Coal used in coke ovens gives off a high percentage of volatile, effluent gas that can be dried and conditioned, resulting in purified coke oven gas (COG). This byproduct gas provides a clean, environmentally friendly and valuable fuel source that can be used in many processes, including as fuel gas in the making of iron and steel.
For the coke oven gas to be most useful, moisture and other contaminants — for instance, tar aerosols, ammonia and naphthalene — must be removed to prevent fouling and corrosion of the gas lines and energy-recovery equipment. Extracted byproducts in the purification process such as ammonia, sulfuric acid and coal tar are cleaned, purified and sold as additional plant revenue streams. Also, hydrogen sulfide is removed from the coke oven gas to meet local emission regulations when it is burned.
In processes for drying and conditioning of coke oven gases, heat exchangers are used to cool circulated process fluid as efficiency increased at lower temperatures. In this application, the process fluids are partly polluted with aggressive components that can corrode metals and swell rubber elastomers used in gaskets. The process liquid also can be loaded with solids, and the risk of units becoming blocked needs to be addressed in their design. Ease of cleaning also is required to ensure thermal efficiency is maintained.
Case in Point: Increase Coke Oven Gas Treatment Capacity
Located in Dortmund, Germany, thyssenkrupp Industrial Solutions offers — apart from its ovens for coke production — advanced processing equipment for coke oven gas treatment. The company’s heavy plant equipment for coke oven processing helps ensure efficient operation.
A large steel plant in Taiwan had installed thyssenkrupp coke oven gas treatment equipment some time ago. In the face of increasingly stringent environmental and emissions regulations, the steel plant needed to upgrade the ovens, but they had to remain within the current plant footprint.
The existing system partly used shell-and-tube heat exchangers. The exchangers offered a robust heat transfer solution but could not be expanded within the restricted space available. A compact solution with a design that helped prevent fouling and handled the aggressive fluids within coke oven gas treatment was needed. Key attributes included the following:
- To be able to handle greater circulation volumes and deliver the necessary improved gas purification, the new heat exchangers needed to be more efficient (by volume) compared with the existing units.
- While the system was being upgraded, the existing pipework and nozzle orientations also needed to be accommodated.
- The delivery and project commissioning schedule were vital to the project.
Although shell-and-tube heat exchangers work well for the processes involved in treating and conditioning coke oven gas, the size of this unit meant there was not room to increase capacity and efficiency to further purify the gas. Shell-and-tube units are relatively heavy compared with other heat transfer technology and require space around them so that the tube bundle can be removed for cleaning. Spiral heat exchangers offer a more compact solution, but they can be an expensive choice. Traditional block-type heat exchangers and most gasketed-plate heat exchangers offer a compact footprint. The narrow gaps within block-type and plate heat exchangers meant that they would have a high potential to foul in this application. Furthermore, a gasketed-plate heat exchanger would not be suitable because the corrosive fluids would quickly attack and compromise gasket integrity.
After reviewing the options, the steel plant selected the APV hybrid plate heat exchanger. This unit has a tube on one side and plate on the other. The fully welded unit has no rubber or elastomer gaskets that could corrode and is designed to minimize the potential for fouling. It is cleaned with ready access to internal plates and a large enough gap to negate the need for angled cleaning heads. Compared with shell-and-tube heat exchangers, the hybrid plate unit delivers the same capacity but can be as small as one-fifth the size of shell-and-tube units. The smaller footprint means that the steel plant is able to improve gas conditioning and reduce emissions within the boundaries of the existing system.
Even with the tough conditions found in coke gas treatment systems, hybrid plate heat exchangers offer high thermal efficiency that can reduce energy costs. The reduced potential for fouling, along with ease of cleaning and inspection access, further help to lower maintenance overhead and maximize system availability. For this particular application, the hybrid plate heat exchanger offered the correct size, efficiency and cost to meet project needs.
The heat exchangers used have shown their capability to handle harsh environmental conditions. The 0.291” (7.4 mm) diameter of the tube within the unit offers a balance between the need to reduce fouling and to keep the unit footprint compact. The construction material chosen — an austenitic stainless steel, Alloy 254 SMO (1.4547) — offers high resistance against corrosive process fluids compared to standard alloy 316L (1.4404) stainless steel. The technology offers efficiency, capacity and operability.
The plant upgrade was a large project using 45 hybrid heat exchanger units. The heat exchanger manufacturer and thyssenkrupp Industrial Solutions worked closely with the customer to ensure the units were precisely engineered for smooth installation. This included providing templates to ready the site for the new machines.
The improved gas-cleaning capability the new heat exchangers will provide will secure future production at the plant, help it meet increasingly stringent local environmental legislation and reduce maintenance overheads.