[Gasification] combustion math question (help needed)
jim mason
jimmason at whatiamupto.com
Fri Nov 16 14:08:01 EST 2007
On Nov 16, 2007 7:26 AM, Thomas Reed <tombreed at comcast.net> wrote:
> Dear Jim and All:
>
> The energy content of fuels is like the proverbial elephant: Depending
> on your first contact only, you may get a very false impression.
>
> The Chemist prefers to look at the Energy Content per MOLE (molecules
> times 6.02 X 10^23).
> The Engineer looks at energy per kg (or lb), which is the way he
> delivers and burns it and sometimes the way he buys it.
> The Pilot looks at energy per kg AND energy per volume, since storage
> space is dear on a plane
> The Environmentalist looks at energy per unit of emissions.
>
that was good tom. thank you for that. the explanation of LHV and
HHV and the relevance of flame front speed of H2 vs CO were also very
clear and well crafted.
what i was attempting in the previous message was to ask the questions
that follow from understanding of the above and other interesting
factors of H2, CO and CH4 based fuels: namely, their high detonation
resistance. i noted that engines have been largely designed in
relation to specific fuels available. and if we are going to engage
H2, CO and CH4 fuels seriously, we need to consider the modifications
in engine design they might allow.
the most significant here is the much higher compression ratios
possible given the high detonation resistance of H2, CO and CH4.
there are some other interesting heat cycle options that fall out,
which i consider below. but first let's just consider the basics.
my math question was trying to understand how to figure the
theoretical compression limits of an IC based on fuel auto ignition
temps. i listed common fuels auto ignition temps as below:
> gasoline: 232C
> octane: 206C
> diesel: 210C
> methane (nat gas): 537C
> propane: 458C
> ethanol: 363C
> methanol: 464C
> CO: 609C
> H2: 500C
the other variable i didn't mention in the first note that seems very
important is fuel/air "compostion". or rather, the variance in
"explosive flammability" as percentage fuel/air varies. you can have
the needed temp for auto ignition, but below the % limit fuel/air and
still not have ignition. how this factors in, and how autoignition
temp varies with pressure, i need to understand more clearly. any
help in understanding how to figure this would be much appreciated.
however, i'm sure this is a large non-linear mess, and the answer is
likely "go get a phd in combustion science" so maybe let's simplify
the question by asking it as "rules of thumb" that have been learned
by experience with real engines and learned trends from proper
complicated modelling.
therefore to recast the question, are there general answers for the following?
- if i double the compression of an unthrottled IC engine, how much
will my efficiency increase?
- if i double the compression of an unthrottled IC engine, how much
will my shaft power increase?
- what are the relative throttling losses in a spark gas engine, vs an
unthrottled diesel engine at 10%, 25%, 50% and 100% power. at 100%, a
spark and injection/compression fire engine should be equivalent
(assuming fuel/air mix is equivalent)
- a diesel engine is approximately 20% more efficient than a gasoline
otto engine. how much of this is from increased compression and how
much of this is from the elimination of throttling losses?
these questions are motivated by a desire to assess whether a
different type of heat cycle might be possible with H2, CO and CH4
based fuels. these gaseous fuels are so detonation resistant, that
one can run in the diesel realm of compression (high teens to 20:1
range).
if you do not have a detonation problem, it is actually preferable to
run an otto cycle and premix the fuel/air instead of inject it at the
ingnition event like a diesel. a diesel type last fuel injection
leads to all sorts of adequate fuel/air mixing problems, famous with
diesels, and thus the relatively dirty burning of diesel engines. in
a diesel engine, one ends up with a diffusion flame to a significant
extent, instead of flame across an already mixed charge.
note that diesel has an auto ignition temp nearly identical to
gasoline. around 220C or so. one could not run an otto cycle with
diesel fuel at diesel engine compressions. you would get bad
detonation. but you can run H2, CO and CH4 at diesel type
compressions.
my proposal for H2, CO and CH4 based fuels is then twofold:
1. can we run such gaseous fuels in a manner that combines the best of
otto and diesel? namely, can we run the engine unthrottled (lean
burn), with premixed fuel/air, and ignite with a spark plug?
certainly we could do this at higher power amounts, which will have
close to stoich fuel air mix, and thus reasonable ignition, but will
the engine still run at lower fuel amounts? or in formal terms, will
the fuel/air composition of the charge at low power, say 5% power,
still be above the fuel concentration explosive limit so ignition will
still happen? or will the mixture be too lean to go off?
2. could we run such fuels under an HCCI (homogeneous charge
compression ignition) scenario, using the turbo and/or supercharger in
real time to vary the compression, and thus control the ignition
timing? if so, such would allow for efficiencies above both typical
diesel engines, and gasoline based HCCI engines. the possible
compression would be about double that of regular gas engine, and the
fuel/air mix would be much better than diesels, thus better energy
extraction.
historic IC engine design was largely in response to the realities and
opportunities of specific fuels, in combination with machining and
metalurgical specifics of the day. if we are now going to start
running biofuels significantly, we should note the new things they
allow/disallow in engine design, and alter engine design accordingly.
we should not just simply be replacing fuels into engines optimized
for a different fuel. thus the very high detonation resistance of H2,
CO and CH4 seem very important positives that deserve more attention.
also, it seems that one could make a very high performance motor with
these fuels. the turbo boost limits would be very high, like a diesel
engine. a gas engine is limited on turbo boost by the premixed
fuel/air mixture and the resulting auto detonation problems.
jim
>
> (Sorry I couldn't come up with seven wise men).
> ------------------------------------------------------------------------
>
> Hydrogen is the BEST fuel based on weight! Hydrogen is the WORST fuel
> based on volume (and storage and shipping and leaking and .....!) Most
> of us live between these extremes.
>
> In the US solid fuels are measured on a High heating value (HHV) basis
> and liquid and gas fuels on a LOW heating value (LHV) basis, typically
> ~10% lower, depending on hydrogen content. In Europe all fuels are
> based on a LHV basis.
>
> Hydrogen has by far the highest burning velocity - ~3 m/s compared to
> 0.4 m/s for most fuels. This lends some of hydrogen's advantages to
> mixtures with all other fuels.
>
> Hydrogen and Carbon Monoxide both have 280 kJ/mole heat of combustion
> when LIQUID water is the product, but in practice only a few systems
> condense the water to use the heat of condensation (550 cal/g; 2300 J/g;
> 1000 Btu/lb; 41.4kJ/g). So the LHV of hydrogen is only 239 kJ/mole, 15%
> *less* than carbon monoxide!
>
> Fortunately, synthesis gas (CO + H2) and producer gas (CO + H2 + N2)
> utilize BOTH the carbon and hydrogen found in all fuel sources and the
> hydrogen high velocity and wide flammability limits compensate for the
> low combustion velocity of CO. As a result, producer gas (*woodgas*,
> Gengas, Holzgas, moteur gaz, etc.) is a very forgiving fuel in the
> conversion of engines for power and transport while synthesis gas is our
> best hope for methanol, mixed alcohol and diesel fuel production.
>
> Onward with good science and engineering,
>
> Yours truly,
>
> TOM REED THE BEF
>
>
>
>
>
>
> jim mason wrote:
> > i have a math question i need worked out in regards answering the potential
> > efficiency (usability) of syngas related fuels in IC engines. hopefully
> > someone here can help.
> >
> > in figuring out the potential for reasonable operation of H2 and CO gaseous
> > fuels in a spark ignition IC engine, we need to allow for the higher
> > compression ratios that CO and H2 fuels will support. diesel engines are
> > more efficient than gas engines bascially from the higher compression
> > ratios. well, that and the lack of air throttling losses too.
> >
> > when comparing the auto ignition temps of gasoline with CO and H2, clearly
> > much higher compression ratios are possible, thus higher efficiencies, which
> > can compensate for the reduced energy density of the biomass derived syngas.
> > adding a turbo or the like to syngas engines to increase compression to
> > just short of the detonation compression should do much for efficiency. but
> > how much i am unsure. (and sadly i am also unsure of the math).
> >
> > some data on auto ignition temps of common fuels (or how high the
> > compression can get before the fuel with detonate in a premixed air/fuel IC
> > situation, like a gasoline spark ignition engine). most are here:
> > http://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html
> >
> > gasoline: 232C
> > octane: 206C
> > diesel: 210C
> > methane (nat gas): 537C
> > propane: 458C
> > ethanol: 363C
> > methanol: 464C
> > CO: 609C
> > H2: 500C
> >
> > the question then is how high of theoretical compression can a premixed,
> > spark ignition engine run with each fuel before detonation will occur? of
> > course this is a non-linear problem with all sorts of complicating factors,
> > but how does one figure the ideal case, isothermic scenario?
> >
> > and the related question, what is the usual increase in effiency gain per x
> > multiple increase in compression ratio?
> >
> > once the max theoretical compression ra
> > tio is somewhat known, we can figure the theroetical max power from the HHV
> > or LHV of the fuel at x rpm. some common values at HHV
> >
> > gasoline:
> >
> > C + O2 = C02 + 393 KJ/MOL
> > CO + 1/2 O2 = CO2 + 283 KJ/MOL
> > H2 + 1/2O2 = 285 KJ/MOL (241kj/mol without H20 condensation)
> > CH4 + 202 = CO2 + 2H20 + 890 KJ/MOL
> >
> > other values are findable here;
> > http://en.wikipedia.org/wiki/Heating_value
> > http://en.wikipedia.org/wiki/Higher_heating_value
> > http://www.google.com/search?q=lower+heating+value+gasoline&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a
> >
> > i realize this is a rather large question. apologies ahead of time for such
> > a doozie. nonetheless, any tutoring help with the math here is much
> > appreciated.
> >
> > jim
> >
> >
> >
> >
> >
>
> --
> ÐÏ à¡± á
>
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--
--------------------------------------------------------------------------------
jim mason
website: www.whatiamupto.com
current project: mechabolic (http://www.mechabolic.org)
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