Refuse-derived fuel

In the fifties of the last century tyres were used for the first time as refuse derived fuel in the cement industry. Continuous use of various waste-derived alternative fuels then followed in the mid-eighties with “Brennstoff aus Müll“ (BRAM) – fuel from waste – in the Westphalian cement industry in Germany.

At that time the thought of cost reduction through replacement of fossil fuels was the priority as considerable competition pressure weighed down on the industry. Since the eighties the German Cement Works Association (Verein Deutscher Zementwerke e.V. (VDZ, Düsseldorf)) has been documenting the use of alternative fuels in the federal German cement industry. In 1987 less than 5% of fossil fuels were replaced by refuse derived fuels, in 2015 its use increased to almost 62%.

Refuse-derived fuels are used in a wide range of specialized waste to energy facilities, which are using processed refuse-derived fuels with lower calorific values of 8-14MJ/kg in grain sizes of up to 500 mm to produce electricity and thermal energy (heat/steam) for district heating systems or industrial uses.

Materials such as glass and metals are removed during the treatment processing since they are non-combustible. The metal is removed using a magnet and the glass using mechanical screening. After that, an air knife is used to separate the light materials from the heavy ones. The light materials have higher calorific value and they create the final RDF. The heavy materials will usually continue to a landfill. The residual material can be sold in its processed form (depending on the process treatment) as a plain mixture or it may be compressed into pellet fuel, bricks or logs and used for other purposes either stand-alone or in a recursive recycling process.[4] RDF or SRF is the combustible sub-fraction of municipal solid waste and other similar solid waste, produced using a mix of mechanical and/or biological treatment methods such as biodrying.[5] in mechanical-biological treatment (MBT) plants.[3] During the production of RDF / SRF in MBT plants there are solid loses of otherwise combustible material,[6] which generates a debate whether the production and use of RDF / SRF is resource efficient or not over traditional one-step combustion of residual MSW in incineration (Energy from waste) plants.[7]

The production of RDF may involve the following steps:

• Bag splitting/Shredding
• Size screening
• Magnetic separation
• Air classifier (density separation)
• Coarse shredding
• Refining separation by infrared separation

RDF can be used in a variety of ways to produce electricity or as a replacement of fossil fuels. It can be used alongside traditional sources of fuel in coal power plants. In Europe RDF can be used in the cement kiln industry, where strict air pollution control standards of the Waste Incineration Directive apply. The main limiting factor for RDF / SRF use in cement kilns is its total chlorine (Cl) content, with mean Cl content in average commercially manufactured SRF being at 0.76 w/w on a dry basis (± 0.14% w/wd, 95% confidence).[8] RDF can also be fed into plasma arc gasification modules & pyrolysis plants. Where the RDF is capable of being combusted cleanly or in compliance with the Kyoto Protocol, RDF can provide a funding source where unused carbon credits are sold on the open market via a carbon exchange. However, the use of municipal waste contracts and the bankability of these solutions is still a relatively new concept, thus RDF's financial advantage may be debatable. The European market for the production of RDF have been grown fast due to e.g. the European landfill directive, or landfill taxes e.g. in UK and Ireland. Refuse derived fuel (RDF) exports from the UK to Europe and beyond are expected to have reached 3.3 million tonnes in 2015, representing a near-500,000 tonnes increase on the previous year.

The biomass fraction of RDF and SRF has a monetary value under multiple greenhouse gas protocols, such as the European Union Emissions Trading Scheme and the Renewable Obligation Certificate program in the United Kingdom. Biomass is considered to be carbon-neutral since the CO
2 liberated from the combustion of biomass is recycled in plants. The combusted biomass fraction of RDF/SRF is used by stationary combustion operators to reduce their overall reported CO
2 emissions.

Several methods have been developed by the European CEN 343 working group to determine the biomass fraction of RDF/SRF. The initial two methods developed (CEN/TS 15440) were the manual sorting method and the selective dissolution method; a comparative assessment of these two methods is available.[9] An alternative, but more expensive method was developed using the principles of radiocarbon dating. A technical review (CEN/TR 15591:2007) outlining the carbon-14 method was published in 2007, and a technical standard of the carbon dating method (CEN/TS 15747:2008) was published in 2008.[10] In the United States, there is already an equivalent carbon-14 method under the standard method ASTM D6866.

Although carbon-14 dating can determine the biomass fraction of RDF/SRF, it cannot determine directly the biomass calorific value. Determining the calorific value is important for green certificate programs such as the Renewable Obligation Certificate program. These programs award certificates based on the energy produced from biomass. Several research papers, including the one commissioned by the Renewable Energy Association in the UK, have been published that demonstrate how the carbon-14 result can be used to calculate the biomass calorific value.

There are major challenges related to the quality assurance and especially the accurate determination of the RDF / SRF thermal recovery (combustion) properties, due to their inherently variable (heterogeneous) composition. Recent advances enable optimal sub-sampling schemes[11] to arrive from the SRF / SRF sample of say 1 kg to the g or mg to be tested in the analytical devices such as the bomb calorimetry or TGA. With such solutions representative sub-sampling can be secured, but less so for the chlorine content.[12] The new evidence suggests that the theory of sampling (ToS) may be overestimating the processing effort needed, to obtain a representative sub-sample.

• Biodrying
• Cement kiln
• Isle of Wight gasification facility
• Mechanical biological treatment
• Mechanical heat treatment
• Waste-to-energy