Carbon Management in the CO2-Rich Natural Gas to Energy Supply-Chain
Ofelia de Queiroz Fernandes Araujo  Stefano Ferrari Interlenghi
and Jose Luiz de Medeiros
The cumulative amount of carbon dioxide (CO,) that can be emitted is referred to as the “carbon budget”. In the latest Intergovernmental Panel on Climate Change (IPCC) report, the budget for having a 50% chance of keeping the average global warming below 2 °C—the UN Paris Agreement, namely the 2D scenario—is estimated to be approximately 275 Gt of carbon (1008 Gt CO,) (IPCC, 2014a). Clearly, the oil and gas (O&G) industry demands urgent and efficient technologies for CO, management, to mitigate the risk of har ing three quarters of the proven fossil reserves becoming unbumable (IPCC, 2014b)— the Stranded Asset Risk (SAR) scenario. Despite the 2D and SAR scenarios, the O&G industry might be building excess capacity (Musarra, 2017) and, to achieve the expected return on invested capital, production life needs to be extended beyond 2050. and a fossil fuel lock-in is expected to occur to some extent.
Carbon capture format routes
To reduce the impacts of fossil fuels, three formal CO, capture routes are currently being pursued: Pre-combustion, post-combustion and oxy-combustion (Al-Mamoori et al., 2017). These concepts can be applied to all fossil fuels but are highlighted for natural gas (NG). Figure 1 sketches the three carbon capture routes associated to power generation and then main differences. Final destination of removed CO, in Figure 1 is considered to be enhanced oil recovery (EOR).
Pre-combustion can be summarized as a gr oup of processes that remove all carbon (decarbonization) from a determined fuel and replace it with hydrogen before its combustion (Olajire, 2010). For example, NG can be transformed into syngas (CO, + CO + H,) via an endothermic steam reform, where some air or O, is supplied in order to release the uecessaiy heat. In a subsequent stage, CO is converted to more CO, and H, via the Water-Gas-Shift (WGS) reaction by adding steam. CO, is then separated from the gas stream via a post-combustion carbon capture technology, usually physical/chemical absorption (acid-gas removal, AGR), and sent to storage, or used as a feedstock, while H, is used as fuel for power/
Figure 1. Carbon capture routes associated to power generation (NG = Natural Gas; WGS = Water-Gas-Shift reaction;
EOR = Enhanced Oil Recovery).
heat generation (Al-Mamoori et al., 2017). Since burning H, does not produce any carbon emissions, CO, removal before fuel combustion allows for a carbon neutral result if all carbon is captured before
H, firing. A potential advantage of pre-combustion is that CO, is separated from a high-pressure stream, possibly reducing power needs and costs.
In oxy-combustion. pure O, is employed for NG firing, reducing the carbon intensity of the power generation (Al-Mamoori et al., 2017). Oxy-combustion results in a highly CO, concentrated flue-gas and steam (Araujo and de Medeiros, 2017). Water vapor can simply be condensed in order to obtain an almost pure CO, stream, avoiding capital intensive cap true technologies. The main bottleneck in oxy-combustion is the cost of producing pure O, via cryogenic separation from air.
In post-combustion. CO, is separated from a flue-gas after burning NG with air (Araujo and de Medeiros, 2017). The use of air combustion generates a flue-gas with high nitrogen content, hampering CO, removal significantly. Post-combustion has gathered the most attention due to the wide array of possible technologies used and its retrofitting potential in already existing plants. As a result, post- combustion currently has the most mature technologies and application scenarios in CO, management of the NG industry and is the focus of this chapter.
I. 2 Carbon capture in the oil and gas sector
Proven oil reserves are expanding as offshore exploration and production (E&P) is increasingly moving to remote areas and ultra-deepwaters (depth > 2000 m). Particularly challenging is the production of natural gas (NG) in ultra-deepwaters, beyond 250 km from the coast, with high gas-to-oil ratio (GOR) and CO,-rich NG, as in the Brazilian pre-salt O&G reserves; e g., the Libra field, whose gas has more than 40%mol CO, (Arinelli et al., 2017). These unconventional reserves are the focused scenario for being among the hardest to exploit, negatively impacting the energy return on (energy) investment (EROI) (Hall et al., 2014). Ultra-deepwater O&G processing employs Floating Production Storage and Offloading (FPSO) platforms, where, due to high GOR, = 60% of the topside area is dedicated to gas processing (Araujo et al., 2017).
In parallel, to adjust investments in line with emissions reduction targets (e.g., 2D scenario), worldwide economies are enforcing mechanisms to move towards a low carbon economy; e.g., carbon taxes and emissions-trading systems, where total emissions are capped and permissions to emit CO, are traded (Energy Institute at Haas, 2016). The 2D scenario is favoring a shift towards renewable energy, demanding actions from O&G companies to mitigate SAR, extending the O&G lifetime, targeting intense carbon capture and storage (CCS).
NG is the fossil fuel with least environmental impact; In power plants it emits 50% to 60% less CO, per kW than coal, which entails other additional environmental issues, such as mercury, particulate matter
(PM) and nitrogen/sulfur oxides (NETL, 2015). NG is expected to expand at an annual rate of 6% from 2017 to 2020 (EIA, 2018), responding to ~ 30% of the global power generation by 2040 (EIA, 2018). Even though NG has a clean burning, its increasing consumption has resulted in CO, emissions (EIA. 2016). It is also challenging that CO, use as an (EOR) Enhanced Oil Recovery agent is intensifying. Injection of CO, in oil reservoirs recovers extra barrels of oil, improving economic performance but increasing the CO, content in produced fluids along operation tune (Zhao et al.. 2016). Concomitantly, the production curve decreases (Reis et al.. 2017), demanding resilient CO, capture technologies to maintain economic performance while mitigating environmental impacts.
This chapter is driven by the critical role of carbon capture in extending the use of fossil-based energy, with destination to EOR, which combines storage with utilization (CCU. Carbon Capture and Utilization) at a large scale in the near-term (Araujo and de Medeiros, 2017). Although expected to grow in the long-term, monetization of CO, through its chemical conversion to value-added products is in an earlier process-systems development, small-scaled and with market size limitations on targeted products (Araujo et al., 2014), comparatively to CCS. Nevertheless, many authors defend the idea that CCU is a more suitable short-term solution within the constraints imposed by the 2D scenario by providing a monetary incentive to close carbon loops when compared to CCS (Stuardi et al., 2019). Prospects of using CO, as a feedstock for chemical production include carboxylation, reduction reactions, heterogeneously catalyzed hydrogenation and photocatalytic/electrocatalytic conversion (Alper and Orliau. 2017). Despite many chemicals currently being investigated as possible CCU outputs, CO, to methanol or methane has the most potential application in the offshore O&G scenario under high GOR and CO,-rich NG constraints and is briefly included among the CO, management technologies reviewed in this chapter.
Despite generating value and storing CO, in the short term, CCU is expected to contribute less than 1% to greenhouse gas mitigation targets when considering its potential scale of deployment (Stuardi et al., 2019). In addition, current trends indicate that two-thirds of the total NG consumption worldwide occurs in powerflieat applications—either in furnaces of industrial processes or electricity generators (EIA, 2018). In some industries (e.g., bulk chemicals, food, metal-based durables and glass). NG supplied over 40% of their lieat/power requirements in 2017 (EIA, 2018). Projections up to 2050 show that NG used for heat/power is expected to steadily grow, due to the high costs associated with fuel switch, but NG as a feedstock will stagnate (EIA, 2018). Thus, CO, management in the NG to energy chain is. in both the short and the long-term, more critical in its environmental and economic impacts and, as a result, the focus of this chapter.
Scope and structure
Considering CO,-rich NG as starting point, the main CO, capture technologies addressed comprise chemical absorption (CA), physical absoiption (PA), membrane permeation (MP), cryogenic distillation (CD), adsorption (ADS), gas liquid membrane contactors (GLMC), supersonic separators (SS) and hybrid technologies (HYB). Figure 2 shows CCS/EOR as CO, destination in the value-chain, composed of onshore power generation (Gas-to-Pipeline-to-Wire, GTPTW) and offshore power generation (Gas-to- Wire, GTW), both with CCS, which is the main scenario in the present approach.
In the value-chain pictured in Figure 2, CO,-rich NG (CO, > 40%mol) is decarbonated in ultra- deepwater FPSO in order to meet CO, specification (< 3%mol), where the thicker lines correspond to the NG path to energy. While CO, is injected into the source offshore reservoir for EOR, decarbonated NG is compressed and dispatched through subsea pipelines to onshore power plants, where CO, is captured from exhaust gases and transported back to offshore reservoirs for EOR. In the shorter offshore alternative, power is generated on a Floating Power Generation Platform (FPGP), after partial decarbonation to reduce CO, content to the acceptable limit of NG-fired turbines (= 20%mol). CO, from NG upgrading is mixed with CO, from offshore post-combustion capture and injected after compression into the source reservoir for EOR. NG and CO, pipelines are eliminated and a HYDC cable is needed for electricity transmission to onshore facilities.
For each CO, capture technology, separation mechanisms, benefits and shortcomings, recent innovations, stakeholders, and technology manuity level, via the Technology Readiness Level (TRL)
Figure 2. Value-chain of C02-nch natural gas to energy with decarbonation: Floating power generation vs onshore power generation index, are given. Still, with the focus on ultra-deepwater processing of CO,-rich NG, the scenario of post- combustion capnue is discussed, followed by a short discussion on pipeline transportation and chemical conversion of CO,.