Waste Fuels


Traditionally landfill has been extensively used for the disposal of many wastes. In more recent years things have changed and government regulations are encouraging the re-use of wastes that are now considered a valuable commodity. Typically wastes are treated in a Materials Recycling Facility (MRF) where materials that can be readily recycled, e.g. metals and plastics are separated for re-use and the residual waste can then be used for Refuse Derived Fuel (RDF) or Secondary Recovered Fuel (SRF). RDF or SRF can be used as a feedstock for Energy from Waste plants incorporating a variety of technologies to recover energy and generate electricity. These technologies include incineration, gasification and pyrolysis.


SRF


SRF is increasingly being used as a feedstock for cement manufacture as a replacement for conventional fossil fuels to give environmental and cost benefits.

In order to assess the suitability of a waste material as a fuel, testing is required for a number of key parameters. MSSL is able to undertake this testing for a wide range of materials.

The basic tests routinely undertaken for waste fuels are the Proximate and Ultimate Analysis and Calorific Value. Other useful measurements include Heavy Metals, Halides and Biomass Content.

Other tests can include Bulk Density, Particle Size and Distribution and Waste Characterisation by Manual Sorting.



Representative Sampling and Preparation


In order to obtain accurate representative data, selection of the correct sampling method is extremely important. There are a number of factors that need to be considered including frequency of sampling, mode of operation of a plant and volume of materials involved and MSSL can provide advice on the most appropriate method with reference to available reference methods.

One key factor that needs to be taken into account is particle size as waste materials can contain a wide range of different size fractions.

Once the sample size has been selected and the sample has arrived in the laboratory the first stage is to separate any ungrindable materials such as metal cans, large stones etc that may damage the mechanical shredding equipment used for the next stage. Ungrindables are separated manually; weighed, and this data used to correct final analysis data to an "as-received" basis where required. The sample is then dried and milled to provide a sample with a particle size of a few millimetres. Sub-samples of the milled sample are then taken for analysis.

Employing the above type of approach coupled with a carefully designed sampling protocol minimises any risk of an unrepresentative sample being submitted for final analysis.




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Retsch Cutting Mill for SRF particle size reduction




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Determination of Moisture

An important test for waste materials is the moisture content which is determined by oven drying to constant weight with results being reported as % by weight. The moisture content is particularly important as it has a diluent effect on all the other parameters and can also lead to process problems for users of waste as a fuel. It is common for waste fuel suppliers to be penalised if the moisture content of a material is too high.




Determination of Calorific Value


The calorific value (CV) of a fuel is the heat available from that fuel when it is completely burned, expressed as heat units per unit of fuel weight or volume.

The gross, or higher, value is determined in the laboratory using a calorimeter. It can be defined as the total heat liberated by the complete combustion of the fuel. It is determined by measuring the heat removed when cooling the products of combustion to a standard reference temperature, and it includes latent heat recovered from condensation of the water vapour component. This water vapour forms as a result of the combustion of any hydrogen molecules contained within the fuel, and the vaporisation of any moisture present.

The net, or lower, value is determined by calculation and equals the gross calorific value minus the latent heat of the water vapour formed from the combustion of hydrogen and from any moisture present in the fuel.

The net value is more representative of the heat available in practice when fuels are burned in equipment such as furnaces and boilers. The latent heat of the water vapour contained in exhaust gases is not normally recoverable, except where low-temperature heat recovery involving condensation is used.



The Bomb Calorimeter for determination of Gross CV. Net CV is determined by calculation from this value and using other data including the hydrogen content.





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Calorific Values of Common Materials


Listed to the left are the calorific values (Gross CV) of some common materials. A material can burn without supporting fuel when it has a calorific value of minimum 14400 KJ/Kg which is approximately dry wood. To know the overall calorific value of waste you need to measure the calorific value or estimate by analyzing the composition. If there is no chemical reaction by mixing different materials together, a weighted average of the different calorific values is a good approximate value for the CV of the waste mix.


Determination of Halides and Sulphur


In order to measure the concentration of halides (chlorine, bromine, fluorine and iodine) and sulphur in the sample the liquid residue produced from bomb digestion above is analysed. The analysis technique utilised by MSSL is Ion Chromatography. Results produced in mg/litre are then converted to % w/w in the fuel sample.




Ion Chromatograph

Dionex Ion Chromatograph for measurement of Halides


Exeter Analytical CE-440 Analyser

Exeter Analytical CE-440 Elemental Analyser

Elemental Analysis of Carbon, Hydrogen and Nitrogen Content of a Fuel


In order to determine the carbon, hydrogen and nitrogen contents of a fuel a milled sample is prepared and analysed using an elemental analyser. The analyser combusts the sample and the resultant gases are then separated and determined quantitatively. Results are reported as % w/w C, H and N.


Determination of Ash and Heavy Metals Content


The inert inorganic residue remaining after the waste/SRF is combusted is the ash content. The ash is mainly comprised of metallic compounds.

The ash content is measured in the laboratory by high temperature combustion of a weighed portion of waste/SRF and then calculating the % ash from the weight of the remaining residue.

The content of heavy metals is determined by microwave acid digestion of a measured volume of milled sample and then the resultant solution is analysed by spectrometry.




Spectrometer

Analyser for measurement of selected metals


Biomass

Measurement of the Biomass Content


A waste/fuel can be subdivided into 3 fractions

Biomass Fraction: includes materials such as wood , paper, cardboard and vegetable matter

Non-Biomass Fraction: includes fossil fuel derived materials, mainly plastics

Inert Fraction: this will be mainly metal compounds, minerals etc

The biomass content is measured in the MSSL laboratory by selective dissolution of a weighed portion of prepared sample.