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Fire Starting Stick for Prairie Burns

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  1. Jun 10, 2017 #1
    I'm trying to figure out the best approach of creating an electric fire starting stick that we plan to use for prescribed prairie burns in Iowa. The drip torches we currently use are fuel based, and we are trying to move away from this if possible.

    I need help determining how to configure a basic fire starting device using wire (Nichrome?), batteries, and a regulator to generate 500 to 600 degrees. The goal will be to drag the heating element across the base of the dry grass to ignite it in flames.
    • What wire/heating element do you recommend for this application?
    • Is there a way to use an 18-volt rechargeable battery from a weedwhacker to power the heating element? Or should it be powered by a 12-volt battery or something different?
    • What additional components should I include in my circuit to regulate the temperature between 500 and 600 degrees and to ensure the wire doesn't overheat and break?
    Any other suggestions or recommendations you have would be greatly appreciated. The ultimate goal is to create a battery-operated heating element that can reach temperatures of at least 500 degrees and that can withstand continuous use and abuse.
     
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  3. Jun 10, 2017 #2

    anorlunda

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    :welcome:

    Interesting problem. I would drag a propane torch, but I'll accept your desire for an electrical igniter.

    The design challenge is only partially electrical. Heating a wire efficiently is typically done by forcing current through a thin wire. Thinner and shorter the better. But such a thin wire is easily broken if you pull it through grass.

    How long must the hot wire be? How fast are you dragging it? Is the hot wire parallel to or perpendicular to the direction of drag? How many seconds does a 500 degree wire need to be in contact with a blade of grass to ignite it?

    A sparking igniter can be more physically tough. If the spark triggers when there is not a blade of grass in the gap, it won't light. If it does light a single blade, then air movement from dragging the device would easily blow it out again before spreading. You would need experiments to make a sparking draggable igniter.

    Edit: Here is a commercial device close to what you have in mind.
    a1eac114bb06aea461694958fe7271e7.jpg
     
  4. Jun 10, 2017 #3

    jim hardy

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    This sounds like something you'd drag behind a tractor. Big.

    Most folks aren't aware just how much energy it takes to make heat.
    778 foot-pounds make a BTU, the work to heat a pound of water one degree.. Water falling over Niagara Falls , 167 feet high, gets heated 167/778 = just 0.2 degrees F by the work gravity did on it in the tumble.

    That's why electric charcoal lighters invariably have a plug and cord. This one's 500 watts, around only half a BTU per second,

    upload_2017-6-10_9-38-10.png

    which at 12 volts would be over forty amps.

    I hate to sound like an "Abominable No-Man"
    but i think you'll do a whole lot better with propane ..

    If it's a small device you're contemplating for spot fires, why not peel a foot or so of nichrome from an old furnace heater and try your weedeater battery on it ?
    Be aware the high current might wreck the battery or set it afire .
    so do it outdoors and be sure not to strap it to yourself.
    Around Lithium batteries - BE SAFE ! They're vicious creatures.
     
  5. Jun 10, 2017 #4
    I was talking with a friend and my concept may not be practical or possible. This is what he had to say:
    • Cellulose type items, wood, paper, plant material, all will combust at the well-known temperature of 451F (hence the movie with that title). However, it depends on several factors. First the amount of moisture content and even dry looking plant material can have moisture in it.
    • To combust, the material itself must be at that temperature. Heat transfer depends on a temperature difference and there is a time factor involved as well as the method of transferring the heat. Think about a pan of water on the stove. It takes time to bring it to a boil. The hotter the burner is (or the higher the flame of a gas range), the quicker it will boil but it still takes time.
    • You can’t transfer much heat by dragging something hot over dry grass. The contact for conduction heat transfer is very poor. Heat from both the hot device and the material to be heated will also escape to the air which means it takes even more heat.
    • The reason a flame will ignite the dry grass is 1) the flame temperature is very high, propane is around 3600˚F, 2) the flame is hot gases which will easily surround items it is close to where something like a hot wire or plate has very poor contact.
    I think the only way this could work if you had a very hot coil of wire with air blowing over it and that wire had enough heat content that the air temperature was extremely high (like a flame). This would take a huge amount of power and, no, the rechargeable battery would not have near enough power. Try using a hair dryer on high and blow it across a sample of the dry grass. I am sure it won’t ignite the grass. Note that the element in the dryer is probably glowing red (1600-1800 ˚F) and if not, you could block a little of the air inlet until it did glow red. The air probably still wouldn’t be hot enough (not even close to the wire temperature) and that hair dryer on high is probably rated at somewhere between 1300 and 1900 watts. To get 1900 watts from that battery you would have to draw 1900VA/18V=105 amps which I am sure it can’t do. It might have an energy rating of 3 amp hours so even if you could get 105 amps from it, it would only last 3/105=.0286 hours = 1.7 minutes = 102 seconds… but I doubt 1900 watts would be enough either.

    Another problem is that if you just planned on dragging some wires across the grass, you would need something that wouldn’t break which if you drug it with a vehicle and there was any kind of loop or the wire was perpendicular to the path of dragging, it could catch on a weed or stick or rock and nothing would stop it from breaking. Let’s say you were going to try some thick wire, let’s say 14 gauge. Two feet would result in a temperature of 2200F and you would need 53 amps at 18 volts. Two feet isn’t much.

    Any battery does not store much energy compared to a flammable liquid like propane. An 18V 3AH battery contains 3*18=54 watt hours of energy or 184 BTU. One gallon of Propane contains 91,220 BTU’s of energy. That gallon of propane weighs 4.2 pounds. A 5 gallon tank weighs about 16.6 lbs. So 4.7 gallons (max fill) with the tank weighs 36.3 lbs and has 428,734 BTUs or 11,810 BTU/lb. The battery weighs maybe 2.4 lbs so that is 76.7 BTU’s per pound. That means you would need 153 times the weight in batteries as a 5 gallon tank of propane for the same energy (2330 batteries).
     
  6. Jun 10, 2017 #5

    jim hardy

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    He gave you a very good and thorough answer.
     
  7. Jun 16, 2017 #6

    sophiecentaur

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    What is your reason for this? It can hardly be Carbon Footprint limitation. :wink:
     
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