This resource describes Flue Gas Desulfurization, commonly referred to as “FGD” — the set of processes and types of equipment used to remove containments from the waste gas of any fossil-fueled combustion process.
Overview
This lesson includes the following topics:
What is Flue Gas Desulfurization, and where is it applied
Where does FGD fit in a power plant?
Global FGD equipment investments that include expansions and retrofits
Background on fossil fuel emissions and flue gas legislation
The two primary types of FGD absorber technologies, including:
Wet scrubbing and
Dry scrubbing
The key gas-handling equipment in an FGD unit, including:
Ducting,
The stack and the
Gas-to-gas heater.
The FGD process.
What is Flue Gas Desulfurization?
Flue gas or “waste gas” as it is commonly known, is the exhaust by-product of fossil fuel combustion.
The “flue” is the stack, pipe or chimney that carries and delivers this gas from a manufacturing or power plant into the atmosphere.
This is different to the cooling towers which only discharge steam.
Flue Gas Desulfurization, or FGD, units have very large scale pieces of equipment, to remove Sulfur Dioxide, from the flue gas.
FGD is used in industrial facilities that burn lots of coal or oil, or process sulfur containing minerals, including:
- power plants,
- smelters and
- cement plants.
The primary focus of this module is coal-fired power plants, because they currently emit the most contaminants of any fossil-fueled facility.
FGD fit in a Power Plant
As we get started, let’s quickly review how a coal-fired power plant works, which is covered in more detail in a separate module.
Briefly, electricity is generated by burning coal in a boiler to produce high pressure steam.
The steam then flows into a turbine.
As the steam in the turbine expands, the power turns a generator to create electricity, which is then sent to the grid and distributed to end-users.
The FGD unit is positioned downstream of the boiler waste-gas stream.
Global FGD Investments
The biggest market for Flue Gas Desulfurization equipment is coal-fired power plants.
As of 2016, the International Energy Agency reports that there are approximately 62,000 thermal power plants in 115 countries, producing 80% of the global kilowatt capacity.
The balance is made up by nuclear power and renewables.
Even though the number of coal plants has reduced in the last 10 years, they are still a substantial provider of electric power.
As you have seen in the overview module, China has the largest number of coal-fired power plants with numerous FGD installations in the power, cement and steel industries.
China stopped new coal plant construction in 2017.
However in late 2018, certain China regions decided to restart some idle coal plants because of continued power growth in their industrial sectors.
How long this will continue depends on China’s national response to climate change initiatives and investment and growth in renewables.
There is also FGD investment in India, Indonesia, and Vietnam, as they continue to build new coal-fired facilities.
Retrofits and Maintenance
In more mature markets, such as the US and Europe, the number of coal-fired power plants is reducing. They are being replaced by smaller, more flexible gas-fired plants to complement the move to renewables.
However, there is still ongoing maintenance, utility expansions and retrofit FGD projects underway.
A retrofit is the term for FGD equipment added after a plant is built, to allow an operator to meet new air pollution targets.
Many coal-fired power plants are continually being expanded.
As such, there is often insufficient room in what is called the onsite facilities, so the placement of the FGD absorber is not optimal.
From an equipment standpoint, in both retrofits and expansions, the absorbers are the same size; the gas-to-gas heater is the same size, but the amount of ducting needed goes up significantly.
Fossil Fuel Emissions
Roughly two-thirds of all Sulfur Dioxide and one-quarter of all Nitrogen Oxide emissions in our atmosphere comes from fossil-fueled power plants.
Particulate matter emissions exist as well.
They are composed of small solid particles from combustion, often called fly ash, which can give flue gas a smoky appearance
In humans, exposure to Sulfur Dioxide and Nitrogen Oxide can lead to:
- Severe respiratory issues,
- Damage to the immune and reproductive systems, and
- Certain forms of cancer.
Additionally, Sulfur Dioxide emissions produce “Acid Rain” which can severely damage:
- Forests and vegetation,
- The soil, and
- Drinking water sources.
Flue Gas Legislation
Air pollution is a global problem and there have been numerous legislative efforts to reduce toxic emissions, especially from power plants.
The US Clean Air Act of 1970 set up the Environmental Protection Agency, or EPA, which has been controlling Sulfur Dioxide and other emissions from power plants and industrial facilities for almost 50 years.
Many other countries adopted the EPA legislative design, as they began to control their own industrial emissions.
In 1997, the Kyoto Protocol shifted emphasis. It mandates binding emission reduction targets on six greenhouse gasses, including Nitrogen Oxide for its country members.
The Kyoto Protocol was not popular because of monetary penalties. In 2018, it was enhanced by a 20-nation agreement to a more voluntary greenhouse gas emissions program, called the Paris Accord.
FGD Technologies
Let’s now get into specifics on the various types of Flue Gas Desulfurization technologies used.
Flue Gas Desulfurization is the series of processes and equipment by which harmful levels of Sulfur Dioxide are removed from the waste gas stream – prior to it being exhausted to the atmosphere.
Another term used by utilities is an Environmental Control System or ECS, the equipment and processes designed to reduce all toxic emissions.
At the power plant, note that the FGD unit may only be a part of a wider ECS initiative.
Flue Gas Desulphurization is the most important technology available to continue to make coal-fired power generation acceptable to the environment.
The Carbon Dioxide emissions from coal-fired power plants are another story.
The Absorber or Scrubber
The heart of any FGD treatment system is the Absorber or Scrubber.
It is the piece of equipment where chemical reactions are used to neutralize and remove Sulfur Dioxide from the flue gas.
FGD treatment systems can vary depending on the technology of the contractor.
However, the key difference in a FGD facility is the liquid used to treat the dirty gas.
- Wet Scrubbing uses a limestone slurry or seawater process and,
- Dry Scrubbing atomizes concentrated limestone slurry directly into the flue gas and removes the resulting Calcium Sulphate via a fabric filter.
Wet Scrubbers
Today the Sulfur Dioxide removal efficiencies greater than 95% are achieved by wet scrubbers.
The newer designs for dry scrubbers are capable of achieving efficiencies in the order of 90%.
The treatment system, whether wet or dry, determines what equipment will be used and the order of the process.
Let’s start our discussion with wet absorbers and there are two types:
- Limestone Slurry and
- Seawater
Limestone units have unique equipment just to manage the slurry, such as:
- A variety of large lined tanks and filters,
- A powder storage area,
- Tanks for make-up water, and
- A recirculation tank for future use of the used slurry.
Seawater absorber equipment is different. Seawater is just simply not as effective as the limestone slurry.
With seawater absorbers, a large volume of seawater is needed to get the same neutralizing effect.
Seawater absorbers, are mostly built of concrete rather than steel – just to make them bigger and thereby more cost-effective.
Finally, large settling ponds are needed to allow the seawater to return to normal before being discharged back into the sea.
Dry Scrubbing
Dry Scrubbing is also known as Spray Dry Absorption.
The dry chemicals used are some form of highly concentrated slurries of powdered limestone.
The process steps are slightly different versus wet scrubbing:
- The waste gas passes through a series of combustion chambers.
- Then the waste gas is sent to the Spray Dry Absorber. Here a vaporized lime reagent is injected directly into the gas stream to remove the Sulfur Dioxide.
- The toxic particulates are collected in Bag Filters, located in what is called the Baghouse.
Note that there are an increasing number of dry FGD processes coming to the market, such as the NID process from GE.
These are quite different and less capital intensive.
In summary, the wet-scrubbing method is the most widely employed today. However, dry-scrubbing is gaining popularity due to:
- Its efficient design,
- Reduced cost to build and maintain, and
- Simplified process and operation
FGD Gas-Handling Equipment
We will now discuss three major pieces of equipment associated with moving the large quantities of flue gas efficiently in and out of the FGD unit, including:
- Ducting,
- the Stack and
- the Gas-to-Gas heater, or GGH unit.
Ducting
One way to look at ducting in an FGD unit is its similarity to the veins and arteries of your blood system.
The vein-like ducting would be the inlet to the unit and the inlet to the absorber.
Artery-like ducting is the outlet ducts.
Those:
- from the absorber
- to the stack, or in any
- by-pass ducting around the FGD unit.
Ducts in an FGD unit are huge and the amount of ducting can vary widely depending on site layout congestion.
FGD ducts may be square, rectangular or circular in profile and are generally constructed of steel.
Ducting is almost always lined after it is erected at the construction site – to minimize any flexing problems.
Stacks
Our next topic is the two types of stacks used in FGD units.
In what is called a dry stack, flue gas is reheated prior to entering the stack. A dry stack is typically made of metal, or concrete with a metal liner.
Reheating gives more mixing and better dispersion at higher altitudes of any remaining un-scrubbed pollutants – especially Nitrogen Oxide.
However, the reheat process is an expensive and ongoing operating cost.
Therefore, many FGD systems operate with what is called a wet stack.
Since there is no heating of the flue gas, acidic water droplets collect on the stack internals, with higher concentrations of pollutant emissions than in the dry stack design.
However, an increased wet stack height can accomplish the needed emissions dispersion.
Wet stack internals are similar to a dry stack, and they are sometimes located directly above the absorber outlet.
In a wet stack, the corrosion potential and pollution threat must be constantly monitored.
GGH Heat Exchangers
With the high cost of the reheat option in a dry stack, engineers have come up with innovative ways to manage the temperature throughout the FGD unit.
One of the most interesting is the gas-to-gas heater, often abbreviated as GGH.
A GGH is a regenerative heat exchanger.
Heat is transferred through a complex rotating unit – as opposed to a series of stationary shell-and-tube heat exchangers.
The GGH is used to cool the plant exhaust gas prior to entering the absorber, and reheat the clean gas for better emissions dispersal, as previously discussed
Where a GGH is not fitted, the plant exhaust gas is cooled using what is called a quencher.
Additionally, reheating of the clean gas is done directly by a fan-assisted heater prior to discharge, although this is now a less common method.
The FGD Process
Let’s now take a closer look at what occurs inside the FGD unit.
Start by picturing very large volumes of waste gas from a power plant boiler. Then note that any environmental control facility has two primary phases:
First is Solids Removal, which includes:
Bag Filters that gather large particles from the waste gas stream, primarily any ash from combustion,
and/or
An Electrostatic Precipitator – a sophisticated filtration device that applies an electrostatic charge to the gas stream particulates , then collects them on an oppositely-charged plate.
The second and more complicated phase is Flue Gas Desulfurization that we have been discussing.
We will now use a limestone wet scrubber example.
Here, limestone slurry is used as the “reagent” to treat the waste gas stream inside the absorber.
A reagent is a substance or compound added to a process or system to cause a chemical reaction.
The basic Limestone Wet Scrubbing process is as follows:
- The waste gas passes through filters to remove particulates, and can bypass the unit if needed.
- A reagent is added to the gas stream near the boiler to first remove the nitrogen oxide.
- The waste gas is cooled by being routed through large heat exchangers, quenchers or a gas-to-gas heater.
- Then the waste gas is sent to the FGD absorber, where the limestone reagent, injected into the gas stream, removes the Sulfur Dioxide.
- Finally, the treated clean gas is routed to the stack, via the GGH (if installed), where it is safely expelled to the atmosphere.
Summary
The world continues to be keenly aware of the hazardous effects associated with toxic flue gas.
Numerous global and local legislative initiatives are underway to continue to improve air quality.
With operation of coal-fired power plants around the world for the foreseeable future – a proven technology to reduce Sulfur Dioxide emissions is Flue Gas Desulfurization or FGD.
From our discussion today, you should now understand that:
Coal-fired power plants currently emit the most contaminants of any fossil-fueled facility.
FGD equipment removes Sulfur Dioxide from the flue gas – but not the Carbon Dioxide.
FGD units can be installed during initial plant construction, or added later as a retrofit or as part of a plant expansion.
Numerous global legislative efforts exist to reduce toxic emissions, especially from power plants.
The FGD unit is positioned downstream of the boiler waste-gas stream.
The heart of any FGD treatment system is the absorber or scrubber.
The key difference in a wet FGD facility is the liquid used to treat the waste gas.
Wet, limestone slurry units are currently the most prevalent FGD method.
However, dry-scrubbing technologies are gaining popularity.
The amount of ducting in an FGD unit can vary widely.
FGD stacks can be either wet or dry.
The gas-to-gas heater helps manage temperature throughout the FGD unit.