You need to be using NFPA 654 "Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids". This NFPA standard is for systems that convey combustible particulate solids that are produced as a result of a principal or incidental activity, regardless of concentration or particle size, where the materials present a fire or explosion hazard. Appendix B has a section on inerting. You may also wnat to review Chapter 6 of NFPA 85 for some design ideas for your inerting system.