The grate of a downdraft gasifier is a perforated plate or bar grid below the reduction zone that holds the charcoal bed up while ash falls into a pit beneath, and it needs shaking every 4-8 hours to keep gas flowing and to break up clinker. The ash pit must be emptied only when the unit is cold, because opening it on a running or hot gasifier admits air below the bed and lets burning gas out.
Part of Building a Downdraft Gasifier
The grate supports the char bed at the bottom of a downdraft gasifier, and the ash pit and shaker keep the gas path open.
Why The Bottom Of The Bed Clogs
The parent places the grate below the reduction zone, with an ash chamber under it. All the gas passes through the char bed and the grate on its way to the outlet, so anything that blocks the grate blocks the gas. Three things do that:
- Ash and char fines that settle in the holes.
- Clinker, fused ash that forms at high temperature and forms a solid lump over the grate. The parent says a shaker helps to break it up.
- Too fine a bed, where small charcoal packs between the grate and the throat.
Wood ash is a small fraction of the fuel (recalled: generally below about 1% of dry mass for clean wood; bark and agricultural residues carry more). For the parent’s 5-8 kg/h that is under 80 g of ash an hour, so the 4-8 hour interval is set mostly by char and fines, not by ash alone. Fuels rich in silica and alkali, such as straw and husks, fuse at much lower temperatures than wood ash and clinker readily, so a design for wood needs rethinking for them.
What You Need
- A grate of perforated steel plate or a bar grid, with openings smaller than the char you want to keep and big enough to pass ash.
- A shaking arrangement operated from outside, such as a rod or lever through a close-fitting bush, so the unit never has to be opened while hot.
- An ash pit with a sealed door that fits tightly. It is under suction when the engine runs, so a leaking door admits air; and under the start-up blower it can leak gas out.
Method
- During a run, shake gently every 4-8 hours, or when the gas flow or flare weakens. Short strokes: a violent shake drops live charcoal and disturbs the reduction bed.
- After shutdown, let the whole unit cool completely before opening the ash door. Hot embers fall out with the ash and can reignite when air reaches them.
- Remove ash and fines into a metal bucket, not a plastic one or a heap near the fuel store, and leave it outside to cool further.
- Look at what comes out. Hard, glassy lumps are clinker, a sign that the bed is overheating or the fuel carries too much mineral; a lot of charcoal fines suggests the fuel is crumbling or the bed is too deep.
Common Mistakes
- Opening the ash door on a hot unit. It lets air in below the glowing bed and lets burning gas out at your hands.
- Shaking too often. The bed collapses and tar passes.
- Leaving the ash door loose. A leak sucks air in and burns the gas inside the vessel.
Safety
The ash pit holds hot embers and carbon monoxide
The gas around the grate is about 18-22% carbon monoxide by volume and the ash can hold live embers for hours. Never empty the pit on a running or warm unit, never in an enclosed space, and never leave a bucket of hot ash in a shed or vehicle cab. Do not pour water on hot embers over the open pit: it makes steam and more fuel gas. See Carbon Monoxide Risks.
What’s Next
- Learn next: Gas Cleaning & Filtering — the gas train after the grate
- Combine with: Hearth Constriction — the other end of the bed