Seawater/alkaline liquid cascade‐scrubbing desulfurization performance for the exhaust gas of a 162‐kW marine diesel engine

Abstract

A cascade‐scrubbing technology was developed for marine diesel engines to improve the current once‐through desulfurization solutions. To confirm the seawater/alkaline liquid cascade‐scrubbing advantages of higher efficiencies and lower lye consumption compared with current solutions, desulfurization experiments with different scrubbing models were performed for a 162‐kW marine diesel engine. In the open‐loop model with SO2 of 1,000–2,860 mg/Nm3, desulfurization with the allowed liquid–gas ratios ≤8 L/Nm3 for the packing tower failed to fully meet the ECA's requirements, despite the efficiency increasing with liquid–gas ratio, seawater alkalinity, and packing height and decreasing with SO2 concentration. In the closed‐loop solution, to obligate the ECA's requirements with SO2 of 2,860 mg/Nm3, a high liquid–gas ratio of 4.8 L/Nm3 was necessitated at Na/S=2. In contrast, in the seawater/alkaline liquid cascade‐scrubbing model under high‐sulfur and low‐alkalinity conditions (2,860 mg/Nm3 and 0.84 mmol/L), the solution utilized 5–6 L/Nm3 seawater in the main scrubbing section to scrub 80–90% SO2 and an extremely low alkaline‐liquid dose below 1 L/Nm3 in the auxiliary scrubbing section to remove the residual SO2, finally allowing a high efficiency ahead of the open‐loop model and meanwhile cutting down an 80–90% lye consumption compared with the closed‐loop model.

Desulfurization performance of a large‐scale marine diesel engine's scrubber with packing scrubbing: Validation of design parameters based on ASPEN PLUS simulations

“…Again, due to the main scrubbing section accomplishing a 90% desulfurization share based on the seawater scrubbing, the seawater/lye cascade‐scrubbing model thus equals to save 90% lye consumption compared with the reported closed‐loop model fully relying on lye scrubbing, 13 thereby decreasing sharply the desulfurization cost. This lye saving superiority has been also reported in previous lab‐scale cascade‐scrubbing experiments 21 . The seawater/lye cascade‐scrubbing model's superiority in saving both seawater and lye consumption and greatly reducing the scrubber resistance should be more prominent as confronting with higher furl‐sulfur circumstances.…”

“…Although shipping scrubbers have wet and dry modes, 21–25 the high desulfurization efficiency requirement, light equipment weight, and limited space on ships all restrain the use of dry scrubbers on ocean‐going ships 26 . The current wet scrubbers mainly have three types according to the different absorbents used, that is, the open‐loop solution based on seawater scrubbing, closed‐loop solution based on lye scrubbing, and hybrid solution freely switching the agent between seawater and lye 10 .…”

“…20 In light of the shortcomings of an obvious price gap between low-sulfur and high-sulfur oils of the first solution and the third solution with alternative fuels necessitating comprehensive power system modification, the ocean shipping industry pins its economical SO 2reduction hope on the installation of scrubbers with high desulfurization performance. 18 Although shipping scrubbers have wet and dry modes, [21][22][23][24][25] the high desulfurization efficiency requirement, light equipment weight, and limited space on ships all restrain the use of dry scrubbers on ocean-going ships. 26 The current wet scrubbers mainly have three types according to the different absorbents used, that is, the open-loop solution based on seawater scrubbing, closedloop solution based on lye scrubbing, and hybrid solution freely switching the agent between seawater and lye.…”

“…Although the hybrid solution is superior to the previous two versions because of its good operational flexibility, the aforementioned shortcomings related to cost and NaOH storage have yet hindered more application under the present shipping circumstances where ships frequently sailing in ECA and offshore areas with seawater desulfurization forbidden. 29 As a potential solution for the aforementioned limitations of three conventional scrubbing modes, a cascadescrubbing technology was developed in the published work, 21,29,30 where the overall scrubbing procedure was partitioned into (i) the precooling and humidifying section in the fore precooling tower and (ii) the main and auxiliary scrubbing section in the rear scrubbing tower, allowing three kinds of cascade-scrubbing models corresponding to different conditions (i.e., seawater/seawater, seawater/lye, and alkali-lean/alkali-rich lye cascade-scrubbing models). Accordingly, by performing experiments in a lab-scale cascade-scrubbing system for the exhaust gas of a 162-kW high-speed marine diesel engine (fueled by diesel oil and high-purity SO 2 added into the exhaust gas to model various fuel-sulfur content series), the following contribution was obtained in three aspects: (i) the seawater/seawater cascade-scrubbing advantages in reducing seawater consumption and achieving the strict ECA's requirements compared with the once-through open-loop solution, 21 (ii) the higher alkali utilization efficiency and limited alkali consumption reduction of the alkali-lean/alkali-rich lye cascadescrubbing in comparison with the once-through closed-loop solution, 29 and (iii) the seawater/lye cascadescrubbing advantages of a higher desulfurization efficiency and far lower lye consumption (because of a 90% desulfurization share undertaken by seawater in the main scrubbing section).…”

“…However, as mentioned above, these desulfurization results were fully gained from a small-scale high-speed marine diesel engine fueled by low-sulfur oil, instead of a large-scale low-speed marine diesel engine burning high-sulfur HFO where the conditions are close to the real-ship environment. Again, the attention of these lab-scale desulfurization experiments were mainly focused on (i) establishing the required scrubbing parameter setups according to various de-SO 2 requirements at each scrubbing model and (ii) performing a comprehensive comparison between each cascade-scrubbing model and its corresponding once-through open-loop (or closed-loop) model so as to highlight the cascade-scrubbing advantages, 21,29,30 lacking investigations into effects of scrubber's configurational parameters on desulfurization. This means that varying scrubber's configurational parameters for evaluating their effects on the desulfurization performance and again confirming the selected design parameter's availability are particularly necessary to explore the adaptability of the cascade-scrubbing technology used for large-scale marine diesel engines.…”