Maybe some "off" questions about condensing steam generation system.

[fastline12]

Trying to help a friend out with a boiler system he is working on. Though he has the background for the thermodynamics, neither of us have experience with steam systems so some of the hurdles I am certain are already pretty standardized.

He is building is own double acting piston engine and powering this from biomass. There is simply not enough fuel to warrant a turbine design I don't believe and as I understand it, those do not scale very well and need to run very high pressures to be happy.

Anyway, two questions I might have for anyone in the industry, though I will probably have more.

1. Engine design. assuming generation in 20-50KW range, and realizing a double acting piston might have decent volumetric efficiency, is there any other engine designs that come to mind that have an edge either in simplicity, maintenance, efficiency, or cost? This one will be 24/7 and I fear the life of a piston engine will not be ideal.

2. The main point to my visit is in working out some math here, it seems that we really need to bring the exhaust side pressure down to optimize efficiency, and as well, he wants this system to be a closed loop, condensing system. I am at a loss to figure out how to get the low pressure condensed water back to the boiler without using a type of high pressure pump? Then there may be an issue of balancing the return water to the exhausted condensate and you certainly will want to run a condensing cycle to protect the pump I would think. Is there another solution to this problem I am not seeing? All the circuit drawings I have found show a pump, but make it look like no big deal.


This system is designed to run at med pressures 500psi so some of this is getting a bit "involved"

 

[Hoxton]

Someone in UK was developing a steam engine generator system for recovering engine exhaust heat - gas engines on landfill etc

I think they we're using a Scania truck engine as a basis. Lot cheaper to buy an engine and modify it, than to set off and design an engine yourself!

Not as much fun, I admit, but a lot cheaper.

From memory, this was a 100 kW or so. And just as complex as a 1000kw system!

I will try and find the details.

 

[NBrink]

Generally a pump is the way such things are done. There is a magical device called a steam injector that uses steam off the boiler to feed water into a boiler. Not commonly used today, since they're not really suitable for high pressure boilers, but might be something to look into if you really don't want a pump. Very efficient too...

Nathan Brink

 

[zeusfaber]

So far as getting water back into the boiler, have you come across injectors?

A.

 

[fastline12]

Thanks you guys. the injector does seem to be something to look at. However, this system is being designed with "storage' in mind, in which the entire boiler system is held at the same high pressure. I will have to think more about the issue but I think even if we are able to get the condensate picked up in a steam line, we will still need to increase pressure or get some pressure delta to get this back to the boiler.


On the engine, I notice that most have abandoned the double acting piston design and not sure why? Is it that the volumetric and mechanical conversions are so good that it doesn't matter? I realize the heat conversion is pathetic but it is otherwise free energy. I do like to idea of using a gasoline engine for conversion, but I wonder if you can really get good efficiency without variable cylinder sizes, thus multi-stage expansion? Really seems that is where the turbine shines as it beautifully utilizes high pressure gas as it expands.

 

[EdStainless]

You need a copy of B&W steam, back about the 34th ed.
These systems are complex if you are running closed.
You need a condenser, and way to pull vacuum on it to remove non-condesables, and way to deliver feedwater at pressure back into the boiler. A steam driven pump is the most common way to do this. If you want to get fancy you can add a a feedwater heater that uses a little hot bleed steam to raise the feedwater temp before it goes into the boiler.
In commercial applications the feewater enters the boiler at the temperature and pressure, and they just use the boiler to provide the change of state.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[GenB]

Ideally cndensing the exhaust steam and install an adapter for the steam engine to run the feed pump. 'can't push the steam back into the boiler.

General Blr. CA,USA

 

[fastline12]

Yes sir, I think I realize we need to condense the exhaust. I think the reality is he is designing for budget and simplicity and realizing closed circuit, efficient steam is not easy.

I was thinking more about a pump or "feed pump" to return condensate but I also read that water may need conditioned or at least air removed. I am wondering about easy solutions for a pump? I would think this would have to be a positive displacement pump to get the higher pressures.

I agree with sistering off the drive of the steam engine for return but in my thinking, I would then have to match the inlet/outlet flows as the pump would be running constantly. I am thinking for simplicity, maybe just a high pressure electric pump with a reservoir for exhaust condensate?


Any more thoughts on the engine guys? Is piston the way to go? We did talk about using some older inline engines and my facility can design and manufacture a custom head for that, but I think this needs careful thought so we get something that is both efficient and low maintenance. I am not sure a standard piston engine will really be able to provide 24/7 service for very long but maybe 1yr of life is reasonable?

 

[Hoxton]

Well I did some research on the project I mentioned above. The development company seems to have constructed one project, ran into many problems and abandoned the idea!

I had a quick search to see if there were others working in the same field (steam boilers, small steam engines), could not find any. Several working on Organic Rankine Cycle (ORC).

Yours may be ok as a demonstration and development project, but to get into commercial operation you have many obstacles, often these are helped by going for larger projects (1000 kWe +) where you get higher financial returns, but of course greater development and construction cost.

 

[fastline12]

Hoxton, I appreciate you looking into it. My company has a few interests in this sector, thus my interest in helping with this project. I am finding that this area is not very well supported. You either have hobbyists tinkering with very low efficiency open loop automotive engine systems, and full large scale commercial systems running turbines.

I would sure take some advise and references on where to soak up some knowledge on the subject. I believe it is counter productive to mess around building stuff that has already been done and documented.

From some brief reading, it seems there could be an lubrication requirement for a high pressure steam piston engine in which the oil becomes a problem getting in the system. With so many other engine types and steam systems out there, I feel like I don't have a firm enough grasp yet to break ground on anything.

 

[Hoxton]

I thought I saw a link to a uk firm that were working in this area. I will look again.

I do not want to b too negative, but have spent several years dealing with inventors who thought they all had the clue to riches via their ideas, but who just did not have the understanding of basic engineering principles

I am glad that you seem to have your eyes open.

Power generation from biomass is not new. The sugar cane industry for example burns the used cane material (bagasse) in a furnace, which feeds a steam boiler which feeds a steam turbine.

These usually power an ac generator. I have seen them with powers of 500 kW or more.

Now at that sort of size you get economies of scale. It is worth finding a clean source of water, and putting in the water treatment plant you need to get the quality of water you need for a steam turbine. You also have the money for all the design work you need to do, fairly straightforward, since this is proven, mature technology.

What you want to do, is scale this down, possibly by a factor of ten. BUT your design work and your components (such as monitoring and instrumentation equipment) will not reduce in the same proportion.

Also if you con get a P&ID for a 500 kW turbine set and a 5000 kW set, you will not find many difference, since the problems are much the same.

Good luck !

 

[EdStainless]

There are small radial flow turbines that are in the size range that you are talking about.
The use of multiple piston expansions is fairly straight forward.
One problem is that if the expansion ratios are fixed then the unit will have a single operating point for best efficiency.
This is the steam locomotive problem, the engines hauling across the plains could run at the most efficient conditions, but everything else was a compromise.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

Forgive me, but the OP and the responses seem to me to indicate a low level of understanding of what you're trying to do and the issues around it.

to start with "This system is designed to run at med pressures 500psi so some of this is getting a bit "involved" " is just the start of it. 500 psi is a high pressure for such a system - steam locos normally sit some where around 200 psig, though there are higher pressure units. I truly hope your friend isn't building the boiler as well... or if he is that it is properly designed, constructed, tested and certified / registered.

A single cylinder set up will be very inefficient - the key with any expanding gas / vapour is to extract as much mechanical energy as possible from the expanding fluid which can often take two or three sets of turbines or systems to avoid excessive size or velocity on the exhaust.

As well noted above, you really want to look at a fixed power, but maybe your 2- - 50 kW was just to identify a range.

Biomass is a bit of a oose term so difficult to know what it is, but trying to use this to create electricity at that sort of power range is never going to work or be efficient, unless your fuel is free.

For your original system, whilst injectors could be used, boiler feed pumps are very common and power is normally low compared to output of a boiler - a few percent as the amount of water being returned in volume terms s low, thought the pressure rise is high.

Having a buffer storage of condensate would be key to balance the system in terms of balancing mass flows of water / steam.

There is a good reason why this hasn't been done before / is very common - it is very expensive and difficult to get working efficiently for what is a fairly pitiful output. There is a good reason why small biomass systems work best on a space heating issue / supply of hot water / low pressure steam to an domestic system.

Like hoxton, we see here and in general an unending supply of people who think their system will solve the energy demands of the world, but on closer review appear to have missed several vital issues.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[fastline12]

Just a few points.

1. The fuel is basically free, and usually has to pay to remove it. Just using it up is ideal.
2. The thermodynamics do indicate this is not a pointless venture due to the amount of fuel source and the facility utility costs for heat and electric.
3. The boiler will be custom built but by a facility that makes them. It will then be fully proof tested and certified to well beyond operating pressure.
4. I mention 500psi as medium pressure because real commercial systems run in the range of 2000psi and beyond. We won't be touching that.
5. The pressure is being raised because the calculations seem to indicate higher efficiencies can be achieved by raising the super heat.
6. I don't think we are trying to cure cancer here, just that they are currently paying for utilities, and paying to remove a biofuel that can easily cover their energy needs.
7. Some of this heat IS going to be used for direct heating. It seems bad practice to convert it all to electric, just to then run electric heating.

 

[LittleInch]

I realize we're throwing darts at your project, but that's part of what happens here sometimes... sorry.

I can fully see that you want ot do something with this material, especially as it's free and yes, it's true that the efficiency goes up as the temperature goes up, but so does the cost and that's where most of us I think can see the disconnect.

Yes, you can get this system to work, but it will cost so much that even over a 20 year period, you'll end up paying more for your electricity and heat than you do at the moment.

500psi steam is quite some beast to tame and like I said - if you look at any other generating system you'll find tow or three levels of pressure to actually use the steam expansion. That costs a lot of money, but otherwise you'll end up with steam at 350 psig coming out of your piston - what happens to it then??

Ditto the condenser, de-aerator, cooling tower and all manner of control gear.

Far better I think to look at other much lower pressure steam turbines as a once through system or feed the outlet steam into your heating system. Clearly you'll need a water treatment plant and somewhere to send excess steam or hot water, but start simple and cost it before you go the whole hog, especially for such a relatively small power range. There are significant economies of scale when it comes to things like this.

Far better to spend say 200K and get a 15KW system than 1000K and get a 25kW system...

Just my opinion, but I hope all you're investing at the moment is your time.

Let us know how it goes.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[crshears]

Skinner made a marine compound "unaflow" engine designed to expand steam from ~750°F & ~440 psig down to moderate sub-atmospheric vacuum, although at much, much higher horsepower ratings than you are describing; see

http://www.carferries.com/skinner/

to see the individual pages of their sales brochure. As to whether the patents and rights are now public domain or still held by some entity I am uncertain...

The car ferry Badger has a pair of these engines still in active annual service between Ludington, Michigan and - Manitowoc? - Wisconsin; there are some YouTube videos of these engines available on line.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]