[Stoves] [Gasification] combustion math question (help needed)

[Thomas Reed]

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.

(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
>
>
>
>
>   

-- 
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