A downdraft gasifier makes engine fuel from dry wood by burning part of it in restricted air and passing the products over hot charcoal, giving a gas that is about 18 to 22% carbon monoxide. That carbon monoxide has no smell and kills gasifier operators, so the gasifier runs only outdoors, the gas is proved at a flare that is lit before the gas arrives and stood well away from, and a flame is never held to a pipe mouth or lowered into a vessel to test it.
Part of Internal Combustion
The design, operation and hazards of a downdraft wood gasifier used to fuel a spark-ignition engine, with the corpus gasification articles as detail.
A Gasifier Is A Fire Deliberately Not Allowed To Finish
The corpus puts it exactly in Gasification Theory: air is rationed, so only part of the fuel burns and the heat of that part converts the rest into a gas that can still burn in the engine. The combustible part is mostly carbon monoxide and hydrogen, and the carbon monoxide is also the poison. Fuel value and lethality cannot be separated.
In a downdraft unit the wood passes four conditions in order, described in Pyrolysis, Reduction, & Oxidation Zones: drying, pyrolysis, oxidation at a narrow throat where air enters, and reduction over glowing charcoal. The point of the downdraft layout is that tar vapour is forced through the hottest zone and cracked, which is why Gasifier Types says only the downdraft makes gas clean enough for an engine. The Imbert design is in Downdraft (Imbert) Gasifier and a build in Building a Downdraft Gasifier; this page keeps to what the engine tree needs.
Typical downdraft gas on dry wood is 50 to 55% nitrogen, 18 to 22% carbon monoxide, 12 to 18% hydrogen and 8 to 12% carbon dioxide, per Syngas Composition. The nitrogen only dilutes it, which is why the corpus gives producer gas and air 2.46 MJ/m³ against 3.80 for petrol and air, and an engine on it 40 to 60% of its petrol power.
What You Need
- Dry, uniform fuel: chunks of 3 to 5 cm at 20% moisture or below on a wet basis, per Fuel Preparation & Carrying and Seasoning & Drying. Wet fuel makes tar and weak gas.
- A gasifier with a narrow hot throat, air nozzles and a char bed.
- A cleaning and cooling train: a cyclone for dust, a cooler to bring the gas to about 40 °C or below, and filters, per Gas Cleaning & Filtering and Tar Removal (Cyclone & Cooling).
- A start-up blower. Use a blower, never the engine, to draw the first gas.
- A flare stack: a short length of pipe standing up and away from the gasifier and the operator.
- A mixing valve to the engine, per Mixing Valve & Air-Fuel Ratio.
- A bucket of sand, and a second person within call.
Method: Start-Up With The Flare Lit First
Step 1: Load dry fuel and check every joint, with the gasifier outdoors, away from buildings and with the wind known.
Step 2: Light the flare first. Put a flame at the top of the flare stack before any gas is drawn: a torch on a stick, or a burning rag on a wire, held or fixed so you are not leaning over the stack. This is the opposite of testing the gas with a flame after it arrives. The corpus startup order is to light the flare first, then the gasifier under blower suction, as in Startup Sequence (20-30 Minutes).
Step 3: Light the gasifier and run the blower from a distance. The first gas is weak and tarry and burns at the flare as it comes; the lit flare consumes it where you are not standing.
Step 4: Watch the flare, not the pipe. A steady, near-invisible to pale flame that stays alight when the pilot flame is taken away means good gas. A smoky, sooty yellow flame or tarry streaks mean the throat is cold or the fuel is wet; do not connect an engine, as Downdraft (Imbert) Gasifier says.
Step 5: Only after 10 to 15 minutes of steady flame, start the engine on its normal fuel and switch to gas, with the timing advanced 10 to 15 degrees as in Engine Modifications.
Step 6: Shut down cold. Close the air and gas valves, switch to petrol for the last minutes if you have it, and let the unit cool unopened; Operating Procedures explains why a hot unit must not be opened.
Worked Example: Wood And Gas For A Day, And What A Small Leak Does
Fuel for a day. Assume a 3 kW load for 8 hours, with the carrier’s 1.4 kg of dry wood per kWh and the corpus yield of 2.5 to 2.7 m³ of gas per kilogram from Energy Content of Wood Gas.
- Energy: 3 × 8 = 24 kWh.
- Wood: 24 × 1.4 = 33.6 kg.
- Gas: 33.6 × 2.5 = 84 m³ to 33.6 × 2.7 = about 91 m³.
A small leak in a shed. Assume 0.1 m³ (100 litres) of gas escapes into a closed 10 m³ shed, at 18 to 22% carbon monoxide.
- Carbon monoxide: 0.1 × 0.18 = 0.018 m³ to 0.1 × 0.22 = 0.022 m³.
- Concentration: 0.018 ÷ 10 = 0.0018, which is 1,800 ppm, up to 0.022 ÷ 10 = 2,200 ppm.
- The corpus gives 1,200 ppm as immediately dangerous to life in Gasification Theory, so even this hundred-litre leak is above it.
That is a small amount of gas by the standards of a running gasifier. Both figures are arithmetic on assumed quantities.
Common Mistakes
- Starting the engine on gas too early. The first gas is tarry and weak.
- Testing the gas with a flame held at the pipe mouth. Your face is in the gas stream and the flame can flash back.
- Judging the air by smell. The smell is tar and smoke; carbon monoxide has none.
- Warming up in a shed in cold weather. It is the one place the gas collects.
- Opening a hot unit. Hot charcoal and gas in air can flash.
- Putting water on a hot gasifier. Use sand.
Safety
Carbon monoxide is odourless and kills gasifier operators
Producer gas is about 18 to 22% carbon monoxide. It binds to haemoglobin roughly 200 times more strongly than oxygen, so the blood stops carrying oxygen; the first signs are headache, nausea, clumsiness and confusion, and they remove the judgement needed to leave. Run, light, refuel, purge and shut down only outdoors and upwind, never in a garage, shed, lean-to or enclosed yard; never sleep near a lit gasifier; never lean over the unit; and never go back in for someone, move them out first. At the first headache or confusion, get into fresh air before touching the controls. See Emergency Procedures and CO Risk.
Never use a flame as the detector for a flammable gas
Light the flare before the gas arrives, from the side, at a distance and upwind. Do not hold a match to the gas outlet, and do not lower a flame into a vessel, pipe or cab to see whether it holds gas. A gasifier is also a closed vessel of combustible gas with a fire in it, and flashback on restart with a hot bed is a real hazard.
What’s Next
- Learn next: Wood Gas Vehicles — running a vehicle on the gas
- Combine with: Ignition Timing — the advance a slow-burning gas needs