Author Topic: Talking Thermodynamics  (Read 88331 times)

Offline Admiral_dk

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Re: Talking Thermodynamics
« Reply #900 on: May 12, 2018, 08:20:39 PM »
Thank very much for you very informative answer - I did admittedly know some of the answers you gave, it was a kind of comment to Willy - but it is always nice go get serious explanation that is easy to digest  :praise2: and the Butane content was new to me but kind of explains it too ....
I read many comments about month old gasoline being useless (most motorcycles needs at least 92 octane) from America (US+ Can) over the last two decades, and always wondered what they talked about as I never had any problems - that is until two years ago for the fuel injected model and some five years for the models with carburettor ....

I wasn't really criticising the product as such, since the bikes runs fine on it when finally started, but it is rather annoying to have starting issues. The really weird thing is that it is only the fuel after the tank that is bad - ie. in the fuel lines, carb etc. - once the old stuff in the lines are used (without the engine starting during cranking), all is good when "fresh" (or not) fuel from the tank reaches the intake ...  :noidea:

Best wishes

Per

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #901 on: May 12, 2018, 11:11:06 PM »
Hi MJM , I have an old BMW outfit 1960 and i only ride it Aprill to October  and it then lives outside under a tarp. And when i go to ride it it starts within 3 kicks  with the same petrol in it since the previous year !!...It may be something to do with the really simple carbs. I think the modern ones get clogged up as they are built to high tolerances with muliple jets and things.?Also my bike always starts on the last kick !!! :D :D
« Last Edit: May 12, 2018, 11:14:08 PM by steam guy willy »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #902 on: May 13, 2018, 12:23:12 PM »
Hi Admiral DK, thank you for your kind words.  My whole aim is to contribute to understanding of the basic science behind our engines, so thank you.  I am not sure why there would be a big difference between the fuel in the lines and fuel in the tank.  The daily temperature cycle leads to the tank breathing, so some exchange of the vapour space with the surrounding air.  In this case you might expect to loose slightly more of the lighter components, leaving the heavier components behind.  But that does not seem to be the explanation.  I wonder if it is possible that the lighter components are absorbed into the rubber hose sections of the lines, or whether the copper fuel lines act as a catalyst in some way.  But I really don't know.  It is interesting to think about the different locations, and what might affect the starting behaviour when the fuel reaches the engine.  Perhaps others have some more specific explanation.

Hi Willy, our police here used to ride those bikes (without the side cars) back when they were new, and always put on an amazing display of high speed, precision formation riding at the agricultural show each year.  I used to aim for a trackside seat so they flew past only a few feet away, half going each way and crossing just in front of me.  But you might be onto something there in terms of the simpler carburettors.  I would guess that more sophisticated designs might be more fussy about the fuel composition and properties.  I guess that only riding in the warmer months, when you get the bike out from under the covers, at least the remaining old fuel would have been from the right season, which is also the season where there is less light ends in the mix anyway.  I don't know if that helps.  Almost certainly better than using old summer fuel in winter.

By the way, I hope you are passing on your skills with those files to your young protege.

We were talking the other day about butane and the pressure changing with temperature in the same way that water boiling point pressure varies with temperature.  I have wanted to add some figures to the general description. 

Of course it immediately gets a little more complex.  With one, two or three carbons in the chain, there are not many possibilities for the arrangement of the atoms, and all three are classed as straight chains.  But with four carbons, you can continue the straight chain arrangement, or you can arrange three in that same straight chain, then make a branch by adding the fourth carbon to the middle one.  So you have two variations on butane, called isomers, normal butane, or n-butane, and iso-butane, or i-butane.  You will see both these names on your can of butane for the stove.  This difference in arrangement of the same atoms makes quite a difference to the physical properties, for example the atmospheric pressure boiling point of normal butane is -0.49 deg C, while the atmospheric pressure boiling point of isobutane is -11.81 deg C.  A mixture of the two, as is commonly sold in those disposable containers, has a boiling point somewhere between, say around -5 C, depending on the actual proportions of each in the mixture.  So in the allotment on a cool morning, you have a very small temperature difference to keep the mixture above atmospheric pressure.  At 40 degrees C, normal butane has a vapour pressure of 377 kPa, so 277 kPa above atmospheric pressure, while iso-butane has a vapour pressure of 528 kPa, 428 kPa above atmospheric.  Again the mixture will be somewhere between.  It is not much pressure to get the required flow through those tiny jets, especially if your air temperature is less than 20, and the pressure, hence flow, keeps reducing as the temperature gets lower. 

I don't have a full table like the steam tables for butane, but I have scanned the pressure-enthalpy diagram in the attached picture so you can see the general similarities with the similar diagram for steam I have previously posted.  Unfortunately I have had to fold the diagram, as it is on A3 paper, while my scanner is only A4, but I think you can see the important details.

Propane has much higher vapour pressure at any temperature, so you have a bit more margin to ensure enough flow to your burner.  This is why it is included in many of those slightly more expensive cans used with hiking stoves.  The readily available mix here is about 27% propane with 44% n-butane and 29% i-butane mixture, which is the specified fuel for my little gas fired marine boiler.  Even with the propane content, things get a bit slow in winter.  Now I have the thermowell sorted, I need to get back to further testing of that boiler.  I will need to record the ambient temperature with the results to be comparable with tests conducted in summer.  It will be interesting to compare with the earlier tests of my two spirit fired pot type boilers which have much more heating surface.

Thanks for following along,

MJM460


The more I learn, the more I find that I still have to learn!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #903 on: May 14, 2018, 12:01:20 PM »
I recently made up a thermowell to replace the fill plug on my centreflue boiler.  I managed to try out that thermowell on a boiler test run today.

This actually follows on quite nicely from the last few posts, as it is a gas fired boiler.  The gas, as I mentioned yesterday, is a mixture of propane, normal butane and iso-butane.  Such a mixture has a vapour pressure dependent on temperature, just the same as butane or water.  However, there are no tables for such mixtures, and vapour pressures have to be calculated. 
With a mixture of gases, each gas behaves independently of all the others, so long as they are not at extremely high pressure, or very close to condensing conditions, and the pressure in an enclosure is the sum of the individual partial pressures of the gases in the mixture.  When we have a two phase mixture of liquefiable gases, the behaviour is quite different.  The presence of the liquid phase means the vapour pressure of the mixture is dependent on the concentration of each gas in the mixture.  A mixture of two liquified gases has a vapour pressure in between the vapour pressures of the constituents, and the composition of the vapour is slightly richer in the lighter component, while the liquid composition is slightly richer in the heavier component.  In addition, as vapour is released or consumed, because the gas is richer in the lighter component, the composition of the mixture looses a little more of the lighter component, so the composition gradually changes as the gas is consumed.

As I mentioned in response to Willy's question about his tea making adventures, the heat required to evaporate the liquid to replace the consumed gas comes from the remaining liquid, which, as we have all observed, gets cooler.  We see evidence of this in condensation of atmospheric moisture on the outside of the vessel.

As I remember it, my previous trials on this boiler were all conducted in summer.  Yes, the little gas tank became cooler, even a bit of condensation on the outside, but the whole plant worked well.  I could raise pressure, even test the safety valve, and the engine ran at something around 2000 rpm.  Some of you might think that is a bit high, but it will slow down when I connect a load of some kind.

I did run it a week or so ago, so the lubricator has been filled with steam oil, but I did not have the thermowell.  Today is nothing like summer, not to us in this country anyway.  Only 14 C in the shed.  However, I filled the boiler and the gas tank, oiled up the engine, filled the displacement lubricator, fitted all my thermocouples and fired it up.

 I got the temperature up to 110 C, opened the stop valve.  The boiler I purchased included a stop valve, unlike my home built boilers, and the engine ran quite well.  The valve was not gummed up like when I had run it on engine oil in the lubricator.  Started off at about 685 rpm by the digital tachometer.

Interesting to say the least.  If the steam plant was all enclosed in a boat, I am sure it would all warm up and there would be some warmth to help evaporate the liquid gas and keep the pressure up.  But there on the bench in the shed, I had to go and get a coat.  I used the infra red thermometer to check the temperature of the gas tank.  Started at only 10.9 C, obviously not really warmed up after transferring the gas from the container to fill it.  A portent of things to come.  By the time the boiler was up to temperature, the gas tank was down to less than one degree.  And instead of the engine speeding up as the steam pipes warmed up, it barely maintained that initial speed, and in fact soon started slowing.  Thirty minutes later, (the gas should not have lasted that long,) the engine speed was down to 228 rpm, the boiler temperature down to 102, even the stack gas was only 100 C, so I closed the stop valve and called the end of the run.  The gas tank temperature was rising to about 4 degrees, indicating a much slower evaporation rate.  I weighed the gas tank (after drying off the outside) and found there was still 8 of the initial 34 grams in the tank.  I don't have the methods to calculate what the tank pressure would have been, but it was too low to maintain a reasonable steam rate, and would be richer in butane, fortunately minimal change in heating value.

As the burner fuel consumption was obviously reducing continually throughout the run, I don't know what I will be able to make of the other results, however I will make a start on analysis tomorrow.  However, it is clear if anyone wants to run at temperatures around 15 C, some means of providing a little heat to maintain the gas pressure is required.  In a model, enclosing the gas tank next to the boiler may be enough.  Possibly even with a means of opening extra ventilation in warmer weather.  Perhaps a steam jacket fed by the engine exhaust, but obviously with a bypass valve so it could be shut off in warmer weather.  Or perhaps a larger jet, and one of those pressure control valves to effectively control the gas  pressure and hence the fuel flow to the burner.  And a fuel mixture with more propane helps if it is available.

I hope to have some more results to report tomorrow.

Thanks for looking in,

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #904 on: May 15, 2018, 02:37:29 AM »
Hi MJM , interesting things happening here that prompts a few questions... do liquid gasses have different specific gravity's and when you mix them together do they separate out ?  Is the natural gas in our pipes always a gas and can it be liquified. My young friend in the side car got his first steam engine when he was 4 years old !! however he has been living in london since then , but he does get Meccano and things !

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #905 on: May 15, 2018, 01:31:47 PM »
Hi Willy, yes, the different liquified gases do indeed have different densities.  The three in my gas mixture are propane, 508 kg/m^3, normal butane, 584 kg/m^3 and iso-butane, 563 kg/m^3.  So all similar and about half the density of water.  They are all fully mixable in each other.  But whether they actually settle out under gravity is an interesting question that often puzzled others besides me at work.  When we got some impurities in a tank, it was hard to mix them well, even though well mixed they would have been within specification.  I don't know if they would significantly settle, given enough time, or if the normal random molecular motion is enough to keep them mixed.

Natural gas is mostly methane, possibly with a small amount of ethane and it can indeed be liquified, as can air which is a mixture of nitrogen and oxygen with a few minor impurities.  In fact liquefaction of these gases, not in the same plant mind you, is the basis of two quite large industries.  You have no doubt heard of liquid air and liquid nitrogen.  When oxygen or nitrogen have to be transported in large quantities, it is usually as liquid.  Natural gas is also stored as a liquid when large quantities are involved.  Liquified natural gas, or LNG is a large industry here and other places, as LNG is shipped to China and other places where it is vapourised into the gas systems as a utility fuel.

However, these are all much lighter gases than propane and butane, so the boiling point at atmospheric pressure is much lower.  Natural gas boils at -161 C and is transported around the world in ships at that temperature.  You will recognise one if you see one in a port or in a picture, as the tanks are spherical, like very large basket balls, sunk into the hull so only the top little bit is visible.  The tank diameter is enough that three or four in a row makes a very large tanker.

Liquid air boils at -194 C which results from the mixture of oxygen, - 183C, and nitrogen, -195C.  If your doctor treats a sunspot with liquid nitrogen, you get a very localised, but very severe case of frostbite.  You can ask how I know if you really must!

The refrigeration units used to liquefy these gases are probably the largest refrigeration systems in the world.  They use turbo expanders to extract energy rather than just throttling.  I always think of my refrigeration systems as very large, but probably a group or two behind those giant plants.  Smaller plants are just not economical, and everyone has to make a profit these days, even if the product is only air.  And natural gas, after being liquified at such low temperature in some of the hottest parts of the world, shipped half way around the world and revapourised, is still sold relatively cheaply, so the volume has to be large to cover cost of the plant and ships plus a profit.

You might wonder why those LNG tanks are spherical.  It is the same principal as those spherical storage tanks you see if you ever drive past a petrochemical plant or a refinery.  They are used for butane and pentane mostly.  The vapour pressure is too high at atmospheric pressure for a normal cylindrical tank with flat or conical roof.  If you calculate the amount of steel for a sphere compared with a horizontal cylindrical pressure vessel, there is always less steel in a sphere.  Once the sphere is large enough, the difference is enough to make the sphere cheaper even allowing for pressing the more complex shape of the plates.  The ones for LNG will be stainless steel, in order to accommodate the low temperature without becoming brittle like ordinary carbon steels.  In addition, they will actually be two spheres, one inside the other, with very dry nitrogen in between along with insulating material, thick enough to prevent frost on the outside.

The ships for LNG have to have refrigeration plants on board.  The systems are not as large as you might expect, as that insulation is pretty good.  Also some of the boil off is used to power the engines, real gas engines you might say, or just big gas engines, depending on your point of view, so the amount to be condensed and returned to the tanks is less.

I am glad to see the young lad off to the right start quite early.  I think I left it too late with my grandchildren, though I have got a great nephew interested in Meccano.  The others would all rather look at a screen or chase a ball - nothing in between.

I hope that answers the right question.  Didn't get much calculation on done today, I hope to have more to show for it tomorrow.

Thanks for looking in,

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #906 on: May 15, 2018, 11:57:32 PM »
Hi MJM, Thanks for the explanation and yes i think i am getting some ideas about Thermodynamics  !! and a new practical question for you ..... I was having afternoon tea/smoothie  and it was quite cold ..so i was thinking about warming it up .and rather than putting hot water in it i thought i could bring it up to ambient temp by putting an ambient temperature knife into it ......I was then thinking that one end is quite thin and the other quite thick. so would the temperature rise quicker or slower in time depending which way the knife was put in ??? I could do an experiment but the cafe doesn't stay open that long !!!!If you drew  graphs at the beginning would they show a slightly different curves ?..thanks
Willy........
« Last Edit: May 16, 2018, 01:02:40 AM by steam guy willy »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #907 on: May 16, 2018, 12:01:46 PM »
Hi Willy, what I can't figure is why you want to warm a smoothie, it's supposed to be cold.  However, I understand that you guys drink your beer warm as well.  Strange!

However, from the thermodynamics point of view, it is a similar problem to the teaspoons in your coffee, or tea, just a different temperature range.  Obviously the temperature gradient is the other way, and heat flows from the knife to the smoothie whereas the heat flowed from the coffee to the spoons in the previous experiment.  But otherwise the sums are very similar.  Heat will flow from the knife to the smoothie, thus warming the drink, and cooling the knife.  It is a little hard to predict the result, as we would need to know the specific heat of the smoothie, and that would depend a lot on how much ice it contains, as well as the precise composition.

However your question is just a little different from the teaspoon one in that the emphasis this time is on the difference in which way around you place the knife.

If you try and balance the knife on your finger at about the point where it just breaks the surface of the drink, I suspect the heavier end would be the handle.  Obviously two different points to check the balance, one for the blade in the drink and the other with the handle submerged. In addition, the surface area of the metal in contact with the drink will be a little different in each case.  So let's consider each case separately. 

If you submerge the blade, you have large surface are in contact with the liquid, with relatively little mass, and therefore stored heat, so the blade will quickly cool to the drink temperature, while transferring only a little heat to the drink.  Most of the heat stored in the knife is in that heavy handle, so must travel along the knife via that small cross sectional area, so some time delay.  All the while, the handle is gaining heat from the atmosphere, as a finned surface, just like the teaspoon handles lost heat in your coffee experiment.  I don't know how much this warms the drink, especially if there is a lot of ice, requiring latent heat before it actually warms.

If we submerge the handle, let's assume that the surface area of flat blade is about the same as the surface area of the elliptical handle, but most of the stored heat is very close to that surface, so this might be expected to transfer to the liquid a little quicker, with only minimum heat to travel the distance down the blade. And the blade exposed to the air would be gaining heat as before.  So I expect the handle submerged might achieve the result a little quicker, but a very similar result in the end.

I suspect it would be difficult to measure the different curves, partly because the difference is probably very small, but also because until that ice melts, the temperature will not rise much.  But in principal, there might be a difference in the shape of the curve between the same end points.  Possibly the temperature stays constant for longer in one case than the other.   The viscosity of a good smoothie means the temperature will not be in equilibrium for the time the change is occurring, so results will depend very much on the location of the thermocouple.  Vigorous stirring will help, partly by keeping the temperature more uniform, and partly by adding extra energy, as the work done during stirring ends up as heat in the liquid.

The other complication is rather like that experiment touching an insulating plastic compared with a block of aluminium.  If the drink contains lots of finely divided ice, it will absorb a lot of heat from your tongue, and so will feel very cold.  As the ice melts, the temperature will not change much, but the liquid will absorb less heat from your tongue, as there is less ice remaining to be melted.  So it will feel less cold, even though it measures the same temperature.

So while I recommend drinking your smoothie cold, particularly on a hot day, there are many lessons in thermodynamics in your little experiment.  It involves concepts of heat (stored in the knife), specific heats,  latent heat, heat transfer, thermal conductivity, work being converted to heat,  conservation of energy and even a little physiology.  Despite the long list, I am not sure if I have included everything.  No wonder it takes a large number of smoothies to keep you cool while you think it all through.  The cafe might not stay open long enough today, but it will open again tomorrow, and they need your custom.

A long day of other essential activities including a late Mother's Day dinner with our daughter and her family, but still made a little progress on those calculations in the remaining hour or less.  So they will have to continue tomorrow.

Thanks for following along.

MJM460
The more I learn, the more I find that I still have to learn!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #908 on: May 17, 2018, 01:06:13 PM »
Hmmm!  A thermodynamics lesson in a smoothie seems to have left everyone speechless.  But I also have been wondering, have I mixed up a smoothie and a slushie?  I hope any confusion this caused has not clouded the issue.

The calculations on the centre flue boiler are proving a challenge.  The insulation on my boiler is only one layer of timber slats.  With a temperature of 110 degrees inside the boiler, the wood temperature as measured by the infra red device was 80 degrees.  This probably means quite a bit of external loss, which I will eventually be able to reduce by adding a few layers of cork under the slats.  I have some extra slats, so with some luck, I will be able to add the cork, and add the number of slats to complete the timber again for appearance  But that is another project.  In the mean time, I can estimate the heat loss with temperature from the cooling curve, and add this to the heat stored in the copper and the water, or the heat in the produced steam.

The more complex problem is caused by the fall in gas pressure and hence gas flow, so heat release.  Not to mention that I don't have a vapour pressure curve for the propane-butane mix in the fuel can.  So quite a few estimates, perhaps more accurately described as wild guesses, in order to complete the process.

However, with a first pass at the calculations complete, I find I can produce a cooling curve, and a curve of heat loss at various boiler internal temperatures.  Should be able to post that tomorrow, though the home front schedule is looking full of activities.  I have also made two estimates of an average burner heat release, one for the first part of the test, heat up, and a second for the steam generation part.  Of course, the heat tapering off like that means the steam production also tapers off.  A bit more checking to see if I can make sense of the results after allowing for the heat loss from the boiler shell/cladding before I am convinced that it is useful data to publish.  In hindsight, I should have shut off the burner much earlier, before the gas pressure and steam production tapered off so much.  I may just have to run more tests over different times, which will allow me to make a better estimate of the steam production variation as the gas pressure falls.  Possibly even try a water bath to maintain the gas pressure more even.  It is all much more complex when things vary so much during the test.

So please bear with me while I spend a bit more time on calculations before posting results.

Thanks for looking in,

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #909 on: May 19, 2018, 12:42:33 AM »
Hi MJM, a few more questions ...I have been reading about these underground coal seams that are burning, some for a long time ? so where does the oxygen come from to keep it burning and why dosen't the smoke extinguish it ??  Also the council has been cutting the grass and leaving the cuttings on top...I have collected these in the past and have noticed that once in a heap they get really very hot inside ...can one explain exactly how this occurs ..please   thanks
Willy

Offline crueby

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Re: Talking Thermodynamics
« Reply #910 on: May 19, 2018, 12:45:32 AM »
Hi MJM, a few more questions ...I have been reading about these underground coal seams that are burning, some for a long time ? so where does the oxygen come from to keep it burning and why dosen't the smoke extinguish it ??  Also the council has been cutting the grass and leaving the cuttings on top...I have collected these in the past and have noticed that once in a heap they get really very hot inside ...can one explain exactly how this occurs ..please   thanks
Willy
Careful about stacking those cuttings Willy, one of my neighbors did that and the fire department had to put out the fire when it got too hot and caught fire. Decomposition is a warm thing, and the grass is a good insulator.

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #911 on: May 19, 2018, 12:58:35 AM »
Hi Chris, yes will do that ,also , Our Harry and your

Megan are coming to the allotment tomorrow  !! something to do with royal weeding !!!!

Offline Steamer5

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Re: Talking Thermodynamics
« Reply #912 on: May 19, 2018, 09:56:23 AM »
Hi Willy,
 You are privileged! I got a house full of slightly inebriated girls watch it.....beer helps....might just have to have a wee dram as well!
Right on the compost front, itís the bacteria that get that temp to climb along with the decomposition. Last place had a couple of large cherry trees that had to have the leaves cleaned up once a week, I used a vac / blower that cut them up when sucked up, by the following weekend the pile was rather toasty!
My understanding is that coal burning underground generates enuff oxygen to via combustion products & whatís released to coninue the process, remember reading about a coal seem in the States thatís been burning under ground for a large number of years.

Cheers Kerrin
Get excited and make something!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #913 on: May 19, 2018, 12:44:11 PM »
Hi Willy, two interesting questions, possibly even related, the pile of grass being not unlike the very earliest stage of the formation of coal.  Your grass clipping example reminds me of the silage that my uncle used to make for the cows on the farm, as an alternative to hay.  The cut grass was put in a pile with vertical iron sheets around it to keep it orderly.  He used to compact it well by driving the tractor back and forth over it.  He would leave a crowbar thrust through the iron, deep into the pile.  He would pull it out each day to check the temperature.  If it burned his hand, it was too hot, or something like that.  If too hot, he would do a bit more compacting with the tractor. 

As the silage was supposed to have better feed value than hay, I assume there was some biological action going on, converting the grass in some way, but I assume oxygen was also important, and compacting left less air in the pile.  If a silage pile ever got totally out of control, it became a pile of ash with no feed value at all.  So I assume a mix of a little combustion with insufficient oxygen, involving oxygen from air trapped between the vegetation, and some biological action, fermenting the grass.  More a case of observation than real research into the detail I am afraid.

Coal seams that have been burning for many and even thousands of years are spread around the world unfortunately.  In total, a significant contribution to atmospheric carbon dioxide.   Coals contain some oxygen bound into the chemical structure, and the softer brown coals, a significant quantity.  However this is bound in the same -OH radical that defines simpler structures as alcohols.  It is not free oxygen that helps start combustion, though there may be some spontaneous decomposition that releases a little.  All my reading suggests that the coal seam fires start from an ignition source, whether lightning, or surface fires that spread over locations where the seam is exposed at the surface.  However, once started the fires continue, probably with oxygen trapped in the earth, slow burning but notoriously difficult to extinguish.  We had one here recently, and it took huge amounts of water flooding to eventually extinguish it.  Earth and coal are both relatively porous substances with many long fine fissures that allow air to percolate deep through.  So once the fire starts, the heat generated together with the available air, all conspire to keep it going.  The smoke contains a fair bit of unburnt carbon but with enough oxygen, there will be no smoke.  In the absence of enough oxygen, some carbon will only go to carbon monoxide, and some carbon will be totally unburned or just the long molecules broken up a bit, creating that sticky tarry stuff characteristic of smoke, and the deposits in the fire tubes of a coal fired boiler.  In the absence of enough oxygen, the hydrogen grabs all the available oxygen first.  Hydrogen in the combustion products definitely is an extreme indication of not enough oxygen, but occurs in the process for producing syngas as a first step in some chemical processes, for example for producing methanol.

Coal is generally considered as having more than 50% carbon.  In brown coal, I assume that is measured after the water is removed by drying, as that stuff is like a sponge.  Coal also contains a considerable amount of hydrogen so is basically a hydrocarbon with much bigger molecules than the simple series we have been looking at.  The molecules take the form of long cross linked structures, each carbon attaching to other carbon and hydrogen atoms. 

Combustion of hydrogen and carbon produces carbon dioxide and water.  But it also contains very small amount sulphur compounds, nitrogen compounds and some others.  Unfortunately these produce the undesirable pollutants that make the combustion process "dirty".  It is hard to object to water as a combustion product, and we all produce and breathe out carbon dioxide, so personally, I would not call it dirty.  It is the amount in the atmosphere, and its contribution to global warming through the greenhouse effect, that is objectionable.  So clean coal means processes to remove those other nasty compounds, either before or after combustion.  Then in principal, coal is similar to any other combustion process.  So what we all desperately need is an alternative energy source that does not involve combustion of carbon. 

I hope that is clear enough and not too political.  There is too much politics and not enough science in most discussion. The politics tends to obstruct any possible progress towards solving the problem.

Thanks Chris and Steamer for coming in, good to have you joining the discussion.  By the way, combustion consumes oxygen.  Some of the oxygen required for combustion might come from the chemical structure, but the combustion products always contain less oxygen than the input to the combustion process.  Any oxygen left over in the combustion process is due to a deficiency of fuel.  But coal does contain some oxygen bound into its chemical structure, which reduces the additional oxygen needed from air to complete combustion of the hydrogen and carbon.

So it seems we are not the only ones in the world where the only available TV program is something about a wedding.  I suppose these days we have on demand services, if we want to watch repeats.    However we will make up for it next week with three days of Brian Cox observing the universe.  I watched it last year, and it was really great.  So patience tonight, and explore the universe next week, as compensation.

I have at last made some progress on the calculations on that boiler test.  I do not mean to imply that they really take so long, it's just that I need an uninterrupted go at it.  Life has not allowed much of that until today, but at last some progress has at last been made.

I think this post is already long enough, so I will summarise my conclusions tomorrow.

Thanks for following along,

MJM460 
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #914 on: May 20, 2018, 12:05:16 PM »
Hi MJM, So how does the pile of grass actually ignite ?......
Willy