Page 12 - North American Clean Energy January/February 2020 Issue

wind power
The Ultimate
Recycling
Process
Regenerating transformer oil so it’s good as new
by Frank May
From their invention in the late 19th century to present day, transformers devices have become essential to everyday life. A transformer “transforms” voltage; they’re all over your house, inside the black plastic cases you plug into the wall to make your electronics work properly, and all over your neighborhood, atop utility poles to run the grid.
In wind turbines, transformers connect the generator to the distribution device.
Over the 25 years of their normal life span, a vital step of transformer maintenance is the collection and disposal of the transformer oil. This oil insulates energized components, dissipates heat, and protects the paper insulation from moisture.
Hazards of transformer oil
Used transformer oil contains 38 chemicals with carcinogenic and mutagenic effects, including benzopyrene, PCB, dioxin, and furan. The Stockholm convention included two of these - PCBs and dioxins - on the list of the twelve most hazardous contaminants, and especially harmful to groundwater; under no circumstance should waste oil be allowed into the environment.
Recycling of used oil
Incineration is one of the most common ways to recycle oil. Oil can be added to liquid fuel and used for combustion, which is converted into heat. The biggest problem with this method is that hazardous contaminants, such as lead and sulfur, are released into the atmosphere. This means the oil should be prepared before burning, to minimize the negative environmental impact.
It’s also possible (in theory) to use waste oil directly as fuel. However, since it doesn’t meet fuel quality standards, the contaminants may damage fuel systems.
Some recycling methods can make oil products from waste oil, including thermal cracking, catalytic hydration, and vacuum distillation. But these require complex equipment; investing in the machinery is only viable for large facilities dedicated to oil recycling.
A rarely-used biological method involves bacteria that consumes hydrocarbons. The resulting inactive microorganisms can serve as fertilizer in parks and flower beds.
Figure 1. Transformer oil before and after regeneration
Transformer oil regeneration process
Oil regeneration can significantly reduce the amount of waste, decrease environmental footprint, increase oil service life, improve transformer reliability, and reduce waste oil replacement costs. Regeneration is a process of removing contaminants and products of aging from transformer oil, stabilizing the oil with additives, and restoring its quality so it can be re-used in electrical equipment.
Components that must be removed before the oil can be reused include: • solid particles;
• water (emulsified and solved);
• air and gaseous products of oil decomposition;
• asphalt and resins;
• acids;
• sludge;
• metal organic compounds (soap); • polyaromatic compounds;
• unsaturated hydrocarbons;
• remaining products of additive decomposition;
• other products of oil aging.
In order to remove all of these contaminants, regeneration must occur in stages.
Although the terms “purification” and “regeneration” are often used interchangeably, each process leads to different results. Purification is a stage of the regeneration process - it cannot restore the oil to its original quality on its own.
Deciding on the method of regeneration depends on economic and environmental considerations. The chosen process must not only efficiently restore the properties of the oil, but also generate minimum waste
This can be achieved by combining several processes in one machine:
1. preliminary filtration to remove particulate matter;
2. adsorption to remove acidic compounds and aging products from the oil;
3. thermal vacuum drying and degassing;
4. mixing an additive into the oil to inhibit oxidation.
Preliminary filtration is straightforward, involving a mechanical barrier trapping solid
particles larger than the openings in the barrier.
Table 1: The results of used oil regeneration
No
Parameter
Before Regeneration
After Regeneration
Specific Standards
1
Appearance
Turbid, dark brown
Clear, transparent
Clear, no sludge or suspended particles
2
Acid number, mg KOH/g, max
0.63
0.01
0.01
3
Corrosive sulfur
Present
Absent
Absent
4
Water content, ppm, max
170
5
30-40
5
Dielectric strength, kV, min
11
73
70
6
Dissipation factor at 90 oC, %, max
4.0
0.001
0.005
7
Gas content, %
12
0.1
-
8
Solid particle size, micron
50
0.2
Absent
9
Stability oxidation state: – acid number, mgKOH/g, max
-
0.2
1.2
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