Modern large reciprocating engines for power generation applications typically have gross electrical efficiencies of up to 47% for engine sizes bigger than about 3 MWe. They are, therefore, very efficient, and such engines are available in diesel, gas and dual- (or multi-) fuel variants in outputs up to around 6 MW more for large two-strokes, and 60 MW or more for multi-engines. For cogeneration applications, diesel and gas engines predominate, though dual-fuel engines are occasionally used because of their inherent fuel flexibility. But which type to use and why? Is reciprocating engine development keeping pace with legislative requirements and the need to cut down the use of increasingly expensive fossil fuels?
Reciprocating engines are not generally designed specifically for use in cogeneration. Cogeneration requires lots of heat in the exhaust, so a chosen engine (gas, diesel or dual-fuel), will simply be optimized for the application. This is comparatively easy to achieve by programming different electronic control parameters or through fuel/air system changes, so that a little thermal efficiency is sacrificed to obtain more exhaust heat. The engines themselves remain virtually identical to their simple-cycle brethren.
The diesel engine
The diesel engine is the most fuel-efficient reciprocating engine available. A good example is Tognum GmbH’s MTU 20 V 4000 engine. This uses only 192 gm/kWh of fuel, representing 44% thermal efficiency, excluding large cogeneration benefits. Indeed, efficiencies, in some cases, now approach 50%. High fuel efficiency is crucial because, at current diesel fuel prices, fuel costs can make up well over 80% of a plant’s life cycle costs, and every 1% reduced fuel consumption results in (typically) 0.8% lower life cycle cost. Diesel combined-cycle plant has, for example, been successfully used as an efficient solution for electricity production around the world.
Other benefits include operational flexibility, easy maintenance, extreme reliability, very long life, extended intervals between servicing and life cycle costs are still being reduced. Of increasing importance is the fact that diesels are reasonably fuel tolerant. The type is, therefore, ideal for a wide range of stationary power applications, particularly where there is no gas supply infrastructure.
Recent technical advances that have improved diesel engine fuel-efficiency and emissions include common-rail fuel injection equipment (FIE), electronically controlled jerk pumps and unit injectors, highly desirable variable injection timings, and re-designed combustion systems. Other advances have been made in turbochargers. Types include two-stage, high-pressure, sequential and variable turbocharging. Improvements have also been made in engine control, electronic management and diagnostic systems.
The latest diesel engines boast very low exhaust emissions compared with earlier engines. For example, Wärtsilä’s brand new and very powerful (23,000 kW) 20V46F engine is especially environmentally friendly, having NOx emissions down to 710 ppm NOx at 15% oxygen. This 18-cylinder engine also complies with ever more stringent World Bank environmental requirements. In another, rather different example, MAN B&W Diesel’s 12K80MC-S slow-speed two-stroke engine develops 43.9 MW at 109.1 r/min. Equipped with Selective Catalytic Reduction System (SCR), Electrostatic Precipitator (ESP) and Flue Gas Desulphurization (FGD) equipment, such very large engines are among the most fuel efficient and cleanest available, but require very large power plant buildings.
Wärtsilä’s new 20V46F electrical power engine being built on the company’s new modular production line
SCR, for exhaust after-treatment, can reduce NOx emissions by up to 80% (to <2 g/kWh), but excellent electronic engine management is necessary to make it work efficiently. Catalytic after-treatment equipment adds cost, however, so diesel power has been at a comparative disadvantage compared with lean burn gas engines, with their much cleaner exhaust, at least in many European countries, and it has become difficult to meet increasingly stringent emissions legislation. It is true that smoke has been almost eliminated and exhaust emissions greatly reduced, but NOx, diesel particulate matter (DPM) and CO2 greenhouse gas emissions are problems for diesel engine operators.
However, big strides are still being made in engine emissions, and longer-term solutions include successful electronic valve actuation (EVA), allowing cam timing independence, and alternative fuels for ultra-lean burn combustion. Hot components made from modern ceramics have potentially very large benefits (higher operating temperatures improve fuel efficiency), but ceramics often fail unpredictably and catastrophically more development work is needed.
Diesel engine research
Research is continuing. For example, a multinational team of over 40 European companies and research institutions, led by Wärtsilä and MAN Diesel, has recently successfully completed the major HERCULES co-operative research project (www.ip-hercules.com) into the technologies necessary to achieve higher-efficiency engines with ultra-low emissions. Though this work was carried out for the marine power sector, many of the benefits will be applicable to stationary power. HERCULES work should allow drastically reduced lower gaseous and particulate emissions, yet with better fuel efficiency and reliability. The project covered the following interrelated work:
- Extreme design parameters extreme pressure conditions were studied using advanced high BMEP (brake mean effective pressure) research engines that coped with severe mechanical and thermal loads. Examples included advanced working cycles (eg the Miller cycle), and examining fuel spray and combustion processes using lasers.
- Advanced combustion concepts 3D computer fluid dynamic (CFD) simulation tools optimized combustion systems. Fundamental investigative work on FIE at high pressures was carried out.
- Better turbocharging systems the potential benefits of variable-geometry turbocharger systems, as well as power take-in/take-out and multi-stage turbochargers, were investigated. New variable turbocharging concepts were developed for four-stroke and two-stroke engines.
- Turbocompounding this looked at the potential benefits of combined cycle systems. Various alternatives were simulated in computer models. The potential for improved combined efficiency is 3%5%.
- Emissions reduction (internal water) methods of using water inside engine cylinders to reduce NOx emissions were developed.
- Emissions reduction (internal exhaust gas) particulate matter emissions from two- and four-stroke diesel engines were characterized; the data obtained will allow more systematic investigation.
- Emissions (after-treatment) the after-treatment of engine exhaust gases was studied, in part using non-thermal plasma (NTP) techniques to demonstrate NOx reduction in two-stroke engine simulations.
- Reduced friction reducing internal engine friction losses through better lubrication and tribology will improve engine efficiency.
- Adaptive and intelligent engine this involved self-learning systems based on monitoring with reliable measuring equipment, together with engine mode changes based on manual or self-detected requirements.
A typical MAN Diesel product. The company has recently been involved in the major HERCULES co-operative research project
Engines for fuel flexibility
Fuel flexible engines able to burn sustainable fuels successfully will become increasingly important. Such machines can accommodate low cost fuels, or convert from one fuel to another often while running. Dual-fuel (DF) engines allow operators to switch to whichever is the cheapest or more convenient fuel gas or diesel fuels so operational flexibility is high. This transfer from gas to diesel mode can, with some engines, take place at any load, automatically and almost instantly. Like gas engines, such DF engines operate ‘lean-burn’ so that there is more air than needed for complete combustion. This increases efficiency and reduces NOx emissions.
Adjustments to Wärtsilä’s new 20V46F engine
Alternatively, some DF engines can burn a combination of natural gas and diesel fuel simultaneously, using a very small amount of diesel fuel as the ignition source (common rail ‘micropilot’). This provides combined diesel and gas engine benefits. Using multi-point, port-injected NG delivery valves, together with an electronic diesel injection system, diesel power and efficiencies can be achieved with significantly reduced base diesel emission levels for NOx and particulates. Unlike gas engines, no ignition equipment is needed, increasing reliability and availability, and possibly cutting costs.
Caterpillar’s DF engines are typical. These burn natural gas and diesel fuel together, the latter being the combustion pilot. Two independently controlled fuel systems communicate via a software interface based on the diesel fuel injection signal from an engine management system (ADEM). The engine control unit (ECU) takes ADEM’s desired diesel fuel injection signal and ‘asks’ for a specific quantity of diesel injection to be delivered at a precise time. The system simultaneously sends a pulse width modulated (PWM) signal to the NG gas injectors operating at rail pressure, metering gas delivery quantities. This signal varies according to manifold and charge air temperatures and pressures, while fuel mapping duplicates base diesel performance. Start-up takes place on 100% diesel fuel after warm-up, diesel substitution occurs. The cycle is effectively diesel, so the total fuel energy combustion event remains essentially unchanged.
Testing Wärtsilä’s new 20V46F engine. This 18-cylinder engine complies with difficult World Bank environmental requirements
Fairbanks Morse opposed piston (O-P) two-stroke DF engines use a broadly similar principle, and equipped with the company’s Enviro-Design technology, there is only a nominal 1% pilot fuel requirement. This equates to lower fuel costs and inherently low NOx emissions to levels, it is claimed, previously achievable only with lean-burn spark ignited gas engines.
Wärtsilä’s tri-fuel 32DF and 50DF engines are claimed to offer ‘the ultimate in fuel flexibility’, being able to run on natural gas, light fuel oil (LFO) or heavy fuel oil (HFO). Furthermore, they can switch smoothly from gas, via LFO to HFO and back during operation. In gas operation, a common-rail micropilot injection system allows stringent emission regulations to be met, impossible if a normal injection system were used. A second, conventional, injection system is used when the engine is run on LFO or HFO. Fuel flexibility and high efficiency are the main advantages. The thermal efficiency is 47%.
Showing the general arrangement of Wärtsilä’s new and very powerful 20V46F electrical power engine
DF engines can, therefore, minimize worries about volatile fuel prices and supply problems, as they can switch to another fuel depending on availability, pricing or environmental legislation. The cost of large diesel storage tanks can also be reduced. Some DF baseload power plants can run on heavy or light fuel oils, as well as crude oil, gas or emulsified fuels, as required. Some manufacturers offer a multifuel capability natural, digester and biogases, plus light, heavy and crude fuel oils, and emulsified fuels.
A number of engine manufacturers now produce fuel flexible engines. Among them are Fairbanks Morse, Caterpillar, MAN B&W, Kybota and Wärtsilä.
While oil and gas are still the main sources of energy, gas is the most CO2 friendly fossil fuel. The lean-burn gas reciprocating engine, available with similar power outputs to diesel engines, is ideal for making best use of natural gas. Such engines have been increasingly seen in Europe as being ideal for distributed power generation, which requires clean, reliable power for long, sometimes intermittent periods of operation, at lowest cost. Other applications include standby power for critical loads and cogeneration systems.
MAN B&W Diesel’s 12K80MC-S slow-speed two-stroke engine develops 43.9 MW at 109.1 r/min. Such very large engines can be among the most fuel efficient and cleanest available
The main drivers of gas engine development have been the demand for ever lower exhaust emissions, coupled with improving fuel efficiencies and reliability, while reducing maintenance costs. For an equivalent amount of heat, burning natural gas produces about 30% less CO2 than burning liquid fossil fuels, so gas engine emissions are lower than those of diesel engines. The lean (weak) mixture of gaseous fuel and air means that a typical 5MW gas engine will have NOx emissions of around 1g/kWh (or better) half that of diesel engines using Selective Catalytic Reduction (SCR). Adding a catalytic converter enables gas engines to operate even in cities. However, formaldehyde emission is now a concern; Danish authorities have been monitoring a gas engine powered CHP plant to establish future limits.
Gas turbines have been the obvious choice for cogeneration applications, but gas reciprocating engines provide better flexibility for frequent starting, stopping and load changes. In these modes, gas engine electrical efficiencies are clearly higher, and emissions are generally lower in relation to produced power. Fuel costs may be up to 30% lower than turbines, though actual economy depends on application, heat recovery, and fuel price. However, because modern gas engines are very fuel efficient, waste heat temperatures are somewhat low for a steam bottoming cycle in combined cycle operation. Even so, high plant efficiencies can be obtained.
The very large Selective Catalytic Reduction System (SCR) for use with MAN B&W Diesel’s 12K80MC-S slow-speed two-stroke engine
Although the gas engine may now be reaching the limits in terms of thermal efficiency improvements with 50% the probable limit development work is still being carried out. Manufacturers are developing advanced machines with the aim of achieving such thermal efficiencies with a 95% reduction in NOx emissions all with major maintenance cost reductions. This can only be achieved using advanced fuel-handling systems, more developed ignition and combustion systems, and advanced high temperature materials.
Individual ignition adjustments (by cylinder, by speed/load/emissions/energy requirement), and individual timings/cylinder are also significant areas of development. For example, adaptive load balancing enables wider operating fields for the engine, since the cylinders’ operating points can be adjusted automatically much closer to each other than with more conventional control systems. Automated load sharing between cylinders allows higher loads, compression ratios and efficiencies.
A Rolls-Royce Bergen gas engine. The company’s K-gas G4.2 gas engine is claimed to have highest electrical efficiency in its range
Further advances are likely to come about through reducing friction (special piston ring pack designs and better tribology / hydrodynamic design), and improving the Miller Cycle with early intake valve openings to increase effective compression ratio and, therefore, cycle efficiency.
In another development, MAN Diesel’s new 58 MW 32/40PGI power generation gas engine has ‘a completely new’, high-energy ignition system (PGI) that operates without spark plugs. It is claimed to combine the advantages of a diesel engine, such as high power density and high efficiency, with the low emissions of a gas engine. The first engine has been operating very efficiently in the company’s Augsburg base CHP plant since 2005. In the PGI process, a small quantity of ignition gas is injected into a pre-chamber, to be ignited on a hot surface. This initiates lean air-gas mixture ignition in the combustion chamber. The mixture contains a lot more air than normally needed, so, combined with the effective ignition, efficiencies approach those of state-of-the-art diesel engines, it is claimed. No elaborate after-treatment to reduce NOx is needed, and the 32/40PGI engine achieves efficiencies above 46%, with NOx emissions of less than 250 mg/Nm3 at 5% O2.
Rolls-Royce’s recently introduced K-gas G4.2 gas engine (12, 16 or 18 cylinders, 24253640 kWe = 200 kW of electrical power / cylinder) is claimed to have highest electrical efficiency in its range and is ideal for cogeneration applications. It offers a 5.5% fuel saving, equivalent to £85,000 / year for an 18-cylinder engine operating for 7000 hours / year compared with the previous G4 model. Improvements derive from a bore-cooled cylinder liner with higher compression ratio. A lower carbon-cutting ring gives reduced ‘dead space’ for the air/gas mixture around the piston’s top land, improving thermal efficiency and reducing unburned hydrocarbon (UHC) emission.
Another good example is Tognum’s MTU long-stroke 12 V 4000 L61 low emissions high-power-density gas engine. The exhaust heat is easily recovered for trigeneration applications, with high overall efficiencies to 84% or more.
Conclusions and biofuels
All forms of reciprocating engine for power generation (and other) applications, are still being developed. Although the maximum achievable fuel efficiencies are now probably being approached, there is still room for significant improvement in all performance variables, but exhaust emissions will become the main driver.
Electricity is getting more expensive, partly because of the ever-greater need to reduce power plant stack emissions, though high-quality low sulphur fossil fuels avoid catalyst poisoning and ‘bad eggs’ hydrogen sulphide (H2S) and sulphur dioxide (SO2) generation. While residual fuels commonly used in diesel engine power plants are competitively priced compared with distillate fuels and natural gas, unfortunately there is also an increasing fuel availability problem. This is partly because of the growing global demand, but also because worldwide peak oil production is already with us or soon will be so energy consumption is rising at the same time as the sources are diminishing. ‘Easy oil’ is over. Moreover, man-made climate change is now a recognized fact, and CO2 emissions are still rising because of fossil fuel dominance.
Therefore, development of alternative and renewable fuels will be essential. Biomass has great potential bio-oils and bio diesel can be produced, as can ethanol, methanol, DME (dimethyl ether), bio-methane and hydrogen. All of these are suitable, but palm oil and Jatropha seed are ideal sources for diesel engines and from them can be produced crude vegetable oil. However, certain biofuels grown as crops may not actually save much CO2 emission (and may even increase it), and land taken away from much-needed crops for food growth will become a significant difficulty. Recycled animal and frying fats, and recycled vegetable oils are better in this respect, but there will never be enough of it. Engines burning a range of biofuels effectively will, therefore, be at an advantage.
James Hunt is a UK-based writer on energy and electrotechnical issues.
The EVE research diesel engine
The Internal Combustion Engine Laboratory of the Helsinki University of Technology has built the EVE medium-speed large bore (200 mm) single-cylinder research engine. The EU and Wärtsilä are among the partners.
For research flexibility, this highly loaded engine features an electro-hydraulic valve actuating system all four valves (two inlet; two exhaust) can be opened and closed independently of each other and of crank angle (as far as avoidance of mechanical disaster allows). The complex interrelated timings possible allow cycles to be achieved that were difficult or impossible previously. Valve control is overseen using a high-speed control system developed at Institute of Hydraulics and Automation of Tampere University of Technology. The cylinder head is a modified Wärtsilä W20 unit, and the piston is a standard W20 unit.
Using EVE, research has been carried out into the following areas:
- Optical measurements of fuel spray and combustion
- Working cycles and fuel efficiency
- High power density
- Fuel injection
- Charging techniques
- Exhaust gas emissions
The target maximum cylinder pressure is 400 bar (50 bar indicated mean effective pressure) yet to be achieved because combustion chamber modifications will be needed. Even so, it has been successfully shown that the 200 mm bore medium-speed diesel engine can be run reliably with an electro-hydraulic valve mechanism. This is potentially very important for future diesel engine development, and indeed, the development of reciprocating engines using other fuels.