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Wasteful

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Tweefo
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« on: April 05, 2014, 09:36:50 AM »

Driving past Sasolburg the other night I noticed the big flame on top of a chimney again. I live on the Highveld and there are a lot of these industries in towns like Secunda, Witbank and my home town of Middelburg to name a few. I've heard it's burning off poisonous gasses, but can't this be used as an energy generating source? It seems to be very wasteful to me. I am not a tree hugger, we need those places, but to just release all of that energy into the atmosphere looks downright criminal to me. Especially in the light of dwindling resources, not to mention load shedding on Escom's part.
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Brian
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« Reply #1 on: April 05, 2014, 09:53:19 AM »

Don"t know abt the gases but to me it's energy. If they're burning methane or similar it can be piped to industries etc for heating boilers etc...maybe too costly, but there are many of these.
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brianvds
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« Reply #2 on: April 05, 2014, 10:23:48 AM »

My guess is that at present it is cheaper to burn it than to use it for anything else. Perhaps, as fossil fuels become scarcer, we won't see those flames anymore.
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Mefiante
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« Reply #3 on: April 05, 2014, 10:29:26 AM »

For reasons of thermodynamic efficiency, a turbine or internal combustion engine is the optimal solution for power generation from combustible fuel.  The problem is that you can’t just feed any old combustible stuff into a turbine or engine and hope that it will run.  Those waste gases would need to be refined by separation into usable constituents (“scrubbed” is the technical term) and therein lies the major problem:  It’s expensive both in terms of capital outlay and running costs, especially as a retrofit to existing machinery.  Another significant problem is that if those waste gases aren’t of the right sort, it may end up requiring more energy to scrub them than can be usefully recovered from power generation from their combustion — i.e. the process has a negative overall efficiency.  Finally, the industrial process may be of such a nature that waste gas production is sporadic so that huge storage tanks are required as a fuel buffer, or power generation is intermittent.

In short, it’s an engineering problem, but not at all a straightforward one.  Still, probably the best use of such raw gases would be for heating, particularly water, for use by people but again there may be severe logistical constraints that prevent such an implementation.

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Rigil Kent
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« Reply #4 on: April 05, 2014, 11:21:20 AM »

Outside of Mossel Bay is a refinery sporting a similar flare stack. Except for the problem of its ridiculous height, I don't see why it can't be fitted with a boiler feeding into a steam driven dynamo.

On a somewhat related issue, has anyone driven past our lovely new local wind-farm?

Rigil
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Tweefo
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« Reply #5 on: April 05, 2014, 11:40:55 AM »

Outside of Mossel Bay is a refinery sporting a similar flare stack. Except for the problem of its ridiculous height, I don't see why it can't be fitted with a boiler feeding into a steam driven dynamo.

On a somewhat related issue, has anyone driven past our lovely new local wind-farm?

Rigil

No, but I went past Cookhouse (Bedford to Somerset East) two weeks ago and saw a big one on a hill. They were all standing still though, Maybe still under construction?
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Rigil Kent
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« Reply #6 on: April 05, 2014, 12:06:17 PM »

Not sure about the Bedford/ Cookhouse one, but the sixty odd units near Jeffreys are spinning happily. They were switched on recently to provide a stiff breeze for the National Windsurfing Championships. Wink

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Mefiante
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« Reply #7 on: April 05, 2014, 16:26:24 PM »

… I don't see why it can't be fitted with a boiler feeding into a steam driven dynamo.
Would you spend R100,000.00 to buy a device that saves you R500.00 a month in power costs and lasts 15 years?  I know I wouldn’t.  It simply doesn’t make economic sense, NPV calculation or no.

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Tweefo
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« Reply #8 on: April 05, 2014, 18:31:49 PM »

… I don't see why it can't be fitted with a boiler feeding into a steam driven dynamo.
Would you spend R100,000.00 to buy a device that saves you R500.00 a month in power costs and lasts 15 years?  I know I wouldn’t.  It simply doesn’t make economic sense, NPV calculation or no.

'Luthon64
It's a very big flame, so it still sounds wasteful to me. Don't know what the cost would be to change it, so maybe you are right and you won't save anything. When these plants were designed and build, back in the day, energy was not as costly as it is today. In a new plant it might be cost effective to design it with this in mind, a little like a turbocharger, use the waste energy.
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Rigil Kent
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« Reply #9 on: April 05, 2014, 18:44:47 PM »

Would you spend R100,000.00 to buy a device that saves you R500.00 a month in power costs and lasts 15 years?

Oh, I've done a lot worse than that!

In a new plant it might be cost effective to design it with this in mind, a little like a turbocharger, use the waste energy.
Well, if it's not economical to turn it into electricity via the steam route, then maybe the flame could make itself useful by helping to heat up whatever it is that they are distilling.

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Mefiante
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« Reply #10 on: April 05, 2014, 19:45:10 PM »

Oh, I've done a lot worse than that!
Probably most of us have at one time or another, but shareholders don’t care how cool (or hot!) your gear is.  They want returns on their investment — the sooner and the more, the better.

Don't know what the cost would be to change it, so maybe you are right and you won't save anything. … In a new plant it might be cost effective to design it with this in mind, a little like a turbocharger, use the waste energy.
Sure.  As I wrote earlier, retrofitting is very costly, not only because it entails modifications to existing systems but also because there are disruptions to normal production, a double whammy for shareholders who aren’t in it for the charity.  Modern plants are designed with a bunch of energy-saving and recovery systems, which raise their cost by a smallish fraction but which don’t suffer from any of the problems and limitations of retrofitting.  (BTW, it’s a mistake to think that a turbocharger significantly improves the efficiency of an internal combustion engine.)

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Tweefo
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« Reply #11 on: April 06, 2014, 06:40:54 AM »

Quote
(BTW, it’s a mistake to think that a turbocharger significantly improves the efficiency of an internal combustion engine.)
I don't like them myself. The engine's got a shorter lifespan, but you do get more power. I am more of the "get there as cheaply as I can" as opposed to the "fast as I can" travelers.
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« Reply #12 on: April 07, 2014, 08:27:51 AM »

Well I think they look cool, so they have that use.
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BoogieMonster
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« Reply #13 on: April 07, 2014, 10:04:45 AM »

BTW, it’s a mistake to think that a turbocharger significantly improves the efficiency of an internal combustion engine.


It would seem counter-intuitive to me then that in the wake of EU and US emissions regulations many manufacturers are going the "smaller engine with a turbo strapped on" route. It would appear to me that they're doing this because they can get away with a smaller displacement putting out the same, or better, power figures whilst lowering emissions. Perhaps this is where I'm making the mistake, equating efficiency with lower emissions. To me they would be directly related. Perhaps the real fuel savings are to be had in other parts of the engine and then the turbo is used to get the power back? *confused*

I will say this though, having driven a couple I'm stunned by some of these small displacement turbo engines. They are really impressive.

On a somewhat related issue, has anyone driven past our lovely new local wind-farm?


I did drive past Jeffreys, heading to PE, in Dec and did note a wind farm next to the highway somewhere along the way. Is this what you are referring to?
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Mefiante
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« Reply #14 on: April 07, 2014, 13:03:04 PM »

My point is that, yes, turbocharging a given engine does improve fuel efficiency but that it’s not an enormous saving because of the way cars are normally used.  Car manufacturers are going the small-engines-with-turbo route because they get green points for doing it, for comparable power output smaller turbo engines’ fuel efficiency is better, and because they can charge more for small-engined vehicles with impressive performance stats.  What they’ve done with diesels illustrates the point:  A diesel is a simpler, less expensive engine to manufacture (even with a turbo and ECU) than a similarly specced petrol engine, and yet diesel cars are pricier than their petrol counterparts.  (The claim that it’s demand-driven isn’t convincing in view of manufacturers’ ability to adjust production volumes in response to demand.)

There are two main aspects to the turbo/efficiency question.  The first is the engine’s thermodynamic efficiency, which is basically the useful work output it delivers, expressed as a fraction of the total input energy.  The second aspect has to do with the performance characteristics of the engine, in particular the torque and power at different engine speeds.

For an internal combustion engine, the input energy is in the chemical potential energy of the fuel.  By using the thermal and the kinetic energy of the exhaust gases, a turbo forces a greater volume of air into the engine’s combustion chamber, ensuring more complete combustion and reduced harmful emissions (partially combusted fuel).  However, since the fuel-air ratio must be correct (determined by the absolute pressure of the mixture), an additional amount of fuel is also needed.  The engine’s overall thermodynamic efficiency is increased slightly because a bit more of the fuel’s chemical energy is converted to useful work (manifested by the exhaust gases of a turbocharged engine being cooler and slower-moving).  The maximum thermal efficiency increase is from about 20% – 25% for a normally aspirated engine to 25% – 30%, which typically occurs at about 75% of the RPMs at which the engine develops its peak power.

So, yes, the engine’s thermal efficiency does increase somewhat with turbocharging.

However, the fuel saving and lower emissions benefits are predominantly the result of the fact that a turbocharged engine has a significantly different torque and power characteristic to a normally aspirated one.  Specifically, its torque is higher (the pressure ratio of the turbocharger determines how much) and, equally importantly, more uniform across a wider RPM range.  Torque is what’s needed to get a car moving, and the turbocharged engine reaches its torque peak at lower RPM, which translates into less incomplete fuel combustion (= wasted fuel) on pull off.  Also, the wider maximum torque range means fewer gear changes in stop-start traffic.

On the open road, the picture is similar.   Suppose you have two cars.  Car A has a 1500 cc normally aspirated engine, and car B has a 1500 cc turbocharged engine.  Car A cruises at constant speed in top gear along a highway so that its engine is running in the optimal torque band (i.e. under constant load in the RPM range around the torque peak), and car B cruises at the same speed.  Car B’s fuel efficiency will be slightly lower, in line with the improvement in thermal efficiency of about 5%–10%, which isn’t huge, but it’s still lower.  This reduction is of course offset by the increased capital and usable lifetime cost of car B.

ETA: In this article, a detailed technical comparison is made between a normally aspirated and a turbocharged version the same engine.  Combined cycle fuel efficiency and CO₂ emissions were both about 5% lower for the turbocharged engine, while peak torque and power were about 35% higher.  This suggests that judged from the perspective of fuel utilisation efficiency, for the same peak power, a smaller turbocharged engine would have a fuel consumption just 70% that of the normally aspirated model, which is a significant reduction.  But the fact remains that the same engine in turbocharged guise delivers a fuel consumption improvement of only 5%–10%.

'Luthon64
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