Ethanol production 9

[JLSeagull]

The press speaks of agricultural methanol as a solution for our energy dependence. What is the yield in terms of volume of ethanol per bushel of corn? What energy is required to process corn to ethanol? What is the likelyhood that ethanol from corn could replace significant portions of the crude oil based refining or petrochemical industry?

 

[owg]

fvincent - Thanks for the link to the additional P&P study.

HAZOP at

http://www.curryhydrocarbons.ca

 

[owg]

Here is an Appendix that was added to the CRPS paper when it was published in "Critical Reviews in Plant Sciences". Sorry about the fonts, they got scrambled during a copy and paste. In the original h sub 1 etc. was eta sub 1 designating efficiency. The bottom line is that the author estimates that a fuel cell vehicle running on ethanol and converting it to hydrogen and then to shaft power would have an energy efficiency of only 38% not 60% as assumed in some studies (guess which ones>). Footnote 68 is a bit of a mess but I left it in. I gave up trying to fix "Delta H superscript 0 subscript f".

Dave: If the following violates any site rules please delete it.

APPENDIX: EFFICIENCY OF A FUEL CELL SYSTEM

In their Science paper, Deluga et al. (2004) claim the
following:
. ..Further, combustion used for transportation has .20% efficiency
as compared with up to 60% efficiency for a fuel cell . . . The
efficiency of these processes for a fuel cell suggests that it may be
possible to capture >50% of the energy from photosynthesis as electricity
in an economical chemical process that can be operated at
large or small scales. (p. 996)
Following Deluga et al.,Patzek (2004) used 60 percent as
an estimate of the overall efficiency of a hydrogen fuel cell car.
Even this optimistic estimate could not make the industrial corn-ethanol
cycle sustainable to within a factor of two. Not so with
sugarcane ethanol. It might be called somewhat sustainable if
the path from the ethanol to electric shaft work were 60 percent
efficient.
First, we assume that the cane ethanol-water mixture used to
generate hydrogen is analytically pure C2H5OH and H2O. Thus,
there are no other contaminants to poison 65 the delicate catalyst
that will convert this EtOH-H2O mixture to hydrogen, carbon
dioxide and carbon monoxide (Deluga et al., 2004). The catalyst
is made of a rare-earth metal, rhodium,66 and a Lanthanoid,
cerium. 67 The catalytic reaction is claimed to have 100 percent
selectivity and >95 percent conversion efficiency. We assume
the conversion efficiency h1=0.96.

After Bossel (2003) we summarize efficiency of a Proton
Exchange Membrane (PEM) fuel cell as follows. In fuel cells,
gaseous hydrogen is combined with oxygen to water. This process
is the reversal of the electrolysis of liquid water and should
provide an open circuit voltage of 1.23 V (volts) per cell. Because
of polarization losses at the electrode interfaces the maximum
voltage observed for PEM fuel cells is between 0.95 and
1.0 V. Under operating conditions the voltage is further reduced
by ohmic resistance within the cell. A common fuel cell design
voltage is 0.7 V. The mean cell voltage of 0.75 V may be representative
for standard driving cycles. Consequently, the average
energy released by reaction of a single hydrogen molecule is
equivalent to the product of the charge current of two electrons
and the actual voltage of only 0.75 V instead of the 1.48 V corresponding
to the hydrogen high heating value.68 Therefore, in
automotive applications, PEM fuel cells may reach mean voltage
efficiencies of
h2=0.75 V/1.48 V =0.50 [10]
However, there are more losses to be considered. The fuel cell
systems consume part of the generated electricity. Typically, automotive
PEM fuel cells consume 10 percent or more of the
rated stack power output to provide power to pumps, blowers,
heaters, controllers, etc. At low power demand the fuel cell efficiency
is improved, while the relative parasitic losses increase.
The small-load advantages are lost by increasing parasitic losses.
Let us assume optimistically that for all driving conditions the
net power output of an automotive PEM fuel cell system is about
h3=0 9 of the power output of the fuel cell stack.
Depending on the chosen drive train technology, the DC
power is converted to frequency-modulated AC or to voltage-adjusted
DC, before motors can provide motion for the wheels.
Energy is always lost in the electric system between fuel cell
and wheels. The overall electrical efficiency of the electric drive
train can hardly be better than h4=0 9.
By multiplying the efficiency estimates, one obtains for the
maximum possible tank-to-wheel efficiency of a hydrogen fuel
cell vehicle
h=h1*h2*h3*h4=0.96 ×0.50 ×0.90 ×0.90 =0.38 [11]
or 38 percent. This optimistic estimate agrees exactly with an-other
analysis (31 to 39 percent) (Fleischer and Ørtel, 2003), and
is significantly less than the 60 percent used by the promoters
of a hydrogen economy and hydrogen fuel cell vehicles.

65 The commercial ethanol fuel is very dirty by chemical catalysis
standards, but we will ignore this unpleasantness.
66 Rhodium is a precious metal whose price is about
US$30 000/kg, 3 ×more expensive than gold,

http://www.kitco.com/

charts/rhodium.html.
67 The nanoparticles of cerium dioxide are called ceria, and cost
$250/kg,

http://www.advancedmaterials.us/58N-0801.htm

68 According to Faraday’s Law, the standard enthalpy of combustion
of hydrogen, H 0
f =.285 9 kJ/mol, can also be expressed as an elec-trochemical
potential (“standard potential”) U 0
=.H 0
f /ne F =1.48
V with ne =2 being the number of electrons participating in the con-version
and F =96485 Coulomb/mol the Faraday constant.



HAZOP at

http://www.curryhydrocarbons.ca

 

[bimr]

fvincent (Mechanical)

It seems that you are overly critical. Patzek states that the information on sugarcane ethanol production has not been well described. You on the otherhand seem to be trying to get more precision out of the limited information that is available.

Rather than being focused on the efficiencies of a few processes in the ethanol cycle, you should look at the big picture.

You are also throwing out a lot of terms. Cogeneration is a good example. Cogeneration (also combined heat and power or CHP) is the use of a power station to simultaneously generate both heat and electricity. It remains a mystery as to who you will sell the steam to that you are generating in your power station.

Modern power plants are not orders of magnitude more thermally efficient than older plants.

You also seem to agree with Patzek's theory: No matter how efficient the engine is that transforms the industrial ethanol cycle’s output into shaft work, the cycle remains utterly unsustainable and unattractive as a source
of fossil fuel.

And further. that "Locally-produced electricity from biomass seems to be the best option that could make a prolific acacia and sugarcane plantation “sustainable,” if their immediate environments were not degraded by the toxic ash and air emissions."

 

[moltenmetal]

owg: the electrochemical analysis boggles me a bit. Does the analysis of the fuelcell system presented also take into account the chemical conversion of hydrogen across the fuelcell itself, or does it assume that 100% of the hydrogen fed is put to beneficial electrochemical use? I understand that real PEM fuelcells, especially those fueled by reformate-sourced hydrogen (i.e. containing more than a trace of CO) generally do not convert 100% of the hydrogen they're fed? They leave a fair bit of unconverted hydrogen in the anode tailgas from what I've heard.

 

[fvincent]

bimr

Old sugar cane ethanol plants have a very small surplus of power generation (let's assume a value as high as 3,3 GJ/ha-yr). Modern ones have higher surplus (I'd say 20 GJ/ha-yr). So the question is not only the power generation but the comparison between surplus supplied to the grid which should be accounted as credits for the ethanol production. The reason for the increase in the surplus is in the fact that a pure Rankine cycle operating between 22 bar/300oC with a few stages turbine (50-55% efficiency) generates half of the power produced by a 60-80 bar / 480-520oC pure Rankine cycle with 75-80% efficiency turbine. That is the trend.

As to the question of the use of the steam, if I have understood it clearly, you probably know that part of the steam is extracted from the turbines and sent to the ethanol plant and the rest is condensed after leaving the turbine exhaust, generating extra power.

I could add some other mistakes: vinasse is not simply treated. It is a source of biogas (0,1 Nm3/ liter of ethanol) or nutrients. Besides all the power used to run the waste water plant comes from the power plant.

I am not overly critical: if the study intends to be a reference on the use/production of ethanol, the numbers must be obtained from modern plants. The methodology used by Patzek is very interesting. No doubt about that. But why to take a low efficiency plant as reference instead of a modern plant? Why should one assume that the extra exergy consumption could be compared directly to the mechanical work at the shaft of an engine fueled with ethanol? Doesn't it bias the conclusions?

I do look at the big picture but I notice some blurred points. After removing or correcting these spots the picture is suddenly other... Just that..

Regards



fabio vincent

http://www.figener.com.br

 

[owg]

moltenmetal: It looks like Patzek only gets 50% conversion of hydrogen into power. If the result is not mainly unconverted hydrogen, then I would expect a lot of useless heat to be generated. Ballard provided data to the Pembina Institute for their paper which I made available a couple of years ago. However that paper just quotes 84.2 miles per US gallon equivalent for the fuel cell vehicle, and the comparable internal combustion car in that study is the Mercedes-Benz Class A at 17.7 miles per US gallon. If the Merc is running at 20% efficiency??, the Ballard (1999) data suggests a very high efficiency for the fuel cell vehicle, like 95%. There is a lot of funny data going around.

HAZOP at

http://www.curryhydrocarbons.ca

 

[JStephen]

An interesting thread that I just now ran across.

I'm not an expert on the topic. But a question for you guys. Doesn't it seem like an awfully convoluted process to grow a crop, throw 95% of the plant away, ferment what's left, throw 80% (I'm guessing here) of that crap away, distill whats left, etc.? Isn't there some woody plant where they could just grow the plant, harvest it, chop it, and throw it in a unit coal train and run it to the power plant? That sounds so much simpler.

By the way, someone up there mentioned they don't grow sugar in the US- last I heard, they were growing sugar beets in CO and MT among other places.

Another issue that rears its head is available water. There is real estate in the US to grow a lot of crops, only groundwater is being exhausted in a lot of dry areas.

 

[moltenmetal]

JStephen: you've hit the nail on the head. As I've said before, going after transportation fuels with renewables makes zero sense until you've taken care of all you STATIONARY power needs with renewables.

It would seem to be more sensible to use plants to capture atmospheric CO2 and solar energy, dry them using solar energy, and then burn them to make electricity and/or heat. Return the ash to the land. That's as close to renewable as you're going to get, and makes the most energetic sense- but only if you use the energy quite close to the point of agricultural production. And even this process isn't sustainable at high yields because it will require energy input in the form of nitrogenous fertilizer, whether that comes by means of crop rotation or by gasifying the same biomass to make hydrogen (to make ammonia to make nitric acid to make fertilizer) instead of using natural gas for that purpose.

If transportation costs are figured into the equation, doing something to get rid of the enormous amount of useless bound water and to increase the energy density of the biomass is required. Anything you do there, whether that be fermentation, pyrolysis or merely drying and pelletizing, will tend to reduce the net conversion efficiency of solar energy capture you end up with. In essence, that's what harvesting the corn is about- you don't need to transport the wet, bulky stalks. But to then do a wet fermentation of the grain in an attempt to make anhydrous ethanol makes little sense to me, even if a proper juggling of the numbers can make it appear to be marginally better than doing nothing and using fossil fuels instead.