How Long could an "Eternal" Fire last?
At some point there is a fire very spiritually important to a certain people - so important that they build a city around it, and begin developing an empire with one of their main motivations being to provide fuel to the fire. They believe that if it burns strong, they will achieve success, if it falters, they will face hardships, and that if the fire ever goes out, the end of the world will soon follow.
Assuming that the fire is in a pit around half the size of an Olympic swimming pool. There's protection from the elements, disposal of ash, and expanding the pit are all easily doable. Fuels available are wood and coal (dung is blasphemous). Is it possible for this "eternal fire" to be kept burning for decades or centuries? If it can, at what point do the fuel requirements become exceedingly unreasonable (i.e. deforesting the average continent)? Are there any other obvious problems with the existence of this fire?
The nature of your question makes it sound like the fire was already there, that these people found it rather than lit it. In which case Will's answer is perfect (and you could easily see why a primitive people could be very impressed by something like that and consider it spiritually important).
But as someone who works with wood fuelled fires on a regular basis (specifically a wood burning pizza oven) I thought I'd provide an alternative angle.
What you want here is efficiency, to minimise fuel consumption while keeping your flame burning. You don't get much more efficient than pizza ovens when you're talking about burning wood.
Our oven is roughly 1m in diameter and probably 30 - 40cm high in the middle and to keep a fire burning that maintains a temperature of 500 - 600 ° C only requires 1 medium sized log (kiln dried hardwood) every half an hour. Of course you're looking at a much bigger fire than this, half an Olympic swimming pool is 25 metres long, so let's say a 25m diameter dome with about a 6m high point (I don't think scaling the height up 1:1 is ideal). And a hotter fire burns faster. You're also presumably looking for more spectacle than heat, which means maintaining a bigger flame then you normally would in a pizza oven.
Even so I think you could say 75 logs every half an hour, which would be about 56kg (assuming a 750g average for a log), so 112kg an hour. Which comes to 981 tonnes a year.
EDIT: Thanks to Joe in the comments for pointing out an error in my calculations here. I've redone them from scratch. The end result is actually a lot less space required as I really made a mess of these the first time round.
Growth rates and sizes for trees are a bit tricky, but the live Oak (from the US) apparently could reach a height of 18m and width of 24m on average in 75 years.
If you assume that only 10% of that 10368³m is actually wood, then you get 767kg of wood [density of 0.74 (103 kg/m3)], which means you would need 1279 oak trees a year to keep your fire burning.
Turns out these calculations were all wrong, based on the information in this page it would appear a single oak is likely to yield a total of 7860kg of wood, about ten times what my original calculations (corrected for errors) would yield. This means you would need 124 oak trees a year to keep your fire burning. Growing this many trees would take roughly 18 acres. Of course, these trees take 75 years to reach maturity, so you should have 1,350 acres of trees in varying states of growth that you replace as you go.
Post Edit: That's still probably on the high side, as Joe mentioned deliberate farming and management can probably drastically reduce the space required, but 1,350 acres really isn't a lot.
Ash and smoke wouldn't be as big a problem as many people have suggested, a pizza oven produces very little of either due to a combination of the kiln dried fuel (more on that in a moment) and the heat. I would estimate from experience that from 72kg of burned wood you end up with about 3 - 4kg of ash. This may improve further due to the hotter nature of your eternal flame, but even at that rate you are clearing out 54.5 tonnes of ash a year (140kg a day). That's a lot, but I don't think it's unmanageable, particularly considering it is actually usable for a variety of things.
I would envision rather than a single large fire you keep burning, that instead your fire would move around the dome like a clock. If you divide the circumference by 24 hours you get 3m slices. Every hour you fill one of these slices with wood (or half of one every half hour) so that the flame is constantly moving around the structure, this allows the other sides to be cleaned (still with some difficulty as the dome will retain it's heat and still be very hot) with long brushes and rakes etc.
It also means that there can be multiple chimneys and entrances for putting the wood in which can also be cleaned in turn as required.
You would use the heat of the fire to effectively kiln dry your wood, there are a number of ways you could do this, which increases the efficiency of it for burning and reduces smoke.
The other advantage of this method is that you end up with an elaborate sacred dome structure for your worshippers to admire (which also protects the Eternal Flame from the elements too), it could also be used as a massive bakery if that wasn't considered too sacrilegious.
Overall I don't think it seems to difficult a logistical feat and should be well within the bounds of plausibility.
Wikipedia has a good article on eternal flames which are fed by underground natural gas deposits. Those deposits can be immense, as we now know because we suck the gas out and use it for our hot water heaters. They can feed a flame a long time. The linked article notes several known to have burned for thousands of years.
I propose you model your eternal flame on the Darvaza gas crater.It is a collapsed natural gas deposit that the Soviets set on fire, because they thought it would be so cool and they were right. It has been burning for 70 years.
It is big.
Yes. They plant a forest of trees ( a renewable resource) to be used only for the holy flame. Faithful can also come and throw offerings (wood, coal, hay, etc) into the fire. They could also carve prayers into the logs that will be put into the fire.
If we go by some real world examples two prominent examples are:
However if you are specifically looking for flame fire then it is the Darvaza gas crater fire burning since 1971 or about 46 years. So you got the idea that if there is some fuel source like coal or natural gas beneath the ground then fire can burn for thousands of years or may be even more.
"Half Olympic pool" is a huge fire. The main problem with this is that it's very hard for humans to approach for refuelling. You haven't specified the technology level so this may present a problem.
Radiated heat is going to be high. You'll have to keep a large area around the fire, dozens of meters. The smoke plume also represents a problem. Not only is it a health hazard if it's blown into the city, the hot cinders will be carried hundreds of meters and potentially ignite things. Your city around it needs to be stone with very little wood construction.
Protection from elements is not a problem; a fire that size will burn through days of rain or snow provided that water doesn't pool at the base. For that and other reasons like ash disposal and visibility you might want to keep the fire elevated rather than in a pit, on a massive stone fire-grate.
If you use the fire for communal heating and cooking, you would be diverting domestic fuel consumption to it and its fuel requirements might be manageable. Otherwise, the refuelling again imposes logistic requirements on you - not just the cutting of trees or digging of coal, but the shipping of same. Note that one of the first uses of iron rails for railways was to help ship inputs to the iron furnaces!
Of course, if your city is ever sieged the fire is going to be seriously diminished after a few days ...
You might be tempted to handle the ash problem by dumping it in the river, but that kills the river. Otherwise you're going to end up with Fly Ash Mountain near the city.
Even for a small country, it would not be a problem at all.
Olympic pool has a surface area of 1250 sq m Assuming an average hearth has an area of 0.5 sq m, this fire would be equivalent to 2500 hearths - significant for a village, but negligible for a city. Keep in mind that feeding this fire takes priority over any other hearth. Unless your setting is in treeless area and there are no coal deposits around, you are virtually guaranteed a sufficient supply of fuel.
What can stop this fire is a natural disaster (hurricane, flood etc.) or interruption in service due to war or major unrest.
Like @Will, I too thought of Darvaza as soon as I read your question...
But there are more subtleties to think about, mainly because of the proposed pit size. The idea of keeping a nearby forest is really cute, but:
You claim that eliminating ashes is easy. I think you may have a bit of a problem reaching the center of the pit.
Then you have to figure out how to dispose of the ashes. Food for the forest? Ok, but you have a very alkaline substance (read corrosive when mixed with water), not to mention having to cool so much material on a constant basis.
Just the sheer amount of fuel is flabbergasting. Think of how fast a forest fire consumes acres and acres in a week. Sure, your fire is controlled, but you may want to do some numbers to keep the concept real.
These three points bring you back to using a natural gas deposit, and to an idea like Darvaza. Your main problem then is the uncertainty of the size of the deposit, although it will go on for a long, long time. Just last year, the US Geological Survey announced the largest deposit of gas and oil ever found. It holds 16 trillion cubic feet of gas. You do the math... In any case, think that we will burn all that gas, and that there are thousands of other deposits being exploited, so the impact of one large fire on the atmosphere will likely not be noticeable.
One more thing... just so you can claim that you did your research thoroughly, look for the Centralia Mine Fire in Wikipedia;)
As people have pointed out fuel isn't a major problem if you have even a modicum of religious zeal and common sense forethought in the mix. Therefore sheltering the flame is probably going to be the real issue early on, unless the flame is situated in some part of the world that doesn't experience sudden severe weather events that surprise modern meteorologists let alone peasants of an earlier age. Wood ash is good fertiliser anyway so you have a positive byproduct and in reality you don't want the flame any bigger than it has to be for a variety of logistical reasons, if they switch to burning coal then ash disposal becomes more of an issue too.
Assuming since you've said that shelter from the elements is easy that the peasants go overboard and it withstands freak events then long-term the deciding issue may be the very fact that the Flame is a religious icon and linked to the fortune and prestige of its city-state. I'm reminded of the Chinese Ministry of Fisheries catch numbers; for a number of years scientists thought that world fisheries were in decline but the official numbers out of China kept going up year on year so they dismissed the findings of their studies in other fisheries as aberrant, it turned out that the Ministry had a policy of reporting an increased haul, regardless of the actual numbers which were actually in decline. Now if the Cult of the Flame were to decide for reasons of national pride, security, and prosperity that "The Flame Must Increase!", and be seen by the people to increase, on a regular basis then you're going to have to start worrying because that attitude leads to madness. You get an appetite for increasing large increases on a more and more regular basis until your city is knocking down buildings to make way for the Flame and exhausts the home reserves to the point where fuel imports become the entire point of foreign trade and they strip the treasury to feed their burning god. All it takes is a single lost caravan and the Flame will falter and the people will expect doom to follow, which it will if they believe in it.
Supposedly a ceremonial fire has been burning at Varanasi (India) for ~5000 years and I believe they only use wood to fuel the fire. It suggests your idea is not far fetched if people put a great importance on it.
Incineration as in "Incineration is a waste treatment process that involves the combustion of organic substances contained in waste materials." See https://en.wikipedia.org/wiki/Incineration
Seriously. They sell waste treatment for the neighboring city states. As long as the "flame burns strong", their economy is successful.
The only waste they don't process is dung. Dung is not only blasphemous, it is also smelly.
Note that in real life, certain religious beliefs have originally originated from economic reasons. At some times, it has been easier to enforce certain "trade/economy regulations" by disguising them as religious laws.
The fire can probably last until environment protection law are enforced (possibly though diplomacy/war/violence from other city-states). They might keep it up despite that by employing filters, catalytic converters - or by producing electrical or mechanical energy, which is so valuable to the global economy that the pollution is accepted.
Just wanted to pop in with some snippets as everyone else's ideas have been so inspiring. It's been well established by other answerers that the concept is not only doable, but actually exists IRL. Based on other answers/comments, I've also built up some aspects of daily life in my headcanon. I've already stated most of this in comments, but had enough to say that I figured I should just write a full post and may come back to add more later.
If you went with a dome structure, the fire could be used to provide a constant stream of purified water by utilizing an inverted dome (think: a giant wok) atop the flames to hold water which steamed to the top of the dome and ran down the ceiling into channels around the outer rim of the structure.
The burning ash and coals could also be used in "earth ovens"-- dig a pit, dump the still-blazing remnants at the bottom, a large pot on top (formal brick structure optional -- could just lower the pot in, then top off with an insulating material), and come back to a lovely stew several hours later, THEN repurpose into bricks as someone else proposed. You can also toss whole animal carcasses in there, or individual meals wrapped in bamboo or banana leaves.
Someone else stated bamboo would not necessarily be the best fuel, but it would behoove the society to keep a grove as a backup source (in case the forest should succumb to fire, avalanche, disease, fungus, etc.) as it's quite hardy and it can be used for building, and the leaves for thatching, cooking, and wearing.
There could also be a community bath nearby to offset some of the heat, with some sort of channeling/fanning of heated air into the base of the structure (jacuzzi bubbles optional). A steam room (sauna) too. Basically every aspect of life (buildings from the bricks, heated foods, self-cleansing...) could be considered spiritual in nature.
The structure containing the fire could also be removable, have a retractable ceiling, or contain "skylights" for pyrotechnics on holy days. Through experimentation, your people would discover certain colors and effects (sparks, colored smoke, popping and squealing, etc.) that could be achieved with certain minerals and essences. With the skylight idea, these could be situated around the outer reaches of the cover and on event days, waxed ropes could be used to create tendrils reaching toward them, and with additional controlled fuel sources, priests could create some truly fantastic choreography.
For very long. There are many huge coal seams which have been burning for many decades. The "Burning Mountain" of Australia has had a coal seam burn for 6,000 years. With human intervention (i.e. moving fuel up to the fire, removal of ash frequently, constant ventilation, etc..) a determined society armed with a large supply of something slow-burning is destined to keep a fire lighted for a very long time.
Of course, there are obvious issues; frequent workers on the fire, if not protected properly, will almost definitely develop some sort of lung disease from constantly breathing ash-laced flames, as well as the fire being a constant threat and pollution source. Optimally, the building would be made of highly fireproof materials (such as refractory brick or Kaowool) so that the fire wouldn't need extinguishing if it went out of control.
Another issue. Is this a big bonfire, or a smoldering pit of embers? Both qualify as fire, but one is much more impressive than the other. Optimally, the fuel should be cut up to maximize surface area and thus a more fiery fire. Yes, that was intended.
Another idea (redundancy!) would be to use gas (propane, LNG, etc..) which makes a big ol' fire which extends much higher. Essentially a big blowtorch with horrifying stochiometry. This is much more expensive, as gas drilling is opposed by many. But if this is truly a sacred fire, have fun.
As there are real life eternal fires, I would suggest taking that and COMBINING it with a religion. Have a City of Flame near it preferably surrounding it), where it's a religious obligation to to start a fire froum the source and then keep it going as long as possible. These smaller fires don't have to be big, they just can't go out. Maybe it's shameful or a sign of illuck. Make it a requirement that the fire has to be useful, and letting it die out looses whatever piece of property it was on. Lot's of possibilities.
An eternal flame can be kept going for as long as the rivers flow, the grass grows and the sun signs. Which if you are a First Nation peoples is until the white man finds gold or something else they want in the ground under the fire. Every story needs conflict. Evidently.