Modeling the Distribution of Sulfur Compounds in a Large Two Stroke Diesel Engine

Abstract

In many years large low speed marine diesel engines have consumed heavy fuel oils with sulfur contents in the order of 2.5−4.5 wt %. Present legislation requires that the fuel sulfur is reduced, and in the near future the limit will be 0.5 wt % globally. During combustion most of the sulfur is oxidized to SO 2 from which a fraction is further oxidized to SO 3 . SO 3 may combine with H 2 O and condense as liquid sulfuric acid that promotes corrosive wear on e.g. cylinder liners. To extend engine lifetime and reduce costs for lubrication it is pivotal to identify formation of SO 3 with respect to operational conditions and sulfur feed. This work presents a computational model of a large low speed two-stroke diesel engine where a 0D multizone approach including a detailed reaction mechanism is employed in order to investigate in cylinder formation of gaseous SO 3 where fuel burn rates are based on experimental pressure traces. In contrast to NO the SO 3 does not really form at the highest combustion temperatures, but like NO the formation of SO 3 is very sensitive to the rate that fresh air mixes with hot combustion products. Consequently a simple mixing rate is proposed and calibrated in order to meet experimental results of NO. For a large low speed diesel engine the model shows that 3−5% of the injected sulfur is oxidized to SO 3 that is formed primarily in the temperature range from 2000 to 1300 K during cylinder expansion. In addition the model is used to reduce the full reaction mechanism from 96 to 7 elementary sulfur reactions without compromising the SO 3 to SO 2 ratio.

An experimental investigation of the corrosive influence of SO2 relative to H2SO4 of marine engine cylinder liners

“…Supplementary experimental research confirms the low conversion of fuel sulphur into SO 3 /H 2 SO 4 in light‐ and medium‐duty diesel engines . The conversion may, however, be more than 4% in large low‐speed marine engines partly because of much lower engine speeds. In addition, advanced computational efforts indicate that H 2 SO 4 vapours will condense on the cylinder liners …”

“…When a fuel contains sulphur, and is combusted in a diesel engine, the sulphur rapidly converts into SO 2 in the hot flame that may exceed 2000°C. A fraction of the SO 2 is subsequently converted into SO 3 and ultimately H 2 SO 4 , as the combustion gases cool during the expansion stroke . Adding SO 2 into the charge air of a fired diesel engine has the same effect as increasing the fuel sulphur content‐more SO 3 /H 2 SO 4 will form.…”

“…Several charge gas compositions are introduced into the cylinders of the motored BUKH engine. Since the engine experiences no combustion and therefore produces lower peak pressures than a fired marine engine, the dosing of SO 2 , H 2 SO 4 , and H 2 O is tuned to mimic the partial pressures within the cylinder of a fired marine engine . The steady experimental operating conditions are listed in Table .…”

“…Since the engine experiences no combustion and therefore produces lower peak pressures than a fired marine engine, the dosing of SO 2 , H 2 SO 4 , and H 2 O is tuned to mimic the partial pressures within the cylinder of a fired marine engine. 10,20 The steady experimental operating conditions are listed in Table 3. The matrix in Table 4 lists the individual experiments, that is, the test cases (TCs) that are performed in this work.…”

“…During combustion, the fuel sulphur is primarily converted into sulphur dioxide (SO 2 ). After combustion and during the expansion stroke, a fraction of the SO 2 is converted into sulphur trioxide (SO 3 ) . SO 3 can combine with available water vapour (H 2 O) in the cylinder gas and form sulphuric acid (H 2 SO 4 ) vapour.…”