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Certified reference material for coal in accordance with the PN-EN ISO 17034:2017-03 standard and ISO GUIDE 35:2017

Certified reference material for coal in accordance with the PN-EN ISO 17034:2017-03 standard and... Environmental & Socio-economic Studies DOI: 10.2478/environ-2023-0003 Environ. Socio.-econ. Stud., 2023, 11, 1: 33-44 ________________________________________________________________________________________________ Original article Certified reference material for coal in accordance with the PN-EN ISO 17034:2017-03 standard and ISO GUIDE 35:2017 Katarzyna Więcek*, Aneta Kocela-Jagiełko, Wojciech Szulik, Piotr Celary Central Laboratory for Measurements and Research Limited Liability Company, Rybnicka 6,44-335 Jastrzębie-Zdrój, Poland E–mail address (corresponding author*): kwiecek@clpb.pl ORCID iD: Katarzyna Więcek: https://orcid.org/0000-0003-3870-4743; Aneta Kocela-Jagiełko: https://orcid.org/0009-0001- 3756-6660; Wojciech Szulik: https://orcid.org/0000-0002-8480-8415; Piotr Celary: https://orcid.org/ – 0000 – 0003-2677-279X ______________________________________________________________________________________________________________________________________________ A B S T R A C T Due to an absence of domestic certified reference materials for coal on the Polish market, an attempt was made to manufacture a new and innovative product tailored to its needs. The chosen candidate material was hard coal acquired from Poland’s coal mines. A single reference material unit consisted of 50g of an analytical hard coal sample with a grain size below 0.2 mm. A manufacturing method was developed enabling production of matrix reference materials addressing the needs of the domestic solid fuel market, and was directed at research laboratories carrying out analyses of solid fuels for the energy and coking sectors. The adapted manufacturing scheme of a certified reference material for coal was presented with a description of the chosen critical steps of the process and discussion of the obtained results in terms of homogeneity, stability, characterisation of the reference material as well as assigned values to particular properties and their uncertainty budget. The results obtained during homogeneity, short-term and long-term stability assessments as well as reference material characterisation confirmed the feasibility of the investigated certified coal reference material manufacturing process. The obtained levels of relative expanded uncertainties of the measurements confirmed the feasibility of the manufactured certified reference material for establishing and maintaining metrological traceability of measurement results. The presented research establishes a base for planning out production of additional reference materials as well as providing the know-how for designing manufacturing schemes for reference materials for solid fuels, or waste related materials like fly ash, or furnace waste. KEY WORDS: certified reference materials, hard coal, traceability, homogeneity, stability of reference material ARTICLE HISTORY: received 19 October 2022; received in revised form 1March 2023; accepted 6 March 2023 ______________________________________________________________________________________________________________________________________________ 1. Introduction the requirements of the PN-EN ISO 17034 standard. Other aspects which need to be taken into The PN-EN ISO/IEC 17025 standard requires consideration when choosing suppliers of reference laboratories to establish and maintain the materials are given in the Eurachem guide (ELLISON metrological traceability of measurement results. & WILLIAM, 2019), and include among others: Traceability of the measurement results to the availability of a certificate fulfilling the requirements International System of Units (SI) can be achieved of the ISO Guide 31: Reference materials – using three methods: calibration, certified values of Contents of certificates, labels and accompanying certified reference materials and direct realization documentation as well as key certification procedures of the SI unit. When using certified values of certified given in ISO Guide 35: Reference materials – reference materials with stated metrological Guidance for characterization and assessment of traceability to the SI for assuring traceability, the homogeneity and stability. Reference materials are standard PN-EN ISO/IEC 17025 imposes that they used by research laboratories for various purposes, are provided by a competent producer fulfilling such as: validation/verification of measurement procedures, optimisation of new measurement research laboratories which carry out analyses of procedures or assessment of measurement accuracy solid fuels for the energy and coking sectors. (BULSKA, 2008). The domestic market hasn’t The entire manufacturing process was developed previously featured nationally manufactured based on the requirements of the PN-EN ISO 17034 certified reference materials for coal. All the available standard and ISO Guide 35: Reference materials – products were provided by foreign manufacturers Guidance for characterization. Candidate materials and only a part of which were PN-EN ISO 17034 were chosen from pre-selected hard coals specific certified. Considering the necessity of verifying to key target customers, which most closely matched hard coal parameters as precisely as possible in the levels of the determined parameters. The aim order to fulfil the requirements of the Ordinance of this article was to present the results of the of the Minister of Energy of 27 September 2018 manufacturing process of a particular certified on quality requirements for solid fuels (Journal of reference material for coal, which meet the Laws 2018, item 1890), as well as properly requirements of the PN-EN ISO 17034 standard controlling technological processes in coke and ISO Guide 35: Reference materials – Guidance production and heat management in the power for characterization. industry, the traceability of measurement results has to be assured. Meeting this requirement is 2. Materials and methods achieved at research laboratories by, among others, the use of reference materials (RM) and 2.1. Manufacturing scheme certified reference materials (CRM). The definition of a certified reference material, is given by the The adopted manufacturing scheme consisted ISO/IEC Guide 99 – the International vocabulary of ten stages in accordance with the requirements of metrology, which states that it is a reference of the PN-EN ISO 17034 standard and ISO Guide 35: material one, or more, of whose property values Reference materials – Guidance for characterization, are certified by a procedure which establishes its these were namely: traceability to an accurate realization of the unit  defining the type of reference material; in which the property values are expressed, and  choosing the candidate material; for which each certified value is accompanied by  material preparation; an uncertainty at a stated level of confidence.  material packaging; Thus, taking into account the above, an attempt  homogeneity assesment; was made to manufacture a new and innovative  short-term stability assesment; product, a certified reference material for coal,  material characterisation; which was tailored to the needs of the Polish  assignment of property values and their market. A manufacturing method was developed uncertainties; to enable the production of matrix reference  certificate preparation; materials which addressed the needs of the  stability monitoring. domestic solid fuel market, and was directed at Table 1. Acceptance criteria for measurand levels and their target uncertainties given in accordance with Bettencourt da Silva & Williams (2019) Acceptance criteria for Measurand Acceptance criteria for measurand levels measurand uncertainties Sulphur content (dry state) up to 0.50% (m/m) Ash content (dry state) up to 5% (m/m) Volatile matter content (850°C, dry state) up to 32% (m/m) No more than 10% of the Volatile matter content (900°C, dry state) up to 32% (m/m) measurand value Carbon content (dry state) up to 85.0% (m/m) Hydrogen content (dry state) up to 5% (m/m) Nitrogen content (dry state) up to 1.5% (m/m) Phosphorus content (dry state) up to 0.015% (m/m) No more than 20% of the Chlorine content (dry state) up to 0.15% (m/m) measurand value Mercury content (dry state) up to 0.030% (m/m) 2.2. Reference material characterisation  intended amount of each reference material unit; The aim was to manufacture a certified reference  optionally, additional units in case of the need material as similar as possible to natural materials to carry out supplementary research in response and characterised by parameters addressing the to client inquiries. needs of the domestic solid fuels market. The manufacturing process involved defining assigned 2.4. Preparation and packaging values (assumed as ones intended for certification) for the measurands, as required by ISO/IEC Guide A single reference material unit consisted of 99 – International vocabulary of metrology, as shown 50 g of an analytical hard coal sample with a grain in Table 1 also summarizing acceptance criteria for size below 0.2 mm. The analytical samples were measurand levels as well as their uncertainties. prepared as described in the Polish Standard PN- G-04502. Each unit of the coal reference material 2.3. Acquiring the candidate material was bottled in an amber glass container and placed in an aluminium foil zip lock bag for additional The chosen candidate material was hard coal protection. acquired from the resources of Jastrzębska Spółka Węglowa S.A. The required amount of candidate 2.5. Homogeneity study material, in accordance with the requirements of ISO Guide 35: Reference materials – Guidance for A homogeneity study was carried out for the characterization and assessment of homogeneity candidate certified reference material in its final, and stability, was estimated based on: packaged form, in order to assure its validity for  number of reference material units required its intended use as outlined in PN-EN ISO for distribution within the intended reference 17034:2017-03. The aim of the homogeneity study material shelf life; was to:  number of units required for homogeneity tests;  estimate within-unit and between-unit  number of units required for stability tests; reference material variance;  number of units required for material  investigate analytical trends as well as ones characterisation; resulting from reference material processing;  number of units required for homogeneity tests;  confirm that all of the reference material  number of units required for stability units feature the same set of uncertainty monitoring in the span of the intended values for each parameter. reference material shelf life; Table 2. The employed simple random flowchart for sampling from particular lots as required by ISO Guide 35: Reference materials – Guidance for characterization and assessment of homogeneity and stability Number of reference material unit 1 2 3 4 5 6 7 8 9 10 Number assigned during packaging 135 30 109 130 164 197 136 139 140 141 Analytical sequences 135 109 164 136 140 30 130 197 139 141 141 140 139 136 197 164 130 109 30 135 30 130 197 139 141 135 109 164 136 140 The between-unit homogeneity study was parameters for the between-unit homogeneity performed using 10 units of reference material study was carried out in repeatable conditions, which fulfilled the criteria set by the ISO Guide 35: the exception being events resulting from the Reference materials – Guidance for characterization measuring method specifications (intermediate and assessment of homogeneity and stability. precision conditions). The following reference Each reference material unit was numbered during material manufacturing stages were comprised of packaging to allow for trend analysis. A simple validated analysis methods in line with appropriate random flowchart for sampling from particular lots reference documents (standardized methods/ was employed (Table 2). Determination of chosen internal analysis procedures), approved for use in the Research Centre of the company and as justified by LINSINGER ET. AL. (2001), that the accordance with the PN-EN ISO/IEC 17025:2018-02 property value C decreases linearly from the standard (AB 300 certificate accreditation scope). initial value C₀ with a degradation rate b and the For the certified values, homogeneity was quantified time x as shown in the following equation (1). as the contribution to the certified values uncertainties or was shown to have a negligible (1) impact on the certified values uncertainties, as outlined by VAN DER VEEN ET AL. (2001). Based on Taking into account the above-mentioned the work of van der VEEN & PAUWELS (2000) the equation, the uncertainty contribution of stability one-way variance analysis ANOVA was chosen as was calculated as the ratio of the product’s the most appropriate and efficient statistical chosen shelf-life (intended shelf-life), as proposed analysis tool for the defined task. by LISINGER ET AL. (2001), and uncertainty of the slope of the regression line (b) as per equation (2). 2.6. Stability study * (2) Reference materials should undergo a stability where: testing as it is generally recognized as a prerequisite – relative uncertainty due to potential for certification (LINSIGER ET AL., 2001). The authors degradation during storage; emphasises that certified reference material – proposed shelf life; manufacturers are required to include the x – time point for each replicate; uncertainties derived from stability and – mean results for all time points; homogeneity studies in the complete uncertainty – residual standard deviation (a measurement of statement for assigned values of the reference material. The aim of a stability study is to determine the amount of dispersion of observed experimental maximum storage time and consequently the shelf data points around a regression line). life of the certified reference material in order to allow for proper resource management and informed 2.7. Characterization decision making by the clients. The performed stability study was carried out in order to produce Reference material characterization, as defined evidence supporting reference material stability by ISO Guide 30 Reference materials – Selected under transport conditions and during long-term terms and definitions, is carried out in order to storage by the manufacturer, and to determine determine its property values, or attributes, as part optimal operating, storing and transport conditions of the production process. The adopted strategy of the reference material as well as to estimate was based on the approach of determining non- the stability contribution to the uncertainties of operationally defined measurands using multiple reference material assigned values. The experiment methods of demonstrable accuracy in multiple investigated the impact of time and temperature competent laboratories as outlined in the PN-EN on reference material stability. In the short-term ISO 17034 standard. The laboratories were chosen stability study reference material units were based on the following criteria: stored at 50 degrees Celsius for a period of 1, 2, 3  management system certified for compliance and 4 weeks. At each time point, reflecting the with the PN-EN ISO/IEC 17025 standard; beginning of the periods outlined above, a set of  in case of no certification/accreditation, the two randomly chosen units was moved from option of carrying out a technical assessment manufacturer storage conditions to a climate- of results after statistical analysis of controlled storage with the temperature set to 50 characterisation results; degrees Celsius. Repeatability, or intermediate  assessment of results obtained for a control precision conditions, were applied where sample (of a certified reference material). appropriate. After 4 weeks all the units were Each laboratory received a single unit of coal analysed. reference material and a unit of a control sample. In the long-term stability study reference The 23 laboratories participating in the material units were stored at room temperature. characterization process carried out determinations Every 3 months (after 3, 6, 9, 12 and so on months) of chosen physicochemical parameters for the a randomly chosen set of two reference material provided research materials. Each participating units was analysed under repeatability, or laboratory received the research materials in the intermediate precision conditions, where form of analytical hard coal samples with a grain appropriate. In the stability study it was assumed, size below 0.2 mm. The results were submitted  certified values – were derived from a minimum by the participants on pre-prepared result sheets of 6 data sets with an uncertainty budget of which stated, among others, the type of parameters the assigned value; to be determined, number of required replicates,  indicative values – were derived from less than 6 requirement of presenting the results for dry state, data sets or from cases where the uncertainty of units of measurements, number of significant digits the assigned value was deemed too large, in and other data required in the reference material order to obtain the status of a certified value, characterisation process. uncertainty of indicative values have to be The variance analysis tool (ANOVA) was used for derived as for the certified values; the reference material characterisation assessment  values given as additional information – were incorporating the assumption outlined in the work derived from small data sets e.g. 2–3 or from of VAN DER VEEN ET AL. (2001) as shown in the cases where the uncertainty of the assigned equation (3). value was deemed too large. Total expanded uncertainty for a certified value was calculated as show by VAN DER VEEN ET AL. (3) (2001) using the equation (6): where: s – standard deviation of results from a within laboratory (within-group); = k* (6) s – standard deviation of results between between laboratories (between-group); where: n – number of replicates from a laboratory. U – expanded uncertainty of CRM; CRM k – coverage factor (k=2); The uncertainty component derived from the u – uncertainty component (reference material char material characterisation process was calculated characterisation); as proposed by van der Veen et al. (2001) using u – uncertainty component (reference material bb the following equation (4): homogeneity); u – uncertainty component (long-term stability); lts (4) u – uncertainty component (short-term stability). sts where: k – number of laboratories participating in the 3. Results and discussion reference material characterisation process. 3.1. Homogeneity assessment 2.8. Assignment of property values and their uncertainties Homogeneity between units of manufactured coal reference material was evaluated. The quantitative The assigned value was derived as an homogeneity assessment between reference material unweighted mean as outlined in the ISO Guide 35: units was carried out using the one-way variance Reference materials – Guidance for characterization analysis ANOVA and the obtained results and were and assessment of homogeneity and stability using compiled in Table 3. Relative inhomogeneity (s ) bb the equation (5) below. was used to calculate the uncertainty component u (Table 4). bb The obtained results show that the manufactured = (5) reference material units are sufficiently homogenous. where: The biggest relative standard uncertainty was y – arithmetic mean value of y means of p sets char i noted for mercury content in the dry state. It was of data. decided to include the uncertainty component resulting from reference material inhomogeneity Assignment of property values and their in the total standard uncertainty of a particular uncertainties listed in the certificate has been done measurement. as outlined by LINSINGER ET AL. (2013) namely: Table 3. Results of the quantitative homogeneity assessment between units of coal reference material Ash content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 0.955 Between SB 0.037603 9 0.0042 Calculated F-value = 2.393 Within SW 0.087600 20 0.0044 Result: homogeneity confirmed Total ST 0.125347 29 Total sulphur content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 1.500 Between SB 0.000622 9 0.000069 Calculated F-value = 2.393 Within SW 0.000920 20 0.000046 Result: homogeneity confirmed Total ST 0.001539 29 Volatile matter content (850°C) in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 0.862 Between SB 0.118050 9 0.0131 Calculated F-value = 2.393 Within SW 0.303067 20 0.0152 Result: homogeneity confirmed Total ST 0.421387 29 Volatile matter content (900°C) in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 0.683 Between SB 0.160783 9 0.0179 Calculated F-value = 2.393 Within SW 0.524200 20 0.0262 Result: homogeneity confirmed Total ST 0.685347 29 Carbon content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 1.692 Between SB 2.324280 9 0.2583 Calculated F-value = 2.393 Within SW 3.053333 20 0.1527 Result: homogeneity confirmed Total ST 5.369667 29 Hydrogen content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 0.667 Between SB 0.003845 9 0.0004 Calculated F-value = 2.393 Within SW 0.011600 20 0.0006 Result: homogeneity confirmed Total ST 0.015417 29 Nitrogen content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 1.500 Between SB 0.002604 9 0.0003 Calculated F-value = 2.393 Within SW 0.003533 20 0.0002 Result: homogeneity confirmed Total ST 0.006097 29 Phosphorus content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 2.000 Between SB 0.00000022 9 0.00000002 Calculated F-value = 2.393 Within SW 0.00000011 20 0.00000001 Result: homogeneity confirmed Total ST 0.00000032 29 Chlorine content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 1.000 Between SB 0.000038 9 0.000004 Calculated F-value = 2.393 Within SW 0.000079 20 0.000004 Result: homogeneity confirmed Total ST 0.000117 29 Mercury content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 2.000 Between SB 0.000142 9 0.00002 Calculated F-value = 2.393 Within SW 0.000137 20 0.00001 Result: homogeneity confirmed Table 3. Calculated component uncertainties ubb Measurand content in Relative standard uncertainty – component Measurand reference material, % resulting from reference material (m/m) inhomogeneity u [%] bb Total sulphur content in dry state 0.50 0.57 Ash content in dry state 3.65 0.61 Volatile matter content (850°C) in dry state 29.52 0.14 Volatile matter content (900°C) in dry state 30.54 0.17 Carbon content in dry state 84.5 0.23 Hydrogen content in dry state 4.87 0.16 Nitrogen content in dry state 1.31 0.42 Phosphorus content in dry state 0.010 0.81 Zawartość chloru w stanie suchym 0.13 0.64 Mercury content in dry state 0.020 7.96 3.2. Stability assessment manufactured coal reference material. No statistically significant trends were found for any of the As required by ISO Guide 35: Reference materials measurements. The reference material was confirmed – Guidance for characterization and assessment to be stable at a temperature of 50°C. The results of homogeneity and stability, two types of reference of the short-term stability assessment were compiled material stability were investigated in accordance in Table 5. The relative standard uncertainty with the aforementioned standard, namely: resulting from potential short-term instability of the reference material ranged between 0.15% –  stability in the assumed transport conditions 0.95%. Table 6 presents the relative standard also referred to as short-term stability; uncertainty values obtained during the short-  long-term stability (reference material stability term stability assessment. For the long-term during the span of its shelf life in the required stability assessment, the reference material units storage conditions). were stored at room temperature. Every 3 months A short-term stability assessment was carried out two randomly chosen units of the reference for two temperatures: room temperature (ambient) material (that is after 3, 6, 9, 12 and so on months) and a temperature of 50°C. The assessment was underwent analysis which was carried out under focused on finding adequate temperature conditions repeatability, or intermediate, precision conditions, for shipping the reference material units and where appropriate. The slope of the regression line verifying the quantitative potential degradation for the content of the investigated measurements, during a span of 4 weeks. Linear regression during the storage time, was verified in terms of confirmed sufficient stability in both cases during statistical significance (α = 0.01) in order to assess the investigated time span of 4 weeks. The slope the stability of the manufactured coal reference of the regression line for the content of the material. The reference material was confirmed investigated measurements during the storage time to be stable at room (ambient) temperature. was verified in terms of statistical significance (α = 0.01) in order to assess the stability of the Table 5. Results of short-term stability assessment at 50°C Ash content in dry state %(m/m) Weeks Unit number 1 2 3 4 3.45 3.55 3.45 3.64 1 3.54 3.63 3.59 3.49 3.63 3.50 3.49 3.46 3.43 3.58 3.63 3.60 2 3.51 3.56 3.44 3.46 3.54 3.50 3.55 3.59 Statistical significance of the regression line slope: none Result: stability confirmed Sulphur content in dry state %(m/m) Weeks Unit number 1 2 3 4 1 0.443 0.448 0.433 0.440 0.443 0.445 0.442 0.456 0.456 0.456 0.446 0.448 0.449 0.443 0.463 0.459 2 0.441 0.447 0.448 0.454 0.456 0.454 0.448 0.448 Statistical significance of the regression line slope: none Result: stability confirmed Volatile matter content (850°C) in dry state %(m/m) Weeks Unit number 1 2 3 4 29.85 29.87 29.75 29.74 1 29.46 29.32 29.39 29.17 29.66 29.67 29.08 29.40 29.84 29.67 29.64 29.53 2 29.73 29.42 29.16 29.15 29.38 29.21 29.60 29.67 Statistical significance of the regression line slope: none Result: stability confirmed Volatile matter content (900°C) in dry state %(m/m) Weeks Unit number 1 2 3 4 30.68 31.03 30.90 30.75 1 30.90 30.54 30.53 30.75 31.01 30.72 30.74 30.63 30.75 30.89 30.69 30.46 2 31.04 30.87 30.49 30.40 30.67 30.72 30.55 30.28 Statistical significance of the regression line slope: found Result: stability confirmed Carbon content in dry state %(m/m) Weeks Unit number 1 2 3 4 81.33 78.76 80.81 81.00 1 80.79 79.33 81.37 81.21 81.06 79.13 81.01 80.96 81.86 80.95 81.43 80.63 2 81.75 80.97 81.01 80.58 81.75 80.71 81.17 80.51 Statistical significance of the regression line slope: none Result: stability confirmed Hydrogen content in dry state %(m/m) Weeks Unit number 1 2 3 4 4.81 4.71 4.84 4.85 1 4.77 4.69 4.82 4.79 4.78 4.67 4.79 4.82 4.86 4.82 4.86 4.79 2 4.83 4.80 4.87 4.76 4.83 4.80 4.89 4.78 Statistical significance of the regression line slope: none Result: stability confirmed Nitrogen content in dry state %(m/m) Weeks Unit number 1 2 3 4 1.29 1.26 1.29 1.29 1 1.28 1.25 1.29 1.25 1.26 1.25 1.28 1.27 1.32 1.29 1.30 1.27 2 1.30 1.26 1.30 1.27 1.29 1.27 1.29 1.27 Statistical significance of the regression line slope: none Result: stability confirmed Phosphorus content in dry state % (m/m) Weeks Unit number 1 2 3 4 0.0070 0.0074 0.0070 0.0075 1 0.0073 0.0076 0.0071 0.0073 0.0072 0.0074 0.0072 0.0075 0.0070 0.0074 0.0073 0.0070 2 0.0070 0.0075 0.0073 0.0070 0.0071 0.0076 0.0070 0.0073 Statistical significance of the regression line slope: none Result: stability confirmed Chlorine content in dry state %(m/m) Weeks Unit number 1 2 3 4 0.099 0.102 0.112 0.116 1 0.105 0.098 0.109 0.111 0.102 0.106 0.106 0.106 0.107 0.106 0.103 0.096 2 0.115 0.114 0.108 0.106 0.100 0.108 0.098 0.105 Statistical significance of the regression line slope: none Result: stability confirmed Mercury content in dry state mg/kg Weeks Unit number 1 2 3 4 0.0205 0.0216 0.0243 0.0219 1 0.0215 0.0230 0.0234 0.0223 0.0222 0.0224 0.0263 0.0238 0.0221 0.0218 0.0224 0.0259 2 0.0234 0.0224 0.0216 0.0232 0.0218 0.0238 0.0231 0.0238 Statistical significance of the regression line slope: none Result: stability confirmed Table 6. Obtained component uncertainties usts resulting from short-term stability Relative standard uncertainty – Content of measurand component resulting from short-term Measurand in the reference instability of the reference material material % (m/m) u [%] sts Total sulphur content in dry state 0.50 0.29 Ash content in dry state 3.65 0.37 Volatile matter content (850°C) in dry state 29.52 0.15 Volatile matter content (900°C) in dry state 30.54 0.37 Carbon content in dry state 84.5 0.18 Hydrogen content in dry state 4.87 0.21 Nitrogen content in dry state 1.31 0.27 Phosphorus content in dry state 0.010 0.51 Chlorine content in dry state 0.13 0.95 Mercury content in dry state 0.020 0.87 The relative standard uncertainty resulting 3.3. Characterisation of the reference material from the potential long-term instability of the reference material ranged between 0.55% – The purpose of the characterisation process, 10.44%. The reference material certificate features as stated in the ISO Guide 35: Reference materials recommendations regarding storage and use of – Guidance for characterization and assessment the material in order to ensure its stability after of homogeneity and stability, was to assign purchase. The users were instructed to store the certified property values to the reference material at room temperature. A stability monitoring material. The adopted strategy was based on the program of materials intended for long-term approach of determining non-operationally storage was also launched which will allow for defined measurements using multiple methods of the quick detection of changes in the stability of demonstrable accuracy in multiple competent reference material properties. laboratories, as outlined in the PN-EN ISO 17034 standard. A total of 23 laboratories participated in the interlaboratory comparison. An example of The obtained data sets were used in the process the obtained results are shown in Table 7. The of assigning property values and their uncertainties. data sets obtained from the laboratories The calculated interlaboratory mean values are underwent a technical assessment. All the summarised in Table 8. The relative standard properties were characterised assuring sufficient uncertainty resulting from reference material traceability. characterisation ranged between 0.20% – 27.9%. Table 7. Results for ash content in dry state obtained from the characterisation of reference material Replicate Number of Laboratory code Mean Standard deviation replications 1 2 3 01 3.66 3.70 3.65 3 3.670 0.0265 04 3.52 3.53 3.51 3 3.520 0.0100 05 3.54 3.53 3.50 3 3.523 0.0208 06 3.74 3.73 3.72 3 3.730 0.0100 09 3.73 3.76 3.73 3 3.740 0.0173 11 3.81 3.83 3.85 3 3.830 0.0200 12 3.84 3.85 3.84 3 3.843 0.0058 13 3.69 3.69 3.69 3 3.690 0.0000 14 3.62 3.55 3.56 3 3.577 0.0379 15 3.67 3.60 3.61 3 3.627 0.0379 16 3.45 3.46 3.46 3 3.457 0.0058 18 3.72 3.73 3.74 3 3.730 0.0100 19 3.67 3.61 3.74 3 3.673 0.0651 20 3.86 3.86 3.77 3 3.830 0.0520 21 3.29 3.43 3.36 3 3.360 0.0700 22 3.52 3.49 3.56 3 3.523 0.0351 Table 8. Interlaboratory mean values and their relative standard deviations Mean of data obtained from the Number of data Measurand characterisation process % (m/m) sets Total sulphur content in dry state 0.499 15 Ash content in dry state 3.645 16 Volatile matter content (850°C) in dry state 29.523 15 Volatile matter content (900°C) in dry state 30.535 4 Carbon content in dry state 84.47 16 Hydrogen content in dry state 4.873 11 Nitrogen content in dry state 1.311 3 Phosphorus content in dry state 0.010 3 Chlorine content in dry state 0.129 6 Mercury content in dry state 0.021 3 3.4. Assignment of property values and their means of result presentation with the hypothetical uncertainties assumption that the solid fuel is free of moisture. The values given as additional information were The assignment of property values and their derived as averages of 3 laboratory means obtained uncertainties as shown in the reference material from reference material characterisation. Values certificate was carried out as outlined in Table 9. given as additional information were presented Each certified value was derived as a mean of for the dry state understood as a means of result accepted data sets (≥ 6) each of which was presentation with the hypothetical assumption that obtained from reference material characterisation the solid fuel is free of moisture. The obtained carried out at different laboratories and/or by levels of measurements, their target uncertainties using different methods of analysis. Certified values as well acceptance criteria have been summed up were presented for the dry state understood as a in Table 10. Table 9. Assignment of property values and their uncertainties in the reference material certificate Absolute expanded Certified value Minimum amount of uncertainty* Parameter sample portion % (m/m) Total sulphur content 0.50 0.03 0.3 g Ash content 3.65 0.18 1 g Carbon content 84.5 1.7 50 mg Hydrogen content 4.87 0.34 50 mg Volatile matter content (850°C) 29.52 0.59 1 g Chlorine content 0.13 0.02 1 g * - coverage factor k = 2, providing an approximate 95% confidence level Table 10. Obtained measurand levels and their target uncertainties based on da Silva & Williams (2015) Acceptance Acceptance criteria Obtained criteria for for relative expanded Obtained relative expanded Measurand measurand measurand uncertainty of uncertainty of measurand level level measurand Total sulphur content in up to 0.50% 0.50% 6.0% dry state (m/m) (m/m) up to 5% 3.65% Ash content in dry state 4.9% (m/m) (m/m) Volatile matter content up to 32% 29.52% 2.0% (850°C) in dry state (m/m) (m/m) Data obtained in the Volatile matter content up to 32% 30.54% characterisation process not (900°C) in dry state (m/m) (m/m) No more than 10% of sufficient enough to perform measurand value an uncertainty assessment Carbon content in dry up to 85.0% 84.5% 2.0% state (m/m) (m/m) Hydrogen content in dry up to 5% 4.87% 7.0% state (m/m) (m/m) Data obtained in the Nitrogen content in dry up to 1.5% 1.31% characterisation process not state (m/m) (m/m) sufficient enough to perform an uncertainty assessment Data obtained in the Phosphorus content in up to 0.015% 0.010% characterisation process not dry state (m/m) (m/m) sufficient enough to perform an uncertainty assessment Chlorine content in dry up to 0.15% 0.13% No more than 20% of 15.4% state (m/m) (m/m) measurand value Data obtained in the Mercury content in dry up to 0.030 0.020% characterisation process not state (m/m) (m/m) sufficient enough to perform an uncertainty assessment 4. Conclusions resulting from homogeneity, stability and reference material characterisation tests were included. A The presented results for the homogeneity, thorough analysis of each uncertainty component short-term and long-term stability assessments and its impact on the total standard uncertainty as well as reference material characterisation of the measurements will allow for a more confirmed the feasibility of the investigated certified effective and thought through planning of each coal reference material manufacturing process. step of the reference material manufacturing The obtained levels for the relative expanded process as well as designing a manufacturing uncertainties of the measurements meet the scheme of other reference materials such as other requirements set by the referenced standards solid fuels or waste related materials like fly ash and confirm the feasibility of the manufactured or furnace waste. certified reference material for establishing and maintaining metrological traceability of measurement results. In the uncertainty assessment components Linsinger T., Pauwels J., van der Veen A., Schimmel H., Acknowledgements Lamberty A. 2001. Homogeneity and stability of reference materials. Accreditation and Quality Assurance, 6: 20–25. The presented results are part of the project titled „Opracowanie Linsinger T., Raffaelli B., Oostra A. 2013. Certfication report metod wytwarzania innowacyjnych Certyfikowanych Materiałów The certification of the gross calorific value and mass Odniesienia (CRM) – nowa jakość w analityce paliw stałych” co- fractions of ash. C. H. N. S. Cl. major elements and trace financed from the Regional Operational Program of the Silesian elements in three coal materials: ERM -EF411 (hard coal). Voivodeship for 2014-2020 (European Regional Development ® ® ERM -EF412 (brown coal) and ERM -EF413 (furnace coke). Fund) under grant number UDA-RPSL.01.02.00-24-0656/17-01. Publications Office of the European Union, Luxembourg. van der Veen A., Linsinger T., Pauwels J. 2001. Uncertainty References calculations in the certification of reference materials. 2. Homogeneity study. Accreditation and Quality Assurance, 6: 26–30. Bettencourt da Silva R., Williams A. (eds) 2015. Eurachem/CITAC van der Veen A., Linsinger T., Schimmel H., Lamberty A., Guide: Setting and Using Target Uncertainty in Chemical Pauwels J. 2001. Uncertainty calculations in the certification Measurement. of reference materials 4. Characterisation and certification. Bulska E. 2008. Metrologia chemiczna. Sztuka prowadzenia Accreditation and Quality Assurance, 6: 290–294. pomiarów. Malamut, Warsaw. van der Veen A., Pauwels J. 2000. Uncertainty Calculations in Ellison S.L.R., Williams A. (eds) 2019. Eurachem/CITAC Guide: the Certification of Reference Materials. 1. Principles of Metrological Traceability in Analytical measurement. Analysis of Variance. Accreditation and Quality Assurance, Linsinger T., Pauwels J., Lamberty A., Schimmel H., van der 5, 12: 464–469. Veen A., Siekmann L. 2001. Estimating the uncertainty of stability for matrix CRMs. Fresenius' Journal of Analytical Chemistry, 370: 183–188. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Environmental & Socio-economic Studies de Gruyter

Certified reference material for coal in accordance with the PN-EN ISO 17034:2017-03 standard and ISO GUIDE 35:2017

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Abstract

Environmental & Socio-economic Studies DOI: 10.2478/environ-2023-0003 Environ. Socio.-econ. Stud., 2023, 11, 1: 33-44 ________________________________________________________________________________________________ Original article Certified reference material for coal in accordance with the PN-EN ISO 17034:2017-03 standard and ISO GUIDE 35:2017 Katarzyna Więcek*, Aneta Kocela-Jagiełko, Wojciech Szulik, Piotr Celary Central Laboratory for Measurements and Research Limited Liability Company, Rybnicka 6,44-335 Jastrzębie-Zdrój, Poland E–mail address (corresponding author*): kwiecek@clpb.pl ORCID iD: Katarzyna Więcek: https://orcid.org/0000-0003-3870-4743; Aneta Kocela-Jagiełko: https://orcid.org/0009-0001- 3756-6660; Wojciech Szulik: https://orcid.org/0000-0002-8480-8415; Piotr Celary: https://orcid.org/ – 0000 – 0003-2677-279X ______________________________________________________________________________________________________________________________________________ A B S T R A C T Due to an absence of domestic certified reference materials for coal on the Polish market, an attempt was made to manufacture a new and innovative product tailored to its needs. The chosen candidate material was hard coal acquired from Poland’s coal mines. A single reference material unit consisted of 50g of an analytical hard coal sample with a grain size below 0.2 mm. A manufacturing method was developed enabling production of matrix reference materials addressing the needs of the domestic solid fuel market, and was directed at research laboratories carrying out analyses of solid fuels for the energy and coking sectors. The adapted manufacturing scheme of a certified reference material for coal was presented with a description of the chosen critical steps of the process and discussion of the obtained results in terms of homogeneity, stability, characterisation of the reference material as well as assigned values to particular properties and their uncertainty budget. The results obtained during homogeneity, short-term and long-term stability assessments as well as reference material characterisation confirmed the feasibility of the investigated certified coal reference material manufacturing process. The obtained levels of relative expanded uncertainties of the measurements confirmed the feasibility of the manufactured certified reference material for establishing and maintaining metrological traceability of measurement results. The presented research establishes a base for planning out production of additional reference materials as well as providing the know-how for designing manufacturing schemes for reference materials for solid fuels, or waste related materials like fly ash, or furnace waste. KEY WORDS: certified reference materials, hard coal, traceability, homogeneity, stability of reference material ARTICLE HISTORY: received 19 October 2022; received in revised form 1March 2023; accepted 6 March 2023 ______________________________________________________________________________________________________________________________________________ 1. Introduction the requirements of the PN-EN ISO 17034 standard. Other aspects which need to be taken into The PN-EN ISO/IEC 17025 standard requires consideration when choosing suppliers of reference laboratories to establish and maintain the materials are given in the Eurachem guide (ELLISON metrological traceability of measurement results. & WILLIAM, 2019), and include among others: Traceability of the measurement results to the availability of a certificate fulfilling the requirements International System of Units (SI) can be achieved of the ISO Guide 31: Reference materials – using three methods: calibration, certified values of Contents of certificates, labels and accompanying certified reference materials and direct realization documentation as well as key certification procedures of the SI unit. When using certified values of certified given in ISO Guide 35: Reference materials – reference materials with stated metrological Guidance for characterization and assessment of traceability to the SI for assuring traceability, the homogeneity and stability. Reference materials are standard PN-EN ISO/IEC 17025 imposes that they used by research laboratories for various purposes, are provided by a competent producer fulfilling such as: validation/verification of measurement procedures, optimisation of new measurement research laboratories which carry out analyses of procedures or assessment of measurement accuracy solid fuels for the energy and coking sectors. (BULSKA, 2008). The domestic market hasn’t The entire manufacturing process was developed previously featured nationally manufactured based on the requirements of the PN-EN ISO 17034 certified reference materials for coal. All the available standard and ISO Guide 35: Reference materials – products were provided by foreign manufacturers Guidance for characterization. Candidate materials and only a part of which were PN-EN ISO 17034 were chosen from pre-selected hard coals specific certified. Considering the necessity of verifying to key target customers, which most closely matched hard coal parameters as precisely as possible in the levels of the determined parameters. The aim order to fulfil the requirements of the Ordinance of this article was to present the results of the of the Minister of Energy of 27 September 2018 manufacturing process of a particular certified on quality requirements for solid fuels (Journal of reference material for coal, which meet the Laws 2018, item 1890), as well as properly requirements of the PN-EN ISO 17034 standard controlling technological processes in coke and ISO Guide 35: Reference materials – Guidance production and heat management in the power for characterization. industry, the traceability of measurement results has to be assured. Meeting this requirement is 2. Materials and methods achieved at research laboratories by, among others, the use of reference materials (RM) and 2.1. Manufacturing scheme certified reference materials (CRM). The definition of a certified reference material, is given by the The adopted manufacturing scheme consisted ISO/IEC Guide 99 – the International vocabulary of ten stages in accordance with the requirements of metrology, which states that it is a reference of the PN-EN ISO 17034 standard and ISO Guide 35: material one, or more, of whose property values Reference materials – Guidance for characterization, are certified by a procedure which establishes its these were namely: traceability to an accurate realization of the unit  defining the type of reference material; in which the property values are expressed, and  choosing the candidate material; for which each certified value is accompanied by  material preparation; an uncertainty at a stated level of confidence.  material packaging; Thus, taking into account the above, an attempt  homogeneity assesment; was made to manufacture a new and innovative  short-term stability assesment; product, a certified reference material for coal,  material characterisation; which was tailored to the needs of the Polish  assignment of property values and their market. A manufacturing method was developed uncertainties; to enable the production of matrix reference  certificate preparation; materials which addressed the needs of the  stability monitoring. domestic solid fuel market, and was directed at Table 1. Acceptance criteria for measurand levels and their target uncertainties given in accordance with Bettencourt da Silva & Williams (2019) Acceptance criteria for Measurand Acceptance criteria for measurand levels measurand uncertainties Sulphur content (dry state) up to 0.50% (m/m) Ash content (dry state) up to 5% (m/m) Volatile matter content (850°C, dry state) up to 32% (m/m) No more than 10% of the Volatile matter content (900°C, dry state) up to 32% (m/m) measurand value Carbon content (dry state) up to 85.0% (m/m) Hydrogen content (dry state) up to 5% (m/m) Nitrogen content (dry state) up to 1.5% (m/m) Phosphorus content (dry state) up to 0.015% (m/m) No more than 20% of the Chlorine content (dry state) up to 0.15% (m/m) measurand value Mercury content (dry state) up to 0.030% (m/m) 2.2. Reference material characterisation  intended amount of each reference material unit; The aim was to manufacture a certified reference  optionally, additional units in case of the need material as similar as possible to natural materials to carry out supplementary research in response and characterised by parameters addressing the to client inquiries. needs of the domestic solid fuels market. The manufacturing process involved defining assigned 2.4. Preparation and packaging values (assumed as ones intended for certification) for the measurands, as required by ISO/IEC Guide A single reference material unit consisted of 99 – International vocabulary of metrology, as shown 50 g of an analytical hard coal sample with a grain in Table 1 also summarizing acceptance criteria for size below 0.2 mm. The analytical samples were measurand levels as well as their uncertainties. prepared as described in the Polish Standard PN- G-04502. Each unit of the coal reference material 2.3. Acquiring the candidate material was bottled in an amber glass container and placed in an aluminium foil zip lock bag for additional The chosen candidate material was hard coal protection. acquired from the resources of Jastrzębska Spółka Węglowa S.A. The required amount of candidate 2.5. Homogeneity study material, in accordance with the requirements of ISO Guide 35: Reference materials – Guidance for A homogeneity study was carried out for the characterization and assessment of homogeneity candidate certified reference material in its final, and stability, was estimated based on: packaged form, in order to assure its validity for  number of reference material units required its intended use as outlined in PN-EN ISO for distribution within the intended reference 17034:2017-03. The aim of the homogeneity study material shelf life; was to:  number of units required for homogeneity tests;  estimate within-unit and between-unit  number of units required for stability tests; reference material variance;  number of units required for material  investigate analytical trends as well as ones characterisation; resulting from reference material processing;  number of units required for homogeneity tests;  confirm that all of the reference material  number of units required for stability units feature the same set of uncertainty monitoring in the span of the intended values for each parameter. reference material shelf life; Table 2. The employed simple random flowchart for sampling from particular lots as required by ISO Guide 35: Reference materials – Guidance for characterization and assessment of homogeneity and stability Number of reference material unit 1 2 3 4 5 6 7 8 9 10 Number assigned during packaging 135 30 109 130 164 197 136 139 140 141 Analytical sequences 135 109 164 136 140 30 130 197 139 141 141 140 139 136 197 164 130 109 30 135 30 130 197 139 141 135 109 164 136 140 The between-unit homogeneity study was parameters for the between-unit homogeneity performed using 10 units of reference material study was carried out in repeatable conditions, which fulfilled the criteria set by the ISO Guide 35: the exception being events resulting from the Reference materials – Guidance for characterization measuring method specifications (intermediate and assessment of homogeneity and stability. precision conditions). The following reference Each reference material unit was numbered during material manufacturing stages were comprised of packaging to allow for trend analysis. A simple validated analysis methods in line with appropriate random flowchart for sampling from particular lots reference documents (standardized methods/ was employed (Table 2). Determination of chosen internal analysis procedures), approved for use in the Research Centre of the company and as justified by LINSINGER ET. AL. (2001), that the accordance with the PN-EN ISO/IEC 17025:2018-02 property value C decreases linearly from the standard (AB 300 certificate accreditation scope). initial value C₀ with a degradation rate b and the For the certified values, homogeneity was quantified time x as shown in the following equation (1). as the contribution to the certified values uncertainties or was shown to have a negligible (1) impact on the certified values uncertainties, as outlined by VAN DER VEEN ET AL. (2001). Based on Taking into account the above-mentioned the work of van der VEEN & PAUWELS (2000) the equation, the uncertainty contribution of stability one-way variance analysis ANOVA was chosen as was calculated as the ratio of the product’s the most appropriate and efficient statistical chosen shelf-life (intended shelf-life), as proposed analysis tool for the defined task. by LISINGER ET AL. (2001), and uncertainty of the slope of the regression line (b) as per equation (2). 2.6. Stability study * (2) Reference materials should undergo a stability where: testing as it is generally recognized as a prerequisite – relative uncertainty due to potential for certification (LINSIGER ET AL., 2001). The authors degradation during storage; emphasises that certified reference material – proposed shelf life; manufacturers are required to include the x – time point for each replicate; uncertainties derived from stability and – mean results for all time points; homogeneity studies in the complete uncertainty – residual standard deviation (a measurement of statement for assigned values of the reference material. The aim of a stability study is to determine the amount of dispersion of observed experimental maximum storage time and consequently the shelf data points around a regression line). life of the certified reference material in order to allow for proper resource management and informed 2.7. Characterization decision making by the clients. The performed stability study was carried out in order to produce Reference material characterization, as defined evidence supporting reference material stability by ISO Guide 30 Reference materials – Selected under transport conditions and during long-term terms and definitions, is carried out in order to storage by the manufacturer, and to determine determine its property values, or attributes, as part optimal operating, storing and transport conditions of the production process. The adopted strategy of the reference material as well as to estimate was based on the approach of determining non- the stability contribution to the uncertainties of operationally defined measurands using multiple reference material assigned values. The experiment methods of demonstrable accuracy in multiple investigated the impact of time and temperature competent laboratories as outlined in the PN-EN on reference material stability. In the short-term ISO 17034 standard. The laboratories were chosen stability study reference material units were based on the following criteria: stored at 50 degrees Celsius for a period of 1, 2, 3  management system certified for compliance and 4 weeks. At each time point, reflecting the with the PN-EN ISO/IEC 17025 standard; beginning of the periods outlined above, a set of  in case of no certification/accreditation, the two randomly chosen units was moved from option of carrying out a technical assessment manufacturer storage conditions to a climate- of results after statistical analysis of controlled storage with the temperature set to 50 characterisation results; degrees Celsius. Repeatability, or intermediate  assessment of results obtained for a control precision conditions, were applied where sample (of a certified reference material). appropriate. After 4 weeks all the units were Each laboratory received a single unit of coal analysed. reference material and a unit of a control sample. In the long-term stability study reference The 23 laboratories participating in the material units were stored at room temperature. characterization process carried out determinations Every 3 months (after 3, 6, 9, 12 and so on months) of chosen physicochemical parameters for the a randomly chosen set of two reference material provided research materials. Each participating units was analysed under repeatability, or laboratory received the research materials in the intermediate precision conditions, where form of analytical hard coal samples with a grain appropriate. In the stability study it was assumed, size below 0.2 mm. The results were submitted  certified values – were derived from a minimum by the participants on pre-prepared result sheets of 6 data sets with an uncertainty budget of which stated, among others, the type of parameters the assigned value; to be determined, number of required replicates,  indicative values – were derived from less than 6 requirement of presenting the results for dry state, data sets or from cases where the uncertainty of units of measurements, number of significant digits the assigned value was deemed too large, in and other data required in the reference material order to obtain the status of a certified value, characterisation process. uncertainty of indicative values have to be The variance analysis tool (ANOVA) was used for derived as for the certified values; the reference material characterisation assessment  values given as additional information – were incorporating the assumption outlined in the work derived from small data sets e.g. 2–3 or from of VAN DER VEEN ET AL. (2001) as shown in the cases where the uncertainty of the assigned equation (3). value was deemed too large. Total expanded uncertainty for a certified value was calculated as show by VAN DER VEEN ET AL. (3) (2001) using the equation (6): where: s – standard deviation of results from a within laboratory (within-group); = k* (6) s – standard deviation of results between between laboratories (between-group); where: n – number of replicates from a laboratory. U – expanded uncertainty of CRM; CRM k – coverage factor (k=2); The uncertainty component derived from the u – uncertainty component (reference material char material characterisation process was calculated characterisation); as proposed by van der Veen et al. (2001) using u – uncertainty component (reference material bb the following equation (4): homogeneity); u – uncertainty component (long-term stability); lts (4) u – uncertainty component (short-term stability). sts where: k – number of laboratories participating in the 3. Results and discussion reference material characterisation process. 3.1. Homogeneity assessment 2.8. Assignment of property values and their uncertainties Homogeneity between units of manufactured coal reference material was evaluated. The quantitative The assigned value was derived as an homogeneity assessment between reference material unweighted mean as outlined in the ISO Guide 35: units was carried out using the one-way variance Reference materials – Guidance for characterization analysis ANOVA and the obtained results and were and assessment of homogeneity and stability using compiled in Table 3. Relative inhomogeneity (s ) bb the equation (5) below. was used to calculate the uncertainty component u (Table 4). bb The obtained results show that the manufactured = (5) reference material units are sufficiently homogenous. where: The biggest relative standard uncertainty was y – arithmetic mean value of y means of p sets char i noted for mercury content in the dry state. It was of data. decided to include the uncertainty component resulting from reference material inhomogeneity Assignment of property values and their in the total standard uncertainty of a particular uncertainties listed in the certificate has been done measurement. as outlined by LINSINGER ET AL. (2013) namely: Table 3. Results of the quantitative homogeneity assessment between units of coal reference material Ash content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 0.955 Between SB 0.037603 9 0.0042 Calculated F-value = 2.393 Within SW 0.087600 20 0.0044 Result: homogeneity confirmed Total ST 0.125347 29 Total sulphur content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 1.500 Between SB 0.000622 9 0.000069 Calculated F-value = 2.393 Within SW 0.000920 20 0.000046 Result: homogeneity confirmed Total ST 0.001539 29 Volatile matter content (850°C) in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 0.862 Between SB 0.118050 9 0.0131 Calculated F-value = 2.393 Within SW 0.303067 20 0.0152 Result: homogeneity confirmed Total ST 0.421387 29 Volatile matter content (900°C) in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 0.683 Between SB 0.160783 9 0.0179 Calculated F-value = 2.393 Within SW 0.524200 20 0.0262 Result: homogeneity confirmed Total ST 0.685347 29 Carbon content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 1.692 Between SB 2.324280 9 0.2583 Calculated F-value = 2.393 Within SW 3.053333 20 0.1527 Result: homogeneity confirmed Total ST 5.369667 29 Hydrogen content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 0.667 Between SB 0.003845 9 0.0004 Calculated F-value = 2.393 Within SW 0.011600 20 0.0006 Result: homogeneity confirmed Total ST 0.015417 29 Nitrogen content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 1.500 Between SB 0.002604 9 0.0003 Calculated F-value = 2.393 Within SW 0.003533 20 0.0002 Result: homogeneity confirmed Total ST 0.006097 29 Phosphorus content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 2.000 Between SB 0.00000022 9 0.00000002 Calculated F-value = 2.393 Within SW 0.00000011 20 0.00000001 Result: homogeneity confirmed Total ST 0.00000032 29 Chlorine content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 1.000 Between SB 0.000038 9 0.000004 Calculated F-value = 2.393 Within SW 0.000079 20 0.000004 Result: homogeneity confirmed Total ST 0.000117 29 Mercury content in dry state Analysis of variance F-value Source of variance SS df MS Critical F-value = 2.000 Between SB 0.000142 9 0.00002 Calculated F-value = 2.393 Within SW 0.000137 20 0.00001 Result: homogeneity confirmed Table 3. Calculated component uncertainties ubb Measurand content in Relative standard uncertainty – component Measurand reference material, % resulting from reference material (m/m) inhomogeneity u [%] bb Total sulphur content in dry state 0.50 0.57 Ash content in dry state 3.65 0.61 Volatile matter content (850°C) in dry state 29.52 0.14 Volatile matter content (900°C) in dry state 30.54 0.17 Carbon content in dry state 84.5 0.23 Hydrogen content in dry state 4.87 0.16 Nitrogen content in dry state 1.31 0.42 Phosphorus content in dry state 0.010 0.81 Zawartość chloru w stanie suchym 0.13 0.64 Mercury content in dry state 0.020 7.96 3.2. Stability assessment manufactured coal reference material. No statistically significant trends were found for any of the As required by ISO Guide 35: Reference materials measurements. The reference material was confirmed – Guidance for characterization and assessment to be stable at a temperature of 50°C. The results of homogeneity and stability, two types of reference of the short-term stability assessment were compiled material stability were investigated in accordance in Table 5. The relative standard uncertainty with the aforementioned standard, namely: resulting from potential short-term instability of the reference material ranged between 0.15% –  stability in the assumed transport conditions 0.95%. Table 6 presents the relative standard also referred to as short-term stability; uncertainty values obtained during the short-  long-term stability (reference material stability term stability assessment. For the long-term during the span of its shelf life in the required stability assessment, the reference material units storage conditions). were stored at room temperature. Every 3 months A short-term stability assessment was carried out two randomly chosen units of the reference for two temperatures: room temperature (ambient) material (that is after 3, 6, 9, 12 and so on months) and a temperature of 50°C. The assessment was underwent analysis which was carried out under focused on finding adequate temperature conditions repeatability, or intermediate, precision conditions, for shipping the reference material units and where appropriate. The slope of the regression line verifying the quantitative potential degradation for the content of the investigated measurements, during a span of 4 weeks. Linear regression during the storage time, was verified in terms of confirmed sufficient stability in both cases during statistical significance (α = 0.01) in order to assess the investigated time span of 4 weeks. The slope the stability of the manufactured coal reference of the regression line for the content of the material. The reference material was confirmed investigated measurements during the storage time to be stable at room (ambient) temperature. was verified in terms of statistical significance (α = 0.01) in order to assess the stability of the Table 5. Results of short-term stability assessment at 50°C Ash content in dry state %(m/m) Weeks Unit number 1 2 3 4 3.45 3.55 3.45 3.64 1 3.54 3.63 3.59 3.49 3.63 3.50 3.49 3.46 3.43 3.58 3.63 3.60 2 3.51 3.56 3.44 3.46 3.54 3.50 3.55 3.59 Statistical significance of the regression line slope: none Result: stability confirmed Sulphur content in dry state %(m/m) Weeks Unit number 1 2 3 4 1 0.443 0.448 0.433 0.440 0.443 0.445 0.442 0.456 0.456 0.456 0.446 0.448 0.449 0.443 0.463 0.459 2 0.441 0.447 0.448 0.454 0.456 0.454 0.448 0.448 Statistical significance of the regression line slope: none Result: stability confirmed Volatile matter content (850°C) in dry state %(m/m) Weeks Unit number 1 2 3 4 29.85 29.87 29.75 29.74 1 29.46 29.32 29.39 29.17 29.66 29.67 29.08 29.40 29.84 29.67 29.64 29.53 2 29.73 29.42 29.16 29.15 29.38 29.21 29.60 29.67 Statistical significance of the regression line slope: none Result: stability confirmed Volatile matter content (900°C) in dry state %(m/m) Weeks Unit number 1 2 3 4 30.68 31.03 30.90 30.75 1 30.90 30.54 30.53 30.75 31.01 30.72 30.74 30.63 30.75 30.89 30.69 30.46 2 31.04 30.87 30.49 30.40 30.67 30.72 30.55 30.28 Statistical significance of the regression line slope: found Result: stability confirmed Carbon content in dry state %(m/m) Weeks Unit number 1 2 3 4 81.33 78.76 80.81 81.00 1 80.79 79.33 81.37 81.21 81.06 79.13 81.01 80.96 81.86 80.95 81.43 80.63 2 81.75 80.97 81.01 80.58 81.75 80.71 81.17 80.51 Statistical significance of the regression line slope: none Result: stability confirmed Hydrogen content in dry state %(m/m) Weeks Unit number 1 2 3 4 4.81 4.71 4.84 4.85 1 4.77 4.69 4.82 4.79 4.78 4.67 4.79 4.82 4.86 4.82 4.86 4.79 2 4.83 4.80 4.87 4.76 4.83 4.80 4.89 4.78 Statistical significance of the regression line slope: none Result: stability confirmed Nitrogen content in dry state %(m/m) Weeks Unit number 1 2 3 4 1.29 1.26 1.29 1.29 1 1.28 1.25 1.29 1.25 1.26 1.25 1.28 1.27 1.32 1.29 1.30 1.27 2 1.30 1.26 1.30 1.27 1.29 1.27 1.29 1.27 Statistical significance of the regression line slope: none Result: stability confirmed Phosphorus content in dry state % (m/m) Weeks Unit number 1 2 3 4 0.0070 0.0074 0.0070 0.0075 1 0.0073 0.0076 0.0071 0.0073 0.0072 0.0074 0.0072 0.0075 0.0070 0.0074 0.0073 0.0070 2 0.0070 0.0075 0.0073 0.0070 0.0071 0.0076 0.0070 0.0073 Statistical significance of the regression line slope: none Result: stability confirmed Chlorine content in dry state %(m/m) Weeks Unit number 1 2 3 4 0.099 0.102 0.112 0.116 1 0.105 0.098 0.109 0.111 0.102 0.106 0.106 0.106 0.107 0.106 0.103 0.096 2 0.115 0.114 0.108 0.106 0.100 0.108 0.098 0.105 Statistical significance of the regression line slope: none Result: stability confirmed Mercury content in dry state mg/kg Weeks Unit number 1 2 3 4 0.0205 0.0216 0.0243 0.0219 1 0.0215 0.0230 0.0234 0.0223 0.0222 0.0224 0.0263 0.0238 0.0221 0.0218 0.0224 0.0259 2 0.0234 0.0224 0.0216 0.0232 0.0218 0.0238 0.0231 0.0238 Statistical significance of the regression line slope: none Result: stability confirmed Table 6. Obtained component uncertainties usts resulting from short-term stability Relative standard uncertainty – Content of measurand component resulting from short-term Measurand in the reference instability of the reference material material % (m/m) u [%] sts Total sulphur content in dry state 0.50 0.29 Ash content in dry state 3.65 0.37 Volatile matter content (850°C) in dry state 29.52 0.15 Volatile matter content (900°C) in dry state 30.54 0.37 Carbon content in dry state 84.5 0.18 Hydrogen content in dry state 4.87 0.21 Nitrogen content in dry state 1.31 0.27 Phosphorus content in dry state 0.010 0.51 Chlorine content in dry state 0.13 0.95 Mercury content in dry state 0.020 0.87 The relative standard uncertainty resulting 3.3. Characterisation of the reference material from the potential long-term instability of the reference material ranged between 0.55% – The purpose of the characterisation process, 10.44%. The reference material certificate features as stated in the ISO Guide 35: Reference materials recommendations regarding storage and use of – Guidance for characterization and assessment the material in order to ensure its stability after of homogeneity and stability, was to assign purchase. The users were instructed to store the certified property values to the reference material at room temperature. A stability monitoring material. The adopted strategy was based on the program of materials intended for long-term approach of determining non-operationally storage was also launched which will allow for defined measurements using multiple methods of the quick detection of changes in the stability of demonstrable accuracy in multiple competent reference material properties. laboratories, as outlined in the PN-EN ISO 17034 standard. A total of 23 laboratories participated in the interlaboratory comparison. An example of The obtained data sets were used in the process the obtained results are shown in Table 7. The of assigning property values and their uncertainties. data sets obtained from the laboratories The calculated interlaboratory mean values are underwent a technical assessment. All the summarised in Table 8. The relative standard properties were characterised assuring sufficient uncertainty resulting from reference material traceability. characterisation ranged between 0.20% – 27.9%. Table 7. Results for ash content in dry state obtained from the characterisation of reference material Replicate Number of Laboratory code Mean Standard deviation replications 1 2 3 01 3.66 3.70 3.65 3 3.670 0.0265 04 3.52 3.53 3.51 3 3.520 0.0100 05 3.54 3.53 3.50 3 3.523 0.0208 06 3.74 3.73 3.72 3 3.730 0.0100 09 3.73 3.76 3.73 3 3.740 0.0173 11 3.81 3.83 3.85 3 3.830 0.0200 12 3.84 3.85 3.84 3 3.843 0.0058 13 3.69 3.69 3.69 3 3.690 0.0000 14 3.62 3.55 3.56 3 3.577 0.0379 15 3.67 3.60 3.61 3 3.627 0.0379 16 3.45 3.46 3.46 3 3.457 0.0058 18 3.72 3.73 3.74 3 3.730 0.0100 19 3.67 3.61 3.74 3 3.673 0.0651 20 3.86 3.86 3.77 3 3.830 0.0520 21 3.29 3.43 3.36 3 3.360 0.0700 22 3.52 3.49 3.56 3 3.523 0.0351 Table 8. Interlaboratory mean values and their relative standard deviations Mean of data obtained from the Number of data Measurand characterisation process % (m/m) sets Total sulphur content in dry state 0.499 15 Ash content in dry state 3.645 16 Volatile matter content (850°C) in dry state 29.523 15 Volatile matter content (900°C) in dry state 30.535 4 Carbon content in dry state 84.47 16 Hydrogen content in dry state 4.873 11 Nitrogen content in dry state 1.311 3 Phosphorus content in dry state 0.010 3 Chlorine content in dry state 0.129 6 Mercury content in dry state 0.021 3 3.4. Assignment of property values and their means of result presentation with the hypothetical uncertainties assumption that the solid fuel is free of moisture. The values given as additional information were The assignment of property values and their derived as averages of 3 laboratory means obtained uncertainties as shown in the reference material from reference material characterisation. Values certificate was carried out as outlined in Table 9. given as additional information were presented Each certified value was derived as a mean of for the dry state understood as a means of result accepted data sets (≥ 6) each of which was presentation with the hypothetical assumption that obtained from reference material characterisation the solid fuel is free of moisture. The obtained carried out at different laboratories and/or by levels of measurements, their target uncertainties using different methods of analysis. Certified values as well acceptance criteria have been summed up were presented for the dry state understood as a in Table 10. Table 9. Assignment of property values and their uncertainties in the reference material certificate Absolute expanded Certified value Minimum amount of uncertainty* Parameter sample portion % (m/m) Total sulphur content 0.50 0.03 0.3 g Ash content 3.65 0.18 1 g Carbon content 84.5 1.7 50 mg Hydrogen content 4.87 0.34 50 mg Volatile matter content (850°C) 29.52 0.59 1 g Chlorine content 0.13 0.02 1 g * - coverage factor k = 2, providing an approximate 95% confidence level Table 10. Obtained measurand levels and their target uncertainties based on da Silva & Williams (2015) Acceptance Acceptance criteria Obtained criteria for for relative expanded Obtained relative expanded Measurand measurand measurand uncertainty of uncertainty of measurand level level measurand Total sulphur content in up to 0.50% 0.50% 6.0% dry state (m/m) (m/m) up to 5% 3.65% Ash content in dry state 4.9% (m/m) (m/m) Volatile matter content up to 32% 29.52% 2.0% (850°C) in dry state (m/m) (m/m) Data obtained in the Volatile matter content up to 32% 30.54% characterisation process not (900°C) in dry state (m/m) (m/m) No more than 10% of sufficient enough to perform measurand value an uncertainty assessment Carbon content in dry up to 85.0% 84.5% 2.0% state (m/m) (m/m) Hydrogen content in dry up to 5% 4.87% 7.0% state (m/m) (m/m) Data obtained in the Nitrogen content in dry up to 1.5% 1.31% characterisation process not state (m/m) (m/m) sufficient enough to perform an uncertainty assessment Data obtained in the Phosphorus content in up to 0.015% 0.010% characterisation process not dry state (m/m) (m/m) sufficient enough to perform an uncertainty assessment Chlorine content in dry up to 0.15% 0.13% No more than 20% of 15.4% state (m/m) (m/m) measurand value Data obtained in the Mercury content in dry up to 0.030 0.020% characterisation process not state (m/m) (m/m) sufficient enough to perform an uncertainty assessment 4. Conclusions resulting from homogeneity, stability and reference material characterisation tests were included. A The presented results for the homogeneity, thorough analysis of each uncertainty component short-term and long-term stability assessments and its impact on the total standard uncertainty as well as reference material characterisation of the measurements will allow for a more confirmed the feasibility of the investigated certified effective and thought through planning of each coal reference material manufacturing process. step of the reference material manufacturing The obtained levels for the relative expanded process as well as designing a manufacturing uncertainties of the measurements meet the scheme of other reference materials such as other requirements set by the referenced standards solid fuels or waste related materials like fly ash and confirm the feasibility of the manufactured or furnace waste. certified reference material for establishing and maintaining metrological traceability of measurement results. In the uncertainty assessment components Linsinger T., Pauwels J., van der Veen A., Schimmel H., Acknowledgements Lamberty A. 2001. Homogeneity and stability of reference materials. Accreditation and Quality Assurance, 6: 20–25. The presented results are part of the project titled „Opracowanie Linsinger T., Raffaelli B., Oostra A. 2013. Certfication report metod wytwarzania innowacyjnych Certyfikowanych Materiałów The certification of the gross calorific value and mass Odniesienia (CRM) – nowa jakość w analityce paliw stałych” co- fractions of ash. C. H. N. S. Cl. major elements and trace financed from the Regional Operational Program of the Silesian elements in three coal materials: ERM -EF411 (hard coal). Voivodeship for 2014-2020 (European Regional Development ® ® ERM -EF412 (brown coal) and ERM -EF413 (furnace coke). Fund) under grant number UDA-RPSL.01.02.00-24-0656/17-01. Publications Office of the European Union, Luxembourg. van der Veen A., Linsinger T., Pauwels J. 2001. Uncertainty References calculations in the certification of reference materials. 2. Homogeneity study. Accreditation and Quality Assurance, 6: 26–30. Bettencourt da Silva R., Williams A. (eds) 2015. Eurachem/CITAC van der Veen A., Linsinger T., Schimmel H., Lamberty A., Guide: Setting and Using Target Uncertainty in Chemical Pauwels J. 2001. Uncertainty calculations in the certification Measurement. of reference materials 4. Characterisation and certification. Bulska E. 2008. Metrologia chemiczna. Sztuka prowadzenia Accreditation and Quality Assurance, 6: 290–294. pomiarów. Malamut, Warsaw. van der Veen A., Pauwels J. 2000. Uncertainty Calculations in Ellison S.L.R., Williams A. (eds) 2019. Eurachem/CITAC Guide: the Certification of Reference Materials. 1. Principles of Metrological Traceability in Analytical measurement. Analysis of Variance. Accreditation and Quality Assurance, Linsinger T., Pauwels J., Lamberty A., Schimmel H., van der 5, 12: 464–469. Veen A., Siekmann L. 2001. Estimating the uncertainty of stability for matrix CRMs. Fresenius' Journal of Analytical Chemistry, 370: 183–188.

Journal

Environmental & Socio-economic Studies – de Gruyter

Published: Mar 1, 2023

Keywords: certified reference materials; hard coal; traceability; homogeneity; stability of reference material

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