Safe and efficient boiler operation must be a priority, whatever the chosen method of complying with IMO's regulations
Regardless of the strategy chosen to comply with the 2020 sulphur cap, the impact on boilers will have to be factored in. Sweden’s Alfa Laval has recognised this and issued guidance outlining the impact of the various compliance strategies on boilers.
Alfa Laval’s guidance says: “In preparing to comply with emission limits in 2020 and beyond, the boiler will only partly influence fuel strategy decisions…However, each strategy will have consequences for the boiler…Due attention must therefore be paid to the combustion in the boiler.”
There are two main streams for compliance with the 2020 regulations: continue to use high-sulphur fuel and fit vessels with an exhaust gas scrubber to keep sulphur emissions below the limits, or switch to an alternative fuel.
Alfa Laval’s guidance limits its scope to what it sees as the main strategies, which are scrubbers, LNG, marine gas oil (MGO) and low- and ultra-low-sulphur HFO (ULSHFO).
There are a range of options when it comes to scrubbers: they can be closed- or open-loop systems, and they can be fitted only to the main engine exhaust or via a multi-inlet configuration that also incorporates boilers and auxiliary engines. If an owner uses a scrubber only for the main-engine exhaust there will not be an impact on the boiler - although they will need to formulate a strategy to ensure the boiler’s emissions are 2020-compliant.
If a multi-inlet scrubber system is used, the resulting increased back-pressure will have an effect on the boiler. Exact levels of back pressure will depend on the scrubber design and it is possible to modify most boiler and burner systems to compensate for this. Alfa Laval notes that if retrofitting is required, a tailor-made package can be produced for the specific boiler/scrubber combination concerned.
“Boilers can operate very well on LNG,” Alfa Laval’s guidance explains, but it notes that individual gas-firing studies may conclude steam output is reduced. Precautions will be required due to the highly explosive nature of LNG and in order to prevent gas leaks, piping systems will need to be double-walled with a gas detection system in place between the two walls. This will ensure that should gas be detected in the void space an alarm will sound. The gas valve train, which regulates gas pressure and flow, is another area to which attention must be paid. To ensure safe and reliable system operation extraction air fans, gas detectors and other equipment are needed.
The guidance anticipates no problems in specifying fuel and ensuring correct boiler components are delivered for newbuild LNG projects, but notes that as well as additional LNG-handling equipment, detailed engineering studies are required.
"Precautions will be required due to the highly explosive nature of LNG"
For retrofit projects investment will be required to get boilers ready for LNG operation. “The most important issue will be the availability of space for the gas valve unit, extraction air fans and gas detectors,” the guidance says. The burner and control panel will need to be replaced. Also, gas flames have less radiation heat transfer, so a boiler designed for fuel oil may require a study to be completed to see if its capacity can be maintained at the same rating as using fuel oil.
Exhaust gas boilers connected to the main and auxiliary engines and associated equipment will need to undergo a design review for the revised exhaust gas temperatures and amounts. “This will provide assurance of heat transfer and establish the new estimated levels of heat recovery,” the guidance notes. In addition, a study will be required to determine the total steam balance; the increased heat required to evaporate LNG counters the lower bunker heating requirement, the guidance says.
“MGO has a much lower flashpoint than HFO, which can lead to some issues when using MGO on a continuous basis,” Alfa Laval explains. This low flashpoint means care must be taken to prevent the formation of gases in the furnace or pipelines. Fuel must be prevented from entering the hot furnace without a flame present to this end, and valves kept in good operational condition. As well as the normal purging that takes place for all fuels, a post-operational purge must also be instituted. Should a blackout cause the burner to stop, the boiler must be purged either by putting the forced-draught fan damper into an open position or by locking it in its last pre-blackout position,” the guidance says. After power is restored, the boiler should be manually purged for a cycle before fuel is reintroduced.
Precautions need to be taken to protect against any potential heat sources increasing the temperature of the fuel to above the flashpoint and to stop gasification of the fuel in the piping system leading to the burner. The circulation pump, fuel line and burner are all key areas to pay attention to in this regard.
Operational issues may also arise from the cold-flow properties of MGO if operating in winter conditions. Close attention must be paid to the cloud point (the temperature at which dissolved particles precipitate and form wax), the pour point (the lowest temperature at which the fuel will flow) and the cold filter plugging point (the lowest temperature at which fuel will pass through a filter under specified conditions).
Another area to consider is that MGO burns with a different light spectrum to HFO, so correct flame detection equipment must be used. MGO’s low viscosity also means care must be taken with tightness of shut-off valves.
Some major suppliers have blended HFO with low-sulphur distillates to create low-sulphur, highly viscous fuels known as low-sulphur or ultra-low-sulphur HFO (LSHFO/ULSHFO).
Alpha Laval notes: “The main issue with LSHFO/ULSHFO is that the consistency of the fuel is not very high. This means it is not known how fuels from different suppliers (or bunkering ports) will react with each other.”
This can lead to sub-optimal combustion and subsequently increased levels of fouling in the boiler. Should this occur, the burner and combustion will require adjustment. Alfa Laval also recommends separate day tanks if LSHFO/ULSHFO is being used in combination with MGO and calls for separate fuel lines for MGO and other fuels to the burner as an ideal solution. The guidance also notes that LSHFO and ULSHFO are known for containing wax and other impurities which can irreversibly form at low temperatures.