Semibituminous coal burns with far less smoke than the bituminous type. It ignites with more difficulty than bituminous coal but produces far less smoke.
The central cone method is recommended for firing semibituminous coal. In this method, the coal is heaped onto the center of the bed, forming a cone, the top of which should be level with the middle of the firing door. This allows the larger lumps to fall to the sides and the fine cones to remain in the center and be coked.
The poking should be limited to breaking down the coke without stirring, and gently rocking the grates. It is recommended that the slides in the firing door be kept closed, as the thinner fuel bed around the sides allows enough air to get through.