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Type of Waste to be treated
Source Separated Organic Waste
Source Separated Packaging
Mixed MSW
Min Capacity (in tonnes)
>5.000
>12.500
>50.000
ALL
Secondary Products or Energy Production
FE
AL
Plastic
Paper
Glass
Wood
RDF
SRF
Compost
Biogas
Max Residues (%)
0-10%
10-20%
20-30%
30-40%
Capital Expenditure € tn of total feedstock on wet basis
0-100
101-200
301-400
601-700
Annual operation and maintenance cost € tn of total feedstock on wet basis
21-40
41-60
61-80
81-100
Land requirement m2 per tn of total feedstock
0-0.2
0.21-0.4
0.41-0.6
0.61-0.8
0.81-1
Water consumption m3 per tn of total feedstock
0-0.05
0.051-0.1
0.11-0.15
Thermal Treatment PDF Print E-mail

This section deals with the description of the alternative thermal practices for municipal solid waste management. Thermal methods for waste management aim at the reduction of the waste volume, the conversion of waste to harmless materials and the utilization of the energy that is hidden within waste as heat, steam, electrical energy or combustible material.

 

According to the New Waste Framework Directive 2008/98/EC, the waste treatment methods are categorized as “Disposal” or “Recovery” and the thermal management practices that are accompanied by significant energy recovery are included in the “Recovery”. In addition, the pyramid of the priorities in the waste management sector clearly shows that energy recovery is more desired option in relation to the final disposal.

 

Figure 1: Pyramid of the priorities in the waste management sector

 

That is why more and more countries around the world develop and apply Waste-to-Energy technologies in order to handle the constantly increasing generated municipal waste. Technologically advanced countries in the domain of waste management are characterized by increased recycling rates and, at the same time, operation of a high number of Waste-to-Energy facilities (around 420 in the 27 European Member-States).

 

At European level, there are great variations in the municipal solid waste management practices applied in the 27 EU Member – States. It can be said that on average, 40% of the generated municipal waste is landfilled, 40% is recycled and 20% is incinerated.

Figure 2: Management practices for municipal waste in the EU countries (Eurostat 2008)

 

There are EU countries where more than 90% of the generated municipal waste is landfilled, while the rest 10% is recycled or energetically recovered, while in other EU countries that are advanced in the waste management field only 10% of municipal waste is disposed at landfills, 65% is recycled and the rest is subjected to thermal treatment methods. Most specifically, in relation to the thermal waste treatment the public opinion is against this alternative and there are EU countries where no thermal management practices are applied for the management of the generated municipal waste (Bulgaria, Estonia, Cyprus, Latvia, Lithuania, Malta, Romania, Greece), while in other EU countries thermal management practices are applied at very limited degree (Slovenia (1%), Poland (1%), Ireland (3%). European countries that are characterized as advanced in the field of solid waste management have already achieved very high recycling rates and at the same time use thermal methods for a large part of the generated municipal waste. More specifically, the percentage of thermal waste treatment is 54% in Denmark, while the relevant figure for Sweden, Holland, Luxembourg Belgium, Germany, France (Autret et al. 2007), Austria and Portugal areι 49%, 39%, 36%, 36%, 35%, 32%, 27% και 19% respectively. It has also to be noted that the green cities in Europe (Stockholm Hamburg, etc) have incorporated thermal treatment facilities in their planning for effective solid waste management.

 

Thermal treatment of municipal solid waste includes all processes that result in the conversion of the waste content in gas, liquid and solid products with release of thermal energy.

 

The thermal waste management technologies can be categorized as follows:

  • Combustion - Incineratyion
  • Pyrolysis
  • Gasification
  • Plasma technology.

 

Additional innovative thermal waste management techniques combine incineration, pyrolysis and gasification, which constitute the three basic thermal treatment options for solid waste. The application units include typical constructions of conventional methods. The main reasons justifying the rapid expansion of new methods include the benefits from the applications. These benefits can be ecological (environmentally friendly air emissions, low quantities of inert solid residues), energetic (production of energy and less use of fossil fuels) and economic (lower capital cost).

 

The main targets of the application of thermal management practices in the field of solid waste management are:

  • The minimization of the waste quantities that end up at landfills.
  • The conversion to inert materials that are less harmful for human health and the environment.
  • The reduction of the environmental pollution and particularly the avoidance of the generation and release of volatile substances, such as furans and dioxins.
  • The utilization of the colorific value of waste for energy production (heat, power, fuel).

 

At this point it should be noted a lot of effort has been made to develop models in order to correlate of the net calorific value of the waste to be treated with different characteristics, such as the elemental waste content. For the case of the municipal solid waste, a large numbers have been proposed, but there is no model that could be characterized as generally acceptable and reliable. Indicatively, the following are mentioned:

 

Dulong: HHV = 81C +342,5 (H – O/8)+22,5 S – 6(9H –W)

 

Steuer: HHV = 81 (C – 3 x O/8) +57 x 3 x Ο/8 +345 (Η – Ο/10) +25S – 6(9H+W)

 

Scheurer – Kestner: HHV = 81 (C – 3 x O/4) + 342,5H + 22,5 S+ 57x3x O/4 - 6(9H+W)

 

Chang: HHV=8561,11 C+179,72 H-63,89 S-111,17 O-91,11 Cl-66,94N

 

Wilson: HHV = 7831 Corg+35,932(H-O/8)+2212S -3545Cinorg +1187 O +578N

 

In general, it can be expressed that the calorific value of a material depends on the content in the basic combustible elements, which are C and H and S at lower degree. Moisture and ash are also main parameters for the potential of energy utilization of a material. The moisture included in waste constitutes an obstacle for the easy thermal treatment, since it requires a significant amount of energy so as to be removed allowing for waste to be combusted and provide the thermal load included within the waste. On the other side, ash involves inorganic constituents included in waste (metals, glass and other inert materials, such as soil), which cannot act as energy sources (Kathiravale et al. 2003, Menikpura & Basnayake 2009, Rao et al. 2004, Minutillo et al. 2009, Friedl et al. 2005).

 

The thermal waste treatment facilities have to be combined with collection systems of the generated waste, sanitary landfills, plants for recovery of materials from waste, composting facilities, etc.

 

It has to be noted that Directive 2008/98/EC of the European Parliament and of the Council of 19 November 2008 on waste and repealing certain Directives (commonly known as Waste Framework Directive) clarifies when the incineration of municipal solid waste is energy effective and, therefore, can be considered are recovery process and not disposal one. More specifically, the Waste Framework Directive considers incineration facilities for municipal solid waste as recovery plants in the case that the energy efficiency is above or equal to:

 

  • 0.60 for incineration installations in operation and permitted in accordance with legislation before 1st January 2009
  • 0.65 for incineration installations permitted after 31st December 2008.

 

The energy efficiency is calculated using the following formula:

 

Energy efficiency R1 = [Ep - (Ef + Ei)] / [0,97 × (Ew + Ef)], where:

 

Ep = annual energy produced as electricity or heat, expressed in GJ/year. It is determined by multiplying heat produced for commercial use by the coefficient 1.1 and the energy in the form of electricity by the coefficient 2.6

Ef = energy input to the system from fuels leading to steam production on annual basis, expressed in GJ/year

Ew = annual energy contained in the treated waste calculated by the use of the net calorific value of the waste, also expressed in GJ/year

Ei = energy imported excluding Ew and Ef on annual basis, expressed in GJ/year

0.97 = is a mathematical factor accounting for energy losses due to bottom ash and radiation.

 

In the way, the planning of incineration facilities in the future is encourages to go towards installations characterized by high energy efficiency. The assessment of the energy efficiency of 231 Waste-To-Energy Plants that represent 70% of the relevant total capacity at European level showed that 169 plants out them were characterized by values of energy efficiency higher than 0.60, while 251 plants had energy efficiency lower than the value of 0.60 or did not reply to the question in relation to the energy efficiency. Consequently, 40% of the Waste-toEnergy Plants that are operating within the European Union, Switzerland and Norway already satisfy the criterion of the Waste Framework Directive so that their operation can be considered as recover and not disposal (Stengler 2010).

 

It is noted that the formula referring to the energy efficiency does not constitute performance indicator of the plant. On the basis of the Waste Framework Directive, the aforementioned limits are not committing, since the climatological conditions should also be taken into consideration, since they can influence the energy efficiency of the plants. The limits on the energy efficiency of waste-to-energy plants that are set by EU can be satisfied according to the Confederation of European Waste-to-Energy Plants (CEWEP, www.cewep.eu), even in the case of exclusive production of electrical energy. In order to confirm the aforementioned estimation, a thermal waste treatment medium-size unit with capacity 300,000 Tonnes per year produces 25 MW of electrical power with an indicative performance degree 26.5%, while the value of the energy efficiency is estimated about R1 = 0,697 (www.wtert.gr).

 

The satisfaction of the energy efficiency target from a waste-to-energy plant does not only depend on the energy included within the waste to be treated, but also on other factors, such as the input to the system from fuels leading to steam production and the amount of the other energy forms that is introduced to the installation. Therefore, it is not possible to determine the energy efficiency just using data on the net calorific values of waste. Nevertheless, in the case that the net calorific value is relatively high, then the energy efficiency target can be achieved more easily.

 

Thermal waste management methods should be applied together with separation at source of all materials that can be recycled in order to maximize material recovery from waste. The advantages of thermal methods in waste treatment are summarized as follows:

  • Reduction of the weight and volume of the treated waste: The final solid residues have weight that varies from 3 to 20% in relation to the initial weight of waste, depending on the technology that is used. Gasification and pyrolysis result in lower quantities of solid residues comparing to incineration.
  • Absence of pathogenic factors in the products:
  • The products of thermal treatment, due to the high temperatures that are developed, are characterized from complete absence of pathogenic factors.
  • The thermal treatment units are characterized by low demands for land for their installation.
  • The pyrolysis and gasification processes require less space in relation to incineration.
  • Through the thermal treatment technologies, the exploitation of the energy content of waste is possible.
  • This energy can be either electric or thermal energy.
  • Demand for limited areas:
  • Utilization of the energy content of waste:
  • Reduction of the burden paused to the landfill sites and consequent increase of their lifetime.
  • Extraction of the organic fraction of municipal waste from landfill sites, as required by the relevant legislative framework (Directive 1999/31/EC).

 

Indicative disadvantages of the application of thermal methods are the following:

  • Relatively high capital cost:
  • Higher than that of other technologies for the management of municipal waste.
  • Significant part of the total capital cost, especially for the case of incineration, is spent on antipollution measures.
  • In general, the thermal management techniques are characterized by relatively high operation cost. The cost is reduced substantially as the capacity of the plant increases.
    • Especially for the case of incineration – combustion, a minimum capacity is required so that the units are financially feasible. Estimated minimum served population from incineration facilities is 100,000 inhabitants (around 50,000 tones of waste annually). Gasification and pyrolysis can be applied for much lower waste quantities (around 15,000 tones of waste per year)
  • Increased operation cost
  • Demand for high quantities of waste:
  • Need for specialized personnel.

 

 

 

REFERENCES

Directive 2008/98/EC of the European Parliament and of the Council of 19 November 2008 on waste and repealing certain Directives.

Friedl, A., Padouvas, E., Rotter, H. & Varmuza K., Prediction of heating values of biomass fuel from elemental composition, Analytica Chimica Acta 544 (2005) 191-198.

Menikpura, S.N.M. & Basnayake, B.F.A., New applications of “Hess Law” and comparisons with models for determining calorific values of municipal solid wastes in the Sri Lankan context, Renewable Energy 34 (2009) 1587-1594.

Rao, M.S, Singh, S.P., Sodha, M.S., Dubey, A.K. & Shyam, M., Stoichiometric, mass energy and exergy balance analysis of post-consumer residues, Biomass and Bioenergy 27 (2004) 155-171.

Stengler, E., The impact of energy recovery status for efficient Waste-To-Energy plants, ISWA World Congress, Hamburg, 15th - 18th November 2010.