Revamps 2024 Issue

From waste to clean energy: The acid path to reducing CO 2 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

T he refining sector faces a critical challenge: achieving significant reductions in greenhouse gas (GHG) emis - sions. This article delves into the wet gas sulphuric acid (WSA) process, an established technology for sulphur diox - ide (SO₂) emission reduction with the potential to enhance energy efficiency and reduce the carbon footprint of refin - eries. 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 foresee - able future. National goals necessitate ambitious reductions, with the US aiming for a 35% decrease in refining produc - tion 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 improv - ing the overall energy efficiency of both primary and aux - iliary 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 envi - ronmental 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 technol - ogy. 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

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Figure 1 Chemical and refining production emissions under net-zero scenarios for the US refining and chemical indus - try (Mt CO₂)

Revamps 2024