My suggestion for the walls of a coal bin is use dry stacked concrete blocks. The kind of block to use depends on how high you need the walls. You could use hollow core 8 or 12 inch block, lower courses could be 12 inch, upper 8". It's surprising how high you can go, especially if you put a re-bar in the open core, to tie each course to the next. Fill the core voids with coal. What's nice about this approach is it's easy to change, enlarge, move etc. Works well on a concrete pad. If you are going to have high walls consider the interlocking retaining wall blocks commonly used in landscaping. Want more stability? Use the dry stack block method that bonds the wall together by a surface plaster of portland cement with embedded fiberglass. In all cases an exterior banding strap can hold it all together for even more security.