*return to* *Practical Engineering*

Here's some approximations and rules of thumb for understanding the relationships between common liquid fuels and woodgas, and the HP, KWe or MPG that results from each. There are more complicated ways to derive these numbers, but these rules of thumb are more than close enough to average the many variables of a gasifier-engine system. These rules of thumb will get you within 10% or so of the "correct" answer.

**Relating Gasoline/Diesel to Woodgas to HP to KWe in your vehicle or genset**

1 gal of gasoline or diesel will make about 15HP of shaft power for one hour. If driving a genset, this will produce about 10 kWh of electricity.

1 gal of gasoline or diesel is equivalent to about 20lbs of biomass through a gasifier.

1 ton of biomass to power through a gasifier-engine system is equal to about 100 gal of liquid fuel in a genset, or 1 MWh of electricity.

*Thus, the main rule of thumb (equivalencies) to remember:*

*1 kg of biomass ≅ 2 lbs biomass ≅ 2 m3 woodgas ≅ 1 HP-hour ≅ 0.75 kWh electrical*

### How much HP is your vehicle using at cruise?

60MPG = 1gal/hr or 15HP for one hour

30MPG = 2gal/hr or 30HP for one hour

15MPG = 4gal/hr or 60HP for one hour

### How much woodgas do I need to make x amount of HP?

1HP = 2 cubic meters of woodgas per hour

5HP = 10 m3/h

10HP = 20 m3/h

50HP = 100 m3/h

100HP = 200 m3/h

You can use the above to size a gasifier for your engine and use situation. We find it more accurate to size a gasifier by actual HP needed and used during typical run conditions, not by swept volume and max rpm. Seldom is an engine running at wide open throttle, as such calculations suggest. Thus we suggest you use the equivalencies and rules of thumb to determine how much HP you need, convert this to the related woodgas flow rate in m3/hr, then find the dimensions that relate to this in the standard Imbert Hearth Sizing Chart.

The more complicated and "accurate" way to do this is by starting with the heating value of the fuel, swept volume of the engine, volumetric efficiency and rpm, all of which calculate a specific gas flow rate needed. We've found this to be way too many decimal points for what is really a compromise between many competiting interactions. You can see the details of the other method in the Fluidyne engine tables at http://www.fluidynenz.250x.com

Doug Williams of Fluidyne has also provides the most accurate conversion specifics for Woodgas to BTU to HP to KW. We've used Doug's numbers and other historic literature to derive the rules of thumb above. A big thank you to Doug Williams for authoring the main numbers and relationships.

1kg of wood 15% moisture content produces 2.185 cubic metres of gas

or 3.165 kW heat from burning gas direct

or 0.837 kW of shaft power (i.e engine)

or 1.12 HP of shaft power (i.e. engine)

or 0.754 kW of electric power generated

1lb of wood at 15% moisture content produces 35 cubic feet of gas

or 4,900 BTU heat from burning the gas direct

or 0.51 HP of shaft power (i.e engine)

or 0.342 kW of electric power generated

*Thus, the main rule of thumb to remember for thermal equivalencies:*

*1 kg of biomass ~ 2 lbs biomass ~ 2m3 gas ~ 10,000 BTU*

Note: Power and Energy Units

Common units of power: horsepower (HP), kilowatt (kW), megawatt (MW), Joule/second (1 J/s = 1 W)

Common units of energy: kilowatt-hour (kWh), megawatt-hour (MWh), horsepower-hour (HP hour), British Thermal Unit (BTU), Joule (J)

Energy is power for a given amount of time. 1 kWh (energy) is one kilowatt (power) for one hour (time). 60 Joules (energy) over 1 minute (60 s, time) is 1 W (power).

Online Conversion: Energy & Power

## Comments (2)

## jeremy rutman said

at 3:07 am on Nov 10, 2009

please keep the power / energy notions straight. e.g. "a ton of biomass produces 1MW of electricity" gives a bad impression even if its more or less clear you are basing

figures on 1 hr. so - "a ton of biomass produces 1MWh of electricity" , etc.

## bk said

at 10:10 am on Nov 10, 2009

Thanks for pointing that out Jeremy. I've looked over the text and fixed the errors I could see. Also added notes on common power and energy units.

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