Photos?

Depending on the construction details of the granary and what you'd like to keep as exposed surfaces, rigid insulation can be added to the exterior. This applies to both the roof and the sides of the home.

We have a grain elevator in our town and the cost of getting rid of all the grain dusts was a fortune it is very volatile and all needs to be professionally removed before any one can use the space . So I need pictures and a lot more info

While we don't have photos, the OP's description of a rustic timber interior leads me to believe they're not planning to live in a grain bin or a grain elevator. Most grains are combustible dusts and can serve as fuel for a combustible dust explosion under the right conditions. So can sawdust. It's a manageable hazard and it doesn't require a team of guys in zoot suits to mitigate.

You should be talking to the dealer who provides the heating system; they'll know what is required for their system to work optimally, if the granary is a viable option in reality.

IMO we need more info I so wanted to convert that grain elevator but at 50-60K for the removal of the dust was just not feasible. Our town decided to save the elevator so footed the bill which BTW was 100K . I only asked becuse sometimes what you want and waht you can have are not the same. Since we hear nothing from the OP who knows .

@Patricia Colwell Consulting,

Contractors are usually regulated in how they mitigate certain hazards and dispose of hazardous materials e.g., lead paint, asbestos, etc.) Hiring a specialty contractor--particularly in a remote area--for hazard mitigation usually ups the price and often by a lot when they're the only one available. Those same tasks can usually be performed by a homeowner unencumbered by the same level of regulations.

With regard to combustible dusts, removal can be performed on a DIY basis. I would recommend use of an explosion-proof vacuum cleaner and non-sparking tools for cleaning up dry grain dust, but pressure washing with water may be applicable for some portions of the building.