Application of Dust Removal Technology for Dryer Exhaust Gas

The exhaust gas from dryers has a high airflow volume, high dust concentration, high moisture content, and a high dew point, making it prone to condensation. With its high discharge point and wide pollution range, it poses a significant challenge for dust removal technology. The exhaust gas from the dryer’s tail section varies depending on each plant’s process conditions. Some enterprises use dryers for multiple types of materials, resulting in significant production fluctuations, frequent start-ups and shutdowns, and wide variations in operating conditions. Furthermore, when coal is used as the heat source for drying, the sulfur oxides in the exhaust gas corrode metal structural components, presenting certain challenges in selecting an appropriate dust collector.

Cyclone Dust Collection Technology

Cyclone dust collection technology is characterized by low equipment costs, the ability to handle high-temperature gases, ease of operation, and suitability for gases with high dust concentrations. Its operating principle is as follows: driven by a fan, the dust-laden gas stream enters the cyclone housing at high velocity along a tangential direction from the inlet, moving in a spiral rotation from top to bottom. Under the action of centrifugal force, dust particles are flung toward the outer wall and spiral downward along it. As the cone narrows, the particles turn toward the axis, are deflected by resistance at the bottom, and return to spiral upward along the axis before being discharged through the core tube. Dust particles on the outer wall, driven by gravity and the downward airflow, fall along the wall into the ash hopper, thereby achieving dust removal. Since cyclone separators rely on the inertia of dust particles for separation, their efficiency is directly proportional to particle size: larger particles yield better results, while smaller particles result in poorer performance. Generally, for dust particles larger than 20 micrometers, the separation efficiency ranges from 70% to 90%.

Baghouse Dust Collection Technology

Baghouse dust collectors are widely recognized for their stability, high efficiency, simple maintenance, and reliable operation. They can achieve dust removal efficiencies as high as 99% for dust-laden gas containing particles as small as 0.1 micrometers; when selecting a baghouse dust collector for drying machine exhaust gas, there is no need to worry about emission concentrations exceeding standards. Anti-condensation fiberglass baghouse dust collectors for dryers are currently the ideal dust removal and purification equipment. This equipment features microcomputer control, compartmentalized reverse-jet cleaning, and timed dust removal, and is equipped with temperature detection and display as well as an over-temperature alarm system. It uses CW300—FcA anti-condensation fiberglass filter bags, which effectively prevent condensation on the filter bags and prevent them from burning out. Hebei Taihang Group’s 3×24-meter slag dryer replaced its two-stage dust removal process with a single-stage LFEF7×358—HSY/H anti-condensation baghouse dust collector. According to measurements by environmental protection authorities, the exhaust gas volume was 42,621 standard cubic meters per hour, with an inlet dust concentration of 69,620 milligrams per standard cubic meter. and the outlet dust concentration was 147 milligrams per standard cubic meter, meeting national emission standards. This not only increased production but also enabled the recovery of dry material, yielding annual economic benefits of over 700,000 yuan, while also delivering significant social benefits. However, baghouse dust collectors have the drawbacks of requiring a large footprint and high investment costs. With the advancement of filter bag membrane coating technology, these drawbacks will be overcome, leading to their more widespread adoption. 

High-Voltage Electrostatic Precipitation Technology

High-voltage electrostatic precipitation technology converts 50-hertz, 220-volt AC into direct current of 100 kilowatts or more, which is applied to the corona electrode (cathode) to form a non-uniform high-voltage electric field. This ionizes the gas, generating a large number of negative ions and electrons, which charge the dust particles in the gas as they enter the electric field. Under the influence of the electric field forces, the charged dust particles are drawn toward the opposite electrode—typically the anode, which serves as the collection electrode—and are dislodged by shaking into a hopper for discharge, thereby completing the purification and dust removal process. High-efficiency, low-resistance electrostatic precipitators can be widely used in dust-polluted environments across industries such as building materials, metallurgy, and chemicals. They are capable of handling high dust concentrations, and their dust removal effectiveness is particularly pronounced for fine particles as small as 0.01 micrometers or dust with high specific resistance. The product series accommodates drying equipment with varying airflow rates, offering flexible compatibility and making it suitable for dust control tailored to the characteristics of exhaust gases from dryers.

Wet Dust Collection Technology

In wet dust collection technology, dust-laden gas is drawn by an induced draft fan through ductwork into the lower section of the dust collection tower. As the cross-sectional area increases, the flow velocity decreases; coarse dust particles settle out of the gas stream first, while finer dust particles rise with the airflow. water droplets sprayed from the spray nozzles move in the opposite direction to the dust-laden gas stream. The dust becomes moistened, and as its weight increases, it overcomes the lift force of the gas stream under the influence of gravity and descends as slurry, entering the settling tank through the lower pipeline, thereby achieving dust removal. The slurry generally undergoes 2 to 3 stages of cyclic settling to become clear water, which is then pumped back into the dust collection tower for recirculation, preventing secondary pollution. This dust removal technology, which operates under positive pressure, is generally used for drying clay and various other raw materials; however, the recovered material is often contaminated and unusable, and it causes significant wear on the fan blades. The Suzhou Cement Plant employs wet dust removal technology on a clay dryer measuring 2.2 × 12 meters in diameter, achieving an emission concentration of 132 milligrams per standard cubic meter, which meets national emission standards. The sedimentation tank is cleaned once a year, and the wet soil is sun-dried and stored in a warehouse for use as raw material in production. This dust removal system features high dust collection efficiency, low pressure drop, no secondary pollution, flexible process design, reliable operation, low power consumption, low investment costs, no condensation, and strong adaptability, delivering significant economic and social benefits.

Gravity Sedimentation + High-Voltage Electrostatic Precipitation Technology

The dust removal process, consisting of a sedimentation chamber and a high-voltage electrostatic precipitator, works as follows: dust-laden exhaust gas is delivered at high speed into the sedimentation chamber via a duct driven by an induced draft fan. Upon colliding with the chamber walls, the airflow changes direction, causing the air velocity to drop rapidly. Particulate dust settles and is discharged via conveying equipment, while fine dust particles follow the airflow into the electric field of the high-voltage electrostatic precipitator. Under continuous bombardment by ions, these particles become charged, are drawn toward the collection electrodes, collected, and then discharged. The purified gas is discharged into the atmosphere via ductwork. In the two-stage dust removal process, the primary settling chamber collects large dust particles larger than 40 micrometers, reducing the dust concentration in the gas. This helps improve the dust removal efficiency of the high-voltage electrostatic precipitator, allowing for higher air velocities and smaller equipment, thereby effectively reducing capital investment. The dust removal process for the 2.2 × 12-meter clay dryer at the Zhejiang Lanxi Cement Plant involves the dust-laden exhaust gas being drawn from the dryer’s tail hood by an induced draft fan through ductwork into a square settling chamber made of steel plates. After settling, the exhaust gas enters the high-voltage electrostatic precipitator and is discharged into the atmosphere after purification. The induced draft fan and settling chamber are installed indoors, while the electrostatic precipitator is installed outdoors. The dust collection system has been operating normally for many years without any condensation. According to measurements by environmental protection authorities, the emission concentration is 48 milligrams per standard cubic meter, which is below the national emission standard.

Cyclone + High-Voltage Electrostatic Precipitator Technology

In this dust removal technology, dust-laden exhaust gas from the dryer enters a primary high-efficiency cyclone dust collector via a duct for preliminary dust removal. Dust is discharged from the hopper via ash-discharge equipment, reducing the dust concentration in the gas stream. The gas then proceeds to a secondary high-voltage electrostatic precipitator for further purification. The purified gas is discharged into the atmosphere via a blower, resulting in improved dust removal efficiency, process flexibility, and safety and reliability.

The application of dust removal technology for dryer exhaust gases not only protects the environment and reduces labor intensity but also increases hourly output, reduces pollution discharge fees, and recovers dust as a high-quality raw material for cement production, yielding significant economic, social, and environmental benefits.