Methane as a fuel source

  • #1
I'm working really hard (within my limited knowledge) to figure out the details of using methane as a fuel source. I found https://www.physicsforums.com/showthread.php?t=224486" and would love to have responded there, but the thread was locked (due to age?).

Presently gasoline accounts for the most prevalent portable energy carrier used in the world. It is used because it is easy to burn, produces a significant amount of energy when it burns, it is extremely portable, and because a massive support infrastructure already exists. Any replacement must meet these requirements.

As the previous thread suggests, methane, the primary (and only really useful) component of natural gas, can be easily produced from CO2 and H2 using a sabatier reaction. The resulting methane can then be burned with O2 to form CO2 and H2O. What is lacking in this process is 2 things: energy to produce hydrogen and energy to start the sabatier reaction.

Methane burns easily and produces significant amounts of energy when it burns, it is portable as a gas or as a compressed liquid, and the infrastructure for distribution and use of methane already exists in the natural gas pipeline.

It was suggested on the other thread that if you have a readily available source of hydrogen that this should be used as the energy carrier instead, however hydrogen presents additional problems in terms of storage and transportation that the existing natural gas pipeline already solves for methane.

It was also noted that methane is not a "fuel source" but an energy carrior akin to a battery, and that the production of methane requires more energy than you get out of it (also a significant argument against hydrogen fuel). However, current fuel sources such as gasoline are no different - gasoline is solar energy that was stored by organisms millions of years ago: a very old battery. Solar energy (and wind and water) can be used to produce methane. Methane will NOT replace fuels used in power plants (because of its energy cost), but will only replace the portable fuels such as gasoline and diesel.

The questions I can't answer are: How much energy would it take to produce hydrogen (electrolytically or hydrolytically with zinc) and to power a sabatier reactor compared to the amount of energy stored in the methane? How much methane (and how many production facilities) would be required to replace gasoline for the United States? What other concerns are there in the production and distribution of methane and in conversion from gasoline to methane? What are the efficiency numbers compared to gasoline, ethanol, and electric batteries - specifically: would a "gas tank" sized can of (compressed) methane carry enough fuel to compete with gasoline or batteries (or other alternatives being considered)? How much energy is released by by burning methane compared to gasoline? http://ergosphere.blogspot.com/2005/06/zinc-miracle-metal.html" [Broken] has a breakdown of a possible zinc fuel that is what I'm looking for here.

Note: I'm leaving biofuel out of this because it's a slow process that's not easily controlled and produces low volume. Future investment in research I'm sure could make this an acceptable alternative - some nations already have biofuel production facilities built into their land fills and water treatment plants.
 
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  • #2
mgb_phys
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Methane will NOT replace fuels used in power plants
Methane is used in almost all new thermal power plants.

You do know that methane is a fossil fuel, you drill for it just like oil.
Making Methane from H2 and CO2 doesn't really make sense - especially since you mostly make H2 industrially from methane.
 
  • #3
Methane is used in almost all new thermal power plants.
Natural methane (natural gas) is drilled for and pumped out of swamps and other natural sources, and that has a positive energy value - we get more energy out of the methane than we spend drilling for it. Using that in power plants is fine, even though natural sources are not carbon neutral.

You do know that methane is a fossil fuel, you drill for it just like oil.
Making Methane from H2 and CO2 doesn't really make sense - especially since you mostly make H2 industrially from methane.
IMO, spending energy to make H2 out of an already readily usable fuel source doesn't make sense for use as a fuel. Hydrogen manufactured in this way is usually put to use industrially in things like synthetic medications - or sold to labs and universities. But this is also why I suggested making H2 electrolytically using alternative electrical sources such as solar, wind, and water.

Using methane to produce hydrogen in order to use the hydrogen as a fuel source seems to me to be wasteful and inefficient when you can just use the methane as fuel.
 
  • #4
Ygggdrasil
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Making hydrogen electrolytically to produce methane is even more inefficient. You would burn methane (or worse coal) to produce electricity, and that electricity would be used to produce hydrogen which would then be used to produce methane (again?!). How does that make sense?

Producing hydrogen, whether for use as a fuel or for the production of methane, requires the US (or any other country) to first develop commercially-viable, widespread, domestic sources of green electricity (solar, wind, geothermal, nuclear, etc.). Unless this prerequisite has been met, electrolytic production of hydrogen is not a smart option. (Other options could include using sunlight and some sort of catalyst to produce hydrogen from water. Although research is headed in this direction, no viable solutions have yet been developed).
 
  • #5
mgb_phys
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I think that's why the OP was confused.
The hydrogen 'economy' relies on having an essentially free source of primary energy somewhere where there is no market for the power.
So making H2 from water in a geothermal plant in Iceland or a solar plant in Australia and then shipping the hydrogen to markets essentaily as a battery might make sense.

But the extra step to make CH4 from H2 just to burn the CH4 again doesn't make sense however you look at it. It might be used along with catalytic cracking to make CH4 or lighter LPGs from heavy fuel oil but that's not a large consumer.
 
  • #6
Making hydrogen electrolytically to produce methane is even more inefficient. You would burn methane (or worse coal) to produce electricity, and that electricity would be used to produce hydrogen which would then be used to produce methane (again?!). How does that make sense?
Hence why I suggest "green" electricity sources.

Producing hydrogen, whether for use as a fuel or for the production of methane, requires the US (or any other country) to first develop commercially-viable, widespread, domestic sources of green electricity (solar, wind, geothermal, nuclear, etc.). Unless this prerequisite has been met, electrolytic production of hydrogen is not a smart option. (Other options could include using sunlight and some sort of catalyst to produce hydrogen from water. Although research is headed in this direction, no viable solutions have yet been developed).
I did mention zinc - sunlight has been successfully used with zinc and water to produce zinc-oxide and hydrogen. It's not perfect yet but is a potentially viable industrial manufacturing possibility. Wind energy is growing almost exponentially (I interviewed for a job with Vestas a while back and did some homework). And hydro-electric power is already in widespread use all over the world and is continuing to grow.


I think that's why the OP was confused.
The hydrogen 'economy' relies on having an essentially free source of primary energy somewhere where there is no market for the power.
So making H2 from water in a geothermal plant in Iceland or a solar plant in Australia and then shipping the hydrogen to markets essentaily as a battery might make sense.

But the extra step to make CH4 from H2 just to burn the CH4 again doesn't make sense however you look at it. It might be used along with catalytic cracking to make CH4 or lighter LPGs from heavy fuel oil but that's not a large consumer.
Hydrogen is incredibly difficult to store and has incredibly low energy density by volume compared to hydrocarbons. Additionally, using hydrogen as a fuel would requier manufacturing a completely new and very large system of pipelines and tanks for transporting - essentially making a duplicate of the existing natural gas infrastructure. Alternatively, fuel can be wasted transporting hydrogen by truck or train, as we do now with gasoline.

Methane, on the other hand, has a much higher energy density by volume and has none of the storage problems imposed by pure hydrogen. AND there is an existing pipeline and storage network already in place for the transportation of methane to distribution points (ie gas stations).

Using hydrogen as fuel requires extensive engineering and research into new technologies in storage storage and distribution methods and a large investment creating a new distribution infrastructure.

Using methane requires an investment in production facilities using existing technology and no additional storage or distribution infrastructure.

How does methane not make sense?
 
  • #7
russ_watters
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I'm working really hard (within my limited knowledge) to figure out the details of using methane as a fuel source. I found https://www.physicsforums.com/showthread.php?t=224486" and would love to have responded there, but the thread was locked (due to age?).
That thread was locked because it was going in circles.
However, current fuel sources such as gasoline are no different - gasoline is solar energy that was stored by organisms millions of years ago: a very old battery.
With the notable difference that the solar energy has already been captured and stored for us, unlike....
Solar energy (and wind and water) can be used to produce methane. Methane will NOT replace fuels used in power plants (because of its energy cost), but will only replace the portable fuels such as gasoline and diesel.
Gasoline and diesel need to be phased-out eventually. There is no question about that. And methane may be a viable solution at some point. But my gripe against this and the proposed "hydrogen economy" is the same: it is putting the cart before the horse. Before you can make a recyclable methane energy carrying infrastructure, you must deal with the existing problem of coal power. If you don't deal with coal power first, replacing gasoline with methane makes the problem worse, not better.
The questions I can't answer are: How much energy would it take to produce hydrogen (electrolytically or hydrolytically with zinc) and to power a sabatier reactor compared to the amount of energy stored in the methane?
I don't know the efficiency of the process, but as a starting point, it is less than 100% - so more energy than is available by burning the methane.
What are the efficiency numbers compared to gasoline, ethanol, and electric batteries - specifically: would a "gas tank" sized can of (compressed) methane carry enough fuel to compete with gasoline or batteries (or other alternatives being considered)?
Using methane instead of a battery as your energy storage media and getting the energy back using an internal combustion engine would yield perhaps 1/3 the efficiency. Thermodynamically, methane would provide slightly more efficient internal combustion engines than gas.

For energy density, no, compressed methane wouldn't be good enough. You would need to liquify it - perhaps by converting it to methanol.
How much energy is released by by burning methane compared to gasoline?
Wikipedia has tables of energy densities of common fuels/storage media: http://en.wikipedia.org/wiki/Energy_density
 
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  • #8
mgb_phys
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The principle advantage of hydrogen is that you can use a fuel cell to generate electricity directly from the hydrogen combustion. With the right engineering you can use the same fuel cell to generate the hydrogen (by putting electricity in), battery and charger in the same unit.

Methane does have a lot of advantages for portable power and direct heating and cooking it's a bit of a waste to use it in electrical power plants - but gas fired stations are quick and cheap to build.
 
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  • #9
Ygggdrasil
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Hence why I suggest "green" electricity sources.

I did mention zinc - sunlight has been successfully used with zinc and water to produce zinc-oxide and hydrogen. It's not perfect yet but is a potentially viable industrial manufacturing possibility. Wind energy is growing almost exponentially (I interviewed for a job with Vestas a while back and did some homework). And hydro-electric power is already in widespread use all over the world and is continuing to grow.
Yes, I missed your comment about green electricity sources, and I'm glad we agree on this point. However, while green energy sources are growing (and will hopefully continue to grow), they still provide only a small fraction of our energy needs and that trend does not seem to be reversing yet. Plus, as new green sources of energy come online, it would probably be more beneficial for the environment to use the electricity to displace coal fired plants than to divert the electricity toward methane production. So the electrolysis route is not likely to be viable in the near- or mid-term.

The zinc-sunlight route sounds like it could be promising, though.

Hydrogen is incredibly difficult to store and has incredibly low energy density by volume compared to hydrocarbons. Additionally, using hydrogen as a fuel would requier manufacturing a completely new and very large system of pipelines and tanks for transporting - essentially making a duplicate of the existing natural gas infrastructure. Alternatively, fuel can be wasted transporting hydrogen by truck or train, as we do now with gasoline.

Methane, on the other hand, has a much higher energy density by volume and has none of the storage problems imposed by pure hydrogen. AND there is an existing pipeline and storage network already in place for the transportation of methane to distribution points (ie gas stations).

Using hydrogen as fuel requires extensive engineering and research into new technologies in storage storage and distribution methods and a large investment creating a new distribution infrastructure.

Using methane requires an investment in production facilities using existing technology and no additional storage or distribution infrastructure.

How does methane not make sense?
One good reasons to favor hydrogen over methane is energy efficiency. Directly using the hydrogen to power fuel cells is likely to be much more efficient than converting the hydrogen to methane and burning the methane in an internal combustion engine (an intrinsically inefficient means of extracting energy).

So, even if there is a massive cost associated with constructing a hydrogen infrastructure, it may pay off in the long run to invest in building a hydrogen infrastructure in order to improve the overall energy efficiency and reduce the energy needs of the transportation sector.

However, your points about hydrogen are well put. We currently lack good technology to store and transport hydrogen. Should these hurdles become insurmountable, it may be necessary to go toward a methane route. However, promising research is going into means of storing and transporting hydrogen, so this route is still not out of the questions (especially given the point above that green sources of hydrogen are not likely to be available in the near- or mid-term).
 
  • #10
That thread was locked because it was going in circles.
Probably the destiny of this thread too - but I hope to learn more before that happens.

Gasoline and diesel need to be phased-out eventually. There is no question about that. And methane may be a viable solution at some point. But my gripe against this and the proposed "hydrogen economy" is the same: it is putting the cart before the horse. Before you can make a recyclable methane energy carrying infrastructure, you must deal with the existing problem of coal power. If you don't deal with coal power first, replacing gasoline with methane makes the problem worse, not better.
I agree that coal power has to go. But the transition to wind and solar is making progress (albeit slowly). I'm a big advocate of nuclear power, but waste disposal is still an issue there. Of course whether we're sequestering depleted uranium or sequestering CO2 from hydrocarbon fuels, I think there's little difference.

I don't know the efficiency of the process, but as a starting point, it is less than 100% - so more energy than is available by burning the methane. Using methane instead of a battery as your energy storage media and getting the energy back using an internal combustion engine would yield perhaps 1/3 the efficiency. Thermodynamically, methane would provide slightly more efficient internal combustion engines than gas.
Granted it's less than 100%. That's a thermodynamic no brainer. My primary concern is help getting some numbers to determine how economically viable the option is. I seem to get a lot of feedback that "it's not" without any mathematical or scientific evidence presented to back up that conclusion.

For energy density, no, compressed methane wouldn't be good enough. You would need to liquify it - perhaps by converting it to methanol. Wikipedia has tables of energy densities of common fuels/storage media: http://en.wikipedia.org/wiki/Energy_density
It seems to be good enough in http://en.wikipedia.org/wiki/Bi-fuel_vehicle" [Broken]. "The most common technology and alternate fuel available in the market for bi-fuel gasoline cars is natural gas (CNG)".


Yes, I missed your comment about green electricity sources, and I'm glad we agree on this point. However, while green energy sources are growing (and will hopefully continue to grow), they still provide only a small fraction of our energy needs and that trend does not seem to be reversing yet. Plus, as new green sources of energy come online, it would probably be more beneficial for the environment to use the electricity to displace coal fired plants than to divert the electricity toward methane production. So the electrolysis route is not likely to be viable in the near- or mid-term.
Fair enough, but I'd still like some numbers - ie how many solar panels/kWh would it take to convert x quantity of water to H2?

The zinc-sunlight route sounds like it could be promising, though.
I found one company that's making leaps in this process, although they are, if I recall, focusing on using it in reverse for electrical production.

One good reasons to favor hydrogen over methane is energy efficiency. Directly using the hydrogen to power fuel cells is likely to be much more efficient than converting the hydrogen to methane and burning the methane in an internal combustion engine (an intrinsically inefficient means of extracting energy).
The key point here is that hydrogen technologies aren't developed to a point where they ARE more efficient. I'm not suggesting that hydrogen or other possibilities should not continue to be developed, but I believe that methane could provide an alternative immediately while other technologies may take years or decades to be viable. Also, while burning methane may be intrinsically inefficient it is, as russ_watters has said, more efficient than gasoline.

So, even if there is a massive cost associated with constructing a hydrogen infrastructure, it may pay off in the long run to invest in building a hydrogen infrastructure in order to improve the overall energy efficiency and reduce the energy needs of the transportation sector.
No argument there. But in the mean time....

The main question boils down to: can methane be manufactured in quantities and at a price that will make it competitive with a) existing natural gas supplies and b) petroleum fuels. I suspect the answer is yes, although the startup cost to build a "green" manufacturing facility may be prohibitive. FYI the ideal manufacturing facility I envision in my head would consist of an array of solar collectors (thermal most likely) interspersed with wind turbines powering hydrogen production (from atmospheric water), atmospheric CO2 filtering, and a sabatier reactor. Methane would be compressed at the plant then fed into the local existing gas pipeline. Once constructed such a plant would be completely self sufficient.

The second question boils down to: can methane burn in an internal combustion engine efficiently enough to compete with petroleum fuels. The answer to this one appears to be yes, since it's already in use in some areas.
 
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  • #11
russ_watters
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Probably the destiny of this thread too - but I hope to learn more before that happens.
Well, you're off to a much better start since what you are suggesting isn't a violation of the first law of thermodynamics. Our concerns are about practicality.
But the transition to wind and solar is making progress (albeit slowly).
The word "transition" doesn't really fit becuase neither can provide continuous power and solar is unlikely to be economically viable in the near future. And wind takes a lot of space. So they are only a partial solution to the problem and a small part at that.
I'm a big advocate of nuclear power, but waste disposal is still an issue there.
Well, it's not the issue most people think it is. It's a political issue, not a technical one. Solving it is a simple (in principle) matter of deciding to solve the political problem. People have to accept recycling it instead of disposing of it. If it is reprocessed, recycled, and reused, the eventual waste is so benign you can just dump it in a landfill.
Of course whether we're sequestering depleted uranium or sequestering CO2 from hydrocarbon fuels, I think there's little difference.
No, there's a big difference: one we can do, the other we can't. Whether we will ever be able to actually sequester carbon is still an open question. So far, the potential solutions have not left the drawing board.
Granted it's less than 100%. That's a thermodynamic no brainer. My primary concern is help getting some numbers to determine how economically viable the option is. I seem to get a lot of feedback that "it's not" without any mathematical or scientific evidence presented to back up that conclusion.
Well, that's kinda backwards - before implimenting a new idea, people first have to be convinced that it is viable.
It seems to be good enough in http://en.wikipedia.org/wiki/Bi-fuel_vehicle" [Broken]

That's a stock-looking Honda Civic with a 200mile range and 113 hp. That's enough to be useful for a primarily city car, but not good enough to replace a regular car. That limitation is pretty fundamental, though. Perhaps making a natural gas/electric highbrid might get you more range and performance.
Fair enough, but I'd still like some numbers - ie how many solar panels/kWh would it take to convert x quantity of water to H2?
From the wiki link I posted earlier, the energy density of hydrogen burned in air is 143 MJ/kg. Taken the other way, water is 5x the atomic mass density of hydrogen so that would be 28.6 MJ/kg for water or 8 kWh to split 1 kg of water to produce .2 kg of hydrogen. A decent solar panel generates about .14 kW/sq meter: http://www.affordable-solar.com/sharp-ND-224U1F-224-watt-solar-panel.htm [Broken]

So using that type of panel, you need about 60 square meters pointed directly at the sun for an hour to split 1 kg of water into .2 kg of hydrogen and .8 kg of oxygen.
The second question boils down to: can methane burn in an internal combustion engine efficiently enough to compete with petroleum fuels. The answer to this one appears to be yes, since it's already in use in some areas.
On a mass, per mole, emissions, or thermodynamic basis, methane makes a much better fuel than gasoline (not to mention coal!). The main drawback for cars really just is the density.
 
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  • #12
The word "transition" doesn't really fit becuase neither can provide continuous power and solar is unlikely to be economically viable in the near future. And wind takes a lot of space. So they are only a partial solution to the problem and a small part at that.
http://en.wikipedia.org/wiki/Dye-sensitized_solar_cell Solar electricity is making significant progress. Also, the zinc-zincoxide cycle can be used: http://www.solarpaces.org/Tasks/Task2/SHP.HTM [Broken] The site also suggests ironoxide and cerium as well. Unfortunately the solarpaces site hasn't been updated recently so it's unclear if they've made any additional progress.

Well, that's kinda backwards - before implimenting a new idea, people first have to be convinced that it is viable.
So I'm going against the grain :)

Bi-fuel means both fuels are used at the same time and the reason is limited storage capacity. Fully natural gas vehicles do exist and have been around a while. As it turns out, they are better than I realized: http://en.wikipedia.org/wiki/Honda_Civic_GX

That's a stock-looking Honda Civic with a 200mile range and 113 hp. That's enough to be useful for a primarily city car, but not good enough to replace a regular car. That limitation is pretty fundamental, though. Perhaps making a natural gas/electric highbrid might get you more range and performance.
I was completely unaware of existing methane/natural gas vehicles. That actually effectively eliminates the consumer end of the problem - there's already a market for the product. From Honda's website: Honda civic (gas): 26 city 34 hwy. Honda civic (CNG): 24 city 36 hwy. Comparing a hybrid gasoline car to a non-hybrid CNG is non sequitur :P And 200 miles is good enough range for me, though my testosterone won't accept 113 hp. My Subaru gets a ~300 mile range. 2/3 ain't bad for a new technology. And I'm not sure how 8 "GGE" translates into tank size in such a car, but it may be possible to increase the tank size and/or pressure for greater storage. Being a gas, future advances in storage of methane can lead to greater fuel capacity with the same sized tanks.

From the wiki link I posted earlier, the energy density of hydrogen burned in air is 143 MJ/kg. Taken the other way, water is 5x the atomic mass density of hydrogen so that would be 28.6 MJ/kg for water or 8 kWh to split 1 kg of water to produce .2 kg of hydrogen. A decent solar panel generates about .14 kW/sq meter: http://www.affordable-solar.com/sharp-ND-224U1F-224-watt-solar-panel.htm [Broken]

So using that type of panel, you need about 60 square meters pointed directly at the sun for an hour to split 1 kg of water into .2 kg of hydrogen and .8 kg of oxygen.
That's very helpful. That's something I can use to determine some of the cost of a manufacturing plant.

On a mass, per mole, emissions, or thermodynamic basis, methane makes a much better fuel than gasoline (not to mention coal!). The main drawback for cars really just is the density.
Well since it's already in use as an alternative, I'll leave the density question to the automakers and I'll just focus on production.

The chief purpose in this whole exercise is to determine if methane can be economically viable as a renewable alternative to gasoline. It seems using it as a fuel source has already been demonstrated to be viable, so the only question left is whether or not manufacturing it can be economically practical.

I'm having some trouble with the math - primarily because I don't know how much hydrogen is in 1 kg of methane. A rough estimate (frankly a totally uneducated guest) says that a 60 square meter solar field could produce enough hydrogen to make about 2.3 GGE of methane per 8 hour day (assuming 100% :D efficiency). Not accounting for energy used for the sabatier reactor or for compression and distribution, this suggests to me that building a solar field of sufficient size to manufacture large quantities of methane could be economical. At the very least it encourages me to do some more research.
 
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  • #13
Ygggdrasil
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According to the fueleconomy.gov (DOE and EPA) mileage ratings, a conventional gasoline Honda Civic gets 25/36 miles per gallon (city/highway). The compressed natural gas (CNG)-fueled Honda Civic GX gets 24/36 mpg (using 121.5 ft3 as the equivalent of one gallon of gasoline). The Honda Civic Hybrid gets 40/45 mpg. Thus, it does not appear that natural gas engines are any more efficient than conventional internal combustion engines. Indeed, the mileage ratings for bi-fuel vehicles (vehicles capable of using either gasoline or CNG) support this assertion.

In contrast, the 2008 Honda FCX Clarity, a vehicle powered by hydrogen fuel cells, gets 77/67 miles per kilogram of hydrogen (combustion of 1 kg of H2 releases the equivalent energy as combustion of one gallon of gasoline). Thus, fuel cells do provide a much more efficient means of extracting energy from fuel.

Therefore, the point that many of us had made remains true: if you can create a green, economically viable source of hydrogen, it would be much better to use the hydrogen as fuel for fuel cell vehicles than to convert that hydrogen to methane and burn it. In addition to methane internal combustion engines being less efficient than hydrogen fuel cells, the overall energy efficiency of the methane fuel would be even worse considering the losses in energy in converting the hydrogen to methane.

Also, regarding the solar power, IIRC the cost of producing electricity from solar energy remains very high due to the high cost of photovoltaics. Given the relatively low cost of gasoline, I find it unlikely that you would be able to produce methane at a low enough cost for it to be competitive as an alternative fuel (of course, this is a problem with all alternative fuels). I haven't run the numbers to confirm this, but it is definitely something you should consider.
 
  • #14
russ_watters
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According to the fueleconomy.gov (DOE and EPA) mileage ratings, a conventional gasoline Honda Civic gets 25/36 miles per gallon (city/highway). The compressed natural gas (CNG)-fueled Honda Civic GX gets 24/36 mpg (using 121.5 ft3 as the equivalent of one gallon of gasoline). The Honda Civic Hybrid gets 40/45 mpg. Thus, it does not appear that natural gas engines are any more efficient than conventional internal combustion engines. Indeed, the mileage ratings for bi-fuel vehicles (vehicles capable of using either gasoline or CNG) support this assertion.
I don't know how they get those efficiency numbers, since natural gas doesn't come in gallons, but because of its properties, natural gas is used in higher compression ratio engines and thermodynamic efficiency is a direct function of compression ratio. I'd like to know where that 121.5 cubic feet per gallon comes from.

[edit] From the wiki link, gasoline is 46.4 MJ/kg or 34.2 MJ/L. Methane is 53.6 MJ/kg or .0364 MJ/L.
Methane is then 1.031 MJ/cubic ft and gasoline 129.5 MJ/gal. Divide one by the other and you get 125.6 cubic feet per gallon. That's within a few percent of their conversion factor. So they normalized the two according to energy density. I don't agree with that approach because they are factoring out the primary benefit of methane!

However, next biggest energy benefit, better thermodynamic efficiency, should still allow methane to come out ahead. I'm not sure why it doesn't.
 
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  • #15
mgb_phys
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I think it's from the amount of CNG you need to rive the car the same distance as one gallon of gasoline ! Somewhat circular reasoning!

Bi-fuel vehicles using LPG are very common in europe, it's higher energy density and slightly easier conversion than CNG
 
  • #16
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From: http://www.afdc.energy.gov/afdc/fuels/natural_gas_cng_lng.html

Compressed Natural Gas

To provide adequate driving range, CNG must be stored onboard a vehicle in tanks at high pressure—up to 3,600 pounds per square inch. A CNG-powered vehicle gets about the same fuel economy as a conventional gasoline vehicle on a gasoline gallon equivalent (GGE) basis. A GGE is the amount of alternative fuel that contains the same amount of energy as a gallon of gasoline. A GGE equals about 5.7 lb (2.6 kg) of CNG.
Assuming standard temperature and pressure (273.15K, 101.325 kPa), I calculate that 121.5 ft3 (3,440 L) of methane has a mass of 2.5kg.

So my conclusion before stands. From the mileage data, methane internal comubustion engines do not use energy more efficiently than conventional gasoline internal combustion engines.
 
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  • #18
russ_watters
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So my conclusion before stands. From the mileage data, methane internal comubustion engines do not use energy more efficiently than conventional gasoline internal combustion engines.
The DOE seems to agree with you, but I'm interested in knowing why. Based on thermodynamics, that shouldn't be true. Do you have any opinions about the possible source of this discrepancy?

And like I said, the DOE did its calculation on a per unit of energy basis. IMO, that's not necessarily the best way to make the comparison*. For the same number of joules, methane produces less carbon dioxide (among other things) and costs less than gas. That's a separate issue from why the engines themselves don't do better.

*To give an example of why that isn't the best way to do it, gas and diesel cars are both rated in mpg. But if we were to apply the DOE's logic on methane to diesel, they would need to de-rate diesel cars to account for the fact that diesel fuel has a higher energy density. The point is, they are not using a uniform standard for comparison.

This is a serious issue, too - the DOE/EPA is doing it so they can compare cars with different energy sources. But the comparisons get more and more difficult as the energy sources get more exotic. Trying to make an "mpg" rating for an electric car would be lunacy.
 
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  • #19
Ygggdrasil
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I agree that there are many issues to consider when choosing how to report mileage data. Methane certainly beats gasoline in miles per greenhouse gas emission, miles per fuel cost, and a variety of other metrics. Which metric to use depends on what type of comparison you'd like to make. Certainly if you are comparing which fuel is best for the environment, mileage per unit energy is not the best measure to use. However, since we were discussing comparing the tank-to-wheels efficiency of various types of engines, miles per unit energy seems like a good enough measure.

From the Honda website, I found that the http://automobiles.honda.com/civic-gx/specifications.aspx [Broken] uses a 10.5:1 compression ratio. So, on this basis we would expect the CNG engine to have a higher efficiency. One mitigating factor, however, might be the increased weight of the GX (2910 lbs versus 2692 lbs for the conventional vehicle). The increased weight (likely due to the tank required to hold the compressed natural gas) would increase the rolling friction on the vehicle and could account for the reduced the mileage of the vehicle. However, I don't know enough about the subject to comment on the magnitude of these effects on the efficiency or whether other factors may be at play.

I'll also note that a 2001 report by GM, BP, ExxonMobil, Shell estimates that the tank-to-wheels efficiency (energy out/energy in) of conventional gasoline (16.7%) and conventional methane (16.9%) engines are similar. Because these figure take into account only energy losses to the engine and drivetrain, they suggest that methane engines are not really more efficient than gasoline engines.
 
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  • #20
According to the fueleconomy.gov (DOE and EPA) mileage ratings, a conventional gasoline Honda Civic gets 25/36 miles per gallon (city/highway). The compressed natural gas (CNG)-fueled Honda Civic GX gets 24/36 mpg (using 121.5 ft3 as the equivalent of one gallon of gasoline). The Honda Civic Hybrid gets 40/45 mpg. Thus, it does not appear that natural gas engines are any more efficient than conventional internal combustion engines. Indeed, the mileage ratings for bi-fuel vehicles (vehicles capable of using either gasoline or CNG) support this assertion.

In contrast, the 2008 Honda FCX Clarity, a vehicle powered by hydrogen fuel cells, gets 77/67 miles per kilogram of hydrogen (combustion of 1 kg of H2 releases the equivalent energy as combustion of one gallon of gasoline). Thus, fuel cells do provide a much more efficient means of extracting energy from fuel.

Therefore, the point that many of us had made remains true: if you can create a green, economically viable source of hydrogen, it would be much better to use the hydrogen as fuel for fuel cell vehicles than to convert that hydrogen to methane and burn it. In addition to methane internal combustion engines being less efficient than hydrogen fuel cells, the overall energy efficiency of the methane fuel would be even worse considering the losses in energy in converting the hydrogen to methane.
While creating hydrogen for conversion to methane requires an extra step and an added energy cost of production, the arguments for methane and against direcly using hydrogen have already been made. To summarize, economical hydrogen is hypothetical, methane is real. It's also a LOT more plausible to talk every gas station in America into hooking up their existing natural gas supply to a public metered hose than it is to talk every gas station into buying expensive cryogenic hydrogen storage tanks and hydrogen pumps.

Also, regarding the solar power, IIRC the cost of producing electricity from solar energy remains very high due to the high cost of photovoltaics. Given the relatively low cost of gasoline, I find it unlikely that you would be able to produce methane at a low enough cost for it to be competitive as an alternative fuel (of course, this is a problem with all alternative fuels). I haven't run the numbers to confirm this, but it is definitely something you should consider.
That consideration is the entire point of this thread. The sad fact of the matter is I'm just no good at math (or chemistry) and so I'm asking for help crunching the numbers - and I do want numbers. And as I've discussed, photovoltaics aren't the only option, although it is the most prevalent solar technology. Concentrated solar thermal splitting of water and solar powered metal catalyst (ie zinc as I've mentioned) are other options that still need more research to be economically viable. There's also biological production of hydrogen - but if I were to use microbes to do anything I'd use Equate's cows to make methane directly and save myself some effort.

By the way, if someone has some numbers on biomass conversion to methane (ie quantity of biofuel produced per unit of biomass per unit time) just for comparison, that'd be really awesome.
 
  • #21
Borek
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/OT mode on

Bi-fuel vehicles using LPG are very common in europe, it's higher energy density and slightly easier conversion than CNG
They are. Every second fuel station here (in Poland) sells both gas and LPG. In most cases that means removing your spare tyre and replacing it with torroidal tank. Some other changes are also required - I don't remeber details, but conversion cost would be in the $700 range for my car. On LPG you have lower mpg, but overall cost fuel cost per mile is substantially lower, I can check numbers with my neighbour who converted most of his cars to LPG. And if he did you can be sure it is economically viable, Ebenezer Scrooge was extravagant in comparison :wink:

None of the cars I ever owned was equipped with LPG installation, but I drove such cars. As long as you are not an accelaration/speed freak you won't notice the difference, although before you can switch to LPG you have to run the engine on normal gas till it gets hot enough.

/OT mode off
 
  • #22
mgb_phys
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I don't remeber details, but conversion cost would be in the $700 range for my car. On LPG you have lower mpg, but overall cost fuel cost per mile is substantially lower,
It was popular in the UK, you got a grant for half of the conversion job and there was only 5% tax on LPG (instead of 75% on petrol). Then the grants stopped and there is talk of a "fair and equitable tax rate".
Funnily enough 10years ago, everybody was encourage to buy diesels by lower road tax and lower fuel prices. Then when enough people had diesels the tax on it was raised to more than petrol to take into account of the higher miles/gallon.
 
  • #23
Borek
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Now you are talking politics, not economy :wink:
 
  • #24
russ_watters
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By the way, if someone has some numbers on biomass conversion to methane (ie quantity of biofuel produced per unit of biomass per unit time) just for comparison, that'd be really awesome.
A couple of years ago, I sized an electric generation system for a sewage treatment plant (I'm an HVAC engineer :uhh: ). I changed jobs last year, but I might still have a copy of the analysis on my home computer (I'm cat sitting for my parents...).

From what I remember of the analysis is that the numbers are eye-popping. The sewage treatment plant used several megawatts of power and produce (and burn off!) enough methane to power the entire facility, plus sell a little back to the grid. In the northeast, the economics are good but since a large fraction of the gases are needed to keep the incubators warm in the winter, they are only good. In the south, where no heat is required to keep the incubators warm, the economics would be truly spectacular.

I did the analysis with power generation in mind, but it would be just as good to sell the methane itself to the utility if there is a good place to connect.
 
  • #25
russ_watters
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I agree that there are many issues to consider when choosing how to report mileage data. Methane certainly beats gasoline in miles per greenhouse gas emission, miles per fuel cost, and a variety of other metrics. Which metric to use depends on what type of comparison you'd like to make. Certainly if you are comparing which fuel is best for the environment, mileage per unit energy is not the best measure to use. However, since we were discussing comparing the tank-to-wheels efficiency of various types of engines, miles per unit energy seems like a good enough measure.

From the Honda website, I found that the http://automobiles.honda.com/civic-gx/specifications.aspx [Broken] uses a 10.5:1 compression ratio. So, on this basis we would expect the CNG engine to have a higher efficiency. One mitigating factor, however, might be the increased weight of the GX (2910 lbs versus 2692 lbs for the conventional vehicle). The increased weight (likely due to the tank required to hold the compressed natural gas) would increase the rolling friction on the vehicle and could account for the reduced the mileage of the vehicle. However, I don't know enough about the subject to comment on the magnitude of these effects on the efficiency or whether other factors may be at play.

I'll also note that a 2001 report by GM, BP, ExxonMobil, Shell estimates that the tank-to-wheels efficiency (energy out/energy in) of conventional gasoline (16.7%) and conventional methane (16.9%) engines are similar. Because these figure take into account only energy losses to the engine and drivetrain, they suggest that methane engines are not really more efficient than gasoline engines.
Thanks, that's a good analysis. A little tidbit: the weight issue wouldn't just be about rolling friction, as cars use more power when accelerating if they are heavier and cars are significantly worse in city driving where there is a lot of starting and stopping. The combination of the two may explain the difference.
 
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