Triangle NoTES


February 2019


To continue with the discussion on the 'Ultimate Analysis' of landfill gas with some of the feedback and what we have determined in the interim.


First, it is understood that some of the elements would not be included in the calculations. The trace sulfur and chlorine, for example. There would also not be an inclusion for the hydrogen and oxygen from the organic compounds reported in the Method 25-C analysis. While there can be even more analyses to determine these components, to a greater degree it seems to be a strong potential for the diminishing return on expenditures.


To better determine what the results would resemble for a landfill gas report, I did a couple of groups of calculations based on a set of three samples which showed consistent results over all of the analysis.


On the wet basis, the mid-range sample had 35.15% CO2, 47.36% CH4 , 12.31% N2, 2.14% O2,, and 0.07% NMOC which left a calculated 2.98% H2O to bring everything to 100%. This translated to a weight basis of 55.76% CO2, 27.38% CH4 , 12.43% N2, 2.14% O2,, 1.93% H2O and 0.03% NMOC using the molecular weight of the components.


Converting this to the elemental components and using the atomic weight of the individual atoms for a weight calculation created a difference just from rounding error. The MW of CH4 is different from the accepted weight of four hydrogen atoms and one carbon atom due solely to rounding. The difference is 0.01 lower for the MW of CH4. With the amount of CH4 in the sample, the difference total is 0.02% higher using the atomic weight of the atoms.


Converting the results to a elemental level and taking the analytical variation between the three analyses of each sample the results are 35.74 +0.01% C, 44.72 +0.02% O, 12.43 +0.01% N, and a constant 7.11% H, which does include water vapor. Some of the references I have found for the solid fuel reports include the water content in the elemental percentage, while others list the water separately from the elemental percentage. The difference seemed to be related to the determination of BTU value based on the elemental percentages. Since the BTU value can more easily be determined by the molecular components in the gas samples and the elemental approach would be mainly for gas density, the water being included in the elemental percentage seem more reasonable to me.


If we do not include the Method 25-C results and use only the Method 3-C as seems to have been indicated in the Subpart D reference to fixed gas analysis, the results become 35.71% C, 44.75% O, 12.43% N, and 7.12% H. This loss of NMOC as carbon does not seem to be very significant when compared to the rounding and analytical uncertainty variations and especially when the costs are included. It does, however, help to offset the other elements not included due to even higher analytical costs.



The percentage breakdown of the molecular components by volume and by weight both including and excluding the NMOC carbon portion:


CO2

CH4

N2

O2

NMOC

H2O - by

subtraction

35.15%

47.36%

12.31%

2.14%

0.07%

2.98%

By vol

55.76%

27.38%

12.43%

2.47%

0.03%

1.93%

By wt

35.15%

47.36%

12.31%

2.14%

NA

3.04%

By vol -NMOC

55.75%

27.38%

12.43%

2.47%

NA

1.98%

By wt -NMOC


The percentage breakdown of the atomic components:


C

O

N

H


35.74%

44.72%

12.43%

7.11%

Incl NMOC carbon

+0.01%

+0.02%

+0.01%

+0.00%

Analytical variation

35.71%

44.75%

12.43%

7.11%

-NMOC carbon

Note: The MW of CH4 is 16.04 as opposed to 16.05 when the rounded atomic weights are used. This makes a 0.02% increase in the atomic versus molecular weight comparison.


Now the information I received in feedback on projects which had used Method 3-C for a mass or density calculation can be discussed. There were two projects discussed and one was more of a biogas than landfill gas location.


The first project was one in which the correction for moisture resulted in a total in excess of 100% for the three samples involved. The solution was to normalize the reported concentrations to 100%, which seemed like a good choice at the time. In our recent discussion I pointed out this meant the measured concentrations were adjusted by an assumed moisture content and then normalized to 100% thereby leaving the moisture at the assumed level and adjusting the measured components accordingly. Fortunately, leaving the measured components at the wet basis and assuming the adjustment for moisture would take that to 100% made an insignificant change to the calculations which had been previously used.


The second project was one in which the correction for moisture was fairly accurate given the totals were near 100% with a close over-and-under range. For this project the components were averaged over the three runs and those averages were used in the final calculations. This also included the average of the three analyses of NMOC as carbon in the totals. The average total did not exceed 100% but was very close so normalization was not considered. This is almost the same process which would be used in totaling the components as wet basis and then considering the moisture as being the remaining percentage. The averaging of the three runs would be a very good addition to the calculations as well.




From what has been used in the field already and what I have seen in these calculations of the single test run, there is little impact from the uncertainty of the analysis or the trace compounds which are not included. The conversion to the elemental level adds a slight positive bias due to the rounding issues, which could be considered as an offset to the lost elements. Whether that offset is sufficient or too much will be something a regulatory entity will have to determine if this process is to be used to determine the fuel density. The addition of the NMOC from the Method 25-C does not seem to be critical nor significant, but it is an easy addition which arguably makes the total calculations more accurate. Given the alternative here would be for the unreported percentage carbon from NMOC to be replaced by moisture, the density of water is greater than that of carbon if that were used. If even an ordinary hydrocarbon, such as hexane, were chosen to be used in place of the NMOC only the dry basis density would change as the moisture density would be greater than the hydrocarbons.


How does this all shake out? Just from the cost to benefit comparison it would be like this:


Method 3-C measured concentrations wet basis with only moisture assumed to make the difference to 100% total weight. The lowest cost approach and seems to give ~99+% of the density of the gas .


Methods 3-C and 25-C measured concentrations wet basis with only moisture assumed to make the difference to 100% total weight and converting the NMOC to at least hexane for a slightly better assumption. Higher cost and maybe only the perception of greater accuracy. This approach still seems to give ~99+% of the density of the gas if only just slightly higher.


Methods 3-C, 25-C, and 16 for Total Reduced Sulfurs wet basis with only moisture assumed to make the difference to 100%. Much higher cost and maybe only the perception of greater accuracy. This approach still seems to give ~99+% of the density of the gas.


Any other analytical methods will increase the costs, but really have little impact on the gas density given the insignificant percentages involved. Even the cumulative effect does not seem to be noticeable given the calculation for the NMOC compared to the water variation.


The change to density for the dry basis will be the largest changes, as the moisture density would offset much of these minor additions for gas density on the wet basis.



Wayne Stollings

Triangle Environmental Services, Inc.



Wstollings@aol.com

P.O. Box 13294 122 US Hwy 70 E

Research Triangle Park, NC 27709 Hillsborough, NC 27278

(919) 361-2890 (800) 367-4862 Fax: (919) 361-3474

January 2019

Triangle NoTES


January 2019


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. Scope

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.

1. Scope

    1. 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



    1. 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

    1. 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.

Wayne Stollings

Triangle Environmental Services, Inc.


Wstollings@aol.com

P.O. Box 13294 122 US Hwy 70 E

Research Triangle Park, NC 27709 Hillsborough, NC 27278

(919) 361-2890 (800) 367-4862 Fax: (919) 361-3474

Sept 2018

Triangle NoTES


September 2018

I have been discussing what can only be described as perceptions of analyses based on assumptions. It is hard to not succumb to some of the assumptions based only on “common sense” but there are more things to consider when quality is involved.


One of the biggest points of confusion is that between precision and accuracy. A precise analysis is not always an accurate analysis, and an accurate analysis may not be that precise. Precision is usually the basis for the desire to look for the lowest detection limit possible. It seems logical that the lower the detection limit the better the analysis, but that is not always the case. If you are dealing with low levels near the detection limit, this can be a good move, but if you are looking for higher levels it may not be so. The fact is a detector can become saturated by a compound or compounds, which may require an additional dilution for the analysis to be completed. This means the detection limit for the sample is not the same as the limit at the analyzer. That analyzer limit is what you may be getting when you ask about the detection limit. This is especially true for multiple compounds reported using only one compound as the calibration or in the case of a method such as TO-15 which actually is set to condense larger sample volumes prior to injection.


Method 25 and 25-C make the detection limit very difficult to determine because using just the detection limit for propane ignores the other peaks which may elute. This is why we just use a very low calibration point. To put it into perspective, if you were have an analysis of C1 – C6 hydrocarbons using just one compound for the calibration, the detection limit for one peak would be very low, but the multiplication of that single peak limit by every other normally reported peak not detected would make it much larger. Thus, if you only had one compound detected you would be unable to determine how many other compounds may have been present below that detection limit. It is common to assume there were none, but that is an assumption that can easily be incorrect. Such a situation would impact the accuracy of the results even though the precision may be great.


The accuracy question is why audits are so important. If the results can be checked against known concentrations, the general accuracy can be determined. It does not mean that some of the analyses are without problems, but it is a means to confirm the general accuracy of the laboratory and its procedures. I believe everyone knows the required accuracy for Method 25 is +20% from the known in order to pass, which is also the requirement used for the few Method 25-C type audits. To compare this with the TO-15 methodology, that requirement is +30% at ambient levels. There is no requirement for higher concentrations which are often where the method is used. In the few instances of a Method 25 audit being used in a Method 25-C type analysis with us, I have not been told of any failures, but I do know of two which passed the requirements. I also know that we passed a full Method 25 audit which was slightly above the 50 ppmC limit.

Another aspect of accuracy is whether the results are reproducible, for which a TO-15 analysis duplicate analysis performed from the same canister must indicate a maximum of a 25% variation between the two analyses. As Method 25/25-C reports the average of triplicate injections from the same canister and there is a requirement for a < 5% relative standard deviation for the analysis to be valid, there is no real comparison between the requirements.



The various other types of analyses utilizing single injection from a whole gas sample probably will not have any similar requirements as does TO-15. The ASTM and other non-governmental types of methods seem to be more flexible than the EPA or other governmental reference methods, such as the SCAQMD methods, for example. Even in the case of the relatively common C1-C6 hydrocarbon analysis, there are no audits available through the TNI program for which a basic accuracy check may be performed.


Another aspect of the assumption regarding the use of most methods is the fact that there will be unknown compounds. In many cases what is in the sample stream does not correspond with the target compound list for the method. Even in the C1-C6 type analyses there are differences between the various carbon compounds which cannot be determined unless the compounds are individually identified. For example, early on in the Method 25-C program, we would have what we called a “fingerprint analysis” performed. This was simply a GC/MS scan of the sample to determine the top 10 – 20 compounds present. This resulted in the majority of compounds being reported as merely a C11, C12, or even C13 compound, with no information on the structure or specifics on the compound. Of course, when the analysis included the TO-14 or TO-15 target list, there were compounds reported, but in much smaller concentrations than the major constituents of the “fingerprint analysis”. If you look at the AP-42 for landfill gas, the TO-15 target list is clearly represented, but not that the constituents are identified as it really only represents the hits from the target compound lists used for the analyses.


To put all of this into perspective, there was a large project in the early 1990s where, in addition to the Method 25 analysis on some 15 sources at a facility, it was requested that we determine the exact constituents for each source. Working from the MSDSs to determine what was potentially being emitted, we set up a sampling plan to cover those compounds and any additional compounds which could be added to the analysis. The TGNMOC program analytical cost at the time was about $10,000.00 but the additional program to identify the species of the carbon was in excess of $250,000.00. The costs to run several sampling trains for the different compounds for each of the sources hopefully was not that magnitude of difference, but it was still several times more given the increase in testing crews needed. The final question came down to whether that additional cost really gave proportionally more usable data because there was still the possibility something was missed, whether the concentrations were comparably accurate, and whether the non-detected compounds were properly included since many of them were not expected to be in the samples. Even though this was over a quarter of a century ago, some of the same issues are still present today in sampling programs. Thus, the perceptions of quality information may not give any significant amount of usable data and may just act to increase the costs.


Wayne Stollings

Triangle Environmental Services, Inc.


Wstollings@aol.com


P.O. Box 13294 122 US Hwy 70 E

Research Triangle Park, NC 27709 Hillsborough, NC 27278

(919) 361-2890 (800) 367-4862 Fax: (919) 361-3474