The Barbecue and CO2

In the last post, we talked about respiratory production of carbon dioxide, an environmentally innocuous process. We hear about the contribution of industry to carbon dioxide production, but I want to focus on something we can all relate to: the backyard barbecue grill.

The ubiquity of propane cannot be overstated. The stuff is all over the place and it isn’t limited to the twenty pound tank that fits beneath the barbecue grill. In my part of the world, New England, it’s used for standby electric generators, heating pools, hot water, fireplaces, etc. The use of it creates CO₂ and I want to bring that down to practical focus for the average person.

The combustion of propane (C₃H₈) follows the following chemical equation for complete combustion:

C₃H₈  +   5O₂  →  3CO₂  +  4H₂O

This chemical equation tells us that for every molecule of propane burned, it needs five molecules of oxygen. The gas that combustion produces contains three molecules of carbon dioxide and four molecules of water. Without going through the “chemical math,” what this is telling us is that for every pound of propane combusted, three (3) pounds of carbon dioxide is produced. It also tells us that for that one pound of propane about 3-2/3 pounds of oxygen is required and it yields about 1-2/3 pounds of water vapor in the process. Oh . . . and it produces heat too. All approximate, of course.

The 20 pound propane tank one purchases at the hardware store or supermarket, will put 60 pounds of CO₂ into the atmosphere when combusted. Think about it.

Carbon Dioxide - An Original Source

We hear about carbon dioxide (CO2) as one of a number of atmospheric gases that are responsible for anthropogenic global warming. This is primarily due to the burning of fossil fuels. As a percentage of the atmosphere, CO2 is minuscule, less than four one hundredths of a percent (actually 0.38% or 380 parts per million (ppm)). 

One source of carbon dioxide is, well, us. Every time we exhale, we create CO2. As a matter of fact, every time every living, breathing thing exhales, it produces carbon dioxide. So how much do we produce? On average, a human produces about 2.3 pounds of it per day (based on various web sources). It adds up. Take a look at this table I constructed. 

CO2 per person each day: 2.3 pounds

2008 Earth Population: 6,694,254,040 (World Bank)

Human CO2 Production each day: 15,396,784,292 (Billion) lbs.

Human CO2 Production each year: 5,619,826,266,580 (Trillion) lbs.

Human CO2 Production each year: 2,809,913,133 (Billion) tons

About three billion tons per year. Sounds like a lot. Should we be worried that  human beings, by their mere breaths, are contributing to global warming? Not  really. We humans among all of the animals, plants, oceans, etc. are part of the natural carbon dioxide cycle of the earth. The real issue is the long run and the debate about global warming. But we can  avoid that debate by burning much less fossil fuels and being much more  efficient in our use of major sources of energy---What I call in my book, "Conservation Without Deprivation."

Flexible Fuel Vehicles: The Engine

In the recent post of October 3^rd, we discussed the types of fuel that could be used in multiple-fuel engines. The Open Fuel Standard Act of 2009 (House Bill H.R. 1476) would require 80% of the cars manufactured or sold in the U.S. to be able to burn M85, E85 or Gasoline. The "85" refers to the percent of methanol or ethanol combined with 15% gasoline.

Today’s gasoline engines are made to run on gasoline with octane that ranges from 87 to 93 (87, 89, 91 & 93) on gas pumps in New England, where I live. Methanol has octane that ranges from 105 to 109 (Source: EPA, 2002 Clean Automotive Technology Program) and ethanol has octane ratings that range from 94 to 96 (Source: Renewable Fuels Association). This is much closer to gasoline’s octane ratings.

What does all of this mean? That a flex fuel engine is a compromise in efficiency.  Gasoline engines are built to have compression ratios of somewhere between 9 and 10 to 1 (9:1 to 10:1). Compression ratio means that the piston squeezes the air fuel mixture by a factor of 10, for example, between the intake of fuel and air and compressing it just before the spark plug ignites the fuel to provide power.

 Source: AutoZone Ref. Library

If you use a low octane fuel with a high compression engine, the fuel may combust before the spark plug ignites it. Mechanics call it “knock” and you can hear it when the engine is running because it sounds like popcorn in a microwave oven. Higher compression ratios mean higher pressures and temperatures and temperature drives efficiency in a heat engine, like a car engine.

If you have higher octane fuels like methanol (especially), the engine can operate at a higher compression ratio. A methanol engine can operate at an optimal compression ratio of 19.5:1 (ranging from 17:1 to 22:1 in EPA tests). This yields higher efficiencies than a gasoline engine.

A confession: In my upcoming book (Energy: The Primer, How to Distinguish a BTU from a BLT and Other Stuff You Should Know About Energy), I have a chapter entitled, "Methanol - The Other Motor Fuel." I like methanol better than ethanol as a gasoline substitute for a number of reasons: (1) Methanol can be made from plentiful coal, natural gas and ultimately carbon dioxide combined with hydrogen (when we run out of fossil fuels), (2) It could eliminate our reliance on imported crude oil,  (3) The higher octane rating will allow internal combustion engines to run more efficiently, (4) Methanol can run directly in fuel cells, ultimately displacing the less efficient internal combustion engine and (5) The world eats corn and it's the feedstock for ethanol. I would rather not have a motor fuel compete for use of a foodstuff as a feedstock.