Carbon-14 based determination of the biogenic fraction of industrial CO₂ emissions – Application and validation

Abstract

The ¹⁴C method is a very reliable and sensitive method for industrial plants, emission authorities and emission inventories to verify data estimations of biogenic fractions of CO₂ emissions. The applicability of the method is shown for flue gas CO₂ samples that have been sampled in 1-h intervals at a coal- and wood-fired power plant and a waste incineration plant. Biogenic flue gas CO₂ fractions of 5–10% and 48–50% have been measured at the power plant and the waste incineration plant, respectively. The reliability of the method has been proven by comparison of the power plant results with those based on carbon mass input and output data of the power plant. At industrial plants with relatively low biogenic CO₂ fraction (<10%) the results need to be corrected for sampled ¹⁴CO₂ from atmospheric air.

Introduction

The current net increase of atmospheric CO₂, which enhances global warming and related changes in the Earths climate system (IPCC, 2007), is mainly due to the combustion of fossil fuels, because its carbon was not part of the Earth’s present day carbon cycle before it was combusted. Biomass carbon, on the contrary, is part of the carbon cycle and the combustion of biomass material does therefore not result in a significant net increase of CO₂ in the atmosphere as long as new biomass is grown again.

The interest in the use of biomass materials as fuels for different industrial plants, like power plants, has increased in the previous years. In many countries around the world governments have implemented financial incentives (subsidies, CO₂ trading system (2003/87/EC, 2003), emission taxes), especially for industrial plants, to discourage the emission of CO₂ due to the combustion of fossil fuels, while the combustion of biomass has been made attractive. A consequence of these kinds of regulations, and their financial implications and interests, is that emission data have to be quantified, monitored and reported annually by the companies involved. Moreover, these data should be verifiable by the emission authorities. This requires accurate, reliable and verifiable methods enabling the CO₂ emissions of fossil fuels and biomass to be quantified separately. At an industrial plant, the most direct way to determine the partition of the biogenic and fossil CO₂ emissions is to investigate the flue gas CO₂ itself. If the composition of the fuel mixture is completely unknown it is in fact the only reliable way. Quantification of the biogenic and fossil CO₂ fractions in flue gas has been demonstrated in the literature by using the ¹⁴C method (Mohn et al., 2008, Reinhardt et al., 2008, Staber et al., 2008) and the balance method (Fellner et al., 2007).

The ¹⁴C-based method to determine the biogenic and fossil carbon fractions in samples has been investigated and described since the 1950s for different research fields like atmospheric research (atmospheric carbonaceous gases and aerosols) (Clayton et al., 1955, Currie et al., 1994, Zondervan and Meijer, 1996) and food authenticity research (Simon et al., 1968). It is based on the principle that biogenic and fossil carbon sources have very different ¹⁴C values. Fossil carbon sources, like natural gas, coal, mineral oil and its products, do not contain the radioactive ¹⁴C (half-life 5730 years) anymore due to their age (millions of years). Recent (<200 years) biogenic carbon sources, like biomass, have well measurable ¹⁴C values that equal the average ¹⁴C value of atmospheric CO₂ of the time-period in which the specific organisms lived. The relation between the fraction of biomass in a sample and its ¹⁴C value is linear. This relation is used to determine the biogenic carbon fraction of many different sample types by measuring ¹⁴C.

Regarding the principal simplicity of the use, the well-established analysis of ¹⁴C in CO₂ (of any source) and the routine extraction, validation of the method is from a scientific point of view superfluous. However, industrial applications of the ¹⁴C method for this purpose, especially with legal consequences, require proper standardization. Examples of industrial applications are the ¹⁴C analysis of solid recovered fuels (CEN/TR15591, 2007), petrol and flue gas CO₂ (European patent by Kneissl, 2002, ASTM D6866-08, 2008). So far, very little data has been published in scientific papers about the validation, reliability and applicability of the ¹⁴C method for flue gas samples of industrial plants (Mohn et al., 2008, Staber et al., 2008). More studies have been performed, but these have, unfortunately, not been officially published for a wide audience (Fichtner, 2007, Raber, 2003). Hämäläinen et al. (2007) have investigated the ¹⁴C values in flue gas CO₂ samples of different power plants in Finland, but did not calculate the biogenic CO₂ fraction. The American test method ASTM D6866-08 (2008) is currently the only available standard that can be used for the ¹⁴C-based determination of the biogenic fraction of flue gas CO₂. The ¹⁴C method to quantify the biogenic CO₂ fraction of industrial CO₂ emissions using flue gas CO₂ is therefore still not widely recognized in the field of potential users.

This paper shows the results of a project in which the biogenic fraction of sampled flue gas CO₂ of a power plant and a waste incineration plant in the Netherlands have been determined based on ¹⁴C analyses. The reliability of the ¹⁴C method has been validated by comparing the ¹⁴C-based biogenic CO₂ fractions of the power plant samples with those based on known carbon in- and output mass data of the power plant. On the basis of the results and experiences of this project, the applicability and reliability of the used ¹⁴C method to determine the biogenic fraction of industrial CO₂ emissions are discussed.