Gasification of Municipal Solid Wastes

Gasificationprocesses involve the reaction ofgases can lead to high rates of metal wastage of
carbonaceous feedstock with an oxygen-containingheat exchange tube surfaces due to high
reagent, usually oxygen, air, steam or carbon dioxide,temperature corrosion.
generally at temperatures in excess of 800°C. ItWhile evaluating gasification or other thermal
involves the partial oxidation of a substance whichtechnologies, the degree of pre-processing required in
implies that oxygen is added but the amounts areconversion of MSW into a suitable feed material is a
not sufficient to allow the fuel to be completelymajor criterion. Unsorted MSW is not suitable for
oxidised and full combustion to occur. The process ismost thermal technologies because of its varying
largely exothermic but some heat may be required tocomposition and size of some of its constituent
initialise and sustain the gasification process.materials. It may also contain undesirable materials
The main product is a syngas, which contains carbonwhich can play havoc with the process or emission
monoxide, hydrogen and methane. Typically, the gascontrol systems.
generated from gasification will have a net calorificThe main steps involved in pre-processing of MSW
value of 4 - 10 MJ/Nm3.The other main productinclude manual and mechanical separation or sorting,
produced by gasification is a solid residue ofshredding, grinding, blending with other materials,
non-combustible materials (ash) which contains adrying and pelletization. The purpose of
relatively low level of carbon. Syngas can be used inpre-processing is to produce a feed material with
a number of different ways, for example:consistent physical characteristics and chemical
- Syngas can be burned in a boiler to generate steamproperties. Pre-processing operations are also
which may be used for power generation or industrialdesigned to produce a material that can be safely
heating.handled, transported and stored.
- Syngas can be used as a fuel in a dedicated gasThere are numerous solid waste gasification facilities
engine.operating or under construction around the world.
- Syngas, after reforming, may be suitable for use inGasification has several advantages over traditional
a gas turbinecombustion processes for MSW treatment It takes
- Syngas can also be used as a chemical feedstock.place in a low oxygen environment that limits the
The most important reason for the growingformation of dioxins and of large quantities of SOx
popularity of thermal processes for the treatment ofand NOx. Furthermore, it requires just a fraction of
solid wastes has been the increasing technical,the stoichiometric amount of oxygen necessary for
environmental and public dissatisfaction with thecombustion. As a result, the volume of process gas is
performance of conventional incineration processes.low, requiring smaller and less expensive gas cleaning
MSW is difficult to handle, segregate and feed in aequipment. The lower gas volume also means a
controlled manner to a waste-to-energy facility. MSWhigher partial pressure of contaminants in the off-gas,
has a high tendency to form fused ash deposits onwhich favours more complete adsorption and
the internal surfaces of furnaces and highparticulate capture. Finally, gasification generates a
temperature reactors, and to form bonded foulingfuel gas that can be integrated with combined cycle
deposits on heat exchanger surfaces. The productsturbines, reciprocating engines and, potentially, with
of the combustion of MSW are also very aggressive,fuel cells that convert fuel energy to electricity more
in that the flue gases are erosive and the relativelyefficiently than conventional steam boilers.
high levels of chloride containing species in the flue