A Waste To Energy (WTE) process creates electricity, heat and other consumer products (such as recyclable metals and vitrified glass) by using domestic garbage as fuel. The official term for the garbage used in waste to energy processes is Municipal Solid Waste (MSW). Municipal Solid Waste is comprised from solid waste produced by apartment
building, households, commercial establishments and educational institutions that are collected by the various local authorities. Waste to energy process is a type of energy recovery with two main methods:
- incineration performed by directly combusting MSW by using large quantities of air and
- Gasification that transforms MSW into Synthetic Gas (Syngas) via thermal treatment (usually in temperatures higher than 700c).
The process of gasification as already mentioned starts from the collection of the Municipal solid waste. The waste trucks takes the MSW to the gasification plant where it drops off its content into a waste storage where it waits to be used as fuel. From the waste storage the waste gets transferred to the area where it will undergo thermal treatment. In the heat exchange point the process creates two sources of energy: steam, that is directed to a steam turbine in order to produce electricity, and syngas. The syngas is then treated in order to be cleaned up from sulfur and HCL (syngas is also produced in biogas process see related article what is biogas) . The clean syngas is then transferred into a gas turbine to produce electricity. It should be noted that at the stage of thermal treatment the leftover from the process is used to produce various consumer products as already mentioned above.
Comparison of gasification and conventional incineration
Gasification is considered a clean alternative to incineration because the latter uses air to burn Municipal Solid Waste (MSW) which creates pollutants that have to be cleaned up in order not to be released into the environment. Moreover, the ash that is left after incineration has to be disposed carefully since it might be toxic. Gasification doesn’t burn MSW but breaks their molecules apart with heat that results to syngas which is similar to natural gas and can generate electricity through combustion (combustion of syngas is a much cleaner option energy than fossil fuels combustion) . Currently, the main disadvantage of gasification versus incineration is its higher cost. On the other hand, technological leaps in the future are expected to eliminate the financial gap with plasma gasification appearing at this stage as the most promising method.
Gasification sustainability and advantages
Gasification can be considered renewable energy and a form of biomass energy if the fuel used is purely of organic origin i.e. biomass gasification. On the other hand, municipal solid waste is made up from a diverse mix of materials with some being organic and others not. As a result there is controversy whether waste to energy processes and gasification can be considered biomass energy which is sustainable. Proponents of waste to energy processes argue that the energy produced from them is sustainable since humans will always produce non-organic waste. Moreover, in studies conducted regarding the percentage of organic material that is contained in MSW show that more than 95% thus to a great extend can be considered renewable energy which can reduce our dependency on fossil fuels. Finally an important advantage of gaisification (and waste to energy processes in general) is the fact that energy that can produced by them can be stored since syngas can be placed in tanks and used on demand. Similarly incineration can also be performed on demand. This is a significant advantage over other forms of renewable energy such as solar energy and wind energy that their energy currently cannot be stored and should be used right away.