Co-milling Coal and Biomass

1. Yes, and that point is most likely 0% for untreated biomass. The process of torrefication (a form of low temperature pyrolysis) will modify the moisture content and consequently the HGI (Hardgrove Grindability Index) of the fuel

2. Yes, although the exteriors may look the same the interiors are designed for a particular type of coal, and more importantly the process conditions such as air temperature/humidity and material flow rates.

3. Yes, the moisture content and hardness of the material being ground is critical, and if too high will lead to fouling and reduced output.

But milling is only one part of the equation, once the fuel is pulverized it must be passed through a combustor and into the furnace where the type of fuel and its conditioning are very critical to maintaining a steady flame temperature and shape. If the cumbustor is not tuned for the fuel then the combustion products may include too much NOx, COx, flyash content, etc., all of which will upset the downstream pollution control systems (and the EPA). See this paper for more info:

http://eprints.whiterose.ac.uk/43115/7/jonesJ2.pdf