The shortage of cheap fuels and the great cost of imported oil had affected many countries of the world for the past decades. After the search of low-cost alternative energy, urban wastes were the most promising potential source. It was obvious because of two reasons. First, a great fraction of waste is consist of combustible components, depending on the country. Second, incineration of municipal waste and use of the waste heat produced had been used in Europe for many years. Many of the combustible components are biodegradable which means it can be converted biologically into fuel gas which can be used directly or can be stored and transported for later use. All urban wastes have various energy potential because the energy content and the ease of extraction of the energy are different. There are many methods of energy recovery from municipal wastes. For example, there are various types of pre-processing of wastes which are the process to separate the combustible components and the non-combustible components before converting them into energy such as mechanical, manual, or mechanical/manual. The method without pre-processing converts the wastes in their original form.

Incineration and refuse-derived fuel production

The energy content or heating value of the wastes is based on the chemical elements in them and the most contributing element are carbon and hydrogen. The energy content also determines the value of the wastes in direct conversion. Notwithstanding, moisture content and the presence of non-combustible materials decrease the fuel value of the wastes. The composition of the waste plays a big role in determining the method of disposal as well as the value of the fuel. For example, if the composition of the waste has relatively high in moisture content, it should be decreased before ignition takes place by using the heat from the combustion of dry materials or additional fuel such as fossil fuel to reduce it so that it will have a higher fuel value. According to the UNEP Solid Waste Management, approximately 50% to 70% of the waste in developing countries are putrescible on a wet weight basis. However, the percentage of plastic and paper are relatively small. There is also 60% ash content in the urban waste.  Thus, the percentage of dry, combustible element is quite small. Furthermore, all these characteristics cause the conversion system to use more energy than creating it. As a result, incineration and thermal processing are not convenient for developing countries as the energy production is insufficient. This method is only applicable in some areas with the suitable composition of waste.

Incineration is a procedure to decrease the total volume of waste as well as harnessing energy from the steam generated and it is practiced in Europe for many decades and most nations of the world is using this method. The simplest incineration is open burning. Other than that, there are also stoker-fired, modular, and fluidised bed systems. Nowadays, with modern technologies, incineration is now a more controlled combustion rather than a crude one. There are several pros and cons of incineration of raw waste. The pro is it is useful for an overpopulated area that has no more space and demands energy because incineration will reduce the volume of waste, therefore, the waste will not take much space plus it will generate energy simultaneously. On the other hand, the con is clearly that it creates pollutants that will affect the whole area and the cost of reducing it is undeniably high. Therefore, it is not the best method of energy recovery.

Refuse-derived fuel is useful as it serves as a feedstock for incineration systems. The procedure to produce it has two main operations, manual and mechanical. Generally, the objective of the operations is to segregate the combustible components from the non-combustible components. The manual operations such as sorting the materials can combine with the mechanical such as size reduction, screening, and magnetic separation or it can be singular.

 Thermal gasification and biogasification

Gasification is the conversion of wastes into a gaseous fuel that is used in solid waste management. Nevertheless, the recovered energy are available more in liquid form, solid form, or both than in gaseous form in certain processes. Plus, it is a very complex and expensive solution for energy recovery and should only be considered in special circumstances. There are two ways of accomplishing gasification, biological and nonbiological processes. The biological processes or usually referred as ?anaerobic digestion?, ?methane fermentation? or ?biogasification? takes place in a landfill whenever the conditions appropriate. The biological process involves mutual activities of anaerobes in the conversion of 30% to 40% of the energy bound in the biodegradable fraction of wastes into the chemical energy of methane. The latter phenomenon provides landfill gas recovery and utilisation. Next, the non-biological gasification processes are thermal in nature which makes them better than biological processes in terms of energy recovery. Non-biological gasification or ?pyrolysis? is the process of distilling organic matter without the presence of O2 which gives the end product of gasses, liquids (oils and tars), and solids (char). The gasses can elevate temperatures and at some degree, pressure. Strict pyrolysis reactions result in low gas yields whereas the combination of strict pyrolysis and limited combustion (admitting a small amount of O2 in) gives higher results. This technology is later developed as ?pyrolysis-combustion? process or more known as biogasification or thermal gasification. A more in depth discussion about biogasfication as a waste-to-energy technology will be discussed in the next report.