ForumRe: Reasons behind oxygen injection in feedwater circuit for supercritical boiler?
From: Jim Watts [764]
Date: Friday, July 30, 2010
Time: 9:22:57 AM
Remote Name: 121.217.162.165
Comments
It is certainly true that removal of Oxygen from boiler feed water has been the conventional treatment of boilers for many years with unarguable benefits. It will also remain the case for the majority of boilers.The problem with oxygen removal has only raised its head as steam temperatures have become very high in particular with supercritical. When oxygen is removed by a scavenger such as hydrazine the trace oxygen remaining oxidises steel to magnetite the black scale normally present in most boilers. This is normally considered a protective scale as it slows further attack on metal surfaces. Unfortunately it does not stop completely however and magnetite layers continue to grow eventually needing acid cleaning. In the case of supercritical temperatures the magnetite growth is quite fast and due to the cycling operation of boilers the temperature shocks cause the scale to shed into the bottom bends of tubes or flow on to the turbine.
Experiments carried out mostly in Japan found that it was better to provide sufficient oxyen to the corrosion process to allow the scale to form as Heamatite instead of magntite. This scale is much thinner and stronger and does not cause as many outages or turbine damage as magnetite scale. The only problem is it must be very carefully controlled by providing just enough oxygn and no more or less.
Further info at http://tinyurl.com/2begu6r
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