Dr. Ing. Axel Hanenkamp, MAN Diesel, Germany
MAN Diesel’s new 51/60DF dual-fuel gas engine (bore 510 mm x stroke 600 mm, 8560 to 17 550 kWe) with its advanced pilot injection and gas admission technology has gained it first orders for land-based applications, including both a contract for new engines and the retrofit of an existing engine.
For stationary power generation, the new engine is offered in a nine cylinder inline version and in vee configuration versions with 12, 14, and 18 cylinders, and has mechanical ratings of 1000 kW per cylinder for 60 Hz power generation at 514 rpm and 975 kW for 50 Hz applications at 500 rpm.
The first order for new engines involved the design and construction of a new 22 MW power station at Owen Springs, 25 km south of Alice Springs, for Power and Water Corporation, a major Australian public utility.
In marine applications, the engine cleared all the necessary approval processes (type approval and factory acceptance tests) on its way to powering the largest liquefied natural gas (LNG) carrier to be fitted with the combination of a medium speed dual fuel engine and electric propulsion.
Interestingly, therefore, the 51/60DF dual fuel engine is set to play a dual role in the global energy economy: powering the ships which transport natural gas around the world and burning that fuel in low emissions, high efficiency, high availability power and cogeneration plants.
A Technological Leap
It is these factors plus a technological leap that have created renewed interest in dual fuel engines in the past few years. The critical advance is the reduction in the amount of diesel fuel needed to ignite an air-gas mixture in dual fuel engines. Instead of 5 to 10 per cent in earlier generations, the latest dual fuel engines employ a distillate fuel micro-pilot to ignite a gas air mixture constituting less than 1 per cent of the energy needed to produce the full rated power of the engine.
The converted 51/60DF engine at the TMG SPE combined heat and power plant. The newly installed gas supply lines are clearly visible in their mandatory yellow colour coding
At 5 to 10 per cent, the ignition pilot in earlier generations of dual fuel engines produced far higher emissions of oxides of nitrogen (NOx) and required selective catalytic reduction to achieve compliance with emissions legislation like Germany’s TA-Luft clean air code. With its micro-pilot, on gaseous fuel MAN Diesel’s 51/60DF engine achieves NOx emissions of 500 mg/mn³ at 5 per cent O2, and hence readily complies with both Germany’s TA Luft clean air regulations and the World Bank Pollution Prevention and Abatement handbook.
It is thus interesting to ponder whether, 20 years ago, lean burn gas engines with spark ignition would have established themselves as the gas engine technology of choice had micro-pilot technology been available at the time.
In any event, a prime advantage of dual fuel engines is their capability to switch to 100 per cent liquid fuel charge when gasesous fuel is not available. As proven in type approval tests for the leading Marine Classification Societies, MAN Diesel’s 51/60DF engine can changeover seamlessly – and, significantly, under any load conditions – from gaseous fuel to 100 per cent distillate fuel (diesel oil, gas oil) or heavy fuel oil (HFO).
Equally, the engine can be switched directly at any load from distillate fuel operation to gaseous fuel mode, while the change from HFO operation requires only a short intermediate period on distillate fuel. These multi-fuel modes are equally useful in both land-based power generation and at sea on LNG tankers or cruise ships, another application where dual fuel engines are creating great interest. On land, as well as providing back-up per se, this fuel flexibility gives dual fuel engines an uninterruptible power supply capability which is of considerable interest for both power security (no-break) and power quality applications.
The 48/60DF has been one of the mainstays of the Augsburg, Germany based Power Plant business unit of MAN Diesel and has been sold all around the world. The new dual fuel eingine is based on the very successful type 48/60 engine platform (bore 480 mm x stroke 600 mm) which in its largest, 18 cylinder vee configuration version the 48/60B offers 18.9 MW (1050 kW/cylinder), and is therefore the most powerful medium speed diesel in Man Diesel’s portfolio.
MAN Diesel expects the new 51/60DF to attract great interest in retrofit applications. At present, many regions which rely on diesel engines burning heavy fuel have acquired a natural gas network, as the supply infrastructure of this acknowledged low emissions ‘fuel of the future’ expands. Yet many regions have also become subject to emissions legislation, while others are introducing incentives for high efficiency cogeneration plants with low emissions. These incentives generally take the form of preferential rates for each kWh of electrical power supplied to the local grid.
All these factors are incentives to retrofit existing HFO engines with advanced dual fuel technology, and MAN Diesel completed its first such project in 2008, converting the 12 cylinder 12V 48/60A heavy fuel engine operated at Vila Nova de Famalicão, Portugal by Têxtil Manuel Gonçalves, Sociedade de Producao de Electricidade e Calor SA (TMG SPE).
The retrofit was timed to coincide with a major overhaul involving extensive dismantling of the engine following over 88 000 hours in grid parallel operation at the cogeneration plant where it provides electrical power for TMG’s adjacent textile works and the Portugues grid, and recovered thermal energy to raise steam for processes at the works.
In this application, all the advantages listed above where fulfilled exploiting a natural gas grid installed since commissioning of the HFO engine, emissions compliance and qualification for preferential kW price in a high efficiency application.
Dual Fuel Retrofit
The 51/60DF engine produces 1000 kW/cylinder at 514 rpm or 975 kW at 500 rpm in both gaseous and liquid fuel mode. In the gas mode the engine operates according to the lean-burn Otto combustion process. In the liquid fuel mode, the thermodynamic working process is the diesel combustion process. The creation of the new dual fuel engine thus essentially involved adding gas related components to the 48/60B HFO base engine.
Hence the conversion of the TMG SPE type 12V 48/60 engine to full type 51/60DF dual fuel specification comprised: replacing the original 480 mm diameter cylinder liners and pistons for new 510 mm bore components; the new pistons having crowns (bowls) adapted to gas combustion; new cylinder heads with separate, side mounted pilot injectors; inlet manifolds with one gas admission valve per cylinder; new inlet cams matched to the gas combustion process; charge air and exhaust manifolds equipped with pressure relief valves to avoid any gas operation risks; a turbocharger compressor bypass to precisely control air:fuel ratio; a common rail fuel injection system, with electric driven pump for pilot fuel injection integrated into the cylinder head; installation of the electronic engine management system SaCoS DF (dual fuel) for the whole engine operation range and all fuelling modes.
SaCoS DF is based on the standard safety and control system (SaCOS) for diesel engines, which is well proven and approved by the major marine classification societies. The common rail pilot injection system, with its precise control of injection pressure, timing and duration in combination with the separate gas admission valves and SaCoS DF control system enable very precise control of combustion in the gaseous fuel mode, allowing exact balancing of the outputs of the individual cylinders and response to combustion knock signals (detonation) on a cylinder-by-cylinder basis.
At the same time, as part of the overhaul of the TMG SPE engine, the main, conventional ‘pump-line-nozzle’ fuel injection system was refurbished and the turbocharger overhauled and re-matched for gas engine operation via the fitting of new nozzle rings to change the pressure of the exhaust gases reaching the turbochargers’ turbines.
Dual Fuel Concept in Detail
The optimized control of the air to fuel ratio enabled by the charge air bypass and gas admission valves gives stable operation in gas mode with high efficiency and low emissions. The electronically driven gas valves feed each cylinder individually with its required amount of gaseous fuel. An electrical actuator controls the main injection pumps. An additional phase pick-up delivers crank angle signals in the liquid fuel mode.
For reliable gas operation, the knocking control system plays an important role. Combustion knocking (detonation) is detected for each cylinder unit individually by an acoustic sensor mounted on the cylinder head. The independent acoustic signals are fed into the main sensor unit where the knocking level is determined. The overall engine control system reacts to the knocking level and makes appropriate adjustments to engine operating parameters.
The solenoid driven common rail pilot fuel injectors are separately installed from the existing main fuel injection system, allowing the main and pilot injectors to be replaced individually, according to their respective times between overhaul. The electrically driven common rail pump achieves long lifetimes and can be maintained during operation. In gas operation the required amount of gas is metered individually for each cylinder by the gas admission valves.
A flow control pipe ensures a homogeneous mixture of air and gas entering the combustion space. A double-walled pipework concept with insulation and ventilation of the main gas valve promotes a gas free engine hall concept, designed to allow the installation of several engines in a single engine room.
Engine Management System
The engine management system SaCoS DF provides the control system in both liquid and gas mode. The air to fuel ratio is controlled by a compressor bypass, installed after the charge air cooler under the control of the SaCoS DF system. SaCoS DF system architecture is dominated by two independent working governors. One governor controls the main injection fuel oil pumps and the individual fuel gas admission valves for each cylinder unit. The second governor controls the common rail pilot fuel injectors.
Principal architecture of the SaCoS DF dual fuel control and safety system
The individual knocking levels, collected from each cylinder by the knocking detection unit are processed in the SaCoS DF system. In combination with the cylinder individual control of the pilot injection and the gas admission, the SaCoS DF control ensures a stable operation in gas mode with a sufficient margin to knocking and misfiring border. In addition the control system establishes connections with overall alarm and safety systems.