Updraft gasifiers

Gasification
Gasification is a widely studied and applied technology to produce a mixture of combustible gases. It consists of several sequential processes which include: drying, pyrolysis to give gases, tars and char, cracking and oxidation of tars and, to a certain extent, oxidation of pyrolysis gases and gasification of char. Together with chemical processes and evaporation of moisture, transport phenomena also take place.
Applications
Typical applications might include using producer gas as a substitute for petroleum fuels in the standard gasoline or diesel engines that are commonly used in developing countries for electrical power production and water pumping or in local industries such as sawmills, rice mills and workshops. In addition, the gas can be used in standard heat appliances such as crop dryers and cement, lime, or brick kilns.
Fixed bed reactors are used in small-scale gasification while large biomass gasifiers are usually of the fluidized-bed or entrained –flow type. Fixed bed, updraft and downdraft reactors are, in general, of very simple construction and operation, and avoid the excessive costs of feedstock pulverization.
Principle
In the updraft gasifier the downward-moving biomass is first dried by the up flowing hot product gas. After drying, the solid fuel is pyrolysed, giving char which continues to move down to be gasified, and pyrolysis vapours which are carried upward by the up flowing hot product gas. The tars in the vapour either condense on the cool descending fuel or are carried out of the reactor with the product gas, contributing to its high tar content. The product gas from an updraft gasifier thus contains a significant proportion of tars and hydrocarbons, which contribute to its high heating value. Usually the gases are directly used in a closely coupled furnace or boiler.
Gas quality
The fuel gas requires substantial cleanup if further processing is to be performed. There is interest in the cleaning of the updraft gas for electricity production, as low temperature tars are more reactive and thus easier to be removed, than the high-temperature tars produced in much lower amounts by downdraft and fluidized bed gasifiers.
Advantages
The principal advantages of updraft gasifiers are their simple construction and high thermal efficiency. The sensible heat of the gas produced is recovered by direct heat exchange with entering feed, which thus is dried, preheated and pyrolysed before entering the gasification zone. Updraft gasifiers can be used in the sizes between 2 and 20 MWe.