178- Discussion on the technical plan for the project aimed at addressing the hazards associated with the exhaust gases from the wet flue gas desulfurization towers in the thermal power boilers used in petrochemical enterprises

The thermal and cogeneration boilers installed in petrochemical enterprises usually include gas boilers, oil-fired boilers, coal-fired boilers, or boilers that burn a mixture of oil and gas, as the waste oil and gas generated by petrochemical enterprises, such as catalytic dry gas and heavy oil catalytic slurry, is used directly as fuel for these boilers. In the early years, due to limitations in the scale of refining and processing facilities, boilers that could burn both oil and gas were commonly used; however, fluctuations in the ratio of oil to gas used led to unstable operation of these boilers. Nowadays, gas-fired and oil-fired boilers are generally operated separately. Gas boilers use catalytic dry gas from petrochemical plants as fuel. Due to factors related to the gas source and its calorific value, they generally produce a small amount of steam with low temperature and pressure; they are mainly used to meet summer heating demands. These boilers generate less flue gas, and the flue gas contains low levels of dust, nitrogen, and sulfur, making it easy to treat. In the past, ammonia used for denitration was directly introduced into boilers that employed ammonia-based desulfurization processes, with the resulting ammonium sulfate crystals being exported. Oil-gas fired boilers for fuel oil boilers operate by directly using the catalytic slurry and hot residue oil from petrochemical enterprises as fuel, or by burning a mixture of oil and gas. Due to factors related to the type of oil and its calorific value, these boilers generally produce a moderate amount of steam at medium temperature and pressure. The volume of flue gas generated is also moderate, with moderate levels of dust, nitrogen, and sulfur content. In the past, petrochemical enterprises frequently employed ammonia-based or sodium alkali-based desulfurization processes.

In recent years, as the scale of petrochemical plants has increased by several times or even dozens of times, the heat demand in these plants has risen sharply. The existing gas and fuel boilers, as well as the oil and gas produced as by-products, are no longer sufficient to meet this heat demand; especially during winter when the heat requirements are high, companies often turn to using coal-fired boilers. Coal-fired boilers use crushed coal, pulverized coal, or coal water slurry as fuel. Due to factors related to the source of coal and its calorific value, these boilers generally produce large amounts of steam, with steam temperatures and pressures reaching medium to high levels. Although they generate a large volume of flue gas, this gas contains high levels of dust, nitrogen, and sulfur; as a result, purifying such flue gas is much more difficult compared to gas-fired or oil-fired boilers. Petrochemical companies often employ the sodium-alkali based wet desulfurization process, which is part of the catalytic flue gas purification systems used for heavy oil both domestically and internationally. This post takes the flue gas treatment of coal-fired boilers in the power plants of petrochemical enterprises, which presents the greatest technical challenges, as an example for joint analysis and discussion.

**There are specific standards established for the emission of air pollutants from boilers, and there are also specific **standards** set for the emissions from industries involved in oil refining. For thermal boilers constructed within petrochemical industrial parks, both requirements must be met simultaneously, and the strictest criteria from their intersection shall be applied. Therefore, it is inappropriate for many environmental protection companies to copy the flue gas treatment process technologies and equipment used in thermal power plant boilers and apply them to the flue gas treatment of thermal boilers in petrochemical enterprises. It is necessary to remind petrochemical companies and environmental protection service firms to pay more attention to this and take it seriously; otherwise, various problems will arise during actual operation.

The example cited in this post relates to a petrochemical company whose thermal boilers in the power station were installed between 1980 and 2000. Coal-fired boilers were the last to be built in 2000. Before 2000, **requirements regarding the emission of environmental pollutants were low, and oversight was merely nominal; in many petrochemical companies, the treatment of flue gases from thermal boilers was often carried out by environmental companies that had experience in handling flue gas from coal-fired boilers in power generation plants. The flue gas desulfurization and denitration systems installed by these environmental protection companies, modeled after the flue gas treatment methods used in coal-fired boilers in thermal power plants, suffer from issues such as operational blockages, high pressure drops, high energy consumption and maintenance costs, poor adaptability to different operating conditions, unstable emissions that fail to meet regulatory standards, as well as water-containing flue gas that causes the ground around the units to be slippery in summer and icy in winter. These problems pose risks to ensuring compliant flue gas emissions, maintaining the safety of the units, and protecting the safety of the workers on site.

This post was last edited by luoli519 on 2021-3-3 at 14:30. The flue gas treatment for the coal-fired boilers in the power plant of the petrochemical company discussed in this post follows a process of denitrification first, followed by desulfurization. In its original denitration process, SCR catalytic reaction technology was used for denitration; this required good dust removal of the flue gas before it entered the denitration unit, in order to prevent dust from accumulating and clogging the catalyst bed in the SCR unit, thereby affecting the reaction efficiency, selectivity, and other aspects of proper operation. The environmental protection company has installed two three-field high-voltage electrostatic precipitators at the front end of this flue gas denitration unit. In actual operation, due to the high dust content in the flue gas and significant fluctuations in other operating conditions, as well as limitations arising from the working principle of high-voltage electrostatic precipitators and their structural design, these precipitators have poor adaptability to varying operating conditions, high operational and maintenance costs, and inadequate dust removal efficiency.

This post was last edited by luoli519 on 2021-3-3 at 14:31. An environmental engineering company, in response to the problems encountered in the actual operation of the two three-field high-voltage electrostatic precipitators located at the front end of the flue gas denitration system used by the client, proposed modifying these electrostatic precipitators using \"electro-bag composite systems\". The specific plan is to retain the first electric field and divide it into two, transforming it into two smaller electric fields; meanwhile, the original second and third electric fields are emptied out to accommodate two bag dust collectors. What do you all think? Are there any better suggestions? It is better to adjust the sequence of setting up the bag filter and the electrostatic precipitator, adopting a \"bag-electrostatic combined\" approach.

This post was last edited by luoli519 on 2021-3-3 11:43. Let’s focus on the \"desulfurization\" step again. This environmental protection company utilizes the traditional FGD technology for flue gas desulfurization in thermal power plants; it installs cooling devices in the inlet pipelines of the scrubber towers, and employs a three-stage circulating scrubbing system for dust removal and desulfurization within the desulfurization towers. At the top of the scrubber towers, a \"one-stage tubular demister + two-stage ridge-type demisters\" is used, which is made of PP material through extrusion processing, just as in the traditional FGD technology used in thermal power plants. The following diagram is a schematic of the desulfurization washing and absorption tower provided by the owner:

This post was last edited by luoli519 on 2021-3-3 at 11:45. After entering the washing and absorption tower, the flue gas first passes through three layers of washing and spraying areas. The layout of the spray washing pipes for each tray layer is as shown in the figure below. Its spray nozzles and piping systems utilize the ROOF ridge-type demister spray nozzles and piping systems made of PP, as those used in the FGD process package mentioned earlier.

In the traditional FGD technology used in thermal power plants for flue gas desulfurization, after the flue gas undergoes three-stage cyclic washing for dust and sulfur removal, it enters a demister made of PP material and extruded into a \"Chevron-shaped baffle\" located at the upper part of the washing tower. The FGD purification technology for boiler flue gas used in thermal power plants abroad in the 1990s was adopted based on a lack of sufficient awareness worldwide regarding the impact of air pollution emissions at that time; it was implemented only in the West, where there already existed basic indicator systems and treatment technologies. The demisting internals used in FGD technology are typically low-cost PP-extruded \"Chevron flat-plate baffle demisters,\" which have poor tolerance to flue gas temperatures; therefore, a quenching cooling system as well as an over-temperature bypass discharge system must be installed for the flue gas entering the tower. Nevertheless, due to the poor weather resistance of plastic products, they usually become brittle and damaged after 3-5 years of use, requiring regular replacement and maintenance. In the vast majority of domestic thermal power enterprises, FGD technology and low-cost “Chevron-type corrugated plate mist eliminators” made through plastic extrusion are still used for boiler flue gas purification. For various reasons, their replacement cycle is less than 3–5 years. Incidents such as liquid carryover in flue gases, excessive emissions, and even deformation or collapse of PP mist eliminators due to excessively high flue gas temperatures occur frequently.

The figure below shows the PLAIN-type “Chevron smooth-plate baffle mist eliminator” used in the FGD process, which is still employed by many domestic enterprises in the environmental protection, chemical, and thermal power industries. The separation efficiency of this type of demister is very low; it can generally only perform primary separation on heavy-phase entrainers with sizes ranging from 35 microns to 80 microns, and it cannot be used in applications that require precise separation for environmental protection purposes. However, many property owners are not aware of this.