All charcoal is fired in kilns. The biggest difference between horticultural charcoal and activated charcoal is the nitrogen, argon and other chemicals added during manufacturing. Orchid growers choose horticultural charcoal because it's cheaper, and fired without chemicals. Both types of char add oxygen and absorb bacteria in standing water.
Horticultural charcoal is usually made from fruitwood, bamboo, or coconut shells that are kiln-fired in a low-oxygen atmosphere called: pyrolysis. The oxygen reduction process changes the structure of the charcoal, making it more porous.
Activated charcoal is great stuff. It can be made from soft coal, wood, bamboo, or coconut shells. It's usually kiln-fired "in reduction", but slowly and to higher temperatures, making it more expensive. High quality activated char is used in hospitals to absorb toxins in the body. I buy activated charcoal capsules at the grocery store, and keep them at home for emergencies. Mixed with water, activated charcoal paste removes toxins from bee stings, mosquito and spider bites (so does epsom salt). One can use activated charcoal in orchid cultivation, but growers should know it may be made from soft coal, and likely has a cocktail of undisclosed chemicals added.
Horticultural charcoal is terrific for keeping water clean. We suggest using it in plant medium, and in trays. We change our "tray char" twice a year. It adds oxygen to the hydration water while filtering bacteria, rot and fungus.
Fertilizer burn from charcoal? No, contrary to old school thinking; charcoal doesn't cause 'salt burn'. In fact, it absorbs excess fertilizer in the tray, and stores it until the grower can replace it.
When it comes to high nitrogen in any plant food; I'm a big believer in the 'less is more' method. Over time, 'fertilizer salts' build-up in the sponge-like medium. Eventually, tender roots and pseudobulbs get chemical burns when more fertilizer-water is applied and re-hydrate the concentrated (dried) fertilizer "salts".
Leading members of NWOS always suggest washing drip trays as often as you fertilize, and hydrating the orchid medium with plain water berore fertilizing to avoid salt burn issues.
Happy Growing!
Kate @ Spirit Line Pottery, home of the raku char orchid pots