1 10 100 1000 Particle size, d (mm)

FIGURE 17. Particle size distribution by sieve analysis of Herhof SRF (Stabilat). Fractions with particle size lower than 60 mm exhibit high biomass content. Redrawn from Wengenroth.¹⁶⁰

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co-deco SRF

SRF from MBT plants that use the Italian Eco-deco biodrying process is produced in Italy and the UK. In the London Frog Island plant the 39% w/w of the input waste is processed into an SRF with NCV of ca. 16 KJ kg^-1 (June 2006 data); higher values up to ca 18.5 KJ kg^-1 have been reported for the Italian plants.²²¹ SRF is used as a co-fuel in cement kilns and in an onsite fluidized bed boiler. SRF specification results from two Italian plants indicate that it complies with the standard class quality of the Italian UNI 9903-3 quality control system.⁵ In the UK the SRF outputs have to meet

FIGURE 18. Composition of Herhof SRF (Stabilat). Data from Herhof Environmental

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FIGURE 19. Energy-based concentrations of trace elements in RDF produced in early MBT plants, presented on logarithmic scale. Comparison with the 1998 limit guidance concentra­tions of the Swiss Agency for the Environment, Forrest and Landscape (BUWAL). A Q_p,_net of 18 MJ kg^-1 has been assumed to convert values from mass basis to energy basis. Redrawn from Rotter et al.⁴⁹

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pecifications agreed to by the specific cement industry end-users. Ultimate analysis including measurements made at various points during 2003 at the Montanaso plant showed mean concentrations of carbon (C) 42.22% w/w, hydrogen (H) 6.06% w/w, and nitrogen (N) 0.83% w/w (basis of reporting, d or ar, not denoted).

Content of Trace Elements of Concern in SRF

The quality of MBT-derived SRF has not been extensively evaluated. Rotter et al.⁴⁹ compared the content of trace elements of concern in SRF from early MBT plants against the limit concentrations of the Swiss Agency for the Environment, Forest and Landscape (BUWAL) standard for cement kilns (Figure 19).⁴⁹

Almost all the median values of the examined elements exceed the guide limits, apart from rare elements such as beryllium (Be) and vanadium (V). However, if this SRF was compared with other, less strict standards, it would perform much better. For example, according to the CEN proposed standards, Hg class code, based merely on median value (80th percentile was not provided), would be Class 2 (0.02 < 0.03 mg MJ^-1 ar). According to a CEN published document, a cement kiln may be willing to accept waste up to Hg class code 4.¹⁸⁴ Careful use of standard is necessary in order to

evaluate the suitability of SRF for each specific intended use, because every set of limit values serves different purposes and reflects varying underlying realties. For example, the BUWAL standard is based on strict considerations regarding cement production material flows. Conversely, the CEN standard classification is general and indicative and has incorporated considerations of the achievable SRF quality into the proposed limits.

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omparison of Fossil fuel with Substituted RDF/SRF

Heilmann and Bilitewski¹⁶³ compared coal with RDF produced from German MBT-treated residual waste. Gendebien et al.³² compared the toxic load of SRF produced by biodrying in Germany with other primary and substitute fuels. Heering et al.¹⁵⁶ showed that the per energy unit generated heavy metal concentrations of the Herhof dry stabilate and the range of values variation are of the same order of magnitude with certain fossil fuels it substitutes. Stabilat compares favorably in terms of content in trace elements of concern with coal. Herhof declared an average NCV of 15 MJ kg^-1 for the Stabilat, rendering it comparable to the CV of dried and processed lignite.¹⁶⁰ A comparison of the potential contribution to global warming emissions between a commerically produced SRF and various types of fossil fuels is available (Table 17).

Statistical Analysis of Available MBT-Derived SRF Data Series Input Data Quality and SRF Statistical Analysis Limitations

In this section, statistical analysis of a heterogeneous compilation of existing data on MBT-derived RDF/SRF, derived under diverging boundary measure­ment conditions, is presented. Statistical analysis is challenging for many reasons, including paucity of data and different statistics reported in the lit­erature data (e.g., mean or median to estimate location). Table 18 summarizes background information on data sources. Available data are mainly found in reports,^5,32,183,213 along with a few peer-reviewed publications.^156,203,222 These data series are mostly derived from quality assurance systems inter­nally implemented by MBT plant operators to satisfy end-user contractual requirements and/or to demonstrate compliance with national standards. The RDF/SRF has been produced from varying input materials (different countries/regions and residual or mixed waste collection schemes), treated in MBT plants with different design and operational configurations (but pre­dominantly biodrying), prepared from different fractions of the input waste (e.g., partially including or excluding the biomass fraction), and prepared to different national standards and end-user requirements (mainly cement kilns but also dedicated FBC and power plants).

Different objectives and methodologies applied alongside the entire measurement process, including sampling plans, subsampling and sample preparation, analytical determination techniques, and dissimilar statistical

TABLE 18. Background information on data series used for the statistical evaluation of the MBT-derived European RDF/SRF