All the organisms are generating some kind of waste through their daily lifestyle. Also the amount of waste generated from human activities have increased rapidly during the last decade.
RCRA states that "solid waste" means any garbage or refuse, sludge from a wastewater treatment plant, water supply treatment plant, or air pollution control facility and other discarded material, resulting from industrial, commercial, mining, and agricultural operations, and from community activities. Nearly everything we do leaves behind some kind of waste. Thus, simply waste is anything which is a refuse.
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In the developed countries, waste to energy transformations are going on and such countries are generating eco-friendly electricity. Today I am going to discuss some basics for the transformation and which parameters to be considered for the waste to generate electricity.
Municipal Solid Waste contains organic and inorganic matter. So the first step is to segregate the waste into organic and inorganic matter. Sometimes segregation is to be done at the source only. If segregation is not done properly it might affect the calorific value of the waste. The latent energy from the organic matter can be retrieved through adoption of several techniques.
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Amount of waste, physical and chemical parameters are crucial factors in deciding the potential of recovery of energy from waste. Physical parameters such as size, density and moisture content.
Smaller size of the waste content helps in faster decomposition of the organic content. Thus, sometimes shredders are used to decrease the size of waste.
Density of the waste is useful to determine the type of waste. Generally organic and biodegradable waste have higher density as compared to inorganic waste like papers.
Moisture content of the waste is a bit complex physical property. Generally high moisture content is good as it helps in faster decomposition.
But when the waste is to be incinerated or is processed through thermal techniques, high moisture content increases the amount of heat energy to be supplied to the waste.
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Chemical parameters that determine the suitability of the waste to generate energy are volatile solids, fixed carbon content, inerts, calorific value, C/N ratio and toxicity.
Calorific value is one of the most important attributes that needs to be considered during the waste to energy transformation. It is the amount of heat generated by the combustion of unit weight of the waste material. It is expressed as kcal/kg.
Higher the heat generated by the combustion of per unit weight, higher is the efficiency of material for generating energy. It can be correlated with the moisture content of the material. If the moisture content is more in waste such as the organic waste, the calorific value gets reduced because we have to give the energy input for initially evaporating the moisture. This is one of the main causes of not appropriate waste to energy setups in developing nations. As the segregation is not done properly, dry waste gets mixed with the wet waste and thus the calorific value of the material gets decreased.
The ratio of carbon to nitrogen is essential during the process of composting. Always remember the portion of carbon should be more than Nitrogen. The sources of nitrogen in the compost pit can be the papers and so papers are to be avoided in the compost pit. Moreover, foul smell from the compost pit signifies the partial decomposition of nitrogen. Thus, the compost will not have the appropriate characteristics which might be present in the absence of excess nitrogenous sources.
For the chemical characterization of the material, two important analyses need to be done. The first one is the ultimate analysis and the other is proximate analysis.
Ultimate analysis is done for the analysis of Carbon, Hydrogen, Oxygen, Nitrogen and Sulphur in the waste material. This is to be done to determine the C/N ratio, which is essential for the biological processes. Moreover, ash content also needs to be determined. These all parameters are important considerations during mass balance calculation for thermal or chemical processes.
Proximate analysis is to be done to determine the combustion characteristics of the waste material. The parameters that are to be analysed are- moisture content, fixed carbon, volatile matter and ash content.
Moisture content is responsible for lowering the calorific value of the waste material. If there is moisture present in the waste material that is to be held into thermal processes, some amount of energy is to be utilized to evaporate the moisture content. Thus it increases the input of energy.
Gravimetric method is used to measure the moisture content. Initially, the weight of the waste material is measured and then it is heated into an oven upto 105℃ for one hour. Again the material is weighed. Whatever the decrement in the weight of the material, it is equivalent to the moisture content.
Volatile matters are the components within the waste systems that get vaporized through the application of heat energy. Such components initially increase the weight of the material but as the heating process begins, it gets vapourized and so they reduce the calorific value of the material.
Now after measuring the moisture content of bulk of waste, the next important parameter to measure is the volatile matter. For measuring it, the material is placed inside a closed crucible and is heated upto 950℃. Here also the gravimetric method is to be adopted i.e. pre-weight and post-weight of the material is recorded.
Fixed carbon is the amount of residue which is left after the complete combustion of the material. If more amount of fixed carbon is present in the material, it will take a longer time period for complete combustion and vice versa.
For the measurement of fixed carbon- after the measurement of volatile matter whatever the residue is left in the closed crucible is the fixed carbon content. You simply had to weigh it.
Ash is the residue left after the burning of the material. It is rich in Carbon. It initially increases the weight of the material but does not have any significant contribution to the heat generation.
For the measurement of ash content- the material is to be burned in an open crucible upto the fusing temperature of the ash. Fusing temperature is the temperature above which the ash starts to agglomerate and forms solid clinkers. Fusing temperature of ash is in the range of 1100-1200℃.
Biological parameters are important to check the biodegradability of the waste. Parameters such as water soluble constituents, hemicellulose, cellulose, lignin, fats, carbohydrates and proteins are to be checked.
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