Interpolate variable 0-24v to power a 12v fan and control it's speed
You could theoretically use a simple series resistor as a voltage divider to produce 7-12 volts over the fan but in practice it may get problematic. You should test the fan behavior versus 14-24V input voltage range.  

As a more robust solution you could use a mosfet as a variable resistor. Use a green LED to bias the mosfet base and basically use the mosfet as a source follower. Replace the LED with diodes and/or another LED depending what you have available and what's your gate threshold voltage. You could also use the same idea with a transistor and a single diode, in fact a transistor would work better with the more predictable base voltage.  

The transistor/mosfet will run hot so it should be a chunky TO-220 style, not petite TO-92 package one. 

Edit: I guess a simple 220R series would do the same job but you need at least 3/4W resistor and it'll get piping hot.