Chapter IV: Optimization of the final waste treatment system in the Netherlands

 

Co-authors: Marko Hekkert, Ernst Worrell, Ad van Wijk.

Accepted for publication in 'Resources, conservation and recycling'

 

 Abstract - The potential for optimizing, in both economic and energetic terms, the final waste treatment system in the Netherlands is evaluated in the light of the performance of new technologies. Projections of the final waste supply and waste treatment technologies are combined to construct several scenarios for waste treatment in the year 2010. Technologies include processes currently in the demonstration or pilot phase.

It is concluded that final waste treatment could be performed at lower cost and with substantially greater energy recovery than at present.

In a minimum cost scenario, the final waste treatment might cost 300-600 MECU per year, compared to 1,000-1,600 MECU/year in a reference scenario, on the assumption that conventional but improved waste treatment technologies are used. A maximum energy recovery scenario might save 80-90 PJ primary energy per year compared to 39-47 PJ/yr for the reference case. Two major competing technologies are gasification, both for biomass waste and integral waste, and fluidized bed incineration. Further development of these technologies integrated with electricity production is recommended.

 

1. INTRODUCTION

 Over the period of ten years (1985-1995) the total amount of waste produced in the Netherlands (excluding contaminated soil, sludge and manure) increased from about 46 Mtonnes to about 50 Mtonnes; in the same period the amount of waste that was re-used increased from 50% to more than 70%. Consequently the total volume of waste that had to be treated by final waste treatment facilities (incineration, landfilling, discharge) decreased from 23 Mtonnes in 1985 to 15 Mtonnes in 1995. The amount of waste that was discharged decreased from 3.5 Mtonnes to 1.5 Mtonnes and the amount of waste that was landfilled decreased from 16 Mtonnes to less than 9 Mtonnes. The amount of waste incinerated increased from nearly 3 Mtonnes in 1985 to more than 4 Mtonnes in 1995.45 The figures show that at present the waste treatment system of the Netherlands depends heavily on landfilling and incineration.

 Major policy goals in the Netherlands are the prevention of waste production and the re-use of waste produced such that less final waste treatment is required. Nevertheless it might well be that in the year 2010 the capacity of the final waste treatment system will be about 16 ktonnes.28 Governmental policy is also directed towards putting a stop to the landfilling of organic waste and increasing the waste incineration capacity.

At present the production and use of energy from the incineration of waste results in annual savings of about 17 PJ in fossil fuel consumption.10 Sixty percent of this amount was generated in large scale facilities that produce heat and electricity.

 In the Netherlands the so-called Waste Consultative (Afval OverlegOrgaan; AOO) deals with the planning of the capacity for treating final waste. The planning is presented in a Ten Year Waste Programme (TYWP), which is updated every 3 years. The current TYWP covers the time frame 1995-2005 and indicates that in the year 2005 incineration plants should be able to process approximately 60% of the final waste; this will require a substantial expansion of the incineration capacity. In addition, incineration of waste is to be combined with the production of steam to generate heat and electricity using a steam turbine.

Biomass wastes and residues are not considered in the TYWP, since these streams are not defined as waste. The main point of attention is the treatment of integral waste from e.g. households and the service sector. This is waste that contains a mixture of organic material, plastics, paper, etc. and inorganic materials. In the TYWP the performance of different waste treatment systems, consisting of different mixtures of waste treatment technologies, are evaluated and compared. Also the effects of different policies on the production and treatment of wastes are assessed. In all cases the existing waste treatment structure (and the lifetime of the existing waste treatment capacity) is the starting point. Environmental effects, like emissions to air and water and the handling of residues, are analyzed by means of a Life Cycle Inventory (LCI) approach.

 According to the TYWP, the recovery of energy from wastes will increase substantially. However, the question that arises is whether the incineration of waste to produce electricity is an optimal route to utilize the energy potential of waste. The electrical conversion efficiency of these systems is at present approximately 12-24%. New designs can increase this efficiency to some extent,35,42 but other technologies, like gasification (integrated with electricity production using a combined cycle), may achieve higher conversion efficiencies at lower costs.35,36,42 In addition, other options like increased separation, digestion of (wet) organic materials and various technologies to process specific waste streams (such as plastics), look promising, both from an economic and an energetic point of view.

Therefore, it is not yet known what the optimal configuration of the total final waste treatment system will look like when various new technologies gradually become commercially available. White et al.43 recommend that complete waste treatment systems be studied instead of only the performance of separate technologies, since changing the waste composition by waste separation technologies and replacing waste treatment capacity by other options will effect the performance of the entire system. Such an integral analysis will be the topic of this article.

 In this paper we evaluate, from an energetic and economic point of view, the potential for optimizing the treatment system for final waste (We define final waste as waste which is not re-used or recycled. Main features of a final waste treatment are the total capacity and the applied mix of technologies for treating the final waste.) in the Netherlands in the year 2010. In this time frame a considerable part of the current capacity to treat waste will have to be replaced.

In this study we will evaluate the potential performance and costs of different waste treatment systems without taking the current operational waste treatment facilities into account. This enables us to assess and compare the maximum possible potential effect on energy recovery and the waste treatment costs if specific technologies are introduced on a large scale. We will include all biomass waste streams in our analysis, since they will probably play a substantial role in the production of energy from waste in the near future and thus affect the optimum configuration of the waste treatment system as a whole.

Taking 2010 as a reference year means that some technologies at present under development will probably become commercially available within this time frame. A number of new technologies, like pyrolysis and gasification, are currently being tested and demonstrated. In this study we will review the potential waste treatment costs and energy conversion efficiency of these new technologies in the year 2010.

Because our focus is on final waste treatment, waste collection and logistics are excluded from the analysis. Although the costs of collection can be considerable, it can be argued that in any structure waste has to be collected and transported, which - as a first approach - will result in relatively small differences in energy use and in the costs of transport in alternative waste treatment systems. Further prevention of waste and the recycling of waste materials will not be investigated. The rates at which these options might be applied in the year 2010 are merely used as starting points. However, recycling waste after further separation will be discussed. In these cases the (energetic) value of recyclable products (such as metals and paper, etc.) that result from the final waste treatment will be taken into account.

 In this article, we will first discuss the projected characteristics and supply of final waste in the year 2010. Next an overview is presented of relevant current and future waste treatment technologies, the focus being on the expected performance in the year 2010. In section 4 the results will be integrated and compared. The performance of different final waste treatment systems consisting of different technology mixes is assessed. From the results conclusions are drawn about the energy recovery potential and waste treatment costs in the Netherlands in the year 2010.

  

2. WASTE STREAMS & SUPPLY

 Table 1 presents projections for the year 2010 about the route and the supply of relevant waste streams in the Netherlands. The figures are taken mainly from Nagelhout et al.28 In these figures the effects of increased prevention of waste and the re-use of materials and products, strongly promoted by the government, have already been taken into account. Therefore, the indicated figures for 2010 can be considered as the amounts of waste that will have to be treated by the final waste treatment system. For comparison table 1 also shows figures for the year 1990.

In table 1 three categories of waste are distinguished: integral waste, biomass waste and specific waste. Each category is subdivided into flows defined by the source of the waste (e.g. households, specific process or sector). In this article these flows are called waste streams. The composition of a waste stream is often rather heterogeneous.

In this study we will focus on waste streams that are at present subject to final waste treatment by public utilities. This excludes a number of specific industrial wastes, such as those from the food and beverage industry. Most of this waste is treated by facilities (often digestors) on site. Chemical and nuclear wastes are also excluded from our analysis.

 The inventory of Nagelhout et al. ignores a number of biomass waste streams. Therefore, in table 1 the figures on biomass waste have been derived from an analysis of Faaij et al. for the year 1994.11 No projections of the potential supply of biomass waste in the year 2010 are available. In this study we will assume that the size of the different biomass waste streams will remain roughly constant in the time frame considered. In table 1

  

Table 1. Waste streams subject to final waste treatment as projected for 2010 in the Netherlands (based largely on Nagelhout et al.28).

  

treatment by incineration (ktonne)

treatment by composting (ktonne)a

treatment by landfill

(ktonne)

total final treatment in 2010 (ktonne)

total final treatment in 1990 (ktonne)

LHV of the waste (GJ/tonne)

Integral waste

 

 

 

 

 

Domestic waste

4250

1550

-

5800

5490

 

Coarse domestic waste

800

70

-

870

700

 

Sweeping wastes

600

190

500

1290

1310

9c

Service sector waste

1800

500

-

2300

2700

 

hospital waste

100

10

-

110

130

 

Public utilities

10

-

100

110

140

 

Biomass waste

 

 

 

 

demolition wood

300

-

-

300

150

15

auction waste

-

310

-

310

170

5

verge grassb

 

 

 

400

5

pruningsb

 

 no projections available about treatment routes in 2010

 

230

8

agriculture

 

 

30

5

greenhouse sectorb

 

 

100

2

bulb cultivationb

 

 

260

5

fruit farmingb

 

 

200

8

Specific wastes

 

 

 

 

 

sludge (dry matter)

285

-

-

285

280

13d

car wrecks (mainly plastics)

200

-

-

200

120e

30

construction & demolition waste

-

-

2700

2700

3500

0

mineral extraction wastes

-

-

40

40

50

0

contaminated soil

-

-

250

250

140

0

residues coal power plants

-

-

650

650

140

0

steel grit

-

-

50

50

80

0

Totals

8350

2630

4290

16500

16320

 

a Composting refers to the organic fractions of various integral waste streams. The average lower heating value of these streams is estimated to be 5.5 GJ/tonne; lower values are for example found for swill-like streams with very high moisture content; the heating value can be above average for wood containing garden waste.9,11,27

b Streams not included in the analysis of Nagelhout et al.28; quantities and heating values are discussed and summarized in Faaij et al.11 The volumes presented are for 1994; no substantial changes in the supply are expected for 2010.

c A heating value of 9 GJ/tonne is used as an average value for all combustible integral wastes.39,42 The value applies to the fractions that are given in the column 'incineration'. The organic fraction given in the column 'composting' has lower heating values.

d The heating value can vary between 10 - 15 GJ/tonne dry matter24; here, an average value is presented.

e Figure for 1990 excludes tyres, which are expected to be totally re-used in 2010.28

 

biomass residues like straw and thinnings from commercial forestry are not included since their market value implies that they should not be considered as waste.9,27 Up till recently sludges have been used as fertilizer in the agricultural sector. Stricter standards and increasing contamination of sludges have put a halt to this practice. Therefore, almost all sludge is now considered as waste and is processed accordingly.27,29

 A far more detailed split of waste streams can be made following a directive of the European Union.6 However, in relation to this assessment of final waste treatment systems and technologies considered here, the split made by Nagelhout et al.28 (extended with a number of biomass wastes streams) is considered sufficient.

 The composition and characteristics of a waste stream determine whether or not the stream can be treated by a specific technology. Important parameters are moisture and mineral fraction content, heating value and degree of contamination, e.g. with heavy metals. In table 1 the lower heating value (LHV) of different waste streams is given. For biomass waste this value depends largely on moisture and ash content. The LHV-figures are taken from Van Doorn9 and Faaij et al.11 With respect to the category integral waste, it is considered realistic to make no difference between the various sources of integral waste; no specific facility will be built to process domestic waste separately from waste from e.g. the service sector. Therefore, although differences can be found in the composition of the various streams falling under the heading integral waste, an average lower heating value of 9 GJ/tonne is used for all waste in this category. The figure is based on analyses of the composition of integral waste.42

  

3. WASTE TREATMENT TECHNOLOGIES

 Before making an integral assessment of final waste treatment systems that might be applied in the year 2010, one has to assess the future performance of present and upcoming final waste treatment technologies. In this study we will focus on processes that are at present commercially available, processes that have been demonstrated and processes that are at present in the pilot stage. Improvement options in terms of energy efficiency and costs of currently used technologies are included in the assessment as well. Given the time between now and the year 2010 it would be unrealistic to take into account processes which are currently in the design phase. Some possible waste treatment technologies, such as Hydro Thermal Upgrading and some pyrolyis processes,39 are therefore excluded from the assessment. Our assessment will be based on a critical analysis of the literature. In some cases, when insufficient data are available, a more detailed analysis will be made to assess the potential performance of the technology involved in 2010.

The technologies are divided into two categories: The first category contains technoloiges that are especially suited for processing the biomass waste and most of the specific waste streams such as plastics; they are described in section 3.1. The second category, described in section 3.2, consists of technologies that are designed to process particularly the integral waste. In this overview only the main features of the technologies are distinguished. It should be noted that often a number of manufacturers are able to supply the equipment for each technology. This results in larger or smaller deviations in the performance and the waste treatment costs of these technologies. We will not describe these differences in detail, but will express the performance in average figures or ranges.

 Each technology will be described briefly, together with its field of application. Main characteristics are presented in terms of the waste treatment costs and the net energy conversion efficiency that can be achieved or the input of energy required to use the

  

Table 2. Main parameters of existing and new waste treatment technologies. Ranges in costs, efficiency and energy use express the observed or projected differences in performance.

 

Energy conversion efficiencya

Energy consumption

Treatment

costs (ECU1995/

tonne)

 

elec. eff.

(% LHV)

eff.prim.E

(% LHV)

Electricity

kWh/tonne of waste

Prim.en. MJ/tonne of waste

 

 

min

max

min

max

max

min

max

min

min

max

Improved incineration (increased steam conditions and coupling to CC)

26

30

 

 

 

 

 

 

80

150

Incineration with CHP; 27% waste heat utilisation assumed.

16

21

27

27

 

 

 

 

80

150

Fluid bed incineration

22

25

 

 

 

 

 

 

30

50

Pyrolysis/gasification

12

22

 

 

 

 

 

 

90

140

Pyrolysis/incineration

16

25

 

 

 

 

 

 

80

190

Waste separationb

 

 

 

60

60

230

230

20 +20%f

Separation-digestion-incin.

24

28

 

 

 

 

 

 

100

120

Separation-composting-incin.

11

11

 

 

 

 

 

 

120 +20%f

RDF gasification

40

43

 

 

 

 

 

 

30

60

Biomass gasification

35

43

 

 

 

 

 

 

-10

10

Composting

 

 

 

70

40

 

 

40

50

Anaerobic digestion

10

45

 

 

 

 

50

70

Co-combustion wood

35

45

 

 

 

 

 

 

30

50

Co-combustion sludgec

37

47

 

 

6

6

1500

0

210

330

Sludge incinerationd

 

 

 

 

400

400

280

330

Wet oxidationd

0

0

0

0

 

 

 

 

270 +20%f

Sludge gasificationd

 

 

 

 

 

 

280

330

Plastics pyrolysis

 

83g

83g

 

 

 

 

0

130

Plastics gasification

98g

98g

 

 

 

 

120

140

Landfillinge

 

 

 

 

 

140

90

20

30

aEfficiency is defined as net energy efficiency, including energy requirements of the process. Primary energy is defined as either heat or gas with a subsequent energy content.

b Performance data for waste separation are given for a wet separation process that produces RDF, organic waste, plastic, paper and metal fractions.

c Co-combustion of sludge is preceded by drying. It is assumed that sludge with a moisture content of 50% (obtained after mechanical dewatering) is to be dried to 10% moisture content or less. The costs mentioned present the cost including and excluding natural gas input per tonne of dry matter; the latter are obtained when waste heat is utilised for sludge drying.

d Performance data for sludge incineration, wet oxidation and sludge gasifciation are given per dry tonne of sludge.

e The energy consumption for landfilling includes energy recovery from landfill gas.

f Only one cost estimate was available. To reflect uncertainties in performance, scale, cost of capital, etc. a cost range of + 20% (based on the cost range of incineration) is taken into account.

g The efficiencies given for gasification and pyrolysis of plastic waste include avoided fossil energy carriers (see also section 3.1).

 

 technology. The waste treatment costs will be presented as costs per tonne of waste treated. Those figures includes investment costs and operation and maintenance costs. The level of these costs is influenced by a number of factors: scale of the facility, cost of capital at the time of construction, depreciation period, efficiency of the treatment, type of gas or water cleaning applied, etc. Consequently in this study the costs are presented as ranges. A disadvantage of presenting ranges is that the differences in the cost of specific technologies become more difficult to establish, but an advantage is that the differences in the waste treatment costs observed for a given technology are taken into account. So that we have a common approach for establishing cost figures, our analysis will be based on tariff figures as far as possible. Table 2 summarizes the main results of the evaluations.

 3.1 Dedicated technologies to treat specific waste streams

 In this section we will discuss dedicated technologies for the treatment of biomass waste, plastic waste and sludge.

 Composting of biomass waste

Composting is an aerobic conversion of biomass waste to organic matter by means of biological degradation. In the process electricity is consumed for pre-treatment, ventilation, moving of material and sometimes forced aeration. The energy use varies between 40-70 kWh/tonne of waste, depending on the specific process used and the scale of the facility.20,21 The costs of the process are strongly influenced by the capacity of the facility. Net treatment costs vary between 40-50 ECU/tonne20, on the assumption that the compost can be used at zero cost. These figures apply to the current situation. No substantial development is expected in the technology and costs of composting.39 Therefore we assume similar performance data for 2010.

 Anaerobic digestion of biomass waste

By anaerobic digestion, biomass waste is converted to biogas (by bacteria in the absence of oxygen) and compost. The biogas is mainly a mixture of CO2 and CH4. The biogas is partly utilised to heat the digestion reactors. The rest can be used to generate electricity and/or heat (e.g. with a gas engine) or, after treatment, be fed into the natural gas grid.

Various digestion processes have been developed. In these processes the energy efficiency of the conversion of biomass waste to biogas can vary between 10-45%.20,21 This figure does not include further conversion to heat or electricity.

The costs of the process are strongly influenced by the capacity of the facility and may vary between 50 and 70 ECU/tonne of waste.20 In this figure the savings on conventional energy consumption are taken into account. No substantial developments are expected in the performance and costs of digestion.39 Assuming that the value of the energy saved will not change substantially either, we assume similar performance data for digestion in the year 2010.

 Co-combustion of waste wood in (existing) coal-fired power plants

Waste wood can be co-combusted in coal-fired power plants. Emission standards may limit the extent to which the wood can be added if it is contaminated with e.g. heavy metals. The type of coal plant (e.g. pulverized, fluidized bed) determines what kind of pre-treatment of the waste wood is required. The pre-treatment consists of sizing, sieving and drying, when necessary.

In the Netherlands it is mainly pulverized coal-fired power plants that are operational. In the EPON coal-fired plant in Nijmegen, the Netherlands, co-combustion of waste wood has recently been demonstrated. Wood is dried to 10% moisture content and pulverized. Treatment costs amount to 30-50 ECU per tonne of waste wood, taking into account the savings on coal use. The required pre-treatment contributes substantially to these costs. The net energy conversion efficiency is 35-38% taking into account the energy use of the pre-treatment.34 These performance data can be considered representative for the current state-of-the-art pulverized coal fired power plants. Several of these plants are likely to be in operation in 2010. Higher conversion efficiencies can be expected if new coal plants are built. We assume that for co-combustion of waste wood an overall energy conversion efficiency of 45% is obtainable in 2010 at a comparable cost level.

 Gasification of biomass waste

Gasification is a thermochemical conversion route that can be used to convert biomass waste to fuel gas. The heating value of this fuel gas is relatively low (typically 4-8 MJ/Nm3). Various processes and reactor designs for gasifying biomass are at present available or under development. The main advantage of gasification is that it can be integrated with electricity production using a gas turbine or a combined cycle: the so-called Biomass Integrated Gasification/Combined Cycle plant (BIG/CC) which is expected to result in relatively high conversion efficiency and low capital costs per kW installed. In this paper we focus on such integrated concepts, although BIG/CC systems have not yet been demonstrated commercially. Here we will focus on the gasification of relatively clean biomass waste streams. Gasification of integral waste (MSW or RDF), which is much more heterogeneous, will be discussed later.

Most biomass gasification schemes are designed to convert clean biomass like cultivated wood.7,8,23 Faaij et al. have analyzed the gasification of biomass wastes.12 They selected a system based on Atmospheric Circulating Fluidized Bed gasification and low temperature gas cleaning. It was found that the highest conversion efficiency is obtained with woody materials (such as waste wood). The use of biomass wastes such as verge grass or organic domestic waste results in a relatively low conversion efficiency because of the higher energy use of drying and the lower power output of the gas turbine. Sludge, even if dried beforehand, is a difficult fuel for a BIG/CC system because of its high ash content, which decreases the heating value of the fuel gas produced (Note that the net electrical efficiency is influenced by the fuel composition. Fuels need to be dried up to a moisture content of 10-15% before gasification. Waste heat can be used for this purpose. For very wet fuels (moisture content over 50-60%) the heating requirements may result in reduced availability of steam for power generation. Furthermore, streams with higher inert fractions produce a leaner fuel gas due to the increased energy needed to heat the mineral fraction of the biomass to the gasification temperature. Combustion of leaner gas results in reduced temperatures in the combustor of the gas turbine and thus reduces the electrical output. For more details see Faaij et al.11,12).

Pressurized gasification seems especially suited for the treatment of clean biomass because it involves dry and high temperature gas cleaning. At the current stage this type of gas cleaning is less flexible in dealing with more contaminated fuels. In this study we will use the performance data of an atmospheric BIG/CC as estimated by Faaij et al.12 According to this study, the treatment costs of biomass wastes might range from about minus 10 to plus 10 ECU/tonne. Negative cost figures are achieved if high revenues are obtained from electricity production. The electric conversion efficiency has been estimated to vary between 35-41%.12 Consonni et al.7,8 expect that the efficiency of a similar 30 MWe system using woody biomass could be 43%. We consider a range of 35-43% for the electrical efficiency of a BIG/CC power plant and the above-mentioned biomass treatment costs to be representative for the year 2010. Note however that the expected performance applies to a 30 MWe unit, so scale effects have not been taken into account here. Larger scales will most probably result in higher efficiencies and lower capital costs per kW installed.12,23

 Combustion or pyrolysis of biomass waste

Numerous processes are available or under development for combusing or purolysing especially relatively dry biomass materials. Most of those processes have been developed for clean wood. Their performance is uncertain when used for the treatment of contaminated biomass materials, especially with respect to emission control and related costs. In general, dedicated biomass combustion facilities yield lower energy conversion efficiencies and involve higher investment costs than projected performance data of BIG/CC systems on similar scales (see e.g. van den Broek et al.5). We will therefore not discuss biomass combustion further in this study.

Most pyrolysis processes under development for clean wood obtain energy efficiencies of about 60-70% for conversion of wood to oil.40 When this oil is used in engines or turbines, the net overall electrical efficiency is about 18% rising to 35% if efficient combined cycles were to be used. The costs of pyrolysis processes using contaminated biomass are uncertain, especially since the bio-oil may require further upgrading, such as by the removal of alkalis, before it is used in turbines.40 Because pyrolysis is expected to be less efficient than gasification if combined with electricity production and because the treatment costs are uncertain at the present stage, we do not include pyrolysis as a treatment option for biomass waste in this study. Pyrolysis processes for the treatment of contaminated, integral wastes will be discussed later.

 Pyrolysis of plastics

Pyrolysis (thermochemical conversion of material by high temperature heating in an oxygen-free environment) can be used to convert mixed plastic wastes to oil products, or so called syncrude, combustible gas and heavy residue. The heavy residue is combusted in order to provide process heat. Syncrude can replace mineral oil in e.g. refineries. A representative example of a pyrolysis process that is being developed for plastic waste treatment is the VEBA process. To calculate the energy efficiency of this process it is assumed that the oil produced replaces crude oil and the gas produced replaces natural gas. Taking into account the avoided fossil energy carriers and the energy consumption of the process, the overall energy efficiency is 83%. The projected treatment costs per tonne of waste are expected to vary between 0 and 130 ECU/tonne.37 The lower side of the cost range is expected to be reached as a result of the potentially high revenues obtainable for the oil produced and the favourable future development of the process.37

Smit et al. report another pyrolysis process for plastic waste similar to the VEBA process. Their process is being tested on a pilot scale. The manufacturer INETI expects waste treatment costs of about 60 ECU/tonne, revenues not included. INETI claims the revenues could be up to 90 ECU/tonne resulting in negative waste treatment costs.39 In this study we will use the performance data of the VEBA process since they are based on a demonstration unit.

 Gasification of plastics

High temperature gasification of plastic waste can be used to produce a syngas. An example is the TEXACO gasification process.37 Before gasification the plastic waste needs to be pre-treated by means of screening and sizing. The syngas produced is first cleaned (for instance by the removal of HCl which stems from PVC) and can subsequently be used for methanol or hydrogen production or for the production of other chemicals, thus replacing the use of mineral oil or natural gas. The gross efficiency of converting the plastic waste to syngas is about 70%. If the energy consumption of the process and the avoided fossil energy carriers which would be used in conventional syngas production are taken into account the overall energy efficiency of the TEXACO process is 98 percent. Costs are projected to vary between 120 and 140 ECU/tonne treated.37

 Incineration of sludge

Sewage sludge can be incinerated until only an ash fraction remains. Before being incinerated sludge is mechanically dewatered to a moisture content of 50-60%. This operation is not counted in the waste treatment costs or energy input, because these operations are usually performed at waste water treatment facilities.

Incineration of sludge does not produce a net energy output but requires an additional energy-input (e.g. natural gas) of approximately 400 MJ/tonne of dry sludge because of the substantial volumes of water that have to be evaporated.24 The costs of incineration are 280-330 ECU/tonne of dry sludge.24 Further optimization of this process seems unlikely and no substantial changes in the above-mentioned performance are expected for the year 2010.

 Co-combustion of sludge

Sludge can be also be co-combusted, just like waste wood. In the Netherlands co-combustion of sludge is planned.25 At present special sludge driers are operational; these reduce the moisture content of sludge from approximately 50-60% to less than 10%. This requires a high natural gas input of 1500 MJ/tonne of dry sludge. Some electricity is required to drive the dryer and air vents: 6 kWh/tonne of dry sludge.19 Drying costs amount to approximately 140 ECU/tonne.19 It is assumed that additional costs in the coal plant (such as for modified burners) are more or less compensated by avoided fuel costs.

Utilization of waste heat (from the coal power plant) in order to dry sludge would result in considerable energy savings, but this concept has not yet been demonstrated. We assume however that the use of waste heat for sludge drying will be possible in 2010. Consequently the costs are expected to be lowered to about 60 ECU/tonne dry sludge.19

The overall conversion efficiency is determined mainly by the efficiency of the (coal) power plant. Coal fired power plants currently in operation have an efficiency of 37-42%13. Coal plants with an efficiency of 48% might be in operation in 2010. For this study we will use a cost range of 60-140 ECU/tonne sludge for 2010 and an electrical efficiency range of 37% if sludge is combusted in currently operational coal plants and up to 47% if new, more efficient coal plants are constructed. Energy inputs for the drying process may vary between 0-1500 MJ, plus electricity consumption per tonne dry sludge.

 Wet oxidation of sludge

Wet oxidation is a treatment method in which sludge is processed under high pressure and at high temperature. An example is the VERTECH process, demonstrated in the Netherlands.3 The high pressure is created by leading the sludge to the bottom of a cylinder dug into the ground. A column of water on top creates a high pressure at the bottom of the cylinder. By feeding air the sludge oxidizes and decomposes at high temperatures. Inerts are pumped to the surface. The process is self-sufficient in energy; the costs of a commercial facility are expected to amount to about 270 ECU/tonne dry sludge treated.3 We assume similar performance data for the year 2010, taking a cost range of 270 ECU/tonne + 20% into account. Such a cost range is also observed for incineration.

 Gasification of sludge

Small scale gasification of sludge has been developed and demonstrated by Royal Schelde.18 In this process dried sludge is gasified using an Atmospheric Circulating Fluidized Bed (ACFB) gasifier. The fuel gas produced is used to dry the incoming sludge. The process is almost self sufficient in energy consumption, depending somewhat on the sludge composition. Treatment costs for 2010 are projected to be in the range of large scale incineration (approximately 280-330 ECU/tonne dry matter).18

 3.2 Treatment technologies suited for integral waste streams

 In this section incineration, gasification, pyrolysis and landfilling of integral waste will be discussed. Furthermore separation as a method for handling integral waste will be dealt with, despite the fact that this paper focuses on final waste treatment. Mechanical separation of mixed waste can be considered as a pre-treatment technology for final waste treatment, allowing for re-use of materials. Consequently, separation is part of the integrated concepts for treating integral waste. Examples are integrated separation-digestion/incineration and separation-composting/incineration concepts. These final waste treatment technologies will be discussed as well.

 Incineration of integral waste

There a number of ways to incinerate waste. Various concepts are used, like fixed bed, moving grid, bubbling bed and circulating fluidized bed incineration. Various systems and processes are used to clean off gases from the incineration process, depending on the emission standards applied. All present MSW (Municipal Solid Waste) incineration facilities in the Netherlands produce steam, which is used to supply heat or to generate electricity. The current electrical efficiency of these plants ranges from 12% for older plants to 24% for the latest plants.35 The electrical efficiencies are generally low because of the high energy consumption of the plant, low steam temperature, a high excess air flow, a high moisture content of the waste and large inert fractions of the waste.35,36 At present the total costs of treating waste by incineration vary between about 80 and 150 ECU/tonne of integral waste.1

A specific issue related to incineration is the utilisation of waste heat. The extent to which CHP is applied depends partly on the extent to which waste heat can be used in the proximity of the plant. CHP based incineration of integral waste is included in the overview of technologies presented in table 2. In the case considered, 27% of the energy from the waste is used as heat. As a result the electrical efficiency is decreased with 7 percent, namely from 24% to 17%.35,36,38 The figure of 27% for heat production represents the current average situation in the Netherlands.1,2 The cost range of 80-150 ECU/tonne of integral waste treated is also applicable to waste incineration in a CHP plant.

 The performance of conventional incineration, such as incineration based on the use of a moving grid, can be improved. Examples of improvement options are: higher steam pressure and steam temperature and a reduction of the excess air. By means of these and other overall process improvements, the electrical conversion efficiency can increase to 26%.35,36

The steam system of the incinerator can also be coupled to a (natural gas fired) gas turbine to create a combined cycle. This option allows the steam produced by the waste incinerator to be superheated by the flue gases of the natural gas fired gas turbine. This can result in a net conversion efficiency of the energy content of the waste to electricity of approximately 39%.35,36 On the other hand there is also a loss in efficiency since the natural gas is not used in an optimal way to generate electricity, as at present in a natural gas fired Combined Cycle plant conversion efficiencies of over 55% are obtained. When a correction is made for this loss, the net efficiency of the waste incinerator is approximately 30%.14,35

Waste treatment costs for improved incineration are assumed to be similar to the costs of current incineration (80-150 ECU/tonne). Additional investments to improve the conversion efficiency and potentially higher operation and maintenance costs are assumed to be offset by higher revenues from electricity production.

Circulating Fluidized Bed (CFB) incineration is a variation on the conventional incinerators. In Europe a few commercial CFB installations incinerate waste. The advantage of this technology is the good boiler efficiency and the relatively high burnout of the fuel. (Mechanical) pre-treatment is required to remove metal parts and to obtain the required particle size for the fluidized bed. Furthermore, the electricity consumption of a CFB boiler is generally higher because the bed has to be forced to circulate by fans. Flue gas cleaning can consist of a dry or a wet system. The latter results in lower emissions but is more extensive and results in higher costs. It is assumed that the characteristics of the residues like ashes are similar to the characteristics of the residues from a conventional incinerator. Reported electrical efficiencies are 22-25%41 and treatment costs 30-50 ECU/tonne (based on vendor quotes) depending on the type of gas cleaning.41 An explanation for these low cost levels is the relatively high throughput capacity that can be achieved with a (C)FB furnace, which limits the investment costs per unit of capacity. When the indicated gas cleaning system is not sufficient to meet the Dutch emission standards for waste incineration more extensive gas cleaning is required, leading to higher waste treatment costs. Whether this is the case is not known at the current stage. Therefore in this study we consider the mentioned conversion efficiency (22-25%) and cost (30-50 ECU/tonne) as being representative figures for the year 2010, although it should be borne in mind that a more advanced gas cleaning system might increase these costs.

 Gasification of integral waste (MSW/RDF)

Integral waste is more difficult to gasify than (cleaner) biomass wastes because of its heterogeneous composition. RDF (Refuse Derived Fuel) consists of organic waste, plastics, wood, paper and board. Gasification of RDF has been demonstrated in Greve (Italy) where an Atmospheric Circulating Fluidized Bed gasification process is in use.41 In this plant the syngas from the gasifier is used for generating steam by direct combustion and for firing a lime kiln. The plant uses RDF pellets. The pellets are produced in an extensive and costly pre-treatment scheme.41

It seems reasonable to assume that the demonstration of BIG/CC technology for generating electricity from clean biomass will also allow RDF gasifiers to be integrated with gas turbines in the longer term. There are no fundamental differences between RDF and biomass gasification except that the former requires extensive pre-treatment. Niesen et al. and other authors report that so called fluff (loose waste) feeding of waste to an (atmospheric) gasifier is not expected to cause fundamental technical problems.23,32,33 Thorough screening to remove metal and larger inert parts and sizing however remains necessary. Fluff feeding however, has not yet been proven technically.

For a 30 MWe IG/CC system using MSW (Municipal Solid Waste) as fuel, the conversion efficiency is projected to be 41%.32 MSW has a relatively high heating value which causes the gasifier to produce a richer fuel gas and consequently leads to a higher gas turbine output. Furthermore, the use of MSW may allow plants with a larger capacity (and consequently more efficient) since MSW is often available in large quantities in a small area. In this study we assume that the conversion efficiency of these plants ranges from 40 to 43%.

The treatment costs per tonne of waste are less certain. They depend on the stage of development (number of plants realized) and the required pre-treatment (sieving, grinding, magnetic screening, drying). Niessen et al. report net waste treatment costs (including energy credits) of 35 US$/tonne.32 The costs may be higher when more extensive gas cleaning and especially more extensive pre-treatment of the waste, like pelletizing, are required. Pelletizing may increase the treatment cost significantly.12,33 We assume that the treatment costs will vary between 30-60 ECU/tonne in 2010; the higher value representing the case where both pelletising and extensive gas cleaning are required.

 Combined pyrolysis/gasification to treat integral waste

A combined pyrolysis-gasification scheme for treating integral waste has been demonstrated in Italy by Thermoselect.41 The core of the process is pyrolysis of the waste followed by high temperature gasification using oxygen as oxidant. The process is suitable for treat destroying coarse integral wastes and achieves low emission levels. The energy use of the process itself is relatively high. In the thermoselect approach the remaining fuel gas is combusted in a gas engine to produce electricity.41 The treatment costs are quoted to be 136 ECU/tonne and the electrical efficiency 12%.41 However, those are performance data for a first demonstation unit. It might be possible to apply the process on a larger scale. Moreover it might be coupled to a combined cycle to obtain a higher energy conversion efficiency. For such a concept Niessen et al. estimate the net operating costs at approximately 100 U$/ton and the net electrical conversion efficiency at 22%.32 For this study we consider a performance range of 12-22% for the electrical conversion efficiency and a cost range of 90-140 ECU/tonne of waste treated in the year 2010.

 Combined pyrolysis/combustion schemes to treat integral waste

In a inventorizing study covering innovative solid waste processing techniques Smit et al. briefly summarize a number of other pyrolysis-based processes for treating integral waste which are currently under development.39 Almost all processes are either in the pilot or demonstration phase. Various manufacturers project treatment costs of 40 up to 190 ECU/tonne integral waste. The low costs are obtained for biomass waste.39

The so-called Schwellbrenn process is an example of a combined pyrolysis/combustion scheme.42 In this process waste is first pyrolized at modest temperatures; thereafter the produced oils and chars are combusted. The energy produced is used to generate electricity. The process has been demonstrated in Germany with treatment costs of approximately 140 ECU/tonne. The reported electrical efficiency is about 17%.2,42 No other performance data are known to us.

Smit et al. state that the costs of treating integral waste with pyrolysis processes are expected to be in the same range as the costs of current incineration (i.e. 80-150 ECU/tonne).39 The energy efficiency of the processes depends on the composition of the waste treated and the utilisation of the syncrude produced. Most concepts are described excluding power generation from syncrude. The conversion efficiency of these processes is calculated typically at about 40-50%. If the products of the pyrolysis were to be used for power generation with an efficiency of 40-50%, this would result in an overall electrical efficiency of 16-25%. For this study we will assume a performance range in the year 2010 of 16-25%. This figure is based on the conversion of waste to electricity and treatment costs ranging from 80 to 190 ECU/tonne.

 Separation of integral waste

Separation of integral waste into different components is possible as a preliminary step in the treatment of all integral waste incineration, gasification, or pyrolysis. There are several separation techniques available. The type of waste and the desired composition of the separated fractions determine the process selection. Here we will describe the basics of a process which is applied to separate Municipal Solid Waste (MSW). After receipt, the waste is loosened, crushed and sieved, by which separates the waste into a large and a small particle fraction. Next, metals are removed (magnetic separation), and paper (35% efficiency) and foils and films (90% efficiency) with help of wind sifting. Paper is separated from plastic by adding water which creates paper pulp. The remaining plastic fraction is sieved. The small particle fraction is led through a sieving drum to create again a small and a bigger particle fraction. The small fraction contains approximately 30% inert material which is largely removed by further sieving. The remaining waste of the process is separated in Refuse Derived Fuel (RDF), and an organic fraction which can either be composted, digested or thermochemically treated.2

The volumes of the separated fractions depend on the waste composition. Typical figures for the separated fractions, based on the present average composition of 1000 kg MSW in the Netherlands is: 22.5 kg plastic films (largely PE), 60 kg of ferrous materials, 53 kg of waste paper and 4.4 kg of non ferrous metals (largely aluminium).2 The total costs of this separation are about 20 ECU/tonne treated. The electricity input is 60 kWh/tonne and the heat input 230 MJ/tonne.2 We do not expect this performance to change substantially in the near future and we will consider these performance data as being representative for the year 2010, taking a cost range of 20 ECU/tonne + 20% into account. Such a cost range is also observed for incineration.

 Integrated separation-incineration and digestion & Integrated separation-incineration and composting

As already indicated, separation of integral waste could be a preliminary step in the final treatment of the waste. Here we will focus on separation combined with incineration and digestion or composting of the waste. The main reason for integrating these technologies in one facility is to obtain efficiency benefits and reduce costs.

The process scheme could read as follows: integral waste is separated, which yields, amongst others, RDF and organic waste. The RDF is incinerated, the organic waste is digested. Produced biogas is fed to a gas turbine which in turn feeds its flue gas to a heat recovery steam generator (HRSG) to produce steam. The hot flue gases of the incinerator are also fed to the HRSG. Steam is superheated by the relatively high temperature of the gas turbine exhaust gases and used for driving a steam turbine to generate electricity.

The quality of the compost which is produced in the digestion step is not as good as compost from separately collected streams and it is doubtful whether it will meet applicable standards or can compete with clean compost. Therefore, in these integrated concepts it is usually proposed to incinerate the residue from the digestion step in the incinerator.

Various integrated concepts have been evaluated by Hazewinkel et al.14; these include retrofitting existing waste incinerators, concepts with and without gas turbines and options to co-fire additional natural gas. The use of a gas turbine/combined cycle to utilize biogas produced seems the most promising concept from an energy efficiency point of view. The projected cost ranges from about 100 to 120 ECU/tonne and the projected overall electrical efficiency from 24 to about 28%.14 The concepts evaluated already include potential future improvements of the technology. Therefore we consider the mentioned performance data to be representative for the year 2010.

 A variant on this concept is integrated separation-incineration and composting, which logically results in lower overall efficiency compared to digestion since no energy recovery takes place during the composting process. According to Hazewinkel et al. the net electrical efficiency of the energy content of the incoming waste of this concept may be about 11%.14 The projected costs may be approximately 120 ECU/tonne. We take a cost range of 120 ECU/tonne + 20% into account. Such a cost range is also found for incineration.

 Landfilling

Landfilling requires energy input for groundwork and other activities on site. Energy-use depends somewhat on the density of the waste. High density streams (with larger fractions of inert material) require more energy per tonne of waste for landfills. Expected total energy use for these landfilling activities is approximately 140 MJ/tonne, in the longer term when landfilled material is expected to consist largely of inert and high density material.4,16

On the other hand energy can be recovered by the utilization of landfill gas. Landfill gas can be converted to electricity (and heat) in gas engines, be combusted for heat production or purified to obtain gas of natural gas quality which can be fed into the natural gas grid. Landfilling of organic materials will in principle cease completely in the Netherlands. Although existing landfills will produce landfill gas in longer term it is questionable whether landfill gas production can be accounted to inert streams like ashes. Here we will circumvent this problem by using a range for maximally possible and zero landfill gas utilisation: Average landfill gas production and utilisation was 47.5 MJ/tonne of waste in 1990.10 We assume that landfilling in 2010 will require a net 90-140 MJ per tonne of waste processed. Landfill tariffs amount currently to 60-140 ECU/tonne of waste landfilled.1 Such tariffs are however affected by levies and do not represent the true costs. According to Kuijper et al. the expected future costs of landfilling, which include a number of measures to prevent leakages and smell, are about 20-30 ECU/tonne.22

  

4. EVALUATION OF FINAL WASTE TREATMENT SYSTEMS

In section 2 we outlined the expected supply of waste in the Netherlands in the year 2010. Section 3 gave an overview of waste treatment technologies that might be applied in the year 2010 and their expected performance range in terms of waste treatment costs and energy use or energy conversion efficiency. In this section three different systems for handling all final waste of the Netherlands in the year 2010 are presented and discussed. For each system a selection is made of the technologies applicable. In one system the selection is based on the criterion that the costs should be as low as possible; another system is selected on the criterion that the energy production should be as high as possible. For comparison a third system is also investigated in which conventional technologies that are currently in use dominate the total future waste treatment system.

In this section total waste treatment costs and total primary energy savings are calculated for each mix of selected technologies. However, first we will discuss how the savings on primary fossil fuel consumption - both through energy production and by re-use of materials - and the cost of waste treatment are calculated. Next, details of the selected waste treatment systems are presented. Finally, the overall outcomes for these systems are given.

4.1 Calculation of energy recovered and waste treatment costs

Energy recovery

Waste treatment technologies are compared in table 2; the focus is on the efficiency of converting a tonne of waste to useful energy carriers (electricity, gas, heat). In most cases the figure for the conversion efficiency (or energy consumption) is given as a range. On the basis of these figures for each waste stream the appropriate technology for handling the waste is established.

Given the total volume of waste that must be treated in the year 2010 per technology selected, the total amount of energy that could be produced or might be required is calculated. A figure for the minimum energy recovery (using lowest efficiencies) and for the maximum energy recovery (using the highest values of the performance ranges) is calculated, which results in the widest possible range for energy recovery per technology used. The total energy recovery for an entire waste treatment system then consists of a summation of the energy production of each technology selected per waste stream.

From the energy carriers produced we have calculated the savings (or consumption) in fossil fuels that normally would have been required to produce the same amount of electricity or heat. In these calculations we assume the average conversion efficiency for electricity and for heat production in the Netherlands to be 50% and 95% respectively in the year 2010.13 The gas produced from various processes (e.g. digestion) is assumed to be upgraded to the quality of natural gas; then it could replace the same amount of natural gas.

Energy savings through recovery of materials

Recycling materials saves primary energy in the production of new material. In our final waste treatment systems, recyclable materials are produced by waste separation of integral waste and by the composting and digestion processes that produce compost. Here we will discuss both options in terms of their potential impact on energy savings:

Waste separation: Separation of integral waste is a pre-treatment option for incineration, gasification or pyrolysis. The indirect savings on primary energy consumption through recycling of materials are substantial, as summarized in table 3. The primary energy saved is calculated by means of the energy content of the recyclable materials. Quantities produced during the separation of integral waste were already given in section 3.2. The net potential savings on primary energy consumption are calculated by substracting the energy required to regenerate recycled materials from the energy content of the materials saved. In our calculation GER values from the literature are used (GER (Gross Energy Requirement) values are determined by energy analysis of e.g. production processes and by taking direct and indirect energy consumption into account. For a detailed description see Worrell et al.44). Some details are given in the footnotes of table 3.

Composting: Composting is also an option to re-use material. Composting is currently the main treatment route for organic waste. The energy savings due to compost use are therefore of relevance. The application of compost determines what material is replaced and therefore the savings on primary energy consumption. Compost can be used as a substitute for peat, as a fertilizer and use as a cover layer (e.g. in parks or on landfills). The applications are very different in purpose and replace different materials. Compost is used as a fertilizer and soil improver in agriculture (59% of all compost produced from Organic Domestic Waste in 1994), in the private sector (20%), in the public sector such as public parks (20%) and others, e.g. export (1%).16

 

Table 3. Potential net primary energy savings by recycling of separated materials from integral wastes.

 plastic films

22.5

59a

1.3

paper

25

25b

0.6

ferrous metal

60

20c

1.2

Non-ferrous metal

4.4

188d

0.8

Total

112

 

3.9

a Assuming that mostly Low Density PolyEthylene is produced. The GER of PE is 69 MJ/kg15; mechanical recycling of PE including additional separation, washing and extruding requires approximately 10 MJ/kg15. The net energy yield is 59 MJ/kg.

b Assuming that the separated paper replaces pulp. Pulp production following the sulphite process without bleaching requires 31.6 MJ/kg (including energy content of the wood) and operations like screening and de-inking require 0.27 MJ/kg.15 Paper recycling is restricted by the loss of quality of the fibres. It is assumed that each recycling loop results in a 20% loss of fiber quality.44 The resulting net energy saved is therefore 25 MJ/kg.

c The ferrous metal is expected to be relatively poor quality, which means that its use is most applicable in the Blast Oxygen Furnace iron-making process. The pig iron replaced has a GER of 20 GJ/tonne.15,44

d It is assumed that most non-ferrous metal is aluminium. Recycled aluminium can be used for molded applications. Primary aluminium production requires 198 MJ/kg; melting of (recycled) aluminium requires 10 MJ/kg. The net energy saved amounts 188 MJ/kg.15,44

 

Heijningen et al.16 have indicated that the heating value of peat determines about 98% of the GER value of peat; transport and handling make only a small contribution. However, for our purpose it is doubtful whether peat should be considered as a fuel, since only a few countries use peat on a substantial scale for energy purposes (Finland, Ireland). Moreover, the replacement of peat by compost is very modest in terms of quantities replaced. The dominant application of compost is in agriculture where compost can (partly) offset the use of fertilizer. We therefore calculate the avoided energy use for the production of fertilizers. The content of the main fertilising substances (N, K and P) in compost is given in table 4. The GER values of fertilizers are given as well. Table 4 shows that the energy content of compost represented by fertilising components is modest.

The use of compost as ground cover could replace soil or sand; the GER value of the replaced materials is nearly 100 MJ/tonne.16 In these cases the net energy saved by producing and using compost as a substitute is negative. Note that for all applications of compost the transport of the compost itself is excluded from the energy required.

The lower heating value (LHV) of organic domestic waste is about 5 GJ/tonne (depending on source and time of year).9,11 In terms of primary energy used, utilizing this type of waste with an efficiency of 35% (in case of gasification) is therefore favourable compared to composting. Digestion allows for both energy production as well as production of compost, but does not result in higher energy savings than e.g. gasification (see section 3).

 

Table 4. Fertilising components in compost; content and energy saved.

 component

content (g/kg dry matter)a, 21

kg fertiliser/tonne ODW)

GER values (MJ/kg)16,44

Total energy saved (MJ/tonne ODW)

Potassium

14 (13-19)

3.36 K (6.42 kg KCL)

0.93 (KCl)

6

Phosphor

7 (6-10)

1.86 P (3 kg P2O5)

15.1(P2O5)

45

Nitrogen

17 (13-26)

4.08 N (11.6 kg NH4NO3)

12.1(NH4NO3)

141

 a 1 tonne of organic domestic waste (60% moisture content) produces 400 kg compost (40% moisture content)

 

 Waste treatment costs

In table 2 the expected waste treatment costs per tonne of waste for all technologies considered are summarized. Total waste treatment costs are calculated by multiplying the volumes of waste treated by the waste treatment costs of the selected technology for that stream and adding together the costs for all streams. This is done for both minimum and maximum treatment costs per selected technology and yields in the maximum possible range for the total waste treatment costs.

 4.2 Definition of final waste treatment systems

 A projection of the waste treatment costs and energy recovery in the year 2010 is calculated for final waste treatment systems consisting of different mixes of technologies. The choice of the technologies is discussed below for each system:

 1. 'Reference system 2010'; in this system the present mix of waste treatment technologies is maintained, but improved versions of conventional technologies are implemented. Efficient incineration of integral wastes and plastic waste, as well as co-combustion of waste wood are assumed. Other biomass waste is composted. Furthermore, sludge is incinerated in dedicated facilities. This scenario is a reflection of the current policy to increase incineration of integral waste and to rely on composting for treatment of most organic wastes.

 2. 'Minimum cost system'; the objective of this system is to obtain the lowest possible overall waste treatment costs. The criterion for selecting technologies per waste stream is therefore the lowest possible cost. Landfilling, however, is out of the question for all streams except inert materials. For some technologies the projected costs are low in the minimum cost case but high in the maximum cost case. In such cases the technology with the lowest minimum costs is selected for this system. This results in the selection of fluidized bed incineration to treat integral wastes and gasification to handle biomass waste streams. In this system sludge is treated by co-combustion, whereas plastics are treated separately by pyrolysis (VEBA process).

 3. 'Maximum energy recovery system'; The objective of this system is to obtain maximum energy recovery from waste treatment. The selection of technologies is determined by the overall net efficiencies per waste stream. In this system gasification is selected for the treatment of integral wastes. Biomass waste streams are gasified as well, assuming that streams with relatively high moisture and ash contents are mixed with cleaner and drier streams. Co-combustion of wood may be somewhat more efficient if new and more efficient coal fired power plants come into use, although this seems uncertain if power generation with natural gas remains cheaper than coal based power generation. It should be borne in mind that the efficiencies given here for biomass gasification apply to 30 MWe units. On a larger scale higher efficiencies are likely to be obtained. In this scenario we assume that waste wood can be converted with an electrical efficiency of 45%. Furthermore, sludge is co-combusted in coal fired power plants, whereas plastics are gasified in a separate process.

 For each system a summary of the selected waste treatment technologies and the projected volumes of waste treated per technology is given in table 5.

 

Table 5. Technology choices for the three final waste treatment systems in 2010.

  Minimum costs system

Max energy recovery system

Reference system 2010

* Fluid bed incineration of integral wastes (9400 ktonne)

* Gasification of integral wastes (9400 ktonne)

* Conventional (improved) incineration of integral wastes (9400 ktonne)

* Gasification of biomass wastes (1830 ktonne)

* Gasification of biomass wastes (1830 ktonne)

* Co-combustion of waste wood (300 ktonne)

* Composting of other biomass wastes (1530 ktonne)

* Co-combustion of sludges (285 ktonne)

* Pyrolysis of plastics (200 ktonne)

* Drying and co-combustion of sludge (285 ktonne)

* Gasification of plastics (200 ktonne)

* Sludge incineration (specific facility) (285 ktonne)

* Conventional incineration of plastics (200 ktonne)

* Landfilled waste (4290 ktonne)

* Landfilled waste (4290 ktonne)

* Landfilled waste (4290 ktonne)

 

4.3 Results

 For each system we calculated the potential energy recovery and waste treatment costs. The results are presented in figures 1 - 4. Figures 1a, 2a and 3a show the breakdown of minimum and maximum energy recovery for each system separately. It is concluded that in all cases the treatment of integral waste remains the most important contributor to the total energy recovery from the final waste treatment systems. However, the total amount of energy recovered differs strongly between the systems.

Figures 1b, 2b and 3b show the breakdown of waste treatment costs per major waste stream category for the three systems. The costs for treating integral waste and to a lesser extent for landfilling dominate the total costs. The costs for landfilling are the same for all systems. Large differences are however observed between the costs for integral waste treatment.

Figures 4a and 4b depict the comparison of the overall energy recovery and waste treatment costs of the three systems.

Table 6 summarizes the main outcomes with respect to primary energy saved and waste treatment costs. The increase in energy recovery for all systems compared to the current energy production (situation 1990; 17 PJ/year10) is striking; between 39-47 PJ in the reference case and 80-90 PJ in the maximum energy recovery case can be recovered from the projected waste supply. This excludes possible savings by means of the increased recovery of materials. A major reason for the increased energy recovery is that all organic material is used for energy production. Efficiency improvements in various technologies are a second explanation. With respect to the total waste treatment costs the differences are less clear, although the reference system is clearly the most expensive one. The minimum cost case results in total waste treatment costs ranging from about 300 to 600 MECU/yr. In the reference case these figures as high as 1,000-1,600 MECU/yr.

Figure 1a

Figure 1b

Figure 2a

Figure 2b

Figure 3a

Figure 3b

Figure 4a

Figure 4b

 

Table 6. Overall primary energy savings and waste treatment costs of the three final waste treatment systems.

 

Total primary energy savings (PJ/year)

Total waste treatment costs (MECU/year)

Systems

min

max

min

max

Minimum cost

55

64

310

590

Maximum energy recovery

80

90

310

730

Reference 2010

39

47

980

1,590

 

Some of the ranges between minimum and maximum energy recovery and minimum and maximum waste treatment costs are fairly wide. This is due to uncertainties in the expected performance of potentially attractive technologies from the cost and efficiency point of view. Important are the integral waste treatment technologies since they make a large contribution to both total costs and total energy recovery. In our new systems two main technologies are deployed: Fluidized Bed Combustion (FBC) and gasification. The performance of both of these technologies in the Dutch context (with applicable emissions standards) is however still uncertain. Differences in costs for FBC are caused mainly by the type of gas cleaning required. For gasification, uncertainties in projected costs of biomass waste treatment result from potentially required additional investments for pre-treatment and gas cleaning and the scale at which the technology is realized.

 As discussed in section 3.2, all technologies suited for treating integral waste can be preceded by waste separation. In table 5 the primary energy savings for the treatment of one tonne of integral waste are presented with and without separation. In the latter case energy savings through the re-use of materials have been taken into account. In table 5 also the treatment costs per tonne of waste are presented with and without separation.

Re-use of materials is possible if there is a steady demand for recycled material and lower grade materials can be used as raw material in production processes. Then the separation of metals, plastics and paper is useful as a pre-treatment step for all integral waste treatment technologies from the energy point of view (see table 7). Waste separation does however not result in a net difference in energy recovery or treatment costs between the various systems, since waste separation can be implemented in all systems.

Table 7 also shows that waste heat utilisation (as taken up for incineration) would result in somewhat higher total net primary energy savings. Note that waste heat utilization has not been taken into account in the set-up of the three systems. In practice, however, waste heat will be utilised, depending on location-specific circumstances. It is however reasonable to assume that in each system the degree of waste heat utilisation will be comparable. Therefore it will not fundamentally influence the comparison between the systems.

Finally the results suggest that fluidised bed incineration is on average the treatment route with the lowest expected cost per tonne of waste treated. IG/CC technology gives the highest energy savings due to the expected high efficiency of conversion to electricity.

 

Table 7. Comparison of waste treatment technologies for integral wastes in terms of waste treatment costs and energy recovery, both including and excluding indirect energy savings through recycling of recovered materials.

 

Improved incineration

Improved incineration + CHP

Fluidised bed incineration

IG/CC

Separation-digestion-incineration

 

min

max

min

max

min

max

min

max

min

max

Without separation

 

 

 

 

 

 

 

 

 

waste treatment costs (ECU/tonne)

83

145

83

145

33

48

30

60

 

 

primary energy saved (GJ/tonne waste treated)

4.7

5.4

5.5

6.2

4.0

4.5

6.3

7.6

4.3

5.0

Including separation

 

 

 

 

 

 

 

 

waste treatment costs (ECU/tonne)

91

115

91

115

47

60

45

72

96

120

primary energy saved (GJ/tonne waste treated)

7.5

8.1

8.2

8.9

6.8

7.3

8.9

10.0

8.3

8.9

 

Data on waste treatment costs and energy efficiencies are taken from table 2.

Primary energy saved is calculated as described in the main text. Waste treatment costs including separation are calculated by correcting for the mass fraction of separated materials. Primary energy savings are calculated by assuming an average 50% efficiency of electricity production and 95% for heat production. It is assumed 225 kg plastics, 60 kg ferro, 4.4 kg non-ferro and 22.5 kg paper are produced per tonne of waste separated.2 Inert material can also be extracted. This will result in a smaller volume of combustible waste but with a higher heating value. We assume however that only recyclables are separated and that the remaining waste has a heating value of 9 GJ/tonne.35,42 The net primary energy saved by using recycled material was given in table 3 and is included in the total energy saved when waste separation is performed.

  

5. DISCUSSION

Although substantial re-use and recycling rates are assumed in the waste supply data for 2010, higher recycling rates might prove possible. This aspect has not been investigated here. Nevertheless, the results presented remain relevant for the treatment of any future waste supply that consists largely of integral and biomass waste streams.

The assessment of the projected performance of different waste treatment technologies is based on data obtained from literature sources. In some cases cited data result from limited studies. Moreover, the methods used to calculate waste treatment costs may vary among different sources. Therefore, the figures presented in this article should be considered as indications. Future technological developments and applications will alter the performance range assessment. Processes based on pyrolysis are currently at an early stage of development and their performance (both in terms of costs and efficiency) is therefore especially uncertain. A more detailed study of such concepts as well as more practical experience can reduce such uncertainties and consequently influence the optimum configuration for the final waste treatment system of the year 2010.

The final waste treatment systems considered have been composed in a relatively simple manner. One main characteristic is that the diverse supply of waste was divided into a limited number of categories. This does not reflect the diversity of waste streams and different qualities that occur in practice. An example is the biomass waste fraction which varies from very wet waste streams to dry wood. Very wet waste is less suited or even not suited for gasification. In this case digestion may be a better alternative. In this study, however, we have assumed that wet materials are mixed with dry fuels prior to drying and gasification. This may lead to higher energy recovery than digestion. In specific situations, however, digestion might still be a favourable option.

A further limitation of this study is the fact that logistics have been excluded from the analysis of waste treatment systems. The collection and transport of waste can have a major impact on the total waste removal and treatment costs. On the other hand for all systems evaluated the total quantity of waste to be transported is the same. However, no analysis has been made of treatment plant size in each system, and therefore of the distances over which waste has to be transported. A structure with a larger number of smaller facilities will show different logistic costs from a structure with a limited number of large facilities. Further analysis of this point is desired. Also the nature of the various waste collection systems will influence the results and should be investigated in further studies.

The analysis has been limited to primary energy savings and costs. Effects attributable to differences in emissions of pollutants have not been investigated. The impact of emissions on the outcome of results can be included via a Life Cycle Analysis approach. This is not done here because of the large amount of data required and uncertainties in the environmental performance of technologies on the longer term. Assuming that all waste treatment facilities will meet the very strict (emission) standards for waste treatment in The Netherlands, the differences between various waste treatment systems with respect to the emissions of pollutants may not be too large.

 

6. SUMMARY AND CONCLUSION

Projections for the Netherlands of the future waste supply (to be treated by the current utility sector) indicate that, apart from biomass waste streams, 16 Mtonnes (of which 10 Mtonnes are integral 'mixed' wastes and 1.8 Mtonnes are organic waste) might require final waste treatment in the year 2010. Despite increased prevention and recycling efforts, it is expected that final waste treatment system requirements will be substantial. Between now and 2010 a significant part of the existing final waste treatment facilities in the Netherlands will have to be replaced or will be replaced within a relatively short time after 2010. This situation enables opportunities for improvement of the final waste treatment system, both from the energy recovery and economic point of view. In this paper the improvement potential of the final waste treatment system, assuming that the entire current waste treatment capacity in the Netherlands will be replaced between now and the year 2010.

Three final waste treatment systems have been evaluated. First, a reference system was defined in which the waste is treated by conventional (but improved) technologies. This system leads to total waste treatment costs of about 1,000-1,600 MECU/year and savings on primary energy consumption of 39-47 PJ/year. If maximum energy recovery is the main objective of the treatment system, gasification technology may be preferred, both for biomass wastes, for integral wastes and for plastics. In this system the total primary energy saved might be 80-90 PJ/year and the costs between 300-700 MECU/year. Co-combustion of sludge and waste wood appears to be a promising option, especially if new coal fired power plants, with a high conversion efficiency (i.e. 48%), are put into use. If the objective of the system is minimum waste treatment costs, the primary energy saved could amount to 55-64 PJ with total waste treatment costs of 300-600 MECU/year.

Compared to the reference system, the results suggest that substantially increased energy recovery and lower costs can be obtained simultaneously. The structure selected in the minimum cost case is rather similar to the maximum energy recovery case. In the latter, fluidized bed combustion is used to treat integral waste instead of RDF gasification, although the difference into projected waste treatment costs for these two options is not large.

Mechanical separation of metal, plastic and paper streams from integral waste is favourable from an energy point of view in all cases, provided there are markets for the regenerated material. The electrical conversion efficiency of integral waste treatment technologies remains the dominating parameter for total energy recovery during integral waste treatment and therefore was the main formed focus of attention in this analysis.

Composting is generally not attractive in terms of primary energy savings, since in most applications the energy content of the material replaced by compost is low. The results of this study suggest that separate collection and treatment of organic domestic waste is not the best course from an energy and economic point of view.

Compared to the current situation and the defined reference system for the year 2010, new waste treatment processes allow substantially increased energy recovery from waste as well as lower waste treatment costs. Key technologies are expected to be the gasification of various waste streams and fluidized bed incineration. The relatively wide ranges in the characteristics of these technologies, especially the treatment costs, illustrate current uncertainties about long term performance. A significant factor for cost and efficiency is the required level of environmental performance. Emission standards can substantially influence the cost and efficiency of waste treatment technologies. In this respect gasification has the inherent advantage of gas cleaning before combustion of the fuel gas. Further development of the gasification of integral waste integrated with combined cycle technology is therefore desirable. However, it should be noted that the future performance of many technologies is uncertain due to limited experience (e.g. pyrolysis based processes) and their performance under specific Dutch emissions standards (e.g. fluid bed combustion). More detailed analyses and practical experience of these technologies are therefore needed too.

Acknowledgements - The authors are grateful to Prof. Wim Turkenburg (STS/UU) and Ir. Jo Daemen of the Waste Consultative (Afval OverlegOrgaan; AOO) for critical comments and suggestions. Eva Olsson (TPS) and Maggie Mann (NREL) are thanked for providing useful information. Sheila McNab is thanked for linguistic assistance.

 

7. REFERENCES

1. Afval Overleg Orgaan, Draft Ten Year Program on Waste 1995 - 2005, Utrecht, January 1995 (in Dutch).

2. Afval Overleg Orgaan, Environmental Impact Assessment Ten Year Program on Waste 1995 - 2005, Utrecht, February 1995 (in Dutch).

3. Bekker, de, P.H.A.M.J., Sludge treatment with the Vertech process, chapter of the course Sludge treatment Stichting Postacademisch onderwijs, gezondheidstechniek en milieutechnologie, Delft, March 1992 (in Dutch).

4. W. Blom, A. van Keken, Landfills in The Netherlands, data 1991, Rijks Instituut voor Volksgezondheid en Milieuhygiëne, reportno. 736201016, Bilthoven, March 1992 (in Dutch).

5. Broek, R. van den, A. Faaij, A. van Wijk, Biomass combustion for power generation, In: Biomass and Bioenergy, Vol 11, no. 4 pp. 271-281, 1996.

6. Commission of European Communities, Establishing a list of wastes pursuant to article 1a of Council Directive 75/442-/EEC on Waste, Commission decision of 20 December 1993 In: official journal of the EC, December 1993

7. Consonni, S., E.D. Larson, Biomass-gasifier/aeroderivative gas turbine combined cycles, part A: technologies and performance modelling. Prepared for Cogen Turbo Power '94, The American Society of Mechanical Engineers' 8th congress & exposition on gas turbines in cogeneration and utility, industrial and independent power generation, Portland, Oregon, 25-27 October 1994.

8. Consonni, S., E.D. Larson, Biomass-gasifier/aeroderivative gas turbine combined cycles, part B: performance calculations and economic assessment. Prepared for Cogen Turbo Power '94, The American Society of Mechanical Engineers' 8th congress & exposition on gas turbines in cogeneration and utility, industrial and independent power generation, Portland, Oregon, 25-27 October 1994.

9. Doorn, J. van, Characterization of energy crops and biomass and waste streams, Netherlands Energy Research Foundation, report no. ECN-C-95-047, Petten, July 1995.

10. Jager de, D., K. Blok, The contribution of waste and biomass to the energy supply in the Netherlands, inventory and potential, ECOFYS, report in the framework of the EWAB programme, Utrecht, November 1994.

11. A. Faaij, A. van Wijk, J. van Doorn, A. Curvers, L. Waldheim, E. Olsson, C. Daey-Ouwens Characteristics of biomass wastes and residues in the Netherlands for gasification, Biomass and Bioenergy Vol. 12, No. 4 pp. 225-240, 1997.

12. Faaij, A., R. van Ree, L. Waldheim, E. Olsson, A. Oudhuis, A. van Wijk, C. Daey Ouwens, W. Turkenburg, Gasification of biomass wastes and residues for electricity production., Department of Science, Technology and Society, Utrecht University, Netherlands Energy Research Foundation (ECN) Termiska Processer AB, accepted for publication in Biomass and Bioenergy, May, 1997.

13. Fockens S., A.J.M. van Wijk, Developments in the total electricity supply system of the Netherlands, prepared for Gasunie N.V., Department of Science, Technology and Society, report no. 94055, Utrecht, 1994 (in Dutch).

14. Hazewinkel, J.H.O, R.J.J. van Heijningen, A.R. Jonkers, J.C. Wardenaar, Evaluation of integrated digestion and incineration of waste, INFOPLAN, vHa, KEMA, report prepared for NOVEM/EWAB no. 9204, Delft, May 1992 (in Dutch).

15. Heijningen, R.J.J. van, J. de Castro, E. Worrell, Gross Energy Requirement figures in relation to prevention and re-use of waste, Van Heijningen Energie- en Milieuadvies, Castro Consulting Engineer, Department of Science Technology and Society, Utrecht University, in relation to the National research programme re-use of waste (NOH) report no. 9210, Amersfoort, February 1992 (in Dutch).

16. Heijningen van, R.J.J., J.F.M. de Castro, E. Worrell, J.H.O. Hazewinkel, More Gross Energy Requirement figures in relation to prevention and re-use of waste, Van Heijningen Energie- en Milieuadvies, Castro Consulting Engineer, Department of Science Technology and Society, Utrecht University, INFOPLAN B.V., in relation to the National research programme re-use of waste (NOH) report no. 9272, Amersfoort, December 1992 (in Dutch).

17. Hekkert, M., Energetic and economic optimisation of the Dutch waste treatment structure in 2010 with new waste treatment technologies, Department of Science, Technology and Society, Utrecht University, report no. 95061, Utrecht, October 1995 (in Dutch).

18. Huisman G.H., Royal Schelde BV; Sludge gasification process, written & personal communication, Goes, November 1996.

19. Hulsbos, W.C., Sludge drying processes, chapter of the course Sludge treatment Stichting Postacademisch onderwijs, gezondheidstechniek en milieutechnologie, Delft, March 1992 (in Dutch).

20. Jong de, H.B.A., W.F. Koopmans, A. van der Knijff, Conversion technologies for Organic Domestic Waste, developments 1992, Haskoning Koninklijk Ingenieurs- en architectenbureau, in relation to the national programme re-use of waste, report no. 9273, Nijmegen, February 1993 (in Dutch).

21. Knijff, van der A., Conversion technologies for Organic Domestic Waste, Haskoning Koninklijk Ingenieurs- en architectenbureau, in relation to the national program re-use of waste, reportnr. 9103, Nijmegen, April 1991. (in Dutch).

22. Kuijper, T., A,. Snuverink, R. Veregas-Carbonell, Cost structures of landfills, TEBODIN BV, report prepared for the Ministry of Housing, Physical planning and Environment, The Hague, February 1993 (in Dutch).

23. E. Larson, E. Worrell, J. Chen, Clean fuels from municipal solid waste for transportation in New York city and other major metropolitan areas, Centre for Energy and Environmental Studies, Princeton University, Report No. 293, January 1996.

24. Marskamp, M. Sludge incineration and vitrification, chapter of the course Sludge treatment Stichting Postacademisch onderwijs, gezondheidstechniek en milieutechnologie, Delft, March 1992 (in Dutch).

25. Ministry of Economic Affairs, Third Energy memorandum, Sdu uitgeverij, The Hague, 1995 (in Dutch).

26. Ministry of Housing, physical planning and environment, National Environmental Action Plan II, The Hague 1992.

27. Mocking, E., A. Curvers, C. Daey Ouwens, J. van Doorn, A. Faaij, V. Schaap, Inventory of potential biomass fuels for the Noord-Holland gasification project, Province of Noord-Holland, ECN, Dept. of Science, Technology and Society, Utrecht University, Haarlem, November 1994. (in Dutch)

28. Nagelhout, D., Z. van Lohuizen, Waste treatment 1990 - 2010, background document for the Nationale Milieu Verkenning 2, RIVM report no. 736201010, Bilthoven, January 1992. (in Dutch)

29. Netherlands Statistics Agency, Environmental Statistics for the Netherlands 1994, Sdu uitgeverij/CBS publications, The Hague, 1994.

30. Netherlands Statistics Agency, Wastes, Industrial waste 1994, Voorburg/Heerlen, 1996.

31. Netherlands Statistics Agency, Wastes, waste collected by municipalities, Part A: quantities, Voorburg/Heerlen, 1996.

32. Niessen, W.R., C.H. Marks, R.E. Sommerlad, Evaluation of gasification and novel thermal processes for the treatment of municipal solid waste, Camp Dresser & McKee National Renewable Energy Laboratory, Golden (CO), USA, August 1996.

33. Olsson, E., Additial information to TPS integrated gasification combined cycle technology for waste processing, Termiska Processer AB, Nyköping, Sweden, 7 December 1995.

34. Penninks, F., Coal and wood fuel for electricity production: an environmentally sound solution for waste and demolition wood, NV EPON, in: proceedings of the 5th Dutch solar energy conference; symposium biomass, Veldhoven, 20 April 1995 (in Dutch).

35. Pfeiffer, A.E., J.W.M. Verdijk, A.W.M. van Wunnik, Optimisation and energy utilisation at waste incineration, part B: exploration of optimisation electricity production, KEMA Engineering & Consultancy, Arnhem, 15 January 1992.

36. Ruiten, van, Business plan optimisation of large scale waste incineration, Projectno. 24.34-030.10, VEABRIN, 1988.

37. Sas, H., Removal of domestic plastic waste; analysis of environmental impact and costs, Centrum voor Energiebesparing, Delft, 1994 (in Dutch).

38. Schipper-Zablotskaja, M., Waste treatment facility for domestic and industrial waste, process description, RIVM, report no. 355120/2010, Bilthoven, 1994 (in Dutch).

39. Smit, R., A. Snuverink, Status report on innovative solid waste processing techniques, Tebodin consultants and engineers, prepared for the ministry of Housing, Physical Planning and the Environment, report no. 320237, The Hague, January 1997.

40. Soltes, E.J., T.A. Milne, Pyrolysis oils from biomass. Producing, analyzing and upgrading, ACS Symposium Series, 376, American Chemical Society, Washington D.C., 1988.

41. Venendaal, R., H.E.M. Stassen, F.S. Feil, A.E. Pfeiffer, H.C. Op 't Ende, J.C. Wardenaar, Gasification of waste, evaluation of installations of Thermoselect and TPS/Greve, Biomass Technology Group and KEMA, in the context of the EWAB programme, Enschede, April 1994 (in Dutch).

42. Vereniging van AfvalVerwerkers (VVAV), Comparitive study of thermal treatment of domestic waste, an evaluation of five technologies, study performed by VVAV and KEMA, report no. VVAV95006V.R, Utrecht, August 1995 (in Dutch).

43. White, P., M. Franke, P. Hindle, Integrated solid waste management, a life cycle inventory, Blackie Academic and Professional, Chapman & Hall, London, 1995.

44. Worrell, E., R.J.J. van Heijningen, J.F.M. de Castro, J.H.O. Hazewinkel, J.G. de Beer, A. Faaij, K. Vringer: New Gross Energy-Requirement figures for materials production, Energy, vol. 19 No. 6 pp.627-640, 1994.

45. RIVM, State Institute for Public Health and Environment, Environmental Balance 1996, the Netherlands environment explained, Bilthoven, 1996 (in Dutch).


N


Last update: August 21, 1997
For any question or suggestion, please feel free to contact the Webmaster