HVLC Systems
Non-condensable gases, or NCGs, are a by-product of the kraft pulping process. NCGs are toxic, foul-smelling and potentially explosive. In the distant past, these gases were largely discharged directly to the atmosphere. This is no longer allowed. The purpose of an NCG system is to collect, condition, and safely deliver these gases to a location within the mill where they are disposed of by incineration. This module will focus primarily on the DNCG type collection system. Dilute systems are also referred to as high volume low concentration (HVLC) systems.
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Learning Objectives
• State the purpose of an HVLC system
• Describe the makeup of a typical dilute non- condensable gas stream
• Differentiate between the two methods used to safely convey NCGs
• Identify and describe the major equipment and components of an HVLC system
• List the conditions needed for incineration of NCGs
• List the possible NCG incineration locations within a mill
• Identify and describe safety hazards and safety design features of an HVLC system
Specs
| Course Level | Intermediate |
| Languages | English |
| Compatibility | Audio, Video |
| Based on: | Industry Standards and Best Practices |
Key Questions
What is the lower explosive limit (LEL) of a flammable gas and air mixture?
The lower explosive limit is the gas to air ratio below which the mixture will not burn.
What are NCGs?
NCGs are Non-Condensable gases. They are compounds which have boiling points above the temperature of most mill water cooling systems.
How are the gases from HVLC systems typically disposed of?
HVLC gases are usually incinerated in a boiler or kiln within a mill.
What is flame speed?
Flame speed is the rate at which a front will propagate through a flammable mixture.
What is the primary technique used to keep HVLC systems safe?
Gas mixtures in an HVLC system are kept below the lower explosive limits.
Sample Video Transcript
Before the collected NCGs flow through the fan, they usually flow through a cooler and mist eliminator. The purpose of the NCG condenser is to remove water and condensable organics from the NCGs. This reduces the volume of gas going through the fan and onto the incineration point. This leads to more consistent burning. Removing water from the gas stream also makes it less corrosive to downstream equipment. The condenser is typically a shell and tube heat exchanger with the NCGs on the tube side. As the gases inside the tubes cool, water vapor in the NCGs condenses out as liquid water. That condensate is sent to a foul condensate collection tank.
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