I have again had requests for an 'ultimate analysis' of landfill gas referencing ASTM Methods for coal and coke. Of course, the methods for solids cannot be applied to gases, but there are some possible ways to get close to the concept.
The ultimate analysis of coal and coke using methods in the ASTM D3176 series would report the elemental constituents, which can also be done with some of the gas methods. The problem is there are small gaps which have not been determined to be insignificant by any regulatory agency to my knowledge.
To give a starting basis for comparison, 40 CFR Subpart D 60.46 for fossil fuel fired steam generators, references ASTM D3176 for solid fuels and three series for natural gas fuels. The D1137 series has been withdrawn by ASTM, which leaves the D1945 and D1946 series for this comparison. These ASTM methods are similar to the fixed gas analysis of Method 3-C and NMOC analysis of Method 25-C which are my options of choice in this case.
Method 3-C gives the concentrations of O2, N2, CO2, and CH4,, which can be converted to the elemental composition of O, N, C, and H with some possible variations. The Method 3-C results are converted to a dry basis based on an assumption of moisture content. This adjustment can cause the total of the compounds to vary slightly above or below 100 percent. The carbon from organic compounds is also not reported. This may be a concern although they generally total in the hundreds or low thousands of ppm rather than percentage levels.
The carbon from the organics can be determined accurately by Method 25-C and added, but only if there are no adjustments performed to take the results to an air free basis, the same moisture determination is used for both the Methods 3-C and 25-C, and there are no additional dilutions to the sample. This will be combining two different methods and will result in a total concentration in excess of 100% more often than just the reported Method 3-C concentrations.
Alternatively, both the Method 3-C and 25-C analyses could be reported on a wet basis. Thus, a measured moisture content could be applied rather than the assumption of saturation at the barometric pressure and temperature, but this would change if the conditions were different at the analyzer. There could be instances of condensation in the sample even though dry helium is used to dilute the sample. This could result in totals of all compounds again exceeding 100%.
Another approach could be to report both of the analyses on the wet basis and once the total concentration of both was determined the difference could be assumed to be water vapor and the calculations done accordingly. All of these options will have some potential for minor loss of some of the more trace elements such as sulfur, chlorine, fluorine, or silicon, but these are generally in the low ppm ranges at most and should not have a significant impact on the total density of the gas.
I suspect the only difference for this approach would be the elimination of the >100% totals seen more often using the moisture correction from the Methods 3-C and 25-C. Most of the Method 3-C results seem to hold near the 100% level as it is, but the assumed moisture, analytical variability, and slight errors in pressure and temperature measurements do allow for totals both slightly over and slightly under.
Given this is more of an enforcement issue, rather than a technical issue, all that would be required is an enforcement decision on the application.
To allow a better understanding of how this would appear, I have pulled a recent set of three samples to give as examples.
The three samples showed very similar concentrations of O2, N2, CO2, and CH4, which made the over all comparison very easy.
The totals for just the fixed gases on the dry basis were 100.0576%, 99.6764%, and 100.1880%.
The O2 was ~2.25%, the N2 was ~ 13.25%, the CO2, was ~36.1% and the CH4 was ~48.3% over the three samples.
The NMOC concentrations were ~650 ppmC over the three samples.
As this was on the dry basis, the H2 and O2 from the water vapor would not be applied, but the H2 would be more than offset by the increase in reported CH4 from that adjustment, while the increase in O2 and CO2 would do the same for the associated O2 from the water vapor.
The ASTM methods, while similar, are substantially different from the EPA methods, which arguably have higher QA/QC standards and are possibly more defensible due to their being codified in the Federal Register as a Standard Reference Method. Even then there has to be a modification to the Method 25-C calculations to eliminate the adjustment to an air free basis. This modification could conceivably only be approved by the EPA itself as a alternative method for this application.
The critical aspect would be what is going to be the legal definition of an 'Ultimate Analysis' for the landfill gases if this is to be used or even if such an approach is necessary to determine fuel density.
I have copied a portion of the two ASTM methods in question for comparison to the similar EPA methods specified for landfill gases.
ASTM D1945 - Standard Test Method for Analysis of Natural Gas by Gas Chromatography
Significance and Use
4.1 This test method is of significance for providing data for calculating physical properties of the sample, such as heating value and relative density, or for monitoring the concentrations of one or more of the components in a mixture.
1.1 This test method covers the determination of the chemical composition of natural gases and similar gaseous mixtures within the range of composition shown in Table 1. This test method may be abbreviated for the analysis of lean natural gases containing negligible amounts of hexanes and higher hydrocarbons, or for the determination of one or more components, as required.
ASTM D1946 – 90(2015) - Standard Practice for Analysis of Reformed Gas by Gas Chromatography
Significance and Use
4.1 The information about the chemical composition can be used to calculate physical properties of the gas, such as heating (calorific) value and relative density. Combustion characteristics, products of combustion, toxicity, and interchangeability with other fuel gases may also be inferred from the chemical composition.
This practice covers the determination of the chemical composition of reformed gases and similar gaseous mixtures containing the following components: hydrogen, oxygen, nitrogen, carbon monoxide, carbon dioxide, methane, ethane, and ethylene.
METHOD 3C—DETERMINATION OF CARBON DIOXIDE, METHANE, NITROGEN, AND OXYGEN FROM STATIONARY SOURCES
1.0 Applicability and Principle
Applicability. This method applies to the analysis of carbon dioxide (CO2), methane (CH4), nitrogen (N2), and oxygen (O2) in samples from municipal solid waste landfills and other sources when specified in an applicable subpart.
METHOD 25C—DETERMINATION OF NONMETHANE ORGANIC COMPOUNDS (NMOC) IN LANDFILL GASES
Scope and Application
Analytes. Analyte CAS No. Nonmethane organic compounds (NMOC) No CAS number assigned. 1.2 Applicability. This method is applicable to the sampling and measurement of NMOC as carbon in landfill gases (LFG).
I know that gas density for landfill gas has a purpose, but I do not have any indication as to what has been done in this situation in the various applications. Any feedback on what has been used or accepted in the past would help determine what we can provide for our clients in such cases. Is the density based on the analysis of the fixed gases sufficient? This seems to be the case with the Subpart D reference, given the levels of NMOC as carbon are usually below the error range of the carbon in the fixed gas analysis. I look forward to input on what you think will be an appropriate approach.
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