Gasification of solid wastes involve the reaction of carbonaceous feedstock with an oxygen-containing reagent, usually oxygen, air, steam or carbon dioxide, generally at temperatures in excess of 800°C. It involves the partial oxidation of a substance which implies that oxygen is added but the amounts are not sufficient to allow the fuel to be completely oxidised and full combustion to occur. The process is largely exothermic but some heat may be required to initialise and sustain the gasification process.
The main product of the gasification process is syngas, which contains carbon monoxide, hydrogen and methane. Typically, the gas generated from gasification will have a net calorific value of 4 – 10 MJ/Nm3.The other main product produced by gasification is a solid residue of non-combustible materials (ash) which contains a relatively low level of carbon. Syngas can be used in a number of different ways, for example:
- Syngas can be burned in a boiler to generate steam which may be used for power generation or industrial heating.
- Syngas can be used as a fuel in a dedicated gas engine.
- Syngas, after reforming, may be suitable for use in a gas turbine
- Syngas can also be used as a chemical feedstock.
MSW gasification plants are relatively small scale, flexible to different inputs and modular development. Producing syngas to serve multiple end-uses could complicate delivery of the plants but it could provide a higher degree of financial security. The most important reason for the growing popularity of thermal processes for the treatment of solid wastes has been the increasing technical, environmental and public dissatisfaction with the performance of conventional incineration processes. MSW is difficult to handle, segregate and feed in a controlled manner to a waste-to-energy facility. MSW has a high tendency to form fused ash deposits on the internal surfaces of furnaces and high temperature reactors, and to form bonded fouling deposits on heat exchanger surfaces.
While evaluating gasification or other thermal technologies, the degree of pre-processing required in conversion of MSW into a suitable feed material is a major criterion. Unsorted MSW is not suitable for most thermal technologies because of its varying composition and size of some of its constituent materials. It may also contain undesirable materials which can play havoc with the process or emission control systems.
Advantages of Gasification
There are numerous solid waste gasification facilities operating or under construction around the world. Gasification of solid wastes has several advantages over traditional combustion processes for MSW treatment. It takes place in a low oxygen environment that limits the formation of dioxins and of large quantities of SOx and NOx. Furthermore, it requires just a fraction of the stoichiometric amount of oxygen necessary for combustion. As a result, the volume of process gas is low, requiring smaller and less expensive gas cleaning equipment.
The lower gas volume also means a higher partial pressure of contaminants in the off-gas, which favours more complete adsorption and particulate capture. Finally, gasification generates a fuel gas that can be integrated with combined cycle turbines, reciprocating engines and, potentially, with fuel cells that convert fuel energy to electricity more efficiently than conventional steam boilers.
Gasification with pure oxygen or hydrogen
Gasification with pure oxygen or pure hydrogen (or hydrogasification) may provide better alternatives to the air blown or indirectly heated gasification systems. This depends greatly on reducing the costs associated with oxygen and hydrogen production and improvements in refractory linings in order to handle higher temperatures. Pure oxygen could be used to generate higher temperatures, and thus promote thermal catalytic destruction of organics within the fuel gas.
Hydrogasification is an attractive proposition because it effectively cracks tars within the primary gasifying vessel. It also promotes the formation of a methane rich gas that can be piped to utilities without any modifications to existing pipelines or gas turbines, and can be reformed into hydrogen or methanol for use with fuel cells.
Plasma gasification or plasma discharge uses extremely high temperatures in an oxygen-starved environment to completely decompose input waste material into very simple molecules in a process similar to pyrolysis. The heat source is a plasma discharge torch, a device that produces a very high temperature plasma gas.
Plasma gasification has two variants, depending on whether the plasma torch is within the main waste conversion reactor or external to it. It is carried out under oxygen-starved conditions and the main products are vitrified slag, syngas and molten metal. Vitrified slag may be used as an aggregate in construction; the syngas may be used in energy recovery systems or as a chemical feedstock; and the molten metal may have a commercial value depending on quality and market availability.
Such processes use high-energy microwaves in a nitrogen atmosphere to decompose waste material. The waste absorbs microwave energy increasing the internal energy of the organic material to a level where chemical decomposition occurs on a molecular level. The nitrogen blanket forms an inert, oxygen free environment to prevent combustion. Temperatures in the chamber range from 150 to 3500C. At these temperatures, metal, ceramics and glass are not chemically affected.
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