Biofuel Bacteria Corrosion in Storage Tanks
Over the year’s biofuels have been gaining increased public and scientific attention, driven by factors such as oil prices, the need for increased energy security and the concern around fossil fuels and global warming. Their storage, however, comes with increased problems from bacteria corrosion.
What is Biofuel?
The encyclopaedia Britannica describes a biofuel as “any fuel derived from biomass”. This can be plants, algae or any kind of animal waste. These are resources which can be replaced so biofuels are considered to be a source of renewable energy. Biofuels include fuels derived from biomass conversion, as well as solid biomass, liquid fuels and various biogases.
Bioethanol is an alcohol made by fermentation, mostly from carbohydrates in sugar or starch crops such as corn or sugarcane. Ethanol is generally used as an additive to petrol to increase engine performance by increased octane, decreasing harmful emissions from vehicle exhausts and various economic advantages. In its pure form, it can also be used as a fuel in its own right. The use of Ethanol as an engine fuel or fuel additive is the single largest use it has and used in varying quantities across the World. Bioethanol is widely used in the USA and Brazil, these are also the biggest producers.
Biodiesel is made from vegetable oils and animal fats chemically reacting with an alcohol. Biodiesel can be used on its own in its pure form or blended at various levels with petrochemical-based diesel. A system known as the “B” factor states the amount of biodiesel in any diesel mix. For example, a fuel mix containing 20% biodiesel and 80% petrodiesel is marked as B20 and 100% biodiesel would be B100.
How does biofuel add to the problem of bacteria corrosion?
Biofuels create a nutrient-rich breeding ground for bacteria. This causes major problems for storage tanks. Biofuel is produced from biodegradable material, giving it the characteristics that make it attractive to biodegrading organisms.
Bacteria corrosion, also referred to as microbial induced corrosion or MIC, is a form of corrosion caused by the presence of microorganisms. These microbes can cause product deterioration, sludge and serious corrosion in tanks and pipework. Biofuels are particularly prevalent to bacteria that cause corrosion, in particular, diesel fuel provides ideal conditions for the rapid growth of microbes even in the best-maintained tanks.
There are dozens of known bacteria responsible for microbial induced corrosion. Most of these fall under two main classifications, aerobic where the microbes require oxygen to live and anaerobic which thrive without the presence of oxygen.
An essential ingredient for these microbes to grow and flourish is water. Even a small amount of water is enough to allow a colony to expand.
Water in storage tanks can come from many sources; moisture from the air that enters the tank from vents or floating lids, any dissolved water in the fuel can condense on the walls of the tank, improper drainage within the tank, even allowable quantities of water within the fuel will be more than enough to grow a significant colony as the microbes produce more water during cell metabolism, constantly renewing the process.
The ethanol content in the fuel also produces its own concerns around phase separation, where the ethanol separates from the fuel. Ethanol rich water sinks to the bottom and the fuel rises to the top creating an unbalanced blend, increasing corrosion risks.
Ethanol use is increasing and is a major concern for oil companies.
Sulphate reducing bacteria
Sulphate reducing bacteria, or SRB as it’s also known, is one example of an anaerobic microbe which does not need the presence of oxygen for a colony to blossom. Sulphate reducing bacteria can often be the cause of damage to many fuel storage tanks.
These bacteria are some of the oldest microorganisms on the planet going back some 3.5 billion years, contributing to the sulphur cycle soon after life emerged on Earth. They are capable of withstanding high temperature and pressure.
Sulphate reducing bacteria, as their name suggests, gain their energy by reducing sulphate content of organic matter to corrosive hydrogen sulphide, contaminating stored gas and fuel. SRB is one of the main causes of microbial induced corrosion in storage tanks and leads to severe pitting of the steel.
You’ll often find dark sludge at the bottom of a tank when SRB is present. This is a by-product caused by insoluble metal sulphides created when the hydrogen sulphide reacts with any metal ions in water.
MIC and sulphate reducing bacteria can seriously increase the rate of pitting corrosion within a storage tank.
Pitting corrosion is a localised form of metal deterioration which displays as small pits or holes on the surface of the steel. It’s one of the most common types of corrosion found in storage tanks and can lead to some very serious problems as the metal loses its thickness. If left untreated these pits will cause a breach in the tank and any stored product will enter the surrounding environment.
The pits themselves can appear in all different shapes and sizes, narrow and deep, shallow and wide, they can also undercut into the steel and develop sideways underneath the surface of the metal. The pits themselves can be either uncovered or covered with a semi-permeable membrane.
The extent of pitting corrosion within a storage tank can be difficult to detect before blasting.
Protecting storage tanks from SRB and MIC
When looking at ways we can prevent damage to storage tanks from this serious form of corrosion we need to consider proactive protection of the internal steel.
The best solution is the application of a solvent free resin coating resistant to the stored fuel. Applied either as a single skin or a monitored double skin system.
The bacteria corrosion attack on the steel will be zero as the solvent free coating acts as an internal ‘skin’ within the tank, providing a barrier between the steel, the stored product and any bacteria which may exist.
A solvent free coating is a coating that doesn’t contain, or contains very little, solvents and volatile organic compounds (VOC’s) which are harmful to people and the environment. Solvent free coatings also have a high solid content which means they are fast curing, resulting in minimum tank downtime.
A solvent free coating can be found to protect against most petroleum products as well as many chemicals.
Using a solvent free coating to protect the internal tank steel has many benefits. It’s possible to achieve a greater level of thickness when applying the coating, making solvent free epoxy’s typically one coat systems. The ability to achieve the required film thickness in a single coat, along with the fast curing ability of most solvent free resins, is a massive time saver and allows the tank to be back in use sooner.
These types of coating have excellent adhesion to the steel and dry to form a smooth tile-like finish. They have a long and successful track record in tank lining.
Once protected with a solvent free coating de-watering of the tank can take place if needed to remove any excess water which has separated at the lower level of the tank.
Any activities carried out in the tank should be done with care so as not to damage the coating.
Installing a double skin tank lining system to combat bacteria corrosion
Another option when lining the storage tank is to install a solvent free double skin liner which includes a monitoring system to supervise tank integrity.
A client of Abfad’s is very proactive with storage tank protection on the sites they operate. Always installing a double skin lining system to their storage tanks.
A new forecourt outside of Warrington with 7 storage tanks on-site storing Unleaded, Super Unleaded Petrol and Diesel, required installation of a double skin lining system to all the tanks.
To install a lining system in a series of tanks on one site makes better financial sense for the client as costs spread out more evenly between the tanks.
Once blasted to SA2.5 standard the extent of any corrosion in the tank is visible. Random steel thickness tests determine the remaining thickness of the steel and any necessary repairs made.
A coating of solvent free resin is spray applied using a plural component heated spray machine. This creates a single skin which encapsulates the internal steel and provides initial protection. This is normally applied to a nominal thickness of between 500 – 1000 microns. The plural component heated spray machine guarantees application of the coating to specification and ensures no anomalies can occur.
Creating an interstice
The double skin materials used to create an interstice around the tank are non-hazardous. These are installed on top of the single skin. An interstice is a space between two objects or structure’s and this is what allows for the monitoring element of the double skin system.
The materials used are able to maintain an open flow of air and fluids even under compression. It’s important to maintain monitoring around the whole of the tank and comply with the BS EN 13160-7 European tank lining standard.
The interstitial space holds vacuum pressure which allows the tank integrity to remain under monitoring 24 hours a day, 7 days a week, 365 days a year. If a breach to the storage tank did happen, the vacuum pressure would dissipate and the monitoring system would enter alarm mode, notifying the site operator that there was a problem.
Another layer of solvent free resin is spray applied on top of the double skin materials to complete the double skin lining system inside of the storage tank.
A pressure transducer valve is installed in the tank chamber and routed back to a monitoring unit, installed in the site control room, which gives a visual display of the vacuum pressure within the interstitial space of each storage tank.
Bacteria corrosion going forward
Tank replacement is costly, even without the downtime necessary to remove and install new tanks. The process to install an internal liner, on the other hand, is a relatively simple process.
Biofuel usage is only going to increase, making it important to protect storage tanks from any microbial induced corrosion. Safeguarding valuable assets. Shielding the environment. Futureproofing your storage needs.