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Name:   
Heading: 1.1.1.1.1.1Asphalt carbon footprint
Message:
I am trying to refute a "fact" appearing in a number of places that recycling 1 ton of asphalt shingles into asphalt for roads will prevent the emission of 60 tons of carbon dioxide. This statement is absurd but I need some sources and in particular I need a NUMBER in tons for the carbon dioxide footprint to produce 1 ton of asphalt including all transportation, processing, etc.


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Clark Hamilton


I don't have specifics, but lets consider this from a different angle.

How much fuel burnning would generate 60 tons of CO2?


According to this website
http://www.walkinginfo.org/faqs/answer.cfm?id=3460
one gallon of gasoline generates 19.4 lbs/gal

therefore

60 tons * 2000lbs/ton / 19.4 lbsco2/gal gasoline
~=6600 gallons of gasoline

Can do a similar conversion for diesel.

Typical fuel consumption rates in a batch asphalt plant are 2.25gal diesel/ton asphalt

That still leaves ~6595 gallons of fuel to be consumed in the process/transportation - I find the 60 ton claim very questionable.

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Brian Glackin


Brian i was wondering if you had any thoughts on the difference in cost between using a sand with less than 3% moisture and using a sand with significanly more moisture maybe 10-15%


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trevor mortimer


Trevor,

I'll take a quick swing at your question. If anyone thinks I'm missing something, please correct me.

The specific heat of water is 4.186 kJ/(ºC*kg).

The heat of combustion of propane is 46.35 MJ/kg.

The heat of vaporization (LHV for our purposes) of water is 2,257 kJ/kg.

We use propane at our hot plants, so I'm going to use propane as the energy source. Assuming that the temperature of the sand is about 21ºC in the pile, it takes about 330kJ/kg to bring the temparature to 100ºC, and another 2,257kJ/kg to vaporize the water for a total of 2.588 MJ/kg. This works out to show that for every 1 kg of water, it requires 0.0558 kg of propane to vaporize and remove in the hot plant.

Applying this to a rough example of operating at 400 TPH we can calculate the extra cost of a wetter sand; Compare a 13% Moisture sand to a 3%, for a difference of 10% (to make it easy). We use about 10% sand, so 40 TPH of sand, with the moisture difference of 10% comes out to 4 TPH of water. 202.6 kg/hour of extra propane is required for the wetter sand. Using today's market wholesale price of propane at $1.56/gallon, this yields an extra $169.50/hour for operation, or $0.42/ton of AC.

I simplified in a couple areas of the example, but its pennies at that point. I'm also assuming a very efficient burner operation, and each plant is different when it comes to efficiency.

You can run any scenario armed with knowing 0.0588 kg of Propane is needed to vaporize 1 kg of water at 100% efficienct burner op. One can modify that up or down knowing the efficiency of the plant.

I think its as easy as that. Just the difference in vaporizing the water is all that needs to be considered, as you're spending the same amount of energy to heat the rock with or without more water.

Hopefully this helps.
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Brian Litts



cheers for the reply exactly what i was looking for. I am supprised to note the tonnage of sand being as low as it is. What are the percentages of the remaining constituants?


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trevor mortimer


Trevor,

The percentage of natural sand in our mixes will vary from plant to plant based on the available materials. Some plants use crusher fines exclusively, and others use a blend of natural river sand and crusher fines.
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Brian Litts
 
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