Waste Management Overview :
Solid waste management normally begins with its collection and ending by safe disposal,passing through transportation, segregation and processing. According to the type of the waste, the processing is planned. Solid waste processing is conducted after segregation to organic,which may be composted, recyclables as glass or plastics which may be recycled, and non-recyclables having high calorific value are directed to profitable products are known asrefused derived fuel RDF used as alternative fuels for energy-consuming industries as cement. Processing and uses of agricultural waste vary between animal feed, fertilizers, or energysources. Semi-solids and used oil are physically and chemically treated for reuse, or being used as a renewable energy source through biodiesel production.
Hazardous and non-recyclable wastes are normally incinerated or landfilled. Examples of these wastes are radioactive waste materials, medical wastes, wastes from the leather tanning industry, etc. Medical wastes are first incinerated then the remaining ashes are disposed of.
Energy Problem and the Need for New Resources :
Energy demand is increasing steadily due to the yearly increase of population as well as rise of living standard that leads to increase the demand of new energy sources. Over the previous few years, the world daily consumption of natural gas and oil increased from 261 billion cubic feet and 85.4 million barrels in 2009 to reach about 335 billion cubic feet and 91.2 million barrels in 2013 by an increase of about 28% and 7% of natural gas and oil consumption respectively . By the end of 2015 the daily oil consumption increased by about 4.2% from that of 2013. Based on the above estimation, new and renewable energy resources should be developed to overcome the problem of fossil fuels shortage over the upcoming decades.
Waste to Energy WTE Concept :
Using different wastes for energy production as alternative energy sources instead of conventional sources is the simple definition of waste to energy concept. Waste materials with a reasonable calorific value can be used for energy generation directly after simple processing as sorting and shredding or through more complicated processes such as thermo-chemical processes like transesterification and pyrolysis. It may be observed that products from thermo-chemical processes have higher calorific values due to the elimination of non-combustible content from the waste materials. These products need to be upgraded to match the international standards.
Examples of WTE Processes :
WTE processes may be classified as direct and indirect ones. Direct processes may be conducted by combustion of RDF, activated sludge, used tires or agro-waste while indirect processes deal with the production of fuel alternatives through other thermo-chemical processes such as transesterification, pyrolysis, gasification, digestion, fermentation, etc.
Direct WTE Processes :
A good example of direct WTE processes is the use of RDF for energy production in cement plants. RDF is the remaining non-recyclable part of MSW and it should be further processed to be used for energy production. RDF should be shredded, screened and dried; the energy content per unit mass of RDF is about one third that of natural gas which make it a promising alternative for fossil fuels. Activated sludge, used tires and agro-wastes can be used directly to generate energy but using these materials may have some drawbacks such as the emission of SOx and NOx gases when combusted and in the case of using activated sludge it may be harmful because of the presence of heavy metals . Agricultural and farm wastes are demanded in other industries, e.g. pulp industry and production of animal feed , but this problem can be solved through the optimization between different uses of these valuable wastes.
Indirect WTE Processes :
Waste oil (WO) transesterification
Waste Oil can be converted into biodiesel through transesterification where WO reacts with an alcohol in the presence of a catalyst to produce fatty acid alkyl ester, i.e., biodiesel, and glycerol as a valuable by-product. The most applied method is the homogeneous base-catalyzed transesterification where alkaline base catalyst such as potassium hydroxide is used. If the free fatty acid content is higher than 2% of WO weight, an esterification step should be performed using alcohol in high excess in the presence of mineral acid as a catalyst to decrease the free fatty acids content to make WO suitable for the transesterification step. After the reaction, different phases are separated and further upgraded to match with theASTM and EN standards.
Pyrolysis and Gasification of wastes :
Pyrolysis is an endothermic reaction that takes place at high temperatures in an inert atmosphere in which tree cuts or materials containing cellulose are converted into more valuable products having higher calorific value, such as char, bio-oil and flammable gases . The final product of this process is syngas, carbon monoxide and water vapour, then this gas mixture can be used for synthesis of higher hydrocarbons through the well known Fischer-Tropsch process.