Energy from Waste

Overview

EfW is the name often given to the thermal treatment of waste under controlled conditions in which energy is produced. This energy can either be converted to electricity to boost the National Grid and/or, at times when the EfW plant is a Combined Heat and Power facility, to provide heat in the form of hot water or steam for use by nearby developments. A plant facilitating the generation of electrical power and recovery of usable heat from a combustion process is termed a Combined Heat and Power (CHP) plant and it is more efficient than conventional power generation that is focused on power generation alone.

In the most widely used EfW process, waste is burned on a moving grate. Air is introduced above and beneath the grate in carefully controlled amounts to ensure proper combustion. Good combustion means fewer emissions. The hot gases released are directed to a boiler to recover the heat. The combustion gases are then cleaned in several stages to a strict standard set by the Waste Incineration Directive (WID), which are monitored by the Environment Agency (EA) in England.

Of the material received into the EfW facility only around 3% will not be recovered and will require specialist treatment. This is residual material created as part of ensuring the gases are clean. Energy from Waste is generally a catch all term for three broad types of technology (with a range of different technology providers within each broad type of technology type):

• Incineration or Conventional Combustion
• Gasification
• Pyrolysis

Efficiency

Where an EFW facility meets the standards of efficiency set out in the Europeans Commissions Guidelines on the R1 energy efficiency formula in Annex II of Directive 2008/98/EC, it can be considered an Energy Recovery Facility or R1 facility and to achieve this a plant may need CHP as part of its solution.

Inputs

Often, but not in all cases, before waste is inputted into the EfW process a pre-treatment of this waste is needed. This can include a sorting process that extracts the recyclable material, is able to manipulate the properties of the feedstock such as Calorific Value, Biomass content, Moisture Content and the size of the feedstock particles through processes like shredding. Different types of pre-treatment include subjecting waste to a Mechanical Sorting Plant, Mechanical Biological Treatment (MBT), or Mechanical Heat Treatment (MHT).

The application of EfW and the associated technology is slightly different according to the types of waste fuel that are to be thermally treated. The approaches and the market can be broadly split into:

• Residual Waste EFW (comprising both untreated and treated Black Bag waste)
• Biomass Waste EFW (comprising primarily Wood Waste EFW but including Sewage Sludge EFW and Agricultural Waste EFW)

The Biomass content of the feedstock is an important parameter as the heat/electricity produced from an EfW plant that can be classified as renewable (and therefore eligible for subsidies e.g. ROC, RHI) is derived from the biogenic portion of the feedstock .

Outputs

Aside from the desired electricity, heat, or chemicals (including fuels) generated from the Energy from Waste process; a range of by-products are also produced.

The residual Bottom Ash or IBA left after the combustion process is generally around 20% of the input tonnage by weight but will vary in quantity and chemical composition depending on the composition of the waste feedstock (i.e. bottom ash from Waste Wood EFW will be different to Residual Waste EFW in composition and in percentage terms) . This often contains minerals and metals along with non-combustible ash, which is increasingly being recycled into Secondary Aggregates, with some currently being accepted by The Highways Agency in a processed form known as IBBA.

The combustion gases from EFW are cleaned in several stages to a strict standard set by the Waste Incineration Directive (WID), which are monitored by the Environment Agency (EA) in England. This gas cleaning results in a residual Flue Ash which is around 3% of the overall input to the plant by weight depending on the level of gas cleaning required.

Fly Ash and Air Pollution Control (APC) residues are often combined. APC residues contain ash, carbon and lime in varying quantities and is considered a Hazardous Waste to be disposed of in a Hazardous Landfill, or sometimes to be further processed through washing or stabilisation in order to make it a non-hazardous waste and allow it to be disposed of in a Non-hazardous Landfill.

The Syngas produced from Advanced Conversion Technologies is comprised of hydrogen, carbon monoxide and methane. This is often combusted in order to generate electricity or in Waste to chemicals processes it can be turned into a fuel.

The by-products produced from EfW processes vary in composition and quantity depending on the type of waste used as a feedstock, and the technology implemented.

Conclusion

Energy from Waste processes play a vital role in diverting waste from landfill and in generating energy. EfW is the preferred technique for managing waste over Landfill providing there is sufficient renewable content in the feedstock and the EfW plant is operating at an energy-efficient state (guide to the debate). Recycling is higher up in the Waste Hierarchy and therefore EfW processes should not in principle take feedstocks that include recyclable materials. However, often a synergy exists between EfW and Recycling where the two waste management techniques can complement each other and work together to reduce waste sent to Landfill.