Grantee Research Project Results
Final Report: A Nitric Oxide/Ammonia Sensor for Fossil Fuel Combustion Control Applications
EPA Grant Number: R826164Title: A Nitric Oxide/Ammonia Sensor for Fossil Fuel Combustion Control Applications
Investigators: Vetelino, John F.
Institution: University of Maine
EPA Project Officer: Hahn, Intaek
Project Period: January 21, 1998 through January 20, 2000
Project Amount: $197,761
RFA: Exploratory Research - Environmental Engineering (1997) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Land and Waste Management
Objective:
The main objective of the research project was to engineer a semiconducting metal oxide (SMO) chemiresistive sensor and neural network capable of simultaneously measuring concentrations of nitric oxide (NO) and ammonia (NH3) in a stream of simulated flue gas. Preliminary work has demonstrated that the electrical conductivity of thin films of WO3, doped with metals such as gold (Au) and ruthenium (Ru), changes upon exposure to NO and NH3. This work also indicates that the sensitivity of WO3 to each of these gases is highly dependent upon parameters such as film thickness, dopant type, and operating temperature. Furthermore, operating two or more identical films at different operating temperatures can achieve linearly independent responses to each gas. By training a neural network to recognize these differences in response characteristics, concentrations of each gas can be measured simultaneously. However, several specific technical steps must be completed to achieve this overall objective.
Summary/Accomplishments (Outputs/Outcomes):
Temperature Characterization of Several Candidate WO3 Films Exposed to Various Concentrations of NO and NH3. The electrical conductivity of several candidate WO3 films were measured upon exposure to various concentrations of NO at a variety of temperatures. The candidate films vary in thickness from 500Å to 10,000Å, and were undoped or doped with either ruthenium or gold. Each film was operated at temperatures ranging from 200°C to 500°C, and exposed to NO concentrations ranging from 0 to 100 ppm, and NH3 concentrations ranging form 0 to 20 ppm, both separately and together, in an environment of simulated flue gas-purchased from the Matheson Corporation in Twinsburg, OH. The constituents of this environment are: 16 percent hydrogen, 0.7 percent oxygen, 3 percent methane, 3 percent carbon monoxide, 20 percent carbon dioxide, 0.1 percent propane, 0.15 percent ethylene, 0.3 percent methane, 0.3 percent ethane, and 0.1 percent butane, balanced in nitrogen.
Identification of Two or More Films/Operating Temperatures Providing Independent Responses to NO and NH3. Once the functional relationship between gas concentrations and temperature was measured for all of the candidate films, two more films and operating temperatures were chosen to provide fast, sensitive, linearly independent responses to the two gases as a function of concentration.
Full Characterization of the Films' Response Features Upon Exposure to NO and NH3. Once two or more linearly independent films have been chosen, a much more thorough characterization of their response features must be performed. Each film was operated at its appropriate temperature, and its electrical conductivity was measured as a function of time for many concentrations of NO and NH3 ranging from 0 to 100 ppm and 0 to 20 ppm, in simulated flue gas, respectively.
Training of Neural Network to Measure NO and NH3 Concentrations. A complete characterization of each film's electrical conductivity as a function of time and NO and NH3 concentration has been performed, the results were submitted as training data to a neural network.
Evaluation of Neural Network Response to Concentrations of NO and NH3. After each film was completely characterized at its respective operating temperature, and a neural network was trained with the data acquired from these tests, the entire array was exposed to various test concentrations of NO and NH3, both separately and together, in simulated flue gas, and the sensors' responses were evaluated with the trained neural network.
Evaluation of Results. The NO and NH3 concentrations calculated by the neural network were compared to the actual concentrations of each gas, and the results were critically evaluated to determine whether the overall objective has been accomplished. Strong correlation between neural networks output and actual gas concentrations adequately demonstrates feasibility of the concept of the proposed work to simultaneously measure concentrations of multiple combustion gases with multiple WO3 thin films and a neural network.
References:
Akiyama M, Tamaki J, Miura N, Yamazoe N. Tungsten oxide-based semiconductor
sensor highly sensitive to NO and NO2. Chemistry Letters
1991;1611.
Akiyama M, Zhang Z, Tamaki J, Harada T, Miura N, Yamazoe N. Tungsten oxide-based sensor for detection of nitrogen oxides in combustion exhaust. Sensors and Actuators B 1993;14(1-3):491-776.
Tamaki J, Zhang Z, Fujimori K, Akiyama M, Harada T, Miura N, Yamazoe N. Grain-size effects in tungsten oxide-based sensor for nitrogen oxides. Journal of the Electrochemical Society 1994;141(8):2207-2210.
Sberveglieri G, Depero L, Gropelli S, Nelli P. WO3 sputtered thin films for NOx monitoring. Sensors and Actuators B 1995;26(1-3):89-92.
Bryant A, Poirer M, Lee D, Vetelino JF. Gas detection using surface acoustic wave delay lines. Sensors and Actuators 1983;4:105-111.
Maekawa T, Tamaki J, Miura N, Noboru Y. Gold-loaded tungsten oxide sensor for detection of ammonia in air. Chemistry Letters 1992;639-642.
Maekawa T, Tamaki J, Miura N, Noboru Y. Promoting effects of noble metals on the detection of ammonia by semiconducting gas sensor. New Aspects of Spillover Effects in Catalysis 1993;421-424.
Meixner H, Gerblinger J, Lampe U, Fleischer M. Thin-film gas sensors based on semiconducting metal oxides. Sensors and Actuators B 1995;(2-3):119-125.
Conclusions:
The increased awareness of the potential hazards of various gases emitted into the environment has created a critical need for sensitive and selective sensors to monitor these gases. Among the most dangerous gases are the oxides of nitrogen (NOx) that are emitted from the combustion of fossil fuels in coal-fired power plants, motor vehicles, and other fossil fuel burning systems. NOx emissions adversely affect human health, agricultural vegetation, and the environment. In many fossil fuel systems, such as coal-fired power plants, a selective catalytic reduction (SCR) technique is employed where ammonia (NH3) is injected into the flue gas stream to react with NO to form environmentally safe gases, such as nitrogen and water vapor. Unfortunately, this process is usually incomplete, resulting in either NOx emissions or excess NH3 (NH3 slip) at the exhaust stack. Therefore, a critical need exists for an in situ sensor array to continuously monitor the NOx and NH3 levels at the output of the SCR system near the stack to provide real-time control of the NH3 injection and hence minimize the NOx emissions released into the environment.
SMO film technology has been exploited for a wide variety of sensing applications to create "chemiresistive" gas sensors. Chemiresistive gas sensors exhibit several advantages over other types of sensors. Chemiresistive gas sensors are small, robust, fast, sensitive, potentially selective, low-power, reusable, and capable of in situ real-time monitoring even in harsh environments. This sensor technology can be utilized for a wide range of environmental, industrial, medical, and defense monitoring applications.
In this report, SMO film technology was used to develop an SMO sensor array capable of selectively and sensitively detecting NOx and NH3. In particular, tungsten trioxide (WO3) films were utilized to engineer a two-sensor array to selectively detect NH3 and NOx gas concentrations typically found in coal-fired power plants. To design the chemiresistive sensor array, the effects of substrate material, film thickness, doping, deposition temperature, and operating temperature on the film's electrical response to NH3 and NOx were closely examined. As a result of this examination, two film recipes capable of selectively detecting NH3 and NOx were identified. The resulting two-sensor array was further characterized using graphical analysis techniques to evaluate response and recovery times, rates of response, sensitivities, dynamic ranges, stability, and selectivity to NH3 and NOx. The film growth reproducibility within a batch, and among batches, also was examined. Finally, principal component analysis (PCA) techniques were employed for sensor array data interpretation and visualization. Specifically, the sensor array's response repeatability, selectivity, and film growth reproducibility were evaluated and presented using PCA.
The present work demonstrates that the electrical response characteristics of the WO3 films were highly dependent on the combinations of substrate material, film thickness, doping, deposition temperature, and operating temperature. The two-sensor array demonstrated sensitive, reversible, and repeatable responses to NH3 and NOx. Both sensors exhibited short-term and long-term baseline stability. The sensor array also responded selectively to NH3 and NOx over the concentration ranges typically found in coal-fired power plants. Preliminary results suggested that more work is required to improve reproducibility within a batch, as well as among batches. Furthermore, a third film recipe displayed all of the desired sensing characteristics to become an H2S sensor that eventually could be integrated into the current NH3 and NOx two-sensor array.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 3 publications | 1 publications in selected types | All 1 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Marquis BT, Vetelino JF. A semiconducting metal oxide sensor array for the detection of NOx and NH3. Sensors and Actuators B 2001;77(1-2):100-110. |
R826164 (Final) |
not available |
Supplemental Keywords:
nitric oxide/ammonia sensor, automotive sensor, fossil fuel combustion sensor., RFA, Scientific Discipline, Waste, Environmental Chemistry, Incineration/Combustion, Environmental Engineering, Nitrogen dioxide, sulfur oxides, selective catalytic reduction, air pollution, chemical contaminants, coal, ammonia sensor, Hydrogen sulfide, flue gas emissions, fossil fuel combustionProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.