If tar is the Achilles heel of gasification, then “hearth packing” is its club foot. The tar problem gets lots of attention, as it is always the first and most dramatic failure (i.e. it’s dirty, smelly and sticky). But “hearth packing” is the next and equally major problem that always appears. Keeping the charcoal train moving through the hearth, not packing up with fines and choking the reduction zone and grate, is a persistent problem that must be solved before long run times are possible.
The packing problem follows from gravity and gas going in the same direction in a downdraft gasifier (i.e. down), and packing things into a cake above the grate. This is accelerated by the physical size reduction of charcoal during chemical reduction, and the resulting mass of charash left at the end of the process. It is common for these fines to get stuck at the grate. And once the flow stops, fines will quickly back up through the bed, filling the reduction bell, and ultimately the combustion bowl. The first result is restricted gas flow (as seen via the P-ratio crashing), but soon the concentrated mineral ash in the combustion bowl will start clinkering, and quickly the entire hearth is plugged with glass rocks. That’s game over, and soon you’ll be face down in the charcoal digging out the rocks.
With the v5 GEK Gasifier we’ve solved this problem by reversing the direction of reduction gas flow. The gas now flows upward against the charcoal and charash, keeping it more “fluffed”, not pushing it into a cake. This “Reversed Flow Reduction” is achieved by a compound projected hearth that runs combustion down the center, and reduction rising back upwards around the annular sides of the hearth. Gas flows downward through the inner bed for combustion, then reverses and flows upwards through an elevated char bed on the outside of the hearth for reduction.