Rolls-Royce has developed a new gas engine designed for the growing reciprocating engine market. The B-gas engine is the world’s most powerful spark-ignited gas engine, and the first unit – a 5 MW, 12-cylinder engine – has been operating in Denmark since October 2003.
Gero Di Piazza
Did you know that it takes five honey bees to do the work of two bumble bees? And did you know that you should never sell the bear skin before capturing the bear? These were the enlightened proverbs used at the Rolls Royce (RR) dinner speech in Denmark held to mark the official launch of the company’s latest reciprocating engine product – the B-gas engine.
The meaning behind the proverbs were light-heartedly linked to the new gas engine. Knut Simon Helland, vice president of engineering (and honey bee keeper) made comparisons between the insect and engine, emphasising the importance they each have for the environment, and their work rates.
The new B-gas series is RR’s most powerful medium speed reciprocating gas engine and is also the world’s most powerful spark-ignited gas engine. The design and development of the B-gas started and finished in Bergen, Norway, and the launch marks a major move by RR into the heavyweight division of the reciprocating engine market.
The reciprocating engines in the B-gas type come in 12, 16 and 20 cylinder configurations, with power ratings from 4500 to 8500 kW. Weighing around 55 t for the 12-cylinder engine, or 80 t for the
20-cylinder version, the B-gas is the largest engine – gas turbine or reciprocating – manufactured by RR. With an efficiency of 46 per cent, the engine also benefits from RR’s spark-ignited, lean-burn, pre-chamber technology, which provides a strong overall combination of high efficiency and low emissions.
Rolls-Royce’s first B-gas engine has been operating in Tørring, Denmark since October 2003
The B35:40-V12AB heavyweight ‘blue beauty’ was displayed in Tørring town, Denmark, 20 km west of the port of Horsens. The engine is the first B-gas unit to enter commercial operation, and it has provided heat and power to the town’s 3000-strong population since October this year. The Tørring Kraftvarmevaerk plant was previously equipped with a 3.6 MW K-gas engine, and its owners did not need too much convincing to allow installation of the new B-gas.
“We were fortunate to have a client like Tørring who decided to come on board during the development phase by ordering the first of this new engine type while it was still on the drawing board,” said Steinar Hansen, vice president of sales and marketing in Bergen.
“For the operator, the new power plant will mean more power and greater cost effectiveness than anything else of its type on the market. It is set to exploit the growing demand for larger output reciprocating power units that can operate profitably. It combines high-technology innovation with easily serviceable components.”
The 55 t engine in Tørring has 12 cylinders, stands at 4.6 m in height and produces 5.1 MWe at 750 r/min. It was assembled in early 2003, factory tested over the summer and delivered to the site in August. The old engine has been removed and a new foundation block built to accommodate the new engine.
Tørring Kraftvarmevaerk needed more power to meet growing demand without the expense of expanding its single-engine facility. They knew of Bergen’s work on concepts for a big new engine that would meet its needs. Today they have 20 per cent more demand from consumers compared with when the Tørring plant opened and are now also subject to stringent emissions requirements. Upgrading the older K-gas engine to meet these standards and the demand for output would have been impossible.
The heat produced is used as the base load for the Tørring community district-heating network. Tariff structure requirements mean that heat is stored in a huge accumulator tank during high tariff hours and the engine is stopped during low tariff hours. The plant is expected to operate for some 5000 h/year.
The engine’s emission output is manipulated to regulatory requirements by a mechanically operated valve combination, comprising admission and flow-valves, which is set into the inlet of each cylinder. This combination is set for low-pressure losses and can be adjusted individually for accurate balancing of the engine, based on readings from the cylinder indicating valves. This means lower gas consumption for the power produced and lower operating costs. Key factors in achieving higher efficiency are the engine’s advanced thermodynamic cycle and its optimized combustion system, advanced turbochargers with variable turbine geometry. Efficient mechanical design that runs at relatively low speed gives the operators the easy task for accurate performances and thus a combination of high efficiency and low emissions.
Everyone involved with the engine from the engineers to the executive directors from all the different branches around the world were brimming with pride of what they described was their ultimate machine. One engineer described the long-awaited launch as being like “a father with 14 daughters whose wife just gave birth to their first son”.
Work on the engine began in Norway in early 2002; it took 18 months to complete it. The original implementation was due two years earlier, explains Lutz Liebenberg, director of technology and development in Bergen, Norway: “We had earlier plans for the gas version of our large medium speed diesel engine [in 1999], however, when RR bought the industrial group to which we belonged, the development was postponed for nearly two years. The main reason for the postponement was the endeavour to harmonize the Bergen engine portfolio with that of Allen Diesels of Bedford [UK], who are under RR ownership. After several tests, it was decided to give Bergen the go-ahead.”
“[With regards to] emissions all manufacturers claim that they are fulfilling the actual standards [required by European environmental regulation],” said Hansen, adding: “However, with the B-gas engine we have met the Danish requirements with far better margins than we see even in the K-gas engine. The Danish requirement is the strictest in the EU today when it comes to unburned hydrocarbons, and will be a benchmark for reciprocating engines.
“On site at the Tørring power plant, the engine was achieving 46 per cent efficiency at full load, with the plant itself operating at 94 per cent total efficiency. These figures are impressive and could soon be seen as the target for competitors to top.”
There were no risks taken by RR in engineering and developing the B-gas engine. Liebenberg commented: “We took a conservative approach when it came to designing this engine. We wanted to use tried and tested equipment and we did this by using aspects of the B-diesel engine, which has proven strength in extreme environments.”
Does this now mean that the K-gas engine is going to be replaced? Unlikely, according to RR. The older K models are still very competitive, explains Hansen. There are existing installations around world in Italy, Spain, Denmark, Brazil, Bangladesh and India. At Tørring, the K-gas engine was taken out refurbished and shipped over to Rolls-Royce Energy systems India Pvt Limited with the intention to sell it on. This shows that other markets, such as Asia, where emission rules are not so stringent, will continue to be strong markets for models such as the K-gas. India’s 2003 Electricity Bill allows industries to set up captive power plants without the permission of the State Electricity Boards, and this will act as a boost for the reciprocating engine industry.
Lean burn gas
The B-gas design came from a combination of K-gas and B-diesel engine technology with additional technology developments. The new design areas included the engine block, where integrated fluid channels have replaced pipes. The crankshaft, cylinder heads, charge-air module and exhaust system are all tailored especially. The engine also features RR’s lean burn technology. A lean-burn gas engine is an ‘Otto-cycle’ engine with a mixture of compression and external ignition. By suitable development of a strong ignition source, the pre-chamber and the gas-air mixture in the cylinder can be leaned out, giving much improved engine performance. Once this process happens efficiency is increased, emissions are reduced, particularly NOx, and the specific power of the engine can be significantly increased because the knock limit is extended.
A lean-burn engine is designed to extend the misfiring limit to air excess ratio (a.e.r) of 1.8 and higher, which opens new possibilities for increasing power, efficiency and reducing NOx emissions. To achieve this, a special combustion system has been developed that gives a strong increase in ignition energy capable of firing such lean mixtures reliably. Also, a highly efficient turbo-charging system is used to take advantage of the possible power increase offered by the extended knock limit of lean mixtures.
Air is drawn in by the turbocharger, through the charge air cooler and into the cylinder. A timed mechanical gas valve injects gas under slight over-pressure into the inlet air stream to ensure a homogeneous and lean mixture of air and gas. Air pressure is controlled by the variable turbine geometry while gas flow is controlled by mechanical valves before each cylinder. The gas pressure is set electronically by the pressure-regulating valve on the fuel gas supply module ahead of the engine.
An air flap for each cylinder bank restricts the air supply during start-up and low load operation. As the pressure in the cylinder is low, gas is admitted into the small pre-chambers – one in each cylinder head – electronically controlled by the same pre-chamber pressure unit. During compression, the lean charge in the cylinder is partially pushed into the pre-chamber, where it mixes with the pure gas to form a rich mixture that is easily ignited by the spark plug. Fast and complete combustion of the main charge in the cylinder is ensured by powerful ignition from the pre-chamber.