Steam Science Question

[Anonymous]

Oxygen is said to support combustion, therefore external and internal combustion engines suck in air so the oxygen component of air can support the combustion of fossil fuel.  What kind of effect does superheated steam have on combustion of fossil fuel?  Are the hydrogen and oxygen molecules in superheated steam linked together in the same way they are with water? 

And as a sidebar question:  What is the limit to temperature of superheated steam, if there is a limit?    

[Paul Milenkovic]

Superheated steam is merely water vapor, heated to a temperature beyond the boiling point.  In a steam locomotive, the boiling water, steam, and then superheated steam, does not come in contact with the combustion gases of the burning coal or other fuel except by the transfer of heat across a boiler flue pipe or metal firebox wall.  That is why steam power is called external combustion while engines such as your car engine, a jet engine, or a Diesel engine have the combustion gases as the working fluid and hence are called internal combustion.

If you make steam hot enough, it will disassociate into the component hydrogen and oxygen elements.  The temperature of that breakdown should be the same as the flame temperature of burning hydrogen and oxygen because it is reversing that process.  One of the proposed processes for making large quantities of hydrogen cheaply for use in future fuel cell cars is to use a high-temperature pellet bed nuclear reactor to free hydrogen from water in this way, and then separate the hydrogen from the oxygen by some process that takes into account the light weight of the hydrogen atoms.

Superheated steam also plays a role in some combustion processes or chemical process based on combustion.  Distilled water is often injected into the combustion chamber of gas turbines used in power plants for electric power generation -- this lowers the combustion temperature to prevent the formation of oxides of nitrogen, a regulated pollutant that can cause breathing difficulties in people and can harm lakes and streams with acids.  It also gives a small increase in power in the turbine from the evaporation of the water into steam.

Superheated steam in contact with coal generates a mix of hydrogen and carbon monoxide gas.  Further processing of the hydrogen and carbon monoxide in what is called the Fischer-Tropsch (F-T) process can make everything from gasoline to Diesel fuel -- the chemical plant to do this is expensive, and making the feed gas from coal uses up a lot of the energy in the coal to free the hydrogen from the bonds to oxygen in the water, but the Germans used this process to make gasoline in WW-II, the South Africans used it under anti-apartheid trade sanctions, and people are thinking of bringing it back on account of the high price of oil.  The main worries about F-T gasoline is that the chemical plants are expensive and if the price of oil goes down again people could lose money on it, and it also uses large amounts of coal, contributing to carbon dioxide emissions implicated in global warming.

Livio Dante (L. D.) Porta worked on a number of improvements to steam locomotives, among them was his "gas producer" fire box.  The gas producer fire box was a small-scale coal gasifier on a locomotive, feeding a deep coal bed with a small amount of air and some steam from the boiler, and then injecting combustion air over the coal bed using "overfire jets."  This is perhaps the only application where superheated steam contacts combustion gases in a steam locomotive.

[Railway Man]

Nice job Paul.

[dldance]

And as a sidebar question:  What is the limit to temperature of superheated steam, if there is a limit?    

My reference texts suggest that above about 1100 degrees F, superheated steam becomes very corrosive to steel and that components of a higher temperature system must be made of stainless steel.  This may be the practical limit.

dd

[erikem]

And as a sidebar question:  What is the limit to temperature of superheated steam, if there is a limit?    

My reference texts suggest that above about 1100 degrees F, superheated steam becomes very corrosive to steel and that components of a higher temperature system must be made of stainless steel.  This may be the practical limit.

Paul was mostly correct in stating that the the majority of the hydrogen bonds in steam don't break down until you reach the combustion temperature of hydrogen + oxygen. Some breakage does occur at room temperature (think pH) and that increases with temperature.

 IIRC, the highest temperature used in a steam plant was 1200F and was a maintenance headache. 1050F is what I recall as a typical max steam temperature for power plants (i.e. your 1100F figure is correct).

[Railway Man]

Babcock-Hitachi have supercritical boilers in operation with inlet temperatures at 1112 deg F.  Design evolution is currently teasing out further increases in increments of as little as 2 deg. F., which pretty much tells me we're staring at the upper limit until there's a breakthrough in materials science.  Still, a 1-2 deg. F increase is nothing to scoff at, every degree makes a big difference in the plant's lifetime generating cost per watt-hour.

RWM

[Anonymous]

Thanks Paul and others.  That is basically what I was looking for; especially your information that if the temperature of super heated steam is high enough, it will liberate the oxygen and hydrogen.  Here is the inspiration for my question:  Recently BMW has announced a prototype for a new type of hybrid vehicle powered by an internal combustion engine that makes steam through an exhaust heat exchanger, and then uses the steam to power a steam engine, which contributes to the propulsion of the vehicle. 

http://editorial.autos.msn.com/article.aspx?cp-documentid=434462

Reclaiming waste heat from the exhaust sounds like a good idea, but adding a whole second engine just to utilize that waste heat seems a bit much.  It sounds like they actually added two steam engines for the power reclamation, with the second being smaller than the first.

I was just wondering what effect could be had by making superheated steam with the exhaust heat and then injecting it back into the combustion chambers of the engine that made the exhaust heat in the first place.  Certainly it would put heat energy back into the combustion chambers and may reclaim power by doing so.  But what if the steam were superheated enough to liberate the hydrogen and oxygen?  Could this become fuel to supplement the galsoline or diesel fuel that the engine runs on to begin with?

[erikem]

I was just wondering what effect could be had by making superheated steam with the exhaust heat and then injecting it back into the combustion chambers of the engine that made the exhaust heat in the first place.  Certainly it would put heat energy back into the combustion chambers and may reclaim power by doing so.  But what if the steam were superheated enough to liberate the hydrogen and oxygen?  Could this become fuel to supplement the galsoline or diesel fuel that the engine runs on to begin with?

Steam injected gas turbine. From a 19 year old brochure on the Allison 501-K turbine, SFC at 60F ambient is  0.48 lbm/hp-hr with no steam injection and 0.35 lbm/hp-hr with 5 lbm/sec of steam injection (7500 SHP). The steam in this case provides for extra mass flow and reduces NOx by reducing combustion temperatures.

I don't think there would be any benefit from the small amount of hydrogen produced. 

[wjstix]

Maybe I'm missing something...the steam in a steam engine is in the boiler tubes. Superheated steam is essentially being sent thru tubes back thru the firebox area to heat the steam up some more, the steam isn't sent back into the firebox. The water that becomes steam never is in direct contact with the fire.

[Anonymous]

Maybe I'm missing something...the steam in a steam engine is in the boiler tubes. Superheated steam is essentially being sent thru tubes back thru the firebox area to heat the steam up some more, the steam isn't sent back into the firebox. The water that becomes steam never is in direct contact with the fire.

I asked the queston about superheated steam because of its relationship to steam locomotives and their knowledge base on this forum, but my question is not about locomotives per se.  It is a question about reclaiming heat from the exhaust of any fossil fuel engine, converting it into superheated steam, and injecting the steam back into the combustion chambers.  It is my speculative concept inspired by the BMW turbosteamer hybird vehicle, as an alternative to that BMW concept.

It may not be practical, feasible, or worthwhile, and it may be nothing new.  So I wanted to know if anybody has heard of it being used or considered.  If the concept were to have merit, it could be applied to any internal combustion engine, including locomotives.  As has been pointed out here, the concept has been applied to steam locomotives in the form of the GAS PRODUCER FIREBOX, but the objective in this application is somewhat different than what I have suggested.

[ragnar]

So on average what percentage of what comes out of the stack is steam as opposed to  burnt fuel?

[santone]

Bucyrus,

Here is a link to an article you might find interesting.

 

http://www.damninteresting.com/?p=467#more-467

 

[Anonymous]

Thanks for that link Santone.  That is an interesting concept.  It differs somewhat from what I was suggesting in that he takes the heat away form the combustion whereas I was proposing taking heat away from the exhaust stream where it is a waste product.  It would seem to me that taking heat away from the combustion would reduce power, but still, there might be a net gain as the heat is converted to steam.  In any case, it is interesting that his 6-cycle engine runs so cool that it does not need a cooling system.

[JT22CW]

If you make steam hot enough, it will disassociate into the component hydrogen and oxygen elements

You're going to need to go as high as 4,940°F (higher than copper's boiling point) if you want to break those hydrogen bonds.  If you achieve this in air (no vacuum), the process will reverse itself quite rapidly as the hydrogens react with other available oxygen molecules.  Safe to say that such a process isn't going to occur in a steam locomotive.

Superheated steam in contact with coal generates a mix of hydrogen and carbon monoxide gas. Further processing of the hydrogen and carbon monoxide in what is called the Fischer-Tropsch (F-T) process can make everything from gasoline to Diesel fuel

The first half of that process you're describing is called "gasification", and it requires heat as well as oxygen as a catalyst to make the coal and steam react.  Carbon dioxide is another byproduct.

[erikem]

If you make steam hot enough, it will disassociate into the component hydrogen and oxygen elements

You're going to need to go as high as 4,940°F (higher than copper's boiling point) if you want to break those hydrogen bonds..

A couple of things going on in this discussion - one is whether you can get useful amounts of hydrogen from superheated steam (answer being 'no' for the reason you pointed out) - the other is that bonds do break at temperatures lower than 4,940F, and that limits how much superheat can be used due to corrosion. In a steam locomotive, lubricants will start breaking down at temperatures much lower than the corrosion limits. 

[Anonymous]

If you make steam hot enough, it will disassociate into the component hydrogen and oxygen elements

You're going to need to go as high as 4,940°F (higher than copper's boiling point) if you want to break those hydrogen bonds..

A couple of things going on in this discussion - one is whether you can get useful amounts of hydrogen from superheated steam (answer being 'no' for the reason you pointed out) - the other is that bonds do break at temperatures lower than 4,940F, and that limits how much superheat can be used due to corrosion. In a steam locomotive, lubricants will start breaking down at temperatures much lower than the corrosion limits. 

erikem and JT22CW,  Thanks for that information.  It sounds like using exhaust heat to recover hydrogen and/or oxgen would not be feasible because of the temperature needed, as well as the problem of the corrosivness.  But what about just making superheated steam and injecting back into the combustion chambers to recover the energy of its heat and its pressure?  What does hot, dry, superheated steam do to a separate combustion process that has just begun or is about to begin?

[erikem]

For a piston engine I would expect an injection of superheated steam would increase the power output and cool the flame (which reduces NOx) and conversely reduce exhaust temperature (which would limit the amount of heat for generating steam). It would increase exhaust mass flow which would allow for more power recovery with an exhaust turbine (again at the cost of reducing exhaust heat available for steam generation). One problem with steam injection is the valving.

Some of these benefits can be accomplished by simply injecting water with the intake air - water injection has been used to increase the effective octane rating of gasolene.

I've already mentioned that this works very nicely with gas turbines. 

[dldance]

There are two problems with generating hydrogen in the superheated steam.  One, corrosion has already been mentioned.  The extra hydrogen ions will combine with almost any water impurity to create an acid and acids at high temperature are extremely corrosive.  The other problem is hydrogen embrittlement.  Steel is very porous to hydrogen and any liberated hydrogen will start diffusing through the steel in the boiler and tubes, changing the characteristics of the steel.  If you look at most any old failure analysis reference, you will find a section on hydrgen embrittlement.  Free hydrogen was one of the causes of the Three Mile Island nuclear reactor problem.

dd

 

[Virginian]

Huh?  And here I thought for years Three Mile Island was because an operator over rode an automatic circuit causing the release of radioactive steam/water into a non-contained area.  BTW, that is the only documented "disaster" in history where no one was killed or injured.  Hiroshima Hysteria at work.

[erikem]

The problem at TMI was that the operator error led to the core being uncovered and subsequent partial melting of the fuel from decay heat. The hydrogen came from the reaction of hot zirconium with water. Some of the hydrogen then combined with the radioactive iodine (fission product) to form HI which readily dissolves in water (as does HF and HCl) thus trapping virtually of of the released iodine (and showing that LWR accident scenarios were vastly overestimating the consequences of an accident).

[wjstix]

So on average what percentage of what comes out of the stack is steam as opposed to  burnt fuel?

That's hard to answer, since it varies so much based on the situation. When working uphill with a long train, a steam engine is producing a lot of black smoke...plus the draft can send unburned coal up the stack too. When running easy there is almost no visible steam or smoke coming out of the stack. In between, you can see nice white steam with just some smoke mixed in.

[tomikawaTT]

Back in the late 1920s or British brethren experimented with a diesel-steam hybrid loco.  It started with steam power, shifted over to diesel after it was rolling, and could add steam propulsion on heavy grades or for fast acceleration.

The engine was, in a word, strange.  It was a flat-four arrangement with double-acting pistons, steam on the outboard ends, diesel on the inboard ends.  The boiler was heated by the diesel exhaust, and, to a lesser extent, by the diesel-end cylinder cooling jackets.  Fuel efficiency was high, thanks to recapturing what would usually be waste heat, but it was complicated and the one rail-borne example (a 2-6-2T) only generated about 900HP.

To see what it looked like, Google Kitson-Still locomotive.

Chuck

[JonathanS]

There were a number of North American steam locomotives that did inject superheated steam into the firebox.  They were used as "overfire jets" and the steam was supposed to cause additional turbulance in the combustion chamber.  This was supposed to result in more complete combustion of the coal in certain conditions.  That the use of overfire jets was not common probably says a bit as to how successful the concept proved to be.

[marknewton]

Maybe I'm missing something...the steam in a steam engine is in the boiler tubes.

Only if it's a water-tube boiler. The saturated steam is in the steam space of the barrel, combustion gases are in the tubes.

Superheated steam is essentially being sent thru tubes back thru the firebox area to heat the steam up some more...

In a loco boiler the supererheater elements are located wholly withing large diameter tubes called flues. The elements do not go back into the firebox.

Mark.

[marknewton]

When working uphill with a long train, a steam engine is producing a lot of black smoke...

No, what you're witnessing there is poor firing technique. The time to put a big fire on is before the grade, not on it. If you've seen a loco working hard upgrade and making a lot of black smoke, it means the fireman didn't know the road, and didn't anticipate the steam demand soon enough.

When running easy there is almost no visible steam or smoke coming out of the stack...

An engine being fired properly will exhibit a relatively clean stack when working hard, not "running easy". You have it back-to -front.

Mark.

[marknewton]

Livio Dante (L. D.) Porta worked on a number of improvements to steam locomotives, among them was his "gas producer" fire box.  The gas producer fire box was a small-scale coal gasifier on a locomotive, feeding a deep coal bed with a small amount of air and some steam from the boiler, and then injecting combustion air over the coal bed using "overfire jets."  This is perhaps the only application where superheated steam contacts combustion gases in a steam locomotive.

Porta certainly perfected the gas producer loco firebox, but he had predecessors. Two examples I'm familiar with are the engine modified by the CME of the railway I work for - the then NSWGR - in the mid-1920s. Wagner of the DR also did some experimenting around that period. Prior to that, tram locos built by Kitson and others inadvertantly became primitive gas-producer engines. Exhaust steam from the accesories was piped into the grates or ashpans on many of these locos.

Mark.