Chapter 9

Chapter 9 – Biomass Gasification: Process Overview, History, and Development (pp. 167-180)

Authors:  (Daniel T. Howe, Pacific Northwest National Labortatory, Richland, WA, USA)
The current unsustainable use trends of petrochemical fuels has increased the
demand for renewable energy alternatives, and thermochemical processes such as
gasification will be required to meet these demands. Gasification is a process that
involves the high-temperature partial oxidation of carbon-containing fuels such as
lignocellulosic biomass to produce a product gas consisting of hydrogen, carbon
monoxide, carbon dioxide, methane, and condensable aromatic liquids known as tars.
The non-condensable product gas can be either purified into synthesis gas, a mixture of
hydrogen and carbon monoxide used in Fischer-Tropsch synthesis of gasoline or diesel
fuel, or sent to gas engines for the generation of heat and electricity. Gasification has
been in use for over 180 years, with coal being the primary feedstock. Over the course of
its development gasification technology has resulted in three general types of reactors:
moving bed gasifiers, fluidized bed gasifiers, and entrained bed gasifiers. Moving bed
gasifiers employ a fixed bed of fuel that moves downward under gravity while contacting
an oxidant in either counter-current or co-current flow. Fluidized bed gasifiers use a
bubbling fluidized bed of inert particles as a heat transfer medium while the oxidant
serves as the source of fluidization.
Entrained flow gasifiers use a dense suspension of fine solid particles in the oxidant
to create a high-temperature, high-throughput system. The choice of a specific type of
gasifier is determined by the end use of the gas as well as engineering constraints. While
the use of biomass in gasifiers is a relatively new phenomenon, a number of success
stories do exist.