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Gas-gas heaters
Howden supplied some of the world’s first FGD systems in 1934, and since then we have become the global supplier of gas-gas heaters. Read more about the benefits and features of the gas-gas heater below.


The Gas Gas Heater in a flue gas desulphurisation (FGD) system, used to remove sulphur dioxide from the flue gases, is a lower temperature version of the rotary regenerative heat exchanger.

Key benefits

Capital investment.
  There had been a tendency for FGD plants to be constructed without a gas gas heater to reduce capital investment.  The environmental considerations of a visible wet plume with insufficient buoyancy, coupled to the considerable additional maintenance costs on the wet stack, have reversed this trend.
 
Low leakage level without drift.  Most new gas gas heaters can achieve leakage levels that are close to their design values after initial commissioning or when set up correctly after an outage.  However, after a period of operation of up to 2 or 3 years, the leakage can increase, causing increased SOx emissions.  The Howden VN sealing concept maintains low design leakage between outages.  Active sealing, (purge and scavenge) systems provide extremely low leakage levels.

Extensive choice of heat transfer elements.  Howden have designed and developed an extensive range of heat transfer elements over the years including profiles which are amenable to vitreous enamelling.  Elements are selected to optimise performance, pressure drop and cleanability.

Low maintenance.  Conceptual and detailed design targets the achievement of simplicity and minimal maintenance. Fixed sealing surfaces and centre drive greatly reduces maintenance by removing actuators and pin racks.
 
High availability.  The major cause of reduced availability in gas gas heaters is due to element fouling.  Careful element selection to suit the application and the application of the correct cleaning system maintains very high availability.

Centre column or bottom girder support.  Custom designed and optimised to suit arrangement and client preference.


Standard Features

The Howden VN sealing concept maintains low design leakage between outages. Active sealing, (purge and scavenge) systems provide extremely low leakage levels.
 

Howden has designed and developed an extensive  range of heat transfer elements over the years including profiles which are amenable to vitreous enamelling. Elements are selected to optimise performance, pressure drop and cleanability.



Detailed specifications

Sealing system has evolved to have sealing surfaces formed and set to achieve minimal gaps with seals at operating conditions combined with labyrinth multiple seals.  Fixed sealing surfaces are a major benefit in such a harsh corrosive environment and enable the flake glass lining to be extended to protect the structure.

Flake Glass Lining
Provides protection to the centre section structure, housing and transition ducts.

Purge and Scavenge active sealing systems.  A purge system can be used to greatly reduce direct leakage from untreated gas to treated gas by pressurising the radial seals at the sector sealing plates with treated gas. A scavenge system can also be used to greatly reduce the untreated gas entrained within the elements by blowing treated gas into the rotor just before the sector sealing plates.  The combination of the two active leakage systems can almost eliminate untreated top treated gas leakage.
 
Seal air system
A sealing air fan is provided to pressurise seals at gas heater penetrations with moving parts – rotor shaft seals and cleaning devices.
 
Centre drive system with inverter speed control
The shaft mounted centre drive employs a high ratio gearbox with the option of multiple electric motors and an air motor.  The drive system is completely removed from inside the extremely corrosive gas gas heater environment and doesn’t require the time consuming installation and outage replacement of pin racks.  Pin racks also have the disadvantage of making the axial seal ineffective.  The inverter provides a very soft controlled start of the largest rotors and enables speed reduction for water washing.
 
Standard spherical roller bearings with bath lubrication
Standard proprietary rolling element bearings with bath lubrication using extremely high viscosity oil has been used in Howden air preheaters in an identical application for over fifty years.
  
Cleaning system
Fully retractable multi fluid (air/steam and LP & HP water) sootblowers.  The lances are fully retracted from inside the gas gas heater when they are not operating in a cleaning cycle, thereby protecting them from the corrosive environment.  HP water washing is carried out on load from the hot and cold ends.

Applications

Flue Gas Desulphurisation  (FGD)

 

The Gas Gas Heater in a FGD system, used to remove sulphur dioxide from the flue gases, is a lower temperature version of the rotary regenerative heat exchanger. It removes heat from the untreated flue gas prior to it entering the FGD absorber and transfers it to the cold, saturated treated flue gases which exit from the FGD absorber at around 45°C. The treated gas is reheated to around 80°C to provide sufficient plume buoyancy at the stack exit. Over 280 GGHs have been supplied since 1982.

 

Selective Catalytic Reduction (SCR)

 

In some stations, there is no space available at the economizer outlet to fit the SCR catalyst.  One option is then to install the SCR plant after the flue gas cleaning equipment.

Tail end SCR where the SCR plant is positioned just prior to the chimney after the electrostatic precipitator and ID fans and is used to reduce the amount of gas burn in this application.

  
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