Atmosphere Maintenance Tips

Problems Caused by Peak Shaving

During a cold spell in the winter months, peak shaving is used to stretch the natural gas supply for the duration of the cold spell.  Continuous peak shaving may result in poor performance and soot formation in the retorts of atmosphere generator combustion chambers and coolers.

How is peak shaving affecting the generation of a protective atmosphere?

Most generators in operation operate on a fixed manual adjustment of the regulators and valves in the air and gas trains without any automatic analysis/control system.  This fixed adjustment relies on a constant composition of the fuel gas.  Normally the pressure and volume changes on the system do not affect the final composition of the product gas. The control of the air/gas ratio is based on a pneumatic balance between the air and gas valves, and the ratio regulator.  Thus, under normal circumstances, a normal consistent product gas composition is expected.

The problems from peak shaving come into focus in the main gas orifice. Properly adjusted and based on a given ratio, the orifice is set to pass a given volume of the natural gas.  As the peak shaving begins, the new fuel gas becomes heavier in content of hydrocarbons, higher in specific gravity and density; and the orifice now passes an incorrect amount of gas into the system.

The original fixed air/gas ratio is now out of balance.

What happens to the fuel gas during peak shaving?

The only obligation the utility company has is to provide the customer with a consistent heating value, most often, 1000 Btu per cubic foot.

When demands on fuel gas soar or supplies go down, the supply of natural gas is boosted by additives.  The additives may include propane, butane, air and many other hydrocarbons among the refinery derivative.

One very popular peak shaving mixture contains 75% natural gas and 25% propane/air mixture.

Generators may experience two types of problems from such a mixture:

• The user's plant may be so close to the supply point of the gas line network that the peak shave gas has not been mixed well enough before the gas enters the plant.  Thus, the incoming gas may vary from a high to a low in the amount of natural gas, propane/air mixture.
• The propane/air mixture itself is mixed in a separate mixing unit at a ratio of 54.9% propane to 45.1% air.  This ratio provides 1400 Btu per cubic foot.  If everything is right on control, the unit has about 0.1% too much air, which can easily be ignored as long as it is run at 10:1 ratio, however, there is a shortage of 3% air at 6:1 ratio.

Using this same peak shaving gas for an endothermic generator at 2.7:1 ratio, a loss of 8% air will result.  This will provide an endothermic product gas with a very low dewpoint and carbon formation in the catalyst.

The major problem is that we are not made aware of peak shaving early enough to make a correction. The second and much more severe problem is the propane/air mixing station. Wide swings on the percentage mixing are far too common.  The result is a process gas going rich and lean with the swings on the original propane/air mixture, even if we manage to correct the generator for peak shaving.

Adding to the overall mixing problem is the vapor point of propylene, which may legally be as high as 5% in HD-5 propane.  Propylene does not crack in the generator.  It only forms soot. It vaporizes more rapidly than propane.  Thus, the effect of propylene becomes more severe at peak shaving during very low temperatures.

The best strategy to avoid the peak shaving problem is to install analyzer/controllers onto the generator.

These monitor the dewpoint of the product gas and control the input air/gas ratio to prevent the dewpoint from going too low or high. Our advice to the customer who uses a minimum of analysis or none at all is to burnout the generator more frequently and replace the catalyst just before the season of peak shaving.

When peak shaving of natural gas takes place, more carbon is formed in the retort than usual. In such cases, the burnout should be started at half airflow for 15 minutes and then normal airflow should be used for the completion of the burnout

The latest design, materials and equipment specifications should be obtained from the company before any reliance is placed on the enclosed since changes may occur due to product improvement.