MadSci Network: Physics Query:

Re: If a human body (grownup) was burned up, how many Btu's would it yield?

Date: Wed Mar 22 09:55:23 2000
Posted By: ,
Area of science: Physics
ID: 949856987.Ph
Message:

This question seems a like disturbing at first. But I presume is motivated by a question about units of energy. The BTU (British Thermal Unit) is not well understood even though it is the unit of choice for such things as furnaces, heat pumps, and air conditioners in the United States.

The definition of a BTU from http://www.whatis.com/btu.htm states:

A British thermal unit (Btu) is an English standard unit of energy. One Btu is the amount of thermal energy necessary to raise the temperature of one pound of pure liquid water by one degree Fahrenheit at the temperature at which water has its greatest density (39 degrees Fahrenheit). This is equivalent to approximately 1055 joules (or 1055 watt-seconds).

The Btu is often used as a quantitative specification for the energy-producing or energy-transferring capability of heating and cooling systems such as furnaces, ovens, refrigerators, and air conditioners. The heat output of computer equipment is often specified so that it can be considered when planning the size of climate control systems in buildings. Computer device heat output is expressed in Btus per hour. 3.7 Btus per hour is equivalent to 1 watt of heat dissipation.

Burnable stuff like vegetation, animal dumpings, and wood are usually lumped into the category of fuels called biomass (http://starfire.ne.uiuc.edu /~ne201/1996/panetton/ or Burning Biomass).

It turns out that a pound of dry vegetation yields about 7500 BTU per pound. This gives us a starting point for the energy from buring a person with of material. Some previous answers to energy questions exist on the MSN:
search engine using the keyword BTU.

A particularly appropriate similar response Re: Is it possible to produce energy with chicken manure? submitted by Bob Peeples indicates that burning manure (actually methane and/or ammonia) at about 500 BTU per cubic foot at standard pressure (this is the volume of the gas). The missing link is the amount of methane/ammonia tied up in manure before it becomes gaseous. If it is close to 15 cubic feet per pound of manure then the numbers for dry vegation and gaseous methane are consistent. This seems reasonable to me.

So, in a leap of disgusting approximation, I could assume a person would release at most the same energy as a pile of manure or dry vegatation of the same size. This is after removing the water from the body. Which accounts for about 50% body weight. More moisture leads to less energy release for use because some of the energy goes to heat the water and if it changes to steam a huge amount energy is lost in the latent heat to induce the phase change. I claim, that the burning process will be approximately 50% efficient.

In any case the amount of water or unuseable component is detrimental to the burning process and is the important approximation in my calculation. Therefore if your average person is about 150 pounds, then approximately 75 pounds of it is burnable biomass with a yield of (75 pounds * 7500 BTU per pound) = 0.6 Million BTU's.

http://www.harmanstoves.com/btu_ conv.htm has comparisons between the most common fuels used (wood, coal, fuel oil). A pound of Anthracite coal yields 13000 BTU/pound and wood pellets (which they are pushing for their wood stoves) yield about 8500 BTU/pound. People as fuel is about half as efficicent as wood and 30% as efficient as coal. And is about the same as 4 gallons of fuel oil.

Sincerely,

Tom "Burning Question" Cull

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