Power plant owners operating heavy fuel oil burning engines are under pressure from authorities to adapt them to run on natural gas. When Turkish cement manufacturer Denizli Cimento’s diesel engine was due an overhaul, the choice to convert was obvious.
Denizli’s Wärtsilä 16V46 diesel engine had completed 53 000 hours of operation and a major overhaul was approaching. The plant was also on the wrong side of Turkey’s environmental authorities – placing the plant’s operating licence in jeopardy – while a surge in the price of heavy fuel oil (HFO) threatened the very commercial survivability of the plant.
The options faced last year by Denizli were to replace old for new, build a new power plant or convert the old engine to run on gas. Denizli selected the latter option, but by no means was it the easy route as a 16V46 diesel engine had never before been converted to dual fuel.
A lot of design work was required: the engine block had to be machined as the cylinder bore diameter increased. Pistons, liners, cylinder heads, connecting rods and fuel pumps were all replaced and gas components added.
“The end result is an electronically controlled dual fuel engine that is comparable in every way to a factory-produced dual fuel engine,” says Leif Enlund, service projects manager, Wärtsilä Services. Much of the engineering work focused on modifying the engine block. With a 460 mm cylinder diameter, the original 46 engine cylinder liners and cylinder heads had to be replaced and the engine block machined in order to accommodate the larger 500 mm cylinder components of the Wärtsilä 50DF type.
The engine block was machined in-situ to accomodate the larger 500 mm cylinder components
The machining was done in-situ – to avoid having to remove the engine block – by using a portable milling tool. Machining of one cylinder liner bore took one-and-a-half days in two shifts of 12 hours.
“It was a very time consuming project machining the engine block, but we gained experience along the way and the process quickened when we introduced a second milling machine to the project,” says Enlund.
Wärtsilä has three milling machines, but the third was in Germany being used in another 16V46 to 50DF conversion, for Cerestar, a Cargill company.
After the machining the work engineers – a team of 15 men working in two shifts – could then reassemble the engine with cylinder liners, pistons and connecting rods and cylinder heads and so on. All the parts were new 50DF production components.
Tailor-made gas piping
Focus was then given to the plant’s gas piping and electronic control systems. But due to the various modifications and improvements to the 46 engine type over the years, each engine was different. So when the team embarked on the conversion project it had to tailor the piping to fit the Denizli plant. This was indicative in the work undertaken to exchange the gas supply system, which needed to be completely re-routed. The gas component parts were also taken directly from dual fuel engine production parts.
Unlike Wärtsilä’s GD gas engine type, however, double wall piping was not a requirement for the Denizli conversion as the dual fuel engine operates with a gas pressure of only 5 bar. Neither was there a need for a gas compressor, although, depending on the location of the conversion project compressors may be required. In Pakistan, for instance, the dual fuel engine can be provided with a gas booster compressor as gas pressure in the supply line drops to just 1.5 bar during the winter months. Usually, however, operators just turn the engine over to run on diesel fuel oil when the pressure drops.
A number of safety precautions were introduced to the exhaust gas system to reduce the risk of gas pockets in the exhaust piping. The project team assembled relief valves into the exhaust line in case of ignition in the gas exhaust pipe.
Other safety measures included the introduction of a ventilation system, which purges the exhaust gas line with fresh air before start up. Only after this operation has been completed can the engine be started.
Control and automation systems were totally renewed and optimized for certain functions but at the core of the system is the main control module. This is responsible for ensuring the engine’s reliable operation and for keeping the engine at optimum performance in all operating conditions such as varying load, ambient temperature and gas quality.
The main control module automatically controls the start and stop sequences of the engine and the safety system. Each cylinder was also equipped with a sensor for controlling cylinder-specific air-fuel ratio by adjusting the gas admission individually for each cylinder.
Denzili cement factory in southwestern Turkey, site of the first dual fuel conversion of Wartsila’s 16V46 engine
After the conversion, engine verification tests were conducted over a ten-hour period with the plant operating at full-load. The conversion team followed the plant’s performance to provide full emission testing, fuel consumption trends and to set parameters for optimum performance.
Normal output for a 46 cylinder engine in a power plant application is about 975 kW per cylinder, but 25 kW per cylinder is, in theory, lost after conversion to dual fuel. In practice, as high energy consuming HFO pumps and heaters are removed during the conversion, the plant net electrical output may not be less than it was as an HFO engine.
Conversion a no-brainer
“Replacing the Denizli power plant with a new one would have required demolition of part of the building to remove the old engine block and building a new power plant would have taken about a year. By converting the old 16V46 engine to a 16V50DF, the conversion work could be carried out on site, with the costs being 70 per cent of the price of a new engine and about 35 per cent of the cost of constructing a new plant,” says Leif Enlund.
“Additionally, the time required for the conversion process was 40 days installation, 10 days commissioning, and one day for performance tests. Taking all these factors into consideration, the decision was not a hard one to make.”
“In spite of some technical problems, cold working conditions and a tight schedule, the shared goal and a good team spirit contributed to the completion of the task on time, and Denizli Cimento now has a smoothly running power plant with improved operational efficiency. Both fuel costs and emission levels have fallen and operators at the power plant can look forward to at least another ten years of reliable, greener energy production,” he adds.
Enlund highlights a growing trend in conversions from HFO to gas or dual fuel engines due to the steep increase in crude oil prices.
Pistons, liners cylinder heads, connecting rods and fuel pumps all had to be replaced
Indeed, while engineers were carrying out the conversion work to the Denizli plant, Wärtsilä was also converting two heavy fuel oil burning 18V32 engines to the type 34SG gas engine for the Turkish textile manufacturer Arenko. Service engineers machined the engine block and renewed same components as for Denizli, from 320 mm to 340 mm.
“The experience and knowledge gained from the conversion projects in Turkey has also led to other projects where owners look to benefit from cheaper energy sources,” says Enlund. For example the first commercial conversion of 16V25 generating sets to duel fuel type is nearing completion following a year long project to modify five installations at one of Indonesia’s largest rubber plants.
The contract was awarded by Filamando Sakti, a subsidiary of the Gajah Tunggal group, in the summer of 2005 when Wärtsilä was already involved in a programme to convert the type 25 engine and construct a prototype at its engine test bed facility in Trollhättan, Sweden.
But the contract to convert five of eight 16V25 diesel generating sets to dual fuel at the Filamendo plant, an hours drive north of Jakarta, represents the first commercial contract for the company to convert this engine type.
The engines are now running on gas and supplying power to the manufacturer’s rubber processing facilities and to the grid. The rubber manufacturer approached the Finnish company to convert its diesel engines to dual fuel because of the surge in diesel fuel costs that hit Indonesia in 2005. The price of petrol rose 87.5 per cent a litre, diesel fuel a 105 per cent and kerosene, on which the poor depend for cooking, a huge 186 per cent.
Wärtsilä 25, Vasa 32, Vasa 32LN and now 46 engines can all be converted to dual fuel and are then able to operate on low pressure natural gas in an Otto-cycle, while keeping the possibility to be run with other fuels (light fuel oil and HFO) in a diesel cycle.
“Natural gas operation is becoming more attractive all around the world, and for many installations the only restriction for conversion to gas is the limited availability of gas. However gas distribution systems are continually being expanded and it is anticipated that the use of natural gas in many industries, including power generation will increase rapidly.” concludes Enlund.