4.0 Boiler Operation
One of the advantages of a waste to energy process over a strict incineration process is the ability to generate electricity. Steam produced in the boiler is the focal point of electricity generation.
4.1 Demineralizer
The source of the boiler feedwater is the demineralizer unit. A demineralizer utilizes ion exhcange to eliminate hardness ions (Calcium and Magnesium)and inorganic salts such as NaCl and NaSO4. The demineralized water must contain little contamination prior to entering the boiler unit to prevent the formation of silica and iron oxide vapor in the steam leaving the boiler unit.The boiler feedwater undergoes heat exchange with the flue gas stream in the economizer unit
4.2 Deaeration
The presence of dissolved gases such as oxygen,ammmonia and carbon monoxide in the boiler feedwater can cause corrosion in the boiler drum. The concentration of the dissolved gases is directly related to their partial pressure. The greater the pressure exerted on a gas-liquid system at constant temperature, the greater concentration of dissolved gases in the liquid. The temperature also can impact the ability of the gases to dissolve into the liquid. If the temperature is raised, the concentration of dissolved gases in the liquid will decrease. At the boiling point, the dissolved gases will be at their minimum concentration.This statement is expressed as Henry's Law.
Pressure Deaeration is a process which utilizes both the effect of pressure and temperature on the solubility of dissolved gases. Steam is sent through the boiler feedwater stream to raise the boiler feedwater temperature to within a few degrees of the boiling point. The steam contributes a neglegible partial pressure which at temperatures near the boiler feedwater's boiling point minimizes the solublity of the dissolved gases. There will be approximately 2% percent dissolved oxygen remaining in the boiler feedwater after deareation is complete. To further remove the remaining dissolved oxygen chemical oxygen scavengers such as Hydrazine,Volatile Organic Compounds or Sodium Sulfite need to be added to the solution.
Go with the flow!