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Is a gasifier just a kind of combustion unit?

It is undoubtedly true that the current vogue for gasification and pyrolysis plant in the context of waste to energy is based mostly on political perception. These processes are perceived as offering a means to sidestep the problems involved in getting a hostile public to accept the idea of a waste to energy plant in their locality after a very effective if misguided campaign against waste combustion plant by "green" pressure groups over many years. The developers hope to present their new plant as clean, green and modern; but is this fair, and are these systems really "something different" or are they just plain combustion in a sequinned tuxedo?

Lets have a look at how pyrolysis, gasification and combustion relate to each other, and the similarities and differences between pure direct combustion (or incineration) plants, and those using pyrolysis / gasification and combustion in separated systems.

The diagram shows a simplified and idealised fire comprising a pile of some solid fuel such as coal or wood, perched on a conventional grate and lit from below, just as we used to use to heat our houses. Bear in mind that this is a gross simplification - before you all mail me! In reality the zones shown are diffuse, and rather mixed within the heap depending on factors like temperature, air flow and porosity. Also, the heat from the burning gases above will create zones of oxidation and gasification in the top layers. The diagram seeks only to illustrate a point. What is shown here, is that the processes of DRYING, PYROLYSIS, GASIFICATION and OXIDATION are all components of the overall process termed COMBUSTION

In the combustion process illustrated left, HEAT is evolved by the exothermic reactions as oxygen in the air combines with carbon and hydrogen in the fuel to produce carbon dioxide and water vapour. Some of this heat goes into drying the fuel, as can be seen when fresh coal or wood is placed on a fire. Only when the fuel is dry can the surface heat up and decompose. The decomposition process also absorbs heat, breaking long organic molecules into smaller, simpler more volatile substances that evaporate and burn as a gas above the fire forming blue and yellow flickering flames. If there is a very high volatile content, these gases may further break down into lighter gases and soot before burning away completely. In the base of the fire, heat from the combustion of carbon to CO2 drives the gasification of further carbon to CO in the air starved region just above, and this CO migrates up to join the other combustible gases.

Fire diagram

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