144- The feather leaf separator is used in the design of a specialized gas-liquid separator at the inlet of the electrostatic tar catcher in the desulfurization tower for raw coke oven gas

In previous technical discussions, analysis and exploration have been conducted regarding the design of the specialized vane separator at the inlet of the dry desulfurization tower in the PSA inlet pipeline for hydrogen production using coker gas. This paper analyzes and discusses the design in which an electrostatic tar catcher is installed on the inlet pipeline of the wet flue gas desulfurization tower and is connected in series with a pre-installed vane separator adjacent to it.

This article uses a coke oven plant of a certain enterprise as an example for analysis. The raw gas produced by the company’s semi-coke furnace is cooled in a direct cooling tower and a cross-tube cooler; thereafter, this raw gas, which contains tar droplets, foam, and dust particles, enters an electrostatic tar catcher to remove the heavy-phase impurities from it. The raw gas coming out of the electrostatic tar catcher is pressurized by a fan and then sent to a desulfurization tower for sulfur recovery; the desulfurized raw gas is then sent to a water-sealed storage tank.

The owner further added that the raw gas in the water-sealed storage tank is used directly as fuel in part, while the rest is sent to PSA for hydrogen production. The raw gas sent to the PSA hydrogen production system is further pressurized by a fan; it first enters a dry desulfurization tower where the sulfur content is further reduced to below 1 ppm, before proceeding to the PSA unit. The separator specifically installed at the inlet of the dry desulfurization tower is the special vane separator for the inlet of dry desulfurization towers, which was the subject of technical discussions and analyses in the past. This is because the gas stored in the water-sealed tank, due to fluctuations in system conditions, carries a considerable amount of liquid droplets and foam into the dry flue gas desulfurization tower.

The issue that the owner is eager to resolve this time is that the unstable operation of the electrostatic tar catcher in separating and removing raw gas results in excessive amounts of tar and dust particles entering the desulfurization tower, thereby rendering the existing desulfurization equipment unable to effectively remove sulfur from the gas flow, leading to the formation of a sulfur discharge system.

In a wet desulfurization tower for raw gas, the desulfurization efficiency declines or even becomes ineffective, with no sulfur being produced, when the control parameters of the desulfurization solution are normal. The main reason for this is that the raw gas carries an excessive amount of tar and dust into the tower, where these substances accumulate over time to form a low-density, non-polar tar layer that floats on the surface of the desulfurization solution. This layer separates the weakly alkaline desulfurization solution from the raw gas, making it difficult for an effective reaction between the desulfurization solution and acidic gases such as hydrogen sulfide to take place.

The desulfurization and displacement reactions that take place within this wet desulfurization tower are interfacial reactions between the gas phase and the liquid phase, and such reactions inherently present considerable resistance. Light hydrocarbon layers such as tar float on the surface of the desulfurization liquid, preventing contact between the liquid and gas phases and thereby increasing the resistance to reactions at the gas-liquid interface; dust particles also disrupt the formation of sulfur foam.

So, why does the operation of electrostatic tar collectors for separating raw gas become unstable, resulting in excessive levels of tar and dust? From the perspective of the working principle of electrostatic precipitators, these devices are designed to capture liquid droplets, mist, and dust particles in the airflow, and this process involves several steps: First, it is necessary to charge the liquid droplets, mist, and dust particles in the airflow, that is, to give them an electrical charge ； Secondly, the charges carried by these charged droplets, mist particles, and dust need to remain stable within the separation space ； Third, it is necessary to find a way to generate a stable strong electric field. Relatively speaking, the third point is relatively easier to achieve. The first point involves charging the droplets, aerosols, and dust particles in the airflow, while the second point ensures that these charged droplets, aerosols, and dust particles remain charged as they move through the separation space. These requirements vary depending on the type of system, its properties, as well as the flow pattern and regime of the fluid. For example, if the heavy-phase liquid droplets, foam, and particulates carried in the gas stream are electrolytes or metal dusts, it is difficult for them to become charged. In some air currents, the heavy-phase liquid droplets, mist, and particles carried along are non-electrolytic, non-metallic dusts. Although they can be charged, they lose their charge due to collisions between charges as they move with the airflow. Additionally, factors such as unstable operating conditions that result in high air flow speeds or particles that are too large can cause these charged particles to deviate from their path in the electric field and leave the capture area before reaching the electrodes, thereby preventing effective capture.

In the actual operation of electrostatic precipitators used to treat furnace gas and raw gas from coking units, unstable control of operating conditions often leads to fluctuations in gas flow velocity, momentum, and kinetic energy, which may exceed the capabilities of the electrostatic precipitators – this is a very likely occurrence. It is also highly likely that electrolyte particles are carried in the airflow. The high-voltage electrodes of electric coke oven gas capture devices and the electric field they generate are highly sensitive to the temperature of the coke oven gas as well as the tar dust mixture it contains. Excessive temperatures caused by the rise in temperature of coke oven gas, as well as the accumulation of tar sludge covering the electrodes, both have a significant impact on the efficiency of the electric tar catcher.

For any separator, there is a specific range of applications, with only the size of that range varying. The electrostatic tar catcher is the same. Generally speaking, in applications for purifying airflow with unstable operating conditions, the variety of heavy-phase substances carried by the airflow, as well as its kinetic energy, momentum, flow pattern, and flow state, often result in a significant reduction in the efficiency of particle capture; therefore, electric catchers should not be used alone for gas-liquid separation.