50DF: the world's largest gas recip

Wärtsilä's newly launched 50DF - the 50 denoting bore in cm and the DF indicating dual fuel - is the biggest gas reciprocating engine in the business. The eighteen cylinder version, the 18V50DF, is rated at 17.1 MW mechanical output. Put two or three of these together and you have a decent sized power plant that can take on an area of the market that has in recent times been considered the preserve of the gas turbine.

The idea of addressing this segment of the market has been one of the main drivers behind Wärtsilä's development of the 50DF. Another has been the company's commitment to the concept of distributed generation. At the recent launch event for the 50DF in Trieste, Wärtsilä's business intelligence specialist, Jukka Hakola, went so far as to say that "the era of big centralised power plants is coming to an end".

While that might be overstating the case a little, distributed generation does have many attractions. Among those listed by Jukka were lower transmission costs, lower investment risks, better matching of heat loads for CHP, faster permitting (because of the size) and the opportunity to invest in new capacity in a step by step fashion, in line with load growth. Large centralised plants tend "to be too big at the beginning and too small at the end," he argued.

According to a recent feasibility study on power options for a town of 85000 people with district heating in Finland, a distributed approach, consisting of five decentralised power plants (each equipped with an 18V50DF), with two more added later, turns out to achieve lower electricity production costs overall than building a single 100 MWe CCGT plant.

Among the advantages of the reciprocating gas engine over the gas turbine are higher electrical efficiency, with no deterioration in performance over time, higher efficiency at partial load, ability to use a wide range of fuels (HFO, LFO, gas, even orimulsion and perhaps biofuels), greater investment flexibility (ie the possibility of building a multi unit plant in a stepwise fashion), shorter delivery time, fast start-up (about two minutes from standby to full load), capability for virtually unlimited numbers of stop-starts and the ability to work with lower gas pressure (less than 4 bar, compared with 15 bar for a gas turbine). The reciprocating gas engine comes into its own when coping with extremes in ambient conditions, particularly competitive when its "hot, high and dry," said Jukka.

Dual fuel

The 50DF, producing 950 kW per cylinder, is a four stroke machine with a speed of 500 rpm (50Hz)/514 rpm (60Hz), available in configurations from 6L up to 18V. It can run either on natural gas or light fuel oil (LFO), and with certain modifications also on heavy fuel oil (HFO). Thermal (shaft) efficiency on gas is 47 per cent.

In gas mode the engine is operated on the lean-burn principle, with a very high air-fuel ratio, typically 2.2. Because this means that a given amount of heat generated by combustion heats up a larger mass of air, maximum temperatures and therefore NOx formation are reduced.

With customers these days insisting on reliability above just about everything, a deliberate policy has been to build on proven technology, so the main components of the 50DF have come from the W46 engine. The engine block is basically the same, the crank mechanism is the same. The only real differences are the bigger bore cylinder liners and pistons. The reason for the bigger bore is that the 46 is designed to work at high specific load, with a brake mean effective pressure (BMEP) of 26 bar. The 50DF, representative of the latest lean-burn technology, has a BMEP of 20 bar. Since this is lower than in a diesel engine, it was possible to increase the piston diameter without overstressing the engine structure. The same 20 bar BMEP is also the limit for diesel mode operation in the 50DF.

A particular feature of the 50DF is its ability to switch smoothly from gas to LFO and vice versa while operating. But here again, the aim has been to make maximum use of established technology, so the dual fuel concept is the same as that used successfully in the 32DF, and in fact a key element of the development approach has been to depart as little as possible from the 32.

When operating on gas, ignition is by pilot injection of fuel oil (with the pilot fuel accounting for less than 1 per cent of the total fuel). The pilot fuel, injected at 900 bar, is ignited in a conventional diesel process, providing a high energy ignition source for the main charge.

Each cylinder is individually electronically controlled to ensure the correct air-fuel ratio and the amount and timing of the pilot fuel injection. The engine is effectively constantly seeking the best operating point.

There is automatic and instant transfer from gas to diesel operation in alarm situations (eg gas supply interruption).

In diesel mode the engine operates as an ordinary diesel engine, with rapid transfer to gas operation at any load up to 80 per cent. When operating on diesel, the fuel is fed to a normal camshaft-driven injection pump and injected from a centrally mounted injector. The main diesel fuel injector is in the same body as the pilot injector.

The 50DF is always started in diesel mode using both main diesel and pilot fuel. At 300 rpm the main diesel injection is disabled and the engine transferred to gas mode. Gas admission is activated only when combustion is stable in all cylinders.

The 50DF engine has been extensively tested at Wärtsilä's facility in Trieste, with results well within or better than expectations. In view of the increased bore size particular attention has been focused on the state of the cylinders, which have been found to be in good condition, as have the big end bearings and the crankshaft. No cavitation has been experienced in the injection nozzles and no wear in the gas admission valves.

Gas Plant 50

One option under consideration for the 50DF is the use of three of them to form a modular 50 MW power plant, dubbed "Gas Plant 50". Overall net plant electrical efficiency would be 45 per cent net, while in CHP mode, efficiency would be up to 90 per cent. Emissions would be within TA-Luft, while the overall plant footprint is said by Wärtsilä to be relatively small relative to the competing options.

Construction should also be rapid, as the 50DF uses a prefabricated auxiliary module containing most of the engine related auxiliaries. In line with the overall philosophy of the 50DF, this module is essentially identical to that used for the 46 engine.

The first sale of the 50DF was a 6L version to Gaz de France for a marine application. But sales to the stationary power generation sector are expected in the near future.
Tables

Basic technical data for the Wärtsilä 50DF