- It takes more time and fuel to reach pyrolysis temperatures;
- More fuel means more ash in the fire chamber that can interfere with thermal flows;
- In a retort kiln pyrolysis of the load will be uneven, compromising the quality and yield of the char;
- In a direct-burn kiln, the result is excessive polluting smoke and difficulty flaring-off waste gases.
A word about Moisture Content (MC) of wood and Relative Humidity (RH) of air. Both are percentages of water, which only serves to mislead, since they have about as much in common as apples and Frisbees. MC refers to the amount of water in the wood as a percentage of dry weight. Depending upon the wood species, freshly cut saturated wood will have an MC in the 30's. Relative Humidity is the percentage of the maximum potential water content of air at a given temperature. Here's the tricky bit: the maximum potential water content of air varies with temperature. A lot.
Under most terrestrial conditions, the equilibrium moisture content of wood (EMC) will be in the teens or drier. Wood waste that's allowed to air dry will approach the EMC as a function of surface/volume and how it's cut (end grain dries faster than other surfaces). Just age it a while, and you're good to go. But in the humid tropics, where temperatures are high and the relative humidity hangs in the 90's, typical EMCs will be in the 20's. Residents of the humid tropics are all too familiar with high EMC's; perfectly clean T-shirts get all moldy-smelling, not because they're dirty, but because the material is damp enough to support fungal growth (yuck!). EMC's in the 20's are high enough to complicate pyrolysis. Simple air drying is not dry enough.
The trick is to elevate the temperature and move air over the surface. Heating air from 30 to 60C takes the RH of the air from 100% to 25%. A given parcel of air that's been heated contains the same amount of water, but now its capacity to do the "work" of drying is much greater. Going from 30 to 60C is a piece of cake with a solar drying kiln. Making a drying kiln that's cheap and simple to use with unwieldy mounds of biomass is the design challenge. Stay tuned...