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Air sample analysis for the concentration of CO2/Carbon Dioxide? Answered

I'm working on a high school science fair project which concentrates on the CO2 emmissions of automobiles and anlyzes two methods of filtering CO2 from car exhaust. It's really a pretty simple project - except that i don't have a Carbon Dioxide monitor. Online they range from \$200-800 and i've contacted the enviornmental department at a local university to see if they can help me. I'm still waiting on a reply but i'm wondering if any of you guys (or gals) have any ideas on how i can find the concentration of CO2 in car exhaust.

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Wow! Where to begin?

I want to start by asking how the filter works, but that would probably be rude to just ask that first thing.

I mean supposing I have a filter that can selectively trap CO2 from automobile exhaust...

Then all I would have to do to measure how much CO2 the car was emitting, is measure the empty weight of the filter, then strap it onto my car, drive a few miles (or km depending on where your roads are), then weigh the filter again, and divide by the distance driven.

By the way, I am expecting the CO2 emitted by my car to weigh about three times the weight of the fuel I burned to make it.

I am going to guess a typical boring car being driven sensibly can burn around 56 g of gas (gasoline, petrol, liquid fuel) per km driven.  That works out to a km-age of about 18 km/kg, or:

(18 km/kg)*(0.62137 mile/km)*(0.73722 kg/L)*(3.78541 L/gallon)  = 31.2 mile/gallon

Those units are mentioned for those of us living in the Former United States where units like "miles" and "gallons" are still commonly used.

So I won't have to drive that far to accumulate enough CO2 in the filter to get enough to weigh on a balance with gram resolultion. Driving 10 km (6.2 miles) should burn about 56 g of go-juice and make about 3*56 =168 g of CO2, and that should be easy enough to measure.

By the way, if you are wondering where that factor of about 3 by weight comes from.  This is just based on the balanced equation for combustion of octane, which is a good approximation for clean combustion in a warmed up gasoline burning engine.

2 C8H18 (l)  + 25 O2 (g) ----> 16 CO2 (g) + 18 H2O (g)

2*114g/mol  + 25*32g/mol  =  16*44g/mol + 18*18g mol

228g C8H18 + 800g O2  ---->  704g CO2  + 324g H2O

Also regarding the concentration of CO2 in car exaust gas, since you asked specifically about that,  the concentration of oxygen in air, at standard temp and pressure, is about 9 millimol per liter, and the concentration of CO2 in exhaust gas will be something close to

(16/25)*9 = 5.76 mmol/L

using similar assumptions, i.e. assuming you pull in so many liters of air, then completely reduce all the oxygen in that air to CO2 and H2O per the equation above.

I'm not sure why you want a gadget to measure this.  I  mean its a huge amount of CO2 compared to the CO2 concentration in ordinary air, which is closer to 12 micromol per liter, i.e. 12e-6 mol/L, uh.. if I've done the math right.  That's based on a 0.035% concentration in air by weight, and I found that number here:
http://en.wikipedia.org/wiki/Atmosphere_of_Earth

Anyway, when you shopping for sensors, that's something to keep in mind; i.e. the concentrations you want to measure might  be outside the range of your typical sensor.  But like I've pointed out, you don't need a sensor if a car engine can produce gram quantities of CO2 per minute.  You just need some way of catching the CO2, so you can weight it.

Which makes me think.  I was going to ask:  What are your "two methods of filtering CO2 from car exhaust"?

Oops. I forgot to multiply by 10 there. Driving 10 km (6.2 miles) should burn about 10*56 g = 560 g of go-juice and make about 3*560 =1680 g = 1.680 kg of CO2

Well, my plan is to build two filters out of PVC. One will be a "dry" filter and the other "wet".

The "wet" filter will essentially channel Car exhaust directly into the filter where it will be bubbled through an aqueous solution of sodium hydroxide.

The "dry" filter will do pretty much the same thing but instead of filtering the car exhaust through aqueous sodium hydroxide, it will be forced through layers of solid sodium hydroxide pellets.

The purpose of this project is really to see how they compare to eachother - which one is more practicle, effective, efficient ...

You're kidding ? Jack explains just HOW much CO2 a road vehicle emits. What's the expected carrier capacity of your filter ?

If your engine is a modest 3 litres, then at 4000 RPM, its pushing 40 litres of gas A SECOND through the filters.

This sounds plausible.  I think I see why you want a sensor that can measure CO2 concentration in gas phase.  That way you could just measure the instantaneous CO2 concentration upstream of the filter, and also downstream, after the filter, and the difference in those two measurements would tell you if the filter was working.

This other idea, my idea, of weighing the filter, that will work too, provided you can find a way to keep the masses of other contaminants, like water, from skewing your measurements.

And I think the usual trick for dealing with water, contaminating some chemical sample you want to weigh, is to dry the sample sample first by heating it in an oven.  And there's going to be some subtlety to that.  I mean there will be some temperature at which all the water will leave, but all the chemically bound CO2 will stay.  And of course there will be some temperature higher than that at which the chemically bound CO2 will also be driven off, but I think that higher temperature is going to be like red, or orange, incandescent hot, something outside the range of a typical toaster oven.

Also, while on the subject of temperature, PVC has good chemical resistance, but it starts to get soft at around 100 C or so, near the boiling point of water.
Car exhaust is hot and steamy.  Steamy since it has a lot of literal steam, water vapor, in it.  So it might be necessary to pipe it through something to take the heat out first.

If you piped the exhaust through something really cold, like ice-water cold, you could take out the heat, and the water vapor too, since the water will condense.  Actually that would be very convenient, since you want to remove water anyway, and the condensate would give you another sample to weigh, for the sake of keeping track of where your masses are going.

Also regarding heat, dry NaOH gets  hot when water is added to it.  So a piece of PVC pipe filled with dry NaOH could get really hot if it were taking in a lot of water all at once, and like I was saying before, PVC gets all soft and squishy if you get it hotter than 100 C or so.

The final thing I was going to mention, is sort of a warning about the political nature of CO2.  Carbon dioxide is a gas, but its also a political topic, since CO2 is connected to AGW (anthropogenic global warming) or ACH (anthropogenic climate change) or whatever they're calling it now.
http://en.wikipedia.org/wiki/Global_warming_controversy

So the criticism you'll receive from those who think AGW is bunk, is that your project is pointless.  There is no need to filter CO2 from car exhaust because CO2 is not a real pollutant.

The criticism you'll receive from the proponents of AGW is going to be something along the lines of, "Well, now what do you do with the solid carbonate? How do you dispose of that?"  Or you might get questions about the total impact of the whole process, like, "How much energy does it take to make the chemical absorbent (in this case NaOH) ?"

One way to answer both these criticisms, is to explain that whether a chemical is "valuable" or a "pollutant" depends entirely on context.   There are places in the economy, or ecology, that actually want CO2.  That is to say it may be possible to trap the CO2 emitted by your car, and move it to somewhere it is needed, like a algae farm, where it can be made into "green" fuel.  Also there are places that want CO2 in the form of carbonates, e.g. the glass industry uses huge amounts of sodium carbonate and bicarbonate. Carbonates are used in cleaning products, like baking soda, washing soda, sodium percarbonate, etc.

Also regarding warnings, you know, hot steam and sodium hydroxide can be dangerous too, so remember to wear your safety glasses.
;-)

You MIGHT be able to MAKE a sensor, but you will have a lot of research to do to do it. Make a dual-wavelength infra-red sensor, where the relative levels of two colours of IR passed through a sample tells you the level.

Not such a simple project if you have to spend a good bit of money on the instruments you need. Where do you live? Do they do emission's testing on cars? If so is it a state ran thing or do independent mechanics do the testing? If it's mechanics then talk to one about your project and see if they can help you out.