5. RDF/SRF QUALITY MANAGEMENT INITIATIVES
5.4. SRF product quality standards for specific end-uses
5.4.1. Specifications for end uses vs. classification
2002
Class codes are a tool for identifying and pre-selecting SRF by giving an immediate, but
2003
inevitably simplifying, image of the SRF quality. However, class codes cannot predict the
2004
actual performance of SRF when used (see TABLE 2) for a list of possible RDF/SRF uses).
2005
Definition of specific SRF properties and value ranges, thresholds and limits most relevant to
2006
each SRF utilisation plant in accordance with the particular technical characteristics, and
2007
legal demands of each thermal recovery process, is imperative for its marketability36.
2008
In order to appropriately characterise SRF, physical-mechanical, chemical and biological
2009
descriptors should be identified. Ultimate and proximate analyses are the minimum
2010
prerequisite to assess the thermal recovery behaviour and performance of a fuel197.
2011
Specifying SRF according to the CEN guidance demands a general list of obligatory and
2012
voluntary descriptors to be quantified. Properties should be measured according to
2013
appropriate, existing, or under development, CEN standard methods190. However,
Thomé-2014
Kozmiensky133and Beckmann et al.36stressed that effective use in varying applications
2015
demands the determination of a more complete list of properties (TABLE 14).
2016
2017
<<Table 14>>
2018
2019
For example, characterisation of the reaction-related properties is critical, especially for
2020
co-combustion applications. For instance, Hilber et al.198have recently developed a method
2021
for assessing the process-specific combustion behaviour of low in char-formation RDF/SRF:
2022
the de-volatilisation of SRF at specific temperatures is measured by multi-sample
thermo-2023
gravimetric analysis (TGA). In the case of biofuel QA/QC, which has similarities with
2024
WDFs, the significance and interrelationships of important physical-mechanical fuel
2025
properties have been investigated by Hartmann199; and the chemical properties reviewed by
2026
Obernberger et al.200. Eckardt and Albers46investigated the current use of specification
2027
properties and limits proposed by plant operators in various thermal recovery applications of
2028
SRF.
2029
However, even within each specific category of RDF/SRF end-uses, it can be
2030
challenging to agree upon defensible specifications that are applicable to every end-use. A
2031
wealth of available expertise has been incorporated in the relevant CEN report196. Despite
2032
that it might still be evident that there is limited understanding of RDF/SRF behaviour within
2033
the various possible thermal recovery systems, resulting in the absence of robust technical
2034
and environmental criteria for their use as substitute fuel46. Furthermore, generalisation on
2035
fuel combustion behaviour is not advisable, and plant-specific investigations are preferable,
2036
because, for instance, transfer factors for elements of concern are highly process and
2037
operation mode-specific36, 196, 198
. In addition, it is usual practice for each plant to prepare its
2038
own unique blend of substitute and raw fuels, leading to varying, case-specific contract
2039
specifications5, 46.
2040
In co-combustion of RDF/SRF with fossil fuels (and other WDFs), the actual degree
2041
of substitution varies, depending on the comparable quality of the RDF/SRF with the rest of
2042
the fuels, along with any related legal stipulations. Substitution of the original fuels by
2043
RDF/SRF depends on compatibility of the RDF/SRF properties with the thermal recovery
2044
process, typically designed for fossil fuels. For example, pulverised hard coal-fired plants
2045
with wet bottom boiler types (i.e., with molten slag with cyclones) (WBB) are more tolerant
2046
to the shape and dimensions of SRF, in comparison to plants with dry bottom boilers
2047
(DBB)196. It has been estimated that coal-fired plants may reach up to 20%w/w. substitution
2048
in the long run190; for cement kilns the percentage may vary between 50-100%w/w.
2049
Dedicated fluidised bed combustion (FBC) and gasification/pyrolysis plants are not
2050
constrained by such limitations. However, Beckmann and Thomé-Kozmiensky45stressed
2051
that substitution rates as low as 1%w/w. have been established for various thermal SRF
2052
recovery applications in the German state of North Rhine Westphalia. Even these low
2053
substitution rates have to be proven in future practice and for higher rates process-specific
2054
limit values should be convincingly defined for reaction kinetic properties. For
2055
confidentiality reasons, contract-based specifications do not often fully reach the public
2056
domain. This constrains the development of a wider consensus on what constitutes accepted,
2057
fit-for-purpose RDF/SRF quality.
2058
Nevertheless, it has been argued that maximum acceptable concentrations of trace
2059
elements of concern in SRF may be used to indicate its environmental suitability for a certain
2060
end-user190. Maximum values exist in national legislation regarding blending of wastes with
2061
fossil fuels. They usually apply to the most volatile elements, namely Hg and Cd or Cd+Tl.
2062
Standards also apply to the “sum of other heavy metals.”33. An indicative list of SRF
2063
environmental classes that could be accepted for certain technologies, based on conservative
2064
assumptions for trace elements is presented in TABLE 15.
2065
2066
<<Table 15>>
2067
2068
van Tubergen et al.190calculated estimations for the value ranges of SRF class-coding
2069
properties that could be accepted for different end-uses. For comparison, Eckardt and
2070
Albers46provided data on Cd, Hg and Tl limits specified for SRF by certain thermal recovery
2071
commercial references in Germany.
2072
The most important descriptors and acceptance values/classes for the main SRF
end-2073
users, focusing on potential properties of concern, are discussed below. Beckmann and
2074
Thomé-Kozmiensky36have detailed the experience in Germany. SRF particle form, size and
2075
shape exemplify the differences in the end-user specifications and NVC are discussed
2076
separately.
2077