8.4 Reducing Acid Rain
Reducing emissions of SO 2 and NO x is necessary if we are to reduce acid deposition.
The first attempts at reducing SO 2 took place in 1936 at the Battersea Plant in
London, England. In recent years, we have made considerable progress in finding
ways to reduce emissions of both SO 2 and NO x .
There are two principal methods for reducing sulfur emissions at power plants
and other facilities that burn coal or oil: fuel switching and scrubbing. Fuel switching
means replacing coal or oil, which contains more sulfur, by fuels such as natural gas,
which has little or no sulfur. Scrubbing means removing sulfur by electrostatic or
chemical (wet or dry scrubbing) means. Electrostatic involves placing electrically
charged plates, called electrostatic precipitators, inside the industry’s stack. These
attract the positively charged sulfur particles to the surface. The surface is periodically cleaned, removing the sulfur before it gets into the air. Wet scrubbing means
injecting water or a chemical solution into the exhaust gases. Dry scrubbing involves
a chemical such as lime, which reacts with the gases without the use of water. The
sulfur will react with the water or chemical and fall out. All types of scrubbing do
pose a problem; we must find an environmentally acceptable way of disposing of the
sulfur after we have removed it.
There are several methods of reducing NO x emissions. Some are mechanical:
changing the ratio of air to fuel or changing the temperature of the combustion. The
cooler the flame is, the less NO x the furnace gives off. Others are chemical: injecting
chemicals such as ammonia, which will react with the NO x and convert it back into
nitrogen and oxygen.
US EPA’s Acid Rain Program (ARP) has given the utility industry a reason to
reduce SO 2 and NO x emissions. This program was established by Title IV of the
1990 Clean Air Act Amendments. It set a cap on the amount of SO 2 power plants can
emit. The program also addressed NO x emissions, but only set maximum emission
rates based on the type of boiler [62].
The Acid Rain Program focused on power plants, the largest single source of SO 2
emissions and a major source of NO x emissions. The plants affected by the program
submitted permit applications explaining how they planned to comply with the
program. US EPA issues permits to each facility. The program also requires the
use of continuous emissions monitors (CEMs), which measure their emissions and
transmit the information directly to US EPA.
A unique element of the program is its use of emissions trading as a compliance
option. Although the national cap on emissions limits the total SO 2 released into the
air, companies may decide the most cost-effective method. Each plant is assigned a
number of “allowances,” based on their average annual SO 2 emissions during the
period from 1985 to 1987. These could be bought, traded, sold, or held. At the end of
the year, each facility has to surrender one allowance for each ton of SO 2 it emitted.
US EPA set up an allowance tracking system and also set up auctions and direct sales
to enable plants, which were not assigned allowances to obtain those they needed to
operate. In 1990, the 263 units designated as part of the Phase I program emitted 10.0
million tons of SO 2 . In 1995, the first year in which the units were required to comply
290
L. K. Wang et al.
Reducing emissions of SO 2 and NO x is necessary if we are to reduce acid deposition.
The first attempts at reducing SO 2 took place in 1936 at the Battersea Plant in
London, England. In recent years, we have made considerable progress in finding
ways to reduce emissions of both SO 2 and NO x .
There are two principal methods for reducing sulfur emissions at power plants
and other facilities that burn coal or oil: fuel switching and scrubbing. Fuel switching
means replacing coal or oil, which contains more sulfur, by fuels such as natural gas,
which has little or no sulfur. Scrubbing means removing sulfur by electrostatic or
chemical (wet or dry scrubbing) means. Electrostatic involves placing electrically
charged plates, called electrostatic precipitators, inside the industry’s stack. These
attract the positively charged sulfur particles to the surface. The surface is periodically cleaned, removing the sulfur before it gets into the air. Wet scrubbing means
injecting water or a chemical solution into the exhaust gases. Dry scrubbing involves
a chemical such as lime, which reacts with the gases without the use of water. The
sulfur will react with the water or chemical and fall out. All types of scrubbing do
pose a problem; we must find an environmentally acceptable way of disposing of the
sulfur after we have removed it.
There are several methods of reducing NO x emissions. Some are mechanical:
changing the ratio of air to fuel or changing the temperature of the combustion. The
cooler the flame is, the less NO x the furnace gives off. Others are chemical: injecting
chemicals such as ammonia, which will react with the NO x and convert it back into
nitrogen and oxygen.
US EPA’s Acid Rain Program (ARP) has given the utility industry a reason to
reduce SO 2 and NO x emissions. This program was established by Title IV of the
1990 Clean Air Act Amendments. It set a cap on the amount of SO 2 power plants can
emit. The program also addressed NO x emissions, but only set maximum emission
rates based on the type of boiler [62].
The Acid Rain Program focused on power plants, the largest single source of SO 2
emissions and a major source of NO x emissions. The plants affected by the program
submitted permit applications explaining how they planned to comply with the
program. US EPA issues permits to each facility. The program also requires the
use of continuous emissions monitors (CEMs), which measure their emissions and
transmit the information directly to US EPA.
A unique element of the program is its use of emissions trading as a compliance
option. Although the national cap on emissions limits the total SO 2 released into the
air, companies may decide the most cost-effective method. Each plant is assigned a
number of “allowances,” based on their average annual SO 2 emissions during the
period from 1985 to 1987. These could be bought, traded, sold, or held. At the end of
the year, each facility has to surrender one allowance for each ton of SO 2 it emitted.
US EPA set up an allowance tracking system and also set up auctions and direct sales
to enable plants, which were not assigned allowances to obtain those they needed to
operate. In 1990, the 263 units designated as part of the Phase I program emitted 10.0
million tons of SO 2 . In 1995, the first year in which the units were required to comply
290
L. K. Wang et al.