When you drive past some power stations (Like at Port Talbot - South Wales) or when you see images of some Oil Rigs, there are often pipes or chimneys showing flames produced from the burning of waste gases. I assume this is done for safety reasons, but is it not feasible to re-use the energy from these flames? Why waste it by sending it into the atmosphere? Is it purely not cost effective, or is there a scientific reason that I can't grasp?
It all depends on the value of ypir life or your froends. Most times it takes about 300 ppm. to kill. U do not want to run it near any cities , because a small leak could kill them all. In west Texas many years ago there were many drove his wagon down the road and die .It is so deadlt that OSHA has established the first alarm point as 10 ppm.
Power Stations don't have flare stack; oil refineries and oil platforms do.
These provide a safe release of produce that is classed as waste.
Vents from safety valves are also part of this system, the stack is equipped with a "pilot light" this ensures that releases from these are ignited.
And yes your; right it's deemed not cost effective to put this to better a use.
well at the time they built most of the refineries they did not have any use for the gas they are emitting
It costs a lot to retool the refinery it is like building it again
Again most of the time the gas flair contains harmless natural gas but someother carry other polluntants
Many of them in today age have sprayers in the chimneys which take out the armful gases before the plume is released into the atmosphere. Most of the heat is used in another process. The smoke you see is the waste product but is mostly CO2 and nothing nasty.
While this heat is recoverable it is often not practical because of location and the problem of what to use the heat for. There have been experiments involving greenhouses and such but most have fallen by the wayside because the supply is not steady but fluctuates. The gasses burned off are usually toxic or hydrogen that can only be stored in small amounts until we develop better hydrogen technology that would make use of this fuel. Hydrogen is dangerous to store and the typical storage facilities exist at refineries where a danger of fire and explosion needs to be kept to a minimum.
If you see a stack with flames on it it is because the plant is flaring waste gases due to an imbalance in the process stream within the plant, due to shutting down or starting up various processes within the plant, usually due to maintenance or the addition of new equipment.
These imbalances are just intermitent lasting only several hours or at the most several days, and they really don't occur on a regular schedule, so having a train of expensive equipment available to try to capture the waste heat from these flares usually is not economical.
If the gas that is causing the flare is available on a 24/7 basis or on a regular schedule so that the heat can be reclaimed you can be sure that it would be done since the plant itself is managed to be as energy efficient as economically possible. Energy costs are one of the major expenses in the running of these plants and they are always looking for ways to save energy which = $$$$.
As far as your power station flare is concerned it is probably a plant that operates on gas turbines, and if there is an upset in the gas supply or if one of the turbines goes down, they may end up with excess gas that has to be flared.
 Where was the last debate on wednesday with Mccain and Obama?
 INSTANCE / WAITING FOR SPRING
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Flames from Burning Waste gases?