Progress 06/01/14 to 01/31/15
Outputs
Target Audience:
Nothing Reported
Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?
Nothing Reported
How have the results been disseminated to communities of interest?
Nothing Reported
What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
Significant emissions of environmentally important gases result from manure management systems associated with animal production. Confined livestock and poultry operations are identified as important sources of emissions that are of safety and environmental concern on spatial scales ranging from local to regional, national, and global. Environmental concerns range from the effects of such emissions on the quality of life locally all the way to the impact of emissions on global climate change. Locally, more than thirty deaths have been recorded over the past 20 years due to accidents involving manure storage pits. Effective management of animal production to minimize these impacts requires a better understanding of the gas emissions generated from these operations. Southwest Sciences proposed to develop new laser-based instrumentation for the measurement of methane (CH4), and hydrogen sulfide (H2S) gases, which are two of the major pollutants emitted from animal production operations, particularly manure management systems. Due to the impact of these gases at both the local and global scale, a monitoring device must be sensitive enough to accurately assess emission burdens while also making measurements rapidly enough to protect health and safety. The instrumentation developed based on our approach will provide a versatile combination of high sensitivity and fast time response that will greatly enhance capabilities for measuring concentrations and emission rates of CH4 and H2S associated with animal manure management systems. This approach is based on wavelength modulation spectroscopy, taking advantage of stronger spectroscopic absorption features at mid-infrared wavelengths. The method we propose directly monitors analyte concentrations at high sensitivity (H2S in the sub-ppm range, and CH4 in the low ppm range) with fast time response (one second), making it suitable for real-time field measurements. Additionally, the instrument requires no consumables or cryogens and only modest electrical power. The first key to this advance is the recent availability of interband cascade diode lasers that open up new spectral wavelength regions for high sensitivity optical sensing. In particular, they allow the detection of H2S gas at concentration levels heretofore not possibly with a compact optical system, along with the simultaneous detection of CH4. The second key is the use of a dense multiple pass absorption cell, invented at Southwest Sciences by the PI of this proposal, to achieve a very long optical absorption path in a modest footprint. All Phase I Technical Objectives were successfully completed. It was demonstrated that we could detect hydrogen sulfide with approximately 10 ppb sensitivities at one Hz and could readily monitor methane at relevant concentrations. Based on these results, we developed a preliminary design for a prototype analyzer than would be used in field measurements during Phase II. Task 1 - Design and Construct Phase I Analyzer In this task, we constructed a compact bench top system based on a new Nanoplus DFB laser and dense pattern optical cell, using a commercial laser controller (Stanford Research 501) and LabView-based data acquisition. The output of the diode laser is focused by an aspheric lens and is injected into the multiple pass cell. Light exiting the cell is directed onto a room temperature extended-wavelength InGaAs photodiode. A voltage preamplifier (SRS Model SR560) combines a 50 Hz sawtooth ramp (SRS Model DS335) with a 35 kHz sinewave generated by the SRS Model 830 DSP lock-in amplifier. The photodiode output is conditioned by a Femto transimpedance amplifier and fed to the ac-coupled input of the lock-in. The 2ƒ-demodulated waveforms are observed and recorded using a National Instruments DAQ and Labview. The scans (~ 0.55 nm) of 1000 wavelength steps are co-averaged to 1 Hz. Task 2 - Evaluate instrument sensitivity Basis function spectra of the various components used in the fitting process were determined. These include the highest H2S concentration we had (25 ppm), moist room air used as the H2O basis, dry air for a baseline basis, and 50 ppm methane. Spectrum ofrm 25 ppm to 250 ppb H2S at 200 torr with a small amount of residual water in the cell were acquired. Despite some residual etalons and modest system noise, the spectral line shapes even at the lowest low concentration are readily observed. This spectrum corresponds to an absorbance of ~4×10-4. The noise on this spectrum is equivalent to an absorbance of 2.5 × 10-5. However, broadly rolling etalons appear to be several times larger than the noise. These will be addressed more thoroughly in Phase II. The methane spectrum is included to provide a sense of its relative strength and separation from hydrogen sulfide. While this is perhaps a weaker methane line at its natural abundance of 1.9 ppm, its absorbance in this cell is still a respectable 2 × 10-4. For operation near animal manure operations, this is sufficiently sensitive to be used in eddy correlation flux measurements, as well as for a safety monitor, where only higher concentrations are of concern. Finally, we note that if real air, as opposed to synthetic air, had been used in this study, the isolated CO2 line also would have been observed. Flows of 25 ppm H2S in dry air were combined with pure dry air to create different mixing ratios of H2S in the cell. Multiple measurements over time allowed us to determine detection precision and linearity as a function of concentration. Fits of a series of results over periods of 100 seconds for each mixture appear to be quite stable, with 1σ standard deviations in the results ranging from 7 to 16 ppb. An Allan variance calculation for a five minute data set of 1.6 ppm (316 seconds) also shows similar high precision, with the precision improving as the square root of averaging time, as expected. Data taken at 252 ppb H2S versus time has excellent precision with almost no drift. However, its absolute measured concentration is about 50% higher than expected here, most likely to a residual etalon.. Measured concentrations were compared with to the prepared concentrations. The linearity is decent at higher values, but rolls off slightly below a few ppm. We feel that the errors at low concentrations seen here and at 252 ppb may arise from larger than desired residual etalons and possible electronic offsets that were not accounted for in Phase I. These issues will be addressed in Phase II. Task 3 - Estimate the Size, Cost and Performance of a Field Instrument In this task, the specifications and preliminary design required to build and deploy a real-time hydrogen sulfide and methane analyzer were developed. Factors of importance included electronics design, size, weight, power, ability to operate unattended, data storage and/or transmission methodologies, and operation under field conditions. We also determined that this analyzer needs to operate at reduced pressures. We have identified suitable pumps that can meet these requirements in Phase II. Finally, detailed calculations imply that we also need to reduce the moisture level in the measurement cell by a factor of 10 or so. One of the Phase II tasks will examine the best approach for achieving this level of reduction.
Publications
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