Carbon Footprint for Polypropylene Vs Copper piping
The cost and performance of polypropylene piping and copper piping material is nearly equal. So we started tossing some numbers around comparing the energy required to produce and recycle one kilogram of Copper versus one kilogram of Polypropylene. Our curiosity is partially driven by clients who are looking deeper into both the life cycle and the carbon footprint of their buildings.
At first, I thought that this would be a fairly simple. All I would have to do is Google “Carbon Footprint of Piping Materials”, and out would pop my answer. This did not turn out to be true. As I dove into various references, it became apparent that there are many factors, processes, weights, conditions, and even conflicting estimates from equally reputable sources. A full academic analysis is beyond the scope of this blog post, but perhaps there is some insight to be gained from the literature review on the subject.
The Wikipedia article for “Embodied Energy” offers a table of building materials, energies, and general definitions. The problem with this analysis lies in the type of energy used to manufacture the material; nuclear, wind, oil, hydroelectric, etc. According to the Wikipedia article, for the average composition of World power generation, 1 kg of CO2 is released for every 10.2 million joules of energy. So, we can use this number as a baseline and reality check as we dig a bit deeper into Copper and Polypropylene.
Embodied Energy Polypropylene Vs Copper Pipe
The following summary chart includes data from several sources that were at least corroborated with other sources. If we assume that Polypropylene sequesters the carbon in its feedstock, the net difference between the two is formidable where polypropylene is the environmentally preferable material over copper by at least by an order of magnitude, and likely much more.
All aspects of the copper production require energy, whether in the form of electricity, explosives, or hydrocarbon fuels (diesel, gasoline, natural gas, fuel oil, coal, etc). Each of these energy forms also require material energy to create as the energy equivalent of materials consumed, such as chemicals, trucks, steel grinding media, etc.
According to a paper from Princeton University on Energy Consumption In the Copper Industry; in 1977, the copper industry purchased 121 Trillion BTUs of energy corresponding to 85 million BTUs per ton of copper produced.
This converts to 98.8 million Joules/Kg.
From this we can pull a rough estimate of
9 Kg of CO2 per Kg of copper
The single most energy-consuming step in the petrochemical industry is the steam cracking of hydrocarbon feedstocks to produce ethylene, propylene, butadiene and aromatics (benzene, toluene and xylenes). Since all of these products result from a single process, it was difficult to isolate what contribution of energy goes to producing polypropylene. However, propylene is a simple hydrocarbon molecule consisting entirely of Carbon and Hydrogen atoms. Because the feedstock is sequestered “energy” we need to count that as energy consumed in addition to the energy required to produce Polypropylene.
Energy required to produce 1 kg of Polypropylene = 23 million Joules
The Calorimetric energy of 1 Kg of Polypropylene = 45.8 million Joules
By addition, this is how we arrived at:
The total energy to create 1 Kg of Polypropylene = 69 million Joules
Here things can get a little tricky. To get a better picture of the environmental impact of these two materials, we consider Polypropylene piping as a carbon sequestration device. Energy entombed in polypropylene would otherwise be converted to fuel, which would contribute to carbon emissions. In the case of polypropylene, the fuel is converted to a useful and ongoing service of transporting water. This is the same essential function of copper without the energy consumption of copper.
Propylene contains 3 carbon atoms and 6 hydrogen atoms. Carbon has a molecular weight of 12 and hydrogen has a molecular weight of 1. So, propylene has a total molecular weight of 42. Oxygen has a molecular weight of 16, so CO2 has a total molecular weight of 44. Every molecule of propylene makes 3 molecules of CO2, with a total molecular weight of 132 (44 x 3).
1 kg of Polypropylene Feed Stock sequesters 132/42 = 3.14 kg of CO2.
Now we need to make a few assumptions. By assuming 100% efficient process and hydrocarbon fuel, this energy conversion factor gets us to some quick estimates.
48.5 million Joules/Kg of product sequesters 3.14 kg of C02 or about
15.4 Million Joules per Kg CO2 sequestered
(This is in the ballpark of 10 MJ per Kg CO2 cited above)
Polypropylene: (Feedstock – process energy) = 23 million Joules. As such, a net 1.5 kg of CO2 are sequestered for every Kilogram of Polypropylene produced.
By comparison, Copper releases 9 kg of C02 per kilogram of copper produced.
More on Copper:
One estimate for the carbon footprint of copper is 1049 kg Co2 per Kg Copper by a Japanese researcher. The lowest estimate that I found was 6.0 Kg of C02 per Kg Copper. This spectacular range of estimates demonstrates the complexity for tracing the copper footprint all around the globe.
On the other hand, plastics manufacturing is far more controlled capturing multiple byproducts from the same stream cracking process and likely sourced closer to the location where it is being used.
Additionally, in the final application, Polypropylene pipe is about ½ the weight of copper so not only is it less expensive to transport, half of the mass of material can be formed to accomplish the same outcome as the full weight of copper.
Polypropylene has a melting temperature between 270–370 F (130 – 180 C). Copper has a melting temperature of 1984 F (1085 C). This means that forming and reforming (recycling) polypropylene is far lest energy intensive than copper. Polypropylene can be molded into new forms with simple tooling from irons to 3D printing. Copper must be cast, drawn, alloyed, stamped, etc.
Sequestration is a strong word in the carbon footprint lexicon. To sequester is to permanently make C02 unavailable for the atmospheric release. For example trees sequester carbon only until they fall and decompose on the forest floor, then their carbon is released to the atmosphere for absorption by another tree.
For better or worse, polypropylene is inert in a landfill and its molecular bonds can only be decomposed by photo degradation for which PP is highly resistant. Indeed a common nuisance is that plastics remain forever, in the case of polypropylene; it will sequester its carbon for millennia much like oil left in the ground.
Again, I had to make some broad assumptions and incomplete references to infer the numbers here – I hope someone could point me to better analysis. But lets look at the orders of magnitude of these materials:
North America consumes over 600 Million Kilograms of copper per year. Approximately 30% is used in construction or could be substituted with Polypropylene for an energy savings of about 35 Trillion BTUs – enough to take several dirty coal-fired generators off-line – and reduce carbon emissions by up to 600 Billion Kg per year.
The world consumes well over 3 trillion kg of copper per year for which 30% may be replaced with polypropylene – so these estimates only get more stark.
Embodied Energy Polypropylene Vs Copper Pipe References:
From: Energy Use in the Copper Industry; Princeton University: https://www.princeton.edu/~ota/disk2/1988/8808/880809.PDF
The Calorimetric energy of Polypropylene feed stock according to the FAA paper entitled: Heats of Combustion of High Temperature Polymers is as follows:
An estimate for energy per is 2.3 X 10^7 J/Kg is taken from this document: http://www.lyondellbasell.com/NR/rdonlyres/C2ED0A47-6430-45FA-87A4-D4018108814D/0/AusPPEnvirostatementJan12final.pdf Environmental Impact Statement
Carbon emissions polypropylene http://www.sprayallcorp.com/carbon_pollutant_emissions.htm
CO2 in Metals Production http://sip.vestforsk.no/pdf/Felles/MetalProduction.pdf