Air today, gone tomorrow

The metalfinishing industry uses a wide range of materials and processes to clean, etch, and plate metallic and non-metallic surfaces to provide desired surface properties. The materials include solvents and surfactants for cleaning, acids and bases for etching, and solutions of metal salts and other compounds to plate a finish on to a workpiece. The latest Process Guidance notes for industries using solvents will supersede those on metal coating in place since 1997. They provide statutory guidance on Local Air Pollution Control (LAPC) and Local Air Pollution Prevention and Control (LAPPC) for processes where over five tonnes of solvent is used in the coating process per year.

The main changes are that they incorporate requirements from the Solvent Emissions Directive (SED) and there are stricter requirements for those activities using potentially more harmful VOCs in Risk phases R45, R46, R49 R60, R61 and R40, the halogenated VOCs. Existing installations have until 2005 or 2007 to comply with the regulations but new abatement plant is required to meet the Solvent Emission Directive immediately. In addition, certain VOCs assigned certain risk phases are required to be controlled and, in specific circumstances, replaced with less harmful substances. Where an installation undergoes a substantive change it must also immediately meet the SED. For a small installation, one using between five and 15 tonnes of solvent a year, a substantive change is regarded as a 25 per cent increase in VOC emissions. For larger organisations, using over 15 tonnes, a change of more than 10 per cent is regarded as substantial.

The Guidance Notes cover all metalfinishing processes with VOC emission potential including transfer of VOCs, makeup, pre-treatment of surfaces, application of coating, spraying, dipping, curing, drying and stoving, and final finishing such as sanding or fault repair. For VOCs two compliance methods are available, the use of the ‘reduction scheme’ or compliance with ’emission and fugitive limits’.

In practice, much of the metalfinishing industry should look to use both compliance methods. While water-based coatings are available in certain applications they compromise the quality of the finish. So this is often complemented by ensuring emission limits are met using an efficient oxidiser that can destroy all VOCs cost-effectively. In section six of the Guidance note an efficient oxidation plant is listed as a control technique under Best Available Techniques (BAT). However, while BAT for one sector is usually the same for another, it is important to take into account the size, configuration, locality and individual characteristics of an operation and its relationship to the rest of the site.

Long term costs

While it is important to achieve high VOC destruction efficiency, an equally important consideration is the long term operating costs of the oxidiser. A well designed Regenerative Thermal Oxidiser (RTO) saves money by maximising the amount of primary heat recovery possible while also taking advantage of opportunities for secondary heat recovery, based on its location within the plant and the demand for steam, hot oil or hot water.

The destruction of the VOC in an RTO is achieved by heating the incoming process air to over 800oC, and retaining this temperature as the process air passes through the oxidiser’s retention chamber. An RTO can cope with a wide range of exhaust volumes and operate with high thermal efficiencies, minimising primary fuel requirements. These high thermal efficiencies are achieved using ceramic bed heat exchangers that operate on a cyclic basis. Heat from the hot exhaust gases leaving the oxidiser is recovered and used to preheat the incoming process air. This process is called primary heat recovery. Primary heat recovery reduces the fuel needed to power the burners that maintain the required 800oC operating temperature for efficient VOC destruction. This in turn impacts directly on cost and your obligations under the Climate Change Levy.

Heat recovery

The optimum primary heat recovery is determined by the concentration of VOC in the process stream and understanding how to optimise the RTO. Achieving this optimum allows the oxidiser to be operated auto-thermally. When an oxidiser operates in this mode it becomes self-sustaining with respect to the fuel required to maintain operating temperatures.

Where companies are finding that they are expanding or increasing the use of solvents in response to increasing orders, some companies forget that there is an opportunity to modify their current oxidiser so that it meets legislative requirements. VOC abatement and odour control equipment needs to be integrated into your process and optimised to allow you to reap savings from reduced operating costs.

Moving and optimising an oxidiser’s performance is not inexpensive but is cheaper than building and installing a new oxidiser. While simply moving the oxidiser without modifying it, where modification is necessary, might seem like an even cheaper option this will prove expensive in the long term. We estimate that the average payback on optimising an oxidiser is short as high running costs associated with fuel consumption are predictable.

With tightening legislation the trend for companies to look at optimising their oxidisers, particularly following changes in production capacity, is increasing. There are cost-effective modifications that can be made to improve an oxidiser’s operating efficiency in 2003, better serve your current production environment, and pre-empt forthcoming legislation.