Heat Recovery&

CorrosionCorrosion

Heat-recovery equipment is vital to energy conservation Air pre-heaters and economizer, other means are improved burners and soot blowers for recovering low-level heat at faster paybacks

Potential improvements in energy efficiency for heat-recovery equipment are:

Equipment Operation Improvement Oil and gas burners

Promote flame conditions that result in complete combustion at lower levels of excess air

0.25% for each 1% decrease in excess oxygen

Air preheaters Transfer energy from stack gases to combustion air

2.5% per 55°C drop in stack gas temperature

Economizers Transfer energy from stack gases to feed water

1% for each 55°C increase in feed water temperature

Combustion control systems

Precisely regulate flow of fuel and air flow

0.25% for each 1% decrease in excess oxygen

Soot blowers Remove deposits that retard heat transfer from the external surfaces of boiler tubes

2.5% for each 55°C decline in stack gas temperature

While analyzing requirements for installation of a Economizer, or a similar heat recovery device, there are two important points that must be considered, namely: The sulfur content of the fuel being burned

The temperature of the feedwater at the inlet of the fuel economizer or heat recovery device or air in case of air pre-heater

If sulfur is present in the fuel, it will combine with oxygen to form sulfur dioxide (SO2) and sulfur trioxide (SO3)

Compounds of oxygen, nitrogen, calcium, chlorine, sodium, potassium and vanadium may also play a role sometimes in very complex relationships. 

This is particularly true of high-temperature corrosion, i.e., steam generator tubes operating at temperatures of 400ºC to 650ºC, in the presence of molten ash and oxides. 

If the temperature of the inlet feedwater is not maintained at proper levels, certain byproducts of the fuel being burned have the potential to cause low temperature corrosion.

The temperature of the flue gas in contact with these surfaces will be virtually the same as the temperature of the metal. The metal temperatures, in turn, will be within a few degrees of the feedwater flowing through the tubing.

Deposits (pH = 3) on Eco Tubes

In cold-end or low-temperature corrosion, i.e., at temperatures below acid dew point, H2SO4 formed from fuel sulphur is the primary agent although in some circumstances HCl and H2CO3 may play some role. 

Oxidation of SO2 by atomic oxygen is a major formation mechanism for SO3 SO2 + O SO3

When combined with superheated water vapor, sulfur trioxide forms sulfuric acid vapor (H2SO4).

50 to 100 ppm of SO3 can raise dew point by 50oC to 80oC.

Certain Facts about SO3 formation :Since, in the manufacture of H2SO4, ferric oxide (Fe2O3) was once used to catalyze SO2 to SO3, it was recognized that the large area of oxidized metal in a boiler might generate SO3 in the same way

The more fly ash, the less SO3 present, which suggests that the former absorbed the latterUnder sub-stoichiometric conditions no SO3 is formed, the more the excess air, more the percentage of SO3

Amount of SO3 formed depends more upon the availability of atomic oxygen than upon sulphur concentration

SO3 formation is depended on boiler load as well as excess air level

The more fly ash, the less SO3 present, which suggests that the former absorbed the latter

When using heavy oil, addition of MgO to the oil in a ratio of 1/1000 results in successful neutralization of the acid at any excess air

However, in all cases, SO3 concentration is very low when the excess oxygen is reduced below 2%.

Approximately 1% to 2% of the sulfur dioxide is further oxidized into sulfur trioxide Sulfur dioxide, on the other hand, will dissolve in any free moisture that may be present in the flue gas to form sulfurous acid (H2SO3) -- a powerful corrosive

It is the temperature at which water vapor condenses out of the flue gas, or the "water dew point," that determines when sulfurous acid formation takes place

The water dew point is a function of the partial water vapor pressure. Partial water vapor pressure is equal to moles of water in flue gas divided by total moles of flue gas times the total system pressure. The total system pressure will generally be 1 atm (=1.013 bar)Depending on the type of fuel being burned, the water dew point will range between 40oC and 65oC

Acid dew point on the other hand is in a range between 115oC (for < 0.2% S in fuel) and 140oC (for 3.5% S in fuel)

When sulfur dioxide (SO2) gases pass through the economizer, the minimum feedwater temperatures indicated will maintain the metal surfaces of the heating elements well above the water dewpoint and eliminate the possibility of sulfurous acid (H2SO3) corrosion

This also explains the acid shower at the APH particularly in winter. Steam Coil Air Pre-heaters are quite commonly used to avoid APH corrosion

A good dew point calculator is available at http://www.permapure.com/newweb/Sulfuric%20Acid.htm