Abstract

Fossil fuel based power plants normally uses flue gas stack for discharging combustion products to atmosphere. The flue gas contains harmful pollutants like SOx, NOx etc which will be diluted with ambient air on discharge at varied level based on atmospheric conditions. The height of these stacks would vary according to the power plant capacity and the dispersion level required for SOx, NOx etc to meet the prevailing ambient air quality (AAQ) norms applicable for the plant.

The flue gas can alternatively be discharged through natural draught cooling towers (NDCT), if the power plant is implemented with NDCT. Such installations are being common in Europe. However, there is no such installations present in India until now.

This paper analyses different factors associated with flue gas discharge through NDCT, key advantages, the concerns to be addressed for adapting the same etc with a case study pertaining to a typical for Indian coal based project.

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Intro

The exhaust gas (flue gas) from fossil fuel fired power plants is a potential pollutant which

causes degradation of ambient air quality if not discharged properly. It is normally vented out

to a safe height so as to limit the ground level concentration (GLC) of toxic gases within

allowable limits as per National ambient air quality norms. The GLC depend on a number of

factors like height of venting arrangement, flue gas analysis, flue gas velocity, temperature

and density of flue gas, prevailing wind velocity, relative humidity of air etc. The conveyance

of flue gas from boiler to a safe height in the atmosphere can be done through a dedicated

flue gas stack or using the natural draft cooling tower in the power plant.

In India, the flue gas discharge to atmosphere is normally done through a flue gas stack. The

existing environmental regulation by Central Pollution Control Board (CPCB) on flue gas

discharge from a power plant insist for a fixed height for the flue gas stack based on the

capacity of power plant, besides meeting the GLC of toxic gases as per ambient air quality

norms.

Many of the recently commissioned power plants in Europe have used the natural draft

cooling tower for venting out the flue gas from boiler ( instead of using a dedicated flue gas

stack ). The flue gas discharge pipe is centrally paced inside the NDCT which was originally

designed for cooling the circulating condenser cooling water. This arrangement is reported to

be meeting the GLC norms with better uniform dispersion of flue gas in the atmosphere.

However the NDCTs used for flue gas discharge has lesser height compared to dedicated

flue gas stack used for discharging same quantity of flue gas. There are few niche

advantages associated in using NDCTs for flue gas discharge like the benefit of lower

construction time and risk, lesser capital cost, space reduction etc.