One of the key features of the planned new power station extension at Delimara is the use, for the first time by Malta’s power generation sector, of a so-called Selective Catalytic Reduction de-NOx system (SCR/DeNOx) meant for flue gas denitrification.

It is inevitable for noxious nitrogen oxides to form during the high temperature combustion of fossil fuels in conventional power stations. Heavy oil burning occurs in air which is basically a mixture of nitrogen and oxygen. Despite the relatively inert nature of nitrogen gas, at elevated temperatures nitrogen and oxygen in an air/fuel mixture react together forming nitric oxide which, in the absence of adequate abatement equipment installed within the plant, is ultimately released into the atmosphere oxidising further to produce acidic nitrogen dioxide.

Nitrogen oxide levels in various urban areas around Malta are already a matter of concern, the main culprit being vehicular emissions. Most modern cars are usually equipped with catalytic convertors meant to cut down on these emissions. The problem with emissions from road transport is not just about high traffic density, it is also about Malta’s vehicle fleet being relatively old and, in many cases, poorly maintained.

The environmental impact statement prepared for the proposed Delimara plant extension refers to the installation of SCR/DeNOx equipment to ensure nitrogen oxide emission releases are kept in check as required under EU law given the requirements of two important directives: Directive 2001/80/EC, the Large Combustion Plants Directive, coupled with Directive 2001/81/EC, the National Emissions Ceilings Directive (NECD), which sets an annual ceiling on the maximum amount of nitrogen oxide, among other pollutants, that can be released to the environment from local sources.

Under the NECD, Malta’s nitrogen oxide emissions load is capped at 8,000 tonnes not to be exceeded by 2010.

As with vehicles, the cutting down on emissions from power plants also serves to ensure Malta abides by the ambient air limit values as prescribed under Directive 1999/30/EC. This directive is to be superseded by the legal architecture of the recently adopted Directive 2008/50/EC on ambient air quality and cleaner air for Europe (the CAFE Directive).

In view of the need to ensure optimal emissions performance and using the best available technology, industrial plants such the present and future power plant at Delimara are regulated under Directive 2008/1/EC on integrated pollution prevention and control. This legislative set-up is under review with a major overhaul in the pipeline through the eventual adoption of the Industrial Emissions Directive, as already approved by the European Parliament last July 7.

SCR/DeNOx technology is all about a chemical process. A substance described as aqueous urea is reacted at high temperatures with nitrogen oxides produced in the plant, converting these into innocuous nitrogen gas and water vapour that are finally released from the plant. The aqueous urea serves as a source of ammonia, a pungent smelling gas, with which the nitrogen oxides react. This happens over a catalyst system designed to speed up chemical reaction.

Enemalta’s electricity generation plan 2006-2015 (EGP) estimates are that a variable amount of between 330 and 650 tonnes of ammonia per 100MWe may be emitted annually into the environment as a result of a deNOx system operating at Delimara. Unless adequately contained, this may effectively result into a minor, or major, inconvenience as a result of the typical pungent smell of ammonia. The EGP highlights the fact that an “ammonia slip” on such a scale may even affect Malta’s compliance with Directive 2001/81/EC, the NECD, especially in a more stringent post-2010 scenario.

The EGP, published in June 2006, had also tackled the issue of pollution abatement costs. For dust and nitrogen oxide emissions alone initial capital expenditure was estimated at €11.7 million envisaged to increase by an additional €5.8 million for enhanced nitrogen oxide removal plus a further annual running cost in the order of €2.8 million, clearly worth every cent on health and environmental considerations. However, the competent authorities may still need to reassure that any prospective de-NOx plant at Delimara will not just be state of the art; it must also operate and be regularly maintained state of the art!

Basic research about de-NOx claims that modern systems have conversion rates that are typically quite high, 95 per cent or more, implying that most of the nitrogen oxide gases are in fact eliminated. Some authors give a more modest conversion rate into nitrogen within the 80 – 90 per cent range. There is a technical challenge with regard to the composition ratio of the de-NOx catalysts, essentially a mixture of tungsten and vanadium oxides, in view of the fact that the system may tend to favour the formation of sulphur trioxide in the plant, which is definitely undesirable. In the end, however, this may not be much of an issue as long as the right technology is employed and at the right abatement costs.

In the evolution of Malta’s power generation sector we should witness the investment in modern pollution abatement technology as top priority on the agenda. However, this does not automatically imply this technology is trouble-free or, as some might have it, pollution-free in its own right. It is therefore in the public interest that the competent authorities leave no stone unturned in the sensible management of an enhanced Delimara power station now and as the plant develops in the years to come.

sapulis@gmail.com

The author specialises in environmental management.

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