Processes occurring in the down-draught gasifier
In the down-draught gasifier, schematically illustrated in Fig. 2.8, the fuel is introduced at the top, the air is normally introduced at some intermediate level and the gas is taken out at the bottom.
It is possible to distinguish four separate zones in the gasifier, each of which is characterized by one important step in the process of converting the fuel to a combustible gas. The processes in these four zones are examined below and the design basis will be discussed in the following section.
a) Bunker Section (drying zone)
Solid fuel is introduced into the gasifier at the top. It is not necessary to use complex fuelfeeding equipment, because a small amount of air leakage can be tolerated at this spot. As a result of heat transfer from the lower parts of the gasifier, drying of the wood or biomass fuel occurs in the bunker section.
The water vapour will flow downwards and add to the water vapour formed in the oxidation zone. Part of it may be reduced to hydrogen (see equation (b), paragraph 2.2) and the rest will end up as moisture in the gas.
b) Pyrolysis Zone
At temperatures above 250°C, the biomass fuel starts pyrolysing. The details of these pyrolysis reactions are not well known, but one can surmise that large molecules (such as cellulose, hemi-cellulose and lignin) break down into medium size molecules and carbon (char) during the heating of the feedstock. The pyrolysis products flow downwards into the hotter zones of the gasifier. Some will be burned in the oxidation zone, and the rest will break down to even smaller molecules of hydrogen, methane, carbon monoxide, ethane, ethylene, etc. if they remain in the hot zone long enough.
If the residence time in the hot zone is too short or the temperature too low, then medium sized molecules can escape and will condense as tars and oils, in the low temperature parts of the system.
c) Oxidation Zone
A burning (oxidation) zone is formed at the level where oxygen (air) is introduced. Reactions with oxygen are highly exothermic and result in a sharp rise of the temperature up to 1200 — 1500 °С.
As mentioned above, an important function of the oxidation zone, apart from heat generation, is to convert and oxidize virtually all condensable products from the pyrolysis zone. In order to avoid cold spots in the oxidation zone, air inlet velocities and the reactor geometry must be well chosen.
Generally two methods are employed to obtain an even temperature distribution:
— reducing the cross-sectional area at a certain height of the reactor ("throat" concept),
— spreading the air inlet nozzles over the circumference of the reduced cross-sectional area, or alternatively using a central air inlet with a suitable spraying device.
Guidelines for throat designs are given in the next section.
d) Reduction zone
The reaction products of the oxidation zone (hot gases and glowing charcoal) move downward into the reduction zone.
In this zone the sensible heat of the gases and charcoal is converted as much as possible into chemical energy of the producer gas (see equations (a) (b), section 2.2).
The end product of the chemical reactions that take place in the reduction zone is a combustible gas which can be used as fuel gas in burners and after dust removal and cooling is suitable for internal combustion engines.
The ashes which result from gasification of the biomass should occasionally be removed from the gasifier. Usually a moveable grate in the bottom of the equipment is considered necessary. This makes it possible to stir the charcoal bed in the reduction zone, and thus helps to prevent blockages which can lead to obstruction of the gas flow.