Chapter II: Characteristics and availability of biomass waste and residues in the Netherlands for gasification
Co-authors: Joep van Doorn, Toine Curvers, Lars Waldheim, Eva Olsson, Ad van Wijk, Cees Daey-Ouwens.
Published in 'Biomass and Bioenergy' Vol.12, No. 4, pp.225-240, 1997.
Abstract
- Characteristics and availability of biomass waste streams and residues for power production by means of integrated gasification/combined cycle technology (BIG/CC), are evaluated for The Netherlands. Four main categories are investigated: streams from agriculture, organic waste, wood and sludges. Altogether 18 different streams are distinguished. Gross availability and net availability are inventorized. Various properties (composition, heating value, supply patterns) are analysed and the suitability of these streams for conversion in a BIG/CC unit is studied. The costs at which various streams are likely to be available are assessed.The gross energetic availability amounts annually approximately 190 PJ (HHV) primary energy. Because of competing useful and higher value applications than fuel of various streams, such as fodder and fertilizer, the net availability is slightly less than 90 PJ (HHV). For a number of streams the costs are negative due to present waste treatment costs. Costs of waste streams vary from -10 up to 5 ECU/GJ. For a small fraction the costs are higher than for energy crops (estimated to be approximately 4.5 ECU/GJ). Because there are large variations in properties and contaminants between various streams, the conversion system needs to flexible when a diversity of streams is treated in one installation. Some streams require mixing with cleaner fuels to make them suitable for use in a direct atmospheric Biomass Integrated Gasifier/Combined Cycle system. Important technical limits for the use of biomass fuels in the system studied, are the moisture content (maximum 70% of wet fuel) and ash content (maximum 20% dry matter content) of the fuel.
1. INTRODUCTION
Since the Netherlands is a densely populated country, it produces substantial quantities of organic wastes and residues in numerous streams. The present treatment routes for these streams are re-use as compost, fodder or fibre board, landfilling (which is currently dominant) and incineration. As landfilling of organic materials will be prohibited in the near future, waste treatment capacity needs to be increased. The use of these biomass waste streams for energy production in an integrated gasification/combined cycle (BIG/CC) system, may provide an environmentally friendly and economically attractive alternative, since such systems promise high conversion efficiencies and potentially low capital costs per capacity unit installed. This technology can also contribute to a reduction in CO2 emissions in that it utilises the energetic potential of biomass wastes and residues more efficiently than mass burning.8
With regard to power plants fired with biomass and biomass wastes, the characteristics of the fuel are very important. Because the composition of the biomass materials and degree of contamination vary widely, the pre-treatment and conversion systems need to be flexible. In addition it should be noted that the costs of the fuel and the required pre-treatment can be major factors in the overall electricity production costs.
To allow an assessment of the possibilities and constraints of the use of various biomass wastes and residues in an integrated direct gasification combined cycle plant, insight in the availability, costs, composition and technical limitations of the conversion technology is required. The objectives of the paper are therefore to identify and quantify these characteristics for organic wastes and residues in the Netherlands.
The feasibility of gasification of various biomass waste streams and residues for electricity production will be discussed in general terms. In this paper the focus will be on direct atmospheric circulating fluidized bed gasification coupled to a General Electric LM 2500 gas turbine, as described in Elliott and Booth, and Faaij et al.7,8
First, the applied methodology is described. Results are presented regarding the availability of biomass streams in the Netherlands. This is followed by an evaluation of the cost ranges of the biomass streams. The composition and degree of contamination are given for a representative selection of streams. Then the technical limitations of an integrated atmospheric gasifier coupled to a combined cycle are discussed in relation to fuel properties. Uncertainties are discussed and conclusions are drawn about the gross and net energetic potential of biomass streams that are at present available for gasification in The Netherlands.
2. METHODOLOGY
To determine characteristics of biomass waste streams we investigate the four following aspects: availability and supply patterns, costs, composition and degree of contamination, and technical limitations for applying biomass waste for energy production in a BIG/CC unit. Four main categories of biomass wastes and residues are considered: agriculture, organic waste, wood and sludges. These categories are subdivided into 18 streams. The investigation will be focused on the situation in the Netherlands in 1994.
2.1 Availability and supply
Gross availability of organic waste and residues is investigated by means of production statistics for agriculture, forestry and waste in the year 1994.
Net availability is determined by correcting the gross availability using data on useful applications per stream, such as utilization as fodder or fertilizer and re-use of waste wood. Volumes of waste are translated into energy potential by means of (both higher and lower) heating values per stream.
Time supply patterns are investigated for each stream by the analysis of seasonal effects, such as harvest time and maintenance activities of public parks, which are undertaken only during specific periods. Next, the supply during (part of) the year is described. On this aspect detailed data are not available. Therefore it is assumed that the available volume of a stream is released in certain months in equal quantities per month. The time dependency of supply is an important aspect to take into account when assessing the use of waste in larger BIG/CC units that are expected to function all the year round (baseload power production).
2.2 Costs
An inventory of the waste treatment costs per waste stream and of the market value of the organic material is compiled. The costs are determined either by the present costs of waste treatment (like composting, landfilling, incineration)1,10 which results in negative costs of the biomass stream, or by the market prices of the material if used in other applications (such as fertilizer, raw material, etc.). Costs are given in ECU1994 and derived from literature or personal communication. Market prices can vary strongly over time because of varying supply and demand.3,17,30 Therefore the costs of biomass streams are expressed in ranges compiled of the minimum and maximum costs resulting from the inventory. In the Netherlands the cost of biomass from energy farming is projected to be approximately 4.5 - 6 ECU/GJ (HHV).16 When the cost of a 'waste' stream appears to be higher than energy crops because of its high value for alternative applications, it is considered unavailable for energy production purposes.
Transport costs from producer to conversion plant are not included in this analysis.
The 1994 situation in the Netherlands will be presented as supply curves that illustrate the minimum and maximum costs per (net available) GJ contained in each biomass waste stream. This gives insight in what cost band biomass streams are likely to be obtainable. It should however be kept in mind that a large demand for biomass waste to produce energy could increase prices.
2.3 Composition and degree of contamination
The chemical composition, degree of contamination, moisture and ash content and calorific value of the waste streams are determined. Moisture and ash content are determining parameters for the heating values of the biomass materials. Higher heating values are derived from both characterization experiments and literature.
Experiments were performed to determine the chemical composition of a selection of biomass streams. These streams are verge grass, waste paper, demolition wood, sewage sludge, cacao shell (being a waste stream from the food and beverage industry), organic domestic waste (ODW; separately collected organic waste from households and services) and for comparison Miscanthus and Willow. The experimental characterization is described in detail in van Doorn.6 Carbon, hydrogen, nitrogen and sometimes sulphur are determined in a single analysis step. Chlorine, fluorine and sulphur are measured in a bomb calorimeter. The moisture content and the volatile matter content are determined gravimetrically. Ash is characterized according to standard procedures. Furthermore initial deformation, softening, hemispherical and fluid ash deformation temperatures are measured. To determine metal contents ash samples are dissolved and analyzed according to standardized procedures.
The streams considered represent a large diversity of biomass fuels. In some cases, if data are lacking, some properties are estimated by comparison with similar streams.
The higher heating value (HHV) represents the heating value of the dry biomass material including ash. Lower heating values are calculated from the higher heating values and the moisture content (
The lower heating value of a biomass material is calculated by the following formula: LHV = HHVdry * (1 - W) - Ew * (W + H * mH2O) in which Ew is the energy required for evaporation of water (2.26 MJ/kg), W the moisture content, H the hydrogen content (weight percent of wet fuel) and mH2O the weight of water created per unit of hydrogen (8.94 kg/kg). With respect to contaminants the focus is on nitrogen, sulphur, chlorine and heavy metals, since these are most relevant with respect to emission standards.2.4 Technical limitations
The physical properties of the organic waste are compared to the demands and limitations of options to utilize this waste. Here, focus lies on BIG/CC technology. Fuel criteria relevant for this technology involve moisture content, ash content, density (because of handling and feeding), heating value, ash fusion temperature, halogen content (because of corrosion and emissions), heavy metal content (because of emissions and potential application of residues). Gasification is assumed to take place in an atmospheric circulating fluidized bed with a tar cracker as the first gas cleaning step (similar to TPS technology).7 Lab scale fuel reactivity experiments are performed with a selection of biomass streams and resulting gas compositions of the fuels are derived with the help of model calculations. These experiments are reported in detail in Lassing et al.15 In this paper technical limitations concerning ash and moisture content and various contaminants will be evaluated in general terms only.
3. RESULTS
3.1 Availability
The biomass residues and waste streams that are taken into account for this analysis are described in table 1. Streams are divided into four categories: agriculture, organic wastes, wood streams and sludges. In general, biomass residues from agriculture on arable land are all re-used as fodder or fertilizer.22,23,29 Only some straw remains available.
The category 'organic waste' covers streams that are collected separately, such as organic domestic waste (ODW). Organic waste from the food and beverage industry is also included in this category. Sludges cover sewage sludge and organic material released during the maintenance of waterways. Manure, which is a substantial stream in The Netherlands, is not considered here because its dry matter content is too low for thermochemical conversion. Furthermore, the surplus depends on a number of factors which are due to change in the near future (such as reduction of stock, and different treatment technology).35
The results for all the streams considered regarding the gross and net availability, lower and higher heating values and energy potential are given in table 2. Moisture and ash content are given per stream; the values mentioned represent average values but in some cases they can cover a wide range.6,17
Estimates of ash and/or moisture content are made for the waste streams from bulb cultivation and the greenhouse sector, swill, fruit farming and sludge from the maintenance of waterways. For simplicity values for the food and beverage industry are averaged to avoid great complexity since this category includes a large number of specific streams. The total annual gross energy supply amounts approximately 190 PJ (HHV). The net supply is slightly less than 90 PJ (HHV).
The main periods (months) in which streams are released are indicated in table 1. Figures 1 and 2 present the energy supply (expressed in Higher Heating Value) of the above mentioned biomass residues and wastes in the Netherlands. Figure 1 shows the total energy supply of available biomass streams per month. Figure 2 shows the energy supply for those streams that are especially bound to seasons.
Table 1. Organic waste streams and residues investigated in this analysis. The last column indicates the time of year at which the supply peaks. Main data sources on biomass materials are de Jager et al.9, Mocking et al.17, Siemons et al.29 and Sikkema.30 Other sources are mentioned per stream.
Stream |
Description |
supply pattern |
Agriculture |
In general residues from agriculture are used in their entirety as fodder, fertilizer, etc. A number of residues is therefore not included in this analysis |
|
straw |
Residue from cereal production; considerable fluctuations in supply and price per year can occur. Composition data and prices stem from19,28 |
July-September |
bulb cultivation |
Includes several streams like straw, plant material and peelings from bulbs. Compostion data of this waste stream lack; average moisture and ash contents are given in36; the heating value of the organic fraction is assumed to be similar to straw. Almost all waste is composted in either central facilities or at farm. |
Peaking May-June |
greenhouse |
Residues from greenhouse culture. Remains of crops mixed with some plastics. Composition data are obtained by personal communication. Almost all waste is composted in central facilities.2 |
Peaking from September-November |
fruit farming |
Prunings from fruit trees including uprooted trees and stubs. Compostion of this woody stream is estimated by assuming a somewhat higher ash content compared to clean wood. Other composition data are similar to thinnings. Residues are either left on the land, partly composted or have a market value as fuel wood.11,14,18 |
Peaking June-August |
auction |
Organic waste from auctions, plant-like material (flowers, vegetables) mixed with packaging materials. Composition data with respect to moisture and ash are reported in17. Auction waste is both composted and landfilled at present. |
Irregular, increase in summer |
Organic wastes |
|
|
households |
Separately collected organic fraction of domestic waste. Contains remains of vegetables, fruit, plants and garden waste. Composition data are reported in a background report17, ODW is largely composted and partly digested.
|
All year with increase in spring and autumn |
services |
Separately collected organic fraction. Mainly contains remains of food processing. This stream is, like ODW, largely composted and digested. The composition is assumed to be equal to ODW since they are treated together in the same facilities.17 |
Constant |
swill |
Food remains released at restaurants, hospitals, etc. Very wet material with a moisture content of approximately 80%. Part of this waste is discharged by the sewer system; other fraction is composted or digested.39 |
All year |
verge grass |
Mowed grass released during maintenance of road sides Information about supply, use and compostion is taken from28 Verge grass either has a useful application as fodder or has to be removed and order to be landfilled or composted.24 |
Peaking in May-June and September-October. |
Stream |
Description |
Supply pattern |
waste paper |
Surpluses of separately collected old paper which are not used for paper production. The volume strongly varies with market price and demand for raw material.38 An estimate of the supply is given by5. Composition data are taken from6. |
All year |
food & beverage industry |
Residues from food processing industries. Highly variable composition per industry. Streams cover sludges, remains of food crops, processing waste (e.g. sugar, tobacco and beer production, meat and fish). Several streams are 100% re-used as fodder or in other applications. An overview of this waste stream category is given in de Jager et al.9. For this overview only waste streams are included with a moisture content below 80%. Those streams are often digested or have a useful application that does not require additional treatment.9 |
All year. Some streams are released during seasonal production |
Wood |
|
|
thinnings |
Byproduct of commercial forestry. Compostion data are taken from6,33. Prices vary according to demand for pulp wood and other applications. A cost range is obtained from31,32,33. |
Delivery all year; storage in woods |
prunings |
Woody stream released during maintenance of public parks, belts, etc. Mainly consists of wood, partly leaves and some other waste.4 Composition is assumed to be equal to forest thinnings but with a slightly higher ash content. Most cleaner wood is shredded and left in the parks or has some value as fuel wood.17,29 |
Peaking April-June and September-November |
industrial waste wood |
Wood released during wood processing. Generally clean and dry material. Prices, avialability and utilisation of various waste wood categories are discussed in12,13,21,27,30 Average moisture content for waste wood is 15% and very low in ash. We use a broad price range for all waste wood categories determined by waste treatment costs (incineration and landfilling). High quality waste wood is excluded from this overview since it has a high value and considered unavailable for energy production. |
All year |
demolition wood |
Wood released in the building sector. Demolition wood is partly re-used, but largely landfilled or incinerated at present. |
All year |
wood products |
Discarded wood products, like pellets, furniture, etc. |
All year |
Sludges |
Manure and sludges from industrial processes are not included in the analysis. |
|
waste water treatment |
Sludge produced at waste water treatment facilities. Initially very wet. De-watering and/or drying is applied. High ash content and contaminated with heavy metals. Average composition data are taken from5,6. Sludge is at present incinerated, dried, landfilled and composted. Cost data are taken from35. |
Constant |
maintenance of waterways |
Mowing waste released during maintenance of waterways. Consists of reed, grass, plant material with high fractions of sand and mud which are sometimes contaminated. Information about moisture and ash content is taken from The material is either left on the side of waterways or in some cases removed and composted.22,23 |
Peaking in summer |
Table 2. Potential, availability, energy content and heating values of biomass waste streams and residues.
|
Amount of wasted |
|
|
|
|
Potential energy supply |
||||
|
(kton/year) |
moisture contentd |
ash contentd |
HHVdrye |
LHVwetf |
(PJLHV/yr) |
(PJHHV/yr) |
|||
|
gross |
net |
(% of wet material) |
(% of dry material) |
(GJ/ton) |
(GJ/ton) |
gross |
net |
gross |
net |
Agriculture |
|
|
|
|
|
|
|
|
|
|
Straw |
800 |
400 |
15 |
10 |
18 |
14 |
11.1 |
5.5 |
12.2 |
6.1 |
Bulb cultivation |
260 |
260 |
60 |
10 |
18 |
5 |
1.4 |
1.4 |
1.9 |
1.9 |
Greenhouse |
100 |
100 |
80 |
1 |
20 |
2 |
0.2 |
0.2 |
0.4 |
0.4 |
fruit farming |
200 |
200 |
50 |
5 |
19 |
8 |
1.5 |
1.5 |
1.9 |
1.9 |
auction |
140 |
140 |
60 |
10 |
18 |
5 |
0.7 |
0.7 |
1.0 |
1.0 |
Organic wastes |
|
|
|
|
|
|
|
|
|
|
households |
1000 |
1000 |
60 |
20 |
16 |
4 |
4.4 |
4.4 |
6.4 |
6.4 |
services |
200 |
200 |
60 |
20 |
16 |
4 |
0.9 |
0.9 |
1.3 |
1.3 |
swill |
100 |
100 |
80 |
1 |
19 |
2 |
0.2 |
0.2 |
0.4 |
0.4 |
verge grass |
500 |
400 |
60 |
10 |
18 |
5 |
2.6 |
2.1 |
3.6 |
2.9 |
waste papera |
3000 |
550 |
10 |
15 |
19 |
16 |
47.1 |
8.6 |
51.3 |
9.4 |
f & b industryb |
8070 |
3700 |
60 |
1 |
19 |
6 |
45.5 |
20.9 |
61.3 |
28.1 |
Wood |
|
|
|
|
|
|
|
|
|
|
thinnings |
1600 |
1100 |
50 |
1 |
20 |
8 |
13.0 |
9.0 |
16.0 |
11.0 |
prunings |
330 |
230 |
50 |
5 |
19 |
8 |
2.5 |
1.8 |
3.1 |
2.2 |
Industrial waste wood |
250 |
50 |
15 |
1 |
19 |
15 |
3.7 |
0.7 |
4.0 |
0.8 |
Demolition wood |
425 |
200 |
15 |
1 |
19 |
15 |
6.2 |
2.9 |
6.9 |
3.2 |
wood products |
375 |
300 |
15 |
2 |
18 |
14 |
5.2 |
4.2 |
5.7 |
4.6 |
Sludges |
|
|
|
|
|
|
|
|
|
|
waste water treatmentc |
300 |
300 |
0 |
40 |
14 |
13 |
3.8 |
3.8 |
4.2 |
4.2 |
maintenance of waterways |
2600 |
550 |
60 |
65 |
7 |
1 |
2.2 |
0.5 |
7.3 |
1.5 |
|
|
|
|
|
TOTALS |
152 |
69 |
189 |
87 |
aAvailability varies strongly per year.
b
Average values are given for moisture, ash and heating values.c
Expected to be delivered in dry form. Actual moisture content depends on the degree of dewatering, drying, etc.d
Figures with respect to availability, potential moisture and ash content of various streams stem from numerous sources given in table 1 per stream.e
Higher heating values are derived from literature. In case no values were available an estimate was made by on basis of comparison with a similar biomass material. Differences, however, in Higher Heating Values of dry biomass are largely caused by differences in mineral fraction (ash content) and not by differences in chemical composition (oxygen, carbon and hydrogen content) since these are comparable for almost all biomass materials. Exceptions to this are paper and sludge.f
The lower heating values are calculated by the following formula: LHVwet = HHVdry * (1 - W) - Ew * (W + H * mH2O) in which Ew is the energy required for evaporation of water (2.26 MJ/kg), W the moisture content, H the hydrogen content (weight percent of wet fuel) and mH2O the weight of water created per unit of hydrogen (8.94 kg/kg).Overall the energy supply peaks between spring and autumn. The minimum and the maximum supply varies from approximately 6 to 9 PJ per month. A large fraction of this supply comes from streams of which the supply remains more or less constant during the year such as from the food and beverage industry and organic domestic wastes. When it is considered desirable to use streams from agriculture, verge grass, prunings, etc. for energy production, one has to take into account that the supply of those streams during winter months will be negligible, as illustrated by figure 2.
Figure 1. Potential energy supply from net available biomass wastes and residues in the Netherlands (PJHHV) during the year.
Figure 2. Potential energy supply in the Netherlands during the year of net available biomass wastes and residues of which the supply is bound to seasons.
3.2 Costs of biomass residues and wastes
The results of the cost inventory of biomass waste streams and residues are given in table 3. Negative costs represent the current waste treatment costs, positive values represent the present market value of the residue. For comparison the projected biomass production costs of energy crops (willow and miscanthus) in The Netherlands are given as well.16 In these figures transport costs to the conversion plant are not included.
Table 3. Costs for biomass waste streams in the Netherlands.
|
(ECU/wet ton)a |
(ECU/dry ton)a |
ECU/GJLHVb |
ECU/GJHHVb |
||||
|
min. |
max. |
min. |
max. |
min. |
max. |
min. |
max. |
Agriculture |
|
|
|
|
|
|
|
|
straw |
23.8 |
95.2 |
27.4 |
109.5 |
1.7 |
6.9 |
1.5 |
6.1 |
bulb cultivation |
-61.9 |
-2.9 |
-99.0 |
-4.6 |
-11.8 |
-0.5 |
-5.5 |
-0.3 |
greenhouse |
-40.5 |
-14.3 |
-72.9 |
-25.7 |
-22.2 |
-7.8 |
-3.6 |
-1.3 |
fruit farming |
0.0 |
47.6 |
0.0 |
71.4 |
0.0 |
6.2 |
0.0 |
3.8 |
auction |
-95.2 |
-28.6 |
-152.4 |
-45.7 |
-18.2 |
-5.5 |
-8.5 |
-2.5 |
Organic wastes |
|
|
|
|
|
|
|
|
households |
-66.7 |
-28.6 |
-106.7 |
-45.7 |
-15.0 |
-6.4 |
-6.7 |
-2.9 |
services |
-66.7 |
-28.6 |
-106.7 |
-45.7 |
-15.0 |
-6.4 |
-6.7 |
-2.9 |
swill |
-66.7 |
-28.6 |
-120.0 |
-51.4 |
-41.1 |
-17.6 |
-6.3 |
-2.7 |
verge grass |
-61.9 |
7.1 |
-99.0 |
11.4 |
-11.8 |
1.4 |
-5.5 |
0.6 |
waste paper |
4.8 |
23.8 |
5.2 |
26.2 |
0.3 |
1.5 |
0.3 |
1.4 |
f & b industry |
-95.2 |
0.0 |
-152.4 |
0.0 |
-16.9 |
0.0 |
-8.0 |
0.0 |
Wood |
|
|
|
|
|
|
|
|
thinnings |
28.6 |
33.3 |
42.9 |
50.0 |
3.5 |
4.1 |
2.1 |
2.5 |
prunings |
0.0 |
4.8 |
0.0 |
7.1 |
0.0 |
0.6 |
0.0 |
0.4 |
industrial waste woodc |
-119.0 |
-9.5 |
-136.9 |
-11.0 |
-8.1 |
-0.6 |
-7.2 |
-0.6 |
demolition woodc |
-119.0 |
-9.5 |
-136.9 |
-11.0 |
-8.1 |
-0.6 |
-7.2 |
-0.6 |
wood productsc |
-119.0 |
-9.5 |
-136.9 |
-11.0 |
-8.6 |
-0.7 |
-7.6 |
-0.6 |
Sludges |
|
|
|
|
|
|
|
|
waste water treatment |
-95.2 |
-38.1 |
-95.2 |
-38.1 |
-7.5 |
-3.0 |
-6.8 |
-2.7 |
maintenance of waterways |
-61.9 |
0.0 |
-99.0 |
0.0 |
-73.6 |
0.0 |
-14.1 |
0.0 |
Energy crops d |
35.7 |
47.6 |
53.6 |
71.4 |
4.7 |
6.2 |
2.8 |
3.8 |
a Cost range excluding transport to the conversion facility. Positive, and thus market values are derived from various studies, mentioned in table 1. Negative values (waste treatment costs) are taken from literature1,10 and by evaluating which treatment route (mainly incineration, landfilling and composting) is applied for each organic waste stream. Usually more than one route is applied causing a wider range in waste treatment costs. Cost figures are obtained from literature per wet ton. Cost figures per dry ton are calculated by correcting for the moisture contents given in table 2.
b
Costs per GJ are calculated by dividing the costs by the energy content per ton which was as well given in table 2.c
Cost range for all waste wood categories.d
Cost figures derived from Lysen et al.16Figure 3 shows supply curves of net available biomass streams to illustrate the distribution of costs relating to energy available in biomass streams. The upper curve represents the maximum costs at which waste streams could currently be obtained, the lower curve the minimum costs. In reality the costs of biomass wastes will be somewhere in between the two curves and show a more gradual progressing curve moving from one type of biomass to the other. However, such detailed data were not available.
Figure 3. Cost supply curves of the net available energetic potential of biomass wastes and residues in the Netherlands in PJ/year. The upper curve represents the maximum costs at which various streams could be available, the lower curve the minimum costs.
Negative costs may serve as a source of income to the conversion plant because it serves as a waste treatment facility when it converts such biomass materials into energy.
The costs mentioned generally do not involve pre-treatment (size reduction and drying) which is required for the gasifier coupled to a Combined Cycle and considered part of the conversion.
However, in case of some woody streams (prunings, fruit farming, waste wood) chipping is applied as part of the harvest and collection process of the material. When delivered in chipped form the price of the material will be in the upper part of the cost range presented in table 3.30
A large part of the sewage sludge produced at waste water treatment plants in the Netherlands is and will be dried with conventional rotary driers (to approximately 10% moisture content). Part of the total sludge supply is therefore available in dry form. Other options available for water removal are mechanical dewatering (by means of a sieveband press or filter press) which can reduce the moisture content down to 50%.35 Dried sludge still needs to be landfilled or combusted, thus although the volume is strongly reduced by water removal, the negative value per ton remains. In table 1 figures of the available dry sludge are presented.
3.3 Composition and degree of contamination
The composition of the biomass material has consequences for the capacity and nature of the gas cleaning, the composition of the ashes, the composition of the waste water stream from the scrubber and for the emissions from the entire system. The nitrogen, chlorine and sulphur content and contamination with heavy metals of a representative selection of streams are shown in figures 5 - 8. Those data are largely derived from characterisation experiments and partly from literature. A more detailed description of the composition and characterization experiments of various biomass wastes and residues is given in van Doorn6, and Mocking et al.17 Such extensive sets of data are not available for all the streams mentioned in table 1. However, the selected streams clearly show the range of various components for a wide range of fuels from sludge to clean wood. The most important components with regard to the legislation and standards on emissions to air are heavy metals, nitrogen (NOx), chlorine and sulphur.
Figure 4. Scheme of an integrated direct atmospheric gasification combined cycle system based on TPS-technology. After pre-treatment biomass is gasified. Tars are cracked in a secondary reactor. Further fuel gas cleaning involves cooling in order to condense alkalis which are together with dust removed by a baghouse filter. A wet scrubber removes mainly ammonia and some remaining contaminants. The gas is then compressed and combusted in a gas turbine. The hot flue gases generate steam in a Heat Recovery Steam Generator in order to drive a steam turbine.
More attention is given to the technical and economic consequences of these variations in the evaluation of the conversion system with various fuels in van Ree et al.26
Figure 4 shows a BIG/CC-system based on atmospheric direct gasification. Fuel enters the system in the pre-treatment section where the incoming fuel is reduced to the required particle size and dried to a moisture content of 15% which is considered acceptable for gasification. In the system the fuel gas is cleaned by various gas cleaning steps: tar cracking, cooling, baghouse filter for removal of particles and alkalis and a wet scrubber for the removal of mainly ammonia. The latter also removes the water present in the gas since it condenses during the scrubbing.
The degree of contamination differs widely per stream and between streams. This has consequences for the required capacity of the gas cleaning system, especially when a conversion unit is to be fuelled with various fuels. For some cases additional gas cleaning might be necessary in order to meet emission standards.
Nitrogen
A high nitrogen contents is found in verge grass and sludge (up to 7 weight pecent of dry matter, see figure 5). Miscanthus and Willow have the lowest nitrogen content (up to 2% for Miscanthus). Nitrogen in the biomass is mainly converted to ammonia which is removed in the scrubber. A higher nitrogen content will directly result in a higher ammonia concentration in the waste water stream. This can have some effect on the waste water treatment costs and also on the required capacity of the scrubber. It might be possible to add an acid to the scrubber water to improve ammonia removal.
Figure 5. Nitrogen content of a selection of biomass sources (presented in ranges). Tick marks present average values when available.
Chlorine
Chlorine content is especially high for verge grass and to a lesser extent in organic domestic waste (see figure 6). The chlorine concentration observed vary widely between samples. For verge grass the use of salt on roads to abate icing can be a reason for the high chlorine contents. A higher chlorine concentration can also be found close to the sea.
Chlorine can cause corrosion in the BIG/CC system because of the formation of HCl during gasification. In gasification experiments it is observed that the dolomite which is applied in the tar cracker will react with chlorine to CaCl2 and will lead to a higher dolomite consumption. The adsorption of HCl is considered to be comparable to the level of adsorption in waste incineration facilities and generally high; adsorption levels of more than 90% can be expected. HCl will also react with particulates which are removed later by the baghouse filter. Nearly all of the chlorine can be removed from the gas as a result of reaction with lime.15 Any HCl that remains will dissolve in the scrubber water.
Figure 6. Chlorine content of a selection of biomass sources (presented in ranges)
Sulphur
Generally speaking the sulphur content in biomass is (very) low. However, some streams, especially sludge but also organic domestic waste and verge grass, show a higher sulphur content than woody streams and energy crops (see figure 7). Sulphur can cause corrosion problems when sulphuric acid is formed in the heat recovery steam generator, although this can be solved by keeping the gas temperature sufficiently high. Sulphur (which is largely converted to H2S during gasification) will react to some extent with dolomite to form CaS. In Lassing et al.15 it is concluded that in the case of various fuels (sludge, verge grass, organic domestic waste) the H2S concentration can reach an equilibrium concentration in the fuel gas. This concentration is too high to meet emission standards for waste incineration.26 Additional sulphur removal may therefore be necessary in order to meet SO2 standards for flue gas emissions, especially for sludge. This can be done by adding a basic to the scrubber water (possibly with a two stage scrubber).
Figure 7. Sulphur content of a selection of biomass sources (presented in ranges) In case of organic domestic waste only one value is available and no range can be presented.
Heavy metals
Very wide ranges of heavy metal concentrations are found per biomass material and between biomass streams. Generally speaking sludge contains very high heavy metal concentrations although this depends largely on the source of the sludge. Levels are especially high for zinc, copper, chromium and nickel (see figure 8 and 9). Waste wood shows, again depending on the source and sample, relatively high concentrations of heavy metals too (e.g. for copper, lead and zinc, see figure 8 ). Some of the heavy metals will remain in the gasifier ash, but they will partly evaporate in the gasifier, particularly those with a low melting point (cadmium, mercury, lead). The extent to which heavy metals will end up in the fuel gas during gasification differs from combustion process conditions. The temperature in the fluidized bed of the gasifier is relatively high compared to combustion temperatures. Furthermore the atmosphere is reducing in contrast to the oxidizing conditions that prevail during combustion. This reducing atmosphere promotes the evaporation of heavy metals in metallic form.40
However, metals present in the gas stream, pass the tar cracker and are cooled to 140 oC in the gas cooler. Generally, the metals present will condense (like the alkali metals) on particulates and be removed by the baghouse filter. The distribution of heavy metals over the two ash fractions is related to the behaviour of the metals in the hot reducing atmosphere in the gasifier and has not been studied in detail. Emissions will however be very low due to the cooling and filter followed by wet scrubbing of the fuel gas.
Figure 8. Cu, Pb, Zn concentration ranges of a selection of biomass streams
Figure 9. As, Cd, Cr, Ni concentration ranges in a selection of biomass streams
Ash
The ash production in the gasifier and the baghouse filter differs per fuel. When the ash fraction of a fuel contains a great deal of sand (heavy particles) most of the ash is removed from the gasifier. When the ash content is low (such as for clean wood) more fly ash is produced. Also the dolomite consumption (e.g. by reaction with chloride and sulphur) varies per stream influencing the ash production.
The distribution of the total ash production can be 30% from the baghouse filter and 70% from the gasifier. In the case of materials with a low ash content and little or no sand content, the proportion of the total ash in the fly ash stream will be relatively large. It is assumed that the gas cleaning system proposed, which includes cooling to near ambient temperatures is sufficient to remove alkalis. Therefore limited attention has been given to alkalis in this paper. In van Doorn6 alkali contents (Na+K+Li) are reported ranging between approximately 1,000 mg/kg dry matter for clean wood up to 5,000 mg/kg dry matter for certain sludge samples.
3.4 Suitability of biomass residues and waste streams for gasification
Gasification is possible with a broad range of fuels with varying properties. Generally speaking, fuels with a higher moisture and ash content will give a fuel gas with a lower heating value. With the selected gasification technology the relations between the heating value of the fuel gas and moisture and ash contents are given in figures 10 and 11. The reason why a decrease in heating value occurs with a higher moisture content is that part of the heat released during partial combustion in the gasifier is needed to evaporate the water. This also implies that more air is fed to the gasifier whenever more water evaporates because more energy is needed for evaporation and consequently more fuel needs to be (partially) combusted. Both the evaporated water and additional air (nitrogen) dilute the gas which causes a decrease in the heating value of the gas.
Figure 10. LHV of fuel gas produced by a directly heated ACFB gasifier as a function of the moisture content (using data of poplar wood with an ash content of 1.3%). Wet gas represents the gas composition including water present in the biomass and evaporated in the gasifier and water formated in the gasification process itself by partial oxydation. The graph is compiled with help of a gasifier simulation model (courtesy of TPS).
Decreasing heating values are also observed when the biomass fuel has a high ash content. The ash of the incoming fuel will be heated to gasification temperature (800 - 900 oC). More ash implies that more fuel needs to be combusted to heat the inert material, thus lowering the heating value of the fuel gas.
Graphs 10 and 11 give the relation between the LHV of the fuel gas and moisture and ash content of the fuel respectively. They are composed by using a gasifier model.15 Curves for both wet and dry gas (in which the water is condensed out) are given. For the system described, the wet gas represents the gas stream leaving the tar cracker, the dry gas is the same gas on a dry basis. The moisture content of the gas entering the gas turbine will lie between these values. The exact value will depend on the degree of cooling and resulting condensation of water vapour which in turn will depend on the temperature in the scrubber.
Figures 10 and 11 are composed with composition data of poplar wood. The limits given are approximate and are valid for similar fuels.
Figure 11. LHV of the fuel gas produced by a directly heated ACFB gasifier as a function of the ash content (using data of poplar wood with a moisture content of 15%). Wet gas represents the gas composition including water present in the biomass and evaporated in the gasifier and water formated in the gasification process itself by partial oxydation. The graph is compiled with help of a gasifier simulation model (courtesy of TPS).
The fuel gas quality has to meet strict demands to be suited for gas turbine firing. The most important restriction is the heating value of the fuel gas which is required for stable combustion. A General Electric LM 2500 is the selected gas turbine (described in more detail in van Ree et al.26) in this research project
The reasons for selecting this turbine are: 1. it is under development for low calorific gas firing in connection to the World Bank GEF project in Brazil7; 2. the size of a BIG/CC unit based on the LM 2500 is approximately 30 MWe which is still a reasonable capacity from the point of view of the availability of fuel and transport distances; 3. this turbine, being an aeroderivative, combines a high efficiency with a high turbine outlet temperature, which is required to achieve a high overall combined cycle efficiency.).The GE LM 2500 gas turbine which uses low calorific gas has to satisfy the following conditions25:
1. The heating value of the gas for which stable operation of the turbine is guaranteed must be at least 5.6 MJ/Nm3
2. Acceptable variations in heating value of the fuel gas amount + 5% of the Wobbe index: (5.5 - 5.7 MJ/Nm3).
3. The flue gas must be largely free of particles and alkalis to prevent excessive wear and corrosion of the gas turbine blades (
Maximum allowable concentrations in the flue gas flow to the expander of the turbine are 4 ppbw (parts per billion weight) for alkalis (Na+K+Li), 600 ppbw for particulates <10 micron, 0.6 ppbw for 10 - 13 micron particulates and 0.6 ppbw for >13 micron particulates.26)Figure 11 shows that biomass streams with an ash content higher than 15 - 20% (moisture content of incoming fuel 15%) are not able to fulfill the first requirement. For direct gasification the moisture content of a given fuel needs to be lower than 20%. A higher moisture contents will result in heating values below gas turbine limits, as is shown in figure 10.
Several waste streams have a moisture content of 70% and above. When biomass fuels are dried with waste heat from the stack, there is sufficient heat to process fuels with a moisture content below 70 - 80%8 (
Assuming that energy from the flue gas is used for drying and that a conventional rotary dryer is involved (requiring approximately 3 GJ/tonH2O evaporated). Fuel gas temperature is approx. 250oC, the temperature of the gas leaving the stack is just over 100oC. Overall electrical efficiency: is 40% on LHV basis. More efficient drying systems such as fluid bed dryers and (indirect) steam dryers, can lower the heat demand and can cope with somewhat wetter fuels.). Waste streams with a higher moisture content require a different waste treatment process such as anaerobic digestion, or need to be mixed with drier material. This would be the case for several waste streams from the food and beverage industry, sludges, swill and wastes from the greenhouse sector and bulb cultivation. A fuel that combines a high moisture content with a high ash content is not suited for gasification unless the material is dried beforehand. Such fuel properties can be found in sludges.Besides the moisture and ash content, the elemental composition is a factor that affects the heating value, although the differences between various kinds of biomass are small.
Pre-treatment of streams such as grinding, densification and drying can ensure that the fuel has the required moisture content and physical properties but this will increase the costs. Mixing of fuels can also be seen as a pre-treatment option to achieve the required fuel properties.
4. DISCUSSION
4.1 Availability and supply
Many types of biomass waste and residues have been considered in this paper. Some of them are excluded for energy production. One criterion for exclusion is the market price of the waste stream. When the price is significantly higher than the projected costs for energy crops such streams are not considered to be available for energy production. However, it should be noted that market prices are sometimes subject to strong fluctuations. When there is a surplus of biomass streams with a relatively high value, fractions of such streams can become of interest for energy production. This can occur with some categories of wood and straw.
The wastes produced by the food and beverage industry form a complex category. To deal with this complexity, difference was made between very wet and dryer streams. Very wet streams, with a moisture content over 70 - 80% are not counted to the streams potentially available for thermochemical conversion. However, the diversity in materials is not reflected in the data presented here. Therefore, before application all streams should be studied and analysed separately, but this is beyond the scope of this study.
There is uncertainty about the availability of biomass materials that are derived from nature reserves such as turf from heath and reeds which are harvested for several reasons. All this material is presently used for other purposes such as fertilizer. However, they could represent a future potential.34
The periods mentioned in this study during which a number of biomass residues and wastes are released are not as strict as stated. In practice supply spreads over longer periods and also variations in harvest time occur from year to year. At the same time, however, those variations make a more exact presentation of supply not very useful.
The time dependence of the supply of biomass streams differs strongly per material. The supply of a number of types of biomass such as straw, remains of bulb production and the greenhouse sector, fruit farming, thinnings, verge grass is coupled to the seasonal production in agriculture or to maintenance activities. Most of this material is released from May to September. When such streams are used for conversion in a power plant in base load operation, substitutes must be found from other sources part of the year. Such back up fuel is desired anyway because the supply will differ in time and in quantity from year to year because of weather conditions. Storage is mainly suited for woody material, preferably dry, because otherwise decomposition will lead to significant losses of organic matter when stored over longer periods. The main conclusion is that for a number of streams year round operation of a conversion plant requires input of other fuels at least part of the year. The low supply in winter can be compensated by using energy crops as willow, which is harvested in wintertime, or by using waste wood streams which are available all the year round. Consequently the system should be capable of dealing with the varying properties of a diversity of other biomass material(s).
4.2 Costs
The costs of biomass streams are hard to determine since they partly depend on market conditions and an increase in demand for a given material can cause in increase in prices. The costs given in this paper represent the present situation in The Netherlands. The given ranges, which are often wide, illustrate the large variations that can occur in biomass costs. More insight is needed in the way these costs are influenced by changing and fluctuating demand, especially when these streams are to be converted into energy on a large scale.
The lower values of the costs as presented in figure 6 are the absolute minimum at which at present organic waste streams and residues are available in the Netherlands. These costs are based on the present costs for waste treatment and reflect the current situation in waste treatment in the Netherlands. Waste treatment costs may change substantially in the future.1 Both lower and higher costs are possible. Important in this context is the ban on landfilling organic materials which will come into force in the near future in the Netherlands. In fact this can push up the costs for waste treatment further (and thus lead to more negative biomass costs) because the present alternative is incineration which is considerably more expensive than landfilling.
One criterion selected to evaluate the potential of biomass wastes and residues, was that streams more expensive than the projected costs for energy farming are not available for energy purposes. When developments in energy farming result in a significant decrease in the costs per unit of energy, this can also influence (decrease) the amounts of wastes to be utilised for energy production. On the other hand if energy crops compete economically with streams like wood from thinnings the price of these residues might drop if the supply of energy crops is sufficiently large. A more detailed analysis of this subject is required.
4.3 Composition and contamination
The best way to present our data on the composition and degree of contamination of biomass wastes and residues is to express them in ranges and not as average values since the composition also depends on the time of year and location where the stream is produced (for example chlorine is likely to be present in higher concentrations in verge grass near the sea6). The combination of both literature sources and experiments gives insight in observed ranges of the composition of various biomass streams. It should however be kept in mind that it is not always clear from some sources what characterization procedures have been applied. Furthermore, the origin of the biomass samples for characterization may have strong influence on the compositions found due to the heterogeneity of specific streams. The ranges found in composition data should therefore merely be considered as indications of the possible variations instead of absolute values.
A given conversion plant will have to meet environmental standards irrespective of whether is uses clean or contaminated sources of biomass. The characterization results therefore provide important input for an environmental assessment of energy production from biomass and for the design and selection of the gas cleaning technology that is required to meet specific standards. Chlorine does not seem to be a problem in the gasification process considered. High sulphur and nitrogen contents might mean that there is a need for additional gas cleaning steps. Some wastes when utilised will produce ash streams that are substantially contaminated with heavy metals (sludge).
The ash melting temperature of biomass is not discussed in this paper although it can be an important restriction on its use in gasifiers. However, initial ash deformation temperatures for a wide range of possible biomass streams as inventorized in van Doorn6 and Lassing et al.15 indicate that most fuels have higher ash melt temperatures than the gasification temperature of 800 - 900 oC. More detailed testing may however be required to assess the behaviour of fuels with high ash contents at high gasification temperatures.
4.4 Gasification of waste streams
This paper focuses on direct (atmospheric) gasification in a BIG/CC unit. The limits obtained here for the moisture and ash content of the biomass in order to obtain fuel gas with a high enough heating value for the gas turbine do not apply to indirect or oxygen blown gasification processes because of the medium heating value fuel gas that is obtained from such processes.
The ash and the moisture content are the most important parameters that determine whether gasification and application of the produced fuel gas in a gas turbine is possible. The boundaries for acceptable moisture and ash content of the fuel entering the gasification system depend on one hand on the demands of the gas turbine and on the other hand on drying technology and heat recovery from the ash. The properties of the fuel gas have to satisfy fixed criteria to enable gas turbine firing. The drying has to be such that the fuel entering the gasifier does not exceed 15 - 20% because otherwise the heating value of the gas becomes too low to satisfy gas turbine requirements. Another factor lowering the heating value is the ash content. This may (partly) be prevented by recovery of heat from the ash, which can be achieved by a counter current fuel feeding system, but this optimization option is not investigated further here.
Adaptations to the gas turbine, especially the combustion chamber, can be made to achieve a higher tolerance to fuel gas with even lower heating values. Such modifications will in turn enable gasification of fuels with a higher ash content in BIG/CC systems.
In general the data presented on biomass waste streams and residues represent the current situation in the Netherlands. However, the technical potential, availability and costs of various streams are not static. Future developments can affect the production and availability of various streams. It is beyond the scope of this paper to make projections for the future availability of all the above-mentioned streams since this will depend on the economic activity in various sectors and changes in land use (e.g. the total land used for cereal production and forestry).
5. CONCLUSIONS
The current gross energetic potential of organic waste and residues in the Netherlands is substantial, namely about 190 PJ (HHV) per year. In practice this potential can only be used partially for energy purposes due to alternative applications such as raw material, fodder and fertiliser. However, even if these factors are taken into account, the energetic potential remains significant, namely about 90 PJ (HHV) per year.
Some streams are not available in winter months. Application of such streams in a baseload power plant requires compensation in wintertime. Peaks in the supply of specific types of biomass can be levelled either by storage (mainly suited for drier woody material) or by using other fuels during part of the year. Energy crops, usually harvested at the end of the year, could prove necessary in a situation where a large part of the potential biomass waste and residues is used for energy production, in order to compensate for decreasing supply of biomass wastes and residues in winter months.
An important criterion for selecting a fuel is its cost. The inventory for 1994 showed large differences in the costs per GJ of various organic wastes and residues. These varied from -10 ECU/GJ to above the projected cost levels of energy crops in the Netherlands (5 ECU/GJ).
The conversion system, particularly in relation to the gas cleaning, has to be flexible when a variety of fuels is used in view of the variations found in compositions. In this respect the removal of nitrogen and sulphur is important. Chlorine seems to cause less problems because it is removed along with the ash streams.
When used in a BIG/CC system for power production, fuels with a high ash content ( > 20%) have to be mixed with cleaner material. This is the case for sludges and possibly organic domestic waste. Very wet streams (> 70% m.c.) such as organic streams from the greenhouse and bulb cultivation sector, swill and food and beverage industry need to be mixed with drier materials. Drying is always required since no biomass stream has a moisture content lower than 15% from origin.
The results can serve as input for assessments of the technical, economic and environmental aspects of gasification of organic waste in the Netherlands. The presented methodology could be useful in specific regional studies designed to evaluate the feasibility of new biomass conversion units to be fired (partly) with various types of waste and residues.
Acknowledgements - The authors are grateful to CEC DG XII for the sponsoring this project within the framework of the EC JOULE II+ programme. Co-sponsoring was provided by the Noord-Holland gasification project and NUTEK. They thank Prof. Dr. W.C. Turkenburg for critical comments and suggestions. The authors are grateful to Sheila McNab for linguistic assistance.
6. REFERENCES
1. Afval Overleg Orgaan (AAO), Conceptual design ten year programme for waste 1995 - 2005, Utrecht, April 1995 (in Dutch).
2. Mr. Both, Composting facility Hoek van Holland, Personal communciation + written information on greenhouse sector waste, Hoek van Holland, May 1994.
3. Dijck, F., van, B. van Marle, E. Libourel, Thermal conversion of wood; pre-study, PNEM, PGEM, BFI, September 1993 (in Dutch).
4. Dielen, L.J.M., R. Sikkema, Residual and waste wood in the Netherlands, Stichting Bos en Hout, Wageningen 1992 (in Dutch).
5. Doorn, J. van, , Energy from Dutch biomass, paper presented at the 4th Dutch Solar Energy Conference 1-2 April 1993.
6. 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.
7. Elliott, P., R. Booth, Brazilian biomass power demonstration project, Special project brief, Shell, London, September 1993.
8. Faaij, A., K. Blok, E. Worrell, Gasification of wet biomass waste-streams for electricity production, prepared for the Provence of Noord-Holland, Report no. 92041, Department of Science Technology and Society, June 1992.
9. Jager, D. de, K. Blok, The contribution of waste and biomass to the energy supply in The Netherlands, inventory and potential, ECOFYS, Utrecht, November 1994 (in Dutch).
10. Jong, H.B.A. de, W.F. Koopmans, A. van der Knijff, Conversion techniques for organic domestic waste, developments in 1992, Haskoning, Nijmegen, February 1993 (in Dutch).
11. Mr. Joosse and Mr. Goedgeburen, Informatie en Kenniscentrum Fruitteelt, Personal communication, Wilhelminadorp, November 1994.
12. Knoef, H.A.M., M.E.T. Leenders, Environmentally sound treatment of demolition wood. The availability of demolition wood, Biomass Technology Group, Enschede, December 1991 (in Dutch).
13. Knoef, H.A.M., M.E.T. Leenders, Environmentally sound treatment of demolition wood. Technical, environmental and economic feasibility, Biomass Technology Group, Enschede, December 1991 (in Dutch).
14. Kortleve, C., R. Joosse, Waste wood in fruit farming, Informatie en Kenniscentrum Fruitteelt, Wilhelminadorp, March 1994 (in Dutch).
15. Lassing, K., E. Olsson, L. Waldheim, Laboratory Analyses and Tests and Gasification calculations, Study performed within the framework of the extended JOULE-IIA programme of CEC DGXII project "Energy from biomass: an assessment of two promising systems for energy production" Termiska Processer AB, Nyköping, Sweden, April 1995.
16. 27. Lysen, E.H., C. Daey Ouwens, M.J.G. van Onna, K. Blok, P.A. Okken, J. Goudriaan, The feasibility of biomass production for the Netherlands energy economy, Rapport nr. 71.140/0130, Nederlandse maatschappij voor energie en milieu (NOVEM), Utrecht, 1992.
17. 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)
18. Nederlandse Bond voor Boomkwekers, Environmental aspects of combustion, chipping and composting of wood residues for tree cultivation, November 1993. (in Dutch)
19. Report on the NOVEM workshop 'Energy from straw and verge grass, application in the Netherlands, Utrecht, The Netherlands, 23 November 1993 (in Dutch).
20. Report on the NOVEM contractors' meeting 'Availability of biomass for Energy', Utrecht the Netherlands, 3 November 1994 (in Dutch).
21. Okken, P.A., H.J.A. van den Akker, J.M. Bais, J. van Doorn, A.D. Kant, Wood stoves in The Netherlands, contribution to the energy system and environmental pollution, Netherlands Energy Research Foundation, Petten, May 1992 (in Dutch).
22. Onna, M.J.G. van, Economic potential for application of compost and sludge in agriculture, Part A Agricultural Economic Institute (LEI), The Hague, May 1989 (in Dutch).
23. Onna, M.J.G. van, Economic potential for application of compost and sludge in agriculture, Part B Agricultural Economic Institute (LEI), The Hague, May 1989 (in Dutch).
24. Oranjewoud B.V. Inventory of treatment options for verge grass, study for Ministerie van Verkeer en Waterstaat, Dienst Weg- en Waterbouwkunde, September 1992. (in Dutch)
25. Prins, B., and H. Steenhuis, Thomassen Stewart & Stevenson International (TSSI), Personal communication with respect to the LM2500 gas turbine, Rheden, The Netherlands, October 1994.
26. Ree, R. van, A.B.J. Oudhuis, A. Faaij, A. Curvers, Modelling of a biomass integrated gasifier/combined cycle (BIG/CC) system with the flowsheet simulation programme ASPENplus, Study performed within the framework of the extended JOULE-IIA programme of CEC DGXII project "Energy from biomass: an assessment of two promising systems for energy production", Netherlands Energy Research Foundation, Department of Science, Technology and Society, Utrecht University, reportno. ECN-CX-94-057, Petten, May 1995.
27. Renia, H.M., R. Sikkema, Wood residues in the Netherlands, A study on the quantity, composition, origin, and destination of wood residues, wood waste, demolition wood and not harvested wood, Stichting Bos en Hout, Wageningen 1991 (in Dutch).
28. Siemons, R.V., Thermal conversion options for straw and verge grass, Biomass Technology Group, Enschede, August 1991 (in Dutch).
29. Siemons, G., W. Snijder, Potential of compostable company waste in The Netherlands, Ministry of housing, physical planning and environment, The Hague, June 1992 (in Dutch).
30. Sikkema, R., Power from trees, potential availability of round wood, wood residues and waste wood for energy production, NOVEM/EWAB, Stichting Bos en Hout, Wageningen 1993 (in Dutch).
31. Sikkema, R., Power from trees, potential of round wood, residues and waste wood for energy production, Stichting Bos en Hout, Wageningen, 1993 (in Dutch).
32. Sipkens, J., Spars as an energy carrier, NOVEM/EWAB, Stichting Bos en Hout, Wageningen, January 1994 (in Dutch).
33. Sipkens, J., The availability of wood from the forest, feasibility study for N.V. PEN, Stichting Bos en Hout, Wageningen, October 1994 (in Dutch).
34. Steetskamp, I., A. Faaij, A. van Wijk, Space for biomass, An exploratory study on the space for energy farming in The Netherlands Department of Science, Technology and Society, Utrecht University, NOVEM/EWAB, Utrecht, December 1994 (in Dutch).
35. Stichting Postacademisch Onderwijs Gezondheidstechniek en Milieutechnologie, Manual Sludge Treatment, Delft, March 1992 (in Dutch).
36. Stoop, J.M., Waste streams from bulb cultivation, Centrum voor Landbouw en Milieu, Utrecht, September 1992 (in Dutch).
37. Tagungsband: Thermische Nutzung von biomasse - Technik, Probleme und Lösungsansätze - Stuttgart 14./14.04.1994, Schriftenreihe "Nachwachsende Rohstoffe" Band 2, bundesministerium für Ernährung, Landwirtschaft und Forsten, Fachagentur Nachwachsende Rohstoffe e.V.
38. Waste paper as an energy source? Study of Verdonk, Otten, Dik & Wiegerink for the Province of Noord-Holland, April 1994. (in Dutch)
39. Zaal, H.J., A. Faaij, I. 't Hart, Organic domestic waste in the sink? Wetenschapswinkel Biologie, Universiteit Utrecht, Augustus 1994 (in Dutch).
40. Obernberger, I., TU Graz, Institut für Verfahrenstechnik, Personal communication with respect to behaviour of heavy metals during combustion and gasification, Graz, Austria, January 1995.
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