- Practice · 6 marks
Floating drum and fixed dome biogas plants
Short answer
In a floating drum plant, gas collects in a moving steel drum, providing constant gas pressure; the drum needs painting and is prone to corosion. In a fixed dome plant, gas is stored under a fixed masonry dome, displacing slurry into an outlet chamber, resulting in variable pressure; it has no moving parts, lower cost and longer life, but leaks are harder to detect.
With neat sketches, explain the working of a floating drum type and a fixed dome type biogas plant. Differentiate between them.
A biogas plant has an inlet (mixing tank), a digester where bacteria act on the slurry, a gas holder and an outlet for the digested slurry.
Floating drum plant (KVIC / Indian type)
gas out
|
___ | ___ <- steel drum (moves
| gas | up/down on guide)
---|--------|---
inlet \ / outlet
--->| slurry |--->
|__________|
digester well
Slurry is fed into an underground cylindrical digester. The gas collects in an inverted mild steel drum that floats on the slurry and rises or falls with the amount of gas stored, so the gas pressure stays constant (about 8–10 cm of water). A partition wall in the digester makes the slurry flow around it, so fresh feed does not pass straight to the outlet.
Fixed dome plant (Chinese / Deenbandhu type)
gas pipe
|
_.-'-._ <- fixed brick dome
/ gas \
--->| slurry |---> outlet/displacement
\_______ / chamber
digester
The digester and gas holder form one masonry (brick or concrete) structure. Gas collects under the fixed dome and pushes slurry into the outlet (displacement) chamber, so the gas pressure varies with the amount of gas stored (up to about 1 m of water column).
| Point | Floating drum | Fixed dome |
|---|---|---|
| Gas holder | Moving steel drum | Fixed dome, no moving parts |
| Gas pressure | Constant | Variable |
| Cost | Higher (steel drum) | Lower |
| Life | 8–15 yr (drum corrodes) | 20 yr or more |
| Maintenance | Painting, drum repair | Cracks are hard to find and repair |
| Gas leak | Easy to see | Difficult to detect |
| Temperature | Exposed drum, more heat loss | Fully underground, more stable |
| Skilled labour | Less | More needed for construction |
✓ Answer checked by Claude Opus 5.5 (high effort)
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