Did you know? Part 4.

In this mini-series of fun facts, we’ve looked at the fuel efficiencies of chickens, liposuction fat, and coffee beans. But what about cow farts and cow manure? I’m sure that topic’s been on everyone’s mind so let me address it. As everyone may or may not know, cows produce a lot of methane gas and many believe they’re a major contributor to global warming (especially since methane is a much stronger greenhouse gas than carbon dioxide). In fact, cow flatulence produces about 90 kilograms of methane per cow per year (source). Methane has an energy density of about 52.75 MJ per kg (source) so that’s 4,748 MJ of methane per cow per year or 13 MJ of methane per cow per day. One way to convert methane into diesel fuel is a process called Gas-to-Liquid (GTL). That process requires 10.5 GJ of natural gas (mostly methane) to produce 42 gallons (1 barrel) of fuel (source) – so 250 MJ of natural gas for 1 gallon of diesel. If a cow produces 13 MJ of methane per day and 250 MJ are required for 1 gallon of diesel, you can get 0.052 gallons of diesel per cow per day. Assuming a diesel engine has a fuel efficiency of 30 miles per gallon, you’d be able to drive 1.56 miles per day of cow farts. Stats on bovine flatulence frequency are hard to come by but this source would indicate that cows pass gas every 40-90 seconds. Assuming they do that 24 hours a day (btw, humans fart in their sleep so let’s assume cows do too), that would be about 1,329 farts/belches per day so you can drive only 0.0012 miles per cow fart. [As an aside, while the typical person farts about 14 times a day (source), human farts are much lower in methane content and therefore not nearly as useful for producing diesel fuel via GTL]

At this point, you’re probably asking yourself one of two questions: (a) “what about the methane in cow manure?” or (b) “why am I still reading this crap?” (no pun intended). I’ll assume you’re asking yourself the former question and do my best to answer. This press release says that a herd of 150,000 cattle generate a total 2,500 tons of manure a day. One (short ton) is 907.18474 kilograms so 2,500 tons would be 2,267,962 kilograms of manure for the entire herd – or 15.12 kilograms of manure per cow per day. From this page, we find out that cow dung gas is 55-65% methane. The source doesn’t specify but I’ll assume this is by weight and that all the dung can be converted to cow dung gas. So 15.12 kilograms of manure per cow per day would be about 9.1 kilograms of methane per cow per day. Following the same math as above, that would translate into 1.91 gallons of diesel per cow per day. At 30 miles per gallon, that’s a whopping 57.42 miles per day of cow manure. According to this site, cows poop 11 to 12 times each day. So we can get a very respectable 4.8 miles per cow dump.

With that, I think it’s likely time for a new hobby since I’ve probably taken this one as far as I can. =)