Refuse Derived Fuels - solid recovered fuels for the cement industry

Refuse Derived Fuels (RDF) as substitute fuel in the clinker process of cement plants have become state of the art in the EU. State of the art is described in the Best Available Techniques Reference Document (BREFs) published by the European Commission.

by Univ.-Prof. Dipl.-Ing. Dr. mont. Roland Pomberger, em.O.Univ.-Prof. Dipl.-Ing. Dr. techn. Karl E. Lorber

Definitions: RDF and SRF

In Austria, the definition of "waste fuels” or "Refuse Derived Fuels” (RDF) is given in the legally binding "Waste Incineration Ordinance” as: "...waste that is used entirely or to a relevant extent for the purpose of energy generation and which satisfies the quality criteria laid down in this directive...”.

Therefore, after adequate and extensive (pre-)treatment in different processing plants and applying strictly defined quality assurance measures, various non-hazardous and/or hazardous waste materials from households, commerce and industry can be used as RDF in co-incineration plants (i.e. cement industry etc.): sewage sludge, waste wood, high calorific fractions from mechanical-physical (MPT) or mechanical-biological treatment (MBT) plants, calorific fractions of household and commercial wastes, shredder light fractions (e.g., from old vehicles and waste electric and electronic equipment (WEEE)), scrap tyres, waste oil and used solvents, etc.

In the narrow sense of the definition, only solid waste fuels which are prepared from non-hazardous sorted or mixed solid wastes only (i.e. municipal waste fractions, commercial wastes, production wastes, construction and demolition waste, packaging wastes, lightweight fractions from MBT-plants, etc.) including quality assurance measures and used for the purpose of energy generation are defined as "Solid Recovered Fuels” (SRF).

As shown, in Austria only, but not elsewhere in the EU, the quality assurance measures and limit values for the delivered (unburnt) waste fuels are legally defined and have to be applied for all types of RDF (i.e. solid and liquid) when used in co-incineration plants, whether produced from non-hazardous and/or hazardous waste materials.

As mentioned before, SRF is a subgroup of RDF and means, as defined in CEN (TC 343 standards), "…only solid fuel prepared from non-hazardous waste… used for energy generation at the highest possible energy efficiency…”

The output RDF and SRF are classified by two waste codes in accordance with the European List of Waste:
  • 19_12_10: (quality assured) combustible waste; i.e. SRF (see Figure 1.),
  • 19_12_12: other wastes (including mixtures of materials) from mechanical treatment of wastes…; i.e. RDF.
 
Figure 1. SRF for the Cement Industry (left: fine SRF for Main Burner; right: coarse SRF for Precalciner)
 

Substitution of fossil feedstock in the cement industry

The specific heat consumption in Austrian cement industry is reported with 3,749 MJ/t clinker (Data for 2013; EU 28: 3,770 MJ/t clinker). Most important conventional fossil fuels, used in the clinker process, are:
  • Lignite and Hard Coal, Lower Heating Value (LHV): 29,400 MJ/tOS,
  • Petrol Coke, LHV: 30,760 MJ/tOS,
  • Heating Oil, LHV: 41,700 MJ/tOS, etc.
Additionally, increasing amounts of RDF are utilized, like:
  • Scrap Tyres, LHV: 28,630 MJ/tOS,
  • Animal Fat & Bone Meal, LHV: 18,000 MJ/tOS,
  • Used Solvents, LHV: 22,480 MJ/tOS,
  • Waste Oil, LHV: 34,760 MJ/tOS,
  • SRF (main burner fuel), LHV: 19,160 MJ/tOS (range: 18,000-23,000 MJ/kgOS).
In total, about 9,630,000 t/year of RDF was used in European (EU 28) cement industry, corresponding to a Thermal Substitution Rate of about 37% in 2012. But Austrian cement industry has reached a Thermal Substitution Rate of about 72% in 2013. Even 100% thermal substitution of fossil fuel seems to be basically (technically) possible, when an optimized feedstock mix (e.g.: animal fat & bone meal, used solvents, SRF etc.) is fed at kiln outlet side (main burner) and scrap tyres are fed at kiln inlet side (secondary burner). Figure 2 displays the Thermal Substitution Rate of fossil fuels (e.g., coal, etc.) by use of RDF in the cement industry for selected countries, in the European Union (EU) 28 (with expected increase of 3-5% per year) and worldwide (with expected increase of 1-2% per year) in 2010.
 
 
Figure 2. Thermal Substitution Rate (%) of fossil Fuels by use of RDF in Cement Industry in selected Countries, EU 28 and Worldwide
 

Utilization of RDF in cement industry

Selected process conditions for the use of waste fuels in the clinker process can be summarised (acc. to EC – BREF – Production of Cement…) as follows:
  • Maximum gas temperature of ca. 2,000 °C (main burner, flame temperature) in rotary kilns,
  • Gas retention time of about 8 seconds at temperatures above 1,200 °C in rotary kilns,
  • Gas retention time in the secondary firing system of more than 2 seconds at temperatures of above 850 °C.
There is a technical reason and a legal restriction that restrains the location of the fuel feeding ports in the kiln system:
  • According to the Directive 2010/75/EU on industrial emissions (EC) as well as to the national regulations, the waste fuel has to be burnt at a temperature of >850°C with a minimum retention time of 2 seconds,
  • The raw material is heated gradually from its temperature of 80°C after grinding to ca. 1,000°C at the kiln inlet up to the sintering temperature of 1,450°C in the burning zone of the rotary kiln.
As shown in Figure 3, different feeding ports can be used to charge fuels into the kiln. The potential feeding ports in a cement production plant are:

(1) Via the main burner at the rotary kiln outlet end: SRF: particle size (d95) < 35 mm, LHV: > 18 MJ/kgOS,
(2) Via secondary burners to the riser duct at the kiln inlet: RDF - scrap tyres,
(3) Via precalciner burners to the precalciner: SRF: d95 < 80 mm, LHV: 12-18 MJ/kgOS,
(4) Via a feed chute to the precalciner (for lump fuel): SRF: d95 < 300 mm, LHV: 12-18 MJ/kgOS.
 
 
 Figure 3. Potential Feeding Ports for RDF into the Cement Kiln
 
It has to be noted that the way of feeding fuel into the kiln is very crucial, as this can have a direct effect on the resulting emissions. The safest way to do this is such that the combustion gases from the fuel pass the highest temperature zone (up to 2,000 °C) of the kiln, namely by feeding them through the main burner (EC – BREF – Production of Cement…).
 
References:
  • Pomberger, R. & Sarc, R. 2014: Use of Solid Recovered Fuels in the Cement Industry; Waste Management, Volume 4., pp. 471-488
  • Sarc, R. et al. 2014: Design, quality, and quality assurance of solid recovered fuels for the substitution of fossil feedstock in the cement industry; Waste Management & Research 2014, Vol. 32, pp. 565-585
 

published: , 12|1990
Univ.-Prof. DI Dr. mont. Roland Pomberger
Montanuniversity Leoben
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