Overview of pH electrode used in the desulfurization industry

Contents

• The relationship between pH and desulfurization efficiency
• Current Applications of pH Monitoring in Wet Flue Gas Desulfurization Systems
• Factors Affecting Accurate pH Monitoring in Desulfurization
• Main Selection Factors for a Robust and Durable pH Electrode in the Desulfurization Industry

Wet flue gas desulfurization (FGD) systems offer high desulfurization efficiency and are a common energy-saving and emission-reduction measure in thermal power plants. During the reaction, the pH of the desulfurization slurry is a key factor influencing desulfurization efficiency. Inaccurate pH monitoring can significantly impact the proper adjustment of system parameters, leading to inefficient and environmentally unfriendly operation of the desulfurization system.

The relationship between pH and desulfurization efficiency

A high pH slurry environment favors sulfur dioxide absorption, while a low pH easily leads to the formation of calcium precipitates. These two factors are contradictory. Data shows that when the pH exceeds 5.8, the desulfurization rate not only fails to improve, but actually decreases. At the same time, the pH should not be too low, otherwise it will affect sulfur dioxide absorption. In practical applications, the pH value inside the desulfurization tower is best between 5.6 and 5.8. Above this range, scaling is likely to form inside the desulfurization tower. Below this range, the slurry’s absorption capacity decreases, ultimately affecting the sulfur dioxide removal rate and the quality of the byproduct gypsum.

Current Applications of pH Monitoring in Wet Flue Gas Desulfurization Systems

1. Expensive equipment and high operating costs. 

Typically, a desulfurization system requires two online pH meters to ensure consistent monitoring data. Currently, replacement costs for electrodes used in the desulfurization industry are generally high. However, due to harsh on-site monitoring conditions and outdated installation technology, even expensive pH electrodes need to be replaced every few months. Therefore, in addition to the cost of the controller itself, the annual replacement cost of the electrodes is also very high.1. Expensive equipment and high operating costs. 

2. Inaccurate pH monitoring data impacts the long-term stable operation of the desulfurization system.

During operation, the following cases often occur, preventing the accurate confirmation of the slurry pH value, which in turn affects the precise adjustment of the slurry volume during the desulfurization process and reduces desulfurization efficiency.

• The difference between the two pH controllers is greater than 0.2 pH.
• After adding limestone slurry, the pH monitoring reading does not change significantly.
• The pH controller reading continues to increase, reaching 14.0, etc.

Factors Affecting Accurate pH Monitoring in Desulfurization

1. Harsh Operating Conditions

The slurry has a density of up to 1.1 and contains sharp, angular limestone crystals with large, hard particles, which can easily cause physical damage to the pH electrode‘s glass membrane. The slurry temperature is approximately 55°C, the pH is approximately 5.6 to 5.8, and the conductivity is approximately 60 mS/cm. It also contains high concentrations of chloride and sulfide. Chloride ion concentrations can reach as high as 20,000 to 30,000 ppm, easily leading to electrode failure.

2. Harsh Process Installation Conditions

The pH electrode is typically installed in the outlet pipe approximately 1 meter from the bottom of the desulfurization tower. the pipe is thin, the slurry flow rate is high, and the pipe pressure is approximately 7 to 8 bar. This is a major cause of breakage of the pH electrode’s glass membrane.

Main Selection Factors for a Robust and Durable pH Electrode in the Desulfurization Industry

1. pH Glass Membrane

A thick, alkali-resistant glass membrane with high ionic strength and high corrosion resistance is ideal. This is the type currently used by leading international pH electrode brands. The rugged glass membrane offers strong impact resistance, while its high alkali resistance makes it suitable for high-ionic strength solutions and reduces measurement errors.

Flat glass membranes are also a common choice. Their smooth, non-depression surface prevents the accumulation of particles in the desulfurization slurry, minimizing scaling during operation. They also reduce the risk of damage from mechanical erosion or particle impact.

2. Reference System

A reference system that is resistant to clogging, pressure, and contamination is ideal. A common structure is a specialized porous ceramic double junction, which can slow down contamination of the reference system.

3. Reference Electrolyte

Gel-type reference electrolyte that is pressure-resistant and non-reactive with sulfur and heavy metal ions in the slurry.

4. Other Factors

• Proper electrode insertion depth, such as 120, 150, 225, 325, or 425 mm.
• Correct temperature compensation type compatible with the pH controller, such as NTC10K, PT100, or PT1000.
• Appropriate process connection, such as PG13.5.
• Proper cable connection, such as S8, S8M, S7, VP, K8S, or K2.