Inorganic elements play an important role in the use of petroleum products, fuels, and lubricants in the industry. Similar to other petroleum products and lubricants, biofuels also have some inor- ganic elements as constituents of their composition. However, the elemental concentrations in biofuels are not as high as in many petroleum products and especially as in lubricants. Discussions on the determination of inorganic elements in petroleum products are available in the proceedings of two ASTM symposiums [1,2]. A review of spectroscopic elemental analysis of biofuels and biolubes is also available [3]. More recently, a comprehensive treatise on this subject has been published by Nadkarni [4]. ASTM WK 27610 reviews the elemental analysis test methods for all biodiesel and ethanol fuel specifications [3,5–7].
A list of the test methods used for elemental analysis is given in Table 5.1. Similar to physical or chemical test methods, most of these elemental test methods originated for use in analyzing petro- leum products and lubricants, and their precision specifically for biofuels is not always available. Additionally, a few new test meth- ods have been specifically developed for elemental analysis of biofuels.
Some of the elemental test methods have their counterparts from other standards writing organizations. These are summa- rized where available in Table 5.2. Although the international standards are not exactly identical to each other, if appropriately followed, they should yield equivalent results to their ASTM counterparts. Similar to the chromatographic methods of analy- sis for organic components in petroleum products and lubricants (Chapter 3), research on elemental analysis of these matrices continues to use a diverse variety of elemental analysis tech- niques, and a large amount of information is thus available in the literature.
EN 14538, Calcium, Potassium,
Magnesium, and Sodium Content of Fatty Acid Methyl Esters
Significance
These alkali and alkaline earth elements may be present in biodie- sel as abrasive solids or soluble metallic soaps. Abrasive solids can contribute to injector, fuel pump, piston, and ring wear, as well as
to engine deposits. Soluble metallic soaps have little effect on wear, but they may contribute to filter plugging and engine deposits.
Sodium and potassium are associated with the formation of ash within the engine; calcium soaps are responsible for injection pump sticking. High levels of calcium, magnesium, sodium, and potassium compounds may also be collected in exhaust particulate removal devices and are not typically removed during passive or active regeneration. They can create increased back pressure and reduced time to service maintenance because ash accumulates on the catalyst [5].
Metal ions are introduced into the biodiesel fuel during the production process. Whereas alkali metals come from catalyst residues, alkaline earth metals may originate from hard washing water.
In reality, there is very little calcium or magnesium in the diesel fuel or gasoline at a refinery with modern processing prac- tices. There could be calcium or magnesium in either fuel if they were contaminated by groundwater (which usually contains these metals) or by engine lube oils (from basic detergents in engine oil).
This is occasionally observed in contaminated fuel samples from the field, but it is not a fuel specification issue for either gasoline or diesel fuel. Caustic soda is a key processing aid in the desulfuriza- tion of gasoline, and the presence of trace amounts of sodium or potassium could originate from this step.
Scope
The biodiesel blend stock (B100) for middle distillate fuels (ASTM D6751) includes these four elements with a maximum limit of 5 mg/kg. Due to the biodiesel in B6 to B20 blends, the concentra- tion of these metals should be less than 1 to 2 ppm, making accu- rate measurement difficult. There are also no controls for these metals in ASTM D975 at present and no available database for the potential contributions of these metals from petroleum-based die- sel fuel. The following limits are set in biodiesel specification standards.
Region Na + K Test Method Ca + Mg Test Method
Brazil 10 mg/kg max EN 14108/14109 Report EN 14538 European Union 5 mg/kg max EN 14108/14109 5 mg/kg max EN 14538 United States 5 mg/kg max EN 14538 5 mg/kg max EN 14538 DOI: 10.1520/MNL772015001105
Table 5.1 List of ASTM Test Methods for Elemental Analysis for Biofuels
aSTM Standard Description
ASTM D129 Sulfur in Petroleum Products (General High Pressure Decomposition Device Method)
ASTM D482 Ash from Petroleum Products
ASTM D874 Sulfated Ash from Lubricating Oils and Additives ASTM D1266 Sulfur in Petroleum Products (Lamp Method)
ASTM D1552 Sulfur in Petroleum Products by High Temperature Combustion and IR Detection ASTM D2622 Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry ASTM D3120 Trace Quantities of Sulfur in Liquid Petroleum Hydrocarbons by Oxidative Microcoulometry
ASTM D3227 (Thiol Mercaptan) Sulfur in Gasoline, Kerosine, Aviation Turbine, and Distillate Fuels (Potentiometric Method) ASTM D3228 Total Nitrogen in Lubricating Oils and Fuel Oils by Modified Kjeldahl Method
ASTM D3231 Phosphorus in Gasoline
ASTM D3237 Lead in Gasoline by Atomic Absorption Spectrometry ASTM D3341 Lead in Gasoline—Iodine Monochloride Method
ASTM D4045 Sulfur in Petroleum Products by Hydrogenolysis and Rateometric Colorimetry
ASTM D4294 Sulfur in Petroleum and Petroleum Products by Energy Dispersive X-ray Fluorescence Spectrometry
ASTM D4628 Analysis of Barium, Calcium, Magnesium, and Zinc in Unused Lubricating Oils by Atomic Absorption Spectrometry ASTM D4629 Trace Nitrogen in Liquid Petroleum Hydrocarbons by Syringe/Inlet Oxidative Combustion and Chemiluminescence Detection ASTM D4927 Elemental Analysis of Lubricant and Additive Components—Barium, Calcium, Phosphorus, Sulfur, and Zinc by Wavelength-Dispersive
X-ray Fluorescence Spectroscopy
ASTM D4929 b Determination of Organic Chloride Content in Crude Oil
ASTM D4951 Determination of Additive Elements in Lubricating Oils by Inductively Coupled Plasma Atomic Emission Spectrometry ASTM D5059 Lead in Gasoline by X-ray Spectrometry
ASTM D5185 Multielement Determination of Used and Unused Lubricating Oils and Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
ASTM D5291 Instrumental Determination of Carbon, Hydrogen, and Nitrogen in Petroleum Products and Lubricants
ASTM D5453 Determination of Total Sulfur in Light Hydrocarbons, Spark-Ignition Engine Fuel, Diesel Engine Fuel, and Engine Oil by Ultraviolet Fluorescence
ASTM D5622 Determination of Total Oxygen in Gasoline and Methanol Fuels by Reductive Pyrolysis
ASTM D5623 Sulfur Compounds in Light Petroleum Liquids by Gas Chromatography and Sulfur Selective Detection ASTM D5762 Nitrogen in Petroleum and Petroleum Products by Boat-Inlet Chemiluminescence
ASTM D6920 Total Sulfur in Naphthas, Distillates, Reformulated Gasolines, Diesels, Biodiesels, and Motor Fuels by Oxidative Combustion and Electrochemical Detection
ASTM D7039 Sulfur in Gasoline, Diesel Fuel, Jet Fuel, Kerosine, Biodiesel, Biodiesel Blends, and Gasoline-Ethanol Blends by Monochromatic Wavelength Dispersive X-ray Fluorescence Spectrometry
ASTM D7318a Existent Inorganic Sulfate in Ethanol by Potentiometric Titration
ASTM D7319a Determination of Existent and Potential Sulfate and Inorganic Chloride in Fuel Ethanol and Butanol by Direct Injection Suppressed Ion Chromatography
ASTM D7328a Determination of Existent and Potential Inorganic Sulfate and Total Inorganic Chloride in Fuel Ethanol by Ion Chromatography Using Aqueous Sample Injection
ASTM D7757a Silicon in Gasoline and Related Products by Monochromatic Wavelength Dispersive X-ray Fluorescence Spectrometry EN 14538a Determination of Ca, K, Mg and Na Content of FAME by Optical Emission Spectral Method with Inductively Coupled Plasma WK 21755a Ca + Mg and Na + K by Combustion Followed by ICP-AES (Not Published)
ISO 13032a Determination of Low Concentration of Sulfur in Automotice Fuels Using Energy Dispersive X-ray Fluorescence Spectrometric Method
ASTM UOP 391 Trace Metals in Petroleum Products or Organics by AAS
aThese test methods were specifically developed for the analysis of biofuels.
Table 5.2 International Equivalents of Elemental Analysis Methods for Biofuels
analysis aSTM IP ISO DIN JIS
Sulfur by Bomb Method ASTM D129 61 51577
Ash ASTM D482 4 6245 K 2272
Sulfated Ash ASTM D874 163 3987 51575 K 2272
Sulfur by Lamp ASTM D1266 107
Sulfur by WD-XRF ASTM D2622 51400T6 K 2541
Sulfur by Oxidative Microcoulometry ASTM D3120 16591
Mercaptan Sulfur ASTM D3227 342 3012 K 2276
Lead by AAS ASTM D3237 428
Lead by ICl Method ASTM D3341 270 3830 51769 T2 K 2255
Metals by ED-XRF ASTM D4294 336 8754
Metals by AAS ASTM D4628 308 51391T1
Nitrogen by Chemiluminescence ASTM D4629 379
Metals by WD-XRF ASTM D4927 407 51391T2
Note: Excerpted from Ref. [8]. AAS = atomic absorption spectrometry; ED-XRF = energy-dispersive X-ray fluorescence analysis; ICl = iodine monochloride;
WD-XRF = wavelength-dispersive X-ray fluorescence.
analySiS
In the test method EN 14538, an exactly weighed test portion is diluted with kerosine using a 1:1 weight ratio. The resulting solution is directly injected into the plasma of an inductively coupled plasma atomic emission spectrometry (ICP-AES). The interference-free wavelengths recommended for analysis are:
• Calcium 422.673 (or 317.933, 393.366, 396.847) nm
• Magnesium 279.553 (or 285.213) nm
• Sodium 588.995 (or 589.592) nm
• Potassium 769.897 (or 766.490) nm
For reference and calibration purposes, calibration samples with known amounts of the elements under investigation in the range of 0.5 to 10 mg/kg are used. The sums of the content of cal- cium and magnesium and sodium and potassium are reported.
preciSion
The following repeatability and reproducibility have been found for this test method.
In a separate ASTM interlaboratory study, six biodiesel sam- ples used in the D02.CS 92 Proficiency Testing Program (PTP) were analyzed for these four elements using a method identical to EN 14538. Only a single sample was analyzed in each laboratory so the repeatability cannot be calculated per ASTM D6300 protocol.
However, the reproducibility found for this analysis was (Ca + Mg) = 2.8 X0.6 and (Na + K) = 1.3 X0.5, where X is the mean of two results.
The reproducibility of EN 14538 is somewhat superior than the one found here. We believe, however, that the ASTM interlaboratory
elements Repeatability Reproducibility
Ca + Mg 0.023 X + 0.271 0.149 X + 1.186
Na + K 0.020 X + 0.193 0.191 X + 0.941
Note: Where X is the mean of the two results.
study reproducibility value is more realistic because this calcula- tion is based on data from 42 laboratories collected over a period of two years. No information is available on the details of the Center for European Normalization (CEN) crosscheck to arrive at their precision calculations.
Results from all ASTM PTPs for biodiesel also indicate very poor quality data using the EN 14538 test method. As examples in Table 5.3 show, there is no value whatsoever in such ineffective analysis, where the standard deviation is equal to or higher than the mean value.
The biodiesel specification ASTM D6751 includes ASTM UOP 391 as an alternate suggested test method for the aforementioned metal analysis instead of EN 14538. The test method consists of wet ashing of an organic material with fuming sulfuric acid that is ignited and ashed at 538°C. The residue is then dissolved with aqua regia and, after evaporation, is dissolved in dilute hydrochloric acid containing scandium as an internal standard. In ASTM UOP 389, the trace elements are determined by ICP-AES, and in ASTM UOP 391 they are determined by atomic absorption spectrometry (AAS).
The fuming sulfuric acid required for dissolution of the sam- ple must be of ultra-high purity with no trace elements. Although it was at one time available from VWR Corp, this reagent has now
Table 5.3 Determination of Calcium Plus Magnesium (Ca + Mg) and Sodium Plus Potassium (Na + K) in Biodiesels
biodiesel Sample Ca + Mg, mg/kg Na + K, mg/kg
BIOD 1204 0.05 ± 0.08 (33) 0.31 ± 0.38 (33)
BIOD 1208 0.07 ± 0.10 (31) 0.53 ± 0.51 (34)
BIOD 1304 0.04 ± 0.06 (34) 0.19 ± 0.24 (36)
BIOD 1308 0.05 ± 0.07 (35) 0.10 ± 0.14 (33)
BIOD 1311 0.03 ± 0.05 (37) 0.13 ± 0.20 (39)
Note: All results are expressed as robust mean ± robust standard deviation (number of valid results).
been discontinued. Therefore, this test method can no longer be used for such analysis.
Inductively Coupled Plasma Atomic Emission Spectrometric Determination of Trace Metals in Fuel Ethanol
This proposed test method would allow several trace elements (copper, lead, phosphorus, etc.) to be determined in fuel ethanol and would replace other currently allowed test methods (ASTM D1688 for copper, ASTM D5059 for lead, and ASTM D3231 or ASTM D4951 for phosphorus). A sample is evaporated to a small volume to remove most of its volatile components. The residue is reconstituted with deionized water or dilute nitric acid, and the trace elements are determined with ICP-AES. Aqueous cal- ibration standards are used. Alternatively, the sample may be directly nebulized into the ICP-AES torch and the elements determined.
The elements that would be determined by this proposed test method are calcium (Ca), copper (Cu), iron (Fe), lead (Pb), magne- sium (Mg), phosphorus (P), potassium (K), sodium (Na), and sul- fur (S). Estimated detection limits would vary from 5 to 50 mg/kg.
Although proposed some time ago, ASTM D02 has not completed this ILS.
Agilent: 5991-0771EN (2012), Metals in Biofuels by Microwave Plasma Atomic Emission
Spectrometry
The presence of metals and metalloids in petrochemical products can influence the performance of engines and can contribute to shortening the life of the machinery. The presence of metals may deteriorate the fuel quality by oxidative decomposition reactions.
Additionally, some elements act as a catalyst poison, contributing to increases in the amount of toxic gases and particulate matter emitted by the vehicles. Some elements can be naturally present in ethanol as a result of the soil composition where sugarcane has grown. Alternatively, these elements can be introduced into the fuel during its production, storage, or transport (or combinations thereof). Thus, after fuel combustion, these elements can signifi- cantly increase air pollution.
Some examples of the use of atomic spectroscopy for the analysis of biofuels are given in references using flame AAS (FAAS) for biodiesel [9], FAAS for ethanol [10], electrothermal vaporiza- tion (ETV) ICP-AES for ethanol [11], and so on. Microwave plasma atomic emission spectrometry (MP-AES) has been suggested as an alternative to ICP-AES for metal analysis. Its use for the determi- nation of silicon (Si) in gasoline and diesel has been mentioned earlier in this chapter [12].
Additionally, this technique has been demonstrated for the determination of chromium (Cr), nickel (Ni), Pb, and vanadium
(V) in ethanol fuel using an Agilent 4100 MP-AES instrument [12].
Ethanol fuel samples were diluted tenfold with nitric acid. Aqueous element standards containing 10 % ethanol were used. Based on spiked recovery analyses, between 92 % and 108 % recoveries were obtained indicating low intensity of matrix effects commonly caused by organic compounds and concomitant elements such as Cu, Na, and Fe. See Table 5.4.
Scope
This proposed test method provides an easy and reliable measure- ment to ensure that these biofuels meet the appropriate quality standards prior to blending. The MP-AES technique is especially useful for measurements in remote regions where gas supplies are problematic because the instrument needs only nitrogen gas that can be generated in the laboratory. This test method covers the determination of metals and metalloids in biodiesel or ethanol.
The specific elements that can be determined include Ca, Cr, Cu, Pb, Mg, Ni, P, K, Si, Na, and V. This test method uses soluble metals for calibration and does not purport to quantitatively determine insoluble particulates. Analytical results are particle size depen- dent, and low results are obtained for particles larger than a few micrometres.
Test times are approximately a few minutes per test specimen, and detectability for most elements is in the low mg/kg range.
interferenceS
Spectral interferences can usually be avoided by judicious choice of analytical wavelengths. When spectral interferences cannot be avoided, the necessary corrections should be made using the computer software supplied by the instrument manufacturer.
Differences in the viscosities of the test specimen solution and standard solutions can cause differences in solution uptake rates.
These differences can adversely affect the accuracy of the analy- sis. The effects can be reduced by using a peristaltic pump to deliver solutions to the nebulizer or by the use of internal stan- dardization (or both), with mass flow control on the nebulizer gas line for more reliable flow. Particulates can plug the nebu- lizer, thereby causing low results. Use of a total dissolved solids Table 5.4 Recovery Studies of Spiked Ethanol Fuel Samples
element Wavelength, nm added, μg/l Found, μg/l Recovery, %
Chromium 425.433 20 21.2 ± 1.2 106
100 95.1 ± 1.2 95
500 460 ± 30 92
Nickel 352.454 100 95.3 ± 0.8 95
Lead 405.781 400 430 ± 10 108
1000 990 ± 10 99
Vanadium 437.923 20 19.8 ± 1.6 99
100 98.4 ± 1.4 98
500 460 ± 20 92
From Ref. [12].
nebulizer helps to minimize this effect. Also, the specimen introduction system can limit the transport of particulates, and the plasma can incompletely atomize particulates, thereby caus- ing low results.
analySiS
Bioethanol samples are diluted 1:10 with aqueous dilute nitric acid, and diesel or biodiesel samples are diluted 1:10 with an organic solvent such as ethanol, xylene, monoisobutyl ketone, Di-isobutyl ketone, white spirit, or narrow cut kerosine and are mixed well. The diluted samples are directly introduced into the microwave plasma atomic emission spectrometer. Measurements of emission intensity at the appropriate wavelength for each ele- ment are then compared with measurements made under the same conditions on a standard reference elemental solution.
preciSion
An actual interlaboratory study for demonstrating the precision of the proposed technique has not yet taken place.
Inductively Coupled Plasma Mass Spectrometric Determination of Trace Metals in Biofuels
Inductively coupled plasma mass spectrometry (ICP-MS) is a powerful technique for the determination of trace elements in petroleum products and fuels. It is superior to ICP-AES in its sensitivity. Hwang reviewed the applications of ICP-MS tech- nique for the determination of multiple trace elements in biodie- sel and bioethanol [13]. Lachas et al. compared several sample preparations (including wet ashing and microwave extraction) for trace element analysis in milligram sample sizes of biomass samples using ICP-MS. The accuracy and sensitivity of the mea- surements improved when the dilution rate was decreased from 5,000 to 1,000 and to 500 [14]. Baernthaler et al. developed a reliable ashing method to accurately determine both major and minor ash-forming elements in solid biofuels using ICP-MS.
Wood and bark, straw, and olive residues were analyzed using several digestion and analytical methods [15]. The digestion methods included wet and dry ashing with different acid mix- tures. Analytical techniques surveyed included FAAS, graphite furnace AAS, cold vapor AAS, ICP-AES, ICP-MS, and X-ray fluorescence.
Saint’Pierre et al. used an ignition delay electrothermal vaporization inductively coupled plasma mass spectrometry (ID-ETV-ICP-MS) technique to determine several trace ele- ments in a fuel alcohol sample [16]. The ID proved to be a robust, fast, and simple calibration technique for the analysis of fuel ethanol. Later, Saint’Pierre et al. developed a flow injection sys- tem, coupling it to an ultrasonic nebulizer for direct introduc- tion of fuel ethanol for trace elements analysis by ICP-MS [17].
External calibration versus aqueous solutions, matrix matching, and ID were compared. Both ID and calibration with aqueous solutions improved speed, precision, sensitivity, and agreement
with the results obtained by ETV-ICP-MS. Saint’Pierre et al. also presented the use of many sample preparation and sample intro- duction systems with an ICP-MS for the analysis of biofuels and petroleum fuels [18]. Woods and Fryer used an ICP-MS fitted with an octopole reaction system (ORS) to directly measure the inorganic contents of several biodiesel materials [19]. Following the sample’s dilution with kerosine, the biofuel was analyzed directly. The ORS effectively removed matrix- and plasma-based spectral interferences to enable measurement of all important analytes, including sulfur, at levels below those possible by ICP-AES.
ASTM D5291, Carbon and Hydrogen in Petroleum Products
Significance
Carbon, hydrogen, and nitrogen analyses are useful in deter- mining the complex nature of sample types. These results can be used to estimate the processing and refining potentials and yields in the petrochemical industry. Hydrogen to carbon ratio is useful for assessing the performance of upgrading processes.
ASTM D7566 allows a maximum of 99.5 mass percent car- bon-hydrogen for hydroprocessed synthetic paraffinic kerosine.
ASTM D5291 is specified for carbon-hydrogen determination in ASTM D7566.
analySiS
Four procedures included in ASTM D5291 use combustion fol- lowed by infrared (IR) detection, gas chromatographic separation, or thermal conductivity detection. Each procedure is based on specific commercial instrumentation.
preciSion
Precision obtainable by ASTM D5291 is summarized in Table 5.5. The samples included in this study were not biofuels.
ASTM D4929b, Organic Chloride Content of Crude Oil
Significance
Chloride ions even at low concentrations in denatured fuel etha- nol are corrosive to many metals. Similarly, ionic (i.e., inorganic)
Table 5.5 Precision of ASTM D5291 for Carbon and Hydrogen
element Concentration
Range, mass % Procedure Repeatability Reproducibility
Carbon 75–87 A–C X + 48.48
(0.0072)
X + 48.48 (0.018)
D 0.5644 1.4671
Hydrogen 9–16 A–C X0.5 × 0.1162 X0.5 × 0.2314
D 0.5905 1.9089
Note: Where X is the average of two determinations in mass percent.