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The need to distinguish between the biogenic and fossil-based fractions of carbon dioxide emissions became apparent at the onset of CO2 emission trading not only in Europe but also in North America and other parts of the world. Governments and industries need to comply with regulations capping their annual CO2 emissions. They need to know the biogenic fraction of the fuels they are combusting to comply with regulations and avail of incentives – efforts that aim to mitigate global climate change by reducing greenhouse gas emissions worldwide.
Determination of the biogenic fractions of fuels was difficult for industrial plants using mixed/ heterogeneous fuels like solid recovered fuels (SRF) or refuse-derived fuels (RDF). Different methods were developed to address this issue. One of the first methods developed to measure biogenic fractions of mixed fuels was the selective dissolution method.
Solid recovered fuels or refuse-derived fuels are solid fuels processed from non-hazardous waste used for energy recovery in incineration or co-incineration plants. They can be household wastes or those from commercial or industrial plants, and they are used as fossil-fuel substitutes in cement kilns, power stations, and industrial boilers.
The European Commission’s 2002 mandate to the European Committee for Standardization (CEN) describes SRFs as “may be composed of a variety of materials of which some, although recyclable, may have been made available in such a form that recycling is not environmentally sound. On the one hand materials collected and/or sorted and prepared into a recyclable form should not be considered as SRFs. On the other hand recyclable materials should not be excluded from SRFs because such an exclusion could lead to disposal of these materials and wastage of the resources embedded in them.”
This 2002 mandate led to the development by the CEN of technical specifications called the CEN/TS 15440, which outlined the methods for the determination of biomass-derived carbon content of SRF. The CEN/TS 15440:2006 methods include the selective dissolution method, the manual sorting method, and the reductionistic method. Based on this technical specification, a standard was created. The EN 15440, published in 2011, include SDM, manual sorting, and the Carbon-14 method.
The selective dissolution method assumes that biomass fuel components of SRF will dissolve in sulfuric acid or hydrogen peroxide but the fossil-fuel components will not. SDM was originally developed as a method to determine the biodegradable content of compost. When it was applied to SRF, it was assumed that “biomass” is equivalent to “biodegradable,” which is not exactly accurate.
The SDM assumption holds true in many cases but uncertainties arise when the SRFs have biodegradable components that are not biomass, e.g. nylon, or when biomass materials are present that are not fully biodegradable, e.g. wool. EN 15440 acknowledged that the SDM is not applicable to some materials usually or possibly present in SRF, and its limitations have to be considered.
CEN/TS 15540:2006 noted that carbon-14 analysis is an alternative method to determine the biomass content expressed in percentage by carbon content. In 2006, the C14 method for SRF was in its early stages of development. The merits of the C14 method were already recognized by the CEN, and this led to the development of a technical report called the CEN/TR 15591 – determination of biomass content in SRF based on the radiocarbon dating method. The CEN 15591 was approved in January 2007, and this technical report eventually became a technical specification in 2008.
CEN/TR 15591 acknowledges that the selective dissolution method’s limitations do not make it the most effective method for determining biomass content of SRF with materials having fossil and biomass carbon at the molecular level. Although radiocarbon dating methods are more expensive and require skilled personnel, they are able to resolve analytical problems that would have been impossible to solve with manual sorting and SDM as methods.
The National Greenhouse and Energy Reporting (Measurement) Technical Guidelines released by the Australian Department of Climate Change in 2008 consider the radiocarbon dating technique better than SDM. Carbon dating can be applied to any type of blended fuel including gaseous, liquid, and solid fuels. It can also analyze the mixed fuels directly or their combustion gases, which is particularly useful for facilities with emissions monitoring systems in place. In cases when sampling fuels can be difficult, carbon 14 analysis is indeed more advantageous because it doesn’t require as much samples as SDM.
Despite its limitations, SDM can estimate the biomass content of SRF by calorific value while carbon dating methods cannot because the non-biomass part of samples are not separated during the carbon-14 analysis. Carbon dating methods also cannot determine the biomass content by weight.
The percentage of biomass energy in mixed fuels, however, can be calculated from carbon-14 analysis results. Details of the computation has been shown in a report commissioned by the UK Renewable Energy Association in cooperation with Columbia University, see pages 8-9.
ISO/IEC 17025:2017-accredited Beta Analytic is a radiocarbon dating lab based in Miami, Florida. It provides EN 15440 testing, which is an industrial application of radiocarbon dating, for users of solid recovered fuels in Europe. The company does not offer selective dissolution method testing.