Oct-2024

From waste to clean energy: The acid path to reducing CO2 emissions

Case study explores environmental benefits of the WSA process to capture waste heat, enhance energy efficiency, and reduce the refinery carbon footprint.

Igor Yu Kostromin
Topsoe

Article Summary

The refining sector faces a critical challenge: achieving significant reductions in greenhouse gas (GHG) emissions. This article delves into the wet gas sulphuric acid (WSA) process, an established technology for sulphur dioxide (SO₂) emission reduction with the potential to enhance energy efficiency and reduce the carbon footprint of refineries. By adopting a technical case study format, the WSA process is explored in depth, comparing it to the established Claus process for sulphur recovery. Emphasising facts and figures, the analysis will explore the environmental benefits of WSA, its role in capturing waste heat, and the resulting potential for carbon dioxide (CO₂) emissions reduction.

Decarbonisation landscape
The global refining industry remains a significant contributor to overall GHG emissions. Without swift and decisive action, emissions are projected to remain elevated for the foreseeable future. National goals necessitate ambitious reductions, with the US aiming for a 35% decrease in refining production emissions by 2030 and a staggering 90% reduction by 2050.1 Encouragingly, a 20% reduction by the mid-2030s appears achievable through the implementation of current economic measures without additional government support.

Refineries offer unique opportunities for decarbonisation efforts. A typical 10 million tons of crude oil per annum (MTA) refinery can generate approximately 2 million tons (Mt) of CO₂ equivalent (CO₂eq) of Scope 1 and 2 emissions.2 These emissions can be effectively mitigated by improving the overall energy efficiency of both primary and auxiliary processes within existing refinery footprints. Current technologies and established procedures offer a pathway towards significant CO₂ reductions, with sulphur recovery units (SRU) potentially playing an important role.

Established practices
The modern refining landscape relies heavily on the modified Claus process for sulphur recovery. This well-established technology utilises a series of catalytic stages to convert hydrogen sulphide (H₂S) into elemental sulphur. However, while the Claus process offers a reliable solution, it presents limitations in terms of maximising energy capture and environmental impact reduction.
Technical advancements offer a path towards improved environmental performance. In 1980, Topsoe introduced the WSA sulphur recovery process; initially considered a cost- effective solution for cleaning low-sulphur waste gases, the WSA process quickly demonstrated its broader applicability.

The process of direct H₂S conversion to sulphuric acid, the ultimate product for much of the recovered sulphur, emerged as a challenge to the modified Claus technology. However, while industry recognised the potential for improved economics and reduced environmental impact through sulphuric acid production, logistical constraints, primarily related to local demand, hindered its widespread adoption. Notably, neither WSA nor Claus processes were commonly considered decarbonisation solutions at that time.

Thermodynamic advantages of WSA
The decarbonisation potential of transitioning from the Claus to the WSA process for sulphur recovery is contingent upon the sulphur content of the specific crude oil feedstock. The contrasting thermodynamics of these two processes play a crucial role in determining the overall energy output.

When converting H₂S to elemental sulphur, the sulphur goes from oxidation state -2 to oxidation state 0 (-2 to 0). This is an exothermal process, yielding approximately 222 kJ per mole of H₂S processed.

When converting H₂S to sulphuric acid, the sulphur goes in two steps: from oxidation state -2 first to oxidation state +4 and finally to oxidation state +6. The reactions involved are also exothermal, but with an almost four times higher enthalpy jump, in total yielding approximately 804 kJ per mole of H₂S processed.

The contrasting reaction enthalpies of the Claus and WSA processes, as depicted in Figure 2, result in a substantially higher energy output from a WSA plant compared to a Claus plant. By harnessing this exothermic energy released during the process in the form of high-pressure (HP) steam, refineries can significantly reduce their overall CO₂ footprint.

Sour crude advantage
Refineries processing sour crudes, characterised by higher sulphur content, stand to gain the most significant benefits from transitioning to the WSA process. Table 1 provides a breakdown of the theoretical energy recovery potential and associated CO₂ equivalent reductions between the Claus and WSA processes, assuming complete H2S conversion to either elemental sulphur or sulphuric acid, respectively.

While the theoretical energy yield of the acid route is substantial, practical limitations exist. Complete energy capture from H₂S conversion into sulphur or acid is unattainable. And the wholesale replacement of existing Claus plants with WSA units seems logistically and financially impractical.

Nevertheless, recovering even 90% of the energy released during sulphuric acid production presents a compelling argument for a gradual transition. Modern Claus processes necessitate the use of amine-based tail gas treatment unit (TGTU) regenerator reboilers, which consume a significant quantity of low-pressure (LP) steam. This LP steam consumption hinders the decarbonisation potential of the sulphur route as the LP steam produced within the Claus process is insufficient to meet the demands of the TGTU.

Replacing or revamping older Claus plants with new WSA units emerges as a strategically sound approach to reducing carbon emissions compared to constructing entirely new Claus units. A progressive shift from sulphur to acid production affords refineries valuable experience in acid handling and facilitates the establishment of robust long-term off-take agreements. As refineries incrementally increase acid gas allocation from energy-inefficient sulphur production to the more efficient acid production, the overall carbon footprint reduces.

Case study
To underscore the benefits of this approach, Topsoe conducted a case study comparing the environmental performance of the WSA process to a modified Claus process and amine-based TGTU configuration. Both systems were designed to achieve 99.9% sulphur recovery (see Figure 3).

The case study focused on greenfield oil refinery SRUs processing 90 mol% H₂S amine acid gas and sour water stripper (SWS) off-gas, producing 270 tons per day (TPD) of sulphur and 800 TPD of sulphuric acid. The analysis compared direct CO₂ emissions from flue gas and indirect CO₂ emissions associated with utility production and consumption.

The utility consumption and production data for the Claus SRU and the WSA SRU exhibit significant disparities attributable to distinct process designs and equipment configurations. A detailed examination of these differences follows:

· Electric power: The WSA process has a higher electrical consumption than the Claus process, mainly because of the higher gas flow rate and the use of an air-cooled acid condenser. The volumetric gas flow rates in Claus and WSA processes diverge primarily due to combustion methods. The Claus process employs partial oxidation, resulting in a specific gas volume. In contrast, the WSA process utilises complete combustion, leading to higher process gas volumes.