Aug-2020

Capturing carbon dioxide from refinery streams

Using hydrogen manufacturing units to demonstrate the relative benefits of technologies for capturing carbon from low and high pressure streams.

LAURENT THOMAS and GARY BOWERBANK
Shell Catalysts & Technologies

Article Summary

Governments around the world are increasingly expected to penalise carbon dioxide (CO2) emissions to help fulfil their responsibilities under the 2015 Paris Agreement on climate change. This leaves refiners and chemical plants with a mandate to reduce their CO2 emissions substantially, and many companies are committing to reducing their carbon footprints.

For most businesses, this will involve a mosaic of solutions, including energy efficiency initiatives, fuel switching, and process optimisation. However, serious ambitions to reduce refineries’ carbon intensity are likely to be spearheaded by carbon capture, utilisation and storage (CCUS). Indeed, the UN’s Intergovernmental Panel on Climate Change special report on the impact of 1.5°C global warming concludes that “early scale-up of industry carbon capture and storage is essential to achieve the stringent temperature target”. The International Energy Agency agrees, stating that CCUS is a key technology for reducing CO2 emissions in carbon-intensive industrial processes and offers one of the lowest cost ways of doing so. In today’s capital constrained environment, one of the principal challenges that refiners may face is finding ways to do this economically.

By 2050, 2.8 billion t/y of CO2 needs to be captured and permanently stored to meet the International Energy Agency’s sustainable development scenario, which meets the UN’s sustainable development goals for energy access, emissions, and air quality, and has a 66% probability of limiting global temperatures to 1.8°C.¹ Currently, CCUS projects capture about 40 million t/y of CO2, so many more projects are needed in the coming decades.²

Over the last decade, CCUS capacity has nearly doubled while the project pipeline shrank from 2010 to 2017 in response to the global financial crisis that focused governments on short term economic recovery and the private sector on survival.² However, during the last three years, project momentum has recovered, driven by the Paris Agreement. There is growing interest from refineries and chemical plants because CCUS offers a cost-effective way to enable carbon-intensive industries to continue to operate through the energy transition.

Upgrading the bottom of the barrel to clean fuels requires hydrogen, so it is common for refineries to have a hydrogen manufacturing unit (HMU), most often based on steam methane reforming (SMR) technology, that creates CO2 from both the chemical reactions and from burning fuel to power the process chemistry. Although many refinery units produce CO2 (see Figure 1), this article focuses on carbon capture from the HMU because it generates a large, relatively pure stream of CO2 and provides opportunities to capture CO2 from high pressure, pre-combustion and low pressure, post-combustions streams, thereby enabling a cost—benefit comparison between two mature capture technologies developed by Shell (see Figure 2).

This article showcases two leading technologies with established records for cost-effective carbon capture in a wide range of industries:
-    Shell’s Cansolv CO2 Capture System for capturing CO2 from low pressure streams, including flue gas; and
-    Shell’s ADIP Ultra technology for capturing CO2 from high pressure process streams.

Selection of a retrofitting option for a refinery (pre- or post-combustion) depends on, among other factors, the value assigned to the captured CO2 (from avoided tax, tradable credits or income from its use in enhanced oil recovery [EOR] or other industrial applications).

Leveraging experience
CO2 capture technology is not new; it is established and proven. In the 1930s, carbon capture technologies began commercial operation in the processing of natural gas. In the 1970s, commercial-scale CO2 injection into reservoirs started. To date, more than 260 million tonnes of anthropogenic CO2 has been captured and stored, mostly through EOR projects, and the current CCUS capacity is about 40 million t/y.2
Other energy-intensive sectors, for example coal-fired power generation, oil sands extraction, and cement manufacture, have already been charged with dramatically reducing the carbon intensity of their operations. Refiners can leverage the operational experience, technologies, and expertise from these sectors to do the same.

For example, the coal-fired power generation sector, after a first generation of carbon capture projects with a capture cost of about $100/t CO2, is now targeting costs of half this, about $50/t CO2, for its future projects.¹

Low pressure streams
Shell’s Cansolv CO2 Capture System can capture up to 99% of the CO2 from post-combustion low pressure off-gases. As a tail-end, low pressure CO2 capture technology, it is well suited for retrofitting. It uses a regenerable solvent based on a proprietary amine to capture the CO2, which is released as a pure stream that can be sold, sequestered, or used in EOR.

In refiners’ technical and economic evaluations for capturing CO2 from flue gas, the Cansolv CO2 Capture System may emerge as the preferred option because of key features such as:
-    CO2 purity: the pure CO2 product enables EOR, CCS, or carbon capture and use downstream of the plant
-    Adaptability: the system is highly adaptable to a wide variety of industrial applications, CO2 concentrations (from 3.5% to 25% and higher), and gas flow rates (licensed units treating gas at flow rates of 11000-685 000 Nm3/h are in opera­tion or under construction)
-    Asset integrity: the system has been designed for reliability through its flexible turndown capacity and improved resistance to oxidative and thermal degradation
-    Low waste: the process uses a regenerable solvent, so there are no direct waste by-products, which can reduce project costs since the effluents are minimal
-    Retrofit suitability: as a standalone system, it is ideal for retrofit scenarios and greenfield projects
-    Low operating costs: the system offers cutting-edge performance. For example, its low parasitic energy consumption, fast kinetics, and extremely low volatility help to reduce the costs of operation and amine consumption
-    Track record: the largest application is designed to capture 1 million t/y of CO2 and has been operating successfully for four years (see case study below)
-    Potential for integrated sulphur dioxide (SO2) removal: it can be integrated with the Cansolv SO2 Scrubbing System for near-complete SO2 removal

Process description
The key steps of the Cansolv CO2 Capture System (see Figure 3) are as follows:
1.    The feed gas is quenched and saturated in a circulated water pre-scrubber
2.    The gas contacts the lean amine solution in a counter-current packed absorption column
3.    CO2 is absorbed and the treated gas exits to atmosphere
4.    Midway along the column, partially loaded amine is removed from the tower, cooled, and reintroduced over a layer of mass transfer packing
5.    CO2-rich amine from the absorption column is pumped through a lean-rich amine heat exchanger and then to the regeneration column
6.    Rising, low pressure, saturated steam in the column regenerates the lean amine solution. CO2 is recovered as a pure, water-saturated product
7.    Lean amine is pumped from the stripper reboiler to the absorption column for reuse in capturing CO2
8.    The CO2 is directed to by-product management systems