Hi, I have this project that I have just started, I have to grow microorganisms in order to decompose organic materials faster than the normal decomposing time. But the first problem is I don't know what to use to produce heat. I have searched things I could use: hair blower, thermostat, just wires; but I am still in doubt 
. I am thinking of using a temperature or thermal sensor (are they the same??) to stop the heat producer when it comes to about 25 to 30 degrees Celsius. Now, what can you guys suggest on how to produce heat?? Any suggestion will be a BIG help!!
to stop the heat producer when it comes to about 25 to 30 degrees Celsius. Now, what can you guys suggest on how to produce heat??
It depends on how much heat you need to produce. Light bulbs or popular (from chicken egg incubators to easy-bake ovens. A wide range of sizes and wattages are available. And they're cheap. Power resistors can be used (and even not-very-powerful resistors. I have fond memories of an Amateur Scientist project article that used something like a 1/2W 1500 ohm resistor to boil liquid nitrogen...) Nichrome wire, aquarium/terrarium heaters (from pet stores, etc) Electric blankets or heating pads. Sous Vide cookers (all the rage, these days!)
There are also cute thin film heaters Like this one. Probably useful for spreading the heat out a bit. (For a while, they were all over the surplus market, but I don't see any from my favorite dealers at the moment.)
Note that NONE of these can be connected directly to an Arduino output; you'll need some sort of higher-power switch.
I could use: hair blower, thermostat, just wires
How would you use a thermostat to PRODUCE heat? Rub two together and hope for a spark?
How would "just wires" work? I have a whole box full in the garage, and they don't even get remotely warm.
I'd suggest roof/pipe heating cable. It would allow you to coil the cable around the pile of organic material and keep the heat spread out. Most of these types of cables will also self-limit their temperature so you don't have the worry of the cable burning itself up.