Waste to Energy: The Carbon Perspective

Waste to energy plants are key treatment facilities for municipal solid waste in Europe. The technology provides efficient volume reduction, mass reduction and hygienisation of the waste. However, the technology is highly disputed in some countries. It is crucial to understand the role of waste to energy with respect to potential contributions to CO2 emissions and savings.

Industry park Höchst, Hesse, Germany features a waste to enrgy plant with a capacity of about 675000 tonnes per year Image credit: Norbert Nagel
Waste to energy plants are key treatment facilities for municipal solid waste in Europe. The technology provides efficient volume reduction, mass reduction and hygienisation of the waste. However, the technology is highly disputed in some countries. It is crucial to understand the role of waste to energy with respect to potential contributions to CO2 emissions and savings.

By Thomas H Christensen, Anders Damgaard & Thomas Astrup

Many arguments have been presented for and against Waste to Energy (WtE) technology. Climate change is a key issue in modern waste management, and it is crucial to understand the role of WtE with respect to potential contributions to CO2 emissions and savings.

Heat substitution is highly affected by local conditions, however in most European cases natural gas is a likely alternative corresponding to around 75 kg CO /GJ heat produced from natural gas and up to 157 kg CO/GJ heat for brown coal

The CO2 account includes loads as well as savings. The CO2 load is caused mainly by use of auxiliary materials and energy at the plant and the direct emissions from the plant through the stack. The saving in CO2 is related to the energy and materials recovered at the plant. In the following presentation, the quantification is made in kg CO2-equivalent per tonne (1000 kg) of wet waste treated, and as a reference it is assumed that conversion of organic matter into biogenic CO2 is neutral in a global warming context as argued by Christensen et al. (2009).

Loads

Manufacturing the materials used in the combustion equipment of a WtE plant, as well as the energy used for the construction process, constitute a CO2 -emission to the environment, as fossil fuel is typically used in the upstream processes. Few quantifications of the CO2 loads of the capital goods for WtE plants exist, but it is likely to be in the range of 7-14 kg CO2/tonne of waste treatment capacity, depending on the size and construction materials used.

Electricity is not only generated, but consumed in the operation of a WtE plant, in a particular for cranes, fans and air pollution control. It may also be used for flue gas condensation. The main controlling factor is the actual configuration of the plant and the air emissions values aimed at.

For example, obtaining low emissions values for SO2 requires the use of more electricity, while low emissions of NOx may require an additional supply of gas. The use of electricity is estimated to typically be 65-185 kWh/tonne of waste treated. The contribution of this to the overall CO2 emissions depends on how this electricity is produced. This is discussed later when setting up the CO2 account.

If the WtE plant produces electricity for the grid, then the use of electricity at the plant must be accounted in the same way or a net value must be introduced, only accounting for the net exchange with the grid.

The direct emission of fossil CO2 is caused by the combustion of fossil based materials such as many plastics and some textiles. This means that the actual emission depends on the waste incinerated. In recent years measurements of the fossil CO2 content of the flue gas as well as the application of indirect balancing methods suggest that the fossil CO2 emission is in the range of 250-600 kg CO2/tonne of waste processed. The variation may be large between plants depending on the waste they treat.

Savings

The savings in CO2 emissions appear as the energy and material recovered at the WtE plant off-set the production of energy and material from fossil fuel based technologies, which would otherwise have had to be produced outside the waste system. Thus, the quantity of energy and material recovered, and which avoided their production elsewhere, affect the overall CO2-savings to be attributed to the WtE plant.:

The use of magnetic separation technologies and eddy currents to recover ferrous metals, as well as aluminium and brass can lead to significant CO2 savings  Credit: Indaver
Energy recovery

The energy recovered can be in the form of electricity to the grid, steam delivered to a nearby industry, and heat supplied to a district heating system. The energy recovery depends on the technology of the facility, as well as the calorific content of the waste (a lower heating value of waste reflects the energy content minus the amount of water that has to be evaporated).

The electricity recovery can range from 0% to 30% of the lower heating value or from 0 to 875 kWh per tonne of waste incinerated. Statistical data for 314 European WtE plants shows an average of 22% for electricity production only and 15% for electricity production when heat is also being recovered.

Factors such as construction costs, electricity prices and the local market for steam and heat effect how much electricity recovery a WtE plant will be designed for. The highest value reported is for the Amsterdam WtE plant which produces up to 31% electricity of the lower heating value of the waste received.

Heat recovery is closely linked to local heat markets. The highest heat recoveries are reported for plants located close to large district heating systems. Values typically range from 5% to 85% of the lower heating value without flues gas condensation and may reach higher than 90% with flue gas condensation involving heat pumps.

Flue gas condensation is associated with a significant use of electricity, and electricity and heat recovery must be addressed in combination. Data for Europe shows that the average heat recovery is 37% when electricity is produced and 77% when only heat is recovered.

Material recovery

Material recovery is primarily from the bottom ash where metal scrap as well as a gravel-like material can be recovered. In terms of CO2 savings the former is the most important contribution. Recovery of hydrochloric acid and excess lime from the air pollution control residues is rare and is not addressed here.

Once metals have been recovered from the bottom ash in some countries it can be used in the manufacture of a variety of construction materials, reducing the carbon emissions from the material for which it is substituted  Credit: Indaver

The scrap metal recovery takes place primarily in terms of ferrous metals removed by a magnet. Aluminium and in some cases copper and brass can also be removed by eddy current technology. The metals are often present as very small particles and recovery requires various pre-treatment steps with size fractionation and homogenisation to obtain high recovery.

The actual scrap metal recovery depends on the waste composition as well as the technology available, but in state-of-the-art cases the recovery can be expected to reach 85% of magnetic iron and around 60% of non-ferrous metals in the waste. Typically this corresponds to 15-30 kg of ferrous metals and 1.2-2.5 kg of aluminium per tonne of waste treated.

Recycling metals saves around 1.5 kg CO2 per kg iron scrap and about 10 kg CO2 per kg aluminium. The recovery of metals at WtE plants is on the increase and in Europe the average recovery is estimated to 60%

In some countries, after initial metal sorting and storing, bottom ash can be used in civic works typically as unbound layers in roads as a substitute for gravel. The environmental burden with respect to climate impacts of gravel production or crushing of rock is relative small; typically 1.5-2 kg CO2 per tonne of gravel excluding the transport. Assuming that 1 tonne of bottom ash can substitute on average 0.8 tonne of gravel, a typical saving by recovering bottom ash for civic works is of the order of 0.5 kg CO2 per tonne of municipal waste treated.

The CO2 account for WTE

In quantifying the climate contribution from waste to energy the commonly accepted approach of using three independent quantifications was applied:

'Indirect, upstream' (CO2 loads from production of facilities, use of materials and energy) 'Direct' from the plant (CO2 loads from combustion of waste and fossil carbon in the waste 'Indirect, downstream' (CO2-savings taking place outside the waste management system obtained by energy delivered to the grid and materials sent to recycling or utilisation). The Spittelau waste to energy plant in Vienna supplies some 60 MW of heat to the city's district heating network and processes around 260,000 tonnes of waste each year  Image credit: Cha già José

In the Waste Management & Research paper, Incineration and co-combustion of waste: Accounting of greenhouse gases and global warming contribution, this approach was implemented by Astrup et al. (2009) for MSW and solid recovered fuels. Some of the basic data used there have also been used in Table 1 where the climate contributions are quantified. Astrup showed that contributions from the use of lime, carbon filters etc. were small and they are excluded from the table in order to focus on the main aspects. Rounded numbers have been used assuming a Lower Heating Value of 10 GJ/tonne waste.

The electricity consumption as well as the recovery are important factors in the accounting and we have chosen to use different sources for producing electricity ranging from the EU mix (0.5 kg CO2/kWh) to brown coal (1.3 kg CO2/kWh). The heat recovery is assumed to substitute heat that would otherwise have been produced by fossil fuels. Heat substitution is highly affected by local conditions, however in most European cases natural gas is a likely alternative corresponding to around 75 kg CO2/GJ heat produced from natural gas and up to 157 kg CO2/GJ heat for brown coal.

Table 2 shows the CO2 account for eight different hypothetical WtE plants incinerating the same waste but with different energy recoveries and interactions with different energy systems with respect to electricity and heat. If no recovery took place, the net CO2 emissions would be over 400 kg CO2 per tonne of waste. Even with a moderate recovery of electricity (13%) the WtE plant will constitute a load to the environment in terms of CO2 equivalents.

The more energy recovered and the more ‘dirty' the energy to be substituted by the recovered energy, the more will the WtE plant contribute to reduce the emissions of CO2 equivalents. If electricity and heat are recovered at a WtE plant in an area where the energy otherwise would have been produced from brown coal, the net saving would exceed 1000 kg CO2 equivalents per tonne waste.

Conclusion

Many aspects may contribute to the sustainability of waste to energy plants; however, the CO2 accounts presented here clearly illustrate that the energy recovery efficiency of the plants is a very important factor. Not only is the quantity of the recovered energy important, but the type of energy off-set within the public energy networks is also critical. The more fossil fuels that are substituted in the energy sector by the electricity and heat production from the WtE plants, the larger the CO2 savings offered.

The CO2 accounting shows that WtE plants with little energy recovery may constitute an overall load to the environment with respect to CO2 emissions. However, with efficient electricity and heat recovery waste to energy plants contribute significantly to reducing the climate impacts of modern waste management and appear much more climate friendly than when the waste is disposed of in landfills.

Thomas H Christensen, Anders Damgaard & Thomas Astrup, Department of Environmental Engineering, Technical University of Denmark, Lyngby

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