It all depends on what you have in mind. If you are just trying to make steam, then build a coco shell fire and put a pot on it. The water boils to make steam. If you wish to convert this steam to electricity this is a little more complicated and dangerous. You will need to let the steam build up pressure, which requires stronger materials that won't rupture at high pressure. The steam pressure is released to drive the turbine, which will generate electricity. This cools the steam. To make it all more green friendly, one needs to recycle the water to lower your total energy and water requirements. If it was me I would google "used wood fired" steam generator. Then I would try to substitute coco shells for wood. The design parameters should be close.