176-Analysis of the reasons for reduced efficiency in the wet desulfurization tower for coal gasification to produce high-quality gas, and discussion on the use of vane separators for purifying the inlet gas

This post was last edited by luoli519 on 2021-2-8 at 15:52. In recent years, due to the significantly better technical and economic benefits associated with the dry distillation and pyrolysis of coal for its subsequent utilization, a number of coal pyrolysis and separation facilities with capacities of millions of tons each have been built in China. Projects that offer good technical and economic benefits and are thus favored and **supported** in terms of coal utilization encounter numerous operational challenges in practice. For example, the use of wet desulfurization processes for upgrading gas produced by dry distillation has caused problems in many enterprises. Initially, the desulfurization efficiency of these towers was fairly good, but as they operated for longer periods of time, their desulfurization efficiency declined significantly, causing various issues for the downstream gas-using units. Next, we will discuss and analyze this through a real-world case study.

We once provided technical support for the design of a project to remove oil (and hydrocarbons) from the gas produced by dry distillation in order to improve its quality, for a large-scale integrated coal chemical enterprise. The company produces solid products such as upgraded coal, coal tar, and coal gas by pyrolyzing raw coal. Originally, the pyrolysis gas from coal dry distillation requires high standards for separation technologies. However, the former chief engineers of such companies, unlike those in large petrochemical enterprises who have a formal education, place little emphasis on separation technologies and do not have a deep understanding of them. Their technical skills are outdated, their knowledge is limited, and they tend to be modest; they think that gas-liquid separation is a simple task. As a result, technical shortcomings arise from the very beginning of project implementation, leading to continuous problems during the actual operation of the facilities, which causes great difficulties for those who take over operation and management later on. The purpose of this post is precisely to avoid similar hardships for the actual managers running the enterprise.

Issues reported by the production department need to have technical solutions determined by the technical department. To gain a comprehensive understanding and access to first-hand information, we had frank and detailed discussions with relevant personnel from the technical department, and also went to the installation site to verify the situation with the operators. This enterprise uses two sets of medium- and low-temperature, near-atmospheric-pressure oxygen-free pyrolysis furnaces; each furnace emits 25,000 m^3/h of gas at a temperature of around 450°C. After waste heat recovery, this gas enters a water scrubbing tower where it is cooled to remove heavier hydrocarbons ； Next, the gas is introduced into an evaporative air cooler to be cooled to around 20°C in order to remove light hydrocarbons; thereafter, it enters a pre-gravity separation tank equipped with electric captors (ID 3000mm*SM/SM 5000mm) for sedimentation and separation, before proceeding to one operational and one standby electric captor (ID 6000mm*H 20000mm). The gas treated by electrostatic capture is sent through a pipeline with a diameter of 2000 mm and a length of approximately 60 m to the pre-washing tower (ID 6000 mm * H 25000 mm), where it undergoes three stages of spray washing. The gas coming out of the water washing tower is fed into the pre-desulfurization tower (ID 6000mm * H 35000mm) for primary desulfurization, and then into the main desulfurization tower (ID 7000mm * H 41000mm) ； The gas coming out of the main desulfurization tower is fed into the final washing tower (ID6000mm*H35000mm) ； The gas coming out of the final washing tower is sent to the gas holder ； The gas holder returns the gas from it to the pyrolysis section through a main pipe with a diameter of 2200 mm; this gas is then divided into 10 branch pipes with a diameter of 600 mm, which deliver the gas to the nozzles of the pyrolysis furnace where it is injected and burned for reuse.

According to the on-site operators, when the facility was first built, there were no such large structures as the pre-washing tower (ID 6000mm*H25000mm) and the final washing tower (ID 6000mm*H35000mm). During the subsequent actual operation of the installation, it was found that the combined operation of the pre-desulfurization tower and the main desulfurization tower did not yield good results; it was believed that the electrostatic precipitator was not performing its separation function properly, which led to impure air entering the desulfurization system. As a result, a pre-washing tower (ID 6000mm * H25000mm) was installed between the electrostatic precipitator and the pre-desulfurization tower. However, the gas produced by the main desulfurization tower still exceeded the specified standards, causing frequent failures in the gas utilization unit of the pyrolysis furnace. Therefore, another washing tower (ID 6000mm * H35000mm) was added between the main desulfurization tower and the gas tank, filling up all the existing purification units. The problem is that despite investing a large amount of money twice to install two massive units – the pre-washing tower (ID 6000mm*H25000mm) and the final washing tower (ID 6000mm*H35000mm) – the gas produced by the desulfurization system still does not meet the required standards; it remains the same as before.

What problems arise in the gas-using units due to the poor operation of the desulfurization system? As expected, the recycled gas combustion nozzles suffer severe corrosion, experience rapid coking, have a short lifespan, and result in extremely high operating costs. The gas unit had to find a way to add a filtration separator to the main gas pipeline. Initially, an additional stage of filtration separator was added, but it was found that this did not significantly improve the problems of severe corrosion of the combustion nozzles, rapid coking, and short service life. Therefore, a second-stage filtration separator was connected in series, and it was found that the problem had improved; currently, three-stage filtration separators are used in series.

After using three sets of filtration separators in series, the problems with the burners were significantly improved. However, the issue of clogging of the filtering elements within these separators became a major problem; sometimes the filter elements had to be replaced every 1–2 days. The total cost associated with replacing these filter elements was comparable to the original operating costs of the burners. Yet, the frequent replacements due to clogging severely disrupted the smoothness of production operations. The image below shows photos of the clogged filter elements, taken at the installation site provided by the owner:

The high costs associated with replacing and maintaining filter elements force owners to consider switching to filters in order to reduce operational costs. A 3-stage screen separator was used, but asphalt, tar, and black particles still adhered to the screen internals, causing blockages in the screen separator and high operating pressure drops. Additionally, asphalt, tar, and black particles quickly accumulated on the nozzles as well. The image below shows photos provided by the owner of a clogged screen filter cloth with 300-mesh (48 microns) density:

From the photos taken by the owner showing clogged filter elements and filter media, it can be seen that the gas passes through a series of devices such as the quenching water tower, gravity separation tank, electrostatic precipitator, pre-washing tower, pre-desulfurization tower, main desulfurization tower, final washing tower, and gas holder for treatment. However, heavy hydrocarbons such as asphalt and coal tar, light hydrocarbons, and dust particles in the gas are not effectively separated or removed. It was only then that the owner realized the importance of gas separation, and he turned to us for tailored technical solutions. Of course, at that point, the chief engineer who had been overconfident, believing that gas-liquid separation was easy, was forced to resign in disappointment and return to farming, while the person in charge of the plant’s operation showed obvious satisfaction.

It is necessary for everyone to review again the equipment configuration in the gas purification process: quench water scrubber tower, gravity separation tank (ID 3000mm*SM/SM5000mm), electrostatic precipitator (ID 6000mm*H20000mm), pre-washing tower (ID 6000mm*H25000mm), pre-desulfurization tower (ID 6000mm*H35000mm), main desulfurization tower (ID 7000mm*H41000mm), final washing tower (ID 6000mm*H35000mm), gas holder, etc. Category 1: Water washing towers such as shock water washing towers and pre-water washing towers (ID 6000mm*H25000mm), pre-desulfurization towers (ID 6000mm*H35000mm), main desulfurization towers (ID 7000mm*H41000mm), and final-stage water washing towers (ID 6000mm*H35000mm), as well as gas holders – these types of water washing and purification equipment achieve good results in capturing and separating dust, asphalt, and light and heavy hydrocarbons contained in gas when operating under stable design conditions. However, this is possible only on the condition that: (1) the organic phase separated out must be discharged promptly and not accumulate on the liquid surface inside the equipment ； (2) At the gas outlets of all washing equipment, especially those of the final treatment units, vane separators with high efficiency in separation, resistance to clogging, and low operating pressure drop must be installed to remove organic liquid droplets, mist, and dust carried in the gas flow, thereby preventing their excessive escape and subsequent failures in the equipment that uses this gas. Category 2: Gravity separation tanks (ID 3000mm*SM/SM5000mm), light hydrocarbon separation tanks, gas holders, and other purification devices that rely on gravity sedimentation. The effective capture and separation of dust, asphalt, and light and heavy hydrocarbons carried in gas is ensured only if the operating conditions remain stable and meet the design specifications; otherwise, the gas-liquid separation efficiency is poor, which can lead to excessive escape of substances in the heavier phase and subsequent failures in the equipment that uses the gas. This is because such devices rely on the difference between the airflow velocity within a narrow range and the gravitational settling velocity of the particles in the heavier phase to achieve separation; if this velocity difference is not maintained, separation fails.

Third type of separation equipment: electrostatic precipitator (ID 6000mm * H 20000mm), primarily used for the efficient separation and capture from airflow of heavy-phase particulate matter with sizes ranging from 0.3 micrometers to 3 micrometers – particles that tend to acquire a stable charge under certain flow velocity conditions in narrow flow ranges. Therefore, it is essential to gain a deeper understanding of the operational scenarios and application constraints under which electrostatic precipitators can function efficiently. Electrostatic precipitators are not a panacea; their constraints include: (1) threshold conditions regarding airflow velocity within a narrow range ； (2) Heavy-phase carriers facilitate stable charging ； (3) Size range of 0.3 micrometers to 3 micrometers. All three mandatory conditions are essential. This is because electric capture devices achieve separation by effectively deflecting charged particles within their electric field. If the gas flow velocity exceeds the velocity threshold condition for the narrow-range gas flow valve, the charged particles are carried away from the electrostatic precipitator by the gas flow before they can be deflected to the poles, resulting in failure of separation. If the carryout species carried by the airflow do not satisfy condition (2), that is, if they are not prone to maintaining a stable charge – such as particles that are easily neutralized electrically or particles that lose their charge during deflection – then they cannot be deflected to the poles, and separation fails. If the size of the entrained particles in the gas stream does not fall within the range of 0.3 microns to 3 microns as specified in (3), the particle sizes do not meet the required criteria; as a result, the net force acting on these particles is not sufficient to enable them to be deflected effectively toward the pole, leading to failure in separation. There are even other factors such as different structures of electric precipitators that limit the application of electric precipitators.