~Air Pollution~

-Introduction

-Pollutants
-Sulphur dioxide
-Nitrogen dioxide
-Carbon monoxide
-Ozone
-Particulates
-Lead

-API
-What's API?
-In Hong Kong
-Equipment
-API figures

-Photochemical Smog
-What is it?
-Problems
-Effects

-Acid Rain
-How it is formed?
-Effects

-Global warming
-Greenhouse effect
-Effects
-What can you do?

-Measures
-Part 1
-Part 2
-Part 3
-What can you do../1
-What can you do../2

Diagram showing how a electrostatic precipitator works

Coal always contains a proportion of ash which cannot be burnt. Tonnes of fly ash is generated each day by coal-fire power plants in Hong Kong. To clean the flue gas before its release into the atmosphere, the power plants are equipped with electrostatic precipitators. In electrostatic precipitation, dirty gases pass through a strong electric field, where the particulates become negatively charged. The charged particulates are then attracted and collected on plates of opposite charge. The cleaned flue gas is then discharged from the chimney.

Over 99% of the fly ash from the flue gas is extracted before emission. The fly ash collected is used for making construction materials such as cement, or for landfill purposes.

A dry electrostatic precipitator in the United States

Installation of in low nitrogen oxide burners power plants

The term "nitrogen oxides" (NOx) usually refers to nitrogen monoxide and nitrogen dioxide. They are formed during combustion of fuels in at least two ways:

1. The nitrogen in the combustion air reacts with oxygen at high temperature to form nitrogen monoxide. When released into the atmosphere, nitrogen monoxide reacts with oxygen to form nitrogen dioxide.
2. Fossil fuels may contain nitrogen compounds. When these fuels are burnt, the nitrogen content is oxidized. The nitrogen oxides formed in this way are known as fuel NOx.

Left: A low nitrogen oxide burner in the power plant
Right: A diagram showing how a low nitrogen oxide burner works

Nitrogen dioxide is one of the major pollutants in the air. The power plants in Hong Kong have been equipped with Low Nitrogen Oxide Burners to cut back emission of nitrogen oxides from existing boilers. The Low NOx Burner System utilizes combustion control method in which the operating conditions for combustion are modified to reduce NOx formation. The combustion control method makes use of the following principles to limit the formation of nitrogen oxides in the burners:

1. reducing the flame temperature in the combustion zone, and
2. reducing the availability of oxygen in the NOx formation zone.

Reductions in NOx of up to 50% have been demonstrated in the low NOx burner systems.