Decarbonisation Technology May 2022 Issue

particulates emissions in the design basis for CC units on such ethylene plants. NO₂ is a contaminant that should be reduced for the CC plant, as it degrades the absorbent. Modern ethylene furnaces without a selective catalytic reduction (SCR) are typically designed to achieve NOx emissions in the range of 50-70 ppm (dry basis, corrected to 3 mol% O₂). A typical conservative assumption is to have a split of 85% NO and 15% NO 2 during a normal cracking operation. The addition of an SCR to reduce NOx and consequently NO₂ at the inlet to the capture plant will be a trade-off between amine degradation and the cost and operation of the SCR. This can be analysed in detail for each case. Another interesting point is that the operation of the furnaces in the different modes (decoking, hot steam stand-by) introduces very small changes to the emitted flue gas. Also, ethylene crackers offer an advantage for CO₂ capture as they run at constant loads for long periods, allowing the CC plant to run smoothly over time with little intervention. It is worth noting the CC plant is also very flexible in operation and can handle a more challenging, dynamic type of operation. In modern ethylene plants, each furnace has an individual induced draft (ID) fan to control its arch pressure. When CC is installed, the outlet of the ID fan will discharge to a common duct, which receives the flue gas from each furnace and directs it to the CC unit. A common booster fan upstream of the CC unit pulls the flue gas through the common duct. Dampers will be installed to permit the furnaces to continue to operate when the CC unit is not running and enable the furnaces to be individually isolated from the common duct. Upon trip of the downstream CC unit, the ethylene plant shall remain in operation. This will require an alternate route to atmosphere for the flue gas from the furnaces. Each furnace will retain an individual stack, which is isolated by a damper in normal operation. On failure of the common booster fan (or trip of the CC unit), the stack damper on each furnace would be tripped open to allow the furnace flue gas to discharge directly to atmosphere. Failure modes for utilities are considered: for example, the steam supply to the CC plant could be tripped to conserve steam in the ethylene plant in the event of a power failure. Everything shall be considered

to allow the continuous/smooth operation of the ethylene plant. It should be noted that the CC plant does not rely on the availability of low CO₂ electricity to reduce the CO₂ emissions from the cracker, although the increased utility demand for the CC plant should be met with as low a carbon footprint as possible. A large part of the CC plant utility demand can be met from the cracker. CO₂ utilisation If the ethylene plant is located near depleted reservoirs or next to a CO₂ pipeline, the CO₂ can be sequestered or used for EOR. Only a compression and purification unit will be required (mainly removing water, but a small concentration of oxygen can also be removed if required for pipeline safety). Traditional technologies for the use of CO₂ include the production of urea and using gas/ liquid CO₂ in the food industry, such as dry ice production. Methanation is another ‘old’ technology being proposed to ‘recycle’ CO₂. Emerging technologies for transforming the CO₂ into a ‘usable’ product (IEA, 2019) are utilising hydrogen, produced with renewable electricity, to transform CO₂ into methanol or ethanol. From there, a number of routes can be followed to transform these alcohols into chemicals or fuels (aviation fuel, for example). Ethylene production by ethanol dehydration (Technip Energies’ Hummingbird technology) is an attractive route for conventional ethylene producers. The ethylene produced in this way will be expected to have a higher price, as it could be considered ‘green ethylene’ (depending on the source of hydrogen). Another way of utilising CO₂ is to produce building materials to replace water in concrete: CO₂ curing. This process is perhaps one of the more mature/developed technologies. CO₂ can also be used as a raw material in its constituents (cement and construction aggregates). Both technologies are centred on the reaction of CO₂ with calcium or magnesium to create low-energy carbonate molecules, the form of carbon that makes up concrete. This technology requires further development compared to curing. In both cases, the resultant concrete can be tested on the construction of non-structural units (roads, floors) until these new materials can meet regulations, which can be very


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