Compressed/Uncompressed gas combustion and Emissions

[jon c]

Hello

Maybe this is a question more for chemists but then maybe not...

We are talking about burnt emissions pollution in class. Comparing internal combustion engine emissions with household water heating systems (boilers) emissions.

Please can anyone tell us if petrol/gasoline internal combustion engine type combustion (compressed gas ignition) produces less or more or different chemicals when compared to atmospheric pressure ignition of the same petrochemicals. Production of usable heat wise is it more efficient to burn fuel air mixes when compressed?

Use LNG or LPG or Kerosene 28 as a chemical example if you wish.

thank you

 

[xxChrisxx]

You have to be quite careful when making a comparison like this. An engine is dynamic, a boiler is a static application. So you can't really make a direct comparison.

However, as this is basically homework, here is some stuff to think about:

What are the products of a complete reaction of a hydrocarbon?
Can you realistically expect to achieve a complete reaction in either scenario?
How does running fuel rich and fuel lean affect the combustion?

Have a look at those, then come back and we'll see what you think.

 

[jon c]

Thank you for the reply

They all know the chemistry of the combustion processes involved (including stoichiometry).
They also appreciate that perfect complete combustion is not possible.

Their question specifically concerns emissions from a LPG car exhaust systems compared to those from the exhausts/flues of a modern LPG household water heaters (boilers).

2 students have asked why there are no catalytic converters fitted to household heating boiler flues. I have told them I am looking into it.
I think they are all astonished at the amount of fossil fuel the average house consumes for heating, and how it compares to the average amount burnt in cars.

 

[xxChrisxx]

Hang fire, you are a teacher? sorry I thought you were a student.

Ok well, realistically in boilers you can expect complete combustion as many boilers can run lean. This is also why they don't need catalytic converters. They don't produce any of the gases that cat's work on.

Engines tend to switch quiickly between rich running and lean running depedning on throttle angle. Also depending on engine temerature you will get CO, NOx (NO and NO2) unburnt hyrocarbons, and in diesels (and direct injection petrols) you'll get particulate matter (soot basically). This is on top of the normal CO2 and H2O from combustion.


This hasn't adressed the compression issue, to be honest I've forgotten quite a bit about this so take it with a pinch of salt till I can read up on it, or someone else confirms (or calls me an idiot).

Boilers don't need to compress because the goal output is heat, which mostly comes from burning the fuel anyway. compression doesn't really help. For an engine the goal is to get the highest cylinder pressure possible (to push the pistion), compression makes this this process slightly more efficient. I have completely forgotten why this is so though.

 

[jon c]

Thank you

Boilers run leaner (more o2) OK.
I will look for some figures for LPG boiler emissions.

Some of them are doing projects on CHP units.

Anyone else please add your thoughts.

Regards

 

[cjl]

As stated above, running lean is the best way to ensure complete combustion, and boilers run lean (along with things like gas heaters). Cars have a much more complex interaction, and they do not burn as completely. A better example would be something like a jet engine - they burn hydrocarbons under very high pressure, but under much more steady-state conditions than the engine of a car. Jet engines burn lean as well, just like heaters, and as a result, they don't need anything like a catalytic converter (although they do have some interesting emissions that heaters do not, for several reasons).

 

[jon c]

So an internal combustion engine needs to run at a constant rpm, load and also be running 'lean' to have emissions comparable to domestic boilers. Many thanks.
Clearly there are nitrogen and sulphur oxides created in domestic boiler hydrocarbon combustion but at lower levels than in vehicle engines.
I haven't been able to find any figures for these domestic boiler emissions.

 

[xxChrisxx]

So an internal combustion engine needs to run at a constant rpm, load and also be running 'lean' to have emissions comparable to domestic boilers. Many thanks.

Not really, a constant speed constant load condition will allow an engine to achieve the best emissions it can achieve but you won't get comparable emissions. I was trying to think of a good analogy but I can't.

A boiler has ages to burn the fuel, as it's injected into a constantly burning fire basically. So even if some unburnt fuel that floats around for a bit, it's likely to be combust at some point.

In an engine, even running slowly, say 2000 rpm (4 cylinder).
2000/60 = 33.33 combustion events / 4 cylinders.
= 8.33 combustion events per cylinder per second.
This leaves the fuel air mixture to be burnt in 0.12 seconds.

As you can see this a very short timeframe for combustion to occur (it's not in engine terms, but compared to a boiler). Although you get an overall lean burn, it's possible that with poor mixing of the fuel and air you can have a locally rich area and have the emissions that go with it.

 

[jon c]

I understand the combustion theory, and can explain it. There are animations and even some high speed films clips of internal combustion engine combustion which show this burning process very clearly. I need to find some emissions figures for boilers and efficient running car engines to compare them. I will put some time aside to do search this weekend.

They have seen formulae showing how the compression ratio of an internal combustion engine is related to its power output and efficiency and compared these figures with the efficiency of atmospheric pressure ignition boilers (for steam production) looking at the power outputs from steam turbines for electricity generation.
(Some modern ships are powered by very large internal combustion engines and claims are that they are cleaner and more efficient than oil fired steam turbine powered vessels)

They are a switched on bunch and I want to stay 'fired up' on this one, a really important topic which is related to a lot of different areas of knowledge.

A thankyou to everyone for continuing input.

 

[Curl]

Under high pressures the Gibbs free energy of compounds changes, which can cause certain reactions to become spontaneous.