduct from chemical manufacturing processes (ammonia, ethanol, hydrogen, and natural gas processing). While most of the CO2 is used in enhanced oil recovery (EOR), which is the process of injecting CO2 into the oil reservoir to increase the mobility of the oil, the second most common use of CO2 is for the food/beverage industry (approximately 30% of all CO2 demand). THE VALUE OF BIOGENIC CO2 AND THE FUTURE CO2 MARKET CO2 can be categorized by its sourcing as traditional CO2 or green CO2. Traditional CO2 mainly comes from the burning of fossil fuels and results in an accumulation of CO2 in the atmosphere. As climate-driven initiatives and regulations become more pressing, the CO2 market will need to evolve to more sustainable alternatives. Green CO2 is a sustainable type of CO2 as it is sourced from the natural carbon cycle making it effectively carbon neutral. The two main sources of green CO2 are direct air capture (DAC) CO2 and biogenic CO2. DAC is a technology that separates CO2 from the atmosphere through chemical reactions and is not limited to point of emission locations. However, DAC is both an expensive and energy intensive method of capturing CO2, as it requires concentrating the dilute stream of CO2 from the atmosphere (Figure 2). Biogenic CO2 is a favorable alternative as it comes from the degradation (digestion/fermentation), combustion, or mineralization of organic matter, and tends to result in a higher concentrated CO2 stream. Currently, ethanol plants provide up to one quarter of the existing biogenic CO2 market's supply in the U.S., but its production is limited to the midwest region. For WRRFs, biogenic CO2 is a natural byproduct of both anaerobic digestion and from the combustion of biogas in combined heat and power systems (CHP), making it a prime point of emission location. Over 1,300 WRRFs are equipped with anaerobic digestion systems, and many are located in urban centers. OPPORTUNITIES AND CHALLENGES FOR BIOGENIC CO2 WRRFs can utilize two pathways for biogenic CO2: carbon capture and utilization (CCU) or carbon capture and storage (CCS). Figure 3 shows a general diagram of the two paths. A national survey was performed to examine the prospects for captured biogenic CO2 from WRRFs to be utilized in the food/beverage, e-fuels, or sold to a distributor. Partial details of the survey are included below; full details will be included in the paper: -E-Fuels -- also known as 'electrofuels', are a class of synthetic fuels that are produced from combining the carbon in CO2 with green or blue hydrogen. Twelve, a carbon transformation company and E-Jet fuel supplier, stated that they source their biogenic CO2 for $20-100 per ton from the ethanol industry. - E-Chemistry -- similar to e-fuels, e-chemicals use electrosynthesis to produce chemicals through more sustainable avenues. This sector is still developing but may become a major user of CO2 in the future. Biogenic CO2 would be highly compatible with this end-use. - Cement manufacturing -- biogenic CO2 can be used as a direct replacement for traditional CO2 which is used for the manufacturing of concrete. Scale and size may limit acceptance. CCS is the process in which captured CO2 is compressed and injected in underground geologic formations for permanent storage or sequestration. WRRF would partner or contract with a regional or local CCS service provider, and the credits can be used to offset a utility's emissions or sold in the voluntary or compliance carbon market. FUNDING MECHANISMS AND ECONOMIC INCENTIVES The Inflation Reduction Act (IRA) was passed in 2022 and provided over $369 billion dollars in climate-related funding for domestic clean energy production, decarbonization technologies, climate change mitigation, and resiliency programs. Within Section 45Q of the IRA, provisions for CCU and CCS provide the financial incentives for utilities with bioenergy programs to invest in CO2 capture technologies (Table 1). TECHNOLOGICAL SOLUTIONS The biogas upgrading to RNG process includes cleaning and separation to remove CO2, H2S, siloxanes, VOCs, and moisture, resulting in RNG and a CO2-rich tail gas that contains 93 to 99.9 percent CO2. This tail gas can be further processed to recover biogenic CO2 rather than vented to atmosphere. Each of these technologies have varying levels of compatibility with CO2 recovery in the tail gas as shown in Table 2. TECHNO-ECONOMIC ANALYSIS A TEA was developed to evaluate the financial feasibility of two different CO2 capture program sizes with and without IRA eligibility. This TEA only considered the CO2 capture program as an add on and not a full CO2 and RNG program which may change the economics. Interviews with various CO2 capture equipment manufacturers, CO2 commodity suppliers and distributors, and CO2 end-users provided economic information for this TEA. The key assumptions that drove the economic model are summarized in Table 3. The results of the TEA illustrates that a 'no distributor model' with IRA funding reflected a positive NPV. The other modelled scenarios are illustrated in Figure 5. Note that the current IRA 45Q guidelines provides credits for only 12 years and that CO2 pricing, transportation costs are highly region dependent and may change the economics (Smith et al. 2021). CONCLUSIONS Capturing CO2 from municipal bioenergy programs is a significant opportunity for resource recovery and carbon management and stands out for its practicality and effectiveness. One advantage of CO2 capture from biogas is its high concentration in the airstream, with purity typically ranging from 30-50% and up to 99.9% from biogas upgrading to RNG. There are several end-uses for biogenic CO2 in the CCS (sequestering and selling credits to the voluntary or compliance carbon market) and CCU market (i.e. food/beverage, industrial uses, e-fuels). However, a limitation for WRRFs-sourced biogenic CO2 is its scale and size as many require a production of a minimum of 30,000 to 50,000 tons of CO2 per year for partnership. We anticipate that the competition for green CO2 sources will drive these end-users to seek WRRFs in the future. The future paper and presentation will explore additional sources of CO2 from WRRFs such as from post-combustion capture, and aeration tanks.