Gasification of Municipal Solid Wastes
Gasification of Municipal Solid Wastes
Gasification of Municipal Solid Wastes
Gasificationprocesses involve the reaction of carbonaceous feedstock with an oxygen-containing reagent, usually oxygen, air, steam or carbon dioxide, generally at temperatures in excess of 800C. 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 is a 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.
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. The products of the combustion of MSW are also very aggressive, in that the flue gases are erosive and the relatively high levels of chloride containing species in the flue gases can lead to high rates of metal wastage of heat exchange tube surfaces due to high temperature corrosion.
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.
The main steps involved in pre-processing of MSW include manual and mechanical separation or sorting, shredding, grinding, blending with other materials, drying and pelletization. The purpose of pre-processing is to produce a feed material with consistent physical characteristics and chemical properties. Pre-processing operations are also designed to produce a material that can be safely handled, transported and stored.
There are numerous solid waste gasification facilities operating or under construction around the world. Gasification 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.
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