Hot Thermo Argument

[jackboot]

I am in a big argument over this question:

How much CO2 is released into the atmosphere when 1 gallon of gasoline is burned?

I have seen the statement in some nature magazines saying that 1 gallon of gasoline releases 20 lb of CO2 when burned.

My thought is that:
1. The gasoline weighs about 5 or 6 lbs per gallon.
2. The reaction is converting some of this mass into energy
3. The remaining mass should be less than the orginal.


Can anyone one shed some light on this? If I am wrong please explain.

jackboot

 

[GregLocock]

you are wrong. When you burn the CH2 you add the weight of the oxygen consumed to the combustion products. This greatly exceeds the weight of carbon in the fuel. The loss of mass of the enrgy is completely insignificant, to an engineer. Cheers

Greg Locock

 

[jackboot]

Greg- do you have the balanced chemical equation for this reaction.

jackboot

 

[ivymike]

By my estimates, carbon makes up about 10.8% of the mass of gasoline, and it makes up about 27.3% of the mass of CO2. Thus the mass of CO2 produced should be about 2.53x the mass of gasoline consumed. Since gasoline is between 6.0 and 6.5 lb/gal, you should get between 15.2 and 16.5 lb CO2/gal gasoline.

 

[GregLocock]

Long chain hydrocarbons can be approximated as CH2

CH2+1.5*O2=CO2+H2O
Cheers

Greg Locock

 

[ivymike]

oops, lemme take that back for a sec, I think I might have goofed something.

 

[ivymike]

here's how I was looking at it (previous mistakes&mis-statements aside):

avg gasoline composition: C8H15
86.5% of the mass is C

each C combines with 2 O's, which multiplies the mass by 3.667

so 6 * 0.865 * 3.667 = 19
and
6.5 * 0.865 * 3.667 = 20.6

 

[jackboot]

Thanks-

I hate being wrong, but arguing with the wrong logic can make one look foolish.

Appreciate the help.

jackboot

 

[Speedy]

On a similar subject, this is something I was wondering about recently - how much oxygen is producd by the average tree? (average - difficult to quantify, I know !)

Put it another way - How many acres of forestry equate to say the CO2 produced by 100 average motor cars? Speedy

"Tell a man there are 300 billion stars in the universe and he'll believe you. Tell him a bench has wet paint on it and he'll have to touch to be sure."

 

[ivymike]

I would guess that the amount of carbon from the atmosphere that gets captured by the tree is pretty close to the mass of carbon in the tree, which I would guess is close to the mass of the tree (if you cut it down and dry it first). As a guess, let's say that the tree is 60% carbon. Based on the above guess, and the guess that 100 average motor cars drive 60 miles a day each, at 20mpg, we find the following:

100 cars burn ~300gal gasoline a day
Each gallon of gasoline releases about 5.4lb of C
A tree gets 9lb heavier when it absorbs 5.4lb of C

So by this estimate, we need to grow 2700lbs of trees every day to keep up with the cars.

Since some logs float, I'm going to take a WAG and say that the density of a tree is close to that of water. A SG of 0.7, perhaps. So 2700lb of tree would translate to 1.75m^3 of solid wood. That's 3.8 trees, each with a 10" trunk, each 30ft tall. If it takes a tree only 10 years to reach those dimensions from a seed, then you only need to be growing about 14,000 trees, at various ages between 0 and 10 years old, to take up the CO2 from 100 cars. If the trees are planted on an eight foot by eight foot grid, that translates to 20.5 acres.

But don't forget - when the trees are fully mature, you can't burn them down, or let them rot, because all the CO2 would go back into the atmosphere. You have to do something with them that will keep the CO2 trapped for a very long time...

 

[CoryPad]

Scientific American has had fantastic articles covering carbon sequestration - learn more at:

http://www.sciam.com

 

[ornerynorsk]

Isn't it much more fun to work on the theoretical stuff than doing the work we're supposed to be doing? LOL. Anyway, just thought I'd let you guys know that wood can range from 8 lbs cu/ft to somewhere around 90 lbs cu/ft. I know because my late father was in the millwork trade for many years. The AVERAGE american hardwood when air-dried runs the range of 30 to 40 lbs cu/ft, with some exceptions running higher (hickory, oak, locust, beech, and rock elm) and others running lower (basswood, cottonwood, poplar). Coniferous are almost always under 30 lbs cu/ft. Green weight (live) runs considerably higher, but this is all moisture content. Most woods have an abundance of resinous substances, which in all likelihood are carbon based, in addition to the cellulose base structure, and do not evaporate out when dried.