Sasol to Receive Carbon Credits for N2O Abatement Project

23 July 2007

Installation of the secondary catalyst for N2O abatement in the reactor. Click to enlarge.

Sasol has become the first company globally to register an abatement project using secondary catalyst technology to convert the greenhouse gas nitrous oxide (N₂O) into nitrogen and oxygen gases as a Clean Development Mechanism project under the Kyoto Protocol.

The Sasol Nitro N₂O abatement project for the reduction of emissions from its two nitric acid plants based at Sasolburg and Secunda in South Africa is the first of its kind.

Based in South Africa, Sasol Nitro is a division of Sasol Chemical Industries. Sasol Nitro produces ammonia, explosives and fertilizers. Sasol is a leading technology developer for and producer of synthetic fuels and chemicals from low-grade coal and natural gas.

Nitrous oxide—with a high Global Warming Potential (GWP) of 310—is formed during the catalytic oxidation of ammonia for the production of nitric acid. Over a suitable catalyst, a maximum 98% (typically 92-96%) of the fed ammonia is converted to nitric oxide (NO). The remainder participates in undesirable side reactions that lead to the production of nitrous oxide, among other compounds.

Waste N₂O from nitric acid production is typically released into the atmosphere, as it does not have any economic value or toxicity at typical emission levels.

The abatement project was developed with the assistance of specialist international suppliers MGM International and Heraeus. MGM International is a leader in the development of greenhouse gas emission reduction projects worldwide. Sasol Nitro has a long standing supply agreement with Heraeus, which provides catalyst technology for nitric acid production facilities.

The new catalyst—the sole purpose of which is the decomposition of N₂O—will sit below the oxidation gauzes in the reactor. This approach using a secondary catalyst has the following advantages, according to the partners:

• The catalyst does not consume electricity, steam, fuels or reducing agents (all sources of leakage) to eliminate N₂O emissions; thus, operating costs are negligible and the overall energy balance of the plant is not affected.

• Installation is relatively simple and does not require any new process unit or re-design of existing ones (reactor basket needs some modifications to accommodate the new catalyst).

• Installation can be done simultaneously with a primary gauze changeover; thus, the loss in production due to incremental down time will be limited.

• The approach offers a considerably lower capital cost when compared to other approaches.

The catalyst, developed by Heraeus, decomposes N₂O without affecting nitric acid production. The active element of the catalyst is a proprietary coat of precious metals (the same elements already present in the oxidation gauzes) deposited over ceramic pieces made of alfa-alumina of several shapes and sizes.

The catalyst has a very high activity for N₂O decomposition; in a typical medium pressure plant, a catalyst weight of 30 kg/m₂ of cross sectional surface is sufficient to reduce N₂O concentration well below 100 ppmv.

Theoretically, any given level of N₂O abatement can be reached, but in practice the abatement will be limited by practical and economical considerations. The Heraeus secondary catalyst is not designed as catchment for precious metals, so it does not adsorb any material lost from the primary gauzes.

To create space to insert the new catalyst, the team will remove layers of Raschig rings from the basket. The secondary catalyst will come inside a cage of the same material used for separating screens on the primary gauze pack to secure even distribution of the pieces over the cross section area of the reactor.

The outside rim of the caged catalyst is sealed with the reactor walls to avoid any by-passing of the N₂O-rich gases. Once the secondary catalyst is installed, the primary gauzes are placed on top of the basket, as usual. The secondary catalyst acts as support system for the primary gauze pack and both catalysts are in close contact.

Sasol Nitro commissioned its N₂O emission abatement technology during the first quarter of 2007. It expects to reduce greenhouse gas emissions equivalent to about a million tonnes of carbon dioxide a year (960,322 tonnes CO₂e).

Sasol expects the project to earn significant income through sales of the resulting carbon credits. A share of these carbon credits will be invested to benefit local community-based sustainable development projects.

MGM is currently working on several other N₂O abatement projects worldwide.

Resources:

• Sasol N₂O Abatement Project