2.1.1.1 FIXED-BED GASIFIERS

Fixed-bed gasifiers generally produce low-heating-valued syngas. They are suitable for small or medium-scale thermal applications.

2.1.1.1.1 UPDRAFT (COUNTER-CURRENT) GASIFIERS The updraft gasifier is the simplest type of gasifier. The biomass is fed at the top while the air is injected at the bottom. Biomass and air move in a countercurrent direction. During its downward movement, biomass is firstly dried passing through a "drying zone." In the "distillation zone," biomass under­goes decomposition and is converted into volatile gases and solid char. The gases and char will be further converted into CO and H2 as they pass through "reduction zone". Since some of the char settles down in the bottom of the reactor, heat is generated through its combustion in the "hearth zone" and is transported upward by the upflowing gas to main­tain the pyrolysis and drying processes. In addition, CO2 and H2O vapor is also produced from char combustion.

Updraft gasifiers can accept biomass with relatively high moisture content (up to 60%). However, the resulting product gas has high tar content because the tar, newly formed during pyrolysis, does not have the opportunity to pass through the combustion zone.

2.1.1.1.2 DOWNDRAFT (CO-CURRENT) GASIFIERS The downdraft gasifier is currently one of the most widely used fixed-bed gasification systems. Different from the updraft gasifier, air in the downdraft gasifier is introduced into the reactor from the middle part. This design leads to the reversed order of the hearth zone and the reduction zone. In this gasifier, the injected air and biomass move cocurrently.

2.1.1.1.3 CROSS-FLOW GASIFIERS In a crossflow gasifier, biomass is added at the top of the reactor and moves downward. Air is introduced from one side of the reactor and the gas products are released from the other side of the reactor on the same horizontal level.

2.1.1.1.4 OPEN-CORE GASIFIERS Open-core gasifiers are generally employed to gasify biomass with low bulk density and high ash content. An example of this kind of biomass is rice husk. Instead of the narrow throat characteristic of other gasifiers, the open-core gasifier has a wide mouth for biomass injection to prevent fuel flow inhibition caused by bridging.

2.1.1.2 FLUIDIZED-BED GASIFIERS

Fluidized-bed reactors are widely employed as gasifiers. Fluidized-bed gasifiers can also be further classified into bubbling fluidized gasifiers and circulating fluidized gasifiers. In a bubbling fluidized gasifier, air is injected from the bottom of a grate, above which the moving bed is mixed with the biomass feed. The bed temperature is maintained at 700-900 °C. Biomass is pyrolyzed and cracked through contact with the hot bed material. In a circulating fluidized gasifier, the hot bed material is circulated between the reactor and a cyclone separator. During this circulation, bed materials and char go back to the reactor, while the ash is separated and removed from the system.