Variable Throat Cyclonic Scrubber at Don Pedro, Philippines

THERMAL ENERGY SYSTEMS

CADP's Don Pedro mill is close to the west coast of Luzon island, south of Manila. When the government introduced stack emissions regulations, the company was the first to implement them as part of its social responsibility programme. It immediately ordered a Thermal Energy Systems scrubber for its primary boiler, stipulating a maximum emission of 80 mg/Nm³, only two thirds of that permitted by the new regulation. A TES variable throat cyclonic scrubber was selected for the duty as it is able to achieve control over a wide range of operating conditions. The Client's engineer specified a dry ID fan so the stack is integral with the scrubber :

Emissions of 80 mg/Nm³ are at the limit of scrubber performance, any more stringent requirement almost certainly requiring an [expensive] electostatic precipitator to achieve. High peformance scrubbers must achieve full wetting of the particles in the gas stream and then separate out not only those particles but also dirty water droplets.

Our variable throat cyclonic scrubber provides both a pre-wetting spray and then full wetting in a turbulent spray zone. The turbulent zone is created by a vane which is adjusted to change the width of the zone as gas flow changes and hence maintain turbulence. As scrubber performance is proportional to the pressure drop across the zone, the vane is controlled by measuring draft. Clog-resistant spray heads are required to cope with the recycled spray water.

Primary separation is achieved by the cyclonic action as the wet gas stream enters the main scrubber chamber, most of the water and particles being thrown to the walls and running down to drain. The quiescent zone and an appropriate vortex breaker above allow suspended droplets to separate out before the gas exits through the stack.

The scrubber water, together with the grit transport water, is treated in a pair of settling ponds which are used alternately over a two week cycle. The one is drained and emptied using mobile equipment while the other is in use :

The water is treated with milk of lime using pH control to ensure that corrosion does not occur.