Hmm, 100W means 4A at nominal load, bit too powerful for most Arduino shields I think - you need to measure the winding resistance to calculate the max current at stall (that could be up to 50A!!), then you'll need a decently capable MOSFET H-bridge motor controller that can handle those peak currents.
Also if you are running from 24V batteries select a 30V or higher controller, not 24V as charging batteries brings them up to 28V easily...
Something like the beefier controllers here perhaps:http://www.pololu.com/catalog/category/94
but many other suppliers exist.