Utilising thermal processes to break down waste into a fraction of its original size, these plants generate baseload renewable energy (i.e. steam which can then be used to generate electricity) and use a series of filters, scrubbers, and catalytic reactors to reduce the resultant flue gases well below European Environmental standards.
Thermal treatment of waste has existed since humans first discovered fire and has been used throughout history, with the first large scale thermal plant being built in Nottingham, England in 1874. From there plants were built throughout Europe and America helping solve the chronic waste issue faced during the Industrial Age, bringing in the Age of Sanitation. Though these plants helped reduce the amount of waste filling landfills, they were inefficient, and their outputs not used to generate renewable energy.
In the 1920’s Josef Martin (of Martin GmbH) invented the ‘reverse-acting grate’ that is based on the premise that fuel ignites more easily when an already existing glowing mass is pushed back underneath it.
The concept was developed over time and the grate proved to be the solution to creating efficient combustion of MSW. This system has been in commercial operation since 1959.
The Danish Energy Statistics report published in 2005 shows that Energy from Waste plants in Denmark now produces over 4.8% of the electricity and 13.7% of domestic heat requirements.
According to Pike Research Waste-to-Energy Technology Markets research report (2010), there are over 900 Energy from Waste plants in operation around the globe, of these over 390 plants use the Martin GmbH grate technology, making Martin the world leader in this field designing and creating plants that supply a cleaner and renewable source of electricity, heat, and usable commodities. In the Asia Pacific region, Mitsubishi Heavy Industries Environmental & Chemical Engineering Co., Ltd. holds a license for the Martin grate technology, and Phoenix Energy Australia is proud to be associated with Mitsubishi Heavy Industries in offering this world leading waste to energy technology in Australia and New Zealand.
How it works
In a typical plant, MSW (Municipal Solid Waste) is brought in by waste collection vehicles and handed over to a holding bin containing the ‘waste crane’. This crane will pick up waste and place it into the grate designed by Martin ensuring a constant flow of fuel into the grate. Within the firing chamber though the manner in which the MSW is managed and brought to combustion is critical to the performance and effectiveness of the plant and to its emissions. By its very nature MSW is not an entirely predictable fuel in its consistency or its constitution. It is therefore this component of the process which is required to be unique to MSW creating an optimised, efficient and complete combustion.
Air is supplied via vents below the grate, which ensures the grate is kept at a constant temperature, and also via nozzles directly into the boiler to generate a greater oxygen mix to break down flue gases.
To breakdown organic toxins, during certain parts of the process the flue gas temperature is raised to at least 850 °C. After which it’s then cooled in the superheaters and boiler, transferring the excess heat to generate steam. This steam can then be sent to steam turbines to generate electricity or used directly by industry as a heat source. Finally the remaining flue gasses are passed through a series of filters, scrubbers and catalyst reactors to remove any hazardous material.
Bottom Ash, the residue which remains after combustion (typically <10 vol% of the feedstock, depending on the feedstock composition), can be recovered and used as aggregate or further processed to create bricks or pavers. Alternatively, Plasma Gasifiers can also be used in conjunction with mass combustion type thermal waste to energy plants to further break down any remaining solid waste (ash). To find out more about Plasma Gasifiers click here.
Typical Stack Emissions for modern WTE facilities
Whilst it is indeed true that in the 1970s and 1980s mass combustion type thermal waste to energy plants were seen as polluting, after the introduction of Maximum Available Control Technology (MACT) regulations implemented in the 1990s by the US EPA and European Environment agencies, the US and European WTE industry invested in retrofitting pollution control systems and has now become one of the lowest emitters of high temperature processes.
The Table summarises the criteria and performance of the Martin grate Brescia (Italy) plant as a reference. This plant was required to meet stringent Plant Approval emissions limits at the time of design and has since been compared to the EU 2000 limits.
|Stack Emissions||Plant Approval Limits (1993)||Plant Design Data (1994)||EUROPEAN UNION Limits (2000)||Actual Operating Data|
|Nitrogen Oxides (NOx)||200||100||200||80|
|Chlorine acid (HCl)||30||20||10||5|
|Fluorine acid (HF)||1||1||1||0.2|
|PAH (Polycyclic aromatic hydrocarbon)||0.05||0.01||-||0.001|
|Dioxin (TCDD Teq) ng/Nm3||0.1||0.1||0.1||0.01|