My knowledge is specific to the sulfur mustard formulations ("H," "HD," "HT") used in the U.S. Army's chemical weapons stockpile, so I may be off on some details for other formulations. Where possible, the following is double-checked against my hazard analysis training materials from 2013. All of the following is available in open sources; my work was civilian-side local .gov and I did not hold a clearance.
Mustard has low volatility and high persistence, which means it doesn't evaporate readily and it sticks around for a while. It's not as long-lived as the persistent nerve agent VX but it's not far off. It is denser than water (about 1.3x), freezes at 58ºF, has a flash point of 221ºF, and is fairly viscous. One of my instructors compared its visible physical properties to thin maple syrup.
The HD formulation is purified/distilled H; physical properties are similar. HT is a mixture of HD and a diluent; it has a lower freezing temperature and is even less volatile than H/HD. The mustard/garlic/horseradish odor of H is due to impurities which the distillation process removes from HD.
Mustard exposure may take from 2 hours to upwards of 24 hours to manifest symptoms. As Swag noted, unlike nerve agents, blister agents do not have counter-agents. Decontamination and treatment of symptoms form your medic's course of action. For exposure survivors, it's also a very potent carcinogen - beyond the scope of most campaigns, but perhaps a factor for roleplaying concerns.
Residual U.S. Army inventory as of 2013 was 105mm artillery projectiles (H and HD), 155mm artillery projectiles (H and HD), and 4.2" mortar projectiles (HD and HT). When stored, they were unfused and without propellant but did contain agent and bursting charges. It was noted in my training that in a storage fire, the projectiles would rupture from vapor pressure before the bursting charges detonated from the heat. Other delivery systems may have existed prior to my time in the program, but any such items were out of inventory before the late '80s and so not a factor for us. Likewise, our work was concerned solely with U.S. inventory, so I have no information on other nations' chemical munitions.
The munitions were designed so that when fired, the bursting charge ruptures the case, producing aerosol/droplets. Initial exposure hazard is from skin and eye contact with this splash. There is a possibility of a secondary downwind vapor hazard from evaporation after the aerosol is deposited on surfaces; I believe this is more likely with higher surface temperatures. My training was not overly-concerned with long-distance airborne plumes of mustard due to the droplets' weight; nerve agents tend to go much farther in air than mustard. Mustard evaporation is slow, so surface contamination does linger.
Over long storage periods (i.e., decades), mustard settles and solidifies. One of the technical challenges facing the disposal teams at the last stockpile sites is that their projectiles have been stored in the same orientation for so long that the chemical agent has formed a solid "heel" in each projectile. It can't be poured out - individual rounds have to be destroyed in a blast chamber.
I can't find a reference for it in my materials, but my recollection is that mustard decays faster under direct sunlight (i.e., UV radiation) and can be fiendishly persistent if left in a dark environment without airflow. The specific example given was battlefield contamination of WWI trenches that were subsequently filled in, resulting in soil still contaminated in the modern era.
This 2007 report from the UK MoD goes into some detail on the issue of mustard's persistence when buried.
I am neither a chemist nor a plume modeling expert. However, based on the above, my response to your specific question is that coal exposed to mustard would at least retain surface contamination, particularly if someone popped a mustard munition over a coal pile and the aerosolized droplets settled on the surface and then migrated into the dark center of the pile. Burning the coal would probably release some of the mustard as vapor, yielding vapor exposure in the downwind exhaust plume. In cool air or on cool surfaces, you might also see the vapor re-condense to liquid droplets and generate some level of surface contamination. Your combustion chamber is also likely to be contaminated, and if you're handling the coal with manual labor, gods help your stokers.
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Originally Posted by swaghauler
JJ Keller produces books on HAZMAT for the Trucking and Materials Handling Industries. I'd get a copy of one of their HAZMAT books (as cheap as $5 new at truckstops) for a reference. Also, note that possession of these books is MANDATORY for any Trucker hauling HAZMAT in the US.
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Coincidentally, I had cause to pull my ERG off the shelf today. We had a minor chlorine issue in our AO and I took the opportunity to do an airborne plume education session with my new-ish deputy.
- C.