Gasification and incineration
In recent decades, industrialized countries also included the thermal treatment (incineration, pyrolysis, or gasification) of MSW as an important option for its management. Within thermal treatments, incineration has reached a great interest. However, although this process notably reduces the space required for the disposal of the same amount of residues in landfills (typically by a factor from 4 to 10) , MSW incinerators (MSWI) have been questioned because of the atmospheric emissions of acid gases, heavy metals, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and especially by the emission of the potential carcinogenic agents polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) .
(a) Incineration (combustion): The term ‘incineration’ is used to describe processes that combust waste and recover energy. In mass burning systems, the refuse is burned in an "as received" condition. Generally, in mass burning systems all of the waste entering the facility is dumped into a large storage pit, with bulky items being removed prior to entering the combustion chamber . To allow the combustion to take place a sufficient quantity of oxygen is required to fully oxidize the fuel. Incineration plant combustion temperatures are in excess of 850oC and the waste is mostly converted into carbon dioxide and water and any noncombustible materials (e.g. metals, glass, stones) remain as a solid, known as incinerator bottom ash (IBA) that always contains a small amount of residual carbon. The direct combustion of a waste usually releases more of the available energy compared to pyrolysis and gasification .
(b) Pyrolysis: Pyrolysis is thermal decomposition in the absence of oxygen. This process requires an external heat source to maintain the pyrolysis process. Typically, temperatures of between 300oC to 850oC are used during pyrolysis of materials such as MSW. The products produced from pyrolysing materials are a solid residue and syngas . The solid residue (sometimes described as a char) is a combination of non-combustible materials and carbon. The syngas is a mixture of gases (combustible constituents include carbon monoxide, hydrogen and methane) and condensable oils, waxes and tars. The syngas typically has a net calorific value (NCV) of between 10 and 20 MJ/Nm3. For comparison, natural gas has NCV of around 38 MJ/Nm3 . If required, the condensable (liquid) fraction can be collected, potentially for use as a liquid fuel or a feedstock in a chemical process, by cooling the syngas . By manipulating the environmental conditions within the reactor, the yield of any desired product (gas of low calorific value, liquid oil and carbonaceous char) may be optimized .
Refuse-derived fuel (RDF): Fuel produced from combustible waste is called refuse derived fuel (RDF). RDFs are processed so that all non-combustible materials like recyclables (glass, metals) and inerts (stones, etc.), which do not contribute to the energy content of the waste are removed prior to burning. The waste going into the RDF mainly comprises wastes with significant energy content like plastics, dried biodegradable materials, textiles, etc . In many instances, the waste remaining after processing is shredded into confetti-like particles [7, 8]. Raw MSW typically has an energy content of 9 – 11 MJ/kg, whereas an RDF can have an energy content of 17MJ/kg .
(c) Gasification: When the heat for pyrolysis is provided by combustion of part of the waste in air or oxygen, the term "gasification" is used. In gasification, air (oxygen) is added but the amounts are not sufficient to allow the fuel to be completely oxidized and full combustion to occur. The temperatures employed are typically above 650oC. The process is largely exothermic but some heat may be required to initialize and sustain the gasification process. The main product is a syngas, which contains carbon monoxide, hydrogen and methane. Typically, the gas generated from gasification has a NCV of 4 – 10 MJ/Nm3. The other main product of gasification is a solid residue of non-combustible materials (ash) which contains a relatively low level of carbon.
Necessary conditions for MSW incineration The key requirements in the incineration of MSW are as follows: · A minimum combustion temperature of 850oC for 2 seconds of the resulting combustion products · Specific emission limits for the release of SO2, NOx, HCl, volatile organic compounds (VOCs), CO, particulate (fly ash), heavy metals, dioxins, etc. to the atmosphere. · Bottom ash that is produced has a total organic carbon content of less than 3%.
USES OF ENERGY GENERATED FROM MSW
Energy recovered from waste can be used in the following ways:
(A) Generation of Power (electricity): The energy generation option selected for an incineration facility will depend on the potential for end users to utilize the heat and/or power available. In most instances power can be easily distributed and sold via the national grid and this is by far the most common form of energy recovery.
(B) Generation of Heat: For heat, the consumer needs to be local to the facility producing the heat and a dedicated distribution system (network) is required. Unless all of the available heat can be used the generating facility will not always be operating at its optimum efficiency.
(C) Generation of Heat and Power: The use of combined heat and power (CHP) combines the generation of heat and power (electricity). This helps to increase the overall energy efficiency for a facility compared to generating power only. In addition, as power and heat demand varies a CHP plant can be designed to meet this variation and hence maintain optimum levels of efficiency.
An incinerator with energy recovery comprises of the following process:
[A] Waste reception, sorting and preparation:
[B] Combustion: The combustion is normally a single stage process and consists of a lined chamber with a granular bubbling bed of an inert material such as coarse sand/silica or similar bed medium. The bed is ‘fluidized’ by air (which may be diluted with recycled flue gas) being blown vertically through the material at a high flow rate. Wastes are mobilized by the action of this fluidized bed of particles. There are two main sub-categories of fluidized bed combustors:
[C] Energy recovery plant: The standard approach for the recovery of energy from the incineration of MSW is to utilize 1the combustion heat through a boiler to generate steam. Up to 80% of the total available energy in the waste can be retrieved in the boiler to produce steam. The steam can be used for the generation of power via a steam turbine and/or used for heating
[D] Emissions control: The combustion process must be correctly controlled and the flue gases must be cleaned prior to their release. Generally, ammonia is injected into the hot flue gases for control of NOx emissions. Lime or sodium bicarbonate is injected to control SO2 and HCl. And finally, a filter bed consisting of adsorbents like activated carbon, fly ash and other solids (lime or bicarbonate) is used to control the release of heavy metals, CO, VOCs and dioxins.
[E] Residue handling: Finally, bottom ash and air pollution control residues should be properly handled and disposed off as per the regulations.