Urea Concentration Measurement in Denitration Processes
Strict air quality regulations worldwide require industrial facilities to control nitrogen oxide (NOx) emissions. Urea, a safe and stable substance, is commonly used in denitration systems to reduce NOx. The key is balancing the amount of urea injected with the real-time NOx levels in the flue gas to achieve the desired NOx reduction without issues.
Under-dosing fails to reduce NOx enough, risking non-compliance with regulations. over-dosing wastes reagent, raises costs, and causes "ammonia slip"—unreacted ammonia escaping into the atmosphere. Ammonia slip is costly, environmentally harmful, and can form sticky salts like ammonium bisulfate and ammonium sulfate, which foul equipment, reduce efficiency, and cause damage.
Challenges of Online Urea Monitoring
Fouling, Crystallization, and Corrosion
Fouling is a persistent issue, particularly when hard water is used to dilute solid urea feedstock. The minerals in the hard water can precipitate out of solution, leading to scaling and clogging of critical components, including injection nozzles and sensors. This phenomenon can cause inaccurate measurements and necessitate frequent, costly maintenance and cleaning, significantly reducing system uptime.
Crystallization is likely to occur at low exhaust temperatures (typically below 200−250∘C) and on surfaces where the urea solution impacts the pipe walls, forming a film. A thicker film, often caused by an increase in spray volume or droplet size, makes it harder for urea molecules to fully evaporate, leading to crystal formation. This process is a primary cause of sensor and nozzle blockage.
The corrosive nature of the urea solution itself poses a significant threat to instrumentation. The synthesis of urea involves the formation of ammonium carbamate, a highly corrosive intermediate that can rapidly degrade conventional materials, leading to catastrophic equipment failure. The selection of instrumentation materials must therefore be a primary consideration, as standard components can be rendered inoperative and require constant replacement in this aggressive environment.
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Influence of Dynamic Process Conditions on Measurement
Physical properties of the fluid itself introduce complexities for accurate measurement. The density of an aqueous solution is highly sensitive to both temperature and pressure. Even slight variations in temperature can significantly influence the measured urea nitrogen concentration. Readings may drift widely and provide inaccurate data to the control system without proper temperature compensation. This variability highlights the critical need for a urea concentration sensor that incorporates real-time temperature compensation to correct for these process fluctuations.
Similarly, factors like flow velocity, viscosity, and the presence of entrained air bubbles can introduce significant measurement instability and errors, demanding a sensor design that is inherently robust and reliable under dynamic operational conditions.
The Lonnmeter Solution: Urea Concentration Meter
Working Principle of Urea Concentration Sensor
In-process urea concentration meter is an inline sensor applied to continuous concentration or density measurement of binary liquids in pipelines, tanks and other vessels. A vibrating tuning fork's resonant frequency changes in direct inverse proportion to the mass and density of the fluid surrounding it. The sensor consists of a U-shaped fork that is electronically driven to vibrate at a precise resonant frequency. When this fork is immersed in a fluid, the fluid's mass adds to the effective mass of the fork, causing its vibration frequency to decrease. The sensor's advanced electronics continuously monitor this frequency shift. By correlating this frequency shift with a pre-programmed calibration curve, the instrument can provide an accurate and repeatable measurement of the fluid's density.
The true innovation lies in the transformation from a basic density reading to a functional concentration value. The Lonnmeter achieves this by integrating a high-precision temperature sensor directly into the probe. This sensor provides real-time temperature data to the internal processing unit, which then applies a sophisticated temperature compensation algorithm. This process corrects the density reading back to a standard reference temperature, minimizing effects of process temperature fluctuations. This corrected density value is then converted into a specific concentration, such as a percentage by weight. This two-step process—measurement of a physical property (density) followed by a transformation via a calibration curve and temperature compensation—is the key to providing an accurate and reliable urea concentration measurement.
The inherent design of the tuning fork sensor provides a profound advantage in the challenging denitration environment. With no small orifices, narrow channels, or delicate diaphragms, the sensor is naturally resistant to the fouling and crystallization that plague other technologies. Its robust, open structure allows for fluid to flow freely around the vibrating tines, minimizing the opportunity for mineral deposits or urea crystals to accumulate and compromise the measurement.
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Engineered for the Denitration Environment
Recognizing the extreme conditions of a denitration plant, Lonnmeter has engineered its sensors with material science at the forefront. The instrument's primary wetted components are constructed from robust materials like 316 stainless steel, providing a high degree of resistance to chemical corrosion, particularly from highly aggressive substances like ammonium carbamate. Corrosion resistant materials extend life expectancy of concentration measuring instrument, maintenance intervals, and a reduction in unscheduled downtime.
The integrated temperature sensor and sophisticated algorithms compensate for temperature variations, ensuring a stable and reliable reading regardless of fluctuations in the process fluid.
Seamless Integration and Connectivity
The Lonnmeter’s 4-20mA current loop output integrates easily with PLC or DCS systems because:
- Simple Wiring: As a two-wire transmitter, it uses one pair of wires for both power and signal transmission, reducing complexity.
- Reliable Signal: The 4-20mA signal is immune to voltage drops over long distances and resistant to electrical noise and electromagnetic interference.
- Linear Scaling: For a 0-100% concentration range, 4mA corresponds to 0% and 20mA to 100%, allowing straightforward scaling in the control system.
- Safe and Stable: Proper grounding of the sensor casing ensures signal accuracy and electrical safety, enhancing compatibility with industrial systems.
Optimal Placements and Practical Advantages
Effective implementation of a urea concentration sensor is about more than just accurate measurement; it is about strategic placement to maximize operational benefit.
The Urea Solution Preparation and Storage Stage
The first and most logical point for sensor deployment is at the start of the denitration process: the urea solution preparation and storage tanks. A sensor installed at this stage provides a crucial first-line defense for quality control, verifying that the prepared solution is at the correct concentration before it is even sent to the dosing system. This proactive measurement can immediately detect errors from incorrect manual dilution, variations in solid urea feedstock, or the use of contaminated water, preventing these issues from propagating downstream and compromising the entire process. Monitoring the concentration in the storage tank also provides a valuable inventory management tool, ensuring a consistent and ready supply of correctly formulated reagent.
Monitoring the Injection and Dosing Lines
To enable true closed-loop control, a urea concentration meter should be installed in the high-pressure injection or dosing line just before the injection nozzles. This placement provides the most direct and accurate measurement of the reagent entering the system in real-time. This live data is the foundational input for advanced control strategies that continuously adjust the injection rate based on measured flue gas NOx levels, catalyst temperature, and other operating parameters.
While some control systems infer problems from pressure fluctuations in the dosing line, a direct, continuous concentration measurement provides a more robust and reliable signal. It can proactively detect pump faults, partial blockages, or an over/under-dosing situation, enabling a rapid, automated response before the system's NOx reduction performance is compromised. This approach moves the plant from a reactive maintenance model to a proactive, predictive one.
The Correlation with Ammonia Slip
The value of the urea concentration sensor extends far beyond a single data point. By providing a stable and reliable stream of data, the sensor enables the control system to precisely manage the reagent injection rate, ensuring the optimal stoichiometric ratio is maintained. This precision is directly correlated with the minimization of ammonia slip. An over-dosing event can be prevented in real-time, reducing both reagent waste and the environmental impact of unreacted ammonia emissions.
Value to Customers
- Enhanced NOx Reduction and Regulatory Compliance;
- Reduction in Reagent Consumption and Operational Costs
- Maximizing Uptime and Minimizing Maintenance Burdens
