by Ing Nicholas Sammut
This article explains the process of energy recovery systems for both the disused Maghtab and Ta’ Zwejra Landfills.
First of all, there is a clear distinction between energy recovery systems for uncontrolled dumps (such as the former dump at Maghtab) and for engineered landfills, as is the case for the Ta’ Zwejra and Ghallis facilities. Both the dump at Maghtab and the Ta’ Zwejra engineered landfills are located within the Maghtab Waste Management Complex, and both are no longer operational. However, this is basically where the similarities end. The differences in land filling patterns have resulted in emissions with very different characteristics.
Energy recovery from the Maghtab site
Energy recovery from gas emissions is done by extracting the gases that form within the landfill. The old uncontrolled dump at Maghtab has been installed with around 400 gas extraction wells, all of which tap into the dump’s crust and allow the channelling of its emissions through an intricate network of pipes.
What happens then is that a group of very powerful booster pumps provide for all the flow requirements within the network. This guarantees a constant supply of gas through the Regenerative Thermal Oxidizer (RTO) treatment facility, which is onsite at Maghtab.
In simple terms, this facility may be considered as a very complicated furnace. Delving into the chemistry of the treatment of harmful gases goes beyond the scope of this article, however what is relevant is that it does so at an elevated temperature exceeding 800 degrees Celsius.
The RTO is preheated electrically until it reaches the correct operating temperature, thus allowing for the initial circulation of the gas within the system. Although the vast majority of the landfill gas generated from the Maghtab dump is not combustible, that tiny fraction of flammable gas is sufficient to maintain the high temperatures of the system. Indeed, following preheating, electrical supply is cut off, and the RTO is practically self-sustaining. The energy available in this gas flow is actually being recovered to treat the toxic gases. Although not directly involved in the generation of electricity, the energy recovered from the landfill gas of the Maghtab dump substantially substitutes the electrical loads which would have been required to maintain the heat.
The new project to Rehabilitate the Closed Landfills shall produce the necessary capping to make the surface of the landfill cleaner from gases emissions. The situation within these old landfills is expected to have a degree of change from aerobic to anaerobic, where the air presence is restricted and the generation of methane gas will be encouraged. If recovery in larger quantities becomes possible, the methane gas collected will be directed to the gas system connected to generate electricity from landfill gas.
Energy recovery from
the Maghtab site
Electrical generation from landfill gas is associated with the engineered landfills within the Maghtab Environmental Complex, namely the Ghallis and Ta’ Zwejra.
The energy recovery system for Ta’ Zwejra landfill is somewhat easier to understand. The percentage of combustible (flammable) fuel present in the gaseous emissions averages at about 50 per cent, which allows for direct combustion of the gas. The gas extraction system for Zwejra will be very similar to what is present in the Maghtab dump, however in this case the gas will be fed to electrical power generators which do not differ considerably from normal combustion engines.
It is estimated that energy recovered from the Ta’ Zwejra landfill gas will be in the order of 750kW. This will contribute to the power produced from the whole Maghtab Environmental Complex, which is estimated to feed a total of 3000 average households.
WasteServ has also started demonstrating the potential of using landfill gas for electrical generation. The first experiment of this sort was the lighting of a 50ft tall, 7kW Christmas tree which was fixed on the apex of the Maghtab dump for all to see during the festive season.
Some history on operating engineered landfills locally
In 2004, WasteServ developed the Ta’ Zwejra Engineered Landfill, which was the first engineered facility locally. Since this site was developed with the necessary engineering to be compliant, it had to be distinguished from the closed dump. The area occupied by this cell was known as Habel Ta’ Zwejra, hence the name Ta’ Zwejra Engineered Landfill, limits of Maghtab, was adopted for this facility.
WasteServ submitted the application to have this facility permitted to be operated and managed as an engineered landfill. On 1 December 2005, Mepa approved the first national Integrated Pollution, Prevention and Control (IPPC) permit to authorise WasteServ to operate and manage the non-hazardous, non-inert Landfills at Ta’ Zwejra.
Since the period taken to process development and operational permits was of a longer duration than expected, WasteServ continued to extend the utilisation of Ta’ Zwejra Engineered Landfill to provide the necessary void space until the first cell of the Ghallis Engineered Landfill was developed. Four cells were constructed and engineered at Ta’ Zwejra, and was used between April 2004 to December 2006.
Currently, WasteServ is in the process of capping and extracting gas from the Ta’ Zwejra Landfill. Tests are also being carried out to find the optimum use of the gases generated.
More information about WasteServ can be obtained by visiting website www.wasteservmalta.com or by calling the WasteServ Freephone 8007-2200.
Ing. Nicholas Vella is the project coordinator for the Rehabilitation of Disused Landfills Project within WasteServ Malta Ltd
Source: The Malta Independent - 25/02/2009