OIL SPILLS - Clean-up Technique 2

IN-SITU BURNING

Following up on the previous blog post where I wrote about bio-remediation as a form of oil spill clean-up, this post will focus on using IN-SITU BURNING as an oil clean-up technique.

If you haven’t read it, click here and come back.

In-situ burning (ISB) is an oil clean-up technique that involves burning the oil that has spilled. It is more popular than bio-remediation because it has proven to be more effective, more efficient and cheaper to carry out. WHO WOULDN’T USE THIS OPTION RIGHT ??

HOW DOES IT WORK ???

In-situ burning is simple, quick and requires minimal effort to be carried out. it does however need specialized equipment for it to be carried out, which may involve a fire-resistant boom and igniters. To burn the spilled oil, three elements must be present; oxygen, fuel and a source of ignition. Also, the spilled oil slick must be at a certain temperature for it to be ignited. Because it is at this temperature that the oil slick can produce gases/vapors that can trigger ignition. Back in Sierra Leone we have “coal pots” that we usually use for cooking which is basically a stove/cooker or whatever you might choose to call it, that’s powered by charcoal. We usually pour kerosene or use plastic bags to ignite the charcoal so it will burn at a temperature high enough to cook food. This is basically the same principle at work in in-situ burning. The temperature at which the oil slick will begin to produce vapors that will trigger ignition is called the “flash point”. The igniters used are usually petroleum products as well, the most effective being kerosene and diesel fuel.

When the oil slick has been ignited, it will burn but not for long because the “flash point” temperature is not high enough to continue to produce vapors at a steady pace. Therefore, the temperature needs to be kicked up a notch for the conditions to be feasible. They do this by adding more of the ignition agent which will cause the temperature to be warm enough to be producing vapors to support continuous burning. This temperature is called the “fire point”.

The thickness of the oil slick is also something that determines how effective the process will be. If the oil slick is thick enough, then the burning lasts longer because it acts an insulator in the process – prolonging the burning. To break this down, in Sierra Leone there are different ways to dispose of garbage. The most common being going to dispose of it at the area rubbish dump where its supposed to be collected every week (lol its Sierra Leone and not dreamland so that doesn’t happen) OR burn the rubbish at the back yard. When burning the rubbish in the backyard, if there are lots of plastic bags or plastic products in amongst the garbage, then the fire will burn longer and hotter. Because as much as as they’re being burnt to be disposed of, the plastic is also melting and, hence fueling the fire. If there is a negligible about of plastic mixed with the garbage, then burning will take a considerably shorter period. In-situ burning of oil slicks basically goes by the same principle.

If the oil that has spilled is too thin that it would not be enough to sustain the burning, then fire resistant booms are used to gather the oil into a thicker layer that will sustain burning. The boom is a temporary floating barrier that is used to contain an oil spill. The boom is dragged by a boat after it has been placed in a position just beyond the oil spill. It is sort of trying to mop the floor after water has been spilled. You try to get the water to one corner so you can deal with it easily.

During the burning process, the oil is obviously being burnt but one thing that happens is that there is residue that is left behind. This residue if not collected immediately will sink to the bottom of the ocean which will then suffocate life at the ocean floor. Because when the residue sinks to the bottom, it will fall on plants and other sea animals. This could restrict movement or limit the flow of oxygen needed for survival, ultimately causing them to die.

Towards the final stages of burning, the temperature of the layer of water directly underneath the oil slick becomes extremely hot and sometimes reaches boiling point. This happens because it is no longer insulated by a thick layer of oil slick, so it fire is now in direct contact with the water. The water will boil if there is no current or turbulence disrupting the flow. When this happens, the steam from the boiling water mixes with the remaining oil layer causing a bigger fire and faster burn rate. This is sort of like when you’re frying something, and droplets of water enter the frying pan causing a blaze in and around the frying pan.

IGNITION PROMOTERS….

Like the dispersants used in bio-remediation to create better conditions for it to occur. Ignition promoters are substances added to oil slicks to increase its ignitability or to improve the chances of the flame spreading over the surface of the unignited oil. Petroleum products have proven to be the most effective ignition promoters. These include kerosene, diesel, gasoline or even the unrefined crude oil itself. So during the refinery process, there are products at the top of the distillery and those at the bottom. The use of middle distillates have proved to be more effective than lighter fractions. This is mainly because:

• Lighter fractions of petroleum are very volatile and could ignite at any point in time posing a significant safety hazard.

• Lighter fractions upon addition, evaporate faster than the middle distillates which causes the oil to cool faster.

• Middle distillates burn at a higher temperature than lighter fractions.

LIMITING FACTORS…

Like many other processes, there are usually factors affecting how well the said process can be carried out. With in-situ burning, the limiting factors include:

• Waves and wind

• Oil type

• The thickness of the oil slick

• Emulsification

• Current

WAVES & WIND SPEED _

ISB is effectively carried out under relatively calm conditions. If there is a high wind speed which will lead to waves of more than three feet forming, then the burning won’t be very effective. This is because the oil slick would be mixing with the water causing it to emulsify. The fire will also be difficult to control if there are high wind speeds especially if it’s being carried out on land where there is a high tendency already of the fire spreading to non-affected areas.

OIL TYPE _

ISB is measured by the rate of reduction and the type of oil spilled plays a part in that. If the type of oil spilled contains hydrocarbons that are volatile, then the rate of reduction will be high because the oil slick will burn faster, vice versa.

EMULSIFICATION_

This happens when the oil spilled is mixed with water. It is directly linked to increased wind speeds and waves. For this to happen, there is mixing energy that needs to be reached. The oil takes on microscopic droplets of water, increasing the “oily material” and causing it to be more viscous (ease of flow). This, in turn, makes it difficult to carry out ISB because water puts out the fire and even though the oil will burn, ignition and combustion will be extremely difficult. So in burning emulsified oil, 2 steps are involved:

• The separation of the emulsified layer of oil which involves boiling off the water or using emulsion breakers to form a layer of emulsified oil.

• Burning off the un-emulsified oil slick.

Naturally, the burn rates decline significantly with increased water content and this decreases even further as the oil burns, hence reducing its content.

PROS & CONS…….

There has been a bit of controversy surrounding the use of ISB as a cleanup method because of the adverse risks it is likely to pose to the environment. When the Exxon Valdez spilled in Alaska, on the second day the authorities chose to use ISB as the immediate response tool. They were able to put the oil under control relatively quickly and with a high removal rate. Another reason why ISB was used was that it offered the option where the remaining oil residue would be very low. ISB can also be initiated very quickly and easily as it doesn’t need much machines and equipment to be carried out.

Using ISB as the immediate response does however that nearby plants and vegetation will be destroyed if its not controlled properly. If its carried out on an inland spill then it poses a long term risk because if the fire burns at an extremely high temperature, this causes parts of plants that are below ground to burn and get killed. It also lowers the water level as well. During the Exxon Mobil spill, it was observed that some animals were dislocated, and a species of salmon went extinct after the cleanup process.

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Overall in-situ burning has been the most effective clean up method over the years. I think that if it is properly carried out, the effects on the environment won’t be as much compared to other clean up techniques. The cost of carrying it out is relatively cheap also. Realistically the only major concern has been the minute particulates in the smoke, and this has very little negative effects. The goal remains however to be more careful when transporting and mining the crude oil to prevent the several mishaps that could’ve been avoided.