United States Environmental Protection Agency Air and Energy Engineering Research Laboratory Research Triangle Park, NC 27711 Research and Development EPA/600/SR-93/088 July 1993 EPA Project Summary Development of Sampling and Analytical Methods for the Measurement of Nitrous Oxide from Fossil Fuel Combustion Sources Jeffrey V. Ryan and Shawn A. Karns The combustion of fossil fuels is sus- pected to contribute to measured in- creases in ambient concentrations of nitrous oxide (N2O). Accurate and reli- able measurement techniques would help to assess the relative contribution of fossil fuel combustion N2O emissions to the increase in ambient concentra- tions. The characterization of NO emis- sions from fossil fuel combustion sources has been hindered by the lack of suitable and acceptable grab sam- pling and on-line monitoring method- ologies. Grab samples have been shown to be compromised by a sam- pling artifact where N2O is actually gen- erated in the sample container in the presence of SO2, NOx, and moisture. On-line monitoring techniques are lim- ited and, of those available, instrument costs are often prohibitive, detection levels are often insufficient, and the techniques are often susceptible to in- terferences present in combustion pro- cess effluents. The report documents the technical approach and results achieved while developing a grab sam- pling method and an automated, on- line gas chromatography method suitable to characterize N^O emissions from fossil fuel combustion sources. The two methods developed have been documented as EPA/Air and Energy Engineering Research Laboratory (AEERL) Recommended Operating Pro- cedures (ROPs). This Project Summary was devel- oped by EPA's Air and Energy Engi- neering Research Laboratory, Research Triangle Park, NC, to announce key find- ings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction Nitrous oxide (N2O) has been a great concern to the combustion community largely because the combustion of fossil fuels has been proposed as a potential contributor to measured increases in ambi- ent N20 concentrations. Currently, atmo- spheric N2O concentrations are increasing at nearly 1 ppbv annually from a present level of approximately 303 ppbv. This in- crease is significant because N2O is con- sidered a "greenhouse" gas and is associated with stratospheric ozone deple- tion. Studies tracking atmospheric in- creases of carbon dioxide (CO2) over time reveal that the increases of both N2O and CO2 occur similarly and that increases of both these anthropogenic pollutants corre- late well with increases in industrial activ- ity. Early efforts to characterize N2O emis- sions from fossil fuel combustion sources focused on identifying a relationship be- tween nitrogen oxides (NOX) and N2O emis- sions. Data were nominally collected in a "piggy back" manner, where N20 grab samples were collected during NOX perfor- mance tests. Considerable data exist on N2O emissions from diverse combustion sources and techniques firing on various fossil fuels. Much of the data reported on N2O measurements from fossil fuel com- bustion sources were obtained using grab sampling methods conducive to a sam- pling artifact that biases the measured val- Printed on Recycled Paper ------- ues of actual emissions. The grab sam- pling artifact is a situation where N20 is actually generated in grab sample contain- ers in the presence of NOX, sulfur dioxide (SO2), and moisture (H2O). N2O generation approaching 200 ppm in grab sample con- tainers has been observed. Realizing that accurate and reliable N2O measurements were essential to emissions characterization research, the Combustion Research Branch (CRB) of EPA's Air and Energy Engineering Research Laboratory (AEERL) initiated a program to concur- rently develop grab sampling and on-line monitoring methodologies suitable for char- acterizing N2O emissions from various com- bustion sources and processes. As a result of this program, two AEERL Recommended Operating Procedures (ROP) were gener- ated. ROP No. 45: "Analysis of Nitrous Oxide from Combustion Sources" details a gas chromatography/electron capture de- tector (GC/ECD) method suitable for grab sample analysis as well as on-line monitor- ing. ROP No. 56: "Collection of Gaseous Grab Samples from Combustion Sources for Nitrous Oxide Measurement" details a grab sampling method suitable for collect- ing gaseous grab samples from combus- tion sources to screen N2O emissions. This report documents the approach and re- sults obtained while developing these pro- cedures. Analytical Method The GC/ECD backflush method devel- oped was found to be suitable for measur- ing N2O from a variety of combustion sources and applications. In addition, the method was found to be equally suitable for on-line monitoring or grab sample analy- sis. Analytical interferences, present in com- bustion process effluents, were negated through the use of a backflushing tech- nique. The backflushing method uses a single, 10-port valve to divert/direct the flow of carrier and sample gas streams through the chromatographic system. This technique employs a precolumn to isolate the analyte of interest from slower eluting interferences such as SO2 and moisture. Once the analyte of interest has eluted from the precolumn to the secondary ana- lytical column, the carrier gas flow through the precolumn is reversed, flushing the undesirable components from the precolumn. Other common flue gas com- ponents—oxygen, carbon monoxide (CO), CO2, NOX, total unburned hydrocarbons (THCs), and ammonia (NH3)—were found not to interfere with the analytical proce- dure. To eliminate the need for manual valve switching, an air actuator, controlled by the GC electronic hardware, was used. Electri- cally energized solenoid valves were used to control actuator operation. The valving system was automated by interfacing the GC and integrator to a timed event control module that converted digital commands from the integrator to time controlled elec- trical switches. The integrator could be programmed to turn the solenoid valves on or off at specific times. The solenoid valves, when actuated, allowed compressed air to pressurize the air actuator. When pressur- ized, the air actuator rotates the 10-port valve to the desired position. The developed system is also capable of unattended, continuous operation, by incorporating the programmed timed events into a separate BASIC program capable of loop functions. At the end of the analytical run, the system is capable of automatically re-initiating the sequence of timed events. Using this method for on-line monitoring allows a semicontinuous measurement approximately every 8 min. The system can be easily incorporated into most con- tinuous emission monitoring sample deliv- ery/conditioning systems, with the only requirements being the removal of particu- late and moisture from the sample stream by a refrigeration condenser. The sample stream should be diverted to the analytical system before further moisture condition- ing by a desiccant. Figure 1 is a schematic diagram of the automated system. N2O is quantified from the linear rela- tionship between N2O concentration and integrated peak area. A least squares lin- ear regression of the calibration variables is used to establish the linear relationship. To improve quantitative accuracy as well as expand the linear range of quantitation, alternative mathematical approaches to lin- earize detector response were investigated. The linear regression approach enables the determination of quantitative bias on an absolute basis. With this approach, er- ror can be reported as less than a certain concentration, often reported as a percent- age of full scale or as deviation from the true or known value. A problem arises in that the estimated bias for low concentra- tions will be very large relative to the mea- sured or true concentration. By performing a linear regression of natural log (In) trans- formed calibration variables, error is ca- pable of being reported on a relative basis. Table 1 compares the relative bias of calculated concentrations (relative to the true concentration) using both quantitative approaches. The linear regression of the transformed calibration variables was ef- fective in minimizing the relative error of calculated concentrations. Less than 10% bias was observed over the entire quanti- tative range as opposed to as much as 700% relative bias for the non-transformed quantitative approach. The automated, on-line GC/ECD sys- tem was evaluated extensively on a num- ber of diverse EPA/AEERL fossil fuel combustion test facilities. Initially, the ana- lytical system was used exclusively during the development of the N2O grab sampling method. On-line and grab sample mea- surements were performed on gases gen- erated by the Flue Gas Simulation System (FGSS). The on-line concentrations mea- sured were compared to grab sample mea- sured concentrations to assess artifact generation. For AEERL's Gas Cleaning Technology Branch (GCTB), the N2O moni- toring system was used to measure N2O emissions resulting from the combustion of various coals during parametric SO2 re- moval testing. The N2O concentrations measured 0.5-10 ppm. During these tests, quality control span checks were performed nearly every hour over the 8-h test period. The quality control checks were used to assess method ana- lytical bias and precision over the course of the entire test period. The average bias observed (2.9%) was well within the tar- geted level of less than 15%. Similarly, the precision observed (2.7%), expressed as percent relative standard deviation (RSD), was well within the targeted level of less than 10%. Grab Sampling Method The discovery of the N2O sampling arti- fact attenuated the need for a standard- ized, reliable sampling method in order to accurately assess the N2O emissions from fossil fuel combustion sources. Realizing that consistently eliminating the sampling artifact entirely would be extremely diffi- cult, if not impossible, the AEERL/CRB believed that minimizing N2O generation to consistent levels within aged sample con- tainers would be suitable to screen for high N2O-emitting combustion sources. Specifi- cally, AEERL researchers felt that consis- tently minimizing N2O generation within grab sample containers to less than 10 ppm over a 1-2 week period would be more than acceptable to screen for high N2O-emitting fossil fuel combustion sources. The screening technique could then be used to direct on-line monitoring efforts. The screening of intended fossil fuel combustion sources would require the vol- untary cooperation of commercial and re- search combustion facilities alike. Therefore, the grab sampling equipment and technique must be easy to use and pose minimal imposition to those partici- ------- To CEMs 12:fr Analytical': Column —'- A/2O Span Gas Air or House Air P5 Carrier Gas Sample Event Control Module Integrator (') 1 ft = 0.305 m Figure 1. Automated, on-line GC/ECD N2O monitoring system. Table 1. Comparison of Relative Bias Using Differing Mathematical Approaches Linear Regression (Untransformed Variables) Linear Regression (Transformed Variables) N2O Known ppm 0.5I 0.97 1.99 5.03 9.85 19.4 40.4 80.1 128 N2O Calc. ppm -3.11 -2.13 -0.40 4.58 11.35 23.18 45.74 83.36 123.68 % Bias -705. 1 -319.6 -120.1 -8.9 15.2 19.5 13.2 4.1 -3.4 N2O Calc. ppm 0.47 0.99 2.02 5.36 10.41 20.11 40.45 77.74 120.79 % Bias -8.6 2.1 1.5 6.6 5.7 3.7 0.1 -2.9 -5.6 pating in screening surveys. Specifically, the grab sampling method should not re- quire a great degree of sampling exper- tise. In addition, the grab sample should be obtainable in a manner compatible with commonly employed continuous emission monitoring (CEM) sample delivery systems. SO2, NOX, and H2O, components present in most fossil fuel combustion pro- cess emissions, were identified as the key artifact reactants. The selective removal of any or all of these reactants was targeted as an approach to eliminating the sam- pling artifact. The use of a desiccant alone, phosphorus pentoxide (P2O5), was effec- tive in drastically reducing the artifact gen- eration but was unable to eliminate it completely. A calcium hydroxide/sand mix- ture was found to be extremely effective at neutralizing high concentrations of SO2. The method developed was designed to be used compatibly with continuous emission monitoring sample delivery/con- ditioning systems or as a stand-alone pro- cedure. Specifically, the developed method uses reactant-specific dry sorbents to re- move the gaseous components, SO2 and H2O, to the degree that H2O generation in ------- stored sample containers (1-2 weeks) is consistently minimized to less than 10 ppm. Sequentially, SO2 is neutralized and H2O removed from a fossil fuel combustion pro- cess flue gas sample stream before enter- ing a Teflon-lined stainless steel container. The neutralization of SO2 requires H2O in the flue gas stream. Therefore, the flue gas sample must be collected upstream of any moisture conditioning devices such as condensers and/or desiccants that may be present in CEM sample delivery/condition- ing systems. The flue gas sample is extracted from the combustion source using a vacuum pump which pushes the gaseous-sample through the two-cartridge, solid sorbent system and ultimately through the grab sample container or "sample bomb." The sampling system was designed to be used upstream of any pollution control equip- ment or on combustion facilities where pollution control equipment did not exist. The grab sampling system is shown in F:igure 2. During the development of this sam- pling method, tests were conducted to de- termine the fossil fuel combustion process flue gas nitric oxide (NO), SO2, and H2O concentration ranges where N2O genera- tion in aged (1-2 week) samples would be consistently minimized to less than 10 ppm. This method was found suitable for use on combustion systems with flue gas concen- trations in the following ranges: SO2: 0-2,500 ppm NO: 0-1,000 ppm H2O: 5-25% (by volume) These flue gas concentration ranges were verified under simulated and actual combustion conditions. An example of the method's effectiveness to minimize N2O generation is shown in Figure 3. Flue gas components of CO, CO2, and THCs, typi- cally present in fossil fuel combustion pro- cess streams, were found not to interfere with sampling method performance. Other common flue gas components, such as hydrogen chloride (HCI) and NH3, were not evaluated and may act as interferences. Inlet From Stack Outlet H2O Sorbent Sample Container Meter Sample Pump Figure 2. Sorbent/sample container schematic. Flue Gas Simulation System A/;?O Artifact Generation of Samples Collected 200 150 §: so 165 ppm 0.8 ppm 0.5 ppm I Without Sorbents \Sample Flow = 11 LPM Sample Bombs Aged 6 Days • With Sorbents (X10) Sample Flow = 9 LPM On-Line N#O Cone. (X10) Sorbent (10:1 Sand, Ca(OH)2 and P&$ 1,200 ppm SO2, 600 ppm NO, 10% Moisture NgO Initial = 0.5 ppm Figure 3. Comparison of on-line N2O concentration to N2O sample container generation with and without use of sorbents. •fru.S. GOVERNMENT PRINTING OFFICE: 1*93 - 730-071/80024 ------- ------- J.V. Ryan and SA /Cams are with Acurex Environmental Corp., Research Triangle Park, NC 27709. William P. Linak is the EPA Project Officer (see below). The complete report, entitled "Development of Sampling and Analytical Methods for the Measurement of Nitrous Oxide from Fossil Fuel Combustion Sources," (Order No. PB93-194330; Cost: $17.50; subject to change) will be available only from National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at Air and Energy Engineering Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati, OH 45268 Official Business Penalty for Private Use $300 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 EPA/600/SR-93/088 ------- |