The wood gasifier2
The wood gas generator consists of a unit with sheet iron walls 6 mm thick lined on the inside with bricks having 50 percent alumina (AL203) content. It has a total height of 3600 mm and its external diameter is 1400 mm. The fuel (wood) loading chute has a diameter of 400 mm. The reactor has cast iron grates installed 300 mm from the bottom. These grates are set 20 mm apart and have a lever movement system for removing ashes. Under the grates, 150 mm from the base, is a water cooling system with a three-fold function as follows
1. Hydraulically closing off the area where the ashes fall out to prevent the combustible gas from leaking.
2. Cooling the grate and lever area.
3. Draining off the ashes with water.
Figure 4.2 Sketch of the wood gasifier at the Sapire sawmill
The gasifier has eight two-inch diameter air entrance openings. Four of these are separated at equal intervals of 1800 mm from the bottom. The remaining four are separated in the same way but at 1320 mm from the bottom (see Figure 4.2).
The gasifier, being moderately large, is able to accept pieces of wood varying in size and moisture content. The wood load forms a column in which carbonization takes place at the bottom and the convected heat dries the pieces at the top. One fuel load lasts for approximately fourteen hours and can be replenished during operation since the cover of the chute is always left half open.
The moisture content of the fuel should be low, preferably below 25 percent, so that effective oxidation of the various pyrolysis products can take place in the oxidation zone. A good quality fuel gas results.
The ash content is 0.7 to 0.9 percent of the weight of the dry wood. During gasification most of the ash reaches the grate where it falls into a sheet of water flowing from the cooling apparatus and is disposed of down a drain. A small part of fly ash is retained in the gas and is removed by cooling.