Project 1: Bottles vs. Cans - Which has the greater carbon footprint?

By Michelle Brouwer and Chad Neumann

The idea is simple. What we do in our lives impacts the environment and those around us. We personally like to sit back and enjoy a cold beer, especially in good company. So... which form of beer drinking is actually better for Earth? Bottles versus cans. What kind of effect do we impose by buying bottles versus cans? Glass versus aluminum. Well, let’s look into that and conclude the great debate. Which is better?

To determine the influence of the product, we have to look at the carbon footprint of the raw materials, manufacturing processes, transportation of the product, usage, and end of life.

Raw Materials

Although beer cans used to be made of steel, they are currently made of aluminum. There are two types of aluminum: primary and secondary. Primary aluminum is aluminum that is produced from an ore; secondary aluminum is aluminum made from recycled aluminum. The process of making primary aluminum is very energy intensive. Primary aluminum is made from bauxite. Bauxite is a sedimentary rock with a very high aluminum content, mainly found in Jamaica and Guinea. Bauxite is mined, then alumina is extracted through the Bayer process, and finally aluminum is smelted through the Hall-Heroult process. It takes 4 pounds of bauxite to produce 2 pounds of alumina, which can then produce 1 pound of pure aluminum. In the US, it takes 15.4 kWh of electricity to produce 1 kg of aluminum using the Hall-Heroult process. A single beer can weigh approximately 14.9 g, so the aluminum used in one beer can requires 0.23 kWh of electricity. Electricity makes up 98% of the energy used in the aluminum production process, so the environmental impact of aluminum production depends heavily on how the electricity used is produced. The process of making 1000 kg of aluminum sheets - enough aluminum for about 55,555 beer cans - from bauxite releases 18,000 kg of NOX and 23,000 kg of CO₂ (negating impacts from mining).

23,000 kg CO2 / 55,555 cans = 414.0 g/12 oz. can

Carbon dioxide emissions from processing raw material

There are only 5 aluminum smelters in the United States, mostly located in the Midwest. We are assuming bauxite is hauled 1000 miles from port to a smelter via a heavy duty truck. A Class 8 truck load would hold 23,600 kg of bauxite. That bauxite equates to 5,888 kg of primary aluminum, or enough aluminum for 327,775 cans. With a fuel efficiency between 4.5-10.5 mpg, heavy-duty trucks, Class 8, can expel a lot of CO₂ during operation. They also take an efficiency hit the more they haul. Assuming a Class 8 truck has a fuel economy of 6 mpg at gross vehicle weight, it would take a 26% loss hauling the full capacity (4.44 mpg). During the 1000 mile commute, the dump truck would burn 225 gallons of diesel, producing 2291 kg of CO₂ emissions

1000 miles = 2291 kg of CO2

2291 kg × 1000 g/kg / 327,775= 6.99 g/12 oz. can

CO₂ created from transporting raw material

In total, the transportation and processing of the raw material to make the aluminum for can production accounts for 421 grams of carbon dioxide emitted for each 12-ounce can.

Sand is a naturally occurring, however rapidly depleting, material. It involves no major processing to turn it into a usable product for making glass bottles. Negating the environmental impacts of mining for sand, it in and of itself does not produce any CO₂ gases. There is very little research out there pertaining to CO₂ production from mining or dredging for sand. Almost all the research that was found deals with the environmental impacts of concrete and cement production. However, stating a few clear assumptions, we can estimate the amount of carbon dioxide emitted while extracting sand and hauling it to the glass bottle factory. There are currently 41 glass container factories in the US. If all were relatively evenly disbursed and close to dense population, there seems to be several sand mines within 100 miles of any populated area. We will use this distance while factoring CO₂ emissions for hauling the sand to the factory.

So, the conditions are laid out as the following:

Sand is being excavated with a 1 cubic yard bucket (0.765 m^3) at 90% fill factor
Diesel engine of the excavator produces 155.6 kg/hr of CO2 (average of a few excavator engines)
Wet sand with a density of 1922 kg/m^3
The hauling truck is a Class 8 truck with a capacity of 26 tons or 23,600 kg

One scoop of sand would weigh 1323 kg and would take the excavator 17-18 buckets to fill up the trailers. If it takes the operator 30 seconds to scoop, pivot, dump, and pivot back for the series of sand excavation, it would take the excavator 9 minutes to fill the dump truck. In the process, the excavator would emit 23.34 kg of carbon dioxide. During the 100 mile commute, the dump truck would burn 22.5 gallons of diesel, producing 229.1 kg of CO₂ emissions. Given that it was stated that the truck would be hauling wet sand, not all of it will be used for making beer bottles. Dry sand has a density of 1281 kg/m^3, so of the sand that was hauled (23,600 kg), only 15,729 kg is usable. If it takes 200 grams of sand to make one bottle, one truck load produces 78,646 bottles.

(23.34 + 229.1) kg of CO₂ × 1000 g/kg / 78,646 bottles = 26.3 g of CO₂/bottle

Raw Material Tally:

Cans: 421 g/12 oz.

Bottle: 26.3 g/12 oz.

Manufacturing Processes

The three main process steps to make glass which produce carbon dioxide are melt, heat (reduce thermal shock after forming the bottle), and anneal. Specifically for melting sand into glass, two forms of energy/heat need to be considered. First, is the amount of heat that is needed to heat each gram up by 1°C or specific heat capacity. The other is the amount of energy needed to convert mass into a new state of matter or enthalpy of fusion; in this case, going from a solid sand to liquid sand. To determine the CO₂ amounts created for one 12 oz. bottle, let’s do some math:

	Melting Temperature (°C)	Specific Heat of Sand (J/g°C)	Enthalpy of Fusion (J/g)	12 oz. (355 mL) Mass (g)
Sand	1700	0.290	~2750	~200^

^Glass bottles are 70-74% silica by weight; the main ingredient is sand, assuming all other ingredients have similar specific heats and melting temperatures to simplify

1700 °C × 200 g × 0.290 J/g°C = 98,600 J

(negating room temp.; this is the energy needed to get the sand to the melting temperature)

For the sand to transition from solid to liquid, the enthalpy of fusion needs to be considered.

200 g 2750 J/g = 550,000 J

Approximately 20% of the energy needed to operate a gas furnace goes towards melting the sand/glass. Adding the two energy equations together and using the furnace efficiency:

648,600 J/20% = 3,243,000 J =3.243 MJ

Energy to melt one bottle’s worth of sand/glass

Specific energy of natural gas: 53.6 MJ/kg

3.243 MJ / 53.6 MJ/kg = 0.0605 kg = 60.5 g

Natural gas needed to produce 3.234 MJ of energy

Molecular weight of natural gas: 19 g/mol

60.5 g / 19 g/mol = 3.18 mol

Perfect combustion of natural gas

CH₄ + 20₂ = CO₂ + 2H₂O

3.18 mol of CH4 = 3.18 mol of CO2

Molecular weight of carbon dioxide: 44 g/mol

44 g/mol × 0.48 mol = 140.1 g of CO₂

For every brand new glass bottle produced, the melting process produces 140.1 grams of carbon dioxide. The bottles pass through open, continuous flame at about a rate of 3-5 seconds per bottle. This process helps reduce thermal shock, limiting the rate of the bottle cooling, thus reduces the chances of the bottle cracking. Without knowing the gas flow to produce those flames, it is hard to calculate the amount of carbon dioxide produced. It can be rationalized that in the production of one glass bottle, it is a fairly negligible amount. However, the annealing process does produce about 12% of the gas emissions for making a bottle. Using these similar equations above for the anneal process, it was equated that annealing produces 18.3 grams of CO₂. The total carbon impact of producing a glass bottle is 158.4 grams of CO₂.

Using cullet (recycled glass) will reduce the energy needed to produce a glass bottle. Roughly speaking, every 10% of glass cullet used in replacement of sand, the energy needed is reduced by ~2.5%.

Aluminum cans are made through a two-piece drawing and wall ironing process. First, a sheet of aluminum is cut into blanks that are about 5.5 inches in diameter. Next, the blanks are drawn to a diameter of 3.5 inches. The aluminum is drawn a second time to a diameter of 2.6 inches. After the second drawing operation, the walls of the can are ironed (thinned out) until the height of the can reaches about 5 inches tall. Approximately 17-19% of aluminum sheets are wasted in this process, but the waste can be reused as scrap.

A study conducted on steel stamping found stamping of steel parts in the automotive industry required 1.5 MJ/kg of steel. This is assuming the stamping was done using a hydraulic press, and includes blanking, forming, cutting, and handling. Since the same equipment used to stamp steel is commonly used to stamp aluminum, we will assume that aluminum requires the same amount of energy per kilogram. This is a conservative estimate given the material properties of steel and aluminum. Therefore, one can requires 22.35 kJ of energy, or 6.21 Wh.

1.5MJ/kg × 0.0149 kg = 22.35 kJ

22.35 kJ/1000 × 1/3600 h/s=6.21 Wh

On average, electricity sources in the United States generate 0.9884 lbs. (448.3 g) of CO₂ per kWh, so the manufacturing of one aluminum can generates 2.78 g of CO₂.

After the can is shaped, the labels are imprinted or stamped onto the can and the can is filled. Since both beer bottles and cans get labeled and filled in a similar fashion, the environmental effects of these steps are neglected for this analysis.

Manufacturing Tally:

Cans: 2.8 g/12 oz.

Bottle: 158.4 g/12 oz.

Product Transport

The transportation of the two types of beer containers is a bit tricky to solve because of so many variables at play. Let’s state some assumptions to frame in the story and compare the two equally. Through personal observation and a bit of Internet research, it seems that many breweries utilize medium to heavy duty trucks, Class 5 to 8. The gas mileage of these trucks ranges from 4.5 to 10.5 mpg. For the sake of the following calculations, we are going to assume that the beer is being transported 100 miles from the brewery, distributor, and store. The beer will be traveling in a Class 5 truck with a cargo volume of 8 x 10 x 20 feet, or 45 m^3, and a 10.0 mpg rating with no cargo. For the comparison, we will assume the truck is hauling only one type of beer vessel, all cans or all bottles, with a packing efficiency of 80%. From here, we can get the overall weight of the cargo and equate the mpg hit from the heavy product.

A case of bottles measures out at ~17 x 11 x 11”, 2057 in^3 or 0.034 m^3. However, a case of cans is ~12 x 17 x 6”, 1224 in3 or 0.020 m3. A fully loaded truck of bottles would haul 1059 cases of bottles and the same truck could haul 1800 cases of can. One 12-ounce beer bottle full of suds has a mass of 556 grams (200 g for the bottle stated above and 356 g of beer). That same 12 oz. of beer in a can weighs 371 grams. A full truck of bottled beer hauls 14,131 kg, in comparison to 16,027 kg of canned beer.

For every 1000 pounds (455 kg) a truck carries, its fuel efficiency can drop ~0.5%. A truck hauling only cans would take a 17.6% efficiency hit and have a fuel economy of 8.24 mpg. With a truck carrying bottles, it would have a 15.5% loss and a 8.45 mpg rating. Each truck drives 100 miles, with the can truck burning 12.14 gallons of diesel and the bottle truck consuming 11.83 gallons. Burning one gallon of diesel produces 22.38 pounds (10,356 g of CO₂). Driving the truck full of cans produces 123.5 kg of carbon monoxide versus 120.3 kg from bottles.

However, we are comparing bottles versus cans on a per 12 oz. of beer basis. With that, per beverage of beer, bottles come in a 4.73 g per and cans at 2.86 g of CO₂ created per container.

Transport Tally:

Cans: 2.86 g/12 oz.

Bottle: 4.73 g/12 oz.

Usage

We don’t have to argue here, the usage is the same and wonderful.

End of Life

Aluminum is an infinitely recyclable material. More than 67% of all the aluminum ever mined is still in use today, and the beverage industry is the largest consumer of recycled aluminum. Approximately, 65% of all aluminum cans are reclaimed. When an aluminum can is recycled, it is sorted, cleaned, and then melted down. The melted aluminum is poured into a mold to make an ingot. Recycling aluminum requires about 95% less energy than the smelting process alone. If one can weighs 14.9 g, approximately 9.7 g would be recycled.

421 g of CO₂ (raw) × 0.95 × 0.65 = 260 g CO₂ reduction (raw material)

Out of the 11.38 million tons of glass waste generated, only 3.03 million tons is recycled (US in 2017). The rest ends of it landfills or combusted for energy recovery. In the case of one glass beer bottle, 200 g mass, only 53.3 g would make it back to the plant as recycled glass cullet. Given the statement above about glass cullet reducing energy needed to make about a bottle, it can be equated that 53.3 g of recycled glass would reduce the emission from production by:

26.6% cullet / 10% cullet ratio = 2.66

2.66 × 2.5% energy reduction = 6.65%

140.1 g CO2 (melt) × 0.0665 = 9.32 g CO₂ reduction (manufacturing)

Plus, the 26.6% can be carried over in a reduction of sand mined and transported to the glass factory:

26.3 g × 26.6% = 7.00 g CO₂ reduction (raw material)

End of Life Tally:

Cans: -260 g/12 oz.

Bottle: -16.32 g/12 oz.

Conclusion

The battle lines were drawn and the clash between bottles and cans was calculated. Not all of the analysis was perfect (we looked into CO₂ as the main impact to the Earth, but there are plenty to consider). However, with these boundaries defined, it is clear that drinking out of an aluminum can is better for the environment. The next time you want to tip back a cold adult beverage, maybe you’ll consider cans over bottles.

Grams of CO₂	Glass Bottle	Aluminum Can
Raw Material	26.3	421
Manufacturing	158.4	2.8
Transport	4.73	2.86
Usage	0	0
End of Life	-16.32	-260
Total	173.11	166.66

For comparison, burning one gallon of gasoline produces 19.64 pounds of CO₂ or 8927 grams. That is equivalent to producing 51.6 glass bottles and 53.6 aluminum cans.

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